Modaoya Fm
Type Locality and Naming
The type section of the Modaoya Formation is Changjianggou-Nianziba section, located at Modaoya, north of Changjianggou, Shangsi, Jiange County, Sichuan Province (105°26’ E, 32°24’N). It is the upper portion of the same section shared with the Changjianggou Formation, lying about 11 km northwest of the Shangsi Town or 15 km northwest of Jiange County. The section was measured by the Sichuan Second Regional Geological Survey Team in 1964. In the type section, the formation is 117.6 m thick. The Modaoya Formation was first published by Zhang et al., (1979). The name is derived from Modaoya Yakou (narrow maintain pass) at north of Changjianggou Village in Shangsi Township, Jiange County, Guangyuan City, northeastern Sichuan Province. It was originally appeared in a 1966 manuscript by the Joint Team of Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, and the Sichuan Institute of Petroleum Sciences.
Lithology and Thickness
The Modaoya Formation is a clastic sequence, dominated by gravel-bearing medium- to coarse-grained or lithic sandstone, intercalated with dark grey, thin-bedded fine-grained sandstone and green calcareous shale. It is characterized by developed oblique bedding and by increasing gravel or pebble amount and diameter upwards in section.
Relationships and Distribution
Lower contact
The Modaoxi Fm rests conformably on the Changjianggou Fm.
Upper contact
In the type section, is disconformably overlain by the Middle Ordovician Baota Fm. In Guangyuan City, the disconformable overlying formation are different from place to place. The Modaoxi Fm is overlain by the Zhaojiaba Fm of Lower Ordovician at the area of Damaoshan anticline, by the Baota Fm of Middle Ordovician at Nianziba and at the area Kuangshanliang anticline, and by the Pingyipu Fm of the Lower Devonian at the area of Tianjinshan anticline.
Regional extent
The Modouya Formation is exposed in northwestern Yangtze Area of South China Region, distributed mainly in the northern and middle sections of the Longmenshan Mountain (Shangsi, Nianziba and Tangjiagou of Guangyuan County) with thickness ranging from 30‒340 m. The formation is thickest at Tangjiaba (340 m), thinning southwestward to about 100 m in the middle section of Longmenshan Mountain with decrease of the gravel amount. The formation is completely thin away at Jiulongshan of Guangyuan County and at Yinquanba of Jiangyou County. In the Hanyuan and Shimian counties, south-central Sichuan Province, the formation is dominated by greyish green gravel-bearing sandstone with interbeds of purplish red gravel-bearing sandstone with a thickness of 30−50 m.
GeoJSON
Fossils
No fossils are found yet.
Age
Depositional setting
It represents littoral setting.
Additional Information