Depositional setting for Eocene seat earths and related facies of the Gippsland Basin, Australia

Sedimentary Geology 390 (2019) 100–113

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Depositional setting for Eocene seat earths and related facies of the Gippsland Basin, Australia Vera A. Korasidis a,⁎, Malcolm W. Wallace a, Julie A. Dickinson a, Nick Hoffman b a b

School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia CarbonNet, 121 Exhibition Street, Melbourne, Victoria 3000, Australia

a r t i c l e

i n f o

Article history: Received 17 April 2019 Received in revised form 22 July 2019 Accepted 23 July 2019 Available online 29 July 2019 Editor: Dr. B. Jones Keywords: Seat earths Underclays Coal Kaolinite Pedogenisis Palynology

a b s t r a c t The origin of seat earths (i.e. underclays, seat rocks, fire clays) has been investigated using sedimentological, palynological and mineralogical analysis of clastic-coal successions from the Eocene Traralgon Formation of the Gippsland Basin, Australia. The seat earths of the Latrobe Group are massive, a light grey to white colour, contain abundant slickensided fracture surfaces and isolated organic matter, and mineralogically consist of abundant kaolinite and lesser amounts of 2 M illite. From palynological evidence, the seat earths have paleoenvironments that grade from a fire-prone heath-fern meadow marsh (i.e. Gleicheniaceae and Epacridaceae dominant), to firetolerant shrubs and small trees (i.e. Cyatheaceae, Schizaeaceae and Proteaceae dominant) that fringe raised peatland rainforests. The palynological data also indicate a non-marine origin for the kaolinitic mudstones. The non-marine seat earths were deposited over a foundation of intertidal sediments (containing lenticular, wavy and flaser bedding, tidal rhythmites, extensive burrowing and a diverse assemblage of marine-influenced dinoflagellates). The upward increase in kaolinite, slickensides and rootlets within the seat earth indicates this clay was kaolinitized by pedogenic processes (i.e. weathering by organically derived humic/fulvic acids) prior to and throughout peat formation. The presence of well-preserved and abundant spore-pollen in the kaolinitic mudstones also suggests that the seat earths were deposited in an acidic and relatively reducing setting. The stratigraphic transition from tidal siltstone, to mudstone (seat earth) to coal in ascending order is interpreted as a shallowing-upwards succession. The seat earths of the Gippsland Basin were therefore deposited as a precursor non-marine facies (mostly meadow-marsh) grading into an ombrogenous coal facies, thereby explaining the intimate association between coals and seat earths globally. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Seat earths (or underclays, seat rocks, fire clays) are clay-rich beds that directly underlie coal seams (Logan, 1842) and their enigmatic origin has stimulated considerable discussion. More recently, seat earths have been described as non-bedded, light coloured, slickensided and rootlet-bearing claystones or siltstones (Huddle and Patterson, 1961; Rimmer and Eberl, 1982; Breyer and McCabe, 1986). Earlier researchers proposed an allochthonous origin for the seat earths that involved an inherited mineralogy (i.e. clay) from external source areas (e.g. Grim and Allen, 1938; Schultz, 1958). Staub and Cohen (1978) instead proposed that seat earths might be formed autochthonously, but after the peat had been deposited (i.e. late diagenesis). These authors suggested that acid solutions percolated downwards from the peats during early diagenesis, leaching the clays beneath. Other studies of the clay

⁎ Corresponding author at: School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia. E-mail address: [email protected] (V.A. Korasidis).

https://doi.org/10.1016/j.sedgeo.2019.07.007 0037-0738/© 2019 Elsevier B.V. All rights reserved.

minerals in underclays (Keller et al., 1954; Keller, 1956; McMillan, 1956; Patterson and Hosterman, 1960; Rimmer and Eberl, 1982), following Stout et al. (1923), proposed that the clays underwent varying degrees of alteration. More recent research on seat earths has suggested a pedogenic origin (e.g. Gardner et al., 1988; Driese and Ober, 2005) as originally suggested by Logan (1842). Very few studies of seat earths have investigated the detailed palynology of seat earths and related lithologies (Smith, 1962; Marshall and Smith, 1964; Scott, 1978, 1979). Seat earths have largely been described from Paleozoic black coalbearing successions globally (Logan, 1842; Gardner et al., 1988). Here we describe a series of Cenozoic seat earths from the brown coal deposits of the Latrobe Group, Gippsland Basin, Australia. The clays that directly underlie coal seams in the lower part of the Traralgon 2 (T2) subunit (Holdgate et al., 2000) in the Wulla Wullock-7 (WW7) well have all the characteristics of seat earths described from coalfields elsewhere. Using a multidisciplinary approach involving sedimentological, palynological and mineralogical analysis of seat earths and associated lithologies from the WW7 core, we suggest that the Gippsland Basin seat earths were largely deposited in a non-marine, fire-prone heath-fern meadow marsh setting.

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Fig. 1. Onshore Gippsland Basin, Australia, structure map. Location of the Wulla Wullock-7 well highlighted. Modified from Abele (1988).

