Geologic units in Worcester county, Massachusetts

Paxton Formation (Silurian) at surface, covers 19 % of this area

Undifferentiated biotite granofels, calc-silicate granofels, and sulfidic schist. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Littleton Formation (Lower Devonian) at surface, covers 14 % of this area

Black to gray aluminous mica schist, quartzose schist, and aluminous phyllite.

Partridge Formation (includes Brimfield Schist of Emerson, 1917) (Middle Ordovician) at surface, covers 9 % of this area

Sulfidic mica schist and subordinate amphibolite.

Hardwick Tonalite (Lower Devonian) at surface, covers 6 % of this area

Dark-gray, moderately to strongly foliated biotite tonalite to granodiorite gneiss; intrudes Dl.

Nashoba Formation (Ordovician or Proterozoic Z) at surface, covers 6 % of this area

Sillimanite schist and gneiss, partly sulfidic, amphibolite, biotite gneiss, calc-silicate gneiss and marble. Nashoba Formation occurs in Nashoba zone of eastern MA. Consists of interlayered sillimanite-bearing, partly sulfidic schist and gneiss, calc-silicate gneiss, and subordinate quartzite and marble. Protoliths were probably volcanogenic sediments interlayered with limy marine sediments. Bell and Alvord (1976) divided Nashoba into 10 members on basis of lithology. Amphibolite is most abundant near presumed base, namely in Boxford Member. Skehan and Abu-Moustafa (1976) divided Nashoba into 30 members based on section in Wachusett-Marlborough tunnel. Although Bell and Alvord's and Skehan and Moustafa's sections contain similar lithologies, Bell and Alvord's is much thicker, and Boxford Member is not readily identified in Skehan and Abu-Moustafa's. Subdivision of Nashoba is conjectural south of Marlborough and Shrewsbury. On MA State bedrock map (Zen and others, 1983) only Boxford Member is separated out from the rest of the Nashoba because this unit was the only member clearly recognized in several area. A definite sequence of members probably does not exist anywhere in the Nashoba because of lenticularity of assemblages and repeated rock types, both of which could be accounted for by either sedimentary or tectonic processes. Although Castle (1965) considered Fish Brook to be either a premetamorphic intrusive rock or a core gneiss of intrusive or sedimentary ancestry, Bell and Alvord (1976) considered it to be volcanic or volcaniclastic in origin. Zircons in Fish Brook are certainly volcanic in origin and yield a date of 730 +/-26 Ma (Olszewski, 1980). If the rock were a core gneiss, that date would apply only to the Fish Brook and not to surrounding rocks; but, Bell and Alvord (1976) believe Fish Brook to be part of the Marlboro Formation-Nashoba Formation sequence and therefore the date does apply to the sequence. In addition, a 1500 Ma date for Shawsheen Gneiss [reference not given] helps bracket age of Marlboro-Nashoba sequence. An upper limit for the sequence was established from the 430 +/-5 Ma age of intruding Sharpers Pond Diorite and 450 +/-23 Ma age of the intruding Andover Granite (Zartman and Naylor, 1984). Although age on MA State bedrock map is shown as Proterozoic Z or Ordovician (due to uncertainty regarding actual rocks sampled by Olszewski and a strong belief that rocks of Nashoba zone correlated with Ordovician rocks to the west), author now feels that rocks of Nashoba zone (except for Tadmuck Brook Schist) are all Proterozoic, but that they are unlike the Proterozoic rocks of neighboring Milford-Dedham zone. [no formal age change made in this report] (Goldsmith, 1991).

Scituate Granite Gneiss (Proterozoic Z) at surface, covers 6 % of this area

Gneissic granite containing biotite in small clots. Equivalent to part of former Northbridge Granite Gneiss (usage now abandoned). Gradational with Zhg.

Paxton Formation (Silurian) at surface, covers 4 % of this area

Sulfidic mica schist where mapped separately. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Fitchburg Complex (Lower Devonian or younger) at surface, covers 4 % of this area

Light-gray to white, medium-grained, weakly foliated muscovite-biotite granite; commonly contains white pegmatite bearing muscovite and tourmaline; may include some granite of late Paleozoic age; locally intrudes Dfgrg, Dfgd, and Dl.

Monson Gneiss (Ordovician, Cambrian, or Proterozoic Z) at surface, covers 4 % of this area

Layered to massive biotite-plagioclase gneiss, amphibolite, microcline augen gneiss.

