Geologic units in Rockingham county, New Hampshire

Additional scientific data in this geographic area

Merrimack Group, Berwick Formation (Ordovician? - Silurian?) at surface, covers 27 % 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

Merrimack Group, Eliot Formation (Ordovician? - Silurian?) at surface, covers 17 % of this area

Gray to green phyllite, calcareous quartzite, quartz-mica schist, and well-bedded calc-silicate.

Massabesic Gneiss Complex (Late Proterozoic) at surface, covers 13 % 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, Unnamed member (Ordovician? - Silurian?) at surface, covers 10 % of this area

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

Merrimack Group, Kittery Formation (Ordovician? - Silurian?) at surface, covers 7 % of this area

Tan, graded-bedded, calcareous metasandstone and purple and green phyllite. Grades into the Eliot formation but facing direction is uncertain.

Concord Granite (Late Devonian) at surface, covers 7 % of this area

Gray two-mica granite, locally grading to tonalite.

Exeter Diorite (Early Devonian) at surface, covers 4 % of this area

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

Rye Complex (Ordovician? - Late Proterozoic?) at surface, covers 4 % of this area

Light-colored to gray schists and gneisses, quartzites, and amphibolites. Variably migmatized and mylonized. Contact with Kittery Formation on west is the Portsmouth Fault.

Perry Mountain Formation, undivided (Lower? - Middle? Silurian) at surface, covers 3 % of this area

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

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

Similar to Concord Granite.

Rye Complex, Breakfast Hill Granite of Novotny (1964). (Ordovician? - Late Proterozoic?) at surface, covers 1 % of this area

Blastomylonitic quartz-feldspar granitic gneiss and pegmatite intruded the Rye Complex and formed a migmatite.

Merrimack Group, Berwick Formation, Gove Member (Ordovician? - Silurian?) at surface, covers 1 % of this area

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

Merrimack Group, Eliot Formation, Calef Member (Ordovician? - Silurian?) at surface, covers 0.9 % of this area

Black phyllite at western contact of the Eliot Formation.

Newburyport Complex (early Late Silurian) at surface, covers 0.8 % of this area

Gray, medium-grained tonalite and granodiorite.

Smalls Falls Formation, undivided (Upper to Middle Silurian (Ludlovian and Wenlockian)) at surface, covers 0.7 % 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.

Pink equigranular biotite granite (Late Devonian) at surface, covers 0.4 % of this area

Found in Woodsville and Whitefield quadrangles and in small intrusive units in northern and southeastern New Hampshire.

Littleton Formation, undivided (Lower Devonian; Siegenian) at surface, covers 0.4 % 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.

Newburyport Complex (Late Silurian) at surface, covers 0.4 % of this area

Medium-grained porphyritic granite.

Augite-hornblende diorite and gabbro (Cretaceous) at surface, covers 0.2 % of this area

Gray to black, coarse-grained, porphyritic. Found in Mt. Pawtuckaway quadrangle.

Ayer Granodiorite (Early Silurian) at surface, covers 0.2 % of this area

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

Gray augite-hornblende-biotite monzonite (Cretaceous) at surface, covers 0.1 % of this area

Gray augite-hornblende-biotite monzonite.

