Clastic Rocks Associated with the

Midcontinent Rift System in Iowa

U.S. GEOLOGICAL SURVEY BULLETIN 1989–I

M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE INTERIOR
Chapter I

Clastic Rocks Associated with the

Midcontinent Rift System in Iowa

By RAYMOND R. ANDERSON and ROBERT M. McKAY

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

Gordon P. Eaton, Director

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: 1997

For sale by

U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE INTERIOR

Anderson, Raymond R.

Clastic rocks associated with the Midcontinent Rift System in Iowa / 

by Raymond R. Anderson and Robert M. McKay.

p. cm. — (Strategic and critical minerals in the midcontinent 

region, United States ; ch. I) (U.S. Geological Survey bulletin ; 1989) 

Includes bibliographical references (p. – ).

Supt. of Docs. no. : I 19.3 : 1989–I

1. Geology, Stratigraphic—Proterozoic. 2. Geology—Iowa. 

3. Keweenawan Rift. I. McKay, Robert M. II. Title. III. Series. 

IV. Series: U.S. Geological Survey bulletin ; 1989.

QE75.B9 no.1989–I

[QE653.5]

557.3 s—dc21 96–50394

[551.7’15’09777] CIP

Published in the Central Region, Denver, Colo.

Manuscript approved for publication October 20, 1996.

Photocomposition by Norma J. Maes.
Contents III

CONTENTS

Abstract I1

Introduction I1

The Midcontinent Rift System (MRS) in Iowa I3

MRS clastic rock data I4

The well record in Iowa I4

Geophysical data I4

Gravity anomaly data I4

Magnetic anomaly data I4

Reflection seismic data I5

Clastic units associated with the MRS I7

Reagan Sandstone I7

Mount Simon Sandstone I7

Red clastics I7

History of the term “Red clastics” I7

The M.G. Eischeid #1 deep petroleum test well I9

Stratigraphy of the M.G. Eischeid #1 Red clastics I10

Upper Red clastics sequence I10

Lower Red clastics sequence I12

Red clastics basins flanking the Iowa Horst I16

Western flanking basins I16

Duncan Basin I16

Defiance Basin I16

Eastern flanking basins I16

Wellsburg Basin I17

Ankeny Basin I18

Shenandoah Basin I18

Clastic basins on the Iowa Horst I19

Stratford Basin I19

Jewell Basin I19

Mineola Basin I19

Ames Block I20

Summary of Red clastics in Iowa I23

Depositional history of MRS clastic rocks in Iowa I24

MRS lower Red clastics sequence I24

MRS upper Red clastics sequence I24

Problems associated with differentiation of clastics I25

Investigation of the UPH–2 core I25

Reinterpretation of the #1 Wilson well I26

Petrographic analysis I26

Examination of fossils I26

Examination of chip samples I29

Conclusions I31

Re-examination of the #1 Huntley well I31

Evaluation of criteria for picking the Mount Simon–Red clastics contact I32

Metallic mineral potential of MRS clastic rocks in Iowa I35

Petroleum resources I35

Petroleum potential studies of Eischeid samples I35

Source-rock potential I35
IV Contents

Porosity I36

Conclusions I36

Petroleum potential of MRS clastic rocks I36

Conclusions I37

Acknowledgments I38

References cited I38

Appendix I1. Results of petrographic analyses of thin-sectioned samples collected from 

basal Mount Simon Sandstone and Red clastics interval of the M.G. Eischeid #1 well in 

Carroll County, Iowa I43

Appendix I2. Results of petrographic analyses of thin-sectioned samples collected from 

Mount Simon Sandstone and Red clastics intervals of several wells in 

Iowa and Illinois I44

FIGURES

 I1.–I2. Maps showing:

I1. Location and major lithologies of the Midcontinent Rift System I2

I2. Structural components of the Midcontinent Rift System in Iowa I3

I3. Chart showing comparative Keweenawan stratigraphy of the Midcontinent 

Rift region I5

I4.–I6. Maps showing:

I4. Major features of the Midcontinent Rift System in Iowa displayed on the

“Bouguer Gravity Anomaly Map of Iowa” I6

I5. Major features of the Midcontinent Rift System in Iowa displayed on a 

modified version of the “Aeromagnetic Anomaly Map of Iowa” I8

I6. Locations of Petty-Ray Geosource, Inc., reflection seismic profiles on a 

map of major features of the Midcontinent Rift System in Iowa I9

I7–I8. Generalized lithologic logs of:

I7. Upper Red clastics sequence from M.G. Eischeid #1 deep petroleum test 

well, Carroll County I11

I8. Lower Red clastics sequence from M.G. Eischeid #1 deep petroleum test 

well, Carroll County I13

19. Quartz–feldspar–lithic grain (QFL) diagrams for Keweenawan Supergroup 

clastic rocks from Lake Superior area and M.G. Eischeid #1 well, Carroll 

County I15

I10.–I13. Maps showing thickness of:

I10. Red clastics preserved in basins flanking Iowa Horst I18

I11. Lower Red clastics sequence in Stratford and Jewell Basins I20

I12. Upper Red clastics sequence in Mineola Basin I21

I13. Upper Red clastics sequence, lower Red clastics sequence, and total

Red clastics on Ames Block I22

I14. Diagram of depositional environment of unit B and lower Oronto Group 

 equivalent rocks I23

I15.–I16. Charts showing:

 I15. Stratigraphy and construction history of #1 Wilson oil exploration well, 

 Page County I27

I16. Historical interpretations of stratigraphy near base of #1 Wilson oil 

 exploration well, Page County I28

I17. Lithic log and quartz–feldspar–lithic grain (QFL) diagram for part of #1 Wilson 

oil exploration well, Page County I29

I18. Photographs of selected microfossils recovered from chip samples from #1 

Wilson oil exploration well, Page County I30
Contents V

I19. Back-scattered electron photomicrograph of Cryptostome bryozoan from #1 

Wilson oil exploration well, Page County I31

I20. Chart showing stratigraphy and construction history of #1 Huntley oil 

exploration well, Butler County I33

I21. Lithic and gamma logs and quartz–feldspar–lithic grain (QFL) diagram for 

part of #1 Huntley oil exploration well, Butler County I34

I22. Quartz–feldspar–lithic grain (QFL) diagram of analyses of Mount Simon 

Sandstone and Red clastics from selected Iowa wells I35

I1. Iowa wells penetrating the Midcontinent Rift System Red clastic rocks I10

I2. Dimensions of Red clastics basins in Iowa I17

METRIC CONVERSION FACTORS

Multiply By To obtain

feet 0.3048 meters

miles

square miles

cubic miles

1.609

2.590

4.168

kilometers

square kilometers

cubic kilometers

TABLES
Clastic Rocks, Midcontinent Rift System, Iowa 

Abstract

The Middle Proterozoic Midcontinent Rift System

(MRS) of North America is a failed rift that formed in

response to region-wide stresses about 1,100 Ma. In Iowa,

the MRS is buried beneath 2,200–3,500 ft of Paleozoic and

Mesozoic sedimentary rocks and Quaternary glaciogenic

deposits. An extremely large volume of sediments was

deposited within basins associated with the rift at several

stages during its development. Although the uplift of a

rift-axial horst resulted in the erosional removal of most of

these clastic rocks from the central region of the MRS in

Iowa, thick sequences are preserved in a series of

horst-bounding basins. Recent studies incorporating petrographic analysis, geophysical modeling, and other analytical procedures have led to the establishment of a

preliminary stratigraphy for these clastic rocks and interpretations of basin geometries. This information has allowed

the refinement of existing theories and history of MRS formation in Iowa. Additionally, drill samples previously

interpreted as indicating the existence of early Paleozoic

basins overlying the Proterozoic MRS basins were re-examined. Samples previously interpreted as deep-lying Paleozoic rocks are now known to have caved from upper levels

of the drillhole and were out of stratigraphic position. No

deep Paleozoic basins exist in this area. These investigations led to the development of petrographic parameters

useful in differentiating the Proterozoic MRS Red clastics

from Paleozoic clastic rocks having similar lithologies.

INTRODUCTION

The Middle Proterozoic Midcontinent Rift System

(MRS) extends from eastern Lake Superior, south and west

across northwestern Wisconsin, southeastern Minnesota,

central and southwest Iowa, southeastern Nebraska, and

northeastern Kansas, with related dikes continuing into

Oklahoma (fig. I1). Over most of its length the MRS is

dominated by a series of axial horsts, composed primar

mafic volcanic rocks, and flanked by clastic-filled b

Related clastic rocks are also locally preserved on the c

horsts. MRS clastic rocks are exposed in the Lake Su

region of Minnesota, Wisconsin, and Michigan, but ar

ied beneath Phanerozoic strata south of that region. 

In Iowa the MRS is dominated by a central hor

marily composed of mafic igneous rocks (the Iowa H

flanked by five clastic-filled basins (Anderson, 1988)

rocks of the MRS in Iowa are buried beneath 22,200–

ft of Phanerozoic marine and terrestrial sedimentary 

and glacial drift. MRS rocks are known only from s

drill samples and interpretation of their geophysical 

tures. Drill samples in Iowa include only 14 penetratio

MRS clastic rocks, most of which are shallow. These 

have historically been called Red clastics (Norton, 1

Most of these drill penetrations are shallow, and 

include cored intervals. 

Modeling of gravity and magnetic data over the 

led to the interpretation of clastic-filled basins flankin

Iowa Horst (fig. I2). They include on the west (no

south) the Duncan and Defiance Basins and on the ea

Wellsburg, Ankeny, and Shenandoah Basins (And

1988). Two basins preserving Red clastics were also 

eled on the Iowa Horst, the Stratford and Mineola B

(Anderson, 1988). Recently, two additional areas o

clastics were identified on the Iowa Horst, the Jewell 

and Ames Block (Anderson, 1992). Additionally

modeling of a series of gravity profiles, controlled by r

tion seismic and magnetic data, has led to the delineat

the geometries of these basins and estimation o

volumes of clastics preserved in each (Anderson, 1988

1Iowa Department of Natural Resources, Geological Survey 

Bureau, 109 Trowbridge Hall, Iowa City IA 52242–1319.

2Measurements of drill holes on which most thicknesses are b

were in feet.

STRATEGIC AND CRITICAL MINERALS IN THE MIDCONTINENT REGION, UNITED STATES

Clastic Rocks Associated with the 

Midcontinent Rift System in Iowa

By Raymond R. Anderson1 and Robert M. McKay1
I2 Strategic and Critical Minerals, Midcontinent Region, U.S.

Drilling of the M.G. Eischeid #1 deep petroleum test

well by the Amoco Production Company in 1987 produced

a suite of chip samples and cores that allowed the erection

of an informal stratigraphy for the Red clastics. This

included the identification of two “groups” and seven “formations” (McKay, 1990; Witzke, 1990). A series of studies

of these samples and associated logs (see Anderson, 1990a)

also provided a wealth of information about the depositional

history, thermal maturity, hydrocarbon potential, a

netic history of these rocks.

Study of chip samples collected during the d

the #1 Wilson and #1 Huntly oil tests suggested 

ence of thick sequences of basal Paleozoic cla

(Bunker, 1982; Bunker and others, 1988; Sims, 19

presence of these thick sequences was interprete

dence of Middle Cambrian reactivation and subs

Figure I1. Location and major lithologies of the Midcontinent Rift System (MRS) (from Palacas and others, 1990).
Clastic Rocks, Midcontinent Rift System, Iowa 

the Red clastics basins. These samples were re-examined

using petrographic and SEM techniques to evaluate the earlier interpretations. Additionally, samples of basal Paleozoic clastic rocks from many areas of Iowa and Illinois

were petrographically examined to determine their characteristics. This included a study of the thick sequence of

Cambrian Mount Simon Sandstone recovered in the Consolidated Edison UPH–2 core from northwestern Illinois. 

THE MIDCONTINENT RIFT SYSTEM (MRS) IN 

IOWA

The Iowa segment of the MRS extends about 300 mi

southwestward from the southern end of the Belle Plaine

Fault Zone just north of the Iowa border to eastern Neb

where it is truncated by a second major transfer fa

north- to northeast-trending structure having a trend a

ently coincident with the Big Blue River. The Iowa seg

is dominated by the Iowa Horst (Anderson, 1988), wh

bounded by the Thurman-Redfield Structural Zone o

southeast and the Northern Boundary Fault Zone (And

1988) on the northwest, and flanked by basins that co

clastic sedimentary rocks as much as 30,000 ft thick, in

ing a denser basal and a less-dense upper sequence. 

three clastic basins identified east of the Iowa Horst in 

(fig. I2), the Wellsburg, Ankeny, and Shenandoah B

and the two basins west of the horst, the Duncan and 

ance Basins, are identified by their pronounced gravity 

ima. They are interpreted as depressions filled with c

Figure I2. Structural components of the Midcontinent Rift System (MRS) in Iowa (modified from Anderson, 1992; and And

1995, fig. 1, p. 56, copyright 1995 Kluwer Academic Publishers, used with kind permission from Kluwer Academic Publi

NBFZ, Northern Boundary Fault Zone; TRSZ, Thurman-Redfield Structural Zone; PHFZ, Perry-Hampton Fault Zone; DF, De

Basin; DC, Duncan Basin; SH, Shenandoah Basin; AK, Ankeny Basin; WB, Wellsburg Basin; SB, Stratford Basin; JB, Jewell 

MB, Mineola Basin; MIS, Manson Impact Structure.
I4 Strategic and Critical Minerals, Midcontinent Region, U.S.

sedimentary rocks of relatively low density (Anderson,

1992). The three basins southeast of the Iowa Horst apparently are interconnected, forming a continuous sequence of

clastic rocks. The two basins northwest of the Iowa Horst

are connected in a similar fashion. The areal extent of the

eastern basins appears to be slightly greater than that of the

western basins, but the MRS is nearly symmetrical in cross

section normal to the strike of the feature.

