New Observations on the Age and Structure of 

Proterozoic Quartzites in Wisconsin

U.S. GEOLOGICAL SURVEY BULLETIN 1904-B
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Chapter B

New Observations on the Age and Structure of 

Proterozoic Quartzites in Wisconsin

By GENE L LABERGE, J.S. KLASNER, and PAUL E. MYERS

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF IAKE SUPERIOR REGION

P.K. SIMS and LM.H. CARTER, Editors
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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Library of Congress Cataioging-in-Publication Data

LaBerge, Gene L.

New observations on the age and structure of Proterozoic quartzites in

Wisconsin / by Gene L. LaBerge, J.S. Klasner, and Paul E. Myers.

p. cm. (Contributions to Precambrian geology of Lake Superior 

region ; ch. B) (U.S. Geological Survey bulletin ; 1904-B)

Includes bibliographical references (p.

Supt. of Docs, no.: I 19.3:1904-6

1. Quartzite Wisconsin. 2. Geology, Stratigraphic Proterozoic. 

3. Geology, Structural Wisconsin. I. LaBerge, Gene L. II. Klasner, J. 

S. III. Myers, Paul E. IV. Title. V. Series. 

VI. Series: U.S. Geological Survey bulletin ; 1904-B. 

QE75.B9 no. 1904-B 

[QE475.Q3]

557.3 s dc20 90-3787 

[552'.4] CIP
CONTENTS

Abstract Bl

Introduction Bl

Acknowledgments B3

Description and interpretation of individual quartzite occurrences B3

Hamilton Mounds B3

Necedah area B6

Baraboo area B6

Waterloo area BIO

Vesper quarry Bll

Veedum area Bll

Conglomerate at Hamann Creek B12

Rib Mountain B12

Quartzite at McCaslin and Thunder Mountains B12

Barron Quartzite and quartzite at Flambeau Ridge B14 

Summary of interpretations B15

Age of quartzites B15

Structural summary B16 

Tectonic implications B16 

References cited B16

FIGURES

1. Map showing location of quartzite occurrences in Wisconsin relative to major 

tectonic provinces B2

2. Map and cross section of Hamilton Mounds area B4

3. Lower hemisphere stereographic plots of main structural features at Hamilton 

Mounds and Necedah localities B6

4. Generalized cross section of the Baraboo syncline B7

5. Generalized geologic map of the Baraboo syncline and associated rhyolite B8

6. Lower hemisphere plots of structural features in the Baraboo syncline B9

7. Lower hemisphere plots of structural data from Waterloo and Vesper 

areas BIO

8. Diagram showing interpretation of geologic relationships at the Vesper 

quarry Bll

9. Photographs showing quartzite boulders in deformed conglomerate along Hamann 

Creek B13

10. Diagram showing interpretation of geologic relationships of quartzite at Rib 

Mountain, Wausau area B14

11. Summary diagram of data on age relationships of quartzites at nine localities in 

Wisconsin BIS

12. Interpretive sketch of tectonic relationships of the quartzites in central and 

southern Wisconsin B17

TABLE

1. Index of Proterozoic quartzite localities and previously used historic names B5

Contents III

CONTRIBUTIONS TO PRECAMBR1AN GEOLOGY OF LAKE SUPERIOR REGION

New Observations on the Age and Structure of 

Proterozoic Quartzites in Wisconsin

By Gene L. LaBerge, 1 J.S. Klasner,2 and Paul E. Myers3

Abstract

Proterozoic quartzite is exposed at several isolated 

localities within an area of nearly 13,000 square kilometers in 

Wisconsin. Although early workers proposed that the quartzite 

is of two different ages, more recent workers have suggested 

that the various quartzite bodies are correlative, and that their 

protoliths were deposited between 1,760 and 1,630 Ma.

Structural and stratigraphic studies of the quartzite 

deposits together with new age data indicate that the quartzite 

is at least of two distinct ages. Quartzite at McCaslin and 

Thunder Mountains, in northeastern Wisconsin, is older than 

1,812 Ma, as indicated indirectly by a dated intrusion, and 

quartzite boulders in conglomerates in central Wisconsin are 

at least as old as the rhyolite country rock (=1,840 Ma). 

Deformed quartzite at Hamilton Mounds, in south-central 

Wisconsin, is intruded by undeformed granite that is 1,764 

Ma. The ages of many other quartzite bodies, however, cannot 

be tightly constrained at present.

Quartzite exposed in central and southern Wisconsin, 

south of the Eau Pleine shear zone, is interpreted as remnants 

of a passive margin sequence that was deposited on an 

Archean microcontinent (Marshfield terrane) and subsequently 

deformed in a major south-verging fold-thrust system during 

collision between the microcontinent and oceanic-arc rocks of 

the Pembine-Wausau terrane. The occurrence of quartzitebearing conglomerates in the 1,860 Ma volcanic rocks of the 

Marshfield terrane suggests that the allochthonous quartzite 

bodies are 1,860 Ma or older. Collision occurred at about 

1,840 Ma, and marked the end of the Penokean orogeny.

Manuscript approved for publication, April 12, 1990.

'U.S. Geological Survey and Geology Department, University of 

Wisconsin Oshkosh, Oshkosh, WI 54901.

2U.S. Geological Survey and Geology Department, Western 

Illinois University, Macomb, IL 61455.

3Geology Department, University of Wisconsin Eau Claire, Eau 

Claire, WI 54701.

INTRODUCTION

Proterozoic quartzite occurs in a variety of geologic 

settings at several widely separated localities in Wisconsin 

(fig. 1). In some areas it is possible to determine the age 

relationships between the quartzite and adjacent Precambrian rocks. In other areas few or no data are available 

to constrain the age because of an extensive cover of 

Paleozoic rocks and Pleistocene deposits. As a result, 

stratigraphic and structural relationships of the quartzite 

bodies to one another are not well established.

Numerous workers have studied the quartzite 

occurrences in Wisconsin, and a number of different ages, 

names, and possible correlations have been suggested. In 

this report we have followed the terminology used in table 

1 for the quartzites in Wisconsin. Van Hise and Leith (1911) 

summarized earlier interpretations on the age of Precambrian quartzites in the Lake Superior region, and 

suggested correlation of strata based on (1) relations to 

series or groups of known age, (2) unconformities, (3) 

lithologic likeness of formations, (4) like sequence of 

formations, (5) subaerial or subaqueous origin, (6) relations 

with intrusive rocks, (7) deformation, and (8) degree of 

metamorphism (1911, p. 598). Based on these criteria, they 

suggested that the various quartzite deposits are "Huronian" 

in age. Thus, a general correlation of many of the quartzite 

occurrences based on field relations has a longstanding 

tradition. They suggested that the Barren Quartzite may be 

Keweenawan in age. Thus, they concluded that at least two 

distinctly different ages of quartzite probably are present in 

Wisconsin.

Numerous subsequent studies have contributed 

additional data on the nature, origin, and possible 

correlation of the quartzite bodies. For example, Dalziel and 

Dott (1970) concluded from sedimentological studies of the 

quartzite and isotopic studies of the slate member of the 

Baraboo Quartzite that this quartzite is about 1,500 Ma. Van 

Schmus (1978) concluded from U-Pb zircon ages of

Age and Structure of Proterozoic Quartzites in Wisconsin B1
CONTINENTAL + 

MARGIN ASSEMBLAGE

EAST FORK OF * 

EAU CLAIRE 

RIVER

44° _

ILLINOIS

Figure 1 (above and facing page). Map showing location of quartzite occurrences in Wisconsin relative to major tectonic 

provinces of the region. Modified from Sims (in press).

southern Wisconsin rhyolites that the Baraboo Quartzite is 

younger than 1,760 Ma. Smith (1978a,b) inferred from field 

and chemical studies of southern Wisconsin rhyolites, as 

well as Rb-Sr age determinations by Van Schmus, that the 

Baraboo Quartzite was deformed about 1,630 Ma. An 

important aspect of these interpretations is that the rhyolites 

in the Baraboo area itself have not been dated; instead, the 

rhyolites at Baraboo are correlated on the basis of chemistry 

with dated rhyolites some 25-50 km away. Based on 

sedimentological studies, Dott (1983) suggested that the 

various quartzites in Wisconsin, namely the Baraboo,

Waterloo, McCaslin, Flambeau, and Barron, as well as the 

Sioux Quartzite of southwestern Minnesota and adjacent 

South Dakota, are correlative. He suggested that the parent 

materials were deposited, consolidated, and deformed 

between 1,760 Ma and 1,450 Ma, and proposed the term 

"Baraboo interval" for this interval of time. He suggested 

that rocks of the Baraboo syncline serve as the type section 

for Baraboo interval rocks. Greenberg and Brown (1983, 

1984) and Greenberg and others (1986) redefined Baraboo 

interval to include all anorogenic igneous and sedimentary 

activity between the end of the Penokean orogeny, which

B2 Precambrian Geology of Lake Superior Region
EXPLANATION 

Sedimentary rocks of Paleozoic age

Middle Proterozoic (Keweenawan) mafic 

igneous and sedimentary rocks of 

Midcontinent rift system (1,200-1,000 Ma)

