Paleosols of southern Patagonia as a tool to reconstructing Miocene landscapes

Abstract

The Lower Miocene Santa Cruz Formation in southern Patagonia, Argentina, crops out as a fluvial succession with stacked paleosols that present variable degree of development. The integration of sedimentological, paleopedological, ichnological, and paleobotanical (silica microremains) data of this unit provides a tool to reconstruct past environmental, ecological, and climatic conditions. Also, it helps to recreate ancient terrestrial landscapes and to establish the interplay of controlling factors in the evolution of these stages.On the basis of the spatial distribution of the paleosols of the Santa Cruz Formation throughout the analysed sections, three stages can be defined. The first scenery attests to the existence of an epiclastic distal floodplain bearing Calcisols (paleo-calcic Inceptisols), which record a relatively dense vegetation (trees, shrubs, palms, and short grasses) and a scarce soil fauna. Over this (second stage), both epiclastic distal floodplain and pyroclastic proximal settings bearing Protosols (Entisols and Inceptisols analogues) with scarce Vertisols developed. These Protosols record a C3 herbaceous vegetation of mainly short grasses, and a soil fauna composed of solitary bee larvae, soil beetles and their pupae, cicada nymphs, myriapods, other soil insects, and oligochaete annelids. Vertisols record an ecosystem dominated by C4 plants (grasses and palms), and by solitary bee larvae, adult and nymph soil beetles, cicada nymphs, myriapods, and other soil insects. Lastly (third stage), an epiclastic and pyroclastic distal and proximal floodplain environment bearing Protosols similar to those previously formed was developed. These Protosols supported grass vegetation, oligochaete annelids, and a moderately diverse insect soil fauna composed of solitary bee larvae, soil beetles and their pupae, cicada nymphs, and myriapods. The evolution of these landscapes took place under relatively warm and humid climates with seasonality in water availability.This integrated analysis of abiotic (sedimentology, paleosols) and biotic (soil mesofauna and flora) data of the Santa Cruz Formation allow to reconstruct three different terrestrial stages. These Miocene landscapes were controlled by highly aggrading fluvial conditions, different positions into the floodplain, variations in sedimentation/pedogenesis ratio, differences in volcaniclastic supply, length of landscape stability, changing hydrologic conditions, and fluctuations of wetter and drier phases.