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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 7. (July)

First Page: 953

Last Page: 953

Title: Tectonic Significance of Depositional Patterns in Nonmarine North Horn Formation, Central Utah: ABSTRACT

Author(s): Douglas Wachtell, M. Dane Picard


The Maestrichtian through Paleocene(?) North Horn Formation is a nonmarine sequence of interbedded and intertonguing synorogenic conglomerate, sandstone, algal limestone, and claystone. Deposition occurred within an interior drainage system restricted by the Sevier highlands to the west and the San Rafael swell to the east. It extended south to at least Salina Canyon in central Utah and north to the southern margin of the Uinta basin. The base of the formation is diachronous, unconformably overlying Campanian rocks. Earliest deposition began along the eastern Wasatch Plateau, then shifted westward until late Paleocene beds onlapped deformed Mesozoic rocks associated with Sevier thrusting.

Paleocurrent measurements indicate a general west to east transport direction. Four major depositional facies are recognized: alluvial fan, proximal braid plain, distal braid plain, and lacustrine. Work has been concentrated on the alluvial fan and braid plain facies west of the Wasatch Plateau.

Massive clast-supported cobble to boulder conglomerate characterizes alluvial-fan facies. Deposition on fan surfaces was primarily by sheet-flood or braided stream processes. Conglomerate shows skewed clast populations. Northeast of Mt. Nebo, 60-70% of clasts are carbonate rocks. Clasts range up to 1 m (3 ft) in diameter. Thick Paleozoic carbonate beds associated with the Nebo fold nappe furnished most of them.

Upward-fining sequences of massive to crudely stratified clast-supported conglomerate, conglomeratic sandstone, and sandstone characterize proximal and distal braid plain facies. During transport down the braid plain, mixing of Eocambrian and Cambrian quartzite, Paleozoic carbonate, and Mesozoic sandstone occurred. Conglomerate was deposited in bars within braided stream channels. It displays imbrication of flattened clasts with long axes transverse (^cong40-70°) and intermediate axes subparallel to flow direction. Sandstone displays poorly developed cross-stratification, planar stratification, and massive bedding. Rapidly fluctuating episodes of sedimentation with high sediment loads on the proximal braid plain, and extensive bioturbation by infauna on the distal braid plain, ar indicated.

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