Papers by Jacobus Le Roux
We present an interpretation of the stratigraphy, age and depositional environment of the Bahía I... more We present an interpretation of the stratigraphy, age and depositional environment of the Bahía Inglesa Formation in the Caldera Basin west of Copiapó, based on as yet unpublished stratigraphic sections, which were used to conduct a geohistory analysis and so we can explain the tectonic setting when its deposition took place. The basin, which stretches south along the coastal plain from Caldera to the mouth of the Copiapó River, falls within the so-called flat slab region. Caldera Basin is filled by a Neogene marine succession overlying Paleozoic metamorphic rocks and Mesozoic granitoids. The succession is subdivided into the Angostura Formation, dated at 15.3±0.6 Ma (Henriquez, 2006), Bahía Inglesa Formation, and the Caldera Beds.
We present new data on the paleoichthyology, paleobotany and radiometric results of the Loreto Fo... more We present new data on the paleoichthyology, paleobotany and radiometric results of the Loreto Formation in the Brunswick Peninsula of southernmost Chile, that allow us to propose a Late Eocene age. The rich diversity of fossil cartilaginous fishes (Chondrichthyes, Elasmobranchii) recognized in upper levels of this unit includes the taxa Carcharias aff. 'hopei' (Agassiz), Odontaspis sp., Carcharoides catticus (Philippi), Striatolamia macrota (Agassiz), Anomotodon sp., Macrorhizodus praecursor (Leriche), Galeorhinus sp., Abdounia sp., Hexanchus sp., Squatina sp., Hexanchidae indet., Myliobatis sp., Myliobatoidea indet., and Ischyodus dolloi Leriche. This assemblage has clear ecological affinities with Eocene Tethyan fauna previously described in the Northern Hemisphere, and also has common elements with Eocene cartilaginous fishes from Antarctica. Additionally, a paleobotanic study of this unit identified leaf imprints of Asplenium sp., Pteris sp., Podocarpus sp., and abundant angiosperms including Nothofagus lanceolata Dusén, N. simplicidens Dusén, N. variabilis Dusén, N. cf. alessandri Espinosa, N. subferruginea (Dusén), Hydrangea sp. and Phyllites spp. Wood remains of Nothofagoxylon scalariforme Gothan and Araucariaceae cf. Araucarioxylon Kraus were also identified. Additionally, pollen grains indicate gymnosperms and angiosperms: Podocarpidites otagoensis Couper, Retitricolpites sp., Tricolpites sp., Liliacidites sp., Polyporina sp., Nothofagidites cincta Cookson, and Nothofagidites cranwellae Couper, having affinities with Eocene florae, and being consistent with the age of the fossil fishes. Finally, a SHRIMP U-Th-Pb analysis of two samples collected from the studied beds provided thirty-eight and sixty zircon grains, indicating a clear main peak at 36.48±0.47 Ma (MSWD=1.5) and 36,73±0.50 Ma (MSWD=0.65). The integrated results indicate that the upper part of the Loreto Formation has a minimum Priabonian age, supporting previous reassignations of this part of the formation into the Late Eocene, and differing from the Oligocene age proposed in its original definition.
Bottom sediment samples were collected in June and December 1997 at 166 stations on a rectangular... more Bottom sediment samples were collected in June and December 1997 at 166 stations on a rectangular grid in a small port (Lirquén Harbor, BíoBío Region, Central Southern Chile) facing siltation problems with a view to determining local seasonal sediment transport pathways through the application of three different methodologies based on grain-size trends. Measured winds and near-bottom currents together with hindcast wave allowed to establishing the influence of the prevailing meteorological and hydrodynamic conditions on local sediment circulation. The results suggest that transport of deep and shallow water sediments occur rarely under high-energy conditions. The current data suggest that tidal asymmetry would be an important agent of sediment transport in Lirquen Harbor. The sediment transport pathways inferred from grain-size trends are compared with the measured water circulation of the study area and sediment dispersal patterns on aerial photographs. The results yielded by the grain-size trend methodologies of Gao-Collins (1992) and Le Roux (1994b) indicate a possible seasonal variation in transport pathways agreeing with the prevailing meteo-hydrodynamic conditions, whereas the McLaren-Bowles (1985) approach does not show a significant difference. It is suggested that the latter methodology may represent transport on a spatial macroscale as opposed to the mesoscale patterns yielded by the other two techniques. Based on the results of the three grain-size trend methodologies, the observed current and wind data, the hindcast wave data and aerial photographs a conceptual model of net annual sediment transport is proposed for Lirquén Harbour.
