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NORTHBRIDGE GRANITE (LATE PROTEROZOIC) EXPLANATION OF GEOLOGIC MAP UNITS Selected modal analysis of samples from most of these units given by Dixon (1974) for samples collected just to the south. Pzngn Pznr Pzna Pzn Pzp Pzng Pznga Pzngs Pzm On The geologic mapping of the Oxford quadrangle is part of a study of nine quadrangles in the region around Worcester, Massachsuetts. The project's goals are to unravel the great structural discontinuity through the region, define the stratigraphy present and correlate it across state lines, tie together the work of eight nearby U.S. Geological Survey quadrangle mapping projects and interpret the aeromagnetic data. The preliminary results were presented in 1977 (Barosh, 1976b, 1977), when quadrangle mapping in southeast New England was halted. These remain the latest map information until now. The compilation on the Geologic Map of Massachusetts (Zen, 1983) and its delayed description (Hatch, 1991) used little of the new field data and were based largely on much earlier concepts. Since releasing the preliminary results the area around Oxford has been studied in detail (Barosh, 1996a, 1996b; Barosh and Johnson, 1976)(fig. 1). Great strides also have been made in dating the rocks and events and in understanding the bedrock fracture system. This system is of particular significance to environmental, ground water, tunnel and earthquake studies in the region. The combination of exposure, large-scale aerial photography, local areas of detailed work and drill hole data has produced the first comprehensive view of a bedrock fracture system in southern New England. The Oxford quadrangle lies astride a northwest plunging anticlinal ridge that is the terminus of the uplands of western Rhode Island. Its northeast side drains to the Blackstone River that flows southward into Narragansett Bay, whereas waters of its western flank flow southward into the Thames River drainage system, that empties into Long Island Sound. The landforms closely reflect the geologic structure and lithology. GEOLOGIC SETTING The Oxford quadrangle lies against the southeast side of a suture zone that snakes across southeastern New England (Barosh, 1982, Woodhouse and others, 1991). This is the fundamental structure of the Northern Appalachian orogen, first recognized by Wilson (1966), that here separates rock on the 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 of Grenville time. The zone is marked by the Nashoba thrust belt in southeast New England (Fig. 1). This great west-dipping zone of dislocation separates an enormous pile of metasedimentary strata in the Sturbridge basin to the northwest from a batholithic complex forming the Southeast New England platform on the southeast. The thrust belt itself contains a thick section of largely metavolcaniclastic strata derived from the southeast. All three provinces are of Late Proterozoic age. The Oxford anticline is situated at the northwest corner of a large complex north-plunging arch within the southeast New England platform. This arch forms Rhode Island, west of the Narragansett basin, and extends into southern Massachusetts. The western side of the arch is wrinkled into a series of north-trending folds, the Western Rhode Island fold belt, that lie against the suture zone (Barosh, 1976a, Barosh and others, 1977). The Oxford anticline forms the largest of these folds along western Rhode Island and the one that extends farthest to the north. ROCK IN THE SOUTHEAST NEW ENGLAND PLATFORM Plainfield Formation: A sequence of quartzite-bearing strata overlain by biotite gneiss, amphibolite and metavolcanic rock and underlain by remnants of calcareous and amphibolitic strata with some pillow basalt and conglomerate is invaded by the Northbridge Granite and other batholithic rock in Rhode Island and vicinity (Emerson and Perry, 1907; Warren and Powers, 1913; Emerson, 1917). The quartzite-bearing sequence is intruded in the vicinity of the Oxford quadrangle and separate terminologies for these strata developed in southern Massachusetts, northeastern Rhode Island and western Rhode Island-eastern Connecticut. The Blackstone Series, encompassing several formations, is used for the strata in northeastern Rhode Island, (Warren and Powers, 1913, Quinn, 1971). This name was adopted despite Emerson and Perry's (1907) extension of the Westboro (Grafton) Quartzite from Massachusetts and their rejection of Woodward's (Shaler and others, 1899) suggestion of using Blackstone Series. The term Plainfield Formation was applied to this sequence separately in southheastern Connecticut by Lundgren (1962) and later carried northward. It was thought to be younger than the Blackstone (Goldsmith, 1966, Quinn, 1971, Dixon, 1974). These three separate stratigraphic terminologies have been applied to the same quartzitic sequence that wraps around the Oxford anticline. The west- dipping Plainfield of the southwestern Thompson quadrangle, to the south, can be matched with the east-dipping Blackstone of the northeastern Chepachet quadrangle, to the southeast, after passing through the northerly- dipping Westboro of southern Massachusetts, with only short granite filled gaps. It is here proposed to continue the usage of Dixon (1974) and apply the Plainfield Formation to all of these strata to show their equivalency and to start a more formal and organized description of the regional stratigraphy. The Plainfield Formation appears to have the best modern description. It includes the strata of both the Westboro Formation and underlying unnamed quartzite and gneiss of Bell and Alvord (1976) in Massachusetts and is equivalent in general to the Blackstone Series in Rhode Island. However, the latter was thought by Quinn (1967) to contain more strata than the Westboro and the Plainfield Formation may not constitute all of the Blackstone Series. The specific correlation of the formation with members of the series needed to form a single standard formational nomenclature awaits further work. The remnants of quartzite and quartz schist in the Oxford quadrangle correlate in general with the more massive quartzite unit in the Plainfield Formation to the south and in the Blackstone to the east. However, quartzite is over represented in Oxford as it tends to be relatively more resistant to both assimilation and to weathering. The well-bedded, thin-bedded, well-laminated metasediments of the Plainfield were probably deposited in a subsiding shallow shelf environment that had occasional influxes of volcanic rock and debris and conglomerate. Eventually the basin was overwhelmed by volcanic sediment and flows. Age: The Plainfield (Westboro) Formation is Proterozoic in age (Bell and Alvord, 1976). It is intruded by the Late Proterozoic Northbridge Granite, but shows no earlier deformation. It therefore is probably not too much older and also of Late Proterozoic in age. Northbridge Granite: The granitic rock of the Oxford quadrangle is described as a porphyritic granite-gneiss and designated the Northbridge Gneiss by Emerson and Perry (1907) following Emerson's (1898) definition of the unit. Emerson later described it as the Northbridge Granite Gneiss; a rock with "....a core of coarse, slightly gneissoid, porphyritic microcline-biotite granite and a broad border of completely mashed, stretched, and penciled highly muscovitic gneiss" (Emerson 1917, p.155). The rock is slightly to strongly foliated quartz monzonite whose composition varies locally due to partial assimilation of various rock types and shearing. It is herein designated the Northbridge Granite for clarity. The outer part of the Northbridge has been followed northeastward through the Shrewsbury and Marlboro and Framingham quadrangles (Nelson, 1975; Barosh 1978a, 1978b) where it had been called Milford Granite. Emerson and Perry (1907) also noted some similarity between the two. The rock in these quadrangles is different from the massive granite of the type Milford, which intrudes the Northbridge in Milford, Massachusetts. The southern part of the Northbridge in Rhode Island was renamed Ponaganset Gneiss by Quinn (1967), because he found the granite to invade the Westboro, whereas Emerson and Perry (1907) stated the Northbridge was older. Later, however when Quinn (1971, p. 11) found that the type Northbridge did cut the Westboro, he did not abandon the term Ponaganset. Quinn's designation was extended westward into the Thompson quadrangle, just south of the Oxford, by Dixon (1974) who divided the granite there into the Ponaganset Gneiss, Scituate Granite Gneiss and the Hope Valley Alaskite Gneiss. These correspond respectively to average Northbridge, the coarser "core", and the "mashed--- muscovite gneiss" mentioned by Emerson (1917). The better exposures and clearer relations in the Oxford quadrangle show these divisions to represent gradual changes within a single granitic body and not separate intrusions. These new names in the Chepachet and Thompson quadrangles now can be dropped. Nomenclature more descriptive of the rock would be Northbridge as a medium- to coarse-grained porphyritic facies, an aplitic border zone facies and sheared zones where muscovite has replaced the usual biotite of the granite. The Northbridge is generally a light, pinkish-gray, medium- to coarse- grained, slightly to strongly-foliated porphyritic biotite granite to quartz monzonite with pink microcline phenocrysts. The biotite occurs as tiny flakes in patches, rather than scattered. The anorthite content of the plagioclase is variable. The content given by Dixon (1974) when arranged by position, shows a change from an20 in the core to an8 at the border. Overall, the rock may appear uniform and monotonous in the field, but it has a great many subtle changes and hand specimens may vary considerably. Inward from the border the foliation decreases, the grain size increases, it appears more porphyritic and the biotite is more prominent. Other variations are due to the degree of hydrothermal alteration which causes the pink coloration of the phenocrysts, degree of assimilation of wallrock and amount of granulation and shearing. The general foliation is due to flowage, with super-imposed and parallel shear foliation, especially in zones along the border. The foliation is shown by sub-parallel patches of biotite, lensoid microcline and, in the fine-grained rock, thin lenses of quartz. The shearing causes the biotite to be replaced by fine muscovite and may reduce the apparent grain size locally to mylonite. The microcline phenocrysts are primary minerals that resist granulation better than the quartz and plagioclase. The quartz is usually finely granulated. The microcline is only slightly larger than the quartz and plagioclase in the fine-grained rocks, but becomes relatively larger in the coarser rocks and may reach over two cm in length. It commonly has a thick lenzoid "football" shape. The rock commonly has some lineation. Where it is strongly lineated or "penciled" the rock appears massive perpendicular to it and strongly foliated parallel to it, although the actual foliation may be obscured. The fine-grained aplitic border is present in the northern part of the quadrangle as a thin zone, that gradually widens to the northeast to form the very wide zone seen along the Massachusetts Turnpike in southwestern Westboro. An excellent section of the border zone and porphyritic rock is exposed in large cuts along Route 146 just northeast of the Oxford Quadrangle. The finer grain, higher quartz content and jointing, controlled by the uniform foliation, give the border zone a bedded look. This may cause it to be mistaken for the quartzite it intrudes. The border suffers varying degrees of shearing and may look very similar to the sheared granite elsewhere. The sheared rock in both cases is associated with thrust faults. Age: The Northbridge Granite is considered the oldest part of the batholithic complex (Quinn,1971). It was long known to be Precambrian, then assigned to the mid Paleozoic from miscorrelations and disturbed radiometric dates and now again firmly shown to be Precambrian. Fossiliferous Cambrian strata, that have suffered only slight metamorphism and therefore younger, occur at several places in eastern Massachusetts (Crosby, 1880; Woodward, in Shaler and others, 1899; Warren and Powers 1913; Bell and Alvord, 1976). In addition older fossiliferous Eocambrian strata of the Boston basin (Lenk and others, 1982) unconformably overlie the batholith (Nelson, 1975). These strata yield an age of 595 my (Kaye and Zartman, 1980). More reliable radiometric ages of 614 my (Zartman, in Nelson, 1975) and 620+- 15 my (Naylor, personal communication, 1975) were obtained from the part of the Milford Granite correlated with the Northbridge. The most likely age now seems to be approximately 625 my (Zartman, personal communication, 1991), but other other scattered dates from the region suggest a wide error margin. The granite is therefore Late Proterozoic Z in age. ROCK IN THE NASHOBA THRUST BELT Marlboro and Nashoba Formations: The Nashoba thrust belt was delineated in northeastern Massachusetts where the contained thick section of interbedded gneiss, amphibolite and schist was described by Bell and Alvord (1976). These strata continue into Connecticut (Barosh, 1977). However, of the Marlboro Formation, Shawsheen Gneiss, Fish Brook Gneiss, Nashoba Formation and Tadmuck Brook Schist present to the northeast (Bell and Alvord, 1976) only thin tectonic slivers of the Marlboro and Nashoba Formations survive in the Oxford quadrangle. These correspond, respectively, to the Quinebaug and Tatnic Hill Formations of Dixon (1964) to the south. The described members of the Nashoba Formation are only partially present