Cambrian Pisolites as Paleoenvironment and Paleotectonic Stress Indicators, Rattlesnake Mountain, Wyoming

Douglas G. Neese, James H. Vernon

Pisolitic-rich carbonates occur within the uppermost 0.5 m of the Meagher Limestone member of the lower Gros Ventre formation in exposures near Cody, Wyoming. The Meagher Limestone is overlain by 51 m, and underlain by 63 m of dark gray Gros Ventre shale. Pisolites range in size from 2.0 to 18 mm in diameter and occur in lime grainstones associated with trilobite fragments, peloids, glauconite, fine-grained subangular quartz, and minor oolites. Girvanella grainstones 15-20 cm thick directly underlie the pisolite strata and have contributed to some of the carbonate material within pisolite nuclei. Dolomite and ankerite may occur within pisolitic rocks as finely crystalline irregular patches.

Pisoliths commonly show an oblate ellipsoid shape, with maximum flattening perpendicular to bedding. Long-axis to short-axis ratios of these grains in fracture planes perpendicular to bedding average between 2.5 to 3.5, with the long axis parallel or subparallel to bedding. Grains observed in bedding planes have ratios averaging between 1.5 to 2.0. A paleostress state has produced a strain ellipsoid with long-axis ratios ranging from 1.7 to over 3.0. There appears to be little or no tectonic strain on the bedding plane, so the strain can be described as uniaxial, with maximum compression perpendicular to bedding.

The majority of carbonate rocks in the Meagher Limestone were deposited in a normal marine subtidal setting, while ooid and pisolitic grain types are suggestive of subtidal-peritidal conditions. Because of the strain deformed pisoliths, a subaqueous versus subaerial environment of pisolite genesis is difficult to assess.

A siliciclastic sandstone, 0.6 m thick with low-angle tabular cross-bedding, is present immediately beneath the Meagher Limestone. The sandstone is composed of 94% fine to medium sand-size subangular quartz grains and is associated with glauconite, minor biotite, zircon, and ilmenite. Both quartz overgrowths and calcite are present as cement. Siliciclastic sediments are suggestive of a nearshore shallow marine environment.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.