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Sedimentological control on the clay mineral distribution in the marine and non-marine Palaeogene deposits of Mallorca (Western Mediterranean)
SEDIMENTARY GEOLOGY ELSEVIER
Sedimentary
Geology
94 (1995) 229-243
Sedimentological control on the clay mineral distribution in the marine and non-marine Palaeogene deposits of Mallorca (Western Mediterranean) M. Ingks
a, E. Ramos-Guerrero
b
aDep. Geoquimica, Petrologia i Prospeccio’ Geol&ica, Universitat de Barcelona, Zona Unicersitciria de Pedralbes, 08071 -Barcelona, Spain b Dep. Geologia Dincimica, Geofisica i Paleontologia. Universitat de Barcelona, Zona Uniuersitriria de Pedralbes, 08071-Barcelona, Spain Received
14 February
1994; revised version
accepted
18 July 1994
Abstract During the Middle Eocene-Oligocene a marine and non-marine succession, about 1000 m thick, was deposited on Mallorca. Palaeoenvironmental interpretation of these deposits was obtained from sedimentological and palaeontological data in earlier studies. The non-marine environments recorded are: alluvial, fluvial (channel and flood plain deposits) and lacustrine (prevailing terrigenous, organic-rich or carbonate sedimentation). Marine environments are represented by littoral and shelf deposits. In most of these palaeoenvironments, terrigenous sedimentation prevailed except in the marine shelf, where mixed sedimentation was dominant, and in the carbonate and organic-rich lakes. In general, Palaeogene freshwater lakes were perennial, open and exorheic, with carbonate waters. The Palaeogene deposits were derived from Jurassic and Early Cretaceous pelagic and hemipelagic marls. The clay mineral assemblage of these source-area rocks was mostly smectite, interstratified illite-smectite and illite. The weathering of these materials under high temperature and rainfall favoured the hydrolysis of clay minerals and the formation of kaolinite. X-ray diffraction, TEM and EDX analysis were carried out on 108 samples of palaeogene mudstones. Clay minerals identified were illite, interstratified illite-smectite, smectite, chlorite and kaolinite. The various Palaeogene environments (marine, transitional and non-marine) can be distinguished on the basis of different clay mineral assemblages. These assemblages are related to weathering and diagenetic processes as well as to the distance from the source area. Alluvial sediments contain smectite and kaolinite with subordinate amounts of illite. All these clay minerals are of detrital origin. Smectite and kaolinite are present in all environments but the proportion of kaolinite to smectite increases with the distance from the source. The K-rich interstitial waters derived from illite hydrolysis in the source area favoured the formation of mixed-layer illite-smectite in the alluvial flood plain where the stagnation of waters during dry periods allowed the concentration of dissolved ions. Fluvial deposits contain interstratified illite-smectite, smectite, kaohnite, illite and chlorite in variable amounts. Variations in the proportion of illite and the occasional occurrence of chlorite was attributed to compositional differences in the source area and variations in the sediment supply input. The terrigenous and carbonate lakes 0037-0738/95/$09.50 0 1995 Elsevier SSDI 0037-0738(94)00089-l
Science
B.V. All rights reserved
230
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Geology 94 (199.5) 229-243
preserved the detrital clay mineral assemblage carried by the rivers. Organic-rich smectite with less illite. Littoral deposits contain kaolinite and illite with minor amounts of smectite Shelf deposits are mostly formed by illite and kaolinite.
1. Introduction
Climate has a strong influence on rock alteration and clay minerals are often the main product of this process. Clay minerals deposited in diverse sedimentary environments may be a detrital association which reflects the composition of the source area or may be directly related to the climatic parameters which controlled the weathering of rocks. Moreover, post-depositional changes are not infrequent and the occurrence of authigenic clays provides information about the sedimentary environments. The relationship between clay minerals and environmental conditions has drawn attention to clay minerals as palaeoenvironmental and palaeoclimatic indicators (Keller, 1970). Reviews of the current state of knowledge can be found in Charnley (1989) and Weaver (1989). Nevertheless, the use of clay minerals as palaeoclimatic indicators is limited because factors other than climate, such as lithology or topography, strongly influence the rates and products of weathering processes. Transport and post-depositional changes can also modify the original clay assemblage. For these reasons the limitations of clays as palaeoclimatic indicators have been reviewed by many authors (e.g. Singer, 1984; Charnley, 1989; Curtis, 1990). We have selected a study area where, during the Palaeogene, the depositional environments were mainly detrital, with very active transport processes although in some areas carbonate and mixed sedimentation occurred. The purpose of this paper is to describe the mineralogy and the clay mineral assemblages in the mudrocks and to establish a relationship between the clay mineral assemblages, the weathering processes and the depositional environments.
2. Geological
lakes contain
mainly kaolinite
and interstratified
and
smectite-illite.
setting
Mallorca is the largest island of the Balearic Archipelago, located in the Western Mediterranean over the Balearic Promontory, which constitutes the northeastern prolongation of the alpine Betic Range (Fig. 1A). The island of Mallorca is made up by Mesozoic and Tertiary rocks folded and thrusted during the Alpine compressive orogeny, which probably started in the Oligocene, but reached its maximum intensity during the Early-Middle Miocene (Burdigalian-Langhian). The total shortening was greater than 50% and transport direction was toward the northwest (RamosGuerrero et al., 1989b). This SE-NW shortening obviously affected the Palaeogene palaeogeographic evolution, producing abundant lateral faties change. After the Langhian the tectonism changed, and a distensive phase took place forming horsts and grabens. The horsts are formed by structured pre- and synorogenic rocks and constitute the major reliefs of the island (Sierras de Tramuntana and Llevant) as well as other minor reliefs in the Central Zone. The grabens formed basins that were filled by post-Langhian sediments, and constitute the lower and flat Central Zone of the island (Fig. 1B). 2.1. The Palaeogene of Mallorca On Mallorca, Palaeogene rocks crop out in small and tectonically disconnected areas (Fig. 1B). Two depositional sequences bounded by unconformities were recognized in the Palaeogene of Mallorca by Ramos-Guerrero et al. (1989b) (Fig. 2). The lower sequence (D.S.1) is Middle Eocene in age (Late Lutetian-Bartonian), whereas the upper sequence (D.S.11) is Late Eocene-
M. Ingl&, E. Ramos-Guerrero
/Sedimentary
Oligocene in age. Each depositional sequence is composed of a set of stratigraphic units.
Geology 94 (1995) 229-243
231
deposits, whereas toward the southeast it is formed by littoral and marine shelf sediments. Both the northwestern lacustrine and the southeastern marine domains remained separated by a threshold where no sedimentation occurred during the Middle Eocene. The northwestern lacustrine deposits constitute the Peguera Limestone Formation (Fig. 21, and have been interpreted as palustrine to shallow lacustrine sediments. The lacustrine basins were extensive, with permanent water (Ramos-
2.2. The Lower Depositional Sequence (D.S.Zj This sequence lies unconformably over a Mesozoic basement, mainly composed of Early Cretaceous rocks, and results from the sedimentation on a stable shelf with two clearly differentiated palaeogeographical domains: toward the northwest the sequence is made up of lacustrine
06 MALLORCA
.. ..
:. .. ..
.. ..
. .. ..
. .. .. . .
. .. .. . .
.. . . . . . . . .
. . . . . . . . .. .. .. . .
. . . .
. . .. .. .. .. .. .. .. .. .
Structured Major
rocks
t’aleogene
Postorogenlc
n
Studled
(Mesozoic
to
OUtCroPs
rocks
sections
Fig. 1. (A) Location of the Balearic location of the outcrops studied.
islands
in the Western
Mediterranean
Sea. (B) Geological
sketch
of Mallorca
island
and
M. In&s,
232
E. Ramos-Guerrero
/Sedimentary
Guerrero et al., 1989a). Terrigenous sedimentation was limited to the supply system’s input points. Locally, the lacustrine sequence includes a lower part with mineable coal and coaly facies. The lacustrine carbonate deposits as well as the associated detrital sediments of the Peguera Limestone Formation are logged and sampled (logs l-4 and 6 in Fig. 3). The water of the Eocene lakes was characterized by high carbonate and low sulphate contents. The lack of evaporites suggests the existence of an open lacustrine system where, however, persistent anoxic conditions may have occurred locally (Ramos-Guerrero et al., 1989a).
Geology 94 (1995) 229-243
The marine sediments on southeastern Mallorca constitute the S’Envestida Calcarenites Formation (Fig. 2). This unit is formed by littoral and mixed shelf sediments, in which organic bioclastic production was substantial. The main lithologies are bioclastic calcarenites as the littoral facie% and silts and marls as open shelf marine facies. The silty facies often contain large monospecific populations of non-reworked Nummulites, which have been interpreted as Nummulites banks. The marine facies of the S’Envestida Calcarenites are presented in log 11 (Fig. 3). Remains of turtles and crocodiles which indicate warm climatic conditions have been identiSE
NW
07
--. . . . ..a._.
