Clay mineral assemblages in flysch from the Campo de Gibraltar ...

Clay Minerals (1999) 34, 345-364 

Clay mineral assemblages in flysch from 

the Campo de Gibraltar area (Spain) 

M. D. RUIZ CRUZ 

Departamento de Quimica Inorg&nica, Cristalografia y Mineralogla, Facultad de Ciencias, Universidad de M(tlaga, 

29071, Malaga, Spain 

(Received 17 July 1996; revised 25 February 1998) 

AB STRACT: In order to determine the relative influence of palaeoenvironmental and diagenetic 

processes in clay assemblages, as well as their significance, both fine- and coarse-grained sediments 

from the Campo de Gibraltar flysch have been studied by means of X-ray diffraction, optical and 

electron microscopy, and chemical analysis. Diagenetic modifications appear to be lithologically 

controlled and mainly affect coarse-grained sediments, where Fe-chlorites, illite and kaolinite are the 

more characteristic authigenic clay minerals. The evolution of detrital assemblages, determined in 

fine-grained beds, indicates that, from Cretaceous to Eocene times, clay mineralogy, characterized by 

the opposite kaolinite+smectite and illite + I-S mixed-layer assemblages, was mainly controlled by 

sources, climate and transport processes. On the other hand, from the Oligocene, clay mineral 

assemblages, characterized either by the abundance of kaolinite, or by the illite+chlorite association, 

mainly reflect the petrology of source rocks, as a consequence of climatic cooling and the increasing 

tectonic activity, which impede the development of soils. 

Clay mineral composition of sedimentary sequences 

not related to areas of tectonic activity, mainly 

reflects climate, relief and lithology of source rocks. 

For this reason the clay assemblages have often 

been used in palaeoclimatic reconstructions, as 

extensively reviewed by Chamley (1989). In 

sedimentary formations found in zones of intense 

tectonic activity (flyschl), clay mineral assemblages 

mainly reflect the composition of the parent rocks, 

since tectonic activity usually impedes the devel- 

opment of continental soils. In addition, the 

transport processes include, in these areas, turbidity 

currents, which are characterized by relatively high 

concentration flows, and in which sediments may 

be kept in suspension by turbulence: and deposition 

does not produce well sorted sediments 

characteristic of normal Stokes-law settling 

i The term flysch (Dzulinski & Walton, 1965; Hsu, 

1970) refers to a particular sedimentary facies consisting 

of thick sequences of interbedded shales and graded 

beds, often interpreted as turbidites, deposited in fore 

deep basins (Selley, 1988). 

9 1999 The Mineralogical Society 

(Kranck, 1984). The sediments usually include 

similar proportions of the grain-sizes found in the 

parent suspensions. 

Flysch is therefore useful for identification of 

source terranes from clay mineralogy, although, as 

with climatic interpretations (Chamley, 1971, 1989), 

there are limitations. The basic limitation derives 

from the fact that flysch usually consists of thick 

accumulations of sediments in areas where burial will 

favour diagenetic modifications, mainly in clay 

mineral assemblages containing smectite. In such a 

case detrital signs may be partially or completely 

obliterated. Also, detrital assemblages frequently 

show notable variations in relative clay mineral 

abundance in turbiditic and pelagic or hemipelagic 

beds (Monaco & Mear, 1984), reflecting the effects of 

sorting processes. These changes can be misinter- 

preted in terms of climatic variations or mixing of 

supplies. These difficulties probably explain the 

scarcity of information on the significance of clay 

assemblages in ancient flysch formations. 

This study involved, at first, the characterization 

of the several units from the Campo de Gibraltar

346 M. D. Ruiz Cruz 

flysch, on the basis of their clay mineralogy and, 

subsequently, the analysis of the main factors 

controlling the clay mineral assemblages. With the 

aim of avoiding the limitations enumerated in the 

preceding paragraph, the evolution and significance 

of detrital assemblages were based chiefly on the 

data obtained from fine-grained sediments, since the 

diagenetic processes were more advanced in coarse- 

grained beds. Similarly, the problems associated 

with multiple sediment sources were avoided using 

Aljibe-type sequences for the analysis of the 

general expression of the palaeoenvironmental 

factors. In these sequences detrital supplies were 

continuously derived from a southern, African 

source. The effects of diagenetic processes were 

analysed in sequences with wide lithologic variety 

(Algeciras-type sequences), which experienced 

similar burial temperatures and pressures. 

GEOLOGY 

In the Strait of Gibraltar area, several 'crustal 

domains' are clearly differentiated (Inset, Fig. 1): 

(1) the Internal zones of the Betic-Rif ranges or 

Alboran domain: (2) the External zones of the Betic 

and Rif ranges or the South-Iberian and Maghribian 

domains, respectively; and (3) the Intermediate 

zones or Flysch domain. The Internal zones 

consist of metamorphic complexes (Nevado- 

Filfibride and Alpuj~rride), and sedimentary to 

very low-grade metamorphic materials (Mal~guide 

Complex), which exhibit geographic and geologic 

continuity in the Betic and Rif ranges (Didon et al., 

1973). The External zones are Mesozoic to 

Cenozoic sediments of the Iberian and African 

palaeomargins. In contrast to the Internal zones, 

sedimentation and evolution in both northern and 

southern palaeomargins changed in the late 

Cretaceous. The Flysch domain, currently located 

between the Internal and External zones, consists of 

thick, mainly turbiditic sequences, which offer 

similar facies on both sides of the Strait (Didon et 

al., 1973). 

The Flysch domain includes sediments deposited 

from Cretaceous to early Miocene within a deep 

basin along the boundary of the North African, 

Iberian and Alboran plates. Sediment deposition 

within this complex arrangement of plates was 

controlled by: (i) uplift of shelves; (ii) subsidence 

of sea-floor; (iii) changes in the CaCO3 compensa- 

tion depth (CCD); (iv) changes in sea-level; and (v) 

migration of plates. Maximum volumes of flysch 

were produced during the late Oligocene and early 

Miocene (Uchupi, 1988). 

Several lithostratigraphic units from different 

North African countries and Spain were described 

in the literature at similar times and therefore were 

assigned different names (Fig. 2). The Spanish 

nomenclature will be used in this presentation. In 

the studied area, the main Cenozoic units 

(Algeciras, Bolonia and Aljibe) are well repre- 

sented, whereas the extent of Cretaceous units is 

more limited (Fig. 1). 

The Facinas unit (Fig. 2) comprises sediments 

from Neocomian to late Cretaceous and has been 

frequently interpreted as the basal part of the Aljibe 

unit (Esteras, 1984), although their stratigraphic 

continuity has not been demonstrated. It consists of 

grey shales with sporadic sandy (Lower Cretaceous) 

and calcarenitic (Upper Cretaceous) beds. It also 

contains mm-cm beds and nodules of Mn and Fe 

oxides and carbonates, and abundant 'tubotoma- 

culum' (Pautot et al., 1975). 

The Almarchal unit (Fig. 2) has been ascribed to 

the External zones of the Rif domain (Didon, 1967, 

1969) and comprises a rhythmic alternation of black 

shales, marls and calcareous turbidites, which 

appear locally silicified. The determination of the 

thickness of this sequence is very difficult since it 

is strongly deformed, but it is estimated as >600 m. 

The Algeciras unit (Didon, 1960) is characterized 

by Oligocene to Aquitanian sequences >1000 m 

thick (Fig. 2), which contain an alternation of 

micaceous, feldspathic and calcareous sandstones, 

and marls. The basal sequences show a transition 

from calcareous (late Cretaceous to Eocene) to 

pelitic (Oligocene) successions. 

The Aljibe unit (Fig. 2) contains thick sequences 

(1000-3000 m), mainly ascribed to the Aquitanian, 

made up of rhythmic alternations of quartzose, 

well-sorted sandstones, and clays, both character- 

ized by the lack of carbonates. The basal sequences 

show frequent lateral variations in thickness and 

composition, and includes some well-defined 

formations (Table 1). 

The Bolonia unit (Fig. 2) contains facies inter- 

mediate between the Algeciras and Aljibe ones 

(Didon, 1969). Algeciras-type sequences appear 

interbedded with typical sandstones and claystones 

of the Aljibe unit. 

The whole of the flysch sequences underwent 

progressive burial from the Cretaceous to the early 

Miocene and were intensively deformed during the 

Alpine orogeny (SECEG-SNED, 1995).

