Coorong Lakes, South Australia

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Younghusband Peninsula is the Holocene dune barrier that forms the Coorong Lagoon and various Holocene lake sumps in its hinterland.

The modern coastal dune ridge (Younghushand Peninsula) and the adjacent Coorong Lagoon are ] the latest of a number of offlapping highstand beach-dune ridges and lagoons that have typified the slowly uplifting southeastern coastal zone of South Australia since the Pliocene. Dolomite and other lacustrine carbonates begin to form in the interdunal sumps, once the lagoons lose a hydrographic connection to the ocean. The name Coorong comes from the Aboriginal word Kurangk, which in the local language of the Ngarrindjeri culture means long or narrow neck of water. It may alos be adapted from the Aboriginal word Coorang, meaning sand dune.

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The Coorong Region was first documented as an area of modern dolomite precipitation by Mawson (1929), but was not studied in any detail until the work of Alderman and Skinner (1957), von der Borch (1965, 1976), von der Borch and Lock (1979), Rosen et al. (1988, 1990), Rosen et al. (1989) and Warren (1988, 1990), Wacey et al. (2007). Most early work in the region concentrated on salinas that contained dolomite, but there are many salinas, probably the majority, where the fill is dominated by carbonate mineral phases other than dolomite (Warren, 1990). Fills include low and high Mg-calcite, magnesite, hydromagnesite, aragonite, and less commonly gypsum). Von der Borch (1976) observed that Holocene dolomite occurs in areas of the coastal plain where rainfall is less than 700 mm, encompassing the area between Kingston and Salt Creek. He suggested that south of the 700 mm isohyet, reduced evaporation rates and higher rainfall prevented the concentration of lake waters to salinities where dolomite could precipitate. Regional sampling of lakes across the area tied to climate confirms his postulate and shows that the more magnesian-rich dolomites tend to occur in the more arid northwestern portion of the Coorong coastal plain, centred on lakes in the vicinity of Salt Creek, while Mg-calcite and calcite-filled salinas are increasingly common in the cooler moister parts to the southeast (Warren, 1988, 1990). More recently, papers detailing the organic constituents of selected Coorong lakes, and their geological significance, have been published (Krull et al., 2009; McKirdy et al., 2010).

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Coorong Lakes, South Australia. A) Locality and map of the Coorong region. Geological plan of the Salt Creek region shows how three of the four major lakes are joined by an interdunal corridor and were connected to the Coorong lagoon earlier in the Holocene. Milne Lake, the best dolomite accumulator in the Coorong coastal plain, never had a surface connection with the Coorong Lagoon. Mineralogies: Lake 1 = Milne Lake (dolomite + magnesite), Lake 2 = Halite Lake (gypsum + aragonite overlain by hydromagnesite + aragonite in massive unit and currently covered by ephemeral halite crust), Lake 3 = Pellet Lake (dolomite + hydromagnesite + aragonite), Lake 4 = North Stromatolite Lake (hydromagnesite + aragonite; minor dolomite in basal unit and about lake edge) (after Warren, 1990). B) Typical vertical sequence in Holocene evaporitic lakes of the Coorong coastal plain.

Textures in Coorong salinas and pans are mostly independent of mineralogy and can be related to salinity, brine depth and permanence. If the salina possessed an early Holocene connection to the marine waters of the Coorong Lagoon, then the lowermost Holocene unit is a marine/estuarine skeletal grainstone/packstone, similar to Holocene units flooring the more saline salinas of the Marion lake Complex and Lake Macleod. If there was no marine connection to the Coorong lagoon, then the basal unit is a quartzose packstone to wackestone. These lowermost sediments can contain small percentages of diagenetic calcian dolomite (Rosen et al., 1989). Above the lowermost unit is a variably-developed massive to faintly laminated organic-rich unit. In lakes with an early marine connection, the levels of total organic carbon (TOC) in this unit can be as high as 12%, usually as an oil-prone proto-kerogen. Its formation indicates a seasonally stratified water column where fresh oxygenated water seasonally overlays stagnant marine water, during the transition from a surface connected marine estuary to a totally isolated groundwater-fed salina.

Above this is a mm-laminated unit of pelletal packstone to mudstone, deposited on the floors of perennial, but schizohaline, density-stratified brine lakes, which formed in the Salt Creek region once the estuarine connection to the open Coorong Lagoon was cut off by beach-ridge accretion at the entrance to the chain of salinas around 6400 years ago. The proportion of faecal pellets in the laminated unit is a direct reflection of energy level and organic binding on the subaqueous salina floor. In areas of higher wave energy/bottom currents and little or no algal binding, the sediment is a packstone. Such sediments are more common about the salina edge and in the shallower parts of the more central sediment fill. In areas of lower wave energy in relatively deeper water, or areas of algal binding, the sediment is a wackestone or at times a mudstone. Muddier sediments are more common in the central parts of a salina. In a few coastal salinas with no early Holocene marine connection, such as Milne Lake, the laminated unit is dominated by magnesian dolomite in a unit up to 5 metres thick. More typically the laminated unit in the Coorong ephemeral lake fill is composed of varying proportions of aragonite, hydromagnesite, magnesian calcite and low magnesian calcite. In one marine-seepage lake (Halite Lake), it is made up of laminated gypsum/aragonite couplets.

