Using macrofossils to interpret peatland facies of Miocene brown coals, Gippsland Basin, southeastern Australia

Dr Anne-Marie Tosolini1, Dr Vera Korasidis2, Associate Professor Malcolm Wallace1, Dr  Barbara Wagstaff1, Professor Robert Hill3

1School of Earth Sciences, The University Of Melbourne, Australia, 2Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, U.S.A., 3Environment Institute, University of Adelaide, Adelaide, Australia

Coals rarely preserve fossil leaves due to catabolic, fungal, bacterial, and/or root disturbance mechanisms that degrade leaf material deposited on the surface of swamps.  Well-preserved leaf macro- and meso-fossils (cuticles) have, however, been recovered from the Miocene Morwell 1 brown coal seams in the Gippsland Basin, southeast Australia. Three separate mechanisms of preservation facilitated the accumulation of these floral Lagerstättan, with the unifying condition for all mechanisms that plant material is delivered directly to the anaerobic catotelm and avoids the degrading aerobic acrotelm. Well-preserved leaf material found in these coals is, thus, derived from leaf litter that falls into low-energy acidic and anoxic water-filled depressions that lie below the water table.

Cyclic successions within individual coal seams reflect repeated lithotype cycles in the peat swamps that represent peatland aggredation and record relative drying or terrestrialization events. Six facies occurring within the cycles are defined by colour, texture, gelification and weathering: laminated dark, dark, medium dark, medium light, light and pale. A full lithotype cycle was located within both the lower M1B and upper M1A seams in the Loy Yang Open Cut Mine, Latrobe Valley, where each facies was sampled, analysed for macrofossils where present, which yielded leaves, wood and seeds, then macerated for mesofossils, which yielded leaf cuticles. 

Laminated dark facies contain abundant rushes (Typhaceae/Restionaceae), common coral-ferns (Gleicheniaceae) and a high abundance of charcoal (but lack recognisable cuticles), suggesting deposition in open, well-lit, inundated, emergent to meadow marsh environments. Dark facies contain abundant Kauri leaves (Araucariaceae: Agathis yallournensis) and Blue Quandong endocarps (Elaeocarpaceae: Elaeocarpus); cuticle assemblages are dominated by gymnosperms (abundant Agathis yallournensis; common Podocarpaceae: Dacrycarpus, Dacrydium). These arborescent taxa suggest deposition in periodically inundated environments of a forested bog. Medium dark and medium light facies are dominated by angiosperms: abundant Elaeocarpus endocarps and Oleinites; common Agathis yallournensis and Myrtaceae leaves; and abundant cuticles of Proteaceae (Banksia laevis, possible Orites) and lilly pilly (Myrtaceae: Syzygium); and notably contain tawheowheo (Parachryphiaceae: Quintinia). Deposition of these facies was in an angiosperm-dominated forested bog environment. Light and pale facies contain abundant Casuarinaceae leaves and cuticles (and rare Quintinia) that represent shallowing upwards and drying of the swamps into ombrogenous forest bog, with some open canopy areas, and finally to the formation of oxidised soils.

The macro- and mesofossil elements of these six different facies within repetitive cycles in the M1B and M1A brown coals, thus, represent changing floral composition as a direct result of changes in substrate wetness during peatland aggradation and the evolution of wetland/peatland systems through the Early to Middle Miocene. Facies progression reflects development of the peatland from fire-prone marsh environments to an angiosperm-dominated, ombrogenous forested bog and supports previous palynological, sedimentological and charcoal analyses. These multidisciplinary analyses have overturned previous theories that charcoal represents the driest facies, whereas in fact, charcoal forms from the burning of fire-tolerant and flammable species in the marsh environments. Modern peat swamp analogues of these cyclic facies successions are found on the South Island of New Zealand and support the “dry-light”, shallowing upwards, facies model.


Biography

Dr Anne-Marie Tosolini, palaeobotanist, Lecturer, School of Earth Sciences, University of Melbourne. Dr Vera Korasidis, palynologist and Postdoctoral Fellow, Smithsonian Institution. Associate Professor Malcolm Wallace, Chair in Sedimentology, University of Melbourne. Dr Barbara Wagstaff, Honourary palynologist, University of Melbourne. Professor Robert Hill, Director, Environment Institute, University of Adelaide.

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