the geological and ( palaeo-)biological evolution of lowland amazonia during neogene and quaternary times remains fascinating since the early days of natural sciences ( for summaries of research history see e.g. , loczy , 1963 and wesselingh , 2008 ) .
many , but partly highly controversial models have been introduced to explain its historical development ( for recent compilations see e.g. , lundberg et  al . , 1998 ;
, 2006 ; wesselingh and salo , 2006 ; rossetti and mann de toledo , 2007 ; haffer , 2008 ; hoorn and wesselingh , 2010 ; hoorn et  al . , 2010 ;
generally , it is widely accepted that around the onset of the miocene ( 23  ma ) a mega - wetland (  pebas system  , also called  lake pebas  ) developed in western amazonia due to the subsiding subandean foreland ( e.g. , hoorn , 1994 , 2006 ; wesselingh et  al . , 2002 ; wesselingh and salo , 2006 ; shephard et  al . , 2010 ) .
short - lived marine incursions or even a transcontinental seaway from the caribbean sea through the venezuelan / columbian llanos basin southwards to the argentinean paran basin are proposed ( e.g. , rsnen et  al . , 1995 ;
2009 ) but heavily disputed ( e.g. , campbell et  al . , 2006 ; cozzuol , 2006 ; westaway , 2006 ; latrubesse et  al . , 2007 , 2010 ;
, 2003 ; hoorn and vonhof , 2006 ; wesselingh , 2006 ) . in the late miocene this mega - wetland disintegrated due to enhanced uplift of the northern / central andes .
the drainage pattern of northern south america started to reverse completely to today s easterly course and the modern  amazon system  became established during the early pliocene ( hoorn , 2006 ; figueiredo et  al .
, 2009 ; hoorn et  al . , 2010 ; latrubesse et  al . ,
2010 ) . beside the vast size of amazonia and the still fragmentary regional coverage with field surveys ,
there are considerable inconsistencies in palaeoenvironmental reconstructions and , in particular , in the chronology and correlation of scattered outcrops .
wesselingh ( 2008 , p. 5 ) stated :  the lack of geological data has led to the emergence of many grand theories about the origin of present - day amazon system and its highly diversity , often based on dubious interpretations of the little data available  .
the present paper aims to contribute basic sedimentological data from a barely studied region ( eirunep , 2.000  km sw manaus ; fig . 
1a ) , which is supposed to be placed at the south - eastern margin of the  pebas system  ( wesselingh and ramos , 2010 ) .
we demonstrate that there is no evidence for a long - lived lake ( sensu gorthner , 1994 ) or any marine influx .
conversely , we document a well - structured , aggrading fluvial system of late miocene age , which is in agreement with the sedimentation model and chronology proposed by latrubesse et  al .
the studied sections are located along the juru and tarauac river , ne respectively se of the city eirunep ( state of amazonia , western brazil ; 245  km  s of benjamin constant ; fig . 
delineations and subdivisions of basins in western amazonia diverge notably and several authors attribute the region of eirunep to the solimes basin ( eirunep subbasin ; e.g. , caputo , 1991 ; roddaz et  al . , 2005 ; ramos , 2006 ; wesselingh and salo , 2006 ; wesselingh et  al . ,
however , based on subsurface information , obtained by extensive hydrocarbon and coal exploration campaigns during the 1970 s , this region is situated west of a basement high ( miura , 1972 ;  iquitos arch  ; corresponds to the  carauari arch  of caputo , 1991 ) on the eastern margin of the acre basin ( e.g. , del arco et  al . , 1977 ; maia et  al . , 1977 ; latrubesse et  al . , 2010 ; fig . 
beside quaternary deposits ( terraces , alluvium ) , the scattered outcrops along river banks expose sediments of the upper parts of the solimes formation ( del arco et  al . , 1977 ;
maia et  al . , 1977 ; paz et  al . , in press ) . the solimes fm .
comprises more than 1000  m thick , largely pelitic  sandy alternations with lignitic intercalations and covers a huge part of western amazonia ( fig . 
. uncertainties in its definition , its stratigraphical and geographical extent as well as its depositional environments basically reflect the ongoing debate about amazonia s history through neogene times .
( 1977 ) , hoorn ( 1993 , 1994 ) , latrubesse et  al .
the sections were vertically logged by visual inspection of the lithofacies ( including colour , grain size , bedding planes , sedimentary structures , macrofossil content ) . for lithofacies coding the scheme of miall ( 1996 )
was used : capital letters  =  dominant grain size ( g , gravel ; s , sand ; f , fine sand  clay ) followed by a lowercase letter  =  sedimentary structures and/or biogenic features ( c , clast - supported ; m , massive / faint lamination ; h , horizontal bedding / lamination ; t and p , trough and planar cross bedding ; r , ripple bedding ; s , scour fill ; l , lamination ; r , rooted ; c , coal ; p , pedogenic overprint ) .
additionally the outcrops were mapped laterally as far as possible . due to poor outcrop conditions
an application of the  architectural element concept ( miall , 1996 ) was only loosely possible .
for micropalaeontological investigations bulk samples ( 12  kg ) were taken from all outcrops .
500  g of dried sediment ( 40  c , 24  h ) were washed by using diluted hydrogen superoxide for disintegration through standard sieves ( h2o2 : h2o  =  1 : 5 ; 63/125/250/500  m ) .
wet sieve residuals were washed with ethanol prior to drying ( 40  c , 24  h ) .
an in - depth taxonomic examination of the obtained microfossils ( including the sieve fractions < 250  m ) will be subject of further studies .
for preliminary stable isotope analyses ( o , c ; 18 samples ) two or three ostracod valves ( 4060  g ; cyprideis spp . ) from earlier sampling campaigns were used ( collected by m.i.r ) . for analyses
a thermo - finnigan kiel ii automated reaction system and a thermo - finnigan delta plus isotope - ratio mass spectrometer were used ( university of graz ; standard deviation  =  0.1 relative to nbs-19 ; results in per mille relative to vpdb ) .
more detailed investigations are in preparation ( m. fonseca / belm , m. caporaletti / graz ) .
location : 12.6  km ne eirunep ( s 063355.5/w 0694611.7  , altitude : 108  m ) , left cutbank of the juru river ( fig .  2 ) .
description : the basal 1.8  m ( layers 19 ) of the section consists of partly ripple bedded sands and laminated silts ; some cm - thick clay intercalations were observed . above
follows a 0.65  m thick alternation of clay with incorporated calcareous nodules and laminated silty sand or ripple bedded sand ( 1017 ; fig . 
the top is rich in vertebrate remains ( crocodiles , turtles ; 17 ; fig . 
massive to finely laminated , occasionally bivalves - bearing ( silty ) clay with coaly and sandy layers forms the next 1.14  m ( 1823 ; fig . 
beds 2425 ( 1.85  m thick ) comprise massive , horizontally or trough cross bedded sand and finely laminated or massive ( sandy  silty ) clay / fine sand ( fig . 
grey mottled , rooted and massive clay ( paleosol ) with a calcareous horizon on its top ( 26 ) .
up - section , a 4.2  m ( 2739 ) thick alternation of massive or laminated clay , silt and partly ripple bedded fine sand is developed ( a slight coarsening - upward trend is anticipated ) .
bed 32 displays a slightly erosive base with intraclasts in the lower part ( mud pebbles ) .
layers 3139 are cut by a channel ( 40 ) with a multiphase fill . large scale cross bedded sands ( -cross stratification ) with sporadic intraclasts ( mud pebbles ) on reactivation surfaces and trough cross bedded and ripple bedded fine 
microfossil contents : samples from the basal part of the section ( pd2 ) yielded only scarce valves of cyprideis spp . and rare fish elements .
interpretation : the lithofacies of the basal part of the outcrop ( 117 ) refers to a deposition in a fluvial overbank environment .
the diffuse layers of calcareous nodules ( 17 ) are supposed to be largely a diagenetic feature ( groundwater caliche ) .
however , it may also hint to soil - forming processes at seasonally dry floodplain areas ( retallack , 2001 ) .
up - section the sediments become finer and contain several ( sandy  silty ) coal - rich layers ( 1823 ) .
these beds are assigned to the formation of a floodplain pond , which was inhabited by opportunistic ostracods , freshwater bivalves and fishes .
however , crevasse splays influenced episodically that shallow lacustrine environment ( sandy and organic matter - rich layers ) .
later ( layers 2425 ) , the influx of crevasse splays into the pond , which is still populated by ostracod faunas and fishes increased ( more prominent sandy beds , partly scour - fills ) .
pedogenic processes are indicated by incipient soil formation in layer 26 ( mottled , roots ) .
floodplain pond conditions ended above ( beds 2739 ) due to enhanced clastic input from an approaching channel
. finally ( bed 40 ) , the overbank fines were intersected by a channel of at least 23  m in depth , which was filled up by large scale cross bedded sand .
more or less perpendicular to the main accretion surfaces oriented ripple bedding indicates deposition by lateral accretion of a point bar .
location : 17.9  km ne eirunep ( s 063251.1/w 0694239.4  , altitude : 107  m ) , left cutbank of the juru river ( fig .  4 ) .
description : the lower 5  m of the outcrop are formed by ( section 1 ) : ( 1 ) greenish  grey , mottled clay with ferran cutans and cm - sized carbonate nodules ( fig . 
