Geological framework of the Ebro continental margin and surrounding areas

Marine Geology, 95 (1990) 265 287 Elsevier Science Publishers B.V., Amsterdam

265

Geological History Geological framework of the Ebro continental margin and surrounding areas Juan J. Dafiobeitia,* Bel6n Alonso and Andr6s M a l d o n a d o Instituto de Ciencias del Mar, C.S.1.C., Paseo Nacional s/n, 08039 Barcelona, Spain (Received by publisher July 13, 1990)

ABSTRACT Dafiobeitia, J.J., Alonso, B. and Maldonado, A., 1990. Geological framework of the Ebro continental margin and surrounding areas, ln: C.H. Nelson and A. Maldonado (Editors), The Ebro Continental Margin, Northwestern Mediterranean Sea. Mar. Geol., 95: 265-287. The eastern and southeastern margins of Iberia were affected during Neogene time by a rifting tectonic process superimposed on the Alpine structures. The Valencia Trough, situated off the northeastern coast of Iberia, has been defined as a rift system that began its activity in late Oligocene-Early Miocene time. The opening of the Valencia Trough produced a series of tectonic grabens created by fractures which strike parallel to the coastline. As a consequence of the rifting, subsidence occurred and depositional sequences of Miocene age were subsequently deposited over the Triassic and Mesozoic basement. Some of the Mesozoic faults were reactivated and affected the Tertiary deposits. The depositionaI history was controlled mainly by extensional faulting during the late Paleocene early Miocene, and by rapid subsidence. During the late Pliocene a second extensional period occurred in the Iberian Chain resulting in the present configuration of the continental margin in the study area. The Pliocene-Quaternary evolution of the margin was largely controlled by fluctuation of the Ebro River sediment supply and sea-level oscillations.

Introduction

The Ebro continental margin is located in the northwestern Mediterranean, on the western side of Valencia Trough (Fig. 1). Since the seventies this area has been the subject of numerous geophysical and geological studies and hydrocarbon exploration (Hinz, 1972; Stoeckinger, 1976; Mauffret, 1977; Rios, 1978; Garcia-Sifieriz et al., 1979; Banda et al., 1980; Maldonado et al., 1985a; Nelson and Maldonado, 1988). The Ebro continental margin was developed within the southern (Mesozoic Cenozoic) Alpine megasuture system (Duran-Delga and Fontbot6, 1980), and is related to the rift development of Valencia Trough which controlled the margin evolution. From the Early Miocene (~20 Ma) to the *Now at: Jaime Almera Institute of Earth Sciences, C.S.I.C., Martl Franqu6s s/n, 08028 Barcelona, Spain. 0025-3227/90/$03.50

present day, continuous convergence has been taking place between Africa and Europe (Pitman and Talwani, 1972; Biju-Duval and Montadert, 1977; Olivet et al., 1984). Nevertheless, important extensional events have also occurred in some parts of the Alpine-Mediterranean area. PostAlpine tensional features (mainly Neogene) crosscut the Alpine and older structures of the Iberian Peninsula from northeast (Pyrenees) to southwest (Betics) and form an elongate region of variable width (Vegas et al., 1980). We postulate, from various independent geophysical data, that the Valencia Trough is located on thinned continental crust, this crust being 15 km thick at the trough axis. A summary of the principle geological and geophysical data available for the marine area and its surrounding region is presented. These data include a concise description of the bathymetry and morphology of the basin. The principal geophysical results in terms of crustal

~ 1990--Elsevier Science Publishers B.V.

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Fig.1. Location map of the study area, showing the main geographic and physiographicareas of the northwestern Mediterranean Sea. structure, heat flow measurement, magnetic anomalies, paleomagnetic data and seismicity are also mentioned. We summarize a model based on this geological and geophysical information for the initiation and evolution of this area of the western Mediterranean Sea. G e o g r a p h i c and p h y s i o g r a p h i c setting

The main depositional and physiographic units in the study area are the Ebro continental margin and Valencia Trough (Fig.l). Valencia Trough is a deep depression that separates the Iberian margin from the Balearic Islands. This depression is incised in the central sector by Valencia Valley (Fig.2). The Ebro continental margin is considered as a classic example of a prograding margin. The growth pattern during its recent evolution has been largely controlled by the fluctuation of the Ebro River sediment supply (Maldonado et al., 1981; Aloisi et al., 1981; Got et al., 1985). A recent compilation of accurate bathymetric data on the Ebro continental margin is presented in Fig.2 based on data from several research programs (this issue; Medialdea et al., 1986; O'Connell et al., 1987). The Ebro continental

