A detailed regional facies analysis of the lower part of the Moenkopi Formation (Triassic) is helpful in understanding Sevier tectonic transport along the shelf-miogeosyncline transition in southeastern Nevada. The Virgin Member represents deposition during the last pulse of marine transgression in the Cordilleran geosyncline from the basin onto the shelf in southeastern Nevada and adjacent Utah and Arizona. Because facies changes and corresponding thickness changes assume linear trends along the transition zone, the post-depositional translation and distortion of facies can be determined.

The stratigraphic section in an area comprising the North Muddy Mountains east to Lake Meade is an apparent autochthonous sequence which is not logically underlain by a regional thrust with scores of miles of displacement. The distance measured perpendicular to depositional strike from true shelf facies to the basinward margin of the transition facies is about 35 mi. Included within this span is a thick evaporite sequence that is equivalent to beds deposited in an open marine setting in the basin on the west. On the north, between the Beaver Dam and the Mormon Mountains, the span between the same 2 facies is about 10 mi. The apparent facies change on the north in the area of the Mormon Mountains is ascribed to a major sole thrust that has displaced facies about 20 mi toward the shelf. The areas are separated by an approximate east-west shear zone that has a minimum right-lateral displacement of about 12 mi.

I believe that attractive oil and gas possibilities exist in the North Muddy Mountains-Lake Meade area which was previously interpreted to be a major allochthon.

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