Sedimentology and Mineralogy of Dolomitic Coorong Lakes, South Australia

Abstract

ABSTRACT Coorong dolomites are true primary dolomites; that is, Coorong dolomite is precipitating where there was nothing before and is not replacing an earlier carbonate mineral. Dolomites in the various lakes of the Coorong coastal plain in South Australia occur in three mineralogical associations; Dolomite ± Mg-calcite (widespread) Dolomite ± Magnesite (common in northwest) Dolomite ± aragonite ± hydromagnesite (rare and only in northwest). Most Holocene dolomite occurrences in the near coastal lakes of the Coorong region are associated with Mg-calcite. Only in the more arid settings found at the northwestern end of the Coorong region, near Salt Creek, is dolomite commonly found in association with magnesite and occasionally with hydromagnesite and aragonite. Dolomite in the near surface sediments of lakes near Salt Creek such as Pellet Lake and Milne Lake is often associated with magnesite in the lake center and Mg-calcite about the lake margin. The dolomites associated with magnesite tend to be more magnesian-rich than dolomites associated with Mg-calcite, a direct reflection of the chemistry of the mother waters. Waters which precipitate magnesite are typically more concentrated and more magnesium-rich than waters pre ipitating Mg-calcite, hence co-precipitated dolomites will also be more magnesium-rich. Textures found in vertical sequences from lakes in the Coorong coastal plain are largely independent of mineralogy; similar vertical transitions occur whether the lake is filling with carbonate, magnesian carbonates, or gypsum. Dolomite has been the main mineral of concern in previous geological studies of this area, yet it makes up no more than 10% of the carbonate minerals forming surficial deposits across the coastal plain. The hydrological model of the Coorong lakes should not be used to explain widespread dolomitization in ancient supratidal and shelf carbonates. Coorong counterparts in the rock record are a facies mosaic of lacustrine carbonate sitting in a much more extensive paleoaquifer.