Enigmatic mesozoic paleoforms revisited: the Australian experience
C.R. Twidale
Department of Earth Sciences, School of Physical Sciences, University of Adelaide, Adelaide 5005, South Australia
a b s t r a c ta r t i c l e i n f o
Article history:
Received 1 July 2015
Received in revised form 22 January 2016
Accepted 25 January 2016
Available online 27 January 2016
Contrary to geomorphological convention many landscape elements evidently predate the Neogene. Some are of
exhumed origin but many are of etch type and can be dated by means of stratigraphic and topographic data.
Most appear to occur in Gondwanan remnants in the southern hemisphere but ancient vestiges increasingly have
been reported also from North America and Europe. Features of Early Tertiary and later Mesozoic age-ranges
have been identified and cogent though possibly inadequate survival factors have been cited. Notwithstanding,
most workers consider that forms of Triassic derivation that occur in the contemporary landscape are considered
incredible.
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Dating
Palaeosurface
Mesozoic
Gondwana
Survival
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
2. Conventional age-range of land surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3. Nature of paleosurfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.1. Exhumed forms and surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.2. Etch surfaces and forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4. Ancient Australian landscapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.1. Favorable aspects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.2. Topographic and stratigraphic relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.2.1. Eastern Victoria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.2.2. Flinders Ranges, South Australia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.2.3. Other examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.3. Correlative deposits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.3.1. Gawler Ranges, South Australia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.3.2. Further examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.4. Duricrusts as morphostratigraphic markers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.4.1. Kangaroo Island and the Gulfs region of South Australia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.4.2. Yilgarn Craton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.4.3. Further examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
5. Conservative factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6. Significance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
8. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
1. Introduction
The possible fate of scientific ideas has been summarized in various
ways, but a representative version, that applies for instance to the
Earth-Science Reviews 155 (2016) 82–92
E-mail address: rowl.twidale@adelaide.edu.au.
http://dx.doi.org/10.1016/j.earscirev.2016.01.011
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Darwinian concept of evolution, suggests that: ‘All truth goes through
three stages. First, it is ridiculed. Second, it is violently and rudely opposed.
Third, it is accepted as being self evident.’
With changing attitudes to interpretation and explanation in mind it
is salutary to consider the history of the ancient landscapes concept,
which challenges several fundamental aspects of general geomorphological
theory but which is widely considered to be based in unreliable
evidence and argument and thus beyond belief.
2. Conventional age-range of land surfaces
Commonsense supports the conclusion articulated by James Hutton
(1788) and supported by Charles Lyell (1830) that the Earth's landscapes
have constantly changed under attack by agencies and processes
active at the Earth's surface. They continue to do so, for it is a matter of
observation that rivers, glaciers, and the wind carry huge volumes of
sediment that are deposited in valley floors, enclosed basins, and the
sea. Transport and deposition imply erosion in source areas. Landscapes
are in flux.
The theme of constant change was reinforced by Davis (1899) who
concluding that rivers are responsible for shaping most of the world's
land surfaces, stated that the effects of wash and streams extend to
every part of the landscape. Hence the almost universal view that with
the exception of exhumed forms, few landscape features predate the
later Cenozoic. Brown (1980) suggested that in theory a few forms dating
from the earlier Tertiary may persist, but most workers considered
that landscapes date from the Miocene or Pliocene at the earliest, and
that most are of Quaternary age (e.g. Ashley, 1931; Thornbury, 1954).
This conclusion was partly based, and found corroboration, in recently
de-glaciated and hence youthful areas of western Europe and
northern North America. The midlatitude deserts that so fascinated
early explorers and scientists also are of recent derivation. Measured
erosion rates and estimates of the time taken to reduce certain areas
to baselevel, considered in conjunction with Davis' belief in the ubiquity
of erosion by running water, seemingly ensured the elimination of earlier
surfaces, and confirmed the essential recency of landscapes (e.g.
Linton, 1957; Schumm, 1963). The youthful landscapes mindset became
embodied in the favored models of landscape development (Davis,
1899; King, 1953; Hack, 1960). That landscapes are both youthful and
of epigene origin became a virtually unchallenged and unchallengeable
axiom both in the literature and in the minds of many geomorphologists;
so much so that the occurrence of landforms of possible prePleistocene
age was considered to be worthy of announcement (e.g.
Bierman and Turner, 1995).
However, there has not been universal accord. Kennedy's (1962)
hypothetical model of landscape developments based in the interplay
of tectonics, stream dissection and mass wasting suggests that in some
circumstances summit surfaces may persist, at least for a time. Also,
field evidence has long been cited that points to contemporary landscape
elements that evidently predate the later Cenozoic. Some are of
exhumed type, that is, they have been shaped, then buried beneath sediments
or volcanic materials, and later resurrected; though it is noted
that some structural geologists consider ‘exhumation’ and ‘erosion’ to
be synonymous. In addition, forms and surfaces of etch or two-stage origin
that long have been exposed to the elements have been reported
from many parts of the world.
