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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 54 (1970)

Issue: 3. (March)

First Page: 487

Last Page: 502

Title: Petrology of Cretaceous Mowry Shale of Wyoming

Author(s): John Clements Davis (2)


The Mowry Shale is a dark, siliceous, Lower Cretaceous shale found throughout Wyoming and in parts of adjacent states. The unit is composed of four facies: siliceous shale, nonsiliceous silty shale, sandstone, and cristobalite shale. Bentonite and porcelanite also are present.

Mineral constituents of the Mowry were determined by an X-ray diffraction technique combining internal and external standards. Mowry Shale contains an average of 50 percent quartz, 5 percent feldspar, and 2 percent organic carbon. The rest of the rock is composed of clay minerals, predominantly mixed layer, and minor zeolites. The lower part of the Mowry contains kaolinite and the upper Mowry is enriched in montmorillonite.

Feldspar is most abundant in a wedge-shaped zone extending from the west. Quartz is present in above-average amounts in an arcuate belt through the center of Wyoming. Organic carbon distribution roughly parallels that of quartz.

Mineral-distribution patterns in the Mowry Shale suggest that it is a transgressive clastic unit similar to other Cretaceous fine-grained deposits in the Rocky Mountains. Excess silica and organic carbon in the rock were derived from tests of Radiolaria and other planktonic organisms that proliferated in upper zones of a restricted terminus of the boreal seaway. Radiolarian bloom was promoted by a continual supply of silica to the seaway from the western margin. Deposition of Mowry Shale ceased when the boreal seaway became connected to the Gulf sea. Apparently, increased circulation destroyed environmental conditions necessary for radiolarian growth, and Mowry deposition was supplanted by normal marine deposition.

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