Xiaoshihugou Fm
Type Locality and Naming
It was named by the Party of Regional Geological Survey, Qinhai Bureau of Geology in 1959 and published by Mu Enzhi and Zhang Youkui in 1964. The type section is located at the branch gully (100°55'E, 38°7'N) in upper reaches of Tongziba River in Mingle County, Gansu Province. (The locality of this type section used to be mistaken by some authors as “Erdougou, north of Obo Liang in Qilian County, Qinghai Province”).
Synonym: (小石户沟组)
Lithology and Thickness
The formation, 1726 m in thickness, is dominated by green gray, dark gray sandy shale, siltstone, sandy conglomerate with tuff partings.
Relationships and Distribution
Lower contact
It overlaps disconformably the Ordovician. Regionally, the schematic strat column indicates the next older unit as the Shichengzi Fm (Note that Ordovician lexicon " Ordov lexicion "includes the Hongtuwan Fm and Shichengzi Fm in the Xiehao Fm")
Upper contact
It is conformably overlain by the Angzanggou Fm.
Regional extent
The formation is characterized by considerable lithological variation: longitudinally it composes a great circle of conglomerate-sandstone-sandy shale, while horizontally sandstone alternates with conglomerate, the thickness of which may vary from several dozen to several hundred meters; volcanic rock partings may get thinner and even completely pinch out. The formation is 1625 m thick at Baiyanghe of Yumen in west; eastwards at upper reaches of Xiaoshipaigou Gully in Shandan County the formation, over 2291 m thick, becomes finer-grained with sandy conglomerate beds of 712 m thick at the base; further eastwards in Qinghai Province the formation again becomes thinner.
GeoJSON
Fossils
The formation bears the following graptolite zones: the Glyptograptus kaochiapienensis zone, the Demirastrites triangulatus zone, and the Monograptus sedgwickii zone.
Age
Depositional setting
Additional Information