Multiproxy studies of Early Miocene pedogenic calcretes in the Santa Cruz Formation of southern Patagonia, Argentina indicate the existence of a temperate warm vegetation adapted to a fluctuating water table

RAIGEMBORN, M. SOL; KRAPOVICKAS, VERÓNICA; BEILINSON, ELISA; GÓMEZ PERAL, LUCÍA E.; ZUCOL, ALEJANDRO F.; ZAPATA, LUCIANO; KAY, M. RICHARD F.; BARGO, M. SUSANA; VIZCAÍNO, SERGIO F.; SIAL, ALCIDES N.

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY

ELSEVIER SCIENCE BV

Año: 2018 vol. 500 p. 1 - 23

0031-0182

The Lower Miocene Santa Cruz Formation in southern Patagonia (Austral Basin, Argentina) contains severalhorizons of pedogenic calcretes, which record −17.5 myr old vegetation adapted to a shallow and fluctuatingwater table at paleolatitude of 56°S. To reconstruct the paleoenvironment, paleoclimate and paleoecosystem, weperformed a multiproxy study of the calcretes examining abiotic and biotic components. The calcretes exhibit avariety of morphologies (horizontal and vertical rhizoliths, laminar structure, nodules, massive crusts), microfabrics(Beta- predominant over Alpha-microfabrics), and δ18O and δ13C values that fluctuate within eachmorphotype and throughout the analysed interval. Microfossils and phytoliths in the host material of the calcretesindicate fluctuating terrestrial, freshwater, and marine conditions, and record an ecosystem dominated byherbaceous plants and arboreal elements in association with a typical coastal ?Santacrucian? vertebrate fauna.We propose that the calcretes developed in soils in a coastal/fluvial setting during pauses in floodplain aggradationthat typically lasted between 8?25 ka and 400 ka years. Variable sedimentation rates in different partsof the coastal/fluvial floodplain, the fine texture of the host sediment, and the influence of a fluctuating watertable also influenced the formation of the calcrete. A high water table in low relief areas of the floodplain createdthe conditions necessary to form a horizontally extended rhizolithic system that, jointly with the biotic proxy,can be correlated with a radicular pattern similar to the arboreal elements from coastal settings. Abiotic andbiotic proxies of the studied interval attest to environmental fluctuations recorded at different scales that tookplace under temperate warm and subhumid climates with a marked rainfall seasonality, with a slight increase inthe aridity towards the top of the studied interval. Under these conditions a subtropical fauna and a C3-dominatedecosystem developed coincident with the onset of the Mid-Miocene Climatic Optimum in Patagonia.