Palaeogeographic implications of the Messinian surface in the Valencia trough, northwestern Mediterranean Sea

Tectonophysics, 203 (1992) 263-284

263

Elsevier Science Publishers B.V., Amsterdam

Palaeogeographic implications of the Messinian surface in the Valencia trough, northwestern Mediterranean Sea C. Escutia and A. Maldonado Institute de Ciencias de1 Mar, Paseo National s/n,

08039 Barcelona, Spain

(Received November 12,199O; revised version accepted May 2,199l)

ABSTRACT Escutia, C. and Maldonado, A., 1992. Palaeogeographic implications of the Messinian surface in the Valencia trough, northwestern Mediterranean Sea. In: E. Banda and P. Santanach (Editors), Geology and Geophysics of the Valencia Trough, Western Mediterranean. Tectonophysics, 203: 263-284. Sparker (3000 J and 8000 J) and multichannel seismic reflection profiles across the Valencia trough show a Messinian unconformity incised by numerous valleys. The main feature of this surface is a large valley that generally underlies the present Valencia valley and is deeply entrenched into the Miocene deposits. The size of this palaeo-valley ranges from 0.5 km wide and 15-100 m deep at its western end, to 1.6-2.8 km wide and 200-250 m deep downstream. An important tributary system is observed, with a main canyon (6-8 km wide and 150-200 m deep) draining the Ebro margin, as well as many other smaller valleys draining the Catalan and Balearic margins. Downstream, other tributaries underlie the present canyons of the Catalan margin. The location of the tributary system is controlled by the Early Miocene rift structures. The relief of the Messinian surface is affected by post-Miocene deformation that results from salt diapirism, extensional faulting and related volcanism. Late Neogene to Quaternary volcanic edifices cut the Messinian surface and coincide with large residual magnetic anomalies. Lower Pliocene to Quaternary salt diapirism in the abyssal plain north of Menorca has created a series of structural highs. Between these highs are deep interdiapiric troughs or basins that have become sediment depocentres during the Plio-Quaternary. The complex network of erosional valleys from the Valencia trough continental margin demonstrates that the valley system in the basin was not related to the refilling of the Mediterranean, but to the Iberian and Balearic margin palaeodrainage that developed during the Messinian desiccation. The presence of at least three erosional unconformities suggests that there were alternating periods of flooding and retreat of Atlantic water during Messinian time. The Messinian subaerial margin with erosional valleys contrasts sharply with the Pliocene-Quaternary marine margin with progradational turbidite systems.

Introduction The first drilling campaign (Leg 13) of the Glomar Challenger in 1970 revealed the existence of Late Miocene extensive erosional surfaces developed over most of the Mediterranean continental margins, and a thick (more than 1.5 km) sequence of evaporites including salts in the deeper parts of the present Mediterranean basins (Ryan et al., 1973). Whether these evaporites were deposited in a deep Mediterranean basin with a bathymetry that strongly resembles that of

Correspondence

to: C. Escutia, Instituto de Ciencias de1 Mar, Paseo National s/n, 08039 Barcelona, Spain.

0040-1951/92/$05.00

today, or were deposited in a shallow basin, and whether the sea level was maintained at or dropped below that of the world ocean, has been a subject of major controversy. All the models proposed for the formation of these evaporites require a restricted, shallow portal between the Atlantic Ocean and the Mediterranean Sea during Messinian time. The model of the deep desiccated Mediterranean basin maintains that tectonic closure of the Betic and Rif straits and generally low sea level isolated the Mediterranean Sea from the world oceans during the Late Miocene. This resulted in accumulation of salt within deep depressions that dried out from excess evaporation (Hsii et al., 1973, 1978; Ryan, 1976; Cita, 1982). In a recent study in the

0 1992 - Elsevier Science Publishers B.V. All rights resewed

264

Fig. 1. Location map of the study area showing the main geographic and physiographic areas of the northwestern Sea (from Daiiobeitia et al., 1990).

Fortuna basin (southeast Spain), Miiller and Hsti (1987) found sedimentological and palaeoceanographic evidences in support of the deep desiccated basin model. They demonstrate that except for temporary closures, marine-water inflow through the Betic Strait into the Mediterranean dominated during the Messinian. In contrast, Sonnenfeld (1985) and Dietz and Woodhouse (1988) have proposed deep brine basin models. Another group of authors suggests that the present abyssal plain came into existence by undergoing subsidence after the deposition of the evaporites in shallow restricted basins, not necessarily related to a drop in sea level (Stanley, 1977; Stanley et al., 1976; Debenedetti, 1982). Seismic evidence demonstrates the occurrence of Pliocene-Quaternary subsidence and vertical

Mediterranean

movements in different areas of the continental margins (Morelli, 1975; Stanley et al., 1976; Stanley, 1977; Biju-Duval et al., 1978). Subsidence is induced by isostatic loading of sediment and water, and thermal cooling of the recently created oceanic crust ~Montade~ et al., 1978; Rehault, 1981; Mauffret et al., 1982; Rouchy, 1982). All the Mediterranean basins, however, did not have the same history during and after the Messinian. Many Messinian basins such as those of Sicily, Calabria, the Apennines and the Betics, were uplifted and emerged, while continental margins as the RhGne, Ebro and Balearics underwent subsidence. Geological evidence such as Messinian outcrops, distribution of erosional surfaces, nature of evaporites and biological response to progressive

