Browsing by Subject "Precambrian"

Now showing 1 - 19 of 19

Bulletin No. 41. The Precambrian Geology and Geochronology of Minnesota
(Minnesota Geological Survey, 1961) Goldich, Samuel S.; Nier, Alfred O.; Baadsgaard, Halfdan; Hoffman, John H.; Krueger, Harold W.
This bulletin is an outstanding example of the cooperation of several scientists and scientific organizations. Without the cooperation of all concerned, such a comprehensive correlation of age determinations and regional geology would have been impossible short of many years of work. The results are a particularly important demonstration of cooperation among geologists, chemists, and physicists. Radioactivity dating of a large number of igneous and metamorphic rocks by the potassium-argon and the rubidium-strontium methods is the basis for revision of the classification of the Precambrian rocks of Minnesota. The major divisions of the three-fold classification are made at time boundaries of 2.5 and 1.7 billion years (b.y.), corresponding to the time of two major orogenies, the Algoman and the Penokean, respectively. The eras are referred to as Early, Middle, and Late Precambrian in preference to the older terminology of Earlier, Medial, and Later Precambrian. The division between the Early and Middle Precambrian is placed at the time of the Algoman rather than of the older Laurentian orogeny. The division between the Middle and Late Precambrian is made on the basis of the Penokean orogeny which resulted in a mountain chain that extended from central Minnesota through Wisconsin into Michigan. The Penokean Mountains formerly were assigned erroneously to late Keweenawan time. The Early Precambrian rocks are divided into the Ontarian and the Timiskamian systems. The Ontarian rocks include the Keewatin group of Minnesota and the Coutchiching metasediments which underlie the Keewatin greenstones in Ontario. Some of the gneisses in the Giants Range and the Vermilion granite regions of Minnesota probably were derived by metamorphism of ancient sediments that were deposited prior to the great outpouring of basalt flows assigned to the Ely greenstone of the Keewatin group.
C-41, Geologic Atlas of Hubbard County, Minnesota
(Minnesota Geological Survey, 2018) Lusardi, Barbara A
A County Geologic Atlas project is a study of a county's geology, and its mineral and ground-water resources. The information collected during the project is used to develop maps, data-base files, and reports. This same information is also produced as digital files for use with computers. The map information is formatted as geographic information system (GIS) files with associated data bases. The maps and reports are also reproduced as portable document files (PDFs) that can be opened on virtually any computer using the free Acrobat Reader from Adobe.com.
GEOPHYSICAL AND PETROLOGIC INVESTIGATIONS OF INVERSELY CORRELATED AEROMAGNETIC AND BOUGUER GRAVITY ANOMALIES WITHIN PRECAMBRIAN GNEISS TERRANE NEAR GARVIN, SOUTHWESTERN MINNESOTA
(Minnesota Geological Survey, 2023) Southwick, David L; Chandler, V.W.; McSwiggen, Peter L
Geophysical models and petrologic inferences presented here are consistent with a buried gabbroic to noritic intrusion as the principal source of two geographically coincident, kilometer-scale, inversely correlated gravity and magnetic anomalies (the Garvin anomalies) located within Archean continental crust near the present-day southern margin of the Superior Craton in Minnesota. Two-dimensional profiles modeled from the total magnetic anomaly, reduced to pole, and the Bouguer gravity anomaly, upward-continued to 2 kilometers, are fit to geologically reasonable distributions of source rocks that have density and magnetic susceptibility values within the ranges reported for gabbro or norite intrusions.
Guidebook 12. Field Trip Guidebook for the Precambrian Geology of East-Central Minnesota
(Minnesota Geological Survey, 1979) Morey, G.B.
The bedrock geology of east-central Minnesota --generally bounded by latitudes 45 ON. to 47° N. and longitudes 92° 15' W. to 95° W. --is particularly interesting because the area contains a wide variety of igneous, metamorphic, and sedimentary rocks which span the entire range of Precambrian time. Unfortunately much of the bedrock is not well exposed. Those rock units that do crop out tend to occur as clusters 2 or 3 acres large or as strings 1 or 2 kilometers long; and these outcrop areas are widely separated by vast expanses of Pleistocene and Holocene materials as much as 140 thick. Therefore aeromagnetic and gravity data, and to a lesser extent, water-well and exploration records acquired over the years by the Minnesota and U.S. Geological Surveys were used to prepare a preliminary and somewhat generalized bedrock geologic map of east-central Minnesota at a scale of 1:500,000 (Morey, 1978).
