Abstract

Study of the Upper Jurassic (Oxfordian) sedimentary rocks of Wyoming and Montana indicates that intense storms raked this part of the continent often enough to play a major role in the redistribution of siliciclastic and bioclastic materials within an elongate seaway. These storms are evidenced by the widespread occurrence of three distinctly different types of coquinoid sandstone: channel lags, storm lags, and swell lags. Channel lags occur as basal concentrates of shell fragments and coarse siliciclastic materials in surge channels cutting marine bar sequences. Storm lags are sheetlike lenses of fragmented shells and coarse siliciclastics interbedded with mudrocks or very fine-grained sandstone. They result from the flushing of surge channels during storms and the deposition of storm-transported debris on the leeward flanks of bars and in muddy platform areas. Swell lags consist of concentrations of whole, or nearly whole thin-walled shells set in a dominantly clayey or silty matrix. Swell lags are interbedded with mudrocks and owe their origin to concentration mechanisms resulting from the passage of storm swells over muddy platform areas developed between marine bar ridges. Combinations of both storm and swell lags are not uncommon.

A model is proposed to explain the sedimentary motif of the central portion of the Upper Jurassic seaway during late Oxfordian time. The model consists of marine bar ridges formed and shaped predominantly by tidal currents. Surge channels were cut through the bar ridges during intense storms. Coquinoid sand formed channel and sheetlike lags in response to to the storm-generated currents acting both on marine bar ridges and interridge platform areas. The model implies that individual beds of coquinoid sandstone in Wyoming and Montana cannot be correlated over large areas. Therefore, stratigraphic interpretations based solely on bed-for-bed lithologic correlation over large portions of the Oxfordian seaway may not be valid.

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