196-4 Joint Patterns and History of the Sioux Quartzite

Session: Strain and Displacement: Patterns, Gradients, Partitioning, and Reconstructions (Posters)

Presenting Author:

Authors:

Abstract:

The Precambrian Sioux Quartzite is an orthoquartzite that is equivalent to the Baraboo Quartzite that is often densely jointed. Study sites include: Arrowhead Park, Dell Rapids, Devil’s Gulch, Falls Park, and Palisades State Park in SD, and Blue Mounds State Park in MN. The study sites span ~30 miles from East to West providing insight into the regional pattern. The outcrops include river gorges, cliff lines, and an abandoned quarry. The data is strike and dips of the joints and was analyzed using stereonets. There are 3 dominant joint directions seen at all of the study sites: a ~NS direction at all but one site, and ~EW direction at all sites, and NW-SE direction at most sites. The ~NS and ~EW are mutually truncating, more pervasive, and are interpreted as an orthogonal set. There are also 3 additional subordinate directions seen at multiple sites. This makes three sets which are an orthogonal pattern(~NS, ~EW), and oblique directions at 30°, 60°, 120°, and 150°. The different sites and different areas within sites have combinations of the 6 sets present. Typically, one or both of the orthogonal pair and one or two of the conjugate components are present.(4 oblique directions may represent 2 conjugate sets or 2 orthogonal sets) The ~EW set seems to align with gentle folds. The joint pattern seems to be domainal. Inconsistent truncations between directions can indicate reactivation or a changing stress field during a jointing episode. Irregular joint surfaces are common. Stepping patterns observed on horizontal surfaces are both left and right in the same outcrop for the same direction. There are coatings of pyrophyllite on many of the joint surfaces. For stepped surfaces, the pyrophyllite can coat one step and not the other. Previous work found a pervasive microfracture anisotropy consisting of both fluid inclusion planes and pyrophyllite seams that may have influenced joint growth. There are three deformation episodes that reasonably could have caused joints in the Sioux Quartzite. These are the Picarus-Baraboo Orogeny, Keweenawan Rift, and recent glacial activity. The study sites are in the foreland of the orogeny which is a common site for jointing. The Keweenawan rift runs NE-SW and could explain the non-orthogonal sets. Glacial isostatic response could produce joints in a NW-SE direction.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9356

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