PETROLOGY, STRATIGRAPHY AND SEDIMENTATION OF THE MIDDLE
PROTEROZOIC BAYFIELD GROUP, NORTHWESTERN WISCONSIN
A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY
Kent Fred Adamson
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
December, 1997 © Kent Fred Adamson, 1997 Abstract
The Bayfield Group (Middle Proterozoic, Lake Superior region) is composed of three formations which are, from oldest to youngest, the feldspatholithic to lithofeldspathic Orienta Formation; the quartz-arenitic Devils Island Sandstone; and the feldspatholithic Chequamegon Formation. The average QFL percentages are 69/17 /14 for the Orienta Formation, 95/4/1 for the Devils Island Sandstone, and 79/14/7 for the Chequamegon Formation. The large amounts of quartz and appreciable feldspar suggest that the primary provenance was older Early and Archean rocks. Diagenetic action on the Bayfield Group is evidenced by authigenic quartz, hematite, kaolinite, zeolite, and minor amounts of potassium feldspar. The average pore space of the sandstones in the Bayfield Group decreases with depth. Dissolution of potassium feldspar is the major factor in the development of minor secondary porosity. Accessory minerals show that "normal" zircon is the most prevalent form found within the non-opaque, non-micaceous suite. The presence of "normal" zircons suggests that a potential provenance was Keweenawan volcanics; some residence time in previously deposited rift sediments (such as the Oronto Group) may have occurred. The minor amount of other zircon varieties support the framework grain analyses that older Early Proterozoic and Archean sources were shedding major amounts of sediment into the depositional basin. The Bayfield Group was deposited in a transgressive-regressive lacustrine to fluvial environnment in the waning stages of the Midcontinent Rift System. The Orienta and Chequamegon Formations represent fluvial environments of deposition while the Devils Island Sandstone was likely deposited in a nearshore lacustrine depositional environment. The paleocurrent trend for the Devils Island is toward the east and southeast while paleocurrent trends for the Orienta and Chequamegon Formations are toward the northeast. The exposed sections suggest that the near-shore environment was restricted in comparison to the fluvial systems that were transporting sediment towards the central rift valley.
111 Table of Contents
Abstract 111
Table of Contents lV
List of Figures lX
List of Plates Xl
List of Tables XlV
Acknowledgements xv
INTRODUCTION 1
Purpose of Study 2
Methods 4
Previous Work 5
REGIONAL GEOLOGY 7
Archean Rocks 9
Early Proterozoic 10
Middle Proterozoic 11
Keweenawan Supergroup 11
Grenville Province 13
Paleozoic and Mesozoic 14
Quaternary 14
Regional Structure 15
South Shore Geophysical Data 16
Review of Upper Keweenawan Paleomagnetic Analysis 21
Age of the Bayfield Group - Conclusions 23
lV PETROLOGY - FIELD DESCRIPTIONS 24
Orienta Formation 24
Devils Island Sandstone 28
Chequamegon Formation 33
Fond du Lac Formation 35
Hinckley Sandstone 35
MINERALOGY AND PETROLOGY 38
Introduction 38
Sample Preparation 38
Operational Definitions 39
Quartz 39
Feldspars 40
Lithics 41
Miscellaneous Grains 43
Cement/Matrix 44
General Petrographic Characteristics and Descriptions 45
Orienta Formation 45
Devils Island Sandstone 48
Chequamegon Formation 52
Diagenesis of the Bayfield Group 55
Paragenetic Sequence ------57
Texture 58
Porosity 59
Modal Analysis of Thin Sections 63
v Heavy Minerals 78
Sample Preparation 78
Operational Definitions 80
Modal Analysis of Heavy Minerals 84
Orienta Formation 87
Devils Island Sandstone 87
Chequamegon Formation 88
KEY AREAS IN THE BAYFIELD GROUP 91
Orienta Formation at Middle River 91
South Landing, Devils Island ------97
Northern Sand Island 101
Geologic Well Logs 105
PALEOCURRENT ANALYSIS 115
Introduction 115
Orienta Formation 118
Devils Island Sandstone 118
Chequamegon Formation 118
PROVENANCE 123
Introduction 123
Paleocurrent Evidence 124
Petrologic Evidence and Interpretation 126
Accessory Mineral Evidence and Interpretation 127
Fond du Lac Formation Data 127
Hinckley Sandstone Data 128
Vl Interpretation 128
SEDIMENTATION AND ENVIRONMENTS OF DEPOSITION 130
Fluvial Systems 130
Nearshore Lacustrine Systems 133
Laminae Sands 134
Forms of Evidence Present 135
Interpretation 138
Orienta Formation 138
Devils Island Sandstone 138
Chequamegon Formation 140
Fond du Lac Formation 140
Hinckley Sandstone 141
TECTONic!SEDIMENTARY MODELS 142
Model 1 - Orienta Formation is equal to the Chequamegon Formation 142
A. Synclinal Structure 142
B. Faulting 144
Model 2 - Orienta Formation is not equal to the Chequamegon Formation 145
CONCLUSIONS 148
REFERENCES CITED 153
Appendix A - Field Data. 160
Appendix B - Outcrop Locations. 181
Appendix C-1 - Orienta Formation Modal Analyses. 183
Appendix C-2 - Devils Island Sandstone Modal Analyses. 185
Appendix C-3 - Chequamegon Formation Modal Analyses. 187
VH Appendix C-4 - Converted QFL and QmFL Percentages from Myers (1971b). 189
Appendix C-5 - Pore space variance. 190
Appendix D - Heavy Mineral Modal Analyses. 191
Appendix E. Paleocurrent Rose Plots. 192
Appendix F. Sand Point Geologic Well Log. 200
Appendix G. Sand Point Well modal analyses. 202
Vlll List of Figures
1 Stratigraphic column for Middle Proterozoic rocks of the Midcontinent Rift System in the Lake Superior region. 1
2 QFL diagram of Oronto and Bayfield Groups showing compositional maturity. 3
3 Basement tectonic provinces in the North American midcontinent. 7
4 Geologic map of the Midcontinent Rift System, showing the Bayfield basin and study area. 8
5 Schematic geologic interpretation of seismic profiles across axis of Midcontinent Rift System, offshore Lake Superior. 18
6 Interpreted reflection profile along line C (shown on Figure 4). 19
7 Geologic map of the Midcontinent Rift System. 20
8 Suggested Apparent Polar Wander path from 1500 to 800 Ma. 22
9 Strikes and dips of Bayfield Group strata. 37
10 Classification of sandstones. 63
11 Orienta Formation QmFLt plot of 18 modal analyses. 69
12 Orienta Formation QFL plot of 18 modal analyses. 70
13 Devils Island Sandstone QmFLt plot of 41 modal analyses. _____ 71
14 Devils Island Sandstone QFL plot of 41 modal analyses. 72
15 Chequamegon Formation QmFLt plot of 17 modal analyses. 73
16 Chequamegon Formation QFL plot of 17 modal analyses. 74
17 Bayfield Group mean modes, QmFLt. 76
18 Bayfield Group mean modes, QFL. 77
19 Outcrop locations for heavy mineral analysis. 79
20 Sketch map of the Middle River at Moonshine Road outcrop area. 94
21 QmFLt diagram for Middle River Orienta samples. 95
lX 22 Structure section on steeply dipping Middle River beds. 96
23 Map of Devils Island showing samples from location 110. 99
24 QmFLt diagram for Devils Island Sandstone, location 1 lOD. 100
25 Sketch map of Sand Island. 103
26 QmFLt diagram for Devils Island Sandstone (location 117). 104
27 Percentages of quartz and lithics in Sand Point well. 109
28 Percentages of feldspar in Sand Point well. 110
29 Maps showing paleocurrent trends in pre-volcanic quartz, interflow sandstones, Oronto Group, and Bayfield Group. 116
30 Paleocurrent directions for Orienta Formation. 120
31 Paleocurrent directions for Devils Island Sandstone. 121
32 Paleocurrent directions for Chequamegon Formation. 122
33 Compositional plots of key sandstone suites. 125
34 The eight basic architectural elements in fluvial deposits. 132
35 Sketch map of how the Midcontinent Rift System may have appeared during Keweenawan sedimentation. 136
36 Sketch map of Northwestern Wisconsin showing the location of brownstone quarries and seacaves. 141
37 A cross section depicting the Devils Island Sandstone as the core of a syncline. 143
38 A cross section depicting the Devils Island Sandstone deposited adjacent to a small-throw fault. 144
39 Sketch of a cross section of the Bayfield Group showing a theoretical depositional pattern for the sediments. 147
x List of Plates
Plate ·1 - Orienta Formation 26
A. Large cross-bed sets within the Orienta Formation at Amnicon Falls, location 4.
B. Ripple marks in the Orienta Formation at Copper Creek, location 17.
Plate 2 - Orienta Formation 27
A. Trough cross-bedding in the Orienta Formation at Copper Creek, location 14.
B. Steeply dipping beds of Orienta Formation on Middle River and Moonshine Road, location 33.
Plate 3 - Devils Island Sandstone 30
A. Ripple marks and mudcracks on the same bed, Devils Island Sandstone, Devils Island, location 11 OM2.
B. Mudcracks in the Devils Island Sandstone, Devils Island, location 1 lOPl.
Plate 4 - Devils Island Sandstone 31
Cross-bedding in Devils Island Sandstone, Devils Island, location 11 OAl.
Plate 5 - Devils Island Sandstone 32
Seacaves of Devils Island Sandstone, Devils Island, location 11 OS.
Plate 6 - Chequamegon Formation 34
A. Channel in the Chequamegon Formation, Hermit Island.
B. Large lensoid massive bodies (channels?) on North Twin Island.
Plate 7 - 46
Photomicrograph of sandstone - Orienta Formation.
Plate 8 - 47
Photomicrograph of sandsone - Orienta Formation.
Xl Plate 9 - 49
Photomicrograph of sandstone - Devils Island Sandstone.
Plate 10 - 50
Photomicrograph of sandstone - Devils Island Sandstone.
Plate 11 - 51
Photomicrograph of sandstone - Devils Island Sandstone.
Plate 12 - 53
Photomicrograph of sandstone - Chequamegon Formation.
Plate 13 - 54
Photomicrograph of sandstone - Chequamegon Formation.
Plate 14 - 61
Photomicrograph of sandstone - Orienta Formation.
Plate 15 - 62
Photomicrograph of sandstone - Chequamegon Formation.
Plate 16 - Heavy Minerals 89
A. Photomicrograph of heavy mineral suite from the Devils Island Sandstone.
B. Photomicrograph of heavy mineral suite from the Chequamegon Formation.
Plate 17 - Heavy Minerals 90
A. Photomicrograph of rounded rutile grain, Devils Island Sandstone.
B. Photomicrograph of a recycled tourmaline grain, Chequamegon Formation.
Xll Plate 18 - 111
A. Photomicrograph of grain assemblage from Sand Point well, interval 35-40 feet.
B. Photomicrograph of grain assemblage from Sand Point well, interval 60-65 feet.
Plate 19 - 112
A. Photomicrograph of grain assemblage from Sand Point well, interval 70-75 feet.
B. Photomicrograph of grain assemblage from Sand Point well, interval 75-80 feet.
Plate 20 - 113
A. Photomicrograph of grain assemblage from Sand Point well, interval 80-85 feet.
B. Photomicrograph of grain assemblage from Sand Point well, interval 85-90 feet.
Plate 21 - 114
A. Photomicrograph of grain assemblage from Sand Point well, interval 90-95 feet.
B. Photomicrograph of grain assemblage from Sand Point well, interval 125-130 feet.
Xlll List of Tables
1 Summary of clay mineralogy of the Bayfield Group. 56
2 Paragenetic sequence in Bayfield Group specimens. 57
3 Orienta Formation, 18 modal analyses. 65
4 Devils Island Sandstone, 41 modal analyses. 66
5 Chequamegon Formation, 17 modal analyses. 67
6 Bayfield Group, 25 heavy mineral modal analyses. 85
7 Heavy mineral modal analyses from Tyler and others (1940). 86
8 Sand Point Well, 21 modal analyses. 108
9 Major provenance types and key compositional aspects of derivative sands. 124
10 Facies classification. 131
11 Evidence for braided and meandering fluvial environments of deposition. 137
XlV Aclmowledgements
The writer gratefully acknowledges Julie Van Stappen and the U. S. Park Service -
Apostle Islands National Lakeshore for permission to access and sample restricted areas and additional assistance; James Robertson, Director, and Roger Peters from the
Wisconsin Geological and Natural History Survey for partial support of this project; and the Graduate School of the University of Minnesota for additional monetary support of this project.
I would like to thank all of the great people whom I met during the duration of the project. I appreciated all of the engaging conversations with my project advisor, Richard
Ojakangas. A special thanks goes to my mom, Josefa Adamson, who is always there for support.
xv INTRODUCTION
The termination of volcanism in the Midcontinental Rift System was followed by
the deposition of large amounts of sediment. Two sedimentary groups are distinguished
above the rift volcanic series, the Oronto Group and the overlying Bayfield Group as seen
in Figure 1. The Bayfield Group consists of three units: the Orienta Formation (575 m
thick) the Devils Island Sandstone (90 m thick) and the Chequamegon Formation (150 m
thick), first based on the report of Thwaites (1912).
O E.-ist-ccmral and Norchcasccrn Minnesota Northwestern Nonhern pm of Soucheasrcrn pan southcascern Minnesota and Isle Royale Wisconsin upper Michigan of upper Michigan B Chequamegon
Hinckley { e Devils Island Sandstone .z l? Fond du Lac Orienta Formation Formation Freda Freda Sandsconc Sandstone Jacobsvi lle § Nonesuch Nonesuch 0 Solar Church E g Sandstone z Formation ______. Copper Harbor Copper Harbor Copper Ha U named r- , __ ) _ ) _ ) Cone:lomcracc I formacion · Chengwatana · Ponage Lake Chengwatana Portage Lake Volcanic Group Volcanics Volcanic Group Volcanics
Conglomerate Nonmagnetic rocks Several unics, Numerous units, at Davis Hill of King, 1975 v o..;i"::>l-__,...,.u;..n-.d._iv_id_e-d_...-.i g. undivided .. ? -- ?- ?'- ?--.i-______...______-1 o e e i---.-,.---...-.i § ______- z 0 z 0 1------1 Kallander Creek Powder Mill 1 > Ely's Peak Basalts > Grand Portage Lavas Powder Mill Formacion :E 0.. Group. undivided Group, undivided - => Nopcming 1------1 Formacion Puckwungc 1 Siemens Creek Formacion 5 Bessemer Quarrzice z0
Figure 1. Stratigraphic column for Middle Proterozoic rocks of the Midcontinent Rift System in the Lake Superior region. (Modified from Morey and Van Schmus, 1988).
Sparse outcrops in shore cliffs or deeply eroded river valleys, along with subhorizontal strata, have made interpretation of the Bayfield Group difficult. The
1 Bayfield Group was formed by deposition in the subsiding rift basin. Sedimentation terminated prior to an episode of reverse faulting. Reverse faulting, specifically illustrated by the Douglas fault, is attributed to the emplacement of the Grenville Province to the east and represents a change from an extensional to a compressional regime in the
Midcontinent Rift System (Cannon, 1994).
Purpose of Study
The Orienta Formation is a feldspatholithic to a lithofeldspathic arenite and passes upward into quartz arenites of the Devils Island Sandstone. The uppermost unit, the
Chequamegon Formation, is a feldspatholithic arenite which does not fit into an overall scheme of a maturing-upwards sequence. Figure 2 shows the compositional maturing- upward scheme which typifies sandstones in the Keweenawan sequence, with the exception to the sequence being the Chequamegon Formation. One hypothesis to explain this sequence is that only two units are actually present, with the feldspatholithic Orienta
Formation overlain by the quartzose Devils Island Sandstone. The Chequamegon
Formation would then comprise the eastern exposures of the Orienta Formation where it is repeated. The Minnesota section does not contain a unit equivalent to the
Chequamegon Formation.
The Minnesota equivalent of the Orienta Formation is the Fond du Lac
Formation, which is a feldspatholithic arenite, and the quartz arenite Hinckley Sandstone is equivalent to the Devils Island Sandstone. The Hinckley Sandstone overlies the Fond du Lac Formation in the Minnesota section. The Fond du Lac Formation is similar in nature to the Orienta Formation and is found in close proximity, about 10 km, to the westernmost locations (Black River, location 96 on Appendix B) of the Orienta Formation.
The Hinckley Sandstone is a thin, 150 m (500 feet) thick unit with a maximum exposed
2 thickness of 30m (100 ft). The Hinckley Sandstone is overlain by Cambrian units farther south. There is no Minnesota equivalent to the Chequamegon Formation, which supports the possibility that the stratigraphy in the Wisconsin section should be similar. The purpose of this study is to determine how the Chequamegon Formation fits into the sedimentary sequence.
Q Devils Island Sandstone (and Hinckley Sandstone} Oronto Group
Freda Orienta Formation Sandstone
Formation Bayfield Group
Copper Harbor Conglomerate
F L Arkosic Arenite Li chic Arenite
Figure 2. QFL diagram of Oronto and Bayfield Groups showing compositional maturity. Chequamegon Formation of the Bayfield Group is outlined with the dashed line. (Modified from Ojakangas, 1986).
3 Methods
To test the hypothesis that only two formations are present in the Bayfield Group, field studies focused on internal sedimentary structures, hand sample identification, and overall geometry of the units in question. This included studying outcrops on the Bayfield
Peninsula and the Apostle Islands. Outcrops were located utilizing outcrop maps by
Thwaites (1912) and Myers (197lb). Additional outcrop locations were found in
Ojakangas (1976) and also provided by my project advisor, Richard Ojakangas.
Paleocurrent indicators, mainly trough-cross-bedding, were measured. The few exposed contacts between the Orienta Formation and overlying Devils Island Sandstone were investigated to determine their relationships. Also, the only exposed contact between the
Chequamegon Formation and Devils Island Sandstone was critically examined on the south end of Devils Island. This was also previously described by Thwaites (1912, p. 38) and Myers (197lb, p. 20-21).
The second part of this investigation focused on the petrography of the sandstones.
A total of 224 samples was collected from outcrops. Seventy-seven thin sections were prepared, representing the full geographical and stratigraphical range of the Bayfield
Group. A set of 21 thin sections was prepared from a 140 foot well with sample increments every five feet. Thin sections were examined with a petrographic microscope.
A total of 76 thin sections, 18 from the Orienta Formation, 41 from the Devils Island
Sandstone, and 17 from the Chequamegon Formation were point-counted, with approximately 600 points per section. An additional 21 thin sections made from well cuttings comprising a transition from the Chequamegon Formation to the Devils Island
Sandstone were also point-counted. The rocks were classified after Pettijohn and others
(1987). Heavy minerals were separated from 25 samples - 10 from the Orienta
Formation, 7 from the Devils Island Sandstone, and 8 from the Chequamegon
4 Formation. The ZTR index (Hubert, 1962) (zircon, tourmaline, and rutile) was used to help determine mineralogic maturity.
Previous Work
The elastic rocks on the south shore of Lake Superior were first described by Owen in 1847 (Myers, 1971 b, p. 5) . The first in-depth work on these rocks, formerly called the
"Western Lake Superior Sandstone," was done by Thwaites (1912). Thwaites originally had (1912, p. 50-51) divided the Oronto Group into five units, from oldest to youngest:
Outer Conglomerate (Copper Harbor Conglomerate), Nonesuch Formation, Freda
Sandstone, Eileen Sandstone, and the Amnicon Formation. The Eileen Sandstone, which crops out only at Fish Creek near Ashland (Thwaites, 1912, p. 54), was included by Myers
(197lb, p. 228) as part of the Freda Sandstone. Thwaites (1912, p. 49) also mapped and named outcrops along the St. Louis River on the Wisconsin-Minnesota boundary as being part of the Amnicon Formation, which was later included within the Fond du Lac
Formation by Tyler and others (1940) . White (1972) suggested that the Oronto Group is composed of the Freda Sandstone and Nonesuch Formation, with the Copper Harbor
Conglomerate being a separate entity. The Bayfield Group was originally divided into its three units, which are from oldest to youngest, the Orienta Sandstone, Devils Island
Sandstone, and Chequamegon Sandstone (Thwaites, 1912, p. 26). The refinement of the
Oronto Group meant that the Amnicon Formation was reassigned and classified as Freda
Sandstone while the Eileen Sandstone was reassigned to be basal Orienta (Tyler and others, 1940, p. 1479; Hite, 1968). Later refinement showed that this classification does not follow in all cases where Amnicon-type sediments occur; those at Amnicon Falls were reassigned to the Orienta Formation (Tyler and others, 1940, p. 1479).
5 Further investigation into the Bayfield Group was done by Myers (1971 b). One of
Myers' objectives was to see if the Bayfield Group did indeed have recycled components
from the underlying Freda Sandstone. Regional mapping and outcrop mapping based on
Thwaites (1912) was done by Myers (197lb, p. 3-4). A new outcrop of the Devils Island
Sandstone on the East Fork of the Iron River was found (Myers, 197lb, p. 19-20), further
defining the Devils Island Sandstone as a mappable unit. The relationship between
sedimentation and faulting (specifically, the Douglas Fault) determined that faulting
occurred after deposition of the sandstones, although there is the possibility of movement
before or during deposition as well (Thwaites, 1912, p. 89; Myers, 1971b, p. 187; Morey
and Ojakangas, 1982). Ojakangas and Morey (1982c) suggested that there is an
unconformity between the Bayfield and Oronto Group rocks since there is no clear
evidence of the Orienta Formation at the Freda Sandstone (Eileen) outcrop at Fish Creek,
6 km (3.75 miles) southwest of Ashland as suggested by Thwaites (1912, p. 54-55).
Subhorizontal beds of the Chequamegon Formation are well exposed 8 km to the north of
the Fish Creek location on the shore of Lake Superior (Ojakangas and Morey, l 982c)
which further indicates an unconformity between the Oronto and Bayfield Groups.
Thwaites (1912, p. 38 and p. 41) inferred that the three formations of the Bayfield Group
comprise a conformable sequence even though contacts between the units are not well- exposed.
6 REGIONAL GEOLOGY
The Canadian Shield is a primarily Archean and Proterozoic terraine with younger
Phanerozoic terranes surrounding it (Figure 3). The Midcontinent Rift System, Middle
Proterozoic in age, is a failed rift which cuts across the older rocks of the Superior Province
and Penokean orogen. The study area of the Bayfield Peninsula and Apostle Islands of
Northwestern Wisconsin (Figure 4) is located on the south shore of Lake Superior, placing
it in the Midcontinent Rift System.
Figure 3. Basement tectonic provinces in the North American midcontinent. MCR, Midcontinent rift; KSZ, Kapuskasing Structural Zone. (From Manson and Halls, 1997).
7 Lake Superior
STUDY AREA --J
00
Lake Owen Fault . Clas tics .,j Intrusives (Mainly Gabbro)
,.. 1 I ... ,..' 1 I .... Volcanics (Mainly Basalt) I...... _.::' I
,.- .-c a 1 Reverse Fault Km 5,0 i° I St. Croix Horst O Mi 25
Figure 4. Geologic map of the Midcontinent Rift System, showing the Bayfield basin and study area. C denotes seismic line of Figure 6. (Modified from Dickas, 1986). Archean Rocks
In the Minnesota River Valley of south-central Minnesota and in the Upper
Peninsula of Michigan, small gneissic areas as old as 3500 Ma are present. Northern
Minnesota formations of Middle Archean age are around 2700 Ma, and increase in age
northward into Canada where some rock units are 3000 Ma. Greenstone-granite terranes
account for most of the Archean bedrock in northern Minnesota. The best-exposed
greenstone belt in Minnesota is the Vermilion district. This terrane forms the southern
part of the Superior Province of the Canadian Shield. Crossing the Wisconsin and
Michigan border is another granite-greenstone terrane.
The Superior Province has been subdivided into east-northeast-trending belts which are alternating volcanic-plutonic rocks and gneiss. Three of these belts extend into
Minnesota: the Wabigoon volcanic-plutonic belt; the Quetico gneiss belt; and the
Shebandowan or Wawa volcanic-plutonic belt (Morey and Van Schmus, 1988). The volcanic-plutonic belts are characterized by piles of mafic to felsic volcanic rocks and related synvolcanic intrusions (Schulz, 1980). Banded iron-formation occurs as lenses throughout the pile but is more common towards the top of the dominantly mafic successions (Morey, 1980). The mafic rocks are succeeded upward and laterally by dacites/rhyodacites which are overlain by a volcanogenic graywacke-shale sequence derived from the felsic volcanic centers (Ojakangas, 1972). The pile of interlayered volcanic and sedimentary rocks has undergone multiple folding events and generally has been metamorphosed to greenschist facies.
Plutonic igneous rocks which bound several greenstone belts in northern Minnesota are younger than the greenstone belts. The Vermilion Massif, located northwest of Ely is the largest body of granitic rocks exposed in the state. The massif is bounded by older metavolcanic and metasedimentary rocks to the south and north (Southwick, 1972). It is
9 composed mainly of biotite granite with three other intrusive rock types (hornblende
quartz diorite, hornblende diorite, and biotite granodiorite) also present (Southwick,
1972). The Giants Range batholith, which is located southeast of Ely, is truncated by the
Keweenawan Duluth Complex to the east. The Giants Range batholith is divided into
several units of tonalitic to granitic composition (Morey and Van Schmus, 1988). It also
includes scattered smaller masses of older plutonic rocks and remnants of metamorphosed
sedimentary and volcanic rocks (Sims and Viswanathan, 1972). In central Wisconsin the
Archean terraine is mainly amphibolite, garnetiferous hornblende gneiss, and schist (Morey
and Van Schmus, 1988). The Wisconsin Magmatic Terrane in northern Wisconsin
consists of calc-alkalic rocks and alkali granites (Sims and others, 1994).
Early Proterozoic
The Early Proterozoic of the Great Lakes region consists of two sedimentary- volcanic supracrustal sequences. The sequences are partially stratigraphically separated by
rocks of the Penokean Orogen, which occurred 1850 ±30 m.y. ago (Sims and others,
1994). The Penokean Orogeny is attributed to a southerly terrane, the Wisconsin
Magmatic Terrane, docking with the Superior Province following subduction of ocean floor (Sims and others, 1994).
The Huronian Supergroup, 2400 to 2100 Ma old, is a 10,000 m thick sequence found primarily north of Lake Huron which includes arkosic and quartzose sandstones, carbonates and glaciogenic units. The Marquette Range Supergroup and Animikian
Group include quartz sandstones, dolomites, basal glaciogenic units and 2000 Ma iron- formation. The iron-formation is found in numerous iron ranges in the western Lake
region. The Animikian basin, in which the Rove, Virginia, Thomson, and
Michigamme Formations were deposited, is interpreted as a foreland basin formed during
10 the Penokean orogeny (Southwick and Morey, 1991; Ojakangas, 1994). Loading of the crust by thrusts contributed to the development of the basin. Units of quartz sandstone, including the Baraboo and Sioux Quartzites, possibly fluvial to marine in origin but since metamorphosed to quartzite, were deposited during the time interval of 1950 to 1650 Ma
(Chandler and Morey, 1992).
Middle Proterozoic
The Sibley Group was deposited in a possible aulacogen around 1430 Ma, and includes a basal quartzite and dolomite (Ojakangas and Morey, 1982b). Thin quartzite units were deposited around Lake Superior at about 1100 Ma, just prior to Keweenawan rifting (Ojakangas and Morey, 1982b).
Keweenawan Supergroup
The Keweenawan is defined by the time period associated with extensional tectonism that initiated rifting in the vicinity of present-day Lake Superior. This rifting split the older Archean and Early Proterozoic southern Canadian Shield, with a maximum extension of 80 km (Morey and Green, 1982).
The Midcontinent Rift System involves immense volumes of both volcanic and
(generally younger) sedimentary strata. Radiometric ages for the igneous rocks range from 1108 Ma to 1086 Ma (Davis and Green, 1997). Whereas the earliest flows are pillowed, the rest of the sequence is composed dominantly of subaerial basalts (Green,
1982). Several large rhyolitic flows are also present. The plateau basalt province covers an area of approximately 100,000 km2 and has a volume of well over 400,000 km3 of intrusive rocks and subaerial lavas (Green, 1982). Exposed north and west of the North
11 Shore Volcanic Group in Minnesota is the mafic Duluth Complex. The volcanics on the south shore of Lake Superior comprise the Chengwatana Volcanic Group which is exposed along the St. Croix River in Minnesota and Wisconsin (Wirth and others, 1997), the
Powder Mill Group, and the Portage Lake Volcanics are also found on the south shore
(Morey and Van Schmus, 1988). The Chengwatana Volcanics have been dated using U-
Th-Pb at 1094.6 ±2.1 Ma, the Portage Lake Volcanics at 1094.6 ±2.1 and 1096 ±1.8 Ma, and the Powder Mill Group at 1107 ±1.6 Ma (Zartman and others, 1997).
