Report of Investigations

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RI-01 Geological Interpretation of Magnetic Map of McLeod County, Minnesota
(Minnesota Geological Survey, 1963) Sims, P.K.; Austin, G.S.
A ground magnetometer survey of McLeod County, Minnesota has delineated 3 structural blocks in the buried Precambrian basement rocks. The sources of the anomalies in each of the blocks have not been identified definitely because of a lack of subsurface data, but are interpreted qualitatively from an analysis of the anomalies and considerations of the regional geology. An anomaly in the northwestern part of the county, near Hutchinson, is interpreted to represent a folded, magnetic rock unit. Probably this unit is a magnetite-bearing schist, but the possibility of its being an iron-formation of the Cuyuna-type should be investigated further.
RI-02 Geology of Clay Deposits Red Wing Area, Goodhue and Wabasha Counties, Minnesota
(Minnesota Geological Survey, 1963) Austin, G.S.
Clay deposits that have been used in the ceramic industry occur sporadically over an area of about 50 square miles in eastern Goodhue County, southeastern Minnesota. The deposits are in the Ostrander Member of the Cretaceous Windrow Formation and to a lesser extent in Pleistocene lake deposits. Currently they are being used in the manufacture of vitreous drain pipe and related products. The Windrow Formation in the area overlies gently folded Cambrian and Ordovician strata. It consists of a lower Iron Hill Member, a ferruginous weathered residuum, and an upper Ostrander Member, a clastic unit. The Iron Hill Member was developed on a mature topographic surface; it formed through residual accumulation of limonite, clay, and chert, the relatively insoluble weathering products of the underlying carbonate rocks. The Ostrander Member formed as floodplain deposits, which were derived from a mixed Precambrian and Paleozoic source. The clay deposits in the Ostrander Member are lenses or tabular bodies as much as a few feet thick and several tens of acres in areal extent that are intercalated with ferruginous sands. The only l known deposit of Pleistocene age is laminated and is as much as 30 feet thick. It is interpreted to have been formed in a glacial lake.
RI-05 The Cretaceous System in Minnesota
(Minnesota Geological Survey, 1964) Sloan, R.E.
The Cretaceous rocks of Minnesota are flat-lying, loosely consolidated sediments of variable lithology. Two formations, both showing wide facies variation are recognized, the predominantly marine Coleraine Formation in northern Minnesota and the predominantly nonmarine Windrow Formation in southern Minnesota. Elsewhere in the State the strata are poorly exposed and knowledge of their lithology, thickness, and correlation is fragmentary. The strata rest unconformably and with profound hiatus on a surface with a maximum relief of 1,400 feet, which developed during a long interval of erosion and weathering that extended from sometime after the Devonian into the Cretaceous. In general, marine sandstones and shales deposited in the western and northern parts of the State grade eastward into estuarine, paludal, deltaic, and lacustrine sediments. The sediments lie. on rocks ranging in age from Precambrian to Devonian. The distribution and character of the sediments are interpreted to indicate that they formed in and adjacent to the Late Cretaceous sea that invaded Minnesota from the west and continued to advance eastward over an irregular terrain. In general, the vertical succession at any particular locality consists of a basal regolith developed on pre-Cretaceous bedrock, an unconformity, basal nonmarine stream deposits, and finally marine clastic sediments; but the full succession is not present at all localities. The fossil record indicate s that the strata can be correlated with the entire Colorado Group of the western interior United State s. Sediments that were deposited at lower present- day altitudes are Cenomanian in age, whereas those deposited on bedrock at higher altitudes are Turonian and later in age.
RI-04 Interpretation of Lake Washington Magnetic Anomaly, Meeker County, Minnesota
(Minnesota Geological Survey, 1964) Sims, P.K.; Austin, G.S.; Ikola, Rodney J.
A magnetic anomaly centered at Lake Washington, in southeastern Meeker County, has been outlined by a ground magnetometer survey. The anomaly is arcuate in outline, about 8.5 miles long and a maximum of 5 miles wide, and has a maximum amplitude greater than 3,000 gammas. A gravity traverse was made across the anomaly to aid in interpretation. Combined magnetic and gravity data are interpreted to indicate that the source is a basin-shaped body having a substantially greater density than the adjacent rocks that lies at a shallow depth. The lithology of the source is not known because of a lack of subsurface data, but is inferred to be either Precambrian iron-formation or mafic igneous rock. The anomaly warrants further investigation.
