The lower Proterozoic Sioux Quartzite is exposed at several localities
within an east-trending area 300 km long and 50 km wide between Mitchell,
South Dakota, and New Ulm, Minnesota. The quartzite unconformably
overlies Archean rocks and is overlain by Cretaceous strata and
Pleistocene glacial materials.
The formation consists predominantly of orthoquartzitic sandstone.
Minor quantities of interbedded conglomeratic orthoquartzite occur in the
lower two-thirds of the section, and thin beds of mudstone occur in the
upper third. The orthoquartzite is compositionally and texturally mature
and is composed largely of well-rounded, well-sorted, monocrystalline
quartz and lesser amounts of detrital chert and iron-formation. Grains of
polycrystalline quartz are abundant at several places, including New Ulm
where they were derived from underlying granite, and near Jackson where
they occur in the lower half of the exposed section. The framework grains
are coated with thin films of iron oxide and indurated by an epitaxial
quartz cement. Multicycle grains with abraded quartz overgrowths beneath
the latest cement are present in most orthoquartzite thin sections.
Rounded zircon and minor rounded tourmaline make up the nonopaque detrital
heavy mineral fraction.
Two types of conglomeratic rocks are present--coarse basal conglomerate
and conglomeratic orthoquartzite. The basal conglomerates at two
locali ties contain clasts, as large as 35 cm, of vein quartz, jasper,
iron-formation, quartzite, and rhyolite. The conglomeratic orthoquartzite
units are characterized by scattered pebble-size clasts of quartz in beds
1 to 10 cm thick. The pebble-rich beds are intercalated with units of
cross-bedded, coarse-grained orthoquartzite.
Mudstone within the Sioux is red to dark purple in color, ranges from
almost pure claystone to silty mudstone, and is composed dominantly of
sericite, quartz, and hematite.
Trough cross-bedding, planar cross-bedding, symmetrical and asymmetrical
ripple marks, and mud cracks are the maj or sedimentary structures.
The cross-bedding consists predominantly of narrow troughs 60 to
140 cm wide and 15 to 30 cm thick. Some herringbone cross-bedding is
present in the upper third of the section.
Measurements of 1, 156 cross-beds have a vector mean of 1620 and a
variance of 2,668, implying a paleoslope inclined to the southeast. No
major vertical or lateral changes in cross-bedding trends were observed.
Analysis of 491 ripple marks also indicates a paleoslope inclined to the
southeast. Bimodal-bipolar paleocurrent patterns occur in the upper part
of the formation.
Most of the Sioux Quartzite is interpreted to have been deposited in a
distal braided river-alluvial plain environment. However the upper third
of the formation has sedimentary structures that possibly are indicative
of a shallow marine, tidally influenced environment.
The Sioux is in part a multicycle sediment, with components derived
from older quartzite and iron-formation. However, the great volume of
monocrystalline quartz must have been chiefly derived from an extensive,
deeply weathered, low-relief terrane of dominantly plutonic rocks that
also contained some volcanic and metamorphic components. Paleocurrent
patterns imply that this terrane existed to the north of the Sioux outcrop
Southwick, D.L., 1984, Shorter Contributions to the Geology of the Sioux Quartzite (Early Proterozoic), Southwestern Minnesota, Minnesota: Minnesota Geological Survey Report of Investigations 32, 74 p.
RI-32 Shorter Contributions to the Geology of the Sioux Quartzite (Early Proterozoic), Southwestern Minnesota.
Minnesota Gelogical Survey.
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