STRAIN ANALYSIS OF BOULDERS IN ESTES CONGLOMERATE WITHIN THE NEMO SHEAR ZONE, BLACK HILLS, SD

The Black Hills in South Dakota provide exposure of the southern Trans-Hudson orogen where the Wyoming and Superior cratons are sutured. The final suturing, local D₃, is recognized as NW-striking shear zones and F₃ folds that developed during left-lateral strike-slip, NE-side up transpression The Nemo shear zone is a similarly oriented structure located in the NE Black Hills. Two miles south of Nemo village, this shear zone overprints the Estes unconformity deforming the rocks in both the Nemo group below and the overlying Estes Conglomerate, This project quantitatively describes the strain and interprets shear sense from the boulder-size clasts present within the Estes conglomerate at this location.

This research mapped a 1.5 x .5 km area, adjacent and above the Estes unconformity, focusing on the Estes Conglomerate. At this location this unit is a matrix supported metaconglomerate containing clasts of iron formation and quartzite derived from the underlying Nemo Group rocks. Clasts range in size from pebbles to large boulders. Deformation associated with the Nemo shear zone flattened the boulders parallel to a NW trending, near vertical foliation. By assuming near spherical original clast shapes 3 dimensional strain ellipses are estimated from 2 dimensional outcrop views of 228 boulders in multiple viewing orientations.

Assuming no area loss, 72% of the boulders are flattened within the foliation with a stretch (S) ≤ 0.6 measured in horizontal and vertical planes perpendicular to the foliation. Average stretches of the long axes in these orientations are similar and range from 1.6 to 1.8, with vertical orientations being slightly greater. When measured on vertical surfaces parallel to the foliation, 82% of long axes have S≥ 1.4, and the axes’ average rake is 72° down from the NW. The similarity in stretch for long axes in all three orientations supports triaxial strain with a significant flattening component. Asymmetry in sigma-shaped boulders on horizontal and vertical surfaces supports left-lateral strike-slip and NE-up displacement. Resolving the high angle between the long axis of the strain ellipse with transport direction based on boulder asymmetry is difficult; however, the strong flattening, shear sense, and orientation of the Nemo shear zone supports it is related to the regional D₃ transpressional event.