Abstract The terminal Ediacaran fossil record includes microbial mats and body fossils characterized by simple morphologies, which represents a challenge to understand several aspects related to the paleoecology of the emerging complex life. The marine siliciclastic deposits of the Cerro Negro Formation (~560–550 Ma) contain evidence of different styles of microbially induced sedimentary structures (MISS) and discoidal forms associated with them. Different types of MISS, such as Kinneyia and wrinkle structures, elephant skin, and Arumberia, are reported and related to shallow marine depositional environments. These morphologies are commonly associated with the high quality of preservation of body fossils in Ediacaran deposits. The preservation of Aspidella discoidal holdfast is related to biotic and abiotic processes involving substrate sealing by microbial mats, fluidization, and probably organic matter decay. Both abiotic factors (tidal currents and waves, added to liquefaction and sand injection) and biotic factors (substrate biostabilization by microbial mats favoring sediment cohesion) are considered the main ones responsible for the preservation style in the Cerro Negro Formation. This formation constitutes a remarkable example in SW-Gondwana of how preservation dynamics took place on a seabed sealed by microbial mats and is an important deposit conserving diverse Ediacaran forms of life in South America.

ABSTRACT The well-preserved limestone succession, Loma Negra Formation (~40 m), in the Tandilia System was deposited in a shallow carbonate ramp under low energy conditions. The evolution in the depositional settings of the unit was indicated as deepening upwards varying from shallow-middle to outer ramp environment. The limestone fabric is assumed as the product of biologically controlled precipitation of micrite, where the terrigenous supply was limited. From detailed meso- and microscopic descriptions it is possible to recognize microbially induced sedimentary structures ‘MISS’ represented by typical microtextures related to microbial activity that appear represented throughout the entire formation. In addition, micro-stromatolites are observed in the unit associated with the microbial mats showing micro-columnar conical to domical morphologies. In the basal and middle Loma Negra Formation, hemispherical structures are recognized in the bed-tops and interpreted as bubbles-like and gas escape features associated with the microbial mat interaction. Their morphology is compared with oxygen bubbles produced by modern experimental modelling with photosynthetic cyanobacteria microbial mats. Moreover, hemispherical structures are associated with increasing gas pressure lifting grains and the organic components to the surface. This paper provides evidence to understand the possible causal relationship between microbial activity and seawater oxygenation. The high oxygen production revealed by geochemistry proxies and here proposed as probably associated with photosynthetic microbial activity might be a plausible explanation for the record of the documented Middle Ediacaran Oceanic Oxygenation Event in the Loma Negra Formation.

The facies analysis and the study of the way of life of the organisms have been historically considered as the conventional methods for the study of the sedimentary environment and therefore for paleodepth estimations. The aim of this work is, first to rule out possible diagenetic alterations of the fossil material and to confirm the ability of the organisms to precipitate their shells in equilibrium with the environment. Secondly, to show how geochemical composition of water varies with depth considering previous paleoenvironmental models obtained from the facies analysis and the way of life of organisms, using innovative geochemical methods such as the study of carbon stable isotopes and rare earth elements plus yttrium in fossils. Finally, it aims to establish the reasons that produce variations in the geochemical composition of waters related to depth. In the present work, we present two case studies from contemporary marine successions deposited in different paleodepths: Arroyo Loncoche (Neuquén Basin; Fig. 1a) and Río Guanaco (Austral Basin; Fig. 1b). In both cases, paleodepths were defined based on the facies analysis and the way of life of organisms. The Arroyo Loncoche sedimentary section (Neuquén Basin) includes the complete succession of the Vaca Muerta and Chachao Formations (Fig. 2) and has been defined as ramp deposits that include different environments from basin to proximal middle ramp. The Chachao Formation is composed mainly of oyster biostromes of the genus Aetostreon sp. (Kietzmann et al., 2008, 2014; Kietzmann and Palma, 2009; Gómez Dacal et al., 2018). From the study of the way of life of oysters, Hernández-Ocaña et al. (2015) showed that this genus lived under shallow-water conditions, from the intertidal zone to 30 m depth maximum. The Río Guanaco sedimentary section of the Austral Basin is found in the southeastern area of the Santa Cruz Province, and includes the complete Springhill Formation and the lower member of the Río Mayer Formation (Fig. 2; Richiano et al., 2012). From the fossils and facies analyses, this sequence has been characterized as an external platform composed mainly of black shales, rich in belemnites of the Belemnopsis sp. fauna (Richiano et al., 2015). Jarvis (1980) postulates, that belemnites lived in depths between 120 and 330 m. A representative sedimentary succession of each study area was carried out, selecting the best and most complete outcrops of the formations (Fig. 3). In addition, a systematic analysis of the fossil material was made in order to stablish their petrographic (cathodoluminscence and scanning electron microscopy) and geochemical characteristics (rare earth elements and yttrium and stable isotopes of C and O). From cathodoluminescence and scanning electron microscopy studies it was possible to demonstrate that selected samples of Aetostreon sp. and Belemnopsis sp. were well preserved (Fig. 4). The REY results, obtained from belemnites, show that the Río Guanaco section contains higher HREY, Y/Ho ratios and the La and Eu anomalies than Arroyo Loncoche section (Fig. 5; Table 1). Oysters and belemnites precipitate their shells in equilibrium with the sea-water composition, where the difference in REY values recorded is attributed to the greater influence of continental sediment supply at the time of the formation of the oyster shells. Chemostratigraphic curves were obtained from the isotopic results (Fig. 6; Table 2). The curve of δ13C from the Río Guanaco section shows more negative values and less variations than the curve of the Arroyo Loncoche section (Fig. 6). This difference Is considered as response of the belemnites may have precipitated their shells under deeper environmental conditions, where the export of carbon from the surface to the deep ocean is continuous and generates an enrichment of 12C. From the results of the present study the following conclusions are obtained: • According to the way of life of the organisms and the interpreted environments in the sedimentary successions, we suggest that belemnites lived in a deeper marine environment, while oysters lived from the intertidal to the 30 m depth. • Both cathodoluminescence and scanning electron microscopy studies indicate a good preservation for selected samples of Aetostreon sp. and Belemnopsis sp. • The Río Guanaco section (Austral Basin) shows higher HREY values, Y/Ho rations and Eu and La anomalies than those recorded in Arroyo Loncoche (Neuquén Basin). These differences agree with the environmental conditions in relation to the continental sedimentary supply during the precipitation of the shells. • The chemostratigraphic curve from Río Guanaco section shows more negative and more homogeneous values than those recorded in the curve of Arroyo Loncoche section. This difference is attributed to belemnites having precipitated their shells in deeper environments, where the export of carbon from the surface to the deep ocean is continuous and generates an enrichment of 12C. • Isotopic and REY trends corroborate that belemnites from Austral Basin lived in a deeper marine environment than oysters from Neuquén Basin.

