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SP-01 History of the Minnesota Geological Survey
(Minnesota Geological Survey, 1964) Schwartz, G.M.
Scattered through the writings of the early explorers in Minnesota are notes on various aspects of the geology of Minnesota. Among the earliest and perhaps most notable was Father Hennepin's account of the discovery and naming of Saint Anthony Falls of the Mississippi River. It should be noted that his description was so accurate that later N. H. Winchell was able to use the location in his remarkable contribution on the time required for the migration of the Falls since the retreat of glacial ice from Minnesota. The most important of the early geological surveys which included Minnesota was that by David Dale Owen who worked under instructions from the United States Treasury Department. Owen was assisted by J. G. Norwood, Charles Whittlesey, B. F. Shumard, and Joseph Leidy. The report of this survey, published in 1852, consists of 638 pages and 92 plates and maps and is entitled "Report of a Geological Survey of Wisconsin, Iowa, and Minnesota." This comprehensive report furnished the principal foundation for later work by the Minnesota Geological and Natural History Survey. An effort was made by the first legislature of the state in 1858 to establish a geological survey. The attempt was repeated by the second legislature, but a lack of income and opposition by the Governor caused the effort to be dropped. In 1864 a joint resolution by the legislature authorized the Governor to appoint a State Geologist. A. H. Hanchett was appointed, and he was assisted by Thomas Clark. Both submitted reports, but evidently Hanchett's performance was not satisfactory, and Henry H. Eames was appointed State Geologist by the Governor. Eames made brief reports for 1865 and 1866, but the legislature refused further appropriations. Other sporadic efforts were made until 1870 when the governor appointed Professor Alexander Winchell of the University of Michigan to examine and report on the reputed Salt Spring lands at Belle Plaine in the Minnesota River valley. The law which established the survey on a permanent basis was drawn up by President W. W. Folwell of the University of Minnesota. There is little doubt that President Folwell's prestige was of prime importance in securing passage of the bill, which entrusted the task of organizing the survey to the University. The law was passed by both houses and approved by Governor Horace Austin on March 1, 1872. The complete act was published in the first annual report and again in Bulletin 1. The essential features may be summarized as follows. The title was "The Geological and Natural History Survey of Minnesota." The Survey was entrusted to the University of Minnesota, where it still remains. The Geological Survey was to include all aspects of the geology of the State with emphasis on all economic materials. Other sections provided for botanical, zoological, and meteorological studies and the establishment of a museum.
SP-02 Geology and Origin of the Iron Ore Deposits of the Zenith Mine Vermilion District, Minnesota
(Minnesota Geological Survey, 1968) Machamer, Jerome F.
The Zenith mine, at Ely in the Vermilion iron district of Minnesota, yielded about 21 million tons of high grade iron ore before closing in 1962. The ore that was mined consisted of massive crystalline hematite, brecciated and cemented by a later generation of crystalline hematite. The deposits are steeply dipping tabular bodies enclosed within walls of low-rank metabasalts of the Precambrian Ely Greenstone; they occupy the stratigraphic position of and grade upward into a jaspilitic iron-formation. Geologic and petrographic relations indicate that the deposits are post-metamorphic in age. One of the ore bodies has a mineralogic zoning characterized by a magnetite zone incorporated within and surrounded by a hausmannite zone. Near the bottom of the deposit both the cementing hematite and the early brecciated hematite give way to carbonate minerals. Two zones of alteration can be recognized in the greenstone wall rocks: an outer zone composed entirely of chlorite and an inner zone in which the rocks are stained by hematite. The inner zone has a mineralogic zoning; dominant kaolinite grades outward into dominant 2Ml muscovite. The altered zones contain substantially more iron than the unaltered country rock. The composition of some of the carbonate minerals indicates that they were deposited at a temperature near 400°C. The mineralogic relations indicate that during much of the period of ore deposition the fugacities of oxygen and sulphur fluctuated around the equilibrium values for the coexistence of hematite, magnetite, and pyrite, and that the fugacity of CO2 was on the order of 103 atmospheres. The kaolinitic alteration adjacent to the deposits indicates that the altering fluid may have been acid, and it is postulated that the acidity resulted from a relatively high concentration of CO2 in the fluid. It is concluded (1) that the deposits were formed by the replacement of the silica in the iron-formation by hematite, (2) that the iron probably was transported in large part as free ferrous ions in an acid hydrothermal fluid, (3) that deposition and replacement was largely restricted to the jaspilite because of its brecciation, and (4) that two of the principal causes of ore deposition were an increase in pH of the fluid resulting from the escape of CO2 in the brecciated zone and oxidation of the iron to the ferric state. The oxygen may have been derived either from the dissociation of unstable oxidized agents introduced with the ore fluid or by the downward diffusion of atmospheric oxygen through ground water. The source of the metals and the fluid is unknown; gross spatial relations suggest that possibly both the metals and the fluid were derived from depth, in an environment of more intense metamorphism.
