The Quequén Salado River Basin: geology and biochronostratigraphy of the Mio-Pliocene boundary in the southern Pampean plain, Argentina.

The Quequén Salado river basin has been the focus of several contributions since the first decades of the XX century, namely dealing with the general geological features of the deposits and with the vertebrate remains. In this paper, the Neogene geological history documented by the Quequén Salado river exposures is reconstructed by means of stratigraphic, sedimentological and paleomagnetic studies along with the paleontological analysis of vertebrate remains. The study area is a crucial setting not only to better understand the evolution of the southern Pampas basin during the late Miocene-early Pliocene interval, but also to test the validity of the biochronologic and biostratigraphic schemes, especially the “Irenense”. A geological model for the Quequén Salado river valley is proposed: a case of downcutting and headward erosion that contributes with a coherent interpretation to explain the spatial distribution of facies and fossil taxa: the younger in the distal sector of the Quequén Salado middle basin and the older in the lower basin. The sedimentary record is believed to represent the distal reaches of a distributary fluvial system that drained from the Ventania ranges. The stratigraphic section of Paso del Indio Rico results a key stratigraphic site to fully understand the stratigraphic nature of the boundary between the Miocene and the Pliocene (the Huayquerian and Montehermosan stages/ages). In this sense, two stratigraphically superposed range zones have been recognized in the area:<em>Xenodontomys ellipticus</em>Range Zone (latest Miocene-early Pliocene; late Huayquerian), and<em>Eumysops laeviplicatus</em>Range Zone (early Pliocene; Montehermosan). Taking into account the available geological and paleontological evidences, the “Irenense” would not represent a valid biostratigraphic unit, since, according to the geological model here proposed, it would be represented by elements of the<em>Xenodontomys ellipticus</em>Range Zone in the lower QS basin and by elements of the<em>Eumysops laeviplicatus</em>Range Zone in the middle QS basin

Quaternary tephra from the Valles caldera in the volcanic field of the Jemez Mountains of New Mexico identified in western Canada

A fine-grained, up to 3-m-thick tephra bed in southwestern Saskatchewan, herein named Duncairn tephra (Dt), is derived from an early Pleistocene eruption in the Jemez Mountains volcanic field of New Mexico, requiring a trajectory of northward tephra dispersal of 3c1500 km. An unusually low CaO content in its glass shards denies a source in the closer Yellowstone and Heise volcanic fields, whereas a Pleistocene tephra bed (LSMt) in the La Sal Mountains of Utah has a very similar glass chemistry to that of the Dt, supporting a more southerly source. Comprehensive characterization of these two distal tephra beds along with samples collected near the Valles caldera in New Mexico, including grain size, mineral assemblage, major-and trace-element composition of glass and minerals, paleomagnetism, and fission-track dating, justify this correlation. Two glass populations each exist in the Dt and LSMt. The proximal correlative of Dt1 is the plinian Tsankawi Pumice and co-ignimbritic ash of the first ignimbrite (Qbt1g) of the 1.24 Ma Tshirege Member of the Bandelier Tuff. The correlative of Dt2 and LSMt is the co-ignimbritic ash of Qbt2. Mixing of Dt1 and Dt2 probably occurred during northward transport in a jet stream

Mineralogy and chemistry of Late Pliocene-Early Pleistocene paleosols on Mount Kenya:Weathering indices of relative age and paleoenvironmental reconstruction

Sherpa Romeo green journal. Permission to archive accepted author manuscript.Iron and Al extracts as weathering indices in paleosols have been used in many localities to determine relative age, transformation of total chemical element concentrations to secondary forms, translocation of organicallycomplexed Al and long-standing inundation of soils with groundwater. On Mt. Kenya, a succession of paleosols straddling the Olduvai subchron are here analyzed to determine the degree to which Fe/Al extracts assist paleoenvironmental reconstruction, especially relative age determination, genesis and translocation of alteration products and the effect of paleoclimatic deterioration with the advent of glaciation ca. 2.0 Ma, and possibly before. Warmer/humid climate from the onset of the Plio-Pleistocene to the Olduvai subchron, thereafter reverting to a long episode of oscillating drier/wetter ice age perturbations is written into the profile morphologies, mineralogies and chemistries supporting earlier stratigraphic interpretationsYe