Anorogenic igneous rocks (1,510-1,470 Ma)

Quartzite in central and southern Wisconsin

WISCONSIN MAGMATIC TERRANES 

Quartzite in Wisconsin magmatic terranes

Island-arc metavolcanic and granitoid 

rocks in Pembine-Wausau terrane 

(1,880-1,835 Ma); includes post tectonic 

granitic rocks (1,760 Ma)

Island-arc metavolcanic and granitoid rocks 

(1,890-1,835 Ma) and gneiss (=^2,800 Ma) 

in Marshfield terrane

CONTINENTAL MARGIN ASSEMBLAGE 

Marquette Range Supergroup (2,100-1.820 Ma) 

and gneiss, granite, and greenstone 

(3,550-2,600 Ma)

Contact

High-angle fault

Thrust fault Sawteeth on upthrust block

Shear zone

The purpose of this study was to determine the 

structural and stratigraphic relationships among the various 

quartzite exposures and their country rocks in an attempt to 

further our knowledge of these enigmatic rocks in central 

and southern Wisconsin. Our approach was to map 

structures within the various quartzite bodies and to 

compare the structure within individual quartzite deposits 

with that in associated, or nearby, igneous rocks, some of 

which have been dated. In this report, we present the results 

of our field observations for the quartzite localities shown 

in figure 1. We also comment on the implications of the 

data concerning the age and structural setting of the quartzite, and we present an interpretation of these data in terms of 

the Early Proterozoic plate tectonic history of the Lake 

Superior region.

ACKNOWLEDGMENTS

Several key localities discussed in this paper were 

brought to our attention by B.A. Brown and J.K. Greenberg, 

and we acknowledge their contributions with thanks. 

Faculty Development Grant R837 at the University of 

Wisconsin Oshkosh (to GLL) helped fund the study and is 

gratefully acknowledged. Critical reviews by KJ. Schulz, 

W.F. Cannon, R.W. Ojakangas, P.K. Sims, and B.A. Brown 

improved the manuscript.

they placed at 1,760 Ma, and the emplacement of the Wolf 

River batholith at approximately 1,500 Ma. They also 

concluded that all the Wisconsin quartzites are essentially 

the same age, that is, post-1,760 Ma.

Recent geologic studies (LaBerge and Myers, 1983, 

1984; LaBerge and others, 1984a, 1984b; LaBerge and 

Klasner, 1986, 1988, 1989; Van Schmus, 1976, 1980; Sims 

and others, 1989; Greenberg and Brown, 1983), have led to 

the interpretation that the Early Proterozoic rocks and 

structures in Wisconsin are primarily the result of plate 

tectonic processes. Several major crustal domains have been 

identified, including the Early Proterozoic Wisconsin 

magmatic terranes, which are flanked on the north by 

Archean rocks of the Superior craton, and on the south by 

Archean and Proterozoic rocks of the Marshfield terrane 

(Sims and others, 1989) (fig. 1). The Niagara fault zone 

marks the boundary between Early Proterozoic igneous 

rocks of the magmatic terrane and the Archean Superior 

craton, and the Eau Pleine shear zone marks the boundary 

between the Pembine-Wausau terrane and the Marshfield 

terrane in central Wisconsin. The quartzite bodies in central 

and southern Wisconsin (fig. 1) appear to overlie Archean 

rocks of the Marshfield terrane, although some occur as 

isolated inliers within the Paleozoic cover in southern 

Wisconsin, and their relationship to basement rocks is 

problematical.

DESCRIPTION AND INTERPRETATION OF 

INDIVIDUAL QUARTZITE OCCURRENCES

Hamilton Mounds

Hamilton Mounds (fig. 1) consists of several 

unnamed quartzite outcrops (table 1) that occur as inliers in 

Paleozoic sandstone in southern Wisconsin. The mounds 

are about 75 km north of the Baraboo syncline and have 

special significance regarding the age and structure of the 

southern Wisconsin quartzites. The locality was first 

mapped by Ostrander (1931), who interpreted the structure 

as a series of N. 75° W.-trending folds with gently plunging 

axes (fig. 2A). He also reported a 25-m-thick zone of 

brecciated quartzite cemented by white vein quartz from 

most of the hills that compose Hamilton Mounds. Ostrander 

also recognized that quartz grains in the quartzite are highly 

strained and that associated sericite is aligned (foliated). He 

proposed that these features were developed by dynamic 

metamorphism. In addition, Ostrander noted that pegmatitic 

granite intrudes the quartzite and has some accompanying 

hydrothermal alteration. Ostrander assumed that the quartzite was correlative with the Baraboo Quartzite. Later, 

Greenberg and Brown (1983), and Greenberg and others 

(1986) recognized structures at Hamilton Mounds similar to

Age and Structure of Proterozoic Quartzites in Wisconsin B3
those described by Ostrander (1931). They also interpreted 

the quartzite to be correlative with the Baraboo, and 

assigned it to the Baraboo interval.

We recognize several distinct structural units in the 

main quarry at Hamilton Mounds (fig. 25). The lower part

of the quarry exposes steeply north dipping quartzite beds 

with a prominent, gently north dipping spaced cleavage (fig. 

3A). Crossbedding indicates that the beds top toward the 

southwest and are overturned in that direction. A shallow 

north-dipping fault zone with slickensides on the shear

N

EXPLANATION

Anticlinal axis 

Synclinal axis 

Fault 500 METERS

J

NE Flat-lying 

upright crossbedding

SW

Zone of distorted 

bedding with boudinage

50 METERS

_J

Vertical dip with 

south-facing 

crossbedding

Nonfoliated

1,764 Ma granite dike

Figure 2. Map and cross section of Hamilton Mounds area. A, Topographic hills (contoured in feet) and N. 75° W.-trending 

anticline and syncline axes (modified from Ostrander, 1931). B, Diagrammatic cross section showing major lithologic and 

structural features at the main quarry in the Hamilton Mounds area. Location of cross section is shown in A. Vertical scale 

exaggerated.

B4 Precambrian Geology of Lake Superior Region
Table 1. Index of Proterozoic quartzite localities and previously used historic names

Locality Historic name

Baraboo syncline, Sauk and Columbia Counties 

Barron, Rusk, Sawyer, and Washburn Counties. 

Flambeau Ridge, Chippewa County....................

Hamann Creek, Marathon County........................

Hamilton Mounds, Adams County ......................

McCaslin Mountain, Forest, Oconto, and

Langlade Counties............................................

Necedah, Juneau County.......................................

Rib Mountain, Marathon County.........................

Thunder Mountain, Marinette County .................

Vesper quarry, Wood County..............................

Waterloo area, Jefferson and Dodge Counties....,

Baraboo Quartzite (Irving, 1877). 

Barren Quartzite (Chamberlin, 1882). 

Flambeau Quartzite (Hotchkiss and others, 1915). 

Marathon Conglomerate (Weidman, 1907). 

"Hamilton Mounds Quartzite" (Ostrander, 1931).

McCaslin Quartzite (Mancuso, 1960). 

"Necedah quartzite" (Van Hise and Leith, 1911). 

Rib Hill Quartzite (Weidman, 1907). 

"Thunder Mountain Quartzite" (Mancuso, 1960). 

Powers Bluff Quartzite (Weidman, 1907). 

Waterloo Quartzite (Buell, 1892).

surfaces truncates the overturned quartzite layers. $! 

foliation strikes about N. 88° W. and dips 22° N. (fig. 3A). 

Poles to bedding (fig. 3A) lie along a great circle and define 

a beta fold axis (P) that plunges gently toward the east. 

Slickensides on the foliation surface plunge toward the 

north-northeast (fig 3B).

A 12-m-thick zone of quartzite with highly contorted 

bedding and boudinage structures (fig. 25) overlies the 

slickensided fault zone. The presence of a Z-shaped fold 

(looking southeast) and a boudinaged quartzite layer (fig. 

25) suggests that movement was from northeast to 

southwest.

The breccia zone (figs. 2B, 3C) described by 

Ostrander (1931) overlies this contorted zone. It contains 

common tabular quartzite clasts, some with strong internal 

foliation, and rare exotic granitoid clasts. Although not 

obvious on first examination, poles to the strike and dip of 

the flat surfaces of 12 sample quartzite clasts tentatively 

indicate that they have a preferred orientation. The clasts 

have about the same strike as the breccia zone, but dip 

toward the northeast slightly steeper than the breccia zone 

itself, suggesting that they have an imbricate orientation 

relative to the tectonic zone (sketch within stereoplot of fig. 

