Mineralogy and grain-size distribution of clay-rich rock units of the Algarve Basin (South Portugal)
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Transcript of Mineralogy and grain-size distribution of clay-rich rock units of the Algarve Basin (South Portugal)
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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
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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
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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
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FIG. 2. Schematic stratigraphy for the Algarve region, adapted from Manuppella (1992). Sampled clay-rich rock
units are represented in regular font style.
Clay-rich rock units of the Algarve Basin 63
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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.
64 M. J. Trindade et al.
-
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
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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.
Clay-rich rock units of the Algarve Basin 71
-
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
72 M. J. Trindade et al.
-
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).
Clay-rich rock units of the Algarve Basin 73
-
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.
74 M. J. Trindade et al.
-
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.).
Clay-rich rock units of the Algarve Basin 75
-
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,
76 M. J. Trindade et al.
-
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
Clay-rich rock units of the Algarve Basin 77
-
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
78 M. J. Trindade et al.
-
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
Clay-rich rock units of the Algarve Basin 79
-
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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