METODOLOGIA GPR PER LA SOLUZIONE DI PROBLEMI STRATIGRAFICI.Bollettino Geofisico Anno XXIII, Vol 1-2 Gen-Giu 2000, pp 5-16; Carrozzo M.T., Leucci G., Negri S., Nuzzo L., 2003. GPR SURVEY TO UNDERSTAND THE

STRATIGRAPHY AT THE ROMAN SHIPS ARCHAEOLOGICAL SITE (PISA,

(Puglia centro-meridionale), Mem. Società Geologica Italiana, 106, pp 449-460,

affioranti tra Galatone e S. Maria al Bagno (LE). Tesi di laurea inedita in geologia,

Salento leccese (Southern Italy): Problematic relationships, Bollettino della Società

considerations. In Ground Penetrating Radar in Sediments, Edited by C.S. Bristow and H. M. Jol, Geological Society Special Pubblication, London, 211, 127-142.

Yilmaz O 1987 *Seismic Data Processing* ed E B Neitzel (Tulsa, OK: Society of Exploration Geophysicists)

### **Orbital Control on Carbonate-Lignite Cycles in the Ptolemais Basin, Northern Greece – An Integrated Stratigraphic Approach**

M.E. Weber1, N. Tougiannidis1, W. Ricken1, C. Rolf2, I. Oikonomopoulos3 and P. Antoniadis3 *1University of Cologne 2Leibniz Institute for Applied Geosciences 3National Technical University of Athens 1,2Germany 3Greece*

#### **1. Introduction**

86 Stratigraphic Analysis of Layered Deposits

Geophysicists)

considerations. In Ground Penetrating Radar in Sediments, Edited by C.S. Bristow and H. M. Jol, Geological Society Special Pubblication, London, 211, 127-142. Yilmaz O 1987 *Seismic Data Processing* ed E B Neitzel (Tulsa, OK: Society of Exploration

> Establishing the time frame is crucial for most geoscientific investigation. Without proper time control, past geologic processes cannot be inferred appropriately, nor can their dynamics be understood adequately. Rhythmic changes in sedimentary cycles hold the key to establishing precise and high-resolution chronologies. The concept goes back to theoretical considerations first published by Milankovitch (1941). His calculations showed that changes in earth's orbital geometry lead to changes in the seasonal and latitudinal distribution of incoming solar radiation (insolation). Three main periods are responsible for these insolation changes, eccentricity (the shape of the orbit around the sun; with periods of 413 kyr, 123 kyr, and 95 kyr), obliquity (the tilt of the axis; changing at a period of 41 kyr), and precession (the wobbling spin of the axis with periods of 19 kyr and 23 kyr). He argued that these changes caused the waning and waxing of polar ice sheets. More than three decades later, Hays et al. (1976) and Imbrie et al. (1984) provided proof the cyclic changes of the earth energy budget were large enough to be preserved in marine sediment. Theoretical calculation of Berger (1976) and Berger and Loutre (1991) supported the Milankovitch theory and provided templates for orbital variability for the last couple of million yeas. Henceforth, cyclic changes in sediment strata were used to develop detailed orbital chronologies by assigning sedimentary cycles to orbital cycles.

> Magnetic polarity stratigraphy is a necessary and independent tool to retrieve chronometric information for time series covering millions of years. It is based on polarity changes of the Earth's magnetic field measured over the oceans and correlated to dated magnetic polarity reversals found on land (Heirtzler et al., 1968). The presently accepted geomagnetic polarity timescale (GPTS) is the one from Cande and Kent (1995). Langereis & Hilgen (1991) and Hilgen et al. (1995) provided the first astronomical age scale for the Pliocene Capo Rosello sections in Sicily based on the GPTS. Their work was further substantiated by Krijgsman et al. (1995). For the Late Neogene, the GPTS is rather precise thanks to accurate radiometric dating methods such as 40Ar/39Ar (Kuiper et al., 2004).

A first, low-resolution, orbital-based chronology for the continental Ptolemais Basin (Fig. 1) was introduced by Steenbrink et al. (1999) and van Vugt et al. (2001; 1998). They ascribed cyclic changes of carbonates and lignites to orbital variability: maxima in lake carbonate correlate to maxima in insolation (and minima in precession). Accordingly, lignite maxima correlate to minima in insolation (i.e., beige layers in Capo Rosello). Also, they carried out extensive radiometric dating using 40Ar/39Ar (e. g., Kuiper, 2003) and delivered a highquality composite stratigraphy for the Ptolemais Basin (Steenbrink et al., 2006). Also, Weber et al. (2010) published an orbital-based chronology for the two Upper Miocene lignite quarries Vegora and Lava. Therefore, there is robust and reliable stratigraphic information available we could base our investigations on.

Fig. 1. Stratigraphic positions for the Ptolemais and Komnina Formations, northern Greece (e. g., Steenbrink, 2001), and magnetic chrons (black and white pattern; ages according to Cande & Kent, 1995) for the Upper Miocene to the Upper Pliocene. Curves display orbital variations of eccentricity, obliquity, precession, and the incoming solar radiation (insolation; all parameters calculated according to Laskar et al., 2004).

So far, most stratigraphic information comes from rather short outcrops in quarries and interpretations mainly rely on composite records. We investigated, for the first time, a long and continuous section (drilling KAP-107), covering the entire Lower Pliocene in a single borehole. Our goal therefore was to collect multiple high-resolution and continuous paleoclimate proxy data to establish both the orbital chronology and reconstruct paleoclimate variability during the Lower Pliocene, a time of extensive lignite formation in the Ptolemais Basin. The coal formation is sandwiched between two important events: the Upper Miocene Messinian Salinity Crisis, a time of severe constriction in the Mediterranean realm with multiple evaporation events from 5.96 – 5.33 Ma (Krijgsman et al., 1999), and the Upper Pliocene onset of northern hemisphere glaciation.

#### **2. The Ptolemais basin**

88 Stratigraphic Analysis of Layered Deposits

A first, low-resolution, orbital-based chronology for the continental Ptolemais Basin (Fig. 1) was introduced by Steenbrink et al. (1999) and van Vugt et al. (2001; 1998). They ascribed cyclic changes of carbonates and lignites to orbital variability: maxima in lake carbonate correlate to maxima in insolation (and minima in precession). Accordingly, lignite maxima correlate to minima in insolation (i.e., beige layers in Capo Rosello). Also, they carried out extensive radiometric dating using 40Ar/39Ar (e. g., Kuiper, 2003) and delivered a highquality composite stratigraphy for the Ptolemais Basin (Steenbrink et al., 2006). Also, Weber et al. (2010) published an orbital-based chronology for the two Upper Miocene lignite quarries Vegora and Lava. Therefore, there is robust and reliable stratigraphic information

Fig. 1. Stratigraphic positions for the Ptolemais and Komnina Formations, northern Greece (e. g., Steenbrink, 2001), and magnetic chrons (black and white pattern; ages according to Cande & Kent, 1995) for the Upper Miocene to the Upper Pliocene. Curves display orbital variations of eccentricity, obliquity, precession, and the incoming solar radiation (insolation;

So far, most stratigraphic information comes from rather short outcrops in quarries and interpretations mainly rely on composite records. We investigated, for the first time, a long and continuous section (drilling KAP-107), covering the entire Lower Pliocene in a single borehole. Our goal therefore was to collect multiple high-resolution and continuous paleoclimate proxy data to establish both the orbital chronology and reconstruct paleoclimate variability during the Lower Pliocene, a time of extensive lignite formation in the Ptolemais Basin. The coal formation is sandwiched between two important events: the Upper Miocene Messinian Salinity Crisis, a time of severe constriction in the Mediterranean realm with multiple evaporation events from 5.96 – 5.33 Ma (Krijgsman et al., 1999), and the

all parameters calculated according to Laskar et al., 2004).

Upper Pliocene onset of northern hemisphere glaciation.

available we could base our investigations on.

The Ptolemais Basin (Fig. 2) is a SSE – NNW elongated intramontane Basin. Together with two flanking mountain ranges, the Askion to the west and the Vermion to the east, it belongs to the Pelagonian Zone (Bornovas & Rondogianni-Tsiambaou, 1983). The Ptolemais Basin formed during the Late Neogene in northwestern Greece (Pavlides & Mountrakis, 1987) and contains lacustrine deposits of Upper Miocene to Quaternary age (up to 800 m thick; Anastopoulos & Koukouzas, 1972) with an extended Lower Pliocene alternation of lignites, clays, and marls. The depositional history reflects interaction of orbital forcing and tectonic movement (Steenbrink et al., 2006; Tougiannidis, 2009). Through continued Pleistocene extension, the Basin is further subdivided into the basins of Florina-Vevi, Amynteon-Vegora, Ptolemais, and Kozani-Servia (Antoniadis et al., 1994), where a number open pit mines and active coal mining fields are located (Tougiannidis, 2009). The deposits are highly fragmented due to fault tectonics.

Borehole KAP-107 was drilled for exploration purposes in 2006 by the Public Power Cooperation (PPC) in the Amynteon lignite field, 12 km northwest of the city of Ptolemaida (at 40º37'3" N and 21º37'20" E). It is 233 m long and has variable borehole diameters of 18 – 8 cm (top to bottom). We investigated the lowermost part between 72 m and 230 m in detail.

Fig. 2. 3-D location map of the Ptolemais Basin in northern Greece. The NNW – SSE elongated basin is flanked by the mountain ranges Askion in the west and Vermion in the east. Numbers indicate the four research sections referred to in this study: 1 – Achlada, 2 – Amynteon with borehole KAP-107, 3 – Vegora, and 4 – Lava. Data from sections Vegora and Lava are published in Weber et al. (2010). For digital elevation data see http://srtm.csi.cgiar.org.

#### **3. Analytical methods**

The methods of this study rely primarily on high-resolution non-destructive color measurements. As a fundamental sediment property color is often used for lithologic differentiation and to determine sedimentological structures, facies etc. We measured a total of approximately 16,500 samples non-destructively for color variability using a Minolta Chromatometer CM—2002. This hand-held system is easy to use in the field. There is hardly any maintenance and measurements can be conducted quickly and cost effective. Measurements were made on clean and fresh (scraped with a spatula) surfaces at 1-cm resolution, using the CIELAB color model "L\*-a\*-b\*". The system provides three color values for each measurement (details see Weber, 1998): the L\* axis (the black-white color component), also known as lightness or grey value; the a\*axis (the green/red component); and the b\*axis (the yellow/blue component). Together, the three parameters describe coordinates in a spherical system (16 million possible variations). Measurements of C\* describe the chroma (colorfulness). The difference between two successive color coordinates (ΔE\*ab) was calculated as ΔE\*ab = √(ΔL\*)2+ (Δa\*)2 + (Δb\*)2. Since ΔE\*ab contains the variability of all three color components, we refer to it as the whole color difference.

Paleomagnetic measurements were made on discrete samples in the lab. We determined natural remanent magnetization (NRM) and alternating-field demagnetization (AF) to generate the characteristic remanent magnetization (ChRM) (Tougiannidis, 2009). A total of 200 completely oriented diamagnetic sample cubes of 12 cm3 were retrieved at 80-cm intervals on average. NRM was measured in x and y direction using a cryogenic spin magnetometer (2G Enterprises). We applied AF before inclination and declination was measured and used orthogonal projections (Zijderveld, 1967) to correct the values for noise before using them for magnetostratigraphy.

We used the Analyseries software (Paillard, 1996) to perform astronomical tuning experiments and to construct age-depth models. Orbital parameters for the time frame 6 – 3 Ma were calculated in 1-kyr increments (see Fig. 1) for eccentricity, obliquity, precession, using the solutions provided by Laskar et al. (2004). Orbital insolation was calculated for the month of June at 40°N to reflect the approximate energy budget of the site.

We used ESALAB (Weber et al., 2010) to study of the resulting time series for frequency pattern and to compare them to orbital time series. ESALAB conducts both bulk and evolutionary spectral analysis (ESA). The program relies on the Lomb (1976) and Scargle (1982, 1989) algorithms, and provides an estimate of the spectrum by fitting harmonic sine and cosine components to the data set. This has two decisive advantages: the input data can be unequally spaced and the resulting spectra are rather robust and of high resolution. While performing the Fourier transformation, both window length and step size are freely adjustable. Also, the window type is selectable among Hanning, Haming, Blackman, sin2, and boxcar. The output consists of graphic files for bulk and ESA spectra and tabulated data. The sample increment of 1 cm puts the Nyquist frequency (the highest frequency detectable) at 0.5 cm-1, i.e., the spatial resolution for bulk and ESA is 2 cm.

