The Upper Precambrian Sioux Quartzite is exposed at several locations along an east-west trend 175 miles long and 30 miles wide between Mitchell, South Dakota and New Ulm, Minnesota. It rests unconformably on Lower Precambrian rocks and is overlain by Cretaceous strata and Pleistocene drift. A coarse basal conglomerate is exposed near New Ulm a short distance from the underlying granite but contains no granitic clasts . The Sioux Quartzite is gently folded. It is intruded by diabase at Corson, South Dakota. The formation consists of over 1600 meters of orthoquartzite sandstone with minor interbedded quartzose conglomerate and mudstone. Conglomeratic units are present in the lower two-thirds of the section and minor thin mudstones occur in the upper third. The compositionally and texturally supermature orthoquartzite is composed almost exclusively of well rounded, well sorted monocrystalline quartz. Detrital chert and iron formation grains are present in some samples. Polycrystalline quartz is abundant only near New Ulm, Minnesota, where it was derived from the underlying granite. In all other parts of the Sioux it makes up only a small percent of the total detrital grains. No feldspar is present in any of the 109 thin sections examined. The mean grain size of the samples studied ranged from fine sand to coarse sand but most were medium sand. The grains are coated with a thin film of iron oxide and cemented by quartz overgrowths. In a few samples the overgrowths are partially replaced by sericite. Multicycle grains with abraded quartz overgrowths beneath the present cement were found in 84 percent of the orthoquartzite thin sections. Rounded zircon and tourmaline are the only common nonopaque detrital heavy minerals. Two types of conglomeratic rocks are present, coarse basal conglomerate and conglomeratic orthoquartzite. The basal conglomerate consists of clasts, as much as 35 centimeters in diameter of vein quartz, hematitic chert, iron formation and quartzite. Clasts of rhyolite were found in one outcrop of basal conglomerate. The conglomeratic orthoquartzite consists of 1 to 10 centimeter layers of pebbles interbedded with cross-bedded coarse orthoquartzite. The mudstones within the Sioux are red to dark purple in color. Most are blocky but some show fissility. 'I'he mudstones range from almost pure claystones to silty mudstone and are composed of sericite, quartz, hematite and illite. Trough cross-bedding, symmetrical ripple marks, current ripple marks and mudcracks are the major sedimentary structures. Less common structures include sand waves, planar cross-bedding, load casts, mud clasts, parting lineation and climbing ripple lamination. The cross-bedding consists predominantly of narrow troughs 60 to 140 centimeters wide and 15 to 30 centimeters thick. Symmetrical ripple marks with wave lengths of 2 to 4 centimeters and amplitudes of 4 centimeters are present throughout the formation. Asymmetrical current ripples are the most common ripple type. They have an average length of 8.2 centimeters and an amplitude of 2.5 centimeters. Herringbone cross bedding and reactivation surfaces, sedimentary structures characteristic of tidal sedimentations are found only in the upper third of the section. Measurements of 1156 cross-beds with a vector mean of 162 degrees show a paleocurrent direction to the southeast. No major vertical or lateral changes in trends were observed. The analysis of 491 ripple marks also indicate a southeast paleoslope. Paleocurrent patterns are unimodal throughout most of the unit but some bipolar patterns occ.ur in the upper part of the formation. The Sioux is a multi-cycle sediment derived f om a highly weathered low relief source area in which quartz sands tone and iron formations were the dominant lithologies. The upper third of the formation is interpreted as having been deposited in a shallow marine intertidal environment. The lower two-thirds of the section may have been deposited in either braided fluvial environment or a high energy shallow marine environment.
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Richard Elmo Weber in partial fulfillment of the requirements for the degree of Master of Science, March 1981.
Weber, Richard Elmo.
Petrology and Sedimentation of the Upper Precambrian Sioux Quartzite Minnesota, South Dakota and Iowa.
Retrieved from the University of Minnesota Digital Conservancy,
Content distributed via the University of Minnesota's Digital Conservancy may be subject to additional license and use restrictions applied by the depositor.