Petrography and Correlation of 

Precambrian Clastic Sedimentary Rocks 

Associated with the Midcontinent Rift System

U.S. GEOLOGICAL SURVEY BULLETIN 1989-E
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Chapter E

Petrography and Correlation of 

Precambrian Clastic Sedimentary Rocks 

Associated with the Midcontinent Rift System

By PIETER BERENDSEN and ANDRZEJ BARCZUK

U.S. GEOLOGICAL SURVEY BULLETIN 1989

STRATEGIC AND CRITICAL MINERALS IN THE MIDCONTINENT REGION, 

UNITED STATES

WARREN C. DAY and DIANE E. LANE, Editors
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Published in the Central Region, Denver, Colo. 

Manuscript approved for publication April 21, 1993. 

Photocomposition by Marie Melone

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U. S. Government

UNITED STATES GOVERNMENT PRINTING OFFICE: 1993

For sale by

USGS Map Distribution 

Box 25286, Building 810 

Denver Federal Center 

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Berendsen, Pieter.

Petrography and correlation of Precambrian clastic sedimentary rocks 

associated with the Midcontinent Rift System / by Pieter Berendsen and 

Andrzej Barczuk.

p. cm. (U.S. Geological Survey bulletin ; 1989) 

(Strategic and critical minerals in the midcontinent region, United States; ch. E)

Includes bibliographical references.

Supt. of Docs, no.: I 19.3: 1989E

1. Geology, Stratigraphic Precambrian. 2. Rocks, Sedimentary Middle 

West. 3. Petrology Middle West. 4. Stratigraphic correlation Middle West. 

5. Keweenawan Rift. I. Barczuk, Andrzej. II. Title. III. Series. IV. Series: 

Strategic and critical minerals in the midcontinent region, United States: ch. E. 

QE75.B9 no. 1989-E 

[QE653] 

557.3 s dc20

(551.7'1'0977] 93^722

CIP
CONTENTS

Abstract El 

Introduction El 

Methodology El 

Generalized stratigraphy E2 

Petrographic study of surface samples E2 

Oronto Group E2

Copper Harbor Conglomerate E2 

Nonesuch Shale E2 

Freda Sandstone E4 

Bayfield Group E6

Orienta Sandstone E6 

Devils Island Sandstone E6 

Chequamegon Sandstone E6 

Bayfield Group Equivalent Rocks E9 

Fond du Lac Formation E9 

Hinckley Sandstone E9 

Jacobsville Sandstone Ell 

Petrographic study of subsurface samples Ell 

Minnesota E12 

Iowa E13 

Nebraska El 5 

Kansas E15 

Modal analysis E16 

Heavy-mineral analysis E17 

Discussion E18 

Acknowledgments E19 

References cited E19

FIGURES

El. Chart showing stratigraphic terminology and correlations of upper Keweenawan

clastic rocks E3 

E2. Generalized geologic map of the western Lake Superior region showing the

distribution of and sample localities for Keweenawan surface rocks E3 

E3. Triangular diagram showing petrographic composition of Precambrian sedimentary

rocks of the northern part of the Midcontinent Rift System E8 

E4. Triangular diagram showing petrographic composition of Precambrian sedimentary

rocks of the northern part of the Midcontinent Rift System based on the composition

of their constituent rock fragments E10 

E5. Generalized geologic map showing the subsurface distribution of Keweenawan

rocks in southern Minnesota, Iowa, and eastern Nebraska, and the location of drill

holes discussed in text Ell 

E6. Map showing generalized distribution of Keweenawan-age rocks in Kansas and

approximate location of drill holes from which samples were taken E12 

E7. Chart showing preliminary stratigraphic correlation of drill-hole cuttings from

Kansas with established stratigraphy in the northern part of the Midcontinent Rift

System E16

Contents III
TABLES

El. Petrographic composition of volcaniclastic and clastic sedimentary rocks associated

with the Midcontinent Rift System E5 

E2. Heavy-mineral composition of volcaniclastic and clastic sedimentary rocks

associated with the Midcontinent Rift System E7 

E3. Heavy-mineral composition of core and cuttings from drill holes in Minnesota,

Iowa, and Kansas E14

IV Contents
STRATEGIC AND CRITICAL MINERALS IN THE MIDCONTINENT REGION, UNITED STATES

Petrography and Correlation of 

Precambrian Clastic Sedimentary Rocks 

Associated with the Midcontinent Rift System

By Pieter Berendsen 1 and Andrzej Barczuk2

Abstract

Rocks from exposures in the Lake Superior region and 

from core and cuttings of exploration drill holes in Iowa, 

Nebraska, and Kansas were analyzed to correlate the subsurface clastic sedimentary rocks in the southern extension 

of the Midcontinent Rift System with rocks of the established Keweenawan stratigraphy in the Lake Superior 

region. Petrographic properties and heavy minerals were 

used to characterize the various formations. Rocks similar 

in character to those of the Oronto Group and the Bayfield 

Group are recognized in the subsurface in Iowa, Nebraska, 

and Kansas. For comparative purposes, additional analyses were made on surface samples collected throughout 

the Lake Superior region.

INTRODUCTION

During the past five years cooperative studies by the 

Kansas Geological Survey and the U.S. Geological Survey 

(USGS) as part of the USGS Midcontinent Strategic and 

Critical Minerals Project (MSCM) have significantly contributed to our understanding of the stratigraphy and structure of the Precambrian rocks associated with the Midcontinent Rift System, especially in that part of the rift 

where Phanerozoic rocks overlie the Precambrian. The 

approximately 1,000-mi (l,500-km)-long arm of the Midcontinent Rift System, stretching from the northern end of 

Lake Superior southwestward into south-central Kansas, 

can be divided into three segments that are left-laterally

'Kansas Geological Survey, University of Kansas, Lawrence, KS 

66047.

2Department of Geology, University of Warsaw, Warsaw, Poland.

offset with respect to each other. The initial extensional 

phase of rifting, beginning about 1,100 Ma, was accompanied by emplacement of sizable mafic intrusive bodies 

and subsequent voluminous outpouring of mafic volcanic 

rock. In the ensuing stages of rift development, initially 

coarse and immature sediments, followed by increasingly 

finer and more mature sediments, collected in the basins 

that developed along the trend of the rift. The coarser 

sediments contained a large percentage of debris derived 

from mafic volcanic rocks in the center of the rift, and the 

younger, predominantly fluviatile sediments were dominated by material from the rift shoulders.

This study contributes information on the petrology 

and mineralogy of subsurface Precambrian sedimentary 

rocks encountered in drill holes in Minnesota, Iowa, 

Nebraska, and Kansas. New data from surface samples 

collected as part of this study, together with published 

information on volcaniclastic and clastic rocks in the outcrop area, allow for a comparison and initial correlation 

between the subsurface stratigraphy of the rift and the 

exposed stratigraphic sequence in the Great Lakes region.

METHODOLOGY

During the first phase of this MSCM project 23 outcrops of the major post-volcanic clastic sedimentary rocks 

in Minnesota, Wisconsin, and Michigan were examined, 

sampled, and described with the aid of a binocular microscope (R.R. Anderson, Iowa Geological Survey; R.M. 

McKay; P.Berendsen; A. Barczuk; G.B. Morey; P. 

McSwiggin; and M.P. Carlson, written commun., 1987). In 

addition, representative samples of well cuttings were collected from a selected number of drill holes penetrating the 

pre-Paleozoic sedimentary section in Minnesota, Iowa,

Precambrian Clastic Sedimentary Rocks, Midcontinent Rift System E1
Nebraska, and Kansas, and the rocks were described. As a 

result, a total of 130 thin sections were prepared: 65 of 

rocks from surface exposures, 45 from drill-hole cuttings, 

and 20 from drill core. Of these, 50 thin sections were 

point counted and 30 were stained to estimate porosity. 

Twenty-seven samples were disaggregated, and the heavy 

minerals were separated in bromoform. The heavymineral separates were in turn point counted. Some samples were analyzed by X-ray diffraction where necessary, 

especially clay-rich samples.

GENERALIZED STRATIGRAPHY

The generalized stratigraphic relationships of the rocks 

associated with the Midcontinent Rift System are shown in 

figure El. Recent papers by Dickas (1986), and a number 

of papers in Wold and Hinze (1982), and references 

included therein, provide details about the stratigraphy and 

the difficulties of correlation even within the outcrop belt 

in the Great Lakes region. The initial opening of the rift at 

about 1,100 Ma was accompanied by voluminous outpourings of predominantly mafic volcanic rocks. The rift basin 

was subsequently filled with immature sediments that initially contained large quantities of the nearby volcanic 

rocks, followed by increasingly more mature sediments. 

The sediments may be divided into an older volcaniclastic 

sequence represented by the Oronto Group and Solor 

Church Formation, and a younger clastic sequence represented by the Bayfield Group, the Fond du Lac Formation, 

and the Hinckley Sandstone. The "Red Clastic series" 

(Lugn, 1934) in Nebraska and the Rice Series (Baars, in 

press) in Kansas collectively include all the post-volcanic 

undivided sedimentary rocks.

PETROGRAPHIC STUDY OF 

SURFACE SAMPLES

Oronto Group

Oronto Group rocks, made up of three petrologically 

distinct formations interpreted to have been deposited 

soon after cessation of rift volcanism, are exposed from 

north-central Wisconsin northeastward throughout the 

Keweenaw Peninsula in Michigan, and on Isle Royale in 

Lake Superior.

Copper Harbor Conglomerate

The Copper Harbor Conglomerate (Daniels, 1982) is a 

distinct rock type easily recognized by its mineralogical 

immaturity, high content of various kinds of volcanic rock 

fragments, high content of altered microcline, lack of 

micas, and the occurrence of zeolites.

