Khuff diachronism

The Khuff Formation is one of the most productive natural gas reservoirs in the world with supergiant fields across the offshore Gulf supplying LNG to the rest of the world for many years. But the formation is also fascinating geologically. It can be seen at outcrop all over the Middle East and its precursor beds - known as the ‘basal Khuff clastics’ in Arabia - are well known for their superb plant fossils. These basal Khuff clastics also contain rich fossil pollen and spore assemblages that suggest that the basal Khuff clastics, and by association the base of the Khuff carbonate facies, is highly diachronous.

Khuff palynology

The Guadalupian to Early Triassic Khuff Formation represents a major transgressive-regressive cycle deposited along the northern Gondwanan margin across the Arabian Peninsula, Iran (where it is known as the Dalan Formation), and Iraq (Chia Zairi Formation). Palynomorphs are rarely recovered from the Khuff Formation itself in core or outcrops, due to inimical preservation conditions in limestone, and modern desert weathering. However, the basal Khuff clastics, part of the genetic package of the Khuff Formation (Osterloff et al., 2004), often yield well preserved palynomorphs from both the subsurface and at outcrop. The basal Khuff clastics are also known across the Arabian Peninsula for their unusually well-preserved plant fossils, for example in Oman the ‘Gharif Palaeoflora’ (Broutin et al., 1995), and in Saudi Arabia the ‘Unayzah Plant Beds’ (El-Khayal and Wagner, 1985). The Jordanian Umm Irna plant beds (Kerp et al., 2006) are similar taxonomically and palaeoenvironmentally to the Gharif and Unayzah plant beds.

Stephenson and Powell (2013) showed that the Umm Irna Formation of Jordan and the Arqov Formation of the Negev have closely similar fluvial and paralic depositional environments to those described for the upper Gharif Formation alluvial plain ‘Type Environment P2’ in the subsurface in Oman (Osterloff et al., 2004) and the basal Khuff clastics at outcrop and in the subsurface in central Saudi Arabia. Evidence from the palynology of the Umm Irna Formation and Negev boreholes (Stephenson and Powell 2013, 2014; Stephenson and Korngreen 2020, 2021) shows that markedly younger taxa characterise possible basal Khuff clastics equivalent beds in the Levant (Fig. 1).

Fig. 1. (A) Diachronism of the basal Khuff clastics and its equivalents. (B) Middle Permian continental configuration. 1 = Jordan/Negev ; 2 = Saudi Arabia; 3 = Oman, (map modified after Muttoni et al. 2009)

The distinctive multi-taeniate bisaccate pollen Protohaploxypinus uttingii and the tri-sulcate pollen Pretricolpipollenites bharadwajii (Fig. 2) are particularly significant because they seem to mark the Lopingian (Late Permian) and they only occur in basal Khuff clastics further north in Arabia and the Levant.

Fig. 2. Protohaploxypinus uttingii (upper two rows) and Pretricolpipollenites bharadwajii (lower row)

Cause of possible diachronism

Mapping the occurrences of key species in the palaeogeography of the time indicates the diachronous onset of the basal Khuff clastics appears to occur northwest through the Arabian Plate (Fig. 1) over an extended period of around 15 my, from the early Wordian (base ~269 My) to the Changhsingian (base ~254 My). The basal Khuff clastics may indicate a period of humid high runoff producing river systems draining previously drier hinterlands, that heralded the transgression that brought on the main Khuff Formation carbonate sedimentation. This carbonate sedimentation persisted for 20 My in parts of the Middle East.

Angiolini et al. (2003) suggested that the Khuff Formation is the result of a major transgression of the Neotethys ocean in the Wordian caused by thermal subsidence of adjacent land and rift shoulders. They also suggested that similar transgressions took place during the Wordian in the Himalayas, Tunisia, and the Salt Range of Pakistan. Palynology seems to indicate either slower thermal subsidence or less thermal subsidence in central Arabia and the Levant since this seems to have been transgressed later. It could also indicate persistent ‘Hercynian’ highs such as the central Arabian arch, or in the Levant, the Helez Geanticline (Gvirtzman and Weissbrod, 1984), or the influence of the Arabian Nubian Shield. All of these highs perhaps resisted transgression until later.

Further studies are required to understand the reason for what appears to be a diachronous base for the Khuff Formation and its equivalents. Comparison with similar tectono-stratigraphic events elsewhere in the Phanerozoic may also provide insights.

References

Angiolini, L., Balini, M., Garzanti, E., Nicora, A., Tintori, A., Crasquin, S. and Muttoni, G., 2003. Permian climatic and paleogeographic changes in Northern Gondwana: the Khuff Formation of Interior Oman, Palaeogeography, Palaeoclimatology, Palaeoecology, v. 191, p. 269–300.

Broutin, J., Roger, J. Platel, J.P. Angiolini, L. Baud, A. Bucher, H. Marcoux J. and Al Hasmi H., 1995. The Permian Pangea. Phytogeographic implications of new palaeontological discoveries in Oman (Arabian Peninsula). Compte Rendus de l’Academie des Sciences de Paris, Series IIa, v. 321, p. 1069–1086.

El-Khayal, A.A., and R.H. Wagner, 1985. Upper Permian stratigraphy and megafloras of Saudi Arabia: Palaeogeographic and climatic implications. Dixième Congrès International de Stratigraphie et de Géologie Carbonifére, Madrid, 1983. Compte Rendu, v. 3, p.17–26.

Gvirtzman, G. and Weissbrod, T., 1984 The Hercynian Geanticline of Helez and the Late Palaeozoic history of the Levant. Geological Society, London, Special Publications, v. 17, p. 177–186.

Kerp, H., Abu Hamad, A., Vörding, B., and Bandel, K., 2006. Typical Triassic Gondwana floral elements in the Upper Permian of the paleotropics. Geology v. 34, p. 265–268.

Muttoni G., Gaetani M., Kent D V, Sciunnach D., Angiolini L., Berra F., Garzanti E., Mattei M. and Zanchi A. 2009. Opening of the Neo-Tethys Ocean and the Pangea B to Pangea A transformation during the Permian. GeoArabia, v. 14, p. 17-48

Osterloff, P., A. Al-Harthy, R. Penney, P. Spaak, G. Williams, F. Al-Zadjali, R. Knox, M. Stephenson, G. Oliver and M.I., and Al-Husseini 2004. Depositional sequence of the Gharif and Khuff formations, subsurface Interior Oman. In M.I. Al-Husseini, ed. Carboniferous, Permian and Early Triassic Arabian Stratigraphy. GeoArabia Special Publication 3, Bahrain, Gulf PetroLink, p. 83–147.

Stephenson M. H., Korngreen, D., 2020. Palynological correlation of the Arqov and Saad formations of the Negev, with the Umm Irna Formation of the eastern Dead Sea, Jordan, Review of Palaeobotany and Palynology, v. 274, 104153.

Stephenson Michael H, Korngreen, D 2021. Palynology of the Permian of the Makhtesh Qatan-2, Ramon-1 and Boqer-1 boreholes Arqov Formation, Negev. Rivista Italiana di Paleontologia e Stratigrafia, V. 127 NO. 3

Stephenson, M. H. and Powell, J. H., 2013. Palynology and alluvial architecture in the Permian Umm Irna Formation, Dead Sea, Jordan. GeoArabia v. 18, p. 17–60.

Stephenson, M H, and Powell, J H. 2014. Selected spores and pollen from the Permian Umm Irna Formation, Jordan, and their stratigraphic utility in the Middle East and North Africa. Rivista Italiana di Paleontologia e Stratigrafia, 120, 145-156.