Palaeosols of the Peñas Coloradas Formation: warm and humid conditions in the Late Palaeocene of Central Patagonia

M. KRAUSE, T. WHITE, M. RAIGEMBORN & E. BELLOSI 4

Mendoza, Argentina

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

The Peñas Coloradas Formation (Upper Palaeocene) is a thin (30 m) continental unit from Central Patagonia. It consists of sandstones, mudstones and tuffs deposited into a mixed-load fluvial system. Strongly-developed palaeosols are recognized as modifying floodplain and abandoned channel deposits. Three distinctive palaeosol types are analyzed for providing new palaeoenvironmental and palaeoclimatic interpretations of the Palaeocene of Patagonia. The Colicurá pedotype is a reddish tuffaeous palaeosol characterized by a complex ichnofabric.Three horizons are present, all lacking ped structure and having gradational boundaries. Bioturbation intensity varies through the profile, sparse to absent in the upper and lower horizons, and moderate in the middle one. An undifferentiated b-fabric is recognized in all horizons, but grano-striated fabric is also present in the middle horizon. Amorphous iron oxides are frequently observed. Clay coatings are especially abundant in the middle horizon.Alumina/bases ratio ranges from ~14, in the upper horizon, to ~ 5 in the middle-lower horizons. The Ba/Sr ratio is uniform and <0.7. The CIA-K and S values range from 89 to 96 and from 0.01 to 0.07 respectively. The clay fraction is composed of kaolinite (24-77%), smectite (5-74%), chlorite (<10%) and illite (2-6%). The Keyoit pedotype is a reddish grey palaeosol developed on tuffaceous sandstones. It is characterized by the presence ofmeter scale slickensides. Fine material (<0.062 mm) is almost absent. Two horizons with gradational contact are recognized. The upper one displays a granular structure, and frequent reticulated to polygonal mottles. Soil slickensides are restricted to this horizon. Rhizoliths and meniscate burrows are common. Micromorphological analysis shows abundant argillans, coating planes, pores and biogenic galleries. The lowermost horizon is massive and lacks mottling and recognizable pedofeatures. Alumina/bases ratio ranges from ~3, in the upper horizon, to ~6 in the lower one. The Ba/Sr ratio is ~1. The CIA-K value is higher than 90 and the S value is ~0.02. The Yatenteh pedotype is a brown palaeosol also developed on tuffaceous sandstones. Three horizons are recognized. The upper one shows granular structure with common iron nodules and rhizoliths. Results of micromorphological study show a poro- and grano-striated b-fabric along with abundant illuviated clay. The base is wavy andsharp. The middle horizon also shows granular structure with common passive-filled and meniscate burrows and rhizoliths. The most significant pedofeature is abundant illuviated clay. The basal contact is gradational. The lower horizon is massive, with common burrows and rhizolits in the upper part. Illuviated clay is present but in less abundance. Alumina/bases ratio ranges from ~7, in the upper horizon, to ~3 in the lower horizon. The Ba/Sr ratio ranges from 0.6 to 2. The CIA-K is ~90 and the S value is~ 0.01. Geochemical and clay mineral data suggest that these palaeosols represent deeply weathered soil profiles, typical of strongly-developed Oxisols and Ultisols, probably formed under warm and humid conditions. Weathering molecular ratio, the presence of well defined horizons, dominance of kaolinite, and abundant clay illuviation in some horizons (Bt horizon) are features characteristic of Ultisols. Other orders with argillic horizons such as Alfisols are ruled out because of thevery low base cation contents. The high CIA-K value near or higher than 90 suggests intense chemical weathering. The high concentration of kaolinite in the Colicurá pedotype supports this interpretation. The CIA-K and S values provide an estimate of the range in mean palaeoprecipitation between 1200 and 1400 mm/year, and a range in mean annual palaeotemperature from 15°C to 16°C. The Peñas Coloradas palaeosols are the oldest nonbiotic record of a Palaeocene warming event in Patagonia, probably related to the global Palaeocene-Eocene thermal maximum.