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  • 1. awGEOLOGYOFTHE MEMPHREMAGOG QUADRANGLEAND THE SOU'I'HEASTERN PORTIONOF THE IRASBURG QUAI)RANGLEVERMONTCHARLES G. DOLLVERMONT GEOLOGICAL SURVEYCHARLES G. DOLL, State GeologistPublished byVERMONT DEVELOPMENT COMMISSIONMONTPELIER, VERMONTBULLETIN NO. 3 1951

2. TABLE OF CONTENTSPAGEAbstract . 7Introduction ....................... 8General Statement ........... 8Location ......................... 8Acknowledgments .................... 10Previous work ..................... 10Method of investigation ................. 10Physiography ....................... 11Topography....................... 11Weathering ....................... 12Drainage ........................ 12Glaciation ....................... 13Sedimentary rocks ..................... 15General statement .................... 15Cram Hill formation ................... 16General description .................. 16Type locality ..................... 16Distribution . . . . . . . . . . . . . . . . . . . . . . 16Relative thickness .................... 17Age......................... 17Lithological detail ................... 17Shaw Mountain formation ................ 18General description .................. 18Type locality ..................... 18Distribution . . . . . . . . . . . . . . . . . . . . . . 18Relative thickness ................... 18Age......................... 19Lithological detail ................... 19Northfield slate ...................... 20General description .................. 20Type locality ..................... 20Distribution . . . . . . . . . . . . . . . . . . . . . . 20Relative thickness ................... 21Age......................... 21Lithological detail ................... 21 3. PAGEAyers Cliff formation . . . . . . . . . . . . . . . . . . . 22General description . 22Type locality ..................... 22Distribution . . . . . . . . . . . . . . . . . . . . . . 22Relative thickness ................... 23Age ......................... 23Lithological detail ................... 23Barton River formation . . . . . . . . . . . . . . . . . . 25General description .................. 25Type locality ..................... 25Distribution . . . . . . . . . . . . . . . . . . . . . . 25Relative thickness ................... 26Age ......................... 26Lithological detail ................... 27Slates ....................... 27Phyllites ...................... 28Quartites ..................... 29Limestones ..................... 30Schists ....................... 31Irasburg conglomerate ................. 32Westmore formation ................... 33General description .................. 33Type locality ..................... 33Distribution . . . . . . . . . . . . . . . . . . . . . . 34Relative thickness ................... 34Age ......................... 34Lithological detail ................... 35Phyllites ...................... 35Limestones ..................... 36Quartzites ..................... 36Schists ....................... 36Irruptive rocks ...................... 37General statement .................... 37The Bolton igneous group and associated granites ...... 38Granite plutons ..................... 41Sedimentary inclusions ................. 43Age of the granites . . . . . . . . . . . . . . . . . . . 44 4. PAGESills and dikes ...................... 45Pegmatites ....................... 47Quartz veins ...................... 48Structure ......................... 49General statement ..................... 49Indian Point syncline . . . . . . . . . . . . . . . . . . . 50Indian Point segment ................. 50Brownington syncline .................. 51North Neighborhood anticline .............. 52Ware Brook thrust . . . . . . . . . . . . . . . . . . . . 52En echelon reentrants . . . . . . . . . . . . . . . . . . . 55Black River fault .................... 58Summary of arguments for structural interpretations . 58Minor structures . . . . . . . . . . . . . . . . . . . . . 60Folds . . . . . . . . . . . . . . . . . . . . . . . . . 60Faults........................ 62Bedding ....................... 63Bedding-cleavage relations ............... 64Sinuosity of beds ................... 64Plastic flow of the calcareous rocks . . . . . . . . . . . . 65Boudinage ...................... 66Linear element .................... 70Metamorphism ...................... 71General statement .................... 71Minerals ....................... 71Chlorite ...................... 71Biotite ....................... 72Garnet ....................... 72Andalusite ..................... 72Staurolite ..................... 73Sillimanite ..................... 73Lime-silicate minerals ................ 73Amphibole ..................... 74Historical geology ..................... 74Economic geology ..................... 77References used ...................... 78 5. IllustrationsPLATE PAGE1. Geological map and structure sections of the Irasburg-Memphremagog area . . . . . . . . . . . . . . . In pocket2. Weathering, glaciation ................ 82, 833. Photomicrographs of Barton River formation ...... 