Precordillera Basin Research Articles

Assessing the biodiversity of Ordovician ostracods from the Argentine Precordillera

Ostracod faunas from the Lower to Middle Ordovician rocks of the Argentine Precordillera Basin (Gualcamayo and Las Aguaditas Formations) are studied. A new family, Garcianidae, is erected. One new genus,Jeanvannieria, and six species are recognized, two of which are new (Jachalipisthia bicornataandJeanvannieria bulbosa). The diversity and composition of the Precordilleran ostracods is evaluated on the basis of previous taxonomic analysis and the fauna studied here. The diversity is moderate, with a peak of 50 species during the early Caradoc. The composition of the fauna is characterized by the dominance of podocopes with a high percentage of binodicopes and a lack of palaeocopes, which is in agreement with a deep shelf environment. The carbonate slope setting of the Las Aguaditas Formation is the deepest environment yet found with Ordovician ostracods and records a relatively diverse fauna. The presence ofEctoprimitioidessuggests biogeographic affinities between the Precordillera and Laurentia. The rest of the fauna contains a high percentage of endemic genera and a mixture of genera with several affinities, Baltic, peri-Gondwanan, and Australian.

Hierarchy of Factors Controlling Faunal Distribution: A Case Study from the Ordovician of the Argentine Precordillera

Abstract Limiting factors affecting faunal distribution in the Ordovician rocks of the Precordillera basin, western Argentina, are analyzed and tentatively established. Eight biotic intervals ranging from the Tremadoc to the Ashgill are defined based upon the distribution in time and space of articulate brachiopod, trilobite, sponge, and bryozoan genera. High magnitudes of turnover are recognized on the basis of vertical ranges of taxa; this observation also is supported by low indices of carryover and holdover. Times of high faunal change define boundaries between the biotic intervals. Abiotic constraints largely prevail over ecological factors in controlling most substantial shifts in faunal composition and distribution at the interval boundaries. Taxonomic composition through different intervals is consistent with a change in paleobiogeographic affinities throughout the Ordovician, from markedly Laurentian affinities in the lowermost Ordovician to increasing Gondwanic affinities towards the Middle and ...

C and Sr isotopic evolution of carbonate sequences in NW Argentina: Implications for a probable Precambrian-Cambrian transition

Upper Precambrian-Lower Cambrian sequences in the Tucuman, Salta and Jujuy provinces, NW Argentina, comprise sandstone, slate, conglomerate and black limestone (Las Tienditas/Volcan Fms.) with abundant Vendian/Tommotian trace fossils in the clastic facies rocks. The Precordillera basin, San Juan province, represents a continuous carbonate sequence belonging to the Lower to Middle Cambrian La Laja Fm. The Pie-de-Palo Range, Pampean Range, characterized by carbonates intercalated with greenschist/amphibolite facies metaclastic rocks, forms a part of the Precordillera basement. The δ13C values in carbonates of the Las Tienditas Fm. show a gradual decrease from the base (+3.4 ‰PDB) to the top with a minimum of −1.6‰ observed at ∼15m from the top, the latter having a higher clay content. Carbonates in a 700m thick section within La Laja Fm. is marked by a slightly positive δ13C values at the base (marly) with a negative anomaly (−2.0‰) at ∼20 m above, followed by a small positive anomaly (+0.5‰) ∼100 m from the base. All the values above this point are around −0.5‰ with a negative anomaly (−2.0‰) recorded at ∼240m above the base. Seawater87Sr/86Sr values define a non-monotonic increase (0.70870–0.71082) through the carbonates in Las Tienditas Fm. while the La Laja Fm carbonates vary from 0.70926 to 0.71030, with higher values at the base. C and Sr isotopes, thus suggest that the Las Tienditas carbonates record the Precambrian-Cambrian transition (∼15m from the top of studied section). The same is also evident at ∼30 m from the base of the La Laja Fm. The narrow range of δ13C variation (−1.4 to +1.3‰) and87Sr/86Sr in the 0.709–0.710 range for the Caucete Group carbonates of the Pie-de-Palo Range, although unequivocally, appear to be in consonance with a Vendian to Tommotian age, reinforced by the presence of the trace fossils Didymaulichnus and Gordia.

Caradoc bivalves from the Argentine precordillera: A local radiation-extinction event

