Prepared in cooperation with the OFFICE OF THE MASSACHUSETTS STATE GEOLOGIST COMMONWEALTH OF MASSACHUSETTS BEDROCK GEOLOGIC MAP UNIVERSITY OF MASSACHUSETTS STATE GEOLOGIST OXFORD QUADRANGLE, MASSACHUSETTS Preliminary Copy - Undergoing Review (June 2005)

Pzn Pzn Pzn Pzn EXPLANATION OF MAP SYMBOLS Gneiss. These correspond respectively to average Northbridge, the coarser northwest one. The Oxford anticline and lineation developed approximately 48 CORRELATION OF MAP UNITS Pznr Pzp 25 55 ? "core", and the "mashed--- muscovite gneiss" mentioned by Emerson (1917). normal to this alignment. The compression produces a relative movement LQ Pznr 30 Pznr 17 Pznga Lithologic Contacts WSQ 50 45 85 20 85 The better exposures and clearer relations in the Oxford quadrangle show within and across the thrust belt of the northwest sides slipping over the Pzngn 30 85 25 20 Pzna 20 ? Pznga Location approximate (within 200 m) these divisions to represent gradual changes within a single granitic body and southeast ones with a right-lateral component. These movements cut out the 15 37 METASEDIMENTARY ROCKS IGNEOUS ROCKS 20 40 not separate intrusions. These new names in the Chepachet and Thompson Marlboro Formation between the southwest part of the quadrangle to a point 60 Pznr 55 50 ZONE 40 35 25 65 36 38 28 Pzng quadrangles now can be dropped. Nomenclature more descriptive of the 23 km to the northeast. 25 Pzm 24 ? ? Location uncertain Pzngn 43 65 Pzna 32 rock would be Northbridge as a medium- to coarse-grained porphyritic 20 30 Pznr 47 20 60 70 V I C A N On facies, an aplitic border zone facies and sheared zones where muscovite has The local movement within the thrust sheets changes greatly and the 50 70 55 50 35 Pznga Faults 40 ? 57 30 replaced the usual biotite of the granite. orientation of small-scale structures varies extremely and has little regional ? 73 Only faults traceable in outcrop are shown as solid lines. All dashed faults were ? 65 80 60 30 15 determined though lineament analysis of aerial photographs, and topographic and significance. The numerous small drag folds can have almost every 70 O R D geomorphic interpretation. FAULT Pzng 20 P zng P znga P zngs The Northbridge is generally a light, pinkish-gray, medium- to coarse- orientation along the twists and turns of the thrust faults. Many of these folds ? Pzp 25 Pzna Pzna Location certain, observed in outcrop (within 5-10 m) 45 60 33 grained, slightly to strongly-foliated porphyritic biotite granite to quartz are well exposed in cuts along Route 395, that crosses the western side of the 35 25 Pzng 50 25 Pznga 35 38 Location approximate (within 200 m) monzonite with pink microcline phenocrysts. The biotite occurs as tiny flakes quadrangle. The most common situation is an anticlinal fold of varying 20 25 30 65 in patches, rather than scattered. The anorthite content of the plagioclase is steepness in the hanging wall and a flattening of the beds in the footwall Pzp 85 20 P zngn ? ? Location uncertain Pzp t variable. The content given by Dixon (1974) when arranged by position, beneath the fault, but locally the folds are very complex. BLOODY 45 63 33 l ? 85 u ? 42 Pznga Pzng 30 a 20 shows a change from an20 in the core to an8 at the border. Overall, the rock t 60 35 F l Strike and dip of fault as measured in outcrop Pzp Pzp 30 30 P zn P znr P zp u Pznga Pzna Pznga y 45 32 35 r may appear uniform and monotonous in the field, but it has a great many The initial thrust faults were very ductile with a smearing and almost a NP ? BLUFF 30 85 a or subsurface

25 u

42 60 F

Pznga Pznga ? b ? 80 65 35 85 f subtle changes and hand specimens may vary considerably. Inward from the flowage of minerals and rocks along them, yet still forming distinct breaks f 45 Pzng w Strike - slip fault, arrows indicate direction of offset ? P zna u e

32 l 34 ? border the foliation decreases, the grain size increases, it appears more with offset. Renewed movement, probably under cooler conditions, results in B 32 N 32 ?

Pzp - y 35 ? Pzng 65 n 38 d Thrust fault

o porphyritic and the biotite is more prominent. Other variations are due to the shearing and mylonization. Slickensides form under later brittle reactivation. t T E P R O Z I C Pzp o

35 n o i

Pzng l Pzng Pzng 60 l degree of hydrothermal alteration which causes the pink coloration of the In places gouge and brecciation form under more brittle conditions, but are

