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The Borden Formation in north-central Kentucky consists of 4 widespread units--a basal clay shale, a middle siltstone and silty shale, a glauconite-rich marker bed, and an upper siliceous silty carbonate. The lower 2 units are a clastic deltaic wedge, the upper limit of which is defined by the marker bed. The wedge thins gradually westward at a few feet per mile to the vicinity of Elizabethtown, where it was found by areal geologic mapping to pinch abruptly from 260 to 50 ft southwestward across a zone about 2 mi wide. This more steeply sloping surface represents the clinoform or foreset front of the Borden delta.
Recognition of this part of the fossil delta is an important adjunct to reconstructing the Late Devonian and Early Mississippian geography of the Illinois basin. Within this framework, several extrapolations are possible: (1) time planes within the delta parallel the delta front; (2) the water into which the delta prograded was at least 200 ft deep in this area; (3) the basal clay shale represents prodelta fondoform sediments; (4) the middle siltstone and silty shale represent the clinoform sediments of the delta proper; and (5) the upper silty carbonate represents resumption of sedimentation under somewhat different conditions following a period characterized by thin glauconite-rich sediment of possible nondeltaic origin.
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