PERSONAL DE APOYO
MARTINIONI daniel Roberto
capítulos de libros
Título:
La Formación Rabot (Cretácico Superior, isla James Ross, Antártida): Un ciclo transgresivo-regresivo de plataforma con dominio de procesos de tormenta.
Autor/es:
D. R. MARTINIONI
Libro:
Geología de la isla James Ross
Editorial:
Instituto Antártico Argentino - Dirección Nacional del Antártico
Referencias:
Lugar: Buenos Aires; Año: 1992; p. 101 - 123
Resumen:
EXPANDED ABSTRACT:   Approximately 350 m of Upper Cretaceous sedimentites are exposed in Rabot Point, James Ross Island, Antarctica. The sedimentary succession is included in the Rabot Formation, which is divided into three informal members (I, II, and III). The lower part of Member I is not exposed, and the upper part of Member III is unconformably overlain by the James Ross Island Volcanic Group. This paper deals with a detailed sedimentological study of the Rabot Formation at its type locality. The results are shown in progressively higher scales of analysis, including: a) the description of single facies (beds); b) the analysis of facies associations; and c) the interpretation of the vertical distribution of different facies associations. The analysis in the first hierarchical level -strata- is related to sedimentary processes that generated the lithofacies; the following level -associations- lays a special emphasis on sedimentary environments in which deposition took place; and the highest level - associations distribution- concerns the evolution of the depositional system. Ten lithofacies were identified in the Rabot Formation: coarse conglomerates, laminated and/or massive burrowed sandstones, wave ripple cross-laminated sandstones, fine sandstones with hummocky cross-stratification, lens-like conglomerates and sandstones, cross-stratified sandstones, mudstone, claystone, laminated limestones and tuff. The coarse conglomerates (Facies A) are lenticular beds showing crude stratification and cross-stratification. They have a possible fluvial origin, with subsequent wave rework. Both, clast composition (mainly sedimentary rocks) and age of resedimented faunal elements (lower Campanian), suggest erosion of exposed deposits during a general regressive episode not previously detected in other localities or caused by a localized cretaceous uplift. According to the age of the fossils (lower Campanian) this uplift or regressive episode might have occurred during the lower Campanian – upper Campanian. Laminated and/or massive burrowed sandstones (Facies B) are amalgamated tabular beds with hummocky cross-stratification and parallel lamination. Bioturbation is mostly restricted to concretions. Such amalgamated burrowed sequences are interpreted as storm generated hummocky cross-stratified sequences. The lack of complete tempestites and the evidence of amalgamation suggest deposition in shallow marine environments. Wave ripple cross-laminated sandstones (Facies C) are supposed to be originated by wave action. The fine sandstones with hummocky cross-stratification (Facies D) are typical tempestites. Each bed is usually overlain by a mudstone layer with lenticular bedding. Both, sandstone and mudstone beds, are burrowed and show abundant trace fossils. These sequences record single storm events and show most of their diagnostic features: a) erosive lower contact, representing high energy conditions at the beginning of the storm event; b) a massive lower part; c) a section with parallel lamination and/or hummocky cross-stratification, considered to have been the main deposit during the storm event; d) a wave ripple cross-laminated zone, reflecting decline of storm activity; and e) a mudstone layer, deposited in normal conditions after the storm. Lens-like conglomerates and sandstones (Facies E) are restricted to certain levels. They usually contain mudstone intraclasts and fragmented bioclasts. The lenses are not widely extended and are massive or parallel laminated. The upper part of some beds shows wave ripple cross-lamination or small scale trough cross-stratification. These deposits may be explained as the result of channelized storm events. The cross-stratified sandstones (Facies F) mainly show planar tabular cross-stratification - sometimes with backflow ripples-; and secondarily trough cross-stratification, hummocky cross-stratification and parallel lamination. Beds are moderately burrowed. Such deposits are interpreted as subaqueous sand dunes and sand bars produced by tractive currents. Mudstone facies (Facies G) is mainly massive, but sometimes shows a diffuse parallel lamination. The siltstone beds that overlie the tempestites (Facies D) show lenticular bedding. The mudstone facies is fossiliferous and dominant in Member II. It occurs in lesser amounts in the other two members. Such beds are commonly burrowed. These deposits are interpreted as regular fallout of suspended material, at times interrupted by distal tempestites, and probably homogenized through biological burrowing. The mudstone facies reflects deposition in a low energy environment below the wave base level. The claystone layers (Facies H) are explained as altered ashfall, deposited directly from suspension, after a volcanic eruption. The laminated limestones (Facies I) consist of thin, lenticular beds. These lenses could have been generated through activity of microorganisms, because of their similitude with some biolaminated deposits. The tenth facies consists of only one bed of tuff (Facies J). It shows evidence of reworking due to wave action or currents at its top. Several studies describe a volcanic activity related to the presence of a magmatic arc to the west of the studied region for this period of time. The analysis of the distribution of facies enabled the recognition of two facies associations: a sandstone-dominated facies association and a mudstone-dominated facies association. The sandstone