This formation is as thick as 6,900 ft (2,100 m); it 

thickens and fines to the northwest towards the axis of the 

rift (Daniels, 1982). It also fines upward into a mediumto coarse-grained volcanic sandstone (Daniels, 1982). A 

clast-supported conglomerate facies is common in the 

lower part of the formation, indicating deposition in a prograding alluvial-fan complex. The environment of deposition changed upward into a shallow-water fluvial regime 

in which the volcanic sandstone was deposited.

Two sites on the north shore of the Keweenaw Peninsula were examined, both areas that typify the Copper 

Harbor Conglomerate (fig. E2): one near Eagle River 

(locality 18, fig. E2) and the other at Copper Harbor (locality 19, fig. E2), Mich. The rocks there are very coarse 

grained, conglomeratic volcanic sandstone (samples 18a 

and 18b) and very coarse grained, sandy volcanic conglomerate (sample 19). The sandy cement in sample 18a is 

composed of calcite (9.4 percent), iron minerals (2.8 percent), aleuritic quartz (2.0 percent), and zeolites (2.0 percent; table 1). Sample 19 has a similar sandy matrix in 

which iron minerals (25.6 percent) and zeolites (7.2 percent) predominate. Quartz, most of it showing abraded 

secondary overgrowths, makes up from 2.6 to 5.4 percent 

of the rock by volume. Feldspar (5.7-12.6 percent) consists of slightly to intensely altered microcline and associated minor plagioclase (albite), orthoclase, and perthite. 

Chlorite and other phyllosilicates are rare.

Volcanic rocks make up most of the rock fragments 

(43.0-54.6 percent). In samples 18a and 18b altered rhyolite, basalt, and diabase are prominent. In sample 19 trachyte having fluidal texture and containing potassium 

feldspar fragments predominate; basalt and diabase are 

minor constituents. Some andesite fragments are present, 

and sample 19 shows a large clast of lithoclastic tuff that 

contains voids filled with zeolites. Sedimentary rock fragments (2.2 percent) of chert, shale, ferruginous mudstone, 

and sandstone, and metamorphic fragments of quartzite, 

various types of schist, quartz ferruginous rock, and cataclasite are the most common. The heavy minerals are 

mostly opaque, but include small amounts of apatite, 

zircon, epidote, rutile, and sillimanite (table E2).

Nonesuch Shale

The Nonesuch Shale underlies the Freda Sandstone and 

crops out from just south of Ashland, Wis., northeastward 

to near Calumet on the Keweenaw Peninsula in Michigan, 

more or less parallel to the Lake Superior shoreline. Shale 

constitutes only a small part of the formation, the bulk of 

the rock consisting of siltstone and very fine grained sandstone.

We examined and collected Nonesuch Shale samples at 

Potato River Falls, near Gurney, Wis. (locality 16, fig. E2), 

and at Silver City (locality 24, fig. E2) and in Porcupine

E2 Strategic and Critical Minerals, Midcontinent Region, U.S.
KANSAS AND NEBRASKA

Rice Series (Kansas) 

and 

red elastics (Nebraska)

IOWA AND MINNESOTA

Hinckley Sandstone

Fond du Lac Formation

Solor Church Formation

WISCONSIN AND MICHIGAN

S&f

I2 *

tS^S

Oronto Group (volcaniclastic)

Chequamegon Sandstone

Devils Island Sandstone

Orienta Sandstone

Jacobsville 

Sandstone

Freda Sandstone

Nonesuch Shale

Copper Harbor Conglomerate

Figure E1 . Stratigraphic terminology and correlations of upper Keweenawan clastic rocks.

92° 90° 88°

47 c

46 C

19

0 20 MILES

0 20 KILOMETERS

EXPLANATION

Middle 

Proterozoic

Bayfield Group and 

equivalent rocks

Oronto Group rocks

Portage Lake Volcanics 

and equivalent rocks

Sample locality

Fault Dashed where 

approximately located

Figure E2. Generalized geologic map of the western Lake Superior region showing the distribution of and sample localities for 

Keweenawan surface rocks discussed in the text (modified from Morey and Green, 1982).

Precambrian Clastic Sedimentary Rocks, Midcontinent Rift System E3
State Park at Presque Isle Inlet (locality 26, fig. E2), both in 

Michigan. The rocks examined are fine- to mediumgrained litharenites (sample 26), medium- to coarse-grained 

litharenite (sample 16a), fine-grained to very fine grained 

arkosic arenite (samples 16b and 16c), laminated clay shale 

(sample 24), and laminated muddy-ferruginous shale (sample 24a). Cement consists mainly of clay minerals impregnated with varying amounts of iron oxides. Samples from 

some places (for example, 16a, 16b, and 16c) contain relatively large amounts (15.2-23.2 percent, table El) of an 

unusual ferruginous-zeolitic-chloritic cement. The cement 

in sample 16c also contains chalcedony. Most of the 

cement is believed to be of secondary origin, resulting from 

the alteration of pyroclastic material.

Angular quartz, of pyroclastic origin, has corroded 

embayments and inclusions of devitrified glass. Secondary overgrowths or other signs of recrystallization are rare. 

The feldspars are similar to those in the Freda Sandstone, 

consisting of abundant unaltered microcline and lesser 

amounts of plagioclase, altered microcline, perthite, and 

orthoclase.

The shale contains a large amount of deformed phyllosilicates, mostly chlorite (4.3-20.1 percent; table El). 

Two types of chlorite are recognized. One is a replacement of detrital biotite, and the other is a metasomatic 

replacement of volcanic glass. Brown or green, highly 

altered biotite and muscovite are accessory minerals. 

Quartz-mica-sericite-graphite-epidote schist, quartzite, 

quartz-ferruginous rock, and cataclasite are the most prevalent rock fragments. A lesser amount of sedimentary 

rock fragments consisting of chert, chalcedony, and clayey 

or clayey-ferruginous shale are present. Mafic volcanic 

rock fragments (basalt and diabase) are rare. The heavymineral fraction is predominantly opaque minerals and 

epidote derived from the alteration of plagioclase (table 

E2).

The Nonesuch Shale is generally conformable with the 

underlying Copper Harbor Conglomerate, but a transgressive relationship was observed in some areas (Daniels, 

1982). Economic concentrations of copper and silver 

(Ensign and others, 1968) and some liquid hydrocarbon 

(oil) (Eglinton and others, 1964; Kelly and Nishioka, 

1985) occur in the slightly metamorphosed Nonesuch 

Shale at the underground White Pine copper mine in 

Michigan. Because of the potential for hydrocarbon accumulations in the shale (Dickas, 1984) in the Great Lakes 

area and probable lithologically equivalent rocks in the 

progressively more deeply buried southern extension of 

the rift, the Nonesuch Shale has received attention in 

numerous studies and related exploration activities.

Generally, the shale accumulated in a standing body of 

water (White, 1972; Hubbard, 1975). Regional fluvial 

influences and local periodic subaerial exposure account 

for lateral lithological variations. A progressive increase 

in the maturity of the rocks towards the east, mostly

involving a decrease in the amount of mafic fragments and 

a concomitant increase in the amount of quartz, was noted 

by several workers and reported by Daniels (1982), who 

also suggested that the more mature sediments were 

deposited towards the center of the basin. If this is true, it 

might suggest a shift in the depocenter over time.

Freda Sandstone

The characteristic features of the Freda Sandstone are 

the mineralogical immaturity, the relatively high feldspar 

content (including plagioclase), the variable provenance of 

the rock fragments together with a high content of volcanic rock, the low quartz content, and the elevated amount 

of authigenic epidote.

We examined samples from south of Ashland along the 

White River (locality 14, fig. E2), at Copper Falls State 

Park near Mellon (locality 28, fig. E2), Wis., and at Porcupine Mountain State Park, Presque Isle Inlet (locality 26, 

fig. E2), Mich. The rocks are predominantly reddishbrown, poorly sorted, tuffaceous litharenites containing 

many semirounded volcanic rock fragments. Sample 28, 

from Copper Falls State Park, is a tuffite. The average 

grain size of the litharenites is about 2 mm, but grains as 

great as 4 mm are common. The cement is mostly composed of clay minerals impregnated with iron oxides, probably derived from altered volcanic ash. Sample 14c 

contains as much as 15 percent secondary calcite cement, 

and sample 28 is recognized by its unusually high content 

of zeolites in the calcite cement. Quartz, typically of volcanic origin and having a characteristic angularity, corroded 

embayments, and abundant inclusions of volcanic glass, 

makes up only a small percentage of the rock (table El). 

Feldspar (6.4-17.1 percent), consists mostly of unaltered 

microcline, perthite, and orthoclase.

In contrast to the younger sandstone units, the Freda 

Sandstone contains from 0.7 to 3.3 percent secondary albite 

after primary plagioclase. Volcanic grains account for the 

bulk of the rock fragments present. Basalt having felsitic 

and micro-ophitic textures, diabase showing ophitic and in 

some cases graphic textures, and altered rhyolite are common rock fragments. Trachyte, trachybasalt, tuff, and tuffite are of minor importance. A few grains of serpentinite 

and metavolcanic diabase, rich in epidote, also are present 

Sedimentary rock fragments (5-6.5 percent) consist of ferruginous shale, mudstone, siltstone, and fine-grained micaceous sandstone that commonly shows bedding or parallel 

laminae. Chert and chalcedony are present locally. 

Quartzite, quartz-ferruginous rock together with minor 

quartz-mica sericite schist and cataclasite make up the 

metamorphic rock fragments (4.4-5.7 percent). Granitoids 

(1.0-2.1 percent) occur only in the coarser grained sandstones. Opaque minerals and secondary euhedral epidote 

make up the bulk of the heavy minerals (0.7-3.0 percent).

