Tropical Loess of SE South America

KROHLING, DANIELA; IRIONDO, MARTIN

Lanzarote, Canary Islands, España

Workshop; “Lower Latitudes Loess – Dust Transport Past and Present”; 2006

Universität Bayreuth, UNESCO-IGCP500, INQUA Project 0509 (Lower Latitudes Loess)& INQUA Dryland Dating Group

Eolian fine-grained sedimentary deposits, dark red in colour and bounded at the base by erosive discordances mantle the landscape of large areas in tropical South America (the hilIs of NE Argentina, SE Brazil, eastern Paraguay, northern Uruguay and the lowlands of  Bolivia). This type of deposit, included in the so-called "red earths" and erroneously ascribed to other types of tropical products (e.g., laterites or colluvium), was named "tropical loess" by Iriondo and Kröhling (1997). The tropical loess of NE Argentina was formally defined as Oberá Formation. It is fomed by loam to silty loam, powderish, friable, porous and massive with a dark red colour (10R 3/6). It forms steep slopes in gullies, with  columnar disjunctions. It lies in an erosive unconformity on Cretaceous basalts and sandstones, ferricretes and Tertiary rocks, and has a typical thickness of 3-8 m. A generalized migration of iron occurred under a post-depositional savanna environment. An Ultisol, middle Holocene in age, is preserved in the middle section of the outcrops. The mineralogy of the modal sand fraction is characterized by rounded quartz with scarce volcanic glass, alterites and amorphous silica. Heavy minerals of the very fine sand fraction are dominated by magnetite and ilmenite. The clay fraction is composed of kaolinite and quartz with subordinate contributions of hematite and gibbsite. Tropical loess was generated  by the accumulation of silt-sized particles and aggregates mainly deflated from the alluvial plains of large rivers (Paraná, Paraguay and Uruguay) during the Last Glacial Maximum.