? B L A Pzp 35 C Normal fault Pzna 40 40 phenocrysts, degree of assimilation of wallrock and amount of granulation not commonly seen. Feldspar porphyroblasts, grading locally into pegmatite, Pznga 40 75 ? Structural Symbols and shearing. The general foliation is due to flowage, with super-imposed forms bands along and near some early shear zones and bull quartz is very 20 25 Pznga 37 40 27 85 46 65 30 30 51 35 Planar Features and parallel shear foliation, especially in zones along the border. The locally present along brittle faults. 8540 Pzng 40 43 ? 34 44 foliation is shown by sub-parallel patches of biotite, lensoid microcline and, in 22 38 40 45 33 18 Strike and dip of dominant foliation . 38 55 28 36 the fine-grained rock, thin lenses of quartz. The shearing causes the biotite to The eastern margin of the Nashoba thrust belt is marked by the Bloody Bluff 75 50 ? Compositional layering generally parallels foliation 40 A' 30 37 be replaced by fine muscovite and may reduce the apparent grain size locally fault zone of Cupples, (1961). This fault was followed from its type area in 80 ? Horizontal foliation Pzp 44 to mylonite. The microcline phenocrysts are primary minerals that resist eastern Massachusetts into the Lake Char fault zone of Dixon (1968), which 30 Pzng 35 85 granulation better than the quartz and plagioclase. The quartz is usually is named after the large lake in the Oxford quadrangle (Barosh, 1984). The Pzngs 85 58 EXPLANATION OF GEOLOGIC MAP UNITS Strike and dip of joint 75 Selected modal analysis of samples from most of these units finely granulated. The microcline is only slightly larger than the quartz and Clinton-Newbury fault zone that marks the northwest side of the belt also was 65 40 34 32 38 31 Strike of vertical joint plagioclase in the fine-grained rocks, but becomes relatively larger in the followed into the area. This fault zone is intruded by granite in Connecticut Pzng 41 43 34 given by Dixon (1974) for samples collected just to the south. Pzngs 35 50 55 35 Pzngs Pzng 85 Linear Features coarser rocks and may reach over two cm in length. It commonly has a thick and had not been recognized there. The Bloody Bluff-Lake Char fault zone Fig. 1, Sketch map of the region around the Oxford (OQ), Webster (WQ), 35 28 80 32 32 NORTHBRIDGE GRANITE (LATE PROTEROZOIC) lenzoid "football" shape. The rock commonly has some lineation. Where it is dips westward about 20 degrees in the Thompson quadrangle (Dixon, 1974), Worcester South (WSQ) and Leicester (LQ) quadrangles showing structural 2920 50 Trend and plunge of mineral lineation Pzn Pzng Pzng 36 strongly lineated or "penciled" the rock appears massive perpendicular to it 30 to 40 degrees to the northwest across the Oxford quadrangle and much provinces, major fault zones, folds, distribution of stratigraphic units and 23 Undivided. Light pinkish-gray to light gray or pale salmon-colored and strongly foliated parallel to it, although the actual foliation may be steeper to the northeast. general area of the Northbridge Granite (pattern). Other intrusive rocks are 85 Pzng 50 Trend and plunge of axis of minor fold Pzng finely to coarse-grained porphyritic undivided biotite quartz monzonite 11 Pzng that is slightly to strongly foliated. The granite is variable due to: 1) obscured. not shown separately. Explanation: NP, Northbridge Granite with rim of 35 45 Fold Axes 48 increase in grain size away from border; 2) decrease in degree of flow NASHUA TROUGH: The Clinton-Newbury just brushes the western side Plainfield Fm; M-N, Marlboro and Nashoba Fms., N, Nashua Fm.; O, 32 foliation away from border; 3) degree of tectonic foliation and related Plunging anticline The fine-grained aplitic border is present in the northern part of the of the quadrangle where it contains a sliver of strata, caught up Oakdale Fm.; Px, Paxton Gp.; and B, Brimfield Gp. 29 Pzng 36 replacement of biotite by muscovite; 4)degree of assimilation of country Syncline quadrangle as a thin zone, that gradually widens to the northeast to form the during a reactivation. This sliver is a remnant of the Nashua Trough, a basin 19 35 30 rock and; 5) degree of deuteric alteration. Typically, the rock is a light 28 very wide zone seen along the Massachusetts Turnpike in southwestern bordered by reverse faults that contains a thick section of Cambro-Ordovician 31 pinkish-gray coarse-grained moderately-foliated biotite quartz Minor antiform 35 Westboro. An excellent section of the border zone and porphyritic rock is strata (Barosh, 1999a). The trough impinges upon the Clinton-Newbury 43 monzonite, that weathers same to slightly lighter. A lineation may be Bedrock Exposure REFERENCES exposed in large cuts along Route 146 just northeast of the Oxford fault zone at Worcester from the north and right-lateral movement has strung 26 28 42 locally apparent. The potassium feldspar phenocrysts are pink to 85 34 salmon colored in football shaped lenses 0.5 x 1-2 cm. Medium- Quadrangle. The finer grain, higher quartz content and jointing, controlled out some slivers of the trough in the fault zone southwest of this intersection.. Barosh, P.J., 1972, Structural geology of and southeastern grained quartz forms a fractured groundmass or stretched into lenses by the uniform foliation, give the border zone a bedded look. This may Massachsuetts as revealed by aeromagnetic data: Geological Society of America 33 31 0.2-0.4 x 1-2.5 cm. Plagioclase is light-gray and more equant in shape. Individual Outcrop cause it to be mistaken for the quartzite it intrudes. The border suffers BRITTLE FAULTS: An immense number of small brittle faults are present Abstracts with Programs, v. 4, n. 7, p. 444-445. Biotite occurs in planar sieve-like irregular patches 1-1.5 cm across. Areas of abundant outcrops varying degrees of shearing and may look very similar to the sheared granite in the quadrangle and were noted by Dixon (1968) in outcrops to the south. Pzng Minor magnetite is present as scattered fine grained crystals. Quartz or shallow bedrock (within Barosh, P.J., 1974, Preliminary bedrock geologic map of the Webster Quadrangle, 33 also occurs locally as veinlets along foliation planes or joints. The 10 ft (3 m) of surface) or elsewhere. The sheared rock in both cases is associated with thrust faults. Exposures here allow them to be shown over much of the Oxford quadrangle Massachusetts-Connecticut: U. S. Geological Survey Open-File Report 74-192, 2 p., 27 deuteric alteration that changes the potassium feldspars to pink or 2 and local areas of detailed mapping reveals even a greater density than is Pzng outcrop areas > 10,000 m scale 1:24,000. 35 25 salmon and saussuritizes the plagioclase, is not seen west of the Bloody Age: The Northbridge Granite is considered the oldest part of the batholithic shown. The majority are near vertical, but many have moderate dips. Most 40 45 41 31 Bluff-Lake Char fault zone. The geologic mapping of the Oxford quadrangle is part of a study of nine complex (Quinn,1971). It was long known to be , then assigned have small normal or lateral offsets that are not well defined. These faults Barosh, P.J., 1976a, The Western Rhode Island fold belt: Geological Society of Pzng America, Abstracts with Programs, v. 8, n. 2, p. 127-128. Aplitic border zone. Pinkish light-medium gray to pale salmon- quadrangles in the region around Worcester, Massachsuetts. The project's to the mid from miscorrelations and disturbed radiometric dates form distinct sets and patterns and many sets are of regional extent. Most are 33 Pznga goals are to unravel the great structural discontinuity through the region, and now again firmly shown to be Precambrian. Fossiliferous relatively old and confined east of the Bloody Bluff-Lake Char fault zone, but 38 30 colored fine to medium grained rock. Grades from slightly to Barosh, P.J., 1976b, Preliminary bedrock geologic map of the Oxford quadrangle, Pzng moderately foliated aplite to strongly foliated sheared to mylonitized define the stratigraphy present and correlate it across state lines, tie together strata, that have suffered only slight metamorphism and therefore younger, relatively young ones cross the entire quadrangle. They are well expressed in Massachusetts, Connecticut and Rhode Island: U.S. Geological Survey, Open-File Pzp 20 15 Pzp 24 rock. Strongly foliated variety has minerals stretched out into long thin the work of eight nearby U.S. Geological Survey quadrangle mapping occur at several places in eastern Massachusetts (Crosby, 1880; Woodward, the topography and generally have tectonic joint sets associated with them. Report 76-622, 5 p., scale l:24,000. lenses, 1-3 mm thick, that impart a pinstriped appearance. Unit grades 20 projects and interpret the aeromagnetic data. The preliminary results were in Shaler and others, 1899; Warren and Powers 1913; Bell and Alvord, The principal fault set trends are, from apparent oldest to youngest: into the porphyritic granite and may contain relatively more quartz and Barosh, P.J., 1977, Preliminary map showing bedrock geology superposed on an 24 25 presented in 1977 (Barosh, 1976b, 1977), when quadrangle mapping in 1976). In addition older fossiliferous Eocambrian strata of the Boston basin northwest, east-west, north-northeast, northeast, just west of north, northwest 75 85 less biotite. Has a more slabby and layered look than main body. This aeromagnetic base map of the Worcester region, Massachusetts, Connecticut, and 25 25 southeast New England was halted. These remain the latest map information (Lenk and others, 1982) unconformably overlie the batholith (Nelson, 1975). and north. The early northwest set is mainly in the inner part of the Oxford Pzng 24 unit occurs primarily along the northern border and is the westernmost Rhode Island, U.S. Geological Survey Open-File Report 77-131, 38 p., 2 pl., scale 20 75 Pzng Pzng 15 34 extension of the wide border zone of the granite present to the northeast until now. The compilation on the Geologic Map of Massachusetts (Zen, These strata yield an age of 595 my (Kaye and Zartman, 1980). More anticline and may be associated with late movement related to its formation. 1:125,000. 55 15 45 56 30 20 of the quadrangle. 1983) and its delayed description (Hatch, 1991) used little of the new field reliable radiometric ages of 614 my (Zartman, in Nelson, 1975) and 620+- The east-west set is most noticeable in the central and northeastern portions 24 data and were based largely on much earlier concepts. 15 my (Naylor, personal communication, 1975) were obtained from the part of the quadrangle. Faults of this trend become more prominent Barosh, P.J., 1978a, Reconnaissance bedrock geologic map of the Marlborough 10 85 Sheared and partially assimilated border zone. Pinkish light-gray quadrangle, Massachusetts: U.S. Geological Survey, Open-File Report 78-221, 14 p., 54 57 45 of the Milford Granite correlated with the Northbridge. The most likely age northeastward of the quadrangle and appear related to the Eocambrian and Pzp Pzngs to light-gray fine to medium-grained moderately foliated muscovitic and scale 1:24,000. 50 quartz-rich rock that is lighter in appearance and more slabby than the Since releasing the preliminary results the area around Oxford has been now seems to be approximately 625 my (Zartman, personal communication, Ordovician faults of the Boston basin (Barosh, 1995). The north-northeast 50 16 typical granite. This unit appears to result more from partial studied in detail (Barosh, 1996a, 1996b; Barosh and Johnson, 1976)(fig. 1). 1991), but other other scattered dates from the region suggest a wide error set is very prominent in the quadrangle and in the northeastern corner of the