Sedimentary sequences are found on top of the volcanic groups. The first unit is the Copper Harbor Conglomerate of the Oronto Group. The lower portion of the
Copper Harbor Conglomerate is interfingered with the youngest lava flows. The Copper
Harbor Conglomerate has been dated at 1087.2 ±1.6 Ma (Davis and Paces, 1990) from dates obtained from the interbedded basalt. The Oronto Group is 6100 meters thick
(maximum thickness on the south shore), consisting primarily of red, immature sandstones and shale. The Oronto Group was deposited in a primarily fluvial environment
(Daniels, 1982; Ojakangas and Morey, 1982a) with the Nonesuch Shale a lacustrine environment (Suszek, 1992).
The Bayfield Group is found above the Oronto Group and constitutes the youngest sedimentary sequence of the Keweenawan. This group is the subject of this thesis. The first unit is the Orienta Formation, which is composed of feldspatholithic arenite and shale interbeds. The second unit is the Devils Island Sandstone, which is a quartz arenite. The Chequamegon Formation is a feldspatholithic arenite with shale interbeds and is the uppermost unit of the Bayfield Group. The upper contact of the
Freda Sandstone of the Oronto Group with the overlying Bayfield Group is not exposed and is thought to be an unconformable contact (Thwaites, 1912; Hite, 1968; Myers,
197lb; Daniels, 1982). The presence of reworked, unlithified Oronto sediments in the
12 Bayfield Group implies that sedimentation of the Bayfield and Oronto Groups were separated by a short hiatus (Myers, 197lb; Morey and Ojakangas, 1982).
Grenville Province
The collision of the Grenville Province to the east of the Midcontinent Rift
System created a compressional regime which caused reverse faulting in the Midcontinent
Rift System (Cannon, 1994). This was about the time that thermal subsidence had declined in the Midcontinent Rift System and late stages of sedimentation, the Bayfield
Group, were occurring. Compression caused up to 30 km of crustal shortening. Rifting during the Keweenawan had created a zone of weak lithosphere, and deformation was focused in this previously extensional regime of the Midcontinent Rift System. The thrusting and northward rotation of the upper (southern) plate has resulted in the exposure of steeply dipping strata about 20 km thick, including up to 10 km of underlying Archean basement rocks, on the south side of Lake Superior. This structure, called the Montreal
River monocline exposes, 75% of the crust as found just after rifting (Cannon and others,
1993). The Freda Sandstone (Oronto Group) is the youngest unit exposed in the monocline. The Marenisco fault has a Rb-Sr age around 1060 Ma (Cannon and others,
1993), which means that the Freda Sandstone (Oronto Group) has an older age of deposition. Deposition of the Bayfield Group terminated around 900 Ma, as was determined from paleomagnetic studies of the Chequamegon Formation (McCabe and
Van der Voo, 1983). The structural shortening caused the formation of a central horst in the Midcontinental Rift System which is called the St. Croix horst in Minnesota and northwestern Wisconsin (Craddock and others, 1963). The Douglas fault is the northern boundary of the horst.
13 Paleozoic and Mesozoic
The Mount Simon Sandstone is Cambrian in age and is found in discontinuous outliers in Wisconsin about 80 km south of the Bayfield Peninsula. In east-central
Minnesota it is found above the Hinckley Sandstone. Cambrian sandstones are found as far north as Taylor Falls, Minnesota, where fossiliferous beach deposits are present. This may be the approximate northern extent of transgressive epicontinental seas that covered much of southern Minnesota during Cambrian and Ordovician time and that retreated for the last time in the Cretaceous. The only evidence for Mesozoic deposition in northeastern Minnesota is Cretaceous outliers in the western and central parts of the
Mesabi Iron Range. There are no evidence of fossils within the Bayfield Group, which discounts the possibility of a Paleozoic age for the Bayfield Group.
Quaternary
The topography of today is largely the product of glacial and glaciofluvial processes. Multiple glaciations removed many overlying strata in the Lake Superior area, specifically from the Lake Superior basin, which is defined by a structural syncline. Glacial
Lake Duluth, a predecessor to Lake Superior, had a shoreline which coincided with the
Douglas fault scarp at Pattison Park, Wisconsin (Black River and Copper Creek outcrop locations of Figure 30). There is thin Quaternary sediment on the upthrown block (the St.
Croix Horst). The downthrown blocks are presently covered by thick glacial lake clay and drift. Outlets of Glacial Lake Duluth, such as the Brule River, cut through the Quaternary drift and exposed the Precambrian bedrock along sections of the present-day river channels.
14 Regional Structure
The early history of the rift was one of extensional tectonic processes. The Oronto
Group and Solar Church Formation appear to occur predominantly in basins along the axis of the rift system, so extension appears to have continued for a short time after cessation of volcanism (Morey and Ojakangas, 1982). The presence of both the Oronto
Group and Solar Church Formation in basins that flank the St. Croix horst indicates that the horst did not exist in Oronto Group time (Morey and Ojakangas, 1982; Cannon and others, 1989). This shows a transition from extensional to compressional tectonic processes (Morey and Ojakangas, 1982).
The study area of this project is northwest of the Montreal River monocline, which includes the tilted Oronto Group. The Bayfield Group as a whole does not seem to be included in the monocline and overlies the tilted rocks of the Oronto Group. The upthrown St. Croix Horst contains evidence of Bayfield Group sedimentary rocks as the uppermost layer; this indicates the faulting occurred after or near the termination of
Bayfield Group sedimentation and its Minnesota equivalents, the Fond du Lac Formation and Hinckley Sandstone. It appears that subsidence of the basin or uplift of the horst more or less kept pace with infilling and that very coarse detritus was introduced only periodically into the western flanking basin (Bayfield Basin) (Morey and Ojakangas,
1982). It is possible that a small amount of Freda Sandstone is exposed at the steeply dipping Middle River beds (Figure 20); this inference is based on the higher frequency of shale interbeds and a different heavy mineral suite (Tyler and others, 1940 p. 1470).
Myers (1971b, p. 82) was supportive of the idea that this exposure was indeed Freda
Sandstone and that the Orienta Formation comprised the sub-horizontal outcrops less than a kilometer (1/2 mile) to the north. This is discussed further under the heading of
Key Areas in the Bayfield Group.
15 The wedge-like geometry of the Bayfield Group in Wisconsin and its Minnesota equivalents implies that deposition occurred in a series of half-graben-like basins bounded on the south by the St. Croix horst and on the northwest by pre-Keweenawan rocks
(Morey and Ojakangas, 1982). The majority of movement along the Douglas fault occurred after deposition of the Bayfield Group sediments (Myers, 197lb, p. 187). The
Douglas fault, which bounds the northwestern side of the horst, affected the Orienta
Formation at four exposed locations.
South Shore Geophysical Data
Seismic reflection data were collected in the mid-l 980s for oil and gas exploration in the elastic sequence of the Midcontinent Rift System. The Oronto and Bayfield
Groups are represented as reflection-poor to reflection-transparent zones on land surveys
(Dickas, 1986b) using a vibration source. An airgun survey on Lake Superior had better results in determining the elastic sequence (Dickas, 1986b). The Oronto Group can be distinguished from the overlying Bayfield Group by an apparent angular unconformity
(Dickas, l 986b). The Oronto Group is folded while the Bayfield Group is structurally simple (Dickas, 1986b; Mudrey and others, 1990), see Figure 5.
The Bayfield Group has a lower seismic velocity than the Oronto Group sediments
(Cannon and others, 1989). In a cross section from the north shore to the south shore based on GLIMPCE data (Great Lakes International Multidisciplinary Program on
Crustal Evolution), it can be seen that the Bayfield Group sediments are draped over the upthrown St. Croix horst (Figures 5 and 6).
Allen (1995) used seismic reflection, gravity and magnetic anomalies, seismic refraction, rock physical properties, and geologic data to determine that there is substantial structural heterogeneity of the Midcontinent Rift System in northwestern Wisconsin. The
16 changes in the structural style of the rift are associated with older Precambrian geologic features (Allen, 1995). The lower portion of the Oronto Group pinches out onto White's ridge and the Grand Marais ridge, as shown on Figure 7. The ridges influenced the evolution of the rift during the late-stage compressional stage. The Emerald basin east of the St. Croix horst in Wisconsin contains up to 6 km of sedimentary strata, mainly belonging to the Bayfield Group, as indicated by an intense negative gravity anomaly
(Allen, 1995). The individual units within the Bayfield Group are not revealed by these geophysical studies.
17 Keweenaw N Bg
I - "i':!-3
- _...... A I 8 1ec'a
1-v., - J I v, I I 0 a l I 811 I Ba1ic Vo lean ica Fel1ic Volconlca Oronto Group Bayfield Group
N s
...... 00
? vb ? B ? 1ec'1 Lake Superior seismology court Hy Grant Norpac, 1985
Figure 5. Schematic geologic interpretation of seismic profiles across axis of Midcontinent Rift System, offshore Lake Superior (line Con Figure 4) . (From Dickas, 1986b). AnG a Shot SHORELINE KEWEENAWAN DULUTH COMPLEXSHORELINE 1000 1500 Points AG GAB BRO B NSV OG PLV JS --- I ------? - ? -? _...... / 4 ,,. .... / ... / 5 "-/ 'l"/ .. i'.:'.
10 to
t5 15 .·:.
Shot 500 1000 1500 Points I!
,__. \0 5 5
......
to 10
t5 15 .. O tO 20km LINE C, MIGRATED I F3 f?+3
Figure 6. [Top] Interpreted reflection profile along line C (shown on Figure 4) showing subsurface structure beneath western Lake Superior. Inferred subsurface units are projected updip to their exposed extensions on land in northern Michigan and Minnesota. [Bottom] Migrated seismic reflection profile along line C. M, approximate location of Moho; AG, Archean Gneiss; PLV, Portage Lake Volcanics; pPLV, pre- Portage Lake Volcanics; NSV, North Shore Volcanics Group; OG, Oronto Group; BS , Bayfield Group; AnG, Animikie Group. Vertical scale is seconds of two-way travel time. Vertical exaggeration is 1: 1 for average velocity of 6 km/s. (From Cannon and others, 1989). 94W 92W 90W
48 N + Duluth+
MINNESOTA
46N + Complex
WISCONSIN
1:-:-.-:· I Bayfield Group lSSSSJ Oronro Group Basin Rocks b''"'''" Plutonic Rocks
44 N + +
.. AF
0 50 100 150 200 250 km
Figure 7. Geologic map of the Midcontinent Rift System, E-Central MN to central Lake Superior. Bold hachured lines within White's ridge and the Grand Marais ridge encompass areas where rift volcanic rocks are absent. Reverse faults: AF-Austin fault, CGF-Cottage Grove fault, CRF-Castle Rock fault, DF-Douglas fault, HF-Hastings fault, !RF-Isle Royale fault, KF-Keweenaw fault, LOF-Lake Owen fault, NF-Northfield fault, PF-Pine fault. Cross Faults: BPF-Belle Plaine fault, CF-Chisago fault, EF-Empire fault, LMF-Lake Minnetonka fault. Folds: AS-Ashland syncline, IRA-Isle Royale anticline, LSS-Lake Superior syncline, MA-Montgomery anticline, RS-Rice syncline, TCB-Twin Cities basin. Other structural features: FTMD-Finland Tectono-Magmatic discontinuity normal fault, SCH-St. Croix horst, SFCCR-Schroeder-Forest Center crustal ridge. (From Allen, 1995).
20 Review of Upper Keweenawan Paleo magnetic Analysis
There are three segments on the Precambrian Apparent Polar Wander path in which the data to correlate ages with paleomagnetic data are nonexistent or extremely poor. These occur as gaps which are not represented as gaps on most APW paths. The first discontinuity is from 2500 to 2300 Ma, the second is from 1650 to 1450 Ma, and the third is from 950 to 750 Ma (Irving, 1979). In most cases, the only units which are suitable for paleomagnetic records are sediments, which in the Precambrian do not have as precise an age determination as igneous rock units. The paleomagnetic data obtained from sediments also plot as a more diffuse cluster than those from an intrusive rock. The well- known Baraboo interval occurs during the second discontinuity and the Oronto and
Bayfield Groups were probably deposited during the third discontinuity. These Upper
Keweenawan units are correlated on the basis of lithostratigraphy, which does not constrain the time interval very well.
The majority of the data points for the Middle Proterozoic are from Keweenawan rocks, which define the Logan Loop, and the Grenville rocks represented by the Grenville
Loop and Grenville Track. The early Hadrynian period (1500 Ma to 1400 Ma) is represented by few data points in comparison to the Grenville Track and Logan Loop.
Figure 8 is the APW path for the North American continent during the time interval from
1600 to 800 Ma, and includes paleopole locations for Keweenawan sedimentary units, but does not have the data points for the Logan loop, Grenville loop, and Grenville track listed for ease of reading. The primary NRM (Natural Remanent Magnetization) pole for the
Chequamegon Formation lies at 12.3° S, 177.7° E (McCabe and Van der Yoo, 1983).
This conforms with other late Keweenawan pole locations on the APW paths.
21 ES•
[J]EARLY EOHELIKIAN LATE pALEO HELIKIAN
Figure 8. Suggested APW path from 1500 to 850 Ma which includes Early Hadrynian (1000 to 800 Ma), Neohelikian (1000 to 1400 Ma), and Late Paleohelikian (1400 to 1600 Ma). (Modified from Irving, 1979). ES - Eileen Sandstone (Freda Ss.) CPP - Copper Harbor Conglomerate FNl - Freda-Nonesuch Fm. combined NN - Nonesuch Formation FN2 - Freda-Nonesuch overprint RL - Isle Royale lower flows FS - Freda Sandstone RU - Isle Royal upper flows Jl - Jacobsville Formation, MI PL - Portage Lake lavas ]2 - Jacobsville Formation, ON BY - Beaver Bay Complex JlAB - Jacobsville-MI KE - Keweenawan intrusion-MN JlC - Jacobsville-ON NVR - North Shore lavas reversed-ON JlSA - Jacobsville-MI SIB - Sibley Group redbeds-ON c - Chequamegon Formation
22 The Hadrynian track poles are a representation of the secondary NRM of the
Chequamegon Sandstone, which is not dateable but represents a time interval for which there are few samples. It is possible that the secondary magnetization is a representation of the present-day magnetic field, which also falls into the third discontinuity, 950 to 750
Ma (Figure 8). McCabe and Van der Voo (1983) concluded that the secondary magnetization is recent rather than part of the Hadrynian track.
Age of the Bayfield Group - Conclusions
The Chequamegon Sandstone used to solve the problem of the vague APW path from the Grenville track to the Cambrian. The Chequamegon paleopole is located closer to the Grenville track and other Keweenawan paleopoles such as the Freda Sandstone
(Henry, Mauk and Van der Voo, 1977) and the Fond du Lac Formation (Watts, 1981) than it is to Cambrian paleopoles. There are two NRM's recorded in the Chequamegon.
This was determined from the hematitic carrier with the secondary NRM paleopole located in the Hadrynian track portion of the APW path. The Hadrynian track may not actually exist, since this could be a representation of the present-day magnetic field in the sandstone. It is possible that the Chequamegon Sandstone has acquired the present day dipole field as a NRM. The Chequamegon Formation was assigned an age of 800 Ma by
DuBois (1962), but the precision on this estimate is not known (Roy, 1983). According to
McCabe and Van der Voo (1983), the age which was assigned by DuBois (1962) for the
Chequamegon Formation is in question due to overlapping remanent magnetizations but would be around 970 Ma based on its paleopole location with respect to the APW path as described by Irving (1979).
23 PETROLOGY - FIELD DESCRIPTIONS
The Bayfield Group was originally estimated to be 4,300 feet thick by Thwaites
(1912, p. 26). This was based on the outcrop belt width and projected dip, and includes the vertical beds at the Middle River location (Myers, 197lb, p. 14). Outcrops rarely contain sequences over 10 meters in thickness, with none over 30 meters thick.
Orienta Formation
The Orienta Formation comprises the basal unit of the Bayfield Group. It is found in the western part of the Bayfield Peninsula, north of the Douglas fault, and is thought to be contemporaneous with the Fond du Lac Formation in eastern Minnesota. It was originally named and described by Thwaites (1912) for exposures near Orienta,
Wisconsin. The Orienta Formation is a feldspatholithic arenite which ranges in color from reddish brown to brown. Some white sandstone beds are found within the Orienta
Formation. These may represent local reduction zones. There is some liesegang banding found which suggests these local reduction zones. The formation is mainly a coarse- to medium-grained sandstone with minor shale, siltstone, and conglomeratic interbeds. The shale interbeds are commonly 10 to 50 cm in thickness and do not appear to be very extensive laterally.
Sedimentary structures found within the Orienta Formation are primarily large- scale trough cross-bedding. Parting lineation and current ripple marks are less common
(i.e., Plates IA, lB, and 2A). The Orienta Formation has no exposed basal contact with the underlying Freda Sandstone of the Oronto Group. The exposure of steeply-dipping beds on Middle River was thought to be a potential location of this contact, but there is still some question as to which group the sandstones from this area belong. Myers (197lb,
24 p. 18) suggested that the steeply-dipping beds belong to the Oronto Group and the horizontal beds belong to the Bayfield Group. In this study it was found that these steeply-dipping beds are atypical of the Bayfield Group and are interpreted as an exposure of the Freda Sandstone. Plate 2B shows the steeply-dipping beds near the Douglas fault contact at this location. This location is discussed in further detail below in the section dealing with the Orienta Formation at Middle River (Figure 20). The upper contact of the Orienta Formation is present on Sand Island; here it grades into the basal beds of the
Devils Island Sandstone. This is further discussed below in the section on Northern Sand
Island.
25 A. Large cross-bed sets within the Orienta Formation at Amnicon Falls, location 4.
B. Ripple marks in the Orienta Formation at Copper Creek, location 17. Note 6-inch plastic ruler on lower right for scale.
Plate 1. 26 Plate 1. A. Trough cross-bedding in the Orienta Formation at Copper Creek, location 14. Note two-meter stick for scale.
B. Steeply dipping beds of Freda Sandstone on Middle River, location 33. Note two-meter stick for scale.
Plate 2. 27 Plate 2. Devils Island Sandstone
The Devils Island Sandstone is the middle unit of the Bayfield Group. It is found
in a narrow belt (Figure 9) which separates the other two formations of the Bayfield
Group. The Devils Island Sandstone was named by Thwaites (1912) for exposures of
thin-bedded quartzose sandstone on Devils Island, Wisconsin. Thwaites (1912, p. 39)
estimated the Devils Island Sandstone to have a total thickness of 300 feet.
The Devils Island Sandstone is poorly exposed and shoreline outcrops on the
Bayfield Peninsula contain not over 10 meters of section, with most exposures ranging
from 1 to 2 meters in thickness. Two inland outcrops, one on the Brule River (locations
45-58) and a second on the East Fork of the Iron River (location 101) were examined
(Appendix B). The East Fork of the Iron River outcrop was found by Myers (1971 b, p.
19-20) and it further defines the Devils Island Sandstone outcrop belt outlined by
Thwaites (1912).
The Devils Island Sandstone is a fine- to medium-grained quartz arenite with some
laminae of coarser and finer grain size. It is a generally uniform orange-white to white unit
with the exception being the laminae in the ripple-marked beds found on Devils Island.
Ripple marks are found in every outcrop of the Devils Island Sandstone. Mudcracks are
also evident in several outcrops and are sometimes interbedded with ripple marks (Plate
3). The ripple marks are predominantly oscillatory, with a 2 to 5 cm wavelength and 0.5
to 1.5 cm height (trough to crest). Trough cross-bedding is not as common in the Devils
Island Sandstone as in the other two formations of the Bayfield Group. The coarser- grained beds typically exhibit cross-bedding (i.e., Plate 4). One of the best exposures along the north end of Devils Island is known to tourists as the seacave exposures (Plate 5).
28 The basal contact is exposed on Sand Island and is discussed below in further detail in the section on Northern Sand Island (Figure 25). The upper contact of the Devils
Island Sandstone was described as being gradational and is exposed on Devils Island
(Myers, 197lb, p. 20-21); this contact is further discussed below in the section dealing with South Landing, Devils Island.
29 A. Ripple marks and mudcracks in the Devils Island Sandstone, Devils Island, location 11 OM2. Note 50 cm stick for scale.
B. Mudcracks in the Devils Island Sandstone, Devils Island, location l lOP. Note 50 cm stick for scale.
Plate 3. 30 Plate 3. Cross-bedding in Devils Island Sandstone, Devils Island, location 11 OAl.
Plate 4. 31 Plate 4. Seacaves of Devils Island Sandstone, Devils Island, location 11 OS.
Plate 5. 32 Plate 5. Chequamegon Formation
The Chequamegon Formation is the uppermost unit of the Bayfield Group. It is found in the Apostle Islands and the eastern part of the Bayfield Peninsula. Most of the inland Chequamegon Formation on the Bayfield Peninsula is covered by Pleistocene glacial cover. The exceptions are two exposures: an exposure less than one-meter-thick on the East Fork of the Iron River (location 107) and the other a 30-meter-thick section at
Big Rock Park (location 108) (Appendix B). The Chequamegon Formation was named by Thwaites (1912) for quarry exposures on Houghton Point near Washburn, Wisconsin.
The Chequamegon Formation is mainly a medium- to coarse-grained feldspatholithic arenite. It is typically brown to brownish red with some deep red beds.
Conglomeratic beds occur throughout the formation as do lesser shale beds. Large-scale trough cross-bedding is predominant and a few exposures of channel filling bodies are found which include this cross-bedding (Plate 6A). Large lensoid bodies probably deposited in channels are present on many of the Islands (plate GB). Brownstone beds, such as in the brownstone quarries, are better stratified and consist of fine- to medium- grained sandstone with a few shale beds.
The basal portion of the Chequamegon Formation was previously interpreted to be intercalated with Devils Island Sandstone beds on the southern tip of Devils Island
(Myers, 197lb p. 24). The upper contact is an erosional contact with Pleistocene glacial units. The Chequamegon is found in close proximity to the Oronto Group sediments and possibly rests against these as a result of displacement on the Douglas fault. The nearest exposure (location 91) of the Chequamegon Formation is 6.5 km (4 miles) north of the
Freda Sandstone outcrop on Fish Creek (Myers, 1971 b, p. 25) (Appendix B).
33 A. Channel in the Chequamegon Formation, Hermit Island.
B. Large lensoid massive bodies (channels?) on North Twin Island.
l
I
Plate 6. 34 I I Plate 6. Fond du. Lac Formation
The Orienta Formation is equivalent to the Fond du Lac Formation of eastern
Minnesota. The Fond du Lac was critically examined by Morey (1967). It appears to be essentially the same in outcrop as the western exposures of the Orienta. The Fond du Lac is characterized by lenticular beds of sandstone and siltstone which are intercalated with locally thick sequences of mudstones or shale (Morey, 1967, 1972; Morey and Ojakangas,
1982). The QFL index of the Fond du Lac Formation is 77 percent quartz, 15 percent feldspar and 8 percent lithics (Morey, 1967, 1972; Morey and Ojakangas, 1982). The
Fond du Lac contains many fining-upward sequences which indicate meandering fluvial sedimentation (Morey, 1967; Morey and Ojakangas, 1982). The Fond du Lac is overlain by the Hinckley Sandstone into which it appears to grade over a stratigraphic interval of at least several tens of meters in locations where both units are present (Morey and
Ojakangas, 1982). The Hinckley Sandstone is correlative with the Devils Island
Sandstone.
Hinckley Sandstone
The Hinckley Sandstone contains medium-scale trough cross-bedding and a few asymmetric ripple marks (Morey and Ojakangas, 1982). The beds range in thickness from a few centimeters to a meter (Tryhorn and Ojakakagas, 1972). It is characterized by a buff to white color with yellow and red staining present locally (Tryhorn and Ojakangas,
1972). Minor arkosic and conglomeratic facies are present locally near the base of the unit
(Tryhorn and Ojakangas, 1972). Morey and Ojakangas (1982, p. 143) describe the
Hinckley Sandstone as follows:
35 Except for basal beds on top of the St. Croix horst, the Hinckley Sandstone consists dominantly of thick to very thick, vaguely cross-stratified beds of fine- to coarse-grained sandstone. A few stratigraphic intervals contain scattered, granule- or small pebble-size grains that in places define thin laminae. Regardless of size, individual grains are moderately well-rounded, but sorting varies from poor to moderate. A majority of beds have less than 5 percent clayey matrix. However, a few thin beds of fine-grained sandstone contain as much as 10 percent clay that is concentrated in small pods and laminae.
The Hinckley Sandstone has an average framework grain composition of 96 percent quartz, 2 percent feldspar, and 2 percent lithics (Tryhorn and Ojakangas, 1972).
