STRUCTURAL AND COMPOSITIONAL ASSESSMENT OF THE ESTES UNCONFORMITY, BLACK HILLS, SOUTH DAKOTA

Precambrian metasedimentary rocks in the Nemo area of the Black Hills, South Dakota, consist of the older Boxelder Creek Quartzite (BCQ) and Benchmark Iron Formation (BIF), deposited >2480 ma and overlain by the younger Estes Formation (EF). Redden and DeWitt (2008) interpret these sequences as being separated by the angular Estes Unconformity (EU), requiring deformation of the older rocks prior to deposition of the younger rocks. An evolving model developed by researchers at Winona State University argues the map pattern for this area does not require an earlier deformation, and instead proposes a single left-lateral transpressional shear zone cutting across all rock units. This research explores the validity of the unconformity model and tests the evidence supporting it by comparing structures and composition across the EU.

Detailed mapping at 1:6,000 across the EU has identified identical NW–SE striking shear fabrics in the BCQ and EF. Shear indicators in the EF, including sigma grains, aligned prolate cobbles, and composite planar fabrics, are consistent with shear fabrics and folds within the BCQ, and document left-lateral transpressional shear in all units. Microstructural analysis on samples collected above and below the EU are consistent with field observations. Bedding within the EF is generally obscured by intense shearing, although compositional layering was observed striking NE, parallel to bedding in the BCQ.

Petrologic criteria previously used to support the unconformity include cobbles of BCQ and BIF in the basal EF, varying feldspar content between the BCQ and EF, and metallic mineral phases present. Shear structures in cobbles within basal EF, previously used to argue for BCQ and BIF source, match composite shear fabrics postdating deposition of the EF. Petrography and geochemistry reveal similar bulk compositions, but euhedral sulfide and oxide phases suggest post-shearing mineral growth. This, in addition to intense shearing, masks original accessory mineral phases. Ongoing compositional and structural analysis is expected to support further the model for a single left-lateral transpressional shear zone, without requiring an unconformity.