Browsing by Author "Ojakangas, Richard W."

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    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.
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    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.
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    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.
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    RI-67 Lunisolar Tidal Signatures in the ~1.9 GA Pokegama Formation, Northeastern Minnesota: Implications for the Lunar Orbit
    (2010) Ojakangas, Gregory W.; Ojakangas, Richard W.
    Specimens of siltstone beds from a roadcut in the lower member of the ~1.9 Ga Pokegama Formation at the village of Midway near Virginia, Minnesota contain laminae that record possible neap-spring cycles and a strong diurnal inequality. These laminae provide support for the tidal origin of the formation proposed by R. Ojakangas (1983) on the basis of other sedimentary structures. In order to enhance observed cyclicities in the presence of significant diagenetic alteration, a new algorithm was developed that transforms digital images of lamina sequences to horizontal uniformity, and then averages them horizontally to reduce noise and enhance lamina boundaries. Although too short and noisy to provide results of appreciable confidence, spectral analysis of a lamina sequence from the Pokegama Formation suggests minimum values for the lunar orbit radius (greater than 30 earth radii) and length of day (greater than 10 hours) at the time of deposition, consistent with theoretical expectations and extrapolation of other cyclic tidal data. The analysis presented here illustrates the potential value of identifying well-preserved tidal sequences in Proterozoic sedimentary rocks of the Lake Superior region.