Con el objetivo de conocer como varía la composición geoquímica de las aguas con la profundidad y que factores influyen en estos cambios, se realizaron estudios de tierras raras e isótopos estables sobre sucesiones sedimentarias definidas ambientalmente por los métodos tradicionales. A tal fin, se tomaron como casos de estudio secciones sedimentarias de las Formaciones Vaca Muerta, Chachao (Cuenca Neuquina) y Río Mayer (Cuenca Austral), depositadas en forma contemporánea pero a profundidades distintas. Los depósitos de la Cuenca Austral, corresponden a ambientes profundos (plataforma externa) y los indicadores geoquímicos fueron obtenidos a partir de ejemplares de belemnites del género Belemnopsis sp. En la Cuenca Neuquina, los estudios se realizaron en ostras del género Aetostreon sp. acumuladas en ambientes marinos relativamente más someros (rampa externa proximal a rampa media proximal). Estudios petrográficos han permitido confirmar el carácter primario de los valores isotópicos. En la Cuenca Neuquina, las ostras de ambiente marino relativamente más somero muestran un amplio rango de valores de δ13C desde negativos hasta positivos (-4,93 a 2,66‰ VPDB), en tanto que los belemnites de la Cuenca Austral, de ambiente más profundo, poseen menor variabilidad y tenores negativos de δ13C (-1,77 a -0,33‰ VPDB). Asimismo, estos últimos organismos se caracterizan por un enriquecimiento en tierras raras pesadas (valores más elevados de Y/Ho) y de las anomalías de Y, La y Eu, en comparación con los encontrados en las ostras de la Cuenca Neuquina. Se concluye que el contenido de tierras raras e itrio (REY) e isótopos estables en el agua de mar en las cuencas Neuquina y Austral durante el lapso Berriasiano- Valanginiano varió sistemáticamente con la profundidad. En el caso de las REY, este cambio estaría relacionado con la cercanía a las fuentes de aporte de material sedimentario, mientras que la variación en las curvas quimioestratigráficas de carbono se vincularía con la productividad primaria.

The sedimentary infill of the Austral-Magallanes Basin since the onset of its foreland stage in the Lago Argentino region is dominated by deep-marine and coastal deposits. However, during the Late Cretaceous the basin accumulated a thick and poorly known continental sedimentary succession, which has received different lithostratigraphic names. The aim of this work is to characterize the here defined Uppermost Cretaceous Continental Deposits (UCCD) from a detailed facies and architectural analysis, as well as the resulting stacking pattern. Seven Facies Associations (FAs) were discriminated in order to define the sedimentary paleoenvironments: FA1, gravelly sheet bodies; FA2, tabular bodies of conglomerates with mud rip-up clasts; FA3, complex tabular sandy bodies; FA4, simple tabular sandy bodies; FA5, tabular bodies of structureless sandstones; FA6, heterolithic deposits; and FA7, fine-grained deposits. Three different fluvial styles were recognized: meandering systems dominated by avulsion and meander abandonment processes (fluvial style a), braided systems (fluvial style b), and meandering systems dominated by overbank flood processes (fluvial style c). The stacking pattern of the FAs allowed to divide the UCCD into two major depositional stages related to the accommodation space vs sediment supply (A/S) ratio. Stage I is characterized by the alternation of fluvial styles a and b, while the Stage II is represented by the alternation of fluvial styles c and b, and the Stage III is characterized entirely by fluvial style c deposits. Although the UCCD are considered as a whole within a framework of low A/S ratio, several high frequency variations were recognized. The Stage I records seven high frequency intervals of which four are characterized by high A/S ratio interrupted by three events of low A/S. While the stage II is represented by six high frequency periods of low A/S ratio and other five high frequency events of high A/S ratio. The Stage II is considered as deposited in a relative higher A/S context in comparison with the Stage I, based on the behavior of the moderate to high sinuosity meander fluvial systems. Finally, the Stage III is represented entirely by a high frequency low A/S ratio event.

