SILURIAN STRATIGRAPHY REVISITED: EXTENSIONS FROM NEW HAMPSHIRE!

In an AJS paper thirty years ago, Hatch, Moench, and Lyons, extended the Silurian and Lower Devonian stratigraphy of the Central Maine Belt from Maine into New Hampshire, establishing the framework for regional lithotectonic maps. Our latest mapping along the Androscoggin River in Gilead and Bethel, Maine, suggests that much of what was identified then as the Devonian Littleton Formation is instead the Silurian Rangeley Formation. Twelve metasedimentary units define the stratigraphy that has poor topping control pervaded by migmatite. The presence of rusty weathering schists and quartzites, biotite and calc-silicate granofels, and calc-silicate pods are all hallmarks of the Rangeley Formation in nearby New Hampshire and strengthen the correlation. The stratigraphy contains hints of the elusive Early Silurian Salinic orogeny in the form of: 1) bedded calc-silicate blocks that represent olistostromal facies; 2) rare quartz pebble conglomerates; and 3) offset bedding on either side of a pre-metamorphic fault.

D₁ Acadian isoclinal folding is characterized by a bedding (S₀)-parallel schistosity (S₁). In the few places where bedding is not parallel to schistosity, a macroscopic fold hinge is delineated, but is elsewhere obscured by migmatization. Several newly mapped, weakly foliated, quartz diorite-tonalite sills intruded syntectonically and represent an extension of the Piscataquis Magmatic Arc that developed on the edge of the migrating Acadian front. Migmatization developed during D₁ as leucosomes and melanosomes are parallel to schistosity. As in adjacent New Hampshire, the migmatite front closely follows the boundary between the Rangeley units (migmatized) and the Perry Mountain units (not migmatized) suggesting bulk compositional control on the temperature of partial melting or stratigraphic control on metamorphic fluid flux. Bedding, schistosity, and migmatite layering are folded about NE trending, shallow plunging axes that record the D₂ deformation. Early pegmatites are also folded by this deformation. D₂ is a continuation of the Acadian or the Late Devonian NeoAcadian orogeny. Several newly mapped, discordant, two mica granite and pegmatite plutons have been mapped. These are likely Permian to Carboniferous (?) and correlative to the Sebago batholith in southern Maine.