3C shows the nature of the oriented clasts). This 

configuration also provides kinematic data suggesting 

tectonic transport from northeast to southwest. Above the 

breccia zone, the bedding in the quartzite is subhorizontal 

with upright crossbedding.

The presence of boudinaged beds, ductile folds, and 

contorted bedding indicates that the rocks at Hamilton 

Mounds were subjected to ductile deformation. Spaced 

cleavage and the brecciated zone of quartzite, however, 

indicate brittle deformation. A southward sense of tectonic 

transport is indicated for both the ductile and brittle 

deformation. Interestingly, the main zone of ductile 

deformation lies between zones of brittle deformation with

a fault zone at the base and the breccia zone at the top. 

These structural relations are similar to those shown by the 

Hinesburg thrust in the northern Appalachians near Burlington, Vt.: the Cheshire Quartzite (Lower Cambrian), 

containing prominent ductile deformation features, has been 

thrust over relatively undeformed carbonates of the Lower 

Ordovician Bascom Formation (Stanley and others, 1987). 

As concluded for the Vermont structures by Stanley and 

others (1987), so at Hamilton Mounds the superposition of 

brittle deformational features on ductile deformational 

features likely reflects multiple periods of deformation.

A hydrothermally altered granitoid dike (as described 

by Ostrander, 1931) intrudes the quartzite beneath the fault 

zone (fig. 25). We interpret the numerous aligned mafic 

xenoliths and megacrysts in the dike to define a flow fabric 

that strikes N. 5° E. and dips 35° SE. This orientation differs 

markedly from the cleavage in the surrounding quartzite, 

which strikes N. 88° E. and dips 22° N. (fig. 3A), and 

indicates that the granitoid dike was not deformed with the 

quartzite. W.R. Van Schmus (written commun., 1989) 

reported a U-Pb age of 1,763±7 Ma on zircons from the dike, 

and a U-Pb age of 2,449±84 Ma on detrital zircon in the 

quartzite.

Interpretation. Quartzite exposed at Hamilton 

Mounds exhibits a variety of deformational styles, which 

reflect a complex structural history, indicating the juxtapositioning of ductile and brittle structures. The ductile 

deformational features must have been imposed under very 

different conditions than those that produced the brittle 

deformation features. A southward sense of tectonic 

transport is indicated by both the ductile and brittle deformational features. Because the brittle and ductile structures 

are similarly oriented, we suggest that the superimposed 

structural units were tectonically emplaced by multiple 

coaxial south-verging thrust events. The granitoid dike that 

cuts the quartzite constrains the age of deformation and the

Age and Structure of Proterozoic Quartzites in Wisconsin B5
HAMILTON MOUNDS

HAMILTON MOUNDS NECEDAH QUARRY

Figure 3. Lower hemisphere stereographic plots and 

accompanying diagrams illustrating the main structural 

features at Hamilton Mounds and Necedah localities. A, Dots 

represent poles to bedding (S0) and define a great circle whose 

beta axis ((5) plunges gently east-southeast at Hamilton 

Mounds. Poles to foliation (Sv X) define an average foliation 

surface (stippled S, plane) that is axial-planar to folds in 

bedding (S0). B, Orientation of slickenside lineations on S^ 

surfaces at Hamilton Mounds. C, Dots represent 12 poles to 

strike and dip of quartzite clasts in breccia zone at Hamilton 

Mounds. Nature of quartzite clasts shown diagrammatically. 

D, Combined plot of field measurements from the north and 

south quarries at Necedah. Dots, poles, and stippled plane 

show orientation of spaced cleavage (S,); heavy solid line, 

strike of bedding. Beds are vertical to near vertical. Square, 

mineral lineation on S-, surface at north quarry; a drag fold with 

inferred shear couple suggests southward structural vergence drag fold was observed at south quarry.

age of the quartzite. Deformation of the quartzite preceded 

emplacement of the dike (1,763±7 Ma) indicating that both 

the quartzite and the deformation are older than 1,763±7 

Ma. The 2,449±84 Ma U-Pb age on detrital zircon in the 

quartzite at Hamilton Mounds suggests a probable Archean, 

rather than Proterozoic, provenance for the quartzite.

Necedah Area

Two inliers of Necedah quartzite (table 1) were 

examined in quarries near the village of Necedah (fig. 1), 30 

km west-southwest of Hamilton Mounds. Red-shaded to 

pink brecciated quartzite is exposed in a quarry just

northwest of Necedah. The quartzite dips steeply and has a 

fracture cleavage that strikes northwest and dips gently 

southwest (fig. 3D). Mineral lineations on the cleavage 

surfaces are generally perpendicular to the strike of the 

foliation and possibly indicate a stretch direction. Thinbedded, weakly foliated quartzite is exposed in an easttrending ridge of outcrops at the south edge of Necedah. 

Bedding is vertical and crossbedding indicates that the beds 

face southward. The south-facing crossbeds indicate that 

bedding was rotated toward the south, and a small drag fold 

also indicates a southward sense of structural vergence.

Van Hise and Leith (1911, p. 358) reported that 

"Drilling at Necedah has disclosed the presence of granite, 

probably intrusive into quartzite," and B.A. Brown and J.K. 

Greenberg (Greenberg and others, 1986) reported a quartzite breccia at Necedah with a granitic matrix (an intrusion 

breccia). The granite appears to be undeformed, and must, 

therefore, postdate the deformation. The age of the granitoid 

rock at Necedah is not known.

Interpretation. Although data at the Necedah 

locality are meager relative to Hamilton Mounds, the major 

structure is consistent with a south-verging fold/thrust and 

the igneous rock appears to postdate the structural event. 

The major relationships are thus the same at both localities. 

However, the lack of isotopic age on the granitoid at 

Necedah precludes establishing the timing of the events.

Baraboo Area

The general geology of the Baraboo area (fig. 1) has 

been known since the early studies of Irving (1877), 

Weidman (1904), Van Hise and Leith (1911), and Leith and 

others (1935). More recent studies include those of Dalziel 

and Dott (1970), Dott (1983), Smith (1978a,b), Van Schmus 

(1978), Jenk and Cambray (1986), Kean and Mercer (1986), 

and Hempton and others (1986). Proterozoic rocks at 

Baraboo, which structurally define a northeast-trending 

doubly plunging syncline (figs. 4, 5), consist of massive 

Baraboo Quartzite (table 1) overlain by the Seeley Slate and 

the Freedom Formation, which consists of dolomite, slate, 

and cherty zones, some of which are ferruginous. The 

ferruginous chert of the Freedom Formation is similar to 

iron-formations of the main Lake Superior region to the 

north (Schmidt, 1951). The Dake Quartzite and Rowley 

Creek Slate are interpreted to overlie the Freedom 

Formation (Leith and others, 1935). Morey and Van 

Schmus (1988) implied that the Freedom Formation/Dake 

Quartzite contact is erosional. These units are similar to 

underlying units, and Schmidt (1951) suggested that they 

may be segments of the Baraboo Quartzite and Seeley Slate 

that are repeated by faulting. Volcanic rocks (mainly 

rhyolite) and granitoid rocks are exposed at several 

localities surrounding the quartzite of the Baraboo syncline 

(fig- 5).

B6 Precambrian Geology of Lake Superior Region
Rhyolite pebbles 

in quartzite

North-dipping

cleavage in 

Paleozoic rhyolite.

South-dipping 

cleavage in 

rhyolite pa | eozolc

Rowley Creek Slate 

Dake Quartzite

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; :{;;; ? %>/ * v- ^ ^J^TT^Tvr^T^v: ,\%v^ vV,v:- .-

r J t -> *- * VV 7 Older Precambriani- " -» "tl "A ii' v jj ,7 " T u A i 'I'-j > Pt v < v i v- r

v A r vA"i- rr t^r r L A ^-'"r.vri.ni " * 1 ., u j -, ^ ^- J -> v >< ^ "* " > <. r <

Figure 4. Generalized cross section of the Baraboo syncline. 

Rhyolite underlies the quartzite on both north and south limbs; 

rhyolite pebbles occur in the basal conglomerate. Bedding and 

cleavage in the rhyolite are parallel to that in the quartzite on 

north limb, suggesting that the units were deformed together. 

Rhyolite on the south limb has south-dipping cleavage that is

similar to mylonitic foliation in the quartzite and parallel to 

axial planes of "reverse" drag folds on the south limb. The 

undeformed Baxter Hollow Granite on south limb intrudes 

foliated rhyolite, suggesting that the intrusion postdated the 

deformation. Length of section = 20 km. Vertical, not to 

scale.