The Maastrichtian shallow marine deposits exposed at the south margin of the Lago Argentino within the Austral-Magallanes Basin are known as the Calafate Formation. In order to interpret the depositional systems and reconstruct the sequence stratigraphic architecture for this unit at its type locality (Cerro Calafate), we acquire new data from seven stratigraphic sections. We recognized six facies associations (FA-1 to FA-6) corresponding to shallow marine deposits, which are organized vertically displaying a transition from shallower to deeper conditions, representing a ~90 m thick transgressive succession. The Calafate Formation deposits are differentiated into a lower wave-dominated coast (FA1, FA2 and FA3) and an upper tide-dominated coast (FA4, FA5 and FA6), each marked by the dominance of wave and tidal sedimentary processes, respectively. The Calafate Formation overlies the fluvial deposits of the Chorrillo Formation by a transgressive surface (TS), which is overlaid by a transgressive marine succession characterized by a retrogradational stacking pattern. The latter is finally covered by offshore transition marine deposits marking a progressive deepening of the depositional system that culminates with the maximum flooding surface (MFS). From here, an aggradational stacking pattern dominates the upper sandstones of the unit representing the highstand systems tract (HST), which is interpreted to be the cause of short-term periods when the accommodation space rate was nearly equaled to the sediment supply rate during the Maastrichtian.

ABSTRACT The Tithonian-Valanginian time interval in the Mendoza Shelf (Neuquén Basin, Argentina) is well exposed in the Río Salado, Puesto Loncoche and Cuesta del Chihuido sedimentary sections. From those localities, more than fifty preserved oyster shells of the genus Aetostreon sp. were selected and sampled in order to perform the first δ13C curves for this particular time interval. Mineralogical and cathodoluminiscence properties, inner micromorphology of the valves, added to major and trace element geochemistry were analyzed in order to highlight the best C-O isotopic preservation. The δ13C isotope curves show values varying between 0 and -3‰ VPDB for the Tithonian-Berriasian basal section, and a positive excursion of ~2.4-2.7‰ VPDB in the Valanginian upper section. This δ13C up section trend is here considered in order to reveal eminent correlations with other sections from the Neuquén Basin, as well as the Weissert Event from the Tethys area, also on the basis of their ammonite faunal zones. The palaeotemperatures obtained from δ18O preserved values, added to a detailed sedimentological study suggest that observed δ13C anomaly may responds to a global climatic change from warm and dry to warm and humid conditions.

RESUMEN El intervalo Titoniano-Valanginiano en la Plataforma Mendocina (Cuenca Neuquina, Argentina) se encuentra muy bien representado en las secciones sedimentarias estudiadas de Río Salado, Puesto Loncoche y Cuesta del Chihuido. En estas localidades más de cincuenta valvas de ostras del género Aetostreon sp. fueron seleccionadas y muestreadas con el objeto de obtener las primeras curvas de δ13C de este particular intervalo temporal. Las propiedades mineralógicas, la respuesta a la catodoluminiscencia, así como la micromorfología interna de las valvas, sumado al análisis geoquímico de los elementos mayoritarios y trazas fueron utilizados para indicar las zonas con el más alto grado de preservación isotópica del C y O. La curva de δ13C muestra valores que varían entre 0 y -3‰ VPDB para la sección basal de edad titoniana-berriasiana y una excursión positiva de ~2.4-2.7‰ VPDB para la sección cuspidal de edad valanginiana. Esta tendencia del δ13C hacia el tope de la sección revela una eminente correlación con otras secciones de la Cuenca Neuquina como así también con el Evento Weissert para el área del Tetis, también respaldada por la fauna de amonites presente. Las paleotemperaturas obtenidas aquí a partir de los datos de δ18O, sumado al estudio sedimentológico de detalle sugieren que la anomalía de δ13C podría ser la respuesta a un cambio climático desde condiciones cálidas y áridas a cálidas y húmedas registrado a nivel global.

The Neoproterozoic sedimentary cover of the Río de La Plata Craton in the Sierras Bayas - Olavarría region, located in the northwestern sector of the Tandilia System, is represented by the Sierras Bayas Group (Villa Mónica, Cerro Largo, Olavarría and Loma Negra formations; Fig. 1), which is unconformably overlain by the Cerro Negro Formation (Poiré and Spalletti, 2005). The Sierras Bayas Group is composed of a variety of different marine sedimentary rocks which are characterized by the absence of strong deformation and metamorphism (Gómez Peral et al., 2007). Compared to the Sierras Bayas Group, the Cerro Negro Formation received less attention over the last years. In the Sierras Bayas region (Fig. 2), Iñiguez and Zalba (1974) and Iñiguez et al. (1989) described and named this unit in well-exposed quarries faces. They indicated that it was mostly composed of siliciclastics (reddish mudstones and sandstones), with subordinated marls in its basal section. In the nearby subsurface area, some authors mentioned the presence of a relatively thick succession of dark fine-grained rocks overlying the Sierras Bayas Group (Amos et al., 1972; Cuomo et al., 1983), but the relationship with the Cerro Negro Formation described in outcrops was unclear. The aim of this contribution is to present the sedimentological attributes and stratigraphic relationships of the outcrop and subsurface successions originally reported as Cerro Negro Formation, in order to integrate this upper section of the Neoproterozoic cover in a common evolutionary model. To do so, more than 200 m of cores were described (Fig. 3), and combined with sections measured in the quarries. Conventional sedimentological description (textures, physical structures, bed thickness, etc.), was combined with X-Ray