#### **4. Ground-truth stratigraphy**

Ground-truth stratigraphy for borehole KAP-107 relies on paleomagnetic data (see Table 1). The site shows a flickering pattern of revers and normal polarity (Fig. 3). Inclination and

The methods of this study rely primarily on high-resolution non-destructive color measurements. As a fundamental sediment property color is often used for lithologic differentiation and to determine sedimentological structures, facies etc. We measured a total of approximately 16,500 samples non-destructively for color variability using a Minolta Chromatometer CM—2002. This hand-held system is easy to use in the field. There is hardly any maintenance and measurements can be conducted quickly and cost effective. Measurements were made on clean and fresh (scraped with a spatula) surfaces at 1-cm resolution, using the CIELAB color model "L\*-a\*-b\*". The system provides three color values for each measurement (details see Weber, 1998): the L\* axis (the black-white color component), also known as lightness or grey value; the a\*axis (the green/red component); and the b\*axis (the yellow/blue component). Together, the three parameters describe coordinates in a spherical system (16 million possible variations). Measurements of C\* describe the chroma (colorfulness). The difference between two successive color coordinates (ΔE\*ab) was calculated as ΔE\*ab = √(ΔL\*)2+ (Δa\*)2 + (Δb\*)2. Since ΔE\*ab contains the

variability of all three color components, we refer to it as the whole color difference.

Paleomagnetic measurements were made on discrete samples in the lab. We determined natural remanent magnetization (NRM) and alternating-field demagnetization (AF) to generate the characteristic remanent magnetization (ChRM) (Tougiannidis, 2009). A total of 200 completely oriented diamagnetic sample cubes of 12 cm3 were retrieved at 80-cm intervals on average. NRM was measured in x and y direction using a cryogenic spin magnetometer (2G Enterprises). We applied AF before inclination and declination was measured and used orthogonal projections (Zijderveld, 1967) to correct the values for noise

We used the Analyseries software (Paillard, 1996) to perform astronomical tuning experiments and to construct age-depth models. Orbital parameters for the time frame 6 – 3 Ma were calculated in 1-kyr increments (see Fig. 1) for eccentricity, obliquity, precession, using the solutions provided by Laskar et al. (2004). Orbital insolation was calculated for the

We used ESALAB (Weber et al., 2010) to study of the resulting time series for frequency pattern and to compare them to orbital time series. ESALAB conducts both bulk and evolutionary spectral analysis (ESA). The program relies on the Lomb (1976) and Scargle (1982, 1989) algorithms, and provides an estimate of the spectrum by fitting harmonic sine and cosine components to the data set. This has two decisive advantages: the input data can be unequally spaced and the resulting spectra are rather robust and of high resolution. While performing the Fourier transformation, both window length and step size are freely adjustable. Also, the window type is selectable among Hanning, Haming, Blackman, sin2, and boxcar. The output consists of graphic files for bulk and ESA spectra and tabulated data. The sample increment of 1 cm puts the Nyquist frequency (the highest frequency

Ground-truth stratigraphy for borehole KAP-107 relies on paleomagnetic data (see Table 1). The site shows a flickering pattern of revers and normal polarity (Fig. 3). Inclination and

month of June at 40°N to reflect the approximate energy budget of the site.

detectable) at 0.5 cm-1, i.e., the spatial resolution for bulk and ESA is 2 cm.

**3. Analytical methods**

before using them for magnetostratigraphy.

**4. Ground-truth stratigraphy**

declination values vary -4 – -67° and 359 – 6° for revers polarity, respectively, and 1 – 84° and 358° for normal polarity. Pronounced reversals occur at 141.1 m, 147.55 m, 165.14 m, 167.55 m, 172.88 m, 178.92 m, 191.24 m, 203.35 m, 215.1 m, and 220.93 m. Although the sample resolution of 80 cm (see above) is relatively low, the combination of inclination and declination changes provides a trustworthy reversal pattern that can be applied rather confidently. With the exception of 165.14 – 167.55 m, all remaining sections of normal polarity can confidently be assigned to chrons Gauss/Gilbert, Cochiti, Nunivak, Sidjufall, and Thvera (Fig. 3).

Fig. 3. Paleomagnetic ground-truth stratigraphy for borehole KAP-107. From left to right are magnetic susceptibility (MS), natural remanent magnetization (NRM), visual color, lithology, magnetic chron assignation, inclination, and declination. Age assignations are according to the age scale of Krijgsman et al. (1999). Black star represents the Neritina marker bed. This shell horizon belongs to the Theodoxus Member and has an age of roughly 4.4 Ma (orange star) according to 40Ar/39Ar dating of Steenbrink (2001). Grey star shows a glauconite horizon that has an approximate age of 5 Ma. Details see text.

Our age assignation is corroborated by additional sedimentologic and biostrati–graphic evidence. Unpublished records of the Federal Institute for Geosciences and Natural Resources (BGR, Hannover, Germany) from 1960 – 1970 mention a glauconite horizon just above the onset of sedimentation in the entire Ptolemais Basin, after the termination of the Messinian Salinity Crisis. We detected this layer at borehole KAP-107 in 228 m (Fig. 3), where the magnetic reversal pattern indicates the top of the Thvera chron (dated to 4.98 Ma). Also, we found the Neritina marker (*Theodoxus Macedonicus* WENZ, 1943) bed in roughly 174 – 178 m. This shell horizon belongs to the Theodoxus member and has an age of roughly 4.4 Ma (Steenbrink, 2001), which is in good agreement to 4.48 – 4.29 Ma deduced from our magnetic dating for this depth section.

Borehole KAP-107 was drilled to the depth where lignite deposition commenced in the Ptolemais Basin. According to our magnetic dating, the lowermost part of the core (underneath 222 m) belongs to the Thvera subchron and is hence somewhat older than 4.98 Ma. These stratigraphic results are inline with Steenbrink et al. (2006), who dated the onset of extensive lignite sedimentation approximately to the base of the Thvera subchron at 5.23 Ma. Hence there appears to be a lag time of approximately 100 kyr to the end of the Messinian Salinity Crisis that dates to 5.33 Ma (Krijgsman et al., 1999), and is marked by rapid re-flooding of the entire Mediterranean Basin (Garcia-Castellanos et al., 2009).

In summary, borehole KAP-107 represents continuous sedimentation during the entire Lower Pliocene. It covers a duration of 2 myr from 5.1 – 3.1 Ma. The resulting low-resolution age model comprises nine confident age control points (Table 1) from reversal dating and two additional evidences that support the stratigraphic assignation. Sedimentation rates are 14 – 2 cm/kyr, with an average rate of 6 cm/kyr, and a decreasing tendency towards the top of the borehole.

#### **5. Cyclic variability of high-resolution color data**

We measured a total of five sediment-optical data sets (L\*, a\*, b\*, C, ΔE\*ab). Most of them characterize facies changes and show striking cyclic variability (Figs. 4 and 5). The advantage of using a number of different sediment proxy records is that various aspects of changing sediment composition and supply and the relation to climate forcing can be addressed simultaneously.

Specifically sediment lightness shows a striking pattern of highs and lows with brighter (marl-rich) and darker (clay-rich or lignite-rich) intervals, and the coal seams as the darkest parts. Therefore, L\* is generally a good indicator for either calcium carbonate (high values) contents (see also Weber, 1998) or organic carbon and/or lignite (low values). The similarity to the whole color difference ΔE\*ab reveals that, for the most part, changes in L\* dominate color variability. Initial spectral analysis of L\* in the depth domain revealed sedimentary cycles ranging from 0.4 to 2.4 m, with a dominance centered around 1 – 1.4 m, and a tendency to become shorter with shallower core depths.

Color values a\*, b\*, and C\* show lower-amplitude cycles and a clear increase in values to the top (Fig. 4). Specifically the sections shallower than approximately 92 m indicate an elevated plateau. Higher b\* values reflect elevated contents of yellowish iron oxides (e. g., above 95 m core depth), most likely caused by the presence of hematite and goethite (see Weber et al., 2010). Low b\* values refer to bluish components, abundant in bitumen or coal (mostly the sections underneath 135 m), or provided by sulfides.

The red-green variation (color value a\*) provides an indication for the redox conditions, although the signal might be influenced by diagenetic overprint. Oxic conditions are indicated for core sections shallower than 95 m; the depths underneath show low-amplitude variations between slightly reduced and slightly oxic. The co-variability of color component C\* throughout the record reveals that a\* values primarily determine the chroma (Fig. 4).

Fig. 4. Non-destructive color data from borehole KAP-107. From left to right are paleomagnetic reversals, visual color, lithology, sediment lightness (L\*), red-green component (a\*), yellow-blue component (b\*), chroma (C\*), and whole color difference (∆E\*ab). Note that, only for graphical considerations, all high-resolution data have been smoothed using an 11-point Gaussian filter. Note further that L\* provides an estimate for either organic carbon or carbonate; a\* indicates the redox state, and b\* yields information about the iron oxide content.

#### **6. Astronomical tuning**

92 Stratigraphic Analysis of Layered Deposits

4.98 Ma). Also, we found the Neritina marker (*Theodoxus Macedonicus* WENZ, 1943) bed in roughly 174 – 178 m. This shell horizon belongs to the Theodoxus member and has an age of roughly 4.4 Ma (Steenbrink, 2001), which is in good agreement to 4.48 – 4.29 Ma deduced

Borehole KAP-107 was drilled to the depth where lignite deposition commenced in the Ptolemais Basin. According to our magnetic dating, the lowermost part of the core (underneath 222 m) belongs to the Thvera subchron and is hence somewhat older than 4.98 Ma. These stratigraphic results are inline with Steenbrink et al. (2006), who dated the onset of extensive lignite sedimentation approximately to the base of the Thvera subchron at 5.23 Ma. Hence there appears to be a lag time of approximately 100 kyr to the end of the Messinian Salinity Crisis that dates to 5.33 Ma (Krijgsman et al., 1999), and is marked by

In summary, borehole KAP-107 represents continuous sedimentation during the entire Lower Pliocene. It covers a duration of 2 myr from 5.1 – 3.1 Ma. The resulting low-resolution age model comprises nine confident age control points (Table 1) from reversal dating and two additional evidences that support the stratigraphic assignation. Sedimentation rates are 14 – 2 cm/kyr, with an average rate of 6 cm/kyr, and a decreasing tendency towards the top

We measured a total of five sediment-optical data sets (L\*, a\*, b\*, C, ΔE\*ab). Most of them characterize facies changes and show striking cyclic variability (Figs. 4 and 5). The advantage of using a number of different sediment proxy records is that various aspects of changing sediment composition and supply and the relation to climate forcing can be

Specifically sediment lightness shows a striking pattern of highs and lows with brighter (marl-rich) and darker (clay-rich or lignite-rich) intervals, and the coal seams as the darkest parts. Therefore, L\* is generally a good indicator for either calcium carbonate (high values) contents (see also Weber, 1998) or organic carbon and/or lignite (low values). The similarity to the whole color difference ΔE\*ab reveals that, for the most part, changes in L\* dominate color variability. Initial spectral analysis of L\* in the depth domain revealed sedimentary cycles ranging from 0.4 to 2.4 m, with a dominance centered around 1 – 1.4 m, and a

Color values a\*, b\*, and C\* show lower-amplitude cycles and a clear increase in values to the top (Fig. 4). Specifically the sections shallower than approximately 92 m indicate an elevated plateau. Higher b\* values reflect elevated contents of yellowish iron oxides (e. g., above 95 m core depth), most likely caused by the presence of hematite and goethite (see Weber et al., 2010). Low b\* values refer to bluish components, abundant in bitumen or coal (mostly the

The red-green variation (color value a\*) provides an indication for the redox conditions, although the signal might be influenced by diagenetic overprint. Oxic conditions are indicated for core sections shallower than 95 m; the depths underneath show low-amplitude variations between slightly reduced and slightly oxic. The co-variability of color component C\* throughout the record reveals that a\* values primarily determine the chroma (Fig. 4).

rapid re-flooding of the entire Mediterranean Basin (Garcia-Castellanos et al., 2009).

from our magnetic dating for this depth section.

**5. Cyclic variability of high-resolution color data**

tendency to become shorter with shallower core depths.

sections underneath 135 m), or provided by sulfides.

of the borehole.

addressed simultaneously.

The process by which past variations in earth's orbit are correlated to the cyclic variability of sediment parameters is called astronomical or orbital tuning. In the Ptolemais Basin, we tuned the cyclic lignite marl alternations to orbital time series, using the algorithms of Laskar et al. (2004), and relying on the magnetic polarity time scale of Krijgsman et al. (1999) as ground-truth stratigraphy (Fig. 5).

