Origination of Limestone-Marl Alternations from Qixia Formation of South China

The Middle Permian of South China is a unique carbonate succession, which is rich in organic matter and chert nodule, and is one set of the most important marine source rocks. Limestone-marl alternations are widely distributed in this area. The limestones are rich in skeletal detritus, dominated by algae, foraminifera, and mollusks with subordinate amounts of brachiopods, echinoderms, and ostracods, and minor bryozoans and trilobites. Fragile skeletons such as calcareous algae were well preserved, indicating that cementation took place during early diagenesis with little diagenetic compaction. Marls are bioclastic wackestone or dark mudstone. Bioclasts are diverse, including remnants of brachiopods, ostracods and rare trilobites. Bioclastic flakes are oriented parallel to bedding planes, and thin-shelled fragments, e.g., ostracods, are broken due to compaction of the unlithified sediment. Between the particles are micrite and a large number of dissolution seams. As to the particular features of the limestone-marl alternations, it is inferred that differential diagenesis occurred between limestones and marls. Aragonite in marl layers was dissolved to cement limestone, which took place in shallow-burial realm. Mechanical compaction and then pressure dissolution were concentrated in the less cemented strata. The dissolution of aragonite in the shallow marine burial realm is thought to have been initiated by modification of the pore-water chemistry by bacterial oxidation of organic matter.