El Paramo Transgressive Gravel Spit, Tierra del Fuego, Argentina

Abstract

The northeastern Atlantic coast of Tierra del Fuego is located in the extra-Andean lowlands of the island. This coast experiences a macrotidal regime, moderate energy waves and intense westerly winds. Extensive beaches and other littoral deposits are composed of gravel and coarse sand. This area has been free of ice since 1.8 Ma B.P. Glacigenic deposits were re-worked by littoral processes that formed beaches during sea level highstands of the Pleistocene. Península El Páramo (El Páramo spit) is a 20 km-long gravel spit barrier that closes partially the San Sebastián Bay. It has formed during the last 6,000 years. The growth of the spit has taken place under limited sediment supply. Its elongation is the result of erosion and sediment recycling, resulting in a landward retreat. The Atlantic beach is composed of gravel and coarse sand and exhibits a reflective morphodynamics. Regularly spaced washover channels develop on the crests of the Atlantic beach of El Páramo spit. These channels are active under storm waves or swell during spring tides. The washover channels of the Atlantic beach of El Páramo spit were formed as the result of subharmonic edge waves generated by incident waves with periods of 11 to 17 s. The modal spacing (90 m) can be related to subharmonic edge waves generated by incident waves with periods of 15 s. The waves in the bay rework the sediments of the washover fans at the inner side of the spit, building a bayside beach ridge and plugging the backbarrier mouth of the washover channels. This process helps preserve the spit crest. A fossil gravel beach ridge at El Páramo spit shows regular and periodic spaced washover channels. These are attributed to a single episodic event of high-energy incident waves with a period of 6 s. Such waves are generated within San Sebastián Bay by extreme south-westerly winds, during maximum high tide. Although resonance or standing waves could also be generated by a strong earthquake, or by a tsunami wave entering the bay.