Australia: The Land Where Time Began
Gawler Ranges see
These ranges, red dome-shaped hills, bornhardts, are eroded from acid volcanic rocks dated to 1540 Ma. This is a massif developed on silicic (they exhibit free silica in the form of quartz)3 volcanic rocks (Gawler Range Volcanics) of Mesoproterozoic age. The rocks are thought to be ignimbrite, cooled ash flow deposits laid down by nuee ardente-type eruptions. Some of the hills are isolated, though arranged in ordered rows, their relief varying from 150 - 180 m. Orthogonal fractures define the blocks each bornhardt is developed on. Convex-upward sheet fractures are the basis of the dome shape of the hills. Vertical partings define columns that are prominent in detail.
The crestal flats of the many bevelled domes combine to form an ancient land surface at the summits. It has been suggested that glaciers must have overridden the present area of the Gawler Ranges during the Permian, the glaciers probably removing any regolith that had formed prior to that time, making the resulting planation of Mesozoic age, as it must be younger than the time when the ice withdrew from the area.
In the formation of the Gawler Ranges of the present, by the Jurassic the Beck Surface, a planation surface, had developed, beneath which the volcanics were weathered differentially, a thicker regolith developing along fracture zones. The present landscape was shaped by the subsurface weathering, the valleys of the present being initiated by weathering that was fracture-controlled. The intervening massive blocks were rounded by either weathering that was guided by sheeting fractures, as the result of corners and edges of blocks being preferentially weathered, or by both mechanisms (Twidale & Campbell, 2005).
It has been suggested that there had been an extensive mantle covering the Nott Surface at an earlier time that has since been eroded and removed, as 2 sites on valley-side slopes have preserved a regolith. It has been found that Mt Anna Sandstone, that is exposed to the north of the upland in the southwest part of the Eromanga Basin, is the deposition site for regolith transported from the Gawler Ranges to the south. Lenses in the Mt Anna Sandstone have been found to contain boulders and cobbles from the Gawler Ranges Volcanics, the degree of weathering of the included fragments decreases down the profile, as would be expected of the stripping of a weathered profile, with the top portion being most heavily weathered because it has been subject to weathering for the longest period of time, and the least alteration occurring near the bottom, making this consistent with the stripping of a regolith.
The Nott Surface is considered to be best described as as an etch surface, a former weathering front, not much regolith being present, the age of which is derived from the presence of fossiliferous strata beneath and above it of Neocomian-Aptian, the earliest Cretaceous of 130-120 Ma. The conclusion is that the present landscape was exposed during the later Mesozoic, not changing significantly during the Cretaceous, as there is little volcanic debris in the adjacent basins. There is a skin that is iron-rich that is believed to be the old weathering front base that has been preserved in patches on the crests and slopes of many hills. Within the valley floors of the Gawler Ranges silcrete formed in the Early Tertiary, as well as in the piedmont zone that was marginal to the upland. In the upland the valleys are virtually intact, but dissection has taken place in the marginal plains.
The author3 suggests they may have formed as ash-flows during effusive eruptions, then later welded to form the rock formations of the present, such as dacite, rhyolite, and rhyodacite. Alternative explanations have also been proposed for these puzzling rock formations (e.g., Blissett et al., 1993; Allen et al., 2003).
The Gawler Range Volcanics formed about 1592 Ma, with columnar or cooling joints developed soon afterwards. About 1585 Ma granites were intruded. Orthogonal fractures, or systems of partings that are arranged at right angles to one another, formed in the granites and the volcanics, that probably resulted from shear stresses some time after 1585 Ma but before 1400 Ma, when dolerite sills were injected along some members of the orthogonal fracture system. Arcuate-upwards sheet structures occur within the orthogonal blocks. The formed prior to the deposition riverine sediments that have been physically dated to about 1424 Ma, in the area that is now east of Lake Gairdner (Campbell & Twidale, 1991a). The surface on which they were deposited has since been partially exhumed (Twidale et al., 1976).
Events occurring in the Neoproterozoic and Palaeozoic
The western Gawler Ranges was impacted by the Acraman bolide or meteorite about 600 Ma (Williams, 1994). The weathered and disturbed zone of the impact is marked by a salina, Lake Acraman, and aset of concentric fractures on the pre-existing orthogonal (slightly rhomboidal) fracture systems.
