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 < Agn Gneissic rocks, undivided; o large areas of younger CE intrusive rocks not shown 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 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 1435 oxide seams (predominantly hematite) which occur throughout the weath- Textures indicative of diagenetic quartz dissolution at clastic grain ered rock. In the descriptive terminology for weathered rocks proposed by boundaries are not common in most of the Sioux Quartzite (Ojakangas Ruxton and Berry (1957), the weathering profile beneath the Sioux is and Weber, 1984), although they have been noted in siltstone and mud- similar to zone III, core stones and matrix, and zone IV, partly weathered stone layers (Vander Horck, 1984) and in conglomerate matrix (Miller, bedrock. Totally decomposed rock analogous to the B horizon of modern 1961). Consequently there is some doubt as to whether the large volume weathering profiles was not found in the core holes. It may never have of authigenic quartz cement in the Sioux can be accounted for by intra- formed, or it may have been eroded prior to deposition of basal Sioux stratal processes alone (Ojakangas and Weber, 1984). Southwick and conglomerate. Mossier (1984) have suggested that some diagenetic silica may have been liberated from the intrastratal breakdown of feldspar, as has been docu- Diagenesis mented by Land and Milliken (1981) and Boles (1982) in Tertiary sand- stones of the Texas Gulf Coast. The extent of this contribution is difficult The diagenetic mineral assemblage diaspore-kaolinite-quartz is to assess, given the uncertain amount of feldspar originally present in the widely distributed in the Sioux Quartzite (Berg, 1937,1938). The textural Sioux Quartzite. relationships between these minerals are identical to those observed in the Sericitic matrix is most abundant in sections of orthoquartzite that early Proterozoic Lorrain Formation of Ontario (Chandler and others, contain interbedded mudstone or are relatively rich in the sericite-kaolinite 1969) and indicate that diaspore formed more or less simultaneously with clots interpreted to be feldspar pseudomorphs. This implies that some an early generation of well-crystallized kaolinite (Chandler and others, matrix sericite may have formed from sedimentary illite (Morey, 1983a, 1969; Southwick and Mossier, 1984; Vander Horck, 1984). 1984), and some may be the redistributed product of feldspar diagenesis. Detrital feldspar is decidedly scarce in the Sioux Quartzite, having Vander Horck (1984) has shown that illite and sericite occur as embay- been observed only in the basal few metres of the drill cores from the ments in quartz grains and in interstitial voids where they surround and are Cottonwood County basin (Southwick and Mossier, 1984) and in one intergrown with kaolinite. He therefore interprets the illite-sericite to have outcrop (Beyer, 1897). This scarcity can be ascribed to a scarcity of formed late in the diagenetic sequence, after much of the primary porosity residual sand-sized feldspar in the weathered source terrane, to selective had been occluded by earlier minerals. removal of detrital feldspar by reworking during sedimentation, and to Pyrophyllite occurs only in mudstone units inferred to have been intrastratal alteration of detrital feldspar after sedimentation. The latter buried in deeper parts of the Pipestone and Fulda basins, where pressure process is suggested by the fact that the few feldspar grains in the basal and temperature may have approached conditions of the lower greenschist rocks are residual kernels heavily mantled by sericite and kaolinite (Fig. 4). facies (Morey, 1983a; Ojakangas and Weber, 1984). Morey (1984) has These altered gTains are similar in size and form to sericite-kaolinite clots noted that quartzite adjacent to pyrophyllite-bearing beds has been weakly which occur widely in the quartzite, but which lack feldspar cores; we infer strained after cementation, as shown by strain shadows and deformation that the clots are pseudomorphs of former feldspar grains which have been lamellae that cross grain boundaries into quartz cement. totally transformed to phyllosilicate assemblages. Berg (1937) and Vander Horck (1984) have observed that hematite occurs as fine dust at boundaries between detrital cores and overgrowths, as included material within the overgrowths, and as discrete grains within masses of kaolinite and diaspore. It therefore appears that hematite formed sporadically throughout the diagenetic history of rock. In contrast, the authigenic rutile occurs as microlites completely surrounded by quartz overgrowths (Vander Horck, 1984). Sedimentary Structures Trough cross-bedding is by far the most abundant and characteristic