;IBRALTAR STRAIT 

MALAGA 

I 

Clay mineral assemblages in flysch 347 

EXTERNAL ZONES 

INTERNAL ZONES 

FLYSCH UNITS 

A A FRONTAL THRUST 

0 ~ 0 O 

)O ~ 0 0 O O00~ O 

O O ~ 

)000 0 0 ~ 0 0000 0 

0000 00 0 0 

0 0 0 O~ 

O 0 0 0 0 0 

o 

13 0 0 0 

10 ~M. 

C. 0 0 

0 0 

"1 A 

GIBRALTAR STRAIT 

1B 

/ / 

%, 

GIBRALTAR 

I I POST OROGENIC FORMATIONS 

~--j INTERNAL ZONES 

~ CALCAREOUS DORSAL 

p ~ ALJIBE UNIT 

~ ALGECIRAS UNIT 

BOLONIA UNIT 

~ FACINAS AND ALMARCHAL UNITS 

FAULT 

A, A THRUST FRONT 

FIG. 1. Simplified geologic map (frolIl SECEG-SNED, 1991) of the Strait of Gibraltar area and location of the 

studied sections. (1) Algeciras unit, (2) Aljibe unit, (3) Bolonia unit, (4) Facinas unit, (5) Almarchal unit. 

Triangles mark the location of the measured sections.


MIOCENE 

OLIGOCENE 

EOCENE 

PALAEOCENE 

LATE 

CRETACEOUS 

EARLY 

CRETACEOUS 

Burdigalian 

Aquitanian 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Albian 

Aptian 

Neocomian 

. . . . . . . . . . . . . . . . . . . . . . . . 

il - 

.~176 '~ ~ 

[- 

J 

9 ,..1 I,T,I 

FIG. 2. Age, approximate thickness and stratigraphic nomenclature of the main Gibraltar Units, from the northern 

and southern (parentheses) sides of the Strait. 

MATERIALS AND METHODS 

About 600 samples were collected in several 

sections of the flysch units. The positions of the 

studied sections are marked in Fig. 1 and the 

distribution of sections by unit, thickness and age 

are given in Table 1. 

The general methodology, described in previous 

papers (e.g. Rodrlguez Jim6nez & Ruiz Cruz, 

1988a,b,c, 1989, 1990), is summarized as follows. 

Optical microscopy, combined with point counting 

(>1000 points) was used to determine the composi- 

tion of sandstone and the texture of different 

lithologies. 

X-ray diffraction (XRD) analyses were used to: 

(i) determine the bulk mineralogy of sediments 

(random mounts; intensity factors from Schultz, 

1964); and (ii) investigate two size-fractions 

(2-20 ~tm and

Clay mineral assemblages in flysck 

TABLE 1. Summary of flysch sample distribution and stratigraphic units sampled. 

Unit Sections Age Thickness Number of 

(m) samples 

1. Algeciras A. Punta Tarifa Eocene-Aquitanian 400 26 

B. Punta Carnero Eocene Aquitanian 900 31 

C. El Rinconcillo Late Cretaceous-Aquitanian 350 36 

D. E1 Ferrocarril Late Cretaceous-Palaeocene 35 9 

E. E1 Cobre Oligocene-Aquitanian 140 14 

2. Aljibe A. Sierra de Oj~n Aquitanian 100 12 

B. Presa de Palmones Aquitanian 250 17 

Benaiza formation C. Tfinel de Palmones Aquitanian 130 13 

D. Canal Guadarranque Late Palaeocene~Oligocene 30 14 

Jimena clays E. Jimena 1 Oligocene 200 23 

F. Jimena 2 Late Cretaceous Eocene 200 20 

G. Jimena 3 Late Cretaceous-Eocene 50 8 

H. Colmenar Late Cretaceous Eocene 55 12 

Blocky clays I. Montecorto Burdigalian? ? 7 

J. Short sequences Eocene 5-20 20 

K. Short sequences Aquitanian-Burdigalian 5 20 25 

3. Bolonia A. E1 Cabrito Eocene Aquitanian 260 14 

B. E1 Pulido Oligocene-Aquitanian 120 25 

4. Facinas A. Alcachota Early Cretaceous 120 16 

B. Puerto del Rayo Cretaceous 100 26 

5. Almarchal A. Los Pastores Late Cretaceous 120 19 

B. P. Bolonia borehole Late Cretaceous 200 17 

C. Bolonia shaft Late Cretaceous 80 17 

D. Tarifa gallery Late Cretaceous 100 10 

E. Short sequences Late Cretaceous 5-25 60 

analysis) and fractured surfaces of critical-point 

dried samples provided textural data on the fine- 

grained components of sediments. We used a Jeol 

JSM-840 scanning electron microscope in both 

secondary and back-scattered modes. 

