Publication: Depósitos de ríos trenzados conglomeráticos Plio-Pleistocénicos de la Depresión de Granada.
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1986
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Universidad Complutense de Madrid. Departamento de Estratigrafía
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Abstract
Los materiales de edad Plioceno II-Cuaternario 1 de la Depresión de Granada (Fig. 1) consisten en depósitos aluviales y lacustres cuya distribución relativa está controlada por la actividad geodinámica de los bordes de la cuenca, la litología del área fuente y la propia situación en la cuenta, todos los cuales están íntimamente relacionados entre sí. El sistema aluvial que depositó los Conglomerados de Moraleda de Zafayona era el predecesor del Río Cacín que, al contrario que hoy día, dominaba sobre el Río Genil en lo que a extensión de sus depósitos se refiere. El modelo fluvial de Moraleda de Zafayona está formado por un valle aluvial elongado en dirección sur-norte que se incurva hacia el oeste cerca de la localidad que le da nombre (Fig. 2). En el valle se depositaban facies de llanura de inundación y de relleno de canal. La tendencia al apilamiento de los canales en la misma vertical sugiere que la dirección del drenaje tiene un control tectónico. En los materiales depositados en la llanura de inundación se pueden diferenciar dos asociaciones de facies acordes con la posición del paleo-nivel freático durante el depósito (Fig. 3): a) llanura de inundación subaérea, con niveles de nódulos carbonatados debidos a rizocreciones, y b) llanura de inundación subacuosa o encharcada (backswamp) con niveles de carbonatos con fauna de gasterópodos lacustres y niveles afectados por procesos hidromórficos. El tipo b) domina hacía el norte donde la subsidencÍa es mayor, pero a lo largo del depósito se expandió en dirección sur. Las facies de relleno de canal se depositaron en un cauce que tenía partes activas e inactivas (Fig. 8) algo más elevadas, cuya posición variaba con el tiempo, generando una estructura interna compleja en la que puede reconocerse una megasecuencia granodecreciente constituida por Gms → Gm/Sh → Gt → St Fl (P) (Fig. 4). En los canales se formaban barras cuya evolución sedimentaria genérica incluye varias fases de erosión y depósito causadas por los altibajos de la descarga que quedan reflejados en las secuencias de varias escalas (Figs. 5, 6, 7). El núcleo de la barra se formó en condiciones de alta descarga y alta energía. Luego, durante gran parte de su historia, la barra iba ofreciendo un obstáculo cada vez mayor a la corriente y la disminución de la profundidad causó condiciones de alta energía que no está necesariamente relacionada con descargas elevadas. A partir de cierta altura, sin embargo, la barra se convirtió en un obstáculo demasiado elevado y el agua que la sobrepasaba se movía con poca velocidad a causa del rozamiento sobre el fondo, depositándose fracciones más finas cada vez, que integran una secuencia granodecreciente superpuesta a la anterior. La preservación de las partes altas de las secuencias es problemática, pues son las primeras en ser eliminadas si llega a producirse erosión posteriormente debido a la migración de canales dentro del cauce mayor (Fig. 8). La arquitectura fluvial y la distribución de facies varían con la distalidad del sistema fluvial (Fig. 9). En los áreas más proximales, el sistema estaba confinado en un valle aluvial y los canales eran bastante rectilíneos, sin bordes apreciables. En ellos se realizaba el depósito en condiciones de elevada energía, desarrollándose barras incipientes cuyos frentes presentan superficies de tipo riffle. La llanura de inundación estaba vegetada y tenía una zona vadosa bien desarrollada. Estaba surcada por canalillos trenzados y recibía depósitos en épocas de crecida (sheet flaad). En áreas algo más distales, donde el sistema estaba sólo parcialmente confinado, los canales eran algo más sinuosos y contenían barras que los dividían. La llanura de inundación, vegetada, tenía el nivel freático muy alto y localmente aparecía encharcada, con precipitación de carbonatos lacustres y desarrollo de procesos hidromórficos. La tasa de acreción vertical era muy elevada, fruto de una subsidencia lineal continua. Los controles mayores del depósito eran: 1) el área fuente, situada varios kilómetros hacia el sur; 2) los paleovalles elongados sur-norte; 3) las masas diapíricas y las zonas de fractura causantes de desniveles en la cuenca; 4) las grandes líneas de fractura que controlaban la posición del depocentro y el desagüe de la Depresión, y 5) el clima templado con precipitaciones estacionales.
