Abstract

Recent mapping of the Willard thrust near Ogden, Utah, reveals a recumbent syncline overturned to the east, in Cambrian to Mississippian rocks beneath at least 10 sq mi of the Willard thrust. The syncline, together with widespread minor folds and sharp facies differences across the thrust in rocks from Precambrian to Pennsylvanian age, confirms eastward movement of the upper plate. These data are consistent with the concept that the Willard thrust is the westward extension of the Paris-Woodruff thrust and a part of the Cordilleran foldbelt of King. According to this interpretation, the Cache “uplift” of Eardley is a gently folded and downwarped portion of the allochthon, not a fault-bounded uplift within the craton. Both stratigraphic and structural data support the concept that the Willard thrust bends southward, west of the Wasatch Range, passing just west of Antelope Island, to connect with the Charleston-Nebo thrust exposed near Salt Lake City. The Willard would thus bisect Precambrian outcrops that form Eardley's Northern Utah uplift; thick sections of unmetamorphosed Precambrian rocks forming the west half are allochthonous and have been thrust eastward over the crystalline basement in the east half.

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