Abstract

The east trend of the Uinta Mountains, which contrasts with the north to northwest trends of many other prominent ranges in the Rocky Mountain foreland, is the product of an ancient heritage. Isotopic dating of basement rocks indicates that the margin of the Archean continent trends eastward at the site of the Uinta Mountains. The precise location and nature of the margin are obscured by discontinuous exposures and by the effects of an Early Proterozoic structural and metamorphic overprint.

The east-trending continental margin was the site of later structural events. During Middle Proterozoic time, clastic sediments as much as 8 km thick were deposited in a basin open to the west and perhaps to the south and controlled on the north by an east-trending hinge line. Geophysical evidence suggests that the deposits of the basin, the Uinta Mountain Group, do not extend beneath the present basins north and south of the Uintas; Late Proterozoic faulting on east trends may have narrowed their original extent. During Middle and Late Cambrian and Late Devonian times, an east-trending positive area occupied the site of the Uintas, and in Early Ordovician time an east-trending positive area extended west of the Uintas.

From Late Cretaceous into at least Oligocene time the Uintas were uplifted at various times and probably by differing amounts at various places along the range, and the east-trending Uinta Basin formed to the south.

Some major fault zones in the rocks of the Archean continent in Wyoming and Montana trend east as does the margin of the Archean continent at the Uinta Mountains, and have had long histories of recurrent movement.