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Watchung Outliers

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The Watchung Outliers include six areas of isolated low hills and rock outcrops of volcanic origin in the U.S. states of New York, New Jersey, and Pennsylvania. These geologic outliers are primarily diminutive and detached remnants of the Triassic/Jurassic age Watchung Mountain basalt flows. All six of the outliers are found along the western edge of the Newark Basin, occupying small synclines adjacent to the Ramapo fault system. The outliers include, from north to south: the Ladentown basalt, Union Hill, the New Germantown/Oldwick basalt (including Round Top), Prospect Hill, the Sand Brook basalt, and the Jacksonwald basalt.

Ladentown Basalt

The Ladentown basalt is the most northern of the Watchung Outliers, located along the border of the Town of Haverstraw and Ramapo, New York, approximately seven miles north of the maximum extent of the Watchung Mountains at Campgaw Mountain. The trap rock of the outlier covers a considerable area, running at or beneath the surface for a length of about two miles with a width of about a mile.[1] Originally, because of its close proximity to the Palisades, the Ladentown basalt was thought to be an extension of the intrusive sill. It is now known from magnetic and gravity measurements that the Ladentown basalt is linked to the Palisades by a significant subsurface volcanic dike[2], but it is definitely an extrusive feature, displaying Orange Mountain basalt characteristic of First Watchung Mountain.[3] This makes the Ladentown basalt unique among the Watchung basalts in that it appears to be rooted to an intrusive feature. In this respect, the Ladentown Basalt can be thought of as the remains of a fissure volcano, owing to the extrusion’s slab-like character and ramping profile. New research indicates that the vicinity of Ladentown may have served as the primary vent from which the magma of the Palisades Sill erupted to the surface to form the Watchung flood basalts.[4]

Union Hill

Union Hill is located near the village of Suffern in Ramapo, New York, about three and a half miles north of the Watchung Mountains, or about halfway between the Ladentown basalt and Campgaw Mountain. The hill originally rose 150 feet above the surrounding terrain,[5] but extensive quarrying for trap rock has since obliterated most of the hill except for the slopes of its perimeter. Like the Ladentown basalt, Union Hill was once considered to be an extension of the Palisades. It has since been determined that the hill consists of a flow of Orange Mountain basalt[3] with a minimum thickness of 125 ft (38 m).[1]

New Germantown Basalt

The New Germantown basalt (also known as the Oldwick basalt) is the most northern of the outliers south of the Watchung Mountains. The hills of the New Germantown basalt are located along the border of Tewksbury Township and Readington Township, New Jersey, approximately seven miles southwest of the southern extent of the Watchungs and about three miles north of Cushetunk Mountain. The structure of the New Germantown basalt appears to mirror the construction of the Watchung Mountains on a tiny scale. The eastern side of the feature consists of a horseshoe shaped ridge with prongs facing to the west. This ridge is composed of Orange Mountain (First Watchung Mountain) basalt which is actively quarried. West of this First Watchung-like ridge is a valley occupied by sedimentary layers of the Feltville Formation, characteristic of the valley between the First and Second ridges of the Watchungs.[3] Finally, at the center of the horseshoe is a dome of volcanic rock forming a hill known as Round Top. The hill is composed of Preakness basalt, making it the equivalent of Second Watchung Mountain in the New Germantown complex.[6] The thickness of basalt in this outlier is estimated at close to 425 ft (~130 m).[3]

Prospect Hill

Contact between basalt lava and underlying shale at Prospect Hill

Located along the border of western Flemington and Raritan Township, New Jersey, Prospect Hill constitutes the smallest of the Watchung Outliers, being approximately one fourth of a mile wide and extending north to south for less than half a mile.[7] The hill is composed of Orange Mountain basalt, with no overlying Jurassic sediments apparent.[3] Dikes branching south from the vicinity of the outlier appear to be connected to the intrusive igneous Sourland Mountain, an extension of the Palisades Sill.[8][9] In this respect, Prospect Hill likely marks another location where the Palisades magma found a way to the surface, albeit not as significant as the Ladentown basalt.

