Reconstructing a Miocene pitfall trap: Recognition and interpretation of fossiliferous Cenozoic palaeokarst Derrick A. Arena a,b, ⁎, Karen H. Black a , Michael Archer a , Suzanne J. Hand a , Henk Godthelp a , Philip Creaser a a School of Biological, Earth & Environmental Sciences, University of New South Wales, New South Wales 2052, Australia b Associated Scientific Pty Limited abstract article info Article history: Received 10 November 2013 Received in revised form 27 January 2014 Accepted 28 January 2014 Available online 7 February 2014 Editor: B. Jones Keywords: Cave deposits Palaeokarst Riversleigh Speleogenesis Palaeoecology The middle Miocene cave deposit at AL90 Site in the Riversleigh World Heritage Area of Queensland, Australia, is the largest excavated palaeokarst deposit within the extensive Cenozoic freshwater limestones of the region. Stratigraphic relationships between various lithological components of the cave deposit indicate a dynamic, complex depositional history including dissolution, infill and reworking of cave materials. Euhedral spar, shelf- stone, calcite rafts and desiccation cracks on cave sediments suggest standing water was variably present in the cave environment. The AL90 Local Fauna is moderately diverse comprising at least 20 vertebrate families and 32 species, yet is taphonomically biased towards large marsupial herbivores (families Diprotodontidae, Macropodidae, and Balbaridae) and cave-dwelling hipposiderid bats. Fossil material is generally exceptionally well-preserved with, in many cases, articulated skeletons recovered. The cave entrance appears to have acted as a natural pit-fall trap. The complex (unconformable) depositional structure and lithology, vertebrate taphonomy and faunal composition at AL90 Site are typical of deposits and assemblages formed in the inner environment of the upper vadose zone. In the past, some similar unroofed cave deposits have been interpreted as clastic fluvio-lacustrine sedimentary deposits formed by surface processes. Analysis of characteristic palaeokarst features at AL90 Site has enabled the identification of other less well-preserved, poorly-exposed or relict cave deposits at Riversleigh and has facilitated interpretation of the depositional and palaeoenvironmental history of those deposits. The findings presented here confirm earlier interpretations that specific, highly- fossiliferous early Miocene to early Pliocene fossil deposits at Riversleigh represent relict caves whose roofs and walls are no longer intact. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The Riversleigh Cenozoic carbonate facies, including—but not restricted to—deposits referred to as the Carl Creek Limestone, occur adjacent to, unconformably within and upon Proterozoic siliciclastic rocks and Cambrian marine limestone and chert deposits forming the northeastern edge of the Barkly Tableland in the Riversleigh World Heritage Area (WHA) in northwestern Queensland (Fig. 1). Fossil vertebrate faunas from these deposits range in age from late Ol- igocene to early-late Miocene (approximately 25 Ma to 10.5 Ma); a Pliocene fauna has also been recovered from a cave fill in Cambrian limestone at Riversleigh and Pleistocene fossils have been found in alluvial terraces along the Gregory River (Archer et al., 1989, 1994, 1997). More than 200 fossil-bearing localities have been recognised so far. Once regarded as lacustrine (Tedford, 1967) and/or alluvial (Williams, 1978; Megirian, 1992), the depositional history of the Riversleigh Cenozoic carbonate deposits is now understood to have involved a complex sequence of lacustrine, fluvial and karst processes (Archer et al., 1989, 1994, 1997, Creaser, 1997; Arena, 2004, 2005; Arena et al., in preparation a,b). The modern Riversleigh landscape can be interpreted as a mature karst. Ongoing karst processes in this terrain include corrosion producing typical karst weathering morphology on carbonate outcrops including rills, lapies and kamenitzas; epikarstic development, including formation of deep fissures and chimneys, pipes, grikes and cleftkarren; development of collapse dolines; retraction of residual high-point landforms by erosion and corrosion. Epikarst in high points is commonly characterised by reduced soil and vegetation cover and exposed rock surfaces, providing potential access to subsurface cavities. The aim of this work is to describe the characteristics of palaeokarst deposits at Alan's Ledge 1990 (AL90) Site and use them to identify and interpret other palaeokarst deposits of Cenozoic age. New observations and geological interpretations are applied independently to fossiliferous deposits at Riversleigh, some of which have previously been identified as cave deposits. In some cases previous interpretations are confirmed, whereas others are reinterpreted based on these findings. Additional study of other Cenozoic deposits at Riversleigh is currently under way (Arena et al., in preparation-a,b) and geochemical analysis of the de- posits at AL90 Site has also been conducted separately (Parmeter et al., 2012). Sedimentary Geology 304 (2014) 28–43 ⁎ Corresponding author. E-mail address: r.arena@unsw.edu.au (D.A. Arena). 0037-0738/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sedgeo.2014.01.005 Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo