Australia: The Land Where Time Began
The Flinders Ranges are a part of an upland system that stretches from the southern coast of South Australia to the Eyre Basin, about 600 km. The elevation of almost all of the ranges is above 300 m, and locally peak elevations exceed about 1100 m. In granite and quartzite catchments, such as Yudnamutana Gorge, in the northern Flinders Ranges, is locally at an elevation of more than 600 m. On the Australian continent, it defines one of the most dramatic and topographically youthful landscapes (Célérier et al., 2005). Surrounding the Flinders Ranges are low-elevation, low-relief regions that are dominated by playa lake systems, Lake Frome, Lake Torrens and Lake Eyre, that drain internally, and granitic inselbergs (Fig. 1, Source 7).
The rocks of the Adelaide Geosyncline that were deposited as sediment along a passive margin of the supercontinent Rodinia form much of the Flinders Ranges. About 540 Ma, in the Cambrian, the Delamerian Orogeny thrust up the rocks of the Adelaide Geosyncline, faulting and folding it to form a large range, the eroded remnants of which are now the Flinders Ranges. The rocks of the Ranges are mostly quartzites, limestones, shales and sandstones.
The rocks composing the Flinders Ranges are believed to have been deposited in shallow seas between about 850 and 500 million years ago. The Adelaidean Sea, of similar size to that of the Mediterranean Sea, stretched into central Australia from Kangaroo Island. The subsiding sea floor was a depotcentre for the sediment from the eroding higher ground to its west. As the climate changed, so did the deposition of these sediments. The coarseness and colour of the alternating layers resulting from the climatic changes can now be read in the rocks, allowing a climatic history of the area to be read from the rocks.
About 600 million years ago, while Australia and Antarctica were still a single continent, the seafloor along their east coast began to plunge beneath them along 4,000 km of their coastline. The mountain chain pushed up by this event ran the full length of the Gondwanan east coast. The mountain chain is now in 2 parts, the Transantarctic Mountains in Antarctica and the Flinders Ranges in Australia. During this process the entire eastern part of Australia was added to the much more ancient western section. For most of the length of the Flinders Ranges the rocks they are formed of were sediments that had been accumulating since before 1.0 billion years ago. The far northern end of the chain is composed of 1650 million year old granites and other crystalline rock.
The original ranges had been reduced almost to the level of the surrounding plains by about 60 Ma. Minor earth movements rejuvenated the area, thrusting the area up into a vast plateau that has since had the softer rocks eroded away to to leave the harder rocks such as quartz as folded ridges of the the present ranges.
At places in the Flinders Ranges, such as Tillite Gorge, evidence can be seen of the glaciation that covered much of Australia about 75 million years ago. At the northern end of the ranges the glacial deposits reach a depth more than 1/2 the height of Mt Everest. They were formed where glaciers dropped their load of rocks and rock powder when they reached the central seaway that covered the area at the time.
The ranges extend from 300 km north of Adelaide for 400 km in a series of north-south trending strike ridges, intersected by many gorges. They act as refugia, rising from the arid plains, with rich biodiversity of plants and animals. Their hydrology is the the result of the present regional variable, low rainfall. The rainfall is mainly in winter, and averages about 200 mm/yr. The accumulation of permanent water bodies is prevented by the an average evaporation rate of greater than 2000 mm/yr.
The landscapes of the ranges have changed a lot through time and change continues to the present time.
Some rivers arise in the Northern Flinders Ranges, flowing east to Lake Frome, while others flow to the west to Lake Torrens in the southern sector or to the Frome River and other river systems that flow towards Lake Eyre in the northern sector.
Quartzite and sandstone from the Neoproterozoic and Cambrian, that dip steeply, and limestones that erode more easily, shales and argillites are included in the bedrock, that are underlain by metasedimentary rocks and granite of Proterozoic age (Coats, 1973; Stevens & Corbett, 1993; Preiss, 1999). Erosional competency contrasts between different stratigraphic units, are reflected by many of the large-scale landforms of the Flinders Ranges, with granite, quartzite and/or sandstone bedrock typically underlying areas of high relief, the areas of low-relief being underlain by mixed shale-limestone-sandstone sequences (Fig. 2, Source 7). Erosion rates of lithologies that are the most resistant in the catchment systems, primarily govern the fluvial systems' base level, the elevations of regional summit surfaces and resultant relief maximums.
