Bedrock Geology of the Fayston‑Buels Gore Area
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BEDROCK GEOLOGY OF THE FAYSTON - BUELS GORE AREA CENTRAL VERMONT * : *• * / f 4 By Gregory J. Walsh Vermont Geological Survey Special Bulletin No. 13, 1992 Diane Conrad, State Geologist BEDROCK GEOLOGY OF THE FAYSTON - BUELS GORE AREA CENTRAL VERMONT By Gregory J. Walsh Vermont Geological Survey Special Bulletin No. 13, 1992 Diane Conrad, State Geologist TABLE OF CONTENTS ABSTRACT . 1 INTRODUCTION ............................................................ 2 LITHOL OGlES 2 UNDERHILL FORMATION ............................................ 3 Quartz Laminated Schist (CZuql) .................................. 3 Schist and Metawacke (CZu) ...................................... 3 Greenstone and Ampbibolite (CZug) ............................... 4 BATTELL FORMATION 5 MONASTERY FORMATION ........................................... 5 Schist(CZm) ................................................... 6 Greenstone (CZmg) .............................................. 7 FAYSTON FORMATION .............................................. 7 White Albitic Schist (CZf) ........................................ 8 Quartzo-Feldspathic Granofels (CZfq) ............................. 8 Quartz-Biotite Gneiss (CZfb) ..................................... 9 Quartz - Muscovite - Tourmaline Schist (CZft) ......................... 10 Greenstone (CZfg) .............................................. 10 GRANVILLE FORMATION ............................................ 10 Carbonaceous Albitic Schist (Cg) .................................. 11 Lincoln Gap Member (Cgl) ....................................... 12 MOUNT ABRAHAM SCHIST ........................................... 12 Mount Abraham Schist (CZa1) .................................... 12 Greenstone (CZag) ........................................ 14 Metawacke (CZaw) ........................................ 14 Mount Abraham Schist (CZa2) .................................... 14 Mount Abraham Schist (CZa3) .................................... 14 Mount Abraham Schist (CZa4) .................................... 15 PINNEY HOLLOW FORMATION ....................................... 16 Silvery Green Schist (CZph) ...................................... 16 Greenstone (CZphg) ............................................. 16 Metawacke (CZphw) ............................................. 17 Quartzose Schist (CZphq) ........................................ 17 OTTAUQUECHEE FORMATION ....................................... 18 Black Phyllite (Cobp) ............................................ 18 Gray Carbonate Schist (Cog) ...................................... 18 Thatcher Brook Member (Cotb) .................................... 19 STOWEFORMATION ................................................. 20 Silvery Green Schist (CZs) ........................................ 20 Greenstone (CZsg) .............................................. 20 ULTRAMAFICS (S) ................................................... 20 CRETACEOUS DIKES (Kd) ............................................ 21 CORRELATIONS..................................................... 22 Local Correlations 22 Regional Correlations 24 DEPOSITIONAL ENVIRONMENT ...................................... 26 Noncarbonaceous Rocks .......................................... 26 MaficSchists ................................................... 30 Carbonaceous Schists ............................................ 30 AGEOF UNITS ...................................................... 31 GEOCHEMISTRY ........................................................... 32 Metaigneous Rocks ....................................................32 Gray Carbonate Schist (Cog) ............................................36 STRUCTURE 36 FOLIATIONS ........................................................ 37 Dominant Schistosity (Sn) ........................................ 37 Relict Schistosity (Sn-i) .......................................... 38 Crenulation Cleavage (Sn+1) ...................................... 38 KinkBands .................................................... 39 LINEATIONS ........................................................ 39 FOLDS.............................................................. 40 FnFolds ...................................................... 40 Isoclinal Reclined Folds .................................... 40 Sheath Folds ............................................. 41 Origin of Reclined and Sheath Folds ......................... 41 Fn-1 .......................................................... 42 Fn+i .......................................................... 43 Regional Fn+1 ............................................ 43 Fault-related Fn+1 ........................................ 43 Fn+2 .......................................................... 44 FAULTS............................................................ 44 Pre-Peak Metamorphic Faults (Tn-i) ............................... 44 Syn-Metamorphic Faults (Tn) ..................................... 45 250 (Z-12) ............................................... 46 313 (Z-11) ............................................... 46 429 (R-14) ............................................... 46 413 (Q-16) ............................................... 46 235 (Y-13) ............................................... 47 Post-Peak Metamorphic Faults (Tn+1) .............................. 47 108 (T-12) ............................................... 47 712 (H-12) ............................................... 48 186 (N-33) ............................................... 48 THE GREEN MOUNTAIN ANTICLINORIUM ............................ 48 MESOZOIC FEATURES ............................................... 50 METAMORPHISM ........................................................... 50 ASSEMBLAGES ...................................................... 50 BULK COMPOSITIONS 51 CHRONOLOGY ...................................................... 53 PLAGIOCLASE 55 Twinning...................................................... 56 Chemistry..................................................... 57 WHITE MICAS ....................................................... 58 REGIONAL CONDITIONS ............................................. 58 AGE OF METAMORPHISM ............................................ 59 Taconian...................................................... 59 Acadian....................................................... 59 DISCUSSION 60 CONCLUSIONS ............................................................. 61 INTERPRE TA TIONS ........................................................ 62 DEPOSITION ........................................................62 DEFORMATION AND METAMORPHISM ................................62 ACKNOWLEDGEMENTS ...................................................... 64 REFERENCES CITED .......................................................65 11 LIST OF TABLES 1 Average Modal Compositions of the Underhill Formation ....................... 4 2 Average Modal Compositions of the White River Member of the Battell Formation and Schists of the Monastery Formation ................... 6 3 Average Modal Compositions of the Fayston Formation ........................ 9 4 Average Modal Compositions of the Granville Formation ....................... 11 5 Average Modal Compositions of the Mount Abraham Schist ..................... 13 6 Average Modal Compositions of the Pinney Hollow Formation .................. 17 7 Average Modal Compositions of the Ottauquechee Formation ................... 19 8 Average Modal Compositions of the Stowe Formation .......................... 21 9 Average Modal Compositions of Ultramafics (S) ............................... 22 10 Modal Compositions of Cretaceous Dikes .................................... 23 11 Local Correlation Chart ................................................... 25 12 Regional Correlation Chart ................................................ 27 13 Major and Trace Element Geochemistry of Greenstones and Amphibolite ......... 33 14 Major and Trace Element Geochemsitry of the Gray Carbonate Schist ............ 37 LIST OF FIGURES 1 Geochemical diagrams ...................................................... 34 2 Photomicrographs of unmylonitized and mylonitized schist ................... Plate 4 3 Photomicrograph of east-over-west mica fish .............................. Plate 4 4 Photomicrograph of east-over-west broken and displaced hard grain ............ Plate 4 5 Thompson A-F-M projection ................................................ 52 6 A-C-Fm-K tetrahedron .................................................... 54 7 Photomicrograph of garnet retrograded during Sn+1 development. ............. Plate 4 8 Photomicrograph of Fn+1 aged kyanite ................................... Plate 4 9 Photomicrographs of two types of growth twins in albite ..................... Plate 4 10 Photomicrograph of albite porphyroblast with deformation twins .............. Plate 4 11 Photomicrograph of albite porphyroclast with complex twins ................. Plate 4 12 Photomicrograph of albite porphyroblast with relict twins ................... Plate 4 13 Photomicrograph of optically zoned plagioclase porphyroblast ................. Plate 4 14 Interpretive lithofacies diagram ............................................ 63 LIST OF PLATES Plate 1 Bedrock geologic map of the Fayston - Buds Gore area, central Vermont Plate 2 Cross section A-A' through the Fayston - Buels Gore area, central Vermont Plate 3 Structural data from the Fayston - Buels Gore area, central Vermont Plate 4 Photomicrographs