_
_
,-.
-.
_m
Littoral
Sands
Terngenous
Shelf
Marls
Coaly facles
scheme
of the Palaeogene
\ of Mallorca
\ Gaklent Fm
Marine.
Carbon&e
Unconformity
studied.
_
Gravels
Fig. 2. Stratigraphic sections
_
Lacustrine - oalustnne:
Fluvio - alluvial.
-
_
I/
&&
Reefal facies
Tectonic boundary (modified
from Ramos-Guerrero
et al., 1989b) and location
of the
M. In&s, E. Ramos-Guerrero /Sedimentary
2 33
Geology 94 (1995) 229-243
gressive cycle characterized by an alluvial system prograding toward the southeast. The lower transgressive cycle is made up of littoral and marine shelf sediments, PriabonianEarly Rupelian in age. These sediments lie unconformably on and onlap toward the northwest the Mesozoic or Palaeogene (D.S.1) basement. Toward the northwest, along the Sierra de Tramuntana, the littoral sediments of the Alar6 Calcarenites Formation (Fig. 2) are the most abundant. The main lithologies of this unit are bioclastic calcarenites, conglomerates, marls and
fied in the lacustrine facies (Hugueney and Adrover, 1982; Jimenez-Fuentes et al., 19891990). The fossil association and the lacustrine hydrochemical characteristics allow us to attribute warm and humid climatic conditions to the Eocene period. 2.3. The Upper Depositional Sequence (D.S.II) In the Upper Sequence (D.S.11) two cycles can be distinguished: a marine and transgressive lower cycle, overlain by an upper continental and re-
7
NW
Cala
4 . . . ... .. . .. . . .. . .. . B. . . . . . .
Blanca
Fm
9
6
..i . L. . . . . .
. . . .
Fig. 3. Simplified
stratigraphic
logs of the studied
outcrops
SE
. .
10 Ir
II 1
??
Y
1 .Y
I
Fm
Alar6 \ \\
and samples
position.
Location
in Figs. 1B and 2
234
M. I&%, E. Ramos-Guerrero /Sedimentary
lutites. Toward the south and southeast the Alar6 Formation laterally changes to marine shelf sediments with reef-limestones, bioclastic calcarenites, lutites and marls. These open marine sediments form the Son Sastre Lutites Formation, the Galdent Limestone Formation and the Calvari Marls Formation (Fig. 2). The littoral facies have been studied and sampled in logs 2-6 and 8 (Fig. 3), whereas the marine shelf facies have been studied in log 8 (Fig. 3). The upper regressive cycle is formed by an alluvial system, Oligocene in age and represented by the Cala Blanca Detrital Formation (Fig. 2). This unit reflects a rapid lateral facies variation, from the proximal conglomeratic alluvial deposits to the northwest toward the distal deposits, located to the southeast, which are composed of lutitic flood plain deposits and lacustrine sediments with mineable coal beds (Ramos-Guerrero and Marzo, 1989). The proximal alluvial facies are gravels and other detrital deposits made up of non-confined sheets of debris flows (Ramos-Guerrero and Marzo, 1989). These facies are mainly located in the southern part of the Sierra de Tramuntana and other localities of the Central Zone, where log number 7 was studied and sampled (Fig. 3). Laterally, the proximal alluvial-fan deposits change to fluviatile deposits whose main lithologies are well sorted conglomerates, sandstones and lutites; among the fluviatile deposits channel filling and lacustrine sequences have been identified interbedded between flood plain lutitic sediments and palaeosoil horizons. The fluviatile faties of the Cala Blanca Formation are studied in logs 3 to 7 (Fig. 3). Clast petrology and palaeocurrent analysis prove the existence of a source area, constituted by Middle Jurassic-Early Cretaceous carbonate rocks (Colom, 1975) and located toward the northwest of the island, in the present-day Catalano-Balear Basin (Fig. 1A). Toward the east and southeast, in a distal position, fluvial plain and lacustrine facies are dominant. Ramos-Guerrero et al. (1989a) characterized two main lacustrine facies assemblages in this area: detrital lacustrine and organic-rich lacustrine facies.
Geology 94 (199.5) 229-243
The main Oligocene lacustrine basins were also perennial, with fresh and carbonate-rich water, but their area1 extension was smaller than in the Eocene (D.S.1). Both lacustrine facies, detrital and organic-rich, have been studied and sampled in logs 9 and 10 (Fig. 3), in the Central Zone of Mallorca. Palynological and palaeobotanical assemblages (Alvarez-Ramis and Ramos-Guerrero, 1986; Alvarez-Ramis et al., 1987; Ramos-Guerrero and Alvarez-Ramis, 1989-1990) and crocodilia and turtle (Hugueney and Adrover, 1982, 1989-1990) remains from the lacustrine deposits of the Cala Blanca Formation suggest tropical to sub-tropical (warm and humid) climatic conditions during the Oligocene. Dispersed organic matter is present in all continental Palaeogene sediments, but is associated with the lacustrine facies (Eocene and Oligocene) where it is most abundant, and locally thick accumulations of lignites and coaly facies are present. Organic geochemistry allows us to attribute the kerogen to types II and III, pointing to a substantial macrophytic contribution. Nevertheless minor algal contributions can also be expected (RamosGuerrero et al., 1989a). These data support the hypothesis of the existence of a stable water column in the lacustrine basins.
3. Analytical
procedure
A total of 108 samples were collected representing alluvial, fluvial, lacustrine (carbonate, terrigenous and organic-rich), littoral and marine shelf environments. Sampling took into account the diversity of sedimentary environments in the Palaeogene of Mallorca; the location of samples is show.n in Fig. 3. For comparative purposes eleven samples, of similar lithology to that inferred for the source area during the Palaeogene, were obtained from Jurassic and Early Cretaceous units. A portion of each sample was dispersed in distilled water and washed until deflocculation occurred. The < 2 pm fraction was separated by centrifugation of the suspension. Oriented mounts of the < 2 pm fraction were prepared for X-ray
M. Ingl&, E. Ramos-Guerrero /Sedimentary
diffraction by gravity settling on glass slides. Clay mineralogy was determined from diffraction patterns obtained using samples that were air dried, ethylene glycol solvated and heated to 550°C during 2 hours. X-ray diffraction analyses were carried out on a Siemens D500 X-ray diffractometer using a CuKa radiation. Ten representative samples of the diverse environments and the source area were further studied by transmission electron microscope equipped with a microanalyzer system in order to determine the qualitative composition of smectites and interstratified minerals and also to identify the non-clay minerals in the < 2 pm fraction. Observation and analysis were performed in a Hitachi H-800 MT with H-8010 STEM and SEM system and X-ray microanalyzer.
4. Results 4.1. Mineralogical fraction
composition
of the
< 2 pm
The main constituents of this fraction are the following: Zllite is recorded in 89% of the samples studied. TEM observations revealed that illite occurs as irregularly shaped grains with micaceous aspect (Fig. 4a). Illite and kaolinite are usually the coarsest grains in the < 2 pm fraction. Smectite and mixed-layer illite-smectite: 67% of the samples studied contain one or other mineral. In fact there is a wide range of composition, from pure smectite to diverse degrees of mixed layering illite-smectite. TEM observations reveal that smectites and mixed-layer illite-smectite have a smaller grain size than illites and kaolinites. The energy dispersive X-ray spectra show that the smectites are of the aluminium type and always contain iron (Figs. 4c and 4d). Interstratified clay minerals are recorded in approximately 25% of the samples. In most of them a low and broad diffraction peak, at 4.5-7” 28, changes to lower angles after solvation with ethylene glycol. This peak indicates the presence of a mixed-layer expandable clay mineral. In a few samples several reflections are present and
Geology 94 (1995) 229-243
235
permit the identification of the mixed-layer minerals as illite-smectite type with proportions of illite layers ranging from 40 to 60% calculated according to the method proposed by Moore and Reynolds (1989). When only one diffraction peak appears the shape, broadness and expansion are variable. This has been interpreted as diverse degrees of interstratification. EDX analysis reveal the presence of grains with ratios of aluminium to silicon lower than the illite but with some potassium content (Figs. 4e and 4f). The 7-A minerals chlorite and kaolinite were distinguished using a slow-scan record between 24 and 25.50”28 in order to distinguish the chlorite (3.54 A) from the kaolinite (3.57 A) peaks (Biscaye, 1965). Nevertheless in a few samples with small amounts of 7-A clay minerals the discrimination of the two peaks has not been possible, in this case we use the term 7-A minerals only when it is impossible to know for sure whether we have kaolinite or chorite, or both minerals together. TEM observations show that kaolinite frequently occurs as hexagonal-shaped crystals of variable grain size (Fig. 4g). Chlorite occurs only in a few Oligocene samples. Non-clay minerals: in the < 2 pm fraction, apart from clays, a wide variety of other detrital minerals are present. Among them, quartz, calcite, goethite and K-feldspar are abundant and can be positively identified on the X-ray diffraction patterns. Small amounts of apatite and titanium oxides were identified during TEM and EDX analyses (Figs. 4a, 4c and 4e). 4.2. Clay mineral distribution The clay minerals identified in the Jurassic samples of the source area are illite, interstratified illite-smectite and 7-A minerals. Cretaceous samples contain predominantly smectite with variable amounts of illite and occasional minor amounts of mixed-layer illite-smectite and 7-A minerals. The smectites are of the aluminium-rich type. Representative X-ray diffraction patterns of the source area are presented in Fig. 5. The clay minerals identified in the Palaeogene samples are: illite, kaolinite, smectite, interstratified illite-smectite and chlorite (Table 1; Fig. 6).