diffraction and standard petrography, in order to define mineralogical composition and proportion of main components. In this context, 181 samples were analyzed by XRD using standard procedures, and 50 thin sections, mostly from siltstones and sandstones, were qualitatively and quantitatively described. This information was integrated in a facies analysis. The facies analysis allowed to identify and interpret 10 sedimentary facies, based on texture, structures and colors, which are grouped in four categories (Figs. 4-7): marls (massive and laminated marls), siliciclastic mudstones (dark fissile mudstones, red massive mudstones, and grey massive mudstones), mudstone-sandstone with heterolithic bedding (grey heterolithics with lenticular bedding, grey heterolithics with wavy bedding, and red heterolithics with wavy bedding), and finally, sandstones (cross-laminated sandstones and massive sandstones). Broadly speaking, marls and siliciclastic mudstones are interpreted as the result of settling from suspension, either in oxygen-rich (marls and red massive mudstones) or oxygen-depleted (dark fissile mudstones) sea floors. Heterolithic intervals with lenticular and wavy bedding represent recurrent alternation of settling and traction, the latter mostly as a result of ripple migration. Finally, the sandstones are interpreted as the product of wave-generated ripple migration (cross-laminated sandstones) or deposition under high-concentrated flows (massive sandstones). The ten recognized facies occur in a systematic order in the investigated succession. Therefore, three distinctive facies successions were defined (facies successions I, I, and III), and correlated when possible across subsurface or outcrop areas (Fig. 8). Facies successions I (4-25 m thick) comprises laminated and massive marls at the base, which grade vertically into red massive mudstones. Mudcracks are occasionally observed. Clay minerals are mostly illitic (illite and illite/smectite). This succession overlies the regional erosive karstic surface sculpted at the top of the Sierras Bayas Group (named Barker surface) and marks the resumption of mixed carbonate/siliciclastic sedimentation in the area. This facies succession is interpreted as deposited under supratidal conditions in a tidalflat depositional environment, probably under well oxygenated conditions. Facies succession II (0-150 m thick) conformably overlies the previous one in many of the investigated cored wells and is composed of dark fissile mudstones and massive grey siltstones, as well as grey heterolithics with lenticular and wavy bedding. In the fine fraction, clays are now dominated by chlorite and illite in similar proportions (Fig. 8b). This facies succession is interpreted to represent low-energy subtidal settings, likely with suboxic to anoxic bottom conditions. Facies succession III (up to 75 m thick) unconformably rests upon the previous, and represents an abrupt change to red heterolithics with wavy and flaser bedding, together with cross-laminated and massive, fine- to medium-grained sandstones (Fig. 8b). Mudcracks, scour marks and flutes are common in this succession. In these facies quartz and plagioclases are abundant, with subordinated clay minerals, which are composed mainly of illite and less proportion of chlorite. This succession is interpreted to represent subtidal to intertidal settings, probably with well-oxygenated substrates. The top of this succession has been removed by erosion both in subsurface and outcrops. The spatial (vertical and lateral) distribution of facies successions and their basal boundaries, together with the faults recognized by well correlation, provide the framework for the proposed tectono-sedimentary model for the investigated Neoproterozoic succession (Fig. 9). This begins with the stage I which coincides with the deposition of the facies succession I over the karstic surface of the top of Sierras Bayas Group. Then, the facies succession II is deposited conformably onto the previous one, and is related here to a sea level drop referred as stage II, which is followed by a transgressive event. Stage I was dominated by marly to clay-rich sedimentation under oxic conditions which turns to anoxic and purely siliciclastic during the stage II. At the end of the stage II the complete succession, including the Sierras Bayas Group strata, was affected by the reactivation of vertical to subvertical falls from the basement with modified the relief by uplift, followed by intense erosion and peneplanation. This period is defined as stage III. Finally, the stage IV is constituted by the deposition of the facies succession III in which the top is not preserved. The lithological and stratigraphic relationships defined in this contribution for the Neoproterozoic upper strata of the northwestern Tandilia System make necessary to postulate a new lithostratigraphic scheme for them. As mentioned before, the facies succession II is composed of up to 150 meters of mainly black shales and was almost unknown till the present study. This stratigraphic division is lithologically distinctive compared to facies successions I and III and is truncated at the top by an erosional unconformity. This interval is therefore proposed as a new formation here, complying the provisions of the article 28 of the Argentine Stratigraphic Code. This definition drives necessary the re-location of the facies succession I, not as the base of the Cerro Negro Formation, but as a new lithostratigraphic unit as well. In this sense, the facies succession I is here defined as Avellaneda Formation, while facies succession II is termed Alicia Formation (Fig. 10). The name Cerro Negro is here attributed solely to facies succession III, the reddish heterolithics that represent the uppermost part of the investigated strata. Moreover, these three lithostratigraphic units, conforming a discrete depositional record within a single basin, are grouped into a new lithostratigraphic unit defined as La Providencia Group (Fig. 10).