The chemical composition of clay minerals was 

obtained by transmission electron microscopy (TEM/ 

AEM), using a 200 kV Philips CM-20 scanning- 

transmission electron microscope. The standards used 

were albite for Na; muscovite and biotite for K; 

albite, spessartite and muscovite for Al; olivine and 

biotite for Mg and Fe; spessartite for Mn; and titanite 

for Ca. More complete chemical analyses for major 

and trace elements, either in the


C~T 

FACINAS UNIT 

I-S 

K ~'~ 14Ch K 

Caicarenite: Open circles 

Sandstone: Triangles 

Shale and marl: Filled circles 

ALMARCHAL UNIT 

St I-S 

LATE 

OUS 

l+Ch 

Fl(~. 3. Clay mineral composition of 75% of this 

mineral (Ruiz Cruz & Esteras, 1993) corresponds to 

non-ideal montmorillonite (Schultz, 1969; Newman 

& Brown, 1987). The Fe content and DTA curves 

confirm, after the data by Brigatti (1983) and 

Mackenzie (1972), the nature of these smectites. 

Compared to most Cretaceous and recent deep-sea 

smectites from different locations (Lancelot et al., 

1972; Bouquillon & Debrabant 1987; Leinen, 1987; 

Chamley, 1989) they show a slightly higher Mg 

content that reflects the local development of Mg- 

rich smectites and, perhaps, palygorskite contam- 

ination. Trace elements also display some differ- 

ences in relation to other deep-sea smectites 

(Leinen, 1987; Chamley, 1989). The mineral


Flc~. 4. Scanning electron micrographs of diagenetic minerals in Cretaceous units. (A) lllite in calcareous beds 

from the Almarchal unit. (B) Chlorite partially enclosed by albite, in sandstones from the Facinas unit. (C) 

Vermicular kaolinite fracture-fillings in shale beds from the Almarchal unit. (D) Platy, euhedral dickite filling 

veins in shale beds from the Almarchal unit. 

described here appears depleted in metallic cations, 

such as Cr, Cu or Ni, showing positive anomalies in 

V and Nb. 

The passage from Eocene to Oligocene flysch 

(Benaiza formation) is clearly marked by a decrease 

in kaolinite (


AI~IBE UNIT 

|-S (S) 

IS 

Aqui~afian- 

Burdigalian 

OLIGOCENE 

K I~Ch 

S I-S 

/ ~ ~ LATE CRETACEOUS- 

Calcarenite: Open circles 

Quartzose sandstone: Triangles 

Claystone: Filled circles 

Fl~. 5. Clay mineralogy in the Aljibe unit. (A) Late 

Cretaceous to Eocene flysch; left: pelitic sequences 

(Jimena formation), right: silicified sequences. 

(B) Oligocene flysch (Benaiza formation). (C) Aquita- 

nian flysch; K: kaolinite, S: smectite, I-S: illite- 

smectite mixed-layers, I: illite, and Ch-S: regular 

chlorite-smectite mixed-layers. 

elements in 28 fine-grained samples (Ruiz Cruz & 

Linares, 1992). The statistical treatment of the data 

showed significant correlations between minerals 

and variables as well as significant positive 

anomalies in Y, Nb and P. 

Algeciras unit 

Calcareous flysch contains the clay mineral 

association illite (20-60%) + smectite or I-S 

mixed-layers (40-80%), with scarce kaolinite 

(generally


FIG. 6. Scanning electron micrographs of diagenetic minerals in late Cretaceous to Eocene flyschs: (A) Mg- 

smectite and palygorskite in clays from the Jimena formation; (B) Fe-rich chlorite filling fossil cavities in 

calcarenites from the Jimena formation; and (C) palygorskite in late Cretaceous marls from the Algeciras unit.


FIG. 7. Scanning electron micrographs of diagenetic minerals in Oligocene to Aquitanian flyschs. (A) Kaolinite 

pore-filling in quartzose sandstones from the Aljibe unit. (B) Fine-grained quartz and lathed illite tilting the pores 

in feldespathic sandstones from the Algeciras unit. (C) Blocky dickite pore-filling in quartzose sandstones from 

the Bolonia unit. (D) Fe-rich chlorites in feldspathic sandstones from the Bolonia unit. 

DISCUSSION 

Textural observations as well as chemical data 

indicate that diagenetic processes were lithologi- 

cally controlled, being more advanced in coarse- 

grained beds (Ruiz Cruz, 1994). Consequently, the 

analysis of source of sediments is notably 

simplified if only clay mineral assemblages from 

fine-grained sediments are analysed. 

Evolution of detrital clays 

Figure 10 summarizes clay mineral composition 

in the fine-grained Algeciras, Aljibe and Bolonia 

units from late Cretaceous to Aquitanian. It can be 

used to suggest an evolution in the composition 

with time. 

(1) Late Cretaceous to Eocene flysch. Whereas it 

is almost generally accepted from sedimentological 

criteria that Aljibe sediments derive from the erosion 

of poorly identified African substrates (Durand-Delga 

& Olivier, 1988; Hoyez, 1988), the origin of the 

AIgeciras and Bolonia late Cretaceous to Eocene 

flyschs have been related to diverse sources: 

(i) crystalline rocks from the Internal zones, which 

could supply quartz, feldspar, micas and rock 

fragments (Chiochini et al., 1980; Guerrera, 

1981 1982); (ii) calcareous, intrabasinal sources, 

which could supply biogenic muds (Ouldchalha et 

al., 1995); and (iii) calcareous, extrabasinal sources, 

which could correspond to 'calcareous dorsal' 

(Durand-Delga & Olivier, 1988). 

Compositional fields of clay mineralogy 

(Fig. 10C) confirm the contribution of at least two

ALGECIRAS UNIT 

I-S 

I-S 

Aqui!anian- 

/ @ OLIGOCENE 

K I 


sandstone: Triangles 

Claystone: Filled circles 


LATE CRETACEOUS- 

Fla. 8. Clay mineral assemblages in the Algeciras unit. 

(A) Late Cretaceous to Eocene flysch. (B) Oligocene 

t]ysch. (C) Late Oligocene to Aquitanian flysch. 

K: kaolinite, S: smectite, I-S: illite-smectite mixed- 

layers, P: palygorskite, l: illite, Ch: chlorite. 

different sources, characterized by the input of 

either kaolinite + smectite or illite + 1-S mixed 

layers. Basal sequences of the Aljibe and Algeciras 

units offer typical associations whereas the Bolonia 

unit displays lower kaolinite content than the 

Algeciras unit, and a similar lack of smectite. 

Thus, the clay mineralogy of the Bolonia unit 

K-,D 

K 

BOLONIA UNIT 

I-S 

I-S 

I-S 

Aquitanian 

I+Ch 

I-Ch 

OLIGOCENE 

EOCENE- 

I*Ch 

I-Ch 

K I+Ch 

I-Ch 


Sandstone: Triangles 

Claystone and marl: Filled circles 

FIG. 9. Clay mineral assemblages in the Bolonia unit. 

(A) Late Cretaceous to Eocene flysch. (B) Oligocene 

flysch. (C) Aquitanian flysch. K: kaolinite, D: dickite, 

I-S: illite-smectite mixed-layers, I: illite, Ch: chlorite, 

and I-Ch: illite-chlorite mixed-layers.


I-S I-S 

'~ I-S / X Aquitanian 

/ \ ~ / \ 

FS 

s.~-s / ..... ~ ~ \ ~\ s 

/ \A '~ /h I -_A 

/ ,,,~176 \ , 

l< C I+Ch 

FIG. 10. Evolution of clay mineral assemblages in fine-grained sediments. (A) Late Oligoeene to Aquitanian 

flysch. Variations in clay mineralogy appears to be a result of mixing J]tite + chlorite and illite-smectite at the 

ratio 2:] (point) with increasing quantities os kaolinite. (B) O|igocene flysch. (C) Late Cretaceous to Eocene 

flysch. Variations appear to be a result of mixing of the supplies with compositions marked by points, and 

probably with chlorite. (D) Late OlJgocene Mal~guide sediments. (E) Cretaceous to ]Eocene Ma]~guide sediments. 

(F) Almarchal unit. K: kaolinite, S: smeetite, I-S: J||ite-smectite mixed-layers, 1: illite, and Ch: chlorite. Arrow: 

mixing fines.