236
M. hgh,
E. Ramos-Guerrero
/Sedimentary
b
h
Geology 94 (1995) 229-243
M. Ingl&, E. Ramos-Guerrero
/Sedimentary
Geology 94 (1995) 229-243
237
Fig. 5. X-ray diffraction patterns of the < 2 Frn fraction, ethylene glycol solvated samples. Representative sample from the Jurassic source area (A) and representative sample from the Cretaceous source area (B). S = smectite; I-S = interstratified illite-smectite; I = illite; K = kaolinite. Unlabelled peaks correspond to quartz and calcite.
Illite is present in almost all the samples. Nevertheless the abundance of illite varies depending on the sedimentary environment. All the environments contain expandable clay minerals, either smectite or mixed-layer illitesmectite. Interstratified minerals are recorded in fluviatile, detrital and carbonate lacustrine, and littoral facies. Smectite is present in all the sedi-
mentary environments and it is abundant in the alluvial, organic-rich lacustrine and shelf facies, where mixed-layer clay minerals are absent. Kaolinite is recorded in most of the samples and is very abundant in alluvial, carbonate and organic-rich lacustrine, littoral and marine shelf sediments. Chlorite is only present in a few samples of
Fig. 4. Transmission electron micrographs of different representative clay samples (left column). Energy dispersive X-ray spectra of minerals marked with an arrow are depicted in the right column. Vertical scale bar in EDX spectra is equivalent to 1000 counts. Unlabelled peaks correspond to carbon and copper of the support grid and coatings. (a) Illite and goethite (electrodense aggregate) from a shelf sample. (b) EDX spectrum of the illite. (c) Smectite from an alluvial sample. The prismatic, electrodense grain is a titanium oxide. (d) EDX spectrum of the smectite. (e) Interstratified illite-smectite from a fluvial sample. The electrodense mineral is a quartz grain. (f) EDX analysis of the interstratified illite-smectite. (g) Kaolinite from an organic-rich lacustrine sediment. (h) EDX spectrum of the kaolinite.
M. I&&,
238
E. Ramos-Guerrero /Sedimentary
Geology 94 (I 995) 229-243
\ I-S,
K
0
6
S,I-S, I,K
0
s,
5
10
15
al
25
I,
A K
20
Fig. 6. Representative X-ray diffraction patterns of the < 2 Frn fraction, ethylene glycol solvated Palaeogene samples of: (A) organic-rich lacustrine mudstone; (B) alluvial mudstone; (C) fluvial mudstone; and (D) shelf mudstone. S = smectite; I-S = interstratified illite-smectite; I = illite; K = kaolinite. Unlabelled peaks correspond to quartz and calcite.
M. Inglk, E. Ramos-Guerrero /Sedimentary Table 1 Percentage
of samples
in which the various
clay minerals
are present
239
Geology 94 (1995) 229-243
in the Palaeogene
sedimentary
environments
studied
Alluvial
Fluviatile
Detrital lacustrine
Carbonated lacustrine
Organic lacustrine
Littoral
Shelf
I S I-S
57 100
87 42 45
83 39 22
100 28 43
100 100
83 13 20
100 17 _
K Ch
100
45 29
61 16
100
100
56 16
100 100
7A
_
38
28
_
_
43
fluvial, detrital lacustrine and littoral origin of the D.S.11 (Late Eocene-Oligocene). Although the clay minerals recorded (except
Carbonate Organtc
lacustnne
fmes
rich lacustrme
Marme faces
chlorite) are ubiquitous, some sedimentary environments are characterized by a definite assemblage distinguishable by the constant presence
faCleS
( httoralto shelf)
Fig. 7. Idealized palaeoenvironmental minerals indicates decreasing relative
scheme of the Palaeogene of Mallorca amounts. In brackets are clay minerals
with clay mineral distribution. The order which are of secondary importance.
of the clay
240
M. In&s> E. Ramos-Guerrero
/Sedimentary
and relative proportions of some clay minerals. The distribution of these clay mineral assemblages in the depositional environments (Fig. 7) can be summarized as follows. Allucial. Smectite and kaolinite are consistently present in all the samples. Illite is recorded in 57% of samples and is relatively scarce. Flucial and detrital lacustrine. All the clay minerals recorded are present in these environments although the relative proportions are very different (Fig. 60. The presence of chlorite is noticeable in some Oligocene samples. Carbonate lacustrine. All the samples contain illite and kaolinite in varying proportions. Nevertheless in the lacustrine deposits of the Peguera Limestone (Middle Eocene) illite prevails over kaolinite and in the carbonate lacustrine deposits of the Cala Blanca Formation (Oligocene) kaolinite is more abundant than illite. Most of the samples contain subordinate amounts of smectite or interstratified illite-smectite. Organic-rich lacustrine. These deposits show an homogeneous mineralogical composition. Illite, smectite and kaolinite are recorded in all samples. Smectite and kaolinite are much more abundant than illite. Littoral. These environments are characterized by the presence of illite and kaolinite. Occasionally smectite, mixed-layer illite-smectite, chlorite and 7-A minerals occur in variable amounts. When smectite or mixed-layer illite-smectite do occur, they can be very abundant. Shel$ These samples are characterized by high kaolinite and low illite contents. Smectite is found in some samples and is in some places quite abundant.
5. Discussion In the source area, the clay minerals present are illite, smectite, interstratified illite-smectite and 7-A clay minerals; the first two minerals are the most abundant. In the Palaeogene sediments detrital clay minerals are dominant. This fact is clearly reflected by the high content of smectite in the proximal area. Titanium oxides and apatite are present in
Geology 94 (1995) 229-243
both the source area and the Palaeogene rocks. These heavy minerals, widespread over all the Palaeogene environments, clearly indicate the importance of detrital processes. Kaolinite is the most abundant clay mineral in the Palaeogene samples. Smectite and mixed-layer illite-smectite come next in terms of abundance. Illite is very widespread, but is always present in a lesser proportion and with a lower crystallinity than the source area samples. The absence of dolomite and magnesium in the clay minerals are noticeable. The lack of magnesium may be related to the lithological composition of the source area, consisting exclusively of limestones and marls. The alluvial facies contain kaolinite and smectite with minor amounts of illite. Smectite and illite are of detrital origin. Chemical weathering of pre-existing clay minerals, probably illite, and to a lesser extent smectite, led to the formation of kaolinite. Goethite and potassium ions are other products of the weathering processes. The presence of both kaolinite and goethite (and occasionally hematite) indicates substantial chemical weathering. EDX analyses (Fig. 4) show that illite and smectite are iron rich; this favoured the formation of goethite. The weathering of illite led to a potassium enrichment of the interstitial waters. The drainage of the area carried the K-rich solutions to the lower zones (Fig. 7) where the formation of interstratified illite-smectite was favoured by the entrance of potassium in the smectite interlayers (Singer and Stoffers, 1980). In the fluviatile and lacustrine environments (except the organic-rich lacustrine) the detrital clay minerals were preserved. These environments are characterized by a complex clay mineral assemblage composed of detrital kaolinite, smectite, illite and in some places chlorite, as well as interstratified illite-smectite formed in the flood plain. The detrital clay mineral preservation is consistent with the characteristics of Palaeogene lacustrine systems: freshwater, exorheic, perennial and calcium bicarbonate dominated; these conditions do not favour clay mineral transformations (Jones and Bowser, 1978; Jones, 1986). The interstratified illite-smectite formation mechanism has been described by Eberl et al.
M. Ingl& E. Ramos-Guerrero
/Sedimentary
(1986). These authors suggested that wetting and drying cycles favour the fixation of potassium in smectites and thus the formation of illitic layers. The mudstone deposits of the fluvial flood plain have decolourations, carbonate nodules and root-marks that indicate palaeosoil development under hydromorphic conditions. In these conditions interstitial water stagnation and ion concentration during the dry season are favoured. According to Mohr and Van Baren (19541, in the same climatic conditions, rapid interstitial water circulation favoured the formation of kaolinite, whereas slow circulation with temporary stagnation made possible the reactions between clay minerals and the dissolved cations. In the organic-rich lacustrine environment the clay mineral association is composed of kaolinite and smectite, both very abundant; illite is also present, but in a lesser proportion. In contrast with other lacustrine environments (carbonate and detrital) neither interstratified illite-smectite nor chlorite are reported. The smectite has good crystallinity, similar to that of the smectite from the Cretaceous rocks of the source area and always better than that of the other Palaeogene sediments. So we suspect that in the organic-rich lacustrine environments some smectite-regeneration mechanism was active in spite of the generally held view (e.g. Berner, 1971; Styman and Bustin, 1984) that these environments favour smectite alteration and kaolinite formation. However, an abundance of both kaolinite and smectite in the organic-rich lacustrine facies was also cited by Ingles and Anadon (1991) in the Palaeogene sediments of the Ebro basin. In the distal environments (littoral and shelf) the proportions of smectite and mixed-layer illite-smectite decrease, whereas kaolinite and minor amounts of illite are the most representative clay mineral assemblage. The kaolinite increase can be attributed to the weathering of the smectite in the proximal area. For all the Palaeogene sedimentary environments of Mallorca it should be emphasized that chlorite is only present in the Oligocene samples. Illite is present in both Eocene and Oligocene samples, but the alluvial and lacustrine Eocene samples record the greatest abundance of illite.
Geology 94 (1995) 229-243
241
The fluctuations in the detrital clay mineral proportions must be attributed to the source area composition variations as well as to the change in palaeocurrents. However, we cannot rule out the possibility of the existence, during the Oligocene, of slightly colder periods, during which weathering was less important, and which allowed the preservation of chlorite and illite. The Eocene lacustrine system (Peguera Limestone) developed in a stable plate, where the lacustrine basins occupied wide areas. On the other hand, the Oligocene lacustrine system (Cala Blanca Formation) developed in an area of deformation processes related to the Betic orogen. This context had a strong control over the genesis and later development of the sedimentary basins and sediment supply, mainly terrigenous. The formation of kaolinite and goethite suggests that weathering processes were developed under high temperature and rainfall conditions. On the other hand the formation of interstratified illite-smectite can be related to a seasonal rainfall contrast. This interpretation agrees with palaeoecological data previously mentioned, which reflect the existence in this area of similar palaeoclimatic conditions. In addition, Charnley et al. (1979) deduce the existence of similar palaeoclimatic conditions during the Middle Eocene-Oligocene in the Atlantic edge of the Iberian plate (DSDP site 3981, in a latitudinal position similar to that of Mallorca, and Charnley (1989) proposed for the same area the existence of a seasonal rainfall contrast. 6. Conclusions The clay minerals in the Palaeogene mudstones of Mallorca are mainly detrital and result from weathering in the source area as well as post-depositional weathering in the proximal environments. However, the final clay mineral assemblages are found to be due as much to the weathering processes as to the sedimentary and diagenetic conditions prevailing in the sedimentary environment. The distribution of the clay minerals in the diverse sedimentary environments is shown in Fig. 7.
242
M. Ingl& E. Ramos-Guerrero
/Sedimentary
The alluvial facies are composed of smectite and kaolinite with minor illite. Smectite and illite are detrital, whereas kaolinite has been formed by the chemical weathering of preexisting clay minerals, mainly illite. Hydrolysis of these facies as well as the source area, with abundant smectite, mixed-layer illite-smectite and illite, under high temperature and rainfall, led to the formation of kaolinite and goethite while water was enriched in soluble cations as residual products. In the fluviatile and detrital lacustrine facies all the recorded clay minerals are present: kaolinite, mixed-layer illite-smectite, illite, smectite and, in the Oligocene samples, chlorite. All clay minerals are detrital in origin except the mixedlayer illite-smectite, which was formed in the fluvial flood-plain, where the imprint of hydromorphic processes has been recognized and the stagnation of interstitial water led to periodic increase of ion concentration. In these conditions an increase of potassium concentration occurred and favoured the formation of illitic layers in smectites by the fixation of potassium, leading in turn to the formation of illite-smectite mixedlayers. Potassium was released during the hydrolysis of illite. The carbonate lacustrine facies are characterized by illite and kaolinite, with subordinate amounts of smectite and mixed-layer illite-smectite. All these minerals are of detrital origin. The organic-rich lacustrine facies are marked by the presence of smectite and kaolinite, with minor amounts of illite. Kaolinite and illite are detrital. The lack of interstratified minerals and the high crystallinity of smectites suggest that some sort of regeneration process of these minerals may be related to the hydrological characteristics of these environments. The marine facies (littoral and shelf) are characterized by the presence of detrital clay minerals, mainly illite and kaolinite, with minor amounts of smectite and mixed-layer illite-smectite. In the Oligocene littoral samples, traces of chlorite are also present. No diagenetic processes have been observed in the detrital and carbonate lacustrine and marine environments. The relative abundance of smectite diminishes
Geology 94 (1995) 229-243
with distance-the organic-rich lacustrine deposits are an exception-whereas kaolinite distribution shows the opposite tendency, increasing in proportion with distance. In our opinion this clay mineral distribution results mainly from weathering processes more than transport and sorting (in distal areas smectite is locally abundant). On the other hand, the transportation processes have been very active as is shown by the sedimentological facies association.
Acknowledgements Financial support was provided by CICYT projects GE0 89-0426 and PB 91-0805. X-ray diffraction analysis were carried out in the Institut de Ciencies de la Terra “Jaume Almera” (CSIC). TEM observations and analysis were carried out in the Serveis Cientifico-Tecnics, Universitat de Barcelona. The manuscript has been improved by critical comments and suggestions from P. At-radon, L. Cabrera and reviewers H. Friis and B.F. Jones.
References Alvarez-Ramis, C. and Ramos-Guerrero, E., 1986. Estudio paleobotlnico de1 Paleogeno de Peguera: extremo occidental de la Serra de Tramuntana (Mallorca). Bol. Sot. Hist. Nat. Balears, 30: 83-93. Alvarez-Ramis, C., Ramos-Guerrero, E. and Fernandez-Marron, T., 1987. Estudio paleobotanico de1 Cenozoico de la zona central de Mallorca: Yacimiento de Son Ferragut. Bol. Geol. Min., 98: 349-356. Berner, R.A., 1971. Principles of Chemical Sedimentology. McGraw-Hill, New York, N.Y., 240 pp. Biscaye, P.E., 1965. Mineralogy and sedimentation of recent deep-sea clay in the Atlantic ocean and adjacent seas and oceans. Geol. Sot. Am. Bull., 76: 803-832. Charnley, H., 1989. Clay Sedimentology. Springer-Verlag, Berlin, 623 pp. Charnley, H., Debrabant, P., Foulon, J., Giroud d’Argout, G., Latouche, C., Maillet, N., Maillot, H. and Sommer, F., 1979. Mineralogy and geochemistry of Cretaceous and Cenozoic Atlantic sediments off the Iberian Peninsula (Site 398, DSDP leg 47B). Init. Rep. DSDP, 47B: 429-449. Colom, G., 1975. Geologia de Mallorca. Instituto de Estudios Balearicos, Palma de Mallorca, 2 Vols., 519 pp. Curtis, C.D., 1990. Aspects of climatic influence on the clay mineralogy and geochemistry of soils, paleosols and elastic sedimentary rocks. J. Geol. Sot. London, 147: 351-357.
