fl€J

Stratigraphy of Middle 

Ordovician Rocks in the 

Zinc-Lead District of 

Wisconsin, Illinois, and Iowa

SURVEY PROFESSIONAL PAPER 274-K

Prepared in cooperation with the 

Wisconsin Geological and 

Natural History Survey
Stratigraphy of Middle 

Ordovician Rocks in the 

Zinc-Lead District of 

Wisconsin, Illinois, and Iowa

By ALLEN F. AGNEW, ALLEN V. HEYL, JR., C. H. BEHRE, JR., and E. J. LYONS 

A SHORTER CONTRIBUTION TO GENERAL GEOLOGY

.>,GEOLOGICAL S URVE Y < PROFES SI ON AL PAPER 274-K

Prepared in cooperation with the 

Wisconsin Geological and 

Natural History Survey

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956
UNITED STATES DEPARTMENT OF THE INTERIOR 

Fred A. Seaton, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 60 cents (paper cover)
CONTENTS

Abstract. __________________________________________

Introduction _____________________________________

Purpose of the investigation____________________

Location and geographic setting__________________

Field work.____________________________________

Acknowledgments. ______________________________

Regional geology and stratigraphicrelations_____ ________

Structure ______________________________________

Regional stratigraphic summary __________________

Stratigraphic principles._________________________

Stratigraphic problems. _________________________

Platteville-Decorah boundary _ _______________

Beloit dolomite.____________________________

Unnamed limestone member, Decorah formation. _ 

Subdivisions of Galena dolomite ______________

Origin and application of the names.______________

Madison sandstone-________________________

Pecatonica dolomite member.________________

McGregor limestone member_________________

Quimbys Mill member _______________________

Stratigraphy of the mining district.___________________

Pre-Platteville rocks_ ___________________________

Pre-Cambrian rocks_________________________

Cambrian rocks_ ___________________________

Ordovician rocks____________________________

Prairie du Chien group __________________

St. Peter sandstone_ ____________________

Platteville formation____________________________

General features____________________________

Glenwood shale member.____________________

Lithologic description and stratigraphic relations _______________________________

Distribution.. __________________________

Fauna and correlation.__________________

Economic products______________________

Pecatonica dolomite member.________________

Lithologic description and stratigraphic relations_ ______________________________

Distribution, ___________________________

Fauna and correlation.__________________

Economic products______________________

McGregor limestone member.________________

Lithologic description and stratigraphic relations. ______________________________

Distribution. _ __________________________

Fauna and correlation.__________________

Economic products._____________________

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Quimbys Mill member_______________________

Lithologic description and stratigraphic relations_ ______________________________

Distribution. _ __________________________

Fauna and correlation.__________________

Economic products______________________

Distribution of facies of the Platteville and conditions of deposition__________ _____________

Decorah formation._____________________________

General features._________---_____--___--___

Spechts Ferry shale member _________________

Lithologic description and stratigraphic relations. ______________________________

Distribution. ___________________________

Fauna and correlation.__________________

Economic products______________________

Guttenberg limestone member._______________

Lithologic description and stratigraphic relations. ______________________________

Distribution___________ _________________

Fauna and correlation __________________

Economic products._____________________

Ion dolomite member_______________________

Lithologic description and stratigraphic relations. ______________________________

Distribution, ___________________________

Fauna and correlation.__________________

Economic products._____________________

Distribution of facies of the Decorah and conditions of deposition_______________________

Galena dolomite._______________________________

General features-___________________________

Cherty unit ________________________________

Noncherty unit _____________________________

Distribution. _______________________________

Distribution of facies of the Galena and conditions of deposition ________________________

Fauna and correlation-______________________

Economic products. _________________________

Post-Galena rocks___ ___________________________

Ordovician rocks—Maquoketa shale.__________

Silurian rocks_ ______________________________

Post-Silurian deposits_ ______________________

Literature cited-____________________________________

Stratigraphic sections._______________________________

Index _____________________________________________

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in
IV CONTENTS

ILLUSTRATION

	Page 

FIGURE 31. Upper Mississippi Valley region showing general geology and location of the zinc-lead district_____ __________ 253

32. Generalized stratigraphic column for zinc-lead district__ ________________________________________________ 255

33. Bluff showing Platteville, Decorah, and Galena strata-_______________-____-__----___----_____-______--_ 256

34. Diagrammatic cross section of Platteville, Decorah, and Galena strata eastward across the mining district-___ 257

35. Map of part of upper Mississippi Valley region, showing localities cited in text.___________________________ 258

36. Generalized west-east cross section of Decorah formation and adjacent strata.____________________________ 263

37. Correlation of Platteville, Decorah, and Galena strata with units in Minnesota-__________________________ 264

38. Stratigraphic column of Platteville, Decorah, and Galena strata in zinc-lead district______-____-____________ 268

39. Classifications of Platteville, Decorah, and Galena sequence since 1906-__________________________________ 270

40. Quarry showing Prairie du Chien group.__________________________---___--__-_____------------___--_- 272

41. Crossbedded St. Peter sandstone in roadcut._______-_______________--_-_________-__--__--------__-_-_- 273

42. Pecatonica and McGregor members of the Platteville formation in roadcut______________________________ 274

43. Glenwood shale member and Pecatonica member of Platteville formation above St. Peter sandstone in roadcut__ 275

44. Suggested correlatives of Platteville, Decorah, and Galena strata._______________________________________ 278

45. Comparative stratigraphic terminology of Platteville, Decorah, and Galena rocks in the zinc-lead district----- 280

46. Type section of Quimbys Mill member of Platteville formation._________________________________________ 283

47. Type section of Spechts Ferry shale member of Decorah formation______________________________________ 286

48. Quimbys Mill member of Platteville formation and the overlying Guttenberg member of Decorah formation in

	a quarry at Calmine, Wis_________-________________-__________---_----_-_______---_-_--_---_-_---_ 287

49. Type section of Guttenberg limestone member of the Decorah formation__________________________________ 290

50. Decorah formation in dolomite facies, overlain by Galena dolomite and underlain by Quimbys Mill member of

	Platteville formation, in quarry at York Church, Wis________________________________________________ 290

51. Carbonaceous shale partings between wavy dolomite beds of Guttenberg limestone member of Platteville forma- 

	tion in quarry in Green County, Wis_______________________________________________________________ 291

52. Exposure in mine showing effect of mineralizing solutions on Guttenberg member of Decorah formation______ 291

53. Honeycomb weathering of Galena dolomite in a roadcut in Grant County, Wis____________________________ 297

54. Maquoketa shale and overlying Silurian dolomite in roadcut and quarry._________________________________ 301
A SHORTER CONTRIBUTION TO GENERAL GEOLOGY

STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD 

DISTRICT OF WISCONSIN, ILLINOIS, AND IOWA

By ALLEN F. AGNEW, ALLEN V. HEYL, JR., C. H. BEHRE, JR., and E. J. LYONS

ABSTRACT

Stratigraphic studies, both outcrop and subsurface, of Ordovician rocks of the zinc-lead district in Wisconsin, Illinois, and 

Iowa form the basis for a more accurate means of delimiting 

potentially ore-bearing areas from less favorable localities. 

Detailed stratigraphic subdivisions of the Platteville, Decorah, 

and Galena strata have permitted the mapping of folds with 

structural relief of 30 feet or less in areas smaller than a quarter 

of a mile square. Such relatively small flexures and their 

associated fractures have caused the localization of the orebearing solutions.

Lithologic characteristics provided the basis for subdividing 

the rocks into mappable units—formations and members. No 

systematic paleontologic study has been made; age assignments, 

therefore, are only general

Although the investigation initially dealt with relatively local 

areas, certain members of the rock units and their subdivisions 

are mappable regionally.

Rock units below the Platteville formation include sandstone 

and shale of the St. Peter sandstone; shale, dolomite, and sandstone of the Prairie du Chien group, sandstone, siltstone, and 

dolomite of Cambrian age, and the Pre-Cambrian granitic rocks. 

The Galena strata are overlain by the shale and dolomite of 

Maquoketa age, upon which rests Silurian dolomite.

Because the Platteville, Decorah, and Galena strata include 

the currently producing ore zone, the detailed investigation was 

restricted to those units.

The Platteville formation regionally includes the green sandy 

Glen wood shale member at its base, as much as 3 feet thick; 

the brown Pecatonica dolomite member, 20—24 feet thick; the 

gray limestone and dolomite of the McGregor limestone member, 

totalling 30 feet; and the uppermost member, the brown limestone 

dolomite, and shale of the Quimbys Mill member, as' much as 

18 feet thick.

In the mining district the Decorah formation includes at the 

base green shale and limestone of the Spechts Ferry shale member, with a maximum thickness of 8 feet; the brown limestone, 

dolomite, and shale of the Guttenberg limestone member, 12-16 

feet thick; and the uppermost member, the grayish-green limestone, dolomite, and shale of the Ion dolomite member, 20 feet 

thick. In the western part of the mining district and to the 

west is an unnamed limestone member that underlies the Spechts 

Ferry.

To the east, the Platteville. and Decorah strata are dolomite 

of similar lithologic character, and have been called the Beloit 

dolomite.

The Galena dolomite is generally coarsely crystalline massive 

vuggy dolomite or limestone divisible regionally into two units, 

a cherty lower unit 105 feet thick and a noncherty upper one 120 

feet thick. The cherty unit is divided into four zones, A, B, C, 

and D in descending order, based primarily upon relative chert 

content and the presence of Receptaculites. The noncherty unit 

is divided on the basis of thinness of bedding and the amount of 

interbedded shale into Dubuque at the top and the more massive, 

less shaly Stewartville and zone P of the Prosser below; the 

latter is divided less precisely on the basis of Receptaculites which 

are abundant in the Stewartville, but not in zone P, below. The 

name Dubuque is applied much as it has been in the past; the 

name Stewartville is applied more or less as it has been in the 

past; and the name Prosser includes the cherty unit and probably 

all of zone P of the overlying noncherty unit. Because of 

paleontologic deficiencies and because of the distinct lithologic 

differences, in this paper the Galena is subdivided into the 

cherty and noncherty units rather than into the Prosser, Stewartville, and Dubuque members of common, although not precise 

usage.

A bentonite layer is present near the base of the Spechts Ferry 

shale member; less commonly bentonite seams are found 30-32 

feet below and 18 feet above the contact between the cherty and 

noncherty units. West of the mining district the unnamed 

lower member of the Decorah formation includes bentonite at 

its base. Locally bentonite is present in the Decorah.

All members of the Platteville formation except the Glenwood 

decrease in thickness toward the west, whereas all members of 

the Decorah formation thicken toward the west.

In general the Platteville, Decorah, and Galena strata represent 

a marine environment of a relatively shallow-water platform.

Facies analysis of the Platteville, Decorah, and Galena formations shows a regional change from shale (Decorah formation) 

and limestones (Galena and Platteville formations, excluding the 

Glenwood member) west of the mining district eastward into 

dolomite (all three formations) east of the district. Chert is 

characteristic of the upper two members of the Platteville formation to the east. To the northwest, in southeastern Minnesota 

the Decorah formation is green shale, and the lower two zones of 

the Galena dolomite are interbedded shale and limestone; the 

Galena has very little chert in Minnesota. The increase in dolomite and chert to the east is probably due to the influence of the 

Wisconsin arch.

Leaching, dolomitization, and silicification, due to the zinc-lead 

mineralizing solutions, have altered some of the carbonate strata

251
252 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

especially the Quimbys Mill member ("glass rock" of local 

usage) and the Guttenberg member (oil rock of local usage); in 

places either of these units may have been reduced from 15 feet 

of argillaceous limestone with a small amount of shale, to less 

than 5 feet of shale and argillaceous residuum.

Zinc-lead bearing strata include the cherty unit of the Galena 

dolomite, the Decorah formation, and the Quimbys Mill and 

McGregor members of the Platteville formation; the zinc-lead 

ore occurs in veins and breccia along inclined fracture zones 

("pitches and flats"), in more or less interconnected vugs 

("brangle"), and disseminated in the more shaly beds. Lead ore 

is found also in vertical joints ("crevices") especially at favorable 

horizons ("openings") in the upper or noncherty member of the 

Galena dolomite.

INTRODUCTION 

PURPOSE OF THE INVESTIGATION

In late 1942, when lead and zinc were in great demand 

because of the requirements of World War II, the U. S. 

Geological Survey began a project in Wisconsin, 

Illinois, and Iowa, hoping thereby to aid prospectors 

and mine operators in the discovery of additional lead 

and zinc ore.

Production from the mining district, which was considered by most geologists to be almost completely 

worked out, was being maintained in a limited way by 

several small mining operations; most of the ore was 

being concentrated in one relatively small custom mill, 

and two smaller mills shipped the rest of the concentrates from the district. None of the companies had 

geologists on their staffs, nor were consulting geologists 

working in the area. It seemed plausible that a detailed restudy of the geology of the district might result 

in a renewed interest in its ore-bearing potentialities, 

and thus might contribute materially to the supply of 

lead and zinc at that critical wartime period.

LOCATION AND GEOGRAPHIC SETTING

The zinc-lead district in Wisconsin, Illinois, and Iowa 

lies mainly within the Driftless Area (fig. 31), on generally southwestward-dipping lower Paleozoic strata. 

As a result of the absence of glacial drift and because of 

the mature dissection, there are many natural exposures; 

they are supplemented by quarries, by roadcuts, and 

by railroad cuts and tunnels. Because of the regional 

dip the outcrop pattern of the Platteville, Decorah, and 

Galena strata (Middle Ordovician) is mapped as a 

generally northwestward-trending band that is limited 

on the southwest by the apron of Maquoketa rocks 

(Upper Ordovician) flanking the cuesta formed by 

rocks of Silurian age and on the northeast by the 

exposure of pre-Platteville beds. Within this band of 

outcropping Platteville, Decorah, and Galena strata 

are a few "mounds"—knobs of Silurian rocks with their 

aprons of Maquoketa shale; in addition some upland 

areas contain relatively thin but rather extensive patches

of Maquoketa under the omnipresent mantle of loess 

and residual material. Pre-Platteville strata are exposed along parts of the valleys especially in the more 

deeply dissected areas in the district.

FIELD WORK

The geologic study of the zinc-lead district in Wisconsin, Illinois, and Iowa was begun by the U. S. 

Geological Survey in October 1942, and was still in 

progress in 1955. The initial investigation was an 

analysis of the structural control of the ore deposits in 

an area of many open mine workings and drill holes 

and some exposures and included a cooperative prospecting program with the U. S. Bureau of Mines. 

This study was soon extended and expanded to a general geologic mapping program of the mining district, 

including the areal geology, stratigraphy, structure, 

economic geology, and the larger geologic features, 

which was started in 1943 and has continued without 

interruption since that time.

It was soon evident that the rock units of Platteville, 

Decorah, and Galena ages, which contain the ores, 

should be restudied because the published stratigraphic 

work of the preceding 10 to 15 years did not agree with 

the rock units recognized underground by the miners and 

in drill cuttings by prospect drillers. Furthermore, 

the miners and drillers were not always consistent in the 

recognition of the rock units in a restricted area, nor 

was there adequate correlation of rock units from area 

to area within the mining district. The writers therefore made a detailed examination of the Platteville, 

Decorah, and Galena formations and subdivided them 

into relatively thin units so that the rather small orelocalizing structures could be mapped more accurately.

A. F. Agnew has been responsible for the direction of 

the stratigraphic study, the description of most of the 

sections, the regional stratigraphy, and the preparation 

of the present report. The other writers described 

stratigraphic sections and in addition contributed much 

helpful discussion. The writers jointly described 

stratigraphic sections and acquired additional stratigraphic1 information in the course of the mapping of 

geologic structure.

The writers' stratigraphic study included the examination of exposures, mine workings, and drill-hole 

cuttings and cores within the mining district, by hand 

lens and binocular microscope. Later, more information of a regional nature was desirable; as a result 

exposures and cuttings from holes drilled in nearby 

areas of Wisconsin, Illinois, Iowa, and Minnesota were 

investigated in 1944 and 1945, mainly by Agnew. The 

stratigraphic description presented herein was originally 

written in 1945 and is a result principally of the work 

from 1942 through 1945, although additional studies by
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 253

EXPLANATION

Younger rocks

ELKHORN CREEK

Outcrop of Platteville, Decorah

and Galena rocks

Outline of zinc-lead district 

Outline of driftless area Data in part from State geol'bgic maps, 

Willman and Templeton, \V*>\'/////////,.

250 Miles

FIGURE 31.—Upper Mississippi Valley region, showing Driftless Area, zinc-lead district, and bedrock distribution of Platteville, Decorah, and Galena strata.

Agnew since 1949, based in part upon his work dealing 

with the subsurface Ordovician rocks of Iowa in 1946 

and 1947 while engaged in ground water studies for the 

U. S. Geological Survey in cooperation with the Iowa 

Geological Survey, have modified considerably some 

of the earlier concepts and terminology. The report 

was completely rewritten in 1953-54, to incorporate a 

revised approach to the problem and to include pertinent information and published references acquired 

since 1945.

Comprehensive reports on the geology, structure, 

ore deposits, and mineral resources of the district have 

been prepared for readers interested in other phases 

of the study (Heyl, Lyons, Agnew and Behre, 1955; 

Heyl, Agnew, Lyons, Behre, in preparation).

The geologic study was instigated in 1942 by C. H. 

Behre, Jr., who had been supervising research in the 

mining district by students of Northwestern University 

since 1933. A. F. Agnew was in charge of the work 

from 1942 to the end of 1945, and again since July
254 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

1950; A. V. Heyl, Jr., was in charge from 1946 to 

July 1950, except for part of 1948-49 when E. J. 

Lyons was in charge. A. E. Flint was in charge of the 

work in Iowa in 1951-1953, and since that time, C. E. 

Brown. The members of the party consisted of A. F. 

Agnew from October 1942 to December 1945 and also 

during the summer of 1948 and since June 1949; A. V. 

Heyl Jr., from February 1943 to August 1950; E. J. 

Lyons as a part-time member of the party from February 1944 to June 1948, and a full-time member from 

June 1948 to October 1949; C. H. Behre, Jr., as a parttime member and project advisor from October 1942 

to October 1945; A. E. Flint, 1949-53; R. M. Hutchinson, 1944-45; Dorothy J. West, 1948; J. W. 

Allingham, 1950-55, C. E. Brown since 1951; Percy 

Crosby, 1953-54, J. E. Carlson, since 1951; W. A. 

Broughton 1950-55; L. G. Collins, 1953. Harriette 

A. Burris, R. W. Chartraw, J. F. Coulthard, R. P. 

Crumpton, D. C. Dixon, Catherine M. Fulkerth, H. L. 

Hefty, Maxine L. Heyl, M. A. Husted, J. H. Moor, 

D. W. Ressmeyer, H. F. Seeley, J. C. Spradling, C. W. 

Tandy, Jr., J. J. Theiler, Mary C. Wheeler, L. A. 

Ziech served as field assistants, and Marge B. Hake 

and Mary J. MacCulloch as scientific aids, during various stages of the program until 1954. A. E. Flint and 

R. P. Crumpton, in particular, added to the knowledge 

of the Platteville, Decorah, and Galena sequence during 

their mapping of the geologic structure,

ACKNOWLEDGMENTS

The study of the zinc-lead district in Wisconsin, 

Illinois, and Iowa has been done in cooperation with 

the Wisconsin Geological and Natural History Survey 

since 1945 and the Iowa Geological Survey since 1951. 

The Wisconsin Survey, directed by E. F. Bean and later 

by G. F. Hanson, contributed funds for the work in 

Wisconsin and a large amount of data including stratigraphic information and logs of deep wells, supplied by 

F. T. Thwaites. The Iowa Geological Survey gave 

financial support to the work in Iowa and permitted the 

use of subsurface data that had been used in groundwater studies by Agnew for the U. S. Geological Survey in 1946-47. A. C. Trowbridge and H. G. Hershey, 

State Geologists of Iowa, freely contributed information.

Our work was aided by the free interchange of data 

with the field party of the Illinois State Geological 

Survey during the summer of 1943 and from the spring 

of 1944 until 1954.

The U. S. Bureau of Mines maintained a party from 

December 1942 to 1954, except for a few months in 

1946, to explore deposits in the district. The cores 

and churn-drill samples were logged by U. S. Geological Survey geologists, and in most instances the explorations were planned and interpreted in close cooperation

by members of both organizations. We wish to 

acknowledge their invaluable aid in carrying out our 

studies.

Acknowledgment for their valuable aid and contributions is made to Paul Herbert, Jr., and R. R. Reynolds, 

formerly of the Illinois State Geological Survey; H. B. 

Willman, of the Illinois State Geological Survey; W. C. 

Bell, formerly of the University of Minnesota; J. R. 

Ball, formerly of Northwestern University; representatives of mining companies, especially the Vinegar Hill 

Zinc Co., The New Jersey Zinc Co., and the American 

Zinc, Lead, and Smelting Co.; to C. W. Stoops, an 

ore buyer, mine operator, and geologist at PlatteviUe; 

and drillers of water wells and prospect holes and the 

many mine operators who supplied useful data. Particular thanks is due to M. A. Melcher, president of 

the Wisconsin Institute of Technology. The school 

furnished office and laboratory space for many years 

and gave us much valuable data and use of its facilities. 

We wish to thank the Geology Departments of the 

University of Wisconsin and Northwestern University 

for use of binocular microscopes and theses.

REGIONAL GEOLOGY AND STRATIGRAPHIC RELATIONS 

STRUCTURE

The zinc-lead district in Wisconsin, Illinois, and 

Iowa lies on the southwestern slope of the Wisconsin 

arch (fig. 31).

The regional clip of the rocks is to the south-southwest 

but it has been modified by folds which are directed 

easterly, northwesterly, and northeasterly. Joints and 

generally high angle faults are associated with the folds, 

and in the mining district bear zinc-lead ore.

REGIONAL STRATIGRAPHIC SUMMARY

Because this report deals primarily with rocks related to the major zinc-lead ore deposits—the Platteville, Decorah, and Galena strata (Middle Ordovician)— 

pre-Platteville and post-Galena rocks are discussed 

only briefly.

Pre-Cambrian rocks underlie the mining district, as 

granite gneiss and other granitic rocks have been 

penetrated in wells at several localities. Cambrian 

strata consist of sandstone, siltstone, and dolomite, of 

Late Cambrian age (fig. 32). The Mount Simon 

sandstone, which unconformably overlies the preCambrian, is succeeded conformably by sandstone and 

siltstone of the Eau Claire sandstone, and that by the 

Dresbach sandstone; the latter is overlain conformably 

by the glauconitic sandstone and siltstone of the 

Franconia sandstone, which is succeeded conformably 

by sandstone, siltstone, and dolomite of the Trempealeau 

formation. There are many facies changes. The
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 255

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Group or 

formation

Maquoketa shale

Galena dolomite

Decora h formation

Platteville formation

St. Peter sandstone

Prairie du Chien group 

(undifferentiated)

Trempealeau 

formation

Franconia sandstone

Dresbach sandstone

Eau Claire sandstone

Mount Simon 

sandstone

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Description

Dolomite, buff, cherty; Pentamerus at top,

Dolomite, buff, cherty; argillaceous near base

Shale, blue, dolomitic; phosphatic depauperate 

fossils at base

Dolomite, yellowish-buff, thin-bedded, shaly

Dolomite, yellowish-buff, thick-bedded; Receptaculites 

in middle

Dolomite, drab to buff; cherty; Receptaculites 

near base

/ Dolomite, limestone, and shale, green and brown; 

phosphatic nodules and bentonite near base

Limestone and dolomite, brown and grayish; green, 

sandy shale and phosphatic nodules at base

Sandstone, quartz, coarse, rounded

Dolomite, light-buff, cherty; sandy near base and 

in upper part; shaly in upper part

Sandstone, siltstone, and dolomite

Sandstone and siltstone, glauconitic

Sandstone

Siltstone and sandstone

Sandstone

Average 

thickness, 

in feet

90

110

200

108-240

40

80

105

35-

225

40

55-75

40+

0- 

240

280- 

320

120-150

110-140

GO140

70- 

330

440- 

780

700- 

1050

FIGURE 32.—Generalized stratigraphic column for zinc-lead district.

389000—5(
256 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

thickness of the Cambrian rocks increases southward 

across the district from 960 to 1,310 feet.

Rocks of the Prairie du Chien group (Lower Ordovician) in many places are dolomite, with a sandstone 

formation or sandy zone near the middle; in other 

localities, however, this group is represented by a 

large amount of red and green shale, silicified limestone, 

and limestone. The overlying St. Peter sandstone 

(Middle Ordovician) is normally 40 feet thick, but 

unconformable relations between it and the Prairie du 

Chien below, coupled with leaching of the dolomite 

in the latter, cause sandstone of this type to be present 

to a depth of at least 320 feet below the top of the St. 

Peter. Because of this the Prairie du Chien reaches 

a maximum thickness of 250 feet; together the two 

units aggregate 280-320 feet.

The outcropping band of the southwesterly dipping 

Platteville, Decorah, and Galena strata (Middle Ordovician), which contain the zinc-lead deposits, strikes 

generally northwesterly across the district; facies 

changes are evident both toward the northwest and 

toward the southeast. These changes are marked by 

differences in the lithologic character of the equivalent 

rocks, and by differences in thickness.

As viewed within the confines of the mining district, 

especially the eastern part, the beds designated Platteville, Decorah, and Galena by earlier writers fall 

naturally, because of general lithologic similarity, into 

three groups, as follows:

Grouping of Platteville, Decorah, and Galena rocks in district 

[Local terminology in parentheses]

Group 1

Group 2

Group 3

Noncherty unit (buff) 

Cherty unit (drab) 

Ion dolomite member (gray and blue)

Guttenberg limestone member (oil rock) 

Spechts Ferry shale member (clay bed) 

Disconformitv

Quimbys Mill member ("glass rock")

McGregor limestone member (Trenton) 

Pecatonica dolomite member (quarry beds) 

Glenwood shale member (shale)

Because the stratigraphic guides for the practical 

miner are the different lithologic characteristics, such 

a classification might be the most practical locally, 

insofar as the eastern, or main part of the mining 

district is concerned.

Group 2 is a lithologic entity within this part of the 

mining district, for the rocks composing all three of 

its subdivisions were deposited under generally similar 

conditions and they also reacted similarly under the 

stress of later geologic solutional and structural processes. Furthermore, in the mining district the basal 

beds of Group 2 are distinct lithologically from the

uppermost beds of Group 3, and the upper beds of 

Group 2 are equally distinct lithologically from the 

lowest beds of Group 1.

When considered regionally, on the other hand, to 

the east the dolomite of the Quimbys Mill member is 

lithologically more similar to that of the McGregor 

member below than it is to the Spechts Ferry above. 

Moreover, the Quimbys Mill pinches out to the west 

as does the Pecatonica, whereas the Spechts Ferry 

pinches out and the Guttenberg thins to the east. 

Furthermore, in its western facies the shale character 

of the Ion member is more similar lithologically to that 

of the Guttenberg and Spechts Ferry below than it is 

to the limestone of the Galena above.

In addition to the facies and convergence factors 

mentioned above, a disconformity marked by a corrosion surface and associated closely with a bentonite 

layer lies within, rather than at the top or bottom of 

Group 2. Thus the available regional evidence suggests a different grouping of the beds, as follows 

(fig. 33):

Regional grouping of Platteville, Decorah, and Galena rocks

Galena dolomite

Decorah formation

Platteville formation

Noncherty unit 

Cherty unit

Ion dolomite member 

Guttenberg limestone member 

Spechts Ferry shale member

Quimbys Mill member 

McGregor limestone member 

Pecatonica dolomite member 

Glenwood shale member

FIGURE 33.—Platteville (Op), Decorah (Od), and Galena (Og) strata in 200 

foot bluff west of State Route 81, in Grant County, Wis. (fig. 35, loc. 9).
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 257

Similarly, 300 miles away in Missouri, Groshskopf 

(1948, p. 354) has shown that the Decorah as used in 

eastern Missouri is coincident in extent with the overlying Kimmswick and overlaps various parts of the 

Plattin; this suggested to him that the Decorah is 

related more closely to the Kimmswick than to the 

underlying Plattin and that a hiatus exists between the 

Decorah and the Plattin.

For these reasons the classification given on the 

preceding page is the one used in the upper Mississippi 

Valley region, which includes the zinc-lead mining 

district.

Except for the Glenwood shale member the Platteville formation is limestone and (or) dolomite, with

thin shale partings. The dolomite facies as mapped 

generally includes part of the mining district and 

extends toward the east, whereas the limestone facies 

extends toward the west (fig. 34).

Significant regional changes in the stratigraphic character of the Platteville formation are not present in the 

Glenwood shale (basal) member. The overlying Pecatonica dolomite member thins northwesterly from 20-24 

feet in the mining district, to 10 feet in northeastern 

Iowa, and less than 2 feet in southeastern Minnesota; 

it maintains a thickness of 26 feet in the vicinity of 

Beloit (fig. 35) and Janesville, Wis., along and east 

of the Wisconsin arch, although Bays and Raasch (1935, 

p. 297) reported only 18 feet of Pecatonica dolomite 

member over the Wisconsin arch.

< WEST

uu Guttenberg, Iowa

CENTRAL 

Platteville, Wis.-Galena,

EAST

Monroe, Wis.

200'-

150'-

100'H

50-

50-

100-

Noncherty unit ~ ~~-~^z-C--^~~ ~ ~-^~ ~ ~ _ — __ _- -^~^~r^ ~\

A ~ — AA .A A A A T-i—— A A — — 

AAAA AAA A A A A A, A A AAAAAAAAA A A AAAAAAAAA AA AA AA 

AA AAAAAAA AAAAAAAAAAA A AAAAAAAAA A AAAA A 

A A AAAAAA A AAAAAAAAAA AAA A A A A A ^ A A A A A A A 

A A AAAAAAA AAA AA A AA A AA.A AA AAAAAAA

X X ^ .A X A AA X X X A A A A X ^ A XXX AA_X AAAAA A A A

AAAAAAAAAAAA.A A A A A A A A A A A. A A 

* „ * n /\ /VAAAAAAAAAAAAAAAAAA

Pecatonica dolomite member

Sandstone

75 Miles

EXPLANATION

Bentonite Phosphatic pebbles Chert Receptaculites 

FIGURE 34.—Diagrammatic cross section of Platteville, Decorah, and Galena strata eastward across the mining district, showing facies relationships.
258 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

, HOUS TON

MNNESOTA 

IO|WA~

14 ALLAMAK 

Decorah

FIG TEE 35.—Map of part of the upper Mississippi Valley region, showing outline of zinc-lead district. Localities cited in the text are listed below.

1 Borden Whey Plant well 2, sec. 33, T. 8 N., R. 3 W., Boscobel, Wis.

2. Roadcut, sec. 22, T. 8 N., R. 3 W., junction of State Routes 60 and 61, Wisconsin.

3. Quarry, sec. 31, T. 6 N., R. 6 W., and sec. 36, T. 6 N., R. 7 W., north edge of 

Wyalusing, Wis.

4. Crow Branch diggings, sec. 22, T. 5 N., R. 1 W., Wisconsin.

5. Roadcut, sec. 15, T. 2 N., R. 2 W., U. S. Highway 151, Wisconsin.

6. Roadcut, sees. 1 and 12, T. 2 N., R. 2 W., U. S. Highway 151, Wisconsin.

7. USGS-Raisbeck hole 2, sec. 21, T. 2 N., R. 1 E., Wisconsin.

8. City well, sec. 18, T. 86 N., R. 5 E., Bellevue, Iowa.

9. Bluff, sec. 36, T. 4 N., R. 5 W., near State Route 81, Wisconsin.

10. Quarry, sec. 4, T. 90 N., R. 2 E., Spechts Ferry station, Iowa.

11. Eagle Point Quarry, sec. 7, T. 89 N., R. 3 E., Dubuque, Iowa.

12. Roadcut, sec. 7, T. 2 N., R. 2 W., U. S. Highway 61, Wisconsin.

13. Quarry, sec. 3, T. 2 N., R. 3 E., Darlington, Wis.

14. Type section of Qlenwood, sec. 6, T. 98 N., R. 7 W., Iowa.

15. Roadcut, sec. 18, T. 5 N., R. 2 W., U. S. Highway 61, Wisconsin.

16. Clayton County Home well, sec. 7, T. 93 N., R. 4 W., Iowa.

17. Cheese factory well, sec. 6, T. 4 N., R. 5 W., North Andover, Wis.

18. Village well, sec. 5, T. 4 N., R. 1 E., Rewey, Wis.

19. School well, sec. 26, T. 6 N., R. 1 E., Cobb, Wis.

20. City well, sec. 12, T. 6 N., R. 6 E., Mount Horeb, Wis.

21. City well 1, sec. 4, T. 90 N., R. 3 W., Colesburg, Iowa.

22. City well, sec. 29, T. 29 N., R. 2 W., East Dubuque, 111.

23. Roadcut, near cent?r NJi sec. 10, T. 4 N., R. 2 W., County Road A, Wisconsin.

24. Roadcut, near center of Sl-2 sec. 35, T. 5 N., R. 1 E., Wisconsin.

25. Type section of McQregor, NEM sec. 28, T. 95 N., R. 3 W., Iowa.

26. Type sections (of Bays and Roasch 1935) of Magnolia, NWM sec. 26, T. 3 N., 

R. 10 E., Wisconsin.

27. Type section of Quimbys Mill, SE corner, sec. 11, T. 1 N., R. 1 E., Wisconsin.

28. Roadcut, center E}4 sec. 1, T. 4 N., R. 3 W., County Road A, Wisconsin.

29. Quarry, near center sec. 23, T. 4 N., R. 5 E., County Road F, Wisconsin.

30. Bautsch mine, sec. 10, T. 27 N., R. 1 E., Illinois.

31. Quarry, NWJ4 sec. 30, T. 3 N., R. 5 E., Wisconsin.

32. Ravine, center of east line sec. 4, T. 1 N., R. 1 E., Wisconsin.

33. Type section of Quttenberg, SWJ4 sec. 5, T. 92 N., R. 2 W., Iowa.

34.-East bank of Galena River, near center sec. 34, T. 29 N., R. 1 E., Illinois.

35. Ravine, NEJ4 sec. 32, T. 97 N., R. 5 W., State Route 51, Iowa.

36. Type section of Ion, NWJ4 sec. 35, T. 96 N., R. 4 W., Iowa.

37. Roadcut, NWJ4 sec. 33, T. 6 N., R. 5 W., U. S. Highway 18, Wisconsin.

38. Quarry, SWJ4 sec. 24, T. 5 N., R. 5 W., Wisconsin.

39. Quarry, NEJ4 sec. 32, T. 6 N., R. 4 W., Wisconsin.

40. Roadcut, center sec. 21, T. 6 N., R. 3 W., U. S. Highway 18, Wisconsin.

41. Roadcut, SWJ4 sec. 31, T. 5 N., R. 3 W., County Road A, Wisconsin.

42. Quarry, SWJ4 sec. 9, T. 3 N., R. 3 E., Calamlne, Wis.

43. Quarry, SEJ4 sec. 19, T. 6 N., R. 1 E., Montfort, Wis.

44. Quarry, sec. 5, T. 4 N., R. 6 E., York Church, Wisconsin.

45. Quarry, along Honey Creek, SWJ4 sec. 23, T. 1 N., R. 6 E., Wisconsin.

46. Qinte mine, sec. 30, T. 29 N., R. 1 E., Illinois.

47. Quarry, SEJ£ sec. 18, T. 3 N., R. 3 W., County Road U, Wisconsin.

48. Roadcut, NWJ4 sec. 34, T. 1 N., R. 2 W., State Route 11, Wisconsin.

49. Quarry, SEJ4 sec. 3, T. 6 N., R. 5 E., U. S. Highway 151, Wisconsin.

50. Old railroad tunnel, SEJ4 sec. 21, T. 1 N., R. 1 E., Wisconsin.

51. T. T. Redfern well, SEJ4 sec. 34, T. 29 N., R. 2 E., Illinois.

52. Quarry, NWJ4 sec. 5, T. 3 N., R. 1 E., Platte Mound, Wis.
STRATIGRAPHY OP MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 259

In southeastern Minnesota, where the Pecatonica is 

less than 2 feet thick, it consists of very sandy limestone 

above calcareous sandstone that contains phosphatic 

nodules. This phase is exposed in the long readout 

along U. S. Highway 16 in sec. 35, T. 103 N., R. 10 W., 

just south of Lanesboro, Minn. (about 40 miles north 

of Decorah, Iowa; fig. 35).