Accordingly, we first generated a low-resolution age model for borehole KAP-107 by using the nine magnetostratigraphic fix points of Table 1 (see blue dots in Fig. 5) and converting core depths into ages linearly between tie points. As a tuning target, we used the insolation curve for the month of June at 40°N according to Laskar et al. (2004). Since magnetostratigraphy was only available for the lignite-bearing sections (135 – 223 m), we conducted the experiments only for the time interval 5.3 – 3.1 Ma.

Given the magnetostratigraphic boundaries and sedimentary cycles of 0.4 to 2.4 m with a dominance of 1 – 1.4 m (see above), calculated sedimentation rates vary from 2 to 12 cm/kyr with an average of 6 cm/kyr. This translates into a cycle length of roughly 1.2 m per precession (insolation) cycle, and 5 – 6 m per eccentricity cycle. Exactly these two frequencies are dominant in the L\* and ΔE\*ab records as lower and higher amplitude cycles, respectively (Fig. 5).

Fig. 5. Orbital tuning of borehole KAP-107. Sediment lightness (L\*) of borehole KAP-107 tuned versus orbital insolation From left to right are magnetic reversal pattern, visual color, lithology, sediment lightness L\* versus orbital eccentricity, and L\* versus orbital insolation at 40°N for the month of June (data from Laskar et al., 2004). Histograms to the right give age-depth relationship (top) and linear sedimentation rates (LSR) between tuned tie points (bottom). Note that carbonate maxima were tuned to insolation maxima (red dots) to concur with the procedure introduced by Steenbrink et al. (2006; 2000). Blue dots give ages of ground-truth stratigraphy.

Steenbrink et al. (2000) pointed out that dark-colored marls correspond to relatively dry periods, whereas light-colored marls represent more humid periods (lake-level highstands). Since humid climate in the Mediterranean occurred during insolation maxima (e. g., Emeis et al., 2000), we used the insolation curve for the month of June at 40°N according to Laskar et al. (2004) and tuned individual carbonate maxima between the magnetostratigraphic tie points to insolation maxima. Thereby, we increased the resolution of the age model to an additional 70 age control points without violating the ground-truth boundaries provided by Table 1. Weber et al. (2010) followed the strategy of Steenbrink et al. (2006; 2000) and tuned insolation minima to dark intervals. However, the KAP-107 record contains broad lows and

Fig. 5. Orbital tuning of borehole KAP-107. Sediment lightness (L\*) of borehole KAP-107 tuned versus orbital insolation From left to right are magnetic reversal pattern, visual color, lithology, sediment lightness L\* versus orbital eccentricity, and L\* versus orbital insolation at 40°N for the month of June (data from Laskar et al., 2004). Histograms to the right give age-depth relationship (top) and linear sedimentation rates (LSR) between tuned tie points (bottom). Note that carbonate maxima were tuned to insolation maxima (red dots) to concur with the procedure introduced by Steenbrink et al. (2006; 2000). Blue dots give ages of

Steenbrink et al. (2000) pointed out that dark-colored marls correspond to relatively dry periods, whereas light-colored marls represent more humid periods (lake-level highstands). Since humid climate in the Mediterranean occurred during insolation maxima (e. g., Emeis et al., 2000), we used the insolation curve for the month of June at 40°N according to Laskar et al. (2004) and tuned individual carbonate maxima between the magnetostratigraphic tie points to insolation maxima. Thereby, we increased the resolution of the age model to an additional 70 age control points without violating the ground-truth boundaries provided by Table 1. Weber et al. (2010) followed the strategy of Steenbrink et al. (2006; 2000) and tuned insolation minima to dark intervals. However, the KAP-107 record contains broad lows and

ground-truth stratigraphy.

Table 1. Age model for borehole KAP-107. Integer numbers refer to ground-truth stratigraphy from paleomagnetic reversal dating (age scale according to Cande & Kent, 1995; Krijgsman et al., 1999), from 40Ar/39Ar dating of the Neritina bed and a glauconite horizon (van Vugt et al., 1998). Remaining age control points result from tuning color compent L\* to orbital insolation (data from Laskar et al., 2004). Sedimentation rates are given in cm/kyr. Ga/Gilb refers to magnetic chrons Gauss/Gilbert. ITP stands for insolation tuning point.

sharp highs in the L\* record, so the tuning the other half of the insolation cycle, i. e., insolation maxima to bright intervals, seemed more appropriate and accurate. In any way, we obtained about the same resolution in the tuned age models as in previous studies and the age models are congruent. To establish the correlation between climate proxy variability and orbital parameter in the time domain was rather easy and straight forward; hence, the resulting stratigraphic model appears robust and reliable with tuned age control points at (ideally) every insolation maximum (every 20 kyr; see Table 1). The only uncertainties resulted from some missing core sections, where we had to interpolate the ages (see Fig. 5).

The resulting high-resolution age model comprises an additional 70 age control points and indicates a preservation period for KAP-107 of almost precisely 2 myr (5.12 – 3.09 Ma). Sedimentation rates were quite variable over this rather long period. Specifically the lower part of the Ptolemais Formation (for formation names see Fig. 7) shows elevated and variable values during the Kyrio member, followed by very low values during the Theodoxus member, intermediate values during the Notio member, and low to intermediate values during the Anargyri member (Fig. 5). Compared to Upper Miocene sections Lava and Vegora, borehole KAP-107 indicates that the Amynteon Basin received only 20 – 30 % of the sediment material during the Lower Pliocene.

#### **7. Spectral analysis**

Spectral analysis in the depth domain is ambiguous when changes in sedimentation rate occur, because the amplitude of the response will be reduced or even be absorbed. Borehole KAP-107 exhibits quite some variability in sedimentation rates; hence we transformed the depth series first into the time domain using the ground-truth age control points (see blue dots in Fig. 5). Only then, we conducted bulk and evolutionary spectral analyses to decide about the following tuning procedure.

In the spectra we mainly found eccentricity and precession cycles (Fig. 6). This is not surprising because precession contributes a substantial part to the insolation forcing in lowto-mid latitudes. Orbital eccentricity modulates the amplitude of the precession cycle (Imbrie et al., 1993) and is therefore also important for the Ptolemais Basin (Steenbrink et al., 2006), specifically in the lower sedimentation-rate sites (Tougiannidis, 2009). Steenbrink et al. (2006) also found a robust obliquity signal in some parts of section Lava. However, obliquity has virtually no impact for borehole KAP-107, which is also not surprising since this frequency is related to the changing tilt of the earth and therefore mainly observable at higher latitudes (e. g., Weber et al., 2001), where it is mostly associated with the presence of larger ice sheets (e. g., Ruddiman, 2004).

Given the average sedimentation rates of 6 cm/kyr and the sampling increment of 1 cm, the average sample resolution is roughly 170 years. This resolution is enough to also be able to detect suborbital (millennial-to-centennial-scale) signals. However, the precession signal is so dominant that higher frequencies are not apparent in the time series, although rhythmic bedding in the cm to dm band can be observed in the outcrop. Either this type of bedding is autocyclic, i. e., it results from basin-internal processes that are not linked to orbital or solar forcing, or, more likely, post-sedimentary compaction operates different on the various facies types, thereby altering the depth-age relationship that has originally been established. Alternatively, different facies types may have been deposited at different rates. In any case, the age control points are only precession-controlled and hence cannot resolve these highfrequencies variations.

sharp highs in the L\* record, so the tuning the other half of the insolation cycle, i. e., insolation maxima to bright intervals, seemed more appropriate and accurate. In any way, we obtained about the same resolution in the tuned age models as in previous studies and the age models are congruent. To establish the correlation between climate proxy variability and orbital parameter in the time domain was rather easy and straight forward; hence, the resulting stratigraphic model appears robust and reliable with tuned age control points at (ideally) every insolation maximum (every 20 kyr; see Table 1). The only uncertainties resulted from some missing core sections, where we had to interpolate the ages (see Fig. 5). The resulting high-resolution age model comprises an additional 70 age control points and indicates a preservation period for KAP-107 of almost precisely 2 myr (5.12 – 3.09 Ma). Sedimentation rates were quite variable over this rather long period. Specifically the lower part of the Ptolemais Formation (for formation names see Fig. 7) shows elevated and variable values during the Kyrio member, followed by very low values during the Theodoxus member, intermediate values during the Notio member, and low to intermediate values during the Anargyri member (Fig. 5). Compared to Upper Miocene sections Lava and Vegora, borehole KAP-107 indicates that the Amynteon Basin received only 20 – 30 % of the

Spectral analysis in the depth domain is ambiguous when changes in sedimentation rate occur, because the amplitude of the response will be reduced or even be absorbed. Borehole KAP-107 exhibits quite some variability in sedimentation rates; hence we transformed the depth series first into the time domain using the ground-truth age control points (see blue dots in Fig. 5). Only then, we conducted bulk and evolutionary spectral analyses to decide

In the spectra we mainly found eccentricity and precession cycles (Fig. 6). This is not surprising because precession contributes a substantial part to the insolation forcing in lowto-mid latitudes. Orbital eccentricity modulates the amplitude of the precession cycle (Imbrie et al., 1993) and is therefore also important for the Ptolemais Basin (Steenbrink et al., 2006), specifically in the lower sedimentation-rate sites (Tougiannidis, 2009). Steenbrink et al. (2006) also found a robust obliquity signal in some parts of section Lava. However, obliquity has virtually no impact for borehole KAP-107, which is also not surprising since this frequency is related to the changing tilt of the earth and therefore mainly observable at higher latitudes (e. g., Weber et al., 2001), where it is mostly associated with the presence of

Given the average sedimentation rates of 6 cm/kyr and the sampling increment of 1 cm, the average sample resolution is roughly 170 years. This resolution is enough to also be able to detect suborbital (millennial-to-centennial-scale) signals. However, the precession signal is so dominant that higher frequencies are not apparent in the time series, although rhythmic bedding in the cm to dm band can be observed in the outcrop. Either this type of bedding is autocyclic, i. e., it results from basin-internal processes that are not linked to orbital or solar forcing, or, more likely, post-sedimentary compaction operates different on the various facies types, thereby altering the depth-age relationship that has originally been established. Alternatively, different facies types may have been deposited at different rates. In any case, the age control points are only precession-controlled and hence cannot resolve these high-

sediment material during the Lower Pliocene.

about the following tuning procedure.

larger ice sheets (e. g., Ruddiman, 2004).

frequencies variations.

**7. Spectral analysis**

Fig. 6. Spectral analysis of color component L\* for borehole KAP-107 for two time slices 4.2 – 3.7 Ma (left) and 3.6 – 3.35 Ma (right). Bulk spectra (top) were calculated using REDFIT software (Schulz & Mudelsee, 2002). Green line shows the 99 % confidence interval. Evolutionary spectral analyses (bottom) were calculated with ESALAB software (Weber et al., 2010) with a window size of 100 kyr and a shift of 5 kyr from one analysis to the next.

#### **8. Age model discussion and chronostatigraphic correlation**

Rhythmic bedding of sedimentary sequences reflects mostly the response to cyclic changes in earth's orbital geometry, namely eccentricity, obliquity, and precession. Changes in orbital parameters lead to changes in insolation with respect to season and latitude. For a given repetitive sedimentary succession we can use this relationship to establish a highresolution and precise chronology. This requires, however, other means of stratigraphic control such as biostratigraphic markers or the orientation of magnetic grains, which are locked immediately after the time of deposition. The magnetic reversal pattern, on the other hand, can precisely be dated and yields the foundation of the geomagnetic polarity time scale (GPTS, Cande & Kent, 1995). At least for the Late Neogene, the dating accuracy for magnetic reversals is sufficient to establish orbital chronologies (e. g., Hilgen et al., 1995).

Obtaining independent stratigraphic information is crucial because there might also be autocyclic processes (see above) that create rhythmic bedding. Here, single crystal 40Ar/39Ar dating using the laser fusion technique provided an essential step forward (e. g., Kuiper et al., 2004).

Once a low-resolution age control is established, spectral analysis can reveal whether or not rhythmic bedding is related to orbital frequencies. Because this was the case, we were able to "tune" sedimentary cycles to orbital frequencies. This rather simple and straightforward approach yields a very powerful stratigraphic tool – the resulting time series provide age models of previously unmatched resolution and precision.

We also followed this strategy and dated borehole KAP-107 using magnetic polarity changes and additional stratigraphic evidence. The resulting low-resolution age model indicated spectral power on the eccentricity band and thereby showed that orbital forcing is the likely cause for rhythmic bedding. We used this information to tune the greyscale (L\*) variability to orbital insolation. As a result, we obtained a precession-controlled age model over 2 myr of deposition in the Lower Pliocene, from 5.1 to 3.1 Ma. The age model provides, ideally, one control point every 1.2 m – enough for a very detailed reconstruction of the depositional history of any given environment.

