Evolution of an aggradational wave-dominated delta: Sediment balance and animal-substrate dynamics (Upper Cretaceous La Anita Formation, Southern Patagonia)

Abstract

Aggradational deltaic successions are uncommon in the stratigraphic record since deltas are progradational by nature. However, deltaic coastlines fed by multiple small distributaries rather than by a well-defined trunk fluvial distributary system might result in aggradational successions if the sediment is effectively redistributed from river mouths by active basinal processes (e.g., tides, fairweather waves, and/or storm waves). The wave-dominated deltaic succession in the lower part of the La Anita Formation accumulated during the foreland stage of the Austral-Magallanes Basin (Argentina). Through a sedimentological and ichnological study of the interval, we evaluate and discuss the potential controls on the evolution of aggradational wave-dominated deltaic successions. The studied stratigraphic interval is up to 150 m thick and includes facies deposited in wave-dominated prodelta, wave-dominated distal delta-front, wave-influenced mouth bar, terminal channel, and distributary channel settings. These facies associations stack vertically, forming clear thick aggradational sets. Fairweather wave processes largely dominate the delta-front and prodelta deposits, although discrete beds point to the periodic importance of linked storm-wave and river-flood processes. Owing to the alternation of beds generated during fairweather shoaling waves with those of coupled storm-wave and river-flood cycles, it is proposed that the overall aggradational stacking pattern of the delta is the response to the sediment balance. Clastic material supplied by the distributary channels of the delta was easily transported alongshore during fairweather conditions by longshore currents and basinward via storm-generated waves, and/or hyperpycnal currents and hypopycnal plumes generated by river floods. The alternation of these depositional processes strongly affected the benthic faunal communities, which is well expressed by the resulting ichnological suites. Storm-wave and river-flood events led to the widespread defaunation of tracemaking organisms, leading to generation of unburrowed beds. Between these events, short-lived colonization windows appeared locally, allowing sparse, facies-crossing dwelling and detritus-feeding burrows to be constructed by the recovering benthic communities. By contrast, during prolonged periods of fairweather shoaling, river-induced physico-chemical stresses were greatly reduced, and the resulting beds display elevated bioturbation intensities with greater numbers of burrows that record a wide range of behaviors.