Stable oxygen (δ18O) and carbon (δ13C) isotope data obtained from outcrops of the Middle Jurassic Dhruma Formation in central Saudi Arabia were investigated to assess paleoenvironments and determine impacts of diagenesis on the primary isotopic signals archived in the sedimentary rocks. Eleven identified facies in the succession are interpreted as inner to middle carbonate ramp deposits. These facies are grouped into low- to moderate-energy lagoonal, intertidal, moderate-to-high-energy shoal complex, and low-energy open marine settings. The δ13C variations facilitate the identification of six isotopic intervals and two negative excursion peaks in the sequence. The δ18O is less variable than the δ13C, and its record shows a long-term positive trend in the Late Bajocian Niortense through the lower part of the Parkinsoni Zone. Bivariate plots of δ13C and δ18O records reveal that most studied samples are within the northwestern Tethyan marine isotope region. However, a few samples clustered within the burial diagenesis zone. Low values of correlation coefficients (< 0.5) between δ13C and δ18O and heavier values of both isotopes compared to their average Jurassic meteoric water values rule out the influence of meteoric water diagenesis. Higher concentrations of Sr than Mn, low Mn/Sr ratios, and lack of any strong relationship between δ18O and elemental ratios, such as Sr/Ca, Fe/Ca, and Mn/Sr, support the absence of meteoric water influence on the isotope values. Coeval negative excursions at the Niortense/Garantiana boundary are likely due to eustatic sea-level fall. Likewise, the coeval positive excursions and covariance of both isotopes in the dolomite-dominated intervals are attributed to syndepositional diagenesis.
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- Middle Jurassic
- Stable isotopes
ASJC Scopus subject areas
- General Earth and Planetary Sciences