5a ) ; ( 2 ) dark grey , massive or indistinctly laminated clay with cm - sized carbonate concretions ( fig . 
5b ) , thin , pyrite - rich coaly layers , plant fragments , root traces and vertebrate remains ( crocodiles , turtles ) ; ( 3 ) greenish  yellowish , mottled clay with ferran cutans ( passes gradually into bed 4 ) ; ( 4 ) violet - red  green - grey , mottled clay / paleosol ( fig . 
5c ) ; and ( 5 ) green  grey , mottled clay with vertebrate remains ( reptiles ) on top .
a trough cross bedded sandy layer with coaly interlayers in the se part of the site mentioned by paz et  al .
( in press ) was not exposed during our observation ( section 4 ) . in the nw part ( section 1 ) , above layer 5 , ripple bedded fine sand with intraclasts ( mud pebbles ) at its base is recorded , which contains bivalves and coalified wood remains ( 6 ) .
laminated sand alternations ( 7 ) and massive sandy clay , which is rich in molluscs in its lower and upper part ( 8) . up - section ( 9 ) , an alternation of clay and coal layers , rich in molluscs ( i.e. , mycetopoda sp . ) , as well as a thinly bedded succession of laminated sand and thin clay interlayers ( 10 ) follow . towards the se ( section 24 )
an up to 2  m thick , large scale cross bedded sandy unit is developed ( fig . 
occasionally , intraclasts and vertebrate fragments are found at the base of this element or on accretion surfaces , which dip around ne .
the dip of ripple foresets is approximately perpendicular oriented to these surfaces ( sw ) .
this unit is overlain by a layer of calcareous nodules ( 11 ; section 3 ) and laminated clay ( silt)coal alternations with bivalve and vertebrate remains ( 13 and 14  =  equivalent to 9 ) .
bed 12 ( plastic , strongly weathered clay ) is an equivalent of layer 10 .
microfossil contents : samples mn1 and mn2 lack microfossils , in mn8a and mn8b only scattered valves of cypria sp . and heterocypris ?
mn9 and mn 14 yielded many fish , gastropod and bivalve ( sphaeriids ) remains as well as abundant ostracod valves ( cyprideis spp . ) .
earlier reports on fossil material : celestino and ramos ( 2007 ; see also wesselingh and ramos , 2010 ) documented the following ostracod taxa : cyprideis graciosa , cyprideis lacrimata , cyprideis longispina , cyprideis machadoi , cyprideis pebasae , cyprideis olivencai , cyprideis sp . 1 and 2 .
further findings ( plants , fishes , reptiles ) are mentioned in del arco et  al .
the later authors also reported the occurrence of cyathecidites annulatus , echitricolporites spinosus and grimsdalea magnaclavata pollen .
greenish to yellowish and dark grey colouration might indicate more or less water - logged conditions , whereas reddish colours may hint to rather well - drained , oxidizing environments .
however , diagenetic alteration can hamper such an interpretation significantly ( retallack , 2001 ) .
layer 2 is less affected by pedogenic processes ( faint lamination , coaly layers , plant and reptile fragments ) and perhaps represents floodplain lake / backswamp deposits .
the paleosols ( overbank fines ; 15 ) are overlain by a sandy channel fill ( point bar ) with lag deposits at its base and repeatedly occurring intraclasts at its lateral accretion surfaces ( channel depth > 2  m based on the preserved height of this unit ) . in the nw part of morada nova layers
68 represent an abandoned channel fill , which could be the fill of a subordinate channel or the fill of a chute channel .
up - section , the influx of the active channel decreased and a floodplain lake developed ( 9 , 13 , 14 ) , which enabled the establishment of plentiful ostracod and aquatic molluscs faunas .
the pond itself or its surroundings were richly vegetated ( coaly layers ) and settled by semi - aquatic reptiles . finally , enhanced input of sandy matter points to an increasing influence of the channel , probably due to crevasse splays .
