Mineralogy and grain-size distribution of clay-rich rock units of the Algarve Basin (South Portugal)

Mineralogy and grain-size distribution ofclay-rich rock units of the Algarve Basin

(South Portugal)

M. J . TRINDADE 1 , 3 , * , F . ROCHA2 , 3 , M. I . DIAS1 , 3 AND M. I . PRUDENCIO1 , 3

1 Campus Tecnologico e Nuclear, Instituto Superior Tenico, Universidade Tecnica de Lisboa, EN 10, 2686-953

Sacavem, Portugal, 2 Departamento de Geociencias, Universidade de Aveiro, Campus Universitario de Santiago,

3810-193 Aveiro, Portugal, and3 GeoBioTec - GeoBiociencias, GeoTecnologias e GeoEngenharias, Universidade de Aveiro, Portugal

(Received 20 April 2011; revised 11 October 2012; Editor: John Adams)

ABSTRACT: A detailed survey of the most clay-rich rock units of the Meso-Cenozoic geological

section exposed in the Algarve Basin (South Portugal) was performed and data were analysed for the

grain-size distribution and mineralogy (whole rock and clay fraction), aimed at a compositional study

of the clay-rich sediments and their depositional environment. Granulometry was obtained using wet

sieving and laser diffraction by attenuation of X-rays, and the mineralogical study was carried out by

X-ray diffraction.

Most clay-rich rock units of the Algarve are classified as silty clays and clayey silts, and only a

minority is coarser. The mineralogical study enabled us to define two main types of clays: (1) non-

calcareous clays, consisting largely of quartz and clay minerals, with goethite as the typical Fe-rich

phase (sediments of Carboniferous, Neogene and Quaternary age and Cretaceous siliciclastic clays);

and (2) calcareous clays, which can be calcite-rich clays (Middle and Upper Jurassic) or dolomite-

rich clays (Triassic and Lower Jurassic), the latter typically containing hematite as an accessory

phase. Plagioclase, K-feldspar, and Ti-oxides are often accessory phases, whereas ankerite,

anhydrite, gypsum and opal are rare.

In the clay fraction illite generally predominates, resulting probably from weathering of

preexisting rocks, as well as the less frequent Fe-chlorite, pointing to incipient chemical alteration

under semi-arid climatic conditions. Kaolinite occurs in diverse proportions, being especially

abundant in Cretaceous and Cenozoic units; it is mainly related to chemical weathering in continental

environments under humid conditions. As the Atlantic Ocean opened during Triassic and the

continental environment evolved to a shallow-marine environment with evaporitic conditions,

smectite became more expressive, being sometimes accompanied by other Mg-rich phases (chlorite,

sepiolite, corrensite and palygorskite). Especially during the Cenozoic the proportion of different

phases in the clay mineral association of the sediments reflects the control of tectonic movements

and fluctuations in sea level during their deposition.

KEYWORDS: Algarve Basin, Portugal, granulometry, clays-rich rock units, clay minerals, mineralogy,palaeoenvironments.

The Algarve basin is relatively well characterized in

terms of sedimentology, stratigraphy, palaeontology

and tectonics but specific information on the clay-

rich rock units is sparse; the outdated work of

Manuppella et al. (1985) is still the main source,

and some other information concerning the clay-* E-mail: [email protected]: 10.1180/claymin.2013.048.4.04

ClayMinerals, (2013) 48, 5983

# 2013 The Mineralogical Society

rich rock units is usually found integrated in more

general works about the Algarve geology (Pereira,

1970; Moreira, 1991). In particular, studies on the

clay mineral associations characterizing the diverse

clay-rich stratigraphic units are few and normally

restricted in terms of area, age and/or number of

formations studied (Prates, 1986; Hendricks et al.,

1988; Prudencio et al., 2002; Trindade et al., 2006,

2010; Heimhofer et al., 2008). This type of study is

essential in complementing other investigations, as

the clay mineral associations are controlled by pre-

and post-burial conditions, enabling clarification of

some palaeoenvironmental questions and assisting

in the reconstruction of the evolutionary history of

sedimentary basins (Millot, 1964; Singer, 1984;

Velde, 1985; Rocha, 1993; Rocha & Gomes, 1995;

Thiry, 2000; Ahlberg et al., 2003; Jeans, 2006a,b,c).

Pre-burial controls include source area lithology,

depositional environments, palaeoclimate and topo-

graphy, among others (Singer, 1984; Chamley,

1989), whereas the post-burial or diagenetic

processes can modify the original detrital composi-

tion of clays or even erase the primary depositional

features (e.g. Hower et al., 1976). The clay mineral

assemblages are significantly influenced by the

dominant weathering processes and provide infor-

mation on changes in aridity/humidity patterns.

In this work, our purpose is to provide a detailed

mineralogical and granulometric characterization of

the most clay-rich rock units from the Algarve

Basin, including geological units of diverse age and

location. Ultimately we aim to study the deposi-

tional environment of the Meso-Cenozoic clay-rich

rocks and contribute to the understanding of the

evolutionary history of the Algarve Basin, based on

the palaeoenvironmental information provided by

the interpretation of clay mineral associations.

The characterization of the Algarve clay-rich

rock units is also behind the objectives proposed in

this work, as it may serve as a basis for future

studies in different areas. For example, it has

application in ceramic industry investigations since

some of the units discussed are currently being

extracted as common clays or they may have the

potential to be exploited in the future. On the other

hand, the compositional fingerprints of any

particular clay-rich unit can be used in archaeo-

metric studies to locate the source of raw materials

used in the manufacture of archaeological ceramics,

as already has been done to establish the origin of

clays used in amphorae production from the Manta

Rota kilns (Dias et al., 2009).

GEOLOGICAL SETT ING

The sedimentary Algarve Basin is located in South

Portugal (Fig. 1a). It is formed of two superimposed

Mesozoic and Cenozoic basins (Terrinha, 1998)

developed on a Carboniferous low-grade meta-

morphic basement (Munha, 1990) that consists of

alternating slates and greywackes metamorphosed

during the Variscan orogeny (Oliveira, 1990).

The Mesozoic basin was related to subsidence

along the Algarve margin, which was controlled by

extensional tectonics associated with the breakup of

Pangea and development of the westernmost Neo-

Tethys (Terrinha, 1998). The Cenozoic basin was

first developed in the Late Palaeogene (Manuppella,

1988) or Early Miocene (Cachao, 1995a). The

hiatus separating the two basinal cycles was caused

by the tectonic inversion and uplift of the Mesozoic

rift basin (Terrinha, 1998).

The sedimentary environments evolved from

continental in the Triassic through confined littoral

and evaporitic in Upper Triassic to Lower Jurassic

(up to Sinemurian) times, to open marine in the

Early Pliensbachian (Rocha & Rey in Terrinha et

al., 2006). Triassic sediments consist of red

terrigenous conglomerates, sandstones and shales;

Upper Triassic to Sinemurian sediments consist of

red shales, dolomites and evaporites; and Early

Pliensbachian sediments consist of limestones,

dolomites and marls. A tholeiitic volcanic event at

the Hettangian-Sinemurian transition signals the

rifting phase (Martins & Kerrich, 1998). The

volcano-sedimentary complex is composed of

basaltic lavas and pyroclastic rocks intercalated

with clays, dolomites or limestones.

Jurassic and Cretaceous sedimentation was

essentially marine (limestones and marls predomi-

nate), displaying important facies variations related

to pronounced sea-level fluctuations. Transgression

cycles of the Lower Cretaceous were sometimes

interrupted by intense tectonic movements that

provoked siliciclastic fluvial discharges during the

Berriasian in Central Algarve (Sobral sandstones

unit) and Barremian in Eastern Algarve (shales and

sandstones of the Wealden facies unit) (Rocha &

Rey in Terrinha et al., 2006).

After a period of intense tectonics from Upper

Cretaceous to Early Miocene, the sedimentary

deposition occurred during two transgression

cycles (Middle and Upper Miocene) separated by

a hiatus that represents a generalized uplift of the

Algarve sector (Cachao & Silva, 1992). The first

60 M. J. Trindade et al.

sequence of sediments is carbonate-rich and the

second is siliciclastic (Cachao et al., 1998; Cachao

& Silva in Terrinha et al., 2006).

Jurassic to Miocene calcareous rocks have been

karstified, being the palaeokarsts fossilized by

Pliocene to Pleistocene fluvial/marine detrital

sediments of the Faro-Quarteira Fm. The youngest

sediments are Holocene beach sand and dunes

forming the Ria Formosa island-barrier system and

alluvium terraces and gravels. Holocene deposition

was influenced by frequent climatic oscillations and

sea-level fluctuations (Moura et al., 2007).

MATER IALS AND METHODS

Sampling was carried out on the most clay-rich

rock units of the Algarve. Eighty-four samples from

various units of Triassic to Holocene age were

collected within the basin; in addition, seven

samples of residual clay developed by weathering

FIG. 1. Simplified geological map of the Meso-Cenozoic Algarve Basin (a) and sampling location on the map (b).

Clay-rich rock units of the Algarve Basin 61

of Carboniferous rocks were also sampled, as they

could be the source of detrital clay during the

Triassic and later deposition (Fig. 1b). The sche-

matic stratigraphy of the Algarve region (based on

Manuppella, 1992) is shown in Fig. 2. In this

figure, the sampled units are represented by regular

font.

The granulometric study of samples was done by

wet sieving, using ASTM standard sieves for grain

sizes >63 mm and using laser diffraction byattenuation of X-rays (Micrometrics Sedigraph

5100) for grain sizes

FIG. 2. Schematic stratigraphy for the Algarve region, adapted from Manuppella (1992). Sampled clay-rich rock

units are represented in regular font style.


primary modal particle size, the measure of

distribution spread defined by the difference

between the 90th and 10th percentile values (D90-

D10), and a description of the shape of the particle

size distribution curve. Table 1 presents the

summary of the particle size data.

Laser granulometry of the fine-grained sediments

reveals polymodal distribution in frequency curves

for the majority of samples, which has long been

recognized for most sediments (Ashley, 1978;

Bagnold & Barndorff-Nielsen, 1980), representing

different transport or depositional processes. The

clay-rich rock units of the Algarve exhibit high

dispersion of particle size, being distributed on

almost all Shepards diagram fields (Fig. 3), with

special incidence of clayey silts and silty clays.

Carboniferous clays are generally richer in the

silt fraction, corresponding mainly to clayey silts.

The fine-grained fraction has a distribution spread

around 20 mm, with a median size and main modeof about 3 and 4 mm, respectively.Triassic to Hettangian red clays have a large clay

and silt content, varying mostly from silty clays to

clayey silts; they have two or more modes and

present very low median values and a relatively low

distribution spread on average. Clayey silts

predominate in the volcano-sedimentary complex,

which exhibits a more poorly sorted bimodal/

polymodal distribution and greater primary modal

particle size.