Clay mineral assemblages in .17ysch 

~ gxr k 

ca uo 

II1 

| 

,3 

n 

-Z 

2q 

-0 

t.-- 

->- 

~. 

~-~. o.~ 

o 

i2


cannot be explained simply as a result of the 

mixing of the Aljibe- and Algeciras-type supplies. 

Although the accurate identification of sources is 

difficult on the basis of clay mineralogy alone, the 

similarity between the basal sequences of the Aljibe 

unit and the Almarchal shales (Fig. 10F) (the latter 

belonging to the Rif domain), confirms a mainly 

southern-related origin for this unit. On the other 

hand, Algeciras clay mineralogy is difficult to 

explain if we take the Internal zones substratum 

as the main source, because Eocene sediments from 

the Internal zones of the Betic range (Ruiz Cruz & 

Serrano, 1991) show clay assemblages characterized 

by a lack of I-S mixed-layers, a large smectite 

content (30 90%), and a kaolinite content of up to 

60% (Fig. 10E). 

Additional evidence for the source of the clays is 

provided by trace element data from selected 

samples of flysch and Internal zone sediments 

(Ruiz Cruz & Serrano, 1991; Ruiz Cruz & Esteras, 

1993). Figure llA reveals a great uniformity and 

indicates a common, stable source for all the flysch 

sequences, which are characterized by low contents 

of metallic cations (Cu, Cr) with respect to the 

mean values of shales and deep-sea sediments 

(Turekian & Wedepohl, 1961). This composition, 

very similar to that exhibited by some smectites 

described by Mosser (1980), suggests an origin 

related to chemical weathering of acid rocks. On 

the contrary, the trace element patterns of Internal 

zones sediments show an anomalous V content 

(mean content = 262 ppm) and an anomalous Cr 

content (mean content = 277 ppm) which confirms 

the presence of ultrabasic rocks in the source. These 

results appear to indicate, together with clay 

mineralogy, that there was little contribution from 

the crystalline substrates of the Internal zones and 

that the calcareous dorsal was influential (Durand 

Delga & Olivier, 1988), although its contribution 

has not yet been evaluated. 

(2) Oligocene flysch. In contrast to late 

Cretaceous Eocene flysch, the Oligocene sediments 

show a remarkable uniformity in clay mineralogy in 

the Aljibe and Algeciras units (Fig. 10 B); this 

despite lithological differences, marked mainly by 

very different carbonate content. Bolonia clays 

show a similar compositional area but illite is 

replaced by the assemblage illite+chlorite (or 

chlorite-like minerals), which probably indicates 

the additional contribution of a chlorite-rich source. 

The general decrease of swelling minerals, the lack 

of smectite and the parallel decrease of kaolinite in 

African-type supplies may be globally related to the 

decrease of hydrolysis on land, due to the world 

cooling from the Eocene-Oligocene boundary 

(Frakes, 1979), and the increased reworking of 

rocky substrates, favoured by lowering of the sea- 

level during the Oligocene (Vail et al., 1987). The 

input of northern materials derived from the rapid 

erosion of Internal zone Palaeozoic substratum is 

now confirmed by clay mineralogy in the Bolonia 

unit. In fact, chlorite and chlorite-type minerals 

(illite-chlorite and chlorite-vermiculite mixed- 

layers) are common in greywackes and schists of 

low metamorphic grade of the Internal zones (Ruiz 

Cruz 1995). 

(3) Late Oligocene to Aquitanian jqysch. It is 

unanimously accepted from petrological and sedi- 

mentological data that Aljibe-type sandstones have 

an African origin whereas Algeciras sediments have 

a northern, Internal zone-derived origin (Penddn, 

1978; Chiochini et al., 1980; Guerrera, 1981 1982, 

Hoyez, 1988)). Clay mineral assemblages in the 

Aljibe and Algeciras units confirm the presence of 

two, well differentiated sources (Fig. 10A). Aljibe 

clays are characterized by the high input of 

kaolinite, whereas Algeciras clays occupy two 

separated areas with very different kaolinite 

content within the compositional diagram, indi- 

cating the presence of sporadic Aljibe-type beds in 

these sequences. Bolonia clays also show two 

different assemblages, one belonging to the 

Aljibe-type beds, and the other intermediate 

between Aljibe-type and Algeciras-type clays, the 

latter including chlorite-rich sediments. The clay 

mineral content of sediments derived from Internal 

zones (Ruiz Cruz & Serrano, 1991) with similar 

age, is characterized by the association illite+ 

chlorite and confirms the mainly northern origin 

of detrital assemblages in the Algeciras and Bolonia 

units. Nevertheless, both the compositional field of 

the Bolonia clays and the high kaolinite content in a 

set of Algeciras samples, suggests that together with 

well-differentiated Aljibe-type and Algeciras-type 

beds, a general mixing of supplies occurred in distal 

areas. 

