Palaeoenvironmental setting of lacustrine stromatolites in the Miocene Wudaoliang Group, northern Tibetan Plateau

摘要

Lacustrine stromatolites were widespread in the Miocene Wudaoliang Group(stromatolites of the Wudaoliang Group), northern Tibetan Plateau;but only at one location nearby the Wudaoliang Town, they occurred intensively in thick, laterally traceable beds(Wudaoliang stromatolites). Although deposited in lacustrine environment, the lack of fossils in these rocks hampers determining whether the stromatolites formed in freshwater or saline conditions.To address this problem, and in an attempt to identify criteria to distinguish differences of freshwater and saline conditions, we studied the laminae microfabrics, stable carbon and oxygen isotope ratios, rare earth element patterns and biomarkers of the stromatolites. These stromatolites can be divided into fenestral stromatolites and agglutinated stromatolites. The fabric of fenestral stromatolites is formed by microcrystalline carbonate enclosing spar-cemented, angular crystal traces. Essentially, this fabric is interpreted as pseudomorph after former formed evaporite crystals. Faecal pel ets identical to that of the present-day brine shrimp Artemia, lack of other eukaryotic fossils,and stable isotopic signals point to a shal ow, evaporation-dominated hypersaline lake setting. Covariation of carbon and oxygen isotopes indicates hydrological y closed conditions of the Miocene lake on northern Tibetan Plateau.However, if compared to other lacustrine carbonates of the Wudaoliang Group, the high δ13 C values of the investigated Wudaoliang stromatolites reveal an additional photosynthetic effect during the deposition of the stromatolites.Furthermore, although no direct evidence is available from field observations and microfabrics, a positive europium anomaly of Wudaoliang stromatolites indicates that a palaeo-hydrothermal inflow system had existed in the outcrop area.These new results favour a hypersaline lake setting subject to hot spring inflow for the Wudaoliang stromatolites, in contrast to earlier interpretations suggesting a freshwater lake setting(e.g. Yi et al., Journal of Mineralogy and Petrology28: 106-113, 2008;Zeng et al., Journal of Mineralogy and Petrology 31: 111–119, 2011). This approach may be appropriate for other lacustrine, unfossiliferous microbialites in settings where the environmental conditions are difficult to determine.