margin extends from Cap Salou in the north (41°N 05') to the Columbretes Islands in the south (39°N 45'). The bathymetric chart illustrates the following features: (1) a wide shelf (up to 70 km), which is one of the largest in the western Mediterranean (Got et al., 1985; Medialdea et al., 1986), (2) an extremely narrow slope (10 km) with steep gradients (4-7°), (3) a base-of-slope region, also with steep gradients (1 2°), and (4) the deep axis of Valencia Valley that incises the Ebro continental rise (Alonso et al., 1985; O'Connell et al., 1985; Nelson and Maldonado, 1988). Except for near the present Ebro Delta (Diaz et al., this issue; Farrfin and Maldonado, this issue) the shelf is flat (0.1°). The shelf break is located in 160_+20 m of water. On the inner shelf, the main physiographic unit is the prodelta (Maldonado, 1972). Seaward of the northern prodelta deposits, on the sea bottom there is a paleorelief of fluvial fan deposits, dunes, terraces and some outcrops (Medialdea et al., 1986). On the middleouter shelf northeast of the delta, there are undulate surfaces corresponding to sand ridges and submarine terraces (Fig.3.1). The slope and base-of-slope are cut, respectively, by several generally short submarine canyons and channels. These distributary systems, which are shown on the bathymetric chart, have been named after nearby coastal cities, towns and geographic features (Fig.2) (Alonso, 1986; Medialdea et al., 1986). According to the main processes controlling their development, these canyons can be classified into two types - tectonic and depositional (Alonso and Maldonado, 1988). Tectonic canyons (Foix, Almera and Pedruell) have a V-shaped profile and dissect the Quaternary deposits. These canyons originate on the shelf and cross the base-of-slope to Valencia Valley (Figs.2 and 3.2). Depositional canyons are asymmetrical, with V or U shapes (Figs.3.3 and 3.4). Some of these canyons, such as Francoli Canyon (52 km long), cut the upper slope and cross the entire base-of-slope. In contrast, the Ebro system canyons (Figs.2 (E Q) and Fig.3.4) are short (<20 km) and restricted to the slope (Martinez del Olmo, 1984; Alonso et al., 1985; Field and Gardner, 1990). This second group bears characteristics of deposition/erosion in the

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growth patterns of its canyons (Martinez del Olmo, 1984; Alonso et al., 1985; O'Connell et al., 1987).

The base-of-slope is occupied by channel-levee complexes, interchannel areas, debris flow deposits and apron deposits, which form the Ebro turbidite systems (Nelson et al., 1983/1984; Nelson and Maldonado, 1988; Field and Gardner, 1990; Alonso and Maldonado, this issue). The channel-levee corn-

plexes (Fig.3.5) are the seaward extension of the majority of the Ebro system canyons (Alonso et al., this issue). The northern sector channels cross the base-of-slope, and reach a water depth of 1700 m. Most of them intersect the Valencia Valley floor (Alonso et al., 1985). In comparison, the southern sector channels do not reach depths of more than 1300 m and terminate before reaching Valencia Valley (Field and Gardner, 1990)•

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Valencia Valley begins at the southwestern edge of Valencia Trough and it is deeply entrenched (400 500 m) into Plio-Quaternary bedded sediments (O'Connell et al., 1985). It receives sediment from the Ebro turbidite system channels (Alonso

et al., 1985) and from canyons eroded into the Eastern Iberia continental margin. The Ebro turbidite systems constitute the most important depositional system that drains into Valencia Valley (Nelson, this issue). The eastern end of

GEOLOGY OF THE EBRO MARGIN AND SURROUNDING AREAS

this valley is defined by a break in slope (Maldonado et al., 1985a). At this point the valley bifurcates into two distributary channels that extend into Valencia Fan (Palanques, 1983; Maldonado et al., 1985b).

Geological and structural setting The Ebro continental margin constitutes the western margin of Valencia Trough. From north to south, it is flanked landwards by the following geological structural elements: (1) the E - W trending Eastern Pyrenees, (2) the Catalan Coastal Ranges, which trend NE-SW, parallel to the coastline, and (3) the NW SE trending eastern end of the Iberian Range (Fig.4). On its southern side, Valencia Trough is flanked by the easternmost sector of the External Zones of the Betic Chain, which extends towards the northeast into the Balearic Islands. The Balearic Islands delineate the southeastern edge of the Valencia Trough (Fig.4). Neogene tectonic activity in the Eastern Pyrenees is characterized by significant block faulting (Fontbot6 and Guitard, 1958). Most of the faults

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269

display E-W, NNW-SSE and ENE-WSW trends. Previous studies have shown that several of the Neogene faults are reactivated Paleogene and older faults (Vegas et al., 1980; Fontbot6 et al., 1986). The Catalan Coastal Ranges are built on top of Hercynian basement overlain by Mesozoic sedimentary sequences (Fig.4). The principal structures of the Catalan Coastal Ranges are nearly vertical basement faults which display right-lateral offsets and strike between ENEWSW and NE-SW (Guimerfi, 1984: Anad6n et al., 1985). The most important and intense Alpine deformation took place along these faults. Tectonic activity controlled sedimentation to a great extent during the Paleogene on the Catalan margin of the Ebro Basin. Thick alluvial fan systems were developed from uplifted blocks along the fault-controlled basin boundary, and progressive and angular syntectonic unconformities formed (Anad6n et al., 1985). The basement faults behaved mainly as normal faults during the Neogene (Fontbot6, 1954; Julivert et al., 1972). The zone between the Catalan Coastal Ranges and the Iberian Range consists of overthrust nappes displaying an E W trend.