SP-03 Ostracoda of the Dubuque and Maquoketa Formations of Minnesota and Northern Iowa
(Minnesota Geological Survey, 1965) Burr, J.H. Jr.; Swain, F.M.
The ostracodes of the Dubuque Formation at Wubbles Ravine, Fillmore County, Minnesota and from the depauperate zone of the Maquoketa Formation at Bellevue State Park, Iowa comprise twelvecgenera and seventeen species, four of which are new. On the basis of the ostracode data assembled here it is concluded that: (1) the Dubuque Ostracoda differ slightly from those of other Middle and Upper Ordovician rocks in the Mississippi Valley; (2) no equivalent of the Eden and Maysville Groups can at present be recognized in ostracode assemblages from Minnesota; (3) an unconformity exists between the Dubuque and Maquoketa Formations, recognizable on both faunal and lithologic grounds; and (4) the ostracode species of the depauperate zone of the Maquoketa Formation are not as a whole characterized by reduced dimensions of the shell as are some of the macrofaunal species.
SP-04 The Middle and Upper Ordovician Conodont Faunas of Minnesota
(Minnesota Geological Survey, 1966) Webers, G.F.
About thirty-five thousand identifiable conodonts were recovered from samples of Middle and Upper Ordovician sedimentary rocks of Minnesota. One section was sampled in detail for each formation except the Glenwood, which was sampled at three localities in east-central and southeastern Minnesota. Thirty-three form species or groups of form species
SP-05 Geology of Precambrian Rocks, Granite Falls-Montevideo Area, Southwestern Minnesota
(Minnesota Geological Survey, 1968) Himmelberg, Glen R.
Precambrian rocks exposed in the Granite Falls-Montevideo area, within the Minnesota River valley, consist of interlayered metamorphic rocks that are intruded by mafic dikes and a small adamellite body. Lithologically the metamorphic rocks are granitic gneiss, hornblende- pyroxene gneiss, garnet-biotite gneiss, and a heterogeneous sequence of interlayered gneisses containing variable proportions of biotite, hornblende, pyroxene, feldspar, and quartz. The mafic dikes are tholeiitic diabase, hornblende andesite, and olivine diabase. Dynamothermal metamorphism approximately 2500-2700m.y. ago produced an inclined, cylindrical fold system that plunges approximately 15° N. 85 W. Most mineral assemblages resulting from the metamorphism belong to the granulite facies. Mineral assemblages characteristic of the amphibolite facies are interlayered with those of the granulite facies, and there is no indication of metamorphic zoning. Coexisting mineral assemblages indicate that there was an approach to chemical equilibrium and that there were no significant variations in physical conditions during metamorphism. Common retrograde metamorphic textures are "serpentine" veins in orthopyroxene, rims of cummingtonite on orthopyroxene, and rims of sea-green actinolite-hornblende on clinopyroxene and hornblende. Intrusion of tholeiitic diabase dikes along a northeast-trending fracture system occurred after the metamorphism and folding. A minimum age for the tholeiitic diabase is 2080 m.y. Cataclastic deformation, represented by narrow northwest- trending shear zones and by granulation, took place after intrusion of the tholeiitic diabase but before intrusion of the 1700-1800 m.y. old hornblende andesite dikes. The 1800 m.y. event is also represented by intrusion of a small adamellite body that was contemporaneous with a thermal event that resulted in the resetting of the biotite ages in the metamorphic rocks.