84, 854. Photomicrographs of Barton River formation ...... 86, 875. Photomicrographs of Barton River formation ...... 88, 896. Photomicrographs of Barton River formation ...... 90, 917. Photomicrographs of Westmore formation ........ 92, 938. Photomicrographs of Westmore formation ....... 94, 959. Bolton igneous group, granites ............. 96, 9710. Granites . . . . . . . . . . . . . . . . . . . . . . 98,9911. Brecciated granite, folded sill, pegmatite, quartz vein . 100, 10112. Quartz mass, folds ................. 102, 10313. Drag-fold, shearing in limestone, marginal fault in granite,longitudinal fault . . . . . . . . . . . . . . . . . 105 . 104,10514. Wedge-shaped fracture in phyllite occupied by limestone,bedding ..................... 106, 10715. Bedding, boudinage . . . . . . . . . . . . . . . . 109 . 108,10916. Bedding-cleavage relationships, sinuous limestone bed,sheared flow fold .................. 110, 11117. Folds, schist inclusion in limestone .......... 112, 113FIGURE1. Index map ......................92. Disrupted granite streamers ...............463. Pegmatite-granite age relationships ...........474. Ptygmatic folds ....................615. Boudinage . . . . . . . . . . . . . . . . . . . . . . . 676. Boudinage and faulted bed ...............69 6. GEOLOGYOFTHE MEMPHREMAGOG QUADRANGLEAND THE SOUTHEASTERN PORTIONOF THE IRASBURG QUADRANGLEVERMONTByCHARLES C. I)OLLABSTRACTAmong six formations mapped, three are new and, therefore, describedin greater detail. They are the Ayers Cliff, Barton River, and Wcstmoreformations, and embrace the belt occupied by the Waits River formation.More than 21,000 feet of strata ranging in age from Ordovician topossibly I)evonian, constitute a variety of rock types exhibiting middle-andhigh-grade metamorphism.A quartz conglomerate basal to the Cram Hill formation has beenlocated, and the stratigraphic position of the Irasburg conglomerate isidentified with the base of the Barton River formation. The Northfieldslate continues as a distinct unit in Canada and lies below the Tomifobiaslates and limestones of Clark.Prominent structural features of late Devonian origin are the plutonscomposed of dark granite invaded by a light, coarser granite. On thewest shore of Lake Memphremagog the Devonian granites are intrudedby metabasic rocks consanguineous with those in the Owl Head Moun-tairiregion in Canada. The frequency of sills is related to the structuresin the sedimentary rocks. Milky quartz veins of large dimensions areespecially distinctive in the Barton River formation.Northeast-trending folds and faults are related to the Acadian dis-turbance.Northwest of the axis of the Brownington syncline and in-cludingthe tract containing the North Neighborhood anticline and 7. Indian Point syncline, the beds are predominantly overturned. Thesymmetry of the folds has been disturbed by the piercement of intrusives.The Ware Brook thrust, lying between the Northfield late and theAyers Cliff formation, continues as the Bunker thrust in Canada and istraceable for more than 100 miles. A feature of importance associatedwith this fault, is a window in the Cram Hill formation exposing AyersCliff limestone. Genetically related to the Ware Brook thrust are theen echelon reentrants representing flaws in fanlike arrangement withrespect to a major tectonic arc swinging toward the Gasp from northernVermont. The Black River fault merges with a fault mapped in theTomifobia River valley in Canada which, projected northeastward,coincides with the flaw in the Massawippi reentrant. Noteworthy arethe igneous masses on the southwest sides of the reentrants.Superb examples of minor structures are impressive of their significancein the structural development of the region. Their effects are cumulativeand are indicated by folds, faults, plastic flow, boudinage, and lineation.INTRODUCTIONGeneral StatementAlthough small parts of the area described in this report receivedearly attention, much of the geology remained unknown. The area,therefore, afforded an opportunity for detailed work which would serveas a beginning in the correlation of Vermont rocks with those on theother side of the International Border in Canada, thus adding to theknowledge of the Appalachian folded belt in this region. A prime con-siderationis the location of the area within a great tectonic are whosegeology is much better known to the southwest and northeast than itis here at the greatest bend. The present work might serve as a nucleusand reference for subsequent geological research in the region.LocationThe mapped area covers the Memphremagog quadrangle and a partof the adjacent Irasburg quadrangle, in northeastern Vermont (Fig. 1).On the Memphremagog quadrangle it touches upon the InternationalBoundary between longitudes 72 00' and 72 15' west, extending southto latitude 44 45' north. The adjoining portion on the west in theIrasburg quadrangle, lies southeast of a northeast diagonal in the eastre

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