Challenges and uses of palynological reworking
Palynomorphs are of a similar size to silt particles, and being relatively tough and durable, are capable of being eroded from unconsolidated sediments or solid sedimentary rocks (like silt sized clastic particles) and then included into younger rocks deposited above. In the early days of palynology, this kind of reworking was considered a hindrance to dating and correlation because it was often difficult to tell between reworked and indigenous (i.e. in situ) palynomorphs. However more modern studies have shown the uses of reworking. In this short article I review some of the problems that reworking causes for stratigraphers and geologists, consider some of the uses of reworked palynomorphs helping geologists to understand basin history, and look forward to useful research on reworking .
Reworking problems and solutions
Reworking – where palynomorphs are transferred by erosion and then deposition into younger sedimentary rocks - has long been known (e.g. Streel & Bless 1980). In many cases when the stratigraphy is well known and understood, it is obvious when palynomorphs have been reworked because they are clearly anachronistic – that is - they are in a rock that is younger than they are. There are often other clues to their reworked origin, for example they may be darker than the indigenous palynomorphs because they have experienced higher levels of maturation. When there are enough reworked palynomorphs to be called an assemblage, we also might find that as an assemblage, they are heterogeneous in age. What I mean by this is that they appear to represent different ages, rather than be part of an assemblage with a broadly similar age. This would be impossible in an in situ assemblage.
In early studies, reworking was considered a major problem, particularly in new basins where stratigraphy was not known, so that it was difficult to know what was reworked and what was indigenous (even with the indicators above). This is particularly true in industry where palynological studies of cuttings samples dominate, and a combination of ‘caving from above’ and ‘reworking from below’, make the dating and correlation of rocks particularly hard (see later).
A further complication in certain clastic dominated successions is the phenomenon of reworking of palynomorphs into otherwise palynologically barren rocks. This problem is seen in the pre-Khuff of the Arabian plate where the Hercynian unconformity separates similar clastic sedimentary rocks, usually sandstones and siltstones. In some parts of the Plate, the unconformity may represent a very large hiatus and a very large amount of missing section, often sedimentary rocks that are seals, sources or reservoirs (e.g. Stephenson 2024, Al Husseini 2004). When there is no reworking, and rocks above and below the unconformity yield indigenous palynomorphs, palynology is a very useful technique to pinpoint the unconformity and guide the drill so that important units are not missed, or that hiatuses are identified. But a problem occurs where rocks above the unconformity naturally contain no palynomorphs - in other words they would have been barren if reworking had not occurred (Fig. 1). The very nature of the boundary (being an unconformity) would tend to mean that material from sediments from below would be reworked into the sediments above, for example as rip-up clasts. But these reworked palynomorphs – if occurring within an otherwise palynologically barren sedimentary layer can easily be taken for indigenous. So an anomalously old age could be suggested, as much as a couple of stages or even systems too old (Fig. 1).
Fig. 1. Reworking into rocks that otherwise would be barren
An even more perplexing problem is where rocks above the unconformity are barren of indigenous palynomorphs but are also only available for study in cuttings form. Where samples are only available as cuttings, it is likely that they contain caved palynomorphs that are much younger that the rocks being studied. If this situation is combined with post-unconformity barrenness then a mix of ages of palynomorphs could be apparent in cuttings samples. There could be older reworked palynomorphs from below and younger caved palynomorphs from above. In this situation, it’s particularly hard to date rocks.
How are these problems overcome? Having independent knowledge of the age or stratigraphic position of the stratigraphic unit helps. If you have evidence from nearby wells of the age of a unit, or age by correlation derived from other biostratigraphy or radiometric dating then any reworking will be clear. Similarly in the case of unconformities, if the unit is demonstrably above an unconformity from seismic, sequence stratigraphic or other evidence, then reworking can be implied. In the case of the Hercynian unconformity in the southern Arabian plate, the kind of facies may be an indicator of the position of a rock unit in relation to the unconformity and therefore help in the determination of reworking. This is because in the southern part of the Arabian plate, glacigenic rocks occur above the unconformity whereas those below are not glacigenic. These contextual methods can be used alongside identification of reworked palynomorphs by their different colour, their heterogeneous nature if in an assemblage, and by using various geochemical methods.
Uses of reworking
However as knowledge of basins develops and ages and palynological successions are established, then reworking becomes more obvious and less of a problem. In fact in many studies in the last few decades reworking has been shown to be quite valuable. Some examples from the British Carboniferous include McLean and Chisholm (1996) where reworked palynomorphs allowed the elements of the provenance of the Haslingden Flags to be determined as including outcrops of unmetamorphosed (i.e. extra-Caledonide) Lower Palaeozoic rocks and marine Devonian strata, and/or marine Devonian sediments containing reworked Lower Palaeozoic palynomorphs. McLean (1995) used reworked palynomorphs to determine that the ‘Greenmoor Rock’ is made up of sediment derived from two possible areas: the Wales-Brabant Massif, or Lower Palaeozoic and/or Devonian rocks of the Mid-North Sea High. In the Middle East, Eshet et al. (1988) showed that the stratigraphic variation in the amounts of reworked palynomorphs indicates that they are most abundant in regressive intervals and that stratigraphic variation in the abundance of reworked organic detritus may be useful for identifying depositional cycles that are not otherwise evident in the rock record.
Further work
Given the control that large unconformities have on rocks useful for groundwater, hydrocarbons and CCS, it’s surprising how little systematic mapping of reworked palynomorphs has taken place across key basins, at least in the public domain. This might be a useful approach for the Devonian and Silurian of the Palaeozoic basins of the Arabian Plate. It would be an interesting exercise to map the presence of reworked Devonian and Silurian palynomorphs in post Hercynian unconformity rocks, for example in the Al Khlata Formation and Unayzah Group. Though the geographical distribution of proven Devonian rocks is well known, an understanding of the distribution of reworking and its age may yield a better understanding of the controls on sedimentation at basin margins as well as an understanding of the amounts of sedimentary overburden that might have been removed which could contribute to source rock maturity studies. Similarly maps of the distribution of reworking might help to understand Hercynian intra-basinal faulting which influenced the geography of the Hercynian unconformity and the extent of missing section in areas of the Gulf which are presently being explored.
Note: This article contains only information from public domain peer-reviewed papers.
Prof Mike Stephenson is available for consulting.
References
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.
Eshet, Yoram, Yehezkeel Druckman, Harold L. Cousminer, Daniel Habib, Warren S. Drugg; Reworked palynomorphs and their use in the determination of sedimentary cycles. Geology 1988;; 16 (7): 662–665. doi: https://doi.org/10.1130/0091-7613(1988)016<0662:RPATUI>2.3.CO;2
McLean D. and J. I. Chisholm (1996). Reworked palynomorphs as provenance indicators in the Yeadonian of the Pennine Basin: Proceedings of the Yorkshire Geological Society Volume 51, Pages 141 – 151
McLean D. 1995. Provenance of reworked palynomorphs from the Greenmoor Rock (Langsettian, Late Carboniferous) near Sheffield, England, Review of Palaeobotany and Palynology, Volume 89, Pages 305-317
Streel, M., Bless, M.J.M., 1980. Occurrence and significance of reworked palynomorphs. 703 Mededelingen Rijks Geologische Dienst 32-10, 69–80.
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