E4 Strategic and Critical Minerals, Midcontinent Region, U.S.
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or Formation 

name hole drill 

No. Sample 

Quartz

(Q) Vein 

(Q) Total 

K-Feldspar

Plagioclase

(F) Total 

Sedimentary

Igneous

Volcanic

Metamorphic

(R) Total 

QUARTZ (Q)

FELDSPAR (F)

ROCK FRAGMENTS (R)

Fe-aggregates

Fe-muddy-clayey 

lithoclasts

Muscovite

Biotite

Chlorite

micas Total 

Iron

Clay

Quartz

Carbonate

Other

(C) Total 

MICA AND CHLORITE

CEMENT (C)

MATRIX

PORES

OTHERS

o 

o

I

o 

&>

D

Q. 

n_ 

to

a.

o 

nto' 

f? 

CL

Q. 

n 

O

D

rt>

D
Apatite and pyrochlore are considered to be epigenetic. 

Detrital garnet, zircon, and rutile are rare (table E2). The 

composition compares well with results reported by others, 

including Daniels (1982). A considerable variation in the 

lateral composition of the sandstone is expected as a result 

of local variation in sediment sources.

The contact of the Freda Sandstone with the underlying 

Nonesuch Shale is gradational and marked by an increase 

in the state of oxidation, grain size, and bedding thickness 

(Daniels, 1982). The upper contact with rocks of the 

overlying Jacobsville Sandstone or the Bayfield Group is 

nowhere exposed. The Freda Sandstone increases in 

thickness in a southerly direction from about 1,500 ft (455 

m) on the northern Keweenaw Peninsula to about 4,000 ft 

(1,215 m) in Wisconsin. In general, the rocks may be 

characterized as a fining-upward sequence containing 

minor conglomeratic units in the lower part of the formation and some siltstone units throughout. A fluvial origin 

for the deposition of the Freda Sandstone is favored by 

Daniels (1982), but lacustrine, flood plain, tidal flat, or a 

combination of these have been proposed by earlier 

authors (Hamblin, 1961a,b; Noble, 1965; Hite, 1968).

Bayfield Group

The Bayfield Group consists mainly of fluviatile and 

minor lacustrine sandstones that are exposed along the 

Wisconsin shore of Lake Superior. Rocks considered to 

be of equivalent age in Minnesota and Michigan have 

been referred to by different names and will be discussed 

separately.

Orienta Sandstone

Buff to red feldspathic to arkosic sandstone makes up 

the bulk of the Orienta Sandstone (Morey and Ojakangas, 

1982). The average composition is 85 percent quartz, 11 

percent feldspar, and 4 percent labile rock fragments 

(Myers, 1971). The Orienta Sandstone is mineralogically 

immature and has a high content of microcline and metamorphic rock fragments. It contains muddy-clayey intraclasts and pyroclastic materials, deformed minerals, and 

recrystallized quartz.

Samples were collected along the Iron River near Port 

Wing (locality 12), in Amnicon Falls State Park (locality 

29), and in Pattison Falls State Park (locality 30), all in 

Wisconsin (fig. E2). They show the sandstone to be a 

fine- to coarse-grained, angular to rounded subarkose and 

in some places very coarse grained to conglomeratic sublitharenite (fig. E3), similar in composition to that 

recorded by Morey and Ojakangas (1982). Recrystallized, 

deformed, and corroded quartz grains make up from 46 to 

62 percent of the rock. In the coarser grained rocks the 

quartz grains are generally well rounded, as opposed to the

poorly rounded grains in the finer grained rocks. Some of 

the quartz grains show signs of a possible pyroclastic origin, such as corroded embayments, increased elongation, 

and inclusion of devitrified glass. Some quartz grains 

contain abundant rutile needles. Other quartz grains may 

have been recycled several times, the grain edges being 

abraded and recrystallized each time. Feldspar, which 

makes up from 9 to 14 percent of the rock, is mainly unaltered microcline and minor amounts of altered microcline, 

perthite, and orthoclase (table El).

Rock fragments, which make up from 4 to 11 percent 

of the sandstone, are similar to those found in the Fond du 

Lac Formation (table El). Metamorphic rock fragments 

consisting mainly of quartzite, cataclasite, quartz-mica-, 

quartz-sericite-, quartz-chlorite-, and quartz-graphite schist 

are the most common. Sample 12a, from near Port Wing, 

Wis., contains a large percentage of muddy-clayey intraclasts, whereas sample 29a, from Amnicon State Park, 

Wis., is enriched in volcanic fragments of basalt clasts that 

possess felsitic and micro-ophitic textures and diabase 

clasts of ophitic, granophyric, and graphic textures.

In the Iron River, Wis., samples, highly deformed mica 

and chlorite make up the bulk of the heavy minerals (table 

E2). In the other samples, opaque and semi-opaque minerals predominate. Zircon, tourmaline, garnet, and hornblende are common constituents. Some of the apatite and 

epidote grains are authigenic.

Devils Island Sandstone

The Devils Island Sandstone is a thinly bedded, wellsorted, white to buff quartzose sandstone (Morey and Ojakangas, 1982) characterized by subangular to wellrounded, medium-sized quartz grains (Tyler and others, 

1940). Dickas (1986) considers it to be an almost pure 

orthoquartzite. The sample collected for this study at 

Siskiwit Falls (locality 11, fig. E2), near Cornucopia, 

Wis., on the west side of the Bayfield Peninsula, is a wellsorted, fine- to-medium grained, well-rounded quartz arenite in which quartz makes up about 95 percent of the 

major rock-forming components. This is similar to values 

published by Morey and Ojakangas (1982). Quartz grains 

are recrystallized and have abraded secondary overgrowths. Rock fragments (table El) consist of chert, 

quartzite, and schist. Muscovite is the predominant heavy 

mineral (75 percent). Opaque and semi-opaque minerals 

make up about 14 percent. Tourmaline is more abundant 

than zircon (table E2).

Chequamegon Sandstone

The Chequamegon Sandstone is massive to thick 

bedded (10-15 ft, or 3-5 m), red, brown, or gray-andwhite-mottled arkosic to feldspathic sandstone, containing

E6 Strategic and Critical Minerals, Midcontinent Region, U.S.
Table E2. Heavy-mineral composition of volcaniclastic and clastic sedimentary rocks associated with the Midcontinent Rift Sys

[Values in percent by volume of heavy minerals. Ss, sandstone; Fm, formation; Cgl, conglomerate; , not detected]

Formation

Hinckley 

Ss

Chequamagon 

Ss

Devils 

Island Ss

JctoUUo

ville Ss

Orienta

Ss

Fond du 

Lac Fm

Freda 

Ss

Nonesuch 

Shale

Copper 

Harbor Cgl

o"

z

<D 

Q.

CO 

CO

2 

3a

10a 

10b 

13a

11

9Oa ££d

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12b

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6a 

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1 *TU

14c 

28

16c

18a

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CO

MN

Wl

Wl

Wl

Wl

MN

Wl

Wl

MN

Rock 

type

Quartz 

arenite

Subarkose 

Subarkose 

Sublitharenite

Quartz 

arenite

Oi tl^*\i4ss\f\f\

Sublitharenite

Subarkose 

Subarkose 

Sublitharenite

Qi ihtori^fioQ

Subarkose

Subarkose 

Lithic arkose 

Lithic arkose

Lilharenite

Litharenite 

Litharenite

Lithic arkose

Conglomerate

Opaque minerals

14.0 

83.8

20.8 

46.6 

80.0

8.7

14.8

24.5

5.1 

1.8

50.7 

ce o

26.1

1.3 

22.5 

23.7

50.0

19.1 

23.8

14.6

49.2

Semi-opaque 

minerals

12.0 

4.0

11.8 

9.0 

6.4

5.7

11^

29.2

6.7

1.1

O*T.O

22.7

24.0

79.8 

54.6 

22.0

27.1

27.6 

53.6

50.1

47.2

Muscovite

1.5 

0.1

0.7 

0.4 

0.1

74.8

^ n O.VJ

4.0

16.9 

55.6

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1.5

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5.5 

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7.9

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15.3

7.6 

7.6 

2.3

0.5

0.8 

8.1

19.9

0.1

Carbonate minerals

0.2

 -

...

4.6

0.2

29.5

2.1

18.1 

2.1

0.9

Phosphate

...

...

...

...

0.3

...

0.1 

0.1

...

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Glauconite

0.2

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0.1

0.1

0.4

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0.3

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47.7 

7.5

53.6 

33.4 

5.8

1.9

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2.2

9.6 

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1.4

5

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7.6

0.8 

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2.5

0.4

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0.1

0.4

Tourmaline

5.7 

2.5

5.6 

6.7 

4.1

3.8

14.7

3.7

4.1

1.1 

10

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2.0

2.3

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1.2

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1.5

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0.8 

0.1

0.3 

0.4 

0.5

0.3

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1.1 

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4.9

t.O

0.3 

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1.2

Hornblende

2.8

2.2 

0.5 

0.7

0.5

Q Q

2.0

0.9 

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,v7 

f\ R

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Staurolite

0.2 

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0.2

0.2 

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30

5?

</> «<

SS. n
Subarkose

Quartz arenite

Sublitharenite

EXPLANATION

Minnesota

(^j Hinckley Sandstone

^^ Fond du Lac Formation

( ) Drill core Lonsdale #65-1

Michigan-Wisconsin

Jacobsville Sandstone 

Freda Sandstone 

Nonesuch Shale 

Copper Harbor Conglomerate

Wisconsin

I I Chequamegon Sandstone

[] Devils Island Sandstone

 Orienta Sandstone

Iowa

Drill core Precambrian "red elastics" 

(Lehman #1, McCallum #A-1)

Figure E3. Petrographic composition of Precambrian sedimentary rocks of the northern part of the 

Midcontinent Rift System (diagram after Pettijohn and others, 1972), Q, quartz; F, feldspar; R, rock 

fragments.

minor interbedded red shale and some conglomerate 

(Morey and Ojakangas, 1982; Tyler and others, 1940). 

