The use of clay minerals and microfossils in palaeoenvironmental reconstructions: The Holocene...

The use of clay minerals and microfossilsin palaeoenvironmental reconstructions:The Holocene littoral strand of Las Nuevas

(Donana National Park) SW Spain

M. I . CARRETERO1 , F . RUIZ2 , A . RODRIGUEZ-RAMIREZ2 , L . CACERES2 ,

J . RODRIGUEZ VIDAL2 AND M. L . GONZALEZ REGALADO2

1Departamento de Cristalografıa, Mineralogıa y Quımica Agrıcola, Universidad de Sevilla, Apdo. 553, Seville, and2Departamento Geodinamica y Paleontologıa, Universidad de Huelva, 21819-Palos de la Frontera, Huelva, Spain

(Received 7 December 2000; revised 16 May 2001)

ABSTRACT: Three steps have been established during the Holocene formation of the bar-built

estuary of Las Nuevas (Donana National Park, Spain), on the basis of the clay mineralogy variations

and the palaeontological record. The first step is characterized by the presence of ostracodes and

homogeneous quantities of clay minerals (17�20% illite, 25�29% smectites), values of smectite(0.64�0.70) and illite (0.60�0.70) crystallinity indexes, and the ratio of AlVI/(FeVI + MgVI) in illite(0.46�0.47). This zone is interpreted as a very shallow lagoon with euryhaline conditions. Thepresence of roots, the progressive disappearance of foraminifers and an increase in the smectite

content (up to 35%) define the second step. A salt-marsh environment with low-energy

hydrodynamic conditions is deduced for this zone. The third step is characterized by an increase

in illite content (up to 35%), and a decrease of the smectite content (up to 21%). The smectite

crystallinity index decreased to 0.38, whereas the illite ratio AlVI/(FeVI + MgVI) decreased to 0.36. In

this zone, the ostracode assemblage contains numerous juvenile stages of coastal species coinciding

with lumachelle accumulations of the estuarine bivalves, abundant foraminifers and the presence of

charophytes. This zone represents a strong marine input, probably caused by storms.

KEYWORDS: clay minerals, palaeoenvironmental reconstructions, Holocene, Donana National Park,Spain.

A classical application of clay minerals is as

palaeoenvironmental indicators (Singer, 1984;

Galan, 1986; Chamley, 1989). In most cases, clay

minerals are stable in their formation environment;

therefore, there is a direct correlation between the

type, chemical composition and crystallinity of a

specific clay mineral and the palaeoenvironmental

conditions under which the mineral was formed

(Keller, 1970). Clay minerals have been used over

the last few decades in the interpretation of

palaeoclimatic and palaeoenvironmental changes,

e.g. to reconstruct palaeoclimatic conditions

(Kuzvart & Konta, 1968; Vanderaveroet et al.,

2000); to determine the palaeotopography in arid

and semi-arid zones from the Cambrian up to the

present (Callen, 1984); in the analysis of hydro-

dynamic processes in coastal environments

(Gutierrez Mas et al., 1997); for palaeogeographic

reconstructions (Ortega-Huertas et al., 1991;

Deconinck & Chamley, 1995; Pletsch et al.,

1996), and to determine sedimentological evolution

in lacustrine (Lopez Aguayo & Gonzalez Lopez,

1992, 1995; Mayayo et al., 2000), alluvial* E-mail: [email protected]: 10.1180/0009855023710020

ClayMinerals (2002) 37, 93–103

# 2002 The Mineralogical Society

(Tomadin & Varani, 1992, 1998) and marine

(Delgado et al., 1992) environments.

In the geological record, correlations between

clay minerals and microfossils have been used for

palaeoclimatic reconstructions (Rotschy et al.,

1972; Sancetta et al., 1985; Vanderaveroet et al.,

1999) or palaeoenvironmental changes (Jones,

1984). In recent environments, the analysis of

both mineralogical and biogenic composition is

useful, for example, to identify transport

phenomena of sediments in littoral areas

(Shaghude & Wannas, 2000).

