Papers by Jacobus Le Roux
Andean Geology
The Sierra Baguales Mountain Range, forming the eastern foothills of the Southern Patagonian Ande... more The Sierra Baguales Mountain Range, forming the eastern foothills of the Southern Patagonian Andes, has well-developed alpine-glaciated landforms which present an ideal opportunity to study climatic and non-climatic factors that control cirque development and morphology. One hundred and forty-three glacial cirques were studied with reference to 14 morphometric attributes which were analyzed using statistical analysis and GIS methodologies. The cirques were classified into two types using cluster analysis complimented with a composite map based on the attributes, the latter technique that is applied to glacial cirque analysis for the first time. Type 1 cirques are associated with glacial processes isolated from the Southern Patagonian Ice Field (SPIF), developed under locally cold and dry climatic conditions. Type 2 glacial cirques are associated with older, more extensive glacial processes controlled by regional-scale climate variables and the presence of the Pleistocene Ice Sheet. ...
Journal of Mountain Science
The glacial morphology of southern South American presents invaluable evidence to reconstruct for... more The glacial morphology of southern South American presents invaluable evidence to reconstruct former glacier behaviour and its relation to climate and environmental changes. However, there are still spatial and temporal gaps in the reconstruction of the Holocene Patagonian glacial landscape. Here we present the first geomorphological record for the Sierra Baguales Mountain Range (SBMR), forming the eastern foothills of the Southern Patagonian Andes 200 km from the Pacific coast. This area is topographically isolated from the Southern Patagonian Ice Field (SPIF), and is affected by the Westerly Winds. The study area shows evidence of ice sheet and alpine glaciations related to Andean uplift, which caused a marked climatic contrast between its western and eastern flanks since the Last Glacial Maximum (LGM). The regional rock mass strength and precipitation gradient acted as a controlling factor in the glacial cirque distribution and sizes, as well as in the development of glaciation types. We report new radiocarbon dates associated with warm/dry to cold/wet climatic changes during the middle Holocene, when former small alpine glaciers were located in the uppermost section of the SBMR basins, and eventually converged to form a small ice field or a composite valley glacier at lower elevations. This can be explained by an estimated regional temperature drop of 3.8°C±0.8°C, based on a 585±26 m Equilibrium Line Altitude (ELA) descent, inferred by geomorphological evidence and the Accumulation Area Ratio (AAR), in addition to a free-air adiabatic lapse rate. Subsequently, the glaciers receded due to climatic factors including a rise in temperature, as well as non-climatic factors, mainly the glacier bedrock topography.
Tectonophysics
The Sierra Baguales, situated north of the Torres Del Paine National Park in the Magallanes regio... more The Sierra Baguales, situated north of the Torres Del Paine National Park in the Magallanes region of southern Chile, shows a well-exposed stratigraphic sequence ranging from the Late Cretaceous to late Pliocene, which presents a unique opportunity to study the evolution of sedimentological styles and trends, palaeoclimate changes, and tectonic events during this period. The depositional environment changed from a continental slope and shelf during the Cenomanian-Campanian (Tres Pasos Formation) to deltaic between the Campanian-Maastrichtian (Dorotea Formation) and estuarine in the Lutetian-Bartonian (Man Aike Formation). During the Rupelian, a continental environment with meandering rivers and overbank marshes was established (Río Leona Formation). This area was flooded in the early Burdigalian (Estancia 25 de Mayo Formation) during the Patagonian Transgression, but emerged again during the late Burdigalian (Santa Cruz Formation). Measured palaeocurrent directions in this Mesozoic-Cenozoic succession indicate source areas situated between the northeast and east-southeast during the Late Cretaceous, east-southeast during the middle Eocene, and southwest during the early Oligocene to early Miocene. This is confirmed by detrital zircon age populations in the different units, which can be linked to probable sources of similar ages in these areas. The east-southeastern provenance is here identified as the Antarctic Peninsula or its northeastern extension, which is postulated to have been attached to Fuegian Patagonia during the Eocene. The southwestern and western sources were exhumed during gradual uplift of the Southern Patagonian Andes, coinciding with a change from marine to continental conditions in the Magallanes-Austral Basin, as well as a decrease in mean annual temperature and precipitation indicated by fossil leaves in the Río Leona Formation. The rain shadow to the east of the Andes thus started to develop here during the late Eocene-early Oligocene (~ 34 Ma), long before the “Quechua Phase” of Andean tectonics (19–18 Ma) that is generally invoked for its evolution at lower latitudes.
Sedimentary Geology
ABSTRACT An improved method is presented to determine the threshold boundary velocity required to... more ABSTRACT An improved method is presented to determine the threshold boundary velocity required to entrain sediments under waves, using the non-dimensional group settling velocity of sediments ranging from very fine sand to granules (0.1 – 3.3 mm), together with a dimensionless boundary velocity. In combination with a more accurate method to calculate the actual boundary velocity under linear as well as non-linear waves, this allows sediment entrainment to be predicted from deep water up to the breaker zone.
