Mud volcanoes in the Alboran Sea:
evidence from micropaleontological and geophysical data
A. Sautkin
a;, A.R. Talukder
b, M.C. Comas
b, J.I. Soto
b, A. Alekseev
a a Moscow State University, Geological Faculty and UNESCO Center of Marine Geology and Geophysics, Moscow, Russia b Instituto Andaluz de Ciencias de la Tierra (C.S.I.C. University of Granada), Campus Fuentenueva s/n, 18002 Granada, SpainReceived 5 June 2001; accepted 5 November 2002
Abstract
During the BASACALB-TTR9 cruise of the R/V Professor Logachev (1999), two mud volcanoes (called Marrakech and Granada) were discovered in the southern sector of the mud diapir province in the West Alboran Basin (WAB). This paper presents micropaleontological and geophysical data on these mud volcanoes from gravity core samples, sidescan sonar (OKEAN) images and high-resolution seismic lines. Mud breccia recovered from the Granada mud volcano is matrix-supported with well-consolidated clasts of limestone, marlstone, claystone, siltstone, sandstone and mudstone, whereas mud breccia from the Marrakech mud volcano contains unconsolidated clasts. The mud breccia matrix contains abundant Miocene calcareous nannofossils (CN), together with Pliocene^Pleistocene species and reworked late Cretaceous and Paleocene^Eocene species. CN dating indicates that clasts in the mud breccia derive from late Cretaceous, Paleocene, Eocene, and probable Miocene sediments. These data suggest that the mud diapirs and mud volcanoes in the WAB can be derived from the olistostromes of Unit VI, the basal stratigraphic sequence in the Alboran Sea basin. Unit VIconsists of lower Miocene sediments that incorporated late Cretaceous and Paleocene^Eocene materials and basement-derived rock fragments. The mud volcanic deposits are covered by a thin drape of pelagic marls, suggesting that these two volcanoes are currently inactive. Structures determined on high-resolution seismic profiles across mud volcanoes and surrounding diapirs correspond to the late-stage, Pliocene-to-Quaternary diapir development. This stage is thought to have developed during a compressional tectonic setting that produced folding and wrench tectonics throughout the basin. Mud ascent at that time resulted in active diapirism and mud volcanoes on the seafloor.
< 2002 Elsevier Science B.V. All rights reserved.
Keywords: mud diapirs; mud volcanoes ; calcareous nannofossils ; Mediterranean; Alboran Sea
1. Introduction
Mud volcanism is widespread in the Mediterra-nean, Black Sea, and Atlantic Ocean, as well as in
many other locations around the world (e.g. Iva-nov et al., 1996 ; Robertson et al., 1996 ; Robert-son and Kopf, 1998; Ivanov, 1999 ; Milkov, 2000). In mud volcanoes, plastic, clayey materials or matrix from deep source strata are extruded to the sea£oor by various driving forces (e.g. Akh-manov and Woodside, 1998; Robertson and Kopf, 1998; Kopf et al., 2000). During mud
as-0025-3227 / 02 / $ ^ see front matter < 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0025-3227(02)00691-6
* Corresponding author.
E-mail address:fu@msu.geol.ru(A. Sautkin).
cent through layers of di¡erent ages, it mechani-cally assimilates fragments of rocks surrounding the feeder channel and transports them to the sea£oor. These fragments of rocks can be partial-ly disintegrated and become incorporated in the matrix. By this means mud volcanic deposits, called mud breccia, usually consist of clay or silty-clay matrix with rock clasts that are hetero-geneous in composition, shape and size (Cita et al., 1981 ; Sta⁄ni et al., 1993). Mud volcanoes can root several kilometers below the sea£oor ( Hig-gins and Saunders, 1974 ; Fowler et al., 2000 ; Aslan et al., 2001) and can therefore be consid-ered ‘windows’ in the sedimentary basins that al-low the sampling of deep rocks.
In the Alboran Sea basin, mud diapirism and mud volcanism occurred in the West Alboran Ba-sin (WAB), where major sedimentary depocenters occurred (up to 7^8 km thick) (Fig. 1). The sedi-mentary sequence in the WAB has been the sub-ject of numerous papers based on seismic re£ec-tion interpretare£ec-tion tied to commercial well data (e.g. Comas et al., 1992, 1999; Jurado and Co-mas, 1992; Watts et al., 1993 ; Soto et al., 1996 ; Chalouan et al., 1997 ; Pe¤rez-Belzuz et al., 1997). Two commercial wells in the Spanish shelf (An-dalucia G1 and Alboran A1 ; Fig. 1), drilled through the complete sedimentary cover, provide information on the lowermost units in major de-pocenters. Ocean Drilling Program (ODP) Leg 161 (Comas et al., 1996) drilled the sedimentary sequence in the WAB, sampling sediments from the middle Miocene to Holocene on top of the metamorphic basement at Site 976 (Figs. 1 and 2). According to seismic interpretation, it has been postulated that mud diapirs in the WAB have their origin in the lowermost sediments ( Ju-rado and Comas, 1992 ; Chalouan et al., 1997 ; Pe¤rez-Belzuz et al., 1997) but no sedimentological or paleontological studies on diapiric materials have been done yet to con¢rm this interpretation. During the BASACALB cruise, leg 3 of the Training Through Research (TTR) 9 cruise, on-board the R/V Professor Logachev, the mud dia-pir province was surveyed in the southern WAB to encounter mud volcanoes by means of high-resolution seismic, sidescan sonar (OKEAN), and gravity coring. In this area, called the mud
volcano area (Fig. 1), two mud volcanoes named Granada and Marrakech were observed on the sidescan images within the diapiric structures. Three gravity cores were taken from the top of the volcanoes, which gave us the opportunity to study the lowermost sediment units of the WAB, where the mud diapir province is considered to be rooted (e.g.Comas et al., 1992, 1999 ; Chalouan et al., 1997 ; Pe¤rez-Belzuz et al., 1997). It was the ¢rst time that mud volcanoes were directly ob-served and sampled on the Alboran sea£oor.