PALEOGEOGRAPHIC

IMPLICATIONS

OF THE

MESSINIAN

SURFACE

isolation, have been investigated in numerous studies to allow a reconstruction of the Mediterranean basin palaeogeography during the Messinian. Erosional discordances are visible in seismic reflection profiles of numerous Mediterranean basins and they are summarized in Ryan and Cita (1978). These erosional surfaces have been interpreted as the product of subaerial erosion (Ginsburg et al., 1975; Ryan, 1976; Rizzini and Dondi, 1978). In addition, subaerial erosional features in deep marginal areas (desiccation cracks, stromatolite layering and fossil drainage systems) along with sedimentological and palaeontological data have been described over the entire Mediterranean basin by Cita and Ryan (1978). In other Mediterranean basins such as Murcia, Lorca, Vera and Cyprus, erosional surfaces are not observed and evaporite deposits lie conformably over the underlying deposits (Montenat, 1973; Rouchy, 1979). In the Valencia trough margin, the top of the Messinian evaporite passes laterally in the landward direction to an erosional surface (Palanques and Maldonado, 1983). This surface has been drilled offshore near the Ebro Delta and separates the non-evaporitic pre-Messinian deposits from post-Messinian marine deposits (Maldonado and Riba, 1974; Stoeckinger, 1976). Recent studies provide new evidence for a subaerial character of these erosional surfaces. In the Ebro continental margin, a 3-D seismic survey in addition to well data in the Castellon Shelf shows not only the existence of fluvial deposits in deep erosive valleys, but also suggests a minimum Messinian sea-level drop in this area of 2000 m (Stampfli and Hacker, 1989). Farran and Maldonado (1990) and Nelson and Maldonado (1990) suggest that the large, deep Messinian canyon underlying the present Ebro continental shelf was eroded under subaerial conditions. Southwest of our study area, Field and Gardner (1990) recognize a Valencia valley palaeochannel cut into the top of the M surface that they believe to be formed by subaerial erosion. In this paper, we provide an integrated analysis of the entire Valencia trough Messinian surface (Fig. 1). The analysis of the Messinian unconformity, combined with the study of seismic

IN THE

VALENCIA

TROUGH

265

facies distribution and geometry, below and above the unconformity, provides more detailed information that permits us to evaluate the palaeogeography of this area during Messinian time. This evaluation of a complete western Mediterranean basin allows us to reach a conclusion about which main model for the Messinian “salinity crisis” is more appropriate for the Valencia trough. Methods For this study, approximately 4472 km of seismic reflection profiles have been interpreted (Fig. 2) including: (1) 2432 km of 3000 J and 8000 J high-resolution Sparker seismic profiles collected by the B/O Cornide de Suavedru during two different cruises, MCB-79 and CO-82-2, in 1979 and 1982 (navigation used during this survey consisted of satellite and Loran C); and (2) 2040 km of a regularly spaced seismic grid from commercial multichannel seismic data. In addition, data were used from D.S.D.P. Sites 122 and 123 and from a number of exploration drill holes along the Iberian margin (Lanaja, 1987). Seismic reflection profiles and drill-hole data were used to produce an isobath map of the Messinian surface by mapping the unconformity, in terms of two-way travel time, beneath the sea surface. Geologic setting The Valencia trough has been defined as an aborted rift system that started at the end of the Oligocene and the beginning of the Miocene following the Alpine orogeny (Biju-Duval et al., 1978; Mauffret et al., 1982; Daiiobeitia et al., 1990). The rift underwent thermal and tectonic subsidence, but failed before oceanic crust was emplaced. The opening of the trough produced a series of tectonic grabens generated by fractures that strike parallel to the Iberian continental margin. The growth patterns of the northwestern margin in the Valencia trough were controlled by this tectonic setting (Julivert et al., 1974; Maldonado and Riba, 1974; Stoeckinger, 1976; Soler et al., 1983; Medialdea et al., 1987). The southern

margin of the Valencia trough has a more complex origin reflecting overthrusting of the Betic erogenic belts or perhaps major transcurrent faulting (Maldonado, 1985; Rehault et al., 1985; FontbotC et al., 1990). The general evolution of the Valencia trough continental margin during the Miocene post-rift period, was characterized by the subsidence of the basin and the deposition of a thick sedimentary cover. Early to Middle Miocene marine deposits filled structural depressions and subdued the pre-existing topography (Daiiobeitia et al., 1990). During the Late Miocene, the Messinian “salinity crisis” resulted in an extensive unconformity in the margins and in deposition of evaporites and salts over the former abyssal plain (Montadert et al., 1970; Mauffret et al., 1973; Hsii et al., 1978). The deposition of the post-Messinian marine sequences took place since the Early Pliocene, prograding the Iberian margin as

t

I

0

l

-

l

The Messinian trough

unconformity

/

,’

w

:\ ,’ ,r, /. /’

41

L

\ /’ A)\

20 Km

in the Valencia

The Messinian unconformity has been traced from onshore outcrops to offshore seismic reflection profiles in the shelf and down across the Valencia trough continental margins to the modern abyssal plain (Maldonado and Riba, 1974; Cita et al., 1978; Mauffret, 1979; Palanques and Maldonado, 1983; Medialdea et al., 1989; and others). We identify it in our seismic records with the following seismic characteristics: over the shelf and continental margins the unconformity is generally well defined because of its markedly erosional character; there is truncation of the bedded Miocene deposits, and a difference in reflec-

\

/’

40”

much as 80 km seaward, and depositing 12002500 m of sediment over the Messinian surface (Alonso et al., 1990).

