DEPOSITIONAL RATES ARE REFLECTED IN MICROTAPHONOMIC AND MINERALOGIC PRESERVATION OF MARINE VERTEBRATE REMAINS: COMPARING DIAGENETIC TRENDS ACROSS SEDIMENTARY BASINS USING THE VALLEY OF THE WHALES (EOCENE, EGYPT) AND CALVERT CLIFFS (MIOCENE, MD)

Although bonebeds are a rich record of vertebrate remains, they can reflect complicated overprinting by time-averaging processes such as erosional winnowing or prolonged exposure before permanent burial, with potential to affect the original biologic signal. Microtaphonomic and diagenetic features such as microboring, staining, cracking, and authigenic infills, which are often rarely considered, could provide a valuable additional means of recognizing taphonomically complex histories. Bones from rapidly deposited sediments (minimal time averaging) should exhibit little alteration, as they’ve experienced comparatively brief opportunities for bioerosion, biogeochemical interactions in the taphonomically active zone, and other processes; bones from erosional settings have potential for overprinting by oxidized minerals and for evidence of drying and/or abrasion related to reworking and subaerial exposure; and bones from sediment-starved or other hiatal settings (delayed burial) should reflect the maximum opportunity for alteration, with heavily tunneled bone margins and complex infilling minerals related to redox cycling in the surface mixed layer. To test this hypothesis, cetacean bones were sampled from well-known Cenozoic marine records where the duration of pre-burial conditions can be estimated from an already-established sequence stratigraphic context, and bones are known to display variable macroscopic preservation. Bones from both stratigraphic records display marine and terrestrial forms of microboring, microcracks from both swelling and drying, and distinctive variation in authigenic infill; all of which follow expectations from their sequence stratigraphic (depositional) context. Microtaphonomic and authigenic alteration of bones thus captures the early diagenetic, pre-burial conditions of bone accumulation in marine settings, correlative with the predicted duration of time averaging and persisting despite pervasive late diagenesis and weathering. Such features thus represent a valuable tool to understanding the nature of bone assemblages.