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Precambrian metasedimentary rocks in the Nemo area of the Black Hills, South Dakota are currently interpreted as having been deposited during two distinct depositional events with an angular unconformity seperating them, requiring deformation of the older rocks prior to deposition of the younger rocks. An evolving model developed by the Winona State Black Hills research group argues that the map pattern in this area does not require an earlier deformation. This research aimed to replicate work done by prior Winona State researchers in a more focused area immediately surrounding the Estes Unconformity. Extensive field mapping and sample collection were performed in the Nemo area of the Black Hills, South Dakota. Significant samples were processed into thin sections to observe microstructures and identify textures and mineral phases. Corresponding powdered samples were sent out to be processed via laser ablation, and the resultant bulk geochemical data was used to confirm petrographic mineral identification and modal abundance calculations. Due to COVID restrictions on lab access and the uncertainty of additional geochemical and thin section processing, this portion of the overall project was incomplete. Nonetheless, compositional assessment proved to be an important supporting argument in the structural assessment of the Estes Unconformity. Uncertainty was introduced to the compositional criteria previously used to support prior deformation and an unconformity. Structural and compositional evidence were both used to support a single extended deformational event affecting the Nemo area rocks. This research establishes a good foundation for future work in the area. Samples collected in the Nemo area could be used for banded iron chemical signature analysis or detrital zircon age dating.

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Stephen Allard



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