Preakness Basalt

(Olsen, 1980) - Dark-greenish-gray to black, very-fine-grained, dense, hard basalt composed mostly of intergrown calcic plagioclase (An55-60) and clinopyroxene (pigeonite and augite). Crystals are generally less than 1 mm (0.04 in) long, but locally feldspar crystals are larger than 1.3 cm (0.5 in.). Small spherical to tubular cavities (gas-escape vesicles) may be filled by zeolite minerals or calcite. Consists of at least three major flows. Prominent amydaloidal zones occur at most contacts between flows. A thin, 2 to 8 m (6.6-26 ft) bed of siltstone (Jps) separates the lower flows. The basal 20 m (66 ft) of the lowest flow is commonly highly vesicular or brecciated. Radiating slender columns 20 to 71 cm (8-28 in) wide, caused by shrinkage while cooling, are most abundant in the highest flow. The small, circiular extrusive body forming Round Top west of Oldwick is identified as Preakness Basalt by geochemistry and position above the Orange Mountain Basalt (Houghton and others, 1992). Thickness ranges from 250 m (820ft) (Olsen and others, 1989) to 320 m (1,050 ft).
State New Jersey
Name Preakness Basalt
Geologic age Lower Jurassic
Lithologic constituents
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)Dark-greenish-gray to black, very-fine-grained, dense, hard basalt composed mostly of intergrown calcic plagioclase (An55-60) and clinopyroxene (pigeonite and augite).
Comments Newark Supergroup, Brunswick Group (Lyttle and Epstein, 1987). Units Jp and Jps are described together on printed map. The units were split into separate records and appropriate descriptions for each were used.

Dalton, R.F., Herman, G.C., Monteverde, D.H., Pristas, R.S., Sugarman, P.J., and Volkert, R.A., 1999, New Jersey Department of Environmental Protection, Bedrock Geology and Topographic Base Maps of New Jersey: New Jersey Geological Survey CD Series CD 00-1; ARC/INFO (v. 7.1), scale 1:100,000.

Drake, Avery A. Jr.,Volkert, Richard, A., Monteverde, Donald H., Herman, Gregory C., Houghton,Hugh F., Parker, Ronald A., and Dalton, Richard F., 1996, Bedrock Geologic Map of Northern New Jersey: U.S. Geological Survey Miscellanenous Investigations Map I-2540-A, 4 cross sections, 2 sheets, size 56x40; 58x41, scale 1:100.000.

Olsen, P.E., 1980, The latest Triassic and Early Jurassic formations of the Newark basin (eastern North America, Newark Supergroup); stratigraphy, structure, and correlation: New Jersey Academy of Science Bulletin, v. 25, no. 2, p. 25-51.

Lyttle, P.T., and Epstein, J.B., 1987, Geologic map of the Newark 1 x 2 degree quadrangle, New Jersey, Pennsylvania, and New York: U.S. Geological Survey Miscellaneous Investigations Series Map I-1715, scale 1:250,00.

Olsen, P.E., Schlische, R.W., and Gore, P.J.W., eds., 1989, Sedimentation and basin analysis in siliciclastic rock sequences; volume 2, tectonic, depositional, and paleoecological history of the early Mesozoic rift basins, eastern North America: Washington, D.C., American Geophysical Union, 28th International Geological Congress, Field Trip Guidebook T351, 174 p.

Houghton, H.F., Herman, G.C., and Volkert, R.A., 1992, Igneous rocks of the Flemington fault zone, central Newark basin, New Jersey; geochemistry, structure, and stratigraphy, in Puffer, J.H., and Ragland, P.C., eds., Eastern North American Mesozoic magmatism: Geological Society of America Special Paper 268, p. 219-232.

NGMDB product
Counties Bergen - Essex - Hunterdon - Passaic - Somerset - Union