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The AAPG/Datapages Combined Publications Database

Wyoming Geological Association

Abstract


Stratigraphy Of Wyoming; 31st Annual Field Conference Guidebook, 1980
Pages 101-116

Depositional Environments of the Lower Cretaceous Smiths Formation Within a Portion of the Idaho-Wyoming Thrust Belt

Steven Kenneth Durkee

Abstract

Strata of the Smiths Formation form an important marker unit in the Lower Cretaceous sequence of the Idaho-Wyoming Thrust Belt. These strata produce a couplet of thick black shales at the base, overlain by quartzitic sandstones, which make a distinctive marker horizon useful for determining stratigraphic position within the predominately fine-grained deposits of the Lower Cretaceous. These black shales and quartzitic sandstones are fairly uniform throughout the northern sector of the Idaho-Wyoming Thrust Belt. The laterally equivalent and lithologically similar Bear River Formation, as defined to the southeast, is more variable and does not exhibit this characteristic couplet. Contact relationships between the shales and sandstones are generally gradational, and coarsening upward; however, sharp erosional contacts can be observed locally which may represent channel fills. The formation, in general, exhibits a coarsening upward sequence of grain size. Worm burrows, along with oscillation ripple marks and ripple cross-stratification are common on the basal portions of the sandstones and suggest shallow water deposition. Fresh water molluscan fossils have been collected from horizons near the gas, middle, and near the top of the sequence, these generally occurring in silty and limey interbeds. Contact relations, sedimentary structures, lithology, and fossils indicate that the Smiths Formation was deposited in a fresh water lacustrine environment. These rock units are as widespread as the underlying Gannett Group, and are not a simple facies of either the underlying or overlying units.


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