The unit is fairly mature, containing an average of 83 percent quartz, 14 percent feldspar, and 3 percent labile rock 

fragments (Myers, 1971). For this study, samples were 

collected from two localities near Washburn, Wis., on the 

east side of the Bayfield Peninsula (localities 10 and 13, 

fig. E2). One of the samples (table El) contains about 20

percent labile constituents, but the other constituents fall 

within the range of reported values (fig. E3). Rock fragments are rare in the subarkoses (0.8 percent in sample 

lOb), but are common in the sublitharenitic phases of the 

unit (as much as 15.8 percent in sample 13). The fragments consist primarily of metamorphic rock (fig. E4), but 

also include quartzite, cataclasite, quartz-graphite-, quartzsericite-, and quartz-muscovite schist; quartz-ferruginous

E8 Strategic and Critical Minerals, Midcontinent Region, U.S.
rocks (iron-formation?), and minor, slightly metamorphosed quartz-sericite mudstone. Volcanic rock fragments 

consist of altered basalt having an ophitic and microophitic texture. Only a few fragments of rhyolite and diabase, which has an unusual granophyric texture, are 

present. Sedimentary rock fragments consist mainly of 

chert and agate.

The heavy-mineral fraction in the subarkosic samples 

consists of resistant minerals, such as zircon and opaque 

minerals, and lesser amounts of tourmaline, epidote, and 

hornblende. The sublitharenitic sample contains mostly 

opaque minerals, but other heavy minerals similar to those 

found in the subarkoses are present. The heavy-mineral 

composition of four samples from the same formation 

from near Bayfield, Wis. (about 12 mi (18 km) north of 

Washburn) collected and analyzed by Tyler and others 

(1940) is mainly ilmenite (67-80 percent), leucoxene (10- 

20 percent), and zircon (4-11 percent), and lesser amounts 

of tourmaline, anatase, apatite, and rutile.

Bayfield Group Equivalent Rocks

Sandstone units that are believed to correlate with formations exposed in Wisconsin have different names in 

the outcrop area in Minnesota. Nondeposition or erosion 

of rocks equivalent to the Chequamegon Sandstone of 

Wisconsin is the reason that these rocks apparently do not 

occur in Minnesota (Morey and Ojakangas, 1982).

Sandstone units in Michigan to the east and south of 

the Keweenaw fault (fig. E2) were assigned to the 

Jacobsville Sandstone by Lane and Seaman (1907). Most 

workers consider the unit to correlate with the Bayfield 

Group because of studies of the heavy-mineral suite by 

Tyler and others (1940), similarities of the sediment, mineral, and clay matrix to the Fond du Lac Formation as 

noted by Kalliokoski (1982), and analyses of paleomagnetic pole positions (Roy and Robertson, 1978) suggesting 

that the unit is younger than the Freda Sandstone.

Fond du Lac Formation

The Fond du Lac Formation is characterized by lenticular beds of very fine grained to coarse-grained, poorly 

sorted arkosic sandstone and siltstone interbedded with 

locally thick units of shale and mudstone (Morey, 1967, 

1972). Conglomerate is scattered throughout the formation.

Sandstone in the Fond du Lac Formation is characterized by its low rank of maturity, the high content of feldspar and rock fragments, muddy-clayey intraclasts, 

deformed mica flakes, quartz grains recrystallized in preferred directions, complex composition of the cement, and 

relatively high content of authigenic minerals.

This rock unit was examined and sampled at Fond du 

Lac Park (locality 7), at Jay Cook State Park (locality 8), 

and along Mission Creek (locality 6), all in the Duluth 

area, Minnesota (fig. E2). At Jay Cook State Park a 60-ft 

(18-m)-thick conglomeratic horizon is present above an 

angular unconformity with the underlying Penokean-age 

(Early Proterozoic) Thomson Formation of the Animikie 

Group. Clasts of milky-white vein quartz, as much as 6 in. 

(15 cm) in diameter predominate, but clasts composed of 

chert, quartzite, graywacke, and slate, some of it derived 

from the underlying Thomson Formation, are also present. 

The rocks sampled as part of this study consist of fine- to 

medium-grained subarkose, lithic arkose, coarse-grained to 

very coarse grained conglomeratic sublitharenite, and basal 

conglomerate. Our data plot on a triangular Q-F-R diagram (fig. E3) in a cluster in the fields assigned to these 

rock types and compare well with data reported by Morey 

and Ojakangas (1982). The framework mineralogy ranges 

from 38 to 53 percent for quartz, from 9 to 19 percent for 

feldspar, and from 8 to 14 percent for rock fragments (table 

El). With the exception of quartz, these values are similar 

to those reported by Morey and Ojakangas (1982) on the 

basis of earlier studies of surface and subsurface samples in 

Minnesota. They report an average of 77 percent quartz in 

the framework. Quartz grains are well rounded in the 

coarse-grained and conglomeratic sandstone. They are 

poorly rounded and angular in the fine- to medium-grained 

sandstone. Only a few quartz grains have abraded, thin 

secondary overgrowth rims. The feldspars consist mostly 

of unaltered microcline, together with orthoclase and perthite. No plagioclase has been found in any of the samples. 

Rock fragments are mostly of metamorphic origin and consist of quartz-mica and quartz-sericite schist, fine-grained 

quartzite, and quartz-ferruginous rock; the latter may have 

been derived from iron-formation. Chert and clay intraclasts make up the bulk of the sedimentary rock fragments. 

Basaltic, diabasic, and devitrified volcanic rock fragments 

are minor components. Only a few fragments of granitic or 

gneissic rocks were identified. Phyllosilicates (2.9-3.7 

percent by volume), nearly all of which are strongly 

deformed, consist mostly of secondary chlorite, with minor 

muscovite, and both fresh and altered biotite. The heavymineral fraction is mostly semi-opaque and opaque minerals (table E2). Other heavy minerals are zircon, tourmaline, garnet, epidote, hornblende, and apatite, part of which 

are authigenic.

Hinckley Sandstone

In outcrop the formation is a thick- to massive-bedded, 

fine- to coarse-grained, pale-red to light-pinkish-gray or 

brownish-gray quartzose sandstone (Morey and Ojakangas, 

1982). The rocks are well exposed in a number of localities 

along the Kettle River in eastern Minnesota. Outcrops of 

Hinckley Sandstone were examined and sampled at

Precambrian Clastic Sedimentary Rocks, Midcontinent Rift System E9
Volcanic 

rocks

Sedimentary 

rocks

. VT

o

Phylarenite

50 50

Metamorphic 

rocks

EXPLANATION

Minnesota

Hinckley Sandstone 

Fond du Lac Formation 

Drill core Lonsdale #65-1

Michigan-Wisconsin

Jacobsville Sandstone 

Freda Sandstone 

Nonesuch Shale 

Copper Harbor Conglomerate

Wisconsin

I I Chequamegon Sandstone

Ql Devils Island Sandstone

 Orienta Sandstone

Iowa

Drill core Precambrian "red elastics" 

(Lehman #1, McCallum #A-1)

Figure E4. Petrographic composition of Precambrian sedimentary rocks of the northern part of the Midcontinent 

Rift System based on the composition of their constituent rock fragments (diagram after Pettijohn and others, 1972).

Hinckley (locality 3a), and in a roadcut near Askov (locality 3b), all in Minnesota (fig. E2). At the last location the 

basal Hinckley Sandstone, exposed along a creek at the city 

park, is a fine- to medium-grained quartz arenite in which 

the quartz grains are rounded to angular. Rock fragments

and feldspar grains are noticeably more abundant at this 

location than at other places, where the rock is a more 

mature quartz arenite, the quartz averaging about 96 percent of the framework grains (Tryhorn and Ojakangas, 

1972). Feldspar and rock fragments generally constitute

E10 Strategic and Critical Minerals, Midcontinent Region, U.S.
less than 5 percent of the rock (fig. E3, table El). Rock 

fragments are chert and quartzite. Heavy minerals are 

opaque minerals, zircon, tourmaline, hornblende, and minor 

rutile, kyanite, and pyroxene. Secondary xenotime and 

pyrochlore make up 3.4 percent of the sample from Robinson Park, near Hinckley.

Jacobsville Sandstone

The Jacobsville Sandstone is a thick fluvial sequence 

(2,800 ft (850 m) or more) of feldspathic and quartzose 

sandstone, conglomerate, siltstone, and shale. The unit 

occurs east and south of the Keweenaw fault (fig. E2) and 

does not contain any lava flows or cross-cutting dikes 

(Kalliokoski, 1982). Generally, the unit varies from subarkose to sublitharenite, in which quartz is derived from 

both metamorphic and volcanic sources, rock fragments 

are mafic or felsic rocks and schist, and microcline is 

fresh, while plagioclase is fresh to altered (Kalliokoski, 

1982).

For this study we sampled and examined rocks from 

the east side of the Keweenaw Peninsula at Gay (locality 

22) and at Baraga (locality 23), both in Michigan. The 

rocks are fine- to medium-grained subarkose, fine- to 

medium- to coarse-grained sublitharenite, and very coarse 

grained conglomeratic litharenite (fig. E3). The cement 

(16.9-17.7 percent) consists mainly of clay, and locally 

additional quartz, iron oxide, and calcite-dolomite. In the 

two samples (22a and 23a) that were point counted, quartz 

makes up from 42.8 to 48.3 percent, feldspar from 12.7 to 

13.3 percent, and rock fragments from 6.8 to 16.8 percent 

(table El). Hamblin (1958) reported a larger percentage 

of quartz (75 percent or more) in the rocks he examined, 

but the reported feldspar content is similar (15 percent).