The structural and stratigraphic relationship of the 

quartzite to the adjacent volcanic and plutonic rocks is of 

major importance in resolving the age and origin of the 

rocks in the Baraboo area. However, the nature of the 

relationship has been debated for nearly a century. Weidman (1904) discussed the schistose nature of the rhyolite 

near the contact with the quartzite and suggested that the 

rhyolite was part of the sedimentary sequence. He also 

reported the presence of rhyolite pebbles in conglomerates 

in the basal quartzite. Van Hise and Leith (1911, p. 360) 

stated that granitic rocks on the south side of the Baraboo 

syncline are intrusive into the quartzite. Stark (1930, 1932) 

concluded that the rhyolites are tuffs and flow breccias and 

that they are older than the Baraboo Quartzite, and further, 

he questioned whether the "Otter Creek granite" was 

intrusive into the quartzite. Gates (1942), however, 

concluded that the Baxter Hollow Granite ("Otter Creek 

granite" of Stark, 1932) intrudes the quartzite. Dalziel and 

Dott (1970) interpreted the quartzite to unconformably 

overlie the rhyolite and the granite surrounding the Baraboo 

syncline. Smith (1978b) suggested that an angular 

unconformity exists between the rhyolite and quartzite in 

the Baraboo area, where he reported that the rhyolite strikes 

at an angle to the quartzite. However, he did not report dip 

directions or the presence of foliation in the rhyolite.

Numerous structural studies of the quartzite and 

associated metasedimentary rocks have been carried out 

(Riley, 1947; Hendrix and Schaiowitz, 1964; Dalziel and 

Dott, 1970; Hempton and others, 1986). Although points of 

disagreement remain, the major aspects of the structure 

appear well established. The major structure in the Baraboo 

area is an east-northeast-trending doubly plunging syncline 

overturned toward the south (fig. 4). The axial plane of the 

syncline strikes N. 75° E. and dips 60° N., with the north

limb vertical, or locally overturned toward the south, and 

the south limb dipping gently toward the north.

The overall structural configuration of the Baraboo 

syncline as shown in Dalziel and Dott (1970) indicates that 

it was formed by south-verging tectonic transport. Dalziel 

and Stirewalt (1975) suggested that major and minor 

structures formed in a single progressive deformational 

event, whereas Hendrix and Schiaowitz (1964), Riley 

(1947), and Hempton and others (1986) suggested at least 

two tectonic events, the first being a south-verging event 

that formed the syncline and its major deformational fabric. 

Cambray (1987) suggested that the syncline is a sheath fold 

caused by southerly directed shearing.

We have focused on resolving the controversy 

regarding the stratigraphic relationships of the Baraboo 

Quartzite and the adjacent igneous rocks, because of the 

longstanding nature of the problem and the importance of 

determining these relationships.

At the lower narrows on the north limb of the syncline 

(fig. 5), the rhyolite is brick red near the contact with the 

quartzite, but it turns to black within a few hundred meters 

of the contact. Conglomeratic layers near the base of the 

quartzite (outcrops in SWV4SWV&NEV4 sec. 22, T. 12 N., R. 

7 E.) contain clasts of rhyolite, vein quartz, jasper, and 

minor granitoid rocks. Primary layering in the rhyolite is 

expressed locally as layers of volcanic clasts as much as 

several centimeters in diameter. Orientation of this layering 

(fig. 6A) is approximately the same as the orientation of 

bedding in the adjacent quartzite on the north limb (compare 

with stereoplot 1A, pi. V of Dalziel and Dott, 1970). A 

prominent N. 72° E., 46° NW. foliation that is developed 

locally in the rhyolite near the quartzite is compatible with 

the foliation in the quartzite of the north limb (see stereoplot 3 A, pi. V, of Dalziel and Dott, 1970). Bedding/cleavage

Age and Structure of Proterozoic Quartzites in Wisconsin B7
EXPLANATION

Quartzite 

Granite 

Porphyritic rhyolite

Rhyolite

Approximate contact 

Axis of Baraboo syncline

Figure 5. Generalized geologic map of the Baraboo syncline and associated rhyolite. Locations of areas discussed in text are 

also shown. Heavy solid lines, boundaries between belts of igneous rocks. Dated igneous rocks are at Montello, Observatory 

Hill, Marquette, and Utley. Modified from Smith (1978a).

intersection lineations in the rhyolite define a gently westsouthwest plunging fold axis (fig. 6B\ roughly parallel to 

the fold axis of the Baraboo syncline, and similar to the 

bedding/cleavage intersection lineations of Dalziel and Dott 

(stereoplot 4A, pi. V, 1970). Therefore, the rhyolite and 

quartzite on the north limb of the syncline appear to have 

been deformed together. Furthermore, the presence of red 

rhyolite clasts as much as 3 cm in diameter in 

conglomerates near the base of the quartzite suggests that 

the quartzite may stratigraphically overlie the rhyolite on

the north limb. It should be noted, however, that the rhyolite 

within approximately 30 m of the contact has a strong 

foliation parallel to the contact with the quartzite.

Structural relationships between the quartzite and 

adjacent igneous rocks on the south side of the Baraboo 

syncline are more complex than those on the north limb. 

Rhyolite in the southernmost outcrops at Caledonia Church 

(fig. 5) is a gray to black plagioclase phyric rock with 

well-preserved primary features suggesting densely welded 

tuff and flow-breccia; it has no visible tectonic foliation.

B8 Precambrian Geology of Lake Superior Region
However, rhyolite closer to the quartzite has a prominent 

south-dipping foliation (fig. 6C); the northwest-dipping 

foliation, which is well developed in the rhyolite on the 

north limb, is absent in the rhyolite on the south limb. The 

nearby Baraboo Quartzite has a steeply northwest dipping 

cleavage (Dalziel and Dott, 1970), and locally, near the 

foliated rhyolite, a south-dipping foliation expressed by 

highly recrystallized (mylonitized) quartzite a fine mosaic 

of quartz that largely obscures the original detrital grains 

(figs. 4, 5, locations).

LaBerge's studies of engineering drill cores from the 

Badger Ordnance area just south of the south limb of the 

Baraboo syncline (fig. 5) show that Baraboo Quartzite 

overlies dark-reddish-brown to greenish-gray volcanic rock, 

which in turn is intruded by a granitoid breccia (Baxter 

Hollow Granite?). The basal quartzite in most of the drill 

cores is a breccia consisting of clasts of various size. In 

some cores, the quartzite forms lensoidal slabs surrounded 

by branching and recombining fractures, and in still other 

cores the quartz in the quartzite is highly polygonized and 

has a prominent mylonitic foliation. White clay (kaolinite?) 

forms the matrix between the clasts and coats fracture 

surfaces, and drusy quartz lines some fractures and cavities. 

In several cores, the base of the quartzite is several meters 

of clay that may be a fault gouge.

The volcanic rock in the engineering drill cores at 

Badger Ordnance is highly chloride andesitic to basaltic 

rock with highly foliated, slickensided zones as well as 

relatively nonfoliated zones. Thus, it contrasts with the 

felsic volcanic rocks in the Caledonia Church area. 

However, near Denzer, approximately 13 km west of 

Badger Ordnance, black undeformed rhyolite is exposed 

just south of the Baraboo Quartzite (fig. 5). Therefore, a 

variety of volcanic and intrusive rocks with varying degrees 

of deformation are present on the south side of the Baraboo 

syncline, whereas only red to gray rhyolite is exposed on the 

north limb.

The granitoid rock (Baxter Hollow Granite?) 

encountered in the drill cores is an intrusion breccia that is 

highly variable in texture and color, contains numerous 

greenish-gray volcanic xenoliths, and is undeformed. 

Quartzite xenoliths are absent. Some intervals of the quartzite in the core are highly bleached and (or) epidotized, 

suggesting hydrothermal activity.

