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

Abstract

The Peñas Coloradas Formation (Upper Palaeocene) is a thin (30 m) continental unit from Central Patagonia. It consists of sandstone, mudstone and tuff deposited into a mixed-load fluvial system. Well-developed paleosols are recognized as modifying floodplain and abandoned channel deposits. Three distinctive palaeosol types are analyzed for providing new palaeoenvironmental and palaeoclimatic interpretations of Palaeocene Patagonia. The Colicurá pedotype is a reddish tuffaeous palaeosol characterized by a gruyere-like ichnofabric. Three horizons are present, all lacking ped structure and having gradationalboundaries. Bioturbation intensity varies through the profile, low to absent in the upper and lower horizons. In contrast the middle horizon shows a moderate to high abundance, reaching near 30%. 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 paleosol developed on tuffaceous sandstone. It is characterized by the presence of meter scale soil slickensides. Fine material (<0.062 mm) is almost absent. Two horizons are recognized in gradational contact. The upper 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 paleosol also developed on tuffaceous sandstone. Three horizons are recognized. The upper horizon shows granular structure with comon 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 and sharp. The middle horizon 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. In its upper part burrows and rhizolits are alsocommon. 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 the paleosols formed from deeply weathered soil profiles typical of strongly-developed Oxisols and Ultisols, probably formed under warm and humid paleoclimate conditons. . 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 the very 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 mm and 1400 mm and a range in mean annual palaeotemperature from 15°C to 16°C. This interpretation along with the age of the Peñas Coloradas Formation provides the evidence of a Palaeocene warming event in Patagonia, perhaps related to the Palaeocene-Eocene thermal maximum.