270

The Iberian Range is an intraplate chain which developed during the Alpine orogeny (Fig.4). However, from its tectonic style, sedimentary evolution and scarcity of magmatism it is not considered, as an Alpine chain sensu stricto (Julivert et al., 1972). The Iberian Range shows a NW-SE trend controlled by a late Hercynian fault system (GuimerS, 1984). The start of the faulting in the Middle Miocene and the ensuing development of Valencia Trough and the Provencal Basin is considered as part of the general rifting process that affected the Western Europe rift system and propagated southward (Vegas et al., 1980). The transition from a compressive to an extensional regime occurred gradually in the Eastern Iberian Chain (Sim6n Gomez, 1983). During the late Pliocene a second extensional period affected the Iberian Chain, giving the nearby Mediterranean Sea its present configuration. The intraplate emergence of the Eastern Iberian Chain is associated with the Neogene rifting process, which raised the crust mantle boundary, thereby thinning the crust towards the Mediterranean Sea (Sim6n Gomez, 1984). There is some evidence from seismic refraction data that suggests an incursion of the rifting process towards the Iberian Chain (Zeyen et al., 1985). In the Eastern Betics, important extensional structures which developed during Neogene are observed (Fontbot6, 1957; Bousquet, 1977; Sanz de Galdeano, 1978). Basins that began forming in the Tortonian are located within a left-lateral shear zone trending NE SW (Julivert et al., 1972; Bousquet, 1979; Montenat et al., 1987). This fault system has been active since Miocene-Pliocene times and divides the region into two crustal blocks of differing structure. The thickness of the crust varies from 24 to 34km in the western block (Central and Southern Betics) to 23 km in the eastern block (Banda and Ansorge, 1980). An extension of this fault system to the southwest crosses the Alboran Sea and extends to the eastern Rif in Morocco (HernS,ndez et al., 1987). However, its northeasterly extent has yet to be precisely defined. The faults in this zone can be grouped into a conjugate fault system (NE-SW and NW SE) and E-W trending faults (Vegas et al., 1980). Most of these large faults have been inherited from previous structural phases, and have played an

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important role in controlling deposition of Miocene sediment (Montenat et al., 1987). The Tortonian extension took place after the crust had already been thickened as a result of the formation of the Betic Cordillera. This situation can be observed in the present-day constitution of the crust beneath the Betic Cordillera. Volcanism was common in the northwestern Mediterranean region during Cenozoic time (Fig.4) (Bellon and Brousse, 1977; Bellon and Letouzey, 1977). The earliest record in the area is found in the deep waters northwest of Mallorca, where Early Miocene andesitic basalts have been located (DSDP 122 and 123) (Ryan et al., 1973). Some basalts dredged near DSDP 122 have an age of 4.5 Ma (Mauffret, 1977). On the Ebro continental margin, quite recent (middle Pleistocene) volcanics outcrop on the Columbretes Islands (Fig.4) (Farr~n and Maldonado, this issue). On the Ebro margin, offshore from the present Ebro Delta, Mesozoic formations were lifted above sea level by the Alpine orogeny. The top of the Mesozoic appears as an erosional surface developed over blocks bounded by NE-SW and ENE-WSW faults. Some of the pre-Tertiary faults were reactivated and affected the Tertiary deposits. In some places above Triassic and Hercynian basement Jurassic and Cretaceous rocks are absent. Tensional events have occurred in several domains of the Alpine Mediterranean area, whereas continuous convergence has been taking place between Eurasia and Africa from Early Miocene (~20 Ma) up to the present day (Pitman and Talwani, 1972; Biju-Duval et al., 1978; Olivet et al., 1984). Valencia Trough developed during this period as one of the extensional structures that characterized the evolution of the Eastern Iberian margin (Roca and Fernandez-Ortigosa, 1989). The Ebro continental margin originated when Valencia Trough opened over thinned continental crust. The margin therefore consists of subsiding continental crust that has undergone extension. The history and evolution of this margin is closely related to that of the nearby Alpine structural units, mainly the Eastern Pyrenees, the Catalan Coastal Range, the Betics and the Iberian Chain (Fig.4).

GEOLOGY OF THE EBRO MARGIN AND SURROUNDINGAREAS

271

Geophysical investigations into the Ebro continental margin and Valencia Trough Deep seismic refraction data The principal characteristics of the lithosphere in Valencia Trough have been defined by seismic refraction experiments carried out over the last two decades, although the fine structure remains to be established by future work. From the seismic refraction data (Hinz, 1972) an abnormal type of crust has been found in the central sector of Valencia Trough (Fig.5.1). It consists of a 0.8 2.5 km thick Pliocene-Quaternary sedimentary

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Seismicity in Valencia Trough is fairly moderate, what activity there is being mainly concentrated in the area northeast of Barcelona and south of Valencia (Gallart et al., 1989b). The available results show no evidence for normal faulting. Moreover, two earthquakes of magnitude between 4 and 5 located in the central and southern part of the basin exhibit a reverse faulting mechanism with a strike-slip component and a major N-S pressure axis (Susagna et al., 1990; Udias et al.,

272

1986). Another two mechanisms in the Catalan Pyrenees are associated with local N E - S W and E - W fault sets. Their focal solutions show orthogonal N W - S E horizontal axes (Gallart et al., 1985; Olivera et al., 1986). Po ten tial fields

The main information on these comes from the gravity map compiled by Morelli et al. (1975). The gravity field shows a positive Bouguer anomaly across the entire basin, with values of up to 150 reGal, and a relative maximum along the central part of the basin. This indicates a thinned crust and thus shallow upper mantle material. While these data cannot be considered as being very precise, they are the only marine gravity data available on a regional scale. A recent interpretation of these data together with some seismic lines suggests that the mass excess beneath the axis of Valencia Trough could be related to a transition zone between the lower crust and upper mantle (Torn6 and Banda, 1988). The top of this zone is proposed at a depth of about 13 km, and its maximum thickness is 27 kin. Another regional map is the one compiled by Haxby (1983) from Seasat data, and we have extracted information relevant to our study area from this map (Fig.6). The free-air anomaly map obtained from a grid of 5'x 5' shows a main regional pattern represented by a high positive anomaly for practically the entire basin, except in the axis of the Valencia Trough and the South Balearic Basin. The highest gradients are observed north of Menorca and south of Mallorca on the Emile Baudot Escarpment (Fig.6). In contrast, there is considerable information on the magnetic structure of the area. The principal data set is the aeromagnetic map of Galdeano and Rossignol (1977). The magnetic anomalies are very helpful in determining basement depth and magnetic styles. Two main regions can be differentiated in the map (Fig.7), (a) the Balearic Promontory, with complex and very low magnetic amplitudes (continental type), and (b) Valencia Trough, with high magnetic amplitudes ( > 5 0 0 nT). Some of these magnetic anomalies display a short wavelength, with a high magnetic amplitude (Fig.7)

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related to the shallow volcanics of the Columbretes Islands and Valencia Seamount (Barone and Ryan, 1987). The latter have been interpreted as the local expression of Neogene volcanism. The short-wavelength anomaly is superimposed on a broader anomaly that is probably the expression of deeper magmatism. It has also been observed that the volcanism in the study area is generally concentrated near the axis of Valencia Trough, thus supporting the idea of a rifting episode (Bellon and Brousse, 1977; Bellon and Letouzey, 1977). Heat flow measurements

The heat flow in Valencia Trough is not well understood. Limited temperature data acquired by hydrocarbon exploration companies have suggested, assuming standard thermal conductivity values, high estimated heat flow values (100-110 mW/m2). Albert-Beltrfin (1979) explained this high heat flow in terms of recent crustal thinning. However, the data should be considered only as a first approximation owing to the lack of thermal conductivity measurements. The interpretation of a lithospheric thermal model along a profile from the Aquitaine Basin in France to the Balearic Promontory predicts a maximum value of 90 mW/m 2 within Valencia Trough for the superficial heat flow, a relative minimum of 71 mW/m 2 in the Balearic Promontory and some indications of increasing heat flow southwest of the trough (Fernfindez et al., 1990). Foucher et al. (1989) were the first to measure thermal conductivity in Valencia Trough and the surrounding areas. Their work has shown heat flow values of higher than 90 mW/m 2 in the central sector of Valencia Trough, which confirms the validity of the model mentioned above. Foucher et al. also found a fairly uniform value of 65 mW/m 2 northwest of Menorca which indicates a distinct thermal province. In contrast, recent geothermal studies onshore indicate that northeastern Spain is not a geothermal province of high heat flow (Fern~indez, 1988). Fernfindez and Banda (1989) have published a map showing a regional normal geothermal gradient (30 35 mK/m). However, there are some conspicuous anomalies, such as a very cold area near the Ebro mouth and a nearby

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anomalous warm area offshore, and these are interpreted in terms of groundwater circulation through the Mesozoic sequence (Fernfindez, 1988).

