Geology of the Mount Mansfield Quadrangle Vermont
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GEOLOGY OF THE MOUNT MANSFIELD QUADRANGLE VERMONT By ROBERT A. CHRISTMAN VERMONT GEOLOGICAL SURVEY CHARLES G. DOLL, Stale Geologist Published by VERMONT DEVELOPMENT COMMISSION MONTPELIER, VERMONT BULLETIN No. 12 1959 CONTEN1 'S PAGE ABSTRACT 7 INTRODUCTION ...................... 8 General Statement ................... 8 Location of the Area ................... 8 Regional Geologic Setting ................ 8 Topography and I )rainagc ................ 10 Previous Work ..................... 11 Present Study ..................... 12 Acknowledgments .................... 12 METAMORPHOSED SEDIMENTARY AND VOLCANIC ROCKS . . 13 General Statement ................... 13 Tibbet Hill Schist .................... 14 General Statement .................. 14 Amphiholitic Greenstone ................ 15 Feldspathic Greens tone ................ 18 Calcareous Greenstone, Phyllite and Metagraywacke . 20 Distribution and Age ................. 22 Camels Hum1) Group .................. 23 General Statement .................. 23 Rocks in the Western Area ............... 24 Rocks in the Eastern Area ............... 30 Rocks in the Intervening Area . . . . . . . . . . . . 36 Source and Age of Sediments .............. 36 Ottauquechee Formation ................. 37 General Statement .................. 37 Description of Rocks ................. 37 Correlation and Age .................. 38 INTRUSIVE IGNEOUS ROCKS ................ 40 Serpentinite, Talc-Carbonate Rock and Steatite ....... 40 Basic Dikes ....................... 42 METAMORPHISM ...................... 42 STRUCTURE ........................ 43 Large Folds ...................... 43 Green Mountain Anticlinorium ............. 44 Fletcher Anticline ................... 45 Cambridge Syneline .................. 46 Bedding 48 l3edcling Schistosity 48 I )rag F1ds 49 Fracture Cleavage .................... .53 Crinkle Lineation .................... .58 Joints ......................... .58 Flowage Folds ..................... .59 Quartz Rodding and East-West Folds ........... 60 East-West Crinkle Lineation ............... 62 Origin of Major Features ................. 63 Age of Deformation ................... 67 SURFICIAL GEOLOGY .................... 67 General Statement ................... 67 Glaciation ....................... 67 Glacial and Recent Deposits ............... 71 Lake Lamoilic Deposits . . . . . . . . . . . . . . . . 72 REFERENCES CITED .................... 74 Illustrations PLATE PAGE 1. Bedrock Geology, Mount Mansfield quadrangle, Vermont . . . . . . . . . . . . . . . . . . . (In pocket) 2. Structural Map, Mount Mansfield quadrangle, Vermont . . . . . . . . . . . . . . . . . . . (In pocket) 3. Surficial Geology, Mount Mansfield quadrangle, Vermont . . . . . . . . . . . . . . . . . . . (In pocket) 4. Amphibolitic greenstone with actinolite (?) porphvroblasts 16 5. Feldspathic greenstone with weathered plagioclase pheno- crvsts ....................... 21 6. Graded bedding, southwest of Fairfax Falls ....... 2() 7. Detail of graded bedding from near Huntsville ...... 27 8. "Shale conglomerate" in metagraywacke sequence 28 9. Small fold in bed of "shale conglomerate" ........ 29 10. Drag folds, west of The Chin, Green Mountain anticlinorium 49 11. Dextral drag fold in mctagraywacke .......... .50 12. Crinkles in metagraywacke at Fairfax Falls ....... .54 13. Fracture cleavage in metagraywacke near Fairfax Falls . 55 14. Fracture cleavage cutting across bedding, east of West Fletcher ...................... .56 15. Bedding cut by fracture cleavage .57 16. Cave of the Winds viewed from east side of. The Chin 60 17. Flowage folds, west flank of Mount Mansfield ......61 IS. Small recumbent folds marked by quartz stringers . 62 19. Glacial erratics and striate, Mount Mansfield .......69 Figures FIGURE PAGE 1. Index map of northwestern Vernnnt...........9 2. Locality map, Mount Mansfield quadrangle .......17 3. Outcrop pattern of grapliitic limestone of Ottauquechee formation ...................... 