Petrology and Sedimentation of the Upper Precambrian Sioux Quartzite

PETROLOGY AND SEDIMENTATION OF THE

UPPER PRECAMBRIAN SIOUX QUARTZITE

MINNESOTA, SOUTH DAKOTA AND IOWA

A THESIS

SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL

OF THE UNIVERSITY OF MINNESOTA

RICHARD ELMO WEBER

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

MASTER OF SCIENCE

MARCH 1,1981 Frontispiece--PALISADES OF SIOUX QUARTZITE SPLIT ROCK CREEK NEAR GAJ.(RETSON, SOUTH DAKOTA i

ABSTRACT

The Upper Precambrian Sioux Quartzite is exposed at several

locations along an east-west trend 17 5 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 cl:1,3ts .

Quartzite is gently folded. It is intruded by diabase at Corson,

South Dakota.

The formation consists of over 1600 meters of o:-thoquartzite sandstone with minor interbedded quartzose conglomerate and stone. Conglomeratic units are present in the lower two-thirds 0£ the section .:!nd minor th.in mudstone.s occur in the up;:ier third.

The compositionally and texturally supermature orthoquartzite is

composed almost exclusively of well rounded, well sorted 1 mcno- crystalline quartz. Detrital chert and iron formation grains are present in some samples. Polycrystalline quartz is abundant cnly near New Ulm, Minnesota, wher-e it was derived from the underlying granite. In all other parts of the Sioux it makes up only 2 small percent of the total detrital grains.

No feldspar is present in any of th2 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 ii

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 orthoquartzi te. The basal conglomerate consists of clasts, as much as 35 centi:n2ters in

diameter of vein quartz, hematitic chert, iron formation and quartzite. Clasts of rhyolite were found in one outcrop of basal conglomerate. The conglomeratic orthoquartzi te consists of 1 to

10 centimeter layers of pebbles interbedded with cross-bedded coarse orthoquartzite.

The m.udsr.ones within the Sioux are red to dark purple in color. Most are blocky but some show fissility. 'I'h·2 muds tones range from almost pure claystones to silty rnudstone and are composed of sericite, quartz, he1rlatite and ill:f.te.

Trough cross-bedding, symmetrical ripple marks, current ripple marks and mudcracks are the major sedimentary structures.

Less common struc tu.res include sand waves, planar cross-·bedding, load casts, mud clasts, parting lineation and climbing ripp• 1... e lamination. The cross-bedding consists predominantly of narrow troughs 60 to 140 centimeters wide and 15 to 30 centimeters thick.

Symmetrical ripple m.?.rks with wave lengths of 2 to 4 centimet2rs and of 4 centimeters are present throughout the forme- tion. 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 iii

reactivation surfaces, sedimentary strucrures charact eristic 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 rom 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 e i ther braided fluvial environment or a hig h energy shallow marine environment. TABLE OF CONTENTS

ABSTRACT ..... i

TABLE OF CONTENTS iv

TABLES. vi

FIGURES . vii

INTRODUCTION •.

Acknowledgements • 3

AREAS OF OUTCROPS . . . 4

New Ulm, Minnesota Area. 6 Jeffers, Minnesota Area. . . . • ...... 6 Southwest Minnesota and Eastern S0uth Dakota 6

AGE AND CORRELATIONS. 8

METHODS OF 13

Field Work ., ".) Laboratory Work •. ..L_i

REGIONAL GEOLOGY. • 15

PREVIOUS WORK 18

STRUCTURE • •

Bedding Attitudes .• 23 Initial Dip. 23 Faulting 28 Joints ...... 29 Jeffers, Minnesota Area. 30 Southwest Minnesota, . . .- . 33 South Dakota and Iowa. • 34 Dell Rapids and Split Rock Creek 35 New Ulm, Minnesota 36 Summary. 36

LITHOLOGY .AND STRATIGR..A.PHY. 38

Lithology ....• 38 Qu.a:::-tzite •• 39 Conglome:rates 39 Hudstones • 41 Int!:"usives •• 1.- 1 v

Stratigraphy . • ...... , 48 New Ulm, Minnesota Column 48 Jeffers Column. . 50 Jasper Colum1"1. . . . . • so Garretson Column. 51 Correlation of Generalized Colum..J.s :J_-?

SEDIMENTARY STRUCTURES. 53

Bedding...... 53 Trough . Cross-Bedding . 56 Tabular Cross-Bedding. 63 Herringbone Cross-Bedding. . 66 Ripple Harks . • • • • • • 66 Climbing-Ripple Lamination • 79 Parting Lineation •• 79 Mudcracks .••.• 79 Mud Chips...... • . 80 Load Structures. 84 Summary. 84

PALEOCURRENTS 87

Sampling and Data Reduction. . 87 Symmetrical Ripples .. 90 Asymmetrical Ripples . • . . 94 Cross-Eedding. • • . . . • • • % Other Paleocurrenc Indications 102 Summary of Paleocurrent Indicators . J.04

PETROLOGY ., j_Q/

Orthoq•Jartzites. J.07 Heavy Mine;.-als 1_..:_.;, q..., Conglomerates. 124 Hudstones .•. 130 Weathering and Alteration. 133 Conclusions. 135

SEDIMENTATION . • • 1 -.-1

Environment of Deposition. . 137 Source Area. 142 Tectonics. 143

CONCLUSIONS J.46

147

REFERENCES CITED .• 150 vi

TABLES

Table 1 SUMMARY OF ISOTOPIC DATING RELEVANT TO THE POSSIBLE CORRELATIONS OF THE SIOUX QUARTZITE • . • • . . . • . 10

2 MINERP.L CONPOSITIONS OF UPPER PRECAl1BRIAN QUARTZITES IN THE LA..T

3 OCCURRENCE OF SEDIMENTARY STRUCTURES IN THE SIOUX QUARTZITE . . 55

4 GRAPHICAL TEST OF SIGNIFICAl.1"CE .• 89

5 CHEMICAL COMPOSITIONS OF HUDSTONE . 134

6 MODAL ANALYSES OF ORTHOQUARTZITES Vii Figure

Frontispiece PALISADES OF THE SIOUX QUARTZITE ALONG SPLIT ROCK CREEK NEAR GARRETSON, SOUTH DAKOTA

1 UPPER QUARTZITES OF THE LAKE SUPERIOR REGION 2

2 OUTCROP AREA LOCATION MAP OF THE SIOUX QUARTZITE . . • . . • . . . 5

3 DIPPING BEDS OBSERVED IN THE SIOUX QUARTZITE ...... 24

4 GENTLY DIPPING BEDS OF ORTHOQUARTZITE ALONG SPLIT ROCK CREEK...... 25

5 STRUCTURAL MAPS OF THE NEW ULM .AND JEFFERS AREAS ...... 26

6 STRUCTURAL MAP OF SOUTHWEST MINNESOTA EASTERN SOUTH DAKOTA...... 27

7 CLOSELY SPACED JOINTS IN A FLAT-LYING BSD OF ORTHOQUARTZITE. • . 31

3 LENS OF PEBBLES ON BEDDHlG PLANE. 40

9 BASAL CONGLOMERATE OF THE SIOUX EXPOSED NEAR NEW ULH, MINNESOTA. 42

10 CONGL011"cRATIC ORTHOQUA ..-q_TzITE. 42

11 BLOCKY MUDSTONE 43

12 SILTY HUDSTONE. 44

lJ LOCATION HAP OF GENERALIZED COLUHNS L;6

14 GENERALIZED COLUMNS . . 47

15 BASE OF THE SIOUX QUARTZITE NEAR NEW ULM. 49

16 HORIZONTAL BEDDING AND CROSS-BEDDING EXPOSED IN WEATHERED ORTHOQUARTZITE . . . 54

17 LIESEGANG BANDING IN ORTHOQUARTZITE 57

18 TROUGH CROSS-BEDDING EXPOSED IN THREE DIMENSIONS ...... 58 Figure

40 PLOTS OF VECTOR MEANS OF CURRENT RIPPLES BY TOWNSHIPS. 95

41 STRATIGRAPHIC PLOTS OF CURRENT RIPPLE VECTOR MEANS 97

42 PLOTS OF VECTOR MEANS OF CROSS-BEDDING BY TOWNSHIPS ...... 99

43 DISTRIBUTIONS OF CROSS-BEDDING BY TOWNSHIPS 100

44 MOVING AVERAGE OF CROSS-BEDDING BY TOWNSHIPS. 101

45 STRATIGRAPHIC PLOTS OF CROSS-BEDDING VECTOR MEANS •...... 103

46 PALEOCURRENT TRENDS IN UPPER PRECA!v.!BRIAN QUARTZITES OF THE LAKE SUPERIOR REGION ...... 106

47 PHOTOMICROGRAPH OF -TYPICAL . 108

48 GRAIN SIZE DISTRIBUTIONS FOR ORTHGQUARTZITES. 111

PHOTOMICROGRAPH OF ORTHOQUARTZITE SHOWING DEFORl1ATION LA..""1ELLAE IN QUARTZ GRAINS • 112 so PHOTOMICROGRl1.PH OF COARSE ORTHOQUARTZITE SHOWING MINOR FR.A.11EWORK CONSTITUENTS (STRETCHED YillTAQUARTZ AND CHERT) . . . . • . . . . LL4

51 PHOTOMICROGRAPH OF QUARTZ GRAIN W:TH SECONDAK{ OVERGROWTH. HEM.A.TI TE COATING BO'lrI BOUNDARIES J.l6

52 PHOTONICROGRAPH OF A MULTICYCLE GRAIN WITH ABRADED SECONDARY OVERGROWTH...... 116

SJ PHOTOMICROGRAPH OF ORTHOQUARTZITE SHOWING GOOD AUTHI GENI C OVERGROW'THS. . . 118

54 PHOTOMICROGR.J\.PH OF AUTHIGENIC DIASPORE .. 120

55 PHOTOMICROGRAPH OF ORTHOQUARTZITE SHOWING SERICITE REPLACING A QUARTZ GRAIN...... 122

56 PHOTOMICROGRAPH OF WELL ROUNDED, ZONED ZIRCON IN ORTHOQUARTZITE . . 125

57 PHOTOI1ICROGRAPH OF MUDSTONE FROM PIPESTONE, MH1""NESOTA 132 1

INTRODUCTION

The upper Precambrian Sioux Quartzite is a thick, gently

dipping sequence of sedimentary orthoquartzite with minor inter-

bedded siliceous conglomerate and muds tone. Cross-bedding and

ripple marks are the common sedimentary structures. The Sioux is

exposed at several localities in southwestern Minnesota and

adjacent areas of South Dak cta and Iowa and is present in the

subsurface over a much larger area (Figure 1). White named the

Sioux Quartzite for outcrops along the Big Sioux River in South

Dakota and Iowa in 1870, but a type section has never been estab-

lished. The Sioux is the most widespread and the westernmost

quartzite of a number of similar-appearing Upper Precambrian

quartizites in the Lake Superior region.

No study of the petrology and sedimentation of

the Sioux Quartzite has been iliade to date. This study of the

Sioux Qua te was undertaken to gain an unders tandi.ng of the

petrology, pa.leoc-urrer.t patterns, the source areas and environ·- ments of deposition. It is hoped that information in this study will aid other geologists in piecing together the correlacions, mechanisms of deposition and basii-1 orientations of the several

Upper Precambrian Quartzites in the Lake Superior region. 1.n 2 recent study of uranium potential in the Precam!Jrian of

Minnesota, Ojakangas (1976) suggested that the unconformity between the 3-Loux and underlying rocks is a favorab l e location for possible •.lranium mineralization. this study may serve as a base upon which further economic evaluation can be made. SIBLEY. J\ \ <&' -

\ //1..-.( BESSEMER

BARRON {] \

•, -- t_.. .--

1--t---

ii OUTCROP [) SUBSURFACE ------tv \ __ __ Figure 1--urPER 1>RECAMBRlAN QU/\KfZ1TES OF THE LAKE S\Tl'ERlOR REGION Dott 1972; 1971. '---- 3

Acknowledgements

The writer wishes to gratefully acknowledge the guidance of

Dr. Richard W·. Ojakangas of the University of Minnesota-Duluth,

who suggested this study and served as my advisor for the project.

Discussions with him throughout the entire course of the study

were extremely helpful.

Dr. David G. Darby of U.M.D. and Dr. G. B. Morey of the

Minnesota Geological Survey seried as members of the advisory

committee; they made valuable suggestions and critically read the

manuscript.

My sincere thanks is extended to the entire faculty of the

University of Minnesota-Duluth, Geology Department . for their

professional assistance and to my fellow graduate students for

their helpful suggestions.

I wish to thank the Minnesota Geological Survey for their

financial support of the research. I also wish to thank my

parents, Mr. & Mrs. Robert Weber of St. Paul, for the use of a vehicle during the field research, and Mr. Robert Halter of St. Paul

for constructing some of the scientific equipment used in this study.

I am deeply indebted to my wife, Mary Pat, for typing drafts

of this paper and for valuable assistance and support during the

field and laboratory phases of this study. 4

AREAS OF OUTCROP

The Sioux Quartzite is exposed in southwestern Minnesota, southeastern South Dakota and the northwestern corner of Iowo..

(Figure 2). The outcrops trend eastward about 190 miles from

Mitchell, South Dakota, to just east of New Ulm, Minnesota. The outcrop belt has a maximum width of 33 miles in eastern South

Dakota and is less than 3 miles wide in the vicinity of New Ulm,

Minnesota. Within this belt the outcrops occur in three areas, separated by broad covered areas. The westernmost outcrop area, extending from Hitchell, South Dakota, to Luverne, Minnesota is connected in the subsurface with outcrops 50 miles to the east near Jeffers, Minnesota. The easternmo::;t Sioux outcrops at New

Ulm, however, are not continuous in the subsurface with those near

Jeffers, 60 miles to the southwest.

The best natural exposures of the Sioux occur along major streams where deep joint-controlled g0rges have been cut by glacial runoff. Nowhere is a complete section exposed, but these outcrops provide the best stratigraphic information. Many of the outcrops, however, are merely small glacially-polished patches exposing only a few feet of beds.

Cuts along railroads and highways and in quarries provide the only opportunity to study the mudstone within the Sioux; it weathers easily and natural outcrops are rare. The muds tone or pipestone, as it is known by the Indians thethave quarried it for centuries at the pr.esent site of the Pipestone National Monument, is highly valued for its deep red color and ease of carving. For 'rllON

'1'109N . r µ\

\ 'l'l08N ---{

Scale 'I'l07N L I 1 mile R37W R36N R35W R34W R33W R32W R31W R30W

R59W R58W R57W R56W R55W R54W R53W R52W R51W R50W R49W R48W R47W R46W R45W R44W

Tl06N Pip stone - Tl05N ------Har wick Tl04N - 'I'l03N e -- - I 'I'l02N - 'rlOlN -\

'l'lOON IOWA

'I'99N VI - Figure 2--0lJ'l'CROP r,EEA LOC.!

this reason many small pits and quarries have been dug into the

mudstones of the Sioux. The quartzite quarries commonly expose

the thickest stratigraphic intervals in any given area, however,

sedimentary structures which are best seen on weathered surfaces

are difficult to see in the quarry walls. At this time (1977),

quarries at New Ulm, Minnesota, Spencer, Del Rapids and Sioux

Falls, South Dakota are crushing the quartzite for aggregate. A

small quarry at Jasper, Minnesota produces dimension stone.

Abandoned quarries are common throughout the outcrop area.

New Ulm, Minnesota Araa

The easternmost exposures of the Sioux Quartzite lie along

the north side of the Hinnesota Ri·,,er one mile east of New Ulm along U.S. Highway 14. The quartzite here is exposed over a one square mile area and is being quarried by the New Ulm Quarry

Company at this location.

Jeffers, 'Minnesota Area

The exposures near Jeffers, Minnesota occur along an east trending ridge which extends for more than 20 miles and is as w.uch as 3 miles wide. (Figure 2) The southward dipping quartzite crops out along the north side of this ridge and along streams which have cut six deep gorges into the quartzite.

Southwest Minnesota and Eastern South Dakota

The Sioux Quartzite is well exposed in easternmost South

Dakota and southwestern Minnesota. Between Luverne and Pipestone and west to Jasper, the Sioux forms bold outcrops along topographic highs and drainageways. In eastern South Dakota the most extensive outcrops are related to the major drainageways, Split 7

Rock Creek and the Big Sioux River. Along Split Rock Creek, the quartzite forms good outcrops for a distance over 8 miles. For much of this distance it forms vertical cliffs 10 to 15 mete-.:-s high. The Sioux also forms a steep gorge at Dell Rapids where the Big Sioux River is divided into two channels by the resistant quartzite and also forms a sequence of waterfalls for which the city of Sioux Falls, South Dakota, is named. East of Sioux Falls and into the northwest corner of Iowa many scattered outcrops occur along the Big Sioux River and its tributaries. Large operating quarries at Dell Rapids and Sioux Falls have also exposed considerable amounts of the Sioux.

West of Sioux Falls only a few scattered outcrops are present. t outcrops in this c-.rea are small and low in relief. 8

AGE AND CORRELATION

The Sioux Quartzite is considered to be Late Precambrian in age. Although the base of the Sioux Quartzite is exposed only at

New Ulm, Minnesota, it is inferred to overlie lower Precambd.an rocks everywhere. At New Ulm a red porphyritic granite is exposed

110 meters west of the basal conglomerate of the Sioux Quartzite.

This granite is similar in appea:cance and composii:ion to the 2 .6 b.y. Fort Fidgely Granite, 17 miles away, but is too weathered to date at New Ulm (Miller, 1961). The nature of the contact wlll be discussed later (See Stratigraphy).

