Paleoenvironmental implications of two phosphogenic events in Neoproterozoic sedimentary successions of the Tandilia System, Argentina

摘要

Two phosphogenic events of potential economic importance (preserving P_2O_5 abundances up to 25 and 35%, respectively) are recognized in the Neoproterozoic Tandilia System. The older of the two lies atop a quartz-arkosic facies association of the Villa Mónica Formation. The age of this unit remains controversial, but has previously been considered as Tonian to Cryogenian based on stromatolite assemblages and carbon isotope trends; strontium isotope abundances of <0.7071 currently support an earliest Cryogenian age for the formation. The younger phosphate level lies at the base of the Cerro Negro Formation above a widespread karstic surface in a succession containing Cloudina, so is most-likely to be Ediacaran in age. In both cases, the phosphatic concretionary levels are related to relative sea-level fall and exposure that may be related to glacial eustacy. In order to reconstruct the paleoenvironment associated with the formation of these concretionary phosphates, we combined field observations with thin-section petrography, XRD, SEM and geochemical analyses. Total rare-earth element (REE) contents range from 311 to 1010 ppm in concretions from the Villa Mónica Formation and from 290 to 1471 ppm those from the Cerro Negro Formation. Villa Mónica concretions reveal no anomalies in Ce abundance, but clearly positive Eu anomalies (ranging from 1 to 1.4). These results suggest reduced conditions in the marine depositional environment. In contrast, Eu anomalies are not recorded in Cerro Negro concretions, while negative Ce anomalies, rangingfrom -0.14 to -0.18, are noted. These results are consistent with oxic seawater conditions in mixed platform facies. This study suggests that depositional conditions where phosphate was concentrated in the Neoproterozoic Tandilla System were markedly different.