2. Geological setting The Cretaceous to Miocene Latrobe Group of the onshore and offshore Gippsland Basin, Australia (Fig. 1) consists of sandstones, siltstones, shales, coals and some volcanics that host major discovered coal and petroleum accumulations (Holdgate, 2003). The enormous brown coal reserves of the onshore Gippsland Basin also occur within the Latrobe Group (Fig. 2). The stratigraphic terminology of the Latrobe Group has had a complex history, with the succession being first known as the Latrobe Valley Coal Measures (Thomas and Baragwanath, 1949) from the onshore Latrobe Valley region. In the onshore Gippsland Basin, this sequence was subdivided into groups, which included the major known coal seams (Yallourn and Morwell groups, Thomas and Baragwanath, 1949). Later work recognized an important sequence of older coal-bearing sediments, known as the Traralgon Group (Gloe, 1960, 1967, 1975). Each of these groups was further subdivided into the major coal seams and interseam sediments (e.g. Morwell 1B, Morwell 1A etc., Gloe, 1960, 1967, 1975). The onshore succession later became known as the “Latrobe Valley Group”, and the offshore as the Latrobe Group (James and Evans, 1971). Here, we use the term “Latrobe Group” for both the onshore and offshore successions. More recently, the Traralgon, Morwell and Yallourn groups were also assigned formation status (i.e. Traralgon, Morwell and Yallourn formations) (Hocking, 1972; Hocking et al., 1976). The oldest (Middle Eocene to earliest Oligocene) of the onshore coal-bearing units is the Traralgon Formation and this has three major coal-bearing intervals, known as the Traralgon 2 (T2), Traralgon 1 (T1) and Traralgon 0 (T0) seams in order of decreasing age (Korasidis et al., 2019a). These subdivisions, though not formally defined, could be viewed as members of the Traralgon Formation. Here, we will refer to these subdivisions as informal sub-units (e.g. T2 sub-unit). The

Wulla Wullock-7 (WW7) core contains the thickest record of the onshore Traralgon Formation, in particular the T2, T1 and T0 seams and represents the focus of this study. 3. Methodology X-ray diffraction analysis on kaolinitic mudstones and related samples was carried out on a Bruker D8 Advance Diffractometer at the Materials Characterisation and Fabrication Platform (University of Melbourne) using Ni-filtered Cu K alpha radiation on powdered unoriented samples (step size 0.020 degrees, count time 0.5 S). The clay minerals present in the samples were identified using the methods of Carroll (1970). The degree of order for kaolinite was determined using the procedure of Hinckley (1963). Palynological analysis was completed on kaolinitic mudstones collected from the WW7 core (Fig. 3). Six kaolinitic mudstone samples were processed using palynological techniques that involved a 10% hydrochloric acid solution, a coarse sieve (150μm) to reduce the large fraction, heavy liquid separation (lithium heteropolytungstate) and concentrated (48%) hydrofluoric acid. Twenty-eight previously prepared palynology slides from the siltstone and mudstone lithofacies in the WW7 core from Holdgate et al. (2000) and twenty-two previously prepared slides from the coals in the WW7 core from Korasidis et al. (2019a) were also used in this study. The state of pollen preservation in the siltstone, mudstone and coal lithofacies from the Latrobe Group has been estimated based on a detailed examination of palynomorphs in transmitted light. Microscopic charcoal was also quantified to assess the distribution of charcoal in the lithofacies from the T2 sub-unit. Microscopic charcoal particles, within the palynological slides, were counted using the methods and definitions outlined in Mooney and Tinner (2011).

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Fig. 2. Generalized chronostratigraphic diagram for the Paleogene-Neogene Gippsland Basin. (Modified from Norvick et al. (2001) and Hoffman et al. (2012)).

4. Biostratigraphy Samples examined from the T2 sub-unit (i.e. from the siltstone, mudstone and coal lithofacies) are assigned to the Middle Eocene Lower Nothofagidites asperus spore-pollen Zone (Fig. 3), based on the presence of Tricolpites simatus Partridge in Stover and Partridge, 1973, Nothofagidites falcatus (Cookson) Stover and Evans, 1973, Nothofagidites vansteenisii (Cookson) Stover and Evans, 1973, Tricolpites leuros Partridge in Stover and Partridge, 1973 and Proteacidites reflexus Partridge in Stover and Partridge, 1973, which first appear in this zone (Partridge, 1999). Samples from the T2 sub-unit also include Proteacidites asperopolus Stover and Evans 1973 (Fig. 4) and Myrtaceidites tenuis Harris (1965), which last appear in the Lower N. asperus Zone (Partridge, 1999; Korasidis, 2018). A lack of indicators from younger zones (i.e., Tricolpites magnificus, Anacolosidites sectus and Aglaoreidia qualumis) is consistent with the zone assigned. The ages assigned to the T2 sub-unit are also in agreement with previous works concerning the age of the T2 coal seams (i.e., Partridge, 1994, 1997; Holdgate et al., 2000; Korasidis et al., 2019b). In addition, the T1 coal seam and the lower portion of the T0 coal seam in WW7 are allocated to the Late Eocene Middle N. asperus Zone while samples from the upper portion of the T0 coal seam in WW7 are assigned to the Early Oligocene Upper N. asperus Zone (Korasidis et al., 2019b).