La cobertura neoproterozoica del Cratón del Río de La Plata en el área de Sierras Bayas - Olavarría (noroeste del Sistema de Tandilia) constituye el relleno sedimentario más antiguo conocido en Argentina, e incluye al Grupo Sierras Bayas y a la Formación Cerro Negro. El objetivo de este trabajo es presentar un estudio sedimentológico y estratigráfico de detalle sobre las sedimentitas que hasta el presente trabajo eran conocidas como Formación Cerro Negro, al que se agrega aquí el análisis de un paquete sedimentario de más de 150 metros de espesor, conformado por sedimentitas finas silicoclásticas de color gris oscuro a negro. Esta sucesión, identificada sólo en subsuelo, se ubica estratigráficamente entre la sección basal margosa y la sucesión netamente heterolítica que caracteriza a la sección superior de la Formación Cerro Negro en los afloramientos típicos de la unidad. Para este trabajo se realizó un análisis de datos obtenidos de 17 secciones de subsuelo en dos yacimientos en actividad, más dos perfiles en frentes de canteras que corresponden a localidades típicas de la unidad. La descripción y caracterización de las sedimentitas de la Formación Cerro Negro permitió la definición de diez facies sedimentarias, las cuales fueron agrupadas en tres sucesiones de facies (I: margas y arcilitas rojas, II: lutitas negras, limolitas grises e intervalos heterolíticos grises y III: sucesiones heterolíticas y arenosas), que se interpretaron como depositadas en un ambiente de planicie de marea. Los análisis petrográficos y de difracción de rayos X permitieron caracterizar mineralógicamente a las unidades litológicas presentes. En las facies silicoclásticas finas se reconoce un predominio de cuarzo, acompañado de plagioclasas y arcillas, mientras que en las facies margosas se observan contenidos variables de calcita, variedades de la sílice y arcillas. Con respecto a las margas, la illita es el argilomineral que predomina, acompañada de clorita en proporciones moderadas a escasas. Por otra parte, los niveles de areniscas finas a medianas, señalan una buena selección y escasa a ausente matriz. Las mismas se componen de cuarzo monocristalino, plagioclasas, arcillas (illita y clorita), micas detríticas (muscovita y biotita) y minerales opacos (pirita y hematita). Integrando las relaciones estratigráficas de las sucesiones reconocidas en superficie con la información adicional de subsuelo se elaboró un modelo evolutivo de los diferentes estadios tecto-sedimentarios, el cual contribuye a comprender mejor la evolución de los procesos depositacionales ocurridos en el sector central y austral de las Sierras Bayas. El reconocimiento de una nueva sucesión constituida por sedimentitas finas de coloración gris oscura a negra (sucesión de facies II) conlleva a modificar el esquema estratigráfico existente para el sector noroeste del Sistema de Tandilia. En esta propuesta, a la sucesión de facies I se le asigna el nombre de Formación Avellaneda, la cual queda restringida a las sedimentitas del sector inferior del relleno sedimentario que se encuentra sobre una discontinuidad regional. A la sucesión de facies II, que se ubica por encima de la Formación Avellaneda, se la denomina con el nombre de Formación Alicia. Finalmente, la Formación Cerro Negro sensu stricto queda circunscripta al tope de la sucesión sedimentaria y es equivalente a la sucesión de facies III de este trabajo. En este sentido, y a partir de los resultados obtenidos, se propone redenominar a las unidades que suprayacen al Grupo Sierras Bayas como Grupo La Providencia, el cual está integrado por las formaciones Avellaneda, Alicia y Cerro Negro.

The Neoproterozoic sedimentary sequences of the Tandilia System Basin are represented in the Olavarría area by interstratified shallow marine, silicoclastic and carbonate units comprising, from oldest to youngest, for the Villa Mónica, Cerro Largo and Olavarría formations of the Sierras Bayas Group (Fig. 1). These almost undeformed and unmetamorphosed siliciclastics sedimentary units allowed studying the different diagenetic regimes due to the preservation of postdepositional features and, also, of many depositional and eodiagenetic ones. In the study area de Sierras Bayas Group is composed of the Villa Mónica (conglomerates, sandstones, shales and dolostones), Cerro Largo (chert breccias, quartz-sandstones and siltstones), Olavarría (mudstones) and Loma Negra (limestones) Formations with a thickness up to 185 m (Figs. 2 and 3; Iñiguez, et al., 1989; Poiré, 1987a, 1993; Gómez Peral, 2008). The siliciclastics facies of the Sierras Bayas Group are composed of conglomerates, sandstones and mudstones, represented in the basal section of the Villa Mónica Formation and the Cerro Largo and Olavarría formations (Fig. 4). The objective of the present study was to reveal the sequence of diagenetic processes represented in the silicoclastic facies from a detailed petrological analysis. It included the recognition of the allogenic and authigenic components, the characterization of the diagenetic microfabrics (chemical and compactational), the definition of microfacies and the interpretation of the postdepositional burial history from the determination of paragenetic sequences. Besides, a revision of the classical depositional facies chart was performed. Considering that different factors may influence the diagenesis of sediments at different times during its evolution (Kantorowicz, 1985), the general controls on diagenesis include the depositional mineralogy, the depositional-water chemistry, the change in pore fluid composition, the inferred burial pressures and the temperatures and subaerial exposure. From the lithofacial analisys we remark the presence of some facies with potential glacial origin here described as laminated gravelly mudstone (Fig. 5; facies FGh) in the basal section of the Villa Mónica Formation and the "Colombo" diamictite (Fig. 6; Ds) in the base of the Cerro Largo Formation. Other interesting level is present in the base of the Olavarría Formation where we recognized an intraformational conglomerate (Fig. 6; facies MCh) likely associated with a volcanic arc affinity by geochemical data (Zimmerman et al., 2011). The quartz-arkosic facies of the lower section of the Villa Mónica Formation can be separated in three petrofacies according with its detrital mineralogy: i) subarkosic sandstones (arenites and wackes) at the base; ii) lithic and sublithic sandstones rich in polycrystalline quartz (arenites and wackes) in the middle part and iii) quartz sandstones in the upper section (Table 1 and Fig. 7). A detailed analysis of the microfacies allowed the interpretation of the interaction of diagenetic processes with sediments during an important geological time interval (Tables 2, 3 and Figs. 8, 9 and 10). In the same sense, the identification of diagenetic facies permitted the differentiation of sections of the succession characterized by different diagenetic modifications. Clay minerals were analyzed since illite crystallinity, identification of interestratified illite/smectite and its smectite proportion help to assess the relative depth of burial reached for these sedimentary rocks. The main detrital and authigenic components, as well as the definition of their microstructures, allowed identifying 25 microfacies in the silicoclastic section of the Villa Mónica Formation (Table 2) and the interpretation of the evolution of the processes occurred during the different diagenetic regimens. The eodiagenetic regime involved degradation of K-feldspars to