Middle and Upper Jurassic clays have a very low

percentage of sand, corresponding mostly to clayey

silts, silty clays and clays; the finer samples,

classified as clays, came from Telheiro clay-pit

(samples Te). Middle Jurassic sediments present a

more well sorted size distribution and lower median

and mode values than the Oxfordian ones.

The siliciclastic Cretaceous (Berriasian and

Barremian) units exhibit different shapes of the

particle size distribution curve, with poorly sorted

distributions, especially for the Sobral Fm., and

consist mainly of clay and silt, with an additional

sand component, sometimes significant; they are

usually clayey silts, and less frequently sand silt

clays or silty clays. The Aptian calcareous clay is

very rich in clay and poor in sand (silty clay); it

FIG. 3. Grain-size classification of the clay-rich rocks units from the Algarve in Shepards (1954) diagram. Fields

indicate: (1) clay, (2) sandy clay, (3) silty clay, (4) sand silt clay, (5) clayey sand, (6) clayey silt, (7) sand,

(8) silty sand, (9) sandy silt, and (10) silt.


shows a very well sorted bimodal distribution, with

very low values for the median and primary mode.

Miocene and Quaternary samples have a highly

variable grain-size distribution, with a tendency to

be coarser than Mesozoic/Palaeozoic samples.

Usually, clay and sand components predominate

and are therefore often classified as sandy clay,

sand silt clay or clayey sand. A polymodal

distribution curve with a wide distribution spread

is common, but a few samples (Sa1, Fa1 and Pt1)

show a well sorted unimodal (asymmetrical)

distribution.

Mineralogy

Taking into account the main mineral compo-

nents that characterize the clay-rich rock units from

the Algarve, they are notable for the existence of a

large group of non-calcareous clays (Fig. 4); they

show highly variable percentages of phyllosilicates

(clay minerals) and quartz, the latter being most

abundant in samples of Cretaceous and Neogene

age, whereas the clay minerals are generally more

abundant in samples from Triassic and Lower

Jurassic units. About half of the samples studied,

especially those from Triassic and Jurassic units,

are characterized by the presence of carbonates in

varying amounts, sometimes much higher than the

percentages of quartz and clay minerals.

A detailed mineralogical study of samples from

the main clay-rich units of the Algarve is presented

in Table 2. The illite crystallinity index results,

measured whenever was possible, are shown in

Table 3. Table 4 illustrates the average mineralo-

gical composition (bulk rock and clay fraction) of

the various stratigraphic units studied.

The composition of the illite octahedral sheet was

studied by obtaining 5 A/10 A peak intensity ratios

in the diffractograms for clay fraction (Fig. 5). Al-

rich (muscovitic) illite has >0.4 values, Fe- and Mg-

rich (biotitic) illite has

TABLE1.Averageparticlesize

distributiondatafor91samplesfrom

the16clay-richunitsstudied.Standarddeviationis

given

inparenthesis.

Age/units

Sam

ples

Sand

Silt

Clay

0.7

and poor crystallinity to smectites with v/p < 0.1.

Residual clays of Carboniferous units

The secondary clays analysed, which result from

weathering of the Carboniferous substrate (slates

and greywackes of the Mira and Brejeira forma-

tions) of the Algarve Basin, consist mainly of clay

minerals and quartz, the Mira Fm. showing a

greater quartz/clay mineral ratio; K-feldspar,

plagioclase, goethite, anatase and rutile are

common accessory phases; dolomite and opal

were rarely identified.

Clay minerals consist of illite (60100 %), minorkaolinite and Fe-rich chlorite, the latter being

abundant in one sample of the Mira Fm. Traces

of smectite, vermiculite and mixed-layered illite-

smectite are occasionally found in the Mira Fm.

Illites are the 2M1 polytype, with high crystallinity

(IC = 0.30.5), being generally muscovitic (Al-

rich), especially in the Mira Fm., and containing

Mg in the octahedral position rather than Fe (10 A/

5 A < 2; White, 1962). Chlorites are trioctahedral

Fe-rich.

Triassic clay-rich units

Triassic sediments represent the initial filling of

the Algarve Basin. The shale layers of these

siliciclastic units are mainly composed of clay

minerals with minor quartz, carbonates (dolomite

and calcite), feldspars, anatase and hematite.

Abundant dolomite and gypsum were found in

two samples from the Upper Triassic, and in one

sample from the Lower Triassic, respectively. The

clay mineralogy is almost exclusively illite withCARBONIFEROUS:Westphalian

BrejeiraFm.(H

Br)

BSJc1

28

61

21

2

51

79

17

4

BSJc2

26

64

24

2

11

86

10

4

BSJc3

39

48

34

3

tr2

1(O

p)

99

1

RP1

24

62

23

4

32

86

14

tr

CARBONIFEROUS:Nam

urian

-M

iraFm.(H

mi)

Az1

62

26

3

5

4

63

14

23

tr

SCB1

50

35

34

53

88

11

1

Ill-Sme

Cp1

29

52

15

12

tr

1

95

5

tr

Abbreviations:Anh-anhydrite;Ank-ankerite;

Ant-anatase;

Cal

-calcite;

Dol-dolomite;

Gt-goethite;

Gp-gypsum;Hem

-hem

atite;

Kfs-potassium

feldspar;

Op-opal;Phy-phyllosilicates;Pl-plagioclase;Py-pyrite;Qtz

-quartz;Rt

rutile;Chl-chlorite;Chl-Sme-mixed

layeredchlorite-smectite;Chl-Vrm

-mixed

layered

chlorite-verm

iculite;Cor-corrensite;Ill-illite;Ill-Chl-mixed

layeredillite-chlorite;Ill-Sme-mixed

layered

illite-smectite;Ill-Vrm

-mixed

layered

illite-vermiculite;Kln

-kaolinite;

Plg

-palygorskite;

Sme-sm

ectite;Sep

-sepiolite;Vrm

-vermiculite.


traces of kaolinite. Illites are the 2M1 polytype and

have a lower crystallinity (IC = 0.50.7) and are

less aluminous than those from residual clays; the

10 A/5 A peak intensity ratios are between 4 and 7,

suggesting that the octahedral sites are Fe-rich.

