Bell and Alvord (1976) placed Tadmuck Brook Schist above Nashoba Formation. Tadmuck Brook is truncated at top by Clinton-Newbury fault. Underlies Silurian rocks in Merrimack synclinorium in and west of fault zone. However, in places in fault zone, Tadmuck Brook is difficult to distinguish from schist and phyllite of synclinorium sequence. In addition, Alvord and others (1976) suggested that Tadmuck Brook Schist might lie unconformably on the Nashoba Formation because it truncates units of the Nashoba. Therefore, age of Tadmuck Brook Schist is changed to Proterozoic Z, Ordovician, or Silurian(?).

Source: GNU records (USGS DDS-6; Reston GNULEX).

Used as Tadmuck Brook Schist of Proterozoic Z, Ordovician, or Silurian(?) age. Consists of andalusite phyllite and sillimanite schist, partly sulfidic, with local quartzite at top.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Tadmuck Brook Schist occurs in Nashoba zone of eastern MA. Bell and Alvord (1976) gave this name to largely pelitic (schist and phyllite) rocks lying west and presumably above Nashoba Formation. Amphibolite and quartzite appear near its base to the east. According to Bell and Alvord, Tadmuck Brook intertongues locally with quartzofeldspathic layers of Nashoba, but, from Littleton to vicinity of Lawrence, Tadmuck Brook appears to truncate units of the Nashoba, suggesting an unconformity or disconformity between the two formations. They also noted that the contact is sometimes a fault. Tadmuck Brook rocks are probably represented by Skehan and Moustafa's (1976) unnamed units U1-U9 (mostly quartzite lying unconformably on Nashoba) in Wachusett-Marlborough tunnel. Hepburn (1978) was unable to identify Tadmuck Brook south of Shrewsbury where Boylston Schist of Merrimack belt lies against Nashoba. To the north, Tadmuck Brook is truncated at the top by Clinton-Newbury fault system. Skehan and Murray (1980) suggested a Cambrian to Ordovician(?) age from Tadmuck Brook on basis of unconformity at its base and the Ordovician to Silurian age for Ayer Granite that intrudes Tadmuck Brook and Nashoba rocks in and west of Clinton-Newbury fault zone. Age shown as Proterozoic Z to Silurian in figure 2. Report includes geologic maps and correlation charts. [Chapters A-J in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to 1:250,000-scale MA State bedrock geologic map of Zen and others (1983).]

Source: GNU records (USGS DDS-6; Reston GNULEX).