A study of Mesozoic (Early Cretaceous) radiolaria from DSDP Hole 534A
Niebuhr, Randy R.
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Continuous coring was attempted at Site 534 during leg 76 of the Deep Sea Drilling Project, in order to recover Middle and Upper Jurassic sediments and basalt basement rocks from the Blake-Bahama Basin in the western North Atlantic. Geographically, Site 534 is located between Florida and Bermuda at 28° 20.6' N, 75° 22.8' W and at a water depth of 4,976 m. The occurrence of Mesozoic radiolarians reported by the shipboard scientists constituted the oldest radiolarian- bearing submarine deposits yet drilled in the Atlantic Ocean. The main objectives of my research were: (1) to examine the radiolarian taxonomy and biostratigraphic distribution; and (2) to compare their geological occurrence with previously established biostratigraphic zones. It was a surprise, therefore, to find that only 13 out of 45 samples examined yielded radiolarians of any significant abundance, and all are Early Cretaceous in age. Core 51 appears to belong to the Eucyrtis tenuis zone of Barremian age, first recognized by Riedel and Sanfilippo in 1974, and Gores 69 through 82 to the Sethocapsa trachyostraca zone of Valanginian (Neocomian) age, first reported by Foreman in 1975. The absence of Dibolachras tytthopora, which defines the top of the Sethocapsa trachyostraca zone, places Cores 70 through 82 in the lower portion of that zone. Radiolarian assemblages of correlative age have been reported from several outcrop sections from Japan, thus providing the basis for future paleoceanographic consideration. The majority of radiolarians recovered from Hole 534A have had their siliceous skeletons replaced by pyrite. The replacement of silica by pyrite probably indicates a reducing environment during the early diagenesis of the sediments. A similar phenomenon was observed from Early Cretaceous sediments recovered during legs 1 and 44 in the same area.