Evolución geomorfológica de la plataforma continental del golfo de Valencia durante el Cuaternario
Resumen Abstract Índice Conclusiones
Silvia Albarracín González
2014-A
El golfo de Valencia (Mediterráneo occidental) está localizado en el margen orien-tal de la península Ibérica entre los 0° 25′ E y 0°18′ W, siendo sus límites latitudi-nales el cabo de la Nao (38° 45’ N) y Sagunto (39° 39’ N). La plataforma continental del Golfo de Valencia, con profundidades de hasta 140 m, presenta una anchura media de 65 km en la zona más septentrional y tan sólo 15 km en el cabo de la Nao. Dicha plataforma se caracteriza por ser una zona de transición entre la plataforma del delta del Ebro, de tipo pasiva y progradante debido a la intensa sedimentación, y la plataforma del margen bético, de tipo activa y menor amplitud. Así, la zona norte del golfo muestra perfiles topográficos muy suaves con relleno de material muy fino, mientras que en el margen sur se manifiesta en superficie la influencia tectónica, con numerosos escarpes de falla. Entre sus principales rasgos geomorfo-lógicos destaca la existencia de barras, ondas de arena y depósitos de fango de gran extensión (Rey y Díaz del Río, 1983).
Esta tesis presenta los resultados de análisis geomorfológicos así como del análisis de la sísmica de alta resolución de la plataforma continental, con el objetivo de definir un modelo de evolución geomorfológica de los ambientes litorales y de la plataforma en relación a las oscilaciones del nivel del mar durante el Cuaternario.
Durante la realización de campañas oceanográficas para la obtención de los datos necesarios para esta tesis se emplearon tantos métodos directos, con toma de muestras de sedimento y roca, como indirectos, donde podemos englobar los mé-todos geofísicos desde sistemas acústicos multihaz hasta la sísmica de reflexión.
La primera campaña oceanográfica donde se recogieron datos para la tesis fue a bordo del buque oceanográfico Isla del Alborán, del día 1 al 6 de julio de 2009. En ella se tomaron muestras de sedimentos con el muestreador HAPS box-corer. Además, se realizó un estudio geofísico con un perfilador de fangos de alta fre-cuencia (3,5kHz) para la caracterización del substrato subsuperficial en la zona del campo de las ondas de arena.
Posteriormente, en la campaña oceanográfica del 19 al 22 de julio de 2010 se lleva-ron a cabo varias inmersiones de buceo para la obtención de la muestra de roca con martillos manuales en la barrera más somera, frente al cabo de Cullera, que posteriormente fueron datadas con 14C.
En la tercera campaña se realizó una batimetría de alta resolución del fondo ma-rino con una ecosonda multihaz Seabeam 1050 D (ELAK NAUTIK) a bordo del B/O García del Cid del 24 al 30 de septiembre de 2010, empleando una frecuencia de 180 kHz para aguas someras en transectos paralelos a la costa. Las velocidades acústicas fueron calibradas mediante perfiles con CTD. Se obtuvieron perfiles sís-micos a lo largo de la plataforma interna y media, desde la zona infralitoral (-10 m) y hasta una profundidad de 70 m, con un Geopulse 205J trabajando con un rango de frecuencias de 200 – 2000 Hz.
Para la caracterización y cartografía de las morfologías superficiales que faltaba cubrir en anteriores campañas, se realizó una cuarta campaña oceanográfica a bordo del B/O Sarmiento de Gamboa del 16 al 18 de julio de 2011. Principalmente se obtuvieron perfiles sísmicos de alta resolución con sonda paramétrica TOPAS que opera con dos frecuencias 0,75 kHz y 15,5 kHz, con una resolución vertical centimétrica.
Los datos no adquiridos en campañas oceanográficas se han obtenido gracias a la colaboración de distintas instituciones del estado español. Así, los datos de clima marítimo (oleaje y corrientes) se obtuvieron de Puertos del Estado. En el centro de descargas del Instituto Geográfico Nacional se dispuso del catálogo de terremotos para la zona del Golfo de Valencia desde 1258 al 2013. En lo que se refiere a los perfiles sísmicos de la campaña de 1983 se tomaron de la base de datos del Institu-to de Ciencias del Mar (CSIC) en Barcelona.