The igneous and sedimentary rocks that were

emplaced during the formation of the MRS in Iowa display

pronounced gravity and magnetic anomalies that extend

continuously to the Lake Superior region; in the Lake Superior region these rocks are assigned to the Keweenawan

Supergroup (King, 1978). MRS rocks sampled during drilling in Iowa are very similar to rocks exposed in the Lake

Superior area (fig. I3). Therefore, the name “Keweewnawan

Supergroup” also was applied to all igneous and sedimentary rocks associated with the Midcontinent Rift System in

Iowa by Anderson (1992). Most of the MRS clastic sedimentary rocks that were deposited in the axial areas of the

structure were subsequently erosionally removed during

uplift of the Iowa Horst. These clastic rocks were, however,

preserved on a step-faulted block on the eastern edge of the

horst in central Iowa (the Ames block), in the Stratford and

Jewell Basins in the center of the horst, and in the Mineola

Basin on the axis of the horst at the Nebraska border

(Anderson, 1992).

MRS CLASTIC ROCK DATA

The clastic rocks associated with the MRS in

Iowa—informally called Red clastics—are buried beneath

2,200–5,500 ft of Phanerozoic strata. Since they are not

known to contain economic levels of mineralization and are

not used as aquifers in Iowa, few wells penetrate them.

Study of the few well samples of Red clastics and analysis

of geophysical data provide the only data on these rocks. 

The Well Record in Iowa

Fourteen wells are known to penetrate Red clastic strata

in Iowa (table I1). Petroleum exploration wells account for

five of these penetrations (total of 19,197 ft) and water wells

for four penetrations (total of 1,064 ft); three drillholes (total

of 75 ft) were drilled during the development of underground natural gas storage structures, and one hole was a

research core into the Manson Impact Structure (370 ft).

Cores of Red clastics in Iowa include the three gas storage

structure wells (total 75 ft), four short intervals of the M.G.

Eischeid #1 oil test (totaling 63 ft), and the M–4 research

core into the Manson Impact Structure (370 ft). The deepest

and most significant Red clastics penetration in Iowa is the

M.G. Eischeid #1 deep petroleum test well, drilled by the

Amoco Production Company in 1987. The Eischeid well

penetrated more than 14,898 ft of MRS clastics fr

chip samples were collected at intervals of 20–3

M–4 core encountered a Red clastics section that 

graphically thinned and overturned by the impact.

GEOPHYSICAL DATA

Much of our current understanding of the vo

areal distribution of Red clastics in Iowa is based 

pretation of geophysical data. These data inclu

data, with a 6-mi grid of points covering almost a

supplemented by areas of 1-mi-spaced coverage

aeromagnetic data was collected over the entir

Iowa between 1953 and 1972. This data was obta

1-mi-spaced flightlines, most at an altitude of 50

final set of geophysical data over the MRS is refle

mic data, collected over the MRS in the early to m

by exploration and geophysical companies as a p

grams to explore MRS clastics. Data collected al

lines of profile by Petty-Ray Geosource, Inc., 

available for investigations, as was one short line 

the AMOCO Production Company.

Gravity Anomaly Data

The “Bouguer Gravity Anomaly Map of Iowa

son, 1981) clearly displays anomalies produce

major lithologies that characterize the MRS in 

I4). Ranging from north-central Iowa to the sout

ner of the state, the strongly positive axial anom

result of the thick sequence of dense, mafic-domi

canic and plutonic rocks that dominate the Iowa 

its shallow location at the Precambrian surface. 

ing gravity minima are produced by the thick seq

less dense, rift-related clastic sedimentary rock

preserved in the basins marginal to the horst. T

rocks associated with the MRS have modeled 

ranging from 2.40 to 2.75 g/cm3, much lower th

ated crustal rocks whose densities range from 2.

g/cm3 (Anderson, 1992). Five clastic-filled flank

are defined by closed contours of gravity minima 

flanks of the Iowa Horst. The very steep gravit

between the axial high and the flanking lows att

abrupt contact between the two features, their de

trast, and the very large volumes of the disparate l

Magnetic Anomaly Data

Over most of its trend in Iowa, the magnetic 

of the MRS (fig. I5) is characterized by a series 

anomalies following the trend of the gravity 

Magnetic intensities are generally within a few 

gammas (nanoteslas) of regional values, as com

anomalies for typical intrusions, which can exc
Clastic Rocks, Midcontinent Rift System, Iowa 

gammas (Carmichael and Black, 1986). The lack of a strong

magnetic signature associated with the generally highly

magnetic mafic igneous rocks of the Iowa Horst can probably be attributed to a combination of the strong remanent

components in these rocks and the vertical sequence of normal and overlying reversely polarized rocks. The clastic

rocks that fill the flanking basins are magnetically transparent. The increase in the depth of the underlying magnetic

basement results in a lowering of the magnetic surface and

observed aeromagnetic field intensity. This creates, in

effect, a filtering out of the shorter wavelength components

of the field. On the magnetic map of Iowa (Zietz and others,

1976), this filtering produces a series of smooth, low-intensity magnetic minima having a general trend parallel to the

rift axis and flanking the Iowa Horst. A close examination of

the more detailed (20-gamma contour interval) “Aero

netic Map of Central Iowa” (Henderson and Vargo, 

clearly shows the contact of the magnetic (igneous) ro

the Iowa Horst with the nonmagnetic (clastic) rocks 

flanking basins. The shallow depth of the magnetic ro

the Iowa Horst produces a high-relief anomaly surface, 

acterized by closely spaced, tightly curving contours

the horst, the greater depth of the magnetic base

beneath the clastics produces much more subdued relie

ing more widely spaced, smoothly curving contours.

Reflection Seismic Data

Reflection seismic data originally collected 

Petty-Ray Geophysical Geosource, Inc., were releas

Figure I3. Comparative Keweenawan stratigraphy of the Midcontinent Rift region. Modified from Green (1982), And

(1992), and Anderson (1997, fig. 5, modified with permission of the publisher, the Geological Society of America, Bo

Colo., U.S.A., copyright  1997 Geological Society of America).

Clastic Rocks, Midcontinent Rift System, Iowa 

Halliburton Geophysical Services, Inc., for investigation by

Anderson (1992). These seismic lines are currently available from the Wisconsin Geologic and Natural History Survey in Madison, Wisc. Data released included final stack

and migrated profiles and profile location maps along seven

lines (fig. I6). Three east-west lines (profiles 7, 12, and 9)

completely cross the Iowa Horst and extend over both flanking basins (although profile 7 displays no useful information

below Phanerozoic reflectors). One more east-west line

(profile 11) and three north-south lines (profiles 8, 10, and

13) extend from on the Iowa Horst over one flanking basin.

In his study, Anderson (1992) utilized interpretive stick diagrams that were manually constructed from the migrated

profiles. Interpretations were constrained by general rift

geometry, lithologies, and seismic velocities described by

earlier workers in the Lake Superior outcrop belt, Iowa drill

data (especially information from the M.G. Eischeid #1

well), magnetic data displayed on maps by Henderson and

Vargo (1965) and Zietz and others (1976), the “Bouguer

Gravity Anomaly Map of Iowa” (Anderson, 1981), and by

co-modeling with two-dimensional gravity data along

extended seismic profiles.

CLASTIC UNITS ASSOCIATED WITH THE 

MRS

The Middle Proterozoic Red clastics were deposited in

two stages during formation of the MRS in Iowa, between

about 1.1 and 1.0 Ma. Similar-appearing clastic rocks of the

Cambrian Mount Simon and Reagan Sandstones overlie the

Red clastics in Iowa. These units have traditionally been

very difficult to differentiate in hand specimen or under a

binocular microscope.

Reagan Sandstone

Taft (1902) assigned the name “Reagan Sandstone” to

the Cambrian-aged basal Phanerozoic sandstone in southern

Oklahoma. The term is also used to identify similar str

Kansas. The Reagan Sandstone overlies sandstones o

Proterozoic Rice Formation in some area of Kansas. 

Reagan Sandstone is generally considered younger tha

Mount Simon (Kurtz and others, 1975). Recent res

(Carlson and others, 1990) suggests that the basal Cam

sandstone in southeastern Nebraska and extreme south

ern Iowa is younger than the Mount Simon and shou

assigned to the Reagan.

Mount Simon Sandstone

The Mount Simon Sandstone, named by Ulrich 

cott, 1914), is the basal Cambrian sandstone over mu

the region, including Wisconsin, Minnesota, Iowa, Ill

and Nebraska. The Mount Simon is consistently less 

300 ft thick over much of Wisconsin, Minnesota, Iowa

Nebraska, but in northern Illinois it reaches a max

thickness of about 2,600 ft (Buschbach, 1964). Pre

interpretations of thick half-graben basins of Mount S

clastic rocks in central Iowa (Bunker, 1981; Bunker an

ers, 1988) are now considered incorrect on the ba

results from this study. The Mount Simon sandstone is 

relative with the Lamotte Sandstone of Missouri.

Red Clastics

History of the Term “Red Clastics”

The term “Red clastic series” was first applied t

shales and sandstones beneath Paleozoic rocks in the 

surface of Minnesota by Hall and others (1911). The re

term “Red clastics” was used by Norton and others (

to describe the red sandstones encountered beneath 

brian strata in Iowa drillholes. At the time, these rocks 

differentiated from the overlying white Cambrian 

stones by their red to pink coloration. He correlate

Iowa strata to similar rocks in the subsurface of Minn

and suggested an Algonkian age for the rocks. Thiel (

described the Red clastics of the southern Minnesota 

surface as making up the lower part of the Lake Sup

series and applied the name “Fond du Lac beds” to th

Minnesota. The term “Red clastics” is still used in Io

define the post-volcanic MRS clastic rocks in Iowa 

example, Anderson, 1990a), although their contact wi

overlying Cambrian sandstones is now defined by litho

and textural criteria instead of coloration, as will be de

later in this report. The Nebraska Geological Survey 

the term “Red clastics” or “Cornhusker clastics” to re

similar rocks in Nebraska. 

Figure I4 (previous page). Major features of the Midcontinent Rift System (MRS) in Iowa displayed on the “Bouguer

Gravity Anomaly Map of Iowa” (from Anderson, 1992; and

Anderson, 1997, fig. 8, modified with permission of the publisher, the Geological Society of America, Boulder, Colo.,

U.S.A., copyright  1997 Geological Society of America).

NBFZ, Northern Boundary Fault Zone; TRSZ, Thurman-Redfield Structural Zone; PHFZ, Perry-Hampton Fault Zone; DF,

Defiance Basin; DC, Duncan Basin; SH, Shenandoah Basin;

AK, Ankeny Basin; WB, Wellsburg Basin; SB, Stratford Basin;

JB, Jewell Basin; MB, Mineola Basin; AB, Ames Block; MIS,

Manson Impact Structure.

Clastic Rocks, Midcontinent Rift System, Iowa 

The M.G. Eischeid #1 Deep Petroleum Test Well

In the mid-1980’s, the Amoco Production Company

was engaged in a program to investigate the petroleum

potential of the MRS. Their geological and geophysical

studies led to the identification of a site for a deep petroleum

test well, which was drilled in 1987 in southwestern Iowa on

the M.G. Eischeid farm near the town of Halbur, in Carroll

County. It reached a total depth of 17,851 ft, penetrating

14,898 ft of MRS clastic rocks, the deepest penetration of

MRS clastic rocks anywhere along the trend of the rif

the entire succession of the clastic rocks in the Defi

Basin, the basin north of the Iowa Horst in western Iow

I1). An excellent suite of sample chips was recovered 

erally at 10-ft intervals), and cores of the clastic rocks 

recovered at four intervals (8,834–8,844, 11,381–11

15,096–15,120, and 16,043–16,058 ft). Splits of the sa

chips and cores were reposited at the Iowa Geologica

vey Bureau (IGSB) Sample Repository and were 

confidential until October 26, 1989.

Figure I5 (previous page). Major features of the Midcontinent Rift System (MRS) in Iowa displayed on a modified version 

“Aeromagnetic Anomaly Map of Iowa” (Zietz and others, 1976). NBFZ, Northern Boundary Fault Zone; TRSZ, Thurman-Re

Structural Zone; PHFZ, Perry-Hampton Fault Zone; DF, Defiance Basin; DC, Duncan Basin; SH, Shenandoah Basin; AK, A

Basin; WB, Wellsburg Basin; SB, Stratford Basin; JB, Jewell Basin; MB, Mineola Basin; AB, Ames Block; MIS, Manson I

Structure.

Figure I6. Locations of Petty-Ray Geosource, Inc., reflection seismic profiles on a map of major features of the Midcontine

System (MRS) in Iowa (from Anderson, 1997, fig. 9, modified with permission of the publisher, the Geological Society of Am

Boulder, Colo., U.S.A., copyright  1997 Geological Society of America). NBFZ, Northern Boundary Fault Zone; TRSZ, 

man-Redfield Structural Zone; PHFZ, Perry-Hampton Fault Zone; DF, Defiance Basin; DC, Duncan Basin; SH, Shenandoah 

AK, Ankeny Basin; WB, Wellsburg Basin; SB, Stratford Basin; JB, Jewell Basin; MB, Mineola Basin; MIS, Manson Impact Stru
I10 Strategic and Critical Minerals, Midcontinent Region, U.S.

At the request of the IGSB, the Eischeid samples were

released about two months prior to the end of the period of

confidentiality for study by IGSB and external researchers

in advance of preparation of a report on the well. The

report, “The Amoco M.G. Eischeid #1 Deep Petroleum

Test, Carroll County, Iowa—Report of Preliminary Investigations” (Anderson, 1990a) was published by the Iowa

Department of Natural Resources, Geological Survey

Bureau as Special Report Series #2. The IGSB (supported

in part by the U.S. Geological Survey’s Midcontinent Strategic and Critical Minerals Program) produced 51 thin sections from chips collected at about 500-ft intervals and from

additional intervals of interest, and 16 thin sections from

selected core samples. All thin sections were stained to aid

in the identification and differentiation of potassium feldspar and plagioclase and analyzed petrographically. The

results of this petrographic examination were discussed and

interpreted by Ludvigson and others (1990), McKay (1990),

and Witzke (1990) (see appendix I1). Additionally, thin sections from the cored intervals were analyzed (Barnes, 1990).