RI-03 Kaolin Clay Resources of the Minnesota River Valley Brown, Redwood & Renville Counties, A Preliminary Report
(Minnesota Geological Survey, 1964) Parham, W.E.; Hogberg, R.K.
Kaolin clays of potential economic significance occur in the upper part of the thick regolith developed on Precambrian crystalline rocks and in overlying shales of late Cretaceous age in the Minnesota River valley, in Redwood, Brown, and Renville counties. Preliminary tests indicate that the clays in the regolith warrant further testing for use as filler and coating clays in the paper industry and for the manufacture of refractories. Ball clays, previously unknown in Minnesota, and a few thin bentonite beds in Cretaceous strata appear to be of limited tonnage but should be investigated further.
RI-07 Stratigraphy and Petrology of the Type Fond du Lac Formation Duluth, Minnesota
(Minnesota Geological Survey, 1967) Morey, G.B.
The Fond du Lac Formation of Upper Keweenawan (Precambrian) age is exposed along the St. Louis River near Duluth, Minnesota. It is inferred to be more than 800 feet thick in this area, but only 300 feet are exposed. The formation consists predominantly of lenticular beds of red sandstone and siltstone and interbedded shale, but conglomerate beds containing clasts of vein quartz, basalt, felsite, chert, and quartzite are common. Physically isolated exposures of quartz-pebble conglomerate previously assigned to the Lower Keweenawan are reassigned to the Fond du Lac Formation because the conglomerate (1) grades into sandstone and shale that are similar in lithology to definite Fond du Lac strata, and (2) it contains heavy minerals identical to those found in the Fond du Lac sandstones. The sandstone of the Fond du Lac Formation is arkosic or subarkosic, consisting of 36-68 percent quartz, 5-29 percent feldspar, 1-10 percent rock fragments, 1-15 percent matrix material compos ed of quartz, illite, chlorite, and rare kaolinite and biotite, and 1-20 percent cement of hematite, calcite, quartz and dolomite. Heavy minerals of the Fond du Lac Formation are leucoxene aggregates, apatite, tourmaline, zircon, magnetite- ilmenite, and garnet. The siltstone and shale, although finegrained, are mineralogically equivalent to the sandstone. Analysis of cross - bedding and other sedimentary directional indicators imply that the formation was derived from a terrane consisting of igneous, high-grade metamorphic, and sedimentary rocks that probably was situated to the west of the Fond du Lac basin of deposition. Although the basal quartz-pebble conglomerate was deposited on an irregular surface during a transgressive sea in early Fond du Lac time, most of the formation was deposited by fluvial-deltaic processes, as indicated by the presence of filled channels, intraformational fragments, mud cracks, ripple marks, rain imprints, and extensive large - and small-scale cross - bedding.
RI-06 Ground-Water Contribution to Streamflow and Its Relation to Basin Characteristics in Minnesota
(Minnesota Geological Survey, 1967) Ackroyd, Earl A.; Walton, William C.; Hills, David L.
Estimates of annual ground-water contribution to streamflow in 38 drainage basins of Minnesota by use of standard streamflow hydrograph separation methods permit determination of relations between ground-water runoff and such basin characteristics as geologic environment, precipitation and temperature, and percentage of lake and wetland cover. Generalized conclusions derived from analysis of the data are that ground-water runoff is (1) least from glaciated basins that have surficial lake bed sediments or gray-drift ground moraine immediately underlain by relatively impermeable bedrock and (2) greatest from glaciated basins that have surficial depos its immediately underlain by permeable bedrock or that have thick surficial loess deposits immediately underlain by permeable bedrock. Ground-water runoff is much greater from glaciated basins having red drift than from basins having gray drift. The rate of ground-water increases as annual precipitation increases. Lakes and wetlands sustain and regulate streamflow during rainless periods; if they were absent sustained streamflow from northern parts of the state would be greatly reduced. Recharge to aquifers in the state is difficult to ascertain. Because many aquifers are deeply buried by glacial materials of varying characteristics, not all ground-water runoff can be diverted into cones of depression, for there is some lateral as well as vertical movement of water in surficial deposits. Data on ground-water runoff can be useful in estimating the rate of recharge to aquifers and in evaluating the potential yields of ground-water reservoirs. However, no simple re- 1ation exists between ground-water runoff and ground-water recharge or the potential yields of aquifers. Studies of basin characteristics were handicapped because of a lack of detailed geologic information. Collection and study of data intended to describe the dimensions and water -yielding properties of unconsolidated deposits in the basins of the state are urgently needed to support hydrologic studies involving ground-water development and management decisions.