kaolinite, smectite, cementation with opal and hematite, as the result of the interaction with meteoric waters between surface and shallow burial depths. Subsequently, basinal brines controlled the diagenetic evolution of the sandstones and resulted in the initial transformation of smectite into interestratified illite-smectite, precipitation of quartz overgrowths, and recrystallization of siliceous cements. During further burial the most important alterations included transformation of interestratified illite-smectite in ordered type (R1) containing 70-80% of illite in relation with authigenic growth of illite; scarce authigenic chlorite as replacement of biotite and amphibole, illitization of kaolinite (inferred), pervasive stylolitization added to the formation of sutured contact grains, and later with hematite replacements and late calcite cements. All these features indicate an equilibrium of the fluids with very deep burial and high temperatures (Fig. 11; > 5 km and >150ºC). An important uplift is registered related to a period of intense erosion and weathering with the generation of a karstic surface on top of dolostones of the Villa Mónica Formation which constitutes a telodiagenetic surface. This important unconformity was situated in 595 Ma according to paleomagnetic studies, (Rapallini et al., 2008) and was characterized by dedolomitization, intense dissolution and precipitation of goethite, hematite, chert and calcite, which produce a typical reddish to pink coloration (Gómez Peral, 2008). Sandstones of the Cerro Largo Formation were divided in two petrofacies (Table 1; Fig. 7); the quartzose lower one is mostly composed of monocrystalline quartz with low proportion of matrix (< 10%), very scarce policrystaline quartz, chert and mudstone intraclasts. The analysis of detrital and diagenetic mineralogy, and the different microstructures allowed the recognition of 5 microfacies (Table 3). The diagenetic evolution of this unit began with eodiagenetic processes related to the formation of euhedral pyrite, infiltration of smectite, authigenic glauconite and chert cementation. The most important mesodiagenetic modifications included quartz cementation, which complete the pore-filling, followed of partial dissolution of quartz and replacement by subhedral hematitic cement. Progressive burial drove the transformation of detrital and eodiagenetic smectite, first into poorly and then into better-ordered, mixed-layer illite/ smectite (I/S). In the Cerro Largo Formation the I/S reaches a low ordered (R0) containing near of 40-50% of illite, added to the authigenic generation of illite with crystallinity index typical of diagenetic zone. Furthermore, these features can be related to a middle-deep mesodiagenesis (Fig. 11; < 4 km and < 120°C). However, this process was restricted to intercalated mudstones with clayminerals. Sparse stylolites and tangential to planar grain contacts are the most conspicuous chemical compaction indicators. Uplift and incursion of meteoric waters constrained the telodiagenetic alterations including kaolinitization and smectite generation as well as degradation of mixed-layer illite/smectite which increment their smectite proportion. One prominent telodiagenetic products in this quartz-sandstones was the dissolution of quartz cements in presence of pervasive fluids and its replacement by hematite, goethite and chalcedony. In the mudstones levels the presence of a clay association of smectite, kaolinite and pyrofillite was related to an advanced argillic alteration due to possibly hydrothermal influence. One of the most important inferences was the recognition of different burial diagenetic histories in the siliciclastic units of the lower part of the Sierras Bayas Group (lower section if the Villa Mónica Formation) respect to the upper successions (Cerro Largo and Olavarría formations) which implies that the discordance on top of the Villa Mónica Formation could represent a long period of erosion and subaerial exposure.

El estudio de las características petrológicas y microfaciales de las unidades silicoclásticas marinas del Neoproterozoico del Grupo Sierras Bayas, (en orden estratigráfico ascendente: formaciones Villa Mónica, Cerro Largo y Olavarría), se presenta en este trabajo con la finalidad de evaluar la evolución e importancia de los controles producidos durante los regímenes eodiagenético, mesodiagenético y telodiagenético. En la sección basal de la Formación Villa Mónica (800-900 Ma) se reconoce entre los componentes producidos durante el régimen eodiagenético, la presencia de caolinita, esmectita, clorita, ópalo y/o chert, goethita y hematita. Durante la mesodiagénesis se originaron interestratificados de illita-esmectita (con alta proporción de illita ~70%), cementos cuarzosos y recristalización de cementos silíceos generados en el régimen eodiagenético. A mayores profundidades se habría favorecido el crecimiento autigénico de illita, formación de estilolitas y de contactos suturados, disolución y reemplazo de cementos cuarzosos por hematita y calcita, como así también se infiere la transformación de caolinita a illita. Esto último indica que la unidad habría alcanzado el equilibrio a profundidades de 5 km o mayores y temperaturas elevadas (>150ºC). Luego, esta unidad fue expuesta a la telodiagénesis, reflejado por la degradación de argilominerales, formación de caolinita y esmectita en los niveles pelíticos y por el desarrollo de una superficie cárstica en las dolomías del tope de la formación. La evolución de la diagénesis en las areniscas de la Formación Cerro Largo se inicia con productos de la eodiagénesis vinculada a un ambiente marino de condiciones reductoras con la formación de pirita, esmectita y glauconita. Luego, durante la mesodiagenésis el cemento predominante fue el cuarzoso, con fuente de sílice externa y la esmectita fue transformada en interestratificados de illita-esmectita (con moderada esmectita ~50-40%), sumado al crecimiento autigénico de illita. Tales evidencias permiten inferir que esta unidad habría alcanzado un régimen meosodiagenétco profundo (<4 km de profundidad y temperatura <120ºC), aunque menor que el observado en la unidad subyacente. La posterior exhumación de estas rocas favoreció las alteraciones telodiagenéticas como la caolinitización, formación de esmectita y degradación de los interestratificados de illita-esmectita. El cuarzo fue disuelto y reemplazado por una asociación de cementos de hematita-goethita-calcedonia, vinculados, desde el punto de vista diagenético, a condiciones subsuperficiales. En las pelitas de la Formación Olavarría los rasgos diagenéticos indican que las mismas fueron afectadas por una mesodiagénesis temprana, donde se destaca el crecimiento autigénico de illita en la matriz arcillosa. Se observa también la preservación de rasgos eodiagenéticos, como escasa deformación de la matriz, predominio de fábrica flotante (abierta), formación de concreciones de calcita de posible origen microbiano, concreciones estratales multiepisódicas de hematita y venillas de ópalo. Este estudio permite señalar la existencia de una marcada diferencia en el grado de enterramiento y modificación diagenética registrados en las facies silicoclásticas inferiores con respecto a las suprayacentes del Grupo Sierras Bayas, cuyo espesor total es actualmente inferior a los 200 m. Por lo tanto, se sugiere que la sección inferior habría sido exhumada y fuertemente erodada con la pérdida de parte del registro rocoso, por lo que la discordancia que las separa representaría un lapso temporal importante.

