Sea-Level Drop Contrasted with Peripheral Bulge Model for Appalachian Basin During Mid-Ordovician
John M. Dennison, 1994. "Sea-Level Drop Contrasted with Peripheral Bulge Model for Appalachian Basin During Mid-Ordovician", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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Middle Ordovician strata record the early Taconic orogeny in the Appalachian basin and also exhibit an imprint of sea-level history. The Bays-Moccasin-Bowen redbeds and the Colvin Mountain-Greensport Formations result from the Blount Phase of the Taconic orogeny. Geographic distribution, geometry, and age relationships of these strata and their equivalents indicate also that a sea-level drop occurred in the Appalachian basin in latest Wilderness time, and its effects were superimposed on the longer lasting accumulation of the siliciclastic wedge of the Blount delta.
Evidence for brief sea-level drop then is present from Alabama to Virginia and Kentucky, and also in New York. Lithostratigraphic evidence for sea-level drop includes thin redbeds, sun-cracked shale sandwiched between marine carbonates, dolomite sandwiched between deeper water limestones, erosional truncation of bentonites on the Nashville dome, and Fincastle Conglomerate transported into deep-water shales in Virginia. Associated bentonite chronology and conodont biostratigraphy provide the best means for measuring time precisely.
Time constraints for this event make it sufficiently contemporaneous that migration of a peripheral bulge is precluded as an explanation. The geographic extent is too large, and it does not migrate through time, as would be expected from an orogenic mechanism.
Details of the Bays-Moccasin-Bowen redbeds suggest two sea-level minima in a short time span, recorded by the distal end of the Moccasin and Bowen redbeds and with the later one representing the greatest drop in sea level.
In northern West Virginia, Maryland, Pennsylvania, and Ohio there is not such clear evidence for this sea-level drop. This may be because deeper carbonate facies accumulated there, or simply because these strata have not been studied in sufficient lithologic detail.
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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.