About 330 Ma in the Permo-Carbiniferous (BMR Palaeogeographic Group, 1992) ice sheets covered the region. Previously developed regoliths were stripped away by the ice sheets leaving behind in many places bedrock surfaces that had been scoured and debris deposits that were ill-sorted. This glacial event provides a valuable marker in landscape chronology in the Gawler Ranges, as well as in many other areas, as, apart from the minor exhumed forms from the Neoproterozoic to the southeast of Lake Gairdner, and features that are associated with the impact of the Acraman bolide, the landscape of the present began with the melting and disintegration of the ice sheets of the Late Palaeozoic.
The Gawler Ranges massif is higher in the south than their north. It is comprised of ordered rows of dome-shaped hills, or bornhardts, each being developed on a block that is fracture-defined that is orthogonal or rhomboidal in plan-shape. The rounded form of the hills is determined by the sheet fractures. The bornhardts (domincal hills) are bevelled, and a prominent summit surface is sloping down to the north is formed together with flattish crests (Campbell & Twidale, 1991b). This is the Nott Surface, which includes bornhardts and bevelled crests.
There is little to no cover on the bald, rocky hills, though there had been a regolith in the past. Evidence of this regolith is found far to the north in the Eromanga Basin (Wopfner, 1969, pp. 152-156; Wopfner et al., 1970), where cobbles and small boulders of Gawler Range Volcanics are found in the Mt Anna Sandstone, dating to the Early Cretaceous (Neocomian-Aptian). It is suggested by sedimentological evidence, the current bedding, thickness and strata, that it was rivers draining the uplands at this time. Therefore a weathered mantle that had developed in association with a plain, the Beck Surface, had been carried by the massif of the present prior to 130 Ma. The Beck Surface was named after Lance Beck (1907-1997) an amateur geologist who was well known who had a long-term interest in the Gawler Ranges. In the present landscape there is no remnant of the Beck Surface that has been recognised. The mantle of regolith consisted of corestones and boulders embedded in a finer clayey grit or sand that developed in well-jointed but impermeable rocks, such as granite, basalt), as is the case with many regoliths that developed on such jointed, impermeable rocks. Hill slopes in the Ranges have a single patch of regolith that has been found which contained spherical corestones set in weathered country rock, though there are others of kaolinised dacite, and that has been preserved to the east of Nonning Homestead, and to the north, occurring near the eastern shore of Lake Gairdner.
The Beck Surface is suggested by the evidence to predate the Early Cretaceous, following the retreat of the ice sheets which had covered the area during the Permian. The stripping of the Beck Surface, which occurred during the Early Cretaceous that the author1 suggests was possibly the result of river rejuvenation that was caused by upfaulting of the southern margin of the present upland along the Corrobinnie Fault Zone, the Corrobinnie Depression (Bourne et al., 1994; Binks & Hooper, 1984) having developed along the fracture zone. During the Eocene and Pliocene it was eroded and drained by the Narlaby drainage. It is suggested by the author1 that the dislocation along the Corrobinnie Fault Zone may have been related to the the separation of Australia from Antarctica.
Since the Early Cretaceous the Gawler Ranges massif has remained virtually unchanged, as suggested by the preservation of siliceous duricrusts from the Early Tertiary - Eocene (Firman, 1983), Eocene or Oligocene (Hou et al., 2003), at the margins of the upland. The author1 suggests the presence of alluvia from the Eocene in the Corrobinnie Depression corroborates the suggested Cretaceous age of upland. The absence of any significant volume of sediments from the Tertiary in adjacent basins to the east and north also suggests stability.
Rejuvenation and associated landscape revival has influenced the margins, but not the interior, of the massif, though the drainage of the Gawler Ranges has been affected by tectonics and the changing climate. Silcrete surfaces from the Early Tertiary are dissected at the eastern margin, though nowhere else. The southern piedmont is the only part of the Ranges where there are minor flared slopes and associated rock platforms. At a few locations in the interior remnants of a pre-Cretaceous regolith have been reserved.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|