GEOLOGICAL CONTEXT FOR THE ONSET OF THE SHURAM CARBON ISOTOPIC ANOMALY, KHUFAI AND SHURAM FORMATIONS, SULTANATE OF OMAN

Ediacaran-age rocks of the Huqf Supergroup, Oman, preserve a major anomaly in the carbon isotopic composition of sedimentary rocks. This anomaly, which is marked by a rapid drop from +6 ‰ to -12 ‰, has been the subject of growing scrutiny and debate. While geochemical models for diagenetic origins have been presented, the geological context of the formations is not sufficiently well characterized. Here we present sedimentological, stratigraphic, and isotopic data from the Khufai Formation that record the initial decline to negative δ¹³C values. In the Huqf outcrop belt of central Oman the anomaly begins in peritidal carbonates of the upper Khufai formation. The anomaly continues through a minimum of -12 ‰ in the carbonate-cemented siltstones of the middle Shuram Formation; shows initial recovery in sandstone/carbonate oolite couplets of the upper Shuram; and recovers fully to values of 0 ‰ in the overlying Buah Formation. This pattern is repeated in cores through the South Oman Salt Basin, as well as in the thermally overmature basinal facies of the Oman Mountains. In the latter case the Khufai and Shuram were buried deeply yet retain the same values expressed in the shallowly buried rocks of the Huqf area (1-2 km), and the moderately buried salt basins (3-5 km). In sequence stratigraphic context, the initial anomaly is recorded by a transgressive system tract, with the maximum flooding interval occurring in the middle Shuram before shallowing to the oolitic highstand systems tract of the upper Shuram. Additionally, hopane and sterane ratios from Huqf samples indicate low thermal maturity inconsistent with deep burial diagenesis. Secondary incorporation of depleted carbon into carbonates requires recrystalization by either C-rich fluids during burial, or surficial meteoric fluids in equilibrium with soil CO₂: both are inconsistent with our observations. Regardless of their timing, these models require a large reservoir of organic carbon; a condition that must have increased in the Phanerozoic with expansion of the terrestrial biosphere. In this context it therefore appears paradoxical that isotopic excursions of similar magnitude and global extent to the Shuram anomaly do not recur during Phanerozoic history when extensive reservoirs of depleted carbon existed in both soils and petroleum source rocks.