Upper Paleozoic Basin Reconstruction in Central Idaho: Preliminary Stratigraphic Correlation of Allochthonous Permian Strata

Brian J. Mahoney, J. K. Geslin, P. K. Link

Pennsylvanian-Permian strata in the Smoky Mountains, west of Ketchum, Idaho, occur as tectonostratigraphic units bounded by low-angle faults of uncertain displacement. Reconstruction of facies within the Pennsylvanian-Permian Wood River basin will constrain displacement along these low-angle faults.

These tectonostratigraphic units consist of calcareous sandstones and carbonaceous siltites and shales of the partially coeval Wood River, Grand Prize, and Dollarhide formations. The carbonaceous lithologies of these formations are part of the Idaho Black Shale Mineral Belt, commonly containing anomalous concentrations of trace metals (zinc, lead, silver).

Sedimentary structures such as cross-stratification, convolute structures, parallel laminae, cyclic fining-upward sequences, and turbidite assemblages, along with lithologic variations, are used to establish facies relationships within the formations. The upper portions (Virgilian-Wolfcampian) of the Wood River Formation contain a thick sequence of locally cross-bedded calcareous sandstones which represent shallow marine shelf deposits. The Wolfcampian(?) Dollarhide and Wolfcampian(?)-Leonardian Grand Prize formations both contain sedimentary features, such as parallel laminae and turbidite sequences, which are indicative of deeper water assemblages. These formations are thought to have been deposited in contiguous portions of the Wood River basin. Stratigraphic and petrographic studi s suggest lateral continuity and possible vertical succession of these formations. The Dollarhide formation represents a submarine slope and basin environment, while the Grand Prize formation is interpreted as an outer shelf to upper slope transitional facies. Vertical facies changes in both formations indicate deposition during local progradation coeval with regional sea level rise. Structural relations between the tectonostratigraphic units suggest deposition of these formations in a westward-deepening basin, although available paleocurrent data is sparse and contradictory.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.