--> ABSTRACT: Episodic Sedimentation on a Lower Silurian Storm-Dominated Carbonate Ramp, Anticosti Island, Quebec, Canada, by Terry Sami and Andre Desrochers; #91022 (1989)

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Episodic Sedimentation on a Lower Silurian Storm-Dominated Carbonate Ramp, Anticosti Island, Quebec, Canada

Terry Sami, Andre Desrochers

The 130-170-m thick Becscie Formation represents continuous deposition on a shallow, open-marine carbonate ramp across the Ordovician-Silurian boundary. The sequence reflects a generally quiet, shallow marine environment punctuated by episodic, storm-generated, high-energy events. These events deposited individual storm units, or tempestites, which occur as fining-upward sequences ranging from 0.5 to 80 cm thick. A complete ideal storm deposit consists of a sharp erosional base overlain by intraclastic to bioclastic rudstone fining upward into calcarenite and then into finely laminated calcisiltite grading upward into shale.

Tempestites exhibit a variety of storm-generated structures which are today exposed on extensive bedding planes. Gutters and gutter casts occur throughout the sequence and show a range of morphologies and fill/substrate combinations. Hummocky cross-stratification is widespread and restricted exclusively to the calcisiltite-rich tempestites. Flat-pebble conglomerates (intraclastic rudstone) occur through most of the sequence and contain clasts of mudstone, packstone, and grainstone, indicating extensive early sea floor lithification.

Tempestite sequences display lateral and vertical variations controlled by water depth and distance from shore. Construction of proximality trends permits recognition of lower order sea level changes within the overall regressive sequence. Sea level changes are believed to be eustatic, yet diastrophic-tectonic influences should not be dismissed due to regional tectonic activity.

Paleocurrent data suggest sediment transport by predominantly southwest-oriented geostrophic currents. Together with sedimentologic evidence, this supports a combined-flow model for storm sediment transport in the Anticosti basin.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.