Foundering of the Cambro-Ordovician Shelf Margin: Onset of Taconian Orogenesis or Eustatic Drowning
Paul A. Washington, Steven A. Chisick, 1994. "Foundering of the Cambro-Ordovician Shelf Margin: Onset of Taconian Orogenesis or Eustatic Drowning", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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Onset of the Taconic Orogeny is generally dated by the age of the last shelf strata beneath Ordovician pelitic sequences. Recent stratigraphic analysis of the shelf sequence in the Champlain Valley, however, suggests that timing of the end of shelf sedimentation in a given area was controlled by eustasy as well as tectonics. Following major eustatic lowstands in Cambrian and Early-Middle Ordovician time, the shelf margin abruptly jumped cratonward and large areas of outer shelf foundered. Only by analyzing subsidence (stratigraphic accumulation) rates can the timing of the orogenic event be determined. Analysis of subsidence rates indicate a simultaneous start of the initial phase of orogenic subsidence in late Early Ordovician time for the entire shelf from Newfoundland to Pennsylvania. Arrival of the orogenic wedge varied from Middle Ordovician (Newfoundland) to early Late Ordovician (Champlain Valley) 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.