Palaeobio Palaeoenv (2014) 94:229–235 DOI 10.1007/s12549-013-0134-8
ORIGINAL PAPER
Palynological records of the Early Permian postglacial climate amelioration (Karoo Basin, South Africa) Annette E. Götz & Katrin Ruckwied
Received: 22 May 2013 / Revised: 26 August 2013 / Accepted: 26 September 2013 / Published online: 28 November 2013 # Senckenberg Gesellschaft für Naturforschung and Springer-Verlag Berlin Heidelberg 2013
Abstract The Permian coal-bearing formations of the South African Karoo Basin play a crucial role in the study and interpretation of Gondwana’s climate history and biodiversity in this time of major global changes in terrestrial and marine ecosystems. Here, we report on new palynological data from the No. 2 coal seam of the Witbank coalfield, documenting the switch from icehouse to greenhouse conditions in the Early Permian. The studied postglacial fluvio-deltaic deposits of a highly proximal setting comprise coarse-grained to pebbly sandstones, partially with an abrupt upward transition into fine-grained sediments and coal, trough cross-stratified medium- to coarse-grained sandstones and horizontally laminated fine- to medium-grained sandstones and siltstones. The sedimentary organic matter content clearly documents stratal changes in the palynomorph assemblage and variations in the amount and in the type, size and shape of plant debris. Generally, palynofacies is characterized by a high amount of opaque phytoclasts. Amorphous organic matter is characteristic of laminated siltstones and coals. The palynological record indicates a cold climate, fern wetland community, characteristic of lowland alluvial plains, and an upland conifer community in the lower part of the coal seam. Up section, these communities are replaced by a cooltemperate cycad-like lowland vegetation and gymnospermous upland flora. The data presented herein, provide important This article is a contribution to the special issue “Green planet—400 million years of terrestrial floras. Papers in honour of JHA van Konijnenburg van Cittert. A. E. Götz (*) Department of Geology, Rhodes University, Grahamstown 6140( P.O. Box 94, South Africa e-mail:
[email protected] K. Ruckwied Shell International Exploration and Production, 200 North Dairy Ashford, Houston, TX 77079, USA
information for (1) reconstructing palaeoenvironmental and climate change within the Lower Permian of the NE part of the South African Karoo, (2) intercontinental comparison of palaeofloral diversity patterns in adjacent sub-basins and (3) intra-Gondwanic correlations of coal-bearing basins (e.g. Australia), recording a major postglacial change associated with climate amelioration. Keywords Palynology . Climate Change . Permian . Witbank Coalfield . Karoo Basin . South Africa
Introduction Coal deposits are unique sedimentary archives of climate change from the late Palaeozoic to the Cenozoic. The Permian coal-bearing formations of the South African Karoo Basin play a crucial role in the study and interpretation of Gondwana’s climate history and biodiversity in this time of major global changes in terrestrial and marine ecosystems. A continuous climate amelioration succeeding the Permo– Carboniferous glaciation was first inferred from palynological data by Falcon et al. (1984) and Falcon (1986), showing that subsequent to the melting of the Dwyka ice, cold to cooltemperate climate conditions prevailed during the Early Permian and a continuous change to hot and dry climate conditions occurred during the Late Permian and Triassic. To date, palaeofloral evidence of climate change during the Permo–Triassic is based on data from these few sources. On the other hand, no recent works address high-resolution palynostratigraphy of the Permian–Triassic coal-bearing formations in the South African Karoo. Whereas new palynostratigraphic zonation schemes have been established in other parts of southern Africa (d’Engelbronner 1996; Nyambe and Utting 1997; Stephenson and McLean 1999; Modie and Le Hérissé 2009), our knowledge of the Permian
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and Triassic palynology of the South African Karoo Basin is based on fundamental research carried out in the 1970s and 1980s by Anderson (1977) and Falcon (see Falcon 1989 and references therein); as such, coal seam detection and correlation are still based on these data. Subsequent palynological studies were carried out only in a few selected sites of Early and Middle Permian age in the Waterberg and Pafuri coal basins (MacRae 1988), the Witbank and Highveld coalfields (Aitken 1994) and near Vereeniging (Millsteed 1993, 1999). Given this background, it seems imperative to study the palynological record of coal seams in much more detail with respect to establishing a high-resolution climate history of the Permo–Triassic of the Karoo. Here, we report on new palynological data from the No. 2 coal seam of the northern Witbank coalfield, documenting the switch from icehouse to greenhouse conditions in the Early Permian (Lower Ecca Group).
