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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 965

Last Page: 965

Title: Permian and Triassic Boundary of Southwestern Utah: ABSTRACT

Author(s): R. Larell Nielson

Article Type: Meeting abstract


The Permian and Triassic boundary, southwestern Utah, is marked by a topographic unconformity associated with a number of minor unconformities. The Harrisburg Member, Kaibab Formation, is the uppermost Permian unit. At its base a chert limestone is overlain by a gypsum in the west and a collapse breccia in the east. A fossiliferous limestone overlying the gypsum and collapse breccia is contorted over the collapse breccia, and is of a constant thickness suggesting that the evaporites were dissolved after its deposition. Above the medial limestone a siltstone is overlain by a limestone that partly fills topographic depressions in the medial limestone. Dissolution of the gypsum continued producing additional relief that was filled by another siltstone and limestone sequence. In the Beaver Dam Mountains gypsums are present above the medial limestone. Conglomeratic lenses (Rock Canyon Conglomerate) derived from the west and southwest are equivalent to both the Permian and Triassic sediments representing a major erosional cycle after the last retreat of the Permian seas and the advance of the Middle Triassic seas. The Timpoweap Member, Moenkopi Formation, was deposited on top of the underlying limestones and conglomerates developing a horizontal plane. It thins to a featheredge west of the Hurricane Cliffs and east of the Utah-Arizona state line suggesting that a positive area was present west of the Hurricane Cliffs during the Early Triassic. Thinning of the lower Red Member of the Moenkopi Formation also occurs west of the Hurricane Cliffs but in places it is absent, reflecting the topographic nature of the Permian and Triassic boundary. It was not until the deposition of the Virgin Limestone member of the Moenkopi Formation that Triassic seas covered the western part of Utah.

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