Erosional History and Possible Passive Uplift of Paris-Willard Thrust Allochthon, Wyoming-Idaho-Utah Thrust Belt: ABSTRACT

Abstract

Stratigraphic distribution of clast lithologies in Sevier foreland basin synorogenic conglomerates in the southwestern Wyoming-southeastern Idaho-northeastern Utah thrust belt provides evidence of the erosional history of the Paris-Willard thrust allochthon. Conglomerates of the Upper Jurassic-Lower Cretaceous Gannett Group were deposited in response to initial movement along the Paris-Willard thrust system and are comprised of cobbles and pebbles of Ordovician through Jurassic strata, with upper Paleozoic and lower Mesozoic clasts most common. Thus, by conclusion of initial Paris-Willard thrust movement, the allochthon had been affected by widespread erosion of lower Paleozoic through lower Mesozoic strata. Evidence for subsequent erosion to deeper stratigraphic levels is contained in the basal Hams Fork Conglomerate Member of the Upper Cretaceous-Paleocene Evanston Formation. This unit contains abundant Precambrian quartzite cobbles, which must have been derived from the Paris-Willard allochthon, the only thrust sheet that contains Precambrian quartzite units. Genesis of the Hams Fork Conglomerate has been related to latest Cretaceous major movement along the Absaroka thrust. Thus, either the Paris-Willard thrust was fortuitously reactivated at the same time as major Absaroka thrust movement or uplift associated with movement along the Absaroka thrust resulted in coeval uplift of the Paris-Willard allochthon. One mechanism that could elevate the older Paris-Willard allochthon and allow for erosion to deep stratigraphic levels during major displacement along the Absaroka thrust would involve antiformal arching of the Paris-Willard allochthon as it was carried passively over a major footwall ramp of the Absaroka thrust. Folding and uplift of older passive thrusts carried piggyback by younger active thrusts might be an important mechanism in formation of source terranes for synorogenic foreland basin clastics. End_of_Article - Last_Page 865------------