Geological setting The Late Carboniferous to Middle Jurassic Karoo Basin (Fig. 1) covers approximately one-third (i.e. 700,000 km2) of South Africa (Johnson et al. 2006), extending into Lesotho and into parts of Swaziland and Mozambique (Cole 1992). A total thickness of 12 km of sediment infill is reached
Coal Seams
Stormberg Group
Stormberg
Beaufort Group Ecca Group Dwyka Group
Johannesburg
Karoo Basin
Cape Fold Belt M
0
100
200
300 km
6 5
Bloemfontein
4
Durban
Middle
Ecca
P E R M I A N
Upper
L
Beaufort
U
TRIASSIC
Stratigraphy
within the southeastern part of the Main Karoo Basin. This sedimentary succession, known as the Karoo Supergroup, is subdivided into the Dwyka Group [Late Carboniferous (Westphalian) to Early Permian (Artinskian); Visser 1996], the Ecca Group of Permian age, the Permian–Triassic Beaufort Group (Johnson et al. 2006) and the Stormberg Group (Late Triassic to Early Jurassic; Catuneanu et al. 1998). Glacio-marine sediments of the Dwyka Group are overlain by marine-deltaic-dominated deposits of the Ecca Group and fluvial-deltaic sandstones of the Beaufort Group, grading into fluvial-lacustrine and aeolian sandstones of the Stormberg Group (Johnson et al. 1996). The Karoo succession is capped by 1.4 km of basaltic lavas of the Drakensberg Group and intruded mafic dyke swarms (Veevers et al. 1994; Johnson et al. 1996). The Karoo Basin forms part of a major series of Gondwanan foreland basins, including the Paraná Basin in South America, the Beacon Basin in Antarctica and the Bowen Basin in Australia (De Wit and Ransome 1992; Veevers et al. 1994; Catuneanu et al. 1998; Catuneanu and Elango 2001; Catuneanu and Bowker 2002). These basins formed mainly as result of collision and terrain accretion tectonics along the southern edge margin of Gondwana (De Wit and Ransome 1992; Veevers et al. 1994). Recent interpretations of the basin evolution and tectonic setting of the Karoo range from a retro-arc foreland basin (Johnson et al.
3
Lower
Beaufort West 2
Queenstown
Dwyka
CARB.
1
East London
N
Cape Town PortElizabeth Elizabeth Port
Study area
Fig. 1 Stratigraphy and distribution of the coal-bearing sedimentary series (Dwyka, Ecca, Beaufort, Stormberg groups) of the Main Karoo Basin and location of the study area in the northeastern part of the basin. Map modified from Catuneanu et al. (2005)
Palaeobio Palaeoenv (2014) 94:229–235 Origin
Group
higher plant debris
phytoclasts
higher plants
sporomorphs
degraded plant debris degraded algae
Constituent
231 preservation potential low high
opaque phytoclasts translucent phytoclasts pollen grains spores degraded organic matter amorphous organic matter
fresh water algae
Tetraporina spp. Botryococcus spp.
Fig. 2 Classification of sedimentary organic matter and approximate preservation potential of the different particles identified in the No. 2 coal seam
2006), a transtensional foreland system created by subsidence and tilting in a strike-slip regime (Tankard et al. 2009) to a thin-skinned fold belt that developed from collisional tectonics and distant subduction to the south (Lindeque et al. 2011). The Karoo Basin hosts important coal resources of South Africa (Johnson et al. 1997), namely, the Witbank coalfield comprising the basin’s northeastern most economic coal
Fig. 3 Sedimentary facies of the No. 2 coal seam exposed in the Inyanda Coal Mine south of Witbank. Proximal deposits of a fluvial-alluvial depositional setting are characterized by coarse-grained to pebbly sandstones, partially with an abrupt upward transition into fine-grained sediments and coal (a), trough cross-stratified medium- to coarse-grained
deposits (Snyman 1998). One of the most important coal seams of the Witbank coalfield, the No. 2 coal seam, represents postglacial fluvio-deltaic deposits of the Lower Ecca Group. The sedimentary succession is built up by coarse-grained to pebbly sandstones, trough cross-stratified sandstones, organic-rich siltstones and coal, pointing to a highly proximal setting of the northern basin margin. A palynostratigraphic framework of the major Permian coal seams of the Witbank Basin was first established by Falcon et al. (1984) who distinguished four palynozones (I–IV) divided into nine subzones (A–H1). These authors placed the No. 2 coal seam into Biozone IID. However, our new palynological data show that the lower part of the coal seam still contains elements of Biozone IC. The age interpretation and stratigraphic significance of these data for cross-basin correlation will be published elsewhere (Ruckwied et al. submitted).