Core and chip samples and logs were sent to U.S. Geological Survey geologists for evaluation of their potential as

source rocks (Palacas and others, 1990), clay mineralogy

and bulk rock composition (Pollastro and Finn, 1990),

porosity (Schmoker and Palacas, 1990), and fluid inclusions

(Barker, 1990). Additional samples were sent to Iowa State

University for calcite veinlet studies (Ludvigson and Spry,

1990) and to the University of Kansas for age dete

(Van Schmus and others, 1990). 

Stratigraphy of the M.G. Eischeid #1 Red Clastics

Beneath the Phanerozoic sequence, the M.G

#1 well encountered 14,898 ft of clastic rocks 

with the MRS and assigned to the Keweenawan S

(Anderson, 1990b). These clastic rocks were di

two sequences, informally named the upper Re

sequence and lower Red clastics sequence (Witz

These units are group-level stratigraphic su

(Anderson, 1992) but will be referred to as 

sequences in this paper. The upper Red clastics 

has been subdivided into four informal units, fro

down, H, G, F, and E (fig. I7). The lower Re

sequence (fig. I8) is divided into three informal un

Eischeid well these informal subdivisions are u

and B (Witzke, 1990). The informal units of both 

are formation-level subdivisions (Anderson, 1992

Upper Red Clastics Sequence

The clastic rock sequences encountered at 

2,802–10,510 ft in the M.G. Eischeid #1 well are d

the upper Red clastics sequence. The litholog

sequence, as interpreted from the mud log, down-

Table I1. Iowa wells penetrating the Midcontinent Rift System (MRS) Red clastic rocks.
Clastic Rocks, Midcontinent Rift System, Iowa 

and petrographic and other studies, was described by

Witzke (1990). He divided the sequence into four formation-level units (from the top): unit H, unit G, unit F, and

unit E.

Unit H (2,802–3,615 ft) (fig. I7) is dominated by two

primary lithologies a clear- to light-reddish-brown,

fine-grained to very coarse grained arkosic sandstone, and a

reddish-brown, micaceous, shaly to sandy siltstone (McKay,

1990). The common occurrence of loose quartz and feldspar

grains in the chip samples indicates that much of the unit is

poorly indurated. Petrographic study of 11 thin sections produced from chips collected from the upper half of unit H

yielded a mean framework composition of 3Q78.5F13.8L7.7

and documented all the feldspar grains to be potassium feldspar. The study also revealed the presence of trace am

of microbrecciated rock fragments, identical to those f

down-hole throughout a large part of the Red clastics 

tion. The presence of microbreccia chips probably im

that the uppermost preserved part of the Red clastics se

was affected by structural movements similar to thos

created similar tectonized fabrics throughout much o

Eischeid well sequence (Ludvigson and others, 1990).

Unit G, encountered at depths of 3,615–4,690 ft 

I7) is dominated by pale-gray (clear) to red-brown, fin

3In Q78.5F13.8L7.7, Q=quartz, F=feldspar, and L=lithic fragm

numbers are percentages by volume.

Figure I7. Generalized lithologic log of upper Red clastics sequence from M.G. Eischeid #1 deep petroleum 

test well (modified from Witzke, 1990), Carroll County, Iowa.
I12 Strategic and Critical Minerals, Midcontinent Region, U.S.

medium-grained sandstone interbedded with red-brown

(and minor green) siltstone and shale (Witzke, 1990). The

sandstone is dominated by quartzose grains and has a mean

framework-grain composition of Q76F17L7). The volcanic

rock component of the lithic grain population in this interval

(35 percent) is the highest observed in any clastic unit in the

Eischeid well (Ludvigson and others, 1990).

Unit F (4,690–7,020 ft), the third Red clastics unit

encountered in the Eischeid well (fig. I7), was subdivided

by Witzke (1990) into three intervals that generally fine

upwards, F–3, F–2, and F–1 (from the top down). These

intervals grade from basal red-brown to pale-gray (clear),

very fine grained to fine-grained sandstones upward to

red-brown (in part mottled light green to gray-green) to red

(in part mottled light green to gray-green) shales and siltstones (Witzke, 1990). The sandstones have a mean framework grain composition of Q69F22L9 (Ludvigson and

others, 1990).

Unit E (7,020–10,510 ft), the basal unit of the upper

Red clastics sequence (fig. I7), is a thick, sandstone-dominated unit that is characterized by the highest percentage of

lithic fragments, having a mean of Q67F20L13, in the Eischeid well (Ludvigson and others, 1990). The unit was subdivided by Witzke (1990) into two descriptive intervals, E–2

(upper) and E–1.

Interval E–2 is the uppermost of the sub-units that

make up unit E. It is dominated by red to brown, generally

very fine grained to fine-grained sandstones interbedded

with red to brown (some mottled grayish-green) siltstones

and shales (Witzke, 1990). Core 1 (8,834–8,844 ft) sampled

10 ft of this interval and displays horizontal bedding and

two facies, one well stratified with low-angle cross-stratification and containing red mudstone rip-up clasts, the second

a massive, nonstratified sequence containing smaller mud

clasts (Ludvigson and others, 1990).

Interval E–1 contains more shale than E–2 and is composed of four relatively thin, fining-upward sequences, each

grading from a basal very fine grained to fine-grained sandstone upward to a red to dark-brown shale and siltstone

(Witzke, 1990).

The upper Red clastics sequence occupies the same

relative stratigraphic position as the Bayfield Group in the

Lake Superior region (fig. I3). Both units are apparently

dominated by fluvial deposits, with the possible exception

of the Devils Island Sandstone of the Bayfield Group,

which may be a lacustrine deposit. There are, however,

some compositional differences between the clastic rocks of

the two sequences. The Bayfield Group is more mature,

both in texture and mineralogy, than the underlying Oronto

Group (Ojakangas, 1986), whereas the rocks of the upper

Red clastics sequence are mineralogically less mature than

the lower Red clastics sequence. Volcanic rock fragments

contributed less to Bayfield Group rocks than to the Oronto

Group (Ojakangas and Morey, 1982b). However, in the

Eischeid well volcanic rock fragments are more common in

the upper Red clastics sequence than the lower. Finally,

Eischeid drilling did not penetrate any Proterozoic quartz

arenites, such as in the Devils Island and Hinck

stones in the Lake Superior area.

The Bayfield Group and upper Red clastics 

do, however, share many characteristics. Both s

apparently overlie the initial MRS clastic units 

Group and lower Red clastics sequence). The 

Group, as described by Ojakangas and Morey (1

dominated by fluvial deposition (excluding the 

lacustrine Devils Island Sandstone). The lithologi

in the upper Red clastics sequence, as described b

(1990), and the petrology and sedimentary stru

established from the cored intervals by Ludvigson 

ers (1990), also suggest a fluvial origin. The diffe

the composition of the units of the upper Re

sequence and the Bayfield Group can be explaine

ferences in the lithologies of source terranes.

Although quartz arenites were not encounter

Eischeid well, unit F of the upper Red clastics sequ

plays the highest content of siltstone and shale. Th

energy deposits may be related to the possible 

deposits observed in the middle unit (Devils Islan

Bayfield Group, perhaps deposited by a low-grad

that flowed into a nearby lake.

Several trends are evident in the compositi

sandstone component of the upper Red clastics 

The QFL composition of the framework grains is 

to the composition of units in the Lake Superior a

ure I8. Quartzose grain content increases upwa

sequence, from 67 percent in unit E to 79 percent 

At the same time, the average percentage of lith

decreased up section from 13 percent in unit E to 

in unit H; the volcanic rock component of t

increased from 22 percent in unit E to 62 percent 

and sedimentary and metamorphic grains decrease

percent in unit E to 37 percent in unit G. The com

of the lithic fragments in the two intervals examin

H were quite disparate, and additional analyses are 

The relative increase in the concentration of 

rock grains moving up section in the upper Re

sequence in the Eischeid well may record the local 

erosional unroofing of the basalts on the Iowa Ho

data from the trend of the MRS in Iowa and seis

pretations indicate that Keweenawan sedimenta

were erosionally removed from most areas of 

Horst, exposing underlying volcanic rocks (A

1988).

Lower Red Clastics Sequence

The rocks encountered at depths between 10

17,700 ft in the Eischeid well are informally c

lower Red clastics sequence (fig. I8). The sequ

divided by Witzke (1990) into three component un

the top downward): units D, C, and B.

The uppermost unit in the lower Red clastics 

unit D (10,510–14,980 ft), was described by Witz

as a thick, generally upward-fining sequence that 

nated by sandstones in the lower half and siltston
Clastic Rocks, Midcontinent Rift System, Iowa 

in the upper half. He subdivided the unit into four intervals

(from the top downward): D–4, D–3, D–2, and D–1. The

sandstones in unit D are dominated by quartz (having a

mean framework grain composition of Q76F21L3), and the

lithic component includes the highest percentage of

sedimentary and metamorphic rock fragments (average 74

percent) observed in any unit in the Eischeid well (Ludvigson and others, 1990).

Interval D–4 is dominated by red to brown (with

minor, light-green to gray) shale in its upper half and

red-brown siltstone in the lower half (Witzke, 1990). Core 2

(11,381–11,395 ft) was recovered from this interval and was

described by Ludvigson and others (1990) as displaying two

facies, one composed of red, very fine grained to

fine-grained, cross-stratified sandstone, and the second a

very fine grained, horizontally stratified sandstone

containing minor interbedded mudstone. These sedi

are interpreted as representative of a fluvial setting ch

terized by shallow channel fills, subsequent subaerial 

sure, inundation by over-bank deposits, and then a ret

shallow channel deposition (Ludvigson and others, 

The cored interval also displays fault-related deformat

The underlying interval, D–3, is dominated by sil

containing minor sandstone and shale. This interva

contains gray to dark-gray siltstone and shale and the u

most occurrences of black siltstone and black carbona

specks. The lower part of this interval is dominat

light-gray, fine- to medium-grained sandstone interb

with minor shales and siltstone (Witzke, 1990).

The upper part of interval D–2, as described by W

(1990), is dominated by varicolored siltstone cont

some dark-gray to black shaly laminations (some py

Most of the interval is dominated by sandstone and 

Figure I8. Generalized lithologic log of lower Red clastics sequence from M.G. Eischeid #1 deep petroleum test well (mod

from Witzke, 1990), Carroll County, Iowa.
I14 Strategic and Critical Minerals, Midcontinent Region, U.S.

interbedded red-brown to gray-green siltstone and shale.

Coarse sand grains are abundant in the basal parts of

interval D–2.

The basal interval in unit D, D–1, was described by

Witzke (1990) as a sandstone-dominated sequence

conatining varicolored shale and siltstone in the upper part.

The sandstone is predominantly light gray to red-brown and

fine to medium grained, containing some minor coarse

grains. Shales are red-brown to gray (some black) and contain siltstone interbeds. The mud log indicated traces of

methane and ethane in this interval, and Witzke (1990)

reported intergranular black residues, possibly hydrocarbon

residues.

Unit C (14,980–16,450 ft) is the most distinctive

Keweenawan Supergroup clastic unit encountered in the

M.G. Eischeid #1 well (fig. I8). It is unique in its abundance of gray to black siltstones and shales, calcite cements,

calcite vein-fills, and structural deformation. The unit is

also the most thoroughly cored, two cores totaling 39 ft having been taken during the drilling. The unit was subdivided

into two intervals, an upper interval (C–2) and a lower

interval (C–1).

Interval C–2 is an interbedded sequence of sandstone,

siltstone, and shale, gray to black siltstone and shale being

more common in the upper part of the interval and

red-brown to green-gray rocks more common in the basal

part (Witzke, 1990). Methane and ethane were detected

throughout the interval, and black intergranular residues,

possibly relict hydrocarbons, were reported. Also reported

on the mud log in this interval (15,440 ft) were traces of

chalcopyrite or native copper. A careful examination of

samples from this interval, however, failed to confirm the

presence of copper or chalcopyrite. Core 3 (15,096–15,120

ft), taken in the upper part of the interval, consists of horizontally laminated, millimeter- to decimeter-thick, interlayered, medium- to dark-gray shale and lighter gray siltstone

to fine-grained sandstone (Ludvigson and others, 1990).

This stratum was interpreted as having been deposited in a

lake or other body of standing water having a fluctuating

depth and intermittent influxes of coarse detritus. Petrologic study of two samples of coarse detritus from interval

C–2 yielded a mean composition of Q72F24L3 (Ludvigson

and others, 1990). The 24-percent feldspar observed in this

interval is the greatest concentration observed in any

interval in the Eischeid well.

Interval C–1 is dominated by light- to dark-gray siltstone and shale containing dark-brown to black shaly interbeds (Witzke, 1990). Calcite cements, spar, and vein fills

are common in this interval, and a carbonate-rich region

near the bottom of the interval was identified as limestone

on the mud log. Core 4 (16,043–16,058 ft) showed a

high-angle, tectonically derived dip ranging from 65° to

vertical and slightly overturned. The cored interval is

composed of well-laminated, interbedded, light-gray, micaceous siltstone and medium-gray to black shale (L

and others, 1990). Calcite veinlets and slickens

surfaces showing calcitic rough facets are comm

core, providing evidence of deformation in a rev

ing mode requiring lateral compression (Ludv

Spry, 1990).

Unit B, the basal clastic unit in the Eisc

(16,450–17,700 ft), informally named by Witzke 

dominated by white to light-gray and red, very fin

to fine-grained sandstone possibly containing sil

shale partings. Petrographic studies of unit B sand

Ludvigson and others (1990) indicated that it con

highest percentage of quartzose grains observed i

clastics sequence, Q87F12L1, and a small lithic c

dominated by sedimentary and metamorphic 

percent).

