TEMPORALLY AND SPATIALLY CONSTRAINING A DOUSHANTUO-STYLE MICROFOSSIL-BEARING PHOSPHORITE, KHUBSUGUL GROUP, MONGOLIA

One of the largest known phosphorite deposits in the world, phosphorites in the Khesen Formation of the Khubsugul Group (Khubsugul Province, northern Mongolia) preserve a diverse Doushantuo-style fossil assemblage. The Khesen Formation presents an excellent opportunity to observe and constrain the evolution and preservation of post-Cryogenian microbiota, as well as investigate the genesis and emplacement of major end-Neoproterozoic/early-Paleozoic phosphorite deposits.

While previous workers have interpreted the Khesen phosphorite to directly postdate and be genetically linked to the termination of the Marinoan glaciation, initial δ¹³C chemostratigraphic correlation with the Zavkhan basin of central Mongolia suggests a late Ediacaran depositional age. Newly developed geochronology, high-resolution chemostratigraphy, and geologic mapping provide direct constraints on the age and duration of phosphorite deposition, as well as the geometry and tectonic evolution of the basin in which it was deposited. Detailed mapping has revealed significant facies changes in phosphorite deposition throughout the basin: while the development of phosphatic hardgrounds and fossiliferous phosphorite textures occurs primarily in the shallower eastern-central reaches of the basin, thick accumulations of granular phosphorite (putatively sourced from hardground-forming regions in the shallow portions of the basin) dominate the western and southern basinal extent. Shallower, fossiliferous sections in the basin, many of which contain highly negative δ¹³C values (up to -16‰), are examined with respect to bioproductivity, taphonomy, basinal circulation, and processes of phosphate and carbonate production, while coeval sections from across the basin are compared in order to further elucidate the paleoenvironmental conditions in which the voluminous phosphorites of the late Ediacaran/early Paleozoic were deposited.