Tectonic Control on Formation and Cyclicity of Major Appalachian Unconformities and Associated Stratigraphic Sequences
Frank R. Ettensohn, 1994. "Tectonic Control on Formation and Cyclicity of Major Appalachian Unconformities and Associated Stratigraphic Sequences", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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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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Tectonic and Eustatic Controls on Sedimentary Cycles
The collected volume begins with a brief perspective by one of the conveners, followed by articles in order of increasing stratigraphic age. Eustatic sea-level changes and tectonic warpings of basins are competing mechanisms for explaining many stratigraphic patterns. The model for sea-level changes should be developed first for a basin, since it is allocyclic and leads to a series of time bands in the strata. The residual effects should then be modeled for tectonic patterns affecting the depositional processes. Doing the reverse limits time constraints on the tectonic warping models and will blur the resolution of detailed time surfaces in the strata. Case histories of situations with both tectonic warping and time surfaces marked by sea-level events will lead to improved interpretations of earth history.