Pablo Pazos

The southern part of the Ñirihuau basin at the valley of the Cushamen River region, is deformed and uplifted forming a prominent Precordillera Patagónica fold and thrust belt, east of the Patagonian Andes. The Ñirihuau Basin infill is represented by Oligocene to Miocene sedimentary to volcaniclastic rocks gathered in eleven lithotypes belonging to four members. These foreland sequences are exposed by a combination of thin-skinned deformation whose shortening is transferred to the basement in the west beneath the Cordón del Maitén. The entire Ñirihuau section as well as the overlying Collón Cura Formation is characterized by the occurrence of progressive unconformities, fact that implies synorogenic sedimentation at the time of the Cordón del Maitén range uplift. This belt is formed by an east-verging basement wedge associated with a series of backthrusts. The reactivation of the deformation associated with growth strata, indicates that the of a foreland system in different pulses of...

The association of trace fossils from the late Silurian Río Seco de los Castaños Formation in central-western
Argentina (San Rafael Block) is mainly composed of Dictyodora, including Dictyodora scotica, Dictyodora tenuis
and a new ichnospecies named Dictyodora atuelica. The latter shows a tendency to migrate to a deeper level –
analogous to Dictyodora liebeana's – which reflects more sophisticated behavioural patterns. The Dictyodora
ichnospecies (one of the most diverse assemblages in Gondwana) coexists with Nereites. SEM observations
and EDS analyses suggest that the wall structure of Dictyodora could be the result of an activity other than the
traditional suggestion of respiration, e.g., feeding. The succession, including some of the levels containing
Dictyodora, presents microbially induced sedimentary structures (MISS) that would indicate a photic zone.
This discovery favours previous deltaic interpretations instead of the deep-sea fan deposits traditionally
suggested for the Siluro-Devonian greywackes of the Cuyania terrane, and particularly for this unit. The abundant
Dictyodora, Nereites, Zoophycos and undermat miners ichnofossils, as well as the scarce arthropod trackways,
resemble older records in Europe and North America. The studied section is very well age-constrained, and the
autochthonous Gondwana ichnofauna permits to cast doubt on previous interpretations, suggesting that after
the Ordovician, microbial mats retreated to marginal marine environments and then spread later in the
Carboniferous. More importantly, the occurrence of Dictyodora and Nereites in beds that bear abundant wrinkle
structures or records of biofilms in someplaces is challengingwith regard to deep-sea interpretations. Dictyodora
is probably not a good stratigraphic indicator, and the provincialism suggested for this ichnogenus may be an
artefact of the lack of more complete records in Gondwana.

The U–Pb and Lu–Hf isotopic analyses of the different sedimentary sequences of the Ventania System, an old
Paleozoic orogenic belt exposed in the southern region of the Río de la Plata Craton in the province of Buenos
Aires, Argentina, provide new evidence for the understanding of the tectonic evolution of the western sector of
the Gondwanides mountain belt. These ranges formed as result of the late Paleozoic collision of the Patagonia
terrane against the continental margin of Gondwana. The provenance analysis together with the sedimentary
paleocurrents confirm a dominant source from the Tandilia System, a Paleoproterozoic mountain belt formed
during the amalgamation of the Río de la Plata Craton at about 1800–2200 Ma, and incorporated to Western
Gondwana during the Brasiliano Orogeny at 550–530 Ma. The local dominant source at the base of the early
Paleozoic changed to more distant supplies toward the top of the sequences, when an increasing participation
of detritus from first, Cambrian (560–520 Ma) zircons from the Pampean Orogen, and later on Ordovician
(480–460 Ma) zircons from the Famatinian Orogen is recorded. The detrital zircon patterns and the maximum
age of the units shed light on some previous discrepancies in the early Paleozoic stratigraphy. The Balcarce Formation,
an early Paleozoic sedimentary cover of the Tandilia metamorphic and igneous basement, shows striking
differences when compared with the new data from the Ventania System. The two data-sets reveal different
sources for the two regions. The late Paleozoic foreland basin deposits mark an abrupt change of 180° in the
paleocurrent directions, in the petrographic composition of the sediments, and in the provenance of detrital zircons.
These data indicate a southern provenance with the first evidence of Carboniferous and Permian magmatic
zircons. The oldest Archean zircons together with the finding of clasts with archeocyathids support the provenance
from Patagonia, which was derived from Eastern Gondwana. The U–Pb ages of the ash-fall tuffs in the
Tunas Formation confirm the Early Permian age of the Eurydesma Fauna in the Ventania System. The U–Pb
data together with the Lu–Hf isotopic data enhance the comprehension of the tectonic evolution of the Ventania
System as part of the larger Gondwanides Belt that amalgamated to Western Gondwana during Late Permian
times with some independent pieces derived from Eastern Gondwana.