Abstract The Mongolian People's Republic in central Asia(frequently called Mongolia, but often referred to in the past as Outer Mongolia in English language literature)encompasses some 2 million km2 of steppe, desert, and mountains. Knowledge and understanding of Mongolian geology is limited, and in particular very little is known outside of Mongolia and Russia. The purpose of this paper is to highlight the development of lake basins, lakes, and lacustrine sequences during the geologic evolution of Mongolia and thereby stimulate further interest and research. Many characteristic features and apparent key controls on lake basin and lacustrine sequence development are described. Lake sediment scan be identified in all geologic periods from the Carboniferous to Quaternary. Collectively, they include examples of all the principal types of tectonically formed lake basins and lacustrine depositional environments. The geology of Mongolia is not known in any detail in western literature, partly due to the country's remoteness and inaccessibility. Pioneering work reported by Berkey and Morris (1927) remains the single most important source of documentation in the English language. Following the changes in Mongolia and the former Soviet Union during the late 1980s,geoscientists can now access the remotest areas of the country. Mongolian and Soviet surface geology maps, made from the 1950s onward, are a key source of data. Most of the stratigraphic units and nomenclature in this paper are based on these maps. The maps, most of which are not officially published, do notably contain some inconsistencies in the use of terms such as "early" vs. "lower"

Abstract Orthochemical sediments associated with Neoproterozoic glaciation have prominence beyond their volumetric proportions because of the insights they provide on the nature of glaciation and the records they hold of the environment in which they were precipitated. Synglacial Fe formations are mineralogically simple (haematite jaspilite), and their trace element spectra resemble modern seawater, with a weaker hydrothermal signature than Archaean–Palaeoproterozoic Fe formations. Lithofacies associations implicate subglacial meltwater plumes as the agents of Fe(II) oxidation, and temporal oscillations in the plume flux as the cause of alternating Fe- and Mn-oxide deposits. Most if not all Neoproterozoic examples belong to the older Cryogenian (Sturtian) glaciation. Older and younger Cryogenian (Marinoan) cap carbonates are distinct. Only the younger have well-developed transgressive cap dolostones, which were laid down during the rise in global mean sea level resulting from ice-sheet meltdown. Marinoan cap dolostones have a suite of unusual sedimentary structures, indicating abnormal palaeoenvironmental conditions during their deposition. Assuming the meltdown of ice-sheets was rapid, cap dolostones were deposited from surface waters dominated by buoyant glacial meltwater, within and beneath which microbial activity probably catalysed dolomite nucleation. Former aragonite seafloor cement (crystal fans) found in deeper water limestone above Marinoan cap dolostones indicates carbonate oversaturation at depth, implying extreme concentrations of dissolved inorganic carbon. Barite is associated with a number of Marinoan cap dolostones, either as digitate seafloor cement associated with Fe-dolomite at the top of the cap dolostone, or as early diagenetic void-filling cement associated with tepee or tepee-like breccias. Seafloor barite marks a redoxcline in the water column across which euxinic Ba-rich waters upwelled, causing simultaneous barite titration and Fe(III) reduction. Phosphatic stromatolites, shrub-like structures and coated grains are associated with a glacioisostatically induced exposure surface on a cap dolostone in the NE of the West African craton, but this appears to be a singular occurrence of phosphorite formed during a Neoproterozoic deglaciation.