Upper TriassicLower Jurassic clay-rich unitsThe Silves red shales unit from Upper Triassic to

Lower Jurassic consists mainly of clay minerals and

lesser amounts of quartz and carbonates (dolomite,

calcite and rarely ankerite); the carbonate content

may vary significantly. Small amounts of hematite,

K-feldspar, plagioclase, anatase and less frequently

anhydrite were also found. The clay fraction is

largely dominated by illite, with minor amounts of

chlorite, smectite and kaolinite. In general, greater

amounts of smectite and chlorite were observed in

the Western Algarve (Trindade, 2007). Sepiolite,

corrensite and irregular mixed-layered chlorite-

smectite and chlorite-vermiculite may occur in

trace amounts. Illites are the 2M1 polytype, have

variable Al2O3/(FeO+MgO) ratio (biotitic to

muscovitic illite) and variable crystallinity, from

high to moderate (IC = 0.30.9) and most of them

are Fe-rich. The position of the (060) reflection

peaks (d060 & 1.53 A) indicates the trioctahedralcharacter of the chlorite. The peak overlap was not

significant, two types of chlorite being identified,

one Fe-rich (samples PTe1, VBoi1-2, SCB5) and

TABLE 3. Illite crystallinity (IC) of the clay-rich units from the Algarve.

Sample IC Sample IC Sample IC

Holocene Pc1 0.25 BSJ3 0.87Ba1 0.42 Pc2 0.26 CM2 0.40Ba2 0.50 Va1 0.43 CM3 0.46Ba3 0.44 Va2 0.26 CM4 0.37Ca1 0.54 Va3 0.31 Crd1 0.80Sa1 0.58 Va4 0.24 Es1 0.31 Pleistocene Va7 0.45 Es2 0.38Fa1 0.56 Va8 0.18 Es3 0.26MT1 0.34 VS1 0.44 Pte1 0.43MT2 0.28 VS2 0.21 RA1 0.58MT3 0.31 VS3 0.25 S1 0.75PF1 0.35 VS4 0.44 SCB5 0.76PoA1 0.46 Oxfordian SCB6 0.49Pt1 0.61 Bt1 0.26 Sv1 0.49Qa1 0.61 Sg1 0.22 Tor1 Miocene Callovian VA1 0.27

Cba1 0.26 RS1 0.33 VBoi1 0.36Cba3 0.34 RS4 0.16 VBoi2 0.43VNC1 0.21 RS5 0.22 VlB1 Aptian Te1 0.27 Upper Triassic PL1 0.31 Te4 0.35 FS4 0.48 Barremian Bathonian FS5 0.51

CL1 0.35 PM1 0.30 FS6 0.53CL2 0.24 Hettangian Lower Triassic Et1 0.33 SC2 SBM1 0.64Et2 0.35 SCB3 WestphalianSL1 0.28 Tor2 BSJc1 0.39SL2 0.36 VB1 0.47 BSJc2 0.42SL3 0.24 Vboi3 BSJc3 0.47 Berriasian VlB2 RP1 0.50

MM1 0.41 Trias.-Hettang. Namurian MM2 0.42 Al3 0.40 Az1 0.50MM3 0.22 Be1 0.45 SCB1 0.28MM4 0.54 BSJ1 0.61 Cp1 0.39MM5 0.27 BSJ2 0.62


the other Mg-rich (e.g. samples BSJ3, S1 and

Cdr1). Mg-rich chlorite occurs in samples where

illite has high IC values. Smectites are dioctahedral,

moderately crystallized (v/p = 0.50.7) and in atleast in two samples (PTe1 and VlB1) was

identified as low-charged due to its incomplete

collapse to 12 A with K-saturation, suggesting an

authigenic origin (Thorez, 1976).

Clay-rich sediments from the volcano-sedimen-

tary complex have a variable composition, espe-

cially in terms of clay minerals and carbonate

proportions. Quartz, feldspars, Ti-oxides and

goethite may occur in small amounts. The clay

fraction commonly consists of illite and smectite in

variable proportions, with minor amounts of kaolin-

ite and chlorite and rarely with palygorskite and

irregular mixed-layered chlorite-smectite; however,

the sample SC2 is an exception, with Fe-rich

chlorite as the major component. The measurement

of illite crystallinity was difficult, due to the large

content of smectite, but well-crystallized illite with

a low Al2O3/(FeO+MgO) ratio (< 0.4) was observed

in sample VB1. Smectites are low charged and have

high crystallinity (v/p > 0.7). The (060) reflection

indicates both dioctahedral (samples Tor2 and

SCB3) and trioctahedral (samples VlB2 and

VBoi3) varieties.

Middle and Upper Jurassic clay-rich units

These marly units consist mainly of calcite and

clay minerals, with minor amounts of quartz and

accessory feldspar and Ti-oxide; rarely gypsum,

dolomite and Fe-oxyhydroxides may occur. Illite

predominates in the clay fraction, being accom-

panied by smectite and kaolinite in Bathonian and

Oxfordian samples, and by kaolinite, chlorite and

smectite in Callovian samples. Traces of vermicu-

lite and mixed-layered illite-vermiculite were

observed in Oxfordian samples. Illites have high

crystallinity (IC = 0.20.4) and are very aluminous

(I(002)/I(001) = 0.4 to 0.6). Smectites are low

charged, dioctahedral and have moderate crystal-

linity (IC = 0.20.7). When it was possible to

observe chlorite diffraction peaks and make

inferences about its composition, we detected Fe-

rich chlorite.

Lower Cretaceous clay-rich units

Clay-rich sediments from the Lower Cretaceous

can be separated into siliciclastic clays and marly

FIG. 5. Illite crystallinity (IC) vs. I(002)/I(001) for illites of the clay-rich rock units from the Algarve: a, biotite;

b, biotite+muscovite; c, phengite; and d, muscovite (Esquevin, 1969).


TABLE 4. Average mineralogy of bulk rock and clay fraction for the various clay-rich geologic units studied from

the Algarve. Abbreviations are as in Table 1.


clays; these different facies indicate that they were

formed under distinct environmental conditions

(Berthou et al., 1983; Prates, 1986). The siliciclastic

units of shales, arenites and conglomerates of the

Wealden facies (Barremian age) and the Sobral Fm.

(Berriasian age) consist predominantly of quartz

and clay minerals. Accessory minerals include

goethite, plagioclase, K-feldspar, anatase, rutile,

calcite and pyrite, the last two were rarely found

in the Sobral Fm. The clay fraction is dominated by

illite and kaolinite (1060 %) with subordinateamounts of chlorite and vermiculite in samples of

Berriasian age, and traces of vermiculite, smectite,

chlorite, and illite-rich mixed-layered phases in

samples of Barremian age. Illites have high

crystallinity (IC = 0.20.5) and variable composi-

tion, but the majority are aluminous (muscovite-

type).

The only marly clay (sample PL1) analysed

consists mainly of clay minerals and lesser amounts

of quartz, calcite, feldspars, anatase and goethite.

The clay fraction has abundant illite and minor

amounts of kaolinite.

Neogene clay-rich units

Miocene clays from the Cacela Fm. are

composed mainly of quartz and clay minerals,

with minor feldspar (especially plagioclase) and

anatase. The clay fraction consists of kaolinite, illite

and smectite in similar proportions. Illites are the

2M1 polytype, muscovitic and with high crystal-

linity (IC = 0.20.5). Smectites are moderately tohighly crystalline (v/p = 0.60.8), dioctahedral and

low charged.

Pleistocene and Holocene clays have similar

mineralogical compositions, consisting mostly of

quartz and clay minerals, with accessory goethite,

feldspars and Ti-oxides. The clay fraction is

dominated by illite and kaolinite in varying

proportions; smectite occurs frequently in minor

amounts, and traces of irregular mixed-layered

TABLE 4. (contd.).


chlorite-vermiculite and illite-vermiculite were

found in Pleistocene samples. The illites are the

2M1 polytype, with IC varying from 0.3 to 0.6 and

tend to be less aluminous than Miocene illite.

Smectite crystallinity was difficult to measure due

to its low abundance, but it appears to have low

crystallinity and to be dioctahedral and low

charged.

D I SCUSS ION

Mineralogical results, particularly the clay mineral

associations, are discussed in the context of the

evolutionary history of the Algarve basin, taking

into account that the changing average amounts of

detrital and neoformed clay minerals reflect the

control of synsedimentary tectonic movements, as

well as of transgressions and regressions, on

deposition. Given the lack of information for a

continuous sedimentary record in this work, the

palaeogeographical and palaeoenvironmental inter-

pretations of the mineralogical record made here are

not exhaustive and are based on previous works

(e.g. Prates, 1986; Hendricks et al., 1988;

Heimhofer et al., 2008), contributing to reinforce

some of the conclusions drawn and give new

insights into the evolution of Algarve Basin.

Residual clays of Carboniferous units

The high crystallinity of illites from residual clays

of Westphalian and Namurian age (Table 3; Fig. 5)

suggests they suffered advanced diagenesis or even a

very low grade of metamorphism (Kisch, 1991). The

presence of illite and Fe-chlorite in these clays have

been interpreted as chemically unaltered detrital

minerals derived from weathering of Carboniferous

strata, and their association as being inherited from

parent rocks (Hendricks et al., 1988). The presence

of small amounts of kaolinite in the residual clays

points to later hydrolysis in a well drained

continental environment, under a warm and humid

climate. The association illite-chlorite-kaolinite

indicates a strong influence of emerged lands

(Lopez Aguayo & Caballero, 1973; Singer, 1984).

Tectonic movements during Permian times

promoted the gradual uplift of Carboniferous

rocks (Marques, 1983) followed by distensive

tectonics during Triassic to Cretaceous times

associated with Pangea fragmentation (Terrinha,

1998). Deposition of the Mesozoic sediments in a

tectonically active setting favoured detrital clay

mineral assemblages (Chamley, 1989) that can

mainly be explained by the source rock mineralogy

and palaeoflow pattern (Net et al., 2002).

Palaeozoic rocks were the most important source

of clay minerals in the Mesozoic receiving basin.

