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Recurrent movement along Precambrian-age shear zones throughout the Pennsylvanian and Mississippian has been a significant factor controlling depositional patterns during these periods. The displacement, although subtle and superposed on the epeirogenic movement, appears to have affected locally the bathymetry or paleotopography in the vicinity of the Las Animas arch. As a result, lithofacies distribution of either clastic or carbonate rock is related to proximity to the shear zones.
Surface lineations derived from Landsat analysis, coupled with lithofacies mapping based on subsurface data, identify the location of several shear zones in the region. The northeast and northwest orientations of these fault zones is subparallel to mapped faults in the Rocky Mountain region. Lithofacies mapping on a regional scale indicates the fault zones may have controlled the orientation and location of middle Morrowan sandstones. Similarly, the fault zones are thought to control Mississippian facies and subsequent dolomitization.
In addition, differential movement of fault blocks bounded by the shear zones is thought to have influenced formation of the Sierra Grande and Apishapa uplifts as well as the Las Animas arch. Isopach and lithofacies mapping show that precursors to the present Las Animas arch were present intermittently during the Mississippian and Pennsylvanian. Although these ancestral folds were not as positive as the present arch, they were oriented in the same general direction and as a result were influenced by the same set of shear zones.
The significance of the shear zones in controlling stratigraphy and structural development of the arch has been a factor largely overlooked during past petroleum exploration in the region. Further study of these shear zones is expected to lead to significant future petroleum discoveries.
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