In the next step, we applied the tuning method to a number of mining fields and outcrops from the Ptolemais Basin (see Fig. 7). Our chronology, again, is tied into the chronology of the composite record that has been established by Steenbrink et al. (1999) for the Ptolemais Basin. As a result, we obtained a complete sedimentary record from the Upper Miocene section Achlada (this study) and sections Lava and Vegora (Weber et al., 2010) to the Upper Pliocene (borehole KAP-107; this study), from roughly 7 to 3 Ma at precession-scale age control, with the exception of the Messinian Salinity Crisis from 5.9 to 5.33 Ma (Krijgsman et al., 1999).

Combined biomagnetostratigraphy and orbital tuning revealed that lacustrine sections Vegora and Lava from the central and southern Ptolemais Basin represent the period 6.85 – 6.57 Ma and 6.46 – 5.98 Ma at sedimentation rates of roughly 14 and 22 cm/kyr, respectively (Weber et al., 2010). Section Achlada from the northern edge of the Ptolemais, however, contains fluvial influence and shows less-convincing paleomagnetic evidence. According to Koukouzas et al. (2010; 2009) the Achlada deposits belong to the Komnina lignite sequence, which is why we tentatively correlated them to chron C3An.1n (Fig. 7).

Times of sediment deposition and lignite formation in the Ptolemais Basin were related to large-scale and global events. The Upper Miocene represented a global cooling (Billups, 2002; Billups et al., 2008) with lacustrine and fluvial input in the Ptolemais Basin (Steenbrink et al., 2006) and occasional lignite formation. The Messinian Salinity Crisis is characterized by at least partial desiccation in the Mediterranean realm (CIESM Workshop, 2007). Reworked alluvial to fluvial deposits dominated in the Ptolemais Basin. During the Lower Pliocene greenhouse warming accompanied the emerging Isthmus of Panama and enhanced oceanic overturning circulation (Brierley et al., 2009; Ravelo et al., 2004; Ravelo et al., 2006). Borehole KAP-107 covers this time of extensive lignite formation in the Ptolemais Basin.

During the Upper Pliocene continental ice sheets built up in the Northern Hemisphere between 3.6 and 2.4 Ma (Mudelsee & Raymo, 2005). A threshold towards full glacial to interglacial conditions occurred near 2.7 Ma (e. g., Ruggieri et al., 2009; Shackleton et al., 1984). This is broadly the time when lignite formation ceased in the Ptolemais Basin and coarser-grained, more oxic sediments were again deposited.

Once a low-resolution age control is established, spectral analysis can reveal whether or not rhythmic bedding is related to orbital frequencies. Because this was the case, we were able to "tune" sedimentary cycles to orbital frequencies. This rather simple and straightforward approach yields a very powerful stratigraphic tool – the resulting time series provide age

We also followed this strategy and dated borehole KAP-107 using magnetic polarity changes and additional stratigraphic evidence. The resulting low-resolution age model indicated spectral power on the eccentricity band and thereby showed that orbital forcing is the likely cause for rhythmic bedding. We used this information to tune the greyscale (L\*) variability to orbital insolation. As a result, we obtained a precession-controlled age model over 2 myr of deposition in the Lower Pliocene, from 5.1 to 3.1 Ma. The age model provides, ideally, one control point every 1.2 m – enough for a very detailed reconstruction of the

In the next step, we applied the tuning method to a number of mining fields and outcrops from the Ptolemais Basin (see Fig. 7). Our chronology, again, is tied into the chronology of the composite record that has been established by Steenbrink et al. (1999) for the Ptolemais Basin. As a result, we obtained a complete sedimentary record from the Upper Miocene section Achlada (this study) and sections Lava and Vegora (Weber et al., 2010) to the Upper Pliocene (borehole KAP-107; this study), from roughly 7 to 3 Ma at precession-scale age control, with the exception of the Messinian Salinity Crisis from 5.9 to 5.33 Ma (Krijgsman et

Combined biomagnetostratigraphy and orbital tuning revealed that lacustrine sections Vegora and Lava from the central and southern Ptolemais Basin represent the period 6.85 – 6.57 Ma and 6.46 – 5.98 Ma at sedimentation rates of roughly 14 and 22 cm/kyr, respectively (Weber et al., 2010). Section Achlada from the northern edge of the Ptolemais, however, contains fluvial influence and shows less-convincing paleomagnetic evidence. According to Koukouzas et al. (2010; 2009) the Achlada deposits belong to the Komnina lignite sequence, which is why we tentatively correlated them to chron C3An.1n (Fig. 7).

Times of sediment deposition and lignite formation in the Ptolemais Basin were related to large-scale and global events. The Upper Miocene represented a global cooling (Billups, 2002; Billups et al., 2008) with lacustrine and fluvial input in the Ptolemais Basin (Steenbrink et al., 2006) and occasional lignite formation. The Messinian Salinity Crisis is characterized by at least partial desiccation in the Mediterranean realm (CIESM Workshop, 2007). Reworked alluvial to fluvial deposits dominated in the Ptolemais Basin. During the Lower Pliocene greenhouse warming accompanied the emerging Isthmus of Panama and enhanced oceanic overturning circulation (Brierley et al., 2009; Ravelo et al., 2004; Ravelo et al., 2006). Borehole KAP-107 covers this time of extensive lignite formation in the Ptolemais Basin.

During the Upper Pliocene continental ice sheets built up in the Northern Hemisphere between 3.6 and 2.4 Ma (Mudelsee & Raymo, 2005). A threshold towards full glacial to interglacial conditions occurred near 2.7 Ma (e. g., Ruggieri et al., 2009; Shackleton et al., 1984). This is broadly the time when lignite formation ceased in the Ptolemais Basin and

coarser-grained, more oxic sediments were again deposited.

models of previously unmatched resolution and precision.

depositional history of any given environment.

al., 1999).

Fig. 7. Correlation of the four sites investigated in the Ptolemais Basin, spanning the time from roughly 7 Ma (chron C3Bn) to 3.3 Ma (Gauss/Gilbert boundary). From left to right are tuning targets (red – orbital eccentricity; black – orbital insolation at 40°N for the month of June (data from Laskar et al., 2004); stratigraphic classification according to Steenbrink et al. (1999), and Sites Amynteon (KAP-107), Vegora, Lava, and Achlada. For simplicity, only paleomagnetic reversal pattern is shown for each section. For detailed orbital tuning see Fig. 5 and Weber et al. (2010).

#### **9. Photospectrometry as stratigraphic and paleoclimate tool**

Initial attempts to identify rhythmic bedding and its relation to orbital forcing concentrated on counting individual beds manually and measuring their thicknesses (e. g., Hilgen et al., 1995). With the implementation of high-resolution rapid scanning techniques, photospectrometry has become very important (e. g., Weber, 1998). It provides a precise digital fingerprint of various geochemical and mineralogical properties that are related to climate and orbital variability. The data yield objective measurements and can be treated mathematically to analyze cyclic behavior and amplitudes of the climatic response. The use of color as highresolution stratigraphic and paleoenvironmental tool has become increasingly important in recent years (e. g., Debret et al., in press).

The foundation for the KAP-107 tuning is sediment lightness (L\*) or greyscale. Variations mimic rhythmic alternations of relatively bright marls and relatively dark clays or lignites. These changes indicate changes in humidity on precessional time scales. The Ptolemais Basin is a continental setting in low- to mid-latitudes. Hence, changes in orbital insolation as the ultimate driver of humidity changes have a strong precessional component. Sediments were deposited in shallow water (Kaouras, 1989; van de Weerd, 1983) of an intramontane lacustrine basin. According to the pollen studies of Kloosterboer-van Hoeve et al. (2006) dark-colored marls of the Ptolemais Basin, which are mostly enriched in clay and/or organic carbon, correspond to relatively dry periods, whereas light-colored marls represent more humid periods.

Changes in humidity are expressed by color component L\* because of the facies change from carbonate-rich (more humid) to clay- or lignite-rich (dryer). More arid conditions, on the other hand, exhibit increased redness (enhanced oxic conditions), whereas humid phases were likely more reduced. This relation is clearly shown in Fig. 8 (right), with separate populations for carbonate-rich, clay-rich, and lignite-rich sediment.

Fig. 8. Sediment color as high-resolution stratigraphic and paleoenvironmental tool. Left shows Q7/4 diagram (Debret et al., in press), i. e., the ratio of reflectance at 700 and 400 nm versus sediment lightness L\*. Note separate populations for carbonate-rich, iron-rich, organic-rich, and clay-rich strata. Right shows L\* versus a\*. Note separate populations for clay-rich, carbonate-rich, and lignite-rich strata.

Debret et al. (in press) introduced the so-called Q7/4 diagram, displaying the ratio of the reflectance between 700 nm and 400 nm versus sediment lightness L\* (Fig. 8, left). Here, lower ratios characterize clays and carbonates, whereas higher ratios indicate sediments rich in organic matter and iron. For the Ptolemais Basin this plot allows to distinguish separate populations for clay-rich, carbonate-rich, and lignite-rich strata with respect to the facies.

#### **10. Summary and conclusions**

This study presents a combined approach to establish geological time in sedimentary strata from magnetostratigrahy and orbital tuning. Polarity changes of the Earth's magnetic field provide the basis for the geomagnetic polarity timescale which is well dated for the Late Neogene using radiometric methods. Changes in earth's orbital geometry, on the other hand, cause changes in insolation, which can also be calculated rather precisely for the Late Neogene. Sedimentary successions are capable of archiving these cyclic changes as rhythmic bedding. We applied this concept of correlating sedimentary cycles to orbital cycles with the aid of magnetic polarity changes to Late Neogene strata from northern Greece.

Lower Pliocene borehole KAP-107 from the Ptolemais Basin shows alternations of marl-rich and clay- or lignite-rich strata. Using ground-truth stratigraphic markers and the magnetic reversal history, we established a low-resolution age model for the borehole spanning 5.1 – 3.1 Ma. Spectral analysis indicates that rhythmic bedding shows two distinct cycles, orbital eccentricity and precession. We used this information to tune the sedimentary record to orbital insolation. As a result, we obtained a high-resolution age model with, ideally, control points at every precession cycle (every 20 kyr). Through correlation of four research sections covering the Ptolemais Basin in a N – S transect, we obtained a complete sedimentary record from the Upper Miocene (7 Ma) to the Upper Pliocene (3 Ma), with the exception of the Messinian Salinity Crisis (5.96 – 5.33 Ma). Conducting the research with high-resolution photospetrometric scanning methods proved very helpful and allows for precise depth age assignations.

#### **11. Acknowledgments**

100 Stratigraphic Analysis of Layered Deposits

Basin is a continental setting in low- to mid-latitudes. Hence, changes in orbital insolation as the ultimate driver of humidity changes have a strong precessional component. Sediments were deposited in shallow water (Kaouras, 1989; van de Weerd, 1983) of an intramontane lacustrine basin. According to the pollen studies of Kloosterboer-van Hoeve et al. (2006) dark-colored marls of the Ptolemais Basin, which are mostly enriched in clay and/or organic carbon, correspond to relatively dry periods, whereas light-colored marls represent more

Changes in humidity are expressed by color component L\* because of the facies change from carbonate-rich (more humid) to clay- or lignite-rich (dryer). More arid conditions, on the other hand, exhibit increased redness (enhanced oxic conditions), whereas humid phases were likely more reduced. This relation is clearly shown in Fig. 8 (right), with separate

Fig. 8. Sediment color as high-resolution stratigraphic and paleoenvironmental tool. Left shows Q7/4 diagram (Debret et al., in press), i. e., the ratio of reflectance at 700 and 400 nm versus sediment lightness L\*. Note separate populations for carbonate-rich, iron-rich, organic-rich, and clay-rich strata. Right shows L\* versus a\*. Note separate populations for

Debret et al. (in press) introduced the so-called Q7/4 diagram, displaying the ratio of the reflectance between 700 nm and 400 nm versus sediment lightness L\* (Fig. 8, left). Here, lower ratios characterize clays and carbonates, whereas higher ratios indicate sediments rich in organic matter and iron. For the Ptolemais Basin this plot allows to distinguish separate populations for clay-rich, carbonate-rich, and lignite-rich strata with respect to the facies.

This study presents a combined approach to establish geological time in sedimentary strata from magnetostratigrahy and orbital tuning. Polarity changes of the Earth's magnetic field provide the basis for the geomagnetic polarity timescale which is well dated for the Late Neogene using radiometric methods. Changes in earth's orbital geometry, on the other

clay-rich, carbonate-rich, and lignite-rich strata.