The McGregor member, a limestone in the western 

and central parts of the district, changes laterally, 

becoming progressively more dolomitic eastward near 

the crest of the Wisconsin arch, where it is dolomite 

(fig. 34). This dolomitization affects the upper beds 

of the McGregor farther west than it does the lower 

strata. Chert is present in the McGregor from Shullsburg, Wis., eastward.

The Quimbys Mill (upper) member of the Platteville 

formation is limestone or limestone and dolomite in the 

central part of the mining district, and is cherty dolomite to the east (fig. 34). The unit wedges out to the 

west, as is shown by its thickness of only 1 foot in the 

roadcut 10 miles southwest of Platteville, Wis., (fig. 35, 

loc. 12), and of only a few inches in a roadcut 10 miles 

northwest of Platteville, in sec. 1, T. 4 N., R. 3 W. 

(loc. 28). The Quimbys Mill averages 14 feet thick in 

localities several miles east of the type section (loc. 27) 

and at Mount Horeb (loc. 20), and is more than 18 feet 

thick in an area southeast of Shullsburg.

All members of the Platteville formation are conformable except the Quimbys Mill. The Quimbys Mill 

strata do lie conformably on the McGregor, and a dark 

shale bed commonly marks the contact. However, 

the Quimbys Mill member and the overlying Decorah 

strata, whether the Spechts Ferry shale member or 

the unnamed limestone member (p. 264), regionally 

show disconformable relationships.

In the central part of the mining district the upper 

surface of the Quimbys Mill is corroded and pitted by 

solution; clots of greenish shale and of sandy limestone 

similar to that in the overlying Spechts Ferry fill these 

pits, giving the appearance of involutions in vertical 

cross section. East of the mining district, where the 

Spechts Ferry is absent, the pits are filled with sandy 

dolomite and phosphatic pebbles of the Guttenberg 

that farther west mark the upper part of the Spechts 

Ferry. Similar features were noted by Chamberlin 

(1882, fig. 8), Sardeson (1898, p. 322), and Ulrich 

(1924, fig. on p. 96). This corrosion surface marks the 

upper limit of the Quimbys Mill member.

The Decorah formation at its type locality, 80 miles 

northwest of the mining district, is 25-30 feet of greenish calcareous shale with nodular limestone, overlying 

a 2-foot basal limestone unit that wedges out to the east. 

In the western part of the mining district the Decorah 

is divisible into the following three members: at the

base is the 8-foot thick greenish shale with limestone 

nodules of the Spechts Ferry, next above is the brown 

limestone and some shale of the Guttenberg 15% feet 

thick, and at the top is the grayish limestone and shale 

of the Ion, 19 feet thick (fig. 34). In the eastern part 

of the mining district the Spechts Ferry has wedged out, 

the brown dolomite of the Guttenberg has thinned to 

8 feet thick, and the overlying grayish buff dolomite 

of the Ion is only 15 feet thick and difficult to distinguish from the lower zone of the overlying Galena 

dolomite.

The Decorah rests disconformably on the Platteville; 

it grades upward into the Galena (Agnew, 1950), contrary to previous statements (Kay, 1932; Kay and 

Atwater, 1935) that an unconformity exists at the 

latter contact.

The Galena dolomite is rather uniform in thickness 

across the mining district—averaging 225 feet—and is 

generally a dolomite. The lower part, however, is 

limestone in the western part of the district and to the 

west, and also locally in the central part of the district; 

The formation is divisible regionally into two subdivisions of about equal thickness, a cherty unit below 

and a noncherty one above. The cherty unit includes 

the lower two-thirds of the Prosser member of established stratigraphic usage, whereas the noncherty unit 

consists of the upper third of the Prosser, the Stewartville, and the Dubuque members. As the Prosser, 

Stewartville, and Dubuque members are not everywhere recognizable in the regional area embraced in 

this study, and because the noncherty and cherty units 

are distinctive throughout its extent, the twofold subdivision of the Galena based on the presence or absence 

of chert is employed in the present report (p. 265).

The Maquoketa shale (Upper Ordovician) is generally grayish dolomitic shale and dolomite in the mining 

district, although its lower one-third is commonly 

brownish in color. The Maquoketa is as thin as 110 

feet in the southern part of the mining district, and 

thickens toward the northeast to 240 feet in a locality 

25 miles west of Madison, Wis. (fig. 31); it likewise 

thickens to the northwest, 220-260 feet being common 

in the area northwest of Dubuque, Iowa. The Maquoketa is characterized by facies changes, especially west 

of the mining district. It appears to lie conformably 

on the Galena, although regionally the contact seems 

to be disconformable.

Dolomite of Early Silurian age is present up to 

200 feet thick in the mining district. The lower 20 

feet or so is argillaceous dolomite; this is succeeded 

by 65 feet of cherty strata, 20 feet of noncherty dolomite, and the upper 90 feet is somewhat cherty and 

contains Pentamerus. Dolomite of the Silurian gener-
260 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

ally lies unconformably on the Maquoketa, although 

locally the contact appears to be gradational.

Post-Silurian deposits found locally in the mining 

district include boulders of quartz sandstone, boulders 

of hematite, boulders of quartzite and greenstone, and 

poorly cemented conglomerate. In addition, in the 

southern, eastern, and western fringes of the district, 

glacial drift occurs generally in local patches (fig. 31). 

Pleistocene terrace deposits and loess are common in 

the mining district.

STRATIGRAPHIC PRINCIPLES

Because the geologic investigation that gave rise to 

the present report had as its major purpose the delineation of geologic structure, the rocks were studied in 

terms of mappable units—formations and members. 

Initially the investigation was concerned with relatively local areas (maximum width 10 miles), but more 

regional relationships were scrutinized as areas 15 or 

20 miles or more distant from the initial one were 

mapped. In local areas 5 to 10 miles wide, differences 

in lithofacies were observed, and the recognition of 

facies relationships was found to be even more important in the regional correlation of the strata than in 

local correlation..

Despite these lateral changes in lithologic features, 

from the standpoint of the structural mapping it was 

desirable to picture the rocks as correlative units, provided that upper and lower limits of the mapped unit 

were recognizable. Thus a liberal interpretation of 

the facies concept was applied.

Regionally and even locally all lithologic criteria 

were evaluated and a top or base was assigned to a 

particular unit, although the gross lithology of that 

unit in places may have been atypical, as for example 

the regional lithology of the Guttenberg limestone 

member, and locally the Ion member of the DecorahGalena contact. As another example, the Decorah 

formation at its type locality is indivisibly green 

shale with limestone nodules, overlain by limestone of 

the Galena, and underlain by limestone of the Platteville. Thirty-five miles closer to Platteville, at the 

type locality of the Ion the upper (Ion) member of 

the Decorah is still green shale and limestone in almost 

equivalent amounts; it is overlain by buff limestone 

of the Galena dolomite and underlain by tan limestone 

of the Guttenberg (middle member of the Decorah). 

At Platteville the Ion is light-brown dolomite with 

greenish argillaceous mottliiigs and partings; it is 

overlain by light-brown dolomite of the Galena and 

is underlain by light-brown Guttenberg limestone. In 

gross lithology the Ion at Platteville is more like the 

overlying than it is like the underlying strata, although 

at Decorah, Iowa, the reverse is perhaps true. The

recognition of one persistent and other less-persistent 

bentonite beds aided in the regional tracing of the 

strata across facies changes.

A "formation" in this study is a locally mappable 

unit consisting of rocks mainly of one dominant lithologic type, with boundaries based upon objective criteria. It is extended regionally, despite facies changes, 

from local areas on the basis of all the lithologic criteria (including fossils) available; and its boundaries, 

although in this extension somewhat subjective, are 

drawn at the same stratigraphic positions relative to 

units above and below, as where the formation was 

initially mapped. Stratigraphic purists may object, 

for example, to calling the Ion a part of the Decorah 

formation at Platteville, Wis., where actually it is 

more similar in lithology to the overlying Galena dolomite. However, the philosophy presented herein permits the use of workable stratigraphic units locally, 

permits the correlation of these units regionally, and 

permits the stratigraphic nomenclature to be held at 

a minimum.

Geologic time rock classification is another matter, 

entirely (see the discussion of this problem as related 

to Cambrian strata of the upper Mississippi Valley by 

Bell, Feniak, and Kurtz, 1952, p. 175-176).

Fossils in the Platteville, Decorah, and Galena 

strata of the upper Mississippi Valley have not been 

adequately studied, nor have most of the many fossil 

collections been gathered with adequate attention to 

principles of stratigraphic geology. As a result the 

faunal characteristics of the strata are as yet not 

completely known.

No systematic paleontologic study of the strata has 

been attempted during the course of the present project. 

However, fossils have been observed as rock characters, 

and it is believed that because the faunas are related 

closely to environmental conditions, and because they 

recur in succeeding strata of similar lithologic facies, 

they are generally facies faunas. Thus only general 

assignments to the larger time-stratigraphic subdivisions (systems and series) is made herein.

Furthermore, dolomitization can destroy evidence of 

fossils originally entrapped in the limestone (Bucher, 

1953, p. 292); therefore, relatively unfossiliferous dolomitic rocks such as the Pecatonica may originally have 

been characterized by a much greater abundance and 

diversity of forms.

Most of the fossils observed in the Galena, Decorah, 

and Platteville beds seem to be facies faunas (as should 

be expected, but commonly is not). As Allan so concisely stated (1948, p. 7),

the practice of matching faunas—usually faunas determined in 

a museum by one person but collected by someone else—is quite 

illogical; and the widely accepted views that dissimilarity of
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 261

faunal content indicates difference of age, and that similarity of 

faunal content establishes chronological equivalence, are extremely unsafe generalizations.

Allan's views were forecast by Sardeson (1907, p. 

190), who stated that "the identity of fossils does not 

prove strictly contemporaneous deposits, but only 

related faunas." (See also Williams, 1952; Rich, 1951, 

p. 17.) Regionally these faunas transgress lithologic 

units, they recur with ecologically recurring rockdepositing environments, or they are absent where 

their particular environment was absent. For example, 

Bell (1950, p. 493) ventures the

suspicion that the widespread Eden-Maysville "hiatus," the 

physical evidence for which is so infrequently demonstrated, 

records the absence of a biotic environment—not the absence of 

a depositional record.

The writers agree heartily with Bell, for there is no 

physical evidence that a "hiatus" exists between the 

Stewartville and Dubuque members of the Galena; yet 

because the fauna of the limestone and dolomite of the 

Stewartville is Trenton in aspect, and the fauna of the 

shale and limestone of the Dubuque is Richmond in 

appearance, a hiatus representing Eden and Maysville 

"time" is commonly postulated between the Stewartville and Dubuque members (see also Lattman, 1954, 

p. 268; McFarlan and White, 1948, p. 1642). Because 

the object of this study was the mapping of geologic 

structure, therefore, and because of the reasons stated 

above, the fossils have not yet been studied in detail 

and only general assignments to the geologic time scale 

are made.

The faunal relationships of these strata remain to be 

worked out; this constitutes an extremely interesting 

problem whose solution will contribute materially to 

our knowledge of stratigraphy, paleontology, ecology, 

and general geologic history of the Middle Ordovician 

of the upper Mississippi Valley.

STBATIGBAPHIC PROBLEMS 

PLATTEVILLE-DECORAH BOUNDARY

Bain's (1905, p. 19) original description of the beds 

of Platteville age at the type locality recorded an 

average thickness of 60 feet, consisting of:

Feet 

4. Thin beds of limestone and shale___________________ 10-20

3. Thin-bedded brittle limestone, breaking with a conchoidal fracture-_______________________________ 25-30

2. Buff to blue magnesian limestone, heavy bedded,

(frequently a dolomite) _____^_^_________^_______ 20-25

1. Shale, blue__________--__-_---__^______________-. 1-5

According to Bain, unit 4 consists of limestone and 

shale commonly in thin alternating beds, but

there is comparatively little uniformity in this member in different parts of the district * * * the shale layers are better developed toward the west, while the limestone is the main part 

or the whole of the member in the eastern portion of the lead 

and zinc district. The shale beds are usually green or blue in 

color, though some of the beds are at times yellow, chocolate 

colored, or even black. The green shales are especially developed 

in Iowa. The chocolate-colored and black shales are highly 

carbonaceous, and are locally termed "oil rock," though the main 

bed of chocolate-colored shale, or the "oil rock" of the lead and 

zinc district, lies just at the base of the Galena. The limestone 

is commonly a thin-bedded, fine-grained, blue rock, rich in 

fossils. At times it is subcrystalline, and, while usually nonmagnesian, becomes markedly magnesian toward the east, where 

it seems to be the equivalent of the "upper buff" of the Wisconsin 

section. The most marked beds of No. 4 in the type locality of 

the Platteville occur near the base of this member, and are 

termed "glass rock." These are the typical glass-rock beds of 

the lead and zinc district.

These "green or blue" shales and the "thin-bedded, 

fine-grained, blue rock [limestone] rich in fossils" 

belong in the Spechts Ferry, which is here referred to 

the Decorah formation, as is discussed later (p. 286). 

The "chololate-colored and black shales" and the 

subcrystalline limestone that becomes dolomitic toward 

the east belong in the Quimbys Mill member of the 

Platteville formation. It is therefore evident that the 

grouping of the two members together as facies of the 

same unit led to Bain's impression that unit 4 of his 

description lacked uniformity.

Bain (1905, p. 21) described the Galena as follows:

Generalized section of the Galena limestone, Illinois

Feet

5. Dolomite, earthy, thin-bedded.____________________ 30

4. Dolomite, coarsely crystalline, massive to thick

bedded____________________________________ 60

3. Dolomite, thick to thin bedded, coarsely crystalline,

chert bearing_-____-___-___-_----_-_-__--______ 90

2. Dolomite, thick bedded, coarsely crystalline.________ 60

1. Thin bedded limestone with shaly partings which are 

highly fossiliferous and, in part, at least, carbonaceous—the 'oil rock' of the miners._______________ 2

The basal member of the Galena, No. 1 of the above section, is 

well known throughout the zinc district. It receives its name 

from the large amount of carbonaceous material which it contains, often sufficient to cause it to burn when lighted with a 

match. In the mining district it is everywhere recognized as 

the oil rock; and as there are usually several bands of shale 

interbedded with thin-bedded brittle limestone, the most 

important band is there discriminated as the main oil rock. 

Away from the mining district the horizon is occupied by a soft 

green clay. Usually the shaly element is most important at 

the top, and ordinarily—in southwestern Wisconsin, at least— 

a particularly bituminous parting is recognized as belonging to 

the oil rock.

It is apparent that Bain considered the carbonaceous 

oil rock and the "soft green clay" as different facies, 

thus implying that the "glass rock," "clay bed," and 

"oil rock" are all facies of the same stratigraphic unit. 

They belong to three separate members (Quimbys 

Mill, Spechts Ferry, Guttenberg) of two different
262 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

formations (Platteville, Decorah), as is demonstrated 

in the discussions of those units in succeeding pages.

In a later publication Bain (1906, p. 22, 23) placed 

all of the "bluish or greenish" shale at Spechts Ferry, 

Dubuque County, Iowa (fig. 35, loc. 10) in the Platteville formation, and stated that in a quarry about 2 

miles northwest of Platteville, Wis., "the equivalents 

of the main oil-rock horizon which marks the base of 

the Galena" were present, and that the unit immediately 

below "represents the clay bed usually found beneath 

it." It is not clear from the above statement whether 

he meant the clay bed to be assigned to the Galena or 

to the Platteville, but presumably the latter.

Two pages later, however, he (1906, p. 25) wrote as 

follows:

General section of basal Galena beds

Ft. In.

4. Thin-bedded magnesian limestone, variable in 

thickness, which depends upon the extent of 

dolomitization___ __________________________ 0-15 15

3. Thin-bedded limestone or dolomite with partings

of oil rock_________________________________ 5-8

2. Brown, shaly material, with minor lenses of limestone ; the main oil-rock horizon.______________ %-2

1. Shale or blue clay containing black phosphatic

pebbles.__________________________________ >4-3

In places dolomitization extends down to the top of No. 3 of the 

above section; in other places No. 1 of the section cannot be 

distinguished from the shaly limestone beds at the top of the 

Platteville, in which black and brown shale, indistinguishable 

from the oil rock of the Galena, occurs.

Bain"s earlier concept that the green shales and 

brown carbonaceous shales are merely different fades 

of the same beds is thus again evident. His described 

section (1906, p. 28) at Eagle Point quarry, Iowa, 

shows 3 feet of green argillaceous fossiliferous shales 

as the basal unit of the Galena.

Ellis (1905, p. 313), who worked closely with Bain 

on the Wisconsin and Iowa parts of the study, wrote 

as follows:

The base of the Galena is marked by a constant shale bed varying from 1 to 3 feet in thickness. This shale consists of blue 

clay and of brown carbonaceous material, which gives it the 

name of "oil rock" * * * [The underlying "glass rock," 

which comprises the upper beds of the Platteville] carries 

varying amounts of magnesium carbonate, although not 

approaching a dolomite.

Cox (1911, p. 431) likewise interpreted this relationship as one of facies:

The base of the main body of oil rock occurring from 8 to 12 

feet above the glass rock is taken as the division between the 

Platteville limestone below and the Galena dolomite above. 

* * * Interbedded with, or just below, there is often a greenish 

clay bed a foot or so in thickness.

It is uncertain whether he meant the "greenish clay 

bed" to be basal Galena or uppermost Platteville.

Grant (1903, p. 35-36; 1906, p. 33) had likewise 

noted the stratigraphic recurrence of the oil rock, with 

the bluish or greenish shale between. However, he 

termed the oil rock above the blue shale the "main 

oil rock." He found that this bed of oil rock in every 

case overlies the clay bed, which in turn overlies the 

"glass rock." He signified the "main oil rock" as the 

basal unit of the Galena, and the clay bed as the uppermost unit of the underlying Platteville. Later he was 

not consistent, however, for he and Burchard (1907, 

p. 6) placed within the Galena the green shale beds 

occurring below the oil rock in the Eagle Point quarry 

at Dubuque, Dubuque County, Iowa (fig. 35, loc. 11), 

although in other sections (1907, p. 5) the blue and green 

shale was consistently put in the Platteville.

Although Davis (1908, p. 548) had noted that the 

clay bed and the oil rock are separate and superimposed 

units and not facies, his lead was not followed until 

Trowbridge and Shaw (1916, p. 39) and Boericke and 

Garnett (1919, p. 1218-1219) likewise distinguished 

them.

The Platteville-Decorah boundary was shown recently (Agnew, Heyl, 1946) to be one of regional disconformity. The upper member of the Platteville 

formation pinches out to the west, whereas the members 

of the Decorah formation thin and are absent to the 

east. Furthermore, locally the upper surface of the 

Quimbys Mill is pitted and corroded, and contains 

involutions of sedimentary rocks of the overlying unit. 

The presence of a bentonite layer near the base of the 

Spechts Ferry is evidence that these relationships are 

not facies, because the bentonite layer remains just 

above the contact of the limestone and shale as the 

beds are traced eastward beyond the area of Spechts 

Ferry strata (fig. 36).

BELOIT DOLOMITE

In the eastern part of the mining district and farther 

east the strata of Platteville, Decorah, and Galena age 

are dolomite, and the gross lithology of the three formations is somewhat similar. A marked difference, 

however, is the fact that the Galena maintains its 

characteristic honeycomb (wormy) weathered appearance, which the lower two formations do not share. 

This eastern area is the type for Sardeson's Beloit 

formation, which name he (1896b, p. 360) suggested 

to replace " 'Trenton' or Lower Trenton," as the 

overlying Galena strata are more probably the "equivalent to the Trenton limestone of New York." The 

Beloit was made up of the four divisions: "lower buff 

beds," "lower blue beds," "upper buff beds," and 

"upper blue beds" (Chamberlin, 1877, p. 293, 295, 296;.

The Beloit was distinguished first on lithologic 

criteria, having "thinner, more compact strata" in
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 263

WESTERN PART 

South-central Grant County

CENTRAL PART 

Western Lafayette County

EASTERN PART 

Eastern Lafayette County

10- <a

c

_0>

ra 

C3

0

Formation Member

Limestone, buff; chert

Limestone, buff

A/A Dolomite, buff; chert

Dolomite, buff

Dolomite, buff; chert

Dolomite, buff

3.0-

20'-

30'-

40'-

50'-1

P/P

Dolomite, grayish-buff; trace 

of olive shale

Limestone and shale, 

greenish-gray

Limestone, gray, dolomitic; 

greenish-gray shale

Shale, green; limestone lenses

__————^

Dolomite and shale, brown

Limestone, gray

Shale and limestone, olive

^

Dolomite, brown; trace of brown shale

Limestone and shale, brown

^-^

Limestone, grayish-buff

Dolomite, buff;trace 

of olive shale

Dolomite, light-brown; 

trace of brown shale

Dolomite, buff; trace 

of brown shale

Dolomite, buff

EXPLANATION

Shale

Bentonite 

FIGURE 36.—Generalized west-east cross section of the Decorah formation and adjacent strata.

contrast with the overlying Galena's "thick porous 

ones" (Sardeson, 1897a, p. 23). Sarcleson felt that a 

faunal distinction also was needed, however, as the 

lithologic differences best recognizable in the "lead region" are not quite satisfactory, because "local alteration of the rock has produced the typical Galena facies 

in the top of the Beloit formation." The addition of 

faunal criteria made the Beloit formation the "zone of 

Orthis subaeguata Con. (0. perveta Con., etc.), and the 

Galena is the zone of Receptaculites oweni H." Sardeson's (1897a, p. 29) correlation placed the top of the 

Beloit at the top of the Fucoid bed (Ion) of Minnesota 

(fig. 37), which he had traced to the vicinity where the

389000—56———3

Beloit is exposed by the presence of fossils including 

Prasopora contigua Ulrich.

This makes the Beloit equivalent to the Platteville 

and Decorah formations of the present classification, 

as noted first by Sardeson (1907, p. 187, 193). The 

lithologic unity of the Beloit formation is characteristic 

eastward from the mining district to the vicinity where 

the Beloit is exposed. The lithologic subdivisions of 

the Beloit are likewise most characteristic in that area. 

However, certain miscorrelations (especially of the 

lower blue beds) which Sardeson perpetuated (1896b, 

p. 362) causes one to avoid those subdivisions as stratigraphic units.
264 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Classification used 

in this paper

Formation

BJ

c

BJ

a

Decora h

'latteville

Member

Dubuque

Stewartville

Prosser

Ion

Guttenberg

Spechts Ferry

Quimbys Mill

McGregor

Pecatonica

Glenwood

Minnesota

Winchell and Ulrich, 

1897, p. Ixxxvi

Maquoketa (Utica)

Maclurea zone

Fusispira

^~~^~~^^^Nematopora 

Clitambonites ^~~~^~~~~^^^

Phylloporina,^^^^ 

^_^-—-^^"^ Ctenodonta

Rhinidictya bed

Stictoporella bed

TmrnrrmrnrmnT

Vanuxemia bed

Buff limestone

Sardeson, 1897a; 

1907, p. 185

Tripleda bed *

Maclurea bed

Lingulelasma bed

Camarella bed

Orthisina bed

Fucoid bed

Stictopora bed

Stictoporella bed

TmTTTTTTfTnmT

Blue, or 

Bellerophon bed

Buff limestone

Hall and Sardeson, 

1892, fig. 5

Maquoketa (Utica)

Maclurea

Lingulasma

Camarella

Orthisina

Zygospira 

Fucoid

Stictopora

Stictoporella

\ IIIIIIIIIIIIIMIIIIII!

Blue

Buff

* Sardeson (1897a) also referred to the Tripleda bed as the'

FIGURE 37.—Correlation of Platteville, Decorah,

^ Furthermore, in the area where the Beloit is exposed 

it is difficult to distinguish the uppermost strata of the 

Beloit from the overlying Galena strata. Conversely, 

the Platteville and Galena formations are distinct in 

the area of their type localities in the mining district; 

the Decorah formation, though mainly shale in its type 

locality 80 miles northwest of the mining district, and 

mainly dolomite at the eastern edge of the mining district, nevertheless is traceable lithologically across these 

facies changes; therefore, the Decorah is a distinct unit 

in the mining district.

Moreover, the terms Platteville and Decorah have 

been in use consistently since 1906, whereas the term 

Beloit has been virtually forgotten. The names 

Platteville and Decorah are therefore used in this 

paper.

UNNAMED i LIMESTONE MEMBER, DECORAH FORMATION

Northwest of the mining district is a thin limestone 

unit that is similar in lithologic character to the underlying McGregor there, but separated from that member 

by bentonite. It is typically exposed in the quarry at 

the north end of Ice Cave Bridge (called Halloran's

1 Although the writers originally intended to name this distinctive unit the Ice 

Cave Bridge member from an exposure in Decorah, Iowa, because of the greater 

thickness in Fillmore County, Minn. (30 miles to the northwest) M. P. Weiss is 

naming and defining the unit from exposures near Carimona hi that county.

Transition formation" and as the Maquoketa shales

and Galena strata with units in Minnesota.

quarry by Calvin, 1906, p. 85), in NE#NW# sec, 15, T. 

98 N., E. 8 W., in Decorah, Winneshiek County, Iowa 

(see section 6, p. 306), where it consists of 2^ feet of 

limestone above half a foot of bentonitic shale and 

siltstone. Other exposures near Decorah that show 

this member well are the Coon Creek ravines in SW% 

sec, 13, T. 98 N., R. 7 W., the Conners roadcut along 

the north line of sec, 13, and the roadcut along the 

south line of the SW/4 sec. 13, all about 8 miles east of 

the Ice Cave Bridge quarry. The thickness has diminished to 2 feet at these localities, but otherwise the 

strata are similar to those at Ice Cave Bridge quarry. 

Twelve miles to the southeast, in a ravine 2 miles 

north of Forest Mills (fig. 35, loc. 35) these limestone 

strata are thinner and have changed somewhat in color 

to bluish gray (see section 7, p. 307). Similar lithology 

was observed in the type section of the Ion, 11 miles 

farther east, in NW# sec, 35, T. 96 N., E. 4 W. (loc, 

36), except that the limestone bed is only 0.9 foot 

thick and the bentonite bed is overlain and underlain 

by dark brown shale. Near McGregor, 5-8 miles 

farther southeast, roaclcuts, in the NE% sec, 9 and the 

}i sec. 34, and the (type locality of the McGregor) 

% sec, 28, T. 95 N., R. 3 W. (loc, 25) showed 0.4-0.7 

foot of purplish limestone overlying 0.3-0.5 foot of 

bentonite and bentonitic siltstone.
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 265

The bentonite bed in the Spechts Ferry member in 

the McGregor-Ion area is 0.3-0.5 foot above the top of 

the limestone of the unnamed member, whereas in the 

Coon Creek to Conners area east of Decorah it is 

0.8-1.3 feet above the limestone; at Decorah the 

bentonite layer is 2.5 feet above.

East of the Mississippi River the relations of this 

unnamed member of the Decorah with the overlying 

Spechts Ferry in Grant County, Wis., are not completely clear, as the tabulation below shows:

Relations of unnamed member of Decorah formation in Grant 

County, Wis.

Exposures

Roadcut, U. S. Highway 18 (loc. 37) L-.

Readout, U. S. Highway 18 (loc. 40) L._

Shriners Park quarry, SEJ4SWJ4 sec. 

34, T. 5 N., R. 3 W . (midway between 

Iocs. 41 and 28) .

Liberty Ridge quarry, NWJ4SE1 ! sec. 

19, T. 5 N., R. 2 W. (1 mile south of 

loc. 15).

Lithologic character

Shale, olive, and brown 

limestone.

Limestone, buff to brown_-_

Thickness 

between 

bentonite 

beds (feet)

Units

0.5

1.6

.2

.8

.6

.1

.7

.6 

.6

.7 

.1 

.2 

.5

.1

.8 

.3

Total

0.5

1.6

1.0

.6

.8

1.2

1.5

1.2

1 Locality numbers and section, township, and range are given on figure 35.

At the Liberty Ridge quarry lenses of limestone 

similar in lithology to the Quimbys Mill are present in 

the basal part of the lower bentonite bed. Six miles 

east of this quarry (roadcut, County Trunk E, center 

E% sec. 18, T. 5 N., R. 1 W.) 1.5 feet of typical limestone of the Quimbys Mill member is overlain by 0.9 

foot of pinkish limestone, which may possibly represent 

the eastern feathering out of the unnamed member, 

although the lower bentonite bed is absent. Moreover, 

4 miles southeast of the Bloomington quarry in a roadcut along State Route 35 (SEK sec. 8, T. 4 N., R. 4 

W.), where the Quimbys Mill is absent, the McGregor 

is overlain in ascending sequence by a 0.1-foot mottled 

brown and green shale, a 0.1-foot olive shale, a 0.1-foot 

greenish limestone, a 0.3-foot olive shale, and that by 

the bentonite bed of the Spechts Ferry. As the bentonite bed at the base of the unnamed limestone member is elsewhere overlain by a mottled brown and green 

shale, the unnamed limestone and its basal bentonite

bed are considered to be absent at this roadcut south 

of Bloomington.

The western edge of the Quimbys Mill is an approximate north-south line about 5 miles west of the east 

line of Grant County, Wis. (fig. 35). Recognizable 

beds of the unnamed limestone unit are seen as far east 

as the mouth of the Wisconsin River, which is 30 miles 

west of the Quimbys Mill pinchout. Between the 

mouth of the Wisconsin River and the central part of 

Grant County, there are beds of indeterminate assignment, although presently they are recognized as having 

some of the characteristics of the unnamed limestone 

member.

Although these limestone beds at Decorah were 

excluded from the Decorah of Calvin, in their eastern 

occurrences (Grant County, Wis.) they are more 

characteristic of Decorah strata than of the underlying 

Platteville. Furthermore, the members of the Platteville formation thin to the west whereas those of the 

Decorah formation thin to the east. The unnamed 

limestone member, therefore, is here referred to the 

Decorah formation. This was apparently the assignment made by Stauffer (1925, p. 616-619) at an exposure in St. Paul, Minn., as he determined the top of 

the Platteville to be below 1.5 feet of brown dolomitic 

limestone, which was overlain by a thick bentonite 

bed that was succeeded above by 2 feet of gray to 

bluish limestone. These limestone beds, which Stauffer placed in the Decorah, rested on a corrosion surface, 

probably the same as the one that Sarcleson (1898, 

pi. 9) recognized at the top of the Bellerophon bed 

(fig. 37).

No systematic paleontologic study has been made of 

these strata (p. 289). Such a study should be valuable, 

as would similar studies through the Platteville, Decorah, and Galena sequence of the upper Mississippi 

Valley. Because the unnamed member is better exposed in Winneshiek County, Iowa, and Fillmore 

County, Minn., than in the mining district, further 

study of this member should be concentrated in that 

general area.

SUBDIVISIONS OF GALENA DOLOMITE

Galena is the name applied by Hall (1851, p. 146) 

to the rocks exposed in the vicinity of the town of 

Galena, 111.

In the upper Mississippi Valley region the Galena has 

been divided into the following three members on the 

basis principally of paleontologic criteria:

Dubuque (Sardeson, 1907, p. 193)—limestone and 

shale, bounded below by the "cap rock" of the 

Stewartville; named from Dubuque, Iowa (fig. 35).

Stewartville (Ulrich, 19lib, pi. 27)—the Madurea
266 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

zone (Winchell and Ulrich, 1897, p. Ixxxiii-lxxxvii) Galena Formation—Continued.

of Minnesota; named from Stewartville, Fillmore Stewartville Member-Continued Feet

/->( , AT- nr- -i ^i . f TTT i 14. Dolomite, gray to brown, weathering yellow. County, Mmn., 65 miles northwest of Waukon. n f •-> » , /•* • » *

' Common fossils are HeceplacuMes owem, Hajilowa. nesquina alternata, Madurina manitobensis, M.

Prosser (Ulrich, 191 la, p. 257)—The Fusispira (above), crassus, Hormotoma major, H. trentonensis,

Nematopora and Clitambonites (below) beds of Illaenus americanus, Isotelusgigas—_________ 14.5

Minnesota (Winchell and Ulrich, 1897, p. Lxxxiii- 13 - Shale, argillaceous, yellow to gray________________LO

Ixxxvii); named from Prosser's ravine, near Wykoff, _. „

Miiin., 50 miles northwest of Waukon, Iowa. Prosser MemberThe Prosser member was named by Ulrich (191 la, 12 - Limestone, compact, drab, subcrystalline, with

orr-rx -IT ii s-17-4 i. -j. /T7- 77 \ some thin, argillaceous layers____-_-_--______ 11.0

p. 257) as including the Ciitamoonites (Vellamo), 11T - *. J.T,JJU J.U-UJJJ *T ' ° -, T r- i 11. Limestone, compact, hard, drab, thin-bedded,

Nematopora, and Fusispira beds (fig. 37) of the Mm- with numerous graptolites_ _________________ 1.0

nesota reports, apparently from the exposures in 10. Limestone, compact, hard, drab, very fossilifer-

"Prosser's ravine, near Wykoff," (sec. 20?, T. 103 N., ous. Common fossils include Rafinesquina delE, 12 W ) toidea, Catazyga uphami, Plectambonites gibmi i ,1 , i i -u j • lA-ii bosus, fiysssnychia, intermedia,-. _____________ 8.0 ihe member was not adequately described in 1911, _ T . ' U1 • u . ^ . ,,.

, ", 9. Limestone, bluish, compact, partly crystalline,

nor is the type section properly located. However, thin-bedded to shaly. Common fossils include

Winchell and Ulrich (1897, p. Lxxxv-Lxxxvii) had Streptelasma corniculum and Rafinesquina

previously given a section at "Prosser's ravine," near defto«fes__________________________________ 8.0

Wykoff, Fillmore County, "which Ulrich undoubtedly 8- Limestone, compact, drab, with some chert or

intended as the type section" (Stauffer and Thiel, 1941, flint' Fossiliferous________________________ 13. 3

p. 86). About 185 feet of limestone occurs at this 7 ' Limestone, compact, drab with numerous fossils.

f ,. i • ^ • Common forms are Ischadites lowensis, Hesper- locality, of which the lower 110 feet or so were assigned orthis iricenarta> Plectambonites sericeus, Rato the Prosser. As described by Stauffer and Thiel the finesquina alternata, Vellamo diversa, Endoceras

beds are fairly thick, composed of hard compact drab annulatum, Isotelus gigas——________________ 19.3

limestone with a small amount of shale. Several zones 6. Limestone, thick-bedded, compact, drab, with

are chertv some chert. Common fossils include HesperThe name Stewartville was applied by Ulrich (1911b, orthis tricenaria and P^orthis plicatella treni n>-,\ , ^1 UT\ r 7 jt e ,i T TIT- , tonensis---------------------------------- 10.3 pi. 27) to the Maclurea zone ol the earlier Minnesota „ T . ,,.,,,,,,,

, „ , .5. Limestone, compact, drab, with shaly beds at the

reports, apparently from exposures near the town of base, forming bottom of small caves and line of

Stewartville, Mmn. (sec. 34?, T. 105 N., R. 14 W.). springs. Graptolitescommon_______________ 9. 2

In southeastern Minnesota, as elsewhere in the upper 4. Limestone, argillaceous, massive, but weathering

Mississippi Valley region, the Stewartville is a dolomite into thin, knotty beds, blue to gray in color.

or dolomitic limestone. It was originally described Receptaculitesowenicommon- ._______.___-__ 17.8

as having a thickness averaging 50 feet. Stauffer and 3 - Limestone, compact, drab, passing into argilThiel (1941, p. 87) described a section from the type laceous beds " R*<xpta*ulte* oweni and Pfoc .

„,,-„ , --p. /->t i " m/ tambomtes sericeus common, ________________ 10.3 region ol the Prosser member, m Prosser Creek, 2% T .,,,,*,,

M , f .-. . f TTT i n? /i - -i ^i . f 2. Limestone, compact, light drab in color. Abund- miles west of the town of Wykoff (15 miles southeast of ant fauna indudes Dinorthis meedsi germanttj

otewartville) as lOllOWS: Rafinesquina alternata, Rhynchotrema increbesMaquoketa Formation: cens_ ________-___-_-_________-__-_--___--

Dubuque Member- Feet ^- Covered interval to level of Deer Creek at outlet

18. Limestone, shaly, gray to buff, fossiliferous______ 20.0 of Prosser Creek___ —— ___ — _____ —— _—— 5.3

Galena Formation:

Stewartville Member: The strata above the Stewartville and underlying the

17. Dolomite, cavernous, yellow to buff, fossiliferous. Maquoketa were named Dubuque by Sardeson (1907,

Common fossils include Madurina mani- p 193 ) from t^eir outcrops at Dubuque, Iowa.

tobensis and Hormotoma major ____________ 16. 0 Sardeson described the Dubuque as consisting of those

16. Dolomite, very porous, yellow from weathering, . -. ,. i • . i

in thin, knotty layers. Common fossils are "strata of irregular limestone and mterlammated

Receptaculites oweni, Madurina cuneata, M. carbonaceous shales" which lie above the "cap rock"

mrMitobensis, Hormotoma major, Endoceras sp., of the Stewartville and below the "blue shales of the

7«aenMsaTOer;canMs_-_-___-________________ 10.5 Maquoketa proper." Kay (1935c, p. 571) stated that

15. Dolomite, thick-bedded, gray to yellow Com- ^ „ YQ^,, m& ^^ ^ Stewartville both

mon fossils are Recepiaculites oweni, Madurina . c ,, , , ,, -. ,,

manitobensis, Hormotoma major, Westonoceras llthologically and faunally, and as the cap rock

minnesotense_.-- ______________________ 12.3 occurs within bed 14 of the following section (Calvin,
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 267

Bain, 1900, p. 429), Kay redefined the Diibuque, 

restricting it to bed 15:

Feet.

15. Thin-bedded Galena limestone, earthy, noncrystalline, the layers ranging from ten or twelve 

inches near the base to l^ss than three inches in 

thickness near the top; upper part of this member 

very shaly; carries as fossils Lingula iowensis, 

Liospira lenticularis, and Conularia trentonesis---- 30

14. Well dolomitized Galena in layers ranging from one 

to two and a half feet in thickness; with softer 

beds near the middle, which frequently disintegrate 

so as to form caverns; basal part only, of this 

member, represented above the Receptaculites beds 

at Eagle Point_______________.________________ 30

Recent work by the writers (see also Flint and 

Brown, 1955), however, has lent support to a subdivision based upon the description by Calvin and 

Bain.

Because the descriptions of the Prosser and Stewartville in their type areas (southeastern Minnesota) are 

incomplete, these names cannot be used precisely in 

stratigraphic work in the general upper Mississippi 

Valley region without further study of the stratigraphy 

and paleontology of the Galena in southeastern Minnesota. Such a study should result in the validation of the 

names Prosser and Stewartville and thus in their 

continued use, because they are well-established in the 

literature. This is preferable to renaming these strata 

from different, although perhaps more representative 

localities regionally, as Templeton and Willman (1952, 

p. 6 and fig. 3) have done. During the past few years 

W. C. Bell and his students have been examining the 

rocks of this area, and M. P. Weiss' report, The stratigraphy and stratigraphic paleontology of the upper 

Middle Ordovician rocks of Fillmore County, Minii.,2 

contains descriptions of type sections and additional 

material that give precise stratigraphic meaning to the 

units described by Ulrich.

In the zinc-lead district the beds of the Galena are 

dolomite and are only sparsely fossiliferous; the fossils 

that are present in exposures are poorly preserved. 

Furthermore, where seen in the mines and in cuttings 

from prospect drill holes, local rock alteration and the 

pulverization by drilling have destroyed most of the 

fossils that survived the effects of regional clolomitization. As a result, during the current study it was 

necessary to subdivide the Galena on the basis of 

lithologic criteria. The units that can be recognized 

in exposures in the mining district are as follows, in 

descending order:

» Unpublished Ph. D. thesis, Minn. Univ., Minneapolis, 1953.

,_.... ' Thickness JNoncherty unit: (feet)

Dolomite and dolomitic limestone, yellowish-gray, 

finely granular, argillaceous, thin- to medium-bedded; 

thin partings of dolomitic yellowish-gray shale; 

lower contact gradationaL_______^___.___________ 35-45

Dolomite, yellowish-buff, coarsely granular to crystalline, vuggy, medium- to thick-bedded; Receptaculites 

common 35-55 ft above base_____________________ 75-85

Cherty unit:

Zone A. Dolomite, buff to drab, otherwise as above; 

chert bands common in upper 44 ft and 50-56 ft 

below top; locally a thin bentonite about 32 ft below 

top; Receptaculites 35-40 ft below top and 10 ft 

above base.____________________________________ 70

Zone B. Dolomite as above, except more brownish;

chert bands rare; Receptaculites common.__________ 15

Zone C. Dolomite as above; chert bands common____ 10

Zone D. Dolomite as above, except with streaks of

greenish argillaceous material; no chert.___________ 10

Total. 225

By definition the Prosser extends upward to the base 

of the upper Receptaculites zone, which has been called 

the base of the Stewartville member (fig. 38). However, 

it has been found extremely difficult to place this 

boundary with any degree of reliability, especially in 

subsurface work. The contact is normally not characterized by a marked and easily recognizable lithologic 

change. In fact, the delimitation of the Prosser member 

in the mining district depends solely upon the finding of 

Receptaculites oweni Hall, a difficult task in weathered 

or largely covered outcrops and in drill cuttings and 

cores. Furthermore, Receptaculites individuals have 

been found downward to within a few feet above the 

topmost chert, or about 25 feet lower than the normal 

position of the base of the upper Receptaculites zone.

The highest appearance of the chert bands within the 

described Prosser affords a useful lithologic key horizon 

(see also Willman and Reynolds, 1947, p. 10). In 

many places an observable color change in the dolomite 

occurs at the appearance of the chert bands. This 

change in lithology, striking in all outcrops and even 

more so in subsurface study, is found in southwestern 

Wisconsin, northeastern Iowa, and northwestern Illinois. The highest chert horizon is readily recognized 

farther south and west in Iowa (Agnew, 1955) and 

farther south in Illinois. (Workman, L. E., and 

Herbert, Paul, Jr., formerly with the Illinois Geological 

Survey, oral communication, August 18, 1945.) The 

stratigraphic continuity of the cherty and noncherty 

units is well established, and the contact between them 

is so dependable as a key horizon that structure contour 

maps using it as a datum have been prepared during
268 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Formation

Maquoketa

Galena

Decorah

Platteville

St. Peter

Member and 

subdivision

Stewartville Dubuque j •>» 

Prosser

Noncherty unit

'c

(U

.c 

O

P

A

B

C

D

Ion

Guttenberg

Spechts Ferry

Quimbys Mill

McGregor Magnolia (of Bays and 

— -^_^__^Raasch, 1935)

Mifflm (of Bays, 1938)

Pecatonica

Glenwood

Local 

terminology

Shale

Buff or 

sandy

n..

Drab

Gray

Blue

Oil rock

Clay bed

Glass rock

Quarry beds

Shale \

Sand rock

3-1^-:^

/ gr

— ^~7 ——

/ /

/

/

/ /

/

/

/ /

/

/

/ / *

/ /

/ /'

/

/ /

/ /

/

/

/ /

/

/

/ /

/

/

/ /

/ /

/

/

/ /

/ /

& / A

/ * / 4

/ A /

A / A.

*\ / A

/ ^ /

A / A

A / A

/ A / A

/ /

/

/ -a /

— ^~~T — ^

/ J / ——

/ /

•* /

/ /

/ -^ /

/

/

/ ^ /

/

/

^7^

/

/ /

/ /

/ —

/

/ — /

/

i — i

/, .^..

i

i i

^P4i

/ /

/ /'

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/

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,v:->W;-^

Description

Shale, blue or brown, dolomitic; with dolomite 

lenses; phosphatic depauperate fauna in 

lower few feet

Dolomite, yellowish-buff, thin- to mediumbedded; with interbedded dolomitic shale

Dolomite, yellowish-buff, thick-bedded, vuggy; 

Receptaculites in lower part

Dolomite as above; bentonite rarely at 

midpoint

Dolomite, drab to buff, thick- to thinbedded; cherty; bentonite at base

Dolomite as above; Receptaculites at top

Dolomite as above; cherty

Dolomite as above; some chert; Receptaculites 

at midpoint

Dolomite as above; little chert; Receptaculites 

abundant

•^^ Dolomite as above; much chert

/ Dolomite as above;

/ Dolomite and limestone, light-gray, argillaceous; 

i' grayish-green, dolomitic shale

/ Dolomite, limestone, and shale as above, 

/ but darker

./ Limestone, brown, fine-grained, thinbedded, nodular, conchoidal; dark- brown 

shale

_x/^ Shale, green, fossiliferous; greenish-buff, 

\ fine-grained limestone; phosphatic nodules 

N^ near top; bentonite near base

\ Dolomite and limestone, dark-brown, fine- 

\ grained, sugary, medium-bedded, conchoidal; 

\ dark-brown shale especially at base

\ Limestone and dolomite, light-gray, fine-grained

x Limestone, light-gray, fine-grained, thin- 

\^ bedded, nodular, conchoidal

Dolomite, brown, medium-grained, sugary, 

thick-bedded

N. Shale, green, sandy

Sandstone, quartz, medium- to coarse-grained 

poorly cemented, crossbedded

Unaltered 

thickness, 

n feet

1

35- 

45

37- 

47

38

32

6

6

26

15

10

10

11

15

5-9

12

0-

120

105

20

16

8

0 18

13- 

18

12- 

17

30

20 24

0-3

10

225

32 

44

55-

75

40+

FIGURE 38.—Stratigraphic column of Platteville, Decorah, and Galena strata in zinc-lead district.
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 269

the past several years by members of the Iowa Geological Survey.

West of the mining district the Stewartville and 

Dubuque members can generally be distinguished by 

lithologic features in the outcrop; however, in the 

mining district and to the east the differences are more 

subtle. Furthermore, in drill cuttings, which constitute 

a major source of information in the mining district, 

the separation of the Dubuque and Stewartville is 

generally difficult.

ORIGIN AND APPLICATION OF THE NAMES

The origin of most of the stratigraphic terms employed in this report is discussed by Wilmarth (1938); 

names that have been applied since 1938 or older names 

that for various reasons merit additional treatment are 

discussed in the paragraphs that follow.

MADISON SANDSTONE

The term Madison (Howell, 1944; Raasch, 1935) is 

not used in the present report, as the writers believe 

that it is not consistently recognizable in the zinc-lead 

district.

PECATONICA DOLOMITE MEMBER

Hershey (1894, p. 175) named the Pecatonica from 

exposures in the Pecatonica River valley "near the 

Wisconsin line, and northward," in southwestern Green 

County (fig. 35). As originally defined the Pecatonica 

included the sandy and shaly "insensible gradations" 

(beds of Glenwood age) downward into the St. Peter 

sandstone. Although Hershey (1897, p. 67) intended 

the name to be applied chiefly to the Elk Horn Creek 

area (fig. 31), and although Sardeson (1897c, p. 333) and 

Bays and Raasch (1935, p. 297) objected to its use, the 

name Pecatonica as restricted by Kay (1935a, p. 286) 

to exclude the "insensible gradations" at its base has 

received wide adoption, even by Bays (1938). This 

unit was called the "Lower Buff" by early writers 

because of its color when weathered.

MCGREGOR LIMESTONE MEMBER

The McGregor limestone member was named by 

Kay (1935a, p. 286) from beds exposed in a ravine a 

mile west of McGregor, Clayton County, Iowa (fig. 35, 

loc. 25). The name was applied to the "limestone 

succeeding the Pecatonica member of the Platteville 

formation, and underlying the Spechts Ferry member." 

This is unit 3 of Bain's (1905) type section of the Platteville, as described on page 261.

The McGregor was divided by Bays and Raasch 

(1935, p. 298) into the McGregor, sensu stricto, below 

and the Magnolia above. Later Bays (1938) renamed 

the lower beds Mifflin from their exposure in "the roadcuts and stream banks of the Pecatonica River, at

Mifflin, Iowa County, Wis." in the NEK sec. 34, T. 5 

N., R. 1 E. (1 mile northeast of loc. 18, fig. 35). The 

Mifflin (of Bays, 1938) is characterized by thinly 

bedded nodular limestone that weathers light gray, and 

is 17% feet thick at Mifflin, Wis. The type exposure of 

the Magnolia (of Bays and Raasch, 1935) is "on and 

near Highways 13 and 14," 1 mile south of the town of 

Magnolia, Wis., in the NWK sec. 26, T. 3 N., R. 10 E., 

Rock County (loc, 26).

In the type outcrops the Magnolia is reported (Bays 

and Raasch, 1935, p. 298) to consist of 39 feet of lightbuff, moderately thick bedded dolomite, with conspicuous fucoidal markings on the bedding planes in the 

upper part of the member. The type section consists 

of several roadcuts and a quarry as much as a quarter 

of a mile apart, each exposing only a small part of 

the Magnolia (of Bays and Raasch, 1935); thus a 

measurement of thickness can be only an estimate. 

Other disadvantages of this selection are the lack of a 

complete exposure of the typical lithology, and the 

lack of exposure of the stratigraphic relations with the 

Mifflin (of Bays, 1938) beds below. No acceptable 

section in the vicinity of Magnolia, Wis., could be 

found during the course of the present study.

QUIMBYS MILL MEMBER

Aguew and Heyl (1946, p. 1585) named the Quimbys 

Mill member from its exposure in the quarry at Quimby's Mill, 5 miles west of Shullsburg, Wis. (fig. 35, 

loc. 27), including all the beds between the top of the 

McGregor (Trenton) and the base of the Spechts Ferry 

(clay bed) there. The Quimbys Mill strata are the 

"glass rock" of the miners.

Kay (1928) included the "glass rock" beds in his 

description of the type section of the Spechts Ferry 

which more or less agreed with Shaw and Trowbridge 

(1916, p. 4). Later Kay (1929, p. 644) described this 

assignment as possible, but shortly thereafter he (1931, 

p. 370) felt it desirable to omit these beds from the 

Spechts Ferry classification. He retained this point 

of view in later publications (1935a, p. 287) but at no 

time defined the exact position and nature of the "glass 

rock" (fig. 39).

Bays and Raasch (1935, p. 298) apparently included 

the "glass rock" beds in the Spechts Ferry but without 

describing them. These authors appear to have referred to the "glass rock" as a lithologic facies, for they 

stated:

The Spechts Ferry varies from one district to another. It may 

be a soft, calcareous shale, a limestone, or a dolomite.

Bays (1938) stated that it "passes laterally from shales 

to limestones to dolomites to cherty dolomites." The 

latter two are facies of the "glass rock" or Quimbys Mill 

member, as recognized in this report.
270 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 271

STRATIGRAPHY OF THE MINING DISTRICT 

PBE-PLATTEVILLE BOCKS

Because this report deals primarily with rocks related to the ore deposits, pre-Platteville strata will be 

discussed only cursorily. Furthermore, as a detailed 

investigation of pre-Platteville strata was not attempted, information regarding those rocks is taken 

partly from the work of others, from rather brief studies 

of outcrops by the writers, and from recent drilling 

data.

PRE-CAMBRIAN ROCKS

Pre-Cambrian rocks were reported (Thwaites, 1923, 

p. 553) in a well at Platteville, Wis. (City well 2, sec. 

15, T. 3 N. t R. 1 W.), where the drill penetrated "granite" at a depth of 1,714 feet (fig. 35). Wells at the 

north margin of the district, in Richland Center, Wis. 

(sec. 16, T. 10 N., R. 1 E.), reached similar pre-Cambrian material at depths ranging from 665 to 678 feet, 

and the Borden Whey Plant well 2 al Boscobel, Grant 

County, Wis., reportedly struck "granite" at 837 feet 

(loc. 1). Information for these wells is in the files of 

Wisconsin Geological and Natural History Survey, 

Madison, Wis.

Pre-Cambrian rocks in the subsurface of this general 

area have been discussed by Thwaites (1931) and Grogan (1949). In the Baraboo area just northeast of the 

zinc-lead district, and in northern Wisconsin the preCambrian rocks are separated from the overlying Cambrian sedimentary rocks by an unconformity of considerable relief.

CAMBRIAN ROCKS

The oldest Paleozoic rocks known in the upper Mississippi Valley (fig. 32) are sandstone, siltstone, and 

dolomite of Late Cambrian age (Twenhofel, Raasch, 

Thwaites, 1935; Trowbridge, Atwater, 1934). These 

rocks are exposed only along the northern edge of the 

zinc-lead district, and farther south they are found in 

deep wells drilled for water and as oil tests. An excellent exposure of much of the upper 150 feet of Cambrian is seen in the bluff at the intersection of State 

Routes 60 and 61 in Crawford County, 2 miles north of 

Boscobel, Wis. (fig. 35, loc. 2).

The thickness of the Cambrian increases southward 

from about 1,000 feet at the north fringe of the district 

(Iocs. 1, 2) to 1,284 feet at Platteville.

The Mount Simon sandstone, which rests unconformably upon the pre-Cambrian rocks, ranges from 

440 to 780 feet in thickness in the mining district. It 

is generally light gray, but locally reddish. In the 

mining district few wells have intersected this sequence; 

however, Templeton (1950) has shown that near Rock389000—56— —4

ford, 111., (fig. 31) and to the south, different lithofacies 

are characteristic of the formation.

The Eau Claire shale overlies the Mount Simon sandstone conformably. In the mining district the Eau 

Claire is mainly sandstone, and its relations with the 

underlying and overlying strata are gradational. It, 

ranges from 70 to 330 feet in thickness.

The Dresbach sandstone is light gray, and is 60-140 

feet thick. The Mount Simon, Eau Claire, and Dresbach together aggregate 700-1,050 feet in thickness in 

the mining district. Part of this variation is due to 

the unconformity at the base of the Mount Simon; part 

to a southward increase in the thickness of the Mount 

Simon; and part is due to the gradational lithologic 

relationships of the three formations.

Above the Dresbach is the Franconia sandstone, 

110-140 feet thick. The glauconite in the Franconia 

is the principal feature by which these strata can be 

distinguished from the similar overlying and underlying sandstone. Recently Berg (1953, 1954) has 

published the results of a thorough study of the facies 

of the Franconia; because the Cambrian strata are discussed only briefly in this report, however, no attempt 

was made by the writers to apply these subdivisions of 

the Franconia to the rocks in and bordering the mining 

district.

The Trempealeau formation, which overlies the 

Franconia, is principally sandstone and siltstone although commonly the lower strata are dolomite. The 

uppermost beds of the Trempealeau are called the 

Jordan sandstone, which is composed of clean wellsorted coarse quartz grains that are subangular to 

round. The Trempealeau formation is 120-150 feet 

thick.

Locally within the mining district and along its 

margins a sandstone unit called the Madison or Sunset 

Point (Raasch, 1951, p. 150) is said to overlie the Jordan 

sandstone of the Trempealeau formation. Raasch regards the Madison as a separate formation, basing this 

opinion primarily upon sedimentary and lithologic 

criteria, which Twenhofel and Thwaites (Twenhofel, 

Raasch, Thwaites, 1935, p. 1711, footnote 45) consider 

as having little weight. Raasch states that the Madison 

is poorly sorted silty or conglomeratic quartz sandstone, 

as much as 60 feet thick. It is not consistently recognized in the mining district.

In most places beds transitional in lithology fill the 

interval between the Jordan sandstone and the overlying dolomite of the Prairie du Chien group (Schuldt, 

1943, p. 404). These transition beds consist of alternating dolomitic sandstones and arenaceous dolomites 

and are as much as 27 feet thick. In areas where the 

transition beds are lacking the contact of the Jordan 

and the Prairie de Chien appears conformable.
272 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Lead minerals have been found in Cambrian rocks 

near Lansing, in northeastern Allamakee County, Iowa, 

sec. 10, T. 99 N., R. 4 W., and 35 miles to the north, 

at Dresbach, in southeastern Winona County, Minn., 

sec. 18, T. 105 N., R. 4 W.; both localities are marginal 

to the mining district (fig. 35). The galena at the 

Lansing occurrence is in the uppermost beds of the 

Cambrian, probably the dolomitic transition beds, 

whereas in the Minnesota locality the minerals were 

found in a shale 350-400 feet below the top of the Cambrian, probably in the Eau Claire sandstone.

Zinc minerals were seen (Heyl, Lyons, and Agnew, 

1951, p. 9) in cuttings from a prospect hole near Montfort, in west-central Iowa County, Wis., sec. 30, T. 6 

N., R. 1 E. (one mile south of loc. 43, fig. 35), from 

149 to 159 feet below the top of the Cambrian in glauconitic sandstone called Franconia.

Evidences of iron mineralization in Cambrian strata 

are abundant, especially just north of the zinc-lead 

district. Iron sulfide estimated at 1—3 percent iron 

was found in the prospect hole at Mont-fort, from 141 

to 179 feet below the top of the Cambrian, in the 

Franconia.

Water supplies ample for large industrial plants and 

municipalities are obtained from the Cambrian strata, 

especially from the Mount Simon and Dresbach.

ORDOVICIAN ROCKS

PRAIRIE DU CHIEN GROUP

The Prairie du Chien strata, which are commonly 

known in the upper Mississippi Valley by the old name 

of Lower Magnesian limestone, are seen above the 

Cambrian strata along the north fringe of the mining 

district and crop out in the more dissected areas in the 

central part of the district. Perhaps the most nearly 

complete exposure is in the quarry at the north edge 

of Wyalusing, Grant County, Wis. (fig. 35, loc. 3).

Rocks of the Prairie du Chien group are extremely variable in lithology. In places the group is divisible into 

three formations—the Oneota dolomite, which overlies 

the Cambrian, the Xew Richmond sandstone, and the 

Shakopee dolomite; the name Root Valley has been 

applied to a sandstone at the New Richmond position 

(Stauffer and Thiel, 1941, p. 59) and the name Willow 

River is used by many in place of Shakopee (Powers, 

1935, p. 390). In other places, however, no recognizable sandstone unit is found; no threefold division could 

therefore be made. Furthermore, rocks that occupy 

the interval represented by the Prairie du Chien in 

places are sandstone, red shale, green shale, silicified 

limestone, and limestone. These strata have been 

assigned to the basal part of the St. Peter sandstone 

(Heyl, Lyons, Agnew, 1951, p. 5). Recent detailed

study by Flint 3 ,011 the other hand, has caused him to 

place them in the Prairie du Chien.

The dolomite of the Prairie du Chien group is light 

buff to light gray, finely to medium crystalline, in part 

vuggy, and thin to thick bedded, or irregularly bedded 

or massive. The dolomite is commonly oolitic, sandy 

with clear rounded quartz grains, and cherty. Locally, 

especially where evidences of iron-zinc-lead mineralization are abundant, the dolomite has been silicified. 

Thin green glauconitic shale lentils are found along1 

bedding planes and in the dolomite. Mound-shaped 

Cryptozoon colonies are common at certain horizons. 

The sandstone is similar to that in the Cambrian below, 

and commonly contains stringers and beds of greenish 

glauconitic shale. The sandstone in most places has 

dolomitic cement. Because of the variation in lithology 

the bedding is irregular, and generally poor (fig. 40).

FIGURE 40.—Exposure of typical dolomite of Prairie du Chien group in quarry in 

NE14NWJ4NWJ4 sec. 32, T. 7 N., R. 1 E., Iowa County, Wis. (4 miles north of 

loc. 43, fig. 35).

In local outcrops of the lower 40 feet of the Prairie 

du Chien, that rock can be zoned by lithology into 

several units, as Starke 4 has shown in the area north 

of the mining district; to these units Raasch (1952) has 

applied names in the Stoddard quadrangle (50 miles 

northwest of Platteville, Wis.), and Raasch has likewise given names to the succeeding 10-40 feet of strata 

in the Stoddard area. Because of the brief discussion 

of the Prairie du Chien in the present report, and because the correlative value of Starke's and Raasch's 

units and the value of Raasch's names have not been 

established in the mining district, the names and subdivisions are not used by the writers.

The Prairie du Chien attains a maximum thickness 

of about 240 feet in the zinc-lead district; it and the

s Flint, A. E., 1953, Stratigraphic relations of the St. Peter sandstone and the 

Shakopee dolomite in southwestern Wisconsin: unpublished Ph. D. thesis, Chicago 

Univ.

4 Starke, O. W., 1949, Persistent litholcgic horizons of the Prairie du Chien 

formation from the type section eastward to the crest of the Wisconsin arch: unpublished M. S. thesis, Wis. Univ., Madison.
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 273

overlying St. Peter sandstone, from which it is separated by an unconformity, aggregate 280-320 feet.

Prairie du Chien strata have been correlated with the 

Canadian series of Missouri (Ulrich, Foerste, and 

Bridge, 1930; Twenhofel, and others, 1954).

The Prairie du Chien strata constitute one of the potential mining zones in the zinc-lead district. Although 

this potential has not been properly evaluated as yet, 

evidence of former mining of lead in these beds and 

recent testing by the U. S. Geological Survey, 1949- 

1951 (Heyl, Lyons, Agnew, 1951; Agnew, Flint, Ailingham, 1953, p. 7-11) show that, with favorable economic conditions, these strata might possibly become a 

lower producing zone and thus prolong the industry in 

the district. Lead mining in the Prairie du Chien has 

been restricted to areas of its exposure along the 

northern fringe of the district. The drilling by the 

U. S. Geological Survey confirmed the presence of 

lead and iron minerals in these beds as far south as 

Mineral Point, Wis., and the Crow Branch diggings, in 

Grant County, Wis., (fig. 35, Joe. 4). Although zinc 

mineraJs have been found in these beds, the only 

locality known to contain any great abundance is that 

10 miles north of Waukon, Iowa (sec. 13, T. 99 N., 

R. 6 W.), where both zinc and lead were mined.

The permeability of the Prairie du Chien is such that 

it is a source of an adequate supply of water for farm 

use, especially where the overlying St. Peter sandstone 

is exposed and dry. Where the St. Peter sandstone is 

water bearing, commonly the Prairie du Chien is not 

a good source of water. Locally, the dolomites of the 

Prairie du Chien are permeable enough to be a reservoir 

rock and during drilling operations are able to absorb 

water rather than contribute it.

ST. PETER SANDSTONE

The St. Peter sandstone is exposed along the Wisconsin River and its tributaries, along the Mississippi 

River southward alomost to Dubuque, Iowa (fig. 35), 

and in areas of deep dissection within the zinc-lead 

district. Good exposures are seen along U. S. Highway 

151 a few miles southwest of Platteville, Wis. (Iocs. 5 

and 6).

The St. Peter consists of clear, fine to coarse, subangular to round quartz grains, as a rule poorly cemented; where the rock is indurated the cement is 

dolomitic, calcareous, or siliceous. Greenish argillaceous material is present in the upper few feet of the 

unit and near its base, particularly where the sandstone 

is abnormally thick; otherwise, it is relatively clean. 

The sandstone is thin-bedded to massive; crossbedding 

is characteristic- (fig. 41). In many places, a variety 

of colors, brown and red being the most common, in

FIGURE 41.—Cross-bedded St. Peter sandstone in readout along State Route 39, 

near center of NWJ4 sec. 1, T. 4 N., R. 5 E., Iowa County, Wis. (2 miles west of 

loc. 44, fig. 35).

the St. Peter is due to oxidation of iron sulfide in the 

cement.

The St. Peter is normally 40 feet thick. However, 

sandstone of this type has been found (fig. 35, loc. 7) 

at least 320 feet below the top of the formation (Heyl, 

Lyons, and Agnew, 1951, p. 33). WTiere green and 

reddish shales are present with the sandstone this 

association of sediments also is known to be more than 

300 feet thick and to rest on strata as old as the Fraiiconia, shown in the following wells (also see p. 272):

Abnormal thicknesses of the St. Peter sandstone

[Data in files of Wisconsin Geological and Natural History Survey, Madison, Wis.; 

Illinois Geological Survey, Urbana, 111.; and Iowa Geological Survey, Iowa City, 

Iowa]

Wells

Dodgeville city well 3, sec. 28, T. 6 N., R 3 E., central

Iowa County, Wis.

Linden city well 2, sec 8, T. 5 N., R. 2 E., 10 miles 

west of Dodgeville, Wis.

Belmont city well, sec. 14, T. 3 N., R. 1 E., northwestern Lafayette County, Wis.

Shullsburg city well 3, sec. 10, T. 1 N., R. 2 E., Lafayette County, Wis. (fig. 35).

Hanover city well, sec. 9, T. 26 N ., R. 2 E., southwest

corner Jo Daviess County, 111.

Bellevue city well, Jackson County, Iowa (fig. 35,

loc. 8).

Thickness, St.

Peter

(feet)

315

385

303

397

340

345

Underlying stre

Trempealeau.

Franconia.

?*

?*

ita

Prairie du Chien.

Do.

*Not reached.

At least part of this difference in thickness is due to an 

unconformity at the base of the St. Peter, which is well 

shown in a quarry in Clayton County, Iowa (sec. 1, 

T. 93 N., R. 3 W".), directly across the Mississippi River 

from locality 3 in Grant County, Wis. (fig. 35).

Nearly everywhere the St. Peter is unfossiliferous. 

However, Sardeson (1896a, p. 79) found fossils in the 

formation in Minnesota and Wisconsin that showed 

greater similarity with the overlying Platteville than 

with the underlying Prairie du Chien. Because of its 

position, he (1896a, p. 83) correlated the St. Peter with 

the Chazy (see also Twenhofel, and others, 1954).
274 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

: Lead and zinc minerals are almost unknown in St. 

Peter rocks. Small amounts have been found where 

mineralized fractures connect with zones of mineralization in higher beds, as at Mineral Point, Wis., and at 

Crow Branch, Grant County, Wis. (fig. 35, loc, 4). 

Iron minerals are characteristic of the St. Peter especially in the uppermost few feet, where pyrite cements 

the quartz sand grains. This evidence of iron mineralization appears to be more abundant in local areas that 

show zinc-lead deposits in overlying beds, and in larger 

areas where the iron minerals appear to be related to 

major structural features as at Red Rock, Wis. (sec. 

17, T. 2 N., R. 4 E.) 10 miles northeast of Shullsburg, 

Lafayette County (fig. 35).

In most places the St. Peter sandstone provides an 

ample supply of water for small towns, for small 

industrial plants, and for farms.

PLATTEVILLE FORMATION 

GENERAL FEATURES

Platteville strata are known throughout the mining 

district by exposures at the surface and in mines, and 

by cuttings from wells and prospect drill holes. Exposures are numerous; some of the best outcrops of the 

beds of Platteville age can be seen in the quarry al 

Spechts Ferry station, Dubuque County, Iowa (fig. 35, 

loc. 10); along U. S. Highway 151 southwest of Platteville, Wis. (loc. 6); along U. S. Highway 61 southwest 

of Platteville (loc. 12); and in the city quarry at Darlington, Wis. (loc, 13).

In the mining district the Platteville formation consists of the following four members in descending order 

(fig. 38): Quimbys Mill member, McGregor limestone 

member, Pecatonica dolomite member, and Glenwood 

shale member.

The Platteville formation ranges in thickness from 55

feet, in the western part (Heyl, Lyons, Theiler, 1952; of 

the district, to 75 feet near Shullsburg. It apparently 

lies conformably on the St. Peter sandstone within the 

mining district, and is overlain disconformably by the 

Decorah.

There is an excellent exposure of the lower three 

members of the Platteville in the western part of the 

mining district about 8 miles southwest of Platteville, 

Wis. (fig. 35, loc, 6). This section, hereafter designated 

the reference for the Platteville formation, appears as 

follows (see fig. 42):

Roadcut, U. S. Highway 151, NWftNEfa sec. 12, T. 2 N., R. 2 W., 

Grant County, Wis.

[Described by A. F. Agnew and A. V. Heyl, Jr., Apr. 1, 1943; revised by Agnew, 

Oct. 4, 1945]

Decorah formation: Th(f^ss 

Spechts Ferry shale member (clay bed), in part 

slumped:

Shale, bluish-green.____________________________ 0. 5+

Bentonite, white; weathering orange brown_______ . 2

Shale, yellowish-green above to bluish-green below_ _ . 2

Shale, brown and olive, soft_____________________ . 1

Platteville formation:

Quimbys Mill member ("glass rock"):

Limestone, dark-purple, fine-grained, dense, conchoidal fracture; very wavy upper surface; thin, 

dark-brown to black, fossiliferous platy shale 

parting at base_ __________________________ .3- .5

McGregor limestone member (Magnolia of Bays and 

Raasch, 1935):

Limestone, light-gray, very fine grained, very dense, 

conchoidal fracture like "glass rock" above, fairly 

massive, very fossiliferous; wavy upper surf ace __ 0. 9

Limestone as next above but less dense, medium 

bedded above to thin bedded below, fossiliferous; 

wavy xipper surface._______________________ . 7

Dolomite, light-olive-drab, fine-grained, "sugary,"

argillaceous, very thin bedded; nodular________ 1. 6

Dolomite as next above but thick-bedded; calcite 

near middle______________ ___________________ 3. 0

Limestone, thin-bedded yet stands massively as one 

unit; light-greenish-gray-brown, weathering 

brown, with a few argillaceous streaks; sparingly 

fossiliferous, but with fossils and fucoids on top 

surface_____________________________________ 2. 6

Limestone, thin-bedded as next above but the beds 

are distinct; nodular beds and shaly partings; argillaceous in upper 0.3 ft, which is very fossiliferous_ 3. 4

Limestone, light-buffish-gray, in medium to thick

beds; in places gradational into unit next below_ 3. 6

Total, upper McGregor____________________ 15.

FIGURE 42.—Pecatonica (Opp) and McGregor (Opm) members of Platteville formation in roadcut, U. S. Highway 151, 8 miles southwest of Platteville, Grant 

County, Wis. (fig. 35, loc. 6).

McGregor limestone member (Mifflin of Bays, 1938): 

Limestone, light-greenish- to bluish-gray, in massive 

beds but composed of thin beds which are not 

separated; ample shaly material in wavy bands; 

fairly fossiliferous, argillaceous; a peculiar mottled light-gray and darker gray 0.1-ft zone, 1 ft 

below top__________________________________ 3. 9
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 275

Platteville formation—Continued

McGregor limestone member (Mifflin of Bays)—Con.

Limestone, light-gray, very fine-grained, very dense, 

sublithographic, in extremely thin and nodular beds 

with thin calcareous shaly partings which become 

thinner below; the shale beds are light grayish 

blue, mottled, very fossiliferous; weathers slightly 

recessed,___________________________________ 4. 0

Limestone as next above, but beds are not quite so 

thin; fossiliferous; poor gastropod zone 1.7 ft 

above base; shaly zone at base________________ 3. 6

Limestone, dolomitic, light-gray, fine-grained, very 

slightly argillaceous, very fossiliferous, mediumbedded; indistinct argillaceous partings, not wavy; 

calcite and limonite, especially in basal 0.6 ft__ 3. 6

Total, Mifflin___________________ 15.1

Pecatonica dolomite member:

Dolomite, light-grayish-brown, very coarse grained 

and vuggy, upper 2 ft are a mixture of this 

lithology and a somewhat argillaceous fine-grained 

"sugary" laminated dolomite; a 1-ft bed of very 

vuggy dolomite from 1.8 to 2.8 ft above base, shaly 

in lower part; stylolitic partings 1 ft above base_ _ 4. 8

Dolomite, medium-gray, laminated, somewhat argillaceous, fine-grained "sugary", fossiliferous, especially in lower 0.9 ft; medium- to thick-bedded; 

shaly at top; weathers brownish in lower 2.5 ft__ 6. 9

Dolomite, medium-gray, laminated, argillaceous;

very fossiliferous partings.___________________ 3.6

Dolomite, light-grayish-brown, very coarse grained 

and vuggy; thin brownish gritty dolomitic and 

platy shale parting at top___________________ 1. 4

Dolomite, medium-gray, laminated, somewhat argillaceous, fine-grained_________________________ 2. 8

Dolomite, medium-gray, laminated, argillaceous, 

silty and sandy with fine to coarse quartz grains 

similar to those of the St. Peter sandstone, phosphate nodules abundant (especially in two zones, 

one at base, the other 1 ft above base)________ 2. 0

Total, Pecatonica__________________________ 21. 5

Glenwood shale member:

Shale, sandy with rounded quartz grains, khaki to 

drab, soft; phosphate nodules________________ 0.4

Shale, sandy, olive to grayish brown; mottled yellowish brown, friable.________________________ .2

Shale, sandy, medium- to dark-gray, olive, blocky,

very hard__________________________________ . 6

Shale, medium-gray, blocky, hard, sandy; streak of 

carbonaceous material at top_________________ . 3

Total, Glenwood_________________________ 1. 5

Total, Platteville- _ _ ____________ _____ 54. 3

St. Peter sandstone:

Sandstone, red and white; rounded, frosted, coarse to 

medium grains- _______________________________ . 1

Sandstone, gray, pinkish, very friable^_____________ . 2

Sandstone, brown, iron-cemented, hard____________ 0-0. 1

St. Peter sandstone—Continued

Sandstone, yellow to gray, very friable, with irregular

lower surface________________________________ 1.3

Sandstone, light-gray, very friable.________________ . 1

Sandstone, yellow to dark-brown, laminated, hard___ . 2 

Sandstone, gray and yellow; hard irregular lower

surface.______________________________________ 1. 1

Sandstone as next above, but medium- to fine-grained;

spalls_____-_______ ____________________________ 7 -\-

The Plattoville strata have been correlated with the 

Black Kiver of New York (Kay, 1935a, p. 288).

GLENWOOD SHALE MEMBER

The Glenwood is unit 1 of Bain's (1905) Platteville 

(p. 261). The type section in Glenwood township, Iowa 

(fig. 35, loc. 14), where Calvin (1906, p. 74-75) described 15 feet of shale beds, is no longer well exposedHis description, which is generahV applicable to exposures in Glenwood township, is as follows:

The sandstone phase [of the St. Peter] is overlain by a bed of 

shale fully fifteen feet in thickness, and the lower eight or ten 

feet of this is highly arenaceous. Some thin bands at intervals 

of a foot or more are almost pure sand. The sand grains in the 

shale are of the same clear, worn and polished type as those 

making up the main body of the sandstone deposit. The upper 

part of the shale bed is quite free from sand and resembles the 

"basal shale" referred to the Trenton series in Allamakee and 

Dubuque [Counties]. The lower arenaceous part should, without any doubt, be regarded as the closing phase of the St. Peter 

stage.

IITHOIOGIC DESCRIPTION AND STRATIGRAPHIC RELATIONS

In the zinc-lead district the Glenwood consists of 

greenish sandj^ and dolomitic shale (fig. 43), varying 

locally with thin shaty or sandj^ dolomite beds. The 

grains are rounded frosted white quartz sand similar

FIGURE 43.—Glenwood shale member (Opg) of Platteville formation, in contact 

with the Pecatonica (Opp) above and the St. Peter (Osp) below, in roadcut. U. S. 

Highway 61, Grant County, Wis. (fig. 35, loc. 15).
276 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

to those of St. Peter sandstone. They are unsorted 

and range from fine to coarse. Almost everywhere 

pyrite cement is present in the Glenwood, and the 

glittering pyritic sandstone fragments are very conspicuous in well cuttings. Many large black or darkbrown phosphatic pebbles and fossils are found near 

the top of the member; these pebbles are dull, subround, 

arid smooth.

The green sandy shale beds maintain an average 

thickness of 1—3 feet in the mining district, although in 

a few places (ledges, east bluff Little Platte River, in 

KEK sec. 4,T.3N.,K.lW.,2 miles north of Platteville, Wis.; roadcut, County Trunk M, SW}£ sec. 10, 

T. 2 N., R. 6 E., 8 miles northwest of Monroe, Green 

County, Wis., see fig. 35) the basal sandy dolomite 

bed of the Pecatonica rests directly on somewhat 

argillaceous relatively unsorted sandstone that in turn 

rests on the cleaner sandstone of the St. Peter. In the 

study of subsurface samples it is not clear whether 

this sandy dolomite bed is truly Pecatonica or is a 

facies of the Glenwood.

Similarly, in the subsurface studies it has been found 

that a variable thickness of the upper part of the sandstone bed immediately underlying the green sandy 

shale bed is related almost as closely in lithology to the 

Glenwood shale member as it is to the St. Peter sandstone below. This sandstone, containing some greenish 

argillaceous lentils, is unsorted in the upper part in 

contrast with the more or less well-sorted St. Peter 

below, which moreover does not contain so much 

coarse sand. This statement is supported by the findings of Thiel (1937, p. 113), who related that in southeastern Minnesota the petrographic constituents of 

the upper part of the St. Peter are more characteristic 

of the Glenwood than of the major part of the St. Peter. 

He found that the upper 3-10 feet of St. Peter are thin 

bedded and not as well sorted and consist of coarser 

sand and more fine silt and clay than does the greater 

part of the formation. Thiel noted that garnet is very 

abundant in the upper third of the Glenwood and decreases toward the base, where there is none; on the 

other hand, the zircon content increases downward. 

Zircon is dominant in the underlying St. Peter beds.

Thiel's studies led him to conclude that the sea of 

early Mohawkian age reworked the upper part of the 

St. Peter sandstone and that additional clastic sediments from new source areas, differing texturally and 

petrographically from the St. Peter, were transported 

to the basin of deposition and mixed with the St. Peter. 

He concluded that the sandstone thus formed was 

therefore an early phase of the Glenwood and should be 

classified as such. Bevan's (1926, p. 13) earlier work 

had resulted in similar conclusions.

Studies with the binocular microscope of subsurface 

samples from wells in southwestern Wisconsin and 

northeastern Iowa show that the thickness and relationships of these lithologic facies are very irregular, as 

follows:

Thickness of different facies of Glenwood shale member

Location 

No. 

(fig. 35)

16 

17

18 

19 

20 

21 

22

Name and location of well

Clayton County Home well, Elkader, 

Iowa, sec. 7, T. 93 N., R. 4 W__.___ __ _

Cheese Factory well, North Andover, 

Grant County, Wis., sec. 6, T. 4 N., 

R. 5 W.-_ — -—— — _.-—_ — - —

Rewey village well, Iowa County, Wis., 

sec. 5, T. 4 N., R. 1 E______. ___________

Cobb School well, Iowa County, Wis., 

sec. 26, T. 6 N., R. 1 E___ _ __ __ _____

Mount Horeb city well, Dane County, 

Wis., sec. 12, T. 6 N., R. 6 E_ __ . .

Colesburg city well 1, Delaware County, 

Iowa, sec. 4, T. 90 N., R. 3 W_._ _______

East Dubuque city well, Jo Daviess 

County, 111., sec. 29, T. 29 N., R. 2 W__.

Shullsburg city well 3, Lafayette County, 

Wis., sec. 10, T. 1 N., R. 2 E________ __

Monroe city well 4, Green County, Wis., 

sec. 34, T. 2 N., R. 7 E_ ____ ... _ ._

Coltman mine well, Lafayette County, 

Wis., sec. 10, T. 1 N., R. 1 E. (8 miles

Thickness (in feet)

Upper

part — 

Shale

9 

4

7

8

5

M

Middle 

part — 

Sandy 

dolomite

4

8 

5

K

Lower 

part — 

Sandstone

5 

5 

3+ 

10

14

10 

18

Likewise Grant 5 recorded the following section "at 

the water reservoir on the spur of the bluff just east of 

Prairie du Chien," Wis. (fig. 35):

Quarry rock, buff____________________

Limestone, like quarry rock_____

Sandrock, green___________________^_

Sandrock, white.______________^_____

Limestone, yellowish brown___________

Sandrock, white- ___________________

Limestone, gray-brown_______________

Sandrock, green___________________

Sandrock, brown to white, cemented to 

quartzite.

Thickness Present writers'

(feet) term

2 Pecatonica.

0. 1-0. 2 Pecatonica.

. 1 Glenwood.

. 5 Glenwood.

. 3 Glenwood.

3. 5 Glenwood.

1 Glenwood.

. 4 Glenwood.

6 St. Peter.

This section shows 5.8 feet of sandstone and limestone 

which represents the Glenwood interval, and is rather 

similar to that at the type locality, 35 miles to the west. 

Whether these differences in lithology are due to 

facies, as Bevan (1926), Bays (1938), and the present 

writers believe, or are due to the intermittent presence 

of one or more of the four members of the Glenwood 

formation of Templeton (1948)—or the four formations 

of the Glenwood subgroup of Templeton and William 

(1952, p. 21 and fig. 11)—the fact is that the Glenwood 

is rather inconsistent in lithology. Criteria for differentiating the four subdivisions of the Glenwood are not 

adequate nor are they useful in field mapping in the 

zinc-lead district. Sardeson (1933, p. 82, 90) recoms Field notes, U. S. Grant, September 3, 1902; in files of TJ. S. Geological Survey, 

Platteville, Wis.
STRATIGRAPHY OP MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 277

mended that the term Glenwood be dropped, as the 

strata are vertically transitional and laterally variable. 

However, because the green shale beds themselves are 

easily distinguishable it would seem advisable to confine the term Glenwood in the zinc-lead district to that 

mappable unit consisting of green sandy and in part 

dolomitic shale that underlies the buff dolomite of the 

Pecatonica and overlies the sandstone containing the 

rounded frosted unsorted quartz sand grains.

The so-called bentonitic clays of the Glenwood 

(Sardeson, 1926a, p. 385-392, pi. 26), which are mainly 

recrystallized orthoclase, contain well-rounded detrital 

grains of garnet, zircon, and tourmaline (Thiel, 1937, 

p. 121). According to Thiel this fact, coupled with the 

facts that X-ray analyses fail to show bentonitic 

clay minerals and that chemical analyses show more 

potassium than normal in the average volcanic rock, 

does not favor a volcanic origin for the shale.

The Glenwood shale member is overlain apparently 

conformably by the Pecatonica dolomite member; the 

writers do not agree with Elder's (1936) view that a 

disconformity exists. Evidences for the transitional 

contact are the lenses of greenish sandy dolomite in the 

upper part of the Glenwood, and the abundance of 

rounded quartz sand grains in the base of the overlying 

Pecatonica.

The lower contact of the Glenwood is likewise transitional, for the shale grades downward into an argillaceous sandstone very similar to the underlying 

St. Peter except in the lack of sorting, the finer grain 

size, and the presence of argillaceous material.

Templeton (1948) found that, particularly in northcentral Illinois (in the vicinity where Rock River crosses 

the LaSalle anticline and the Wisconsin arch; fig. 31), 

the Glenwood consists of four subdivisions, (1) sandstone and siltstone, (2) dolomite and sandstone, (3) 

sandstone, and (4) very argillaceous sandstone and 

shale, in ascending order. Whether Templeton's superposition of 4 members, or the facies concept of Bevan 

(1926, p. 10) is accepted, both of those authors recognized an unconformity at the base, separating sandstone of the Glenwood from the underlying St. Peter. 

This unconformity represents a greater span of geologic 

time farther east, for Pecatonica and even Mifflin 

beds of local usage rest directly on St. Peter sandstone 

(Willman and Payne, 1942, p. 61-62) in Ogle County, 

111., 20 miles southwest of Rockford (fig. 35).

In addition to differences in lithology, the presence 

of the unconformity at its base is physical evidence 

against including the Glenwood in the upper part of 

the St. Peter (Hershey, 1894, p. 174). Thus the Glenwood even as early as 1855 (Percival, 1855, p. 17; 

Chamberlin, 1877, p. 287, 293; Strong, 1877, p. 683;

Sardeson, 1897a, p. 26) was included in what is now 

known as Platteville.

DISTRIBUTION

Because of the shaly and friable sandy character of 

the Glenwood, the strata are usually not well exposed. 

However, excellent outcrops are in the roadcut southwest of Platteville, Grant County, Wis., along U. S. 

Highway 151 (fig. 35, loc. 6), in the roadcut along U. S. 

Highway 61 northwest of Platteville (loc. 15), in a 

roadcut along County Trunk A northwest of Platteville 

(loc. 23), and in a roadcut in Iowa County, northeast of 

Platteville (loc. 24).

FAUNA AND CORRELATION

Stauffer (1935, p. 128) presented a list of genera of 

megafossils reported from the Glenwood in the Minneapolis-St. Paul region, and described 85-90 species 

of conodonts and scolecodonts from outcrops in southeastern Minnesota. According to him many of these 

are certainly forms of Mohawkian age, although there is an 

element of the conodont fauna that is decidedly older 

than the Platteville and never found indigenous in the 

Mohawkian; these may be residual forms weathered 

out of older beds and incorporated in the younger 

(Glenwood) beds as they were being deposited.

The Glenwood has been considered to be of Lowville 

(fig. 44) age (middle Black River of the standard New 

York section) by Kay (1935a, p. 288) because of its 

conodonts of Mohawkian age and stratigraphic position 

with regard to the Pecatonica (see also Twenhofel and 

others, 1954).

ECONOMIC PRODUCTS

Commonly the sand grains of the Glenwood are 

cemented by pyrite, and less commonly by iron oxide. 

The pyrite appears to be more abundant in areas where 

zinc, lead, and iron minerals are present in the beds 

above. Locally, minor amounts of zinc and lead minerals have been found in the Glenwood. None of these 

occurrences has proven commercial.

PECATONICA DOLOMITE MEMBER

The Pecatonica is unit No. 2 of Bain's (1905) Platteville (p. 261). There is no type locality described for 

the Pecatonica dolomite member, but its lithology is 

substantially uniform regionally. The lithology in the 

mining district is similar to that shown in the 20 feet 

of Pecatonica dolomite member exposed in the bluff 

along the Pecatonica River at Lattice Bridges (NW%- 

NW% sec. 21, T. 1 N., R. 6 E.), Green County, Wis. 

(2 miles west of loc. 45, fig. 35), which may be designated as the type outcrop. The section about 8 miles 

southwest of Platteville, Wis. (loc. 6), will serve to 

convey a general picture of the sequence (p. 275).
278 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

New York-Ontario

Trenton

Black 

River

Collingwood

Upper Cobourg

Lower Cobourg

Sherman Fall

Hull

Rockland

Chaumont

Lowville

Pamelia

Chazyan

Kay, 1935b 

fig- 10

Dubuque

Stewartville

7

Prosser 

Pros

_ . Deco Decorah

_L

Platte 

Platteville

III III 1 II 1 II 1 II

St. Peter

Kay, 1935a 

fig. 211

Dubuque

Stewartville

7

Fusispira

ser Nematopora

Vellamo

Ion

Guttenberg

H__ __ _±_^__

Spechts Ferr

McGregor

miff

St. Peter

Bays-Raasch 

1935, p. 300-301

Galena*

Decorah

y

— ' Platteville

Stewartville 

Prosser

Ion 

Guttenberg

Spechts Ferry 

Magnolia

McGregor 

Pecatonica 

Glenwood

Kay and Atwater 

1935, fig. 1

Galena

Decorah

Platteville

Prosser

Ion

Guttenberg

Spechts Ferry

Lower Buff

Glenwood

|| mi || 1 1|

Kay, 1948, 

p. 1402

Trenton ian

Bolarian

Chazyan

: Bays and Raasch consider the Dubuque member of the 

Galena formation to be early Cincinnatian in age

FIGURE 44.—Suggested correlatives of Platteville, Decorah, and Galena strata, by various authors.

IITHOIOGIC DESCRIPTION AND STRATIGRAPHIC RELATIONS

The beds constituting the Pecatonica dolomite 

member of the Platteville formation are dolomite that 

is medium grained, sugary, buff to gray where freshly 

broken, weathering-brown, as a rule in fairly thick beds. 

Subordinate amounts of light-brownish platy shale 

occur along the partings, especially in the upper more 

thinly bedded part.

The Pecatonica is known locally by the name "quarry 

rock" (Strong, 1877, p. 682) because in almost all parts 

of the district building-stone blocks were obtained 

from quarries in these strata.

The thickness of the Pecatonica within the mining 

district is 20-24 feet.

The basal foot or so of the Pecatonica is usually 

marked by sandy dolomite with many phosphatic 

nodules and pebbles. The sand is clear quartz grains, 

coarse to medium in size, well-rounded, and somewhat 

frosted, typical of the St. Peter. A good exposure of 

this contact is in the readout in the SE%NE%SW% 

sec. 35, T. 5 N., R. 1 E., Iowa County, Wis. (fig. 35, 

loc. 18). Pettijohii (1926) has described the phosphatic 

pebbles in detail.

In the mining district the upper strata of the Pecatonica are, in most localities, thin and nodular bedded; 

in this respect they appear much like the lower beds of 

the McGregor above. This fact tends to make precise

determination of the contact between the Pecatonica 

and McGregor difficult in certain areas; on the whole, 

however, the beds are distinctive, because throughout 

most of the mining district the Pecatonica is a mediumbedded dolomite whereas the overlying McGregor is 

generally a thin-bedded limestone (fig. 42).

Bays 6 stated that the dominant heavy minerals in 

the Pecatonica are garnet, tourmaline, and zircon in 

order of decreasing abundance, thus similar to the 

heavy mineral suite found in the Glenwood. Dake 7 

found the Pecatonica to have a low percent of insoluble residue; the insoluble material is dolomoldic chert 

(Ireland, 1947, p. 1481), angular quartz, and feldspar.

DISTRIBUTION

The Pecatonica is normally resistant and forms cliffs 

and bluffs; for this reason, and because it has been 

quarried extensively, there are many good exposures. 

Some of the better outcrops are listed below:

Roadcut, U. S. Highway 151, Grant County, Wis. (fig. 35, loc. 6). 

Roadcut, U. S. Highway 61, Grant County, Wis. (loc. 12). 

Quarry, Potosi station, NW}iNEK sec. 9, T. 2 N., R. 3 W.,

Wis., Grant County, 3 miles southwest of above. 

Quarry, 3 miles southwest of Mineral Point, NEJ4NWJ4 sec. 34,

Iowa County, T. 4 N., R. 2 E., Wis.

6 Bays, C. A., 1937, Stratigraphy of the Platteville formation. Unpublished 

Ph. D. thesis, Wis. Univ., Madison.

7 Dake, L. F., 1937, Insoluble residues of the Mohawkian series of Wisconsin. 

Unpublished Ph. D. thesis, Wis. Univ., Madison.
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 279

City quarry, Mineral Point, SE#SE# sec. 31, T. 5 N., R. 3 E.,

Iowa County, Wis.

Roadcut, 1 mile southeast of Mifflin, Iowa County, Wis. (loc. 24). 

Ravine, 2 miles southeast of Meekers Grove, SE%NE}£ sec. 27,

T. 2 N., R. 1 E., Lafayette County, Wis. (1 mile southeast of

loc. 7).

FAUNA AND CORRELATION

Contrary to its unfossiliferous appearance on the 

weathered surface at the outcrops, the Pecatonica is 

moderately fossiliferous. However, because the Pecatonica is dolomite throughout the district, its fossils 

are likewise dolomitie, and are preserved as molds and 

casts. The coarseness of grain and sugary nature of 

the dolomite result in most of the fossils being very 

poorly preserved, and they tend to crumble. The megafossils are mainly molluscan but have not been adequately described. No microfossils have been reported 

from the Pecatonica.

The Pecatonica has been correlated with the Black 

River of the standard New York section by Kay 

(1935a, p. 288), who stated that "inconclusive evidence 

suggests the correlation of the Glenwood and Pecatonica" with the Lowville, while Bays and Raasch 

(1935, p. 300) said that "the Lower Buff [Pecatonica] 

member is definitely pre-Lowville and equivalent to a 

part of the Stones River" of Tennessee. Bays (see p. 

278) correlated the Pecatonica with part of the Plattin 

of Missouri, and Twenhofel, and others (1954) joined 

him in correlating the Pecatonica with part of the 

Pamelia (lower Black River) of New York (fig. 44).

ECONOMIC PRODUCTS

The Pecatonica strata were a poor host for lead and 

zinc minerals, and only locally are even small quantities 

of galena and sphalerite found. Evidence of iron mineralization is similarly uncommon, although significant 

amounts were seen (Agnew, Flint, Crumpton, 1954) in 

samples from holes drilled at the Alderson mine in SE% 

sec. 32, T. 2 N., R. 2 E., 2 miles northwest of Shullsburg 

(fig. 35). As noted earlier (p. 278), the Pecatonica has 

been quarried for building stone.

McGREGOR LIMESTONE MEMBER

Kay's (1935a, p. 287) description of the type section 

of the McGregor recorded:

* * * 21'6" of gray-blue, rather fine-textured, fossiliferous limestone with greenish shale partings and interbeds, the latter most 

prevalent in the middle part of the section. The thickness is 

normally from 20 to 25 feet, and is about 27 feet in the type 

section of the Platteville formation.

The type section is rather poorly exposed, as it is in 

a rather shallow ravine; thus measurements of thickness may not be accurate. A more complete description 

of the exposure is given in section 1, p. 305.

389000—56———5

LITHOLOGIC DESCRIPTION AND STRATIGRAPHIC RELATIONS

The representative lithology of the beds of the 

McGregor limestone member in the central part of the 

district can be taken from the typical Platteville section 

(p. 274). A dolomite facies of the McGregor begins in 

the central part of the mining district (fig. 34) and continues eastward to the vicinity of Beloit, Wis. and Rockford, 111.

These beds, called the Trenton in early reports (Hall, 

1862, p. 33) and Blue and Lower Blue in others, are 

still known as Trenton by the well drillers and mining 

men of the district (fig. 45).

In describing the Mifflin beds of local usage Bays 

(1938) said that the unit consists of thin-bedded limestone that passes laterally into dolomitie limestone and 

dolomite, and at the type locality is \1% feet thick. 

Furthermore, the rock is characterized by fossils preserved on bedding planes, and by a blue color when 

fresh (footnote p. 278). Weathering produces bluishgray to grayish-buff colors. The thickness ranges 

between 16 and 18 feet in the mining district, and becomes greater to the east of the Wisconsin arch where, 

near Beloit, Bays found it "nearly 25 feet" thick. The 

dominant heavy mineral of the Mifflin is zircon, he 

said. Dake (footnote p. 278) found that the amount 

of insoluble material in the eastern facies of the Mifflin 

(of Bays 1938) is greater than that in the Pecatonica, 

and that this material consists almost wholly of lightgray shale, partly dolomoldic.

Referring once more to the typical Platteville section 

(p. 274), the limestone is typically light gray, very fine 

grained, very dense, and very fossiliferous; it is in thin 

nodular beds plastered with shale partings of light 

grayish blue. In many places these shales are brownish and resemble somewhat the oil-rock type of shale, 

discussed under the Guttenberg member (p. 290).

Bays (p. 278) stated that the basal contact of the 

Mifflin is well marked "in many quarries in the Lead 

and Zinc District" by a conglomerate bed about 1 foot 

thick, composed of "irregular and subangular pieces of 

buff dolomite in a bluish-gray, clayey, fine-grained 

limestone matrix." Bays stated further that over 

parts of the Wisconsin arch, in Dane, Green, and Iowa 

Counties, Wis., a calcareous green or blue shale, 10 

inches to 1 foot thick, occurs as the basal bed of the 

Mifflin.

Observations made in the course of the present 

study, based on both outcrop and subsurface data, 

tend to corroborate the sporadic occurrence of the 

shaly bed at the contact of the McGregor and 

Pecatonica, but 110 exposure has been found that 

illustrates the "conglomerate," despite the fact that 

several of the conglomerate localities listed by Bays 

were visited by the present writers. We feel that the
280 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Maquoketa

Galena

Decorah

Platteville

St. Peter

Owen 

1840

Upper limestone Lower Magnesian of Clifpart or 

Arenaceous

Arenaceous, stratified; chert

Blue 

fossiliferou*s 

limestone

Buff 

limestone

Saccharoid 

sandstone

Phillips 

1854

\m^

J-^^Shale beds

Cap rock 

(Calico rock and 

shingle rock)

f

Arenaceous 

limestone 

(Sand rock and 

crevice rock)

Flint-bearing 

limestone

^;>

Blue 

limestone

Sandstone

Percival 

1855-1856

Blue shale

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0

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0)

c

0

</>

0)

E

0) 3

00

Upper 

bed

Mid 

be

a 

\ *-° V

IAQ. 0) r>-°

13 

0)

.a 

a>

13 

13'^.

die

d

/

Ner i 

ed /

Lower 

bed

/Upper 

sandstone

Whitney 

1862

Hudson 

River group

mestone

(D

c

_0) 

(D

O

\

a>

c

0 

I/) 

a>

E

a>

3 

00 J

Bu 

limes

Thin-bedded, argillaceous

Massive

^9\ 

^^»\ lithology Irregular chert ;

\^ •r 

i»£ o 00 *"

^ 

o

0

ff 

tone

St. Peters 

sandstone

Miners' 

terms

Shale or 

slate

Buffer 

sandy

Drab

Gray

Blue

Oil rock

Clay bed

Glass rock

Trenton

Quarry beds

Sand rock

Classification used 

in this paper

Maquoketa

Galena

Decora h

(Platteville 

Dubuque •> 

Stewartville J •

0) </> </> o

ol

c 

Ci

unit Noncherty 

'c

3 

>x 

t 

0) 

.G 

CJ

P

A

B

C

D

Gray

Blue

Guttenberg

Spechts Ferry

--• — *- —— - —— - —— --— -^x- —— ^> — - —— s — ^ — -~_^- —— * —— ^

Quimbys Mill

McGregor

Magnolia (of 

Bays and Raasch, 1935)

Mifflin 

(of Bays, 1938)

Pecatonica

Glenwood ^

St. Peter

FIGTIEE 45.—Stratigraphic terminology of early geologists compared with that used by miners and drillers and with the classification used in

this paper.
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 281

bed is not a sedimentary conglomerate; it might be due 

to secondary dolomitization along a favorable horizon, 

or might be the corrosion zone noted by Sardesou 

(1898, p. 318 and pi. 9) at the top of his Buff limestone 

in the St. Paul-Minneapolis area.

The lithologic distinction between the brownish 

argillaceous dolomite beds of the Pecatonica and the 

light-gray dense fossiliferous generally calcareous beds 

of the Mifflin (of Bays 1938) in the mining district is 

somewhat offset by the similarity in thickness of 

bedding in the upper part of the Pecatonica (p. 278; 

fig. 42).

The Mifflin (of Bays 1938) of local usage contains a 

fauna rich in gastropods; in the limestone facies 

bryozoa and microfossils are particularly abundant, 

but, except for ostracodes, are nearly always absent 

in the dolomitic facies.

In the lower part of the Magnolia (of Bays and 

Raasch 1935) the surfaces of the beds bear great 

numbers of the ostracode (Leperditia sp. aff. L.fabulites 

Conrad and large specimens of LarnbeophyUum projundum (Conrad). An abundant molluscan fauna, 

remarkably similar to that in the Pecatonica, has been 

obtained from the Beloit region. Bays and Raasch 

(1935, p. 298) stated that in western Wisconsin the 

Magnolia is thinner and less conspicuous, and that in 

the "Lead Region it tends to be more limey and more 

thinly bedded than in the type area."

Referring to the typical Platteville section (p. 274), 

the limestone and dolomite of the Magnolia (of Bays 

and Raasch 1935) are typically light to medium gray, 

slightly mottled with dark-gray markings, fine to 

medium grained, and contain some soft dove-gray to 

olive-brown shale. The strata are thin or of medium 

thickness, and in the lower part look not unlike the 

Mifflin (of Bays 1938). In many places the upper 1-2 

feet are very fine grained, very dense, light-gray 

fossiliferous limestone that breaks with a conchoidal 

fracture and is difficult to distinguish from the "glass 

rock" beds above, the difference in color being the 

principal distinctive feature, as the "glass rock" beds 

are purplish brown on a freshly fractured surface. 

Further stratigraphic study of this part of the Platteville formation to the east and southeast may show 

these "glassy" McGregor strata to be a distinctly 

recognizable lithologic unit.

Chert is present about 12 feet below the top of the 

Magnolia (of Bays and Raasch 1935) in the Calumet 

and Hecla mines in sec. 22, T. 1 N., R. 2 E., 2 miles 

south of Shullsburg, Wis. (fig. 35), and in exposures to 

the east.

Bays has recorded two localities in the mining 

district in which limestone conglomerate occurs at the 

base of the Magnolia (of Bays and Raasch 1935), one

of these being in the small quarry south of the city 

quarry at Mineral Point. Nowhere was any such 

conglomerate observed during the studies here reported, all observations -in the mining district pointing 

rather to a gradational and obscure contact between 

the Mifflin (of Bays 1938) and the Magnolia (of Bays 

and Raasch 1935).

According to Bays (p. 278) the heavy minerals of the 

Magnolia in order of decreasing abundance are garnet, 

tourmaline, and zircon. Dake (p. 278) found less 

insoluble material in the eastern facies of the Magnolia 

(of Bays and Raasch 1935) than in the Pecatonica. 

The lower part of the Magnolia has more residue than 

does the upper; the residue is principally quartz and 

feldspar.

The fauna of the Magnolia has not been adequately 

described. These strata are far less fossiliferous in the 

mining district than are those of the Mifflin (of Bays 

and Raasch, 1935).

It has been noted that the Magnolia is only poorly 

exposed in the type locality at Magnolia. However, 

where seen in and near the mining district, the thin 

dense nodular limestones of the Mifflin (of Bays 

1938) are very distinctive, as are the thicker bedded 

less dense dolomite and limestone strata termed the 

Magnolia by Bays. But, commonly no definite 

lithologic break is present between the two units, and 

the beds just above or below the contact are transitional 

through a zone of 2-3 feet. Moreover, two adjacent 

outcrops may show the contact at somewhat different 

stratigraphic horizons.

These views regarding the transitional nature of the 

rocks are supported by the fact that in the mining 

district the combined thickness of the two sets of beds 

rarely departs appreciably from the average of 28-31 

feet, yet the two lithologic types commonly show 

complementary thickness variations within these limits. 

The following table illustrates this relationship :

Thickness of Mifflin (of Bays, 1938), Magnolia (of Bays and

Raasch, 1935), and McGregor

[Localities shown on fig. 35]

Location of outcrop

Ravine, west side valley, in SEJ4NEJ-4 sec. 25, T. 3 

X., R. 3 W., Grant County, Wis. (3 miles north of 

loc. 12) _________________________________________

Quarry, Spechts Ferry station Dubuque County, 

Iowa (loc. 10) __________ _ _________ _________ _

Roadcuts, U. S. Highway 61, Grant County, Wis. 

(loc. 12)_________________________________ __________

Roadcut, U. S. Highway 151, Grant County, Wis. 

(loc. 6)_____ _______________________________________

Cltv quarry, Mineral Point, in 8EJ.iSEi.iSEJ.4sec. 

31, T. 5 N., R. 3 E., Iowa County, Wis____-___--_

Quarry, Darlington, Lafayette County, Wis. (loc. 

13) — —— ———— —— - ———— _„ ——— - ——— — —

Thickness, in feet

Mifflin 

(of Bays 

1938)

19.5

16.5 

15.7 

15 

19 

18 

13

Magnolia 

(of Bays 

and 

Raasch 

1935)

9

11 

13.3 

14.5 

12 

12 

15

Total

28.5

27.5 

29.0 

29.5 

31.0 

30.0 

28.0
282 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Thus it would appear that, although both the Mifflin 

and Magnolia have distinctive lithologic characteristics, 

the boundary between them is somewhat arbitrary. 

The retention in the mining district of the member term 

McGregor with the two facies, rather than substituting 

two member terms is therefore preferable for the beds 

above the Pecatonica member and below the Quimbys 

Mill member of the Platteville formation.

DISTRIBUTION

The beds of the McGregor are not so well exposed as 

the Pecatonica because of the thin-bedded character, 

but form bluffs where overlying and underlying strata 

are present. Quarries supplement outcrops along 

streams. Representative exposures are at the localities 

given on page 281.

FAUNA AND CORRELATION

The Mifflin and Magnolia faunas of the McGregor 

limestone member are clearly of Black River age, but 

there is disagreement among authors as to the precise 

position within the Black River. Kay (1935a, p. 288) 

stated that the McGregor (and the Spechts Ferry 

above) is equivalent to the Chaumont (upper Black 

River), whereas Bays and Raasch (1935, p. 300) implied that the McGregor should be placed in the Lowville (middle Black River). Bays (p. 278) later went 

even farther, in correlating his Mifflin with the upper 

Pamelia (lower Black River) and asserting that the 

Magnolia transgresses the Pamelia-Lowville time line 

(fig. 44). In the Missouri section a part of the Plattin 

is the equivalent of the McGregor (Twenhofel, and 

others, 1954). As for the Tennessee-Kentucky sequence, according to Bays the nearest equivalent of the 

Mifflin is found in the Stones River there. The McGregor also includes the Vanuxemia bed of Minnesota 

(fig. 37) as used by Winchell and Ulrich (1897).

ECONOMIC PRODUCTS

McGregor strata constitute a potential zinc-lead 

mining zone. Zinc-lead ore has recently (1948-1952) 

been found in these beds south of Shullsburg, Wis., and 

south of Galena, 111. Within 2 miles northwest of Shullsburg these beds have been mined for zinc and lead at the 

old Mulcahy and the Lucky Hit mines (Agnew, Flint, 

Crump ton, 1954). Isolated occurrences of zinc-lead 

ore have been found at other places—Etna mine, a mile 

north of locality 27, figure 47 (Agnew, Flint, Crumpton, 

1954) and Last Chance mine, 2 miles south of locality 

4, (Lincoln, 1947). The ore-bearing potential of the 

McGregor member appears to be better in the eastern 

and central parts of the district than in the western 

part. In mineralized areas beds of the McGregor have 

been leached to a grayish clayey mass, with an accompanying reduction in thickness.

QUIMBYS MILL MEMBER

At its type locality (fig. 46) the Quimbys Mill member 

consists of 6 feet of dolomite above 6 feet of limestone, 

as follows:

Quarry, Quimby's mill, southeast corner, sec. 11, T. 1 N., R. 1 E., 

Lafayette County, Wis.

[Described by A. F. Agnew, August 25, 1944; revised November 20, 1945]

Thickness Decorah (feet) formation:

Spechts Ferry shale member (clay bed) :

Shale, olive, calcareous- ________________________ 0. 3

Limestone as below, weathers very ropy __________ .8

Limestone, light-grayish-buff, fine-grained, dense, 

very fossiliferous; phosphatic nodules____---____ 1. 5

Bentonite, a white plastic clay, weathers orangeLimestone, light-olive-gray, very dense, very thin

. 2

Total, Spechts Ferry_________________ ______ 2. 9

Platteville formation:

Quimbys Mill member ("glass rock"):

Dolomite, light-brown, fine grained, "sugary,"

dense, thin-bedded___________________________ 6. 0

Limestone, dark-purplish-brown, very fine grained, 

very dense, conchoidal fracture; in thin, nodular 

beds, with dark-brown shale partings; half an 

inch of platy fossiliferous dark-brown shale at 

base______________-______-___--_-_---_------ 6. 0

Total, Quimbys Mill_____________-_-------- 12. 0

McGregor limestone member (Magnolia beds of Bays

and Raasch, 1935; "Trenton"): 

Limestone, light-gray, pinkish, fine-grained, dense;

conchoidal fracture (the "glassy" McGregor)____ 1. 5 

Limestone, light-gray, fine-grained, thin-bedded._ 5 +

LITHOIOGIC DESCRIPTION AND STBATIGBAPHIC GELATIONS

Typically a densa sublithographic limestone, the 

Quimbys Mill is the "glass rock" of the miners, a clearly 

recognizable unit present in the eastern and central 

parts of the district.

Quimbys Mill strata in the western part of the mining 

district are limestone and subordinate amounts of shale, 

whereas in the eastern part of the district dolomite and 

a small amount of shale are characteristic. In a northeastward-trending area across the central part of the 

mining district the limestone and dolomite beds intermingle.

The name "glass rock" is probably derived from the 

characteristic fracture. The designation is found in 

the description of the quarry at Quimby's mill in early 

reports on the district (Whitney, 1862, p. 163). Until 

the geologic study (Grant, 1903, 1906; Bain, 1905, 

1906) about the beginning of the present century, the 

term "glass rock" was loosely applied to a lithologic
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 283

FIGURE 46.—Type section of Quimbys Mill member (Opg) of Platteville 

formation, with Spechts Ferry (Ods) above and McOregor (Opm) 

below, in quarry at Quimbys Mill, Lafayette County, Wis. (fig. 35, 

loc. 27).

facies, sometimes to what is here called the Quimbys 

Mill member, sometimes to the underlying part of the 

McGregor called Mifflin (of Bays 1938), and often to 

the lower lirny beds of the overlying Guttenberg. The 

surveys of Grant and Bain in the early 1900's applied 

the term "glass rock" especially to the Quimbys Mill 

beds but mentioned the fact that the lower beds of the 

McGregor had been and occasionally still were referred 

to as "glass rock."

From that time until the present, however, the 

custom has prevailed among the mining men of the 

district to designate as "glass rock" only those beds 

now known as Quimbys Mill. Thus in practice the 

"glass rock" beds are a generally recognized stratigraphic entity.

In the western part of the mining district the Quimbys 

Mill is less than a foot thick and is overlain by bentonite 

and interbedded limestone and shale of the Decorah 

formation (see section 2, p. 305). East of the mining 

district, where Spechts Ferry strata are absent, the 

Quimbys Mill is overlain by dolomite of the Guttenberg 

member of the Decorah formation. A regional disconformity therefore exists at the contact of the Quimbys Mill (Platteville) and Decorah. The Quimbys 

Mill rests conformably on the McGregor member.

Regional dolomitization eastward is shown by the 

complete dolomite section at Shullsburg and to the east 

(Agnew, 1950; Herbert, 1946).

The Quimbys Mill thickens locally to more than 18 

feet in an area just southeast of Shullsburg, Wis. 

However, eastward from the central part of the mining 

district the unit is 13—14 feet thick and becomes somewhat cherty (see section 3, p. 305).

Locally the mineralizing solutions leached the calcareous elements from the Quimbys Mill, increased the 

relative brown shale content- -called "oil shale" or "oil 

rock" by many geologists (Scott, Behre, 1935), although 

this term should be reserved for the Guttenberg member 

of the Decorah formation—and noticeably diminished 

the thickness of the unit (Agnew, 1950). For example, 

in diamond-drill holes (U. S. Bureau of Mines 20 and 

17) only 75 feet apart at the Bautsch mine south of 

Galena, 111. (fig. 35, loc. 30), thicknesses of 13 feet (near 

normal) and 4 feet, respectively, were recorded.

Dolomitization accompanying the zinc-lead mineralizing solutions notably affected the Quimbys Mill, 

resulting in a more granular or sugary texture (Agnew, 

1950). In such dolomitized rock other primary lithologic characteristics still are present. Less commonly, 

silicification accompanying the zinc-lead-bearing solutions caused parts of the beds to be replaced by silica 

so faithfully that the only megascopic difference is in 

the hardness (Agnew, 1950). Rarely, secondary chert 

nodules were formed.

Bays (p. 278) stated that the cherty dolomite (eastern) 

facies contains a suite of heavy minerals with garnet 

dominant, similar to that in the underlying Magnolia 

(of Bays and Raasch 1935); this is in contrast to the 

overlying Spechts Ferry shale member, which is characterized by zircon. Dake (p. 278) found that the 

Quimbys Mill contains less insoluble material than does 

the Magnolia; the residue is silicified fossils and some 

buff to brownish-gray shale. Aberdeen 58 found almost 

no residue in Quimbys Mill strata near Platteville.

DISTRIBUTION

Because of their dense and resistant character the 

Quimbys Mill commonly crops out; moreover, 

because these beds are valuable as building stone, 

quarries in them are common. Several of the better 

exposures of the Quimbys Mill are listed below, together with the thickness and general lithology:

58 Aberdeen, Esther J., 1931, The location of the break between the Galena and 

the Platteville limestones: unpublished M. S. thesis. Northwestern Univ., Evanston, 

111.
284 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Thickness and general liihology of Quimbys Mill member

Thickness 

Location Lithology (feet)

Bluff, north side valley, in NWJ4NEJ4 Limestone and 12

see. 7, T. 1 N., R. 2 E., Lafayette dolomite

County, Wis. (2 miles east of loc.

27, fig. 35). 

Quarry, Darlington, Lafayette Dolomite___ 10+

County, Wis. (loc. 13). 

Quarry 1 mile north of Darlington, _____do_______ 14. 2

in bWJ4SE% sec. 27, T. 2 N., R.

3 E., Lafayette County, Wis. (2

miles north of loc. 13). 

Quarry, }i mile east of Calamine, _____do_______ 12

Lafayette County, Wis. (loc. 42). 

Quarry, Lafayette County, Wis. (loc. ____do ______ 8. 4

31). 

Quarry, }i mile north of York Church, ____do_______ 13

Green County, Wis. (loc. 44). 

Quarry, along Honey Creek, Green _. -_do_______ 13.2

County, Wis. (loc. 45). 

City quarry, Mineral Point, in SE}4 __ do_______ 11

SEJ4 sec. 31, T. 5 N., R. 3 E., Iowa

County Wis. 

Small ravine from north, at bend of Limestone and 8±

Fever River, in SWINE'4 sec. 22, dolomite. 1

T. 2 N., R. 1 E., Lafayette County,

Wis. (1 mile northeast of loc. 7). 

Ravine from east, east of farm house, Limestone.__ 8±

in SWj4NW}4 sec. 26, T. 2 N., R.

1 E., Lafayette County, Wis. (1

mile southeast of loc. 7). 

fcteep ravines from east, in SE}4 sec. Limestone and 8±

27, T. 2 N., R. 1 E., Lafayette dolomite. 1

Countv, Wis. (1 mile south of loc.

7). 

Quarry, west bluff of valley, near Limestone____ 3

center NE}£ sec. 8, T. 3 N., R. 1 W.,

Grant County, Wis. (2 miles northwest of Platteville). 

Roadcut, County Trunk E, near ___ do_______ L 5

center W%E>£ sec. 18, T. 5 N., R. 1

W., Grant County Wis. (3 miles

northwest of loc. 4). 

Roadcut, County trunk O, in NW}£ ____do_______ 2

NE# sec. 35, T. 3 N., R. 2 W.,

Grant County, Wis. (2 miles northwest of loc. 6). 

Roadcut, U. S. Highway 61, Grant Limestone and 1

County, Wis. (loc. 12). dolomite. 

1 This dolomite is attributed to the lead-zinc mineralizing solutions.

FAUNA AND CORRELATION

Strata of the Quimbj^s Mill are fossiliferous, particularly in the limestone facies; the fauna has not yet been 

described, although Aberdeen (p. 283), Bays (p. 278), 

and Kajf (1929, p. 657) prepared fossil lists.

Bays correlated beds of the Quimbys Mill with the 

Lowville (middle Black River) of the standard New 

York section (fig. 44), although Kay (1935a, fig. 11) 

shows its position to be equivalent to part of the 

Chaumont. Until further paleontologir work has been

completed only tentative correlation of these strata 

with those in other areas can be accomplished.

ECONOMIC PRODUCTS

The Quimbys Mill ("glass rock") constitutes one of 

the ore-bearing zones and has been prospected and 

mined particularly since 1940 (Agnew, and Heyl, 1947, 

p. 228).

The limestone of the "glass rock" is suitable for a facing stone and has been used for that purpose in many 

areas, particularly in the central part of the district.

DISTRIBUTION OF FACIES OF PLATTEVILLE AGE AND CONDITIONS OF DEPOSITION

The Platteville strata are rather uniform in 

except for the Glenwood shale member whose facies 

relationships are not completely known as yet, and 

were products of a marine environment.

The Pecatonica is a dolomite or dolomitic limestone 

throughout the area of study. On the other hand , the 

McGregor and the Quimbj^s Mill are limestone to the 

west, limestone and dolomite in the central part of the 

mining district, and dolomite to the east. Both the 

Magnolia beds of the McGregor member, and the 

Quimbj^s Mill become cherty to the east. The chertification and dolomitization are apparently related to the 

major structural axis, the Wisconsin arch; similar 

relationships of dolomite and structure have been discussed for north-central Illinois by Willman and Pajnie 

(1942, p. 64-65), and in nearby states by Cohee (1948, 

p. 1432).

The clastic quartz sandstone, dolomite, and shale of 

the Glenwood shale member show evidence of relatively 

coarse deposition under rather shallow open-water 

"platform" conditions (Krumbein, 1947). The alternation of coarse clastic material with the finer clays and 

the thin bedding, which is commonly somewhat obscure, are both characteristic of shallow-water environment ; the tendency toward crossbeddiiig seen in southeastern Minnesota and the area south of Rockford, 111., 

marks deposits formed under conditions prevalent in 

an environment of very shallow water.

Pecatonica strata may have been laid down as a 

clastic limestone platform deposit (Sloss 1947), although 

the massive bedding characteristic of the Pecatonica is 

cited by Rich (1951) as a criterion for deep-water 

deposition. The general absence of fossils also tends to 

support Rich's deeper water origin. Furthermore, 

although the Pecatonica is now a granular sugary 

dolomite, the original limestone may have been fine 

grained; if so, Rich's relatively deep water origin for the 

Pecatonica sediments gains credence.

Miffliii strata (of Bays 1938) of the McGregor member were generally of shallow-water platform origin
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 285

(Sloss, 1947), as shown by the thin nodular beds of Decorah formation—Continued ""uSf

limestone separated by silty and clayey fragments, Limestone, bluish-gray, medium-to coarsely crystal-

,,, i j , - . • , j i line, fossiliferous_____________---_______------ 1.0 although deeper water environment is suggested by ____

fineness of grain and the sublitliographic type of lime- Total, Ion ____________„________________ 17. 4+

stone, coupled with the slightly bituminous character of —

some of the shale partings. The highly fossiliferous Guttenberg limestone member ("oil rock"):

nature of the Mifflin strata is characteristic of shallow- Transition beds-limestone, buffish, medium-

, . crystalline, fossiliferov:s__ _____________________ .9

\v ater deposits. Limestone, brown, thin-bedded, fine-grained, fosMagnoha strata (of Bays and Kausch, 1935) of the siliferous, band of chert nodules 3.5-feet below

McGregor member are similar to the Mifflin strata top_____ -------_--------------------------_ 5.0

except that the deep-water characteristics are absent; Limestone, brown, fine-grained, dense, nodular;

shallow-water environment is therefore indicated. interbedded brown platy shale—— — ——— 6.3

In the Quimbys Mill strata we find the same type of Total Guttenber ________ ______ 122

conflicting evidence as that seen in the Mifflin except ' ====_

that in these platform (Sloss, 1947) deposits shallow- Spechts Ferry shale member (clay bed):

water criteria are less abundant and apparently of less Shale, olive, calcareous; trace of orange bentonimportance than the deep-water characteristics. The ite ( ?) --------------------------------------

T • -I -i -i • , • , f ,-, T i IT.I i • Limestone, light-brown to cream; fossil fragments highly bituminous nature of the shale, the lithographic ^ phosphate nodules_______________________ . 8

type of limestone, and the general absence of fossils Shale7 olive; brown and greell mottled fine-grained

(except on the shale partings) are dominant character- argillaceous limestone; brown platy shale. ______ . l

istics. However, thinness and evenness of bedding, Limestone, light-brown, fine-grained, dense,

reeflike concentrations of fossils, and interbedded shale nodular— ______ _________________________ 1.1

, . i n , • • f ,1 T , Limestone, light-brown, dense, nodular, wavy- strata suggest a shallow-water origin for these sediments. bedded . parting of tan platy shale at top_ ______ . l

DECORAH FORMATION Limestone and thin platy tan shale_______________ . 1

	Beiitonite__-___---____-----------------_---_- . 1-. 2

GENERAL FEATURES Limestone, greenish-buff, nodular, argillaceous, and

. n . -i x i , f . i • light-brown interbedded shale_ ________________ .3 An excellent exposure in the central part ol the mm- ________

ing district of all but the upper few feet of Decorah is Total, Spechts Ferry------_____________ 3. 0

seen in a steep ravine from the west into the Galena =====

River (locality 32, fig. 35) as follows: Total, Decorah. ___________________________ 32. 6+

	Platteville formation: 

Ravine, west side of Galena River, center of east line, sec. 4, T. 1 N., Quimbys Mill member ("glass rock"):

R. 1 E., Lafayette County, Wis. Limestone, light-brown to brown, thin- to medium-

[Described by A. F. Agnew, Aug. 22,1949] bedded, very fine grained and dense, conchoidal

Galena dolomite- Thickne** fracture; dolomitic shaly zone at base—— —— _ _ _ _ 8. 0

(feet)

Cherty unit (zone D): Westward from the mining district the Decorah

Dolomite, brownish, medium-crystalline, thin- .„ . . , , , . , ,. bedded; mottled with calcareous areas__________ 2. 2 becomes rather uniformly greenish shale with limestone

Limestone, buff to flesh-colored, thin-bedded ______ 1. 0 nodules, as is seen at Decorah, Iowa (fig. 35).

Covered interval.______________________________ ____ 6. 8 Calvin's (1906, p. 85) "typical exposure of the shale

Decorah formation: at the foot of the bluff on the left of the 'Dugway' "

Ion dolomite member (gravbeds): „ ., -, Limestone, light-buffish-gray, argillaceous, thin- today shows only the upper 15 feet of what corresponds

bedded________________ ___________________ 9. 4 to the Ion dolomite member, of the Decorah formation.

Limestone, grayish-buff, coarsely crystalline, very Calvin stated further that "the shales overlie the limefossiliferous; a 0.1-ft platy grayish shale at base__ 2. 0 stones in the west quarry of Mr. Halloran, east of the

Ion dolomite member (blue beds): Ice Cave Md „ JR tmg ry Qnl ^ lower 5 feet

Limestone, bluish-gray alternating with gravish- „ , ,-r , , . , " - .

buff; thin-bedded in lower 0.5 ft; upper 0.7 ft is 1 OT SO °f the Decorah> whlch corresponds in position to

bed______________________________________ i. 2 the Spechts FemT shale member, is visible above 3 feet

Limestone, greenish-gray, shaly, platy ____________ . 7 of limestone, here assigned to the unnamed member at

Limestone, fossiliferous_ ________________________ .7 the base of the Decorah.

Limestone, grayish-buff and bluish, crystalline, A secijon that exposes the full thickness of the

mottled, argillaceous; upper 0.4 ft verv fossilifer- ^ ^ ,, . , , , rr /i^r»n ^n\

ous_____ _______________ _'_________ 1 4 Decorah formation was noted by Kay (1929, p. 651),

Limestone, thin-bedded, lower part bluish-gray and because it is only 8 miles from the city of Decorah

upper part flesh-colored____________________ 1. o it may be considered typical (see section 4, p. 306).
286 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

The Decorah formation changes in lithology across 

the mining district so that at the east border of the 

district the formation consists wholly of dolomite. In 

like manner the thickness decreases easterly, the 

unnamed member is absent east of Platteville, Wis., and 

the Spechts Ferry is absent in the eastermost part of 

the mining district. A typical section of the Decorah 

formation in this eastern facies is given in section 5, 

page 306.

In the western part of the mining district the lithology 

of the Decorah is limestone and shale, and the formation 

is approximately 44 feet thick. In the central part of 

the zinc-lead district the Spechts Ferry has about the 

same lithology as to the west; the Guttenberg and Ion 

members, however, contain less shale, and regional 

dolomitization has affected the upper part of the Ion 

member (Agnew, 1950). The thickness of the Decorah 

formation in the central area is approximately 41 feet, 

owing principally to a decrease in thickness of the 

Spechts Ferry shale member. Farther east, at Darlington (fig. 35, loc. 13) the Spechts Ferry is almost absent, 

and the Guttenberg and Ion are both dolomite because 

of regional dolomitization; the Decorah in this area is 

only about 30 feet thick.

Gocd exposures of the Decorah formation can be seen 

as follows: western area, roadcut along U. S. Highway 

52 at north edge of Guttenberg, Clayton County, Iowa 

(fig. 35, loc. 33); central area, east bank of Galena. River, 

Jo Daviess County, 111. (loc. 34); eastern area, quarry 

in south part of Darlington, Lafayette County, Wis. 

(loc. 13).

SPECHTS FERRY SHALE MEMBER

IITHOIOGIC DESCRIPTION AND STEATIGEAPHIC RELATIONS

Kay (1928) noted that in the "ravine southwest of 

the C. M. & St. P. railroad station at Spechts Ferry," 

Dubuque County, Iowa (loc. 10, fig. 35), the Spechts 

Ferry consists of

* * * the eight and one half feet of shales and interbedded 

limestones [which] form a lithologic unit lying above the "Platteville" limestone; the "Platteville" of Iowa does not include the 

uppermost beds of the typical Platteville of southwestern 

Wisconsin. The Spechts Ferry member includes the "glass 

rock" and overlying shales at the top of the typical Platteville.

Later, Kay (1929, p. 645) stated that in the type 

outcrop of the Spechts Ferry

* * * true "glass rock" beds are not present in the base of the 

Spechts Ferry member, though there are conspicuous limestones.

Further:

* * * there is no evidence to prove that these limestones are 

the same, and it is possible that they are younger than the 

"glass rock" and that those beds are absent at Spechts Ferry.

Although the "glass rock" unit (Quimbys Mill member) 

is virtually absent at this locality, its presence is

FIGURE 47.—Type section of Spechts Ferry shale member (Ods) of 

Decorah formation, overlain by Guttenberg limestone member (Odg 

overhang); underlain by McGregor limestone member (Opm) of 

Platteville formation, in quarry 200 yards southeast of Spechts Ferry 

station, Dubuque County, Iowa (fig. 35, loc. 10).

indicated by a thin dark-brown green-mottled fissile 

shale. The quarry 200 yeards east of the ravine shows 

the following section (see fig. 47; section 8, p. 307). 

This description is a slight revision of Kay's (1929, 

p. 646, table III) description because he included the 

dark brown shale—herein called Quimbys Mill member—in the Spechts Ferry shale member. This shale 

represents the western edge of the "glass rock" unit.

The Spechts Ferry is green or slightly bluish-green 

shale with subordinate thin beds of light-greenish-buff 

fine-grained sugary fossiliferous dense limestone.

The thickness of the Spechts Ferry at the type 

locality is 8.8 feet, but this decreases to the east so 

that a short distance west of Shullsburg, Wis. only 

3 feet of Spechts Ferry remain (section 9, p. 307). The 

green shale pinches out farther east, as noted previously; 

an interesting section in this respect is that in the 

quarry at Calamine, Lafayette County, Wis., section 

10, page 308 (fig. 35, loc. 42; fig. 48).
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 287

Quarry along road %-mile south of Mifflin in SW%NE% sec. 34, 

T. 5 N., R. 1 E., Iowa County, Wis.

P'IGDEE 48.—Quimbys Mill member (Opq) of Platteville formation overlain by Guttenberg limestone member (Odg} of Decorah formation, in 

quarry at Calamine, Lafayette County, Wis. (fig. 35, loc. 42).

The Spechts Ferry shale member, referred to in the 

mining district as the "clay bed," ranges from 1 to 5 

feet thick there. Of this thickness perhaps half is 

limestone which, in zones of mineralization, has been 

almost wholly leached away, there being left only the 

argillaceous residues and the original shale beds. The 

shale is blocky in the outcrop but becomes quite plastic 

and claylike in the presence of water, as in the mines.

The commonly recognized bentonite layer of the 

mining district occurs in this member 0.3-0.4 feet above 

the base. This bentonite is the yellowish "pipe clay" 

of the miners.

In the following section the bentonite layer is present; 

in addition, effects of the mineralization on the Guttenberg and Quimbys Mill strata are a striking feature 

of this quarry at Mifflin, Iowa County, Wis., (2 miles 

northeast of loc. 18, fig. 35).

[Described by A. F. Agnew and A. V. Heyl, April 21, 1943] 

Decorah formation: Thickness 

(feet)

Guttenberg limestone member (oil rock):

Limestone, brownish-gray, mottled brown, sugary, 

dolomitized______________________________ 4. 0-6. 0

Gougelike material; brown to chocolate-colored shaly 

clay_____________________________ 1. 0-3. 0

Spechts Ferry shale member (clay bed):

Shale, greenish-brown; a 0.3-ft limestone at base____ . 9

Shale, grayish-green, blocky____________________ . 4

Bentonits, cream-colored; weathers yellow_________ . 4

Shale, green, blocky____________________________ .4

Total, Spechts Ferry_ _______________________ 2. 1

Platteville formation: Quimbys Mill member ("glass 

rock"): Dolomite, light-buff, fine-grained, granular; 

contains chert nodules of secondary origin_ _________ 3 +

The bentonite layer has been noted in many other 

outcrops; some of the better ones are the following:

Quarry, north end of Ice Cave Bridge, in NE^NW}4 sec. 15,

T. 98 N., R. 8 W., Winneshiek County, Iowa. This is one

of Calvin's type sections of the Decorah (fig. 35) . 

Ravine along side road, Allamakee County, Iowa. This is the

Ion type locality (loc. 36). 

Ravine from south, Clayton County, Iowa. This is the McGregor type locality (loc. 25) . 

Roadcut, west side U. S. Highway 52, Clayton County, Iowa,

This is the Guttenberg type locality (loc. 33) . 

Ravine from west, SEX sec. 30, T. 4 N., R. 4 W., Grant County,

Wis. (8 miles southeast of loc. 17) . 

Roadcut, County trunk A, north side of road, Grant County,

Wis. (loc. 28).' 

Ravine from west, in SW^ sec. 4, T. 2 N., R. 3 W., Grant

County, Wis. (5 miles west of loc. 12). 

Roadcut, south side County Trunk O, in NW%XE>4 sec. 35,

T. 3 N., R. 2 W., Grant County, Wis. (2 miles north of loc. 6). 

Roadcut, northeast side U. S. Highway 61, Grant County, Wis.

(loc. 12).

Roadcut, west side U. S. Highway 151, Grant County, Wis. (loc. 6) . 

Ravine from east, in NWJ4 sec. 20, T. 3 N., R. 1 W., Grant

County, Wis. (1 mile west of Platteville). 

Roadcut, County Trunk E, just east of center sec. 18, T. 5 N.,

R. 1 W., Grant County, Wis. (3 miles northwest of loc. 4). 

Outcrop, west bank Fever River, in SWJ4SW}{ sec. 14, T. 2 N.,

R. 1 E., Lafayette County, Wis. (2 miles northeast of loc. 7.) 

Montfort quarry, north side of road, Iowa County, Wis. (loc. 

43).

The heavy mineral suite of the Spechts Ferry is 

characterized by zircon (Bays, see footnote p. 278). 

Insoluble residues from limestones of the Spechts Ferry 

show brownish silt that differs from the honey-colored 

resinous silt of the overlying Guttenberg limestone 

member. Quimbys Mill strata show almost no residue 

(Aberdeen, see footnote p. 283), and its heavy minerals 

are characterized by garnet.
288 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Near the top of the Spechts Ferry member are minute 

phosphatic pebbles, nodules, and fossils. Aberdeen 

attempted to estimate their stratigraphic value, both 

laterally and vertically. She stated that in the vicinity 

of Platteville on the basis of fossils two breaks of 

diastem proportions were recognizable, one at the base 

of a blue shale bed and another at its top; this bed she 

referred to the uppermost Spechts Ferry. She found 

phosphatic nodules in the limestone bed immediately 

overlying this blue shale bed and the presence of these 

nodules seemed to justify drawing the line between the 

Spechts Ferry and Guttenberg at the base of this limestone. Kay and Atwater (1935, p. 101) noted the 

presence of the phosphatic nodules in this limestone 

but assigned the limestone to the Spechts Ferry, as Kay 

had done in his description of the type section in 1929.

The field work associated with the present study has 

shown that the phosphatic nodules are not restricted 

to this limestone bed, which in the Platteville area is 

referred by the writers to the basal Guttenberg. 

In places (as at the Spechts Ferry type locality) phosphatic nodules are found in one or two limestone beds 

of the Spechts Ferry member below the upper bed, 

but separated from it by an interval of shale. Thus it 

can be said only that the phosphatic nodules occur 

in a zone near the contact of the two members.

The phosphatic pebbles are significant not because 

they depict a sedimeiitational break at their horizon, 

but because they are second only to glauconite in 

abundance at horizons "within a few feet above stratigraphic breaks" (Goldman, 1921, p. 4); the PlattevilleDecorah disconformity occurs 5-10 feet below the 

phosphatic zone being discussed, and the less obvious 

Glenwood—St. Peter unconformity occurs 1—8 feet below 

a similar phosphatic zone with glauconite, at the contact of the Glenwood and Pecatonica.

Trowbridge and Shaw (1916, p. 39) found in the 

Spechts Ferry shale member "Dalmanetta subaequata 

which show evidences of having been rolled or worn" 

and concluded that, as this species occurs abundantly 

in the upper part of the Platteville, an unconformity at 

the contact of the Galena and Platteville (marked by 

the clay bed) is suggested. Broken fossil shells are 

common in the coquina lenses of the Spechts Ferry, 

but are here interpreted as being one of the results of 

fairly shallow-water environment rather than being 

reworked from earlier deposits.

The Spechts Ferry member west of the mining district lies conformably on the unnamed limestone member 

of the Decorah. In the central and eastern parts of 

the mining district it rests disconformably on the corrosion surface at the top of the Quimbys Mill member 

of he Platteville formation. Farther east the Spechts

Ferry is absent, and Guttenberg strata rest disconformably on Quimbys Mill.

Bays (1938) stated that the Spechts Ferry and 

Quimbys Mill are facies. Evidence bearing on this 

problem is as follows:

(1) The bentonite layer near the base of the Spechts 

Ferry is consistently present, despite the inverse convergences of the Spechts Ferry and the underlying 

Quimbys Mill strata. This feature was also noted 

by Kay and Atwater (1935, p. 101), who stated, "it 

has been believed that the thinning of the member is 

due to convergence rather than to disconformity [at the 

top]."

(2) The Spechts Ferry and Guttenberg thin to the 

east; on the other hand, to the east and southeast the 

Quimbys Mill increases in thickness.

(3) In all cases where the two units are exposed together, the Spechts Ferry overlies the Quimbys Mill.

(4) There is no known iiiterbedding or iiiterfiiigering 

of the two types of lithology typified by the green shale 

and limestone of the Spechts Ferry, and the purplishbrown limestone and brown shale of the Quimbys Mill.

(5) In the pitted upper surface of the Quimbys Mill 

the irregularities contain phosphatic pebbles and fillings 

of greenish argillaceous dolomite similar to that of the 

Spechts Ferry.

However, features that might be interpreted as 

arguing for contemporaneity and thus lateral facies are:

(1) Faunal contrasts between strata of the Quimbys 

Mill and the Spechts Ferry may be due solely to different environments and not to different ages.

(2) Bays' (footnote, p. 278) facies changes of the 

Spechts Ferry from green shale in the west to brown 

limestone (Quimbys Mill) to buff dolomite (Quimbys 

Mill) in the east have their parallel in the facies changes 

of the Guttenberg from green shale in the west to lightbrown limestone to buff dolomite in the east, and in 

those of the Ion from green shales in the west to lightgreenish-gray limestones and shales, to greenish buff 

dolomite in the east. Nevertheless, the zone of phosphatic nodules at the contact of the Spechts Ferry and 

Guttenberg carries across these changes in facies.

The bentonite layer is therefore of the utmost importance because other evidence might be termed not 

entirely conclusive. This bed of plastic clay was discussed as follows by (X S. Ross (written communication, 

June 14, 1945):

Victor Alien * * * has examined the material and says that 

it resembles bentonite of that age [Middle Ordovician] which he 

has studied. Preservation of ash structures is very rare in both 

the eastern and the Mississippi Valley regions and so their 

absence in your material is not surprising. I * * * find abundant orthoclase which strongly suggests bentonite * * * also 

zircon, which Alien says is very characteristic. Therefore, it 

seems very probable that this material is bentonite.
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 289

This bentonite, originally correlated with the Hounsfield (Kay, 1931) and later shown (Kay, 1935b, p. 229) 

to be younger in age, has been reported from the 

Ordovician of New York; Ontario; from the Minnesota, 

Iowa, and Wisconsin region; and from Missouri. 

Alien's (1932) study of the bentonites in the Mississippi Valley provided several conclusive arguments 

for the truly bentonitic nature of the bed under discussion. Alien found crescent-shaped shards and 

sanidine feldspar as well as euhedral apatite and zircon 

grains. The occasional rounded grains of minerals such 

as garnet, which are foreign to volcanic deposits, he 

explained as having been "added by wave action during 

or shortly following the deposition of the volcanic material." This and other evidences suggested to him 

"active agitation and some reworking of the volcanic 

material before ordinary Decorah sedimentation was 

resumed."

Within the mining district, the contact of the Sperhts 

Ferry arid Guttenberg members is easily discernible, 

as it shows greenish shale and limestone overlain by 

brownish limestone and shale. To the east the Spechts 

Ferry is absent; to the northwest, however, as near 

Decorah, Iowa, and farther northwest, the basal green 

shale (Spechts Ferry) member of the Decorah thickens 

at the expense of the overlying limestone (Guttenberg) 

member.

DISTRIBUTION

Because of their shaly character the Spechts Ferry 

strata are commonly not well exposed. The good exposures are usually in quarries or roadcuts that include 

the limestone or dolomite beds above and below (p. 287).

FAUNA AND CORRELATION

Microfossils and megafossils are abundant. Kay 

(1929, p. 658) listed lower and upper Spechts Ferry 

faunas. His lower fauna came from

* * * gray fine-textured, soft, vertically-jointed limestone beds 

from 1 to 4 feet from the bottom of the member. It has been 

recognized in Winneshiek County [Iowa] and as far to the 

southeast as Patch Grove township, Grant County, Wis. [fig. 

35, loc. 37].

This collection is apparently from the unnamed limestone member, previously described (p. 264). His 

upper fauna came from the upper foot of "dark blue 

hard, pyritic shale."

Spechts Ferry strata have been correlated with the 

Stictoporella bed (fig. 37) of Minnesota (Bays and 

Kaasch, 1935, p. 300; Kay 1940, p. 235), although 

earlier Kay (1935a, p. 287) had believed the Spechts 

Ferry to be the equivalent of the Rhinidictya bed above.

Kay (1929, p. 666) thought that strata of the Spechts 

Ferry (together with the lower Guttenberg strata) 

could be correlated with the Glenburnie member of the

Chaumont (upper Black River) of New York (fig. 44) 

because of the bryozoa. Furthermore, he (1934, p. 

330) believed that the bentonite near the base of the 

Spechts Ferry is "identical with the Hounsfield bentonite in the Glenburnie."

Bays and Raasch (1935, p. 301) correlated the 

Spechts Ferry faunas with both the Tyrone of Tennessee and the Leray of Ontario (see also Twenhofel, 

and others, 1954).

Kay (1935a, p. 288) noted that the "so-called 

'Decorah shale' of * * * Missouri is synchronous with 

the Spechts Ferry member," and Bays and Raasch 

agreed.

ECONOMIC PRODUCTS

Disseminated zinc and lead minerals are common in 

the Spechts Ferry shale member, especially in places 

where ore minerals occur in the overlying beds. Because 

the unit is thin, however, and because it is difficult to 

mine and mill for the zinc and lead content, the Spechts 

Ferry is not normally considered an ore zone. Disseminated pyrite crystals are even more widespread than 

zinc and lead minerals. The Spechts Ferry in former 

3'ears was almost without exception the lowest zone 

penetrated in prospecting and mining, as the miners 

believed that such a body of shale (which they called 

the clay bed) was impervious to the descending (according to views then current) solutions that deposited 

the zinc and lead minerals; furthermore, as the underlying Quimbys Mill is an aquifer, the miners were 

apprehensive about tapping this source of additional 

water because it might pose new problems of mine 

drainage.

The bentonite in the Spechts Ferry is said to have 

been used in a few places in the middle 1800's as a 

pipe clay.

GUTTENBERG LIMESTONE MEMBER

Kay's (1928) original description stated that the 

Guttenberg "consists of about fifteen and one half feet 

of brownish fine-textured limestone," and he (1929, p. 

648) subsequently published the type section as seen 

in a "ravine a mile north of Guttenberg, Iowa." This 

ravine is now obscured by U. S. Highway 52; the roadcut, however, may be said to preserve the type section. 

(See section 11, p. 308; fig. 49.)

1ITHOLOGIC DESCRIPTION AND STRATIGRAPHIC RELATIONS

A typical section of the Guttenberg in the mining 

distr'ct is that along the west bank of the Galena River, 

in sec. 4, T. 1 N., R. 1 E., Lafayette County, Wis., 

given on page 285.

Throughout most of the district the thickness of the 

unaltered Guttenberg is 12-14 feet. On the other 

hand, east of Mineral Point and Shullsburg the unit
290 SHOETER CONTRIBUTIONS TO GENERAL GEOLOGY

FIGURE 49.—Type section of Guttenberg limestone member (Odg) of 

Decorah formation; overlain by Ion dolomite member (Odi), underlain by Spechts Ferry shale member (Ods), in roadcut, U. S. Highway 52, 1 mile north of Guttenberg, Clayton County, Iowa (flg. 35, 

loc. 33).

begins to thin and at Blanchardville (fig. 35, loc. 29) 

it is only 6.5 feet thick. Slightly less than 2 feet remain 

near Rockford, 111.

The Guttenberg becomes dolomitic east of Shullsburg 

and Mineral Point (see section 12, p. 308; fig. 35, loc. 44; 

fig. 50).

Herbert 9 found in some exposures a thin bentonite 

layer along a shale seam 2-3 feet above the base of the 

Guttenberg. The limestone below the bentonite layer 

is grayish brown, that above is tan or light brown. 

This bentonite layer was found at Spechts Ferry, 

Dubuque County, Iowa (fig. 35, loc. 10), northeast of 

Galena, 111. (loc. 34), and midway between these two 

localities, in U. S. Bureau of Mines diamond-drill holes 

at Fairplay, Grant County, Wis. (sees. 25, 26, T. 1 N., 

R. 2 W.).

Insoluble residues of the Guttenberg show (Dake, 

footnote p. 278) an increase in quantity over those from 

the Quimbys Mill; the residue is mainly brownish chert 

with some quartz sand. Herbert related that the lower

9 Herbert, Paul, Jr., 1949, Stratigraphy of the Decorah formation in western 

Illinois: unpublished Ph. D. thesis, Chicago Univ., Chicago, 111.