Posteriormente a la obtención de muestras y datos brutos se procedió al análisis y procesado para una correcta interpretación de los resultados. Las muestras reco-gidas en las campañas se prepararon para realizar los análisis granulométricos del sedimento con un Laser Coulter del Instituto de Ciencias del Mar (CSIC). Una vez preparadas las muestras de roca se procedió al corte para obtener la lámina delga-da de la roca y realizar la descripción litológica. La datación por radiocarbono de dicha roca se realizó en el laboratorio de Beta Analytic (Miami, EEUU).
En cuanto al procesado de los datos sísmicos, se procedió a cargar los archivos brutos SEGY, definir sistemas de coordenadas, datum y aplicar la velocidad del sonido para la conversión de tiempos dobles en distancias. El dibujado, escalado de los perfiles sísmicos e interpretación de los reflectores se ha realizado con el soft-ware Kingdom suite®.
Los datos brutos obtenidos con la ecosonda multihaz se procesaron con el progra-ma CARIS® aplicando factores de corrección como la velocidad del sonido en el agua entre otros factores, obteniendo un modelo digital batimétrico 5×5 m de resolución. La interpretación geomorfológica de los datos batimétricos se realizó mediante la visualización y generación de entornos 2D y 3D obteniendo modelos digitales del terreno, lo cual permitió deducir características geomorfológicas como la pendiente, orientación, red de drenaje y volumen.
La integración de todos los datos de situación y el análisis espacial se llevó a cabo en un sistema de información geográfica (SIG) con el software Arcgis 10®, el cual permitió unificar espacialmente todos los datos geomorfológicos y de posición.
Entre los resultados obtenidos, cabe destacar que la plataforma continental del Golfo de Valencia durante el Cuaternario se caracteriza por la presencia de anti-guos sistemas de barreras costeras y lagunas asociados. La obtención de una car-tografía geomorfológica de los depósitos relictos como el afloramiento de la barra costera desde las proximidades de la ciudad de Valencia hasta el cabo de La Nao y de palecauces fluviales cortando esta barrera más somera que correspondería una antigua paleolínea de costa del Tirreniense, sugieren un carácter similar a la actual restinga y Albufera de Valencia.
La estratigrafía de las secuencias sedimentarias encontradas, el cortejo progradan-te y la posterior retrogradación de las barreras, son la respuesta a las variaciones del nivel del mar durante el Pleistoceno Superior
Por otra parte, frente a la Albufera de Valencia, en la plataforma media del golfo de Valencia, entre las isobatas de -60 y -80 metros, se ha cartografiado un sistema de 27 ondas de arena con crestas orientadas NNE–SSW, es decir, oblicuas a la actual línea de costa. Estas morfologías presentan alturas de hasta 10 m y longitudes de hasta 3 km, además de una pendiente en la cara de avalancha de hasta 6°. La presencia de sedimentos gruesos relacionados con ambientes próximos a la costa implican un nivel del mar más bajo para la formación de estas ondas de arena. Éstas podrían haber sido afectadas por la deriva litoral Norte-Sur y por tormentas. De acuerdo a la secuencia estratigráfica y a la curva relativa del mar se debieron formar a partir del Younger Dryas (˜ 12 ky BP). Por consiguiente, han sido clasificadas como ondas de arena holocenas asociadas en su génesis a procesos sedimentarios costeros.
Con todo ello se ha obtenido un modelo de evolución de los ambientes sedimenta-rios de la plataforma en relación a las oscilaciones del nivel del mar, analizando la alternancia de ambientes sedimentarios de plataforma, litorales y emergidos como consecuencia de combinar las oscilaciones eustáticas del nivel del mar, la subsi-dencia regional e inferir una curva de oscilación relativa del mar.
The Gulf of Valencia (western Mediterranean) is located in the Iberian Peninsula Continental Margin between 0° 25’E and 0° 18′ W, with the latitudinal limits de-fined by La Nao Cape (38° 45’N) and Sagunto (39° 39′ N). The continental shelf of the Gulf of Valencia, with depths up to 140 m, has an average width between 15 and 65 km, at La Nao Cape. This shelf is characterized by a transition zone between the shelf of Ebro’s Delta, passive progradational type, due to intense sedimenta-tion, and the shelf of the Betic margin, active type and with a smaller width. Thus the northern Gulf area shows very gentle topographic profiles filled with fine sed-iments, while in the southern margin manifestation of tectonic influence are more significant, with numerous fault scarps. The main geomorphological features are bars, sand ridges and mud sediments of great extent.
