Recognition of Regional (Eustatic?) and Local (Tectonic?) Relative Sea-Level Events in Outcrop and Gamma-Ray Logs, Ordovician, West Virginia
Richard J. Diecchio, Brett T. Brodersen, 1994. "Recognition of Regional (Eustatic?) and Local (Tectonic?) Relative Sea-Level Events in Outcrop and Gamma-Ray Logs, Ordovician, West Virginia", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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Plots of the cumulative aggradation of cyclically repetitive strata were generated by a method similar to that for Fischer plots. Plots were generated for three outcrop sections of the upper part of the Upper Ordovician Juniata Formation, eleven wells penetrating the Juniata Formation, and four wells penetrating the entire Ordovician System. The resultant plots are inferred to represent relative changes in sea level for each locality over the time interval represented.
Cycles of relative sea-level change with a periodicity of approximately 5 my are apparent throughout the Ordovician. These cycles are superimposed upon a larger scale relative sea-level curve that reflects the sea-level pattern associated with the Creek holostrome (Tippecanoe I sequence). Each of these sea-level cycles can be correlated from well to well and, because of their regional extent, are possibly the effect of an eustatic cause.
Other relative shallowing and deepening events are apparent in some of the wells, and at various stratigraphic positions. These local events are interpreted as tectonic uplifts and downwarps. Many of these localized sea-level events are more long-lived than the regional 5-my sea-level signal, but have a shorter duration than the Creek holostrome.
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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.