OROGRAPHIC CONTROL ON THE AREAL DISTRIBUTION AND ABUNDANCE OF NODULAR MICRITE IN THE LOWER CRETACEOUS CLOVERLY FORMATION OF WYOMING

The nonmarine Cloverly Formation accumulated during the incipient development of the Sevier fold-thrust belt and adjacent foreland basin in the Early Cretaceous. Coarse, siliciclastic detritus derived from the fold-thrust belt was distributed by Cloverly rivers up to 600 km into the basin. Simultaneously, widespread lakes and floodplains were centers for deposition of mudstone, fossiliferous wackestones and packstones, and extensive formation of micrite nodules. Some micrite nodules occur in laminated mudstone also containing barite nodules and gypsum pseudomorphs, suggesting periods of increased lake salinity due to high rates of evaporation in an arid setting. In contrast, other nodular micrite is encased in granular, bioturbated mudstone containing abundant slickensides caused by expansion and contraction of smectitic mud during pedogenesis. The presence of pedogenic carbonate also indicates supersaturation and precipitation of calcium carbonate from groundwater under relatively arid conditions. Petrographic examination of all carbonate lithofacies reveals several episodes of recrystallization of micrite and truncation by spar-filled veins attesting to at least three diagenetic events. Consequently, genetic interpretation of a potential climatic signature from carbon and oxygen isotope data is virtually impossible.

The areal abundance and distribution of nodular micrite within the Cloverly Formation changes systematically across the foreland basin. The thickest accumulation of pedogenic carbonates and playa-lake deposits occur in proximal parts of the foreland basin; the nodules are rare or absent in the distal parts of the basin. This distribution may be related to the eastward decreasing intensity of an orographic "rain shadow" induced by the rising Sevier Mountains in eastern Idaho and western Wyoming during the Early Cretaceous.