MICROBIAL FABRIC FORMATION IN SPRING MOUNDS (MICROBIALITES) OF ALKALINE SALT LAKES IN THE BADAIN-JARAN SAND SEA, PR CHINA

The large sandy desert of Badain Jaran Shame, Inner Mongolia, has seve ral salt lakes Located among megadunes with crests as high as 400 m. M ost of the lakes are hypo- to hypersaline and alkaline, with pH values between 8.5 and 10. Because of their reef-lihe aragonitic pinnacles r ising from the lake floor, Lake Nuoertu and Lake Huhejaran have been i nvestigated in detail. The porous limestones of the tufa pinnacles ('' spring mounds'') result from a hydrochemically forced, exopolymer-medi ated calcification of cyanobacteria-dominated microbial mats at sublac ustrine springs. The development of their enigmatic fabrics is documen ted for the first time. Sickle-cell-like and bubble-shaped fabrics ref lect successive mineralization. during degradation, shrinkage of the o rganic mucus, and gas formation by bacteria. Fossil analogues are know n from the Pleistocene and Miocene (Ries-crater Lake), but older non-m arine, Phanerozoic examples are expected. To the extent that these tex tures are associated exclusively with alkaline environments, they can be wed to test the hypothesis of an alkaline ocean early in Earth hist ory.