Abstract

The stratigraphy and structure of Tertiary rocks along the topographic divide between Box Elder and Cache Counties, Utah, are extremely complex. Detailed field and photogeologic mapping of Tertiary rocks integrated with geochronological and geophysical techniques indicate that a Miocene episode of extension preceded Late Cenozoic Basin-and-Range normal faulting. This early extension may have induced rapid, localized onset of deposition of the Salt Lake Formation. However, later sedimentation rates in the Salt Lake Formation were very slow, and most deformation of these rocks appears to be due to slip on north-striking Basin and Range normal faults.

The study area lies between the lower Bear River basin on the west and Cache Valley on the east, from the Wellsville Mountains north almost to Clarkston Mountain. The two mountain ranges are cored by folded and thrusted Paleozoic rocks. Both ranges are bounded on the west by the Wasatch fault zone and on the east by the West Cache fault zone. Between these two normal-fault zones, poorly consolidated Tertiary rocks of the Wasatch Formation and Salt Lake Formation overlie Paleozoic rocks in the foothills and the low divide between the north-trending ranges. Tertiary rocks are folded and are cut by a complex array of normal faults.

Dramatic changes in the Tertiary stratigraphy between the northern Wellsville Mountains and the low hills to the north probably reflect active normal faulting early during deposition of the Salt Lake Formation (approximately 13 Ma). The Miocene to Pliocene Salt Lake Formation accumulated above thick to absent conglomerates of the Paleocene to Eocene Wasatch Formation. The Salt Lake Formation in the study area consists of two members. The Collinston Conglomerate comprises the basal member, and, like the Wasatch Formation, is present only in the northern Wellsville Mountains. The overlying tuffaceous Cache Valley Member extends across the area of study. The Cache Valley Member was subdivided into a local basal conglomerate, a thick and widespread tuffaceous subunit, and an overlying oolitic limestone subunit. The basal conglomerate and tuffaceous subunits of the Cache Valley Member are approximately 13 to 5 Ma. The oolitic subunit may be younger than 5 Ma.

Normal faults in the study area can be subdivided into three groups. North-striking normal faults are the youngest, and include major range-bounding faults. East-striking normal faults are less numerous, generally have small displacements (<200 m =656 ft), and end against the north-striking faults. The southwest-dipping low- to moderate-angle Beaver Dam fault is the oldest normal fault in the study area and was the site of up to 4 km (2.5 mi) of dip separation. This major normal fault was active during deposition of the Collinston Conglomerate member of the Salt Lake Formation. The Collinston Conglomerate has an inferred age of 16 to 13 Ma, but may be younger. The Beaver Dam fault may have accommodated early westward collapse of the region. Later reactivation of the Beaver Dam fault, possibly as a segment of the Wasatch fault zone, was probably minor.