SEDIMENTARY FEATURES AND FOSSIL ACCUMULATION IN MUDSTONES OF THE KOPE FORMATION, LOWER MAQUOKETA GROUP (UPPER ORDOVICIAN), INDIANA

The Maquoketa Group of Indiana consists of thinly interbedded black to gray silty mudstones and limestones. The Kope Formation is the shaly interval that comprises the lower Maquoketa Group, and is characterized by interbedded gray and brown silty shale and mudstone. Common sedimentary features of this section include knife-sharp contacts, homogenized (massive) mudstone beds, laminated shale beds, and lag deposits. The succession is also quite fossiliferous, with common occurrences of graptolites (1 cm to > 3 cm) and brachiopod shells (< 0.5 cm). In this study, graptolites and shell hash have been thin sectioned and observed under a light microscope. Most of the fossils are surrounded by a fine-grained, shaly matrix, and pyritic halos (scattered pyrite framboids) extending outward from pieces of fossil debris. All of the halos are flattened. Because framboidal pyrite typically forms in surficial sediments, it probably means that the pyritic halos formed early in diagenesis, prior to any significant compaction. With this assumption we can use these halos to estimate the original water content of Kope Formation muds to have been approximately 70-80%. Thus, the surface sediments of the Maquoketa sea were soupy muds. During periods of swifter currents, the soupy surface sediments were eroded until stiffer and more deeply buried muds were exposed. This led to knife-sharp contacts between mud beds after deposition resumed. These erosive events are most likely linked to occasional storms that reworked the seabed, and also produced the observed lag deposits and layers of reworked fossils.