( s 063140.8/w 0693952.0  ; altitude : 106  m ) , left cutbank of the juru river ( fig . 
description : section 1 ( w part ) starts with a 1.6  m thick succession of laminated , ripple and cross bedded silty sand with coaly intercalations ( layers 16/1 ) .
up - section ( 714/1 ) , a 2.5  m thick sequence of ripple bedded silty fine sand layers with commonly erosive basal boundaries ( incl .
convolute bedding and m - thick concretions are frequent in its upper part ( 1114/1 ) .
silty clay ; 1520/1 ) and comprise coal and mollusc - rich pelitic ( 16/1 , 19/1 ) and coaly layers ( 18/1 ) as well as carbonate - cemented clay ( 20/1 ) .
layer 21/1 ( 0.75  m thick ) is composed of greenish  violet , mottled or poorly laminated clay with scattered bivalve shells .
intercalated are several layers ( 10  cm - thick ) , which consist of 12  mm large , rounded clay clasts ( fig . 
it is overlain by 1  m of massive clay with abundant mollusc ( e.g. , sheppardiconcha septencincta ; fig . 
leaves ; 22/1 ) and a > 0.5  m thick ( weathered ) coaly layer ( 23/1 ) with mollusc fragments ( e.g. , anodontites ?
section 2 ( e part ; 120  m downstream of section 2 ) starts with > 0.35  m thick , laminated fine sand with coaly interlayers ( layer 1/2 ) .
it is cut by a 4  m wide and 0.8  m deep channel fill ( 25/2 ; fining - upward , from base to top : laminated , coal - rich fine sand with rare mollusc and plant remains ; ripple bedded fine sand , rich in plant fragments ; massive clay , rich in bivalves ( sphaeriids ) , several wood remains ; fig . 
this channel fill is topped by a 0.5  m thick , massive  poorly laminated clay with thin fine sand interlayers ( 6/2 ) and 0.2  m of massive , partly indurated ( cemented ) clay ( 7/2 ) .
coalified wood remains are rare ; bivalves ( sphaeriids ) are abundant ( especially upper part of 6/2 ) .
layer 6/2 and 7/2 taper off towards the w. up - section ( 1  m thickness ) , follow massive , silty  sandy clays ( 89/2 ) and ripple bedded silty fine sand with silt interlayers ( 1014/2 ) .
especially layer 8/2 is rich in molluscs ( sphaeriids ) , 1314/2 contain coaly fragments ; clay intraclasts are observed at the base of 13/2 .
these beds are topped by 2.7  m of mottled silty fine sand with coaly fragments in the lower 0.3  m ( 15/2 ) .
. layer 16/2 ( > 1.2  m thick ) comprises coal  coaly fine sand  finely laminated clay alternations ( fig . 
7f ) and is rich in bivalves ( sphaeriids ) in the lowermost 57  cm .
whereas samples aq5/1 and aq6/1 contain only rare ostracod remains , aq16/1 yields , beside fish fragments , some valves of cytheridella sp . and darwinulids , accompanied by rare specimens of cypria sp . , cyprideis spp . and
, darwinulids , cypria sp . , ilyocypridids ) and fish bones . several ( aq22a/1 ) respectively rare ( aq22b/1 ) cyprideis spp .
valves are recorded in the samples of section 1 above , together with fish remains and gastropods ( aq22b/1 ) .
sample aq3/2 of section 2 delivered only rarely cyprideis spp . and some fish remains . in the samples aq 5 - 6 - 8 - 13/2 plentiful ostracod faunas ( cytheridella sp . , ilyocyprinids , darwinulids , cyprideis spp .
are documented along with fish remains and sphaeriid bivalve shells ( aq8/2 and aq13/2 ) . aq15/2 and aq16/2 are almost barren of ostracods but contain fish and bivalve ( sphaeriids ) fossils . earlier reports on fossil material : celestino and ramos ( 2007 ; see also wesselingh and ramos , 2010 ) recorded the following ostracod taxa : alicenula ( darwinula ) fragilis , cypria aqualica , c.  longispina , c.  pebasae , cyprideis sp . 3 , cytheridella purperae , heterocypris ?
( 2006b ) re - examined mollusc material studied by roxo ( 1937 ) from aquidab and described : ampullariidae sp .
sheppardiconcha septemcincta . additional findings ( including vertebrates and plants ) are mentioned by del arco et  al .
the basal , sandy  silty layers ( 16/1  =  1/2 ) possibly represent crevasse splay deposits , which are followed by a series of crevasse splay and/or crevasse channel sediments ( 714/1 respectively 25/2 ) .
alternatively , these layers could represent channel deposits of an avulsive river arm . in section 2 a subordinate abandoned channel fill ( floodplain pond ; 67/2 ) is developed .