36 Chequamegon Fm. [§] Devils Island Ss.
N OrientaFm. Devil:_! sland North T wm· Island
/ - , ' "' Figure 9. Strikes and dips of Bayfield Group strata. Additional data from Mudrey and Brown (1988). MINERALOGY AND PETROLOGY Introduction All three Bayfield Group formations are classified as arenites. The units range from feldspatholithic to quartzose with the Orienta being lithofeldspathic in some areas. About 600 points per thin section were counted using a Swift Model F mechanical counting stage. An additional 21 thin sections were prepared from well cuttings and point-counted (Appendix B). The thin sections of well cuttings are discussed below under Geologic Well Logs. Sample Preparation Thin-section chips were prepared using a water-cooled diamond saw to minimize contamination of pore spaces (versus using an ethylene glycol-cooled diamond saw) prior to having these chips impregnated. The chips were cut with the long direction normal to bedding in most cases. The thin sections were made commercially. All thin sections were stained for potassium feldspar to simplify identification of feldspars; rhyolitic rock fragments also became stained due to their high groundmass composition of potassium feldspar and this complicated differentiation between feldspar and rhyolite rock fragments. The thin sections were impregnated with blue epoxy to enhance pore spaces; they appear blue in plane polarized light. Thin sections from the well cuttings were prepared by impregnation of the loose sample material with standard epoxy. These thin sections were prepared by the author and were not stained with sodium cobaltinitrite as were the commercially prepared thin sections. 38 Operational Definitions Point-counting identified a number of constituents which are described so that the reader will be able to follow the information that is presented in Tables 3 through 5. Quartz Common Single crystal to slightly polycrystalline (up to four or five small subcrystals); few inclusions; sharp to undulose extinction. Includes single crystals with undulose extinction and vein quartz (commonly with planes of bubbles and a "d us k y II appearance ) . Polycrystalline-recrystallized Polygonal subcrystal boundaries; equidimensional fabric of individual subgrains. Polycrystalline-stretched Sutured crystals; elongate fabric of individual subgrains with sutured boundaries. Recycled Common quartz grains with well-rounded primary quartz overgrowths beneath secondary overgrowths. 39 Volcanic Single crystal with sharp extinction; embayments containing fine-grained volcanic groundmass. Feldspars Microcline Equidimensional crystals;. cross-hatched twinning; rounded grains common; both altered and unaltered. Orthoclase Subhedral crystals; uniform extinction; low birefringence; biaxial indicatrix; both unaltered and altered. Perthite Cloudy orthoclase crystals with numerous small albite inclusions; commonly altered. Plagioclase Subhedral crystals with Albite and Carlsbad twinning; commonly altered. Undifferentiated K-feldspar K-feldspar grains so stained with sodium cobaltinitrite that they are nearly opaque; optical properties which would differentiate between the different types of K-feldspar are masked. 40 Lithics Felsic volcanic Well-rounded to subrounded grains; subrounded to subangular grains less common; dominantly potassium feldspar groundmass; some are porphyritic with quartz phenocrysts. Felsic hypabyssal Fragments containing equidimensional quartz and feldspar grains; larger grain sizes than in volcanics but smaller than in felsic plutonic rock fragments. Mafic hypabyssal medium-grained mafic rock containing pyroxene, olivine or plagioclase crystals that are commonly highly altered. Mafic volcanic Porphyritic texture common with plagioclase phenocrysts; replacement of groundmass with clay common; subhedral crystals of plagioclase replaced with clay; dark brown groundmass common. Felsic plutonic, P-Q Large grains consisting of large single units of quartz and smaller amounts of plagioclase feldspar. 41 Felsic plutonic, K-Q Large grains consisting of large single units of quartz and smaller amounts of potassium feldspar. Mafic plutonic Coarse-grained mafic containing pyroxene or olivine crystals, commonly altered to clay. Metamorphic, argillite-slate Very fine-grained groundmass with aligned grains and small . . mica grams. Metamorphic, schist Fine- to medium-grained quartz and mica with an elongated fabric. Metamorphic, quartzite Polysutured quartz grains with evidence of being a quartzite (i.e. deformed silica cement between deformed subcrystals); multi-grained polysutured quartz. Chert Microcrystalline quartz texture; cloudy appearance. Sedimentary, mudstone-siltstone Quartz grains with clay matrix, tabular grains. 42 Sedimentary, sandstone Fragments containing quartz grains with silica cement between grains; not deformed or metamorphosed. Sedimentary, carbonate Fragments of carbonate, either calcite or dolomite. Other Rock Fragments Partially to nearly completely dissolved grains; original composition impossible to determine. Miscellaneous Grains Mica Individual biotite and muscovite grains, tabular form. Zircon Small rounded to angular grains; high birefringence and high index of refraction. Tourmaline Rounded to subrounded; pleochroic green to brown; moderate index of refraction. Opaques Magnetite and/or ilmenite with gray-metallic luster in reflected light; hematite with a characteristic reddish color in reflected light. Limonite is yellow-brown in reflected light. Leucoxene is white in reflected light. 43 Cement/Matrix Silica Continuous or semicontinuous rims of silica overgrowths on quartz grains, creating angular appearance of crystals. Extinction with grains on which cementation occurs (syntaxial). Iron-Oxide Reddish color in reflected light (hematite), opaque. It is commonly found coating quartz grains and their silica overgrowths. Clay Low-birefringence kaolinite books, stringers or patches. Higher-birefringence illite. Zeolite Optically-continuous, clear crystals with a radial crystal habit. Not common but occurs as patches in several samples. 44 General Petrographic Characteristics and Descriptions Orienta Formation The average framework grain composition of the Orienta Formation found in this study from 18 samples using QmFLt is 66% quartz (monocrystalline), 17% feldspar grains and 17% lithics. A QmFLt plot includes monocrystalline quartz on the Qm pole, feldspar on the F pole, and total lithics which includes polycrystalline quartz and chert on the Lt pole. The average amount of pore space is 12.6%; porosity ranges from 4.2% to 25.3%. Typical grain sizes range from fine to coarse. The Orienta Formation typically consists of over 50% common quartz which is subrounded to well-rounded (Plate 7). Feldspar grains are typically rounded to well- rounded (Plate 7) . The most common rock fragment is rhyolite exhibiting a poikilitic 11 snowflake 11 texture (Plate 8). Clasts found in conglomerate laminae are typically vein quartz. These conglomeratic beds vary in thickness; they are typically not more than one to several clasts thick and are sporadic in geographical distribution. Small amounts of basalt and felsic clasts are also found in the conglomerates. The Orienta Formation has no specific trend in the QmFL amounts with respect to geographic location, which is also true for the conglomeratic beds. Typically, a higher percentage of feldspar in a sample is associated with a higher abundance of rock fragments. Iron-oxide is the most abundant cement, with silica being the next-most abundant. Laminations with abundant hematite are commonly present. 45 Photomicrograph of rounded quartz grain (center) with silica cement. Rounded microcline grain is in upper left. Snowflake-textured rhyolite fragments are in lower right and lower left. Upper photo is in crossed-polarized light. Lower photo is in plane-polarized light. The field of view is 2.5 mm across. Orienta Formation, sample 16. Plate 7. 46 Plate 7. Photomicrograph of a large rhyolite fragment on right. Opaque cement (iron-oxide) can be seen on left. Upper photo is in crossed-polarized light. Lower photo is in plane-polarized light. The field of view is 2.5 mm across. Orienta Formation, sample I 04A. Plate 8. 47 Plate 8. Devils Island Sandstone The Devils Island Sandstone is nearly a pure quartz arenite. It has an average porosity of 14.1 % with a range in porosity of 2.3% to 22.5%. The average framework grain composition from 41 samples in this study using QmFLt is 88.8% quartz 3.8% feldspar and 7.5% lithics. Monocrystalline quartz is the most prevalent quartz type (plate 9), poly-recrystallized quartz accounts for most of the remaining percentage of quartz present. Laminae of coarser and finer grain distributions contain a higher percentage of feldspar. The finer laminae tend to contain the feldspar. Quartz grains are more subangular than subrounded in the finer grained fraction (0.062 to 0.088 mm) than in the coarser grained fraction (0.250 to 0.500 mm) that has well-rounded quartz grains and an abundance of quartz overgrowths (plate 10). The laminae sands are dominant in the thinly bedded ripple-marked outcrops that typically form sea caves on the lakeshore. This is visible on Devils Island, Sand Island and Squaw Bay. These laminated sands were identified as bimodal sands in a previous study by Myers (1971 b, p. 78-81). Higher percentages of iron-oxide cement are found both above and below the finer grained sequences, which further defines the rippled zones in outcrop. The crests of ripples are commonly finer-grained than the troughs. Felsic volcanic fragments are rare throughout the Devils Island Sandstone. A few vesicular basalt fragments are found in a location at the south end of Devils Island (location l lOD, Figure 23). Plate 11 shows an example of a recycled quartz grain. Silica and hematite cement are present in nearly equal amounts, with kaolinite also present as a cement and matrix. Some feldspar grains are partially dissolved and appear as skeletal crystals. 48 Photomicrograph of typical Devils Island Sandstone. Note rounded to angular common quartz grains along with silica cement. Upper photo is in crossed-polarized light. Lower photo is in plane-polarized light. The field of view is 2.5 mm across. Devils Island Sandstone, sample 46. Plate 9. 49 Plate 9. Photomicrograph of typical Devils Island Sandstone laminae sands. Note rounded common quartz grains along with silica cement in the coarse fraction and subangular quartz grains in the fine fraction. Top of bedding is on the left side of photos. Upper photo is in crossed- I polarized light. Lower photo is in plane-polarized light. The field of view is 2.5 mm across. Devils Island Sandstone, sample 11 OB2. l Plate 10. 50 Plate 1O. Photomicrograph of a recycled quartz grain (left-center). Note high percentage of pore space present (blue in lower photo). Upper photo is in crossed-polarized light. Lower photo is in plane-polarized light. The field of view is 2.5 mm across. Devils Island Sandstone, sample 11OI1. Plate 11. 51 ....,.. , " . •.: .. •. , . ( ' .. • . • ..t Plate 11 . Chequamegon Formation The average framework grain composition from 17 samples in this study using QmFLt is 74.8% quartz, 13.7% feldspar and 11.5% lithics. The average amount of pore space is 16.2%; the range is 5.2% to 24.0%. Grain sizes range from fine to coarse. Outcrops and hand samples are typically a brown to reddish brown. Hand samples are the same color as in the Orienta Formation and are indistinguishable when compared. The thin sections are also similar to those from the Orienta Formation. Felsic volcanic rock fragments are the main type of rock fragments. The typical rhyolite 'snowflake' rock fragments as found in the Orienta Formation samples are also present (Plate 12). The outcrop at Little Sand Bay, which is relatively near the Devils Island Sandstone, contains a large percentage of feldspar and rock fragments including good examples of 'snowflake' texture (Plate 12). The grains are rounded to angular and include a substantial amount of feldspar and rock fragments (Plate 13). Hematitic cement is present in most samples with silica being found in lesser quantities. 52 Photomicrograph of grain assemblage which contains snowflake-textured rhyolite fragments (bottom center and top center). Upper photo is in crossed-polarized light. Lower photo is in plane-polarized light. The field of view is 2.5 mm across. Chequamegon Formation, sample 108. Plate 12. 53 Plate 12. Photomicrograph of grain assemblage showing K-feldspar grains which have been stained yellow. Note high porosity in the sample (blue in lower photo). Upper photo is in crossed- polarized light. Lower photo is in plane-polarized light. The field of view is 2.5 mm across. Chequamegon Formation, sample 120B. Plate 13. 54 Plate 13. Diagenesis of the Bayfield Group The Bayfield Group contains evidence of diagenetic processes including cementation, compaction, dissolution, and replacement. The diagenetic characteristics of 77 Bayfield Group thin sections were petrographically examined and the diagenetic mineralogy and paragenetic sequences were determined. Authigenic minerals found within the Bayfield Group include hematite, quartz, kaolinite, zeolite, and trace amounts of potassium feldspar. Hematite is the dominant authigenic mineral closely followed by quartz; kaolinite is found in much lesser quantities. Authigenic feldspar is not common even though many detrital feldspar grains, as well as rhyolitic lithics containing a feldspar-rich groundmass, have been dissolved, which would provide potential components for authigenic-feldspar-forming minerals. Plate 14 shows a skeletal rhyolite crystal in the Orienta Formation. A Chequamegon Formation specimen which is a fine-grained to coarse-grained (0.25 to 1.41 mm) sandstone with dissolution of feldspar grains is shown in plate 15. Authigenic quartz occurs as overgrowths. It is found in nearly all of the specimens examined. Its abundance ranges as high as 8% (sample 11 OB2) of the whole rock. The Devils Island Sandstone contains the highest amount of quartz overgrowths, the Chequamegon Formation contains fewer, and the Orienta Formation contains the least. Trends in the quartz overgrowths seem to parallel the decrease of feldspar grains through dissolution in many of the Devils Island Sandstone specimens. The higher the quartz average, the lower the feldspar average. There are more quartz overgrowths in samples with much dissolution. Iron-oxide in the form of hematite occurs in most samples, commonly in conjunction with quartz overgrowths. The abundance of iron-oxide ranges as high as 23.2% (sample 33A). Differing abundances of iron-oxide produce the hand specimen 55 color of orange-white for the Devils Island Sandstone and brown to reddish-brown for the Orienta and Chequamegon exposures. The most prevalent liesegang banding is found at Amnicon Falls (locations 1-14). The day mineralogy of the Bayfield Group was studied by Myers (I 971 b, p. 88- 106). This was to determine the general mineralogic composition of shale beds and the day mineralogy of the sandstone bed matrix, and to see if lateral variations in day mineralogy exist. Clays identified within the Bayfield Group were illite, kaolinite, chlorite, and mixed layer illite/chlorite (Myers, 197lb, p. 90). Illite was identified as the most abundant day present, with kaolinite also present in all of the specimens studied by Myers (1971b, p. 91). It was also found that kaolinite is much more abundant in the Bayfield Group than in the Freda Sandstone, providing a means by which to differentiate between the two (Myers, 197lb, p. 91). The amount of kaolinite present suggests that the Bayfield Group was deposited in a nearshore environment (Myers, 1971 b, p. 96). The day mineralogy from the Bayfield Group is summarized in Table I. Conclusions made by Myers (1971b, p. 105-106) on the day mineralogy are that no systematic lateral variation pattern could be determined, and that high illite values are present in the steeply-dipping beds on the Middle River, which contrasts with lesser quantities in the rest of the Bayfield Group. Chequamegon Formation: Illite, chlorite, mixed-layer illite/chlorite (some expandable), chlorite (trace) Devils Island Sandstone: Kaolinite, illite (trace) Orienta Formation: Illite, mixed-layer illite/ chlorite, kaolinite, chlorite Table 1. Summary of day mineralogy of the Bayfield Group. Clay minerals are listed in order of decreasing abundance. (From Myers, 1971b, p. 92). 56 Paragenetic Sequence The order of cementation is similar in all of the specimens examined. Stratigraphic and lateral variations are minor, with the major differences being the amount of quartz overgrowths and iron-oxide (hematitic) cementation present. In some specimens a total of six distinct stages in cementation history were determined (Table 2). The majority of the specimens display four stages starting with syntaxial silica overgrowths, then iron-oxide (hematitic) cementation, dissolution of feldspar and lithic grains, and finally the replacement and infilling of pore spaces with kaolinitic clay. Stage 4 and stage 5 appear to occur simultaneously since there are examples where a lithic grain contains clay infilling. The dissolution stage in some cases may occur during the silica overgrowth stage, as evidenced by a greater volume of pore space that seems to be secondary. 1) Compaction; evidenced by deformed muscovite or biotite grains. 2) Syntaxial silica overgrowths on quartz grains. 3) Iron-oxide cementation. 4) Dissolution of feldspar and lithic grains, commonly incomplete. 5) Clay infilling of pore spaces, mainly kaolinitic. 6) Zeolite infilling of pore spaces; minor amounts common. Table 2. Paragenetic sequence in Bayfield Group specimens. 57 Texture Four primary diagenetic textures were identified in samples of the Bayfield Group, that result from incomplete replacement processes. These are (I) irregular contacts between the detrital grains and cement; (2) the margin of a detrital grain being replaced by a replacement mineral; (3) ghost outlines resulting from impurities in the original replaced detrital grain; and (4) authigenic minerals containing inclusions of the original detrital grain. The irregular contacts occur where grain replacement is incomplete. It occurs mainly as kaolinite replacing potassium feldspar grains where a potassium feldspar core is present and the rim has been converted to clay. This also occurs with some of the rhyolitic rock fragments that contain high amounts of potassium feldspar in the groundmass. Ghost outlines of crystals occur in samples which contain high amounts of quartz overgrowths and are feldspar-deficient; partially dissolved feldspar grains are more prevalent in these specimens as well. Many of the specimens exhibit inclusions of the detrital grain that is being replaced. This occurs mainly as the replacement of rhyolitic rock fragments where the quartz groundmass remains while the potassium feldspar portion has either been replaced with kaolinite or totally dissolved (Plates 14 and 15). Fracture filling and fracture lining textures are found mainly in conjunction with hematitic cementation. 58 Porosity The amount of pore space generally decreases with depth. The Chequamegon Formation has an average of 16.2% porosity; porosity decreases to 14.1 % in the Devils Island Sandstone, and to 12.6% in the Orienta Formation. The variance on these percentages is 1.27 for the Chequamegon Formation, 0.81 for the Devils Island Sandstone, and 1.30 for the Orienta Formation (Appendix C-5). This trend is compatible with burial diagenesis where porosity decreases with depth of burial. There appear to be minor amounts of secondary porosity present in the specimens. In the Orienta Formation the lowermost portion of the unit has the least amount of porosity {samples found near the Douglas fault, samples 1, 7 and 33A). Also, specimens from exposures with parting lineation tend to have a smaller amount of porosity (sample l l 7Kl). The Devils Island Sandstone does not appear to have a definite trend of porosity with respect to location. Similarly, the Chequamegon Formation does not show a definite trend of porosity with respect to location. Even two samples from the same area in the Chequamegon Formation have amounts of porosity that vary by almost 10% {samples 107 and 107A). Quartz overgrowths are commonly the most significant porosity-reducing feature in the diagenesis of quartz arenites (Hutcheon, 1983). Hutcheon (1983) summarized that the most obvious feature of sandstone diagenesis is the modification and general reduction of porosity by compaction, pressure solution, and authigenic cementation by mineral reactions. The dominant type of porosity observed in major reservoir sandstones worldwide is secondary porosity (Shanmugam, 1985). Secondary porosity was difficult to differentiate from primary porosity in the Bayfield Group thin sections. Criteria for secondary porosity recognition were established by Shanmugam (1985). Extensive dissolution of framework grains may lead to the misinterpretation of the original sandstone composition (Shanmugam, 1985). 59 Petrographic criteria that were used to determine secondary porosity (Schmidt and McDonald, 1979) were I) partial dissolution, 2) molds, 3) inhomogeneity of packing, 4) oversized pores, 5) elongate pores, 6) corroded grain margins, 7) intraconstituent pores, and 8) fractured grains. The most easily recognized secondary porosity in the Bayfield Group is the partial or nearly complete dissolution of potassium feldspar grains. In most cases secondary porosity is not significantly reduced by later quartz cementation. The Devils Island Sandstone has a number of specimens {samples l IOHI, l IOII, l IOMI) in which feldspar is nearly non-existent or missing altogether, and there is a related increase in the volume of quartz overgrowths. 60 Photomicrograph of a skeletal grain, most likely a rhyolite grain which has had the feldspars dissolved away with some clay replacement (center). Upper photo is in crossed-polarized light. Lower photo is in plane-polarized light. The field of view is 2.5 mm across. Orienta Formation, sample 76. I Plate 14. 61 Plate 14. Photomicrograph of skeletal feldspar grain (top center). Quartz grains are subrounded and have silica overgrowths. Upper photo is in crossed-polarized light. Lower photo is in plane- polarized light. The field of view is 2.5 mm across. Chequamegon Formation, sample 89. Plate 15. 62 Plate 15 . Modal Analysis of Thin Sections The Bayfield Group shows an increase in textural and mineralogical maturity going upsection from the Orienta Formation into the Devils Island Sandstone and then decreases again going into the Chequamegon Formation. QmFLt indices were calculated utilizing the counts of only quartz, feldspar and lithic framework grains. QmFLt indices differ from QFL indices in that only monocrystalline quartz is plotted in the quartz field and polycrystalline quartz is included in the lithic field. Plutonic rock fragments are separated into 1/3 feldspars and 2/3 quartz and not plotted as lithics. The QmFLt classification shows differences in composition more clearly in Bayfield Group samples than does the QFL, as most samples are bunched together at the quartz pole on a QFL plot. The data are classified according to Pettijohn and others (1987) (Figure 1O). Figure 10. Classification of sandstones. (From Pettijohn and others, 1987). 63 Table 3 presents modal analyses of the Orienta Formation. Locations for all samples can be correlated using Appendix B; there is no stratigraphic or geographic significance in the numbering system. Totals for quartz, feldspar and lithics are also included for individual samples. Table 4 presents modal analyses of the Devils Island Sandstone and Table 5 shows modal analyses of the Chequamegon Formation. Appendices C-1 to C-3 are complete tables for each unit within the Bayfield Group, including raw point-count data along with the calculated QmFLt and QFL percentages of each sample. 64 Ouam: Fcldsoar Rock F ra2mcms Miscellaneous Ccmcm and Macrix u c ] 8 ..,,, u u c a ::: 0 c c ..,,, ..,,, 0 1 q - Table 3 - Orienta Formation, 18 modal analyses. Percentages are listed for each category. Counts based on an average of 600 points per thin section, counted normal to bedding. x = trace amount {less than 0.5% of 600 points). louarn Fddsp:u Rock Fr:aemcnts Miscellaneous Cement :ind M:urix ] = ll 'li il. 1l '' ¥ u :::i :::i c ... ] lll g_. 