Sedimentology of coorong dolomite in the Salt Creek region, South Australia

Abstract

Lakes in the Salt Creek area of the Coorong region, South Australia, contain a diverse suite of Holocene carbonates including aragonite, calcite, Mg-calcite, magnesite, hydromagnesite, and dolomite. Dolomite has been the main mineral of concern in previous geological studies of this area, yet it makes up no more than 5% of the carbonate minerals forming surficial deposits across the coastal plain. The hydrological setting of the Coorong lakes is such that the area should not be used to generate a model to explain the occurrence of widespread ancient supratidal and shelf dolomites. Coorong counterparts in the rock record are a facies mosaic of lacustrine carbonate sitting in a much more extensive paleoaquifer. Textures found in vertical sequences from lakes in the Salt Creek region are largely independent of mineralogy; similar vertical transitions occur whether the lake is filling with carbonate or gypsum. If the lake possessed an early Holocene connection to the marine waters of the lagoon then thebasal unit is a marine/estuarine skeletal grainstone/packstone. If there was no marine connection the basal unit is often a quartzose packstone to wackestone. Basal facies can contain a basal dolomite. Above the basal facies in some lakes is a massive to faintly laminatedorganic-rich unit. In lakes with an early marine connection the levels of T.O.C. in this unit can be as high as 12%, usually as an oil-prone proto-kerogen. Overlying this unit is amm-laminated unit of pelletal packstone to mudstone deposited once the estuarine connection to the open Coorong Lagoon was completely cut off by beach-ridge accretion. In a few lakes that were never connected to the estuary, such as Milne Lake and Lake 5, this laminated unit is composed of evaporative dolomite up to 4 meters thick. More typically it is composed of varying proportions of aragonite, hydromagnesite, magnesite, and/or magnesian calcite. In Halite Lake, it is composed of a laminated gypsum/aragonite. Capping the lake sediments is a“massive” unit of poorly layered packstone/mudstone usually less than 60 cm thick and showing a varying degree of induration. This unit usually holds the bulk of the evaporative dolomite found in the Coorong but can also be composed of aragonite, hydromagnesite, magnesian calcite and magnesite. The massive unit contains a characteristic suite of sedimentary structures including: bioturbation structures, stromatolites, mud cracks, extrusion structures, tepees, and breccia fragments; all these features indicate at least occasional desiccation. The presence of this capping unit in conjunction with laminated sediments provides the most reliable diagnostic for an ancient Coorong counterpart. Coorong dolomites are true “primary dolomites”; dolomite is precipitating as dolomite and not replacing an earlier carbonate mineral. Coorong dolomite occurs as two mineralogically distinct (type A and type B) forms, which are deposited as three stratigraphically distinct forms (evaporative, margin, and basal dolomite).Type A dolomites tend to be Mg-rich with a crystalline but heterogeneous microstructure, the unit cell is contacted in the ao dimension, and it has a heavier carbon isotope value and a wellclustered oxygen signature compared to type B.Type B dolomites tend to be calcian-rich to near-stoichiometric, with expanded unit cell dimensions (ao and co), they are more crystalline with a more homogenous microstructure compared to type A, and show isotopically lighter carbon values, as well as a larger variation in oxygen isotope values. Volumetrically the most important type of dolomite is anevaporative dolomite, a type A dolomite deposited as the last episode of sedimentation in those Coorong lakes which contain dolomite. In most lakes it is a capstone unit no more than tens of centimeters thick, although in Milne Lake it has infilled the lakes to form dolomitic units up to 4 – 5 meters thick. In some lakes there is a second type of surficial dolomite, a more calcian-richmargin dolomite (a type B dolomite) about the edges of the lake. Like the evaporative upper dolomite this dolomite is not intergrown with other carbonate phases and defines areas where resurging continental groundwaters first enter the lake margin. The third type of dolomite in some Coorong lakes is abasal dolomite (a type B dolomite) which is more crystalline than the other two forms of dolomite. It formed some 6000 years ago when the rising Pleistocene water table (driven by a transgressing sea) first caused continental groundwaters to emerge and evaporate at the surface. The Coorong Dolomites form by the evaporation of continental meteoric waters, and this controls the occurrence of dolomite versus other lacustrine carbonates.