Latin American Journal of Sedimentology and Basin Analysis E-ISSN: 1851-4979 [email protected] Asociación Argentina de Sedimentología Argentina Cuadrado, Diana G.; Gómez, Eduardo A.; Ginsberg, S. Susana LARGE TRANSVERSE BEDFORMS IN A MESOTIDAL ESTUARY Latin American Journal of Sedimentology and Basin Analysis, vol. 10, núm. 2, diciembre, 2003, pp. 163-172 Asociación Argentina de Sedimentología Buenos Aires, Argentina Available in: http://www.redalyc.org/articulo.oa?id=381747745006 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative AAS Revista (2003), vol. 10 nº 2:163-172 ISSN 0328 1159 Asociación Argentina de Sedimentología LARGE TRANSVERSE BEDFORMS IN A MESOTIDAL ESTUARY Diana G. CUADRADO, Eduardo A. GÓMEZ y S. Susana GINSBERG Instituto Argentino de Oceanografía. CONICET. CC804. 8000 Bahía Blanca. Argentina Depto. Geología. UNS. San Juan 670. 8000 Bahía Blanca. Argentina. Email: [email protected] - [email protected] - [email protected] ABSTRACT Large transverse bedforms occur on shallow continental shelves, tidal inlets and large estuaries as a result of the strong currents in water sufficiently deep. Although Bahía Blanca Estuary is not a large estuary, its topographical and hydrodynamical characteristics allow the development of a dune field including, among other bedforms, very large dunes. Such large bedforms are rare in this kind of estuary. Large, submerged dunes of Bahía Blanca Estuary were studied to acquire additional knowledge of the different morphologies found in nature, their relationships with the dynamics of the area and the mechanisms involved in their formation. To do this, the dunes field was surveyed in detailed and classified according to height, spacing, superposition, sediment characteristics, flow structure and history. Side scan sonar and echo-sounder records were used to analyze the height and spacing and shape of the dunes. The results of several current meters deployed in the study area were used to analyze the flow structure. The current velocities and the characteristics of the bottom sediments were relat- ed to the bedforms in order to understand the factors controlling their formation. Keywords: subaqueous dunes, side scan sonar, tidal currents, bottom sediments, Bahia Blanca Estuary. Palabras claves: dunas subácueas, sonar de barrido lateral, corrientes de marea, sedimentos de fondo, Estuario de Bahía Blanca. RESUMEN EXPANDIDO que controlan su formación. El tamaño de las geoformas reconocidas varía Grandes geoformas en un estuario mesomareal desde muy grandes a pequeñas dunas. En la trayectoria donde se encontraron las dunas muy grandes, la forma de las mismas cambia desde dunas Las grandes formas de fondo que se encuentran en con crestas relativamente rectas en las zonas profun- las plataformas continentales someras, entradas de das (hacia la boca de estuario) a dunas sinuosas en marea y en grandes estuarios, son resultado de las aguas menos profundas (hacia el interior del estuario). fuertes corrientes en aguas suficientemente profundas. Todas las geoformas encontradas, inclusive las dunas El Estuario de Bahía Blanca es un estuario con carac- muy grandes, se desarrollan en profundidades me- terísticas topográficas e hidrodinámicas que permiten nores a 20 m, y sus modificaciones tanto en forma el desarrollo de un campo de dunas compuesto, entre como en tamaño pueden estar relacionadas con un otras geoformas, por dunas muy grandes. Tales geo- cambio en la velocidad de las corrientes. Las dunas formas son raras en este tipo de estuarios. muy grandes se desarrollan en arena gruesa, mientras que donde predomina la arena muy gruesa no hay Las grandes dunas sumergidas presentes en el presencia de geoformas. Existe una superposición de Estuario de Bahía Blanca fueron estudiadas con el fin formas de fondo, encontrándose dunas medias sobre la de adquirir un conocimiento de las diferentes rampa de las dunas muy grandes. morfologías encontradas en la naturaleza, su relación con la dinámica del área y de los mecanismos Este trabajo confirma que una sola variable no involucrados en su formación. Para ello, el campo de explica la existencia o ausencia de geoformas, ya que dunas fue relevado detalladamente y clasificado de el tamaño de una duna está determinado por una acuerdo a la altura, espaciamiento, superposición, compleja interrelación entre la profundidad, la características del sedimento y la estructura del flujo. velocidad de la corriente y el tamaño y disponibilidad de los sedimentos. Registros de sonar de barrido lateral y sonda ecográfica fueron utilizados para analizar la altura, el espaciamiento y forma de las dunas. Los resultados de INTRODUCTION varios correntógrafos fondeados en el área de estudio fueron utilizados para conocer la estructura del flujo. Las velocidades de las corrientes y las características In the symposium "Classification of Large-Scale- de los sedimentos de fondo fueron relacionadas con Flow-Transverse Bedforms" held in 1987 as a meeting las geoformas