Abstract: Lacustrine Pseudovarves from Cretaceous Peterson Limestone, Western Wyoming

Steven W. Glass, Bruce H. Wilkinson

Finely laminated rhythmites are ubiquitous structures in marginal lacustrine facies exposed at the base of the Peterson Limestone in western Wyoming and southeastern Idaho. Nearly identical structures have been reported from other ancient lacustrine limestones, and have been interpreted as varves representing significant seasonal changes in temperate and/or evaporation rate resulting in increased carbonate precipitation during summer months. As with varved sediments forming in many modern temperate region lakes, individual laminae within the Peterson rhythmites are markedly graded. The lower part of each exhibits a sharp basal contact, is composed of fine sand-sized calcite, dolomite, and terrigenous silicate minerals, and grades vertically into a mud-sized fraction.

Unlike recent varved carbonate sediments, Peterson rhythmites exhibit invariant calcite to quartz ratios vertically through individual laminae, and an absence of organic material. Slump structures, penecontemporaneous microfaults, microload casts, and convolute bedding commonly are associated with these fine laminations which are restricted in their distribution to a 2-m thick interval exposed along a 25-km outcrop belt of thin marginal lake facies. They are noticeably absent from thicker lake-center sections. On the basis of their uniform mineralogy, associated sedimentary structures, and restriction within the Peterson Limestone, these laminae are interpreted as current-lain units which formed as turbid waters transported material derived from nearshore areas in a downslope directio . Although marked similarities exist between these laminations and true varves, their origin was not related to seasonal variations in carbonate precipitation within the Peterson lake.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma