Formation of the Lake
The processes that led to the formation of Lake Eyre began about 200 million years ago when a large band of land between the Gulf of Carpentaria and the area of the South Australian salt lakes began to sink, becoming a depocentre for river and lake sediments. This subsidence has continued to the present. Then about 100 million years ago the whole area was inundated by the sea. 20-30 million years later the sea receded leaving only several large rivers that are thought to have flowed to the southern coast. About 1 million years ago the land to the south of the salt lakes was tilted up, the faulting blocking the flow of the rivers to the coast. The result was a huge lake, Lake Dieri, named after one of the Aboriginal tribes living east of Lake Eyre. The existence of Lake Dieri that was up to 10 times the size of the present lake has been disputed on the basis of the lack of geological evidence.
Major climatic oscillations connected with the glacial-interglacial phases through the Quaternary Period have resulted in the size of Lake Eyre varying between a perennial lake much larger than the present playa, with a depth of 25m, to completely drying up, when it underwent extensive deflation of its sediments, as the winds of the glacial peaks scoured the surface, carrying away the sediment. Groundwater controlled this deflation as evaporation lowered the water table, producing the morphology of the present day playa. It is now in a relatively stable ephemeral lake stage, the deflation of the dry playa being roughly balanced by the inflow of sediments brought by the floods in its catchment.
Study of the sediments of Madigan Bay, the largest of the Lake Eyre North bays, has demonstrated the lake's changes during the last full glacial cycle of 130,000 years.
35,000 years ago Lake Dieri (or the ancestral Lake Eyre) was 3 times the size of the present Lake Eyre and had a depth of at least 17m. At that time, lush vegetation surrounded the lake. From 20,000 years on the climate changed so much that the rivers that fed the lakes diminished and then stopped flowing, apart from the occasional flood, and the area became as arid as it is today, the lakes shrinking until only salt lakes remained. Most of the salt delivered to the lakes was leached from the ancient marine sediments that underlie the catchment.
Lake Dieri, was at least 3 times the size of the present lake, at least 28,000 km² in area, compared to the 9,330 km², at about 600,000, 60,000, and finally at 40,000-20,000 years ago. At these times it was 17 m deep. These were secular or long-term stands of the lakes. There are ancient shingle beach ridges around Lake Eyre, at 280, 160, and 70 cm higher than the 1974 lake bed. The 1970s filling of Lake Eyre showed that the beach ridges may not represent long-term lake levels. In the 1970s it was found that beaches, spits and bars could form in a few weeks. The arid zone of Australia reflects the present conditions as well as past climates.
It has been estimated that about 125,000 BP, when lake Eyre was a deep, permanent megalake of close to 35,000 km², the combined Lake Eyre-Lake Frome system held 430 km³ of water, compared to about 30 km³ when full in historical times.
The evaporation rate at Lake Eyre, and all the other dry salt lakes of arid Australia, is much higher than the precipitation rate and the inflow from flooded channels combined. Under such conditions it would be expected that the bed would have long ago dried out to bedrock, but there is permanent mud under the crust, as there is also at Lake Frome, and probably other such lakes. The explanation is probably that, as has been found at Lake Frome, evaporation from the surface draws up water from the saline aquifers underlying the area through the salt-rich marine sediments left by the huge sea that covered the area in the past. The crust would be constantly added to from this mineral-rich water.
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.
The indoor part:
1) The name of this cache.
2) The names of other cachers you send the answers for. (If you do.)
The outdoor part:
The coordinates will bring you close to the lake. Close to the car park you'll find some information panels. Have a look.
3) How long ago did the lake has a depth of seventeen meters?
Now please make your way to the lake.
4) What do you see? Please describe colour/texture and so on.
5) Pick up a bit of salt and describe, how it differs from the regular table salt you buy in stores.
Please stand at the edge of the salt and look towards the lake.
6a) IF there is water:
-How far is it from the given coordinates to the shore?
-What do you think, is the lake filling or emptying at the moment?
-Why do you think so?
6b) 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.