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Sedimentary Features and Depositional Setting 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-dominated succession. The terrigenous fine clastic rocks derived from the Taconic Orogeny show an upward increase in carbonate content. 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 in northeast Indiana and northwest Ohio. Eight different mudstone facies were differentiated in 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, suggesting 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. The middle portion of the examined interval shows multiple banded silty mudstone cycles, with calcareous silt laminae and lags at the base (starvation surfaces), and grading upwards into dark crypto-bioturbated mudstones. The upper portion of the interval consists of 150 ft of greenish strongly bioturbated mud with macroscopically visible burrows. The study interval is overlain by a limestone-dominated unit that reflects diminished clastic input. Higher energy deposits (tempestites) suggest overall shallowing. A 640 ft thick core from Seneca County in northwestern Ohio shows a similar succession of mudstone packages. The banded pyritic mudstone at the base grade into gray and dark homogenized mudstones with pyrite lags and nodules. Above the middle part of the study interval, silt laminae and silt lags increase, possibly due to its more proximal setting. Strongly bioturbated mudstone and carbonate concretionary mudstone dominate the upper portion of the interval. Current ripples and hummocky cross stratification (HCS) indicate a shallower environment with storm influence. The greenish silty mudstones of the top of the succession also suggests increased sediment supply.