Through.out the outcroo area the Sioux is overlain by

Cretaceous strata and Pleistocene drift. However, Kirwin (1.965) > reported that Upper Keweenawan sediments overlie the Sioux. in a well at Glencoe, Minnesota.

Granular-textured iron formation, characteristic of the

Middle Precambrian, is present as cobbles and boulders ir: the basal conglomerate et New Ulm and in lesser amounts in other conglomeratic zones within the Sioux Quartzite. On the basis of these iron formation clasts, Grout tentatively placed the Sioux between the Animikie and Keween2wan (Grout, 1951).

The mudstone from the pipestone quarries at Pipestone,

Minnesota has been

Goldich (1961). He suggests this date marks the time of folding rather than deposition.

Alternating layers of rhyolite and quartzite have been penetrated at a depth of 231 meters in a well at Hull, Iowa 9

(Figure 2). The rhyolite has an apparent Rb-Sr age of 1470

+ 50 m.y., which because of slight alteration, is regarded as a rT L:; minimum age (Lidiak, 1971). Is it not clear whether the r-hyolites represent flows or sills. However, with either interpretation, the Sioux Quartzite is at least 1470 m.y. old. Datt and Dalziel

(1972) report a whole rock Rb-Sr date of 1530 + 50 m.y. on the

Hull Rhyolite.

Winchell (1884), reported "fossils" from the rnudstones of the

Sioux. He identified these fossils as a brachiopod and a trilc- bite, but after examining Winchell's specimens, Darby (1972), concluded they were probably non-organic in origin. No evidence of fossils was observed during the course of this study.

Because of the lack of fossils, methods of correlating the

Sioux Quartzite are limited to gross lithology and ra.diometric ages. On these criteria, the Sioux Quartzite can be correlated with the Barron, Baraboo, Waterloo, Flambeau and Bessemer zites of Wisconsin and the Puckwange of Minesota (Figure 1).

The isotopic dates available indicate all of these quartzites were deposited between 1,800 and 1,650 m.y. ago. Although accur-· ate age determinations of sedimentary rocks are difficult to obtain, the data seem to ind:Lcate these formations were deposited at approximately the same time. Table 1 summarizes the isotopic age data.

The litbologies of these fo>.."!nations are remarkably similar.

They are all orthoquartzites with varying amounts of interbedded siliceous conglomerate and mudstone. Clasts of iron formation are known from the conglomerates of the Sibley, Barron, Flambeau, Age (m.y.)

1,000------1,000----- North Shore Keweenaw an Volcan:l.cs and ----1,109?---- Duluth Gabhro Basalts i,200 ---1,200--- ? ?

-----1,376-----

1,400 Sioux Sibley Barron "Lower Baraboo Keweenawan" ---1,410-1,440--- Puckwunge or Sedimentary Bessemer "Waterloo Quartzites Sequence

Quartzite j Qunrtzi.te Quartzite Quartzite" 1,600 -----1,635------1,M·O------1,660-- ? Rhyolite ---1,700------1,720---- Complex

1,800 ANIMIKEAN BASEMENT

Table--1 SUMMARY OF DATING RET...EVANT TO THE POSSIBLE CORRELATIONS OF 'l'HE SIOUX QUARTZITE. (After Datt and Dalziel, 1972)

I-' 0 11

Bessemer and the Sioux Quartzite (Hattis, 1972). A summary of the petrology of these quartzites is given in Table 2.

Paleocurrent data on these quartzites indicate the quartzites were all derived from sources to the north and northwest. A discussion of the overall sedimentation of this group of quartzites will be presented in a later section. 01 T-i

TABLE 2·--MINERAL COMPOSITIONS OF UPPER PRECAMBRTAN QUARTZITES IN THE LAKE SUPERIOR REGION

Quartzites below Baraboo Barron Sioux Keweenawan Basalts

Pebbles (>2mm) Vein quartz, Vein quartz, Vein quartz, Quartzite, vein chert (jasper) quartzite, chert (jasper) quartz, jasper, jasper, slate quartzite argillite Quartz sand types Strained, Unstrained, Strained, Unstrained polycrystalline, strained, polycrystalline, sttained, chert polycrystalline, unstrained, polycrys taLline, chert chert chert Principal heavy acces- sory minerals Zircon, magnetj.te, Zircon, ilmenite, Zircon, rutile, Ilmenite, zircon, . pyrite, rutile leucoxene, iron oxides leuc.oxene, barite rutile, apatite (no other data) apatite, biotite, tourmaline Phyllosilicate minerals known to be present Muscovite, Kaolinite, Pyrophylli te, Chlor:i. te pyrophylU.te, illite muscovite, (no other data) kaolinite illite, kaolinite? 13

METHODS OF STUDY

Field work was conducted during the summer of 1976 with

additional visits to the outcrop area in the fall of 1976, and

spring of 1977. Outcrop locations were established by reference

to previous publications ( expecially Baldwin, 19 51, and Miller,

1961), and by consulting with residents in the field 2rec:. Every

major reported outcrop was visited. Due to drought conditions

during the summer of 1976, it was possible to examine many out-

crops in stream channels that would normally have been inacces-

sible. Over 150 outcrops and five active qua.rry operatiocs wE::re

visited and examined. Rocks of the best outcrops were measured

and described in detail.

The description of each outcrop consisted of: location;

physical occurrence; bedding thickness; description_,

including grain size, sorting, color and degree of cementation;

stratigraphic thickness; and occurrence of structural features

such as joints and small scale offsets. Particular attention was

paid to the occurrence and orientation of sedimentary structures

because of their value in ascertaining pe.leocurrent t ".cends and

envirorm1ents of deposition. In the course of this study, measurements were made on 1, 156 cross-beds and Lf91 ripple marks at 141

locations within the outcrop area. One or more samples was

taken at each outcrop for petrographic study.

Laboratory Work

In the laboratory, 109 thin sections and 15 heavy mineral mounts were examined and a model analysis was determined for each. 14

All thin section heels were stained for potassium feldspar. Slabs of six conglomerates were examined under a binocular microscope to determine the lithology of the clasts.

From information recorded in the field, four generalized stratigraphic sections were constructed. The occurrence and abundance of sedimentary structures were plotted on each generalized section. Frequency diagrams and vector means of paleocurrent indicators were plotted on regional maps and on the stratigraphic sections in order to examine both regional and stratigraphic variation in paleocurrent directions. 15

R3GIONAL GEOLOGY

Precambrian

In the Lake Superior Region, the Precambrian is divided into

three time-stratigraphic intervals; Lower Precambrian (older than

2.6 b.y.), Middle Precambrian (2.6 - 0.8 b.y.), and Upper

Precambrian (1.8 - 0.6 b.y.). The Lower Precambrian rocks of

northern and western Minnesota consist of linear belts of meta-

morphosed sedimentary rocks, volcarrics and granitic intrusives

with minor ma fie intrusive rocks. A sequence of intensely meta-

morphosed Lower Precambrian granitic gnei.sses is exposed along the

Minnesota "liver Valley. Some of these rocks are greater than 3

b.y. in age (Grant, 1972). Although outcrops are poor, this

intensely metamorphosed terrane is inferred from geophysics to be

present to the north of the Sioux and probably underJ.i8s it as

well.

The rocks of Middle Precambrian age in consist of

two sedimentary sequences and numerous intermediate intrusive

rocks. The lower sequence is very poorly exposed. It consists of

quartzite, slate:s and carbonate rocks. The upper sequence con-

sists of quartzi.te, iron formation and graywacke. Middle

Precambrian iron formation is well exposed in northern Minnesota,

northern Wisconsin and northern Michigan and has been mined

extensively. Sedimentary rocks of Middle Precambrian age are

inferred to cover a large part of northern and east-central

Hinnesota and may have served as sources for the Sioux. 16

The Upper Precambrian of the Lake Superior region is characterized by two distinct groups of rocks, a elastic sequence of mature orthoquartzites, including the Sioux Quartzite and a narrow zone of younger Keweenawan volcanic 2nd sedimentary rocks. The thick orthoquartzites (Figure 1), which include the Sioux,

Baraboo, Barron, Sibley, Flambeau and Waterloo Quartzi tes were probably deposited in shallow water environments on the edge of a stable craton before the eruption of Keweenawan lavas.

The Keweenawan volcanics and sediments are confined to a relatively narrow band exposed along lake Superior and are trace- able through eastern 'Minnesota, trending southwestward to Kansas in the subsurface. This zone, which is characterized by ma fie volcan:i.c rocks ,, is commonly referred to as the

Gravity High." It may have formed as a result of continent.:±1 rifting (Chase and others, 1975).

Intrusions

Within the outcrop area, the Sioux Quartzite is intruded by two ma.fie igneous rock bodies and interbedded rhyolite is known from a well at Hull, Iowa. Although the contacts are not explosed, nearby quartzites and mudstones show little or no alteration due to the intrusions.

Post-Precambrian

The Sioux is intermittently overlain by Cretaceous strata and

Pleistocene drift within the outcrop area. The Cretaceous rocks are underlain by a residum of clays developed on Precambrian rocks. The weathered zone is best developed on granitic rocks, but has been reported on the Sioux near New Ulm, Minnesota 17

(Austin, 1970). In western Minnesota and eastern South Dakota the

Cretaceous rocks consist of poorly consolidated marine and non- marine shales, sandstones and chalk. As these rocks are easily eroded, outcrops are rare. Four meters of Cretaceous or Cenozoic tuff and chert overlie the Sioux Quartzite along Split Rock Creek in Section 26, T. lOlN, R. 48W (Baldwin, 1951).

The entire area has been glaciated by one or more advances of

Pleistocene continental ice sheets, resulting in the deposition of varying thicknesses of glacial drift over the quartzite and

Cretaceous formations. Most outcrops are striated and polished.

Chatter marks and large glacial er!'.'atics are common place. 18

PREVIOUS WOR...1<

No comprehensive study of the sedimentation, petrology,

source area, paleocurrents and environment of deposition has been

made on the Sioux Quartzite. Much of the previous work to date, with the exception of work done by Baldwin (1949, 1951), consists

of either general overviews relating the Sioux Quartzite to the

other Upper Precambrian Quartzites of the Lake Superior Region, or

detailed studies on small areas within the outcrop belt. The main works are summarized below.

Catlin and 1839

The earliest recorded geological observations of the Sioux

Quartzite were made by George Catlin during his Visit to the

Indian pipestone quarries near Pipestone, Minnesota. Catlin recognized the st!'atified nature of the pipes tone and overlying quartzite, bl,l t incorrectly a3 sumed the pipes tone to be an individual mineral. Charles Jackson, who published a chemical analysis of Catlin's pipestone sample i.n 1839, named it catlinite in his honor.

White, 1870

C. A. White named the Sioux Quartzite in 1870 for outcrops found along the Big Sioux River in Iowa and South Dakota. He attribute:d its uniform texture and splintery fracture to meta- mcrphism, but -recognized its original sedimentary nature and the presence of well pre:::eP1ed stratification and ripple marks (White, l.870). White examined the outcrops at New Ulm, southwestern

Minnesota and eastern South Dakota concluding that they were all 19

part of one unit. White hypothesized that the Sioux Quartzite was of Azoic age on the basis of its lack of fossils, complete metamorphic character and stratigraphic relationships at New Ulm,

Minnesota.

Irving and Van Hise, 1884

The first petrographic work on the Sioux Quartzite was done by Irving and Van Hise (1884). They discussed secondary enlarge- ments in perfect optical continuity with the original grains and attributed the overgrowths to "the interstitia.l deposition of sileceous cement by circulating ground waters." They also men- tionerl the similarities between the Sioux and the Baraboo Quart- zite of Wisconsin.

W. H. Winchell and Upham, 1882-1885

The most extensive early study of the Sioux Quartzite was published in the first and second volumes of The Geology and

Natural History Survey of Minnesota (1882 and 1885).

In his reports on Cottonwood, Brown and Nicollet Counties,

Upham described in detail the lithology and structure of the outcrops near New Ulm and along the ridge near Jeffers, Minnesota.

He noted the presence of ripple marks, mud clasts and mudcracks.

He correctly described the relationship of the quartzite conglomerate and the granite at New Ulm and reported finding no granite pebbles in the conglomerate. Upham realized the quartzite exposed in this area was the same formation that was exposed in the southwestern Minnesota and eastern South Dakota. He suggested the

Sioux Qua r tzite was a southwestward continuation of the Keweenawan series near Lake Superior and of Potsdam age. 20

The results of Winchell's investigation in Rock and Pipestone

Counties were published in these same volumes. Winchell paid

particular attention to the pipstone quarries north of

Minnesota, describing in detail the outcrops in this area. He

concluded the pipestone was not a mineral and reported two fossils

from it in The Thirteenth Annual Report for the Year 1884.

Winchell reported the occurrence of conglomerates in Rock County;

however, he incorrectly concluded they occur near the top of the

formation. Maps constructed by Winchell and Upham show areas where the Sioux Quartzite is exposed in Minnesota.

Keyes, 1893

Keyes in his description of the Sioux Quartzite in Iowa,

reported the occurrence of rhyolite interlayered with the Sioux in

a well at Rull , Iowa.

:Beyer, 1897

Beyer published an account of the structural relations between the Sioux Quartzite and an unnamed diabase'l\1 exposed one mile cf Corson, South Dakota. In his report, he concluded that the coarse holocrystalline olivine diabase had intruded the

Sioux Quartzite. He reasoned that the coarse grain size and absence of pyroclastic and other characteristics of near surf ace crystallization indicated intrusion at depth. No actual contact between the diahase and the quartzite was observed.

Sa.rdeson, 1 908

F. W. Sardeson studied the quartzite near New Ulm, Minnesota, and considered it to be a separate formation with a type locality at Courtland, Minnesota. He considered the "Courtland Quartzite" 21

to be Lower Cambrian in age and that the conglomerate at New Ulm overlaid the quartzite and was Middle Cambrian in age. In his

report, Sardeson describes an altered feldspar porphyry dike

intruding into the quartzite. (Note: this dike has never been found by other workers, including this author.)

Berg, 1937 and 1938

E. L. Berg published a paper describing the occurrence of diaspore in the quartzite, as irregular masses between the framework grains of the quartzite. Berg recognized that the overall character of the Sioux suggests the quartzite has not been sub- jected to deep burial or high temperature metamorphism. The following year Berg described the field and petrographic relationships of mudstone and quartzite exposed at Pipestone, Minnesota.

He identified sericite, pyrophyllite, diaspore, hematite and pyrite in the mudstone.

Baldwi.n, 19!;.9 to 19 51 .

In 1949, B. W. Baldwin published a preliminary report on the

Sioux Quartzite in South Dakota. This report summarized the current knowledge of lithology, structure and locations of outcrop for the purpose of promoting economic development of the Sioux

Quartzite in South Dakota. A generalized stratigraphic section along Split Rock Creek near Garretson, South Bakota was included and more detailed information was presented in his Ph.D. dis-· sertation at Columbia. University (1951).

In his thesis, Baldwin examined the Sioux in South Dakota and visited the major outcrops in Minnesota. The major emphasis of

Baldwin's work was to delineate the structure of the unit and 22

describe the interbedded conglomerates and mudstones. Many of the details of Baldwin's work will be presented later.

Miller, 1961

T.P. Miller did a M.S. thesis at the University of ninnesota entitled A Study of the Sioux Formation of the New Ulm Area. His study concentrated on the basal conglomerate and the adjacent granite and overlying quartzite.

Miller came to no definite conclusions as to the relationship of the conglomerate and granite, but stated that other than an absence of granitic clasts in the conglomerate, there is no evidence for faulting between the conglomerate and granite. Many of the details of Miller's work will be presented later.

Austin, 1972

G. S. Austin summarized the current knowledge of the Sioux

Quartzite in Geology of Minnesota: A Cent.l>nnial Volume, P. K.

Sims and G. B. Morey, editors. This paper is largely based on work by Miller (1961) and Baldwin (1951). 23

STRUCTURE

The objectives of this study did not include a detailed

examination of the structural geology of the Sioux Quartzite. In

conjunction with the measurement of paleocurrent indicators,

bedding attitudes were measured and the presence of shear zones

and joint systems were noted, but no detailed structural analysis

was undertaken. A summary of structural features of each of

the outcrop areas is presented, although the paucity of outcrops

and the lack of key marker horizons make structural interpre-

tations tenuous.

Previous work by Baldwin (1949; 1951) and others have

presented a general structural picture for the outcrop areas.

Baldwin considered the typical of the Sioux to be a

gentle, slightly plunging elongate basin 10 to 20 miles long and

about half as wide. The basins he delineated show a preferred

elongation in a director between north-northwest and west-south-

Reference to Figure 5 and Figure 6 will be useful in

the following discussion.

Bedding Attitudes

Throughout the outcorp area the dip of individual beds is

slight. The greatest dip recorded was 20 degrees (Figure 3), but most dips ar2 between 4 degrees and 8 degrees (Figure 4), with

dips greater than 12 degrees being extremely rare. Figure 4 shows

that the predominate dip direction is to the south.