Oakdale Formation (Silurian) at surface, covers 2 % of this area

Metamorphosed thin-bedded, pelitic and calcareous siltstone and muscovite schist, probably low-grade equivalent of Paxton Formation. The Oakdale Formation is here revised to include strata previously mapped in CT and adjacent MA as the Hebron Formation and the Scotland Schist. The Scotland Member (Pease, 1980) is renamed the Scotland Schist Member of the Oakdale. The Oakdale is a homogeneous, calcareous metasiltstone at the base of a thick stratigraphic sequence in a geosyncline terrane and extends from NH to the Honey Hill fault in eastern CT. In central eastern CT it underlies the Hebron Formation; in northeast CT and adjacent MA it underlies conformably the Dudley Formation of the Paxton Group; in central MA it underlies the Paxton Group undivided. The lower part of the Oakdale is cut out along the Clinton-Newbury fault zone. Thickness in type area is about 1500 m. Correlative with the Gove Member of the Berwick Formation in NH and the Gonic Formation in ME. Age is Late Proterozoic(?) based on intrusion of 440 Ma Hedgehog Hill gneiss in the upper part of the Brimfield Group at the top of the stratigraphic sequence, and an age of 1188 Ma for detrital zircons from the Paxton in north-central MA (Pease, 1989).

Ponaganset Gneiss (Proterozoic Z) at surface, covers 2 % of this area

Gneissic biotite granite containing megacrysts of microcline; biotite in coarse streaks and patches. Equivalent to part of former Northbridge Granite Gneiss (usage now abandoned). Sterling Plutonic Suite is here restricted to the Hope Valley terrane. (The Hope Valley together with the Esmond-Dedham terrane make up the Avalon superterrane of this report.) The Ponaganset Gneiss and the Ten Rod Granite Gneiss lie within the Esmond-Dedham terrane and are therefore removed from the Sterling. Ponaganset lies east of the Hope Valley shear zone and extends from RI into MA. [Apparently does not occur in CT.] Predominantly granite, ranges to tonalite. Gray to light gray, some pink, generally medium- to coarse-grained, but ranges from fine-grained to porphyritic. Alkali K-feldspar, plagioclase, quartz, and biotite are major minerals. The Ponaganset is interpreted as an intensely deformed phase of the Esmond Plutonic Suite. Strong lineation is defined by elongate aggregates of quartz and feldspar and trains of biotite and hornblende. Age is Late Proterozoic based on correlation with Northbridge Gneiss (MA), which has been dated at 557+/-4 Ma by Zartman and Naylor (1984) (Skehan and Rast, 1990).

Blackstone Group, Undivided (Proterozoic Z) at surface, covers 2 % of this area

Quartzite, schist, phyllite, marble, and metavolcanic rocks.

Hope Valley Alaskite Gneiss (Proterozoic Z) at surface, covers 2 % of this area

Mafic-poor gneissic granite, locally muscovitic. Gradational with Zsg. Late Proterozoic Hope Valley Alaskite Gneiss occurs as one of several plutonic rocks in Milford antiform. Forms tabular masses along west side of Rhode Island anticlinorium from southern RI and eastern CT to northwestern RI; flanks west side of Milford anticlinorium and terminates at north end of anticlinorium in MA. Color is light pink to tan. Intrudes Plainfield Formation in CT and Blackstone Group rocks in RI. Isotopic age of 630 Ma by U/Pb methods on zircon is reported by Zartman and Naylor (1984) from a sample in MA. Age of 601 +/-5 Ma by U/Pb methods on zircon is reported by Hermes and Zartman (1985) from a sample in RI (Wones and Goldsmith, 1991).

Fitchburg Complex (Lower Devonian or younger) at surface, covers 2 % of this area

Light-gray, strongly foliated biotite-muscovite granite to granodiorite gneiss; common small to very large inclusions of Dl, some mapped separately.

Marlboro Formation (Ordovician, Cambrian, or Proterozoic Z) at surface, covers 2 % of this area

hinly layered amphibolite, biotite schist and gneiss, minor calc-silicate granofels and felsic granofels.

Ayer Granite (Lower Silurian) at surface, covers 2 % of this area

Granite to tonalite, partly porphyritic; locally gneissic, locally muscovitic; may include rocks older than Silurian; intrudes Sb and So. Ayer Granite is divided into the Clinton facies and the Devens-Long Pond facies (Gore, 1976). In addition, there are some masses not assigned to either facies that intrude Berwick Formation west and northwest of Lawrence, and that intrude Paxton and Oakdale Formations south of Worcester and west of probable southern continuation of Wekepeke fault. Radiometric ages obtained for facies of Ayer pose problems in assigning ages to unfossiliferous sedimentary rocks they intrude. Clinton facies has a well-defined U-Pb zircon age of 433 +/-5 Ma (Zartman and Naylor, 1984) that authors cite as Early Silurian; Devens-Long Pond facies has a similar age. This age greatly compresses the time available for deposition, burial, deformation, and metamorphism of Berwick and Paxton if these units are truly Silurian. Some of the Ayer not assigned to a facies may have been more properly correlated with Early Devonian Chelmsford Granite and muscovite-biotite granite at Millstone Hill. Bodies south of Worcester may be more properly correlated with Canterbury Gneiss of CT, which lies on strike with Ayer and has Early Devonian age of 329 +/-9 Ma (Zartman and Naylor, 1984). Zartman and Naylor (1984) believe Ayer Granite has same age range as Newburyport Complex. It is quite possible, based on textural and mineralogical differences that the two facies should be separate units, representing different magmatic events (Wones and Goldsmith, 1991).

