SEDIMENTARY FEATURES AND DEPOSITIONAL SETTINGS OF ORGANIC-RICH SHALE FACIES IN THE MIDDLE TO UPPER ORDOVICIAN OF NORTHEAST INDIANA AND NORTHWEST OHIO

The Ordovician Maquoketa Group in Indiana and the Cincinnatian Series in Ohio, were deposited as part of westward-thinning shale-dominant succession. The terrigenous fine clastic rocks derived from the Taconic Orogeny show an upwards increase in carbonate content, probably caused by overall sea level rise and shoreline retreat. In this study small-scale sedimentary features, facies types, and depositional cycles are examined to interpret the depositional setting of the lower half of the succession (Utica equivalent) in northeast Indiana and northwest Ohio.

Eight different mudstone facies were differentiated in a detailed description of a 440 ft thick core and 35 thin sections from Allen County in northeastern Indiana. The bottom of the succession is a banded pyritic mudstone representing slower sediment accumulation after initial flooding. A 1 cm thick altered volcanic ash bed (K-bentonite) was observed 15 ft above the Trenton/Maquoketa contact, probably derived from explosive silicic arc volcanism on the eastern margin of North America. Upsection, the succession grades from gray homogenized mudstone into darker organic-rich homogenized mudstone. This interval suggests deposition under variably dysoxic conditions. Phosphatic fossil debris, spherical to irregularly shaped phosphate nodules, and pyrite nodules in brownish mottled mudstone suggest low sediment accumulation rates and may indicate marine flooding surfaces.

The middle portion of the examined interval shows multiple banded silty mudstone cycles, with calcareous silt laminae and lags at the base (starvation surfaces) grading upwards into dark crypto-bioturbated mudstone at the top. The upper portion of the interval is characterized by 150 ft of olive-green strongly bioturbated mud with macroscopically visible burrows. The studied interval is overlain by a limestone-dominated unit that reflects diminished clastic input. Higher energy deposits (tempestites) suggest an overall shallowing trend.