Maokou Fm

The type section is located on the western bank of the Maokou River 22.5 km to the southwest of the Langdai County, Guizhou Province (105°20′E, 26°10′N). It was named by Yue Senxun in 1929.

Dolomitic limestone. In the typical region the Maokou Fm is generally composed of light-grey bioclastic limestone, and is divisible into three members in ascending order: The Xianrenmiao Member, the Dazhai Member, and the Honglakong Member. The Xianrenmiao Member is composed of light-grey thick-bedded and massive dolomite, dolomitized sparite and micrite, with a thickness of 350-480 m. This member can be subdivided into three sub-members. The upper and lower sub-members consist of dolomite or strongly dolomitized bioclastic limestone, and the middle sub-member consists already of weakly dolomitized bioclastic limestone, or essentially of bioclastic limestone. The Dazhai Member is composed of dark-grey and grey medium- and thick-bedded cherty-band-bearing bioclastic micrite, locally being dolomitized, with a thickness of 100-170 m. The Honglakong Member is composed of light-grey thick-bedded weakly dolomitized bioclastic limestone, with a thickness of 60-130 m, with its top commonly containing a 0-20 m-thick layer of grey-black to dark-grey wavy lenticular compressed bedding, containing carbonaceous clayey limestone, intercalated with organic calcareous shale.

In East Yunnan, West Guangxi, South Guangxi and the Ceheng and Ziyun areas of South Guizhou the strata corresponding to the Maokou Fm represent a suite of light-grey massive sponge-reef-facies and Fusulinida-rich biogenic beach-facies limestones. This suite of platform-marginal-facies limestones is assigned in the South Guizhou to the Houziguan Limestone (Xiao Weimin et al., 1986), which yields abundant reef-facies biogenic sponge, algae and bryozoan. Fusulinida fossils represented by Cancellina, Neoschwagerina, Yabeina and Metadoliolina are extremely abundant. Northerly till the North Guizhou region the Maokou Fm is subdivided into five lithological members in ascending order as follows: the Lower Leopard Limestone Member, the Augen Limestone Member, the Upper Leopard Limestone Member, the Cherty Limestone Member, and the Massive Limestone Member, which are corresponding to the increase in the amount of clayey substance and decrease in dolomitic matter of the lower three sub-members of the Xianrenmiao Member, as compared with that in the typical region, meanwhile the fossil features are characterized by the development of brachiopods and the predominance of the euryfacies Fusulinida represented by the Schwagerina and Chusenella to replace the Cancellina, Pseudodoliolina, Neoschwagerina and Afghanella.

Still northerly to the Sichuan Basin, or easterly to Hubei, Hunan, Central Guangxi, North Guangxi and Northwest Jiangxi, the Maokou Fm in the North Guizhou region can only be subdivided into three members, with the lower member universally containing clayey substance, but in Hubei, Northwest Jiangxi, it has locally a higher content of clayey substance in the form of chert essentially. Yet in the Laibin and Yishan areas of Guangxi it is the limestone that predominates, and salient features of its fossils tend to lie in the development of brachiopods represented by Cryptospirifer, etc. In Hubei Province it is the Verbeekina of Fusulinida that is developed the most, with a local concentration of Presumatrina of Fusulinida. The middle member of the formation is corresponding to the Upper Leopard Limestone Member and Cherty Limestone Member in North Guizhou, or to the Upper Sub-member of the Xianrenmiao Member and Dazhai Member in South Guizhou. The particular member is known as the Siliceous Member in Hunan Province. Except for the Sichuan Basin, the particular member is rich in cherty nodules and siliceous bands, and the fossils which it yields are characterized by the development of Fusulinida represented by the Chusenella, while in the Sichuan Basin it yields abundant fossil calcareous algae, among which the lobate algae layers are developed in South Sichuan and North Guizhou with a stable stratigraphic horizon, and the Neoschwagerina is very common in this region. The upper member is corresponding to the Massive limestone Member in North Guizhou or to the Honglakong Member in South Guizhou. The particular member is composed largely of light-grey massive bioclastic limestone, being rich in Fusulinida represented by Metadoliolina and Yabeina, with a local concentration of sponge and bryozoan, but in North Sichuan and its neighboring Southwest Hubei the particular member consists already mainly of dark-grey thin-bedded limestone, or siliceous rocks, yielding ammonoids represented by Altudoceras and Shouchangoceras, etc.

In the type section, the Xianrenmiao Member middle sub-member has as its main fossils Fusulinida such as Cancellina, Afghanella and Neoschwagerina, and Corals such as Ipciphyllum, Tetraporinus, etc. The Dazhai Member yields Fusulinida such as Yabeina, Chusenella, and Corals such as Ipciphyllum, etc. The Honglakong Member yields Fusulinida such as Metadoliolina, Lantschichites and Corals such as Huangophyllum, etc.

The fossils of the Maokou Fm comprise essentially Fusulinida. In the typical region there occur the Cancellina zone, the Neoschwagerina simplex zone, the Afghanella schencki zone, the Yabeina gubleri zone and the Metadoliolina douvillei zone ("Regional Geology of Guizhou Province", 1987). In Hubei Province there occur the Verbeekina grabaui zone, the Pseudodoliolina-Chusenella conicocylindric zone and the Neoschwagerina-Yabeina zone ("Regional Geology of Hubei Province", 1990). The Corals in North Guizhou are subdivided into the Polythecalis yangtzeensis-Tetraporinus planotabulatus zone, the Metasinopora carassa zone, the Ipciphyllum-Iranophyllum zone, the Endamplexus lateradissepimentes zone and the Ipciphyllum-Huangophyllum zone ("Regional Geology of Guizhou Province", 1987). The Brachiopods in Guizhou Province are subdivided into the Cryptospirifer assemblage and the Neoplicatifera huangi assemblage, of which the former assemblage is widely distributed in North Guizhou, Sichuan Basin, West Hubei and Northwest Hunan. The Conodonts are found to occur mainly in Guangxi, West Hunan and North Sichuan, which are subdivided in ascending order into the Mesogondolella (now classified as Jinogondolella) asserata zone, the M. (now J.) postserrata zone, the M. (now J.) altudaensis zone, M. (now J.) prexuanhanensis zone and the M. (now J.) xuanhanensis zone (Mei Shilong et al., 1994). The non-Fusulinida Foraminifera fossils are abundant, for which Lin Jiaxing et al. have created the following assemblages: the Pseudovidaline delicate-Langella lepida-Neodiscus maopingensis assemblage and the Hemigordiopsis-Postendothyra scabra assemblage, of which the former assemblage tends to extend from the Qixia Fm to the Maokou Fm.

It is interpreted as a suite of shallow-water platform facies, including sponge-reef-facies and Fusulinida-rich beach-facies,