Title

Shell Concentrations and Their Genesis: Case Studies in the Middle Devonian of Ohio and the Lower Pliocene of Baja California Sur

Degree Year

1993

Document Type

Thesis

Degree Name

Bachelor of Arts

Department

Geology

Abstract

Two case studies are used to address the question of what processes control the formation of shell concentrations in the stratigraphic record. Shell beds from both carbonate and clastic shallow marine sedimentary environments were utilized to assess Kidwell's (1986) model, which predicts that shell concentrations predominantly form during periods of low net sedimentation. The Middle Devonian Columbus Limestone located in Marblehead Quarry, Ohio, contains shell beds that were deposited during storm events over a carbonate bank occurring within a broad epeiric sea. This is indicated by the sharp basal contacts of the shell beds, fining-upward sequences within, and the disturbance of fossils from life position. The San Marcos Formation, a Lower Pliocene sequence of interbedded marine and non-marine deposits located in Baja California Sur, contains shell beds which formed as sediment starved transgressive lags of subtidal facies during repeated transgressiveregressIve cycles. This interpretation is supported by the high degree of lateral variability, several examples of depositional onlap, the association of scallops with volcaniclastics pebbles and boulders, and by the depositional facies of the other, non-fossiliferous units. Shell concentrations in the two locations formed under very different conditions, and by different processes. This demonstrates that shell concentrations form within a wide variety of depositional environments and that while Kidwell's model often predicts accurately the occurrence of shell beds, it does not fully encompass this diversity of depositional conditions.

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