A MINERALOGICAL APPROACH TO DISTINGUISHING DIAGENETIC ALTERATION FROM STRATIGRAPHIC CHANGE IN THE COWAN BASIN, WESTERN AUSTRALIA

Lake Aerodrome is an ephemeral, gypsum-dominated acid salt lake located on the southeastern margin of the larger Lake Cowan basin near Norseman, Western Australia (WA). The modern depositional environment is strongly influenced by repeated episodes of flooding, evapoconcentration and desiccation, as well as eolian transport. As a result, the surface sediments are largely Quaternary lacustrine, alluvial and eolian sands, and muds; however, these deposits are thin and bedrock can be observed in multiple local outcrops. Two sediment cores were collected from Lake Aerodrome during a 2009 drilling campaign. The first (LA1-09; 21.62 m) was located within the modern lake boundary, while the second was located approximately 100 m north, in the modern sandflat (LA2-09; 59.64 m). Analysis of the Lake Aerodrome core samples includes lithologic description of the sediments, reflectance spectroscopy of the ferric iron and Al-OH (phyllosilicate) components, and X-ray diffraction analysis of the bulk mineralogy. Using previous sediment core data acquired and described by Western Mining Corporation of Victoria, Australia, the subsurface formations recovered in the Lake Aerodrome cores likely include the Late Eocene Werrilup Formation (59-33 m), the Late Oligocene-Middle Miocene Revenge Formation (33-4.6 m) and Cowan Dolomite (30-28 m) and the Pliocene-Holocene Polar Bear Formation (4.6-0 m). Changes in lithology are characterized by varying amounts of several mineral constituents including quartz, feldspar, palygorskite, dolomite, and chlorite. In addition to detecting the mineralogical variations associated with these formations, the analysis of the Lake Aerodrome cores has also identified diagenetic changes in mineralogy, including a notable change that occurs at approximately 20m and is detectable in all of the analyses used in this study. This change is interpreted as a major diagenetic reaction front associated with the evolution of acidic surface conditions. Ongoing geochemical and petrographic analyses will aid in evaluating the diagenetic mechanism that have influenced these sediments.