5. Results 5.1. Laminated siltstone lithofacies The siltstone lithofacies that underlie the mudstone lithofacies in the WW7 well (Fig. 3) are generally thin (b3 m thick) and most typically have transitional contacts with sandstones and/or mudstones. In core from WW7, siltstone lithofacies display a large variety of sedimentary structures, the most prominent being lamination and bioturbation. The most common lithology consists of alternating laminae of fine sands with dark mudstones. The fine scale alternation of quartzdominated fine sand laminae with dark mudstone laminae is averaged on the generated logs (Fig. 3), producing an intermediate “siltstone” signature between sandstones and mudstones (even though the dominant grainsizes in this facies are fine sand and clay). These lithologies have previously been referred to as “heterolithic” facies (Davis, 2012). The coarser quartz-dominated laminae are commonly lenticular and/or loaded and vary in thickness from b0.5 mm to 0.8 cm thick (Fig. 5). The interlaminated mudstones are generally organic-rich, with abundant small intraclasts of coal commonly present. In the lower portion of the WW7 well, this facies grades into a dark mudstone facies by the gradual thickening of dark mudstone laminae.

Alternating fine-sand and mud laminae commonly have rhythmic thickness variations and, in some cases, resemble tidal rhythmites (Kvale and Archer, 1990). Small-scale (b 5 mm of erosional truncation) erosion surfaces are also commonly present in this lithology. Smallscale isolated ripples (b 2 cm wide and 5 mm high) are commonly present and show evidence of sediment loading. Vertically-oriented silt and sandstone filled fractures are sporadically present in this lithofacies with these structures generally showing evidence of deformation during compaction (i.e. indicating their early synsedimentary origin). These features appear to be either sand-filled syneresis cracks or true dessication cracks. Small-scale synsedimentary faulting is also common in this lithofacies. Palynologically the siltstone lithofacies in the T2 and T1 sub-units of the WW7 core contain a diverse assemblage of dinoflagellates including Operculodinium centrocarpum, Spinidinium spp., Gippslandia spp., Spiniferites spp., Corrudinium spp. and the acritarch Michrystridium spp. (Fig. 6). In the basal T2 sub-unit of the WW7 core the palynoflora of this lithofacies includes relatively high abundances of Araucariaceae, in particular Araucariacites spp. and Dilwynites granulatus, the Podocarpaceae Dacrydiumites florinii, Phyllocladidites mawsonii and Podocarpidites spp., and high relative abundances of Casuarinaceae and the Nothofagaceae subgenera Brassospora and Nothofagus (Figs. 89).Holdgate et al. (2000) also reported that the siltstone lithofacies from many onshore interseam facies (including the T0 and T1 subunits) consistently contained dinoflagellates. Holdgate et al. (2000) further noted that this lithofacies was associated with a high abundance of Nothofagaceae. Dinoflagellates have also been recorded from numerous offshore petroleum wells of the Gippsland Basin within the siltstone facies (Partridge, 1976). Macphail (1985) also recorded dinoflagellates (including Areosphaeridium diktyoplokus) sporadically throughout the T2 sub-unit within the nearby Barracouta 5 well. The majority of palynomorphs in the siltstone lithofacies are also well preserved. 5.2. Light coloured mudstone lithofacies Mudstone intervals from the Traralgon Formation are typically thin (usually b3 m thick) and generally occur in direct association with coals. Mudstones generally display a gradational contact with both coals and siltstones. In the basal T2 sub-unit in WW7, mudstones have a distinctive unlaminated character with a light grey to white colour (Fig. 5) and on logs display high natural gamma (N110 API units), and high neutron porosity. Slickensided fracture surfaces are abundant (Fig. 5) while isolated and vertically oriented fragments of organic matter, that resemble plant rootlets, are infrequently present in the mudstones (Fig. 5). These light-grey-white clays grade into the overlying coals by a gradual increase in organic content (Fig. 7). Downwards, the mudstones

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Fig. 3. Stratigraphic section (in metres) derived from electric logs (gamma and neutron) and core log through the Traralgon 2, Traralgon 1 and Traralgon 0 sub-units in Wulla Wullock-7. Distribution of spore-pollen zones in WW7 derived from data presented herein and in Korasidis et al. (2019b). Missing core in the core log largely corresponds to unconsolidated sand intervals.

