Hekou Gr
Type Locality and Naming
Xining-Lanzhou-Minhe Basin. Meng Zhaoyi, Wang Shangwen et al erected the Hekou series in 1947. The section for the designation is at Wazhazi in Shibangou of Hekou, Lanzhou City, Gansu (103°27′E; 36°12′N). Other stratigraphic work divided the Hekou Group into different stratigraphic units. Cai Xiongfei (1998) divided the Hekou Group of the Lanzhou-Minhe basin into the upward succession of the Zhujiatai Fm, Yanguoxia Fm, Hongkucheng Fm and Huazhuang Fm. The China University of Geosciences (Wuhan) (2003) separated the group into 8 stratigraphic units.
Lithology and Thickness
The Hekou Group is built up by variegated (mainly purple red, as well as gray green, tangerine, brown and blue gray) clastic rock series. The lower rock formation is represented by variegated mudstone, silty shale, siltstone and sandstone with a thickness exceeding 2177 m; while the upper rock formation tangerine and brown red mudstone and siltstone with gypsum, measuring about 100 m in thickness. The entire formation is 2277 m thick.
Relationships and Distribution
Lower contact
The base of the group is exposed incompletely. Shown as unconformity with next older unit being Xiangtang Fm by Dangpeng Xi, Xiaoqiao Wan, et al. (2019; Integrated Stratigraphy and TimeScale of China).
Upper contact
The top is bounded by the unconformable termination of tangerine mudstone and the appearance of conglomeratic sandstone of the Gansu Gr. Shown as unconformity with next younger unit being Minhe Fm by Dangpeng Xi, Xiaoqiao Wan, et al. (2019; Integrated Stratigraphy and TimeScale of China).
Regional extent
The present group mainly occurs in Lanzhou, Tianzhu, Yongdeng, Baiyin and Jingyuan of the Gansu Province, and Minhe, Hualong and Ledu of the Qinghai Province. It varies considerably in thickness, ranging from more than 100 m to 5000 m, and varies remarkably in facies. It is commonly coarse-grained on the margin of the basin and is dominated by piedmont-fluvial macroclastic rocks. In the Xiayangwa-Shaotangou area of Quanzui of Zhongbao of the Yongdeng County, Gansu, the lower rock formation is built up by dark purple and purple red interbedded conglomerate and sandstone of different thicknesses, measuring 722 m thick; while the upper rock formation purple red and break red interbedded sandstone, conglomerate and breccia, with a thickness of 237 m. In the Baiyin area, the lower rock formation is built up by purple red breccia and conglomerate which grade upward to variegated silty mudstone with sandstone, measuring 627 m in thickness, while the upper rock formation tangerine sandstone and clayey siltstone with conglomerate, being of 41 m thick. In the Jingyuan area, the lower rock formation is represented by variegated shale with siltstone, with a thickness of 445 m, while the upper rock formation tangerine and purple red mudstone and sandstone intercalated with gray white feldspar fine sandstone. In Weiyuan and Lintao, the lower rock formation is represented by dark brown and purple red conglomerate and sandy conglomerate which become fine-grained upward, dominated by sandy and silty mudstone, ranging from 2100 to 3000 m in thickness. In the Shipogou area of Ledu County of Qinghai, the Hekou Group is represented by dark red sandstone interbedded with mudstone with a small amount of conglomerate, with the base made up of conglomerate lying unconformably on the Huangyuan Gr and with the top lying unconformably under the Minhe Fm. It is only 385 m thick. However, it is 1026-1738 m in Minhe, Datonghe, on the south slope of the Laji Mountain, on the north slope of the Nanshan Mountain along the Hexi Corrido and Heihe. The present Group may lie unconformably on the Upper Jurassic Hengtang Fm or older strata.
GeoJSON
Fossils
The Hekou Group bears Pisces Lycoptera sp. and Sinamia sp. and Bivalves Nakamuranaia chingshanensis, etc. In the Hekou-Minhe area, the lower rock formation also yields bivalves Nakamuranaia cf. subrotunda, N. elongata, Margaritifera lacustris, Unio grabaui, Nippononaia tetoriensis; gastropods Viviparus cf. onogoensis, Valvata transbaicalensis, V. cf. helicoides, Bithynia leachioides; estherias Eosestheria sp., Orthestheria leduensis, Yanjiestheria kyongsangensis; Ostracods Cypridea koskulensis, C. vitimensis, C. prognata; Charophytes dominated by Mesochara and also including M. symmetrica, M. amoera, M. xiagouensis, Sphaerochara verticillata; floras Podozamites lanceolatus, Onychiopsis sp. and sporopollen grains represented by the Cicatricosisporites- Schizaeoisporites assemblage. The upper rock formation yields ostracods Ziziphocypris simakovi, Cypridea cf. skeeteri and Rhinocypris jurassica; Charophytes Aclistochara huihuibaoensis, Flabellochara harris, Hornichara stipitata, Sphaerochara parrula, S. verticillata; and sporopollen grains dominated by Schizaeoisporites, Lygodiumsporites and Classopollis as well as Angiosperms.
"Dinosaur body fossils have also been recovered from the Hekou Group, including the iguanodont Lanzhousaurus and the titanosaurs Daxiatitan, Huanghetitan and Yongjinglong, and the nodosaur Taohelong. Fossil eggs are rare, but one oogenus, Polyclonoolithus, was discovered in the Hekou Group." (Wikipedia entry, citing Xie et al., 2016, Verterata PalAsiatica 54:1-10). Includes the Liujixia National Dinosaur Geopark site at Yanguoxia, which has numerous theropod tracks.
Age
Depositional setting
The Hekou Group is of fluvial-lacustrine facies from alluvial fan, fan-delta and lacustrine depositional systems.
Additional Information