The Newbury Volcanic Complex of northeasternmost Massachusetts is encompassed, except for one small occurrence, in a northeast-trending wedge-shaped belt bounded by regional faults. Ten stratified members, aggregating more than 4,400 m in thickness and intruded by subconcordant podlike masses of micrographic rhyolite, are disposed in a homoclinal sequence that is overturned and broken into blocks by minor faults. In contrast to previous descriptions, this report proposes that the sequence has been inverted and that it neither unconformably overlies the granitic pluton that borders it on the southeast nor is intruded by that pluton, as previously has been suggested. The lower 2,900 m of the complex are entirely volcanic materials. In this interval, the great volume of strata is contained in three of the members: a fine-grained basalt flow member; a thick flow-banded rhyolite vitrophyre member that locally includes at its base as much as 120 m of ash-flow tuff, and a porphyritic andesite member that contains numerous flows but is dominated by flow breccias and tuffs. Most of the andesites accumulated subaerially, but andesitic detritus was also laid down in a shallow marine environment. Two lesser members of rhyolitic tuff, a minor lens of dense basalt(?), and a fine-grained andesite(?) member partly of epiclastic materials complete the lower part of the Newbury. The upper 1,500 m of the complex is composed of three very poorly delineated sedimentary members: a siliceous siltstone, a red sandy mudstone, and a calcareous mudstone. The red mudstone member contains some volcanic detritus. The calcareous mudstone member is ostracodal, and the porphyritic andesite member contains at several sites a shelly fauna that provides specific correlation with the Leighton Gray Shale Member of the Pembroke Formation of the Eastport area of Maine. In the latter locality, ostracodes indicate affinities with fauna of the Baltic-British region rather than those of the Appalachian region and provide the basis for suggesting that the porphyritic andesite member of the Newbury may be wholly of Pridoli (latest Silurian) age. Ostracodes of the calcareous mudstone member may be longer ranging; therefore, the possibility that higher strata of the Newbury are as young as Gedinnian (earliest Devonian) cannot be ruled out. Recent paleontologic and radiometric dating indicates that the uppermost part of the Ames Knob Formation and the Thorofare Andesite in the southern Penobscot Bay area and the Castine Volcanics in the northern Penobscot Bay area (all more than 200 km up the coast in Maine) are also temporal equivalents.
The Newbury was not affected by mild regional metamorphism, nor was it invaded by granitic plutons during the Acadian orogeny, as were some of the Silurian-Devonian terranes of coastal Maine. The lithologically comparable Lynn and Mattapan Volcanic Complexes, of the nearby Boston area, apparently were similarly unaffected by metamorphism; correlation of the Newbury with these formations is not yet proved.