Initial Dip

The significance of initial dip on the present structure of 24

Figure 3--STEEPEST DIPPING BEDS OBSERVED I N THE SIOUX QUAH.TZ ITE . 25

Figure 4--GENTLY DIPPING BEDS OF ORTHOQUARTZITE ALONG SPLIT ROCK CREEK. Note erosion along joints. Beds dip to the south (right). HEV/ ULM 26

31 32

T 110N I r ; 09N

.. . '

RJ<:W RJ5W' ....,..,,, Yi I ' 31 32 3J 35 36 3 ilal N I I Gf(7I -5 }f. I I Ii I I .., /.I I f I I i ,. 1 -;' 2 1 T107N 5 6 5 b3 6 ..... ,,yi. I ...... 4/ 1: I. I I J .,. , kl I l-1 s ,...... i 'l' I I 7 a 9 10 11 r- 12 ,.,, 7 10 ·1 12 7 I I I. .. I I I ... ,,...----. ' '<' 17 ·16 15 I 14 13 18 17 f < I

20 21 22 23 24 19 lt'20 I I I /

30 29 26 27 26 25 I I I I I I I I f I I;.; 6 I k .. T10oN I l I I _L__I i

--=lm1lq

Figure-- 5 STRUCTURAL M.!'>..PS OF THE NEW ULM AND JEFFERS AREAS 27

'< \'

Pipest011e 1-- r-

..-(

"\"" 1'" >.. Dell "',..-\ ..\ -\- >-...... \ >- ..I >- -j I Hartwick, '/ >-I y ,..\.. "-/ .J..,. J.. y

( Luverne · ./

Figure-- 6 STRUCTURAL NAP OF SOUTHWEST MINNESOTA AND EASTERN SOUTH DAKOTA 28

the Sioux Quartzite cannot be properly evaluated because of the absence of basal contacts. Limited subsurface information suggests the Sioux was deposited on an irregular surface. The granite adjacent to the basal conglomerate at New Ulm is at an elevation of 815 feet above sea level but 35 miles to the southwest in Cottonwood County a granite underlies the quartzite at an elevation cf only 550 feet suggesting the Sioux was deposited on an irregular basement. Thiel (1944) showed that the elevation of the granite surface at New Ulm drops sharpl.y to the south and east.

Figures 5 and 6 show that the general dip of the Sioux is to the sou th. This southerly dip is in close agreement with the paleoslope direction (See Paleocurrents). D:i.ps toward the north we.re observed in only four locations: New Ulm and Luverne,

Minnesota and Parker and Rowena, Sotith Dakota.. The deposition of over 1600 meters of sediment requires that some relief be maintained during sedimentation, but in the absenc2 of basal contacts and more extensive subsurface information, the effect of initial dip on present structure cannot be evaluated.

Faulting

Two a;:-eas of possible faulting have been discussed by previous workers. One is along the outcrop belt in Cottonwood

County, Minnesota and the other is between the basal conglomerate and the adje.cent granit2 at New Ulm. In western Minnesota and eastern South Dakota where the most extensive and continuous outcrops are located, no evidence of faulting has been observed. 29

Both Baldwin (1951) and Miller (1961) discussed the possi-

bility of faulting between the conglomerate and the adjacent

granite near New Ulm. The lack of granite fragments in the

conglomerate is cited as evidence for possible faulting. The actual contact between the two units is covered and neither

Baldwin nor Miller came to any definite conclusion, but both considered an unconformable contact more likely than a faulted contact. In this report the conglomerate is considered to be basal, resting unconformably upon the granite. Evidence for thii

interpretation will be discussed later. (See Petrology )

Faulting is certainly an important aspect of +.:he structure along the ridge of outcorps in Cottonwood County. This faulting according to Baldwin (1951), has offset a five mile long block about a mile or a mile and one-half to the north. Brecciated

201v2s, minor offsets along joints and abrupt changt::s in strike accompany the major displacement. The possible movements have been discussed by Baldwin (1951) as follows: (1) dropping of the central block a distance of less than 500 feet; - (2) moving the block laterally by the amount . of the offset; or (3) a combination of vertical and horizontal displacement. The close agreement of the bedding attitudes between t h e offset block and the remainder of the ridge suggests that the displacement is most likely horizontal.

Joints

Joints and. fractures are developed in almost all outcrops.

Vertical, horizontal and bedding-plane joints are present but the vertical ones dominate. At most outcrops two vertical or near 30

vertical joint sets intersect at 70 degrees to 110 degrees, giving

the quartzite a blocky appearance. The joint3 are usually spaced

30 to 100 centimeters apart, but some surfaces show jointing on 5

to 10 centimeter spacings (Figure 7). Baldwin (1951) reported

that in areas where the quartzite is noticeably dipping, the

joints are perpendicular to bedding rather than perfectly vertical, suggesting a tilting of the beds after the joints were formed. Irregular fractures and oblique joints are found in a few outcrops.

From more than 1100 readings, Baldwin determined that three significant joint directions are present over the entire area of outcrop; North 75 to 35 degrees West, North 10 degrees West to

North 15 degrees East, and North 50 to 70 degrees East.

The erosion of steep narrow gorges along the ridge near

Jeffer.s, along Split Rock Creek and along the Big Siou:" River is controlled by the joint patterns. These joint systeffis also allow for the movement of ground water through the otherwise impermeable quartzite.

Jeffers, Minnesota Area

The Sioux Quartzite forms a 20 mile long, east-west trending ridge in northwest Cottonwood County, Minnesota and adjacent sections of Brow-n and Watonwan Counties. This 100 to 150 feet high ridge is maintained by the south dipping resistant quartzite.

The structure of the quartzite is generally a gentle, south

.. ipping. homoeline com.pl icated by north-northwest trending faults and/or sharp flexures. The dips range from 3 to 14 degrees 31

Figure 7--CLOSELY SPACED JOINTS IN A FLAT LYING BED OF ORTHOQUARTZITE. Joints trend north-northwest. 32

throughout the area; but away from the faults and flexures, the

dips are consistently 4 to 6 degrees southward.

Topographic evidence suggests that a 5-mile long central

block of the ridge has been offset approximately one mile to the

north. The central block is composed of 460 meters of well-

exposed quartzite consistently dipping 4 to 6 degrees to the

south. The eastern boundary of this block is marked by an abrupt

change in the strike from N. 85 W. to N. 23 W. in less than one

mile in Section 13, T. 107 N., R. 35 W. (Figure 5). This change

in strike is accompanied by a similar change in the trend of

the ridge. Southeast of this flexure Section 20, T. 107 N., R.

35 w., the ridge returns to a westerly trend with the beds strik-

ing N. 87° w. and dipping 5 degrees to the south. At thi s loca-

tion, 'Saldwin (1951) suggested that a no·cth-northwest trending

fault may be present. The wes te:.:-n boundary of the cer.tral block

in Section 12, T. 107 N., R. 36 w., is also marked 1ry a sharp

flexure. Here the beds change from a strike of N. 88 w. and

4-degree southerly dips, to a N. 14° W. strike and a 10-degree

northeast dip. To the northwest of this flexure, local faulting with minor displacements along joints, breccia zones, and shearing

and crumpling of mudstones occurs. The best exposure of these

structural features is in a deep intermittent stream gorge in

Sections 2 and 3, T. 107 N., R. 36 w., where the rock is shattered by closely-spaced f ractures and joints. The gorge trends N 70 E

for approxi:nately 500 meter s. The beds show anomalous dips to the

north, east, and southwest. A mudstone exposed along the west end of the gorge is intensely brecciated and crumpled . 33

At the western end of the outcrop belt, t.hree outcrops have

dips to the east. On this basis, Baldwin (1951) interpreted the

overall structure of the outcrops near Jeffers as a "trough that

plunges gently to the east or southeast." Because east and

northeast dips occur in conjunction with the faults and flexures

and there are no exposures on what would be the south limb, I feel

the structure is best explained as a south-dipping homocline with

sharp flexures and faults superimposed on it.

Southwestern Minnesota

In southwest Minnesota, the Sioux quartzite forms a gently-

dipping structural basin which trends north-·northwest (Figure 6).

The basin, centered near Jasper, Minnesota, is approximately 18

miles long and 10 miles wide. 'fT,,ro conglomeratic zones separated

stratigraphically by 1800 feet of quartzite serve as marker and

can be traced two-thirds of the way around the basin. The basin

stands out as a circular feature on both topographic maps and

aerial photographs.

In the southeast corner of the basin in the vicinity of Blue

Mound State Park, the beds dip 7 to 11 degrees to the wast and northwest. The "Mound" itself is a sharp flexure that marks the

southwest corner of the basin. On the east edge of the park, the quartzite strikes N .20° E. and forms high bluffs and vertical cliffs that trend south for a mile and then swing westward with an accompanying change in strike to N.80° E. along the southern boundary of the park. No faulting of any kind was observed along this flexure which is well exposed along its entire length. 34

Beds along the eastern and northern margins of the basin dip

gently towards the center at 4 to 7 degrees; and within 2 miles of

the center, the dips flatten to 2 to 3 degrees. The southwest

corner of the basin is marked by only a few outcrops. A north-

east-dipping outcrop in Section 6, T. 103 N., R. 46 w., shows a

dip of 19 degrees and may indicate that the southwest flank is more steeply dipping than the other flanks.

Nine miles north-northwest of the center of the basin, a

coarse conglomerate striking north and dipping 26 degrees east is exposed in Section 36, T. 106 N., R. 47 w. Two miles to the northeast, there are two outcrops of coarse quartite that strike

N. 5° to 13° E. and dip 15° E in Sections 13 and 24, T. 106 N., R.

48 w. These beds are interpreted as underlying the beds exposed

in the basin.

An anomalous section of quartzite and mudstone forms a 2

1/2-mile long north-northwest trending ridge in the Pipestone

National Monument. Along this rdige, the quartzite strikes N. 19° to 27° w. and dips east 6 degrees. This bedding attitude is difficult to reconcile with the southerly-dipping lower conglomerate zone of the basin exposed only 4 miles to the south. The structural and stratigraphic relationships of this ridge of quartzite and mudstone to the outcrops forming the flanks of the basin are unknown.

South Dakota

The outcrops the east fork of the Vermillion River and Hitchell, South Dakota, are too sparse to give a reliable picture of the structure, and no marker beds are present. In 35

general, the beds dip less than 5 degrees to the south or south-

east; but at Parker, South Dakota, there are low-angle dips to the

north.

Southeastern South Dakota and Northwestern Iowa

The structure of the Sioux between Sioux Falls, South Dakota

and the Iowa border (Figure 6) has been interpreted as a

" •.• trough that plunges westward or west-southwestward" (Baldwin,

1951). The outcrop at the falls at Sioux Falls and a quarry

2 miles south of Brandon form the north limb, dipping south 5 to 8

degrees. The outcrops west of Rowena form the eastern end of the

trough, dipping 4 degrees west. The south limb is exposed along

the Iowa-South Dakota border where the beds dip north.

Dell Rapids and Split Rock Creek, South Dakota

The most extensive exposures of the Sioux quartzite are at

the Dells of the Big Sioux River at Dell Rapids . and along Split

Rock Creek (Figure 6). The beds dip to the south and southwest in

this area. The greatest dip observed was 11 degrees in the northernmost outcrop along Split Rock Creek, but most beds dip less than 7 degrees.

Along Split Rock Creek in the vicinity of Garretson, South

Dakota, the quartzite is continuously exposed for a distance of 7 miles. The quartzite dips to the south and southwest. At the base of the exposed section, the dip is 11 degrees, but it becomes progressively flatter to the south. Near Garetson, just north of

Palisades State Park, the beds dip only 2 degrees south but steepen to 6 degrees in the gorge of the Palisades. Near Corson,

South Dakota, the dips are shallow, less than 3 degrees, but show 36

anomalous dip directions to the southeast, southwest, and west over a small area in Section 14, T. 102 N., R. 48 w. These outcrops are within one miles of a diabase intrusion and may have been structurally affected by it. Baldwin (1951) explained the minor variation in dip along Split Rock Creek from south-southeast to southwest as resulting from "gentle undulations" and "minor warping."

At Dell Rapids, the quartzite dips to the south and southwest. Horizontal and vertical joints are well developed at the dells, giving the vertical faces a blocky appearanc e. The major joint directions at both Dell Rapids and along Split Rock Creek

0 are: N. 63° to 77° w., N. 0° to Lf E., and N. 58° to 78° E

(Baldwin, 1951).

New Ulm, Minnesota

At New Ulm, Hinnesota (Figure 5), the Sioux dips gently to the northeast at 5 to 20 degrees. The basal conglomerate exposed a mile and one-half to the west of the main body of quartzite strikes N. 15°to 45° E., with dips of 15 to 20 degrees southeast.

These bedding attitudes seem to define a trough or basin plunging to the northeast.

Summary

The structure of the Sioux quartzite is characterized by

relatively undeformed strata. Dips to the south and southeast predominate, but north-dipping strata are observed in a few outcrcrs. The clcse correlation of present dip and inferred paleoslcpe suggests that at least in part, the present

of the Sioux reflects initial dip. However, the struc- 37

ture of the Sioux near New Ulm and near Jasper cannot be explained

by initial dip. In both areas, bedding attitudes of up to 2 0

degrees and the tilting of vertical joints indicate minor warping.

Vertical joints, relatively consistent in orientation over

the entire outcrop area, give the outcrops a blocky appearance.

Slight movement and brecciation along joints is only observed near

Jeffers, Minnesota. This is the only area where inferred faults

have been mapped. Faulting is considered to be a very minor

factor in the observed structure cf the Sioux quartzite.

The absence of key marker beds makes correlation between major

outcrop areas difficult to impossible.

\ 38

LITHOLOGY AND STRATIGRAPHY

The lithologic variations and stratigraphy of the Sioux quartzite are only partially known because of poor outcrop and the lack of subsurface information. Nowhere is a complete stratigraphic section exposed. The base of the Sioux is exposed only at

New Ulm, Minnesota, and the original top has eroded away. Most wells end and: at the top of the quartzite or penetrate only a few tens of meters into it. The information presented in the follow-

Jng sections has been compiled from scattered outcrops and well data. Petrography will be described in a following section.

Lithology

Sedimentary orthoqua rtzi te is the dominate litholog y in the

Sioux, but minor amounts of conglomerate and mudstone also occur.

In general, the conglomerate beds are limited to the lower two- thirds of the section, and the muds tones are more c0r;1mon in the upper third. At the present level of exposure, the Sioux is composed of 90 percent orthoquartzi te, 8 percent conglomerate and conglomeratic quartzite, and approximately 2 percent mudstone.

Inaccuracies in estimating the relative amounts of each lithology are related to exposure. The quartzite and conglomerate are both highly resistant to weathering and erosion, but the muds tones are readily eroded and rarely form outcrops. Thus, the estimate of the amount of mudstone is probably low. In large vertical exposures, as much as 40 meters thick in quarries, mudstones were not observed. It seems that mudstone comprises only a minor part of the formation. 39

Quartzite

Medium-grained (0.25 to 0.15 mm) orthoqua.rtzitic sandstone so

well cemented with quartz that it exhibits conchoidal fracture i.s

the most common rock type. The color is typically pink: but

ranges from white to deep purple. The grains are typically

well-sorted and well-rounded. Areas of uncemented sands tone were

observed at a few locations, but all were related to fractures or

surface alteration. These will be discussed later in the section

on petrology. A few very thin beds, less than 3 centimeters

thick, of very fine-grained quartzite and siltstone are present.

These thin laminae are less well cemented and stand in negative

relief. Scattered lenses and layers of small pebbles and coarse

sand on bedding planes occur throughout the section, as shown in

Fig"Jre 8.

Conglomerates_

Two types of conglomerate are found in the Sioux, a coarse

basal conglomerate arid numerous zones of conglomeratic quartzite.

The basal conglomerate is exposed at New Ulm, Minnesota; another

outcrop of similar conglomerate near Pipestone, Minnesota is

inferred to be basal . Both of these conglomerates are very

coarse, with clasts a8 much as 35 centimeters in c1iameter at New

Ulm and 16 centimeters in diameter at Pipestone. The coarser

clasts are at the bases of these units, and the coarse ates pass upward into pebble conglomerates. They are poorly

stratified and poorly sorted at the base, but are well stratified and better sorted up-section. 40

Figure 8--LENS OF PEBBLES ON BEDDING Pen gives scale. 41

All other conglomerate layers in the Sioux consist of 1 to

10-centimeter layers of moderate to well-sorted, 1 to 3-centimeter

pebbles interbedded with cross-stratified coarse quartzite. The

differences in fabric and size distribution of the two types of

conglomerates are shown in Figure 9 and Figure 10. " Mud stones

Mudstones were observed at only 12 localities, and are

considered to make up only a minor part of the formation. The

muds tones range in color from brick red to purple (Figure 11).

Most are massive or blocky, but some of the muds tone exposed at

New Ulm show fissility (Figure 12). The thickest mudstone exposed

is 12 feet thick and contains no intervening sand. The top of

this mudstone, located in Section 31, T. 103 N., R. 47 w., is not

exposed, and it may be much thickec. The contacts between the

base of the muds tones and the underl y:i.n.g quartzi tes a.re generally

sharp. The upper contact is commonly erosional, with a thin

mudchip conglomerate developed at this level. The lithology of

these mudstones ranges from almost pure claystone to silty mud-

stone. The mineralogy and alteration of these mudstones will be

discussed later (see Petrology).

Intrusions

Within the outcrop area, three occurrences of igneous rocks

are known. The largest of these is an olivine diabase exposed in

Sections 22 and 15, T. 102 N., R. 48 W., one mile north of Corson,

South Dakota. The diabase is exposed intermittently for 1-1/2

miles along the west bank of Split Rock Creek. The maximum

vertical exposure is approximately 18 feet near the south edge of 42

Figure 9--BASAL CONGLOMERATE OF THE SIOUX EXPOSED NEAR NEW ULM, MINNESOTA.

Figure 10--CONGLOMERATIC ORTHOQUARTZITE. Note layers of pebbles are interbedded with trough cross-bedded orthoquartzite. 43

Figure 11--BLOCKY MlJDSTONE OF THE SIOUX QUARTZITE. Note small reduction spots near center of photograph. 44

Figure 12--SILTY MUDSTONE WITH FISSILITY EXPOSED IN A QUARRY NEAR NEW ULM, MINNESOTA. 45

Section 22. At most locations, the rock is so completely weathered it can be knocked apart by hand; but in the NW 1/4 of Section

22, a rapids is formed by a low ridge of fresh diabase. Horizon- tal and vertical joints cutting the diabase are filled by iron- stained calcite. The diabase is capped by a layer of residual clay 2 or 3 inches thick; and at one location, the Cretaceous

Niobrara Chalk lies on top of the clay (Beyer, 1897, p. 81).

