Abstract The Olduvai Basin of northern Tanzania contains well-known Pleistocene archaeological sites in a palaeoenvironmental setting of interfingering paleolake and volcanic fan deposits. This study documents the mineral assemblages, as determined qualitatively through bulk X-ray Diffraction (XRD) of systematically sampled lacustrine sediments from Olduvai Gorge Coring Project (OGCP) Cores 1A, 2A, and 3A, from about 45 m below the surface (mbs) to the base of all three cores. This interval includes fluvio-lacustrine sediments below and between two major progradations of Ngorongoro volcanic deposits recorded in Core 2A, and all of Olduvai Bed I and the lower part of Bed II in all three cores. Changes in mineral assemblage track changes in sediment sources and lake water chemistry. Abundant quartz indicates sediment sources derived from eroded Precambrian basement quartzite and gneiss exposures to the west, southwest, and north. In contrast, the presence of anorthoclase feldspar (sometimes associated with minor augite) indicates southerly and easterly sources, derived from the then active volcanoes (Ngorongoro, Olmoti) of the Ngorongoro Volcanic Highlands. The presence and relative abundance of specific zeolites provides information about Paleolake Olduvai's pH conditions and the relative activities of various dissolved cations (K+, Na+, Ca2+, Si) at the time of alteration, and the presence and abundance of authigenic K-feldspar indicates post-depositional exposure to saline-alkaline pore fluids. The mineral assemblages recorded in the cores reveal both large-scale trends in sediment composition over time, and smaller-scale changes related to fluctuating lake levels. In the lacustrine deposits near the base of Core 2A (interval NSF-1), analcime dominates the zeolite assemblage (with minor clinoptilolite), whereas carbonates and authigenic K-feldspar are rare. This interval is followed by more lacustrine deposits in which phillipsite and chabazite are the dominant zeolites (and analcime is absent). Lacustrine deposition was interrupted by the first of two pulses of coarse Ngorongoro volcaniclastic material (NVF-1). In the lacustrine interval between the two Ngorongoro volcanic pulses (NSF-2, recovered only in Core 2A), chabazite and erionite dominate the zeolite assemblage, carbonate and K-feldspar are rare, and analcime is absent, indicating less extreme saline-alkaline conditions and greater Ca2+ availability for zeolite formation. Volcanic glass is preserved, along with abundant zeolites, including phillipsite, chabazite, and erionite, in altered volcanic materials from the upper Ngorongoro volcanic pulse (NVF-2). In the lacustrine deposits of Lower Bed I (below the Bed I basalts but above NVF-2, only recovered in Core 3A), zeolites and K-feldspar are rare to absent, consistent with wetter conditions and fresher lake waters. Carbonates are ubiquitous throughout the lacustrine intervals of Beds I and II. In lacustrine deposits above the Bed I basalts (in Upper Bed I), phillipsite is the dominant zeolite, and quartz is rare. Alternating intervals of zeolite and K-feldspar-rich and poor sediments suggest variations between wetter and drier conditions throughout Upper Bed I and into Lower Bed II. Aragonite and associated dolomite in Bed I during the deposition of a claystone Mg-anomaly points to high magnesium concentrations and elevated Mg2+/Ca2+ ratios during that time. A major change between lowermost Bed II and overlying Bed II lacustrine sediments includes an abrupt increase in quartz coupled with an increase in the zeolite analcime, consistent with the onset of more saline-alkaline conditions, contemporaneous with overall drier conditions.

中文翻译：

---

使用岩心 X 射线衍射分析重建坦桑尼亚奥杜威古湖的湖泊状况和碎屑来源

摘要 坦桑尼亚北部的奥杜威盆地在古湖泊和火山扇沉积物相互交错的古环境中包含着名的更新世考古遗址。本研究记录了矿物组合，这些矿物组合是通过从奥杜威峡谷取芯项目 (OGCP) 岩心 1A、2A 和 3A 中系统采样的湖相沉积物的整体 X 射线衍射 (XRD) 定性确定的，距离地表以下约 45 m (mbs)到所有三个核心的基础。该层段包括在岩心 2A 中记录的恩戈罗恩戈罗火山沉积物的两个主要进积之下和之间的河湖沉积物，以及所有三个岩心中的所有奥杜威岩床 I 和岩床 II 的下部。矿物组合的变化跟踪沉积物来源和湖水化学的变化。丰富的石英表明沉积物来源来自侵蚀的前寒武纪基底石英岩和西部、西南和北部的片麻岩暴露。相比之下，斜​​长石（有时与少量辉石有关）的存在表明来自南风和东风的来源，来自恩戈罗恩戈罗火山高地当时的活火山（恩戈罗恩戈罗，奥尔莫蒂）。特定沸石的存在和相对丰度提供了有关 Paleolake Olduvai 的 pH 条件和各种溶解阳离子（K+、Na+、Ca2+、Si）在蚀变时的相对活性的信息，以及自生钾长石的存在和丰度表明后- 沉积暴露于盐碱孔隙流体。岩心中记录的矿物组合揭示了沉积物成分随时间的大尺度趋势，以及与湖泊水位波动相关的小规模变化。在核心 2A 底部（NSF-1 层段）附近的湖相沉积物中，方沸石在沸石组合（含有少量斜发沸石）中占主导地位，而碳酸盐和自生钾长石很少见。该间隔之后是更多的湖相沉积物，其中菱沸石和菱沸石是主要的沸石（没有方沸石）。湖相沉积被粗恩戈罗恩戈罗火山碎屑物质 (NVF-1) 的两个脉冲中的第一个中断。在两次恩戈罗恩戈罗火山脉冲（NSF-2，仅在岩心 2A 中发现）之间的湖相层段，菱沸石和毛沸石在沸石组合中占主导地位，碳酸盐和钾长石稀有，方沸石不存在，表明不太极端的盐碱条件和更大的 Ca2+ 可用于沸石形成。火山玻璃以及丰富的沸石（包括菱沸石、菱沸石和毛沸石）都保存在来自恩戈罗恩戈罗火山脉冲 (NVF-2) 上部的蚀变火山物质中。在下层 I 的湖相沉积物中（在层 I 玄武岩以下但在 NVF-2 以上，仅在岩心 3A 中回收），沸石和钾长石很少甚至不存在，这与较湿润的条件和较新鲜的湖水相一致。碳酸盐在 I 和 II 层的整个湖相层段中无处不在。在Ⅰ层玄武岩（上层Ⅰ层）之上的湖相沉积物中，沸石以菱沸石为主，石英少见。沸石和富含钾长石的沉积物和贫瘠沉积物的交替间隔表明整个上床 I 和下床 II 的湿润和干燥条件之间存在差异。在粘土岩 Mg 异常沉积期间，层 I 中的文石和伴生白云石表明在此期间镁浓度高且 Mg2+/Ca2+ 比率升高。最下层的第二层和上层的第二层湖相沉积物之间的主要变化包括石英的突然增加以及沸石方沸石的增加，这与更多盐碱条件的开始一致，同时整体条件更加干燥。