UNDERSTANDING DIAGENETIC VARIATION IN FOSSIL ASSEMBLAGES: LEVERAGING THE SEQUENCE STRATIGRAPHIC CONTEXT OF VERTEBRATE REMAINS WITHIN THE CALVERT CLIFFS (MIOCENE OF MARYLAND)

Vertebrate fossils can vary strongly in preservational state, even within a single formation, displaying different colors of bone tissue and extent of void infilling. To investigate the cause of local variation, fossil cetacean bones from each major depositional unit (Shattuck Zones) within the Calvert Cliffs were analyzed petrographically for bioapatite preservation and authigenic mineral infills.

Bones should be most vulnerable and reactive in their immediate post-mortem environment, leading to the hypotheses that: (1) bones found in rapidly deposited units should show minimal alteration and a high degree of similarity between samples; (2) bones associated with hiatal surfaces should display a high degree of disparity ranging up to high alteration, due to time-averaging; and (3) bones associated with sequence boundaries should show signs of exhumation and subaerial exposure (abrasion, oxidized minerals, microcracked tissue).

Thin-section analysis yielded the surprising result that many specimens lacked extensive authigenic infill. Bones from two major sequence boundaries differ strongly. A bone from the base of Zone 21 (SM0 surface) has an unaltered outer surface but internal voids are lined with discoloration and partially infilled by siliciclastics, peloidal micrite, and barite. A bone from the base of Zone 17 (CT0 surface) has extensive microboring, which is infilled by iron oxides, producing a darkened exterior rim. The bone also has adhering material at the exterior surface, but mostly unfilled internal voids. Samples from minor sequence boundaries often show microcracks and a discolored rim at the outer surface of the sample. The marine hiatal (SMT; Zone 12) samples reveal extensive exterior microboring and geopetal clay infills. Samples from aggradational intervals lack siliciclastic or authigenic infill but do show microboring of the exterior and rims of discolored bioapatite.

These findings highlight that preservational disparities among fossils can be related to their early depositional context -- indeed fossils capture environmental variation in early diagenesis that is not otherwise detectable -- and support the theory that early diagenetic processes play a significant role in long-term fossil preservation.