Paleosols and related soil-biota of the early Miocene Santa Cruz Formation (Austral-Magallanes Basin, Argentina): a multidisciplinary approach to reconstructing ancient terrestrial landscapes

A. F. ZUCOL; N. TOLEDO; L. MARTEGANI; V. KRAPOVICKAS; E. BEILINSON; S. LIZZOLI; E. PASSEGGI; M. S. RAIGEMBORN; L. ZAPATA; J. PERRY; D. E. TINEO

Latin American Journal of Sedimentary and Basin Analysis

ASOCIACIÓN ARGENTINA DE SEDIMENTOLOGÍA

Lugar: Buenos Aires; Año: 2019 vol. 25 p. 117 - 148

The middle and upper parts of the lower Miocene Santa Cruz Formation (~17?15.9Ma) in the southeastern Austral-Magallanes Basin (southern Patagonia, Argentina)crop out as a fluvial succession that in parts is pedogenically modified. The studyof the paleosols of this unit combined with the study of ichnofossils, microremains,and fossil vertebrates present in these allows us to reconstruct past environmental,ecological, and climatic conditions, as well as paleolandscape evolution of theSanta Cruz Formation during ~1 my. These reconstructions demonstrate threedifferent stages during which very weak to moderate pedogenesis took place. Thefirst one (middle part of the unit) is an epiclastic distal floodplain bearing Calcisolsor paleo-calcic Inceptisols, which record a relatively dense vegetation integratingtrees, shrubs, palms, and short grasses. Soil fauna is scarce and it is in associationwith a vertebrate fauna typical of coastal ?Santacrucian assemblages?. The secondlandscape stage (the bottom of the lowermost upper part of the unit) is composedof epiclastic distal floodplain areas and minor pyroclastic proximal floodplainsettings that contain mainly Vertisols. Vertisols record an ecosystem dominated bygrasses and palms adapted to variable conditions in hydric availability (C4 plants)and by solitary bee larvae, and adult and nymph soil beetles. Finally, up-section,the third stage attests to the existence of an epiclastic and pyroclastic distal andproximal floodplain over which Protosols developed. Past ecosystems supportedgrass vegetation (C3 plants), oligochaete annelids and a moderately diverse insectsoil fauna (interpreted by their trace fossils) composed of solitary bee larvae, soilbeetles and their pupae, cicada nymphs, and myriapods. This combined abiotic(paleosols) and biotic (ichnofossils, microremains, and vertebrates) study indicates that several factors controlled the landscape evolution during the early Mioceneof southeastern Patagonia. Highly aggrading fluvial conditions, variations in theposition in the floodplain and in sedimentation/pedogenesis ratio, the inputof pyroclastic materials, the length of landscape stability, changing hydrologicconditions, and fluctuations of wetter and drier phases in a context of relativelywarm and humid climate, seem to be the main factors controlling the landscape.