Geology of the New York City Region

The Green Pond Outlier

The Green Pond Outlier is a complex northeast-trending belt of Paleozoic sedimentary rocks that bisects the Precambrian crystalline rocks of Reading Prong (Highlands Province), and extends for 65 miles between I-80 in New Jersey and the New York Thruway in New York (Figure 67). The sedimentary rocks within this belt are folded into a complex synclinorium in which the beds are locally completely overturned, particularly in the New York end. The trough is locally broken by faults that run in parallel to the axis of this great fold, locally giving it a graben-like character in portions of the New Jersey region.

Map of the Green Pond Outlier, New Jersey and New York
Figure 67. Map Silurian and Devonian rocks of the Green Pond Outlier, New York and New Jersey (modified after Drake et al., 1996 and Fisher et al., 1995). Green is Devonian sedimentary rocks; purple is Silurian sedimentary rocks; and white is older Paleozoic and Precambrian metamorphic and granitic rocks.

The sedimentary formations which crop out in the Green Pond Outlier correlate to equivalent units in the Valley and Ridge region, however there are some notable differences in the character of the rock. The oldest Silurian strata consists of the Green Pond Quartzite, a alluvial conglomerate which accumulated unconformably on an irregular surface of Precambrian, Cambrian, and Ordovician rocks that were truncated by erosion following the Taconic Orogeny. This massive conglomerate and cross-bedded sandstone is equivalent in age to the Shawangunk Conglomerate to the north. It forms the core of Green Pond Mountain in New Jersey (see Figure 67).

By Late Silurian time the coarse clastic deposition gave way to finer-grained mud (represented by the Longwood Shale), and eventually to shallow marine carbonate deposition (Poxono Island and Bershire Valley Formations). The total thickness of the Silurian section is approximately 1,500 feet.

In the Green Pond Outlier, the Early Devonian is not represented (the entire Helderberg Group is missing). Middle Devonian strata rest unconformably on the surface top of the Silurian section. The base of the Devonian sequence is represented by the Connelly Conglomerate, an equivalent stratigraphic unit of the Oriskany Sandstone which crops out throughout the Appalachian Basin region. The Connelly Conglomerate is overlain by a sequence of Middle Devonian shale and sandstone formations that locally bear marine invertebrate fossils. In ascending order, the Middle Devonian units include Esopus Formation, Kanouse Sandstone, Cornwall Shale, and Bellvale Sandstone. This Middle Devonian sequence approaches about 3,000 feet thick.

The Bellvale Sandstone grades conformably into the overlying Shunnemunk Conglomerate of Late Devonian age. This massive conglomerate forms the resistant caprock of Schunnemunk Mountain (pronounced as an "sk"), a ridge that rises to 1,564 feet just to the west of the Thruway near Highland Mills. The Skunnemunk Conglomerate approaches 3,000 feet thick; it also forms the resistant hogback ridge of Bearfort Mountain in New Jersey. The Skunnemunk Conglomerate is lithologically unique in that it is a classic "puddingstone," consisting of well-rounded quartz and red sandstone cobbles in a fine-grained red ironstone matrix. Pieces of Skunnemunk Conglomerate are easy to recognize in glacial deposits throughout the lower Hudson Valley region. The Skunnemunk Conglomerate is equivalent to the Late Devonian sequence of the Catskills region.

It is unclear of the exact age of the structure of the Green Pond Outlier. The similarities of stratigraphic units within the regions to the north and west suggest that the structure was proximal to sediment source areas in the late stages of the Taconic Orogeny and again during the Acadian Orogeny. In addition, the structural trend of the Green Pond Outlier is clearly consistent with the overall trend of regional structures associated with the Alleghenian Orogeny as well as with the western border faults of the Newark Basin. It is likely that the rocks of Green Pound Outlier and the surrounding Highlands have endured many structural modifications throughout their long geologic history.

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Last modified: 3/11/2019