FIGURE 50.—Decorah formation, consisting of Ion dolomite member 

(Odi) and Guttenberg limestone member (Odg), overlain by Galena 

dolomite (Og), underlain by Quimbys Mill member (Opg) of Platteville formation, in quarry at York Church, Green County, Wis. (flg. 

35, loc. 44).

2 to 3 feet of the Guttenberg has grayish argillaceous 

residues, whereas the rest of the member has tan argillaceous material with red and orange specks. The 

Guttenberg contains silicified fossils and quartz silt.

The Guttenberg unit contains carbonaceous shale 

partings (fig. 51). Locally the mineralizing solutions 

leached the limestone from this member as well as from 

the Spechts Ferry and Quimbys Mill members below, 

leaving a reduced thickness of beds that consist mostly 

of shale and argillaceous residuum. The chocolatecolored shaly residues of the Guttenberg found in mineralized areas are known as the oil rock. The following 

section illustrates markedly this feature, for it shows 

less than 5% feet of Guttenberg (fig. 35, loc. 11):

Eagle Point Quarry, U. S. Highway 151, near southeast corner 

sec. 7, T. 89 N., R. 3 E., Dubuque County, Iowa

[Described by A. F. Agnew, spring, 1943]

Thickness 

Decorah formation: (feet)

Ion dolomite member (blue beds): Limestone, blue,

coarsely crystalline, recrystallized_ _______________ 4.0

Guttenberg limestone member ("oil rock"):

Limestone, pink, crystalline, mottled; dark-brown

shale laminae______________-___---___-_---_-- 1. 2

Shale, dark-brown, with thin lenses of pink limestone; nodules of chertified limestone in lower 

0.2 ft__.________-________________ .9 

Limestone, light-brownish-pink, nodular, chertified;

chocolate shale seams________________________ 3.0
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT

Eagle Point Quarry, U. S. Highway 151, etc.—•Con.

„ , r ,. ^ ,. , Thkkness Decorah formation—Continued (jeet)

Guttenberg and Spechts Ferry members undifferentiated: Shale, chocolate-covered above, grading 

downward into greenish and yellowish-brown; 

phosphate nodules in lower 0.2 ft ___________ 0.3-0.5

Spechts Ferry shale member (clay bed):

Shale, gray-green_____________________________ .9

Shale, bluish-green______________________________ . 9

Locally the oil rock is termed "rich" in the zones of 

mineralization because there its shaly residue phase is 

most abundant and the calcareous elements have been 

leached away, thus making the hydrocarbon content 

more conspicuous. Fairly well leached Guttenberg is 

called "medium oil rock" by the miners, and that 

which has been only slightly leached is designated 

"poor oil rock." Where no leaching has taken place 

and the original limestone and shale relations have 

been preserved, the rock is called "bastard oil rock" 

or "limy oil rock." (Fig. 52).

The above discussion suggests how an incorrect 

interpretation of the structure and of the relations 

between thickness of oil rock and localization of ore 

deposition came to be the accepted one. Geologists 

had found that the hydrocarbon-rich facies of the oil 

rock was thicker in the basins and near ore bodies, 

and thus they postulated irregularities in deposition 

to account for this feature; such views were held as 

late as 1934, when Scott 10 said that lateral variations 

such as 6 inches of oil shale terminating against limestone are due to sedimentary environment.

Actually, the stratigraphic unit as a whole had 

thinned greatly in such places because more of the

10 Scott, E. R., 1034, Structural control of ore deposition: Unpublished M. S. thesis, 

Northwestern Univ., Evanston, 111.

FIGTIRE 51.—Carbonaceous shale partings between wavy dolomite beds 

of Guttenberg limestone member (Odg) of Decorah formation, which 

rests on Quimbys Mill member (Opq) cf Platteville formation, in 

quarry on Honey Creek, Green County, Wis. (fig. 35, loc. 45).

FIGURE 52.—Effect of mineralizing solutions on Guttenberg limestone member of Decorah formation. Dark material is 

brown oil rock—argillaceous and shaly residuum derived mainly from leaching of material similar to the adjoining calcareous 

rock. Liberty mine NEJ4NEJ4 sec. 16, T. 2 N., R. 1 E., Lafayette County, Wis. (1 mile north of loc. 7, fig. 35).
292

calcareous beds had been removed. The thinning 

caused by such leaching not uncommonly reduced the 

Guttenberg locally to 6-8 feet in thickness; indeed, 

in an extreme case in the Ginte Mine in Illinois (fig. 

35, loc. 46), a reduction in thickness was found from 

the normal 13 feet to a minimum of 2 feet; the 2 feet 

consisted wholly of oil-rock shale. As another example, 

two adjacent drill holes (U. S. Bureau of Mines 58 and 

59) in the Bautsch ore body (loc. 30) gave thicknesses 

of 13 feet and 7 feet, respectively.

The oil rock was called the "chocolate brown rock" 

and "brown rock" in the early reports (Strong, 1877, 

p. 695), and these names are still used by older miners 

near Montfort, Highland, and Dodge ville, in the 

northern part of the district. The unit was first 

described in detail as oil rock by Grant (1903, p. 34), 

as follows:

"[It] is a compact, very finely laminated, soft shale which 

varies in color from a very light-yellowish gray to a dark chocolate brown color, and even becomes perfectly black in places

* * * It contains considerable percentage of carbonaceous 

matter * * * [and] when dry, particles of this rock will usually 

burn with a thick smoky flame."

Later Grant (1906, p. 40) stated that "when burning 

it gives off the peculiar petroleum odor and consequently has received the name 'oil rock.' "

Partial analyses of three samples of oil rock were 

reported by Strong (1877, p. 680). A sample taken 

from the Oakland level, in SW# sec. 5, T. 1 N., R. 2 E. 

(2 miles northwest of Shullsburg, Lafayette County, 

Wis.), gave 40.60 percent of "carbonaceous" matter; 

two samples from the Silverthorn mine, in NE% sec. 

31, T. 2 N., R. 2 E. (2 miles farther northwest), 

showed 18.31 percent and 15.76 percent. Tests made 

by F. F. Grout on samples from the Dugdale prospect 

west of Platteville (Bain, 1906, p. 25) showed 20.85 

percent of volatile matter and 7.96 percent of 

true carbonaceous material in thoroughly air-dried shale. 

Leaching the shale with ether gave a thick, heavy oil * * * 

[containing] an appreciable amount of sulfur.

The more detailed analysis by R. T. Chamberliri 

(Bain, 1906, p. 26) gave 39.98 percent hydrocarbon, 

largely methane, and 6.79 percent hydrogen sulfide. 

One volume of the rock gave 57.46 volumes of gas.

David White (Bain, 1906, p. 26) made a microscopic examination of slides of the same material, 

concluding that there were present bodies

* * * corresponding to the contours of collapsed and flattened 

unicellular plants, * * * interpreted as the fossil remains of 

microscopic, unicellular, gelosic algae * * *

The soft brown shales of the oil rock, commonly 

convoluted with the yellowish bentonite of the underlying "clay bed" (Spechts Ferry), were locally termed 

"bull dum" by the miners.

In part of the district (T. 1 N., R. 1-2 E., southwestern Lafayette County, Wis.) the Guttenberg contains a bed of discrete chert nodules, 1.5—2 feet below 

its top.

Dolomitization accompanying the zinc-lead mineralizing solutions has caused confusion in the correct 

identification of the Guttenberg. In some places this 

dolomitization resulted in bleaching, as in the Trego 

mine (NE# sec. 10, T. 3 N., R. 1 W., at north edge of 

Platteville, Wis.), where the otherwise brownish rock 

is a light-cream color. In most dolomitized areas, 

however, the Guttenberg retained its original color, 

but became granular and sugary upon dolomitization, 

as in the Hoskins mine, Lafayette County, Wis. 

(SWtf sec. 18, T. 1 N., R. 2 E., 2 miles southeast of 

loc. 27, fig. 35).

Silicification likewise was caused by the mineralizing 

solutions and notably affected the Guttenberg, where 

the replacement preserved the characteristics of the 

limestone and shales so faithfully that until the hardness is tested the rock appears normal; it is this rock 

that was named "bastard oil rock" originally, although 

more recently that term has been applied also to the 

hard unleached limestone phase of the Guttenberg. 

In many places the silicification progressed far enough 

that brown chert nodules were formed from limestone 

nodules, or grew between the shale laminae, seeming 

to force them apart.

As previously mentioned (p. 288, 289), the contact 

between Guttenberg and Spechts Ferry members is 

well-marked in the mining district.

To the southeast the Spechts Ferry is absent and 

the Guttenberg lies with apparently slight regional 

disconformity on the pitted surface of the uppermost 

bed of the Quimbys Mill below, whereas northwest of 

the district the Guttenberg passes into a greenish shale 

and argillaceous limestone facies, and in Allamakee 

and Winneshiek Counties, Iowa, is poorly distinguished, 

or indistinguishable from the overlying Ion.

In the mining district the upper boundary of the 

Guttenberg is almost as distinct as the lower; lightbrown dense limestone and less-abundant chocolatebrown shale beds are overlain by gray-blue more 

coarsely crystalline limestone and greenish-gray fossiliferous shale. This change in lithology is even more 

striking in the areas of zinc-lead mineralization, where 

an abrupt change from chocolate-brown shale upward 

into bluish-green shale is seen.

Southeast of the mining district the light-gray dolomite of the Ion conformably overlies the light-brown 

dolomite of the Guttenberg, and the contact is distinct.

DISTRIBUTION

Despite the thin-bedded and nodular character of 

the Guttenberg, it usually is exposed as ledges and
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 293

small cutbanks in streams and also in quarries where 

the lower or higher beds have been taken as quarry 

stone. Some good exposures are listed on page 287.

FAUNA AND CORRELATION

Kay (1929, p. 659) listed species of brachiopods, 

mollusks, and trilobites from Guttenberg strata along 

the Galena River 2 miles north of Galena, 111. The 

fauna gains a bryozoan element in the shaly facies to 

the northwest, as shown by Kay's list of bryozoa 

from an exposure south of Waukon, Iowa. Kay 

(1935a, p. 290) stated that many of the species are 

common to the Rockland limestone (lowest Trenton) 

of Ontario (fig. 44).

In a later publication Kay (1940, p. 235) stated 

that the "Guttenberg member is believed to be represented in the Ehinidictya and Ctenodonta zones of 

Minnesota" (fig. 37), and he repeated his 1934 correlation of the Guttenberg with the Rockland (lower Trenton) of Ontario (see also Twenhofel, and others, 1954).

ECONOMIC PRODUCTS

The Guttenberg or oil rock contains zinc and lead 

ore in many places. In areas of rich oil rock the minerals are normally disseminated, as they are also in 

the underlying Spechts Ferry. However, in contrast 

to the Spechts Ferry (p. 289), mining- and milling 

problems in the Guttenberg are not a deterrent. 

Furthermore, zinc-lead veins are present in many 

places and are mineable particularly where the overlying strata are mineralized.

ION DOLOMITE MEMBER

Kay (1929, p. 650) described the type section of the 

Ion, but field work by Paul Herbert, Jr., in 1944 made 

it appear advisable to redescribe the type section. The 

following section (fig. 35, loc. 36), is presented as a 

result of this study of outcrops and of a field conference, 

held August 18, 1945, in which the participants were 

Paul Herbert, Jr., H. B. Willman, and L. E. Workman of 

the Illinois State Geological Survey; A. F. Agnew, 

C. H. Behre, Jr., and A. V. Heyl/Jr. of the U. S. 

Geological Survey.

Outcrop in ravine along road in NW}i sec. 35, T. 96 N., R. 4 W., 

Allamakee County, Iowa

Outcrop in ravine along road in NW}i sec. 35, etc.—Continued

Thickness 

(feet}

[Described by Paul Herbert, Jr., November 8, 1944]

Thickness 

(feet)

Galena dolomite:

Cherty unit (zone D):

Limestone, light-grayish-buff, slightly mottled 

grayish in lower part, finely to medium-crystalline, dense, fossiliferous_______________________ 6 +

Decorah formation: 

Ion dolomite member:

Shale, olive-brown ____________________________ .3

Limestone, grayish, coarsely crystalline, fossiliferous, numerous Prasopora_ _____________________ 1.0

Shale, green, P/-asopora__ _______________________ . 5

Shale, greenish, calcareous, and thin grayish-green 

shaly limestone beds; the ledges 2-3 ft above base 

contain Glyptorthis and Dinorthis in abundance__ 15. 5 

Limestone, thin-bedded, argillaceous; interbedded 

greenish shaly partings; ledge containing Glyptorthis 0.5 ft below top______________________ 3. 2

Limestone, gray or greenish-gray, gray-mottled, 

thin-bedded; weathers buff to brown, wavy-bedded, 

fossiliferous_________________________________ 1. 3

Total, Ion_____.___________________________ 22

Guttenberg limestone member: Limestone, lightbrownish-buff, finely crystalline, dense, fossiliferous; 

partings of brown to chocolate-colored shale______ 8. 3

Rocks of the Spechts Ferry and the McGregor are also 

exposed at the type section of the Ion. The section 

crops out in a shallow ravine and is accurately measured only with difficulty. The dips of the beds and 

the low gradient of the stream make the thickness 

measurements, especially of the Ion, not very precise. 

The differences between this described section and the 

description of Kay is that the upper 4.5 feet of Guttenberg (as described by Kay 1929) are here placed in the 

Ion. This interpretation is favored because this 4.5- 

foot zone is more similar lithologically to the local 

Ion, as well as to the Ion in the mining district to the 

southeast.

LITHOLOGIC DESCRIPTION AND STRATIGRAPHIC RELATIONS

Sections (see sections 11, 13, p. 308) showing Ion very 

similar to that at its type locality are exposed at Guttenberg, Iowa, and a short distance northwest of Potosi, 

Grant County, Wis. (fig. 35, loc. 47).

In contrast, about 10 miles southeast of the above 

locality the Ion is dolomitic, and this lithology is 

characteristic of the central and eastern part of the 

mining district, as follows: (fig. 35, loc. 12).

Roadcut, U. S. Highway 61, in NW% sec. 7, T. 2 N., R. 2 W., 

Grant County, Wis.

[Described by A. F. Agnew, Apr. 22, 1945]

Thickness 

(feet) Galena dolomite:

Cherty unit (zone D):

Dolomite, buff with slight greenish mottling, mediumcrystalline to coarsely granular; thick-bedded.___ 9. 5 

Decorah formation:

Ion dolomite member (gray beds):

Shale, dolomitic, olive-green; with casts of PrasoporaC?) __________.._____-_-____----_--__--__- . 1
294 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Roadcut, U. S. Highway 61, etc.—Continued

Decorah formation—Continued

Ion dolomite member—Continued Thickness 

Dolomite, olive-gray, mottled, medium- to thick- (^ 

bedded; grayish-green shale stringers and partings________________________________________ 14. 0

Total, gray beds___________________________ 14. 1

Ion dolomite member (blue beds):

Shale, dolomitic, olive-gray, somewhat fossiliferous_ . 3 

Shale, olive-gray; lenses of coquinalike limestone____ 1. 0 

Limestone, olive-gray, medium-crystalline, mediumbedded; in wavy beds with olive shale partings__ 3 +

In the eastern part of the mining district the Ion, 

still a dolomite, has lost the distinctive characteristics 

of its upper part so that it is inseparable from the basal 

strata of the overlying Galena formation (see section 14, 

p. 309, fig. 35, loc. 13).

The lithology of the Ion as seen from these sections 

consists of two facies: western facies of light- to darkbluish-gray finely to coarsely crystalline fossiliferous 

limestone beds, with abundant greenish-gray shale beds; 

and eastern facies of light- to medium-grayish-blue, 

crystalline to granular, vuggy medium- to massivebedded less fossiliferous dolomites, with subordinate 

amounts of olive-gray argillaceous patches and dolomitic 

shale beds.

The thickness of the Ion is consistently 20-22 feet 

in outcrops and in many drill holes.

In places minute phosphatic nodules (smaller and 

more sparse than those in the upper part of the Spechts 

Ferry) are present near the base of the Ion. The Ion 

contains scattered rounded grains of clear quartz sand 

that are relatively abundant near the base; such 

grains are likewise present, but are less common, in the 

Guttenberg below, and in the overlying beds of the 

cherty unit of the Galena formation.

The divisions of the Ion, which are known in the 

miners' terminology as the blue and the overlying gray, 

can be differentiated in most parts of the district. 

The blue beds consist of about 7 feet of gray dolomitic 

limestone whereas the gray beds contain approximately 

14 feet of lighter gray dolomitic limestone. The blue 

beds have relatively more of the clear rounded grains of 

quartz sand, are more mottled, and in most places 

are more argillaceous and shaly than the gray beds. 

Minute phosphate granules are common near the base 

of the blue. Outside the district these separate units 

are not distinct, and in different parts of the district 

they show some variation. It is therefore best, as with 

the Mifflin (of Bays; 1938) and Magnolia (of Bays and 

Kaasch 1935) subdivisions of the McGregor member, 

to use these terms only locally. In the northern part

of the mining district the lower, or blue beds of the Ion 

are referred to as green rock (Strong, 1877, p. 695).

Herbert reported the insoluble residues from the Ion 

to be greenish clay. He also noted at the top of the 

blue beds, southward from the mining district, an 

erosion surface that was not seen by the writers.

A bentonite bed was observed at the base of the Ion 

at a quarry in SE# sec. 27, T. 3 N., K. 3 E., Lafayette 

County, Wis. (2 miles north of loc. 13, fig. 35), where it 

is the basal 0.2 foot of the Ion and rests on dolomite of 

the Guttenberg that is 8 feet thick.

As was already mentioned (p. 292), the contact of the 

Guttenberg and Ion is conformable and, although in 

most of the mining district the differentiation of the 

beds is relatively simple, to the northwest where the 

green shale and limestone facies of the Guttenberg 

occurs it is not so obvious. Likewise, southeast of the 

district where both the Guttenberg and the Ion are 

dolomite the boundary between these two units is 

drawn with some difficulty.

The upper contact of the Ion is conformable with 

the base of the Galena. In most places the contact 

is easily distinguishable although locally it is indistinct, 

especially in the eastern area of the outcrop, where 

strata near the contact are a grayish dolomite mottled 

with green areas that are argillaceous in part, but 

include no definite shale beds (see section 14, p. 309).

Studies of many outcrops in the region from Rockford, 111., on the southeast to Decorah, Iowa, on the 

northwest, amply supplemented with the examination 

of cuttings from wells have shown the contact between 

the Decorah and Galena to be a conformable one 

regionally; and detailed, studies of closely spaced 

outcrops in certain areas of the mining district indicate 

that these relations remain constant. Thus the writers 

do not agree with Kay (1932) and Atwater (Kay and 

Atwater, 1935, p. 109-110), that

* * * in the Upper Mississippi Valley Lead and Zinc District, 

there is a distinct disconformity at base of the Galena dolomite, 

bringing that formation in contact with beds of the Ion and of the 

Guttenberg ("Oil rock member") members of the Decorah 

formation.

The same authors continued that

* * * this is in contrast to the stratigraphic relations in 

sections northwest of the district, where the Prosser limestone 

of the Galena group lies conformably on the Ion; this conformity 

continues into Minnesota.

Kay later (1939, p. 27) repeated the former statement 

even more emphatically.

It is the considered opinion of the writers that the 

sections of Kay and Atwater were incorrectly described 

and that the "clastic beds" which are said to mark this 

disconformitv are actually altered strata in mineralized
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 295

outcrops or weathered exposures heavily covered with 

loess and other surficial material.

For example, the two cases cited by Kay and Atwater 

(1935, table I, p. 103 and table II, p. 104) oneeighth of a mile apart at Spechts Ferry, Iowa, which 

they give as a major illustration of the so-called disconformity, are actually not at all dissimilar. They 

state (1935, p. 104) correctly that in the quarry the 

"Ion limestone has been so metasomatized as to become 

a dolomite." The effects of this "metasomatism" have 

been not only to alter the lithology of the affected strata 

in both exposures, but in addition it reduced materially 

their thicknesses, as is illustrated by similar changes in 

limestones of the Quimbys Mill and Guttenberg, 

described previously (p. 283, 292). The table below 

lists the observed thicknesses of the various units at 

the two Spechts Ferry localities, together with the 

normal thicknesses:

Effects of alteration on thickness at two exposures only 600 feet 

apart at Spechts Ferry, Dubuque County, Iowa

Rock units

Decorah:

Thickness, in feet

Quarry

6.0

16.3 

14.8 

8.1 

.3

Ravine

5.8

13.0 

13.6 

8.0 

.2

Normal

10

21 

15 

8-8.1 

0.2-0 3

This alteration of lithology and thickness is a common 

feature in the mines to the east, and has been noted as 

far west as the vicinity of Beetown, Wis., (10 miles 

northwest of the Spechts Ferry localities), where it was 

accompanied by significant zinc ore mineralization.

Kay and Atwater described another exposure (1935, 

table IV, p. 105), which shows ocher-yellow silty clay 

with a "poorly defined" 3-inch bed of dolomite. This is 

nothing more than a weathered exposure in a stream 

bank. A similar condition is presented in their figure 4; 

the left part of the figure evidently shows a weathered 

and rubbly upper part of an outcrop. A detailed 

study of the outcrop by the present writers clearly 

established this conclusion.

It is thought that the facies changes of the Ion have 

helped cause this misinterpretation, as the Ion strata 

are irregularly dolomitic in the area where Kay and 

Atwater noted the so-called disconformable relations. 

The misinterpretation was aided by the secondary 

effect of the alteration by mineralization as previously 

described.

The Ion grades upward into the basal Galena. The 

rocks in the basal part of the Galena, however, contain 

slightly less greenish argillaceous material than the Ion

does in its dolomite facies in the mining district and to 

the southeast. Similarly, the basal Galena strata in 

the limestone facies northwest of the mining district 

contain less green fossiliferous shales than the Ion beds 

do in that area.

DISTRIBUTION

Similar to the Guttenberg member, the Ion is exposed 

mainly as ledges in streams and in quarries (see p. 287).

FAUNA AND CORRELATION

At the top of the Ion is a zone of Prasopora insularis 

Ulrich. These bryozoa are abundant in the northwestern part of the area of outcrop, but become less 

common to the southeast so that only along the western 

edge of the mining district specimens of Prasopora are 

observed. This fossil is usually found in the upper foot 

and a half of the Ion. A zone containing abundant 

Clyptorthis bellarugosa (Conrad) occurs in the Ion 

2-6 feet above its base.

Kay (1935a, p. 290) stated that the Ion contains 

typical Trenton forms (see Twenhofel, and others 

1954) and the ostracodes, Kay (1934, p. 331) found, 

have a "striking similarity" to those of the lower Hull 

(of Raymond 1914) of Ontario (fig. 44); (Kay, 1940, 

p. 235) correlated the Ion with the Phylloporina 

(Chasmatopora) and Fucoid zones of Minnesota (fig. 37).

ECONOMIC PRODUCTS

The Ion beds are ore-bearing and, together with the 

overlying Galena dolomite, comprise the host rock of 

most of the zinc-lead vein ("pitch and flat") and breccia 

deposits of the mining district. The "pitch" of local 

miners refers to an inclined ore-filled fracture, and the 

"flat" is a vein along bedding; the two types of fracture 

are closely associated structurally, as the "flats" 

terminate against the "pitches."

DISTRIBUTION OF FACIES OF THE DECORAH AND CONDITIONS OF DEPOSITION

The Decorah strata are mainly shale with thin limestone bands and nodules west of the mining district; at 

Decorah, except for the lower, unnamed limestone 

member, it is difficult to distinguish subdivisions. In 

the mining district, however, the main part of the 

Decorah is limestone with relatively little shale, and 

only the Spechts Ferry shale member contains more 

shale than limestone. East of the mining district shale 

in the Decorah is very rare; the rocks are dolomite.

Not only is there a change in facies from dominantly 

shale in the west to dominantly carbonate rock in the 

east, but in addition the lower, unnamed limestone 

member and the Spechts Ferry shale member pincb out 

toward the east—the former in the western part of the
296 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

mining district, the latter in the eastern part; furthermore, the Guttenberg member thins to the east of the 

district. This is generally in contrast with Platteville 

strata, as the Quimbys Mill and upper McGregor strata 

thin toward the west. Coupled with the evidence for a 

hiatus between the Platteville and Decorah afforded 

by the bentonite layers in the lower Decorah strata, 

this indicates an incursion of the Decorah sea from the 

west over the corroded surface of the uppermost Platteville beds, in contrast to the eastward retreat of the sea 

at the end of the Platteville.

The shale and exceedingly shaly limestone beds of the 

Decorah were deposited under generally shallow-water 

conditions of a platform environment (Sloss, 1947). 

The shale beds are unfossiliferous; however, fossils 

are found as floods in coquinalike layers, commonly 

broken and washed, and are found in extremely fossiliferous fine-grained limestone beds. Thus, although 

the environment of the shale was generally hostile to 

life, the shells were contributed during the deposition 

of clay at times stormy conditions, and the 1- to 

2-inch coquinoid layers therefore represent only 

instants in geologic time (see also Bucher, 1919).

The limestone beds and interbedded shale partings 

of the Decorah in the mining district were deposited 

under conditions generally similar to those prevailing 

when McGregor and Quimbys Mill strata were accumulating—that is, generally shallow-water environment.

The more coarse-grained dolomitic rock east of the 

mining district is due to subsequent dolomitization 

apparently related to the Wisconsin arch (see p. 284).

The bentonite beds, the result of volcanic activity 

probably in the southern Appalachian area (Nelson, 

1922), may have caused disruption of animal and plant 

life development, although not so great as Sardeson 

(1926b) believed.

GALENA DOLOMITE

The beds called the upper magnesian limestone or 

cliff limestone by Owen (1840, p. 19, 24) were designated 

"Galena" by Hall (1851, p. 146) from their exposures 

in the vicinity of the town of Galena, Jo Daviess County 

111., (fig. 35). The strata making up the Galena in its 

type area were described by Hall as "gray, or drabcolored limestone, and very friable." As exposed in 

the vicinity of Galena, 111., they consist of light-grayish 

or drab to light-brownish or buff medium- to thickbedded medium crystalline to coarse-grained, "sugary," 

extremely vuggy dolomite. Many chert bands are 

present in the lower half of the formation, and the beds 

normally show a honeycomb type of weathering.

GENERAL FEATURES

Regionally it has been found useful to divide the 

Galena into two lithologic units, a cherty, and a nonchertyone (Agnew, 1950).

The total thickness of the Galena in the mining 

district generally ranges between 215 and 230 feet. 

The following table shows the thickness of the Decorah 

formation, and the Galena dolomite and its subdivisions 

in wells irregularly distributed in and near the mining 

district:

Thickness of the Decorah formation, Galena dolomite, and its subdivisions in, wells irregularly distributed in and near the mining 

district

[Localities shown on fig. 35]

County Home well, Clayton County, 

IowaQoc.16)— ---- — -- — --.-_----

Colesburg city well 1, Delaware County, 

Iowa(loc.21)__ ____ ________ __ ____

Wiest Brothers farm well, sec. 12, T. 3 N., 

R. 5 W., Grant County, Wis. (2 miles

USBM Pikes Peak DD 7, sec. 33, T. 89 

N., R. 2 E., Dubuque County, Iowa (5

Swanson farm, sec. 26, T. 1 N., R. 2 E., 

Lafayette County, Wis. (2 miles south

Decorah

47

46

39+

49

25

Cherty

125

107

119

113

103

Galena

Noncherty

87

113

102+

118+

113

Total

212

220

221+

231+

216

CHERTY UNIT

Workable subdivisions of the cherty unit, arranged 

(Paul Herbert, Jr., Illinois Geological Survey, oral 

communications, 1944) in subsurface studies at the 

Bautsch mine, Illinois (fig. 35, loc. 30), were modified 

and applied to outcrops both locally and regionally in 

the course of the studies for the present report. Because 

these units are not stratigraphic entities large enough 

or definite enough regionally to require formal names, 

letter designations similar to those applied by Fowler 

and Lyden (1932) in the Tri-State zinc district are used.

The following points regarding the above classification are significant:

(1) Zone D is lithologically transitional between the 

Ion beneath and zone C.

(2) Zone C contains the lowest widespread chert of 

the Galena formation; this chert is abundant both as 

beds and nodular bands, and as separate nodules.

(3) Zone B is the lower Receptaculites zone; it contains 

Receptaculites oweni Hall abundantly, although not 

uniformly; chert bands and nodules are not nearly so 

common as in zone C, below.

(4) In exposures in the area bounded by the towns of 

Galena, Platteville, Gnttenberg, and Dubuque, zone A 

(see p. 267) can be divided generally as follows, in 

descending order:
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 297

Subdivisions of zone A of ckcrty unit of Galena dolomite

Thickness 

(feet)

Unit 1. Dolomite, light-buff, massive above to thinbedded below; many chert bands; thin shale or 

shaly zone, locally with bentonite, at base____ 32

Unit 2. Dolomite, thin-bedded, as above; chert rare,

Receptaculites and Ischadites sparse_ _________ 6

UnitS. Dolomite as above, chert common^____________ 6

Unit 4. Dolomite, light-gray to drab, thick-bedded, 

sparsely cherty near top but chert common near 

base; Receptaculites and Ischadites present 

sparsely in a thin zone near middle. _________ 26

Total, zone 70

From subsequent field work in the correlation of these 

subdivisions of the cherty unit, several facts are evident, 

namely:

The shale parting at the base of zone A, unit 1 is in 

places more or less obscure; in some localities it is only 

a wafer-thin parting of dolomitic shale, whereas in 

others it is bentonite and shale;

The less-cherty zones differ somewhat in the abundance of chert present; the value of this feature is complicated in well cuttings by caving from overlying cherty 

beds;

The occurrence of Receptaculites and Ischadites in 

units 1 and 3 is helpful in outcrop studies, and the gray 

to drab color of unit 4, while not uniformly noteworthy 

in the outcrops, is nevertheless of value in the subsurface studies;

Strata in all the above units are commonly marked 

by extremely honeycomblike weathering (fig. 53); yet 

in some areas the same beds present a fairly massive 

appearance. Fucoidal surfaces are generally present 

and are especially characteristic of zone A, unit 4 and 

zone B.

The four major zones (A-D) described above can be 

distinguished in outcrops, diamond-drill cores, and 

churn-drill samples mainly on the basis of the relative 

abundance of chert, because in churn drill cuttings 

Receptaculites can not be recognized. As all four of the 

zones generally have only one type of weathering—and 

thus the color and texture of the weathered rock are 

not useful—the relative amount of chert present and 

subtle differences in bedding are the distinctive features 

in exposures.

In the western part of the district bentonite has been 

noted at a horizon normally represented by a shale 

parting near the base of unit 1, zone A. Localities 

showing this bentonite include the roadcut along U. S. 

Highway 61 in SE^XEtf sec, 26, T. 3 N., R. 3 W., 

Grant County, Wis. (3 miles northwest of loc, 12, 

fig. 35), and the roadcut along U. S. Highway 52, at the 

north edge of Guttenberg, Clayton County, Iowa (fig. 

35, loc. 33).

FIGURE 53.—Honeycomblike weathering of zone P of noncherty unit, and zone A 

of cherty unit of Galena dolomite in roadcut, State Route 11, Grant County, Wis. 

(fig. 35, loc. 35).

The thickness of the cherty unit remains fairly constant at 100-105 feet, although in the western part of 

the mining district isolated chert nodules and a thin 

band of nodular chert are found as much as 8 feet 

above the top of the very cherty sequence that marks 

the top of zone A of that unit. The cherty unit is 

dolomite in the mining district and to the southeast. 

Some distance northwest of the district, however, beyond the mouth of the Wisconsin River the facies 

consists almost wholly of limestone, and in the same 

area green shale is also present in the lower three zones. 

The cherty unit includes units 2 through 8 of Stauffer 

and Thiel's section west of Wykoff, Minn. (p. 266).

KOKCHERTY UNIT

The noncherty unit includes the upper, noncherty 

part of the Prosser cherty member, the Stewartville 

massive member, and the Dubuque shaly member. 

The first two of these three are similar in lithology and
298 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

are thus difficult to differentiate, although the base of 

the Stewartville is supposedly marked by the base of 

the upper Receptaculites zone. Unfortunately, Receptaculites individuals are found sporadically at least 

25 feet below the base of the zone; in poorly exposed 

strata their proper stratigraphic assignment is therefore 

exceedingly difficult. Furthermore, in drill cuttings 

the Receptaculites fossils have been destroyed, so the 

criterion is nonexistent.

On the other hand, in areas of excellent exposures a 

bedding plane at the base of a zone of abundant Receptaculites can be taken as a datum for mapping geologic structure locally. This is the boundary selected 

between the Stewartville and zone P of the noncherty 

unit.

The lower two subdivisions of the noncherty unit are 

light-yellowish-buff medium- to coarse-grained crystalline to granular dolomite in medium to thick beds; the 

strata weather brownish, and with honeycomblike surfaces that mask the bedding, making the rock appear 

massive. Minute 3"ellowish to cinnamon-colored specks 

are characteristic of parts of the Stewartville.

Bentonite is found locally along a bedding plane that 

normally contains only shale, about 18 feet above the 

base of the noncherty unit, as in the roaclcut along U. 

S. Highway 20 at the east edge of Galena, 111. (sec. 

20, T. 28 N., K. 1 E.).

Zone P is 35-40 feet thick, and the rocks of the overlying Stewartville total 37-47 feet.

The Dubuque member in the mining district is a 

light-gray to buff fine grained sugary and silty clolomitic limestone that weathers to a yellowish-buff color. 

This rock is medium- to thin-bedded and contains thin 

interbeds of platy clolomitic shale; as a result, it was 

called "shingle rock" by the early miners (Phillips,

1854, p. 129). The clolomitic limestone beds become 

thinner and more calcareous and shale is more abundant, 

in the upper part of the member. The contact of the 

Stewartville and Dubuque is graclational from massive 

honeycombed dolomite below to medium-bedded dolomite and interbeclded clolomitic shale above. The 

Dubuque as a whole is more calcareous than the underlying Stewartville. The brachiopocl Lingula (Pseudolinfjula) iowensis Owen is common, the shell standing 

vertically in the dolomite. The contact between the 

Dubuque and the overlying Maquoketa is regionally 

clisconformable (DuBois, 1945, p. 15), and locally a 

corrosion zone shows pits in the Galena dolomite that 

are filled with the rocks of basal Maquoketa. 11

The thickness of the Dubuque is fairly constant, 

ranging from 35 to 45 feet in the outcrop.

The noncherty unit is dolomite in the mining district 

and to the east. Toward the west, however, the lower 

one-third and upper one-third become limestone, as is 

seen in Stauffer and Thiel's section west of Wykoff, 

Minn.

DISTRIBUTION

The Galena strata generally form bluffs; the rock is 

commonly quarried for road material and agricultural 

limestone. The location and thicknesses in good exposures of parts of the Galena and adjacent strata are 

shown in the following table.

Because of its more thinly bedded and shaly character, the Dubuque does not form bluffs as does the 

more massive rock below. Nevertheless the unit is 

resistant enough to stand fairly well, and it is locally

11 Rail, R.W., 1951, Ostracods from the depauperate zone of the Maquoketa shale: 

unpublished M. S. thesis, 111. Univ., Urbana, 111.