in fault slices and are not seperated. The lithologic divisions made are merely local ones. The strata present probably represent parts of the higher members as the overlying Tadmuck Brook Schist is present only a few km to the north (Barosh, 1996b). The Yantic member of the Tatnic Hill in the Thompson quadrangle (Dixon, 1974) just to the south appears to be a zone of deformed strata along the west side of the formation and not a stratigraphic unit. Faults also appear to cut out part of the section in Connecticut as the section only reaches about 1,700m (Dixon, 1964) compared to the 10,890 m of the Nashoba to the northeast (Bell and Alvord, 1976). The lower part of the Connecticut section is reduced even more towards the Oxford area. A few blocks of laminated clean quartz-rich gneiss, similar to the Fish Brook Gneiss are present as float, at the east edge of the Nashoba in the southern Oxford quadrangle. These could possibly represent a tiny sliver of that unit or, perhaps, a higher lens of similar lithology. The type Nashoba Formation is from a much more complete section than the Tatnic Hill and is described in greater detail. Nashoba should thus, replace the term Tatnic Hill in the region. The lower, more amphibolite-rich part of the section is referred to as Marlboro Formation (Emerson and Perry, 1907), in Massachusetts and Quinebaug Formation in Connecticut (Dixon, 1964). Both sections are similar in thickness: the section in Massachusetts 2,140 m (Bell and Alvord, 1976) and that in Connecticut about 2,250 m. The thin-bedded, very well- bedded upper member of the Quinebaug (Dixon, 1964) is similar to the upper Sandy Pond member of the Marlboro, especially its lower part. The upper member reaches 300 m in thickness, but is locally less than 100 m, whereas the Sandy Pond is 640 m in thickness. Faulting at the top of the Quinebaug in Connecticut may account for the difference. The well described Marlboro takes precedence and should be used in place of Quinebaug, while retaining the type Quinebaug section as a reference section. Age: The Marlboro-Tadmuck Brook sequence is known to be pre-Silurian from overlying unmetamorphosed fossiliferous red beds and volcanic rock (Bell and Alvord, 1976, Bell and others 1996). Furthermore, it can be inferred to be pre-Late Cambrian in age from the age of the base of a mildly metamorphosed sequence, described to the north by Peck (1976) in the Nashua trough.. The individual formations and the sequence closely match a fossiliferous Upper Cambrian-Ordovician strata in Maine (Barosh, 1999a). These beds post-date the intense deformation and metamorphism that affects the Nashoba. In addition, this intense deformation is intimately asssociated with the dated similar Late Paleozoic deformation in the batholithic complex to the east, where no later deformation as intense has been found. The Marlboro and Nashoba Formations must be Proterozoic in age and seemingly of post-Grenville Late Proterozoic Z age. A radiometric age date from the Fish Brook Gneiss, that lies between them gave 730 my (Zartman and Marvin, 1987). ROCK IN THE NASHUA TROUGH Nashua Formation: A sliver of phyllite, that is exposed in the Clinton- Newbury fault zone at Hodges Village dam just off the northwest corner of the quadrangle, probably extends beneath a covered zone at the western edge of the quadrangle. This dark-gray, thin-bedded, graded-bedded phyllite was previously named the Hodges Village Phyllite (Barosh, 1974) and found to be part of Unit 3 of Peck's (1976) section in the Clinton quadrangle to the north (Barosh, 1977, 1996a). Nashua Formation herein replaces this temporary unit term and is named for outcrops near the Nashua river in Lancaster, Massachusetts in the central part of the Clinton quadrangle. The formation occurs there in a 1,830 to 2,440 m thick section (Peck, 1976) that lies in the Nashua trough. The formation lies between Peck's Units 2 and 4 and overlies the much more deformed Proterozoic strata and granite of the Sturbridge basin. Age: The Nashua formation is part of a sequence that closely correlates with a fossiliferous Ordovician section in Maine (Barosh, 1999a). STRUCTURAL GEOLOGY OXFORD ANTICLINE: The dominating structure of the quadrangle is the Oxford anticline that juts northwestward into and under the Nashoba thrust belt (Barosh, 1976b, 1982) (Fig. 1). The granite-cored fold is revealed by both the bedding in the Plainfield Formation and the paralleling flow foliation in the Northbridge Granite. The anticline is well expressed in the topographic, aeromagnetic and gravity data (Barosh and others, 1977). The aeromagnetic expression allows mapping of portions of its border beneath a cover of glacial debris and water. A faint north-plunging lineation, that is more prominent to the south, parallels the axis and the orientation of the block jointing in the granite rotates around the axis. The axis of the anticline bends and continues southward approximately along the border between the Thompson and Chepachet quadrangles. The fold is slightly asymmetrical and the axis lies closer to its southwestern side. The outer part of the northeastern limb strikes to the east and is affected by a slight warp, shown with a synclinal axis. The inner part is more tightly squeezed, and in its northeastern part strikes to the southeast. A zone of thrust faults allows for adjustment between the inner and outer parts. Greater deformation adjacent to the Nashoba thrust belt, especially on the southwest side, caused more shearing and mylonization and thrust faulting. The shears and thrust faults were generally controlled by the bedding and flow foliation. These faults seem to cause some repetition of the Plainfield Formation and to have broken the nose of the anticline and moved it eastward with a right-lateral shift. NASHOBA THRUST BELT: The Oxford anticline impales the Nashoba thrust belt causing it to thin tectonically from 22 km in width to the northeast to a half km in the Oxford-Webster area. The thinning appears largely accomplished by progressively overriding the lower thrust sheets, thus first eliminating the lower part of the Nashoba Formation and adjacent units and then working upwards. The overall alignment of compression in the region was east-northeast-west- southwest with the southeast side shoved west-southwest beneath the northwest one. The Oxford anticline and lineation developed approximately normal to this alignment. The compression produces a relative movement within and across the thrust belt of the northwest sides slipping over the southeast ones with a right-lateral component. These movements cut out the Marlboro Formation between the southwest part of the quadrangle to a point 23 km to the northeast. The local movement within the thrust sheets changes greatly and the orientation of small-scale structures varies extremely and has little regional significance. The numerous small drag folds can have almost every orientation along the twists and turns of the thrust faults. Many of these folds are well exposed in cuts along Route 395, that crosses the western side of the quadrangle. The most common situation is an anticlinal fold of varying steepness in the hanging wall and a flattening of the beds in the footwall beneath the fault, but locally the folds are very complex. The initial thrust faults were very ductile with a smearing and almost a flowage of minerals and rocks along them, yet still forming distinct breaks with offset. Renewed movement, probably under cooler conditions, results in shearing and mylonization. Slickensides form under later brittle reactivation. In places gouge and brecciation form under more brittle conditions, but are not commonly seen. Feldspar porphyroblasts, grading locally into pegmatite, forms bands along and near some early shear zones and bull quartz is very locally present along brittle faults. The eastern margin of the Nashoba thrust belt is marked by the Bloody Bluff fault zone of Cupples, (1961). This fault was followed from its type area in eastern Massachusetts into the Lake Char fault zone of Dixon (1968), which is named after the large lake in the Oxford quadrangle (Barosh, 1984). The Clinton-Newbury fault zone that marks the northwest side of the belt also was followed into the area. This fault zone is intruded by granite in Connecticut and had not been recognized there. The Bloody Bluff-Lake Char fault zone dips westward about 20 degrees in the Thompson quadrangle (Dixon, 1974), 30 to 40 degrees to the northwest across the Oxford quadrangle and much steeper to the northeast. NASHUA TROUGH: The Clinton-Newbury just brushes the western side of the quadrangle where it contains a sliver of Ordovician strata, caught up during a reactivation. This sliver is a remnant of the Nashua Trough, a basin bordered by reverse faults that contains a thick section of Cambro-Ordovician strata (Barosh, 1999a). The trough impinges upon the Clinton-Newbury fault zone at Worcester from the north and right-lateral movement has strung out some slivers of the trough in the fault zone southwest of this intersection.. BRITTLE FAULTS: An immense number of small brittle faults are present in the quadrangle and were noted by Dixon (1968) in outcrops to the south. Exposures here allow them to be shown over much of the Oxford quadrangle and local areas of detailed mapping reveals even a greater density than is shown. The majority are near vertical, but many have moderate dips. Most have small normal or lateral offsets that are not well defined. These faults form distinct sets and patterns and many sets are of regional extent. Most are relatively old and confined east of the Bloody Bluff-Lake Char fault zone, but relatively young ones cross the entire quadrangle. They are well expressed in the topography and generally have tectonic joint sets associated with them. The principal fault set trends are, from apparent oldest to youngest: northwest, east-west, north-northeast, northeast, just west of north, northwest and north. The early northwest set is mainly in the inner part of the Oxford anticline and may be associated with late movement related to its formation. The east-west set is most noticeable in the central and northeastern portions of the quadrangle. Faults of this trend become more prominent northeastward of the quadrangle and appear related to the Eocambrian and Ordovician faults of the Boston basin (Barosh, 1995). The north-northeast set is very prominent in the quadrangle and in the northeastern corner of the Uxbridge quadrangle to the east. These faults commonly have left-lateral offsets. The just west of north set appears to follow a broad zone that crosses the east side of the quadrangle. The above sets appear restricted to east of the Bloody Bluff and probably formed prior to the Silurian before the Bloody Bluff-Lake Char fault zone closed (Bell and others, 1996), whereas the following sets cut across the fault zone. The northeast set is composed of several faults, with approximately a one km spacing, that cut the anticline and tend to form cross valleys. Drilling into the prominent Reid Smith fault of this set showed a moderately steep dip to the northwest. The northwest set is a relatively young set with a half to one km spacing, that is prominent throughout the region. The north set appears the youngest and has a relatively wide spacing. One zone passes through Oxford center, a prominent one through the middle of the quadrangle, a less defined one two km to the east and a probable, poorly exposed one between Wallum lake and Stevens pond. The middle two appear to be the splayed end of a very prominent but unmapped fault that crosses all of eastern Connecticut. It is herein named the Tetreault fault after a pond which lies in its fault trough in the East Killingly quadrangle to the south. Many of these northwest and north-trending faults in southeastern New England are post-Triassic in age, some cut Cretaceous strata offshore and a number remain active (Barosh, 1999b). "HOPE VALLEY SHEAR ZONE": A major north-trending, east-dipping structural break called the Hope Valley Shear Zone has been hypothesized to pass approximately through and east of the southeast corner of the Oxford quadrangle by O'Hara and Gromet (1985) and Hermes and others (1994). This is an extension of a fault found along the west side of the Oxford anticline to the south and named the Oneco fault (Barosh, 1972, 1976a; Barosh and Hermes, 1981; Hermes and others, 1981). Similar rock occurs on both sides of this fault, which rolls over to dip to the west, diminishes in offset and eventually dies out to the north in the Lake Char zone. The hypothesized extension and great offset of the Hope Valley shear zone would violate the known continuity of the stratigraphy, igneous rock bodies, fold axes, and both the aeromagnetic and gravity data in this region. GEOLOGIC HISTORY The recorded history in the Oxford area began with the deposition of the Plainfield Formation in the Proterozoic. Shales, dolomite and clean well- bedded and finely laminated sand were deposited in a subsiding shelf area, that gradually became overwhelmed with volcanic debris and flows as the environment changed. The basin then had a relative position somewhere northwest of the present coast of northwestern Africa. The immense thickness of the volcaniclastic debris of the Nashoba and adjacent formations was deposited from the southeast (Bell and Alvord, 1976) into a rapidly subsiding basin farther seaward contemporaneously or slightly later. This region drifted towards the Sturbridge basin offshore of an earlier North American craton, as it was constituted in Grenville time. The early intervening ocean, referred to as Iapetus, narrowed and the basins containing the Nashoba and Plainfield collided against and slid under the offshore basin. The strata were dragged down, heated, partially melted to granitic magma and began to wrinkle into folds along the leading edge of the granitic mass. The Plainfield strata folded, while granite formed below, into the Oxford anticline. Thrust slices within the volcaniclastic strata of the collision zone fitted themselves around this growing anticlinal protuberance into the thrust belt. Continued movement sheared and finely ground many deformed zones, perhaps as the strata, that were at elevated temperatures and pressures began to cool. These events occurred roughly 625 my ago at the time of the Cadomian orogeny, which deformed much of southern Europe. The area rose up and was eroded rapidly nearly down to sea level. The uplift initiated brittle faulting as extensional movement began to form the Boston basin by 595 my and volcanic deposits poured into it. Marls and mud overlapped the eroded batholithic complex east of Oxford in the Early Cambrian. A narrow sea still existed between here and the North American mainland to the west during Cambrian, as shown by differing faunas on either side; a sea that was reduced further during the Ordovician (Wilson, 1966). Northwest of the suture zone a quiet water basin received turbidity flows of mud during the Ordovician, forming the Nashua Formation and other deposits, whereas southeast of it volcanoes were active around the Boston area. Intrusion, volcanism and renewed thrust faulting occurred towards the end of the Ordovician to mark the Taconic orogeny. This movement formed the Nashua trough and injected granitic rock into some of the reactivated faults in the Nashoba thrust belt just north of Oxford. The strata in and northwest of the thrust belt were moderately metamorphosed whereas the old granites to the southeast may have been only hydrothermally altered. The Taconic activity closed any gap along the Clinton-Newbury fault zone. Silurian and Early Devonian volcanic rock and red beds were deposited, along a shore line over the eroded surface of the earlier rock on both sides of the fault (Bell and others, 1996; Barosh, 1995). These strata remain unmetamorphosed and later events, which are relatively weak in the Oxford area, are best discussed elsewhere (Woodhouse and others, 1991). The Clinton-Newbury fault continued to reactivate and move with right-lateral motion at times in the Paleozoic and locally with normal offset later (Barosh, 1995). Many of these events were associated with more widespread brittle faulting. Movement along northeast-trending normal faults occurred as the North Atlantic ocean basin opened. Their movement tapered off and northwest and north-trending faults became active as the ocean basin widened. These two fault sets are still active locally in the region, but no earthquakes have been recorded in the Oxford quadrangle. RESOURCES The area has considerable resources for crushed rock and dimension stone, but these are not being exploited at present. Small stone quarries existed in the past in the southern part of the Northbridge Granite, but have all closed. It makes a very handsome decorative rock, but the block jointing limits the size of blocks. Most rock, other than rusty-weathering layers, would serve as crushed rock for general fill and rock material. The Northbridge Granite is good for crushed rock for concrete, except in the strongly foliated parts. These may have too highly strained quartz for such use. Marble lenses in the Nashoba Formation were mined in the past for lime. The exposed lenses are tiny and unexposed ones would be too small for exploitation today. Fig. 1, Sketch map of the region around the Oxford (OQ), Webster (WQ), Worcester South (WSQ) and Leicester (LQ) quadrangles showing structural provinces, major fault zones, folds, distribution of stratigraphic units and general area of the Northbridge Granite (pattern). Other intrusive rocks are not shown separately. Explanation: NP, Northbridge Granite with rim of Plainfield Fm; M-N, Marlboro and Nashoba Fms., N, Nashua Fm.; O, Oakdale Fm.; Px, Paxton Gp.; and B, Brimfield Gp. REFERENCES Barosh, P.J., 1972, Structural geology of Rhode Island and southeastern Massachsuetts as revealed by aeromagnetic data: Geological Society of America Abstracts with Programs, v. 4, n. 7, p. 444-445. Barosh, P.J., 1974, Preliminary bedrock geologic map of the Webster Quadrangle, Massachusetts-Connecticut: U. S. Geological Survey Open-File Report 74-192, 2 p., scale 1:24,000. Barosh, P.J., 1976a, The Western Rhode Island fold belt: Geological Society of America, Abstracts with Programs, v. 8, n. 2, p. 127-128. Barosh, P.J., 1976b, Preliminary bedrock geologic map of the Oxford quadrangle, Massachusetts, Connecticut and Rhode Island: U.S. Geological Survey, Open-File Report 76-622, 5 p., scale l:24,000. Barosh, P.J., 1977, Preliminary map showing bedrock geology superposed on an aeromagnetic base map of the Worcester region, Massachusetts, Connecticut, and Rhode Island, U.S. Geological Survey Open-File Report 77-131, 38 p., 2 pl., scale 1:125,000. Barosh, P.J., 1978a, Reconnaissance bedrock geologic map of the Marlborough quadrangle, Massachusetts: U.S. Geological Survey, Open-File Report 78-221, 14 p., scale 1:24,000. Barosh, P.J., 1978b, Reconnaissance bedrock geologic map of the Shrewsbury quadrangle, Massachusetts: U.S. Geological Survey, Open-File Report 78-394, 19 p., scale 1:24,000. Barosh, P.J., 1982, Structural relations at the junction of the Merrimack province, Nashoba thrust belt and the Southeast New England platform in the Webster-Oxford area, Massachusetts, Connecituct and Rhode Island: in Joesten, R., and Quarrier, S.S., eds., Guidebook for fieldtrips in Connecticut and south-central Massachusetts: Connecticut Geological and Natural History Survey Guidebook 5, p. 395-418. (New England Intercollegiate Geologic Conference, 74th Annual Meeting., University of. Connecticut). Barosh, P.J., 1984, The Bloody Bluff fault system in Hanson, L.S., editor, Geology of the coastal lowlands from Boston, MA to Kennebunk, ME: New England Intercollegiate Geologic Conference, 76th annual meeting guidebook, Salem State College, MA. p. 310-324. Barosh, P.J., 1995, Paleozoic rifting in New England, New Brunswick and Nova Scotia, U.S.A. and Canada: in Ojakangas, R.W., Dickas, A.B. and Green, J.C., editors. Proceedings of the Tenth International Conference on Basement Tectonics: Kluwer Academic Press, p. 73-101. Barosh, P.J., 1996a, Bedrock geologic map of the Webster quadrangle, Worcester County, Massachusetts and Windam County, Connecticut: U.S. Geological Survey, unpublished Geologic Quadrangle, 30 p., scale l:24,000. Barosh, P.J., 1996b, Bedrock geologic map of the Worcester South quadrangle, Worcester County, Massachusetts: U.S. Geological Survey, unpublished Geologic Quadrangle, 28 p., scale 1:24,000. Barosh, P.J., 1999a, Ordovician turbidite basin in southeastern New England: Geological Society of America, Abstracts with Programs, v. 31, no. 2, p. A-3. Barosh, P.J., 1999b, Post-Triassic faults and extensional basins in the northern Appalachian region: Geological Society of America, Abstracts with Programs, v. 31, no. 2, p.A-3. Barosh, P.J., and Hermes, O.D., 1981, General structural setting of Rhode Island and tectonic history of southeastern New England, in Boothroyd, J.C., and Hermes, O.D., editors, Guidebook to geologic field studies in Rhode Island and adjacent areas: New England Intercollegiate Geologic Conference, University of R.I., Kingston, R.I., p. 1- 15, also in Barosh, P.J., and Smith, P.V., editors, 1983, New England Seismotectonic Study activities during fiscal year 1981: U.S. Nuclear Regulatory Commission Report NUREG/CR-3253, p. 98-115. Barosh, P.J., and Johnson, C.K., 1976, Reconnaissance bedrock geologic map of the Leicester quadrangle, Massachusetts, U.S. Geological Survey Open-File Report 76- 814, 9 p., scale 1:24,000. Barosh, P.J., Pease, M.H., Jr., Schnabel, R.M., Bell, K.G., and Peper, J.D., 1977, Aeromagnetic lineament map of southern New England showing relation of lineaments to bedrock geology: U.S. Geological Survey, Misc. Field Studies Map, MF-855. Scale l:250,000. Bell, K.G., and Alvord, D.C., 1976, Pre-Silurian stratigraphy of northeastern Massachusetts, in Page, L.R., ed., Contributions to the stratigraphy of New England: Geological Society of America, Memoir 148, p. 149-216. Bell, K.G., Shride, A.F., Cupples, N.P., Dennen, W.H. and Barosh, P.J., 1996, Bedrock geologic map of the Georgetown quadrangle, Essex County, Massachusetts: U.S. Geological Survey, unpublished Geologic Quadrangle, scale 1:24,000. Crosby, W.O., 1880, Contributions to the geology of eastern Massachusetts: Boston Society of Natural History, Occasional Papers 3, 286 p. Cupples, N.P., 1961, Post-Carboniferous deformation of metamorphic and igneous rocks near the northern boundary fault, Boston Basin, Massachusetts: U.S. Geological Survey, Professional Paper, 424D, p. D46-D48. Dixon, H.R., 1964, The Putnam group of eastern Connecticut: U.S. Geological Survey, Bulletin 1194-C, 12 p. Dixon, H.R., 1968, Bedrock geology of the Plainfield-Danielson area, Connecticut: unpublished PhD thesis, Harvard University, Cambridge, MA, 336 p., scale 1:24,000. Dixon, H.R., 1974, Bedrock geologic map of the Thompson Quadrangle, Windam County, Connecticut, and Providence County, Rhode Island: U.S. Geological Survey, Geologic Quadrangle Map, GQ-1165, scale 1:24,000. Emerson, B.K., 1898, Geology of old Hampshire County, Massachusetts: U.S. Geological Survey, Monograph 29, 790 p. Emerson, B.K., and Perry, J.H., 1907, The green schists and associated granites and porphyries of Rhode Island: U.S. Geological Survey, Bulletin 311, 74p. Emerson, B.K., 1917, Geology of Massachusetts and Rhode Island: U.S. Geological Survey Bulletin 597, 289 p., map scale 1:25,000. Goldsmith, R., 1966, Stratigraphic names in the New London area, Connecticut: U.S. Geological Survey, Bulletin 1224-J, 9 p. Hatch, N.L., editor, 1991, The bedrock geology of Massachusetts: U.S. Geological Survey, Professional Paper 1366. 132 p. Hermes, O.D., Gromet, P.L. and Murray, D.P., 1994, Bedrock geologic map of Rhode Island: Office of the Rhode Island State Geologist: Scale 1:100,000. Hermes, O.D., Barosh, P.J., and Smith, P.V., 1981, Contact relationships of the Late Paleozoic Narragansett Pier Granite and country rock, in Boothroyd, J.C., and Hermes, O.D., editors, Guidebook to geologic field studies in Rhode Island and adjacent areas: New England Intercollegiate Geologic Conference, 73rd Annual Meeting, Dept of Geology, Univ. of R.I., Kingston, R.I., p. 125-151. Lenk, L., Strother, P.K., Kaye, C.A., and Barghoorn, E.S., 1982, Proterozoic Z age of the Boston basin, Massachusetts; new evidence from mircrofossils: Science, v. 216, p. 619-620. Lundgren, L., Jr., 1962, Deep River area, Connecticut: sratigraphy and structure: American Journal of Science, v. 260, p. 1-23. Nelson, A.E., 1975, Bedrock geologic map of the Framingham quadrangle, Middlesex and Worcester Counties, Massachusetts: U.S. Geological Survey, Geologic Quadrangle, GQ-1274, scale 1:24,000. O'Hara, K., and Gromet, P.L.,1985, Two distinct late Precambrian (Avalonian) terranes in southeastern New England and their late Paleozoic juxtaposition: American Journal of Science, v. 285. p. 673-709. Peck, J.H., 1976, Silurian and Devonian stratigraphy in the Clinton Quadrangle, central Massachusetts in Page, L.R., editors, Contributions to the stratigraphy of New England: Geological Society of America, Memoir 148, p. 241-252. Quinn, A.W., 1967, Bedrock geology of the Chepachet quadrangle, Providence County, Rhode Island: U.S. Geological Survey, Bulletin 1241-G, 26 p. scale 1:24,000. Quinn, A.W., 1971, Bedrock geology of Rhode Island: U.S. Geological Survey, Bulletin 1295, 68 p., map scale 1:125,000. Shaler, N.S., Woodworth, J.B., and Foerste, A.F., 1899, Geology of the Narragansett Basin: U.S. Geological Survey, Monograph 33, 402 p. Warren, C.H. and Powers, S., 1913, Geology of the Diamond Hill-Cumberland district in Rhode Island: Geological Society of America, Bulletin, v. 25, p. 435-476. Wilson, J.T., 1966, Did the Atlantic close and then reopen?: Nature, v. 211, p. 676. Woodhouse, David and Barosh, P.J., editors, 1991, Geology of the City of Boston, Massachusetts, Association of Engineering Geologists, Bulletin, v. 28, p. 375-512. Zartman, R.E., and Marvin, R.F., 1987, Radiometric ages on file in the radiometric age data bank (RADB) of rocks from Massachusetts: U.S. Geological Survey, Open- File Report 87-170, 421 p. Zen, E-an, editor, 1983, Bedrock geologic map of Massachusetts: U.S. Geological DISCUSSION INTRODUCTION The Oxford quadrangle is situated at a knot of structural and lithologic complexity along the suture zone of the Northern Appalachian orogen. The excellent exposures in the area not only reveal details of the structure and its history, but provide a rare opportunity to understand the general bedrock fracture system that is of great practical importance in the region. NP LQ WSQ Location certain, observed in outcrop (within 5-10 m) Location approximate (within 200 m) Location uncertain Lithologic Contacts Bedrock Exposure Individual Outcrop Areas of abundant outcrops or shallow bedrock (within 10 ft (3 m) of surface) or outcrop areas > 10,000 m 2 Faults Location approximate (within 200 m) Location uncertain 50 50 Structural Symbols Strike and dip of dominant foliation . Compositional layering generally parallels foliation Planar Features Linear Features Strike and dip of joint Trend and plunge of mineral lineation Trend and plunge of axis of minor fold EXPLANATION OF MAP SYMBOLS Strike and dip of fault as measured in outcrop or subsurface 85 18 ? ? Strike - slip fault, arrows indicate direction of offset Thrust fault Horizontal foliation Strike of vertical joint Fold Axes Plunging anticline Syncline Normal fault Minor antiform 20 Only faults traceable in outcrop are shown as solid lines. All dashed faults were determined though lineament analysis of aerial photographs, and topographic and geomorphic interpretation. ? ? Comments to the Map User SEA LEVEL 600' 200' 1000' 1000' SEA LEVEL 600' 200' 1000' 1000' Rt. 395 Rt. 16 Douglas Sutton Douglas Webster MA CT Lake Char - Bloody Bluff Fault Zone Oxford Anticline A' A SW NE Pzngn Pznr Pzna Pzm Pzp Pzngs Pzng Pzng Pzng Pzng 500' 500' ORDOVICIAN LATE PROTEROZOIC METASEDIMENTARY ROCKS IGNEOUS ROCKS On Pzn Pzngn Pznr Pzna Pzp Pzng Pznga Pzngs C linton-Newbury F ault B loody B luff F ault CORRELATION OF MAP UNITS NASHOBA FORMATION (LATE PROTEROZOIC) Undivided. Interbedded light to medium-gray gneiss, medium to dark-gray amphibolitic gneiss, dark-gray amphibolite, rusty- weathering gneiss, medium gray schistose gneiss and minor marble. Generally thin-bedded and commonly laminated. Garnet-bearing in part. High metamorphic grade with some beds containing sillimanite. Porphyroblasts of feldspar or thin-layers of feldspar and quartz that grade to pegmatite where relatively greater deformation. Foliation parallel to bedding. Commonly sheared parallel to bedding and locally mylonitized. Cut by a variety of granitic rocks just to west and may contain few thin sills or lenses. Too complex structurally to separate members of Bell and Alvord, (1976), but some general lithologic assemblages recognized locally. The formation is described elsewhere in Connecticut as the Tatnic Hill Formation, but mapping here shows its equivalency to the Nashoba, which takes precedence. Amphibolitic Unit. Medium to dark-gray and greenish dark-gray, fine to medium- grained amphibolite and subordinant amount of medium-gray, medium-grained gneiss. Thin-bedded and locally very well laminated. Sedimentary structures locally obscured by shearing, grades upward into rusty gneiss unit. Rusty Gneiss Unit. Medium to dark-gray gneiss that weathers yellow gray to rusty to dark-brown interbedded with dark-gray amphibolite and medium-gray gneiss, that weathers slightly lighter and rare light-gray marble and garnetiferous beds. Commonly laminated and usually thin-bedded, but may have medium beds locally. Commonly sheared. Unit grades upward. g; Lense of dark- gray basalt or gabbro? at Route 39 and Thompson Road. Gneiss Unit. Light to medium-gray fine to medium-grained gneiss and medium to dark- gray amphibolitic gneiss. Gneiss is garnet- bearing locally, and contains some schistose layers. Usually laminated and thin-bedded. In places contains porphyroblasts of feldspar and thin sheets of feldspar and quartz, especially where relatively more sheared. MARLBORO FORMATION (LATE PROTEROZOIC) Dark-gray to black amphibolite and medium to dark-gray amphibolitic gneiss. Locally has few to abundant thin lenses of light-gray fine to medium-grained feldspar and quartz that produce a banded rock. The formation is well-bedded into thin beds and commonly well laminated elsewhere, but here deformation has obscured much of this and results in a thinly layered rock with varying degrees of irregularities, wavyness and local mylonitic layers. The unit is a valley former and is very poorly exposed. Thin fault slivers could be present farther to the northeast. It is better exposed and less deformed to the south where Dixon (1974) describes mineralogic variations. The formation is described in Connecticut as the Quinebaug Formation, but mapping here shows its equivalency to the Marlboro, which takes precedent. PLAINFIELD FORMATION (LATE PROTEROZOIC) Light to medium-gray quartz schist and white to light-gray to very pale tan, fine to medium grained quartzite. Contains variable amounts of sericite to muscovite that commonly produce lustrous surfaces. In well-laminated, thin to medium beds, but these are usually obscured or obliterated by shearing and alteration from intrusion of the Northbridge Granite. The slight to moderate foliation parallels bedding and is also obscured by shearing. The quartz schist is assimilated by the granite first and there is relatively more quartzite along the contact than in the formation. The quartzite locally grades into granite. The strata here are remnants of the formation and are equivilant to one of the quartzite rich units mapped in the Plainfield Formation by Dixon (1974) to the south. Quartz schist usually sheared into thin irregular lenses, 1-3 cm thick, sub-parallel to bedding. Quartzite beds are sheared less and more representative of original bedding. Undivided. Light pinkish-gray to light gray or pale salmon-colored finely to coarse-grained porphyritic undivided biotite quartz monzonite that is slightly to strongly foliated. The granite is variable due to: 1) increase in grain size away from border; 2) decrease in degree of flow foliation away from border; 3) degree of tectonic foliation and related replacement of biotite by muscovite; 4)degree of assimilation of country rock and; 5) degree of deuteric alteration. Typically, the rock is a light pinkish-gray coarse-grained moderately-foliated biotite quartz monzonite, that weathers same to slightly lighter. A lineation may be locally apparent. The potassium feldspar phenocrysts are pink to salmon colored in football shaped lenses 0.5 x 1-2 cm. Medium- grained quartz forms a fractured groundmass or stretched into lenses 0.2-0.4 x 1-2.5 cm. Plagioclase is light-gray and more equant in shape. Biotite occurs in planar sieve-like irregular patches 1-1.5 cm across. Minor magnetite is present as scattered fine grained crystals. Quartz also occurs locally as veinlets along foliation planes or joints. The deuteric alteration that changes the potassium feldspars to pink or salmon and saussuritizes the plagioclase, is not seen west of the Bloody Bluff-Lake Char fault zone. Aplitic border zone. Pinkish light-medium gray to pale salmon- colored fine to medium grained rock. Grades from slightly to moderately foliated aplite to strongly foliated sheared to mylonitized rock. Strongly foliated variety has minerals stretched out into long thin lenses, 1-3 mm thick, that impart a pinstriped appearance. Unit grades into the porphyritic granite and may contain relatively more quartz and less biotite. Has a more slabby and layered look than main body. This unit occurs primarily along the northern border and is the westernmost extension of the wide border zone of the granite present to the northeast of the quadrangle. Sheared and partially assimilated border zone. Pinkish light-gray to light-gray fine to medium-grained moderately foliated muscovitic and quartz-rich rock that is lighter in appearance and more slabby than the typical granite. This unit appears to result more from partial assimilation of quartzite and shearing than being aplitic, although some aplite is probably present. The areas in which these processes operated tend to overlap and the rocks affected are not separated. The shearing of the porphyritic granite results in the biotite being replaced by muscovite and a reduction in grain size. Grades to both Plainfield Formation and porphyritic granite. Small unmapped bodies probably present elsewhere and contacts are very approximate. NASHUA FORMATION (ORDOVICIAN) Dark gray fine grained phyllite, that weathers medium gray with a slight olive cast. Very well bedded in thin, 0.3 to 2 cm thick graded beds. Grading is from siltstone base to mudstone and mica tops, that maybe slightly ankeritic and produce browninish streaks. Locally bedding surfaces are lustrous from fine sericite and may contain staurolite nodules, 2 mm in diameter. Fine foliation is present, that is oblique to bedding and is deflected across grading in beds. Joint surfaces commonly weather rusty. Medium metamorphic grade and cut by few quartz veins and lenses, but not pegmatite or granitic rock. Exposures are just west of the quadrangle. Previously informally designated Hodges Village phyllite (Barosh, 1974). It correlates with Unit 3 of Peck (1976) and is named herein the Nashua Formation from the Nashua Rriver in the type area of Unit 3 in the Clinton quadrangle to the north. Pzngs Pzngs Pzngn Pzn Pzn On Pzn A A' ? ? ? ? ? ? ? ? ? 40 20 57 70 FAULT CHAR LAKE FAULT ZONE CLINTON-NEWBURY FAULT ZONE ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 85 80 85 45 53 43 80 50 75 75 45 65 50 70 ? ? ? ? ? ? ? ? ? ? ? OXFORD ANTICLINE 10 6 5 30 85 12 40 36 30 20 9 9 8 35 25 45 60 47 20 25 60 51 35 55 35 32 35 80 80 50 23 24 83 39 20 55 45 75 65 65 32 58 45 28 45 30 35 45 60 23 50 40 60 85 70 35 44 30 27 12 10 33 36 45 20 13 22 50 16 45 75 57 55 50 48 40 42 45 43 35 75 56 38 43 51 65 15 40 33 65 45 25 10 40 45 40 28 31 54 7 8 9 8 8 50 30 58 75 25 40 38 80 45 60 32 30 45 35 25 40 33 20 35 10 40 36 18 60 30 33 45 60 32 63 60 30 35 30 80 65 65 30 30 20 10 15 12 12 15 25 15 20 24 24 24 45 28 26 31 33 35 20 30 35 35 31 29 32 30 19 28 29 28 41 31 38 32 32 35 32 30 30 28 20 22 34 35 35 32 45 25 38 40 35 40 30 44 40 40 32 20 25 32 46 43 37 33 35 44 32 30 38 33 15 30 25 17 50 35 30 25 60 45 35 50 28 50 37 40 55 15 30 36 60 40 10 20 23 11 33 55 35 30 72 55 57 10 15 12 15 10 10 42 37 35 35 35 30 33 25 8 5 7 8 5 20 10 35 18 35 30 30 25 25 15 12 25 11 10 20 25 38 30 20 20 34 30 20 17 15 20 25 25 20 15 10 22 15 11 41 24 15 27 33 31 25 34 34 35 45 35 43 35 50 36 48 35 38 27 40 36 45 42 40 38 40 45 40 30 30 35 34 32 24 25 20 20 25 25 45 50 60 55 60 47 38 55 30 20 30 35 20 70 73 65 48 25 80 43 20 25 25 15 30 20 10 42 8 40 34 20 85 64 80 80 80 72 50 80 80 60 75 85 77 45 80 75 80 85 85 85 85 85 85 80 85 85 75 85 65 85 65 80 85 60 72 57 85 75 62 85 70 85 85 45 58 55 65 60 65 50 60 85 65 80 85 85 63 70 85 85 80 80 80 85 85 80 80 85 85 85 75 31 85 85 75 88 75 55 28 16 10 25 Pzng Pzp Pzm Pzp Pzm Pzp Pzp Pzm Pzngn Pzm Pzm Pzm Pzna Pzm Pznr Pzp Pzm Pzm Pzngn Pzngn Pznr Pznr Pzna Pzna Pzp Pzp Pzp Pzp Pzp Pzp Pzp Pzp Pzp Pzp Pzp Pzp Pzp Pzp Pzp Pzp Pzp Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzng Pzngs Pzngs Pzngs Pzngs Pzngs Pzngs Pzngs Pzngs Pzngs Pzngs Pznga Pznga Pzp Pznga Pznga Pznga Pzp Pzna Pzna Pzna Pzna Pzna Pzna Pzngn Pzngn Pznr Pzp Pzp Pzp Pzm Pznga Pznga Pznga Pznga Pznga Pznga Pzp Pzn Pzn Pzn Pznr Pznr Pznr Pznr Pzp Pzn BLUFF ZONE BLOODY OFFICE OF THE MASSACHUSETTS STATE GEOLOGIST UNIVERSITY OF MASSACHUSETTS Topographic base scanned from U.S. Geological Survey, 1969 Polyconic projection. 1927 North American Datum Digitally reprojected to 1983 North American Datum 10,000 foot grid ticks based on Massachusetts state plane coordinate system, mainland zone 1000-meter Universal Transverse Mercator grid ticks, zone19 Geology mapped by P. J. Barosh, 1974-1975, 1993- 1994 and revised 2004, assisted by P. V. Smith 1982, and digitized by S. B. Mabee, K. W. Christle, and J. Felis, 2004 - 2005. Prepared in cooperation with the COMMONWEALTH OF MASSACHUSETTS STATE GEOLOGIST BEDROCK GEOLOGIC MAP OXFORD QUADRANGLE, MASSACHUSETTS Preliminary Copy - Undergoing Review (June 2005) MAGNETIC NORTH APPROXIMATE MEAN DECLINATION, 1999 TRUE NORTH 14 / 1 2 MASS. QUADRANGLE LOCATION BEDROCK GEOLOGIC MAP OF THE OXFORD QUADRANGLE, WORCESTER COUNTY, MASSACHUSETTS, PROVIDENCE COUNTY, RHODE ISLAND, AND WINDAM COUNTY, CONNECTICUT By Patrick J. Barosh 2005 A geologic map displays information on the distribution, nature, orientation and age relationships of rock and deposits and the occurrence of structural features. Geologic and fault contacts are irregular surfaces that form boundaries between different types or ages of units. Data depicted on this geologic quadrangle map are based on reconnaissance field geologic mapping, compilation of published and unpublished work, and photogeologic interpretation. Locations of contacts are not surveyed, but are plotted by interpretation of the position of a given contact onto a topographic base map; therefore, the accuracy of contact locations depends on the scale of mapping and the interpretation of the geologist(s). Any enlargement of this map could cause misunderstanding in the detail of mapping and may result in erroneous interpretations. Site-specific conditions should be verified by detailed surface mapping or subsurface exploration. Topographic and cultural changes associated with recent development may not be shown. We recommend reading Reading Maps with a Critical Eye: Becoming an Informed Map Reader by the Maine Geologic Survey to make the best use of a geologic map (http://www.maine.gov/doc/nrimc/mgs/mapuse/informed/informed.htm). This Open-File map is a progress report of mapping in this area, and is preliminary in nature. Revision of the map is likely because of the on-going nature of work in the region. It has not been peer reviewed or edited to conform with editorial standards of the Massachusetts State Geologist or with the North American Stratigraphic Code. The contents of the report and map should not be considered final and complete until reviewed and published by the Office of the Massachusetts State Geologist. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the University of Massachusetts, Commonwealth of Massachusetts, and the United States Federal Government. This research was supported by U.S. Geological Survey, National Cooperative Geologic Mapping Program, under assistance Award No. 04HQAG0028. Citation: Barosh, P.J., Bedrock geologic map of the Oxford quadrangle, Worcester County, Massachusetts, Providence County, Rhode Island and Windham County Connecticut. Office of the Massachusetts State Geologist, (in review, June 2005) This map was produced on request directly from digital files (PDF format) on an electronic plotter. A digital copy of this map (PDF format), including GIS datalayers, is available at http://www.geo.umass.edu/stategeologist

