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Nashoba Formation (MAOZn;0)

Geologic Formation in MA

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Lithology

Marble

Amphibolite

Schist

Other (201 mi²)

Geologic Age

Neoproterozoic – Ordovician (1000 Ma – 445.3 Ma ago)

Geologic Unit Label

OZn

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).

— USGS

Taxonomy

Classified As

Geologic Formation