i . , ..
'Jhi torrtpkiton & this 1}LtStz .suypo-r-tzd 1\an4y fiward
. in :his ft'i.ef1tOYE 1 °fo attotJteY ouhiartdt:Jtf:j s:r11ditate sittdt:itt . t}tt Gto 1o,,qB 1)ltpaA1rtt:1tt,
1bttvets1t,,y tf 1rltn1te$ofa1 l)u1uti
of Univer o.itv of .Minnesota. Dulutli THE QUATERNARY STRATIGRAPHY AND GLACIAL
HISTORY OF THE DULUTH-SUPERIOR AREA
A THESIS
SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL
OF THE UNIVERSITY OF MINNESOTA
BY
PATRICK MICHAEL LANNON
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
MASTER OF SCIENCE
NOVEMBER 1986 ABSTRACT
Three major lithostratigraphic units of Pleistocene age are exposed in the Duluth - Superior area, all of which are the result of
Late-Wisconsin glacial activity.
The lowermost unit, a reddish-brown, sandy-textured diamicton
(sand/silt/clay ratio: 52/38/10) containing abundant clasts of
Precambrian red sandstone, is interpreted to be subglacial till of the
Lower Cromwell Formation, which was deposited by the Superior lobe during the St. Croix phase of glaciation approximately 20,000 years B.
P. Its main exposure within the study area is at the base of stream valleys and roadside cuts in the southern portion of the Eska
Quadrangle.
Overlying the Lower Cromwell Formation is a very compact reddish-brown, silt-rich diamicton (sand/silt/clay ratio: 30/43/27_) interpreted to be subglacial till of the Upper Cromwell Formation, which was deposited by the Superior lobe during the Automba phase of glaciation, approximately 14,000-18,000 years B. P. This unit is the dominant stream valley and roadside exposure in the southern portion of the Eska Quadrangle.
The Upper Cromwell Formation also contains a supraglacial facies composed of flow tills and outwash s ediments deposited during the
i retreat of Automba phase ice. These flow tills have been previously interpreted as subglacial tills associated with a later "Split Rock" advance. This supraglacial facies is the dominant surficial deposit in the northern portion of the Esko Quadrangle.
The youngest lithostratigraphic unit grades verticall y from a basal laminated silt and clay into massive r ed clay in its upper portions, and grades laterally into sands and gravels. Topographically, it is confined to elevations below 1115 feet, and is found throughout the southern portion of the Esko Quadrangle and most of the Berea Quadrangle. Based on its areal extent, facies relationships, grain-size trends, stratigraphic relationships, geomorphic expression, sedimentary structures, and engineering properties, the entire unit is interpreted to be glaciolacustrine sediment and is assigned to the Wrenshall
Formation. It was deposited in Glacial Lake Duluth approximately 10,000 years B. P. The massive clay facies of this unit is not the equivalent of the Douglas Till Member of the Miller Creek Formation deposited during a late-glacial ice expansion referred to as the "Marquette" phase approximately 9900 years B. P. That advance affected portions of northern Wisconsin and Upper Michigan, but did not reach as far southwest into the Superior Basin as the Duluth - Superior area.
ii ACKNOWLEDGEMENTS
This study was funded in part by a grant from the Department of
Geology, University of Minnesota - Duluth.
Many people have assisted with this study, including numerous property owners, geological survey staff members, state department of transportation workers, and various county officials. A special thanks is extended to each one.
Assistance in the field was provided in part by Colin Reichhoff,
Doug Davis, and John Heine.
A special thanks is extended to Beth Moyer of RREM Engineering, Inc. for drafting many of the figures; to Glenn Evavold, for his help in many aspects of this study (and for being a very understanding employer); to
Mary Nash and Joan Hendershot of the geology department (who serve as every graduate student's unofficial "secondary advisors"); to the geology department faculty, who were always willing to help in any manner needed, especially Dr. John Green, who reviewed and offered very helpful suggestions for the manuscript; and finally, to my thesis committee members - Dr George Ahlgren (Biology), Dr. Dave Darby, and most of all, to my advisor, Dr. Charlie Matsch.
Lastly, I want to thank my parents for all they have done for me. It is to them that I dedicate this thesis.
iii TABLE OF CONTENTS
ABSTRACT • •••••••••••••••••••••••••••••••••••••••••••.••••••••••• ... i
ACKNOWLEDGEMENTS • ••••••••••••••••••••••••••••••••••••••••••••••••••• iii
TABLE OF CONTENTS . ....•....••.•••..••.••...... •••.•••...... ••••.••• iv
LIST OF ILLUSTRATIONS ..•.•••••••••••••.••••.•••..•.•..•...•.••...•••• vi
Figures •• .vi Plates. .vii Tables. .vii
INTRODUCTION ...... 1
Location ..•...... •••••••••• 1 Field Methods ..••...••• ...... 3 Laboratory Methods ••.• . ... 3 Bedrock Geology .••.••• • • 4 Previous Work and Problems ••• • • 4
QUATERNARY STRATIGRAPHY AND SED TI1ENTOLOGY ••••.••••••..•...... •••••••• 12
Introduction •••••••...•••••••••••••.••••••.••••• • 12 Quaternary Stratigraphy of Northeast Minnesota •• • 12 Till Genesis .••••. • •• 16 Till Units •••••••• . •• 20 Possible Pre-St. Croix Phase Unit. ..20 General Characteristics ••••••••• . .•.... 20 Origin and Regional Correlation • ...... 22 Lower Cromwell Formation •••••••••• ...... 24 General Characteristics ....•...••••.....••....••.•.....••..•. 24 Origin and Regional Correlation •••••••••••••••••••••••••••••• 31 Upper Cromwell Formation: Subglacial Facies •••••••••••••••••••• 31 General Characteristics •••.••••. .31 Origin and Regional Correlation. .34 Upper Cromwell Formation: Supraglacial Facies •••••••••••••••••• 35 Site-Specific Characteristics •••••••••••••••••••••••••••••••••• 38 Glaciolacustrine Units ••••••••••••••••••••••.••••••••••••••••.••• 46 Wrenshall Formation: Shallow-Water Facies •••••••••••••••••••••• 46 Wrenshall Formation: Bedded Silt and Clay Facies ••••••••••••••• 55 General Characteristics ..•..•.•...... ••.....••.•••.....•• 55 Origin and Regional Correlation •• ...... 57
iv • • • • · • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •••••••••••••••• •Page Wrenshall Formation: "Massive Red Clay" Facies ••••••••••••••••• 60 General Characteristics •••••••••••••••••••••••••••••••••••••• 60 Criteria used to determine the origin of the red clay •••••••• 65 Areal Extent ...... 66 Facies Relationships and Grain Size Trends ••••••••••••••••• 68 Stratigraphic Relationships ••••••••••• .- •••••••••.•••.•••••• 70 Geomorphic Expression •••••••••••••••••••• ...... 70 Fabric ...... 7 3 Sedimentary Structures ••••••• • 76 Fossils ...... 78 Engineering Properties (Bulk Density, Consolidation) •• .79 Origin and Regional Correlation •••••• .82 Depositional Model ••••••••••••••••••• . .• 82 Other Quaternary Sediments ••• ...... 86 Introduction ••••••••••••••• . ....•.....•.•... 8 6 Deposits of the Nickerson Moraine •••••••• ...... 86 Sublacustrine Outwash •••••••••••••••• ...... 87 Holocene Alluvium ••••.•••••.••...... 88
LATE WISCONSIN AND HISTORY. .89
Introduction ...... 89 History of Late-Wisconsin Glaciation ••••. • •• 90 Pre-St. Croix Phase ...... 90 St. Croix Phase •• • •• 90 Automba Phase ••.• • • 91 Split Rock Phase. • • 93 Nickerson Phase •• ,95 Agassiz Phase •••• • • 95 Problems With the Late-Glacial Sequence of Events •. • . 97 Post-Glacial History •....•...... •.....•...... • •••••• 101
REFERENCES • ••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 10 6
APPENDIX A •••••••••••••••••••••••••••••••••••••••••••••••••••••••••• A-1
APP END IX B . •.••.•..•••••.•••.••••••••.••••••••••.••..••.•.•••.••.••. A-3
v LIST OF ILLUSTRATIONS
Figures
1 • Location Map . •..•.•.•..••..•.....•..•••...•.•· •.•..••.•..•..••..•.. 2
2. Bedrock Geology ...... 5
3. Post-Nickerson Ice Margins ••.•••.••••••••••••••••••••••••.•••.••• 11
4. Quaternary Stratigraphy of Northeastern Minnesota .•.•.••.••.•.•.• 14
5. Depositional Model for Multi-layered Sequences .••••••..••.••••••• 21
6. Cobble Lithology of Tills ••.••••••.•.••.•••••••••..••.••••••.•.•• 23
7. Lower Cromwell Formation XRD . ....•...... •...... 27
8. Cromwell Formation Fabric Diagrams ••..•...... ••.••...•••••.•..•. 30
9. Upper Cr omwell Formation XRD .•...••..••.•....•••••.••.••••••••••• 33
10. Upper Cromwell Format ion Textural Diagram •.••.••.•.•••••.....•.•. 36
11. Upper Cromwell Formation (Supraglacial Facies) Fabric Diagrams ••• 37
12. Exposure of Till Un i ts in Jay Cooke State Park •••••••.••••••.•.•• 40
13. Supraglacial Ridge-Development Model ••••••••••••••.•••.•.••••..•• 43
14. Exposure of Wrenshall Formation in Jay Cooke State Park •••••••••• 45
15. Cross-bed Measurements from Lagro Pit •••••••••••••••••••••••••••• 49
16. Proctor Stratigraphic Section ••••••••••.•••••••.••••••••••.•••••• 51
17. Proctor Diamicton Fabric Diagram ••••••••••••••••••••••••••••••••• 53
18. Wrenshall Formation Gray Silt XRD •••••••••••••••••••••••••••••••• 58
19. Wrenshall Formation Red Clay Grain-size Curve •••••••••••••••••••• 62
20. Wrenshall Formation Red Clay XRD •••.••••••••••••••••••••••••.•••• 63
21. Facies Assemblage of Zarth (1977) •.••.••••••.•••••••••••••••••••• 69
22. Depositional Model for the Red Clay •••••••••••••••••••••••••••••• 85
23. Sub-lacustrine Moraines in Western Lake Superior •••••••••••••••• 100
vi Figures Page
24. Schematic Section of Study Area: Agassiz Phase •••••••••••••••••• 104
25. Schematic Section of Study Area: Present-day •••••••••••••••••••• 105
Plates
1 • Exposure of Cromwell Formation in Jay Cooke State Park ••••.•.•••• 29
2. Exposure of Till Units in Jay Cooke State Park •••.••••••••.•.•••• 39
3. Exposure of Upper Cromwell Formation Supraglacial Facies •••..•••• 42
4. Exposure of Wrenshall Formation in Jay Cooke State Park •••••••••• 44
5. Deltaic Exposure in Lagro Pit •••••.•.•.••.•••••.••.•..••••••••••• 47
6. Bedded Sands and Gravels in Lagro Pit •••••••••••••.•••••.•••.•••• 48
7, Proctor Stratigraphic Section (Figure 16) ..••••••••••••••...... •. 50
8. Sand/Diamicton Contact in Proctor Exposure ••.••••••••.•••.•.•.••• 54
9. Wrenshall Formation Bedded Silt and Clay ••...... ••••.••.•.•. 56
10. Exposure of Wrenshall Formation Along St. Louis River .••.••••••.• 71
11. Glacial Lake Duluth Plain .•.•••.•••••••.•••••••••.••••.•••••..••• 74
12. Bedded Sand Within Massive Matrix (Near Lagro Pit) .•••.••.••••••• 77
Tables
1. Cromwell Formation Sand Fraction Lithology ••••••••••••••••••••••• 25
2. Wrenshall Formation Red Clay Mineralogy •••••••••••••••••••••••••• 64
vii INTRODUCTION
The Late Wiscionsin glacial history of the Duluth - Superior area is incompletely understood. Questions involving late-stage ice advances and the origin of surficial clay deposits in the area have not adequately answered by previous work, owing primarily to the fact that detailed investigation of the Quaternary geology of this area has not been carried out. The present study concerns itself the general
Quaternary stratigraphy of the area, the lithology and ger.esis of the
Quaternary deposits, and a geologic history based on their interpretation.
LOCATION
The study area (Figure 1) is located along the Minnesota - Wisconsin border and includes portions of Carlton and St. Louis counties
(Minnesota) and Douglas county (Wisconsin). The Esko and Borea
Quadrangles, United States Geological Survey 7.5 minute topographic map series, comprised the area of concentrated study. The West Duluth and
Frogner Quadrangles (along with portions of adjacent quadrangles) were also included in the study, although to a lesser degree. CANADA
\ MINNESOTA / / / "'
\ ""' I
\I
« .:Y" LAKE 0 '" o" $ 0..,.. .p k--;; SUPERIOR '"=-,.<... :¥ ,0 -'<" oo \ <(} cY /f' l "" •'f. 0- # ,,_--"'·' ,CJ & :;:;;o ;J;;:: C:)
.:::> ()6' g 1-. l<. CJ.."' .!
HOLYOKE
FIGURE 1: Study area location map.
2 FIELD METHODS
Field work was carried out during the Fall of 1984 and the Summer of
1985. United States Geological Survey 7.5 minute topographic maps were used as base maps. Stratigraphic studies included fabric measurements (Holmes, 1941), and stone counts. Cross-stratification attitudes were measured at several delta exposures, and bedrock striae were measured at several outcrop locations. Water-well logs and bore-hole data on file with the Minnasota Geological Survey, the
Wisconsin Geological and Natural History Survey, RREM Inc., and Lakehead Testing were used to construct stratigraphic and to aid in the overall interpretations.
LABORATORY METHODS
Grain-size characteristics of the various units were oy standard sieve and pipette analysis (Folk, 1980).
Color of the samples was recorded using the Munsell color chart according to the method described by Raukas (1982).
Grain counts of the fine- and coarse-sand fractions of the diamictons were done according to the method described by Anderson
(1957).
X-ray diffraction analysis of the <.002mm fraction of the various units was done on a Siemens diffractometer using Ni-filtered CuK(Alpha)
3 radiation. Preparation of samples is described in Appendix A.
Oriented samples and small cores of massive red clay were impregnated (Johnson, 1980) and cut into thin slabs for radiographic examination in a Torrex 150 X-ray unit. However, the results were not helpful as the machine did not detect any sedimentary structures not already visible.