The aim of this investigation was to detect

possible palaeoenvironmental changes during the

evolution of the previous salt marshes to the littoral

strand of Las Nuevas (Donana National Park,

Spain), using variations in clay minerals and

microfossils as indicators.

DO NANA NAT IONAL PARK

The present-day outlet of the Guadalquivir river is

an estuary generated since the last Holocene sea-

level rise (~6500 y BP, according to Zazo et al.,

1994) and is composed of salt marshes and littoral

spits (the so-called Donana National Park).

Geomorphological mapping has enabled us to

differentiate two morphogenic systems in the

present-day Donana National Park: littoral and

estuarine (Rodrıguez Ramırez, 1996). Various

spits and sandy strands that tend to seal up the

Guadalquivir estuary form the littoral system. On

the right bank is the Donana spit. This comprises

the most extensive system of spits, which has

grown towards the E and SE. Today they are partly

covered by active dunes.

The estuarine system consists of marshes filling

the extensive area behind the littoral spits. This

filling has taken place gradually as the littoral

formations have sealed the estuary. Waves, tides

and the intense fluvial action lead to the shape of

the marshes. Thus there is a direct relationship

between the littoral formations and the estuarine

ones.

The littoral spit of Donana includes four

progradation phases (Zazo et al., 1994; Lario,

1996; Rodrıguez Ramırez et al., 1996). These

sedimentary phases are interrupted by rapid rises

in the sea level, with erosion of the previously

constructed littoral barriers. Littoral strands in the

marshes represent these erosive events: Carrizosa-

Veta La Arena, Vetalengua, Las Nuevas (Fig. 1).

The littoral strand of Las Nuevas is a small ridge

with an elevation of up to 2 m with respect to the

adjoining salt marshes. On the surface, this

sedimentary bed is composed of a lumachelle

level of bivalves enclosed in a sandy matrix,

which was deposited between 1900 and 1500 y

BP (Rodrıguez Ramırez, 1996). This paper analyses

its origin from a multidisciplinary perspective.

METHODOLOGY

In the bar-built estuary of Las Nuevas, a sediment

core was sampled with a hand coring device

capable of recovering cores up to 1 m in length

and 2 cm in diameter. Twelve samples (4 cm in

length) were collected and analysed (Fig. 1).

Initially, the lithostratigraphy of the core was

tested and the molluscan shells included in the

samples identified.

In a second step, the total and clay mineralogy

were studied by X-ray diffraction (XRD). The

equipment used was a Philips PW 1130/90, with an

automatic slit, at 20 mA, 40 kV and using

Ni-filtered Cu-Ka radiation. The <2 mm fraction

was separated by a standard sedimentation method

(Barahona, 1974). Before the separation, carbonates

were eliminated using 0.6 N acetic acid.

Identification of clay minerals was performed by

routine methods, which involved the standard

solvation with ethylene glycol and dimethylsulph-

oxide, and heating at 5508C. The semi-quantitativecomposition was calculated using data from Tank

(1963), Schultz (1964) and Biscaye (1965).

Correction factors were applied for a diffractometer

with an automatic slit.

The crystallochemical parameters of clay minerals

determined were the smectite and illite crystallinity

indexes, and the ratio of AlVI/(FeVI + MgVI) in illite.

The smectite crystallinity index was determined by

the ratio of C/A, where A is the maximum height of

the 001 basal reflection and C is the height of the

same reflection at �1/282y, both measured onaggregates solvated with ethylene glycol (E. Galan,

pers. comm.). The ratio of AlVI/(FeVI + MgVI) in

illite was determined from the ratio of the 002 and

001 basal reflections on aggregates solvated with

ethylene glycol, according to Klingebiel and

Latouche (1962), and Esquevin (1969). The illite

crystallinity index was measured by the width of the

peak at half-height of the first basal reflection at

10 A, according to Kubler (1968), and following the

recommendations by Kisch (1991), using the <2 mm

94 M. I. Carretero et al.

size-fractions after solvation with ethylene glycol,

expressing the result in terms of the D82y, using ascan speed of 28/min, at a time constant of 2, and apaper speed of 1 cm/82y.For the micropalaeontological analysis, a stan-

dard weight (15 g) of sediment was washed in

63 mm sieves, then dried at a temperature of 608C.If possible, 300 ostracodes (valves and tests) were

separated from the residues by handpicking. The

number of individuals per gram was calculated, as

well as the proportions of the marine species. In

addition, the presence/absence of other fossils with

palaeoecological interest (molluscs, foraminifers,

charophytes) was examined.