SP-06 The Cryptostome Bryozoa from the Middle Ordovician Decorah Shale, Minnesota
(Minnesota Geological Survey, 1969) Karklins, Olgerts L.
The Middle Ordovician Decorah Shale in Minnesota is a distinctive fonnation containing an abundant diversified fauna; this study is concerned with the cryptostome Bryozoa, a group that constitutes a large part of the fauna. I have used a new approach in the interpretation of cryptoston1e zoarial structures emphasizing the dark boundary zones and their spatial arrangement in zoaria as distinct morphological features in genera of the families Rhinidictyidae, Stictoporellidae, and Ptilodictyidae. The dark boundary zones are formed by abutting or adjoining laminae and, when present, they outline zooecia or other structural segments of zoaria. In thin sections the boundaries appear as dark lines representing the edge views of planar to curved zones between zooecia and extend for different lengths throughout zoaria. Sixteen species distributed among six genera are described and illustrated. One species, Stictopora lita, is new. On the basis of the boundary zones in association with other morphological features two new genera, Astreptodictya, type species Pachydictya acuta (Hall), and Athrophragma, type species Pachydictya foliota Ulrich, are proposed. Emended generic defmitions are presented for Stictopora, Escharopora and Graptodictya. The wall structure of each genus is described in detail and, where the material permits, previously described Ordovician species are reassigned to new genera. The infonnal stictoporid and escharoporid groups of Ross (Phillips, 1960) are redefmed and three additional infonnal groups are proposed: "stictoporellid," athrophragmid, and astreptodictyid. The genera are reassigned to these groups as appropriate. The stictoporid group is defined as having zoaria with approximately linear zooecial ranges in which adjacent zooecia are separated by range boundaries laterally and zooecial boundaries longitudinally. In the "stictoporellids" the zooecial boundaries are polygonal in tangential views and the range boundaries are lacking. The escharoporid group is characterized by having well-defined wall laminae that are continuous between adjacent zooecia. There are no boundary zones in the exozone of zoaria. In the athrophragrnids the zooecial boundaries descnbe a cylindrical form in the exozone and appear approximately oval in tangential views. The walls between adjacent zooecia in the exozone may contain numerous intermittent dark zones, but there are no range boundaries. The astreptodictyid group is distinguished in having range partitions between adjacent zooecial ranges in exozone that extend throughout zoaria and are at about right angles to the zoarial swface. The zooecial boundaries are like those in the athrophragmids and the range boundaries, similar to those in the stictoporids, are located along the median of the range partitions. Geographic and stratigraphic distribution of species permits the division of the Decorah Shale into three zones from bottom to top: 1. Stictoporella angularis zone, with three restricted species, 2. Stictopora mutabilis zone, with one restricted species, and 3. Stictopora minima zone, with two restricted species. One of the remaining species, Stictopora lita, occurs primarily in the Stictoporella angularis zone, but ranges into lower part of Stictopora mutabilis zone; seven species occur primarily in the S. mutabilis zone, but range into S. minima zone, and two were found throughout the section at all seven localities. Cryptostomata are abundant throughout the Decorah Shale except in the lower part of the Stictopora mutabilis zone. The three cryptostome zones approximately coincide with ostracode zones suggested by Swain and others in 1961. On the basis of present knowledge, the Stictoporella angularis zone is the approximate biostratigraphic equivalent of the Spechts Ferry Shale Member of the Decorah Formation in Illinois, Wisconsin, and Iowa. Evaluation at the species level suggests that the cryptostomes of the Decorah Shale are more closely related to those of the Trenton Group of New York than to those of the Black River Group of New York.
SP-07 The Geology of the Middle Precambrian Rove Formation in northeastern Minnesota
(Minnesota Geological Survey, 1969) Morey, G.B.
The Middle Precambrian Rove Formation, the upper part of the Animikie Group, is estimated to be at least 3,200 feet thick and is exposed between northwestern Cook County, Minnesota and the Thunder Bay district, Ontario. It is a sequence of graywacke, argillite, locally abundant intraformational conglomerate, quartzite, and carbonate rocks. The formation was deposited some time between 2.0 b.y. and 1.7 b.y. ago in a northeast-trending basin, the configuration of which probably was controlled by a pre-existing structural grain. Detailed mapping in the 7 1/2-minute South Lake quadrangle combined with a field and laboratory study of approximately 150 other scattered stratigraphic sections provide a basis for the recognition of three informal lithic units. From oldest to youngest these are: (1) lower argillite, 400 feet thick; (2) transitional beds' of interbedded argillite and graywacke, 70 to 100 feet thick; and (3) thinbedded graywacke, as much as 2,700 feet thick. It is concluded that the argillite and associated graywackesandstone and graywacke-siltstone units were deposited in moderately deep, quiet water. Repeated graywacke sedimentation units indicate sediment transport and deposition by turbidity currents. A sedimentation unit reconstructed from composite sections consists of (1) a basal conglomeratic graywacke, (2) a structure less unit that grades indistinctly into (3) a graded graywacke that is overlain by (4) a laminated graywacke, which may be modified by (5) small-scale cross-bedding, or (6) contorted bedding. Anyone or several of these may be absent, but the unit is always overlain by (7) an argillite. Post-depositional soft-sediment structures such as load casts, flame structures, clastic dikes, bed pull-aparts, overfolds, and microfaults indicate rapid deposition of Rove sediments, active bottom currents, and post-depositional deformation, implying a significant paleoslope. A detailed analysis of paleocurrent directional indicators such as groove casts, flute casts, dendritic ridges, and cross-bedding shows that the turbidity currents had a southerly trend about perpendicular to the axis of the Rove basin. However, ripple marks, winnowed lag deposits at the tops of many graywacke beds, and possibly some festoon-type cross-bedding show that the turbidities were later modified by bottom currents that trended southwesterly or parallel to the axis of the basin. The heavy minerals of the Rove are characterized by epidotegroup minerals, apatite, sphene, and tourmaline, and are typical of older Precambrian igneous rocks now exposed north of the present Rove outcrop area. Thin-section and X-ray analyses of 200 samples show that the graywackes consist of angular, poorly sorted grains of clastic quartz and plagioclase (An10-An25) embedded in an argillaceous matrix that now consists of quartz, chlorite, and muscovite. The fine-grained, fissile argillite and mudstone have the same mineralogy and microtextures as the graywacke. Erosion subsequent to pre-Keweenawan tilting removed an unknown amount of the formation prior to the deposition of Lower Keweenawan sedimentary rocks. The intrusion of Middle Keweenawan mafic igneous rocks caused local metamorphism of the Rove Formation to a variety of mineral assemblages now assigned to the pyroxeneand hornblende-hornfels facies, but the remainder of the formation is essentially unmetamorphosed.
SP-08 The Geology of the Isaac Lake Quadrangle, St. Louis County, Minnesota
(Minnesota Geological Survey, 1969) Griffin, W.L.; Morey, G.B.
