CARBONATE MICROBIALITES OF LAKE BULLENMERRI, AUSTRALIA, PROVIDE NEW INSIGHT INTO LATE HOLOCENE LAKE HISTORY

The Western Plains District of Victoria in southern Australia is a large region of Quaternary volcanics with low relief and poorly integrated surface drainage. There are a large number of small cones and eruption centers in the area, some of which contain relatively deep, perennial lakes. The limnology of these closed-basin “rain-gauge” lakes have received considerable attention, and their offshore Holocene stratigraphic sequences have played a pivotal role in helping better understand hydrologic and climatic change in southern Australia. Many of the basins also contain a variety of organogenic shoreline and nearshore structures, which have been comparatively poorly studied.

Lake Bullenmerri is a hyposaline (~9 ppt TDS) maar lake located about 200 km west of Melbourne, Australia. Although the late Holocene offshore stratigraphic record in this 60 m deep lake is complacent, a 20 m water level decline since the 1880’s has exposed a variety of well-indurated carbonate hardgrounds and pavements deposited over the past several millennia. Morphologies range from beachrocks, boulder encrustations and small (30-40 cm high) tubular and branching cauliflower-form microbialites to meter-scale biohermal structures. Most of the rocks are massive to faintly laminated mudstones and packstones composed mainly of aragonite and poorly-ordered Ca-rich dolomite, and small amounts of magnesian calcite and monohydrocalcite. Although many of the saline and hypersaline lakes of the Western Plains region are sites of modern and Holocene dolomite formation, the CaMg(CO₃)₂ in Bullenmerri likely formed in relatively low salinity waters. Detailed investigation of these late Holocene carbonates offers important insight into the recent water level and associated hydrochemical changes that are not readily resolved from the offshore sediments.