BHCC 2013: When Geology and Offroading Meet

This post covers three trails that were part of the BHCC 2013: (1) Jake; (2) Iceman; and (3) Full Size. Each trail required most of a day and was located several miles from the main camp at Wild Bill’s near Deadwood. I am only going to discuss the relationship between the geology and the off-roading experience today. The maps below are approximately the same region; the projections are different so they don’t match exactly but I have labeled important locations that are shown in later photos.

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The star indicates the location of the campground where the event activities and camping were located. Unlike some events, this one occurred in a national forest instead of an ORV (Off-Road Vehicle) park. Wild Bill’s is at the top of a long climb up from the mineralized zone around Lead and Deadwood. Many faults and dikes are visible in these roadcuts. This image shows 30 MA dikes (light colored) along bedding/structural weaknesses in >2 GA metasediments, which hit the angular unconformity at the Great Unconformity that separates ~1.5 GA rocks from 600 MA rocks. These Cambrian sediments are not metamorphic (no great heat or pressure) and they are nearly flat; the hot molten magma turns horizontal to follow their bedding surfaces.

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Before hitting the trails, I drove south to Harney Peak and hiked to the fire tower at the top (~7300 feet). This is the backbone of the Harney Granite that was intruded ~1.6 GA. This image shows the trail and the overall shape of the pluton. The fire tower is above the word “Black” in the GoogleEarth image.

harney_peak

The complex relationships between the surrounding metasedimentary rock and the Harney Granite are seen in this image from the Mt. Rushmore monument.

granite_relationships

The first trail we followed was Jake (see image above). We drove into the pC metamorphic rocks and followed a forest service trail to a narrow gully that began climbing to the east. These rocks were originally deposited as mud/sand in nearshore environments like Louisiana ~2.3 GA and were buried. At ~1.5 GA the Harney Granite was emplaced (over tens of millions of years) and they were warped during the coeval orogeny. This occurred at about 8 miles below the surface. When they were uplifted at ~30 MA, they fractured into a common hexagonal fracture pattern that leaves their edges like knives. They are also very dense because of the high pressure. They form a truly tiring trail that sliced my aluminum wheels like butter.

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