Abstract

A mechanical analysis and heavy-mineral study were included as part of an investigation of the stratigraphy of the Morrison (late Jurassic) and Cloverly (early Cretaceous) rocks in the southern Big Horn Mountains, Wyoming. A total of 131 samples were mechanically analyzed and 34 slides were examined for heavy minerals. The heavy-mineral study was confined to the 62-micron fraction (very fine sand grade). The results show that the Cloverly sandstones are somewhat coarser than those of the Morrison, but have essentially the same relatively high degree of sorting, roundness and sphericity. The Morrison and Cloverly formations have approximately the same major heavy mineral assemblage, namely magnetite-ilmenite, leucoxene, hematite, pyrite, zircon, tourmaline, rutile, and garnet. They also contain many of the same minor minerals. However, of the 32 minerals and varieties identified, only colorless zircon and colorless garnet appear to be stratigraphically diagnostic. Specifically, it is suggested that a low zircon-high garnet content characterizes the Morrison whereas a high zircon-low garnet content characterizes the lower member of the Cloverly. If this distinction is real, the relationship should be useful in picking the contact between these 2 lithologically similar formations, especially in subsurface work. The abundance of the stable heavy-mineral suite, zircon-tourmaline-rutile, and the high degree of rounding, indicate that both formations were derived largely from preexisting sedimentary rocks.

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