sedimentary structure in the Sioux Quartzite. It is generally of the festoon type, and even the master bedding planes in cross-bedded sequences have gentle rolls or warps of diverse size and orientation (Morey, 1984). Planar cross-bedding is present in only about 10% of the studied outcrops, and herringbone cross-beds have been observed at four localities (Ojakangas and Weber, 1984). Other characteristic sedimentary structures are ripple marks (both symmetric and asymmetric types), mud cracks, and zones of orthoquartzite with scattered to abundant intraformational rip-up clasts of mudstone. Scour-and-fill deposits and lag deposits of granule-sized quartz grains also occur. The typical composite sequence of sedimentary textures and struc- tures at the Pipestone National Monument in the Fulda basin near Pipe- Figure 4. Photomicrograph of kaolinized and sericitized plagio- stone (Morey, 1984, p. 63-68) begins with a basal, intraclast-strewn, clase sand grains from near the base of the Sioux Quartzite, Cotton- scoured surface overlain by a unit of trough-cross-bedded orthoquartzite wood County basin (DDH no. SQ-6). Of the three prominent grains in that is typically 5-10 m thick. This is succeeded locally by a planar cross- the center of the photograph, the grain on the right is slightly altered, bedded orthoquartzite unit as thick as 1 m, then by horizontally laminated the one in the middle is partly altered, and the one on the left is almost and ripple-bedded fine sand and muddy units a few centimetres thick, totally altered to kaolinite and sericite. The matrix material is kaoli- and finally by thin clay drape deposits. Similar fining-upward sequences nite, sericite, hematite, and chert. Middle grain is 0.4 mm long. were noted in the Cottonwood County drill cores (Southwick and 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 1436 SOUTHWICK AND OTHERS Mossier, 1984), but in addition the cores reveal many small-scale internal sequences, typically less than a metre thick, that coarsen upward individu- ally. These generally begin with a mudstone or fine orthoquartzite at the base in scoured contact with subjacent beds, which can vary in texture from coarser orthoquartzite to mudstone, and pass upward into very coarse-grained orthoquartzite. The lowermost 300 m of Sioux Quartzite in the Cottonwood County basin finesupwar d overall, indicating a long-term temporal decrease :'n depositional energy, but the internal juxtaposition of fining-upward and coarsening-upward sequences suggests that the rocks Cross - bedding Symmetrical Ripples were deposited in a. regime of waxing and waning currents. Paleocurrent Analyses Regional paleacurrent studies of the Sioux by Pettijohn (1957) and EXPLANATION Ojakangas and Weber (1984) have shown that major flow was toward the south-southeast (vector mean of 162°; Fig. 5). A principal anomaly is the New Ulm basin, where the basal conglomerate was deposited from cur- rents that flowed southeast (az. 148°), and the overlying quartzite was deposited by currents that flowed northeast and northwest (Ojakangas and Weber, 1984). Pakiocurrent azimuths in the west and northwest parts of 10 20 30% the Cottonwood County basin follow the regional southeast trend, but they shift to west o r northwest directions in the east. This implies that the MEAN CURRENT DIRECTION Cottonwood County basin may have been a depocenter fed by a crudely centripetal current regime (Southwick and Mossier, 1984). MEAN RIPPLE TREND Asymmetrical Ripples The erosional remnants of Sioux Quartzite are elongated in a south- eastward direction, parallel to the dominant paleocurrent azimuth. This Figure 5. Rose diagrams of cross-bedding, asymmetrical ripples, situation would be expected if the rocks were deposited from alluviating and symmetrical ripples in Sioux Quartzite. N, number of measure- river systems in elo agate, structurally controlled basins. In modern basins ments. Adapted from Ojakangas and Weber (1984). of this type, the through-going trunk streams tend to be parallel to struc- tural grain, whereas, the tributary streams are transverse to structural grain and are generally shorter and smaller (Miall, 1981). Suttner and others (1981) have pointed out that a rare combination of tropical climate, low relief, and low sedimentation rate is required to INTERPRETATION produce a first-cycle quartz arenite. These unique circumstances contrib- uted to the mineralogical maturity of the Mesozoic Nubian Sandstone of The Sioux Quartzite is a texturally and compositionally mature red- North Africa (Klitzsch and others, 1979; Van Houten, 1980), a unit de- bed sequence which rests unconformably on a weathered basement of rived from a source area composed of Precambrian crystalline and sedi- metamorphic and igneous rocks (Fig. 6). The