细菌岩及其分类OA

The classification of microbialites constitutes a fundamental basis for investigating their origin and distribution.Owing to ambiguities in the definition of microbialites,these rocks are,in essence,predominantly bacterial in origin.Consequently,the term"bacterialite"is considered more pre-cise than"microbialite"when referring to sedimentary rocks formed through bacterial activity.Bacterialites may be recognized as an independent category within the conventional classification of carbonate rocks,positioned alongside reef carbonates.Based on mineral composition,bacterialites can be divided into seven primary types:bacterial carbonates,siliceous,ferrous,phosphate,saline,bauxitic,and clastic bac-terialites,each of which allows for further subdivision.Taking bacterial limestones as a representative example,they are classified according to macrostructural features into thrombolites,stromatolites,coatites,and leiolites.Thrombolites are subdivided into framework-bearing and non-framework types.Framework thrombolites can be further categorized into sparry,micritic-sparry,and micritic types,reflecting a progressive decrease in hydrodynamic energy and a corresponding increase in water depth from sparry to micritic forms.Stromatolites are classified based on lamination clarity into lamination-clear and lamination-obscure types;the former typically form in intertidal to supratidal(subaerial)environments,while the latter occur in subtidal(subaqueous)settings.Additionally,stromatolites are categorized mor-phologically into columnar,wavy,planar laminar,domal,conical,and wall-like forms,with each morphotype further differentiated by size.Coatites are similarly classified according to the distinctness of concentric lamination into lamination-clear and lamination-obscure types,which correspond to intertidal and subtidal depositional environments,respectively.They may also be subdivided based on size into oncolites and bacterial oolites.Leiolites are classified into microcrystalline and granular varieties.This classification system presents several innovative aspects:(1)mineral composition is introduced as the principal criterion for defining major bacterialite categories;(2)the nomenclature is revised by replacing"microbialite"with the more accurate term"bacterialite";(3)the texture-based classification of frame-work thrombolites provides enhanced environmental interpretation;(4)stromatolites are classified both by lamination clarity-indicative of depositional conditions-and by morphology and size;(5)coatites are systematically categorized according to the degree of concentric lamination,which serves as an environ-mental indicator;and(6)a formal classification of leiolites is proposed,accompanied by clearly defined diagnostic criteria.