Sedimentary Geology, 76 (1992) 135-153 135 Elsevier Science Publishers B.V., Amsterdam

Recognition of lake-level changes in Miocene lacustrine units, Madrid Basin, Spain. Evidence from facies analysis, isotope

geochemistry and clay mineralogy

A . B e l l a n c a a, j . p . C a l v o b, p . C e n s i a, R. N e r i a a n d M. P o z o c

a Istituto di Mineralogia, Petrografia e Geochimica, Universit?J degli Studi, Via Archirafi 36, 90123 Palermo, Italy b Departamento de Petrologia y Geoquimica, Universidad Complutense, 28040 Madrid, Spain

c Departamento de Geologia y Geoquimica, Universidad Autonoma, 28049 Madrid, Spain

Received May 16, 199l; revised version accepted November 4, 1991

ABSTRACT

Bellanca, A., Calvo, J., Censi, P., Neri, R. and Pozo, M., 1992. Recognition of lake-level changes in Miocene lacustrine units, Madrid Basin, Spain. Evidence from facies analysis, isotope geochemistry and clay mineralogy. Sediment. Geol., 76: 135-153.

The Miocene lacustrine sequence exposed near Esquivias, in the central area of the continental Tertiary Madrid Basin, is formed of two main lithostratigraphic units that represent two different stages of a palaeolake system.

Unit I consists of five lithofacies: (A) nodular carbonates and marls, (B) massive dolostones, (C) bioclastic limestones, (D) mudstones, and (E) chert, which are characteristic of sedimentation and early diagenesis in a shallow lacustrine environment. Two main stages of the same general Iowstand situation are reflected in mineralogy and isotope geochemistry results. An initial stage is suggested by the presence of non-stoichiometric dolomite and trioctahedral smectites. High 6180 and /~13C values of the dolomites (+5.5 and +2.9 6%0, respectively) indicate strongly evaporative conditions. A second stage is characterized by the presence of calcite, fibrous clay minerals and by generally lower 6180 (from + 1.5 to -6.5 •%o) and 613C (from - 1 to -9.8 8%o) values for carbonates, which suggest more diluted waters during this lowstand phase of the lake.

Unit II comprises seven lithofacies: (A) conglomerates, (B) intraclastic mudstones, (C) laminated marls, (D) massive mudstones, (E) diatomaceous marls, (F) laminated bioclastic limestones, and (G) root-bioturbated limestones. Both conglomerates and intraclast mudstones fill sharply eroded contacts in the underlying deposits of Unit I. We demonstrate how the arrangement of facies, supported by isotopic results from the carbonates 6180 (from -0.7 to -7.2 ~%o; ~13C from -4.2 to -8.6 ~ %o), is clearly indicative of deeper lake conditions. The basal reworked facies mark the initial phase of a lake transgression. In contrast with Unit I, the clay mineral assemblage from unit II is mainly detrital. Clay minerals thus also support the interpretation of a dilute water body, consistent with a highstand of the palaeolake.

Introduction

T h e s e d i m e n t a r y r eco rd of c losed bas in lakes

is usual ly cha r ac t e r i z ed by m a r k e d l i thologica l

changes in ver t ica l sequences . T h e s e var ia t ions

a re due to the de l ica te ba l ance be tween gains

( inflow plus p r ec ip i t a t i on on the l ake sur face) and

losses by evapo ra t i on and inf i l t ra t ion, and resul t

in r ap id and f r equen t shif t ing of the shore l ines as

a r e sponse to changes in l ake wa te r level (Eugs t e r

and Kelts , 1983; Gore , 1989). Hydro log ica l ly

c losed bas ins e m b r a c e a wide s p e c t r u m of set-

t ings which range f rom annua l ly des icca ted playas

of ar id c l imates (H a rd i e et al., 1978) to pe renn ia l ,

commonly shal low lakes o f severa l l a t i tudes

(Fouch and Dean , 1982; Eugs t e r and Kelts, 1983).

P r imary p rec ip i t a t i on of ca rbona t e s as well as

fo rma t ion of clay minera l s in c losed bas in lakes is

closely r e l a t ed to the chemis t ry of the lake waters

which in tu rn var ies to some extent dur ing per i -

ods of high- or low-s tand lake level. Thus, the

i sotopic compos i t ion of c a r b o n a t e lake deposi ts ,

which prov ides clues abou t the c a r b o n a t e forming

condi t ions , is cons ide red to be re levan t for char-

0037-0738/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

136 A. BELLANCA ET AL.

acterizing different stages of the evolution of a closed lake (Gasse et al., 1987; Janaway and Parnell, 1989). In addition, the clay mineralogy of mudstones deposited in lakes is commonly sensi- tive to hydrological changes in such a sedimen- tary environment (Millot, 1970; Jones, 1986).

Middle Miocene strata of lacustrine origin oc- curring in the central parts of the Madrid Basin, Spain (Fig. 1) were formed in a closed shallow- lake environment which extended Over a wide area (about 4000 km 2) of the basin (Calvo et al., 1989b). The lake deposits consist of a variety of mudstones, carbonates and, in places, gypsum which interfinger with clastic marginal lake facies and alluvial deposits (Calvo et al., 1989a). The vertical arrangement of the lacustrine facies clearly reflects changes in the lake palaeoenviron- ment that are thought to have been related to climatic fluctuations resulting in changes of water supply to the lake. An arid to semiarid climate has been suggested for the Madrid Basin during the MiddLe Miocene (Daams and Van der Meulen, 1984; L6pez Martlnez et al., 1987).

A geographically restricted area located to the south of Madrid, near the locality of Esquivias (Fig. 1), was selected to characterize the evolu- tion of the lacustrine sedimentary units on the basis of facies analysis, clay mineralogy, and de- termination of stable isotopes from carbonates. One of the reasons for the selection of the study area was the occurrence of thin diatomite de- posits, which constitute a rather rare feature in the Tertiary Madrid Basin (Bustillo, 1984; Calvo et al., 1988). Information from isotope geochem- istry relative to diatomites and associated rocks has proved very useful in reaching meaningful conclusions about depositional environments and/or diagenetic changes in them (Pierre and Fontes, 1978; McKenzie, 1985a; Bellanca et al., 1986; Bellanca et al., 1989). On the other hand, the clay mineralogy of lake deposits has been demonstrated to be a good indicator both of the weathering conditions in the surrounding drain- age areas and of the diagenetic reactions taking place in the lake water (Jones, 1986; Chamley, 1989).

D I A T O M I T E S . CLAYS

A N

SIL ~ AI

AN

MIC SA

ST SE

' ~ 0 o,5

80"

60-

40-

Lominoled carbonates and dio~omHes

Massive green and purple (sepiolite) mudstones. Silicified carbonates

Mudsione - c a r b o n a t e

cycles

Coarsening - thickening 20- upward cycles

(deltaic)

E S ~ , v , . ,~..~

Om

Micoceous sandstones

Green massive mudstones

Fig. 1. Geographic location and outcrop map of the Esquivias area with indication of the position of the general stratigraphic section (drawn at left) and the sections (ESA, ESQ, EST, ESR, ESP) referred to in this report (see Fig. 2 for correlation).

RECOGNITION OF LAKE-LEVEL CHANGES IN MIOCENE LACUSTRINE UNITS, MADRID BASIN 137

Stratigraphical sequence

North of Esquivias (Fig. 1), the stratigraphic sequence is composed of massive mudstones, mi- caceous sandstones, marls, chert, and carbonates. The lithostratigraphic log shown in the figure is interpreted to represent the transition from a marginal lake environment (sandstone-mudstone deltaic cycles) to a carbonate mudflat and to open lake facies. This investigation focuses on the upper levels of the section. Two different groups of materials are recognized: (1) massive green and purple mudstones which intercalate silicified carbonate beds, and (2) well-bedded carbonates, marls (most of them diatom-rich) and roughly laminated mudstones. The former group is widely extended in the area (Fig. 1), whereas the latter is restricted to small outcrops. Both of them have been studied from quarries where these materials are extracted for cement. The two groups will be referred to in further sections as Units I and II, respectively.

Materials and methods

More than seventy samples of carbonates, clays, marls and chert levels were studied from five sections located northeast of Esquivias (Fig. 1). Detailed logs of these sections and the corre- lation between them are shown in Fig. 2. The location of the samples is indicated in a summary log in Fig. 6.

The selected outcrops were analysed in terms of facies and facies associations, whole rock and clay mineralogy, and isotope geochemistry. Stan- dard petrographic thin sections supplemented the sedimentological information obtained from the outcrops. The mineralogical composition of the samples was studied by means of XRD analysis on both randomly oriented samples and ho- moionic Mg, ultrasonic treated suspensions ori- ented on glass slides.

