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 School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia b CarbonNet, 121 Exhibition Street, Melbourne, Victoria 3000, Australia abstract article info 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 The origin of seat earths (i.e. underclays, seat rocks, fire clays) has been investigated using sedimentological, pal- ynological 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 ka- olinite 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 fire- tolerant 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 dino- flagellates). 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 precur- sor 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. Keywords: Seat earths Underclays Coal Kaolinite Pedogenisis Palynology 1. Introduction Seat earths (or underclays, seat rocks, fire clays) are clay-rich beds that directly underlie coal seams (Logan, 1842) and their enigmatic or- igin 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 pro- posed 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 minerals in underclays (Keller et al., 1954; Keller, 1956; McMillan, 1956; Patterson and Hosterman, 1960; Rimmer and Eberl, 1982), fol- lowing 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 li- thologies (Smith, 1962; Marshall and Smith, 1964; Scott, 1978, 1979). Seat earths have largely been described from Paleozoic black coal- bearing successions globally (Logan, 1842; Gardner et al., 1988). Here we describe a series of Cenozoic seat earths from the brown coal de- posits of the Latrobe Group, Gippsland Basin, Australia. The clays that di- rectly underlie coal seams in the lower part of the Traralgon 2 (T2) sub- unit (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, palyno- logical and mineralogical analysis of seat earths and associated litholo- gies 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. Sedimentary Geology 390 (2019) 100–113 ⁎ Corresponding author at: School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia. E-mail address: verak04@gmail.com (V.A. Korasidis). https://doi.org/10.1016/j.sedgeo.2019.07.007 0037-0738/© 2019 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo