Marion Lake complex, South Australia

Gypsum-filled coastal lakes in southern Australia are located mostly to the west of the Coorong coast and are best documented in the region of Marion Lake on the southwestern tip of Yorke Peninsula. Individual gypsum-filled salinas measure up to 20 x 12 km (Lake Macdonnell on the coast of the Great Australian Bight), with centres filled by laminated gypsum units up to 10m thick. Most of these gypsum salinas occur within a Mediterranean-style Csb Köppen climate regime; although the largest of the exploited gypsum salinas, Lake Macdonnell, lies in a BSk climate.

Groundwater feeds to these salinas are dominated by seawater, so the current hydrological equilibrium surfaces (Stokes surface) in all the gypsum salinas are still up to 50 cm below sea level. In plan view, surface sediments in a gypsum salina define bull’s-eyes, consisting of a carbonate seepage rim and a more central gypsum zone.

A carbonate unit forms a seepage rim about the salina edge and is composed of a boxwork limestone unit (up to 4 to 5 m thick) overlain by a fenestral limestone sheet or crust, which is typically less than 1 m thick. Boxwork limestone is a diagenetic unit created by the cannibalisation of earlier marine-lacustrine carbonates and sulphates by inflowing “fresher,” but still saline, marine-derived groundwaters. The “boxwork” contains relict algal structures, evaporite pseudomorphs and other profoundly altered bedded fabrics in proportions depending on pre-existing sediment types. Some areas of the limestone sheet above the boxwork limestone consist of tepee-overprinted crusts, others of mm-laminated subaqueous stromatolites and algal mats.

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Gypsum salinas. A) Seepage fed coastal salinas of the Marion Lake complex at the southwest tip of Yorke Peninsula, South Australia. B) Surface geology and cross section of Marion Lake are after Warren, 1982a,b. (Landsat image courtesy of NASA).

Gypsum-filled coastal lakes in southern Australia are located mostly to the west of the Coorong coast and are best documented in the region of Marion Lake on the southwestern tip of Yorke Peninsula. Individual gypsum-filled salinas measure up to 20 x 12 km (Lake Macdonnell on the coast of the Great Australian Bight), with centres filled by laminated gypsum units up to 10m thick. Most of these gypsum salinas occur within a Mediterranean-style Csb Köppen climate regime; although the largest of the exploited gypsum salinas, Lake Macdonnell, lies in a BSk climate.

Groundwater feeds to these salinas are dominated by seawater, so the current hydrological equilibrium surfaces (Stokes surface) in all the gypsum salinas are still up to 50 cm below sea level. In plan view, surface sediments in a gypsum salina define bull’s-eyes, consisting of a carbonate seepage rim and a more central gypsum zone.

A carbonate unit forms a seepage rim about the salina edge and is composed of a boxwork limestone unit (up to 4 to 5 m thick) overlain by a fenestral limestone sheet or crust, which is typically less than 1 m thick. Boxwork limestone is a diagenetic unit created by the cannibalisation of earlier marine-lacustrine carbonates and sulphates by inflowing “fresher,” but still saline, marine-derived groundwaters. The “boxwork” contains relict algal structures, evaporite pseudomorphs and other profoundly altered bedded fabrics in proportions depending on pre-existing sediment types. Some areas of the limestone sheet above the boxwork limestone consist of tepee-overprinted crusts, others of mm-laminated subaqueous stromatolites and algal mats.

Taller stromatolites in Marion Lake grew as current-aligned mm-laminated elongate domes (up to 40 cm relief) in a brine-filled feeder channel, which focused the flow of marine seepage waters into the perennial gypsum lake of the salina centre. Other smaller domal and encrusting stromatolites define the water-saturated cyanobacterial terrace of the lake strandzone, where indurated biolaminites represent areas of very shallow and sometimes ephemeral surface waters. Stromatolite domes cap tepee structures in some marine-fed seepage areas.

Like stromatolites, tepee structures are confined to the capping carbonate unit and form an extensive fenestral limestone sheet or capstone, up to 60 cm thick. Tepees form readily in areas of resurging groundwater, where seasonal changes in the groundwater head encourage tepee growth (Warren, 1982b). Most of the limestone sheets caught up in the tepee structures are capillary crusts composed of fenestral lime mudstones containing stromatactic-like voids, pisolites, laminar cements, and geopetal internal sediment partially filling pores and vugs.

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Sedimentary structures of the carbonate (aragonite) fringe in salinas in the southern Yorke Peninsula. A) Slabbed section showing a mm-laminated Holocene stromatolite coating a coral head. in Marion Lake. B) Tepee structure (50 cm relief) growing on the edge of Deep Lake. C) Slabbed section showing fenestrate geopetal structure as seen in the aragonite crust caught up in the tepees of Deep Lake.

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Gypsum facies in South Australian salinas, after Warren 1982b, showing domes overlain by laminites and the eolian reworking of upper portions of the lake sediments into gypsite-capped lunettes. For simplicity, the carbonate rim is not shown.

Salina gypsum in the central portions of Marion Lake is coarse-grained, porous, bottom nucleated and growth-aligned; it retains intercrystalline porosity of 15-25%; pore infilling gypsum cement is rare. South Australian salinas are one of a few Holocene occurrences where subaqueous gypsum is deposited as widespread units of coarsely crystalline bottom-aligned swallowtail and palmate crystals. Worldwide, gypsum in most modern salinas is a layered gypsarenite. The cm-scale gypsum crystals are usually more than 90% pure and laid down as characteristic shallowing-upward growth-aligned crystal aggregates.

At the base of an idealised succession are massive, poorly layered domes of coarsely crystalline palmate and swallowtail crystals (aka selenite). Modern growing examples of this style can be found regrowing on the floors of flooded mined-out salinas such as Lake Inneston. The elongate gypsum prisms in the early stages of dome growth show little or no preferred orientation, with carbonate pellets and flakes distributed randomly through the gypsum porosity. Active domes on the floor of Lake Inneston are coated by microbial slime and can be considered to be a type of coarsely-layered gypsum stromatolite. 

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Gypsum in salinas of southern Yorke Peninsula. A) Gypsum domes regrowing on the mined-out floor of Lake Inneston. B) Close up of Inneston gypsum dome. C ) Layered and aligned gypsum prisms in a dome. D) mm-laminated gypsum aggrading through aragonite lamine via subcrystral growth.

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Mechanically reworked gypsum, southern Yorke Peninsula. A) Wave-rippled subaqueous gypsum - Marion Lake. B) Cross-bedded aeolian gypsum dune with a white pedogenic (gypsite) cap - Lake Fowler lunette.

Gypsum is not only deposited as coarse-grained gypsum (selenite) in South Australian coastal lakes. Some salinas are filled with laminated gypsarenites and are probably as common, if not more common, than those filled with the geologically more interesting coarsely-crystalline gypsum. 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.

Parts of the horizontally laminated gypsarenite unit, especially near the salina strandzone, can be reworked into wave-oscillation ripples. Laminated and rippled gypsarenite is in turn overlain by a thin, massive, poorly-bedded gypsarenite unit deposited under seasonally vadose or subaerial conditions and represents accumulation in the salina capillary fringe.

Topping the whole succession is a unit of cross-stratified eolian gypsum and, in areas stabilised by vegetation, a pedogenic cap of gypsite (silt-sized). Throughout coastal and inland Australia, this gypsum soil is a degradational profile that is slowly cannibalising depositionally inactive regions of both lacustrine and eolian gypsum.

Carbonate laminae in the various gypsum units form by the precipitation and accumulation of aragonite pellets during the spring and early summer. The pellets come from the faeces of ostracods and brine shrimp, mixed with the micritised remnants of algal tubules. At first, the pellets mantle any underlying gypsum and often are captured by a cyanobacterial mat that covers the gypsum. These aragonite laminae can be caught up in a growing gypsum pavement in two ways.

If a stratified brine column forms above a growing crystallisation surface, solar-driven crystal growth of gypsum or halite in the bottom waters first slows then stops. Once a water body is stratified, no solar mechanism is available to concentrate the bottom waters further. Subaqueous crystallisation can only continue in a stratified system in zones of brine mixing, or zones of bottom temperature change near subaqueous spring seeps. Once the freshened inflow ceased, ongoing evaporation concentrated the upper brine body until its density matches that of the lower brine body. The two water bodies then mix (holomixis) and crystal growth on the bottom can begin anew.

This type of holomictic bottom crystal precipitation, where the crystallisation surface is never in contact with undersaturated bottom water, deposits layered euhedral growth-aligned crystal beds on the brine floor (e.g. chevron halite or aligned swallowtail gypsum.

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Growth-aligned gypsum shows internal textures are controlled by the gypsum growth surface interacting with the stability and salinity of bottom brines. In the early stage of fill, when the brine lake is deeper, the bottom brines are stable, and salinities do not ever decrease to where they become undersaturated with respect to gypsum. “Zig-zag” textures result. When the sediment surface aggrades to where bottom brines freshen, then the periodic dissolution of the upper part of the growth surface dissolves, and mm-laminated textures form. The present stage of sediment fill is where the brine lake has been filled to equilibrium with the local watertable, the eolian reworking of seasonal gypsarenite precipitates is the dominant mode of deposition (after Warren, 1982b).

If, on the other hand, the freshened upper water body does come into contact with the crystallisation surface of the salt bed, then subaqueous horizontal planation surfaces develop. Subhorizontal planation occurs both in saline basins subject to complete desiccation and in those parts of evaporite seaways covered by perennial brine sheets (tens of cm deep) that were subject to periodic freshening. A desiccation stage, followed by a freshened waterflood, creates a near-horizontal planation surface with intercrystalline karst, which marks a time when the watertable was lowered into the salt bed immediately before a subsequent freshening event.

In contrast, freshening of perennial bottom brine creates a horizontal planation surface atop the salt bed with little or no intercrystalline karsting (laminated coarse-grained gypsum in Marion Lake. In both cases the freshened surface water sheet only dissolves the uppermost portion of the salt bed, it does not displace the much denser pore brines that saturated the bulk of the underlying salt bed. A planation surface defines the top of saturated pore brines. Vadose karst in a desiccated salt pan can only extend down to the top of this dense pore brine layer. Below it, the pore brines are saturated, above it, the waters are undersaturated or nonexistent. These flat, laterally extensive dissolution surfaces typically truncate the tops of previously growing crystals, be they trona, halite, gypsum or any other bottom nucleating salt.

As the freshened water body begins to concentrate it may precipitate a less saline mineral phase, which settles onto the planation surface. It may also be the time when clays, formerly suspended in the floodwaters or blown in by the wind, begin to flocculate and sink through increasingly saline surface waters. As the freshened water body continues to concentrate into salinities that precipitate the dominant salt phase the aggrading crystals that underlie the planation surface can now poikilitically enclose the less-saline precipitates (e.g. laminae of aragonite pellets in laminated coarse-grained gypsum).

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Lake Inneston (reflooded, once-mined, salina with new gypsum domes precipitating on the subaqueous floor)

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