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<ul><li> 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</li></ul><p> 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 eastrectangle and includes the southeast rectangle.8 8. 74 73 72 71-I- - - iI / GUECC 91,.-- /._/ I.. C' T / ___PBLIRLING TON dO /I(tB RYI .4/ MOMTPELIER.- ..rMOIVT 'I R(/TI-AND R/vLRt/cr I N.H.I :CF//CORDI's GTO OAEENC 2 ' I I M14S3. Ig * __ I9 MILESARIAS MAPPING IN IRA$SURG -MAPPED PROGRESS /.IEMPMQEMA GOGFIG. 1INDIX MA. AREA4746454443 9. AcbnowLedgments The writer is extremely grateful to the members of the Departmentof Geology in Columbia University, who have shown an interest in thisproblem. He is especially thankful to Professor Walter H. Bucher for hisstimulating and valuable guidance and to Professor Marshall Kay forhelpful suggestions. Dr. Robert K. Doten gave appreciable aid in theearly part of the study of the thin sections and, in this connection, thewriter wishes to express his thanks for the kindly advice given by Pro-fessorS. James Shand. Professor Elbridge C. Jacobs has made available,through the Vermont Geological Survey, financial aid for a part of onefield season; his interest in the work is appreciated. Clarence G. Baileyand Harrison W. Cooke, Jr., assisted ably in the field for two seasons.Appreciation is expressed to Mr. L. S. Walker for rock analyses.Previous Worb'F he earliest work pertaining to the geology east of the Green Moun-tains,appears in the first and second reports of the Vermont GeologicalSurvey, by Adams (1845 and 1846, respectively). Thompson (1848)describes anew the "calcareo-mica slate" of Adams. A more compre-hensivestudy of the rocks was made by Hitchcock (1861), who termedthem "calciferous mica schist." Incidentally, Hitchcock (1861, p. 573)conceived the granite to be metamorphosed stratified rock. Early studiesof the geology of the northern part of this region were made by Hall(1871, and in Hitchcock, 1861, p. 719). Parts of the area mapped wereinvestigated by Richardson (1905-1906, 1907-1908, and 1911-1912) andby Jacobs (1921-1922).In the terrane bordering on the International Boundary in Canada,pioneer work was done by Logan (1852 and 1863). Further studies werecompleted by Ells (1894), Kerr (1923), and Clark (1934). More recently,a geologic map accompanied by a brief description, by Ambrose (1943),shows the rocks equivalent to the three new formations herein proposed,undifferentiated.Method of IiwestigationThe greater part of six summers was spent in detailed mapping of theMemphremagog and one-sixth of the adjacent Irasburg quadrangles,comprising in all an area of 262 square miles. Reconnaissance studieswere made in surrounding regions, including Canada where some workhas been done.10 10. The standard topographical map, enlarged to a scale of 3 inches to themile, served as a base in the field work. Most points and outcrops werelocated by the method of resection, excepting in wooded tracts wherelocations were established by means of pace-and-compass. Contactsbetween formations were traced in detail. A total of 172 thin sectionswere studied.PHYSIOGRAPHYTopographyThe area is hilly to mountainous, rising from an elevation of 682 feetat the shore of Lake Memphremagog in the northwestern part to thegreatest elevation of 2935 feet on the summit of Goodwin Mountain inthe southeastern corner. The southeasterly increase in elevation is con-trolledlargely by the progressively more resistant lithologies of thesedimentary formations in that direction. The lowest sedimentarysurface is underlain by the relatively weak limestones of the Ayers Cliffformation and the highest sedimentary surface by the dominantlysiliceous Westmore formation. By and large, stream dissection hasattained a mature stage in a topography modified by continental glacia-tion.The topography reflects the rock types and structures, as, for example,the domical granite hills extending northwestward from Elm Hill toSalem Hill in the central igneous body, and the hogback ridges exempli-fiedby Shattuck and Dowling hills. The former reveal the massivestructure of the more resistant igneous rocks and the latter the beddedstructure of the more resistant among the steeply-dipping metamor-phosedsediments. Barton Mountain is a conspicuous monadnock ofgranite near the southern margin of the area.Elevated terraces in the vicinity of Lake Memphremagog recordpost-glacial uplift of the region. Two well-marked planes occur atelevations of about 700 feet and 1000 feet to the north and east of thecity of Newport. The lower terrace is well-developed on the east side ofthe lake at Indian Point and Lake Park. The higher level is present onPine Hill, directly east of Newport City, from whence it can be followedaround the north end of the central igneous body and southeasterly toWest Charleston. It probably represents, at least in part, an elevateddelta of the Clyde River which was built into glacial Lake Memphrema-gogand has subsequently been dissected by stream erosion. Its surface11 11. contains the lakes peripheral to the central igneous body and is deeplyincised by the Clyde River downstream from Salem Pond. Levels aremanifest on the east side of th...</p>

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