Paleomagnetic data Preliminary paleomagnetic data are from recent studies carried out mainly on the boundaries of Valencia Trough (Freeman et al., 1989; Paras et al., 1988). Study and comparison of the Tertiary paleomagnetic data from the emergent lands around Valencia Trough indicates a clockwise rotation of the Catalan Coastal Range relative to the stable Iberian Peninsula and a similar clockwise rotation of Mallorca relative to the Catalan Coastal Range. This is interpreted as a diachronous rotation of the eastern (Mallorca) and western (Catalan Coastal Range) margins of Valencia Trough during the Tertiary (Paras et al., 1988). The relative motion of Mallorca comprises two partial rotations: (1) after the Burdigalian (Miocene, syntectonic), and (2) in the Late MiocenePliocene. On the western margin a dextral rotation took place during the middle Eocene Oligocene, before the initial stages of development of Valencia Trough and the formation of the continental margin of the northwestern Mediterranean.

Depositional sequences of the Ebro margin

Basal surface of the synrfft units The base of the synrift units is defined by Reflector H3 mapped by the IGME (1987) (Fig.8). Lower Miocene rocks (Aquitanian) onlap a regional erosional unconformity, which shows a very rough topography on the Ebro margin. This topography exhibits many irregularities, such as three grabens ( > 4000 m) located in the continental shelf north of the Ebro Delta, a graben (> 4000 m) located in the rise off the Ebro Delta, and a structural high (shallower than 3000 m) situated on the slope off the Columbretes Islands. Most structural patterns at the base of the Neogene display NE-SW trending elongate shape subparallel to the Ebro continental margin (Fig.8). This structural trend is an offshore continuation

of the grabens and horsts of the Catalan Coastal Ranges (Fig.4) (Anad6n et al., 1985). Another major structural lineation is observed along the axis of the Valencia Valley (Fig.8). This lineation separates the generally deep basement of the Ebro margin to the northwest, from the basement of the Balearic margin to the south, which is shallower and more regular (Fig.8).

Seismic stratigraphy of the northwestern Mediterranean The acoustic basement of the northwestern Mediterranean Sea ranges in age from Paleozoic to Oligocene. Above this, the sedimentary sequence (reaching 7 km in thickness) can be divided into three main depositional sequences (Fig.9): (1) a lower sequence of detrital and marly deposits of Early Miocene age, (2) a middle sequence of thick evaporitic formations of Messinian age, and (3) an upper sequence of mainly marls but including turbidite beds of Pliocene and Quaternary age (Alla et al., 1972; Rehault et al., 1985). These sequences are separated by several reflectors, which have all been given different names by various authors. (1) The Lower and Middle Miocene sequence is made up of a group of units that include all the pre-Messinian deposits and it corresponds to the Seismic Unit D of Alla et al. (1972). The base of Unit D is represented by the acoustic basement. This unit exhibits a mean thickness of 3-4 km in the abyssal plain, but less than 3 km on most of the Ebro margin. (2) The Messinian evaporitic sequence is subdivided into three units (Rehault et al., 1985). (a) The lower evaporitic beds (500-700 m) which represent the first evaporitic event (Montadert et al., 1978). These beds overlie the Miocene pelagic layers, although some of the latter may also be interbedded with the evaporitic beds (Cita, 1974; Pastouret et al., 1975; Bizon et al., 1975). (b) The salt layers (600-1000 m) correspond to Seismic Unit C (Hersey, 1965). This unit is bounded at its base by Reflector L of Montadert et al. (1970) (Reflector A of Burollet and Byramjee, 1974), and is totally transparent due to the low impedance of the halite (Alia et al., 1972). Unit C

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is absent from the continental margin but reaches a thickness of more than 1000m in the distal sector of Valencia Trough (Leenhardt, 1969). (c) The upper evaporite beds (500-600m) include marly beds and dolomitic and gypsiferous layers (Ryan et al., 1973; Mauffret, 1977; Hsfi et al., 1978). These beds correspond to Seismic Unit B, with Reflector K as its lower boundary (BijuDuval and Montadert, 1977). This unit is characterized by several continuous reflectors of high acoustic impedance that can be traced laterally over long distances. On the Ebro continental margin the evaporitic sequence is developed only in local depressions, where it reaches a maximum thickness of a few tens of meters. On most of the

Ebro margin, however, the evaporites are replaced by an erosional surface (Hsii et al., 1978). (3) The Plio-Quaternary sequence has a thickness of about 500 1500 m in the northwestern Mediterranean and corresponds to Seismic Unit A (Alla et al., 1972). It is separated from the underlying units by a reflector called H by Alinat et al. (1966), M by Ryan (1973), and J by Alla et al. (1972). The Plio-Quaternary sequence exhibits a transparent lower unit and a well-stratified upper unit. The boundary between the two units is defined by Reflector G (Alla et al., 1972; Mauffret, 1977). A Pliocene age has been attributed to the lower unit and a Quaternary age to the upper one (Hsfi et al., 1978; Mauffret et al., 1981). Along the

GEOLOGY OF THE EBRO MARGIN AND SURROUNDING AREAS

central sector of Valencia Trough, Unit A (400-500 m) overlies a Late Miocene erosional surface showing a strong relief locally associated with evaporitic, lacustrine and alluvial deposits (Hsfi et al., 1978; Maldonado et al., 1985a; O'Connell et al., 1985). This erosional surface has been modified by strong differential subsidence.

279

Closure of the Mediterranean-Atlantic connection during the Messinian salinity crisis resulted in the emergence of the region (Hsfi et al., 1978; Howarth and Berckhemer, 1982; Stampfli and H6cker, 1989). During this time, the overall drainage pattern of the Ebro Basin may have been altered as a result of the drastic lowering of sea level (Stampfli and H6cker, 1989).

Lithostratigraphic sequences over the Ebro margin Lower Miocene sequence

The initial depositional sequence over the Triassic and Mesozoic basement is of Miocene age (Fig.9). Most of the Early Miocene (Aquitanian) sequences (Alcanar Breccia) are terrestrial to shallow marine in nature (Figs.9 and 10) (Stoeckinger, 1976; Watson, 1982). These sequences and their growth pattern are controlled by the extensional fault system that cut the margin during the late Paleogene-Early Miocene (Anad6n et al., 1985). Other controlling factors are the rate of subsidence and the position of the coastline undergoing transgression. During the Burdigalian, a generalized diastrophism over the whole region produced: (1) an overthrust-sheet structure and gravity-emplaced olistostromes in the Betic Orogenic Belt, (2) formation of oceanic crust to the east of the Balearic Platform and (3) strong subsidence over the Ebro continental margin (Fig. 10) (Rehault, 1981; Mauffret et al., 1981). Middle-Upper Miocene sequence

The Castellon Marls developed during the Serravallian (Stoeckinger, 1976) (Fig.10). The young relief of uplifted fold belts around the western Mediterranean provided a large sediment supply to the basin (Mauffret et al., 1981; Rehault et al., 1985). This situation favored the development of coarse-grained deposits in nearshore environments alternating with coral reef platforms. The slight diastrophism and relatively stable highstand of sea level (Middle Miocene), together with a high terrigenous input, controlled the development of finegrained prograding wedges (Biju-Duval et al., 1978). The Upper Miocene depositional sequence includes the Castellon Sandstone, which developed during the Tortonian (Stoeckinger, 1976) (Fig.10).

Lower Pliocene sequence The depositional units of this sequence are characterized by ponding in the fluvial valleys and sediment drape over the margin. The fluvial valleys were filled with transitional deposits and, seaward, a relatively thin sediment drape (Ebro Clays) (Figs.9 and 10). The Ebro Clays comprise plastic clays and marls equivalent to the acoustically transparent unit of the distal margin. This unit has been located on the shelf in industrial boreholes (Garcia-Sifieriz et al., 1979; Watson 1982) and in Valencia Trough during DSDP 122 (Ryan et al., 1973). During the early Pliocene, deposition was reduced and controlled by tectonic activity and the Messinian erosional surface (Field and Gardner, 1990). The reduced sediment supply to the margin can be attributed to the rapid rise in sea level after the opening of the Strait of Gibraltar and to the global eustatic highstand of the early Pliocene (Haq et al., 1987). Upper Pliocene-Quaternary sequence During the late Pliocene-Pleistocene, global climatic changes caused a change in eustatic sea level. Low sea level and glaciation caused a large sediment supply to be introduced from the Ebro River directly to the Mediterranean Sea. Thus, the style of deposition of the sedimentary sequences changed. The following units were developed in the Upper Pliocene-Quaternary sequence: (a) a thick progradational unit over most of the proximal margin (Diaz et al., this issue; Farr~n and Maldonado, this issue), (b) shelf margin deltas at the shelf/slope transition (Alonso et al., 1989; Farr~n and Maldonado, this issue) and (c) slope channel-levee complex wedges and base-of-slope aprons on the distal margin (Nelson and Maldonado, 1988; Alonso et al., this issue). These units correlate with the Ebro Sandstones which consist of varying amounts of

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carbonate-rich plastic clays interbedded with sand and silt layers (Figs.9 and 10). Geodynamic evolution of the western Mediterranean Sea

The Mediterranean Sea is an example of a relatively small and deep marine basin surrounded by continents that were developed within mobile

belts characterized by differential lateral movement. Three main phases have been defined to describe the relative motion between Eurasia and Africa: (1) from Early Jurassic to Late Cretaceous, Africa moved left-laterally with respect to Eurasia, (2) from the late Santonian to the late Eocene, a right-lateral motion of Africa follows, and (3) finally the main collision between Africa and Eurasia begins in the late Eocene.

GEOLOGY OF THE EBRO MARGIN AND SURROUNDING

281

AREAS

The Tethys Sea separated Eastern Europe from Africa-Arabia during the Mesozoic and was consumed due to the convergence of Africa with respect to Eurasia. The opening of the Atlantic Ocean in the Middle Jurassic (Sheridan et al., 1982) changed the original motion of the Tethys to convergence. The resulting tectonism involved several stages of continental collision and seafloor spreading that continuously rearranged the relative position of the continents and oceans. The resulting tectonism involved several stages of continental collision and seafloor spreading that continuously rearranged the relative positions of the continents and oceans (Le Pichon et al., 1977; Srivastava and Tapscott, 1986; Klitgord and Schouten, 1986). It has been suggested by several authors (Le Pichon and Sibuet, 1971; Searle and Whitmarsh, 1978; Kidd et al., 1982; Whitmarsh et al., 1982; Grimaud et al., 1982) that the PyreneesKing's Trough acted as a primary plate boundary from mid-Cretaceous time up to the Paleogene

(Fig. 11). Recent reconstructions of the plate kinematics of the North Atlantic show evidence of this Pyrenean plate boundary (Srivastava et al., 1990). However, it is difficult to identify the main plate boundary in the Western Mediterranean because two sutures coexist during the Eocene - - one to the south in the Internal Zone of the Betic Cordillera and along the Kabylian Ranges (Fig. 12) (Duran-Delga and Fontbot6, 1980; Rehault et al., 1984) and another in northern Iberia following the so-called Pyrenean Axial Zone (Fig. 12) (Van der Voo and Boessenkool, 1973; Fontbot6 et al., 1986). In the Alpine orogeny during Cretaceous and early Tertiary times, most of the marine basins of the Tethys were destroyed. Africa ceased to move left-laterally and started a right-lateral motion of limited extent until the early Eocene. The western Mediterranean Sea reflects the structural relationship between the adjacent fold belts of northern Africa and Southern Europe, the associated basins, and the evolution of the North

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282

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Atlantic Ocean (Fig. 12). Dynamic reconstruction models of this basin show a changing mosaic of small and large plates which produced ridges, backarc basins and island arcs (Dewey et al., 1973; Biju-Duval et al., 1978; Cohen, 1980; Savostin et al., 1986). In general, two major phases can be differentiated in the evolution of the basins. The first phase involves the breaking and thinning of the continental crust to create continental margins, followed by seafloor subsidence and widening of the basin. The timing and style of this first phase clearly differentiated several basins in the western Mediterranean, i.e. the Alboran, Balearic and Tyrrhenian Seas. The second phase may partially coexist with the previous one. It is represented by the subsidence of the basins and the deposition of thick sedimentary sequences, both on the continental margins and in deeper domains. Seismic reflection profiles (Biju-Duval et al., 1978), deep-sea drilling (Ryan et al., 1973; Montadert et al., 1978), and conventional deep sampling (Mauffret, 1977; Mauffret et al., 1981) have all provided major evidence throughout the western Mediterranean for this second phase. The main features of the northwestern Mediterranean resulted from extensional rifting that began in late Oligocene time and propagated westward during the Late Miocene within the Alpine oroI LATE PALEOGENE

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genic belts forming backarc basins (Rehault et al., 1985; Maldonado, 1985). The development of the basins involved stretching continental crust and magmatic intrusion of new oceanic crust (Le Douaran et al., 1984). The oceanic crust in the western Mediterranean was developed following the climax of the Alpine orogeny (Auzende et al., 1973). The Balearic Basin is the largest of the western Mediterranean basins. It is divided into the North and South Balearic Basins. These sectors are distinguished on the basis of the nature of their crust and their main tectonic trends. The South Balearic Basin is the oldest, dating back to latest Oligocene or Early Miocene. The best studied sector, however, is the North Balearic Basin, which developed in a pre-existing block of folded Paleozoic formations overlain by Mesozoic platform deposits. This basin includes Valencia Trough (Mauffret, 1977; Aloisi et al., 1981; Maldonado, 1985; Dafiobeitia et al., 1989), and the Provencal Basin, which has been defined as a backarc basin created by the rotation of the Corsica-Sardinia block (Biju-Duval et al., 1978; Durfin-Delga and Fontbot6, 1980). The Provencal Basin includes the Ligurian Basin and the Gulf of Lyon (Rehault, 198! ; Burrus, 1984; Le Douaran et al., 1984). The separation of the Corsica-Sardinia Calabria microplate (Channell et al., 1979) from the European mainland created the Balearic Sea (Provencal Basin and South Balearic Sea). The evidence for such a drift is based on geological correlation on the margins of the Ligurian Sea (Alvarez, 1972), paleomagnetic data (Nairn and Westphal, 1968; Westphal, et al., 1976) and seafloor spreading anomalies identified in the Ligurian and Balearic Seas (Bayer et al., 1973; Burrus, 1984). The oceanic crust formed during the opening of the Balearic Sea is found northeast of Menorca. It is bounded by a large fracture zone striking NW-SE that probably traces the rotation of the Corsica Sardinia block (Mauffret et al., 1972). Auzende et al. (1972) have drawn attention to a series of horsts and grabens which trend in the same direction. A general NE-SW trend is observed in the morphology of the continental margin south of the Balearic Islands (Mauffret et al., 1972).

283

GEOLOGY OF THE EBRO MARGIN AND SURROUNDING AREAS

In contrast to the Tyrrhenian Sea, Valencia Trough is characterized by the absence of a welldeveloped tectonic arc. Conclusions and synthesis of the Ebro continental margin evolution The Ebro continental margin and Valencia Trough must be considered as a small fragment of the various elements that should meet the constraints imposed by reconstruction models of the northeastern Atlantic Ocean and Mediterranean Sea. During the Cenozoic, the major trends of the evolution of the Mediterranean Sea were controlled by the relative motion between Africa and Eurasia. This resulted in the Alpine orogeny and the creation of intraorogenic depressions. After the Oligocene, when the Iberian Peninsula joined the Eurasian plate, an extensive rifting episode affected most of the western Mediterranean and initiated the creation of the basins and margins which are seen today. A southward propagation of this extensional event cut across the eastern and southeastern boundary of Iberia from the Pyrenees to the Alboran Sea. This extension generated the opening of Valencia Trough in late Oligocene-Early Miocene time by crustal thinning and ascent of upper mantle. This is corroborated by the moderately high values of heat flow (Foucher et al., 1989) and the anomalously low seismic velocities found beneath the western Mediterranean basin (Banda et al., 1980). In addition, a recent compilation of the Moho depth for the Mediterranean Sea shows a thin crust (< 20 kin) in the area (Geiss, 1987). Faulting and thinning were active until the Middle Miocene (Serravallian) in the Valencia Trough. Then, the rifting episode must have slowed down or been aborted, because no oceanic crust exists along the axis of the trough. The Ebro continental margin was initiated during the late Paleogene at the western edge of Valencia Trough as a consequence of the rifting of the basin. This margin was developed over a thinned continental crust that was part of a Mesozoic-Cenozoic megasuture of the southern Alpine system (Durfin-Delga and Fontbotd, 1980). The development of the margin progressed by extensional tectonism involving both Paleozoic

basement and undeformed Mesozoic cover. This tectonism produced a series of E-dipping faults trending between NE-SW and N S that were responsible for the subsidence of the faulted central section of the Valencia Trough (Fernfindez-Ortigosa and Roca, 1989). Thermal subsidence may also have been important during this period in assisting in the collapse of the basin, The rifting generated tectonic grabens that strike parallel to the coastline (Sold Sugrafies, 1978). Some of these grabens correspond to the southern extension of the Tertiary basins of the Catalan Coastal Range located north of the Ebro Delta (Julivert et al., 1972; Maldonado and Riba, 1974; Anaddn et al., 1979). These grabens have controlled the morphology and the main depositional growth patterns of the Ebro margin. The most important factor controlling the evolution of the margin in the Late Miocene was the drastic lowering of sea-level produced by the Messinian salinity crisis, this condition contrasting with the global highstand at this time (Haq et al., 1987). In Plio-Quaternary times, variations in the sediment supply and sea-level fluctuations related to global changes were the main factors controlling the depositional patterns. Some other factors, such as local normal faults that cut Plio-Quaternary deposits (Alonso et al., this issue) were of lesser significance. Thermal and lithostatic subsidence were active until the Quaternary (Farrfin and Maldonado, this issue).

Acknowledgements We dedicate this paper to the memory of Dr. Josd Maria Fontbotd (Barcelona University) who kindly reviewed this article and provided constructive suggestions, but who unfortunately died before completion of the article. We would also like to thank the scientific staff of the "Instituto de Ciencias del Mar" and the "Instituto Jaime Almera" of the CSIC (Barcelona) for helpful discussion. Kim Kastens reviewed the paper and made useful recommendations. This study was supported by the US Spain Joint Committee for Scientific and Technological Cooperation (project CCA 8309/074).

284

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