The lower Red clastics sequence is very s

and may correlate with, the Oronto Group in 

Superior area (fig. I3). The biggest difference be

two sequences is the coarser grained facies pre

the Copper Harbor Conglomerate, the basal u

Oronto Group, and the lack of the coarse subfac

B, the basal unit of the lower Red clastics seque

Copper Harbor is dominated by a conglomera

composed primarily of mafic and felsic volc

clasts. Associated sandstone facies also are dom

volcanic rock fragments. Unit B is dominated b

stone facies that displays the highest quartzose 

tent (87 percent) of any Red clastics unit in the 

well (fig. I9). This disparity can be explained by 

tions of the two units relative to the central ho

Midcontinent Rift. The Copper Harbor Con

exposures are located on the central horst, whic

axial graben at the time of Copper Harbor deposi

volcanic-rock clasts that predominate in the Co

bor were deposited by alluvial fans that were d

erosion of Keweenawan volcanic rocks capping 

walls of normal faults that bounded the graben. 

heid well, and unit B, are located in one of t

basins that flank the central horst and is therefor

that was outside of the central graben at the t

deposition. It is dominated by quartzose sedime

from granitic rocks, the most common litholo

region in which the MRS developed, and was de

rivers that flowed towards the axis of the rift. 

rivers neared the axial graben they apparently cu

footwall-capping volcanics, depositing the Copp

lithologies in the graben.

Unit C is lithologically and sedimentologically 

ilar to the Nonesuch Formation of the Oronto Gr

fine-grained components of unit C display unid

bidirectional, symmetrical, and trough cross-stratifi

small-scale, amalgamated, hummocky cross-stra

Shrinkage cracks are also present in this interval (L
Clastic Rocks, Midcontinent Rift System, Iowa 

and others, 1990). Milavec (1986) identified similar features

in the Nonesuch Formation.

The Nonesuch has been interpreted as a lake deposit by

most workers (for example, Elmore, 1981; Daniels, 1982;

Milavec, 1986), but some workers have suggested that it

may have been deposited in a marine environment (Burnie

and others, 1972). In either case, the Nonesuch appears to

have been deposited in a standing body of water. Structures

observed in cores of unit C also suggest deposition in a

standing body of water (Ludvigson and others, 1990). The

presence of these lake deposits in the Eischeid well indicate

that the body of water was not confined to the central horst,

the position where correlative Nonesuch deposit

observed. Formerly confidential petroleum industry se

data suggest that unit C may extend at least 7 mi west 

Iowa Horst in the Defiance Basin, and perhaps as mu

15 mi or more (Anderson, 1992).

The black coloration in unit C, like the Nonesuch 

mation, is primarily the product of disseminated or

carbon, along with minor pyrite. Palacas and others (

reported total organic carbon (TOC) values as high a

percent (averaging 0.6 percent) in the dark shales 

siltstones of unit C. The Nonesuch Formation has yi

maximum TOC values of almost 4 percent (Hieshim

Figure I9. Quartz–feldspar–lithic grain (QFL) diagrams for Keweenawan Supergroup clastic rocks from (A)

Lake Superior area (from Ojakangas, 1986) and (B) M.G. Eischeid #1 well (from Anderson, 1990b; Ludvigson

and others, 1990; and Anderson, 1997, fig. 4, modified with permission of the publisher, the Geological Society of America, Boulder, Colo., U.S.A., copyright  1997 Geological Society of America), Carroll County,

Iowa.
I16 Strategic and Critical Minerals, Midcontinent Region, U.S.

others, 1989). The petroleum source-rock potential of unit

C is discussed later in this report.

Unit D occupies the same stratigraphic position in the

Eischeid well as the Freda Formation in the Oronto Group

of the Lake Superior area. Interpretation of core 1 from unit

D suggested to Ludvigson and others (1990) that the unit

was deposited in a fluvial environment. A similar interpretation has been proposed for the Freda Formation by many

workers (for example, Daniels, 1982; Morey and Ojakangas, 1982). Petrographic studies of unit C revealed a mean

detrital composition of Q76F21L3 compared to a mean

composition of Q52F18L30 for seven Freda exposures

reported by Daniels (1982). The lithic component of the

Freda is dominated (72 percent) by volcanic rock fragments.

The higher volcanic rock lithic component indicates that

volcanic rocks were exposed in the Freda source area, and

similar lithologies were not exposed in the source area of

unit D.

Red Clastics Basins Flanking the Iowa Horst

The Red clastic rocks deposited during the formation

of the Midcontinent Rift System in Iowa are most extensive

and reach their maximum thickness in the series of basins

that flank the Iowa Horst to the west and east (fig. I10).

The dimensions of these basins and the volumes of Red

clastics preserved in them are given in table I2. The rocks

of the flanking clastic basins are buried beneath

2,200–3,500 ft of Phanerozoic strata, and only six drillholes

penetrate these rocks, which cover an estimated 13,400 mi2

of the Iowa Precambrian surface and total more than 34,000

mi3 (Anderson, 1992). With this limited well control, most

of our understanding of these basins is derived from interpretation of geophysical data (Anderson, 1992). A reflector

within the clastic sequence as interpreted on the reflection

seismic profiles appears to be coincident with the approximate contact between the upper and lower Red clastics

sequences. The depth to this reflector was extrapolated

over each clastic basin and was used by Anderson (1992) to

delineate the limits of each of these clastic sequences.

Western Flanking Basins

The thick sequence of Middle Proterozoic Red clastics

that flank the Iowa Horst on the west forms a belt from 22 to

32 mi wide. Although the belt is apparently a continuous

blanket of sedimentary rocks covering an estimated 4,400

mi2 of the Precambrian surface, it is divided into two basins,

best delineated on the “Bouguer Gravity Anomaly Map of

Iowa” (fig. I4). The central areas of these basins, the Duncan Basin on the north and the Defiance Basin on the south,

are located by closed gravity minima. The Manson Impact

Structure is located on the saddle that separates the basins.

Modeling (Anderson, 1992) suggests that a total of about

12,000 mi3 of Red clastics is present in the weste

(table I2).

Duncan Basin

The northwesternmost of the MRS clastic bas

ing the Iowa Horst in Iowa, the Duncan Basin, ex

southwesterly direction for about 70 mi from the M

border (fig. I2). The basin ranges in width from ab

20 mi, an area of about 1,500 mi2, but the zero ed

clastics is difficult to define and may extend as m

mi beyond the mapped limits (Anderson, 1992). 

reaches a maximum modeled depth of 27,700 ft a

Iowa Horst (fig. I10). Modeled overthrusting of t

sequence by the volcanic rocks of the Iowa Ho

from 3 to 8 mi, generally greater on the southern 

basin. Lower Red clastic sequence rocks and u

clastic sequence rocks are preserved in the Dunc

No wells penetrate Duncan Basin clastic rocks in I

Defiance Basin

The southernmost of the western flanking b

Defiance Basin, includes an area of about 2

extending northwest for about 120 mi from the 

border (fig. I2). The basin ranges in width from ab

in the north to about 33 mi in the south. The ze

difficult to define and may extend as much as 10 m

its model location. The Middle Proterozoic R

rocks that fill the basin reach a modeled maximum 

of about 34,600 ft near the Iowa Horst in the sout

of the basin and total about 7,700 mi3. The volca

of the Iowa Horst are thrust 5 to 10 mi over Defia

clastics. Only two wells penetrate into these clas

One well, the M.G. Eischeid #1, has presently 

complete section of MRS clastic rocks. The sec

the Martin Augustine #1, produced only poor 

(table I1). Rocks of both upper and lower Re

sequences are preserved in the Defiance Basin (tab

Eastern Flanking Basins

On the eastern side of the Iowa Horst, an a

continuous belt of flanking Proterozoic Red cla

stretches from Minnesota to Nebraska. It has bee

into three basins, from north to south, the W

Ankeny, and Shenandoah Basins, their centers d

closed isogal minima on the “Bouguer Gravity 

Map of Iowa” (fig. I4). The eastern flanking basi

than the western basin, with clastics covering a

9,000 mi2. The total volume of Red clastic roc

basins flanking the eastern margin of the Iowa H

I2), based on modeling, is about 23,300 mi3.
Clastic Rocks, Midcontinent Rift System, Iowa 

Wellsburg Basin

The northernmost of the three eastern clastic basins,

the Wellsburg Basin (fig. I2), extends south from the Minnesota border for about 100 mi to central Marshall County,

where it abuts the Ankeny Basin. The basin covers an area

of about 2,700 mi2 and ranges in width from 13 to 47 mi,

but its eastern zero edge is poorly constrained and 

extend as much as 10 mi farther east than is mapped

model indicates that about 7,400 mi3 of MRS uppe

lower Red clastics sequences rocks fill the basin (tabl

reaching a maximum thickness of about 36,300 ft. 

rocks are penetrated by only one well, the #1 Hunt

Butler County. The Red clastics in the Wellsburg Bas

Table I2. Dimensions of Red clastics basins in Iowa.

[n.a., not applicable]
I18 Strategic and Critical Minerals, Midcontinent Region, U.S.

overthrust by the Iowa Horst, from about 3 mi on the north

to as much as 8 mi in the area of the Ames Block (fig. I10).

Ankeny Basin

The central one of the three eastern flanking MRS

basins in Iowa is the Ankeny Basin (fig. I2). The basin covers an area of about 2,500 mi2, extending for about 70 mi

between the Wellsburg and Shenandoah Basins. It ranges in

width from about 23 to 50 mi, but the eastern zero edge of

the clastics is poorly defined. Modeling (Anderson, 1992)

indicates a maximum thickness in excess of 37,000 ft and a

total volume of about 5,300 mi3 of MRS upper and lower

Red clastics sequences rocks are preserved in the Ankeny

Basin. The Red clastic rocks are penetrated by only one

well, the Story City Town Well #3. The Iowa Horst is thrust

from 5 to 8 mi over the Red clastic rocks in the basin.

Shenandoah Basin

The Shenandoah Basin, the southernmost o

ern flanking basins in Iowa (fig. I2), has two co

lobes. The lobes are separated by an area of pos

on the crystalline basement, coincident with th

Iowa Arch pluton (Anderson, 1988). The basin 

area of about 3,800 mi2, extending for about 10

the Nebraska border to the Ankeny Basin. The 

plays a maximum width, in the northern lobe, of 

mi and a minimum width of 25 mi between 

lobes—but the eastern zero edge is not well co

Proterozoic upper and lower Red clastics sequenc

the Shenandoah Basin range to as much as about 

in thickness (fig. I10), and modeling suggests the 

of about 9,700 mi3 of the clastic rocks preserv

basin. The clastic rocks are overthrust from 5 to 

the Iowa Horst.

Figure I10. Thickness of Red clastics preserved in basins flanking Iowa Horst (from Anderson, 1992; and Anderson, 

10, modified with permission of the publisher, the Geological Society of America, Boulder, Colo., U.S.A., copyright  

logical Society of America). MIS, Manson Impact Structure; contour interval 2 km (1.24 mi).
Clastic Rocks, Midcontinent Rift System, Iowa 

Two wells penetrate into the Red clastics of the

Shenandoah Basin, the #1 Wilson in Page County, which

sampled 1,605 ft of the sequence, and the Gulf Energy

16–13 Poetker in Mills County, which penetrated 49 ft of

Red clastics. Additionally, a third well, the Ohio Oil

Company Wisnom #1, penetrated 200 ft of quartzarenite at a

depth below 3,150 ft. The only quartz arenite in this part of

the geologic column is in the basal Cambrian sandstone.

This basal sand, however, rarely exceeds 100 ft in thickness

in this region. The anomalous quartz arenite in the Wisnom

well may represent rocks related to the Bayfield Group

Hinckley or Devils Island Sandstone of the Lake Superior

region.

Clastic Basins on the Iowa Horst

Stratford Basin

The Stratford Basin (fig. I2) was identified as the

“Stratford Geophysical Anomaly” by Osweiler (1982), who

interpreted the feature as an elliptical, clastic-filled basin

having an area of 650 mi2 that formed along the axis of the

MRS, similar to the Twin City Basin in Minnesota (King

and Zietz, 1971). Using a gravity contrast of 0.6 g/cm3

between the clastics and the underlying mafic volcanic

rocks, Osweiler modeled the basin as elongate (about 35 mi

long) trending northeasterly (along the axis of the MRS),

and about 23 mi wide. The basin reaches a maximum depth

of about 3,000 ft below the base of overlying Phanerozoic

rocks. 

The basin was studied by Anderson (1992) using data

from Petty-Ray Geosource, Inc., seismic profile 9. With

this data he calculated a maximum depth of about 5,500 ft.

To produce a gravity model using this depth required a density of 2.65 g/cm3 (density contrast of only 0.2 g/cm3 with

the underlying basalts) for the Red clastics. This density is

higher than the average 2.55 g/cm3 density assigned to the

lower Red clastics sequence by Anderson (1992) when

modeling the horst-flanking basins along profile 9. He suggested that the density of the clastics in the Stratford Basin

may be increased by a higher percentage of mafic volcanic

rock fragments (similar to clasts in the Copper Harbor Conglomerate) in the clastic rocks of the Stratford Basin. This

theory is strengthened by a description (Norton, 1928) of

the rocks encountered during the drilling of the Ogden City

#2 well in 1929 (table I1). Although no samples exist for

the basal 7 ft of the well, the lowest samples, from a depth

of 2,845 ft, were described as a litharenite containing abundant feldspar, mafic igneous rock fragments, and mafic minerals. This lithology is similar to the Copper Harbor

Formation and in Iowa is part of the lower Red clastics

sequence. The well encountered the lower Red clastics at a

depth of 2,750 ft and penetrated 102 ft of the clastic

sequence. Additionally, the Boone City #1 well also

apparently penetrated the clastics of the Stratford Basi

Boone City #1, drilled in 1890, encounter

sequence of red shales and soft red sandstones, interp

as the lower Red clastics sequence, at a depth of about 

ft. A total of 300 ft of these rocks was penetrated. 

tionally, deep burial along the axis of the MRS and tho

cementation may combine to form an anomalously 

unit, similar to the dense clastic rocks encountered 

Amerada Schroeder well, located in a similar positi

the Iowa Horst in Nebraska. 

