C. R. Twidale

Landforms and geology of granite terrains.

a geomorfologia granitica constituye un campo de estudio de indudable interes en aquellos territorios asentados sobre importantes escudos continentales hercinicos. Existen innumerables estudios en practicamente todos los continentes, pero sin duda, Africa, Australia y en el Escudo hercinico de Europa occidental son las zonas mejor estudiadas, a partir de las cuales se han elaborado la mayor parte de las hipotesis sobre la evolucion y desarrollo de paisajes y formas graniticas. En lo que respecta a la Peninsula Iberica, el Macizo Hesperico o Iberico, y muy especialmente las zonas de Galicia, Portugal, Castilla y Leon, Extremadura y NO de Andalucia constituyen zonas de referencia para este campo de estudio de la geologia y de la geomorfologia. La trayectoria investigadora, y nivel internacional, de los autores de este libro avalan de antemano la calidad y proyeccion teorico-aplicada de sus contenidos.

The two-stage concept of landform and landscape development involving etching: origin, development and implications of an idea

Abstract Two-stage development of landforms has been appreciated for more than two centuries with respect to minor features, and major forms and landscapes have been viewed in similar terms for almost a hundred years. Early workers understood the significance of fractures as passages for water and, hence, as avenues of weathering, the tendency for weathering to produce rounded forms, the progression of weathering from the surface downwards, weathering contrasts between wet and dry sites, contrasted erosive susceptibility of weathered and unweathered rock, and reinforcement effects. Forms of deep and shallow derivation can be differentiated. By whatever name it is known—etch, double planation, subcutaneous, or two-stage—the concept is one of the most fruitful developed in the last century, for it bears not only on the origin of a wide range of landforms, but also on the crucial role of water and weathering, the age of landforms and landscapes, palaeogeographic reconstructions, climatic geomorphology and theories of landscape development.

Episodic exposure of inselbergs

Flared slopes, breaks of slope, tafoni, and gently inclined platforms occur together in the piedmont zone of many inselbergs. Similar assemblages of forms at various levels above the present hill-plain junction are interpreted as former piedmont zones that have been abandoned as the inselbergs have been episodically exposed. Correlation of these old piedmont assemblages with paleosurface remnants of adjacent plains and uplands suggests that parts of the inselbergs are tentatively dated as Mesozoic, and many of the lower whalebacks are considered to be of Tertiary age. Most of the discussion is concerned with Eyre Peninsula in South Australia, a region that diastrophically is and has been relatively stable and that has suffered progressive erosion.

Gondwanan (Late Jurassic and Cretaceous) palaeosurfaces of the Australian craton

Abstract Cretaceous, and especially Early Cretaceous, marine transgressions played an important part in the evolution of the Australian landscape. They covered at least 40% of the present Australian Craton and about 45% of the entire present continent. In so doing they preserved in unconformity extensive areas of the pre-existing or Gondwana landscape, though substantial Gondwana elements have been ehumed at the margins of the depositional basins as a result of the Cainozoic stripping of the sedimentary cover. Clear examples are demonstrated in the Isa Highlands, the Peake and Davenport Ranges, and the northermost Flinders Ranges. The basinal areas subsided under the weight of water and sediment, while adjacent land areas were uplifted (flexure continentale or Randschwelle). Those that were already of low relief were dissected, and regoliths were eroded but substantial areas of the Gondwana plains in etch form were preserved on divides, as in the Gawler and Hamersley ranges; others were reduced to low relief by rivers graded to Cretaceous shorelines, as in the southern and central Flinders Ranges. Elsewhere the Gondwana landscapes were preserved more or less intact, as in the western Yilgarn Craton, the Mt Lofty Ranges and possibly the higher ridges of the Macdonnell Ranges. The nature of the Gondwana landscape varied according to structure, but inselberg landscapes were widely developed, and have been exhumed, for instance in the western Pilbara and in the eastern Musgrave Block (Kulgera region). The Gondwana landscape was already complex with stratigraphic conditions suitable for the exhumation of Archaean, Preterozoic, and Cambrian forms, for instance, already in place; re-exposure later took place so that very old elements form integral parts of the present landscape. Exhumed Gondwana landscapes have been preserved by burial, and the stripping of some of the etch forms may have taken place in the later Cainozoic so that their survival poses few difficulties. Other etch surfaces, like those of the Gawler and the Hamersley ranges demonstrably were stripped in the Early Cretaceous and Eocene respectively, and they, like epigene earliest Tertiary and Mesozoic features, call for explanation. Various factors, like resistant bedrock, unequal erosion, surfaces which stand high and dry, interior situation, and reinforcement effects, go some way to relieving, though they do not resolve, the problem.

