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22-3/I54-03/2-2/1 Contour interval: 50

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In updating the calcareous microplankton stratigraphic record of the lower part of the Browns Creek Clay (the Turritella clays and Notostrea greensand), Otway Basin of southeastern Australia, unexpected problems with significant consequences were encountered. Assemblages from the base of the formation include two key species, the nannofossil lsthmolithus recurvus (hitherto unknown from this level) and the foraminiferid Acarinina collactea (unknown from this formation until fairly recently)- in association with Neococcolithes dubius, Chiasmolithus oamaruensis, Cyclicargolithus recticulatus, Discoaster saipanensis, Globigerinatheka index, Tenuitella aculeala, T gemma and T insolita. The stratigraphic range of I. recurvus is disjunct (in this respect, I. recurvus resembles the two Eocene foraminiferids T aculeata and G. index in southern Australia): in its lower range I. recurvus is rare, but it is common in its upper range-with a substantial gap in between, within which N. dubius disappears. Elsewhere, including other southern high-latitude sections, A. collactea is known to disappear well below the first appearance datum (FAD) of I. recurvus, and the split stratigraphic range of I. recurvus has not been explicitly reported. Reworking could be the cause of the overlap in the stratigraphic ranges of I. recurvus and A. eollactea at Browns Creek, although a seemingly wide geographic distribution of the association of these two species and N. dubius in southern Australia has been indicated herein. Evidence (including the presence of rare T gemma) has been discussed , favouring I. recurvus (in preference to A. collactea) as a prime evidence for dating the base of the Browns Creek Clay. This base has been correlated with the sequence boundary between the third order cycles 4.1 and 4.2, and dated late Eocene, at about 36.2 Ma. Cyclicargolithus reticulatus disappears near the Turritella clays/Notostrea greensand boundary - within the foraminiferid Hantkenina alabamensis primitiva interval, which has long been accepted as isochronous across southern Australia. The event of disappearance of C. reticulatus is thought to be at 34.9 Ma, in agreement with a previously given K-Ar age limits for the Hantkenina interval. The combined evidence of I. recurvus and C. reticulatus assigns the lower part of the Browns Creek Clay a late Eocene age, equivalent to low either Zone NP19/20 or Subzone CP15b. A correlation with the foraminiferal zonal interval high P15 to within P16 of the tropics is possible.

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Meteorites are associated with five impact structures in Australia. Three of them are group IIIAB irons (Wolf Creek, Henbury, and Boxhole), Veevers is a group IIAB iron, and material recovered from the crater at Dalgaranga is a mesosiderite stony-iron. The impacts range in age from a few thousand years (Dalgaranga, Henbury, Veevers, and Boxhole) to 300,000 years (Wolfe Creek Crater). Metallographic studies of the surviving fragments at some of the craters show that impact damage ranges from simple fracturing, through shock-hardening of metal, to plastic and shear deformation, reheating and attendant recrystallisation, and, ultimately, melting. Details of the microstructures of surviving fragments of iron meteorite from the craters suggest that shear deformation may have been an important mechanism in the disruption of the projectiles. Frictional heating from viscous drag between projectile and target, and from rapid shear deformation within the projectile, may be sufficient to melt and vaporise significant portions of the projectiles and account for the large deficit of meteoritic material from Australian impact craters.

The Acraman ejecta horizon is a thin (0-40 cm ) sand/breccia unit consisting almost entirely of angular volcanic fragments. The horizon occurs within Neoproterozoic (~590 Ma) deep-water (below storm-wave-base) shales of the Adelaide Geosyncline (Bunyeroo Formation) and Officer Basin (Rodda beds). Much evidence points to the derivation of the horizon from Australias largest meteorite impact feature, the Acraman structure, in the Gawler Ranges. The ejecta horizon contains anomalous quantities of Ir, Au, Pt, Pd, Ru, and Cr, consistent with a meteoritic source for the horizon. Other evidence for an impact-related origin is the presence of shattered mineral grains, multiple sets of planar shock lamellae within quartz phenocrysts, the occurrence of small shatter cones on clasts, and the presence of altered melt particles. Two major sedimentological styles of ejecta occur in the Adelaide Geosyncline: type 1 ejecta sequences are characterised by a distinctive lonestone/breccia-sandy mudstone-graded-sandstone sequence whose clasts are almost perfectly sorted and normally graded; and type 2 sequences are characterised by weak normal grading, poor sorting, and common cross-lamination. Type 1 ejecta sequences are widespread in the central and eastern portions of the Adelaide Geosyncline, while type 2 sequences are more common in the western part of the Geosyncline. In the Officer Basin, the ejecta horizon consists of thin (0-0.7 cm) layers of sand-size angular volcanic clasts. We interpret type 1 sequences as primary fallout deposits whose perfect sorting and normal grading reflects suspension settling; and type 2 sequences as ejecta that have been completely reworked by impact-induced tsunamis. When treated as a single sediment sample, the grain size distribution in the type 1 ejecta sequences displays two quite distinct populations: the basal breccia forms a very poorly sorted population; and the graded sand constitutes a moderately well sorted population. These two grainsize populations might represent different transport modes: the breccia by fireball processes or ejecta flows, and the sand by atmospheric processes such as impact-induced air-blasts.