Guidebook 14. Field Trip Guidebook for the Precambrian Terrane of the Minnesota River Valley
(Minnesota Geological Survey, 1982) Weiblen, Paul W.
The gently rolling farmland of southwestern Minnesota is a deceptive blanket over a rich record of the evolution of the earth's crust. The effects of Pleistocene glaciation dominate the area, and the landscape is characterized by a wide variety of glacial deposits which cover most of the bedrock geology. However, many outcrops are a product of glacial erosion and all the Precambrian outcrops in the Minnesota River Valley (MRV) were exposed by steambed erosion in the Glacial River Warren. This river drained Glacial Lake Agassiz prior to the disappearance of the ice sheet which prevented northward drainage to Hudson Bay. As can be seen in Figure 2, the Precambrian rocks of Minnesota occupy a central position in the North American continent. This position is reflected in the present day drainage pattern of the continent. Despite the low topographic relief (600-2,000 ft), there are three major drainage divides in Minnesota. The gentle relief and low elevation are also deceptive indicators of the crustal thickness of the North American continent in the region, which is on the order of 40 km. Despite its thickness and antiquity, the crust in Minnesota has remained geologically active. This is attested to in part by the fact that the Mesozoic north-south hinge line of the sedimentary basins involved in the formation of the Rocky Mountains crossed Minnesota. During the Paleozoic the region was involved in a number of oscillations of epicontinental seas. The distribution of the thin veneer of Phanerozoic rocks (maximum thickness less than 600 m), which were produced in these two episodes of crustal evolution, is shown in Figure 3 and in the geologic map of Minnesota on the frontispiece of the guidebook The geologic record is briefly outlined on the back cover. The thick, "stable" crust in the region is currently involved in the isostatic rebound associated with the disappearance of the Pleistocene ice sheets over North America. The locations of 12 recorded earthquakes in Minnesota are shown on Figure 3. The distribution of the earthquakes appears to be related to Precambrian tectonic features (Mooney and Morey, 1981), and may be related to reactivation of old fault systems in the course of isostatic rebound (Dutch, 1981). The Precambrian rocks in Minnesota define five distinct geologic terranes. The geology of these terranes is briefly outlined on the last page of the guidebook, and further details may be found in Sims and Morey ( 1972) and in more recent papers listed in Morey and others (1981). The outcrops in the MRV are part of the oldest terrane (I). They contain a record of a broad spectrum of igneous and metamorphic rocks which span a time interval from at least 3,500 to 1,800 m.y. Terrane I extends into central Minnesota where it is separated from the greenstone-granite terrane (II) to the north by a complex tectonic zone.
Guidebook 17. Field Trip Guidebook for Selected Areas in Precambrian Geology of Northeastern Minnesota
(Minnesota Geological Survey, 1987) Balaban, N.H., Editor
CONTENTS STRUCTURAL GEOLOGY OF THE BOUNDARY BETWEEN ARCHEAN TERRANES OF LOW-GRADE AND HIGH-GRADE ROCKS, NORTHERN MINNESOTA, P.J. Hudleston, R.L. Bauer, D.L. Southwick, D.D. Schultz-Ela, and M.E. Bidwell GEOLOGY OF THE KEWEENAWAN (UPPER PRECAMBRIAN) BEAVER BAY COMPLEX IN THE VICINITY OF SILVER BAY, MINNESOTA, James D. Miller, Jr. ROADLOG AND STOP DESCRIPTIONS FOR THE BEAVER BAY COMPLEX, James D. Miller, Jr., Paul W. Weiblen, and John C. Green
Guidebook 3. Field Trip Guide Book for Precambrian North Shore Volcanic Group Northeastern Minnesota
(Minnesota Geological Survey, 1972) Green, John C.