Interpretation. The data presented raise some 

questions regarding the relationship of the rhyolites to the 

quartzite in the Baraboo area. Although the quartzite may be 

stratigraphically younger than the rhyolite on the north 

limb, where both quartzite and rhyolite were affected by the 

deformational event that formed the Baraboo syncline, the 

relationship of the quartzite to the rhyolite on the south limb 

of the syncline is ambiguous. The rhyolite near Caledonia 

Church and at Denzer lacks the Sj fabric related to 

formation of the Baraboo syncline, but near the contact with 

the quartzite in the Caledonia Church area the rhyolite has

Figure 6. Lower hemisphere plots of structural features in the 

Baraboo syncline. A, Seven poles to layering (S0, dots) in 

rhyolite near the Lower Narrows, north limb. Stippled area, 

approximate orientation of bedding plane. B, Thirty-nine poles 

(dots) to ST foliation in rhyolite at the Lower Narrows and five 

measured lineations (x) resulting from intersection of cleavage 

(S,) and bedding (S0). Stippled area, approximate orientation of 

S, plane. C, Seventeen poles to foliation in rhyolite (R), 

Caledonia Church area, and nine poles to foliation in quartzite 

(Q) near the rhyolite. Stippled area, approximate plane of 

orientation of foliation in rhyolite in this area. Difference in 

orientation of foliation between rhyolite (R) and quartzite (Q) is 

interpreted to be caused by refraction resulting from different 

lithologies. D, Eighteen poles to bedding on south limb of 

Baraboo syncline; stippled area, plane of approximate 

orientation of bedding. E, Twenty poles to S, on south limb of 

the Baraboo syncline; stippled area, plane of approximate 

orientation of bedding. F, Three poles to spaced axial plane 

foliation of chevron folds in S^ foliation at Skillet Creek (see 

fig. 5), and orientation of four fold axes (x). Stippled plane, 

approximation of the axial plane foliation.

a post-Si fabric. This suggests that deformation in the 

rhyolite on the south limb may be younger than the 

structural event that formed the Baraboo syncline, and that

Age and Structure of Proterozoic Quartzites in Wisconsin B9
the igneous rocks may be younger than the quartzite. 

Alternatively, the rhyolite and other igneous rocks on the 

south limb lie stratigraphically below the quartzite, but an 

S, fabric simply did not form in this area during formation 

of the Baraboo syncline. (It should be noted that quartzite 

within 100 m of the rhyolite does have an Sj fabric.) The 

structural differences between the volcanic rocks on 

opposite limbs of the syncline suggest the possibility of 

more than one age of rhyolite in the Baraboo area. If more 

than one rhyolite is present, then the proposed post-1,760 

Ma age of the Baraboo Quartzite, based on dating of 

rhyolite some 25-50 km northeast of the Baraboo area, is 

uncertain.

A post-1,760 Ma age for the Baraboo Quartzite is 

based on the following assumptions: (1) the quartzite 

stratigraphically overlies the adjacent rhyolite (Dalziel and 

Dott, 1970), and (2) the rhyolite in the Baraboo area is the 

same age as the rhyolite from 25-50 km northeast of it 

(dated igneous rocks noted in fig. 5 are samples dated by 

Van Schmus, 1978; Smith, 1978a). Smith (1978a, 1983) 

used major- and trace-element geochemistry to suggest the 

existence of a series of northeast-trending belts of igneous 

rocks in south-central Wisconsin, and based on Van 

Schmus' (1978, 1980) U-Pb zircon age of 1,760 Ma, he 

concluded that the rhyolite at Baraboo can be inferred by 

chemical similarity to correlate with the dated rocks. 

However, new compositional data (K.J. Schulz, written 

commun., 1988), on granite and rhyolite at Gary Mound, 10 

km north of Veedum in central Wisconsin, are similar to 

those described by Smith (1978a,b, 1983), yet they yield a 

U-Pb zircon age of 1,833±4 Ma (W.R. Van Schmus, written 

commun., 1989). This suggests that chemical similarity 

alone may not be a valid basis for inferring the age of the 

rhyolite in the Baraboo area; we suggest that the age of the 

rhyolite in the Baraboo area is still in question.

Waterloo Area

In the Waterloo area, approximately 50 km eastsoutheast of the Baraboo syncline (fig. 1), occur two 

distinctly different quartzites, which historically were 

named the Waterloo Quartzite (table 1) in Jefferson and 

Dodge Counties. One exposure, in a farmyard in the 

NE1/4SE'/4 sec. 35, T. 9 N., R. 13 E., is a thin-bedded, 

red-brown, prominently crossbedded quartzite. The other is 

a gray quartzite exposed at Portland quarry (E'/aSEV^ sec. 

33, T. 9 N., R. 13 E.) and near Reeseville (NW^SE^ sec. 

3, T. 9 N., R. 14 E.). Our studies indicate that the red and 

gray quartzites have had different deformational histories 

and could be of different ages.

The red quartzite consists of thin (=1 cm) layers of 

buff to red quartzite with ~1 -cm-thick pelitic layers. 

Bedding in the quartzite is oriented N. 4° E., 30° SE. (fig. 

1A). The quartzite lacks a distinct deformational fabric, but 

the pelitic layers have a penetrative cleavage that strikes N.

19° W. and dips 20° NE. Bedding/cleavage intersection 

lineations plunge 12° N. 28° E. (fig. 1A).

The gray quartzite at Portland quarry is conglomeratic, containing pebbles of maroon to red quartzite, 

vein quartz, chert, and volcanic rocks. Bedding is oriented 

approximately N. 20° E., 54° SE. and cleavage is oriented 

N. 59° W., 60° SW. (fig. IB}; bedding/cleavage intersection 

lineations plunge 53° S. 42° E. Pebbles within some layers 

in the gray quartzite are stretched and have longrshort axial 

ratios of as much as 4:1, suggesting constrictive strain. 

Stretch lineations defined by the long axis of the pebbles 

plunge about 70° WSW.

Red quartzite 

Waterloo

Gray quartzite 

Reeseville

D

Gray quartzite 

Portland quarry

Vesper quarry

Figure 7. Lower hemisphere plots of structural data from 

Waterloo and Vesper areas. A, Red quartzite at Waterloo. Solid 

dots and stippled plane indicate orientation of bedding (S0); x 

and lined plane indicate orientation of cleavage (S-,) in thin 

pelitic layers. Solid squares are measured lineations formed by 

intersection of cleavage and bedding (Lbc). B, Cray quartzite at 

Portland quarry. Solid dot, pole to bedding (S0); stippled plane, 

approximate orientation of bedding; x, pole to spaced 

cleavage (S,); lined plane, approximate orientation of spaced 

cleavage (S,) in quartzite. Solid square, orientation of lineation 

(intersection of bedding and cleavage, Lbc). Open circle, 

orientation of long axis of stretched clast (Ls). C, Gray quartzite 

near Reeseville. Solid dot, pole to bedding; stippled plane, 

approximate orientation of bedding (S0); x, pole to S, foliation; 

lined plane, approximate orientation of foliation (S,). Solid 

triangle, pole showing orientation of joints (J); open circle, pole 

to a spaced cleavage/joint system (SN). D, Stereoplot of spaced 

cleavage in quartzite at Vesper quarry. Contours 5, 10, 20 

percent are on 21 poles to cleavage (S,). Solid dot, area of 

highest concentration. Stippled plane, orientation of S, 

cleavage.

B10 Precambrian Geology of Lake Superior Region
The quartzite at Reeseville is dark gray (like that at 

Portland quarry), thin bedded with abundant dark- and 

light-gray bands, crossbedded, and locally conglomeratic; it 

contains minor pelitic layers. Crossbedding indicates that 

the beds are upright. Northwest-trending joints abound, and 

some are in closely spaced zones. An irregular, northwesttrending, 10-m-wide zone of brecciated quartzite cemented 

with white vein quartz cuts the quartzite. Bedding is 

oriented N. 50° E., 42° SE., and a spaced cleavage is 

oriented N. 78° W., 50° SW. (fig. 7C). Joints are near 

vertical and strike about N. 40° W. The breccia zone is 

roughly parallel with the joints. A vague, spaced fracture 

(joint?) system SN oriented approximately N. 50° W., 36° 

NW. is also present; however, its relative age and 

significance are not known.

Interpretation. Our data indicate that the structural 

features in the red quartzite are distinctly different from 

those in the gray quartzite. The red quartzite lacks a strong 

deformational fabric, but cleavage in pelitic layers dips 

gently east-northeast, and fold axes (determined from bedding/cleavage intersection lineations) plunge gently northnortheast. In contrast, foliation in the gray quartzite strikes 

northeast and dips moderately to the southeast; fold axes 

plunge steeply southeast. Not only do these attitudes 

suggest that the structural history of the quartzites is 

different, but also the presence of pebbles of red quartzite in 

the gray quartzite at Portland quarry suggests in particular 

that the gray quartzite may be younger than the red, even 

though the gray quartzite is more deformed.

The gray quartzite at Waterloo, the rhyolite and 

adjacent quartzite near Caledonia Church, and the reverse 

drag folds (chevron folds) at the Skillet Creek locality on 

the south limb of the Baraboo syncline (Adair, 1956; Dalziel 

and Dott, 1970; Hendrix and Schaiowitz, 1964) have a 

south-dipping deformational fabric. The south-dipping 

fabric may thus be of regional significance, and it could 

represent a structural event that postdates formation of the 

major structure of the Baraboo syncline. However, this 

fabric apparently is absent in rocks on the north limb of the 

Baraboo syncline, and does not occur in the red quartzite at 

Waterloo. Consequently, the event that produced the southdipping fabric apparently did not affect all of the rocks in 

the region.