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E. Ramos-Guerrero
/Sedimentary
Eberl, D.D., Srodon, J. and Northrop, H.R., 1986. Potassium fiiation in smectite by wetting and drying. In: J.A. Davies and K.F. Hayes (Editors), Geochemical Processes at Mineral Surfaces. Am. Chem. Sot. Symp. Ser., 323: 296-326. Hugueney, M. and Adrover, R., 1982. Le peuplement des Baleares (Espagne) au Paleogbne. Geobios, M&m. Spec., 6: 439-449. Hugueney, M. and Adrover, R., 1989-1990. Rongeurs (Rodentia, Mammalia) de I’Oligocene de Sineu (Baleares, Espagne). Paleontol. Evol., 23: 157-169. Ingles, M. and Anadon, P., 1991. Relationship of clay minerals to depositional environment in the non-marine Eocene Pontils Groups, SE. Ebro basin (Spain). J. Sediment. Petrol., 61: 926-939. Jimenez-Fuentes, E., Ramos-Guerrero, E., Martin de Jesus, S., Perez-Ramos, E. and Mulas-Alonso, E., 1989-1990. Quelonios de1 Eocene medio de Mallorca. Paleontol. Evol., 23: 153-1.56. Jones, B.F., 1986. Clay mineral diagenesis in lacustrine sediments. In: F.A. Mumpton (Editor), Studies in Diagenesis. U.S. Geol. Surv. Bull., 1578: 291-300. Jones, B.F. and Bowser, C.J., 1978. The mineralogy and related chemistry of lake sediments. In: A. Lerman (Editor), Lakes: Chemistry, Geology, Physics. Springer-Verlag, Berlin, pp. 179-235. Keller, W.D., 1970. Environmental aspects of clay minerals. J. Sediment. Petrol., 40: 788-814. Mohr, E.C.J. and Van Baren, F.A., 1954. Tropical Soils. Interscience, New York, N.Y., 498 pp. Moore, D.M. and Reynolds R.C., 1989. X-Ray Diffraction
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and the Identification and Analysis of Clay Minerals. Oxford Univ. Press, Oxford, 332 pp. Ramos-Guerrero, E. and Alvarez-Ramis, C., 1989-1990. Paleoflora de1 Oligocene de Mallorca. Boll. Sot. Hist. Nat. Balears, 33: 141-158. Ramos-Guerrero, E. and Marzo, M., 1989. Sedimentologia de un sistema fluvio-aluvial en el Oligocene de las Baleares: la Formacidn Detritica de Cala Blanca. In: S. Robles, V. Pujalte and P.A. Fernindez-Mendiola (Editors), Comunicaciones de1 XII Congreso Espaiiol de Sedimentologia, Grupo Espafiol de Sedimentologia, Bilbao, pp. 47-50. Ramos-Guerrero, E., Cabrera, L. and Marzo, M., 1989a. Sistemas lacustres paleogenos de Mallorca (Mediterraneo occidental). Acta Geol. Hisp., 24: 185-203. Ramos-Guerrero, E., Rodriguez-Perea, A., Sabat, F. and Serra-Kiel, J., 1989b. Cenozoic tectosedimentary evolution of Mallorca island. Geodin. Acta, 3: 53-72. Singer, A., 1984. The paleoclimatic interpretation of clay minerals in sediments-a review. Earth Sci. Rev., 21: 251-293. Singer, A. and Stoffers, P., 1980. Clay mineral diagenesis in two east african lake sediments. Clay Miner., 15: 291-307. Styman, W.B. and Bustin, R.M., 1984. Sedimentology of Frasser delta peat deposits: a modern analogue for some detritic coals. In: R.A. Rahmani and R.M. Flores (Editors), Sedimentology of Coal and Coal-bearing Sequences. I.A.S., Spec. Publ., 7: 241-271. Weaver, C.E., 1989. Clays, Muds and Shales. Developments in Sedimentology 44, Elsevier, Amsterdam, 819 pp.
Sedimentary
Geology
94 (1995) 229-243
Sedimentological control on the clay mineral distribution in the marine and non-marine Palaeogene deposits of Mallorca (Western Mediterranean) M. Ingks
a, E. Ramos-Guerrero
b
aDep. Geoquimica, Petrologia i Prospeccio’ Geol&ica, Universitat de Barcelona, Zona Unicersitciria de Pedralbes, 08071 -Barcelona, Spain b Dep. Geologia Dincimica, Geofisica i Paleontologia. Universitat de Barcelona, Zona Uniuersitriria de Pedralbes, 08071-Barcelona, Spain Received
14 February
1994; revised version
accepted
18 July 1994
Abstract During the Middle Eocene-Oligocene a marine and non-marine succession, about 1000 m thick, was deposited on Mallorca. Palaeoenvironmental interpretation of these deposits was obtained from sedimentological and palaeontological data in earlier studies. The non-marine environments recorded are: alluvial, fluvial (channel and flood plain deposits) and lacustrine (prevailing terrigenous, organic-rich or carbonate sedimentation). Marine environments are represented by littoral and shelf deposits. In most of these palaeoenvironments, terrigenous sedimentation prevailed except in the marine shelf, where mixed sedimentation was dominant, and in the carbonate and organic-rich lakes. In general, Palaeogene freshwater lakes were perennial, open and exorheic, with carbonate waters. The Palaeogene deposits were derived from Jurassic and Early Cretaceous pelagic and hemipelagic marls. The clay mineral assemblage of these source-area rocks was mostly smectite, interstratified illite-smectite and illite. The weathering of these materials under high temperature and rainfall favoured the hydrolysis of clay minerals and the formation of kaolinite. X-ray diffraction, TEM and EDX analysis were carried out on 108 samples of palaeogene mudstones. Clay minerals identified were illite, interstratified illite-smectite, smectite, chlorite and kaolinite. The various Palaeogene environments (marine, transitional and non-marine) can be distinguished on the basis of different clay mineral assemblages. These assemblages are related to weathering and diagenetic processes as well as to the distance from the source area. Alluvial sediments contain smectite and kaolinite with subordinate amounts of illite. All these clay minerals are of detrital origin. Smectite and kaolinite are present in all environments but the proportion of kaolinite to smectite increases with the distance from the source. The K-rich interstitial waters derived from illite hydrolysis in the source area favoured the formation of mixed-layer illite-smectite in the alluvial flood plain where the stagnation of waters during dry periods allowed the concentration of dissolved ions. Fluvial deposits contain interstratified illite-smectite, smectite, kaohnite, illite and chlorite in variable amounts. Variations in the proportion of illite and the occasional occurrence of chlorite was attributed to compositional differences in the source area and variations in the sediment supply input. The terrigenous and carbonate lakes 0037-0738/95/$09.50 0 1995 Elsevier SSDI 0037-0738(94)00089-l
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B.V. All rights reserved
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Geology 94 (199.5) 229-243
preserved the detrital clay mineral assemblage carried by the rivers. Organic-rich smectite with less illite. Littoral deposits contain kaolinite and illite with minor amounts of smectite Shelf deposits are mostly formed by illite and kaolinite.
1. Introduction
Climate has a strong influence on rock alteration and clay minerals are often the main product of this process. Clay minerals deposited in diverse sedimentary environments may be a detrital association which reflects the composition of the source area or may be directly related to the climatic parameters which controlled the weathering of rocks. Moreover, post-depositional changes are not infrequent and the occurrence of authigenic clays provides information about the sedimentary environments. The relationship between clay minerals and environmental conditions has drawn attention to clay minerals as palaeoenvironmental and palaeoclimatic indicators (Keller, 1970). Reviews of the current state of knowledge can be found in Charnley (1989) and Weaver (1989). Nevertheless, the use of clay minerals as palaeoclimatic indicators is limited because factors other than climate, such as lithology or topography, strongly influence the rates and products of weathering processes. Transport and post-depositional changes can also modify the original clay assemblage. For these reasons the limitations of clays as palaeoclimatic indicators have been reviewed by many authors (e.g. Singer, 1984; Charnley, 1989; Curtis, 1990). We have selected a study area where, during the Palaeogene, the depositional environments were mainly detrital, with very active transport processes although in some areas carbonate and mixed sedimentation occurred. The purpose of this paper is to describe the mineralogy and the clay mineral assemblages in the mudrocks and to establish a relationship between the clay mineral assemblages, the weathering processes and the depositional environments.
2. Geological
lakes contain
mainly kaolinite
and interstratified
and
smectite-illite.
setting
Mallorca is the largest island of the Balearic Archipelago, located in the Western Mediterranean over the Balearic Promontory, which constitutes the northeastern prolongation of the alpine Betic Range (Fig. 1A). The island of Mallorca is made up by Mesozoic and Tertiary rocks folded and thrusted during the Alpine compressive orogeny, which probably started in the Oligocene, but reached its maximum intensity during the Early-Middle Miocene (Burdigalian-Langhian). The total shortening was greater than 50% and transport direction was toward the northwest (RamosGuerrero et al., 1989b). This SE-NW shortening obviously affected the Palaeogene palaeogeographic evolution, producing abundant lateral faties change. After the Langhian the tectonism changed, and a distensive phase took place forming horsts and grabens. The horsts are formed by structured pre- and synorogenic rocks and constitute the major reliefs of the island (Sierras de Tramuntana and Llevant) as well as other minor reliefs in the Central Zone. The grabens formed basins that were filled by post-Langhian sediments, and constitute the lower and flat Central Zone of the island (Fig. 1B). 2.1. The Palaeogene of Mallorca On Mallorca, Palaeogene rocks crop out in small and tectonically disconnected areas (Fig. 1B). Two depositional sequences bounded by unconformities were recognized in the Palaeogene of Mallorca by Ramos-Guerrero et al. (1989b) (Fig. 2). The lower sequence (D.S.1) is Middle Eocene in age (Late Lutetian-Bartonian), whereas the upper sequence (D.S.11) is Late Eocene-
M. Ingl&, E. Ramos-Guerrero
/Sedimentary
Oligocene in age. Each depositional sequence is composed of a set of stratigraphic units.
