Sequence Stratigraphy and Chemostratigraphy Across the Permo-Triassic Extinction Event, Upper Khuff Carbonates, Ghawar Field, Saudi Arabia

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Virginia Tech


Logging of cores of the Upper Permian and Lower Triassic Khuff Formation, Ghawar, Saudi Arabia, has allowed a high resolution sequence stratigraphic framework to be generated. The lithofacies of this huge, arid epeiric ramp succession include: subaqueous -and supratidal anhydrite, tidal flat laminites, lagoonal mudstone, ooid-peloid grainstone, and subtidal off-shoal open marine mudstone.

Third order sequences include the Late Permian upper Khuff C, the Early Triassic Khuff B and the Khuff A sequences, which corrrelate with global cycles. Seven high frequency sequences (HFSs) make up the Changhsingian upper Khuff C. These HFSs are ~400 k.y. duration and probably driven by long term eccentricity. The Early Triassic Khuff B and A sequences are made up of 4 HFSs each, which appear to be ~100 to 200 k.y. duration and not easily tied to eccentricity forcing. The HFSs are in turn composed of parasequences, which appear to be 10 to 20 k.y. average durations, suggesting precessional and half precessional forcing. However, many thin locally developed cycles may be autocycles or subprecessional cycles.

Sequence stratigraphic cross sections and facies maps document progradation directions on the platform, reflecting the subtle interplay between the Ghawar structure and regional paleoslopes. Anhydrites are rare in the Permian Upper Khuff C except near the base of the studied interval. Anhydrites are well developed in the Triassic Khuff B and Khuff A where some form transgressive deposits while others are highstand deposits of high frequency sequences.

The Permian-Triassic boundary (PTB) on the Arabian Platform marks a significant relative sea-level drop, that exposed from the outcrop belt to somewhere east of Ghawar. This contrasts with transgressive PTB settings elsewhere. Across the PTB the mass extinction is marked by a major decrease in biotic groups. The extinction was followed by development of subtidal thrombolites and increased microbial calcification due to decreased bioskeletonization.

The dominant reservoirs in the Permian Upper Khuff C occur in oolite in the uppermost high frequency sequence. In the Triassic Khuff B and A the reservoir facies are commonly non-dolomitized oolitic facies associated with open lagoon carbonates distant from evaporitic tidal flats. Within dolomitized units, best reservoirs are associated with oomoldic porosity, but oolite units proximal to evaporitic tidal flats have porosity plugged by anhydrite.

Carbon and oxgyen isotope profiles up to 150 m long were obtained from cored wells of the Khuff Formation, Ghawar Field, Saudi Arabia, across the Permian-Triassic boundary. Major global excursions are at the Changhsingian-Wuchiapingian boundary and the Permian-Triassic boundary, but several smaller excursions also appear to correlate with excursions elsewhere. The presence of the negative C-isotope excursions globally in both δ¹³Ccarbonate and δ¹³C organic as well as in deeper water sections lacking emergence surfaces, strongly supports the idea of these excursions being global phenomena related global C cycling.

Over 75% of the negative carbon isotope excursions in Ghawar occur beneath emergence surfaces, including the two major excursions at the Wuchiapingian and Changhsingian stage boundaries. The δ¹³C profiles beneath the boundaries resemble those iii associated with early diagenesis associated with isotopically light soil gas. The δ¹⁸O profiles beneath the surfaces are variable, perhaps reflecting variable effects of evaporation on the meteoric input, mixing or overprinting by burial diagenesis. This suggests that the C-isotope excursions on the Arabian Platform, although global in origin, appear to have been modified by early diagenesis.

U depletion across the boundary is compatible with the postulated origins of the PTB event with bottom waters becoming stagnant and reducing, as a result of warming induced by volcanogenic CO₂ released by Siberian trap volcanism, methane release from thermal metamorphism of coals and destabilization of clathrates in the deep sea due to ocean warming. The global extent of the C-isotope and U excursions provides a high resolution correlation tool for Late Permian and Early Triassic successions.



Uranium depletion, C/O isotopes, Sequence stratigraphy, Carbonates, Khuff Fm., Permo-Triassic boundary, Ghawar Field