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Comment on: "Dry and cold climatic conditions in the formation of the present landscape in Southeastern Brazil. An interdisciplinary approach to a controversially discussed topic" (Authors: M. Lichte and H.Behling, Z. Geomorph. N. F. 43 (3), 341-358. Sept. 1999)
Iriondo, Martin; Kröhling, Daniela
Zeitschrift für Geomorphologie Volume 45 Issue 3 (2001), p. 401 - 402
5 references
published: Oct 9, 2001
ArtNo. ESP022004503017, Price: 29.00 €
Abstract
We would like to make a few comments to the interesting paper by Lichte & Behling (1999) on two controversially tropical issues. The authors made a bibliographic revision of the German literature on the Quaternary of Southeastern Brazil, arriving at the correct conclusion about the origin of tropical Quaternary covertures. In fact, the question is more general and appears in most tropical regions in the World. Persistently, the aeolian activity as a geomorphological agent in the tropics has been neglected and even denied. That is particularly mandatory when materials are red in colour. According to a dogma originated in the French pedological literature in the twenties “red = alterations”, no other processes are admitted. However, the sedimentary action of the wind is important in the tropics. Just think in the trade winds, harmattan and others. A rationale on this theme was published by Iriondo & Kröhling (1997) in the proceedings of the 30st International Geological Congress. Complementary advances produced by our group are: Iriondo et al. (1997), Iriondo (1999), Iriondo & Molinas (2000) and Iriondo & Kröhling (in press). Aeolian fine grained mantles, massive, yellowish ochre to dark red in colour, covering discordantly the landscape in some tropical lowlands are a distinct sediment type named “tropical loess”. Such sediments exist with other red bodies as alteration mantles, alluvial fans, colluvium, etc. Each of them can be (and normally is) red in colour. In short, fine aeolian sediments covering hills are widespread in SE Brazil, NE Argentina and north of Uruguay. The unique geological process able to put sediments on top of hills is the wind. The tropical loess covering Cretacic basalt in Argentina contains a sizable proportion of Zircon, which is forbidden in the geochemistry of basalt. That confirms the allochtonous origin of the sediment postulated by Lichte and Behling. On the other hand, the term stoneline is particularly unfortunate. And probably it is the origin of the intrincate discussion about its genesis since several decades: In all outcrops studied by us, and also in the photographs published by Lichte and Behling, the stonelines are simply basal conglomerates, a very common occurrence in Sedimentology. Besides Sedimentology, the accumulation mechanisms of the materials {stoneline and tropical loess) are confirmed by independ disciplines, such as Thermoluminescence Dating, Palinology, Geochemistry, etc. According to our observations, tropical loess in the region is generated by the accumulation of silt-sized particles and aggregates deflated from floodplains during dry seasons at the Last Glacial Maximum. It was proved that most of the present suspended load of the Paraná river is composed of silt-sized aggregates of colloids and clay particles.