con el fin de comprender los factores of SEPM in Texas, the flow-transverse bedforms were Recibido: 23 de octubre de 2003 163 Aceptado: 05 de enero de 2004 Diana G. CUADRADO, Eduardo A. GÓMEZ y S. Susana GINSBERG defined as repetitive structures that develop on a sedi- coastal-plain estuaries and rias and they are ment bed under a unidirectional current. The consensus particularly widespread in the tidal inlets and tidal of the participating symposium members was that, channels. However, very large-scale bedforms (spacing despite the wide spectrum of bedforms (fluvial, coastal 100-300 m) occasionally occur in estuaries (Ashley, and shallow marine environments), all large-scale 1990). Dalrymple et al. (1978) studied bedform flow-transverse bedforms are sufficiently similar in the hydraulic stability in the Bay of Fundy, Boothroyd formative processes as to be assigned the single term (1985) and Knebel et al. (1999) described several "dune" (Ashley, 1990). Perillo (2001) in his study bedforms in different estuaries of North America. about the searching of a nomenclature of bedforms, Aliotta and Perillo (1987) found a dune field in the agree with the term "dunes" for the transverse geo- mouth of Bahía Blanca Estuary characterized by dunes forms generated by water flows in comparison with up to 6 m in height and spacing between 80 and 200 m. those generated by wind. The present investigation is focused in another large bedform field recently discovered in the Bahía Blanca Large-scale bedforms usually occur on shallow Estuary, located northwestward the previous one. continental shelves and epicontinental platforms that are affected by strong geostrophic currents, occasional Where these features are present is important to storm surges and tidal currents. In these environments, carry out intense studies in order to get an adequate the water depths allow development of bedforms of understanding of the mechanisms involved. Thus, the impressive dimensions (Flemming, 1978; Stride, 1982; aim of the study carried out in the recently field found Harris et al., 1986). The shape of the large bedforms in Bahía Blanca Estuary is to acquire additional reflects the relative strengths of the dominant and the knowledge of the different morphologies found at this opposing currents and, in some cases, wave modifica- site and their relation to the dynamics of the area. This tion. It is common to find in the literature the detection study follows the classification scheme recommended of these bedforms in the Belgian continental shelf by Ashley (1990) (Table 1), which by adequately (Lanckneus et al., 1994; Gao et al., 1994, Collins et al., describing the different morphologies found in nature, 1995, Hennings et al., 2000), the North American shelf allow more precise comparisons among different (Goff et al., 1999) and on the Spanish shelf (Lobo et al., bedforms and places. Accordingly, the First Order 2000). descriptors, shape and size, were determined for the bedforms in the Bahía Blanca Estuary as well as Another environment where large-scale bedforms Second Order descriptors, that is superposition can develop is the tide-dominated coastal embayment (simple or compound) and sediment characteristics in which water movement is fast and deep enough (grain size and sorting). The Third Order descriptors (Ashley, 1990). There, coastal water bodies are include bedform profile, dune orientation and flow partially enclosed by topography and have a free characteristics. Besides the description of the dunes connection to the sea with low fresh water run-off field, the factors controlling spatial and temporal volume relative to the tidal volume. In general, the variation within the estuary were also investigated. greater the tidal range the greater the maximum flow strength. STUDY AREA The distribution of dunes within estuaries was widely presented by Dalrymple and Rhodes (1995). They affirm that dunes are particularly abundant in The Bahía Blanca Estuary is located on the eastern tide-dominated estuaries where they are widespread on coast of Argentina (Fig. 1). The tides are semi-diurnal the elongate sand bars which occur at the mouth. They with a tidal range varying between 2.3 to 1.4 m at the are also abundant at the mouth of wave-dominated, estuary mouth and between 3.8 and 2.7 m at the Table 1. Descriptive classification of dunes (from Ashley, 1990). Height is calculated from the spacing using the equation H=0.0677 L 0.8098 (Flemming, 1988). Tabla 1. Clasificación descriptiva de dunas (extraído de Ashley, 1990). La altura es calculada a partir del espaciamiento utilizando la ecuación H=0.0677 L 0.8098(Flemming, 1988). 164 AAS Revista 10(2), 2003 Large transverse bedforms in a mesotidal estuary Figure 1. Satellite Image showing the Bahía Blanca Estuary. The study area is marked with a frame. Figura 1. Imagen satelital del Estuario de Bahía Blanca. El área de estudio está marcada por un recuadro. estuary head, spring and neap tides respectively.