Calibrating palynostratigraphy

Palynology is used throughout the world to correlate and date clastic sedimentary rock successions. In some parts of the world, particularly Palaeozoic Gondwana basins, it is the main tool to correlate subsurface formations, where sub-anhydrite seismic is poor and where there is extreme lithological variation between wells.

Many palynological zonations are of very high resolution, allowing thin subsurface units to be correlated and identified. This is of great value in field scale correlation. However these zonations are often of limited value beyond the region in which they were developed because it is difficult to compare them to other fossil zonations, and therefore to zonations that are used to define the standard subdivisions of global stratigraphy at system (e.g. Permian) or stage (e.g. Sakmarian) level.

This might not seem important. Why is it useful to establish correlations outside your basin or region, or to be able to date to stage or substage level?

The answer is that correlation helps palynologists, stratigraphers and geologists to relate sedimentary rocks deposited in one place to those in another, and to relate geological resources or events to each other and to other important geological or scientific phenomena, such as climate and environmental change. Without this ‘contact with the outside world’, some of the predictive value attached to stratigraphy is lost (Stephenson 2018).

In this short article I describe three examples of how palynological zonations can be calibrated to the standard scale, the official arrangement of systems and stages.

Radiometric dating of a palynological zone

The first is perhaps the most direct. In the first example a palynological zone is related to the standard subdivisions of global stratigraphy simply by yielding a radiometric date.

In this case the zone is the Converrucosisporites confluens Oppel Zone which is assigned to part of the Ganigobis Shale Member in Namibia, the lowest unit of the glaciogenic Dwyka Group, part of the Karoo Supergroup in southern Namibia (Stephenson 2009; Fig. 1). But the Ganigobis Shale Member also contains thin volcanic ash layers (Fig. 2). Ash layer IIb of the Ganigobis Shale Member was radiometrically dated at 302.0 ±3.0 Ma. This date, though imprecise, indicated the possibility that the Converrucosisporites confluens Oppel Zone may range earlier than previously thought, because it had been originally assigned entirely to the Permian (Stephenson 2009).

Fig. 1 Lower Ganigobis Shale at the Fish River section

Radiometric dates for Palaeozoic Gondwana basins are getting more precise. For example, recent radiometric dating of the Permian of Australia (Laurie et al. 2016) has relatively high resolution allowing isolated uncorrelated palynological biozonations to be related to the standard scale and the bigger picture in regional and continent-wide geological evolution.

Fig. 2 Volcanic ash layers in the Ganigobis Shale

Dating using an independent fossil group

The next example of calibration of palynological biozones is less technological in that it involves simply using another independent fossil group, namely the fusulinids. Fusulinids are very scarce in the Permian of the former Gondwana continents. In fact the lack of any kind of fossil apart from palynomorphs in continental Upper Palaeozoic successions is one of the main reasons why zonations developed in Gondwana are so hard to relate to the standard stratigraphic scale. For example, even now it is difficult to correlate the Carboniferous – Permian boundary into Gondwana.

In some Permian Gondwana succession however thin limestone beds do occur, particularly in deglaciation successions, where the rocks contain evidence of warming conditions following the main Carboniferous-Permian glaciation (Stephenson et al. 2007). One of these limestone units is the Haushi Limestone of Oman. Very detailed study of the limestone in wells Hasirah-1 and Wafra-6 in Oman (Angiolini et al. 2006; Stephenson et al. 2008) revealed fusulinids including species of Pseudofusulinaallowing direct correlation of the Haushi Limestone with the stages of the Urals of Russia which in the past were partly defined on the basis of fusulinid biostratigraphy. This allowed a date of Sakmarian to be applied directly to part of the Haushi Limestone (Angiolini et al. 2006). In turn, the occurrence of palynomorphs of OSPZ3c age in the Haushi Limestone in the same successions suggested a Sakmarian age for the OSPZ3c zone.

Strontium method

The final example is more indirect, using strontium. The ratio of radiogenic to unradiogenic strontium (87Sr/86Sr), as measured in carbonates and evaporate minerals precipitated from seawater, changes through geological time and can be used as a tool to date ancient marine sediments. In Oman, 87Sr/86Sr was obtained from brachiopods of the Khuff Formation of a part which is assigned to the OSPZ6 palynological zone (Fig. 3). Samples from the five Oman pristine brachiopod ranged from 0.707217 to 0.707280 (Stephenson et al. 2012). A steep 87Sr/86Sr gradient exists between the Asselian and the Capitanian stage of the Permian potentially allowing for quite precise dating, though the overall coverage of reference data is rather sparse between those two dates.

Fig. 3. Khuff Formation in Oman

The 87Sr/86Sr values from the Khuff Formation brachiopods allowed an approximate correlation within the late Early to early Middle Permian. As the density of reference samples increases (e.g. Korte and Ullman (2018) the 87Sr/86Sr technique should become more accurate.

This brief article shows three ways to calibrate palynological biozones to the standard systems and stages. Although it concentrates on the Carboniferous and Permian, similar correlation solutions occur elsewhere in the stratigraphic column.

References

Angiolini, L., Stephenson, M H & Leven, Y. 2006. Correlation of the Lower Permian surface Saiwan Formation and subsurface Haushi Limestone, central Oman. GeoArabia, 11, 17-38.

Korte, C. & Ullmann, C. (2016). Permian strontium isotope stratigraphy. Geological Society London Special Publications. 450, 105-118.

Laurie, J.R., Bodorkos, S, Nicoll, R.S., Crowley, J.L., Mantle, D.J., Mory, A.J., Wood, G.R., Champion, D.C., Holmes, E. and Smith, T.E., 2016. Calibrating the middle and late Permian palynostratigraphy of Australia to the geologic time-scale via U–Pb zircon CA-IDTIMS dating. Australian Journal of Earth Sciences, 63, 701-730.

Stephenson, M H, Angiolini, L, Darbyshire, F, and Leng, M J. 2012. Age of the Khuff Formation in Saudi Arabia and Oman from brachiopod strontium. GeoArabia, 17, 61-76.

Stephenson, M H. 2018. Permian palynostratigraphy: a global overview. In: Lucas, S G, and Shen, S Z. (eds) The Permian Timescale. Geological Society, London, Special Publications, 450; 321-347.

Stephenson, M. H., Angiolini, L. & Leng M. J. 2007. The Early Permian fossil record of Gondwana and its relationship to deglaciation: a review. In Williams, M. & Heywood, A. Climate Change in Deep Time, Special Publication of the Geological Society, London, 103 -122.

Stephenson, M. H.; Angiolini, L.; Leng, M.; Brewer, T.S.; Berra, F.; Jadoul, F.; Gambacorta, G.; Verna, V.; Al Beloushi, B. 2008. Abrupt environmental and climatic change during the deposition of the Early Permian Haushi limestone, Oman. Palaeogeography, Palaeoclimatology, Palaeoecology, 270, 1-18.

Stephenson. M.H. 2009.The age of the Carboniferous-Permian Converrucosisporites confluens Oppel Biozone: new data from the Ganigobis Shale Member, Dwyka Group, Namibia. Palynology 33, 55-63.

Prof Mike Stephenson is available for consultancy

Email: mikepalyno@me.com

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