Origin of Late Ordovician (mid-Mohawkian) temperate-water conditions on southeastern Laurentia: Glacial or tectonic?

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Abstract

In mid-Mohawkian time at the Turinian-Chatfieldian (Blackriverian-Trenton) transition, the extensive, shallow-, warm-water Blackriverian carbonate platform, in a low-latitude, subtropical setting across east-central Laurentia, underwent an abrupt change to temperate-water, sedimentary and faunal regimes. The change occurs across a regional unconformity, the formation of which is coincident with inception of a major Taconian tectophase and the related breakup of the Blackriverian Platform along old zones of structural weakness into smaller and higher platforms and shelves separated by a linear low area called the Sebree Trough. This breakup is interpreted to reflect far-field, foreland deformation, and temperate-water carbonates soon predominated across the resulting shelves and platforms, some 1000 km from the nearest open continental margin. Yet, paleogeographic models, the presence of warm-water carbonates distal to the trough as well as warm-water faunal and lithologic inliers in protected back-shoal settings on the Lexington Platform still indicate the presence of warm surface waters in a subtropical setting. Most interpretations support the necessity of upwelling from the Sebree Trough to explain this seeming anomaly, reflecting the significance of coeval tectonism in generating necessary conditions for the upwelling. Others have called upon glaciation for generating cooler oceanic waters and the enhanced oceanic circulation that would have developed coastal upwelling, sufficiently intense and widespread to have penetrated far across the open continental margin. Although latest Ordovician Hirnantian glaciation is well supported, evidence for the timing, extent, and likelihood of earlier Late Ordovician (Chatfieldian) glaciation is uncertain and contradictory. Although it is not possible to preclude the effects of such glaciation, the synergetic effects of tectonics and paleogeography may offer a more plausible explanation for upwelling during the time of syn-Taconic, far-field, foreland deformation on southeastern Laurentia; these effects may have even accentuated any glacial influence. Inasmuch as the Late Ordovician was a time of global tectonism, there may have been other low-latitude Ordovician cratonic settings in which changes in nearby orogens offer explanations for abrupt changes in sedimentary and faunal regimes on the adjacent forelands.

Original languageEnglish
Pages (from-to)163-175
Number of pages13
JournalSpecial Paper of the Geological Society of America
Volume466
DOIs
StatePublished - 2010

ASJC Scopus subject areas

  • Geology

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