Pzp 40 Pzng Barosh, P.J., 1978b, Reconnaissance bedrock geologic map of the Shrewsbury ? 85 75 20 assimilation of quartzite and shearing than being aplitic, although some quadrangle, Massachusetts: U.S. Geological Survey, Open-File Pzngs 11 10 22 Great strides also have been made in dating the rocks and events and in margin. The granite is therefore Late Proterozoic Z in age. Uxbridge quadrangle to the east. These faults commonly have left-lateral 35 aplite is probably present. The areas in which these processes operated Pzn understanding the bedrock fracture system. This system is of particular offsets. The just west of north set appears to follow a broad zone that crosses Report 78-394, 19 p., scale 1:24,000.

On 75 20 tend to overlap and the rocks affected are not separated. The shearing ? 15 50 15 significance to environmental, ground water, tunnel and earthquake studies ROCK IN THE NASHOBA THRUST BELT the east side of the quadrangle. 28 31 28 10 17 85 of the porphyritic granite results in the biotite being replaced by Barosh, P.J., 1982, Structural relations at the junction of the Merrimack province, 43 in the region. The combination of exposure, large-scale aerial photography, 20 Pzng muscovite and a reduction in grain size. Grades to both Plainfield Nashoba thrust belt and the Southeast New England platform in the Webster-Oxford

15 ? 8 8 80 85 Formation and porphyritic granite. Small unmapped bodies probably 45 48 15 85 local areas of detailed work and drill hole data has produced the first Marlboro and Nashoba Formations: The Nashoba thrust belt was The above sets appear restricted to east of the Bloody Bluff and probably area, Massachusetts, Connecituct and Rhode Island: in Joesten, R., and Quarrier, 42 40 12 80 Pzngs 20 present elsewhere and contacts are very approximate. comprehensive view of a bedrock fracture system in southern New England. delineated in northeastern Massachusetts where the contained thick section formed prior to the Silurian before the Bloody Bluff-Lake Char fault zone S.S., eds., Guidebook for fieldtrips in Connecticut and south-central Massachusetts: 8 ? 7 22 Connecticut Geological and Natural History Survey Guidebook 5, p. 395-418. (New 15 of interbedded gneiss, amphibolite and schist was described by Bell and closed (Bell and others, 1996), whereas the following sets cut across the fault Pzng 80 80 85 NASHUA FORMATION (ORDOVICIAN) England Intercollegiate Geologic Conference, 74th Annual Meeting., University of. 33 36 45 40 20 Pzngs The Oxford quadrangle lies astride a northwest plunging anticlinal ridge that Alvord (1976). These strata continue into Connecticut (Barosh, 1977). zone. The northeast set is composed of several faults, with approximately a 11 25 On Connecticut). 43 10 Dark gray fine grained phyllite, that weathers medium gray with a slight is the terminus of the uplands of western Rhode Island. Its northeast side However, of the Marlboro Formation, Shawsheen Gneiss, Fish Brook Gneiss, one km spacing, that cut the anticline and tend to form cross valleys. Drilling 10 olive cast. Very well bedded in thin, 0.3 to 2 cm thick graded beds. ? 85 Pzng 85 75 5 Pzng Grading is from siltstone base to mudstone and mica tops, that maybe drains to the Blackstone River that flows southward into , Nashoba Formation and Tadmuck Brook Schist present to the northeast (Bell into the prominent Reid Smith fault of this set showed a moderately steep dip Barosh, P.J., 1984, The Bloody Bluff fault system in Hanson, L.S., editor, Geology of 45 85 88 slightly ankeritic and produce browninish streaks. Locally bedding whereas waters of its western flank flow southward into the Thames River and Alvord, 1976) only thin tectonic slivers of the Marlboro and Nashoba to the northwest. The northwest set is a relatively young set with a half to the coastal lowlands from Boston, MA to Kennebunk, ME: New England 8 Intercollegiate Geologic Conference, 76th annual meeting guidebook, Salem State Pzm 85 25 surfaces are lustrous from fine sericite and may contain staurolite drainage system, that empties into Sound. The landforms Formations survive in the Oxford quadrangle. These correspond, one km spacing, that is prominent throughout the region. The north set 70 85 12 nodules, 2 mm in diameter. Fine foliation is present, that is oblique to College, MA. p. 310-324. 38 30 closely reflect the geologic structure and lithology. respectively, to the Quinebaug and Tatnic Hill Formations of Dixon (1964) to appears the youngest and has a relatively wide spacing. One zone passes Pzngs 62 bedding and is deflected across grading in beds. Joint surfaces the south. The described members of the Nashoba Formation are only through Oxford center, a prominent one through the middle of the 12 25 commonly weather rusty. Medium metamorphic grade and cut by few Barosh, P.J., 1995, Paleozoic rifting in New England, New Brunswick and Nova