Beyer made a detailed study of this intrusion in 1897 and concluded that the diabase is intrusive into the Sioux. No actual contact with the quartzite is exposed, but an outcrop of mudstone approximately a quarter of a mile from the diabase has not been affected. Goldich suggested the intrusion could be of Keweenawan age, but it has never been dated (Gol

Twelve miles south west cf the diabase intrusion, a small

Patch of gabbro is reported to crop out along the floodplain of the big Sioux River in Sioux Falls, South Dakota. This rock, according to Todd (1904) is composed of plagioclase, augite, biotite, and magnetite. The rock is exposed in Section 11, T. 101

N., R. 49 W., according to Baldwin (1951), but could not be located during this study. No contact with the Sioux has been reported, and the nearest outcrop one-third of a mile away shows no sign of alteration of either quartzite or raudstone.

The third occurrence of igneous rocks is known only from well cuttings at Hull, Iowa. Alternating layers of rhyolite and quartzite were penetrated at a depth of 234 meters. Whether this rhyoli te represents flows or sills is unknown (Lidiak, 1971). 46

SOUTH DAKOTA 0 50

MILES

MINN ESOTA

IOWA

3: \ rn .,, -i rn m m -i' ::0 [JJauARTZITE IJ) \\ 1500 150 0 0 \ II MUDSTONE El CONGLOMERATE I 14 0 0 0 \\ 1200 \ \ \\ I 3000 900 I ·.. ·:·.·...

12000 600 _;Pie

.. '·":- . ·,. 300 •• • • y,. • lOOO 11

0 0 Ii II

l . Figure 13--LOCATION MAP OF GENERALIZED COLUNNS 47 JASPER

t::'··.:::,I QCJ.f._RTZITE

1000 MUD STONE 300 138 - ' CONGLOMERi\TE (/) CJ a:: f- 145 w L!.J 124 THIN SECTION ;; f- w 143 w 200 LL :E: 130 z z 131 ..... 500 .... w 140A w _J 1408 --' 100 <( <( u u (f) CJ)

0 0 137 JEFFERS

21:3

SPLIT ROCK CREEK 220 116 219 124 117 212

182 214 211 114 216 188 179 176 215 210 175

J 78 106 172 181

ULM 256 179 253 169

165 ".;)"'' .... , 0 "·_(. 110 l 64 254 o• ·, .o .• '• 1.1 2 :. ·-'"•. , . ' ... 0 009 252

013

157 101 155 251 102 155

Fi£ure 14--GENERALIZED COLUHNS 48

Stratigraphy

A generalized stratigraphic column was constructed for each

major outcrop area. The locations of the four generalized columns

are shown on Figt.'re 13. It often was necessary to project beds as

much as 2 miles along strike into the line of section because of

the intermittent nature of the outcrops. The thicknesses shown

were calculated from the formational dip and the outcrop width on

the map, with appropriate corrections for changes in elevation.

As these sections are similar to those constructed by Baldwin

(1951) and are intended only as a base on which sedimentological,

petrographic, and paleocurrent data are presented, only a short

summary of each follows (Figure 14).

New Ulm Column

At New Ulm, Minnesota, 19 meters of conglomerate crop out 110 meters east of a reddish-orange, somewhat weathered, porphyritic granite. The actual contact between the two units is obscured by floodplain deposits (Figure 15). This conglomerate has been subdivided by Miller ( 1961) into three stratigraphic intervals.

From bottom to top, these are: a basal conglomerate, a cobble conglomerate, and a conglomera tic orthoquartzi te. However, the entire conglomerate will be referred to as basal conglomerate in this paper.

A covered interval separates the basal conglomerate from 215 meters of orthoquartzite exposed 1-1/4 miles to the east. The lower 30 meters of orthoquartzite contain several beds of mudstone and argillaceous orthoquartzite, and the upper 40 meters of section contain scattered pebbles of siliceous rock types. Two 49

Figure 15--THE BASE OF THE SIOUX QUARTZITE NEAR NEW ULM, MINNESOTA. THE GRANITE IN THE FOREGROUND IS SEPARATED FROM THE BASAL OF THE SIOUX QUARTZITE IN THE BACK- GROUND BY 110 METERS OF COVER. Used by permission of R. W. Ojakangas. 50

10-centimeter mudstone beds separated by 2.5 meters of ortho-

quartzite occur approximately half-way through the section.

Jeffers, Minnesota Column

The outcrops near Jeffers are structurally complicated by

faulting, so a generalized column including all of the beds

present could not be developed. A generalized column was pieced

together from the central block (see Structure), but outcrops to

the east and west are too sparse to work out a sequence. Approxi-

mately 460 of orthoquartzite are exposed along the line of

section. The lower 60 :neters consist of very coarse o!'thoqua rt--

zite with abundant thin layers of small pebbles 5 to 12mm in size.

The rest of the section is composed of orthoquartzite with

several thin argillaceous and silty beds. Clayey films mark many

of the bedding surfaces. Two thirds of a meter of mudstone and 4

meters of conglomerate are exposed to the west of the line of

section, but their stratigraphic positions are unknown.

Jasper, Minnesota Column

At least 1540 meters and probably 2100 meters of orthoquart-

zite are exposed in the structural basin centered near Jasper.

The upper 1540 meters are well exposed. The possibility of the

lower 600 meters is based on only three outcrops -- the conglom-

erate inferred to be basal in Section 36, T. 106 N., R. 47 w.; and

two small outcrops of orthoquartzite in Sections 13 and 24, T. 106

N., R. 46 W. The lower 600 meters of section are not included in

the generalized column for the Jasper area because a total of only

45 meters of the 600 meters of section is exposed. If the

structural interpretation that these beds do project beneath the 51

basin is correct, the Sioux is at least 2100 meters thick here.

The 1540 meters of well-exposed section are shown in Figure

14. The lower 200 meters consist of medium to coarse-grained orthoquartzite with a few thin seams of siltstone less than 3 centimeters thick. This 200-meter thickness of orthoquartzite is overlain by 125 meters of conglomeratic orthoquartzite. The lower

15 meters contain clasts up to 7 centimeters in diameter; but in

the upper 100 meters, the clasts are 2.75 centimeters or less in diameter. A thickness of 550 meters of orthoquartzite separates this zone from a similar zone of conglomeratic orthcquartzite 60 meters thick. The pebbles of this second zone are commonly less than l centimeter, but a few as large as 2 centimeters were noted.

The remaining 650 meters of section are orthoquartzite except for two thin muds tone beds. The lowennos t mud stone is approximately

185 meters from the top of the section. This deep red mudstone is exposed in a small pit in Section 21, T. 105 N., R. 46 w.

The other mudstone is poorly exposed in a creek that follows the outcrop pattern of the soft mudstone in Section 6, T. 104 N., R.

46 w. It lies approximately 45 meters from the top of the section. This section of 1540 meters is reasonably well exposed and is probably characteristic of the entire formation.

Garretson, South Dakota Column

Several writers, including Todd (1904) and Baldwin (1949), have constructed generalized stratigraphic columns along Split

Rock Creek near Garretson, South Dakota. The Sioux is exposed more or less continuously along the creek for approximately 7 miles as it flows southward almost perpendicular to strike. The 52

lowermost 25 meters of this 830-meter section are composed of medium and coarse-grained orthoquartzite, commonly with a few scattered pebbles on the bedding planes. This is overlain by 50 meters of poorly-exposed medium to coarse-grained orthoquartzite.

Above this are 25 meters of cong.lomeratic orthoquartzi te with pebbles 1 to 2 centimeters in diameter. Approximately 525 meters of orthoquartzite with abundant scattered pebbles in the lower 150 meters separate the conglomeratic orthoquartzite from the lowest of four mudstones in the section. This red medstone is very poorly exposed in Section 17, T. 103 N., R. 48 W. This muds tone is separated from a deep purple mudstone at least 4 meters thick by 90 meters of orthoquartzi te. Two more mudstone layers and approximately 30 meters of orthoquartzite are poorly exposed in the top part of the section.

Correlation of Generalized Columns

No correlation between the generalized will be proposed in this paper because of the large distances between major outcrop areas and the lack of key marker beds. 53

SEDIMENTARY STRUCTURES

One of the major objectives of this research was to study the

occurrence and orientation of primary sedimentary structures in

the Sioux Quartzite. Primary sedimentary structures, or those

st rue tures whose formation is dependent upon the conditions of

deposition, provide the most accurate information on the agents

and environments of deposition of the units in which they occur.

Directional structures such as cross-bedding and ripple marks are

particularly important because they commonly can be used to

determine paleo-cur.rent and paleoslope directions.

Twelve different types of sedimentary structures were observ-

ed during this study. The relative abundances of these 3tructures

and the lithologies in which they are preserved are summarized in

Table 3. In the following sections, each structure will be

described as to its occurrence in the Sioux. A short discussion

of the process involved in the formation of each structure is

included.

Bedding is one of the most important characteristics of a

sedimentary rock. A single bed is separated from adjoining beds

by bedding surfaces visible because of some textural or slight:

compositional differences. Both the texture and composition of

the Sioux are fairly constan:: throughout the unit, making obser-

. vatj ans on bedding difficult. In addl tion, weathered surfaces on

which bedding planes are best obseTved are practically nonexistent

in outcrops of the Sioux Quartzite because of the extensive

glacial polish. 54

Figure 16 --HORIZONTAL BEDDING AND CROSS-BEDDING EXPOSED IN WEATHERED ORTHOQUARTZITE. SS

TABLE 3

SEDIMENTARY STRUCTURES IN THE SIOUX QUARTZITE

Sedimentary Structure Abundance Lithology

Horizontal Bedding A Q M Cg

Trough Cross-Bedding A Q Cg

Planar Cross-Bedding c Q Cg

Symmetrical Ripples u Q M

Asymmetrical Ripples A Q M

Clim.bing-Ripple Lamination A Q

Sandwaves u Q

Mud Cracks u M

Mud chips Clas ts u Q Cg

Load Structures u Q M

Parting Lineation u Q

A (abundant) c (common) u (uncommon)

Q Quartzite

M Muds tone

Cg Conglomerate 56

Three types of bedding have been observed in outcrops of the

Sioux; these are cross-bedding, horizontal bedding, and ripple bedding. Cross-bedding is by far the most common type and ripple bedding the least common. Horizontal bedding is present in most outcrops. Bedding surfaces are generally marked by thin clay films or scattered lenses of coarse sand and small pebbles. The beds range in thickness from very thin (less than 1 centimeter) to thick (30 to 100 centimeters), but beds 1 to 10 centimeters thick seem to be the most common. Sedimentary structures such as mud cracks, ripple marks, and load structures are found on some beddir.g planes. The measurement of bedding attitudes is complicated by the fact that cross-bedding is commonly the only type of bedding that can be observed. Both cross-bedding and ripple bedding will be discussed in the fellowing sections.

Liesegang color banding is present in many outcrops of the

Sioux (Figure 17). The deep red color bands are the result of ground water diffusion of iron oxides that form a coating on the grains. Although the color banding is sometimes conformable with bedding or cross-stratification, it commonly transects bedding.

Because true bedding planes are commonly difficult to see, care must be taken so as not to confuse liesegang banding with bedding.

Trough Cross-Bedding

Of the three types of cross-bedding present in the Sioux, trough (or festoon) is the most common. The other two types, planar and herringbone, are found in less than 20 percent of the outcrops. Typical trough cross-stratification in the Sioux is illustrated in Figure 18. Each trough-shaped set consists of an 57

Figure 17--LIESEGANG BANDING IN ORTHOQUARTZITE. 58

Figure 18--TROUGH CROSS-BEDDING EXPOSED IN THREE DIMENSIONS. 59

elongate scour filled with curved laminae. The thickness of most sets is 4 to 8 centimeters, and the width ranges from less than a half a meter to 2 meters. The individual laminae range in thickness from less than 1 millimeter to more than 1 centimeter.

Placers of heavy minerals and post-depositional iron stain- ing commonly mark the laminae. The traces of the laminae are markedly curved and tangential to the lower surface of the trough.

The cross-beds dip from 14 to 27 degrees in the downcurrent direction.

On a bedding plane exposure (Figure 19), the traces of the laminae are markedly curved and concave downcurrent. The bisectrix of these laminae is the direction of current flow (Pettijohn, 1963). In many glaciated outcrops, the long axes of the troughs can be traced as far as

3 meters. The troughs almost always occur in sets; isolated troughs are extremely rare. Trough cross-bedding occurs in sediment ranging in size from fine sand (0.2 millimeters) to pebble conglomerate (2 to 4 centimeters) in the Sioux Quartzite. It was not found in beds which contain clasts coarser than 4 centimeters.

Dunes are the migrating bed forms that deposit trough cross- stratification (Figure 20). Groups of trough cross-strata indicate unidirectional flows with appropriate ranges of velocity, grain size, and flow depth needed to form dunes (Figure 21). The thickness of the troughs suggests a minimum flow depth of approximately twice the thickness of the individual sets (Harms, et al,

1975). With this interpretation, the trough cross-beds in the s1·oux were formed at depths greater than 30 to 40 centimeters and under flow velocities _greater than 60 centimeters per second. 60

Figure 19--TROUGH CROSS-BEDDING EXPOSED ON A BEDDING PLANE SURFACE. Compass points in paleocurrent direction. 61

Figure 20--THE FORl1ATION OF TROUGH CROSS-BEDDING BY MIGRATING DUNES From Reineck and Singh, 1973. 62

i l I 4

3 8 0 J t:i

2 R!P?LES 0 \-/ \-1 0 e I \-/ 8 \'/ \-/ E 8 :r: 0 0 a... llJ 0.8 DUNES c t- El r:l \-/ 3 Q.6

0-I u.. -----..---- UPPER LIM IT Or Fl ur.,E 0.4 DATA

0.2

20 80 100 200 FLOW VELOCITY {cmhec)

Figure 21--DEPTH-VELOCITY DIAGRAM OF BED CONFIGURATIONS Triangles, no movement; squares, ripples; inverted triangles; sandwaves; circles; dunes. From Reineck and Singh, 1973. 63

Tabular Cross-Stratification

Tabular cross-stratification was observed in 10 percent of

the outcrops but is never the dominant type. This type of

cross-bedding is found in both orthoquartzites and conglomerates.

The tabular cross-sets consit of straight laminae commonly dipping

27 or more degrees in the downcurrent direction. The tabular sets

range in thickness from a few decimeters to a meter or more, but

the individual laminae are almost always 1 to 2 centimeters thick.

Symmetrical and asymmetrical ripples oriented obliquely to the

strike of the tabular sets were observed on the face of some laminae (Figure 22).

The tabular cross-sets observed in the Sioux can be divided into two types based on the thickness of the complete set, those with a thickness of less than 50 centimeters and those with a thickness of greater than a meter. The smaller tabular cross-sets occur in groups, and generally are closely associated with ripple marks. This type is found only in orthoquartzi te. The larger type with thicknesses up to 2 meters was found in both orthoquartzi te and the basal conglomerate. These thicker sets commonly occur in the Sioux as isolated, rather than grouped sets, and are not associated with any other structure.

Migrating sand waves like those illustrated in Figures 22 and

23 are the bed forms that are responsible for the deposition of tabular cross-stratification. Formed under unidirectional flow, sand waves indicate lower flow velocity than the dunes that form trough cross-stratification (Figure 21). 64

,Figure 22--SANDWAVE IN ORTHOQUARTZITE WITH TRANSVERSE Hammer sits on a thin remnant of mudstone de- :posited on the sandwave. 65

Lgure 23--THE FORMATION OF PLANAR CROSS-BEDDING BY MIGRATING SANDWAVES From Reineck and Singh, 1973. 66

Exceptionally thick, isolated tabular sets (Figure 24)

suggest a variety of interpretations (Harms, et al, 1975). Those

found in the orthoquartzites and basal conglomerates may be

fluvial, representing accretion on oblique or longitudinal bars.

Thick,isolated, tabular cross-bedding is only found in the lower

two-thirds of the section.

Herringbone Cross-Bedding

A special form of cross-stratification known as herringbone

cross-bedding was observed at four locations in the Sioux Quart-

zite. This term is applied to cross-bedded units in which the

foreset laminae in adjacent layers dip in opposite directions

(Figures 25 and 26). The difference in the direction of dip

between adjacent layers of this type of cross-bedding in the Sioux

is approximately 180 degrees. True herringbone cross-bedding can

only be recognized in three-dimensional sections, as trough

cross-bedding looks similar in diagonal sections.

The individual sets of laminae range from 3 to 12 centimeters; and in one case, has a width of at least 1 meter. In a

few cases, such as the one shown in Figure 26, evidence of four flow reversals is preserved, but most sets consist of only two or three adjacent cross-sets with opposite dip directions.

The individual sets of cross-laminae can be formed by migrating dunes or sand waves, but the important factor is that flow direction has been reversed between deposition of adjacent layers.

This type of cross-bedding is typical of tidal environments.

Ripple Marks

Ripple marks are the second most common sedimentary structure preserved in the Sioux Quartzite, after cross-bedding. Both 67

Fiigure 24--LARGE PLANAR CROSS SETS IN ORTHOQUARTZITE. 68

Figure 25--HERRINGBONE CROSS-BEDDING IN ORTHOQUARTZITE. 69

Figure 26--HERRINGEONE CROSS SETS WITH REACTIVATION SURFACES. 70

current ripples (asymmetrical) and oscillation ripples (symmetrical) are found on bedding planes of the orthoquartzite and muds tone. Current ripples dominate in the mudstones. · The size and form of the ripples are consistent throughout the outcrop area. The five types of bedding plane patterns observed are shown in Figure 27. The ripple indices, amplitude divided by wave length, of both current and oscillation ripples fall between 1/5 to 1/18, formation in water. The internal structures of ripples could not be seen either in outcrop or in thin

The trends of oscillation ripples and the current direction of current ripples were recorded and provide valuable paleocurrent information (see Paleocurrents).

