Seismites as a tool in the palaeoenvironmental reconstruction of fluvial deposits: The Cambrian Guarda Velha Formation, southern Brazil

Soft-sediment deformation (SSD) is widely described in the literature, but there is no clear consensus regarding its origin and significance. Existing models for SSD in fluvial sediments do not clearly demonstrate a relationship between the structures, preserved facies expression, and larger-scale depositional architecture. In this study several types of SSD structures are recorded in Cambrian fluvial deposits and these occur interbedded with undeformed strata throughout the entire stratigraphic interval. The random distribution of these features in relation to primary facies types and fluvial forms indicates that they have neither a direct nor indirect relationship with any depositional processes or bedform type. We propose that the relationship of SSD at the bed-set-scale to larger-scale depositional architecture, combined with tectono-stratigraphic analysis allows the determination of both short-term fluvial hydraulic conditions in ancient stream systems, such as the nature of the flow regime responsible for depositing ancient fluvial stream successions, and the long-term subsidence rates, in the form of mean recurrence interval of the seismic events responsible for triggering the generation of SSD in tectonically active basins. (c) 2012 Elsevier B.V. All rights reserved.

Morphology and surface sedimentation on the upper and middle continental slope off Uruguay

This paper presents for the first time a morphological and surface sediment characterization of the Uruguayan outer continental shelf and slope. The study is based on a high-resolution coverage using hydrographical, geomorphological and sedimentological sampling and several textural and productivity proxies. Along slope terraces and an important canyon system characterizes continental slope morphology, indicating that across- and down-slope sedimentary processes control large-scale sedimentation. Terraces represent the prolongation of the Argentinean Contouritic Depositional System that vanishes in the study area, presumably as a result of the dynamic of the Brazil-Malvinas confluence. Canyons incised in the upper slope are likely related to low-stand sea level conditions. At the outer shelf and shallow upper slope (170-250 m depth), off-shelf sand transport is inferred from the distribution of relict sand and reworked biogenic gravel. In the upper continental slope, the northern region is characterized by an erosive environment controlled by a steep slope and the southward flowing Brazil current. In the south, a depositional environment is enhanced by the presence of a gentler slope and seaward incised canyons and is mainly controlled by hemipelagic processes associated with nutrient-rich Sub-Antarctic Waters (SAW), by its confluence with South Atlantic Central Waters (SACW) and by the Rio de la Plata’s (RdlP) influence. Additionally, within the upper slope, the occurrence of igneous-metamorphic cobbles and pebbles in canyon and mound lag deposits suggests the influence of glacial fluvial discharge and/or iceberg transport processes. In the middle slope, sedimentation is controlled by thermohaline-induced deep-water bottom currents. The decreasing influence of the erosive Antarctic Intermediate Water (AAIW) is evident in a northward diminution in grain size. The variety of transport and sedimentary processes identified reflect the control of the Brazil-Malvinas confluence zone and the Rio de la Plata’s discharge.

Indicios de antecedencia en la red fluvial del sureste de Galicia

[ES] El presente trabajo ofrece un análisis preliminar de la red fluvial con antecedencia terciaria del sureste gallego. Las observaciones geomorfológicas de campo se centran en la cartografía de terrazas erosivas, canales abandonados, meandros colgados, codos de captura y redes anómalas. Para su interpretación se confrontaron con los thalwegs de los cursos principales y las fracturas alpinas cartografiadas por otros autores. Se propone una cronología para los procesos fluviales identificados; cronología que apunta una antigüedad de la red fluvial mayor a la estimada hasta el momento. De las siete tendencias identificadas, tres presentan una entidad regional (ENE-WSW, NE-SW, N-S), y cuatro local (NW-SE, SW-NE, SE-NW, S-N). Se confirma el carácter principal de la paleorred ENE-WSW (caracterizada por el río Sil) y como hipótesis se propone, para la Sierra de Queixa-San Mamede, el carácter de paleorrelieve positivo de herencia mesozoica. Este relieve habría sufrido varios procesos de levantamiento isostático y también tectónico durante la Orogenia Alpina. Estos levantamientos habrían provocado la superposición de capturas en las estribaciones surorientales de la Sierra de Queixa-San Mamede.

Evolución geomorfológica del Macizo Hespérico peninsular:estudio de un sector comprendido entre las provincias de Lugo y Ourense (Galicia, NW de España)

[EN] This paper deals with the relief generation in Ourense, an interior territory of the Galicia Country, at NW Spain, after the breakdown of Pangea 200 million years ago. The rupture of supercontinent causes the main effects in the outer part of Galicia, the present coast line and the shelf, but also the inner parts of Galicia where the landscape changes dramatically mainly ruled by fluvial incision connected with uprising, (orogenic, epirogenic, or isostatic origin), or even with eustatic oscillations, that shaped the previous old mesozoic landscape. Various things complicate the correct understanding of Galician geomorphology:1) the prevalent hercynian structure, (presumably reactivated during the Alpine Orogeny), causes that the epigenic processes, (fluvial, glaciar, marine and etching), acting on Galicia from Mesozoic to present times, produce end forms identified erroneously at the previous literature as tectonic and not as etch forms profiting from lithological or structural contrasts. 2) the common morphotectonic model accepted by all previous researchers establishes for the whole of Galicia a blocky pattern, (horst and graben like), due to extensional tectonic regime. This model is proved as no longer valid because the Galician tertiary basins, even were described at the past as graben depressions never have this origin. 3) big differences exist between the north and western sides of Galicia that show contrasted tectonic regime: compressional (with forms as the so called raised platforms), at the northern coast border, and extensional (with forms so typical as the Rias), at the western side. The study area is located at the confluence of two tectonic domains where the above mentioned effects are coincidents and specially well showed through different effects: prominent assimetry of fluvial captures (west facing), pronounced river incision and different kinds of tertiary basins: either strike slipe faults (Maceda, Xinzo de Limia, etc), or overslipped by inverse faults, (Quiroga, A Rúa, etc.), or even corresponding with depressions never, (or anywise passively), affected by tectonic movements, (Monforte). The paper include a detailed inventory of surfaces and terrace levels and their incision sequence which allow stablish a relative chronology of geomorphic evolution at this area of NW Spain during meso-cainozoic times.