Scielo RSS <![CDATA[Latin American journal of sedimentology and basin analysis]]> http://www.scielo.org.ar/rss.php?pid=1851-497920110002&lang=es vol. 18 num. 2 lang. es <![CDATA[SciELO Logo]]> http://www.scielo.org.ar/img/en/fbpelogp.gif http://www.scielo.org.ar <![CDATA[Sedimentología de la formación Carrizal (Triásico) en el Depocentro Marayes - El Carrizal, provincia de San Juan, República Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792011000200001&lng=es&nrm=iso&tlng=es Se efectúa el análisis sedimentológico de la Formación Carrizal (Triásico) del Depocentro de Marayes - El Carrizal, ubicado en el sector oriental de la provincia de San Juan. Sobre la base de muy contrastantes características faciales y arquitecturales, la Formación Carrizal es dividida en dos miembros: Arroyo Seco y Rickard. El Miembro Arroyo Seco se caracteriza por el predominio de conglomerados, a los que se asocian areniscas, areniscas conglomerádicas y escasas intercalaciones pelíticas. Estos depósitos se asignan a un sistema fluvial desarrollado en áreas de marcado gradiente y caracterizado por cambios en la disponibilidad de agua y de materiales detríticos, los que fueron esencialmente transportados como carga de lecho. El Miembro Rickard consiste en una sucesión en la que predominan las areniscas y pelitas en litosomas que poseen marcada continuidad lateral, las que aparecen bruscamente cortadas por cuerpos lenticulares de conglomerados medianos. Estas facies permiten definir un sistema fluvial de carga mixta con buen desarrollo de depósitos de planicie de inundación y rellenos de canales compuestos esencialmente por sedimentitas psefíticas. El importante cambio en la arquitectura fluvial que se produce entre los miembros de la Formación Carrizal se relaciona con una marcada variación en la acomodación sedimentaria. El Miembro Arroyo Seco constituye un cortejo de baja acomodación, mientras que el Miembro Rickard se define como un cortejo de alta acomodación. La superficie de retrogradación que limita a ambos miembros señala un cambio importante en el perfil gradado del sistema fluvial, tiene dimensión regional y puede ser considerada como un horizonte de valor cronoestratigráfico.<hr/>The Carrizal Formation is one of the Triassic units of the Marayes - El Carrizal rift located in the eastern San Juan Province, Argentina (Figs. 1, 2). The facies analysis and the architectural study of the Carrizal Formation (Table 1) allowed recognising two facies associations. Facies association 1 corresponds to the Arroyo Seco Member (Fig. 3), and facies association 2 to the Rickard Member (Fig. 4). The Arroyo Seco Member (Figs. 3, 5) comprises vertical stacked channel belt conglomerates, associated with sandstones, gravelly sandstones and scarce mudrocks. Channel belt conglomerates are composed of extraformational rounded to subangular pebbles of gneiss, migmatite, schist, granitoid, anphibolite and quartz derived from the local crystalline basement. These deposits are encased in amalgamated lenticular beds composed of sandstones, gravelly sandstones and fine-grained conglomerates, in which trough cross-bedding is the most conspicuous sedimentary structure (Fig. 6). Thin and laterally continuous fine- and medium-grained sandstones and mudstones are less common. A monotypic corystospermaceae forest with 38 stems resting in live position was identified in one of the basal levels of the Arroyo Seco Member (Fig. 6). The deposits of this facies association are essentially the result of bedload accumulation in a braided fluvial system (cf. Smith, 1970; Miall, 1978; Bluck, 1979; Steel and Thompson, 1983; Ramos et al., 1986; Smith, 1990; Bristow and Best, 1993; Bristow, 1993b), characterised by marked changes in discharge and sediment supply. The Rickard Member (Fig. 4) consists of discrete conglomerate channel fill deposits cut into laterally extensive tabular deposits (sandstones, heterolithic intervals, mudstones, coal beds). Three sub-associations are recognised. The first sub-association is composed of discrete thick and lenticular conglomerate beds showing a well defined imbrication structure, and composed of subangular to rounded pebbles and cobbles derived from the