Worcester Formation (Lower Devonian and Silurian) at surface, covers 1 % of this area

Carbonaceous slate and phyllite and minor metagraywacke.

Milford granite (Proterozoic Z) at surface, covers 0.9 % of this area

Light-gray to pale orange-pink biotite granite; biotite tends to be in clots or short streaks, quartz granular; locally gneissic. Intrudes Zb. Occupies an area of about 100 sq km. Central mass near Milford is elliptical and is divided into and mapped as a light-colored phase and a dark-colored phase; dark-colored phase defines an irregular border for largest of light-colored plutons. Intrudes Blackstone Group rocks and Ponaganset Gneiss, but was deformed with them at some later unknown time. Isotopic age of 630 +/-15 Ma was determined using U-Pb methods on zircon by Zartman and Naylor (1984). Characterized by salmon-pink color, bluish quartz on weathered surfaces, and lineations defined by lenticular mosaics of quartz and oriented patches of biotite; texture contrast strongly with that of Dedham Granite (Wones and Goldsmith, 1991).

Ayer Granite, Clinton facies (Lower Silurian) at surface, covers 0.8 % of this area

Porphyritic biotite granite with a non-porphyritic border phase; intrudes Sb. Ayer Granite is divided into the Clinton facies and the Devens-Long Pond facies (Gore, 1976). In addition, there are some masses not assigned to either facies that intrude Berwick Formation west and northwest of Lawrence, and that intrude Paxton and Oakdale Formations south of Worcester and west of probable southern continuation of Wekepeke fault. Radiometric ages obtained for facies of Ayer pose problems in assigning ages to unfossiliferous sedimentary rocks they intrude. Clinton facies has a well-defined U-Pb zircon age of 433 +/-5 Ma (Zartman and Naylor, 1984) that authors cite as Early Silurian; Devens-Long Pond facies has a similar age. This age greatly compresses the time available for deposition, burial, deformation, and metamorphism of Berwick and Paxton if these units are truly Silurian. Some of the Ayer not assigned to a facies may have been more properly correlated with Early Devonian Chelmsford Granite and muscovite-biotite granite at Millstone Hill. Bodies south of Worcester may be more properly correlated with Canterbury Gneiss of CT, which lies on strike with Ayer and has Early Devonian age of 329 +/-9 Ma (Zartman and Naylor, 1984). Zartman and Naylor (1984) believe Ayer Granite has same age range as Newburyport Complex. It is quite possible, based on textural and mineralogical differences that the two facies should be separate units, representing different magmatic events (Wones and Goldsmith, 1991).

Coys Hill Porphyritic Granite Gneiss (Lower Devonian) at surface, covers 0.8 % of this area

Coarse-grained porphyritic microcline granite gneiss, commonly containing garnet and sillimanite with or without muscovite; continuous with the Cardigan and Ashuelot plutons of Kinsman Quartz Monzonite in New Hampshire; appears to be an early quasi-concordant intrusion within Dl.

Biotite granite (Proterozoic Z) at surface, covers 0.7 % of this area

Light-gray to grayish-pink, biotite granite, locally foliated. Mafic minerals less prominent than in Milford Granite but granular quartz common. Includes mafic-poor granite similar to Zhg. Intrudes Zdi, Agb, and Zv.

Paxton Formation (Silurian) at surface, covers 0.7 % of this area

Rusty-weathering sulfidic quartzite and sulfidic schist. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Biotite granitic gneiss (Devonian) at surface, covers 0.7 % of this area

Biotite granitic gneiss.

Tadmuck Brook Schist (Silurian?, Ordovician, or Proterozoic Z) at surface, covers 0.6 % of this area

Andalusite phyllite and sillimanite schist, partly sulfidic; local quartzite in upper part.

Paxton Formation (Silurian) at surface, covers 0.6 % of this area

Sulfidic magnesian biotite and magnesian cordierite schist and sillimanite quartzite. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Fitchburg Complex (Lower Devonian or younger) at surface, covers 0.6 % of this area

Dark-gray, strongly foliated biotite granodiorite to tonalite gneiss; resembles Dht; intrudes and contains inclusions of Dl, some mappable; locally cut by sills identical to Dfgrg.

Partridge Formation (includes Brimfield Schist of Emerson, 1917) (Middle Ordovician) at surface, covers 0.5 % of this area

Sulfidic mica schist and abundant amphibolite.

Boylston Schist (Silurian or Ordovician) at surface, covers 0.5 % of this area

Carbonaceous phyllite and schist, locally sulfidic; quartzite; calc-silicate beds.

Milford granite (Proterozoic Z) at surface, covers 0.5 % of this area

Mafic phase. Gray, seriate to sub-porphyritic granite to granodiorite, mafic minerals tend to be in clots; locally gneissic. Intrudes Zb. Occupies an area of about 100 sq km. Central mass near Milford is elliptical and is divided into and mapped as a light-colored phase and a dark-colored phase; dark-colored phase defines an irregular border for largest of light-colored plutons. Intrudes Blackstone Group rocks and Ponaganset Gneiss, but was deformed with them at some later unknown time. Isotopic age of 630 +/-15 Ma was determined using U-Pb methods on zircon by Zartman and Naylor (1984). Characterized by salmon-pink color, bluish quartz on weathered surfaces, and lineations defined by lenticular mosaics of quartz and oriented patches of biotite; texture contrast strongly with that of Dedham Granite (Wones and Goldsmith, 1991).

