No abstract

Palaeosols of the Koluel‐Kaike Formation, a red colour‐banded, pyroclastic succession from southern Argentina, constitute a proxy for Eocene climate changes. Reticulated and vertically elongated ferric mottles, along with iron and manganese nodules are the most significant climate indicators, which originated by alternating cycles of waterlogging and drying conditions causing Fe‐Mn mobilization and fixation. Clay minerals vary from a kaolinite > smectite suite in the lower and middle sections, to a smectite > kaolinite one in the upper part. High concentrations of iron oxides/hydroxides and kaolinite, lack of exchangeable bases, absence of carbonate cement, presence of ironstone and redness of hue in most of the palaeosols suggest intense chemical weathering related to leaching and lateritization processes. Five pedotypes, ordered in a stratigraphic sense, were identified. Strongly developed, red to orange Chornk (Fragiaquult) and Kápenk (Plinthaquult) pedotypes display argillic horizons, abundant ferric nodules and slickensides; they are dominant in the lower and middle sections, and formed in seasonal humid and megathermic (tropical) conditions with a mean annual precipitation of 1200 to 1300 mm and a mean annual temperature of 15 °C. Weakly developed, less structured Ornek (Vitrand) and Pólnek (Placaquand) pedotypes occur in the middle and upper sections, and originated in sub‐humid and mesic‐megathermic conditions with a mean annual precipitation around 1000 mm and a mean annual temperature around 12 °C. The brownish Soorsh (Argialboll) pedotype exhibits a granular structure and is present at the uppermost part of the unit. It developed in sub‐humid/semi‐arid and mesic conditions, with a mean annual precipitation of 600 to 700 mm and a mean annual temperature around 10 °C. This pedotype succession and clay mineral distribution indicates a decrease in chemical weathering and degree of soil development with time. Koluel‐Kaike palaeosols from Central Patagonia are some of the first continental non‐palaeobiological data linked to the Early Palaeogene global warming in South America; they show an especially close relationship with the Early Eocene Climatic Optimum and the following long‐term cooling and drying initiated by Middle to Late Eocene time.

The Río Chico Group in the San Jorge Basin of central Patagonia (Argentina) preserves some of South America's most significant Paleogene records of biotic and climatic change. Three of its constituent formations, the Peñas Coloradas, Las Flores, and Koluel-Kaike, host vertebrate faunas referred to the "Carodnia faunal zone," the Itaboraian South American Land Mammal Age (SALMA), and the Riochican SALMA. However, the precise absolute ages of these units, and thus their asso ciated faunas and paleoclimate records, are poorly resolved. Herein, we report new paleomagnetic and geochronologic results from these formations in south-central Chubut Province, Argentina. U-Pb dating of four volcanic ashes, using both laser abla tionmulticollector-inductively coupled plasmamass spectrometry and high-resolution chemical abrasion-isotope dilution-thermal ionization mass spectrometry, indicates ages of igneous crystallization of 51.403 ± 0.037 (0.045) [0.071] Ma for a level within the middle Las Flores Formation and 46.714 ± 0.012 (0.026) [0.057] Ma, 44.579 ± 0.013 (0.026) [0.054] Ma, and 42.856 ± 0.017 (0.027) [0.054] Ma for levels in the lower, middle, and upper Koluel-Kaike Formation, respectively. Combining these with previous isotopic ages in our new magnetostratigraphic framework, we correlate the Peñas Coloradas Formation to chrons C27n-26r (ca. 62.5 to ca. 61.6 Ma; late Danian) and the section from the middle Las Flores to the uppermost Koluel-Kaike to chrons C23n to C19r (ca. 51.4-42.2 Ma; mid Ypresian-late Lutetian). We combine these data with other recently published chronostratigraphic results from Paleogene units in Patagonia to better constrain the ages of noteworthy Paleogene plant and mammal fossil sites in Patagonia and to develop a revised temporal calibration of the Las Flores, Vacan, and "Sapoan" faunas.