Pondville Conglomerate - Quartz conglomerate having abundant sandy matrix; boulder conglomerate, arkose; fossil plants. Pondville Conglomerate is present in Narragansett and Norfolk basins. Although Chute (1964, 1966, 1969) recognized a lower boulder conglomerate member and an upper sandstone to pebble conglomerate member in the Pondville in the northeast part of the basin, such a division is not readily made to the southwest because of facies changes. Upper member grades into and interfingers with Wamsutta Formation. Basal beds nonconformably overlie Dedham Granite in northern part of Narragansett basin. Age of deposits in Narragansett and Norfolk basins ranges from Early to Late Pennsylvanian; however, Skehan and Murray (1980) assigned lower part of Pondville to Mississippian. Further study may reveal even older Paleozoic sediments in Narragansett basin (Goldsmith, 1991).
Rhode Island Formation - Sandstone, graywacke, shale and conglomerate; minor beds of meta-anthracite. Fossil plants. Rhode Island Formation is thickest and most extensive formation in Narragansett basin. Does not extend to Norfolk basin. Consists of gray sandstone and siltstone and lesser amounts of gray to black shale, gray conglomerate, and coal beds 10 m thick. Interfingers with Wamsutta Formation in Narragansett basin. In places overlies Dedham Granite. Age is Middle and Late Pennsylvanian (Goldsmith, 1991).
Wamsutta Formation - Red to pink, well-sorted conglomerate, graywacke, sandstone, and shale; fossil plants. Wamsutta Formation occurs in Narragansett and Norfolk basins. Consists of conglomerate, lithic graywacke, sandstone, and shale. Also contains rhyolite and basalt horizons near Attleboro. Northwest of Attleboro, Wamsutta overlies Diamond Hill Felsite as used by Skehan and Murray (in Skehan and others, 1979). Volcanic rocks similar to Diamond Hill Felsite crop out west of Lake Pearl, between Franklin and Wrentham, on west flank of Norfolk basin. These are shown within Wamsutta Formation on MA State bedrock map of Zen and others (1983) because of their proximity to Diamond Hill. They also resemble Proterozoic Z Mattapan Volcanic Complex. Chute (1966) described lenses of carbonate rock in red and green shale in Wamsutta in Norwood quad. Limestone also observed in rocks mapped as Wamsutta adjacent to exposed Dedham Granite at Manchester Pond Reservoir (J.P. Schafer, 1982, oral commun.). Red and green shales may actually be Cambrian. Upper member of Pondville Conglomerate grades into and interfingers with Wamsutta; in turn, Wamsutta interfingers with Rhode Island Formation in northwest part of Narragansett basin. Nonconformably overlies Dedham Granite. Partly equivalent to Rhode Island Formation. Age is Early and Middle Pennsylvanian. Contains a few plant fossils (Goldsmith, 1991).
Wamsutta Formation - Rhyolite and mafic volcanic rocks in the Wamsutta Formation. Wamsutta Formation occurs in Narragansett and Norfolk basins. Consists of conglomerate, lithic graywacke, sandstone, and shale. Also contains rhyolite and basalt horizons near Attleboro. Northwest of Attleboro, Wamsutta overlies Diamond Hill Felsite as used by Skehan and Murray (in Skehan and others, 1979). Volcanic rocks similar to Diamond Hill Felsite crop out west of Lake Pearl, between Franklin and Wrentham, on west flank of Norfolk basin. These are shown within Wamsutta Formation on MA State bedrock map of Zen and others (1983) because of their proximity to Diamond Hill. They also resemble Proterozoic Z Mattapan Volcanic Complex. Chute (1966) described lenses of carbonate rock in red and green shale in Wamsutta in Norwood quad. Limestone also observed in rocks mapped as Wamsutta adjacent to exposed Dedham Granite at Manchester Pond Reservoir (J.P. Schafer, 1982, oral commun.). Red and green shales may actually be Cambrian. Upper member of Pondville Conglomerate grades into and interfingers with Wamsutta; in turn, Wamsutta interfingers with Rhode Island Formation in northwest part of Narragansett basin. Nonconformably overlies Dedham Granite. Partly equivalent to Rhode Island Formation. Age is Early and Middle Pennsylvanian. Contains a few plant fossils (Goldsmith, 1991).
