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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 52 (1968)

Issue: 3. (March)

First Page: 520

Last Page: 521

Title: Shelf-to-Basin Lower Triassic Sediments Across Las Vegas Hingeline, Nevada: ABSTRACT

Author(s): H. J. Bissell

Article Type: Meeting abstract

Abstract:

Lower Triassic Moenkopi (Formation of McKee, 1954; Group of Poborski, 1954) changes facies with increase in thickness from mostly nonmarine and minor marine character east of Las Vegas, Nevada, to progressively more marine character toward the west as the Las Vegas hingeline is crossed. The thickness of Moenkopi strata east of this hinge in the Rainbow Garden area (east of Frenchman Mountain) is 1,810 ft, of which about 350 ft is the marine Virgin Limestone. About 20-22 mi farther east, Moenkopi aggregates about 1,250 ft; this section contains more redbed but less carbonate and gypsiferous facies than at Rainbow Gardens.

Outcrops west of the hinge in the expanse between Las Vegas and east Blue Diamond Mountain show a progressive increase in Moenkopi thickness, with more marine sediments; Virgin Limestone is about 500 ft thick. A much more striking change takes place westward,

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toward the west side of Blue Diamond Mountain, where the Virgin Limestone consists of 900 ft of marine limestone, dolostone, claystone, and shale; weaker units in the Moenkopi are covered largely by alluvium. About 6-8 mi farther south, in the Cottonwood Wash-Bird Spring Range area, total Moenkopi is about 2,000 ft, of which Virgin Limestone consists of 1,025 ft of cyclically accumulated marine limestone, dolostone, claystone, and shale.

The leading edge of the Keystone overthrust is 2-3 mi west of Triassic outcrops of the Blue Diamond Mountain-Bird Spring Range localities; Triassic strata on the upper plate are exposed superbly in the Lovell Wash-Lovell nose area, and in the Grapevine Springs locality near Kyle Canyon, about 10-15 mi west and northwest, respectively, from the Blue Diamond locality. Total thickness of interbedded and cyclically accumulated marine limestone, shale, and claystone is at least 2,435 ft; west-dipping strata are beneath Quaternary alluvium.

Lower Triassic strata of the autochthonous sections east of the Keystone overthrust are disconformable on the Harrisburg Member (gypsiferous) of the Kaibab Formation in most localities, and in turn are overlain disconformably by Shinarump Conglomerate. In the allochthonous block, however, the Harrisburg Member is absent, and only locally do Lower Triassic sediments lie on the Kaibab; elsewhere, channels were cut through the Kaibab and Toroweap, and the Mesozoic sediments have thick cobble and boulder conglomerate resting with marked unconformity on Permian redbeds.

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