SEDIMENTARY FACIES IN THE UPPER ORDOVICIAN MAQUOKETA GROUP OF INDIANA: A PRELIMINARY ANALYSIS

The Upper Ordovician Maquoketa Group is a mixed carbonate-clastic succession found in Indiana. Previous work on this succession has largely ignored small-scale (i.e., cm-scale) heterogeneities within the fine-grained mudstones. The purpose of this study is to identify facies distribution along more stringent sedimentary guidelines and how they relate to geophysical properties in the sub-surface using wire-line logs. Describing lithofacies variation of the Maquoketa from proximal paleo-sediment source (i.e., Taconic mountains) to distal depositional zones in the Illinois Basin will be the primary objective.

Detailed descriptions were made of three cores, from northeast, north central, and northwestern Indiana. In the proximal setting of Wells County, we find that, vertically, the lower Maquoketa Group consists of slightly darker mudstones, with macroscopic organic flakes (1-2mm), pyrite nodules (3-10mm in diameter) and cemented pyritic horizons, as well as distinct graptolite-rich horizons. Upsection, light off-color cm-scale mudstone banding is observed, along with minor erosional surfaces and calcareous silt lags. Brachiopods, and later bivalves, crinoids, and trilobites become common in the upper Maquoketa Group, but gradually decline towards the top of the studied section. Alternating black/grey shale cycles, varying in thickness from 2cm and building towards 70cm upsection, interpreted as parasequences, are present in the upper half of the group, with variable macroscopically visible bioturbation. Before fading into a carbonate-dominated succession, the Maquoketa is capped by light-gray mudstone, with horizontal and vertical burrows. In the distal setting of Fulton and Lake Counties, the variety of mudstone sub-types decreases dramatically (less than half of the sub-types observed in the proximal setting).

The lithofacies variation in terms of shale types becomes quite apparent upon moving westward (i.e., distal), as the shale types vary in thickness and frequency, as well as changes in the overall matrix composition of the group with regards to calcite cementation. This probably reflects depositional environment, indicating a shift to a deeper marine setting. Future work will focus on petrographic characterization using polished thin sections and SEM of ion-milled surfaces.