Investigaciones Geográficas, 2013
Composite maps have previously been applied to paleogeographic reconstructions and as an aid to m... more Composite maps have previously been applied to paleogeographic reconstructions and as an aid to mineral exploration. However, because they can combine different criteria that are used in risk and feasibility assessment into a single map with equal or weighted input from all the parameters, decisionmaking can be optimized. The methodology is particularly suitable for the evaluation of landslide hazard and susceptibility assessment, as it can combine the unfavourable factors typically associated with mountainous terrains. Composite maps also have the potential to standardize prediction and prevention criteria for different areas. The methodology is described together with an hypothetical example and a real case study in Patagonia, the results of which are compared with a susceptibility analysis of the same area using stochastic methods
En la paleobahía de Tongoy (30°S), ubicada en Chile centro-norte, se encuentran cuatro terrazas d... more En la paleobahía de Tongoy (30°S), ubicada en Chile centro-norte, se encuentran cuatro terrazas de abrasión marina, denominadas T V , T IV , T III y T II , las cuales se asocian a los estadios isotópicos marinos (MIS, por sus siglas en inglés) MIS 1, MIS 5e, MIS 7 y MIS 11. Sobre las tres terrazas más antiguas, se desarrollan suelos carbonáticos cementados, que se conocen en la literatura como calcretas. Las calcretas de Tongoy alcanzan un estado de desarrollo maduro en las tres terrazas donde se presentan, llegando en la terraza T II al estado VI en la secuencia de desarrollo de Machete (1985), el cual corresponde al más avanzado. La formación de estas calcretas responde a una serie de procesos pedogénicos, los cuales dan cuenta de una ciclicidad en el paleoclima de la zona. Las calcretas estudiadas, se subdividen en calcreta masiva, calcreta laminar y calcreta brechosa, esta última presente solo en la terraza T II que corresponde a la más antigua. La calcreta masiva posee una serie de rasgos que muestran la participación de organismos en la precipitación del carbonato, presentando una fábrica de tipo "beta". La calcreta laminar presenta una fábrica de tipo "alfa", que corresponde a la precipitación de calcita por procesos físico-químicos. La calcreta brechosa, corresponde al fracturamiento de la calcreta laminar, y se da en estados avanzados de desarrollo de este tipo de suelos.
Sediment Geol, 2008
An exceptionally large tsunami affected the coastline of southern Chile during the Pliocene. Its ... more An exceptionally large tsunami affected the coastline of southern Chile during the Pliocene. Its backflow eroded coarse beach and coastal dune sediments and redistributed them over the continental shelf and slope. Sandstone dykes and sills injected from the base of the resulting hyperconcentrated flow into underlying cohesive muds, assisted in plucking up large blocks of the latter and incorporating them
South African Journal of Geology, 1991
Sediment Geol, 2007
Grain size trends have been applied in many diverse sedimentary environments to determine sedimen... more Grain size trends have been applied in many diverse sedimentary environments to determine sediment transport paths, generally coinciding with information from tracer studies, current measurements and the orientation of sedimentary structures. The different methods proposed to date are critically analysed and compared with reference to recent field studies. It is concluded that the twodimensional methods produce comparable results and may in fact complement each other. In spite of the advances, several problems still exist, which include the sampling method and density, the choice of trend types, the relative weight of grain size parameters and the interpretation of results. These are discussed together with possible solutions.
Sedimentary Geology, 2015
The stratigraphy of the Bahía Inglesa Formation in the Caldera Basin west of Copiapó, (north-cent... more The stratigraphy of the Bahía Inglesa Formation in the Caldera Basin west of Copiapó, (north-central Chile) is revised , based on hitherto unpublished stratigraphic sections and 87 Sr/ 86 Sr dating. The depositional environment varied from a rocky shoreline to the upper continental slope, with sea-level oscillations and tectonic movements causing numerous local unconformities as well as lateral and vertical facies changes. Geohistory and sedimento-logical analyses show that the area was close to the concurrent sea level at about 15.3 Ma, but at the same time being elevated about 100 m above the present sea level. Although the basin then subsided at least 350 m until around 6 Ma, marine deposition was only recorded after 10.4 Ma. This suggests that the sea level initially dropped faster than the rate of subsidence so that subaerial erosion occurred. The period of subaerial exposure before 10.4 Ma can be attributed to the presence of a NE-trending branch of the Juan Fernández Ridge below the continental crust at this time, whereas the ensuing subsidence was due to subduction erosion and crustal accommodation in its wake as it migrated south along the South American coastline. The subsequent uplift of at least 250 m can be explained by an acceleration in plate expansion and isostatic rebound of the continental crust after being partially submerged in the upper mantle. The uplift–subsidence–uplift pattern mirrors those recorded around the Nazca Ridge in Peru, as well as in similar basins to the south of Caldera. However, a higher southward migration rate of the Juan Fernández Ridge against the edge of the South American Plate and less intense uplift– subsidence–uplift cycles are recorded in the latter basins. This can possibly be attributed to oroclinal bending of the ridge due to friction with the overlying continental plate, which diminished the angle of incidence and the intensity of the stress field, but increased the migration velocity of the ridge relative to the coastline.
Sediment Geol, 2003
The wave friction factor f w is an important dimensionless parameter used to estimate wave-induce... more The wave friction factor f w is an important dimensionless parameter used to estimate wave-induced bed shear stress which, together with current-induced bed shear stress, controls sediment transport in the marine and lacustrine environment. However, f w is either overestimated or underestimated by existing equations. The matter is complicated by the fact that different equations must be used for hydrodynamically smooth or rough flows, but the limit between these two boundary conditions is still poorly defined. Some equations also require estimated values for the Nikuradse roughness length and the wave boundary layer thickness, neither of which can be derived accurately. It is, therefore, not surprising that f w values as calculated by existing equations differ by as much as a factor of 3. In the present paper, f w is related to the Shields parameter b for unidirectional currents, which not only facilitates the direct comparison and summation of bed shear stresses, but also yields a more accurate equation valid for grains of different density in any fluid irrespective of the boundary condition.
Ore Geology Reviews, 1994
Uploads
Papers by Jacobus Le Roux