Key Hercynian palynomorphs

The subsurface distribution and duration of the hiatus associated with the Hercynian Unconformity (HU) is important for the Palaeozoic hydrocarbon - and more recently carbon dioxide storage - potential of the Arabian Plate.

Although the HU is important for exploration because of its association with missing reservoir and source rock, it is difficult to detect on seismic because it often separates similar sandstone lithological types (though of radically different ages), and because, apart from palynomorphs, it is generally barren of fossils. The ability to image the HU on seismic has improved, but palynology is still the main way that the HU can be pinpointed, and the size of the hiatus it represents, determined. Palynology at the wellsite can be used almost in real time to guide the drill and prevent penetrations into much older rocks than intended.

This short article and accompanying diagram (a higher resolution version is available on request) summarises the key palynological taxa associated with the Carboniferous and Permian beds above and below the HU across the Arabian Plate. Since the HU is mainly a subsurface feature, the basic stratigraphic data comes from key wells in the public domain that span the subcontinent in both basinal and basin margin positions, and public domain wells that contain core-based palynological information that covers the pre- and post HU. However, information on a few key outcrops is also used.

More details of the palynostratigraphy of the HU is given in Stephenson (2024). An upcoming paper will describe the age calibration for the late Guadalupian and Lopingian palynostratigraphy shown here, and descriptions and definitions for new OSPZ zones (OSPZ7 to 9). A longer term project will provide the detailed ranges of the key palynomorphs illustrated here, including their age calibrations.

The key palynomorphs here have stratigraphic ranges across the time period of the HU hiatus. Where the hiatus is large at or close to the Levant, Al Batin, and Oman-Hadhramaut arches, many of these palynomorphs will be missing because the rocks within which they are contained have been removed or never deposited due to the actions of the Hercynian orogeny. Where the hiatus is small for example in the Nafud-Ma’aniya and Faydah-Jafurah basins in western Iraq and the central Gulf respectively a fuller representation of the key palynomorphs will be present. The KH-5/1 well in western Iraq (Nafud-Ma’aniya Basin) appears to show the most complete section (at least in the public domain) with a maximum hiatus duration of part of the Serpukhovian and Bashkirian  (Carboniferous). The key palynomorphs in KH-5/1 and in similar sections elsewhere that occur only above the HU include monosaccate pollen and G. confluens, while others e.g. A. multiseta and V. agadesi, only appear to occur below the HU. So even in areas where the HU is relatively subtle a skilled palynologist can still detect the hiatus. A few key taxa have fairly limited stratigraphic ranges, but still span the minimum duration of the HU, e.g. A. saharaensis and S. triangulus; in other words they occur above and below the HU.

Reference

Stephenson, M H 2024. Palynology of strata associated with the Hercynian Unconformity across the Arabian Plate, from the Levant to southern Arabia, Geological Society, London, Special Publications Volume 550 https://doi.org/10.1144/SP550-2023-182

Prof M H Stephenson is available for consulting