Rocky Mountain Section - 61st Annual Meeting (11-13 May 2009)
Paper No. 4-13
Presentation Time: 8:00 AM-5:00 PM


KEELEY, Joshua A., Department of Geosciences, Idaho State University, 921 S. 8th Ave. Stop 8072, Pocatello, ID 83209, and RODGERS, David W., Department of Geosciences, Idaho State University, 921 South 8th Ave., Box 8072, Pocatello, ID 83209

New geologic mapping and kinematic analysis was completed in the southern Portneuf Range of southeast Idaho to characterize the Valley Fault, a large-offset normal fault interpreted as the breakaway for the regionally extensive Bannock detachment system. The Valley Fault separates Miocene Salt Lake Fm from underlying Neoproterozoic to Cambrian Brigham Group and Cambrian rocks. The fault underlies several klippen within the range, contrary to previous mapping that interpreted a depositional contact between Miocene and Brigham Group strata. Three-point problems indicate the Valley Fault strikes NNW and dips 10-15o WSW, in stark contrast to previous work which interpreted the fault as steeply W-dipping. Hanging wall strata, consisting of conglomerate with rare tuff, are undated but lithologically correlative to the late Miocene upper Salt Lake Fm. These rocks strike NNW and dip 11o ENE adjacent to the fault and steepen progressively downsection to ~60 ENE. Footwall strata strike NNW and dip 4515 ENE. Four fault sets cut the footwall. Chronologically they include 1) a localized, gently E-dipping thrust fault that places Camelback Quartzite on Twin Knobs Limestone; 2) a subhorizontal fault with top-to-the-west normal offset of ~600m; 3) the Valley Fault; and 4) NE-striking normal faults with up to 740m of offset. Top-to-the-west offset across the Valley Fault, using the Tertiary unconformity as a marker, is 5800 m.

The initial dip of the Valley Fault is interpreted to have been steeper than its present 10-15 dip. If footwall strata were subhorizontal prior to extension, as indicated by reconstruction of the Tertiary unconformity (Rodgers & Janecke, 1992), then the Valley Fault had an initial dip of 6015 WSW based on measured bedding-to-fault angles. Hanging wall bedding-to-fault angles diminish upsection from ~70 to 20. Combined with the modern low dip of the Valley Fault, these data are interpreted to indicate the Salt Lake Fm accumulated adjacent to the active Valley Fault while the fault as well as footwall and hanging wall strata tilted NE through time. The Valley Fault's planar to mildly listric shape, the progressive domino-style tilting, and the moderate amount of total slip suggest the Valley Fault is a typical Basin-Range normal fault rather than a low angle breakaway fault for the Bannock detachment system.

Rocky Mountain Section - 61st Annual Meeting (11-13 May 2009)
General Information for this Meeting
Presentation Handout (.pdf format, 7359.0 kb)
Session No. 4--Booth# 13
Geologic Mapping Supported by EDMAP and STATEMAP in the Rocky Mountains Region (Posters)
Utah Valley University: Library 4th Floor
8:00 AM-5:00 PM, Monday, 11 May 2009

Geological Society of America Abstracts with Programs, Vol. 41, No. 6, p. 9

© Copyright 2009 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.