1999 GSA Annual Meeting -- Denver, Colorado
1999 GSA Annual Meeting -- Denver, Colorado
Author(s): BENNINGTON, J Bret and BONELLI, James, Dept. of Geology, 114 Hofstra University, Hempstead, NY 11549, geojbb@hofstra.edu; CHANDLER, Jen, W. C. Mepham High School, Bellmore, N.Y., 11710; SELSS, Mark, John F. Kennedy High School, Bellmore, N.Y., 11710.
Keywords: Navesink, paleoecology, biocorrosion, backlap, gryphaeidae
The Navesink Formation in central New Jersey is an approximately 8 m thick transgressive sequence consisting of bioturbated, predominantly quartz sands in the lower part, changing upsection to bioturbated, muddy glauconite sands. Oyster-dominated fossil assemblages are present in the upper Navesink and were replicate sampled along a single prominent fossiliferous horizon at two localities (Poricy Brook and Big Brook, Monmouth Co.). Sediment analysis using a magnetic separator shows that the upper 5 meters of the Navesink Fm. consists of 95% authigenic glauconite sand with accessory carbonate grains and scarce detrital quartz sand. The fossil assemblages collected are numerically dominated by the small ostreid genus Agerostrea with lesser numbers of the larger gryphaeids Pycnodonte and Exogyra, and the brachiopod Choristothyris. Other taxa include small pectinid bivalves, and regular and burrowing echinoids (represented by spines). Of the large oysters, almost 100% show evidence of biocorrosion, primarily in the form of clionid borings, but also present are borings attributable to lithophagid bivalves, acrothoracican barnacles, and polychaete annelids. Encrusting organisms are also common and include several species of bryozoa, serpulid annelids, and small oysters. Of the smaller Agerostrea, approximately 50% are encrusted or biocorroded. Many large oyster valves are almost completely biodegraded and some show evidence of having remained partially buried for extended periods of time. Most bivalved specimens are disarticulated, although approximately even valve ratios are present in samples. These observations suggest a benthic environment undisturbed by wave activity or pulses of substantial sediment input, where shells remained exposed on the sediment surface for long periods of time or are buried slowly by bioturbation. The Navesink oyster assemblage is thus an example of a backlap shellbed, developed within a condensed section deposited near maximum sea-level rise under sediment-starved conditions at middle- to outer-shelf depths.
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