Date of Award:

1999

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

Robert Q. Oaks, Jr.

Abstract

This study integrates detailed mapping of Tertiary deposits along the divide between the lower Bear River basin and the Cache Valley basin with several other techniques to generate a depositional model, define extension-related structures, and compile a geologic history for this part of the northeastern Basin and Range province. The study area is situated along the topographic divide between Box Elder and Cache Counties, Utah, from the Wellsville Mountains north almost to Clarkston Mountain. These ranges are cored by folded and thrusted Paleozoic rocks. They are bound on the west by normal faults of the Wasatch fault zone and on the east by the West Cache fault zone. Between these two fault zones, poorly consolidated Tertiary deposits of the Wasatch Formation and Salt Lake Formation overlie Paleozoic rocks in the foothills and low divide between the north-trending ranges.

The Miocene to Pliocene Salt Lake Formation accumulated above non-tuffaceous conglomerates of the Paleocene to Eocene Wasatch Formation, up to 0.5 km thick in the Wellsville Mountains, but thin or absent northward. The Salt Lake Formation in the study area consists of an apparently non-tuffaceous lower conglomerate member, up to 0.5 km thick in the Wellsville Mountains, and a widespread younger tuffaceous and lacustrine member, at least 1 km thick. The traditional names of Collinston Conglomerate and Cache Valley Member were used for these two lithologies. The Cache Valley Member was further subdivided into a local tuffaceous basal conglomerate, a widespread tuffaceous subunit, and an overlying oolitic subunit.

Normal faults in the study area comprise three groups. North-striking normal faults are the youngest, and include major range-bounding faults. East-striking normal faults are less numerous, and are cut by the north-striking faults. The southwest-dipping low-to moderate-angle Beaver Dam fault separates the Cache Butte Divide and Junction Hills from the Wellsville Mountains. It may be unique within the area of study, and may comprise a newly identified segment of the Wasatch fault zone. Most of its displacement appears to pre-date the late Miocene, at the time that previous authors have suggested for the onset of Basin-and-Range normal faulting.

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