Geology 94 (1995) 229-243
231
deposits, whereas toward the southeast it is formed by littoral and marine shelf sediments. Both the northwestern lacustrine and the southeastern marine domains remained separated by a threshold where no sedimentation occurred during the Middle Eocene. The northwestern lacustrine deposits constitute the Peguera Limestone Formation (Fig. 21, and have been interpreted as palustrine to shallow lacustrine sediments. The lacustrine basins were extensive, with permanent water (Ramos-
2.2. The Lower Depositional Sequence (D.S.Zj This sequence lies unconformably over a Mesozoic basement, mainly composed of Early Cretaceous rocks, and results from the sedimentation on a stable shelf with two clearly differentiated palaeogeographical domains: toward the northwest the sequence is made up of lacustrine
06 MALLORCA
.. ..
:. .. ..
.. ..
. .. ..
. .. .. . .
. .. .. . .
.. . . . . . . . .
. . . . . . . . .. .. .. . .
. . . .
. . .. .. .. .. .. .. .. .. .
Structured Major
rocks
t’aleogene
Postorogenlc
n
Studled
(Mesozoic
to
OUtCroPs
rocks
sections
Fig. 1. (A) Location of the Balearic location of the outcrops studied.
islands
in the Western
Mediterranean
Sea. (B) Geological
sketch
of Mallorca
island
and
M. In&s,
232
E. Ramos-Guerrero
/Sedimentary
Guerrero et al., 1989a). Terrigenous sedimentation was limited to the supply system’s input points. Locally, the lacustrine sequence includes a lower part with mineable coal and coaly facies. The lacustrine carbonate deposits as well as the associated detrital sediments of the Peguera Limestone Formation are logged and sampled (logs l-4 and 6 in Fig. 3). The water of the Eocene lakes was characterized by high carbonate and low sulphate contents. The lack of evaporites suggests the existence of an open lacustrine system where, however, persistent anoxic conditions may have occurred locally (Ramos-Guerrero et al., 1989a).
Geology 94 (1995) 229-243
The marine sediments on southeastern Mallorca constitute the S’Envestida Calcarenites Formation (Fig. 2). This unit is formed by littoral and mixed shelf sediments, in which organic bioclastic production was substantial. The main lithologies are bioclastic calcarenites as the littoral facie% and silts and marls as open shelf marine facies. The silty facies often contain large monospecific populations of non-reworked Nummulites, which have been interpreted as Nummulites banks. The marine facies of the S’Envestida Calcarenites are presented in log 11 (Fig. 3). Remains of turtles and crocodiles which indicate warm climatic conditions have been identiSE
NW
07
--. . . . ..a._.
_
_
,-.
-.
_m
Littoral
Sands
Terngenous
Shelf
Marls
Coaly facles
scheme
of the Palaeogene
\ of Mallorca
\ Gaklent Fm
Marine.
Carbon&e
Unconformity
studied.
_
Gravels
Fig. 2. Stratigraphic sections
_
Lacustrine - oalustnne:
Fluvio - alluvial.
-
_
I/
&&
Reefal facies
Tectonic boundary (modified
from Ramos-Guerrero
et al., 1989b) and location
of the
M. In&s, E. Ramos-Guerrero /Sedimentary
2 33
Geology 94 (1995) 229-243
gressive cycle characterized by an alluvial system prograding toward the southeast. The lower transgressive cycle is made up of littoral and marine shelf sediments, PriabonianEarly Rupelian in age. These sediments lie unconformably on and onlap toward the northwest the Mesozoic or Palaeogene (D.S.1) basement. Toward the northwest, along the Sierra de Tramuntana, the littoral sediments of the Alar6 Calcarenites Formation (Fig. 2) are the most abundant. The main lithologies of this unit are bioclastic calcarenites, conglomerates, marls and
fied in the lacustrine facies (Hugueney and Adrover, 1982; Jimenez-Fuentes et al., 19891990). The fossil association and the lacustrine hydrochemical characteristics allow us to attribute warm and humid climatic conditions to the Eocene period. 2.3. The Upper Depositional Sequence (D.S.II) In the Upper Sequence (D.S.11) two cycles can be distinguished: a marine and transgressive lower cycle, overlain by an upper continental and re-
7
NW
Cala
4 . . . ... .. . .. . . .. . .. . B. . . . . . .
Blanca
Fm
9
6
..i . L. . . . . .
. . . .
Fig. 3. Simplified
stratigraphic
logs of the studied
outcrops
SE
. .
10 Ir
II 1
??
Y
1 .Y
I
Fm
Alar6 \ \\
and samples
position.
Location
in Figs. 1B and 2
234
M. I&%, E. Ramos-Guerrero /Sedimentary
lutites. Toward the south and southeast the Alar6 Formation laterally changes to marine shelf sediments with reef-limestones, bioclastic calcarenites, lutites and marls. These open marine sediments form the Son Sastre Lutites Formation, the Galdent Limestone Formation and the Calvari Marls Formation (Fig. 2). The littoral facies have been studied and sampled in logs 2-6 and 8 (Fig. 3), whereas the marine shelf facies have been studied in log 8 (Fig. 3). The upper regressive cycle is formed by an alluvial system, Oligocene in age and represented by the Cala Blanca Detrital Formation (Fig. 2). This unit reflects a rapid lateral facies variation, from the proximal conglomeratic alluvial deposits to the northwest toward the distal deposits, located to the southeast, which are composed of lutitic flood plain deposits and lacustrine sediments with mineable coal beds (Ramos-Guerrero and Marzo, 1989). The proximal alluvial facies are gravels and other detrital deposits made up of non-confined sheets of debris flows (Ramos-Guerrero and Marzo, 1989). These facies are mainly located in the southern part of the Sierra de Tramuntana and other localities of the Central Zone, where log number 7 was studied and sampled (Fig. 3). Laterally, the proximal alluvial-fan deposits change to fluviatile deposits whose main lithologies are well sorted conglomerates, sandstones and lutites; among the fluviatile deposits channel filling and lacustrine sequences have been identified interbedded between flood plain lutitic sediments and palaeosoil horizons. The fluviatile faties of the Cala Blanca Formation are studied in logs 3 to 7 (Fig. 3). Clast petrology and palaeocurrent analysis prove the existence of a source area, constituted by Middle Jurassic-Early Cretaceous carbonate rocks (Colom, 1975) and located toward the northwest of the island, in the present-day Catalano-Balear Basin (Fig. 1A). Toward the east and southeast, in a distal position, fluvial plain and lacustrine facies are dominant. Ramos-Guerrero et al. (1989a) characterized two main lacustrine facies assemblages in this area: detrital lacustrine and organic-rich lacustrine facies.
Geology 94 (199.5) 229-243
The main Oligocene lacustrine basins were also perennial, with fresh and carbonate-rich water, but their area1 extension was smaller than in the Eocene (D.S.1). Both lacustrine facies, detrital and organic-rich, have been studied and sampled in logs 9 and 10 (Fig. 3), in the Central Zone of Mallorca. Palynological and palaeobotanical assemblages (Alvarez-Ramis and Ramos-Guerrero, 1986; Alvarez-Ramis et al., 1987; Ramos-Guerrero and Alvarez-Ramis, 1989-1990) and crocodilia and turtle (Hugueney and Adrover, 1982, 1989-1990) remains from the lacustrine deposits of the Cala Blanca Formation suggest tropical to sub-tropical (warm and humid) climatic conditions during the Oligocene. Dispersed organic matter is present in all continental Palaeogene sediments, but is associated with the lacustrine facies (Eocene and Oligocene) where it is most abundant, and locally thick accumulations of lignites and coaly facies are present. Organic geochemistry allows us to attribute the kerogen to types II and III, pointing to a substantial macrophytic contribution. Nevertheless minor algal contributions can also be expected (RamosGuerrero et al., 1989a). These data support the hypothesis of the existence of a stable water column in the lacustrine basins.
