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

VERÓNICA KRAPOVICKAS; M SOL RAINGEMBORN; ELISA BEILINSON; ZAPATA LUCIANO; SABRINA LIZZOLI; JONATHAN PERRY; ESTEBAN PASSEGGI; ALEJANDRO FABIÁN ZUCOL; NESTOR TOLEDO; LUCÍA MARTEGANI

Latin American Journal of Sedimentology and Basin Analysis

Asociación Argentina de Sedimentología .

Lugar: La Plata; Año: 2018 vol. 25 p. 117 - 148

1669-7316

The middle and upper parts of the Lower Miocene Santa Cruz Formation (~17?15.9 Ma) in the southeastern Austral Basin (southern Patagonia, Argentina)crops out as a fluvial succession that in parts? is pedogenically modified. Thestudy of the paleosols of this unit combined with the study of ichnofossils,microremains, and fossil vertebrates present in these allows us to reconstructpast environmental, ecological, and climatic conditions, as well aspaleolandscape evolution of the Santa Cruz Formation during ~1 my. Thesereconstructions demonstrate three different stages during which very weak tomoderate pedogenesis took place. The first one (middle part of the unit) is anepiclastic distal floodplain bearing Calcisols or paleo-calcic Inceptisols, whichrecord a relatively dense vegetation integrating trees, shrubs, palms, and shortgrasses. It is a scarce soil fauna, in association with a vertebrate fauna typicalof coastal ?Santacrucian assemblages?. The second landscape stage (thebottom of the upper part of the unit) is composed of both epiclastic distalfloodplain and pyroclastic proximal settings that contain mainly Protosols orEntisols and Inceptisols analogues, with scarce Vertisols. The first group of soilsrecords a microthermic (C3) herbaceous vegetation of mainly short grasses, anda soil fauna composed of solitary bee larvae, soil beetles and their pupae,cicada nymphs, myriapods, and other soil insects, as well as oligochaeteannelids. However, the Vertisols of this stage record an ecosystem dominatedby grasses and palms adapted to arid conditions (C 4 plants) and by solitary beelarvae, adult and nymph soil beetles, cicada nymphs, myriapods, and other soilinsects.Finally, up-section, the third stage attests to the existence of an epiclastic andpyroclastic distal and proximal floodplain over which Protosols similar to thoseof the previous stage developed. Past ecosystems in this landscape resemblethose of the second stage: they supported grass vegetation, oligochaeteannelids and a moderately diverse insect soil fauna composed of solitary beelarvae, soil beetles and their pupae, cicada nymphs, and myriapods. Thiscombined abiotic (paleosols) and biotic (ichnofossils, microremains, andvertebrates) study indicates that several factors controlled the landscapeevolution of the Santa Cruz Formation during the early Miocene of southeasternPatagonia. Highly aggrading fluvial conditions, variations in the position in thefloodplain and in sedimentation/pedogenesis ratio, the input of pyroclasticmaterials, the length of landscape stability, changing hydrologic conditions, andfluctuations of wetter and drier phases in a context of relatively warm and humidclimate, seem to be the main factors controlling the landscape.