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Dolomite cements in Cenomanian continental sand deposits: Time evolution and significance (Zoovch Ovoo U-deposit, East Gobi Basin, Mongolia)
Sedimentary Geology ( IF 2.8 ) Pub Date : 2020-07-01 , DOI: 10.1016/j.sedgeo.2020.105688
Dimitrios Rallakis , Raymond Michels , Marc Brouand , Olivier Parize , Michel Cathelineau

Abstract Shallow buried unconsolidated sands generally provide very little information about diagenesis as most detrital minerals remain unchanged. However, in rare cases carbonate cemented nodules or sandstone layers may occur inside unconsolidated series. These cements could help to reconstruct the chronology of events from early to late phase stages of diagenesis. The Late Cretaceous sequence of the Zoovch Ovoo depocenter in East Gobi Basin is represented by 600 m of clastic deposits. The 60–80 m of Cenomanian unconsolidated sands and clays, deposited in alluvial-deltaic to lacustrine settings, compose the upper part of the post-riftmedium-grained siliciclastic reservoir, the Sainshand Formation which hosts uranium roll-front systems. Dolomite cemented sandstone layers with 10–20 cm thickness occur among the unconsolidated rock facies. Calcite is absent from this formation, but is present only in the overlying, also outcropping, Bayanshiree Formation. Samples from the dolomite cemented sandstone layers were investigated in detail to uncover their origin and diagenetic history. Four dolomite cement types were recognized that indicate recrystallization episodes and were classified based on the size and shape of the crystals, namely: (i)microcrystalline dolomite frequently associated with siderite, (ii) euhedral dolomite also associated with siderite, (iii) subhedral dolomite and (iv) finally anhedral dolomite. Their REE content varies significantly from dolomite I to IV, in particular by a strong depletion in LREE about 30 times. The contrasted precipitation of calcite in Bayanshiree and dolomite cements in Sainshand formations could be attributed to different Mg/Ca ratio of the circulation fluids in the two aquifers. Both carbonates display however rather homogenous oxygen and carbon isotopic compositions for δ18O (−10 ± 1‰ V-PDB) and δ13C (−7 ± 1‰ V-PDB). The δ18O values are interpreted as inherited from typical meteoric waters quite close to present day waters. The δ13C values indicate a mixed source of both organic and mineral carbon. All data taken into account, a full paragenetic succession was constructed. It includes the evolution of dolomite during burial diagenesis and the effects of the oxidizing roll-front uranium rich waters in the system. The latter induces partial dissolution followed by precipitation of a dolomite phase typical of roll-front zones. Carbonate cements can be thus considered as the best and rather unique geochemical indicators for the recognition of a burial history framework to paleofluid circulations and fluid-rock interactions in these intracontinental series of unconsolidated sands.

中文翻译:

Cenomanian 大陆砂矿床中的白云岩胶结物:时间演化和意义(Zoovch Ovoo U 矿床,东戈壁盆地,蒙古)

摘要 由于大多数碎屑矿物保持不变,浅埋松散砂通常提供很少的关于成岩作用的信息。然而,在极少数情况下,未固结系列内可能会出现碳酸盐胶结结核或砂岩层。这些胶结物可以帮助重建从成岩作用的早期到晚期阶段的事件年表。东戈壁盆地 Zoovch Ovoo 沉积中心晚白垩世层序以 600 m 的碎屑沉积物为代表。沉积在冲积三角洲至湖相环境中的 60-80 米 Cenomanian 松散砂岩和粘土构成了裂谷后中粒状硅质碎屑岩储层的上部,Sainshand 组拥有铀滚动前沿系统。松散岩相中出现10-20 cm 厚的白云岩胶结砂岩层。该地层中不存在方解石,但仅存在于上覆且出露的 Bayanshiree 地层中。对来自白云岩胶结砂岩层的样品进行了详细调查,以揭示它们的起源和成岩历史。四种白云岩胶结物被识别出表明再结晶事件,并根据晶体的大小和形状进行分类,即:(i) 常与菱铁矿伴生的微晶白云岩,(ii) 也与菱铁矿伴生的自形白云岩,(iii) 半自形白云岩(iv) 最后是自形白云岩。它们的 REE 含量从白云岩 I 到 IV 变化很大,尤其是轻稀土的强烈消耗约 30 倍。Bayanshiree 中方解石和 Sainshand 地层中白云石胶结物的对比沉淀可归因于两个含水层中循环流体的不同 Mg/Ca 比。然而,两种碳酸盐都显示出相当均匀的 δ18O (-10 ± 1‰ V-PDB) 和 δ13C (-7 ± 1‰ V-PDB) 的氧和碳同位素组成。δ18O 值被解释为从非常接近现代水域的典型大气水域继承而来。δ13​​C 值表明有机碳和矿物碳的混合来源。考虑到所有数据,构建了完整的共生序列。它包括埋藏成岩过程中白云岩的演化和系统中氧化性滚锋富铀水的影响。后者引起部分溶解,然后沉淀出典型的滚前区白云石相。
更新日期:2020-07-01
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