**10. Summary and conclusions**

populations for carbonate-rich, clay-rich, and lignite-rich sediment.

humid periods.

We thank the Public Power Corporation (Ptolemaida) for granting permission to work in their quarries. Also, we wish to thank Greek colleagues Mr. Panagiotis Nikolakakos (Director-general) and Dipl.-Geol. Antonis Nikou (Drilling Section), as well as Daniela Sprenk and Andreas Holzapfel (both University of Cologne) for technical assistance. Supplementary data to this paper are available at doi:10.1594/PANGAEA.770293 (thanks to Hannes Grobe and Rainer Sieger).

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### **The Muhi Quarry: A Fossil-Lagerstätte from the Mid-Cretaceous (Albian-Cenomanian) of Hidalgo, Central México**

Victor Manuel Bravo Cuevas1, Katia A. González Rodríguez1, Rocío Baños Rodríguez2 and Citlalli Hernández Guerrero2

*1Area Académica de Biología, Instituto de Ciencias Básicas e Ingeniería, 2Licenciatura en Biología, Instituto de Ciencias Básicas e Ingeniería, Universidad Autónoma del Estado de Hidalgo,*

*México*

#### **1. Introduction**

The German word *lagerstätten* (singular, *lagerstätte*) describes sedimentary deposits with an important concentration of fossil material in a variable state of preservation. According to Seilacher (1970), there are two types of *lagerstätte*: Concentration deposits (*Konzentratlagerstätte*) and conservation deposits (*Konservat-lagerstätte*). Concentration deposits are originated by sedimentological and biological processes that promote the accumulation of a high density of fossil remains, particularly hard parts such as shells and bones. Conservation deposits are distinguished by uncommon depositional conditions (for example, rapid burial, anoxia, and/or hypersalinity) that favor exceptional preservation of an organism (Seilacher, 1970; Seilacher et al., 1985). Conservation deposits have received more consideration in the literature and in several instances are regarded as "snapshots" of a particular moment in the history of life (Benton & Harper, 2009; Nudds & Selden, 2008). Although concentration deposits are not mainly distinguished by a high-quality preservation of organic remains, its fossil richness provides information suitable to elaborate paleobiological inferences about an ancient community.

A few Mexican marine Cretaceous deposits are distinguished by important concentrations of fossil remains, usually in a high state of preservation. The localities which have been considered as *Fossil-lagerstätten* in México include: Vallecillo quarry, Nuevo León (northern México) of early Turonian age (Blanco-Piñón et. al. 2002); Múzquiz quarries, Coahuila (northern México) of late Turonian-early Coniacian age (Stinnesbeck et al., 2005); Xilitla quarries, San Luis Potosí (central México) of Turonian age (Blanco-Piñon et al*.*, 2006); and Tlayúa quarry, Puebla (central México) of Albian age (Alvarado-Ortega et al*.,* 2007). A sedimentary deposit with comparable characteristics was discovered about 13 years ago in northwestern sector of the state of Hidalgo, central México; the locality is formally known as Muhi Quarry. Paleontological work carried out in the site since its discovery, has allowed recovering an important sample of fossil remains, which represents a diverse marine community of mid-Cretaceous age. Because of the importance of the site, we discuss taphonomic evidence to consider the Muhi Quarry as a *Fossil-lagerstätte*.

### **2. Historical review**

The Muhi Quarry is geographically located at 20º49'21.5" N and 99º15'38.3" W, set in northwestern sector of the state of Hidalgo, central México (Figure 1). The quarry has been worked for more than 30 years for flagstone by the Yánez family inhabitants of the San Pedro town in Zimapán. Before the first visit by personal of the University of Hidalgo in 1998, Sergio and Ignacio Yánez did not understand the presence of fish in the stones. Once they were instructed about the importance of the discovery, they started paying attention on the specimens during the exploitation. Previously they used to sell them together with the slabs for building. At the same time, kids living around the place became hard collectors (Figure 2). The first specimen formally recovered from the quarry corresponds to a beautifully preserved pachyrhrizodontid head. Since that moment, more than 2,200 specimens have been collected including invertebrates and fish.

The real diversity of the quarry is still unknown since new taxa are discovered every year. Although the fauna is not completely described because of the newness of the discovery, it is remarkable the presence of new species and new records for America, including the shrimp *Aege*r *hidalguensis* (Feldman et al., 2007), which only Cretaceous sister species *Aeger libanensis* Roger, 1946, comes from the Cenomanian of Lebanon (Feldman et al., 2007); and the fishes *Ichthyotringa mexicana* Fielitz & González-Rodríguez, 2008, which sister species were also found in the Cenomanian Lebanese localities of Namoura, Hakel and Hajula; undescribed halecoid fishes previously known from some localities of Western Tethys; agonids earlier found in Cenozoic localities of Europe (González-Rodríguez & Schultze, 2010); and tetraodontiforms (González-Rodríguez et al., 2011).

Fig. 1. Index map showing the location of the Muhi Quarry at the Zimapán Area, northwestern sector of Hidalgo, central México.

#### **3. Litostratigraphy, sedimentary environment, and age**

The Muhi Quarry is a calcareous rock sequence belonging to the La Negra Facies of the El Doctor Formation, consisting of micritic limestones, bedded and/or nodular cherts, and laminae of barely consolidated mixture of siliciclastic clay and calcium carbonate. The sedimentary sequence shows a lithological variability indicative of an outer sea shelf setting, which received temporal pulses of pelagic waters, and occasionally influx of near-shore waters maybe during storms (Bravo-Cuevas et al., 2009). The age of the Muhi Quarry hinges on its stratigraphic relationships with other Mexican Mesozoic rock units and on the fish fauna that it bears; the deposit is considered of mid-Cretaceous (Albian-Cenomanian) age (Bravo-Cuevas et al., 2009; González-Rodríguez & Bravo-Cuevas, 2005).

Fig. 2. The Muhi Quarry at Zimapán Area, northwestern sector of Hidalgo, central México. A. Panoramic view of the quarry; B. Exploitation of the quarry for flagstone and gravel; C. The Yánez brothers, workers of the quarry; D. Prospecting and collecting of fossil material by kids of the Zimapán area.

#### **4. Material and methods**

108 Stratigraphic Analysis of Layered Deposits

The Muhi Quarry is geographically located at 20º49'21.5" N and 99º15'38.3" W, set in northwestern sector of the state of Hidalgo, central México (Figure 1). The quarry has been worked for more than 30 years for flagstone by the Yánez family inhabitants of the San Pedro town in Zimapán. Before the first visit by personal of the University of Hidalgo in 1998, Sergio and Ignacio Yánez did not understand the presence of fish in the stones. Once they were instructed about the importance of the discovery, they started paying attention on the specimens during the exploitation. Previously they used to sell them together with the slabs for building. At the same time, kids living around the place became hard collectors (Figure 2). The first specimen formally recovered from the quarry corresponds to a beautifully preserved pachyrhrizodontid head. Since that moment, more than 2,200

The real diversity of the quarry is still unknown since new taxa are discovered every year. Although the fauna is not completely described because of the newness of the discovery, it is remarkable the presence of new species and new records for America, including the shrimp *Aege*r *hidalguensis* (Feldman et al., 2007), which only Cretaceous sister species *Aeger libanensis* Roger, 1946, comes from the Cenomanian of Lebanon (Feldman et al., 2007); and the fishes *Ichthyotringa mexicana* Fielitz & González-Rodríguez, 2008, which sister species were also found in the Cenomanian Lebanese localities of Namoura, Hakel and Hajula; undescribed halecoid fishes previously known from some localities of Western Tethys; agonids earlier found in Cenozoic localities of Europe (González-Rodríguez & Schultze,

specimens have been collected including invertebrates and fish.

2010); and tetraodontiforms (González-Rodríguez et al., 2011).

Fig. 1. Index map showing the location of the Muhi Quarry at the Zimapán Area,

The Muhi Quarry is a calcareous rock sequence belonging to the La Negra Facies of the El Doctor Formation, consisting of micritic limestones, bedded and/or nodular cherts, and laminae of barely consolidated mixture of siliciclastic clay and calcium carbonate. The

**3. Litostratigraphy, sedimentary environment, and age**

northwestern sector of Hidalgo, central México.

**2. Historical review**

In order to characterize the fossil assemblage of the Muhi Quarry, we used taphonomic indicators. The fossil assemblage was classified considering life habitat and state of preservation of the record, including autochthonous, parautochthonous or allochthonous concentrations; a mixed assemblage is referred to autochthonous-parautochthonous, parautochthonousallochthonous, or autochthonous-allochthonous (after Kidwell et al., 1986). In addition, we considered the "time averaging" involved in its formation (Kidwell & Bosence, 1991).

The taphonomic features evaluated comprise: (1) Anatomical completeness, (2) disarticulation, and (3) fragmentation. The deformation of the axial skeleton and closure/openness of jaws were evaluated in fishes. Anatomical completeness (1) refers to the percentage of bones preserved in an individual specimen (Brett & Baird, 1986). We considered three states, as follows: complete (100% of skeletal elements present); partially complete (at least 50% of skeletal elements present); and incomplete (less than 50% of skeletal elements present). Disarticulation (2) was evaluated in crinoids, echinoids, crustaceans, and fishes. This feature refers to the number of hard parts scattered and/or displaced from their original anatomical position (Hill, 1979). Degree of disarticulation was measured as: slightly disarticulated (less than 25% of skeletal elements scattered and/or displaced); moderately disarticulated (25%- 50% of skeletal elements scattered and/or displaced); and highly disarticulated (more than 50% of skeletal elements scattered and/or displaced). Thus, articulated specimens mean that all hard parts are in their original anatomical position. Fragmentation (3) refers to the breaking of the skeletal elements, caused by scavengers, predators and/or physical factors (for example, transport produced by currents or wind) (Benton & Harper, 2009; Speyer & Brett, 1988). Three states were recognized: low fragmentation (0-30% of skeletal elements broken), medium fragmentation (30%-70% of skeletal elements broken), and high fragmentation (70-100% of skeletal elements broken) (Brett & Baird, 1986).

According to Bienkowska (2004), deformation of fish axial skeleton includes: vertebral series straight, slightly arched, arched, or S-shaped; this condition is related to a post-mortem effect. Moreover, openness/closure of jaw was used as a relative indicator of level-oxygenation.

#### **5. Fossil concentration of the Muhi Quarry**

The fossil concentration of the Muhi Quarry represents a polytypic association defined by an accumulation of invertebrates, vertebrates, and trace fossils. The sample consists of about 2,200 specimens in a variable state of preservation. Invertebrates include ammonites, crustaceans, echinoids, and crinoids. Vertebrate groups comprise cartilaginous fishes and bony fish; remains belonging to an unidentified reptile are also present. Class-level distribution of this record is shown in Figure 3. There are numerous coprolites, probably produced by some fossil groups, such as fish, crustaceans, and/or cephalopods.

Fig. 3. Class-level distribution of fossil concentration in the Muhi Quarry, mid-Cretaceous (Albian-Cenomanian) of Hidalgo state, central México.

Crinoids are the most abundant group, including about 1,000 un-stalked forms of the Order Comatulida, which are comparable to the Upper Jurassic to Lower Cretaceous genus *Saccocoma* (Turek et al., 1989). Numerous disarticulated echinoid spines (about 200 specimens) are present. Crustaceans and ammonites are less abundant in comparison with other invertebrates of the quarry. Crustaceans include specimens belonging to Aegeridae and Palinuridae families (Feldmann et al., 2007), consisting of about 10 specimens. Ammonites include five poorly preserved specimens tentatively assigned to ?*Mortoniceras* sp. (Bravo-Cuevas et al., 2009; González-Rodríguez & Bravo-Cuevas, 2005).

Fishes are the second most abundant group, consisting of several complete specimens, and numerous skulls, fins, and vertebral series; the sample includes about 800 specimens in a variable state of preservation. The diversity recorded at the moment holds isolated shark teeth and vertebrae, two incomplete bodies of Rajiformes, and 15 Neopterygian families. Majority of the fish groups are under detailed taxonomic study, in addition to an unidentified reptile.

### **6. Taphonomy: A preliminary approach**

#### **6.1 Fishes**

110 Stratigraphic Analysis of Layered Deposits

preserved in an individual specimen (Brett & Baird, 1986). We considered three states, as follows: complete (100% of skeletal elements present); partially complete (at least 50% of skeletal elements present); and incomplete (less than 50% of skeletal elements present). Disarticulation (2) was evaluated in crinoids, echinoids, crustaceans, and fishes. This feature refers to the number of hard parts scattered and/or displaced from their original anatomical position (Hill, 1979). Degree of disarticulation was measured as: slightly disarticulated (less than 25% of skeletal elements scattered and/or displaced); moderately disarticulated (25%- 50% of skeletal elements scattered and/or displaced); and highly disarticulated (more than 50% of skeletal elements scattered and/or displaced). Thus, articulated specimens mean that all hard parts are in their original anatomical position. Fragmentation (3) refers to the breaking of the skeletal elements, caused by scavengers, predators and/or physical factors (for example, transport produced by currents or wind) (Benton & Harper, 2009; Speyer & Brett, 1988). Three states were recognized: low fragmentation (0-30% of skeletal elements broken), medium fragmentation (30%-70% of skeletal elements broken), and high fragmentation (70-100% of

According to Bienkowska (2004), deformation of fish axial skeleton includes: vertebral series straight, slightly arched, arched, or S-shaped; this condition is related to a post-mortem effect. Moreover, openness/closure of jaw was used as a relative indicator of level-oxygenation.

The fossil concentration of the Muhi Quarry represents a polytypic association defined by an accumulation of invertebrates, vertebrates, and trace fossils. The sample consists of about 2,200 specimens in a variable state of preservation. Invertebrates include ammonites, crustaceans, echinoids, and crinoids. Vertebrate groups comprise cartilaginous fishes and bony fish; remains belonging to an unidentified reptile are also present. Class-level distribution of this record is shown in Figure 3. There are numerous coprolites, probably

Fig. 3. Class-level distribution of fossil concentration in the Muhi Quarry, mid-Cretaceous

Crinoids are the most abundant group, including about 1,000 un-stalked forms of the Order Comatulida, which are comparable to the Upper Jurassic to Lower Cretaceous genus *Saccocoma* (Turek et al., 1989). Numerous disarticulated echinoid spines (about 200 specimens) are present. Crustaceans and ammonites are less abundant in comparison with other

produced by some fossil groups, such as fish, crustaceans, and/or cephalopods.

skeletal elements broken) (Brett & Baird, 1986).

**5. Fossil concentration of the Muhi Quarry**

(Albian-Cenomanian) of Hidalgo state, central México.

The fish assemblage mainly consists of incomplete skeletons highly disarticulated, whereas partially complete skeletons show a variable degree of disarticulation; a low frequency of complete articulated skeletons or slightly disarticulated is observed (Figure 4A). This

Fig. 4. Frequency distribution of taphonomic features evaluated in bony fish. A) Taphonomic grades of anatomical completeness and disarticulation. B) Deformations of axial skeleton. C) Specimens with jaws opened or closed. Abbreviations: ART= articulated, SD= slightly disarticulated, MD= moderately disarticulated, HD= highly disarticulated, S= straight, SA=slightly arched, A= arched, SS= S-shaped.

pattern points to a slight degree of anatomical completeness and a high state of disarticulation, which suggests that the majority of fish carcasses experienced a full flotation related to water warm conditions (Elder, 1985; Elder & Smith, 1988). In full floating conditions fishes commonly lose dermal skull bones and/or the skull is completely disarticulated from the body (Wilson 1980, 1988a, 1988b); fish thanatocenosis of the Muhi Quarry exhibits this pattern. Moreover, the low degree of anatomical completeness observed, implies a relatively high post-mortem transport, turbulence, decay, and/or bioturbation (McGrew, 1975).

Scavenging frequently occurs in high or sufficient oxygenated waters (Elder, 1985). In particular, the scavenging effect on fish bones produces a pattern of disarticulation that is distinguished by skeletal parts scattered in all directions and without orientation (Smith & Elder, 1985). This condition is rarely observed in the fish skeletons recovered from Muhi Quarry, thus suggesting low or none scavenging activity at the site of deposit, which in turn is indicative of low levels of water oxygenation (Smith & Elder, 1985).

Majority of fish specimens do not show axial skeleton deformation, but some exhibit an arched or S-shaped body (Figure 4B). The bending of the vertebral column is explained as a natural post-mortem effect (Bienkowska, 2004; Jerzmanska, 1960); but, a straight condition of the vertebral column suggests a slow rate of decay which is delayed in conditions of high salt concentrations (Hecht, 1933) and /or low levels of water oxygenation (Schäfer, 1972). Based on these evidences, it is possible that changes in the salinity and oxygenation of the mass water produced the post-mortem effect pattern observed in the Muhi Quarry fishes.

Proportion of fish specimens with an opened and closed jaw is almost equal (Figure 4C, Figure 5H-G). Openness of the jaws indicates sudden death of the fish caused by asphyxiation, poisoning, heat shock or choking up of the gills produced by suspended particles in the water (Ciobanu, 1977, as cited in Bienkowska, 2004; Elder & Smith, 1988; Wilson, 1988b). This suggests that the cause of death of the Muhi Quarry fishes was diverse; it is probable that differential causes of death were related to environmental fluctuations of salinity and oxygenation at the site of deposit.

#### **6.2 Crinoids**

Several strata of the Muhi Quarry are covered by comatulids (Figure 6A); its anatomical completeness varies from complete to partially complete individuals, and the majority of the specimens are articulated (Figure 7, Figure 6B-D). Complete disarticulation of recent comatulids occurs relatively fast after a few days, unless they are rapidly buried or removed from the normal cycle of post-mortem disintegration (Blyth Cain, 1968; Liddell, 1975; Meyer, 1971). It is remarkable that majority of the comatulids have their arms coiled, indicating dehydration caused by hypersaline waters (Seilacher et al., 1985). Hence, we propose a sudden death promoted by hypersaline waters, accompanied by a rapid burial in the Muhi Quarry comatulids; but it is also probable that a low-level of oxygenation additionally promoted their death.

pattern points to a slight degree of anatomical completeness and a high state of disarticulation, which suggests that the majority of fish carcasses experienced a full flotation related to water warm conditions (Elder, 1985; Elder & Smith, 1988). In full floating conditions fishes commonly lose dermal skull bones and/or the skull is completely disarticulated from the body (Wilson 1980, 1988a, 1988b); fish thanatocenosis of the Muhi Quarry exhibits this pattern. Moreover, the low degree of anatomical completeness observed, implies a relatively high post-mortem transport, turbulence, decay, and/or

Scavenging frequently occurs in high or sufficient oxygenated waters (Elder, 1985). In particular, the scavenging effect on fish bones produces a pattern of disarticulation that is distinguished by skeletal parts scattered in all directions and without orientation (Smith & Elder, 1985). This condition is rarely observed in the fish skeletons recovered from Muhi Quarry, thus suggesting low or none scavenging activity at the site of deposit, which in turn is indicative of low levels of water oxygenation (Smith & Elder,

Majority of fish specimens do not show axial skeleton deformation, but some exhibit an arched or S-shaped body (Figure 4B). The bending of the vertebral column is explained as a natural post-mortem effect (Bienkowska, 2004; Jerzmanska, 1960); but, a straight condition of the vertebral column suggests a slow rate of decay which is delayed in conditions of high salt concentrations (Hecht, 1933) and /or low levels of water oxygenation (Schäfer, 1972). Based on these evidences, it is possible that changes in the salinity and oxygenation of the mass water produced the post-mortem effect pattern

Proportion of fish specimens with an opened and closed jaw is almost equal (Figure 4C, Figure 5H-G). Openness of the jaws indicates sudden death of the fish caused by asphyxiation, poisoning, heat shock or choking up of the gills produced by suspended particles in the water (Ciobanu, 1977, as cited in Bienkowska, 2004; Elder & Smith, 1988; Wilson, 1988b). This suggests that the cause of death of the Muhi Quarry fishes was diverse; it is probable that differential causes of death were related to environmental fluctuations of

Several strata of the Muhi Quarry are covered by comatulids (Figure 6A); its anatomical completeness varies from complete to partially complete individuals, and the majority of the specimens are articulated (Figure 7, Figure 6B-D). Complete disarticulation of recent comatulids occurs relatively fast after a few days, unless they are rapidly buried or removed from the normal cycle of post-mortem disintegration (Blyth Cain, 1968; Liddell, 1975; Meyer, 1971). It is remarkable that majority of the comatulids have their arms coiled, indicating dehydration caused by hypersaline waters (Seilacher et al., 1985). Hence, we propose a sudden death promoted by hypersaline waters, accompanied by a rapid burial in the Muhi Quarry comatulids; but it is also probable that a low-level of oxygenation additionally

bioturbation (McGrew, 1975).

observed in the Muhi Quarry fishes.

salinity and oxygenation at the site of deposit.

1985).

**6.2 Crinoids**

promoted their death.

Fig. 5. States of preservation in enchodontids, most abundant fish group from the Muhi Quarry (except D). (A-B) Well-preserved specimens. A) UAHMP-1059, complete and

articulated individual with vertebral series straight.  (C-F) Moderate to poorly preserved specimens. C) UAHMP-3166, slab showing a partially complete enchodontid specimen (lower large arrow) with the head bones disarticulated (small arrows), which can be attributed to scavenging activity; a juvenile indeterminate specimen is also preserved in the same slab (upper large arrow). D) UAHMP-2968, isolated vertebral series of unidentified specimen. E) UAHMP-2202, skull slightly disarticulated. F) UAHMP-2203, skull bones highly disarticulated. (G-H) Openness/Closure of jaws. G) UAHMP-2170, specimen with jaws closed. H) UAHMP-784, specimen with jaws opened. Scale bars equal 1 cm.

Fig. 6. States of preservation in the comatulids of the Muhi Quarry. A) Complete stratum covered by comatulids in a fine state of preservation (Scale bar equals 5 cm). (B-D) Wellpreserved specimens. B) UAHMP-2892, complete and articulated individual. C) UAHMP-3099, partially complete and articulated individual. D) UAHMP-3685, incomplete and slightly disarticulated individual. Scale bars in B-D equal 1 cm

Fig. 7. Frequency distribution of taphonomic features evaluated in crinoids, including anatomical completeness and disarticulation. Abbreviations are as in Figure 4.

#### **6.3 Other invertebrates**

114 Stratigraphic Analysis of Layered Deposits

articulated individual with vertebral series straight.  (C-F) Moderate to poorly preserved specimens. C) UAHMP-3166, slab showing a partially complete enchodontid specimen (lower large arrow) with the head bones disarticulated (small arrows), which can be attributed to scavenging activity; a juvenile indeterminate specimen is also preserved in the same slab (upper large arrow). D) UAHMP-2968, isolated vertebral series of unidentified specimen. E) UAHMP-2202, skull slightly disarticulated. F) UAHMP-2203, skull bones highly disarticulated. (G-H) Openness/Closure of jaws. G) UAHMP-2170, specimen with

jaws closed. H) UAHMP-784, specimen with jaws opened. Scale bars equal 1 cm.

Fig. 6. States of preservation in the comatulids of the Muhi Quarry. A) Complete stratum covered by comatulids in a fine state of preservation (Scale bar equals 5 cm). (B-D) Wellpreserved specimens. B) UAHMP-2892, complete and articulated individual. C) UAHMP-3099, partially complete and articulated individual. D) UAHMP-3685, incomplete and

slightly disarticulated individual. Scale bars in B-D equal 1 cm

Crustaceans belonging to Aegeridae are well preserved (Figure 8A-B); majority of the specimens are complete or partially complete individuals, a small proportion are disarticulated and/or broken (Figure 9). The presence of non-fragmented fragile skeletal elements (such as the lightly sclerotized exoskeleton of crustaceans) is indicative of a lowenergy environment (Brett & Baird, 1986). It has been shown that scavengers, bacteriological decay, and infaunal bioturbation rapidly destroyed fresh carcasses of decapods; therefore, they occasionally persist more than one to two weeks, whereas in a setting without any physical disturbance, disintegration occurs after one week (Plotnick, 1986). Hence, the presence of well-preserved crustaceans in the Muhi Quarry suggests a rapid burial, and non-physical damage of their carcasses. Only one specimen belonging to *Palinurus* is poorly preserved; a similar condition also is observed in ammonites referable to ?*Mortoniceras* sp. (Figure 8C-D).

On the other hand, echinoids are represented by numerous isolated spines (Figure 8E-F). This is a common condition in fossil echinoids preservation, due to spines are separated from the body almost immediately after their death; implying post-mortem transport. Echinoid spines are heavy enough for avoiding individual or collective transport over long distances; thus, they are usually found near or at the original habitat of the individual (Schäfer, 1972).

