Desarrollo secuencial de formas de fondo en un régimen macromareal

Abstract

En el extremo NO del golfo San Matías (sector exterior de la bahía de San Antonio) grandes bancos arenosos forman, hasta una profundidad de 14 m, un gran delta de marea, el cual constituye la característica morfológica general. La circulación del área es gobernada por un régimen semidiurno macromareal con amplitudes de sicigias de hasta 9 m. Por medio de métodos sísmico-acústicos (sonar de barrido lateral y ecosonda) y muestreo de sedimentos del fondo se analizaron rasgos morfológicos subácueos (ondas de arena y megaóndulas, lineaciones sedimentarias, marcas de cometa), conducentes a inferir las condiciones físicas ambientales referidas a su generación y a completar el conocimiento de la dinámica sedimentaria regional. Ondas de arena de hasta 4 m de altura se presentan agrupadas en campos bien definidos o en cubetas sedimentarias delimitadas por material rocoso con una configuración tipo "barján". En general, el talud de las mismas se orienta hacia el sur-sureste. Sobre las rampas de las ondas de arena y en sectores de menor profundidad es común el desarrollo de megaóndulas. La composición de todas estas geoformas varía desde arena gruesa a fina (0,28 phi-2,31 phi). Las lineaciones sedimentarias se encuentran a partir de los 8 m de profundidad sobre la plataforma adyacente al delta de marea. Se agrupan en dos tipos: 1) franjas con espaciamiento del orden de los 10 m; 2) franjas con espaciamiento de 30-40 m. Estas acumulaciones arenosas, cuyo relieve no supera el metro, alcanzan los 200 m de longitud. Asociadas a las mismas se producen marcas de cometa ("comet marks"). La generación de ambas geoformas se relaciona a un sedimento arenoso movilizado sobre un sustrato gravoso. La estrecha relación entre la dinámica sedimentaria y formas de fondo es evidente y refleja un transporte neto como carga de fondo hacia el exterior de la bahía de San Antonio. El análisis morfológico determina la coexistencia de las diferentes formas mencionadas, las que se ordenan en sucesión lateral según el siguiente patrón: 1) ondas de arena y megaóndulas, 2) lineaciones sedimentarias de pequeño espaciamiento y marcas de cometa, 3) lineaciones de gran espaciamiento. La disponibilidad de sedimento es el factor preponderante que define la generación secuencial de las diversas formas de fondo. Así, los sectores que reciben mayor aporte del interior de la bahía de San Antonio se caracterizan por el desarrollo de ondas de arena. En cambio en aquellas zonas intermedias con menor disponibilidad de sedimento, las lineaciones arenosas y marcas de cometa resultan el rasgo característico.

n the north patagonian coast (NW corner of San Matías Gulf), at the inlet of San Antonio Bay, an extended tidal delta develops to depths of 14 m. A semidiurnal tidal regime, with spring ranges of about 9 m, is the main circulation factor. The more important morphological features are the presence of intertidal banks attached to the main channel and the asymmetry of the ebb tidal delta resulting from the interactions of waves coming from the east and wind-driven currents from the west. By means of seismo-acoustic methods (side-scan sonar and precision depth surveys) and sampling of bottom sediments, an analysis of the bedform sequence (sand waves, megaripples, sand ribbons, comet marks) was carried out in order to recognize the environmental conditions responsible for their generation and to improve the knowledge of the regional sedimentary dynamics. Sand waves develop at different sectors (Fig. 2). the larger ones, up to 4 m high, are found isolated at the deepest parts and have a "barchan type" configuration (Fig. 3). Asymmetrical sand waves up to 2 m high are also observed on depressions confined to rocky bottoms; their slopes are oriented to the south. Over the stoss sides, oblique megaripples run 30º-40º with respect to the sand wave crests (Fig. 4). On the outer platform, sand waves do not exceed 1.5 m in height and are grouped in well defined fields. They generally slope to the S-SE, although their crests pattern is variable (Fig. 2). Megaripples are found on the shallower areas. All these bedforms are composed of coarse to fine sands (mean size from 0.28 phi to 2.31 phi). The sedimentary lineations are mostly sand ribbons, (according to Kenyon, 1970) and develop on the adjacent shallow shelf, at minimum depths of 8 m (Fig. 2). Their heights are less them 1 m, and can be grouped into two types, according to their spacing: 1) ribbons with spacing in the order of 10 m; and 2) ribbons with 30-40 m spacings (Fig. 5). Their lengths reach more than 200 m and their orientations vary between N140º and N170º, depending on location (Fig. 2). Sonographs also permitted to establish the presence of comet marks (Werner & Newton, 1975), mainly associated with sand-ribbon fields. Both bedforms are composed of sand moving over a reflective substrate mostly composed of gravels. The asymmetrical sand waves developed on the bottom of the main channel, have straight or "barchan-like" crests, and indicate a net offshore sediment transport. The detailed morphologic analysis of the shallow shelf adjacent to San Antonio Bay establishes the coexistence of different bedforms occupying contiguous sectors. The scheme of figure 10 shows the lateral sequence of the bedforms: 1) sand waves; 2) short-spaced sand ribbons; 3) long-spaced sand ribbons. The mostly sharp, although slightly transitional in some places, boundaries between different bedforms suggest that these differences cannot be assigned to variations in flow intensity. On the other hand, the bedforms are not dependent upon depth, as within a few tens of meters they change without significant depth variations. Sediment availability is therefore the prime factor in the development of bedform types. The sectors with higher sediment availability are characterized by sand waves. The source material comes from the coastal system, from the inner environments of San Antonio Bay (Fig. 10). At intermediate zones, with less sediment input, sand ribbons occur. In sum, sand waves develop in sectors having a thick sediment layer which allows their development; sand ribbons and comet marks are formed in areas with a thin bedload transport layer (Fig. 11).