Oscillation Ripples

Symmetrical ripples of the Sioux are remarkably similar throughout the outcrop area. These ripples have sharp or rounded crests and rounded troughs, with an average wave length of 2.5 centimeters and average amplitude of 0.5 centimeters. Most symmetrical ripples have straight crests (Figure 28), but some are catenary, as shown in Figure 29. Symmetrical ripples are preserved in both orthoquartzite and mudstone, but in the Sioux are never found in sediment coarser than medium sand. No significant difference in amplitude or wave length was observed between ripples formed in mudstone and those in orthoquartzite.

Symmetrical ripples in orthoquartzite are commonly associated with current ripples. The two different ripple types were commonly observed on adjacent beds but never found on the same bedding surface. When found together, the crests of these two ripple ------71

Catena;y Straight

linguO:::I Sim:ous

,...-- .' . c(_r Lunole

.·. "------··. ·.:

Figure 27--BEDDING PLANE PATTERNS OF RIPPLE MARKS FOUND IN THE SIOUX QUARTZITE 72

Figure 28--STRAIGHT CRESTED SYMMETRICAL RIPPLES. 73

Figure 29--SYMMETRICAL RIPPLE WITH CATENARY SHAPED CRESTS. 74

types are usually paralle or subparallel, as shown in Figure 30.

In the muds tones, symmetrical ripples are much more common than current ripples. These two ripple types were never found together in outcrops of muds tone. Symmetrical ripples were found on the horizontal bedding planes of strongly trough cross-bedded intervals of orthoquartzite, but were never found on the foresets of planar cross-sets as were current ripples.

In addition to the small symmetrical ripples described above, larger symmetrical ripples with wave lengths as much as 30 centimeters and amplitudes of 4 to 8 centimeters were observed at four locations (Figure 31). These ripples have straight or slightly curved crests that can be traced 2 to 3 meters across the outcrops.

Both the crests and troughs are rounded. These larger ripples were only observed in orthoquartzite; and in three of the four localities,they were associated with trough cross-bedding. At the fourth location, the large ripples were associated with current ripples.

Ripples with symmetrical cross sections are formed as a result of wave oscillation. The amplitude and wave length of symmetrical ripples is related to the wave size, water depth, and grain size, but no unique relationship is known (Harms, et al,

1975). According to them, symmetrical ripples in fine to medium sand with wave lengths of 10 to 30 centimeters are common in many shallow marine environments; and smaller ripples with wave lengths of 1 to 3 centimeters are typical of shallow ponds. The short wave lengths of the great majority of symmetrical ripples in the

Sioux indicate formation in extremely shallow water under slight to moderate wave action. 75

Figure 30--SYMMETRICAL AND ASYMMETRICAL RIPPLE MARKS ON ADJACENT BEDS OF ORTHOQUARTZITE. Note parallelism of crests. 76

Figure 31--LARGE-SCALE SYMMETRICAL RIPPLES IN ORTHO- QUARTZITE. 77

Interference ripples, formed by two superimposed ripple

patterns, were observed in a few outcrops. They probably only mark a local change in wave direction.

Current Ripples

Asymmetrical current ripples are the most common ripple type in the Sioux (Figure 32). They are found throughout the outcrop area and provide valuable paleocurrent information. In the Sioux, asymmetrical ripples are found in orthoquartzite and mudstone, but those in mudstone are rare. No current ripples were observed in sediments coarser than coarse sand (2 millimeters). This type of ripple is found associated with symmetrical ripples and planar cross-bedding. On the basis of bedding plane configuration, the current ripples of the Sioux can be divided into five forms: straight, sinuous, catenary, linguloid, and lunate (Figure 27).

Straight and sinuous forms are dominant, but lunate and linguloid forms are common. Catenary forms were observed in few current ripples but are more common in symmetrical ripples. The current ripples have an as;"'ID.metrical cross section with the downcurrent face having the steepest slope. Most current ripples in ,the Sioux have amplitudes of 1 to 2 centimeters and wave lengths of 3 to 7 centimeters, but these measurements become difficult in less regular forms such as the linguloid and lunate types.

Current ripples are the bed form produced by unidirectional flow and low flow strengths (Figure 21). Current ripples can co-exist with other larger bedforms such as sand waves where they are commonly distributed on the deeper parts of sand wave profiles

(Harms, et al, 1975). This relationship has been discussed and is 78

Figure 32--CURRENT RIPPLE M..\RKS IN ORTHOQUARTZITE. 79

illustrated in Figure 22. Because current ripples can form in a

variety of water depths, no maximum or minimum depth can be

estimated.

Climbing-Ripple Lamination

In-phase asymmetrical, climbing-ripple lamination was observ-

ed in one outcrop of the Sioux. This type of climbing-ripple

consists of superimposed undulating laminae possessing a symmetri-

cal, sinusoidal profile and has been called sinusoidal ripple

lamination by Jopling and Walker (1968).

Climbing-ripples require that abundant sediment be available

to wave or current action so that ripples are built upward, rather

than migrating downcurrent. Although climbing-ripple lamination

is a ccmmon fluvial feature, Wunderlich (1969) stated that they

also occur locally on intertidal flats at places of high rates

of sedimentation.

Parting Lineation

Parting lineation or primary current lineation was observed

in two outcrops in the Sioux. This structure appears as parallel

ridges and' grooves shown by McBride and Yeakel (1963) to be the

result of an orientation of grain elongation parallel to the

current direction. This structure was only observed in thin silty

beds or in films of clay on the bedding planes of the orthoquart-

zi tes. Parting lineation is interpreted by Harms, et al (1975),

to form at higher unidirectional flow velocities than ripples,

sand waves, or dunes.

Mudcracks

Mudcracks are found on many bedding planes in the Sioux. The 80

size of the polygons, width of the desiccation cracks, and methods

of preservation vary widely. The polygons range in size from a

few centimeters to more than one-half meter across, and the

desiccation cracks are from less than 0.5 centimeters to 2 . 5

centimeters wide. In some cases, both the mudstone and the sand

filling the cracks are preserved, as shown in Figure 33. The sand

filling the cracks is sometimes preserved on top of bedding

planes. This occurs when the desiccation cracks extended through

the clay bed the underlying sand layer. The sand filling the

cracks is then cemented to the underlying bed and stands in

positive relief after the mudstone has been eroded away, as shown

in Figure 34. Mudcracks also are found preserved as negative

molds on tops of becls of orthoquartzi te. Because mudcracks are

formed by shrinkage, they cannot be formed in pure sand, as the

sand undergoes no volume decrease on drying. These mudcracks

must have formed in an overlying raudstone and both the mud and the

sand filling the cracks have been removed by erosion.

Mudcrack systems are developed as a result of shrinkage

caused by loss of water by drying, which implies subae!"ial expo-

sure. In order to be preserved, the cracks must be filled by coarser materials as the overlying bed is deposited. The environments most favorable to mudcracking are the intertidal zone, ephemeral playa lakes, and overbank mud flats of the floodplain

(Pettijohn, 1975).

Mud Chips

Mid-chip clasts are fairly common throughout the Sioux

(Figure 35). Although most occur in orthoquartzite, the basal 81

Figure 33--MUDCRACKS IN MUDSTONE OVERLYING ORTHOQUARTZITE. 82

Figure 34--MUDCRACKS CASTS CEMENTED TO THE UNDERLYING ORTHO- QUARTZITE BED. 83

Figure 35--MUDCHIPS IN ORTHOQUARTZITE. 84

conglomerate on the Minnesota-South Dakota border has abundant mud-chip clasts in the upper beds. The mud chips are angular, 1

to 6 centimeter, tabular clasts similar to the red mudstones of

the Sioux. They often occur in well-sorted medium to coarse

orthoquartzi te that lacks any other coarser material. In some

cases, the mud chips can be traced back to an underlying bed of mudstone; but in others, no local source is apparent. In Section

24, T. 101 N., R. 53 w., a mud chip conglomerate 10 centimeters

thick overlies 4 feet of mudstone and grades upward into orthoquartzite with occasional mud chips.

As the mud chips stand in sharp contrast to the highly resistant nature of all other coarse material in the Sioux, they must have been derived from local intraformational sources. These mud chips were probably formed when desiccated mud beds were covered by coarser sediment. The cohesion of the mud resulting from drying would allow the mud to be deposited as clasts rather than become dispersed as matrix in the orthoquartzite.

Load Structures

In some cases, the orthoquartzi te overlying a mud stone bed exhibits irregular bulbous or mammillary features on its lower surface. These structures, shown in Figure 36, are the result of unequal loading of the mud by the overlying sand. According to

Pettijohn (1975), these structures form when sand is deposited on a water-saturated hydroplastic mud.

Summary

Primary sedimentary structures are extremely common and well preserved in the Sioux. The structures appear to be distributed 85

Figure 36--LOAD CASTS ON THE SOLE OF A ORTHOQUARTZITE BED. 86

throughout the outcrop area. None of the structures are restricted geographically, and no major stratigraphic variations were noted. However, herringbone cross beds were only found in the upper one-third of the rock column, and large planar cross-sets are only present in the lower two-thirds.

Sedimentary structures in the Sioux current ripples, planar cross-bedding, and trough cross-bedding are the structures predicted by experimental work to form with successively increasing current velocity as shown in Figure 21. It is somewhat anomalous, however, that ripples (the low-energy bed form) and trough cross- bedding (the high-energy bed form) are abundant, whereas planar cross-bedding is relatively rare. This relationship, and the interrelationships of all the sedimentary structures in the Sioux, will be discussed later in terms of environments of deposition

(see Sedimentation). 87

PALEOCURRENTS

In this study of the Sioux quartzite, over 1600 paleocurrent indicators (mostly cross-bedding and ripple marks) were measured in order to determine the paleocurrent directions and presumably also the paleoslopes that existed during deposition. Paleocurrent indicators throughout the outcrop area were examined to determine if geographic or stratigraphic variations occur. Although most of the conclusions presented in this section are based on large numbers of cross-bedding and ripple mark measurements, other indicators such as parting lineation and pebble orientation were also utilized.

Sampling and Data Reduction

In order to get random samples of paleocurrent directions over the entire outcrop area, paleocurrent measurements were made at all available outcrops. Sampling on a predetermined grid was impossible because of the sparse and unequal distribution of outcrops, but every effort was made to keep the sampling unbiased.

A number of measurements were made at each outcrop, but not more than a single measurement was taken from any cross-set. No more than 50 measurements of any one structure were made in a square mile area. In addition, an effort was made to distribute measurements evenly over the entire thickness so as not to bias the measurements stratigraphically.

For each major current indicator -- cross-bedding, current ripples, and symmetrical ripples -- a vector mean, vector magnitude, and a frequency distribution in 30-degree class intervals were calculated. This was done for each of three levels of 88

factual generalizations -- by outcrop, by township, and a grand total over the entire area.

The vector mean or vector resultant is the most accurate measure of preferred paleocurrent direction from a group of observations. The vector magnitude is a measure of dispersion about the mean where the magnitude of the resultant vector varies from 0 to 100 percent. Zero means complete randomness (i.e., maximum dispersion) and 100 percent means perfect orientation of all indicators. A graphical test of significance (Curray, 1956) was performed for each vector mean at all three levels of generalization. It is based on the number of observations and the vector magnitude. The 0.05 level is commonly the dividing line between nonsignificant (greater than 0.05) and significant (less than or equal to 0.05). The results of the graphical test of significance of the overall vector means for cross-bedding, current ripples, and symmetrical ripples are given in Table 4.

All calculations and statistical tests were done on a Control

Data 3200 computer. Computer-construe ted rose current diagrams were examined at each level of generalization to identify the presence of bimodal or polymodal current distributions. These paleocurrent data were then plotted on maps and stratigraphic sections at the various levels of generalization. 89

TABLE 4

GRAPHICAL TEST OF

Indicator Vector Magnitude N Significance *

20 Trough Cross-Bedding 47.63% 1109 1.0 x 10- significant 5 Planar Cross-Bedding 43. 72% 47 1.0 x 10- significant

Symmetrical Ripples 65.23% 178 1.0 x 10-15 significant 20 Asymmetrical Ripples 74.55% 223 LO x 10- significant ·

*less than or equal to .05 is significant 90

Symmetrical Ripples

Ripples with symmetrical cross sections are formed by

waves moving at right angles to the crests. Although symmetrical

ripples give a line of movement or direction, but not a sense of

movement, they provide valuable information on wave direction and

possible shoreline orientation. The trends of 17 4 symmetrical

ripples from 16 different townships were measured.

The vector mean of the perpendicular to the crests of

all symmetrical ripples is 72 degrees, indicating that waves

moving to the southeast or northwest were responsible for their

formation. The distribution of the sylllmetrical ripples measured

is shown in Figure 37.

A plot cf the vector means of symmetrical ripples by township

(Figure 38) shows that while some minor geographic variations do

occur, the trends of the ripples are generally orientated north-

east to south west. Symmetrical ripples show a very strong

orientation in any given outcrop. In most individual outcrops, more than 50 percent of the measured trends fall in the same

30-degree interval. However, wheVi they are grouped by township,

generally fewer than 30 percent of the measurements fall in a

30-degree interval. This indicates that while relatively constant conditions may have existed over a period of time at any given

location, conditions were not as uniform over large areas. To examine the stratigraphic variations in symmetrical ripple trends, the vector means were plotted on the generalized stratigraphic columns (Figure 39). Minor fluctuations do occur through the sections, but no major changes were observed. 91

LEGEND N

MEAN CURRENT DIRECTION

MEAN RIPPLE TREND Symmetrical Ripples

Asymmetrical Ripples Cross- bedding

Figure 37--SUMM.ARY OF PALEOCURRENT DISTRIBUTIONS FOR THE SIOUX QUARTZITE ...... TllON K-"-- Tl 091'! / v------TlOBN /Tr / I v I T1071,I ?

R 37W R3 6W R35VJ R3 l+W RJ3W R32W R3 l W R3 OW

R 5 9 W R 5 8 W R 5 7 W R 5 6 W 5 W R 5 t+ W R 5 3 W R 5 2 W R 5 1 W R 5 0 W R '• 9 WI'< 4 8 W Rt+ 7 WR 4 6 W R 4 5 W I'< 4 4 W

T106N \ f ' l 1 - Tl 05f'l .... \ (\ i --

T104N \ if '-if/ - Tl03N z _, r ,, . \ '1 ---Jlft / Tl02M "'Y , -,_ .I d ! - TlOlN I I \ li'y'IY : '.. I ) '{_ - K I . MINNESO, A - TlOOt..J "\ .I f r-...._ 'OUTH OAKOT" /f ' "-'IJ "' ', IOWA - T99N !. Figure 38--PLOT MEANS OF SYMMETRICAL RIPPLES BY TOWNSHIP N 93 JASPER

QUARTZITE MUD STONE 300 ITl ooo TE !/) c: f- I(' UJ UJ f- w w 200 u. 8 ::2: z z 500 ..... 8 w w _J _J 100 < Iii < u u !/) UJ

0 0 JEFFERS

ULM

.... ·. -. ·· f'

Figure 39--STRATIGRAPHIC PLOTS OF SYMMETRICAL RIPPLES 94

Asymmetrical (Current) Ripples

Ripples with asymmetrical crests are formed un

(see Sedimentary Structures). The direction of flow is perpendicular to the crests and toward the direction of the steeper- faced side of the ripple. The trends of 217 cur rent ripples were measured in 16 townships. For each current ripple, the trend of the ripple crest was measured and a sense of current direction was noted. In the statistical analysis, the actual calculated value of the current direction was used.

The vector mean of the current direction from all 217 asymmetrical ripple measurements is 137 degrees, with a vector magnitude of 74.55 percent. The distributions of the current directions and the ripple trends are shown in Figure 37. When grouped by outcrop, asymmetrical ripple measurements have an average vector magnitude of 85 percent. When grouped by township, it is 80 percent. These average vector magnitude values indicate t hat the amount of variation of current direction over a small outcrop area, anywhere from a few square meters to a quarter of a section, is about the same as the variation in current direction over an entire township. No systematic change in the amount of variation of current direction was detected over the outcrop area or in the stratigraphic sections.

A plot of vector means of current ripples generalized by township is shown in Figure 40. The currents show consistent directions to the south-southwest to south-southeast. Minor variations across the outcrop area do occur, but no major VECTOR MEANS OF CURRENT RIPPLES BY TOWNSHIP T 1101'-l '-... T109N / t MILE _,,,- / - T108N m-/ ""- / J I T107N -/ " VECTOR MEAN \ CURRENT DIRECTION R37W fB 6\'JR35W R34W R33W R32W R31 W R 30W

R59W R58W R57WR56WR55W R54W R53W F<52WR 5 1W R 5 0 W R49W R48W R47WR46WR45 W R44W

T106N .\ \ { t \

T105N \ \ _; !\ Tl04N \ / \ \ \ I \ I T103N \,,/ \ I I IA \ N I T102N / j II" I \

TlOlN \ y \ K MINNE SOTA IOWA '(_ KOTA TlOON ( ...... __ sou D.l J ·-- "\.. I\ ' T9 9 N \0 """ Vl '\ ' ·1

Figure 40- -PLOT OF VECTOR MEANS OF CURRENT RIPPLES BY TOWNSHIP 96

geographical changes in ripple direction are present. Similarly, no major changes occur stratigraphically (Figure 41).

