The Stratigraphy of the Amsden Formation of Wyoming: ABSTRACT

Abstract

The Amsden Formation was studied in 1) western Wyoming, 2) the Wind River Mountains, and 3) the Big Horn Mountains area. Thirteen sections were measured in detail and representative samples collected for petrographic analysis. Based upon the range of percentage of clastic quartz and clasticity, the Amsden Formation is divisible, in ascending order, into: Subunit 1--the Darwin Sandstone and, where present, the overlying siltstone/shale; Subunit 2--a quartz-poor, predominantly carbonate sequence; Subunit 3--many thin, quartz-poor, cyclic pairs of carbonate/non-carbonate beds; and Subunit 4--many thin, quartz-rich, cyclic pairs of carbonate/non-carbonate beds. The greater amount of clastic quartz in the Wind River Mountains compared with areas to the north and west suggests a source area south of the Wind River Mountains. The clasticity and percentage of clastic quartz, in conjunction with lithologic curves, indicate that shallow water environments persisted in the area of the Wind River and northern Big Horn Mountains, whereas the central and southern Big Horns and western Wyoming areas were deeper water environments. Subunits 1 and 2 represent a general transgression, and Subunits 3 and 4 a general regression. The presence of many diastems and clastic/carbonate pairs of cyclic sediments in the Wind River Mountains and northern Big Horn Mountains sections and their absence in the deeper water environments suggest many minor oscillations in sea level during both the transgressive and regressive phases. End_of_Article - Last_Page 341------------