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

Key words: 
• calcareous concretions /
• crack /
• Qiongzhusi Formation /
• genetic mechanism /
• northern Sichuan Basin

Abstract: 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.