CIG   05423
CENTRO DE INVESTIGACIONES GEOLOGICAS
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Sedimentology and sequence stratigraphy of the fluvial-to-tidal transition zone in the upper Lajas Formation (Neuquen Basin, Argentina)
Autor/es:
GUGLIOTTA, M.; FLINT, S.S.; HODGSON, D.; VEIGA, G.D.
Reunión:
Conferencia; 8th International Conference on Tidal Environments; 2012
Resumen:
The change from fluvial to estuarine settings is marked by the interaction of unidirectional fluvial currents and tidal currents. In modern systems the sedimentary process changes in the transition zone can be monitored. However, our understanding of how the fluvial-to-tidal transition zone is recorded in stratigraphic successions is poorly constrained. The lower reaches of many rivers are influenced by marine process, including tidal currents and brackish water. Because the resulting interaction of fluvial and marine processes occurs in low-lying areas near the coast, there is a high likelihood that these facies will be preserved in many sedimentary basins. This study is focused on the sedimentological characteristics of the fluvial-to-tidal transition zone in order to develop a better set of criteria for determining palaeogeographic position. Specific points to be addressed include: how does grain size change through the transition?; in what order do the various marine indicators begin to be expressed moving down palaeoriver?; how does the transition change as a function of parasequence stacking pattern (forward-stepping vs aggradational vs back-stepping)? Preliminary results from a detailed outcrop case study of the Middle Jurassic Lajas Formation of the Neuquén Basin are presented. The Lajas Formation comprises about 600 m of succession, spanning approximately 4.5 My and it is distinctive as tide-influenced sedimentation persisted through complete base level cycles and was not restricted to transgressive systems tracts or to the fills of incised valleys (McIlroy et al., 2006). The Lajas Formation contains fluvial deposits and is overlain transitionally by the fluvial Challacó Formation, providing an ideal opportunity to explore the stratigraphic distribution of preserved sedimentary facies, and reservoir and seals, in the fluvial-to-tidal transition. The study is focused initially on the “Bajada de los Molles” area, in which the transition between tidal and fluvial deposits is clearly and continuous exposed for about 0.5 km along the outcrop. Several erosion surfaces, correlatable across the entire outcrop, mark abrupt stratigraphic changes in facies that allow interpretations of changes in palaeoenvironment. Multiple measured sections have been correlated to constrain strike-parallel change in facies. In general, tidal and fluvial channel deposits are characterized by rather different grain size ranges. Tidal channels are filled by fine to medium sandstone and some abandoned mud-filled channels are also present. Fluvial channel fills are coarser (medium to coarse sandstone) and may have basal pebbles layers. Palaeocurrent data indicate westerly flowing rivers but the marginal marine/marine deposits show a wide range of paleocurrent directions and scales of crossbedding. Generally the Lajas Formation prograded from the southern and the eastern margins of the basin, but the subordinate tidal current and wave processes generated centimetre-scale cross-bedding in different directions. As well as grain size and palaeocurrents additional indicators allow the distinction of fluvial-dominated from tidal-dominated deposits and better define the range of the fluvial-to-tidal transitional zone. These indicators include tidal bundles, drapes of mud and coaly material, shells, bioturbation (marine) and large-scale cross bedding, abundant silicified wood, plant debris and thin carbonaceous shales (fluvial).