4. Geologic map of the Waterville Talc deposit .......41 5. Low-grade marble in drag fold ..............52 6. joint diagram, Mount Mansfield, Vermont .........59 7. Diagrammatic sketch showing usual structural relationships 64 S. Possible effect of a shifting deformational force ......66 lables TABLE PAGE 1. Estimated modes and petrographic data for amphibolitic greenstones ....................19 2. Data on amphibole from amphiholitic greenstone in the Tibbet Hill schist ..................20 3. Estimated modes and petrographic data for some meta- gravwackes from the western part of the Mount Mansfield quadrangle .....................3 1 4. Estimated modes and petrographic data for schist from Mount Mansfield ..................33 5. Data on garnets from Mount Mansfield .........34 GEOLOGY OF THE MOUNT MANSFIELD QUADRANGLE VERMONT By ROBERT A. CHRISTMAN* ABSTRACT The rocks in the Mount Mansfield quadrangle were deposited in a Cambrian eugeosyncline principally as graywacke, siltstone and shale with minor amounts of volcanic, calcareous and carbonaceous material. Rocks in the western part are coarser and include local beds of quartz conglomerate, "shale conglomerate," and many coarse gravwackes associated with graded beds. Eastward across the quadrangle the rocks become finer grained. Within this sequence older strata containing lava and volcanic detrital rocks belong to the Tibbet Hill unit and younger rocks containing carbonaceous material and limestone belong to the Ottauquechee formation. The remaining portions of the sequence belong to the Camels Hump group which could not be differentiated. These rocks have been metamorphosed principally to metagraywacke and phyllite on the west where metamorphism was less intense and to quartz-albite-muscovite schist on the east where metamorphism was more intense along the Green Mountain anticlinorium. All the rocks belong to the greenschist facies, mostly to the chlorite and biotite zones but some rocks along the anticlinorium may be in the lower part of the almandine zone. Amphibolitic greenstone and feldspathic greenstone are two distinctive metamorphosed lavas in the Tibbet Hill unit. In the Ottauquechee formation the rocks are principally graphitic phyllite and schist with minor amounts of quartzite and recrystallized black lime- stone. The ridge along the crest of Mount Mansfield closely coincides with the axis of the Green Mountain anticlinorium. Here, the average bedding schistosity dips gently to the south, corresponding with the plunge of the anticlinorium. The drag folds and fracture cleavage indicate that the anticlinorium is slightly overturned to the west. Farther to the west * The author is Assistant Professor of Geology at Cornell University. the rocks are isoclinally folded so that the Cambridge syncline is recog- nized principally by the Ottauquechee formation in its core and the Fletcher anticline is recognized by the Tibbet Hill volcanic rocks. It is postulated that the area suffered one principal deformation and that the direction of the deforming force may have shifted from S. 70 °E. to east-west. INTRODUCTION General Statement Despite the geographic prominence and easy accessibility of Mount Mansfield little detailed geologic work has been done in its vicinity. Probably the principal deterring factors have been the uniformity of rock types, the apparent complexity of structures of the metamorphosed rocks and the lack of economic deposits. The present study was undertaken to ff1 in this void of geologic information and to obtain fundamental data on the Green Mountains in northwestern Vermont. A particularly interesting problem is the lack of correlation of the rocks on the west side of the Green Mountains with the well-established sequence of rocks on the east side. Location of the Area The Mount Mansfield quadrangle is located in northwestern Vermont (Fig. 1) between 440 30' and 44° 45' north latitude and 72° 45' and 730 00' west longitude. It includes parts of Chittenden, Lamoille and Franklin counties and portions of Bakersfield, Cambridge, Essex, Fairfax, Fairfield, Fletcher, Jericho, Johnson, Morristown, Stowe, Underhill, Waterville and Westford Townships (P1. 1). Regional Geologic Setting The axis of the Green Mountain anticlinorium extends across the southeastern part of the quadrangle so that, except for the southeastern corner, the area lies on the western flanks of the Green Mountains. The Green Mountains are part of the Taconic allochthone in which the rocks are postulated to have reached their present position by thrusting from the east (Cady, 1945). The location of the roots of the thrusts is not known, although Cady's palinspastic map (1945, p. 568) indicates that the rocks may have originated in New Hampshire more than 100 miles to the east. Traces of the thrust planes beneath the allochthone have QUEBEC VERMONT Miss issquol iv Jay Peak x EnosburqT, 2 Falls 2 ST ALBANS ost to Lomoille iver — )Milton Cambridge wetters n Ville Westford Manstield X"\ Morrisville /r0r fRLINGTON z 7 SN iver Waterbury MONTPELIE Camels Hump B AR RE Lincoln Peak 9 Lincoln Figure 1. Index map of northwestern Vermont. Mount Mansfield quadrangle is shaded. The quadrangles are numbered as follows: 1. Enosburg Falls 4. Hyde Park 7. Plainfield 2. Jay Peak 5. Camels Hump 8. Lincoln Mountain 3. Mount Mansfield 6. Montpelier 9. East Barre 9 been mapped one to five mllcs west of the qua(Irangle (Stone, 1951) No faults have been recognized in the Mount Mansfield quadrangle and