The Newark Group, also known as the Newark Supergroup, is an assemblage of Late Triassic and Early Jurassic sedimentary rocks which outcrop intermittently along the United States East Coast; the exposures extend from Massachusetts to North Carolina, with more still in Nova Scotia. It is named for the city of Newark, New Jersey.
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The Newark Group consists largely of poorly-sorted nonmarine sediments; typical rocks are breccia, conglomerate, arkose sandstone, siltstone, and shale.[1][2] Most of the strata are red beds that feature ripple marks, mud cracks, and even rain drop prints; dinosaur footprints are common, though actual body fossils are very rare.[3] Some of the strata are detailed to the level of varves, with indications of Milankovitch cycles.[4] In preserved lake sediments, Semionotus fossils are especially common.[5]
The Newark sediments are extremely thick (up to 6 kilometers); they were deposited in a series of half-grabens that were themselves faulted into block mountains.[6] The beds dip to the east, while the faults dip westward.[7] The beds are intruded by numerous dikes and sills, indicative of considerable igneous activity; a superb example is the New Jersey Palisades sill.[8]
The Newark Group's lithologies and structure are the classic hallmarks of a rift valley; the fault-blocking illustrates the crustal extension forces in play during the breakup of Pangea during the late Triassic Period.[9] The Appalachian Mountains had already been nearly eroded flat by the end of the period; the uplift and faulting that was the first part of the rifting provided new sources of sediment for the vast thicknesses deposited in the Newark Group; the igneous intrusions are similarly diagnostic of a rift valley.[10] Coarse sediments were deposited near the eastern mountain front, while progressively finer ones were deposited farther west.[11]
Evidence suggests the climate at the time was subtropical and rainy, though divided between wet and dry months.[12] A few organic-rich deposits suggest patchy or intermittent swamps and lakes.[13]
Accumulation of Newark sediments continued from the late Triassic into the early Jurassic.[14]
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