3. Nature of paleosurfaces
3.1. Exhumed forms and surfaces
The age of a resurrected surface is bracketed by the age of the youngest
rock exposed in the eroded and buried surface and the oldest of the
cover material. It usually can be taken as immediately predating the
cover. Exhumed forms and surfaces are widely distributed both in
space and time. They range in age from Late Archean to Late Pleistocene.
Some have been exhumed more than once. The Mesozoic surface that
persists on the Armorican Massif of western France is a case in point
(Bessin et al., 2015) and the Lake District of northwestern England
and Enchanted Rock and associated bornhardts located in the Llano of
central Texas offer similar possibilities (Marr, 1906; Barnes, 1981).
Their persistence poses no problems unless they were exposed during
or prior to the Cenozoic.
Some exhumed surfaces and associated regoliths provide evidence
of the environments in which they were shaped. For instance, the mineralogy
of the thick (circa 200 m) regolith derived from the alteration of
a Paleoproterozoic granitic surface and now exhumed in the northern
Yilgarn Craton of Western Australia indicates extreme aridity
(Lascelles, 2014). Charnwood Forest is an inlier of Precambrian rocks
in a Triassic desert terrain (Watts, 1903; but also Bridger, 1981). Barchan
dunes are preserved beneath Triassic lavas in the Parana Basin of
southern South America (Almeida, 1953) and a field of similar forms
was overwhelmed by flood basalts of Early Cretaceous age in Namibia
(Jerram et al., 2000). Friable regoliths as well as anomalous sedimentary
structures and morphological decorations also are preserved on exhumed
bedrock surfaces (e.g. Twidale, 1984; Battiau-Queney, 1997).
3.2. Etch surfaces and forms
Etch surfaces evolve in two stages. First a regolith is formed by
weathering, and predominantly by water-related alteration, or etching,
of the country rock. The junction between the weathered mantle and
cohesive country rock is the weathering front or Tiefenfront (Büdel,
1957, Mabbutt, 1961). Then, and second, the stripping of the mantle
or regolith exposes the weathering front as an etch surface (Fig. 1). Subsurface
weathering and formation of a regolith and its subsequent exposure
are the most recent and evident processes in the evolution of etch
surfaces and justify their being referred to as ‘two-stage’ features. However,
contrasts in the resistance to etching at the weathering front
consequent on variations in rock fabric show that many etch forms reflect
rock fabric (structural forms sensu lato) and are examples of
Gefugerelief (Sonder, 1948). Thus, they are multistage in character
(Gagny and Cottard, 1980; Twidale and Vidal Romani, 1994).
It is not suggested that exposed etch surfaces remain untouched by
weathering and erosion after the evacuation of the regolith. Former
weathering fronts may be dissected (Fig. 2) and otherwise modified,
and also are subject to seismic joggling and dislocation (e.g. Twidale
and Bourne, 2000a, 2003). Notwithstanding, the essential features
shaped in bedrock remain intact, and their nature remains evident. As
Hills (1975, p. 300) stated, these old land surfaces ‘have naturally suffered
some reduction and modification in detail during the long periods
of time to which they have been exposed to weathering and erosion but
this is relatively minor….’
But whether two- or multi-stage, establishing the age range and accounting
for the survival of very old etch forms poses complex
problems.
4. Ancient Australian landscapes
4.1. Favorable aspects
The dating of ancient landscapes can be illustrated by reference to
specific criteria and by consideration of principles illustrated by reference
to Australian examples (Fig. 3). This is not to suggest that very
old landscapes are older or more widely developed in Australia than
in other parts of the world, but the dating of such surfaces is facilitated
by several factors (Twidale, 2007a).
The Australian continent is relatively stable in the sense that there
has been no post Paleozoic orogeny though epeirogenic movements
and isostatic adjustments continue to this day, even in the relatively stable
shield lands (Twidale, 2011a). The lowland character of much of the
continent has allowed marine transgressions such as those of the
83C.R. Twidale / Earth-Science Reviews 155 (2016) 82–92
Cretaceous (Frakes, 1987) to extend widely over the then continent
thus facilitating the dating of adjacent and re-exposed surfaces. Widespread
Cenozoic hotspot volcanic activity in the Eastern Highlands has
assisted dating of topography as have volcanic events associated with
the separation of Australia and Antarctica during the Mesozoic. The development
and conservation of stratigraphically-dated duricrusts have
proved critical in some areas. The compact shape of the continent plus
the lack of high mountains and its relatively low latitude also have
served to minimize the impacts of Quaternary cold periods and dissection
by rejuvenated streams related to low sea levels, thus enhancing
the possibility of old land surfaces being preserved. Also, the disappearance
of the Permian ice sheets that occupied most of the south and center
of the present Australian continent provides a baseline, and an age
limit for any remnants of epigene or etch surfaces (e.g. Paine, 1990,
p. 48).
