English: Fluvially-polished quartzite in the Precambrian of South Dakota, USA.

Extensive outcrops of pinkish, Paleoproterozoic-aged quartzites are present at Falls Park along the Big Sioux River in the city of Sioux Falls, South Dakota. The quartzites here have nicely water-worn, sculpted surfaces. These rocks are part of the Sioux Quartzite, which consists of 1.65 to 1.70 billion year old metamorphosed sandstones. Despite the metamorphism, original sedimentary features such as horizontal stratification, cross-bedding, and ripple marks are still preserved.

The Sioux Quartzite is an erosion-resistant unit in America’s midcontinent. It has formed a long-lived paleotopographic high since Precambrian times - the Sioux tectonic core. This high is part of a northeast-to-southwest trending series of paleotopographic highs & depressions known as the Transcontinental Arch, which extends from Arizona to Minnesota (see Carlson, 1999).

Quarries of Sioux Quartzite occur in southeastern South Dakota and southwestern Minnesota. The rocks are used as building stone, road gravel, sidewalk and paving gravel, and erosion control material.

The slightly shiny areas on this Sioux Quartzite surface are fluvial polish. During times of energetic flow by the nearby Big Sioux River, abrasion from sediments have eroded, scoured, smoothed, and polished the rocks. Naturally polished rocks can also form by glacial action (glacial polish), faulting (structural polish or fault polish), and wind action, which results in ventifacts.

The arcuate cracks are percussion marks (percussion cracks; percussion cones). These features formed by clast impacts during riverine high flow.

Stratigraphy: Sioux Quartzite, upper Paleoproterozoic, 1.65-1.70 Ga

Locality: Falls Park, near Sioux Falls along the Big Sioux River in the town of Sioux Falls, southeastern South Dakota, USA

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Reference cited:

Carlson (1999) - Transcontinental Arch - a pattern formed by rejuvenation of local features across central North America. Tectonophysics 305: 225-233.