Abstract

The Beaver Dam Mountains in southwesternmost Utah expose a Phanerozoic stratigraphic sequence more than 6 mi thick that rests on a Precambrian complex of gneiss, schist, and pegmatite. The sequence includes more than 13,000 ft of Paleozoic strata, 12,000 ft of Mesozoic strata, and as much as 7,000 ft of Cenozoic sedimentary and volcanic rocks. Major structural features are the northwest-trending, asymmetric, doubly-plunging Beaver Dam Mountains anticline some 10 mi long, bounded on the northeast by the north-plunging Shivwits syncline. These large folds are truncated on their east side by a major north-trending fault zone which includes three sectors called, from south to north, the Grand Wash fault, the Reef Reservoir fault, and the Gunlock fault. The Grand Wash fault enters the area at the Utah-Arizona border with normal down-to-the-west displacement of about 1500 ft; this displacement decreases to almost zero 5 mi north of the Utah-Arizona border. The Reef Reservoir fault, a west-side-up reverse fault, continues northward for 5 mi from where the Grand Wash fault dies out, extending nearly to Shivwits where it, in turn, dies out. The fault zone continues northward from Shivwits as the normal, down-to-the-west, Gunlock fault which attains a maximum stratigraphic displacement of about 3000 ft near Gunlock.

Along the southwestern base of the Beaver Dam Mountains, detached and brittlely attenuated Paleozoic rocks rest on Precambrian, Paleozoic and Tertiary bedrock in extensional denudation fault relationships. At the northwestern edge of the range and adjacent to Square Top Mountain and Jackson Peak, Permian strata have been thrust over folded Mesozoic rocks, and produced late Cretaceous-early Tertiary synorogenic conglomerates which, in turn, are covered by post-orogenic Paleocene?, Oligocene, and Miocene volcanic and sedimentary deposits. Major extensional faulting which began about 13 million years ago resulted in the development of the Mesquite basin along the west side of the Beaver Dam Mountains. This basin was filled with several thousand feet of erosional debris comprising the Muddy Creek Formation, of which only the upper few hundred feet have been exposed as a result of headward erosion by the Virgin River and its tributaries within the last few million years. High-level alluvial deposits and inverted valley topography under the Pleistocene Gunlock basalt flows attest to recent erosion leading to the development of the present topography.

In summary, the major folds and compressional faults were produced by eastward directed Sevier orogenic forces. The deformed rocks were then eroded and covered by Paleocene(?), Oligocene, and early Miocene volcanic and sedimentary deposits. East-west crustal extension began in the area in later Miocene time about 13 million years ago and created the Mesquite graben-basin in which the Muddy Creek Formation accumulated and into which some local bedrock masses along the west side of the Beaver Dam Mountains slid along denudation faults. Miocene deposits in the Mesquite basin are now being exhumed by headward erosion of the Virgin River, a major tributary to the Colorado River.