Polyconic projection. 1927 North American Datum 14 / 2 ... Northbridge is generally a light, pinkish-gray, medium- to coarse-grained, slightly to strongly-foliated porphyritic biotite

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Page 1: Polyconic projection. 1927 North American Datum 14 / 2 ... Northbridge is generally a light, pinkish-gray, medium- to coarse-grained, slightly to strongly-foliated porphyritic biotite

NORTHBRIDGE GRANITE (LATE PROTEROZOIC)

EXPLANATION OF GEOLOGIC MAP UNITSSelected modal analysis of samples from most of these units given by Dixon (1974) for samples collected just to the south.

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The geologic mapping of the Oxford quadrangle is part of a study of nine quadrangles in the region around Worcester, Massachsuetts. The project's goals are to unravel the great structural discontinuity through the region, define the stratigraphy present and correlate it across state lines, tie together the work of eight nearby U.S. Geological Survey quadrangle mapping projects and interpret the aeromagnetic data. The preliminary results were presented in 1977 (Barosh, 1976b, 1977), when quadrangle mapping in southeast New England was halted. These remain the latest map information until now. The compilation on the Geologic Map of Massachusetts (Zen, 1983) and its delayed description (Hatch, 1991) used little of the new field data and were based largely on much earlier concepts.

Since releasing the preliminary results the area around Oxford has been studied in detail (Barosh, 1996a, 1996b; Barosh and Johnson, 1976)(fig. 1). Great strides also have been made in dating the rocks and events and in understanding the bedrock fracture system. This system is of particular significance to environmental, ground water, tunnel and earthquake studies in the region. The combination of exposure, large-scale aerial photography, local areas of detailed work and drill hole data has produced the first comprehensive view of a bedrock fracture system in southern New England.

The Oxford quadrangle lies astride a northwest plunging anticlinal ridge that is the terminus of the uplands of western Rhode Island. Its northeast side drains to the Blackstone River that flows southward into Narragansett Bay, whereas waters of its western flank flow southward into the Thames River drainage system, that empties into Long Island Sound. The landforms closely reflect the geologic structure and lithology.

GEOLOGIC SETTING

The Oxford quadrangle lies against the southeast side of a suture zone that snakes across southeastern New England (Barosh, 1982, Woodhouse and others, 1991). This is the fundamental structure of the Northern Appalachian orogen, first recognized by Wilson (1966), that here separates rock on the 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 of Grenville time.

The zone is marked by the Nashoba thrust belt in southeast New England (Fig. 1). This great west-dipping zone of dislocation separates an enormous pile of metasedimentary strata in the Sturbridge basin to the northwest from a batholithic complex forming the Southeast New England platform on the southeast. The thrust belt itself contains a thick section of largely metavolcaniclastic strata derived from the southeast. All three provinces are of Late Proterozoic age.

The Oxford anticline is situated at the northwest corner of a large complex north-plunging arch within the southeast New England platform. This arch forms Rhode Island, west of the Narragansett basin, and extends into southern Massachusetts. The western side of the arch is wrinkled into a series of north-trending folds, the Western Rhode Island fold belt, that lie against the suture zone (Barosh, 1976a, Barosh and others, 1977). The Oxford anticline forms the largest of these folds along western Rhode Island and the one that extends farthest to the north.

ROCK IN THE SOUTHEAST NEW ENGLAND PLATFORM

Plainfield Formation: A sequence of quartzite-bearing strata overlain by biotite gneiss, amphibolite and metavolcanic rock and underlain by remnants of calcareous and amphibolitic strata with some pillow basalt and conglomerate is invaded by the Northbridge Granite and other batholithic rock in Rhode Island and vicinity (Emerson and Perry, 1907; Warren and Powers, 1913; Emerson, 1917). The quartzite-bearing sequence is intruded in the vicinity of the Oxford quadrangle and separate terminologies for these strata developed in southern Massachusetts, northeastern Rhode Island and western Rhode Island-eastern Connecticut. The Blackstone Series, encompassing several formations, is used for the strata in northeastern Rhode Island, (Warren and Powers, 1913, Quinn, 1971). This name was adopted despite Emerson and Perry's (1907) extension of the Westboro (Grafton) Quartzite from Massachusetts and their rejection of Woodward's (Shaler and others, 1899) suggestion of using Blackstone Series. The term Plainfield Formation was applied to this sequence separately in southheastern Connecticut by Lundgren (1962) and later carried northward. It was thought to be younger than the Blackstone (Goldsmith, 1966, Quinn, 1971, Dixon, 1974).

These three separate stratigraphic terminologies have been applied to the same quartzitic sequence that wraps around the Oxford anticline. The west-dipping Plainfield of the southwestern Thompson quadrangle, to the south, can be matched with the east-dipping Blackstone of the northeastern Chepachet quadrangle, to the southeast, after passing through the northerly-dipping Westboro of southern Massachusetts, with only short granite filled gaps. It is here proposed to continue the usage of Dixon (1974) and apply the Plainfield Formation to all of these strata to show their equivalency and to start a more formal and organized description of the regional stratigraphy. The Plainfield Formation appears to have the best modern description. It includes the strata of both the Westboro Formation and underlying unnamed quartzite and gneiss of Bell and Alvord (1976) in Massachusetts and is equivalent in general to the Blackstone Series in Rhode Island. However, the latter was thought by Quinn (1967) to contain more strata than the Westboro and the Plainfield Formation may not constitute all of the Blackstone Series. The specific correlation of the formation with members of the series needed to form a single standard formational nomenclature awaits further work. The remnants of quartzite and quartz schist in the Oxford quadrangle correlate in general with the more massive quartzite unit in the Plainfield Formation to the south and in the Blackstone to the east. However, quartzite is over represented in Oxford as it tends to be relatively more resistant to both assimilation and to weathering.

