The hidden passive margins from the birth of SW Gondwana

Abstract

The final consolidation of southwestern Gondwana during the Ediacaran-Cambrian resulted in the formation of a large landmass originally surrounded by the newborn Panthalassic Ocean. The Río de la Plata Craton is one of the last pieces that complete the geotectonic puzzle along the austral part of this supercontinent. However, the sedimentary record corresponding to the interval between the consolidation of SW Gondwana until the initial deposition in a large Ordovician cratonic basin is apparently missing. In this context, the Ediacaran-Cambrian epiclastic shallow-marine ramp of the Cerro Negro Formation, the uppermost unit of La Providencia Group, Tandilia System, is envisaged as the first known stratigraphic record of the hidden pericratonic basins of Gondwana. Geochronological and provenance data, coupled with robust stratigraphic and sedimentologic background, support that this unit keeps more tectono-sedimentary affinity with the Ordovician Balcarce Formation than the Neoproterozoic Sierras Bayas Group. The presence of similar Neoproterozoic source areas for both units, the absence of any metamorphism and deformation related to the Brasiliano cycle, and shallow marine deposition strongly suggest that the paleogeography and physiography of the original southernmost continental platform of Gondwana were similar to the Ordovician ones. Thus, the Cerro Negro Formation appears as the missing link between the ultimate stages of the Brasiliano Orogenic Cycle and the final stabilization of the continental platform from the SW part of Gondwana.

Deciphering the Importance of Mineralogical Changes in the Neoproterozoic Epeiric Seas through the Sedimentary Succession of Tandilia System: A Brief Review

“…In previous geochronological studies [63], sandstones showed a dominant concentration of Paleoproterozoic detrital zircons, accompanied by other populations of Archean to lowermost Paleoproterozoic and Mesoproterozoic ages [4]. Archean detrital zircons of this succession indicate a source of detritus from the Nico Perez Terrane (Uruguay), which is closer to Tandilia up to the present [64]. In addition, the lack of Neoproterozoic detrital zircons confirms the deposition in a cratonic margin, probably in an epeiric sea deepening to the east and south [4].…”

“…Geochronological data based on mineralogical, geochemical, and geochronology of detrital zircons, coupled with robust stratigraphic and sedimentologic background, support that this unit keeps more tectono-sedimentary affinity with Paleozoic sedimentary rocks of the Mar del Plata area (SE of Tandilia System) than the Neoproterozoic Sierras Bayas Group (NW Tandilia System, Figure 1). In addition, the absence of any metamorphism and deformation related to the Brazilian cycle and shallow marine deposition strongly suggest that the paleogeography and physiography of the original southernmost continental platform of Gondwana were similar in the latest Ediacaran and in the Paleozoic [64] (Figure 12).…”

“…The proportion of monocrystalline quartz also increased relative to polycrystalline quartz. Polycrystalline quartz is an abundant component of these rocks that can be considered a stable component when it comes from the cratonic igneous basement rocks as is in this case [62][63][64][65][66]. Opaque minerals such as hematite were identified as accessory minerals, along with rutile and rounded and fractured detrital zircons.…”

“…In addition, in Sequence VII subtle changes in the chemistry and mineralogy between the two facies successions were recently interpreted as a change from anoxic to suboxic seawater conditions [51]. The Cerro Negro Formation, Sequence VIII, is thought to be the first known stratigraphic record of the hidden passive margins from Gondwana, corresponding to an epiclastic shallow-marine ramp developed in the southern part of this supercontinent during the Ediacaran-Cambrian transition [64]. Geochronological data based on mineralogical, geochemical, and geochronology of detrital zircons, coupled with robust stratigraphic and sedimentologic background, support that this unit keeps more tectono-sedimentary affinity with Paleozoic sedimentary rocks of the Mar del Plata area (SE of Tandilia System) than the Neoproterozoic Sierras Bayas Group (NW Tandilia System, Figure 1).…”