Marlboro Formation (Ordovician, Cambrian, or Proterozoic Z) at surface, covers 0.5 % of this area

Homogeneous light-gray feldspathic gneiss.

Plainfield Formation (Proterozoic Z) at surface, covers 0.4 % of this area

Quartzite, pelitic schist, minor calc-silicate rock and amphibolite. Plainfield Formation extend into MA in Webster-Oxford area from eastern CT and western RI along west flank of Rhode Island batholith, along same strike as elongate lenses of Westboro Formation (as shown on State bedrock map of Zen and others, 1983). Lithology of Plainfield in New London area of southeastern CT (Goldsmith, 1966, 1976) and in eastern CT (Harwood and Goldsmith, 1971), and its structural and stratigraphic position indicate Plainfield and Westboro are equivalent. Plainfield is truncated along CT-RI border by Lake Char fault; however, in New London area, Plainfield is overlain by suite of largely mafic metavolcanic rocks (Waterford Group of Goldsmith, 1980), a relationship similar to that of the Westboro Formation and overlying metavolcanic rocks. Base of Plainfield is unknown, but gneiss and schist in center of Lyme dome may lie below it. Probably equivalent to quartzite and schist of Blackstone Group on basis of similar lithology and structural relations with Rhode Island batholith rocks (Goldsmith, 1991).

Partridge Formation (includes Brimfield Schist of Emerson, 1917) (Middle Ordovician) at surface, covers 0.4 % of this area

Mafic and felsic gneisses of volcanic derivation with calc-silicate granofels.

Light-gray muscovite granite (Precambrian to Silurian) at surface, covers 0.4 % of this area

Light-gray muscovite granite.

Westboro Formation (Proterozoic Z) at surface, covers 0.3 % of this area

Quartzite, schist, calc-silicate quartzite, and amphibolite. Consists of quartzite and argillite in Saugus and Lynnfield areas. Westboro Formation consists primarily of orthoquartzite and subordinate mica schist, calc-silicate rock, amphibolite, and quartzitic biotite gneiss and schist. Westboro as portrayed by Nelson (1974), Bell and Alvord (1976), and Hepburn and DiNitto (1978) are correlative [with varying certainty]. West and south of Boston, disconnected masses of quartzite and associated rocks are shown on State bedrock map by Zen and others (1983) as Westboro although not continuous with belts mapped by Nelson (1974) or Hepburn and DiNitto (1978). Includes isolated quartzite masses mapped by Castle (1964) in Reading area. On State bedrock map, arbitrarily includes thin quartzite mapped as Burlington Formation by Bell and Alvord (1976) because the units are similar and to reduce number of small units on State map. Rocks mapped as Rice Gneiss by Nelson (1974) were included in unnamed metamorphosed mafic and felsic volcanic unit on State bedrock map, but author now feels it should be either a part of Westboro, or a separate unit below it. Westboro in Framingham area and to the northeast is overlain by unnamed assemblage of metamorphosed mafic and felsic volcanic rocks. South of town of Westborough, Westboro is truncated by Bloody Bluff-Lake Char fault system. Intruded by Proterozoic Z batholithic rocks. Equivalent to Plainfield Formation of eastern CT because it lies in same strike belt, and is probably equivalent to Quinnville Quartzite and unnamed mica schist and phyllite of Blackstone Group. Contact between Westboro and Blackstone is arbitrary on State bedrock map on basis of proximity of isolated exposures of the two units to their respective type areas. No area of continuous exposure exists between Westboro and Blackstone (Goldsmith, 1991).

Tower Hill Quartzite (Silurian) at surface, covers 0.3 % of this area

Quartzite and phyllite.

Partridge Formation (includes Brimfield Schist of Emerson, 1917) (Middle Ordovician) at surface, covers 0.3 % of this area

Biotite gneiss.

Intimately interfolded Littleton and Partridge Formations (Lower Devonian and Middle Ordovician) at surface, covers 0.2 % of this area

In areas of poor exposure and incomplete mapping.

Biotite-hornblende diorite and quartz-bearing diorite (Devonian) at surface, covers 0.2 % of this area

Mostly foliated; intrudes Dl.

Tatnic Hill Formation (Ordovician or Proterozoic Z) at surface, covers 0.2 % of this area

Sulfidic sillimanite schist, sillimanite schist and gneiss, biotite gneiss; minor amphibolite, calc-silicate gneiss and marble.

Biotite-garnet-feldspar gneiss of Ragged Hill (Devonian) at surface, covers 0.2 % of this area

Although extremely narrow, shows local cross cutting relations with Ops, Sfs, and Dl.

