New WSGS maps define fault lines, landslide risk, and 8 million years of "missing time" - Casper, WY Oil City News
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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 Range in the background. (Courtesy of WSGS)

CASPER, Wyo — The Wyoming State Geological Survey (WSGS) released five new geological maps of areas in southwestern and northwestern Wyoming. The agency published the 1:24,000-scale maps through its STATEMAP program.

 Three are bedrock maps of quadrangles in Sweetwater County:

Two are surficial maps of quadrangles in Lincoln County (combined in a single publication):

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Lincoln County Mapping:

“The purpose of this mapping project was to revisit prior mapping that identified a suspected unrecognized portion of the Greys River fault,” said geologist Seth Wittke. “With additional study, this new information will help scientists better understand the seismic risk in the area.”

The Greys River fault, which bounds the west side of the Wyoming Range, extends roughly 65 kilometers (40 miles) from the headwaters of the Greys River north to its newly recognized intersection with the Little Greys River. Geologists were able to extend tracing of the fault by more than 6 miles.

Numerous landslides are visible along the west-dipping slopes of the Wyoming Range south of Deadman Mountain. View is to the northwest. (Courtesy of WSGS)

Since the area is prone to landslides, geologists wanted to investigate and map landslides in the area. At least two significant landslides—Blind Bull and Porcupine Creek—occurred in the area over the past five years, damaging roads and causing economic losses to nearby communities.

The results of the study may aid in regional seismic hazard modeling and protection of infrastructure in the Greys River valley and surrounding drainages from landslide hazards.

SGS geologist Seth Wittke maps in the Little Greys River drainage, Wyoming Range. (Courtesy of WSGS)

Other outcomes of the project include mapping of previously undifferentiated Pinedale and Bull Lake glacial deposits and fluvial terraces along the Greys River.

Sweetwater County Mapping:

The three Sweetwater County maps are on the southern end of the Rock Springs Uplift, a large folded and faulted structure with a complex array of geology. The new mapping provides key information on the timing of deformation of the Rock Springs Uplift.

“When you look at all three of the maps together, they illustrate how structurally complex the southern Rock Springs Uplift is,” says geologist Kelsey Kehoe.

Kehoe was the lead investigator on the mapping of the Lion Bluffs quadrangle on the south-central portion of the Rock Springs Uplift.

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

“We suspect that the alteration is due to the interaction of a low-sulfidation epithermal system with thick Cretaceous marine deposits, like the Baxter Shale.”

The Earnest Butte quadrangle, on the southwestern flank of the Rock Springs Uplift, is full of interesting geology, says geologist Dr. Ranie Lynds.

“For example, the unconformity between the Upper Cretaceous Almond Formation and the Paleocene Fort Union Formation represents about 8 million years of ‘missing’ time that was removed as strata were eroded from the flanks of the rising Rock Springs Uplift,” she says.

Lynds says geologists on this project revised the axis of the Rock Springs Uplift to lie further west of where it had previously been drawn on smaller-scale maps.

Geologist Patty Webber worked on mapping the South Baxter quadrangle, along the southeastern flank of the Rock Springs Uplift.

“There is significantly more faulting and deformation than is apparent from the surface,” Webber said. “This is in part due to the syn-depositional nature of the Upper Cretaceous to Eocene units found in the Rock Springs Uplift, which were eroding and depositing as the uplift developed,” Webber said.

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

All three new maps will contribute to the Firehole Canyon 1:100,000-scale bedrock geology map the WSGS is working toward completing in the coming year.

Dr. Erin Campbell, WSGS director and state geologist., said “STATEMAP’s emphasis on resources and natural hazards complements WSGS mapping priorities of mineral and energy resources, hydrogeology, and geologic hazards.” 

The STATEMAP program is a cooperative agreement with the U.S. Geological Survey’s National Cooperative Geologic Mapping Program. 

Maps, including accompanying technical reports, are available as free downloads and as hard copies for purchase on the WSGS website.