Revision of Ancestral Rocky Mountain Paleogeography

Charles Kluth

Paleogeography of the Ancestral Rocky Mountains (ARM) has been revised based on recent data and re-examination of older data. Criteria for revision include presence of pre-Penn rocks in basins or absence on uplifts. The uplifts and basins are oriented NW-SE and there is no good evidence for N-S trends that show up on previous maps. The Front Range was a NE-tilted fault block that developed early in the Pennsylvanian. The Front Range was separated from the Ute Pass Block to the south by a shallow marine trough. The narrow Ute Pass Block shed coarse debris both to the N and S. The San Luis Uplift was a SW tilted block that was coeval with the Front Range. The Uncompaghre Uplift was a NE-tilted fault block that did not develop until the Early Permian. The Paradox and Eagle Basins were continuous during salt deposition. There was a, low uplift between the San Luis and the Cimmeron Uplift in northern New Mexico. The Brazos uplift and related structures were oriented NW-SE. The Apishapa Uplift in southern Colorado is a NW-SE uplift but its relationship to the Sierra Grande Uplift in New Mexico in not yet clear. The Las Animas Arch in SE Colorado, the Sierra Grande Uplift and the Pedernal Uplift in central NM are part of a different more N-S structural trend that formed from middle Pennsylvanian to early Permian time. Reactivation of ARM faults by the Laramide is rare. Excess sand in the ARM sections is probably derived from the Devonian age Appalachian Caledonides.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012