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-28- REFRACTION SEISMIC INVESTIGATIONS OF THE NORTHERN MIDCONTINENT GRAVITY HIGH HAROLD M. MOONEY, CAMPBELL CRADDOCKt, PAUL R. FARNHAM2, STEPHEN H. JOHNSON3, AND GARY VOLZ4 Department of Geology and Geophysics University of Minnesota Minneapolis, Minnesota A B S T R A C T Eighty—seven seismic refraction profiles have been obtained to define the geologic structure in the upper crust associated with the Midcontinent Gravity High in Minnesota and Wisconsin. The seismic measurements were taken across a fixed spread of seven geophones from distances up to 13 km. A structural section was prepared for each profile by interpretation of the travel—time graph, and the individual sections were compiled into regional cross sections. Measured seismic velocities in bedrock fall in the 2.5 — 7.1 km./sec. range. Observed velocities can be assigned to seven groups corresponding to Paleozoic, upper, middle, and lower Upper Kewee— nawan strata, Middle Keweenawaivolcanics, pre—Keweenawan felsic intru— sives, and pre-Keweenawan mafic intrusives. These groups display good continuity through the area and allow tentative correlation of rock bodies between geologic provinces. The St. Croix Horst and its flanking basins underlie the Midcontinent Gravity High and its parallel gravity lows north of Minneapolis. Minimum throw along the western and eastern boundary fault zones reaches about 3.0 and 2.0 km. Sedimentary rocks in the Eastern Basin reach a thickness of at least 2.6 km. A complex horst—like structure also underlies the Midcontinent Gravity High in southern Minnesota; an uplifted basaltic bady is bordered by sedimentary basins about 3.0 km. thick. Middle Keweenawan basalts are nresent lncilly in the Eastern and Western Basins underlying the Upper Keweenawan strata. Rocks probably equivalent to the Oronto Group are rare in the Western Basin, conmion in small basins on the St. Croix Horst, and abundant in the Eastern Basin. Rocks probably enulvalnt to the Bavfield Group are extensive in the Western and Eastern Basins, but they have not been found on the St. Croix Horst. The Bayfield Group seems to be several km. thick across Douglas County north of the Douglas Fault, and it does not appear to increase in thickness under the Bayfield Peninsula.
-29- 1. Department of Geology and G°oohystcs, University of Wisconsin, Madiqon Wisconsin, 53706. 2. Department of Geology, College of St. Thomas, St. Paul, Minnesota. 3. Department of Oceanography, Oregon State University, Corvallis, Oregon, 97331. 4. Chevron Oil Comnany. Houston. Texas. 77027.
Page 1 and 2: -ç — — I: .t. F Geology Superi
Page 3 and 4: TABLE OF CONTENTS INSTITUTE DIRECTO
Page 5 and 6: -2- "P R 0 G R A M" Tate day, Ma9 5
Page 7 and 8: -4- SESSTOM 2 1.00 Business Meeting
Page 9 and 10: -6- F'viday, May Slit, 1970 Page No
Page 11 and 12: -8- SYNTHESIS OF EARLY PRECAMBRIAN
Page 13 and 14: -10- THE SOUTHERN ASSOCIATED PART O
Page 15 and 16: -12- THE STRUCTURE OF THE NORTHERN
Page 17 and 18: -14- MT. ST. JOSEPH - AN ARC}IAEAN
Page 19 and 20: -16- A MODEL FOR TECTONIC VARIATION
Page 21 and 22: -18- EARLY PRECAMBRIAN GEOLOGY OF T
Page 23 and 24: DEFORMATION OF THE SEINE CONGLOMERA
Page 25 and 26: -22- MINERAL POTENTIAL IN CREENSTON
Page 27 and 28: -25-- ROOT SEVERANCE AND TECTONIC T
Page 29: -27- STRUCTURE OF ThE DULUTH GABBRO
Page 33 and 34: -31-. KEWEENANWAN COPPER DEPOSITS I
Page 35 and 36: —33- The youngest rocks in the ar
Page 37 and 38: -35- STRUCTURAL AN!) METAMORPHIC HI
Page 39 and 40: -37.- ARCHAEAN VOLCANIC STRATIGRAPH
Page 41 and 42: ______ __________ _____ IDEALIZED S
Page 43 and 44: —41- (3) The upper liimit of 30 p
Page 45 and 46: -43- Fjgire 2 The revised classific
Page 47 and 48: —45— EVIDENCE FOR A TROPICAL CL
Page 49 and 50: —47-- WIDESPREAD OCCURRENCE OF AL
Page 51 and 52: —49— PROTEROZOIC ROCKS IN THE T
Page 53 and 54: —51— Guide to the Proterozoic R
Page 55 and 56: WEST EAST f * . . .. . . S S S S 5S
Page 57 and 58: ________ — ;—=- — —-- —
Page 59 and 60: —53— The uppermost submember of
Page 61 and 62: —55— Morey notes that sediment
Page 63 and 64: —57— (c) layered mafic bodies,
Page 65 and 66: —59— DESCRIPTION OF STOPS Milea
Page 67 and 68: —6 P- Note downwarping of beds on
Page 69 and 70: —63— Note the lenticular chert
Page 71 and 72: —65— 0.0 From the intersection
Page 73 and 74: —67— STOP 16 In the roadcut to
Page 75 and 76: —69— ThE BEARDMORE-GERALDTON BE
Page 77 and 78: —72— the Quetico belt by Stockw
Page 79 and 80: —72 B— ABITIBI BELT —ø.jsour
Page 81 and 82:
—74— Most of the metavolcanic
Page 83 and 84:
—76— SELECTED REFERENCES Anonym
Page 85 and 86:
—78— Stockwell, C. II., 1964; F
Page 87 and 88:
—80— ae DESCRIPTION OF STOPS 0.
Page 89 and 90:
—82— STOP 8 (Paint Lake Fault).
Page 91 and 92:
-84— Outcrops near the road are o
Page 93 and 94:
—87— Guide to the Port Coldwell
Page 95 and 96:
AGE RELATIONSHIPS The volcanosedime
Page 97 and 98:
.• ____________________ . — .._
Page 99 and 100:
-I- + -4.- -I- - - - - — - 4- ; E
Page 101 and 102:
—93— (3) 'flow—layering' exhi
Page 103 and 104:
GOSSAN ZONE IN SULPHIDE BEARING GAB
Page 105 and 106:
—95— (3) feldspars variably fol
Page 107 and 108:
—97— Fig. 6c Fig. 6b ;' TRAIL T
Page 109 and 110:
Pig. 7 West contact of CROSS SECTIO
Page 111:
PRINCE AR THUR HOTEL ® 0 ® SHOREL
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