local metamorphic-igneous basement (Fig. 7). The second sub-association consists of massive, trough and low-angle cross-bedded, medium- to coarse-grained sandstones and granule sandstones. Remains of reworked stems, very abundant in some levels, were identified in both sub-associations (Fig. 7). The stems in one of the most conspicuous levels are oriented in a NE-SW direction (Fig. 8). The third sub-association is characterised by tabular and laterally extensive massive, laminated and rippled beds of fine- to medium-grained sandstones (Fig. 7), heterolithic intervals, greenish to dark (carbonaceous) mudstones, and thin coal beds. Bioturbation is common (Fig. 7), and several fossiliferous beds composed of Cladophlebis sp., Dicroidium sp., Yabeiella sp., Kurtziana sp. and Johnstonia sp. have been identified in this sub-association. The deposits of the Rickard Member indicate a mixed-load fluvial system (Collinson, 1986; Spalletti and Barrio, 1998; Spalletti, 2001b) characterised by isolated channels encased in rapidly aggrading and extensive floodplains (Smith et al., 1989; Jensen and Pedersen, 2010). The Carrizal Formation shows a drastic change in the evolution of fluvial architecture (Fig. 9). The lower Arroyo Seco Member is dominated by tractional coarse-grained multistorey deposits, formed in gravelly and sandy alluvial plains with a discrete degree of incision. Instead, the upper Rickard Member was originated in a mixed load fluvial system, characterised by rapidly aggrading floodplain deposits in which the main channels are confined by sharp erosional bases and side banks. The vertical transition from bedload to mixed-load fluvial systems upwards suggests a marked change in the rate of creation of accommodation (McLaurin and Steel, 2007; Leleu et al., 2010). The Arroyo Seco Member is thus characterised as a low-accommodation systems tract and the Rickard Member as a high-accommodation systems tract (Fig. 9). The contact between these two members is a chronostratigraphic retrogradational surface which can be traced for several kilometres along strike. <![CDATA[A preliminary analysis of flat-gravel transport over a Sandy Beach, Pehuén Co, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792011000200002&lng=es&nrm=iso&tlng=es Pehuén Co Beach, on the southwest coast of the Buenos Aires Province (Argentina), provides an example of an heterogeneous (mixed sand-gravel) sediment beach with gravels of the most diverse size and origin. Over 40% of the pebbles are carbonate flat gravels. Field observations included beach profiles, measurements of wave generated currents with an acoustic currentmeter (ADV), videos of 5 x 5 m grid located on the swash zone during inundation from a nearby tower. Video films where rectified and analyzed using PIV and wavelets. Based on the results, we demonstrated that flat gravels can be easily transported upbeach distances in excess of 5 m in about 1.5 h. This period corresponds to the high tide situation and covering the steepest beachface segment. Wave conditions during the experiment corresponded to low breaker height and short periods, but by no means they could be considered as storm waves demonstrating that gravels can be transported even by saltation during fair weather conditions.<hr/>La playa de Pehuén Co, localizada en la costa sudoeste de la provincia de Buenos Aires (Argentina), es un ejemplo de playa con sedimentos heterogéneos (mezcla de arena y grava) con gravas de diversos tamaños y orígenes. Más del 40% de los clastos son gravas planas carbonáticas. Se efectuaron estudios en un sector de la playa que incluyeron perfiles, mediciones de corrientes generadas por olas con un correntómetro acústico (ADV) y videos de una grilla de 5 x 5 m en la zona de lavado durante el período de inundación desde una torre cercana. Los videos fueron rectificados y analizados usando PIV y wavelets. Basados en los resultados, se puede demostrar que las gravas planas puede ser transportadas fácilmente hacia la parte alta de la playa distancias mayores de 5 m en 1,5 hs. Este período corresponde a la condición de pleamar cuando el segmento más empinado de la playa frontal queda cubierto. Las condiciones de olas durante el