Quinebaug Formation (Ordovician, Cambrian, or Proterozoic Z) at surface, covers 0.2 % of this area

Amphibolite, biotite and hornblende gneiss, felsic gneiss, and calc-silicate gneiss.

Tower Hill Quartzite (Silurian) at surface, covers 0.2 % of this area

Gray phyllite associated with the Tower Hill Quartzite.

Metamorphosed mafic to felsic flow, and volcaniclastic and hypabyssal intrusive rocks (Proterozoic Z) at surface, covers 0.1 % of this area

Includes some diorite and gabbro north and northwest of Boston.

Belchertown Complex (Devonian) at surface, covers 0.1 % of this area

(Intrudes De) - Intrusive breccia, mafic and ultramafic fragments in quartz diorite matrix.

Belchertown Complex (Devonian) at surface, covers 0.1 % of this area

(Intrudes De) - Hornblendite.

Paxton Formation (Silurian) at surface, covers 0.1 % of this area

Diopside calc-silicate granofels. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Fitch Formation (Upper Silurian) at surface, covers 0.1 % of this area

Sulfidic calc-silicate and minor sulfidic schist. Although the text and figures of this report show the Fitch as Silurian, a footnote [added just before this report went to press] cites a change in age from Silurian to Early Devonian based on conodonts found at the Bernardston, MA, locality, as reported in Elbert and others (1988). In Bronson Hill anticlinorium in MA, Fitch occurs as lenses between Clough Quartzite and Littleton Formation. Most common rock types in MA are gray, massive to weakly bedded, quartz-labradorite-biotite granulite containing a moderate amount of some combination of calc-silicate minerals (calcic amphibole, zoisite or clinozoisite, diopside, sphene, and microcline); commonly interbedded with biotite-free granulite that contains same calc-silicate minerals. One small exposure consists of nearly pure calcite marble. Larger lenses of Fitch consist of varieties of schist, similar to Partridge Formation. Best exposures are in low hills west of village of Orange, northeast of junction of MA Hwys 2A and 78. As shown on MA State bedrock geologic map, Fitch everywhere overlies Clough Quartzite and is never in contact with Partridge. Fossils dating the Fitch as Pridolian (Harris and others, 1983) are all from Littleton, NH, area [however, see mention of footnote, above]. [Papers presented as chapters in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to MA State bedrock geologic map of Zen and others (1983) at scale of 1:250,000.] (Hatch and others, 1988).

Reubens Hill Formation (Silurian or Ordovician) at surface, covers < 0.1 % of this area

Amphibolite, hornblende-chlorite schist, and feldspathic schist. Includes metamorphosed diorite.

Littleton Formation (Lower Devonian) at surface, covers < 0.1 % of this area

Quartz-feldspar-garnet gneiss, probably originally felsic volcanic rocks.

Hardwick Tonalite (Lower Devonian) at surface, covers < 0.1 % of this area

Porphyritic microcline-biotite granite gneiss in sills intruding Dht.

Diabase dikes and sills (Lower Jurassic) at surface, covers < 0.1 % of this area

Diabase dikes and sills.

Biotite-muscovite granite (Devonian) at surface, covers < 0.1 % of this area

Slightly foliated.

Paxton Formation - Bigelow Brook Member (Silurian) at surface, covers < 0.1 % of this area

Biotite granofels, sulfidic schist, and minor calc-silicate granofels. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Esmond Granite (Proterozoic Z) at surface, covers < 0.1 % of this area

Light-gray to pink biotite granite, generally slightly altered, locally foliated. Gradational with Zgmgd. Intrudes Zsg. Esmond Granite occupies nearly 100 sq km in RI and forms an elongate pluton southeast of Woonsocket, RI, of which only the northern part is in MA. Occurs in Dedham batholith. Consists of mottled red and green, mainly massive granite. Coarse-grained facies intrudes Blackstone Group rocks and a related tonalite; fine-grained facies intrudes the tonalite and coarse-grained facies (Hermes and Zartman, 1985). Contact relations with Milford Granite not known, although phases of Milford resemble Esmond. Age of 621 +/-8 Ma (U/Pb methods on zircon by Hermes and Zartman, 1985) is close to age of Milford (Wones and Goldsmith, 1991).

Muscovite-bearing granite (Lower Devonian) at surface, covers < 0.1 % of this area

At Millstone Hill; intrudes So.