RESULTS

Lithostratigraphy

The lower interval (�1.62 m to �1.29 m) ischaracterized by massive, blue clays with very

sparse fauna. Over this zone, very similar sediments

present abundant fragments of phanerogams and

numerous vertical roots (�1.29 m to �0.63 m).This homogeneous lithology is only interrupted at a

core-depth of �0.91 m to �0.79 m by a fine levelof sandy silts with scattered fragments of the

bivalve Cardium edule. The samples between

�0.63 m and �0.43 m depth consist of yellow,

sandy silts with abundant macrofauna. Finally, the

San Jacinto

La Algaida

Marismilla

Guadalquivir

El Abalario

Ocean

Atlantic

0 10 km

Carrizosa-Veta la Arena

Las Nuevas

Vetalengua

ba

67

8

9

3

4

5

10

11

12

1

2

M P

c d e

0

20 cm

6 7

1 2 3 4 5

8 9 10

River

Huelva

Guad

iana

Riv

er

Odie

lR

iver

Tinto

River

Gua

dalq

uivi

rR

iver

Cádiz

Gulf

0 30 km

N

Spain

Sea

Sevilla

PO

RT

UG

AL

Doñana National Park

FIG. 1. Geomorphological map of the Guadalquivir River mouth, showing the location of the drill site and

indicating samples taken for mineralogical (M) and palaeontological (P) studies. Symbols: (a) silty sands, (b)

sandy silt, (c) clays, (d) macrofauna, (e) vegetation. Map: (1) Plio-Quaternary substratum, (2) Marshes, (3) The

2nd phase of the littoral spit progradation (H2), (4) The 3rd phase (H3), (5) The 4th phase (H4), (6) Quaternary

dune systems, (7) Littoral ridges, (8) Estuarine ridges, (9) Clayey levees, (10) Drill site location.

Clay minerals and microfossils in palaeoenvironmental reconstructions 95

upper 0.43 m of the core presents yellow silty sands

with abundant and well-preserved molluscs (mainly

C. edule). This macrofauna was disposed in

lumachellic beds, massive or in slightly plane-

parallel strata (Fig. 1).

Palaeontology

Palaeontological analysis enables three zones to

be established (A, B and C). In zone A (samples 1,

2, 3), the number of ostracodes decreases towards

the top. The ostracode assemblage is dominated by

Cyprideis torosa and Loxoconcha elliptica, with

Leptocythere castanea and Leptocythere tenera as

additional species (Table 1). Juvenile specimens of

C. torosa and L. elliptica are very abundant in the

lower two samples (subzone A1), whereas adults of

these species and some occasional species

(Palmoconcha turbida, Leptocythere macallana,

Carinocythereis whitei) were only found in sample

3 (subzone A2). In this sample, the micropalaeonto-

logical record includes numerous benthic foramini-

fers and rare specimens of marine, planktonic

foraminifers (Globigerina bulloides) and charo-

phytes, whereas some fragments of bivalves

(Cardium edule), gastropods (Rissoidae) and

echinoderms characterize the macrofauna.

A second zone (B: samples 4, 5, 6) is defined by

the absence of ostracodes or the presence of sparse

valves and carapaces (<0.2 individuals per gram)

belonging to very juvenile specimens (A-4 to A-6)

of C. torosa or L. elliptica. Benthic foraminifers

(mainly Ammonia inflata) are abundant in sample 4

and very rare in the two following samples,

coinciding with a generalized absence of macro-

fauna. In these samples, roots are very frequent.

In the upper four samples studied (zone C),

ostracodes are well represented only in sample 12.

