川北地区下寒武统筇竹寺组钙质结核特征及成因机制

不同成因的钙质结核在研究沉积水体、早期成岩环境及泥页岩的压实程度方面具有重要意义。通过对川北地区旺苍县下寒武统筇竹寺组钙质结核抛光，发现结核内部具有层理、暗色圈层和张裂缝构造。结核和围岩的层理关系和具有漂浮状的微观结构特征表明，该地区结核是形成于压实作用之前的成岩早期结核。对结核中Al、Ti、Ca、K、Na元素分析表明，结核的元素分布受到结核内部裂缝的影响。结核内部富含生物碎屑和其形成深度为数十米的特征，推测结核主要是在硫酸盐还原带生长。结核具有暗色圈层之间层理不弯曲和元素移动的特征，揭示了结核为透入性生长，在整个结核生长阶段就开始大量同时结晶。Mg/Ca和Sr/Ca的比值说明，结核内部钙质成分为富镁方解石-生物成因方解石。结核中富含钙质成分的原因可能是围岩中的生物成因方解石大量向结核运移所致。裂缝的形态特征表明，裂缝为张裂缝。对结核上部地层的沉积特征研究表明，张裂缝的成因很可能与筇竹寺顶部的浊积岩和沧浪铺组底部的似瘤状灰岩快速搬运沉积有关。

Characteristics and Genetic Mechanism of Calcareous Concretions in the Early Cambrian Qiongzhusi Formation of Northern Sichuan Basin

Calcareous concretions of different genesis have a great significance in the study of early diagenetic environment, the sedimentary water body and shale of compacted degree.By polishing, the calcareous concretion shows the interior textures including bedding, dark circles structure and tension cracks during the lower Cambrian Qiongzhusi stage in Wangcang, north of Sichuan Basin.The relationship of concretions and surrounding rock of bedding and the preservation of sedimentological micro-features in the concretion matrix point to an early diagenetic concretions and forming before compaction.Not obvious systematic geochemical variations including Al, Ti, Ca, K, Na from concretion center to edge suggests that the element distribution of concretions are affected by cracks.The rich bioclastic and forming about tens meters under the sediment-water interface suggest the concretions may primarily grow during sulfate-reduction.The two characteristics which the beddings are not deformed among the concentric circles and the move of elements reveal that the concretions are pervasive growth which the calcium crystallites of the concretions nucleated at virtually the same time across the entire concretion.The ratios of Mg/Ca and Sr/Ca indicate that the internal calcium components of concretions is magnesium-rich calcite and biogenic calcite.The large migration of calcium in the biogenic calcite of the surrounding rock lead to the rich calcium component in the concretions.Fracture morphology shows the crack is tension cracks.The rapid transportation and deposition of turbidite at the top of Qiongzhusi Formation and analogous-nodular limestone at the bottom of Canglangpu Formation may be the cause of tension cracks.