Lower Permian continental deposits of the fault-controlled Orobic Basin (Central Southern Alps; Northern Italy) include alluvial fan facies interfingering with muddy basin-floor deposits, consisting of three facies associations: heterolithic fine-grained siliciclastic facies, laminated sandstone facies, and oncoidal limestone facies. Besides oncoidal and microbial limestones, carbonates occur as nodules in sandy tabular beds within the laminated sandstone facies association. Microfacies analyses distinguish several types of oncoidal carbonate (consisting of an alternation of microbial carbonate and fibrous calcite) and carbonate nodules. Each type of carbonate has been characterized in terms of δ18O and δ13C. The two types of carbonate in the oncoids record a stable δ18O and a slightly varying δ13C, whereas the isotope composition of the calcite in nodules is completely different. Carbonate nodules in sandy beds of the laminated sandstone facies association have a diagenetic origin as indicated by cross-cutting relationships between nodules and lamination; the nodules are interpreted as groundwater calcrete, formed in the subsurface at the top of the unconfined water table. The exclusive sedimentation of oncoidal carbonate facies within siliciclastic deposits indicates that when oncoids were formed in ephemeral shallow ponds, siliciclastic input was minimal. The sedimentological and geochemical characteristics of the studied succession and the stable isotopic composition of the oncoids (the absence of covariance between δ18O and δ13C excludes deposition in evaporating basins) indicate persistent stable conditions for sufficient time to permit growth of centimeter-sized oncoids. Oncoids are interpreted to have formed in spring-fed ponds and outflow channels, with flowing, clean water, at the toe of major alluvial fans. Episodes of rapid delivery of sand and silt-sized sediments by flash floods, with an oscillating water table, caused the observed facies alternation. The precipitation of calcareous cements close to the water table surface produced nodules in sandy layers. Carbonate precipitation within laminated sandstone reduced porosity and permeability, causing a strong compartmentalization in the well-bedded continental succession.

中文翻译：

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断层控制盆地（早二叠世，意大利北部）中大陆硅质碎屑岩层序中的 Oncoids 和地下水钙质

断层控制的奥罗布盆地（中南阿尔卑斯山；意大利北部）的下二叠纪大陆沉积包括冲积扇相与泥质盆底沉积相交叉，由三种相组合组成：异质细粒硅质碎屑相、层状砂岩相和盘状砂岩相石灰岩相。除了盘状和微生物石灰岩外，碳酸盐在层状砂岩相组合内的砂质板状层中以结核形式出现。微相分析区分了几种类型的盘状碳酸盐（由微生物碳酸盐和纤维方解石的交替组成）和碳酸盐结核。每种类型的碳酸盐都根据 δ18O 和 δ13C 进行了表征。oncoids 中的两种碳酸盐记录了稳定的 δ18O 和略有变化的 δ13C，而结核中方解石的同位素组成则完全不同。层状砂岩相组合砂层中的碳酸盐结核具有成岩成因，如结核和层状之间的横切关系所示；结核被解释为地下水钙质混凝土，形成于无承压地下水位顶部的地下。硅质碎屑沉积物内的粒状碳酸盐相的独有沉积表明，当在短暂的浅水池中形成粒状沉积物时，硅质碎屑输入量很小。所研究序列的沉积学和地球化学特征以及类核生物的稳定同位素组成（δ18O 和 δ13C 之间没有协方差，不包括蒸发盆地中的沉积）表明持续稳定的条件有足够的时间允许厘米大小的类核生物生长。Oncoids 被解释为在泉水池塘和流出通道中形成，在主要冲积扇的脚趾处有流动的清洁水。山洪暴发快速输送沙子和淤泥大小的沉积物，地下水位波动，导致观察到的相交替。靠近地下水位表面的钙质水泥的沉淀在砂层中产生了结核。层状砂岩内的碳酸盐沉淀降低了孔隙度和渗透率，