#### **6.4 General taphonomic consideration**

The preliminary taphonomic evaluation performed in the macrofossils of the Muhi Quarry, suggests that fluctuations of oxygenation-level and salinity occurred at the site of deposit, related to temporal lowering of dissolved oxygen and to a salinity increment in the mass water. Based on the depositional environment proposed by Bravo-Cuevas et al. (2009), it is probably that these stagnant conditions were controlled by alternated influx of neritic waters and open oceanic waters, further occasionally influx of coastal waters. Additional detailed taphonomic and geochemical studies will lead to precise and corroborate this contention.

Fig. 8. States of preservation in other groups of invertebrates from the Muhi Quarry. (A-B) Aegeridae well preserved specimens. A) UAHMP-711, complete and articulated individual. B) UAHMP-824, complete and slightly disarticulated individual. (C-D) Ammonoid specimens poorly preserved. C) UAHMP-1465, isolated fragment of shell. D) UAHMP- 2032, mold of an ammonite referable to ?*Mortoniceras* sp. (E) UAHMP-702, isolated and partially articulated and (F) UAHMP-741, partially articulated spines of echinoids. Scale bars equal 1 cm.

#### **7. Characterization and classification of the fossil assemblage**

The fossil assemblage from the Muhi Quarry is mainly composed of nektonic animals, such as bony fish, sharks, crustaceans belonging to *Aeger* (Clarkson, 1998), and ammonites referable to ?*Mortoniceras* sp. [this genus probably had a deep-nektonic mode of life (Reboulet et al., 2005)]; floating organisms like the comatulids are also present. It is reasonable to expect that several members of these groups may potentially live at or near the site of deposit; the occurrence of fishes in different ontogenetic stages (for example, enchodontids and berycoids) (Fielitz & González-Rodríguez, 2010; González-Rodríguez, & Fielitz, 2009) is consistent with this argument. Specimens from the Muhi Quarry show a variable degree of disarticulation, which is related to a differential movement of the specimen from its original source; nevertheless, it does not imply that they are out of its life habitat. Both considerations are related to a parautochthonous association (Kidwell et al., 1986; Martin, 1999). Additionally, it has been observed that carbonate sediments normally consist of the autochthonous or parautochthonous remains of organisms living at or near the site of the deposit (Maiklem, 1968).

Fig. 8. States of preservation in other groups of invertebrates from the Muhi Quarry. (A-B) Aegeridae well preserved specimens. A) UAHMP-711, complete and articulated individual. B) UAHMP-824, complete and slightly disarticulated individual. (C-D) Ammonoid specimens poorly preserved. C) UAHMP-1465, isolated fragment of shell. D) UAHMP- 2032, mold of an ammonite referable to ?*Mortoniceras* sp. (E) UAHMP-702, isolated and partially articulated and

The fossil assemblage from the Muhi Quarry is mainly composed of nektonic animals, such as bony fish, sharks, crustaceans belonging to *Aeger* (Clarkson, 1998), and ammonites referable to ?*Mortoniceras* sp. [this genus probably had a deep-nektonic mode of life (Reboulet et al., 2005)]; floating organisms like the comatulids are also present. It is reasonable to expect that several members of these groups may potentially live at or near the site of deposit; the occurrence of fishes in different ontogenetic stages (for example, enchodontids and berycoids) (Fielitz & González-Rodríguez, 2010; González-Rodríguez, & Fielitz, 2009) is consistent with this argument. Specimens from the Muhi Quarry show a variable degree of disarticulation, which is related to a differential movement of the specimen from its original source; nevertheless, it does not imply that they are out of its life habitat. Both considerations are related to a parautochthonous association (Kidwell et al., 1986; Martin, 1999). Additionally, it has been observed that carbonate sediments normally consist of the autochthonous or parautochthonous remains of organisms living at or near the

(F) UAHMP-741, partially articulated spines of echinoids. Scale bars equal 1 cm.

**7. Characterization and classification of the fossil assemblage**

site of the deposit (Maiklem, 1968).

Fig. 9. Frequency distribution of taphonomic features evaluated in crustaceans. A) Taphonomic grades of anatomical completeness and disarticulation. B) Taphonomic grades of fragmentation. Abbreviations are as in Figure 4.

The record includes fossil material belonging to lepisosteids and to the spiny lobster *Palinurus*. Extant lepisosteid species are primarily freshwater fishes that occasionally swim into coastal marine or brackish environments, whereas fossil species are known from freshwater deposits or are thought to have been temporary immigrants from nearby freshwater tributaries (Grande, 2010). On the other hand, *Palinurus* is an epibenthic form typical of rocky, muddy and/or sandy bottoms (Alvarez et al., 1996). Considering the site of deposit that represents the Muhi Quarry, both groups occur in a foreign life habitat; furthermore, specimens referable to these taxa exhibit a low degree of preservation. Thus, they would be considered as allochthonous forms. Given this, the association of the Muhi Quarry is representative of a mixed assemblage, integrated mainly by parautochthonous forms and in a lesser degree by autochthonous forms, which is referred as a parautochthonous-allochthonous association (following Kidwell et al., 1986).

The fossil assemblage from the Muhi Quarry shows an effect of time-averaging. The presence of skeletal elements at the site of deposit with a differential durability (teeth, bones, and chitinous exoskeletons), which persist it at or near the sediment surface for variable periods of time, evidences an accumulation of specimens that not necessarily were alive at the same time (Flessa, 2008). Taphonomic evidence may also indicate time-averaging; particularly, specimens belonging to enchodontids, the most abundant group of fishes in the quarry, show a mixture of states of preservation, indicating some degree of time-averaging. By the same token, variation in preservation among taxa is also indicative of time averaging; the co-occurrence of fish skeletons in a variable state of preservation, with delicate articulated chitinous exoskeletons belonging to crustaceans supports this argument. This variability in preservation within and among taxa leads to classified the fossil assemblage as a *within-habitat assemblage*, suggesting a time-averaging over time intervals ranging from years to thousand years (Flessa, 2008); specifically in marine fossil assemblages it would be expected at least centuries of extent of time-averaging (Kidwell & Bosence, 1991; Martin, 1999).

#### **8. The Muhi Quarry: A lagerstätte**

There exist various *Fossil-Lagerstätten* in México, characterized by diverse fauna of invertebrates and vertebrates, coming from different ages and several paleoenvironments: The Tlayúa quarry of Albian age is a locality frequently compared with Solnhofen because of the great diversity of fauna and the excellent state of preservation, including details of muscles, gills, digestive tracts, and stomach contents with fishes making up 70%–80% of the macrofossils deposited in a restricted basin with marine and freshwater influence (Alvarado-Ortega et al., 2007; Applegate et al., 2006); the El Chango and El Espinal quarries of Albian-Cenomanian age (Alvarado-Ortega et al., 2009) with plant remains, mollusks, crustaceans, insects and different fish taxa. Vega et al. (2006) suggested that these deposits were accumulated within a shallow lagoon or estuary with occasional freshwater influence; the Vallecillo quarry of early Turonian age (Blanco-Piñón et al., 2002) where the fauna consisting of invertebrates, sharks, neopterygians, and latimerioids was deposited on an open shelf, which is supported by the pelagic assemblage and the absence of submarine barriers in the region around Vallecillo, Nuevo León (Ifrim et al., 2005, 2010); the El Rosario quarries of late Turonian-early Coniacian age containing plant remains, invertebrates and vertebrates with anatomical details of soft tissues preserved and some specimens conserved in 3D, that were deposited in an open marine shelf environment, at least 100 km away from coast line (Stinnesbeck et al., 2005); and the Xilitla quarries of Turonian age containing invertebrates and fishes deposited in an open shelf environment with low energy influx (Blanco-Piñón et al., 2006). Although there are some similarities among supraspecific fish taxa within Muhi Quarry and other Late Cretaceous localities of México, at species level the Muhi fish are different, most of them representing new species. Moreover the paleoenvironment condition where the biota was deposited is dissimilar in most cases; although the proposed paleoenvironment for the Muhi quarry seems to be comparable to those of El Rosario and Xilitla quarries, the lithology, and paleobiota are quite different. We have not found inoceramids that are common in both localities and either plant remains.

The Muhi Quarry fauna has been previously compared with some Cenomanian fossil *lagerstätten* of the eastern Tethys such as Namoura, Hakel and Haula in Lebanon, Jebel Tselfat, Kem Kem beds and Daura in Morocco, Ein-Yabrud in Jerusalem, English Chalk in England, and Comen in Slovenia (Bravo-Cuevas et al., 2009; González-Rodríguez & Bravo-Cuevas, 2005), but there is a closer similarity with the lower Cenomanian fish fauna of Hakel and Haula in Lebanon which paleobiota also includes crustaceans and land plants. In these localities the fish beds consist of thin-bedded, siliceous limestone alternating irregularly with more massive limestone, nodules and lenses of impure chert occur occasionally throughout the beds (Patterson, 1967). The fish beds were deposited in small basins, 250 m across, which Hückel (1970) interpreted as sinkholes formed by tectonic activity on the contemporary seafloor, at the outer margin of the continental shelf (Forey et al., 2003). The main difference among the Muhi Quarry and Hakel and Hajula quarries is the presence of land plants and terrestrial fauna in the Lebanese localities. This condition does not occur in the Mexican outcrop, suggesting that the Muhi fauna was deposited offshore.

#### **9. Conclusion**

118 Stratigraphic Analysis of Layered Deposits

variation in preservation among taxa is also indicative of time averaging; the co-occurrence of fish skeletons in a variable state of preservation, with delicate articulated chitinous exoskeletons belonging to crustaceans supports this argument. This variability in preservation within and among taxa leads to classified the fossil assemblage as a *within-habitat assemblage*, suggesting a time-averaging over time intervals ranging from years to thousand years (Flessa, 2008); specifically in marine fossil assemblages it would be expected at least centuries of extent

There exist various *Fossil-Lagerstätten* in México, characterized by diverse fauna of invertebrates and vertebrates, coming from different ages and several paleoenvironments: The Tlayúa quarry of Albian age is a locality frequently compared with Solnhofen because of the great diversity of fauna and the excellent state of preservation, including details of muscles, gills, digestive tracts, and stomach contents with fishes making up 70%–80% of the macrofossils deposited in a restricted basin with marine and freshwater influence (Alvarado-Ortega et al., 2007; Applegate et al., 2006); the El Chango and El Espinal quarries of Albian-Cenomanian age (Alvarado-Ortega et al., 2009) with plant remains, mollusks, crustaceans, insects and different fish taxa. Vega et al. (2006) suggested that these deposits were accumulated within a shallow lagoon or estuary with occasional freshwater influence; the Vallecillo quarry of early Turonian age (Blanco-Piñón et al., 2002) where the fauna consisting of invertebrates, sharks, neopterygians, and latimerioids was deposited on an open shelf, which is supported by the pelagic assemblage and the absence of submarine barriers in the region around Vallecillo, Nuevo León (Ifrim et al., 2005, 2010); the El Rosario quarries of late Turonian-early Coniacian age containing plant remains, invertebrates and vertebrates with anatomical details of soft tissues preserved and some specimens conserved in 3D, that were deposited in an open marine shelf environment, at least 100 km away from coast line (Stinnesbeck et al., 2005); and the Xilitla quarries of Turonian age containing invertebrates and fishes deposited in an open shelf environment with low energy influx (Blanco-Piñón et al., 2006). Although there are some similarities among supraspecific fish taxa within Muhi Quarry and other Late Cretaceous localities of México, at species level the Muhi fish are different, most of them representing new species. Moreover the paleoenvironment condition where the biota was deposited is dissimilar in most cases; although the proposed paleoenvironment for the Muhi quarry seems to be comparable to those of El Rosario and Xilitla quarries, the lithology, and paleobiota are quite different. We have not found inoceramids that are common in both localities and either plant remains.

The Muhi Quarry fauna has been previously compared with some Cenomanian fossil *lagerstätten* of the eastern Tethys such as Namoura, Hakel and Haula in Lebanon, Jebel Tselfat, Kem Kem beds and Daura in Morocco, Ein-Yabrud in Jerusalem, English Chalk in England, and Comen in Slovenia (Bravo-Cuevas et al., 2009; González-Rodríguez & Bravo-Cuevas, 2005), but there is a closer similarity with the lower Cenomanian fish fauna of Hakel and Haula in Lebanon which paleobiota also includes crustaceans and land plants. In these localities the fish beds consist of thin-bedded, siliceous limestone alternating irregularly with more massive limestone, nodules and lenses of impure chert occur occasionally throughout the beds (Patterson, 1967). The fish beds were deposited in small basins, 250 m across, which Hückel (1970) interpreted as sinkholes formed by tectonic activity on the contemporary seafloor, at the outer margin of the continental shelf (Forey et

of time-averaging (Kidwell & Bosence, 1991; Martin, 1999).