Sand Brook Basalt

The Sand Brook basalt, located in the eastern part of Delaware Township, New Jersey, appears nearly identical in construction to the New Germantown basalt. A low ridge composed of Orange Mountain basalt forms a horseshoe around the eastern edge of a belt of sedimentary strata from the Feltville Formation. In the middle of the outlier is a dome-like structure of what is likely Preakness Mountain basalt, not unlike Round Top in the New Germantown outlier.[3][8][9] The thickness of the basalt in this outlier is estimated at nearly 400 ft (~120 m), similar to the New Germantown basalt.[3]

Jacksonwald Basalt

The Jacksonwald basalt is the only Watchung outlier in Pennsylvania. It is located over 100 km southwest of the actual Watchung Mountains[3] in Exeter Township. The outlier shares some similarity with the Sand Brook and New Germantown basalts. As in the other two outliers, the Jacksonwald basalt features a low, horseshoe shaped ridge of Orange Mountain basalt that embraces an area composed of sedimentary strata from the Feltville Formation. However, unlike in the Sand Brook and New Germantown outliers, the Jacksonwald basalt lacks a central dome of Preakness basalt. The thickness of the basalt in this outlier is estimated to be in excess of 325 ft (~100 ft).[3]

An interesting trait of the Jacksonwald outlier is that it is surrounded by a ridge of intrusive diabase on its eastern side. The diabase, which lies stratagraphically below the basalt, is thought to have been intruded into the syncline containing the Jacksonwald outlier during or after some degree of folding had affected local strata. The diabase is therefore considered to constitute a phacolith.[10]

See also

References

  1. ^ a b Kummel, Henry B. Annual Report of the State Geologist for the year 1898 – The Extension of the Newark System of Rocks. MacCrellish & Quigley, State Printers, Trenton, NJ, 1899. See Pages 54. Available via Google Books
  2. ^ Kodama, K.P., 1983. Magnetic and gravity evidence for a subsurface connection between the Palisades sill and the Ladentown basalts. Geological Society of America Bulletin, v. 94, no. 1, p. 151-158.
  3. ^ a b c d e f g h i 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.
  4. ^ Puffer, J.H., Block, K.A., Steiner, J.C., 2009. Transmission of Flood Basalts through a Shallow Crustal Sill and the Correlation of Sill Layers with Extrusive Flows: The Palisades Intrusive System and the Basalts of the Newark Basin, New Jersey, U.S.A. The Journal of Geology, volume 117, p. 139–155.
  5. ^ Darton, Nelson Horatio. Bulletin of the United States Geological Survey No. 67 – The Relations of the Traps of the Newark System in the New Jersey Region. Government Printing Office, Washington, 1890. See Pages 53, Union Hill Trap. Available via Google Books
  6. ^ Mineral Resources Online Spatial Data – Preakness Basalt. U.S. Geological Survey. Accessed December 22, 2009.
  7. ^ Darton, Nelson Horatio. Bulletin of the United States Geological Survey No. 67 – The Relations of the Traps of the Newark System in the New Jersey Region. Government Printing Office, Washington, 1890. See Page 66. Available via Google Books
  8. ^ a b Lewis, Volney J. Annual Report of the State Geologist of New Jersey for the Year 1907 – Part IV: Petrography of the Newark Igneous Rocks of New Jersey. John L. Murphy Publishing Company, Printers, Trenton, NJ, 1908. See Pages 166-167 Available via Google Books.
  9. ^ a b Kümmel, Henry B. Annual Report of the State Geologist of New Jersey for the Year 1897 – Part II: The Newark System. John L. Murphy Publishing Company, Printers, Trenton, NJ, 1898. See Pages 92-93 Available via Google Books.
  10. ^ LeTourneau, Peter M. & Olsen, Paul E. (editors). “The Great Rift Valleys of Pangea in Eastern North America – Volume One: Tectonics Structure and Volcanism”. Columbia University Press, New York, 2003. ISBN 0231111622. See Page 41. Limited Preview available via Google Books.