Maotian Fm
Type Locality and Naming
The type section of Maotian Formation is located at Maotian Village, about 3.5 km southwest of Ganxi Town, or 25 km of southwest of the seat of Yanhe County (108°24’E, 28°22’N). The type section was measured by the No. 3 Brigade of Guizhou Regional Geological Survey Team in 1960. In the type section, the Maotian Formation is 200.21 m thick. The Maotian Formation was first published by No. 3 Brigade of Guizhou Regional Geological Survey Team (1970). The name is derived from Maotian Village in Ganxi Township, Yanhe Tujia Autonomous County, Tongren City, northeastern Guizhou Province. It was originally appeared in a 1966 manuscript by the No. 3 Brigade of Guizhou Regional Geological Survey Team. This formation spans the Cambrian-Ordovician boundary.
Synonym: （毛田组）
Lithology and Thickness
The Maotian Formation is a carbonate sequence. It consists of grey and dark grey, medium- to thick-bedded limestone, dolomitic limestone, and dolomite, alternated in unequal thickness and intercalated with oolitic, laminated, and edgewise limestone. In the type section, the formation is divided into five lithologic beds, from bottom up: 1, light grey, grey, thick-bedded to massive dense limestone with two interbeds of light grey, thick-bedded, fine-grained dolomite (21.4 m); 2, light to dark grey, massive limestone and dolomitic limestone with oolitic limestone at the base (50.4 m); 3, light grey and grey, thick-bedded, massive fine-grained dolomite (5.07 m); 4, light grey and grey, dolomitic limestone with oblique lamination in the lower part and light grey and grey, thick-bedded edgewise limestone in the upper part (3.65 m); 5, light grey and grey, massive limestone, dolomitic limestone, and dolomite with thick-bedded dolomite at the base (119.69 m). The Maotian Formation is fairly stable in lithology, and it represents tidal flat sedimentation setting.
Relationships and Distribution
Lower contact
The Maotian Fm is in conformable contact with the underlying Loushankuan Fm. The lower boundary of the formation is defined by the disappearance of light grey and greyish white, thick bedded dolomite of the Loushankuan Formation and appearance of light grey and grey, massive limestone.
Upper contact
The Maotian Fm is in conformable contact with the overlying Tongzi Fm. The upper boundary is defined by the appearance of dark grey and greyish black bioclastic limestone of the Tongzi Fm.
Regional extent
The Maotian Formation has similar distribution as the Pingjing Fm. It is limitedly exposed in eastern Yangtze Area, South China Region, distributed in Yanhe, Dejiang, Yinjing, and Jiangkou counties, eastern Tongren City, northeastern corner of the Guizhou Province and in the Shizhu,Youyang and Xiushan counties at southeastern corner of Chongqing Municipality. The formation increases dolomitic content eastward and southwestward. The thickness of the formation ranges from 105‒300 m. In northeastern Guizhou, it is 105‒214 m, and in southeastern Chongqing it is 170‒300 m.
GeoJSON
Fossils
The Bed 2 of the Maotian Formation in the type section yields trilobite Metacalvinella sp. and brachiopod Nisusia sp. In other localities, the formation yields also trilobites Metacalvinella typica, M. elongata, M. yanheneis, and Mictosaukia striata, and nautiloid Yanheceras endogastrum. Conodonts Monocostodus sevierensis and Teridontus nakanurai is known from the top part of the formation, which lies immediately below the trilobite Wanliangtingia-bearing base of overlying Tongzi Fm. Wanliangtingia, known from the third Ordovician trilobite zone, the Wanliangtingia lobata Zone in southern Sichuan Province (Lu, 1975), indicates the formation ranges up into Lower Ordovician.
Age
Depositional setting
The Maotian Formation represents tidal flat sedimentation setting.
Additional Information