Detailed mapping of the 85th Minnesota shore of Lake Superior began with A. E. Sandberg's study (1938) of the section between Duluth and Two Harbors. Grout and Schwartz (1939) and Gehman (1957) studied the intrusions and flows in eastern Lake County; Grogan (1940) mapped the lakeshore between Two Harbors and Split Rock River; Schwartz (1949) studied the Duluth area; and Grout and others (1959) mapped most of Cook County. James Kilburg (1972) has recently mapped the wedge of lavas just west of Duluth. Most of the data reported in this account derive from studies by the writer who, starting in 1965, has mapped the shoreline between Silver Bay and Grand Portage, with considerable reconnaissance inland and to the southwest (Green, 1966; 1968a; 1968b; 1970). The report does, however, also lean considerably on Grout and others (1959) and, for the Duluth-Two Harbors area, on Sandberg (1938). The field studies have been supported by the Minnesota Geological Survey, and most of the laboratory studies have been .supported by the National Science Foundation (Grant No's GP-5865 and GA-134ll). Sincere gratitude for this support is extended to both agencies. The writer's ideas have benefited from discussions with many other geologists concerned with Keweenawan rocks, especially including Bill Bonnichsen, D. M. Davidson, Jr., H. Hubbard, G. B. Morey, W. C. Phinney, P. W. Weiblen, and W. S. White. Trip will leave Duluth and head up-section in the southwestern limb of the basin.
Guidebook 5. Field Trip Guide Book for Precambrian Migmatitic Terrane of the Minnesota River Valley
(Minnesota Geological Survey, 1972) Grant, J.A.; Himmelberg, Glen R.; Goldich, S.S.
The Minnesota River Valley provides a tantalizing window onto the Canadian Shield on the eastern margin of the Great Plains, tantalizing because of the high grade of the metamorphism, and especially because of the antiquity of the rocks there exposed. Essentially, this is a migmatitic terrane of granitic gneisses with lesser amphibolitic gneisses, commonly with pyroxene, and biotite-rich gneisses, which may contain garnet, cordierite, sillimanite, anthophyllite, or hypersthene. Some of the rocks are greater than 3.0 b.y. in age, and they have been involved in metamorphism and deformation at least 2.6 b.y. ago. These events left rocks with a metamorphic grade in the upper amphibolite or granulite facies, and with a major structure that is similar throughout most of the exposed area. Later minor intrusions, dominantly mafic, cut the older rocks, and conglomerate and quartzite of the Sioux Formation of Late Precambrian age locally overlie them. Deep weathering of the gneisses formed a regolith about 100 feet thick, a part of which was reworked in the formation of Cretaceous deposits of sand and clay. Over this came the glacial deposits of the Pleistocene. With the formation of Lake Agassiz, drainage via Glacial River Warren scoured out the precursor of the present valley leaving an underfit present-day Minnesota River and the glimpse of the Precambrian described in the following pages. The granitic gneisses in the vicinities of Morton, Granite Falls, and Montevideo are among the oldest known crustal rocks. Like very ancient rocks in other parts of the world the gneisses have had a complicated history, and metamorphic changes have masked their original characters and obscured their age. Conservatively the age may be given as 3200 or 3300 m. y. Goldich and others (1970) have attempted to probe the metamorphic history and concluded that the gneisses date back to 3550 m.y. ago. Similarly old, or older gneisses (3600 to 4000 m.y.) have been reported from the Godthaab district, West Greenland (Black and others, 1971). Field and more detailed geochronological and geochemical investigations are being continued, and the nature of this work is briefly indicated in following sections.
Guidebook 6. Field Trip Guide Book for Precambrian Geology of Northwestern Cook County, Minnesota
(Minnesota Geological Survey, 1972) Weiblen, P.W.; Davidson, D.M. Jr
An exceptionally complete record of Precambrian history is recorded in the rocks exposed in Cook County, Minnesota. In northwestern Cook County, in the vicinity of the Gunflint Trail the Lower Precambrian is represented by a metavolcanic succession, which was intruded by the somewhat younger Saganaga Tonalite. These rocks are unconformably overlain by the Middle Precambrian Animikie Group, consisting of the Gunflint Iron Formation and the Rove Formation. In northeastern Cook County, a gently dipping angular unconformity separates Middle Precambrian and Upper Precambrian strata. There, a thin basal sandstone, the Puckwunge Formation, is overlain by volcanic rocks of the North Shore Group. The Logan intrusions and the Duluth Complex intrude and truncate Middle and Upper Precambrian rocks and comprise the major part of the Upper Precambrian section in northwestern Cook County. Although the geology of Cook County was summarized by Grout and others (1959), geologic mapping since 1962 has considerably revised the earlier geologic interpretation. Because much of this work is unpublished as yet, a comprehensive summary is presented here. The discussion is meant to provide a framework for the specific aspects of the geology which the chosen stops illustrate.Mileages for this trip are listed by stop as distances in miles along Minnesota 12 (The Gunflint Trail) going both northwest from Grand Marais and southeast from Trails End Campground, a round-trip distance of about 120 miles. Figure 1 indicates the location of the Gunflint Trail as well as the general geology of the area. A larger scale geologic map of the field trip area together with all the field trip stops is shown in Figures 2 and 3, while the cross section on Figure 2 and the block diagrams of Figure 4 represent the gross structural relationships between the units encountered on the field trip.
