Newbury Volcanic Complex - Lower Members

Basalt, andesite, rhyolite, and tuff.
State Massachusetts
Name Newbury Volcanic Complex - Lower Members
Geologic age Lower Devonian and Upper Silurian
Lithologic constituents
Igneous > Volcanic > Felsic-volcanic > Rhyolite
Igneous > Volcanic > Mafic-volcanic > Basalt
Igneous > Volcanic > Mafic-volcanic > Andesite
Igneous > Volcanic > Felsic-volcanic (Pyroclastic-tuff)
Comments Part of Milford-Dedham Zone (Tertiary and Older Rocks). Additional unit description from ref. 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. 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).]
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Counties Essex