The smaller quartz grains are poorly rounded, and the 

larger quartz grains and pebbles are well rounded. Feldspar is mostly fresh microcline, with lesser amounts of 

altered microcline, orthoclase, and perthite. Several 

plagioclase feldspar grains were found in sample 22a 

(table El). Metamorphic rock fragments, consisting of 

various types of schist and quartz-ferruginous rocks (ironformation?), predominate. Minor amounts of volcanic and 

sedimentary rock fragments are also present. Similar findings were reported by Kalliokoski (1982).

The heavy-mineral fraction consists of opaque and 

semi-opaque minerals, and appreciable amounts of zircon, 

tourmaline, garnet, hornblende, rutile, epidote, and apatite. 

A characteristic mineralogical feature of the Jacobsville 

Sandstone is the high content of secondary xenotime and 

pyrochlore. Sample 22a (table E2) contains the highest 

concentration of these minerals encountered in this study.

Figure E5 (facing column). Generalized geologic map showing 

the subsurface distribution of Keweenawan rocks in southern 

Minnesota, Iowa, and eastern Nebraska, and the location of 

drill holes discussed in text.

PETROGRAPHIC STUDY OF 

SUBSURFACE SAMPLES

Samples of core and cuttings from exploration drill 

holes in Minnesota, Iowa, Nebraska (fig. E5), and Kansas 

(fig. E6) were collected in the second phase of the study. 

The petrographic characteristics of the samples are described below for each state.

96° 94° 92°

46°

44°

42 o

40°

Lake Superior^ 

150 MILES

200 KILOMETERS

Kansas

03 Drill hole

1.Lonsdale#65-l

2. Schuette #1

3. B. Lehman #1

4. McCallum #A-1

5. Fitzgerald #1

6. Eischeid #1

7. Koutney #1

8. Schroeder #1

9. Ihde #1 

10. Sullivan #1

EXPLANATION

NW1/4SW1/4SWV4 sec. 14, T. 112 N., R. 21 W. 

WV2NW1/4NE1/4 sec. 6, T. 108 N., R. 22 W. 

SWV4NW1/4NW1/4 sec. 18, T. 79 N., R. 27 W. 

NW1/4NW1/4SWV4 sec. 5, T. 79 N., R. 27 W. 

WV2 sec. 21, T. 71 N., R. 41 W. 

NWV4NW1/4SE1/4 sec. 6, T. 83 N., R. 35 W. 

SE1/4SE1/4NW1/4 sec. 11, T. 15 N., R. 7 E. 

El/2 sec. 26, T. 11 N., R. 12 E. 

SE1/4NW1/4 sec. 14, T. 10 N., R. 4 E. 

NWV4NW1/4NW1/4 sec. 36, T. 9 N., R. 5 E.

Sedimentary rocks (Bayfield 

Group and equivalent rocks)

Volcanic and associated sedimentary rocks 

(Oronto Group and Solor Church Formation)

Precambrian Clastic Sedimentary Rocks, Midcontinent Rift System El 1
WOODSON J ALLEN ' 

 'BOURBON I

50 100 MILES

50 100 KILOMETERS

EXPLANATION

Area underlain by predominantly 

granitic rocks

Area underlain by predominantly 

mafic volcanic rocks

Area underlain by predominantly 

sedimentary rocks

Drill hole locations

Figure E6. Generalized distribution of Keweenawan-age rocks in Kansas and approximate 

location of drill holes from which samples were taken (for legal description of the drill-hole 

locations, see fig. E7).

Minnesota

In southern Minnesota, Keweenawan rocks are buried 

beneath younger sedimentary rocks. Petrographic studies 

of subsurface samples by Morey (1977) enabled him to 

make stratigraphic correlations with Keweenawan rocks in 

Wisconsin and Michigan. As part of the MSCM Project, 

selected segments of core from five drill holes were examined and described (R.R. Anderson, Iowa Geological Survey; R.M. McKay; P. Berendsen; A. Barczuk; G.B.

Morey; P. McSwiggin; and M.P. Carlson, written commun., 1987). Samples from two drill holes were collected 

for petrographic studies and heavy-mineral analysis to aid 

in tentative stratigraphic correlations with rocks farther to 

the south. One of the drill holes, the Lonsdale #65-1 

(fig. E5), is located in the central part of the rift (St. 

Croix horst) and penetrates rocks of the Solor Church Formation (fig. El). These rocks were designated as the type 

locality of the formation by Morey (1977) and are 

nowhere exposed in the state. The other drill hole,

E12 Strategic and Critical Minerals, Midcontinent Region, U.S.
Schuette #1 (fig. E5), is located on the rift shoulder and 

penetrates younger rocks equivalent to the Bayfield 

Group, possibly the Orienta Sandstone.

Core samples were collected from the Lonsdale #65-1 

drill hole in Rice County (fig. E5) from the following 

depths: 900, 918, 934, 947, 1,279, 1,310, 2,507, 2,524, 

and 2,774 ft. Detailed petrographic analyses were performed on samples from the 918- and the 1,310-ft depths 

(table El).

At 918 ft the rock is a fine- to medium-grained, 

weakly cemented, clayey subarkose having a bimodal 

grain-size distribution. Fine-grained, angular quartz is 

associated with the clayey matrix, and medium-grained 

quartz grains occur in the very porous primary matrix. 

Feldspar (mainly euhedral), authigenic microcline, and a 

small amount of altered muscovite are also present. Rock 

fragments consisting of quartzite, gneiss, and some chert 

make up less than 1.0 percent. Heavy minerals such as 

zircon, tourmaline, and epidote are present. Thin sections 

of the rocks at 900 and 934 ft show them to be quartz 

arenite, richer in quartz than the rock described above. 

The rock at 947 ft is a ferruginous shale interbedded or 

intermixed with mudstone, which is enriched in detrital 

quartz and altered potassium feldspar.

The rock at 1,310 ft is a very fine grained, laminated, 

crossbedded lithic arkose. It is poorly cemented with 

porous clay. Angular quartz having secondary overgrowths is the principal mineral. Unaltered microcline 

and minor kaolinized plagioclase make up as much as 

17.2 percent of the rock by volume. Metamorphic rocks 

(quartz-mica and quartz-chlorite schist) and lesser amounts 

of sedimentary and volcanic rocks are the major fragments. The rock contains altered biotite and muscovite as 

well as detrital or authigenic chlorite. Heavy minerals are 

rare. Authigenic epidote predominates over zircon, tourmaline, and garnet (table E3).

The rock at 1,279 ft is a dark-green, green, and red 

shale containing irregular intercalations of mudstone, 

which may be classified as very fine grained arkose and 

arkosic wacke containing altered feldspar (as much as 20 

percent), and as much as 20 percent muscovite and chlorite. The three samples from below 2,500 ft are shale, 

mudstone, and siltstone. The rock is as much as 30 percent quartz, and potassium feldspar and plagioclase make 

up as much as 10 percent. The heavy-mineral fraction 

commonly contains opaque minerals, zircon, tourmaline, 

rutile, sphene, and epidote.

Our analysis indicates that the Keweenawan clastic 

unit in the Lonsdale #65-1 drill hole decreases in maturity 

with depth. The petrological composition of the rocks 

tentatively suggests to us that they are similar to those of 

the Oronto Group in Wisconsin and Michigan. Morey 

(1977) assigned the rocks penetrated by the drill hole to 

the stratigraphically equivalent Solor Church Formation 

and designated these rocks as the type locality. Morey

(1977) suggested that the rocks were deposited in an 

alluvial-plain-lacustrine environment. Seismic evidence 

indicates that an additional 1,300 ft (395 m) of older 

Keweenawan rocks are present at this location (Mooney 

and others, 1970). Some constituents of the heavymineral suite (especially epidote) and the occurrence of 

shale in the section may also indicate a similarity to the 

Solor Church Formation. On the basis of the total amount 

of quartz and the absence of plagioclase feldspar, we think 

that the rocks above about 1,200 ft may belong in the 

Bayfield Group.

The Fond du Lac Formation overlying the Solor 

Church Formation (fig. El) was examined in the Schuette 

#1 drill hole, in Waseca County. A heavy-mineral concentrate was prepared from a sample from 2,305.0 ft deep. 

The rock is a dark-red subarkose containing abundant 

small white spots (kaolinite), similar to equivalent-age 

rocks that crop out farther north in the basin west of the 

horst. The composition of the heavy-mineral concentrate, 

including the small amount of epidote and larger concentrations of tourmaline and zircon, suggests that the rock 

unit may indeed be the correlative of the older rocks in the 

Bayfield Group, possibly the Orienta Sandstone (table E3). 

Sedimentary structures described by Morey (1977) indicate that the rocks were deposited in a fluvial, shallowwater, deltaic environment.

Iowa

Keweenawan clastic rocks, informally referred to as 

"red elastics," were penetrated in five holes drilled prior to 

1987. The Amoco M.G. Eischeid #1 (Eischeid #1) drill 

hole, completed in 1987 and located in the Defiance Basin 

west of the Iowa horst (fig. E5), is the only drill hole that 

penetrated the complete Keweenawan "red clastic" 

sequence (Witzke, 1990). Witzke (1990) was able to correlate the informally named upper "red clastic" sequence 

with rocks of the Bayfield Group and the lower "red clastic" sequence with rocks of the Oronto Group.

