CHARACTERISTICS OF SEDIMENTARY BASINS FORMED ABOVE LOW-ANGLE DETACHMENT FAULTS: EXAMPLES FROM THE BASIN AND RANGE PROVINCE, WESTERN U.S.

Sedimentary basins formed as a result of low-angle normal faulting in highly extended areas have been the focus of many studies. Models for these so-called “supra-detachment basins” describe shallow basins with gently dipping strata and depo-centers distal from the surface breakaway of the basin-bounding fault (e.g.Friedmann,1995). However, geologic mapping and observations of sedimentary sequences formed adjacent to currently low-angle detachment faults show that upon palinspastic restoration, these basins do not differ significantly in scale from typical high-angle-bound half-graben basins.

This study draws from field investigations of two supra-detachment basins: the Sacramento Pass Basin in the Snake Range (SPB), east-central Nevada, and the sedimentary section on the western flank of the Grouse Creek Mountains, NW Utah (GCM). These low-angle fault-bound basins have several features in common. Interfingering of coarse alluvial sediments and fine-grained lacustrine or playa sediments is typically only a few kilometers from the fault, similar to that observed seismically in modern half-graben basins. At the base of the sections, mega-breccia sheets were deposited catastrophically as rock avalanches, indicating significant footwall topography early in basin history. Geochronology data constrain the timing of deposition of these breccias. In the GCM, rock avalanches occurred about 15Ma, pre-dating a period of rapid uplift and inferred slip on the bounding fault at about 13Ma (apatite fission track dating, Dumitru et al., 2000). Rock avalanches in the SPB were deposited at 20Ma, predating a period of rapid uplift and inferred slip along the basin-bounding fault at about 17Ma (Miller et al., 2000). As faulting progressed, the sedimentary depo-center shifted basinward and megabreccia deposits are absent. This may indicate rotation of the fault to low angles.

Widespread tuff layers provide control points for detailed palinspastic reconstructions of basin cross-sections, e.g. in the SPB, a tuff dated as 20.05 ± 0.19 Ma was used to restore faults that cut and uplift the section. When the reconstructed cross-sections of these basins are compared to cross-sections from seismically imaged high-angle fault bound basins, the two types have similar geometries and thicknesses of sedimentary sequences.