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Mineral Resources > Online Spatial Data > Geology > by state > Massachusetts Geology

Geologic units in Worcester county, Massachusetts

Partridge Formation (Middle Ordovician) at surface, covers 0.3 % of this area

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Biotite gneiss.

Lithology: biotite gneiss

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

Hardwick Tonalite - Porphyritic microcline-biotite granite gneiss in sills intruding Dht.

Lithology: granitic gneiss

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

Hornblende-olivine gabbro - Intrudes Dl.

Lithology: gabbro

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

Ayer Granite - Clinton facies. 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).

Lithology: granite

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

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

Lithology: biotite gneiss; amphibolite; augen gneiss

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

Spaulding Tonalite (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.

Lithology: quartz diorite; tonalite; granodiorite; granite

Partridge Formation (Middle Ordovician) at surface, covers 0.5 % of this area

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Sulfidic mica schist and abundant amphibolite.

Lithology: mica schist; amphibolite

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

Paxton Formation - 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).

Lithology: schist; quartzite

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

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

Lithology: diorite; gabbro; metavolcanic rock; granite; granodiorite

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

Biotite granitic gneiss.

Lithology: granitic gneiss

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

Granite - Mostly nonfoliated.

Lithology: granite

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

Granodiorite - Mostly nonfoliated, intrudes Dl.

Lithology: granodiorite

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

Milford Granite - 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).

Lithology: granite; granodiorite

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

Hornblende-plagioclase gneiss .

Lithology: mafic gneiss

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

Coys Hill Porphyritic Granite Gneiss - 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.

Lithology: granitic gneiss

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

Biotite-garnet-feldspar gneiss of Ragged Hill - Although extremely narrow, shows local cross cutting relations with Ops, Sfs, and Dl.

Lithology: gneiss

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

Paxton Formation - 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).

Lithology: amphibolite

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

Marlboro Formation - Thinly layered amphibolite, biotite schist and gneiss, minor calc-silicate granofels and felsic granofels.

Lithology: amphibolite; mica schist; biotite gneiss; granofels

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

Muscovite-bearing granite - At Millstone Hill; intrudes So.

Lithology: granite

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

Granodiorite of the Indian Head pluton - Light-gray to pinkish-gray, fine- to medium-grained biotite granodiorite, and gray fine-grained hornblende-biotite tonalite. Intrudes OZm.

Lithology: granodiorite; tonalite

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

Fitch Formation - 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).

Lithology: calc-silicate rock; schist

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

Upper part of Rangeley Formation - 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.

Lithology: pelitic schist; metasedimentary rock; calc-silicate rock

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Lenses of ultramafic rock, commonly hornblendite with or without olivine, orthopyroxene, spinel, cummingtonite, anthophyllite, ilmenite and chlorite.

Lithology: ultramafic intrusive rock; hornblendite

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

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

Lithology: slate; phyllite; metasedimentary rock; conglomerate; arkose

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

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

Lithology: felsic gneiss

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

Nashoba Formation - Boxford Member - 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).

Lithology: amphibolite; biotite gneiss

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

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

Lithology: granitic gneiss

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

Tatnic Hill Formation - Yantic Member - Gray mica schist.

Lithology: mica schist

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

Ayer Granite - 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).

Lithology: granite; tonalite

Hope Valley Alaskite Gneiss (Proterozoic Z) at surface, covers 0.8 % of this area

Hope Valley Alaskite Gneiss - 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).

Lithology: alkali-granite (alaskite)

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

Westboro Formation - 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).

Lithology: quartzite; schist; amphibolite; argillite

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

Worcester Formation - Carbonaceous slate and phyllite and minor metagraywacke.

Lithology: slate; phyllite; metasedimentary rock

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

Paxton Formation - 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).

Lithology: granofels

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

Biotite granite - 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.

Lithology: granite

Ponaganset Gneiss (Proterozoic Z) at surface, covers 3 % of this area

Ponaganset Gneiss - 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).

Lithology: granite

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

Paxton Formation - Bigelow Brook Member - 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).

Lithology: granofels; schist

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

Paxton Formation - 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).

Lithology: granofels; schist

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

Paxton Formation - 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).

Lithology: quartzite; schist

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

Tower Hill Quartzite - Gray phyllite associated with the Tower Hill Quartzite.

