LATE SILURIAN TO EARLY DEVONIAN MAFIC VOLCANISM AND THE CHANGE TO EARLY DEVONIAN SEDIMENTATION IN THE CONNECTICUT VALLEY – GASPÉ TROUGH, NORTHERN MAINE

Previous tectontic models for selected Late Silurian to Early Devonian mafic rocks of the Piscataquis volcanic belt in northern Maine have been interpreted as being related to crustal rifting, slab detachment during subduction, or other subduction - related models. Recent mapping at the northern end of the Picataquis volcanic belt and the adjacent Connecticut Valley – Gaspé Trough suggest eruption and deposition within a rifted environment. In this area mafic volcanics are consistent with effusive and explosive volcanism in both shallow marine and subaerial environments. Rocks erupted into the marine environment exhibit pillow structures of varying size and shape, with locally present agglomerates or breccias. In the marine setting, sedimentary rocks of various compositions, some of which are fossil bearing, were deposited between flows. Non marine flows exhibit of a range of thicknesses from approximately 2 to 20 or more meters. Amygdules or vesicles are common in these rocks and locally are concentrated near the top of the flow. Baked contacts and/or reddish gray slates interpreted as soil horizons are locally present. Non marine mafic flows are commonly porphyritic while pillowed flows lack phenocrysts. Peperite is locally present between flows.

In northern Maine, the primary mafic volcanic unit is the Spider Lake Formation whose outcrop belt is a minimum of 65 km long and 15 km wide. To the south and west of the Spider Lake outcrop belt are additional units such as the Allagash Lake and Five Mile Brook formations, and other minor units. These outliers suggest that the mafic volcanic province was wide spread.

Regional subsidence followed the eruptive episodes. Mafic volcanic rocks, and the sedimentary rocks of the Connecticut Valley – Gaspé trough unconformably overlie, or are in fault contact with a variety of Late Ordovican arc – related units. On the southeastern - most side of the Connecticut Valley – Gaspé outcrop belt a basal conglomerate overlies Late Ordovician arc volcanics. Further to the northwest, slates of the Connecticut Valley – Gaspé trough disconformably overlie subaerial to marginal marine rocks of the Spider Lake Formation. Silurian and Devonian age subduction related complexes are lacking in this region.