This PhD dissertation presents the results of the geomorphological and seismic analysis of high resolution data, and aims to define a model of evolution of the sed-imentary environments, relative to sea level oscillations, during the Quaternary.
The oceanographic cruises to obtain data of this thesis can be divided into direct methods as sediment and rock sampling and indirect methods including acoustic techniques such as echosounder, multibeam and seismic reflection.
During the first oceanographic cruise on board the R/V Isla de Alboran, from 1 to 6 July, 2009, sediment samples were taken with a box-corer sampler and a geophysi-cal survey with mud penetrator (3.5 kHz) was used to characterize the subsurface substrate in the sand ridges field. From 19 to 22 July 2010 several dives were car-ried out to recollect rock sample with manual hammers in the shallower barrier, in front of the Cullera cape. A high-resolution bathymetry of the seabed were con-ducted by multibeam Seabeam 1050 D (ELAK NAUTIK) installed aboard the R/V García del Cid from 24 to 30 September 2010, using a frequency of 180 kHz for shallow water, in transects parallel to the coast. Acoustic velocities were calibrated
using CTD profiles also acquired during this cruise. Seismic profiles were obtained along the inner and middle shelf by Geopulse 205J, working with a frequency range of 200-2000 Hz, for characterizing and mapping the surface of seafloor. The last oceanographic survey was conducted aboard the R/V Sarmiento de Gamboa from 16 to 18 July, 2011. During this cruise, high-resolution seismic profiles were ac-quired with TOPAS, operating at two frequencies, 0.75 kHz and 15.5 kHz, resulting in a centimeter vertical resolution of the substrate.
Other data was obtained in collaboration with other Spanish Institutions. Oceano-graphic data was collected from Puertos del Estado, providing current and wave data to characterize the marine clima in the Gulf of Valencia. The earthquake cata-log from 1258 to 2013 was retrieved from the Instituto Geografico Nacional. The Instituto de Ciencias del Mar (CSIC) contributed with seismic profiles from an oceanographic cruise in 1983 that were re-processed and used in this study.
Processing and analysis of all the acquired data was done for a correct interpreta-tion of the results. Sediment samples were prepared for grain size analysis using a Coulter Laser granulometer at the Instituto de Ciencias del Mar (CSIC). Once pre-pared, rock samples were cut into thin sections for descriptions. Rock samples were analysed for radiocarbon dating at the Beta Analytic (USA), to determine the age of the samples.
Concerning the seismic data, the raw files were converted to SEGY format, defined their navigation coordinates and converted the double time vertical scales to depth in meters. The interpretation of the reflectors was done using the Kingdom Suite ® software, for drawing and scaling of seismic profiles.
The raw data obtained with the multibeam echosounder were processed using CARIS ® software applying correction factors as the speed of sound in water, ob-taining a Digital elevation model (DEM) with a grid resolution of about 5 × 5 m. The geomorphological interpretation of the bathymetric data was performed using visualization and generation of 2D and 3D environments, obtaining digital eleva-tion models to deduce the geomorphological characteristics such as slope, aspect, drainage and volume. The integration of geographic data and spatial analysis was carried out on the Arcgis 10 ® geographical information system (GIS) software.
The continental shelf of the Gulf of Valencia during the Quaternary is characterized by the presence of a coastal barrier-lagoon system. The geomorphological map of the offshore from the city of Valencia to La Nao Cape shows the presence of relict deposits with linear outcrops of sand barriers, cut by palaeochannels of rivers that correspond to a Tyrrhenian shoreline, suggesting similar characteristics to the present-day sand barrier in La Albufera.The stratigraphy of the sedimentary se-quences shows progradation tract and retrogradation barriers, as response to the sea level oscillations in the Upper Pleistocene.
On the other hand, a total of 27 bedforms were located in front of the La Albufera of Valencia in the central region of the Gulf of Valencia ranging from 60 to 80 m water depth, with a NNE-SSW orientation, oblique to the present-day coastline. These morphologies have maximum height and length of 8.5 m and 3 km respectively, with a slope in the lee faces of 6°. The presence of coarse sediments associated with near-shore environments indicates a lower sea level for the formation of these sand ridges. They could also have been affected by the littoral drift and storms. According to the sedimentary sequences and the relative sea level curve, sand ridges were formed from Younger Dryas (˜ 12 ky BP). Therefore, they have been classified as Holocene sand ridges associated with coastal sedimentary pro-cesses.
