Late Pleistocene and Holocene geomorphologic evolution of Laguna Las Vueltas area, Tierra del Fuego (Argentina)

Laguna Las Vueltas (LLV) area retains the morphology of a late Pleistocene watershed that was flooded during a mid-Holocene marine transgression. Sediments associated with a paleosol dated at 22,582 cal yr BP reflect subaerial exposure of the area prior to the submergence during the marine transgression. This transgression produced an extensive tidal flat near the mouth of the former LLV watershed by 7,477 cal yr BP. Subsequent decoupling of the Las Vueltas valley from the sea occurred through the growth of a baymouth barrier and a beach-ridge plain to the east. This decoupling turned the lagoon into a pan environment in which subsequent lake-level fluctuations were controlled by climate. A lunette dune developed at the pans in the former lagoon, providing a narrow corridor where humans trapped, killed and processed guanacos as early as 3,402 cal yr BP. Changes in aeolian sedimentation hint at increased aridity during the past 500 years.

Long-term entrenchment and consequences for present flood hazard in the Garona River (Val d'Aran, Central Pyrenees, Spain)

On 18 June 2013, a damaging flood of the Garona River (Val d'Aran, Central Pyrenees, Spain) caused losses exceeding EUR 100 million. Few studies have related flood events to the geologic, tectonic and geomorphologic context. This study deals with both short- and long-term processes by studying the upper reach of the Garona River on different timescales and space scales. There has been a clear entrenchment tendency of the drainage network since the Miocene. Post-orogenic exhumation and uplift of the Axial Pyrenees determines the recent and active tectonics of the area and leads to fluvial incision. The last Upper Pleistocene glaciation affected Val d'Aran and gave rise to a destabilization period during the glacial–interglacial transition, marked by a postglacial incision tendency. Mean entrenchment rates between 0.68 and 1.56 mm yr−1 since deglaciation have been estimated. The assessment of the 2013 flood, characterized by the predominance of vertical incision and bank erosion, suggests that the long-term tendency of the fluvial system is reflected in short-term processes. The study of the geologic and geomorphologic evolution, combined with the analysis of this 30–50-year return period flood event, helps to improve flood risk management by providing contextual information that can constrain predictions and help guide choices and decisions. In fact, the millennial entrenchment tendency is shown at the human scale, which is considered useful for river management, but could be imperceptible in detailed hydrodynamic and channel morphology studies that describe river dynamics mostly at the 10–15-year timescale.

FATORES CONTROLADORES DOS PROCESSOS DESNUDACIONAIS GEOQUÍMICOS NO ESPINHAÇO MERIDIONAL (DEPRESSÃO DE GOUVEIA, MG)

O presente trabalho investiga os fatores controladores dos processos desnudacionais geoquímicos naDepressão de Gouveia e seu entorno (Espinhaço Meridional/MG). Foram utilizadas análises físicoquímicas de águas superficiais coletadas em 21 (vinte e um) pontos de monitoramento, localizados emduas bacias hidrográficas que drenam a área de estudo. A coleta das amostras de água foi realizada emtrês períodos distintos do ano: seco, úmido e semi-úmido. Os parâmetros analisados foram: pH,condutividade elétrica, sólidos totais dissolvidos, sílica, ferro, alumínio, sódio, cálcio, magnésio epotássio. Os resultados obtidos indicam que o arcabouço litoestrutural corresponde à principal variávelambiental capaz de explicar a dinâmica desnudacional geoquímica da área investigada, a qual é seguida,em grau de relevância, pelo débito do canal fluvial amostrado. Os resultados apontam ainda para aocorrência de dois comportamentos desnudacionais geoquímicos distintos: (i) elevadas taxas dedesnudação geoquímica naquelas áreas topograficamente deprimidas que ocupam o piso da Depressãode Gouveia, modelada nos litotipos arqueanos do Complexo Gouveia; (ii) baixas taxas de desnudaçãogeoquímica notadamente nos planaltos que circundam essa depressão, modelados, sobretudo nasunidades quartzíticas do Supergrupo Espinhaço. A evolução geomorfológica da área investigadaapresenta elementos preconizados em algumas teorias que tratam da evolução do modelado continental,embora não seja possível enquadrar, de forma inequívoca e absoluta, a evolução da área investigadaem uma única dessas teorias.ABSTRACTThis paper is a research about the controls factors of chemical denudation process in Gouveia’sDepression and around (Espinhaço Meridional/MG). The Gouveia’s Depression is a depression ongranites and a gneissic rock that has in it’s around the Espinhaço Mountains that is on quartzite’srocks. The research is based in physical-chemical analysis of superficial water. Was checked 21(twenty one) points in two rivers basins that draining this area. The water levy was in three periodsof the year: humid, dry and semi-humid. The physical-chemical analysis was: pH, electric conductivity,total load, Fe, Al, Na, Ca, Mg, K and silica. The results show that the litology and the structure arethe most important environment variables that can explains the denudation dynamic in the area. Insecond place is the water discharge of the rivers. The results show too that has two chemicaldenudations situations in the area:(i) highest in the depression and (ii) lowest in the around. Thearea’s geomorphologic evolution has elements of many models of relief evolution, however cannotbe explain by only one.