Gannett Group of Western Wyoming and Southeastern Idaho

Abstract

The Jurassic and Cretaceous Gannett Group in the thrust belt of western Wyoming and southeastern Idaho, primarily of freshwater deposition, consists of five units (in ascending order): the Ephraim Conglomerate, Peterson Limestone, Bechler Formation, Draney Limestone, and an upper mudstone unit here raised to formation status and designated the Smoot Formation. All the formations are thickest in the type area, along the Idaho-Wyoming line, where the total thickness exceeds 5,000 ft. The Gannett is underlain, with apparent conformity, by the marine glauconitic sandstone of the Jurassic Stump Sandstone and is overlain conformably by the Cretaceous marine lower Tygee Member of the Bear River Formation. The Ephraim Conglomerate and Bechler Formation consist of conglomerate, sandstone, and red mudstone containing beds with abundant calcareous nodules. Grain size is coarsest in the type area and becomes finer eastward, and both formations disappear abruptly westward. Charophytes and freshwater ostracods are present in the upper calcareous parts of these formations. The lower part of the Ephraim contains a marine molluscan fauna. The Peterson and Draney Limestones consist of limestone with interbedded gray shale containing abundant charophytes and freshwater ostracods. The newly designed Smoot Formation consists of red mudstone, siltstone, and calcareous, nodular beds which also contain charophytes and freshwater ostracods. Except for the lower Ephraim, all the units contain a fluvial and lacustrine biota. Nine species of charophytes and 11 species of ostracods were found in the Gannett. One species of charophyte and three species of ostracods are new. The ostracods Cypridea A and Theriosynoecum angularis and the charophyte Clypeator corrugatus are known only from the Peterson. The ostracods Cypridea anomala, C. nitidula, C. wyomingensis, C. brevicornis, C. compta, Theriosynoecum persulcatum, and Cypris purbeckensis and the charophyte Amblyochara latifasciata are present in the upper Bechler, throughout the Draney, and in the lower and middle parts of the Smoot. The charophyte Mesochara A is known only from the Draney Formation. The ostracods Cypridea diminuta, Metacypris A, and Eucandona A, and the charophytes Flabellochara harrisi, Atopochara trivolvis, Stellatochara mundula, Mesochar voluta, Charaxis striatus, and Obtusochara cylindrica are present throughout the nonmarine part of the Gannett Group. Lower Ephraim mollusks are of Jurassic age. The Early Cretaceous Neocomian cannot be distinguished from present fossil evidence, but may be represented by coarse clastic rocks in the middle Ephraim. Charophytes and ostracods of the Gannett are of Aptian age; however, the middle Smoot contains a biota more like that of the overlying Albian Bear River Formation. Amblyochara latifasciata, Theriosynoecum angularis, and Metacypris A are present in the Draney but are very sparse. All three are abundant in the Smoot and the first two also are present in the overlying Bear River. Three species of charophytes and three species of ostracods are restricted to the lower parts of the Gannett and are absent in the Smoot. Four species of charophytes and eight species of ostracods which are found in the Gannett are absent in the overlying Bear River Formation. Therefore, the Smoot seems to contain a transition biota and is interpreted to be partly of Albian age.