In this interval, the number of species increases

towards the top, C. torosa being the main species

(20�40% of adults). Planktonic foraminifers are

f requen t (Globiger ina , Glob iger ino ides ,

Globorotalia, Orbulina), whereas charophytes

were only found in sample 11. The macrofauna is

well represented, with bivalves (C. edule, Mactra,

Crassostrea, Donax), gastropods (Gyraulus laevis,

TABLE 1. Relative abundance (%) of ostracodes in the samples studied.

Species Samples 1 2 3 4 5 6 8 9 11 12

Aurila convexa ** 0.3Callistocythere flavidofusca ** 0.6Callistocythere rastrifera ** 7.1Carinocythereis whitei ** 4 3.5 4.8 1.6Costa edwardsii ** 0.6Cyprideis torosa 82.8 40.3 76 100 80.7 66.7 64.3 65.4Cytheretta adriatica ** 9.5Hiltermannicythere emaciata ** 2.4 0.3Leptocythere bacescoi ** 1.9Leptocythere castanea 4.4 2.4 2.4Leptocythere macallana ** 4Leptocythere pellucida ** 4.8Leptocythere tenera 2 0.7 4Loxoconcha elliptica 10.8 56.7 4 100 5.3 4.8 3.8Loxoconcha rhomboidea ** 0.3Microcytherura angulosa ** 4.8Neocytherideis subulata ** 0.3Palmoconcha guttata ** 2.2Palmoconcha turbida ** 4 1.8 4.8 11.9 9.7Paracytheridea depressa ** 0.3Pontocythere elongata ** 4.8 2.4 4.4Semicytherura incongruens ** 1.8 1.9Semicytherura sulcata ** 1.6Urocythereis oblonga ** 7 4.8 2.5

(**) marine species


Rissoa sp.), scaphopods (Dentalium vulgare) and

spines of echinoderms (Table 1).

Mineralogy

The most abundant minerals in all samples are

clay minerals (>45%), calcite (~20�30%) andquartz (~10�20%). Besides these, feldspars (<5%,except sample 1), dolomite and halite (<5%), and

traces of hematite in some samples, were detected

(Table 2). In the bulk sample analysis, the main

clay minerals are smectites (20�35%) and illite(17�35%), with an inverse correlation betweenthem (Fig. 2). Other clay minerals found are

kaolinite and chlorite, with traces of mixed-layer

illite-smectite and illite-chlorite in some samples

(Table 2).

Taking into account the variations of the contents

of clay minerals (Fig. 2), three zones in the profile

studied were established, coinciding with the zones

deduced from the palaeontological analysis. These

variations indicate a progressive contribution of

clay minerals from sample 3 onwards, with the

greatest contribution in sample 7. Minor variations

were observed in the contents of calcite and quartz.

Zone A is characterized by slight changes in the

contents of illite (17�20%) and smectites

(25�29%) (Table 2, Fig. 2), and homogeneousvalues of both the illite (0.60�0.70) and smectite(0.64�0.70) crystallinity indexes, and illite ratioAlVI/(FeVI + MgVI) (0.46�0.47) (Fig. 3). In Zone Ban increase of the smectite contents (up to 35% in

samples 5 and 6) and homogeneous contents of

illite (19�20%) can be observed. In this zone, thesmectite and illite crystallinity indexes, and the

illite ratio AlVI/(FeVI + MgVI) show practically no

change (Fig. 3). Zone C is characterized by drastic

changes, mainly in sample 10. A significant

increase of illite content (from 20% in sample 6,

to 35% in sample 10) and a significant decrease of

smectite content (from 35% in sample 6, to 21% in

sample 10) were observed (Fig. 2). Likewise, the

smectite crystallinity index (from 0.66 in sample 6,

to 0.38 in sample 10) and illite ratio AlVI/(FeVI +

MgVI) (from 0.56 in sample 6 to 0.36 in sample 10)

decreased drastically (Fig. 3).