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Fig. 4. Well-preserved pollen grains/indicators from the seat earths of the Latrobe Group. Pollen grains photographed in transmitted light and are illustrated at the same magnification (= 20 μm scale). 1) Proteacidites asperopolus Stover & Partridge 1973 at 638.6 m. 2) Proteacidites pachypolus Cookson & Pike 1954 at 649.32. 3) Triporopollenites ambiguus Stover & Partridge 1973 at 638.6 m. 4) Tricolporites leuros Stover & Partridge 1973 at 638.6 m.

show a gradual increase in quartz and display a transition into the laminated siltstone facies. X-ray diffraction data indicate that the clays predominantly consist of kaolinite, with lesser amounts of 2 M illite (muscovite) and quartz. In the two continuously cored sections studied in detail from WW7, there is a gradual increase in kaolinite upwards towards the coal. In the uppermost section (642–637 m), the sample immediately beneath the coal shows a very high proportion of wellordered kaolinite (Hinckley Index 1.3). However, most kaolinites are moderately disordered (Hinckley Index 0.5 to 0.9). These light-greywhite mudstones from WW7 therefore appear to have all the characteristics of “seat earths” or “underclays” that have been described from coal successions globally (e.g. Logan, 1842). Unlike the underlying siltstone facies, the mudstones from the WW7 core do not contain dinoflagellates but have a well-preserved and abundant spore-pollen assemblage. The light-grey-white clays from the

lower portions of the WW7 core (Fig. 3) are characterised by high relative abundances of Gleicheniaceae (coral or tangle ferns), Cyatheaeceae (tree ferns) and Schizaeaceae (curlygrass or comb ferns) spores, high relative abundances of Epacridaceae (heaths) pollen and relatively high micro-charcoal concentrations. Moderate relative abundances of Casuarinaceae, the Podocarpaceae P. mawsonii and Podocarpidites spp., Sparganiaceae pollen in addition to low relative abundances of Nothofagaceae subgenus Brassospora pollen are also observed in the lower portions of the mudstone lithofacies. Stratigraphically higher in the mudstone lithofacies (Figs. 8–9) and directly underlying the coal, the palynofloral assemblage is characterised by relatively high abundances of Proteaceae pollen, in particular Proteacidites asperopolus, Proteacidites pachypolus, Proteacidites spp. and Banksieaidites spp. Sporadically high abundances of Cyatheaeceae and Schizaeaceae are also common in the stratigraphically higher mudstone lithofacies.

Fig. 5. Core photographs illustrating lithologies from the siltstone and mudstone lithofacies in Wulla Wullock-7. A) Interlaminated siltstone and organic-rich mudstone with small synsedimentary faults present at 641.2 m. B) Interlaminated siltstone and organic-rich mudstone with small synsedimentary faults present at 641.15 m. C) Typical slickenside found within kaolinitic mudstones (seat earth) beneath coal at 638.8 m. D) Light-grey to white kaolinitic mudstones (seat earth) beneath coal. Strings of vertically-oriented organic material are interpreted as rootlets at 638.6 m.

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Fig. 6. Dinocysts and acritarchs from Wulla Wullock-7. Specimens photographed in transmitted light and under differential interference contrast and are illustrated at the same magnification (=40 μm scale). 1–2) Operculodinium centrocarpum (Deflandre & Cookson) Wall 1967 at 466.9 m, 3–4) Operculodinium centrocarpum (Deflandre & Cookson) Wall 1967 at 495.7 m, 5) Spiniferites spp. at 530.3 m, 6) Spiniferites spp. at 641.3 m, 7) Spinidinium spp. at 516.7 m, 8) Spinidinium spp. at 516.7 m, 9) Gippslandica extensa (Stover) Stover & Williams 1987 at 530.3 m, 10) Corrudinium spp. at 641.3 m, 11–12) Michrystridium spp. at 641.3 m, 13) Dinocyst sp. at 641.3 m, 14) Dinocyst sp. at 641.3 m.

Dinoflagellates or acritarchs of any type are completely absent from the mudstone lithofacies in agreement with the findings of Holdgate et al. (2000). The majority of palynofloras in the mudstone lithofacies are well preserved, with a very minor proportion of degraded grains (Fig. 4). 5.3. Coal lithofacies The Traralgon Formation coals in the WW7 well occur at depths of between 440 and 670 m from the surface. These coals have a dark brown colour and a poorly consolidated nature. Woody material and plant fragments are readily identifiable in the coal. The sulfur content of Traralgon Formation coals varies from 1 to 6 wt%, and the ash content from 2 to 10 wt% (Holdgate et al., 2000). Calorific value and vitrinite reflectance for the offshore coals of the upper Traralgon Formation indicate a high-rank brown coal classification (Suggate, 1974; Smith and Cook, 1984). Moisture content (in-situ or bed moisture) from the onshore coals in WW7 average between 45 and 50 wt% at a depth of 500 to 600 m (Holdgate et al., 2000; Holdgate, 2005). These coals commonly have gradational contacts with the underlying siltstone or mudstone lithofacies. Palynologically the coals underlying the siltstone facies and overlying the mudstone facies in the WW7 core are characterised by particularly high relative abundances of Casuarinaceae, namely Haloragacidites

harrisii, and Myrtaceae including Myrtaceidites tenuis, Myrtaceidites parvus-mesonesus and Myrtaceidites spp. pollen (Figs. 8-10). Moderate relative abundances of Proteaceae including Propylipollis annularis and Proteacidites spp., the Podocarpaceae Dacrydiumites florinii and Podocarpidites spp., and the Nothofagaceae subgenera Brassospora and Nothofagus characterise the coal lithofacies in the WW7 core. Notably, Epacridaceae (heaths), Schizaeaceae (curlygrass or comb ferns), Cyatheaceae (tree ferns) and dinoflagellates were not recorded in the coals overlying the seat earths examined. The majority of palynofloras in the coal lithofacies are well preserved, with a very minor proportion of degraded grains.

