CICTERRA   20351
CENTRO DE INVESTIGACIONES EN CIENCIAS DE LA TIERRA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
The Late Devonian glaciation in western Argentina: expanding the frontiers of Gondwanan glacial epoch.
Autor/es:
EZPELETA M. Y ASTINI, R.A.
Lugar:
Bochum, Alemania
Reunión:
Congreso; SEPM-CES Annual Meeting 2008; 2008
Resumen:
Unquestionable Devonian glacial record has
not been documented in Argentina. The
absence of Upper Devonian Lower
Carboniferous deposits has been attributed to
tectonic causes, linked with the collision of
Chilenia Terrane along the Gondwana margin,
or to the combined action of tectonism and
glaciation during the late Early Carboniferous.
In the Las Minitas Ranges, northern
Precordillera, a notable record of diamictites
within a strongly folded marine sequence
(Jagüel Formation) is recorded. Bimodal
igneous rocks bracketed Late Devonian-Early
Carboniferous (Ar-Ar ages ca. 346-364 Ma,
Coughlin 2000), where folded together with
these rocks and both have been truncated in
angular unconformity before deposition of
Late Carboniferous deltaic systems (Río del
Peñón Formation). Devonian trilobites
recorded elsewhere in the Precordillera and
associated brachiopod and bivalve faunas
occur below and above this interval and
certifie the Devonian age of the interval.
Gray-greenish massive and stratified
diamictites and pebbly mudstones (~70 m)
including a variety of suspended boulders, are
sharply overlain by a well stratified interval of
bioturbated silty shales and graded beds
yielding marine faunas. Diamictites possess a
massive and meter scale interval at the base
with subtle textural changes and different
concentrations of suspended clasts inside the
matrix (Dmm). Suspended subrounded to
subangular blocks (> 1m) yield polished and
striated surfaces. Well rounded boulders with
flattened faces (flat-iron) or bullet-shapes are
also common. All these features indicate that
the sediment was partly involved within the
subglacial load . Stratified diamictites (Dms),
interbedded with the Dmm develop as
interbedded intervals. Dms show fine-grained
partings and contain abundant dropstones that
commonly exceed bed thickness. The facies
may be indicating water-lain processes where
thin fine-grained partings are the product of
residual settling after more massive rain out.
Oversized clasts are clearly dropstones and
show impact associated structures. Facies
association also contain intervals (<2.5 m) of
laminated mudstones with dispersed
dropstones and sandy intervals (1.5 m)
composed by groupings of tabular graded to
massive layers separated by thoroughly
bioturbaded mud partitions. Internal features
are compatible with gravity flows (turbidity
current deposits). Some of the sandy beds
show subtle coarser grains on the top or pebble
concentrations indicating winowing effects and
rework by diluted bottom currents.
Diamictites are locally involved within
slumped and folded intervals (~17 m) linked
with coherent flow processes (flow tills?).
Clast composition is dominated by highly
compacted sandstones, limestones, and basic
rocks derived from underlying Ordovician
units cropping in the nearby region. These
compositions clearly differ from Nammurian
diamictites where high-grade metamorphic
rocks and granitic boulders are dominant.
Paleontological and geochronological record
allow constraining this glaciomarine sequence
to Late Devonian and contribute to separate
this glacial record from the overlying Río del
Peñón Formation recording the Nammurian
glaciation, immediately above the angular
unconformity. The deformation and the
bimodal igneous dykes that affect the
Devonian record do not affect the Río del
Peñón Formation, indicating an important
tectonic event also explaining the angular
unconformity between both.
Glacial tillite included in the Jagüel Formation
allows expanding the area covered by the Late
Devonian Glaciation in South America,
previously recorded in Brasil and along the
Central Andes.