BEDROCK GEOLOGY
Bedrock geology of the study area is predominantly slate and graywacke of the middle Precambrian Thomson Formation (Morey and
Ojakangas, 1970; Clark, 1985), Late Precambrian (Keweenawan) basaltic lava flows of the North Shore Volcanic Group (Green, 1972), and slightly younger Keweenawan red sandstone of the Fond Du Lac Formation (Morey,
1967). A generalized bedrock map of the region is shown in Figure 2. The reader is referred also to Schwartz (1949); Taylor (1964); Sims and
Morey (1972); and Wold and Hinze (1982), for detailed information on the bedrock geology of the area.
PREVIOUS WORK AND PROBLEMS
Detailed studies of the Quaternary geology of Northeastern
Minnesota have been few in number; as a result, results of regional
4 Crysrol I ine rocks
Patcny Cretaceous sandstone ond cl ay
Gr an ite
Crystalline rocks
IOWA WISCONSIN
FIGURE 2: Generalized bedrock map of Minnesota and parts of adjacent states. From Wright, 1972b.
5 studies of a reconnaissance nature are more commonly reported in the literature. Only those reports which are particularly relevant to the present study will be discussed here.
Some of the most important early work was done by Frank Leverett of the United States Geological Survey. In 1929, "Moraines and Shore Lines of the Lake Superior Basin" was published, followed in 1932 by
"Quaternary Geology of Minnesota and parts of Adjoining
Although some of his interpretations were originally made by earlier workers (Upham, 1894; Taylor, 1897; Winchell, 1899a,b; 1901), Leverett added several new ideas and also synthesized the state of knowledge of the time in this area. He outlined the evidence for ice advances, distinguished by several moraine systems, as we l l as evidence for post-glacial lakes (including what is now recognized as Glacial Lake
Duluth).
More recently, E. Wright, Jr. (1969; 1972h; 1973; 1975) and
Wright and others (1965; 1969; 1970; 1973) have outlined a glacial history of Northeastern Minnesota which has remained largely unchanged to the present time. The scenario involves four separate advances of the
Superior lobe into the region.
The oldest ice advance for which there is evidence in the Lake
Superior region is called the St. Croix phase. The term "phase" as used here is interpretive, and refers to an event-stratigraphic unit corresponding to the advance as well as the subsequent stillstand and/or retreat of an ice margin. In the St. Croix phase, the confluent basinal
Superior and upland Rainy lobes advanced from the northeast to a
6 terminus at the St. Croix Moraine. The Rainy lobe deposited a sandy, stone-rich till named the Independence Till by Wright et al. (1970) and more recently included in the Sullivan Lake Formation by Gross (1982).
The Superior lobe deposited a red, sandy till referred to as the lower
Cromwell Formation in Minnesota (Wright et al., 1970), and the Copper
Falls Formation in Wisconsin (Mickelson et al., 1984). Radiocarbon dates of organic lake sediment overlying St. Croix phase till place this advance at about 20,000 years B. ?. (Wright et al., 1973). However, some workers in Wisconsin (C l ayton and Moran, 1982) argue that this date may be too old, because of possible contamination by older carbon. They propose an age of 15,000 years, based on dates using only wood.
Following a retrea t several hundred miles from its maximum position at the St. Croi x moraine, the Superior lobe readvanced during the Automba phase. The Automba phase in Minnesota is correlative with the Tiger Cat phase in northwestern Wisconsin (Clayton, 1984) . Advancing westward cut of the Superior basin, the Superior lobe flowed to the
Mille Lacs Moraine at its farthest margin, forming the Automba drumlin field along the way. On the North Shore Highland it deposited the
Highland Moraine. Till deposited during this phase is similar to that of the St. Croix phase, except that in texture it is slightly more silt-rich. This till comprises the upper Cromwell Formation in Minnesota
(Wright et al., 1970) and is part of the Copper Falls Formation in
Wisconsin (Mickelson et al., 1984). Radiocarbon dates are not available for this advance. However, an age of 18,000 years B. P. is reasonable, based upon radiocarbon dates for the preceeding St. Croix phase and the
7 subsequent Split Rock phase.
Following ice wastage at the close of the Automba phase, a readvance termed the Split Rock phase occurred, which was contemporaneous with the
Pine City phase of the Grantsburg sublobe of the Des Moines lobe (Wright et al., 1973). Prior to this advance, the ice must have retreated far enough into the Superior basin to allow the formation of proglacial lakes, which received clayey and silty because till of this advance is of a clayey texture (Wright, 1972b). It is informally referred to as the "Barnum Till", as is till of the subsequent Nickerson phase (Baker, 1964; Matsch and Schneider, in press). This Split Rock phase till forms a thin, discontinuous mantle over earlier landforms.
Eskers and outwash fans southwest of Cloquet, Minnesota and a small, weakly developed drumlin field in the Split Rock Valley mark the extent of this advance. A small area northeast of Esko, Minnesota was also supposedly covered by this advance (Wright et al., 1969), but was never examined in detail until the present study. In Wisconsin, this advance is represented by the Swiss ice margin (Clayton, 1984). Radiocarbon dates of organic sediments overlying glacial sediments of the Split Rock phase place this advance at 16,000 years B. P. (Wright et al., 1973).
However, Clayton and Moran (1982) argue that this date may be too old and place this advance at 12,300 years B. P. based on correlation with ice margin positions in Wisconsin and Michigan.
A final advance of the Superior lobe, referred to as the Nickerson phase, occurred after retreat from the Split Rock maximum contemporaneously with the Alborn phase of the St. Louis sublobe. It
8 involved the advance of a narrow tongue of ice to the Thomson -
Nickerson moraines and the deposition of a clay-rich till (Wright,
1972b). Radiocarbon dates of lake sediment overlying the Nickerson
Moraine as well as those of sediment overlying correlative Alborn phase till place this advance at 10,500-12,000 years B. P. (wright et al.,
1973). In Wisconsin, the Nickerson phase is correlative with the Winegar
- Athelstane phases, and its limit is referred to as the Nickerson,
Airport, and Lake Ruth ice margins (Clayton, 1984).
Upon retreat of the Nickerson ice, small proglacial lakes formed along the ice front and eventually coalesced to form Glacial Lake
Duluth. The history of the subsequent glacial lakes which formed as the ice front retreated to the northeast is discussed by various workers
(Farrand, 1960; 1969; Saarnisto, 1974; Landmesser et al., 1982; Farrand and Drexler, 1985). These events occurred between approximately 11,500 and 9900 years B. P. (Farrand and Drexler, 1985). What previously had been interpreted as two distinct lal<:e levels, Glacial Lake Nemadji and
Glacial Lake Duluth, has been shown to be features associated with a single Glacial Lake Duluth level (Zarth, 1977). This period is also referred to as the Agassiz phase of glaciation (Wright, 1972b), although active (i.e. advancing) glacial ice was not present in Minnesota at this time.
Deposits of red clay present in the southeast portion of the study area (and present inland along the southern shore of Lake Superior) have presented a problem. The clay has been interpreted as a lacustrine sediment associated with Glacial Lake Duluth (Leverett, 1929; Mengel,
9 1973; Moss, 1977; Zarth, 1977; Moss et al., 1979; Gross, 1982). Wright et al. (1970) defined it as the glaciolacustrine Wrenshall Formation.
Others contend it is of subglacial origin (Johnson, 1980; 1983; Need,
1980) and refer to it as the Douglas Member of the Miller Creek
Formation (Mickelson et al., 1984). Johnson (1980) and Need (1980) had originally correlated the Douglas till with till of t he Nickerson phase.
However, recent work in Michigan (Hughes, 1971; 1978; Drexler, 1981) has shown that a late-glacial ice expansi on termed the Marquette advance at
9900 years B. P. refilled the Lake Superior basin following wastage at the close of the Nickerson phase (Figure 3). The validity of the
Marquette advance has been further confirmed by studies relating discharge events from the Lake Agassiz basin with glacial lake stages in the Lake Superior basin (Clayton, 1983; Drexler et al., 1983; Teller and
Thorleifson, 1983). As a result of this work, the Douglas till is now correlated with till of the Marquette advance (Clayton and Moran, 1982;
Clayton, 1984; Need and Johnson, 1984; Attig et al., 1985), which is referred to as the Lake View advance in northwestern Wisconsin. However, a major problem which has not yet been resolved is the location of the
Marquette ice margin at the time of its maximum extent. Thus, the origin of this red clay in the present study area and the relation between the
Wrenshall Formation and the Douglas Till is a final problem addressed by this study.
10 I I I I I 11,800 - 11,500 BP co 11,000 BP I I I I _____ J-iI ·- •, \,-, - - ...... ,. / ...... _ ...... _ ...... I SUPERIOR LO BF -, '-. '-. ... ,. " / ( Mareniscn Phose) I ----..... _ , \ __ .,_ I I \ ,_..._ MtC lllGMJ LOO E \ •. ·. .· St River 0
Lake .//1 Af9 onquin
i I . I I 10 ,700 BP I 9,900 BP LAKE L il Kf I f--' ,1GilSSIZ : f--' ! - j ''1 \ I '-. 'I '---./ "'"" ?:-' "" """•") \ .. i ; --._ ./ ';;1 - - ...... \ Lake ': r·--- ·-- SUPERIOR LOBE I -. .' , '-, : lake Pha se) ' ... \ '"" """'""' -.' fl \ .. , : Vulut/ •h' ,,.,,.. ... ,' \ '"'"" " / : /Ir-, " co "CF\\.. . \ \ ( ()) ,,,/ ""/ J. ,' ',I ,<---,< ·?(______:_,, ___. '-,) (;;,- __ ,.• \ ', j ..•.. ··1" l.'_ ,-__ r 1,_.-../
' I --- -, .- -- -''· . " ) ... --- ... 0 • 'I 1'/J I • \\ ., /7." r-=c:_ l'oSI · I ' ' 'l/ ' ( v 1 Lo.lie - ' '(I / /1/ 1 I /) "''I'"'"""' / Clurpc"n I ' : -- ,, 1' I' ' . /1 ____ ' ------
Figure 3. Ice margins following the Nickerson maximum. From Farrand and Drexler, 1985. QUATERNARY STRATIGRAPHY AND SEDIMENTOLOGY
INTRODUCTION
Glacial sediments of the Esko-Superior area are the result of a
variety of depositional processes associated with subglacial,
supraglacial, ice-marginal, and proglacial environments. Basal tills,
flow tills, glaciofluvial sediments, and glaciolacustrine sediments are
all present in the study area. Before discussing the characteristics and
genesis of the individual units, a discussion of the general Quaternary
stratigraphy of northeastern Minnesota will be presented, followed by a
brief review of the mode of deposition and general characteri stics of
basal tills and flow tills.
QUATERNARY STRATIGRAPHY OF NORTHEASTERN MINNESOTA
Based on the known glacial history of northeastern Minnesota, multiple till sequences and associated glacial sediments should be
present in the region. In addition, surficial geomorphic evidence would also be expected to reflect the multiple ice advances which occurred
during late Wisconsin time. The few detailed studies which have been
12 done in this region do indicate that such stratigraphy and geomorphic features are indeed present (Wright et al., 1969; 1970; Moss, 1977;
Zarth, 1977; Gross, 1982).
A generalized diagram of the Quaternary stratigraphy and geomorphic features of northeastern Minnesota is shown in Figure 4. The deposits and geomorphic features reflect the activity of the Rainy and Superior lobes and the St. Louis sublobe of the Des Moines lobe during late
Wisconsin time.
The oldest deposits of the region belong to the Sullivan Lake
Formation (Gross, 1982) and the Cromwell Formation (Wright et al.,
1970). The Sullivan Lake Formation includes till of the Rainy lobe previously referred to as the Independence Till by Wright et al. (1969).
The Cromwell Formation is composed of reddish-brown sandy- to silty-textured till and associated sand and gravel deposited by the
Superior lobe and its meltwater streams (Wright et al., 1970) during the
St. Croix (Lower Cromwell Formation) and Automba (Upper Cromwell
Formation) phases. Red Precambrian sandstone derived from the Lake
Superior basin is a diagnostic lithologic indicator for this unit.
The Sullivan Lake Formation (Gross, 1982) is a grayish-brown to yellow-brown colored sandy-textured till containing abundant stones throughout. Gabbro and granite are diagnostic lithologic indicators for this unit. It is the surficial sediment of the Toimi drumlin field and was deposited by the Rainy lobe during the St. Croix phase. This unit was not observed in the study area.
Along its eastern edge, the Toimi drumlin field is buried by the
13 GLACIAL LAKE HIGHLAND MORAINEl DULUTH NW TOIMI DRUMLINS HIGHLAND FLUTES SHORELINE [3 P-WAT R F CIES)
WRENSHALL FM. D (SHALLOW-WATER FACIES)
1·.1 NICKERSEN (BARNUM) TILL A [::;] ALBORN DRIFT OUTWASH AND ESKERS AUTOMBA SW DRUMLINS THOMSON /NICKERSON MORAINE E;l SPLIT ROCK (BARNUM) TILL NE
UPPER CROMWELL FM. RED SS
1· .1 LOWER CROMWELL FM . 8
SULLIVAN LAKE FM.
BEDROCK
Figure 4. General Quaternary stratigraphy and geomorphic features of northeastern Minnesota: A) NW-SE section northeast of Duluth (after Gross, 1982); B) NE-SW section southwest of Duluth. Diagrams not to scale. Highland Moraine, an ice-stagnation complex made up of material assigned to the upper portion of the Cromwell Formation. This unit has also been referred to as the Cloquet Till by Baker (1964). It was deposited by the
Superior lobe during the Automba phase (Wright et al., 1969; Wright,
1972b; Moss, 1977; Gross, 1982).
These units are equivalent to the Copper Falls Formation of northern
Wisconsin, which includes till and glaciofluvial sediments deposited by the Superior lobe and its associated meltwater streams. It is similar in lithology to the Cromwell Formation of northeastern Minnesota and was deposited between 19,000 and 11,000 years B. P. (Mickelson et al.,
1984).
Southwest of Lake Superior the Cromwell Formation is overlain by a red, clay-rich till referred to as the Barnum Till (Baker, 1964). This unit was deposited by the Superior lobe during the Split Rock and
Nickerson phases of the Superior lobe (Wright et al., 1970).
