Abstract

Macrofossil associations have proved to be vital aids for resolving the sequence architecture of Castlecliff cyclothems (5th and 6th order, mid-Pleistocene depositional sequences) because of a close correlation between particular faunal associations and systems tract character. Associations from transgressive systems tracts are typically derived from mixed and transported assemblages, which, in turn, commonly occur within cross-bedded units (Type A shell-beds) at the base of Castlecliff sequences. These associations commonly represent reworked sandy shoreface and estuarine paleocommunities that are not otherwise preserved in the Castlecliff section. Associations from mid-cycle condensed shellbeds (Type B shellbeds) comprise well-preserved, near-situ, dispersed to closely-packed macrofossil assemblages in mud-rich facies. These faunas represent inner to middle shelf, shell-ground to shelly soft-ground paleocommunities. Associations from high-stand systems tracts also comprise well-preserved near situ faunas. These assemblages, however, are variably dispersed within shelf siltstones and silty fine sandstones, and represent muddy soft-ground paleocommunities. Abrupt shifts in taxonomic composition and taphonomic attributes occur across systems-tract bounding disconformities. The nature of these changes, measured against an "idealised continuum" of macrofossil associations, is used to gauge the magnitude of these disconformities. This approach is especially useful in determining the nature, of mid-cycle disconformities in Castlecliff sequences, which are difficult to evaluate from lithofacies characteristics alone.

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