Oxfordian microbial laminites from La Manga Formation, Neuquén Basin, Argentina: Remarkable nanobacteria preservation

Exceptionally preserved stromatolites have been found in the shallow marine carbonate facies of the Callovian-Oxfordian La Manga Formation, in the Neuquén Basin (Argentina). The stromatolites exhibit planar and crinkle lamination, often disrupted by sheet-cracks, mudcracks, and fenestral structures, which indicate periodic subaerial exposure. These and other evidences suggest that these stromatolites grew in low energy upper intertidal to lower supratidal environments. They consist of fine micrite/microsparite crystal fabrics (with a remarkable lack of allochems) that define submillimiter alternations of dense laminae. Extensive SEM examinations of polished samples of the stromatolites reveal exceptional preservation of rod-shaped bacteria, coccoid like microorganisms, and abundant aggregates of framboidal pyrite. The rod-like bacteria consists of a network of irregular distributed filaments, which range from 150 nm to an uncommon 640 nm in length; diameters range from 54 nm to 90 nm. Subspherical bodies range in size between 70 and 89 nm. The presence of abundant framboidal pyrites is interpreted as the result of the metabolic activity of sulfate–reducing bacteria and decay of organic matter.Se describen estromatolitos excepcionalmente preservados en facies carbonáticas marinas someras en la Formación La Manga, de edad Calloviense-Oxfordiense, en la Cuenca de Neuquén (Argentina). Los estromatolitos muestran una laminación tanto planar como ondulada, frecuentemente alterada por estructuras de tipo fenestral, y sheet y mud-crack, que indican etapas de exposición subaérea. Estas y otras características sugieren que estos estromatolitos crecieron en ambientes de baja energía, intermareales altos y supramareales. Están constituidos por una fábrica de micrita-microesparita (con una destacada ausencia de aloquímicos) que constituyen alternancias submilimétricas de laminación densa. Estudios detallados con SEM sobre muestras pulidas revelan una preservación excepcional con morfología alargada, microorganismos tipo cocoide, y abundantes agregados de pirita framboidal. Las bacterias con morfologías alargadas están constituyendo una red irregularmente distribuida de filamentos que oscilan en tamaño desde 150 nm hasta, excepcionalmente, 640 nm en longitud; el diámetro oscila entre 50 nm y 90 nm. Las morfologías subesféricas oscilan entre 70 nm y 89 nm. La presencia de abundantes piritas framboidales es interpretada como resultado de una actividad metabólica de bacterias sulfato-reductoras y la descomposición de materia orgánica

Osagids-types oncoids from La Manga Formation and their paleoecological significance, La Vaina Creek, Mendoza

Microbial oncoids have been found in the Oxfordian limestones of the La Manga Formation in the La Vaina section at Potimalal River, Mendoza province. The oncoids ocurr either in packstone or floatstone-rudstone or are scattered in the wackestones. They are mostly elliptical, ameboidal and subordinately spherical in shape. Different types of oncoids were recognized, according to their features of envelopes: (1) micritic laminations, (2) grumose laminations, and (3) organism-bearing laminations. Molluscs and echinoid fragments, peloids, and intraclasts acted as oncoid nuclei. Laminae follow the shape of nuclei, especially in the inner zone of the cortices. However, in the outer zones the laminae contain encrusting organisms dominated by nubeculariids and serpulids. Some oncoids, especially the ameboidal or elliptical forms, are characterized by multiple nuclei, represented by small oncoids. The oncoids are associated with bivalves, echinoderms, forams, and serpulids. The fauna is indicative of calm, shallow conditions and the excellent preservation of echinoderms suggests minimal transport prior to burial. Growth histories are in evidence. The oncoids grew in a shallow, low energy, slightly to moderate agitated subtidal normal sea water environment. The limited rolling growth oncoids was probably accompanished by intermittent currents that reoriented the oncoids parallel to stratification. The discontinuous organisms-bearing laminations reflect periods of non-agitation and litification, which facilitated the growth of encrusting organisms on static oncoid particles during a period of low sedimentation rate. Deposition of the oncoid-bearing limestones took place during shallowing of the carbonate interval and associated with emersion, subaerial exposition and paleokarst as consequence of sea level fluctuations.Fil:Palma, R.M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Bressan, G.S. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Kietzmann, D.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Crustacean microcoprolites from the Upper Jurassic-lower cretaceous of the Neuqun Basin, Argentina: Systematics and biostratigraphic implications

As a result of a microfacial study in the outer and middle ramp deposits of the Vaca Muerta Formation (lower Tithonian-upper Valanginian), four ichnotaxa of crustacean microcoprolites are described: Palaxius azulensis Kietzmann isp. nov., Palaxius caracuraensis Kietzmann isp. nov., Helicerina? isp. A. aff. Helicerina siciliana and Helicerina isp. B. They represent one of the first records of crustacean microcoprolites for the Neuqun Basin and Argentina. Helicerina is reported for the first time from the Upper Jurassic and Lower Cretaceous of South America. It is inferred that Palaxius ichnospecies were produced by callianassids, while Helicerina ichnospecies could be produced by decapods of Mecochiridae, Erymidae, and/or Nephropidae affinity. Two assemblages of crustacean microcoprolites are recognised, a middle Tithonian to lower Berriasian Palaxius-dominated assemblage and an early to late Valanginian Helicerina-dominated assemblage.Fil:Kietzmann, D.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Fernández, D.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Palma, R.M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

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