C L I N T O - E W B U R Y F A Z 13 10 quartz veins and lenses, but not pegmatite or granitic rock. Exposures GEOLOGIC SETTING partially present in fault slices and are not seperated. The lithologic divisions quadrangle, a less defined one two km to the east and a probable, poorly Scotia, U.S.A. and Canada: in Ojakangas, R.W., Dickas, A.B. and Green, J.C., 40 Pzng editors. Proceedings of the Tenth International Conference on Basement Tectonics: LAKE Pzp are just west of the quadrangle. Previously informally designated made are merely local ones. The strata present probably represent parts of exposed one between Wallum lake and Stevens pond. The middle two Pzng Pzngs Hodges Village phyllite (Barosh, 1974). It correlates with Unit 3 of The Oxford quadrangle lies against the southeast side of a suture zone that the higher members as the overlying Tadmuck Brook Schist is present only a appear to be the splayed end of a very prominent but unmapped fault that Kluwer Academic Press, p. 73-101. 53 Peck (1976) and is named herein the Nashua Formation from the few km to the north (Barosh, 1996b). The Yantic member of the Tatnic Hill crosses all of eastern Connecticut. It is herein named the Tetreault fault after 9 snakes across southeastern New England (Barosh, 1982, Woodhouse and Barosh, P.J., 1996a, Bedrock geologic map of the Webster quadrangle, Worcester Nashua Rriver in the type area of Unit 3 in the Clinton quadrangle to in the Thompson quadrangle (Dixon, 1974) just to the south appears to be a a pond which lies in its fault trough in the East Killingly quadrangle to the 80 the north. others, 1991). This is the fundamental structure of the Northern Appalachian County, Massachusetts and Windam County, Connecticut: U.S. Geological Survey, 47 orogen, first recognized by Wilson (1966), that here separates rock on the zone of deformed strata along the west side of the formation and not a south. Many of these northwest and north-trending faults in southeastern unpublished Geologic Quadrangle, 30 p., scale l:24,000. NASHOBA FORMATION (LATE PROTEROZOIC) stratigraphic unit. Faults also appear to cut out part of the section in New England are post-Triassic in age, some cut strata offshore 45 12 7 15 southeast that once joined the present northwest Africa from rock on the northwest that lay offshore of North America as it was constituted at the end Connecticut as the section only reaches about 1,700m (Dixon, 1964) and a number remain active (Barosh, 1999b). Barosh, P.J., 1996b, Bedrock geologic map of the Worcester South quadrangle, Pzngs Undivided. Interbedded light to medium-gray gneiss, medium to Pzp Pzn compared to the 10,890 m of the Nashoba to the northeast (Bell and Alvord, Worcester County, Massachusetts: U.S. Geological Survey, unpublished Geologic dark-gray amphibolitic gneiss, dark-gray amphibolite, rusty- of Grenville time. Quadrangle, 28 p., scale 1:24,000. Pzp 10 weathering gneiss, medium gray schistose gneiss and minor marble. 1976). The lower part of the Connecticut section is reduced even more "HOPE VALLEY SHEAR ZONE": A major north-trending, east-dipping Pzn Pzng 9 10 Generally thin-bedded and commonly laminated. Garnet-bearing in Pzm 25 The zone is marked by the Nashoba thrust belt in southeast New England towards the Oxford area. A few blocks of laminated clean quartz-rich gneiss, structural break called the Hope Valley Shear Zone has been hypothesized to Barosh, P.J., 1999a, Ordovician turbidite basin in southeastern New England: CHAR 85 part. High metamorphic grade with some beds containing sillimanite. (Fig. 1). This great west-dipping zone of dislocation separates an enormous similar to the Fish Brook Gneiss are present as float, at the east edge of the pass approximately through and east of the southeast corner of the Oxford Geological Society of America, Abstracts with Programs, v. 31, no. 2, p. A-3. 30 45 Porphyroblasts of feldspar or thin-layers of feldspar and quartz that Nashoba in the southern Oxford quadrangle. These could possibly represent quadrangle by O'Hara and Gromet (1985) and Hermes and others (1994). 20 25 grade to pegmatite where relatively greater deformation. Foliation pile of metasedimentary strata in the Sturbridge basin to the northwest from 5 77 a tiny sliver of that unit or, perhaps, a higher lens of similar lithology. Barosh, P.J., 1999b, Post-Triassic faults and extensional basins in the northern 45 parallel to bedding. Commonly sheared parallel to bedding and a batholithic complex forming the Southeast New England platform on the Appalachian region: Geological Society of America, Abstracts with Programs, v. 31, Pzngn Pzng 30 Pzng locally mylonitized. Cut by a variety of granitic rocks just to west and southeast. The thrust belt itself contains a thick section of largely This is an extension of a fault found along the west side of the Oxford no. 2, p.A-3. 75 may contain few thin sills or lenses. Too complex structurally to metavolcaniclastic strata derived from the southeast. All three provinces are The type Nashoba Formation is from a much more complete section than the anticline to the south and named the Oneco fault (Barosh, 1972, 1976a; 25 separate members of Bell and Alvord, (1976), but some general 30 of Late Proterozoic age. Tatnic Hill and is described in greater detail. Nashoba should thus, replace Barosh and Hermes, 1981; Hermes and others, 1981). Similar rock occurs Barosh, P.J., and Hermes, O.D., 1981, General structural setting of Rhode Island and 30 70 85 lithologic assemblages recognized locally. The formation is described 30 the term Tatnic Hill in the region. on both sides of this fault, which rolls over to dip to the west, diminishes in tectonic history of southeastern New England, in Boothroyd, J.C., and Hermes, O.D., 85 elsewhere in Connecticut as the Tatnic Hill Formation, but mapping editors, Guidebook to geologic field studies in Rhode Island and adjacent areas: New 35 40 Pzng 35 The Oxford anticline is situated at the northwest corner of a large complex offset and eventually dies out to the north in the Lake Char zone. The 18 80 here shows its equivalency to the Nashoba, which takes precedence. England Intercollegiate Geologic Conference, University of R.I., Kingston, R.I., p. 1- 50 Pzp north-plunging arch within the southeast New England platform. This arch The lower, more amphibolite-rich part of the section is referred to as hypothesized extension and great offset of the Hope Valley shear zone would 30 Pzngs 15, also in Barosh, P.J., and Smith, P.V., editors, 1983, New England Seismotectonic Pznr 35 Amphibolitic Unit. Medium to dark-gray and greenish dark-gray, forms Rhode Island, west of the Narragansett basin, and extends into Marlboro Formation (Emerson and Perry, 1907), in Massachusetts and violate the known continuity of the stratigraphy, igneous rock bodies, fold Study activities during fiscal year 1981: U.S. Nuclear Regulatory Commission Report 80 Pzna 60 10 fine to medium- grained amphibolite and subordinant amount of Pzna southern Massachusetts. The western side of the arch is wrinkled into a series Quinebaug Formation in Connecticut (Dixon, 1964). Both sections are axes, and both the aeromagnetic and gravity data in this region. NUREG/CR-3253, p. 98-115. 