Compaction-formed platy limestone from the Middle Cambrian Zhangxia Formation (Western Hills, China): towards a new classification for bedded limestone

摘要

Remarkable bedding features occur in Middle Cambrian platy limestone of the Western Hills close to Beijing in NE-China, which are intercalated in a sequence of shallow water carbonates (mudstones, storm deposits, oolitic grainstones). The platy limestone beds (up to 5 cm thickness) have undergone complex diagenetic compaction and pressure solution. Varying facies types are characterized by wavy, stylolitic boundaries with different thickness of clay accumulation and common lateral pinch out. Cross-cutting relationships of stylolites commonly destroy primary bed-surfaces. This indicates an intimate interfingering resulting in an indenting fabric of primary separated facies types. Nevertheless, primary sedimentary boundaries can be recognized. There occur varying types of compaction features documented by different stylolite types with varying amplitudes and thickness of clay-enrichments (parallel clay seams, stylolamination, stylo-nodular and stylo-brecciated structures with multi-grained seams). Bedded limestone of the type documented, generally belong to the limestone family of Plattenkalk, Lithographic Limestone or platy limestone, which can form in different environments. Consequently, using these names without detailed data on some specific parameters (e.g. thickness, surface morphology, composition of allochems, particle and crystal size) results in more confusion and hinders the comparison of Plattenkalk, Lithographic Limestone and platy limestone from different locations throughout the earth history. Therefore, a classification is proposed here which is based on macroscopic, microscopic, and sub-microscopic parameters. Plattenkalk and platy limestone are considered to form the two main groups. Plattenkalk beds are laterally consistent and have parallel, horizontal surfaces. Platy limestone can pinch out laterally and reveals irregular and inclined bed surfaces. Single beds in both can have different thickness, internal structure (e.g. micritic, microsparitic) and fabric (e.g. homogeneous, nodular), particle content and other variations (e.g. chemical, mineralogical). These parameters should be added to the basic name and used in a system similar to Folk’s limestone classification. Lithographic Limestone is defined as a subgroup of Plattenkalk with well-defined parameters. A consequent use of this classification will also help to understand fossil preservation and/or non-preservation in different types of Plattenkalk, Lithographic Limestone, and platy limestone.