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AAPG Bulletin

Abstract


Volume: 50 (1966)

Issue: 10. (October)

First Page: 2082

Last Page: 2111

Title: Pre-Pennsylvanian Paleotectonics--Key to Basin Evolution and Petroleum Occurrences in Paradox Basin, Utah and Colorado

Author(s): D. L. Baars (2)

Abstract:

A large northwest-trending graben-faulted anticline consisting of late Precambrian through Mississippian rocks is exposed in the core of the San Juan Mountains near Silverton, Colorado. The structure was formed before Late Cambrian time when younger Precambrian quartzite beds were down-faulted extensively into the older Precambrian basement complex. The structure stood as a high topographic feature because of differential erosion during Late Cambrian to Late Devonian time, but was largely buried by the Late Devonian upper Elbert Formation. Renewed activity occurred in latest Devonian or earliest 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 Mis issippian, because the Leadville Formation is preserved only as tidal-flat dolomite and weathered residual blocks within the overlying regolithic Molas Formation. Pennsylvanian and later fault movement occurred, as Pennsylvanian Hermosa and Permian Cutler strata were displaced progressively in the graben.

The use of this paleotectonic feature as a model makes other areas more readily understood. A similar ancient fault block south of Ouray, Colorado, extends northwest into the subsurface of the eastern Paradox basin. This structure joins a major northwest-trending pre-Pennsylvanian system of faults along each of the major salt anticlines, which are parallel with the adjacent Uncompahgre uplift. Isopachous and lithofacies studies reveal that these structural lineaments already were present in Late Cambrian time, and that they 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 high on the upthrown fault blocks. Late Devonian linear sand bars and Early Mississippian biogenic crinoid banks, which are associated with all production from the Devonian (McCracken) and Mississippian (Leadville), formed in the shallow-water environments that developed along these structural flanks. However, where paleotectonic relief was too great, pre-Pennsylvanian rocks are missing either because of non-deposition or subsequent erosion.

The downfaulted paleotectonic troughs were the site of thick salt deposition during Pennsylvanian time. When Middle Pennsylvanian to Early Permian clastic wedges from the Uncompahgre uplift apparently triggered salt flowage by differential loading, the fault blocks probably acted as buttresses that deflected the plastic salt upward. Consequently, the salt anticlines grew along the linear trends created by the faults.

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