Cosmopolitan Early Jurassic Marine Gastropods from West-Central Patagonia, Argentina

A new, relatively diverse gastropod fauna is reported from the Chubut province of west−central Patagonia. The gastro− pod association at the “El Córdoba ” fossiliferous locality (Lower Toarcian of Osta Arena Formation) consists of three new species: the eucyclid Amberleya? espinosa sp. nov. and two procerithiids Cryptaulax damboreneae sp. nov. and Cryptaulax nulloi sp. nov. Other members of the association are the ataphrid Striatoconulus sp., discohelicid Colpom− phalus? sp., and an undetermined zygopleurid. Knowledge on Early Jurassic gastropods from South America and other southern continents is reviewed to show that the taxonomic composition of the El Cordoba association strongly resem− bles other gastropod associations of this age (even those from Europe), suggesting a wide distribution of cosmopolita

Early Cambrian archaeocyathan limestone blocks in low-grade meta-conglomerate from El Jagüelito Formation (Sierra Grande, Río Negro, Argentina

Massive grey limestone blocks containing a fairly diverse but poorly preserved archaeocyath fauna were recovered from a meta-conglomerate bed in the El Jagüelito Formation (Sierra Grande area, Eastern North Patagonian Massif, Río Negro, Argentina). This is the first documented reference of the presence of archaeocyaths in continental Argentina. Seven different taxa were identified, preliminary described and figured. Recrystallization of the skeletons due to regional low-grade metamorphism and deformation of the unit does not allow observation of key detailed features and prevents identification to genera and species. Nevertheless, the specimens studied show general affinities with archaeocyathan assemblages from the Australia-Antarctica palaeobiogeographic province and indicate a middle Early Cambrian (Atdabanian-Botomian) maximum age for the deposition of the El Jagüelito Formation protoliths. The similarities between the North Patagonian Early Paleozoic El Jagüelito Formation and those rocks from Antarctica suggest a geologic and biologic common history of these regions on the same southwest margin of Gondwana during that time

A Comparison of Jurassic Marine Benthonic Faunas from South-America and New-Zealand

Fil: Damborenea, Susana Ester. División Paleontología Invertebrados. Facultad de Ciencias Naturales y Museo. Universidad Nacional de La Plata; ArgentinaFil: Manceñido, Miguel Oscar. División Paleontología Invertebrados. Facultad de Ciencias Naturales y Museo. Universidad Nacional de La Plata; Argentin

First record of the Early Toarcian Oceanic Anoxic Event from the Southern Hemisphere, Neuquén Basin, Argentina

<p>The first record of the Early Toarcian Oceanic Anoxic Event (<em>c</em>. 183 Ma) from the Southern Hemisphere is described from the Neuquén Basin, Argentina, identified chemostratigraphically on the basis of a relative increase in marine organic carbon and a characteristic negative carbon-isotope excursion (δ¹³C_org) in bulk rock and fossil wood. The negative excursion of −6‰ in bulk organic carbon (falling to −31.3‰) crosses the boundary of the <em>tenuicostatum</em>–<em>hoelderi</em> Andean ammonite Zones, equivalent to the <em>tenuicostatum</em>–<em>falciferum</em>/<em>serpentinum</em> zones of Europe. These data indicate that the Early Toarcian Oceanic Anoxic Event was a global phenomenon. </p

The Vaca Muerta transgression (Upper Jurassic), Neuquén Basin, Argentina: Insights into the evolution and timing of aeolian-marine transitions

Considering the evolution of aeolian to marine transitions for the geological record, either catastrophic or gradual transgressive scenarios showing high or low rates of coastal migration have been proposed. A critical evaluation of modern analogues suggests that a catastrophic transgression shares many characteristics with Holocene transgressions, yet they are caused by different rates of sea-level rise. The present study provides insights into the evolution of aeolian to marine transitions in order to discuss these alternative scenarios of sea-level rise. For this purpose, a sedimentological and ichnological analysis was carried out on ten stratigraphic sections of the Picún Leufú area, Argentina. There, marine deposits of the Vaca Muerta Formation accumulated over the aeolian deposits of the Quebrada del Sapo Formation during the early Tithonian. The sedimentary evolution of the transition can be summarized in: (i) a shutdown of aeolian dune field deposition, generating a planation surface in somewhat elevated areas and reworked megadunes in lowlands; (ii) beach sedimentation caused by episodic marine flooding that contributed to megadune reworking; and (iii) deposition in an embayed marginal-marine setting at the coast, recorded by bay margin bindstone, proximal bay and distal bay sedimentation. This transition indicates very rapid coastline migration and a condensed Transgressive Systems Tract succession throughout the study area. Rates of sea-level rise similar to Holocene ones (millimetres to centimetres per year) may have produced the transition between the Quebrada del Sapo and Vaca Muerta formations. The Late Jurassic represents a non-glacial time, and the global sea-level maximum highstand pre-dated the Vaca Muerta transgressive event. Thus, part of the sea-level rise has to be attributed to tectonic/thermal subsidence and compaction of underlying strata, which may have generated these atypical rapid rates of sea-level rise