PALAIOS, 2020, v. 35, 22–36 Research Article DOI: http://dx.doi.org/10.2110/palo.2019.073 THE ORIGIN OF FLORAL LAGERST ¨ ATTEN IN COALS VERA A. KORASIDIS, 1,2 MALCOLM W. WALLACE, 1 ANNE-MARIE P. TOSOLINI, 1 AND ROBERT S. HILL 3 1 School of Earth Sciences, University of Melbourne, Victoria 3010, Australia 2 Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20013, USA 3 Environment Institute, University of Adelaide, South Australia, 5005, Australia email: korasidisv@si.edu ABSTRACT: Floral Lagerst¨ atten deposits (i.e., fossil sites with exceptional preservation and diversity) are preserved within the Miocene brown coals of the Latrobe Group, Gippsland Basin, Australia. Three independent mechanisms are conducive to their accumulation. Throughout the coal seams the conversion of plant material into charcoal (fusain) and its accumulation in a subaqueous setting provides one means of near-perfect preservation. A second and more uncommon example occurs in the form of a 20 cm thick leaf-litter horizon that extends for over two kilometers. In this case, flooding of freshwater tributaries and lakes during the early stages of low-gradient peat development resulted in an extensive, shallow, acidic and water-filled depression that subsequently accumulated and preserved the surrounding plant material. The third and most common form results from the deposition of plant material into small, isolated pools that formed as depressions on the ombrogenous (i.e., rain-fed) and domed surface of the peatlands. In all of these settings an essential component allowing detailed floral preservation is the delivery of plant material directly to the anaerobic catotelm (i.e., below the water table), hence avoiding the physical and chemical processes of degradation that typically occur in the surficial aerobic acrotelm (i.e., above the water table). Leaf litter that falls into low-energy acidic and anoxic water-filled depressions that lie below the acrotelm will likely be well-preserved. INTRODUCTION Peatlands are considered unlikely settings for Lagerst¨ attan (i.e., fossil deposits, which, show a great diversity of organisms preserved, or excellent preservation, or both), to develop and accumulate. Despite the generally acidic (pH , 3.5) and anoxic nature of peat swamps—conditions that promote the preservation of well-preserved plant material in peat and coal after burial—most leaf material that settles onto peat surfaces is degraded via catabolic, fungal, bacterial, and/or root disturbance mechanisms, thereby preventing preservation (Gastaldo and Staub 1999; Gastaldo 2010; Gastaldo and Demko 2011). Because well-preserved fossil leaf material is uncommon in coals (Winston 1989; Gastaldo and Staub 1999) paleoeco- logical studies of coals have largely relied upon permineralized plants in coal balls (e.g., Phillips et al. 1985; DiMichele and Phillips 1988; Raymond et al. 2014), high-ash paper coals (e.g., Neavel and Guennel 1960; DiMichele et al. 1984), cuticular analysis (e.g., Schneider 1980, 1995), wood and bark tissue (e.g., Moore and Swanson 1993; Van de Wetering et al. 2013), or the study of spores and pollen (e.g., Smith 1967; Anderson and Muller 1975). Rare leaf-litter lenses have, however, been recovered from low-ash coals with their preservation attributed to the establishment of small acidic depressions of water (Gastaldo and Staub 1999; Ferguson et al. 2010). The origin of these small leaf-litter bearing water-filled depressions, that vary in extent and longevity but preserve floral Lagerst¨ atten, has previously been attributed to the displacement of local rootstocks as trees either die and fall over or are blown down in severe storms (Gastaldo 1990; Moore 1989; Diessel 1992; Gastaldo and Staub 1999; Gastaldo and Demko 2011) and the development of hollows produced by lightning strikes or the spontaneous combustion of the upper and drier portion of the peat (Diessel 1992). Three types of floral Lagerst¨ atten occur within the Miocene Latrobe Group coals and are described collectively for the first time. A detailed depositional model for leaf-litter preservation in coal, based on new macrofloral (i.e., leaves and seeds), palynological (spores, pollen and charcoal), and sedimentological analysis is presented. The Lagerst¨ atten described provide unparalleled insight into the early through middle Miocene floral diversity and paleoecology of southeastern Australia. GEOLOGICAL SETTING The Gippsland Basin of southeastern Australia (Fig. 1) contains the thickest terrestrial succession in the world, with multiple brown coal seams exceeding 100 m in thickness (Holdgate 2003). The coal-bearing Latrobe Group is divided into three formations and from oldest to youngest include the Traralgon, Morwell, and Yallourn formations (Hocking et al. 1976). The Morwell and Yallourn formations, the focus of this paper, are further subdivided into individual seams. In decreasing age, these are the Morwell 1B (M1B), Morwell 1A (M1A), and Yallourn (Y) seams (Hocking et al. 1976). Through palynostratigraphy, samples examined from the lowermost M1B seam (Fig. 2) are assigned to the early Miocene Middle Proteacidites tuberculatus Zone (Korasidis et al. 2018). The uppermost M1B seam and lowermost M1A seam samples are assigned to the early Miocene Upper P. tuberculatus Zone whereas the uppermost M1A seam and the Yallourn seam are assigned to the late early-middle Miocene Triporopollenites bellus Zone (Korasidis et al. 2016, 2018). In the coals of the Latrobe Group, lithotype refers to color layering visible on the dried coal faces (Holdgate et al. 2014). Variation in color, texture, wood abundance, charcoal concentration, gelification, and weathering patterns produce distinct layers (George 1982). Six lithotypes Published Online: January 2020 Copyright Ó 2020, SEPM (Society for Sedimentary Geology) 0883-1351/20/035-022 Downloaded from https://pubs.geoscienceworld.org/sepm/palaios/article-pdf/35/1/22/4915251/i0883-1351-35-1-22.pdf by Vera Korasidis on 17 January 2020