XRD diagrams were obtained by a Philips 1040 equipment using CuKa radiation. X-ray diffraction traces of unoriented powders ranged from 2 to 60 ° (20). The range covered for ori-

EST

1oi

ESP

/ /

/ / ~3v ~

BIOCLASTJC LIMESTONES

DOLOSTONES

NODULAR CARBONATES AND MARLS

MUDSTONES

CONGLOMERATE AND INTRACLASTIC MUDSTONES

ESR

/

,-I

ES¢

MARLS

DIATOMACEOUS MARLS

~ ROOT BIOTURBATED LIMESTONES

: ~ LAMINATED BIOCLASTIC LIMESTONES

CHERT

ESA

Fig. 2. Schematic illustration of the stratigraphic relationship among the studied sections. See Fig. 1 for location in the sketch map.

138 A. BELLANCA ET AL.

ented samples was 2-30 ° (20). The clay fraction (< 2 ~m) was separated by standard sedimenta- tion methods (Tucker, 1988). Four different treat- ments were made for each sample: (1) air-dried mounts, (2) solvation with ethylene glycol vapour for 24 h, (3) heating at 550°C, (4) unoriented sample for do6 o parameter determination.

Semiquantitative estimates of relative percent- ages of both bulk and clay mineralogy in the samples were made using the methods of Schultz (1964), Van der Marel (1966) and Barahona (1974).

The oxygen and carbon isotopic composition of CaCO 3 minerals and dolomite were deter- mined by the following procedure: the samples to be measured were first heated under vacuum at 400°C and treated with 100% H3PO 4 at 25°C for variable periods (12 h for calcite, 72 h for dolomite) according to the mineral present. After the beginning of the reaction, CO 2 gas was col- lected at time intervals of 20 min for calcite and 72 h for dolomite. Isotopic analyses were per- formed with a Varian Mat 250 mass spectrome- ter. The results are expressed in 3%0 units and reported against the PDB-1 standard. The repro- ducibility for the isotope determinations was _+0.1%o (ltr) for 3180 and +0.05%0 ( lg) for 313C.

Results

Both mapping and detailed sedimentological analysis of the outcrop sections allow the conclu- sion that two facies associations can be differenti- ated in the study area (Fig. 2). Stratigraphically, the lower facies association (called Unit I in this paper) is separated from the upper one (Unit II) by an erosion surface directly overlain by carbon- ate and soft mudstone clasts (Fig. 3). This usually thin conglomerate bed is not present in all the area, but in many places the limit between the two units is a hummocky surface developed on chertified nodular carbonates.

Facies description Unit I

The facies that form this unit have been anal- ysed in section ESA, ESQ, and ESR (Fig. 2). They include five main lithofacies types:

Nodular carbonates and marls This facies comprises white to pale brown,

strongly brecciated, nodular to massive carbon- ates and marls. The carbonates and marls form 20-135 cm thick beds which usually show lower and upper irregular surfaces. Two types of nodu-

Fig. 3. Outcrop photograph of the EST section showing the erosion surface between Units I and II as well as the basal

conglomerate deposits of this latter unit. Height of the hammer is 31 cm.

RECOGNITION OF LAKE-LEVEL CHANGES IN MIOCENE LACUSTRINE UNITS, MADRID BASIN 139

lar structures occur in these beds: (1) irregular, cube-shaped nodules, and (2) closely packed, ver- tically oriented prismatic nodules. Rootmarks are commonly observed within this facies. The car- bonate content varies between 45 and 100%. The predominant carbonate mineral is low-Mg calcite.

Under the microscope, the carbonates consist of dense to crumby micrite with fractures due to both desiccation and roots (Fig. 4 a). The frac- tures are usually filled by fine sparite mosaics and/or phyllosilicate aggregates. The micrite contains minor amounts of silty to sandy feldspar and quartz grains as well as sparse fossils (plant debris, ostracods, gastropod debris). Palaeoe- daphic features, such as small pisoliths, man- ganese rims, and clay cutans are frequently recog- nized. In some levels, the nodular carbonates and marls are partially replaced by chert. The nature of this chert is described below.