6. Discussion 6.1. Depositional setting of lithofacies in Wulla Wullock-7 The seat earths from the Wulla Wullock-7 well occur in direct association with the underlying laminated siltstone lithofacies and overlying coal lithofacies. The transitional relationships between these three lithofacies suggest that they are lateral facies equivalents of each other. Hence, it is important to establish the depositional setting of these related lithofacies in order to fully understand the origin of seat earths.

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Fig. 7. Stratigraphic sections and mineralogical data (x-ray diffraction) for light-grey-white kaolinite-rich mudstones (seat earth) beneath coals in Wulla Wullock-7. There is a significant increase in kaolinite upwards towards the coal in each section. These sections are from the basal portion of the T2 sub-unit.

6.1.1. Depositional setting for the laminated siltstone lithofacies Sedimentological, palynological and geochemical data indicate that this lithofacies was deposited in a marginal-marine setting. The alternation of mud and sand laminae (couplets) within the laminated siltstone lithofacies indicates a short-term alternation of energy in the environment. Modern environments that have this pattern of energy change include subtidal and intertidal settings (Reineck and Wunderlich, 1968). The presence of a diverse assemblage of dinoflagellates (Fig. 6 and Holdgate et al., 2000), including Operculodinium centrocarpum, Spinidinium spp., Spiniferites spp. and Michrystridium spp. that are all associated with marginal-marine and marine environments (Brinkhuis, 1994; Kurita and McIntyre, 1995; Lord et al., 2014), in the siltstone lithofacies also supports a tidal or marginal-marine depositional setting

for this lithofacies. The presence of abundant upland-sourced Araucariaceae pollen, in particular Dilwynites granulatus (Crouch and Visscher, 2003), in the siltstone lithofacies is also consistent with a tidal or marginal-marine depositional setting. Higher proportions of upland palynofloras, in association with diverse dinoflagellate assemblages, likely result from fluvial-transported palynofloras mixing with marine assemblages in a marginal-marine setting (Muller, 1959; Traverse, 2007). 6.1.2. Depositional setting for seat earths The clays present beneath the coals in the lower part of the T2 subunit of the WW7 well have all the characteristics of seat earths described from coalfields elsewhere. This includes an unlaminated

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Fig. 8. Stratigraphic section (in meters) and palynology of siltstones, seat earths and coal from 637 to 642 m, Wulla Wullock-7.

character with a light grey to white colour, abundant slickensided fracture surfaces, isolated fragments of organic matter (i.e. plant rootlets) and a mineralogical composition of kaolinite, with lesser amounts of 2 M illite (muscovite) and quartz. Notably, the lower portion of the seat earths are characterised by a palynofloral assemblage that resembles a meadow marsh (Fig. 11) with abundant Gleicheniaceae (coral ferns) and Epacridaceae (heaths) present. This palynofloral assemblage is very similar to that documented in the laminated dark lithotype coal facies of the onshore Oligo-Miocene Morwell Formation (Korasidis et al., 2016, 2017a). Therefore, the lower portions of the seat earths are interpreted to represent a heath-fern meadow marsh that established in the fine-grained mudstone lithofacies. The relatively higher charcoal concentrations recorded in the seat earths, relative to the underlying and overlying strata, is also consistent with higher charcoal concentrations previously documented in the laminated dark lithotype facies of the onshore Latrobe Group coal seams. This floral assemblage and the abundant charcoal have been interpreted as being characteristic of a fire-prone marsh environment, where frequent wildfires were caused by the inherent flammability of the fine vegetation (i.e. this flora is particularly susceptible to drying after short periods without precipitation) (Holdgate et al., 2014; Korasidis et al., 2016, 2019b). The upper portion of the seat earths are characterised by an abundance of Myrtaceae and Proteaceae pollen (Fig. 11). This palynofloral assemblage is very similar to that documented in the dark lithotype facies