35 medium-gray, medium-grained gneiss. Thin-bedded and locally very 12 of north-trending folds, the Western Rhode Island fold belt, that lie against similar in thickness: the section in Massachusetts 2,140 m (Bell and Alvord, 44 ? well laminated. Sedimentary structures locally obscured by shearing, Barosh, P.J., and Johnson, C.K., 1976, Reconnaissance bedrock geologic map of the F 65 ? 1976) and that in Connecticut about 2,250 m. The thin-bedded, very well- AULT 10 65 grades upward into rusty gneiss unit. the suture zone (Barosh, 1976a, Barosh and others, 1977). The Oxford GEOLOGIC HISTORY Leicester quadrangle, Massachusetts, U.S. Geological Survey Open-File Report 76- 30 60 Pzp 5 10 ? anticline forms the largest of these folds along western Rhode Island and the bedded upper member of the Quinebaug (Dixon, 1964) is similar to the 27 814, 9 p., scale 1:24,000. Pzm Pzp 6 ? Rusty Gneiss Unit. Medium to dark-gray gneiss that weathers one that extends farthest to the north. upper Sandy Pond member of the Marlboro, especially its lower part. The The recorded history in the Oxford area began with the deposition of the ? Pznr Pzngn ? yellow gray to rusty to dark-brown interbedded with dark-gray Barosh, P.J., Pease, M.H., Jr., Schnabel, R.M., Bell, K.G., and Peper, J.D., 1977, Pzp 15 Pzng Pzng Pzng upper member reaches 300 m in thickness, but is locally less than 100 m, Plainfield Formation in the Proterozoic. Shales, dolomite and clean well- 23 45 ? amphibolite and medium-gray gneiss, that weathers slightly lighter Aeromagnetic lineament map of southern New England showing relation of and rare light-gray marble and garnetiferous beds. Commonly ROCK IN THE SOUTHEAST NEW ENGLAND PLATFORM whereas the Sandy Pond is 640 m in thickness. Faulting at the top of the bedded and finely laminated sand were deposited in a subsiding shelf area, 85 45 Pzngs lineaments to bedrock geology: U.S. Geological Survey, Misc. Field Studies Map, 60 33 Pzngs laminated and usually thin-bedded, but may have medium beds Quinebaug in Connecticut may account for the difference. that gradually became overwhelmed with volcanic debris and flows as the ? MF-855. Scale l:250,000. 35 locally. Commonly sheared. Unit grades upward. g; Lense of dark- Plainfield Formation: A sequence of quartzite-bearing strata overlain by environment changed. The basin then had a relative position somewhere ? 30 gray basalt or gabbro? at Route 39 and Thompson Road. biotite gneiss, amphibolite and metavolcanic rock and underlain by remnants The well described Marlboro takes precedence and should be used in place northwest of the present coast of northwestern Africa. The immense Bell, K.G., and Alvord, D.C., 1976, Pre-Silurian stratigraphy of northeastern 65 33 Pzna Pzp 24 85 Massachusetts, in Page, L.R., ed., Contributions to the stratigraphy of New England: 65 75 55 of calcareous and amphibolitic strata with some pillow basalt and of Quinebaug, while retaining the type Quinebaug section as a reference thickness of the volcaniclastic debris of the Nashoba and adjacent formations ? Gneiss Unit. Light to medium-gray fine to medium-grained gneiss section. Geological Society of America, Memoir 148, p. 149-216. 30 45 10 Pzngn and medium to dark- gray amphibolitic gneiss. Gneiss is garnet- conglomerate is invaded by the Northbridge Granite and other batholithic was deposited from the southeast (Bell and Alvord, 1976) into a rapidly 75 Pzm ? 63 32 ZONE 16 bearing locally, and contains some schistose layers. Usually laminated rock in Rhode Island and vicinity (Emerson and Perry, 1907; Warren and subsiding basin farther seaward contemporaneously or slightly later. 45 12 Bell, K.G., Shride, A.F., Cupples, N.P., Dennen, W.H. and Barosh, P.J., 1996, 60 85 Pzp 15 and thin-bedded. In places contains porphyroblasts of feldspar and Powers, 1913; Emerson, 1917). The quartzite-bearing sequence is intruded Age: The Marlboro-Tadmuck Brook sequence is known to be pre-Silurian 20 18 75 Bedrock geologic map of the Georgetown quadrangle, Essex County, Massachusetts: Pznr 64 20 thin sheets of feldspar and quartz, especially where relatively more 32 45 55 85 80 in the vicinity of the Oxford quadrangle and separate terminologies for these from overlying unmetamorphosed fossiliferous red beds and volcanic rock This region drifted towards the Sturbridge basin offshore of an earlier North U.S. Geological Survey, unpublished Geologic Quadrangle, scale 1:24,000. 57 28 sheared. 60 8 80 strata developed in southern Massachusetts, northeastern Rhode Island and (Bell and Alvord, 1976, Bell and others 1996). Furthermore, it can be American craton, as it was constituted in Grenville time. The early 60 58 50 80 72 20 inferred to be pre-Late Cambrian in age from the age of the base of a mildly Crosby, W.O., 1880, Contributions to the geology of eastern Massachusetts: Boston 45 8 western Rhode Island-eastern Connecticut. The Blackstone Series, intervening ocean, referred to as Iapetus, narrowed and the basins containing 31 MARLBORO FORMATION (LATE PROTEROZOIC) Society of Natural History, Occasional Papers 3, 286 p. 15 72 Pzm encompassing several formations, is used for the strata in northeastern metamorphosed sequence, described to the north by Peck (1976) in the the Nashoba and Plainfield collided against and slid under the offshore A Dark-gray to black amphibolite and medium to dark-gray amphibolitic 83 gneiss. Locally has few to abundant thin lenses of light-gray fine to Rhode Island, (Warren and Powers, 1913, Quinn, 1971). This name was Nashua trough.. The individual formations and the sequence closely match a basin. The strata were dragged down, heated, partially melted to granitic Cupples, N.P., 1961, Post- deformation of metamorphic and igneous 39 Pzp 9 Pzng Pzp ? Pzng medium-grained feldspar and quartz that produce a banded rock. adopted despite Emerson and Perry's (1907) extension of the Westboro fossiliferous Upper Cambrian-Ordovician strata in Maine (Barosh, 1999a). magma and began to wrinkle into folds along the leading edge of the granitic rocks near the northern boundary fault, Boston Basin, Massachusetts: U.S. 10 The formation is well-bedded into thin beds and commonly well (Grafton) Quartzite from Massachusetts and their rejection of Woodward's These beds post-date the intense deformation and metamorphism that affects mass. The Plainfield strata folded, while granite formed below, into the Geological Survey, Professional Paper, 424D, p. D46-D48. 80 ? ? laminated elsewhere, but here deformation has obscured much of this 80 OXFORD (Shaler and others, 1899) suggestion of using Blackstone Series. The term the Nashoba. In addition, this intense deformation is intimately asssociated Oxford anticline. 