Gustavo González Bonorino

Drainage and base-level adjustments during evolution of a late Pleistocene piggyback basin, Eastern Cordillera, Central Andes of northwestern Argentina

In northwestern Argentina, mid-Pleistocene out-of-sequence thrusting further disrupted the Andean foreland, giving rise to the Lerma piggyback basin within the Eastern Cordillera. Emergent topography along the eastern edge of the basin, as well as in its interior, interfered with preexisting eastward-flowing river courses. In the northern part of the Lerma basin, rivers temporarily incised across the new topography, yielding wind and water gaps, and in places preserving straths with treads tilted upstream, revealing their tectonic origin. Application of Hack’s law shows correspondence between the active and the abandoned channel profiles. Bed profiles of wind gaps are distinct from those of nearby consequent streams. Defeated and deflected northern streams coalesced into a trunk stream, which maintained an open channel across the eastern bounding ranges. River defeat may have been related to rain shadowing by the growing topographic barrier and retention of easterly derived moisture outside the Lerma basin. In the southern Lerma basin, river courses were not generally capable of sustaining active water gaps. Instead, they were deflected southward, and their discharges fed ancient Lake Lerma. Lake expansion ensued until water level reached a structural low, through which lake waters outflowed from the basin and subsequently incised across the eastern bounding ranges. The relative success of northern rivers in incising across the rising topographic barrier is mainly attributed to their greater channel gradients at the point of incision, combined with high rainfall levels. Longitudinal channel profiles show that rivers in the northern Lerma basin had approached their base level of erosion before uplift of the eastern bounding ranges. After this disturbance, the system continues to approach a new base level of erosion, modified by sediment aggradation within the basin. Speculatively, mid-Pleistocene out-of-sequence thrusting is attributed to basement uplift in the distal foreland.

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.