Lithology: phyllite

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

Other metasedimentary rocks - metaclastic rock, undivided - Red to gray to green polymict conglomerate, sandstone, and shale. Variably metamorphosed. Includes rock mapped formerly as Bellingham Conglomerate.

Lithology: meta-conglomerate; metasedimentary rock

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Biotite gneiss of volcanic derivation, minor amphibolite and sulfidic schist.

Lithology: biotite gneiss; amphibolite; schist

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

Rangeley Formation, Pink to green calc-silicate and purple biotite granofels - 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.

Lithology: calc-silicate rock; granofels

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

Littleton Formation - Black to gray aluminous mica schist, quartzose schist, and aluminous phyllite.

Lithology: schist; phyllite

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

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

Lithology: granite; granodiorite; tonalite; quartz diorite

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

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

Lithology: tonalite

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

Marlboro Formation - Homogeneous light-gray feldspathic gneiss.

Lithology: granitic gneiss

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

Monson Gneiss - Amphibolite where mapped separately.

Lithology: amphibolite

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

Milford Granite - 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).

Lithology: granite

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

Granodiorite - Mostly nonfoliated.

Lithology: granodiorite

Mattapan Volcanic Complex (Proterozoic Z or younger) at surface, covers < 0.1 % of this area

Mattapan Volcanic Complex - Rhyolite, melaphyre, agglomerate, and tuff. Mattapan Volcanic Complex is found in west and southwest part of Boston basin and beyond, and to the south in Blue Hills. Similar in lithology to Lynn Volcanic Complex. Both units consist largely of partly porphyritic rhyolite and rhyodacite flows, welded ash-flow tuffs, vitric tuff, lapilli tuff, lithic tuff, flow breccias, breccia pipes, and extrusion domes. Mattapan's rhyolite and rhyodacites are thinner and less varied in composition and texture than Lynn's, and volcanic breccias are absent in Lynn. Both units are reported to lie nonconformably on Dedham Granite and unnamed plutonic-volcanic complex of eastern MA; however, Mattapan has been observed as dikes and stocks cutting Dedham Granite (Billings, 1976; Kaye and Zartman, 1980; Chute, 1966), and other workers have pointed out evidence that some of Mattapan may be penecontemporaneous with younger phases of Dedham batholith. Westwood Granite may be intrusive equivalent of Mattapan. Mattapan is conformably and fairly continuously overlain by Roxbury Conglomerate of Boston Bay Group. LaForge (19832) cautioned against identifying rocks interbedded within Mattapan that look like Roxbury, but are still part of Mattapan. Metavolcanic rocks in Blue Hills resembling Mattapan are assigned to Mattapan on State bedrock map (Zen and others, 1983); for years they were considered to be either Silurian and Devonian, or Carboniferous; however, they are chemically and mineralogically distinct from Ordovician and Silurian Blue Hills [sic] Granite Porphyry. Author follows usage of Chute (1966) who could find no difference between volcanic rocks in Blue Hills area and Mattapan rocks, and thus these rocks are assigned to Mattapan. Proterozoic Z age is based on U-Th-Pb zircon date of 602 +/-3 Ma (Zartman, in Kaye and Zartman, 1980). Although Billings (1979) questions reliability of zircon ages from volcanic rocks, discovery of Proterozoic Z acritarchs in overlying Cambridge Argillite indicates zircon age is appropriate (Goldsmith, 1991).

Lithology: rhyolite; mafic volcanic rock; volcanic breccia (agglomerate); tuff

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

Concord Granite - Gray two-mica granite, locally grading to tonalite.

Lithology: granite; tonalite

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

Tatnic Hill Formation - Fly Pond Member - Calc-silicate gneiss and marble.

Lithology: gneiss; marble

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

Tadmuck Brook Schist - Andalusite phyllite and sillimanite schist, partly sulfidic; local quartzite in upper part.

Lithology: phyllite; schist; quartzite

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

Diabase dikes and sills.

Lithology: diabase

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

Ayer Granite - Devens-Long Pond facies. 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).

Lithology: granite; granodiorite

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.

Lithology: bimodal suite

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

Fitchburg Complex - 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.

Lithology: granite; pegmatite

Partridge Formation (Middle Ordovician) at surface, covers 0.3 % of this area

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Mafic and felsic gneisses of volcanic derivation with calc-silicate granofels.

