Abstract

The Cottonwood Limestone, a thin (7 feet or less), laterally persistent, fossiliferous, marine limestone and calcareous shale, has been traced in outcrop from southeastern Nebraska to north-central Oklahoma. Detailed field and laboratory analyses reveal 5 distinct facies. From southern Nebraska to central Kansas the lower half of the Cottonwood Limestone is a fine-grained buff limestone composed of finely comminuted fossils, especially the algal-foraminiferal intergrowth Osagia, echinoderms, and bryozoans (bioclastic facies). The upper half of the unit in this region is fine-grained buff limestone with abundant small fusulines; fossils typical of the bioclastic facies are also present (fusuline facies). In central Kansas the Cottonwood Limestone is a fine-grained, gray, massive limestone having abundant broken thalli of the calcareous alga, Anchicodium, and some Osagia (platy algal facies). The Cottonwood in southern Kansas and northern Oklahoma is an interbedded limestone and calcareous shale with a very diverse, abundant, and well-preserved fauna, especially brachiopods, molluscs, and ammodiscid foraminifers (shelly facies). At the southern limit of its outcrop the Cottonwood Limestone is a medium crystalline limestone with thin, shaly layers; molluscs, Osagia, and quartz silt are common (silty Osagia facies).

The bioclastic and fusuline facies were deposited in shallow, well-lit, moderately turbulent waters having very small amounts of terrigenous influx; water circulation was variably restricted. The shelly facies was deposited in a less turbulent and deeper-water environment, having good circulation and relatively large amounts of terrigenous material entering from the south; the silty Osagia facies was formed in a shallower, more turbulent, nearer-shore environment marginal to the shelly facies. Separating these two facies provinces was a broad shoal where the platy algal facies was deposited; here waters were moderately turbulent, well-lit, shallow, and somewhat restricted in circulation.

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