Cambridge Argillite(Proterozoic Z to earliest Paleozoic)at surface, covers < 0.1 % of this area
Cambridge Argillite - Gray argillite and minor quartzite; rare sandstone and conglomerate. Contains acritarchs. Cambridge Argillite of Boston Bay Group contains sandy horizons which are in some places quartzite. Most prominent are Milton quartzite unit of Billings (1976), and Tufts Quartzite Member (described by Billings, 1929, and LaForge, 1932) in northern part of basin. Red sandstone and sandy argillite in Chelsea, Revere, and Milton-Quincy areas intertongue with green argillite (Kaye, 1980). Red beds lie above cleaner quartzites such as Tufts and Milton units. Core analysis by D.A. Ashenden (Metropolitan District Commission, 1980, written commun.) indicates that Cambridge and Braintree Argillites are identical. Age of Boston Bay rocks has been controversial and was once thought to be Cambrian to Pennsylvanian (the latter based on lithologic similarity to rocks of Narragansett basin and now discredited plant fossils). Age of Cambridge and of Boston Bay Group as a whole is Proterozoic Z and possibly Early Cambrian based on presence of acritarchs in Cambridge. Acritarchs are diagnostic species that ranges in age from Proterozoic Z to Early Cambrian, but is most abundant in Proterozoic Z time (Lenk and others, 1982; [also see Goldsmith and others, 1982]). Age is also supported by the following: 1) plant fossils so numerous in strata of Narragansett basin are absent in the Boston Bay Group strata, 2) Late Ordovician and Early Silurian Quincy Granite contains argillite inclusions that are on strike with Cambridge Argillite, and 3) Boston Bay Group stratigraphy is primarily marine, not similar to terrestrial stratigraphy of Narragansett basin (Goldsmith, 1991).
Roxbury Conglomerate - Conglomerate, sandstone, siltstone, argillite, and melaphyre. Consists of Brookline, Dorchester, and Squantum Members. Roxbury Conglomerate forms base of Boston Bay Group. Divided into Brookline, Dorchester, and Squantum Members. Conglomerate in Brookline Member contains clasts of Dedham Granite, quartzite (possibly from Westboro Formation), and volcanic rock from underlying Mattapan Volcanic Complex. Dorchester Member consists of interbedded argillite and sandstone and forms an intermediate unit between Brookline Member and overlying Cambridge Argillite. Uppermost Squantum Member is a distinctive diamictite which appears to pinch out in northern part of basin. Brighton Melaphyre lies within Brookline Member and consists of mafic volcanic rocks (quartz keratophyre, keratophyre, and spilite). Roxbury clearly lies nonconformably on Dedham Granite near Hull, MA; can be traced continuously over Mattapan Volcanic Complex. Age is Proterozoic Z and possibly Early Cambrian (Goldsmith, 1991).
Roxbury Conglomerate - Melaphyre in the Roxbury Conglomerate. Brighton Melaphyre lies within Brookline Member and consists of mafic volcanic rocks (quartz keratophyre, keratophyre, and spilite). Roxbury clearly lies nonconformably on Dedham Granite near Hull, MA; can be traced continuously over Mattapan Volcanic Complex. Age is Proterozoic Z and possibly Early Cambrian (Goldsmith, 1991).
Blue Hill Granite Porphyry - Microperthite-quartz porphyry. [Used mistakenly as Blue Hills Granite Porphyry; the plural Hills was not intended and should not be used.] Described as dark gray to dark blue, weathering buff brown to salmon. Assigned a Late Ordovician and Early Silurian age because of its close similarity, both chemically and mineralogically, to Quincy Granite. Cobbles of the rock are found in Early Pennsylvanian Pondville Conglomerate. Bounded by Quincy Granite to the north and east, and by Neponset fault on the west; unconformably overlain by Pondville Conglomerate to the south (Wones and Goldsmith, 1991).
