EOCENE CHAROPHYTE FLORA FROM THE XIA GANCHAIGOU FORMATION IN ZONGMAHAI LAKE AREA, QAIDAM BASIN, NORTHWEST CHINA
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Abstract:
The Qaidam Basin is the largest terrestrial basin with the most continuous Cenozoic sediments in the northeastern Tibetan Plateau. It is located in the northwestern Qinghai Province, northwestern China, triangular in shape, bounded by the Altyn-Tagh to the northwest, the Qilian Mountain-Nan Mountain to the northeast, and the Eastern Kunlun Mountain to the south. The basin has an area of ~200000 km2, with altitude between 2600 and 3000 meters. The Cenozoic consists of 7 for-mations, in ascending order: Lulehe, Xia Ganchaigou, Shang Ganchaigou, Xia Youshashan, Shang Youshashan, Shizigou, and Qigequan formations. The charophyte fossils are very abundant and display different floral assemblages with different ages in the Qaidam Basin, according to previous charophyte biostratigraphy studies for oil exploration over the last 60 years. The Eocene Xia Ganchaigou Formation is well exposed in the Yidaogou Section in the Zongmahaihu area, the northern Qaidam Basin. The lithology was described in detail, dominated by brownish-red mudstone, interbedded with light gray-ish-green gypsum-containing fine sandstone, calcareous silty fine sandstone, and a small amount of gravel-containing coarse sandstone and brownish red limestone. Two species belonged to two genera of charophytes were found from this formation including Lamprothamnium ganchaigouensis (Tang and Di) Li et al. comb. nov. and Lychnothamnus vectensis (Groves) Soulié-M?rsche. L. ganchaigouensis was newly combined. L. vectensis, previously known from the Eocene to Oligocene in Europe, is reported in China Eocene deposits for the first time, and its intraspecific variation is discussed based on population, including size range and continuous variations of apical ornaments. The population of L. vectensis has a tendency to increase in gyrogonite size with the increase of latitude. On the contrary, the latitude of the Qaidam Basin is lower than that of the Ebro and Paris basins in Europe, but the population from the Qaidam Basin is larger in gyrogonite size than those from the latter two basins. It was speculated that higher water clarity and/or higher temperature of palaeolakes produced larger gyrog-onites in the Qaidam Basin.