Bellingham Conglomerate (Pennsylvanian, Cambrian or Proterozoic Z) at surface, covers < 0.1 % of this area

Red and gray metamorphosed conglomerate, sandstone, graywacke, and shale. Bellingham Conglomerate consists of conglomerate and lithic graywacke interbedded with chlorite phyllite and is confined to Bellingham basin. Also contains some volcanic rocks (rhyolite porphyry in roadcut on MA Hwy 146 at Premisy Hill west of Woonsocket, and felsite porphyry in the Franklin area east of Bellingham). Conglomerate contains pebbles of quartzite from adjacent Blackstone Group rocks and typical blue quartz of Milford Granite, so sediments are locally derived. Exposures on east side of Woonsocket Hill, southeast of Woonsocket, RI, show cliffs of steeply dipping, thin-bedded, white to gray quartzite of Blackstone Group standing above green schistose conglomerate containing many flattened white to gray quartzite pebbles and interbedded green calcareous quartz schist. Contact is probably a fault, but source of pebbles is quite obvious. In the same area, schist of Blackstone Group is difficult to distinguish from those of the Bellingham because of low-grade metamorphism of Blackstone rocks; it is probable that some of the low-grade Blackstone Rocks mapped northwest of Woonsocket in Blackstone River valley are part of Bellingham. The two rock units have been traditionally distinguished in the past by presence or absence of epidote (Warren and Powers, 1914), but this needs further study. Age is uncertain. Rocks have customarily been correlated with those of Pennsylvanian Narragansett basin; however, rocks in some exposures, such as the one at River St and Blackstone St in Woonsocket resemble Proterozoic Z Roxbury Conglomerate in Boston basin. Skehan and others (1979) suggest that Bellingham may have a similar age to that of Roxbury. This is supported by observation that Bellingham is a structural trough extending southwest from Boston basin and separating primarily Proterozoic Z granitoids from altered, but nongneissic, Proterozoic granitoids (Wones and Goldsmith, 1991). In deference to tradition, and because Proterozoic Z age has not been proven, age is shown on MA State bedrock map of Zen and others as Proterozoic Z to Pennsylvanian [map actually has age of Proterozoic Z, Cambrian, or Pennsylvanian, which differs from age stated in this report.] (Goldsmith, 1991).

Metamorphosed felsic metavolcanic rocks (Proterozoic Z) at surface, covers < 0.1 % of this area

Metamorphosed felsic metavolcanic rocks .

Upper part of Rangeley Formation (Lower Silurian (Llandoverian)) at surface, covers < 0.1 % of this area

Rusty-weathering, pelitic schist, metasandstone, and local coarse-grained metasandstone lentils; calc-silicate pods common; minor coticule. Probably equivalent to member C of Rangeley Formation of Maine.

Ayer Granite - Devens-Long Pond facies (Lower Silurian and Upper Ordovician?) at surface, covers < 0.1 % of this area

Equigranular to porphyritic gneissic biotite granite and granodiorite. Ayer Granite is divided into the Clinton facies and the Devens-Long Pond facies (Gore, 1976). In addition, there are some masses not assigned to either facies that intrude Berwick Formation west and northwest of Lawrence, and that intrude Paxton and Oakdale Formations south of Worcester and west of probable southern continuation of Wekepeke fault. Radiometric ages obtained for facies of Ayer pose problems in assigning ages to unfossiliferous sedimentary rocks they intrude. Clinton facies has a well-defined U-Pb zircon age of 433 +/-5 Ma (Zartman and Naylor, 1984) that authors cite as Early Silurian; Devens-Long Pond facies has a similar age. This age greatly compresses the time available for deposition, burial, deformation, and metamorphism of Berwick and Paxton if these units are truly Silurian. Some of the Ayer not assigned to a facies may have been more properly correlated with Early Devonian Chelmsford Granite and muscovite-biotite granite at Millstone Hill. Bodies south of Worcester may be more properly correlated with Canterbury Gneiss of CT, which lies on strike with Ayer and has Early Devonian age of 329 +/-9 Ma (Zartman and Naylor, 1984). Zartman and Naylor (1984) believe Ayer Granite has same age range as Newburyport Complex. It is quite possible, based on textural and mineralogical differences that the two facies should be separate units, representing different magmatic events (Wones and Goldsmith, 1991).

Diorite and gabbro (Proterozoic Z) at surface, covers < 0.1 % of this area

Complex of diorite and gabbro, subordinate metavolcanic rocks and intrusive granite and granodiorite.

Vaughn Hills Quartzite (Silurian or Ordovician) at surface, covers < 0.1 % of this area

Quartzite, phyllite, conglomerate, and chlorite schist.

Tatnic Hill Formation - Yantic Member (Ordovician or Proterozoic Z) at surface, covers < 0.1 % of this area

Gray mica schist.

Ammonoosuc Volcanics (Middle Ordovician) at surface, covers < 0.1 % of this area

Amphibolite, felsic gneiss, garnet-amphibole quartzite, and marble too thin to show separately at map scale. Gedrite, anthophyllite, cummingtonite locally abundant in amphibolite layers.

Berwick Formation (Silurian) at surface, covers < 0.1 % of this area

Thin- to thick-bedded metamorphosed calcareous sandstone, siltstone, and minor muscovite schist. In New Hampshire: Used as Berwick Formation of Merrimack Group. Consists of purple biotite-feldspar granofels or schist. Contains interbeds of calcsilicate granofels and minor metapelites. Includes Gove Member, mapped separately. Stratigraphic sequence with respect to Eliot Formation is uncertain. Age of all formations in Merrimack Group changed to Ordovician(?) to Silurian(?) based on isotopic age determinations of approx 440 and 420 Ma from detrital zircons from Berwick by J.N. Aleinikoff (oral commun., 1994) (Lyons and others, 1997).

