Geologic units in Hillsborough county, New Hampshire

Additional scientific data in this geographic area

Spaulding Tonalite (Early Devonian) at surface, covers 23 % of this area

(Spaulding Quartz Diorite of Fowler-Billings, 1949) - Weakly foliated to nonfoliated, spotted biotite quartz diorite, tonalite, granodiorite, and granite; garnet and muscovite may or may not be present.

Kinsman Granodiorite (Early Devonian) at surface, covers 17 % of this area

(Kinsman Quartz Monzonite of Billings, 1955) - Foliated granite, granodiorite, tonalite, and minor quartz diorite; large megacrysts of potassium feldspar characteristic; garnet locally abundant.

Massabesic Gneiss Complex (Late Proterozoic) at surface, covers 12 % of this area

Quartzose-feldspathic gneiss and biotite schists (locally rusty), granofels, and cal-silicate rocks closely intruded by, and grading into, a pink gneissic granite (623 Ma) that produced a migmatite.

Merrimack Group, Berwick Formation (Ordovician? - Silurian?) at surface, covers 8 % of this area

Purple biotite-quartz-feldspar granofels or schist and interbeds of calc-silicate granofels and minor metapelites. Stratigraphic sequence with respect to Eliot Formation uncertain

Upper part of Rangeley Formation (Lower Silurian (Llandoverian)) at surface, covers 7 % of this area

Rusty-weathering, pelitic schist, metasandstone, and local coarse-grained metasandstone lentils; calc-silicate pods common; minor coticule. Probably equivalent to member C of Rangeley Formation of Maine.

Merrimack Group, Berwick Formation, Unnamed member (Ordovician? - Silurian?) at surface, covers 6 % of this area

Contains more calc-silicate (15 percent) than does the remainder of the formation (5 percent).

Gray biotite granite (Permian) at surface, covers 6 % of this area

Contains minor muscovite. Found in Milford quadrangle.

Lower part of Rangeley Formation (Lower Silurian (Llandoverian)) at surface, covers 5 % of this area

Gray, thinly laminated (5-25 mm) metapelite with local lentils of turbidites and thin quartz conglomerates in western New Hampshire. Sparse calc-silicate pods and coticule. Probably equivalent to member B of Rangeley Formation of Maine.

Perry Mountain Formation, undivided (Lower? - Middle? Silurian) at surface, covers 5 % of this area

Sharply interbedded quartzites, light-gray nongraphitic metapelite, and "fast-graded" meta-turbidites. Coticule layers common.

Concord Granite (Late Devonian) at surface, covers 3 % of this area

Gray two-mica granite, locally grading to tonalite.

Two-mica granite of northern and southeastern New Hampshire (Early - Late Devonian) at surface, covers 2 % of this area

Similar to Concord Granite.

Littleton Formation, undivided (Lower Devonian; Siegenian) at surface, covers 2 % of this area

Gray metapelite and metawacke and subordinate metavolcanic rocks; generally, but not everywhere, conformable with underlying Fitch or Madrid Formations. Fossiliferous in western New Hampshire.

Rangeley Formation, undivided (Lower Silurian (Llandoverian)) at surface, covers 2 % of this area

Rangeley Formation, undivided.

Smalls Falls Formation, undivided (Upper to Middle Silurian (Ludlovian and Wenlockian)) at surface, covers 2 % of this area

Very rusty weathering, thinly bedded sulfidic-graphitic schist and pyrrhotitic calc-silicate granofels. Eastern facies equivalent to lower part of the Fitch Formation. Locally mapped as Francestown Formation of Nielson (1981) in southern New Hampshire.

Ayer Granodiorite (Early Silurian) at surface, covers 0.6 % of this area

Gneissic granite to tonalite, locally coarsely porphyritic and muscovitic, southeastern New Hampshire.

Madrid Formation (Upper Silurian?) at surface, covers 0.6 % of this area

Massive to weakly foliated, purple biotite-feldspar granofels, layered calc-silicate, and dark pelitic-sulfidic schist containing calc-silicate pods in upper member; an eastern facies equivalent to the upper part of the Fitch Formation. Locally mapped as the Warner Formation of Nielson (1981) in southern New Hampshire.

Exeter Diorite (Early Devonian) at surface, covers 0.3 % of this area

Includes associated intrusive rocks of southeastern New Hampshire; pyroxene and pyroxene-hornblende diorite and gabbro, along with minor granodiorite and granite.

Merrimack Group, Berwick Formation, Gove Member (Ordovician? - Silurian?) at surface, covers 0.2 % of this area

White muscovite schist. Equivalent to the Gonic Formation of Hussey (1962).

Rangeley Formation, Pink to green calc-silicate and purple biotite granofels (Lower Silurian (Llandoverian)) at surface, covers 0.1 % of this area

Thinly bedded. Close to transition from lower to upper parts of the Rangeley Formation. Probably equivalent to part of Paxton Formation of Zen and others (1983) in Massachusetts.