The main aim of this paper is to determine the age of the material brought up by the mud volca-noes in the WAB. Special attention was paid to the micropaleontological study of matrix and rock clasts from the mud breccia. As there exists a close relationship, both spatial and genetic, be-tween mud volcanoes and mud diapirs (Silva et al., 1995), the age of the mud volcano samples would have important implications for the age of the mobilized sediments and clasts in the mud diapirs.
2. Geological setting
The Alboran Sea represents a basin of about 400 km long and 200 km wide with a maximum water depth of 2 km. It has complex sea£oor morphology, with several ridges, seamounts, troughs and three main sub-basins : the WAB, the East Alboran Basin and the South Alboran Basin (Fig. 1). The Alboran Ridge is the most prominent NE^SW linear relief across the Albo-ran Sea, and emerges locally forming the small Alboran Island. The Xauen Bank is situated at the southern extremity of the WAB at the junc-tion with the Alboran Ridge. The bank is formed by close folds trending ENE^WSW (Bourgois et al., 1992 ; Comas et al., 1992, 1999).
the present day. The basin itself formed by late orogenic extension and crustal thinning, coeval with thrusting and shortening in the peripheral mountain belt. Extensional tectonics was active from at least the early Miocene (about 22 Ma) to the late Tortonian (about 9^8 Ma). Since then, contractional tectonics produced inversion of previous normal faults, reverse and strike-slip faults, and folding (see Comas et al., 1999, for references).
Six litho-seismic units (labeled VI^I from bot-tom to top ; Fig. 2), tied to the commercial wells o¡ the Spanish coast, have been recognized within the sedimentary record of the Alboran Sea basin (Comas et al., 1992 ; Chalouan et al., 1997). Ac-cording to these data, the older deposits overlying the basement (Unit VI) are marine sediments of probable latest Aquitanian ( ?)^Burdigalian age,
and consist of olistostromes containing polymictic rocks (olistoliths and rock breccia) embedded in an under-compacted shale matrix. Unit VIhas been drilled only at the Alboran A1 well and the precise age of this unit is still under debate. It consists of clays with interbedded sandy and sandy-pebbly intervals. Low values for sonic ve-locity, density, and resistivity, shown by logging data, are consistent with the occurrence of under-compacted shales in Unit VI(Alboran A1 well) and also at the base of Unit V (Andalucia G1 well) (Jurado and Comas, 1992). The under-com-pacted shales of Unit VIand probably from the base of Unit V have been suggested as the source layer for the mud diapirs and volcanoes in the WAB (Comas et al., 1992 ; Jurado and Comas, 1992 ; Chalouan et al., 1997 ; Pe¤rez-Belzuz et al., 1997). Western Alboran Basin (WAB)
Andalucia G-1
Unit IV (middle Serravallian to lower Torto-nian) mainly consists of sand-silt-clay turbidite and turbiditic muds. Sediments from Unit III (Tortonian) comprise sandstone intervals, with claystone and silty clay beds, also corresponding to turbidite facies. Unit II (Messinian deposits) consists of marine siliciclastic and shallow carbon-ate facies, with occasional gypsum and thin anhy-drite intervals (Jurado and Comas, 1992). Unit I (Pliocene to Holocene sediments) was sampled in its entirety at ODP Site 976 (Fig. 2). It mainly consists of pelagic marls, muddy turbidites, hemi-pelagic clays and rare silty-sand turbidites (Comas et al., 1996). Neogene calcareous nannofossils (CN) in middle to upper Miocene and Pliocene sediments in the WAB were comprehensively de-scribed in samples from ODP Site 976 (Siesser and de Kaenel, 1999).
3. Mud volcano morphology
The surface expressions of mud volcanoes and diapir highs were analyzed on the sidescan sonar mosaic, composed of seven OKEAN pro¢les (9.5 kHz) recorded during the BASACALB cruise (TTR-9, leg 3) in the mud volcano area (Fig. 1). The OKEAN pro¢les are 8 km wide and overlap each other, giving a full coverage of an area of about 1120 km2 (Fig. 3).
Most of the area is draped with uniform sedi-mentation that provides a relatively uniform backscatter. In three places, high backscatter in-tensity in comparison with the general level of background backscatter is observed (Fig. 3). Two of these high backscatter features have been proved to correspond to mud volcano cra-ters. The variation of backscatter intensity among 1000
1200
800
6 00
Granada mud volcano Marrakech mud volcano
0
Recovery: 210 cm Recovery: 144 cm of muddy mixed sediments
carbonate clay
sample of carbonate clay (N 4) sample of crashed rocks (N 5) sample of carbonate clay (N 1)
sample of carbonate clay (N 2) sample of carbonate clay (N 3) TTR9 - 258G TTR9 - 259G
the di¡erent mud volcanoes is probably caused by variation in thickness of pelagic sediments over-lying the mud breccia, which produces an acoustic attenuation of the sidescan sonar signal (Volgin and Woodside, 1996).