Guidebook 9. Field Trip Guidebook for Stratigraphy, Structure and Mineral Resources of East-Central Minnesota
(Minnesota Geological Survey, 1979) Morey, G.B.; Davidson, D.M. Jr
Early in the 20th century, east-central Minnesota became the source of appreciable quantities of iron and ferromanganese, and even earlier, the source of a variety of granite products (Morey, 1977). Because of the obvious economic importance of the commodities to the state, most of the geologic work in east-central Minnesota focused on the Cuyuna iron-mining district or on the St. Cloud area where there are numerous granite quarries. Less attention was given to the geology of other parts of east-central Minnesota and to the possible presence of other mineral resources. This was true mainly because a fairly ubiquitous mantle of Quaternary materials made it difficult, time consuming and expensive for a company to establish the basic geologic information necessary to a successful exploration program. However, recent geologic work (Morey, 1978) has led to the recognition of several geologic environments that are similar to mineral-producing districts elsewhere in the world (Morey, 1977). Although these studies have shown that a variety of mineral occurrences may exist, most attention to date has focused on environments that may contain uranium. This road log starts at the Minnesota-Wisconsin border along the st. Louis River near Fond du Lac, the westernmost suburb of Duluth, Minnesota, and terminates near Sturgeon Lake on U.S. Interstate Highway 35 some 50 miles southwest of Duluth. Note that the mileages in this road log are approximate.
Identification of primary and diagenetic mineralogy preserved in silica-cemented horizons of the Biwabik Iron Formation, Minnesota, using petrography and electron microprobe analysis
(2020-11) Duncanson, Samuel
The primary mineralogy of iron formations, iron and silica-rich chemicalsedimentary rocks, are crucial archives of Precambrian seawater chemistry. Post-depositional alteration from diagenesis and metamorphism commonly obscure the original mineralogy in many iron formations. Recent studies of well-preserved iron formations have identified putative primary mineral phases preserved in silica-cemented horizons. Silica cement aids in mineral preservation by sealing pore space with quartz, a stable mineral on the Earth’s surface. These previous studies focus on iron formation precipitation during the initial rise of oxygen in Earth’s atmosphere and oceans from ~2.5 - 2.3 Ga. Following this rise, ocean oxygenation remains poorly understood. The ~1.9 Ga Biwabik Iron Formation in northeastern Minnesota provides an opportunity to study well preserved (sub-greenschist facies) iron formation following the ~2.5 Ga rise in oxygen. Minerals were identified in silica-cemented horizons and non-silica-cemented horizons with petrography and electron microscopy. Cross-cutting relationships and mineral compositional data inform a paragenetic sequence and distinguish diagenetic minerals from texturally earlier minerals. Observations from petrography and electron microscopy suggest silica-cementation preserves textures not present in adjacent banded horizons. Diagenetic mineral compositions are influenced by their relative spatial proximity between silica-cemented and banded horizons. Within different silica-cemented horizons, the texturally earliest mineral phases were greenalite or <5 μm hematite. These two minerals suggest the initial sediment of the Biwabik Iron formation was a Fe(II)-Si greenalite-like gel and/or an oxidized hematite precursor.
Information Circular 34. Precambrian Geology of the Southern Canadian Shield and the Eastern Baltic Shield
(Minnesota Geological Survey, 1991) Ojakangas, Richard W.
