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The Middle Ordovician Ancell Group is the basal transgressive deposit of the Tippecanoe sequence in the Illinois Basin region. The mixed carbonate and siliciclastic composition of its constituent lithologies is a reflection of depositional conditions related to regional tectonism, the lithic character and nature of erosional processes in adjacent source areas, and changes in relative sea level and paleoclimate. Differentiation of lithofacies, their inferred depositional environments, and their vertical and lateral relationships provide a basis for identification of three major depositional cycles. Each cycle is made up of small-scale, shallowing-upward sequences that tend to reflect increasingly subtidal conditions upsection. The cycles are regionally correlative; consequently, they provide a basis for subdividing the Ancell Group into genetic units and for interpreting regional sedimentological, paleoclimatic, and tectonic processes during early-Middle Ordovician time in the Illinois Basin region.

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