Using a density of 2.65 g/cm3 for the lower Red 

tics, Anderson (1992) modeled the Stratford Basin as h

a width and length similar to values determined by Osw

(1982), but reaching a greater maximum depth (fig. 

about 12,000 ft. The total volume of lower Red cl

sequence rocks in the Stratford Basin is approximatel

mi3.

Jewell Basin

The Jewell Basin was defined by Anderson (199

the area of the Iowa Horst that is capped by lower Red 

tics sequence rocks between the elliptical Stratford 

and the Perry-Hampton Fault Zone (fig. I11); it has an 

of about 420 mi2. Seismic profile 8 (fig. I6) crosse

Jewell Basin and displays a continuously thickening c

sequence, ranging in thickness from about 1,500 ft o

north to 12,500 ft on the south, at its intersection wi

Perry-Hampton Fault Zone. Interpretation of availabl

suggests that the Jewell Basin was never a discrete de

tional basin. It is apparently a remnant of lower Red cl

sequence rocks that were originally deposited unif

within the central graben of the MRS in Iowa, but then 

structurally preserved in the area called the Jewell Ba

differential uplift of the Iowa Horst. The Jewell Basin 

tains approximately 410 mi3 of lower Red clastics seq

sedimentary rocks.

Mineola Basin

The southernmost of the clastic basins identifi

the Iowa Horst in Iowa is the Mineola Basin (fig. I1

was originally referred to as the “Mineola Graben,” 

early gravity modeling suggested a clastic-filled graben 

ing its center near the northern Mills County town of M

ola. However, interpretation of seismic profil

(Anderson, 1992) clearly displays a basin structure h

no prominent bounding faults. On seismic profile 1

basin appears to be about 9 mi wide. It proved diffic

match the gravity values observed along profile 13 by 

eling using lower Red clastics sequence densities in a 

6,000 ft deep, as required by the seismic interpre

(Anderson, 1992). However, using upper Red cl

sequence densities, the seismic data show a basin 

5,000 ft deep and yield a gravity model that fits the obs
I20 Strategic and Critical Minerals, Midcontinent Region, U.S.

data very well. Therefore, Anderson concluded that the Mineola Basin is filled with sedimentary rocks displaying upper

Red clastics sequence densities.

The limits of the Mineola Basin are best delineated on

the “Aeromagnetic Map of Iowa” (Zietz and others, 1976).

Interpretation suggests that the basin is about 40 mi long,

extending up the axis of the rift from the Nebraska border to

eastern Pottawattamie County, Iowa. The basin displays a

relatively continuous width of about 9 mi over most of its

length and a total area of about 280 mi2. Having a maximum depth of about 5,000 ft, the basin contains about 120

mi3 of upper Red clastics sequence sedimentary rocks. 

The presence of upper red clastics sequence sedimentary rocks in the Mineola Basin suggests that the feature

must have formed after uplifting of the Iowa Horst and the

unroofing of horst volcanics by the erosion of lower Red

clastics sequence sediments (Anderson, 1992). Th

that the Mineola Basin is the youngest MRS 

feature on the Iowa Horst. The much younger 

(Pennsylvanian) Glenwood Syncline (Hershey a

1960) is nearly coincident with the Mineola Basin 

represent structural reactivation of this feature.

Ames Block

Although the uplift of the Iowa Horst is r

uniform, as documented by the long, continuous, 

reflectors seen in the seismic profiles of the volca

several major structural features were identified on 

by Anderson (1992). Key among them is the Am

an elongate feature that trends along the eastern e

horst (fig. I2) and that was first identified and de

Figure I11. Thickness of lower Red clastics sequence in Stratford and Jewell Basins, Midcontinent Rift System (MRS) of Iowa (from Anderson, 1992; and Anderson, 1997, fig. 14,

modified with permission of the publisher, the Geological Society of America, Boulder, Colo.,

U.S.A., copyright  1997 Geological Society of America). Contour interval 1 km (0.62 mi).
Clastic Rocks, Midcontinent Rift System, Iowa 

Anderson (1992). It is bounded on the west by the

Perry-Hampton Fault Zone and on the east by the Thurman-Redfield Structural Zone. The Ames Block is apparently floored by a thick sequence of Keweenawan volcanic

rocks, indicating that it is a part of the Iowa Horst (Anderson, 1992). Above the volcanic rocks, both lower and upper

Red clastics sequence rocks have been structurally preserved (fig. I13). Both sequences have thicknesses nearly

identical to those modeled in the flanking Wellsburg 

Ankeny Basins. The Ames Block has been uplifted 

respect to the flanking basins. This uplift has maximum 

ues of about 2 mi at its northern (profile 12) and south

tral (profile 11) regions, to only about 1 mi between 

(profile 9) (see fig. I6).

The Ames Block is down-dropped with respect 

main body of the Iowa Horst, which is west o

Figure I12. Thickness of upper Red clastics sequence in Mineola Basin, Midcontinent Rift

System (MRS) of Iowa (from Anderson, 1992; and Anderson, 1997, fig. 15, modified with permission of the publisher, the Geological Society of America, Boulder, Colo., U.S.A., copyright

 1997 Geological Society of America). Contour interval 1 km (0.62 mi).
I22 Strategic and Critical Minerals, Midcontinent Region, U.S.

Figure I13. Thickness of (A) upper Red clastics sequence, (B) lower Red clastics sequence, and (C) total R

clastics on Ames Block, Midcontinent Rift System (MRS) of Iowa (from Anderson, 1992; and Anderson, 1997, fi

13, modified with permission of the publisher, the Geological Society of America, Boulder, Colo., U.S.A., cop

right 1997 Geological Society of America). Contour interval 1 km (0.62 mi).
Clastic Rocks, Midcontinent Rift System, Iowa 

Perry-Hampton Fault Zone. Anderson (1992) modeled apparent displacements, controlled by seismic interpretations,

ranging from a minimum value of 1.9 mi along the southern

end of profile 11, to 4.2 mi along profile 9, and 3.7 mi near the

northern end of the block along profile 12.

The upper surface of the volcanic rocks on the Ames

Block displays an apparent synclinal structure. There is no

apparent vertical displacement along the Perry-Hampton

Fault Zone on the southern end of the structure. Downward

displacement of the block increases to the north, to a

maximum near profile 9 (fig. I6), then decreases to its

northern intersection with the Thurman-Redfield Structural

Zone.

Both the upper and lower Red clastic sequence rocks

are preserved on the Ames Block. Both sequences are

apparently erosionally beveled along the southern end of

the Ames Block, where drill data confirm the presence of

volcanic rocks at the Precambrian surface. A total volume

of about 3,400 mi3 of Red clastics are preserved on the

Ames Block.

Available information suggests that the uplift of the

Iowa Horst included reactivation and reversal of the normal

faults that bounded the central graben of the MR

Iowa—the Northern Boundary Fault Zone on the we

the Thurman-Redfield Structural Zone on the east. 

ever, along the central area of the eastern margin of th

ben, a new structure formed, the Perry-Hampton Fault 

The thick Upper red clastics group rocks on the Ames 

suggest that deposition of the unit on the block occur

the same time as in the adjoining flanking basins. Th

some late stage, the Ames Block was uplifted, pro

along with the remainder of the Iowa Horst. Addi

complexity is added by the absence of clastic rocks 

southern end of the block, as confirmed by well data. 

southern end probably remained attached to the main p

the Iowa Horst, as it moved upwards (Anderson, 1992

Summary of Red Clastics in Iowa

MRS Red clastic rocks cover about 16,100 mi2 

Iowa basement (table I2). The two sequences, infor

named “upper and lower Red clastics sequences” by W

(1990), are probably equivalents of the Oronto and Ba

Figure I14. Depositional environment of unit B and lower Oronto Group equivalent 

rocks (from Anderson, 1992; and Anderson, 1997, fig. 6, modified with permission of 

the publisher, the Geological Society of America, Boulder, Colo., U.S.A., copyright 

1997 Geological Society of America).
I24 Strategic and Critical Minerals, Midcontinent Region, U.S.

Groups (and related rocks) of the Lake Superior area. Anderson (1992) estimated that a total volume of 39,800 mi3 of

these rocks are preserved in five basins flanking the Iowa

Horst and three basins and a structural block on the central

horst in Iowa. The Red clastics in Iowa are restricted to the

subsurface and have been sampled by only 14 drill penetrations. 

Depositional History of MRS Clastic Rocks in 

Iowa

MRS Lower Red Clastics Sequence

Much of the early stages of formation of the MRS was

characterized by the emplacement of thick sequences of

mafic-dominated volcanic rocks. Periods of magmatism

were separated by periods of quiescence marked by erosion

and the deposition of interflow clastic rocks. None of these

interflow clastic rocks is known to be present in Iowa, but it is

probable that they exist and they are similar to those

described in the Lake Superior region (see Jirsa, 1984). Studies in the Lake Superior region by several geologists, including Daniels (1982), indicate that as rift volcanism waned,

clastic sedimentation became the predominant depositional

process along the axis of the rift. Although rift volcanism

ceased, probably due to a relaxation of the tensile stresses,

subsidence of the rift zone continued, driven in part by isostatic adjustments to the thick, dense pile of rift-filling volcanic rocks. In the Lake Superior area, the Oronto Group and

related units are known primarily from exposures of deposits

in the central graben. In the Eischeid well in Iowa the lower

Red clastics sequence was deposited outside of the central

graben. The basal unit of the Oronto Group, the Copper Harbor Conglomerate, is dominated by mafic volcanic rock fragments, apparently derived from volcanic rocks in the footwall

of the graben-bounding normal faults (fig. I14). This suggests that deposition of Copper Harbor Conglomerate sediments was primarily limited to the area of the central horst.

This unit and all other Oronto Group units are known only

from this area, but related rocks may be buried beneath thick

Bayfield Group sedimentary rocks as modeled by Mooney

and others (1970) and Chandler and others (1989). Unit B in

the M.G. Eischeid #1 well, the basal unit of the Red clastics

in the horst-flanking basins in Iowa, was interpreted by

Anderson (1992) as the distal equivalent of the Copper Harbor Conglomerate. These fluvial sediments were deposited

by rivers that eventually flowed into the central graben. Two

wells, the Ogden City #2 and Boone City #1, on the Iowa

Horst in Boone County, encountered clastic rocks, probably

Copper Harbor Conglomerate equivalent units.

Standing bodies of water, probably lakes but possibly an

arm of the sea, that developed in the MRS grabens hosted the

deposition of a sequence of siltstones and shales, the Nonesuch Formation and equivalent units. Although Hie

Pratt (1991) used sulfur/carbon isotope ratios 

marine environment for Nonesuch sedimentation

(1992) concluded that regional structures argued 

connection to the open ocean.

The water bodies in the MRS were apparen

algae and fungae (Moore and others, 1969), wh

an organic-rich sediment. The Nonesuch Forma

Lake Superior area, having a maximum known th

about 700 ft (Daniels, 1982), was apparently limite

sition to the area of the central graben. In Iowa, h

equivalent rock sequence (unit C) in the Eischeid 

onstrates that in this area the lacustrine en

extended beyond the limits of the central graben 

1990c). The 1,470-ft thickness of unit C was 

about 15 mi outside the graben and about 55 m

axis of the graben. Additionally, dark-gray shale

were encountered in a number of research cores 

the central peak of the Manson Impact Structure

mi west of the Iowa Horst. These shales, as lar

clasts in an impact melt-rock matrix, are appare

posed against underlying brecciated crystalline r

gesting that unit C may over-step unit B near the 

the flanking basins. 

If interpreted graben margins in Iowa are c

lakes that filled the subsiding MRS were probably 

possibly more than 60 mi wide and may have exte

most of the length of the Iowa segment of the MR

sibly the entire length of the rift. The thickness 

near the axis of the graben was probably severa

thickness of unit C (Anderson, 

The rift-axial lakes were eventually filled by 

ing alluvial fans that deposited the fluvioclastic ro

Freda Formation in the Lake Superior area and 

units, including Eischeid well unit D. Deposit

rift-related clastic rocks of the Oronto Group proba

only a few million to a few tens of millions of y

most after the cessation of volcanism (about 1,08

Schmus and Hinze, 1995). The end of rift sedimen

probably indicative of the end of rift subsidence an

the initiation of compressive tectonics. This m

beginning of a period that was apparently domina

sion of the recently deposited clastics.

MRS Upper Red Clastics Sequence

Following a period of regional erosion, the 

MRS was affected by increasing regional shorteni

to a reversal of movement along the fault system 

the central grabens. The central grabens w

upwards, ultimately forming axial horsts. Oronto 

iments were eroded from the rising horsts and d

basins that formed along the horst flanks. The r

Oronto Group sediments, winnowed during fluvia

were also apparently enriched in quartz by m
Clastic Rocks, Midcontinent Rift System, Iowa 

clastics derived from the granitic terranes in which the MRS

was emplaced (Ojakangas and Morey, 1982a). The sandstone-dominated sequences that were deposited by these

processes, the Bayfield and equivalent units, reach a maximum of about 7,000 ft (Ojakangas and Morey, 1982a). In

the Lake Superior area, the Bayfield Group is composed of

three units (fig. I3). Two are feldspathic, fluvial sandstones

(the basal Orienta Sandstone and upper Chequamegon

Sandstone), and the intervening unit is a quartz arenite (the

Devils Island Sandstone), interpreted as lacustrine in origin

(Ojakangas and Morey, 1982b).