Regarding the main palaeoflow to the south

(Oliveira, 1990), the sediment path to the Algarve

basin was related to fluvial systems that received

sediments from the drainage basement rocks, which

should have mostly supplied illite; indeed, illite

abounds in Mesozoic sediments. The good crystal-

linity of illites, similar to those of the source rocks,

sustains the hypothesis of significant source rock

control over the clay mineral assemblages of the

basin sediments. The compositional variability of

illite between the Mira and Brejeira formations

suggests some heterogeneity of the source area.

Triassic clay-rich units

Ferric illite is frequently the sole component of

the clay assemblage of Triassic fine-grained

siliciclastic units from the Algarve, as is often

found in Permian to Triassic sediments of Western

Europe (Jeans et al., 1994), and is most probably

related to the climatic conditions. The Pangea

climatic regime has been described as megamon-

soonal, characterized by a pronounced long dry

season with temporally concentrated rain seasons

(Parrish, 1993). The combination of high erosion

rate of the rejuvenated Palaeozoic rocks with an

arid climate promoted advanced physical alteration,

enabling the preservation of detrital minerals and

leading to the formation of the typical red beds

(Millot, 1964; Daoudi & Deconinck, 1994). This

siliciclastic unit widespread all over the world has

variable granulometry and consists mainly of illite

associated with hematite.

The low IC values obtained for Triassic samples

and their 2M1 polytype, suggest that the illites are

detrital. Illites were most probably formed due to

in tense wea the r ing of the source a rea

(Carboniferous rocks) under semi-arid seasonal

conditions, associated with long fluvial transport

in repeated erosion-sedimentation cycles, with total

destruction of other clay minerals (e.g. chlorite)

beyond the resistant illite in an aqueous environ-

ment (Lippmann & Berthold, 1992; Alonso-

Azcarate et al., 1997). However, Jeans et al.

(1994), basing their findings on chemical, miner-

alogical and radioisotope (K/Ar) data of Permo-

Triassic illite clay assemblages of Western Europe,


refute the detrital origin of illite as being derived

from pre-existing rocks, suggesting that illite is

pedogenic, originally formed in coeval desert soils

that were later eroded and deposited as fine grained

detritus in adjacent areas. Indeed, the authorsrecords indicate that pedogenic clay mica was

formed in large quantities during the Permo-

Triassic when the climate was arid over much of

Europe. For the Algarve Triassic illites we cannot

draw conclusions on its pedogenic origin with the

insufficient data we have, in particular with the lack

of K/Ar age data.

Upper Triassic-Lower Jurassic clay-rich units

From Upper Triassic to Lower Jurassic times, the

early breakup of Pangea was accompanied by

increased onshore humidity as seaways opened

into the Pangean interior with the installation of

epicontinental seas (Ahlberg et al., 2002). The

Silves shales, dolomites and evaporites unit were

deposited in this transitional environment. It

consists mainly of illite and minor chlorite and

smectite, the first two being most likely detrital.

The attribution of a continental origin to smectite

by pedogenesis of detrital illite or chlorite is

improbable due to the rarity of mixed-layer illite-

smectite or chlorite-smectite, which may reflect the

degradation of the inherited minerals (Hendriks et

al., 1988). Indeed, the low-charge smectite found in

this unit points to an authigenic origin (Thorez,

1976) and is regarded as symptomatic of marine

environments (Gibbs, 1977; Thiry, 2000). As the

evaporitic conditions prevailed during deposition of

the pelitic sequence, smectite was probably

neoformed due to a strong influx of ion (Mg, Ca,

Si, Na and K)-rich solutions from continental areas

(Chamley, 1989, Meunier, 2005).

Beyond well crystallized Fe-chlorite of probable

detrital origin, poorly crystallized Mg-chlorite was

also identified in a few samples that also show

poorly crystallized illite with moderate IC values

(Fig. 5), which have been attributed to its degrada-

tion rather than being an original feature (Hendriks

et al., 1988). This suggests some sort of transforma-

tion of illite into Mg-chlorite, as the most degraded

illites may fix Mg and evolve to chlorite, which is

relatively common in shallow-marine hypersaline

environments (Alonso-Azcarate et al., 1997).

However, the low amount of Mg-chlorite in the

samples studied may be a consequence of the high

dolomite content, which fixes the Mg from

inhibiting the formation of chlorite.

Other Mg-rich minerals (sepiolite and corrensite)

occur rarely in the Silves red shale unit. Their

occurrence has been commonly reported in

evaporitic environments for the early Mesozoic

sediments from Western Europe (Chamley &

Debrabant, 1984; Castano et al., 1987), but not

that much in Portugal (Rocha, 1993). These Mg-

rich phases may form under special chemical

conditions induced by high evaporation rates,

depending on pH and Mg, Si and Al activities in

solution (Hillier, 1995; Birsoy, 2002). The modes of

occurrence and genesis of sepiolite could be diverse

(Dias, 1998; Birsoy, 2002), but most frequently take

place by direct precipitation from Mg enriched

solutions (Velde, 1985; Jones & Galan, 1988;

Meunier, 2005) under the 8.59.5 range of pH(Galan & Castillo, 1984). Corrensite is usually an

intermediate product in the formation process of

Mg-chlorite by reaction of Mg-carbonate with

dioctahedral phyllosilicates (Barrenechea et al.,

2000; Meunier, 2005).

The red shale unit characteristics, consisting of

both detrital (illite and Fe-chlorite) and neoformed

(smectite, Mg-chlorite, sepiolite and corrensite) clay

minerals, in addition to the presence of dolomite

and anhydrite, point to their formation in an

environment with continental and marine control.

This mineral assemblage found in Algarve red

shales is in accordance with those formed in

epicontinental regions of Western Europe, domi-

nated by arid climates with enhanced seawater

evaporation (Rocha, 1993; Weaver, 1989). The

coexistence of detrital and neoformed minerals in

evaporitic lagoons, showing zonation where Mg

minerals occupy a central position, have been

frequently reported (Krumm, 1969; Millot, 1964;

Lopez-Aguayo & Caballero, 1973; Meunier, 2005).

The South Iberian continental rifting and

subsequent overturn of the basin to sea is

represented by a Hettangian volcanic event that

was mixed with sediments, forming a volcano-

sedimentary complex (VSC) in which the type and

proportion of clay minerals depend on the nature of

the sediments, but illite and smectite generally

predominate.

The most probable origin for the smectite is

diagenitic alteration of ash-derived volcanic glass,

through a mechanism involving devitrification of

ashes, hydration and subsequent crystallization of

smectite (Ortega-Huertas et al., 1995; Jeans et al.,

2000; Shoval, 2004; Meunier, 2005). Huge amounts


of water and large contact surfaces with magma are

necessary to transform glass into smectite (Meunier,

2005), such conditions being consistent with the

phreato-magmatic system existing in the Algarve

during the volcanic episode that produced powerful

eruptions with abundant ashes mixed with high-

temperature water vapour (Martins et al. in Terrinha

et al., 2006).

Trioctahedral Mg-smectite and palygorskite,

frequently reported in evaporitic lagoons (Weaver,

1989), were also identified in the VSC. The

palygorskite sample was collected in the same

area of the Central Algarve where the sepiolite

sample was found, suggesting the existence of

suitable Mg-rich conditions (Jones & Galan, 1988;

Hillier, 1995) in the past to generate Mg-rich

fibrous phases in the Tor region. A three-step

transformation of Al-Fe-rich dioctahedral smectite,

through Mg-rich smectite into palygorskite, is

commonly proposed for their generation (Velde,

1985; Dias, 1998; Meunier, 2005).

In Portugal, fibrous clay minerals like palygors-

kite and sepiolite are mainly associated with

Tertiary sediments from the main river basins

(Dias, 1998; Dias & Rocha, 2001); their presence

in clay-rich sediments of Triassic to Hettangian age,

associated with evaporitic conditions, was first

pointed out by Rocha (1993) when studying the

clay mineral associations of sediments from the

Aveiro basin (North Portugal). In this work, we

show that although rare, fibrous clays can be found

associated with the Algarve red shales and VSC

units of Upper Triassic and Hettangian age.

Middle and Upper Jurassic clay-rich units

After the Sinemurian the sediments were

deposited on a carbonate platform due to a

generalized transgression. In the clay fraction of

these calcite-rich clays the illite predominates,

containing minor amounts of smectite, kaolinite

and chlorite. The presence of kaolinite suggests

influence of continental lands, while smectite points

to a marine control. As sediments were deposited

on a carbonate platform, a higher smectite/kaolinite

proportion would be expected; the abundance of

kaolinite observed in the samples studied is

probably related to the lowering of sea level due

to the Upper Callovian to Middle Oxfordian

compressive tectonic event, thus explaining a

higher continental fingerprint in the sediments

(Hendriks et al., 1988). Illites of Jurassic clay-rich

units have generally high crystallinity, which is

probably due to a pressure and temperature increase

during the tectonically active period referred to. A

tectonic control on changes of illite crystallinity,

independent of lithology and depth of burial, is

often suggested (Fernandez-Caliani & Galan, 1992;

Roberts et al., 1991; Alonso-Azcarate et al., 1997).

Lower Cretaceous clay-rich units

The majority of Lower Cretaceous sedimentation

took place in carbonate-rich environments, varying

from lagoonal to marine, along three transgressive-

regressive cycles that were twice interrupted (late

Berriasian and Barremian) due to compressive

tectonics. As a consequence, gaps on the strati-

graphic record arise in the Western Algarve and

siliciclastic fluvial discharges due to destabilized

terranes occurred in the Central Algarve (Sobral

Fm.) and Eastern Algarve (shales, arenites and

conglomerates of Wealden facies).

The clay fraction of siliciclastic clay-rich units,

consisting mainly of illite and kaolinite, suggests a

strong continental influence. Illite is most likely a

detrital mineral, indicating active mechanical

erosion of the source area and limited soil

formation, whereas kaolinite is usually a residual

mineral formed due to leaching of most cations

from pre-existing rocks under humid, tropical to

subtropical conditions (Chamley, 1989). The

association illite-kaolinite-goethite has been inter-

preted following Millot (1964) as a siderolithic

facies related to erosion and transport of laterites in

tropical climates. Indeed, during the major regres-

sion of the Lower Cretaceous, vast areas were

exposed to intense chemical weathering under

humid conditions, resulting in the formation of

abundant kaolinite all over Western Europe

(Molina-Ballestreros et al., 1997; Blanc-Valleron

& Thiry, 1997). However, the predominance of

illite in the samples studied suggests the hydrolysis

was not extreme. The presence of vermiculite in a

few samples points to pedogenetic degradation of

chlorite under continental influence (Hendriks et

al., 1988) and the occurrence of mixed-layer illite-

smectite and illite-vermiculite indicate some degra-

dation of illite.

The Late Aptian marl sample of the Luz Fm. has

higher illite/kaolinite proportions than the silici-

clastic facies, suggesting more intense mechanical

weathering processes in the source area than the

chemical processes. This is probably related to


lower precipitation rates and a semi-arid to arid

setting in combination with tectonically enhanced

erosion of Palaeozoic rocks (Heimhofer et al.,

2008), due to progressive acceleration of the

Algarve Basin subsidence by the expansion of the

North Atlantic during the Middle Aptian to Albian

period (Rey, 1986).