At elevations below 1100 feet, deposits assigned to the Wrenshall
Formation overlie the Cromwell Formation or the Barnum Till. The
Wrenshall Formation consists of sands, silts, and clays deposited in
Glacial Lake Duluth (Wright et al., 1970), which formed along the retreating margin of the Superior lobe following the Nickerson phase.
The Wrenshall Formation is the youngest of the glacial deposits found in the region.
In northwestern Wisconsin, the Copper Falls Formation is overlain by the Miller Creek Formation, which consists of lacustrine silt and clay and clay-rich till. An older member, the Hanson Creek Member, is a
15 clay-rich till equivalent to the Barnum till of northeastern Minnesota
(Matsch and Schneider, in press). However, the origin and correlation of the younger Douglas Member is controversial. Some workers contend it is a clay-rich till deposited during a late-glacial advance which occurred
10,000 years B. P. (Johnson, 1980; Mickelson et al., 1984), while others contend it is laterally continuous with and thus equivalent to the glaciolacustrine Wrenshall Formation (Gross, 1982). Both the Douglas
Member of the Miller Creek Formation and the Wrenshall Formation consist of reddish-brown, clay-rich sediment and form the surficial unit wherever present.
The reader is referred to Matsch and Schneider (in press) for a regional correlation of the glacial deposits found throughout Minnesota and northwestern Wisconsin.
TILL GENESIS
Glacial deposits can be grouped into two general categories according to their mode of deposition: primary and secondary. Primary processes are those which deposit sediment directly by glacier ice; these include lodgement and melt-out processes. Primary deposits or ortho-tills consist of lodgement till and melt-out till (Dreimanis,
1982). Secondary processes are those which rework and resediment primary deposits. These processes include sediment flow as well as fluvial, lacustrine, and eolian processes and are not restricted to the
16 ice-contact environment as are primary processes. Secondary deposits include allo-tills, which are formed from glacial debris which has undergone redeposition shortly after its release from glacier ice
(Dreimanis, 1982). An example of an allo-till is sediment flow deposits, commonly referred to as flow tills (Boulton, 1968).
Since each type of glacial sediment has a unique mode of deposition, each commonly has unique properties which allow the interpretation of its depositional history. The two major types of glacial depositional environments are subglacial and supraglacial.
Lodgement till accumulates at the base of a moving glacier as a result of pressure melting, which releases sediment from the ice and allows the "plastering" of the material onto the bed, or by basal drag, whereby particles are released from the ice onto the bed when the frictional resistance between the bed and the entrained sediment exceeds that between the sediment and the glacier ice (Sugden and John, 1976;
Boulton, 1982; Shaw, 1985).
Some distinguishing properties of lodgement till include: 1) high degree of compactness; 2) shear planes with incorporated sand and/or silt; 3) fissility; 4) fabric commonly parallel to ice flow direction with a-axes dipping upglacier (transverse fabrics may develop in zones of compressive flow); and 5) clasts dominated by local lithologic types
(Dreimanis, 1976). In addition, "bullet boulders", which are large clasts showing abraded stoss sides and plucked lee sides resembling miniature roches moutonnees, are commonly found embedded within lodgement till (Boulton, 1978).
17 Melt-out till accumulates as a result of the release of englacial sediment by the melting of supporting or enclosing glacial ice, primarily in the basal and surface zones of the glacier. Melt-out till formed at the surface tends to be resedimented, whereas basal melt-out till does not (Boulton, 1971). Thus, the original fabric and structures of basal melt-out till have a much greater chance of preservation (Shaw,
1985). Because the melt-out process involves the release of meltwater, which tends to sort and redeposit material, layers and lenses of sorted sediment enclosed within diamictons are common features of melt-out tills (Shaw, 1985).
Some distinguishing properties of basal melt-out till include: 1) less compactness than lodgement till; 2) layers and lenses of sand and silt laminae; 3) transverse or parallel fabric with a-b planes sub-parallel to the surface of deposition; and 4) mixed lithologic composition from local to distant sources (Dreimanis, 1976).
In most cases, distinction between lodgement till and basal melt-out till is very difficult; many workers prefer instead to use the term basal till or subglacial till, which encompasses both of the above types of till (Dreimanis, 1976). This practice will be followed in the present study: the term subglacial till will be used for any sediment interpreted to have been deposited directly from glacier ice in its basal zone. Moderate to high degrees of compaction with strongly-developed clast fabrics will be considered as strong evidence for subglacial deposition.
Sediment flow deposits are commonly referred to as flow tills
18 (Boulton, 1968). However, Lawson (1979) considers this term misleading.
His definition of till is "a sediment, deposited directly from glacier
ice, which has not undergone subsequent disaggregation and
resedimentation11 (Lawson, 1979, p. 28). Because a flow till has
undergone resedimentation after deposition by glacier ice, he does not
consider such a sediment to be a till, and instead refers to such
deposits as sediment flows. However, the definition of the term till as
proposed by the International Union for Quaternary Research (INQUA) is
11 a sediment that has been transported and deposited by or from glacier
ice, with little or no sorting by water" (Dreimanis, 1982, p. 21). Thus,
according to the INQUA definition of till, the term flow till is an
acceptable term, and is considered to be a type of allo-till. The
present study will use this broader definition of till as proposed by
INQUA, and will use the term flow till for till that has been
resedimented within the proximal glacial environment, with little or no
disaggregation or sorting apparent.
Flow tills commonly develop as a result of water saturation of
supraglacial debris (Boulton, 1968; 1971) or by ablation of active basal
ice exposed at the glacier margin or stagnant basal ice exposed in
ice-cored slopes (Lawson, 1979). The saturation or removal of ice
support results in the resedimentation of the material and redeposition as a flow till.
Flow tills are most commonly found around the glacier margin, where
there are exposed ice-cored slopes and where compressive flow and ablation result in a relatively large amount of supraglacial debris
19 (Shaw, 1985). They are commonly found interbedded with associated meltwater sediments and generally have limited horizontal distribution
(Marcussen, 1975).
Boulton (1972) has shown that multiple till sequences are common products of glacial deposition associated with a single episode of advance and retreat of a glacier. Thus, multiple till sequences may represent a single phase rather than a succession of such events. A generalized diagram depicting the development of a multiple till sequence of subglacial till, flow till, and meltwater sediments associated with a single advance/retreat is shown in Figure 5.
Subglacial tills, flow tills, meltwater sediments, and glaciolacustrine sediments make up the bulk of the glacial deposits present within the study area. Each major unit will be described below, along with its origin and correlation to other described lithostratigraphic units of the region.
TILL UNITS
Possible Pre-St. Croix Phase Till
General Characteristics
At one location within the study area, in the NE 1/4, SW 1/4, SE
1/4, Section 2, T.48N., R.16W. (Eske Quadrangle), a sandy-textured
20 flowtill released at glacier surface
builds up
(b)
till forms as containing ice melts / (c)
. . . . , .. ·
, ··... $ ......
(d) -;;:Siiiil!ii fl ow ti 11 : !!!!!!..... me It-out ti 11 <, .. _ .. ·.:-;., till deposited ·"' · subglacially from active ice
Figure 5: Diagram showing how till and meltwater sediments may be interstratified after ice wastage, producing multilayered sequences related to a single glacial advance/retreat (from Boulton, 1971).
21 diamicton rich in red sandstone fragments was observed to underlie the
Lower Cromwell Formation (described below). Only five feet of vertical
exposure was observed. Its upper contact with the Lower Cromwell
Formation was sharp and planar where observable, but was mostly
slump-covered.
Grain-size analysis of the matrix yielded 52% sand, 38% silt, and
10% clay, which is identical to that of the Lower Cromwell Formation.
Lithologic composition of the sand-size fraction is similar to that of
the Lower Cromwell Formation as well. However, it is distinctly more
rich in clasts of red sandstone, as shown in Figure 6. Furthermore, its
appearance was different from that of the Lower Cromwell Formation; it
was less compact and lacked the cohesive structure commonly observed in
the Lower Cromwell Formation.
Origin and Regional Correlation
This lowermost unit may represent a portion of the Lower Cromwell
Formation that has an unusually high content of red sandstone clasts.
Another possibility is that it represents a pre-St. Croix phase till,
possibly correlative with the Hawk Creek Till of Matsch (1972),
deposited by the Superior lobe in southwestern Minnesota. However,
because it was only observed at this single location, no definite
conclusion can be made at the present time as to the origin of this unit
or its correlation with other described units.
22 STONE COUNTS LOWERMOST UNIT
D LOWER CROMWELL FM.
"OTHER S" include s porphy ritic an d es ite, fe l sit e , i ron-fo rma t i on , gneiss , .. red rock" , and gray""•acke .
..c .2 iii 1:: CD .c.. 0 N w 0
E z:t
BASALT RED SS GRANITE GABBRO OTHERS
Figure 6. Cobble lithology of a possible pre-St. Croix phase till (Lowermost Unit) and the overlying Lower Cromwell Formation. Lower Cromwell Formation
General Characteristics
The lowermost Quaternary unit of significant areal extent observed in the study area is a reddish-brown, sandy-textured diamicton, with a
Munsell color of 5YR 5/3. It is very compact, and displays a weak fissility. It occurs at the base of a few axposed sections in stream valleys and roadside cuts; it does not form the surficial unit anywhere else within the study area.
Clasts >32 mm in diameter make up 15-20% of the total volume. The matrix (<2 mm) has an average sand/silt/clay ratio of 52/38/10.
Cobble lithology is dominated by red sandstone and basalt, with various other rock types present in much smaller amounts.
The coarse- and fine-sand lithology is shown in Table 1. Red sandstone and basalt comprise the majority of rock types present in the coarse-sand size fraction, just as for the cobble-size fraction. Some monominerallic grains are also present in this size fraction in minor amounts. The fine-sand fraction is dominated by quartz and feldspar grains.
The change in composition between different size fractions is explained by the fact that till composition is reflective of two major lithologic components: rock fragments, and their constituent minerals
(Dreimanis and Vagners, 1969; 1971a,b). This results in a bimodal or even multimodal distribution of rock and mineral fragments. Coarse
24 TABLE 1: Sand fraction lithologies/mineralogies of the Upper and Lower Cromwell Formation. "Heavies" include horneblende, pyroxene, and magnetite. "Other" includes calcium carbonate concretions and unidentifiable grains. LC=Lower Cromwell Formation coarse-sand fraction; LF=Lower Cromwell Formation fine-sand fraction; UC:Upper Cromwell Formation coarse-sand fraction; UF:Upper Cromwell Formation fine-sand fraction.
LC LF UC UF
Slat e/Graywacke 8 3 6 Iron-Formation 3 6 Red Sandstone 43 2 22 5 Rhyolite 14 2 17 3 Basalt 29 35 Red Rock/Granite 4 7 15 5 Gab bro 12 4 18 4 Quartz and Feldspar 26 127 21 120 Heavies 2 5 5 13 Other 9 5 Totals 150 150 150 150
25 particle sizes (>1 mm) are dominated by rock fragments, whereas the finer particle size fractions are dominated by their constituent mineral grains. The data in Table 1 illustrate this relationship quite well.
An important point illustrated by these data that coarser-sized particles may be more useful in determining the provenance of different tills in cases where bedrock mineralogy is similar throughout a region.
X-ray diffractograms of a sample of the <.002 mm size fraction of the lower diamicton are shown in Figure 7.
The first sample was run untreated. This resulted in major peaks which correspond to montmorillonite [(001) at 12.7A], illite/mica [(001) at 10A], chlorite/kaolinite [ (002)/(001) at 7.1A and (004)/(002) at
3.5A], quartz [(101) at 3.34A], and feldspar [(040) and (002) at 3.2A].
Other minor peaks were present which were integral series of the (001) spacings of the clay minerals or weak reflections related to the other planes of the quartz and feldspar crystals.
The second sample was placed in a dessicator with ethylene glycol and heated at 6o 0 c for one hour. This treatment causes the basal spacing of the montmorillonite lattice to increase to 17A. Following this treatment, the original 12.7A spacing shifted to 17A. Other peaks remained unchanged, but the shift of the montmorillonite peak did allow the 10A illite/mica peak to be more easily observed.
The third sample was heated at 550°c for one hour and allowed to cool in a dessicator before analysis. This treatment causes several changes in the diffractogram: 1) kaolinite peaks will be destroyed; 2) chlorite peaks may be destroyed, except for the 14A peak which may be
26 KEY
-A J. .w UNTREATED
GLYCOLATED
HEATED
30° 25° 20° 15° 10° 50
FIGURE 7: X-ray diffractograms of the Lower Cromwell Formation, <.002mm size fraction. Symbols: F:feldspar; Q:quartz; Ch:chlorite; K=kaolinite; I=illite; M:montmorillonite. Numbers in parentheses represent Miller indices of respective peaks.
27 shifted to 13.6-13.8A; 3) the (001) montmorillonite peak will shift to
9A; and, 4) the 10A illite/mica peak may intensify.
After running the heat-treated sample, the (001) chlorite peak was isolated and contracted very slightly, whereas other chlorite peaks were destroyed. This behavior is characteristic of an iron chlorite (Carroll,
1970; Thorez, 1976). The montmorillonite peak shifted to 9A and overlapped the illite/mica peak at 10A, which became sharper. Quartz and feldspar peaks remained unchanged.
The term illite is commonly used to indicate all 10A non-expanding clay material such as illite and muscovite mica (Thorez, 1976). In the absence of chemical analyses, this term must be used to denote any 10A mineral present. However, because mica usually gives much sharper, more intense reflections than those observed (Carroll, 1970), it is likely that the 10A mineral present is illite rather than muscovite.
To summarize the diffractogram data, the bulk of the <.002mm particle-size fraction consists of montmorillonite. Lesser amounts of quartz, chlorite/kaolinite, and illite are present. Feldspar is present only in minor amounts.
Macrofabric analysis was done in an exposure of the lower diamicton located along highway 210 in Jay Cooke State Park (NW 1/4, NE 1/4, SW
1/4, Section 1, T.48N., R.16W; Plate 1); the results are shown in Figure
8a. A strongly-developed NNW-SSE fabric is present at this site.
28 PLATE 1: Exposure of the Lower and Upper Cromwell Formation in the NW 1/4, NE 1/4, SW 1/4, Section 1, T48N, R16W (Esko Quadrangle). Shovel (26" length) is along contact.
29 n=50 n •50 N N
:
.. w 0
. . .