In Iowa, upper Red clastic sequence units E, F, and G

in the Eischeid well were interpreted as equivalent to the

Bayfield Group (fig. I7) by Witzke (1990). Additionally,

Eischeid well unit H is also considered a part of the upper

Red clastics sequence (McKay, 1990) and also equivalent to

the Bayfield and related rocks (Anderson, 1990c). All upper

Red clastic units in the Eischeid well are feldspathic sandstones. Unit G, however, is dominated by very fine grained

to fine-grained sandstones and has a large component of siltstone, possibly indicative of deposition in a standing body

of water. This is similar to the interpreted environment of

the Devils Island Sandstone in the Lake Superior area.

Units E through G show a progressive increase in the mafic

volcanic content of the lithic fragment component of the

sandstones and may record the progressive unroofing of the

volcanic rocks of the central horst (Anderson, 1990c; 1992).

Seismic data were interpreted by Cannon and others

(1989) to indicate the presence of Bayfield Group sedimentary rocks overlying the Oronto Group beneath central Lake

Superior. On land, however, no Bayfield Group rocks have

been identified on the central horsts. The exception may be

found in Iowa, where the Mineola Basin, centered on the

Iowa Horst in southwestern Iowa and modeled on profile 13,

is filled with sediments interpreted as low-density upper

Red clastic sequence rocks.

In Iowa, rocks of the Bayfield Group equivalent, the

upper Red clastics sequence in the Eischeid well, totals

7,708 ft in thickness. Along several of the seismically controlled profiles, the upper sequence reaches a thickness of

about 13,000 ft in the Wellsburg Basin and on the Ames

Block.

Problems Associated with Differentiation of 

Clastics

Historically, geologists working in Iowa have had difficulty differentiating the Red clastics from the basal Paleozoic sandstones (traditionally called Mount Simon

Sandstone). This problem arises from the similarity of the

lithologies (sandstones, siltstones, and shales), color, and

the scarcity of diagnostic fossils. Additionally, since the

basal clastic units apparently contain no mineral resources

or petroleum and are only rarely utilized as an aquifer, samples are rare and the units are not well understood. Previous

examinations of samples and logs from the two deepest

early penetrations of these clastic sequences (#1 Wilso

#1 Huntly oil tests) identified Cambrian fossils deep in

clastic sequence, leading to the interpretation of 

Mount Simon Sandstone basins above the Red clastic b

on the flanks of the MRS Iowa Horst (Bunker, 1982; B

and others, 1988). Such “reactivated” basins ar

observed in similar positions along the MRS in Minn

or Kansas (Sims, 1990). This interpretation of deep M

Simon basins was investigated by additional studies 

petrology and other characteristics of strata in thick M

Simon sequences. This information, combined with 

from the study of the Red clastics in the M.G. Eische

well (see Anderson, 1990a) led to the development of 

ria for distinguishing the two units.

Investigation of the UPH–2 Core

The first step in this stratigraphic investigation w

determine the characteristics of thick Mount Simon 

stone strata. Such a thick Mount Simon sequence is p

in northern Illinois, but no cores of this material had 

described in detail. To provide this information the 

Mount Simon interval of the Consolidated Edison U

core drilled in Stephenson County, in northern Illinoi

the town of Lena, was logged. The core, one of three

recovered from the northwest flank of the Mount S

Basin, mapped by Buschbach (1975) and known to e

2,460 ft in depth.

The Mount Simon Sandstone in the UPH–2 c

present at a depth between 1,310 and 2,178.5 ft and ov

Proterozoic granite porphyry. It can be characterized 

very fine to very coarse grained sandstone containing 

ules and pebbles. The unit contains less than 5 percent 

and includes many upward-fining sequences characte

by a poorly stratified base grading upward to better stra

upper units. The coarse-grained and pebbly beds gen

increase in frequency with depth. The texture, grain 

and sorting are highly variable from bed to bed. The 

of the sandstones of the Mount Simon is also highly va

from white (clear to frosted) to yellow and red, an

shales are predominantly red with some green and m

colors.

Body fossils were not observed in the Mount S

Sandstone in the UPH–2 core, but some trace fossils 

observed, including Skolithos, Arenicolites, and Cruzia

Rusophycus, generally above 1,584.5 ft. Bioturbati

most common in the upper 220 ft of the Mount Simon 

totally absent below 1,600 ft.

The sandstones in the Mount Simon are well stra

individual strata generally being less than 1 ft thick. 

tures observed include parallel, horizontal, small- 

large-scale cross-stratification. Clay drapes were obs

over small-scale, cross-stratified intervals, mudstone r

clasts, minor flaser structures, and minor lenticular 

stone bodies.

A qualitative mineralogical assessment during lo

determined that the sandstones were quartz-rich. S
I26 Strategic and Critical Minerals, Midcontinent Region, U.S.

were silty, sandy, and micaceous. Nine thin sections, representing samples collected at about 100-ft intervals, were

point-counted (appendix I2). These thin sections are quartz

arenites, the quartz ranging from 91 to 100 percent of QFL

grains. Feldspar grains compose as much as 8 percent (all

potassium feldspar) of the rocks; lithic fragments were not

observed. The mean composition for the framework grains

in the nine sections was Q99F1L0 (see appendix I2). The

cement in the rock is predominantly quartz, with minor

clay/oxides and rare carbonate minerals.

The UPH–2 core is reposited at the Iowa Geological

Survey Sample Repository near Iowa City. A detailed log is

on file at the Iowa Geological Survey Bureau.

Reinterpretation of the #1 Wilson Well

Only chip samples were collected during drilling of the

#1 Wilson well. Sandstones interpreted as Reagan Sandstone were encountered at 3,555 ft, beneath an undifferentiated Arbuckle Group or Upper Cambrian (Carlson and

others, 1990) dolomite sequence (fig. I15). Beneath the

Reagan Sandstone, the top of the MRS Red clastic sequence

is placed at a depth of 3,700 ft. The sandstone is dominated

by coarse sand (with subordinate very coarse to very fine

sand, minor green shale fragments and free dolomite rhombohedrons) to a depth of 3,935 ft. An 18-ft thick sequence

of red silty, sandy shale was encountered at that depth. At

3,953 ft the well passed back into the sandstone, which continued to a depth of about 4,130 ft, where an alternating

series of red shales and sandstones was encountered to

4,670 ft. Below a 30-ft sample gap, the nature of the samples changed to thick, dark-brown silty shale sequences

interbedded with fine-grained to very fine grained sandstone

to about 4,805 ft, light-brown siltstone interbedded with

dark shale and sandstone to about 4,985 ft, interbedded red

and dark-brown sandy, silty shales to 5,180 ft, and

coarse-grained to very fine grained sandstone to 5,270 ft.

Samples were not recovered from the interval 5,270–5,305

ft. The most recent previous interpretation of the clastic

rocks in the Wilson well (Bunker, 1982; Bunker and others,

1988) inferred that the Mount Simon–Red clastics contact

was within the sample gap from 4,670 to 4,700 ft, on the

basis of the major change in lithology from a sandstone-dominated sequence containing subordinate red

shales to a dark-brown and red-shale-dominated sequence

containing subordinate brown siltstone. Figure I16 shows

previous interpretations of the strata at the base of the Wilson well. Bunker’s interpretation was strengthened by the

presence of dolomitic green shale between the depths of

3,565 and 4,670 ft and by fossil fragments associated with

minor green shales between 4,205 and 4,235 ft and at 4,380

ft. Most of the fossil fragments and dark-green grains of

probable biogenic origin were not identifiable; however, a

brachiopod and a trilobite fragment were noted. This interpretation of the basal contact of the Mount Simon

Sandstone infers a unit thickness of 1,090 ft and w

the key pieces of evidence that led to the interpr

the deep Mount Simon basins flanking the Iowa H

To evaluate this interpretation, samples from 

intervals of the #1 Wilson well were petrographic

ined to identify the constituent framework g

cements. Fossil fragments and associated matrix 

lected and analyzed to identify the fossils and 

that the material was in place. Historic well 

reviewed to determine if any aspect of the drilling 

struction history of the #1 Wilson well might h

enced sample integrity. Finally, the well ch

re-examined to determine a new position for t

Simon–Red clastics contact if the previous inte

was found to be in error.

Petrographic Analysis

Samples were selected for petrographic ana

18 intervals in the clastic rock sequence at the b

Wilson well, and thin sections were prep

point-counted; both framework grains and cem

inventoried. The point-count data (appendix I2), 

lyzed to display framework grain data in the Q

showed a clustering into three groups that decreas

eralogical maturity with depth. The upperm

samples (3,655 ft) are the most mature (fig. I17), 

a framework mode of Q99.5F0.5L0.0. The second 

included 10 samples from depths of 3,703–4,50

samples display a mean QFL mode of Q91.6F8.3

third group of three samples recovered from 4,81

are the least mature, having a mean QFL comp

Q72.6F27.0L0.4. The sample gap that previously 

preted as the contact between the Mount Simon an

clastics falls within the interval between the se

third maturity group.

Examination of Fossils

Samples from the intervals in the Wilson we

been found to contain fossil fragments (between 

4,213–4,221 ft and 4,385 ft were examined under 

lar microscope. Representative fossil fragments, 

ments that displayed the best potential for iden

were collected from depths of 4,213 and 4,221 ft

sils included medium- to dark-green pellets, tubes

ments of gastropods and bryozoans, all having a p

appearance and less than 2 mm in length. Also, f

molds of brachiopods in gray siltstone were 

Additionally, samples from selected overlying in

Paleozoic strata, which had lithologies similar to 

associated with the deep fossil fragments, were a

ined. Fossil fragments similar in appearance to 

fossil fragments were recovered from a sample o
Clastic Rocks, Midcontinent Rift System, Iowa 

of the Ordovician St. Peter Sandstone (3,021–3,026 ft).

Fossil fragments were photographed through the binocular microscope (fig. I18), then 13 fossils, selected from

each of the 3 intervals, were mounted in epoxy and a polished thin section produced. Upon petrographic analysis

many fragments displayed an organized internal wall and

chambered structure typical of microfossils. External walls

and chambers were also observed, and the fossils displayed

petrographic characteristics typical of finely crystalline apatite (phosphate). The 13 fossil fragments mounted on the

polished thin section, 2 free individual fossil fragments, and

2 siltstone rock chips that contained phosphatic grains were

prepared for analysis on a scanning electron microscope.

Individual fossil fragments were examined, with seco

and backscattered electrons collected (fig. I19). Semi-

titative microanalysis was conducted.

Electron dispersion (EDS) analysis showed the co

sition of most fossils to be predominantly calcium 

phate (apatite), and some secondary pyrite and dolo

The bryozoans and gastropods showed moldic preserv

and calcium phosphate composition, and brachiopod

played moldic composition and possibly replaced or

shell material. Other grains included indeterminate o

irregularly shaped calcium phosphate pellets. Bryo

were the predominant fossil fragment recovered. Som

ozoans appear to be Cryptostomes, and one was tenta

Figure I15. Stratigraphy and construction history of #1 Wilson oil exploration well, Page County, Iowa. FMS, Formations

Group; SH, Shale; SS, Sandstone.

Clastic Rocks, Midcontinent Rift System, Iowa 

identified as a Ptilodictya sp (fig. I18G), but others were not

identified. Ptilodictya sp was reported to be present in the

St. Peter Sandstone in the Minneapolis area by Sardeson

(1892, 1896) and Sloan and others (1987). Neither the brachiopods nor the gastropods could be identified.

Bryozoans do not appear in the geologic record below

the base of the Ordovician, brachiopods appear at the base

of the Cambrian, and gastropods appear in the Cambrian

(McKerrow, 1978). The presence of bryozoans in the samples recovered from depths of 4,213 and 4,221 ft in the Wilson well, deep within a thick sandstone sequence that is

unlike any lithology within the Ordovician strata of the

region, and below documented Cambrian strata, indicates

that the fossils are not indigenous to the sample intervals

from which they were recovered. They appear to be s

in every way to the fossil fragments recovered and ana

from the base of the Ordovician St. Peter Sand

(3,021–3,026 ft). The fossils reported in the #1 Wilson 

ples from depths of 4,205–4,235 ft and 4,385 ft are 

preted as contamination that fell down the borehole fro

base of the St. Peter Sandstone at a depth of 3,021–3,0

Examination of Chip Samples

A re-examination of samples from depth

3,000–5,305 ft in the #1 Wilson well revealed tha

sandstones between 3,555 and 4,700 ft are contami

due to caving with green dolomitic shales and phosp

Figure I17. Lithic log and quartz–feldspar–lithic grain (QFL) diagram for part of #1 Wilson oil exploration well, Page Cou

Iowa. GP, Group; SS, Sandstone.
I30 Strategic and Critical Minerals, Midcontinent Region, U.S.
Clastic Rocks, Midcontinent Rift System, Iowa 

microfossils. Identical dolomitic green shales first occur in

the Wilson samples at 3,465 ft, and this interval in the middle Arbuckle Group is confidently identified as the source of

the dolomitic shale in the deeper sample sets. The fossils

originate from the overlying St. Peter Sandstone. There are

no reported occurrences of dolomitic shales in the Mount

Simon of the northern midcontinent and very few occurrences of dolomitic cements. Finally, rounded to

well-rounded, frosted quartz grains found throughout the

contaminated interval may, in part, be derived from the Jordan, St. Peter, or other mature Cambrian or Ordovician

sandstones.

Investigation of the drilling records for the #1 Wilson

disclosed that the well was drilled over a 2-yr period

(1929–31) with a cable tool drill rig. The hole was cased to

the top of the Ordovician Maquoketa Shale (depth of about

2,012 ft) and then left open until the well reached 4,708 ft,

the next casing point. The drilling of this 2,700-ft interval

was estimated from well file records to have taken about 1

yr, and over that period of time the caving intermittently

contaminated the newly drilled samples. At a depth of 4,700

ft the well was cased, and the lithologies observed in the

samples below this depth change dramatically. This change

was apparently due to the casing sealing upper intervals of

the borehole, preventing contamination of the samples

below the casing. The contamination of the interval above

4,700 ft indicates that the samples collected are not representative of the lithologies in the borehole and point-count

data from these intervals are compromised. The samples

collected below 4,700 ft are probably representative of the

lithologies in the borehole.

