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Upper Devonian of Western United States
The Late Devonian geologic history of the conterminous United States, west of the Transcontinental arch, is interpreted through a series of nine paleobiogeographic lithofacies maps. A sequence of 17 eustatic and epeirogenic events, representing a 17 m.y. long depophase, is dated and timed in terms of the worldwide standard conodont zonation. The nine mapped time slices, representing intervals of no more than 1 million to as little as 200,000 years, were chosen to depict the major sedimentological and paleogeographic changes produced by major rises and falls of sea level. Maps 1–3 demonstrate successively early Frasnian (early Late Devonian) onlap and later Frasnian construction and drowning of a carbonate platform. Maps 5–8 demonstrate four major transgressive pulses that interrupted a Famennian (late Late Devonian) worldwide regression. Maps 4 and 9, respectively, demonstrate major regressions caused by severe eustatic falls that preceded and accompanied extinction events terminating the Frasnian and Famennian Stages. Paleotectonic settings are partly reconstructed on maps 2, 3, and 5, and paleoenvironments are partly reconstructed on maps 7 and 8. Additionally, map 3 restores the original depositional limits for a 200,000 year time slice, representing maximum Late Devonian transgression. The history of the Pilot basin, which developed within the carbonate platform in an outer-shelf trough (Cordilleran miogeosyncline), is traced through four successive maps and is interpreted by means of a biostratigraphic cross section based on 207 conodont samples. Five columnar sections and three correlation diagrams illustrate the most significant stratigraphic units and their regional correlation. This paper represents a summation and interpretation of more than 3,000 lithostratigraphic and biostratigraphic data points studied during the authors’ 30 years of publications on Upper Devonian rocks.
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