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

Abstract

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 pellets identical to that of the present-day brine shrimp Artemia, lack of other eukaryotic fossils, and stable isotopic signals point to a shallow, evaporation-dominated hypersaline lake setting. Covariation of carbon and oxygen isotopes indicates hydrologically closed conditions of the Miocene lake on northern Tibetan Plateau. However, if compared to other lacustrine carbonates of the Wudaoliang Group, the high δ 13 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.may be appropriate for other lacustrine, unfossiliferous microbialites in settings where the environmental conditions are difficult to determine.

A multi‐scale approach to laminated microbial deposits in non‐marine carbonate environments through examples of the Cenozoic, north‐east Iberian Peninsula, Spain

“…2.3.2 | Structure, texture and stable isotopes Most studied microbialite deposits formed in closed lake systems correspond to micritic stromatolites (sensu, Riding, 2000). There are few cases that describe agglutinated stromatolites in saline lacustrine environments, for example, in the Miocene Wudaoliang Group, northern Tibetan Plateau (Zeng et al, 2019) and in the Lower Cretaceous, lacustrine-coastal environment of the Cameros Basin, Spain (Suárez-González et al, 2014). The examples considered here refer mainly to micritic microbialites.…”

Stromatolites and calcitized evaporite in a hypersaline playa lake: Rossport Formation (Mesoproterozoic, Ontario)

“…Indeed, of the two best‐known occurrences of modern marine stromatolites, Shark Bay (Suosaari et al, 2016) and the Bahamas (Andres & Reid, 2006; Suosaari et al, 2019), only the Bahamian stromatolites grow in open marine waters of normal salinity (Dill et al, 1986; Reid et al, 1995). In contrast stromatolites occur in a wide variety of modern and Phanerozoic lakes (Chidsey et al, 2015; Gomez et al, 2014; Martin‐Bello et al, 2019; Zeng et al, 2019) and may present opportunities as analogues for ancient stromatolites. For example, modern lacustrine stromatolites share textural features with Proterozoic stromatolites, including clotted micritic to microsparitic fabric, botryoidal texture and radial‐fibrous fabric (Gomez et al, 2014; Keim et al, 2020), each of which are common in Proterozoic marine stromatolites (Bartley et al, 2000; Knoll et al, 2013) across a wide range of macroscopic forms.…”