Conclusions

On the basis of the results of this research, the contact

between the base of the Reagan Sandstone and the top of the

Red clastics is placed at a depth of about 3,700 ft. QFL

analysis of framework grains from point-count data show a

substantial increase in coarse feldspar grains over this

interval (from 0.52 percent to 18.75 percent). This

coarse-grained feldspar content is much higher 

observed in any interval of the Mount Simon in the U

core. However, the presence of two metamorphic rock 

ments in the 3,655-ft interval leads to some uncertainty 

this interpretation. Metamorphic and volcanic rock 

ments are extremely rare in the Mount Simon Sand

except in the very lowest strata where clasts of the un

ing lithology are sometimes incorporated into basal s

Additional petrographic studies will be undertaken i

near future in an attempt to define the contact more 

rately.

Re-examination of the #1 Huntley Well

The #1 Huntley well in Butler County, Iowa

drilled in 1963–65 and penetrated almost 1,900 ft of 

tic rocks in the basal half of the well. The clastic 

penetrated included Cambrian Lone Rock, Wonewoc

Figure I18 (previous page). Photographs of selected microfossils recovered from chip samples from #1 Wilson oil exploration well (W–00017), Page County, Iowa. Specimens from

the interval 3,021–3,026 ft were collected from basal St. Peter

Sandstone (Ordovician). Specimens from 4,213–4,221 ft are

from two intervals interpreted as Red clastics in this report.

All specimens are preserved as calcium phosphate molds.

Bar scale A–I=0.25 mm; J=1.0 mm. A, Cryptostome bryozoan:

3,021–3,026 ft. B, Cryptostome bryozoan: 4,213 ft. C, Gastropod: 4,213 ft. D, Gastropod: 3,021–3,026 ft. E, Curved

tube (gastropod fragment?): 4,221 ft. F, Cryptostome bryozoan (?Ptilodictya sp.): 4,213 ft. G, Cryptostome bryozoan

(?Ptilodictya sp.): 3,021–3,026 ft. H, Cryptostome bryozoan:

4,213 ft. I, Cryptostome bryozoan: 4,213 ft. J, Brachiopod

impression in siltstone: 4,221 ft.

Figure I19. Back-scattered electron photomicrogra

Cryptostome bryozoan from #1 Wilson oil exploration 

(W–00017), Page County, Iowa. Image collected from 

bon-coated polished thin section. Photograph 

well-defined wall and chamber structure. Both wal

chambers are composed principally of calcium phos

Bright, high-relief spots are pyrite; dull, low-relief spo

dolomite. Slightly darker material along left margin is 

num oxide polishing compound. Sample 

W–00017–2–D3–BS; sample depth, 4,221 ft; acceleratin

age, 20 kV; magnification, 250×; bar scale, 120 μm.
I32 Strategic and Critical Minerals, Midcontinent Region, U.S.

Claire, and Mount Simon Formations overlying a thick

sequence (about 1,280 ft) originally interpreted as Red

clastics. A number of brachiopod fragments were reported

to be present at depths of 3,505–3,525 ft near the bottom of

the well and were originally interpreted as contamination

due to caving (IGSB unpublished log by D.L. Koch, 1965).

Re-examination of the chips in the late 1970’s led to a

revised interpretation of the stratigraphy (Bunker, 1981;

1982; and Bunker and others, 1988). The new interpretation

placed the basal 1,465 ft of the clastics in the Mount Simon

Sandstone. This was based on the lack of a distinct contact

with the Red clastics, the coarse to very coarse sand grains

present throughout the unit, dolomitic cement, and the fossils near the base of the well that were thought to be imbedded in a matrix that was unlike any lithology observed in

overlying units. The fossils were, therefore, interpreted to

be in place.

For this study, the history of the drilling of the well

was investigated (fig. I20); samples were collected from 18

intervals of chips, and grain-mount thin sections were produced and point-counted. The data collected (appendix I2)

was analyzed to determine QFL framework mineralogy

(fig. I21). The QFL analysis of the data produced two populations. The upper interval (2,150–2,305 ft) was mineralogically mature, having a composition of Q98.4F1.6L0.0; no

lithic grains were encountered. This composition is typical of the Mount Simon Sandstone. Point counts of 13 thin

sections showed the underlying intervals to have a mean

QFL composition of Q78.3F16.2L5.5. This is similar to the

composition of unit H of the Eischeid well. On the basis

of the maturity and lithic grain composition of the framework grains, examination of the natural gamma-ray log,

and feldspar lithic grain size, the contact between the

Mount Simon and the Red clastics is judged to be at 2,305

ft in the #1 Huntley well. This interpretation results in the

Mount Simon being 175 ft thick, much thinner than the

previous interpretation, which suggested a unit thickness of

1,400 ft.

Fossil fragments and their matrix from chip samples of

the depth interval between 3,505 and 3,575 ft were examined. Fragments of inarticulate brachiopods were identified

in a greenish, argillaceous, glauconitic, very fine grained to

fine-grained sandstone matrix. Using existing lithologic

logs of the well as a guide, chips from selected intervals of

Cambrian shales in the well were re-examined, and several

samples were identified that may have been the source of

the brachiopods and their matrix. Identical brachiopods

and matrix were observed in the basal Eau Claire and

uppermost Mount Simon Sandstones in the depth interval

from 2,020 to 2,040 ft. This higher level, brachiopod-rich

interval remained uncased, and the open hole caved during

the drilling to its final depth of 3,595 ft (see fig. I20).

Examination of the well file records indicates that the well

was affected by severe caving through this interval, and

since this caving-prone, brachiopod-bearing interval was

never cased off, it is concluded, therefore, that th

late brachiopods identified between 3,505 and 3

the Huntley well results from contamination from 

Cambrian Eau Claire Sandstone. On the bas

restudy, the top of the Proterozoic Red clastics is 

a depth of 2,305 ft.

Evaluation of Criteria for Picking the M

Simon–Red Clastics Contact

This study of the Mount Simon–pre-Mou

contact in Iowa has led to the identification 

lithostratigraphic correlation characteristics that a

ful. Traditionally, several criteria have defined th

the Mount Simon and the top of the Red clastics

among these criteria is the change in color from 

sandstones and gray-green shales of the Mount Sim

pink, orange, or red sandstone and red siltstone an

the Red clastics. Other criteria include the lack o

the Red clastics, the first appearance of red shale

stones, and an increase in consolidation with de

Red clastics over the Mount Simon. This study, 

by Robert McKay, suggests that these criteria m

valid. Examination of the UPH–2 core indicate

Mount Simon sandstone can be either red or orang

include interbedded red shales. Additionally, th

core shows that the Mount Simon can be wel

While it is true that no fossils are known to be 

MRS sandstones, caving within a drillhole can br

from Paleozoic units down the hole to mix with R

samples. The study by McKay identified sever

that appear to be useful in delineating the contact:

Feldspar size and shape.—Whereas the Mou

Sandstone is predominantly a quartz arenite, som

may contain 30 percent or more feldspar (fig. I

Mount Simon feldspar is limited to potassium 

many grains display well-defined potassium feld

growths on detrital potassium feldspar grains that 

hedral to euhedral, pore-filling crystals. Detrita

grains in the Mount Simon are always smaller 

grained to medium grained) than associated qua

and are rounded to subrounded. Feldspar is a com

stituent of the Red clastics; potassium feldspar is m

mon, but minor plagioclase can also be found. A

feldspar is also present in the Red clastics but 

common than detrital feldspar. Most diagnos

observation that detrital feldspar grains in Red c

as large or larger than the quartz grains with whic

associated. Additionally, detrital feldspar grains 

monly angular to subangular and generally dis

nounced cleavage faces.

Cement mineralogy.—The mineralogy of th

observed in the Mount Simon and the Red clastic

be helpful in differentiating the units. The Moun
Clastic Rocks, Midcontinent Rift System, Iowa 

dominated by quartz and potassium feldspar cement and

contains minor carbonate, hematite and clay (kaolinite and

illite) cement. Red clastics sandstones are predominantly

cemented by quartz, calcite, hematite, and clay, and minor

authigenic feldspar.

Detrital rock fragments.—Detrital rock fragments are

very rare in the Mount Simon Sandstone. Detrital rock fragments were not observed in point counts of nine thin-sections from the UPH–2 core; rock grains were identified in

only one thin section of samples collected from seven intervals of Mount Simon Sandstone from other wells. Detrital

rock fragments constitute an important constituent of the

Red clastics rocks studied. Lithic fragments were observed

to constitute as much as 19.8 percent (appendix I1) of some

Red clastic intervals studied, with felsic volcanic and

metamorphic rock fragments being the most common

lithologies of the lithic fragments. The presence of com

lithic rock fragments is indicative of Proterozoic 

clastics.

Sandstone-to-mudstone ratio.—Although not as u

in defining the contact, the sandstone-to-mudstone 

and siltstone) ratio can be helpful in determining if a de

tional sequence should be assigned to the Mount Sim

the Red clastics. More than 90 percent of observed M

Simon rocks are sandstone, mudstone representin

percent of the section. The Red clastics display a 

more variable sandstone-to-mudstone ratio, ranging 

about 20 percent to greater than 80 percent mudstone. 

discriminator may be rendered invalid if Proterozoic 

equivalent to the Hinckley Sandstone of Minnesot

observed in the Iowa subsurface. The Hinckley, lik

Mount Simon, is strongly dominated by sandstone.

Figure I20. Stratigraphy and construction history of #1 Huntley oil exploration well, Butler County, Iowa. DOL, Dolomite

Formation; SS, Sandstone.
I34 Strategic and Critical Minerals, Midcontinent Region, U.S.

Presence of quartz arenites.—All quartz arenites (>90

percent quartz) that have been encountered near the Mount

Simon–Red clastics contact in Iowa have been assigned to

the Mount Simon. Petrographic examination of the Mount

Simon has shown it to be strongly dominated by quartz

arenites. Quartz arenites have not been observed in the Red

clastics of Iowa. 

The comparative study of the Mount Simon Sandstone

and Red clastics also identified a number of criteria that are

not useful in defining the contact between these units. They

include:

Color.—Color has not been shown to be an effective

tool for the differentiation of Mount Simon and Red clastic

rocks. Sandstones in both units can range in color 

to pink, orange, or red. While Mount Simon shale

stones are commonly gray to grayish-red in the s

red shales are not uncommon. The shales obser

Red clastics range in color from red to green m

brown, dark gray, or even black.

Grain size.—The Mount Simon Sandstone 

observed to range from very fine to very coars

granules being frequently observed. The Red cl

play a similar variability in grain size.

Induration.—Both the Mount Simon and the 

clastics sequence are generally poorly indurated

may be well indurated in selected intervals. While 

that the Red clastics may, as a unit, be sligh

Figure I21. Lithic and gamma logs and quartz–feldspar–lithic grain (QFL) diagram for part of #1 Huntley oil explorati

Butler County, Iowa. SS, Sandstone.
Clastic Rocks, Midcontinent Rift System, Iowa 

Figure I22. Quartz–feldspar–lithic grain (QFL) diagram of

analyses of Mount Simon Sandstone and Red clastics from

selected Iowa wells.

indurated, both units show sufficient variability to invalidate

the usefulness of this criterion.

METALLIC MINERAL POTENTIAL OF MRS 

CLASTIC ROCKS IN IOWA

Examination of the lithologic logs from 9 of the 14

wells that penetrate MRS clastic rocks (Red clastics) in

Iowa, and examination of the 481 ft of Red clastics core

from 6 wells (table I1), have not disclosed any metallic

mineral occurrences. Only one log, the mud log for the

M.G. Eischeid #1 well, identified metallic mineral occurrence. The log identified “copper or chalcopyrite” at a

depth of 15,440 ft. We carefully examined samples from

that interval under a binocular microscope, but we found no

copper, chalcopyrite, or any other metallic mineral.

PETROLEUM RESOURCES

Potentially valuable petroleum resources may be

present in MRS clastic rocks. The Nonesuch Formation is

dominated by dark-gray to black, organic shale and siltstone

that were apparently deposited in a lake or other oxygen-starved euxinic body of standing water. Oil seeps from

the Nonesuch ceiling rock in older parts of the Copper

Range (White Pine) Mine, in northern Michigan, have been

known for many years. This oil was shown to be indigenous

to the Nonesuch Formation by Barghoorn and others

(1965), and Rb-Sr dating of calcite containing primary

petroleum inclusions by Kelly and Nishioka (1985) yi

a minimum age of 1,047±35 Ma for the hydrocarbons

petroleum is a paraffinic, low-sulfur oil of 32° API g

(Dickas, 1987).

Petroleum Potential Studies of Eischeid 

Samples

Since the target of the Amoco drilling was to sear

economic deposits of petroleum and to learn more 

petroleum potential of the sedimentary rocks o

Keweenawan Supergroup, core and chip samples fro

M.G. Eischeid #1 well were examined by several wo

Logs of the well did not indicate any oil shows; how

trace amounts of methane and ethane were detected in 

B, C, and D–1, and intergranular black residues (sugg

of oil movement) were reported in units B, C–2, and 

Black shales were reported by Witzke (1990) in units 

and D–1, unit C being dominated by black to dark

shale and siltstone, all organic-rich and potential petro

source rocks. Finally, the large sandstone component 

upper and lower Red clastics sequences and interb

finer grained units offer the potential for petroleum acc

lation and stratigraphic or unconformity traps, as w

structural traps that may have formed as a result of th

compressional phase of the MRS.