Neogene clay-rich units

During the Early Cenozoic, the first Alpine

tectonic movements together with the drying of

the climate promoted physical weathering of

kaolinite palaeosols, causing the onset of the most

important detrital discharge in Western Europe,

which was not synchronous everywhere. The input

of abundant kaolinite to sedimentary basins

occurred several million years after the formation

of thick kaolinitic profiles on the continent, and at

the time of its reworking the climate was no longer

warm and wet, but had well-defined dry seasons

(Thiry, 2000; Simon-Coincon et al., 1997).

The main source area of sediments to the Algarve

Cenozoic basin was the siderolithic succession

(kaolinite-rich) developed during the Cretaceous,

which explains the kaolinite enrichment in the

Miocene sediments from the Cacela formation. This

unit was deposited at low energy in a confined

shallow marine environment during the beginning of

a sea level decrease after an extensive transgression

(Cachao, 1995b; Cachao et al., 1998). The smectite

found in a high proportion of the samples studied

from this unit seems to be of marine origin (Hendriks

et al., 1988). Hence, the clay mineral association of

Miocene clays can be explained by both marine

control, represented by smectite neoformation, and

continental control, with illite and kaolinite carried

out by the rivers. The same illite-kaolinite-smectite

association found in the Betic Cordilleras (SE Spain)

sediments was attributed to karstification and

pedogenesis in the source areas, suggesting marine

deposition punctually affected by local emerging

areas (Palomo, 1987; Vera et al., 1989).

The Pleistocene (sands and gravel from the Faro-

Quarteira unit) and Holocene clay-rich sediments

(gravel and terrace units) consist mainly of detrital

minerals (illite and kaolinite), indicating that

sedimentation was essentially continental.

Glaciations and the consequent the sea level

fluctuations played a major role during this period

and variations in the clay mineralogy may be linked

to short-term palaeoclimatic changes (Thiry &

Jacquin, 1993; Gibson et al., 2000) and subsequent

different degrees of chemical weathering of the

source area.

CONCLUDING REMARKS

Clay-rich rock units of the Algarve have a wide

range of grain-size distributions, being predomi-

nantly classified as silty clays and clayey silts. In

general, the Cretaceous siliciclastic sediments and

the Cenozoic units are richer in the sand fraction.

The detailed mineralogical study presented here

enabled us to distinguish two main types of clay-

rich rocks units for the Algarve region: (1) non-

calcareous clays, consisting mainly of quartz and

clay minerals, with goethite as the typical Fe-rich

phase; in general it corresponds to clay-rich

sediments of Carboniferous, Neogene and

Quaternary age and to Cretaceous (Berriasian and

Barremian age) siliciclastic clays; and (2) calcar-

eous clays, which in agreement with the type of

carbonate can be separated into: (a) calcite-rich

clays, which are characterized by high percentage

of calcite and include the marly clays from the

Middle and Upper Jurassic; and (b) dolomite-rich

clays, which are characterized by variable propor-

tions of clay minerals and quartz, with the

ubiquitous presence of hematite and may contain

minor calcite; they correspond to the clay-rich

sediments of Triassic and Lower Jurassic

(Hettangian) age. Plagioclase, K-feldspar and Ti-

oxide (most frequently anatase) were observed as

accessory phases in almost every sample analysed.

Illite is the most abundant clay mineral and is

most probably detrital, formed by the weathering of

preexisting rocks, as well as Fe-chlorite that occurs

in small amounts in some units, suggesting incipient

chemical alteration in the adjacent continental areas

due to the prevailing arid to semi-arid climatic

conditions. Kaolinite is generally a subordinate

phase in the Mesozoic clay-rich units, except in

the Cretaceous siliciclastic sediments where it is

more abundant. In Cenozoic sediments, kaolinite is

as abundant as illite, which seems to be a

consequence of the mechanical erosion of kaolinitic

profiles formed during the Cretaceous under

tropical to sub-tropical conditions. The presence

of Mg-rich minerals (Mg-chlorite, Mg-smectite,

corrensite, sepiolite and palygorskite) in Triassic

and Lower Jurassic units signalizes the rifting phase

where a shallow marine environment with strong

evaporitic conditions dominated. The marine


influence on clay mineralogy due to subsidence and

development of the basin was mainly indicated by

the presence of neoformed smectite. However, in

the volcano-sedimentary complex, smectite is most

likely to be associated with hydrothermal alteration

of volcanic ashes.

Changes in the proportion of detrital and

neoformed minerals in the clay-rich rock units

reflect the control of synsedimentary tectonic

movements and fluctuations in sea level during

their deposition. Source area composition, palaeo-

climate and different degrees of weathering also

contributed to differences in the clay mineralogy.

The study of clay mineral associations, in addition

to bulk mineralogy performed in this work,

contributes to a better understanding of the regional

geology and evolutionary history of the Algarve

Basin, in association with previous sedimentolo-

gical, palaeontological and structural studies.

ACKNOWLEDGMENTS

Financial support for this work was provided by the

Foundation for Science and Technology as a PhD grant

(SFRH/BD/11020/2002) to M.J. Trindade, which is

gratefully acknowledged. The authors would like to

thank the reviewers for comments, suggestions and

corrections that improved the manuscript.

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Mineralogy and grain-size distribution of clay-rich rock units of the Algarve Basin (South Portugal)

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