of the offshore T1 coal seam (Korasidis et al., 2019a). These groups may represent fire-tolerant shrubs and small trees that, in modern settings, fringe the boundary between the rainforest communities that dominate the peatlands and the meadow marsh assemblages growing on a peat or clay substrates. This interpretation is consistent with the high relative abundances of Cyatheaeae (tree ferns) in the upper portion of the seat earth that in modern settings characterise rainforest margins (Jones and Clemesha, 1976). It is likely that this floral assemblage grew on a substrate at or near the water table in partially emergent conditions. 6.1.3. Depositional setting for the coal lithofacies The overlying coal is dominated by relatively low diversity palynofloras (i.e. Casuarinaceae, Myrtaceae, Proteaceae) known to occur in ombrogenous coal-forming swamps (Korasidis et al., 2019b). The observed palynofloral assemblage from the coals is characteristic of the medium lithotype coals in the offshore Traralgon Formation (Korasidis et al., 2019b). The low moisture (i.e. b50–55%) content of the examined coal seams (Holdgate, 2003), their floristic composition and calculated pollen and charcoal concentration support their assignment as medium lithotype coals as defined in Korasidis et al. (2017a). 6.2. A non-marine origin for Gippsland Basin seat earths Overall, the palynological data point to an initial non-marine origin for the seat earth mudstones as first proposed by Grim and Allen

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Fig. 9. Stratigraphic section (in meters) and palynology of seat earths and coal from 648 to 652 m, Wulla Wullock-7.

Fig. 10. Palynological data (expressed as a percentage of the total sum) for the characteristic spore-pollen of the tidal, seat earth and coal facies in the Traralgon 2 sub-units in Wulla Wullock-7. The central box represents the middle 50% of the data while the whiskers represent the extreme values that are not outliers. A thick black line represents the median while a black circle represents the mean. The number of samples represented by the facies is 22 (coal facies – data derived from Korasidis et al., 2019b), 6 (seat earth facies) and 2 (tidal facies).

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Fig. 11. Depositional model for seat earth development. Palynological data indicate the seat earths were deposited in a non-marine setting.

(1938), because they have a non-marine palynofloral assemblage (e.g. heath-fern meadow marsh in the lower portions). Taken together, the palynological, mineralogical and sedimentological data suggest that the seat earths were first deposited in a non-marine setting over a foundation of intertidal sediments (i.e. a tidal-flat setting) prior to peatland progradation (Fig. 11). The upward increase in kaolinite (a product of intense acid leaching/chemical weathering), slickensides and rootlets suggests this non-marine clay was kaolinitized by pedogenic processes (i.e. weathering) that occurred both prior to and perhaps during peat formation (i.e. continued leaching by swamp water) in agreement with Gardner et al. (1988). It is also possible that acidic fluids from the overlying coal have further altered the clays during later diagenesis (e.g. Staub and Cohen, 1978). The presence of abundant kaolinite, in addition to well-preserved and abundant palynofloras, in the seat earths indicates a relatively acidic diagenetic setting (Driese and Ober, 2005; Korasidis et al., 2017a). The acidic nature (pH b 4) of modern raised bogs is well documented (see Becking et al., 1960; Anderson, 1983; Cecil et al., 1985) and occurs because of plant decay, namely the release of hydrogen ions and humic/fulvic acids in aerobic conditions (Anderson, 1983). The well-preserved nature of the palynofloras in the seat earths examined is also consistent with their deposition in a relatively anoxic environment (White, 1933; Twiddle et al., 2012; Korasidis et al., 2017a). The primary non-marine origin of the seat earths from WW7, based on palynological data, explains the intimate association between coals and seat earths, from the Gippsland Basin and globally. The seat earth clays form in an environment that is a precursor to domed peatland development. 6.3. Palynoflora of seat earths and associated coals globally Despite the widespread recognition of (carbonized) plant rootlets in seat earths (i.e., Logan, 1842; O'Brien, 1964; Breyer and McCabe, 1986; Driese and Ober, 2005), there is limited information on the palynoflora preserved in seat earths. Notable exceptions include the early miospore work of Smith (1962), Marshall and Smith (1964) and Scott (1978, 1979) on the underclays of the Pennsylvanian (Late Carboniferous) in the United Kingdom. More recently, Lindström and Erlström (2006) and Petersen et al. (2013) conducted palynological analysis on a grey structureless mudstone (underclay) with abundant rootlets, clay cutans and kaolinite enrichment, that underlies a thin Late Triassic coal seam in southern Sweden. Bartley et al. (2010) also carried out palynological work on the Miocene Middle Fork Eel River coal-bearing beds (California, United States of America). Pennsylvanian studies indicated that the seat earth floras, i.e. Calamites (Equisetum, horsetail), differed from the coal floras and formed in floodplain environments. The Calamites flora was interpreted as

