CONGLOMERATES IN AND NEAR THE IDAHO-WYOMING THRUST BELT

Conglomerates have long been studied to determine diverse source areas. Conglomerates also exhibit diverse styles of deformation depending on P-T conditions. Four conglomerates are described from different burial depths to illustrate different structural behavior. The Neogene Salt Lake Formation overlaps thrusts and has been involved in normal faulting. Large and small clasts of Swan Peak Quartzite are abundant in diamictite and conglomerate beds. Touching clasts exhibit small solution pits and common fractures radiating from contact points. The rocks were buried no more than 2 km during normal faulting. At Togwotee Pass, 40 km east of Jackson Hole and east of the thrust belt, the Upper Cretaceous Harebell Conglomerate exhibits thrust-related deformation features in competent quartzite pebbles. Radial clast fractures and microfracturing are due to grain-to-grain contact, whereas parallel, sub-horizontal planar fractures are fault-related. Burial depth at time of deformation is largely uncertain but no more than 1 to 2 km. Lower Cretaceous chert conglomerate along the Idaho-Wyoming border was buried 10 km under the Meade thrust plate. Mary Beth Gray showed that the pebbles and cobbles have pressure solution indentations, and many clasts have numerous parallel, through-going, vertical fractures parallel to tectonic transport. The oldest rocks in the thrust belt were buried beneath at least 12 km of Proterozoic and Paleozoic rocks during thrusting. A thick quartzite conglomerate on Scout Mtn. exhibits little deformation, but the same unit has stretched pebbles in an adjacent mountain. Differing groundwater circulation patterns may be the cause of this differential behavior.