No direct (physical or numerical) method of dating pre-Quaternary
land surfaces is known at present. Yet for more than two centuries, reasoning
from observed data has made it possible to establish the relative
age of landforms and later to provide a stratigraphic age. Surfaces and
forms can be dated relatively according to their position in the landscape,
and geologically by reference to local and regional stratigraphy.
In many instances multiple lines of evidence and argument can be adduced
in an effort to resolve the age of regional landscapes.
4.2. Topographic and stratigraphic relations
Rütimeyer (1769) and Baulig (1928) long ago deduced that in a tectonically
undisturbed landscape older elements stand higher in the relief
than the younger. Clearly the highest ‘riser’ displayed on the
stepped northwestern slope of Yarwondutta Rock (Fig. 4), a granite inselberg
on northwestern Eyre Peninsula, is older than the lower. A
known concavity, already shaped beneath the present piedmont plain
and awaiting exposure, is a potential still younger landform. Similarly,
superposition demonstrates that the granite hill shown in Fig. 5 predates
the late Pliocene or early Pleistocene basalt (age less than 2 Myr:
Stephenson et al., 1980) that flowed around and isolated it.
At a regional scale the present scenery of Tasmania has developed
through ‘the successive inward transgression of landscapes formed at
progressively lower levels’ (Davies, 1959, p. 20). The value of such relative
dating was immeasurably enhanced with the realization that dated
valley fills, of whatever kind, provide a minimum stratigraphic age for
bevels preserved on the adjacent range. Thus in Tasmania basalts of
Early Tertiary age occur in the valleys of several lowland rivers (Owen,
1954; Banks, 1962), showing that their valleys and the central Plateau
and high plains into the flanks of which they are incized, are of earlier
age. Some low level duricrusts appear to be of Eocene age. Thus, the
high central plateau of the island appears to be of earliest Tertiary or
Mesozoic age.
4.2.1. Eastern Victoria
Rather earlier, Hills (1934, 1938) observed that many of the beveled
crests of ridges eroded in deformed but resistant Paleozoic rocks in the
uplands of eastern Victoria are separated by valleys occupied by lavas
(Fig. 6). Stratigraphic evidence, later confirmed by physical/numerical
analyses, dated the volcanic rocks as of Eocene–Oligocene age. From
Fig. 1. (a) The laterite-capped plateau that forms part of The Granites, near Mt. Magnet,
Yilgarn Craton, Western Australia, has been dissected to the base of the regolith, exposing
granitic blocks and boulders in the valley floor at the weathering front. The laterite and the
shaping of the weathering front date from the later Mesozoic, but the exposure of the front
occurred in the Tertiary, and probably the middle Tertiary. (b) Granitic corestones and intrusive
vein exposed in road cutting, Rocky Mountains, Colorado. Note exposed corestone,
or boulders, standing on the natural surface.
Fig. 2. Part of the Kuiseb Canyon, central Namibia, showing duricrust-capped mesa and
beveled remnants of weathering front preserved on dissected land surface shaped in
dipping strata.
84 C.R. Twidale / Earth-Science Reviews 155 (2016) 82–92
this Hills deduced that the crestal bevels are remnants of a planation
surface of at least Cretaceous age.
Later clarification of the regional tectonic setting caused this agerange
to be extended (Hills, 1955a, 1955b). The Jurassic strata bordering
Bass Strait are down-warped, suggesting that the highlands to the north
of any hinge line and on which are preserved Hills' summit bevels, may
have been tilted upwards. This was taken to imply that the low relief
remnants previously dated as Cretaceous were older than first thought,
and were possibly of ‘pre-Jurassic, and at least Triassic in age’ (Hills,
1955a, p. 32; see also Neilson, 1962; Jenkin, 1988, pp. 408 and 417).
Geological mapping of the region led Orth et al. (1995, p. 31) to conclude
that the Middle Devonian (Tabberabberan) orogeny ‘began a
long process of erosion that continued to the late Mesozoic, and
which, by the mid Cretaceous, had produced a land surface of low relief
over most of southeastern Australia’. These analyses caused a revision of
the Pliocene age ascribed to the high plains recognized in the adjacent
region (e.g. Craft, 1932, 1933).
Fig. 3. Location maps of (a) Australia featuring the cratons and basins referred to in text,
together with the Eastern Uplands, and the area covered in (b) part of South Australia,
with various topographic regions and sites, including three Triassic basins that occur in
the vicinity of Leigh Creek, and those named Boolcunda (B) and Springfield (S) further
to the SSE.
Fig. 4. Yarwondutta Rock is a low granite inselberg located near Minnipa on northwestern
Eyre Peninsula, South Australia. (a) Low oblique aerial view from the ENE, showing reservoir,
prominent Kluftkarren or fracture-controlled clefts and flared marginal slopes, (b) a
stepped northwestern slope of the hill with a large residual boulder, and (c) the ‘reservoir’
in summer without water, exposing a concave subsurface slope that represents an early
stage of development of a flared slope, like that seen in the wall beyond.