Harvard Conglomerate (Pennsylvanian) at surface, covers < 0.1 % of this area

Conglomerate and chloritoid-hematite phyllite.

Tatnic Hill Formation - Fly Pond Member (Ordovician or Proterozoic Z) at surface, covers < 0.1 % of this area

Calc-silicate gneiss and marble.

Granodiorite (Precambrian to Paleozoic) at surface, covers < 0.1 % of this area

Mostly nonfoliated.

Coal Mine Brook Formation (Middle Pennsylvanian) at surface, covers < 0.1 % of this area

Fossiliferous, carbonaceous slate and garnet phyllite with a lens of meta-anthracite; conglomerate and arkose.

Granite (Precambrian to Paleozoic) at surface, covers < 0.1 % of this area

Mostly nonfoliated.

Spaulding Tonalite (Early Devonian) at surface, covers < 0.1 % of this area

(Spaulding Quartz Diorite of Fowler-Billings, 1949) - Weakly foliated to nonfoliated, spotted biotite quartz diorite, tonalite, granodiorite, and granite; garnet and muscovite may or may not be present.

Fitchburg Complex (Lower Devonian or younger) at surface, covers < 0.1 % of this area

Dfgd containing many zones of foliated biotite-muscovite granite gneiss and inclusions of mica schist and feldspathic granulite.

Fitch Formation (Upper Silurian) at surface, covers < 0.1 % of this area

Sulfidic mica schist. Although the text and figures of this report show the Fitch as Silurian, a footnote [added just before this report went to press] cites a change in age from Silurian to Early Devonian based on conodonts found at the Bernardston, MA, locality, as reported in Elbert and others (1988). In Bronson Hill anticlinorium in MA, Fitch occurs as lenses between Clough Quartzite and Littleton Formation. Most common rock types in MA are gray, massive to weakly bedded, quartz-labradorite-biotite granulite containing a moderate amount of some combination of calc-silicate minerals (calcic amphibole, zoisite or clinozoisite, diopside, sphene, and microcline); commonly interbedded with biotite-free granulite that contains same calc-silicate minerals. One small exposure consists of nearly pure calcite marble. Larger lenses of Fitch consist of varieties of schist, similar to Partridge Formation. Best exposures are in low hills west of village of Orange, northeast of junction of MA Hwys 2A and 78. As shown on MA State bedrock geologic map, Fitch everywhere overlies Clough Quartzite and is never in contact with Partridge. Fossils dating the Fitch as Pridolian (Harris and others, 1983) are all from Littleton, NH, area [however, see mention of footnote, above]. [Papers presented as chapters in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to MA State bedrock geologic map of Zen and others (1983) at scale of 1:250,000.] (Hatch and others, 1988).

Littleton Formation (Lower Devonian) at surface, covers < 0.1 % of this area

Orthopyroxene-biotite gneiss, probably originally intermediate volcanic rocks.

Clough Quartzite (Upper Silurian) at surface, covers < 0.1 % of this area

Quartz-pebble conglomerate, quartzite, and minor mica schist and calc-silicate rocks. Fossils at Bernardston are similar to those at Croyden Mountain, New Hampshire which indicate late Llandoverian age. Parts of the Littleton and Partridge Formations, and Clough Quartzite in MA are here reassigned to the Rangeley Formation [here geographically extended to MA]. The four mapped areas of Clough Quartzite in the Amherst area west of the Connecticut Valley border fault are now interpreted as conglomerate lenses in the Rangeley. Clough is considered the key stratigraphic unit in Bronson Hill anticlinorium because 1) it is dominated by distinctive, readily recognizable rock types, 2) where present, it is base of Silurian-Devonian sequence, resting with detectable unconformity on older rocks, and 3) it contains late Llandoverian fossils at several localities in western NH and adjacent VT, and at Bernardston, MA. Consists mostly of quartz-pebble conglomerate in which pebbles are typically deformed; other lithologies are quartz grit or white to pink, well-bedded quartzite. Locally contains some mica schist beds. On the MA State bedrock geologic map (Zen and others, 1983), thickness is locally exaggerated because at many localities, the unit was only a few meters or less thick and could not be shown at a scale of 1:250,000. Maximum thickness is 200 m on west limb of Northfield syncline. Unconformably overlies Fourmile Gneiss in Pelham dome and in Kempfield anticline, or Ammonoosuc Volcanics over most gneiss domes. Partridge Formation occurs along Clough-Ammonoosuc contact as lenses in many areas (Hatch and others, 1988).

Littleton Formation (Lower Devonian) at surface, covers < 0.1 % of this area

Thinly laminated calcitic marble.

Lower part of Rangeley Formation (Lower Silurian (Llandoverian)) at surface, covers < 0.1 % of this area

Gray, thinly laminated (5-25 mm) metapelite with local lentils of turbidites and thin quartz conglomerates in western New Hampshire. Sparse calc-silicate pods and coticule. Probably equivalent to member B of Rangeley Formation of Maine.

