Permian palynology since 2007

I was commissioned by the Subcommission on Permian Stratigraphy (SPS) to write a review of advances in Permian palynology since 2007 for the SPS publication Permophiles. The review was published in Permophiles, 2023, volume 75, p. 22-26 (https://permian.stratigraphy.org/files/permophiles/Permophiles%2075.pdf). But the review is published here also.

Background and introduction

Since this is a review concerned mainly with the chronostratigraphy of the Permian, it will concentrate on the use of palynology in stratigraphy and in the elucidation of aspects of Permian geological history and geology, rather than on palynological taxonomy. Palynological biostratigraphy (or palynostratigraphy) is the use of palynomorphs (defined as organic-walled microfossils 5–500 microns in diameter) in correlating and assigning relative ages to rock strata. As such, it is a branch of biostratigraphy and follows the rules of biostratigraphic practice: for example, those set out by Rawson et al. (2002).

The Permian has a number of distinct and recognisable events related mainly to the development of land plants. Amongst the most important changes in land plants is the replacement, near the end of the Carboniferous, of arborescent lycophytes by arborescent tree ferns, with arborescent lycophytes only persisting into the Guadalupian in China. The arborescent horsetails also declined by the end of the Carboniferous. In the Permian, a great variety of new seed plant groups appeared such as cycads, ginkgos, voltzialean conifers and glossopterids. The latter are important palaeobotanical biostratigraphic markers for the Permian of Gondwana and include several hundred species. It is estimated that by the Lopingian about 60% of the world’s flora consisted of seed plants (Gradstein and Kerp 2012).

These big evolutionary changes in plants, modified by local and regional effects, are responsible for the palynological succession that provides opportunities for subdivision on which palynostratigraphic schemes are built. However, the pronounced phytogeographical differentiation of the Permian has an effect on palynostratigraphy, such that schemes differ considerably across Pangea and correlation between schemes is even now tentative or incomplete. In the Gondwana phytogeographical province, for example, it is difficult to correlate to the standard Permian stages; and the Carboniferous–Permian and Permian–Triassic boundaries are not precisely correlateable into Gondwana basins using palynology (Stephenson, 2008, 2016). Until recently, progress in correlation was hampered by the lack of fundamental stratigraphic standards such as stage Global Stratigraphic Sections and Points (GSSPs); however, since 1997 (Jin et al. 1997; Henderson et al. 2012) important  GSSPs have been established within the Pennsylvanian – Permian succession, including all the Permian GSSPs, except for the Kungurian (SPS website: https://permian.stratigraphy.org/gssps).

Developments since 2007

The main developments in palynostratigraphy since 2007 relate to the radiometric dating of palynological biozones which has gone a long way to resolving the problem of calibrating palynozones, mainly in Gondwana. Most progress has been made in two geographical regions: South America and Australia. In other areas the main progress has been in taxonomic - palynostratigraphic studies of key areas; examples include palynology of southern African coal seams (Goetz and Ruckwied 2014; Ruckwied et al., 2014, Barbolini and Bamford, 2014); and Permian-Triassic palynology in the key sections of the Salt Range of Pakistan (Hermann et al., 2012).

Updates in Australian palynostratigraphy

Australia has some of the best documented Permian basins in Gondwana, but much of the succession is nonmarine. In the past, calibration of the most widely used local Australian palynostratigraphic scheme (Price, 1997) to the global timescale was indirect and very difficult, having traditionally relied on correlations from relatively sparse, high-latitude, marine strata, within which ammonoids and conodonts are rare, fusulinids are unknown, and much of the other fauna (brachiopods, bivalves) is endemic. Tie points are rare and often tenuous: one example is the record of a single specimen of the ammonoid Cyclolobus persulcatus from the Cherrabun Member of the Hardman Formation, in the Canning Basin, Western Australia, dated as ‘post-Guadalupian’ by and ‘Capitanian–Dzhulfian’ (see Foster and Archbold, 2001 for details). However in eastern Australia, the Permian succession contains felsic ash beds, many of which contain zircons. Ash beds are rare in Western Australia, but some have been found in the Canning Basin. In the last decade fieldwork has involved sampling ash beds for radiometric dating, coupled with sampling of adjacent sedimentary rocks for palynomorphs, mostly from cores and coalmines in the Sydney, Gunnedah, Bowen and Galilee basins in eastern Australia, and core in the Canning Basin in Western Australia (Fig. 1). Dating zircons involved Chemical Abrasion-Isotope Dilution Thermal Ionisation Mass Spectrometry (CA-IDTIMS) for U-Pb dating. The resultant radioisotopic dates, with associated palynostratigraphic determinations, permit the direct calibration of the Price (1997) scheme to the numerical timescale.

Fig. 1. Map of Australia showing Permian sampling locations. From Smith et al., 2017.

Several papers and reports describe early results (Bodorkos et al., 2016; Laurie et al., 2016; Mantle et al., 2010; Smith and Mantle 2013; Nicoll et al., 2015, 2017), but a convenient summary is that of Smith et al. (2017). In broad terms the effect on Permian Australian palynozones has been significant with some zonal boundaries in the Permian shifting by as much as six million years. Revised dates for the Permian palynozones can now be applied to all Permian basins across Australia, including the Perth, Carnarvon, Canning and Bonaparte basins (along the western and northern continental margins), the Cooper and Galilee basins (in central Australia), and the Bowen, Gunnedah and Sydney basins (in eastern Australia).

In summary, the following changes are suggested for the Cisuralian of the Permian:

·        APP3 (Price, 1997) zone is younger than previously calibrated

·        APP2 zone has a greater duration, starting earlier and ending later, than previously determined.