Material and methods During a field campaign in 2011, we sampled an 8-m-thick succession of the No. 2 coal seam exposed in the Inyanda Coal Mine south of Witbank to study facies and sedimentary
sandstones (b) and horizontally laminated, fine- to medium-grained sandstones and organic-rich siltstones (c). An up to 1-m-thick sandstone marker bed, representing a channel fill deposit, separates the Lower and Upper Coal Seam
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Fig. 4 Palynofacies of the No. 2 coal seam is dominated by opaque and translucent phytoclasts of different sizes and shapes, characteristic of a highly proximal depositional setting
organic matter content. Samples comprise fine- to mediumgrained sandstones, organic-rich siltstones and coals. For the palynofacies analysis we prepared 22 samples using standard palynological processing techniques, including HCl (33 %) and HF (73 %) treatment for the dissolution of carbonates and silicates and a saturated ZnCl2 solution (D≈2.2 g/ml) for density separation. Residues were sieved at 15 µm mesh size. Slides were mounted in Eukitt, a commercial, resin-based mounting medium. Sedimentary organic matter was grouped into phytoclasts, sporomorphs, fresh water algae, degraded organic matter and amorphous organic matter (Fig. 2). Determination of the relative percentage of these components was based on counting at least 300 particles per slide.
Facies and sedimentary organic matter content The northern Witbank coalfield is characterized by proximal deposits that include coarse-grained to pebbly sandstones, partially with an abrupt upward transition into fine-grained sediments and coal, trough cross-stratified medium- to coarsegrained sandstones and horizontally laminated, fine- to medium-grained sandstones and organic-rich siltstones. An up to 1-m-thick sandstone marker bed separates the Lower and Upper Coal Seam (Fig. 3). In this fluvial–alluvial depositional setting, peat swamps developed on broad abandoned alluvial plains.
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Sedimentary organic matter of these proximal deposits is highly degraded and poorly preserved, but stratal changes in the palynomorph assemblage and variations in the amount and in the type, size and shape of plant debris are clearly documented. Generally, palynofacies is characterized by a high amount of phytoclasts (opaque and translucent) showing a high variability in size and shape throughout the section (Fig. 4). The lower part of the section (Lower Coal Seam) is characterized by a high amount of opaque particles, and the upper part (Upper Coal Seam) is rich in translucent plant debris (Fig. 5). Amorphous organic matter (AOM) is characteristic of laminated siltstones and coals. Degraded organic matter (DOM) is preserved in all sampled lithologies. Freshwater algae (Tetraporina spp., Botryococcus spp.) are common in the Upper Coal Seam (Fig. 5). The lower part of the section (Lower Coal Seam) is dominated by horsetail and fern spores (Calamospora spp., Microbaculispora spp., Punctatisporites spp., Verrucosisporites spp.) and gymnospermous monosaccate pollen grains (Florinites sp., Potonieisporites sp., Monosaccates indet.), whereas the upper part of the section (Upper Coal Seam) is totally dominated by gymnospermous bisaccate (non-taeniate and taeniate) pollen grains (Alisporites spp., Protohaploxypinus sp., Striatoabieites sp.) with only very few fern spores preserved, showing an increase in colpate pollen grains (Cycadopites sp.). The sporomorph assemblages indicate a fern wetland community, characteristic of lowland alluvial plains, and an upland conifer community dominated by monosaccateproducers in the lower part of the coal seam. Up section, these communities are replaced by a cycad-like and lycopsid (Lundbladispora sp.) lowland vegetation and a gymnospermous upland flora of bisaccate-producers.