The well-bedded, thin-bedded, well-laminated metasediments of the Plainfield were probably deposited in a subsiding shallow shelf environment that had occasional influxes of volcanic rock and debris and conglomerate. Eventually the basin was overwhelmed by volcanic sediment and flows.

Age: The Plainfield (Westboro) Formation is Proterozoic in age (Bell and Alvord, 1976). It is intruded by the Late Proterozoic Northbridge Granite, but shows no earlier deformation. It therefore is probably not too much older and also of Late Proterozoic in age.

Northbridge Granite: The granitic rock of the Oxford quadrangle is described as a porphyritic granite-gneiss and designated the Northbridge Gneiss by Emerson and Perry (1907) following Emerson's (1898) definition of the unit. Emerson later described it as the Northbridge Granite Gneiss; a rock with "....a core of coarse, slightly gneissoid, porphyritic microcline-biotite granite and a broad border of completely mashed, stretched, and penciled highly muscovitic gneiss" (Emerson 1917, p.155). The rock is slightly to strongly foliated quartz monzonite whose composition varies locally due to partial assimilation of various rock types and shearing. It is herein designated the Northbridge Granite for clarity. The outer part of the Northbridge has been followed northeastward through the Shrewsbury and Marlboro and Framingham quadrangles (Nelson, 1975; Barosh 1978a, 1978b) where it had been called Milford Granite. Emerson and Perry (1907) also noted some similarity between the two. The rock in these quadrangles is different from the massive granite of the type Milford, which intrudes the Northbridge in Milford, Massachusetts.

The southern part of the Northbridge in Rhode Island was renamed Ponaganset Gneiss by Quinn (1967), because he found the granite to invade the Westboro, whereas Emerson and Perry (1907) stated the Northbridge was older. Later, however when Quinn (1971, p. 11) found that the type Northbridge did cut the Westboro, he did not abandon the term Ponaganset. Quinn's designation was extended westward into the Thompson quadrangle, just south of the Oxford, by Dixon (1974) who divided the granite there into the Ponaganset Gneiss, Scituate Granite Gneiss and the Hope Valley Alaskite

Gneiss. These correspond respectively to average Northbridge, the coarser "core", and the "mashed--- muscovite gneiss" mentioned by Emerson (1917). The better exposures and clearer relations in the Oxford quadrangle show these divisions to represent gradual changes within a single granitic body and not separate intrusions. These new names in the Chepachet and Thompson quadrangles now can be dropped. Nomenclature more descriptive of the rock would be Northbridge as a medium- to coarse-grained porphyritic facies, an aplitic border zone facies and sheared zones where muscovite has replaced the usual biotite of the granite.

The Northbridge is generally a light, pinkish-gray, medium- to coarse-grained, slightly to strongly-foliated porphyritic biotite granite to quartz monzonite with pink microcline phenocrysts. The biotite occurs as tiny flakes in patches, rather than scattered. The anorthite content of the plagioclase is variable. The content given by Dixon (1974) when arranged by position, shows a change from an20 in the core to an8 at the border. Overall, the rock may appear uniform and monotonous in the field, but it has a great many subtle changes and hand specimens may vary considerably. Inward from the border the foliation decreases, the grain size increases, it appears more porphyritic and the biotite is more prominent. Other variations are due to the degree of hydrothermal alteration which causes the pink coloration of the phenocrysts, degree of assimilation of wallrock and amount of granulation and shearing. The general foliation is due to flowage, with super-imposed and parallel shear foliation, especially in zones along the border. The foliation is shown by sub-parallel patches of biotite, lensoid microcline and, in the fine-grained rock, thin lenses of quartz. The shearing causes the biotite to be replaced by fine muscovite and may reduce the apparent grain size locally to mylonite. The microcline phenocrysts are primary minerals that resist granulation better than the quartz and plagioclase. The quartz is usually finely granulated. The microcline is only slightly larger than the quartz and plagioclase in the fine-grained rocks, but becomes relatively larger in the coarser rocks and may reach over two cm in length. It commonly has a thick lenzoid "football" shape. The rock commonly has some lineation. Where it is strongly lineated or "penciled" the rock appears massive perpendicular to it and strongly foliated parallel to it, although the actual foliation may be obscured.

The fine-grained aplitic border is present in the northern part of the quadrangle as a thin zone, that gradually widens to the northeast to form the very wide zone seen along the Massachusetts Turnpike in southwestern Westboro. An excellent section of the border zone and porphyritic rock is exposed in large cuts along Route 146 just northeast of the Oxford Quadrangle. The finer grain, higher quartz content and jointing, controlled by the uniform foliation, give the border zone a bedded look. This may cause it to be mistaken for the quartzite it intrudes. The border suffers varying degrees of shearing and may look very similar to the sheared granite elsewhere. The sheared rock in both cases is associated with thrust faults.

Age: The Northbridge Granite is considered the oldest part of the batholithic complex (Quinn,1971). It was long known to be Precambrian, then assigned to the mid Paleozoic from miscorrelations and disturbed radiometric dates and now again firmly shown to be Precambrian. Fossiliferous Cambrian strata, that have suffered only slight metamorphism and therefore younger, occur at several places in eastern Massachusetts (Crosby, 1880; Woodward, in Shaler and others, 1899; Warren and Powers 1913; Bell and Alvord, 1976). In addition older fossiliferous Eocambrian strata of the Boston basin (Lenk and others, 1982) unconformably overlie the batholith (Nelson, 1975). These strata yield an age of 595 my (Kaye and Zartman, 1980). More reliable radiometric ages of 614 my (Zartman, in Nelson, 1975) and 620+-15 my (Naylor, personal communication, 1975) were obtained from the part of the Milford Granite correlated with the Northbridge. The most likely age now seems to be approximately 625 my (Zartman, personal communication, 1991), but other other scattered dates from the region suggest a wide error margin. The granite is therefore Late Proterozoic Z in age.

ROCK IN THE NASHOBA THRUST BELT

Marlboro and Nashoba Formations: The Nashoba thrust belt was delineated in northeastern Massachusetts where the contained thick section of interbedded gneiss, amphibolite and schist was described by Bell and Alvord (1976). These strata continue into Connecticut (Barosh, 1977). However, of the Marlboro Formation, Shawsheen Gneiss, Fish Brook Gneiss, Nashoba Formation and Tadmuck Brook Schist present to the northeast (Bell and Alvord, 1976) only thin tectonic slivers of the Marlboro and Nashoba Formations survive in the Oxford quadrangle. These correspond, respectively, to the Quinebaug and Tatnic Hill Formations of Dixon (1964) to the south. The described members of the Nashoba Formation are only partially present in fault slices and are not seperated. The lithologic divisions made are merely local ones. The strata present probably represent parts of the higher members as the overlying Tadmuck Brook Schist is present only a few km to the north (Barosh, 1996b). The Yantic member of the Tatnic Hill in the Thompson quadrangle (Dixon, 1974) just to the south appears to be a zone of deformed strata along the west side of the formation and not a stratigraphic unit. Faults also appear to cut out part of the section in Connecticut as the section only reaches about 1,700m (Dixon, 1964) compared to the 10,890 m of the Nashoba to the northeast (Bell and Alvord, 1976). The lower part of the Connecticut section is reduced even more towards the Oxford area. A few blocks of laminated clean quartz-rich gneiss, similar to the Fish Brook Gneiss are present as float, at the east edge of the Nashoba in the southern Oxford quadrangle. These could possibly represent a tiny sliver of that unit or, perhaps, a higher lens of similar lithology.

The type Nashoba Formation is from a much more complete section than the Tatnic Hill and is described in greater detail. Nashoba should thus, replace the term Tatnic Hill in the region.

The lower, more amphibolite-rich part of the section is referred to as Marlboro Formation (Emerson and Perry, 1907), in Massachusetts and Quinebaug Formation in Connecticut (Dixon, 1964). Both sections are similar in thickness: the section in Massachusetts 2,140 m (Bell and Alvord, 1976) and that in Connecticut about 2,250 m. The thin-bedded, very well-bedded upper member of the Quinebaug (Dixon, 1964) is similar to the upper Sandy Pond member of the Marlboro, especially its lower part. The upper member reaches 300 m in thickness, but is locally less than 100 m, whereas the Sandy Pond is 640 m in thickness. Faulting at the top of the Quinebaug in Connecticut may account for the difference.

The well described Marlboro takes precedence and should be used in place of Quinebaug, while retaining the type Quinebaug section as a reference section.

Age: The Marlboro-Tadmuck Brook sequence is known to be pre-Silurian from overlying unmetamorphosed fossiliferous red beds and volcanic rock (Bell and Alvord, 1976, Bell and others 1996). Furthermore, it can be inferred to be pre-Late Cambrian in age from the age of the base of a mildly metamorphosed sequence, described to the north by Peck (1976) in the Nashua trough.. The individual formations and the sequence closely match a fossiliferous Upper Cambrian-Ordovician strata in Maine (Barosh, 1999a). These beds post-date the intense deformation and metamorphism that affects the Nashoba. In addition, this intense deformation is intimately asssociated with the dated similar Late Paleozoic deformation in the batholithic complex to the east, where no later deformation as intense has been found. The Marlboro and Nashoba Formations must be Proterozoic in age and seemingly of post-Grenville Late Proterozoic Z age. A radiometric age date from the Fish Brook Gneiss, that lies between them gave 730 my (Zartman and Marvin, 1987).

ROCK IN THE NASHUA TROUGH

Nashua Formation: A sliver of phyllite, that is exposed in the Clinton-Newbury fault zone at Hodges Village dam just off the northwest corner of the quadrangle, probably extends beneath a covered zone at the western edge of the quadrangle. This dark-gray, thin-bedded, graded-bedded phyllite was previously named the Hodges Village Phyllite (Barosh, 1974) and found to be part of Unit 3 of Peck's (1976) section in the Clinton quadrangle to the north (Barosh, 1977, 1996a). Nashua Formation herein replaces this temporary unit term and is named for outcrops near the Nashua river in Lancaster, Massachusetts in the central part of the Clinton quadrangle. The formation occurs there in a 1,830 to 2,440 m thick section (Peck, 1976) that lies in the Nashua trough. The formation lies between Peck's Units 2 and 4 and overlies the much more deformed Proterozoic strata and granite of the Sturbridge basin.

Age: The Nashua formation is part of a sequence that closely correlates with a fossiliferous Ordovician section in Maine (Barosh, 1999a).

STRUCTURAL GEOLOGY

OXFORD ANTICLINE: The dominating structure of the quadrangle is the Oxford anticline that juts northwestward into and under the Nashoba thrust belt (Barosh, 1976b, 1982) (Fig. 1). The granite-cored fold is revealed by both the bedding in the Plainfield Formation and the paralleling flow foliation in the Northbridge Granite. The anticline is well expressed in the topographic, aeromagnetic and gravity data (Barosh and others, 1977). The aeromagnetic expression allows mapping of portions of its border beneath a cover of glacial debris and water. A faint north-plunging lineation, that is more prominent to the south, parallels the axis and the orientation of the block jointing in the granite rotates around the axis. The axis of the anticline bends and continues southward approximately along the border between the Thompson and Chepachet quadrangles.