RI-08 K-Ar Ages for Hornblende from Granites and Gneisses and for Basaltic Intrusives in Minnesota
(Minnesota Geological Survey, 1968) Hanson, G.N.
K-Ar ages have been determined for hornblende concentrates from granitic intrusive and metamorphic rocks from the Minnesota River valley, northeastern Minnesota, and an adjacent area in Ontario, which were involved in the Algoman orogeny. Nine determinations giving ages ranging from 2570 to 2770 m. y. have an average of 2670 + 30 m.y. This is significantly older than the age of about 2500 m.y. indicated by the Rb-Sr and K-Ar methods for biotite from these rocks. The age, however, agrees with the U -Pb ages of about 2700 m. y. for coexisting zircons. Mineral and whole-rock K-Ar ages for basic rocks that invaded the granitic and metamorphic rocks show two periods of intrusion in Minnesota--at 1600-1800 m. y. and at or before 2100 m. y. --in addition to the late Keweenawan igneous activity at about 1100 m. y.
RI-09 The Duluth Complex in the Gabbro Lake Quadrangle, Minnesota
(Minnesota Geological Survey, 1969) Phinney, W.C.
RI-10 Clay Mineralogy, Fabric, and Industrial Uses of the Shale of the Decorah Formation, Southeastern Minnesota
(Minnesota Geological Survey, 1969) Parham, Walter E.; Austin, George S.
The Decorah shale varies both vertically and laterally in its clay mineral assemblages. It is principally an illitic shale in Minnesota, but kaolinite is approximately of equal abundance in the basal part to the southwest, reflecting nearness to the source area, most probably the Transcontinental Arch. Lateral variations in clay mineral assemblages of the Decorah are similar to that of the coextensive older Ordovician Glenwood Formation, suggesting that the source was the same and that both were deposited under similar conditions. The vertical variation in clay mineral assemblages in the Decorah shale, from kaolinite and illite toward the base to only illite in the upper part, reflects the transgressive nature of the Decorah sea. Certain engineering properties of the Decorah shale are related to the orientation of clay minerals, The shale is relatively impermeable perpendicular· to bedding where the clay minerals are arranged with their shortest axis normal to the bedding surface, as in a deck-of-cards arrangement, but is permeable parallel to bedding. In contrast, a random clay mineral arrangement, as in the card-house structure, is permeable in all directions. Both types of clay mineral arrangements are present in the Decorah shale, Maintaining the natural moisture content of the Decorah shale during and after construction aids in stabilizing slopes and foundation bases. The clay mineral data suggest that the Decorah shale may be satisfactory for use in ceramic products such as face brick, sewer pipe, lightweight aggregate, structural tile, and drain tile. Numerous areas along the shale's outcrop belt in southeastern Minnesota would be satisfactory for open pit mining.
RI-12 Deep Stratigraphic Test Well Near Hollandale, Minnesota
(Minnesota Geological Survey, 1970) Austin, G.S.
A deep stratigraphic test well, the Hollandale No.1, in Freeborn County, Minnesota was drilled in rocks that range in age from Devonian to Precambrian in search for reservoirs suitable for the underground storage of natural gas. The porous St. Peter Sandstone of Ordovician age and the Mt. Simon Sandstone of Cambrian age are potentially good reservoir rocks. The Ordovician Decorah Shale overlies the St. Peter Sandstone and probably is the most suitable cap rock in southeastern Minnesota. However, the Decorah is restricted geographically and generally lies relatively near the surface. The Eau Claire Formation, above the Mt. Simon Sandstone, also has been considered as a possible cap rock, but it is composed of shales that are interbedded with sandstones and is vertically permeable. As this was the first cored well in this part of southeastern Minnesota, a study was undertaken of the lithic units penetrated during drilling, and interpretations were inferred for the depositional environment of each unit. The rocks of Paleozoic age were deposited in a shallow-water environment in cyclic fashion. Four recurrent lithotopes that characterize the rocks are (1) quartzarenite, (2) poorly sorted lithotope with strata composed of clastic particles ranging in size from silt to granule or with arenaceous carbonate strata, (3) shale or argillaceous sandstone, and (4) carbonate rock. The recurrence of these lithotopes in the Paleozoic column in southeastern Minnesota has led to the identification of nine sedimentary rock cycles. Several of the cycles are bounded on their upper surfaces by unconformities; in others, rocks occur that appear to have been deposited by both transgressional and regressional seas. The general trend in the depositional cycles of Cambro-Ordovician rocks of southeastern Minnesota is from predominant sandstone and subordinate carbonate in the older cycles to subordinate sandstone and predominant carbonate in the younger cycles. This progression reflects the gradual degradation of the Precambrian surface and the covering of these basement rocks and older Paleozoic rocks by younger sediments.