Andean Geology, 2012
We present new data on the paleoichthyology, paleobotany and radiometric results of the Loreto Fo... more We present new data on the paleoichthyology, paleobotany and radiometric results of the Loreto Formation in the Brunswick
Andean geology, 2009
New remains of plesiosaurs (Diapsida; Sauropterygia) found in a transported block correlated with... more New remains of plesiosaurs (Diapsida; Sauropterygia) found in a transported block correlated with upper levels of the Dorotea Formation, Late Cretaceous (Maastrichtian) are described herein. They were collected on the southern slopes of Sierra Dorotea located northeast of Puerto Natales (51°41'20,4"S, 72°26'07,4"W), Magallanes Region, Chile. This is the first discovery of the family Elasmosauridae in high latitudes of South America, complementing the previously known paleodistribution of this group in the eastern Pacific Ocean and the Antarctic during the latest Cretaceous.
Andean Geology, 2012
A number of glacial moraines are distributed from the eastern margin of the Torres del Paine drai... more A number of glacial moraines are distributed from the eastern margin of the Torres del Paine drainage basin to near the present margin of the Patagonian Ice Fields, together with a set of regionally continuous lacustrine terraces related to glacial fluctuations. The geomorphology, supported by lake sediment evidence, indicates the existence of a single proglacial paleolake in this area, here referred to as the Great Tehuelche Paleolake. This concept helps to clarify the chronology of glacial events and leads to a better understanding of the evolution of the hydrologic system in the Torres del Paine area. Glacial advances previously referred to as A, B and C occurred during the Last Glacial Maximum and fed the Great Tehuelche Paleolake with meltwater, allowing it to reach its maximum extension. The discovery of thrombolites at Laguna Amarga suggests that the drainage of the paleolake towards the Última Esperanza Fjord took place at 7,113 Cal. yr BP, after the melting of an ice barrier that existed during the earlier glacial advance. This gave rise to the development of a complex fluvio-lacustrine hydrologic system that persists to the present day.
Palaeogeography, Palaeoclimatology, Palaeoecology, 2010
Journal of South American Earth Sciences, 2005
The stratigraphy, sedimentology, and paleontology of the Coquimbo Formation in a coastal section ... more The stratigraphy, sedimentology, and paleontology of the Coquimbo Formation in a coastal section approximately 100 km north of La Serena was studied to establish relative low-order sea-level changes during the Neogene. During the Early-Middle Miocene, a wide coastal platform was bevelled onto Palaeozoic basement rocks, and marine sedimentation may have taken place within a deep coastal inlet later transformed into a submarine canyon. Transgression over this platform commenced during the late Burdigalian-early Langhian (16-15 Ma) and culminated in a sea-level highstand near the end of the Serravalian (11.8 Ma). This transgression was followed by regression during the rest of the Serravalian and Tortonian until approximately 9 Ma. Renewed transgression during the Messinian was succeeded by regression lasting until 5.5 Ma, when another transgressive cycle bevelled a ravinement surface into the underlying deposits. This cycle terminated in a marine highstand during the Zanclean at 4.9 Ma. Thereafter, the succession changed from outer platform to middle shoreface deposition. Rapid flooding at 3.9 Ma resulted in the formation of an extensively bored phosphatic hardground, which represents a condensed section. A lowstand during the Piacenzian is represented by inner-shelf deposits, followed by a return to outer-shelf deposition with contemporaneous upper continental slope sedimentation below the ancient shelf break. The succession is capped by Gelasian upper shoreface deposits dated at 2.0-1.8 Ma.
