Some concepts on gondwana landscapes: Long-term landscape evolution, genesis, distribution and age

Abstract

The concept of “Gondwana Landscape” was defined by Fairbridge (The encyclopedia of geomorphology. Reinhold Book Corporation, New York, p. 483, 1968) as an “ancestral landscape” composed of “series of once-planed remnants” that “record traces of older planation” episodes during the “late Mesozoic (locally Jurassic or Cretaceous)". This has been called the “Gondwana cyclic land surface” in the continents of the southern hemisphere, occurring extensively in Australia, Southern Africa and the cratonic areas of South America. Remnants of these surfaces are found also in India, and it is assumed they have been preserved in Eastern Antarctica, underneath the Antarctic ice sheet which covers that region with an average thickness of 3,000 m. These paleolandscapes were generated when the former Gondwana supercontinent was still in place and similar tectonic conditions in its drifted fragments have allowed their preservation. In Pangaea, remnants of equivalent surfaces, though of very fragmentary condition, have been described in Europe and the United States, south of the Pleistocene glaciation boundary. These Gondwana planation surfaces are characteristic of cratonic regions, which have survived in the landscape without being covered by marine sediments along extremely long periods, having been exposed to long-term subaerial weathering and denudation. Their genesis is related to extremely humid and warm paleoclimates of “hyper-tropical” nature, with permanently water saturated soils, or perhaps extreme climates, with seasonal and long-term cyclic fluctuations, from extremely wet to extremely dry. Deep chemical weathering is the dominant geomorphological process, with the development of enormously deep weathering profiles, perhaps of up to many hundreds of metres deep. The weathering products are clays, in some cases kaolinite, pure quartz and other silica types sands, elimination of all other minerals and duricrust formation, such as ferricretes (iron), silcretes (silica) and calcretes (calcium carbonate). Mean annual precipitation in these periods would have been perhaps higher than 10,000 mm, with extremely high, mean annual temperatures, such as 25-30 °C. These deep weathering processes can be achieved only under extremely stable tectonic and climatic conditions. The geomorphological processes continued with fluvial removal of the weathering products in wet climates and with hydro-eolian deflation in the areas with strong climatic seasonality. The final landform products of these deep weathering systems are planation surfaces, inselbergs, bornhardts, duricrust remnants covering tablelands, associated pediments, granite weathered landscape, etc. Some concepts related of these ancient landform systems were theoretical, developed by Walther Penck in the early twentieth century. The Gondwana Landscapes were studied by Alexander Du Toit and Lester C. King in Africa and more recently, by Timothy Partridge and Rodney Maud in South Africa, C. Rowland Twidale and Cliff Ollier in Australia and Lester C. King and João José Bigarella in Brazil, among others. Both in Australia and Southern Africa, these landform systems have been identified as formed in the Middle to Late Jurassic, throughout the Cretaceous and, in some cases, extending into the Paleogene, when Gondwana was still only partially dismembered.