(a) ( b)
Figure 8. Fabric diagrams of the Lower Cromwell Formation (a) and the Upper Cromwell Formation (b). Points represent the intersection of the long axes of clasts on a lower hemisphere equal-area net. Origin and Regional Correlation
The highly compact nature of this unit, its textural characteristics, and its strong fabric suggest that it is a glaciogenic sediment deposited by lodgement and/or melt-out processes. These characteristics, along with its lithology and mineralogy, and its stratigraphic position suggest that it is the equivalent of the Lower
Till of Moss (1977), which was assigned to the lower portion of the
Cromwell Formation of Wright et al. (1970), deposited by the Superior lobe during the St. Croix phase. These units are referred to as the lower part of the Cromwell Formation by Matsch and Schneider (in press) and are correlative with the Sylvan Lake Member of the Copper Falls
Formation of northwestern Wisconsin (Johnson, 1984). They are also correlated with the Sullivan Lake Formation of Gross (1982), found inland from the North Shore Highland in northeastern Minnesota, and the
Pierz and Brainerd Tills of central Minnesota (Schneider, 1961), all deposited by the Rainy lobe during the St. Croix phase.
Upper Cromwell Formation: Subglacial Facies
General Characteristics
Immediately overlying the Lower Cromwell Formation in the Esko area is a light reddish-brown diamicton which is rich in silt. Its Munsell color is 5YR 6/3 but it develops a weak red weathering zone of color 10R
31 5/4. This unit is very compact and commonly jointed, with joint surfaces typically dark colored and spaced several cm apart, giving the unit a
"blocky" structure. Carbonate concretions several cm or more in length are scattered throughout its weathered zone. This unit is observable along most stream and roadside exposures within the Eska Quadrangle and forms the surficial deposit in the northern portion of the quadrangle.
Clasts >32mm in diameter make up 10-15% of the total volume of the sediment. The matrix (<2mm) has an average sand/silt/clay ratio of
30/43/27.
Cobble lithology is similar to that of the Lower Cromwell Formation, and is dominated by basalt and red sandstone, with other non-local rock types comprising the remainder.
Coarse-sand lithology, shown in Table 1, is generally similar to the pebble lithology, with basalt and red sandstone being the most abundant lithologic types present. Minor amounts of quartz, feldspar, and heavy minerals (hornblende, magnetite, and pyroxene) are also present.
Fine-sand mineralogy, also shown in Table 1, is dominated by quartz and feldspar grains rather than by rock fragments. This illustrates once again that composition is dependent on size: coarser-sized fractions are dominated by rock fragments, whereas finer particle-size fractions are dominated by their constituent mineral grains.
X-ray diffractograms of untreated, glycolated, and heated samples of the <.002mm size fraction of the upper diamicton are shown in Figure g.
Composition is identical to that of the Lower Cromwell Formation except for the fact that calcite is present in minor amounts, as shown by the
32 KEY
_,,,._,..., UNTREATED
..JA.,,,l-_.1 GLYCOLATED
HEATED
0
30° 25° 20° 15° 50
FIGURE 9: X-ray diffractograms of the <.002mm size fraction of the Upper Cromwell Formation. Symbols as in Figure 7; C:calcite.
33 minor (104) reflection at 3.035A, which is not observed in the
diffractogram of the Lower Cromwell Formation.
Macrofabric analysis of this unit was performed in an exposure shown
earlier (Plate 1), taken several feet above the contact with the Lower
Cromwell Formation. Results are shown in Figure 8b; a strong ENE-WSW
fabric is present at this site.
Origin and Regional Cc.-rrelation
This unit's highly compact nature, along with its textural
characteristics, and strong fabric indicates a subglacial origin, with
deposition by lodgement and/or melt-out processes. These properties,
along with its stratigraphic position and lateral continuity, allow it
to be correlated with the Cloquet Till of Baker (1964) and the Upper
Till of Moss (1977), both assigned to the Upper Cromwell Formation of
Wright et al. (1970) deposited by the Superior lobe during the Automba
phase. These units are referred to as the upper part of the Cromwell
Formation by Matsch and Schneider (in press). It is also correlated to
that portion of the Copper Falls Formation deposited by the Superior
lobe in northwestern Wisconsin during the Tiger Cat advance (Clayton,
1984).
The siltier texture (as compared to the Lower Cromwell Formation) may be the result of (1) incorporation of proglacial lake sediments into
the ice during its readvance; and (2) comminution of incorporated
sediment by crushing and abrasion processes (Dreimanis, 1971a).
34 Upper Cromwell Formation: Supraglacial Facies
In the northern portion of the Esko Quadrangle, the Upper Cromwell
Formation is the surficial deposit. This unit includes subglacial till
(described above) as well as outwash sediments and flow tills which are found in close association with each other.
These flow tills and glaciofluvial sediments of the area have been misinterpreted as deposits of a younger ice advance, the Split Rock phase (Wright et al. , 1969). However, textural analyses of these tills show that they are not a more clay-rich unit (Figure 10); they are identical in grain-size characteristics to the Upper Cromwell Formation.
Fabric data for two samples of this unit are shown in Figure 11. A very weak to random fabric is present, arguing against a subglacial origin.
The difference in the appearance of this facies (as compared to its subglacial counterpart) is the result of the disaggregation and resedimentation which it has undergone. This results in the decrease in compactness and loss of preferred clast orientation which is otherwise present.
The association of tills and glaciofluvial sediment found in the area is in agreement with the ice advance/retreat depositional model of
Boulton (1972; shown in Figure 5), which suggested that multiple sequences of interbedded till and outwash sediments are commonly the result of a single ice advance/retreat rather than products of successive glacial advances.
35 . MATRIX TEXTURE
clay
0 • 0 • 00 •• o. o• ••• oo•
D • D •Do • D • • D
sand ,______.},jt______:i sill
o Upper Cromwell 29/44/27 (n:14) •"Split Rock" 30/43/2 7 (n: 15)
FIGURE 10: Textural data for samples of the Upper Cromwell Formation and the so-called "Split Rock Till", taken from the northern Eska Quadrangle.
36 n=50 n s50 N N
w .. -..,J
(a) (b)
Figure 11. Fabric diagrams of the lower sandy diamicton (a) and the upper silty diamicton (b) in a ridge exposure at the center of the eastern edge of Section 21, T49N, Rl6W (Eska Quadrangle). Site-Specific Characteristics
An exposure of both the Upper and Lower Cromwell Formation is found in the NW 1/4, NE 1/4, SW 1/4, Section 1, T.48N., R.16W. (Eska
Quadrangle) and is shown in Plate 1. This exposure consists of approximately 8 feet of the Lower Cromwell Formation overlain by 10 feet of the Upper Cromwell Formation. The contact between the two units is sharp and planar. Boulders showing striated upper surfaces are scattered along the plane of contact. The lower unit shows a weak fissility and poorly-developed blocky structure. The upper unit displays the prominent jointing and strongly-developed blocky structure characteristic of the
Upper Cromwell Formation. Fabric data for both units were taken at this site and were shown in Figure 8.
Another exposure of these units is found in the NE 1/4, SW 1/4, SE
1/4, Section 2, T.48N., R.16W. (Esko Quadrangle). Their contact is slump-covered and not directly observable, but is discernable based on textural analysis and examination of sequential samples (Plate 2 and
Figure 12). Approximately 33 feet of the Upper Cromwell Formation overlies 10 feet of the Lower Cromwell Formation at this location. In addition, 5 feet of the sandstone-rich, sandy-textured diamicton underlies the Lower Cromwell Formation and is in sharp and planar contact with it. This lowermost unit, which may represent a possible pre-St. Croix phase till, was observed only at this single location.
An exposure of the supraglacial facies of the Upper Cromwell
Formation is found ·along the center of the eastern edge of Section 21,
38 PLATE 2: Exposure shown in Figure 12; NE 1/4, SW 1/4, SE 1/4, Section 2, T48N, R16W Eska Quadrangle). Base of measuring stick (scale in feet) is along contact between the Lower Cromwell Formation and a possible pre-St. Croix phase till.
39 NR-7 Sequential Samples
Upper Cromwell Formation • 25/44/31
• 27/44/29 5 ft. ] • 19/44/37 '
• 24/43/33
• 36/42/22 ---?---? -- ?- - ?- -- • 43/37/20 Lower Cromwell Formation - ?- - - ? . • 44/38/18 ..· . - - -?-. -- ?-. - - ? . - • 52/38/10 Possible Pre-St. Croix Phase Unit
FIGURE 12: Stratigraphic section showing three subdivisions based upon lithologic differences at sampled points (squares). Contacts slump-covered and not observable except where shown by a solid line. Numbers represent the sand/silt/clay rati o of the sample.
40 T.49N., R.16W. (Eska Quadrangle), and is shown in Plate 3. This exposure consists of approximately six feet of sandy diamicton, which in places shows faint bedding features, overlain by four feet of massive, silt-rich diamicton. The exposure is in a ridge which resembles an esker, and was previously thought to be an Automba phase esker with a thin cap of Split Rock till (C. L. Matsch, personal communication,
1984). However, textural and fabric analyses indicate a supraglacial origin.
This exposure is thought to represent supraglacial till or glaciofluvial sediment, which accumulated in a trough on the ice surface, with a capping of flow till derived from the adjacent melting ice. Similar deposits have been described by Boulton (1971; 1972) at the margins of active glaciers. A depositional model for this type of feature is illustrated in Figure 13.
In the SW 1/4, SE 1/4, NW 1/4, Section 10, T.48N., R.16W. (Eska
Quadrangle), approximately 17 feet of the Upper Cromwell Formation is overlain by one to two feet of sandy gravel succeeded by 15 feet of bedded silt and clay of the Wrenshall Formation (Plate 4 and Figure 14).
Contacts between the units are generally sharp and planar. The Upper
Cromwell Formation here is similar in appearance to its other exposures described above. The sandy gravel unit and the Wrenshall Formation are described in later sections.
Numerous other exposures of the Cromwell Formation are found within the Eska Quadrangle along road-cuts and stream valleys. However, slumping commonly obscures contacts and masks the true stratigraphy.
41 PLATE 3: Ridge exposure at the center of the eastern edge of Section 21, T49N, R16W (Eska Quadrangle). Tip of shovel blade is along contact between crudely-bedded sandy diamicton and overlying massive, silt-rich diamicton.
42 A
c ®
FIGURE 13: Ridge development by relief inversion. (A) shows initial system of ridges and troughs in which outwash and flow till accumulate, as shown in (B). (C) shows the inversion of the topography, with ridges resulting from trough fillings. Some ridges contain both outwash and flow till (1); others contain mostly outwash (2); and some contain mostly flow till (3). From Paul, 1983. 43 PLATE 4: Exposure of the Upper Cromwell Formation and Wrenshall Formation in the SW 1/4, SE 1/4, NW 1/4, Section 10, T48N, R16W (Esko Quadrangle). Contact is marked by a change in color and in slope angle. Scale of measuring stick in feet.
44 SOIL HORIZON IOft. j / .l WRENSHALL Fl FORMATION
Oft. SAND AND SGm, t :·:··:::.:· :-·-·: :_: ··:·:·:·:·:·: :·: ...:· .:_:_.: =· ._. ._.. : ·:-:..:·.=-.:·_:_.: :-:; ...... GRAVEL • • • • • • • • • • • • • • • • • • • • • • • UPPER Dmm .. • • • • A CROMWELL . . FORMATION . .. • • .. • • • . • .• . • . • . • • .• . • . • • •
FIGURE 14: Stratigraphic diagram of the exposure shown in Plate 4. Coding of units (left of profile) according to Eyles et al., 1983.
45 GLACIOLACUSTRINE UNITS
Wrenshall Formation: Shallow-Water Facies
Overlying the Upper Cromwell Formation at elevations between approximately 1000 to 1150 feet are sorted and bedded sands and gravels and associated flow tills. The sands and gravels show a general coarsening-upward trend in vertical exposures.
A fan-shaped deposit of bedded sand and gravel is located in the north 1/2, Section 28, T.47N., R.14W. in the Borea Quadrangle (Plate 5).
The uppermost surface of this deposit occurs at an elevation of approximately 1100 feet. An exposure in this deposit is shown in Plate
6. The sequence consists of approximately 20 feet of planar-bedded and rippled sands and gravels capped by 5 feet of trough cross-bedded sandy gravel. Sands typically showed gradations in bedding from type A cross-lamination to type B cross-lamination to draped lamination. The planar-bedded sands and gravels show dips up to 20° in a general northward trend (Figure 15).
This deposit is interpreted as a delta of Glacial Lake Duluth, possibly an abandoned delta of the Black River (Farrand, 1960).
At another location, in the West Duluth Quadrangle (NE 1/4, NW 1/4,
Section 14, T.49N., R.15W.), a sequence of clayey silts, sands, gravels, and flow till is found at an elevation between approximately 1150 and
1065 feet (Plate 7 and Figure 16). This sequence is interpreted as a regressive sequence of Glacial Lake Duluth, representing a shift from
46 PLATE 5: Sand and gravel pit in the N 1/2, Section 28, T47N, R14W (Berea Quadrangle).
47 PLATE 6: Close-up of sands and gravels shown in Plate 5. Scale of measuring stick in feet.
48 N
w
s
FIGURE 15: Cross-bed measurements (dip direction of foreset beds) from Lagro gravel pit, NE 1/4, Section 28, T47N, R14W (Borea Quadrangle). Each block represents one measurement.
49 PLATE 7: Exposure of the Wrenshall Formation in the NE 1/4, NW 1/4, Section 14, T49N, R15W (West Duluth Quadrangle). Plate corresponds to Figure 16.
50 PROCTOR SECTION
1150 ft. Slhi SGm . -· : : ·; .. ' ...... • ...... Dmm(r) ...... • ... • ...... · .. .,, .. SGh ······ -· .. ·.•.·•·····.··· - SGm 10 ft. FSh Sh
FSh 0
Stones
SGp
FSh
FIGURE 16: Proctor stratigraphic section (NW 1/4, Section 14, T49N, R15W, West Duluth Quadrangle). Coding of units according to Eyles et al., 1983.
51 deep-water lacustrine deposition of silt and clay (represented by units
"Fld" and "Fl" in Figure 16) to shallow-water sedimentation of sand and gravel (units "Sm", "SGm", "Sh", "Gm", and "FSh") followed by deltaic deposition of planar cross-bedded sand and gravel· (unit "SGp"). Above the deltaic deposit, a channel lag deposit (represented by the stone layer) and fluvial deposits of silt, sand, and gravel (units "FSh",
11 11 "Sh", SGm , and "SGh") are found, and are capped by a massive diamict unit ("Dmm(r)").
