DOES SEQUENCE STRATIGRAPHY CONTROL THE DIVERSITY OF CONTINENTAL FOSSILS? A GASTROPOD CASE STUDY FROM THE KAIPAROWITS FORMATION, SOUTHERN UTAH

Changes in relative sea level and sedimentation rates associated with systems tracts are known to exert significant control on the taphonomy of shallow marine shelly faunas, and therefore influence metrics such as diversity and evenness. To what extent sea level and systems tracts exert similar control on continental taphonomy remains largely unexplored, especially for shelly mollusk-dominated faunas. This study collected and identified nearly 2,500 pulmonate and aquatic gastropods (37 morphospecies) from 30 localities in the alluvial to coastal plain deposits of the Upper Cretaceous Kaiparowits Formation in southern Utah. Overbank pond (mud) and fluvial channel lag (sand) facies were sampled and categorized on the basis of stratigraphic height in section and systems tract (late-TST, MFZ, and HST), and rarefaction curves were generated to identify diversity and evenness patterns by category. No significant difference in diversity is observed among systems tracts except when they are also categorized by facies. In channel sand facies, diversity and evenness decrease from l-TST to HST, from 22 to 14; whereas in the muddy pond facies, diversity and evenness increase over the same systems tracts from 9 to 22. The inverse response of diversity and evenness between the channel and pond gastropods likely reflects different taphonomic processes influenced by sedimentation patterns. Species diversity in channel sands is likely enhanced by mixing and reworking of robust shells, a process that is associated with amalgamated channels of the l-TST. By contrast, gastropods recovered in pond sediments are gracile and unlikely to survive reworking. Here, higher diversity observed during the HST may relate to the general aggradational style of sedimentation wherein long-lived ponds accumulated time-averaged communities of gastropods.