Pre-Pennsylvanian Stratigraphy and Paleotectonics of the San Juan Mountains, Southwestern Colorado

A large northwest-trending graben-faulted anticline involving all pre-Pennsylvanian rocks is exposed in the core of the San Juan Mountains near Silverton, Colorado. The feature was formed before Late Cambrian time when younger Precambrian quartzites were extensively down-faulted into juxtaposition with the older Precambrian basement complex. The quartzites emerged topographically high and supplied local talus to Ignacio deposits (Late Cambrian) adjacent to the faults. Ignacio rocks do not cover the fault block, but thicken and become finer-grained away from the feature.

As the Late Devonian seas advanced, the McCracken Sandstone Member of the Elbert Formation was deposited along the flanks of the structural feature, but apparently did not cross it. The upper Elbert intertidal dolomites were the first sediments to be deposited across the faulted fold where they lie directly on Precambrian quartzites. Latest Devonian or earliest Mississippian stromatolitic dolomites of the Ouray Formation overlie Elbert strata on the flanks of the old feature, and equivalent normal marine limestones are locally present within the down-faulted block.

The Early Mississippian Leadville Formation thins abruptly onto the flanks of the graben-faulted anticline, where the lithologic aspect is one of stromatolitic dolomite formed in an inter-tidal zone. Within the graben, Leadville strata are thin or missing except as remnants in the overlying Molas regolith, suggesting that intense post-Leadville weathering removed those rocks across the regionally high structure. A small horst is present within the large graben where Pennsylvanian sedimentary rocks rest directly on upturned Precambrian quartzites. The graben was again down-faulted at some undetermined post-Desmoinesian time, for Middle Pennsylvanian beds are now in fault contact with Precambrian rocks along the flanking faults.

A similar ancient fault block south of Ouray, Colorado, extends northwesterly into the sub-surface of the eastern Paradox basin. This feature parallels the nearby Uncompahgre uplift and may be genetically related to it.