‘, -\

P

sDp Sites . HOLE

DRILL

l

MIX-79

I---

co-82-2

-

MULTICHRNNEL

4”

-

- -

5”

Fig. 2. Chart showing track lines of seismic reflection profiles and location of D.S.D.P. Sites and exploration drill holes. MCB-79 and CO-82-2 correspond to two different data sets of high-resolution Sparker seismic lines. Location of profiles in Figs. 3-5, 8, 10-12, 14 and 16 are shown by bold-numbered sections.

PALEOGEOGRAPHIC

IMPLICATIONS

OF THE

MESSINIAN

WNW

ESE

SURFACE

o

IN THE

VALENCIA

267

TROUGH

WNW

ESE

o

Fig. 3. Seismic line across the continental shelf: (A) uninterpreted, (A’) interpreted. Notice erosional character of the Messinian unconformity (ES), and the badland topography. G refers to the seismic reflector that separates the Lower from the Upper Pliocene seismic sequences. See Fig. 2 for location of profile.

tor geometries between pre- and post-Messinian sequences (Fig. 3). Basinward, in the Valencia trough, this Messinian erosional surface passes laterally to a horizon with two or three strong parallel reflectors of high amplitude (Figs. 4 and 10). We trace these reflectors into the basin to a position where they extend over and/or they are onlapped by the upper evaporite sequence (Fig. 8B). Beneath the onlap there has been some controversy as to how far the erosional surface extends (Cita and Ryan, 1978). In our seismic

records, we recognize an erosional surface beneath the evaporitic sequence, and channelling at its top. Erosion at the top and at the base of the evaporite sequence has been shown in this area at D.S.D.P. Site 372 (Cita et al., 1978) and in the Valencia trough (Mauffret, 1979). In the basin plain, we trace a strong group of parallel reflectors that are continuous over the basin and are underlain by a transparent flowing unit (Fig. 5). We correlate these reflectors with sections described by D.S.D.P. Leg 13 (1970) and Leg 42A SE

NW

0

LEUEE COMPLEH

Fig. 4. Seismic profile in the Valencia trough: (A) uninterpreted, (A’) interpreted. Note the different stratigraphic style between the Messinian (erosional) and the Pliocene-Pleistocene (progradational). See Fig. 2 for location of profile.

(

3.0

HA AND

I:X‘lJ

A. MAI.I)O>AI)O

NW -I

2.6 3.0-

NUJ

SE

3.4

I

L

I= _

3.8

if 2

4.2

f

cg z; 3

4.6 t

5.0 5.4 5.86.2-

Fig. 5. Seismic profile

in the Balearic

0

I’

2 Km

I

Abyssal

Plain showing

Interpreted.

the seismic characteristics

See Fig. 2 for location

(1975) which show these parallel reflectors to be evaporite deposits (Ca-sulfate layers), underlain by the transparent unit that corresponds with a sequence of Messinian salt layers (halite layers). The upper evaporite sequence pinches out at the base of the slope (about 3.1 s) against the ero-

of the Horizon

HORIZONM (I .*I

Ryan

HORIZON

A2

(91

UNIT

(8)

S

(10.11.5)

et

al. (1973) Allnat et al., (1966) Burollet and ByramJee (1974) tlontadert et ai., (1978) Maulfret al., (1973) (6) Alla et al., (1972) (7) Sell1 and FaDr( (1971) (8) Beaufort et al., (1954) (9) Flnettl and Norelll (1973) (IO) Allfnat et al, (1978) Montadert et al.. (1970)

(5)

et

(12) Harsey (1965) (13) Biju-Duval and Montadert (*) Escutla and Baldonado (this

Fig. 6. Different

nomenclatures

(A’)

sional surface. In the Catalan and Balearic margins, the evaporitic deposits are very reduced or absent, but we observe them locally filling lows of the pre-existing topography, as has been previously shown by Mauffret (1979) and Palanques and Maldonado (1983).

-y-10-

(1) (2) (3) (4)

M. (A) Uninterpreted.

of profile.

used to refer to the top of the evaporitic

(1977) study)

sequence

in the Mediterranean

basin.

PALEOGEOGRAPHIC

IMPLICATIONS

OF THE

MESSINIAN

SURFACE

IN THE

VALENCIA

TROUGH

270

(‘

WSW

ENE

t~S(‘Ll’I’IA

URLENCIA

UALLEY

SSE

NNUI URLENCIR

A MA1 IX)NAl)O

_._ I

ENE

WSW

AND

2.4

URLLEY

‘M 3.2

\

\

B’

3.4 2.0

NW

NW

SE URLENCIR

2.4

VALLEY

!/ _

acoustic

basement 0?

C’

Km

4.8

3.2

Fig. 8. Selected upper

seismic profiles

Messinian

Uninterpreted

Valencia

across

Valley.

(C) and interpreted

(D’) cross-section

of the Messinian

the Messinian

Uninterpreted

(C’) cross-section Valencia

Valencia

valley. Uninterpreted

(B) and interpreted

(A) and interpreted

(B’) cross-section

of the lower Messinian

Valencia

valley seismic expression underneath location of profilc-.

of the middle valley.

the present

(A’) cross-section Messinian

Uninterpreted lower Valencia

Valencia

of the valley.

(D) and interpreted valley. See Fig. 2 for

PALEOGEOGRAPHIC

IMPLICATIONS

OF THE

MESSINIAN

SURFACE

IN THE

I

VALENCIA

TROUGH

NNE

_._ I

ssw

271

-2.6 UBLENCIA

--My

UALLEY

) -3.8 0 I

0’

2 Km

Fig. 8 (continued).

The complete evaporite sequence is as much as 2 km thick in the Balearic Abyssal Plain (Ryan et al., 1973; Ryan and Cita, 1978; Montadert et

al., 1978). The top of the Messinian evaporite sequence correlates with a prominent seismic reflector usually referred as the “M” reflector (Ryan \ I

\

/

CONTOUR

1

UPPER

MIDDLE

UALLEY

VALLEY

I

INTERURL

4.5

: 0.5 SEC.\

LOWER UALLEY

2100

I

I

39n ni

r1g.

L

an

,Fig. 8B

160

5-O

DISTANCE

Ii0

2tio

(in kilometers)

Fig. 9. Longitudinal profile of the Messinian Valencia valley. Depths in m (below sea level) measured from seismic reflection profiles. Distances in km measured from the mouth of the valley.

272

(

et al., 1971), although it has been identified throughout the Mediterranean with several names (Fig. 6). We can assign stratigraphic ages of Miocene or older below the Messinian unconformity and Early Pliocene above from the D.S.D.P. drill holes (Ryan, 1978). Magnetostratigraphy of the Lower Pliocene section at O.D.P. Leg 107, Site 652, places the Miocene/Pliocene boundary at 4.8 Ma (Kastens and Mascle, 1990; Channel et al., 19901. Based on the seismic characteristics of the M surface, we correlate between adjacent track lines and map the unconformity throughout the study area (Fig. 7). The surface formed by the Messinian unconformity in the Valencia trough margin shows a rugged erosive topography that in many places parallels the present bathymetry. Basinwards, the M reflector consists of a flat-lying surface that slopes gently down to the axis of the trough. At the western end of the study area, the most remarkable characteristics of this surface are the absence of faults and its smoothness; irregularities are limited to erosional canyons (Fig. 4). The Messinian surface is incised by numerous canyons and valleys that form an intricate drainage network with a main central valley, the palaeo-Valencia valley. The seismic stratigraphy of the Pliocene-Quaternary deposits show that this valley has been acting since the Messinian as the main bypass for sediment transported from the continent to the Balearic basin plain (Maldonado et al., 1985; Nelson and Maldonado, 1990). At the eastern end of the Valencia trough, the M surface consists of a series of topographic highs and lows that were created from the deformation of the salt layer (Fig. 5) as described by Stanley et al. (1976) and Bellaiche et al. (1981). Northeast of Menorca, the Messinian surface often appears completely disrupted by salt diapirs. The Messinian surface also is affected by volcanic intrusions scattered over the study area. Physiography of the Messinian surface

erosional

valleys

in

the

The Messinian Valencia valley

The palaeo-Valencia valley is a sinuous valley below the present axis of the Valencia trough

t-X‘117 IA

ANI>

\

M,\l I)OhAl)O

that can be traced for at least 184 km. The widths and depths of the valley increase basinward from a narrow valley 0.5-l km wide and 15-100 m deep at the western end of the valley (Figs. 8A and 41, to 2-2.8 km wide and 200-250 m deep in the middle valley (Fig. 8B), and 3-4 km wide and 250 m deep at the lower valley (Fig. 80. Beneath the present lower valley, the palaeo-Valencia valley decreases in size from 2.5 to 3-4 km wide and relief from 50 to 100 m deep (Fig. 8D). In the distal regions of the Valencia trough, beneath the present upper and middle Valencia Fan, well-defined Messinian channels are not observed, but locally small shallow channels and/or narrow Vshaped incisions which may correspond to distributary channels can be identified. Our seismic profiles do not permit detailed mapping of these palaeo-channels because they are disrupted by the volcanic intrusions and salt structures. At the western end of the studied area, the Messinian Valencia valley is sharply incised in the Miocene deposits and has a V-shape in cross-section (Fig. 8A). Downstream, the cross-section becomes U-shaped and exhibits a flat valley floor (Figs. 8B and 80 The longitudinal profile of this Messinian valley calculated from seismic reflection profiles is irregular (Fig. 9). The valley floor has an average gradient of 1: 200, similar to those of middle fan channels (Nelson and Kulm, 1973; Nelson and Nilsen, 19841. In detail the gradient pattern is that of a steep-flat-steep sequence. The gradient is steepest (1: 45) along the Ebro margin, but decreases to as low as 1: 1256 in the area of confluence between the Messinian Ebro and Foix canyons. After meeting these major tributaries there is a new steep-flat section that corresponds to the middle and lower Messinian Valencia valley. It starts with a gradient of 1 : 182, followed by a section with a gradient of 1: 343 and ending with a flatter section with a gradient of 1: 684. A third steep to flat section can be recognized beneath the present lower Valencia valley. In this section the gradient in the steep part is 1: 96, and in the flat area is more than 1: 1000. Overall, the Messinian valley has a narrower cross-section profile, less steep walls and steeper gradients than the present valley (Maldonado et al., 1985; O’Connell et al., 1985). The Messinian

PALEOGEOGRAPHIC

273

lMPLlCATIONS OF THE MESSINIAN SURFACE IN THE VALENCIA TROUGH

Valencia valley does not exhibit weld-d~v~l~p~d levees in the classical sense of depositional bodies, but it locally shows asymmetry in cross-section presenting steep northern walls and gentle southern slopes (Figs. 8B and 80. The palaeo-Valencia valley parallels an old Neogene fault zone (Nelson and Maldonado, 1990). The Location of the valley corresponds with

the NE-SW and ENE-WSW trends of the basement.

regiunaf faulting

The tributary system At the western end of the study area there is a complex Messinian tributary system that enters the palaeo-Valencia valley (Fig. 7). Numerous

1.0

N&J

SE EBROCRNYON

I

: _L-

-

acoustic

basement

B

$ km

6.0

Fig, 10. Seismic profile across the Messinian Ebro canyon: (A) uninterpreted, CA’)interpreted. Note the cut-and-fill structure: first (El) and second (E2) fill generations. See Fig. 2 for lacation of profile.

(‘

FOIX

ES(‘UTIA

AND

A. MAI.IX)NAIX)

ENE

CANYON

\

c2.2

2.6

2.6

4.2

4.2 0

1 Km

t

A’ 4.6

Fig. Il. Seismic profile across the Messinian

Foix canyon:

(A) uninterpreted,

(A’) interpreted.

See Fig.

1.6

for location

of profile.

1.6

NW

BLRNES CANYON

acoustic

basement

2.4

~2.4

NW

BLANES CRNYON

SE

\

,I

-

acoustic

.2.8

s

M-

-3.2

.3.6

basement 5.6 0 I

1 Km

6.0

6.4

Fig. 12. Seismic profiles canyon

when it enters

across

the Messinian

Blanes

the study area; uninterpreted

canyon:

uninterpreted

(B) and interpreted Fig. 2 for location

(A) and interpreted

(B’) cross-section

of profiles.

(A’) cross-section

of the Blanes canyon

of the Blanes near its end. See

IMPLICATIONS OF THE MESSINIAN SURFACE IN THE

PALEOGEOGRAPHK

275

VALENCIA TROUGH

MIIH. UALLEV RlJfRI)M WlDTH(Km) DfPW 1ENGR 6fWDlENi (Km) M ‘E Ml-In. URlLEV

p:

E t ~MLENCIR ,

250

84

1:200

8

200

24

127

4

160 80

1:46

40

1:115

lRLLEV

BRO RNYON

OIH RNVON

0

32 Km MRP SCRLE

4Km

Cl?NVUN’ISCf?lE U.E. x 3.1)

/’

2

P

2.5

144

Fig. 13. Comparative geometries of selected Messinian canyons.

SE 2.4

3.2 3.6

1

A,

acoustlcbssement

t

Km

Fig. 14. Seismic profiles across relict Messinian canyons on the Balearic margin. Uninterpreted (A) and interpreted (A’) cross-section showing one Messinian fill sequence. Uninterpreted (B) and interpreted (B’) cross-section with two fill sequences; 1 corresponds to the fill after the first erosional phase, and 2 after the second erosional phase.

210

canyons and valleys drain the Iberian and Balearic margins of the Valencia trough. Many of the present canyons and valleys north of the Ebro delta (Foix, Sant Feliu, Blanes etc.) developed canyons during the Messinian. However, there is an important number of Messinian canyons that have no modern bathymetric expression. Overall, the Messinian canyons have smoother cross-section profiles and less steep walls than the present submarine canyons. Some of these canyons are part of a local drainage system that did not always reach the main central palaeo-Valencia valley; they are gully-like erosional features forming incised V-shaped depressions and a badland topography (Fig. 3). Others are part of a more regional drainage system that is sometimes bigger than the palaeo-Valencia valley itself. They usually exhibit broad U-shaped profiles in cross-section and flat valley floors with high-amplitude subparallel reflectors underlain by high-amplitude hyperbolic and chaotic or discontinuous reflection configuration referred to as HAR (high amplitude reflectors) by many authors (Kastens and Shore, 1986; Nelson and Maldonado, 1988; Weimer, 1989) (Figs. 10 and 80 A main canyon drained the Ebro margin in the region of the present delta (Fig. 10). This Messinian canyon, previously identified with drill-hole data on the shelf north of the present Ebro Delta (Farran and Maldonado, 1990), is mapped in detail on the shelf across the Ebro margin using exploration drill holes and seismic data. The Messinian Ebro canyon is 124 km long, several km wide and 500 m deep at its head (Farran and Maldonado, 1990; Nelson and Maldonado, 1990), and 6-8 km wide and 200 m deep near its end at the palaeo-Valencia valley (Fig. 10). It is U-shaped in cross-section with a flat valley floor that in the multichannel records exhibits the HAR’s reflection configuration. The longitudinal profile of this canyon is steep with an average gradient of 1: 77. Another two important Messinian tributary canyons are the Foix canyon, also known as Sitges or Tarragona canyon, and the Blanes canyon. The drainage area at their heads is incompletely mapped because of a lack of closely spaced seismic profiles or drill holes. The Foix canyon enters

(

t~S(‘IIt‘lA

AND

A MAt

l)O~AtlO

the survey area where the canyon floor is about 1076 m deep and from there the canyon is more than 80 km long. The size of the valley ranges from 2 km wide upstream to 4 km wide and 180 m deep near its end (Fig. 11). The cross-section profile changes from a V-shape at the head of the canyon to a broad U-shape close to its end. The average gradient is steeper I1 : 46) in comparison to the present canyon (1: 67) (O’Connell et al., 1985). The Messinian Blanes canyon, from where it enters the study area at 2437 m deep, is a sinuous 144 km long canyon that merges with the lower Messinian Valencia valley at a low angle (Fig. 7). The palaeo-Blanes canyon is erosional where it enters the survey area. Downstream it becomes a broad shallow channel. The size of this channel ranges from 2 km wide and 40 m deep (Fig. 12A) to 2.5 km wide and 30 m deep close to its end (Fig. 12B). In general it exhibits a U-shaped

Fig. 15. Map showing volcanic Messinian outcrops kssociated with the magnetic anomalies in this area. Contour interval SO nT (modified from Gatdeano and Rossignol, 1977, Daflobeitia et al., 1990). Continuous lines correspond to positive values and dashed lines to negative ones.

PALEOGEOGRAPHIC

271

IMPLICATIONS OF THE MESSINIAN SURFACE IN THE VALENCIA TROUGH 2.4

2.8-

SE

NW

3.24

SE EHTENSIONRL

FAULTS

SRLT STRUCTURES

2Km IFI’

P

Fig. 16. Seismic profile showing the different structural components

formed during &static adjustment of salt to sediment loading. Note in the interdiapiric basins the thinning of the lower Pliocene deposits towards the flank of the salt structures, indicating that movement of salt was contemporaneous to sedimentation. See Fig. 2 for location of profile.

cross-section profile with low relief and subdued canyon walls. The palaeo-Blanes canyon has an overall gradient of 1: 115. The gradient is as steep as 1: 28 upstream and 1: 117 downstream. A graphic comparison of the geometries of the palaeoBlanes and other main canyons of the Messinian drainage system described above can be seen in Fig. 13. Prior to the formation of the tributary system incised at the top of the Horizon M, we find in our seismic records a number of relict canyons entrenched in Miocene and/or Messinian de-

posits. These ancient canyons drain mainly the Ebro and the Balearic margin. Each canyon contains at least one fill sequence deposited prior to formation of the Messinian unconformity and the deposition of the Pliocene sediment (Fig. 14A). Some of these relict tributary fills have been re-eroded at the end of the Mess~nian as is the case in the Messinian Ebro and two canyons from the Balearic margin (Figs. 14 and 10). The lack of closely spaced seismic data and/or borehole information does not allow detailed reconstruction of this pre-Messinian tributary system.

27x

The location of most of the canyons forming the Messinian palaeodrainage network is related to the Miocene depositional depocentres that correspond to the structural trends of the basement. The Foix and Blanes canyons, among others, follow deep faults in the basement (Medialdea et al., 1989). Pre- and post-Messinian deformation structures The Messinian erosional surface is affected by two types of post-Miocene deformation: volcanism and salt diapirism, both related to neotectonic and extensional faulting. Volcanism

Numerous intrusions are observed in the study area that are interpreted to be of volcanic origin. The exposed and buried edifices correspond to large magnetic anomalies (Fig. 15). Volcanic edifices outcrop in the Columbretes Islands and in seamounts that have been mapped in the Valencia trough (Barone and Ryan, 1987). Volcanic samples were collected from D.S.D.P. Sites 122 and 123 situated on the flanks of two seamounts (Ryan et al., 1973). Fragments of vesicular basalt, sampled at Site 122, and the volcanic ash recovered at Site 123 are both low-magnesium, high-alkali basalts and andesites (Weibel and Hsti, 1973). Ash from Site 123 was dated radiometrically and with fission tracks, and gave a Early Miocene age (Ferrara et al., 1973). Volcanic activity in northeastern Spain (Olot, Gerona) has occurred until recently during the Late Quaternary. Offshore, the Columbretes Islands south of the study are also interpreted to have formed during Pliocene to Middle Pleistocene time (Farran and Maldonado, 1990). Volcanic intrusions have affected the Messinian surface in different ways: locally the intrusions pierce or disrupt the Messinian surface, while in other locations, the surface is only deformed over the topography created by the protruding seamounts. The volcanic intrusions also disrupted the Messinian drainage patterns. The existence of these volcanic structures along the paths of some of the Messinian valleys resulted in

deflection of the streams. An example can be seen at the Messinian Valencia valley just before it meets the Messinian Blanes canyon (Fig. 71. The presence of the volcanic intrusion at Site 122, the Valencia Seamount (Barone &rd Ryan, 19871 and another one north of it resulted in a steep-walled valley (Fig. 71. The flanks of the intrusive bodies also exhibit drainage systems. Barone and Ryan (1987) found gullies forming major re-entrants, spurs and peaks in the flanks of the Valencia Seamount. Salt diapirism

In the abyssal plain north of the Balearic Islands, numerous salt structures of the Messinian salt or halite layer deform the Messinian surface. The salt layer is identified by the following seismic characteristics: (1) an undulating upper surface, and (2) a planar lower surface with strong diffractions marking the top and bottom due to velocity anomalies. The internal seismic characteristics of the salt layer are the transparency of the layer itself due to the low acoustic impedance of the halite (Alla et al., 19721, and the reflection-free diapirs due to strong deformation in the layer. There are strong diffractions marking the upper surfaces of diapirs and other mobilized salt bodies that make it difficult to precisely define the salt limits. Various theories are invoked for initiation of salt domes including a pre-salt irregular surface, variation of thickness and density of the overburden, faulting of the bedded salt, and externally applied compressive stresses (Humphris, 1978). Where a salt structure begins to grow, the density contrast between the salt and the heavier sediment on top is sufficient to maintain growth (Humphris, 1978). Th e salt diapirs in the study area follow NNE-SSW and a E-W directions corresponding to the regional distribution of faults. This suggests that the diapirs are formed by reactivation of faults in a compressive stage during the Quaternary (Le Cann, 1987). Three structural components are formed during isostatic adjustment to sediment loading on the salt (Berryhill, 1986): (1) domes or uplifts caused by the mobile salt diapirs; (2) faults caused

PALEOGEOGRAPHK

IMPLICATIONS

OF THE

MESSINIAN

SURFACE

by differential loading around the diapir and by cracking and displacement above; and (3) interdiapiric basins. All three of these types of features are observed to have the following characteristics in the Valencia trough (Fig. 16). (1) We can differentiate four salt provinces by the different salt structures deforming the Messinian surface: (a) undeformed salt; (b) saltcored anticlines or pillows; (c) salt domes; and (d) salt diapirs. Transition from one salt structure province to an other is gradational (Fig. 16). (2) The Messinian surface is affected by faults directly or indirectly related to diapirism. They usually are of the growth type in which progressive increase in stratigraphic offset at depth indicates repeated movements (Fig. 16). (3) The interdiapiric basins have the morphologic expression of a “trough”. They correspond to structural lows formed between the salt domes and salt diapirs. Post-Messinian deposits usually thin towards the flanks of these basins (Fig. 16). Horizon M is stratified and conforms to the shape of the structural lows and the salt structures except in the case of salt diapirs where it is interrupted or deformed resulting in rim synclines and overturned reflectors (Fig. 16). Palaeogeographic significance of the Messinian surface Subaerial drainage patterns and deposits

Palaeogeographic features of the Messinian events can be inferred from the morphology of the Messinian surface, and from the nature and seismic expression of the channel fill in the Messinian canyons and valleys. The erosional surfaces observed in the Valencia trough margins, and the overall morphology of the Messinian surface, especially the intricate tributary pattern, with streams that tend to grow by meeting of various tributaries, and the central valley incising the Messinian unconformity surface, suggest subaerial erosion during the Late Miocene. The seismic characteristics of the Messinian valley fill support these observations. In the seismic records, the Messinian canyons, such as the

IN THE

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219

Ebro and Foix, show subparallel, high-amplitude reflectors (HAR) under the channel floor. We interpret these to be coarse-grained fluvial channel floor deposits, because they occur in incised erosional valleys, in contrast to lenticular depositional turbidite channel-levee complexes that have formed on the Valencia trough floor in the Pliocene-Pleistocene marine sequences (Nelson and Maldonado, 1988; Alonso et al., 1990). Our interpretation is reinforced by fluvial deposits reported from D.S.D.P. site 122 (Ryan et al., 1973), that are represented by rounded volcanic cobbles and Lower Pliocene littoral sediments. Hsii et al. (1978) also recovered lacustrine muds within the Messinian interval at D.S.D.P. Site 372, located et the edge of the Balearic basin. A recent study using 3-D seismic profiling grids and drill holes in the Castellon Shelf (Stampfli and Hacker, 1989) shows detailed subaerial erosion patterns in Messinian canyons and provides evidence for subaerial deposits in canyon floors. The different generations of palaeochannels during the Messinian indicate that there was more than one phase of erosion (Figs. 10 and 14). In the Valencia trough continental margin, erosional surfaces represent places of non-deposition. In this case, the discordance separates pre-evaporitic Miocene deposits from Messinian continental or Pliocene marine deposits (Fig. 3). Basinward, the first erosive phase separates pre-evaporitic Miocene deposits from Messinian evaporitic and/or continental deposits (Figs. 4 and 14). The last erosive episode is represented by channelling at the top of the evaporite sequence (Figs. 4 and 8). In between these two phases we find in our seismic records at least one intra-Messinian discordance (Fig. 14B). This erosional episode may have been less extensive than the first and last ones, since we only find evidence of the intervening erosional surface in the Valencia trough margin. These three erosional phases represent alternating episodes of flooding and retreat of Atlantic waters through the Betic Portal and/or the Rif Strait. The fact that we do not see evidence of terraces in our seismic records may result from the rapid fluctuations of base level that did not permit development of valley infilling and terrace

280

formation (Thornbury, 1958). Erosion in more than one phase is also supported by the steepflat-steep valley axis morphology observed in the Messinian Valencia valley (Fig. 9). This irregular profile corresponds to the concept of dynamic metastable equilibrium of Schumm (19771, that stresses periodic or episodic erosion resulting from exceeding threshold conditions for a graded profile. The decrease in gradient between the confluence of the Ebro and Foix canyons, however, may be related to a temporary base level. The morphological character between these tributaries and the main stream is that of a Piedmont plain where a large volume of unconsolidated sediment was eroded, brought into the main stream by the tributaries during Messinian time and accumulated to disrupt the base level. Several lines of evidence support previous hypotheses that the Valencia trough was already a deep basin in Messinian time. One of them is the distribution of the evaporitic sequence that is thicker in the deeper basin and then thins towards the margins until it pinches out at the base of the slope. The evaporite deposits observed that fill depressions in the Balearic and Catalan margins, apparently were deposited in small marginal basins which had become isolated during the Messinian, when sea level changed. Additional evidence for a deep Messinian basin is the location of the Messinian drainage network that correlates with the Miocene depocentres and with the structural trends of the Valencia trough, both controlled by the Early Miocene rift grabens. The onlapping of the pre-Messinian Miocene sequences and the fact that the upper Messinian deposits are not affected by faults, indicate that the generalized rifting stage finished before the Messinian time. Consequently, a deep structural Valencia trough basin was formed by Messinian time and has undergone limited subsidence during the Pliocene and Quaternary. The overall Messinian morphology is very similar to the present relief. Most of the present canyons of the Valencia trough margin have been active since the Messinian. However, there are major differences in the margin depositional styles that we will discuss in more detail in the following section.

(

1-S II I’IA

.ANIl

A.

MAI

IJOI\RI)O

Comparison of Messinian erosional with PliocenePleistocene progradational sequences

The Messinian and the Pliocene-Quaternary show a well developed drainage netwdrk. However, the regional Pliocene-Quaternary stratigraphy and turbidite facies association of the Ebro margin (Nelson et al., 1983/1984; O’Connell et al., 1987; Nelson and Maldonado, 1988; Field and Gardner, 1990; Alonso et al., 1990) compared to that of the Messinian shows a marked change between Late Miocene and Pliocene-Pleistocene depositional patterns. During the PliocenePleistocene, the depositional style is that of a prograding depositional margin, with rather small turbidite canyons and channels or erosional gullies. Offshore from the present Ebro Delta, two kinds of canyons and associated base of the slope turbidite systems are recognized by their morphologic character (Nelson et al., 1983/1984; Alonso et al., 1985; O’Connell et al., 1987): (1) slope canyons leading to channel-levee systems, and (2) erosive and gullied slope terrain leading to unchannelized base-of-slope apron deposits. The Pleistocene canyons generally do not reach the shelf, show concave-upwards axial profile gradiaggradational/progradational ents, exhibit canyon-wall levees (Alonso et al., 1990), and prograde across the rift structures. In contrast, deep erosional canyons following the rift structures, are the most characteristic features of the Messinian geological record (Fig. 4). These canyons do not develop levees, although locally there is asymmetry of the canyon walls. Consequently, in the young continental margin of the Valencia trough, subaerial erosion (Messinian) and subaqueous deposition (Pliocene-Quaternary) result in two completely different types of margin development. Deformation of the Messinian surface Volcanism

The volcanic intrusions observed in our seismic records are associated with the magnetic anomalies (Fig. 15). Daiiobeitia et al. (1990) differentiate two types of anomalies which correspond to two volcanic episodes. The volcanic Columbretes

PALEOGEOGRAPHIC

IMPLICATIONS

OF THE

MESSINIAN

SURFACE

Islands are correlated with magnetic anomalies that have a short wavelength with a high magnetic amplitude, interpreted as the local expression of the Neogene volcanism. Short wavelengths are superimposed on low-amplitude anomalies that are thought to be the expression of a deeper magmatism (Dafiobeitia et al., 1990). The volcanic intrusions of the Valencia trough follow the central trough axis which suggest that they may be associated with the rifting episode of the Early Miocene. The volcanic edifices affect Messinian and Pliocene-Pleistocene depositional patterns. The Valencia Seamount and another volcanic edifice to the north, could be responsible for the upstream flat gradient observed in the Valencia valley longitudinal profile between the confluence of the Ebro and Foix canyons (Fig. 9). Downstream, the gradient becomes steeper as a result of the erosion related to increasing stream velocities in the confined area between the volcanoes. The gullies of the flanks of the Valencia Seamount are interpreted to have been formed by subaerial erosion during the Messinian (Barone and Ryan, 1987). The spurs and peaks are interpreted to be erosional remnants of flank gullies excavated by mass-wasting. Salt diapirism

Horizon M is stratified and uniform in thickness, indicating that movement of salt postdates the Messinian. The stratigraphic and structural relations observed in the seismic records suggest that most movement of salt began during Early Pliocene time. Lower Pliocene deposits wedge towards the flanks of salt structures (Figs. 5 and 161; this suggests that deposition and rise of adjacent diapiric uplift are essentially simultaneous. Some of these salt structures had periods of activity during the Quaternary. In these cases, salt is exposed or almost exposed at the sea floor and is disrupting the Quaternary deposits. Other salt structures were not active during deposition of the Quaternary, and the Pleistocene deposits onlap the diapiric structure. Interdiapiric basins are of importance because they become sediment depocentres during the deposition of the PlioceneQuaternary sediment.

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281

Conclusions The overall morphology and seismic characteristics of the Messinian surface revealed by the seismic reflection survey in the Valencia trough, continue to substantiate the previous hypothesis of subaerial erosion and formation of evaporites in a deep and largely desiccated basin during Messinian time. In the Valencia trough, we found evidence for at least three erosive episodes, as demonstrated by the existence of three generations of erosional canyons. These three erosive episodes represent alternating opening and closing of the Betic Portal and the Rif Strait. The first erosive phase separates Miocene marine deposits from Messinian evaporitic and continental deposits. An intra-Messinian erosional phase is indicated by geophysical evidence in the Balearic margin. During this period, continental and evaporite sediments were eroded, reworked and altered. The final Messinian episode resulted in a major erosional surface that covers most of the Valencia trough margins and forms a well-developed subaerial drainage system. This system includes numerous large canyons draining mainly the Ebro and Catalan margins and a main valley, the Messinian Valencia valley, that parallels the axis of the Valencia trough. The location of the Messinian drainage system and the depositional patterns of the Valencia trough are largely related to the local structure. Most of the Messinian canyons and valleys follow depressions controlled by the Early Miocene rift structure. Other canyons, such as Blanes and Foix, follow deep faults in the basement. The Messinian erosional surface was rapidly buried by Pliocene marine deposits during reflooding of the Mediterranean basin. The subaerially eroded continental margin type where drainage patterns were controlled by rift structures, evolved to a progradational PlioceneQuaternary continental margin, where subaqueous turbidite channel-levee complexes were formed across the rift structures. Volcanic intrusions affected Messinian drainage patterns and valley morphology by entrenching and deflecting streams and by contributing to the disrupted gradients of the palaeo-Valencia

2x3

valley axial profile. Salt diapirism deformed the Messinian surface in the Balearic abyssal plain since the Early Pliocene. The basins that developed between the salt structures as a result of salt halokinesis became large sediment depocentres during the Plio-Quaternary.

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