Formas mareales de la Ria del Deseado, Santa Cruz

Abstract

El río Deseado tiene la particularidad de haber labrado su desembocadura en rocas piroclásticas de edad Jurásica, y poseer cañadones o gargantas que actualmente no fluyen con caudales necesarios que justifiquen el carcavamiento observado. Hacia la boca del estuario, la ciudad de Puerto Deseado se emplaza, como muchas ciudades del litoral patagónico, sobre playas fósiles, también conformadas entre afloramientos de piroclastitas. La ria del Deseado posee una forma de embudo orientado hacia el noreste en sus sectores mas distales hacia el este en el sector de la desembocadura. Hacia el interior de la ria se identificaron y muestrearon extensas planicies de marea y marismas, con canales mareales que las drenan. Bancos mareales (delta de flujo) y playas elevadas con restos de "sambaquis" ("concheros"] en el tope, fueron reconocidos en las porciones medias del estuario. Diferentes zonas fueron reconocidas en la ria con distintos tipos de fondo, dinámica física y régimen salino; en síntesis, características sedimentarias y parámetros ambientales bien definidos: A) Una zona de dinámica marina con acción de olas y compuesta por bancos de arenas medianas a finas y restos de conchillas representando un delta de flujo de marea disectado. B) zona de fangos estuarinos donde la fracción arenosa está en menor proporción. C) zona de mezcla de aguas, con sedimentos limo-arcillosos y parámetros ambientales característicos. D) Zona fluvial de baja salinidad con sedimentos muy gruesos en canales anastomosados. De las observaciones morfológicas se comprobó que la onda de marea se extiende 40 km hacia el interior de la ria. No obstante ello, la presencia de planicies no operativas en el interior de cañadones como el Cañadón del Puerto constituye un rasgo relicto sedimentario muy reciente que obedecería a variaciones en el régimen del río o en el régimen de mareas, posiblemente debidas a la regresión holocénica de los últimos 5.000 años.

Patagonian rivers flow across desert lands and between two plateaux of Mesozoic, Tertiary and Quaternary rocks. The original wide valleys were reduced by the deposition of terminal moraines which blocked some Atlantic watersheds at the headlands and were responsible for the changes in the outflow of the rivers. Strong westerly winds increase the evaporation. Those rivers that arrived to the Atlantic coast are subject to the action of large tidal ranges that contribute to change their inlets twice a day moving towards the headlands along manykilometers. These estuaries are called “rías” in the partial sense, and worlwide used, equivalent to “drowned river valleys” from Von Richtoffen, but where a structural control should not be discarded, as stated originaly for the “Rías Baixas” of Galicia. Patagonian rías have particular characteristics: river outflows and valley capacities diminished, and a Holocene sea-level fluctuation that affected the morphosedimentary evolution of the estuaries. The Deseado River estuary has a funnel shape oriented towards the northeast and changing to an eastward direction at the inlet. The river has excavated this inlet into piroclastic rocks of Jurassic age. Gorges and small canyons are indicating erosion features that cannot be justified by present-day processes. A sediment sampling was carried out at this estuary in order to recognize present sediment dynamics previous to any alteration induced by mariculture activity or harbour enviroments. At the inner estuary, extended tidal flats, marshes and tidal channels were recognized. Tidal-sand banks conforming a flood delta and elevated beaches (with shell middens on top) characterize the middle portion of the estuary. Salinity measurements and morphologic analysis indicate that the tidal excursion extends about 40 km. into the estuary. Non-operating tidal flats were recognized at the bottom of some canyons. The Cañadón del Puerto is a relict tidal flat originated by changes in the river dynamics or to the Mid-Holocene sea-level drop. Towards the estuary mouth, Puerto Deseado is emplaced on Pleistocene beach systems, composed mainly by gravel, and the ignimbrites, riolites and tuffs of the piroclastic Jurassic complex. Towards the headlands, at Paso Gregores, the river is recognized in an anastomosing design, with little freshwater flowing between bars composed of gravel and sand. Four different zones were discriminated along the ria with different kind of bottomcomposition, dynamics and salinity regime. The Outer Marine zone is characterized by the action of waves with tidal banks composed of medium to fine sands with shells, constituting a dissected flood-tidal delta. At the Estuarine Zone muds dominate over sands, but salinity is still indicating marine waters. At the Zone of Water Mixing, silts and clay dominate. The presence of non-operative tidal flats on the bottom of small canyons (cañadon del Puerto) is a relict feature indicating changes in the fluvial or tidal regimes, possibly associated to the sea-level regression of the last 5000 years.