The Piedras de Afilar Formation (Neoproterozoic, Uruguay): Sedimentology and provenance of a key unit for SW-Gondwana paleogeography

Abstract

The Piedras de Afilar Formation (PAFm) is a late Neoproterozoic sedimentary succession autochthonous to the Tandilia Terrane, part of the Río de la Plata Craton. It overlies with erosional unconformity a Paleoproterozoic igneous and metamorphic basement. According to facies analysis and lithostratigraphy, the PAFm is divided into three members for which detailed petrography, X-ray diffractometry and geochemistry studies were carried out: (1) the Cuchilla Alta Member, made up of yellowish-white quartz arenites and subarkoses with through, planar and hummocky cross-stratification, ripples, interference ripples and flute casts; (2) the Arroyo del Junquito Member, comprising dark grey, red and green shales and siltstones, with wavy lamination and wrinkle structures; and (3) the Arroyo de la Tuna Member, made up of grey limestones, grey to green marls, rhythmites and dark shales. Dolerite sills, up to 50 m thick, intrude the succession, and affect clay mineralogy and rare earth composition near the contact. A deepening, fining- and thining-upward sequence in a shallow marine paleoenvironment is evidenced by facies analysis. Ce anomalies are negative at the base of the Arroyo del Junquito Member and become positive up section, indicating increasingly anoxic conditions. Paleocurrents show a bimodal oblique pattern, with predominant paleocurrents to the north-east and subordinately to the east, suggesting a provenance from the west, which agree with detrital zircon interpretation. Sm–Nd analyses of shales yielded TDM model ages between 2.53 and 1.55 Ga, which indicate a mixed provenance from Paleoproterozoic units of the Río de la Plata Craton and from a proto-Andean, Mesoproterozoic belt at its western border, in accordance with available detrital zircon ages for sandstones of the PAFm. The influence of a recycled orogen as source area is also shown by sandstone petrography and trace element composition of shales. The correlation with other Ediacaran successions of the Río de la Plata Craton, such as the lower Arroyo del Soldado Group in Uruguay and lower Sierras Bayas Group in Argentina, is strengthened by the new data. All units were deposited on a stable shelf developed at the eastern margin of the Río de la Plata Craton.