Articles

 

Boletín de la Sociedad Geológica Mexicana


Volumen 62, núm. 2, 2010, p. 281-304

http://dx.doi.org/10.18268/BSGM2010v62n2a7

Sediment transport trends in Bahía Concepción, Baja California Sur, Mexico, based on textural parameters and heavy mineral concentrations

Oscar González-Yajimovich1,*, José Luis Perez-Soto1, Guillermo E. Avila-Serrano1, Keith Meldahl2

1 Facultad de Ciencias Marinas, Universidad Autónoma de Baja California, Ap. Postal 76 Ensenada, Baja California México 22800.
2 Department of Physical Sciences, Mira Costa College, 1 Barnard Drive, Oceanside, CA 92056.

* This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

The dispersion pattern of the modern sediments of Bahía Concepción, Baja California Sur, Mexico was obtained based on the spatial trends of their textural properties and heavy mineral content. The McLaren and Bowles and LeRoux models were applied to a sediment texture data set. The McLaren and Bowles method was the most appropriate for the present study due to the elongated nature of the bay and sampling design, but it was complemented by the LeRoux model, providing in some cases a lateral transport component. Seven heavy minerals were identified and their sources were located with the aid of Principal Component Analysis. Two main sources were evident based on the spatial textural trends; one located between Playa Santispac and Punta Santa Bárbara, and the other adjacent to the Cadejé Basin. A bi-directional transport pattern was found at the mouth of the bay with sediment being exported near the margins and imported through the deeper central portion. Transport along the western margin was mostly towards the head of the bay, and towards the north along the eastern margin. In both cases the transport trends indicate a component of transport toward the deeper parts of the bay. A good agreement was found among the three methods in identifying the clastic-sediment sources and their main transport directions.

Keywords: Gulf of California, bathymetry, modern sediments, heavy minerals, provenance, modeling of sediment load.

 

Resumen

Se obtuvo el patrón de dispersión de los sedimentos superficiales de Bahía Concepción, Baja California Sur, con base en las tendencias espaciales de sus propiedades texturales y contenido de minerales pesados. Se aplicaron los modelos de McLaren y Bowles y LeRoux a un conjunto de datos de textura de sedimentos. El método de McLaren y Bowles fue el más apropiado para el presente estudio debido a la naturaleza elongada de la bahía y diseño de muestreo, pero el método de LeRoux lo complementa pues provee en algunos casos una componente lateral. Se identificaron siete minerales pesados y sus fuentes fueron localizadas con la ayuda de Análisis de Componentes Principales. Basado en las tendencias espaciales de la textura, se hicieron evidentes dos fuentes principales; una localizada entre Playa Santispac y Punta Santa Bárbara, y la otra adyacente a Cuenca Cadejé. Se identificó un patrón de transporte bidireccional en la boca de la bahía con sedimentos exportándose cerca de las márgenes e importándose a través de la parte central más profunda. El transporte a lo largo de la margen oeste fue en su mayor parte hacia la cabecera de la bahía y a lo largo de la margen este hacia el norte. En ambos casos las tendencias de transporte indican una componente de transporte hacia las partes profundas de la bahía. Se encontró que los tres métodos tienen buena concordancia al identificar las fuentes de sedimento clástico y las direcciones principales de transporte.

Palabras clave: Golfo de California, batimetría, sedimentos modernos, minerales pesados, proveniencia, modelado de transporte sedimentario.

 

1. Introduction

Bahía Concepción (Figure 1) is a small rift basin in the borderland of the Gulf of California (Nava-Sánchez et al., 2001) and has been proposed as a potential analog for Neogene shallow marine rift basins, for example, Loreto basin, which developed during the early stages of the Neogene (Meldahl et al., 1997; Pérez-Soto, 1998) and has similar stratigraphic facies. The results of studies of sedimentary dynamics of these rift basins could be a useful tool in interpreting stratigraphic facies.