Geiger and others (1981) concluded that the pelitic 

rocks intercalated with the quartzite in the Waterloo area 

underwent a static metamorphism that postdates the 

deformation. Inasmuch as undeformed granitic dikes cut the 

quartzite on Rocky Island in the Crawfish River northeast of 

Waterloo (Brown, 1986), we suggest that the metamorphism is related to the igneous activity in the area, and 

that both postdate the deformation. Unfortunately, the age 

on the igneous rocks in the Waterloo area is poorly 

constrained.

Probable intrusive contact

'Hi "

Deformed 

quartzite

V + + ^+ + + Undeformed 1,835 Ma

+ + + + red granite + 4- + + + +

Figure 8. Interpretation of geologic relationships between 

undeformed granite and deformed quartzite at Vesper quarry. 

Length of section =50 m. Vertical, not to scale.

Vesper Quarry

The Vesper quarry in Wood County (fig. 1) contains 

exposures of pink quartzite historically named the Powers 

Bluff Quartzite (table 1), containing minor pelitic zones and 

massive red granite (fig. 8). The quartzite has a pervasive, 

steeply dipping, spaced cleavage; pelitic zones in the 

quartzite have a penetrative cleavage subparallel with that in 

the quartzite. A stereoplot of poles to fracture cleavage and 

penetrative cleavage at Vesper quarry shows that the 

cleavage is oriented N. 85° E., 80° NW. (fig. ID}. The red 

granite, which crops out within several meters of the 

quartzite, has no discernible deformational fabric. Although 

the contact between the granite and quartzite is not exposed, 

a highly altered, iron oxide-rich zone of quartzite lies within 

the contact zone. Inasmuch as the granite lacks any 

penetrative fabric, we suggest that it was emplaced after 

deformation of the quartzite.

The red granite at Vesper is typical of the widespread 

alkali-feldspar granites in central Wisconsin (LaBerge and 

Myers, 1984; Sims and others, 1989). They are undeformed 

and posttectonic. The granite at Vesper is mineralogically, 

texturally, and chemically similar to the red alkali-feldspar 

granite at Gary Mound about 15 km to the southwest (KJ. 

Schulz, written commun., 1989), which has a U-Pb zircon 

age of 1,833±4 Ma (Sims and others, 1989).

Interpretation. Assuming that the red granites at 

Vesper and Gary Mound are coeval, then the deformed 

quartzite at Vesper quarry must be older than 1,833+4 Ma. 

This interpretation is supported by the occurrence of large 

blocks of quartzite at a red granite quarry on Gary Mound 

(Greenberg and Brown, 1983, p. 27).

Veedum Area

Exposures in quarries at Veedum in Wood County 

(fig. 1), about 10 km south of Gary Mound, consist of highly 

sheared unnamed quartzite that contains a green chromerich mica (probably fuchsite) on nearly horizontal slickensided fault surfaces. Foliation appears to be nearly

Age and Structure of Proterozoic Quartzites in Wisconsin B11
horizontal in the northernmost pit, but the outcrop is badly 

broken and structural observations are difficult to obtain. 

B.A. Brown (Greenberg and others, 1986, p. 46) reported 

that Archean gneiss is exposed in Turner Creek just east of 

the Veedum quarries, and that drilling encountered Archean 

gneiss 3 to 4 m below the floor of the southernmost quarry. 

Structures in Archean gneiss in nearby quarries are steep to 

vertical, markedly different from those in the overlying 

quartzite.

Interpretation. The slickensides and near-horizontal 

foliation suggest that the quartzite was emplaced on the 

gneiss by low-dipping (possibly thrust) faults. Alternatively, 

the quartzite could have been deposited directly on the 

Archean rocks.

volcanic rocks of the Marshfield terrane, which has been 

dated at 1,858±5 Ma (Sims and others, 1989).

Interpretation. These quartzite occurrences define 

an easterly trending belt of conglomeratic rocks in central 

Wisconsin that roughly parallels the trend of the Eau Pleine 

shear zone to the south (fig. 1). The east-trending zone of 

quartzite-bearing conglomerate indicates that an older 

quartzite and iron-formation unit was being eroded during 

deposition of the =1,860-1,840 Ma volcanic rocks. Thus, 

quartzite with a minimum age of =1,840 Ma existed in 

central Wisconsin during deposition of the conglomerate.

Rib Mountain

Conglomerate at Hamann Creek

Four occurrences of conglomerate interbedded with 

volcanic rocks of the Wisconsin magmatic terranes have 

been mapped in central Wisconsin, and several other 

occurrences were recorded and named the Marathon 

Conglomerate (table 1) by Weidman (1907). The largest 

exposure is along Hamann Creek in southwestern Marathon 

County (SEV4 sec. 35, T. 28 N., R. 3 E.). Other exposures 

are in the western margin of Marathon City, along the 

Wisconsin River just west of Brokaw, and along the East 

Fork of the Eau Claire River in eastern Eau Claire County 

about 100 km west of Wausau (fig. 1). The conglomerate 

along Hamann Creek contains boulders as much as 1 m long 

of both red and gray quartzite, jaspilitic iron-formation, 

volcanic rocks, and granitoid rocks; it is interbedded with 

graywacke and intermediate and felsic volcanic rocks. The 

boulders are matrix supported, and the conglomerate has 

been intensely deformed (fig. 9) in local zones that are as 

much as 20 m wide (LaBerge and Myers, 1984). 

Deformation in the conglomerate is similar to that in the 

surrounding graywacke and volcanic rocks. The volcanic 

rocks are part of a succession (Pembine-Wausau terrane) 

that has been dated at =1,840 Ma (Sims and others, 1989). 

About 3 km south of the Hamann Creek exposures the 

major zone of deformation is cut by a posttectonic granite 

(LaBerge and Myers, 1983).

Relatively undeformed conglomerate containing 

quartzite and granitoid boulders as much as 25 cm in 

diameter in a volcanic sandstone matrix is exposed along 

the Wisconsin River at Brokaw, just north of Wausau. Both 

red and gray quartzite clasts are present. A deformed 

conglomerate with boulders of quartzite and volcanic rocks 

was exposed in excavations in Marathon City, west of the 

syenite plutons at Wausau. These conglomerates are 

enclosed in volcanic and plutonic rocks of the PembineWausau terrane. A similar deformed conglomerate 

containing gray quartzite cobbles and boulders is also 

exposed along the East Fork of the Eau Claire River (Myers, 

1980). This conglomerate is associated with deformed

Quartzite xenoliths are common in the ring-shaped 

1,470 Ma alkalic plutons near Wausau. Most of these 

xenoliths are white quartzite; some, however, are red 

quartzite. The largest of the white quartzite xenoliths forms 

the crest of Rib Mountain, historically named the Rib Hill 

Quartzite (table 1) by Weidman (1907) (this report, fig. 1). 

The alkalic plutons intrude calc-alkaline volcanic and plutonic rocks of the Pembine-Wausau terrane (LaBerge and 

Myers, 1983). Although the volcanic rocks surrounding the 

plutons at the present erosion level are metamorphosed only 

to greenschist facies, several of the quartzite xenoliths 

contain highly flattened quartz grains and strongly lineated 

sillimanite.

Interpretation. LaBerge and Myers (1984) 

interpreted the relations at Rib Mountain to indicate that the 

quartzite was subjected to amphibolite-grade regional metamorphism, and that the quartzite xenoliths were carried up 

through the 1,840 Ma greenschist facies metavolcanic rocks 

during intrusion of the syenitic magma (fig. 10). This 

interpretation requires that the quartzite be older than 1,840 

Ma.

Quartzite at McCaslin and 

Thunder Mountains

The quartzite at McCaslin Mountain and adjacent 

areas, historically named the McCaslin Quartzite (table 1) 

by Mancuso (1960), is exposed in hills along the northern 

margin of the Wolf River batholith in northeastern 

Wisconsin (fig. 1) (Sims, 1990). Olson (1984) reported that 

the quartzite at McCaslin Mountain is at least 1,220 m thick. 