The Plioccne II . Quaternary I sediments of the Granada Basin (Fig. 1) eonsist of alluvial and fluviolacustrine deposits, and their distribution is controlled by geodinamics along the borders of the basin, the source areas and the place that they oceupy into the basin. The so-called Moraleda de Zafayona eonglornerates were laid down by the paleo-Cacín river, the deposits of whieh were much more widely spread than those of the paleo-Genil river. The present-day situation is the reverse: The assumed fluvial model consists of an alluvial valley elongated south-north, that bends towards the west near the village of Moraleda de Zafayona (Fig. 2). Flood plain and channcl fiU deposits are found into the valley. The prominent tendeney of the channel deposits to pile up along the same vertical plane suggests a teetonically-controlled drainage pattern. Aecording to the position of the ground water level as related to the ground of the fIood plain. two facies associations have been distinguished: a) subaerial fload plain, including reddish fine sediments and pedogenic carbonate nodules and b) greyish backswamp deposits. The latter is best represented in the northern area were the subsidence was more pranauneed, but it expanded towards the south with time (Fig. 3). Channel-fill facies correspond to pebbly braided streams having a broad and complex river bed. There were active and temporary-inactive (these assumed to lay at higher levels) courses that moved across with time generating multistorey conglomerate layers with a complex fining-upwards sequence made up of Gms→Gm/Sh→Gt→St→FI (P) (Fig. 4). Bars forming inside the channels share a eommon sedimentary evolution that ineludes suceessive episodes of deposition and erosion as well as sequences of various scales due to the fIuctuations of the river discharge (Figs. 5, 6, 7). There is an initial stage of high energy related to high discharge, where the core of the bar is laid down. High-energy conditions are also recorded in the part of the sequence that forms during the progressive vertical growth, as the bar increasingly interferes with the fIow and the generalized sweeping of the sediment resting on the surface of the bar causes the formation of pavements and reactivation surfaees. After the bar reaches a certain height (as compared to the eontinously-changing water level) it severely interferes the flow and the water depth over the top of 1he bar becomes very low and deposition of fine sediments occurs. The upper part of the internal structure of the bars shows a fining upwards trend with common rootling. The preservation of the upper parts of the sequences remains problematic because they are the first that will be removed if eros ion by migrating channels occurs (Fig. 8). The fluvial architecture and facies distribution change along with the proximality or distality of the system (Fig. 9). In proximal areas the river was confined to an alluvial valley, there were more or less straight channels, with no recognizable levees, under low-energy conditions, that formed very flat bars with riffle-type fronts. Vegetated flood plain with well-developed vadose zones, minar ephemeral channeis and sheet flood sandy or gravelly layers extended up to the borders of the valley. In more distal arcas, where the alluvium was only partially confined, the channels adquire a more sinuous tendency with braided bars. The ground level in the flood plain was quite high leading 10 backswamp or even locally lacustrine realms. Lacustrine gastropod limes tone and hydromorphism are common place. A rather high rate of vertical accretion is assumed due to the continued lineal subsidence. The five majar controls for the alluvial deposition were: 1) the source area, placed a score of kilometres towards the south; 2) the existence of paleovalleys, trending southnorth; 3) the diapiric structures and zones of faulting responsiblc for local arcas of subsidence; 4) the major faul systems that controlled both the locatíon of the depocenters and the drainage of the basin, and 5) the warm climate with seasonal rains..
The Plioccne II . Quaternary I sediments of the Granada Basin (Fig. 1) eonsist of alluvial and fluviolacustrine deposits, and their distribution is controlled by geodinamics along the borders of the basin, the source areas and the place that they oceupy into the basin. The so-called Moraleda de Zafayona eonglornerates were laid down by the paleo-Cacín river, the deposits of whieh were much more widely spread than those of the paleo-Genil river. The present-day situation is the reverse: The assumed fluvial model consists of an alluvial valley elongated south-north, that bends towards the west near the village of Moraleda de Zafayona (Fig. 2). Flood plain and channcl fiU deposits are found into the valley. The prominent tendeney of the channel deposits to pile up along the same vertical plane suggests a teetonically-controlled drainage pattern. Aecording to the position of the ground water level as related to the ground of the fIood plain. two facies associations have been distinguished: a) subaerial fload plain, including reddish fine sediments and pedogenic carbonate nodules and b) greyish backswamp deposits. The latter is best represented in the northern area were the subsidence was more pranauneed, but it expanded towards the south with time (Fig. 3). Channel-fill facies correspond to pebbly braided streams having a broad and complex river bed. There were active and temporary-inactive (these assumed to lay at higher levels) courses that moved across with time generating multistorey conglomerate layers with a complex fining-upwards sequence made up of Gms→Gm/Sh→Gt→St→FI (P) (Fig. 4). Bars forming inside the channels share a eommon sedimentary evolution that ineludes suceessive episodes of deposition and erosion as well as sequences of various scales due to the fIuctuations of the river discharge (Figs. 5, 6, 7). There is an initial stage of high energy related to high discharge, where the core of the bar is laid down. High-energy conditions are also recorded in the part of the sequence that forms during the progressive vertical growth, as the bar increasingly interferes with the fIow and the generalized sweeping of the sediment resting on the surface of the bar causes the formation of pavements and reactivation surfaees. After the bar reaches a certain height (as compared to the eontinously-changing water level) it severely interferes the flow and the water depth over the top of 1he bar becomes very low and deposition of fine sediments occurs. The upper part of the internal structure of the bars shows a fining upwards trend with common rootling. The preservation of the upper parts of the sequences remains problematic because they are the first that will be removed if eros ion by migrating channels occurs (Fig. 8). The fluvial architecture and facies distribution change along with the proximality or distality of the system (Fig. 9). In proximal areas the river was confined to an alluvial valley, there were more or less straight channels, with no recognizable levees, under low-energy conditions, that formed very flat bars with riffle-type fronts. Vegetated flood plain with well-developed vadose zones, minar ephemeral channeis and sheet flood sandy or gravelly layers extended up to the borders of the valley. In more distal arcas, where the alluvium was only partially confined, the channels adquire a more sinuous tendency with braided bars. The ground level in the flood plain was quite high leading 10 backswamp or even locally lacustrine realms. Lacustrine gastropod limes tone and hydromorphism are common place. A rather high rate of vertical accretion is assumed due to the continued lineal subsidence. The five majar controls for the alluvial deposition were: 1) the source area, placed a score of kilometres towards the south; 2) the existence of paleovalleys, trending southnorth; 3) the diapiric structures and zones of faulting responsiblc for local arcas of subsidence; 4) the major faul systems that controlled both the locatíon of the depocenters and the drainage of the basin, and 5) the warm climate with seasonal rains..