UNRAVELING PATTERNS OF REWORKING IN LOWSTAND SHELF DEPOSITS USING AMINO-ACID RACEMIZATION AND RADIOCARBON DATING

Amino acid racemisation calibrated by radiocarbon dating (liquid scint illation and accelerator mass spectrometry) has provided a chronostrat igraphic framework in which to evaluate the later Quaternary evolution of mollusc-rich carbonate sediments on the outer continental shelf of New South Wales, Australia. Three vibracores (Cores 112/VC/130, 112/V C/121 and 112/VC/134) from the outer continental shelf in present wate r depths of 123, 139 and 150 m, respectively, each contain alternating successions of pedogenically-unmodified, fine-grained mixed quartz-ca rbonate sand. and densely-packed mollusc-dominated sediments, set in a matrix of carbonate sand. The extent of racemisation for several amin o acids and radiocarbon dates on the shallow-water molluscs Pecten fum atus and Placamen placidium reveals the presence of sediments deposite d during three successive glacial maxima (i.e. Stages 8, 6, and 2 of t he Marine Oxygen Isotope record). Three chronostratigraphically distin ct aminozones (Oxygen Isotope Stages 8, 6 and 2) are identified in Cor e 134. Samples in Cores 121 and 130 record only two aminozones collect ively (Stages 6 and 2). Amino acid racemisation indicates that fossil molluscs with 'background' radiocarbon results are in fact significant ly beyond the range of radiocarbon dating, by analogy with other depos its from southern Australia. A thin veneer of early Holocene fine calc areous sand occurs at the top of each core. All three cores reveal var ying degrees of reworking of fossil molluscs, which is not immediately apparent on the basis of lithostratigraphy or biostratigraphy. Rework ing of macrofossils is more pronounced closer to the inferred location of the palaeoshoreline of Pleistocene glacial maxima. Copyright (C) 1 996 Elsevier Science Ltd