The Isaac Lake quadrangle lies between the Mesabi and Vermilion ranges, and includes a segment of each. The Lower Precambrian rocks that characterize the Vermilion range underlie the northern 2/3 of the quadrangle, and include both the Ely Greenstone (with interbedded iron-formation) and an overlying sequence of interlayered metasedimentary and meta-volcanic rocks. These rocks strike northwestward and dip steeply northeast or southwest; they were folded and intruded by the Giants Range Granite during the Algoman orogeny (2.5 b. y.). The grade of regional metamorphism increases southeastward along strike from Tower into the Isaac Lake quadrangle; the Ely Greenstone and meta-sedimentary and meta-volcanic rocks within the quadrangle contain mineral assemblages characteristic of the upper amphibolite facies. The well known Middle Precambrian Animikie Group of the Mesabi range uncomformably overlies the Giants Range Granite in the southeastern corner of the quadrangle and these rocks have been intruded by dikes of Keweenawan (?) age and metamorphosed by the Middle Keweenawan Duluth Complex. The basalts and tuffaceous sedimentary rocks of the Ely Greenstone are metamorphosed to massive or schistose amphibolites, and contain thin lenses of cherty iron-formation. The overlying highly metamorphosed sedimentary and volcanic rocks have been informally divided into two units, the layered gneiss and the (overlying ?) Argo gneiss. The Argo gneiss consists of fine-grained weakly foliated biotite gneisses with rare thin mafic layers; these gneisses are metamorphic equivalents of greenschist facies slates and graywackes which lie along strike to the northwest. The layered gneiss comprises amphibolite and leucocratic biotite and hornblende gneisses, interlayered on all scales and well-foliated parallel to the compositional layering. Thick layers of very coarse-grained biotite gneiss within the layered gneiss apparently were partially mobile during metamorphism. The layered gneisses are correlative with less-metamorphosed volcanic and volcaniclastic rocks to the northwest. The higher-grade parts of the layered gneiss are migmatized by fme-grained leucotrondhjemites that were foliated by the regional metamorphism, recrystallized to gneissic textures, and cut by granitic dikes. The Algoman Giants Range Granite intrudes the older rocks and underlies the southern half and the northeastern corner of the quadrangle. It is predominantly a coarse-grained hornblende granodiorite or monzonite. The granite is strongly foliated parallel to its contacts and is generally conformable to the structure of the country rocks. The batholith is interpreted as a late-kinematic forceful intrusion, emplaced at relatively shallow depths. Two major transcurrent faults strike northeastward across the quadrangle. The western one, here named the Waasa fault, offsets the granite-gneiss contact three and three-fourth miles. The eastern, or Camp Rivard fault offsets this contact about two miles. Contacts and fold axes within the gneisses appear to be offset even further, and other evidence also suggests that the faults were active both before and after emplacement of the granite. Numerous smaller faults are sub-parallel to the larger ones. Rocks north of the Waasa fault dip steeply northeast; those between the two faults generally dip steeply south west. Statistical analysis of foliations and lineations suggests non-cylindrical folding about a northwest-trending axis. In the northwestern corner of the quadrangle there has been minor later crossfolding about an axis perpendicular to the axis of the major folding. The Middle Precambrian Animikie Group, in the East Mesabi district of the Mesabi range consists of three comformable sedimentary formations, the Pokegama Quartzite at the base, the Biwabik Ironformation, and the Virginia Formation at the top. They are exposed in the southeastern part of the quadrangle where they lie unconformably on the Giants Range Granite. The Pokegama Quartzite fills minor topographic irregularities in the older granite surface; accordingly, it is variable in thickness, ranging from near zero to approximately 30 feet. The Biwabik Iron-formation is approximately 400 feet thick and is subdivided into seven cartographic units on the basis of bedding characteristics, texture, and gross mineralogy. These units are readily correlated with those previously recognized by Wolff (1917) and Gundersen (1960). The Virginia Formation is not exposed in the quadrangle and is known only through diamond drilling. The structure of the Animikie Group is relatively simple. The beds dip 5° - 15° SE, but the lower part of the Biwabik Iron-formation is warped by several small-scale folds whose axes trend north-northwest and plunge south-southeast at low angles. Structural closure on each fold seems to decrease upward so that near the top of the iron-formation the small-scale folds are no longer apparent. The Animikie Group was intruded by gabbro dikes of possible Middle Keweenawan age, and metamorphosed by the Middle Keweenawan Duluth Complex. The formation is characterized by abundant prograde and retrograde cummingtonite with minor amounts of prograde fayalite, orthopyroxene, hedenbergite, and diopside.
SP-09 Geological Map of the Minneapolis Quadrangle, Minnesota
(Minnesota Geological Survey, 1970) Hogberg, R.K.
Plate 1, Surficial Geology ; Plate 2, Bedrock Geology, scale 1:48,000 (Minneapolis 15' quadrangle)
SP-10 Clay Mineralogy and Geology of Minnesota's Kaolin Clays
(Minnesota Geological Survey, 1970) Parham, Walter E.