afterwards ( 1523/1 and 816/2 ) the influx of crevassing respectively the active channel successively decreased and led to the formation of a floodplain lake , which was only influenced by flash floods in more proximal settings ( section 2 ; e.g. , 12/2 ) . aquatic faunas ( bivalves , ostracods ,
fishes ) and semi - aquatic crocodilians inhabited that lake , which was surrounded by a densely vegetated backswamp .
these beds display a mottled appearance and contain ( 21/1 ) layers of crumb peds ( clast - supported layers of rounded but not interlocked tiny mud balls ) , which indicate bioturbation due to invertebrate activity ( retallack , 2001 ) .
finally , the lake became replaced by a swampy environment ( 23/1 and 16/2 ) .
location : 27.4  km ne eirunep ( s 063122.0/w 0693542.8  ; altitude : 105  m ) , left cutbank of the juru river ( fig . 
description : at the base > 2  m of slightly southwards inclined ( < 10 ) rippled bedded silty fine sand ( layer 1 ) are followed by 2.7  m thick violet - brown ( 2 ) and yellowish ( 3 ) mottled paleosols with calcareous nodules ( between 2 and 3 a pedogenically overprinted clay plug is intercalated ) .
layers 13 are cut by a channel , whose fill starts ( 4 ) with > 1.5  m thick , massive  poorly laminated silty clay with gastropods , rare bivalves and plant ( leaves ) remains .
bed 4 grades into a 0.8  m thick , brownish - grey , blocky structured silty clay ( 5 ) with a calcareous crust on its top .
it is overlain by 2.4  m of violet - brown , mottled clay ( 6 ) with intercalated layers of calcareous nodules .
strata 13 as well as the channel fill ( 46 ) are topped by a 0.4  m thick alternation of laminated silty clay and laminated or rippled bedded fine 
medium sand with a high amount of plant detritus ( also wood remains ) and abundant vertebrates ( turtles ; 7 ) .
the badly exposed , 7.2  m thick beds up - section ( 8) consist of cross and ripple bedded fine sand with intercalations of clayey lenses ( 2  m lateral extension , 20  cm thickness ) , which are formed by clay intraclasts ( mud pebbles , 1  cm diameter ) .
repeatedly coaly layers are intercalated and rarely vertebrate ( crocodiles ) remains were found in the intraclast conglomerate .
layer 9 represents a 1.2  m thick alternation of horizontally or ripple bedded fine 
the topmost 1.2  m thick layer 10 consists of ripple bedded silty fine sand and silty clay alternations , which form cm - thick , fining - upward couplets .
microfossil contents : sample re4 yielded only a few valves of cyprideis spp . as well as some fish and gastropod remains .
interpretation : layer 1 could represent a crevasse splay or levee deposit , which is overlain by pedogenically altered floodplain fines ( 23 ) with intercalated minor channel fills .
these overbank deposits are cut by a channel ( ? 20  m wide and 5  m deep ) , which became filled by pelitic , afterwards pedogenically overprinted sediments .
the rare ( a matter of preservation ? ) mollusc and ostracod record indicates at least at the beginning ( 4 ) aquatic life in the abandoned channel . above , a crevasse splay ( 7 ) points to rising proximity of the active channel .
large wood remains and vertebrate fragments were accumulated by this splay . the sandy succession up - section (
8) can only be attributed to a sandy bedform sensu miall ( 1996 ) due to insufficient outcrop conditions .
intercalated shallow scours , filled up with mud balls indicate deposition of reworked material and variations of the flow regime .
the more or less rhythmically stratified , sandy - pelitic alternations at the top of remanso ( 910 ) mark a shift towards overbank environment and might represent a bar - top assemblage . location : 17.5  km se eirunep ( s 064923/w 0694704  , altitude : 117  m ) , left cutbank of the tarauac river ( fig . 
description : the basal 1.7  m ( layers 13 ) consist of massive or laminated pelite- and fine sand with rare bivalve remains .
they are topped by 0.2  m of reddish - brown , massive clay ( 4 ) .
the sandy  silty layers 59 display a general coarsening - upward trend and contain only rarely bivalve fragments .
up - section ( 10 ) , a 0.25  m thick intraclast conglomerate ( mudstone pebbles ) with abundant mollusc ( e.g. , ampullariidae ?
this stratum is overlain by thin sand and clay layers ( 1117 ) with abundant mollusc remains ( 12 , 14 , 16 , lower part of 17 ) .
layer 14 is rich in coal fragments ( wood , leaves ) at its top .
the 2.57  m thick succession above ( 1823 ) starts with thin , ripple bedded fine sand beds ( 1819 ) , followed by massive silty / sandy clay with coaly fragments ( 20 ) and cm - thick alternations of fine sandy silt and silty fine sand ( 21 ; forming couplets of small - scale fining - upward cycles ) .
beds 22 and 23 comprise alternations of massive , fine sandy silt and ripple bedded , silty fine sand .
