Magnetic Polarity and Fission-Track Chronology of a Late Pliocene–Pleistocene Paleoclimatic Proxy Record in the Tropical Andes

Two sections exposing a Late Pliocene–Pleistocene sedimentary sequence in the marginal valleys of the Bogotá Basin (Colombian Andes, South America) were sampled for paleomagnetic analysis. Magnetostratigraphy and fission-track dates of tephra beds provide a detailed geochronologic calibration for the sedimentary basin. Measurements of magnetic susceptibility complement the regional environmental record provided by lithological and palynological evidence. Sedimentation in the Bogota Basin started in the early part of the Gauss Chron at ca. 3.2 myr. The oldest recorded sediments belong to the Guasca Member of the Upper Tilatá Formation. They were deposited in a lacustrine/paludal environment, near the end of the tectonic uplift in the Bogotá area, and/or under Pliocene climatic conditions that were warmer than today. Repeated climate cooling associated with glaciations in the surrounding mountains resulted in the deposition of a fluvial–lacustrine complex referred to as the Subachoque Formation. The first glaciation is placed near the Gauss/Matuyama polarity reversal at 2.6 myr. A lithologic change in the Subachoque Formation marked by coarser-grained fluvial deposits and a possible increase in amplitude of the magnetic susceptibility signal occurs near the Matuyama/Brunhes boundary at 0.8 myr, indicating a shift toward higher magnitude climate oscillations.