This massive diamict shows a random fabric (Figure 17). Its lower contact is sharp and undulating, and is marked by "squeeze-up" features composed of the underlying sand (Plate 8). The upper contact is gradational with massive, sandy gravel. It is texturally identical to the Upper Cromwell Formation. Based on the fabric characteristics and stratigraphic relationships, this unit is interpreted as a flow till.
Its proposed origin is as follows: as the Superior lobe withdrew into the basin of Glacial Lake Duluth, blocks of debris-rich stagnant ice remained on land. Melting of these ice blocks resulted in the accumulation of fluvial deposits. Concurrent with this sedimentation was the sporadic deposition of sediment flows, which account for the flow till occurring in close association with stratified sediments. This depositional model was also proposed by Gross (1982) for similar deposits found in the Two Harbors area.
Sequences such as those described above are common in the 1000-1150 ft. zone; sand and gravel pits are also commonly located within this zone. They represent nearshore and shore-zone deposits of the epi-Duluth
52 n=50 N
FIGURE 17: Fabric diagram for uppermost diamicton in the Proctor stratigraphic section (Dmm(r) in Figure 16).
53 PLATE 8: Contact between the silt-rich diamicton and underlying bedded sand (units Dmm(r) and SGh) found near the top of the stratigraphic sequence shown in Plate 7 and Figure 16.
54 and Glacial Lake Duluth lakes (Farrand, 1960). Thus, they are assigned to the shallow-water facies of the Wrenshall Formation, which is the name given to glaciolacustrine deposits associated with Glacial Lake
Duluth (Wright et al., 1970). These deposits are generally confined to zones below 1150 feet in elevation in the southwestern end of the Lake
Superior Basin. They consist of two major sedimentary facies: shallow-water sands and gravels (and associated flow tills) found near the former strandline, and deeper-water laminated to massive silts and clays which are found basinward of this shallow-water facies.
Wrenshall Formation: Bedded Silt and Clay Facies
General Characteristics
Overlying the Upper Cromwell Formation at elevations below approximately 1000-1050 feet is a laminated to thinly-bedded sequence of silt and clay. The clay, in laminations 2mm thick, alternates with silt beds which vary between 4 and 10cm in thickness (Plate 9). The clay layers are reddish brown (Munsell color of 2.5YR 4/4), but the silt is gray with a Munsell color of 2.5Y 6/2. However, oxidation of surface exposures causes a color change to a very pale brown to yellow with a
Munsell color of 10YR 7/3 to 10YR 7/6.
Irregular-shaped carbonate concretions up to several cm in length are present throughout the weathered zone of this unit.
Textural analysis of the thin clay beds show that they are composed
55 PLATE 9: Laminated silt and clay of the Wrenshall Formation.
56 of 85% clay-size particles, whereas the gray silt beds consist of 80% silt-size material (Wright et al., 1970). The silt beds are not graded; a rather abrupt grain-size transition occurs at each silt-clay boundary.
Results of x-ray analysis of the <.002mm fraction of the gray silt beds are shown in Figure 18. Minerals present are montmorillonite, illite, chlorite, kaolinite, quartz, feldspar, and calcite. The carbonate content is 10 per cent higher than that of the thin interbedded red clay laminae (Wright et al., 1970).
Origin and Regional Correlation
The alternate type section for the Wrenshall Formation is located in the study area, where U. S. Highway 210 crosses the 960-foot contour north of the St. Louis River in the Esko Quadrangle. Wright et al.
(1970) described the following stratigraphy at this location:
Wrenshall Fm.
3.2 feet. Clay; red, with irregular concretions.
9.3 feet. Silt; yellow, with thin red clay interbeds
and regular concretions.
2.6 feet. Silt; gray.
Cromwell Fm.
1.4 feet. Gravel; very coarse.
22.0 feet. Sand; brown to red-brown, with thin interbeds
of silt.
57 KEY
--" .J...; UNTREATED 6 .,r..,,J"...,,. J G LYCO LAT ED ...... s 6 0 ...... \ 'g0 I H } '
30° 25° 20° 15° 10° 50
FIGURE 18: X-ray diffractograms of the <.002mm size fraction of the gray silt of the Wrenshall Formation. Symbols as in Figure 7; C=calcite.
58 A somewhat similar stratigraphic assemblage was observed by the author at a nearby location and is shown in an earlier section (Plate 4 and
Figure 14). The laminated silt and clay has been oxidized to the characteristic buff-yellow color. Scattered, irregular-shaped carbonate concretions up to several cm in length are common within this unit.
Slumping has occurred, partially obscuring contacts and resulting in the underlying Upper Cromwell Formation being covered with several cm of this buff-yellow material.
A problem with this unit has involved the color of the silt beds.
Most of the sediments of this area are red to reddish brown in color, owing to the oxidation of iron minerals present in the local rock units
(Meyers, 1977). The gray color of these silts has been explained by the following theories: 1) reduction of red iron oxide in the deeper portion of the lake basin soon after its deposition (Swain and Prokopovich,
1957); and, 2) deposition by inflowing water of the St. Louis River, which has a source in gray calcareous drift derived from northwestern-source rocks (Leverett, 1929; Wright, 1955; Wright et al.,
1970).
Of these two explanations, the second seems to be the most probable, for several reasons. All geochemical studies of gray sediments found within the Lake Superior basin indicate that they are in an oxidized state (Dell, 1971). This result is not surprising: ice-contact and proglacial lakes generally have very low organic productivity and are of an early oligotrophic state, whose waters are well-oxygenated throughout the entire basin. Thus, reducing conditions are highly unlikely in such
59 lakes. Furthermore, mineralogical analysis supports the second explanation. The higher carbonate content found in the gray silt is not characteristic of other sediments found in the area; the diffractograms show a relatively strong calcite peak at 3.035A, which is dissimilar to any other samples analyzed. The diffractogram pattern is also unique in that it is the only one with a distinct absence of the (001) chlorite peak following heating. This suggests that the chlorite present in this sample is different from that present in the other units; its behavior is characteristic of a type of septochlorite known as greenalite or chamosite (Deer et al., 1966; Carroll, 1970; Thorez, 1976). This mineral is a unique component of the Biwabik Iron Formation located to the north-northwest of the study area, but is not present in the source rocks of the red tills found in the area (Dell, 1975).
These features indicate a different source region for the gray silt as compared to that for the Cromwell Formation and the red clay of the
Wrenshall Formation. Its composition suggests a north-northwest source region, in drift of the St. Louis sublobe. The clay-size fraction of that drift, the Alborn drift of Baker (1964), has a composition similar to that found for the gray silt (Arneman and Wright, 1959).
Wrenshall Formation: "Massive Red Clay" Facies
General Characteristics
The uppermost unit present in the study area is a reddish brown clay
60 of Munsell color 2.5YR 5/4. It is the surficial unit throughout most of the areas below 1000 to 1050 feet in elevation. Undisturbed vertical sections of this unit are very rare because the deposit has been subject to extensive slumping.
This unit is very clay-rich; textural analysis of 11 samples averaged 10.1% sand, 14.5% silt, and 75.4% clay, with particles >2mm comprising <1% of the total mass. A typical particle-size curve for this unit is shown in Figure 19.
Results of x-ray analysis of the <.002mm size fraction of the red clay are shown in Figure 20. Except for minor differences in the intensity of several components, the mineralogy is similar to that of the Upper Member of the Cromwell Formation (Figure 9); quartz, feldspar, montmorillonite, illite, chlorite, kaolinite, and calcite are present in this unit. These results are in agreement with those obtained by Mengel and Brown (1976) and Meyers (1977); quantitative data obtained by Mengel and Brown (1976) are listed in Table 2 (average for 9 samples). The difference in mineralogy between the coarse- and fine-clay fractions is striking: non-clay mineral types are present in significant amounts in the coarse-clay size fraction but are absent in the fine-clay fraction.
This feature is interesting to note because, considering the fact that this sediment is derived from glacial deposits, it further illustrates the concept of terminal grade size. Of all mineral types present in this unit, only the clay minerals are capable of being comminuted to the fine-clay size range by glacial processes (Dreimanis, 1971). This fact is reflected in the mineralogical data for the red clay.
61 CUMULATIVE CURVE PROBABILITY ORDINATE 99.99
99.9 99.8
99 98
95 90
80 70 60 a:: I.LI 50 en ...... ,. a:: 40 5 2 0.1 0.01 I I 4 5 6 7 8 9 10 II PARTICLE DIAMETER UNITS FIGURE 19: Grain-size analysis curve (probability ordinate) of the massive red clay. A majority of the sample weight is composed of particles finer than 11¢ and is unsuitable for pipette analysis (Fo1k, 1980). 62 KEY UNTREATED .../\,..1\..¥11 GLYCOLATED HEATED ...... - C\I 0 -== 0;::: ooo= .Qo -- 0.-. ct:5 H--o 0 ::!:rg.r:...... v (.) 0 .Q g 30° 25° 20° 15° 10° 50 FIGURE 20: X-ray diffractograms of the <.002mm size fraction of the massive red clay of the Wrenshall Formation. Symbols as in Figure 7; C=calcite. 63 TABLE 2: Quantitative mineral abundance for the .002-.0002mm and the <.0002mm particle-size fractions of the red clay. Figures in weight percent. Data from Mengel and Brown, 1976 • • 002-.0002mm <.0002mm Quartz 16 Plagioclase 10 K-Feldspar 5 Calcite 1 Dolomite 4 Illite 15 23 Chlorite 35 19 Smectite 9 51 Kaolinite 5 7 Total 100 100 64 Additional properties of the red clay are described below. These consist primarily of properties observed in the field rather than those determined by laboratory analyses. Criteria Used to Determine the Origin of the Red Clay The origin of the surficial red clay unit in the southeast portion of the study area has posed a problem to workers of this region. Two different origins have been proposed for this unit: subglacial and lacustrine. The problem with genetic interpretation lies in the fact that subglacial till composed of reworked lacustrine sediment can have many characteristics similar to massive glaciolacustrine deposits. Because of these potential similarities, no single set of criteria can be used by itself to determine the origin of such a deposit. Rather, a combination of several independent characteristics should be examined before any conclusions are made regarding the genesis of the material. A discussion of the various methods used in determining the origin of the red clay is presented below. These methods consisted of an examination of the unit's areal extent, facies relationships and grain-size trends, stratigraphic relationships, geomorphic expression, fabric, sedimentary structures, fossil content, and engineering properties (bulk density and consolidation). 65 Areal Extent Deposits of different origin commonly have characteristic areal expressions, related to their mode of deposition. For example, beach deposits of a glacial lake are generally confined to a particular small range of elevation (near the shoreline). Because of this fact, the areal extent of a unit can often help indicate its genetic origin. The red clay present in the study area is confined to elevations below 1100 feet. At no location was this unit observed above this elevation. This type of topographic control is one characteristic which is common to lacustrine deposits. Other workers have also observed a similar trend for this unit, and this was used by them to support a lacustrine origin for the red clay (Leverett, 1929; Mengel, 1973; Zarth, 1977). Conversely, Need (1980) has shown that a red clay similar to that found in the present study area is present above 1100 feet at a location several miles north of Lake Nebagamon in Douglas County. His evidence was based on the isolated occurrence of a red clay in two water-well logs at an elevation of 1200 feet, and was not field-checked. Based on this isolated occurrence, he argued that the red clay could not be of lacustrine origin, as it is present above the 1100-foot strandline, which is supposedly the highest level of post-glacial lakes in this area. He therefore named the red clay the Douglas Till and ascribed its presence to a glacial advance. There are two problems with this reasoning. The first is that 66 correlation of the red clay in the present study area with the red clay observed at the 1200-foot elevation is highly uncertain. The second problem is the contention that the highest post-glacial lake levels do not exceed 1100 feet in elevation. The 1100-foot is the highest level of Glacial Lake Duluth in this area, but small, proglacial lakes referred to as the Epi-Duluth stage were located along the retreating ice margin prior to the development of Glacial Lake Duluth (Farrand, 1960; 1969). It is very possible that such small, proglacial lakes could be the source of isolated occurrences of red clay above the 1100-foot elevation. A clay-rich sediment similar to the so-called Douglas Till was observed by Gross (1982) in the area of Two Harbors, Minnesota at elevations of 1220 to 1240 feet. She proposed that a localized ice advance overrode lacustrine sediments and deposited this material as a till. However, because there is a lack of continuity in the areal extent of this sediment, as well as a lack of additional independent evidence for such an advance, it seems more likely that this sediment is lacustrine, accumulating in small ice-marginal lakes, rather than deposited by glacier ice. To summarize, the areal extent of the red clay strongly supports a lacustrine origin for this unit. Small, isolated occurrences above 1100 feet in elevation are consistent with the idea of small ice-marginal lakes forming along the retreating ice front prior to the development of Glacial Lake Duluth. 67 Facies Relationships and Grain Size Trends Zarth (1977) documented a lacustrine facies assemblage in the area of Wrenshall, Minnesota (several miles west of the present study area) which consisted of bedded sands above 1000 feet in elevation grading into massive and laminated silts and clays at lower elevations (Figure 21). This assemblage was interpreted by her as a prograding shoreline facies grading into deep-water (offshore) silts and clays. These lacustrine silts and clays are laterally continuous from Wrenshall into the present study area and form the surficial red clay unit throughout the area. This facies assemblage is identical to that found in the present study area. Bedded sands and gravels are found at elevations above 1000 to 1050 feet; below this level, silts and clays form the surficial unit. Mengel and Brown (1976) documented a similar assemblage in the vicinity of Little Balsam Creek in the present study area. It also consisted of sands laterally grading into silts and clays at an elevation of about 1000 feet. They also interpreted this assemblage as a nearshore/offshore lacustrine facies. The persistent trend of sands grading into and interfingering with silts and clays at lower elevations strongly indicates a lacustrine depositional environment for these sediments (Reineck and Singh, 1973), reflecting the decreasing energy within the local environment of deposition as water depth increases. 