24 consists of 10  cm - thick alternations of indistinctly ripple bedded silty fine sand and massive or poorly laminated pelite . the upper part ( 1.5  m ) is mottled and primary sedimentary structures are obscured due to pedogenic processes .
these samples contain several fish remains and in to12 rare characean gyrogonites were found additionally . the samples to20 and to 23 delivered only rare ostracod ( cypria sp .
earlier reports of fossil material and facies : ramos ( 2006 ) mentioned the following ostracod taxa : c.  aqualica , c.  graciosa , c.  lacrimata , c.  longispina , c.  pebasae , c.  purperae , cytheridella sp . , darwinula fragilis as well as additional vertebrate and crab findings .
interpretation : the lowermost sandy - pelitic layers ( 14 ) might be deposited in a floodplain pond / lake , which was influenced by crevasse splays .
the coarsening - upward trend of layers 59 hints to a progressive influx of a crevasse channel ( layer 10 ) .
it remains unclear if the ostracods of this layer are allochthonous or if they are incorporated in that layer due to subaquatic inflow of the crevasse splay ( crevasse delta ) into a floodplain pond / lake .
up - section ( 1124 ) , plentiful ostracod and mollusc faunas ( especially in the lower part : 1117 ; compare ramos , 2006 ) indicate the return to an aquatic environment and the abandonment of the crevasse channel .
small - scale fining - upward alternations of ripple bedded sand and pelite document individual flooding events or surges .
location : 22.1  km sse eirunep ( s 065218.3/w 0694705.1  ; altitude : 120  m ) , left cutbank of the tarauac river ( fig . 
description : layers 16 represent a 3.2  m thick succession of greenish ( 1 ) , reddish ( 2 ) , yellowish ( 3 , 5 , 6 ) or violet grey ( 4 ) coloured , mottled , frequently calcareous nodules - bearing paleosols . in layer 6 vertebrate remains ( crocodile teeth ) and gastropods were found .
above follows a 1.1  m thick , indistinctly bedded clay to fine sand ( 7 ) , which contains in its lower half coaly plant fragments and becomes towards the top and laterally progressively mottled ( paleosol ) .
the 0.65  m thick sediment column up - section ( 811 ) is composed of massive clay ( 8) , ripple bedded fine sand ( 9 ) , laminated silty fine sand to silt ( 10 ) and fine sand ( 11 ) .
each of these layers displays a slightly erosive base and an internal fining - upward trend .
the beds 8 , 9 and 11 each start with a basal layer of concretionary mud clasts in which ( layer 8 and 11 ) abundant vertebrate remains were found .
layer 12 is formed by 1.2  m thick , ripple bedded fine sand with cm - thick , laminated silty clay interlayers .
up - section a 0.7  m thick , orange - grey , mottled paleosol with mud clasts and rare vertebrate remains at its base ( 13 ) as well as a > 1.5  m thick , violet - purple coloured , mottled paleosol are developed ( 14 ) .
microfossil contents : the samples ba7a and ba7b contained several ostracod valves ( mainly cyprideis spp . , some specimens of cypria sp .
interpretation : the basal layers ( 16 ) represent a succession of paleosols , which could be seriously altered floodplain deposits . due to the occurrence of ostracods , probably bed 7 was deposited in a floodplain lake environment .
afterwards , a series of crevasse splay ( maybe partly also crevasse channel ) deposits follows ( 813 ) , which is topped by pedogenically overprinted floodplain fines ( 14 ) .
the sedimentary record of the investigated outcrops documents vertically as well as laterally highly variable fine - grained clastic successions ( predominantly clay  fine sand , along with fine  medium sand ) . based on the lithofacies and , as far as possible , lateral observations , these sediments represent fluvial deposition within active channels as well as in overbank environments .
macroforms , generated within the active channel include lateral accretion deposits ( sandy point bars ; figs .  2 and 4 ) and undifferentiated sandy bedforms ( stacked dune fields ? ; fig . 
8) , which are probably partly intersected by chute channels or are overlain by overbank fines .
these comprise pelite  sand - alternations of levee , crevasse splay and crevasse channel deposits .
frequently , a lag of mud pebbles and vertebrate fragments is found at the base of crevasse channels / splays . in part
, some splays may have entered floodplain ponds / lakes via crevasse deltas ( e.g. , figs .  6 and 9 ) .
abandoned , fine - grained channel fills ( mud plugs ) were observed ( fig . 
decimetre - thick , clay  silt beds with rich ostracod and mollusc faunas indicate the presence of short - lived , shallow ponds or lakes within the floodplain ( floodbasin ; e.g. , fig . 
