Abstract

An integrated study of outcrop and subsurface stratigraphy and petrography, of the Cloverly group of the Casper arch area, Wyoming, shows that the Lakota conglomerate was probably deposited by eastward- and northeastward-flowing streams, as evidenced by cut-and-fill structures, trough cross-stratification, and the trend of channel sandstone bodies. The mineralogy of the Fuson shale suggests that it was dominantly derived from a residual soil cover. The Fall River sandstone, which overlies the Fuson shale, is a marginal marine sandstone body which consists of one thick sandstone unit in the SE. portion of the Casper arch. This sandstone body, as traced westward and northwestward, is separated by a thin shale unit into 2 distinct sandstone units; the lower sandstone unit thins and disappears about half of the way across the Casper arch, and its stratigraphic interval is replaced by siltstone, whereas, the upper sand body continues to the western flank of the arch. The Lakota conglomerate is composed largely of a mixture of well rounded detrital chert grains and multi-cycle quartz grains. The Lakota detritus contains approximately 23% black and 18% white chert grains. The black phosphatic chert was derived from the Rex chert member of the Phosphoria formation W. and SW. of the Casper arch area, possibly in eastern and central Idaho. The Lakota detritus consists predominantly of multi-cycle, subrounded to well rounded grains of plutonic quartz, with subordinate metamorphic and vein quartz. Subangular to angular quartz grains make up a small proportion of the quartz grains. The nonopaque heavy-mineral assemblage of the Lakota conglomerate is mature; 97% of it consists of numerous varieties of subrounded to well rounded zircon and tourmaline. The subrounded to well-rounded ultra-stable quartz, zircon, and tourmaline are multi-cycle grains, derived largely from preexisting sandstones. Angular hornblende, staurolite, rutile, zircon, and tourmaline form 3% of the nonopaque heavy-mineral assemblage. The angular nonopaque heavy-mineral grains are first-cycle detritus and were apparently derived from crystalline remnants of the ancestral Rocky Mountains on the S. and SW.

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