In T. 102 N., R. 48 W., the distribution of current rip-

ple measurements is bimodal. Current ripples were measured

on only two outcrops in this township, but both outcrops show

this bimodal distribution. The two modes, one to the sotith-

east and the other to the northeast, document two current direc-

tions approximately 160 degrees apart. It is significant that herringbone cross-bedding and a bimodal cross-bedding distribution were also observed at these two outcrops. Current ripples giving northerly current directions were observed in two other townships, but not enough measurements could be obtained to determine a statistically valid bomodal distribution. In both of these

townships, bimodal cross-bedding distributions substantiate the existence of a current system orientated at 180 degrees to

the dominant south-southeast system. All three areas showing opposite current ripple systems are located in the upper third of the stratigraphic column.

Cross-Bedding

Cross-bedding is the most abundant type of paleocurrent indicator in the Sioux. Two main types of cross-bedding, planar and trough, are present. Planar cross-sets are rare, and only 47 paleocurrent measurements were ·obtained from them. Trough cross- bedding, on the other hand, is very abundant, allowing paleocurrent measurements of this kind to be made at almost all outcrops. The curved nature of the laminae of trough cross-beds, when viewed on a bedding plane surface, allows for fast and accurate 97 JASPER

Bill QUARTZITE 1000 STONE 300 mm [] CO NGLO:!ER.:l..'!'E: (/) 11110 a:: I- w w I- w w 200 u... :i ....z 500 w w _J _J 100 <( Ill§ <( u u (/) Ul

0 0 JEFFERS

SPLIT RGCK CREEK

!'JEW ULM

:. . _.; . ·.·.. ·

1<'fonre 41--STRATIGRAPHIC PLOTS OF CURRENT RIPPLE VECTOR MEANS 98

paleocurrent measurements. Because most measurements were of the axes of troughs, rather than of individual cross-set dip directions, the problem of unrepresentative measurements is virtually eliminated. A total of 1156 cross-bed axes were measured in 28 different t;.6wnships.

The vector mean of trough cross-bedding is 162 degrees, with a vector magnitude of 47 .63 percent (Figure 37). Although the vector magnitude for trough cross-bedding is much lower than those of either the symmetrical or asymmetrical ripples, it is based on a much larger sample size and is significant.

Trough cross-bedding shows more variation in current direction than do ripples, but current directions toward the south- sou thwe st and sou th-sou the as t predominate in all townships except two (Figure 42).

The exceptions are T. 101 N., R. 53 W. in South Dakota and in T. 110 N., R. 30 w. near New Ulm, Minnesota. At both of these locations, well-defined unimodal current distributions give paleocurrent means toward the north-northeast.

Figure 43, a map of the distributions of cross-bedding

'by township, shows that no systematic variation in the degree of dispersion is present in the outcrop area. A two-dimensional moving average of the cross-bed measurements in adjoining townships is shown on Figure 44. This method smooths out local variations and indicates a good regional trend of paleocurrents toward the south, but with the two northerly-trending anomalies previously mentioned. '- PLOT OF VECTOR MEANS OF T 11 01\J CROSS-BEDDING BY TOWNSHIP I\< ...... T109N .,,,-...... v / TlOSN MILE ...... , /"TT / / .' '....VECTOR MEAN T107N / " /

1137W R34WR33\'J R32W R31W R30W

R 5 9 W R 5 8 W R 5 7 W I< 5 6 W R 5 5 W R 5 Lf W R 5 3 W R 5 2 W R 5 1 W R 5 0 \\I R 4 9 W R 4 8 W R 4 7 W R 4 6 W R 4 5 W R 4 4 W Tl06N ( \. ( / / \

T105N \ \ ./ I 7 T104N \..., ,_, \ I K r I \. "' \ \ T103N ' \( / l,1 \ I/ \ J N\ l T102N " ,,, ,}_ \ -- - I r< I TlOlN vf y \ i M NNESC TA \" - \ 10 WA TlOON '(_ so IJTH C Al

T109t'1

MILE Tl OBN

DtSTRIBUTION Tl07N

R36WR35W R34W R33W R32W R31W

R 5 9 W R 5 8 W R 5 7 W R 5 6 W R 5 5 W R 5 W R 5 3 W R 5 2 W R 5 1 W R 5 0 W R 4 9 W R 4 8 W R 7 4 W R 4 6 W R 4 5 W R 4 W

T106N I :;] \ f\ r • I Tl05N \ Ll ,.,..,.. I I I/ '-f \, T104N --' \ ( .\ "< .. T103N \ ) c1J7 tj. ' %.1.::.\ , '< I Tl02N c r---., 1 1' \ " I I I

TlOlN vI I/) I\ J MINNI SOTA I ' IOWA TlOON I / I\ f OUTH DAl

TlOBN I MILE "

Tl07N '\. ' CURRENT DIRECTION R 3 7 W · l"B 6 t'J R 3 5 \.'J R 3 4 W R 3 3 W R 3 2 W R 3 1 \'J R 3 0 W

R59\'I R58W R57W R561/IR55WR54W R53W R52W R51WR50WR49WR48W Rl•7WR46WR'+5W R'•4W

Tl06N

Tl05N

Tl04N

Tl03N

Tl02N

TlOlN

TlOON

T99N ..... 0 J ..... Figure 44--MOVING AVERAGE OF CROSS-BEDDING BY TOWNSHIP 102

Plots of cross-bed paleocurrent directions on stratigraphic sections generally show no changes (Figure 45). In the New Ulm area, however, a major paleocurrent change is observed between the basal conglomerate and overlying orthoquartzi te. The cross-beds in the basal conglomerate have a vector mean of 148 degrees, with a vector magnitude of 87 percent, whereas the overlying orthoquartzi te shows palceocurrent directions toward the northeast and northwest. This northerly paleocurrent direction is anomalous relative to the dominant southerly trend of the entire area, and may be the result of local topographic variation near the base of the formation.

Bimodal cross-bedding distributions were observed in five outcrops. In each of these outcrops, a northerly paleocurrent system is present in addition to the normal southerly direction.

Four of the outcrops contain herringbone cross-stratification, indicating the current reversals were periodic (see Sedimentary

Structures). In the other outcrop, the current ripple data also record the presence of a northerly current system. Stratigraphi- cally, these bimodal outcrops are all in the upper third of the section.

Other Paleocurrent Indicators

Two minor sedimentary structures of value in determining paleocurrent directions were observed in the Sioux. Parting lineation was observed in three outcrops. Five measurements were made giving an average current direction toward either the northwest or southeast. The orientation of the long axes of a few layers or lenses of pebbles found on the bedding planes showed a 103 CJ

300 ,1000 UJ • I [] wc:: I wf- 200 I I : I Isoo w _J <( u UJ

0 0

Q G)

Figure 45--STRATIGRAPHIC PLOTS OF CROSS-BEDDING VECTOR MEANS 104

parallism to the paleocurrent direction given by cross-bedding.

Because of the low number of measurements, neither of these

structures were included in the statistical analysis of paleo-

current indicators.

Summary of Paleocurrent Indicators

Symmetrical ripples, current ripples, and cross-bedding all show that the dominant paleocurrent trend in Sioux

Quartzite is to the south east. The close agreement of these three separate indicators is important because it is evidence that the paleocurrent data are reliable. In outcrops where two or more of the structures occur together, the expected relationships exist; the trends of symmetrical ripples and current ripples are parallel and the current directions indicated by cross-bedding and current ripples are similar.

Statistical tests show that significant vector means can be calculated for outcrops, townships, and the entire area for each of the three types of paleocurrent indicators. Geographical and stratigraphic analyses of paleocurrent direction and paleocurrent dispersion generally revealed only minor fluctuations. One major change in paleocurrent trend with stratigraphic position is evident at New Ulm where the basal conglomerate has a southerly paleocurrent trend and the overlying orthoquartzites have a northerly trend.

Biomodal paleocurrent distributions indicating a northerly flow, in addition to the dominant southerly flow, were observed in both the current ripple and cross-bedding data.

Stratigraphically, these bimodal patterns are limited to the 105

upper third of the section.

The depositional strike of the Sioux quartzite, as given by the southeasterly paleocurrent trend, is approximately parallel to the east-west trend of the outcrop belt. This makes analysis of changes in paleocurrent direction and paleocurrent dispersion in the downslope direction impossible, as nowhere are data available for more than a few tens of miles in the downcurrent direction.

The results of this detailed study of paleocurrent indicators in the Sioux are in agreement with two previous general paleocurrent studies. Pettijohn (1957) obtained a mean of 162 degrees for the Sioux on 39 cr_oss-beds measured at an unspecified number of locations. Dott and Dalziel (1972) analyzed and plotted 355 cross-bedding measurements and 53 ripple marks

(type unspecified) from seven locations in the Sioux. They calculated vector means of 269 degrees for trough axes, 178 degrees for cross-sets, and 42 degrees for the ripple crests. The poor agreement of measurements of the Datt and

Dalziel study with those found in this study can be explained by the fact that 27 percent of the measurements made in the former were located at New Ulm, Minnesota, an area shown by this study to have anomalous paleocurrent orientations with respect to those in the rest of the Sioux.

Figure 46 is a plot of mean paleocurrent directions for all of the Upper Precambrian orthoquartzi te units discussed as possibly correlative with the Sioux (see Age and Correlation). The southeast paleocurrent trend for the Sioux is generally similar to the paleocurrent patterns of the other Precambrian units. 106

SIBLEY

SOUIX •

a OUTCROP 0 SUBSURFACE

Figure 46--PALEOCURRENT TRENDS IN UPPER PRECAHBRIAN QUARTZITES OF THE LA.KE SUPERIOR REGION 107

PETROLOGY

One hundred and four samples of the Sioux were selected for microscopic study. Emphasis was placed on selecting those samples which would provide coverage of geographical, stratigraphical and lithological variations. The samples selected for sectioning were distributed among the three lighologies orthoquartzi te, mudstone and conglomerate, according to their proportions in the

Sioux. (See Lithology) All thin sections were studied for composition and texture. Six random traverses of 100 points perpendicular to bedding were counted on each of the conglomerate and or thoquartzi te thin sections. A complete scan at 200 power was made of all point counted thin sections to verify the presence of grains of multicycle origin. The major framework constituents of the orthoquartzi tes and conglomerates are quartz, chert and siliceous rock fragments, cemented by optically continuous overgrowths of quartz.

Orthoquartzites

Ninety thin sections of the orthoquartzi tes were examined.

Size analysis was performed on five typical thin sections to supplement estimates on grain size and sorting. A photomicrograph of a typical orthoquartzite is shown in Figure 47. The term orthoquartzite will be used throughout this report as the dominant lithology is sedimentary orthoquartzite with more than 90 percent quartz grains. According to the classification of Pettijohn,

Potter and Siever (1973), the rocks are quartz arenites. 108

Figure 47--PHOTOMICROGRAPH OF TYPICAL ORTHOQUARTZ. Sample 193a, crossed nicols. 109

Texture

Where the distinction between original grain and authigenic

cement is clear, the orthoquartzites of the Sioux are composed of

90 to 95 percent framework grains and 5 to 10 percent authigenic cement. No porosity or void space of any type was observed in any of the thin sections. The contacts between framework grains have undergone little or no modification. Point contacts between grains are the most common with some concave-convex contacts usually developed. Sutured grain contacts, such as are charac- teristically developed during pressure solution of framework grains, were observed in only one thin section. No preferred orientation, primary or secondary, of the framework grains -was observed.

The mean grain size of the sections examined ranged from very fine sand (0.088 millimeters), to very coarse sand (1.0 millimeter). All but seven of the samples were considered to be well sorted and 29 were considered to be very well sorted under criteria set forth by Folk (1974). Most of the thin sections contain only sand sized material, but two contain minor detrital silt and clay. These four argillaceous orthoquartzites are located at New Ulm, Minnesota and near Jeffers, Minnesota; argillaceous orthoquartzites were not observed a.t any other location.

Scattered granules and very small pebbles are common in some thin sections. They usually occur in distinct layers and have been discussed as seen in outcrop. (See Lithology) The distributions of grain sizes from five thin sections in which 300 grain size llO

determinations were made show the five samples are well to very- well sorted and skewed slightly to the coarse end (Figure 48).

The framework grains are in almost all cases well to very- well rounded. Grains less than 0.25 millimeters (fine sand) in diameter are subrounded to subangular in some · thin sections.

There is no apparent difference in the degree of rounding between different grain compositions. The framework grains are generally spherical, but the coarser grain sizes, coarse sand and granules, are more spherical than other sizes.

Texturally, all the orthoquartzites studied are very mature sedimentary rocks. The individual grains of the orthoquartzites have not been recrystallized, but post-depositional undolosity has been developed in many grains, probably due to strain after cementation. Vacuoles and inclusions arranged in lines represent incipient, healed fractures that sometimes stretch across two or more grains and their Qvergrowths. Strain lamellae, as shown in Figure 49, were observed in a few sections but are rare.

Fr8mework Grains. Quartz is the major constituent of the orthoquartzites of the Sioux. It occurs as the most abundant detrital grain species, as well as forming the major cementing material.

Detrital quartz grains range in size from silt (less than

0.0625 millimeters) to granules (greater than 2 millimeters) in the orthoquartzites. Both monocrystalline and polycrystalline quartz grains are present, but polycrystalline grains seldom compose more than 3 percent of total quartz. Polycrystalline 111

75%

50% 50%

25% 25% I

o,,;;25 0.25 o.5· i.o um. 0.625 1).25 0.5 1.0 2.0 urn.

75%

50%

25%

0.625 1),25 0.5 l.O 2.0 nm.

75% .

50%

25% 25%

0.625 1).25 0.5 1.0 2.0 ""'· 0.625 •).25 0.5 · 1.0 2.0 mn.

Figure--48 GRAIN SIZE DISTRIBUTIONS FOR ORTHOQUARTZITES 112

Figure 49--PHOTOMICROGRAPH OF ORTHOQUARTZITE SHOWING DEFOID-rATION LAMINAE IN QUARTZ GRAINS. Sample 189, crossed nicols. 113

quartz grains are almost exclusively restricted to sizes coarser than medium sand (0.5 millimeters).

The monocrystalline quartz grains show all variations of extinction from very sharp to extremely undulatory. An attempt to analyze the relative amounts of undulose versus nonundulose quartz contributed by the source area was unsuc- cessful because of wide variations in the amounts of post-depositional strain.

Polycrystalline quartz grains of two types were observed.

The more common type is composed of two to six subequal crystals joined along smooth to irregular boundaries. The individual crystal units may be undulose or nonundulose. Stretched metaquartz with elongate, lenticular individual crystal units is the other common polycrystalline quartz type. These grains are composed of five to twenty or more individual crystal units, commonly showing a bimodal size distribution and strongly sutured intercrystalline boundaries. A grain of this type is illustrated in Figure 50. Both polycrystalline quartz types are found throughout the outcrop area, but the stretched metaquartz reaches very high concentrations in the matrix of the basal conglomerate

(68 percent) and the overlying orthoquartzite (34 percent at New

Ulm, Minnesota. This anomaly will be discussed later.

The quartz grains contain numerous small, irregular vacuoles, as well as rare mineral inclusions. In most grains, the inclusions are but some grains show extremely abundant inclusions arranged along parallel traces. Folk (1974) interprets this type of inclusion pattern as diagnostic of hydrothermal vein 114

Figure 50--PHOTOMICROGRAPH OF COARSE ORTHOQUARTZITE SHOWING MINOR CONSTITUENTS, STRETCHED METAQUARTZ AND CHERT. Sample 25lc, crossed nicols. 115

quartz. A few grains were noted to contain abundant hair-like

acicular rutile neeldes. Inclusions of small crystals of zircon

and tourmaline are preserit and one grain contained a crystal of

euhedral biotite.

Because the maturity of the Sioux suggests derivation

from older sediments, each slide was carefully checked to detect

the presence of grains showing evidence of recycling. Grains with

abraded secondary overgrowths were found in 84 percent of the thin

sections of orthoquartzite (Figures 51 and 52). Thin films of

iron oxide are present on both the original grains and on the

abraded overgrowths. Some thin sections contain as many as 25

grains showing good multicycle evidence, but others contain only

a few. Although a worn secondary overgrowth on a quartz grain is

diagnostic of derivation of that grain from an older sediment

(Blatt, et al, 1972), there is no way of determining how many of

the other grains in the rock also are multicyclic. Certainly if one grain is recycled, others may well be. At least one grain has

two abraded overgrowths, suggesting- some of the quartz is third cycle.

Detrital chert is present in almost all thin sections of the orthoquartzite, although it seldom makes up more than a few percent (Figure 50). Two types of chert were recognized:

1) "Normal chert," essentially monomineralic, microcrystal-

line quartz showing pinpoint extinction and lacking

colored inclusions; and

2) Iron-stained chert containing inclusions of hematit·e

(red) or magnetite (black). 116

51--PHOTOMICROGRAPH OF QUARTZ GRAIN WITH SECONDARY OVERGROWTH. HEMATITE COATING MA .. BOTH BOlJNDRIES . Sample 137, crossed nicols.

Figure 52--PHOTOMICROGRAPH OF A MULTICYCLE GRAIN WITH ABRADED SECONDARY OVERGROWTH. Sample 125, crossed nicols. 117

Both types are found as well-rounded grains ranging in size

from fine sand to granules in the orthoquartzi tes. The amount

of chert does not vary geographically or stratigraphically.

A single grain of chalcedonic quartz with a radiating micro-

crystalline structure was observed in thin section. For purposes

of modal analysis, it was counted as chert. Two types of sili-

ceous rock fragments, iron-formation and quartzite, were found in

the orthoquartzites. Fragments of both of these sedimentary rock

types occur as subrounded to rounded, coarse sand grains.

