Recent advances in the paleontology, geochemistry and geochronology of terminal Neoproterozoic successions worldwide have contributed substantially to the calibration of the Ediacaran-Cambrian timescale. The majority of new data is reported from Ediacaran successions in North America, Asia and Africa. South American successions, in contrast, still lack reliable radiometric ages, hampering intercontinental correlations and impeding the detailed characterization of the Ediacaran-Cambrian transition for SW Gondwana. Here, we present several SHRIMP zircon ages from the Itapucumi Group, Paraguay, tied to a solid stratigraphic, palaeontological and isotopic framework.

The Itapucumi Group occurs extensively in northeastern Paraguay close to the Paraguay River, with the thin sedimentary package deposited directly on the Paleoproterozoic basement of the Rio Apa Craton. The entire stratigraphic thickness of this unit is about 400 m, comprising siliciclastic/volcanic rocks of the basal Vallemí Formation, overlain by limestones and dolostones of the Camba Jhopo and Tagatiya Guazu formations, and capped by carbonates and siliciclastic rocks of the Cerro Curuzu Formation. The Itapucumi Group is interpreted as deposited in a homoclinal, rimmed carbonate ramp that faced a (paleo) western ocean, presenting an extensive protected lagoonal environment in which microbialites and Ediacaran skeletal organisms thrived (Warren et al., 2017). Recently published carbon isotope data from unaltered carbonates of the Itapucumi Group show mean values of +1.9% δ¹³ VCPDB and ⁸⁷Sr/⁸⁶Sr ratios of 0.708604 (Warren et al., 2019). The consistent positive δ¹³C plateau in the entire stratigraphic succession and the ⁸⁷Sr/⁸⁶Sr values are typical of the end of Ediacaran Period ca. 550 Ma. In addition, the presence of Cloudina index fossil in the Tagatiya Guazu Formation reinforces this age and confirms a depositional age between 550 and ~539 Ma (Linnemann et al., 2019) for the oldest carbonate rocks of the Itapucumi Group. However, fossils are apparently absent from the overlying Camba Jhopo and entire Cerro Curuzu formations, so the depositional age of the uppermost Itapucumi Group is constrained only by chemostratigraphic data. Similarly, the depositional age of the siliciclastic rocks of the basal Vallemi Formation is unconstrained by biostratigraphy. In order to provide new geochronological constraints for these rocks, we analyzed 78 zircon grains using U-Pb Sensitive High Resolution Ion Microprobe (SHRIMP), targeting three stratigraphic intervals of the Itapucumi Group (Fig. 1). A total of 37 detrital rounded zircons were analyzed from coarse sandstones (8 zircons from sample VLM28C) and red fine sandstones (29 zircons from sample VLM53D) samples from the base and top of the Vallemi Formation, respectively. Several small, euhedral volcanic zircons were also collected from an ash bed (41 zircons from sample VLM11) located in the top of the Cerro Curuzu Formation (Figs. 2A and B). The rock samples were processed by standard methods for zircon grain separation and the U and Pb isotope ratios in zircon grains were measured in a High Resolution Ion Micro Probe (SHRIMP II) equipment housed at Curtin University, Australia. Reported age constraints are based on analyses of zircon with <10% discordance.

The detrital zircon ages obtained for the Vallemi Formation vary between minimum and maximum of 598±13 Ma and 2590±49 Ma, respectively, with a well-defined age peaks around 600 Ma and 900 Ma, indicating the predominance of Neoproterozoic source regions. Paleoproterozoic and Archean zircons correspond to material sourced from igneous and high-grade metamorphic basement units from the Rio Apa Craton (e. g. Alumiador and Centurion suites and Apa Basal Complex, Cordani et al., 2010).

The maximum depositional age of the Vallemi Formation (base of the Itapucumi Group) corresponds to the minimum age obtained for the detrital zircon assemblage (598 ± 13 Ma).

The volcanic zircon grains from the ash bed located at the top of the Cerro Curuzu Formation present ages varying between a maximum of 2092 ± 15 Ma and a minimum of 498 ± 19 Ma. The main population (29 grains) of Archean and Palaeoproterozoic zircons were considered as inherited from units of the regional basement of the Rio Apa Craton.

A total of 12 volcanic zircon grains presenting early Cambrian to Ediacaran ages varying between 537±10 Ma and 565±10 Ma provided a “concordia” age of about 545±4.5 Ma (Fig. 1), placing the deposition age of the top of Itapucumi Group close to the Ediacaran-Cambrian transition. If the youngest analyzed zircon is taken as significant (498 ± 19 Ma), it would indicate deposition in the late Cambrian. Considering the regional deformation of Itapucumi and Corumbá basins took place at approximately 528-490 Ma ago (Tohver et al., 2010), it is plausible that the upper part of the Itapucumi Group was deposited into a foreland basin near the end of the Brasiliano. Thus, the paleontological, isotopic and geochronological data support the paleogeographical and temporal correlation between Itapucumi (Paraguay) and Corumbá (Brazil) groups, suggesting a coeval sedimentary evolution of these units in the margins of the Amazon Craton and the Rio Apa Block.

Last but not least, the presence of volcanic zircons constraining an Ediacaran age at the top of the Itapucumi Group opens an innovative perspective for paleontological and ichnological investigations in this unit and help to calibrate the Ediacaran-Cambrian transition in the SW Gondwana.

Several SHRIMP zircon ages from the Itapucumi Group, Paraguay, tied to a solid stratigraphic, palaeontological and isotopic framework, are presented here. A total of 12 volcanic zircon grains presenting early Cambrian to Ediacaran ages varying between 537±10 Ma and 565±10 Ma provided a “concordia” age of about 545±4.5 Ma, placing the deposition age of the top of Itapucumi Group close to the Ediacaran-Cambrian transition.