THE NEOPROTEROZOIC-LOWER CAMBRIAN IN NORTHERN GOVI-ALTAY, WESTERN MONGOLIA: REGIONAL SETTING, LITHOSTRATIGRAPHY AND BIOSTRATIGRAPHY

Abstract

This paper provides a framework for the stratigraphy and correlation of key sections through the Neoproterozoic–lower Cambrian of the Govi-Altay area, western Mongolia, studied by members of IGCP Project 303. Dzabkhan Formation volcanic rocks at the base of the succession are suggested to have formed during rifting of the Baydrik–Tarbagatay microcontinent in the Baykalian interval (c. 850–650 Ma). Pre-Vendian accretion transformed this rift and adjacent microcontinental fragments into a marginal marine back-arc basin, in which accumulated relatively uninterrupted Neoproterozoic and earliest Palaeozoic sediments. Lithostratigraphy for the latter is defined, with detailed maps, at five sections (Bayan Gol northern and southern blocks, Tsagaan Gol, Salaany Gol and Tayshir). The Tsagaan Oloom Formation lower member contains two diamictites of possible Sturtian age. Middle and upper carbonates are compared with the Yudomian (Vendian) of Siberia, the former ending with a unit of Boxonia grumulosa stromatolites. In the upper member, phosphorite-shale is followed mainly by limestones, with the earliest skeletal fauna. The Bayan Gol Formation comprises siliciclastic–limestone intercalations with increasingly diverse invertebrate biota. Correlation of lithological marker beds across the study area produces a composite stratigraphic section to which the earliest skeletal fossil occurrences are tied. The Anabarites trisulcatus Zone of the Nemakit-Daldynian Stage begins near the base of the Tsagaan Oloom Formation upper member (unit 11). The Purella antiqua Zone begins near the base of the Bayan Gol Formation (unit 18). Assemblages typical of the Nochoroicyathus sunnaginicus Zone (base of the Tommotian Stage) range through units 19 to 23. Elements typical of the Dokidocyathus regularis Zone (middle Tommotian) appear in unit 24 near the top of the Bayan Gol Formation.