Hercynian unconformity

Across the Arabian Plate a stratigraphic gap is present within the Palaeozoic, sometimes known as the Hercynian unconformity. In this small article I explain why the Hercynian unconformity is so important geologically and why palynology is useful in identifying the unconformity, and the beds missing because of the unconformity.

Why is the Hercynian unconformity important to explorationists?

The areal extent of the unconformity and the hiatus it represents are important economically because typical Arabian source and reservoir rocks are often not present because of erosion (or non-deposition) related to the unconformity. Rocks affected by this include the Berwath, Jubah, Jauf, Wajid and Qalibah formations in Saudi Arabia and other Gulf states; the Wajid Formation in Yemen; and the Haima Supergroup in Oman. The unconformity therefore affects access to geological formations that could hold oil and gas, or could be suitable for CCS targets, for geothermal, or for heat and coolth storage.

The duration of the stratigraphic gap associated with the Hercynian unconformity varies hugely across the Arabian plate. Well sections provide very important evidence as to the biostratigraphy of the unconformity. Northern well sections containing stratigraphy that is useful in defining the unconformity include those in ABSF-29 (offshore Arabian Gulf), ST-8 (northern Saudi Arabia) and KH-5/1 (Iraq, see map). These well sections have stratigraphic coverage of both the oldest post-unconformity beds and the youngest pre-unconformity beds, and have been used to most narrowly constrain the age of the event that caused either the non-deposition or erosion associated with the Hercynian unconformity. Although the name implies a relationship with the European Hercynian orogeny, there may be no connection.

Why is palynology important to identifying and determining the missing beds associated with the Hercynian unconformity?

Much of the Palaeozoic of the Arabian plate is made up of clastic sedimentary rocks that contain palynomorphs (defined as organic-walled microfossils 5–500 µ in diameter) as the only fossil content. Often the best way of distinguishing these beds, which are otherwise very similar in appearance, is through their palynomorph content. The succession of palynological assemblages is quite well established as you rise through the stratigraphy of the Palaeozoic succession from older to younger, and so missing strata can be spotted if certain assemblages are absent.

Perhaps the most obvious use of this succession of palynological assemblages is in guiding drilling. A problem for drillers is drilling unknowingly through the Hercynian unconformity into sometimes much older rocks, missing their target. Palynology is very important in preventing this because a palynologist, consulted by the drillers and given up to date information, can tell you whether you have drilled through the target formation and the Hercynian unconformity and have entered a deeper and perhaps much less permeable or less economically useful stratum. An example of this in Oman where in parts of the country, the Hercynian unconformity separates the prospective and productive Carboniferous-Permian Al Khlata Formation from the much older (Ordovician-Cambrian) Haima Supergroup, or the Proterozoic Huqf Supergroup. In the case of the Al Khlata-Haima transition, the palynological distinction is very clear because the palynomorph assemblages above and below the unconformity are so different. Similar situations are present in central Saudi Arabia where the Carboniferous-Permian Unayzah Formation is separated from the Silurian Qalibah Formation by the Hercynian unconformity.

Palynological work of this type focussed on missing strata, has allowed the duration of the hiatus associated with the Hercynian unconformity to be deduced at key locations across the Arabian Plate. Perhaps the most precise delineation of the hiatus where it covers a relatively short period of geological time is in the fully cored water well KH-5/1 in western Iraq (Stephenson et at. 2017; see map).

Location of KH-5/1 deep water well

The part of the KH-5/1 well section in question contains the Raha Formation overlain by the Ga’ara Formation (see section). The boundary between the Raha Formation and the Ga’ara Formation has been picked previously at 670m on the basis of a change in sedimentary facies. This change in sedimentary facies could be an indicator of the presence of the Hercynian unconformity.

Palynologically, there are two broad assemblages between 796 and 486m in the KH-5/1 well section (see section; Stephenson et al. 2017). The lowest (between 796 and 739m) called ‘Assemblage A’ consists of spongeous zonate spores mainly assignable to the genus Lunbladispora, species of Vallatisporites, and Spelaeotriletes (mainly S. triangulus); also present are Schopfipollenites ellipsoides, Apiculiretusispora multiseta, Rugospora sp. and Retusotriletes incohatus. Assemblage A is likely to be Tournaisian to earliest Namurian in age.

Between 665 and 486m is the more diverse Assemblage B which is distinct from that below, chiefly because it contains abundant bisaccate and monosaccate pollen in addition to common Lunbladispora (e.g. L. braziliensis), and related taxa (e.g. Densoisporites truswellii). From 615.5m taeniate bisaccate pollen also becomes quite common. A few horizons between 615.5 and 593.5m yield quite diverse spores including Vestispora spp., Reticulatisporites polygonalis, Raistrickia saetosa and Aratrisporites saharaensis. Taeniate bisaccate pollen (an example is pictured) includes Protohaploxypinus amplus, P. limpidus and Complexisporites polymorphus. Assemblage B is likely to be Westphalian in age.

A taeniate bisaccate pollen grain (about 100µ across) from the assemblage above the unconformity in KH-5/1

An unconformity situated around 670m in KH-5/1 could therefore represent non-deposition or erosion of rocks corresponding approximately in age to part of the Serpukhovian and Bashkirian (see section). However it is also possible that the palynologically barren section (of maximum thickness 74m) between 739 and 665m, containing the unconformity, could represent part or all of this time period, depending on the duration of the hiatus related to that unconformity. To precisely delineate the hiatus related to the unconformity would require productive palynological samples right above and below the unconformity. This was not possible in this case.

Palynology of the Hercynian unconformity in KH-5/1 deep water well. For details see Stephenson et al. (2017)

This kind of analysis shows how powerful palynology can be in both identifying the presence of an important unconformity, but also in being able to quite precisely delineate the hiatus in time represented by that unconformity.

References

For more details of the palynology of the Hercynian unconformity in KH-5/1 have a look at:

Stephenson, M H, Kader Al-Mashaikie, S Z A, and Kurukchi, W M. 2017. Palynological assemblages across the Hercynian unconformity in Western Iraq. Revue de Micropaleontologie, 60, 417-432.

For more general discussion of the Hercynian unconformity have a look at:

Al-Husseini, M.I., 2004. Pre-Unayzah unconformity, Saudi Arabia. In: Al-Husseini, M.I. (Ed.), Carboniferous, Permian and Early Triassic Arabian Stratigraphy. GeoArabia Special Publication 3, Gulf PetroLink, Bahrain, 15-59. https://pubs.geoscienceworld.org/gpl/books/book/2224/chapter/125392445/Pre-Unayzah-unconformity-Saudi-Arabia

Acknowledgement

Dr Sa’ad Zeki A. Kader Al-Mashaikie of the Department of Geology, College of Sciences, University of Baghdad for obtaining the KH-5/1 samples.

Available for consulting

Prof Mike Stephenson is Director of Stephenson Geoscience Consulting

Web: https://www.stephensongeoscienceconsultancyltd.com/

Email: mikepalyno@me.com