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 Geological studies in the northern sector of the Chaco foreland Basin, Bolivia, yielded new fossils coming from late Oligocene-late Miocene of the Petaca Formation. Few fossil mammals were known from the Subandean Region of Bolivia. We report a partially complete mandible of a hegetotheriid Hegetotheriinae (Notoungulata, Typotheria) from Abapó (Río Grande River). The specimen (YPFB-LIT-PAL-005) is very close in size and dental morphology to the late Oligocene (Deseadan South American Land Mammal Age, SALMA) — Santacrucian (early Miocene) Prohegetotherium schiaffinoi (Kraglievich 1932) from Fray Bentos (Uruguay and Argentina), Salla (Bolivia), Divisadero Largo-Potrerillos and Quebrada Fiera (Argentina). However, mandibular characteristics, as the shape with a marked change in height along its length, increasing towards the back, a prominent masseteric crest, a deep mandibular groove, and a remarkable thickening of the ventral rim of the mandible, indicate differences between this specimen and Prohegetotherium schiaffinoi and the other Hegetotheriinae. The affinity of YPFB-LIT-PAL-005 with P. schiaffinoi suggests a late Oligocene to early Miocene for the upper bearing horizon of the Petaca Formation (in Abapó), an older age than previously assigned to the top of this unit (late Miocene), and confirms the taxon distribution between ∼ 36 ° to ∼ 17 ° south latitude.

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.

Sediments comprising the Agrio Formation (Weaver, 1931) were deposited in the Neuquén Basin (Argentina) during the late Tithonian - early Barremian cycle embracing the Mendoza Group (Stipanicic et al. 1968). This formation extends throughout the Neuquén Basin, and it is characterised by different facies associations. In the Neuquén Province the Agrio sediments are siliciclastic and skeletal sandstones, sandy limestones, micritic limestones and carbonate shales. In the Mendoza Province they are mainly shales, marlstones, mudstones and wackestones; bituminous shale is the most common lithology (Mombrú et al., 1978). The study area is located in the south of Mendoza Province, about 18 km southwest of Malargüe. A detailed section through the Agrio Formation was measured along the Loncoche creek (Fig. 1). Ammonites present throughout the sequence demonstrate its late Valanginian to late Hauterivian age. At this locality, the Agrio Formation overlies oyster bank facies of the Chachao Formation, and it is succeeded by shallow marine limestones of the Chorreado Member (Huitrín Fo

En la sección basal de la Formación Agrio (Cretácico inferior, Cuenca Neuquina, Argentina) se verificó la relación entre las facies, trazas fósiles y evolución del nivel del mar. Por otra parte se introdujo el concepto de asociaciones icnolitológicas, entendiéndose como tal al conjunto genética y espacialmente relacionado de trazas fósiles y facies sedimentarias. En el arroyo Loncoche, al sur de la localidad de Malargüe (provincia de Mendoza), la Formación Agrio (222 m) está compuesta por una sucesión cíclica de pelitas laminadas, margas y calizas micríticas, depositadas en un ambiente marino de rampa externa. Para el intervalo basal de la unidad (77,6 m) se definieron tres asociaciones icnolitológicas: 1- Wackestones masivos con Thalassinoides de relleno grueso, 2- Mudstones con Thalassinoides de relleno fino y 3- Wackestones entrecruzados con Palaeophycus y Teichichnus. Asimismo se observó que la presencia de cada una de ellas responde a condiciones de depositación particulares que pueden vincularse con la evolución del nivel del mar. Utilizando como herramienta las asociaciones icnolitológicas definidas, las variaciones en el apilamiento de facies y en el contenido faunístico se distinguieron cuatro intervalos dentro del tramo de secuencia analizado. El primero está compuesto por wackestones masivos con Thalassinoides de relleno grueso (asociación icnolitológica 1); el segundo por pelitas laminadas sin bioturbación; el tercero por intercalaciones de margas y mudstones con Thalassinoides de relleno fino (asociación icnolitológica 2) y el cuarto por wackestones masivos con Thalassinoides de relleno grueso, mudstones con Thalassinoides de relleno fino y wackestones entrecruzados con Palaeophycus y Teichichnus (asociaciones icnolitológicas 1; 2 y 3 respectivamente) con delgadas intercalaciones de margas. El primer intervalo constituye el cortejo transgresivo, el segundo corresponde a la zona de máxima inundación y los dos últimos conforman el cortejo de mar alto.