RI-11 Seismic Studies Over the Midcontinent Gravity High in Minnesota and Northwestern Wisconsin
(Minnesota Geological Survey, 1970) Mooney, Harold M.; Farnham, Paul R.; Johnson, Stephen H.; Volz, Gary; Craddock, Campbell
Seismic refraction data from 87 profiles contribute to the delineation of the geologic structures which cause gravity and magnetic anomalies associated with the northern part of the Midcontinent Gravity High. Interpretations of the seismic data provide knowledge about structure within the sedimentary section as well as depth to the igneous basement, which reaches a maximum of about 10,000 feet in this area. The area investigated extends from the Minnesota-Iowa border on the south to Lake Superior on the north, and includes parts of eastern Minnesota and northwestern Wisconsin. Gravity and magnetic maps compiled from published data and 12,000 additional gravity stations occupied as part of the present investigation were used in combination with geologic control from outcrops and drill holes as a guide in the location and later interpretation of the seismic profiles. The seismic data were obtained by shooting across a fixed geophone spread for shot distances up to a maximum of eight miles. Results for each profile are presented separately and include observed seismic travel-time data, an inferred geologic structure section, and a discussion. The discussion describes the geologic setting and purpose of the seismic profile, the reliability of the seismic interpretation, and the correlation of the seismic results with gravity, magnetic, and geologic control. Tabulated values for the observed seismic velocities fall in the range of 9,000-23,000 feet/second. The velocities can be assigned to seven groups of geologic strata, corresponding to Paleozoic, upper, middle, and lower Upper Keweenawan sedimentary strata, Middle Keweenawan volcanics, pre-Keweenawan felsic intrusives, and pre-Keweenawan mafic intrusives. Good velocity correlations can be established between similar strata in different geologic provinces. A second paper (Mooney and others, 1970) synthesizes data reported here into a regional geologic picture in the form of six structural cross-sections across the Midcontinent Gravity High.
RI-14 Stratigraphy of the Lower Precambrian Rocks in the Vermilion District, Northeastern Minnesota
(Minnesota Geological Survey, 1970) Morey, G.B.; Green, J.C.; Ojakangas, R.W.; Sims, P.K.
The system of stratigraphic nomenclature used previously for the Lower Precambrian rocks in the western part of the Vermilion district, northeastern Minnesota, is replaced by a formal nomenclature based on increased data gained from recent geologic mapping. The resulting changes in stratigraphic nomenclature are the following: 1. Previously recognized lithostratigraphic units -- the Ely Greenstone, Soudan Iron-formation, and Knife Lake Group -- are redefined and restricted in usage. 2. The Lake Vermilion Formation is formally established for rocks exposed in the vicinity of Lake Vermilion that were previously considered part of the Knife Lake Group. Four informal lithologic members are recognized in the Lake Vermilion Formation. These include a metagraywacke-slate member, a feldspathic quartzite member, a volcaniclastic member, and a mixed metagraywacke-felsic conglomerate member. Each is characterized by dominant and distinctive lithologies and may contain several mappable rock units, such as iron-formation and pillowed metabasalt, that can be recognized and delineated on the ground, but whose geographic extent and/or stratigraphic relationships are not known completely. 3. A second unit -- the Newton Lake Formation, a metavolcanic formation -- also is formally recognized. It is inferred to stratigraphically overlie rocks assigned to the Knife Lake Group and to consist of two informal lithologic members, a dominantly mafic volcanic member and a dominantly felsic-intermediate volcanic member. The two members interfinger in the Vicinity of the type locality at Newton Lake. 4. A variety of hypabyssal intrusive rocks are intimately associated with all the volcanic and sedimentary rocks in the Vermilion district. The hypabyssal rocks were emplaced over an interval of time as a consequence of the igneous cycle in the district. Accordingly the time term "Laurentian" should no longer be applied to these rocks in the Vermilion district. 5. The recognition that (1) mafic volcanism was not confined to a single period and (2) that a major unconformity separating an effusive volcanic episode ("Ely Greenstone" of the older literature) from a younger sedimentary series ("Knife Lake Group" of the older literature) is lacking raises serious doubts about the validity ofregionaJ correlations previously accepted in northern Minnesota. Accordingly, it seems necessary to abandon the terms "Keewatin" and "Coutchiching" as time-stratigraphic units for strata of Early Precambrian age in northern Minnesota.
