|Name||Merrimack Group, Berwick Formation, Unnamed member|
|Geologic age||Ordovician? - Silurian?|
Metamorphic > Schist
Metamorphic > Granoblastic > Granofelspurple biotite-quartz-feldspar granofels, interbeds of calc-silicate granofels (Contains more calc-silicate (15 percent) than does the remainder of the formation (5 percent)).
Metamorphic > Metasedimentary > Calc-silicate-rock
|Comments||Part of the Nashoba-Casco-Miramichi Terrane (Merrimack Trough). Secondary unit description from USGS Geologic Names lexicon (ref. NH047): Used as Berwick Formation of Merrimack Group. Consists of purple biotite-feldspar granofels or schist. Contains interbeds of calcsilicate granofels and minor metapelites. Includes Gove Member, mapped separately. Stratigraphic sequence with respect to Eliot Formation is uncertain. Age of all formations in Merrimack Group changed to Ordovician(?) to Silurian(?) based on isotopic age determinations of approx 440 and 420 Ma from detrital zircons from Berwick by J.N. Aleinikoff (oral commun., 1994). Onway tholeiite dike (point within formation) = 177 K/Ar, whole rock per NH042.|
Lyons, J.B., Bothner, W.A., Moench, R.H., and Thompson, J.B., Jr., 1997, Bedrock Geologic Map of New Hampshire: Reston, VA, U.S. Geological Survey Special Map, 2 sheets, scale 1:250,000.
Sundeen, D.A., and Huff, M.C., 1991, K-Ar ages and characterization of Mesozoic mafic and felsic dikes in southeastern New Hampshire [abs.]: Geological Society of America Abstracts with Programs, v. 23, p. 136.
USGS Geologic Names lexicon found at: http://ngmdb.usgs.gov/Geolex/
|Counties||Hillsborough - Rockingham|