3. Analytical
procedure
A total of 108 samples were collected representing alluvial, fluvial, lacustrine (carbonate, terrigenous and organic-rich), littoral and marine shelf environments. Sampling took into account the diversity of sedimentary environments in the Palaeogene of Mallorca; the location of samples is show.n in Fig. 3. For comparative purposes eleven samples, of similar lithology to that inferred for the source area during the Palaeogene, were obtained from Jurassic and Early Cretaceous units. A portion of each sample was dispersed in distilled water and washed until deflocculation occurred. The < 2 pm fraction was separated by centrifugation of the suspension. Oriented mounts of the < 2 pm fraction were prepared for X-ray
M. Ingl&, E. Ramos-Guerrero /Sedimentary
diffraction by gravity settling on glass slides. Clay mineralogy was determined from diffraction patterns obtained using samples that were air dried, ethylene glycol solvated and heated to 550°C during 2 hours. X-ray diffraction analyses were carried out on a Siemens D500 X-ray diffractometer using a CuKa radiation. Ten representative samples of the diverse environments and the source area were further studied by transmission electron microscope equipped with a microanalyzer system in order to determine the qualitative composition of smectites and interstratified minerals and also to identify the non-clay minerals in the < 2 pm fraction. Observation and analysis were performed in a Hitachi H-800 MT with H-8010 STEM and SEM system and X-ray microanalyzer.
4. Results 4.1. Mineralogical fraction
composition
of the
< 2 pm
The main constituents of this fraction are the following: Zllite is recorded in 89% of the samples studied. TEM observations revealed that illite occurs as irregularly shaped grains with micaceous aspect (Fig. 4a). Illite and kaolinite are usually the coarsest grains in the < 2 pm fraction. Smectite and mixed-layer illite-smectite: 67% of the samples studied contain one or other mineral. In fact there is a wide range of composition, from pure smectite to diverse degrees of mixed layering illite-smectite. TEM observations reveal that smectites and mixed-layer illite-smectite have a smaller grain size than illites and kaolinites. The energy dispersive X-ray spectra show that the smectites are of the aluminium type and always contain iron (Figs. 4c and 4d). Interstratified clay minerals are recorded in approximately 25% of the samples. In most of them a low and broad diffraction peak, at 4.5-7” 28, changes to lower angles after solvation with ethylene glycol. This peak indicates the presence of a mixed-layer expandable clay mineral. In a few samples several reflections are present and
Geology 94 (1995) 229-243
235
permit the identification of the mixed-layer minerals as illite-smectite type with proportions of illite layers ranging from 40 to 60% calculated according to the method proposed by Moore and Reynolds (1989). When only one diffraction peak appears the shape, broadness and expansion are variable. This has been interpreted as diverse degrees of interstratification. EDX analysis reveal the presence of grains with ratios of aluminium to silicon lower than the illite but with some potassium content (Figs. 4e and 4f). The 7-A minerals chlorite and kaolinite were distinguished using a slow-scan record between 24 and 25.50”28 in order to distinguish the chlorite (3.54 A) from the kaolinite (3.57 A) peaks (Biscaye, 1965). Nevertheless in a few samples with small amounts of 7-A clay minerals the discrimination of the two peaks has not been possible, in this case we use the term 7-A minerals only when it is impossible to know for sure whether we have kaolinite or chorite, or both minerals together. TEM observations show that kaolinite frequently occurs as hexagonal-shaped crystals of variable grain size (Fig. 4g). Chlorite occurs only in a few Oligocene samples. Non-clay minerals: in the < 2 pm fraction, apart from clays, a wide variety of other detrital minerals are present. Among them, quartz, calcite, goethite and K-feldspar are abundant and can be positively identified on the X-ray diffraction patterns. Small amounts of apatite and titanium oxides were identified during TEM and EDX analyses (Figs. 4a, 4c and 4e). 4.2. Clay mineral distribution The clay minerals identified in the Jurassic samples of the source area are illite, interstratified illite-smectite and 7-A minerals. Cretaceous samples contain predominantly smectite with variable amounts of illite and occasional minor amounts of mixed-layer illite-smectite and 7-A minerals. The smectites are of the aluminium-rich type. Representative X-ray diffraction patterns of the source area are presented in Fig. 5. The clay minerals identified in the Palaeogene samples are: illite, kaolinite, smectite, interstratified illite-smectite and chlorite (Table 1; Fig. 6).
236
M. hgh,
E. Ramos-Guerrero
/Sedimentary
b
h
Geology 94 (1995) 229-243
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/Sedimentary
Geology 94 (1995) 229-243
237
Fig. 5. X-ray diffraction patterns of the < 2 Frn fraction, ethylene glycol solvated samples. Representative sample from the Jurassic source area (A) and representative sample from the Cretaceous source area (B). S = smectite; I-S = interstratified illite-smectite; I = illite; K = kaolinite. Unlabelled peaks correspond to quartz and calcite.
Illite is present in almost all the samples. Nevertheless the abundance of illite varies depending on the sedimentary environment. All the environments contain expandable clay minerals, either smectite or mixed-layer illitesmectite. Interstratified minerals are recorded in fluviatile, detrital and carbonate lacustrine, and littoral facies. Smectite is present in all the sedi-
mentary environments and it is abundant in the alluvial, organic-rich lacustrine and shelf facies, where mixed-layer clay minerals are absent. Kaolinite is recorded in most of the samples and is very abundant in alluvial, carbonate and organic-rich lacustrine, littoral and marine shelf sediments. Chlorite is only present in a few samples of
Fig. 4. Transmission electron micrographs of different representative clay samples (left column). Energy dispersive X-ray spectra of minerals marked with an arrow are depicted in the right column. Vertical scale bar in EDX spectra is equivalent to 1000 counts. Unlabelled peaks correspond to carbon and copper of the support grid and coatings. (a) Illite and goethite (electrodense aggregate) from a shelf sample. (b) EDX spectrum of the illite. (c) Smectite from an alluvial sample. The prismatic, electrodense grain is a titanium oxide. (d) EDX spectrum of the smectite. (e) Interstratified illite-smectite from a fluvial sample. The electrodense mineral is a quartz grain. (f) EDX analysis of the interstratified illite-smectite. (g) Kaolinite from an organic-rich lacustrine sediment. (h) EDX spectrum of the kaolinite.
M. I&&,
238
E. Ramos-Guerrero /Sedimentary
Geology 94 (I 995) 229-243
\ I-S,
K
0
6
S,I-S, I,K
0
s,
5
10
15
al
25
I,
A K
20
Fig. 6. Representative X-ray diffraction patterns of the < 2 Frn fraction, ethylene glycol solvated Palaeogene samples of: (A) organic-rich lacustrine mudstone; (B) alluvial mudstone; (C) fluvial mudstone; and (D) shelf mudstone. S = smectite; I-S = interstratified illite-smectite; I = illite; K = kaolinite. Unlabelled peaks correspond to quartz and calcite.
M. Inglk, E. Ramos-Guerrero /Sedimentary Table 1 Percentage
of samples
in which the various
clay minerals
are present
239
Geology 94 (1995) 229-243
in the Palaeogene
sedimentary
environments
studied
Alluvial
Fluviatile
Detrital lacustrine
Carbonated lacustrine
Organic lacustrine
Littoral
Shelf
I S I-S
57 100
87 42 45
83 39 22
100 28 43
100 100
83 13 20
100 17 _
K Ch
100
45 29
61 16
100
100
56 16
100 100
7A
_
38
28
_
_
43
fluvial, detrital lacustrine and littoral origin of the D.S.11 (Late Eocene-Oligocene). Although the clay minerals recorded (except
Carbonate Organtc
lacustnne
fmes
rich lacustrme
Marme faces
chlorite) are ubiquitous, some sedimentary environments are characterized by a definite assemblage distinguishable by the constant presence
faCleS
( httoralto shelf)
Fig. 7. Idealized palaeoenvironmental minerals indicates decreasing relative
scheme of the Palaeogene of Mallorca amounts. In brackets are clay minerals
with clay mineral distribution. The order which are of secondary importance.
of the clay
240
M. In&s> E. Ramos-Guerrero
/Sedimentary
and relative proportions of some clay minerals. The distribution of these clay mineral assemblages in the depositional environments (Fig. 7) can be summarized as follows. Allucial. Smectite and kaolinite are consistently present in all the samples. Illite is recorded in 57% of samples and is relatively scarce. Flucial and detrital lacustrine. All the clay minerals recorded are present in these environments although the relative proportions are very different (Fig. 60. The presence of chlorite is noticeable in some Oligocene samples. Carbonate lacustrine. All the samples contain illite and kaolinite in varying proportions. Nevertheless in the lacustrine deposits of the Peguera Limestone (Middle Eocene) illite prevails over kaolinite and in the carbonate lacustrine deposits of the Cala Blanca Formation (Oligocene) kaolinite is more abundant than illite. Most of the samples contain subordinate amounts of smectite or interstratified illite-smectite. Organic-rich lacustrine. These deposits show an homogeneous mineralogical composition. Illite, smectite and kaolinite are recorded in all samples. Smectite and kaolinite are much more abundant than illite. Littoral. These environments are characterized by the presence of illite and kaolinite. Occasionally smectite, mixed-layer illite-smectite, chlorite and 7-A minerals occur in variable amounts. When smectite or mixed-layer illite-smectite do occur, they can be very abundant. Shel$ These samples are characterized by high kaolinite and low illite contents. Smectite is found in some samples and is in some places quite abundant.
