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Allostratigraphy, a practical solution to the Gharif/Khuff boundary discrepancies in the Lower Permian of Oman

Allostratigraphy, a practical solution to the Gharif/Khuff boundary discrepancies in the Lower Permian of Oman

Newsletters on Stratigraphy 35(2): 83-95

In the Lower Permian of North Oman, continental clastics of the Gharif Formation pass upwards into shallow marine carbonates and redbeds of the Khuff. Due to the gently dipping shelf, facies are laterally continuous and changes are gradual. As a result, wireline logs indicate many markers which can be correlated for many kilometers. Cyclicity is obvious and is believed to be related to global eustatic sea level changes. Dual lithological criteria are used to define the Gharif/Khuff boundary: base carbonates in North Oman and top sands in South Oman. This boundary was assumed to be approximately synchronous. However, regional correlation of data proved that the dual criterion created major inconsistencies. Correlations using a sequence stratigraphic timeframe indicate the top Gharif and base Khuff carbonate to be diachronous, particularly in Central and South Oman. A conventional stratigraphic solution to this problem would be to accept the diachroneity of the units, restrict all the carbonates to the Khuff Fm. and include all the Khuff Redbeds in the Upper Gharif as a new U. Gharif Shale Member. However, in wireline log interpretation markers, defined as correlatable log picks, are considered to constitute synchronous events. The same applies for seismic interpretation where synchronous seismic reflections are often translated into lithostratigraphic units. To reconcile both these and stratigraphic requirements, it is here proposed to apply Allostratigraphy, as it provides: (1) a chronostratigraphic framework based on sequence boundaries and maximum flooding surfaces, and (2) in this framework, to identify lithological units, resulting in a comprehensive facies mosaic. Allostratigraphy of the Gharif/Khuff transition sequence results in a series of Cycles, each characterized by a Khuff Carbonate Allomember and a Khuff Shale Allomember. Top Gharif, defined as top sands, would occur at variable positions, but mainly in the PA 3.1 Cycle.

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