Thank you to juc_cacher for alerting us to the condition of the road, in wet weather it has become very slippery and boggy, possibly a problem for those not in a 4 x 4 or those towing, Please use caution on the road and park in a safe place back up at the intersection if needed.
WELCOME TO LAKE TYRRELL
Lake Tyrrell is a large ephemeral salt lake, Victoria’s Largest salt lake covering 20860 hectares.
Palaeomagnetic investigations by previous studies have demonstrated that Lake Tyrrell represents a remnant of a Pleistocene mega-lake, Lake Bungunnia. This mega-lake is known to have been relatively fresh, and as such is indicative of generally wetter climatic conditions in southeastern Australia during its existence. The drying of Lake Bungunnia commenced between 0.7 Ma and 1.2 Ma, and signaled the onset of aridity in southeastern Australia. Indications from this and other sites point to multiple cycles of wetting and drying, correlated to global glacial-interglacial cycles; as such, this period represents the greatest environmental transformation of the last 20 million years.
In modern times, Lake Tyrrell is a terminal lake of the Avoca River/Tyrrell creek system. Tyrrell creek flows every three to five years but often does not reach the lake and is not the only source of water for the lake. The Lake’s level is controlled by climate and groundwater. Up to a metre of water fills the basin during the wetter and cooler winter season, but evaporates during the summer, precipitating up to 10 cm of halite. Each year the same pool of ions is redissolved by this annual freshening.
The small percentage of gypsum precipated < 2% in the surface salt crust reflects the low calcium content of the brine which, in turn, is a function of the negligible net discharge of calcium from the groundwater system. The small influx of fine grained clastic sediment to the lake floor comes from surface runoff, wind, and reworking of older sediment from the shoreline.
The Lake Tyrrell basin lies in a setting in which three different groundwater types, identified by distinct salinities, interact with surface waters. A refluxing cycle that goes from discharging groundwater at the basin margin, to surface evaporation on the lake floor, to recharge through the floor of the lake, controls the major chemical characteristics of the basin. In this process, salts are leached downward from the lake floor to join a brine pool below the lake. This provides an outlet from the lake, especially under conditions that have been both drier and wetter than those of today. Enhanced discharge occurs under drier conditions, when the enclosing regional groundwater divide is lowered, whereas a rise in lake level increases the hydraulic head over that of the sub surface brine and promotes an increase in brine loss from the lake.
Sulphate reducing bacteria in a zone of black sulphide rich mud beneath the salt crust help prevent gypsum from being incorporated into the recent sedimentary record. However, below the upper 5 to 10 cm zone of bacterial activity, discoidal gypsum is being precipitated within the mud from the groundwater. These crystals have grown by displacing the mud and typically “float” in a clay matrix; in some zones, they form concentrations exceeding 50% of the sediment. The occasional laminae of more prismatic gypsum that occur within the upper metre of mud have crystallised from surface brines. The scarcity of these comparatively pure prismatic‐crystal concentrations probably is a function of unfavourable chemical conditions in the lake brine and of the role that sulphate‐reducing bacteria have played.
Salt is commercially extracted by Cheetham Salt Works at the northern end of the lake. See reference point.
To log this Earthcache we require you to read the cache notes provided and to explore the area listed on WP2, then, to the best of your ability message us with the following answers;
1. What is below Lake Tyrrell?
2. What causes the lake to be sometimes Pink?
3. Does the presence of gypsum promote clear or cloudy water?
4. At WP2 (use trailhead listed, don't block the walkway with your car) Walk down to the lake and find some evaporite, describe its colour and texture, this will vary with the seasons.
5. A photo of your Team, GPS near GZ with your log and answers. (optional)
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers within a couple of days. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted.
Acknowledgments; http://www.cheethamsalt.com.au, http://laketyrrell.com.
