Reducing-condition reworked phosphorite sedimentary model: Petrographic and geochemical evidence from the Lower Cretaceous in the Thaniyat area, northern Saudi Arabia

Abstract

The Thaniyat phosphorite, occurred in northern Saudi Arabia, is a component of a massive phosphorite deposit that was formed in the Late Cretaceous to Paleogene era within the southern Neo-Tethys Ocean. To gain insights into the sedimentary environments and formation processes of these phosphorites, samples were collected from the Thaniyat Phosphorite Member in northern Saudi Arabia and analyzed using petrological and geochemical methods. To ensure a comprehensive understanding, the obtained results were compared with results from nearby sites. The Thaniyat phosphorite is composed of phosphatic grains and quartz grains with distinctive granular textures. The phosphatic grains were derived from pristine phosphorite present along the shelf margin, while the quartz grains were supplied by terrestrial input. The presence of this dual provenance is furtherly supported by the negligible correlation observed between P2O5 content and the content of Al2O3 and TiO2. Trace element proxies provide additional insights into the sedimentary setting of the phosphorite, indicating that it occurred in an upwelling environment, which caused relative enrichment of Cr and a positive correlation between the value of Co × Mn and Al2O3 content of the phosphorite. Proxies related to Fe and Mn were found to be unsuitable for predicting the redox conditions in this area. A combination of V, Mo, and U proxies suggests a prevailing suboxic environment during the deposition of the Thaniyat phosphorite. A comparative analysis with contemporaneous phosphorite samples from Jordan and Iran demonstrated consistent characteristics of the sedimentary environment. Building upon these findings, a sedimentary model was proposed to elucidate the depositional processes of the phosphorite in a suboxic environment, where an upwelling with an extensive oxygen minimum zone dominated the sedimentary environment. The extensive reducing sedimentary environment likely facilitated carbon fixation and subsequently climate change. These findings contributed to a better understanding of the role of phosphorite in the paleoenvironmental context of the southern Neo-Tethys Ocean.