Abstract

The Brazer limestone (Mississippian) 12 mi. W. of Mackay in Custer County, Idaho, has sedimentary features that aid in interpreting the Mississippian history of the region. The rocks, approximately 7,400 ft. thick, range in age from Osage to late Meramec, possibly Chester, as determined from endothyrid Foraminifera. They are underlain by about 4,000 ft. of siliceous argillites (probably Lower Mississippian) and overlain by Challis volcanics (Tertiary). In this area, the Brazer consists of limestones with intercalated quartzose clastic rocks. The limestones change from fine-grained, impure, nonfossiliferous or foraminiferal in the lower part of the section to calcarenitic, pure, and very fossiliferous in the upper part. This change is accompanied by a corresponding increase in coarseness of the associated quartzose clastic rocks from fine siltstones to pebble conglomerates. The conglomerate beds, all lenticular and quartzose, are restricted to the upper 3,000 ft. of the section. They can be divided into those that have characteristics of beaches and those that resemble turbidity current deposits. Petrographic studies, faunal assemblages, and the great thickness of the Brazer in this area suggest that it was deposited in a marginal miogeosynclinal environment. The lower part was laid down in shallow, quiet marine waters, the middle in freely circulating marine waters of moderate depth, and the upper in a fluctuating marine environment near a low landmass probably on the W. or SW. Orogenic movements involving the landmass in Late mississippian time are recorded in the conglomerates.

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