Finally, an evolution model of the sedimentary environments was proposed for the continental shelf, reflecting the sea level oscillations of the last 22 ky BP. The alter-nation of shelf, coastal and continental sedimentary environments, resulting of the combination of eustatic oscillations, regional neotectonics and local subsidence allowed the deduction of a curve of relative sea level changes in the Gulf of Valencia.
RESUMEN15
ABSTRACT19
INTRODUCTION23
1.Pleistocene coastal barrier-lagoon systems26
2.Sand ridges28
STUDY AREA33
1.Regional setting35
2.Seismicity40
3.Oceanographic Setting42
OBJETIVOS, INTERÉS Y PRESENTACIÓN DE LA TESIS45
OBJECTIVES, INTEREST AND THESIS PRESENTATION45
METODOLOGÍA53
1.Análisis de información previa55
2.Campañas oceanográficas56
3.Métodos de procesado de muestras y datos geofísicos59
4.Análisis geomorfológico de las sand barrier61
5.Análisis sísmico del sistema de barreras Pleistoceno63
6.Análisis geomorfológico y sísmico de las ondas de arena64
RESULTS67
1.Geomorphological analisis of the Pleistocene coastal barrier69
2.Seismic analysis of the Pleistocene coastal barrier – lagoon system76
3.Geomorphological and seismic analysis of the relict Holocene sand ridges82
DISCUSSION97
1.Geomorphological analisis of the Pleistocene coastal barrier99
2.Seismic analysis of the Pleistocene coastal barrier – lagoon system103
3.Geomorphological and seismic analysis of the relict Holocen sand ridges105
4.The reconstruction of the geomorphologic evolution for the coast and inner shelf of the Gulf of Valencia during the Quaternary113
CONCLUSIONS119
REFERENCES125
PUBLICATIONS, EXTENDED ABSTRACTS TO CONGRESS RELATED WITH THESE THESIS147
A reconstruction of the Quaternary geomorphological evolution in the continental shelf of the Gulf of Valencia has been carried out by analyses of the detailed ba-thymetry obtained with multibeam echo-sounder data, high-resolution seismic profiles, sediment and rock samples, the analysis of the dominant oceanographic and climatic conditions, and through the spatial analysis of all the geographic in-formation. The subsequent conclusions have therefore been derived.
Seafloor geomorphology of the Gulf of Valencia continental shelf
– A Linear rocky outcrop, located between10 and 40 m water depth, has been iden-tified as having a length of approx. 70 km, a width of at least 1.7 km, with NNW–SSE orientation, parallel to the present-day shoreline, at a constant distance offshore of about 1.5 km. It is interpreted as a Pleistocene coastal submerged barrier island that extends between Valencia and La Nao Cape.
– Two incisions in this indurated coastal sand barrier suggest locally-strong ero-sion corresponding to fluvial palaeo-channels formed during the last glacial maxi-mum (LGM), according to the 14C ages of 21,090 to 20,440 cal. years BP (2 sigma calibrated, 95% probability), which correspond to marine isotope stage 2. The inlet located NE of Cullera Cape confirms that the ancient mouth of the Jucar River was previously northwards of the cape showing a change in the pattern of fluvial discharge in the study area.
– The presence of several Quaternary coastal sand barrier/lagoon systems has been discovered within the modern continental shelf deposits. Their sedimentary sequence comprises four seismic facies: coastal sand barriers, lagoonal deposits, ancient continental shelf deposits and modern continental shelf deposits.
– A total of 27 sand ridges have been identified in the central region of the Gulf of Valencia continental shelf. They cover an area of at least 65 km2 and are located in ten parallel lines with a NNE–SSW orientation of their crests. The morphology of these bedforms corresponds to subaqueous dune with backward orientated horns. The presence of coarse sediments suggests that the origin of this sedimentary de-posit is within the nearshore environment.
