The role of tidal, wave and river currents in the evolution of mixed-energy deltas: Example from the Lajas Formation (Argentina)

Many modern deltas show complex morphologies and architectures related to the interplay of river, wave and tidal currents. However, methods for extracting the signature of the individual processes from the stratigraphic architecture are poorly developed. Through an analysis of facies, palaeocurrents and stratigraphic stacking patterns in the Jurassic Lajas Formation, this paper: (i) separates the signals of wave, tide and river currents; (ii) illustrates the result of strong tidal reworking in the distal reaches of deltaic systems; and (iii) discusses the implications of this reworking for the evolution of mixed-energy systems and their reservoir heterogeneities. The Lajas Formation, a sand-rich, shallow-marine, mixed-energy deltaic system in the Neuquen Basin of Argentina, previously defined as a tide-dominated system, presents an exceptional example of process variability at different scales. Tidal signals are predominantly located in the delta front, the subaqueous platform and the distributary channel deposits. Tidal currents vigorously reworked the delta front during transgressions, producing intensely cross-stratified, sheet-like, sandstone units. In the subaqueous platform, described for the first time in an ancient outcrop example, the tidal reworking was confined within subtidal channels. The intensive tidal reworking in the distal reaches of the regressive delta front could not have been predicted from knowledge of the coeval proximal reaches of the regressive delta front. The wave signals occur mainly in the shelf or shoreface deposits. The fluvial signals increase in abundance proximally but are always mixed with the other processes. The Lajas system is an unusual clean-water (i.e. very little mud is present in the system), sand-rich deltaic system, very different from the majority of mud-rich, modern tide-influenced examples. The sand-rich character is a combination of source proximity, syndepositional tectonic activity and strong tidal-current reworking, which produced amalgamated sandstone bodies in the delta-front area, and a final stratigraphic record very different from the simple coarsening-upward trends of river-dominated and wave-dominated delta fronts.