Earth and Planetary Sciences ETDs

Publication Date

Summer 7-13-2020

Abstract

The modern Salt River flows southwest from the Colorado Plateau, through the Arizona Transition Zone and into to the Basin and Range. Rivers in this area flowed northeast during the Paleogene from the Laramide Mogollon Highlands into structural basins of the topographically lower southern Colorado Plateau area. One of these rivers carved the Salt River paleocanyon through a portion of the Mogollon Highlands known as the Apache Uplift to similar depths as the modern Grand Canyon. This study refines the timing of this drainage reversal in the Salt River area by constraining the ages of paleoriver sediments deposited during the time of northeast-flowing rivers, internal drainage and southwest-flowing rivers. These results provide insights into the evolution of the southern Colorado Plateau.

U-Pb dating of detrital zircon is used for maximum depositional ages and provenance analyses. 40Ar/39Ar dating of detrital sanidine is used for higher precision maximum depositional ages. The majority of paleoriver samples that were analyzed with both detrital zircon and sanidine show similar maximum depositional ages. Minimum or direct depositional ages are determined by overlying or interbedded volcanic units.

The Mogollon Rim Formation is composed of river gravel, sandstone and mudstone on the southern rim of the Colorado Plateau that were deposited by northeast-flowing rivers. Results indicate deposition began after 59.38 Ma in the Flying V Canyon area and from 37 – 33.55 Ma in the Trout Creek section. The Whitetail Conglomerate represents the transition from northeast-flowing rivers to internal drainage in the Salt River paleocanyon. Whitetail Conglomerate is as old as the interbedded 37.6 Ma dacite in Canyon Creek fault side drainages, but deposition in the axis of the Salt River paleocanyon occurred between 30 and 21.8 Ma. The transition from northeast-flowing rivers to internal drainage occurred between 33.55-30 Ma marking the age of initial development of the southern edge of the Colorado Plateau. Apache Leap tuff flowed NE down Salt River paleocanyon nearly to Canyon Creek fault at 18.6 Ma. Internal drainage is documented by the 14.67 Ma Black Mesa basalt that flowed onto underlying lake beds. The first southwest-flowing river system in the Salt River paleocanyon deposited the Dagger Canyon conglomerate after incising at least 200 m deep into the Whitetail Conglomerate. The lower Dagger Canyon conglomerate is present in tilted fault blocks on the eastern side of Tonto Basin and the western portion of the Salt River paleocanyon. The dip of bedding increases down-section from 0 to 27o providing evidence that deposition occurred while the faults were active. Lower Dagger Canyon conglomerate deposition began after 12.49 Ma, presumably due to base level fall associated with Basin and Range extension. The upper Dagger Canyon conglomerate is composed of flat-lying river gravels and sandstone located at higher elevations than the lower Dagger Canyon conglomerate in Tonto Basin and the Salt River paleocanyon. Deposition of the upper Dagger Canyon conglomerate began after 7.34 Ma and a possible a lag in fluvial deposition occurred between the two facies. If a lag in fluvial deposition occurred then the upper Dagger Canyon conglomerate represents a rejuvenation of the southwest-flowing river system in the Salt River paleocanyon after Basin and Range faulting waned. A likely driver for this rejuvenation would be southern Colorado Plateau uplift by the building of Mount Baldy volcanic field 12-8 Ma.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Karl Karlstrom

Second Committee Member

Laura Crossey

Third Committee Member

Louis Scuderi

Fourth Committee Member

Matthew Heizler

Language

English

Keywords

Salt River, Arizona, Colorado Plateau

Document Type

Thesis

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