85C.R. Twidale / Earth-Science Reviews 155 (2016) 82–92
4.2.2. Flinders Ranges, South Australia
The southern Flinders Ranges is a region of ridge and valley developed
on folded Cambrian and Proterozoic strata (Preiss, 1987). Uplift
of the western margin of the upland caused the pre-existing westerly
drainage to be blocked and a Middle Eocene lake inundated the northern
part of the intermontane Willochra Basin (Fig. 3b; Johnson, 1960).
It tongued up the valleys, which clearly predate the lacustrine deposits,
as do the adjacent ridges. The ridge and valley assemblage was in existence
some 60 Mya (Twidale and Bourne, 1996). A pre-Tertiary age for
the ridge and valley topography of the northern Flinders Ranges is suggested
by the occurrence of silcrete of putative Eocene age (Wopfner
et al., 1974) in local and regional piedmonts (e.g. Campana et al.,
1961; Coates, 1973).
The beveled crests of the quartzite/sandstone ridges are higher and
older and probably date from the Cretaceous. They may be relics of a
planate surface shaped by rivers graded to the Early Cretaceous marine
transgression and shoreline, of which the Mt. Babbage outlier and the
sequences preserved beneath adjacent down-faulted lowlands, are a reminder
(Woodard, 1955; Sheard, 2001).
In addition to surfaces of definite or putative Cretaceous ages there
are hints of possible earlier relics. Several intermontane Triassic basins
occur within the upland (Fig. 3b). Considering the regional setting
Parkin (1953, p. 16) stated that prior to Triassic down-warping and
sedimentation in the Leigh Creek area, the Precambrian and Cambrian
rocks of the northern Flinders Ranges had been ‘reduced virtually to peneplain
level.’ This planation must have occurred during the earlier Triassic
or later Permian following the end of the earlier Permian
glaciation. Kwitco (1995, p. 99) stated that the basins stood in an area
of subdued relief and sluggish drainage and that the sediments and
contained plant remains indicate deposition in a warm humid climate
(also Wopfner, 1969, p. 137).
The regional planation evidently extended to more southerly areas
because independent indicators of Triassic planation were reported
from the Springfield Basin (Johnson, 1960). In particular, several cores
drilled through the sequence indicate an upward fining of sediments.
This can be construed as the result of a decrease of relief amplitude
through time in the catchment of this particular down-warped depression.
However, though the bevels preserved on some of the higher
peaks are suggestive, no surfaces or forms that can firmly be linked by
field evidence to the Triassic deposits have been located adjacent to
these basins.
4.2.3. Other examples
An Early Cretaceous marine incursion that surrounded Kakadu, located
adjacent to the north coast of the continent, converted the massif
into an island. It follows that the summit high plain of the residual,
though complex (Nott, 1994) and transecting various strata, was
already in existence prior to the Early Cretaceous (Needham, 1982).
Similarly, the Everard Ranges of northern South Australia evidently
existed prior to the ‘Upper Triassic to Lower Cretaceous’ (Wells et al.,
1970) when the deltaic De Souza Sandstone, of possible Jurassic age,
was deposited in the broad valleys between the beveled granitic
bornhardts of the eastern part of the upland.
4.3. Correlative deposits
Whether transported by rivers, glaciers, wind or waves, deposits
imply erosion in the source areas. Dating the deposits provides an
age-range for the associated destructional surface.
4.3.1. Gawler Ranges, South Australia
The bornhardt massif of the Gawler Ranges is developed on a series
of Mesoproterozic dacite and rhyolite (Blissett et al., 1993; Allen et al.,
2003). It has been attributed to the preferential subsurface weathering
of a major orthogonal and rhomboidal fracture system. The corners
and edges of the fracture-defined compartments were preferentially
weathered to produce an ordered series of rounded or domical bedrock
forms, the crests of several of which are beveled (Fig. 7). The present
valley floors are underlain by weathered country rock, but only one
small regolithic remnant consisting of corestones set in a matrix of
weathered dacite has been located on a high plain remnant. Notwithstanding,
it can be presumed that a regolith formed on a planation surface
that formerly extended over the whole region. Uplift of the
southern margin of what is now the Gawler Ranges massif along the
Corrobinnie Fault Zone caused rejuvenation of streams flowing north
Fig. 5. Granite hill surrounded and partly submerged by Pleistocene basalt flow,
Einasleigh, north Queensland.
Fig. 6. The Nunniong Plateau, part of the dissected upland of eastern Victoria. The beveled
crests predate the Early Tertiary (mostly Eocene) volcanic flows, which occupy many of
the valley floors (Geological Survey Victoria).
86 C.R. Twidale / Earth-Science Reviews 155 (2016) 82–92
to the present Eromanga Basin. Fluvial deposits derived from the upland
and including cobbles of Gawler Range Volcanics – construed as
corestones derived from the previously developed regolith – were laid
down as the Mt. Anna Sandstone (Wopfner et al., 1970). Thickness variations
and cross-bedding indicate the southerly provenance for the
Sandstone, which was interbedded with fossiliferous marine beds of
the Cadna–Owie Formation, showing it to be of Early Cretaceous
(Neocomian–Aptian) age.