RI-13 Sedimentology of the Middle Precambrian Thomson Formation, East-Central Minnesota
(Minnesota Geological Survey, 1970) Morey, G.B.; Ojakangas, Richard W.
The Thomson Formation is exposed in parts of Carlton, Pine, and southern St. Louis Counties of east-central Minnesota. The formation was folded and metamorphosed during the Penokean orogeny 1,700 million years ago, but primary sedimentary textures and structures are well-preserved in the Cloquet-Carlton area. The formation is characterized by intercalated slate, siltstone, and graywacke. In two measured sections at the type locality, graywacke comprises 34 percent, siltstone 35-43 percent, and slate 23-31 percent of each section. Most beds are less than one foot thick. Because of abundant graded beds, lateral continuity of individual beds, well-defined internal structures common to turbidite sequences and consistent directional structures, the graywacke and siltstone beds are interpreted as individual sedimentation units, apparently deposited by waning, sediment-laden turbidity currents. An analysis of cross-bedding suggests that much of the sediment was deposited by southward-flowing currents moving down a regional paleoslope. However, the presence of flute and groove casts which trend eastward and westward implies that some currents probably flowed parallel to the strike of the inferred paleoslope. X-ray and thin section studies reveal that the graywackes are composed of 4-35 percent quartz, 2-28 percent feldspar, 1-10 percent rock fragments, 15-85 percent matrix material consisting of muscovite, chlorite, and quartz, and 1-17 percent calcite. Mineralogically, the siltstones are fine-grained equivalents of the graywackes. The correlation of the Thomson Formation with other similar rocks in the Lake Superior region has been debated since Irving in 1883 first suggested a Middle Precambrian age, but the formation's physical isolation has left correlations in doubt. The marked similarity of the mineralogic and sedimentologic aspects of the Thomson Formation with those observed in the Middle Precambrian Rove Formation shows they were derived from a similar source terrane and were deposited by similar mechanisms. This, coupled with paleogeographic data, strongly suggests that they can indeed be correlated with each other.
RI-15 Quaternary Geologic Map Index of Minnesota
(Minnesota Geological Survey, 1976) Goebel, Joseph E.
The following list of selected references is a guide to the maps, figures, plates, sheets and data being used to compile a new Quaternary Geologic Map of Minnesota. Topics include glacial geology, rock outcrops, bedrock topography, recent sediments, soils, geomorphic and physiographic provinces, and other information pertinent to the surficial geology of the State. These references are identified and located on the Quaternary Geologic Map Index of Minnesota (Plate 1).
RI-16 Revised Keweenawan Subsurface Stratigraphy, Southeastern Minnesota
(Minnesota Geological Survey, 1977) Morey, G.B.
