Abstract

Southern Cache Valley occupies one of several large north-trending valleys in the northeastern part of the Basin and Range Province. It lies between the Wellsville Mountains in the west and the Bear River Range in the east. Paleozoic rocks exposed in the two mountain ranges dip easterly adjacent to the north part of the study area and in a narrow horst along the valley center in the south, but curve to a northerly dip in the southwest. A complex depositional and tectonic evolution of Tertiary strata in southern Cache Valley was determined in this study. Detailed photogeologic and field mapping, supplemented by geochronologic and industry seismic-reflection data, show that southern Cache Valley is a complexly folded and faulted half-graben in the hanging wall of the East Cache normal-fault zone. The antithetic West Cache normal-fault zone bounds the western part of the study area, and dips east-northeast, but curves to an east strike near its southern end, where it is correlated with the newly identified West Valley fault (Smith, 1997; Smith and others, 1997b). Normal faults within the hanging wall of the Cache Valley basin dip both east and west in the eastern half of the basin, but form a conjugate set of closely spaced easterly striking faults in the western half of the basin. The latter faults offset both the Tertiary strata and underlying Paleozoic rocks.

Extensional folds between the East and West Cache normal faults deform strata younger than 4.4 to 5.1 Ma in the Salt Lake Formation. Most trend northerly, parallel to the East Cache fault zone. Three folds in the southwest part of the study area show abrupt bends or small lateral offsets (less than 400 feet, 120 meters) across mapped faults. These folds are thus older than, or coeval with, the faults. One fold parallels the eastward bend in the West Valley fault, then turns abruptly south as it approaches the north-trending horst along the valley center.

In southern Cache Valley, the oldest Tertiary unit is poorly consolidated, distinctively red, conglomeratic Wasatch Formation (Paleocene to Eocene), capped by a white, oncolitic limestone (Cowley Canyon Member). It is absent in the south, but thickens to 200 feet (60 meters) northward. It overlies a profound angular unconformity on beveled Paleozoic rocks. Above the Wasatch Formation are Eocene to Oligocene deposits coeval with the Fowkes Formation (44.2 ± 1.7 Ma, determined by the ⁴⁰K/⁴⁰Ar method), and with the Norwood Tuff (30.6 ± 0.6 Ma, determined by tephrochronology). The contact between the Wasatch Formation and these overlying units is an angular unconformity. It is marked by a sharp lithologic change to white tuffaceous strata, many of which are unconsolidated. Maximum thickness of the Fowkes Formation - Norwood Tuff equivalents in the study area is about 600 feet (180 meters). These units may be absent 11 miles (18 kilometers) northwest of the study area, where the Salt Lake Formation onlaps Paleozoic rocks (Goessel, 1999; Goessel and others, 1999b). It consists of tuffaceous sandstone, conglomerate, micritic or oolitic limestone, tuff, and sandstone. Most of these sediments were deposited in lakes, impounded either by volcanic ash that dammed stream outlets of tectonic basins (Adamson, 1955) or by tectonics combined with paleorelief. Conglomerates deposited about 8.5 to 10.5 Ma and also after 4.4 to 5.1 Ma are dominated by Paleozoic carbonate, chert, and sandstone, with minor amounts of Precambrian quartzite, whereas conglomerates deposited from about 7.9 Ma to somewhat younger than 4.4 to 5.1 Ma typically contain fewer carbonates and up to 40% Precambrian quartzite. Thickness of the Tertiary deposits in the study area probably approaches 10,000 feet (3,050 meters) (Evans and Oaks, 1999).

Following folding and initial faulting, a period of tectonic quiescence permitted widespread erosional development of a rather planar, bench-type pediment sloping west from the East Cache fault zone toward the center of the valley. This McKenzie Flat pediment truncates the faulted and folded strata of the Salt Lake Formation. Although uplifted by subsequent faulting, and dissected by perennial streams, extensive undissected remnants of this pediment surface remain. Thus, it may be Quaternary in age.