TRILOBITE TAPHONOMY OF SHALY SHELF CARBONATES IN THE ELGIN MEMBER OF THE MAQUOKETA FORMATION (LATE ORDOVICIAN; NE IOWA AND SE MINNESOTA)

Abundant trilobites in the Elgin Member of the Maquoketa Formation (Katian 1), a transgressive-regressive sequence deposited in a density-stratified epeiric sea, exhibit facies dependence in the distribution of both species and taphonomic attributes. Outer shelf, trilobite-graptolite lime mudstones (Isotelus “zone”) near the IA/MN border are dominated by I. gigas, holaspid carcasses of which are prone or gently flexed and lie parallel to the tabular bedding. Farther south, more distal lime mudstones deposited in deeper waters are dominated by the smaller species, I. iowensis, again with carcasses (predominantly holaspids) outstretched parallel to bedding. Isotelus carcasses and thoracopyga commonly are inverted, and some carcasses form monospecific pairs or clusters of mutually overlapping individuals. Both may show partial disarticulation and occur mainly within sparse lime mudstone between very thin concentrations or hashes of unsorted, randomly oriented sclerites, though some carcasses occur on upper bedding planes in shale. Associated carcasses of Anataphrus vigilans and Gravicalymene fayettensis also are prone or slightly flexed. These two species dominate argillaceous, outer shelf lime wackestones in NE Iowa, where carcasses are outstretched, while I. gigas, represented by scattered sclerites, dominates laterally equivalent, brachiopod/echinoderm-bearing lime wackestones in the IA/MN border area. In NE Iowa, argillaceous, middle shelf lime wackestones with brachiopods and echinoderms (Anataphrus “zone”) are dominated by A. vigilans, carcasses of which are mostly fully enrolled and inclined at various angles to bedding. G. fayettensis carcasses in middle shelf beds likewise are mostly fully or partially enrolled, as are other trilobites such as Bumastus, Ceraurinus, Ectenaspis, and Pterygometopus. Additionally, some A. vigilans carcasses show sigmoidal flexure and/or unusual attitudes, including subvertical with the cephalon directed downward. Together, these observations indicate (1) that carcasses and molt ensembles in outer shelf settings underwent gentle burial, in situ, by suspended lime mud or clay; (2) that carcasses in middle shelf settings were first transported and then buried, suddenly, by slumps or mud flows; and (3) that enrollment was triggered by entrainment.