The fold is slightly asymmetrical and the axis lies closer to its southwestern side. The outer part of the northeastern limb strikes to the east and is affected by a slight warp, shown with a synclinal axis. The inner part is more tightly squeezed, and in its northeastern part strikes to the southeast. A zone of thrust faults allows for adjustment between the inner and outer parts.

Greater deformation adjacent to the Nashoba thrust belt, especially on the southwest side, caused more shearing and mylonization and thrust faulting. The shears and thrust faults were generally controlled by the bedding and flow foliation. These faults seem to cause some repetition of the Plainfield Formation and to have broken the nose of the anticline and moved it eastward with a right-lateral shift.

NASHOBA THRUST BELT: The Oxford anticline impales the Nashoba thrust belt causing it to thin tectonically from 22 km in width to the northeast to a half km in the Oxford-Webster area. The thinning appears largely accomplished by progressively overriding the lower thrust sheets, thus first eliminating the lower part of the Nashoba Formation and adjacent units and then working upwards.

The overall alignment of compression in the region was east-northeast-west-southwest with the southeast side shoved west-southwest beneath the

northwest one. The Oxford anticline and lineation developed approximately normal to this alignment. The compression produces a relative movement within and across the thrust belt of the northwest sides slipping over the southeast ones with a right-lateral component. These movements cut out the Marlboro Formation between the southwest part of the quadrangle to a point 23 km to the northeast.

The local movement within the thrust sheets changes greatly and the orientation of small-scale structures varies extremely and has little regional significance. The numerous small drag folds can have almost every orientation along the twists and turns of the thrust faults. Many of these folds are well exposed in cuts along Route 395, that crosses the western side of the quadrangle. The most common situation is an anticlinal fold of varying steepness in the hanging wall and a flattening of the beds in the footwall beneath the fault, but locally the folds are very complex.

The initial thrust faults were very ductile with a smearing and almost a flowage of minerals and rocks along them, yet still forming distinct breaks with offset. Renewed movement, probably under cooler conditions, results in shearing and mylonization. Slickensides form under later brittle reactivation. In places gouge and brecciation form under more brittle conditions, but are not commonly seen. Feldspar porphyroblasts, grading locally into pegmatite, forms bands along and near some early shear zones and bull quartz is very locally present along brittle faults.

The eastern margin of the Nashoba thrust belt is marked by the Bloody Bluff fault zone of Cupples, (1961). This fault was followed from its type area in eastern Massachusetts into the Lake Char fault zone of Dixon (1968), which is named after the large lake in the Oxford quadrangle (Barosh, 1984). The Clinton-Newbury fault zone that marks the northwest side of the belt also was followed into the area. This fault zone is intruded by granite in Connecticut and had not been recognized there. The Bloody Bluff-Lake Char fault zone dips westward about 20 degrees in the Thompson quadrangle (Dixon, 1974), 30 to 40 degrees to the northwest across the Oxford quadrangle and much steeper to the northeast.

NASHUA TROUGH: The Clinton-Newbury just brushes the western side of the quadrangle where it contains a sliver of Ordovician strata, caught up during a reactivation. This sliver is a remnant of the Nashua Trough, a basin bordered by reverse faults that contains a thick section of Cambro-Ordovician strata (Barosh, 1999a). The trough impinges upon the Clinton-Newbury fault zone at Worcester from the north and right-lateral movement has strung out some slivers of the trough in the fault zone southwest of this intersection..

BRITTLE FAULTS: An immense number of small brittle faults are present in the quadrangle and were noted by Dixon (1968) in outcrops to the south. Exposures here allow them to be shown over much of the Oxford quadrangle and local areas of detailed mapping reveals even a greater density than is shown. The majority are near vertical, but many have moderate dips. Most have small normal or lateral offsets that are not well defined. These faults form distinct sets and patterns and many sets are of regional extent. Most are relatively old and confined east of the Bloody Bluff-Lake Char fault zone, but relatively young ones cross the entire quadrangle. They are well expressed in the topography and generally have tectonic joint sets associated with them. The principal fault set trends are, from apparent oldest to youngest: northwest, east-west, north-northeast, northeast, just west of north, northwest and north. The early northwest set is mainly in the inner part of the Oxford anticline and may be associated with late movement related to its formation. The east-west set is most noticeable in the central and northeastern portions of the quadrangle. Faults of this trend become more prominent northeastward of the quadrangle and appear related to the Eocambrian and Ordovician faults of the Boston basin (Barosh, 1995). The north-northeast set is very prominent in the quadrangle and in the northeastern corner of the Uxbridge quadrangle to the east. These faults commonly have left-lateral offsets. The just west of north set appears to follow a broad zone that crosses the east side of the quadrangle.

The above sets appear restricted to east of the Bloody Bluff and probably formed prior to the Silurian before the Bloody Bluff-Lake Char fault zone closed (Bell and others, 1996), whereas the following sets cut across the fault zone. The northeast set is composed of several faults, with approximately a one km spacing, that cut the anticline and tend to form cross valleys. Drilling into the prominent Reid Smith fault of this set showed a moderately steep dip to the northwest. The northwest set is a relatively young set with a half to one km spacing, that is prominent throughout the region. The north set appears the youngest and has a relatively wide spacing. One zone passes through Oxford center, a prominent one through the middle of the quadrangle, a less defined one two km to the east and a probable, poorly exposed one between Wallum lake and Stevens pond. The middle two appear to be the splayed end of a very prominent but unmapped fault that crosses all of eastern Connecticut. It is herein named the Tetreault fault after a pond which lies in its fault trough in the East Killingly quadrangle to the south. Many of these northwest and north-trending faults in southeastern New England are post-Triassic in age, some cut Cretaceous strata offshore and a number remain active (Barosh, 1999b).

"HOPE VALLEY SHEAR ZONE": A major north-trending, east-dipping structural break called the Hope Valley Shear Zone has been hypothesized to pass approximately through and east of the southeast corner of the Oxford quadrangle by O'Hara and Gromet (1985) and Hermes and others (1994).

This is an extension of a fault found along the west side of the Oxford anticline to the south and named the Oneco fault (Barosh, 1972, 1976a; Barosh and Hermes, 1981; Hermes and others, 1981). Similar rock occurs on both sides of this fault, which rolls over to dip to the west, diminishes in offset and eventually dies out to the north in the Lake Char zone. The hypothesized extension and great offset of the Hope Valley shear zone would violate the known continuity of the stratigraphy, igneous rock bodies, fold axes, and both the aeromagnetic and gravity data in this region.

GEOLOGIC HISTORY

The recorded history in the Oxford area began with the deposition of the Plainfield Formation in the Proterozoic. Shales, dolomite and clean well-bedded and finely laminated sand were deposited in a subsiding shelf area, that gradually became overwhelmed with volcanic debris and flows as the environment changed. The basin then had a relative position somewhere northwest of the present coast of northwestern Africa. The immense thickness of the volcaniclastic debris of the Nashoba and adjacent formations was deposited from the southeast (Bell and Alvord, 1976) into a rapidly subsiding basin farther seaward contemporaneously or slightly later.

This region drifted towards the Sturbridge basin offshore of an earlier North American craton, as it was constituted in Grenville time. The early intervening ocean, referred to as Iapetus, narrowed and the basins containing the Nashoba and Plainfield collided against and slid under the offshore basin. The strata were dragged down, heated, partially melted to granitic magma and began to wrinkle into folds along the leading edge of the granitic mass. The Plainfield strata folded, while granite formed below, into the Oxford anticline.

Thrust slices within the volcaniclastic strata of the collision zone fitted themselves around this growing anticlinal protuberance into the thrust belt. Continued movement sheared and finely ground many deformed zones, perhaps as the strata, that were at elevated temperatures and pressures began to cool. These events occurred roughly 625 my ago at the time of the Cadomian orogeny, which deformed much of southern Europe.

The area rose up and was eroded rapidly nearly down to sea level. The uplift initiated brittle faulting as extensional movement began to form the Boston basin by 595 my and volcanic deposits poured into it. Marls and mud overlapped the eroded batholithic complex east of Oxford in the Early Cambrian. A narrow sea still existed between here and the North American mainland to the west during Cambrian, as shown by differing faunas on either side; a sea that was reduced further during the Ordovician (Wilson, 1966). Northwest of the suture zone a quiet water basin received turbidity flows of mud during the Ordovician, forming the Nashua Formation and other deposits, whereas southeast of it volcanoes were active around the Boston area. Intrusion, volcanism and renewed thrust faulting occurred towards the end of the Ordovician to mark the Taconic orogeny. This movement formed the Nashua trough and injected granitic rock into some of the reactivated faults in the Nashoba thrust belt just north of Oxford. The strata in and northwest of the thrust belt were moderately metamorphosed whereas the old granites to the southeast may have been only hydrothermally altered.

The Taconic activity closed any gap along the Clinton-Newbury fault zone. Silurian and Early Devonian volcanic rock and red beds were deposited, along a shore line over the eroded surface of the earlier rock on both sides of the fault (Bell and others, 1996; Barosh, 1995). These strata remain unmetamorphosed and later events, which are relatively weak in the Oxford area, are best discussed elsewhere (Woodhouse and others, 1991). The Clinton-Newbury fault continued to reactivate and move with right-lateral motion at times in the Paleozoic and locally with normal offset later (Barosh, 1995). Many of these events were associated with more widespread brittle faulting. Movement along northeast-trending normal faults occurred as the North Atlantic ocean basin opened. Their movement tapered off and northwest and north-trending faults became active as the ocean basin widened. These two fault sets are still active locally in the region, but no earthquakes have been recorded in the Oxford quadrangle.

RESOURCES

The area has considerable resources for crushed rock and dimension stone, but these are not being exploited at present. Small stone quarries existed in the past in the southern part of the Northbridge Granite, but have all closed. It makes a very handsome decorative rock, but the block jointing limits the size of blocks.

Most rock, other than rusty-weathering layers, would serve as crushed rock for general fill and rock material. The Northbridge Granite is good for crushed rock for concrete, except in the strongly foliated parts. These may have too highly strained quartz for such use.

Marble lenses in the Nashoba Formation were mined in the past for lime. The exposed lenses are tiny and unexposed ones would be too small for exploitation today.

Fig. 1, Sketch map of the region around the Oxford (OQ), Webster (WQ), Worcester South (WSQ) and Leicester (LQ) quadrangles showing structural provinces, major fault zones, folds, distribution of stratigraphic units and general area of the Northbridge Granite (pattern). Other intrusive rocks are not shown separately. Explanation: NP, Northbridge Granite with rim of Plainfield Fm; M-N, Marlboro and Nashoba Fms., N, Nashua Fm.; O, Oakdale Fm.; Px, Paxton Gp.; and B, Brimfield Gp.

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Wilson, J.T., 1966, Did the Atlantic close and then reopen?: Nature, v. 211, p. 676.

Woodhouse, David and Barosh, P.J., editors, 1991, Geology of the City of Boston, Massachusetts, Association of Engineering Geologists, Bulletin, v. 28, p. 375-512.

Zartman, R.E., and Marvin, R.F., 1987, Radiometric ages on file in the radiometric age data bank (RADB) of rocks from Massachusetts: U.S. Geological Survey, Open-File Report 87-170, 421 p.

