CORRELATION OF SUPRACRUSTAL ROCKS IN THE LITTLE ELK TERRANE, BLACK HILLS, S.D

The Little Elk Terrane (LET) is an erosional window that exposes Archean basement gneisses and detrital supracrustal rocks in the NE Black Hills 5 km to the south, near Nemo, Proterozoic supracrustal rocks deposited during two distinct periods are separated by an angular unconformity, requiring a deformational event prior to the younger depositional period. Gneisses and supracrustal rocks in the LET contain a strong NW-striking, shear fabric with left-lateral, east-side up shear sense. Although this deformation is comparable to a 1750-1715 Ma shear-fold event identified in the younger Proterozoic sediments farther south, a direct age on the fabric in the LET has been difficult to attain and it is unknown whether it is the same event or perhaps a pre-unconformity event. This project attempts to determine if the shearing in these two areas are the same event or two separate but similar events by correlating the LET supracrustal rocks to either the older or younger sequence near Nemo.

This project describes the stratigraphy of the LET in detail and identifies distinct marker beds for comparison to well-defined stratigraphic units in the Nemo area. In the LET the supracrustal rocks range from mica-schist to mica-rich pebble conglomerate to matrix supported cobble conglomerate. Three characteristics were chosen to be used as markers within the LET sequence; a fuchsite-rich mica schist layer, a micaceous quartz-pebble conglomerate with minor fuchsite, and a matrix-supported cobble conglomerate with milky quartz cobbles up to 20cm in diameter.

All three markers were identified 4 km SE of Nemo in a single outcrop mapped as the Boxelder Creek formation, a unit in the older Proterozoic sequence. Because this unit has a pre-unconformity deformational history it remains possible the shearing is part of the older event. However, concurrent field mapping near Nemo along the unconformity (Heim, this meeting) determined the 1750-1715 Ma shearing event clearly involves rocks above and below the unconformity. Without directly dating the shearing in the LET it remains possible these are separate events; nevertheless, we propose these observations strengthen a model for a single NW-striking, Left-Lateral shearing event at 1750-1715 Ma.