Abstract

Virgin River depression is a Neogene basin, with a surface area that exceeds 1,500 km2 in the Basin and Range structural province of south-eastern Nevada and northwestern Arizona. The depression formed within the foreland of the Sevier orogenic zone, a region that was characterized in Paleogene time by a flat-lying section of Cambrian to Cretaceous platform strata about 5 km thick. Well data from Mobil Virgin 1A on Mormon Mesa reveal 2,000+ m of Neogene basin fill that consists mostly of the Muddy Creek Formation (4-10 Ma), the red sandstone unit (10-12 Ma) of Bohannon (1984), and the Lovell Wash Member of the Horse Spring Formation (12-13 Ma). Seismic reflection data from six primacord and two vibroseis lines show that the Muddy Creek Formation uniformly fills Virgin River depression to a depth of 1-2 km. Two older and less-extensive basins, the Mormon and Mesquite, lie beneath the Muddy Creek and are separated from one another by a complex buried ridge. The basins are mostly filled with rocks of the red sandstone unit and Lovell Wash Member to depths locally exceeding 6 km. Two older members of the Horse Spring Formation, the Rainbow Gardens and Thumb, also occur in Mormon basin (the western one), where they rest disconformably on the pre-Tertiary strata. The basins are east-tilted half grabens that are bounded on the east and southeast by large listric normal fault systems. The faults that bound Mormon basin are buried by the Muddy Creek Formation, but the Piedmont fault, on the east side of Mesquite basin, cuts Quaternary alluvium.

The Virgin River depression formed in three stages. The period from 24-13 Ma is characterized by slow subsidence in Mormon basin and little noticeable deformation of the basin substrate. The Mormon and Mesquite basins became fully differentiated during the period from 13-10 Ma. This stage is associated with large displacements on the normal faults bounding both basins and the buried ridge. Proterozoic crystalline rocks were exposed locally, providing a source for part of the red sandstone unit deposited in the basins. Tectonic denudation during the 13-10 Ma stage locally removed large amounts of the pre-basin section. By 10 Ma, most of the fault activity had ceased, the ridge between the basins was overlapped, and Virgin River depression began to subside uniformly over a wide area. This stage lasted until the commencement of the modern period of dissection associated with the Colorado River. Our structural analysis suggests that upper crustal extension within the basin, mostly during the 13-10 Ma stage, might have exceeded 60%. The basin subsidence was partly due to extension in the upper crust and partly due to viscous flow in the deeper crust beneath the basin. It is not clear to us what caused the uplifts that flank the depression, but isostatic rebound due to tectonic denudation remains aviable possibility.

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