Gustavo Gabriel Bujalesky

Holocene raised beaches along the northern coast of the Beagle Channel, Tierra del Fuego, Argentina

Abstract Raised beaches developed during Holocene times along the northern Beagle Channel coast. These deposits contain a rich marine fauna, especially shelled organisms such as mollusks. Four Holocene marine sites along the northern coast of the Beagle Channel have been analysed; from west to east they are: Bahia Ensenada, Playa Larga, Bahia Brown and Cutalataca airfield. They were studied in relation to relative altitude, sediment type, faunal composition and radiocarbon dates. The area is characterized by several discontinuous terraces with elevations varying from about 1.5 to 10 m above sea level. Their possible origin (1) due to tectonic uplifting, in comparison with raised beaches of the northern part of Tierra del Fuego, and (2) resulting from glacio-isostatic recovery, taking into account the ice retreat, is discussed. Seventeen species of mollusks (eight bivalves, nine gastropods) were collected from the study sites. All of them are living species from the Beagle Channel and have been interpreted as representative of Holocene paleocommunities in this channel. These species show no evidence of substantial climatic changes during the Holocene, although minor temperature fluctuations cannot be ruled out yet.

Gravel drift and wind effects on the macrotidal San Sebastian Bay, Tierra del Fuego, Argentina

Abstract In northeastern Tierra del Fuego, the Holocene mean sea-level (MSL) fluctuation reworked much pre-Wisconsin glacial drift into sand/gravel beaches and spits, and sand/mud tidal flats. For the last 7000 years, sea level has been falling. A southwards wave-induced longshore transport of sand and gravel has constructed the 17 km long El Paramo spit, which partly encloses the wide San Sebastian Bay. Semidiurnal tides with a range of up to 10 m enter the bay from the south, causing a dominance of flood currents and wave action in the southern part of the bay; ebb currents predominate in the north. This segregation of flows conditioned sedimentation and is thus responsible for the south-north transition of welded gravel ridges, sand flats, mixed flats and mud flats. On the southern coast of the bay, wave action constructed discrete sets of gravel ridges which diverted the San Mar′in River inlet northwards over the tidal flats. North of this inlet, where gravels are not yet available, storms produce chenier systems (discrete deposits of shells and sands), the lengths of which progressively shortened as the inlet of the bay became narrower and the bay infilled. Because storms affect both the bay and the Atlantic beach, washover fans cut these cheniers and the gravel ridges on the narrower part of the spit. The fossil gravel ridges in the northern and southern parts of San Sebastian Bay are not the only supratidal environments. Fossil and active marshes evolve, conditioned by the strong and persistent wind action. Seasonal ponds are produced by intense deflation. Salicornia marsh occurs as circular structures (2–3 m diameter) clustered over the supratidal flats. On the intertidal mud flats, wind causes deflation during neap tides and silting during spring tides. North of the bay, the eastward migration of straight tidal channels or rills is conditioned by winds. The effects of strong and persistent winds from the Pacific influence not only the tidal flats, but also the gravel spit. If ocean waves induce a southward drift of gravels within the bay, the wind-induced gravel drift is to the north. During winter there are no aeolian effects over the iced tidal flats.

Coastal Geology and Morphology of Patagonia and the Fuegian Archipelago

Publisher Summary This chapter focuses on the coastal geology and morphology of Patagonia and the Fuegian Archipelago. Patagonia and Tierra del Fuego show a wide and varied record of coastal evolution for the Quaternary period, comprising the complicated relationships among glacial deposits, interglacial highstands, and different uplift trends. The sea-level highstands that occurred during the Middle Pleistocene are difficult to distinguish because it is necessary to consider the precision of the dating method and if the uplifting rate permitted a vertical displacement between two consecutive highstands. In northern Patagonia, Holocene beach plains originated because of gravel abundance, between capes or related to river inlets. On the Atlantic coast of Tierra del Fuego there are regressive-like sequences of beach ridge plains in protected areas, and transgressive-like beach plains developed at exposed areas are subjected to fluctuations in gravel availability.

Palynofacies analysis of surface sediments from the Beagle Channel and its application as modern analogues for Holocene records of Tierra del Fuego, Argentina

Palynofacies analysis of surface sediments from the Beagle Channel, Tierra del Fuego, Argentina, was carried out to establish modern analogues for comparison with other Holocene marine records in southern Isla Grande de Tierra del Fuego. Our results show the dominance of highly degraded translucent phytoclasts, associated with amorphous organic matter (AOM) and palynomorphs, while opaque phytoclasts are poorly represented. The organic constituents indicate the proximity of the continental source area to marine environments, with distances and/or times of relatively short transport. The predominance of translucent phytoclasts associated with pyrite suggests reducing conditions, probably associated with marginal-marine environments. Among the terrestrial palynomorph group, the predominance of Nothofagus pollen reveals the presence of forests along the channel. The high terrestrial organic matter input to the depositional area are consistent with a marginal-marine environment. The aquatic palynomorphs, mainly dinoflagellate cysts show assemblages characterised by low species diversity and low concentration values. The dominance of Peridiniales over Gonyaulacales suggests inner neritic environments. Comparison with two fossil sections of Mid-Late Holocene age (Albufera Lanushuaia and Río Ovando) shows similar distribution of the total palynological matter.