**8. The Muhi Quarry: A lagerstätte**

The fossil concentration of Muhi Quarry represents a mixed assemblage integrated by a parautochthonous-allochthonous association; furthermore, it is evidenced that the fossil assemblage may be considered as a *within-habitat assemblage*. A preliminary taphonomic approach suggests that at site of deposit, occurred conditions of hypersalinity and/or lowlevel oxygenation, favoring an important accumulation of organic remains and its eventual differential preservation; further detailed taphonomic and geochemical studies will lead to precise and corroborate this argument. Nevertheless, stagnant conditions are commonly associated to the formation of a *Fossil-lagerstätte*. The Muhi Quarry is comparable in preservational and environmental conditions to other mid-Cretaceous Mexican quarries considered as *lagerstätten*, such as El Rosario and Xilitla, northern and central México, as well as to the Hakel and Hajula quarries of eastern Tethys, although the fish fauna seem to be endemic for the Atlantic Ocean and the Paleo- Caribbean province.

#### **10. Acknowledgment**

We thank to Sergio and Ignacio Yánez, workers of the Muhi quarry for collecting fossil material since 1999. Also we want thank the Biologists Jorge Alberto González and Jaime Priego Vargas for editing the figures of these report.

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### **Pliocene Mediterranean Foraminiferal Biostratigraphy: A Synthesis and Application to the Paleoenvironmental Evolution of Northwestern Italy**

Donata Violanti *Earth Science Department, Turin University, Italy*

#### **1. Introduction**

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In this chapter biostratigraphic concepts and methods are applied to the Pliocene foraminiferal assemblages of the central Piedmont (Northwestern Italy), with examples of correlations at regional and Mediterranean scale. The order of discussion of topics is as follows: foraminifers and biostratigraphy and ecobiostratigraphy methods, a review of previous definitions of the Pliocene series and its stages, the Mediterranean foraminiferal biozonation, brief description of the Northwestern Italy Pliocene succession and of Piedmont selected sections, correlations and applications to the paleoenvironmental and tectonic history of the area. Locations of the stratotype sections, of cited sites, and of the studied sections are reported in Fig. 1.

Foraminifers are one of the most widespread marine protozoans, inhabiting both the water colomn with planktonic forms and bottom sediments from the inner neritic zone to bathyal depths with very diversified taxa. Planktonic species, known from the Jurassic, are characterized by a very high evolutive rate and provide the biostratigraphic markers in pelagic, deep sea sediments of Upper Mesozoic and Cenozoic. Appearance or occurrence, as well as disappearance of a taxon represent biostratigraphic data.

Moreover, other data could be obtained by environmentally controlled parameters, such as frequency peaks or coiling changes of a taxon occurring nearly simoultaneously in the same area. In the Mediterranean Pliocene, ecobiostratigraphical data, recognized all over the region, are the delayied re-entry of some planktonic (*Globorotalia scitula*) and benthic species (*Cibicidoides robertsonianus, Siphonina reticulata*) during the lower Early Pliocene, slightly later than the basin infilling after the Mediterranean Salinity Crisis. Ecobiostratigraphy based correlations are obviously more dependent from environmental parameters than biostratigraphical events, but are useful to improve the stratigraphic resolution almost at the regional scale.

The peculiar geological history of the Mediterranean was at the origin of a distinct bioprovince, and justifies the necessity of a regional biostratigraphic zonation. During the Neogene, the closure of marine connections with the Indian Ocean, to the East, and the reduction of comunications with the Atlantic, to the West, led to the progressive decrease of exchanges with oceanic waters. From the Oligocene and Miocene, the latitudinal northward shift was the main cause of the disappearance of tropical macroforaminifers, whereas the stronger climatic deterioration during the Upper Pliocene and the Pleistocene affected also the warm or warm-temperate water planktonic and benthic foraminifers. Aims of this paper are to document the application of foraminiferal biostratigraphy and ecobiostratigraphy to the Pliocene succession of Piedmont, that provide new data on the evolution of the Northwestern Mediterranean margin and its relation with the central basin.

#### **2. Biostratigraphy and ecobiostratigraphy methods**

From the second half of the last century, planktonic foraminifers provide fundamental tools for biostratigraphy, the part of stratigraphy using the fossil distribution to construct relative time scales and to correlate time-equivalent sediment layers. From the Jurassic, and mainly from their explosive diffusion in the Cretaceous, many planktonic foraminiferal species have had the quality for being successfully applied as index fossils: a world-wide diffusion in the marine realm, good preservation in sediments due to their calcareous test, common to abundant specimens in marine deposits, fastly evolving taxa with many short-lived species, and various morphological features, allowing taxonomic identification of different species and subspecies. In the last decades, the integration of micropaleontological, geomagnetic and radiometric data with orbital periodicities provided an accurate calibration of biostratigraphic units and events.

The biozone, formal unit of biostratigraphy, is defined as the stratigraphic interval characterized by a typical fossil content, different from those of the adjacent rock bodies (Salvador, 1994). Primary data for the definition of biozones and biostratigraphic scales are bioevents, such as:

Fig. 1. Location of the Rossello Composite Section (Zanclean and Piacentian GSSP = Global Sections and Points), of cited sites and studied sections. 1: Ventimiglia, 2: Ceriale, Rio Torsero.

Neogene, the closure of marine connections with the Indian Ocean, to the East, and the reduction of comunications with the Atlantic, to the West, led to the progressive decrease of exchanges with oceanic waters. From the Oligocene and Miocene, the latitudinal northward shift was the main cause of the disappearance of tropical macroforaminifers, whereas the stronger climatic deterioration during the Upper Pliocene and the Pleistocene affected also the warm or warm-temperate water planktonic and benthic foraminifers. Aims of this paper are to document the application of foraminiferal biostratigraphy and ecobiostratigraphy to the Pliocene succession of Piedmont, that provide new data on the evolution of the

From the second half of the last century, planktonic foraminifers provide fundamental tools for biostratigraphy, the part of stratigraphy using the fossil distribution to construct relative time scales and to correlate time-equivalent sediment layers. From the Jurassic, and mainly from their explosive diffusion in the Cretaceous, many planktonic foraminiferal species have had the quality for being successfully applied as index fossils: a world-wide diffusion in the marine realm, good preservation in sediments due to their calcareous test, common to abundant specimens in marine deposits, fastly evolving taxa with many short-lived species, and various morphological features, allowing taxonomic identification of different species and subspecies. In the last decades, the integration of micropaleontological, geomagnetic and radiometric data with orbital periodicities provided an accurate calibration of

The biozone, formal unit of biostratigraphy, is defined as the stratigraphic interval characterized by a typical fossil content, different from those of the adjacent rock bodies (Salvador, 1994). Primary data for the definition of biozones and biostratigraphic scales are

Fig. 1. Location of the Rossello Composite Section (Zanclean and Piacentian GSSP = Global Sections and Points), of cited sites and studied sections. 1: Ventimiglia, 2: Ceriale, Rio Torsero.

Northwestern Mediterranean margin and its relation with the central basin.

**2. Biostratigraphy and ecobiostratigraphy methods**

biostratigraphic units and events.

bioevents, such as:

All these bioevents are applied to define biozone boundaries or as datum planes or datum levels for the definition of biohorizons. Among the different types of biozones used by paleontologists, one of the more frequently used by micropaleontologists is the Interval Zone, defined as the interval, or body of strata, between two specified bioevents (FAD, LAD, FO, etc.). Other biozones of common application are the two type of Range Zones: the Taxon-Range Zone or Total Range Zone, defined as the total range of stratigraphic and geographic occurrence of the selected marker, and the Concurrent Range Zone, defined as the body of strata including the overlapping parts of the distribution of two marker taxa. Abundance Zones, frequently called Acme or Peak Zones, are defined as the sedimentary interval in which an unusual abundance of a peculiar taxon is registered. The species abundance is strongly influenced by local environmental factors and can occur at different times in different sites. Acme zones are more properly ecobiostratigraphical zones, strongly dependent on environmental factors (water depth and temperature, trophic resources, etc.), that influence the species diffusion and abundance (Iaccarino, 1985). Lineage Zones are defined as the sedimentary interval yielding specimens of a peculiar (total or partial) segment of an evolutionary lineage. Lineage Zones are considered the most reliable units for biostratigraphic correlations and their boundary can approximate those of chronostratigraphic units. They require confident philetic relations between the selected taxa and therefore their application is limited to few well known lineages. For the Pliocene, an example is provided by the evolution of *Globorotalia margaritae*. The taxon is an index species for the Mediterranean Pliocene foraminiferal biozonation and evolved in the equatorial Atlantic during the Messinian (Late Miocene), probably from the ancestral forms *Globorotalia scitula* and *G. juanai* (Kennett & Srinivasan, 1983). The species, originally described from sediments of Venezuela of dubious Miocene age (Bolli & Bermudez, 1965), was later entirely ascribed to the Pliocene (Bolli, 1970). Cita (1973) documented the morphological evolution of *G. margaritae* in Mediterranean deep-sea Pliocene assemblages and recognized two new subspecies, *Globorotalia margaritae primitiva* and *Globorotalia margaritae evoluta*, besides the typical *Globorotalia margaritae margaritae*. On the basis of their stratigraphical distribution, the author defined two lineage zones, comprised with the younger *Sphaeroidinellopsis* acme zone in a super zone, named *Globorotalia margaritae* Total Range Zone, that covered the entire Lower Pliocene (in the twofold division in Lower and Upper Pliocene).

Cita (1973) zonal scheme was substituted by the Mediterranean Pliocene zones, (MPl1 to 6), numbered from bottom to top (Cita, 1975a). Nevertheless, the *G. margaritae* lineage zones remain successfully applied to Mid-Atlantic (Bolli & Saunders, 1985) and Southern Atlantic successions (Coimbra et al., 2009). Cita (1973) also hypothesized that the *Globorotalia margaritae* Total Range Zone would cover an undefined interval of the late Miocene. In the upper Messinian, the closure of communications with the Atlantic prevented the diffusion of the taxon within the Mediterranean. The hypothesis of a Miocene origin of *G. margaritae* was confirmed by following researches, dating its FAD at 6.2 Ma (Chaisson & Pearson, 1997) in the central Atlantic Ocean. A taxon related to the G*loborotalia margaritae* plexus is *Globorotalia praemargaritae*, described in Early Messinian sections of Sicily (Catalano & Sprovieri, 1969; 1971), recognized also in coeval interval of land sections in Spain (Berggren et al., 1976). The presence of this unkeeled species testified influxes of the *G. margaritae* lineage ancestrals in the Mediterranean, before the onset of the Messinian Salinity Crisis.

Ecobiostratigraphy applies to stratigraphy paleobiological signals derived or influenced by the ecosystem, so increasing the biostratigraphic resolution and allowing detailed correlations. The FO, FCO, LO and LCO, as well as fluctuations in abundance of a taxon, are diachronous in different paleobioprovinces or in distant areas of the same geographical province and are influenced by paleoenvironmental factors, such as temperature, changes in water-mass circulation, basin depth, etc. Also variations of ecophenotypical characters such as the coiling pattern are successfully applied as bioevents and for local or world-wide correlations. This is the case of the coiling change of *Neogloboquadrina acostaensis* from sinistral to dextral, registered worl-wide in the Late Miocene (Messinian) (Bolli & Saunders, 1985). In the Mediterranean successions it marks the upper boundary of the *Globorotalia conomiozea* Zone (pre-evaporitic Messinian) (Iaccarino, 1985). Other coiling shifts of *N. acostaensis,* recognized in the lowermost Pliocene (Di Stefano et al., 1996), provide tools for correlations in the Mediterranean area (Iaccarino et al., 1999a; Pierre et al., 2006). A high resolution sampling and detailed quantitative studies are needed to obtain a reliable ecobiostratigraphic record. The most important feature of ecobiostratigraphic events is that they occurred in the same stratigraphic order in different sites. Integrated micropaleontological, cyclostratigraphic, isotopic and paleomagnetostratigraphic analyses precisely calibrated many ecobiostratigraphic events occurring in the Pliocene, that will be discussed in the following paragraphs.

#### **3. The Pliocene chronostratigraphy**

It is beyond the scope of this work to propose a detailed review of former biostratigraphic and chronostratigraphic researches on the Pliocene and its subdivisions. Wide discussion and references can be found in Berggren et al. (1985), Cita (1975b), Gradstein et al. (2004), Iaccarino et al. (1985), Rio et al. (1991) and Vai (1997). Here the present state-of-the-art will be presented, with a synthesis of selected previous studies.