ABSTRACT. The Candeleros Formation is represented by a succession of deposits that represent terminal fans starting to fill a foreland basin (back-arc) during the Albian. The different sub-environments of the terminal fan suggest low gradients during the onset of deposition of the Río Limay Subgroup (Neuquén Group). The terminal fan system is represented by distributary zone (TF-DZ), distributary zone-base zone (TF-DZBZ) and base zone deposits (TF-BZ). Its most notable feature is the presence of bitumen in "pockets" and scattered in the sandstone channel fill and crevasse splays deposit. In this system the feeder zone of the terminal fan system has not been identified, however, the distribution of architectural elements and paleocurrent data allow us to infer that the catchment area of the backbone would have been located to the NNW/NW. Distributary zone deposits are characterized by thick deposits of low sinuosity channel, with a high degree of connectivity and strong vertical aggradation of channel belts. Deposits of distributary zone-base zone are characterized by a higher density of channel deposits/distributary plain, but with belt channel less thick, longer laterally, greater width/thickness ratio and connectivity in the braided deposits. The distal parts of the terminal fan are well represented and vary only in the channelized deposits and mantle/floodplain ratio, which is abruptly reduced towards the flood basin connection. The importance of Candeleros Formation deposits is that they are remnants of accommodation that preserved the original architecture and represent exhumed reservoirs. RESUMEN. Estratigrafía y sedimentología del abanico terminal de la Formación Candeleros (Grupo Neuquén), Cretácico Inferior, Cuenca Neuquina, provincias de Neuquén y Mendoza, Argentina. La Formación Candeleros se encuentra representada por una sucesión de depósitos de megabanicos terminales que representan el inicio del relleno de la cuenca de antepaís (retroarco) durante el Albiano. La identificación de los distintos subambientes del abanico terminal permite inferir bajos gradientes regionales topográficos durante el inicio de la depositación del Subgrupo Río Limay (Grupo Neuquén). El sistema de abanico terminal está representado por depósitos de la zona distributaria (TF-DZ), zona distibutaria-zona base (TF-DZBZ) y zona base (TF-BZ). Su rasgo más destacable es la presencia de bitumen en "bolsones" y de manera dispersa en las areniscas del relleno de canal y en los depósitos de abanicos de desborde. En este sistema no se ha identificado un sistema de alimentación del abanico terminal, sin embargo, la distribución de los elementos arquitecturales y datos de paleocorrientes permiten inferir que el área de captación de la red troncal se habría ubicado al NNW/NW. Los depósitos de la zona distributaria están caracterizados por potentes depósitos de canales de baja sinuosidad, con escasa participación de canales sinuosos, con un alto grado de conexión y fuerte agradación vertical de los cinturones de canal. Los depósitos de la...
RESUMEN.En el sector centro-este de la Cuenca Neuquina, suroeste de Neuquén, Patagonia Argentina, el Subgrupo Río Neuquén de origen continental ha sido dividido en dos secuencias estratigráficas incompletas (SI y SII) sobre la base de discontinuidades que limitan arreglos de litofacies y elementos arquitecturales característicos. Parte de la sucesión (Formación Portezuelo-SI) se formó a partir de sistemas fluviales de baja y alta sinuosidad que drenaban hacia el noreste. La acreción lateral y oblicua es un rasgo importante de los sistemas de alta energía con carga mixta. Los depósitos de planicie de inundación dominan la estratigrafía, contribuyendo con más del 45% de los sedimentos. La Formación Plottier (SII) se acumuló a partir de un sistema de baja sinuosidad perenne con un buen desarrollo de los depósitos de planicie aluvial con eventos de inundaciones. La unidad se acumuló sobre una topografía con un gradiente general hacia el este-noreste. La arquitectura aluvial y patrones de apilamiento en el Subgrupo Río Neuquén estuvo controlada por la combinación de factores alogénicos, el más significativo fue la tectónica seguida por el clima. La sucesión del Cretácico Tardío en la cuenca de antepaís andina, depositada en la cuenca periférica, consiste de ciclos tectónicos que comprenden dos componentes: estratos depositados en una cuenca subalimentada (Formación Portezuelo, AA-SI) y sedimentación en una cuenca sobrealimentada sin un abultamiento periférico definido topográficamente (Formación Plottier, BA-SII). ABSTRACT. Sedimentology and high resolution stratigraphy of Río Neuquén Subgroup (Upper Cretaceous) department Confluence, Province of Neuquén, Argentina.The Upper Cretaceous exposid in the northeastern part of the Neuquén Basin, southeast of Neuquén, Argentina. Río Neuquén Subgroup of continental origin, has been divided into two incomplete stratigraphic sequence units (SI y SII) on the basis of lithofacies assemblages and architectural elements. Parts of the succession (Portezuelo Formation SI) formed in northeasward-draining low and high sinuosity fluvial systems. Those are analyzed in detail in terms of macroform-scale and stratigraphic-level architecture to assess fluvial styles and the evolution of non-marine depositional environments. Lateral and oblique accretion is a significant process of deposition in high-energy systems with mixed load. The overbank deposits dominate the floodplain stratigraphy; they contribute more than 45% of floodplain sediments. The Plottier Formation (SII) was accumulated from a low sinuosity perennial system with well developed deposits of alluvial plain with flood events. The unit was accumulated on topographic slopes dipping from west-southwest to east-northeast. The alluvial architecture and stacking patterns in the Río Neuquén Subgroup were controlled by a combination of allogenic factors, most significantly tectonics followed by climate. The Upper Cretaceous succession in the Andean foreland basin, deposited in a backbulge zone, consists of tectonically-driven cycles ...
The La Chilca Formation is exposed at the Poblete Norte and Ancha sections, Talacasto, Central Precordillera. Detailed facies analysis on field outcrops suggests a tide-dominated tidal plain model on the Silurian siliciclastic ramp, integrated by supratidal to subtidal plain deposits. The lower part of the La Chilca Formation (upper supratidal plain) is herein attributed to reworked glaciomarine deposits that occurred during interglacial Hirnantian age, under conditions of sea rise and the action of tidal amplification. The intertidal-supratidal depositional environments are represented by tidal channels, scour and fill, tidal dunes, and hummocky cross-stratified sandstones. The estimated tidal conditions for the tidal plain were mesotidal to macrotidal, under action of combined flows, attributed to the interaction of tidal currents and orbital low-velocity waves. The identification of microbially induced sedimentary structures (MISS) in these deposits, such as petee polygonal structures, pitted texture, imploded sand volcanoes, Astropolithon sp., Manchuriophycus sp., domal build-ups, biofilms, elephant skin, and trace fossils such as Chondrites sp., Zoophycos sp., Undichnia sp., Planolites sp., Monomorphichnus sp., and gastropod traces, may have played a protection and stabilization role on the clastic sediment surfaces against erosion of the La Chilca Formation deposits. The prolific growth of microbial mat on the intertidal to supratidal zone occurs explicitly at the turning points of transgression-regression cycles in shallow-water environments under stable base-level conditions. The middle-upper section of the La Chilca Formation shows non-cyclic rhythmites that are replaced by the cyclic rhythmites, suggesting an open tidal plain subject to an orbital/climatic rather than a tectonic control of cyclicity (Milankovitch cycles). The aggradation in this section indicates a shallowing towards the top of the formation. The La Chilca Formation records two third-order transgressive events and a shallowing event. The transgressive events are found in the base and top of the formation and are marked by the presence of Fe-oolites, while the shallowing event is determined by the presence of MISS.
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