·        the top of the Pseudoreticulatispora confluens (APP1.22) zone lies in the late Asselian;

·        the top of the Pseudoreticulatispora pseudoreticulata (APP2.1) zone lies in the middle Artinskian;

·        the top of the Microbaculispora trisina (APP2.2) zone lies in the early Kungurian;

·        the top of the Phaselisporites cicatricosus (APP3.1) zone lies in the late Kungurian.

As detailed by Laurie et al. (2016) and Bodorkos et al. (2016), the results for the Guadalupian and Lopingian of Australia indicate that these Middle and Late Permian palynozones are significantly younger than previously suggested. The recalibrations indicate:

·        the top of the Praecolpatites sinuosus (APP3.2) zone lies in the early Roadian;

·        the top of the Microbaculispora villosa (APP3.3) zone lies in the middle Roadian;

·        the top of the Dulhuntyispora granulata (APP4.1) zone lies in the Wordian;

·        the top of the Didecitriletes ericianus (APP4.2) zone lies in the early Wuchiapingian;

·        the entire Dulhuntyispora dulhuntyi (APP4.3) zone lies within the Wuchiapingian; and

·        the top of the Dulhuntyispora parvithola (APP5) zone lies at or near the Permian−Triassic boundary

These recalibrations are summarised in Fig. 2.

Fig. 2. Revised calibration of Permian palynostratigraphy in Australia. Each arrow represents a biostratigraphically controlled U–Pb zircon date obtained via CA-IDTIMS and interpreted to represent  the depositional age of the sampled tuff layer. From Smith et al., 2017.

South America

Calibration of palynostratigraphic zones by radiometric dating has progressed recently in four basins: the Tarija and Chacoparana basins in northern Argentina, the Paganzo in central western Argentina, the Claromeco Basin in eastern Argentina, and the Paraná and Amazonas basins in Brazil. There are a number of basin-specific palynostratigraphic schemes, but in general, the biostratigraphy of the basins is difficult to relate to the international stages of the Carboniferous and Permian because of the scarcity of marine faunas. Since 2007, the most marked progress has been made in integrating radiometric dates with palynological biozones, allowing limited – not always reconcilable – calibration of the latter with the international scale. Amongst the most important of these studies since 2007 are those of Césari (2007), Guerra-Sommer et al. (2008), Césari et al. (2011), Mori et al. (2012) and di Pasquo et al. (2015). In the first of the studies, Césari (2007) noted radiometric dates in the San Rafael Basin in central western Argentina and in the Paraná Basin in southern Brazil that suggested numerical ages for biozones established by Césari & Gutiérrez (2000) and Souza & Marques-Toigo (2003) in those basins, respectively. So the Lueckisporites–Weylandites Assemblage Biozone of Césari and Gutiérrez (2000) in the San Rafael Basin contains a horizon dated at 266.3 + 0.8 Ma (Wordian), while the Lueckisporites virkkiae Interval Biozone of Souza & Marques-Toigo (2003) in the Paraná Basin contains a dated horizon of 278.4 + 2.2 Ma (Kungurian). Guerra-Sommer et al. (2008) reported an age of 285.4 + 8.6 Ma (Artinskian) within the Paraná Basin Faxinal coal seam, which is assigned to the Hamiapollenites karooensis Sub-biozone of the Vittatina costabilis Interval Biozone of Souza and Marques-Toigo (2003). Mori et al. (2012) noted a date of 281 + 3.4 Ma (Kungurian) for another horizon within the Lueckisporites virkkiae Interval Biozone of the Paraná Basin in the Candiota coal mine. Césari et al. (2011) summarised the palynostratigraphy and radiometric dating of the Carboniferous and Cisuralian sequence across Argentina and Brazil correlating the San Rafael and Paraná basin biozones and using radiometric dates to relate South American palynological biozones to those of Namibia and Australia. di Pasquo et al. (2015) gave radiometric dates from five volcanic ash beds within the Cisuralian Copacabana Formation in central Bolivia (Tarija Basin). The five dates (cited as preliminary and published only in the non-peer reviewed Permian ICS Newsletter Permophiles, 53, Supplement 1) are 298, 295.4–295.1 and 293 Ma (for two ash layers approximately 25 m apart stratigraphically), and 292.1–291.3 Ma. According to di Pasquo et al. (2015), these dates suggest an Asselian age for the Vittatina costabilis assemblage and an Asselian – Sakmarian age for the Lueckisporites virkkiae assemblage of di Pasquo et al. (2015).

Conclusion

This review indicates the considerable progress that has been made in palynostratigraphy since 2007 in relation to the radiometric dating of palynological biozones, mostly in the former continents of Gondwana, where ash layers have facilitated CA-IDTIMS for U-Pb dating. This has resulted in spot calibration for palynozones in several basins in South America. Perhaps the most systematic and significant progress has however been made in the Gondwana basins of Australia, in several cases moving zonal boundaries in the Permian by as much as six million years. The implications of these changes in Australia and South America are mainly still to be realised but are very likely to change our view of Permian glaciation, palaeophytogeography, and other Permian events.

To continue some of these advances, a SPS Working Group, the Euramerica-Gondwana correlation Working Group, has been set up to deal with issues such as difficulties in identifying Euramerican defined GSSPs (including the C/P boundary) in Gondwana, different provincial palynological ‘taxonomies’ and issues over the quality of data and information variation in different parts of Gondwana and Euramerica.

In the coming decades it is likely that radiometric dates will continue to be the most important ‘glue’ between palynostratigraphic schemes which reflect considerable phytogeographic provinciality, perhaps ultimately providing a basis for worldwide relatively high resolution palynostratigraphic correlation and dating.

Acknowledgement to Alex Wheeler for useful discussions.

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