As deposition implies erosion, this stratigraphic age determination
applies also to the source area. The regolith that contributed to the Mt.
Anna Sandstone was stripped and the bornhardt landscape exposed
during the Early Cretaceous. It also attests a phase of planation and
weathering predating the Cretaceous but postdating the Permian glaciation
(Campbell and Twidale, 1991). It was during this Triassic–Jurassic
interval that the present bornhardt landscape was etched as a topographically
differentiated weathering front.
4.3.2. Further examples
Woodward (1914) considered the possible source areas of adjacent
basin deposits, and suggested that the bevels that cut across Paleozoic
strata in the Geraldton area of Western Australia and the crystalline
Archean terranes to the east, in the northern Yilgarn Craton, are of Mesozoic
age. Very old landscape events were identified by Jones (1931,
p. 58 et seq.) who, considering the topography of the Bristol Channel
and adjacent regions, stated that in his view probably ‘no single episode
has contributed so materially to the shaping of the surface of this region
as the intense erosion, which succeeded the post-Carboniferous or Armorican
earth movements’. He attributed the shaping of the beveled
summits of the Welsh massif, later referred to as the ‘hill-top surface’
(Jones, 1951), and the resultant sedimentary sequences deposited in adjacent
basins, to planation during the Triassic. Poag and Sevon (1989)
linked flights of planation surfaces recognized in the Appalachians of
eastern North America with three post-Triassic depositional phases
evidenced in stratigraphic sequences beneath the adjacent continental
shelf. Referring to the same areas, Cleaves (1989) suggested an interpretation
that included a Late Triassic–Early Jurassic event.
4.4. Duricrusts as morphostratigraphic markers
As well as dated sediments and volcanic lavas, and as mentioned
with reference to the Central Plateau of Tasmania, duricrusts frequently
have proved invaluable as morpho-stratigraphic markers. Silcretes of
stratigraphically determined Eocene age (Fig. 8) are widely preserved
as cappings on dissected plateaux in central Australia (Wopfner et al.,
1974).
4.4.1. Kangaroo Island and the Gulfs region of South Australia
The broad features of the Gulfs region of South Australia have been
determined by recurrent and continuing faulting (Fig. 9). Like the Lincoln
Upland of southern Eyre Peninsula and the Fleurieu Peninsula of
the southern Mt. Lofty Ranges, Kangaroo Island is dominated by a
laterite-capped plateau developed on Proterozoic, Cambrian and Permian
strata but the age of which is in dispute.
Sprigg et al. (1954) observed that what was taken to be part of a laterite
profile is overlain by, and is therefore older than the volcanic rock
that later became known as the Wisanger Basalt (Milnes et al., 1982). At
that time the basalt was correlated with the later Quaternary volcanic
rocks of the South East district of South Australia and adjacent parts of
western Victoria (e.g. Sprigg, 1952), suggesting a pre-Quaternary age,
possibly Pliocene (Northcote, 1946), for the laterite. Potassium/argon
dating (McDougall and Wellman, 1976), however, showed that the
Wisanger Basalt is some 174–175 Myr old, i.e. of Middle Jurassic age.
This extended the time scale of landscape evolution, but the stratigraphic
relationships were not. The laterite still appears to be overlain by, and
therefore predates, the basalt. It probably indicates weathering under
torrid conditions and a putative Triassic age (Daily et al., 1974).
Numerical dating methods applied to different mineral phases have
produced contrasting results. Oxygen isotope dating supports an older
(pre-Late Mesozoic) age for the Kangaroo Island site under review
(Bird and Chivas, 1993), whereas paleomagnetic and K–Ar (potassium–argon)
dating indicate Late Tertiary (Miocene) ages (Schmidt
et al., 1976; Pillans, 2005).
Though the kaolinized sediment overlain by basalt near Wisanger
predates the basalt, no ferruginous pisolitic or other zone of iron concentration
occurs in the weathered rock exposed at the critical site. Accordingly
it might not be a truncated lateritic weathering profile. On the
other hand, a small consanguinous occurrence of basalt west of
Penneshaw (Tilley, 1921) appears fresh, despite the site standing
lower in the local landscape than the lateritic plateau located a few kilometers
to the south. This surely implies that the basalt postdates the period
of intense weathering during which the laterite was formed.
Also, if the Kangaroo Island laterite were of the Miocene or Pliocene
age indicated by paleomagnetic analyses and other considerations, earlier
sediments ought to be lateritized. They are not. For instance, on
southern Eyre Peninsula, the Eocene sedimentary sequences preserved
in the Cummins Basin, west of the lateritized Lincoln Uplands, are not
themselves lateritized or otherwise intensely altered, though they are
discolored white and red as to be expected of sediments derived from
an intensely weathered source area (Johns, 1958, p. 65, 1961, pp. 26
and 27; R. K. Johns, pers. comm. 2013).
