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ItemDeformation and the thermobaric history of the eastern coast of Williams IslandMarks, Bianca ( 1997)Williams Island is located off the southern coast of the Eyre Peninsula of South Australia where the Palaeoproterozoic rocks of the Lincoln Batholith intrude a portion of an Archaean basement complex. The structures of the eastern coast of Williams Island are controlled by the rheological contrast between the mafic dykes and the felsic granite gneisses that comprise the batholith. Planes of rheological weakness exist at the dyke margins along which strain is localised. The plane of failure and the kinematics along it depends upon the orientation of the dyke with respect to the stress field. Displacements at cross-cutting dyke margins indicates the occurrence of three significant deformation events, D 1, D2 and D3. By comparison, the D1 is localised to a region of outcropping Jussieu Dykes, the D2 is pervasive and the D3 is confined to the discrete Northern and Southern Shear Zones. Associated with the latter two deformations is an increase in temperature and strain rate which controls the relative strength of the metabasic and the granite gneiss rocks. Brittle extensional structures, such as boudinage, form when the mafic dykes behave in a more competent manner relative to the host, whereas ductile extensional features, like pinch and swell, infer a greater homogeneity between the rock types. The rheological contrast is inverted with a preferential increase in strain resulting in granite boudinage. The D2 fabrics arc predominantly defined by a granulite two-pyroxene assemblage and the structural elements of D3 are characterised by minerals associated with amphibolisation. Average pressure calculations of representative assemblages give 7 ± 1 kbar for M2/D2 and 12 ± 2 kbar for M3/D3, which suggests crustal thickening over D2 - D3 time. Exhumation of the crustal block therefore occurred after peak D3.
ItemGeology of the lookout area, Eyre Peninsula, South AustraliaAnnear, Joshua A. ( 1996)The Lookout Area, southern Eyre Peninsula, South Australia forms part of the Lincoln Batholith with outcropping Donington Granitoid Suite units of Palaeo-Proterozoic age (l840-1800Ma) and both syn-plutonic and intrusive mafic dykes. The area was surveyed and mapped at 1:500 to produce a detailed geological map including the form surface trend of structural features. The units, incorporating felsic, intermediate and mafic gneisses intruded by the mafic dyke sequences, preserve high strain mylonitic deformational features with well developed fabrics in the felsic units. The Mafic dykes are boudinaged and display asymmetries attributed to antithetic back-rotation due to extensional shear. Kinematic indicators including shear band formation, parasitic folding and strain shadows suggest a kinematic history of initial sinistral deformation post-dated by dextral west-up oblique shear associated with the formation of the principal foliation. This is in turn post-dated by local sinistral shearing. The strain associated with the deformation can be measured by use of porphyroclast shape and distribution. These results indicate that the strain is locally variable, from constrictional to flattening, but generally constrictional with an extensional ratio of approximately 6:1. The metamorphic conditions preserved by the mineral assemblages analysed through the average PT method using THERMOCALC are; 700-850°C and 5.8-6.8 kbar. This mineral assemblage indicates metamorphism in the area has achieved upper-arnphibolite to lower granulite facies metamorphism at some stage in the PT history of the area.
ItemLate Cainozoic climatic and eustatic record from the Loxton-Parilla Sands, Murray Basin, Southeastern AustraliaKotsonis, Andrew ( 1995)A series of ancient shoreline ridges in the western Murray Basin of southeastern Australia preserve a detailed legacy of Pliocene marine retreat. The 157 subdued NNW trending coastal ridges of the Loxton-Parilla Sands, mapped using conventional techniques and night-time thermal imagery from the NOAA and the ERS-l satellites, extend in a parallel series from 400 km inland to the present coastline, and provide a virtual contour plan of the Pliocene landscape. Coastal ridges of the Loxton-Parilla Sands range in age from 6:6 Ma in the east, to 3.5 Ma towards the west, where they are tectonically deformed by the uplift of the Pinnaroo Block. The deposition of the Loxton-Parilla Sands at 6.6 Ma is correlated with high global sea levels, with the distribution of the sands suggesting deposition at a topographic level comparable to an ice-free earth (i.e. complete deglaciation of the polar regions). Coastal ridges consist of beach-barrier and near-shore sediments deposited in conditions of fluctuating sea levels. The absence of aeolian sediments within the ridges implies a significantly weaker wind-wave regime and/or permanent vegetation cover existed throughout the Pliocene. Eustatic oscillations recognized within the shoreline sequence correlate well with glacio-eustatic changes modulated by the axial precession of the earth with a periodicity near 20, 000 years. Following retreat of the sea, the Loxton-Parilla Sands were subject to deep weathering, with the resultant profile termed the Karoonda Regolith. Following cessation of coastal deposition the Karoonda Regolith developed diachronously, with the oldest pedogenic exposures in the east to the youngest towards the west. Ferric and silicic weathering profiles developed in late Miocene to Plio-Pleistocene times. Pedogenic silcretes formed by downward movement of acidic soil waters with saturation and deposition at the soilwater-groundwater interface under alternating wet and dry conditions. High water tables probably ensured accumulation of silica in the near surface environment. By the Mid Pliocene (3.5 Ma) weathering changed from predominantly silica to iron mobilization with development of ferricrete profiles. Late Pleistocene (0.7-0.4 Ma) ferricrete development ceased when arid climates developed as represented by calcareous soils across the basin. Addition of calcareous parna on the Karoonda Regolith buffered soil water pHs, and switched off ferricrete development. Extensive opaline silica dissolution under alkaline conditions resulted in the development of karstic-type solution pipes that were infilled with pisoliths and clasts of sandstone. Lowered groundwater tables probably contributed to the removal of silica from the near-surface permitting transfer to deep aquifers within the Loxton-Parilla Sands. The change from ferricrete to calcrete formation marks the onset of arid climates in Australia. Correlatives can be drawn between this continental record of sea level changes with those of the deep sea oxygen isotope curves which reflect Milankovitch-type changes in the ice budget of the world.