A flexural model for the Paradox Basin: implications for the tectonics of the Ancestral Rocky Mountains

The Paradox Basin is a large (190 km x 265 km) asymmetric basin that developed along the southwestern flank of the basement-involved Uncompahgre uplift in Utah and Colorado, USA during the Pennsylvanian-Permian Ancestral Rocky Mountain (ARM) orogenic event. Previously interpreted as a pull-apart basin, the Paradox Basin more closely resembles intraforeland flexural basins such as those that developed between the basement-cored uplifts of the Late Cretaceous-Eocene Laramide orogeny in the western interior USA. The shape, subsidence history, facies architecture, and structural relationships of the Uncompahgre-Paradox system are exemplary of typical 'immobile' foreland basin systems. Along the southwest-vergent Uncompahgre thrust, similar to5 km of coarse-grained syntectonic Desmoinesian-Wolfcampian (mid-Pennsylvanian to early Permian; similar to310-260 Ma) sediments were shed from the Uncompahgre uplift by alluvial fans and reworked by aeolian-modified fluvial megafan deposystems in the proximal Paradox Basin. The coeval rise of an uplift-parallel barrier similar to200 km southwest of the Uncompahgre front restricted reflux from the open ocean south and west of the basin, and promoted deposition of thick evaporite-shale and biohermal carbonate facies in the medial and distal submarine parts of the basin, respectively. Nearshore carbonate shoal and terrestrial siliciclastic deposystems overtopped the basin during the late stages of subsidence during the Missourian through Wolfcampian (similar to300-260 Ma) as sediment flux outpaced the rate of generation of accommodation space. Reconstruction of an end-Permian two-dimensional basin profile from seismic, borehole, and outcrop data depicts the relationship of these deposystems to the differential accommodation space generated by Pennsylvanian-Permian subsidence, highlighting the similarities between the Paradox basin-fill and that of other ancient and modern foreland basins. Flexural modeling of the restored basin profile indicates that the Paradox Basin can be described by flexural loading of a fully broken continental crust by a model Uncompahgre uplift and accompanying synorogenic sediments. Other thrust-bounded basins of the ARM have similar basin profiles and facies architectures to those of the Paradox Basin, suggesting that many ARM basins may share a flexural geodynamic mechanism. Therefore, plate tectonic models that attempt to explain the development of ARM uplifts need to incorporate a mechanism for the widespread generation of flexural basins.