NEW INTERPRETATION OF THE MERRIMACK GROUP IN SOUTHWESTERN MAINE AND SOUTHEASTERN NEW HAMPSHIRE

The Merrimack Group of southwestern Maine and southeastern New Hampshire, as originally defined by Billings (1956), included, in ascending stratigraphic order, the Kittery, Eliot, and Berwick Formations. However, the compilation a decade ago of the geological map of New Hampshire (Lyons and others, 1997) resulted in a reversal of that stratigraphic order. More recent mapping and reinterpretation in the compilation of the geology of the Kittery 1:100,000 map sheet, supported by the Maine Geological Survey, suggests further revision of the Merrimack Group. In New Hampshire the contact between the Eliot Formation and the Berwick Formation is a fault with early ductile and later brittle fault characteristics. Correlatives of the Berwick Formation in the Portland, ME, area are separated from sheared Eliot by the Nonesuch River Fault, a post-metamorphic high-angle fault marked by local zones of silicification. Direct data suggesting stratigraphic continuity between these two formations is lacking. Further supported by recent detrital zircon ages (Wintsch and others, in press) that indicate a somewhat younger Silurian (Wenlock or younger) provenance age, the Berwick Formation is separated from the Merrimack Group. It is probably more closely related to the lithically similar Vassalboro Formation in central Maine and the Oakdale and Paxton Formations in north-central Massachusetts. Rocks of the Macworth Formation, well-exposed in the Casco Bay area, are correlated with the Eliot Formation and are interbedded conformably with the uppermost Jewell Formation of the Casco Bay Group. This relationship supports the view that the Eliot Formation, not the Kittery (nor Berwick as argued above), is the basal unit of the Merrimack Group. No evidence has emerged from recent mapping suggesting other than a conformable stratigraphic contact between the Eliot and overlying Kittery Formation. A correlation of the Kittery and Eliot Formations with lithically similar rocks of the Bucksport Formation of the Fredericton Trough is reaffirmed. Ages of the Eliot and Kittery formations, as well as the Bucksport Formation, are probably latest Ordovician to early Silurian.