being rooted subaqueously in seat earth, consistent with the presentday ecological niche for horsetails. Lycospora tree-sized club mosses (i.e. Sigillaria and Lepidodendron) were partly rooted in seat earth before peaty debris, derived of fallen club mosses, rapidly accumulated in anoxic conditions (Smith, 1962; Stach, 1982; Haszeldine, 1989). Petrographically, this peat is associated with bright coal layers (vitrain and clarain). Stunted and low diversity Densospora flora subsequently colonized the increasingly thick and elevated peat. The raised nature of the peat excluded clastic input, thereby lowering the ash content of the constituting coal, resulting in the accumulation of partly oxidized peat (Stach, 1982; Haszeldine, 1989). Petrographically, this peat is associated with dull coal layers (durain). This reaffirms that the Pennsylvanian seat earths of the United Kingdom underlie dulling-upwards coal cycles that result from doming of the swamp surface in response to seral buildup of peat. Previous studies of Triassic underclay floras also documented that seat earth palynofloras differed from the overlying coal palynofloras. The Late Triassic mudstones (underclays) from the Triassic in southern Sweden are also characterised by relatively high abundances of Equisetopsids and rare dinoflagellate cysts, and stratigraphically higher are dominated by fern spores, notably Deltoidospora spp. (Lindström and Erlström, 2006). In contrast, the overlying black and bright coals are dominated by high relative abundances of conifer pollen, notably Perinopollenites elatoides, and bryophyte spores (Lindström and Erlström, 2006; Petersen et al., 2013). This underclay has been interpreted as a vertisol (Arndoff, 1994) and a lagoonal mud, deposited in a marginal marine setting, that turned into a palaeosol via pedogenic overprinting from the overlying coal (Lindström and Erlström, 2006). The topmost sample, which is devoid of marine palynomorphs and instead dominated by spores, is interpreted to reflect the colonization of an exposed lagoonal mud and the initial stage of mire establishment (Lindström and Erlström, 2006). Thick grey-brown mudstones (underclays) from the Miocene Middle Fork Eel River coal-bearing beds have also been interpreted as seat earths based on an abundant palynomorph assemblage and the presence of two distinct zonations in the underclays in outcrop (Bartley et al., 2010). 6.4. Floral succession and cyclicity in the Gippsland Basin The floral and lithofacies succession, from tidal siltstone to seat earth to coal (black or brown) represent a shallowing-upwards succession. The palynological similarities between the seat earths and the darker lithotypes of the onshore Latrobe Group (Fig. 12) indicate that both represent meadow marsh and rainforest margin facies. We therefore suggest these two facies represent approximately the same depositional setting; one having a clastic substrate, the other having a peat substrate.

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Fig. 12. A comparison of shallowing-upwards cycles from onshore (coal dominated) to offshore (clastic dominated) in the Latrobe Group, Gippsland Basin, Australia.

In the onshore Gippsland Basin, the thick coal seams have developed behind the coeval and regionally extensive back barrier complex known as the Balook Formation (Thompson and Walker, 1982; Holdgate and Sluiter, 1991). Likewise, in the offshore Gippsland Basin, the coal seams have also developed behind regionally extensive barrier systems (Fig. 2). In the onshore Latrobe Valley coals, the shallowing-upwards succession typically has a basal erosion surface and is overlain by laminated dark lithotype coal. This is overlain by a dark lithotype coal and terminates with a medium lithotype coal (Fig. 12). This type of shallowing-upwards succession is best developed in the onshore Morwell 1 coal seams of the Latrobe Group where numerous lightening-upwards lithotype cycles are present (Holdgate et al., 2014; Korasidis et al., 2019a). Closer to the palaeoshoreline, the succession grades from burrowed and laminated silt and clay to a kaolinitic seat earth. This is overlain by a medium lithotype coal. This type of shallowingupwards succession is best developed in the lowermost T2 sub-unit in the WW7 well. 6.5. Modern analogues The degree of substrate inundation (i.e. water table depth) and nutrient availability dictates the composition and succession of flora that develop in modern and ancient peat-forming environments (Cameron et al., 1989; Page et al., 2006; Korasidis et al., 2017b). The distinct floral zonations documented in modern ombrogenous peats highlight this predisposition whereby structurally complex and floristically diverse forest develops around the nutrient-rich peat dome margins while less diverse and stunted forest develops over the thicker nutrientdepleted peat towards the center (Anderson, 1963; Cameron et al., 1989; Page et al., 1999, 2006). In modern peat-forming environments, large ombrogenous (i.e. oligotrophic) peats, the precursor to the Latrobe Group coals, are rain-fed resulting in acidic and low plant nutrient conditions (Burrows and Dobson, 1972; Cameron et al., 1989; Page et al., 2006; McGlone, 2009; Holdgate et al., 2014). The downward filtering of acidic waters (pH <5), from peat through to the underlying clays in modern peatforming environments in South Carolina, also results in nutrient-poor conditions and kaolinite-rich clays (Staub and Cohen, 1978). Further, despite differences in substrate (i.e. peat vs kaolinitic clays) the same

