High Frequency Eustatic and Siliciclastic Sedimentation Cycles in a Foreland Basin, Upper Devonian, Appalachian Basin
Jonathan K. Filer, 1994. "High Frequency Eustatic and Siliciclastic Sedimentation Cycles in a Foreland Basin, Upper Devonian, Appalachian Basin", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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During the Late Devonian, a thick clastic wedge, derived from a tectonically active source area to the east, was deposited in the Appalachian basin. The basinal facies of the wedge is composed of blackish-gray or dark-brown organic carbon-rich shales alternating with gray or greenish-gray non-organic shales. Five large (third-order or approximately 2 Ma or longer) black/gray shale cycles have been previously recognized in the New York outcrop belt, as well as in the subsurface throughout the basin. Eleven newly defined higher frequency basinwide organic/inorganic shale cycles occur within the relatively inorganic portion of one of the larger cycles. These higher frequency lithologic cycles represent the basinal expression of fourth-order (0.1-0.3 Ma) cycles of parasequence scale.
The stratigraphic position of these cycles can be traced into nearershore silty and sandy facies using subsurface gamma-ray logs, revealing a synchronous cyclicity. These correlations show that the eleven cycles can be subdivided into two stacked sets of basinwide progradational parasequence sets. This pattern is confirmed by correlation into the most distal portions of the basin and examination of patterns of thinning and convergence. A previously recognized major transgression occurs at the top of the interval which rapidly initiated a major period of organic-rich shale deposition (Huron/Dunkirk Shales). These stratigraphic patterns are interpreted to result from a series of minor eustatic sea-level cycles during Upper Devonian time.
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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.