STRATIGRAPHY AND STRUCTURE OF THE MT. PAWTUCKAWAY QUADRANGLE, SOUTHEASTERN NEW HAMPSHIRE

Abstract

The chief problem in the Mt. Pawtuckaway quadrangle was to determine the relation of the early and middle Paleozoic rocks of western and central New Hampshire to supposed Pennsylvanian rocks of southeastern New Hampshire. A related problem was to determine the stratigraphic relations of the Berwick “gneiss” which Katz considered pre-Algonkian. The Mt. Pawtuckaway quadrangle is an area of metamorphosed sedimentary rocks intruded by plutonic rocks of the late Devonian (?) New Hampshire magma series and the Mississippian (?) White Mountain magma series. The metamorphic rocks are assigned to three stratigraphic units: the Eliot, Berwick, and Littleton formations. The Eliot formation consisted originally of interbedded shale, sandstone, and dolomitic limestone; these rocks were metamorphosed to phyllite, quartz-mica schist, quartzite, and lime-silicate rocks. The Calef member at the top of the Eliot formation was originally a carbonaceous shale; it is now a dark phyllite. The Berwick formation was composed of interbedded shale, sandstone, shaly sandstone, and dolomitic limestone; these rocks have been metamorphosed to phyllite, quartzite, quartz-mica schist, and lime-silicate rock. The Littleton formation consisted originally of sandstone, shaly sandstone, and shale; these rocks were metamorphosed to quartzite, mica-schist, mica-garnet schist, mica-staurolite schist, and mica-sillimanite schist. The New Hampshire magma series, which is rather widespread in the central and northwestern parts of the quadrangle ranges from gabbro to granite. It has been divided into five units on the basis of lithologic characteristics and distribution: Exeter diorite, quartz diorite, quartz monzonite, binary granite, and microcline granite. They are dominantly massive rocks, but locally foliation is conspicuous. A small, circular pluton of the White Mountain magma series, ranging from gabbro to monzonite, was emplaced by forceful injection and cauldron subsidence in the Pawtuckaway Mountains in the central part of the quadrangle. Three smaller bodies of the White Mountain magma series at the southern border of the quadrangle in Raymond township were emplaced by forceful injection and volcanic eruption. They are composed of coarse-grained rocks such as gabbro, hypersthene gabbro, and hypersthene monzonite, and fine-grained rocks such as andesite, quartz latite, keratophyre, and rhyolite. Minor folds, lineation, rare fracture cleavage and a structural terrace considered as a drag fold on Saddleback Mountain indicate that the top of the beds is dominantly to the northwest. Thus the metamorphic rocks form a northwesterly dipping homocline within the limits of the quadrangle. Metamorphism decreases southeast and northwest from the granitic pluton. Chlorite, biotite, garnet, staurolite, and sillimanite zones are present. An actinolite isograd occurs in the biotite zone. The Berwick formation conformably overlies the Eliot formation and underlies the Littleton formation. The Devonian Littleton formation has been traced to the Mt. Pawtuckaway quadrangle from western New Hampshire, where it is dated by fossils. Correlation with the mid-Silurian Waterville slate in Waterville, Maine, indicates that the Eliot and Berwick formations are probably Silurian.