Mississippian Carbonate-Siliciclastic Eolianites in Southwestern Kansas
C. Robertson Handford, Benjamin J. Francka, 1991. "Mississippian Carbonate-Siliciclastic Eolianites in Southwestern Kansas", Mixed Carbonate-Siliciclastic Sequences, Anthony J. Lomando, Paul M. Harris
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Previous investigations of Mississippian St. Louis and Ste. Genevieve limestones in southwestern Kansas have suggested deposition in shallow marine environments exclusively. However, recent work has determined that these formations also contain a heretofore unrecognized eolian facies. Cross-bedded eolianites, made up of well-sorted ooid, skeletal, and peloid carbonate grains mixed with siliciclastic sands, reach 100 ft (30.5 m) thick in an area encompassed by seven southwestern Kansas counties. Cross-bed sets average about 1 to 3 ft (0.3 to 0.9 m) thick, but some reach 9 ft (2.7 m) thick. Foreset laminae, which are inclined approximately 16° to 20°, were formed by eolian grainfall processes. Thin, continuous laminae with inversely graded (texturally and compositionally) grains are abundant and interpreted as subcritically climbing translatent strata formed by migrating wind ripples. St. Louis subtidal grainstones commonly contain fragmented, large skeletal grains and are both cross-bedded to burrowed and moderately sorted. In contrast, the eolianites are well sorted and lack any coarse-grained bioclasts. All carbonate grains in the eolianite deposits are extremely abraded and well rounded. Indirect evidence suggesting an eolian origin includes features interpreted to represent rhizocretions, paleocaliche crusts, and a brecciated eolianite thought to have formed by surface weathering of a partially cemented dune ridge.
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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.