Groundwater Dynamics on Macrotidal Gravel Beaches of Tierra del Fuego, Argentina*

Abstract On bimodal microtidal beaches, there are grain-size segregations and the relationship between mean size and foreshore slope is not clear. Dealing with macrotidal beaches composed of gravel and sand, the dynamics vary in response to the tide: reflective conditions at high tide, and a dissipative behaviour during low tide. These dynamic changes are related to grain-size segregations: the upper beach is dominated by gravel (or gravel with coarse sand) and the low-tide terrace is exclusively composed of fine sand. Along the Atlantic coast of Tierra del Fuego, nineteen beaches analysed vary in mean grain size between −4 and 4 phi units. The slope of the foreshore (reflective portion) is linearly related to grain size. The slope of the low-tide terrace (dissipative portion) has no relationship to grain size (fine to very fine sand). These segregations cause temporal changes in the composition of the groundwater sceping at the low-tide terrace. Test experiments performed at the beach permitted to recognise the relationships between coastal and ground water. Salt groundwater similar to the sea (31 practical salinity units; psu) seeps at the initial stages of the dropping tide. Seeping groundwater salinity diminishes progressively, and at low tide it arrives to a minimum of 10 psu. Tidal effects are therefore conditioning (chemically and physically) the water exchange between the beach and the nearshore areas.

Saltation on gravel beaches, Tierra del Fuego, Argentina

Abstract Saltation is a common phenomenon on gravel beaches. Sediment traps across two beaches of the El Paramo Spit (Tierra del Fuego, Argentina) yielded different results on mean-grain size, sorting, saltating load, shape and height above the bed. Mean-clast size increases with height above the bed to an altitude where the saltating size diminishes. The sorting of the saltating populations also increases until an altitude where it begins to diminish. Spheres are preferentially set into saltation, although the bed is dominated by platy, bladed and elongated gravels. Much of the behavior of the saltation population is related to local wave dynamics, beach steepness and stage of the tide. The processes of gravel segregation (size and shape) on beaches obey to particular phenomena not considered in present-day sediment transport equations, but of much importance in relation to coastal defense improvements as dynamic revetments.

Tsunami Overtopping Fan and Erosive Scarps at Atlantic Coast of Tierra Del Fuego

Abstract BUJALESKY, G.G., 2012. Tsunami overtopping fan and erosive scarps at Atlantic Coast of Tierra Del Fuego. The Ensenada de la Colonia embayment, NE Tierra del Fuego, presents an extensive and wide Holocene beach ridge plain composed of gravel and coarse sand. These deposits have revealed a regressive behavior since 6000 years BP. A Pleistocene gravel beach ridge plain was recognized in the area. Several erosive scarps are present at the embayment, indicating strong and episodic erosive events. Seaward and immediately behind the present storm berm, a high and unique overtopping fan develops. Tierra del Fuego is a seismic area, and the subduction zone of the South Sandwich Islands constitutes a potential source of tsunamis. The abrasion platform of embayment shows a glacifluvial carved paleomorphology. This feature conditions the wave refraction pattern, concentrating the orthogonal rays in its central section. In this setting, a tsunami backwash could generate megavortexes in the nearshore zone of the bay linked to the depressed areas of the abrasion platform. The combined action of large seepage, tsunami backwash currents, and tangential currents of the vortex applied on saturated gravel beaches would cause carving of the lower section of the beach and instability of the entire profile. Then collapses of relevant volumes of gravel and sand would lead to the formation of semicircular erosive scarps. The overtopping fan architecture displays a sequence of three to five landward onlapping radar facies of overwashing and overtopping events. The semicircular erosive scarps and the overtopping fan were created by tsunamis that reached the Atlantic coast of Tierra del Fuego. At least three pulses of a unique tsunami episode constructed the overtopping fan.

Holocene coastal environments and processes in subantarctic/temperate cold Tierra del Fuego, Argentina-Chile

Abstract Holocene coastal environments in the Tierra del Fuego archipelago, southernmost South America, evolved in a temperate cold climate characterized by strong and persistent mid-latitude westerly winds, and under stable to falling relative sea-level. Southern and western coasts are rocky, microtidal and generally devoid of Holocene accretionary coastal landforms. They are young, having originated from flooding by the early Holocene. Northern and eastern coasts are lined by receding cliffs that supply abundant sediment to macro/mesotidal embayments lodging extensive accretionary coastal landforms. Spit growth on the Atlantic shore played a major part in modifying the development of other landforms by barring the shoreline from wave attack. Sediment was mainly supplied from Pleistocene glacial deposits exposed at eroding headlands and, more locally, obtained by cannibalization and from elevated Pleistocene beaches. In places, westerly winds combined with Atlantic swell to foster spit growth under high-energy wave power on both bay and ocean flanks – a unique occurrence. In addition, the westerlies caused widespread deflation on supratidal marshes and delivered fine-grained sediment inland and to the sea. Recent acceleration of glacio-eustatic rise is gradually changing the coastal zone to a condition of relative sea-level rise. The generally high and steep coastlines will not be greatly affected. A few gently-dipping lowlands might be flooded and biodiversity will be negatively affected.

El Paramo Transgressive Gravel Spit, Tierra del Fuego, Argentina

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. Peninsula El Paramo (El Paramo spit) is a 20 km-long gravel spit barrier that closes partially the San Sebastian 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.