The influence of tectonics on flank margin cave formation on a passive continental margin: Naracoorte, Southeastern Australia Susan White ⁎, John A. Webb Environmental Geoscience, La Trobe University, Victoria 3086, Australia abstract article info Article history: Received 31 December 2013 Received in revised form 26 August 2014 Accepted 4 September 2014 Available online 16 September 2014 Keywords: Southeastern Australia Coastal karst Flank margin caves Mixing corrosion Speleogenesis Tectonics Intensive cave development within the highly porous and permeable Eocene–Middle Miocene Gambier Limestone in southeastern South Australia is restricted to a 1 × 11 km area at Naracoorte. The caves are over- whelmingly horizontal, consisting of large solutional domes connected by smaller passages, with bell holes, small pendants and large non-directional scallops on the walls and ceilings. Orientation is strongly controlled by NW/SE joints. Cave entrances have been opened by subsequent collapse, and breakdown is common. The caves are located on the Kanawinka Fault escarpment, which was uplifted in the Late Miocene–Early Pliocene and then overlain by a series of Pleistocene carbonate strandline dunes, deposited as the sea retreated following a Late Miocene transgression. The coastline lay along the Kanawinka escarpment at ~0.9–1.1 Ma, when the caves formed just inland of the shoreline within the zone of enhanced dissolution at the seaward margin of the fresh- water lens. They have the typical flank margin cave morphology, except that joint development adjacent to the fault caused the strong linear orientation parallel to the coastline. The beach dune deposited to the west of the Kanawinka escarpment shows that sea-level had dropped sufficient- ly to completely drain the caves at 780–880 ka. Cave development was therefore confined to a period spanning ~0.8–1.1 Ma. Continuing gradual uplift through the Pleistocene means that the caves are now N 100 km inland, obscuring the essentially coastal nature of the Naracoorte karst. Speleogenesis at Naracoorte reflects the interaction of neotectonics with coastal dissolution, and emphasizes the role that tectonism can play in Pleistocene karst development even on passive continental margins, where glacio/ eustatic sea-level fluctuations are generally ascribed as the dominant role. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Highly porous and permeable Paleogene–Neogene marine carbonates outcrop extensively along the passive continental margin of southern Australia, but within this region zones of intensive cave devel- opment are relatively uncommon, and are interspersed with extensive areas of limited karst formation. In southeastern South Australia and southwestern Victoria, the Paleogene–Neogene limestones comprise the Gambier Karst Province (Marker, 1975; White, 2005)(Fig. 1). Within this province the densest concentration of caves is found at Naracoorte, where over 150 caves occur in a 1 km wide, 11 km long band. This band has a conduit density of 44 km/km 3 and a cave density of 0.14%, relatively high by world standards (Worthington et al., 2000). The Naracoorte caves contain a diverse Pleistocene vertebrate fauna (Wells et al., 1984; Reed, 2003), which is World Heritage-listed. The presence of Pleistocene strandline dunes in the Naracoorte area indicates that the karst probably formed under coastal conditions in the mid-Pleistocene (Marker, 1975; Moriarty et al., 2000), but the inter- relationship between the water table levels that formed the caves and the well-dated Pleistocene sea-levels has not been elucidated, and previous studies have not considered the significance of the coastal high porosity, high permeability limestone setting. There is therefore a need for an integrated cave and landscape evolution model for the Naracoorte region, incorporating all the available evidence on sea- levels and speleogenesis. 2. Geological setting The Naracoorte area lies within the western part of the Otway Basin, a rift basin that developed in the late Mesozoic across the passive south- eastern margin of Australia during the breakup of Gondwana, as Australia separated from Antarctica. Siliciclastic deposition in the Otway Basin in the Cretaceous changed during the Paleocene to cycles of marine transgression and regression, depositing cool-water shelf carbonates and deeper-water marls up until the Middle–Late Miocene (Benbow et al., 1995; Holdgate and Gallagher, 2003). In southeastern South Australia, during the latest Eocene to early Middle Miocene the essentially flat-lying Gambier Limestone Geomorphology 229 (2015) 58–72 ⁎ Corresponding author. E-mail addresses: s.white@latrobe.edu.au (S. White), john.webb@latrobe.edu.au (J.A. Webb). http://dx.doi.org/10.1016/j.geomorph.2014.09.003 0169-555X/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph