Paleocene-Eocene boundary in Salta Basin, northwest Argentina: stratigraphic and paleoclimatologic significance of a paleosol marker horizon

T, W.; CECILIA EUGENIA DEL PAPA; I, S.; M, A.; R, B.

Bariloche

Congreso; IV Congreso Latinoamericano de Sedimentología y XI Reunión Argentina de Sedimentología; 2006

Recognition of the Paleocene-Eocene boundary has become a priority in many sedimentary basins globally because of the importance for understanding warm episodes in Earth history, and because it was a time of widespread hydrocarbon source rock formation. In the Salta Basin, northwest Argentina, the Maiz Gordo Formation has long been known to contain the Paleocene-Eocene boundary but the boundary’s precise location within a sequence stratigraphic and facies construct has not been well constrained. Here we identify the Paleocene-Eocene boundary as existing within amalgamated lateritic gleysols in the upper part of the Maiz Gordo Formation. The amalgamated-paleosols marker zone is distinct from the calcic aridisols above and below it and is widespread in the basin across at least 200 kilometers in well-exposed sections. The paleosol zone also corresponds to coeval lacustrine highstand facies in the middle of the basin. We present two measured sections. At Quebrada de Los Conchos, approximately 1.0 meter of lateritic gleysol containing prominent pedotubules (likely rhizoconcretions) is split by 0.5 meter of pebbly sandstone/conglomerate, and immediately overlain by pebbly sandstone; a thick succession of calcic aridisols and fluvial sandstones overlies this interval. At Tin Tin, the interval is much thicker, consisting of a succession of up to thirty lateritic intervals culminating in 6 meters of lateritic gleysol containing the same prominent pedotubules we observed at Los Conchos; here too the succession is overlain by calcic aridisols and fluvial sandstones. We base our boundary pick on similar, well-studied, and in many cases well-dated, sections throughout North America and parts of Europe, and on a detailed literature compilation that includes sections on all continents except Antarctica. Our conceptual model for the development of this geosol (widespread coeval paleosol) is paleoclimatologically based; the late Paleocene-early Eocene represents the warmest event during the past at least 70 million years of Earth history during which time tropical soil-forming conditions expanded poleward in both hemispheres. The importance of our work is three-fold: 1)       provides a new site in the southern hemisphere for studying these globally widespread ancient warm Earth paleosols; 2)       establishes a continental marker horizon for use in other basins in South America where the Paleocene-Eocene boundary has also not been well constrained; 3)       aids understanding of any paleoclimatological controls on Paleogene Lake Maiz Gordo in which the lake highstand was attained during an episode of maximum planetary warmth and acceleration of the atmospheric hydrologic cycle.