Massive dolostones Dolostones are not common in the studied

sections. The occurrence of white, massive dolo- stones is restricted to the lower part of the ESA section (Fig. 2). The dolostones form 25-80 cm thick beds and are interbedded with greenish mudstones. The dolostone beds are massive to nodular and exhibit root-bioturbation. They are characteristically formed of non-stoichiometric dolomite. The texture consists of crumby dolomite (average size of crystals about 5-7/zm) which is disrupted by bioturbation traces. The fractures display phyllosilicate veneers and are filled with dolomicrosparite. Minor amounts of quartz, feldspar, chert and fine-grained mudstone clasts are included in the fine crystalline carbonate groundmass.

Bioclastic limestones Bioclastic limestones form a 75 cm thick bed

located at the lowermost part of the ESA section (Fig. 2). This is a unique lithofacies within Unit I, that, in general, is formed of deposits barren of fossil skeletons. The bioclastic limestone contains abundant gastropods as well as ostracod shells within a carbonate matrix. Both micrite and skeletal components are widely recrystallized to microsparite and/or sparite mosaics. The carbon-

ate content of this facies ranges from 85 to 100%, the noncarbonate fraction being made of clays and quartz traces.

Mudstones Mudstones occur interbedded with nodular

carbonates and marls. Contacts between these lithofacies are usually gradual. The thickness of the single mudstone beds is within the range of 10-155 cm. Beds containing more than 75% of phyllosilicates and/or very fine-grained sediment are scarce. Two types of mudstone facies have been recognized: (1) dark green (light green upon weathering) massive clays exhibiting a soapy ap- pearance; (2) pink to purple, conchoidally fractur- ing, massive clays. Both types of clays include small amounts (5-10%) of silt-grained quartz.

Under the microscope, the pink to purple clays consist of a groundmass of phyllosilicates (Fig. 4b) characterized by a desiccated texture. The fractures are usually recrystallized. Sparse plant debris and ostracods are the only fossil remains in this facies.

As discussed below, the mineralogy of the green clays and the pink-purple clays is domi- nated by smectites and fibrous clays, respectively. Palygorskite is especially abundant in the upper levels of Unit I whereas sepiolite occurs both in these upper levels and towards the bottom of the section (Fig. 6).

Chert Chert occurs in different beds in Unit I (Fig.

2). Two chert beds are traceable over much of the study area. In addition, other chert occurrences are as nodules which are areally restricted. In both cases chert replaces limestones and marls (Fig. 4c). The extensive chert beds display irregu- lar upper and lower surfaces. The thickness varies from 45 to 75 cm, though changes in thickness can also be observed along the same bed. The chert is pale brown to greenish and shows a massive to strongly brecciated structure. A minor amount of the primary carbonate material is pre- served within the beds. The texture of the chert consists of massive opal (opal-CT) that usually displays fracturing (Fig. 4d). In that case, the

r:l

Z

RECOGNITION OF LAKE-LEVEL CHANGES IN MIOCENE LACUSTRINE UNITS, MADRID BASIN 141

fractures are filled by both microcrystalline quartz and chalcedony.

The discrete nodules that are spaced out along some beds show similar textures and mineralogi- cal features. They grade into the enclosing car- bonate and exhibit either a massive or a brec- ciated structure. The shape of the nodules varies from spherical to elongated.

Interpretation of the facies associations in Unit I

The deposits of Unit I show sedimentary fea- tures that suggest a very shallow lacustrine origin. The widespread occurrence of desiccation struc- tures, rootmarks and other pedogenic features (pisoliths, cutans) within the nodular carbonates and marls as well as in dolostones clearly indi- cates that the deposits underwent emersion after deposition in the lake system. A model of fluctu- ating lake water level as well as changing hydro- chemical conditions is supported by variations of mineralogy and facies upwards in the sections (see Fig. 6 for an integrated view). Bioclastic limestones are envisaged as the fresher, and per- haps deeper, deposit in the lake during the sedi- mentation of Unit I. Dolostones were deposited in more concentrated waters as is also deduced from the formation of magnesium-rich smectites which occur interbedded with the dolostones. The interpretation of these clays is discussed below.

Conglomerate A laterally discontinuous conglomerate deposit

is located at the basal part of Unit II in the study area. The conglomerate occupies depressions that were dug out on the underlying nodular carbon- ates and marls. The lateral extent of the depres- sions reaches a few tens of metres. The maximum thickness of the conglomerate is 1.5 m. The com- position of the clasts is essentially carbonate and soft mudstone pebbles. The diameter of the clasts ranges between 2 and 10 cm. The fabric varies from clast- to matrix-supported (Fig. 3).