Upper Pleistocene and Holocene stratigraphic sequences
– Coastal paleo-reconstructions based on seismic records from the inner continen-tal shelf of the Gulf of Valencia provide evidences of two sedimentary systems tracts. The older corresponds to a prograding highstand systems tract (HST) formed by a gently rising of the relative sea level. Landward of the HST, and sepa-rated by a first-order erosion surface is identified a younger transgressive system tract (TST), which was deposited in the course of a relative sea level rise in the Pleistocene. The most modern coastal sand barrier of this TST should be formed during the Tyrrhenian sea level highstand (MIS 5e, 120 ky BP). The fact that it is currently submerged is an evidence of the influence of the regional tectonic sub-sidence in the geomorphological evolution of the Gulf of Valencia. Therefore, a tec-tonic subsidence rate of 30 cm/ky has been deduced for the last 120 ky.
– The stratigraphic sequence on the central continental shelf is characterized by the presence, over the previously defined TST, of Upper Pleistocene forced regres-sive wedges (stages FRW1 to FRW2) followed by transgressive systems (stages St1 to St3; of the Upper Pleistocene and early Holocene). Stage St2 is composed by a transgressive surface (T2), by an aggradational deposit (A2) and by progradational deposits (P2). Overlying P2, there is a ravinement surface (RS) which defines the irregular morphology being the base to the later formation of the sand ridges unit (SU). According to the seismic stratigraphy data and the relative sea level curve reconstructions, the sand ridges started to form in the Younger Dryas (˜10 ky BP) with the sea level located 30 to 50 m below present-day sea level.
Environmental conditions in the Quaternary and sea level fluctuations
– The geomorphological configuration of the Gulf of Valencia during the Tyrrhenian sea level highstand was characterized by a very long coastal sand barrier and an associated coastal lagoon, i.e. similar to that of the present-day where the Albufera de Valencia coastal lagoon is confined by an Holocene coastal sand barrier. This scenario is supported by the preservation of lagoonal deposits landward of the sand barrier currently outcrouping in the inner shelf. Thus, this high preservation of littoral facies, and particularly of sand barriers, was due to the cementation of the littoral sands through the interaction between freshwater and seawater in the back-barrier lagoonal environment. The presence of truncated surfaces at the top of the sand barriers reflects strong erosion during subsequent marine transgres-sions.
– Two mechanisms are proposed for morphogenetic formation of the sand ridges by coastal processes, which are not necessarily incompatible: the influence of storm waves and the longshore littoral drift. Lateral sediment transport and redis-tribution processes induced by storm flows favour the seaward transport of coarse sediments and the generation of bedforms. Nevertheless, a lower sea level also implicates that these bedforms were formed in shallow water, and consequently, they could be strongly affected by the longshore littoral drift. Nowadays these sand ridges are relict, and therefore, only extreme storm waves can influence them, eroding the thin layer of fine modern sediments covering their surface.
– The coastal geomorphology of the Gulf of Valencia has been characterized by i) the formation of coastal sand barriers with the associated wetland systems from the Upper Pleistocene to the Present, associated to transgression and stillstands of the sea level, and ii) the formation of sand ridges associated with the initial phase of the Younger Dryas transgression.
The reconstruction of the geomorphologic evolution of the coast and inner shelf of the Gulf of Valencia during the Quaternary
Sea level Oscillations have led to continuous and alternating changes in the geo-morphology of coastal area and continental shelf, over the last 200,000 years in the Gulf of Valencia:
-The oldest identified barrier-lagoon deposit systems (sedimentary systems tracts) were formed at isotopic stage 7. At a later stage, during the regression of the MIS 6, the barrier-lagoon deposit systems became cemented as they were exposed. The transgressive erosional surface identified above these deposits is considered as formed by a rapid initial rising before isotopic stage 5. These elements make up a significant area of paleo-relief, upon which sediment deposition during the Holo-cene has taken place.
– The surfaces FRW2 and FRW1 and the regressive depositional units in downlap has been situated before the transgressive Flandrien rising. Therefore, they corre-spond to different fluctuations in the descending phase before the last regressive stabilization of the sea-level in isotopic stage 2. During the Last Glacial Maximun, rivers cut through the cemented coastal outcrops as well as the result of pale-ochannels.
– In the Holocene transgression during the last warm isotopic stage (isotopic stage 1) occurred the landward migration that destroyed the majority of morphologies formed during the last regressive Pleistocene stage, forming T1 and T2 transgres-sive to correspond baseline for the Holocene rising in the Gulf of Valencia. Howev-er, Holocene transgression did not take place continuously as there were pauses in the rising process related to climatic-eustatic changes as occurred in the last neo-glacial period of Younger Dryas. During that period the sea-level stabilized (still-stand) favouring the formation of sand ridges.