The geologic histories of the Canadian and Baltic Shields in North America and Europe, respectively, are broadly similar, and the topic was discussed during a conference and field trip involving North American and Russian participants in the late summer of 1990. During a two-day meeting prior to the field trip, twelve North American and eleven Soviet geologists presented papers, and participants discussed a variety of problems and ideas in Precambrian stratigraphy, sedimentology, tectonics, magmatism, industrial minerals, and metallogeny. Special emphasis was placed on problems of correlation. All papers were simultaneously interpreted by Senior Translator and Interpreter Grigori Sokolov of the Institute of Geology, Karelian Branch, U.S.S.R. Academy of Sciences, who accompanied the Russian delegation. His ability contributed greatly to the meeting's success. In addition to the speakers, thirty-eight geologists attended the conference: four Canadians, two Finns, and thirty-two Americans, including eight graduate students. As a result of the seminar and field trip, exciting and promising opportunities for continued cooperation were identified. Specific proposed activities include meetings, field excursions, short courses, joint publications, individual research-oriented exchanges, and joint projects. Involvement of young geologists was especially encouraged to promote long-term cooperative relationships. Opportunities also were identified for cooperation with other international projects, such as existing bilateral programs and the International Geological Correlation Program. It was mutually agreed that in 1991-1992, the Institute of Geology, Karelian Research Center, and the Kola Research Center of the USSR Academy of Sciences will host conferences and field trips on Proterozoic and Archean geology and metallogeny in the eastern Baltic shield. In 1991, the field program will emphasize Proterozoic geology, and in 1992, Archean geology. Other joint activities in the future will depend on the outcome of the 1991 and 1992 meetings. It was the intent of the organizers to bring this joint activity to the attention of officials involved in relevant international programs. Toward that end this proceedings volume has been published by the Minnesota Geological Survey. The body of this report consists of two parts; the first is a series of short papers that provide an overview of the Precambrian geology in the Great Lakes Region; the second part consists of a similar overview of the eastern part of the Baltic Shield.
M-194 Bedrock Geology of the Twin Cities Ten-County Metropolitan Area, Minnesota
(2013-08-01) Mossler, John H.
This regional map is partly a compilation of existing maps of bedrock geology in the metropolitan area and partly a remapping of bedrock in areas where the existing maps were out of date because of the acquisition of new subsurface data.
M-199, Bedrock Geology of the Mark Lake Quadrangle, Cook County, Minnesota
(Minnesota Geological Survey, 2018) Boerboom, Terrence J; Green, John C
Portrays the bedrock geology of the Mark Lake quadrangle which prior to this effort was largely unmapped. The map shows the distribution of the various rock types, locations of bedrock outcrops, and structural attributes of the bedrock. Mapped outcrops were used to constrain the geology for the most part, but mapping was augmented by the use of geophysical maps, and lidar imagery. Lidar was especially useful in locating bedrock outcrops during field work, and also for delineating the various bedrock units during the map compilation stage following fieldwork.
Midcontinent Rift System Bibliography
(University of Minnesota Duluth, 1995-12) Hauck, Steven A
The co-chairs of the IGCP Project 336 field conference on the Midcontinent Rift System felt that a comprehensive bibliography of articles relating to a wide variety of subjects would be beneficial to individuals interested in, or working on, the Midcontinent Rift System. There are 2,543 references (>4.2 MB) included on the diskette at the back of this volume. PAPYRUS Bibliography System software by Research Software Design of Portland, Oregon, USA, was used in compiling the database. A retriever program (v. 7.0.011) for the database was provided by Research Software Design for use with the database. The retriever program allows the user to use the database without altering the contents of the database. However, the database can be used, changed, or augmented with a complete version of the program (ordering information can be found in the readme file). The retriever program allows the user to search the database and print from the database.
Minnesota at a Glance Precambrian Geology
(Minnesota Geological Survey, 2020; 1994) Boerboom, Terrence J.
What do the cliffs along the North Shore of Lake Superior, the smooth outcrops in the Boundary Waters Canoe Area Wilderness, the immense iron mines on the Mesabi Iron Range, and the knobby outcrops within the Minnesota River valley have in common? They are part of the very old bedrock that underlies all of Minnesota. Minnesota is situated at the southern edge of the Canadian Shield (Fig. 1)—the nucleus of the continent of North America that formed during Precambrian time. This period of time encompasses about 85% of Earth's history. Geologists consider Precambrian time to have begun with the formation of planet Earth about 4,550 million years (Ma) ago and to have ended about 541 Ma, when organisms with hard parts, such as shells, rapidly diversified. The rocks formed in Minnesota during this enormous span of time record a complicated geologic history that involved volcanoes, ocean islands, mountain chains, earthquakes, and unstable geologic conditions that were very different from the Minnesota of today. Precambrian Minnesota resembled modern-day Indonesia for a while; later, it resembled modern-day California; and still later it resembled parts of the Middle East and eastern Africa.