Pebbles with a diverse shelly fauna, including bivalves, brachiopods, gastropods, and bryozoans havebeen found in the basal diamictite of the Don Braulio Formation, cropping out in the Sierra de Villicum, Argentine Precordillera. The Don Braulio Formation is of Ashgill age, but according to the brachiopods the fauna of the pebbles is of early Caradoc age. On the basis of this age and some elements in common, pebbles probably come from the underlying La Cantera Formation. Bivalves include nine genera, seven of which are new. The relatively high diversity and endemism of this fauna contrasts with the low diverse and widespread Caradocian associations from other regions of South America. The life span of this association is short. The Ashgillian beds of the Don Braulio Formation have yielded a different, low diverse association of bivalves. Causes of this brief radiation-extinction event affecting the bivalves are not clear. According to the recently proposed palaeobiogeographic reconstructions, during the early Caradoc the Precordillera attained the “pre-accretion stage”, and was placed near to the Gondwana continent. Consequently, a geographic isolation cannot be advocated to account for the high endemism of the bivalves. The principal cause of the endemism could be linked to climatic conditions. According to the global and local temperature curves, during the Caradoc an increase in water temperature occurred. Caradoc units of the Precordillera (Las Plantas and Las Aguaditas formations) include some carbonate deposits suggesting temperate waters. This could be the reason for the rapid diversification of the bivalves. Conversely, the dominantly clastic sedimentation of other Gondwanan areas such as the Central Andean basin of NW Argentina and Bolivia suggests that water temperature could have been cooler than in the Precordillera basin and that cold waters could have been a climatic barrier precluding expansion. The rapid extinction of the fauna could be linked to the progressive cooling during the Ashgill which culminates with the latest Ordovician glacial event.

“Fauna cámbrica” versus “fauna paleozoica” en el Ordovícico temprano del Oeste de Argentina: Interacción entre provincialismo y ambiente

The limestones of the San Juan Formation (Upper Tremadoc-Lower Llanvirn) in the Precordillera basin bear a group of communities dominated by sessile-suspension feeder organisms, including mainly brachiopods and sponges. The broadly contemporaneous siliciclastic Acoite (Lower-Middle Arenig) Formation in the Northwestern basin yield a succession of trilobite dominated communities. According to the global model of the three evolutionary faunas the former associations fit in the “Palaeozoic Fauna” and the latter could be referred to the “Cambrian Fauna”. Comparative analysis of structure and function of both types of communities, taking in account tiering strategies and the development of different guilds, demonstrates similar values of dominance diversity and richness as well as number of occupied guilds and tiers. Besides, trilobite dominated communities extend along different bathymetric ranges in the shelf environment -from offshore to the shoreface and even are represented in estuarine settings- without significant shifts in higher taxa proportions, showing a pattern of ecological stasis through time. Possible explanations for this pattern are derived from the ecological properties underlain by individualistic community histories, governed by the paleogeographic position of both areas during the Arenig times. We conclude that the three evolutionary faunas concept could be applied to equatorial and subequatorial areas. Cold water Gondwanan and peri-Gondwanan associations could be included in a new model of evolutionary faunas. For this purpose further tests in other Ordovician Gondwanan communities would be necessary.

An Ordovician sponge fauna from the San Juan Formation, Precordillera Basin, western Argentina

An Ordovician sponge fauna from the San Juan Formation, Precordillera Basin, western Argentina

Descripción y génesis de los bancos de oolitas ferruginosas en la base del Silúrico de la precordillera Argentina

Detailed outcrop and microscopic descriptions and current ideas about factors controlling the origin of oolitic ironstones coming from the Lower Silurian of the western Argentina Precordillera Basin are given. Chamosite and goethite comprise around 20 to 98 % of the oolitic ironstones. Phosphatic nodules are common components in these deposits. Lithofacies arrangement, outcrop architecture and bioturbation patterns allow to interpret them as shallow marine bars, which typically constitute parasequences boundaries. They are associated with widespread detrital facies. Their origin is related to sea-level fluctuations and a good correlation with global events can be established. The origin and distribution of the ironstones in the Precordillera basin is related with cool-water seas in high southern latitudes, during a high stand of sea-level, occurring after the Late Ordovician glacial event.

Tectofacies ordovícicas y evolución de la cuenca eopaleozoica de la precordillera Argentina

After a typical passive margin setting during the Cambrian, two basically different tectofacies evolved simultaneously and mostly independently during the Ordovician in the Argentinean Precordillera basin. The eastern domain characterizes epicontinental depositional systems developed on a stable margin, while the western domain is composed of deep-sea turbiditic systems which developed on a high subsidence «cuasioceanic» substratum. The two domains were set aside during the Ocloyic movements and later tectonic phases of the Caledonic cycle. A regional unconformity developed by upwarping of the former platform (eastern domain) conforms the downlaping surface of the Late Ordovician and Silurian siliciclastic shallow marine sequences. This regional doming is interpreted as a peripheral bulge in an incipient foreland basin caused by a type A subduction to the present west. This hypothesis is quite different from former ones and solves most of the stratigraphic and architectural problems during the Ordovician and Silurian Silurian and Devonian depocenters are mainly localized on the eastern domain and conform a succession of shallow marine sequences, which commonly characterize forelands. The different unconformities affecting the Silurian and Devonian deposits are interpreted as basin relaxation periods and as global sea-level oscillations. The Late Devonian Chanic diastrophysm involved an important shortening (definitive annexation of the two domains) and caused the main paleogeographic rearrangement, supported by the regional angular unconformity which separates the Lower Palaeozoic from the Upper Palaeozoic continental deposits.