New lithostratigraphic approach and regional correlation significance of the Piedras de Afilar Formation, Neoproterozoic of Uruguay

Abstract

The Piedras de Afilar Formation is part of the Tandilia Terrane, Precambrian of Uruguay. It is a Neoprotero- zoic sedimentary sequence, overlying with erosional unconformity the Soca granite (2054 ± 11Ma) and the Mon- tevideo Formation (ca. 2200 Ma). Three new members are defined from base to top: (1) Cuchilla Alta Member, which comprises whitish conglomerates, quartzitic sandstones and limestones, with fining- and thinning-upward cycles, asymmetric ripples, interference ripples, hummocky cross-stratification (HCS), trough cross stratification (TCS), as main sedimentary structures. Type area is located in the Cerros Piedras de Afilar; (2) Arroyo Junquito Member, which is made up of dark shales with plane-parallel and wavy bedding; and type area is at Imhoff Quarry (6143602.90 N, 64190.14 E UTM); and (3) Arroyo de la Tuna Member, which is composed by marls, limestones and shale/limestone rithmites with shales interbeded; its type area is at Heide Quarry (6142063.90 N, 63784.38 E UTM). The depositional environment of the Piedras de Afilar Formation is a shallow marine plat- form. Whereas the lower siliciclastic member is related to the backshore, the pelitic member is associated to a marine transgression and the calcareous member is related to a shallow carbonatic ramp. In respect to the age of the Piedras de Afilar Formation, the available data suggest a late Neoproterozoic age. U-Pb LA ICPMS data for detritic zircons of the Cuchilla Alta Member yielded ages of 990 Ma for the youngest zircon, constraining the maximum age of the sedimentary sequence. C isotopic results suggest that the Arroyo de la Tuna Member is Cryogenian Ediacaran in age, according with encountered values ranging between +5.05% and +5.80 %‰; 813C(PDB). These also suggests that this unit is probably correlative of the 813C results reported for lower Po- lanco Formation (+2.6 to +5.3 %%); (PDB); and for Loma Negra Formation (+2.7 to +4.5%). 87Sr/86Sr values around 0.7146, recently obtained for these limestones are too radiogenic and not suitable for age interpretation. The presence of a dolerite sill overlying the carbonates and the consequent fluids migration might possibly have affected the original contents of Sr in the limestones. Non carbonatic units of the Piedras de Afilar Formation are probably correlative of Yerbal Formation (lower Arroyo del Soldado Group).