Compositional Variation In Cincinnatian Mudstone Communities Upper Ordovician Of Ohio Indiana And Kentucky

Author : Nathan T. Marshall
Publisher :
ISBN 13 :
Total Pages : 86 pages
Book Rating : 4.:/5 (777 download)

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Download or read book Silt in the Upper Ordovician Kope Formation (Ohio, Indiana, Kentucky) written by Nathan T. Marshall and published by . This book was released on 2011 with total page 86 pages. Available in PDF, EPUB and Kindle. Book excerpt: The Edenian Age (Caradoc or upper Katian Stage of international usage; ~450 MA) Kope Formation has long been recognized as a significant natural laboratory for understanding depositional processes in Paleozoic and cyclic clastic-carbonate successions. In recent decades the Kope Fm. has been the focus of many sedimentological and paleontological studies aimed at understanding the mechanisms that drive cyclicity. There is ongoing debate whether the limestone represents the shallowest part of the cycle and should cap the shallowing-up shale interval cycle, or deeper part of cycle and should be construed as the base of a given cycle. Previous studies focus on the alternating fossiliferous limestone and shale, but ignore a third lithologic component: siltstone, which is assumed to be random intrabasinal calcareous silt. A closer study of siltstone petrology, using carbon content analysis to find the carbonate content, x-ray diffraction and thin-section studies to determine mineralogy and stable carbon isotope analysis to determine the diagenetic history of carbonate has shown that siltstone in the Kope Formation is actually siliciclastic, made of mostly quartz silt. The silt must have traveled a great distance from a terrigenous source, making its presence an indicator of an important change in sediment source and/or input rate. Detailed study of the stratigraphic distribution of siltstones shows that siltstones are cyclically distributed on two levels of cyclicity. They are commonly found to increase in frequency or thickness upward in mudstone intervals and appear in four distinctive pulses within the Kope Formation as a whole. Siltstone seems to record a relative rise in sedimentation rate of siliciclastics upward in cycles. The abrupt change from an upward coarsening siliciclastic succession to a thin bundle of shell-rich limestone upward in the cycles further supports the hypothesis that the limestone represents the period of sediment starvation, in which sediment does not drown out the carbonate factory. Further geochemical analysis using stable sulfur isotopes as a proxy for sedimentation rate and immobile trace elements for fingerprinting different provenances add validity to the conclusions found in the petrology and stratigraphic study. Sulfur isotopic analyses of siltstones indicate an increasing sedimentation rate from the start of a cycle to just below limestone beds, which have a very low sedimentation rate. Trace element data shows that siltstones and shale were derived from similar sources. On the contrary, the siliciclastic fraction of limestones were derived from a different source, indicating that the source for the mud and silt shuts off, and a different, source remains.