Dolomite Occurrence in Coorong Region, South Australia

Michael Rosen, John K. Warren

Lakes in the Coorong region are filled with a diverse suite of Holocene carbonates; mineralogies include aragonite, calcite, magnesium-calcite, magnesite, hydromagnesite, and dolomite. Dolomite is the main mineral of concern in this paper, yet it makes up no more than 5% of the carbonate minerals forming surficial deposits in the coastal plain of southeast South Australia. Coorong dolomite occurs as three stratigraphically and mineralogically distinct forms. Volumetrically, the most important type of dolomite is an evaporative dolomite laid down as the last episode of sedimentation in those Coorong Lakes that contain dolomite. In most lakes, dolomite is a capstone unit no more than a meter thick, although in a few lakes it has infilled the lakes to form dolomitic units up to 4-5 m thick. Evaporative dolomite is usually magnesian-rich. In some lakes, a calcian-rich dolomite occurs along the edges of the lake. Like the evaporative upper dolomite, this dolomite is not intergrown with other carbonate phases and appears to define areas where continental ground waters first enter the lake. A third type of dolomite occurs in some Coorong lakes. A basal dolomite, which is more crystalline than the other two forms of dolomite, appears to have formed some 6,000 years ago when the rising Pleistocene water table (driven by a transgressing sea) first caused continental ground waters to outcrop and evaporate at the surface.

Textures found in vertical sequences from Coorong lakes are largely independent of mineralogy. In lakes with an early Holocene connection to the marine waters of the lagoon, the basal unit is a marine/estuarine skeletal grainstone/packstone. If no marine connection exists, the basal unit is often a quartzose packstone to wackestone. Basal facies can contain small percentages of basal dolomite. Above the basal facies is a massive to faintly laminated organic-rich unit. In lakes with an early marine connection, the levels of TOC in this unit can be as high as 12%, usually as an oil-prone proto-kerogen. Overlying this unit is a millimeter-laminated unit of pelletal packstone to mudstone deposited once the estuarine connection to the open Coorong Lagoon was completely cut off by beach-rid e accretion. The proportion of grains in the laminated unit is a direct reflection of energy level and organic binding when the lake sediments were deposited. In areas of higher wave-energy/bottom currents and little or no algal binding, the sediment is a packstone, more common around the lake edge. In areas of lower wave-energy/bottom currents in relatively deeper water and algal binding, the sediment is a wackestone, or at times a mudstone. These muddier sediments are more common in the central parts of the lakes. In a few lakes, such as Milne Lake, this laminated unit, composed of evaporative dolomite up to 5 m thick, often is composed of varying proportions of aragonite, hydromagnesite, magnesian calcite, and rarely composed of laminated gypsum/aragonite couplets. Capping the lake se iments is a "massive" unit of poorly layered packstone/mudstone, usually less than 60 cm thick. 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 cap contains bioturbation structures, mud cracks, extrusion tepees, and breccia fragments, all features indicative of at least occasional desiccation.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.