RI-69 Reexamination of the Minnesota River Valley Subprovince with Emphasis on Neoarchean and Paleoproterozoic Events
(2014-03-06) Southwick, David
The Minnesota River valley subprovince (MRV) is a fragment of Mesoarchean continental crust that was sutured to the southern margin of the Superior craton about 2,600 m.y. ago. The suturing event induced widespread regional metamorphism and local anatexis in a dominantly orthogneissic crust and ended with the emplacement of numerous granite plutons. In the Paleoproterozoic era the MRV was a tectonically rigid part of the cratonic foreland with respect to Penokean (geon 18), Yavapai (geon 17), and Mazatzal (geon 16) accretionary events. As such, it was affected by crustal extension and the emplacement of mafic dikes associated with the ca. 2,070 Ma opening of the pre-Penokean ocean. Subsequently, internal shear zones that had formed during Neoarchean docking of the MRV crustal block were reactivated in response to stresses applied during cycles of Paleoproterozoic stretching and subsequent compression from the south and southeast. Most of this reactivation is inferred to have taken place between 2,000 and 1,750 Ma. The Minnesota segment of the Great Lakes tectonic zone, the Neoarchean suture, was not significantly reactivated, whereas the Appleton shear zone and the Yellow Medicine shear zone both were. Six sets of mafic dikes were emplaced in the interval between 2,070 and ca. 1,750 Ma. Two sets that were emplaced early in the interval are the southwesternmost members of the pre-Penokean Kenora–Kabetogama/Fort Frances dike swarm. Two and perhaps four younger dike sets were emplaced during a period of vigorous crustal heating and magmatic activity that affected much of the MRV in early- to mid-geon 17. Numerous plugs and small plutons also were emplaced in early- to mid-geon 17. These intrusions range in composition from peridotite to granite and are comparable to rock types within and satellitic to the East-Central Minnesota batholith; they are most abundant in the eastern and southern parts of the MRV, relatively near the inferred Penokean and Yavapai tectonic fronts. Transtensional stress during the extensional stage of the Mazatzal orogenic cycle generated differential subsidence of crust south of the Yellow Medicine shear zone and produced en echelon fault-bounded depressions that became depocenters filled by supermature clastic sediment ancestral to the Sioux Quartzite. The Sioux Quartzite was deposited, lithified, and hydrothermally altered over a prolonged time interval that may have begun as early as ca. 1,730 and ended as recently as ca. 1,280 Ma.
S-21 Geologic Map of Minnesota-Bedrock Geology
(Minnesota Geological Survey, 2011) Jirsa, Mark A.; Boerboom, Terrence J.; Chandler, V.W.; Mossler, John H.; Runkel, Anthony C.; Setterholm, Dale R.
This map is a new construct that incorporates existing geologic maps where prior mappers had adequate ground control, and new interpretations based on drill hole, geophysical, and unpublished data where they did not. The interpretation differs significantly from previous maps to reflect new data and accommodate scale. It portrays our current geologic understanding of the temporal and geographic distribution of units within major Precambrian terranes and of the Phanerozoic strata. The western part of the mapped Precambrian terrane is inferred largely from geophysical maps, anchored locally by drilling. In many places, contacts are drawn between units of the same or similar apparent rock type (and same unit label); these are recognized as geometrically distinct, though geophysically or lithologically similar. Digital files corresponding to this map allow removal of Cretaceous, Paleozoic, and some parts of Mesoproterozoic strata to reveal an interpretation of the underlying Precambrian bedrock. For additional data see: (http://hdl.handle.net/11299/98043 [select, copy and paste into browser]) which contains files associated with Bedrock Topography, Depth to Bedrock, and locations of Outcrop and Geochronologic analyses.
S-22, Geologic Map of Minnesota, Precambrian Bedrock Geology
(2012) Jirsa, M.A.; Boerboom, T.J.; Chandler, V.W.
This map of the Archean and Proterozoic (Precambrian) geology of Minnesota is identical with MGS State Map S-21, except it portrays an interpretation beneath Phanerozoic (Paleozoic and Mesozoic) strata inferred from geophysical maps and drill core.

University Digital Conservancy

Browse by Subject

Browsing by Subject "Precambrian"