|Name||Newbury Volcanic Complex|
|Geologic age||Lower Devonian and Upper Silurian|
Igneous > VolcanicUndivided sedimentary and volcanic rocks.
SedimentaryUndivided sedimentary and volcanic rocks.
|Comments||Part of Milford-Dedham Zone (Tertiary and Older Rocks). Secondary unit description per MA023 as reported in USGS Geologic Names lexicon (ref. MA024): Newbury Volcanic Complex of Late Silurian and Early Devonian age consists of sedimentary and volcanic rocks confined to two wedge-shaped and lenticular fault-bounded basins aligned between Newburyport and the Middleton-Topsfield area. Volcanic materials are composed largely of basalt flows; flow-banded rhyolite, vitrophyre, and ash-flow tuff; and porphyritic andesite flows, breccias, and tuff, partly waterlain. Upper part of complex consists of shallow marine siliceous siltstone, red sandy mudstone containing some volcanic detritus, and calcareous mudstone. Porphyritic andesite contains shelly fauna indicating latest Silurian (Pridolian) age. Calcareous mudstone of upper part contains ostracodes having a greater age than the fauna in andesite, and could be as young as earliest Devonian (Gedinnian). Nether the top or bottom is known. Equivalent to Leighton Formation of Pembroke area, ME, on the basis of correlation of shelly fauna. Ames Knob Formation and Thorofare Andesite of Penobscot region, ME, are also temporal equivalents. On basis of similar lithologies, Newbury has been equated in the past to Mattapan and Lynn Volcanic Complexes, but they are now believed to be Proterozoic Z in age (Goldsmith, 1991).|
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., and 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.
USGS Geologic Names lexicon found at: http://ngmdb.usgs.gov/Geolex/