We compare the litho-, bio- and chemostratigraphy of Neoproterozoic sedimentary successions of the Río de la Plata Craton, namely the Arroyo del Soldado Group (ASG) of Uruguay and the Sierras Bayas Group -Cerro Negro Formation (SBG-CNFm) of Tandilia (Argentina). Both successions are characterized by alternation of carbonates and siliciclastics (Poiré, 1987; Gaucher, 2000), as well as a geotectonic setting corresponding to a passive continental margin. The Yerbal, Polanco and Cerro Espuelitas Formations (ASG) are correlated lithostratigraphically to the Cerro Largo, Loma Negra and Cerro Negro Formations of Tandilia. Whereas the ASG is up to 5 km thick, the SBG-CNFm represents a strongly condensed section showing numerous erosive surfaces and only 350 m in thickness. This difference may be explained by the shallower, epicontinental sedimentary environment of the SBG-CNFm. The predominantly siliciclastic Yerbal and Cerro Largo Formations represent the transgression of the Vendian (Ediacaran) sea onto the Río de la Plata Craton. The overlying Polanco and Loma Negra Formations are characterized by thick and pure carbonate (especially limestone) deposits currently mined in both countries for use in the cement, lime and fertilizer industries. An expressive palaeokarst is developed on top of these carbonates, indicating sea-level drop and platform exposure (Barrio et al., 1991; Gaucher et al., 2004b). The lower Cerro Espuelitas Formation and the Cerro Negro Formation are made up of marls at the base, passing into shales and heterolithic facies up section. We report for the first time the occurrence of Cloudina cf. C. riemkeae Germs (1972) in micritic limestones of the Loma Negra Formation. This supports the correlation of this unit with the lower Polanco Formation, where these shelly fossils also occur (Gaucher y Sprechmann, 1999; Gaucher et al., 2003b). Moreover, Cloudina is currently regarded as an index fossil of the late Ediacaran (Grant, 1990; Amthor et al., 2003). Low diversity acritarch assemblages occur both in the ASG and SBG-CNFm. In the former, two assemblages were distinguished: a Bavlinella faveolata- Soldadophycus assemblage occurring mainly in the siliciclastic units, and a slightly more diverse Leiosphaeridia-Lophosphaeridium assemblage characteristic of the Polanco Formation (Gaucher, 2000). In the SBG-CNFm, the acritarch asemblage is sphaeromorph-dominated (mainly Leiosphaeridia), and similar to the Leiosphaeridia-Lophosphaeridium microflora of the ASG. For the first time we report acritarchs occurring in greenish pelites of the Villa Mónica Formation, assigned here to Leiosphaeridia minutissima, L. tenuissima and Synsphaeridium sp. These represent the oldest fossils so far reported from Argentina. Pothe de Baldis (1983) described Chuaria olavarriensis, Leiosphaeridia sp. and Paleorivularia ontarica from the upper Cerro Largo Formation. The former two taxa can be reassigned to Chuaria circularis and Leiosphaeridia minutissima, respectively. Palynological macerations of shales of the Cerro Largo Formation carried out in the present study yielded Leiosphaeridia tenuissima as dominant species, and a single vesicle assigned to Leiosphaeridia minutissima. A detailed micropalaeontological study is due to Cingolani et al. (1991), which describe the following acritarch species from the Cerro Negro Formation: Leiosphaeridia sp., Trachysphaeridium sp., Synsphaeridium sp. and granular sphaeromorphs grouped by the authors under «type A» acritarchs. The application of the currently accepted taxonomic criteria to these acritarchs allows assigning them to Leiosphaeridia jacutica, L. minutissima, L. tenuissima, Lophosphaeridium sp. and Synsphaeridium sp. In the present study, acritarchs assigned to Leiosphaeridia tenuissima were isolated from shales of the Cerro Negro Formation. Throughout the SBG-CNFm, a tendency towards smaller sphaeromorphs from base to top is observed: from 450 µm for Villa Mónica Formation to 380 µm for the Cerro Largo Formation and finally 200 µm for the Cerro Negro Formation. This trend has been reported for other shallow-water, marine Ediacaran successions elsewhere (Butterfield, 1997). On the basis of low diversity and dominance of sphaeromorphs, the acritarch assemblage of the SBG-CNFm can be assigned either to the Simple Leiosphere Palynoflora of Grey et al. (2003) or the younger Kotlin-Rovno assemblage of Vidal y Moczydlowska (1997). The palynoflora occurring in the lower and middle ASG has been correlated to the Kotlin-Rovno assemblage (Gaucher, 2000), supporting the lithostratigraphic correlation depicted above. Available chemostratigraphic data for the ASG and SBG-CNFm also suggest that the Loma Negra Formation correlates with the Polanco Formation. Whereas 87Sr/86Sr values for limestones of the Loma Negra Formation range between 0.7069 and 0.7087 (Kawashita et al., 1999b), limestones of the Polanco Formation yielded ratios between 0.7072 and 0.7085 (Kawashita et al., 1999a; Gaucher et al., 2004c). The Loma Negra Formation is further characterized by d13C values around +4 ‰ PDB (Gómez Peral et al., in press), which are similar to d13C relationships between +5.5 and 0 ‰ PDB reported for the lower Polanco Formation (Unit A of Gaucher et al., 2004b). Regarding the Villa Mónica Formation, available data are ambiguous regarding its age assignment and correlatios. On the one hand, stromatolite assemblages described from the unit (Poiré, 1987, 1993) suggest an upper Riphean age (Tonian or Cryogenian). On the other hand, the similarity of acritarch assemblages preserved throughout the SBGCNFm suggests that there is not a large time hyatus between the Villa Mónica and Cerro Largo Formations, raising the possibility of the Villa Mónica Formation being of Ediacaran age. The correlations here suggested imply that an extensive marine shelf developed on a large area of the Río de la Plata Craton in the late Ediacaran, possibly under tropical climate. According to Gaucher et al. (2003b), this shelf also included the Corumbá Group (Mato Grosso, Brazil), being thus at least 2500 km long.

