Temporal and Spatial Distribution of the Wufeng (Late Ordovician) and Lungmachi (Early Silurian) Black Shales in the Yangtze Region of South China
Xu Chen, Yuandong Zhang, and Junxuan Fan
Key Laboratory of Economic Stratigraphy and Palaeogeography of CAS, State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Nanjing, China
The black shales of the Wufeng Formation (late Ordovician) and the Lungmachi Formation (early Silurian), dominated by graptolitic shales, siliceous shales and cherts, are extensively distributed over the Yangtze Region, and cover almost the entire Yangtze Platform. The Wufeng Formation consists of black shale and chert, and is ca. 7-8 meters thick on average, while the lower, black shale, part of the Lungmachi Formation (its upper part being typified by dark grey shale and siltstone) is commonly less than 80 meters thick. The extent of the two formations is marked in the east at Kunshan (Jiangsu), in the west at Guangyuan-Erlangshan (Sichuan), in the north by the Lianyungang (Jiangsu) ― Hefei (Anhui)― southern Dabie Mts ― Daba Mts ― Qinling Mts, and in the south by the boundary with the Jiangnan Slope, which extends along Jurong (Jiangsu) ― Jinxian (Anhui) ― Yixian (Anhui) ― Boyang Lake ― Dongting Lake ― Fenghuang (western Hunnan) ― Tongren (northeastern Guizhou) ― Zunyi (northern Guizhou).
The Wufeng Formation conformably overlies the contemporary Linhsiang, or Chientsaokou, or Tantou formations, which are composed of limestone, nodular limestone and calcareous mudstone and contain shelly faunas. The base of the Wufeng Formation coincides with the base of the Dicellograptus complanatus graptolite Biozone (late Katian), and the top falls within the Normalograptus extraordinarius graptolite Biozone of the Hirnantian. At the top of the formation, and below the overlying Lungmachi Formation, there is commonly a bed of limestone containing the shelly Hirnantia Fauna, namely the Kuanyinchiao Bed. The Kuanyinchiao Bed is less than half a meter in thickness, and spans a short time interval within the Hirnantian. The age of the top of the Lungmachi Formation varies geographically; in most places it is no younger than the Lituigraptus convolutus Biozone of the Aeronian, but in a few localities the formation continues as late as the Spirograptus guerichi Biozone of the earliest Telychian.
The development and the temporal-spatial distribution of the black shales of the two formations are affected and largely controlled by the contemporary tectonic evolution of South China. At the time corresponding to the Diplacanthograptus spiniferous Biozone of the early Katian (Late Ordovician), the Kwangsian Orogeny, widely regarded as an uplift movement, initiated to the south of the Nanling Mts and the Jinggang Mts (southern Jiangxi). The orogeny resulted in the progressive, northwestward expansion of the Cathaysian Land towards the Yangtze Platform, and to the northwestern Xuefeng Mts of the Jiangnan Slope in late Katian, and eventually enclosed a huge, semi-restricted, gulf-type basin on the extensive Yangtze Platform. In the deep of the basin an anoxic environment was developed and the black shale of the Wufeng Formation was deposited. This specific, semi-restricted and widely-anoxic environment, though briefly interrupted by the global Hirnantian glaciation event, continued into the early Silurian and accounts for the global development of Rhuddannian black shale, as exemplified by the Lungmachi Formation in South China. Although the two formations are lithologically rather similar, they were formed in different geological settings.
In the latest Ordovician and earliest Silurian, the Kwangsian Orogeny proceeded to affect the Yangtze Platform and led to the regional Yichang Uplift, which accounts for a significant hiatus in the Ordovician-Silurian transition. As a result of the uplift, the strata of various intervals, from the Wufeng Formation up to the Demirastrites triangularis Biozone of the Lungmachi Formation, are absent in six areas of different extent on the Yangtze Platform, which are (from west to east): (1) western Sichuan, (2) central Chongqing, (3) northeastern Guizhou, (4) western Hunnan and Hubei, (5) Dongting Lake, and (6) Boyang Lake.
In the earliest Telychian, the Cathaysian Land expanded to the central Yangtze Platform, and as a result deposition of the black shale of the Lungmachi Formation ceased. On the northwestern margin of the Yangtze Platform, i.e. the marginal area with the West Sichuan Land, some marine red beds (Lower Red Bed) were deposited. In the latest Telychian, the entire Yangtze Platform was uplifted to be largely subaerial, as shown by the extensive absence of stratigraphic records for that interval.
The Yangtze Region witnessed the global end-Ordovician biotic mass extinction, which was represented by the extinction of the Dicranograptidae– Diplograptidae– Orthograptidae graptolite fauna (DDO fauna) as recorded in the Wufeng Formation. As a result, the formerly dominant DDO fauna of the late Katian was rapidly replaced by a small, expanding group of graptolites, the Normalograptidae (N fauna), the latter being the source of the subsequent bioradiation in the early Silurian. The Rhuddanian graptolite fauna in the Lungmachi Formation started to recover from the mass extinction from the beginning of the Silurian (Akidograptus ascensus Biozone), and diversified in the late Rhuddanian (Coronograptus cyphus Biozone). The development of Rhuddanian graptolitic black shale in the Yangtze Region coincides with that of the well-known “hot shale” in North Africa, and has drawn great attention from petroleum geologists.
Aiming to help assess the petroleum and gas potential of the Wufeng Formation, one of the present authors (FJX) reconstructed the three dimensional model for the extent of the formation using the data of 142 localities retrieved from the Geobiodiversity Database (GBDB, the official database of the International Commission on Stratigraphy), and estimated a volume of some 6.5*1012 cubic meters for the formation. A similar analysis and estimate for the Lungmachi Formation in the Yangtze Region of South China is being conducted.
AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013