Lithostratigraphy and Depositional Environments of the Ancell Group in Central Illinois: A Middle Ordovician Carbonate-Siliciclastic Transition
T. H. Shaw, B. C. Schreiber, 1991. "Lithostratigraphy and Depositional Environments of the Ancell Group in Central Illinois: A Middle Ordovician Carbonate-Siliciclastic Transition", Mixed Carbonate-Siliciclastic Sequences, Anthony J. Lomando, Paul M. Harris
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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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Mixed Carbonate-Siliciclastic Sequences
The study of carbonate-siliciclastic mixed sequences has seen an increase in the number of investigations that focus on mixed settings as part of the continuum between the carbonate and clastic end members. Cyclic deposition in mixed basins by reciprocal sedimentation has become one of the foundation blocks for sequence stratigraphy. In addition, these mixed sequences have a variety of distinctive petroleum reservoir characteristics, important for both exploration and development programs. The emphasis now is on reevaluating ancient sequences in the light of a more dynamic understanding of spatial and temporal variations and controls on these sequences. Examples in this volume are subdivided under the following headings: Shelf Wide, Coastal and Inner Shelf, Middle to Outer Shelf, Slope to Basin and Paleokarst. Many mixed sequences have been described in the literature, but understanding the controls of these sequences from a process approach in now in an adolescent stage.