6 ) point to partly swampy , poorly - drained conditions and a densely vegetated floodplain , which is , however , influenced by high clastic input .
intensively mottled paleosols with root casts and calcareous nodules are ubiquitous ( e.g. , fig . 
differences in colour and occasional calcic horizons may reflect diverging water - logging due to e.g. , local topography and/or seasonal fluctuations in discharge or climate ( kaandorp et  al .
nevertheless , diagenesis may have altered significantly primary features and more  specific investigations are required ( retallack , 2001 ) .
we are aware that restricted outcrop conditions and the limited areal and stratigraphical extent ( 30  m stratigraphical thickness ) of the current investigation hamper conclusions about the nature of fluvial style .
reconstructions of fluvial geomorphology , based solely on lithofacies assemblages and fragmentary known fluvial elements , remain to some degree tentative ( miall , 1996 ; bridge , 2003 )
. however , it seems obvious that we are dealing with a suspended load river system due to the high amount of overbank fines , probably within a tropical to subtropical wet  dry climate ( kaandorp et  al . , 2005 ; latrubesse et  al .
we observed : ( a ) a large proportion of overbank deposits , ( b ) laterally extensive , heterogeneous , fine - grained avulsion deposits ( including crevasse splays ) , ( c ) predominantly sandy channel deposits , which change laterally and vertically rapidly into overbank deposits as well as ( d ) lacustrine and coaly deposits . some sandy point bars ( of secondary channels ? ) indicate subordinate lateral sediment accretion .
these features are not exclusive to anastomosing river deposits , however , they largely coincide with the  standard  model of an anastomosing river sensu miall ( 1996 ) as well as with characteristics of such a system as reviewed by makaske ( 2001 ; compare also smith , 1983 ) .
the general setting ( subsiding foreland basin , extensive sediment input from the andes , possibly incompletely framed by tectonically active basement highs towards the east ) as well as a wet  dry seasonal climate ( seasonal floods , channel banks stabilized by vegetation ) would provide the conditions for anastomosing river systems ( smith , 1983 ; nanson and knighton , 1996 ; makaske , 2001 ; latrubesse et  al .
( late miocene ) in the state of acre ( sw brazil ) led to similar assumptions ( latrubesse et  al .
these authors proposed a megafan system , which originates in the andes and encompass a complex mosaic of avulsive rivers and associated wetlands ( including abandoned channels , floodplain lakes , floodbasin deltas , backswamps and well - drained floodplains ; see also wilkinson et  al .
this system is suggested to reach to the east at least as far as 67 w ( iquitos arch  sensu del arco et  al . , 1977 ) and probably also beyond (  purus arch  ; latrubesse et  al . , 1997 , 2010 ; fig
according to that , the sections around eirunep were located in a more distal position , which could be the reason for a more pronounced anastomosing - anabranching pattern due to a lower gradient ( compare also interpretations of the late miocene tariquia formation , southern bolivia ; uba et  al .
( 2007 , 2010 ) compared the depositional environment of the late miocene solimes fm . in acre with i.e. , the quaternary megafans of the chaco plain and the pantanal wetland ( iriondo , 1993 ; horton and decelles , 2001
the mollusc fauna , recorded from floodplain pond / lake deposits , is dominated by sphaeriids and the pachychilid sheppardiconcha ( fig . 
7b ) and indicates freshwater conditions within a fluvio - lacustrine environment ( wesselingh et  al . , 2006b ; wesselingh and ramos , 2010 ) .
typical freshwater ostracods ( darwinulids , ilyocyprinids , cypria , cytheridella ) are associated with diverse species of cyprideis .
this genus is a holoeuryhaline taxon , able to cope with fluctuating salinities , aberrant water chemistries , variable temperatures and oxygenation ( e.g. , meisch , 2000 ; keyser , 2005 ) . as it could adapt to freshwater conditions ( e.g. , lake tanganyika ; wouters and martens , 2001 ) and co - occurs here with exclusively freshwater taxa
, there is no constraint to conclude a brackish water environment ( ramos , 2006 ) .
preliminary stable isotope data ( o , c ) measured on cyprideis valves from aquidab , morada nova and torre da lua , yielded very negative values ( o :  5.7 to  9.7 , c :  10.3 to  12.5 ) , which exclude any marine influx .
there is still intensive debate on definition and timing of palynological biozonation concepts ( e.g. , hoorn , 1993 , 1994 ; latrubesse et  al . , 2007 , 2010 ; jaramillo et  al . , 2010 ; silva - caminha et  al . ,
however , palynostratigraphy remains a cornerstone for correlation across amazonia up to now and provides at least a rough stratigraphical framework .