For our study, core from two drill holes (B. Lehman #1 

and McCallum #A-1) in the Shenandoah Basin, east of 

the Iowa horst, and cuttings from the Fitzgerald #1 drill 

hole on top of the Iowa horst, in the southwestern part of 

the state (fig. E5), were analyzed. Rocks of the Cambrian 

Mount Simon Sandstone (Witzke, 1990) and of the upper 

part of the Bayfield Group (Chequamegon Sandstone) 

were penetrated by the drill holes. The Mount Simon 

Sandstone, the basal Cambrian sandstone present in eastern Iowa, may be more than 1,000 ft thick in the basins 

east of the Iowa horst (Witzke, 1990). These rocks may 

be difficult to distinguish from the sandstone in the upper 

part of the Chequamegon Sandstone and equivalent rocks 

elsewhere along the rift. On top of the Iowa horst, the 

Mount Simon Sandstone is generally less than 100 ft thick 

(Witzke, 1990).

Precambrian Clastic Sedimentary Rocks, Midcontinent Rift System E13
Table E3. Heavy-mineral composition of core and cuttings from drill holes in Minnesota, Iowa, and Kansas.

[Values in percent by volume of heavy minerals. , not detected]

Sample type

Core

Core

Core

Core

W1/2

NWNE

Cuttings

CD

E

CO

c

CD 

O

Q

McCallum

#A-1

Lonsdale

#65-1

B. Lehman

#1

#1

Finn #1

O

z

CD 

CL

CO 

(O

M o

M-4

M-5

Lo-6

Lm-2

Lm-3

S-1

FC5/6

*-.

H

CD 

Q

OQQA

OA-IO e

3014

1310

2927

2947

2305

3225

CO

CO

CD

c 'E

CD

cr

CO

1 C C

^w. 1

12.5

31.5

8.0

37.0

2.5

474

CD

Semi-opaqu minerals

OC 0

21.2

18.2

43.2

9.0

39.3

26.2

?66

Muscovite

9.4

7.2

12.5

2.7

16.5

2.0

0.6

07

TJ

C 

CO CD 

CD -H 

~ 0

o Z 

CO °

0.4

?0.4

2.0

45.0

7.8

0.5

1 ?

Carbonate minerals

4.8

...

0.2

1.2

54

Phosphate

0.4

0.1

...

Glauconite

1.1

...

0.9

0.2

01

c 

o

N

10 n

11.1

14.7

2.1

5.2

3.8

1.2

59

Tourmaline

1.5

2.7

1.9

0.4

2.3

1.7

1.0

OR

"CD

c

CO

o

0.7

0.1

0.1

1.5

...

0.1

1.7

OR

CD 

-a O

'o.

1.7

0.7

2.0

12.7

0.9

1.1

0.4

1 5

CD 

jt±

CO

8.6

3.0

8.3

4.6

3.7

60

Hornblende

0.1

0

0.

0.1

0.3

0.3

0.1

01

CD 'c 

CO

...

Staurolite

and Sphene 

anatase

0.1

0.1

c

Sillimanite aandalusite

...

-o

Pyroxene an 

monazite

0.1

...

c CO 

 w

 i.|

...

0.1

CD

o

CD

X,

1 °.

1.5

4.5

0.4

2.4

1.3

9.8

2.0

Py h loreroc 

19 Q 1 £..%/ 

00.

1.2

0.4

0.1

0.3

0.2

In the B. Lehman #1 drill hole in Dallas County, Precambrian rocks from 2,924 to 2,964 ft were cored. The 

contact between the Cambrian Mount Simon Sandstone 

and the Precambrian "red elastics" was picked at 2,924 ft 

(R.R. Anderson, Iowa Geological Survey; R.M. McKay; 

P. Berendsen; A. Barczuk; G.B. Morey; P. McSwiggin; 

and M.P. Carlson, written commun., 1987). Thin sections 

were prepared from samples from depths of 2,922, 2,927, 

2,947, and 2,956.5 ft. The first three samples are fine- to 

coarse-grained, poorly sorted subarkose, some intermixed 

with varicolored mudstone and conglomeratic sandstone. 

The quartz content of the rock varies from 20.0 to 70.0 

percent, and it is commonly recrystallized (table El). 

Altered feldspar (as much as 20.0 percent) consists of 

microcline and perthite, and minor orthoclase. Mica and 

chlorite (as much as 5.0 percent) is present at 2,947 ft. 

The red and greenish siltstone at 2,956.5 ft contains as 

much as 30.0 percent muscovite, biotite, and detrital chlorite. In addition to opaque and semi-opaque minerals, zircon, apatite, epidote, tourmaline, and glauconite are 

present in the samples (table E3). The petrographic and 

heavy-mineral composition indicate that these rocks are 

similar to those in the lower part of the Bayfield Group, 

possibly the Orienta Sandstone.

The McCallum #A-1 drill hole is located 2 mi (3.2 km) 

northeast of the B. Lehman #1 drill hole in Dallas County. 

The contact between the Upper Cambrian Mount Simon 

Sandstone and the Keweenawan "red elastics" was picked 

at 3,009.3 ft (R.R. Anderson, Iowa Geological Survey; 

R.M. McKay; P. Berendsen; A. Barczuk; G.B. Morey; P. 

McSwiggin; and M.P. Carlson, written commun., 1987). 

Thin sections of samples from depths of 2,967, 2,990, 

3,008.5, 3,012.5, and 3,014 ft were studied. The first three 

samples are from the Mount Simon Sandstone and are

fine- to coarse-grained subarkose to arkose, in some places 

conglomeratic. Quartz, most of it recrystallized, predominates, whereas microcline and orthoclase are subordinate 

(as much as 10 percent; table El). The sample from 2,967 

ft contains only a few zircon and tourmaline grains. The 

rock at 3,012.5 ft is a medium-grained subarkose very similar to that at 3,008.5 ft, above the angular unconformity in 

the Mount Simon Sandstone.

The contact between the Mount Simon Sandstone and 

the Chequamegon Sandstone is interpreted to be at about

3.013 ft. The underlying Chequamegon Sandstone at

3.014 ft is a fine-grained subarkose to arkose, mixed with 

minor subarkosic wacke or mudstone. Phyllosilicates, 

preferentially concentrated along bedding planes, make up 

as much as 10 percent. Zircon, tourmaline, epidote, and 

rutile grains are important constituents (table E3). The 

amount of feldspar, phyllosilicates, and fragments of metamorphic and sedimentary rocks also suggests a similarity 

to the Chequamegon Sandstone.

We examined two samples of cuttings from a depth of 

2,595-2,605 ft in the Fitzgerald #1 drill hole, located on 

top of the Iowa horst in Mills County (fig. E5), southwestern Iowa. The rock is a fine- to coarse-grained arkose 

cemented with clay. Microcline, orthoclase, and a few 

grains of albite make up as much as 20 percent of the 

rock.

In the lower 5 ft of the drill hole, basalt having a microophitic or insertal texture and minor basaltic tuffs and tuffites are present. The volcanic rocks have a high content 

of opaque minerals (as much as 5 percent). The clastic 

sedimentary rocks were reported to constitute the lower 

part of the Mount Simon Sandstone directly overlying the 

Keweenawan volcanics (R.R. Anderson, Iowa Geological 

Survey; R.M. McKay; P. Berendsen; A. Barczuk; G.B.

E14 Strategic and Critical Minerals, Midcontinent Region, U.S.
Morey; P. McSwiggin; and M.P. Carlson, written commun., 1987). Our preliminary analysis of the cuttings indicates that the sedimentary rocks are similar to the 

Chequamegon Sandstone.

Sedimentary rocks showing petrographic characteristics 

similar to those of the Oronto Group were not encountered 

in any of the samples studied, even in the cuttings from 

the Fitzgerald #1 drill hole on top of the Iowa horst. The 

McCallum #A-1 and the B. Lehman #1 are located in the 

Thurman-Redfield tectonic zone separating the Iowa horst 

from the Shenandoah Basin to the east. Because of possible complex structural relationships, the preserved 

Keweenawan sequence in the two holes may differ 

slightly. Because we did not study the petrography of the 

Mount Simon Sandstone, the contact with the equivalent 

underlying rocks of the Bay field Group is difficult to 

interpret and should be considered tentative.

Nebraska

The basal Paleozoic rocks in Nebraska are sandstone 

that in the past has been referred to as the "Dresbach Sandstone" (Upper Cambrian), but more recently as the "Mount 

Simon Formation" in "Correlation Of Stratigraphic Units 

of North America" (COSUNA) correlation charts. Precambrian clastic sedimentary rocks associated with the 

Midcontinent Rift System are collectively referred to as the 

"red elastics." Some attempts have been made to subdivide the sequence on the basis of the primary rock type 

present and the color of the rock. Differentiation between 

Paleozoic basal sandstone and underlying Precambrian 

sedimentary rocks is to a large degree based on the degree 

of rounding and frosting of quartz grains and the scarcity 

of other mineral grains.

Cuttings from four drill holes were examined. Three of 

the holes (Koutney #1, Ihde #1, and Sullivan #1) are 

located in the basin northwest of the central horst in 

Nebraska (fig. E5). The fourth hole (Schroeder (Amerada) 

#1) is located on top of the southwestern extension of the 

Iowa horst close to the Iowa border (fig. E5).

We examined samples from depths of 1,690-1,700, 

1,730-1,740, 1,790-1,800, 1,890-1,895, 2,260-2,270, 

2,300-2,310, 2,570-2,580, and 2,650-2,660 ft in the Koutney #1 drill hole, in Saunders County (fig. E5).

The rocks above 2,300 ft are characteristic of the lower 

Paleozoic rocks in the area, consisting of lithic arkose, 

clean quartz sandstone, dolomite, and oolitic ironstone in 

the lower part of the section. The cuttings below 2,300 ft 

are quartz arenite resembling rocks of the Hinckley Sandstone. Illitic and calcareous shale and some dolomite and 

limestone also are present in the cuttings and probably are 

cavings from higher up in the section.

