Earth and Planetary Sciences ETDs

Publication Date

Fall 12-13-1979

Abstract

The objective of this investigation is to study the provenance of the Westwater Canyon and Brushy Basin Members of the Morrison Formation between Gallup and Laguna, New Mexico. The coarse, subarkosic Westwater Canyon Member is separated from the quartzitic sandstones of the underlying Recapture Member by an unconformity that provides a recognizable datum for correlation and a definite base for the Westwater Canyon Member.

The very pale orange, cliff-forming Westwater Canyon Member consists of Oto 137 m of very coarse- to very fine-grained, poorly sorted, subrounded to angular, arkosic to quartzitic sandstone with numerous lenses of arkosic granules and pebbles of plutonic and volcanic rocks, quartzite, chert, and sandstone. The Westwater Canyon Member intertongues laterally with the Brushy Basin Member.

The Brushy Basin Member consists of Oto 90 m of slope-forming, greenish-gray, bentonitic mudstone interbedded with white, fine-grained sandstone, cross-bedded arkosic sandstone and thin discontinuous nodular limestone. The Jackpile sandstone of economic usage intertongues with the Brushy Basin Member and is a northeast-trending body of white, subangular to subrounded, poorly to moderately well sorted, friable, fine- to medium-grained arkosic to quartzose sandstone.

The dominant sedimentary structures in the Westwater Canyon Member are tangential to concave, medium to large trough cross-beds and minor scattered large-scale angular cross-beds. Mean paleocurrent transport directions are northeast in the Gallup area, east in the Grants area and southeast in the Laguna area. Isopach maps illustrate that the main thickness of the Westwater Canyon Member lies to the north of the outcrop area and that the unit thins stratigraphically to the south in the Grants and Laguna areas. Paleocurrent directions and isopach maps indicate that the source area of the Morrison Formation is to the southwest of Gallup, probably in central Arizona.

Light and heavy minerals indicate that the source area of the upper Morrison Formation consists of both acidic and basic igneous rocks, sedimentary rocks, and low- to medium-grade metamorphic rocks. Rb-Sr dates of calcalkaline granitic pebbles from the Westwater Canyon Member suggest the source area contains 1.3 b.y. old igneous rocks. Pebbles also suggest that sandstone and chert bearing sedimentary units and low-­to medium-grade metamorphic rocks were common in the source area.

Model analyses, mineralogy, texture, and preliminary age dates of pebbles collected from the Westwater Canyon Member suggest that central Arizona is the source of the granitic material in the Morrison Formation. Late Precambrian, Paleozoic, and Mesozoic strata in central Arizona provided mainly quartz, chert, sandstone and volcanic rock fragments.

A northwest-trending volcanic are existed in southern Arizona between 155 to 190 m.y. and provided a possible mechanism for uplifting the ancestral Mogollon Highland. Age dates on authigenic clay minerals in the Westwater Canyon Member provided a minimum age of 149 - 17 m.y. (Moon Lee, personal communication, 1975) for the deposition of the upper Morrison Formation.

Outcrops between Gallup and Laguna represent the distal facies of a large alluvial fan that makes up the Westwater Canyon Member; this unit was deposited in a humid climate. The Brushy Basin Member was deposited in a large flood plain that graded into and intertongued with the Westwater Canyon alluvial fan. A possible modern analog of the Westwater Canyon alluvial fan is the alluvial fan of the Kosi River of India.

Level of Degree

Doctoral

First Committee Member (Chair)

Lee A. Woodward

Second Committee Member

Sherman Alexander Wengerd

Third Committee Member

Barry Stephen Kues

Document Type

Dissertation

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