Rangeley Formation, Felsic metavolcanic rocks (Lower Silurian (Llandoverian)) at surface, covers < 0.1 % of this area

Found in south-central New Hampshire.

Massabesic Gneiss Complex (Late Proterozoic) at surface, covers < 0.1 % of this area

Migmatite consisting of pink, foliated biotite granite intruding gneissic and granulose metasedimentary and metavolcanic rocks in southeastern New Hampshire.

Oakdale Formation (Silurian) at surface, covers < 0.1 % of this area

Metamorphosed thin-bedded, pelitic and calcareous siltstone and muscovite schist, probably low-grade equivalent of Paxton Formation. The Oakdale Formation is here revised to include strata previously mapped in CT and adjacent MA as the Hebron Formation and the Scotland Schist. The Scotland Member (Pease, 1980) is renamed the Scotland Schist Member of the Oakdale. The Oakdale is a homogeneous, calcareous metasiltstone at the base of a thick stratigraphic sequence in a geosyncline terrane and extends from NH to the Honey Hill fault in eastern CT. In central eastern CT it underlies the Hebron Formation; in northeast CT and adjacent MA it underlies conformably the Dudley Formation of the Paxton Group; in central MA it underlies the Paxton Group undivided. The lower part of the Oakdale is cut out along the Clinton-Newbury fault zone. Thickness in type area is about 1500 m. Correlative with the Gove Member of the Berwick Formation in NH and the Gonic Formation in ME. Age is Late Proterozoic(?) based on intrusion of 440 Ma Hedgehog Hill gneiss in the upper part of the Brimfield Group at the top of the stratigraphic sequence, and an age of 1188 Ma for detrital zircons from the Paxton in north-central MA (Pease, 1989).

Berwick Formation (Silurian) at surface, covers < 0.1 % of this area

Thin- to thick-bedded metamorphosed calcareous sandstone, siltstone, and minor muscovite schist. In New Hampshire: Used as Berwick Formation of Merrimack Group. Consists of purple biotite-feldspar granofels or schist. Contains interbeds of calcsilicate granofels and minor metapelites. Includes Gove Member, mapped separately. Stratigraphic sequence with respect to Eliot Formation is uncertain. Age of all formations in Merrimack Group changed to Ordovician(?) to Silurian(?) based on isotopic age determinations of approx 440 and 420 Ma from detrital zircons from Berwick by J.N. Aleinikoff (oral commun., 1994) (Lyons and others, 1997).

Diorite and tonalite (Devonian and Silurian) at surface, covers < 0.1 % of this area

Includes Dracut Diorite, tonalite near the Ayer Granite, and equivalents of the Exeter Diorite of New Hampshire; intrudes Sb.

Paxton Formation (Silurian) at surface, covers < 0.1 % of this area

Undifferentiated biotite granofels, calc-silicate granofels, and sulfidic schist. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Chelmsford Granite (Lower Devonian) at surface, covers < 0.1 % of this area

Light-gray, even and medium-grained, muscovite-bearing granite; locally foliated; intrudes Sb.

Ayer Granite (Lower Silurian) at surface, covers < 0.1 % of this area

Granite to tonalite, partly porphyritic; locally gneissic, locally muscovitic; may include rocks older than Silurian; intrudes Sb and So. Ayer Granite is divided into the Clinton facies and the Devens-Long Pond facies (Gore, 1976). In addition, there are some masses not assigned to either facies that intrude Berwick Formation west and northwest of Lawrence, and that intrude Paxton and Oakdale Formations south of Worcester and west of probable southern continuation of Wekepeke fault. Radiometric ages obtained for facies of Ayer pose problems in assigning ages to unfossiliferous sedimentary rocks they intrude. Clinton facies has a well-defined U-Pb zircon age of 433 +/-5 Ma (Zartman and Naylor, 1984) that authors cite as Early Silurian; Devens-Long Pond facies has a similar age. This age greatly compresses the time available for deposition, burial, deformation, and metamorphism of Berwick and Paxton if these units are truly Silurian. Some of the Ayer not assigned to a facies may have been more properly correlated with Early Devonian Chelmsford Granite and muscovite-biotite granite at Millstone Hill. Bodies south of Worcester may be more properly correlated with Canterbury Gneiss of CT, which lies on strike with Ayer and has Early Devonian age of 329 +/-9 Ma (Zartman and Naylor, 1984). Zartman and Naylor (1984) believe Ayer Granite has same age range as Newburyport Complex. It is quite possible, based on textural and mineralogical differences that the two facies should be separate units, representing different magmatic events (Wones and Goldsmith, 1991).

Massive to weakly foliated, pink and gray, fine- to medium-grained biotite granite (Pennsylvanian) at surface, covers < 0.1 % of this area

In the Townsend area; commonly contains pink magnetite-bearing pegmatite identical to granite of Milford, New Hampshire; intrudes OZma and Sp.