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North Stromatolite Lake, Coorong area, South Australia . A. Domal stromatolites. B. Extrusion tepees in a slightly siliceous crust (after Warren 1988, 1990). Scale bar in both images is 15 cm long.

Capping the lake sediments is a “massive” unit of poorly layered packstone/mudstone. It is usually less than 60-80 cm thick and shows varying degrees of induration from brecciated crusts about the salina edges to domal leathery stromatolites in some of the more central water-covered areas. Domal stromatolites seem to flourish in Coorong salinas filling with a mixture of hydromagnesite and aragonite. This, the uppermost part of the sediment fill, usually holds the bulk of the evaporative dolomite in the majority of the Coorong carbonate salinas containing dolomite, especially in the vicinity of Salt Creek. But the Massive Unit can also be composed of aragonite, hydromagnesite, magnesian calcite and magnesite. This uppermost unit in the Coorong lake fill stratigraphy contains numerous terrestrial burrows and root traces, as well as mud cracks, extrusion tepees and breccia fragments - all features indicative of at least occasional desiccation and thorough bioturbation.

Whatever the mineral assemblage, the dominant textural feature in the sediment column of a carbonate salina is lamination passing up section into a more massive unit with evidence of seasonal subaerial exposure of the sedimentation surfaces. Tepee-overprinted carbonate crusts are especially prevalent in the desiccated strandzones of the lake margins, while domal stromatolites and cyanobacteria are common in areas of more permanent brine cover or saturation, especially in salinas precipitating hydromagnesite and aragonite. Brackish lakes, such as Lake Fellmongery, in the southeastern part of the Coorong coastal plain, have well-developed algal tufa fringes, as described earlier, with some tufas first precipitated as monohydrocalcite and quickly altering to calcite. Surfaces of all the carbonate-filled salinas in the Coorong coastal zone are half a metre or more above sea level indicating the current nonmarine supply of waters (as groundwater cannot flow uphill). Some Coorong lakes had a marine seepage connection in their earlier stages of infill (e.g. Halite Lake, with a laminated unit composed of gypsum and aragonite).

Organic content and biomarker stratigraphy in North Stromatolite Lake defines five discrete organic facies, each with a distinctive mineralogy (McKirdy et al., 2010). The organic-rich unit (6−12% TOC) may be subdivided into facies 1 (Type I/II kerogen) and 2 (Type II kerogen), whereas the organically-leaner laminated and massive units are distinctly bimodal with respect to organic content, hosting both facies 3 (Type II/III kerogen) and 4 (Type III kerogen). The latter two facies together define an inverse relationship between hydrogen index and TOC content, which is a geochemical signature attributed to differences in the extent of pelletisation of carbonate muds by a diverse halotolerant fauna including brine shrimp, gastropods and ostracods during the shallowing perennial and ephemeral phases of the lake’s history. One sample from the basal unit is representative of organic facies 5 (Type IV kerogen).

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Plot of homohopane/cholest-2-ene ratio versus depth in a core from North Stromatolite Lake recording the up-section addition of faecal cholesterol to the mud by benthic microfauna that grazed on algae and cyanobacteria. The right-hand panel highlights a parallel increase in cholest-2-ene as a proportion of the total eukaryotic input of C27−C29 Δ2-sterenes.

Aliphatic hydrocarbon distributions in these lacustrine sediments are dominated by C20 and C25 highly branched isoprenoids; and C12−C33 n-alkanes displaying marked odd/even predominance above, and even/odd predominance below, C20. This biomarker assemblage reflects the respective major contributions of Bacillariophyceae (diatoms), Chlorophyceae (green algae) and eubacteria (including cyanobacteria) to their preserved organic matter. Its passage through the guts of the aforementioned grazers and excretion as faecal pellets has dramatically enhanced the relative abundance of cholest-2-ene, thereby imparting to the pelletised upper sapropel, laminated and massive units a molecular signature indicative of mud ingestion and excretion.

Biomarker work across the sapropel underlines significant distinctions, not generally recognised in sedimentological studies, namely (McKirdy et al., 2010; Edwards et al., 2006): 1) Absence or presence of lamination in a fine-grained lacustrine succession does not necessarily mean that the sediment has or has not been ingested; however the absence or presence cholest-2-ene does. 2) If pellets are deposited as soft aggregations of micrite, then the pelleted texture will be quickly lost during the physical compaction associated with early burial and diagenesis, yet the organic signature of ingestion may remain. 3) In lacustrine associations, high levels of hydrogen-rich Type-1 protokerogen may have accumulated beneath relatively-shallow bottom waters (<5m deep) that, although meromictic, were not necessarily perennially anoxic. We shall return to the discussion of organics in the Coorong lakes in Chapter 9.