To test this hypothesis, which presumes the 

presence of important bottom currents in the 

basin, the chemical analysis of 45 samples from 

the three units, with similar compositional range 

(kaolinite content from 0% to 60%) was carried out 

(Ruiz Cruz & Linares, 1992). On a Si/K vs. A1/K 

diagram (Fig. 12), similar to that proposed by de 

Caritat et al. (1994), the regression lines for the

3O 

2O 


~0~ p~O0 9 9 

l 0 nn~o~ 9 

, 

Aljibe unit : Filled circles 

Algeciras+Bolonia units: Open circles r = 0.9 2 

0 4 8 12 16 

F]~. 12. Si/K vs. A1/K variations in the Aljibe and Algeciras-Bolonia samples. The mean value of Internal zone 

sediments is marked by a square. 

Aljibe and Algeciras-Bolonia samples coincide (the 

differences in slope being


Age Lithology Clay mineralogy 

< 

Z 

r..) 

9 

9 

J 

< 

b-.. 

c~ 

[..., 

i I 

[i 0- 

Clay minerals 

Litholog~ 

Kaoltmtc 

|llite 

Smectlte 

PMygorsktte 

Chlor~tc 

I-S nuxcd-!ayers 

Sands[ones 

Calcarenites 

Fe-Mn beds 

Claystones 

FIG. 13. Lithology and clay mineral evolution in three partial sequences of the Aljibe unit. 

diagrams. In late Cretaceous-Eocene claystone 

beds, illite content is relatively low, whereas 

kaolinite and smectite display opposite, random 

variations, which can be related to climatic 

fluctuations and consequently to high but variable 

hydrolysis intensity and drainage in soils (Latouche 

& Maillet, 1982). The decrease of hydrolysis 

processes on land, related to the main cooling 

event from the boundary Eocene-Oligocene (Frakes, 

1979) determines the parallel, rapid decrease of 

both minerals in Oligocene sediments. 

The important change in mineralogy from 

Oligocene to Aquitanian sediments cannot be 

explained on the basis of climatic cooling alone, 

which would lead to a higher illite content, but 

rather to a drastic change in detrital sources, related 

to tectonic activity. The maturity of sandstones 

together with texture and morphology of quartz 

confirm the aridity of climate and point, together 

with clay mineralogy, to physical reworking of 

ancient lateritic profiles (Ruiz Cruz & Linares, 

1992) an&'or erosion of sedimentary series as was

suggested by Chiochini et al. (1980) and Hoyez 

(1988). 

Diagenetic modifications 

Clay mineralogy has been modified to a very 

variable extent during the diagenetic processes, 

reflecting the various controls acting during deposi- 

tion, burial and tectonic events. Diagenetic reactions 

+.a 

< 

i 

Z 

9 

9 

< 


Clay mineralogy 

t I 

V/.,"//, 1 I 

Y////J I 

liiii~iii~!![-.:-.'V///A 

...... r 

l~iiii!~i~-.T,,'/////~ 

I~:-~rj,-_-v///////A 

l- - - ~,,'//7/./'/J~ u 

..... == . . . . . . . . . . . . 

have been important in coarse-grained beds and are 

thus mostly controlled by lithology. The Algeciras 

sandstone beds show wide compositional variety 

(Figs. 8 and 14) and offer a good example for 

analysing the diagenetic evolution. A typical 

Algeciras sequence shows a vertical transition from 

calcareous- to argillaceous- and/or feldspathic 

sandstone, with local intercalations of Aljibe-type, 

quartzose sandstone, which experienced similar 

CLAY MINERALS 

Kaolinite 

illite 

I-S mixed-layers 

[ee~ Chlorite 

LITHOLOGY 

F 

L 

................. 0 

Feldspathic sandstone 

Argillaceous sandstone 

i ~ ~' Calcareous sandstone 

Quartzose sandstone 

Claystone and marl 

50 m 

FIG. 14. Lithology and clay mineralogy of sandstones in the Punta Tarifa section (Algeciras unit), showing the 

strong lithological control on the clay mineralogy.