In the northern Flinders Ranges, fluvial sequences from the Cretaceous locally overly summit surfaces, which indicates that the relief must have formed later than the Mesozoic. It has been suggested by some (e.g. Veevers & Conaghan, 1984) that the topography dates from the early Cainozoic, because of the lack of apparently recent tectonic activity. A change to coarse sedimentation in the terminal Miocene, that has been interpreted to mark the initiation of uplift in the Flinders Ranges, as indicated by range-bounding alluvial fan sequences of Pliocene age that overlie low-energy lacustrine sequences from the Miocene. A number of range bounding reverse faults with displacements that are demonstrable and significant from the Quaternary have been found (Sprigg, 1945; Williams, 1973; Belperio, 1995; Sandiford, 2003, Célérier et al., 2005; Quigley et al., 2006a). Some attribute 30 % (Quigley et al., 2006a) to 50 % (Sandiford, 2003) of the present relief to tectonic uplift in the Late Miocene to Recent.
On the Australian continent, the Flinders Ranges form one of the most seismically active regions at the present, hundreds of small earthquakes being recorded every year, and in the past century there have been 5 earthquakes that were greater than magnitude 5 (Greenhalgh et al., 1994). Inferred palaeo-stress orientations derived from kinematic analysis of faults from the Pliocene and Quaternary have been corroborated by maximum compressive stress orientations, oriented approximately east-west, derived from focal plane solutions of historical earthquakes (Clark and Leonard, 2003).
Fluvial incision has outpaced erosion of hillslopes and summit surfaces in the Yudnamutana Catchment in the late Quaternary, according to erosion rate estimates based on cosmogenic beryllium (10Be). When these rates are linearly extrapolated over longer time scales the rugged topographic relief of the northern Flinders Ranges of the present is indicated to have possibly formed in about 4 million years. From the start of the Pliocene evidence along the range front of a change to coarse, high-emery sedimentation, found in the sedimentary record, and the neotectonic faulting record, suggest there was tectonic uplift over this time of more than 150 m. This supports the conclusion that the topographic relief of the present could have formed in as little as 4 million years. A gradient that was steep enough to sustain fluvial incision was sustained by the relief generated by tectonic uplift, and by uplift of adjacent summit surfaces relative to the floor of the valley. In situ bedrock weathering rates and the magnitude and frequency distribution of floods of large magnitude, resulting from episodic climate change, led to increased bedload transport and fluvial incision. Although most of the land surface of the Australian continent is low, flat and old, places such as the Flinders Ranges, that have experienced tectonic activity in the Plio-Quaternary, are pockets of more youthful and dynamic regions.
see Source 7 for more detailed information
Brachina Gorge is in the central Flinders Ranges. Brachina Creek flows westward to Lake Torrens from its source in the centre of the ranges, flowing through the km of Brachina Gorge, a meandering sandy channel in the wider sections, but in the Bottleneck section of the gorge is is confined to a narrow rocky bed. River red gums grow in the creek beds. Many of the ranges are covered with white cypress pine and gum-barked coolabah, while red mallee cover many of the ridges. The surrounding plains have a cover of low shrubs and grasses, and after rain, ephemeral herbs.
It has been found that a lake existed in the area in the past. In the creek-bank sediments, the sediment column has a basal, clay-rich deposit, separated from lacustrine sediments by a weakly-developed palaeosol horizon. This demonstrates how local conditions modified the response to the climatic changes occurring in the surrounding region.
High ground, such as these ranges, in the midst of flat arid region, tends to create its own local microclimate. The record of sedimentation in Brachina Creek shows an anomalous history of sedimentation when compared with that of the surrounding region.
It showed that rapidly developing dust mantles reduced run-off and increased infiltration, thereby altering the local hydrology. The fine-grained clay-rich sediment in the lower layer is believed to be reworked aeolian dust, that accumulated during dust storms as aridity set in at the approach of the last glacial maximum. These dust mantles were re-mobilised and accumulated as deep uniform silt and clay deposits in regional drainage channels. Sedimentation at this site occurred at about the same time as deposition at other inland sites. At 28,000 BP deposition at this site stopped, which again accords with the end of deposition at the other sites, resulting from decreasing rainfall in the region.
The following palaeosol zone indicates that although rainfall in the region had diminished there was enough water in the area to allow soil formation at some time between 28,000 and 21,000 BP. The approaching glacial maximum didn't result in a drastic increase in aridity locally. A lake formed, Lake Brachina, when the bottleneck in Brachina Gorge in the ABC Range was blocked, and it remained there throughout the 4500 years of the glacial maximum. In the basal lacustrine sediment there is evidence of a sudden event leading to the blockage of the bottleneck, but the cause of the blockage is not known, whether a land slide, tectonic activity, sedimentation or vegetation plugging the outlet.