Cuttings from a depth of 1,570-1,575 ft in the 

Schroeder (Amerada) #1 drill hole, in Cass County (fig.

E5), were examined. The rock is a fine-grained arkose 

cemented with calcite. Quartz of probable volcanic origin 

makes up the bulk of the rock, but a considerable amount 

of feldspar consisting of microcline, altered plagioclase, 

orthoclase, and a few authigenic albite crystals, are also 

present. Minor volcanic rock fragments consist of basalt, 

altered volcanic glass, and tuff. Abundant heavy minerals 

include epidote and opaque minerals, together with minor 

rutile, zircon, and hornblende. The petrographic and 

heavy-mineral composition of the cuttings resembles those 

of the lower part of the Bayfield Group or upper part of 

the Oronto Group.

Cuttings from a depth of 2,830 ft in the Ihde #1 drill 

hole, in Seward County (fig. E5), consist of sandstone, red 

hematitic shale, and a few fragments of granite-gneiss, 

typical of rocks of the Rice Series in Kansas.

Cuttings from a depth of 2,310-2,319 ft in the Sullivan 

#1 drill hole, in Lancaster County (fig. E5), consist of 

orange to pink, partially kaolinized feldspar and mediumto coarse-grained, well-rounded quartz grains. They are 

typical of a rock referred to in Kansas as "granite wash," a 

term used to describe mostly arkosic debris directly overlying Precambrian basement rocks. These rocks are erosional detritus derived from the predominantly granitic 

basement and are not restricted to the area of the rift, but 

occur throughout the state. The granite wash ranges in 

age from Precambrian to Middle Pennsylvanian and may 

be as much as several hundred feet thick.

Kansas

Cuttings from 15 drill holes (fig. E6) were examined 

and described. Tentative correlations with established 

Stratigraphic units in the Great Lakes region were made on 

the basis of composition of most of the cuttings in the 

samples (fig. E7).

Most Precambrian and, possibly, some Lower Cambrian sedimentary rocks in central and eastern Kansas 

have been assigned to the Rice Series. On the basis of 

examination of drill-hole cuttings and minor core samples 

from a large number of drill holes, we have found the 

rocks are mostly oxidized, fine- to coarse-grained arkose 

and subarkose, siltstone, and conglomerate. Medium- to 

dark-gray, partly calcareous siltstone and shale, interbedded with limestone, also are part of the sequence. Underlying the clastic rocks are middle Keweenawan mafic 

volcanic rocks.

Poorly cemented, fine- to medium-grained, varicolored 

sandstone (quartz arenite) makes up most of the section in 

the upper part of the Livingood #1 drill hole (Dickinson 

County; figs. E6 and E7). The quartz grains in this sandstone are poorly rounded. Microcline, orthoclase, and perthite, altered to varying degrees, and minor mica are 

present. Some mudstone or shale is locally interbedded.

Precambrian Clastic Sedimentary Rocks, Midcontinent Rift System E15
Drill 

hole 

name

Livingood #1 

Finn #1 

Ireland #1 

Beach #1 

Blaney #1 

Neal #1

Wood #1 

(Crawford)

Sedlacek #1 

Suggett #1

Meyers #1 

(Seneca)

Fagerberg #1 

Sherman #1 

Weakley #1 

Kratchovil #1 

Winkler#1

Drill hole location, 

(sec., township, 

and range) 

county

Sec. 13, T. 13 S., R. 2 E 

Dickinson

Sec. 4, T. 4 S., R. 7 E. 

Marshall

Sec. 8, T. 5S., R. 7 E.

Marshall 

Sec. 3, T. 4S., R. 8E.

Marshall

Sec. 3, T. 5 S., R. 7 E. 

Marshall

Sec. 5, T. 4 S., R. 8 E.

Marshall

Sec. 1,T. 7S., R. 3E. 

Clay

Sec. 31, T. 4S., R. 8 E. 

Marshall

Sec. 34, T. 2 S., R. 8 E. 

Marshall

Sec. 19, T. 3S., R. 11 E 

Nemaha

Sec. 31 , T. 6 S., R. 8 E. 

Pottawatomie

Sec. 13, T. 5S., R. 3 E. 

Washington

Sec. 26, T. 5S..R. 7 E. 

Marshall

Sec. 30, T. 5 S., R. 7 E. 

Marshall

Sec. 8, T. 6 S., R. 6 E. 

Riley

Granite 

wash

2,295

2,420

1,623

707 750

2,015 2,030

CLASTIC ROCKS

BAYFIELD GROUP AND JACOBSVILLE SS

Mount Simon Ss, 

Chequamegon Ss

3,260

2,360

1,454

1,715

1,920

Devils Island Ss, 

Hinckley Ss

3,900

2,570

2,264

1,780

2,114

Orienta Ss, 

Fond du Lac Fm

4,865

2,164

2,305

2,150

VOLCANICLASTIC ROCKS

ORONTO GROUP AND 

SOLOR CHURCH FM

Freda Ss

2,245

3,162

3,140

Nonesuch 

Shale

3,800 3,900

3,255

3,245

Copper Harbor 

Conglomerate

VOLCANIC 

ROCKS

2,500 2,526

2,585 2,728

Figure E7. Preliminary stratigraphic correlation of drill-hole cuttings from Kansas with established stratigraphy in the northern 

part of the Midcontinent Rift System. Numbers at the end of arrows indicate the depth penetrated into the Precambrian; other 

numbers are approximate depths of contacts. Ss, sandstone; Fm, formation.

These rocks are assigned to the Rice Series in Kansas. 

Petrographically, they are similar to sedimentary rocks of 

the Bayfield Group or the Jacobsville Sandstone (figs. El 

and E7) and may be equivalent to the Chequamegon Sandstone or the lower part of the Mount Simon Sandstone.

Similar rocks occur in the Blaney #1, Sedlacek #1, 

Weakley #1, Ireland #1, Beach #1, and Neal #1 drill holes 

and in the upper part of the Suggett #1 drill hole, all in 

Marshall County (figs. E6 and E7). The cuttings from the 

Fagerberg #1 drill hole (Pottawatomie County) indicate 

that the rocks may belong to the lower part of the Bayfield 

Group (fig. E7). The rocks in the upper part of the Meyers 

#1 drill hole (Nemaha County), above the granite basement, are most likely granite wash (fig. E7).

Cuttings below about 3,800 ft in the Livingood #1 drill 

hole are similar to rocks of the Bayfield Group or the 

Jacobsville Sandstone and are tentatively correlated with 

rocks of the Hinckley Sandstone and the Fond du Lac Formation (figs. El and E7).

The cuttings at 3,800 ft in the Finn #1 drill hole (Marshall County), the Sherman #1 drill hole (Washington 

County), and the Wood #1 drill hole (Clay County) contain 

significant amounts (more than 20 percent) of volcanic 

rock fragments, suggesting that they may be equivalent to 

rocks in the upper part of the Oronto Group (figs. El and 

E7). The cuttings from about 2,500 ft in the Kratchovil #1 

drill hole (Marshall County), which are of poor quality, 

may be derived from volcanic rocks or from fragments of 

volcanic rock in a clastic sedimentary rock.

MODAL ANALYSIS

There are major petrographic differences between the 

older volcaniclastic sedimentary rocks (Bayfield Group) 

and the younger arkosic rocks (Oronto Group). The psammitic volcaniclastic sedimentary rocks are exclusively litharenite and lithic arkose, whereas the arkosic rocks are

E16 Strategic and Critical Minerals, Midcontinent Region, U.S.
mainly subarkose. According to the classification scheme 

of Pettijohn and others (1972), the clastic rocks plot in the 

quartz-rich sector of the triangular diagram (fig. E3), and 

most fall in the left, or arkosic, part of the sector. Rocks 

of the Fond du Lac Formation tend to be transitional 

between arkosic and volcaniclastic sedimentary rocks and 

plot in intermediate positions on the triangular diagram. 

In addition, they are also slightly enriched in volcanic and 

sedimentary rock fragments (fig. E4).

Classifying the rocks according to the composition of 

the rock fragments shows trends similar to those of the 

petrographic classification (fig. E4). The volcaniclastic 

sedimentary rocks are mostly volcanic arenite, whereas the 

arkosic rocks containing metamorphic fragments are classified as phylarenite (only rocks that contain more than 5 

percent rock fragments were included). The Nonesuch 

Shale is an exception. The points plotted on the triangular 

diagram (fig. E4) reveal the pelitic to aleuritic character of 

the Nonesuch Shale. The fine-grained volcanic rock fragments in the shale have already been kaolinized, zeolitized, and chloritized, and this accounts for a large 

proportion of the cement and the chlorite content. In addition, both quartz and iron in these rocks are mainly of 

volcanic origin.

Metamorphic rock fragments are important components 

of the arkosic rocks examined. Their texture and composition are such that they are resistant to weathering. Plutonic rocks are generally coarser grained and weather 

easily, and consequently they contribute more to the mineral fraction of the newly formed rocks.

The characteristic features of the quartz and the composition of the feldspar grains in the upper part of the 

arkosic sequence suggest that the amount of material 

derived from older volcanic and volcaniclastic rocks 

diminished upward. Quartz grains are well rounded in the 

coarse-grained sandstone and poorly rounded to angular in 

the finer fractions. Quartz grains commonly show evidence of secondary overgrowths and one or more abrasion 

rims, indicative of several cycles of sedimentation. Quartz 

of possible volcanic origin, showing corroded bays, inclusions of devitrified glass and needles of rutile, occurs 

mostly in the volcaniclastic rocks. They are also characterized by their low quartz content, ranging from 2.6 to 

12.4 percent (except sample 16a, which contains 27.3 percent quartz). The quartz content of the arkosic rocks is 

markedly higher, ranging from 37.8 to 71.3 percent. The 

quartz content is, to a large extent, determined by the 

provenance of the rock. The lower quartz content is 

mainly compensated for by a higher content of pelitic and 

aleuritic material.