Over 125 thin section heels and slabs were stained for

K-feldspar, but none was found. In thin section, all ques-

tionable grains were checked for biaxial interference figures

but no feldspar of any kind was observed in any of the sec-

tions.

One thin section of fine-grained orthoquartzite contains

trace amounts of well-rounded detrital grains of muscovite. No other detrital micas were observed in the orthoquartzite, but they do occur in the mudstones.

Magnetite, hematite, and ilmenite are the only opaque minerals that form detrital grains. They are commonly found placered along the laminae of cross-beds. Zircon and tourmaline are the only common non-opaque heavy minerals and will be discussed separately (see Heavy Minerals).

Cement. Authigenic quartz overgrowths in optical continuity with the quartz grains comprise the major cement (Figure

53). A coating of hematite on the original grains forms a demar- cation line between the grains and cement in most sections. The li8

Figure 53--PHOTOMICROGRAPH OF ORTHOQUARTZITE SHOWING GOOD •AUTHIGENIC OVERGROWTHS. Note simple grain contacts. Sample 193a, crossed nicols. 119

pore space has been completely filled with cement, forming an interlocking fabric. No inclusions of calcite are present. The formation is uniformly cemented throughout, although some decemen- tation has taken place along fractures. Three thin sections from the New Ulm area have chert cement rather than quartz overgrowths.

The characteristic pink color of the Sioux is due to a thin coating of hematite on the detrital grains. This coating was observed in all thin sections and always lies between the grain and the overgrowths. In some rocks, abundant detrital grains of magnetite or hematite after magnetite give the rock a deep purple color.

Authigenic minerals were found in small quantities in most of the orthoquartzites. Diaspore, a hydrous aluminum oxide with a composition of AlO (OH), is present in 73 percent of the orthoquartzi te thin sections, and ranges in amount from a tracE to 6 percent. It occurs as subhedral grains and irregular masses in the intersticies of the orthoquartzites

(Figure 54). It was identified optically by its perfect cleavage, strong birefringence, and high relief. It e x hibits parallel extinction and is biaxial positive with a 2V of 84 degrees.

Diaspore also occurs as tabular crystals up to ois millimeters in length and is commonly observed replacing the quartz cement. It is often associated with sericite, but diaspore is also found in the purest of orthoquartzites showing no trace of sericite or argillaceous material.

The occurrence of diaspore does not appear to be stratigraphically or geographically controlled but may be related 120

Figure 54--PHOTOMICROGRAPH OF AUTHIGENIC DIAPORE. Sample 102, crossed nicols. 121

to freshness of exposure. Samples form the lower levels (anywhere from 5 to 30 meters below the surface) of the five major

operating quarries contain no diaspore, although surface samples

from nearly all outcrops contain diaspore. The possible origin of

the diaspore will be discussed later (see Alteration). Berg

(1938) found that diaspore did not extend deeper than 5 to 6 feet from the surface at a quarry near Pipestone.

Authigenic sericite was observed in almost all thin sec-

tions of orthoquartzite (Figure 55). Sericite, which is fine- grained, slightly impure, K-defficient muscovite, was identified by its low relief and straw-yellow birefringence. It occurs as fine crystalline aggregates replacing quartz overgrowths and quartz grains. The amount of sericite ranges from a trace to 5

percent and does not decrease in abundance in deep quarry samples as does di as pore. Sericite is always younger than the quartz overgrowths, but its age relationship to the diaspore is not clear.

Berg (1938) reported the presence of pyrophyllite in the orthoquartzite at Pipestone, Minnesota. In a thin section from this location, no pyrophyllite was observed and x-ray diffraction showed no pyrophyllite.

Modal Analysis. Six-hundred points were counted on each of 109 thin sections. Table 5 summarizes the results of these analyses. The exact stratigraphic position of the samples is given on Figure 14. There are no significant changes in the composition of the Sioux orthoquartzite, except at New Ulm. At

New Ulm, there is a noticeable decrease in the percentage of 122

Figure 55--PHOTOMICROGRAPH OF ORTHOQUARTZITE SHOWING SERICITE REPLACING A QUARTZ Sample 184, crossed nicols. 123

polycrystalline quartz from 68 percent in the matrix of the basal

conglomerate to 40 percent in the lowermost coarse orthoquartzite

bed. Approximately 215 meters higher in the section in an ortho-

quartzite of similar grain size, only 9 percent of the quartz is

·polycrystalline. The polycrystalline quartz in this column is

almost exclusively the stretched metaquartz type described

earlier. A thin section of the granite underlying the basal

conglomerate at New Ulm was examined and found to contain an

abundance of similar stretched metaquartz. Therefore, the high

percentage of polycrystalline quartz found in the New Ulm samples

can be probably attributed to the adjacent basement sources.

The "average" orthoquartzi te is composed of 90 to 95 percent

framework grains and 5 to 10 percent quartz cement. The framework

grains consist of 95 ot 97 percent monocrystalline quartz; 1 to 3

percent polycrystalline quartz; 1 percent chert; and trace amounts

of opaques, heavy minerals, and siliceous rock fragments. The

cement is almost exclusively quartz with minor amounts of authi-

genic diaspore, sericite, and rutile.

According to the classification system of Pettijohn, Potter,

and Siever (1973), all sandstones examined in this study are

quartz arenites. Chemically, the orthoquartzi tes are composed

almost exclusively of silica.

Heavy Minerals

Heavy minerals were separated from 15 samples of ortho-

quartzite using 1,1,2,2, tetrabromoethane. Only non-magnetic, non-opaque detrital heavy mineral grains were examined. No

point counts were made due to lack of variability. Very well- 124

rounded grains of tourmaline and zircon were the only detrital

minerals encountered. Clear or slightly cloudy, colorless, high

birefringence zircon makes up almost 100 percent of the heavy

mineral content (Figure 56). The well-rounded zircon grains are

commonly zoned with well-developed, sharp crystal angles •

. The heavy mineral separate from the basal conglomerate

at New Ulm contains several euhedral prismatic zircons that

show no indication of rounding. The same type of small, cloudy,

euhedral zircons are present in the underlying granite. The close

similarity and lack cf rounding of these zircons suggest they were

derived from the underlying granite.

Well-rounded, brown and blue tourmaline grains were found in

six samples. They are considered to make up only a very minor

part of the heavy mineral assemblage. One blue tourmaline grain was composed cf a rounded core with a rounded, clear, secondary overgrowth indi.cating recycling.

Rutile was found in two samples as knee-shaped, twinned prismatic crystals. As this form is not likely to be preserved in a detrital grain, it probably is authigenic. The heavy mineral assemblage described above indicates the Sioux was derived from a mature sedimentary source, but it is possible that the assemblage has been modified by intrastralal solution.

Cong'omerates

The conglomerates of the Sioux are too coarse to be examined in thin section. Twenty-five sawed slabs were examined for cl as t composition and six thin sect ions of the ma tr ix material were analyzed. In addition, three clasts were thin- sectioned to obtain more detailed information. 125

Figure 56--PHOTOMICROGRAPH OF WELL ROUNDED ZONED ZIRCON IN ORTHOQUARTZITE. Sample 110, crossed nicols. 126

Basal Conglomerates. The basal conglomerate at New Ulm

was intensively studied by Hiller (1961). This 20-meter thick

conglomerate is composed of 6 meters of poorly sorted, very

coarse, matrix-supported congomerate with clasts as large as 30

centimeters. The base of this coarse conglomerate is not exposed.

Overlying this unit is a clast-supported, moderately sorted cobble

conglomerate that was estimated by Miller (1961) to be 10 meters

thick. Resting on top of the cobble conglomerate are 4 meters of

well-sorted pebble conglomerate and conglomeratic orthoquartzite.

No compositional changes of the clasts or matrix were noted

through the 20 meters.

The clasts of the basal conglomerate are composed ·of several

siliceous rock types. No clasts of granite were found by Baldwin

(1951), Miller (1961, or the author. More than 50 percent of the

clasts are milky white to gray translucent vein quartz. These

clasts range in size form 2 to 15 centimeters, but do not make up

the largest clasts. The vein quartz clasts are rounded and are

· usually elliptical, rather than spherical. Tt:-1enty to 30 percent

of the clasts consist of two types of hematitic chert that Miller

(1961) described as green-gray granular chert and red hematitic

chert, as follows:

"The green-gray, granular chert has abundant

irregular to crudely elliptical, chert granules

averaging 0. 3 millimeters in diameter. Commonly,

these granules are part hematite and part chert.

They average between 0.2 and 0.3 millimeters in

diameter and are outlined by dusty iron oxide in a 127

fine-grained cherty groundmass. Some of the

granules of chert are bordered by subhedral hema-

tite which protrudes into the center of the granule

in some cases. The interior of the granule is

usually structureless and consists of coarser

grained chert than the very fine-grained ground-

mass.

The red, granular, hematitic chert consists pri-

marily of hematite and chert granules in a chert or

microcrystalline jaspery quartz ma tr ix. The gran-

ules are subrounded to rounded, equidimensional to

ellipsoidal grains. They have a wide range in size

up to 1.5 millimeters. The granules consist of

hematite, chert, or a combination of hematite and

chert. The granules of hematite and chert usually

have hematite outlining the granule with prominent

euhedral blades protruding into the center of the

granule. In a few cases the euhedral blades of

hematite are in the center of the granule. Al-

though the hematitic blades project into the

interior of the granule forming a jagged interior,

they usually unite to form a smooth outer rim."

Miller has stated that the cherts found in the conglomerate are similar to those of Middle Precambrian iron-formations of the Lake Superior region (see Regional Geology). A few clasts of banded iron-formation characteristic of Middle Precambrian iron-formation were also found. The hematitic chert clasts are 128

well-rounded and moderately spherical. They form clasts as much as 30 centimeters in diameter.

Approximately 10 percent of the clasts are fine to medium- grained orthoquartzi te. A thin section of this orthoquartzi te shows the grains to be subrounded to well rounded and moderately well sorted. The grains are cemented by quartz overgrowths and the original detrital grain boundaries are marked by thin films of iron oxide. It occurs as rounded clasts from 2 to 25 centimeters in diameter. This orthoquartzite was noted to be low (less than

10 percent) in polycrystalline quartz.

The matrix of this basal conglomerate is composed of moderately well-sorted, medium to very coarse sand with scattered granules. The grains show bimodal rounding, with most grains being subrounded to subangular, while others are very well rounded. Roundness appears to be a function of composition rather than size, with monocrystalline quartz 9 rains showing better rounding than polycrystalline grains.

The major framework grains of the conglomerate matrix are monocrystalline quartz, polycrystalline quartz, opaques, and hematitic chert. Polycrystalline quartz grains make up

68 percent of the grains in the matrix. This is the highest percentage of polycrystalline grains encountered in the Sioux.

The grains have been cemented by precipitation oi quartz overgrowths, accompanied by pressure solution and slight inter- penetration as noted by sutured grain contacts. This is the only location· where this texture was observed. Later authigenic sericite has replaced much of the quartz cement. Zircon is the 129

.only non-opaque heavy mineral found in the conglomerate. The presence of euhedral prismatic, unzoned zircons in the basal conglomerate is significant because these zircons resemble the zircons observed in the granite (see Heavy Minerals).

A second conglomerate, inferred on structural and lithologic evidence to be basal, is poorly exposed near Pipestone in Section 36, T. 106 N., R. 47 w. This conglomerate is composed of boulders and cobbles of orthoquartzi te, vein quartz, hematitic cherts, mudstone, and rhyolite in a matrix of coarse to medium sand and argillaceous material. The conglomerate is coarsest at the base and grades upward into argillaceous orthoquartzite with scattered beds of coarse cobble conglomerate. The quartzite clasts are subrounded to well rounded, and range in size from 2 to 16 centimeters. They are the most abundant clasts. Clasts of vein quartz and hematitic cherts are rounded to subrounded and are about equally abundant.

Mudstone fragments predominate in some beds. They are similar in coaposition to the mudstone within the Sioux and occur as angular, tabular fragments 20 to 60 millimeters across.

As with other mudchips within the Sioux, they were probably locally derived.

Although rare, rounded 4 to 6-centimeter clasts of rhyoli te are very significant, as this is the only location in the Sioux that contains any evidence of f l edspathic material. The clasts consist of an aphanitic groundmass with phenocrysts of euhedral hexagonal quartz. The quartz phenocrysts have straight sides and deep enbayments. The groundmass is completely altered 130

to sericite and no trace of fledspar remains. One 3-centimeter

clast slabbed for identification consisted of banded white agate.

The matrix of the conglomerate is poorly cemented by quartz overgrowths and contains much original argillaceous material

possibly formed upon disintegration of some of the mudstone clasts.

Other Conglomerates. The conglomeratic orthoquartzites found within the Sioux are composed of 1 to 6 centimeter pebbles and cobbles of vein quartz, quartzite, and hematitic chert with sparse clasts of banded iron-formation. No feldspathic rock fragments were found. The clasts are well rounded and well sorted. The conglomerate beds are clast-supported and contain a matrix of coarse sand. Heavy minerals, commonly magnetite and lesser amounts of zircon, are placered in streaks in the conglomeratic layers. No major geographical or stratigraphic changes in mineralogy were observed in these conglomera tic orthoquartzi tes, but the percentage of chert and quartzite clasts decreases higher in the section.

Mud stones

The mudstones of the Sioux are not particularly well suited to petrographic study. Small grain size and deep hematite stain- ing make compositional determinations difficult.

Eight thin sections were examined for composition and texture, but no point counts were made. Five samples were also x-rayed to determine the dominant mineralogy.

Quartz, sericite, hematite, and diaspore are visible micro- scopically. The quartz occurs as angular silt grains and 131

subrounded to rounded fine sand grains; it generally comprises 10 to 25 percent of the mudstones. Quartz grains were present in all samples of mudstone except the pipestone at Pipestone, Minnesota

(Figure 57).

The hematite stain is disseminated throughout the matrix of the mudstones, except along fractures and spherical spots where bleaching has occurred. A few scattered hematite clumps or grains 0.01 millimeter in diameter were observed. Coarse grains of muscovite (0.25 millimeter) orientated parallel to bedding are present in two mudstones; these grains are considered to be detrital because no recrystallization was observed in the matrix of these mudstones. Very fine-grained, unoriented sericite makes up the bulk of the matrix in all of the mudstones. A few very small (less than 0 .01 millimeter) irregular patches of diaspore were also recognized in the matrix.

Minerals identified by X-ray defraction include muscovite, quartz, diaspore, pyrophyllite, hematite, and possibly illite. Pyrophyllite was present only in the samples from the mudstone or "pipestone" located at the Indian quarries at Pipestone, Minnesota. The mineral identified as pyrophyllite occurs in the matrix of the deeply hematite-stained mudstone as subrounded to angular patches lacking hematite stain. Pyrophyl- lite has low relief, high birefringence, and parallel extinction; but when fine-grained, it is very difficult to distinguish from sericite by optical means (Kerr, 1959). The occurrence of pyrophyllite as scattered silt and fine sand-sized aggregates and as layers parallel to bedding is textural evidence that pyrophyllite 132

Figure 57--PHOTOMICROGRAPH OF MUDSTONE FROM PIPESTONE, MINNESOTA. PYROPHYLLITE (WHITE) HAS REPLACED QUARTZ GRAINS. Sample 127b, crossed nicols. 133

has replaced detrital quartz.

The available chemical analyses on the mudstones of the

Sioux were completed by Miller (1961) and are shown in Figure

58. Only the · mudstone from Pipestone, Minnesota was analyzed.

Because this unit is anomalous compared to other mudstones in that

it lacks quartz and contains pyrophyllite, the sample is not representative. The aluminum content of this mudstone is approximately twice that of a normal shale, as shown in Figure 59.

Miller (1961) attributed this high aluminum content to lateritic weathering in the source area. However, the occurrence of authigenic diaspore, apparently related to the present surface, suggests that the high aluminum concentration of this mudstone may be related to a later weathering event.

Weathering and Alteration

Poorly-cemented orthoquartzite and areas of loose sand are rarely observed in outcrop and in quarry walls. Austin

(1970) stated that these areas are related to weathering along joints and fractures. Uncemented beds were not observed. In weathered orthoquartzite, the original grains have been freed by the removal of all or part of the silica cement by intense chemical weathering. The weathered orthoquartzite is bleached white and stands in sharp contrast to the commonly unweathered, dark pink orthoquartzite. Austin proposed that prior to deposition of the Cretaceous sediments, intense chemical weathering produced a sandy, kaolin rich residuum on the orthoquartzite of the New Ulm area. He stated that "humid tropical climatic conditions" must have existed in order to produce the two-layer clay minerals of 134

"Average Shale"1 Mudstone!! 1 2 3 4 5

SiO:: 5S.01 49.01 4S.20 57.43 53.25 50.40

AI:.P3 15.40 35.17 2S.20 25.94 35.'90 33.30

Fe::03 4.02 3.06 5.00 8.70 2.80 FeO 2.45 none MgO 2.44 0.23 6.00 0.17 Cao 3.11 . 0.05 2.60 0.60 Na 0 1.30 2 0.06! 4.10 K..O 3.24 5.62 5.63 8.40 7.44 6.43 9.60 H;O 5.00 H:!O-i 0.24"

Ti02 0.65 0.44 Li::O 0.16

P20;; 0.17

C02 2.63

S03 0.64 Bao 0.05 c O.SO MnO . 0.60 Ignition, less total H::O 0.24 100.00 99.91 99.00 99.51 100.63 100.97

1 Analysis of average shale taken from Clarke, 1924 •Analyses of catlinite from Pipestone, Minnesota: 1, Berg, 1938 2-5. Winchell and Upham, 1834

CHEMICAL COMPOSITIONS OF MUDSTONE 135

the kaolin group found in the residuum.