specialised flora establishes on acidic and nutrient-poor substrates in modern peat-forming environments (Cameron et al., 1989). As a result, the seat earths and darker lithotypes of the onshore Latrobe Group have similar palynological characteristics because both sediments developed in acidic and nutrient-poor settings; one having a peat substrate (e.g. the darker lithotype facies), the other having a clastic substrate (e.g. the seat earth facies). Modern Australian taxa that establish in nutrient poor (i.e. nitrogen and phosphorus) substrates include Proteaceae, Cyperaceae and Restionaceae (Lamont, 1993). Likewise, New Zealand taxa that persist in strongly acidic and nutrient deficient soils, in addition to acidic (pH 5.6) lagoonal soil settings, includes Gleicheniaceae, Cyperaceae, Leptospermum and Sphagnum (Mark and Smith, 1975; Robertson et al., 1991; Dickinson and Mark, 1994). In New Zealand, the forest trees Lagarostrobos, Dacrycarpus and Phyllocladus also establish in nutrient poor substrates (Robertson et al., 1991). The aforementioned New Zealand taxa, that are adapted to nutrient-deficient substrates, also thrive in poorly drained substrates (Mark and Smith, 1975; Wilmshurst et al., 2002). Relatively high abundances of the plants described above, adapted to nutrient-poor conditions, characterise the seat earth and darker lithotype palynofloras of the Latrobe Group. This suggests that nutrient availability and substrate wetness, as opposed to substrate lithology, controlled the distribution of floral communities in the Latrobe Group coals and associated sediments. Modern marshes and lagoons from New Zealand may therefore provide appropriate floral analogues for the palynological facies identified in this study (Fig. 13). Supratidal freshwater facies are here interpreted as the best sedimentological analogues for the seat earths examined. Freshwater tidal flats are documented in select modern estuaries and are best developed in the most upstream (i.e. landward) portion of the estuary (Reineck and Singh, 1980). Floristically, freshwater tidal flats such as the Weser Estuary of Germany are characterised by a high abundances of reeds (Poaceae), reedmace (Typhaceae) and club-rush (Cyperaceae) (Reineck and Singh, 1980). These floral communities share many similarities with the meadow marsh flora documented in the seat earth and darker lithotype facies in this study. In modern prograding coastlines, this supratidal lagoonal environment is also commonly overlain by freshwater marshes with peat formation (Reineck and Singh, 1980), consistent with our interpretation of the tidal siltstone to seat earth to coal cycles as shallowing-upwards successions.

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Fig. 13. Modern settings from southern New Zealand that provide floral and environmental analogues for seat earths. A) Emergent marsh facies of the Okarito Lagoon, South Westland. B) Meadow marsh with abundant Gleichenia (bright green fern) and sedge of the Okarito Lagoon, South Westland. Leptospermum (Myrtaceae) forms a fringe between the rainforest and the meadow marsh. C) Dismal Swamp, south of Haast, with meadow marsh in foreground. Leptospermum (Myrtaceae) forms a fringe between the tall podocarp rainforest and the meadow marsh. D) Abundant tree ferns (Cyatheaceae/Dicksoniaceae) grow at the transition from forest margin to tall podocarp forest in the Ship Creek area, Haast Ecological District.

7. Summary and conclusions The origin of seat earths from the Gippsland Basin has been investigated through sedimentological, palynological and mineralogical analysis, on coal-bearing successions from the Traralgon 2 Formation subunit of the Gippsland Basin, Australia. The seat earths of the Latrobe Group are unlaminated, have a light grey to white colour, contain abundant slickenside fracture surfaces and isolated organic matter, and contain abundant kaolinite and lesser amounts of 2 M illite. The seat earth palynofloras include a fire-prone heath-fern meadow marsh flora with abundant Gleicheniaceae (coral ferns) and Epacridaceae (heaths). In the upper portion of the seat earths, abundant Cyatheaceae (tree ferns), Schizaeaceae (curlygrass or comb ferns) and Proteaceae replace this flora. These groups represent fire-tolerant shrubs and small trees that fringed the boundary between the rainforest communities that dominated the peat and the meadow marsh heaths and ferns that grew on the seat earth. The palynological data indicates a non-marine origin for the kaolinitic mudstones and this suggests that the seat earths were first deposited in a non-marine setting over intertidal (i.e. a tidal-flat marsh) sediments as evidenced by lenticular bedding, extensive burrowing and a diverse assemblage of marine-influenced palynomorphs in the underlying siltstone lithofacies. These observations also indicate that portions of the Eocene Latrobe Group were deposited in marginalmarine and tidal settings, rather than being of fluvial origin. The upward increase in kaolinite, slickensides and rootlets suggests this non-marine clay was kaolinitized by pedogenic processes (i.e. weathering by organically derived humic/fulvic acids) occurring prior to and during peat formation. The presence of well-preserved and abundant spore-pollen in the kaolinite mudstones also suggests that the seat earths were deposited in an acidic and reducing diagenetic setting. The stratigraphic transition from siltstone, mudstone (seat earth) and coal lithofacies upwards is interpreted as a shallowing-upwards succession, similar to

that found in the onshore coals of the Latrobe Group. The seat earths of the Gippsland Basin were therefore deposited as a precursor facies (predominantly a meadow-marsh facies) to the ombrogenous coal facies. This facies interpretation explains the intimate association between coals and seat earths from the Gippsland Basin and globally.

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