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DEPARTMENT OF THE INTERIOR Prepared in cooperation with the OPEN-FILE REPORT 2006-1011

U. S. GEOLOGICAL SURVEY POMPERAUG RIVER WATERSHED COALITION PLATE 1 OF 1

Southbury, CT 

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7 02 

73 09

' 38" ' " 

41 35 06 ' " 

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DESCRIPTION OF MAP UNITS 

(minerals in pre-Mesozoic rocks listed in ascending order of abundance)

(For more complete discussion and description of units, see Burton and others, 2005) 

South Brook Basalt. Dark gray to black, amygdaloidal Js basalt exposed only in South Brook in Woodbury.

Primary texture is intergranular, with abundant 

microlites of plagioclase. Strongly altered due to 

proximity to eastern border fault and associated 

mineralization, which has produced secondary 

CORRELATION OF MAP UNITS 

30 

64 mineralization in amygdules and groundmass of 45 secondary chlorite and calcite. Locally at least 35 m

(well SB-33)

Mesozoic 50

sedimentary and 83 Jw White Oaks Formation. Exposed only in South Brook, as volcanic rocks Paleozoic Paleozoic Dg a conglomerate immediately underlying and in contact

metamorphic of the intrusive igneous 35 40 

46 86 

with South Brook Basalt (Js). Contains subrounded, 78 Dg 29 

rocks Pomperaug basin rocks 23 66 Dg equant to elongate clasts up to 2 cm length of quartzite, 40 83 80 siltstone, shale, marble, and garnet-mica schist, in finerOtb grained matrix consisting mostly of subangular to 

subrounded grains of quartz, feldspar, and garnet in a 30 34

88 26 88 47 

43 80 secondary calcite cement. Southern extent defined by 

Js South Brook Basalt 55 

45 

47 well records and quarried blocks in Southbury. Locally 40 

43 

77 Dp 46 51 at least 31 m (well SB-33) 34 

39 58 88

Jw White Oaks Formation 

46 

47 

60 80 35 Otbs 38 Orenaug Basalt. Dark gray, massive basalt that is well 8 Jo 

exposed and dominant ridge-f in Pomperaug 

-deve oped co

and cons

78 74 88 ormer

Jo Orenaug Basal 53 t 32 basin. Locally contains well

j i ing. Texture is i

30 72 79 Dg umnar

nt ntergranu i f

aths up to 2 mm long, 

60 

o ar sts o Dg 31 plagioclase l interstitial

Jc Cass Formation 75 78 73 clinopyroxene, 43 43 indicate at least 2

is about 80 m 

and opaques. Quarry exposures

flow units; total thickness of basalt 11

Je East Hill Basalt 82

Otba 47 45 

ORDOVICIAN SILURIAN DEVONIAN TRIASSIC JURASSIC 

Cass Formation. Fine-grained, thin-bedded, gray to

Trs South Britain Formation 17 Jc green sandstone and siltstone, consisting mostly of 62 67 subangular grains of quartz and minor plagioclase and 

unconformity 37 detrital mica in a matrix of calcite and lesser hematite.

Ot 

45 Total thickness about 35-40 m. In Woodbury and Dp Pegmatite 78 87 

70 1 Southbury quarries exposed upper section reveals a 3 

m-thick, fine-grained, well-sorted sandstone of aeolian 

origin, overlain by a 1 m thick cross-bedded white

Dg Granite 36 

sandstone at contact with overlying basalt. In Cass

Brook, formation contains a thin (<1m) interval of buff, 54 2

DSt The Straits Schist 3 fossiliferous limestone and black slate. Pebbly arkose 

and fanglomerate near top of section formerly exposed

in Southbury quarry 

32

Oca Occ 28

Oc Collinsville Formation

Ocb Ocq

Je East Hill Basalt. Green to gray weathering, highly 

vesicular basalt. Poorly exposed, with characteristic 

Ot Taine Mountain Formation 25 4 spheroidal weathering. Commonly altered, with 

25 calcite-filled vesicles and chloritized matrix. Total 

38 thickness about 7 m

Otba Otb Otbs Taine Mountain Formation, basal member 68 

32 

thrust 36 South Britain Formation. Light to dark red arkose, Trs 

Ora commonly medium-grained, but with local pebbly

Ork Orb 24 horizons and fine-grained zones. Pebbles consist Or Ratlum Mountain Schist 25

Orq 20 

mostly of white quartz and lesser plagioclase and Orc