Recently it has been suggested that the source-sink approach in this type of environment provides a framework for the understanding and prediction of flows and chemical transformations of the sediments and solutes in the system and the resulting sedimentary sequences. In this way, the interaction of the source, transport and destination of the sedimentary components with the climate and tectonic processes establishes the distribution of the sedimentary constituents in these environments (Sánchez et al., 2008). Therefore it is relevant to establish the dynamic sedimentary processes and the sources of modern materials. Bahía Concepción is considered an area of great potential for tourism development but at this time only small communities occupy its shores, and this provides a good opportunity to implement an urban planning strategy. There are however very few studies on the environmental significance that it would have on this area (Lechuga-Devéze et al., 2000; López-Cortés et al., 2003; Camprubí et al., 2008; Gutierrez de Velasco et al., 2006; Meldahl et al., 1997; Morquecho and Lechuga-Devéze, 2003; Ponte et al., 2006; Rodríguez-Meza et al., 2009). Sediment transport paths and sea floor mapping are essential and required in environmental studies to determine, for example, the possible dispersal patterns of contaminants (Duman et al., 2004), dispersion of larvae in aquiculture programs or of red tide producing dynoflagellate cysts (Morquecho and Lechuga-Devéze, 2003), management and conservation of shores and ecological reserve design (Wright and Scholz, 2005). An excellent discussion of the sediment-transport environmental implications can be found in Sánchez et al. (2008).

Sediment transport models have been proposed that identify the direction of particle movement by using the spatial trends of their textural properties (e.g., McLaren and Bowles, 1985; Gao and Collins, 1992, 1994; LeRoux, 1994a, b; Sánchez et al., 2008). This type of analysis has been used to determine the sediment transport trends in different environments such as coastal lagoons estuaries and the continental shelf. Two textural trends models were applied to a sediment texture data set in this investigation (McLaren and Bowles, 1985; LeRoux, 1994a, b).

Heavy mineral contents and trends were identified to provide an independent control on the textural models. By quantifying the relative concentrations of different heavy minerals (accessory components of detrital sediments with densities above 2.89), one may be able to reconstruct and to interpret the history of sediment from the initial erosion of parent rocks to the final burial of their detritus, i.e., to unravel the line of descent or lineage of the sediment under investigation (Mange-Rajetzky, 2003). The ultimate goal is to deduce the characteristics of source areas from measurements of compositional and textural properties of sediments, supplemented by information from other lines of evidence (Wright and Scholz, 2005; Mange-Rajetzky, 2003). Van Andel’s (1964) monumental work on the sediment sources and their dispersal patterns focused on the heavy mineral content in order to define provenance and found that from the Bahía de La Paz, in the southern portion of the peninsula, to just north of Bahía Concepción the volcanics of the Comondú Group are a source of heavy minerals with a dominance of augite. Van Andel (1964) named this area “The augite province of Baja California”. Baba et al. (1991) identified the sources and dispersal of sediments in the Gulf of California and found patterns similar to those described by Van Andel (1964).

We report here on the dispersion of surface sediments by means of sediment textural trends and heavy mineral dispersion in Bahía Concepción, and provide a high resolution bathymetric survey of the bay.

The aim of this study is to provide a sediment transport scheme to be use for future studies, particularly those of environmental monitoring and management, and as a potential tool for the interpretation of stratigraphic facies.

 

2. Study area

Bahía Concepción is an enclosed shallow bay about 40 km long and varies from 5 to 10 km wide. The deepest parts are associated with a main channel that switches from the east at the northern part of the bay to the west in the southern part of the bay (Gonzalez-Yajimovich and Pérez-Soto, 1998). The bay is open to the Gulf of California at its north end (Figure 1). The eastern side of the bay is formed by the 10 to 15 km-wide Concepción Peninsula, which rises to elevations of over 700 m. Distinct geomorphic differences characterize the eastern and western shores of the bay. The eastern bay’s shore (western side of the Concepción Peninsula) has a well-developed, 30 km-long continuous alluvial slope or “bajada” backed by short, steep canyons that rarely extend more than 5 km back into the Concepción Peninsula. The western shore of the bay is dominated by steep rocky shorelines rising abruptly from the water. This rugged shoreline is interrupted in several places by long deep canyons, the largest of which extend inland 10 to 25 km. In distinct contrast to the extensive bajada on the east side, alluvial fans on the west side occur only at the mouths of these large canyons. In embayments along the western shore there are several sheltered pocket bays rich in carbonate sand and several mangrove swamps.

 
Figure 1. Location of Bahía Concepción, Baja California Sur, showing sample stations. Sediments were collected with a Petit Ponar grab sampler from a boat along east-west transects (dots) and by hand at the arroyos (squares).

 

The southern end of the bay is a low-lying alluvial plain that rises gradually away from the bay toward the southeast.

