EARLY PENNSYLVANIAN DEVELOPMENT OF THE OQUIRRH BASIN IN SOUTHERN IDAHO

Strata of the Carboniferous-Permian Oquirrh Group in the Sublett Range of south-central Idaho show that this portion of the Oquirrh basin began major subsidence early in the Pennsylvanian. This area was considered an area of relatively limited shallow deposition on the Oquirrh Shelf, a northeastern portion of the basin in northern Utah and SE Idaho, but recent geologic mapping to resolve uncertainty about the lower Oquirrh in the Sublett Range has shown that subsidence and deposition of large quantities of siliciclastic sediments began by the Atokan in this area. The lowest Oquirrh unit in the Sublett Range is the 304m limestone-quartzite Heglar Canyon Fm, dated as mid Morrowan (by conodonts). The overlying 1300m Tussing Fm contains mid Atokan (dated by conodonts) deposits in the lower part of the formation, at 114m above the base in North Heglar Canyon (420m above the Oquirrh base). The Tussing contains sandstones, siltstones, conglomerates and a few laterally extensive limestones. The conglomerate sheets and lenses occur in a unit with Zoophycus facies trace assemblage, suggesting slope deposition. The conglomerates are similar to chert-rich conglomerates in the Hailey Mbr of the Wood River Fm, which are derived from Antler orogenic belt highlands, and were probably transported south to the Sublett area along a basin low. This indicates the Oquirrh basin extended north across the Snake River Plains. Also, depositional trends and facies belts of the Wood River deposits in the Sun Valley-Hailey area are similar to those south of the Snake River Plains. Major subsidence began earlier in the northern portion of the Oquirrh basin than in the southern portion of the basin.