Lithology: mafic gneiss; felsic gneiss; granofels

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

Meladiorite and norite - Weakly to moderately foliated; intrudes Dl.

Lithology: diorite; norite

Blackstone Group (Proterozoic Z) at surface, covers 1 % of this area

Blackstone Group - Undivided - Quartzite, schist, phyllite, marble, and metavolcanic rocks.

Lithology: quartzite; schist; phyllite; marble; metavolcanic rock

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

Light-gray muscovite granite.

Lithology: granite

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

Berwick Formation - 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).

Lithology: metasedimentary rock; mica schist

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

Metamorphosed mafic to felsic flow, and volcaniclastic and hypabyssal intrusive rocks - Includes some diorite and gabbro north and northwest of Boston.

Lithology: metavolcanic rock; mixed clastic/volcanic; diorite; gabbro

Intimately interfolded Littleton and Partridge Formations (Lower Devonian and Middle Ordovician) at surface, covers 0.1 % of this area

Intimately interfolded Littleton and Partridge Formations - In areas of poor exposure and incomplete mapping.

Lithology: mica schist; biotite gneiss; felsic gneiss; mafic gneiss; augen gneiss; gneiss; amphibolite; schist; marble; ultramafic intrusive rock; granofels; calc-silicate rock

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

Harvard Conglomerate - Conglomerate and chloritoid-hematite phyllite.

Lithology: conglomerate; phyllite

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

Biotite-muscovite granite - Slightly foliated.

Lithology: granite

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

Paxton Formation - 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).

Lithology: mica schist

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

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

Lithology: granitic gneiss; mica schist; phyllite; granulite

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

Littleton Formation - Orthopyroxene-biotite gneiss, probably originally intermediate volcanic rocks.

Lithology: mafic gneiss

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

Tower Hill Quartzite - Quartzite and phyllite.

Lithology: quartzite; phyllite

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

Belchertown Complex (intrudes De) - Hornblendite.

Lithology: hornblendite

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

Fitch Formation - 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).

Lithology: mica schist

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Amphibolite where mapped separately.

Lithology: amphibolite

Partridge Formation (Middle Ordovician) at surface, covers 9 % of this area

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Sulfidic mica schist and subordinate amphibolite.

Lithology: mica schist; amphibolite

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

Clough Quartzite - 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).

Lithology: conglomerate; quartzite; mica schist; calc-silicate rock

Scituate Granite Gneiss (Proterozoic Z) at surface, covers 7 % of this area

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

Lithology: granite

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

Fitchburg Complex -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.

Lithology: granitic gneiss; mica schist; phyllite

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

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

Lithology: granitic gneiss; mica schist; phyllite

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

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

Lithology: phyllite; schist; quartzite; calc-silicate rock

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

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

Lithology: quartz diorite; breccia; mafic rock; ultramafic intrusive rock

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

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

Lithology: amphibolite; felsic gneiss; quartzite; marble

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

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

Lithology: schist; gneiss; biotite gneiss; amphibolite; marble

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

Quartz diorite - Mostly nonfoliated.

Lithology: quartz diorite

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

Coys Hill Porphyritic Granite Gneiss - Hornblende gneiss inclusions in granitic gneiss of the Coys Hill.

Lithology: mafic gneiss

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

Biotite-hornblende diorite and quartz-bearing diorite - Mostly foliated; intrudes Dl.

Lithology: diorite; quartz diorite

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

Oakdale Formation - 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).

Lithology: metasedimentary rock; mica schist

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

Lower part of Rangeley Formation - 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.

Lithology: meta-argillite; meta-conglomerate; calc-silicate rock

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

Vaughn Hills Quartzite - Quartzite, phyllite, conglomerate, and chlorite schist.

Lithology: quartzite; phyllite; conglomerate; greenschist

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.

Lithology: breccia; metamorphic rock

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

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

Lithology: slate; schist; metasedimentary rock

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

Littleton Formation - Thinly laminated calcitic marble.

Lithology: marble

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

Metamorphosed felsic metavolcanic rocks .

Lithology: felsic metavolcanic rock

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

Nashoba Formation - 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).

Lithology: schist; gneiss; amphibolite; biotite gneiss; marble