( 2006a ) are highly linked to the concept established by hoorn ( 1993 ) .
based on the presence of some index taxa ( c.  annulatus , e.  spinosus , g.  magnaclavata ) a late miocene age ( probably asteraceae zone sensu lorente , 1986 ) is proposed by paz et  al .
( in press ) for the outcrops morada nova and torre da lua recently . the vertebrate fauna from torre da lua ( ramos , 2006 ) is related to the so - called  acre fauna  , which is one of the best documented faunas of northern south america ( cozzuol , 2006 ; latrubesse et  al .
the acre vertebrate assemblage displays significant affinities with the  mesopotamian  faunas of argentina and uruguay as well as with the urumaco assemblages ( venezuela ) and is dated to the late miocene ( huayquerian south american land mammal age ; cozzuol , 2006 ; latrubesse et  al .
is still poor and has to be extended by later works ( wesselingh and ramos , 2010 ; see roxo , 1937 ; wesselingh et  al .
, 2006b ; wesselingh , 2008 , and outcrop descriptions herein ) . from a stratigraphical point of view the mollusc faunas of aquidab and torre da lua
( 2006b ) to be of late miocene origin and post - date the  pebas fauna 
( compare celestino and ramos , 2007 ; wesselingh , 2008 ; wesselingh and ramos , 2010 , which suggested a late middle miocene age based on the ostracod record ) .
a late miocene age seems to be in agreement with the lack of typical endemic pebasian molluscs ( wesselingh and ramos , 2010 ) . the recorded cyprideis - species from aquidab , morada nova and torre da lua ( ramos , 2006 ; celestino and ramos , 2007 ; wesselingh and ramos , 2010 ) have generally long ranges according to muoz - torres et  al .
( 1998 , 2006 ; see also whatley et  al . , 1998 ) .
the occurrence of c.  lacrimata and c.  pebasae might be indicative for an age not younger than the ( early ) late miocene .
nonetheless , due to some contradictions ( e.g. , c.  lacrimata is supposed to appear first after the last appearance of c.  longispina but both are co - occurring at morada nova and torre da lua ) and the lack of key taxa , the current ostracod zonation needs some adjustment and is not further stressed here .
however , to date , a late miocene age seems most plausible for all outcrops described herein by considering the palaeontological record .
moreover , this fits well to the palaeogeographic context of this region as proposed by latrubesse et  al .
sedimentological observations derived from outcrops around eirunep ( south - western part of the state of amazonia ) document various subenvironments of a fluvial system ( upper part of the solimes formation , late miocene ) . beside sandy channel deposits ,
the main part comprises overbank sediments of levees , crevasse splays / channels / deltas , abandoned channels , backswamps and floodplain paleosols .
based on the lithofacies and the general geological setting , this system can be possibly related to an anastomosing river style .
there is not any indication for a long - lived lake (  lake pebas  ) or any marine influx in this region during the late miocene .
( 1997 , 2007 , 2010 ; compare also cozzuol , 2006 ; westaway , 2006 ) .
 in miocene times a vast wetland existed in western amazonia . 
 whereas the general development of this amazing ecosystem is well established , many questions remain open on sedimentary environments , stratigraphical correlations as well as its palaeogeographical configuration . 
 several outcrops located in a barely studied region around eirunep ( sw amazonas state , brazil ) were investigated to obtain basic sedimentological data . 
 the observed deposits belong to the upper part of the solimes formation and are biostratigraphically dated to the late miocene . 
 vertically as well as laterally highly variable fine - grained clastic successions were recorded . 
 based on the lithofacies assemblages , these sediments represent fluvial deposits , possibly of an anastomosing river system . 
 sand bodies formed within active channels and dominant overbank fines are described ( levees , crevasse splays / channels / deltas , abandoned channels , backswamps , floodplain paleosols ) . 
 lacustrine environments are restricted to local floodplain ponds / lakes . 
 the mollusc and ostracod content as well as very light 18o and 13c values , measured on ostracod valves , refer to exclusively freshwater conditions . based on palaeontological and geological results the existence of a long - lived lake (  lake pebas  ) or 
 any influx of marine waters can be excluded for that region during the late miocene . 
Introduction
Working area and geological background
Field and laboratory work
Results  sedimentology of the outcrops
Discussion
Conclusions