1! g-= . £ e t.i ,; ,; t.i 1 ] ·• a a a .s l -¥ .s .. ""iJ u.. > "-= 1"' ""ii. =1 ] ] ] 1 e. :.c :.c ·- "' c: c5 c5 i ..:: ::; 1 al i ! N o 31 31 46- 74 .8 2.0 76.8 l.O 1 ] 1.0 2.0 2.7 1.7 16.4 58- 73.3 x 5.5 79.2 1.2 3.21 4.310.5 0.7 1.2 2.3 3,3 0.7 1.2 8.8 101- 73.3 1.8 75.2 0.3 x 0.8 11.2 1.0 1.2 0.5 20.7 llOBI 64.2 1.9 66.0 4.6 4.6 0.5 0.5 x 15.8 2.2 10.5 3.6 66.4 2.2 1 IOB2 62.8 2.21 x 0.9 11.5 8.0 x 2.9 118.7 llOB3 I70 .8 4.5 75.3 3.5 3.5 0.8 0.8 3.0 4.0 1.8 11.4 llOCI 69.9 4.8 74.7 0.2 x 0.5 0.5 x 1.6 6.9 1.4 15.2 11001 55.7 2.2 57.9 0.5 15.0 15.5 1.5 1.8 18.5 3.7 2.3 69.I 0.8 x 2.0 3.01 1.0 4.9 11002160.0 9.1 x 16.3 0.5 3.2 1.5 116.4 11003 62.9 5.5 68.4 4.7 x 6.2 2.2 0.8 1.0 4.7 x 7.0 3.0 10.4 11004 55.9 1.2 5.4 62.4 10.0 10.010.8 3.5 x 1.5 x 0.7 x 7.4 x 13.2 4.0 2.7 11005 58.2 12.0 70.2 0.5 7.0 7.5 x 2.8 3.2 1 x 2.0 1.8 11.0 3.8 llOOL 166.8 6.5 73.4 3.5 3.510.5 2.2 0.5 0.7 13.9 1.0 3.0 6.5 18.7 llOE I 69.5 x 6.3 76.0 4.2 4.2 1.4 0.5 1.9 1.9 0.7 3.7 11.7 1 IOFI 64 .2 0.7 5.9 70.8 3.5 3.5 1.0 0.7 1.7 2.9 1.7 3,7 2.5 13.1 I IOGI 65.4 6.2 71.6 6.7 6.7 1.8 x 12.2 2.3 x 2.2 14.6 G\ 1 IOHI 171.1 x 4.6 x 176.2 1.0 11.0 6.6 x 116.0 G\ 11011 69.5 4.0 x 73.8 0.5 0.5 5.4 1.0 19.2 110)1 69.7 5.2 74 .8 2.3 1.5 3.8 0.2 1.3 2.7 0.5 16.5 I !OKI 75.8 0.5 4.0 80.3 0.7 0.7 2.5 4.7 1.5 1.2 9.2 I IOK2 73.5 6.9 80.4 0.2 0.5 x I 1.3 1.7 1.5 114.1 llOLl I68.4 5.5 73.9 0.5 6.2 6.7 0.2 x 0.7 3,7 14.3 llOMI 76.7 1.2 10.7 88.5 0.2 2.3 x x 8.3 llOM2 65.2 2.3 9.2 76.7 0.2 o.5 I o.5 1.3 x 20.8 llONI 179.1 0.7 1.7 81.4 0.3 1.7 x 0.7 115.4 0.21 x llON2 75.7 x 2.0 77.8 0.2 0.5 0.7 4.8 x 16.3 llON5 76.7 0.8 77.5 x 0.3 x 0.5 11.0 3.2 x 3.5 0.8113.3 llOPI 78.2 1.3 79.5 o.81 o.8 o.8 0.5 1.5 0.5 6.0 3.8 7.8 76.7 1.5 4.0 0.7 x 116.7 llOP2 I73.3 0.7 2.7 . I 0.2 1.2 117A I 67.3 x 1.8 69.31 x x 0.7 x 1.0 11.2 0.7 2.7 2.3 1.2 1.3 0.8119.9 117A2 73.3 x 3.2 76.7 0.7 0.7 x 0.5 0.8 3.2 1.3 x 17.2 117BI 74.1 0.7 74 .8 0.5 0.7 1 x 0.3 1.8 2.8 10.2 0.8 8.7 80.0 3.0 0.5 15.4 117B2 176.8 3.2 0.2 0.5 1.0 11701 72.3 1.5 73.8 0.5. I 0.7 0.8 1.2 0.7 3.8 0.5 19.2 11702167.2 x 2.0 69.3 0.7 0.8 1.8 x 1.3 '·7 l.O 2.8 0.7 22.5 117 El 66.3 0.3 66.7 l.O 1.7 x 6.7 9.7 1 1.0 1.3 I2.3 x 11.3 1.7 7.7 117Fl 163.5 x 1.8 65.7 1.0 1.3 4.3 6.7 0.8 x 1.5 x I 5.0 2.8 1.8 0.5115.7 117GI 61.7 2.0 63.7 0.5 1.2 4.0 5.7 1.7 2.01 x 4.7 x 1.2 x 22.2 117HI 66.1 1.8 67.9 x 0.5 1.5 2.5 1.0 0.5 0.5 2.3 1.2 1.7 0.7 2.5 21.1 117H2 70.6 x 2.9 73.7 0.7 0.7 0.7 x 11.2 4.3 l.O x 18.8 11711 I 68.2 1.5 69.7 0.7 0.8 x 0.7 0.5 x I 3.3 5.5 x n.8118.2 Table 4 - Devils Island Sandstone, 41 modal analyses. Percentages are listed for each category. Counts based on 600 points per thin section, counted normal to bedding. x=trace amount (less than 0.5% of 600 points). Quartz Feldspar Rock Fra2mcnts Miscellaneous Cc:mc:nt and Mauix .,,, u c Jl 8 il lJ E u ..c c lJ ti ·2 0 c c '§ " u .,,, .,,, 9 9 :a -il _,,0 1l l!:: !:: ..c !:: ..c ] .,,, c.. ::<: g .ac -b :J lJ :J u u u u u "E 5 r: :. z Table 5 - Chequamegon Formation, 17 modal analyses. Percentages are listed for each category. Counts based on 600 points per thin section, counted normal to bedding. x =trace amount (less than 0.5% of 600 points). The modal analyses are split into two plots for each unit, in order to be consistent with the current convention (Dickinson and Suczek, 1979; Dickinson, 1985). The first plot is a QmFLt, which only has monocrystalline quartz on the quartz pole (Qm), all feldspars on the feldspar pole (F), and all rock fragments including chert and polycrystalline quartz on the lithic pole (Lt). The second plot is a QFL, which contains all quartz with the exception of chert in the quartz pole (Q), all feldspars in the feldspar pole (F), and rock fragments excluding polycrystalline quartz in the lithic field (L). This is for comparison purposes with other studies and for provenance analysis with respect to QFL (Dickinson and Suczek, 1979; Dickinson, 1985). The use of these plots as an indicator of plate-tectonic setting may result in errors in tectonic interpretation. For example, an older tectonic setting may be shedding detritus into an area with a different setting, thereby confusing the tectonic interpretation (Mack, 1984). The use of these plots is discussed further under the section on provenance. Figures 11, 13, and 15 are QmFLt plots and Figures 12, 14, and 16 are QFL plots of thin sections point-counted in this study from the Orienta Formation, Devils Island Sandstone, and Chequamegon Formation, respectively .. 68 Qm 100 F Lr Figure 11. Orienta Formation QmFLt plot of 18 modal analyses. Point 7 is from Amnicon Falls, 102A and 106 are from the Iron River north of Devils Island Sandstone belt and 114A is from Roman Point which is also north of the Devils Island Sandstone belt (see Appendix B). 69 Q 100 F L Figure 12. Orienta Formation QFL plot of 18 modal analyses. Point 7 is from Amnicon Falls, 102A and 106 are from the Iron River north of Devils Island Sandstone belt and l l 4A is from Roman Point which is also north of the Devils Island Sandstone belt (see Appendix B). 70 Qm F Lt Figure 13. Devils Island Sandstone QmFLt plot of 41 modal analyses. 71 Q F L Figure 14. Devils Island Sandstone QFL plot of 41 modal analyses. 72 Qm 100 F Lt Figure 15. Chequamegon Formation QmFLt plot of 17 modal analyses. 73 Q 100 F L Figure 16. Chequamegon Formation QFL plot of 17 modal analyses. 74 The average QmFLt index for the Orienta Formation is 66/17 /17 compared to 74.8/13.7/11.5 for the Chequamegon Formation (Figures 11, 15). Using QFL, the Orienta Formation is 68.9/I 7.3/13.8, which classifies it as a feldspatholithic arenite and the Chequamegon Formation also is a feldspatholithic arenite with a QFL of 78.7/14/7.4 (Figures 12, 16). The Devils Island Sandstone has a QmFLt of 88 .8/3.8/7.5 and a QFL of 94.6/4/1.4 which classifies it as a quartz arenite (Figures 15, 16). Previous studies of the Bayfield Group by Myers (1971 b) defined the Orienta and Chequamegon formations as feldspathic arenites and the Devils Island Sandstone as quartz arenite. The major cementing agent was identified as silica (Myers, 1971 b, p. 64). The major rock fragment constituent was found to be felsic volcanics. Myers (197lb, p. 50-51) had an overall QFL, from at least 300 points per section, of 84.5/11.2/4.1 for the Orienta Formation, 98.9/0.5/0.5 for the Devils Island Sandstone, and 82.7/13.7/2.6 for the Chequamegon Formation. Myers (1971b) used a QFL which included quartzite plotted on the quartz pole. This would cause the average to be closer to the quartz pole than averages from this study. Myers' (1971b, p. 50-53) averages are from 19 specimens from 15 localities in the Orienta Formation, 7 specimens from 6 localities in the Devils Island Sandstone, and 26 specimens from 14 localities in the Chequamegon Formation and are plotted in Figures 17 and 18. Figure 18 is a QFL showing the means of each unit and includes the Fond du Lac Formation from Morey (1967) as well as the Hinckley Sandstone from Tryhorn and Ojakangas (1972). A QmFLt of the data is shown in Figure 17. This study has 18 specimens in the Orienta Formation, 41 from the Devils Island Sandstone, and 17 from the Chequamegon Formation. This study differs in that the both the Orienta and Chequamegon Formations contain more lithics than were found in the study by Myers (197lb, p. 53), and the Devils Island Sandstone does not plot on the quartz pole. The data used to construct Figures 17 and 18 (Myers, 1971 b, p. 50-51) are presented in Appendix C-4 for purposes of comparison with plots of this study. 75 Devils Island Sandstone, Devils Island Sandstone (this study) Qm /Myers, (197lb) 100 Hinckley Sandstone, .___-Myers, (197lb) 'V 90/10 Chequamegon Formation, Myers, (197lb) Fond du Lac Formation, Myers, (197lb) Chequamegon Formation, (this study) Orienta Formation, (this study) F Lt Figure 17. Bayfield Group mean modes, QmFLt. Based on 18 modal analyses in the Orienta Formation (66/17/17), 41 in the Devils Island Sandstone (88.8/3.8/7.5), and 17 in the Chequamegon Formation (74.8/13.7/11.5). Data of Myers, (197lb) is based on 15 modal analyses in the Orienta Formation (77.8/11.3/10.9), 7 in the Devils Island Sandstone (95/0.5/4.4), and 22 in the Chequamegon Formation (78.9/13.8/7.3). Fond du Lac Formation 75.6/7.1117.3) and Hinckley Sandstone (92.7/1.1/6.2) are also shown. 76 Devils Island Sandstone, Hinckley Sandstone, Myers, (1971b) Myers, (1971b) Q / Hinckley Sandstone, Tryhorn Devils Island Sandstone (this study) 100/ ./ and Ojakangas, (1972) Orienta Formation, Myers, (1971b) Chequamegon Formation, Fond du Lac Formation, Myers, (1971b) Myers, (1971b) Chequamegon Formation, (this study) Orienta Formation, (this study) Fond du Lac Formation, Morey, (1967) F L Figure 18. Bayfield Group mean modes, QFL. Based on 18 modal analyses in the Orienta Formation (68.9/17.3/13.8), 41 in the Devils Island Sandstone (94.6/4.0/1.4), and 17 in the Chequamegon Formation (78.6/14.0/7.4). Data of Myers, (1971b) is based on 15 modal analyses in the Orienta Formation (84.7/11.3/4.1), 7 in the Devils Island Sandstone (99/0.5/0.5), and 22 in the Chequamegon Formation (83.6/13.8/2.6). Fond du Lac Formation (85.8/7.117)- Myers (1971b), based on 3 modal analyses; (77/15/8) - Morey (1967), based on 19 modal analyses and Hinckley Sandstone (98.2/1.1/0.7) - Myers (1971b), based on 6 modal analyses; (95 .9/2.2/1.9)- Tryhorn and Ojakangas (1972), based on 20 modal analyses are also shown. 77 Heavy Minerals Sample Preparation A total of 25 samples were prepared for heavy mineral analysis (Figure 19). Each sample, typically fist-sized, was first broken up into individual grains by using a steel mortar and pestle. Using a "hammering" rather than a "grinding motion," the majority of a sample can be easily broken up into discrete grains. Each sample was then washed in water and decanted to remove the clay particles and other very fine fractions. The samples were air dried prior to sieving. The samples were sieved for five minutes on a shaking device and the size fraction under 0.250 mm was utilized for heavy mineral separation. The actual separation was done in separatory funnels using tetrabromoethane as the heavy liquid (d=2.95). Each sample was periodically stirred to ensure a good separation. Each heavy separate was well-washed with acetone to remove any remaining tetrabromoethane. The light fraction was also washed with acetone. The samples were then dried. Magnetite was removed with a hand magnet after separation. The remainder was split to obtain an appropriate amount for mounting on a glass slide with Canada balsam under a cover slip. The splitting process involved repeatedly splitting the heavy fraction until an appropriate amount for mounting was obtained. For each sample, all non-opaque, non-micaceous, detrital grains were identified and counted from three slides. Each slide was examined using a polarizing microscope equipped with a mechanical stage. A continuous grid pattern was established in counting which effectively covered the entire area of the slide without missing grains or double counting. 78 Chequamegon Fm. Devils Island Ss. N Orienta Fm. Devils Island 0 10 15 20 Miles -..._ Formation Contact D D 0 10 15 20 25 30 Km A Outcrop Location r/o <--·- D 0 - Fault Contact K< >// 0 _Jf / l!SCJ 0 IY"'\ ('\_ '1-1/'")Q 9 \_) 120 . {) /\ d \.0 \.:?J.¥.-e .\'. ' '\. Figure 19. Outcrop locations for heavy mineral analysis. Operational Definitions Point-counting identified a number of constituents which are described so that the reader will be able to follow the information that is presented in Table 6. Apatite Colorless to pale yellow, well-rounded tabular-like grains which are generally free of inclusions. Low birefringence. Augite Colorless to very pale green. Moderate birefringence. Inclined extinction of 30-45°. Cassiterite (?) Brown to dark brown. Rounded grains present. Extreme relief and slight pleochroism present. Chlorite Greenish to yellowish green. Rounded to irregular cleavage fragments. Epidote Light yellowish-green to light medium-green. Weakly pleochroic. Moderate birefringence. High relief and generally equant grains that are rounded to angular. 80 Garnet Isotropic, colorless, high relief, subangular to dominantly angular grains (probably due to breakage during dissaggregation). Rare rounded grains present. Glaucophane (?) Blue to pale blue with intense pleochroism present. Inclined extinction but nearly parallel extinction on cleavage fragments. Hornblende Brownish-green to green, strong pleochroism, subrounded to rounded tabular grains. Moderate to strong birefringence. Inclined extinction. Opaques Heavy mineral grains which are too dark to classify. No attempt was made to identify these in this study but the prevalent types are listed since they were identified in thin section and previous studies by Tyler and others (1940, p. 1473) and Myers (197lb, p. 86). Ilmenite Much of the ilmenite shows partial alteration to leucoxene. The surfaces of the grains are generally pitted and rough, and their shape ranges from angular to round. Rounded grains are common (Tyler and others, 1940, p. 1473). 81 Leucoxene Porous white to yellowish-brown grains. Subangular to well- rounded. Pumpellyite Pale green to bluish green. Radial fibers arranged in a fan shape is characteristic of this mineral. Highly pleochroic, equant grains which are angular. Rutile Dark to light reddish-brown. Subrounded to commonly rounded grains. High relief. Sphene Yellow to pale yellow. Weakly pleochroic. Strong birefringence and high relief. Tourmaline Ranges from brown to green to blue. The brown variety is the most common with green the next; bluish varieties are rare. Rounded to subrounded, some angular grains. Grains are generally larger than other heavy minerals, especially in the Devils Island Sandstone. Zircon Three general types are found with "normal" zircon dominant, malacon common and hyacinth rare. Iron-oxide stains grains a yellowish-brown. Prismatic grains with 82 rounded terminations are the most abundant. Zoning is common (Tyler and others, 1940, p. 1473). Hyacinth Slightly pleochroic from dark to light purple. Commonly shows faint zoning and contains a few crystalline inclusions. High birefringence, second or third order. (Tyler and others, 1940, p. 1441) Malacon Abnormally weak birefringence, first order gray and yellow but large grains show first order red. Dull luster and cloudy altered appearance are also characteristics. Usually rounded prismatic to spherical outline, some euhedral elongated prisms with steep pyramids (Tyler and others, 1940, p. 1441) "Normal" Have a well-developed crystal form with sharp crystal angles. Commonly zoned with the inner zones following the external crystal form. Grains contain numerous acicular, crystalline, and irregular gaseous and opaque inclusions, most of which have a random orientation. Most of the zircons are water clear, although some are light tan to yellow. Have strong birefringence characteristic of common zircon (Tyler and others, 1940, p. 1465). 83 Modal Analysis of Heavy Minerals The heavy mineral suite is dominated by opaque grains. Previous studies of the heavy mineral suite were done by Tyler and others (1940) and for the Chequamegon Formation by Myers (1971 b), all of which identified ilmenite as the dominant heavy mineral present. The magnetite was removed by a hand magnet in this study. This study focused on non-opaque heavy minerals. The Bayfield Group heavy mineral suite is composed of tourmaline and zircon with small amounts of rutile and very little garnet, whereas the Oronto Group contains more epidote and smaller amounts of tourmaline and zircon in comparison to the Bayfield Group (Tyler, et al., 1940 p. 1475). Table 6 is a tabulation of the 25 heavy mineral separates that were counted, plus their ZTR indices; ten are from the Orienta Formation, seven from the Devils Island Sandstone, and eight from the Chequamegon Formation. Table 7 presents data from Tyler and others (1940, p. 1470-1472), and includes ZTR indices of maturity calculated during the present study from their data. The ZTR index of maturity is the amount of zircon, tourmaline, and rutile with respect to other non-opaque, non-micaceous, detrital heavy minerals (Hubert, 1962). As these are heavy minerals that are resistant to weathering and abrasion, it is a measure of the maturity level of a sedimentary unit. The data from Table 7 are based on multiple samples from one location each in the Chequamegon Formation and Devils Island Sandstone and two locations from the Orienta Formation. 84 II -0 -0 u -0 1l -0" -0 c " -0 " c ]:" -0 ::J ]: -0 -0 1l c ::J c :; -0 :; 1l 1l ::J g 1l -il ::J -0 g -0 -0 -0 -0 c c g c 0 c" " "c " ::J 1 " c " u ::J ;, ;, c " ::J ] ::J c ::J ;, ::J 1 c ::J c g " g g -0 -0 c x g c c g c g "c g u " " " " 5 -0 " " " -a0 i u ;, c c E u 1{ c c ::0 ::0" u 0 0 u 0" 0" " c c -;;; :; ::J " " ::J -0 -0 0 E E E" ·=<:.: 0 0 :; :; ::J ll. :a ::J .g 0 0 0 N N I- I- <:.: <:.: <: <: <: G" u u c.. ien en ::r: ::r: (.)" (.)" I- Ch equamegon Formanon 88 64.0 14.0 9.0 5.0 3.0 2.0 1.0 2.0 100 97.0 938 84.2 1.6 10.9 0.5 2.2 0.5 184 99.5 107 10.0 25 .0 25.0 18.3 5.0 1.7 1.7 13.3 60 85.0 1080 7.9 7.9 34.2 15.8 23.7 2.6 2.6 2.6 2.6 38 92.I 116LS81 52.6 5.3 10.5 5.3 15.8 5.3 5.3 19 89.5 1188 38.0 17.4 26.4 5.0 1.7 1.7 0.8 0.8 0.8 2.5 5.0 121 90.I 119A 81.8 9.1 9.1 II 100.0 1208 80.9 10.4 5.0 1.4 1.4 0.4 0.4 278 99.6 Devils Island Sandstone 56- 51.0 21.8 14.7 3.1 0.7 0.4 0.2 0.2 1.8 0.7 0.7 4.8 455 93.3 98- 57.8 39.6 0.6 0.6 0.2 0.6 0.4 0.2 510 98.6 IOI- 35.2 3.4 43.2 0.8 15.9 0.8 0.8 264 98.5 llOCI 10.5 0.5 66.3 19.9 0.5 0.3 I.I 0.5 0.3 371 97.3 11011 5.1 66.0 25.6 0.6 0.6 1.9 156 96.8 117AI 40.I 1.0 46.0 9.7 2.2 0.5 0.5 411 96.8 11711 16.7 37.5 33.3 12.5 24 87.5 Orienta Formation 3- 50.0 21.9 18.8 6.3 3.1 32 96.9 27A 32.6 25.I 9.4 1.9 0.7 0.9 0.9 0.2 0.2 0.9 1.2 4.2 21.8 427 70.5 50 65.9 9.8 4.9 4.9 4.9 2.4 2.4 4.9 41 90.2 760 66.7 I I.I 6.7 2.2 I.I I.I I.I 4.4 5.6 90 86.7 960 72.5 10.0 10.0 2.5 2.5 2.5 40 95.0 100- 48.6 14.3 20.0 8.6 2.9 1.4 4.3 70 94 .3 102- 63.0 4.3 23.9 6.5 2.2 46 97.8 1120 56.1 24.4 14.6 2.4 2.4 41 95.I 113- 50.0 11.5 7.7 7.7 7.7 3.8 11.5 26 84 .6 117KI 73.7 3.8 18.I 3.2 0.3 0.3 0.6 342 99.I Table 6. Bayfield Group, 25 heavy mineral modal analyses. Percentages are listed for each category. 85 l., -c., " ., c0 ., -c c:0 .,c: .,c: -c ..., 1:! ., >R0 >< 1:! .5 c: 0 -;: :a 0 ti 0.. u ., c: i5 -c E a: § i5 :a ·a E .. V) N j " :@ :r: u t.r..l (.'.)" Chequam egon Form auon, Bay1e Ii Id 440 II 3 0.5 17 68 1 14.5 100.0 439 7 2 0.5 11 80 9.5 94.7 438 8 5 20 67 0.5 13.0 100.0 437 4 I 10 85 5.0 100.0 D CVIS·1 I san I dS an els cone, C ornucoo1a 448 16 10 0.5 9 64 0.5 1 27.0 98.l 447 26 8 0.5 13 53 0.5 34.5 100.0 446 21 4 1 11 63 0.5 26.0 100.0 445 17 12 0.5 9 60 0.5 2 30.0 98.3 444 19 20 21 38 2 39.0 100.0 443 18 16 15 50 0.5 1 34.5 98.6 442 13 8 0.5 24 55 21.5 100.0 441 27 16 1 36 20 0.5 44.0 100.0 Orienta Formarion, Orienta Falls 0.0 454 5 3 4 0.5 30 58 0.5 12.5 68.0 453 5 1 3 19 72 9.0 66.7 452 6 0.5 7 1 30 55 1 14.5 51.7 451 3 2 3 21 71 8.0 62.5 450 7 2 3 0.5 32 54 2 12.5 76.0 449 6 3 5 22 62 2 14.0 64.3 Orienta Formation, Midclle River 0.0 11245 2 1 5 4 88 8.0 37.5 11241 2 0.5 3 7 88 0.5 6.0 41.7 11 240 2 1 3 5 89 0.5 6.5 46.2 11239 2 1 5 5 87 0.5 8.5 35.3 11237 1 1 4 3 91 0.5 6.5 30.8 11236 2 0.5 0.5 5 92 0.5 1 4.5 55 .6 11235 0.5 I 3 6 87 3 7.5 20.0 11234 1 0.5 3 4 87 5 9.5 15.8 11233 2 2 3 90 3 7.0 28.6 Onenta Formation, M1dclle River (Amnicon Sandstone) 0.0 11232 0.5 1 6 4 76 6 6 19.5 7.7 11229 3 2 0.5 3 69 20 3 28.5 12.3 1234R 2 0.5 2 5 50 0.5 38 2 45.0 5.6 Table 7. Heavy mineral modal analyses from Tyler and others (1940; modified from Table 6, p. 1470-1472). Percentages are listed for each category. 86 Orienta Formation Ten heavy mineral separates were counted from the Orienta Formation. The non- opaque heavy mineral fraction is dominated by zircon and tourmaline. In most cases the grains are well-rounded. Garnet is present in minute amounts with the exception of sample 27A that has a relatively high garnet content and is from the steeply dipping beds at Middle River (Figure 20). This outcrop has been debated as possibly belonging to the Freda Sandstone. The data from Tyler and others (1940, p. 1470-1472) led them reclassify the Amnicon Formation as Freda Sandstone and the Eileen Sandstone as basal Orienta Formation. The Orienta Formation heavy mineral suite differs from that of the Oronto Group mainly in its lack of epidote (Tyler and others, 1940, p. 1476). The stratigraphic problem on the Middle River is examined with respect to heavy minerals and other petrographic data in the Key Areas section below. The prevalent zircon varieties found in the Orienta Formation are "normal" and malacon. The "normal" zircon is only found in Keweenawan or younger rocks in the Lake Superior region (Tyler and others, 1940). Devils Island Sandstone Seven heavy mineral separates were counted from the Devils Island Sandstone. The unit is characterized by larger tourmaline grains. There is a greater abundance of zircon and tourmaline in the Devils Island Sandstone than in the Orienta and Chequamegon formations (Plate 16). There is a lesser percentage of opaques in the Devils Island Sandstone than in the other two units, although the total weight percent of heavies present is less than in the other two units (Tyler and others, 1940, p. 1480). The dominant 87 zircon type present is "normal" with malacon the next in abundance. There is also a greater amount of rounded rutile present, suggesting that this unit is the product of the reworking of older sediment. Some rutile grains have a pitted surface (Plate 17A). The amount of reworking is evidenced by the degree of rounding present on most of the grains. Angular grains are minor within the Devils Island Sandstone. Chequamegon Formation Eight heavy mineral separates were counted. The Chequamegon Formation has a similar heavy mineral distribution to the Orienta Formation, but there are more rounded rutile and tourmaline grains present in the Chequamegon relative to the Orienta Formation that tends to have a higher percentage of angular grains. The Chequamegon samples have a very large amount of opaque grains that accounts for the low counts of non- opaque grains. Tyler and others (1940, p. 1481) suggested that the Chequamegon Formation heavy mineral suite appears to be identical with that of the Orienta Formation. Ilmenite and leucoxene are the dominant heavy minerals present (Tyler and others, 1940, p. 1481) and they interpreted the amount of rounded grains as similar to that of the Orienta Formation, which is different than the just-stated finding of this study. "Normal" zircon is the most abundant non-opaque heavy mineral present, with tourmaline being the second-most-abundant non-opaque. Some of the tourmaline is recycled (Plate l 7B). The tourmaline is dominantly brown and green, with other colors comprising a small minority. The same relationship of tourmaline varieties is present in the Orienta Formation. 88 / \ \ 1 A. Photomicrograph of heavy mineral suite, composed mainly of rounded tourmaline and zircon. Photo is in plane-polarized light and the field of view is 2.5 mm across. Devils Island Sandstone, sample 98. B. Photomicrograph of heavy mineral suite, composed of rounded tourmalines and a zircon grain (left). Photo is in plane-polarized light and the field of view is 2.5 mm across. Devils Island Sandstone, sample 11OC1. Plate 16. 89 • ..•.. -·' .... t ••e . •• - ·-·- ····· ----···-···- ·-····· · ··----··-··-· ····· - . Plate 16. A. Photomicrograph of rounded rutile grain (center). Rounded opaque is also present. Photo is in plane-polarized light and the field of view is 1 mm across. Devils Island Sandstone, sample 98. B. Photomicrograph of a recycled tourmaline grain (center). Angular opaque is also present. Photo is in plane-polarized light and the field of view is 1 mm across. Chequamegon Formation, sample 107. Plate 17. 90 Plate 17. KEY AREAS IN THE BAYFIELD GROUP There are key areas in which stratigraphic relationships have not been resolved in previous studies. Three locations were chosen which could change the stratigraphic boundaries within the Bayfield Group (Northern Sand Island and Devils Island South Landing) and between the Bayfield Group and Oronto Group (steeply dipping beds on the Middle River). Previous studies of the Bayfield Group have also focused on these locations (Thwaites, 1912; Tyler and others, 1940; and Myers, 1971b). A fourth key area is the stratigraphic sequence found from cuttings from a water well at Sand Point. This study attempts to clarify the stratigraphy at these four locations. Orienta Formation at Middle River Locations 33 and 26 are from the Middle River at the Moonshine Road area (Figure 20), (SE 114 Sec. 24, NE 1/4 Sec. 25 T. 48 N., R. 12 W., Poplar 7.5 minute quadrangle) where what has been called the Orienta Formation is truncated by the Douglas fault. The outcrop at this location consists of steeply dipping to overturned beds, based on ripple marks (Plate 2). There is also horizontal jointing found in the overturned beds just north of the Douglas fault contact that can be mistaken for bedding. Location 36 (Figure 20) is from the Middle River at Hwy. 13, which is approximately 1 mile north of location 26, the locality from which thin section 16 of this study was obtained. Thin section 33A of this study was collected from beds assigned to the Freda Sandstone of the Oronto Group by Myers (1971b p. 19-20). This is the only location where Oronto Group sediments can be found on the up-thrown block of the Douglas fault, according to Myers (1971b p. 19-20 and 162). This sample (33A) has a QmFLt index of 60/23.4/16.5. The nearby sample 16 of this study (from location 26) is similar, with a QmFLt index of 91 62.9/12.7/24.4. Sample 33A shows an inverse relationship with the feldspar and rock fragment percentages when compared to sample 36A which has a QmFLt index of 48.8/13.2/38. Samples of the Orienta Formation from steeply dipping beds at Middle River are some of the most immature Orienta Formation samples in this study (Figure 20). No contact between the Freda and Orienta is observed at this location. Because more immature sandstone of the Orienta Formation is found farther north in a location that is clearly within the boundaries of the Orienta Formation (i.e., sample 36A), more information would be necessary to classify the steeply dipping Middle River beds as Freda Sandstone. Previous studies of heavy minerals by Tyler and others (1940, p. 1479) concluded that the heavy mineral suites of the Middle River location are similar to the Eileen Sandstone (since assigned to the Orienta Formation) at Fish Creek (4 miles south of location 91 and approximately 35 miles east of the Middle River, as found in Appendix B) and are the same unit. This led to the recommendation by Tyler and others (1940, p. 1479) to drop the name Eileen and that the exposure at Fish Creek be assigned to the Orienta Formation. Subsequently the Amnicon Formation at Fish Creek was assigned to the Freda Sandstone, while the Eileen Sandstone at this type section was thought to be Orienta in contact with the Freda (Myers, 197lb, p. 34). The only agreement with Freda Sandstone heavy mineral data is the high amount of garnet found in the heavy mineral separate of Sample 27A in this study. This is the only heavy mineral separate examined in this study that contains a high percentage of garnet (a third more than the next closest, sample 107). Tyler and others (1940, p. 1470-1472) showed that the samples that were previously classified as the Amnicon Formation by Thwaites (1912) should be classified as Freda Sandstone since they have similar heavy mineral suites (Table 7). This contrasts with petrographic evidence from sample 36A which demonstrates that the Orienta Formation contains lithofeldspathic beds which are comparable to those found within the 92 Freda Sandstone (Myers, 197lb, p. 68). A QmFLt plot of samples from the Middle River area indicates that the Oronto Group is indeed present north of the Douglas fault (Figure 21). The internal sedimentary features found within the sandstone beds at this location are atypical of Orienta Formation outcrops. Shale beds are more abundant at this location than is typical of the Orienta Formation. The outcrop near the fault is also more thinly bedded and does not contain the larger cross-bed sets that are normal for the majority of the Orienta Formation. The contact between the Freda Sandstone and the Orienta appears to be abrupt and is not exposed, and may be either a faulted contact or an unconformity. Figure 22 shows cross sections of previous and present interpretations of the Middle River exposure. In summary, the Freda Sandstone of the Oronto Group is indeed present north of the Douglas fault. The contact between the steeply dipping beds which are interpreted to be Freda Sandstone and the flat-lying Orienta Formation beds further to the north is not well-exposed but appears to be approximately 1/2 km (1/3 miles) to the north of the Douglas fault (Figure 20). The nature of this contact is also unknown, but has been interpreted to be a gradational contact (Figure 22A), contrary to other interpretations that there is an unconformity (Figure 22B) between the Oronto and Bayfield Groups (this study, Thwaites, 1912; Hite, 1968; Myers, 197lb; Daniels, 1982; Morey and Ojakangas, 1982; Ojakangas and Morey, 1982a). 