burial temperatures and pressures. The determined 

diagenetic sequence (Ruiz Cruz, 1994) indicates that 

early dissolution was controlled mainly by detrital 

mineralogy and primary porosity, decreasing in the 

sense: quartzose- ~ feldspathic- -* argillaceous- -~ 

calcareous sandstone, whereas early carbonate 

cementation was mainly developed in sandstone 

samples containing calcareous rock fragments. 

During this stage a limited development of clays 

(kaolinite and I-S mixed-layers) occurred in felds- 

pathic and calcareous sandstones, while Fe- and 

Si-A1 oxides and hydroxides developed widely, in 

argillaceous and quartzose sandstone, respectively. 

During burial, textural and mineralogical changes 

were also controlled by lithology (Ruiz Cruz, 

1994). Compaction and pressure solution are 

common in feldspathic and quartzose sandstones. 

Extensive alteration of detrital feldspar and subse- 

quent formation of illite is only well-developed in 

feldspathic sandstone. The Fe and Mn carbonates 

and chamosite developed in calcareous and felds- 

pathic sandstone respectively, whereas quartzose 

sandstone is characterized by the notable develop- 

ment of kaolinite and/or dickite. The latest 

diagenetic stage, characterized by the development 

of calcite and quartz cements, feldspar and fibrous 

illite, also affected, to a variable extent, the 

different lithologies. 

The diagenetic processes are less evident in fine- 

grained sediments. Early processes are only 

recognizable in Cretaceous-Eocene clays, in 

which ferromanganese carbonates and oxides 

developed extensively, and where some Mg- 

smectite and palygorskite probably formed during 

this stage. Late processes probably occurred during 

postsedimentary overburden and tectonic structura- 

tion (after Burdigalian time), affecting mainly the 

late Cretaceous shales from the Almarchal and 

Facinas units (Ruiz Cruz et al., 1995; Ruiz Cruz & 

Reyes, 1998), where authigenic kaolinite and 

dickite, that originated from smectite dissolution, 

infilled fractures associated with shale beds. 

CONCLUSIONS 

Clay mineral assemblages in fine-grained sediments 

from the Campo de Gibraltar flyschs are mainly 

detrital in origin and reflect the contribution of 

different factors: type and evolution of sources, 

climatic changes, tectonic activity, transport and 

deposition processes, and current activity, whose 

respective influences vary with time. 

The contribution from two sources, characterized 

by the input of either kaolinite + smectite or illite + 

I-S mixed-layers appears as the main factor 

controlling detrital clay assemblages of late 

Cretaceous to Eocene flysch. Climatic changes, 

which determine the introduction of more illite and 

chlorite, appear to control the detrital clay 

mineralogy of Oligocene flysch. From late 

Oligocene time, the influence of at least two well- 

differentiated sources, determines the clay mineral 

association of flysch. Nevertheless, both the 

mineralogy and geochemical data reflect the 

mixing of northern- and southern-derived supplies 

and highlights the importance of bottom currents in 

the flysch basin. The importance of the tectonic 

activity in the Gibraltar area is, on the other hand, 

revealed by the differences, in clay mineralogy, 

between Campo de Gibraltar flysch and Atlantic 

deep-sea sediments, the former also reflecting the 

lack of sorting. These factors determine clay 

assemblages with large kaolinite or illite contents 

that reflect the type of sources better than deep-sea 

sediments, where smectite is the dominant species 

in spite of the differences in sources. 

Diagenetic modifications in fine-grained beds 

appear to be controlled by the smectite content 

and they are restricted to late Cretaceous sediments. 

On the other hand, these processes widely affected 

coarse-grained sediments, and were lithologically 

controlled, indicating the importance of chemical 

microenvironments in specific mineral formation. 

ACKNOWLEDGMENTS 

The study of these sequences has been carried out since 

1985 in collaboration with SECEG (Spanish Society 

for the Study of Fixed Link across the Strait of 

Gibraltar). The author is grateful to Prof. H. Chamley, 

Prof. R. Ferrell and Prof. E. Galen for help and critical 

review of the manuscript. 

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