Document Type
Contribution to Book
Journal/Book Title/Conference
Geology and Resources of the Wasatch: Back to Front
Publisher
Utah Geological Association
Publication Date
2017
Journal Article Version
Version of Record
First Page
295
Last Page
360
Abstract
Years of drought conspired to produce the present historic low stand in Gunnison Bay of Great Salt Lake. The lake has receded enough to expose the Great Salt Lake fault zone, and reveal its geometry through the shallow and clear lake water. We mapped the newly visible escarpments along the fault zone, upgraded the Utah fault map, and revised the regional correlation of the Great Salt Lake, Rozel, and North Promontory fault zones. We explored the origin of wave-cut platforms in the footwall of the faults, and identify geometrically unique types of microbial communities along faults and fault-related springs and seeps.
Mapping and analysis show that the North Promontory fault is more likely to be the northward continuation of the Great Salt Lake fault zone than the 40-60-km-long Rozel fault zone, which is 5-15 km to the west and uplifted a separate horst block. The North Promontory fault and Great Salt Lake fault zone are directly along strike of one another, share a continuous footwall block composed of the same Paleozoic and older formations, have similar long-term slip rates, and last ruptured in the latest Pleistocene to Holocene. The Promontory segment of the Great Salt Lake fault zone probably connects with the North Promontory fault across an ~25 km long mountain front with no fault scarps in Bonneville or younger deposits. The unfaulted part of the mountain front is comparable to the length of rupture segments along the Wasatch and Great Salt Lake fault zones. The Oquirrh-Great Salt Lake fault zone contains nine fault segments in our revised interpretation, an increase of one segment and lengthening of the fault zone of ~25 to 30 km.
The west-dipping strands of the Rozel fault zone are separated from the Promontory segment of the Great Salt Lake fault zone by an intervening sedimentary basin that persists north from the hanging wall of the Promontory segment. The Rozel fault zone is distributed across a 4-8 km wide zone composed of many closely spaced normal faults with small-displacements. The normal faults cut late Cenozoic basalt flows and underlying Paleozoic rocks in a ridge from Rozel Point to Hansel Valley, and part of the fault zone steps east of the Hansel Valley fault zone and part interfingers within the Hansel Valley fault zone across an accommodation zone. In the south, at Rozel Bay, the Rozel fault zone is composed of ~15 small, west- and fewer east-dipping normal faults. The fault zone is likely wider, based on a seismic reflection profile that images small-displacement faults in the hanging wall of the largest-slip master fault (Mohapatra, 1996).
We propose that distributed extension across many closely spaced, widely distributed, small-displacement normal faults is common within faulted volcanic fields because the high heat flow in the fields raises the brittle-ductile transition in the crust. The Rozel and Hansel Valley fault zones both exhibit this volcanic-fault-style within the coeval Plio-Quaternary Northwest Utah volcanic fields. These fault geometries differ significantly from widely spaced, localized fault zones with large displacement that are typical of Basin-and-Range geometries along the Great Salt Lake-North Promontory fault zone. A thin mechanical sheet breaks up internally along many small normal faults in the volcanic fields. The extensional geometry matches that in the thin hanging wall of detachment faults.
Unusually wide footwall terraces lie along the Great Salt Lake and Rozel fault zones. Uplifted rocks in the footwalls of the normal faults of Fremont Island, the Promontory Mountains, and Rozel Hills are composed of hard crystalline quartzite, carbonate, and basaltic bedrock, and we propose that the gently inclined platforms are the result of cliff retreat and erosion by waves. Previous seismic reflection data show that a thin(?) cover of halite, carbonate microbialite mounds, mirabolite, and clay beds overlies these eroded strath terraces.
Because of the time required for waves to erode hard Precambrian to Paleozoic bedrock beneath the 1-7 km wide platforms east of the Great Salt Lake and Rozel fault zones, the Great Salt Lake fault zone may have experienced a long period of quiescence in the past, most likely in past interglacial times, with a slight freshening of its trace during the Holocene. More rapid slip in the Holocene is consistent with large temporal changes in slip rate ascribed to secondary rebound-related forces during loading and unloading of Lake Bonneville along the Wasatch fault zone (Hetzel and Hampel, 2005). Temporally changing slip rates have also been documented by Perouse and Wernicke (2016) throughout the northern Great Basin.
Springs and seeps along normal faults localize microbial mounds in Great Salt Lake. Many geometric differences exist between solitary, ~ 1m wide, disk-shaped microbial mounds that arrange themselves into polygonal colonies, and smaller, amalgamated, blob-like microbial mounds that form along faults, springs, and seeps. These differences could reflect biological, environmental, or other differences between fault-loving microbialites and the much more common polygon-forming ones.
Recommended Citation
Janecke S.U., and Evans, J.P., 2017, Revised structure and correlation of the East Great Salt Lake, North Promontory, Rozel and Hansel Valley fault zones revealed by the 2015-2016 low stand of Great Salt Lake: in Lund, W.R., Emerman, S.H.; Wang, W., Zanazzi, A., editors, Geology and Resources of the Wasatch: Back to Front; Utah Geological Association Publication 46, p. 295 to 360.