Parham (1970) suggested the possibility of a peneplain

having been developed on the Precambrian surface before Cretaceous

time in southwestern Minnesota. He stated that a residuum of

kaolinite up to 200 feet thick had been developed on Precambrian

granite and gneiss, but that it was probably not as thick on the

Sioux. Above this residuum lies Cretaceous on which a second weathered zone developed which contains pisolitic kaolinite,

gobbsite, and boehmite. Because the Sioux probably formed prominent ridges and highlands and is known to have contributed sediment to the Cretaceous sands, the Cretaceous sediment separating

the two weathering zones on the granite probably did not exist on

the Sioux. This second weathering event that produced the gibbsite (AlOH) and boehmite (AlO(OH)) probably was responsible for the formation of diaspore (ALO(OH)) in a surface zone 1 to 3 meters thick on the orthoquartzite and the mudstones.

Chandler and others (1969) have suggested a different origin for the formation of diaspore and pyrophyllite in the

Middle Precambrian quartzites of Ontario. They suggest that diaspore was formed by leaching of feldspars before cementation; diaspore then reacted with the quartz during regional metamorphism to form pyrophyllite.

Conclusions on Petrography

From the preceding data, the following conclusions are drawn. · The terrane which formed the source area of the Sioux quartzite was primarily composed of sedimentary rocks; in 136

particular, orthoquartzite, iron-formation, and chert. Direct contributions from plutonic and metamorphic rocks are considered minor. The sediments are mineralogically and texturally very mature, typical of a long history of reworking in a high-energy environment. Two separate lines of petrographic evidence, quartz types, and zircon shape and roundness suggest the granite underlying the Sioux at New Ulm has contributed to the lower Sioux beds.

After deposition, the grains were coated with hematite and thoroughly cemented with quartz overgrowths. Later slight physical deformation produced deformation laminae in some grains.

An intense chemical weathering event caused the formation of diaspore near the present surface. 137

SEDIMENTATION

ENVIRONMENT OF DEPOSITION

Both marine and continental environments have been suggested

by previous writers (Miller, 1961; and Baldwin, 1949) to explain

the characteristics of the Sioux Quartzite. Based on this study

of sedimentary structures, paleocurrent patterns and petrology, a

continental, predominantly fluviatile depositional environment is

postulated for the Sioux Quartzite.

The sedimentary s true tures, paleocurrent patterns and

petrology of the Sioux Quartzite suggest that deposition took

place on the edge of the craton in a high energy, shallow stream

system. Given the age of the Sioux at approximately 1500 m.y.,

the lack of vegetation of any type would allow rapid erosion and

resedimentation of sand size material and removal of silt and clay

size particles by wind erosion. In the absence of banks stabiliz-

ed by vegetation and/or a high silt and clay content, runoff would

collect in shallow braided stream systems allowing for uniform

sedimentation of sand size and coarser material over extensive

areas.

In Harm's et al. (1975) model of a braided stream, four

major sedimentary facies are defined: channel floor lag, in-

channel nearly filled channel sediment and vertical

accretion deposits. Channel floor deposits are well developed in

the lower section of the Sioux. The thin, but coarse conglomeratic beds found in Jasper section along with the coarse, one layer

pebbly lenses found throughout the Sioux were accumulated as 138

channel floor lag during the removal of finer material and concentration of coarser clasts by rapidly flowing streams. The bulk of the sedimentary sequence consists of in-channel sediment characterized by abundant, large scale trough cross-bedding. This facies probably represents channel in-filling by migrating bars and dunes; Current ripples and symmetrical ripples would be expected to form in the shallow water of nearby filled channels.

The thin, silty mudstone units showing limited lateral extent, probably were formed as overbank deposits or in temporarily abandoned channels. Later desiccation or exposure and subsequent rises in water level would have formed the observed mud cracked surfaces and mudchip conglomerates.

The coarseness, poor sorting and fining upw2rd sequence within both outcrops of basal conglomerate, suggest they are fluvial. The large clast size (up to 35 cm) and matrix-supported fabric of the lower beds of the basal conglomerate, suggest they were deposited on a substantial slope and never reworked.

The complete absence of fossils requires that several lines of evidence be followed in order to ascertain the depositional environment of this sandstone. None of these individually provides conclusive evidence that the Sioux Quartzite is predominantly a fluviatile deposit but collectively they seem to indicate that this is the case.

The major sedimentary structures of the Sioux Quartzite, cross-bedding, ripple marks, mud cracks, while not unique to a fluviate environment are probably more characteristic of fluviatile deposition on a coastal plain than they are of deposition in 139

a shallow marine environment. The lack of thick sedimentation

units and well defined fluvi tial sequences indicates the stream

systems were shallow and constantly fluctuating.

The cross-bedding of the Sioux is predominantly trough

cross-sets indicating intense scour and fill action associated with migrating dunes (Figure 20). As previously mentioned,

experimental data suggest that the trough cross-bedding of the

Sioux were formed at water depths greater than 30 to 40 centimeters and under flow velocities greater than 60 centimeters per

second. Rapid sedimentation under these conditions is more

typical of a stream environment than of shallow marine deposition.

Mud cracks and mud chip conglomerates also indicate shallow water conditions. The formation of mud cracks and mud chip conglomerates requires subaerial exposure and subsequent disrup- tion by strong currents. These conductions would be common in the overbank areas of fluctuating streams.

Sedimentary structures found in the Sioux indicative of paleocurrent direction, current ripples, planar cross-bedding and trough cross-bedding are the structures predicted by experimental work to form with successively increasing current velocity as shown in Figure 21. It is somewhat anomalous, however, that asymmetrical ripples (the low energy bed form) and trough cross- bedding (the high energy bed form) are quite abundant in the Sioux with one or the other or both being observed in almost every outcrop, whereas planar cross-bedding (the medium energy bed form) is relatively rare. 140

While deposition in a stream system with seasonal flooding

would explain the dominance of high and low velocity bed forms, a

more complete range of current velocity and water depths would be

expected in a shallow marine environment.

The occurrence of herringbone cross-bedding, however,

suggests that deposition was affected by tidal action at the

coastal margin. Klein (1970) has shown that herringbone cross-

stratification, reactivation surfaces and associated bipolar

distribution of paleocurrent indi·cators are indicative of tidal-

flow processes. While these structures are conclusive indicators

of tidal processes, their occurrence is restrictive, both geo-

graphically and stratigraphically in the Sioux, being found in

four locations (see sedimentary structures) in the upper third of

the exposed section and only along the southern margin of the

outcrop belt. It is probable that this portion of the Sioux was

formed on the margin of the proposed fluvial system at the inter-

face between continental and marine sedimentation.

Paleocurrent -patterns also suggest that deposition of the

Sioux took place in a fluviatile system on a coastal The

cross-bedding data (Figures 41, 42, 43 and 44) indicate a consis-

tent regional pattern of southward transport over an area of about

6,000 square miles. The limited variability, both stratigraphi-

cally (Figure 44) and geographically (Figure 42) (with the excep-

. tion of areas showing bimodal distribution previously noted)

implies a stable system of depositing currents and probably is more characteristic of stream currents than of currents in

shallower marine environments. 141

The close correlation between the direction indicated by

cross-bedding measurements and those from asymmetrical ripples

further suggests a consistent southerly direction of transport.

The distributrion of symmetrical ripple trends (Figure 37) also is

compatible with fluviatal deposition.

While the mean symmetrical ripple trend in the Sioux is at right angles to the mean current directions of other indica-

tors, the rose plot shows a significant number of trends subparallel to current directions. In a stream system ripples parallel

to the shoreline would be common, but orientation of ripples with

trends parallel to main current direction would probably not be common in a marine environment.

The petrology of the sandstones and conglomerates also suggest that the formation was laid down in a fluvial environment.

Texturally, the sands of the Sioux are well-rounded, moderately well sorted, medium to coarse grained sandstones with the sand fraction washed clean of fine material, but often containing coarser material. The presence of multiple overgrowths on many grains suggest that well rounded nature of the sand is a function of the source area rather than the environment of deposition and therefore does not imply extensive reworking to achieve the observed roundness.

Most of the coarser grained sediments of the Sioux Formation show poor sorting and pronounced lensing of coarser with finer materials (Figure 10). In many cases, pebbly lenses and individual pebbles are distributed irregularly in a cross-bedded, predominantly sandy matrix as shown in Figure 8. Such material 142

probably represents lag deposition in beds of rapidly fluctuating

streams.

Source Area

Paleocurrent analyses suggest the source area of the Sioux

Quartzite was to the north of the Sioux outcrop belt. The nearly

complete lack of exposure of Precambrian rocks in the presumed

source area prohibits positive identification of the source.

Paleocurrent indicators studied suggest that the major paleoslope was to the southeast. The minor variations observed in this

consistent pattern are probably due to local topography within the

environment of deposition.

The petrology is indicative of a source composed dominantly

of mature elastic sedimentary rocks and siliceous chemical sediments. Orthoquartzite, chert and iron-formation were probably the dominant lithologies.

The low content of polycrystalline quartz and the very mature heavy mineral suite indicate that granitic gneiss and other high grade metamorphic rocks were minor or absent in the source area. Deep weathering of the Lower Precambrian granitic gneiss terrain could produce large volumes of quartz-rich sediment free of feldspar in one cycle of sedimentation, but this sediment would be expected to be high in polycrystalline quartz types. The lack of polycrystalline quartz types, the presence of quartz grains with abraded overgrowth beneath younger overgrowths, and the well developed rounding and sorting suggest the Sioux is predominantly a multicycle sediment. 143

Tectonics

The textural and mineralogical maturity of the Sioux Quartz-

ite suggests that the sand grains underwent considerable rework-

ing and chemical weathering before their final deposition. The

complete removal of all traces of feldspar implies low relief and

deep weathering in the source area, but the . accumulation of over

1600 meters of sediment suggests the depocenter was slowly subsid-

ing and hence a tectonically negative element. The uniform

occurrence of the same types of primary sedimentary structures

throughout the entire section and the lack of stratigraphic or

geographical changes in paleocurrent directions suggest general

stability of conditions. The minor beds of conglomeratic ortho-

quartzite in the lower two-thirds of the section and the increased

abundance of mudstone in the upper third may indicate that relief

had been further reduced to a stage where only sand size and finer material was being transported into the depocenter.

The supermature mineralogy of all parts of the Sioux indicates a lack of major orogenic or volcanic activity in either the source area or the depocenter.

The Sioux is one of several late Precambrian orthoquartzites deposited in before the Keweenawan event, including the Athabasca

Formation of Saskatchewan.

The remarkable similarity of these sandstones suggest that they were laid down under similar environmental conditions. These orthoquartzi tes all contain typical shallow water sedimentary structures, such as trough cross-bedding and r i pp 1 e marks • Shallow water marine environments have been 144

postulated for most of these units based on their textural

maturity, thickness and overlying marine carbonates and iron

formations.

The Athabasca Formation of Saskatchewan, very similar to

the Sioux in the occurrence of sedimentary s true tures, has been

interpreted by several authors as being deposited in a fluvial

environment. Fahrig (1961) considered that the trough cross-

bedding and the occurrence of mudchip conglomerates and pebble

lenses were more characteristic of deposition in a fluvial envir-

onment than a shallow marine environment. Ramaskers and Dunn in a

more recent study (1976), suggested that the Athabasca was depos-

ited in an extensive, shallow braided stream system. Rarnaskers

and Dunn also pointed out that in the absence of vegetation, rates

of sedimentation under braided stream conditions are likely to be

high, thus accounting for the great thickness of both the Sioux

and Athabasca formations.

The extreme mineralogic and textural maturity of each unit

indicates a long period of stability preceded the final deposition

of the sandstones. The large amount of almost pure quartz sediment that makes up these units would require the weathering and

erosion of enormous amounts of crustal rocks. As evidenced by the presence of multi-overgrowths in the Sioux, this sediment was the product of several cycles of erosion and sedimentation, rather

than a single erosional episode.

The paleocurrent evidenced from the Upper Precambrian quartzites of the Lake Superior region indicates deposition of the edge of an area that lay on the southern edge of the Precambrian Shield 145

(Figure 45). The westward paleocurrent directions of the

Athabasca suggest that orthoquartzi te sandstones were deposited

along the coastal margins of the entire craton during early,

late-Precambrian time.

An encroachment of the seas onto the craton marked the

end of elastic on both the northern and southern margin of the

Canadian Shield prior to the Keweenawan event. This encroachment

is marked by the deposition of chemical marine sediments, con-

formally above both, the Athabasca is overlain by the Carswell

Dolomite, and dolomite and f a-rruginous slates overlie the Baraboo

Quartzite. 146 146

CONCLUSIONS

1. The Sioux Quarzite is a multicycle sediment derived from

older orthoquartzites, cherts and iron-formations. The Sioux

Quartzite is a sedimentary unit and has not been metamorphosed.

2. An intense post-depositional weathering event, probably

Cretaceous in age, is responsible for the occurrence of diaspore

in surface samples of the orthoquartzite.

3. The unexposed contact between the conglomerate of the Sioux

Quartzite and the "New Ulm Granite" at New Ulm, Minnesota is an

unconformable basal contact. Material derived from the under-

lying granite probably has contributed detritus to the Sioux

at this location.

4. Paleocurrent indicators show that the dominant paleoslope during the deposition of the Sioux Quatzite was generally to

the southeast.

5. Primary sedimentary structures indicate that the upper one- third of the Sioux was deposited in a shallow marine environment in which tidal currents were present. The sedimentary structures of the lower two-thirds of the section suggest deposition in a fluvial braided stream environment. 147

TABLE 6--MODAL ANALYSES OF ORTHOQUARTZITES

aJ aJ i:: i:: •r-i •r-i r-l r-l r-l r-l Cll Cll aJ .µ .u tJ .µ Ul UJ •r-1 aJ Ul •r-l aJ :>-. :>-. .µ .u r-l tJ !-< !-< N H N ·r-1 N .U •r-l aJ Cll ·r-l 0 tJ .u tJ .u .u .µ .u .µ i:: .w :l !-< !-< 0.. 0 !-< :>-. !-< H Cll !-< !-< OJ Cll O' OJ OJ UJ i:: C'j r-l Cll aJ i= aJ ell i= s ell i:: (/) cu Sample 0 :l 0 :l ..c OJ.= :l Q) Q) °" ·M -.-! ::a: O' p.. O' u ::i:: u O'U ::i:: 0 ::a: 0 102 77 15 1 Tr 7 104A 82 6.5 1. 5 1 1 6 1 106 77 15 3 2 1 1 1 109A 78 14 .5 Tr 1 .5 5 109C 76 15 1.5 Tr 2 Tr Tr llO 89 5 Tr Tr 4 2 BM13 78 15 Tr 3.5 1 .5 2 112 89 4.5 Tr 2.5 1 1 2.5 114 94 5 Tr .5 .5 117C 75 20 .5 . 5 3 .5 .5 ll7B 87 6.5 Tr 2.5 Tr 4.5 120 90 1.5 Tr 1. 5 1 1.5 4 124A 88.5 4.5 Tr Tr 5 Tr 1 125 88.5 4 Tr Tr 4 3 127A 70 9 1.... Tr 1 19 128A 89 5 1 Tr 128C 90 3.5 Tr Tr 3.5 1 Tr 1 128E 87 6.5 1 .5 3.5 .5 1 130A 91 4.5 1 2 2 136 93 2 . 5 3 1 1 137 89 5 Tr Tr 4 2 138 93.5 3 Tr Tr 2.5 1 140 93 2 3 2.5 141 94 5 Tr 1 150 93.5 3 Tr Tr 2.5 .5 152 85 9.0 .5 Tr 2 4 155A 93 3 Tr Tr 2.5 1 Tr 155B 92 4 Tr .5 2 1 157 91 7 2 160 95 1.5 2 1.5 164 90.5 3 Tr Tr 2 3 1 165 94 2 .5 3.5 Tr Tr 169 85 10 Tr 3 1 Tr 170 86 9.5 .5 3.5 .5 .5 .5 172 95 2 Tr 3 .5 Tr 175B 95 3 Tr Tr 1 .5 179 95 1.5 2 1.5 181 94 1.5 .5 2.5 1.5 182B 85 3 Tr 3.5 3 4.5 183D 94 2 .5 3 Tr Tr 183A 92 4 3 Tr .5 183C 92.5 4 2.5 1 184B 94 2 .5 3.5 Tr Tr 148

TABLE 6--MODAL ANALYSES OF ORTHOQUARTZITES (Continued)

... :>... .µ .µ r-l ·r-l H H N H N •r-1 N .µ ·r-l ... H H ctl H .5 Tr 1. 5 203A 85 3 Tr 3.5 4.5 3 203B 86 4 3.0 4 2 203C 84 4 1 Tr 2.0 1 4 3 203P 89 5.5 3.5 2 204 92 4 Tr .5 2 1 205 95 1.5 2.5 1 Tr 206 94 2 .5 3 Tr Tr 207 92 4 Tr 2.5 1. 5 214 82 6 2 . 5 4 Tr 4.0 215A 87 5.5 Tr 3 .5 216B 86 3.5 .5 7 6 218 82 4 3 .5 10 Tr 219 77 3 1 8 8.5 Tr 220 83 6 1. 5 8 Tr 1 .5 250 23 61 1 6 10 .5 3 1.5 251 16 65 Tr 3 15.5 2 251C 13.5 58.5 1 4.1 10 1.5 252B 76 14 3 1 1 .5 5 253 60 26.5 4 .5 12 2.4 255 47 40 3.5 2 7 1 Tr 258 72.5 9.3 1 .5 3.6 2 8.6 Tr 260 56 16 4 3 3 15 3 149

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