texture and composition of mentary rocks. The Nubian rocks consist primarily of sand-sized quartz the deposit are entirely consistent with derivation from a deeply weathered and kaolinite. Warm, humid climate and extended exposure and rework- source area of low to moderate relief on which the rate of erosional ing of sediment in an alluvial-plain environment apparently led to weather- downcutting did not exceed the rate of weathering. The source area prob- ing and destruction of feldspar, mica, and ferromagnesian minerals ably was mantled with a grus-like residuum dominated by quartz, kaoli- (Klitzsch and others, 1979). An analogous situation may have contributed nite, and illite, and the landscape may have resembled present-day to the mineralogical maturity of the Sioux and may partly alleviate the northwestern Austnilia or parts of the Canadian Arctic Archipelago. problem of accounting for the great volume of quartz sand in the Sioux We interpret the Sioux as a first-cycledeposi t derived predominantly Quartzite and its correlatives (see Ojakangas and Weber, 1984). from a complex terrane of metamorphic and igneous rocks and to a lesser In Early Proterozoic time, the mechanical maturation of loose surfi- extent from older quartz-rich sedimentary rocks. First-cycle deposition, cial material would have been facilitated by the absence of terrestrial vege- subsequently modified by extensive alluvial reworking, is indicated strati- tation. Periodic episodes of sheet wash, alternating with dry times when graphically and texi:urally by the fact that basal, gritty conglomerate and eolian action was prevalent, may have been particularly effective at win- quartz arenite, inter Dedded with mudstone and characterized by polycrys- nowing clay and silt, rounding residual quartz, and destroying micaceous talline quartz, pass u pward into thick units of cross-bedded, uniform quartz minerals (Reineck and Singh, 1975, p. 262). An ideal climatic scenario for arenite that are poor in both associated mudstone and polycrystalline these processes might be the alternating wet (for weathering) and dry (for quartz. The basal de posits lie on a basement of weathered granitoid gneiss, winnowing and disaggregating) seasons typical of the present-day a ready source of monocrystalline and polycrystalline quartz grains (for sub-Sahara. gritty arenite) and clay minerals (for mudstone). Subsequent reworking of these materials by streams would hydraulically separate quartz and clay Nature of the Source Rocks minerals and would tend to disaggregate polycrystalline quartz into smaller, monocrystiilline grains (Blatt, 1967). The Sioux, however, con- The contribution of granitoid gneiss to the Sioux Quartzite is indi- tains a substantial fraction of quartz-sand grains with abraided epitaxial cated by the presence of gneiss boulders in the basal conglomerate of the overgrowths. These clearly indicate quartz-rich sedimentary source rocks, Cottonwood County basin, the fact that granitoid gneiss immediately un- and the Sioux therefore is multicyclic to some degree. derlies the base of the Sioux at New Ulm and in Cottonwood County, and 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 100 100 Km. < N I Prg Rhyolite-granite terrane of southern Wisconsin 1,760 m.y.) GLTZ: Great Lakes tectonic zone N N xM Rocks of the Central Plains Orogen Figure 6. Generalized tectonic map of the Upper Midwest, showing major Pre- V. Pa a cambrian terranes and terrane boundaries. Q) ^ The Central Plains orogen (southwestern Early Proterozoic supracrustal o E c O area of map) contains metamorphic rocks rocks of Animikie Basin ® ID O- CO deformed at about 1630 m.y. B.P. Rela- y Pvg /\ tively undeformed red quartzite units "I (Sioux, Barron) lie on Archean basement; Early Proterozoic rocks of "O a> >> deformed quartzite units (Flambeau, Bar- Wisconsin magmatic terrane; e § aboo, Washington County) lie on various includes large areas of younger Proterozoic terranes southeast of the intrusive rocks and Storm Lake and Niagara trends. Modified reworked Archean rocks from Sims (1985). 