Qixia Fm

The type section is located at the Qixia Mt. in the eastern suburb of Nanjing City (119°E, 31°20′N). It was named by Richthofen F.V. in 1882.

Synonym: Chihsia Fm; Qixia Epoch (regional Chinese epoch) or also spelled Chihsian Epoch

Limestone. The typical Qixia Fm represents restricted carbonate platform strata composed largely of dark-colored chert-bearing limestone. The Qixia Fm in the Ningzhen Mt. area can be subdivided into the following six ascending lithologic members of the Clastic-rock Member, the Fetid Limestone Member, the Lower Siliceous-rock Member, the Original Limestone Member, the Upper Siliceous-rock Member, and the Top Limestone Member (Geological Memoir of the middle and lower reaches of the Changjiang River, 1989). The Clastic-rock Member is composed chiefly of interbeds of calcareous shale and micrite, with a thickness of tens of centimeters to several meters. In the area of development of limestone there occur animal fossils represented by Schwagerina, while in the area of development of shale it yields bryozoan represented by Fenestella, etc.; brachiopods represented by Athyris, etc.; and Ostracoda represented by Bairdia, etc. The Clastic-rock Member represents essentially littoral deposits formed at the initial stage of marine transgression. In most parts of South China region, the stratigraphic horizon of the Clastic-rock Member is corresponding to the Brevaxina zone of Fusulinida, but in such areas of the Upper Yangtze Region as the North Guizhou, West Hubei and Sichuan Basin, the stratigraphic horizon of the Clastic-rock Member is corresponding to the Misellina claudiae zone of Fusulinida. The Fetid Limestone Member is composed of dark-grey bituminous limestone, and yields Fusulinida represented by Misellina claudiae, corals represented by Wentzellophyllum volzi, westerly up to the Upper Yangtze Region it changes facies into the Liangshan Fm. The Lower Siliceous-rock Member is composed of dark-grey chert, siliceous shale, calcareous shale and medium- and thin-bedded cherty-nodule limestone; in the areas of Tonglu of Zhejiang Province, Daye of Hubei Province, Laibin of Guangxi and Huaying Mt. of Sichuan Province it is still possible to distinguish the above-mentioned cherty-nodules or bands-rich lithologic member above the Misellina claudiae zone. The Original Limestone Member is composed of medium- and thick-bedded cherty-nodule-bearing bioclastic micrites, yielding abundant Fusulinida represented by Nankinella and Schwagerina, and corals represented by Hayasakaia and Polythecalis, and is distributed extensively in most areas of South China. The Upper Siliceous-rock Member is composed of dark-grey siliceous rocks, silica layer and thin-bedded limestone, yielding Chusenophyllum. The Top Limestone Member is composed of medium-, and thick-bedded limestone, and is rich in Parafusulina multiseptata of Fusulinida. The Upper Siliceous-rock Member and the Top Limestone Member are hard to be distinguished from one another in most regions of South China, and in the major parts of the Yangtze Platform Region to the west of Jiangxi Province the carbonate-rock strata which are corresponding to the above-mentioned two members have been incorporated into the Maokou Fm. In the area of Central Hunan and Guangdong Province the limestones above the Clastic-rock Member are as a whole similar to those occurring in the Ningzhen Mt., but their subdivision is not distinct. The Qixia Fm in Fujian Province is greatly different lithologically from that in the standard region, which contains a great amount of clastic-rock intercalations. In the marginal belts, as well as on the isolate carbonate platforms within the basin there is developed a “White Qixia” Fm composed of light-grey massive bio-limestone, which is in an indistinct contact with both the underlying Maping Fm and the overlying Maokou Fm, thus resulting in its being incorporated into the Houziguan Limestone due to the lacking in a distinct lithological boundary between them. In the Cangshan Mt. and Longmen Mt. areas of Sichuan Province the upper part of the Qixia Fm is also composed of light-grey massive bioclastic limestone. The formation is commonly 100-400-m thick in most regions of South China, but in individual areas as in the eastern part of Yunnan Province it is locally only 3.4-m thick, with the greatest thickness of 600 m being seen in the Laibin area of Guangxi.

The Qixia Fm yields mainly such fossils as Fusulinida, corals, brachiopods, algae, and non-fusulinid foraminifera. In the standard locality the Fusulinida fossils are subdivisible in ascending order into the Misellina claudiae zone, the Nankinella orbicularia zone, the Schwagerina chihsiaensis zone and the Parafusulina multiseptata zone, among which it is the S. chihsiaensis zone that is distributed in a fairly restricted area, and the P. multiseptata zone comprises also Praesumatrina and Verbeekina in the Tonglu area of Zhejiang Province, in Fujian Province and in south Anhui Province. In south Guizhou Province, Southeast Hunan Province, as well as in the Laibin area of Guangxi below the M. claudiae zone there occurs also the Staffella vulgaris zone. The Coral fossils are divisible into the Wentzellophyllum volzi zone, the Hayasakaia elegantula zone and the Polythecalis yantzeensis zone. With the latter being subdivided into the P. yangtzeensis zone and the Chusenophyllum zone in south Anhui Province. The brachiopods can be divided into the Orthotichia chekiangensis-Liraplecta richthofeni assemblage and the Chaoina reticulata assemblage. The Conodonts are often represented by the Sweetognathus whitei, which can be subdivided into the S. whitei zone and the Mesogondolella idahoensis zone in north Sichuan Province.

It is mainly a restricted carbonate platform.