Volumetrically, most salina carbonates in the various Coorong Lakes formed by the evaporation of unconfined magnesium-rich continental groundwaters, driven to the surface along the coastal zone as they float up over a more dense seawater wedge. That most dolomite forms where large volumes of resurging continental waters can evaporate, explains why the thickest Coorong dolomites form adjacent to the present coast. However, mineralogically similar dolomites are also precipitating 20-40 km further inland as thin capping units in entirely continental ephemeral lakes. These areas are well away from the seawater wedge, but still in areas where the regional watertable intersects the land surface, and continental groundwaters can pond and evaporate. 

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Isotopic signatures of Type A, Type B dolomite and sapropel carbonates across various Coorong Lakes

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Lattice dimensions of Type A and B dolomites

Dolomites in salinas in the Salt Creek region precipitate as two geochemically and isotopically distinct types (Type-A and -B), typically in association with other carbonate minerals. Type-A dolomite has a slightly heavier oxygen isotope signature than type-B, and is 3 - 6‰ heavier in 13C. Type-A dolomite also has distinct unit cell dimensions. It tends to be magnesium-rich with up to 3-mole percent excess MgCO3, while type-B is near stoichiometric or calcian-rich. Type-A dolomite typically occurs in association with magnesite and hydromagnesite, Type B with Mg-calcite. Transmission electron microscopy (TEM) shows that Type A dolomites have a heterogeneous microstructure due to closely spaced random defects, while type B dolomites exhibit a more homogeneous microstructure implying excess calcium ions are more evenly distributed throughout the lattice. TEM studies show that the two types of Coorong dolomite are distinct and are not intermixed with other mineral phases; they are primary precipitates, and not replacements and are not transitional (Miser et al., 1987).

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Sapropel carbonates are typically mixtures of aragonite and Mg-Calcite, indicative of the somewhat lower ambient salinities when these sediments precipitated (McKirdy et al., 2010). As such, sapropel carbonates tend to occupy a distinct isotope plot field compared to type A and B dolomite. Their somewhat more negative carbon isotope values may indicate a more significant organic contribution to the carbonate precipitate in the sapropel. The separation of isotope fields for dolomite A from dolomite B is also mainly due to differences in carbon values, with dolomite B tending to have more negative values. Rosen et al. (1989) attributed this to differences in water chemistry (more evaporitic versus more meteoric-influenced feeder brine). In later papers, Wright and Wacey (1999) Wacey et al. (2007)argued dolomite formation in the Coorong salinas is bacterially-mediated based on a combination of SEM and sulphur isotope observations. Laminated gypsarenite fills large interdunal corridors near Streaky Bay and Point Fowler in South Australia and Hutt and Leeman Lagoons in Western Australia. In salinas with coarse-grained fills, the coarsely-crystalline laminated gypsum (selenite) unit, punctuated by carbonate laminae, is in turn overlain by a mm-laminated, sand-sized gypsarenite accumulation

Bacteria can play a role in shaping all isotope signatures. However, neither type A and B dolomites show the negative carbon values typically associated with the bacterial sulphate reduction (BSR) and Coorong dolomite plotfields lie outside the documented sulphate reduction or methanogenic dolomite fields). Wacey et al. (2007) argue this is because bacterially-mediated precipitates in the Coorong lakes incorporate carbon primarily from the inorganic, lakewater reservoir, partially diluted by an organic component. However, it is essential that any proposed precipitation process does not oversimplify or overgeneralise a complex set of precipitation mechanisms across a range of evolving carbonate mineralogies. The figure clearly shows dolomite mineral phases occupy separate isotope plot fields and so likely involve a variety of feeder chemistries. The research on the relative importance of bacteria versus concentration on the formation of Coorong dolomite continues.

Dolomites in Coorong salinas are evaporitic carbonates, once thought to only form in areas free of preserved evaporites (von der Borch and Lock, 1979). Yet, Milne Lake, a salina filled with 4 metres of laminated dolomite-magnesite, lies less than a kilometre from Halite Lake, a depression filled with 2-3 metres of laminated gypsum-aragonite (Warren, 1990). Milne Lake was fed by seaward-flowing continental groundwaters throughout its infill history. Halite Lake, during the gypsum stage of its fill, was fed by landward-flowing marine groundwaters (evaporative drawdown stage). Today Halite Lake is filled to its hydrological equilibrium level; its present depositional surface lies above the level supplied by marine seepage, and it no longer accumulates laminated gypsum-aragonite. Above its laminated gypsum-aragonite unit is a 10-50 cm thick capstone bed of pelleted aragonite-hydromagnesite, a mineral association that indicates a nonmarine groundwater feed dominates the current surface hydrology, but with a distinct isotope plot field. Other Coorong Lakes in the same climatic zone, but located further inland, and more isolated from resurging marine groundwaters, are entirely filled with this aragonite-hydromagnesite association (Figure; North Stromatolite Lake and South Stromatolite Lake). Historically, before it became part of the Coorong National park, the seasonal halite crust that formed each year in Halite lake, was mined in late summer. 

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Causeway across Halite Lake, Coorong region

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