Bedrock geology surrounding the bay consists of a basement complex of Cretaceous schistose and granitic rocks overlain unconformably by Oligocene and Miocene volcanics of the Comondú Group (McFall, 1968; Ledesma-Vázquez and Johnson, 2001; Camprubí et al., 2008). McFall (1968) recognizes six vertically stacked Comondú formations in the region (Figure 2). The ~300 m Salto Formation is the oldest and consists of red tuffaceous cross-bedded sandstone and interbedded tuffs. On top of the Salto

 
Figure 2. Geologic map of Bahía Concepción, modified from McFall (1968), showing the main drainages (arroyos). Heavy lines are faults; light lines are shorelines or contacts between major rock units. Qal = quaternary alluvium; Pi = Pliocene Infierno Formation; Mr = Miocene Ricason Formation; Mi = Miocene intrusions; Mp = Miocene Pelones Formation; Ms = Miocene Salto Formation; Kg = Cretaceous basement.

 

Formation dominating the bay is the Pelones Formation that consists of 2,000 m of volcanic agglomerates, basalt flows, and tuffs, with some conglomerate and tuffaceous sandstone beds. In some areas the Pelones Formation is topped by one or more of the following formations: the Minitas Formation (30 to 150 m of coarse tuffaceous conglomerate), the Pilares Formation (100 m of aphanitic and porphyritic basalt), and the Hornillos Formation (150 m of coarse tuffaceous conglomerate). The Ricasón Formation dominates the western side of the bay and consists of more than 1,500 m of interbedded basalt flows, agglomerates and tuffs.

Gabbro stocks and dykes intrude the Pelones Formation and older rocks. Several tonalite intrusions also occur in the area, the largest being the Cerro Blanco stock (McFall, 1968).

A thin veneer of conglomerates, sandstones, siltstones and coquinas can be found over the Comondú Group. These are of the Pliocene Infierno Formation (Ledesma-Vázquez et al., 1997) and crop out widely over the southeastern area of the bay and at the northern end of the Concepción Peninsula.

Meldahl et al. (1997) found that Holocene sediments accumulate in seven distinct depositional environments in the bay: alluvial fans, fan-deltas, coastal interfan flats, pocket bays, mangrove swamps, nearshore shelf, and offshore shelf.

The central basin of the bay is mainly composed of fine green mud (Meldahl et al., 1997; Rodríguez-Meza et al., 2009), and the currents produced by tides are of low intensity (Obeso Nieblas et al., 1996; Gutierrez de Velasco et al., 2006; Rodríguez-Meza et al., 2009). These characteristics suggest that Bahía Concepción acts as a large sediment trap and thus the bathymetry must play an important role in the distribution of sediment.

Caliskan and Valle-Levinson (2008) reported surface waves produced by winds that blow predominantly from the northwest in the winter and from south to southeast in the summer. They recorded surface waves with periods of ~3, 7 and 15 seconds at the mouth of the bay, but only waves with periods of ~3 seconds at the head of the bay. They observed that energetic long-period waves entering at the mouth were dissipated within the bay and were not observed at the head of the basin, concluding that these energetic waves were attenuated by wave blocking owing to the change of coastline orientation of the bay. Ponte et al. (2006) used drifters at three different levels and found a vertically stratified circulation close to the entrance of the bay and a vertically homogeneous cyclonic circulation in the south of the bay. Close to the mouth, the surface currents were to the south. At a depth of 10 m, however, the drifters oscillated around their release positions and along the axis of the channel while being slowly advected to the south. At 20 m, the currents were to the north, out of the bay. On the south transect, drifters deployed at the same horizontal position but at different depths follow similar trajectories: a cyclonic pattern around the end of the bay, with most drifters ending their trajectories at the southwest corner.

Obeso Nieblas et al. (1996) measured and modeled the tides in Bahía Concepción and found a semi-diurnal type with a range of 59 cm at the entrance and 75 cm at the head during April of 1992. The bay responded almost instantly to the tide with no lag between the entrance and the head. The current recorded at the entrance during February of 1994 had a maximum magnitude of 29.5 cm/s, and a northwest-southeast direction. During December of 1992 it had a maximum magnitude of 30 cm/s, and a northwest-southeast direction. Northwesterly winds were recorded at 3 m/s during February 1994 and 4.3 m/s during April 1992.

Gutierrez de Velasco et al. (2006) investigated the tidal and wind forcing and found a diurnal character tide with little vertical variation throughout the bay and a 1 m range during spring tide. They found an almost instantaneous tide along the bay that can be explained by the ratio of the Gulf of California diurnal tide wavelength to the bay’s length. Their results of wind measurements show fall and winter seasons being dominated by northwesterly wind events with magnitudes in excess of 10 ms-1 and 3 to 7 day duration, with most of these events having a strong diurnal component. Spring and summer conditions exhibit 2 ms-1 southeasterly wind with daily periodicities and occasional 10 ms-1southeasterly events of 3 to 7 day duration.

