Cretaceous calcareous nannofossils of DSDP Leg 77 holes

Five of the six sites drilled during Leg 77 of the Deep Sea Drilling Project yielded Cretaceous sediments. Two of these sites, 535 and 540, form a composite section that spans the upper Berriasian through most of the Cenomanian. Olive black marly limestones in this interval yield relatively rich, well-preserved nannofossil assemblages that allow biostratigraphic subdivision of the sequence. This composite section provides important information on the Early Cretaceous history of the Gulf of Mexico, as well as additional information on tropical Lower Cretaceous nannofossil assemblages. The post-Cenomanian nannofossil (and sedimentary) record is limited to a thin, condensed section of Santonian through lower Maestrichtian pelagic sediments at one site (538) and is absent or represented by redeposited material at the other sites. Two new genera, Perchnielsenella and Darwinilithus, are described. Two new taxa, Darwinilithus pentarhethum and Lithraphidites acutum ssp. eccentricum, are described; and two new combinations, Rhagodiscus reightonensis and Perchnielsenella stradneri, are propose.

深海钻探计划（Deep Sea Drilling Project）第77航次所钻取的6个站位中，有5个站位产出白垩纪沉积物。其中535号与540号站位组成一套复合地层剖面，地层跨度覆盖贝里阿斯阶（Berriasian）上部至森诺曼阶（Cenomanian）大部分层位。该层段内的橄榄黑色泥灰岩产出相对丰富且保存完好的超微化石（nannofossil）组合，可实现该序列的生物地层细分。这套复合剖面为墨西哥湾早白垩世的演化历史提供了重要参考数据，同时也为热带下白垩统超微化石组合的研究补充了额外信息。森诺曼阶之后的超微化石（及沉积）记录仅在538号站位可见一套薄而凝缩的圣通阶（Santonian）至马斯特里赫特阶（Maestrichtian）下部远洋沉积物剖面，其余站位要么缺失该记录，要么仅见再沉积物质。本次研究描述了两个新属：Perchnielsenella与Darwinilithus；描述了两个新分类单元：Darwinilithus pentarhethum与Lithraphidites acutum ssp. eccentricum；并提出了两个新组合：Rhagodiscus reightonensis与Perchnielsenella stradneri。