80 and results in a thinly layered rock with varying degrees of Pzngn Pzm with the dated similar Late Paleozoic deformation in the batholithic complex Dixon, H.R., 1964, The Putnam group of eastern Connecticut: U.S. Geological irregularities, wavyness and local mylonitic layers. The unit is a valley Plainfield Formation was applied to this sequence separately in southheastern Survey, Bulletin 1194-C, 12 p. 80 former and is very poorly exposed. Thin fault slivers could be present Connecticut by Lundgren (1962) and later carried northward. It was thought to the east, where no later deformation as intense has been found. The Thrust slices within the volcaniclastic strata of the collision zone fitted 80 ANT Pzm Pzng farther to the northeast. It is better exposed and less deformed to the to be younger than the Blackstone (Goldsmith, 1966, Quinn, 1971, Dixon, Marlboro and Nashoba Formations must be Proterozoic in age and seemingly themselves around this growing anticlinal protuberance into the thrust belt. Dixon, H.R., 1968, Bedrock geology of the Plainfield-Danielson area, Connecticut: Pzna 80 IC 35 55 50 LI south where Dixon (1974) describes mineralogic variations. The unpublished PhD thesis, Harvard University, Cambridge, MA, 336 p., scale 1:24,000. ? 1974). of post-Grenville Late Proterozoic Z age. A radiometric age date from the Continued movement sheared and finely ground many deformed zones, 25 51 35 65 ? NE formation is described in Connecticut as the Quinebaug Formation, 50 20 57 35 23 Fish Brook Gneiss, that lies between them gave 730 my (Zartman and perhaps as the strata, that were at elevated temperatures and pressures began 35 but mapping here shows its equivalency to the Marlboro, which takes 36 Dixon, H.R., 1974, Bedrock geologic map of the Thompson Quadrangle, Windam 10 35 These three separate stratigraphic terminologies have been applied to the Marvin, 1987). to cool. These events occurred roughly 625 my ago at the time of the 25 32 precedent. County, Connecticut, and Providence County, Rhode Island: U.S. Geological Survey, 70 Pzm Pzp Pzp same quartzitic sequence that wraps around the Oxford anticline. The west- Cadomian orogeny, which deformed much of southern Europe. Geologic Quadrangle Map, GQ-1165, scale 1:24,000. 30 ? 8 33 40 ? 25 Pzngn 25 Pzng PLAINFIELD FORMATION (LATE PROTEROZOIC) dipping Plainfield of the southwestern Thompson quadrangle, to the south, ROCK IN THE NASHUA TROUGH 45 Pzm Pzp P zp Light to medium-gray quartz schist and white to light-gray to very pale 40 60 can be matched with the east-dipping Blackstone of the northeastern The area rose up and was eroded rapidly nearly down to sea level. The uplift Emerson, B.K., 1898, Geology of old Hampshire County, 35 Pzp tan, fine to medium grained quartzite. Contains variable amounts of 60 Pzng 25 Chepachet quadrangle, to the southeast, after passing through the northerly- Nashua Formation: A sliver of phyllite, that is exposed in the Clinton- initiated brittle faulting as extensional movement began to form the Boston Massachusetts: U.S. Geological Survey, Monograph 29, 790 p. 30 Pzng Pzng sericite to muscovite that commonly produce lustrous surfaces. In 58 Newbury fault zone at Hodges Village dam just off the northwest corner of 32 ? 30 well-laminated, thin to medium beds, but these are usually obscured or dipping Westboro of southern Massachusetts, with only short granite filled basin by 595 my and volcanic deposits poured into it. Marls and mud Emerson, B.K., and Perry, J.H., 1907, The green schists and associated granites and 30 obliterated by shearing and alteration from intrusion of the gaps. It is here proposed to continue the usage of Dixon (1974) and apply the quadrangle, probably extends beneath a covered zone at the western overlapped the eroded batholithic complex east of Oxford in the Early porphyries of Rhode Island: U.S. Geological Survey, Bulletin 311, 74p. 55 85 60 45 Northbridge Granite. The slight to moderate foliation parallels the Plainfield Formation to all of these strata to show their equivalency and to edge of the quadrangle. This dark-gray, thin-bedded, graded-bedded phyllite Cambrian. A narrow sea still existed between here and the North American 72 35 80 bedding and is also obscured by shearing. The quartz schist is Pznr 38 start a more formal and organized description of the regional stratigraphy. was previously named the Hodges Village Phyllite (Barosh, 1974) and found mainland to the west during Cambrian, as shown by differing faunas on Emerson, B.K., 1917, and Rhode Island: U.S. Geological 75 40 ? Pzm assimilated by the granite first and there is relatively more quartzite Survey Bulletin 597, 289 p., map scale 1:25,000. 80 20 along the contact than in the formation. The quartzite locally grades The Plainfield Formation appears to have the best modern description. It to be part of Unit 3 of Peck's (1976) section in the Clinton quadrangle to the either side; a sea that was reduced further during the Ordovician (Wilson, north (Barosh, 1977, 1996a). Nashua Formation herein replaces this into granite. The strata here are remnants of the formation and are includes the strata of both the Westboro Formation and underlying unnamed 1966). Northwest of the suture zone a quiet water basin received turbidity Goldsmith, R., 1966, Stratigraphic names in the New London area, Connecticut: equivilant to one of the quartzite rich units mapped in the Plainfield quartzite and gneiss of Bell and Alvord (1976) in Massachusetts and is temporary unit term and is named for outcrops near the Nashua river in flows of mud during the Ordovician, forming the Nashua Formation and U.S. Geological Survey, Bulletin 1224-J, 9 p. Formation by Dixon (1974) to the south. Quartz schist usually sheared equivalent in general to the Blackstone Series in Rhode Island. However, the Lancaster, Massachusetts in the central part of the Clinton quadrangle. The other deposits, whereas southeast of it volcanoes were active around the Topographic base scanned from U.S. Geological Survey, 1969 Geology mapped by P. J. Barosh, 1974-1975, 1993- into thin irregular lenses, 1-3 cm thick, sub-parallel to bedding. 141 /2 latter was thought by Quinn (1967) to contain more strata than the Westboro formation occurs there in a 1,830 to 2,440 m thick section (Peck, 1976) that Boston area. Intrusion, volcanism and renewed thrust faulting occurred Hatch, N.L., editor, 1991, The bedrock geology of Massachusetts: U.S. Geological Polyconic projection. 1927 North American Datum 1994 and revised 2004, assisted by P. V. Smith 1982, Quartzite beds are sheared less and more representative of original and the Plainfield Formation may not constitute all of the Blackstone Series. lies in the Nashua trough. The formation lies between Peck's Units 2 and 4 towards the end of the Ordovician to mark the Taconic orogeny. This Survey, Professional Paper 1366. 132 p. M and digitized by S. B. Mabee, K. W. Christle, and J. T bedding. A Digitally reprojected to 1983 North American Datum R G U N E The specific correlation of the formation with members of the series needed and overlies the much more deformed Proterozoic strata and granite of the movement formed the Nashua trough and injected granitic rock into some of E MASS. Felis, 2004 - 2005.