The Granada mud volcano presents an elliptical feature de¢ned by an outer ring with moderate backscatter and an inner elliptical patch with high backscatter compared to the low uniform background backscatter intensity. The patch of high backscatter is o¡set towards the northeast of the outer circle and is about 1.8 km in diameter (Fig. 3). The Marrakech mud volcano shows a circular feature with irregular boundaries. To the northwest of this mud volcano, the patch of high backscatter with very irregular boundaries is interpreted as a diapiric high with a positive mor-phologic expression. On the southeastern £ank of this diapiric high, a series of patches with moder-ate backscattering are observed, which can be in-terpreted as mud £ows on the £ank of the mud diapiric volcanic structure (Fig. 3).
4. Nature of breccia and clasts
The Granada mud volcano was sampled by gravity cores 258G and 259G, taken from its cra-ter at wacra-ter depths of 583 m and 580 m, respec-tively (Fig. 3). Lithological studies of the cores from the Granada mud volcano indicate that the materials consist of matrix-supported mud breccia with sedimentary rock clasts. Core 258G recov-ered 144 cm of sediments, the uppermost being 18 cm of brown pelagic marls, whereas the rest is a matrix-supported breccia with clasts of vary-ing lithology (Fig. 3). Rock clasts were randomly distributed throughout the core sections and make up approximately 5% by volume of the mud brec-cia deposits. The average size of the rock clasts is 2^3 cm. Core 259G recovered 119 cm of similar mud breccia with rock clasts (Fig. 3). The upper part of the succession is characterized by intervals with abundant sandy admixture. Larger rock clasts are found at the top of this core (the largest clast is formed of cemented siltstone, 10 cm in diameter) although their maximum size is limited by the diameter of the core (14.8 cm). Much
larg-er clasts (1^2 m) wlarg-ere obslarg-erved at the cratlarg-er of the Granada mud volcano by the underwater TV sys-tem (Comas et al., 2000a). The rock clasts from each core were classi¢ed according to their com-position, color, grain size, structure, reaction with HCl and hardness (Table 1). In the Granada mud volcano the more abundant clast lithologies are limestones (15 clasts) and claystones (10 clasts), whereas marlstones (four clasts), sandstones (three clasts), siltstones (four clasts) and mud-stones (three clasts) are less common (Table 1). The recovered mud breccia was similar to that described in the eastern Mediterranean (Kopf et al., 2000 ; Sta⁄ni et al., 1993).
The Marrakech mud volcano was sampled by a Table 1
single gravity core (262G) taken from its slope at a water depth of 1086 m (Fig. 3). The core recov-ered 210 cm of sediments, which is composed of pelagic marl (0^30 cm) ; gray, structureless muddy mixed sediments (30^150 cm) with numerous un-consolidated clasts of claystone (varying in size from 0.05 to 1 mm) ; and patchy/cloudy clayey sediments (150^210 cm) (Fig. 3). The clasts have a similar consistency to the matrix. Samples of muddy mixed sediments, di¡erent-colored carbon-ate clays observed in the patchy/cloudy interval, and a crushed fragment of soft unconsolidated rocks pressed into the sediments were studied (Fig. 3). The muddy mixed sediments with numer-ous millimetric and unconsolidated clasts might be considered mud breccia. The lowermost
inter-val is similar to the patchy/cloudy mud breccia described by Sta⁄ni et al. (1993) in volcanoes from the eastern Mediterranean.
5. CN biostratigraphy
CN assemblages were carefully examined on smear slides for all lithological varieties of the rock clasts. Smear slides of matrix and pelagic marl from the Granada mud volcano and from the various types of sediments encountered in the Marrakech mud volcano were prepared for CN identi¢cation. The smear slides were prepared by standard techniques (Perch-Nielsen, 1985). No cleaning or concentration of the materials was Table 2
undertaken in order to retain original sediment composition and gather data on the biogenic and inorganic composition of the ¢ne fraction. Smear slides were studied using an Olympus po-larizing light microscope with a magni¢cation of U1000. A semiquantitative method of investiga-tions was performed on the smear slides (Silva et al., 1995).
For a more precise analysis of CN, 19 rock clasts were studied under the scanning electron microscope (SEM). A settling technique described by Shumenko (1987), which allows the elimina-tion of particles under 2 Wm and above 30 Wm, was used to prepare samples for SEM. This study provides additional data for CN biostratigraphy and SEM microphotographs of CN.
The abundance of individual species and the total abundance of CN species are indicated in
Tables 2^8 as follows : RR: very rare : one speci-men in 2^20 ¢elds of view; R: rare : one or two specimens in 1^2 ¢elds of view ; C : common : 2^10 specimens in each ¢eld of view ; A: abundant: 10^ 20 specimens in each ¢eld of view ; AA: very abundant: more than 20 specimens in each ¢eld of view.
The estimation of CN preservation is based on
Table 4
Summary information on distribution of Paleogene and Neo-gene CN
Table 3
the comments ofSiesser and de Kaenel (1999)and indicated in Table 2^8 as follows : P: poor pres-ervation : severe dissolution, fragmentation and/or secondary overgrowths destroying primary struc-ture. Most specimens cannot be identi¢ed to the species or generic level ; M: moderate preserva-tion : dissolupreserva-tion and/or secondary overgrowths alter primary morphological features. Most speci-mens can be identi¢ed to the generic level and often to species level ; G: good preservation : mi-nor evidence of dissolution and/or secondary overgrowths. Diagnostic features are fully pre-served and almost all specimens can be identi¢ed to species level.