In addition, the Mt. Lofty horst that forms the eastern flank of the
Gulfs region was initiated, and the high plain uplifted, prior to the Middle
Eocene for sediments of that and later Tertiary ages that were
Fig. 7. The bornhardt landscape of the Gawler Ranges, South Australia. Note the beveled
crests and hence even skyline. Sheet structure developed in dacite can be seen in the
foreground.
Fig. 8. Dissected plateau capped by Eocene silcrete, southwest Queensland.
87C.R. Twidale / Earth-Science Reviews 155 (2016) 82–92
deposited in the fault-angle depressions or half-graben developed at the
western margin of the Mt. Lofty Ranges. Lateritic debris occurs in the
Tertiary marine sequences (Glaessner and Wade, 1958, p. 117).
In summary, the age of the lateritized plateau of Kangaroo Island
remains controversial. The stratigraphic setting is suggestive but some
of the evidence is inconclusive or contradictory. As a result most authors
oppose the suggestion of great antiquity as unreasonable (e.g. Bourman,
1995). It would, perhaps, be preferable to consider the question as ‘still
open’.
4.4.2. Yilgarn Craton
The landscapes of the Craton proper revealed elements of similar
antiquity for Jutson (1914) noted that the so-called New Plateau – really
a high plain – is the result of the dissection and partial stripping of the
largely lateritic duricrust that forms and underlies the Old Plateau
(Fig. 1a). Relative uplift of the lateritized surface of the Old Plateau induced
rivers to incise their beds. The valleys so formed merged to
become the New Plateau, which continues gradually to extend
headwards at the expense of the Old. Near the coast the alluvia (the
‘deep leads’ of Jutson, 1914) are of Eocene age (Van de Graaff et al.,
1977; also, Clarke, 1994), but inland are younger. For instance, the alluvium
of the Salt River, south of Kellerberrin, is of Miocene age (Salama,
1997; Twidale et al., 1999). Thus, the New Plateau or high plain is an
etch plain of Cenozoic age. The duricrusted Old Plateau clearly predates
the Eocene (Twidale and Bourne, 1998).
Possible confirmation for this pre-Eocene age of the duricrusted Old
Plateau occurs just north of Perth, where the fluviatile Bullsbrook Beds,
of Neocomian or Early Cretaceous age, were deposited in valleys scored
in the Darling Fault scarp, below the level of the duricrusted Plateau.
They are described as very weathered and partly lateritized (Playford
et al., 1976, pp. 174–175), but this can be interpreted as confirming
that lateritization postdated the Neocomian, but still predated the
Eocene. Alternatively, the reported discoloration of the Bullbrook Beds
can be attributed to runoff charged with iron salts washed from a preexisting
lateritized Plateau.
As with Kangaroo Island, paleomagnetic dating has complicated dating
of the Yilgarn Plateau with several ferruginous occurrences dated as
Cenozoic, but a few as Mesozoic (Pillans, 2005). However, the possible
Cretaceous age of the Old Plateau surely cannot be doubted if only because
of the many dissected valleys carrying alluvia of Eocene age in
the in lower, earlier incised, valley floors and Miocene and younger
deposits laid down in the headwater sectors.
The many inselbergs, such as Hyden Rock, that project above the
level of the New Plateau were initiated by differential weathering
beneath the Old. Some however stand as uplands on the lateritic Old
Plateau. Residuals like The Humps, which stands over 420 m above
sea level and about 80 m above the adjacent lateritized plain (Fig. 10),
are clearly part of the former (?) Cretaceous landscape (Twidale and
Bourne, 2004). But the residual is stepped (cf. Twidale and Bourne,
1975) and displays an upper dome about 10 m high. It is flanked by
Fig. 9. Lateritic plateau of faulted Gulfs region, South Australia. 1, Cummins Basin; 2, Adelaide Plains sub-basin; 3, Clarendon Basin; and 4, Willunga Basin.
88 C.R. Twidale / Earth-Science Reviews 155 (2016) 82–92
flared sidewalls that identify a former piedmont (e.g. Twidale 1962) and
possibly a higher and hence older plain level. Several other bornhardts,
such as McDermid Rock (Bourne and Twidale 2002), located on the
southeastern Yilgarn Craton, display a similar ‘dome-on-dome’
morphology.
4.4.3. Further examples
Working beyond the southern margin of the Yilgarn Craton,
Fairbridge and Finkl (1978) recognized old valley systems dating from
the Permian – presumably later, postglacial, Permian – and from the
later Jurassic to early Cretaceous. They occupy upland divides and
imply local relief inversion.
The Macdonnell Ranges of central Australia consists of ridges and
valleys developed on folded and faulted strata of Precambrian and Paleozoic
ages. The assemblage was already in existence by the Eocene because
the bevels preserved on the summits of the low ridges can be
correlated with adjacent low hilltop bevels cut across steeply dipping
strata and with low level beveled quartzite ridges and silcrete-capped
piedmont and valley mesas. Older relics of a summit bevel shaped by
streams graded to Cretaceous seas occupying adjacent basins also
have been recognized (J. A. Mabbutt, in Brown et al., 1968, p. 304).
