The upper Palaeocene–lower Eocene Umm er Radhuma Formation in the subsurface of Qatar is dominated by subtidal carbonate depositional packages overlain by bedded evaporites. In Saudi Arabia and Kuwait, peritidal carbonate depositional sequences with intercalated evaporites and carbonates in Umm er Radhuma have been previously interpreted to have been dolomitized via downward reflux of hypersaline brines. Here, textural, mineralogical and geochemical data from three research cores in Qatar are presented which, in contrast, are more consistent with dolomitization by near‐normal marine fluids. Petrographic relationships support a paragenetic sequence whereby dolomitization occurred prior to the formation of all other diagenetic mineral phases, including chert, pyrite, palygorskite, gypsum, calcite and chalcedony, which suggests that dolomitization occurred very early. The dolomites occur as finely crystalline mimetic dolomites, relatively coarse planar‐e dolomites, and coarser nonplanar dolomites, all of which are near‐stoichiometric (50.3 mol% MgCO₃) and well‐ordered (0.73). The dolomite stable isotope values (range −2.5‰ to +1‰; mean δ¹⁸O = −0.52‰) and trace element concentrations (Sr = 40 to 150 ppm and Na = 100 to 600 ppm) are compatible with dolomitization by near‐normal seawater or mesohaline fluids. Comparisons between δ¹⁸O values from Umm er Radhuma dolomite and the overlying Rus Formation gypsum further suggest that dolomitization did not occur in fluids related to Rus evaporites. This study provides an example of early dolomitization of evaporite‐related carbonates by near‐normal seawater rather than by refluxing hypersaline brines from overlying bedded evaporites. Further, it adds to recent work suggesting that dolomitization by near‐normal marine fluids in evaporite‐associated settings may be more widespread than previously recognized.

中文翻译：

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近乎正常的海洋流体引起的早期和普遍的白云石化：卡塔尔始新世蒸发环境的新教训

卡塔尔地下的上古新世-下始新统Umm er Radhuma地层以潮汐碳酸盐岩沉积包裹物为主，并由层状蒸发物覆盖。在沙特阿拉伯和科威特，以前曾解释过在Umm er Radhuma中夹杂有蒸发物和碳酸盐的碳酸盐沉积碳酸盐岩沉积序列是通过高盐盐水的向下回流而白云石化的。这里介绍了卡塔尔三个研究核心的结构，矿物和地球化学数据，相比之下，这些数据与近乎正常的海洋流体的白云石化作用更加一致。岩石学上的关系支持一种共生序列，即白云石化发生在所有其他成岩矿物相形成之前，包括石，黄铁矿，坡缕石，石膏，方解石和玉髓，这表明白云石化发生得很早。白云岩以细晶状模拟白云岩，较粗糙的平面e白云岩和较粗糙的非平面白云岩出现，所有这些都是接近化学计量的（50.3 mol％MgCO₃），井井有条（0.73）。白云石稳定同位素的值（范围-2.5‰〜+ 1‰;平均δ ¹⁸ O = -0.52‰）和微量元素浓度（SR = 40至150ppm和Na = 100至600ppm）通过与白云石化兼容的近正常的海水或中盐溶液。之间的比较δ ¹⁸Umm er Radhuma白云岩和上覆的Rus层石膏的O值进一步表明，与Rus蒸发岩有关的流体未发生白云石化作用。这项研究提供了一个例子，即与近乎正常的海水相比，蒸发了与蒸发岩有关的碳酸盐的早期白云石化作用，而不是使上覆层状蒸发岩中的高盐盐水回流。此外，它增加了最近的工作，表明在与蒸发岩有关的环境中接近正常海洋流体的白云石化作用可能比以前认识的更为广泛。