STRUCTURAL ANALYSIS OF PROTEROZOIC SHEARING AND FOLDING; NORTHEAST BLACK HILLS, SD

The Estes Unconformity seperates the supracrustal rocks in the NE Black Hills, SD into two depositional events. The older, >2480 Ma, Nemo sequence includes Box Elder Creek Quartzite and Benchmark Iron Fm. The Estes Fm is the base of the younger sequence above the unconformity. Elsewhere along the eastern margin of the hills, rocks of the younger sequence host a 1750-1715 Ma shear-fold event coupling NW-trending, upright, near-vertically plunging F₃ folds with a NW-trending left-lateral, east-side up shear zone. Although Archean basement and >2480 Ma rocks in the NE contain similarly oriented structures, a debate exists whether deformation in >2480 Ma supracrustal rocks record the 1750-1715 Ma shear-fold event or only pre-unconformity deformation. Mapping along and adjacent to the Estes unconformity investigated this controversy.

Detailed field mapping at 1:6,000 along and near the Estes Unconformity identified strong NW-striking shear fabric in both younger and older supracrustal rocks. Boulders in the Estes Conglomerate are deformed asymmetrically reflecting east-side up, left-lateral shear. Shear fabrics and clast asymmetry in the older quartzite and iron formation adjacent and <10 meters from the unconformity are parallel to this fabric and contain the same shear sense. Nearby, northwest-trending, upright, near-vertically plunging folds, similar in style and orientation to F₃ folds elsewhere, are mapped in gabbroic sills and supracrustal layers. Microstructural analysis of samples from hinge areas in these folds identified folding and crenulation of an earlier fabric (S₂) and retro-grade metamorphism that replaces late-syn porphyroblasts with unstrained minerals, a texture identical to textures associated with the shear-fold coupled event recognized elsewhere.

Shearing and folding near the Estes Unconformity has abundant evidence illustrating that the rocks both above and below are identical in deformational structure, style, and orientation to each other and to deformation elsewhere along the eastern margin of the hills. This includes to the south near Rockerville, elsewhere in the Nemo sequence, and rocks in the Little Elk Terrane. We interpret it is more likely a single 1750-1715 Ma event is responsible for shearing and folding of all rock units in the NE of the Black Hills.