T Hermes, O.D., Gromet, P.L. and Murray, D.P., 1994, Bedrock geologic map of Rhode N 10,000 foot grid ticks based on Massachusetts state plane coordinate I C O

R Sturbridge basin. to form a single standard formational nomenclature awaits further work. The the reactivated faults in the Nashoba thrust belt just north of Oxford. The N Island: Office of the Rhode Island State Geologist: Scale 1:100,000. T O system, mainland zone H R T remnants of quartzite and quartz schist in the Oxford quadrangle correlate in strata in and northwest of the thrust belt were moderately metamorphosed H 1000-meter Universal Transverse Mercator grid ticks, zone19 Hermes, O.D., Barosh, P.J., and Smith, P.V., 1981, Contact relationships of the Late AP P R OXIMATE ME AN general with the more massive quartzite unit in the Plainfield Formation to Age: The Nashua formation is part of a sequence that closely correlates with whereas the old granites to the southeast may have been only DE C LINATION, 1999 DISCUSSION the south and in the Blackstone to the east. However, quartzite is over a fossiliferous Ordovician section in Maine (Barosh, 1999a). hydrothermally altered. Paleozoic Narragansett Pier Granite and country rock, in Boothroyd, J.C., and QUADRANGLE LOCATION Hermes, O.D., editors, Guidebook to geologic field studies in Rhode Island and represented in Oxford as it tends to be relatively more resistant to both adjacent areas: New England Intercollegiate Geologic Conference, 73rd Annual INTRODUCTION assimilation and to weathering. STRUCTURAL GEOLOGY The Taconic activity closed any gap along the Clinton-Newbury fault zone. Meeting, Dept of Geology, Univ. of R.I., Kingston, R.I., p. 125-151. Silurian and Early Devonian volcanic rock and red beds were deposited, The Oxford quadrangle is situated at a knot of structural and lithologic complexity The well-bedded, thin-bedded, well-laminated metasediments of the OXFORD ANTICLINE: The dominating structure of the quadrangle is the along a shore line over the eroded surface of the earlier rock on both sides of Lenk, L., Strother, P.K., Kaye, C.A., and Barghoorn, E.S., 1982, Proterozoic Z age of along the suture zone of the Northern Appalachian orogen. The excellent Plainfield were probably deposited in a subsiding shallow shelf environment Oxford anticline that juts northwestward into and under the Nashoba thrust the fault (Bell and others, 1996; Barosh, 1995). These strata remain the Boston basin, Massachusetts; new evidence from mircrofossils: Science, v. 216, p. 619-620. exposures in the area not only reveal details of the structure and its history, but that had occasional influxes of volcanic rock and debris and conglomerate. belt (Barosh, 1976b, 1982) (Fig. 1). The granite-cored fold is revealed by unmetamorphosed and later events, which are relatively weak in the Oxford provide a rare opportunity to understand the general bedrock fracture system that Eventually the basin was overwhelmed by volcanic sediment and flows. both the bedding in the Plainfield Formation and the paralleling flow foliation area, are best discussed elsewhere (Woodhouse and others, 1991). The Lundgren, L., Jr., 1962, Deep River area, Connecticut: sratigraphy and structure: is of great practical importance in the region. in the Northbridge Granite. The anticline is well expressed in the topographic, Clinton-Newbury fault continued to reactivate and move with right-lateral American Journal of Science, v. 260, p. 1-23. Age: The Plainfield (Westboro) Formation is Proterozoic in age (Bell and aeromagnetic and gravity data (Barosh and others, 1977). The motion at times in the Paleozoic and locally with normal offset later (Barosh, Alvord, 1976). It is intruded by the Late Proterozoic Northbridge Granite, aeromagnetic expression allows mapping of portions of its border beneath a 1995). Many of these events were associated with more widespread brittle Nelson, A.E., 1975, Bedrock geologic map of the Framingham quadrangle, Middlesex and Worcester Counties, Massachusetts: U.S. Geological Survey, Geologic but shows no earlier deformation. It therefore is probably not too much older cover of glacial debris and water. A faint north-plunging lineation, that is faulting. Movement along northeast-trending normal faults occurred as the Quadrangle, GQ-1274, scale 1:24,000. and also of Late Proterozoic in age. more prominent to the south, parallels the axis and the orientation of the North Atlantic ocean basin opened. Their movement tapered off and block jointing in the granite rotates around the axis. The axis of the anticline northwest and north-trending faults became active as the ocean basin O'Hara, K., and Gromet, P.L.,1985, Two distinct late Precambrian (Avalonian) Northbridge Granite: The granitic rock of the Oxford quadrangle is bends and continues southward approximately along the border between the widened. These two fault sets are still active locally in the region, but no terranes in southeastern New England and their late Paleozoic juxtaposition: American Journal of Science, v. 285. p. 673-709. dy described as a porphyritic granite-gneiss and designated the Northbridge Thompson and Chepachet quadrangles. earthquakes have been recorded in the Oxford quadrangle. A e b s t r W