 

3. Methods

During March 1994, 58 superficial sediment samples were collected from different sedimentary environments (fan-deltas, pocket bays, mangrove swamps, nearshore shelf and offshore shelf) in Bahía Concepción, Baja California Sur, México (Figure 1). The samples were obtained with a Petit Ponar Grab from a small inflatable boat along east-west transects across the bay, and by SCUBA diving along transects perpendicular to the shore. Samples were also obtained by hand at the mouths of 13 arroyos. The sampling site coordinates were recorded with a Garmin GPS, model 2000. At each station ~500 cm³ of surface sediment were collected to a depth in the sediment of about 10 cm.

The analyses of the sediments consisted of grain size and heavy mineral identification. Grain size was obtained using the sieving method for the sand fraction and the pipette method for the silt-clay fraction, as described by Lewis (1984). Standard textural parameters were calculated using the method of moments. The heavy mineral composition analysis was performed only on the 3.0 phi fraction as recommended by Rendón-Márquez (1995) using sodium polytungstate (3Na2WO4-9WO3-H2O) calibrated to a density of 2.85 g/cm³ as the heavy liquid (Callahan, 1987). The separated minerals were mounted on glass slides with Hystoclad (η=1.54) mounting medium for identification using a petrographic microscope, and their relative abundances were recorded. For each sample 200 to 250 grains were counted using the “ribbon” method (Mange and Maurer, 1992). To characterize the spatial distribution of the heavy mineral combinations, a principal components analysis (PCA) was applied to heavy mineral abundance data. PCA is a multivariate statistical method that aims at reducing the number of variables. A detailed description of the method is provided in Davis (1986). In this study it was prepared based on a Pearson lineal correlation matrix and its numerical resolution presented, based on the association between the calculated components and the original variables. The results of this characterization were associated to the geology surrounding the bay to locate the sediment sources and/or areas that affect some control over sedimentation, by identifying those components responsible for the mineralogical variability of the deposits (Leinen and Pisias, 1984).

As noted, two models were applied to the sediment grain size data to infer the residual sediment transport by analyzing the spatial trends of the sediment textural properties based on a general four step procedure: 1) the analysis of textural data; 2) the combination of the grain size trends; 3) the transport vectors; and 4) the significance statistical test of the transport vectors. Detailed descriptions of the methods are provided in McLaren and Bowles (1985) and in LeRoux (1994a, b).

Semi enclosed basins are influenced by bathymetry and since no detailed bathymetric chart of Bahía Concepción was found in the literature at the time of the investigation, one was prepared based on 18 transects across and parallel to the bay axis obtained with a Raytheon DE-719B echo sounder and positioned with a Magellan NAV5000 DX GPS receiver on board a small Panga type boat. Isobath interval was 1 meter. Detailed bathymetric charts have been published more recently (Rodríguez-Meza et al., 2009; Cheng, et al., 2010).

 

4. Results

Bahía Concepción was found to be a relatively shallow bay. Maximums depths are 38 m at the southern end and towards the western shore, and 34 m in the northern part and towards the eastern shore. An interesting feature is the occurrence of a channel along the main axis of the bay that is close to the eastern shore in the northern half, and gradually changes towards the western shore in the southern half. The central portion of the bay is relatively flat, but with the presence of islands and seamounts (Figure 3).

Of the 58 samples obtained in Bahía Concepción, 14 were compromised due to improper labeling and storage after the heavy mineral analyses. Therefore grain size was only analyzed on the remaining 44. The results are shown in Table 1 and in figures 4, 5 and 6. The grain sizes ranged from coarse sand to very fine silt (0.84 to 7.64 phi), their dispersion ranged from moderately to poorly sorted (0.52 to 3.17 phi) and the asymmetry (skewness) values ranged from –0.94 to 1.79. Typically areas deeper than 20 m contain the finer sediments. The grain-size analysis data was used to obtain sediment transport trends using the two methods, and the results are shown in figures 7 and 8.

Table 2 shows the proposed transects and their results when a Z test was applied and figure 7 and shows the results of the sediment transport vectors using the McLaren and Bowles (1985) method. This method indicated two main sources of sedime