Quincy Granite(Lower Silurian or Upper Ordovician)at surface, covers < 0.1 % of this area
Quincy Granite - Alkalic granite containing riebeckite and aegirine. Intrudes CAbw and PZZc (?). Quincy Granite intrudes Middle Cambrian Braintree Argillite. Southern contact is with chemically and mineralogically similar Blue Hills Granite Porphyry. Fresh rock is dark gray to dark green, weathering to buff brown or salmon. Bounded on the northwest by Blue Hills thrust and on the west by Neponset fault. No clasts of Quincy observed in Early Pennsylvanian Pondville Conglomerate, but clasts of overlying and probably related Blue Hills Granite Porphyry are common in Pondville. Combined area of Quincy and Blue Hills rocks is 55 sq km. Small mass of "Quincy Granite" east of Woonsocket, RI, near RI-MA border is similar in texture to Quincy Granite at Quincy, MA, but is more peralkaline. The mass near Woonsocket was thought at the time of compilation of the MA State geologic map (Zen and others, 1983) to be same general age as granite at Quincy. Age of mass at Woonsocket has been more recently determined to be Devonian or possibly Carboniferous, rather than Late Ordovician and Silurian (Hermes and Zartman (1985) (Wones and Goldsmith, 1991).
Dedham Granite - Light grayish-pink to greenish-gray, equigranular to slightly porphyritic, variably altered, granite south and west of Boston. Includes dioritic rock near Scituate and Cohasset and Barefoot Hills Quartz Monzonite of Lyons (1969) and Lyons and Wolfe (1971). Intrudes Zdi, Zgb, Zb, Zv. Extensive calc-alkaline plutons separated by Boston basin have long been mapped as Dedham. Those to the north of Boston and studied in this report, are referred to as Dedham North. Crystallization ages for the Dedham North suite (based on titanites and zircons) have been determined at 607+/-4 Ma, while ages for the Lynn are slightly younger at 596+/-3 Ma. Both are clearly part of the Late Proterozoic magmatic event. Dates on two samples from Sheffield Heights indicate that the diorite and granite are part of the Dedham North suite. The Dedham south and west of Boston has been dated at 630+/15 Ma (Zartman and Naylor, 1984). Dedham North Granite has a compositionally highly variable suite ranging from leucogranites to granodiorites, tonalites, and quartz diorite. The granites originated by partial melting of a sedimentary protolith, while the intermediate members show a mixing of granitic magma and mafic magma (Hepburn and others, 1993).
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.
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).
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).
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).
Sharon Syenite - Gray to dark-gray syenite containing microperthite, oligoclase and clinopyroxene, mixed with ferro-gabbro. Proterozoic Z Sharon Syenite forms long narrow mass along southeast side of Norfolk basin. Consists of gray to dark-gray syenite and minor ferrogabbro. Both rock types are intruded by Proterozoic Z Dedham Granite, but relationship to other Proterozoic Z gabbros is unknown. Near Sharon, unit is intruded by unnamed Proterozoic Z diorite (Wones and Goldsmith, 1991).
Westwood Granite - Light-gray to pinkish-gray, fine- to medium-grained granite. Intrudes Zdgr. Proterozoic Z Westwood Granite forms small lenses of light-colored granite intruding Proterozoic Z Dedham Granite and older rocks. Occurs within Dedham batholith. Some rocks resembling Westwood, such as those exposed in Plymouth quarries near Weymouth, have been mapped as part of Dedham Granite on MA State Geologic Map (Zen and others, 1983). Extensive intrusion breccias occur at contacts of Westwood with older mafic rocks. Contacts with Dedham are commonly abrupt; dikes of Westwood cut Dedham and rare inclusions of Dedham are found within Westwood (Chute, 1966). No reports of cobbles of Westwood within Roxbury Conglomerate; therefore, it is possible that Westwood is intrusive equivalent of Mattapan Complex that underlies Roxbury and that Westwood was not exposed to erosion at time of deposition of Roxbury. Also may be an intrusive equivalent of Lynn Volcanic Complex. A somewhat questionable Rb/Sr whole-rock age of 579 +/-28 Ma (Fairbairn and others, 1967) obtained from Westwood indicates it may be younger than Dedham and may be about same age as Mattapan (Wones and Goldsmith, 1991).
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.