It contains lenticular conglomerates in the lower 300 m, but 

consists mainly of a mature quartz arenite. Although the 

base is not exposed, the quartzite is interpreted to unconformably overlie mafic to felsic volcanic rocks, which are 

assumed to be part of the older (1,889-1,860 Ma) volcanic 

succession of the Pembine-Wausau terrane (Sims and 

others, 1989). The quartzite was folded into a south westB12 Precambrian Geology of Lake Superior Region
Figure 9. Quartzite boulders in deformed conglomerate along Hamann Creek. A, 

Relatively undeformed conglomerate with abundant volcanic clasts in the matrix. B, 

Flattened quartzite boulders in conglomerate about 30 m from locality of A.

plunging syncline (Olson, 1984) after regional deformation 

of the underlying volcanic rocks (Sims and others, 1989). 

Olson reported an increase in metamorphic grade from 

epidote-albite hornfels to hornblende hornfels facies from 

west to east in the quartzite, and she attributed the metamorphism to intrusion of phases of the Wolf River batholith. 

Sims and others (1988) suggested that quartzite clasts in the 

nearby Baldwin Conglomerate were derived from this 

quartzite, and that the Baldwin Conglomerate was deformed 

in the Mountain shear zone prior to intrusion of the Hines

Quartz Diorite, which has been dated at 1,812.7±3.6 Ma 

(Sims and others, 1990).

Interpretation. If the quartzite clasts in the Baldwin 

Conglomerate were derived from nearby sources at McCaslin and Thunder Mountains, as seems almost certain, 

these quartzites are definitely older than 1,812 Ma. The 

Mountain shear zone is a post-Penokean discrete ductile 

deformation zone (Sims and others, 1990) that was intruded 

by the 1,812.713.6 Ma Hines Quartz Diorite (Sims and 

others, 1988).

Age and Structure of Proterozoic Quartzites in Wisconsin B13
Lineated sillimanite-bearing quartzite

Deformed

greenschist facijss

j 1,840 Ma "

'-yplpanic rocks

Figure 10. Interpretation of geologic relationships of quartzite at Rib Mountain, Wausau area. Lineated sillimanite-bearing 

(amphibolite facies) quartzite is at the same present topographic level as greenschist facies volcanic rocks, suggesting 

that the quartzite has been transported upward to its present 

position. Length of section ~25 km. Vertical, not to scale.

Barron Quartzite and Quartzite at 

Flambeau Ridge

The Barren Quartzite (table 1) crops out in an area of 

about 480 km2 in parts of Barron, Rusk, Sawyer, and 

Washburn Counties in northwestern Wisconsin (fig. 1). 

According to Hotchkiss and others (1915) and Rozacky 

(1987), the major lithology is pink to red quartz-cemented 

quartz arenite, with interbedded red argillite and a thin basal 

conglomerate. Pebbles in the conglomerate are mainly 

white quartz, but include volcanic rocks, granitoid rocks, 

and iron-formation. Drill cores show that the formation is at 

least 198 m thick, and may be as much as 400 m thick, 

although the upper contact is not exposed (Rozacky, 1987). 

Rozacky suggested that crossbedding indicates deposition 

on a braided alluvial plain succeeded upward by a marine 

shelf environment; the postulated source area is to the north 

and comprises mainly sedimentary rocks. The Barron 

Quartzite is almost undeformed, with dips averaging 

5°-10°; it is unmetamorphosed (Hotchkiss and others, 1915; 

Rozacky, 1987). It uncomformably overlies a regolith 

developed on igneous and metamorphic rocks of the 

Pembine-Wausau terrane.

The quartzite at Flambeau Ridge forms a much 

smaller area than the Barron Quartzite. It is located in 

northern Chippewa County (fig. 1) and lithologically 

resembles the Barron Quartzite (Campbell, 1986). 

Exposures comprise approximately 700 m of conglomeratic 

quartzite, divisible into three major units: (1) A 50-m-thick 

basal conglomeratic unit with abundant subangular clasts (5 

cm) of iron-stained slate (probably derived locally from an 

underlying regolith), overlain by approximately 100 m of

crossbedded maroon quartzite containing scattered slate 

clasts. The slate clasts are embedded in a matrix of 

subrounded iron-stained quartz. (2) A 450-m-thick middle 

conglomeratic unit that contains subrounded clasts (5 cm) of 

vein quartz, granular iron-formation (jasper), pale-yellow 

metachert, and minor pale-green metavolcanic rocks. (3) An 

upper 100-m-thick well-cemented pale-orange crossbedded 

quartzite. Campbell (1986) reported that the matrix in the 

quartzite is dominantly sericite and kaolinite; therefore, the 

rocks are virtually unmetamorphosed. The contact between 

the quartzite and underlying rocks is not exposed; however, 

drill holes in the surrounding area (Campbell, 1986; 

LaBerge, 1988) encountered volcanic and plutonic rocks 

typical of the Pembine-Wausau terrane. The penetration of 

these rocks suggests that the quartzite unconformably 

overlies rocks of Early Proterozoic age, as does the Barron 

Quartzite.

The quartzite at Flambeau Ridge, historically named 

the Flambeau Quartzite (table 1) by Hotchkiss and others 

(1915), differs from the Barron Quartzite in several ways. It 

is thicker, contains far more conglomeratic zones than the 

Barron, and lacks interbedded argillite (Campbell, 1986). 

Additionally, the quartzite has been tightly folded into an 

asymmetric syncline with a nearly vertical south limb and a 

more gently dipping north limb; the fold axis plunges 

55°-65° WNW. The north limb is truncated by a branch of 

the Jump River fault zone, a major N. 60° E. lineament in 

the Wisconsin magmatic terranes (Myers, 1974). Sheared, 

unmetamorphosed (Keweenawan?) diabase intersected in 

drill cores along the Jump River fault zone near Jump River 

(LaBerge, 1988) suggests reactivation of this major fault 

during Keweenawan (or later) time.

Interpretation. Similarity in lithology, bedding 

features, relationship to underlying Early Proterozoic 

igneous rocks, and lack of metamorphism all suggest that 

the Barron Quartzite and the quartzite at Flambeau Ridge 

are correlative, and that they are younger than the volcanic 

rocks of the Pembine-Wausau terrane (1,889-1,860 Ma). 

One of the distinctive features of the Barron Quartzite and 

the quartzite at Flambeau Ridge is the presence of clasts of 

granular iron-formation in the conglomerates. Granular 

iron-formations are mainly restricted to Early Proterozoic 

sedimentary sequences (Gross, 1965), and the presence of 

such clasts suggests a source area of Early Proterozoic 

sedimentary rocks such as those in the Gogebic iron range 

to the north. We suggest that these quartzite formations are 

demonstrably younger than the Penokean orogeny. Similar 

granular iron-formation clasts are present in the Sioux 

Quartzite in southwestern Minnesota, and their presence, 

along with that of kaolinitic argillite ("pipestone") layers, 

suggests a correlation between the Barron, the Flambeau 

unit, and the Sioux Quartzite.

B14 Precambrian Geology of Lake Superior Region
Barren and Flambeau Unconformably overlie rocks 

areas of Pembine-Wausau terrane

Inferred belt of

quartzite-bearing

conglomerate

Deformed quartzite intruded by 

undeformed granite

Rib Mountain

Pre-1,840 Ma

Foliated, sillimanite-bearing quartzite 

xenolith in 1,500 Ma syenite that 

intrudes greenschist-facies 1 ,840 Ma 

volcanics

Quartzite deposited and metamorphosed 

to sillimamte grade prior to 1 ,840 Ma

Clasts of McCaslin Quartzite in Baldwin

McCaslin Mountain and Conglomerate, which is deformed in 

Thunder Mountain Mountain shear zone

Pre-1,812 Ma Mountain shear zone cut by 1,812 Ma 

pluton

Quartzite boulders in conglomerate 

interbedded with deformed 1,860 Ma 

Hamann Creek volcanics

Pre-1,860 Ma Quartzite predates 1,860 Ma

Deformed quartzite inferred to be 

intruded by undeformed 1,832 Ma 

Vesper quarry red granite

Pre-1,835 Ma Quartzite and deformation predate 

1,835 Ma

Deformed quartzite intruded by 1,764 

Hamilton Mounds Ma granite

Pre-1,764 Ma Quartzite and deformation predate 

1,764 Ma

Waterloo area 

Uncertain

Deformed quartzite intruded and 

metamorphosed by undeformed granite

Age of igneous event not well established

Quartzite in contact with igneous rocks 

Baraboo syncline of more than one age

Uncertain Age of igneous rocks unknown

Figure 11. Summary of data on age relationships of Proterozoic quartzite at nine localities in Wisconsin.

SUMMARY OF INTERPRETATIONS

Age of Quartzites

The data presented herein clearly indicate that some 

quartzite bodies are older than 1,760 Ma (fig. 11), the lower 

age limit of the Baraboo interval, as defined by Dott (1983). 