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 1438 SOUTHWICK AND OTHERS the similarity of polycrystalline quartz grains in the basal Sioux at New Many fault-controlled, nonmarine basins contain alluvium on the order of Ulm to quartz grains in the underlying granitoid gneiss (Miller, 1961). The 600-2,500 m in thickness, or more (Lorenz, 1976; Hempton and Dunne, cobbles of rhyolite a nd welded rhyolite tuff in conglomerate west of Pipe- 1984; Speksnijder, 1985). stone imply that felsic volcanic source rocks were present locally. The thickness argument relate!! directly to point 2, the uniformity of Sedimentary source rocks are indicated directly by the clasts of iron- thickness. Indirect evidence suggests that the northwest trend of the Cot- formation, chert, arid quartzite in conglomeratic beds of the Sioux, and tonwood County and Fulda basins in Minnesota are determined by base- indirectly by the common occurrence of quartz sand grains with abraded ment faults (Southwick and Mossier, 1984), which well may have been overgrowths in orthoquartzite. active during sedimentation. The paleocurrent data support this inference In all likelihood, the cobbles in the basal conglomeratic units were to some degree. It is quite possible, therefore, that the Sioux Quartzite derived from nearby sources because of the high stream competency re- filled a set of en echelon, fault-bounded basins that evolved slowly as quired to transport I hem. Until recently, the large clasts of iron-formation, sedimentation progressed; sedimentary thicknesses may have been trivial chert, and quartzite posed serious interpretational problems because no outside these depressions, and the deposit as a whole may not have been nearby basement so urces of these rocks were known (Miller, 1961). Re- blanket-like in form. cent drilling and geophysical investigations, however, have identified The question of regional correlations among the several red-bed metavolcanic and netasedimentary rocks, including iron-formation with quartzites of the Upper Midwest is related to the questions of thickness and granular texture (Jirsa, 1986) in parts of southwestern Minnesota that geometry. We cannot argue with the data of Van Schmus (1980) which were formerly map ped entirely as granitoid gneiss (Fig. 2). These rocks of bracket the age of the Baraboo Quartzite in Wisconsin between 1,760 and Late Archean or Early Proterozoic time are up the paleoslope from the 1,630 m.y., nor can we argue against the time-honored logic of extrapolat- Sioux basins and ¡ire much closer than alternative sedimentary source ing this depositional age to other lithologically similar quartzite formations rocks in east-central and northeastern Minnesota. (Dott, 1983; Bergstrom and Morey, 1985). If these rocks were deposited in separate fault-controlled basins, however, there is no reason why they Environment of Deposition could not be of diverse age, even if they all were deposited during the same broad time interval (compare Dott, 1983; Greenberg and Brown, 1984). Historically the Sioux Quartzite has been interpreted as a nearshore, The well-indurated red and purple quartzite clasts in basal Sioux Quartzite shallow-marine deposit (Baldwin, 1951; Austin, 1972). More recently, are unlike any quartzite known among older Proterozoic rocks of the Lake Weber (1981) and Ojakangas and Weber (1984) have advocated a mar- Superior region. Were these clasts derived from an older red quartzite of ginal marine setting in which the lower two-thirds of the formation were "Baraboo interval" age? We do not know where the red quartzite clasts deposited by fluvisJ processes, and the upper one-third was deposited in came from, but their presence makes us uneasy about rigorously correlat- tidal estuaries or on a shallow-marine shelf. Morey (1983a, 1984) and ing the Sioux with other quartzite units to the east (Fig. 6). In this connec- Southwick and Mossier (1984) have favored a purely fluviatile environ- tion, recent paleomagnetic studio of "Baraboo interval" quartzites in ment of deposition by braided streams. Wisconsin appear to indicate that the Barron quartzite is younger than the The shallow-inarine and marginal-marine interpretations of the Sioux Baraboo (Kean, 1986). Quartzite rest primarily on the following four points: (1) The deposit is Although herringbone cross-strata are common and perhaps charac- supposedly very thick (2,400 m or more); (2) the deposit is supposedly teristic features of tidal sedimentation (Klein, 1977), they are known to blanket-like in form, and its present distribution is determined more by form under specific conditions in fluvial systems. Boersma (1967) has postdepositional deformation and erosion than by original geometry; shown that reversely dipping cross-sets may be deposited from back-flow (3) the Sioux is supposedly a time correlative of the Baraboo, Barron, eddies on the lee slope of mega-ripples. They also are likely to form from Flambeau, and other smaller quartzite units of Wisconsin (Fig. 6) and is large, reverse-flow eddies at bank margins. In addition, Alam and Crook the westernmost remnant of a formerly continuous deposit; and (4) the (1985) have documented their occurrence at the confluence of a low- herringbone cross-bedding in the Sioux is supposedly clear evidence for gradient", braided trunk stream with a low-gradient, braided tributary. The tidal currents and, by extension, for a marginal marine setting (Ojakangas herringbone pattern develops as currents are reversed in either stream and Weber, 1984). Points 1 through 3 have been taken as evidence for a during periods of high water, the net upstream or downstream current formerly continuous wedge of sedimentary rock that extended from cen- depending on the timing of peak-flow arrivals. It thus appears that the tral South Dakota to Lake Michigan; this wedge has been interpreted as sparse herringbone cross-bedding in the Sioux can be interpreted as a having formed on a passive continental margin that subsequently was fluvial feature and is not necessarily indicative of tidal currents, especially deformed by Early Proterozoic subduction (Dalziel and Dott, 1970; Dott, in the absence of other evidence for a tidal environment. 1983; Anderson and Ludvigson, 1986). We suggest that points 1 through 3 In summary, we are not convinced by the evidence previously mar- are little more than reasonable assumptions, and that point 4, while obser- shalled for interpreting the Sioux Quartzite as a marginal-marine deposit. vationally correct, may have an alternative interpretation. In addition to the points mentioned above, we note that the dominance of We already have commented on the thickness of the Sioux in the quartz sand over carbonate and pelitic sediments in the Sioux appears to introductory section of this paper and have pointed out that the widely set it apart from deposits on Phanerozoic marine shelves on passive conti- cited thickness estimates of Baldwin (1951), which were based on assump- nental margins, which typically have approximately equal proportions of tions of planar stratigraphy, may be substantially too large. Residual thick- sandstone and shale (for example, The Atlantic Coastal Plain; Brown and nesses on the order of 600 to 1,000 m may be closer to the truth. Such others, 1972). It is also inconsistent with the composition of so-called thicknesses certainly do not require a marine setting in which to accumu- marine platform deposits in an epicratonic setting (Potter and Pryor, 1961; late, nor, for that matter, do the larger thicknesses Baldwin estimated. Sloss, 1963, 1981). Moreover, the dominance of sandstone in the Sioux 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 1439 BRAIDED-STREAM FACIES MODELS COMPOSITE, SIOUX f Laminated to . unbedded mudstone SAND FLAT MIXED INFLUENCE CHANNEL Rippled siltstone Planar X-bedded fine sandstone Vertical accretion: Vert icaI accret ion : Vert ica1 accret ion : mud mud mud Final channel fill: X-1aminated fine Sand flat: Alternating sand sand, silt; mud in accretion and flat and channel backwaters Trough X-bedded aggradation conditions: sandstone; local deposi ts of interbedded intervals with planar X-lam- trough and planar planar X-bedding Inated fine sand, X-laminated sand, silt; coarser fine sand, silt. Continuing channel sand in channels depos i tIon: dominantly trough Cross-channel X-bedded sand bar, grading to with local planar Cross-channel bar: sand flat: planar X-bedded cross- planar X-bedded X-bedded sand channel bars medium to coarse sand Trough X-bedded sandstone, gritstone Channel deposits: trough X-bedded Channel depos1ts: sand, gravel Channel deposits: trough X-bedded trough X-bedded sand, gravel sand, gravel Figure 7. Comparison of integrated vertical sequence in Sioux Quartzite at the Pipestone National Monument (D) with published facies models (A, B, C) for a braided fluvial system. Sequence D is built up from observations of several incomplete sections. Models A, B, and C are modified from Walker and Cant (1984, p. 81). does not square with the composition of earlier Proterozoic sequences that cut-and-fill structures. This is consistent with the braided-stream interpre- have been interpreted as marginal-marine in origin (Young, 1983; Morey, tation, because the braided channels in sandy, cohesionless material are 1983b). broad and shallow, and are prone to lateral migration more than vertical We interpret the Sioux Quartzite as having been deposited by downcutting. Thin, lenticular sediment bodies with unscoured or weakly braided streams. The dominant braided streams flowed southeastward in scoured contacts are the typical product of braided-stream deposition broad, fault-controlled valleys cut into a gentle landscape on deeply (Dott, 1983), and these are well represented in the Sioux Quartzite. weathered metamorphic and igneous rock. Whether these valleys merged The nearly structureless mudstone layers in the Sioux are interpreted into a marginal-marine coastal plain southeast of the present outcrop area as either clay drape deposits over underlying beds, which most likely is not known but is thought to be unlikely. formed at times of very low water in pools or abandoned channels, or as As evidence favoring the braided-stream interpretation of the Sioux