experimento eran de rompientes bajas y períodos cortos, pero en ningún caso se pueden considerar como olas de tormenta, lo cual demuestra que estas gravas pueden ser transportadas, aún por saltación, durante condiciones de buen tiempo. <![CDATA[Findings of intraformational striated pavements in the late carboniferous glacial deposits of the Andean Precordillera, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792011000200003&lng=es&nrm=iso&tlng=es Pehuén Co Beach, on the southwest coast of the Buenos Aires Province (Argentina), provides an example of an heterogeneous (mixed sand-gravel) sediment beach with gravels of the most diverse size and origin. Over 40% of the pebbles are carbonate flat gravels. Field observations included beach profiles, measurements of wave generated currents with an acoustic currentmeter (ADV), videos of 5 x 5 m grid located on the swash zone during inundation from a nearby tower. Video films where rectified and analyzed using PIV and wavelets. Based on the results, we demonstrated that flat gravels can be easily transported upbeach distances in excess of 5 m in about 1.5 h. This period corresponds to the high tide situation and covering the steepest beachface segment. Wave conditions during the experiment corresponded to low breaker height and short periods, but by no means they could be considered as storm waves demonstrating that gravels can be transported even by saltation during fair weather conditions.<hr/>La playa de Pehuén Co, localizada en la costa sudoeste de la provincia de Buenos Aires (Argentina), es un ejemplo de playa con sedimentos heterogéneos (mezcla de arena y grava) con gravas de diversos tamaños y orígenes. Más del 40% de los clastos son gravas planas carbonáticas. Se efectuaron estudios en un sector de la playa que incluyeron perfiles, mediciones de corrientes generadas por olas con un correntómetro acústico (ADV) y videos de una grilla de 5 x 5 m en la zona de lavado durante el período de inundación desde una torre cercana. Los videos fueron rectificados y analizados usando PIV y wavelets. Basados en los resultados, se puede demostrar que las gravas planas puede ser transportadas fácilmente hacia la parte alta de la playa distancias mayores de 5 m en 1,5 hs. Este período corresponde a la condición de pleamar cuando el segmento más empinado de la playa frontal queda cubierto. Las condiciones de olas durante el experimento eran de rompientes bajas y períodos cortos, pero en ningún caso se pueden considerar como olas de tormenta, lo cual demuestra que estas gravas pueden ser transportadas, aún por saltación, durante condiciones de buen tiempo. <![CDATA[Las tempestitas peloidales de la formación Vaca Muerta (Tithoniano-Valanginiano) en el sector surmendocino de la Cuenca Neuquina, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792011000200004&lng=es&nrm=iso&tlng=es Se analizan los depósitos de tormenta de dos secciones estratigráficas de la Formación Vaca Muerta (Tithoniano-Valanginiano) en el área de Bardas Blancas, sur de la provincia de Mendoza. Estos depósitos incluyen: a) depósitos con estratificación entrecruzada de bajo ángulo; b) depósitos con estratificación entrecruzada hummocky de pequeña y gran escala; y c) depósitos laminados con gradación normal intralámina. Las secuencias completas con HCS están compuestas por tres unidades (Sa, Sb y Sc) asociadas al evento de tormenta y una unidad que incluye depósitos de post-tormenta y de buen tiempo (Sd). La comparación con ejemplos actuales y ejemplos del registro, permite estimar un rango batimétrico de 25 a 50 m para los depósitos con estratificación entrecruzada hummocky. Asimismo se discuten posibles orígenes alternativos relacionados con inestabilidades en la interfase de la picnoclina y los efectos de la compactación sobre la preservación de las texturas y estructuras primarias de los depósitos de la Formación Vaca Muerta.