Source-Rock Potential

Samples collected from 53 intervals of Proterozo

Paleozoic units in the Eischeid well were analyzed fo

organic carbon (TOC) by Palacas and others (199

determine their source-rock potential. Selected sa

were also analyzed for the maximum pyrolysis temper

(Tmax), the hydrogen index (HI), the oxygen index 

the production index (PI), the free hydrocarbons (S1

the pyrolytic yield (S3). All upper Red clastics seq

samples contained less than 0.1 percent TOC, and mo

ues were in the range of 0.03–0.05 percent. The lowe

clastics sequence showed higher values of TOC, unit D 

ues all being less than 0.1 percent. Unit C was the 

organic-rich, reaching a maximum value of 1.4 percen

averaging 0.6 percent. The only analysis from unit B 

played a TOC of 0.4 percent. These TOC values are 

but many petroleum geologists consider a TOC of 0.

cent a minimum for a source rock (J.G. Palacas, oral 

mun., 1990). By this definition the carbonaceous roc

unit C can be considered a marginal potential source ro

Although the TOC content of unit C in the Eis

well might suggest that it is a marginal source rock, 

minations of Tmax (the temperature at which the yie

pyrolysis products is maximized) for samples col

from six intervals of unit C core by Palacas and o
I36 Strategic and Critical Minerals, Midcontinent Region, U.S.

(1990) indicate that the organic material in the unit is

overmature with respect to oil generation. They identified

Tmax values ranging from 497 to 508 °C (averaging 503

°C), considerably higher than the 460–465 °C considered

the maximum temperature at which liquid petroleum is produced. The 503 °C Tmax values for unit C samples places

the unit in the lower end of the wet gas zone and possibly

the dry gas (metagenetic) zone (Palacas and others, 1990).

This advanced stage of thermal maturity was corroborated

by several other studies including homogenization measurements on two-phase fluid inclusions in calcite and quartz

veins in unit C by Barker (1990). He identified two populations of temperatures, indicating an earlier 200 °C event and

a later 140 °C event, which he attributed to deep burial, elevated heat flow, or hydrothermal activity. He verified the

200 °C event by measuring the peak bitumin reflectance

(vitrinite reflectance equivalent) of unit C rocks. Ludvigson

and Spry (1990) measured homogenization temperatures of

two-phase liquid-vapor fluid inclusions in calcite veins near

the top of unit C ranging from 125 to 178.6 °C. Pollastro

and Finn (1990) used clay geothermometry to calculate a

minimum paleotemperature of 175–180 °C for samples

from the top of unit C, and to estimate the minimum bottom

hole (at 17,851 ft) paleotemperature of 192–197 33 °C.

Reflectance measurements on vitrinite-like particles, measurements of thermal-alteration index, and indices for chloroform-extractable organic matter determined by Palacas

and others (1990) indicated that the organic matter in unit C

has advanced well into the gas-generation zone. Palacas

and others (1990, p. 119) also calculated genetic potentials

of 0.1–0.4 HG/g and hydrogen indices from 20 to 80 mg

HC/g TOC leading them to conclude that “at present, these

shale beds have no potential of generating commercial

petroleum.” They concluded, however, that significant

amounts of hydrocarbon may have been generated in the

geologic past.

Porosity

The porosity of the Proterozoic Red clastic rocks in the

M.G. Eischeid #1 well was investigated by Schmoker and

Palacas (1990) and Ludvigson and others (1990).

Schmoker and Palacas utilized a variety of down-hole geophysical logs to calculate the porosity of sandstones

between depths of 11,450 and 17,340 ft (units B and C and

the lower part of unit D). They determined porosity ranging

from 1 to 6 percent (averaging 2.3 percent) in this interval,

with “better” porosity (3.5 percent or more) in about 14 percent of the section distributed in their upper (in unit D) and

lower (in unit B) zones. The Eischeid mud log noted the

detection of methane and ethane gases in these two zones

and reported intergranular black residue, possibly a bitumen

or pyrobitumen-like material (Schmoker and Palacas,

1990), in the lower zone.

Ludvigson and others (1990) and Barnes (19

however, that optically resolvable porosity was no

in samples below 8,000 ft. The disparity betwee

porosity reported by Schmoker and Palacas and 

absence of porosity noted by Ludvigson and Barn

the presence of micropores (not observable b

microscopic techniques) or pores as inclusions 

gas and (or) liquid in calcite and quartz veins and 

cements. Also, chips from many intervals inclu

dant loose (not cemented) grains, indicating poss

of incomplete cementation and higher porosity.

Conclusions 

It appears that some intervals of the lower R

sequence in the M.G. Eischeid #1 well, especial

once contained high levels of organic carbon and 

produced significant volumes of petroleum. 

hydrocarbon production probably reached its peak 

early in the rock’s history, about 800 Ma (Palaca

ers, 1990). Black intergranular residue reported on 

heid mud log may identify paths of petroleum 

through overlying and underlying rocks, but no ar

petroleum accumulations were identified. The or

rocks in the Eischeid well have realized most of t

leum potential and are presently overmature and 

potential for producing commercial petroleum. P

also very low in the Proterozoic clastic rocks in th

well. Much of the original pore space may h

destroyed by compaction and the precipi

intergranular cements.

Petroleum Potential of MRS Clastic Ro

The absence of commercial petroleum an

petroleum potential in the Proterozoic clastic ro

M.G. Eischeid #1 well does not preclude the pos

ence of commercial quantities of petroleum in oth

the MRS. The advanced state of maturity observe

by Palacas and others (1990) is the result of high 

tures affecting the Proterozoic rocks in the Eisc

probably early in their history. The methane-fi

inclusions in tectonic veinlets described by Ludv

Spry (1990) indicate that late Keweenawan deform

coeval with petroleum migration in the Defian

Pollastro and Finn (1990) estimated minimum pal

atures between 175 and 197 °C for this thermal e

siderably higher than the present estimated bo

temperature of 105 °C (Barker, 1990). It seem

that this thermal regime was the product of dee

alone. The heat source was probably elevated 

from the igneous activity along the trend of the 

nent Rift, and possibly by hydrothermal activity 
Clastic Rocks, Midcontinent Rift System, Iowa 

with later stages of this activity. The elevated heat flow and

geothermal activity were most pronounced nearest the axis

of the rift. 

At more distal areas the maximum temperatures would

probably have been lower. Interpretations of seismic data

acquired over the Defiance Basin by Anderson (1992) suggest that reflectors assigned to unit C lithologies continue

for at least 7 mi and possibly as far as 15 mi from the Iowa

Horst (originally the central graben). If unit C also contained significant amounts of organic carbon away from the

central graben, these organic-rich strata may not have been

exposed to the high paleotemperatures affecting rocks

closer to the axis of the rift. Consequently, these rocks

might not have released their petroleum as early as rocks

closer to the center of the MRS, and petroleum may still

remain, trapped in stratigraphic and (or) structural traps.

Citing fluid inclusion and isotopic evidence, Ludvigson and

Spry (1990) suggested that during uplift of the axial horst,

fluid transport in the Keweenawan sedimentary basins was

away from the rift axis. The petroleum migrated up dip,

generally away from the axis of the rift, suggesting that the

best potential exploration targets may exist on the outer

flanks of the MRS (Ludvigson and Spry, 1990).

It appears that some intervals of the lower Red clastics

sequence in the M.G. Eischeid #1 well, especially unit C,

once contained high levels of organic carbon and may have

produced large volumes of petroleum. However, hydrocarbon generation probably reached its peak relatively early in

the rock’s history, about 800 Ma (Palacas and others, 1990).

Black intergranular residue reported on the Eischeid mud

log may identify paths of petroleum migration through surrounding rocks, but no areas of past petroleum accumulations were identified. The organic-rich rocks in the Eischeid

well have realized most of their petroleum potential and are

presently overmature, retaining no potential for producing

commercial petroleum. Porosity is also very low in the Proterozoic clastic rocks in the Eischeid well. Much of the original pore space may have been destroyed by compaction and

the precipitation of intergranular cements.

CONCLUSIONS

This study of the clastic rocks associated with the Midcontinent Rift System in Iowa has yielded significant information that is useful in understanding these rocks, their

relationships to the units in their Lake Superior exposure

area, and their mineral potential. Significantly, petrographic

characteristics have been identified that can be used to

descriminate Midcontinent Rift clastics rocks and lithologically similar basal Paleozoic strata. Some of the major conclusions of this investigation are as follows:

• Cutting samples and cores from the Amoco M.G. Eischeid #1 deep oil test were studied. The well penetrated

15,049 ft of MRS clastic rocks that were assigned to

two sequences, the upper Red clastics sequence and the

lower Red clastics sequence. The upper sequenc

subdivided into four units; the lower sequence 

divided into three units. The lower Red cl

sequence is similar in many ways to the Oronto G

of the Lake Superior area, but a direct correlation 

possible. The upper Red clastics sequence bears 

similarities to the Bayfield Group of Wisconsin bu

cannot be directly correlated.

Unit C in the lower Red clastics sequence 

tained abundant dark-gray and black shale and sil

beds and is tentatively correlated with the Non

Formation of the Oronto Group of Wisconsin 

Michigan. They are presently relatively low in 

(averaging about 0.6 percent) and are overmature 

respect to their source-rock potential. They pre

do not constitute a potential source of economic q

ties of petroleum, but probably produced large am

of petroleum early in their geologic history. Pre

of black intergranular residues in a number of sa

strongly suggests the migration of liquid petrole

some time in the geologic past.

Reflectors interpreted as the result of strata 

lar to unit C have been identified in deep seismic d

southwestern Iowa. These reflectors indicate th

unit may extend for 15 mi or more away from the 

Horst. This interpretation was strengthened by th

covery of unit C rocks on the central peak of the 

son Impact Structure. In the more distal area

effects of the elevated heat flow and possible hydr

mal effects associated with the axial regions of th

may be less pronounced and the organics in th

may not have reached thermal maturity as early in 

histories. Consequently, the possibility of encoun

economic concentrations of petroleum, derived 

MRS clastics rocks, may be greater in these 

areas.

• Red clastic rocks are preserved in a series of bas

the Precambrian surface of Iowa. Five large hal

ben basins flank and deepen towards the Iowa Ho

the MRS. These basins—two west of the hors

three to the east—preserve an estimated volum

35,400 mi3 of upper and lower Red clastic strata

clastics are also preserved in several areas on the 

Horst. These include three basins and 

down-dropped block that preserve an additional 

mi3 of Red clastics. 

• A reinvestigation of core and chip samples of MR

clastic rocks in Iowa did not yield any evidence o

nomic mineralization. Copper or chalcopyrite rep

from the Eischeid well was not confirmed by our 

tigation. The lack of mineral occurrences in these 

does not preclude the possibility of its presence i

clastics in other parts of Iowa, since the ava

sample set is extremely limited.
I38 Strategic and Critical Minerals, Midcontinent Region, U.S.

• Well samples previously interpretated as evidence of

deep basal Cambrian basins overlying the Red clastics

basins that flank the Iowa Horst were re-examined with

petrographic and SEM techniques. Cambrian fossils

previously thought to be in place deep in the basal clastic sequence in two wells were proven to be contamination by caving of stratigraphically higher units during

the drilling. Chip samples of the clastics from these

deeper intervals were shown to be petrographically

similar to Red clastics, not basal Cambrian units. No

evidence of the deep Cambrian basins was found.

• Following restudy of samples from a number of wells

in Iowa and Illinois, a series of lithostratigraphic correlation characteristics were identified that will prove

useful in determining the contact between the basal

Cambrian sandstone and the underlying Red clastics in

the region. These distinguishing characteristics were

described earlier. One of the most useful characteristics is the very mature mineralogy of the framework

grains in the Mount Simon Sandstone, which is composed of quartz arenites (>90 percent quartz) in almost

all intervals, and the very limited number of lithic fragments encountered, especially volcanic and metamorphic grains. The Red clastics framework grains always

display a higher feldspar content, and lithic rock fragments are a common component of the framework.

ACKNOWLEDGMENTS

The authors wish to acknowledge the persistant

encouragement and logistical assistance provided by Warren

Day and the extremely helpful reviews by P.K. Sims, both of

the U.S. Geological Survey. Additionally, the authors

would like to acknowledge the funding assistance provided

by the U.S. Geological Survey Midcontinent Strategic and

Critical Minerals Program.

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APPENDIXES
I42 Strategic and Critical Minerals, Midcontinent Region, U.S.

Appendix I1. Results of petrographic analyses of thin-sectioned samples collected from the basal Mount Simon Sandstone 

Red clastics interval of the M.G. Eischeid #1 well in Carroll County, Iowa. 

[Q, quartz; F, feldspar; L, lithic grains; Qm, monocrystalline quartz; Qp, polycrystaline quartz; P, plagioclase; K, potassium feldspar; Ls, sedimentary 

grains; Lm, metamorphic lithic grains; Lv, volcanic lithic grains; M, mica; H, heavy minerals; Qtz cem, quartz cement; Carb cem, carbonate cement

samples of Mount Simon Sandstone; units H, G, F, and E are from upper Red clastics sequence; units D, C, and B are from lower Red clastics sequ
Clastic Rocks, Midcontinent Rift System, Iowa 

Appendix I2. Results of petrographic analyses of thin-sectioned samples collected from Mount Simon Sand

stone and Red clastics intervals of several wells in Iowa and Illinois. 

[Petrographic data from study of samples from the M.G. Eischeid #1 well are reproduced in appendix I1. WNUMBER, Iowa wel

identification number; SPLTYPE, sample type (CUT, cuttings/chip samples; CORE, core samples); NUMCOUNTS, number of points

counted; Q, percent quartz grains; F, percent feldspar grains; L, percent lithic grains;: #1 Wilson, W00017, Page Co., Iowa; #1 Huntley

W14428, Butler Co., Iowa; M.G. Eischeid #1, W27933, Carroll Co., Iowa; UPH–2, W27606, Stephenson Co., Ill.]
I44 Strategic and Critical Minerals, Midcontinent Region, U.S.

Appendix I2. Results of petrographic analyses of thin-sectioned samples collected from Mount Simon Sandstone and Red clastics intervals of several wells in Iowa and Illinois—Continued 

Online version prepared by Joan Nadeau
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