Plagioclase (as much as 5.0 percent) occurs only in the 

volcaniclastic sedimentary rocks, except for one sample of 

Jacobsville Sandstone (sample 22a). The feldspar content 

of the upper Keweenawan sedimentary rocks ranges from 

5.7 to 21.3 percent, averaging about 10-15 percent. Only

two samples contain less than 5.0 percent feldspar (table 

El). Microcline, orthoclase, and perthite are the predominant feldspars in the clastic rocks.

Opaque iron-rich aggregates of magmatic or metamorphic origin are important components of volcaniclastic sedimentary rocks, whereas clayey and muddy intraclasts are 

more common in ferruginous arkosic rocks. The elevated 

amount of metamorphic ferruginous rock fragments in the 

Jacobsville Sandstone may be derived from nearby ironformation.

Except for the Nonesuch Shale, the amount of phyllosilicates is generally higher in the arkosic rocks. Secondary metasomatic alteration and, to a certain extent, the 

depositional environment contribute to the enrichment of 

the Nonesuch Shale in phyllosilicates, especially chlorite. 

Cement in the majority of samples consists mainly of iron 

oxides and clay (table El). Quartz and carbonate are the 

predominant cement in a few samples. Zeolite is an 

important cement-forming mineral in the Nonesuch Shale 

and is as high as 13.2 percent (sample 16a; table El, 

under "other cement"). These variations are believed to 

be controlled primarily by the depositional environment.

The phosphorite and glauconite content of all Precambrian rocks is low (less than 0.5 percent), as opposed to 

that of the Lower Cambrian rocks studied. The porosity 

increases with maturity of the rocks and reaches a maximum in quartz arenite of the Hinckley Sandstone and the 

Devils Island Sandstone.

HEAVY-MINERAL ANALYSIS

There is a noticeable difference in the heavy-mineral 

composition of volcaniclastic sedimentary rocks of the 

Oronto Group as compared to the increasingly more arkosic rocks of the Bayfield Group. The older volcaniclastic 

sedimentary rocks, such as the Freda Sandstone and the 

Nonesuch Shale, contain as much as 3 percent (by volume) heavy minerals, most of which are opaque and semiopaque ferroan metamorphic minerals, iron titanium 

oxides, and some sulfides, as well as a small amount of 

mostly authigenic epidote. The epidote group of minerals 

consists of pistacite, normal epidote, and clinozoisite. 

Small amounts of garnet, zircon, and rutile are present, but 

tourmaline is absent.

On the basis of studies by Tyler and others (1940), and 

also reported by Dickas (1986), we know that the ratio of 

combined tourmaline and zircon to epidote in the Bayfield 

Group is the reverse of that in the Freda and Nonesuch 

assemblages. We found the same pattern in our studies 

(table E2) and used this information as a discriminating 

factor in correlating subsurface samples with known 

stratigraphy.

In the younger arkosic rocks of the Bayfield Group, the 

combined amounts of zircon, tourmaline, garnet, rutile, and

Precambrian Clastic Sedimentary Rocks, Midcontinent Rift System E17
hornblende, with few exceptions, make up a larger fraction 

of the heavy-mineral suite than in the more mature and 

older rocks of the Oronto Group. Elevated amounts of 

partly authigenic apatite occur in the transitional rocks of 

the Orienta Sandstone and the Fond du Lac Formation. 

Inconsistencies in the expected makeup of the heavymineral fraction in some of the rocks may be attributable 

to differences in the local environment of deposition. The 

amount of opaque and semi-opaque minerals in the heavymineral fraction varies from 3 to 90 percent, and in most 

cases exceeds 40 percent.

The heavy-mineral composition of the drill core samples is characterized by generally elevated amounts of zircon and tourmaline (table E3) and low contents of epidote 

(except for sample LO-6), which is similar to the heavymineral composition of the upper clastic rocks. The apatite 

content is characteristic of rocks of the Orienta Sandstone 

and the Fond du Lac Formation.

DISCUSSION

Until recently, when two deep oil exploration wells 

were drilled into the buried southern extension of the rift in 

Iowa and Kansas, most of our knowledge about the history 

and development of the rift was derived from studies in the 

outcrop belt in Minnesota, Wisconsin, and Michigan. The 

1,281-Ma age of the gabbro (Van Schmus and others, 

1990) below the sedimentary section in the Eischeid #1 

drill hole in Carroll County, Iowa, is evidence (Van 

Schmus and others, 1990) that the complete Keweenawan 

sedimentary sequence was penetrated at that location. 

Stratigraphic (Witzke, 1990) and petrologic (Ludvigson 

and others, 1990) studies of the Keweenawan section in the 

basin west of the Iowa horst, enabled Witzke (1990) to 

tentatively correlate rocks of the Oronto and Bayfield 

Groups with those in the Eischeid #1 well. The gabbro in 

the Texaco Poersch #1 drill hole, in Washington County, 

Kansas, was dated at 1,097.5 Ma (Van Schmus and others, 

1990). The important inference is that rifting was initiated 

at the same time as in the Lake Superior region. Thus, 

even though the rift in Kansas is physically separated from 

the rift farther north, Stratigraphic correlations may be 

made between the rocks in Kansas and those in Nebraska, 

Iowa, and Minnesota.

The initial opening of the rift (graben formation) at 

about 1,100 Ma was characterized by the eruption of large 

volumes of mafic volcanic rock and shallow mafic intrusive rocks over a time span of about 100 million years 

(McSwiggen and others, 1987; Behrendt and others, 

1988; Chandler and others, 1989). After cessation of the 

main phase of volcanic activity, during which time the 

area apparently remained above sea level, the central graben began to fill with coarse volcaniclastic rocks and

coarse interflow conglomerate. This sequence was overlain by rocks of the Copper Harbor Conglomerate, representing a prograding alluvial-fan complex (Daniels, 1982). 

The overlying Nonesuch Shale was deposited in a riftflanking lacustrine environment, probably caused by disruption and damming of existing drainages (Daniels, 

1982).

The clastic sedimentary succession overlying the volcanic rocks was divided into the Oronto (lower) and the 

Bayfield (upper) Groups (Thwaites, 1912), each of which 

exhibits distinct characteristics (fig. El). We refer to the 

older Oronto Group as a volcaniclastic sequence because 

the rocks contain abundant debris derived from the underlying volcanics. In addition, Dickas (1986) mentioned an 

upward and distal decrease in grain size. Elmore and 

Daniels (1980) viewed the depositional environment of the 

Oronto Group as a transgressive-regressive alluvial-fan, 

lacustrine, and fluvial system, which Morey and Ojakangas (1982) considered characteristic of an extensional 

regime.

During the evolution of the rift, contributions from 

rocks exposed in the nearby highlands east and west of the 

graben increased (Merk and Jersa, 1982). The rocks of 

the younger Bayfield Group are compositionally more 

mature. The heavy-mineral suite of the group as a whole 

is different from that of the underlying Oronto Group. 

Individual formations such as the Orienta Sandstone and 

the Fond du Lac Formation contain large amounts (as 

much as 12.1 percent) of authigenic apatite. Plagioclase is 

a less common mineral constituent; orthoclase and microcline feldspar instead became prominent. Metamorphic 

fragments predominate over volcanic fragments.

The rift is characterized by an axial horst flanked by 

clastic-filled basins (Anderson, 1990; Dickas, 1986). 

Axial basins are known to occur in Minnesota, Iowa, and 

Nebraska. Anderson (1990) mentioned that late-fill rift 

sediments are locally preserved in the axial basins 

(Anderson, 1990).

Geologic studies by Morey and Ojakangas (1982) and 

gravity and magnetic modeling by McSwiggen and others 

(1987) suggest that some of the axial basins contain older 

and denser sedimentary rocks of the Oronto Group, which 

are as much as several miles thick. If this is true, then the 

central horst must have developed after deposition of the 

Solor Church Formation-Oronto Group, but not necessarily before deposition of the Bayfield Group.

Rift-related faults were periodically reactivated 

throughout geologic time (Berendsen and others, 1990), 

resulting in extensive erosion from topographic highs and 

preservation of the sedimentary section in downdropped 

blocks along the axial trace of the rift. High-angle normal 

and reverse faults and overthrusting juxtaposed rocks of 

different lithologies and ages. Woelk and Hinze (1991) 

interpreted seismic data to show a major reverse fault in

E18 Strategic and Critical Minerals, Midcontinent Region, U.S.
the stratigraphic succession in the Texaco Poersch #1 deep 

drill hole into the Precambrian in Washington County, 

Kansas (Berendsen and others, 1988). In this drill hole a 

4,600-ft-thick unit of primarily basalt and gabbro (1,097.5 

Ma) overlies a 3,900-ft-thick package of presumably 

younger, primarily arkosic rocks.

ACKNOWLEDGMENTS

This study was made possible through the cooperation 

and support of the USGS through the Midcontinent Strategic and Critical Minerals Project. We also acknowledge 

the cooperation of geological survey personnel in the 

states of Minnesota, Wisconsin, and Michigan. They were 

very helpful in providing us access to core and leading us 

to outcrops.

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E20 Strategic and Critical Minerals, Midcontinent Region, U.S.

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State catalogs, "List of U.S. Geological Survey Geologic and Water-Supply Reports and Maps For (State)," may be purchased by mail and 

over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet 

form only.

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" is available free of charge by mail or may be 

obtained over the counter in paperback booklet form only. Those wishing 

a free subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 National Center, Reston, VA 22092.

Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and publications.