Zen, E-an, editor, 1983, Bedrock geologic map of Massachusetts: U.S. Geological

DISCUSSION

INTRODUCTION

The Oxford quadrangle is situated at a knot of structural and lithologic complexity along the suture zone of the Northern Appalachian orogen. The excellent exposures in the area not only reveal details of the structure and its history, but provide a rare opportunity to understand the general bedrock fracture system that is of great practical importance in the region.

NP

LQ WSQ

Location certain, observed in outcrop (within 5-10 m)

Location approximate (within 200 m)

Location uncertain

Lithologic Contacts

Bedrock Exposure

Individual Outcrop Areas of abundant outcrops or shallow bedrock (within 10 ft (3 m) of surface) or

outcrop areas > 10,000 m2

Faults

Location approximate (within 200 m)

Location uncertain

50

50

Structural Symbols

Strike and dip of dominant foliation .Compositional layering generally parallels foliation

Planar Features

Linear Features

Strike and dip of joint

Trend and plunge of mineral lineation

Trend and plunge of axis of minor fold

EXPLANATION OF MAP SYMBOLS

Strike and dip of fault as measured in outcropor subsurface

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18

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Strike - slip fault, arrows indicate direction of offset

Thrust fault

Horizontal foliation

Strike of vertical joint

Fold AxesPlunging anticline

Syncline

Normal fault

Minor antiform

20

Only faults traceable in outcrop are shown as solid lines. All dashed faults were determined though lineament analysis of aerial photographs, and topographic and

geomorphic interpretation.

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Comments to the Map User

SEA LEVEL

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CORRELATION OF MAP UNITS

NASHOBA FORMATION (LATE PROTEROZOIC)

Undivided. Interbedded light to medium-gray gneiss, medium to dark-gray amphibolitic gneiss, dark-gray amphibolite, rusty-weathering gneiss, medium gray schistose gneiss and minor marble. Generally thin-bedded and commonly laminated. Garnet-bearing in part. High metamorphic grade with some beds containing sillimanite. Porphyroblasts of feldspar or thin-layers of feldspar and quartz that grade to pegmatite where relatively greater deformation. Foliation parallel to bedding. Commonly sheared parallel to bedding and locally mylonitized. Cut by a variety of granitic rocks just to west and may contain few thin sills or lenses. Too complex structurally to separate members of Bell and Alvord, (1976), but some general lithologic assemblages recognized locally. The formation is described elsewhere in Connecticut as the Tatnic Hill Formation, but mapping here shows its equivalency to the Nashoba, which takes precedence.

Amphibolitic Unit. Medium to dark-gray and greenish dark-gray, fine to medium- grained amphibolite and subordinant amount of medium-gray, medium-grained gneiss. Thin-bedded and locally very well laminated. Sedimentary structures locally obscured by shearing, grades upward into rusty gneiss unit.

Rusty Gneiss Unit. Medium to dark-gray gneiss that weathers yellow gray to rusty to dark-brown interbedded with dark-gray amphibolite and medium-gray gneiss, that weathers slightly lighter and rare light-gray marble and garnetiferous beds. Commonly laminated and usually thin-bedded, but may have medium beds locally. Commonly sheared. Unit grades upward. g; Lense of dark-gray basalt or gabbro? at Route 39 and Thompson Road.

Gneiss Unit. Light to medium-gray fine to medium-grained gneiss and medium to dark- gray amphibolitic gneiss. Gneiss is garnet-bearing locally, and contains some schistose layers. Usually laminated and thin-bedded. In places contains porphyroblasts of feldspar and thin sheets of feldspar and quartz, especially where relatively more sheared.

MARLBORO FORMATION (LATE PROTEROZOIC) Dark-gray to black amphibolite and medium to dark-gray amphibolitic gneiss. Locally has few to abundant thin lenses of light-gray fine to medium-grained feldspar and quartz that produce a banded rock. The formation is well-bedded into thin beds and commonly well laminated elsewhere, but here deformation has obscured much of this and results in a thinly layered rock with varying degrees of irregularities, wavyness and local mylonitic layers. The unit is a valley former and is very poorly exposed. Thin fault slivers could be present farther to the northeast. It is better exposed and less deformed to the south where Dixon (1974) describes mineralogic variations. The formation is described in Connecticut as the Quinebaug Formation, but mapping here shows its equivalency to the Marlboro, which takes precedent.

PLAINFIELD FORMATION (LATE PROTEROZOIC)Light to medium-gray quartz schist and white to light-gray to very pale tan, fine to medium grained quartzite. Contains variable amounts of sericite to muscovite that commonly produce lustrous surfaces. In well-laminated, thin to medium beds, but these are usually obscured or obliterated by shearing and alteration from intrusion of the Northbridge Granite. The slight to moderate foliation parallels bedding and is also obscured by shearing. The quartz schist is assimilated by the granite first and there is relatively more quartzite along the contact than in the formation. The quartzite locally grades into granite. The strata here are remnants of the formation and are equivilant to one of the quartzite rich units mapped in the Plainfield Formation by Dixon (1974) to the south. Quartz schist usually sheared into thin irregular lenses, 1-3 cm thick, sub-parallel to bedding. Quartzite beds are sheared less and more representative of original bedding.

Undivided. Light pinkish-gray to light gray or pale salmon-colored finely to coarse-grained porphyritic undivided biotite quartz monzonite that is slightly to strongly foliated. The granite is variable due to: 1) increase in grain size away from border; 2) decrease in degree of flow foliation away from border; 3) degree of tectonic foliation and related replacement of biotite by muscovite; 4)degree of assimilation of country rock and; 5) degree of deuteric alteration. Typically, the rock is a light pinkish-gray coarse-grained moderately-foliated biotite quartz monzonite, that weathers same to slightly lighter. A lineation may be locally apparent. The potassium feldspar phenocrysts are pink to salmon colored in football shaped lenses 0.5 x 1-2 cm. Medium-grained quartz forms a fractured groundmass or stretched into lenses 0.2-0.4 x 1-2.5 cm. Plagioclase is light-gray and more equant in shape. Biotite occurs in planar sieve-like irregular patches 1-1.5 cm across. Minor magnetite is present as scattered fine grained crystals. Quartz also occurs locally as veinlets along foliation planes or joints. The deuteric alteration that changes the potassium feldspars to pink or salmon and saussuritizes the plagioclase, is not seen west of the Bloody Bluff-Lake Char fault zone.

Aplitic border zone. Pinkish light-medium gray to pale salmon-colored fine to medium grained rock. Grades from slightly to moderately foliated aplite to strongly foliated sheared to mylonitized rock. Strongly foliated variety has minerals stretched out into long thin lenses, 1-3 mm thick, that impart a pinstriped appearance. Unit grades into the porphyritic granite and may contain relatively more quartz and less biotite. Has a more slabby and layered look than main body. This unit occurs primarily along the northern border and is the westernmost extension of the wide border zone of the granite present to the northeast of the quadrangle.

Sheared and partially assimilated border zone. Pinkish light-gray to light-gray fine to medium-grained moderately foliated muscovitic and quartz-rich rock that is lighter in appearance and more slabby than the typical granite. This unit appears to result more from partial assimilation of quartzite and shearing than being aplitic, although some aplite is probably present. The areas in which these processes operated tend to overlap and the rocks affected are not separated. The shearing of the porphyritic granite results in the biotite being replaced by muscovite and a reduction in grain size. Grades to both Plainfield Formation and porphyritic granite. Small unmapped bodies probably present elsewhere and contacts are very approximate.

NASHUA FORMATION (ORDOVICIAN)Dark gray fine grained phyllite, that weathers medium gray with a slight olive cast. Very well bedded in thin, 0.3 to 2 cm thick graded beds. Grading is from siltstone base to mudstone and mica tops, that maybe slightly ankeritic and produce browninish streaks. Locally bedding surfaces are lustrous from fine sericite and may contain staurolite nodules, 2 mm in diameter. Fine foliation is present, that is oblique to bedding and is deflected across grading in beds. Joint surfaces commonly weather rusty. Medium metamorphic grade and cut by few quartz veins and lenses, but not pegmatite or granitic rock. Exposures are just west of the quadrangle. Previously informally designated Hodges Village phyllite (Barosh, 1974). It correlates with Unit 3 of Peck (1976) and is named herein the Nashua Formation from the Nashua Rriver in the type area of Unit 3 in the Clinton quadrangle to the north.

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OFFICE OF THE MASSACHUSETTS STATE GEOLOGISTUNIVERSITY OF MASSACHUSETTS

Topographic base scanned from U.S. Geological Survey, 1969Polyconic projection. 1927 North American DatumDigitally reprojected to 1983 North American Datum10,000 foot grid ticks based on Massachusetts state plane coordinate system, mainland zone1000-meter Universal Transverse Mercator grid ticks, zone19

Geology mapped by P. J. Barosh, 1974-1975, 1993-1994 and revised 2004, assisted by P. V. Smith 1982, and digitized by S. B. Mabee, K. W. Christle, and J. Felis, 2004 - 2005.

Prepared in cooperation with the COMMONWEALTH OF MASSACHUSETTS

STATE GEOLOGIST

BEDROCK GEOLOGIC MAPOXFORD QUADRANGLE, MASSACHUSETTS

Preliminary Copy - Undergoing Review (June 2005)

�MA

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MASS.

QUADRANGLE LOCATION

BEDROCK GEOLOGIC MAP OF THE OXFORD QUADRANGLE, WORCESTER COUNTY, MASSACHUSETTS, PROVIDENCE COUNTY, RHODE ISLAND, AND

WINDAM COUNTY, CONNECTICUTBy

Patrick J. Barosh2005

A geologic map displays information on the distribution, nature, orientation and age relationships of rock and deposits and the occurrence of structural features. Geologic and fault contacts are irregular surfaces that form boundaries between different types or ages of units. Data depicted on this geologic quadrangle map are based on reconnaissance field geologic mapping, compilation of published and unpublished work, and photogeologic interpretation. Locations of contacts are not surveyed, but are plotted by interpretation of the position of a given contact onto a topographic base map; therefore, the accuracy of contact locations depends on the scale of mapping and the interpretation of the geologist(s). Any enlargement of this map could cause misunderstanding in the detail of mapping and may result in erroneous interpretations. Site-specific conditions should be verified by detailed surface mapping or subsurface exploration. Topographic and cultural changes associated with recent development may not be shown.

We recommend reading Reading Maps with a Critical Eye: Becoming an Informed Map Reader by the Maine Geologic Survey to make the best use of a geologic map (http://www.maine.gov/doc/nrimc/mgs/mapuse/informed/informed.htm).

This Open-File map is a progress report of mapping in this area, and is preliminary in nature. Revision of the map is likely because of the on-going nature of work in the region. It has not been peer reviewed or edited to conform with editorial standards of the Massachusetts State Geologist or with the North American Stratigraphic Code. The contents of the report and map should not be considered final and complete until reviewed and published by the Office of the Massachusetts State Geologist. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the University of Massachusetts, Commonwealth of Massachusetts, and the United States Federal Government.

This research was supported by U.S. Geological Survey, National Cooperative Geologic Mapping Program, under assistance Award No. 04HQAG0028.

Citation:Barosh, P.J., Bedrock geologic map of the Oxford quadrangle, Worcester County, Massachusetts, Providence County, Rhode Island and Windham County Connecticut. Office of the Massachusetts State Geologist, (in review, June 2005)

This map was produced on request directly from digital files (PDF format) on an electronic plotter.

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

OPTICAL BRIGHTENERS C.I.NO. USESOPTICAL BRIGHTENING pH (1%) : About 9±. Shade : Pinkish. Solubility : Readily miscible with water. STABILITY Hydrogen peroxide : Very good Sodium chlorite

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