EVIDENCE OF MARINE CONDITIONS IN THE UPPER PART OF THE DEGONIA SANDSTONE (ELVIRAN STAGE, CHESTERIAN SERIES) IN THE ILLINOIS BASIN

摘要

The purpose of this study is to document and describe newly observed marine fossils from the upper part of the Degonia Sandstone and to infer the depositional setting of this horizon. Previous studies have shown that the Degonia Sandstone (Elviran Stage, Chesterian Series, Mississippian Subsystem) contains sedimentological evidence of several depositional environments (i.e. terrestrial, fluvial, nearshore deltaic, tidal, and possible marine zones); however, no body fossils had been observed. The only fossils identified in the Degonia are non-diagnostic trace fossils and Carboniferous plant remains such as Lepidodendron trunks and Stigmaria casts, suggesting a terrestrial environment. In 2010, Mary Seid and Joseph Devera of the Illinois State Geological Survey found marine fossils in the upper Degonia Sandstone in a stream bed located within the Wolf Creek Fault Zone. They associated these fossils with a marine environment, contradicting earlier assessments. Four study localities were found throughout the study, one each in the Gorham, Cobden, Glendale, and Raddle Quadrangles of southern Illinois. Sampling localities were found using geologic maps to locate the Degonia-Kinkaid contact, specifically areas where large stream valleys cut through the Kinkaid Formation and into the Degonia Sandstone throughout southern Illinois. The boundaries between the Degonia Sandstone and the overlying Kinkaid Formation were walked in order to find indicators of the marine zone (i.e. the presence of the shaly layer of the upper Degonia Sandstone). The reference section (the Gorham locality) contains the largest diversity of fossils and represents a storm deposit. The Cobden locality appears to contain a zone of dwarfed specimens and a zone of normal sized specimens, and represents a storm deposit. The Glendale locality is dominated by a single bivalve and represents brackish water conditions. The Raddle locality is non-fossiliferous, but was within the intertidal zone. The fauna identified by this study consists of four Phyla: Arthropoda, Brachiopoda, Bryozoa, and Mollusca. The only Arthropoda observed was a burrowing barnacle (Acrothoracica). The Brachiopoda observed consist of Diaphragmus nivosus, Orthotetes kaskaskiensis, Anthracospirifer occiduus, and Composita sp. The Bryozoa observed include Fenestrate and Trepostome. The Mollusca are the most diverse phyla observed, consisting of bivalves (Wilkingia walkeri, ?Edmondia sp., Aviculopecten winchelli, Promytilus illinoisensis, Myalina sp., ?Septimyalina sp., Myalinella meeki, ?Sphenotus monroensis, and four species of unknown bivalves), cephalopods (Reticycloceras sp., Endolobus sp., Liroceras sp., Metacoceras sp., and Domatoceras sp.), and gastropods (Euconospira sturgeoni, ?Eotrochus cf. marigoldensis, and an unknown gastropod). Other fossils observed were crinoid stem molds and plant material. The characterization of invertebrate fossils occurring in the upper Degonia supports the previous suggested marine sedimentological features of the Degonia Sandstone.