68 NW SE 300 m ·""···· ······· .,, ··"· ff a O"' '° D SILT/ Cl/\Y s AND rw-1 SAND/GRAVEL 0 1.6 km I DRIFT & BEDROCK Figure 21. Stratigraphy duc:11111e11t e d hy Zar-th, 1'177, jn the vicinity of Wrenshall, M.l.1111 e suta, appr-ox.ima lL!ly 15 km west of the llorea Quadrangle. Stratigraphic Relationships The red clay forms the surficial unit throughout the southeast portion of the study area. To the northwest, it has been deeply dissected by the St. Louis River and its tributaries. Clean or undisturbed exposures of the red clay are extremely rare owing to extensive slumping or lack of relief. However, several exposures are present along the St. Louis River. In all observed sections containing massive red clay, it is the uppermost unit. In addition, zones of interbedded, laminated silts and clays are commonly present within the massive red clay, with gradational contacts (Plate 10). If the massive red clay is a subglacial till, one would not expect gradational contacts with underlying sediments but rather, sharp unconformable contacts (Shaw, 1985). Furthermore, the lack of any glaciotectonic structures in the underlying bedded sediments argues against a subglacial origin for this unit. Such features would be expected if it had been deposited subglacially (Eyles et al., 1985). Geomorphic Expression Land surfaces underlain by sediments of different origin commonly show distinct geomorphic features related to their mode of deposition. As a result, the geomorphic expression of a deposit may give an indication of its mode of origin. Glaciolacustrine sediments generally accumulate from the settling of 10 PLATE 10: Laminated silt and clay which grades upward into massive red clay; SW 1/4, Section 30, T49N, R14W (West Duluth Quadrangle). 71 suspended particles within the water column, resulting in blanket-like deposits with subdued topography (Smith and Ashley, 1985). Subglacial sediments are predominantly of two major types: those deposited directly by glacial ice (lodgement till and melt-out till) and those deposited by running water (stratified drift). Lodgement till accumulates at the base of a moving glacier as a result of pressure-melting, whereby particles are lodged onto the glacier bed and the till surface is built up by accretion; basal melt-out till is deposited as the sole of the glacier melts, gradually releasing its debris load onto the underlying till surface (Sugden and John, 1976). The melt-out process involves large volumes of meltwater that must be evacuated from the till (Muller, 1983). This water tends to redistribute and sort the subglacial sediment and deposit it as stratified drift. Landscapes composed of subglacial till commonly show a streamlined topography, with drumlins and flutes being common landforms (Clayton and Moran, 1974; Sugden and John, 1976; Fookes et al., 1978; Boulton, 1982; Shaw, 1985). However, in zones of compressive flow, especially near the frozen terminus of a glacier, subglacial thrusting and compressional folding may occur, resulting in landforms which are perpendicular to ice flow direction. Ribbed (Rogen) moraine, DeGeer moraine, and glacial thrust masses are characteristic landforms in these zones of compressive flow (Clayton and Moran, 1974; Sugden and John, 1976; Shaw, 1985). Subglacial landforms composed of stratified drift consist predominantly of eskers (Sugden and John, 1976; Shaw, 1985) which are generally situated parallel to ice-flow direction and reflect a 72 decreasing hydrostatic gradient toward the glacier margin (Wright, 1973). Subglacial till surfaces associated with the Superior lobe commonly show these characteristic morhological features (Wright, 1972a; 1973), for example, the Automba Drumlin Field. However, surficial landforms which are characteristic of subglacial deposition are strikingly absent on the surface of the red clay. Its surface is remarkably flat, showing little relief except for modern drainage networks (Plate 11). Although Clayton (1984) has indicated that low drumlins are present in the study area southwest of the city of Superior in townships T.48N, R.15W and T.49N, R.14W, they are not observable on USGS 7.5 1 topographic maps or on 1:12,000 USDA air photos, nor were they observable in the field. Interestingly, Clayton has recently stated (personal communication, 1985) that it is very likely that the drumlins do not actually exist within the study area. The strikingly flat surface and total absence of any subglacial landforms in the area indicate a lacustrine, rather than subglacial, origin for the red clay. Fabric The alignment of clasts within till, referred to as fabric, has been used as evidence for subglacial deposition and as an indicator of ice flow direction (Holmes, 1941; Harrison, 1957; Kruger, 1970; Lindsay, 1970; Boulton, 1971). The alignment of sand- and silt-sized grains 73 PLATE 11: Flat plain of Glacial Lake Duluth. View is looking SE from the center of the northern edge of Section 33, T47N, R15W (Borea Quadrangle). 74 within a clayey matrix, referred to as microfabric, has also been used for the same purpose (Sitler and Chapman, 1955; Ostry and Deane, 1963; Evenson, 1971). However, fabric data by itself is not indicative of the origin of a sediment. Sediments of different origin, deposited in different environments, can exhibit preferred grain as a result of (1) magnetic factors - the earth's magnetic field can cause magnetically-susceptible grains in a suspension to become aligned parallel to this field and eventually reflect this alignment after deposition (Ellwood and Ledbetter, 1979); (2) currents - currents can cause the long axes of particles to become aligned parallel to the direction of flow (Domack and Lawson, 1985); and, (3) viscous flow dynamics - mass-flow deposits can develop a fabric as a result of laminar flow forces (Lindsay, 1968). Microfabric analyses of the red clay from the present study area were not carried out because undisturbed samples could not be obtained. However, Johnson (1980) did perform microfabric analysis on undisturbed samples of the red clay obtained several miles northeast of the present study area. His work has shown that a well-developed microfabric is present in most samples of the red clay. He attributed that fabric to subglacial lodgement processes and concluded that the red clay is therefore a subglacial till. It is important to note that he also analyzed several samples of glaciolacustrine sediments for comparison and found that they also exhibited a well-developed microfabric. Thus, it is apparent that microfabric analysis results are inconclusive with respect to the origin of the red clay because it is present in both 75 subglacial tills and glaciolacustrine sediments. Sedimentary Structures Johnson (1980) observed rare occurrences of sorted and, in some places, bedded lenses of sand within the red clay. He believed that they were due to subglacial meltwater sorting the sediment and depositing this material in cavities within the ice, resulting in the enclosure of sand within the red clay. These types of structures have been observed in subglacial tills (Kruger, 1979). However, such features can also be found in other types of sediments. In a delta exposure in the southeast portion of the Borea Quadrangle (Section 28, T.47N., R.14W.), a block of sorted and bedded sand was observed within a matrix of crudely-bedded sand and gravel (Plate 12). The only method by which such a structure could be emplaced is by deposition as a frozen block. Rafting by icebergs could account for similar structures within the red clay in offshore zones, such as the isolated occurrences observed by Johnson (1980). An exposure of the red clay along the St. Louis River in Section 30, T.49N., R.14W. (West Duluth Quadrangle) revealed interbedded clay and silt laminae which graded upward into massive red clay (Plate 10). The laminated nature of the lower portion of this deposit indicates that it was not deposited subglacially. Instead, it resembles deep-water, current-deposited glaciolacustrine sediment (Ashley, 1975; Smith and Ashley, 1985). 76 PLATE 12: Block of bedded sand within a massive deposit of sand; from a gravel pit in Section 28, T47N, R14W (Borea Quadrangle). 77 Fossils The proglacial lacustrine environment is generally regarded as being almost devoid of any life because of extremely low water temperatures, high sedimentation rates, and the temporal nature of the lakes. Some studies have reported trace fossils from lake sediments (Gibbard and Stuart, 1974; Ashley, 1975) but these findings are rare. Reports of actual remains of organisms in glaciolacustrine sediments are very rare in the literature. Ostracodes of the genus Candona have been reported in massive red clay from cores of Lake Superior Sediments (Dell, 1971). If this unit is the equivalent of the massive red clay present in the study area, this informati on NOuld help confirm a lacustrine origin for this unit. However, not enough information is known at the present time to make any correlation between these two units. Trace fossils were not observed in the red clay, possibly due to the lack of undisturbed exposures of this unit which are needed for this type of observation. Sand fractions (>.0625mm) of the red clay were examined with a binocular microscope for any organic remains. Preservation of completely intact, delicate structures is highly unlikely in the subglacial environment; their presence would help confirm a lacustrine origin for the red clay. However, no such remains were observed. Based on these results, fossil data are inconclusive with respect to determining the origin of the red clay in the present study area. 78 Engineering Properties Geotechnical properties of a sediment can sometimes aid in the genetic interpretation of a material, especially when other parameters are inconclusive. Two such properties which have often been used for this purpose are consolidation and bulk density. Consolidation is a measure of the reduction in volume a sediment has undergone as a result of compressive stress. In order for consolidation to take place, drainage must occur as a stress is applied, otherwise some of this stress will be supported by pore fluids rather than the sediment itself, resulting in erroneous laboratory consolidation data. Results of consolidation test data are standardized for comparative purposes by using the overconsolidation ratio (OCR) , which is the ratio of the preconsolidation pressure (the experimentally-determined maximum pressure to which a sediment has been subjected to) to the present overburden pressure exerted on the material. Bulk density is the ratio of the weight of a sample to its volume. Its value depends on 1) the density of the individual particles of the sample; 2) porosity; and, 3) the sample's particle-size distribution. Consolidation and bulk density are related properties because as a sediment becomes more consolidated, it also becomes more dense. (For a more thorough discussion of this topic, see Taylor, 1948). Overconsolidation, a case in which the preconsolidation pressure is greater than that currently exerted on the sample, can result from one or more of the following processes (Dreimanis, 1976): 1) removal of an 79 overlying load; 2) fluctuations in the water table; and, 3) dessication. Previous studies have usually interpreted overconsolidation to be the result of past loading by glacial ice (Harrison, 1958; Aario, 1971; Boulton, 1975; Edil et al., 1977; Kemmis et al., 1981). However, dessication and fluctuations in the water table have also been shown to be a cause of apparent overconsolidation in glacial lake clays (Rominger and Rutledge, 1952; Soderman and Kim, 1970). Fluctuations in the water table have also been proposed as a possible cause of apparent overconsolidation of the red clay in an area several miles east of the present study area (Johnson, 1980). Bulk density data have been used to differentiate subglacial till frcm water-laid deposits (Easterbrook, 1964; Moss, 1977; Johnson, 1980; Gross, 1982). However, since differences in grain-size distribution influence bulk density values, differences between till and till-like sediments are significant only if the samples have similar particle-size distributions. Bulk density values for proven massive glaciolacustrine clays with a particle-size distribution similar to the red clay of the present study area are unavailable. However, such values are known for subglacial tills, and range between 1.8-2.0 g/cc (Edil et al., 1977). Information on file with the U. S. D. A. Soil Conservation Service indicates that the bulk density values for the red clay (Ontonagon soil series) vary between 1.2-1.5 g/cc. These values are much lower than those for subglacial till having a similar particle-size distribution and thus do not support a subglacial origin for the red clay. Consolidation tests were not performed on samples of the red clay, 80 due to the lack of undisturbed exposures in the study area. However, consolidation test results for the red clay were obtained from the Wisconsin Department of Transportation and show OCR values which range between 1.7-13.4. These data indicate that the red clay is slightly overconsolidated, similar to the values shown by Lake Agassiz clays, which show overconsolidation ratios as high as 11 (Rominger and Rutledge, 1952). Subglacial tills along Lake Michigan's shoreline show much higher overconsolidation ratios, ranging between 18-35 (Edil et al., 1977). If the red clay is a basal till, one would expect higher overconsolidation ratios than those reported. One could argue that subglacial drainage was impeded as the red clay was deposited, thus resulting in the slight overconsolidation ratios observed. The problem with this explanation is that such a case was not likely. Birks (1976) has shown that a change in vegetation from tundra to spruce forest occurred in northern Minnesota about 14,700 - 13,600 years B. P. If this change corresponds to a permafrost/non-permafrost transition (as suggest ed by Edil et al., 1977), subglacial drainage would occur. In addition, subglacial drainage has been documented for the Superior lobe and was especially prominent in its later phases (Wright, 1973). Thus, consolidation of subglacial materials would occur under these conditions and as a result, higher overconsolidation values for the red clay would be observed if it were of subglacial origin. It is proposed that the slight overconsolidation values shown for the red clay are instead the result of a fluctuation in the water table and dessication. Both of these processes have occurred as the result of 81 fluctuations in glacial lake levels and drainage of Glacial Lake Duluth (Farrand, 1960; 1969; Farrand and Drexler, 1985). As a result, even though the red clay is believed to be of lacustrine, and not subglacial, origin, slight overconsolidation values for this unit would be expected. The consolidation data are consistent with this interpretation and correlate well with similar interpretations (Rominger and Rutledge, 1952; Soderman and Kim, 1970). Origin and Regional Correlation The areal extent, facies relationships, gr ain-size trends, str atigraphic relationships, geomorphic expression, sedimentary struct ures, and engineering properties all strcngly support a lacustrine origin for the red clay within the study area. It is laterally continuous with Zarth's (1977) offshore facies. Because of these characteristics, this unit is correlated with the deep-water facies of the Wrenshall Formation of Wright et al. (1970) and is interpreted as an offshore deposit of Glacial Lake Duluth. Consequently, the term "Douglas Till" is considered to be incorrect, and its use should be abandoned. Depositional Model A property of the red clay which has posed problems regarding its origin is the fact that in many exposures it is massive and lacks any bedding features. The question arises as to how these massive portions 82 can be deposited in a glacial lake, because most glaciolacustrine deposits tend to exhibit bedding features that reflect the seasonal sediment input caused by a spring/summer melt season, which is accompanied by an increased sediment input into the lake basin. A common type of glaciolacustrine deposit thus formed is varves, originally defined by DeGeer (1912), which consist of an alternation of light and dark colored laminae, each pair representing one year of deposition. The lighter-colored portion is generally thicker, consisting of coarser particle sizes (silts); this layer represents the spring/ summer sedimentation. The thinner, darker-colored portion is generally composed of finer grain sizes (clays) and represents the winter sedimentation. Other varve-like deposits can be produced by a variety of depositional processes (Smith and Ashley, 1985), but the term "varve" is restricted to this silt/clay couplet deposited during one year of time. Although the existence of varves is widely reported in the literature (see Sturm, 1979), massive glaciolacustrine deposits are much less commonly mentioned. Such massive deposits can form if the lake basin receives a continuous influx of suspended matter (Sturm, 1979), or if deposits are reworked in some way (for example, bioturbation). Massive glaciolacustrine deposits have recently been described in the Great Lakes Region by Eyles and Eyles (1983). These workers have described stratigraphic sequences consisting of laminated silts and clays interbedded with massive, muddy diamicts. A depositional model involving the rain-out of suspended fines with a coarse component derived from floating ice is proposed. The similarity between these 83 deposits and sequences containing the massive red clay allows for the proposal of a similar depositional model. A continuous rain-out of suspended fines within the water column, derived from inflowing rivers as well as from distal subglacial discharge, as well as failure of subaqueous deposits (Shaw et al., 1978) all contribute to the year-round supply of sediment, resulting in the massive deposits of red clay. In many exposures, gravelly sand is overlain by laminated silt and clay, which grades upward into the massive red clay. This type of stratigraphic sequence may reflect the progressive retreat of the ice margin from the local area and a change from ice-contact and proglacial sedimentation, which is characterized by tills, subaqueous outwash, and bedded silts and clays (Smith and Ashley, 1985), to non-contact or distal sedimentation involving continuous deposition of suspended fines delivered to distal locations by density inflows, resulting in the massive deposits. These inflows result from meltwater discharge along the glacier margin as well as from inflowing sediment-laden streams. Because of the characteristically high suspended sediment concentration of these inflows, interflows and underflows are the dominant dispersal mechanisms in ice-contact lakes (Smith and Ashley, 1985). A depositional model illustrating these concepts is shown in Figure 22. 84 PROGLACIAL A. LAKE ,. ·· .:.··: ICE RETREAT B. Stratified In f low . • . · ...· ·.·.. -__.. OVERFLOW .··· ·· --.. INTE RFLOW FIGURE 22: Proposed depositional model for the red clay and associated units: (A) Ice-contact and proximal deposition (from Landmesser et al., 1982); (B) Density inflows of suspended sediment which deliver red clay to distal locations (from Smith and Ashley, 1985). 85 OTHER QUATERNARY SEDIMENTS Introduction Other deposits, in addition to those described above, are present in the study area. Although they are generally not as extensive as the Cromwell or Wrenshall Formations, they are important in terms of · understanding the geological history of the area. These deposits will be described briefly below, in terms of their general characteristics and probable origin. Deposits of the Nickerson Moraine Sandy and gravelly deposits of the Nickerson moraine are found In the extreme southeastern portion of the Borea Quadrangle. These vary from bedded sands and gravels to massive, sandy diamictons. These deposits mark the southeast flank of the ice margin during the Nickerson advance (Wright et al., 1969; Wright, 1972a; 1972b). From this location southwest to Nickerson, the moraine is somewhat indistinct. Between Nickerson and Moose Lake, however, the moraine is characterized by irregular topography in stone-poor clayey till. Several miles southwest of Moose Lake, the moraine swings northeastward and continues as the Thomson moraine to the village of Mahtowa. From this point to Carlton, however, the moraine is no longer marked by the presence of red, clayey till. The sandy nature of these portions of the Thomson-Nickerson 86 moraine has posed a problem, because till deposited during the Nickerson phase is very clay-rich rather than sandy-textured. It is possible that these portions of the moraines represent deposits of collapsed outwash rather than subglacial till; the bedded sands and- gravels of the moraine in the study area would support such an origin. However, more detailed work must be done on the moraine before such questions can be answered with any certainty. Sublacustrine Outwash A thin layer of bouldery and gravelly sand is present between the Upper Cromwell Formation and the Wrenshall Formation, as shown in Plate 4 and Figure 14. This unit is averages two feet in thickness and contains particles from cobble- to sand-size, with massive to crude horizontal bedding and trough cross-bedding structures present. The upper and lower contacts of this unit are sharp and planar. It has been suggested (Zarth, 1977) that this unit represents part of a transgressive facies of Glacial Lake Duluth. However, if such an origin were the case, one would expect to see a more gradational upper contact rather than a sharply-defined plane separating this unit from the overlying bedded silt and clay. Furthermore, trough cross-bedding, which is present in this unit, is an unlikely feature in a transgressive, nearshore lacustrine deposit. Finally, this unit occurs at elevations at least as low as 950 feet: if it were part of a transgressive sequence, representing shoreline/nearshore deposits, this 87 would imply that the shoreline of the glacial lake was at or near this level prior to transgression. However, this is not likely. Farrand (1960; 1969) has shown that ice-marginal lakes above 1100 feet in elevation coalesced to form Glacial Lake Duluth as the ice margin retreated into the Superior basin. This implies that an 1100-foot strandline was maintained for a time as the ice margin retreated northeastward. It is proposed that this deposit instead represents sublacustrine outwash deposited by the ice lobe as it retreated into the Superior basin. Similar deposits have been described by Rust and Romanelli (1975) and Shaw (1985). This origin is consistent with the ice-magrin retreat/depositional model illustrated in Figure 22. Holocene Alluvium River deposits of sand and silt are found along some of the stream valleys within the study area. Channel fills as well as lateral and vertical accretion deposits are more common along the larger stream channels. These deposits are relatively minor features of the overall landscape and have formed post-glacially, as a result of the erosion of Pleistocene sediments and subsequent sorting and redeposition by normal stream processes. 88 LATE WISCONSIN and HOLOCENE HISTORY INTRODUCTION Evidence for several glacial phases, as as for the existence of a large proglacial lake, is present within the study area. However, this investigation has led to the conclusion that some of the previous interpretations of the glacial history of this area are partially incorrect. These errors include the existence of a Split Rock till northeast of Esko, Minnesota (which is now considered to be supraglacial sediment associated with the Automba phase) and the subglacial origin of the surficial red clay (which is considered by the author to be a lacustrine deposit of Glacial Lake Duluth). An outline of the late Wisconsin and Holocene history of the area is presented below. This outline is based on both stratigraphic and geomorphic evidence. The term "phase" is an event-stratigraphic unit and will refer to the advance as well as the subsequent retreat and/or stillstand of an ice margin (Attig et al., 1985). 89 HISTORY OF LATE-WISCONSIN GLACIATION Pre-St. Croix Phase Whereas no extensive stratigraphic evidence exists within the Superior Basin for a pre-St. Croix phase ice advance, one exposure, described in a previous section (and shown in Plate 2 and Figure 12), does suggest that pre-St. Croix phase glacial deposits are indeed present in the area. Because this possible pre-St. Croix phase till was observed at only one location within the study area, it is not interpreted as representing an unequivocal pre-St. Croix phase deposit. However, pre-St. Croix phase deposits attributed to the Superior lobe have been observed in southwestern Minnesota (Matsch, 1972) and thus indicate that pre-St. Croix phase Superior lobe activity did occur. In any case, evidence (although sparse) for a pre-St. Croix phase ice advance is present and must be noted. St. Croix Phase The earliest ice advance for which there is extensive evidence within the study area is represented by the Lower Cromwell Formation. This unit consists of subglacial till deposited during the St. Croix phase of glaciation approximately 15,000-20,000 years B. P. Fabric data within the Esko Quadrangle indicate a NNW-SSE ice flow direction (Figure Ba). However, this may be a local feature, because the regional ice flow 90 direction for the Superior lobe during the St. Croix phase is known to have been to the southwest (Wright, 1969; 1972b). This unit and its correlatives are present throughout northern Minnesota and northern Wisconsin and extend southward to the St. Croix Moraine, a terminal ice-margin position marking the limits of St. Croix phase glaciation (Wright, 1972b). The Toimi drumlins, composed of the Independence Till of Wright et al. (1969) and later redefined as the Sullivan Lake Formation by Gross (1982), serve as geomorphic evidence for this ice advance (by the Rainy lobe) in northeastern Minnesota. Wastage following the St. Croix phase maximum is not fully understood. However, the ice retreat must have been fairly extensive, owing to the fact that the subsequent Automba advance involved deposition of a distinctly more silt-rich till. Because this silt-rich till is present throughout the study area and is in sharp contact with the underlying Lower Cromwell Formation, it is likely that the retreat following the St. Croix maximum progressed at least as far northeastward as the present study area. Automba Phase Immediately overlying the Lower Cromwell Formation within the study area is the more silt-rich Upper Cromwell Formation. This unit consists of subglacial till and supraglacial sediments deposited during the Automba phase of glaciation. This unit and its correlatives extend throughout northeastern Minnesota and northern Wisconsin; its maximum 91 extent is marked by the Highland Moraine and Mille Lacs Moraine in Minnesota and the Bluff-Karlsborg ice margin in northwestern Wisconsin (Wright et al., 1969; 1972; Clayton, 1984; Attig et al., 1985). The Highland and Mille Lacs moraines are strong features; however, the correlative ice-margin features in northwestern Wisconsin are not nearly as distinct, and their correlation is somewhat uncertain. Fabric data obtained from the Esko Quadrangle indicate an ENE-WSW ice flow direction for the ice lobe at this time (Figure 8b), which is perpendicular to that of the preceeding St. Croix phase. Two explanations for this more westerly flow direction have been proposed (Wright, 1972b): (1) During wastage of the confluent Superior and Rainy lobes at the end of the St. Croix phase, the Superior lobe portion retreated less rapidly (owing to its greater ice thickness as compared to that of the Rainy lobe). As the Rainy lobe portion retreated to the Vermillion Moraine, space became available for the Superior lobe to expand westward into this newly-vacated area. Thus, in this interpretation, the Automba phase is simply a localized readvance which occurred during St. Croix phase wastage. (2) Stagnant ice from the St. Croix phase remained in the area of Sandstone (to the southeast) and provided a barrier which blocked the readvancing Superior lobe and diverted it westward. Evidence which would support this second theory involving stagnant ice has been found in Minnesota (Florin and Wright, 1969). Geomorphic evidence also supports this latter explanation. There is a distinct absence of any well-developed moraine in northwestern Wisconsin which can be attributed to this ice advance, yet such features 92 are present in Minnesota. Because of this lack of a well-developed moraine on the southeast flank of the ice lobe, as well as the presence of a silt-rich till distinctly different from that of the St. Croix phase, the second theory is favored as an explanation for the westward ice flow of the Automba phase. The silt-rich texture of this till can be explained by the incorporation of silty proglacial lake sediments into the ice during its readvance, and the further comminution of particles by crushing and abrasion (Dreimanis, 1971a; 1971b). Split Rock Phase There is no evidence for a Split Rock phase within the study area. However, several miles to the southwest, a clayey till referred to as the Barnum Till (Baker, 1964) forms the surficial geologic unit and is attributed to the Split Rock (and subsequent Nickerson) advance. This unit is confined to the narrow lowland at the southwest end of the Lake Superior Basin. It forms a discontinuous thin mantle over older landforms, as well as a small drumlin field in the Split Rock Valley north of Denham. The maximum extent of this advance is marked by the limit of this red, clayey till as well as a series of eskers and outwash fans and ice-contact slopes southwest of Cloquet, Minnesota. This ice margin is correlated to the Swiss ice-margin position in northwestern Wisconsin (Clayton, 1984). This advance was contemporaneous with the Pine City phase of the Grantsburg sublobe of the Des Moines lobe (Wright, 1972b); 93 their maxima occurred between 13,000 and 16,000 years B. P. It is not known whether the ice retreat following the Automba maximum progressed into the study area prior to the Split Rock readvance: no Split Rock features are present within the study area. This lack of evidence may simply reflect the possibility that the ice retreat did not reach this far to the northeast, or that the retreat did reach this area, but Split Rock features were destroyed by the subsequent Nickerson advance and/or Glacial Lake Duluth processes. In any case, wastage following the Automba maximum must have progressed well into the Superior Basin, thus allowing the formation of proglacial lakes which were ultimately responsible for the clayey texture of the till deposited by the Split Rock ice. Because of the lack of evidence supporting climatic changes which could be responsible for this advance, it has been proposed (Wright, 1969; 1973; 1980) that a surge was responsible for the Split Rock (and subsequent Nickerson) advance. Further study has led other workers to report similar conclusions (Clayton et al., 1985). These workers propose that subglacial water permitted increased ice flow and the build-up of ice behind a frozen terminus, which acted as an ice dam. As the pressure 11 of this ice exceeded the strength of the 11 dam , failure occurred , and a rapid flow or surge of the ice lobe then followed. Stagnation immediately followed the surge event, with extensive outwash sediments being deposited in front of the ice margin. The abundant outwash deposits southwest of Cloquet, Minnesota and those of the correlative Swiss surface in northwestern Wisconsin support this hypothesis. 