Sedimentology of a Shallowing-Upward Sequence in Middle Cambrian Carbonate-Siliciclastic Associations, Western Wyoming: ABSTRACT

Abstract

The Middle-Upper Cambrian succession in western Wyoming comprises a series of interbedded siliciclastics and carbonates, some of which were deposited in a variety of shallowing-upward sequences. Although the overall succession of basal Flathead Sandstone-Gros Ventre Formation-Gallatin Limestone suggests a classic transgressive package, minor and major oscillations of the strandline resulted in several regressive phases. Carbonates of the upper Death Canyon Limestone member and siliciclastics and carbonates in the lower Park Shale member (middle and End_Page 448------------------------------ upper members, respectively, of the Gros Ventre Formation) contain evidence of shoaling carbonate-clastic associations within a major transgressive sequence. Death Canyon carbonates accumulated far offshore in relatively shallow water of restricted circulation. The basal portions of the Death Canyon represent subtidal blanket carbonates consisting of burrowed and mottled biomicrites and peloidal biomicrites. Strandline stability resulted in prolonged periods of vertical aggradation of this carbonate platform into progressively shallower water. Evidence for this shoaling includes: (1) increase in abundance of ooids and intraclasts up section; (2) increase in the relative abundance of siliciclastic debris up section; (3) coarsening upward of the siliciclastics; (4) occurrence of cryptalgal structures and large algal stromatolites as well as desiccation features near the top of the Death Canyon. These stromatolites are closely spaced circular mounds from 1 to 3 m (3 to 9 ft) in basal diameter and 0.5 to 2 m (1.6 to 6.6 ft) in height. The mounds consist of a thick inner faintly laminated zone and a thinner outer zone of discrete columns composed of curved laminations. Pits and channels occur on the outer surface and probably represent areas where low-water stage runoff was concentrated. Carbonate production diminished with the influx of fine-grained siliciclastics near the Death Canyon-Park contact. Although the lower Park is predominantly dark, micaceous shale, it also contains cryptalgal micrites, algal stromatolites, and lenticular beds of feldspathic and arkosic arenites. These deposits all accumulated in shallow water a considerable distance from the Middle Cambrian shoreline. The siliciclastics were derived from nearby source areas that were probably offshore islands of Precambrian crystalline basement. Facies associations indicate deposition of the coarser clastics in subtidal settings. End_of_Article - Last_Page 449------------