Lake George in the Southern Tablelands of New south Wales, a much less arid environment, also contained water throughout the glacial maximum. It is believed that Lake Brachina was able to survive the arid times because of a number of factors, lower evaporation, the winter cloud base was lower, and winter rainfall was higher in the surrounding areas, and probably higher than at present.
The Willochra Plain, drained by Willochra Creek, is an intermontane topographical basin formed when a block of terrain was lowered between adjacent ridges. The Willochra Basin is an important underground and artesian water source, another in the Southern Flinders Ranges is the Walloway Basin. The northern Willochra Plain was a large lake during the Middle Eocene, its arms extending up the valleys of Mt Arden and the Kanyaka Creek and Boolcunda Creeks.
About 580 million years ago a 4 km-wide meteorite impacted at the site of Lake Acraman, the impact crater, on the Eyre Peninsula north of the Gawler Ranges and about 300 km west of the Flinders Range. A layer in the sediments of the Flinders Range is composed of the debris of this impact. It had a big effect on the Ediacaran biota, there was a complete change of the acritarch fauna.
Both regions of the Flinders Range and the Mid North are part of the same geosynclinal belt as the Mt Lofty Ranges, the belt is crossed by 2 major lineament corridors (4A and 8 in O'Driscoll, 1986,1989), expression of which is seen in the Norwest-McDonald Fracture Zone that trends NW-SE and the Paralana Fault System that runs NE-SW.
The steeply inclined faults are generally simple and open, and even slightly overturned strata are evident in places. Dissection has occurred to a much greater degree in the uplands than occurs in the Mt. Lofty Ranges (Preiss, 1987; Twidale & Bourne, 1999), the author8 suggesting possibly as they lack a protective capping, as remains prominent in the south. In many areas there are remnants of a summit high plain that have been preserved, though the dominant topography is ridge and valley topography, with small plateaux, small or table-like mountains, standing where the strata of the crests of arches and troughs of basins are flat-lying locally. The pattern of folds is largely reflected in the topography, though it is disturbed in some areas, or the occurrence of diapers or mudstone emplacement or shale that were injected from below, most of which are present in the cores of domes and anticlines.
Ridge and valley topography8
The development of ridge and valley landscapes involves 2 factors - variations in the composition or lithology of strata or tectonism, namely folding. Thick layers of sandstone, limestone, clays and muds were deposited in the Adelaide Geosyncline, between and offshore from, the Gawler Craton and the Curnamona Craton, forming sequences of strata. The sedimentary layers that were deposited offshore from the then continent at any given time 1,000-600 Ma varied in character as do the shorelines of the present that consist variously of shingle beaches, sands and mudflats. The location of the shoreline also varied over time as the sea level fluctuated, and the changes in the level of the land that resulted from earth movements. A given sedimentary stratum may have varied laterally, though of the same age, in composition, or lithology following burial and lithification. It may have consisted of various types of stone in different places, such as sandstone in one place, mudstone in another, and conglomerate or puddingstone in yet another, etc. When the sequence is later uncovered it is exposed to weathering the stratum was of varied resistance to attack by moisture and rivers, possibly because of its varied composition. This is one reason for differential weathering and erosion leading to the development of valleys and therefore ridges.
Also, about 550 Ma the area was affected by folding that introduced stresses, the crests of arches, such as anticlines and domes, in particular, were in tension as a result of being stretched, high in the structures while the structures were compressed at depth. In the high part of the structure troughs, such as synclines and basins, were in compression while at depth they were in tension. Fractures are opened by stretching or tension which leads to the penetration of water, weathering and erosion, and fractures are closed by compression so the penetration by water is inhibited therefore the rocks involved are protected.
In fold mountain belts relief inversion is common (e.g., Derruau, 1965, p. 320) as a result of this, with synclines and basins, which are structural lows, often standing high in the topography, while domes, anticlines or arches, that are structural highs, are reduced to valleys or plains. This process occurs at various scales. Structural basins, such as Wilpena Pound and Mainwater Pound, or natural amphitheatres, while the Dutchman's Stern, Gammon Ranges and Angepena Range are pitching or tilted synclines underlying positive relief features. Willochra Plain, an intermontane plain of regional extent occupies an anticline that is denuded regionally, though it is really an anticlinorium, as it is comprised of several arches and troughs within the overall structure. The Willochra anticline is also faulted, as is the case with other similar structures, with the development of a graben, rift valley or downfaulted block in the stretched crest of the fold (Milton and Twidale, 1977).
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|