94 Nickerson Phase After retreat of the ice from the Split Rock maximum, the Superior lobe advanced once more as a narrow lobe to the Thomson-Nickerson moraine. Nickerson phase deposits are absent in the Esko Quadrangle but are found in the southeastern portion of the Berea Quadrangle. Nickerson phase deposits consist of collapsed outwash and flow till and red, clayey deposits belonging to the Barnum Till (Wright et al., 1969; Wright, 1972a). The Nickerson ice margin has been correlated to the Lake Ruth ice margin in northwestern Wisconsin (Clayton, 1984). This advance was contemporaneous with the Alborn advance of the St. Louis sublobe of the Des Moines lobe in northern Minnesota; their maxima occurred approximately 12,000 years B. P. (Wright et al., 1973). As with the Split Rock phase, the Nickerson phase is thought to have been a surge of the Superior lobe (Wright 1969; 1973; 1980). The extensive collapsed outwash and flow till deposits of the Nickerson moraine indicate ice stagnation and support the surge hypothesis. Agassiz Phase The Agassiz phase of glaciation is marked not by a readvance of the ice margin but by the formation of several glacial lakes. Within the study area, shoreline features and lacustrine deposits belonging to the Wrenshall Formation are evidence for the existence of Glacial Lake Duluth, which formed as the Superior lobe retreated northeastward into 95 the Superior Basin (Glacial Lake Algonquin occuppied the Michigan and Huron basins at this time). The initial outlet channel for the lake was the Portage (formerly referred to as the "Moose Lake") outlet, at an elevation of approximately 1050 feet above sea level, and later, the Brule outlet, at an elevation of 1024 feet. These outlets have not been significantly affected by glacial rebound since they were last functioning (Farrand and Drexler, 1985). Lower outlets were opened up as the ice margin retreated northeastward, resulting in a lowering of the lake levels; these events are discussed in detail by Farrand (1960) and Drexler (1981). The retreat of the ice from the region was rapid; the time spanned by the existence of the post-Duluth lakes was only 100 years or so (Farrand and Drexler, 1985). This explains why shoreline features of lower lake levels are weak or absent in the study area; a rapid lowering of the lake level did not allow adequate time for the formation of shoreline features or for the deposition of a regressive sequence. These post-Duluth lakes discharged eastward via outlets in the Huron Mountains-Marquette area, and are detailed by Drexler (1981). Farrand and Drexler (1985) suggest that the strength of the Glacial Lake Duluth shoreline features in the study area may be due to the fact that the lake was at the Duluth level twice: both after ice retreat following the Nickerson maximum (phase B of Clayton, 1983) and later, during the Marquette readvance, which occurred approximately 9900 years B. P. (phase D of Clayton, 1983). Throughout the rest of the basin, only the later Duluth levels (phase D) would be present, as earlier features 96 would have been destroyed by the readvancing Marquette ice. The end of the Agassiz phase, and thus the beginning of the Holocene epoch, can be considered as that point in time when the lake occupying the Superior Basin ceased to be a glacial lake; that is, when its level was no longer controlled by an ice barrier at some point on its periphery (Farrand and Drexler, 1985). Thus, with the development of Lake Minong, which was the first late- or postglacial lake (for which shoreline evidence still remains) to approximate the present coastal configuration of Lake Superior, the Agassiz phase came to a close. This occurred approximately 9500 years B. P. (Drexler et al., 1983). The lake level at this time was 450 feet above sea level (Farrand, 1969). The lacustrine sediments of the study area were undergoing rapid erosion by the St. Louis River and its tributaries at this time. PROBLEMS WITH THE LATE-GLACIAL SEQUENCE OF EVENTS Several problems remain with the current late-glacial sequence of events for the region: a possible pre-St. Croix phase unit; the validity of the Split Rock phase; and the maximum extent of the Marquette advance are all areas which need further study. Although the study area has undoubtedly been subjected to numerous glaciations during the Pleistocene, only unequivocal St. Croix phase and younger-age glacial deposits have been observed in the area. However, the presence of the sandstone-rich diamicton observed underlying the 97 Lower Cromwell Formation (Plate 2 and Figure 12) presents the possibility that a pre-St. Croix phase deposit is present in the area. It is similar in texture and lithology to the "Old Red Drift" of earlier workers in southeastern Minnesota and to the Hawk Creek Till of Matsch (1972). Both were deposited by the Superior lobe and both predate the St. Croix phase. Although it is interesting speculation that the lowest till in the study area may be correlative with those tills, a definite correlation cannot be made. Further study is needed in order to determine if this unit is areally significant and if it does indeed represent a pre-St. Croix phase deposit rather than a localized sandstone-rich portion of the Lower Cromwell Formation. The validity of the Split Rock advance is also questionable. Although evidence for both the preceeding Automba phase and the subsequent Nickerson phase is present, no evidence exists within the study area for a Split Rock phase. The evidence for this phase is found southwest of Cloquet, Minnesota and is based on geomorphic features such as drumlins, outwash fans and ice-contact slopes; at no location is a Nickerson till observed to overlie a Split Rock till. However, some of these geomorphic features attributed to the Split Rock phase are inconsistent with the surge hypothesis proposed for the Split Rock advance. Most notable among these inconsistencies are the Split Rock drumlins. Every case of drumlins reported in the literature has been attributed to non-surging glaciers. A legitimate problem which is posed is whether drumlins are capable of being formed under surge conditions. 98 Additionally, these drumlins are found in the marginal zone of the proposed Split Rock advance. Surge glaciers normally have marginal zones characterized by stagnation-type features and supraglacial sediments rather than subglacial features such as drumlins, which are normally found further up-glacier (Sugden and John, 1976). It is quite possible that the Split Rock drumlins are actually Automba drumlins mantled by a cover of clay-rich Nickerson till, and that other deposits and geomorphic features attributed to the Split Rock advance are actually due to the Nickerson ice advance and its subsequent retreat. In any case, more detailed work is needed in the Split Rock area before these questions can be answered and the existence of the Split Rock phase can be proven/disproven. A final problem that remains with the postulated late-glacial sequence of events in the region is the location of the Marquette ice margin at the time of its maximum extent. The Marquette advance did not reach as far southwest into the Lake Superior Basin as the present study area, but was instead marked by a terminus located somewhere to the northeast. An interesting speculation is that the "Unit 1" moraine of Landmesser et al. (1982) beneath Lake Superior may represent a terminal moraine formed during the readvance of the Marquette ice (Figure 23). Recessional moraines within the Lake Superior Basin are absent southwest of this position, but are present to the northeast. Furthermore, this moraine has been correlated to the Glacial Lake Duluth level (Landmesser et al., 1982). However, the idea that this "Unit 1" moraine represents the terminal position of the Marquette ice advance is merely speculative 99 0 50 100 150 Km , '•• ,,,- .. '!Ii \ ,' ,',.' \ .. ,/.':::· '. , ,, \ Marks ' } ;" ·-,·,• ICE '- '•-· ---·I ' ' ,, ____ ... _,. ,.. _. ,' I ICE ,-. ,- ' ' I I '':' ,., .. .. 1\11 ' A. Glacial Lake Duluth C. Glacial Lake Beaver Bay Finlayson Mor. Dog Lake Mor. 'll,. .... ¥ I:.. -. I-' i!:O..' .. -...... ,,1 • 0 . ... 0 , .. ; '"' \ ,, " \ - \ ICE \ ' ...... __ .. - -,.. ! .::: I B. Glacial Lake Washburn D. Glacial Lake Minong Figure 23. SUlrunary of Lake Superior deglaciation showing the location of major recessional moraines identified on seismic-reflection profiles and their correlation with Glacial Lakes Duluth, Washburn, Beaver Bay, and Minong. From Landmesser et al., 1982. at this point. Only more detailed work in northern Wisconsin will help resolve this problem. POST-GLACIAL HISTORY The post-glacial history of the region can be considered to begin with the development of Lake Minong approximately 9500 years B. P. Its outlet was controlled by a bedrock sill in the vicinity of Sault Ste. Marie, which at that time was 200 feet below its present altitude (Farrand, 1969). The following summary of the post-glacial history of the lakes in the Superior Basin is based on the accounts given by Farrand (1969) and Farrand and Drexler (1985). Following the development of Lake Minong, the North Bay, Ontario area was deglaciated, which opened up the North Bay outlet channel by which the three upper Great Lakes were drained to very low levels. The retreating ice margin also allowed eastward discharge from Lake Agassiz to enter the Superior Basin (Lake Minong), which in turn resulted in a rapid downcutting of its outlet to the Houghton Lake level (Teller and Thorleifson, 1983). The Houghton level was approximately 375 feet above sea level, and discharged through a proto-St. Mary's River into Lake Stanley (180 feet) in the Huron basin. Rebound of the North Bay outlet to the level of the Sault barrier resulted in a confluent level of lakes in the Huron-Michigan and Superior basins at 7500 years B. P. The water level continued to rise due to isostatic rebound for several thousand 101 years until the North Bay outlet reached the level of the abandoned Chicago and Port Huron outlets to the south, which brought into existence the Nipissing Great Lakes stage common to the three upper Great Lakes around 4700 years B. P. with discharge through three outlets: the North Bay outlet in the northeast, and the Chicago and Port Huron outlets in the southern region. This stage persisted until approximately 4000 years B. P. when rebound raised the North Bay outlet above the lake level, leaving discharge to the two southern outlets. Post-Nipissing history is marked only by minor changes in lake level. Downcutting of the Port Huron outlet brought into existence the Algoma Lake stage in the Superior, Michigan, and Huron basins. Approximately 2200 years B. P. rebound raised the bedrock sill at Sault Ste. Marie above the level of lakes Huron and Michigan, forming the St. Marys Rapids at 590 feet above sea level and bringing into existence the Sault Lake level in the Superior Basin and an independent Lake Superior. Continued rebound has raised the level of the rapids to the present level of 600 feet above sea level. Isostatic rebound is presently continuing in the basin, with the northern corner of the basin rising 27 cm per century and the Duluth area subsiding 21 cm per century relative to Point Iroquois at the head of the St. Mary's River (Farrand and Drexler, 1985). Within the present study area, dissection of the Glacial Lake Duluth plain by the St. Louis and Nemadji River systems has been the principal active geological process occurring since the lowering of the lake from the Duluth level. The southern portion of the Eska Quadrangle has been 102 affected to a great degree by this stream dissection; the Berea Quadrangle has been affected less severely due to the relatively flat topography and greater distance from the St. Louis River system, which is the major agent of erosion in the area. Profiles of the local northwestern zone of the lake basin, both as it existed during Glacial Lake Duluth time as well as for the present, are shown in Figures 24 and 25. As can be seen from the profiles, a large amount of glacial sediment has been removed by river systems. The process of headward erosion and mass-wasting of valley side-slopes can be expected to continue in the future, eventually transforming the area into a system of stream valleys separated by intervening ridges. 103 NNW SSE 0 lmi. ,/' ...... / ESKO QUADRANGLE""' FROGNER QUADRANGLE 1300ft. WRENSHALL rM. 600fl. [3 (DEEP-WATER FACIES) k<\I SAND AND GRAVEL I-' 0 .i:-- ll UPPER c ROMWELL FM. I: •I LOWER CROMWELL FM. BEDROCK Figure 24. Topographic profile and sche matic section across the southern portion of the Esko Quadrangle and northeastern Frogner Quadrangle during Glacial Lake Duluth time. Bearing of section is approximately 150°. NNW SSE ./ 0 lmi. 7 ESKO QUADRANGLE""' FROGNER' QUADRANGLE Hwy. 210 Hwy. 23 1300ft. St. Louis River I WRENSHALL FM. 600ft. (DEEP-WATER FACIES) cg I-' SAND AND GRAVEL 0 Vl L3 UPPER CROMWELL FM. l:=3 LOWER CROMWELL FM. BEDROCK Figure 25. 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P., ed., Till: A Symposium, Columbus, Ohio State University Press, p. 41-72. , 1972, Modern Arctic glaciers as depositional models for former ice sheets: Journal of the Geological Society of London, v. 128, p. 361-393. 1975, The genesis of glacial tills - A framework for geotechnical interpretation, in Proceedings of Symposium "The Engineering Behavior of Glacial Materials", Midland Soil Mechanics and Foundation Engineering Society, p. 53-59. 106 , 1978, Boulder shapes and grain-size distributions of debris as indicators of transport paths through a glacier and till genesis: Sedimentology, v. 25, p. 773-799. , 1982, Subglacial processes and the development of glacial bed forms, in Davidson-Arnott, R., Nickling, W., and Fahey, B. D., Research in Glacial, Glaciofluvial, and Glaciolacustrine Systems: Norwich, Geo Books Norwich, p. 1-31. Carroll, D., 1970, Clay Minerals: A Guide to their X-Ray Identification: United States Geological Survey Special Paper 126, 80p. Clark, R. 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J., 1973, Superior and Des Moines Lobes: Geological Society of America Memoir 136, p. 153-185. Zarth, R. J., 1977, The Quaternary Geology of the Wrenshall and Frogner Quadrangles, Northeastern Minnesota: Unpub. M. S. thesis, University of Minnesota, 93p. 115 APPENDIX A Preparation of Samples and Analysis of the Fine Clay Fraction Preparation of samples for X-ray analysis of the <.002mm fraction consisted of adding several ml of dispersing agent (100 g sodium hexametaphosphate and 10 g sodium carbonate in one liter of deionized water) to 20-25 g of sample material soaked in 200-250 ml of deionized water. The sample was then mixed in a malt mixer for approximately ten minutes and then filtered through a #230 mesh (.0039 mm) screen to remove the size fraction greater than 4 phi. The remaining silt and clay was then put in a beaker and allowed to sit undisturbed for ten minutes. An eye dropper was then brought into contact with the surface of the suspension and a sample was drawn off and placed onto a clean glass slide. The particle size of this sample is <.002 mm. Three slides were made of each sample. They were allowed to dry at room temperature for 24 hours. These mounts result in strongly-oriented samples with {001} faces of the clay minerals parallel to the slide, thus resulting in intense reflections under X-ray analysis (Carroll, 1970). Three X-ray analyses were done on each sample at a scanning speed of 1° 28 per minute from 4° to 40° 29. The first set of samples was run untreated. The second set of samples was placed in a dessicator with ethylene glycol and heated at 6o 0 c for one hour, then allowed to cool before being run. This treatment causes the expansion of the A-1 montmorillonite lattice to increase to 17A. The third set of samples was heated at 55ooc for one hour and allowed to cool in a dessicator before analysis. This treatment causes several changes in the diffractogram: 1) any kaolinite peaks will be destroyed; 2) chlorite peaks may be destroyed, except for the 14A peak which may be shifted to 13.6-13.8A and may intensify slightly; 3) the (001) montmorillonite peak will shift to 9A; and, 4) the 10A illite peak will intensify. A-2 APPENDIX B Important Stratigraphic Exposures 1. Location: NE 1/4, SW 1/4, SE 1/4, Section 2, T48N, R16W (Esko Quadrangle - Jay Cooke State Park). Descriotion: Several feet of a possible pre-St. Croix phase till overlain by approximately 5-10 feet of the Lower Cromwell Formation and 20-30 feet of the Upper Cromwell Formation (see Plate 2 and Figure 12 of text for reference). 2. Location: NW 1/4, NE 1/4, SW 1/4, Section 1, T48N, R16W (Esko Quadrangle - Jay Cooke State Park). Description: Approximately 8 feet of Lower Cromwell Formation overlain by 10 feet of the Upper Cromwell Formation (see Plate and Figure 8 of text for reference). 3. Location: Center of eastern edge of Section 21, T49N, R16W (Eska Quadrangle). Description: Approximately 10 feet of interbedded outwash and supraglacial till of the Upper Cromwell Formation (see Plate 3 and Figure 11 of text for reference). A-3 4. Location: SW 1/4, SE 1/4, NW 1/4, Section 10, T48N, R16W (Esko Quadrangle - Jay Cooke State Park). Description: Approximately 17 feet of Upper Cromwell Formation overlain by 1-2 feet of sandy gravel (sublacustrine outwash) and 15 feet of bedded silt and clay of the Wrenshall Formation (see Plates 4 and 9 and Figure 14 of text for reference). 5. Location: NW 1/4, NE 1/4, NW 1/4, Section 14, T49N, R1?w (West Duluth Quadrangle). Description: Approximately 85 feet of interbedded gravels, sands, silts, and clays of the Wrenshall Formation (see Plate 7 and Figure 16 of text for reference). 6. Location: SE 1/4, NE 1/4, Section 28, T47N, R14W (Berea Quadrangle). Description: Approximately 25 feet of bedded sand and gravel of the Wrenshall Formation shallow-water facies (see Plates 5 and 6 of text for reference). 7. Location: SW 1/4, SW 1/4, Section 30, T49N, R14W (West Duluth Quadrangle). Description: Approxi mately 15 feet of Wrenshall Formation laminated silt and clay grading upward into massive red clay (see Plate 10 of text for reference). A-4