THE GEOLOGY OF THE LYNDONVILLE AREA, VERMONT By JOHN G. DENNIS VERMONT GEOLOGICAL SURVEY CHARLES G. DOLL, Stale Geologist Published by VERMONT DEVELOPMENT COMMISSION MONTPELIER, VERMONT BULLETIN NO. 8 1956 Lake Willoughby, seen from its north shore. TABLE OF CONTENTS ABSTRACT ......................... 7 INTRODUCTION 8 Location 8 Geologic Setting ..................... 8 Previous Work ...................... 8 Purpose of Study ..................... 9 Method of Study 10 Acknowledgments . 11 Physiography ...................... 11 STRATIGRAPHY ....................... 16 Lithologic Descriptions .................. 16 Waits River Formation ................. 16 General Statement .................. 16 Distribution ..................... 16 Age 17 Lithological Detail .................. 17 Gile Mountain Formation ................ 19 General Statement .................. 19 Distribution ..................... 20 Lithologic Detail ................... 20 The Waits River /Gile Mountain Contact ........ 22 Age........................... 23 Preliminary Remarks .................. 23 Early Work ...................... 23 Richardson's Work in Eastern Vermont .......... 25 Recent Detailed Mapping in the Waits River Formation. 26 Detailed Work in Canada ................ 28 Relationships in the Connecticut River Valley, Vermont and New Hampshire ................... 30 Summary of Presently Held Opinions ........... 32 Discussion ....................... 32 Conclusions ...................... 33 STRUCTURE 34 Introduction and Structural Setting 34 Terminology ....................... 34 Major Structural Features ................. 35 Related Major Structures in Adjoining Areas ........ 37 Minor Structural Features ................. 39 Geometry ....................... 39 Planar Elements ................... 39 (a) Foliation ................... 39 (1) Bedding ................... 41 (2) Schistosity ................. 41 (3) Slip Cleavage ................ 42 (h) The Pattern of the Secondary Foliations, and its Relation to the Minor Folds ........... 44 (c) Joints ..................... 47 Linear Elements ................... 50 (a) Fold Axes ................... .50 (b) Intersections of s-Planes ............. .S1 (c) Boudinage ................... .Si Grain Fabric .................... 53 Interpretation of Grain Fabric ............. 60 The Movement Pattern ................. 61 (a) Criteria here Accepted as Valid ........... 61 (b) Recapitulation of Geometric Premises ........ 62 (c) Kinematic Interpretation .............. 62 (d) Sequence of Movements .............. 66 Tectonic Synthesis .................... 67 METAMORPHISM ....................... 69 Introduction ....................... 69 Description of Rocks ................... 70 Pelites . . . . . . . . . . . . . . . . . . . . . . . . 70 Siltstones ....................... 74 Siliceous Marbles . . . . . . . . . . . . . . . . . . . . 75 Mafic Volcanics .................... 75 Evaluation of Metamorphism ............... 75 Siltstones ....................... 75 Siliceous Marbles . . . . . . . . . . . . . . . . . . . . 78 Mafic volcanics .................... 78 Pelites . . . . . . . . . . . . . . . . . . . . . . . . 79 Discussion ........................ 82 Summary of Metamorphic History ............. 83 THE GRANITES ..................... 84 Introduction ....................... 84 Mineralogy 85 The Contacts ...................... 85 Structural Relations of the Granites ............ 88 SUGGESTIONS FOR FURTHER WORK .............. 90 APPENDIX I: A Few Interesting and Easily Accessible Exposures 91 APPENDIX II: Explanations for Plates 1-3 ........... 92 BIBLIOGRAPHY ....................... 94 List of Illustrations FIG. 1. Status of detailed mapping in northeastern Vermont and ad- joining region ..................... 9 2. Reference letters of quadrangle divisions .......... 10 3. Domal Structures in Northwestern New England ...... 38 4. Generalized diagram showing relations between major and minor structures .................... 40 S. Minor folding in marble ................. 42 6. Generalized strikes of cleavages in the Woodsville quadrangle 48 7. Typical minor folds ................... 49 8. Post-kinematic basalt in en echelon tension joints ...... 50 9. Petrofabric diagrams . . . . . . . . . . . . . . . . . 58-60 10. History of a mantled gneiss dome ............. 68 11-14. Facies diagrams ................... 81 15. Probable relationship of the Al2Si05 polymorphs to the muscovite stability curve ................ 83 List of Plates Frontispiece: Lake Willoughby, seen from its north shore . 2 PLATE 1. Geologic map and cross-sections of the Lyndonville quadrangle, Vermont . Pocket 2. Tectonic map . Pocket 3. Generalized geologic map, eastern Vermont and adjoining region . Pocket 4. Correlation chart .................... 12 5. Topographic expression of the Perry fault ......... 13 6. Saw-tooth scarp pattern of granite sheets . 15 7. Exposure illustrating the later structural history of Gile Mountain rocks . 43-44 8. Some minor structures in the Littleton quadrangle . 45-46 9. Bedding-cleavage relations, and b-lineation, pelite-marble con- tact ......................... 47 10. Divergence of axis of minor fold and lineation ....... 52 11. Boudinage ....................... 52 12. 'Boudinage" in granitic sill . . . . . . . . . . . . . . 54 13. Foliation in thin section ................. 55 14. Photomicrograph of siliceous marble ........... 56 15. Minor folds in the Waits River formation ......... 64 16. Steeply plunging minor folds ............... 65 17. Photomicrographs . . . . . . . . . . . . . . . . . 72-73 18. Standing Pond lava below St. Johnsbury dam ....... 76 19. Pillow lava . . . . . . . . . . . . . . . . . . . . 76-77 20. Minor fold in Barton River at Glover Village ....... 78 21. Granite contact phenomena ............... 86 22. Photomicrographs of granite ............... 87 23. Contacts of the Mt. Pisgah granite and calcareous sediments of the Waits River formation ................ 89 THE GEOLOGY OF THE LYNDONVILLE ARE& VERMONT By JOHN G. DENNIS ABSTRACT The Lyndonville quadrangle is situated in northeastern Vermont, between the Green Mountain anticlinorium and the New Hampshire plutonic belt. Two sedimentary formations crop out within the quadrangle, the Waits River formation (Barton River member) and the Gile Mountain formation. Both belong to the St. Francis group which extends from Quebec into Massachusetts and which is here tentatively assigned Devonian age. The rocks consist of an alternation of graywackes, quartzites, siliceous limestones and volcanics, in which are emplaced a number of cross-cutting "granite" plutons. The Gile Mountain formation lies at the core of a syncline, the Brown- ington syncline, which is flanked on its east by an arch-like structure, the Willoughby arch. Two phases of deformation are recorded in the rocks: an early one marked by a sericite schistosity essentially parallel to the bedding and isoclinal drag folds; and a later one dipping away from the crest of the arch, marked by slip cleavage in some rocks and schistos- ity in others, and by minor folds which face away from the crest of the arch (with certain exceptions). There is evidence for thinning of the limestones at the crest of the arch, and thickening on its eastern flank. Considered in conjunction with the presence of gneiss domes to the south along the same trend, these structural features lead the writer to in- terpret the Willoughby arch as a domal structure caused by the diapir- like rise of a subjacent mass. The arch-like pattern of the slip-cleavage and schistosity is not interpreted as deformation of a pre-existing hori- zontal structure but as a flowage phenomenon initiated by the doming. The cleavage and schistosity are a structural "reference grid" used to distinguish at least two phases of metamorphism: an early sericite phase and a later garnet-staurolite phase. The loss of water in the early low-grade phase caused unusual textures in the later, higher grade phase. The "granites" have the composition of granodiorites. They show intrusive relationships toward the country rock, but some of the evidence can be interpreted in favor of replacement. The two phases of structural and metamorphic evolution recorded in the rocks may represent two distinct cycles, or stages of an essentially continuous history at the time of the Shickshockian orogeny. INTRODUCTION Location The Lyndonville quadrangle is situated between latitudes 44°30' and 44°45' and longitudes 72°00' and 72°15'. Its northern limit is less than 20 miles south of the Canadian border, and its southeastern corner is some 13 miles northwest of the New Hampshire state line at Littleton dam. U.S. Route 5, Vermont Routes 5A and 12, and the Canadian Pacific Railway's main line from White River Junction to Montreal traverse the area. Geologic Setting The Lyndonville quadrangle lies in a belt of Lower Paleozoic meta- morphic rocks occupying the Connecticut River depression between the Green Mountains and the White Mountains. Only two stratigraphic units occur within the quadrangle: the Barton River member of the Waits River formation, and the lower part of the Gile Mountain forma- tion. The Waits River formation includes a lens of Standing Pond lava. Two granodiorite plutons and numerous related dike rocks occur within the area; these are part of a plutonic belt extending from Massa- chusetts into the Eastern Townships