Humid tropical weathering during the latter part of the Mesozoic Era, probably during early Late Cretaceous time, produced a thick kaolinitic residuum (unit 1) over much of Minnesota, mainly from Precambrian metamorphic and igneous rocks. The weathered zone is now covered by younger Cretaceous sedimentary rocks and Pleistocene glacial deposits except locally along the Minnesota River Valley in southwestern Minnesota and between St. Cloud and Little Falls in the central part of the state. As much as 100 feet of residuum is exposed along a 45-mile long section of the Minnesota River Valley between Granite Falls and Fort Ridgley State Park; marginal to the valley, the residuum is overlain by 40 or more feet of clays and shales. and glacial deposits. The clay minerals of the residuum-unit 1 of this report-that were formed from weathering of felsic rock types are composed primarily of kaolinite. In the least weathered parts of the profile, the kaolinite has an irregular platy form. Tubular halloysite is present in minor amounts, especially in the lower part of the weathering profile. Mafic rock types weathered first to montmorillonite and under progressively more intense weathering to kaolinite. Two Upper Cretaceous units of kaolinitic sedimentary rocks (units 2 and 3) overlie the residuum. The lowermost of these (unit 2), which was derived from erosion of the weathered residuum and which also underwent tropical weathering, has a maximum observed thickness of 45 feet, and is composed of varying proportions of kaolinite and quartz, with trace amounts of halloysite. A three- to five-foot, generally iron-rich, kaolinitic, pisolitic clay that contains small amounts of gibbsite and boehmite lies at the top of unit 2. Sedimentary rocks of unit 3 disconformably overlie unit 2, and consist of gray to black, organic-rich clays and shales, thin beds of lignite. and at least one thin bed of bentonite. Kaolinite is abundant in the basal part of unit 3 but gives way upward progressively to montmorillonite and illite, suggesting that the humid tropical climate under which units 1 and 2 had formed had been replaced by more temperate conditions by unit 3 time. Some of the kaolinitic clays of units 1 and 2 are potentially important as raw material for paper coating and filler. However, the presence of minor amounts of halloysite in some of these kaolin clays might adversely affect the flow properties of clay-water suspensions during paper-coating operations by increasing the suspensions' viscosity; some kaolinite having an irregular particle form may also produce a similar effect. It may be difficult to improve the whiteness and brightness of some of the clays of unit 1 if their natural color is pale green; the clays of unit 2, on the other hand, generally respond better to chemical bleaching. In addition, some of the kaolin clays present in all three stratigraphic units may be satisfactory for use in the ceramics and refractories industries. Ball clays of unit 3, which are very plastic and burn white, could be mixed with less plastic kaolin clays of units 1 and 2 for the production of a variety of refractory products. Most of the kaolin clays of the three units when fired become tan, pink. or white and could be used in common types of light-colored ceramic products.
SP-11 Glacial and Vegetational History of Northeastern Minnesota
(Minnesota Geological Survey, 1969) Wright, H.E. Jr.; Watts, William A.; Jelgersma, Saskia; Waddington, Jean C.B.; Ogawa, Junko; Winter, T.C.
