FYI, Subject: Dec 18 Brown Bag - John Webb 0n Mt Gambier karst - Australia at 12:03 PM
Edwards Aquifer Philosophical Society - aka brown bag luncheon Friday, December 18 at 12:03: PM Edwards Aquifer Authority 1615 N. St. Mary's Street, San Antonio, TX We are pleased to announce that Dr. John Webb, LaTrobe University, Victoria, Australia, will be making a presentation titled The Tertiary Karst Plain Near Mt. Gambier, Southeastern Australia, and the Origin of the Cenotes There. Mt. Gambier is a recent volcano located in southern Australia that has emerged through a limestone terrain. The presentation will be held on Friday, December 18, at the Edwards Aquifer Authority. These are informal presentations so please feel free to bring your lunch. For directions or further information, the Authority may be contacted at 210-222-2204 or www.edwardsaquifer.org<http://www.edwardsaquifer.org> Thank you. Geary Schindel Chief Technical Officer Edwards Aquifer Authority The Tertiary karst plain near Mt Gambier, Southeastern Australia, and the origin of the cenotes there John A. Webb Environmental Geoscience, La Trobe University, Victoria, Australia Mt Gambier in southeastern Australian lies on a broad, low relief, coastal limestone plain only 30-40 m asl, lacking any surface drainage network. The plain is composed of essentially flat lying Eocene - Miocene limestone deposited on a cool-water open marine shelf, and is made up of sand-sized fossil fragments with substantial granular porosity. It overlies a calcareous mudstone aquitard. The limestone thickens to the south and has been offset along several northwest/southeast faults. Joints in the limestone trend generally parallel to the faults. During the Pliocene and Pleistocene the sea covering the area retreated, leaving behind a series of coastal dunes (former shorelines) ranging in age from 860,000 years to ~125,000 years old (Last Interglacial). In the Late Miocene - Early Pliocene a period of active deformation that affected southeastern Australia caused the northern part of the limestone plain to be uplifted by 30-40 m. This was followed by a period of volcanic activity that formed the Mt Burr Range (?mid-Pleistocene) and Mt Gambier (~28,000 years BP) and Mount Schank (~5,000 BP). The Mt Gambier limestone plain is punctured by a number of cenotes, that are circular, cliffed, collapse dolines containing water-table lakes up to 120 m deep, floored by large rubble cones. Most of the deepest cenotes are concentrated in two small areas located along trends parallel to the main joint direction in the limestone. The cenotes do not connect to underwater phreatic passages, and water chemistry data confirm that they are not part of an interconnected karst network. They formed by collapse into large chambers (up to >1 million m3) that extended 125 m or more below the land surface, probably to the base of the limestone. Several cenotes have actively growing stromatolites on the subvertical walls that started growing at ~8000 years BP. The caves that collapsed to form the deep Mt Gambier cenotes did not have an epiphreatic origin, as they are deeper and much larger than shallow phreatic caves in the area, and do not connect into deep phreatic systems. They were not formed by freshwater/seawater mixing, responsible for the well-known Yucatan cenotes, because they are not associated with locations of the mixing zone during previous higher sea levels, and are much larger than phreatic caves forming along the mixing zone at present in the Mt Gambier area. Instead dissolution was most likely due to a hypogene process whereby acidified groundwater containing large amounts of volcanogenic CO2 ascended up fractures from the magma chambers that fed the Pleistocene-Holocene volcanic eruptions in the area; deep reservoirs of volcanogenic CO2 occur nearby. Cave dissolution could have been a single event during one of the eruptions. Short biography John Webb is Associate Professor of Environmental Geoscience and co-ordinator of the Environmental Science program at La Trobe University, Melbourne, in southeastern Australia. He obtained his PhD from University of Queensland. He teaches courses in hydrology, hydrogeology and water geochemistry at undergraduate and postgraduate level, as well as courses on landscape and climate change, and remote sensing and GIS. He has supervised 21 PhD students and 79 Honours students in a variety of geological and hydrogeological projects. He has participated in a number of consulting projects, has been an invited member of expert panels to assess groundwater and contaminated site management, and has acted as an expert witness in hydrogeology and hydrogeochemistry in several court cases. He works on karst geomorphology and neotectonics, particularly of the Southeastern Australian Highlands, as well as hydrogeology and environmental geochemistry, mostly relating to dryland salinity and remediation of acid mine drainage. He has written a number of chapters in general texts about the geology and geomorphology of Victoria, including karst areas. He has worked on karst development and karst water chemistry in several areas in Australia: Nullarbor Plain, southeast SA (Mt Gambier), eastern Victoria (Buchan) and north Queensland (Chillagoe). He currently has a major hydrogeological project on the effect of climate and land use change on surface and groundwater resources in western and central Victoria, supported by the new National Groundwater Centre for Groundwater Research and Training. His acid mine drainage research has concentrated on neutralisation using limestone and increasing the chemical stability (resistance to leaching) of neutralisation sludges. Contact details: Environmental Geoscience, La Trobe University, Victoria, 3086. Phone +61 3 9479 1273 Email [email protected]<mailto:[email protected]> www.latrobe.edu.au/envsci/profiles/jwebb.htm
