Deposition and Diagenesis of a Marine-Swamp Margin; The Providence Limestone and Adjacent Coals, Western Kentucky
Carol B. Dewet, Stephen O. Moshier, James C. Hower, Susan M. Rimmer, 1991. "Deposition and Diagenesis of a Marine-Swamp Margin; The Providence Limestone and Adjacent Coals, Western Kentucky", Mixed Carbonate-Siliciclastic Sequences, Anthony J. Lomando, Paul M. Harris
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Marine limestones, terrigenous siltstones, shales, and coals exhibit complex stratigraphic relationships in the western Kentucky coal fields at the southern margin of the Illinois Basin. The Providence Limestone member of the Sturgis Formation (Pennsylvania) is a mixed siliciclastic-carbonate unit bounded by the No. 11 (Herrin) and No. 13 (Baker) coal seams. The coals contain fractured and brecciated horizons, cemented by sparry calcite and dolomite. Shallow-marine limestones and clastic rocks are interbedded with coals; lateral thickness and lithologic changes are abrupt. Up to 6.6 ft (2 m) thick beds of limestone are brecciated, with abundant intraclasts and disrupted fabrics. Slickensided claystones containing plant debris (underclays) directly underlie the coal seam deposits. The coals are overlain by thin shales with limestone nodules, followed by normal marine limestones. The limestones are generally disrupted, with pedogenic features overprinting the original marine fabric. Hard claystones overlie the disrupted limestones.
The coals and Providence lithologies have complex diagenetic histories. Early lithification of brecciated coal fragments prevented complete organic decay, and the brecciated coals have well-preserved macerals. Petrographic examination of the Providence shows an original marine faunal assemblage in wackestones, packstones, and grainstones. Multiple episodes of carbonate cementation, silicification, and pedogenic alteration have altered the primary textures, creating a diverse suite of microfabrics.
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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.