Australia: The Land Where Time Began
Ellery Creek - The long Grand Canyon
Ellery Creek is a tributary of the Finke River. Along a 10 km stretch of Ellery Creek the bank is layered rock that has been tilted to 90° from its original horizontal position. This 10 km section of bank is like the layers of rock of the Grand Canyon, but tilted on its side, and covers 1800 million years of the geological history of central Australia. Fossil stromatolites can be found along the course of Ellery Creek.
Part of Ellery Creek runs across the Central Australian shield, known as the Arunta Block.
The same sand that formed the Heavitree Quartzite, gradually blocked the connection of the seaways with the ocean, the water of the inland seaways evaporating to form landlocked lakes. With insufficient inflows from rain to replenish them, the saline water became more and more concentrated, different minerals crystallising out to form layers on top of the sand as the super-saline water continued to evaporate. Over the following eons silt and debris washed in to bury the sand. The resulting shale and siltstone slabs, the Bitter Springs Formation, from this period can still be seen at this site.
Further along Ellery Creek are the remains of stromatolites that lived in the super-saline lakes. On first sight they look like marbled limestone shot through with purple. The curved purple lines are fossilised domes of the stromatolite columns.
At around 700 million years ago the climate cooled, as the area moved into a polar region, thought to be the North Pole. Adjacent to the Bitter Sprigs Formation are deposits laid down during this glacial period. At this time another period of uplift pushed parts of the previously level land of central Australia up as a range of mountains. The jagged crests and U-shaped valleys are characteristic of glaciated surfaces. The U-shaped valleys are formed by the scouring effect of glaciers. The pebbles and rock-dust swept up by the ice is deposited as the ice melts from the end of the glacier. The sediment accumulating was composed of the debris from the Arunta shield granite, the Bitter Springs Formation limestone and the Hevitree Quartzite. The debris dropped by the glaciers eventually raised the lake beds above sea level. The resulting dry land was a maze of hills, lakes and ridges. Rocks remaining from this period can also be seen in the Mt Conner tabletop along the road to Uluru, Lasseters Highway.
As the continent drifted to lower latitudes, the climate gradually improved. Large deltas formed and 2000 m of sediment built up in the shallow seas still covering much of Australia at this time. Even individual tides from this time can be seen in the sandstone of the creek bed near the crumbling moraine in this section. The 2000 m is now shale. It can be seen in the creek bed near tide marks in the sandstone.
By the end of this period sediment almost filled the shallow marine shelf. As a result of the concentration of the brine another limestone layer, the Julie Formation, was deposited. This is a thin dark-grey 4-m high ridge.
The next section records the Petermann Orogeny, when the Petermann Ranges were thrust up. A collision on the far side of Antarctica, on a similar scale to the collision of India with Asia, is believed to have occurred about 600 Ma. Somewhere near the Northern Territory-South Australian border, a massive range of mountains rivalling the Himalayas in size was forced up from the thinner crust between cratons. As usual when such mountains arise, there is a great increase in erosion from their sides. The result was a vast alluvial fan on the northern side of the range. As the mountain range rose, the inland sea that was at their base was gradually forced further away and then the sediments washing into the already shallow sea reduced it even further. After millions of years of consolidation and heating, as they were buried by later sediment, the heat and pressure eventually produced the red Arumbera Sandstone, on the southern side of the limestone of the Julie Formation.
The next chapter is a dark shale. At the time this shale formed Australia was on the equator, and the shale was the muddy bed of the sea. The warm, shallow sea swarmed with life. In this shale are found such signs of life as the tunnels of burrowing worms, jellyfish, the first brachiopods (lamp shells), early ancestral mussels and oysters.
The next sequence is another sandstone, this time interbeded with limestone. This indicates that another transgression of the sea occurred at this time. For the first time in the sequence trilobites appear in the deposits, though fossils are few in number. Their fossils are found in the limestone in this sequence.
After passing through this shale ridge, Ellery Creek flows into a sandstone gorge. Then followed a sequence of shifts in the level of the sea floor, there were a number of marine transgressions and regressions, during which the site must have been on the edge of a far different sea. This time waves were breaking on the continental shelf. Longshore bars formed at this time, and with the addition of coarse sand from the sea pounding on the rocks, a depth of over 1000 m of sand was deposited. On the bed of this sea were large numbers of benthic organisms (benthos). This is the orange-yellow Pacoota Sandstone, in the form of a high escarpment. In this sandstone are found fossils of many of the creatures living in this sea. Some include the coprolites (faecal pellets) of the trilobites and the markings of the trilobites and other creatures moving across the sea bed. There are also sections of rock called pipe-rock. These are vertical and U-shaped worm burrows. There are also ripple marks from the sea of 500 Ma.
The next in the sequence is the Horn Valley Siltstone. This siltstone was deposited in the Ordovician ocean. In this fine silt were deposited many of the creatures from the upper water layers, their remains settled to the sea bed when they died. Most would have been eaten by the hordes animals they had to pass on their way to the silt, those that were missed by the scavengers were covered by the constant rain of silt in the deeper parts of the ocean. Once buried, the anoxic (oxygen deficient) conditions inhibited bacterial decomposition allowing time for them to be fossilised before they could be completely broken down.
The most numerous organism found here are graptolites, but they occur as faint impressions. There is a very rich assemblage of species fossilised in this deposit.
After this formation the sea level changed. This level outcrops as a ridge of pale sandstone. In this deposit of beach sand are the fossils of the first fish to appear, Arandaspis. They are the first known vertebrates to evolve.The Meerenie Sandstone, laid down about 350 Ma, is a rare example of a Silurian aeolian sandstone (deposited under desert conditions). This is one of the largest deposits in the Amadeus Basin. It was deposited over a long period, up to about 100 million years. Over this period of time the deposits were in several different environments, marine, rivers and lakes. The final deposit was in what is believed desert dunes. It is thought the large flat area of desert reached to the shore of a shallow sea that gradually moved in from the west.
Following the formation of the Meerenie Sandstone a period of mountain building occurred resulting in the rise of the third phase of Macdonnell Ranges formation. The last 3 rock formations along the Ellery are a siltstone, a sandstone, and finally a very thick layer of conglomerate rocks, the Brewer Conglomerate, formed from the debris eroded from the Macdonnell Ranges. The combined depth of the 3 deposits is about 6 km. The are made up of sediment from all the other formations along Ellery Creek.
The Ellery Creek sediments also record a period of glaciation, and one of a marine transgression. The first occurred during the Permian. At this time Australia was situated at the South Pole. The transgression occurred during the Cretaceous. This transgression covered a vast area of the continent, covering the area now occupied by the Great Artesian Basin, including Lake Eyre. This inland sea was eventually filled in by sediment and the sea withdrew from the continent for the last time.
Timeline of the Ellery Creek deposits as the creek flows from north to south, beginning with the Arunta Block
Era Age Formation
Penny Van Oosterzee, 1993, The Centre - The Natural history of Australia's Desert Regions, Reed Australia.
|Author: M. H. Monroe Email: email@example.com Sources & Further reading|