93 located approximately 2 miles to the north Sec. 24 & 25, T48N, R. 12W @ Orienta Formation Horizontal beds - Orienta Formacion --Freda Sandscone--- N @)--1 Moonshine Road D Steeply dipping overturned Middle River beds - Douglas Flows 0 Scale 2000' Figure 20. Sketch map of the Middle River at Moonshine Road outcrop area. Circled numbers are locations from this study and smaller circled numbers are from the heavy mineral study by Tyler and others (1940). (Modified from Myers, 197lb). 94 Qm 100 F Lt Figure 21. QmFLt diagram for Middle River Orienta samples. Note that sample 16 is from location 26 on Figure 20. Sample 33A is interpreted to be Freda Sandstone while 16 and 36A are interpreted as Orienta Formation. 95 s N ...... " . . : .. . . -- A s N B Figure 22. A: Structure section on steeply dipping Middle River beds, Douglas County, Wisconsin with a conformable contact between the Orienta and Freda Sandstone. {From Tyler and others, 1940). B: Structure section that has an unconformable contact between the Freda Sandstone and Orienta Formation. (This study; interpretation of Thwaites, 1912; Hite, 1968; Myers, 197lb; Daniels, 1982; Morey and Ojakangas, 1982; Ojakangas and Morey, 1982a). 96 South Landing, Devils Island The contact between the Chequamegon Formation and Devils Island Sandstone was placed at the south end of Devils Island by Thwaites (1912, p. 38) (Figure 23). The types of rocks found on the east and west sides of the island are on strike. Location 11 OD of this study (South Landing) was placed in the Chequamegon Formation by Thwaites (1912, p. 38) and again by Myers (197lb, p. 20-21and52) (Figure 23). Taking a closer look at this location, sections l lODl through l 10D5 and l lODL (all samples are from location Don Figure 23) show that this outcrop has an average QmFLt of 75.8/9.8/14.5, which indeed is closer to the Chequamegon Formation mean QmFLt index of 74.8/13.7/11.5 than the QmFLt index of 88.8/3.8/7.5 for all of the Devils Island Sandstone samples (Figure 24). In particular, sample llODL has rare 1 to 2 cm clasts of mafic volcanic rock fragments (vesicular basalt), indicative of a nearby minor source area of immature materials. However, these large fragments are not present in thin section, felsic volcanics are the dominant rock fragment present. As seen on Figure 23, location E has previously been mapped in the same subunit as location D. Sample l lOEl has a QmFLt of 84.7/5.2/10.l, and sample l lOFl (farther to the northeast) has a QmFLt of 84.5/4.6/10.8, both of which are similar to 11 OD3 and l lODL bur close to the Devils Island average. The South Landing location is found to be more closely related, mineralogically, to the Chequamegon than the Devils Island and suggests that there is a gradational contact with the Devils Island Sandstone, as previously reported by Myers (197lb, p. 20-21) and Thwaites (1912, p. 38). The heavy mineral analyses from 1 lOil and 1 lOCl are almost identical. Sample 1 lOil is from the northwestern South Landing location and shows a high percentage of non-opaque grains which is consistent with Devils Island Sandstone heavy mineral separates, not 97 Chequamegon. The contact between the two formations then is fairly abrupt and occurs within a distance of 3 meters (10 feet) stratigraphically, or over a distance of 20 meters (65 feet) geographically along the shore. In summary, a gradational contact between the Devils Island Sandstone and overlying Chequamegon Formation is inferred to be present on Devils Island. The Chequamegon Formation is present at location D (Figure 23) on Devils Island. Mineralogically, the samples from location D are similar to other Chequamegon Formation samples. The heavy mineral suite found near the south end of Devils Island is more similar to that found within the Devils Island Sandstone, and is likely caused by its proximity to the transitional boundary between the two units. 98 s N E Contact between Devils Island and Chequamegon (this study). Similar to contact location as described by Myers, 1971. Scale 1 :24000 1 2 0 I MILE 1000 0 1000 2000 3000 4000 5000 6000 7000 FEET I .5 0 I KILOMETER E""3 E3 E"'3 &=:c: F3 Figure 23. Map of Devils Island showing samples from location 110. Sea cave localities are found at R, S, and T. Contour interval is 10 feet, trails are dashed single and double lines. (Map is modified from USGS Rocky Island 7.5 minute series quadrangle, 1975). 99 Qm 100 F Lt Figure 24. QmFLt diagram for Devils Island Sandstone, locations l lOD through l lOF. (South Landing, Devils Island). 100 Northern Sand Island The northeast end of Sand Island from the center of Justice Bay northward has been mapped as the Orienta Formation, while outcrops south of Justice Bay were placed in the Devils Island Sandstone (Myers, 197lb, p. 18) (Figure 25). Samples 117£ through l 17K of this study would be in the Orienta Formation and 117A through 117D in the Devils Island Sandstone (Figure 26). Locations 117E through 117] have a QmFLt Index of 89.3/5.8/4.7 and the QmFL index of 117A through 117D is 94.8/1.114.2. Both have more mature indices than the average for Devils Island Sandstone. Sedimentary structures, ripple marks and mudcracks associated with samples 117E to 117] are most typical of the Devils Island Sandstone, based on the regional study of this project. The first outcrop which is compositionally similar to the Orienta Formation is not found until location 117K (proceeding in a northerly direction along the shoreline from previous samples), where there is a thinly bedded sandstone with parting lineation; this location has a QmFLt of 66.7/27.1/6.5, which is clearly typical of the Orienta Formation. The transition on Sand Island shows a conformable sequence from the Orienta to the Devils Island. The heavy mineral suite from this location indicates that there indeed is a transitional zone between the Orienta and Devils Island. Sample 117Il contains almost all opaque minerals, which is not characteristic of the Devils Island Sandstone. Sample 117Kl contains a high amount of non-opaque heavy minerals in comparison to other Orienta Formation samples, although it has a higher zircon percentage than the other Sand Island separates. Also problematic is the higher ZTR index of sample 117K (not disputed to be Orienta Formation) compared to the that from the transition zone. This transition zone is also evident in some of the point-counts of Orienta Formation samples on the Bayfield Peninsula; samples that are nearest the Devils Island 101 Sandstone on the peninsula have a higher percentage of quartz than the Orienta samples. This demonstrates, along with the better-exposed boundary on Sand Island, that the upper contact of the Orienta Formation with the Devils Island Sandstone is conformable. No visible erosional surface or was found in the outcrop. In summary, the Orienta Formation grades upwards into the Devils Island Sandstone at Northern Sand Island over a stratigraphic thickness of approximately 10 to 15 meters. The primary sedimentary structures change to those found predominantly in the Devils Island from those typical of the Orienta Formation (parting lineation). The sandstone becomes more mineralogically mature as it grades upward into Devils Island- type sandstone from Orienta-type. 102 Proposed Orienta/Devils Island contact, based on this study Orienta/Devils Island contact (Myers, 197lb) N East Bay Scale 1:24000 -B c Figure 25. Sketch map of Sand Island {location 117) showing sample locations and previous and proposed Devils Island Sandstone/Orienta Formation contacts. {Map is modified from USGS Sand Island 7.5 minute series quadrangle, 1975). 103 Qm 90/10 90/10 G ++ Fl + El 60/40 F Lt Figure 26. QmFLt diagram for Devils Island Sandstone (location 117), A through D (triangles) and E through J (pluses) on Sand Island. Sample Kl from the Orienta Formation is also plotted. 104 Geologic Well Logs Geologic well logs with cuttings were obtained from the Wisconsin Geological and Natural History Survey (WGNHS) for Bayfield County, Wisconsin. Ten geologic well logs were examined. The locations of the wells are found in Appendix B. In general the glacial cover of the area is between 5 and 230 feet thick, but is typically around 50 feet in thickness. The wells near Bayfield and Washburn bottom out in the Chequamegon Formation and do not penetrate the Devils Island Sandstone. There are no geologic well logs in the entire area from Port Wing to Bayfield, which is where penetration of all three units would likely occur in a deep well. The well at Port Wing is entirely in the Orienta Formation, beneath the glacial cover. Two wells are found within the outcrop belt of Devils Island Sandstone. One of these is a 170-foot well on Sand Island that penetrates 55 feet of glacial cover and then immediately encounters Orienta Formation. It is presumed that a thin stratigraphic section of Devils Island Sandstone at this location was removed prior to deposition of the glacial cover, and hence no transition zone is present. The second well is on Sand Point (Appendix B). It is a 140-foot well that penetrates 35 feet of glacial cover before encountering the Chequamegon Formation. The cuttings in the interval from 35 to 80 feet, collected at 5 foot intervals, are characteristic of Chequamegon-type sandstone. Only loose samples (sand) remained after the drilling process, so well cuttings are in fact disaggregated samples. The 85-135 foot depth interval shows characteristics of the Devils Island Sandstone. The dominant color changes from a light red brown to a pink or light yellow red brown in the 80-85 foot interval, and finally to an orange red in the 85-90 foot interval (Massie, 1987). The deeper interval contains more frosted grains (Massie, 1987). This is the only well that penetrated the transition from one formation to another within the Bayfield Group. It is also the only information 105 that confirms that the Chequamegon - Devils Island contact exposed on Devils Island is also present elsewhere. This well log from the WGNHS is found in Appendix F. The Sand Point well samples were obtained from the WGNHS and 21 thin sections were produced by the author between the 35-40 foot interval and the 135-140 foot interval. Each thin section was point-counted, with an average of 600 points per section. Table 8 presents modal analyses of the well cuttings. Figure 27 shows the change in percentages of quartz and lithics with respect to the QFL classification and monocrystalline quartz and total lithics with respect to the QmFLt classification. Appendix G is a complete table which includes raw point-count data along with the calculated QmFlt and QFL percentages of each sample . . There is a trend of increasing mineralogic maturity with depth in the well. Near the top of the well the percentage of quartz is approximately 91 % and it increases to approximately 97% at the 85-90 foot interval in the QFL frame of reference. The percentage of lithics varies inversely with the quartz percentage with approximately 4.5% near the top of the well, decreasing to approximately 1.5% at the 80-85 foot interval in the QFL frame of reference. The feldspar percentage near the top of the well is approximately 5% and decreases to approximately 0.5% at the 85-90 foot interval (Figure 28). The lower interval samples exhibit well-rounded quartz grains with silica overgrowths. Samples near the top of the well are more texturally immature and contain more angular to subangular grains. This change in textural maturity can be seen in a sequence of photomicrographs (Plates 18-21). The well samples from Sand Point thus show a transitional boundary from the Chequamegon Formation, at the top of the well, to the Devils Island Sandstone at the bottom of the well. Samples from the bottom of the well contain more silica cement than do those at the top of the well, which is consistent with what is found in other Devils 106 Island samples. The transitional boundary found within the well at Sand Point occupies a greater interval, approximately 40 feet (12 meters) than that found at the South Landing of Devils Island, which occurs over a stratigraphic interval of 3 meters (I 0 feet). 107 Ouam. Feldspar Rock Miscellaneous Cement and Matrix u .,, c: .,, 8 u u ll u u ..c: c: I'.? ·g 8 c: c: B u 0 .,, .,, .,, :-.<: .,, ..c u ..c: ..c: -uc: u .,, .,, 01 :. :e "g. .:, -bu ll l'.l l'.l ll u u u u u "E 5 !:! 01 .!:; 01 .!:; 01 u u " u. .:: .!:! ll ·c: ..c 1..c ·c: ·c: ·c: u "uE 0 !:! .; .; ii E 0 0 8 c: u c: =;;j =;;j c: c: .!:; 01 .,, rl :; :; l! l! l! ii ..c: u u 0 u .,, " .; OJ 0 0 -e0 0 0 c: c: c: 0 u u '?. "' ..u u u u =-a =-a u. > ..c: ..c: > Q c. c." u u c:.: E E ·c: & 0 0 0 u u u E c: E iu iu iu >< u 0.. rl § e :.;; <.c: <.c: § l! E E E u 01 0 C1 0 E fl -s -s u fl u u :.;; :.;; :.;; u " E E E E E 0 1-0 1-0 0 * u u c: ".ii ..c: -s 0 0 1l. 0 -s u "'il "'il u u u :a " u "- "- > ic:.: ;:;: 0 0 ::E ::E "- "- :J u. u. u. ::E ::E ::E u 0 f... Table 8 - Sand Point Well, 21 modal analyses. Percentages are listed for each category. Counts based on an average of 600 points per thin section. x=trace amount (less than 0.5% of 600 points). QmFLt lithics percent QmFLt quartz percent -30 -30 -35 -35 -40 -40 -45 -45 -50 -50 -55 -55 -60 -60 -65 -65 -70 -70 -75 -75 -80 -80 -85 -85 -90 -90 -95 -95 -100 -100 -105 -105 -110 -110 -115 -115 -120 -120 -125 -125 -130 -130 -135 -135 -140 ...... 1--..---+-....--+---.--l---,---+---.r--+--140 0 2 4 6 8 10 12 75 80 85 90 95 100 Percent Percent QFL lithics percent QFL quartz percent -30 -30 -35 -35 -40 -40 -45 -45 -50 -50 -55 -55 -60 -60 -65 -65 -70 -70 -75 -75 -80 -80 -85 -85 -90 -90 -95 -95 -100 -100 -105 -105 -110 -110 -115 -115 -120 -120 -125 -125 -130 -130 -135 -135 -140 4-.....-..--...... -140 0 2 3 4 5 6 75 80 85 90 95 100 Percent Percent Figure 27. Percentages of quartz and lithics in Sand Point well. Upper graphs show lithics and quartz percentages based on the QmFLt classification. The lower graphs are based on the QFL classification. 109 Feldspar percent -30 -35 -40 -45 -50 -55 -60 -65 -70 -75 ..., "' -80 -85 -s0.. -90 Cl"' -95 -100 -105 -llO -115 -120 -125 -130 -135 -140 0 2 3 4 5 6 7 8 Percent Figure 28. Percentage of feldspar in Sand Point well. Percentage of feldspar is the same in QmFLt and QFL classifications. 110 A. Photomicrograph of grain assemblage containing angular to subrounded quartz grains and rhylolite fragments (center). Photo is in crossed-polarized light and the field of view is 2.5 mm across. Sand Point well, interval 35-40 feet. B. Photomicrograph of grain assemblage containing angular to subrounded quartz grains and a rhyolite grain (left center). Photo is in crossed-polarized light and the field of view is 2.5 mm across. Sand Point well, interval 60-65 feet. Plate 18. 111 Plate 18. I \I A. Photomicrograph of grain assemblage containing subangular to subrounded quartz grains with silica overgrowths. Photo is in crossed-polarized light and the field of view is 2.5 mm across. Sand Point well, interval 70-75 feet. B. Photomicrograph of grain assemblage containing subrounded quartz grains. Photo is in crossed-polarized light and the field of view is 2.5 mm across. Sand Point well, interval 75-80 feet. Plate 19. 112 Pla re l 9. I \ t A. Photomicrograph of grain assemblage containing subrounded to rounded quartz grains with silica overgrowths. Photo is in crossed-polarized light and the field of view is 2.5 mm across. Sand Point well, interval 80-85 feet. B. Photomicrograph of grain assemblage containing subrounded quartz grains with silica overgrowths. Photo is in crossed-polarized light and the field of view is 2.5 mm across. Sand Point well, interval 85-90 feet. Plate 20. 113 ,. Plare 20. A. Photomicrograph of grain assemblage containing rounded quartz grains with silica overgrowths. Photo is in crossed-polarized light and the field of view is 2.5 mm across. Sand Point well, interval 90-95 feet. B. Photomicrograph of grain assemblage containing subrounded quartz grains with silica overgrowths. Photo is in crossed-polarized light and the field of view is 2.5 mm across. Sand Point well, interval 125-130 feet. Plate 21. 114 Plate 2 1. PALEOCURRENT ANALYSIS Introduction Measurements of crossbeds suggest that throughout the depositional history of the sediments associated with the Midcontinent Rift, the paleocurrent direction was basinward (Ojakangas and Morey, 1982c). A factor in measuring paleocurrent directions is that most paleocurrent indicators in ancient deposits are relatively small features such as trough cross- bedding (Le Roux, 1992). These bedforms are of a smaller scale than the channel, which means they will have orientations that depend on the local bottom flow and thus can be expected to produce a higher dispersion of flow directions (Le Roux, 1992). Paleocurrents of the Bayfield Group were studied in each of the three units. The pre-rift sandstones and the interflow sandstones have paleocurrent trends indicating that the Lake Superior syncline was a depocenter throughout the early sedimentational history of the rift (Figure 29). The interflow sandstones are synvolcanic sandstones deposited between flow events during rifting. This trend is also seen in the paleocurrent measurements from the Oronto and Bayfield Group sediments (Figure 29). Although several sub-basins may have been present and may have locally diverted the currents, the major trend is toward the center of the basin, approximately the center of present-day Lake Superior. On Isle Royale, paleocurrent trends are towards the east and east-southeast and along the South Shore there is a paleocurrent trend towards the north (Ojakangas and Morey, 1982c). The paleocurrent measurements made during this study for the Bayfield Group are listed in Appendix A and the averages given are vectoral averages. 115 \ J / B Figure 29. A: Map showing paleocurrent trends in pre-volcanic sandstones of the Midcontinent Rift, from earlier work. Units shown in light gray are the Bessemer Quartzite in Michigan and Wisconsin, the "Nopeming Formation" at the west end of Lake Superior, the Puckwunge Formation in northeastern Minnesota, and the Pass Lake Formation (Sibley Group) and the sandstones of the lower Osler Group in Ontario on the north shore of Lake Superior. B: Map showing paleocurrent trends in interflow sandstones. The volcanics that contain the interflow sandstones are shown by the checked pattern. The dashed line is the axis of the Lake Superior syncline, and the solid heavy lines are major faults. (From Ojakangas and Morey, 1982c). 116 c Figure 29 continued. C: Map showing paleocurrent trends in the Oronto Group on the Upper Peninsula of Michigan and on Isle Royale and in the "Mica Bay Sandstone" at the east end of Lake Superior, from earlier work. Dotted areas in southeastern Minnesota are subsurface occurrences. D: Map showing paleocurrent trends in the Bayfield Group of Wisconsin, the Fond du Lac Formation and the Hinckley Sandstone of Minnesota, and the Jacobsville Sandstone of Michigan and Ontario. The dotted areas in southeastern Minnesota and Wisconsin are subsurface occurrences. The dashed line is the axis of the Lake Superior syncline, and the solid heavy lines are major faults. (From Ojakangas and Morey, 1982c). 117 Orienta Formation The Orienta Formation paleocurrent measurements made during this investigation show a strong northeasterly trend (Figure 30), with the exception of the westernmost locations, Black River and Copper Creek. A total of 359 measurements were taken in this study. Individual rose diagrams for each measurement location can be found in Appendix E. Devils Island Sandstone The Devils Island Sandstone exhibits oscillatory ripples that formed by waves in shallow water (Figure 31). An easterly direction of transport is inferred from 80 measurements of paleocurrent indicators at a few individual locations. At Lenfoot Ledges on the Brule River {westernmost exposure of the Devils Island Sandstone), large trough cross-bedded strata give a wide distribution in trends. It is possible that the plotted Devils Island trends may not be representative of the unit, because of the limited number of Devils Island Sandstone outcrops. Individual rose plots for the Devils Island Sandstone are found in Appendix E. Chequamegon Formation The Chequamegon Formation was emphasized in a previous study in which 181 measurements were taken (Myers, 197lb, p. 263, 239-252). Only nine measurements were taken in this study, and the majority of these were from Big Rock Park. The paleocurrent trend for the Chequamegon Formation is towards the northeast with a few 118 divergent directions (Figure 32). Except for the Washburn area, this again matches the Orienta Formation paleocurrent trend. This indicates a return to the depositional system by which the Orienta Formation was deposited. Individual rose plots for the Chequamegon Formation can be found in Appendix E. 119 Chequamegon Fm. Devils Island Ss. N Orienta Fm. 0 10 15 20 Miles --- Formation Contact 0 10 15 20 25 30 Km A r Outcrop Location -----iL, 0 -..._ Faulc Contact D /f0 0 c net\ot o \;a.Re .:>'1t: Bark Poi!!!_____ Roman p9·' . Quarry Point d ...... P onW mg / () /\ N ./ 17 0 -22 26 Lake Creek__.. - Arrl Orienta Falls Iron River " Figure 30. Paleocurrent directions for Orienta Formation based on 359 measurements. Arrows with number of measurements listed indicate vector average direction of flow for each locality. [§] Chequamegon Fm. Devils Island Ss. N OrientaFm. Devils Island 0 10 15 20 Miles -..._ Formation Contact ..- v 0 10 15 20 25 30 Km A T Outcrop Location / -- 13rl7 - / v 0 - Fault Contact 0 0 O V 0 D \,a\ze Sti.\'er\ot . d ...... 0 /\ ...... N Iron River Lenfoot & May Ledges " Figure 31. Paleocurrent directions for Devils Island Sandstone based on 80 measurements. Arrows with number of measurements listed indicate vector average direction of flow for each locality. ro;::;:-i egon Fm. . 5 Cheqmm North Twin Ourer Island ! '\ Washburn Figure 32. Paleocurrent directions for Chequamegon Formation based on 190 measurements. Arrows with number of measurements listed indicate vector average direction of flow for each locality. (181 measurements from Myers, 1971 b). PROVENANCE Introduction The tectonic setting usually exerts primary control on sandstone composition (Dickinson, 1985). Other factors, typically secondary, are relief, climate, transport mechanism, depositional environment, and diagenetic change (Dickinson, 1985). Table 9 shows the four major provenance types and their tectonic settings as outlined in Dickinson (1985). Modal analyses of the Orienta and Chequamegon Formations plot in the recycled orogen portion of the diagram. This is not the tectonic setting of a rift system, but the sediments are derived from orogenic belts surrounding the Midconrinent Rift System (Figure 3). Mack (1984) pointed out that the Dickinson triangle does not fit all cases. The Devils Island Sandstone plots in the stable craton portion of the triangles, whereas the Chequamegon and Orienta Formations fall within the recycled orogen area but relatively close to the continental block area (Figure 33). These fields are correlated with Table 10 and their corresponding provenance type. 123 Provenance T e Tectonic Settin Derivative Sand Com osition stable craton continental interior or quartzose sands with high Qm/Qp passive platform and KIP ratios basement uplift rift shoulder or transform quartzofeldspathic (Qm-F) sands rupture low in Lt with Qm/F and KIP ratios similar to bedrock magmatic arc island arc or continental arc feldspatholithic (F-L) volcaniclastic sands with high PIK and Lv/Ls ratios grading to quartzofeldspathic (Qm-F) batholith-derived sands recycled orogen subduction complex or quartzolithic (Qt-Lt) sands low in F fold-thrust belt and Lv with variable Qm/Qp and Qp/Ls ratios Table 9. Major provenance types and key compositional aspects of derivative sands. Qm is monocrystalline quartz,Qp is polycrystalline quartz, Qt is total quartzose grains, F is total feldspar grains, K is K-feldspar grains, P is plagioclase grains, L is total unstable lithic fragments, Lv is volcanic/metavolcanic lithic fragments, Ls is sedimentary and metasedimentary lithic fragments, and Lt= L + Qp (From Dickinson, 1985). Paleocurrent Evidence The paleocurrent data suggest that the source area for the Bayfield Group is to the west of Lake Superior, where the bedrock surface at 1.1 Ga included the older Archean granite-greenstone terrane in northern Minnesota and the Animikie basin sediments associated with the Penokean Orogeny. Middle Keweenawan volcanics are also found in this area, within the rift basin. 124 Qt RECYCLED OROGEN SANDS Om • OceAnic Subduction Complex Provenance o Conlinenlel Thrustbell Provenance + Forelond Bosln Sondslono Suites :t Collision Bell Snndslone Suites ...... N 0 VI ) "Ideal' Arkose "Ideal' Arkose Volcaniclaslic Arc Sands F v _ .. ___ ,,._ LF. u -- Lt 0 Chequamegon Formation + Devils Island Sandstone IJ Orienta Formation Figure 33. Compositional plots of key sandstone suites. (From Dickinson, 1985). Compositional plots from this study are shown for comparison. For corners of triangles see caption for Table 9. Petrologic Evidence and Interpretation The lithic fragments give the most insight into the original source rock compositions. The most common rock fragment is rhyolite. Rhyolite is present in the Keweenawan North Shore Volcanic Group. The next most abundant rock fragment is "feldspar-quartz plutonic" (i.e., granitic). Therefore the two most abundant rock fragments provide a contrasting picture as to the source. The clasts within the conglomeratic beds complement the lithologic data derived from thin section study. The conglomeratic beds within the Chequamegon Formation contain mostly quartz and quartzite clasts whereas the Oronto Group has a dominance of volcanic clasts. The Keweenawan volcanics were most likely not readily available as source rocks by Bayfield time, as the Midcontinent Rift System was subsiding, with drainage extending outward into the older terranes. This would have caused the Bayfield Group to have been deposited on top of the volcanics and the Oronto Group. However, topographically higher areas of these older Midcontinent Rift System sediments and volcanics were likely eroded and redeposited into the Bayfield Group. The faulting and uplifting of the St. Croix horst occurred after deposition. Myers (1971 b, p. 78, 87, and 200; Ojakangas and Morey, 1982a) concluded that the Bayfield Group represents sediments that contained some sands reworked from the Freda Sandstone of the Oronto Group; this could partially account for the increased maturity levels of the Bayfield Group relative to the Oronto Group. The heavy mineral assemblage also gives credence to the reworked sediment hypothesis, due to the presence of successively more rounded grains and the maturity of that assemblage. The monocrystalline quartz and feldspars were probably derived in large part from older rocks on the shoulders of the Midcontinent Rift System (see Figure 3). The 126 Midcontent Rift System source does not explain the high volume of these grains present when one takes into account that the Midcontinent Rift System rocks are dominantly mafic in composition. Accessory Mineral Evidence and Interpretation The 11 normal 11 zircons present in the heavy mineral separates indicate that the sediments were mostly derived from Keweenawan igneous rocks or older Keweenawan sediments, specifically, the Oronto Group. The minor malacon and hyacinth varieties present indicate that older rocks are a minor source (Tyler and others, 1940, p. 1481). An Oronto Group source of heavy minerals would require an almost complete elimination of epidote and garnet and a large decrease in feldspar and ilmenite with respect to the heavy mineral assemblage of that Group. Tyler and others (1940, p. 1481) suggested that the Bayfield Group was derived from Keweenawan sediments or felsic igneous rocks which did not originally contain large amounts of epidote and ilmenite, and that the region that had been supplying garnet to the Oronto Group had ceased to contribute sediments to the basin during the deposition of the Bayfield Group. Fond du Lac Formation Data The Fond du Lac Formation of east-central Minnesota, which is correlative with the Orienta, contains abundant angular to sub-angular clasts of felsic and basic extrusive rocks likely derived from a nearby Middle Keweenawan extrusive rock source (Morey, 1967). Morey (1967, p. 17) also found that resistates (well-rounded quartzite, chert, and iron-formation) indicate that Middle Precambrian or older rocks located some distance from the Fond du Lac depositional area also supplied material to the basin. This, together 127 with paleocurrent data, led to the conclusion that the source area was located west of the Fond du Lac area in what is now central Minnesota (Morey, 1967, p. 17). Hinckley Sandstone Data The Hinckley Sandstone, correlative with the Devils Island Sandstone, has a high mineralogic maturity, rounded grains, and reworked quartz grains. This implies that older sandstones were a source for the Hinckley Sandstone (Tryhorn and Ojakangas, 1972). The most likely source is the underlying Fond du Lac Formation. Extensive weathering, erosion, and reworking of the sand-sized fraction of the Fond du Lac Formation would result in the elimination of most of the labile components (Tryhorn and Ojakangas, 1972). Interpretation The sources of the Bayfield Group changed from the sources of the older Oronto Group. The Oronto Group was getting a direct influx of material from the Keweenawan volcanics and contains more lithic grains than the Bayfield Group. The Freda Sandstone is a lithofeldspathic arenite (Hite, 1968). The volcanic source was largely eroded away or covered by the time Bayfield Group sedimentation commenced. There is a substantial amount of volcanic material in the Orienta Formation but less than what is found in the Freda Sandstone. The volume of volcanic grains is fairly consistent within the Orienta and Chequamegon formations; this suggests that the older Keweenawan volcanic source was never completely shut off during Bayfield deposition. The Devils Island Sandstone most likely had the same source material but is more texturally and mineralogically mature due to the sedimentary processes that were involved in the deposition and reworking of this unit. Some reworking of the Orienta Formation may have contributed to the maturity. 128 The Chequamegon Formation contains the same types of lithic fragments as the Orienta. This indicates that there was no significant change in source throughout the depositional history of the Bayfield Group. The volume of felsic plutonic fragments within the Bayfield Group suggests that the source area included rocks outside the Midcontinent Rift System (i.e., the rift flanks) . Early Proterozoic rocks such as those in the large Penokean terrane to the southeast or southwest (Figure 3) could also have been sources. The influx of material from largely intrabasinal Keweenawan volcanic sources was not great enough to mask the Archean source from eastern and northeastern Minnesota, as it was during Oronto Group sedimentation. This leads to a picture of a basin which was receiving a smaller volume of sediment as it was beginning to fill up, and a slower depositional rate than for the Oronto Group. Hence the more labile minerals had greater opportunity to be weathered and abraded, leaving more mineralogically mature sediments relative to the Oronto Group. 129 SEDIMENTATION AND ENVIRONMENTS OF DEPOSITION Myers (197lb, p. 67 and 76-78) hypothesized that a new cycle of sedimentation occurred in the Lake Superior syncline after deposition of the Oronto Group, based on the higher compositional and textural maturity exhibited by the Bayfield Group sandstones. Daniels (1982) outlined the Oronto Group as being composed of a complex collection of facies which have an upward succession of 1) prograding alluvial fan, 2) transgressive lacustrine (a primary reducing environment), and 3) prograding braided-meandering stream environments. The Bayfield Group was inferred to have had a general geologic setting of alluvial plain, characterized by low-gradient, meandering, perennial streams (Myers, 197la, Myers, 197lb, p. 191). Fluvial Systems Stratification and bedforms can be recognized in alluvial deposits and the deposits can be classified at two distinct scales (Miall, 1985, 1988, and 1992). The smaller scale includes facies and at the larger scale they can be grouped into architectural elements (Miall, 1992). In the use of architectural elements, which are also known as facies associations, the approach to facies models is to first subdivide all depositional environments into a relatively small number of basic types and then describe the environments as carefully as possible by comparing recent examples with those interpreted to be their ancient counterparts (Walker, 1990). The outcrops found demonstrate a cross section of environments found in modern fluvial examples. Therefore the internal structure of each element (i.e., trough cross-bedding) is important in placing an outcrop within the fluvial environment (Table 10, Figure 34). 130 Bedform studies show that trough cross-bedding is produced by dunes, which occur within the lower flow regime (Southard and Boguchwal, 1990). The small-scale ripple marks were most likely produced by ripples, which are present in a lower flow regime. A series of phase diagrams that set out the flow depths, flow velocities, and grain sizes that control the formation and stability of the various bedforms has been established by experimental work (Miall, 1992; Ashley and others, 1990). Facies Facies Sedimentary Structures Interpretation code Gms massive, matrix-supported grading debris flow deposits gravel Gm massive or crudely bedded gravel horizontal bedding, imbrication longitudinal bars, lag deposits, sieve deposits Gt gravel, stratified trough cross-beds minor channel fills Gp gravel, stratified planar cross-beds longitudinal bars, delraic growths from older bar remnants St sand, medium to very coarse, solitary or grouped trough cross- dunes (lower flow regime) may be pebbly beds Sp sand, medium to very coarse, solitary or grouped planar cross- linguoid, transverse bars, sand may be pebbly beds waves (lower flow regime) Sr sand, very fine to very coarse, ripple cross-lamination ripples (lower flow regime) may be pebbly Sh sand, very fine to very coarse, horizontal lamination, parting or planar bed flow (upper flow may be pebbly streaming lineation regime) SI sand, very fine to very coarse, low angle ( < 10°) cross-beds scour fills, washed-our dunes, may be pebbly antidunes Se erosional scours with intraclasts crude cross-bedding scour fills Ss sand, fine to very coarse, may be broad, shallow scours scour fills pebbly Fl sand, silt, mud deposits fine lamination, very small overbank or waning flood ripples Fsc silt, mud laminated to massive backswamp deposit Fcf mud massive, with freshwater molluscs backswamp pond deposits Fm mud, silt massive, desiccation cracks overbank or drape deposits c coal, carbonaceous mud plant, mud films swamp deposits p carbonate pedogenic features paleosol Table 10. Facies classification (From Miall, 1978) 131 Channel LA Lateral Accretion ...rt\ ,....If(\ ,...t'(\ =<§? DA Downstream Accretion LS Laminated Sand [ 0.2 - 2.0m] OF Overbank Fines Figure 34. The eight basic architectural elements in fluvial deposits. No vertical exaggeration. Note the variable scale. (From Miall, 1992). 132 N earshore Lacustrine Systems The nearshore facies is made up of parallel-, ripple-, and cross-bedded sand (Howard and Reineck, 1981). Howard and Reineck (1981) note that rounded rock- fragment pebbles are present both individually and as layers in the foreshore and more commonly in the swash zone with alternating layers of coarse and fine sand locally present. Parallel-laminated sand is typically the dominant physical sedimentary structure in the nearshore facies (Howard and Reineck, 1981). The individual laminae commonly pinch out at erosional contacts, suggesting that these are wedge-shaped lamina sets. Cross- bedded sand comprises sets of cross-bedding in which dipping laminae are more than 2 cm thick (Howard and Reineck, 1981). Small-scale (oscillation) ripple lamination is restricted mostly to the nearshore facies, with all of the examples in a modern-day study by Howard and Reineck (1981) found below the low-water line. Preserved sets of ripple lamination are associated with concentrations of heavy minerals, which suggests that wave reworking of the substrate under conditions of minor sedimentation was occurring (Howard and Reineck, 1981). Wave-formed structures are useful in determining the processes which produced them, by comparing them to modern examples. Clifton and Dingler (1984) noted that the character of the flow and the composition of the bed (texture and mineralogy) combine to determine the general configuration of the bed and the size and shape of the bedforms themselves. Oscillation ripples are the predominant wave-generated bedforms (Clifton and Dingler, 1984); these are abundant in the Devils Island Sandstone. The majority of oscillation ripples are transverse to the wave-induced current (Clifton and Dingler, 1984). 133 Laminae Sands The laminae sands found in the Devils Island Sandstone may have been formed by adhesion ripples. Experimentally produced climbing adhesion ripple structures were found to have alternating coarser and finer laminae (Kocurek and Fielder, 1982). This tends to occur at lower water levels, where adhesion-ripple overhanging ridges became less pronounced (Kocurek and Fielder, 1982). Adhesion ripples can occur on surfaces with high water contents but the surfaces do not need to be fully saturated (Olsen and others, 1989). The alternating fine-grained laminae represent the actual adhesion-ripple ridge formed by the adhering of grains placed in saltation by wind, while the coarser laminae represent the filling of cavities beneath the overhanging ridges by grains in creep (Kocurek and Fielder, 1982). The adhesion-ripple climb angle is a function of water content, wind velocity, and the local angle between saltating grain paths and the depositional surface (Kocurek and Fielder, 1982). The adhesion ripples can realign themselves to changing wind directions and form a deposit of diversely oriented sets of pseudo-cross-lamination, since individual adhesion ripples climb upwind over each other causing a set of pseudo- cross-lamination with "foresets" dipping downwind in unidirectional winds (Kocurek and Fielder, 1982). Similar characteristics have been described by Swezey and others (1996) in sandstone of the Upper Permian Champenay Formation, France. Long, continuous, slightly subhorizontal, 1 - 3 mm thick laminae of medium-grained sandstone with moderate- to well-rounded quartz grains are interpreted to be subaqueous upper plane bed laminae and/or possible antidune stratification on a lacustrine beach (Swezey and others, 1996). 134 Forms of Evidence Present Primary evidence for the environment of deposition in the Bayfield Group comes from primary sedimentary structures. The primary structures found are large-scale trough cross-bedding, small-scale trough cross-bedding, ripple marks, mudcracks, mudchip conglomerate, conglomeratic beds and parting lineation. Large channels such as that in Plate SA also provide information showing that the depositional environment could be a fluvial system. The parting lineation indicates flow in the upper flow regime, and hence the current is locally fast and perhaps has a higher gradient. Shale beds occur within the Orienta and Chequamegon Formations. They are minor, only 10 to 50 cm in thickness, and not very extensive laterally. The shale beds indicate local pockets of quiet waters which allowed the settling of clay- and silt-sized particles. Ripple marks and mudcracks of the Devils Island Sandstone are commonly found within the same layers. They are well-preserved; this indicates that the water was shallow and that alternating periods of drying and wetting occurred. The ripple indices (wavelength/amplitude) range from 6 to 13.75 (this study) and 3.5 to 33.3 (Myers, 197lb), indicating deposition by water rather than by wind (Reineck and Singh, 1980). Current ripples are mostly 8 to 15 while wind sand ripples fall in the range of 10 to 70 (Reineck and Singh, 1980). The Midcontinent Rift System provided a natural mechanism for erosion of highlands on either side and deposition in a central basin (Figure 35). The various types of evidence indicate that the Bayfield Orienta and Chequamegon Formations were deposited in a fluvial depositional environment (Table 11) 135 ...... \..).) G'\ ...., v \ < \ r- v ...-i Older Precambrian Rocks /..- (' ...J ...., ' -J <. \j '1 7 I- /'\ '7 '\"" \j \J Figure 35. Sketch map of how the Midcontinent Rift System may have appeared during Keweenawan 'sedimentation. (From Ojakangas and Marsch, 1982, p. 59). Braided flu vial "evidence:" Meanderin Flu vial "evidence:" Conglomerates common (clast-supported) Conglomerates mainly mud-chip (intraformational) Sands coarse (dominant) to fine Sands medium-fine Mineralogically immature? Mineralogically immature? Silt & clay minor (less than 10%, in Silt & clay abundant (overbank = abandoned channels and therefore floodplain = broad) "shoestring") Fining-upward sequences not common Fining-upward sequences common Cross-beds - planar dominant, trough Cross-beds - trough common, planar minor? minor? Planar stratification common Planar stratification - some Paleocurrents strongly unimodal Paleocurrents unimodal Fossils rare Fossils rare Table 11. Evidence for braided and meandering fluvial environments of deposition. (From Ojakangas, unpublished). 137 Interpretation Orienta Formation The large-scale cross-bedding and parting lineation indicate high-velocity currents, probably in a fluvial environment (Myers, 197lb, p. 141; Morey and Ojakangas, 1982). The gravel and the fine fraction (i.e., shale interbeds) fit the idea that there is variability within the river system. This implies a meandering stream, in view of "evidence" present in meandering streams in Table 11. Internal structures observed in the Orienta Formation are trough cross-bedding, channel bodies, massive bedding, parting lineation, ripple marks, mudcracks, parallel bedding, mudchip conglomerate, and conglomeratic sandstone beds. The facies are those associated with large channels, with the channel fills in the thicker- bedded sequences. The Orienta Formation is interpreted herein as a fluvial deposit which exhibits the characteristics of a meandering stream. Devils Island Sandstone The Devils Island Sandstone is composed of well-sorted quartz sand. Compared to the Orienta Formation, the Devils Island Sandstone has thinner beds and abundant ripple marks, indicative of a standing body of water (a large lake?) where feldspathic detritus was reworked and made more mature (Morey and Ojakangas, 1982). Thin sequences of laminae sands (coarse versus fine) occur at three locations: Devils Island (east and west landings), Sand Island (south of Justice Bay and south of East Bay), and the Squaw Bay area on the Peninsula. These laminae sands are perhaps best seen in the Apostle Islands National Lakeshore at the Devils Island sea caves (Figure 36). These are what are 138 interpreted to be adhesion ripples, as seen in plate 10. They represent a depositional environment that likely had wind action in addition to wave action. A shallow lake environment would account for a beach type of deposit that is perhaps what is found in the Devils Island Sandstone. Oscillatory ripple marks are the dominant internal structural feature found in this unit. The amount of rounding and cleaning up of the sands would be better explained by a combination of eolian and shallow-water environments (Myers, 197lb, p. 81). Eolian action on the vegetationless Orienta Formation alluvial plains is perhaps a major factor in the maturation of the sediment (Ojakangas, 1986). The mudcracks are evidence that subaerial conditions existed in the region during Devils Island-type sedimentation (Myers, 1971b, p. 141). Several fining-upward cycles (from 0.5 to 1 cm, and 1 meter thick) are present in the Devils Island Sandstone near the transitional boundary with the overlying Chequamegon Formation on Devils Island (location 1 lOI on Figure 23). This would indicate that the transition zone from the Devils Island to the Chequamegon comprises a change from lacustrine to fluvial processes. The shallow lake environment would account for the ripple marks found, but eolian processes are much more effective as a rounding agent than water (Kuenen, 1959). Therefore the Devils Island Sandstone appears to have been deposited in a nearshore variable-energy environment that fluctuated between subaqueous and subaerial conditions. 139 Chequamegon Formation The Chequamegon Formation indicates a return to fluvial conditions after the temporary lacustrine environment of deposition of the Devils Island Sandstone. The Chequamegon Formation is very similar to the Orienta Formation in that the internal structures found are trough cross-bedding, channel fills which contain trough cross-beds, massive bedding, parting lineation, ripple marks, mudcracks, parallel bedding, and conglomeratic sandstone beds. The Chequamegon Formation also contains brownstone beds, named after the building material which was extracted at the end of the last century. The brownstone trend was first observed by Thwaites (1912, p. 33-37) and confirmed by Myers (1971 b, p. 24). These beds are indicated on Figure 36 that also includes quarry locations in the Orienta Formation. The brownstone beds were described by Thwaites (1912, p. 33) as follows: "Heavily-bedded dark brown ferruginous sandstone with pebbles and clay pockets; between heavy beds are thin-bedded layers and lenses of red, micaceous shale." The channel bodies and cross-bed sets in the Chequamegon Formation are similar to those found within the Orienta Formation and both are interpreted to have the same depositional environment. Thus the Chequamegon Formation is also interpreted to represent a meandering stream depositional environment. Fond du Lac Formation The Fond du Lac Formation was deposited in a shallow, deltaic environment by a system of streams beginning in a western highland and dispersing material to the east (Morey, 1967, p. 21). Morey and Ojakangas (1982) identified 171 fining-upward cycles in a drill core of Fond du Lac Formation near Moose Lake, Minnesota, and interpreted 140 this as evidence for a meandering fluvial environment. Therefore it fits the processes thought to have occurred in the Orienta Formation, its equivalent in Wisconsin. Hinckley Sandstone Tryhorn and Ojakangas (1972) suggested that the textural and mineral maturity of the Hinckley Sandstone required a nearshore, high-energy environment that succeeded the fluvial-deltaic environment of the Fond du Lac Formation. The entire basin stabilized tectonically and was filled by fluvial-deltaic deposits (Tryhorn and Ojakangas, 1972). A large lake probably formed (Figure 35) in the basin and slowly transgressed higher onto the older fluvial-deltaic deposits while the feldspatholithic sands of the Fond du Lac Formation were reworked in a stable, shallow-water lacustrine environment (Tryhorn and Ojakangas, 1972). Figure 37 shows a schematic cross section describing this mechanism and is discussed in detail in the third model of the next section. Figure 36. Sketch map of Northwestern Wisconsin showing the location of brownstone quarries (indicated by squares) and seacaves (indicated by triangles). (Modified from Myers, 197lb). 141 TECTONic!SEDIMENTARY MODELS There are various hypotheses on the generation of the Bayfield Group. Several of these hypotheses will be discussed along with their pros and cons. Information supporting the models will be examined along with the evidence that may cause a hypothesis to be invalid. The first would support the hypothesis put forth in the Purpose of Study section, wherein the Orienta and Chequamegon Formations are one and the same unit. The second model stays with the interpretation put forth by Thwaites (1912) and Myers (197lb) that there are three separate units present in the Bayfield Group, with the Orienta Formation the oldest and the Chequamegon Formation the youngest. Model 1 - Orienta Formation is equal to the Chequamegon Formation A. Synclinal Structure Geographically, the Devils Island is found between the other two units of the Bayfield Group. It is known to have a conformable contact with the underlying Orienta Formation on Sand Island. It also appears that a gradational contact that can be traced on the mainland by petrologic evidence shows that samples of the Orienta Formation within close proximity of the Devils Island Sandstone outcrop belt have a higher QFL index than other Orienta Formation samples. The Devils Island Sandstone would be the youngest unit in this scenario, and thus fit the dominant trend of increasing mineralogic maturity within the sequence of Keweenawan sediments. The rock units within the Bayfield Group are fairly flat-lying, with southeasterly dips of 1 to 6 degrees. The only location which does not follow this trend is the Chequamegon Formation outcrop at Big Rock Park (location 108) on the mainland 142 (Appendix B, Figure 9). At this location, the dip is towards the southwest at 15 degrees. This information suggests that there is a syncline present (Figure 37). However, the lack of outcrops to the southeast of the Devils Island Sandstone on the mainland makes the support of this hypothesis difficult. The Oronto Group, which is adjacent to the southeasternmost outcrops of the Bayfield Group (locations 108, 91), is folded and is part of a monocline (Cannon and others, 1993) (Appendix B) . The folding event which caused this may have also affected the Bayfield Group. Devils Island Sandstone NW SE I I . F . --/ 0 nenta ormat10n ___ 'Chequamegon Formation Figure 37. A cross section depicting the Devils Island Sandstone as the core of a syncline. The main problem with this hypothesis is that no structural data from within the Devils Island Sandstone itself support the conclusion that the Devils Island Sandstone occupies the core of a syncline. The Devils Island Sandstone outcrops have a consistent southeasterly dip of 2 to 6 degrees on the Apostle Islands as well as on the mainland. The Sand Point well appears to encounter Devils Island Sandstone beneath the Chequamegon Formation. This means that it is not probable that a syncline is present, or for that matter, a structure that confines the deposition of the Devils Island Sandstone to the outcrop belt defined in bedrock maps. The folding (i.e., tilting) event which affected the Oronto Group appears to have occurred before the Bayfield Group was deposited (Figure 5) . The interpretation of seismic lines from the western Lake Superior region indicates that the 143 Oronto Group is folded and structurally complex in comparison to the Bayfield Group, which is structurally simple and shows no folding other than a slight tilt to the southeast (Figure 6) . B. Faulting The Chequamegon Formation is similar to the Orienta Formation. It has a similar environment of deposition, and modal analysis shows it to be slightly more mature but otherwise similar mineralogically to the Orienta Formation. The heavy mineral suites of the two formations are very similar as well. What if the Orienta Formation and Chequamegon Formation are the same formation? This would explain many of the similarities between the two formations. Faulting could have caused the Devils Island Sandstone to appear in between the two units of similar nature in the Bayfield Group. A normal fault parallel to the Douglas fault with a minor vertical displacement, on the order of 90 meters (300 feet) would provide a topographic low in which the Devils Island was deposited (Figure 38). Devils Island Sandstone NW SE I ;$ Orienta Formation Chequamegon Formation t t Figure 38. A cross section depicting the Devils Island Sandstone deposited adjacent to a small-throw fault. 144 No major faults other than the Douglas fault are found within or adjacent to the Bayfield Group sediments. The interpretation of seismic data shows that there are no faults present within the Bayfield Group large enough to cause a repetition of the Orienta Formation in what is described as the Chequamegon Formation (Cannon and others, 1989). Furthermore, the Devils Island Sandstone is conformably overlain by the Chequamegon Formation, as seen on the South Landing on Devils Island where the Devils Island Sandstone grades upward into the Chequamegon Formation, and in the well on Sand Point. There is no structural evidence that would indicate that the Orienta Formation is repeated and expressed in outcrop as the Chequamegon Formation. Model 2 - Oi:ienta Formation is not equal to the Chequamegon Formation. The majority of the evidence supports the conclusion that there was a continuous sequence of deposition throughout Bayfield Group time and that there is no hiatus present. The Devils Island was found to overlie the Orienta conformably on Sand Island. The upper contact of the Devils Island is found on the South Landing of Devils Island where it grades upward into the basal beds of the Chequamegon Formation, and in the well on Sand Point. The Devils Island Sandstone appears to have been deposited in a shallow lake that was relatively short-lived. It was shallow enough that it dried up on more than one occasion, as evidenced by mudcracks. Figure 35 indicates that sedimentation was occurring in the central portion of the basin (Ojakangas and Marsch, 1982). Deposition continued with a current movement towards the northeast. During deposition of the Bayfield Group, the region was not very active tectonically. Locally there was still subsidence and therefore the basin geometry would have been constantly changing depending on the influence of the tectonics that created the depocenter. The center of the 145 basin (i.e., the topographic low) would be the location of the Devils Island Sandstone, where a final 'cleaning' or maturing of the sandstone could have occurred. The accumulation of sediments may have caused the deepest part of the basin to shift laterally, the central part of the basin containing quartzose (Devils Island-type) sediments and the borders containing more immature (Orienta-type) sediments. A depositional pattern whereby the quartzose sands were covered by a slightly younger deposit of more immature Orienta-type sediments (Chequamegon Formation) as the basin moved laterally could be hypothesized. In this interpretation, the Bayfield Group is a time-transgressive sequence with the exposures along the South Shore showing but a small portion of the entire basin. Figure 39 is an illustration of this hypothesis. This interpretation is the same interpretation first put forth by Thwaites (1912), and a later study by Myers (197lb) was in general agreement with the findings by Thwaites (1912). The Bayfield Group indeed shows a trend from an immature feldspatholithic/lithofeldspathic arenite, to a much more mature quartz arenite, and then it returns to a mineralogically immature feldspatholithic arenite in the Chequamegon Formation. The Orienta and Chequamegon Formations had similar environments of deposition and could represent the same unit in a time-transgressive depositional sequence in which the Devils Island Sandstone represents a time period during which reworked sands were being deposited under more stable conditions. In Figure 39, the exposed sequence is depicted by the vertical bar. 146 Y unit, time NW transgressive lateral movement (transgression-regression) SE / Devils Island Sandstone ! Formatlo - (I) ...... --:---:::::::: ...... s Orienta Formation ..._. Freda Sandstone '-.) exposed in the western Lake Superior region Figure 39. Sketch of a cross section of the Bayfield Group showing a theoretical depositional pattern for the sediments. The vertical bar shows the sequence of rock units found in the study area. CONCLUSIONS 1) The Orienta Formation is the most mineralogically immature sandstone of the Bayfield Group and is classified as a feldspatholithic to lithofeldspathic arenite. It has an average QmFLt composition of 66/17/17 and an average QFL composition of 69/17/14. 2) The Devils Island Sandstone is classified as a quartz arenite and is the most mature unit mineralogically and texturally within the Bayfield Group. It has an average QmFLt composition of 88/4/8 and an average QFL composition of 95/4/1. 3) The Chequamegon Formation is classified as a feldspatholithic arenite. It has an average QmFLt composition of 75/14/11 and an average QFL composition of 79/14/7. 4) Diagenesis has introduced authigenic minerals, including hematite, quartz, kaolinite, zeolite, and trace amounts of potassium feldspar. 5) The paragenetic sequence found within the Bayfield Group has six sequential elements: 1) compaction, 2) silica overgrowths, 3) iron-oxide cementation, 4) dissolution of grains, 5) clay infilling, and 6) zeolite infilling. The majority of samples exhibit elements 2 through 4. 148 6) Porosity apparently decreases with depth within the Bayfield Group. The Chequamegon Formation has an average porosity of 16.2 percent, the Devils Island Sandstone 14.1 percent, and the Orienta Formation 12.6 percent. The Orienta Formation has lower porosity near its basal beds. The Devils Island Sandstone and Chequamegon Formation do not show any obvious internal trends in porosity. Porosity-reducing features include silica overgrowths and hematitic cementation. 7) Secondary porosity is caused by the dissolution of potassium feldspar grains and rhyolite rock fragments that contain a potassium feldspar-quartz groundmass. In many cases, skeletal remnants of the feldspar crystals are present in partially dissolved grams. 8) The most prevalent lithic grain type found in the Bayfield Group is felsic volcanics. The majority of rock fragments falling under this category are rhyolitic, which typically have a "snowflake" texture and contain a dominantly potassium feldspar groundmass. 9) Accessory mineral studies show that the Bayfield Group contains mainly rounded zircon (plus an appreciable angular component) and rounded tourmaline in the non- opaque, non-micaceous fraction. There is little or no epidote present. The majority of zircon found is of the "normal" variety, which is only present in the Lake Superior region in rocks of Keweenawan age. 10) The Freda Sandstone is exposed on Middle River, although the presumably unconformable contact between the steeply-dipping Freda Sandstone and horizontal beds of the Orienta Formation is not exposed. 149 11) The contact between the Chequamegon Formation and Devils Island Sandstone is present over a thin stratigraphic interval on the south end of Devils Island. The contact between the Orienta Formation and Devils Island Sandstone on the northeast shoreline of Sand Island is gradational. Bedforms normally found within the Devils Island Sandstone (ripple marks and mudcracks) are also present in more mineralogically immature beds that are inferred to be the uppermost Orienta Formation. 12) Cuttings from a public well at Sand Point show that a transitional contact between the Devils Island Sandstone and the overlying Chequamegon Formation is also present at this locality. 13) The paleocurrent data for the Orienta Formation show a strong trend towards the northeast. The Devils Island Sandstone has a more easterly to southeasterly trend, but not at all exposures. The Chequamegon Formation's paleocurrent trend is towards the northeast. 14) The Bayfield Group likely received material from recycled Freda Sandstone as well as a smaller component directly from Midcontinent Rift System volcanics. The large amounts of quartz and microcline suggest that Archean or Early Proterozoic sources contributed the majority of sediment. The presence of a dominant "normal" zircon population in the accessory mineral suite supports the inference that the source rocks included the Keweenawan volcanics, perhaps with an intermediate step of temporary storage within the Freda Sandstone before final deposition in the Bayfield Group. The smaller quantities of other zircon varieties reasserts that there are older Archean or Early Proterozoic sources. 150 15) The Orienta Formation was deposited in a fluvial environment. Primary structures suggest that it was a meandering river system transporting large amounts of sediment. 16) The Devils Island Sandstone was most likely formed by reworking of Orienta Formation sands. It was deposited in a shallow-water, nearshore depositional environment that was also exposed to eolian processes. 17) The Chequamegon Formation environment of deposition a meandering fluvial system. It signifies a return to the same depositional environment as that which formed the Orienta Formation. 18) There was no evidence found that would suggest that a synclinal structure is present such that the Devils Island Sandstone is the youngest unit in the Bayfield Group. There was no evidence that a repetition of the Orienta Formation by faulting is the reason why the Chequamegon Formation is found as the uppermost unit. 19) The Bayfield Group consists of a conformable sequence composed of the Orienta Formation, overlain by the Devils Island Sandstone, in turn overlain by the Chequamegon Formation. 20) The general depositional environment of the Bayfield Group is that of a transgressive- regressive lacustrine shoreline on a broad alluvial plain. The Orienta and Chequamegon Formations represent fluvial sediment transport towards the center of the valley, whereas the Devils Island Sandstone is indicative of a lacustrine system in a topographic low located near the center of the Bayfield basin of the Midcontinent Rift System. 151 21) The sequence in Minnesota contains only the Fond du Lac Formation overlain by the Hinckley Sandstone. There is no equivalent of the Chequamegon Formation found above the Hinckley Sandstone in the Minnesota section. It possibly was never deposited or was deposited and subsequently eroded. 152 REFERENCES CITED Allen, D.]., Hinze, W. J., Dickas, A. B., and Mudrey, M. G., 1995, Integrated geophysical investigations of the Midcontinent Rift: New models for western Lake Superior, Wisconsin, and Minnesota: 1995 IGCP Project 336, Proceedings, Duluth, MN, p. 3-4. Allmendinger, R. W., Charlesworth, H. A. K., Erslev, E. A., Guth, P., Langenberg, C. W., Pecher, A., and Whalley, J. S., 1991, Microcomputer software for structural geologists: Journal of Structural Geology, v. 13, p. 1079-1083. Ashley, G. M., 1990, Classification of large-scale subaqueous bedforms: a new look at an old problem, SEPM Bedforms and Bedding Structures: Journal of Sedimentary Petrology, v. 60, p. 160-172. Cannon, W. F., 1994, Closing of the Midcontinent rift -A far-field effect of Grenvillian compression: Geology, v 22, p 155-158. Cannon, W. F., Green, A. G., Hutchinson, D. R., Lee, M., Milkereit, B., Behrendt, J. C., Halls, H. C., Green, J. C., Dickas, A. B., Morey, G. B., Sutcliffe, R., and Spencer, C., 1989, The North American Midcontinent Rift beneath Lake Superior from GLIMPCE seismic reflection profiling: Tectonics, v. 8, p. 305-332. Cannon, W. F., Peterman, Z.E., and Sims, P.K., 1993, Crustal-scale thrusting and origin of the Montreal River monocline - a 35 km-thick cross section of the Midcontinent rift in northern Wisconsin and Michigan: Tectonics, v. 12, p. 728-744. Chandler, V. W., and Morey, G. B., 1992, Paleomagnetism of the Early Proterozoic Sioux Quartzite, Southwestern Minnesota - Implications for Correlating Quartzites of the Baraboo Interval: U.S. Geological Survey Bulletin 1904-N, 14 p. Clifton, H. E. and Dingler, J. R., 1984, Wave-formed structures and paleoenvironmental reconstruction: Marine Geology, v. 60, p. 165-198. Craddock, C., Theil, E. C., and Gross, B., 1963, A gravity investigation of the Precambrian of southeastern Minnesota and western Wisconsin: Journal Geophysical Research, v. 68, p. 6015-6032. Daniels, P. A., Jr., 1982, Upper Precambrian sedimentary rocks: Oronto Group, Michigan-Wisconsin: in Wold, R. J., and Hinze, eds., Geology and Tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, p. 107-133. Davidson, D. M., Jr., 1982, Geologic Evidence relating to interpretation of the Lake Superior Basin structure: in Wold, R. J., and Hinze, eds., Geology and Tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, p 5-14. 153 Davis, D. W., and Green,]. C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, v. 34, p. 476-488. Davis, D. W., and Paces,]. B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift System: Earth and Planetary Science Letters, v. 97, p. 54-64. DeCelles, P. G., Langford, R. P., and Schwartz, R. K., 1983, Two new methods of paleocurrent determination from trough cross-stratification: Journal of Sedimentary Petrology, v. 53, p. 629-642. Dickas, A. B., l 986a, Comparative Precambrian stratigraphy and structure along the Mid- Continent Rift: American Association of Petroleum Geologists Bulletin, v. 70, p 225-238. _____, 1986b, Seismological analysis of arrested stage development of the Midcontinent Rift: Geoscience Wisconsin, v. 11, p. 45-52. Dickas, A. B., Mudrey, M. G., Ojakangas, R. W. and Shrake, D. L., 1992, A possible southeastern extension of the Midcontinent Rift System located in Ohio: Tectonics, v. 11, no 6 p. 1406-1414. Dickinson, W. R., 1985, Interpreting provenance relations from detrital modes of sandstones: in Zuffa, G. G., ed., Provenance of Arenites: D. Reidel Publishing Company, p. 333-361. Dickinson, W. R. and Suczek, C. A., 1979, Plate tectonics and sandstone compositions: American Association of Petroleum Geologists Bulletin, v. 63, p. 2164-2182. Du Bois, P. M., 1962, Paleomagnetism and correlation of Keweenawan rocks: Geological Survey Canada Bulletin, v. 71, p. 1-75. Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region: in Baragar, W. R. A., Coleman, L. C., and Hall, J.M., eds., Volcanic Regimes in Canada: Geological Association of Canada Special Paper 16, p. 407-422. _____, 1982, Geology of Keweenawan extrusive rocks, in Wold R. J. and W. J. Hinze, eds., Geology and Tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, p. 47-56. Henry, S. G., Mauk, F. J., and Van der Voo, R., 1977, Paleomagnetism of the upper Keweenawan sediments: the Nonesuch Shale and Freda Sandstone: Canadian Journal of Earth Sciences, v. 14, p. 1128-1138. Hite, D. M., 1968, Sedimentology of the upper Keweenawan sequence of northern Wisconsin and adjacent Michigan [Ph.D. thesis]: University of Wisconsin, Madison, Wisconsin, 217 p. 154 Howard, J. D. and Reineck, H., 1981, Depositional facies of high-energy beach-to- offshore sequence: Comparison with low-energy sequence: American Association of Petroleum Geologists Bulletin, v. 65, p. 807-830. Hubert,]. F., 1962, A zircon-tourmaline-rutile maturity index and the interdependence of the composition of heavy mineral assemblages with the gross composition and texture of sandstones: Journal of Sedimentary Petrology, v. 32, p. 440-450. Hutcheon, I., 1983, Diagenesis 3. Aspects of the diagenesis of coarse-grained siliciclastic rocks: Geoscience Canada, v. 10, p. 4-14. Irving, E., 1979, Paleopoles and paleolatitudes of North America and speculations about displaced terrains: Canadian Journal of Earth Sciences, v. 16, p. 669-694. Kocurek, G. and Fielder, G., 1982, Adhesion structures: Journal of Sedimentary Petrology, v. 52, p. 1229-1241. Kuenen, P. H., 1959, Experimental abrasion fluviate action on sand: American Journal of Science, v. 257, p. 172-190. Le Roux, J. P., 1992, Determining the channel sinuousity of ancient fluvial systems from paleocurrent data: Journal of Sedimentary Petrology, v. 62, p. 283-291. Mack, G. H., 1984, Exceptions to the relationship between plate tectonics and sandstone composition: Journal of Sedimentary Petrology, v. 54, p. 212-220. Manson, M. L., and Halls, H. C., 1997, Proterozoic reactivation of the southern Superior Province and its role in the evolution of the Midcontinent rift: Canadian Journal of Earth Sciences, v. 34, p. 562-575. Massie, K., 1987, Geologic well log no. 050-Ba-257: Wisconsin Geologic and Natural History Survey. McCabe, C., and Van der Yoo, R., 1983, Paleomagnetic results from the upper Keweenawan Chequamegon Sandstone: implications for red bed diagenesis and Late Precambrian apparent polar wander of North America: Canadian Journal of Earth Sciences, v. 20, p. 105-112. Miall, A. D., 1978, Facies types and vertical profile models in braided river deposits: a summary, in Miall, A. D., ed., Fluvial Sedimentology: Canadian Society of Petroleum Geologists, Memoir 5, p. 597-604. _____, 1985, Architectural-element analysis: a new method of facies analysis applied to fluvial deposits: Earth Science Reviews, v. 22, p. 261-308. _____, 1988, Reservoir heterogeneities in fluvial sandstones: lessons from outcrop studies: American Association of Petroleum Geologists Bulletin, v. 72, p. 682-697. 155 _____, 1992, Alluvial deposits, in Walker R. G. and James, N. P., eds., Facies Models: Response to sea level change: Geological Association of Canada, p. 119- 142 Morey, G. B., 1967, Stratigraphy and petrology of the type Fond du Lac Formation, Duluth, Minnesota: Minnesota Geological Survey Report of Investigations 7, 35 p. _____, 1972, Petrology of Keweenawan sandstones in the subsurface of southeastern Minnesota, in Sims, P. K., and Morey, G. B., eds., Geology of Minnesota: A Centennial Volume: Minnesota Geological Survey, p. 436-449. _____, 1980, A brief review of the geology of the western Vermilion district, northeastern Minnesota: Precambrian Research, v. 11, p. 247-265. Morey, G. B., and Ojakangas, R. W., 1982, Keweenawan sedimentary rocks of eastern Minnesota and northwestern Wisconsin, in Wold R. J. and W. J. Hinze, eds., Geology and Tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, p. 135-146. Morey, G. B. and Van Schmus, W. R., 1988, Correlation of Precambrian Rocks of the Lake Superior Region, United States, in Harrison, Jack and Peterman, Zell, eds., U.S. Geological Survey Professional Paper 1241-F, 31 p. Mudrey, M. G. and Brown, B. A., 1988, Summary of field mapping in the Superior map sheet: Open-File Report 88-7, Wisconsin Geological and Natural History Survey, 39 p. Mudrey, M. G., Dickas, A. B., McGinnis, L. D., and Cannon, W. F., 1990, Tectonic ramifications of GNI/Argonne Lake Superior seismic data [abstract]: 36th Institute on Lake Superior Geology, Thunder Bay, p. 73-75. Myers, W. D., II, 197la, The sedimentology and tectonic significance of the Bayfield Group, Wisconsin and Minnesota [abstracts]: 17th Institute on Lake Superior Geology, Duluth, p. 54-55. _____, 1971 b, The sedimentology and tectonic significance of the Bayfield Group (upper Keweenawan?) Wisconsin and Minnesota [Ph.D. thesis]: University of Wisconsin, Madison, Wisconsin, 269 p. Nemec, W., 1988, The shape of the rose: Sedimentary Geology, v. 59, p. 149-152. Ojakangas, R. W., 1972, Archean volcanogenic graywackes of the Vermilion district, northeastern Minnesota: Geological Society of America Bulletin, v. 83, p. 429-442. _____, 1976, Uranium potential in Precambrian rocks of Minnesota: U.S. Energy Research and Development Administration Report No. GJBX-62 (76), 259 p. 156 _____, 1986, Reservoir characteristics of the Keweenawan Supergroup, Lake Superior region: Geoscience Wisconsin, v. 11, p. 25-31. _____, 1994, Sedimentology and Provenance of the Early Proterozoic Michigamme Formation and Goodrich Quartzite, Northern Michigan - Regional Stratigraphic Implications and Suggested Correlations: U.S. Geological Survey Bulletin 1904-R. 31 p. Ojakangas, R. W., and Marsch, C. L., 1982, Minnesota's Geology: Minneapolis, Minnesota., University of Minnesota Press, 255 p. Ojakangas, R. W., and Morey, G. B., 1982a, Proterozoic sedimentary rocks: in Wold R. ]. and W. ]. Hinze, eds., Geology and Tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, p. 83-84. _____, 1982b, Keweenawan pre-volcanic quartz sandstones and related rocks of the Lake Superior region: in Wold R. ]. and W. ]. Hinze, eds., Geology and Tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, p. 85-96. _____, 1982c, Keweenawan sedimentary rocks of the Lake Superior region: a summary: in Wold R.]. and W.]: Hinze, eds., Geology and Tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, p. 157-164. Olsen, H ., Due, P. H., and Clemmensen, L. B., 1989, Morphology and genesis of asymmetric adhesion warts-a new adhesion surface structure: Sedimentary Geology, v. 61, p. 277-285. Owen, D. D., 1847, Report of a geological reconnaissance of the Chippewa land district of Wisconsin, etc.: Senate Exec. Document No. 59, 30th Congress, 1st Session, v. VII, p. 49-85. Pettijohn, F.]., Potter, P.E., and Siever, R., 1987, Sand and Sandstone, 2nd edition: New York, Springer-Verlag, 553 p. Reineck, H.-E., and Singh, I. B., 1980, Depositional Sedimentary Environments, with Reference to Terrigenous Clastics, 2nd edition: Berlin; New York, Springer- Verlag, 549 p. Roy,]. L., 1983, Paleomagnetism of the North American Precambrian: A look at the data base: Precambrian Research, v. 19, p. 319-348. Schmidt, V., and McDonald, P.A., 1979, Texture and recognition of secondary porosity in sandstones: in, Scholle, P.A., and Schluger, P.R., (eds.), Aspects of Diagenesis: Society of Economic Paleontologists and Mineralogists Special Publication No. 26, p. 209-226. 157 Shanmugam, G., 1985, Significance of secondary porosity in interpreting sandstone composition: American Association of Petroleum Geologists Bulletin, v. 69, p. 378-384. Shultz, K. J ., 1980, The magmatic evolution of the Vermilion greenstone belt, NE Minnesota: Precambrian Research, v. 11 , p. 215-245. Sims, P.K. and Viswanathan, S., 1972, Giants Range Batholith: in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume: Minnesota Geological Survery, p. 120-139. Sims, P. K., Schulz, K. J., DeWitt, E., Brasaemle, G., 1994, Petrography and Geochemistry of Early Proterozoic Granitoid Rocks in Wisconsin Magmatic Terranes of Penokean Orogen, Northern Wisconsin: U.S. Geological Survey Bulletin 1904-J, 31 p. Southard, J. B. and Boguchwal, L.A., 1990, Bed configurations in steady unidirectional water flows. Part 2. Synthesis of flume data: Journal of Sedimentary Petrology, v. 60, p. 658-679. Southwick, D. L., 1972, Vermilion Granite-Migmatite Massif: in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume: Minnesota Geological Survery, p. 108-119. Southwick, D. L. and Morey, G. B., 1991, Tectonic Imbrication and Foredeep Development in the Penokean Orogen, East-Central Minnesota-An interpretation based on regional geophysics and the results of test-drilling: US Geological Survey Bulletin 1904-C, 17 p. Suszek, T. J., 1991, Petrography and Sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, Western Lake Superior Region, Midcontinent Rift System [MS thesis]: University of Minnesota Duluth, Duluth, Minnesota, 209 p. Swezey, C., Deynoux, M., and Jeannette, D., 1996, Sandstone depositional environments of the Upper Permian Champenay Formation, Champenay Basin, northeastern France: Sedimentary Geology, v. 105, p. 91-103. Thwaites, F. T., 1912, Sandstones of the Wisconsin coast of Lake Superior: Wisconsin Geological and Natural History Bulletin 25, 117p. Tryhorn, A. D.and Ojakangas, R. W., 1972, Sedimentation and petrology of the upper Precambrian Hinckley Sandstone of east-central Minnesota: in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume: Minnesota Geological Survey, p. 431-435. 158 Tyler, S. A., Marsden, R. W., Grout, F. F., and Thiel, G. A., 1940, Studies of the Lake Superior Pre-cambrian by accessory-mineral methods: Geological Society of America Bulletin, v. 51, p. 1429-1538. Walker, R. G., 1990, Perspectives: facies modelling and sequence stratigraphy: Journal of Sedimentary Petrology, v. 60, p. 777-786. Watts, D.R., 1981, Paleomagnetism of the Fond du Lac Formation and the Eileen and Middle River sections with implications for Keweenawan tectonics and the Grenville problem: Canadian Journal of Earth Sciences, v. 18, p. 829-841. White, W. S., 1972, The Base of the Upper Keweenawan, Michigan and Wisconsin: U.S. Geological Survey Bulletin 1354-F, p. Fl-F23. Wirth, K. R., Vervoort, J. D., and Naiman, Z. J., 1997, The Chengwatana Volcanics, Wisconsin and Minnesota: petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent rift: Canadian Journal of Earth Sciences, v. 34, p. 536-548. Zartman, R. E., Nicholson, S. W., Cannon, W. F., and Morey, G. B., 1997, U - Th - Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, p. 549-561. 