The quartzite at McCaslin and Thunder Mountains is older 

than 1,812 Ma, as indicated by the age of undeformed Mines 

Quartz Diorite that cuts quartzite-bearing conglomerate, and 

the quartzite at Hamilton Mounds is older than 1,764 Ma, 

the age of undeformed granite that intrudes it. Also, the 

quartzite at Vesper quarry must be older than 1,760 Ma, and

probably is older than 1,835 Ma, the presumed age of the 

undeformed red granite associated with the quartzite. The 

quartzite-bearing conglomerates in central Wisconsin, such 

as at Hamann Creek which are interbedded with volcanic 

rocks in the Pembine-Wausau terrane predate 1,860 Ma 

volcanic rocks. Another quartzite-bearing conglomerate in 

Eau Claire County (Marshfield terrane) is interbedded with 

1,860 Ma volcanic rocks.

The ages of many other quartzite bodies cannot be 

tightly constrained, however, because of the absence of 

associated igneous rocks of known age. The Barren Quartzite and the quartzite at Flambeau Ridge, for example, are 

only known to be younger than the underlying volcanic 

rocks (1,889-1,860 Ma). The controversial Baraboo

Age and Structure of Proterozoic Quartzites in Wisconsin B15
Quartzite is presumed to be younger than the underlying(?) 

rhyolite (Dalziel and Dott, 1970), which probably is 1,760 

Ma (Van Schmus, 1978), but this should be confirmed by 

dating rhyolite in contact with or near the quartzite.

Structural Summary

Structures present in all the quartzites in central and 

southern Wisconsin south of the Eau Pleine shear zone 

suggest that these rocks were deformed by south-verging 

structural processes. At Hamilton Mounds, structures 

indicate deformation in a south-verging fold-thrust system 

wherein rocks with brittle and ductile deformation features 

were tectonically stacked (fig. 25). As at Hamilton Mounds, 

the quartzite at Waterloo and Vesper also has north-dipping 

cleavage, which suggests southward vergence. At Necedah, 

the quartzite is steeply dipping, has south-facing crossbedding, and a nearly horizontal cleavage; kinematic data 

indicate southward transport. Along the southern margin of 

the exposed Precambrian in Wisconsin, the Baraboo syncline is overturned to the south and has an axial plane 

cleavage that dips north. Cambray (1987) suggested two 

episodes of deformation in the Baraboo area: (1) formation 

of an upright syncline; and (2) south-directed simple shear 

to deform the upright fold to its present symmetry. 

Similarity of structural features indicates that the quartzite 

and the rhyolite on the north limb have been deformed 

together.

We propose that the similarity in structural style and 

evidence of consistent southward vergence together suggest 

that the several quartzite bodies in central and southern 

Wisconsin, in the Marshfield terrane, were deformed by a 

major, complex, south-directed fold-thrust event. Available 

data suggest that deformation predates the 1,835 Ma posttectonic red granite in central Wisconsin and clearly 

indicate that the deformation predates the 1,764 Ma granite/rhyolite event in southern Wisconsin. Structural data at 

Baraboo and Waterloo suggest that a north-verging 

structural event of unknown age and extent postdates the 

south-verging event. This event is expressed as southdipping foliation in rhyolite and quartzite at Caledonia 

Church on the south limb of the Baraboo syncline and in the 

Waterloo area.

TECTONIC IMPLICATIONS

The scattered quartzite bodies in central and southern 

Wisconsin are interpreted as remnants of a once-extensive 

sequence of platform sedimentary rocks that were deposited 

on the passive margin of the Archean minicontinent (Marshfield terrane) that lies south of the Eau Pleine shear zone. 

We propose that all of them were deformed in a major 

south-verging fold-thrust system that extends southward

from the Eau Pleine shear zone for at least 140 km to the 

Baraboo and Waterloo areas in southern Wisconsin. 

Structural and stratigraphic data presented herein indicate 

that these rocks were deposited before 1,860 Ma, inasmuch 

as clasts of quartzite (and iron-formation) are present in 

conglomerates interbedded with the 1,860 Ma volcanic 

rocks of the Marshfield terrane. This interpretation is 

consistent with that proposed recently by Sims and others 

(1989). They proposed that northward subduction moved 

the Archean minicontinent (Marshfield terrane) beneath the 

Pembine-Wausau terrane along the Eau Pleine shear zone, 

resulting in voluminous magmatism (=1,850-1,840 Ma) in 

the southern part of the Pembine-Wausau terrane. 

Following collision, the platform quartzite and associated 

rocks were obducted southward, being deformed for 

distances of at least 140 km, to account for the distant 

Baraboo syncline (fig. 12). Deformation predates the latestage (1,835 Ma) magmatism in central and southern 

Wisconsin.

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Dott, R.H., Jr., 1983, The Proterozoic red quartzite enigma in the 

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p. 21-40.

B16 Precambrian Geology of Lake Superior Region
Baraboo 

syncline Vesper Eau Pleine 

shear zone

Hamann 

'* % Creek

Rib 

Mountain

EXPLANATION

Syenite Age, ^1,500 Ma

Granite

Quartzite

Volcanic rocks

Allochthonous crust

of unknown age 

Archean crust

Contact

Thrust fault Showing 

relative horizontal 

movement

Figure 12. Interpretive sketch of tectonic relationships of the quartzites in central and southern Wisconsin, illustrating nature 

of proposed south-verging deformation. Length of section =175 km. Vertical, not to scale.

Greenberg, J.K., and Brown, B.A., 1983, Middle Proterozoic to 

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50 p.

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J.L., 1986, The Wolf River Batholith and Baraboo interval 

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12, 56 p.

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structures within phyllitic layers of the Baraboo Syncline, 

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p. 1045-1050.

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study of the Baraboo Quartzite, Wisconsin: Geoscience 

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magmatic terrane: U.S. Geological Survey Open-File 

Report 88-536.

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south-directed Early Proterozoic thrust sheet in south 

central Wisconsin: Geological Society of America Abstracts 

with Programs, v. 18, p. 664.

____ 1988, The Baraboo Quartzite A new look at an old 

problem: 34th Annual Institute on Lake Superior Geology, 

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____ 1989, Tectonic implications of the structure and stratigraphy of quartzites in central and southern Wisconsin: 35th 

Annual Institute on Lake Superior Geology, Duluth, 

Minnesota, p. 49-50.

LaBerge, G.L., and Myers, P.E., 1983, The Precambrian geology 

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____ 1984, Two early Proterozoic successions in central 

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Age and Structure of Proterozoic Quartzites in Wisconsin B17
____ 1984b, Early Proterozoic Plate Tectonics Evidence from

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Morey, G.B., and Van Schmus, W.R., 1988, Correlation of

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States: U.S. Geological Survey Professional Paper 1241-F,

31 p. 

Myers, P.E., 1974, General geology Precambrian geology: 38th

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p. 1-3. 

____ 1980, Precambrian geology of the Chippewa valley An

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Ostrander, A.R., 1931, Geology and structure of Hamilton

Mounds, Adams County, Wisconsin: Madison, Wis.,

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Riley, N.A., 1947, Structural petrology of the Baraboo Quartzite:

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Rozacky, W.J., 1987, The petrology and sedimentation of the

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Geological Survey Miscellaneous Investigations Series Map

1-1924, scale 1:250,000. 

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Mountain and Escanaba I°x2° quadrangles, northeastern

Wisconsin and northwestern Michigan: U.S. Geological

Survey Miscellaneous Investigations Series Map 1-2056,

scale 1:250,000. 

____ in press, Geologic map of Precambrian rocks, southern

Lake Superior region, Wisconsin and northern Michigan:

U.S. Geological Survey Miscellaneous Investigations Series

Map 1-2185, scale 1:500,000.

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shear zone, northeastern Wisconsin A discrete ductile 

deformation zone within the Early Proterozoic Penokean 

orogen: U.S. Geological Survey Bulletin 1904-A, 15 p.

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____ 1932, Igneous rocks in the Baraboo District, Wisconsin: 

Journal of Geology, v. 40, p. 119-139.

Van Hise, C.R., and Leith, C.K., 1911, Geology of the Lake 

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641 p.

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the Great Lakes area, North America: Royal Society of 

London Philosophical Transactions, ser. A, v. 280, 

p. 605-628.

____ 1978, Geochronology of southern Wisconsin rhyolites and 

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____ 1980, Chronology of igneous rocks associated with the 

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and lower Proterozoic rocks, Southern Canadian Shield: 

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Weidman, S., 1904, The Baraboo iron-bearing district of 

Wisconsin: Wisconsin Geological and Natural History 

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Geological and Natural History Survey Bulletin 41, 697 p.

B18 Precambrian Geology of Lake Superior Region &U.S. GOVERNMENT PRINTING OFFICE: 1992-673-049/46023


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