The Midcontinent Gravity High is the major tectonic feature of the northern midcontinent region. Numerous geophysical surveys have shown that this structure is mainly a sequence of basaltic lava flows which form steep-sided, en echelon blocks, on the average about 40 miles wide and several miles thick. Clastic rocks occur in graben-like flanking basins and in axial basins on top of the blocks. Of particular interest are the sedimentary rocks of Keweenawan age that flank and overlie the St. Croix horst, an uplifted basalt block in east-central and southeastern Minnesota. Because of their red color, these sandstones and shales have been grouped together under the name "Red Clastic Series," a "temporary" term first proposed in 1911. However, a detailed study of approximately 4,000 feet (1,220 meters) of diamond drill core from a number of localities has demonstrated the presence of at least three distinct lithic units which can be traced laterally for some distance. Accordingly, it is recommended that the name "Red Clastic Series" be abandoned and replaced by a more suitable nomenclature. The three Keweenawan formations recognized in the subsurface are: (1) Hinckley Sandstone, a buff to tan rock containing 95 or more percent quartz; (2) Fond du Lac Formation, consisting of intercalated moderate red shale and reddish-brown sandstone containing quartz, orthoclase, microcline, sodic plagioclase and IIgraniticll rock fragments; and (3) Solor Church Formation, a newly named formation consisting of dark reddish-brown mudstone and pale reddish-brown sandstone, containing variable amounts of quartz, plagioclase of intermediate composition (oligoclase-andesine), and aphanitic rock fragments. The first two formations are named from surface exposures; however, the Solor Church Formation, so far as is known, is confined entirely to the subsurface. A stratigraphiC analysis indicates that, in the flanking basins, the Solor Church Formation is overlain unconformably by the Fond du Lac Formation, which in turn is gradationally overlain by the Hinckley Sandstone. In contrast, the Solor Church Formation overlies basaltic rocks on top of the horst and in turn is unconformably overlain by the Hinckley Sandstone; at places a regolith separates the two formations. Either the Fond du Lac Formation was never deposited on top of the horst, or it was removed prior to the time of Hinckley deposition.
RI-17 Geology, Sulfide Mineralization and Geochemistry of the Birchdale - Indus Area Koochiching County, Northwestern Minnesota
(Minnesota Geological Survey, 1977) Ojakangas, Richard W.; Meineke, David G.; Listerud, William H.
The rocks of the Birchdale-Indus area, in northern Minnesota, are part of a poorly-exposed volcanic-sedimentary sequence within the southwest extension of the Wabigoon Volcanic Belt of Canada. The bedrock is Early Precambrian (or Archean) in age and includes mafic to intermediate lavas and sub volcanic intrusive rocks; felsic dikes, agglomerate, tuff, and volcaniclastic rocks; iron-formation and associated metasediments; and granitic rocks of Algoman age. All rock types are cut by northwest-trending Middle Precambrian mafic dikes. The area is largely covered by Pleistocene glacial deposits. The volcanic-sedimentary sequence has been isoclinally folded and has steeply-dipping northeast-trending bedding and foliation. A doubly-plunging anticline and a syncline have been mapped within the area, and a second generation of folding on more northerly trending axes is suggested by some structural observations. Three sets of faults and fractures have been delineated. All the Lower Precambrian rocks were metamorphosed to amphibolite grade during the Algoman event. Synthesis of the geology of the Birchdale-Indus area with that of the better exposed Emo area in adjacent Ontario (Fletcher and Irvine, 1954-) provides a firm basis for interpretation of the development of the volcanic accumulation. Mafic and intermediate volcanic and intrusive rocks apparently constitute a lower stratigraphic unit. An explosive felsic volcanic center, marked by abundant agglomerate, developed upon the older, mafic platform in the vicinity of Birchdale, Indus, and Emo. Felsic tuff, volcaniclastic rocks, and iron-formation were deposited outward from this center. The Birchdale-Indus area and areas to the west and south have been actively explored for base metal sulfide deposits during the past decade. Thick zones of massive, submassive, and disseminated pyrite and/or pyrrhotite have been penetrated at several localities, but copper and .zinc have not been found in economic quantities. Several of the iron-sulfide bodies that were penetrated are associated with oxide iron-formation. To assist in further exploration, geophysical and geochemical anomalies have been located by ground survey methods by the Minnesota Department of Natural Resources.
RI-21 Lower and Middle Precambrian Stratigraphic Nomenclature for East-Central Minnesota
(Minnesota Geological Survey, 1978) Morey, G.B.
