The hypertidal estuary of the Santz Cruz estuary (SOUTH PATAGONIA, ARGENTINA): A composite RIA-TYPE SYSTEM subject to exceptional hydronamic and sedimentary forcings
Résumé
Few coastal sites are subject to hypertidal tidal regimes, i.e. with tidal ranges greater than 8 m (Archer, 2013). Hypertidal systems are mainly found in the Northern hemisphere, where the continental shelves are the most developed. In the Southern hemisphere, tides reach hypertidal range only in two places, in the North of Australia, and in Southern Patagonia in Argentina. The huge Patagonian continental shelf allows a strong amplification, with tidal ranges reaching up to 12 m at the entrance of the estuaries present along the coasts of the Santa Cruz Province. This is the case of the Santa Cruz estuary (The SCE) located about 350 km North of the Strait of Magellan, at the northern limit of the Atlantic Austral Basin. This ria-type estuary is bordered by cliffs, a few tens of meters high, cut into fine-grained, poorly cemented Miocene sandstones, and covered by pebbly Plio-Quaternary fluvial terraces (the "Rodados Patagónicos"). The SCE is formed at the confluence between the Andean rivers Santa Cruz and Rio Chico, located 28 km from the mouth. The Santa Cruz is the most important river in Patagonia, with an annual flow of 750 m3/s. The Rio Chico has an average annual flow of only 25 m3/s. The tidal range is about 12 m just upstream of the mouth, and still 8 m at the confluence. Tidal currents are powerful, up to 5 knots, with a dominance of the flood between the mouth and the confluence. In the Santa Cruz, due to the strong fluvial stream, the flood is only active for a few kilometers upstream from the confluence; there is no reversal of currents beyond. In the Rio Chico, the flood spreads over a distance of 30 km upstream from the confluence. Atlantic swells induce an active erosion of the coastal cliffs, and the formation of gravel and pebble spits and beach ridges at the estuarine mouth. The very strong W-SW winds, which blow almost permanently in this region of Patagonia, form energetic waves into the estuary. The present works in the SCE aims in defining the characteristics of this hypertidal estuary located in a sub-desertic climate, connected to proglacial rivers with contrasting seasonal flows, in terms of morphodynamic processes and evolution. Our findings are based on a morphosedimentary study of the inter- and supratidal environments, as well as on a seismic survey of the subtidal domain, complemented with GPR investigation of the pebble spits and barriers. They show that the SCE corresponds to a wave-dominated estuary, with a central basin between the mouth and the confluence. A bay-head delta accumulates at the confluence and upstream of the confluence in the Santa Cruz. The lower reach of the Rio Chico constitutes a “tide-dominated” estuary in itself. The central basin, between the confluence and the inlet, is a bedrock, sediment by-passing area, with sparse gravel bars and tidal flood-oriented dunes. All along the estuary banks, from the fluvial segment to the coast, there is a continuum of stepped pebble spits related to the massive reworking of the terrace pebbles, and favoured by the tectonic uplift context. Most of the sand fraction, supplied by the Miocene sandstone erosion, is exported offshore where it forms a huge ebb-tidal delta. In contrast with this strong seaward flushing of sand, the hypertidal dynamics allows large intertidal mudflats to form in the bayhead delta area. These deposits, for part of aeolian origin, host a variety of micro-organisms (thecamoebians and estuarine foraminifers) typical of the Southern Hemisphere, but seem devoid of macrobenthos and related bioturbation. Acknowledgements. Funding provided by the ECOS-Sud – MinCyt and CNRS-INSU-SYSTER programs, M2C, LOG, IGeBA and IPGP labs.