Khuff Unconformities

Problem: The Khuff, Dolan and Chia Zairi Formations span the Permo-Triassic boundary. Paleontologic data identifies this horizon as a major unconformity, but its sedimentologic expression is poorly documented. Here, we provide available documentation for the expression of this important boundary.

The Evidence : Portions of two cores from the Khuff B member shown below exhibit caliche soil profiles. These subarial exposure horizons include corroded brecciated textures, risoliths, and vug porosity plugged by anhydrite cement or locally crystal silt. There are other cored Khuff unconformities that have similar paleosol features, but none other than the unconformity illustrated below, mark a mass extinction of marine biota.

Implication: A subarial exposure surface identifies the Permo-Triassic boundary in Saudi Arabia.

Problem: The Middle or Jir Anhydrite forms an inportant correlation horizon but its position within a sequence stratigraphic framework is debatable. Part of the problem reflects the uncertainty about its relationship to an unconformity. Again, no hard data exists to support one view or another.

The Evidence: Observation on a single core from Saudi Arabia shows that an unconformity marks the carbonate transiton into the overlying anhydrite and another unconformity seperates the anhydrite from the overlying carbonate section. In Iran, the anhydrite also has a basal unconformity but the transition into the overlying carbonate appears to be gradational.

Implication: The unconformity at the base of the anhydrite seems to be regional whereas the one observed in Saudi Arabia is not. Thus, the anhydrite section is genetically linked to the overlying carbonate sequence, not the underlying one. I hope to be able to illustrate these two cores soon.

Problem: Made up of an interbedded succession of carbonate and thick anhydrite over much of the Arabia Platform, the Khuff A and B members define two major carbonate-evaporite cycles (Sharland, et.al. 2001). Currently conseptual models more so than ground truth drive the sequence stratigraphic interpretation of this part of the Khuff Formation. Consequently opposing views exist. Sharland, et.al. (2001) believe the evaporite deposits represent highstand accumulations terminated by an unconformity. Conversely, Johnson, et.al. (1998) submit sequence boundaries should beplaced at the base of the evaporites because they are lowstand accumulations. The question is where are the unconformities actually located in the stratigraphic succession.

Critical Evidence: Core examined from Saudi Arabia reveals evidence of exposure at all carbonate - evaporite boundaries. None of these surface exhibit characteristics of being a major unconformity and therby provide little definitive sequence stratigraphic support. A correlative and entirely carbonate section cored in Abu Dhabi contains a single major unconformity. Regional gamma ray correlation establishes that the unconformity equates with exposure horizons at the top of black anhydrite deposits in Saudi Arabia.

Implication: The black anhydrite deposits represent highstand accumulations. Thus, the observation supports the view of Sharland, etal. (2001).

Core 1

Corel 2