In the course of identi¢cation of CN we have adhered to the criteria presented byPerch-Nielsen (1985)for Mesozoic CN and for Cenozoic CN. In addition, the following publications were used :
Young (1998) for Neogene CN, Varol (1998) for Paleogene CN, Burnett (1998) for Upper Creta-ceous CN, Albian to Pleistocene CN from the Western South Atlantic (Perch-Nielsen, 1977), Neogene CN from the Mediterranean (Muller,
Table 6
Summary information on distribution of CN in pelagic marl (pm), matrix, muddy mixed sediments (mms) and in samples of carbonate clays
Table 5
1978), Oligocene^Miocene CN biostratigraphy and paleoecology from Iberia abyssal plain (De Kaenel and Villa, 1996), Eocene CN from Iberia abyssal plain (Liu, 1996).
Distribution of Paleogene and Neogene CN taxa has been re¢ned based on data from ODP drilling results (Muller, 1978 ; Liu, 1996 ; De Kae-nel and Villa, 1996 ; Siesser and de KaeKae-nel, 1999). The Cenozoic zonation of Martini (1971) and Cretaceous zonation of Sissingh (1977), revised by Perch-Nielsen (1985), were applied.
5.1. Granada mud volcano
5.1.1. Rock clasts
All clasts sampled in cores 258G and 259G
have been classi¢ed into three main groups ac-cording to age (Table 1).
Group 1 (Cretaceous) :
The CN content in this group, although rela-tively abundant, is badly to moderately preserved (Fig. 4), and has low diversity (Table 2). The age of the studied samples is mainly late Cretaceous, but their biostratigraphic zone is often uncertain due to the low diversity of CN and the lack of marker species. Only in some cases was a precise dating of clasts possible.
Samples : TTR9 258G Ic, 258G Ia, 258G IIc. The presence of Aspidolithus parcus expansus in-dicates an age of early Campanian (zones CC18^ CC19) for these clasts (Table 3).
Table 7
Table 8
1 2
3
4
5
6
7
10 11 12
8 9
sample 259G 1s15
sample 259G 1s5
sample 258G VI
sample 259G 2s6
sample 258G II sample 258G III a
sample 258G V
sample 259G 1s1 1 m
1 m
1 m
1 m
1 m
1 m
1 m
1 m 1 m
1 m
1 m 1 m
Fig. 4. SEM microphotographs of late Cretaceous species of CN found in rock clasts.
Samples : TTR9 259G 1s16, 259G 2s10, 258G. IIb, 259G 1s5. The presence of Arkhangelskiella cymbiformis indicates that the age of these clasts could be between late Santonian and late Maas-trichtian (Table 3). But taking into consideration the last occurrence of specimens ofA. cymbiformis
var. NT with thin margin (Fig. 5) in the upper Campanian (Varol, 1989), we can assume that the age of these rocks is late Santonian^late Campa-nian.
CN assemblages found in the other rock clasts show less diversity (Table 2). However, based on these CN associations we can suggest ages for the rock clasts. For most of these rocks the lower age limit is the onset of Arkhangelskiella cymbiformis
(CC17) (Burnett, 1998) or Micula decussata
(CC14) (Perch-Nielsen, 1985) and the upper limit is extinction of late Cretaceous CN (Cretaceous^ Tertiary boundary) (Table 3). The occurrence of
Tranolithus orionatusindicates that the upper age limit is early Maastrichtian (CC23) ( Perch-Niel-sen, 1985 ; Burnett, 1998). The last occurrence of the large forms ofAspidolithus parcusgroups near the Campanian/Maastrichtian boundary ( Perch-Nielsen, 1985), thus the presence of these forms indicates an age older than Maastrichtian. Based on these observations we can assume that most of the rock clasts are Santonian^Campanian or San-tonian^Maastrichtian in age (Table 3). Clast 258G II and clast 259G 1s15 could be dated be-tween middle Cenomanian, based on the ¢rst oc-currence of Gartnerago obliquum (Burnett, 1998) and upper Maastrichtian (Table 3). Samples TTR9 258G VIand 259G 1s2 can be approxi-mately dated as Cretaceous (Table 3), based on the presence of Lithraphidites carniolensis (Berri-asian^Maastrichtian) and Predicosphaera spp. (lower Albian^Maastrichtian) (Perch-Nielsen, 1985).
Group 2 (Paleogene) :
This group represents the smaller proportion of the collected clasts. CN are abundant and moder-ately well preserved (Fig. 5), but have relatively low diversity (Table 4). The following samples contained well-preserved CN.
Sample TTR9 258G III. This clast can be dated as upper Paleocene^lower Eocene (Thanetian^ Ypresian). The lower limit corresponds to the
on-set of Toweus eminens (Fig. 6) in the late Paleo-cene (Perch-Nielsen, 1985) and the upper age limit corresponds to the last occurrence ofEllipsolithus macellus (Fig. 5) in the early Ypresian (top of NN11) (Varol, 1998) (Table 5).
Sample TTR9 258G VII. The presence of Cru-ciplacolithus frequens, Ellipsolithus bolliiand Neo-chiastozygus sp. (Fig. 5) indicates that the age of this rock is middle Paleocene^late Paleocene (Se-landian^Thanetian) (Table 5).
Paleocene^Eocene species were also observed in the others clasts (Table 4), but the preservation of CN is somewhat worse.
Sample TTR9 258G VIII. Based on the pres-ence ofToweiusspp.,Toweius eminens (last occur-rence in early Eocene) (Perch-Nielsen, 1985), and
Reticulofenestraspp. (¢rst occurrence in early Eo-cene) (Perch-Nielsen, 1985; Varol, 1998), this rock can be approximately dated as lower Eocene (Ypresian).
Sample TTR9 259G 2s14. The presence of
Campylosphaera dela (NN10^NN15) ( Perch-Niel-sen, 1985) indicates that the age of this clast could be early^middle Eocene (Ypresian^Lutetian) ( Ta-ble 5).