Intraclastic mudstones This facies comprises well to crudely lami-

nated, brown-greyish mudstones that contain abundant pebbles and sand-grained clasts of car- bonate and clay composition (Fig. 5a). These clast-bearing mudstones occur as either horizon- tally laminated beds (10-45 cm in thickness) or lenticular fine conglomerate bodies with erosive bases. When thinly laminated, the intraclastic mudstones exhibit textures that are characterized by sub-millimetre-thick laminae of mudstone and micrite grains (intraclasts) as well as minor quartz grains (Fig. 5b).

This facies occurs towards the lower part of Unit II (Fig. 2), where it is commonly associated to the basal conglomerate, or is interbedded with marls in upper levels in the sections.

Facies description Unit H

The deposits that form this unit have been studied in sections EST and ESP (Fig. 2). They include the following seven lithofacies types.

Laminated marls This facies comprises white to greyish coloured,

millimetre-thick (0.10-0.46 mm), horizontally laminated marls. The thickness of the beds varies from a few cm to 60 cm. The carbonate content is

Fig. 4. a. Photomicrograph of nodular carbonate facies. It shows a crumby micrite groundmass dissected by microspar-filled cracks,

due to desiccation, and some small pisoliths (arrowed) originated in vadose conditions. Scale bar = 0.5 mm. b. Photomicrograph of massive mudstones which include sparse quartz grains. Phyllosilicates are arranged according to a bimasepic fabric. Fractures are

filled by recrystallized clays. Crossed polars. Scale bar = 0.5 mm. c. Outcrop photograph of nodular carbonates bearing discrete,

separate chert nodules. Shape of the nodules varies from spherical to elongated. Length of the pencil is 15 cm. d. Photomicrograph of typical texture of chert nodules of Unit I. Massive opal-CT (dark) displays pervasive fracturing that is filled by quartz chalcedony.

Crossed polars. Scale bar = 0.5 mm.

t~

RECOGNITION OF LAKE-LEVEL CHANGES IN MIOCENE LACUSTRINE UNITS, MADRID BASIN 143

Fig. 5. a. Close-up view of intraclastic mudstones displaying crude lamination as well as internal small-scale erosive surfaces. Grading is recognizable in some layers (arrowed). Dark and white clasts correspond to fragments of mudstones and carbonates, respectively. Visible length of the pencil is 13 cm. b. Texture of the intraclastic mudstones. The rather well-defined lamination is chiefly due to the orientation of micrite flakes. Light grains are quartz and mudstone clasts whilst the dark ones are carbonate

grains. Scale bar = 0.5 mm. c. Photomicrograph of laminated diatomaceous marls. Oriented diatom frustules are encased in micrite; many of them are calcitized. Light areas in the photograph are fine sparite mosaics. Scale bar = 0.05 mm. d. Photomicrograph of massive diatomaceous marls. The randomly oriented diatom frustutes are encased in a mixture of micrite and phyllosilicates. Small circles correspond to siliceous spicules. Scale bar = 0.25 mm. e. Photomicrograph of laminated bioclastic limestones. The lamination is due to the orientation of ostracod valves and flattened charophyte stems; lamination is enhanced by the alternation of micrite and bioclastic laminae. The crenulated appearance is thought to result from loading in early burial stages. Scale bar = 0.5 mm. f. Outcrop photograph of root-bioturbated massive limestones. Empty root holes cross down both carbonates and thin diatomaceous marly beds. Note the general tabular geometry of the single beds that are topped by irregular upper surfaces. Length

of the pencil is 15 cm.

usua l ly low ( 1 8 - 5 2 % , t h e l a t t e r v a l u e on ly ob -

s e r v e d in o n e s a m p l e ) a n d it is c lose ly r e l a t e d to

t h e p r e s e n c e o f o s t r a c o d s , r a r e c h a r o p h y t e s t e m s ,

d e t r i t a l c a r b o n a t e g ra ins , a n d smal l a m o u n t s o f

mic r i t e . T h e b ioc l a s t s a n d g r a i n s a r e i n c l u d e d in

l a m i n a e t h a t a r e m a i n l y f o r m e d o f phy l los i l i ca t e s .

144 A. BELLANCA ET AL.

In many cases, particularly in the lower part of the section, the phyllosilicates consist of sepiolite and/or palygorskite.