The broad relief features of the northern Minnesota bedrock permitted the digitation of the margin of the Wisconsin ice sheet into several lobes, and the diversity of the bedrock lithology resulted in the drift of each lobe being of different color, texture, and stone content. Moraines, drumlins, outwash plains, pro glacial lake plains, diversion channels, and other glacial features provide the opportunity to check the contemporaneity of the advances of different ice lobes-or lack of it. Of the four ice lobes identified for the Minnesota area, three affected the northeast. The Superior Lobe moved out of the Lake Superior basin. The Rainy Lobe advanced across the upland north of the Lake Superior basin, and the St. Louis Sublobe of the Des Moines Lobe came from far to the west and almost reached the head of the Lake Superior basin. Four phases of ice movement can be delineated for northeastern Minnesota. In the St. Croix phase, the Superior and Rainy lobes together covered most of the eastern half of the state, forming extensive drumlin fields and terminating at the St. Croix Moraine. During wastage, a great series of southwest-trending tunnel valleys was cut through the drift by subglacial meltwater under hydrostatic pressure, and many were later partially filled with eskers. The Superior Lobe then retreated barely into the Lake Superior basin, and the Rainy Lobe withdrew to near the Canadian border. In the Automba phase that followed, the Rainy Lobe advanced slightly on the upland and formed the Vermilion Moraine. The Superior Lobe expanded out of the Lake Superior basin with configuration different from before, because it was not impeded this time by the Rainy Lobe on its right flank. It sent a long tongue southwestward as far as the Mille Lacs Moraine in the center of the state, and it built the Highland Moraine at the crest of the slope leading up from the north shore of Lake Superior. It also formed drumlins and fluted terrain in its progress toward its terminal moraines. Glacial Lakes Upham I and Aitkin I were dammed on the north side of this long finger of ice. The Superior Lobe then retreated farther into the Lake Superior basin, and a glacial lake in front of the ice received red clayey sediment from the wasting ice. When the ice readvanced once again in a narrow tongue during the Split Rock phase, it incorporated the lake sediments and produced a red clayey till, which formed a veneer over the older glacial landforms. Once again the Superior Lobe retreated into the basin, and then it readvanced a still shorter distance than before-to the Nickerson Moraine, made of more red clayey till. Meanwhile, the St. Louis Sublobe extended eastward from the Des Moines Lobe, overrode the silty deposits of Glacial Lakes Upham I and Aitkin I, and reached within 25 miles of the Lake Superior basin. The Superior and St. Louis lobes then began their retreat together. Glacial Lake Upham II formed in front of the retreating St. Louis Sublobe, and its discharge eastward down the newly formed St. Louis River was blocked by the slowly retreating Superior Lobe and diverted southward to the Moose River and ultimately to the St. Croix. Four diversion channels at successively lower elevations can be identified. Finally, with further ice retreat, the St. Louis River entered Glacial Lake Nemadji and then, when the Superior Lobe retreated still more to uncover lower outlets in Wisconsin, the river drained into Glacial Lake Duluth. During these several epochs of ice-margin fluctuation, which occurred between about 16,000 and 12,000 years ago, the exposed terrain was covered with tundra vegetation, as shown by pollen and plant-macrofossil analyses of the sediment at several lake sites north and west of Lake Superior. The pollen stratigraphy has few features that can be correlated with ice-margin fluctuations, however. Either pollen analysis is not a subtle enough technique to reveal climatically induced vegetational changes in a tundra environment, or the vegetation itself did not respond significantly to climatic change, or the ice-margin fluctuations were caused by some factor not directly or immediately related to climatic change. The spruce forest that covered much of central and southern Minnesota during the time of Wisconsin ice retreat spread to northeastern Minnesota. It replaced the tundra west of the head of Lake Superior about 11,500 years ago, but it moved northward slowly, reaching Weber Lake, about 70 miles farther northeast, 10,000 years ago. Meanwhile the spruce forest in the south was replaced by birch and alder about 11,000 years ago and then abruptly by jack or red pine, which entered the state from the east in very great numbers about 10,500 years ago. The pine spread rapidly northward, but its dominance in the newly established spruce forest developed much more slowly, and it was not until about 7200 years ago that the spruce forest was completely gone. Meanwhile, alder spread abruptly in the area about 9000 years ago, and oak spread up from the south about 8500 years ago. At about the same time, white pine invaded in quantity from the east. By 7000 years ago even prairie openings occurred in the region. Reversal of the climatic trend initiated withdrawal of prairie herbs and oak from northeastern Minnesota, and the westward expansion first of white pine, spruce, and larch, and later of red pine and jack pine. Lakes that had previously been intermittently dry (as indicated by macrofossils) became permanent again, and as the lakes became filled with sediment they developed a margin of marsh or bog in the shallow water, and many lakes became converted completely to wetlands. The changing composition of the forest cover since deglaciation was caused basically by a climatic change to warmer and drier conditions and then the reverse. But the effects were not instantaneous: differential rates of migration of major tree types from Pleistocene refuges resulted in successive arrivals of potential dominants, so that the forest was continually changing. The additional factors of progressive leaching of the soil and paludification of lowlands added habitats that were not previously very extensive. These trends have continued to modern times, when lumbering, agriculture, and fire protection have interrupted the natural successions.