Sample TTR9 259 1s6 can be approximately dated as middle Paleocene^early Paleocene based on the rare occurrence of Fasciculithus tympani-formis (NN5^NN9) (Perch-Nielsen, 1985) (Table 5).
Group 3 (Neogene) :
This group includes only two clasts containing poorly preserved and di⁄cult to identify CN (Fig. 6).
Samples TTR9 259G 2s11, 259G2s12. Very similar assemblages of CN are encountered in these two clasts (Table 4, Fig. 6). The presence of Coccolithus miopelagicus indicates that the age of these clasts could be between late Oligo-cene and MioOligo-cene.
The ¢ve samples TTR9 259G 2s8, 259G 2s7, 258G IIa, 259G 1s18, 258G Ib (Table 1) are bar-ren of CN.
5.1.2. Mud breccia matrix
and rare pre-Miocene CN are found. Upper Cre-taceous CN are rare and moderately preserved (Table 6). Together with widely distributed taxa (Table 7), the marker species of late Campanian^ Maastrichtian Quadrum tri¢dum (CC22^CC23) and Arkhangelskiella cymbiformis are found (
Ta-ble 7). Paleocene^Eocene CN are rare and badly preserved (Table 6). The matrix also contains spe-cies common during the late Eocene, Oligocene and early Miocene (Tables 6 and 8).
Miocene to Pliocene CN are well preserved ( Ta-bles 6 and 8). The ages determined are early
Mio-sample 258G III
sample 258G VII
1 2
3
6 5
4
7
10 11
12
8 9
1 m
1 m
1 m 1 m 1 m
1 m
1 m 1 m
1 m
1 m 1 m
cene (Burdigalian) (Discoaster druggii, Sphenoli-thus belemnos), and middle Miocene^middle Plio-cene (Reticulofenestra pseudoumbilicus, Discoaster intercalaris). The most common species are Reti-culofenestra minuta and RetiReti-culofenestra minutula, which are widespread from the Miocene to the late Pliocene. At the same time, Gephyrocapsa
spp., typical for Pliocene sediments, are less com-mon in the matrix.
5.1.3. Pelagic marl
One sample of pelagic marl overlying mud brec-cia in core 258G (Fig. 3) was examined. CN are very abundant in this sample. The large share of
Emiliania huxleyi (Table 6) in the assemblage in-dicates theEmiliania huxleyi acme zone, thus this pelagic marl can be dated as the late Pleistocene^ Holocene.
sample 259G 2s11
sample 259G 2s12
1 2
3
4 5 6
7 1 m
1 m 1 m
1 m
1 m 1 m
1 m
Fig. 6. SEM microphotographs of Neogene species of CN found in rock clasts.
1. Helicosphaerasp. (sample 259 2s11) 2. Discoastersp. (sample 259G 2s11) 3. Pontosphaera multipora(sample 259G 2s11) 4. Reticulofenestrasp. (sample 259G 2s12) 5. Discoastersp. (sample 259G 2s12)
6, 7. Discoastercf.de£andrei(sample 259G 2s12)
Fig. 5. SEM microphotographs of Paleogene species of CN found in rock clasts.
1. Chiasmolithuscf.bidens(sample 258G III) 2. Chiasmolithus solitus(sample 258G III) 3. Ericsonia subpertusa(sample 258G III) 4, 7. Discoasteroidescf.bramlettei(sample 258G III) 5. Toweius eminens(sample 258G III)
5.2. Marrakech mud volcano
In the muddy mixed sediments from this core, CN are abundant or common and their preserva-tion varies from rather poor for older species to good for Miocene to recent forms (Table 6).
Upper Cretaceous CN are rare in this sample (Table 6). In association with widely distributed taxa (Table 7) marker species of Maastrichtian age, Lithraphidites quadratus (zones CC25^ CC26) and late Campanian^Maastrichtian ages,
Quadrum tri¢dum (zones CC22^CC23), are en-countered (Table 7). Paleocene^Eocene and Oli-gocene CN are very rare in this sample (Tables 6 and 8).
Miocene^Pliocene CN are common (Table 6).
Triquetrorhabdulus carinatus is a marker species for earliest Miocene (Aquitanian^Burdigalian) age (NN1^NN2), the presence ofCyclicargolithus £oridanus, common Cyclicargolithus abisectusand
Discoaster de£andrei is also indicative for early Miocene age (Table 7).Discoaster pseudovariabilis
is characteristic for middle Miocene, Discoaster pansus for early Pliocene age. Many species with a relatively wide distribution were observed in the sample (Table 8).
Pleistocene CN are abundant (Table 6). The most abundant are small Gephyrocapsa sp. (63 Wm), Gephyrocapsa caribbeanica and Emiliania
huxleyi.
A mixture of CN of di¡erent ages was observed in the carbonate clays sampled from the lower-most interval (Table 6). Upper Cretaceous CN, poorly or moderately preserved, are rare in these samples (Table 6). Paleocene^Eocene CN are rarer than upper Cretaceous ones and only a few poorly preserved specimens were encountered (Table 6).
The most abundant species in these samples are well preserved and diversi¢ed Miocene^Pliocene CN (Table 6). Among them species indicative for late Miocene to late Pliocene age are encoun-tered (Table 8).
Finely preserved and abundant Pleistocene CN were also identi¢ed in the samples (Table 6), with common forms beingEmiliania huxleyi and small
Gephyrocapsa spp. (63 Wm).
Among the samples of particular interest is a
crushed fragment of soft rock sampled in the core (Fig. 3) containing poorly or moderately pre-served, low-diversity CN assemblage. The most common are upper Cretaceous taxa (Table 6). The presence ofLithraphidites quadratusindicates a late Maastrichtian age (zones CC25 and CC26). Miocene taxa, including early Miocene Triquetro-rhabdulus carinatus (NN1^NN2), were also en-countered in this sample (Table 6).
