Rio De La Plata Craton

Oxygenation of the Earth's atmosphere occurred in two major steps, near the beginning and near the end of the Proterozoic Eon (2500 to 542 Ma ago), but the details of this history are unclear. Chromium isotopes in iron-rich chemical sediments offer a potential to highlight fine-scale fluctuations in the oxygenation of the oceans and atmosphere and to add a further dimension in the use of redox-sensitive tracers to solve the question regarding fluctuations of atmospheric oxygen levels and their consequences for Earth's climate. We observe strong positive fractionations in Cr isotopes (δ 53 Cr up to +5.0‰) in iron-rich cherts and banded iron formation horizons within the Arroyo del Soldado Group (Ediacaran; Uruguay) that can be explained by rapid, effective oxidation of Fe(II)-rich surface waters. These fluctuations are correlated with variations in ratios of highly reactive iron (Fe HR) to total iron (Fe tot) which indicate a predominance of anoxic water columns (Fe HR /Fe tot >0.38) during the onset of oxidation pulses. We favor the scenario by which isotopically heavy Cr(VI) entered the basin after pulses of oxidative weathering on land and in which Fe(II) accumulated in the water column. Neodymium isotopes reveal that these oxygenation pulses were followed by increased influxes to the basin of continental crust-derived detrital components of Paleoproterozoic (Nd T DM model ages=2.1–2.2 Ga) provenance typical of the Rio de la Plata Craton. The association of positive δ 53 Cr–ferruginous (Fe HR /Fe tot >0.38) stratigraphic intervals with low-diversity acritarch assemblages dominated by Bavlinella faveolata strongly support models postulating a stratified, eutrophic Neoproterozoic ocean. Thus, even within a few million years of the Precambrian–Cambrian boundary, paleoceanographic conditions resembled more those of Paleoproterozoic oceans than Phanerozoic and present oceans. This highlights the sheer magnitude of ecological changes at the Precambrian–Cambrian transition, changes which ultimately led to the demise of the Precambrian world and the birth of the metazoan-dominated Phanerozoic.

Mesoproterozoic volcanosedimentary units and
tectonic events occurring in the Rı´o de la Plata Craton
(RPC) are reviewed. A belt consisting of volcanosedimentary
successions exhibiting greenschist-facies metamorphism
is exposed in the eastern RPC (Nico Pe´rez Terrane)
in Uruguay. The Parque UTE Group consists of basic volcanics
and gabbros at the base (1,492 ± 4 Ma, U–Pb on
zircon), carbonates in its middle part and interbedded
carbonates, shales and acid volcanics (1,429 ± 21 Ma,
U–Pb on zircon) at the top. The Mina Verdu´n Group is
made up of rhyolites and acid pyroclastics at its base and
top, and Conophyton-bearing limestones and massive dolostones
in the middle. A U–Pb LA-ICP MS zircon age of
1,433 ± 6 Ma is reported here for lapilli-tuffs at the base of
the Mina Verdu´n Group (Cerro de las Vı´boras Formation).
This age shows that the Mina Verdu´n Group immediately
postdates the Parque UTE Group, a fact supported by carbon
isotope chemostratigraphy. Both units were deformed
and metamorphosed between 1.25 and 1.20 Ga, as shown
by K–Ar and Ar–Ar ages. This tectonic event affected most
of the RPC and led to the accretion of the Nico Pe´rez
Terrane to the remainder of the RPC along the Sarandı´ del
Yı´ megashear. We report a U–Pb LA-ICP MS zircon age
(upper intercept) of 3,096 ± 45 Ma for metatonalites of the
La China Complex (Nico Pe´rez Terrane), which yield a
lower intercept age of 1,252 Ma. A proto-Andean, Mesoproterozoic
belt is envisaged to account for abundant
Mesoproterozoic detrital zircon ages occurring in Ediacaran
sandstones of the RPC. If the RPC is fringed at both sides by
Mesoproterozoic, Grenville-aged belts it is likely that it
occupied a rather central position in Rodinia. A possible
location between Laurentia and the Kalahari Craton, and to
the south of Amazonia, is suggested.

Two main areas in the Río de la Plata Craton (RPC) show important exposures of Neoproterozoic sedimentary successions, comprising both siliciclastic and carbonate rocks: the Nico Pérez Terrane in Uruguay and Tandilia Terrane in Argentina. The aim of this chapter is to describe the lithostratigraphy of the Neoproterozoic and Early Palaeozoic successions occurring there.

The Río de la Plata Craton (RPC) comprises extensive Neoproterozoic sedimentary successions cropping out mainly in Uruguay and Argentina. The lithostratigraphy and depositional environments of the Las Ventanas, Piedras de Afilar, Cerro Negro and Balcarce formations, and Mina Verdún, Sierras Bayas and Arroyo del Soldado groups is reviewed and summarised. Most of these units were deposited in the Ediacaran, except for the Mina Verdún Group and the lower Sierras Bayas Group, which may be assigned to the late Mesoproterozoic-Tonian. The early Ediacaran Las Ventanas and Playa Hermosa formations are interpreted as rift deposits. The Colombo diamictite in the Sierras Bayas Group is a likely equivalent of the latter units. The Arroyo del Soldado and upper Sierras Bayas Group were deposited in an extensive passive margin developed on the eastern RPC in the late Ediacaran. The Late Ordovician to Early Silurian Balcarce Formation was deposited in an epicontinental, shallow-marine shelf.