vertical accretion deposits from standing water on flood plains. The thicker Quartzite, we note (1) the sand-dominated nature of the deposit; (2) the claystones at Pipestone National Monument are interpreted to be flood- assemblage of primary sedimentary structures, especially the dominance of plain deposits (Morey, 1984). trough cross-bedding; (3) the vertical successions of rock types, structures, and textures, which accord well with the braided-stream facies-models of Tectonic Setting Smith (1970), Miall (1978), and Rust (1972); and (4) the paleocurrent patterns, particularly the within-outcrop differences in flow directions in- The Sioux Quartzite and similar red quartz arenites of the Upper dicated by trough cross-bedding and ripples. Midwest have been attributed to deposition along an east-west-trending, Detailed vertical successions from near Pipestone and the Cotton- passive continental margin which was later deformed by subduction (Dal- wood County basin have been described by Morey (1983a, 1984) and ziel and Dott, 1970; Dott, 1983; Anderson and Ludvigson, 1986), and Southwick and Mossier (1984), respectively. These sequences fit very well alternatively, to sedimentation in an intracratonic setting, with deforma- with published facies models for distal braided river/alluvial plain, sand- tion due to anorogenic igneous activity and epeirogenic doming (Green- dominated river systems of lower flow regime, as generalized in Figure 7. berg and Brown, 1984). As a general rule, the Sioux Quartzite lacks well-defined channels or We favor an intracratonic setting for the Sioux because there is no 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 1440 SOUTHWICK AND OTHERS clear-cut evidence for marine sedimentation (see foregoing sections), the near the basal contact of the Sioux in the Cottonwood County basin. They Sioux basins are aligned with en echelon basement faults that are oblique have suggested that these anomalies could be related to a concealed to the east-west, piissive continental margin proposed (Dott, 1983), and uranium concentration of the unconformity-related, vein-breccia type. Sioux sedimentation was associated with rhyolitic, ignimbritic volcanism. They point out, however, that radon anomalies are more closely linked to We note, however, that the Sioux basins and their bounding faults are the distribution and concentration of radium than to uranium, and that the more or less parall el to the trend of the recently defined Central Plains evidence for significant uranium in the contact zone is speculative at best. orogen (Sims, 198:5; Sims and Peterman, 1986; see Fig. 6). The faulting Nonetheless, it is premature to rule out the possibility for uranium deposits and tilting of the Sioux therefore could be related to tectonic activity along at the Sioux unconformity at this time. a plate margin which lay some distance to the southwest and may not be due to purely epeirogenic processes. Suitability for Paleoplacer Gold Because of the ubiquitous cover of Phanerozoic rocks, the nature of the en echelon faulis associated with the Sioux basins is far from certain. A The possibility for discover/ of paleoplacer gold deposits of the growing body of geophysical evidence, however, indicates an abundance "Witwatersrand-type" (Pretorius, 1976) in the Sioux Quartzite is based on of northwest-trending faults in the Precambrian subsurface rocks of west- analogous sedimentary regimes. In the Witwatersrand, placer gold was ern Minnesota and the eastern Dakotas (Chandler, 1985; Sims, 1985), deposited by the repeated reworking of sediments in a braided stream many of which are strike-slip. This raises the possibility that the Sioux environment (Smith and Minter, 1979). Although the distribution of basins could be modified pull-apart structures related to mid-Proterozoic Witwatersrand gold is closely related to hydraulic conditions during trans- transcurrent faultin g (Crowell, 1974), possibly on the Central Plains trend. port and deposition, much gold is associated with carbonaceous material Our knowledge of the associated rhyolites also is sketchy. The in finer-grained parts of the section (Hallbauer, 1972). This carbonaceous rounded clasts of rhyolite and rhyolite welded tuff in Sioux conglomerate material is thought by some geologists to have played a major and perhaps are devitrified, but neither strained nor metamorphosed; these record vol- critical role in the gold-concentrating process (Myers, 1981; Mossman and canism that preceded Sioux sedimentation and may have been related to Dexter Dyer, 1985), whereas it is interpreted as a relatively minor factor the onset of extensional faulting which defined the Sioux basins. Beyer's by others (Minter, 1976; Zumberge and others, 1981; Tankard and others, (1893) report of