New WSGS Maps Define Fault Lines, Landslide Risk and 8 Million Years of “Missing Time”

WSGS geologists Derek Lichtner and Seth Wittke map atop Middle Ridge, with the Salt River mountain range in the background. (Courtesy of WSGS)

CASPER, Wyo – The Wyoming State Geological Survey (WSGS) has released five new geological maps of the southwestern and northwestern regions of Wyoming. The agency released the maps at 1: 24,000 scale through its STATEMAP program.

Three are bedrock maps of the Sweetwater County Quadrangles:

Two are area maps of Lincoln County quadrangles (combined in one publication):

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Lincoln County Mapping:

“The purpose of this mapping project was to review previous mapping that identified an unrecognized part of the Grays River fault,” said geologist Seth Wittke. “With further study, this new information will help scientists better understand the seismic risk in the region.”

The Grays River Fault, which marks the western side of the Wyoming Range, stretches approximately 65 kilometers (40 miles) from the headwaters of the Grays River in the north to its newly recognized intersection with the Little River Grays. Geologists were able to extend the tracing of the fault by more than 6 miles.

Numerous landslides are visible along the westerly dip slopes of the Wyoming Range south of Deadman Mountain. The view is to the northwest. (Courtesy of WSGS)

Since the area is prone to landslides, geologists wanted to study and map landslides in the area. At least two major landslides – Blind Bull and Porcupine Creek – have occurred in the area over the past five years, damaging roads and causing economic loss to neighboring communities.

The results of the study can aid in the modeling of regional seismic risks and in the protection of infrastructure in the Grays River valley and surrounding drainages against landslide risks.

SGS geologist Seth Wittke maps the Little Grays River watershed in the Wyoming Range. (Courtesy of WSGS)

Other project outputs include mapping of the Pinedale and Bull Lake glacial deposits and river terraces along the previously undifferentiated Grays River.

Mapping of Sweetwater County:

The three maps of Sweetwater County lie at the southern end of the Rock Springs Uprising, a large folded and faulted structure with a complex set of geology. The new mapping provides key information on the timing of the Rock Springs uplift deformation.

“When you look at the three maps together, they illustrate the structural complexity of the Southern Rock Springs Uprising,” says geologist Kelsey Kehoe.

Kehoe was the lead investigator on the mapping of the Lion Bluffs Quadrangle on the south-central part of the Rock Springs Uprising.

“One of the most interesting lessons from this project was the degree and extent of weathering across the quadrilateral. It wasn’t just confined to Aspen Mountain, which has historically documented mineralization, ”Kehoe said.

“We suspect that the weathering is due to the interaction of a low sulphidation epithermal system with thick Cretaceous marine deposits, such as the Baxter Shale.”

The Earnest Butte Quadrangle, on the southwest flank of the Rock Springs Uprising, is teeming with interesting geology, says geologist Ranie Lynds.

“For example, the discordance between the Upper Cretaceous Almond Formation and the Paleocene Fort Union Formation represents about 8 million years of ‘missing’ time that was removed when strata were eroded from the flanks of the upward uplift. of Rock Springs, ”she said.

Lynds says the geologists on this project revised the axis of the Rock Springs uplift to be further west of where it was previously drawn on smaller-scale maps.

Geologist Patty Webber worked on mapping the South Baxter Quadrangle, along the southeast flank of the Rock Springs Uprising.

“There are a lot more faults and warping than you might think on the surface,” Webber said. “This is in part due to the syn-depositional nature of the Late Cretaceous to Eocene units found in the Rock Springs Uprising, which were eroding and settling as the uprising developed,” said Webber.

The mapping projects incorporated a range of analytical data collected from the study areas, including new geochronology of detrital zircon, geochemistry of whole rock and stream sediment samples, palynological analyzes and rock data. mothers.

The three new maps will contribute to the 1: 100,000 scale bedrock geological map of Firehole Canyon that the WSGS is working to complete over the coming year.

Dr Erin Campbell, WSGS Director and State Geologist, said that “STATEMAP’s focus on natural resources and hazards complements the WSGS mapping priorities for mineral and energy resources, hydrogeology and natural hazards. geological risks ”.

The STATEMAP program is a cooperative agreement with the National Cooperative Geological Mapping Program of the US Geological Survey.

The maps, including accompanying technical reports, are available as a free download and in hard copy to purchase from the WSGS website.

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