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Changing depocentre environments of Palaeolake Olduvai and carbonates as marker horizons for hiatuses and lake-level extremes
Palaeogeography, Palaeoclimatology, Palaeoecology ( IF 3 ) Pub Date : 2020-12-01 , DOI: 10.1016/j.palaeo.2020.110032
Ian G. Stanistreet , Connor Doyle , Tom Hughes , Elisabeth D. Rushworth , Harald Stollhofen , Nicholas Toth , Kathy Schick , Jackson K. Njau

Abstract Primary carbonate and marl layers and limestone nodular horizons were intersected in OGCP Boreholes 1A, 2A, 3A, 3B, drilled into the depocentre of Palaeolake Olduvai. The various carbonate types were analysed, employing petrographic (including cathodo-luminescence), stable isotope, and sequence stratigraphic techniques, and recorded important information concerning lake evolution. Primary carbonate and marl layers are preserved at the top of lake deepening cycles (lake-parasequences), marking maximum flooding, followed by lake withdrawal, and then fluvial erosion, leading to the next depositional cycle. Presence of pelagites and a meromict lake favouring anoxic conditions, show it to be not a playa lake, but a deeper fault-bound lake, akin to present day Lake Eyasi. Carbonate and marl layers were exclusively deposited when claystone facies show a geochemical magnesium anomaly associated with a phase of basaltic volcanism affecting the basin, marked by mafic tuffs and basaltic lava flows. Calcium was partitioned into evaporites, such as gypsum/anhydrite, together with the nodular horizons and carbonate soil profiles in the sequence. The resulting high Mg2+/Ca2+ ratio in the lake waters promoted dolomite deposition and replacement, and the formation of aragonite. The nodular horizons yield rainfall isotope values and mark times when the lake was empty and the lake-bed fell under meteoric conditions, precipitating limestone nodules just below the sediment surface under pedogenic vadose, groundwater interface, and groundwater phreatic conditions. The nodular limestone horizons sit below erosional/hiatal surfaces and show vadose micritic and fibrous types to phreatic sparry and other accretionary type textures. Enterolithic to chickenwire textured nodules indicate the pseudomorphing of anhydrite nodules that form beneath salt marshes. Thus, the two carbonate types, primary layers and nodular horizons, provided depth gauges, respectively for the extremes of lake expansion and emptying/drying out. In the case of nodular horizons, the maturity of the carbonate profile gives an indication of the magnitude of the hiatal time gap represented, ~ 1 kyr to 2 kyr for a single horizon to ~ 3 kyr to 9 kyr for more mature soil profiles. Emptying/drying out episodes of Palaeolake Olduvai were not uncommon, and often short-lived. A spectrum of hiatal disconformities in the Olduvai Basin vary from multimillennial hiatuses to mega-disconformities of ~ 40 kyr and ~ 75 kyr.

中文翻译:

Palaeolake Olduvai 和碳酸盐岩的沉积中心环境变化作为中断和湖泊水位极端的标志层

摘要 OGCP钻孔1A、2A、3A、3B中原生碳酸盐岩、泥灰岩层和石灰岩结节层相交,钻入Palaeolake Olduvai沉积中心。使用岩相学(包括阴极发光)、稳定同位素和层序地层学技术分析了各种碳酸盐类型,并记录了有关湖泊演化的重要信息。原生碳酸盐岩和泥灰岩层保留在湖泊加深旋回(湖泊-副层序)的顶部,标志着最大洪水,然后是湖泊退缩,然后是河流侵蚀,导致下一个沉积循环。远洋岩的存在和一个有利于缺氧条件的 meromict 湖,表明它不是一个海滩湖,而是一个更深的断层湖,类似于今天的 Eyasi 湖。当粘土岩相显示出与影响盆地的玄武岩火山作用阶段相关的地球化学镁异常时,碳酸盐和泥灰岩层专门沉积,以镁铁质凝灰岩和玄武岩熔岩流为标志。钙被分配到蒸发岩中,例如石膏/硬石膏,以及顺序中的结节层和碳酸盐土壤剖面。湖水中产生的高 Mg2+/Ca2+ 比促进了白云石的沉积和置换,以及文石的形成。结节层产生降雨同位素值和标记时间,当湖泊为空且湖床在大气条件下下降时,在成土包气、地下水界面和地下水潜水条件下,沉积物表面正下方沉淀石灰岩结核。结节状石灰岩层位位于侵蚀/裂孔表面下方,显示渗流泥晶和纤维类型到潜水喷射和其他增生类型纹理。肠石到鸡丝纹理的结节表明在盐沼下形成的硬石膏结节的假形。因此,原始层和结节层这两种碳酸盐类型分别为湖泊膨胀和排空/干涸的极端情况提供了深度计。在结节层的情况下,碳酸盐剖面的成熟度给出了所代表的裂隙时间间隔的大小,单个层为 ~ 1 kyr 到 2 kyr,对于更成熟的土壤剖面为 ~ 3 kyr 到 9 kyr。Palaeolake Olduvai 的清空/干涸事件并不少见,而且往往是短暂的。
更新日期:2020-12-01
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