Late Middle to Late Ordovician Phosphate Accumulation of the Moyero River Section (Siberia): A Record of Upwelling and Global Cooling

The phosphate-bearing rocks (phosphate rocks and phosphorites) were studied over a 45-meter interval of the Moyero river section, covering the upper part of the Darriwilian and the lower part of the Sandbian stages of the Ordovician. These rocks were investigated by field observations and laboratory methods, including optic and scanning electron microscopy, X-ray diffraction analyses. The accumulation of phosphatic matter is manifested in carbonates, sandstones, and aleurolites in the form of grains (ooids and peloids), intraclasts, phosphatic and phosphatized shells. In argillites, it is represented by cryptocrystalline matter. Phosphate matter consists of fluorapatite. The peaks of phosphate accumulation are associated with the formation of layers of physically reworked granular phosphorites (condensation horizons) directly above the depositional sequence boundaries. The formation of the studied phosphate-bearing rocks during the Darriwilian-Sandbian transition was influenced by a combination of global (Great Ordovician Biodiversification Event (GOBE), reduction in sea surface temperature, atmospheric CO2, high sea level, flooding of craton margins) and regional (equatorial position of Siberia, arid climate, facies) factors. Global conditions led to the enrichment of seawater with phosphorus and the effect of upwelling. Regional conditions determined the characteristics of phosphate formation. Studied phosphate-bearing rocks can be considered as a record of upwelling on the Siberian craton during the Middle-Late Ordovician transition and one of the manifestations of long-term global cooling started early in the Middle Ordovician.