SEQUENTIAL HISTORY OF SALT STRUCTURES AND RELATED FEATURES IN THE PARADOX BASIN, USA

Formed during Late Paleozoic Ancestral Rocky Mountain (ARM) compression, the Paradox Basin is distinguished by thick Pennsylvanian, Permian and Triassic strata and the presence of extensive beds of salt in the Pennsylvanian Hermosa Group. The asymmetric basin has large salt structures within and along the northeastern deeper part of the basin (the Deep Fold and Fault Belt - DFFB), and has a shallow (thinner) southwestern portion with essentially autochthonous salt. Halite deposition was greatest during Late Morrowan, Atokan and Desmoinesian, with a total thickness of up to 8000 ft (2400 m) in the DFFB (including interbedded strata within the gross salt interval). Salt lay essentially immobile until the latter part of the Desmoinesian when massive prograding wedges of siliciclastics eroded from the adjacent Uncompahgre Uplift were deposited in the DFFB, causing significant movement of thick underlying salt beds into salt rolls, anticlines, walls and diapirs, while leaving isolated pillows of salt. Major salt growth continued into the Early Triassic. By Late Jurassic, salt had welded out in the adjacent minibasins and upward movement of salt had greatly slowed or essentially stopped. By Late Cretaceous all of the salt structures were deeply buried. During ARM compression, long linear and parallel fault trends in basement rocks formed the buttresses which deflected initial salt rolls upward into anticlines, walls and diapirs. Early salt wall growth was passive as flanking sedimentation pushed more salt toward the wall with strata thinning over the wall or pinching out on the flanks. Diapirism occurred when the wall was high enough to become buoyant; strata above the diapir bulged upward. An erosional event removed overburden from the surface bulge allowing salt to be extruded at the surface, in some cases forming salt plugs (fountains) and namakiers. Prior to extrusion, pressure of the diapiric salt-bearing strata was at or near lithostatic. Major extrusion events occurred during the formation of regional unconformities in the Early Permian, Early Triassic and Neogene.