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Bedrock Geology of the Brattleboro Quadrangle, Vermont-New Hampsiiire

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Bedrock Geology of the Brattleboro Quadrangle, Vermont-New Hampsiiire BEDROCK GEOLOGY OF THE BRATTLEBORO QUADRANGLE, VERMONT-NEW HAMPSIIIRE By J. CHRISTOPHER HEPBURN' NEWELL J. TRASK2 JOHN L. ROSENFELD3 JAMES B. THOMPSON, JR.4 VERMONT GEOLOGICAL SURVEY CHARLES A. RATTE, State Geologist AGENCY OF ENVIRONMENTAL CONSERVATION Department of Water Resources and Environmental Engineering MONTPELIER, VERMONT BULLETIN NO. 32 1984 1. Dept. of Geology and Geophysics, Boston College, Chestnut Hill, Mass., 02167 2. U.S. Geological Survey, Reston, Va., 22092 3. Dept. of Earth and Space Sciences, Univ. of California, Los Angeles, Calif., 90024 4. Dept. of Geological Sciences, Harvard Univ., Cambridge, Mass., 02138 TABLE OF CONTENTS Page ABSTRACT 9 CHAPTER 1, INTRODUCTION ............................................. 10 Location................................................................... 10 Previous Geologic Work .............................................. 11 Regional Geologic Setting ............................................ 12 Eastern Sequence-Western Sequence Division ................. 15 Acknowledgements ..................................................... 16 CHAPTER 2, STRATIGRAPHY OF THE WESTERN SEQUENCE ..... 17 General Sequence ....................................................... 17 Precambrian.............................................................. 20 Mt. Holly Complex .................................................. 20 Late Precambrian (?) or Cambrian (?) ............................ 20 Bull Hill Gneiss ....................................................... 20 Hoosac Formation ................................................... 21 Cambrian.................................................................. 21 Pinney Hollow Formation ......................................... 21 Ottauquechee Formation ........................................... 21 Ordovician................................................................ 23 Missisquoi Formation ............................................... 23 Moretown Member ............................................... 23 Barnard Volcanic Member ..................................... 26 Ordovician or Silunan ................................................. 31 Unnamed Schist-Amphibolite Unit ............................. 31 Silurian..................................................................... 33 Russell Mountain Formation ...................................... 33 Northfield Formation ............................................... 35 Waits River Formation ............................................. 37 Standing Pond Volcanics .......................................... 42 Gile Mountain Formation .......................................... 47 Putney Volcamcs ..................................................... 52 PlutomcRocks ........................................................... 55 Ultramafic Rocks .................................................... 55 New Hampshire Plutonic Series ................................. 56 Post-Metamorphic Diabase Dike ................................ 59 Age and Correlations .................................................. 59 CHAPTER 3, STRUCTURAL GEOLOGY OF THE WESTERN TERRANE.................................................................. 62 General Statement ...................................................... 62 Pre-Silurian Rocks, East Limb, Green Mountain Anticlinorium .................................................... 64 MajorStructure ....................................................... 64 Minor Structures ..................................................... 64 Central and East Central Brattleboro Quadrangle ............. 67 General Statement ................................................... 67 Younger Major Structures ......................................... 68 Early Major Folds ................................................... 69 MinorFolds ........................................................... 72 Lineations.............................................................. 82 Black Mountain Granite ............................................ 83 Gravity Model, Black Mountain Granite ................... 88 Tectonic Synthesis ................................................... 89 An Alternative Interpretation—Back Folding Accompanying Doming ....................................... 93 CHAPTER 4, METAMORPHISM OF THE WESTERN TERRANE ..... 101 Introduction.............................................................. 101 PeliticRocks ............................................................. 102 Volcanic and Mafic Rocks ........................................... 108 Calcareous Rocks ....................................................... 