The age of the lateritized summit surface on Kangaroo Island and adjacent areas of South Australia

Abstract Stratigraphic, geomorphological, and radiometric evidence shows that the laterite of the high plains and plateau of Kangaroo Island is older than the Middle Jurassic but younger than the Early Permian. Palaeoclimatic and palaeontological considerations suggest the Triassic as the most likely age of both the laterite and the surface on which it is developed. High‐level lateritized surfaces in the adjacent Mount Lofty Ranges and southern Eyre Peninsula are of similar age.

Role of Salts in Development of Granitic Tafoni, South Australia

Fresh Proterozoic and Paleozoic granitic rocks in South Australia possess approximately 0.06% soluble salts (chiefly Ca, Na, S, and Cl), possibly in fluid inclusions. Active tafoni developed in these rocks show small-scale flaking on their ceiling and walls. Fresh and flaking granite are similar in all respects (chemistry, mineralogy, degree of alteration, texture, and structures), except that soluble salts are 2-13x more abundant in the flakes, where they occur as gypsum and halite crystals. An external source for these salts is ruled out by their chemical character. They are most likely a concentration of soluble substances present in the fresh rock and leached to a near-surface location when available moisture is evaporated. Tafoni configuration, distribution, and mineralogy argue against either thermal changes or wetting-and-drying being major contributors to the flaking process. We believe that salts cause flaking by crystal growth. Residual strain energy in the granites could help explain how only 0.4% salts can create 10-20% fracture porosity in an otherwise fresh rock.

The evolution of bornhardts in silicic volcanic rocks in the gawler ranges

Most bornhardts (or domical inselbergs) are developed in granitic rocks. Some have evolved in massive sediments (sandstone, conglomerate, limestone) but few, if any, have been reported from volcanic terrains. Yet bornhardts dominate the landscape of the Gawler Ranges, located in the interior of South Australia, where domes of dacite and rhyolite are developed on fracture‐defined blocks. Their profiles are associated with the development of sheet structure. Columns of rock are also prominent components of hillslope profiles. The volcanics were extruded in Middle Proterozoic times (1592 ± 2 Ma) and the columnar cooling joints and the orthogonal sets, the latter at various scales, developed soon afterwards. The sheet fractures developed after the columnar joints and probably later than the regional orthogonal joints; it has not been possible to determine their age but they pre‐date the Late Jurassic planation and deep weathering of the massif. The field evidence suggests that the bornhardts are etch forms th...

Relative and ‘absolute’ dating of land surfaces

Abstract The dating of land surfaces has long posed problems for geomorphologists. Relative methods (stratigraphic, geomorphic, topographic) are sound and convincing. Exhumed forms may complicate identification and relationships, for both epigene and etch forms have been buried, and exhumed, but in tectonically undisturbed areas, the higher surfaces are older than those preserved at lower levels. Also, surfaces have an age range. The relationship of surfaces with volcanic deposits, old shorelines, and genetically related sedimentary sequences provides sound ages, and correlation with dated duricrusts and faults is also useful. There are no temporal limits to relative dating, for the methods are equally applicable to the dating, say, of Proterozoic surfaces as of those of Pleistocene age. The disadvantage of such methods is that the necessary evidence is frequently either not preserved or not exposed. The so-called ‘absolute’ (physical, numerical) methods, and especially those based on exposure age dating with in situ cosmogenic radionuclides, are appealing because they produce direct numerical ages, and appear to be widely applicable, but there are severe temporal limitations, and sampling problems complicate, and may invalidate, interpretation. Absolute age determinations must be consistent with the stratigraphic and geomorphologic settings. The best results are obtained when physicists and earth scientists pool their knowledge and experience. A background in local and regional geology is especially important.

The subsurface initiation of some minor granite landforms

Abstract Flared slopes originate as concave weathering fronts beneath the regolith, and several other minor granite landforms found in association with them (tafoni, Rillen) may be initiated subsurface also. Other features, particularly incipient gnammas and gutters, have been observed on surfaces that have only recently been cleared of granite debris in situ.

On the multistage development of etch forms

Abstract Although the two stage concept is useful in the interpretation of many landforms and landscapes, it is an oversimplication of reality. The subsurface weathering that results in the development of the weathering front, and is thus critical to the evolution of etch forms, exploits magmatic, thermal, tectonic and sedimentary bedrock features at various scales, and of greater or lesser antiquity. Exposure of the weathering front leads to the development, diversification or destruction of bedrock forms. For these reasons it is more accurate and appropriate to view etch forms as complex and multistage features, some of which have their origins in the distant past, as well as in more recent weathering and erosion.