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Linear Surface Anomalies - Bennington, ID

While mapping the geology of the 1:24,000 scale Georgetown Quad near Bennington, ID, structural geologist Jim Coogan discovered linear anomalies in aerial photos. Jim showed Dennis and I these when we were all working in the area together and I've done a bit more snooping around in Google Earth since then.

The linear elements are best seen in harvested hay fields. They are durable features that persist in photo sets from 2013, 2014, and 2016. Earlier photos are of insufficient resolution to see them. Their NE orientation parallels the Preuss Range and its range-bounding faults. Some sets of anomalies appear to align with a prominent NE-trending fault scarp at the mouth of Bennington Canyon.

The features may be erosional rills, cattle/elk tracks, relict en echelon cracks in the Salt Lake Formation translating through the Pleistocene cover, or the surface expression of recent bedrock faulting. Other explanations may exist.

While mapping the surficial geology, I observed small-offset faults in Pleistocene fan gravels in a borrow pit (W 7th Street west of Georgetown) and deformation fabrics in indurated lacustrine-overbank-paleosol deposits beneath undeformed alluvial fan gravels along Georgetown Cemetery Road.

The hamlet of Bennington is located in a half graben between the Bear River and the Preuss Range in southeastern Idaho about 45 minutes from Soda Springs. Hwy 30 runs through town. Montpelier and Bear Lake are to the south. The linear anomalies described here are located between the highway and the western flank of the Preuss Range.

Black circle shows location of linear anomalies. My draft surficial mapping from 2019 is background image.

Bennington, ID is located in the broad valley at top-center of figure. The Bear River is shoved to the west side of the valley by Pleistocene alluvial fans spilling from the Pruess Range. Structurally, its Basin and Range half graben formed by extensional reactivation of older thrust ramps. A thick sequence of east-dipping tuffaceous Tertiary sediment (Salt Lake Fm in light pink) fills the half graben. Complexly folded and faulted Paleozoic and Mesozoic strata lies to the east in the Pruess Range. Figure is an excerpt of a draft cross section by Jim Coogan, who did his PhD work here in the late 1980s-early 1990s.

Location A: Red Canyon Road east of Bennington, ID.

Location B. Two track farm road for scale.

Location C. Farm roads and harvester patterns for scale.

Location D (2013 photo).

Location D (2014 photo).

Location D (2016 photo).

Deformed laustrine-overbank-paleosol beneath undeformed alluvial fan gravel. Possible caused faulting or relict frost heave. Georgetown Cemetery a few miles north of Bennington.

A few NE-trending, small-offset faults like this one cut Pleistocene alluvial fan gravels in a borrow pit west of Georgetown, ID.


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