D o u g l a s A' Bloo Gneiss by Emerson and Perry (1907) following Emerson's (1898) definition Peck, J.H., 1976, Silurian and Devonian stratigraphy in the Clinton Quadrangle, C T M A ne SW o R t . 1 6 S u t o n Z D o u g l a s NE of the unit. Emerson later described it as the Northbridge Granite Gneiss; a The fold is slightly asymmetrical and the axis lies closer to its southwestern RESOURCES central Massachusetts in Page, L.R., editors, Contributions to the stratigraphy of New ult Oxford Anticline

1000' R t . 3 9 5 Fa 1000' rock with "....a core of coarse, slightly gneissoid, porphyritic microcline-biotite side. The outer part of the northeastern limb strikes to the east and is England: Geological Society of America, Memoir 148, p. 241-252. Lake Char - 600' Bluff 600' granite and a broad border of completely mashed, stretched, and penciled affected by a slight warp, shown with a synclinal axis. The inner part is more The area has considerable resources for crushed rock and dimension stone, Quinn, A.W., 1967, Bedrock geology of the Chepachet quadrangle, Providence Pzngn Pzngs Pzng Pzng highly muscovitic gneiss" (Emerson 1917, p.155). The rock is slightly to tightly squeezed, and in its northeastern part strikes to the southeast. A zone but these are not being exploited at present. Small stone quarries existed in 200' Pzna Pzng 200' County, Rhode Island: U.S. Geological Survey, Bulletin 1241-G, 26 p. scale SEA LEVEL Pznr Pzm Pzp Pzng SEA LEVEL strongly foliated quartz monzonite whose composition varies locally due to of thrust faults allows for adjustment between the inner and outer parts. the past in the southern part of the Northbridge Granite, but have all closed. 1:24,000. partial assimilation of various rock types and shearing. It is herein designated It makes a very handsome decorative rock, but the block jointing limits the 500' 500' the Northbridge Granite for clarity. The outer part of the Northbridge has Greater deformation adjacent to the Nashoba thrust belt, especially on the size of blocks. Quinn, A.W., 1971, Bedrock geology of Rhode Island: U.S. Geological Survey, 1000' 1000' been followed northeastward through the Shrewsbury and Marlboro and southwest side, caused more shearing and mylonization and thrust faulting. Bulletin 1295, 68 p., map scale 1:125,000. Framingham quadrangles (Nelson, 1975; Barosh 1978a, 1978b) where it had The shears and thrust faults were generally controlled by the bedding and Most rock, other than rusty-weathering layers, would serve as crushed rock Shaler, N.S., Woodworth, J.B., and Foerste, A.F., 1899, Geology of the Narragansett Comments to the Map User been called Milford Granite. Emerson and Perry (1907) also noted some flow foliation. These faults seem to cause some repetition of the Plainfield for general fill and rock material. The Northbridge Granite is good for Basin: U.S. Geological Survey, Monograph 33, 402 p. similarity between the two. The rock in these quadrangles is different from Formation and to have broken the nose of the anticline and moved it crushed rock for concrete, except in the strongly foliated parts. These may the massive granite of the type Milford, which intrudes the Northbridge in eastward with a right-lateral shift. have too highly strained quartz for such use. Warren, C.H. and Powers, S., 1913, Geology of the Diamond Hill-Cumberland A geologic map displays information on the distribution, nature, orientation and This Open-File map is a progress report of mapping in this area, and is Milford, Massachusetts. district in Rhode Island: Geological Society of America, Bulletin, v. 25, p. 435-476. preliminary in nature. Revision of the map is likely because of the on-going age relationships of rock and deposits and the occurrence of structural features. NASHOBA THRUST BELT: The Oxford anticline impales the Nashoba Marble lenses in the Nashoba Formation were mined in the past for lime. nature of work in the region. It has not been peer reviewed or edited to conform Wilson, J.T., 1966, Did the Atlantic close and then reopen?: Nature, v. 211, p. 676. Geologic and fault contacts are irregular surfaces that form boundaries between The southern part of the Northbridge in Rhode Island was renamed thrust belt causing it to thin tectonically from 22 km in width to the northeast The exposed lenses are tiny and unexposed ones would be too small for with editorial standards of the Massachusetts State Geologist or with the North different types or ages of units. Data depicted on this geologic quadrangle map Ponaganset Gneiss by Quinn (1967), because he found the granite to invade to a half km in the Oxford-Webster area. The thinning appears largely exploitation today. American Stratigraphic Code. The contents of the report and map should not Woodhouse, David and Barosh, P.J., editors, 1991, Geology of the City of Boston, are based on reconnaissance field geologic mapping, compilation of published the Westboro, whereas Emerson and Perry (1907) stated the Northbridge was accomplished by progressively overriding the lower thrust sheets, thus first Massachusetts, Association of Engineering Geologists, Bulletin, v. 28, p. 375-512. and unpublished work, and photogeologic interpretation. Locations of contacts be considered final and complete until reviewed and published by the Office of older. Later, however when Quinn (1971, p. 11) found that the type eliminating the lower part of the Nashoba Formation and adjacent units and are not surveyed, but are plotted by interpretation of the position of a given the Massachusetts State Geologist. The views and conclusions contained in this Northbridge did cut the Westboro, he did not abandon the term Ponaganset. then working upwards. Zartman, R.E., and Marvin, R.F., 1987, Radiometric ages on file in the radiometric contact onto a topographic base map; therefore, the accuracy of contact document are those of the authors and should not be interpreted as necessarily age data bank (RADB) of rocks from Massachusetts: U.S. Geological Survey, Open- Quinn's designation was extended westward into the Thompson quadrangle, File Report 87-170, 421 p. locations depends on the scale of mapping and the interpretation of the representing the official policies, either expressed or implied, of the University of just south of the Oxford, by Dixon (1974) who divided the granite there into The overall alignment of compression in the region was east-northeast-west- Massachusetts, Commonwealth of Massachusetts, and the United States Federal geologist(s). Any enlargement of this map could cause misunderstanding in the the Ponaganset Gneiss, Scituate Granite Gneiss and the Hope Valley Alaskite southwest with the southeast side shoved west-southwest beneath the Zen, E-an, editor, 1983, Bedrock geologic map of Massachusetts: U.S. Geological detail of mapping and may result in erroneous interpretations. Site-specific Government. conditions should be verified by detailed surface mapping or subsurface exploration. Topographic and cultural changes associated with recent This research was supported by U.S. Geological Survey, National Cooperative development may not be shown. Geologic Mapping Program, under assistance Award No. 04HQAG0028.

We recommend reading Reading Maps with a Critical Eye: Becoming an Citation: Informed Map Reader by the Maine Geologic Survey to make the best use of a Barosh, P.J., Bedrock geologic map of the Oxford quadrangle, Worcester County, Massachusetts, Providence County, Rhode Island and Windham County BEDROCK GEOLOGIC MAP OF THE OXFORD QUADRANGLE, WORCESTER geologic map Connecticut. Office of the Massachusetts State Geologist, (in review, June (http://www.maine.gov/doc/nrimc/mgs/mapuse/informed/informed.htm). 2005) COUNTY, MASSACHUSETTS, PROVIDENCE COUNTY, RHODE ISLAND, AND This map was produced on request directly from digital files (PDF format) on an electronic plotter. WINDAM COUNTY, CONNECTICUT

A digital copy of this map (PDF format), including GIS datalayers, is By available at http://www.geo.umass.edu/stategeologist Patrick J. Barosh 2005