dominated facies association (Members I and III) mainly consists of sandstone beds (Facies B, C, D and F); in a lesser proportion of conglomerates (Facies A) and mudstone (Facies G); and secondarily of claystone (Facies H), limestones (Facies I), lenticular sandstones (Facies E) and tuff (Facies J). This facies association is divided into two facies subassociations: laminated or massive sandstones and cross-stratified sandstones, and wave ripple cross-laminated sandstones and fine sandstones with hummocky cross-stratification. The first subassociation mainly consists of sandstones (Facies B and F) and conglomerates (Facies A) generated in a high energy environment under both fair weather and storm conditions. Other facies are present in lesser amounts. This subassociation is interpreted as representing the shallowest milieu for the Rabot Formation consisting of upper shoreface deposits and, perhaps, even foreshore deposits (considering that the conglomerates of Facies A could represent deposition closer to the source area). Amalgamated storm sequences (Facies B) alternate with sedimentites produced during fair weather wave action (Facies F). Wave action prevails over other coastal processes and only a few records of mudstone (Facies G) indicate periods of calm between storms. The second subassociation is made up of sandstones (Facies C and D) produced during fair weather and storm wave action with intercalation of fair weather and storm decline mudstone facies (Facies G). The subassociation is in agreement with a lower shoreface to offshore-transition depositional environment. In its lower part there is a preponderance of amalgamated wave ripple cross-laminated beds (Facies C) deposited above average fair weather wave base. In the upper part, complete tempestite sequences (Facies D) alternate with mudstone layers (Facies G) originated at a slightly higher depth between the average fair weather wave base level and the storm wave base level. The mudstone-dominated facies association -present in the middle of the section (Member II)- is predominantly made up of mudstone (more than 95%); having scattered deposits of unidirectional, channelized lenticular sandstones (Facies E) and only a few tabular sandstone beds (incomplete Facies D), generally lacking evidence of wave action. An offshore shelfal environment is suggested for this association, with prevailing conditions of fallout from suspension of the fine material (silt/fine sand). The intercalation of tempestites (Facies D) and channelized flows (Facies E) without evidence of wave action indicates deposition below the average storm wave base level, mainly in the offshore and, maybe, partially in the offshore-transition zones. The analysis of the vertical distribution of facies associations in the Rabot Formation indicates that Member I is characterized by nearshore deposits of foreshore, upper shoreface and lower shoreface to transition zones. A progressive deepening, which continues in Member II with transition to offshore deposits, stands for the upper part of Member I. Member II is formed by outer shelf deposits and records the maximum depositional depth in the sequence. Nearshore deposits, mainly related to the lower and upper shoreface zones, are again recorded in Member III. The transition from Member II to Member III is interpreted as a progressive shallowing of the basin in this area. This distribution of facies associations indicates, first, a shallow environment followed by a progressive deepening, that reaches a peak during deposition of Member II and, finally, a progressive shallowing. The evolution of the depositional system of Rabot Formation suggests a complete transgressive-regressive cycle, which supposes the following sequence of shelfal zones: foreshoreupper shoreface, lower shoreface-transition-offshore, offshore and shoreface.   -   RESUMEN:   El levantamiento de perfiles sedimentológicos detallados de la Formación Rabot (Cretácico superior) en punta Rabot (isla James Ross, Antártida) ha permitido la caracterización e interpretación de las principales facies sedimentarias que componen esta unidad. Se reconocieron diez facies sedimentarias: conglomerados gruesos, areniscas laminadas y/o masivas con bioturbación, areniscas con laminación entrecruzada ondulítica, areniscas finas con gradación de estructuras, conglomerados y areniscas lenticulares, areniscas con estratificación entrecruzada, fangolitas, arcilitas, calizas laminadas y tufita. Las facies de granulometría más gruesa (conglomerados y areniscas) evidencian depositación bajo influencia de acción de olas y de corrientes. Las facies de granulometría más fina (fangolitas -que dominan gran parte de la secuencia- y arcilitas) responden a procesos principalmente decantativos y en general no están afectados por acción de olas. El análisis de la distribución vertical de facies ha permitido reconocer dos asociaciones de facies; una dominada por areniscas y otra dominada por fangolitas. La asociación de facies dominada por areniscas constituye depósitos marinos de plataforma cercanos a la costa, característicos de las zonas de playa frontal (foreshore), anteplaya (shoreface) superior e inferior y transición. La asociación de facies dominada por fangolitas reúne depósitos típicos de una plataforma externa de poca profundidad, representantes de las zonas de transición y costa afuera (offshore). La distribución de las asociaciones de facies muestra que por encima de la asociación de facies dominada por areniscas se dispone un importante espesor correspondiente a la asociación de facies dominada por fangolitas. Estas últimas, a su vez, están sucedidas nuevamente por la asociación de facies dominada por areniscas. El análisis de distribución vertical de facies y paleoambientes interpretados, sugiere que la Formación Rabot representa un ciclo transgresivo-regresivo completo.