Newburyport Complex (Silurian) at surface, covers < 0.1 % of this area

Gray, medium-grained porphyritic granite with microcline phenocrysts; intrudes SOk. Newburyport Complex was divided into two facies, tonalitic granodiorite and granite, by Shride (1971). Tonalitic facies was originally termed Newburyport Quartz Diorite and included dioritic rocks north of Clinton-Newbury fault zone that are now called Sharpners Pond Diorite in Nashoba zone, and Topsfield Granodiorite in Milford-Dedham zone. These correlations are no longer tenable due to differences in age and composition. Therefore, Newburyport Complex is restricted to the two facies present in Newburyport area. Rocks formerly mapped as Newburyport Quartz Diorite and Salem Gabbro-Diorite, except for gabbros at Salem Neck, MA, are included in undifferentiated diorite and gabbro unit (Zdigb) on MA State Geologic Map (Zen and others, 1983), largely because they could not be mapped separately at 1:250,000 scale. Unit Zdigb also includes mafic dikes and sills that are probably younger or contemporaneous. Most of the dioritic rocks northeast of Boston previously assigned to Newburyport Quartz Diorite are now assigned to an undifferentiated diorite unit (Zdi) on MA State Geologic Map. Newburyport Complex forms a large mass near Newburyport and a small one to its west, both truncated by Clinton-Newbury fault. Tonalite and granodiorite facies occupies core of Newburyport Complex at Newburyport and is intruded to the north by granite facies; described as medium to dark gray in fresh rock, weathering to both green and red, fine to medium grained, and highly variable in mineralogy. A U-Pb zircon age of 455 +/-15 Ma was determined by Zartman and Naylor (1984) for the tonalite. Granite facies intrudes both the Kittery Formation and the tonalite and granodiorite facies and covers an area of about 45 sq km. Described as light gray to dark gray, buff weathering, and porphyritic. No radiometric ages available for granite facies, but it is conceivable that the two facies are different in age. [Papers presented as chapters in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to MA State bedrock geologic map of Zen and others (1983) at scale of 1:250,000.] (Wones and Goldsmith, 1991).

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).

Newburyport Complex (Silurian or Ordovician) at surface, covers < 0.1 % of this area

Gray, medium-grained tonalite and granodiorite. Newburyport Complex was divided into two facies, tonalitic granodiorite and granite, by Shride (1971). Tonalitic facies was originally termed Newburyport Quartz Diorite and included dioritic rocks north of Clinton-Newbury fault zone that are now called Sharpners Pond Diorite in Nashoba zone, and Topsfield Granodiorite in Milford-Dedham zone. These correlations are no longer tenable due to differences in age and composition. Therefore, Newburyport Complex is restricted to the two facies present in Newburyport area. Rocks formerly mapped as Newburyport Quartz Diorite and Salem Gabbro-Diorite, except for gabbros at Salem Neck, MA, are included in undifferentiated diorite and gabbro unit (Zdigb) on MA State Geologic Map (Zen and others, 1983), largely because they could not be mapped separately at 1:250,000 scale. Unit Zdigb also includes mafic dikes and sills that are probably younger or contemporaneous. Most of the dioritic rocks northeast of Boston previously assigned to Newburyport Quartz Diorite are now assigned to an undifferentiated diorite unit (Zdi) on MA State Geologic Map. Newburyport Complex forms a large mass near Newburyport and a small one to its west, both truncated by Clinton-Newbury fault. Tonalite and granodiorite facies occupies core of Newburyport Complex at Newburyport and is intruded to the north by granite facies; described as medium to dark gray in fresh rock, weathering to both green and red, fine to medium grained, and highly variable in mineralogy. A U-Pb zircon age of 455 +/-15 Ma was determined by Zartman and Naylor (1984) for the tonalite. Granite facies intrudes both the Kittery Formation and the tonalite and granodiorite facies and covers an area of about 45 sq km. Described as light gray to dark gray, buff weathering, and porphyritic. No radiometric ages available for granite facies, but it is conceivable that the two facies are different in age. [Papers presented as chapters in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to MA State bedrock geologic map of Zen and others (1983) at scale of 1:250,000.] (Wones and Goldsmith, 1991).

Mesoperthitic granite (Jurassic) at surface, covers < 0.1 % of this area

Contains riebeckite and (or) hastingsite.

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

Phyllite and calcareous phyllite. In New Hampshire: Used as Eliot Formation of Merrimack Group. Consists of gray to green phyllite, calcareous quartzite, quartz-mica schist, and well-bedded calc-silicate. Includes Calef Member, mapped separately. 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 Formation (of Merrimack Group) by J.N. Aleinikoff (oral commun., 1994) (Lyons and others, 1997).

Augite-hornblende-biotite gabbro (Cretaceous) at surface, covers < 0.1 % of this area

Fine- to coarse-grained.

Mesoperthitic biotite granite (Early Cretaceous) at surface, covers < 0.1 % of this area

Pink Conway-type granite of Kingsley (1931).

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

Mica 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).