Petrological and geochemical characteristics of Ordovician cherts in the Qasim Formation, NW Saudi Arabia: Understanding the roles of biogenic and volcanic activities

Abstract

The Upper Ordovician Qasim Formation features bedded cherts with mudstone interbeds, which are overlain by Tertiary volcanic rocks. The origin and formation conditions of the cherts are almost completely unknown. To address these, this study employed thin-section petrography, XRF analysis, REE geochemistry, SEM analysis, and stable oxygen isotopes. Results of the study indicate that the cherts are primarily biogenic, derived from siliceous sponges, with contributions from terrestrial and river sources. Silica from marine upwelling and terrestrial sources enhanced biogenic activity, leading to the formation of the chert. Ratios of Si/(Si + Al + Fe) and Al/(Al + Fe + Mn) support a biogenic silica origin, while REEs suggest formation under oxic marine conditions (Ce/Ce* anomaly 0.3–1.0, avg. 0.7). The cherts also show an average Y/Ho value of 33.3, indicating marine influence with freshwater input. Diagenesis played a vital role in the formation of the cherts, with microcrystalline quartz replacing carbonate bioclasts and subsequently partly recrystallizing into blocky mega-quartz near chert-basalt contact. The microquartz cherts have δ ¹⁸O values of 26.7 to 33.7 ‰ (± 0.6 ‰), corresponding to formation temperatures of approximately 14 to 45 °C (avg. 38 °C; ±3 °C), whereas the mega-quartz chert has a δ ¹⁸O of 20.4 ‰ (±0.6 ‰), reflecting a formation temperature of approximately 115 ± 5 °C). These oxygen isotope values and temperatures reveal that the microquartz chert formed in cooler, shallow marine conditions, while the megaquartz chert was affected by a significant thermal event at the contact with the Tertiary basalt emplaced over the chert layers. These findings provide insights into paleoenvironmental conditions, thermal history, and the impact of later volcanic activity on chert recrystallization.