<hr/>Storm deposits (tempestites) have been extensively studied in the sedimentary record, particularly those deposits with hummocky cross-stratification, knowing their bathymetric ranks, facies models and their hydrodynamic conditions (Harms et al., 1975; Dott y Burgeois, 1982; Dumas y Arnott, 2006). However, detailed studies in carbonate sediments are relatively scarce. Recently, other types of interference generated at the picnocline interface (internal waves) were proposed to interpret similar deposits without being controlled by the position of the of storm-wave level (Pomar et al., 2012). The Tithonian - Valanginian deposits in the southern Mendoza - Neuquén Basin are particularly interesting because of their association with a homoclinal carbonate ramp system. During the Late Jurassic - Early Cretaceous a series of marine sequences were developed throughout the basin that are grouped under the Mendoza Group. In the southern Mendoza area it includes the Vaca Muerta Formation (early Tithonian - early Valanginian), the Chachao Formation (early Valanginian) and the Agrio Formation (early Valanginian - early Barremian) (Figs. 1 and 2). The Vaca Muerta Formation is widely distributed over the entire Neuquén Basin. It consists of dark bituminous shales, marls and limestones deposited in response to a rapid and widespread marine transgression originated from the Pacific Ocean, as consequence of a tectonic phase of compressional relaxation (Legarreta and Uliana, 1991). While initial studies show significant variations in the proportions of different lithologies, sedimentological knowledge is still general, with the exception of contributions from the south of the basin (Spalletti et al., 2000, 2008; Scasso et al., 2002, 2005) and southern Mendoza area (Doyle et al., 2005; Kietzmann et al., 2008, 2011). Storm deposits are well developed in the Vaca Muerta Formation. Based on tempestites characteristics and their associated beds four distinct deposits have been identified in the Bardas Blancas area (Fig. 3): (1) packstones with low-angle cross-stratification, (2) packstones/grainstones with large scale hummocky cross-stratification, (3) rudstones/grainstones with small scale hummocky cross-stratification, and (4) laminated wackestones/ packstones with intralaminar normal gradation (Figs. 5 and 6). Packstones with low-angle cross-stratification can be interpreted as anisotropic hummocks deposits or low relief megaripples generated under combined flows during major storms in the middle ramp. A complete sequence of HCS consists of three divisions (Sa-Sc) associated with the storm event, and an upper unit (Sd) that represents the fair-weather period between major storm events. Unit Sa starts with an erosion base and consists in low angle cross-stratified rudstones/grainstones. Unit Sb The unit begins with a second order surface and is composed by packstones and grainstones with hummocky cross-stratification. Unit Sc is represented by peloidal packstones with ripple marks, and finally the unit Sd consists of laminated wackestones/packstones. Laminated wackestones/packstones with intra-laminar normal gradation consist of bioclastic wackestones/packstones composed of saccocomids remains or a rhythmic alternation of peloidal packstones and sandy layers with current ripple marks, which probably represents the distal expression of the storms. Variations from this sequence allow us to make some comparison with other ancient carbonate storm deposits (Kreisa, 1981; Monaco 1992, Sami y Descorchers, 1992; Molina et al., 1997; Zhou et al., 2011). Although deposits of the Vaca Muerta Formation are compositionally similar to those described by Molina et al. (1997), the ideal sequence is similar to that recognized by Monaco (1992) (Fig. 10). The division proposed by Monaco (1992) seems to be the most accurate, since it is based on the recognition of structures generated under different hydrodynamic conditions rather than genetic interpretations associated with the phases of the storm. Comparing the described storm-influenced deposits with ancient and modern storm deposits examples, a relative water depth of 25 to 50 m is estimated for the formation of the HCS limestone facies. Even assuming its origin from internal waves, the position of the pycnocline in warm seas and small basins would be found within the same depth range (Nordberg et al., 2000), so that the deposits described in this paper may be important in future palaeogeographical studies in the Neuquén Basin.