1000Marinespecies

Estuarinespecies

0

20 cm

67

8

9

3

4

5

10

11

12

1

2

Clay mineralsCalciteQuartz

Smectites Ostrac.Nº ind./gr. Es-Ms

Char.Bent. Plan.

Moll.Foram.

MINERALOGYLITHOLOGY PALAEONTOLOGY

10 65 20 40

Rare

Frequent

Abundant

Clay minerals

0 300

CalciteQuartz

Illite

C

B

A

15 35

Samples

% %%

FIG. 2. Results of the mineralogical and palaeontological analysis. Ostrac.: ostracodes; N8 ind/gr.: number ofindividuals per gram; Es: estuarine species, Ms: marine species; Char.: Charophytes; Foram.: foraminifers; Moll.:

Molluscs; Bent.: Benthic; Plan.: Planktonic.


TABLE2.Mineralogicalcompositionofbulksamplesandthe<2mmfraction.

Bulksamples

<2mmfraction(*)

Sample

Quartz

Calcite

FeldsparsClaymineralsDolomiteHaliteHematite

Smectites

IlliteKaoliniteChlorite

I-S

I-Ch

12

18

21

353

32

Tr

25

23

41

Tr

n.d.

11

10

19

560

32

n.d.

25

30

32

Tr

n.d.

10

921

261

33

n.d.

21

35

41

Tr

n.d.

79

20

265

22

Tr

28

32

41

n.d.

n.d.

611

21

261

33

n.d.

35

20

51

Tr

Tr

510

24

258

33

n.d.

35

19

4Tr

n.d.

n.d.

412

25

455

22

n.d.

30

20

5Tr

Tr

n.d.

313

32

147

33

Tr

26

17

31

Tr

n.d.

216

27

249

33

n.d.

25

20

31

n.d.

n.d.

116

19

951

32

n.d.

29

18

31

n.d.

n.d.

Tr=Traces;n.d.=notdetected;I-S=mixed-layerillite-smectite;I-Ch=mixed-layerillite-chlorite

(*)Valuesrelatedtobulksample


DISCUSS ION

Taking into account the palaeontological analysis,

in Zone A two subzones were distinguished. In

subzone A1, the ostracode assemblage (C. torosa, L.

elliptica) is characteristic of the internal borders in

a very shallow lagoon (0�1 m depth) with euryha-

line conditions (salinity down to 15�20%)(Marocco et al., 1996; Montenegro & Pugliese,

1996; Ruiz et al., 2000). In this quiet environment,

a permanent water layer is deduced by the

abundance of C. torosa, because its eggs cannot

withstand aerial exposure (Anadon et al., 1986). In

sample 3 (subzone A2) small amounts of marine

0,30

0,35

0,40

0,45

0,50

0,55

0,60

0,65

0,70

0,75

12 11 10 7 6 5 4 3 2 1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

12 11 10 7 6 5 4 3 2 1

samples

��

�º

ZONE C ZONE B ZONE A

samples

a

b

c

0,3

0,35

0,4

0,45

0,5

0,55

0,6

12 11 10 7 6 5 4 3 2 1

samples

Illi

tera

tio

Al

/(F

e+

Mg

)V

IV

IV

IIl

lite

cryst

alli

nit

yin

dex

Sm

ecti

tecr

yst

alli

nit

yin

dex

FIG. 3. (a) Smectite crystallinity expressed by the ratio of C/A; (b) ratio of AlVI/(FeVI + MgVI) in illite; and (c)

illite crystallinity index, according to Kubler (1968).


micro- and macrofauna were determined, indicating

a small contribution of marine sediments in a small

marine input (Fig. 4).

From the mineralogical study it is not possible to

distinguish two subzones in Zone A. In this zone

smectites are the main clay minerals present in the

samples. Homogeneous quantities of clay minerals

and values of smectite and illite crystallochemical

parameters can be observed. Therefore, for dedu-

cing slight marine inputs in this case, the use of

palaeontological analysis is better than variations in

the clay minerals.

Zone B is characterized by the presence of roots

and the progressive disappearance of foraminifers,

indicating a low salt-marsh environment, with a

prolonged subaerial exposure (Gonzalez-Regalado

et al., 1996; Ruiz et al., 1997a). The ostracode

record is restricted to very sparse immature moult

stages, indicating a low-energy death assemblage

(Fig. 4). In this zone there are practically no

changes in either the illite contents or in the

crystallochemical parameters of illite and smectite;

but an increase of the smectite contents was

observed (Fig. 2). The relative concentration of

smectite compared to illite in zone B could be due

to the environmental conditions deduced from

palaeontological analysis. The high- and low-tide

phases, marked by low-energy hydrodynamic

conditions, allow the settling of smectites, which

accumulate in surface sediments.

Zone C is characterized by important changes,

both in its palaeontological record and in its

mineralogy. The ostracode assemblage of this zone

contains numerous juvenile stages of coastal species

(Ruiz et al., 1997b), coinciding with lumachelle

accumulations of the estuarine bivalve C. edule,

abundant carapaces of the foraminifer Ammonia and

the presence of charophytes. This conjunction would

indicate a lake shore facies where these fauna were

accumulated by storm waves after death (Anadon et

al., 1986). These waves originated in a coastal

environment and eroded the estuarine bottom when

C. edule lived. Taking into account the mineralogical

study, zone C is characterized by an increase in illite

content and a decrease in smectite content (Fig. 2).

Likewise, an increase in smectite crystallinity in

0

20 cm

67

8

9

3

4

5

10

11

12

1

2

LT

HT

MI

MI

A2

C

B

A1

Samples

FIG. 4. Palaeoenvironmental reconstruction on the basis of the drilling data (grey rectangle) in the surroundings of

the Guadalquivir salt marshes. A1: subtidal (LT: low tide, HT: high tide); A2: intertidal, with some marine

influence (MI); B: aerial exposure; C: similar to B, but with more significant marine influence (high energy).


sample 10, and a decrease in the illite ratio

AlVI/(FeVI + MgVI), was observed (Fig. 3). The

variations of both contents and crystallochemical

parameters of clay minerals could be due to the

contribution of illite from marine deposits caused by

the new, more important, marine input, that was

established over the salt marsh deposits of zone B

and which have been deduced from the palaeonto-

logical analysis. This hypothesis agrees with the

composition of the sediments of the southeastern

Gulf of Cadiz, rich in illite (Gutierrez Mas et al.,

1997; Lopez Galindo et al., 1999). Source mixing

appears to be the main mechanism for explaining the

distribution of clay minerals in zone C.

CONCLUS IONS

Variations in both the contents of clay minerals,

and their crystallochemical parameters, together

with palaeontological studies have allowed the

palaeoenvironmental reconstruction of the bar-built

estuary of Las Nuevas (Donana National Park). It

was formed in three steps: Zone A, a very shallow

lagoon with euryhaline conditions, characterized by

the presence of ostracodes and homogeneous

quantities of clay minerals and values of smectite

and illite crystallochemical parameters; Zone B, low

salt-marsh with low-energy hydrodynamic condi-

tions, characterized by the presence of roots and the

progressive disappearance of foraminifers, and an

increase in smectite contents; Zone C, strong

marine input, caused by storms, and characterized

by a large contribution of marine sediments, with

an increase in illite content and a decrease in

smectite, together with variations in the crystal-

lochemical parameters of clay minerals. Source

mixing appears to be the main mechanism for

explaining the distribution of clay minerals in this

zone. The ostracode assemblage contains numerous

juvenile stages of coastal species coinciding with

lumachelle accumulations of the estuarine bivalve

Cardium edule, abundant tests of the foraminifer

Ammonia and the presence of charophytes.

ACKNOWLEDGMENTS

Financial support was provided by the Spanish Projects

AMB99-0226-C03-03 and BTE-2000-1153, and the

Research Groups RNM-293, RNM-238 and RNM-135

(Andalusian Goverment). It is a contribution to the

IGCP 396 and 437.

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The use of clay minerals and microfossils in palaeoenvironmental reconstructions: The Holocene...

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