Carbonate concretions have been recorded in many recent and ancient marine sediments around the world. The Middle Miocene marl of the Tenes area, situated in the northeast of the Lower Chelif Basin in NW-Algeria, contains such carbonate concretions but with a variety of different structures and morphologies. Three different basic types are distinguished: nodular (spheroidal, ellipsoidal, disc, and irregular), stratiform, and tubular concretions, the last locally have a central conduit. The close association between carbonate concretions and synsedimentary deformation structures (synsedimentary faults, slumps) and normal faults, pronounced in the Ounsour Anhas outcrop, indicates synsedimentary instability related to upward fluid movement. The concretions were formed by precipitation of micritic carbonate within the host marl at shallow burial depth, probably in the active microbial methanogenesis zone. Strongly varying δ13C values (− 9.82 to + 5.85‰ PDB) are interpreted as the result of the balance between 13C-enriched (residual CO2 from methanogenesis) and 13C-depleted (microbial organic matter decomposition) CO2 added to the pore solutions. δ18O values (− 2.39 to + 1.71‰ PDB) indicate that carbonate concretion growth occurred during early diagenesis conditions, from marine-derived pore-water.

中文翻译：

---

阿尔及利亚西北部中新世泥岩中的碳酸盐结核：类型、地球化学和起源

碳酸盐结核在世界各地的许多近代和古代海洋沉积物中都有记录。Tenes 地区中中新世泥灰岩位于阿尔及利亚西北部的下 Chelif 盆地东北部，含有此类碳酸盐结核，但具有多种不同的结构和形态。区分三种不同的基本类型：结节状（球状、椭球状、盘状和不规则状）、层状和管状结核，最后一种局部具有中央导管。碳酸盐结核与同沉积变形结构（同沉积断层、塌陷）和正断层之间的密切关联，在 Ounsour Anhas 露头中很明显，表明与向上流体运动相关的同沉积不稳定性。结核是由泥晶碳酸盐在浅埋深的主体泥灰岩中沉淀形成的，可能在活跃的微生物产甲烷区。强烈变化的 δ13C 值（- 9.82 至 + 5.85‰ PDB）被解释为添加到孔隙溶液中的富含 13C（产甲烷产生的残余 CO2）和耗尽 13C（微生物有机物分解）的 CO2 之间的平衡。δ18O 值（- 2.39 至 + 1.71‰ PDB）表明，在早期成岩条件下，来自海洋孔隙水的碳酸盐结核生长。