Tectonic and Eustatic Influences Upon the Sedimentary Environments of the Upper Ordovician Strata of New York and Ontario
David Lehmann, Carlton E. Brett, Ronald Cole, 1994. "Tectonic and Eustatic Influences Upon the Sedimentary Environments of the Upper Ordovician Strata of New York and Ontario", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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The Upper Ordovician stratigraphic succession in New York and Ontario is superficially similar to a very large eustatically-controlled sequence. These strata are bounded by unconformities, and analogs of systems tracts are present. Submarine fan siltstones, sandstones, and shales comprise the lowstand systems tract analog; the transgressive systems tract analog is represented by a stratigraphically condensed section, and the highstand systems tract analog is characterized by an upward-coarsening, and generally upward-shallowing, succession of strata. The diachroneity of the systems tract analogs and of sequence-bounding unconformities suggest, however, that this stratigraphic succession was most strongly influenced by tectonic forces associated with the Taconic Orogeny.
In the Taconic foreland basin of New York and Ontario, thrust- and sediment-loading drove subsidence so that the Middle to Late Ordovician Trenton carbonate ramp progressively oversteepened and collapsed. In New York, the oversteepening is represented by a stratigraphic succession in which carbonate-dominated deposits are disconformably overlain by flysch. In the Toronto, Ontario region, which was more distal to the thrust sheets and, presumably, more proximal to the Late Ordovician tectonic hinge, the disconformable relationship between the underlying carbonates and the overlying siliciclastics grades to conformity. In New York and Ontario, progressive southeast to northwest oversteepening of the carbonate ramp resulted in a geographically diachronous shift from carbonate-dominated deposition to organic-rich mud deposition; a sediment-starvation surface (a condensed section) is often associated with this shift. The siliciclastic strata that overlie the condensed section record the prograding Queenston clastic wedge. Paleocurrent and stratigraphic data suggest that movement along Taconic, or older rejuvenated, basement normal faults was an important control on basin subsidence and filling.
Smaller scale isochronous stratigraphic changes that cross-cut facies patterns may record eustatic events. These possible eustatic events include: (1) an Early Maysvillian deepening event, recorded by the Collingwood Formation in Ontario; (2) a basal-Pulaski progradational event; and (3) a mid-Queenston transgression.
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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.