Location and thicknesses (in feet) in good exposures of parts of the Galena and adjacent strata

[Localities shown on fig. 35]

Location

Quarry, south side of road XWJ4SEJ4 sec. 20, T. 3 N., R. 1 W., 2 miles southwest of

Roadcut, U. S. Highway 61, SWMSEHNE^ sec. 26, T. 3 N"., R. 3 W., Grant

Quarrv, 500 feet south of County Trunk U, Grant County, Wis. (loc. 47)

Roadcut, U. S. Highway 151, Grant County, Wis. (loc. 6). _ .... . ... ... . . _

Roadcut, near center east line NWJ4NEJ4 sec. 24, T. 1 NT ., R. 1 E., Lafayette

Roadcut, State Route 11, Grant County, Wis. (loc. 48) ... __ ___

Eagle Point Quarry, west side U. S. Highway 151, Dubuque County, Iowa (loc. 11) __

Decorah

Ion

13+

21.5 

10.5+ 

18.5+ 

19+

17.5 

19

Galena

Cherty unit

ZoncD

10

8.5 

10.5 

9.5 

9.5

12 

10.5

Zone C

10

13 

24 

16 

11

3.5+

II 

29

Zone B

15

7+

15+ 

1 

16 

12+

17

5+ 

11+ 

20

.5

Zone A

25+ 

25+ 

36+

10+

4+

40+ 

69.5 

69 

75. 5± 

73.5

Noncherty unit

Zone P

38 

11+ 

37±

21+

Stewartville

8+

17+
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 299

well exposed in road cuts and quarries. Some of the 

better exposures of the Dubuque strata are listed below:

Readouts along new U. S. Highway 20, in sections 27 and 28,

T. 89 N., R. 2 E., Dubuque County, Iowa (6 miles southwest

of loc. 11, fig. 35). 

Quarry at Loras College, in SE^ sec. 23, T. 89 N., R. 2 E.,

Dubuque County, Iowa. This is the Dubuque type locality

(3 miles southwest of loc. 11). 

Ravine from east, in SWJ4NEJ4 sec. 33, T. 27 N., R. 1 E., Jo

Daviess County, 111. (4 miles south of loc. 30). 

Roadcuts along State Route 11, in sec. 30, T. 1 N., R. 1 W., and

in sec. 33, T. 1 N., R. 2 W., Grant County, Wis. (3 miles

east, and 1 mile west, respectively, of loc. 48). 

Roadcut along U. S. Highway 151 at Chicago and Northwestern

Ry. underpass, near center of east line sec. 18, T. 3 N., R. 1 E.,

3}£ miles east of Platteville, Wis.

Wells that intersected representative thicknesses of 

the Galena are:

Locality 

(See fig. 35)

Well, Swanson farm, SE^SW^ sec. 26. 

T. 1 N., R. 2 E., Lafayette County, 

Wis. (2 miles south of Shullsburg) _ __ 

U. S. Bureau of Mines D. D. holes 15-24, 

29, Bautsch lease Jo Daviess County, 

111. (loc. 30)-- — ———— — —— — ——

Roadcut, State Route 11, Grant County, 

Wis. (loc. 48)——— —— —— — — —

U. S. Bureau of Mines D. D. hole 6, 

Pikes Peak lease, SEJ4 sec. 33, T. 89 N., 

R. 2 E., Dubuque County, Iowa (5

Woodward farm well, near southeast 

corner NWM sec. 35, T. 89 N., R. 1 E., 

Dubuque County, Iowa (9 miles west 

of loc. 22)————————————————

U. S. Bureau of Mines churn drill holes 1, 

5, Piquette lease, SEJ£NEJ£ sec. 35, 

T. 3 N., R. 3 W., Grant County, Wis.

Well, Wiest Brothers farm, SE^SWM 

sec. 12, T. 3 N., R. 5 W., Grant County,

Colesburg city well 1, Delaware County,

Well, County Home, Clayton County,

Cherty 

unit

Zone A

(feet)

65+

66-71 

69.5

67 

12+

75±

73± 

62± 

85±

Noncherty unit

Zone P

(feet)

45

71-' 

38

34 

43

36

S 

43

7

Stewartville 

(feet)

35

7.5

8+

49 

32

19+

0 

35 

0

Dubuque

(feet)

33 

42-46

14+ 

33

22+ 

35 

17

DISTRIBUTION OF FACIES OF THE GALENA AND 

CONDITIONS OF DEPOSITION

The Galena in the mining district is mainly dolomite, 

with bands of chert nodules in the lower half. The 

upper part contains interbedded dolomitic shale.

The dolomite (see p. 284) and possibly the chert were 

formed subsequent to the deposition of the strata and 

are related to the Wisconsin arch, although in a more 

general manner than the chert of the Platteville formation. The contact of the dolomite and limestone 

generally ascends in the cherty unit toward the west 

so that along the western fringe of the district the lower 

part of the cherty unit is limestone (see also Calvin and 

Bain, 1900, p. 406-411); limestone islands, such as the 

limestone lithology of zones C and D in the center of the 

mining district (near loc. 32, fig. 35) are present, however. Beds of the Dubuque are limestone northwest of Elkader, Clayton County, Iowa (loc. 16), 

according to Kay (1935c, fig. 8).

The Galena strata were deposited as platform limestone (Sloss, 1947) in a fossiliferous environment of 

relatively shallow water. The limestone and shale 

sediments of the upper subdivision of the noncherty 

unit, and west of the mining district the lower strata of 

the noncherty unit were deposited under somewhat 

deeper water conditions, and Sloss would assign this 

facies also to the platform type of deposit.

The two discontinuous bentonitic seams near the 

contact of the cherty and the noncherty units denote 

volcanic activity similar to that which supplied the 

bentonitic material of the Decorah (see p. 296).

FAUNA AND CORRELATION

In Minnesota the Prosser member has been divided 

into the following three zones in ascending order 

(Ulrich, 191 Ib, p. 488): Vellamo (Clitambonites) zone, 

Nematopora zone, and Fusispira zone (fig. 37).

Kay (1935a, p. 291) stated that the Vellamo zone is 

equivalent to the upper Hull (of Raymond 1914) (basal 

Trenton) in New York, and that the overlying Nematopora zone has bryozoa that are common in the overlying Sherman Fall formation in New York (see also 

Twenhofel, and others, 1954), although Sardeson (1897b, 

p. 190) had warned that these are facies faunas.

Kay (1935a, p. 291) continued,

The Lower Receptaculites zone, in which Receptaculites oweni 

Hall is abundant, persists southward [from Minnesota] at about 

the horizon of the Nematopora zone.

This is true only for southeastern Minnesota and northeastern Iowa; there it represents zone C as well as B of 

the cherty unit. To the south the base of the lower 

Receptaculites zone migrates upward stratigraphically; 

as a consequence, in the mining district zone C contains no Receptaculites individuals, whereas zone B 

above bears that fossil in abundance and is the lower 

Receptaculites zone there. Toward the north, as is well 

shown near Guttenberg, Iowa (fig. 35, loc. 33), the 

fossils are present four feet above the base of zone C 

and upward through all of zone B.

Zone A can probably be correlated with the Sherman 

Fall (fig. 44) of northern Michigan (Kay, 1935a, p. 292). 

The Kimmswick limestone of Missouri is said to be the 

equivalent of the Prosser.

The Stewartville has been correlated with the upper 

Cobourg (of Raymond, 1921) because of fauna, and with 

the McCune limestone (of Keyes, 1898) of eastern 

Missouri (Kay, 1935a, p. 292; Twenhof el, and others, 

1954).

Sardeson (1907, p. 193) correlated the Dubuque with 

the Oxoplecia (Triplecia) beds of Minnesota which, 

however, he placed in the Maquoketa shale. Kay
300 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

(1935c, p. 583) correlated the Dubuque with the 

Collingwood formation of northern Michigan both by 

fauna and because of stratigraphic position (see also 

Twenhofel, and others, 1954), and Stauffer and Thiel 

(1941, p. 90) hold that the Dubuque fauna of Minnesota "seems to lack most of the diagnostic Galena 

fossils." This is undoubtedly a facies fauna from the 

shale and limestone facies of the Dubuque, and thus 

difficulty may be expected in attempts to correlate it 

with the dolomite-limestone facies.

It has been mentioned by several other stratigraphers 

that the Dubuque member is Utica or even Richmond 

in age. On the other hand, Kay (1935c, p. 579; also 

oral communication, December 27, 1945) has stated 

that he now considers both the Stewartville and the 

Dubuque to be of Trenton age, as does Fettke (1948, 

fig. 2).

The existence of a hiatus between the Stewartville 

and the Dubuque, postulated by Kay and others 

apparently because of the dissimilarity between the 

Stewartville and Dubuque faunas, is doubted by the 

present writers, as no physical evidence of this feature 

can be found. Furthermore, the writers believe that 

this dissimilarity is due to facies.

As the Dubuque fauna has not been carefully studied 

throughout all of its facies, the evidence is inconclusive (see Twenhofel, and others, 1954, p. 270 and 

chart; Lattman, 1954).

ECONOMIC PRODUCTS

The Galena, called by the miners the yellow sandy 

(noncherty unit) and the drab (cherty unit), is the 

principal host rock for zinc and lead minerals. In 

the northern part of the district (15 miles northwest of 

Mineral Point) miners call the upper part of the Ion 

and the lower part of the cherty unit the wool rock. 

The main vein (pitch and flat) and breccia deposits are 

in the lower part of the Galena (zones B, C, and D 

of the cherty unit. In the upper part of the Galena 

dolomite (noncherty unit and zone A of the cherty 

unit are the "openings" along vertical joints ("crevices") from which lead and zinc minerals were mined in 

the early days. An "opening'' (Whitney, 1858, p. 

439-440) is a zone of weathered sandy dolomite at a 

favorable stratigraphic horizon, and was the locus for 

lead mineralization. Several openings commonly are 

superimposed along a crevice. An opening normally 

varies between 5 or 6 feet wide and a fraction of an 

inch. Above the water table galena is the principal 

mineral of economic importance, and below it sphalerite is in most places predominant; in the zone near the 

water table smithsonite and galena are most common.

The Galena dolomite is the principal source of water 

for most of the farms in the mining district.

POST-GALENA BOCKS

Because, this report deals primarily with rocks related 

to the ore deposits, post-Galena rocks will be discussed 

only briefly.

Information regarding these strata is derived mainly 

from studies by Agnew before 1947. More detailed 

discussions of the Maquoketa will be forthcoming in the 

publication dealing with the state of Iowa (Agnew, 

1955), further work is contemplated for the Silurian 

rocks, and additional study of the post-Silurian deposits 

is currently under way.

ORDOVICIAN ROCKS—MAQUOKETA SHALE

The name Maquoketa was applied by White (1870, 

p. 181) to the shales exposed along the Little Maquoketa River about 12 miles west of Dubuque, Iowa 

(fig. 31). In the mining district the Maquoketa occurs 

below and at the base of erosional remnants called 

"mounds;" in Wisconsin these are 4 and 7 miles east 

of Platteville, 16 miles south of Platteville, and 25 

miles northeast of Mineral Point. Elsewhere in Wisconsin the Maquoketa shale is in the uplands south of 

Shullsburg and along the Grant-Lafayette County line 

south of Platteville, and in Illinois and Iowa it is exposed 

next to the escarpment of southward-dipping Silurian 

strata, which passes easterly and northwesterly from 

Galena, 111.

The Maquoketa is principally blue or gray dolomitic 

silty shale and some grayish-buff medium-grained 

sugary argillaceous thin-bedded dolomite; the lower 

30-40 feet of the formation, however, is commonly 

brown in color. In the basal few feet phosphatic 

pebbles and minute fossils are present. This is the 

depauperate zone (Ladd, 1929, p. 371-375), a zone of 

"universal smallness" that contains forms which, 

because of the unfavorable environment, do not attain 

the size of other species of these genera; they are distinct from forms that are dwarfed, which under favorable environmental conditions attain larger size. Away 

from the mining district the Maquoketa varies greatly 

in lithology; to the west, south, and east dolomite is 

more abundant than shale. Furthermore, the thickness 

is not constant. A few miles south of Galena, 111., 

in U. S. Bureau of Mines Mougin diamond-drill hole 

(fig. 35, loc. 30), the Maquoketa is only 108 feet thick; 

8 miles south of Shullsburg, Wis., in the T. T. Redfern 

well (fig. 35, loc. 51), it is at least 170 feet thick; (data in 

files of Illinois Geological Survey, Urbana, 111.) at Blue 

Mounds, Iowa County, Wis., NW# sec. 1, T. 6 N., 

R. 5 E. (3 miles east of loc. 49, fig. 35), it is 240 feet thick; 

(data in files of Wisconsin Geological and Natural 

History Survey, Madison, Wis.) and in Fayette County, 

Iowa, it is about 260 feet thick (Savage, 1905, p. 486).
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 301

At the top of the Maquoketa where it is thickest, a 

reddish hematitic clay and pebble zone has been reported (Agnew, 1955; Workman, 1950) outside the 

mining district. This zone (the Xeda formation of 

Savage and Ross, 1916), probably represents a soil zone 

that marks the unconformity between the Maquoketa 

and the overlying Silurian.

A good exposure of the upper part of the Maquoketa 

can be seen along U. S. Highway 67 at the south edge 

of Bellevue, Jackson County, Iowa (fig. 35, loc. 8; fig. 

54); the lowermost beds, including the depauperate 

zone, can be seen in the railroad cut just west of Scales 

Mound, Jo Daviess County. 111., SW# sec. 26, T. 29 

N., R. 2 E. (1 mile northeast of loc. 51, fig. 35).

The Maquoketa strata are a poor host rock for zinc 

and lead minerals. In the Glanville prospect at Scales 

Mound, Jo Daviess County, 111., NW# sec. 24, T. 29 

N., R. 2 E. (3 miles northeast of loc. 51, fig. 35), a 

short drift was driven for sphalerite and barite in 

dolomite at about the middle of the Maquoketa; the 

minerals did not occur in paying quantity. In some of 

the more dolomitic beds pyrite is not uncommon, and 

in many places it is abundant in the phosphatic 

depauperate zone.

FIGURE 54.— Maquoketa shale (Om) overlain by dolomite of early Silurian age (Sd), 

in readout and quarry, U. S. Highway 67, half a mile south of Bellevue, Jackson 

County, Iowa, (fig. 35, loc. 8).

The basal part of the Maquoketa contains abundant 

organic material and in many places gives an oily scum 

to the water bailed during the drilling of wells and 

prospect holes.

SILURIAN ROCKS

Silurian rocks, locally referred to as Niagaran although they are older than Niagaran, are found only 

in the "mounds" and at the southern edge of the mining district, where erosion of the southward dipping 

beds has created an escarpment (p. 300). Excellent exposures of the upper beds are seen in quarries at Platte 

Mound, locality 52, Lafayette County, Wis. (fig. 35) 

and Belmont Mound, Lafayette County, Wis., XW 

corner, sec. 2, T. 3 N., R. 1 E. (3 miles east of loc. 52); 

the lower beds are well exposed at Bellevue, Jackson 

County, Iowa (loc. 8).

The Silurian rocks in the zinc-lead district are mainly 

yellowish-buff medium- to coarse-grained "sugary" 

dolomite, in part vuggy. Near Galena, 111., the thickness totals as much as 200 feet (Willman and Reynolds, 1947, p. 7). Near Galena the uppermost 90 

feet of rock contains chert and is marked at the top 

by Peniamerus; next below occurs approximately 20 

feet of noncherty strata; below that lies about 65 feet 

of cherty beds. The basal 20 feet of strata is argillaceous dolomite. In general the Silurian rocks differ from 

the Galena dolomite in being less vuggy and more 

yellowish; furthermore, the Galena dolomite of the 

mining district possesses no beds comparable to the 

laminated silty and argillaceous dolomite of the lower 

part of the Silurian.

Rocks of Silurian age overlie the Maquoketa unconformably; the basal argillaceous siliy zone appears to 

thicken and thin inversely with the thickness of the 

underlying Maquoketa.

An interesting effect of rock alteration is seen at the 

Blue Mounds (3 miles east of loc. 49, fig. 35), where 

the complete sequence of dolomite of Silurian age has 

been silicified (Whitney, 1862, p. 190. See also well 

for radio station WIBA-FM transmitter, NW% sec. 1, 

T. 6 N., R. 5 E., Iowa County, Wis.; data in files of 

Wisconsin Geological and Natural History Survey, 

Madison, Wis.). This is thought to be due to leaching and weathering of a limestone that contained 

siliceous fossil shells (Hubbard, 1900).

The dolomite of the Silurian, because of its similarity 

to that of the Galena, should be a potential host rock 

for zinc and lead minerals. However, because it is 

present mainly along the southern margin of the mining district and to the south, aw&y from the center of 

mining activity, and as it is covered throughout most 

of its extent near the district by glacial deposits, its 

ore-bearing possibilities have not been appraised. 

Isolated crystals of sphalerite and galena have been
302

found in many places, and mining attempts have followed the discoveries of small amounts of galena at 

Sherrill Mound, 10 miles northwest of Dubuque, and 

near Clinton and Anamosa (Calvin, 1896, p. 110), Iowa, 

which are 30-40 miles southeast and southwest of 

Dubuque, respectively (fig. 31).

South and west of the mining district the dolomite of 

Silurian age is an adequate source of water for farm 

wells.

POST-SILURIAN DEPOSITS

Several types of deposits of post-Silurian age have 

been found locally in the mining district. These 

include:

1. Boulders of quartz sandstone in anomalous stratigraphic positions, mostly in the Galena and 

Decorah. These may be related to sandstone 

"dikes" injected into joints from below, or as 

sedimentary filling from above. They occur at 

many localities in the central part of the mining 

district.

2. Boulders of hematite—near heads of ravines southeast of Hazel Green, Wis. (southwest corner of 

Lafayette County).

3. Boulders of quartzite and greenstone—near head of 

ravine in Wisconsin, 7 miles northwest of Spechts 

Ferry, Iowa (fig. 35, loc. 10).

4. Conglomerate at McCartney —poorly cemented aggregate of local and exotic pebbles, at crest of 

ridge, near No. 3, above.

These four types of deposits may have been laid down 

at one or more times since the end of the Silurian 

period. During Pennsylvanian time sandstone may 

have been deposited in what is now the mining district; 

the present northernmost exposure of sandstone of 

Pennsylvanian age in this general area is only 35 miles 

south of Dubuque. During Cretaceous tune ironcemented gravels or conglomerates were emplaced in 

this general region; the nearest such deposit is at 

Waukon, Iowa (fig. 35). During Pleistocene time it 

is possible that some of these deposits were laid down 

by any of several processes, including ice rafting. 

Further study of these interesting stratigraphic features 

is under way.

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STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 303

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•———— 1897, The term Pecatonica limestone: Am. Geologist, 

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Heyl, A. V., Jr., Agnew, A. F., Lyons, E. J., Behre, C. H., Jr. 

(in preparation), The geology of the upper Mississippi 

Valley zinc-lead district: U. S. Geol. Survey Prof. Paper.

Heyl, A. V., Jr., Lyons, E. J., and Agnew, A. F., 1951, Exploratory drilling in the Prairie du Chien group of the Wisconsin 

zinc-lead district by the U. S. Geological Survey in 1949- 

1950: U. S. Geol. Survey Circ. 131.

Heyl, A. V., [Jr.], Lyons, E. J., Agnew, A. F., Behre, C. H., Jr., 

1955, Zinc-lead-copper resources and general geology of 

the Upper Mississippi Valley district: U. S. Geol. Survey 

Bull. 1015-G, p. 227-245.

Heyl, A. V., Jr., Lyons, E. J., and Theiler, J. L., 1952, Geologic 

structure of the Beetown lead-zinc area, Grant County, 

Wis.: U. S. Geol. Survey Min. Inv. Ser., Field Studies Map 

MF3.

Howell, B. F., and others, 1944, Correlation of the Cambrian 

formations of North America: Geol. Soc. America, Bull., v. 

55, p. 993-1003, chart.

Hubbard, G. D., 1900, The Blue Mound quartzite: Am. Geologist, v. 25, p. 163-168.

Ireland, H. A., and others, 1947, Terminology for insoluble 

residues: Am. Assoc. Petroleum Geologists Bull., v. 31, 

p. 1479-1490.

Kay, G. M., 1928, Divisions of the Decorah formation: Science, 

new ser., v. 67, pt. 1, p. 16.

———— 1929, Stratigraphy of the Decorah formation: Jour. 

Geology, v. 37, no. 7, p. 639-671.

———— 1930, Formations subjacent to the Black River-Trenton 

line [abs.]: Geol. Soc. America Bull., v. 41, p. 201-202.

———— 1931, Stratigraphy of the Ordovician Hounsfield metabentonite: Jour. Geology, v. 39, p. 361-376.

1932, Base of Ordovician Galena formation [abs.]: Geol.

Soc. America Bull., v. 43, p. 268.

———— 1934, Mohawkian Ostracoda, species common to Trenton 

faunules from the Hull and Decorah formations: Jour. 

Paleontology, v. 8, p. 328-343.

———— 1935a, Ordovician system in the upper Mississippi 

Valley: Kans. Geol. Soc. Guidebook 9th Ann. Field Conf., 

p. 281-295.

———— 1935b, Distribution of Ordovician altered volcanic materials and related clays: Geol. Soc. America Bull., v. 46, 

p. 225-244.

———— 1935c, Ordovician Ste wart ville-Dubuque problems: Jour. 

Geology, v. 43, p. 561-590.

———— 1939, Stratigraphic setting, Wisconsin-Illinois district, 

in Bastin, E. S., and others, Contributions to a knowledge 

of the lead-zinc deposits of the Mississippi Valley region: 

Geol. Soc. America Special Paper 24, p. 25-28.

1940, Ordovician Mohawkian Ostracoda: Lower Trenton

Decorah fauna: Jour. Paleontology, v. 14, p. 234-269. 

Kay, G. M., and Atwater, G. I., 1935, Basal relations of the

Galena dolomite in the upper Mississippi Valley lead and

zinc district: Am. Jour. Science, 5th ser., v. 29, no. 170,

p. 98-111. 

Kay, Marshall, 1948, Summary of Middle Ordovician bordering

Allegheny synclinorium: Am. Assoc. Petroleum Geologists

Bull, v. 32, no. 8, p. 1397-1416. 

Keyes, C. R., 1898, Some geological formations of the Cap-auGres uplift [111.]: Iowa Acad. Sci. Proc., v. 5, p. 58-63. 

Krumbein, W. C., 1947, Shales and their environmental significance: Jour. Sed. Petrology, v. 17, no. 3, p. 101-108. 

Ladd, H. S., 1929, The stratigraphy and paleontology of the

Maquoketa shale of Iowa: Iowa Geol. Survey, v. 34, p.

305-448. 

Lattman, L. H., 1954, The sub-Eden beds of the Ohio Valley

around Cincinnati: Am. Jour. Sci., vol. 252, no. 5, p. 257-276. 

Lincoln, F. C., 1947, Last Chance zinc mine, Grant County,

Wis.: U. S. Bur. Mines Rept. Inv. 4028. 

McFarlan, A. C., and White, W. H., 1948, Trenton and preTrenton of Kentucky: Am. Assoc. Petroleum Geologist

Bull., v. 32, no. 8, p. 1627-1646. 

Nelson, W. A., 1922, Volcanic ash bed in the Ordovician of

Tennessee, Kentucky, and Alabama: Geol. Soc. America

Bull, v. 33, p. 605-615.
304 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Owen, D. D., 1840, Report of a geological exploration of part of 

Iowa, Wisconsin, and Illinois, 1839: Congressional Documents, 26th Gong., 1st sess., H. Ex. Doc. 239.

Percival, J. G., 1855, Annual report on the geological survey of 

the State of Wisconsin (1854), Madison.

———— 1856, Annual report of the geological survey of the State

of Wisconsin (1855), Madison. 

Pettijohn, F. J., 1926, Intraformational phosphate pebbles of the

Twin City Ordovician: Jour. Geology, v. 34, p. 361-374. 

Phillips, J. V., 1854, The geology of the Upper Mississippi Lead

region: Mining Mag., v. 2, p. 129-138. 

Powers, E. H., 1935, Stratigraphy of the Prairie du Chien (upper

Mississippi Valley): Ivans. Geol. Soc. Guidebook 9th Ann.

Field Conf., p. 350 (fig. 224), 390-394. 

Raasch, G. O., 1935, Stratigraphy of the Cambrian system of the

upper Mississippi Valley: Ivans. Geol. Soc. Guidebook 9th

Ann. Field Conf., p. 302-315.

———— 1951, Revision of Croixian Dikellocephalids: 111. Acad. 

Sci., Trans. v. 44, p. 137-151. 

-, 1952, Oneota formation, Stoddard quadrangle, Wisconsin: 111. Acad. Sci. Trans., v. 45, p. 85-95. 

Raymond, P. E., 1914, The Trenton group in Ontario and 

Quebec: Canada Geol. Survey Summary Rept. 1912, 

p. 342-350.

———— , 1921, A contribution to the description of the fauna of 

the Trenton group: Canada Geol. Survey Mus. Bull. 31.

Rich, J. L., 1951, Three critical environments of deposition, and 

criteria for recognition of rocks deposited in each of them: 

Geol. Soc. Ajuerica Bull., v. 62, no. 1, p. 1-20.

Sardeson, F. W., 1896a, The St. Peter sandstone: Minn. Acad. 

Nat. Sci. Bull., v. 4, p. 64-88.

———— 1896b, The Galena and Maquoketa series, Pt. I.: Am. 

Geologist, v. 18, pt. 2, p. 356-368.

———— 1897a, The Galena and Maquoketa series, Pt. II: Am. 

Geologist, v. 19, pt. 1, p. 21-35.

———— 1897b, The Galena and Maquoketa series, Pt. IV: Am. 

Geologist, v. 19, p. 180-190.

———— 1897c, Nomenclature of the Galena and Maquoketa 

series: Am. Geologist, v. 19, p. 330-336.

———— 1898, Intraformational conglomerates in the Galena 

series: Am. Geologist, v. 22, p. 315-323.

———— 1907, Galena series: Geol. Soc. America Bull., v. 18, 

p. 179-194.

———— 1926a, Beloit formation and bentonite (1): Pan Am. 

Geologist, v. 45, p. 383-392.

— ———— 1926b, Pioneer re-population of devastated sea bottoms: 

Pan Am. Geologist, v. 46, p. 273-288.

1933, Stratigraphic affinities of Glenwood shales: Pan

Am. Geologist, v. 60, p. 81-90. 

Savage, T. E., 1905, Geology of Fayette County: Iowa Geol.

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Savage, T. E., and Ross, C. S., 1916, The age of the iron ore in

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187-193. 

Schuldt, W. C., 1943, Cambrian strata of northeastern Iowa:

Iowa Geol. Survey, v. 38, p. 379-422. 

Scott, E. R., and Behre, C. H., Jr., 1935, Structural control of

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Shaw, E. S., and Trowbridge, A. C., 1916, Galena Elizabeth

quadrangles: U. S. Geol. Survey Geol. Atlas, folio 200. 

Sloss, L. L., 1947, Environments of limestone deposition: Jour.

Sed. Petrology, v. 17, no. 3, p. 109-113.

Stauffer, C. R., 1925, Mineralization of the Platteville-Decorah 

contact zone in the Twin City region: Geol. Soc. America 

Bull., v. 36, p. 615-622.

————, 1935, Conodonts of the Glenwood beds: Geol. Soc.

America Bull., v. 46, p. 125-168. 

Stauffer, C. R., and Thiel, G. A., 1941, The Paleozoic and related

rocks of southeastern Minnesota: Minn. Geol. Survey Bull.

29. 

Strong, Moses, 1877, Geology and topography of the lead region:

Geology of Wisconsin [Wisconsin Geol. Survey], v. 2, pt. 4,

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Templeton, J. S., 1948, Members of the Glenwood formation in

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————, 1950, The Mt. Simon sandstone in northern Illinois: 111.

Acad. Sci. Trans., v. 43, p. 151-159. 

Templeton, J. S., and Willman, H. B., 1952, Guidebook 16th

Ann. Field Conf. Tri-State Geol. Soc. (central-northern

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Thiel, G. A., 1937, Petrographic analysis of the Glenwood beds of

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1, p. 113-122. 

Thwaites, F. T., 1923, The Paleozoic rocks found in deep wells

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no. 7, p. 529-555.

————, 1931, Buried pre-Cambrian of Wisconsin: Geol. Soc. 

America Bull., v. 42, no. 3, p. 719-750.

Trowbridge, A. C., and Atwater, G. I., 1934, Stratigraphic problems in the upper Mississippi Valley: Geol. Soc. America 

Bull., v. 45, no. 1, p. 21-80.

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Survey, Bull. 26.

Twenhofel, W. H., and others, 1954, Correlation of the Ordovician formations of North America: Geol. Soc. America Bull., 

v. 65, p. 247-298, chart.

Twenhofel, W. H., Raasch, G. 0., and Thwaites, F. T., 1935, 

Cambrian strata of Wisconsin: Geol. Soc. Ameiica Bull., 

v. 46, no. 11, p. 1687-1744.

Ulrich, E. O., 191 la, Bearing of the Paleozoic Bryozoa on paleogeography: Geol. Soc. America Bull., v. 22, p. 252-257.

————, 191 Ib, Revision of the Paleozoic systems: Geol. Soc. 

America Bull., v. 22, p. 281-680. 

-, 1924, Notes on new names in table of formations and on

physical evidence of breaks between Paleozoic systems in 

Wisconsin: Wis. Acad. Sci., Arts, Letters Trans., v. 21, 

p. 71-107.

Ulrich, E. O., Foerste, A. F., and Bridge, Josiah, 1930, systematic 

paleontology [of late Cambrian and early Ordovician formations of Ozark region, Missouri]: Mo. Bur. Geology and 

Mines, 2d ser., v. 24, p. 188-212.

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of Iowa: Iowa Geol. Survey, v. 1.

Whitney, J. D., 1858, Chemistry and economical geology [of 

Iowa] in Hall, James and Whitney, J. D., Report on the 

geological survey of the State of Iowa, v. 1, p. 324-472.

————, 1862, Stratigraphical geology, in Hall, James and Whitney, J. D., Report of the geological survey of the State of 

Wisconsin, v. 1, p. 140-193.

Williams, James Steele, 1952, Discussion in Symposium of evolutionary explosions in geologic time: Jour. Paleontology, v. 

26, no. 3, p. 387-388.

Willman, H. B., and Payne, J. N., 1942, Geology and mineral 

resources of the Marseilles, Ottawa and Streator quadrangles: 111. Geoi. Survey Bull. 66.
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 305

Will man, H. B., and Reynolds, R. R., 1947, Geological structure 

of the zinc-lead district of northwestern Illinois: 111. Geol. 

Survey Rept. Inv. 124.

Wilmarth, M. G., 1938, Lexicon of geologic names of the United 

States: U. S. Geol. Survey, Bull. 896, pts. 1, 2.

Winchell, N. H., and Ulrich, E. O., 1897, The Lower Silurian deposits of the upper Mississippi province: Minn. Geol. Survey, 

v. 3, pt. 2, p. Ixxxiii-cxxviii.

Workman, L. E., 1950, The Neda formation in northeastern 

Illinois: 111. Acad. Sci. Trans., v. 43, p. 176-182.

STRATIGRAPHIC SECTIONS

Section 1.— Type section of McGregor limestone member. Ravine 

from south, one mile west of McGregor, Clayton County, Iowa, 

NE}i sec. 28, T. 95 N., R. 3 W. (fig. 35, loc. 25}

[Described by Paul Herbert, Jr., and A. F. Agnew, Nov. 7,1944; revised by Agnew, 

Aug. 18, 1945]

Thickness Decorah (feet) formation:

Spechts Ferry shale member:

Shale, bluish-green, very fossiliferous; many lenses 

of greenish-buff fine-grained earthy limestone 

(partly covered) ______________________________ 6. 0

Shale, gray_____________________________________ 1. 6

Bentonite, with platy chocolate shale on top _______ .1

Shale, brown and green, mottled._________________ . 3

Total, Spechts Ferry________________________ 8. 0

Unnamed member:

Limestone, dark-brownish-gray, medium crystalline-- . 7 

Shale, dark-brown, platy, and light-gray, orangeweathering bentonite; bentonite grades laterally 

into orange siltstone___________________________ . 4

Total, unnamed member. 1. 1

Platteville formation:

McGregor limestone member:

Limestone, light-grayish-buff with buff mottled

areas, fine-grained, somewhat fossiliferous________ 4. 8

Limestone, light-grayish-pink, very fine grained, very 

dense, in thin nodular beds, very fossiliferous 

(mainly brachiopods and bryozoans); many grayishbrown thin wavy shale partings.________________ 18. 0

Total, McGregor_________________________ 22.

Pecatonica dolomite member:

Limestone, somewhat dolomitic and silty, lightbrown, fine- to medium-grained, sugary, in 

medium to thick beds; basal bed contains phosphatic nodules-.-- — —______________________ _ 18. 3

Section 2.—Roadcut, County Trunk A, center E1/? sec. 1, T. 4 

N., R. 3 W., Grant County, Wis. (fig. 35, loc. 28)

[Described by A. F. Agnew, spring 1943; revised Aug. 11, 1953]

Thkknes Decorah formation: (feet)

Spechts Ferry shale member:

Shale, green to olive, blocky; thin lenses of

coquina.____________________-_--__----_- 4. 5

Limestone, coarsely crystalline, coquinoid, wavy 

base; argillaceous coquina lens plastered 

against base__ —— __- — — _-_. —— — — --- — - 0. 3-0. 5

Limestone, light-gray, fine-grained, dense._____ 0. 5-0. 7

Total, Spechts Ferry 5. 5

Unnamed member:

Limestone, dark-pink, coarsely crystalline____ 0. 1

Shale, brownish-green, mottled_______________ .2

Limestone, buff, fine-grained, dense ___________ .5

Siltstone, dolomitic, brownish-yellow; brown

soft bentonite at base, 0.1 ft thick __________ .7

Total, unnamed member. 1. 5

Platteville formation: 

Quimbys Mill member:

Limestone, purplish-brown, fine- to mediumgrained, dense; hard dark-brown shale at top___ . 3

Total, Quimbys Mill. 0. 3

McGregor limestone member (Magnolia of Bays

and Raasch 1935): 

Limestone, light-gray, fine-grained, dense, conchoidal fracture__--__-__-_-_-_----_---_-_- 2. 0 

Limestone as above, thin- to medium-bedded,

fracture not conchoidaL___________________ 7. 0 +

Section 3.—Quarry just west of Blanchardville, near center sec. 23, 

T. 4. N., R. 5 E. Lafayette County, Wis., (fig. So, loc. 29).

[Described by A. F. Agnew, spring, 1945]

. Thickness 

Decorah formation: (feet)

Guttenberg limestone member:

Dolomite, buff to light-brown, thin-bedded, mediumto coarse-grained, fossiliferous; some reddishbrown shale; a 0.2 ft bed of light gray, fine, dense 

dolomite at base ____________^--_----__------ 6. 5

Platteville formation: 

Quimbys Mill member:

Dolomite, buff, fine-grained, sugary; conchoidal

fracture; very fossiliferous, especially in upper

3.5 ft; band of chert nodules 0.5 ft below top;

thin limonitic fossiliferous parting at base_--__-_ 13. 0

McGregor limestone member (Magnolia of Bays and

Raasch 1935):

Dolomite, slightly grayish-buff, medium- to thickbedded————————————————————— 12 +
306 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Section 4.—Coon Creek section, the second ravine north of road 

along south line SW}i sec. 13, T. 98 N., R. 7 W., Winneshiek 

County, Iowa, (6 miles east of loc. 14, fig. 35).