Se compara la lito- bio- y quimioestratigrafía de sucesiones sedimentarias del Neoproterozoico del Cratón del Río de la Plata, a saber: Grupo Arroyo del Soldado (GAS), Uruguay y Grupo Sierras Bayas - Formación Cerro Negro (GSB-FmCN) de Tandilia (Argentina). Ambas sucesiones se caracterizan por la intercalación de unidades carbonáticas y siliciclásticas, así como por un ambiente geotectónico correspondiente a un margen continental pasivo. Las formaciones Yerbal, Polanco y Cerro Espuelitas inferior se correlacionan litoestratigráficamente con las formaciones Cerro Largo, Loma Negra y Cerro Negro de Tandilia. Las formaciones siliciclásticas Yerbal y Cerro Largo representan la ingresión del mar Vendiano (Ediacarense) sobre el Cratón del Río de La Plata. Las sobreyacentes formaciones Polanco y Loma Negra se caracterizan por importantes depósitos carbonáticos, culminando con una expresiva superficie kárstica indicando regresión y exposición subaérea de la plataforma. La parte inferior de la Formación Cerro Espuelitas y la Formación Cerro Negro se caracterizan por depósitos margosos en la base, pasando a pelitas y facies heterolíticas hacia el tope. Se comunica el hallazgo de Cloudina cf. C. riemkeae de calizas micríticas de la Formación Loma Negra, apoyando la correlación de esta unidad con la parte inferior de la Formación Polanco y la asignación de ambas unidades al Ediacarense superior. Una microflora de acritarcas de baja diversidad ocurre en el GAS y en el GSB-FmCN. En el GAS se distinguen dos asociaciones de microfósiles de pared orgánica: una dominada por Bavlinella faveolata y Soldadophycus que ocurre en las formaciones siliciclásticas, y una de diversidad algo mayor denominada de Leiosphaeridia- Lophosphaeridium, característica de la Formación Polanco. En el GSB-FmCN la asociación está dominada por esferomorfos, principalmente Leiosphaeridia, siendo comparable a la asociación de Leiosphaeridia-Lophosphaeridium del GAS. Se reportan por primera vez para la Formación Villa Mónica las siguientes especies de acritarcas: Leiosphaeridia minutissima, Leiosphaeridia tenuissima y Synsphaeridium sp. Trátase de los fósiles más antiguos que se conocen en territorio de Argentina. En la Formación Cerro Largo superior aparece Leiosphaeridia tenuissima como elemento dominante y una sola vesícula de Leiosphaeridia minutissima, sumándose a previas descripciones de acritarcas de esta unidad. En la Formación Cerro Negro pudieron aislarse acritarcas asignables a Leiosphaeridia tenuissima. Se verifica una disminución del diámetro máximo de las acritarcas hacia el tope: de 450 µm para la Formación Villa Mónica a 380 µm para la Formación Cerro Largo y finalmente 200 µm para la Formación Cerro Negro. La asociación de acritarcas del GSB-FmCN puede correlacionarse con la Palinoflora de Leiosferas Simples o con la asociación Kotlin-Rovno del Ediacarense. La asociación de acritarcas del GAS inferior y medio ha sido asignada a la asociación Kotlin-Rovno y por tanto apoya una correlación entre las formaciones Cerro Largo, Loma Negra y Cerro Negro con la mitad inferior del GAS. Los datos quimioestratigráficos disponibles apoyan esta correlación, verificándose valores de 87Sr/86Sr entre 0.7069 y 0.7087 y variaciones seculares de d13C para los carbonatos entre -3.5 y +5.8 ‰ PDB. Respecto a la Formación Villa Mónica, los datos disponibles son ambiguos en cuanto a su edad y posibles correlaciones. Mientras la asociación de estromatolitos que ocurre en esa unidad sugiere una edad Neoproterozoico temprano, la similitud de la microflora allí preservada con la de las unidades suprayacentes sugiere que no existe un hiato temporal muy importante entre las mismas. Desde el punto de vista paleogeográfico, las correlaciones arriba planteadas implican el desarrollo de una extensa plataforma en el Ediacarense tardío y en condiciones tropicales, que abarcaba una gran parte del Cratón del Río de la Plata e incluía posiblemente al Grupo Corumbá en Mato Grosso, Brasil.