Investigaciones marinas, 2002
Bottom sediment samples were collected in June and December 1997 at 166 stations on a rectangular... more Bottom sediment samples were collected in June and December 1997 at 166 stations on a rectangular grid in a small port (Lirquén Harbor, BíoBío Region, Central Southern Chile) facing siltation problems with a view to determining local seasonal sediment transport pathways through the application of three different methodologies based on grain-size trends. Measured winds and near-bottom currents together with hindcast wave allowed to establishing the influence of the prevailing meteorological and hydrodynamic conditions on local sediment circulation. The results suggest that transport of deep and shallow water sediments occur rarely under high-energy conditions. The current data suggest that tidal asymmetry would be an important agent of sediment transport in Lirquen Harbor. The sediment transport pathways inferred from grain-size trends are compared with the measured water circulation of the study area and sediment dispersal patterns on aerial photographs. The results yielded by the grain-size trend methodologies of and Le Roux (1994b) indicate a possible seasonal variation in transport pathways agreeing with the prevailing meteo-hydrodynamic conditions, whereas the McLaren-Bowles (1985) approach does not show a significant difference. It is suggested that the latter methodology may represent transport on a spatial macroscale as opposed to the mesoscale patterns yielded by the other two techniques. Based on the results of the three grain-size trend methodologies, the observed current and wind data, the hindcast wave data and aerial photographs a conceptual model of net annual sediment transport is proposed for Lirquén Harbour.
Antarctic Science, 2008
The Bransfield Strait, located between the South Shetland Islands and the north-western end of th... more The Bransfield Strait, located between the South Shetland Islands and the north-western end of the Antarctic Peninsula, is a back-arc basin transitional between rifting and spreading. We compiled a geomorphological structural map of the Bransfield Basin combining published data and the interpretation of bathymetric images. Several analogue experiments reproducing the interaction between the Scotia, Antarctic, and Phoenix plates were carried out. The fault configuration observed in the geomorphological structural map was well reproduced by one of these analogue models. The results suggest the establishment of a transpressional regime to the west of the southern segment of the Shackleton Fracture Zone and a transtensional regime to the south-west of the South Scotia Ridge by at least c. 7 Ma. A probable mechanism for the opening of the Bransfield Basin requires two processes: 1) Significant transtensional effects in the Bransfield Basin caused by the configuration and drift vector of the Scotia Plate after the activity of the West Scotia Ridge ceased at c. 7 Ma. 2) Roll-back of the Phoenix Plate under the South Shetland Islands after cessation of spreading activity of the Phoenix Ridge at 3.3 AE 0.2 Ma, causing the north-westward migration of the South Shetland Trench.
Conference Presentations by Jacobus Le Roux
We mapped the spatial and elevation distribution of 143 cirques in southern Patagonia, between 50... more We mapped the spatial and elevation distribution of 143 cirques in southern Patagonia, between 50 º and 51º S, and analyzed their morphometry using simple and multivariate statistical methods. The cirque basins are located east of the Southern Patagonian Ice Field and around the Sierra Baguales Mountain Range, about 200 km from the Pacific coast. The geomorphologic evidence indicates that alpine glaciations have been the predominant style in the study area. The distribution and activity on glacial cirques are influenced by both tectonic factors (Andean uplift) and climate (Westerly Winds). Morphometric characteristics indicates a persistent climate contrasts between the western (maritime, temperate) and eastern (dry and cold) sides of southern Patagonia since the Last Glacial Maximum (LGM). Based on our analyses, we infer that glaciers located at the higher elevations were not affected by post LGM high-frequency climatic variations that would have led to only small glaciers of limited influence on cirque form evolution; on the other hand most glacial cirques analyzed shows an isometric developing trend, which could indicate that the deglaciation process in Sierra Baguales occurred quickly and continuously, and equilibrium line altitudes (ELAs) experienced a relatively rapid rise of about 400 m after the last major occupation.
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Papers by Jacobus Le Roux
Conference Presentations by Jacobus Le Roux