[Described by A. F. Agnew, Paul Herbert, Jr., H. B. Willman, October 1, 1944; 

revised by Herbert, November 7, 1944]

/-, i j T ., Thickness Galena dolomite: (feet)

Cherty unit (zone D):

Limestone, buff to light-brownish, fine-grained, thinbedded, fossiliferous; local greenish argillaceous 

areas and paper-thin green shale partings-______ 15. 0±

Decorah formation: 

Ion dolomite member:

Shale, greenish, calcareous, fossiliferous; Prasopora,, . 4

Limestone, grayish-brown and gray, mottled, 

coarsely crystalline, dense, fossiliferous; local 

green shale partings-_________________________ 2. 1

Shale, essentially green to gray-green; thin lenses of 

nodular limestone; Prasopora at top____________ 10. 9

Limestone, brownish-gray, coarsely crystalline, 

dense, fossiliferous; some greenish argillaceous 

areas; interbedded green shale_________________ 1. 4

Shales, green to gray-green____________________ 7. 9

Total, Ion-_-______________________ 22. 7

Guttenberg limestone member:

Limestone, gray, with dark-gray spots, mediumcrystalline, fairly argillaceous________________ 1. 0

Limestone, greenish-buff, dense, nodular, fossiliferous; green and partly brown shale; brown shale at 

base____-______-___-________________---_____ 4. 0

Limestone, greenish-gray with dark-gray mottlings, 

dense, nearly lithographic, fossiliferous; some 

green calcareous shale ________________________ 1. 5

Total, Guttenberg_______________________ 6.5

Spechts Ferry shale member:

Shale, green to blue-green, with interbedded lenses of 

greenish fossiliferous coquina-like limestone; thin 

orange-weathering bentonite 2.4 ft above base___ 14. 5

Total, Spechts Ferry.__________________ 14.5

Unnamed member:

Limestone, dark-gray, dense, glassy; thin brownish 

shale at top and base_________________________ . 7

Limestone, light-brown, mottled gray, dense______ 1. 1

Bentonite, orange-weathering,___________________ .2

Shale, dark-brown _____________________________ . 1

Total, unnamed member____________________ 2. 1

Total, Decorah____________________________ 45.

Platteville formation: McGregor limestone member: 

Limestone, medium-gray, mottled, glassy, thinbedded-________________________________________ 1. 5

Section 5. Quarry, 1 mile north of Darlington, in SW}iSE}i sec. 27, 

T. 3 N., R. 3 E. Lafayette County, Wis. (2 miles north of loc. 13, 

fig. 35)

[Described by A. F. Agnew, August 11, 1945; revised April 7, 1946]

T-. , .. .. Thickness Decorah formation: (feet)

Ion dolomite member:

Dolomite, light-grayish-blue, coarsely crystalline, 

sugary, thin-bedded; very fossiliferous near base_ 9 +

Shale, white to light-brown; weathers yellowishorange like bentonite but is hard and gritty 

because of dolomite grains. ___________________ .2

Guttenberg limestone member:

Dolomite, light-brownish-buff, otherwise like Ion 

above; in thin nodular beds, but weathers massive; 

fossiliferous_-___-------------------------_-- 3. 0

Dolomite as above, but weathers to thin beds_____ 5. 0

Total, Guttenberg_____________-_-___-_-___ 8.0

Spechts Ferry shale member:

Limestone, like Quimbys Mill below, but greenishgray ; dolomitic shale parting at base ___________ .2

Total, Spechts Ferry_______________________ 0. 2

Platteville formation: 

Quimbys Mill member:

Limestone, buff, fine-grained, greenish-gray mottled 

dolomitic shale parting at top; fucoidal openings 

containing phosphatic pebbles and green shale 

fragments in upper 0.2 ft_____________________ .5

Dolomite, light-grayish-buff, slightly pinkish, finegrained, dense, medium-thick bedded, nodular; 

conchoidal fracture_______--_-_-_------_-__-__ 13. 5

Section 6.—Quarry Ice Cave Bridge, NE}iNW}i sec. 15, T. 98 N., 

R. 8 W., Decorah, Winneshiek County, Iowa (fig. 35)

[Described by Paul Herbert, Jr., Nov. 9, 1944; revised by Herbert and Agnew,

Sept. 1, 1945]

Decorah formation: Thickness 

Spechts Ferry shale member: - (feet) 

Rubble of green shale and limestone blocks; thin

bentonite 0.3 ft above base____-________________ 5 +

Shale, dark-green to olive, blocky; 0.2 ft grayish-buff 

argillaceous limestone beds at top and base______ 1. 3

Shale, green above, dark brownish-gray mottled 

green and olive at base ________________________ .9

Thickness, Spechts Ferry______________._____ 7.2 +

Unnamed member:

Limestone, light-brown, fine-grained, very dense, 

medium-bedded; thin yellowish shale parting at 

top; undulatory lower surface ______________ 2. 4-2. 7

Shale and siltstone, buff to orange, bentonitic; grades 

laterally into bentonite; brown platy shale at 

base____-_____--___-_-_-_---_--_--------- 0. 2-0. 5

Thickness, unnamed member_________________ 2. 9

Platteville formation:

McGregor limestone member:

Limestone, gray, weathers light-brown, mottled

grayish._______-_-______-___---__-_-___---___ 10 +
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 307

Thicknes 

(feet)

Section 7.—Ravine, west of State Route 51, near center 

sec. 82, T. 97 N., R. 5 W., Allamakee County, Iowa (fig. 85, 

loc. 35).

[Described by A. F. Agnew, June 29, 1953]

Decorah formation:

Guttenberg limestone member:

Limestone, grayish-buff, fine-grained, dense, fossiliferous; buff platy iiiterbedded shale.-__________ 10. 0

Covered._________________________________________ 10. 0

Unnamed member:

Limestone, flesh-colored, fine-grained, dense, fossiliferous..- _____________________________________ . 8

Limestone in 2 beds, the upper like that above; the 

lower bluish to flesh-colored____________________ .5

Bentonite, orange, soft-_____________________ 0. 1-0. 2

Thickness, unnamed member.____________ 1. 4—1. 5 +

Platteville formation:

McGregor limestone member:

Limestone, buff to flesh-colored, mottled brown, 

fine-grained, dense ____________________________ .5

Limestone, buff, thin-bedded___ _________________ 22. 0±

Section 8. — Quarry at Spechts Ferry, Iowa, near center sec. 4, T. 

90 N., R. 2 E., Dubuque County, Iowa (fig. 35, loc. 10)

[Described by A. F. Agnew, November 1942; revised October 1945.]

Decorah formation:

Guttenberg limestone member (''oil rock"):

Limestone, tan to light-pinkish-brown, fine-grained, 

dense, conchoidal fracture, nodular; interbedded 

brown shaly partings--- _______________________

Spechts Ferry shale member (clay bed) :

Limestone, light- to medium-brown, fine grained,

crystalline, dense, nodular; phosphatic pebbles

rare near top; olive-brown shale parting at top_ _ _

Shale, greenish and brownish, weathers brown;

blocky ___ __________________________________

Limestone, light-brown to buff, slightly mottled 

greenish, finely crystalline, dense, fossiliferous; 

phosphatic nodules rare at base; basal contact 

wavy_-___ ___________________________________

Shales, greenish-blue and purplish-gray, blocky, 

thin-bedded; with thin coquinas and dark-brown 

weathered zones ______________________________

Limestone, light-greenish-gray, fine-grained, dense___0. 

Shale, greenish-blue, blocky, very fossiliferous_---__ 

Limestone as above, with coquina at top _ _________

Shale, greenish-blue, blocky__ __________ __________

Coquina_ ______________________________________

Limestone as above _____________________________

Shale, greenish-blue, blocky______________________

Limestone as above_ _ ___________________________

Shale, yellowish-olive, soft, clayey ________________

Shale, purplish-gray, thin-bedded, calcareous, fossiliferous ______________________________________

Shale, soft, olive, weathers orange (fossil remains?) __ . 

Shale, dark-grayish-green, blocky _ ________________

Limestone, dark-greenish-gray, fine-grained, dense, 

conchoidal fracture_______._____ _______________

(feet) 

12. 4

.7 

. 3

1. 0

1. 3 

1-. 2 

. 3 

. 2 

. 8 

. 1 

.3 

. 2 

.3 

.1

. 4

0-. 1

. 9

. 3

Section 8.—Quarry at Spechts Ferry, Iowa, etc.—Continued

Thickness Decorah (feet) formation—Continued

Guttenberg limestone member—Continued

Shale, dark-grayish-olive, hard, calcareous, fossiliferous ______-___-_______----_--_--_____--_-_ . 1

Limestone, as next above._______-_-_--__-_______ . 5

Shale, dark-brownish-gray _______________________ .1

Bentonite, weathering yellowish-orange, ___________ . 1

Shale, dark-brownish-gray, blocky ________________ .3

Limestone, as next above; brownish in lower 0.1 ft__ .3

Total, Spechts Ferry_____.____________________ 8. 8

Platteville formation:

Quimbys Mill member ("glass rock"):

Shale, dark-brown, mottled with greenish areas, hard,

thin-bedded___ — — __--___. — - — — - — ---- — - 0. 

Shale, dark-brown, platy, and brown dense lime-

. 2

Total, Quimbys Mill______-__--_--_------___ 0. 3

McGregor limestone member (Magnolia [of Bays and

Raasch 1935] Trenton):

Limestone, light-grayish-buff near top, merging into 

light-greenish-gray below, finely crystalline, dense, 

fossiliferous, conchoidal fracture___-_-__--_-____ 3.1

Section 9.—Bluff, southeast bank Galena River, in NW}iNE}i 

sec. 15 T. 1 N., R. 1 E., Lafayette County, Wis. (one mile south 

of loc. 27, fig. 35)

[Described by A. F. Agnew, September 18, 1944]

Thickiwas Decorah (feet) formation:

Guttenberg limestone member ("oil rock"):

Limestone, light pinkish-buff, fine-grained, dense, 

conchoidal fracture, nodular, fossiliferous; lightbrown, chocolate-colored shale_______________ 13±

Spechts Ferry shale member (clay bed):

Shale, brown and greenish-olive, blocky_________ . 3

Limestone, light-gray, very thin bedded, nodular, 

fine-grained, dense, fossiliferous---------------- . 9

Limestone, mottled pink and greenish, fine-grained,

dense, fossiliferous; phosphatic nodules abundant- . 4

Shales, greenish and brownish, calcareous; limestone 

nodules; greenish and pink coarsely crystalline 

coquina-__-_------------__---------------- 0. 1-. 2

Limestone, light-greenish-gray, very fine grained,

very dense, sublithographic, nodular___________ 1.0

Bentonite, white; weathers orange-brown_______ 0. 1-. 2

Limestone, nodular, as above but unfossiliferous- _ 0. 1-. 2

Total, Spechts Ferry____--_------------- 3. 0-3. 1

Platteville formation:

Quimbys Mill member ("glass rock"):

Limestone, dark-pink, more coarsely crystalline than 

above, nodular, fossiliferous------------------- 0. 1

Limestone, purplish, medium-grained, dense, conchoidal fracture____________ __________________ 6 +
308 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Section 10.—Quarry half a mile east of Calamine, Wis., just 

north of the bend in the road, near the southwest corner sec. 9, T. 

3 N., R. 3 E., Lafayette. County, Wis. (fig. 35, loc. 4%)

Thicknes

(feet)

[Described by A. F. Agnew, August 11, 1945]

Decor ah formation:

Guttenberg limestone member:

Dolomite, light-brown to buff, weathers lightyellow-buff; fine- to medium-grained; fossiliferous; 

in thin, wavy beds with light-brown argillaceous 

and thin platy shale partings; very shaly and 

fossiliferous in lower 0.5 ft_ ______________ _____ 5 +

Dolomite, light-brown to buff, fine-grained, fossiliferous; very sandy with dolomite granules ______ 2

Dolomite, light-grayish-buff, finely granular, dense_ . 1 

Spechts Ferry shale member:

Dolomite, greenish and buff-spotted, argillaceous, 

containing phosphatic nodules; otherwise not 

fossiliferous- ____________--.__-_-.___ ________ 0. 0-. 1

Platteville formation: 

Quimbys Mill member:

Dolomite, light-grayish-buff, slightly pinkish, finegrained, medium-bedded, nodular; light-brownishbuff argillaceous and platy shale partings rare, 

fossiliferous; zone of involutions at top contains 

phosphatic pebbles and green argillaceous dolomite inclusions-____•______-----______-_______ 11. 5

Section 11.—Roadcut, U. S. Highway 53, about 1 mile north of 

Guttenberg, Iowa, in SW]i sec. 5, T. 92 N., R. 2 W., Clayton 

County, Iowa (fig. 3d, loc. 33}

[Described'by A. F. Agnew, Paul Herbert, Jr., H. B. Willman, October 1, 1944: 

revised by Herbert and Agnew, September 3, 1945]

Thickness 

(feet)

Decor ah formation: 

Ion dolomite member:

Limestone, dark-gray to pinkish, crystalline, dense,

thinly bedded__________________"____________ 2. 7

Shale, gray-green, platy, calcareous_______________ • 3

Limestone, as above, but more bluish and fossiliferous _ • 8 

Shale, as above_-_-.--_-_______^_________________ . 8

Guttenberg limestone member:

Limestone, slightly more pink than above, becoming 

darker pinkish brown downward, nodular; reddishbrown shale common__________________________ 6. 9

Shale, brown, wavy-bedded, some reddish cast. _ ^ _ _ .3 

Limestone as above_____________________________ 7. 6

Shale, light-brown----------____^_____________^_ . 1

Limestone as above, but very dense, glassy, with

conchoidal fracture____________________________ . 6

Total, Guttenberg-_________________________ 15. 5

Spechts Ferry shale member:

Shale, greenish-olive, weathering brownish, platy,

fossiliferous__________________________________ . 4

Limestone, light-grayish-brown to grayish-green;

argillaceous phosphatic nodules common-________ . 7

Shale, green, blocky; thin coquina beds.___________ 1. 5

Limestone, blue-green; in thin beds, very fossiliferous_____-_________________________________ 1. 1

Thickness 

(feet)

Section 12.—Quarry }i-mile north of York Church, Wis., near 

center sec. 5, T. 4 N., R. 6 E., Green County, Wis. (fig 35, 

loc. 44)

[Described by A. F. Agnew, spring, 1943]

Decorah formation: 

Ion dolomite member:

Shale, gray-green, gritty, somewhat fossiliferous,

thin-bedded argillaceous greenish-gray dolomite. _ 4. 0 

Dolomite, in thicker beds but otherwise similar to

that above,__________________________________ 5. 5

Guttenberg limestone member:

Dolomite, light-gray to buff, pinkish, medium- to 

fine-grained, somewhat vuggy, thin- to mediumbedded, nodular; a few very thin stringers of 

reddish-brown platy shale; at base a 0.1-ft zone 

of red shale and pinkish dolomite-______-__--_-_ 6. 5

Platteville formation: 

Quimbys Mill member:

Dolomite, light-cream to buff, fine-grained, very 

dense, semiconchoidal fracture; layer of chert 1.5 

ft below top; at base a thin reddish-brown indistinct shale parting________---___..__-_---__-_ 13. 0

Section 13.—Quarry, 500 feet south of County Trunk U, near 

center of SEtf. sec. 18, T. 3 N., R. 3 W., Grant County, Wis. 

(fig. 35, loc. 47)

[Described by A. F. Agnew, July 1, 1945]

Galena dolomite: TMff)t> 

Cherty unit (zone D):

Dolomite, buff, vuggy, coarse-grained-___________ 8. 5

Decorah formation:

Ion dolomite member (gray beds):

Shale parting, olive-gray, dolomitic______________ . 1

Limestone, dolornitic, buff to gray, coarsely crystalline, vuggy_________-------_--_---_-------__- 1. 4

Shale, greenish-gray, argillaceous; greenish-gray 

shaly dolomite_________-__________-__--_--- . 7

Lireestone as above___________-____---_-------_ 9. 3

Limestone, light-gray, coarsely crystalline-_______ 2. 5

Total, gray beds,_____________________ 14. 0

Ion dolomite member (blue beds):

Shale, olive-gray, very fossiliferous; thin coquina

beds----___-____-_---_-----_--------------- 1.6

Limestone, dark-gray, medium-crystalline, dense-__ . 5 

Shale, greenish-gray__-_________________________ . 1

Limestone as above____________________________ .9

Shale as above___________________-_-_-_---_-_- . 3

Limestone as above____________________________ 1. 6

Shale as above_______-_______-__-_____--_-__-- . 2

Limestone, grayish-green, coarsely crystalline;

rounded medium-sized quartz sand grains____-__ . 7

Shale parting, greenish-gray____________________ . 1

Limestone, light-gray to buff, coarsely crystalline,

transitional with Guttenberg limestone below____ 1. 5

Total, blue beds_____________________ 7. 5

Total, Ion.________________-____ 21. 5

Guttenberg limestone member ("oil rock"):

Limestone, light brownish-pink, medium-crystalline, dense, fossiliferous; brownish wavy thin shale 

beds___ ____________________________________ 5 +
STRATIGRAPHY OF MIDDLE ORDOVICIAN ROCKS IN THE ZINC-LEAD DISTRICT 309

Section 14. — Quarry, southwest edge of Darlington, Wis., in 

sec. 3, T. 2 N., R. 3 E., Lafayette County, Wis. (fig. 35, loc. 13)

[Described by A. P. Agnew, summer 1943]

Galena dolomite: Thickness 

Cherty unit (zone C) : (feet) 

Dolomite, grayish-buff, coarse-grained, mottled; 

bands of chert nodules common_____ ___________ 2. 0

Galena and Decorah formations:

Cherty unit (zone D) — Ion ir^ember (gray beds) : 

Dolomite, light creamy-gray to light-olive, coarsely 

granular, mottled- ___________________________ 15. 0

Decorah formation :

Ion dolomite member (gray beds) :

Dolomite, lighter colored than above; a very few 

olive-buff shale beds, including a 0.1-foot shale 2 

feet from top, and a 0.2-ft shaly zone 2.5 ft above 

base_____________________-_-_---__-_-_----_- 10. 5

Ion dolomite member (blue beds) :

Dolomite, dark-bluish-gray to light-blue, fine- to 

medium-grained, with much interbedded shale — 

the upper 0.5 ft is very shaly, fossiliferous. _____ 6. 0

Guttenberg limestone member ("oil rock") :

Dolomite, reddish, very coarse-grained, vuggy_____ 3 +

INDEX

A Page 

Abstract.... - — — — — — — —— —_ — — _ — __ — — — — .. — ._. — — — — 251-252

Acknowledgments...... _ ____ _ _____________________________________________ 254

Allan, R. S., quoted--__________. -___-_-__.._.._.-_.----._..__-__--__--_-_-_ 260-261

Apatite_--_ -_--_-_---_....---__--_-- _ --___-._._.___-_...__-__-. _ __——___ 289

B 

Bain, H. F., quoted..-.....-.- _ ---.-._........ __ ..... _ ._-____-__ 261,262,292

Bays, C. A., quoted-______._. ...... ...__-_-_______--_-_ _ ....... _ ....... 269,279

Bays, C. A., and Raasch, G. O., quoted---..---. _ ....--.-..... _ _______ _ 269

Bell, W. C., quoted................... _ .................................... 261

Beloit dolomite, classification of ---_-----__-_-_______-_-_____..__.._._._.___ 262-264

Bentonite layer— ....... 263, 264-265, 283, 287, 288-289, 290, 292, 294, 296, 297, 298, 299

Bull dum. _ -------------------................_............._._............. 292

O 

Calvin, Samuel, quoted---.--..-.. _ -_______________________-._.-...-_.-__ 275,285

Cambrian age, rocks of-__-----_-_-_---_-__-_ _ .................... 254-256, 271-272

Chert— -------------..-.-...---------.............-......... 259, 267, 281, 290, 301

Cherty unit. See Galena dolomite.

Conglomerate, at McCartiiey _ -__--_--______________-___-__-._...........-_ 302

Cox, G. H. quotcd_ _ .-.---.-.---.-----..........._.......................... 262

D

Decorah formation, boundary of, with Platteville formation... _-_________-- 261-262

condition of deposition ___ _ _ --_-----___-_______________.-_....-..-___- 296

correlation with units in Minnesota, chart----------------------------. _ 264

cross section of _ _________ _ .......................... _ _______________ 263

description. _.__-.-_---_- _ ______ __ _________________________ 255,259,285-286

disconformity with Platteville formation ___ _ _ __________________ 256, 259, 288

distribution of facies--..-...-.--...-----.-...--.-.. ..---......-.-..--.-. 295-296

stratigraphic column __ ____________________________________________ _ .. 268

stratigraphic relations..- _ ............................................. 256,257

in Missouri-. ____________________ _ __________ _ ______________ _ ___. 257

stratigraphic section.. __ _ .......... _ _______ _ ...... 285,287,293-294,305-309

thickness. --__----_____________-____.._.___.________________........._____ 296

unnamed limestone member__-_-_-_____. _______________________________ 264-265

See also Spechts Ferry shale member, Guttenberg limestone member, and

Ion dolomite member. 

Dolomitization, effect of. — —— — — — — — — — — — — — — — — — — — —— —_ 283,292

Dresbach sandstone.. ______________ _ _____________________________ 254, 255, 271, 272

Driftless Area.-— — . — — _ _ _ _ __________________________________________ 252

Dubuque member of Galena dolomite, contact of, with Maquoketa shale_--__ 298 

contact of, with Stewartville member__--_-__----------_-____________.__ 265, 298

correlation _____________________________________________________ 259,264,299-300

stratigraphic column showing __ ________________________________________ 268

thickness--------------------------......................-................ 298

E

Facies relationships, diagrammatic cross section._____________________________ 257

Field work.--_--.--.-----------.------..........................--..-...... 252-254

Flat, definition of term.-----------------------------_-_______________„ 295

Fossils, Petterophon bed....--- — ..--. — .......______....__________ 264,265

Chtambonites beds.— — — ———- — -—_ — — — __ — — — — — — — — — — __ 264,266

depauperate zone....____ ............................................... 300

Fusispira beds______________________________________________ 264,266,278

Madwea zone.... -...- — .-.-..---...-............____________ 264,265-266

need for study of_____.— — — ——— ———— — — — — — — — —_ 260-261,265

Nematopora beds.______________.-...._..__.___._________________ 264,266.278

FfeceptacuUtes zone...--- — - — --------.-_ ... 262-263, 267, 268, 296, 297-298, 299

zone of Orthis subaequata—-.......--.--- — - _—— — — — — — — — —.... 262-263

Vanuxemia bed____ _ ___ _ _ _______________________________ _ ___ _ 264,282

See also Stratigraphic sections and individual formations and members of

formations. 

Franconia sandstone___ _____._____..---.___________________.______. 254, 255, 271, 272

O 

Galena deposits —_ — — - — -- — — — — — -- — — ——— — — --- — — -. — --- ——- — — 300

Galena dolomite, cherty unit, facies of....------------------ — ------------- 257,297

cherty unit, fossils in--------___--___-_--------_---_--__-----------__-__ 297

subdivisions.----_--_-..--___---_------------.---------------------. 296-297

thickness-.----.--------- —-------------------------------------- 297,298

conditions of deposition.—_____ — — — — — —_ ——— — -- — ---------------- 299

correlation.--------..--_- — — — _-_ — — — — — - — — — — — — — ---- — — ——- 299

with units in Minnesota, chart-------------.------------------------- 264

distribution....-----.--------------------------------------------.- 298-299

economic products_— — — — — ____-. — —.-_ — — — — - — — — — -— — -- — — 300

facies--.... — —- — _------ — -_-___ —— ------- — —------- 299

fossils...---..........------.-.-.._-_.--- — ------------------- - 266-267,299

lithologic description----.--- — ------------------.--------- —------- 255,259,267

noncherty unit, bentonite in __..____________-._-____--_--_-__ _— _______ 298

description... —— — — —— - —— . —— — — — —— —— — — --— 297-298

fossils—- — -.- — ---——— — —— - ——- —— - — — — - — — — 297-298

thickness---.-----------------.---.----------------------------------- 298

stratigraphic column.____ — — — — — — _ — — — -_ — — — — — — ——- — -- 268

stratigraphic relations-------------------------------------------------- 256, 257

stratigraphic sections---------------------------------------- 285,293,306-309

subdivisions of—_-- — -- — — — — — ——— -- — - — — - — — ---- 265,269

thickness--.--.------------------------------------------------- 296,298

See also Dubuque member and Stewartville member. 

Garnet----------.------------.-.-.--------------------- 276,277,278,281,283,287,289

Geographic setting-_ — __-_ ———_______------------------ — ____----_____ 252

Glass rock. See Quimbys Mill member of Platteville formation. 

Glenwood shale member of Platteville formation, conditions of deposition.— 284 

correlation.__________-.-.--_-______--_-_-__-_-_-----_---_---------_- 264,277,278

distribution of ----------------_ — ——— — — — — — — — — — — — —— — 277

economic products—------------------------------------------------------ 277

fossils—— —— —— — —— ——— — —— ——— ————— — ——— — — 277

lithologic description._____._.--_____----.-___-_-__-------_--------_ 268,275-276

stratigraphic grouping of—--------------------------- — ---------------- 256

stratigraphic relations of-_-__-.-..-.------------------------------- 257,276-277

stratigraphic sections.-.---____________-_—_-_____ — — ___ — — -_-___— 275

thickness of different facies _________._____ — ______-. — ______. — -____ 276

Grant, U. S., quoted--.------------.--------------- ——------------- 276,292

Greenstone boulders—------------------------------- — — — ---------------- 302

Guttenberg limestone member of Decorah formation, correlation—__._.___ 264,293 

cross section showing stratigraphic relations__-------------------------- 263

distribution.. —————— ———-- — — ——— — — —— — — 292-293

effect of dolomitization of. — —-—-- — — — — -- — — — — — -—- — — — — — — -— 292

effect of siliciflcation on-.------------------„..-------------------- 292

facies relations of _- ———— ———— ——— —— —— ——— ———— 257,286,288,292

fossils —— — —— — — —— — — ——— — ——— — — — ———— — —— — — 293

lithologic description.____---._-_-_---------------.------_------------ 268,289

minerals in._________.____.____._..________---.-_____---_____----------_ 293

stratigraphic grouping of._-_______---_-__---_--_--___-_-- — ------------ 256

stratigraphic sections--....-.---_______--------------_ 285,287,290-291,305-309

thickness--------------------------------------------------------------- 259,295

H 

Hematite boulders-—— ———— — ——— ———— — —— —— — ————— 302

Introduction __- —— — — ————— - ———— ——— ——— ————————— 252-254

Ion dolomite member of Decorah formation, correlation._______ ——_-_-__. 264,295

cross section showing stratigraphic relations._-..____ _—_ — — --._ — — 263

economic products..----------------------------- — ----- — -------------- 295

facies relations..------------------------------------------------ 257,286,288,294

fossils,____-- — .- — - — ----.--____--.__-_.__._ ———— _._-_-.-.____- 295

lithologic description.-.--------.---------------------------- 268,293-294

stratigraphic grouping of_ ———— — — ——— — — - — _ — -------------- 256

stratigraphic relations-_------_--___---------------- — — -------------- 294-295

stratigraphic section---_-____--._----__----------_-__--- 285,290,293-294,306-309

thickness-.----------.-------------.-------------------------- 259,294,295,298

Iron mineralization--_-_-_---------------_-----------_------------ 272, 273,274

J 

Jordan sandstone.----_----------------------------------------------------- 271

311
312 INDEX

Lead minerals_____________________________ 272,273,277,282,289,293,294,300

Literature cited—____-_______________________________ _ _.__.. 302-305

Location of zinc-lead district_-_.--_-----------_-._...................... 252,253

Lower Magnesian limestone. See Prairie du Chien group.

M

McGregor limestone member of Platteville formation, conditions of deposition- __ — ——— — — — — — — — — — — — — _ — — __ 284-285

correlation.-------------------.--------------------.-- 264,270,278,280,282

distribution-- — _...................................................... 282

facies relationships__-_-_____-_-_____-_---__-_____________-_.._..._.-_ 257, 284

fossils — — — — — — — — — — — — — —_ — _ — — — _---------- 281,282

lithologic description--_------_---------_ — - — ----______________________ 259,279

minerals— — _______________________________--..-__.-_-_---_-__ 282

origin and application of name__- — --------------------------------------- 269

stratigraphic grouping oL____________-_-_-_______-___-__-_---_________ 256, 268

stratigraphic relations---_-------------_ — --_--..-____________...... 281-282

stratigraphic sections--______________-_________________-—.. 274-275,282,305-307

Madison sandstone.-- ——— ——— — __- — — __-_--...-_-___-____________ 269,271

Magnolia beds— — — —— — — ———— ———— —— — 269,274-275,279-282,284-285

See also McGregor limestone member of Platteville formation. 

Map, showing localities cited in text--------------__________.___........_. 258

Maquoketa shale, contact of, with Dubuque member of Decorah formation—. 298 

description..... —— ..„... ———..———— — — .__________ 255,259,268,300-301

minerals in.._______________________________.-_................___ 301

Mifflin beds---------.-------------.-..-...--- 269,274-275,279-282,284-285

See also McGregor limestone member of Platteville formation. 

Mounds, in Wisconsin_._-----------_._._.......................... 300

Mount Simon sandstone_..--._._._-...__________________ 254, 255, 271, 272

N

Noncherty unit. See Galena dolomite.

Pecatonica dolomite member of Platteville formation, building stone------- 278,279

conditions of deposition_____._._ — ___..__..__...._.-.___________...._ 284

correlation___.............____.......__................._.. 264,278,279

distribution.-_._._-...............-......................._-......_ 278-279

fossils___.---_.__----_-_-_.__-._-----_-----_-_______-___-..___-____ 279

lithologic description..__-__..._____-___._-__________.... 259, 268, 277-278

origin and application of name__—— — — — . —-—- — — __ — — —. 269

stratigraphic grouping of_...._.----............................--.-..._ 256

stratigraphic relations.--- ——-------------------— — __ — -_-- — _— 278

stratigraphic sections_________________________________________________ 275, 305

thickness_.-_-_______________.-__--__-_______-_-----._-__________.. 257, 259, 278

Phosphatic pebbles, in Maquoketa shale—.----_---_-__---_-___-_.._-_______- 300

Phosphatic podules, in Pecatonica dolomite member___-_..________......_. 278

Phosphatic pebbles, in Quimbys Mill member..____________.__-________.... 288

Phosphatic nodules, significance of, in Spechts Ferry shale member_____.... 288

Pitch, definition of term._-_--.____ — — —-- — -._..-. —______— .__ — — _ 295

Platteville formation, boundary of, with Decorah formation._______________ 261-262

conditions of deposition-.—-_—-._--.- —— _—__ —— — __.— — — _ 284 

correlation with units in Minnesota, chart--------------------_------_--_. 264

disconformity with Decorah formation-...___......-.__-.______.__. 256, 259, 288

distribution of facies of-__ ——---—-_---_- — — —— — - — ___ — — 284

general description__________________________........_.__ 255, 257, 268, 274-275

stratigraphic relations,. —— ___ — —— _—__ —_ — — -- — —______ 256, 257

stratigraphic section...__......_.._...__-_-______ 274-275, 282, 305-308

See also Quimbys Mill member, McGregor limestone member, Pecatonica

dolomite member, and Glenwood shale member. 

Prairie de Chien group..-..................................... 255, 256, 271, 272-273

Pre-Cambrianrocks_ — — — ———-- — — - — — — -——- — -- — — — — _______— 254, 271

Prosser member of Galena dolomite.------------------ 259, 264, 266, 267, 268, 297-298

Purpose of the investigation-- — — — -— — — — __.____-____._.. 252

Quartziteboulders—---_-- — ------ — --- — — ---———------------— 302

Quimbys Mill member of Platteville formation, conditions of deposition..—— 285 

correlation........---.._._-______________-_-----.------------------------- 284

cross section showing. ______________--__--_----_-__--__--_--_-_---__-- — __ 263

distribution..-_.---.-- —_ — — -----—--------- — - ———-----— — — 283-284

economic products_____________— -._.. — ------------------------------ 284

effect of dolomitization on_--_ — _—— — - — — - — --— — — ——— — ---- — .-- 283

facies relationships — — — — — —— - — — — ---- — — — — — 257, 284, 288

lithologic description...———— —— — — — —— —— ———— 259, 268, 282-283

origin and application of name.. ——--_. — _____________________________ 269

stratigraphic grouping of-—------ — — — — --—-- — -_----. —--- — __ 256

stratigraphic relations--------------..__-____________________ 259, 283, 288

stratigraphic sections..__..-_-______________._.----------.- 274, 282, 285, 305-308

thickness-.——- — — -- — -- — — —— —— —— — - — — —— — —— - 284,295

R

Raasch, G. O., with Bays., C. A., quoted. 

Ross, C. S., quoted.--------.---..-.------.

2G9

St. Peter sandstone, description--.-.------------------------------ 255, 256, 273-274

stratigraphic sections.__ _ ___— ____ —— ._--.--_ ——— — -- — — -_-_ 275

Sandstone boulders— — _ — — - — — -- — - — - — -- ——— — ————-—— — ———— 302

Sardeson, F. W., quoted---------------________________.__ 263,266

Shaw E. W., with Trowbridge, A. C., quoted..— —— - — - ———— —— —— -— 288 

Silicification, effect of____________-_----.._____________________________________ 292

Silurian age, rocks of- — -------- — ----------------- — —— — — -— 255, 301-302

Smithsonite-.----.------------------ —------------------------------- 300

Spechts shale member of Decorah formation, condition of deposition,......... 296

correlation..- —------ — -------- — -— — — ------ —— ———— —— 264, 289

facies relationships—.-.--.-.---.----------------------- 257, 286, 288, 295-296

fossils—.-.----------- —--------______.____-._.__.____._.... 289

economic products_____________-___-_-_--__---_---__-_--______-__--_-_-_ 289

lithologic description-- — — — -- — —— - — — ——— —— — ———— 268, 286-287

significance of phosphatic pebbles in..... — __. — — _ —.___-_._ —.......... 288

stratigraphic grouping of_ — — _________-_-_-------_-___ — ——- ——.- 256

stratigraphic relations...__--. — ... — — __ — ____ — .............. — .... 288, 289

cross section showing--. — —— -- — --— — — - — — — — — — — ———_— — — 263

stratigraphic sections-— — __....____._ — __— 274, 282, 285, 287, 291, 305-308

thickness.-.- — ————— — — - — -— — - — — — —— —— 259, 286-287, 295

Sphalerite------------------------------------------------------------------- 300

Stauffer, C. R., and Thiel, G. A., quoted—— — — -___._____________.___ 266

Stewartville member of Galena dolomite, contact of, with Dubuque member- 265, 298

correlation..— _ __ _ ______.___________........._____ 264,265-266,299

stratigraphic column showing_.......................... —___________ 268

stratigraphic section, quoted__ — ———- — -- — ———-— — — — - —...— 266

Stratigraphic charts—------------------------------------ 255, 264, 268, 270, 278, 280

Stratigraphic column, of Platteville, Decorah, and galena strata...---__-._-.- 268 

Stratigraphic cross section— — — — — — — __. — — ___ — — _— — — — — - — - 257, 263

Stratigraphic grouping of the rocks_----- — _____-___-. — ------- — -------- 256

Stratigraphic names, origin and application of.. — — — - — — — — - — — — - — - 269

Stratigraphic principles_.__ — — — — — — — — — — — —— ——— — — — — 260-261

Stratigraphic sections——— — — 261, 262, 266, 267, 274-275, 282, 285, 293-294, 305-309 

Stratigraphy of the mining district....——. —. —— — —————————— 271-302

Structure-.---------------------------------------------------------------- 254

T

Terminology, local, for rocks. — ——————- —— —— — - — — ———— -— 268, 280 

Thiel, G. A., with Stauffer, C. R., quoted—————— —— — — — — — — — 266

Tourmaline------------------------------------------------------- 277, 278, 281

Trempealeau formation,.— .......___.________-_---_-.-._-_____ —... 254, 255, 271

Trenton beds. See McGregor limestone member of Platteville formation. 

Trowbridge, A. C., and Shaw, E. W., quoted-------------------------- 288

W 

Wool rock... — —— ——————————— —— — ———————— —— — — 300

Zinc minerals..----- ....-.____-_.__________- 272, 273, 277, 282, 289, 293, 294, 295, 300

Zircon.—-_.______-_-_--------------------------- 276, 277, 278, 279, 281, 283, 287, 289

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