6. Structures associated with mud diapirs and volcanoes
In the mud volcano area (Fig. 1), 70 km of high-resolution seismic lines recorded during the BASACALB cruise (TTR9, leg 3) were used to study the recent mud diapirism and volcanism (Fig. 7). Multichannel seismic re£ection lines crossing the area were used to correlate the dis-tinguished litho-seismic units.
On the high-resolution seismic pro¢les, mud di-apirs are imaged as transparent to semitranspar-ent seismic facies with a chaotic facies at their nucleus. Seismic pro¢ling only crossed the Marra-kech mud volcano, although the seismic tracking was o¡ center. This mud volcano developed on the £ank of a diapir with a chaotic seismic facies, and has a negative sea£oor expression (Figs. 3 and 8).
from top to bottom, Q1 to Q3, for the Quater-nary ; and P1 and P2 for the Pliocene ;Figs. 8 and 9). Subunit Q2 presents the maximum thickness variation. Folds observed in the studied area are mainly related to the distribution of diapiric highs, so that the anticlines and synclines coincide with the diapiric highs and marginal troughs, re-spectively.
7. Discussion
Lithological and CN data from the studied samples indicate the nature of the materials trans-ported to the sea£oor by the mud volcanoes. The abundance of Miocene CN in the matrix from the Granada mud volcano (Table 6) suggests that the principal sources forming the mud volcano are
1400
1200 1000
800
100 200
400 600
Morocco
262G
5 00’W 4 30’
35 30
’N
35 10
’
Fig. 8
Fig. 9
Conrad 828
258G 259G
MCS lines
single channel seismic lines seismic lines presented in figures
mud volcanoes sampled by gravity coring Granada
mud volcano
Marrakech mud volcano
° °
° °
sediments of Miocene age. The presence of the early Miocene (Burdigalian) marker species indi-cates that the sediments belonging to the lower Miocene unit (olistostrome Unit VI) were the source layer. At the same time, the presence of late Miocene and Pliocene CN indicates that younger sediments were incorporated from the
pierced overlying strata during the ascent of the mud volcano.
The upper Cretaceous (Santonian^Maastricht-ian), Paleocene and Eocene CN observed in the matrix are interpreted as reworked material. The relatively common occurrence of these CN in the matrix (Table 6) results from the redeposition of
Diapir high
Marrakech mud volcano
2km
TWTT(msec)
TWTT(msec)
2200 2200
1800 1800
1400 1400
1000 1000
320 320
400 400
480 480
560 560
640 640
720 720
800 800
NW-SE NW-SE
P2
P2 Q1
Q1
P1+Q3
Q2
Upper Cretaceous and Paleogene materials. The existence of the same set of upper Cretaceous taxa in the matrix as well as in the rock clasts (Table 9) suggests a common sediment source. Most of the rock clasts encountered in the Granada mud vol-cano are from upper Cretaceous sediments, mainly Santonian^Maastrichtian, whereas clasts
derived from Paleocene, Eocene and Miocene sediments are less common (Table 1). The mixing of rock clasts of di¡erent ages is interpreted as derived from material belonging to the basal olis-tostrome Unit VI, drilled by the commercial well Alboran A1 in the WAB. The variety and abun-dance of Miocene CN in the matrix and the scar-city of Miocene rocks in the mud breccia indicate that the Miocene rocks were most probably to-tally disintegrated into muddy lithologies, whereas the older, and more consolidated rocks (Creta-ceous to Eocene) were better preserved as clasts in the mud breccia.
The most common CN assemblage in the Mar-rakech mud volcano deposits is Miocene^Pliocene and Pleistocene in age. The lower Miocene taxa (Aquitanian^Burdigalian) are encountered in this assemblage, so it seems possible that the Marra-kech mud volcano could have the same source as the Granada mud volcano (olistostrome unit VI). However, the common presence of the middle Miocene to early Pliocene species would seem to indicate that younger sediments were extensively involved in the activity of this mud volcano. The presence of reworked upper Cretaceous (Santo-nian^Maastrichtian) and Paleogene CN was also determined in the Marrakech mud volcano. The set of upper Cretaceous CN encountered in the muddy mixed sediments, very similar to that of those from the rock clasts and matrix from the Granada mud volcano (Table 9), implies that these materials could be reworked from the same source. The crushed fragments of rock (sam-ple N5) with upper Cretaceous and lower Mio-cene CN (Table 6) is evidence for the disintegra-tion of upper Cretaceous rocks into a muddy lithology.
If it is correct that the source of mud volcano materials is the olistostrome unit in the WAB (Unit VI), known in the commercial well (Albo-ran A1) o¡ the Spanish coast, then we can spec-ulate that this unit is distributed throughout the WAB and ¢lled the main depressions on both the Spanish and Moroccan margins. Based on our micropaleontological data, we can also demon-strate that this olistostrome unit includes upper Cretaceous and Paleogene blocks.
Gravity cores TTR9 262, taken from Marra-Table 9
kech mud volcano, and TTR9 258G, taken from Granada mud volcano, were found generally cov-ered by a drape of pelagic marls. The local ab-sence of a pelagic blanket (core TTR9 259G) may be due to erosion of the uppermost sediments by surface currents. Study of the CN assemblage from these sediments (recovered on the Granada mud volcano) enables dating of this marl as late Pleistocene^Holocene. On the Granada mud vol-cano there was no evidence of £uid escape seen on underwater TV system during the BASACALB cruise (Comas et al., 2000b). The existence of these pelagic marl blankets, together with the ab-sence of £uid escape and the relatively uniform backscatter intensity across the area, suggests that the two mud volcanoes are probably cur-rently inactive.
