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Cambridge Argillite

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).
StateMassachusetts
NameCambridge Argillite
Geologic ageProterozoic Z to earliest Paleozoic
Original map labelPzZc
CommentsPart of Milford-Dedham Zone (Tertiary and Older Rocks) Secondary unit description per MA023.
Primary rock typeargillite
Secondary rock typequartzite
Other rock typessandstone; conglomerate
Lithologic constituents
Major
Metamorphic > Metasedimentary > Metaclastic > Argillite
Minor
Metamorphic > Metasedimentary > Metaclastic > Quartzite
Incidental
Sedimentary > Clastic > Sandstone
Sedimentary > Clastic > Conglomerate
Map references
Unpublished Digital Geologic Map of Massachusetts received from Rudi Hon at Boston College in 1998.
Unit references
Zen, E-An (ed.), Goldsmith, R. (comp.), Ratcliffe, N.M. (comp.), Robinson, P. (comp.), Stanley, R.S. (comp.), Hatch, N.L., Jr., Shride, A.F., Weed, E.G.A., Wones, D.R., 1983, Bedrock Geologic Map of Massachusetts: U.S. Geological Survey, Reston, VA, scale 1: 250,000.
Goldsmith, Richard, 1991, Stratigraphy of the Milford-Dedham zone, eastern Massachusetts; an Avalonian terrane, Chapter E, IN Hatch, N.L., Jr., ed., The bedrock geology of Massachusetts: U.S. Geological Survey Professional Paper, 1366-E-J, p. E1-E62.
CountiesEssex - Middlesex - Norfolk - Plymouth - Suffolk

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