SIOUX FALLS
Extensive outcrops of
pinkish, Paleoproterozoic-aged quartzites are present at Falls Park along the
Big Sioux River in the city of Sioux Falls, southeastern South Dakota,
USA. The quartzites here have nicely water-worn, sculpted surfaces with
good, fluvially abraded polish in places. These rocks are part of the Sioux
Quartzite (upper Paleoproterozoic, 1.65-1.70 Ga). Despite being
subjected to regional metamorhpism, this unit's original sedimentary features,
such as horizontal stratification and ripple marks, are still preserved.
Sioux Quartzite (1.65-1.70 Ga) along the
Big Sioux River at Sioux Falls, southeastern South Dakota, USA.
Sioux Quartzite (1.65-1.70 Ga) along the
Big Sioux River at Sioux Falls, southeastern South Dakota, USA.
Sioux Quartzite (1.65-1.70 Ga) along the
Big Sioux River at Sioux Falls, southeastern South Dakota, USA.
Sioux Quartzite (1.65-1.70 Ga) along the
Big Sioux River at Sioux Falls, southeastern South Dakota, USA.
Water-worn & sculpted
surface of the Sioux Quartzite (1.65-1.70 Ga) along the Big Sioux River
at Sioux Falls, southeastern South Dakota, USA.
Ripple marks in the Sioux
Quartzite (1.65-1.70 Ga) along the Big Sioux River at Sioux Falls,
southeastern South Dakota, USA. Lens cap for scale.
Polish on an eroded surface
of Sioux Quartzite (1.65-1.70 Ga). The polish is the result of
fluvial abrasion from the Big Sioux River.
Percussion marks (arcuate
cracks) in the Sioux Quartzite (1.65-1.70 Ga). These features
formed as a result of clast impacts during energetic flow in the Big Sioux
River. Ant for scale.
The Sioux Quartzite is a
famous 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 NE-SW trending series of paleotopographic
highs & depressions known as the Transcontinental Arch, which
extends from Arizona to Minnesota (see Carlson, 1999, Transcontinental Arch - a
pattern formed by rejuvenation of local features across central North America.
Tectonophysics 305: 225-233).