rhyolite interlayered with the Sioux in the subsurface of 1982, p. 132). Organic material is altogether lacking in the Sioux Quartz- Iowa implies that felsic igneous activity continued after at least some ite so far as we know. Carbonaceous material also is lacking in the Lorrain sedimentation had occurred. Formation, a quartz arenite sequence of fluvial/alluvial origin in the upper part of the Huronian Supergroup that shows anomalous gold values (Col- ECONOMIC GlEOLOGY vine, 1982). Therefore, the depositional regime of the Lorrain, which involved extensive reworking, apparently was capable of concentrating Precambrian alluvial quartz arenites resting on and derived from finely divided gold in the absence of organic matter. We suggest that the complex terranes of older Proterozoic and Archean rocks have been found sedimentary regime of the Sioux was closely analogous in its hydraulic to contain very significant deposits of gold and uranium elsewhere in the characteristics to that of the Lorrain. world. Although i;he Sioux contains no documented occurrences of eco- Although hydraulic conditions are a critical factor in the genesis of nomic or subeconomic mineralization, comparisons of the Sioux to other gold placers, the availability of gold in the source area is equally as impor- gold- and uranium-bearing Precambrian quartz arenites are nonetheless tant (Mossman and Harron, 1984; Robb and Meyer, 1985). In southwest- appropriate. ern Minnesota, the possibility of paleoplacer gold deposits in the Sioux Quartzite is enhanced by recently recognized remnants of metavolcanic Suitability for Unconformity-Related Vein-Breccia Uranium Deposits and metasedimentary rocks, including iron-formation, in areas up the paleoslope from the Sioux (Jiisa, 1986). These erosional remnants are In 1976, Oakangas proposed that the unconformity beneath the tentatively correlated with Early Proterozoic rocks of the Animikie basin Sioux could be a liavorable place for the development of uranium mineral- in east-central Minnesota, some of which show anomalous gold values ization. Subsequently Cheney (1981) re-emphasized that idea and sug- (Eldougdoug and others, 1984; Morey and others, 1985). gested that the biisal part of the Sioux Quartzite in South Dakota could Mineral exploration of the Sioux Quartzite is hampered by shallow contain a "giant" uranium deposit. Both Ojakangas and Cheney based dips, generally poor exposure, poorly understood stratigraphy, and block- their proposals on the fact that the unconformity beneath the Sioux has faulting of unknown extent. Nevertheless, the geologic history of this red many of the geologic attributes associated with unconformity-related, vein- quartz arenite indicates at least some likelihood of findinga n economically breccia uranium deposits as defined by Mathews (1978). important mineral deposit. No uranium mineralization has been identified along the basal Sioux unconformity or within the quartz arenite itself. Furthermore, the possibil- ACKNOWLEDGMENTS ity for this type cf deposit in southwestern Minnesota is diminished by an apparent absence from the basement of a reductant (Button and Tyler, Some readers may have noticed our frequent citations of the unpub- 1981) that is seemingly required for the vein-breccia model to work. The lished Ph.D. thesis by Brewster Baldwin (1951). Baldwin's high-quality composition of the basement beneath the Sioux, however, is poorly work has been most useful to our investigation, and it deserves recognition. known, and rocks that could serve as a reductant may occur locally. We thank R. W. Ojakangas and R. R. Anderson for many hours of Additionally, Southwick and Lively (1984) have shown that radon, discussion and debate and J. G. McPherson and W. E. Galloway for helium, and dissolved U3O3 are anomalously concentrated in water wells helpful technical reviews. 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 1441 Morey, G. 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D., ed., Sedimentation and seams, Witwatersrand sequence: Organic geochemical and microbiological considerations: U.S. Geological Survey tectonics in alluvial basins: Geological Association of Canada Special Paper 23, p. 1-33. Professional Paper 1161-0,7 p. Miller, T. P., 1961, A study of the Sioux Formation of the New Ulm area [M.S. thesis]: Minneapolis, Minnesota, University of Minnesota, 75 p. MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 3,1986 Minter, W.E.L, 1976, Detrital gold, uraninite and pyrite concentrations related to sedimentology in the Precambrian Vaal REVISED MANUSCRIPT RECEIVED MAY 19,1986 Reef Placers, Witwatersrand, South Africa: Economic Geology, v. 71, p. 157-175. MANUSCRIPT ACCEPTED JULY 11,1986 Printed in U.S.A. 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