A large northwest-trending fault block composed of late Precambrian through Mississippian rocks is exposed in the core of the San Juan Mountains near Silverado, Colorado. The fault block was formed prior to Ignacio (Late Cambrian) time when younger Precambrian quartzite were extensively down-faulted into the older Precambrian basement complex. The structure stood as a high topographic feature during Ignacio to Late Devonian time, but was large buried by the upper Elbert Formation. Renewed activity occurred in Ouray (latest Devonian or earlier Mississippian) time, when tidal flats developed on the high flanks of the fault block while normal marine waters moved into the graben. The entire structure was high during the Early Mississippian, for the Leadville Formation is pres rved only as tidal-flat dolomites and weathering residual blocks within the regolithic Molas Formation above. Pennsylvania and later movement occurred along the graben, as Hermosa and Cutler strata are now involved in the graben.

With this paleotectonic feature as a model, other areas are more readily understood. A similar ancient fault block is present south of Ouray, Colorado, and extends northwest into the subsurface of the eastern Paradox basin. This structure joins a major northwest-trending pre-Pennsylvanian fault system that flanks each of the major salt anticlines which parallel the adjacent Uncompahgre uplift. Isopachous and lithofacies studies reveal that these structural lineaments were already present in Late Cambrian time, and actively controlled sedimentation through Mississippian time. It is possible that the closely related Uncompahgre uplift had a similar early history.

Pre-Pennsylvanian reservoir facies are best developed along the high flanks of the faults. Late Devonian McCracken sandstones occur in linear bars along the structures, and crinoidal biogenic banks, which are associated with all Leadville production, also occur on the shallower structural flanks. Where paleotectonic relief was too high, however, pre-Pennsylvanian rocks are missing either because of non-deposition or subsequent erosion.

The down-faulted paleotectonic troughs were the site of thick Pennsylvanian salt deposition. When Middle Pennsylvanian to Early Permian clastic wedges from the Uncompahgre uplift initiated salt flowage by differential loading, the fault blocks acted as buttresses which deflected the plastic salt upward. Consequently, the salt anticlines grew along the linear trends created by the Precambrian through Mississippian faults.

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