Palaeoenvironmental and palaeoclimatic signatures Since spatio–temporal variations in the composition of land plant communities are directly related to the environmental and climatic changes in the area where the plants lived, plant fossils as well as palynomorphs and plant debris are significant proxies to interpret palaeoenvironmental and climatic changes through time. Coal-bearing series are particularly excellent archives of such changes and have been recently studied to decipher the related vegetational patterns (Ruckwied et al. 2008; Götz et al. 2011). The palynological data from the Witbank No. 2 coal seam document (1) a change in the depositional system and (2) a major climatic shift from cold to cool-temperate conditions (Fig. 5). Palynofacies of the Lower Coal Seam shows the highest amount of opaque, large blade-shaped phytoclasts and spores characteristic of a swamp-dominated depositional environment. Organic particles were deposited in standing water bodies without any indication of long-
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No. 2 Coal Seam
Phytoclasts opaque
Upper Coal Seam
8m
translucent
233 Pollen Grains monosaccate
Spores
bisaccate
Freshwater Algae
AOM/DOM
Environ- Vegetation Climate ment
lake/pond dominated 6m
Cycad-like Lowland/ Gymnospermous Upland Community
cooltemperate
river dominated
Lower Coal Seam
4m
swamp dominated
2m
Fern Wetland/ Upland Conifer Community
cold
0m
Fig. 5 Palynofacies patterns of the No. 2 coal seam and interpretation of the palynological record reflecting a major change in the land plant community. AOM/DOM Amorphous organic matter/degraded organic matter
distance transport and related sorting. The dominance of trilete fern and horsetail spores is indicative of a wet habitat. Sedimentary organic matter of the Upper Coal Seam is characterized by translucent phytoclasts, pollen grains and freshwater algae, pointing to a lake/ponddominated depositional setting. The higher amount of freshwater algae and the preservation of AOM may rather point to hydrologically closed and stratified lake basins than to broad swamps rich in fern and horsetail spores. Fluvial sandstones, representing channel fill deposits in the middle part of the No. 2 coal seam, show the highest amount of degraded particles and opaque, equidimensional plant debris (due to transport and winnowing); pollen grains are quite abundant, amorphous organic matter is not preserved. A prominent vegetational change related to climate amelioration is documented by the different sporomorph assemblages of the Lower and Upper Coal Seam. A postglacial monosaccate-producing upland gymnospermous flora is replaced by a warmer climate bisaccate-producing plant community. A change from a postglacial fern flora to a warmer climate colpate-producing cycad-like and lycopsid flora is detected in the lowland communities. The palynological record of the No. 2 coal seam is interpreted as documenting the switch from icehouse to greenhouse conditions, i.e. from a cold climate, fern wetland community, characteristic of lowland alluvial plains, and an upland conifer community to a cool-temperate cycad-like lowland vegetation and gymnospermous upland flora (Fig. 5). A similar palaeofloral turnover was detected in the Lower Permian of Oman (Stephenson and Osterloff 2002), pointing to an overregional signature in vegetational dynamics
of importance for intra-Gondwanan correlations of Early Permian deposits.
Conclusions The No. 2 coal seam is identified as the crucial period of Permian climate change from icehouse to greenhouse conditions. The palynological data presented herein, from a highly proximal basinal setting, provide important information for (1) reconstructing palaeoenvironmental and climate change within the Lower Permian of the NE part of the South African Karoo, (2) intercontinental comparison of palaeofloral diversity patterns in adjacent sub-basins and (3) overregional, intra-Gondwanic comparison of palynofacies patterns in other localities recording a major postglacial change associated with climate amelioration—for example, in East Africa (Wopfner and Diekmann 1996), Australia (Backhouse 1991, Wopfner 1999), South America (Iannuzzi et al. 2007) and in the Arabian Peninsula (Stephenson and Osterloff 2002; Stephenson et al. 2005). Since the palynostratigraphic correlation of glacigenic successions on a Gondwana-wide scale is still limited (for discussion see Stephenson 2008), the application of palynofacies analysis as a high-resolution correlation tool is very promising. Ongoing studies will address a basin-wide palynofacies analysis of the Karoo and a high-resolution palynostratigraphic framework of the Permian. Furthermore, the detailed investigation of Late Permian–Early Triassic coal deposits will enable the major floral changes to be deciphered within the context of the most severe biodiversity crisis in Earth’s history across the Permian–Triassic boundary. These
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data will contribute to a better understanding of global processes causing major perturbation of marine and terrestrial ecosystems. Acknowledgements This work is based on the research supported by the National Research Foundation of South Africa (Grant No. 85354). We kindly acknowledge Exxaro Resources, South Africa for giving us permission to sample the No. 2 coal seam exposed in the Inyanda Mine. We thank Stuart Clague (Exxaro) and Wlady Altermann (University of Pretoria) who encouraged us to work on the South African Gondwana coals and supported our field campaign in 2011. The constructive comments of Susanne Feist-Burkhardt (SFB Geological Consulting & Services, Germany) and an anonymous reviewer are gratefully acknowledged.
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