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Tectonic Control on Formation and Cyclicity of Major Appalachian Unconformities and Associated Stratigraphic Sequences

By
Frank R. Ettensohn
Frank R. Ettensohn
Department of Geological Sciences, University of Kentucky, Lexington, KY 40506-0053
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Published:
January 01, 1994

Abstract

Recently developed flexural models suggest that lithospheric responses to early craton-margin orogenies should result in at least one major unconformity, both within and beyond the foreland basin, as well as a distinct sequence of lithologies largely restricted to the foreland basin. Consequently, interpretation of tectonic origin is based on (1) the presence of a distinctive overlying flexural sequence, (2) the coincidence of unconformity formation with the inception of established orogenies or tectophases therein, and (3) the distribution of unconformities relative to probable loci of tectonism.

Based on the above criteria, ten of the 13 major interregional and regional unconformities in the Appalachian basin appear to reflect major tectonic control, one is uncertain, one is largely eustatic in origin, and one probably reflects some combination of tectonic and eustatic control. Eight of the ten tectonically related surfaces are concurrent with the initiation of tectophases in the Taconian, Salinic and Acadian orogenies, whereas the other two reflect overlap of Mississippian Ouachita flexural events into extreme southern parts of the Appalachian basin. A widespread Early Pennsylvanian unconformity probably coincides with the initiation of the Alleghanian orogeny but lacks the anticipated overlying flexural stratigraphic sequence. Although this surface could reflect major eustatic influence, the differences in the accompanying sequence might just as likely result from the different style of tectonism accompanying this late-stage orogeny. The only certain, largely eustatically derived unconformity in the Appalachian basin appears to be that at the Ordovician-Silurian boundary, and even it bears some overprint of Taconian tectonic influence. However, a Middle Mississippian unconformity may represent some combination of eustatic lowering and relaxational bulge movement.

Inasmuch as typical Appalachian interregional or regional unconformities recur repeatedly in Paleozoic rocks both within and beyond the Appalachian basin, they must be considered cyclic. The cycles, however, are irregular and appear to be largely related to concurrent phases of tectonism.

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Contents

SEPM Concepts in Sedimentology and Paleontology

Tectonic and Eustatic Controls on Sedimentary Cycles

John M. Dennison
John M. Dennison
University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
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Frank R. Ettensohn
Frank R. Ettensohn
University of Kentucky, Lexington, Kentucky
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SEPM Society for Sedimentary Geology
Volume
4
ISBN electronic:
9781565762275
Publication date:
January 01, 1994

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