Taphonomic processes in the Waldron Shale, Silurian, southern Indiana

The Waldron Shale (Silurian) in southern Indiana contains four lithofacies, ranging from very fossiliferous to nearly unfossiliferous, each with a distinct taphonomic history. Preburial taphonomic processes include primartily disarticulation of multielement skeletons, fragmentation, and limited dissolution. Transport distances were negligible even during storms. Storms resulted in shell-gravel lags in shallower water and wellpreserved smothered-bottom assemblages in deeper water. Taphonomic analysis of the fossils and stable isotope data (C, S) were used to identify a series of post-burial taphonomic events. From earliest to latest those events are: 1) pyrite shell replacement in reducing microenvironments, 2) pyrite mold formation and shell dissolution, 3) dolomitization, 4) precipitation of euhedral pyrite as overgrowths on fossils, and 5) precipitation of sparry calcite in the voids offossils partly replacing internal morphology. The effect of each event varied among taxa such that in any given sample the percentage of pyritized or dolomitized shells may range from 0 to 100% depending on the taxa. The severity of each event also varied among facies. Models based on the taphonomy of Recent shelled invertebrate communities (excluding sedimentological processes) may not be applicable to Paleozoic assemblages because of secular variations in shell mineralogy of the dominant shelled organisms, burrowing rates, and chemical and physical conditions of shallow marine environments. A high proportion of fossils, even juveniles, may have been preserved providing that favorable conditions existed in Paleozoic muddy seafloors.