MICROSTRUCTURAL ANALYSIS OF THE YATES UNIT AT THE DEEP UNDERGROUND SCIENCE AND ENGINEERING LABORATORY

Recent drilling at the Deep Underground Science and Engineering Laboratory (DUSEL) on the 4850 level provides an opportunity to understand the Paleoproterozoic structural evolution in the Lead anticlinorium and the northern Black Hills. Partitioning of late deformation may allow folded rock fabric in the Yates unit of the Poorman Formation amphibolite to have preserved the earliest parts of the Black Hills deformation history. Geochemistry and field relations of the amphibolite in the Yates unit indicate are similar to amphibolite around other areas of the Black Hills. The earliest structural events associated with an episode of NW-vergent, thin-skinned thrusting (D1). The details of this event are still unknown, due to the lack of metamorphism or foliation evidence. The D1 event that led to the foliation the primary bedding occurred as early as ~1785 Ma and is attributed to the NW-directed accretion of the Central Plains island-arcs.

Logging of oriented core from drill holes 3 and D indicate a folded foliation defined by the alignment of hornblende. The latest folds yet identified, plunge gently to the SE and have a similar trend to the Lead anticlinorium. Detailed descriptions of thin-sections from oriented samples taken from upper, middle, and lower sections of the drill holes will show structural and petrologic evidence, characterizing important deformational events and specifically answer questions about how the Paleoproterozoic rock fabrics and microstructures observed in the Yates unit of the Poorman Formation correlate with known regional deformational events.