159 ( Appendix A Field Data. Headers indicate an outcrop location with the abbreviations OR (Orienta Formation), DI (Devils Island Sandstone), and CH (Chequamegon formation). Each outcrop location lists the geographic location based on the Public Land Survey System. Column 1 is the location number from field notes. The second column indicates hand samples from an outcrop. The third column denotes paleocurrent measurements of trough cross-bedding unless otherwise noted. The first number is the azimuth toward which the cross-bed dips, and the second number is the dip of the cross-bed. For example, 298112 means an azimuth of 298 and a dip of 12 degrees towards an azimuth of 298. TCB axis means trough cross-bed axis (i.e., a complete trough was present in the field and therfore only the axis was measured). The very low bed dips eliminated the need for sterographic rotation of beds. The fourth column indicates bedding in the form of strike and dip. CH-Bayfield SE. 114, Sec. 22; Sec. 23; SE. 114, Sec. 14, T. 50 N., R. 4 W. Sec. 6 and Sec. 7, T. 50 N., R. 3 W. Location 77 Location 78 Location 88 95-88-WI 95-88A-WI Location 89 95-89-WI Location 90 CH-Big Rock SE. 114 and NE. 1/4 Sec. 24, T. 49 N., R. 5 W. Location 108 11/2 135/13 262/7 265/22 160 97119 303/7 95-108-WI 95-108A-WI 95-108B-WI 298/16 304/16 95-108C-WI 95-108D-WI CH-Houghton Point Location 83 Sec. 33, T. 48 N., R. 4 W. 95-83-WI Location 84 Sec. 22 and Sec. 27, T. 49 N., R. 4 W. 95-84-WI 95-84A-WI 157/6 Location 85 Sec. 21 T. 49 N., R. 4 W. CH-Iron River NE. 114 Sec. 36, T. 49 N., R. 9 W. Location 107 95-107-WI 95-107A-WI CH-Little Sand Bay N. 1/2 Sec. 32, T. 52 N., R. 4 W. Location 116 95-l 16LSB1-WI 95-l 16LSB2-WI CH-Manitou Island Sec. 24, T. 52 N., R. 3 W. Sec. 19, T. 52 N., R. 2 W. Location 119 95-119-WI 95-l 19A-WI 161 CH-Raspberry Island Sec. 24, T. 52 N., R. 4 W. Location 118 95-118-WI 95-l 18A-WI 95-l 18B-WI 95-118C-WI CH-Stockton Island SE. 114 Sec. 33, T. 52 N., R. 2 W. Location 120 95-120-WI 95-120A-WI 95-120B-WI CH-Van Tassells Point Sec. 33 and Sec. 34 , T. 50 N., R. 4 W. Location 86 95-86-WI Location 87 95-87-WI 95-87A-WI 95-87B-WI CH-Washburn Sec. 19; Sec 18; Sec. 7; Sec. 4, T. 48 N., R. 4 W. Location 82 95-82-WI Location 91 95-91-WI 95-91A-WI 95-91B-WI Location 92 Location 93 95-93-WI 95-93A-WI 95-93B-WI 95-93C-WI 162 DI-Cornucopia W. 1/2 Sec 35 and NE. 1/4 Sec. 34, T. 51 N., R. 6 W. Location 98 150/11 Bedding 196/4 7919 63/6 44/4 77/11 88/10 34/4 172/17 200/3 6215 86/12 95-98B-WI 181/4 85/8 127/8 6516 204/6 190/1 181/2 95-98C-WI 95-98D-WI 95-98£-WI 111/7 114/7 68/24 113/9 76/21 28/5 55/8 88/26 5117 15/4 115/3 84/2 Bedding 13/2 49/4 115/6 11412 103/3 95-98-WI 95-98A-WI 163 DI-Devils Island Sec. 10 and Sec. 15, T. 53 N., R. 3 W. Location 110 162/28 95-110-WI 95-1 lOA-WI Location 11 OA Location 11 OB 95-1 lOBl-WI Bedding 43/2 Bedding 48/2 131/4 134/2 95-110B2-WI 95-110B3-WI 172/1.5 155/4.5 Location 11 OC 240/1 130/4 166/6 95-1 lOCl-WI Location 11 OD 60/1 95-1 lODl-WI 95-110D2-WI 95-110D3-WI 95-110D4-WI 95-11005-WI Location 11 OE 95-1 lOEl-WI Location 11 OF 95-1 lOFl-WI Location 11 OG 95-1 lOGl-WI Location 11 OH 95-1 lOHl-WI Location 11 OI 165/25 95-1101-WI Location 11 OJ 95-110]1-WI Location 11 OK 128/11 95-1 lOKl-WI 95-110K2-WI Location 11 OL 164 95-1 lOLl-WI Location 11 OM 95-1 lOMl-WI 95-110M2-WI Location 11 ON 95-1 lONl-WI 95-110N2-WI 95-110N3-WI Location 1100 Location 11 OP 95-1 lOPl-WI 95-l 10P2-WI 95-110P3-WI Bedding 65/1 Bedding 346/2 Bedding 202/ 1 Bedding 23611 Bedding 217/3 Bedding 24/4 Bedding 344/2 Bedding 14/3 Bedding 19/5 Bedding 254/3 Bedding 181 /2 Bedding 47/3 Bedding 27/1 Bedding 288/2 Location 11 OQ Bedding 102/3 Bedding 96/4 Bedding 346/9 Location 11 OR Bedding 84/2 Location 11 OS Bedding 12/ 6 95-11051-WI Location 11 OT 78/15 134/13 DI-Iron River NE. 1/4 NE. 1/4 Sec. 25, T. 49 N., R. 9 W. Location 101 18/1 7717 210/12 165 152/4 20516 183/7 173/8 165/8 219/12 205/1 194/3 225/10 Bedding 215/12 95-101-WI 95-lOlA-WI Dl-Lenfoot and May Ledges NE. 114 Sec. 22 and SE. 1/4 Sec. 15, T. 48 N., R.10 W. Location 45 Location 46 95-46-WI 95-46A-WI Location 47 284/3 Location 47A 193/2 95-47-WI 199/7 Location 48 17/10 27/13 54/13 350113 359/22 346/23 Location 55 95-55-WI 181/4 Location 56 95-56-WI 66/1 220/6 340/4 112/4 Location 56A 202/5 308/5 29017 Location 57 38/8 166 23116 Location 57A Bedding 320/6 Location 58 95-58-WI DI-Sand Island Sec. 12; E. 112 Sec. 24; Sec 25; E. 112 Sec. 13, T. 52 N., R. 5 W. Sec. 18, T. 52 N., R. 4 W. Location 117 Sand Island, All locations by letter Location 117A 95-117Al-WI 80/7 95-117A2-WI 108/4 113/2 Location 117B 95-117Bl-WI 95-117B2-WI Location 117C Location 117D 95-117Dl-WI 95-11702-WI Location 117E 95-117£1-WI Location 117F 95-117Fl-WI 96128 Location 117G 95-117Gl-WI Location 117H 95-117Hl-WI 15/12 95-117H2-WI Location 117I 95-11711-WI Location 117] DI-Squaw Bay Sec. 19 and Sec. 18, T. 51 N., R. 5 W. Location 115 Bedding 21/3 Bedding 144/8 Bedding 338/5 167 Bedding 102/3 95/4 95-115Al-WI 95-115A2-WI OR-Amnicon Falls Sec. 29, T. 48 N., R. 12 W. Location 1 through 14 91129 TCB Axis 3117 95-0-WI 116113 109/5 142/20 143/6 16/9 159113 155119 26/23 55/19 225/9 107/14 104/12 45/10 2517 151111 288/28 355114 25/18 163/35 92/30 237/26 207/10 136/6 234/7 334/26 92/4 Bedding 222/7 60/12 39/32 343/3 326/15 355/3 114/5 Bedding 32/14 Bedding 124/6 Bedding 155/12 168 86/23 155116 30/9 Bedding 262/10 5715 308/18 22/22 221113 185/13 85/12 239/13 36/4 40/14 115/7 110/6 15/19 117/20 0124 9/32 141/14 96/23 83/8 148/22 158/6 248/13 323/8 154/5 81112 41111 117/10 355/20 102/19 98/8 9614 6017 Bedding 58/10 258/13 42/1 45/2 104/5 104/23 326/11 122/32 119/6 152/22 349/25 125/17 120/10 169 146/8 73/7 221112 234/26 70/11 63/20 95-13-WI 347/27 340/37 326/24 132/12 350/11 33/8 7714 Bedding 127/9 Bedding 11811 2 350/12 95-1-WI 95-lA-WI 95-2-WI 356/21 215/4 60/4 95-3-WI 95-4-WI 71110 94/6 43/10 82/9 82/20 29719 91113 127/11 Bedding 285/5 OR-Bark p 01nt· Sec. 24, T. 51 N., R. 7W. Location 113 96/24 73/27 95-113-WI 106/19 94/21 95-l 13A-WI 95-l 13B-WI 95-l 13C-WI 123/4 170 0 R-Black River E. 112 Sec. 21, T. 47 N., R. 14 W. Locations 79 through 80, 96 through 97 95-79A-WI 95-79B-WI 95-79C-WI 95-79Float-WI 95-79D-WI 56/11 312/10 77/3 165/23 230/10 159/6 6015 205/32 120/19 Bedding 313/8 95-79-WI 40/5 224/6 54/7 201/42 21115 95-80-WI 15/36 44/31 63/21 1147 347/32 72119 20/30 95-96-WI 355/35 32/23 13/24 18/27 95-96A-WI 95-96B-WI 15/26 359/24 357/26 17/26 11/26 7140 8/30 171 17/48 344/32 95-96C-WI 95-96D-WI 95-97-WI 0 R-Brule River W. 1/2 Sec 2 and E. 1/2 Sec. 3, T. 48 N., R. 10 W. W. 1/2 Sec. 35 and E. 1/2 Sec. 34, T. 49 N., R. 10 W. Locations 39 through 44 95-39-WI 95-39A-WI OR-Copper Creek SE. 114 Sec. 15, T. 47 N., R. 14 W. Locations 15 through 24 95-5-WI 95-6-WI 343/6 102/18 9214 169/7 Bedding 168/5 91114 150/3 184/2 111/15 154/4 Bedding 162/6 95-7-WI 95-7A-WI 266/1 Bedding 27113 95-8-WI Bedding 260/7 Bedding 316/2 22616 288/11 242/3 284/6 24113 273/3 Bedding 187 /2 Bedding 109/3 147/7 168/14 172 175/15 95-9-WI 135/6 95-10-WI Bedding 166/12 95-11-WI 159/4 95-12-WI OR-Herbster N. 1/2 Sec. 7; Sec 6; NW. 1/4 Sec. 8, T. 50 N., R. 7 W. N. 1/2 Sec. 12, T. 50 N., R. 8 W. Location 111 139/4 126/12 165/5 70/21 87/14 8017 104/28 95-111-WI 95-11 lA-WI 95-1 llB-WI 95-11 lC-WI Location 112 139/7 164/4 95-112-WI 118/16 108/23 76127 134/14 95-l 12A-WI 95-l 12B-WI 95-l 12C-WI 95-l 12D-WI 95-l 12E-WI 95-l 12F-WI 33/2 100/9 109/23 133/9 0 R-lron River E. 1/2 Sec. 15, T. 49 N., R. 9 W. Locations 102 through 106 173 75/10 9917 95-102-WI 95-102A-WI 108/20 67/17 68/26 96/13 Bedding 99/6 72121 70125 Bedding 32/9 84/23 65120 76/24 21119 15/20 79/22 30/30 32/13 54/14 35/15 44/12 56/22 11114 40/22 47/24 27/17 95-103A-WI 95-103B-WI 95-104-WI 95-104A-WI 69115 44124 128/17 67/14 52/19 74/23 100/14 95-104C-WI 62/10 12/25 75/21 100/4 8117 19/18 143/8 350/19 174 356/19 48/8 38/19 55119 11114 6619 101/19 109/19 119/8 117115 82/11 149/18 124/17 77/12 78/17 166/10 71111 95-106-WI OR-Lake Creek SW. 1/4 Sec. 8 and NW. 114 Sec. 17, T. 48 N., R. 11 W. Locations 49 through 54 63/16 65/20 95-49-WI 95-50-WI 112125 123/23 20/14 95-52-WI 89/2 95-53-WI 95-54-WI 95-54A-WI · OR-Middle River/Hwy 13 SW. 114 Sec. 7, T. 48 N., R. 11 W. SE. 1/4 Sec. 12 and NE. 114 Sec. 13, T. 48 N., R. 12 W. Locations 35 through 38 95-35-WI Bedding 215/5 95-36-WI 95-36A-WI 95-36B-WI Bedding 245/4 Bedding 109/25 175 Bedding 115114 Bedding 324/6 95-38-WI OR-Middle River/Moonshine Road SE. 114 Sec. 24 and NE. 114 Sec. 25, T. 48 N., R. 12 W. Locations 25 through 34 95-14-WI 99/25 142/5 97/23 95-15-WI Bedding 90/ 10 95-16-WI 95-16A-WI 95-16B-WI 95-27-WI 178/66 95-27A-WI 95-27B-WI Bedding 179/75 95-28-WI 95-29-WI 95-29A-WI Bedding 219/69 95-30-WI 95-30A-WI 95-31-WI 95-31A-WI Bedding 181/64 95-32-WI 95-32A-WI 95-32B-WI 95-32C-WI 95-32D-WI 95-33-WI 95-33A-WI OR-Orienta Falls NW. 114 Sec. 10; SW. 114 SW. 114 Sec. 3; SE. 114 Sec. 4, T. 49 N., R. 9 W. Location 75 95-75-WI Location 76 95-76-WI 95-76A-WI 95-76D-WI 176 95-76Clasts-WI 68/17 76/12 332/9 147/3 84/6 57/21 20/9 11111 53/13 126/9 33/21 32/12 6015 138/14 349/12 95-76C-WI Location 99 150/2 89/15 15511 75/14 106119 95-99-WI 42/2 150/5 28/17 205/3 30/4 78/7 OR-Port Wing Sec. 35; NW. 114 Sec. 36; Sec. 25, T. 50 N., R. 9 W. Locations 60 through 73 95-60-WI 95-60A-WI 29017 95-60B-WI 95-61-WI 95-61A-WI 95-62-WI 95-62A-WI 95-62B-WI 95-62C-WI 95-62D-WI 95-62E-WI 95-62F-WI 177 95-62G-WI 95-62I-WI 95-63-WI 95-64-WI 119/13 95-65-WI 95-66-WI 95-67-WI 60120 56/15 89/16 88/20 57/17 71114 102/15 103/12 97/12 132/6 81115 89/17 131111 342/22 100/7 95-70-WI 372/24 338/15 337/21 45/1 42/20 55/12 33114 358/15 338/10 95-72-WI OR-Quarry Point Sec. 19 and Sec. 30, T. 50 N., R. 8 W. Locations 74, 94 through 95 95-74-WI 89/15 32/4 115/26 Bedding 130/25 TCB axis 103 DIP=? TCB axis 83 DIP? 80/22 56/17 178 Bedding 10114 95-74A-WI 64/25 31/23 44/20 85/12 86/25 95-94-WI 119/7 77/10 115/11 103/14 130/11 90/14 95-94-WI 95-94A-WI 99/10 95/17 120/28 124/25 95-95-WI . 95-95A-WI 95-95B-WI 95-95C-WI OR-Roman Point Sec. 29 T. 51 N., R. 6 W. Location 114 12/15 38/28 72/23 95-114-WI 38/18 57/12 97/22 93/12 130/16 102/17 8/21 43/16 7/14 318/26 25/20 9217 84/12 22/18 23/9 179 95-114A-WI 0 R-Sand Island Sec. 12, T. 52 N., R. 5 W. Location 117K 95-117Kl-WI 85/9 OR-Twin Falls NE. 114 Sec. 32, T. 50 N., R. 8 W. Location 100 95-100-WI 95-lOOA-WI 88/4 336/17 82/9 88/11 129/2 57/3 58/13 6118 192/10 7/17 1111 104/10 63/8 84/15 189/14 57/6 65/15 21113 180 \ N 0 5 10 15 20 Miles 0 5 10 15 20 25 30 Km A Chequamegon Fm. Devils Island Ss. Orienta Fm. Appendix B. Outcrop locations. Triangles represent geologic well log locations. 181 Devils Ishµid North Twin Island .,,., / ---11of\_u- . ? / .... 0 D 1-- __ , Formation Contact 1------1 Fault Contact D Outcrop Location Appendix B. 182 Quartz Feldspar Rock Fraements -0 ... .ll c -0 j u u s. !! c -E d 8 !j -0 -0 -0 er .. ] !! ..c !! ..c !! -0 -;; d. c '(' ..,:, -0 g. u !! !! !! !! u "E u u u u .. "e -;; ..c ..c > 0.." c. c. u u "" e 0 0 0 u u "u u 0.. -;; ..c 8 8 :.;:; ·;;; e e e 1 1 -5 u u "B flu flu flu :.;:; :.;:; e 0 0 0 1 0 u u c: 0 u u ':i ':i u u ':i u u V).. j ix: ;:;:: V) V) u "'"' " E- 8 0 ::> E- ::E ::E ::E ::E ::E ::E I- 302 "'"'I 5 308 9 I 54 9 "'"'I "'"' 4 78 103 15 10 6 I 7- 376 5 2 383 4 2 14 I 41 62 12 5 16- 305 9 2 316 3 52 7 62 67 7 II 6 5 4 I 33A 251 2 3 256 6 20 7 65 98 36 12 36A 236 23 259 5 57 2 64 90 23 11 11 2 8 49- 261 I 262 14 3 40 14 21 92 75 21 2 10 2 54- 279 4 283 II 67 I 3 82 81 15 8 8 I 2 66- 262 3 21 286 17 38 I 9 65 63 12 I 5 9 I I I I 76- 353 I II 365 16 22 18 56 41 4 I I 94- 325 18 343 II 4 53 I 16 85 39 6 I 2 96- 267 I 8 276 9 24 91 124 41 I 4 4 IOOA 329 5 334 12 12 3 87 114 II l02A 383 13 396 I 12 13 I 106- 407 10 417 7 18 2 27 54 8 4 1118 233 2 13 248 38 9 57 II 13 128 46 14 9 5 4 2 2 1138 298 12 3 313 19 I 75 9 2 106 60 I 3 4 5 114A 383 8 I 392 4 3 8 3 13 31 7 I I I 117KI 236 I 237 13 9 2 72 96 19 I I- 50.4 0.2 0.8 51.4 1.5 0.2 9.0 1.5 0.2 0.7 13.0 17.2 2.5 1.7 1.0 7- 63.I 0.8 0.3 64.3 0.7 0.3 2.3 0.2 6.9 10.4 2.0 0.8 16- 50.9 1.5 0.3 52.8 0.5 8.7 1.2 10.4 11.2 1.2 1.8 1.0 0.8 0.7 0.2 33A 41.9 0.3 0.5 42.7 1.0 3.3 1.2 10.9 16.4 6.0 2.0 36A 40.3 3.9 44.3 0.9 9.7 0.3 10.9 15.4 3.9 1.9 1.9 0.3 1.4 49- 43.4 0.2 43.6 2.3 0.5 6.7 2.3 3.5 15.3 12.5 3.5 0.3 1.7 0.3 54- 46.6 0.7 47.2 1.8 11.2 0.2 0.5 13.7 13.5 2.5 1.3 1.3 0.2 0.3 66- 43.6 0.5 3.5 47.6 2.8 6.3 0.2 1.5 10.8 10.5 2.0 0.2 0.8 1.5 0.2 0.2 0.2 76- 59.1 0.2 1.8 61.1 2.7 3.7 3.0 9.4 6.9 0.7 0.2 0.2 94- 54.4 3.0 57.5 1.8 0.7 8.9 0.2 2.7 14.2 6.5 1.0 0.2 0.3 96- 44.3 0.2 1.3 45 .8 1.5 4.0 15.l 20.6 6.8 0.2 0.7 0.7 IOOA 55.1 0.8 55.9 2.0 2.0 0.5 14.6 19.l 1.8 102A 65.4 2.2 67.6 0.2 2.0 2.2 0.2 106- 67.7 1.7 69.4 1.2 3.0 0.3 4.5 9.0 1.3 0.7 1118 38.8 0.3 2.2 41.3 6.3 1.5 9.5 1.8 2.2 21.3 7.7 2.3 1.5 0.8 0.7 0.3 0.3 1138 49.7 2.0 0.5 52.3 3.2 0.2 12.5 1.5 0.3 17.7 10.0 0.2 0.5 0.7 0.8 114A 63.9 1.3 0.2 65.4 0.7 0.5 1.3 0.5 2.2 5.2 1.2 0.2 0.2 0.2 117KI 39.3 0.2 39.4 2.2 1.5 0.3 12.0 16.0 3.2 0.2 Appendix C-1 . Orienta Formation modal analyses. Counts are listed in upper field and percentages in lower. 183 Cement and Matrix "c: g 0 c: ""c: .D " a u"0 .... e 0 13 "c: ..c: u "0 c c"' -u " u " :i: w ::> c: c c c c " u 0 e tl u u u e e 5 ..c: 11 g :; " fl .... :.a 0 0 0 e e eu eu " ..c: .s 0 .... 0 Vl" u 0 !-< i:i:i u 8 N !-< 0 u" u" u :::E u .. !-< & ...... -l d u.. ..J 9 143 I I 33 10 25""' 529 599 57.I 14.7 28.2 59.0 15.I 25.9 7 24 I 2 73 51 469 596 80.6 13.2 6.2 82.4 13.6 4.1 9 110 I 10 3 97 488 599 62.9 12.7 24.4 65 .6 13.I 21.3 15 I 64 l 6 139 5 30 418 599 60.0 23.4 16.5 63.1 24 .4 12.4 16 161 2 2 8 25 64 484 585 48.8 13.2 38.0 55 .0 14.0 31.0 12 122 6 I 24 25 69 476 601 54.8 19.3 25 .8 56.4 20.0 23 .5 10 125 3 12 28 66 490 599 56.9 16.7 26.3 58.8 17.3 23.9 II 104 I 7 4 5 19 110 455 601 57.6 14.3 28 .1 64.2 14.9 20.9 7 54 3 44 7 68 475 597 74.3 11.8 13.9 77.0 11.9 11.2 4 5 57 2 30 80 485 597 67.0 17.5 15.5 70.9 17.6 11.5 II 61 I 4 I 40 39 3 54 461 603 57.9 26.9 15.2 60.4 27.2 12.4 8 19 I 25 17 87 467 597 70.4 24.4 5.1 71.5 24.4 4.1 4 5 I II 2 6 152 414 586 92.5 3.1 43 95.8 3.2 l.O 2 14 2 I 5 5 II 92 485 601 83.9 11.1 4.9 86.5 11.4 2.1 I 14 97 I I 50 13 63 473 601 49.3 27.I 23.7 53.I 27.4 19.5 3 76 I 2 8 93 495 599 60.2 21.4 18.4 63.6 21.6 14.7 5 15 I 9 4 16 131 438 599 87.7 7.1 5.3 89.6 7.2 3.2 2 22 12 2 139 19 40 34 354 601 66.7 27.I 6.5 66.9 27.1 6.2 Averages 66.0 17.0 17.0 68 .9 17.3 13.8 l.5 23.9 0.2 0.2 5.5 l.7 4.2 88.3 l.2 4.0 0.2 0.3 12.2 8.6 78.7 l.5 18.4 0.2 l.7 0.5 16.2 81.5 2.5 0.2 10.7 0.2 l.O 23.2 0.8 5.0 69.8 2.7 27.5 0.3 0.3 1.4 4.3 10.9 82.7 2.0 20.3 l.O 0.2 4.0 4.2 11.5 79.2 l.7 20.9 0.5 2.0 4.7 11.0 81.8 1.8 17.3 0.2 1.2 0.7 0.8 3.2 18.3 75.7 l.2 9.0 0.5 7.4 l.2 11.4 79.6 0.7 0.8 9.5 0.3 5.0 13.4 81.2 l.8 10.I 0.2 0.7 0.2 6.6 6.5 0.5 9.0 76.5 1.3 3.2 0.2 4.2 2.8 14.6 78.2 0.7 0.9 0.2 l.9 0.3 1.0 25.9 70.6 0.3 2.3 0.3 0.2 0.8 0.8 1.8 15.3 80.7 0.2 2.3 16.I 0.2 0.2 8.3 2.2 10.5 78.7 0.5 12.7 0.2 0.3 l.3 15.5 82.6 0.8 2.5 0.2 1.5 0.7 2.7 21.9 73.I 0.3 3.7 2.0 0.3 23.I 3.2 6.7 5.7 58.9 Appendix C-1, continued. 184 IQu:ua Fclds ar Rock Fragmenu 46- 12 460 0 58· 33 475 26 101- 140 II '51 11081 378 11 389 27 27 11082 367 21 388 13 13 11083 422 27 449 21 21 I IOCI 404 28 432 I I 11001 334 13 347 90 93 11002 355 54 409 12 18 29 11003 3n 33 410 28 37 13 11004 334 32 373 60 60 21 11005 348 72 420 42 45 17 llOOL 399 39 438 21 21 13 llOE 1 411 37 449 25 25 llOFI 382 35 121 21 21 1 IOGI 390 37 427 40 40 11 llOHI 418 27 448 11 011 401 23 426 0 llOJI 418 31 449 23 1 IOK I 455 24 482 4 1 IOK2 439 41 480 I llOLI 407 33 440 37 40 1 IOMI 460 64 531 llOM2 391 14 55 460 I JONI 472 1 10 486 llON2 -454 12 467 llON5 460 5 465 1 IOPJ 470 478 I IOP2 140 16 460 I 17AI 103 11 415 117A2 140 19 460 11 781 152 4 456 11 782 160 19 479 11701 433 9 442 11702 403 12 416 5 11 117 El 398 2 400 10 40 58 l 17Fl 381 11 394 26 40 117GI 370 12 382 ,. 31 10 I 17Hl 395 11 406 15 6 I 17H2 421 17 439 4 4 11711 10') 9 418 46- 74 .8 2.0 76.8 1.0 58- 73.3 0.3 5.5 79.2 1.2 3.2 4.3 0.5 0.7 101- 73.3 1.8 75.2 0.3 0.3 0.3 11081 64.2 1.9 66.0 4.6 4.6 11082 62.8 3.6 66.4 2.2 2.2 0.3 0.3 11083 70.8 1.5 75.3 3.5 3,5 llOCl 69.9 4.8 74 .7 0.2 0.2 0.2 0.2 0.5 11001 55 .8 2.2 57.9 0.5 15.0 15 .5 0.2 0.2 11002 60.0 9.1 69. 1 0.8 0.2 2.0 3.0 1.0 4.9 11003 62 .9 5.5 68.4 0.3 0.3 0.2 4.7 0.3 0.3 6.2 0.3 2.2 0.3 IJOD4 55 .9 1.2 5.1 62.1 10.0 10.0 0.8 0.2 3.5 0.3 1.5 0.2 11005 58.2 12.0 70.2 0.5 7.0 7.5 0.2 2.8 1IODL 66.8 6.5 73.4 3.5 3,5 0.5 2.2 0.5 I IOE I 69 .5 0.2 6.3 76.0 4.2 4.2 1.4 ll OFI 61.2 0.7 5.9 70.8 3.5 3,5 1.0 llOGI 65.1 6.2 71.6 6.7 6.7 1.8 llOHI 71.1 0.2 4.6 0.3 76.2 11011 69.5 1.0 0.3 73.8 llOJJ 69.7 5.2 74.8 2.3 1.5 3.8 0.2 I !OKI 75.8 0.5 1.0 80.3 0.7 0.7 I IOK2 73.5 6.9 80.1 0.2 0.2 0.2 I JOLI 68.1 5.5 73.9 0.5 6.2 6.7 llOM! 76.7 1.2 10.7 88.5 I IOM2 65 .2 2.3 9.2 76.7 0.2 0.2 I IO N \ 79.1 0.7 1.7 81. 1 0.2 0.2 0.3 l !ON2 75 .7 0.2 2.0 77.8 0.2 0.2 0.5 l ION5 76.7 0.8 n.5 0.2 0.2 0.3 0.3 0.2 I IOP I 78.2 1.3 79.5 0.8 0.8 0.8 0.2 11 OP2 73.3 0.7 2.7 76.7 0.2 0.2 1.2 l 17Al 67.3 0.2 1.8 69.3 0.2 0.2 0.3 0.7 0.2 l 17Al 73.3 0.2 3.2 76.7 0.7 0.7 0.2 0.2 11781 71.1 0.7 74.8 0.2 0.5 0.7 0.2 11 782 76.8 3.2 80.0 0.2 0.2 0.5 0.2 11 701 72.3 1.5 73.8 0.2 0.5 0.7 0.8 11702 67.2 0.2 2.0 69.3 0.7 0.3 0.8 1.8 0.3 117 El 66.3 0.3 66.7 ).0 1.7 0.3 6.7 9.7 1.0 117Fl 63.5 0.3 1.8 65 .7 1.0 1.3 1.3 6.7 0.8 0.2 0.3 117Gl 61.7 2.0 63.7 0.5 1.2 4.0 5.7 1.7 0.2 117HI 66. 1 1.8 67.9 0.3 0.2 0.5 1.5 2.5 1.0 0.2 0.2 11 7H2 70.6 0.2 2.9 73.7 0.7 0.7 0.7 0.3 11711 68.2 1.5 69.7 0.2 0.7 0.8 0.3 0.2 Appendix C-2. Devils Island Sandstone modal analyses. Counts are listed in upper field and percentages in lower. 185 Ccmt'nt :i nd M:i uix i1 2 ] .e . .• & 0 £ \J u • j '§" 2 ::i 8 n -a .• """ " , c g c .. c \J i i a"' ]" § -e ·.s -e 1£ a j j u c5 ,'! o g N c5 .:! .:! u :!1 .:! i. ,'! & .... Cl .... 6 12 I 2 16 10 98" 472 599 94.9 0.0 5.1 97.5 0.0 2.5 7 14 I 20 4 7 53 515 MO BSA 5.0 9.5 92.7 5.J 1.9 5 7 6 7 l 124 4(.0 600 95.7 0.4 J .9 98.0 0.4 1.5 l l I I 93 13 62 419 589 90.2 6.4 3.3 92.8 6.4 0.7 5 9 47 I 17 109 41 0 584 89.S 3.2 7.J 'JS.O 3.3 1.7 5 5 18 24 II 475 596 88.8 4.4 6.7 94 .S 4.4 I.I l I 9 40 8 '"'88 442 578 91.4 0.2 8.4 97.7 0.2 2.0 9 ii I Ill 22 14 451 599 74 .1 20.6 5.J 77.1 20.7 2.2 2 37 l 19 9 97 464 592 76.5 3.9 19.6 92.3 5.9 1.7 5 6 28 I I 42 18 62 475 599 79.4 7.8 12.8 88.1 8.7 3.2 4 I 44 I I 79 24 16 4n 59" 70.0 12.6 17.4 8 1.1 t-4 .0 4.8 I 19 2 12 II ,,. 23 484 598 71.9 9.J 18.8 89.1 10.S 0.4 4 23 6 18 39 52 482 597 82.8 4.4 12.9 92.7 5.2 2.1 l II II 4 22 69 485 591 84 .7 5.2 JO.I 93.7 5.7 0.6 4 10 I 17 10 22 15 78 452 595 84.5 4.6 10.8 94 .0 5.1 0.9 2 l l 14 2 l l 87 480 596 81.3 8.l 10.4 90.5 9.1 0.4 6 6 39 I 94 454 588 92.S 0.0 7.5 98.7 0.0 J.l l l J I 6 Ill 429 5n 93.9 0.0 6.1 99.3 0.0 0.7 I I 8 16 l 99 m 600 88.4 4.9 6.8 94.9 4.9 0.2 0 15 28 9 7 55 486 600 93.6 0.8 S.6 99.2 0.8 0.0 2 l 2 10 9 84 484 597 90.7 0.2 9. 1 99.3 O.J 0.4 I I 2 •I 4 22 85 481 595 84.6 8.J 7.1 91.5 8.l 0.2 I I I 14 I 2 50 532 (.00 86.5 0.0 13.S 99.8 0.0 0.2 l l 2 8 I 125 464 600 84.3 0.2 15 .5 99. 1 0.2 0.6 2 10 2 4 92 489 597 96. S 0.2 J.J 99.4 0.2 0.4 I 4 29 I 98 472 600 96.2 0.2 3.6 98.9 0.2 0.8 l 6 19 2 21 5 80 473 (.00 97.3 0.4 2.3 9R.3 0.4 l.l l 9 l 36 23 47 492 (,O J 95.5 1.0 3.5 97.2 1.0 1.8 2 9 I 24 4 I 100 470 600 93.6 0.2 6.2 97.9 0.2 1.9 6 7 4 16 14 7 8 5 11 9 426 599 94.6 0.9 4.5 97.4 0.9 1.6 3 5 19 8 I 103 469 600 93.8 0.9 5.3 98. 1 0.9 I. I I 2 II 17 6 1 5 I 53 462 li JO 97.8 0.9 1.3 98.7 0.9 0.4 2 6 18 l 92 486 599 94.7 0.2 5. 1 98.7 0.3 1.0 2 7 I 4 23 l 115 453 599 95.6 0.9 3.5 97.6 0.9 1.5 8 10 I 6 17 4 135 437 (.00 92.2 2.5 5.J 95.2 2.5 2.3 14 2 I I 68 10 46 472 600 84.3 12.3 J.4 84 .7 12.3 3.0 •I 9 I I 30 17 II l 94 443 (.00 86.0 9.0 5.0 88.9 9.0 2.0 I 12 I ZR I 7 2 133 .,. (.00 86.4 7.9 S.li 89.3 7.9 2.8 3 3 14 7 I 10 4 15 126 435 598 90.8 3.4 5.7 93.S 3.5 3.0 I 7 26 6 2 112 450 596 93.6 0.9 S.li 97.8 1.0 I.I I 4 l I 20 33 2 5 109 427 (.00 95.8 1.2 J .O 97.9 1.2 0.9 Avt'ragn: 88.8 3.8 7.S 94.6 4.0 IA 1.0 2.0 0.2 0.3 2.7 1.7 16.4 78.8 1.2 2.3 0.2 3.3 0.7 1.2 8.8 85 .8 0.8 1.2 1.0 1.2 o.s 20.7 76.7 0.5 0.5 0.2 0.2 IS.8 2.2 10.S 71.1 0.9 l.S 8.0 0.2 2.9 18.7 70.2 0.8 0.8 3.0 4.0 1.8 11 .4 79.7 o.s 0.2 t.6 6.9 1.4 IS.2 76.S LS 1.8 0.2 18.S 3.7 2.3 75.J 0.3 6.3 o.s 3.2 l.S 16.4 78.4 0.8 1.0 4.7 0.2 0.2 7 .0 3.0 J0.4 79.3 0.7 0.2 7.4 0.2 0.2 13.2 4.0 2.7 7'JJI 0.2 3.2 0.3 2.0 1.8 11 .0 3.8 80.9 0.7 3.9 1.0 3.0 6.S 8.7 80.7 o.s 1.9 J.9 0.7 3.7 11.7 82.1 0.7 1.7 0.2 2.'J t.7 3.7 2.S 13.I 76.0 0.3 2.2 2.3 0.3 2.2 14.6 80.S 1.0 1.0 6.6 0.2 16.0 77.2 0.5 0.5 S.4 1.0 19.2 74.4 0.2 0.2 1.3 2.7 o.s 16.S 78.8 2.5 4.7 1.5 1.2 9.2 8 1. 0 0.3 0.5 0.3 l.l 1.7 1.5 14.1 81.l 0.2 0.2 0.3 0.2 0.7 3.7 14.3 80.8 0.2 0.2 0.2 2.3 0.2 0.3 8.3 88.7 0.5 0.5 0.3 1.3 0.2 20.8 77.3 0.3 1.7 0.3 0.7 15.4 8 1.9 0.2 0.7 4.8 0.2 16.3 78.7 0.5 1.0 3.2 0.3 3.S 0.8 13.3 78.8 0.5 1.5 o.s 6.0 3.8 7.8 81.9 O.J 1.5 0.2 4.0 0.7 0.2 16.7 78.J 1.0 1.2 0.7 2.7 2.3 1.2 1.3 0.8 19.9 7 1.1 0.5 0.8 3.2 1.3 0.2 17.2 78.2 0.2 O.J 1.8 2.8 10.0 0.8 0.2 8.7 75.7 O.J 1.0 3.0 O.S IS.4 HI.I O.J 1.2 0.2 0.7 3.8 o.s 19.2 75.6 J.l 1.7 0.2 1.0 2.8 0.7 22.s 72.8 1.3 2.3 0.3 0.2 0.2 11 .3 1.7 7.7 78.7 0.2 1.5 0.2 0.2 s.o 2.8 1.8 O.S IS.7 73.8 0.2 2.0 0.2 4.7 0.2 1.2 0.3 22.2 7 1.3 0.5 o.s 2.3 1.2 0.2 1.7 0.1 2.s 21 .t 72.7 0.2 1.2 4.4 1.0 0.3 18.8 75.5 0.2 0.7 0.5 0.2 J.3 s.s 0.3 0.8 18.2 71.2 Appendix C-2, continued. 186 Quartz Feldspar Rocle Frae mencs -0 c:u 8 -0... =ii ll ..E ..c: 8. ·2 "c: B ci ci ·g 8 -0 -0 -0 :.a .... -;:) -;:) ll ..c: ll ..c: -0 -;a d.. ::.::: g ::I c: El t -0 IJ -0 ll ll u -a u .; .; u E lil -;a <.':: -;a <.':: -;a ..c: ll ;z, ·a .;" .; .; ;.§ tl ... '§ g "§ ..c ..c ·a ·a ·a 6 6 -;a ; .; .; .; .; .; <.':: -;:) u rl 8 8 8 c: =ii :q :q :q c: c: ::I ::I ::I { i:l i:l u -0 ll u 0 0 0 0 0 c: c: ..... 0 ·a & -0 -0 -0 :.a :.a .... > ..c: ..c: > 0. 0. 0. u u a. E -;a 0 0 0 8 8 u u u u E E E rl -5 ..c: u u 'ti 11 ·;;; a a a 0 1 1 c: u u u 'ti 'ti 0 0 1 0 u u u u u u u VJ u !-< ;:;:: 0 c5 :::> r:: ...... ::E ::E ...... ::E ::E ::E ::E VJ VJ 82- 372 ""'I 15""' 388 6 23 ""' ""' 37 66 7 83- 301 2 303 25 63 8 96 43 I 84- 360 I 7 3 371 17 I 21 39 26 5 86- 38 1 4 21 406 5 5 7 17 4 2 87- 259 2 261 20 2 21 4 40 87 54 I 4 87A 349 2 5 356 3 20 4 36 63 23 3 I 88- 336 I 4 2 343 3 25 9 4 41 31 2 2 2 89- 344 2 6 I 353 3 7 24 12 6 52 14 3 I 91B 361 I 14 376 7 3 50 10 I 71 16 3 93- 364 2 18 I 385 6 51 14 I 5 77 II I 2 107- 346 13 359 5 3 47 55 18 3 107A 395 II 406 3 I 2 50 56 3 3 I 108- 271 50 321 10 9 6 8 1 58 92 26 15 3 2 116LSB 352 4 15 371 10 13 10 10 18 61 40 7 1 2 3 118A 268 16 284 9 22 I 52 84 46 5 5 4 119A 354 14 I 369 6 4 3 29 42 26 2 5 3 I 120B 297 14 14 325 9 18 4 20 51 20 3 2 2 1 82- 62.1 0.2 2.5 64.8 1.0 3.8 6.2 11.0 1.2 83- 50.8 0.3 51.1 4.2 10.6 1.3 16.2 7.3 0.2 84- 60.1 0.2 1.2 0.5 61.9 2.8 0.2 3.5 6.5 4.3 0.8 86- 63.5 0.7 3.5 67.7 0.8 0.8 1.2 2.8 0.7 0.3 87- 43 .1 0.3 43.4 3.3 0.3 3.5 0.7 6.7 14.5 9.0 0.2 0.7 87A 58.3 0.3 0.8 59.4 0.5 3.3 0.7 6.0 10.5 3.8 0.5 0.2 88- 56.5 0.2 0.7 0.3 57.6 0.5 4.2 1.5 0.7 6.9 5.2 0.3 0.3 0.3 89- 57.3 0.3 1.0 0.2 58.8 0.5 1.2 4.0 2.0 1.0 8.7 2.3 0.5 0.2 91B 60.4 0.2 2.3 62.9 1.2 0.5 8.4 1.7 0.2 11.9 2.7 0.5 93- 61.0 0.3 3.0 0.2 64.5 1.0 8.5 2.3 0.2 0.8 12.9 1.8 0.2 0.3 107- 57.6 2.2 59.7 0.8 0.5 7.8 9.2 3.0 0.5 107A 66.1 1.8 67.9 0.5 0.2 0.3 8.4 9.4 0.5 0.5 0.2 108- 45.2 8.3 53.5 1.7 1.5 1.0 1.3 0.2 9.7 15.3 4.3 2.5 0.5 0.3 l 16LSB 58.5 0.7 2.5 61.6 1.7 2.2 1.7 1.7 3.0 10.1 6.6 1.2 0.2 0.3 0.5 118A 45.0 2.7 47.7 1.5 3.7 0.2 8.7 14.I 7.7 0.8 0.8 0.7 119A 59.0 2.3 0.2 61.5 1.0 0.7 0.5 4.8 7.0 4.3 0.3 0.8 0.5 0.2 120B 49.5 2.3 2.3 54.2 1.5 3.0 0.7 3.3 8.5 3.3 0.5 0.3 0.3 0.2 Appendix C-3. Chequamegon Formation modal analyses. Counts are listed in upper field and percentages in lower. 187 Cement and Matrix 1l .,,, c: c: 0"' E " .D .§ E s 0 a :;< :. (._') u" .;:; E 1;- 0 -"' u u 1;- u c: l;! -"'u c: ...c: u ... 0 c: 0 u ·a c: E E E x E E (._') t u 0 E ::; u u u ·.:: u s u ...c: rl 0 :; ...c: E E E E rl :;:; l5 8.. u u u :. u .;:;"' u ...c: l5 ::2 0 l5 _, u c5 E- i:C u 0 N c5 u u u :::E u c... .. E- & u. o-l d u.. "' 3 10 3 4 9 13 106 464 599 80.2 14.2 5.6 83.6 14.2 2.2 8 52 8 6 26 102 451 593 66.7 21.3 12.0 67.3 21.4 11.3 9 3 43 4 25 7 I 109 453 599 80.1 8.6 11.3 82.6 9.0 8.4 1 2 9 41 2 3 122 432 600 88.2 3.9 7.9 94.0 3.9 2.1 II 70 I I I 115 34 31 418 601 62.0 20.8 17.2 63.1 21.1 15.8 8 35 45 6 94 454 599 76.9 13.9 9.3 78.9 14.1 7.0 5 42 25 12 2 130 426 595 79.3 9.6 11.0 80.8 9.8 9.4 5 23 I 24 I 18 128 428 600 80.6 12.1 7.2 82.9 12.4 4.7 I 20 4 2 I 124 467 598 77.3 15.2 7.5 80.9 15.4 3.6 I 15 I I I 3 12 17 85 477 597 76.5 16.l 7.3 81.0 16.3 2.7 3 24 I 12 5 I 144 438 601 79.0 12.6 8.4 82.4 12.8 4.8 4 2 13 I 6 14 14 88 475 598 83.2 11.8 5.1 85.9 12.0 2.1 7 53 39 12 83 466 600 58.2 19.7 22.l - 71.0 20.8 8.2 5 58 5 16 30 2 59 490 602 71.8 12.4 15.7 76.0 12.6 11.4 9 4 73 16 2 I 23 51 7 55 441 596 60.8 19.0 20.2 65 .0 19.3 15.6 4 41 18 I 46 25 58 452 600 78.5 9.3 12.2 82.4 9.7 8.0 8 36 2 54 132 412 600 72.1 12.4 15.5 79.2 12.5 8.3 Averages 74 .8 13.7 11.5 78.6 14.0 7.4 0.5 1.7 0.5 0.7 1.5 2.2 17.7 77.5 1.3 8.8 1.3 1.0 4.4 17.2 76.1 1.5 0.5 7.2 0.7 4.2 1.2 0.2 18.2 75.6 0.2 0.3 1.5 6.8 0.3 0.5 20.3 72.0 1.8 11.6 0.2 0.2 0.2 19.l 5.7 5.2 69.6 1.3 5.8 7.5 1.0 15.7 75.8 0.8 7.1 4.2 2.0 0.3 21.8 71.6 0.8 3.8 0.2 4.0 0.2 3.0 21.3 71.3 0.2 3.3 0.7 0.3 0.2 20.7 78.l 0.2 2.5 0.2 0.2 0.2 0.5 2.0 2.8 14.2 79.9 0.5 4.0 0.2 2.0 0.8 0.2 24.0 72.9 0.7 0.3 2.2 0.2 1.0 2.3 2.3 14.7 79.4 1.2 8.8 6.5 2.0 13.8 77.7 0.8 9.6 0.8 2.7 5.0 0.3 9.8 81.4 1.5 0.7 12.2 2.7 0.3 0.2 3.9 8.6 1.2 9.2 74.0 0.7 6.8 3.0 0.2 7.7 4.2 9.7 75.3 1.3 6.0 0.3 9.0 22.0 68.7 Appendix C-3, continued. 188 Qm F L Q F L Total Chequamegon 4387 770 406 4648 770 145 5563 Devils Island 1751 10 82 1824 10 9 1843 Orienta 3817 552 537 4154 552 200 4906 Hinckley 1696 21 113 1797 21 12 1830 Fond du Lac 709 67 162 805 67 66 938 Chequamegon 78.9 13.8 7.3 83.6 13.8 2.6 Devils Island 95 .0 0.5 4.4 99.0 0.5 0.5 Orienta 77.8 11.3 10.9 84.7 11.3 4.1 Hinckley 92.7 1.1 6.2 98.2 1.1 0.7 Fond du Lac 75.6 7.1 17.3 85.8 7.1 7.0 Appendix C-4. Raw converted point count data (upper field) and QFL and QmFLt percentages (lower field) from Myers' (197lb) study. (Modified from Myers, 1971b, Table 2, p. 50-51). 189 -.. Orienta Formation Devils Island Sandstone x-average (x-avg)"2 x-average (x-avg)"2 4.2 -8.5 72.87 16.4 2.2 4.94 8.6 -4.2 17.24 8.8 -5.3 28.14 16.2 3.5 12.14 20.7 6.5 42.62 5.0 -7.7 59.31 10.5 -3.6 13.04 10.9 -1.8 3.13 18.7 4.5 20.49 11.5 -1.2 1.51 variance 11.4 -2.7 7.45 variance 11.0 -1.7 2.86 1.30 15.2 1.1 1.18 0.81 18.3 5.6 31.28 2.3 -11.8 139.26 11.4 -1.3 1.74 16.4 2.2 5.05 13.4 0.7 0.48 10.4 -3.8 14.34 9.0 -3.8 14.10 2.7 -11.5 131.39 14.6 1.9 3.47 3.8 -10.3 105.92 25.9 13.2 175.00 8.7 -5.4 29.46 15.3 2.6 6.75 11.7 -2.5 6.07 10.5 -2.2 4.96 13.1 -1.0 1.06 15 .5 2.8 7.93 14.6 0.5 0.21 21.9 9.2 83.91 16.0 1.8 3.42 5.7 -7.1 49.74 19.2 5.1 26.00 12.7 average 0.0 30.5 16.5 2.4 5.58 9.2 -5.0 24.71 14.1 -0.1 0.00 Chequamegon Formation 14.3 0.1 0.02 _j x-average (x-avg)"2 8.3 -5.8 33.69 17.7 1.5 2.20 20.8 6.7 44.83 17.2 1.0 0.97 15.4 1.3 1.62 18.2 2.0 3.93 16.3 2.2 4.82 20.3 4.1 16.97 13.3 -0.8 0.65 5.2 -11.l 122.23 7.8 -6.3 39.91 15.7 -0.5 0.27 variance 16.7 2.5 6.39 21.8 5.6 31.75 1.27 19.9 5.7 32.82 21.3 5.1 26.21 17.2 3.0 9.17 20.7 4.5 20.45 8.7 -5.4 29.70 14.2 -2.0 3.91 15.4 1.2 1.49 24.0 7.7 60.00 19.2 5.1 25.61 14.7 -1.5 2.24 22.5 8.4 69.92 13.8 -2.4 5.67 7.7 -6.5 41.88 9.8 -6.4 41.13 15.7 1.5 2.34 9.2 -7.0 48.80 22.2 8.0 64.46 9.7 -6.5 42.87 21.1 6.9 48.06 22.0 5.8 33.48 18.8 4.7 21.66 16.2 average 0.0 27.2 18.2 4.0 16.23 14.1 avera11;e 0.0 27.0 Appendix C-5. Pore space variance