5. Discussion In the source area, the clay minerals present are illite, smectite, interstratified illite-smectite and 7-A clay minerals; the first two minerals are the most abundant. In the Palaeogene sediments detrital clay minerals are dominant. This fact is clearly reflected by the high content of smectite in the proximal area. Titanium oxides and apatite are present in
Geology 94 (1995) 229-243
both the source area and the Palaeogene rocks. These heavy minerals, widespread over all the Palaeogene environments, clearly indicate the importance of detrital processes. Kaolinite is the most abundant clay mineral in the Palaeogene samples. Smectite and mixed-layer illite-smectite come next in terms of abundance. Illite is very widespread, but is always present in a lesser proportion and with a lower crystallinity than the source area samples. The absence of dolomite and magnesium in the clay minerals are noticeable. The lack of magnesium may be related to the lithological composition of the source area, consisting exclusively of limestones and marls. The alluvial facies contain kaolinite and smectite with minor amounts of illite. Smectite and illite are of detrital origin. Chemical weathering of pre-existing clay minerals, probably illite, and to a lesser extent smectite, led to the formation of kaolinite. Goethite and potassium ions are other products of the weathering processes. The presence of both kaolinite and goethite (and occasionally hematite) indicates substantial chemical weathering. EDX analyses (Fig. 4) show that illite and smectite are iron rich; this favoured the formation of goethite. The weathering of illite led to a potassium enrichment of the interstitial waters. The drainage of the area carried the K-rich solutions to the lower zones (Fig. 7) where the formation of interstratified illite-smectite was favoured by the entrance of potassium in the smectite interlayers (Singer and Stoffers, 1980). In the fluviatile and lacustrine environments (except the organic-rich lacustrine) the detrital clay minerals were preserved. These environments are characterized by a complex clay mineral assemblage composed of detrital kaolinite, smectite, illite and in some places chlorite, as well as interstratified illite-smectite formed in the flood plain. The detrital clay mineral preservation is consistent with the characteristics of Palaeogene lacustrine systems: freshwater, exorheic, perennial and calcium bicarbonate dominated; these conditions do not favour clay mineral transformations (Jones and Bowser, 1978; Jones, 1986). The interstratified illite-smectite formation mechanism has been described by Eberl et al.
M. Ingl& E. Ramos-Guerrero
/Sedimentary
(1986). These authors suggested that wetting and drying cycles favour the fixation of potassium in smectites and thus the formation of illitic layers. The mudstone deposits of the fluvial flood plain have decolourations, carbonate nodules and root-marks that indicate palaeosoil development under hydromorphic conditions. In these conditions interstitial water stagnation and ion concentration during the dry season are favoured. According to Mohr and Van Baren (19541, in the same climatic conditions, rapid interstitial water circulation favoured the formation of kaolinite, whereas slow circulation with temporary stagnation made possible the reactions between clay minerals and the dissolved cations. In the organic-rich lacustrine environment the clay mineral association is composed of kaolinite and smectite, both very abundant; illite is also present, but in a lesser proportion. In contrast with other lacustrine environments (carbonate and detrital) neither interstratified illite-smectite nor chlorite are reported. The smectite has good crystallinity, similar to that of the smectite from the Cretaceous rocks of the source area and always better than that of the other Palaeogene sediments. So we suspect that in the organic-rich lacustrine environments some smectite-regeneration mechanism was active in spite of the generally held view (e.g. Berner, 1971; Styman and Bustin, 1984) that these environments favour smectite alteration and kaolinite formation. However, an abundance of both kaolinite and smectite in the organic-rich lacustrine facies was also cited by Ingles and Anadon (1991) in the Palaeogene sediments of the Ebro basin. In the distal environments (littoral and shelf) the proportions of smectite and mixed-layer illite-smectite decrease, whereas kaolinite and minor amounts of illite are the most representative clay mineral assemblage. The kaolinite increase can be attributed to the weathering of the smectite in the proximal area. For all the Palaeogene sedimentary environments of Mallorca it should be emphasized that chlorite is only present in the Oligocene samples. Illite is present in both Eocene and Oligocene samples, but the alluvial and lacustrine Eocene samples record the greatest abundance of illite.
Geology 94 (1995) 229-243
241
The fluctuations in the detrital clay mineral proportions must be attributed to the source area composition variations as well as to the change in palaeocurrents. However, we cannot rule out the possibility of the existence, during the Oligocene, of slightly colder periods, during which weathering was less important, and which allowed the preservation of chlorite and illite. The Eocene lacustrine system (Peguera Limestone) developed in a stable plate, where the lacustrine basins occupied wide areas. On the other hand, the Oligocene lacustrine system (Cala Blanca Formation) developed in an area of deformation processes related to the Betic orogen. This context had a strong control over the genesis and later development of the sedimentary basins and sediment supply, mainly terrigenous. The formation of kaolinite and goethite suggests that weathering processes were developed under high temperature and rainfall conditions. On the other hand the formation of interstratified illite-smectite can be related to a seasonal rainfall contrast. This interpretation agrees with palaeoecological data previously mentioned, which reflect the existence in this area of similar palaeoclimatic conditions. In addition, Charnley et al. (1979) deduce the existence of similar palaeoclimatic conditions during the Middle Eocene-Oligocene in the Atlantic edge of the Iberian plate (DSDP site 3981, in a latitudinal position similar to that of Mallorca, and Charnley (1989) proposed for the same area the existence of a seasonal rainfall contrast. 6. Conclusions The clay minerals in the Palaeogene mudstones of Mallorca are mainly detrital and result from weathering in the source area as well as post-depositional weathering in the proximal environments. However, the final clay mineral assemblages are found to be due as much to the weathering processes as to the sedimentary and diagenetic conditions prevailing in the sedimentary environment. The distribution of the clay minerals in the diverse sedimentary environments is shown in Fig. 7.
242
M. Ingl& E. Ramos-Guerrero
/Sedimentary
The alluvial facies are composed of smectite and kaolinite with minor illite. Smectite and illite are detrital, whereas kaolinite has been formed by the chemical weathering of preexisting clay minerals, mainly illite. Hydrolysis of these facies as well as the source area, with abundant smectite, mixed-layer illite-smectite and illite, under high temperature and rainfall, led to the formation of kaolinite and goethite while water was enriched in soluble cations as residual products. In the fluviatile and detrital lacustrine facies all the recorded clay minerals are present: kaolinite, mixed-layer illite-smectite, illite, smectite and, in the Oligocene samples, chlorite. All clay minerals are detrital in origin except the mixedlayer illite-smectite, which was formed in the fluvial flood-plain, where the imprint of hydromorphic processes has been recognized and the stagnation of interstitial water led to periodic increase of ion concentration. In these conditions an increase of potassium concentration occurred and favoured the formation of illitic layers in smectites by the fixation of potassium, leading in turn to the formation of illite-smectite mixedlayers. Potassium was released during the hydrolysis of illite. The carbonate lacustrine facies are characterized by illite and kaolinite, with subordinate amounts of smectite and mixed-layer illite-smectite. All these minerals are of detrital origin. The organic-rich lacustrine facies are marked by the presence of smectite and kaolinite, with minor amounts of illite. Kaolinite and illite are detrital. The lack of interstratified minerals and the high crystallinity of smectites suggest that some sort of regeneration process of these minerals may be related to the hydrological characteristics of these environments. The marine facies (littoral and shelf) are characterized by the presence of detrital clay minerals, mainly illite and kaolinite, with minor amounts of smectite and mixed-layer illite-smectite. In the Oligocene littoral samples, traces of chlorite are also present. No diagenetic processes have been observed in the detrital and carbonate lacustrine and marine environments. The relative abundance of smectite diminishes
Geology 94 (1995) 229-243
with distance-the organic-rich lacustrine deposits are an exception-whereas kaolinite distribution shows the opposite tendency, increasing in proportion with distance. In our opinion this clay mineral distribution results mainly from weathering processes more than transport and sorting (in distal areas smectite is locally abundant). On the other hand, the transportation processes have been very active as is shown by the sedimentological facies association.
Acknowledgements Financial support was provided by CICYT projects GE0 89-0426 and PB 91-0805. X-ray diffraction analysis were carried out in the Institut de Ciencies de la Terra “Jaume Almera” (CSIC). TEM observations and analysis were carried out in the Serveis Cientifico-Tecnics, Universitat de Barcelona. The manuscript has been improved by critical comments and suggestions from P. At-radon, L. Cabrera and reviewers H. Friis and B.F. Jones.
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