SP-13 Lower Precambrian Rocks of the Gabbro Lake Quadrangle, Northeastern Minnesota
(Minnesota Geological Survey, 1970) Green, John C.
In an approximately 100-square-mile area east of Ely in the center of the Vermilion district, three major stratigraphic units are mapped. The oldest unit, the Ely Greenstone, contains at least 12,000 feet of dominantly metabasaltic rocks, and the base is not exposed. Thin chert-siderite and chert-magnetite iron-formations are interbedded with the lavas. Overlying this unit essentially conformably (though with a basal conglomerate composed of Ely Greenstone clasts) are at least 5,000 feet of graywackes, argillites, slates, and felsic to intermediate pyroclastic rocks and their clastic debris, which constitute the Knife Lake Group. Apparently stratigraphically above the Knife Lake Group, as here defined, is a thick sequence of felsic and intermediate volcanic rocks, mostly pyroclastic, that interfingers to the west with metabasalts; this sequence has been named the Newton Lake Formation. This unit, which may be continuous with rocks to the east included by Gruner (1941) in the Knife Lake Group, consists of at least 8,000 feet of strata, including a thick sequence of mafic clastic rocks and a 500-foot thick lens of recrystallized calcareous chert. Along the North Kawishiwi River another thick group of metaconglomerates and metagraywackes, now gneisses and schists, is faulted against the lower part of the Ely Greenstone. These rocks are tentatively assigned to the Knife Lake Group. The stratified rocks are intruded by a variety of porphyries, including a regionally widespread porphyritic dacite-rhyodacite that was extruded onto the surface in Late Ely Greenstone time. The Ely Greenstone and the Knife Lake metasedimentary rocks along the North Kawishiwi River were metamorphosed and intruded by the granitic rocks of the Giants Range batholith, which is dominated in this area by two facies, a non-porphyritic (Clear Lake) and a porphyritic (Farm Lake) type. Both facies are composed predominantly of hornblende-biotite adamellite, monzonite, and granodiorite, and contain many local variations including mafic types. Most of the contacts within the batholith are unchilled, and many suggest physical mixing of viscous magmas. Fine-grained biotite adamellite is the youngest mappable phase, and aplite and pegmatite dikes are common. A variety of dioritic and gabbroic dikes cut the batholith but they are metamorphosed by it. At the northwestern margin of the quadrangle are several small outlying plutons of the Vermilion batholith, which has metamorphosed the adjacent rocks. Southeast of Fall Lake, granitic rocks have been faulted upward into the Knife Lake metasedimentary rocks, and southeast of Stub Lake small bodies of pink quartz syenite to granodiorite intrude the Ely Greenstone. Keweenawan diabase dikes cut all the major stratigraphic units and the Lower Precambrian structures. The entire structural deformation of the Lower Precambrian rocks in this area is attributable to the Algoman orogeny (2.6-2.5 b.y. ago). The strata are nearly vertical, and depositional structures indicate that tops are generally to the north, away from the North Kawishiwi fault. There is local internal isoclinal folding, however, in all the formations. Faults are of particular significance, and at least some followed intrusion of the batholith. Several eastward- to northeastward-trending faults of regional importance cross the area, and lesser north-northeastward- and northeastward-trending faults with apparent displacements of as much as two miles cut the Ely Greenstone into many blocks. Strong, steeply-plunging lineations are widespread in the northern edge of the area and along the North Kawishiwi River. Kink-folds with gently-plunging or vertical axes, which represent minor displacements and a higher level of deformation, are superimposed on the earlier structures, especially in a zone centered in the Knife Lake belt. Their age is unknown but probably is late Algoman. Metamorphism of the stratified rocks is generally of very low grade (greenschist facies), and a lack of equilibrium is widespread. Near the Vermilion and Giants Range batholiths, epidote-amphibolite and amphibolite facies are attained, and next to the Duluth Complex is a narrow zone of pyroxene hornfels. Although much effort has been spent in the past in prospecting the iron-formations. no economically viable deposits have been found in this area. Sulfides, as disseminations and small veins, are scattered through the greenstones and rarely in other rocks, and exploration for sulfide deposits currently is in progress.