Formation of the Lake
Lake Eyre began forming about 200 million years ago when land between the Gulf of Carpentaria and South Australian salt lakes subsided, creating a basin for sediments. Around 100 million years ago the area was flooded by the sea, which later receded, leaving major rivers flowing south. About 1 million years ago uplift and faulting blocked these rivers, forming a vast inland lake known as Lake Dieri, although its full extent remains debated due to limited geological evidence.
Throughout the Quaternary Period, climatic changes caused Lake Eyre to alternate between a large permanent lake up to 25 m deep and periods of complete drying. During dry phases, strong winds eroded sediments, shaping the present-day playa, while floods later replenished sediments. Studies of Madigan Bay sediments show these fluctuations over the past 130,000 years.
Around 35,000 years ago, ancestral Lake Eyre was three times its current size, at least 17 m deep, and surrounded by lush vegetation. Increasing aridity after 20,000 years ago reduced river inflow, shrinking the lake to salt lakes. Much of the salt came from ancient marine sediments beneath the catchment. At several times over the past 600,000 years, the lake reached long-term high stands, leaving beach ridges, though modern observations suggest such features can form rapidly.
At its peak about 125,000 years ago, Lake Eyre was a permanent megalake of around 35,000 km⊃2;, holding far more water than in historical times. Despite high evaporation exceeding rainfall and inflow, permanent mud remains beneath the salt crust, likely sustained by saline groundwater drawn up from underlying marine sediments.
Lake Eyre filled during past geological periods as a result of major climatic and hydrological changes, particularly increased rainfall in northern and eastern Australia. During these times, the climate was significantly wetter than it is today, with a stronger and more sustained monsoonal system operating over northern Australia. This led to prolonged and reliable rainfall over the large river catchments that drain southwards into the Lake Eyre Basin, including the Diamantina–Georgina river system and Cooper Creek.
As a result of this increased precipitation, these river systems flowed continuously over long periods, rather than episodically as they do under the present arid climate. The sustained river inflow delivered vast quantities of freshwater into the Lake Eyre Basin, allowing the lake to expand and deepen. Geological evidence, such as lake sediments and ancient shoreline features, indicates that during these wet phases Lake Eyre became a large, permanent lake, reaching depths of up to 17–25 metres and covering areas many times larger than today.
In addition to climatic factors, tectonic processes played a critical role in allowing Lake Eyre to fill. Uplift and faulting to the south of the basin blocked former drainage pathways to the Southern Ocean. This prevented river water from flowing out of the basin, effectively trapping water inland. With high inflow from the north and east and limited outflow, water accumulated in the basin rather than being lost to the sea.
Together, the combination of long-term wet climatic conditions, increased monsoonal rainfall, sustained river inflow from the north and east, and restricted drainage due to tectonic uplift caused Lake Eyre to fill repeatedly during the Quaternary Period. These conditions contrast strongly with the modern climate, where irregular rainfall and high evaporation rates prevent the lake from becoming permanently full.
These are the stages believed to have occurred in the formation of Lake Eyre:
Early stage 5 lacustral phase, about 130,000-90,000 BP:
It was a mostly saline, permanent lake throughout this period. It dried at Williams Point. There may have been a brief period of deflation.
Later stage 5 lacustral phase, 90,000-70,000 BP:
It refilled, alternating between ephemeral and more permanent saline conditions. There were periodic brackish to fresh interludes. Before 70,000 BP soil formation occurred as it dried out, when the conditions were approximately as they are today. It refilled by 70,000 BP, probably fluctuating till about 65,000 BP.
Dune-building and probable deflation phase, 60,000-50,000 BP:
This phase is characterised by Aeolian sediments derived initially from beaches, and later gypsum and pelleted clay, derived from playa deflation.
Possible stage 3 lacustral phase, 50,000-25,000 BP:
Laminated sediments in some drill cores suggest a shallow saline lake phase. This stage would correspond to a full stage at the Willandra Lakes, and Lake Frome, which is less certain.
Playa phase - 25,000-10,000 BP:
Prior to the Holocene, groundwater controlled deflation removed earlier lacustrine sediments, excavating the basin to 17.4 m below sea level. This produced the present morphology. Sedimentation in an ephemerally flooded playa environment occurred, once the water table became stable and lowering ceased. During dry stages the soils formed and sediments oxidised. During the arid glacial maximum a thick salt crust formed.
Early Holocene shallow lacustrine phase, 10,000-4,000 BP:
The lake bed had filled by at least 10,000 years ago, and was probably semi-permanent. Part of the halite crust was dissolved, the remainder was sealed by a gypseous clay layer preventing further dissolution.
Modern ephemeral playa lake, 3000-present:
The part of the halite layer that was not dissolved in the previous phase is the modern salt crust, that is relatively thin. The entire surface of the salt crust is dissolved during major ephemeral floods, then as the lake subsequently dries, salt is precipitated as the water evaporates, and migrates up through sediment deposited on it by the flood. The present interglacial seems to be drier than the previous interglacial. During this previous interglacial there was a semi-permanent lake between 130,000 and 70,000 BP. It appears the monsoonal influence was much stronger than at the present in the northern catchments.
Please answer the following questions:
1) The name of this cache.
2) The user names of other cachers you send the answers for. (If you do.)
3) How long ago did the lake have a depth of seventeen meters?
4) What conditions have to be met in the different cardinal direction to cause the lake to fill? Focus on at least two or a general answer summarising the whole area.
5) Do you think this has occurred recently?
6) Why do you think this?
According to recent changes in local laws you are not allowed to move all the way to the Lake anymore. Please only walk to the fence line and, from there, look towards the lake.
7a) IF there IS water:
-Estimate the distance from the given coordinates to the shoreline.
-What do you think, is the lake filling or emptying at the moment?
-Why do you think so?
7b) IF there IS NO water:
-Does it look like there actually is water in the distance?
-What do you think makes the dry lake look like this?
-And how far do you think you'd have to walk to reach it?
Please send the answers via email or preferred message center in English or German. Please send them before or shortly after you log your find and do not post them in your log. If there are any problems, feel free to contact me.