The crests of Uluru (Ayers Rock) and Kata Tjuta (The Olgas) massifs of
central Australia may be of similar antiquity for they rise from and
stand above plains in which are preserved relics of the Eocene silcreted
surface (Fig. 10; Mabbutt, 1965; Wopfner, 1997; Twidale, 2010).
Silcreted surfaces dated as of Eocene age are also recognized in United
States as well as southern Africa (Partridge and Maud, 1987;
Gassaway, 1988; Partridge, 1998).
5. Conservative factors
Very old paleoforms are a global phenomenon, and distinguished
and experienced geologists of earlier times apparently had no difficulty
in accepting the implications of their field evidence and concluding that
considerable tracts of their study areas are of great antiquity. How very
old landforms had withstood the elements over the ages was of less
concern than the self-evident argument that if a thing exists it must
be possible.
Nevertheless, several conservative factors can be invoked (e.g.
Twidale, 1976, 2007b). First, the structure, for most of the old land surfaces
is of etch type in which coherent rock is exposed. Most old surfaces
are preserved on tough resistant rocks of which the Gawler Range Volcanics
can be cited as an example. Many of the granites on which the
Yilgarn Craton bornhardts are shaped are buttressed by sills and dikes.
Some Brazilian morros (bornhardts) are shaped in resistant compartments
based in deep compressed antiforms (e.g. Lamego, 1938), and
the summit surface of the Roraima Plateau of northern South America
is shaped in massive quartzite (Briceño and Schubert, 1990; Schubert
and Huber, 1990).
Second, location is significant because some older remnants persist
on drainage divides. Interior position also is especially significant. For
example, the interior Mesozoic and Early Cenozoic plains and inselberg
landscapes of southern Africa are buttressed against erosion by the great
size and compact shape of the subcontinent. Recurrent uplift has caused
repeated phases of exoreic stream rejuvenation and landscape revival
with nick points and valley side facets working inland from the coast,
and younger plains extending at the expense of the old (e.g. Lister,
1987).
Third, though widely active in shaping the land surface, rivers per se
are not as effective as has been supposed (e.g. Baker, 1988). Potholes,
flutings and concave scoops are scoured and gouged in fresh granite
exposed in the channel of major rivers, especially during floods. The capacity
of minor streams, rivulets and wash, however, is questionable.
Soils and regolith are readily stripped by rivulets and wash but incision
virtually ceases where coherent rocks are encountered (e.g. Jutson,
1934; Noldhart and Wyatt, 1962). As E. S. Hills (pers. comm. 1962) stated,
when rivers cut into a soil-covered terrain developed on cohesive
bedrock, “…there is first of all a period of rapid skimming off of the
soil and rotten rock, followed by a sudden decrease in removal of detritus
when fresh rock is exposed to the eroding agents.” Like some
glaciers (Boyé, 1950; Bird, 1967; Lidmar-Bergström, 1997), minor
streams and diffuse wash act as bulldozers rather than excavators.
Fourth, erosion is differential and not evenly distributed as had been
supposed by Hutton, Davis and others (Behrmann, 1919; Bliss Knopf,
1924; Crickmay, 1976; King, 1970; Twidale et al., 1974; Brunsden,
1993). Accordingly, once in positive relief resistant masses shed water
and become relatively dry sites. Rocks like granite disintegrate in contact
with moisture but they are relatively stable and resistant when
and where dry sites tend to persist (Logan, 1851, p. 326; Alexander,
1959; Eggler et al., 1969).
Episodic periods of uplift, whether epeirogenic or isostatic, have induced
alternations of weathering and erosion, leading to the development
of stepped topography and an increase in relief amplitude
through time (Twidale and Bourne, 1975; Bourne and Twidale, 2000).
In this way, there were formed local or regional uplands not immune
to destructive processes but subject only to slow change. Thus, concatenation,
involving structurally initiated unequal weathering and erosion
plus positive feedback or reinforcement, has contributed significantly to
the enhancement and persistence of the forms (Twidale, 2007b).
Furthermore, though scarp retreat is a real and critical factor in landscape
evolution (but see also, Moss, 1980), its effectiveness is reduced in
time. Gully gravure (Bryan, 1940; Twidale and Bourne, 2000, 2003)
Fig. 10. (a) The Humps, a stepped, dome-on-dome inselberg located some 12 km north of
Hyden, Western Australia, and standing on a lateritized surface of putative Cretaceous age
with laterite exposed in a shallow borrow pit in the foreground, and (b) the flared wall of
an upper dome construed as a former piedmont.
89C.R. Twidale / Earth-Science Reviews 155 (2016) 82–92
delays recession. Diminution in the area of headward catchments induces
a decreased rate of recession (Twidale, 1978) such that late in a
phase of recession virtual standstill of the backing scarp has allowed
weathering to shape a well known suite of piedmont forms around
the bases of surviving upland remnants or inselbergs (Thorbecke,
1927; Clayton, 1956; Pugh, 1956; Twidale and Bourne, 1998).
