--> Abstract: Hardgrounds and Omission Surfaces in Middle Ordovician Deep Water Carbonates: Probable Indicators of Relative Sea Level Fluctuations, by F. D. Siewers and P. A. Sandberg; #91012 (1992).

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ABSTRACT: Hardgrounds and Omission Surfaces in Middle Ordovician Deep Water Carbonates: Probable Indicators of Relative Sea Level Fluctuations

SIEWERS, FREDERICK D., and PHILIP A. SANDBERG, University of Illinois, Urbana, IL

Numerous submarine hardgrounds and sedimentary omission surfaces crop out in several continental margin sections of Whiterockian-age limestones in central and southern Nevada. The hardgrounds cap successions of echinoderm packstone-grainstone (encrinites) and microbioclastic packstone-grainstone and are characterized by phosphatized surfaces, concentrations of orthoconic and coiled cephalopods, and encrusting echinoderm holdfasts. The omission surfaces top intervals of burrow-mottled mudstone to packstone, are commonly phosphatized, and exhibit sediment filled burrows. Unlike the hardgrounds, which were lithified enough to support an encrusting epifauna, the omission surfaces were likely only firm to semi-indurated, as indicated by common soft-sediment deformation of the substrates on which they developed. Both the hardgrounds and the omission surfaces mark vertical lithologic changes from carbonate-dominated sedimentation to either mixed carbonate-siliciclastic or predominantly clastic (shale) sedimentation.

Stratigraphic, lithologic, and biostratigraphic data indicate hardground development occurred in roughly coeval depositional settings during relative sea level rise in early Whiterockian time (Orthidiella

and lower Anomalorthis biozones). Hardground formation appears restricted to the shelf and upper slope, whereas phosphatic omission surface development was limited to middle and lower slope settings. Both features are indicative of low net sediment accumulation during the early Whiterockian transgression. The occurrence of at least two hardgrounds or omission surfaces in each of the measured sections suggests the transgression may have occurred as discrete flooding events rather than as a smooth continuous rise. Additional litho- and biostratigraphic investigations are necessary to constrain those events.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)