Integration of Ground-Based Gravity Data with Geologic Maps, DEMs, Well Control, and Seismic Data for Detailed Basin-Scale Structural Analysis: Examples from Extensional Basins in SE Idaho and Northern Utah

Abstract:

Previously acquired ground-based gravity data along the eastern margin of the Basin and Range Province in southeast Idaho and northern Utah was combined with newly acquired gravity data in and around Marsh and Cache Valleys, Idaho and Utah. The new data were acquired and reduced with the aid of sub-meter to centimeter-scale GPS equipment, a hand-held laser rangefinder and various scale DEM’s. Structural interpretation of the integrated gravity data set, via gravity maps and 2 ½ D gravity models constrained by previous geologic mapping, well control, and seismic data, show detailed structural elements within the basins such as buried faults, horsts, grabens, and half grabens, along with the depth to Paleozoic basement across and along the axes of the basins. These models also show a two to three-phase history of late-Cenozoic extensional deformation within Marsh and Cache Valleys, as well as the surrounding ranges.

 

The earliest pre-detachment phase of extension appears along the western margin of Cache Valley as isolated Eocene to Oligocene basins. The major phase of extension, which began 16 to 13 Ma in Cache Valley and surrounding ranges, and before 10 Ma in Marsh Valley and surrounding ranges, resulted in deposition of the Salt Lake Formation. This phase culminated in widespread top-to-the southwest extension above the Bannock detachment fault in both Cache and Marsh Valleys and surrounding ranges between ca. 10 and 4 Ma, waning or ceasing sometime between 5 and 3 Ma. The northeast tilted basins, formed during and possibly before the detachment phase of extension are preserved in the isolated exposures in the ranges, but are also interpreted from gravity, seismic, and well data to occur as mostly half-graben subbasins beneath the Quaternary fill of both Marsh and Cache Valleys. After 3-4 Ma, predominantly east-west extension along relatively steeper-dipping, widely-spaced north-south Basin and Range block faults cut the detachment fault and earlier formed basins. This phase of faulting resulted in the current physiographic expression of Marsh Valley, Cache Valley, and the surrounding ranges. Gravity and other data suggest that faults formed during this phase of deformation occur not only along the margins of the ranges but also within the more central parts of the basins.