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Fluvial Origin of the Lower Proterozoic Sioux Quartzite, Southwestern Minnesota

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Fluvial Origin of the Lower Proterozoic Sioux Quartzite, Southwestern Minnesota Fluvial origin of the lower Proterozoic Sioux Quartzite, southwestern Minnesota D. L. SOUTHWICK G. B. MOREY Minnesota Geological Survey, 2642 University Avenue, St Paul, Minnesota 55114 J. H. MOSSLER ABSTRACT strata, cropping out only in a few stream cuts and widely scattered upland knobs in Minnesota and South Dakota. This sparsity of exposures has The Sioux (juartzite of inferred Early Proterozoic age (1,760- impeded stratigraphic and sedimentological studies, and the inferred depo- 1,630 m.y.) occurs in southwestern Minnesota, eastern South Dakota, sitional and tectonic models (Weber, 1981; Morey, 1983a, 1984; Ojakan- and adjoining pails of Iowa and Nebraska where it overlies a regolith gas and Weber, 1984; Southwick and Mossier, 1984) remain qualitative developed on an older Precambrian crystalline basement The rocks and somewhat subjective. constitute a red-t»ed sequence that was deposited by braided streams flowing over a deeply weathered land surface of moderate relief. Dep- osition was coniined largely to fault-bounded basins in a cratonic setting; the basins subsided slowly and were rarely, if ever, areas of steep relief. The Sioux Quartzite is a texturally and mineralogically mature quartz arcnite. Sand grains are mainly monocrystalline quartz with rare grains (if chert, granular iron-formation, and quartzite. Scat- tered conglomeratic layers contain lithic clasts that include red quartz- ite, chert, iron-formation, vein quartz, and rhyolite, together with rare welded rhyolite tuff and granitoid gneiss. Stratigraphic intervals of conglomeratic orthoquartzite are interspersed throughout the basal two-thirds of the formation, whereas thin units of sericitic mudstone are most abundant in the upper third. Overall, the upward fining of size grades in the Sioux may indicate diminishing stream gradients and reduction of relief in source areas during deposition. The diagenctic minerals diaspore, kaolinite, and quartz cement indicate an environment of intense leaching, probably under warm, humid climatic conditions, during or closely following deposition. Detrital feldspar, present in trace amounts in deeper parts of the stratigraphic section, may have been more abundant originally but was destroyed by intrastratal reactions. The sub-Sioux regolith is characterized by (1) fine-grained kaolinite and sericite formed from intensively altered coarse-grained metamorphic feldspar and (2) the presence of abundant secondary hematite and silica. 50 100 Miles By virtue of its Early Proterozoic age, its alluvial origin, and its —t- _J unconformable position above older Proterozoic and Archean rocks, 0 50 100 Kilometers the Sioux is infeiTed to have some potential for unconformity-related, K Cretaceous and younger sedimentary rocks Athabasca-type uranium deposits. Its provenance, which includes older Proterozoi c iron-formation and volcanic rocks (greenstone), en- Ull Paleozoic sedimentary rocks hances its potential for paleoplacer gold deposits. Sioux Quartzite (Early Proterozoic) INTRODUCTION The Sioux Quartzite is a sandstone-dominated unit of Early Protero- Archean gneiss, terrone zoic age that extends in an east-trending belt across southwestern GL Minnesota, southeastern South Dakota, and adjacent parts of Iowa and -TZ Great Lakes tectonic zone Nebraska (Fig. 1). It is inferred to have been deposited between 1760 and 1630 m.y. B.P. (Bergstrom and Morey, 1985). The complex crystalline Figure 1. Regional geologic map of southwestern Minnesota, basement beneath the Sioux is of predominantly Archean age in south- eastern South Dakota, and adjacent parts of Iowa and Nebraska, central Minnesota but includes some local remnants of older Proterozoic showing position of Sioux Quartzite relative to surrounding Phaner- rock. The Sioux is covered extensively by Cretaceous and Quaternary ozoic strata. Adapted from Southwick and Mossier (1984). Geological Society of America Bulletin, v. 97, p. 1432-1441, 7 figs., December 1986. 