Lower Cretaceous nannofossils of ODP Holes 113-682B and 113-693A

Organic-rich, moderately to sparsely nannofossiliferous Lower Cretaceous claystones ("black shales") were cored at two Ocean Drilling Program Leg 113 sites on the continental slope of East Antarctica off Dronning Maud Land. A 39 m section at Site 692 yielded a Neocomian assemblage of limited diversity with rare Cyclagelosphaera deflandrei, Diadorhombus rectus, and Cruciellipsis cuvillieri, and is probably Valanginian in age. A 70-m section at Site 693 is assigned to the Rhagodiscus angustus Zone (late Aptian-early Albian in age). The latter zone is represented at DSDP sites on the Falkland Plateau, but equivalents to the Neocomian section are absent there, probably due to a disconformity. Watznaueria barnesae is the dominant species at both ODP sites, but it shares dominance with Repagulum parvidentatum at Site 693, where they total 70%-90% of the assemblage; their dominance is attributed to a paleogeographic setting within a restricted basin rather than to postdepositional dissolution of other species. The evolutionary development of this restricted basin and its eventual ventilation in early Albian times is discussed in terms of the regional stratigraphy and the breakup and dispersal of southwestern Gondwanaland. One new species, Corollithion covingtonii, is described.

富含有机质、中等到稀疏含颗石藻（nannofossil）的下白垩统黏土岩（“黑色页岩”），在东南极洲龙尼-毛德皇后地外大陆坡的两个大洋钻探计划（Ocean Drilling Program，ODP）第113航次站位中被取芯。692站位的39米岩芯段产出多样性有限的纽康姆期化石组合，其中仅见少量*Cyclagelosphaera deflandrei*、*Diadorhombus rectus*及*Cruciellipsis cuvillieri*，其地质年代大概率属于凡兰吟期。693站位的70米岩芯段被归为*Rhagodiscus angustus*带，地质年代为阿普第期晚期-阿尔布期早期。该*Rhagodiscus angustus*带在福克兰高原的深海钻探计划（Deep Sea Drilling Project，DSDP）站位中均有产出，但该处缺失与纽康姆期岩芯段相当的地层，这可能是由沉积间断所致。*Watznaueria barnesae*是两个ODP站位的优势物种，但在693站位，它与*Repagulum parvidentatum*共同占据优势，二者合计占化石组合总量的70%~90%；二者的优势地位被归因于局限盆地内的古地理环境，而非其他物种发生了沉积后溶蚀作用。本文结合区域地层学以及西南冈瓦纳大陆（Gondwanaland）的裂解与分散过程，探讨了该局限盆地的演化历程及其在阿尔布期早期的最终充氧作用。本文还记述了一个新种*Corollithion covingtonii*。