The Pliocene-to-Quaternary diapiric rise, occur-ring duoccur-ring sedimentation, created local uplift and subsequent erosion and thinning of the litho-seis-mic units producing lateral pinchout and onlap towards the diapiric highs, and therefore inducing local unconformities in the sedimentary sequence (Figs. 8 and 9). The angular unconformities at the base of subunits Q1 and Q2 and the highly vari-able thickness of subunit Q2 suggest that there were at least two major pulses of diapiric rise (pre- and post-Q2) during the late Pliocene and Quaternary in the studied sector (Figs. 8 and 9).
The Marrakech mud volcano developed on the £ank of a diapiric high following a fault (Fig. 8), indicating that it was probably formed by £uid migration through the body of the diapir. The Granada mud volcano, on the other hand, shows no direct relationship with any visible diapir. The existence of larger and abundant clasts on the crater of this volcano (seen on the underwater TV system during the cruise) and the surrounding moderate backscatter intensity suggest that it was probably the result of the rise of £uidized sedi-ments through faults from probably deeper dia-pirs. Thus, we suggest that these mud volcanoes can be the result of diapirism (as diapir extrusion) primarily driven upward by buoyancy forces (e.g.
Brown, 1990; Kopf et al., 1998 ; Milkov, 2000). The mud volcano area developed at the north-ern £ank of Xauen Bank, which is formed by post-Messinian close folds (Bourgois et al.,
1992 ; Comas et al., 1992, 1999). High-angle faults, folds and sharp contacts in relation to mud diapirs, together with the geological situation of the mud volcano area, suggest that mud volca-nism developed in a general compressional setting during the Pliocene to Quaternary.
Our CN dating of the mud volcano material demonstrates for the ¢rst time that the source layer of the mud volcanoes and mud diapirs in the WAB is lower Miocene sediments with em-bedded Paleogene and Upper Cretaceous clasts. The mud volcanoes seem to be rooted in olistos-tromic Unit VI, so our data con¢rm the early Miocene age of this unit, and also indicate that the olistostrome contains Paleogene and Creta-ceous materials.
8. Conclusions
(1) Mud volcanoes encountered in the WAB are formed by matrix-supported mud breccia. Based on the presence of lower Miocene CN along with the mixing of diversi¢ed rock clasts in the mud breccia, we determined that the mud volcanoes are likely rooted in olistostromic Unit VI, which should therefore be early Miocene in age, forming the lowermost sedimentary sequence in the WAB. (2) The occurrence of upper Cretaceous to Eo-cene clasts in the mud volcano sediments suggests that olistostromic Unit VIcontains, among other materials, upper Cretaceous and Paleogene blocks.
(3) Drapes of pelagic marl on mud volcanic deposits indicate that the studied mud volcanoes are currently inactive.
(4) The Marrakech mud volcano is interpreted as resulting from £uid migration along the sea-£oor-piercing mud diapiric body. The Granada mud volcano is probably formed from the rise of £uidized mud breccia along a fault on the £ank of a deeper diapir.
Acknowledgements
We are grateful to M.K. Ivanov, of the TTR program, for his all-encompassing help. More-over, we wish to express our gratitude to the BA-SACALB TTR9 scienti¢c party. We thank refer-ees J.A. Flores, E. De Kaenel, A. Kopf and N. Kenyon for their thoughtful comments and de-tailed criticism, which helped to clarify the manu-script. We also thank C. Laurin for her careful and detailed linguistic revision. Funding for the BASACALB Cruise and this work was provided by Project REN2001-3868-C03 (MCYT, Spain).
Appendix. Calcareous nannofossils identi¢ed in
this paper
Group 1. Cretaceous CN listed alphabetically by generic epithet.
Ahmuellerella octoradiata (Gorka, 1957) Rein-hardt (1966)
Arkhangelskiella cymbiformis Vekshina (1959)
Arkhangelskiella specillataVekshina (1959)
Arkhangelskiella cf.A. specillata Arkhangelskiella spp.
Aspidolithus parcus (Stradner, 1963)
Braarudosphaera bigelowii (Gran and Braarud, 1935) De£andre (1947)
Calculites obscurus(De£andre, 1959) Prins and Sissingh in Sissingh (1977)
Chiastozygus litterarius (Gorka, 1957) Manivit (1971)
Cribrosphaerella ehrenbergii (Arkhangelsky, 1912) De£andre in Piveteau (1952)
Ei¡ellithus turrisei¡elii (De£andre in De£andre and Fert, 1954) Reinhardt (1965)
Gartnerago obliquum (Stradner, 1963) Noel (1970)
Glaukolithus diplogrammus (De£andre in De-£andre and Fert, 1954) Reinhardt (1964)
Kamptnerius magni¢cusDe£andre (1959)
Lithraphidites carniolensis De£andre (1963)
Lithraphidites alatus Thierstein in Roth and Thierstein (1972)
Lithraphidites praequadratusRoth (1978)
Lithraphidites quadratus Bramlette and Martini (1964)
Markalius inversus(De£andre in De£andre and Fert, 1954) Bramlette and Martini (1964)
Markalius spp.
Manivitella pemmatoidea De£andre
Microrhabdulus decoratus De£andre
Micula concava(Stradner in Martini and Strad-ner, 1960) Verbeek (1976)
Micula decussata Vekshina (1959)
Placozygus sigmoides (Bramlette and Sullivan, 1961) Romein (1979)
Predicosphaera grandis Perch-Nilsen (1979)
Predicosphaera cretacea (Arkhangelsky, 1912) Gartner (1968)
Predicosphaera spinosa (Bramlette and Martini, 1964), Gartner (1968)
Predicosphaera spp.
