ORIGIN OF THE ATLANTIC

Pangaea did not last very long. Continent fragmentation began in the New York State region in the later part of the Triassic period (245-201 mya). Rifting of Pangaea led to the origin of the Atlantic Ocean in the Jurassic period (201-144 mya). The 120o angle formed by the New York Blight (the edge of the continental shelf south of Long Island) and seen in the trend of the southern Massachusetts-New Jersey shoreline reflects two of the active arms of the rift system along which west Africa broke away from eastern North America. Roughly parallel to the southwest arm is the Ramapo-Canopus fault, which runs southwest from Peekskill (Westchester Co.) to Suffern (Rockland Co.). The small area east of the fault and west of the Hudson River in Rockland dropped down and received a thickness of 20,000 feet (6,000 m) of Upper Triassic and Lower Jurassic sand, mud, and gravel eroded from the Hudson Highlands. These subaerial sedimentary rocks have yielded dinosaur tracks at Blauvelt (Rockland Co.) and continue into New Jersey in the Newark Basin. They are similar to rocks deposited in the Connecticut River Valley, the failed arm along this spreading system that extends under eastern Long Island.

fig 51
Volcanism, Rifting, and Splitting of Pangea into Two Continents

The creation of the Palisades was one result of the breakup of Pangaea. Stretching of the earth’s crust and development of the Ramapo-Canopus fault allowed the release of basaltic molten rock, which moved laterally in the Triassic-Jurassic sediments as a subhorizontal injection called a sill. As it cooled, the rock shrank and fractured into the six-sided columns that are now exposed as the Palisades cliff on the western bank of the Hudson River. The Palisades sill is 390-980 feet (120-300 m) thick and over 40 miles (65 km) long; it extends from Staten Island to High Tor in Haverstraw (Rockland Co.). The second effect of the continental breakup was the uplifting of the region that underwent rifting, as it was underlain by less dense molten rock. This meant that the dinosaurs at Blauvent wandered around in a highland valley that was probably arid. In addition, large areas lateral to the rift were elevated and eroded. It is possible that much of the rock laid down during the Allegheny orogeny across New York was eroded at this time.

fig 52
Continued Rifting

With further spreading along fractures that defined the New York Blight, ocean waters entered the rift, and the Atlantic Ocean was born. Initially, the edges of the New York Blight were elevated because of the heat of their rocks. With time they cooled, became dense, and subsided. This subsidence allowed Jurassic and Cretaceous (144-65 mya) shorelines to creep northward, reaching Staten Island and Long Island by the later Cretaceous. These Cretaceous red shales and sandstones are now easily accessible only along the shore at Garvies Point Preserve on northwestern Long Island. Latest Cretaceous and Cenozoic era (65 mya) rocks occur underwater south of Long Island but can be seen east of the state of Gay Head Cliffs on Martha’s Vineyard, Mass.

fig 58
Atlantic Ocean Continues to Widen

An odd rock type appeared in Ithaca during the Cretaceous small vertical veins of kimberlite (a black igneous rock derived from the mantle of the earth) that fill fractures in Late Devonian sandstone. Diamonds are mined from kimberlite in South Africa and elsewhere, but none have been found in Ithaca. The Ithaca kimberlite may reflect development of a hot spot, a giant plume of molten rock that moved upward through the earth’s crust, comparable to the source of the volcanic rock in Hawaii and Yellowstone Park. Hot spots remain relatively fixed and leave a track of volcanoes as the plate moves above them. The Ithaca kimberlite may be the earliest expression of a plume recorded by a line of younger volcanoes vents. These vents underlie Montreal, continue eastward along the St. Lawrence River, then run southeast through the White Mountains of New Hampshire until they run offshore (and underwater) as the New England Sea Mounts. Movement of this region above a plume during the Cretaceous (and with heating and doming of the crust along the track of the plume) may explain the doming of the Adirondacks and the erosion of thousands of feet of sedimentary rocks that once covered it.


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