1432 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/12/1432/3419462/i0016-7606-97-12-1432.pdf by guest on 28 September 2021 FLUVIAL ORIGIN OF PROTEROZOIC QUARTZITE, MINNESOTA 1433 EXPLANATION Ps Sioux Quortzite . Pvg o o Metavolcanic and o metasedimentary rocks or of Wisconsin magmatic o terrone; includes younger cu intrusive rocks and reworked Archean rocks et SPvs < <t xxxx Metavolcanic and metasedimentary rocks, including granular iron -formation Agn Gneissic rocks, undivided; o large areas of younger CE intrusive rocks not shown <t inferred fault 20 Km. Figure 2. Pre-Paleozoic subcrop map of southwestern Minnesota, showing the outlines of the Pipestone, Fulda, Cottonwood County, and New Ulm basins of Sioux Quartzite (Es) and three unnamed, poorly defined patches of Sioux. Heavy dashed lines indicate possible faults in magnetic basement, as inferred from aeromagnetic maps. Basement rocks at sites 4,10,11, and 12 have cataclastic textures; the rock at site 12 is a refractured blastomylonite. Modified from Southwick and Mossier (1984). This paper summarizes the major sedimentological features of the 7,900 ft) for the Sioux Quartzite in the Pipestone basin and a thickness of formation in Minnesota. The reader interested in greater detail, including 610 to 1,220 m (2,000 to 4,000 ft) in the Cottonwood County basin by modal analyses, structural observations, paleocurrent analyses, and mea- using conglomeratic beds as stratigraphic markers and assuming an aver- sured sections, should refer to papers by Morey (1983a, 1984), Ojakangas age constant dip. This approach to estimating thickness is not very precise, and Weber (1984), and Southwick and Mossier (1984). because the conglomerate marker beds tend to be lenticular in form, are difficult to correlate with certainty across areas of sparse exposure, and DESCRIPTION probably were deposited on a sloping surface. Gries (1983), after restudy- ing evidence in South Dakota, concluded that the Sioux there is generally Recent exploratory drilling has shown that the Sioux Quartzite in not much thicker than 305 m (1,000 ft), considerably thinner than the Minnesota is restricted to four northwest-trending blocks or basins (named original estimate of Baldwin. The maximum thickness transected by drill- in Fig. 2) which are separated from one another by areas where the ing on the north flank of the Cottonwood County basin is 287 m (942 ft). quartzite is patchy or absent (compare Fig. 2 with Austin and others, Nearby outcrops represent a stratigraphic thickness of about 200 m (660 1970). Faults, inferred from aeromagnetic anomaly patterns (Philbin and ft), most of which is above the section cut by drilling (Southwick and Gilbert, 1966), and cataclastic textures in four basement drill cores along Mossier, 1984). The total documented thickness in Cottonwood County is the trends of the anomalies, are located along the divides between basins. therefore about 490 m (1,600 ft), somewhat more than the thickness Differential fault movement may have controlled sedimentation and may estimated by Gries (1983) in South Dakota but much less than Baldwin's have continued long after the Sioux was lithified. Geomorphic inversion estimate of 1,220 m (4,000 ft). caused by the Sioux's superior resistance to weathering and erosion created topographic ridges on the Precambrian bedrock surface, which influenced Petrology patterns of sedimentation in the Paleozoic and Mesozoic (Fig. 1; also Shurr, 1981). The Sioux Quartzite consists of four major rock types: orthoquartzite, Although the Sioux "basins" are erosional remnants, structural clo- conglomeratic orthoquartzite, conglomerate, and mudstone. Orthoquartz- sures and paleocurrent flow-directions (discussed below) imply that the ite is by far the most abundant. Pipestone basin (Baldwin, 1951) and the Cottonwood County basin Orthoquartzite. The orthoquartzite is a tightly cemented, very ma- (Southwick and Mossier, 1984) were depocenters. ture quartz arenite that typically is red or pink in color, owing to the Baldwin (1951) calculated a thickness of 1,600 to 2,400 m (5,300 to presence of disseminated hematite. Petrographic descriptions and modal Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/12/1432/3419462/i0016-7606-97-12-1432.pdf by guest on 28 September 2021 1434 SOUTHWICK AND OTHERS analyses have been given by Morey (1983a) and Ojakangas and Weber (1984). Two textural and compositional varieties of orthoquartzite have D.D.H. Red shale and mudstone, SQ-6 been recognized. In the first, about 90% of the rock consists of rounded and laminated to blocky well-sorted quartz ¡¡and, and about 10% consists of subequal proportions of authigenic quartz cement and phyllosilicate matrix. The second type has a Red mudstone, siltstone, and fine sandstone texturally bimodal framework consisting of coarse sand-sized grains that (locally with dispersed granules) are well rounded and medium sand-sized grains that are subangular to elevation <m) subrounded. This type contains 15% to 25% phyllosilicate matrix material Medium-grained orthoquartzite iLi-fc:- and lacks a quartz lament. It has been identified only near major claystone 239.3 - with many thin interbeds of red shale units at Pipestone National Monument, near Pipestone. A significant fraction of sand grains in both orthoquartzite types have Medium to very coarse red abraded epitaxial overgrowths (Ojakangas and Weber, 1984). Monocrys- orthoquartzite and grit talline quartz is the overwhelmingly dominant framework component ex- cept in proximity to
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