DIAGENETIC HETEROGENEITY OF TIME-AVERAGED VERTEBRATE REMAINS IN MARINE SETTINGS: A TEST CASE OF THE MIOCENE SHARKTOOTH HILL BONE BED, CALIFORNIA

Skeletal remains associated with unconformities and other stratigraphically important surfaces have the potential to be strongly time-averaged, leading to the potential for diagenetic heterogeneity. Diagenetic infillings, alteration, and replacement should, in principle, reflect the burial rate (sedimentation or lack thereof), environmental energy, burial-exhumation cycles, and duration of residence within the most taphonomically active part of the sedimentary column. Diagenetic heterogeneity should thus provide evidence of non-depositional as well as depositional conditions, including cryptic stratigraphic boundaries not evident from sediments alone. To test these ideas, thin-sections of cetacean rib and shark tooth fragments are being examined from the Sharktooth Hill Bonebed (SHBB), a 10-50 cm interval within the Round Mountain Silt that has been intensely studied owing to its wide diversity of vertebrate taxa, from sharks to gomphotheres. The SHBB is thought to mark a surface of maximum transgression and represent 700,000 years of condensation by the starvation or bypassing of siliciclastics during the Middle Miocene Climatic Optimum (15.9-15.2 Mya; Pyenson et al. 2009, Geology). Pyenson et al. reported macroscopic taphonomic damage, including extensive fragmentation, some rounding, and bone cracking akin to subaerial weathering, despite no known occurrence of subaerial exposure. My new petrographic and SEM analysis reveal the way in which cracks propagated and whether cracking was due to water absorption or drying; the sequence of void infilling, apatite replacement, and surface coatings of skeletal elements; and variation among tissue types (cortical and cancellous bone, tooth dentin and enamel). Bone from a largely articulated cetacean from the overlying highstand system tract illuminates the understanding of late versus early diagenesis – if diagenesis is more influenced by residence time in the mixed layer than by pervasive, deep burial conditions, then greater heterogeneity is expected within the SHBB, where the bones experienced up to 700,000 years of varied conditions.