Lithostratigraphy of Holocene Sand Ridges from the Nearshore and Middle Continental Shelf of New Jersey, U.S.A.
Published:January 01, 1986
James M. Rine, Roderick W. Tillman, William L. Stubblefield, Donald J. P. Swift, 1986. "Lithostratigraphy of Holocene Sand Ridges from the Nearshore and Middle Continental Shelf of New Jersey, U.S.A.", Modern and Ancient Shelf Clastics: A Core Workshop, Thomas F. Moslow, Eugene G. Rhodes
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Two sand ridges on the New Jersey continental shelf were cored to determine their lithologic characteristics and possible modes of deposition. The ridge at location 1A is a nearshore, shoreface-connected sand ridge within 3 mi (4.8 km) of shore and in less than 65 ft (20.3 m) of water. The ridge at location 2 is 25 mi (40 km) from shore and in 80-115 ft (25-36 m) of water. Three main lithologic units are present in the cores from both ridges. The cores presented in this manuscript have sedimentary characteristics representative of these lithologic units and display the key differences between ridges observed in cores.
The three main lithologic units are (from bottom to top): (1) "nonfossiliferous" sand and mud, (2) shell-rich poorly sorted sand and mud, and (3) upper ridge sand. The "nonfossiliferous" unit contains no macrofauna, but has traces of microfauna, massive-appearing sand layers, laminated muds, and some pebbly sand layers. The shell-rich unit contains numerous shell fragments and is predominantly bioturbated. Carbon-14 age determinations from the shell-rich unit in the nearshore ridge range from 6130 ± 120 years BP to 6360 ± 90 BP; those from the midshelf ridge range from 12,480 ± 155 years BP to 13,240 ± 180 BP. The upper ridge sand unit consists of stacked beds ranging in thickness from 1 to 28 in. (2.5-70 cm). Within the upper ridge sand unit, most beds in the nearshore ridge are up to 9 in. (22.5 cm) thick, whereas most beds in the midshelf ridge are slightly thicker, 12 in. (30 cm) or less. Within the upper ridge sand unit, both ridges have alternating laminated and nonlaminated (bioturbated) layers, contain fine- to medium-grained sand, and generally coarsen upward. The nearshore ridge has a slightly coarser range of mean grain size (150-400 μ) than that of the midshelf ridge (140 to 360 μ). The youngest carbon-14 age determinations from the upper ridge sand unit are 1480 ± 170 years BP from the nearshore ridge and 1155 ± 85 BP from the midshelf ridge.
Results from this study support the hypothesis that both the nearshore and midshelf ridges are being actively modified and possibly formed at present sea level. These conclusions support past theories on the origins of the nearshore ridges (Duane et al., 1972), but indicate that the midshelf ridge has a much more "dynamic" posttransgressional depositional history than previously surmised.
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Modern and Ancient Shelf Clastics: A Core Workshop
The organizers of this core workshop opted for a broad, permissive definition of shelf clastics and contributors were encouraged to focus on their own interpretations rather than force-fitting their examples to a restrictive theme. As a result, the editors assembled a selection of core examples which range from standline to shelf edge. Given the dominance of petrophysical logs in subsurface studies, contributors to the workshop worked hard to make the necessary core-to-log comparisons which were essential if geologists are to more fully utilize wireline data in facies analysis. Contributors were also encouraged to place their studies within the context of energy prospects associated with environments of deposition. Hydrocarbon potential and reservoir quality dominate these discussions. The high degree of lithofacies variability and the effect of high-energy events on shelf deposits cause sediments formed within this environment to be particularly challenging exploration targets.