Abstract The Olduvai Gorge Coring Project (OGCP) drilled four boreholes 1A, 2A, 3A, 3B at three sites into the central palaeolake of the Olduvai Basin depocentre. Previously known Beds I, II, III, IV, Masek Beds and Ndutu are identifiable in the upper part of the cores, and from these formations predominantly lacustrine facies associations are recognised. These are described and then interpreted in terms of their palaeoenvironmental significance. However, stratigraphy known from natural outcrop exposures is more than doubled by strata intersected below. For example, an extra 135 m of pre-Bed I stratigraphy in Core 2A records the entire volcanic history of a fan-delta sourced from Ngorongoro Volcano. Beneath that fan sequence is an interval of fluvio-lacustrine non-volcaniclastic sediments. For the latter the new stratigraphic term Naibor Soit Formation is introduced, because these strata were discovered for the first time in Boreholes 2A and 3A, to either side of Naibor Soit Inselberg, where it thus laps onto its basement topography. The volcanically sourced unit above is named Ngorongoro Formation, including Marker Tuffs CFCT (Coarse Feldspar Crystal Tuff) and the Naabi Ignimbrite within its topmost portion. Only the dominantly lacustrine sediments above the main Ngorongoro sourced fan body are now attributed to Lower Bed I. The Ngorongoro Formation defined here contains primary Tuff Markers NgA to NgQ, and volcaniclastic fan-delta deposits that interfinger with and overlie claystones and interbedded sandstones of the fluvio-lacustrine Naibor Soit Formation. Boreholes 1A and 2A intersect the Bed I Basalt, but not Borehole 3A, where the position of the lava is marked by three separated volcaniclastic sandstone layers, containing pebble-sized basalt scoriae, correlating to three complex basalt flows at Locality 9A and Orkeri and allowing assessment of syn-basalt sedimentation. Disconformities bounding Beds I, II, III, IV, Masek Beds and Ndutu Beds provide important stratigraphic markers for the novel application of sequence stratigraphic concepts, as they mark lowstands of Palaeolake Olduvai, guiding outcrop to core correlations. Major disconformities are identified in: (1) Core 1A, where the Lower Bed II Crocodile Valley Incision Surface cuts out Tuff IF; (2) Core 1A, where disconformities and facies within Beds II, III, IV, and Masek Beds are identified; and (3) Cores 2A and 3A, where Tuff IF is identified below the Lower to Middle Bed II disconformity; and (4) Lower Bed I claystones and Tuff IA in Core 3A that correlate with similar sediments from Locality 66e in the Western Gorge, but which are absent from Core 2A because of a newly identified major disconformity, which causes the Bed I lava to sit directly on top of the CFCT marker.

中文翻译：

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OGCP 岩心揭示的新奥杜威盆地地层学和地层概念进入坦桑尼亚奥杜威古湖沉积中心

摘要 奥杜威峡谷取心项目（OGCP）在奥杜威盆地沉积中心的中央古湖的三个地点钻了四个钻孔 1A、2A、3A、3B。先前已知的 I、II、III、IV 层、Masek 层和 Ndutu 可在岩心的上部识别，并且从这些地层中识别出主要的湖相组合。这些被描述，然后根据它们的古环境意义进行解释。然而，从自然露头暴露中已知的地层被下面相交的地层增加了一倍多。例如，核心 2A 中额外 135 m 的前层 I 地层记录了源自恩戈罗恩戈罗火山的扇三角洲的整个火山历史。在扇层序之下是河湖相非火山碎屑沉积层。对于后者，引入了新的地层术语 Naibor Soit Formation，因为这些地层是首次在 Naibor Soit Inselberg 两侧的钻孔 2A 和 3A 中发现的，因此它与基底地形重叠。上面的火山源单元被命名为 Ngorongoro Formation，包括 Marker Tuffs CFCT（粗长石晶体凝灰岩）和位于其顶部的 Naabi Ignimbrite。现在只有恩戈罗恩戈罗源扇体上方的主要湖相沉积物归于下层 I。此处定义的恩戈罗恩戈罗组包含原生凝灰岩标记 NgA 到 NgQ，以及与粘土岩和夹层砂岩相交并覆盖的火山碎屑扇三角洲沉积物。河湖型 Naibor Soit 地层。钻孔 1A 和 2A 与层 I 玄武岩相交，但不与钻孔 3A 相交，熔岩的位置以三个分离的火山碎屑砂岩层为标志，其中包含卵石大小的玄武岩渣，与 9A 和 Orkeri 的三个复杂玄武岩流相关，并允许评估同玄武岩沉积。边界层 I、II、III、IV、Masek Beds 和 Ndutu Beds 的不整合为层序地层概念的新应用提供了重要的地层标记，因为它们标记了 Palaeolake Olduvai 的低水位，引导露头到核心相关性。主要的不一致性体现在：(1) 岩心 1A，下层 II 鳄鱼谷切口表面切割凝灰岩 IF；(2) 核心 1A，其中确定了床 II、III、IV 和 Masek 床内的不整合和相；(3) 岩心 2A 和 3A，其中凝灰岩 IF 位于下层至中层 II 不整合下方；