Oligo-Miocene peatland ecosystems of the Gippsland Basin and modern analogues
Introduction
The origin and interpretations of brown coal lithotypes have been controversial. Following the early facies models from the German brown coals (e.g. Teichmüller, 1952, Teichmüller, 1958), the lighter-coloured lithotypes from the Latrobe Valley were initially interpreted as accumulating in permanently inundated, largely stagnant, laterally extensive and relatively deep (> l m) lakes (Luly et al., 1980, Kershaw and Sluiter, 1982, Kershaw et al., 1991). This was based on a lack of identifiable plant macro remains and a predominance of pollen derived from supposed dryland plants with good dispersal ability. The slow accumulation of regionally-derived pollen in these lakes was thought to account for the higher pollen concentrations recorded in the lighter lithotypes (Kershaw et al., 1991). In contrast, Anderson and Mackay (1990) argued that the lack of macroscopic plant debris was evidence that conditions at the time of deposition actually led to high degrees of degradation. More recent studies also suggest that the lighter lithotypes represent the final stages of ombrogenous peat growth and reflect relative drying-upwards in ombrogenous peat domes (Holdgate et al., 2014, Korasidis et al., 2016).
This paper presents a detailed study of the lightening-upwards cycles within the Morwell 1B seam of the Latrobe Valley based on new palynological (i.e. spore-pollen abundance data, a study of pollen grain preservation and average pollen concentration), geological and charcoal data, in conjunction with pre-existing geochemical and palaeobotanical data. In particular, the lightest lithotypes were investigated in detail because the controversial interpretation of these lithotypes resulted in contrasting models for the origin of brown coal lithotypes. Comparisons are also drawn from ombrogenous peats in Southeast Asia (i.e. Grady et al., 1993, Esterle and Ferm, 1994, Moore et al., 1996) and modern vegetation zonations in New Zealand (i.e. Mark and Smith, 1975, Robertson et al., 1991, Dickinson and Mark, 1994). We use modern New Zealand environments as floral analogues to better understand the Latrobe Valley Oligo-Miocene peatland environments.
Section snippets
Geological setting
The Latrobe Valley Depression of the Gippsland Basin in Southeastern Australia (Fig. 1) contains thick and widespread Eocene to Miocene brown coals and associated sediments up to 700 m thick (Holdgate et al., 2000). Individual seams are > 100 m thick, and the vertical multi-seam levels of the coals are without parallel globally (Holdgate et al., 1995). The main coal-bearing sequences in the Latrobe Valley Group of the onshore Gippsland Basin are, in stratigraphic order, the Traralgon Formation,
Lithotypes in the Latrobe Valley and lithotype models
Six lithotypes, defined on dry and weathered coal surfaces by colour, texture, gelification and weathering, are recognized in the Latrobe Valley brown coals: laminated dark, dark, medium dark, medium light, light and pale (George and Mackay, 1991, Holdgate et al., 1995). The lithotypes occur as a series of 10–30 m cycles and are characterised by lightening-upward trends with prominent banding on a 1–3 m scale (Mackay et al., 1985, Holdgate et al., 2014). Various models have been proposed, based on
Methodology
Stratigraphic sections from the M1B seam were measured on the southeastern and eastern face of the Loy Yang Open Cut Mine (Fig. 2). Coal samples were repeatedly collected at 25 cm intervals based on the stratigraphic heights measured using a Jacob's staff. These samples were dried at 40 degrees for 3 days and crushed to a grain size of 0.5–1 mm because the lithotype colour range is dependent on the grainsize of the crushed coal (Attwood et al., 1984). Quantitative colourimetry was subsequently
The Morwell 1B seam
Detailed palynological analyses were carried out on various intervals throughout the M1B seam (see Fig. 3 for the stratigraphic height of samples). Two complete lightening-upwards cycles were studied to investigate the controls of lightening-upwards. These samples are derived from the southeastern side of Loy Yang Open Cut (Fig. 2) where sampling was undertaken perpendicular to the strike of the coal beds so a vertical profile of the M1B seam was collected. Select samples from the more
Discussion
The origin of brown coal lithotypes has been dominated by two contrasting models termed the dry-dark and dry-light models (Holdgate et al., 2014). It was initially thought, based on palynological studies, that the lighter lithotypes in the Latrobe Valley were deposited in long-lived, laterally extensive, relatively deep (> l m) lakes within the coal-forming environment (Luly et al., 1980, Sluiter, 1984, Kershaw et al., 1991). Anderson and Mackay (1990) opposed this ‘open water’ depositional
Conclusion
New palynological, geological and charcoal data presented in this study suggests that the type of peat-forming environment and the associated hydrological regime were the main factors influencing the development of lithotypes in the Latrobe Valley brown coals. Each of the lithotypes examined in detail throughout the Morwell 1B seam has a distinct floral component that is controlled by the peat-forming environment.
The laminated dark lithotype represents a fire-prone emergent marsh that grew on
Acknowledgements
AGL Loy Yang and GHD are gratefully acknowledged for enabling the sampling of the M1B seam from the Loy Yang Open Cut Mine. We would particularly like to thank Ben Jansen (GHD) for facilitating our visits to the Loy Yang Open Cut Mine and to Dr. Michael Shawn-Fletcher (University of Melbourne) for his assistance in developing methods used for palynological processing. We are also grateful to Professor Joan Esterle and Dr. Tim Moore for constructive and helpful comments on the manuscript. An
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