Petrology and Stratigraphy of Nonmarine Upper Jurassic-Lower Cretaceous Rocks of Western Wyoming and Southeastern Idaho

Abstract

The Nevadan orogeny had little influence on sedimentation along the eastern margin of the Cordilleran geosyncline in the Late Jurassic Epoch. Montmorillonite in the Morrison, upper Ephraim, and Cloverly formations is believed to be an alteration product of volcanic ash blown from volcanic centers in western California and Oregon during the Late Jurassic, and from sources farther north during the Early Cretaceous. Thus the presence of such ash indirectly reflects tectonism in the present western Cordillera, but the first changes in physical sedimentation that can be related directly to the Cordilleran orogeny occurred in western Wyoming in Early Cretaceous time, as shown by study of Gannett and Cloverly rocks. Petrologic similarities, especially of feldspar and chert content, between the Morrison Formation in central Wyoming and lower Ephraim formation in the Wyoming-Idaho thrust belt suggest that they were derived from the same sources and probably are time equivalent, if considered within the Late Jurassic tectonic setting. The Wyoming interior basin was stable during Late Jurassic time, and one source of sediment was on the southwest in western Utah and northern Arizona. Sediment was carried northeastward by prograding streams as the Sundance Sea retreated north. Abrupt coarsening and influx of phosphorite and spicular chert detritus at the base of the Cretaceous Cloverly Formation and basal conglomerate of the upper Ephraim formation suggest that a change in provenance accompanied tectonic disturbances. Limestone clasts in the conglomerates came from the Pennsylvanian and Permian rocks of Idaho and northern Nevada. This fact and concurrent textural coarsening suggest that the basal Cloverly is largely time equivalent to the lowest widespread conglomerate in the Ephraim. Regional maps show that the Cloverly and equivalent formations both thicken and coarsen westward. As a result of Early Cretaceous uplift, sediments composed predominantly of chert and carbonate were shed eastward to form a clastic wedge deposit in the western Wyoming basin. Unlike some better known clastic wedges (e.g., Catskill Formation), this one contains little feldspar and few igneous and metamorphic rock fragments.