Dongzhakou Fm
Type Locality and Naming
The type section is located in the Dongzhakou area, Zhangxian County, Gansu Province. The reference section lies at Shangjialing 43 km northeast of Lintan County Seat and at Heidianxia 35 km south of Lintao County Seat, Gansu. It was named by Huang Zhenhui in 1962, and was officially cited by the Compiling Group on the Regional Stratigraphic Chart of Gansu Province in 1980.
Lithology and Thickness
Mixed marine facies. Lower part is composed of purplish-red massive pebble-bearing clayey siltstone and grey-green medium-bedded pebble-bearing fine-grained sandstone, intercalated with variegated calcareous conglomerate. Base of formation is a white-pink medium-bedded bioclastic limestone, Upper part is composed of dark-grey medium-bedded micrite, sandy limestone, calcareous siltstone and calcareous silty siltstone. A ca. 7-m-thick grey-black medium- and thick-bedded bioclastic micrite and purplish-red thin-bedded calcareous silty siltstone is at the top of the formation. Total thickness is 281.48 m.
Regionally, the lithofacies changes greatly, and can be divided into two lithofacies zones: (1) The South Zone lies in the areas of Lintan, Zhuone and Yeliguan, dominated by neritic carbonate rock facies, intercalated with clastic rocks, represented by the Shangjialing section, consisting of gray-white and carnation thick-bedded massive limestone, with quartzose sandstone, shale and conglomerate at bottom. Exposed thickness 320 m. Eastward to the Yeliguan area, the thickness increases to 1000 m. (2) The North Zone outcrops around Kangle to Lintao, dominated by marine-continental clastic rocks. In the Heidianxia section, it is composed of gray-black to gray-green sandstone, sandy conglomerate and siltstone, intercalated with marl and thin coal seams; thickness >232 m.
Relationships and Distribution
Lower contact
Fault contact with the grey-green thin-bedded silty siltstone and fine-grained sandstone of the underlying Dacaotan Gr. Unconformably over the Xiajialing Fm in the South Zone. Unconformably over the Badu Fm in the North Zone.
Upper contact
Conformably under the Qixia Fm in the South Zone.
Unconformable contact with the overlying Cretaceous stratum in the North Zone
Regional extent
South Zone is areas of Lintan, Zhuone and Yeliguan. North Zone is mainly distributed in the northern Qinling Mts.
In the Baishi Mt. area, Kangdo, Zhuony County, the formation has no base being exposed due to the overburden of its lower part, but its upper part can be seen to be in a fault contact with the Yangjiahe Fm. The rocks in its Lower part represent grey massive sparry limestones, while its Upper part consists of light-grey and light-pink massive bioclastic sparry limestones. In the Xiangfangli area of Zhangxian County the formation is composed of pink massive microcrystalline limestone, yielding abundant Fusulinida fossils, and with a thickness of 105 m. Coeval with the Gahia Fm in this region.
GeoJSON
Fossils
South Zone: abundant Fusulinids: Pseudoschwagerina fusulinoides, Zellia sp., Pseudofusulina fecunda, P. cf. solida, Hemifusulina sp., etc. Eastward to the Yeliguan area, it yields Fusulinids Pseudoschwagerina sp., Rugosofusulina sp., Quaisifusulina sp., etc
North Zone: Brachiopod Dictyoclostus taiyuanfuensis assemblage as well as Fusulinid Paraschwagerina sp., Plant Calamites cistii, etc
Age
Depositional setting
It is interpreted as platform-marginal shallow-beach deposits.
Additional Information