55% coverage west 22-2/G53-14/3-4

Legacy product - no abstract available

22-1/H54-7/2 Contour interval: 20

From a study of middle to late Eocene calcareous nannofossil assemblages in four sections in the Otway Basin of southeastern Australia, a sequence of biostratigraphic events has been deduced, spanning the interval from the lowest appearance of Cyclicargolithus reticulatus (middle Eocene) to the disappearance of Discoaster saipanensis (latest Eocene). The sequence is compared with its coumerpart in New Zealand, and is placed against the planktic foraminiferal P. zones of the tropics. The previously determined foraminiferal biostratigraphy of the sections studied has been compared with the nannofossil biostratigraphy, and, as a result, the local highest appearance of the foraminiferid Acarinina primitiva is now placed in zone P. l3 of the tropics , and not P.12 or P.14, as previously. The disappearance up the section of the foraminiferid Acarinina collactea is found to be locally inconsistent with other evidence. During the middle Eocene, marine ingressions, represented by isolated nannofossil assemblages, occurred in the Gambier Embayment of the western Otway Basin, but did not reach the Browns Creek area, eastern Otway Basin, attesting to the diachroneity of Eocene marine sedimentation in the basin . The diachroneity is also indicated by transgressive rock units in the Gambier Embayment. The ingressions seem to coincide with a major change in the sea-floor spreading rate south of Australia. During the latest middle to early late Eocene, a major transgression began synchronously in widely separated areas across the basin. The upper Eocene section in the Gambier Embayment represents condensed sedimentation and ends in a sharp disconformity, indicated by the simultaneous disappearance of Cyclicargolithus reticulatus and Discoaster saipanensis. At Browns Creek, that part of the section between the highest appearances of C. reticulatus and D. saipanensis is thick, suggesting relatively rapid rates of sedimentation. However, in the expanded part of the section at Browns Creek and also at Castle Cove, there is evidence that extreme shoal conditions existed as a result of imbalance between sedimentation and subsidence. During the middle and late Eocene, conditions along the Australian southern margin were generally temperate, with surface-water temperature decreasing eastward, and the depositional environment was essentially shallow marine - nearshore or shelf.

The Proterozoic I-type Kalkadoon and Ewen Batholiths and their comagmatic extrusive equivalents, the Leichhardt suite, form an association covering at least 5000 km2 in the central part of the Mount Isa Inlier. U-Pb zircon data and some Rb-Sr total rock data show that these rocks crystallised from melts emplaced between 1840 and 1870 m.y. ago and are the oldest dated igneous rocks in the Inlier. Chemically and isotopically, these granites are relatively uniform and, compared with most other Mount Isa granites , they have higher Sr and Al2O3 contents, and lower TiO2 , Zr, Nb, and Th contents . These chemical characteristics appear to be restricted to felsic igneous rocks known to be older than 1800 m .y. and may be useful in identifying the older felsic melts of the Mount Isa Inlier. The source for the rocks of the Kalkadoon- Ewen- Leichhardt association is estimated to have had an SiO2 content of 55-60 per cent. Relative to other large Palaeozoic and Mesozoic I-type batholiths elsewhere, this Mount Isa association is enriched in K20, Rb, Th, U, La, Ce, Zr, and Nb, and depleted in CaO, MgO, Ni, and Cr. The least isotopically disturbed granites of the association have relatively low initial 87Sr/86Sr ratios (about 0.704), which implies that the age of the source for these melts was not much older than the age of their emplacement. As chemically and isotopically similar granites occur in most Proterozoic areas of Northern Australia, it is inferred that during the period 1900-2100 m.y. a significant mantle differentiation event took place, during which large volumes of material were accreted to the base of the crust in these areas. Post-emplacement metamorphism and deformation , which have a maximum age of 1640 m.y. , caused significant textural and mineralogical changes in the Kalkadoon Batholith, but had a lesser effect on the Ewen Batholith . Igneous textures are commonly preserved in the Ewen Batholith, but the Kalkadoon Batholith, which has been metamorphosed from lower greenschist to upper amphibolite grade, shows significant isotopic disturbances.

Lake Eliza is a hypersaline coastal lake in southeast South Australia, a region of winter rainfall and summer drought. It is fed by ground waters and has no connection with the sea. Salinity rises from < 100 in winter to > 360%() in summer, with accompanying fall in lake level. The lake contains a biota of generally non-marine lineage. Two areas of the lake margin exposed in summer were studied. One, on the western shore, was protected from prevailing winds, the other, on the eastern shore was exposed to wave attack. The western shore is an area of fine carbonate sediments with high organic content. The eastern shore is an area of moderately sorted quartz-carbonate sand of lower organic content. The sediments of Lake Eliza are similar to some of those described from the Wilkins Peak Member of the Green River Formation, USA, and a comparison between the two systems suggests that the lamosite oil shales of the type found in the Green River Formation may not have been deposited in a fresh to brackish lake floor as has been supposed, but could have formed beneath cyanobacterial mats along a protected margin of a saline lake, in a setting equivalent to the western margin of Lake Eliza.

Boron-bearing kornerupine occurs in phlogopite-albite gneisses of the Arunta Block, southwest of Mount Baldwin and in the western Harts Range. At the second locality, idioblastic sapphirine occurs with the kornerupine, and excellent crystals of both minerals have been collected. Most of the larger kornerupine crystals from the Mount Baldwin locality are , in fact , pseudomorphs of fine-grained phyllosilicates after kornerupine .

50% coverage east 22/E54/47 Vertical scale: 250

22-1/I54-8/11 Vertical scale: 10

A systematic examination of multivariate geochemical data for two predominantly felsic volcanic units from the Proterozoic of the Mount Isa region, the Leichhardt Metamorphics (1865 Ma) and the Argylla Formation (1777 Ma), has established practical criteria to distinguish these units. The criteria are applied to volcanics from the Ewen Block, Wonga Belt, and Bottletree Formation, which have been difficult to correlate using lithologic and structural criteria. Histograms provide limited discrimination, because of the large range of most elements. Variation diagrams using Si02 as the independent variable allow clear distinction of the two formations to be made from their relative abundances of Ti, AI, Fe, K , Ce, Cr, Nb, Pb, Sr, Th, Y, and Zr at any particular SiO content. An examination of ratios of elements with similar chemical properties effectively distinguishes the two formations. The similarity coefficient and discriminant analysis allow classification of the two formations and are of use in assigning analyses of unknown specimens. All techniques indicate that the Ewen Block volcanics are probably Leichhardt Metamorphics and that the Wonga Belt and Bottletree Formation specimens more closely resemble the Argylla Formation. The higher CaO, Sr, and Pb, and lower K20 distinguish the Bottletree Formation from the Argylla Formation and support its status as a separate formation.