6. Significance
Major dating factors are emphasized in these regional accounts but
in most instances multiple lines of evidence and argument are germane
to the problem. Cretaceous surfaces are well represented and though
putatively identified in only a few areas, the possibility of still older Triassic
landscapes is implied by the field evidence in the Gawler and Flinders
ranges of South Australia, in southeastern Australia, notably
eastern Victoria, and on the Yilgarn Craton of Western Australia. That
surfaces of great antiquity are preserved in other continents is confirmed
by reference to various authors additional to those already
cited (e.g. Curtis et al., 1958; King, 1962; Briceño and Schubert, 1990;
Birkenhauer, 1991; Demoulin et al., 2005).
In addition to influencing the way scenery is viewed, the possibility
of there being very old remnants in the contemporary landscape of very
old land surfaces ought also to enhance stratigraphic interpretation by
relating deposits to source areas. With some notable exceptions geological
reports provide accounts of bedrock occurrences but neglect the
erosional side of the depositional coin.
Very old landscapes are of interest to paleobotanists, and to pedologists.
For instance, very old eucalypt species survive in favorable niches
on the slopes of bornhardts of putative Early Cretaceous age on the
Yilgarn Craton of Western Australia (Hopper et al., 1996; Twidale and
Bourne, 2004). As is well known, some elements and minerals are concentrated
in old regoliths.
But above all paleosurfaces pose problems that are challenging and
stimulate the curiosity. In particular, plausible evidence of the survival
of very old – earliest or pre-Tertiary landforms – serves as a reminder
of the implications for a general theory of surfaces that have persisted
despite eons of exposure to the elements.
7. Discussion
The possibility of Triassic landforms having survived to become elements
of the contemporary landscape has been canvassed previously
(Twidale, 2000b). However, the re-evaluation and testing of older evidence,
and in particular the implications of the regional stratigraphic
settings of key areas, justifies this further consideration of possible
very old paleoforms, and indeed it appears that considerable areas of
the continents, and especially the cratons and older orogenic belts that
constituted Gondwana are occupied by very old surfaces and features,
either exhumed or etched.
The great age of some resurrected forms is accepted by all, but the
postulated great age of some etch features is not only questioned and
challenged – which would be proper and welcome – but denied without
consideration of the evidence. Even if in due course some of the evidence
and arguments presented in justification of the postulated antiquity
of these forms are found to be flawed, other examples remain
validated on the basis of topographic and stratigraphic sequences, correlative
deposits, and regional stratigraphy. They are enough to call
into question various aspects of conventional geomorphological theory.
Also the evidence pointing to antiquity, like that referred to here, also
will still call for an explanation.
The degree of antiquity that is considered incredible varies. Cenozoic
landscape elements have endured for 50–60 Myr and are accepted
without demur (Fig. 8), but claims that Mesozoic landforms and landscapes
persist are peremptorily dismissed. Those who condemn the ancient
landscape concept have neither stated their reasons, nor have they
offered alternative explanations for the evidence and argument of the
kind presented here.
Even when a full account has been taken of the various conservative
factors cited above, and bearing in mind Hills' comments on the nature
of such old surfaces, they still pose problems and call for investigation
and explanation. However, if the evidence of their great age proves to
be sound – and it has not so far been refuted or explained in alternative
terms but rather condemned (Twidale and Bourne, 1998; Twidale,
2011b) – then surely it is more rational to accept that if surfaces of
great antiquity exist they must, somehow, be possible, and proceed to
test the evidence and argument on that basis, rather than assert that
they are not real because of a disbelief founded in convention.
8. Conclusions
If some landscape elements are as old as is suggested – and though
some of the evidence and arguments are debatable, some are compelling
and difficult to deny – they change the way we view our scenery.
Their age is such that they have been subject to significant climatic
changes and to the cumulative effects of tectonic stress as well as adjustments
to changing lithostatic pressures. They are palimpsest surfaces
that surely must find a place in the stratigraphic record if only as the
source areas for sedimentary sequences. These old forms and surfaces
have persisted in part as a result of differential erosion, which necessitates
adjustments to previous estimates of rates of exogenous geomorphological
processes. They stand in denial of general landscape
lowering over time for they imply enduring local and region increases
in relief amplitude.
If they are as old as suggested paleosurfaces like those discussed
here are anomalous and as such pose problems for the conventional
models of landscape evolution. Scientific research begins with the identification
of anomalies, and the definition of problems. It has been
claimed that science is merely applied and organized common sense
(which may have been true a century ago), and certainly the simple dating
methods outlined above were conceived and used by early geological
investigators interested to explain not only the rocks and structures
but also the landscapes of the particular areas with which they were
concerned.
Though the evolution of the ancient landscapes hypothesis can realistically
be considered at the second of the stages cited in the introduction
to this essay – it is rudely and violently opposed – it is encouraging
for those who see merit in the ancient landscapes hypothesis to recall
Einstein's suggestion that: ‘If at first an idea is not absurd, then there
is no hope for it.’
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