Braintree Argillite and Weymouth Formation

Argillite, some with rare limestone; contains Middle and Early Cambrian faunae respectively.
State Massachusetts
Name Braintree Argillite and Weymouth Formation
Geologic age Middle and Lower Cambrian
Lithologic constituents
Metamorphic > Metasedimentary > Metaclastic > Argillite
Sedimentary > Carbonate > Limestone
Comments Part of Milford-Dedham Zone (Tertiary and Older Rocks). Secondary unit description per MA023 as reported in USGS Geologic Names lexicon (ref. MA024): Braintree Argillite is present only south of Boston basin in and around Quincy Granite. Consists of noncalcareous green to dark-gray or black massive slate or argillite. Adjacent to Quincy Granite, it is indistinctly bedded greenish-gray hornfels. Thickness estimated at 1000 m (LaForge, 1932). Contact with underlying Weymouth Formation not exposed; top of formation not known. No measured section has been done by any worker. Faunal assemblage is characterized by Middle Cambrian representatives of PARADOXIDES (trilobite) (Goldsmith, 1991). Weymouth Formation consists of greenish-gray and dark-red slate containing calcareous nodules, and subordinate beds and lenses of argillaceous limestone. Locally converted to hornfels by intrusion of Quincy Granite. At Nahant, MA, contains layer of greenish-white limestone. Lithology is similar to Hoppin Formation at Hoppin Hill, MA, except that lower quartzitic part of section has not been recognized in the Weymouth. No complete section of Weymouth has been described by any worker. Base is not exposed and can only be inferred to be at least 100 m thick (LaForge, 1932). Fossils in Weymouth are largely conoidal forms, but include some olenellids. Age is Early Cambrian (Goldsmith, 1991).
NGMDB product
Counties Norfolk