Cambrian Foreland Phosphogenesis in the Khuvsgul Basin of Mongolia

Ediacaran-Cambrian phosphorite deposits in northern Mongolia have been associated with a putative increase in nutrient delivery to the global oceans that drove oxygenation and the rise of animals. However, like many phosphorites from this ~130 Myr interval, the precise age and depositional setting of these deposits remain poorly constrained. Here, we integrate new geological mapping, lithostratigraphy, chemostratigraphy, and U-Pb zircon geochronology to develop a new age and tectonic basin model for the Cryogenian to Cambrian Khuvsgul Group of northern Mongolia. We demonstrate that Cambrian strata were deposited into two composite foreland basins: a ~535–524 Ma pro-foreland basin formed during collision of the Khantaishir-Agardag oceanic arc, and a younger ~523–505 Ma retroarc foreland developed behind the Ikh-Mongol continental arc. The Kheseen Formation phosphorites, which include a Doushantuo-Pertatataka-type microfossil assemblage, were deposited in the pro-foreland basin between 534 and 531 Ma, at least 40 million years later than the phosphatized Weng’an Biota of the Doushantuo Formation of South China. Tectonically-mediated basinal topography associated with foreland development was a necessary condition for phosphogenesis along the Tuva-Mongolia-Zavkhan margin, with different styles of phosphate mineralization associated with sediment starvation and migrating redox boundaries across the margin. The apparent Ediacaran-Cambrian increase in preserved phosphorite deposits was not an event associated with an increase in nutrient delivery to the oceans, but rather represents the opening of a taphonomic window in which a long-term, sustained increase in redox potential enabled increased authigenic phosphate accumulation over a protracted period in marginal marine environments with the requisite tectono-stratigraphic and sedimentological conditions.