108 Retrograde Metamorphism ........................................... 112 Large Crystals in Standing Pond Volcanics ..................... 112 Conditions of Metamorphism ........................................ 113 Relation of Metamorphism to Deformation ...................... 114 Age of Metamorphism and Deformation ......................... 120 CHAPTER 5, STRATIGRAPHY OF THE EASTERN SEQUENCE ..... 120 General Statement ...................................................... 120 Ordovician................................................................ 121 Ammonoosuc Volcanics ........................................... 121 Partridge Formation ................................................. 124 Silunan..................................................................... 126 Clough Formation .................................................... 126 Fitch Formation ...................................................... 128 Devoman.................................................................. 129 Littleton Formation ................................................. 129 Intrusive Igneous Rocks .............................................. 134 Age and Correlations, Eastern Sequence ........................ 135 CHAPTER 6, STRUCTURE OF THE EASTERN TERRANE ............ 136 Introduction.............................................................. 136 VernonDome ............................................................ 138 KeeneDome ............................................................. 138 Wellington Hill Anticline ............................................. 141 Bernardston Nappe .141 Skitchewaug Nappe .................................................... 142 Chicken Yard Line ..................................................... 142 Brattleboro Syncline ................................................... 144 Post-Metamorphic Faulting .......................................... 144 CHAPTER 7, METAMORPHISM OF THE EASTERN TERRANE ..... 145 Introduction .............................................................. 145 Pelitic Rocks ............................................................. 146 Mafic Igneous Rocks ................................................... 148 Calcareous Rocks ....................................................... 149 Retrograde Metamorphism ........................................... 149 CHAPTER 8, EcoNoMic GEOLOGY ..................................... 149 CHAPTER 9, GEoLoGIc HISTORY ...................................... 151 Connecticut River Gaspe-Synclinorium, East Flank Green Mountain Anticlinorium ......................................... 151 Bronson Hill Anticinorium .......................................... 152 REFERENCES CITED ....................................................... 154 ILLUSTRATIONS Figure Page 1- 1 Location of Brattleboro Quadrangle . 11 1- 2 Map of Regional Geologic Features .......................... 13 2- 1 Stratigraphic Column for Brattleboro Quadrangle ....... 18 2- 2 Correlation Chart .................................................. 22 2- 3 Porphyritic Amphibolite in the Barnard Volcanic Member.............................................................. 27 2- 4 Layered Gneiss in the Barnard Volcanic Member ....... 29 2- 5 Outline Map of Standing Pond Volcanics .................. 43 2- 6 Detailed Stratigraphy of Western Belt of the Standing Pond Volcanics .................................................... 46 2- 7 Cross-bedding in the Putney Volcanics ..................... 54 3- 1 Map of Principal Structural Features, Brattleboro Quadrangle.......................................................... 63 3- 2 Cleavages and Minor Folds in Moretown Member ...... 67 3- 3 Stereonet Plot of Minor Fold Hinges, Prospect Hill Fold 71 3- 4 Schematic Block Diagram, Prospect Hill Fold ............ 71 3- 5 Stereonet Plot of Minor Fold Hinges, Fall Brook Anticline............................................................. 72 3- 6 Fi Folds, Standing Pond Volcamcs .......................... 73 3- 7 F2 Folds, Standing Pond Volcamcs .......................... 74 3- 8 Sketch of F2 Fold, Waits River Formation ................ 76 3- 9 Stereonet Plot of Poles to Axial Surfaces of F2 Folds, North End Guilford Dome ..................................... 76 3-10 Minor Folds (F2) in Northfield Formation ................. 78 3-11 Refolded Fold, Gile Mountain Formation .................. 79 3-12 Slip Cleavage Parallel to Axial Surface of F3 Fold Northfield Formation ............................................ 80 3-13 Photomicrograph of Slip Cleavage in Mica Schist from Northfield Formation ............................................ 80 3-14 Open, Flexural Folds, Gile Mountain Formation ........ 82 3-15 Kink-bands in the Putney Volcanics ......................... 83 3-16 Dikes and Sills adjacent to Black Mountain Granite 84 3-17 Sheeting in the Black Mountain Granite ...................
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