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ABSTRACT: Tectonic Origin of Lower Mesozoic Regional Unconformities: Southern Colorado Plateau and Adjacent Basin and Range

John E. Marzolf

Palinspastic restoration of Basin and Range structural blocks to early Mesozoic positions relative to the Colorado Plateau permits correlation of lower Mesozoic regional unconformities of the Colorado Plateau across the southern Basin and Range. These unconformities correlate with tectonic reconfiguration of sedimentary basins in which enclosed depositional sequences were deposited. Lesser recognized intraformational unconformities are related to relative sea level change. Ages are permissive of correlation with eustatic curves; however, age constraints are not definitive.

The Tr-1 unconformity developed on subaerially exposed, karsted, and deeply incised Leonardian carbonates. The overlying Lower Triassic Moenkopi Formation and equivalent strata display a narrow, north-south aligned, passive-margin-type architecture subdivided by Smithian and Spathian intraformational unconformities into three depositional sequences. From basinal to inner shelf facies, Tr-1 truncates folds in Permian rocks. Initial deposition of the lowest sequence began with sea level at the base of the continental slope. Basal conglomerates of the Upper Triassic Chinle Formation were deposited in northward-trending paleovalleys incised within and parallel to the Early Triassic shelf. Distribution of fluvial deposition, orientation of paleovalleys,

paleocurrent indicators, and provenance indicate change from the passive-margin-bordered Early Triassic basin to an offshore active-margin basin. Continental and marine facies suggest two depositional sequences separated by an early Norian type 2(?) sequence boundary. The J-O unconformity at the base of the Lower Jurassic Glen Canyon Group marks a major change in tectonic setting of western North America as evidenced by (1) progressive southwestward downcutting of the unconformity to deformed Paleozoic rocks and Precambrian basement, (2) coincidence in time and space with Late Triassic to Early Jurassic thrust faults, and (3) initiation of calcalkaline volcanism. The J-2 unconformity, which truncates the J-1 unconformity and progressively older strata eastward, correlates with a major ignimbrite outburst in the arc terrane followed by magmatic-load-nduced subsidence.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990