Massive m&stones The massive mudstones occur in lenticular to

laterally discontinuous beds which occasionally

UNIT II

UNIT I

CLAY MINERALOGY

-1 (SP)

COMPOSITE LOG

SAMPLES STABLE ISOTOPES

-8 -4 0 -8 -4 0

-8 -4 0 -8 -4 0

0 CALCITE

. DOLOMITE

T E

t D

+ C

i 8 1 A

J-

Fig. 6. Integrated graph showing the composite log from the sections studied in the Esquivias area, with profiles of isotopic delta

values and clay mineral composition of the different levels. Clay minerals indicated in brackets (Sp = sepiolite; Sm = smectite;

k = kaolinite) have subordinate occurrences (5%). See text for significance of A, B, C, D, and E.

RECOGNITION OF LAKE-LEVEL CHANGES IN MIOCENE LACUSTRINE UNITS, MADRID BASIN 145

exhibit an internal prismatic structure. They are grey to green in colour. Traces of millimetre-thick laminations are occasionally observed. The thick- ness of the beds varies from a few centimetres up to half a metre. The mudstones are chiefly formed of homogeneous clays with minor amount of silty quartz. A few carbonate and/or mudstone intra- clasts are also included in some mudstone beds. As discussed below, the clay minerals that form the mudstones are exclusively dioctahedral smec- tires. The carbonate content is less than 10%.

Diatomaceous marls This facies comprises white to pale yellow marls

that contain a large amount of diatom frustules. The beds have lower and upper planar surfaces and are laterally continuous. Three different lev- els in the section (Figs. 2 and 6) belong to this facies. Two of them are located towards the mid- dle part of Unit II and consist of white to cream, thinly laminated diatomaceous marls (Fig. 5c) which grade upwards into more massive, lime-rich marls (Fig. 5d). The third diatom-rich deposit occurs at the uppermost part of the section (sam- ples 59 to 71 in Fig. 6). In this location, the diatomaceous marls are white and characteristi- cally massive and are interbedded with more in- durated carbonate beds (root-bioturbated carbon- ate facies; see description below).

The diatomite beds are composed of biogenic opal, calcite, and minor amounts of phyllosili- cates, quartz and feldspar. Biogenic opal corre- sponds to the diatom frustules as well as to siliceous spicules, the latter being abundant in some laminae. Chrysophyte remains have also been recognized. Most of the diatoms have pin- nate shapes and they form densely packed aggre- gates mixed with phyllosilicates. The uppermost diatomite level is slightly richer in carbonate (5- 10%) and the diatoms occur mixed with some ostracods and charophyte stems and gyrogonites.

Laminated bioclastic limestones This facies comprises well-bedded, tabular

limestone beds which occur both in the middle and upper part of Unit II. The thicknesses of the

two levels are 75 cm and 1 m, respectively. The limestones are white but when silicified, which is a very common feature in the carbonate beds, they show black to grey colours. The structure of the limestones is characterized by microlamina- tion. The thickness of the single laminae varies between 2 and 7 mm, although sub-millimetric laminae are frequently recognized. The lamina- tion is defined by oriented ostracod shells, bivalve fragments, squashed gyrogonites and partially crushed charophyte stems (Fig. 5e). The lamina- tion is also highlighted by oriented dark carbona- ceous debris. The limestones exhibit a packstone fabric.

Root-bioturbated massive limestones White massive limestones displaying vertical

rootmarks occur at the uppermost part of Unit II (Fig. 2). The limestones are interbedded with massive diatomaceous marls. The thickness of the limestone beds ranges from 10 to 30 cm. The length of the roots within the beds is of the same magnitude as the bed thickness, although the root holes occasionally reach the underlying marls (Fig. 5f).

The fabric of the limestones consists of bio- clastic wackestones to packstones in which ostra- cod shells, gyrogonites and charophyte stems are unoriented. Diatoms are not rare in this facies but they have been typically calcitized and their recognition in thin section is difficult. The root voids disrupting the limestone are filled up with phyllosilicate aggregates and minor quartz and mica grains.

