The fluvial-tidal transition zone in an ancient tide-influenced fluvio-deltaic system (Lajas-Challacó Fms., Neuquén Basin, Argentina)

Abstract

The fluvial-tidal transition zone has been studied in modern as well as ancient systems. Nonetheless the sedimentary facies, the depositional architecture and the spatial extent of this zone can be widely different depending on the system (e.g. delta vs. estuary) and on the intensity of tidal interactions (fluvial-dominated tide-influenced or tide- dominated fluvial-influenced). The longitudinal changes of facies from purely fluvial, through the transition zone, to marine settings has been described extensively, but different patterns can also occur in adjacent depositional settings (e.g. interdistributary overbank vs. distributary channel deposits), increasing the complexity of this depositional setting. This study is focused on the Middle Jurassic Lajas and Challacó Formations (Neuquén Basin, Argentina), which constitute a fluvio-deltaic system influenced by tidal processes throughout most of the 300 m thick succession. The palaeogeographic setting was a large back-arc basin in which structural topography inherited from the earlier syn- rift phase developed a narrow embayment that enhanced tidal range throughout complete depositional sequences. Methods used include collection of several measured stratigraphic sections. Sedimentary facies and stratigraphic analysis are integrated with the interpretation of laterally extensive outcrops and photopanels in order to capture the depositional architecture, the key stratigraphic surfaces, and the lateral and vertical facies variations. Facies associations recognized in the Lajas Fm. consist of purely fluvial or tide-influenced distributary channels, muddy abandoned channels, muddy and heterolithic bayfill, sandy and heterolithic mouth bars, prodelta mudstones, extensive tidal flats/marshes and tidal channels. The overlying Challacó Fm. includes fluvial channels, extensive fluvio-lacustrine floodplains and crevasse-splays. In these facies associations the degree of tidal influence is weak and consists of tidal modulation of fluvial dunes and bars, which is considered to happen in the backwater zone ( sensu Martinius & Gowland, 2011). Conversely, the deltaic Lajas Fm. shows continuous tidal signatures although the abundance of tidal facies may vary. The lower part of the study interval is characterized by extensive interdistributary bay, distributary channel and mouth bar deposits. These facies were deposited in a tide- influenced delta in which the maximum suspended sediment concentration was at the mouth of the river and the currents were predominantly unidirectional and seaward oriented. These facies are overlain by extensive tidal flats and heterolithic tidal channels showing a retrogradational stacking pattern consistent with abandonment of the delta plain and development of tidal facies. The Challacó Fm. consists of a fluvial system deposited in an alluvial plain to upper delta plain. Well-drained overbank deposits were periodically flooded by fluvial fresh water. In the fluvial channels up to approximately 10 km inland of the coeval shoreline, cyclic segregation of grain-size, bundling with increasing-decreasing frequency of carbonaceous drapes and rare mud drapes are interpreted as due to flow retardation and tidal drawdown of the fluvial currents. Although these channels were affected subtly by tidal processes they lack any marine or brackish trace fossils, suggesting a fresh-water tide-modulated setting. The fluvial-tidal transition zone can be up to several hundreds of kilometres in length, thus a significant amount of its deposits can be erroneously interpreted as purely fluvial. The correct identification of sedimentological features of this zone and its migration as recorded in stratigraphic successions will improve understanding of sediment transfer processes from continents to oceans.