--> ABSTRACT: Impact of Paleoceanographic Changes on Carbonate Platform Evolution: Ecologic Influences on Carbonate Sequence Stratigraphy, by M. L. Caplan, M. R. Bustin, and K. A. Grimm; #91021 (2010)

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Impact of Paleoceanographic Changes on Carbonate Platform Evolution: Ecologic Influences on Carbonate Sequence Stratigraphy

CAPLAN, MARK L., MARC R. BUSTIN, and KURT A. GRIMM

Integration of sedimentological, paleontological and geochemical data from a Late Devonian carbonateiblack shale transition in the Alberta subsurface suggests that eutrophication rather than relative sea level change governed the demise of a carbonate platform and its burial in organic-rich mudrocks.

Upper Devonian Big Valley carbonates are conformably overlain by black, organic-rich shales of the Lower Carboniferous Exshaw Formation. Big Valley deposits record the vertical transition of three facies: 1) oncolite-brachiopod- crinoid packstones, dominated by benthic photoautotrophs; 2) phosphate- and glauconite-rich crinoidal-brachiopod-bryozoan grain stones, characterized by benthic heterotrophic suspension feeders; and 3) calcareous mudstones. Demise of the Big Valley platform has been attributed to a major transgressive event. Our data indicate that eutrophication of surface waters led to inhospitable benthic conditions and demise of carbonate producers; changes that culminated in deposition of organic-rich Exshaw mudrocks. Geochemical (delta{13}C[org] and delta{15}N[tot]) trends, becoming lighter toward the basal Exshaw Formation, confirm our paleoecological and lithological interpretation.

New evidence emerging from the application of multidisciplinary studies suggests that genesis of some stratigraphically important surfaces in carbonate successions result from environmental perturbations of the water column that affected ecological composition and carbonate accumulation. Such lithological changes in the carbonate rock record are similar to those widely attributed to relative changes in sea-level; these distinct (yet commonly interrelated?) fabrics must be considered in sequence stratigraphic architecture of ancient carbonate deposits.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.