Quadrum tri¢dum (Stradner in Stradner and Papp, 1961) Prins and Perch-Nilsen in Manivit et al. (1977)
Rhagodiscus angustus Stradner (1963)
Rhagodiscus spp.
Retecapsa crenulata (Bramlette and Martini, 1964) Noel (1970)
Tetrapodorhabdus decorus(De£andre in De£an-dre and Fert, 1954) Wind and Wise (1977)
Thiersteinia cf.T. ecclesiactica Thoracosphaeraspp.
Tranolithus exiguus Stover (1977)
Tranolithus gabalus Stover (1966)
Tranolithus orionatus Reinhardt (1966)
Tranolithus spp.
Watznaueria barnesae Black in Black and Barnes (1959)
Zigodiscus cf.Z. variatus
Group 2. Cenozoic CN listed alphabetically by generic epithet.
Biscutum spp.
Calcidiscus leptoporus (Murray and Blackman, 1898) Loeblich and Tappan (1978)
Calcidiscus macintyrei (Murray and Blackman, 1989) Loeblich and Tappan (1978)
Camylosphaera dela (Bramlette and Sulivan, 1961) Hay and Mohler (1967)
Ceratolithus spp.
Chiasmolithus bidens Bramlette and Sullivan (1961)
Chiasmolithus consuetus (Bramlette and Sulli-van, 1961) Gartner (1970)
Chiasmolithusspp.
Coccolithus eopelagicus (Bramlette and Riedel, 1954) Bramlette and Sullivan (1961)
Coccolithus miopelagicus Burky (1971)
Coccolithus pelagicus (Wallich, 1877) Schiller (1930)
Coronocyclus nitescens(Kamptner, 1963) Brem-lette and Wilcoxon (1967)
Cruciplacolithus tenuis Hay and Mohler (1967)
Cruciplacolithus frequens (Perch-Nielsen, 1977) Romein, 1979
Cruciplacolithus spp.
Cyclicargolithus abisectus (Muller, 1970) Wise (1973)
Cyclicargolithus £oridanus (Roth and Hay, 1967) Bukry (1971)
Dictiococcites perplexa Burns (1975)
Dictiococcites productusBackman (1980)
Dictiococcites bisectus(Hay, Mohler and Wade, 1966) Bramlette and Sullivan (1961)
Discoaster barbadiensis Tan (1927)
Discoaster bollii Martini and Bramlette (1963)
Discoaster brouweri (Tan, 1927) Bramlette and Riedel (1954)
Discoaster de£andrei Bramlette and Riedel (1954)
Discoaster druggii Bramlette and Wilcoxon (1967)
Discoaster hamatus Martini and Bramlette (1963)
Discoaster intercalaris Bukry (1971)
Discoaster multiradiatus Bramlette and Riedel (1954)
Discoaster neorectusBukry (1971)
Discoaster pansus (Bukry and Percival, 1971) Bukry (1973)
Discoaster pentaradiatus (Tan, 1927) Bramlette and Riedel (1954)
Discoaster pseudovariabilisMartini and Worsley (1971)
Discoaster variabilis Martini and Bramlette (1963)
Discoaster cf.D. de£andrei Discoaster spp.
Discoasteroides cf. D. bramlettei Ellipsolithus bollii Perch-Nielsen (1979)
Ellipsolithus macellus (Bramlette and Sullivan 1961) Sullivan 1964
Emiliania huxleyi (Lohmann, 1902) Hay and Mohler (1967)
Ericsonia subpertusa Hay and Mohler (1967)
Fasciculithus tympaniformis Hay and Mohler (1967)
Geminilithella bramlettei (Hay and Towe, 1962) Varol (1989)
Gephyrocapsa oceanicaKamptner (1943)
Gephyrocapsa caribbeanica Boudereaux and Hay (1967)
Gephyrocapsa spp.
Helicosphaera carteri(Wallich, 1877) Kamptner (1954)
Helicosphaera mediterranea Muller, 1981
Helicosphaera euphratis Haq, 1966
Helicosphaera lophota Bramlette and Sullivan, 1961
Helicosphaera spp.
Lanternithus minutus Stradner (1962)
Nanotetrina spp.
Neochiastozygus cf. N. perfectus Neochiastozygus spp.
Pontoshpaera multipora (Kamptner) Roth (1970)
Pontoshpaeraspp.
Reticulofenestra haqii Backman (1978)
Reticulofenestra hillae Bukry and Percival (1971)
Reticulofenestra minutaRoth (1970)
Reticulofenestra minutula (Gartner, 1967) Haq and Berggren (1978)
Reticulofenestra pseudoumbilicusGartner (1969)
Reticulofenestra umbilicus(Levin, 1965) Martini and Ritzkowski (1968)
Rhabdosphaera procera Martini (1969)
Rhabdosphaera clavigeraMurray and Blackman (1898)
Scapholithus fossilisDe£andre in De£andre and Fert (1954)
Scapholithusspp.
Sphenolithus belemnos Bramlette and Wilcoxon (1967)
Sphenolithus elongatusBramlette and Wilcoxon (1967)
Sphenolithus radiansDe£andre in Grasse (1952)
Toweius eminensBramlette and Sullivan (1961)
Toweius spp.
Triquetrorhadulus carinatus Martini (1965)
Umbilicosphaera sibogae (Weber van Bosse) Gaarde (1970)
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