Hornblende-olivine gabbro (Precambrian to Paleozoic) at surface, covers < 0.1 % of this area

Intrudes Dl.

Silicified fault-breccia or strongly silicified metamorphic rocks (Lower Jurassic) at surface, covers < 0.1 % of this area

Silicified fault-breccia or strongly silicified metamorphic rocks.

Concord Granite (Late Devonian) at surface, covers < 0.1 % of this area

Gray two-mica granite, locally grading to tonalite.

Partridge Formation (includes Brimfield Schist of Emerson, 1917) (Middle Ordovician) at surface, covers < 0.1 % of this area

Layered felsic gneiss and schist.

Hornblende-plagioclase gneiss (Precambrian to Paleozoic) at surface, covers < 0.1 % of this area

Hornblende-plagioclase gneiss .

Coys Hill Porphyritic Granite Gneiss (Lower Devonian) at surface, covers < 0.1 % of this area

Hornblende gneiss inclusions in granitic gneiss of the Coys Hill.

Partridge Formation (includes Brimfield Schist of Emerson, 1917) (Middle Ordovician) at surface, covers < 0.1 % of this area

Amphibolite where mapped separately.

Partridge Formation (includes Brimfield Schist of Emerson, 1917) (Middle Ordovician) at surface, covers < 0.1 % of this area

Lenses of ultramafic rock, commonly hornblendite with or without olivine, orthopyroxene, spinel, cummingtonite, anthophyllite, ilmenite and chlorite.

Meladiorite and norite (Devonian) at surface, covers < 0.1 % of this area

Weakly to moderately foliated; intrudes Dl.

Granodiorite (Devonian) at surface, covers < 0.1 % of this area

Mostly nonfoliated, intrudes Dl.

Rangeley Formation, Pink to green calc-silicate and purple biotite granofels (Lower Silurian (Llandoverian)) at surface, covers < 0.1 % of this area

Thinly bedded. Close to transition from lower to upper parts of the Rangeley Formation. Probably equivalent to part of Paxton Formation of Zen and others (1983) in Massachusetts.

Granodiorite of the Indian Head pluton (Precambrian to Paleozoic) at surface, covers < 0.1 % of this area

Light-gray to pinkish-gray, fine- to medium-grained biotite granodiorite, and gray fine-grained hornblende-biotite tonalite. Intrudes OZm.

Monson Gneiss (Ordovician, Cambrian, or Proterozoic Z) at surface, covers < 0.1 % of this area

Amphibolite where mapped separately.

Nashoba Formation - Boxford Member (Ordovician or Proterozoic Z) at surface, covers < 0.1 % of this area

Thin bedded to massive amphibolite, minor biotite gneiss. Of the 10 members of the Nashoba Formation defined by Bell and Alvord (1976), only amphibolitic Boxford Member, at the presumed base of Nashoba is separated out on MA State bedrock map of Zen and others (1983) because it is the only member clearly identified in several locations. Although age on MA State bedrock map is shown as Proterozoic Z or Ordovician (due to uncertainty regarding actual rocks sampled by Olszewski and a strong belief that rocks of Nashoba zone correlated with Ordovician rocks to the west), author now feels that rocks of Nashoba zone (except for Tadmuck Brook Schist) are all Proterozoic, but that they are unlike the Proterozoic rocks of neighboring Milford-Dedham zone. [No formal age change is made in this report.] (Goldsmith, 1991).

Paxton Formation (Silurian) at surface, covers < 0.1 % of this area

Amphibolite. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Kinsman Granodiorite (Early Devonian) at surface, covers < 0.1 % of this area

(Kinsman Quartz Monzonite of Billings, 1955) - Foliated granite, granodiorite, tonalite, and minor quartz diorite; large megacrysts of potassium feldspar characteristic; garnet locally abundant.

Quartz diorite (Precambrian to Phanerozoic) at surface, covers < 0.1 % of this area

Mostly nonfoliated.

Ammonoosuc Volcanics, Bimodal volcanic rocks (Middle - Upper Ordovician) at surface, covers < 0.1 % of this area

Ammonoosuc Volcanics, Bimodal volcanic rocks - Locally includes unmapped Oals.

Partridge Formation, undivided (Middle - Upper Ordovician) at surface, covers < 0.1 % of this area

Black, rusty-weathering sulfidic-graphitic slate or schist and sparse to abundant metagraywacke. Lies stratigraphically between upper and lower parts of the Ammonoosuc Volcanics.

Fitchburg Complex (Lower Devonian or younger) at surface, covers < 0.1 % of this area

Inclusions of massive coarse-grained biotite-hornblende tonalite within Dfgd.

Other metasedimentary rocks - metaclastic rock, undivided (Pennsylvanian? or Late Proterozoic?) at surface, covers < 0.1 % of this area

Red to gray to green polymict conglomerate, sandstone, and shale. Variably metamorphosed. Includes rock mapped formerly as Bellingham Conglomerate.