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Keywords:

  • Arabian Plate;
  • mid-Cretaceous;
  • Sarvak Formation;
  • carbonates;
  • chemostratigraphy;
  • karst;
  • Iran;
  • Zagros;
  • Cenomanian;
  • Turonian

Stable-isotope and trace-element analyses from five surface and subsurface sections of the mid-Cretaceous Sarvak Formation in southern and offshore Iran confirm the presence of the regional-scale Turonian unconformity and of a more local Cenomanian–Turonian unconformity. The geochemical results indicate the presence of previously unrecognized and/or undifferentiated subaerial exposure surfaces. Sarvak Formation carbonates at or near palaeo-exposure surfaces show varying degrees of diagenetic modification, and more extensive alteration is associated with longer periods of exposure. The subaerial exposure and associated diagenetic processes greatly influenced reservoir quality and amplified karstification and evolution of porosity in the Upper Sarvak Formation.

The palaeo-exposure surfaces are identified by their negative δ13C values (as low as – 6.4%) and negative δ18O values (as low as –9.4%), together with low Sr concentrations and relatively high 87Sr/86Sr ratios. These geochemical characteristics are interpreted to be the result of the interaction of the Sarvak Formation carbonates with meteoric waters charged with atmospheric CO2. The meteoric waters also caused karstification and soil formation which in some places extends a few metres below the exposure surfaces. Depleted carbon values were not recorded in areas where palaeosols are not well developed or where the uppermost layers of the Sarvak Formation have been removed by erosion.