This report summarizes stratigraphic nomenclature used by the Minnesota Geological Survey for the Lower and Middle Precambrian rocks that occur in east-central Minnesota. In addition, it provides brief descriptions of dike rocks of possible Middle and Upper Precambrian age. A sequence of Lower Precambrian rocks consisting of an older gneiss terrane and a younger greenstone-granite terrane has been established on the basis of reconnaissance mapping in the area. The gneiss terrane is divided into three lithostratigraphic units of uncertain relative age. These units are the Richmond Gneiss, the McGrath Gneiss, and the Sauk Rapids Metamorphic Complex. The Sauk Rapids Metamorphic Complex is further subdivided into three units of formational status: the Sartell Gneiss, a broadly distributed quartzofeldspathic unit, and the Watab Amphibolite and St. Wendel Metagabbro, mappable mafic enclaves within the Sartell Gneiss. The younger greenstone-granite terrane is poorly exposed, and these rocks are not described in this report. The Middle Precambrian consists of both stratified and plutonic rocks. The stratified rocks lie unconformably on the Lower Precambrian rocks and on the Hillman Migmatite, a hybrid rock of uncertain age, and are divided into two groups. From oldest to youngest these are: (1) Mille Lacs Group, comprising the Denham Formation, Glen Township Formation, Randall Formation, Little Falls Formation, and Trout Lake Formation; and (2) the Animikie Group, comprising the Mahnomen Formation, Trommald Formation, Rabbit Lake Formation, and the Thomson Formation. An iron-rich uni t named the Emily Member is also formally recognized in the Rabbit Lake Formation. The stratified rocks were deformed and metamorphosed during the Penokean orogeny, and were subsequently intruded by igneous rocks of gabbroic to granitic composition. The igneous activity was characterized by the emplacement of several plutons of widely varying size. Small dikes or sills of gabbroic to dioritic composition are inferred to be the oldest Middle Precambrian plutonic rocks. Probably this period of mafic activity was followed by the emplacement of several small stocks of granodioritic composition --the Freedhem and Bradbury Creek Granodiorites of this report-and later by the emplacement of several large granitic plutons of generally sodic composition--the Reformatory, Isle, Warman and Pierz Granites of this report. Plutonic igneous activity culminated with the emplacement of the Stearns Granitic Complex. This dominantly granitic unit is subdivided into the St. Cloud Granite and a border phase named the Rockville Granite. The former is further subdivided into several unnamed facies. The earlier gabbroic to dioritic and granodioritic rocks were emplaced during the waning stages of the Penokean orogeny, whereas the granitic rocks generally are post-tectonic in age. The Lower Precambrian rocks and the Middle Precambrian felsic plutonic rocks are cut by both felsic and mafic rocks of unknown age, whereas the Middle Precambrian stratified rocks are cut by several kinds of mafic dikes emplaced during Early and Middle Keweenawan (Upper Precambrian) time.
RI-18 Cedar Valley Formation (Devonian) of Minnesota and Northern Iowa
(Minnesota Geological Survey, 1978) Mossler, John H.
RI-19 Results of Subsurface Investigations in Northwestern Minnesota, 1972
(Minnesota Geological Survey, 1978) Mossler, John H.
Two test wells were drilled in northwestern Minnesota during a test drilling program conducted by the Minnesota Geological Survey in 1972. Total footage drilled was 851.5 feet (260 m). Together, the wells represent a relatively complete stratigraphic section of the Paleozoic and Mesozoic rocks found in northwestern Minnesota, which are covered by thick Pleistocene overburden and do not crop out. Four stratigraphic units are present. The basal unit is the Winnipeg Formation, a Middle Ordovician unit approximately 160 feet (49 m) thick composed of quartzose sandstone and shale. It is overlain by the Red River Formation, an Upper Ordovician unit about 200 to 300 feet (61 to 91 m) thick composed of dolomitic limestone and dolomite. The other two units are interpreted to be Mesozoic. The lower unit is a reddish-brown shale with a thin layer of dolomite at the top. It is as much as 105 feet (32 m) thick in Minnesota. It may be equivalent to the red beds in adjoining parts of Manitoba and North Dakota that are assigned a Jurassic age, although no paleontologic evidence was found to indicate its age. The uppermost unit is a gray shale that is provisionally interpreted to be Cretaceous. Two tills can be recognized in the Pleistocene section on the basis of the lithology of the sand-size fraction and size-grade distribution. The surficial Pleistocene unit is clay associated with Glacial Lake Agassiz. The Precambrian basement in the area is principally volcanogenic metasedimentary rock and metamorphosed volcanic rock of Early Precambrian age. It was penetrated in one well where it is represented by actinolitic hornblende-plagioclase schist, a mafic, tuffaceous, sedimentary rock that has been metamorphosed.

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