Interpretation of the facies association in Unit H

Both the geometry and the composition of the clasts of the conglomerate at the base of Unit II indicate extensive reworking of the underlying strata. Resedimentation of carbonates and mud- stones from the neighbouring areas continued during the first sedimentary stages of Unit II leading to the deposition of intraclastic mud- stones. This facies shows features that are indica- tive of rapid deposition (sharply defined lower boundaries of the beds, grading) which is thought

146 A. BELLANCA ET AL.

to have taken place in shallow water. A progres- sive deepening of the lake would be indicated by the appearance of marls and carbonates upwards in the section. Both marls and carbonates are thinly laminated. The abundance of broken and oriented charophyte stems and of ostracod shells within the laminae strongly suggests transport from shallower lake areas that were densely colo- nized by freshwater biota.

The deposition of the diatomaceous marls took place under quiet conditions in open lake areas. These areas were submitted to fluctuations of the water level as demonstrated by changes in the diatom flora (S. Servant-Vildary, pets. commun., 1990) and by the occurrence of thin fine-grained clastic deposits interbedded with the marls. To- wards the upper part of Unit II, sedimentation in shallow but fluctuating water prevailed, leading to the formation of root-bioturbated carbonates and massive diatomaceous marls.

Clay mineralogy

The mineralogy of clays has been determined in detail from mudstones (both massive and intra- clastic), laminated marls, and diatomaceous marls. Clay composition of the different levels is sum- marized in Fig. 6. From bottom to top, six clay mineral associations have been recognized:

UNIT I: (a) trioctahedral smectite +illite + sepiolite; (b) sepiolite + trioctahedral smectite + illite; (c) dioctahedral smectite + illite + sepiolite; (d) palygorskite + sepiolite + smectite. The latter association is commonly accompanied by calcite and opal-CT. The italicized minerals correspond to the predominant clay mineral within each asso- ciation.

UNIT II: (e) sepiolite-palygorskite + illite + smectite; (f) dioctahedral smectite + illite + kaolinite + sepiolite-palygorskite.

A brief description follows of the characteris- tics of these clay minerals in the studied sections. Emphasis has been placed on the crystallinity of the clays and its variability in vertical sections.

Although in minor amounts, illite is a common clay mineral in all the studied sections. It is somewhat more abundant in beds made up of green massive mudstones, in which illite reaches

up to 25% of the total clay association. The crystallinity of illite is always very poor.

Kaolinite occurs only in small amounts ( < 5%) in the studied deposits. Its presence is restricted to some levels of Unit II. Characteristically, kaoli- nite displays very poor crystallinity.

Smectites are the main component of the green massive mudstones. Two different groups of smectites have been determined according to d060 values. Trioctahedral smectites are characterized by d060 = 1.52 A~t, whereas in dioctahedral smec- tites d060 is 1.49-1.50 A~i (Despraires, 1983). Tri- octahedral smectites occurring in this area of the Madrid Basin have been characterized as saponites (Galfin et al., 1986).

The trioctahedral smectites occur mainly in the basal levels of Unit I where they are associ- ated with dolostones. Traces of magnesite have also been determined in these levels. In contrast, dioctahedral smectites are mostly present in mud- stones and marls of Unit II, their occurrence being restricted to small amounts in the underly- ing unit. Occasionally they are mixed with triocta- hedral smectites, as suggested by mixed d060 re- flections in the X-ray diffraction traces.

The crystallinity of the smectites remains rather constant along all the section with mean values of 0.5, except for the transition from saponite to sepiolite in the lower part, where saponite has a crystallinity value of 0.28. On the other hand, dioctahedral smectites in the upper part of Unit II attain crystallinity values of 0.77.

Sepiolite is a common fibrous clay in lacustrine sequences of the Madrid Basin (Galfin and Castillo, 1984; Brell et al., 1985; Leguey et al., 1989; Ord6fiez et al., 1991). In the study area, sepiolite occurs in different levels of the strati- graphic section (Fig. 6). The purest sepiolite de- posits correspond to pink-purple, metre-thick mudstone beds located at the lower and middle parts of Unit I. Here sepiolite, which forms up to 95% of the whole deposit, exhibits very good crystallinity, as indicated by pronounced peak sharpness in X-ray diffraction traces. The pres- ence of sepiolite is especially noticeable below silicified carbonates and marls displaying palaeo- edaphic features.

In contrast, sepiolite has been recognized in

R E C O G N I T I O N O F L A K E - L E V E L C H A N G E S IN M I O C E N E L A C U S T R I N E U N I T S , M A D R I D B A S I N 147

minor amounts within the deposits of Unit II, except for the lowermost part of the unit where sepiolite occurs as a significant component of the intraclastic mudstones (up to 65% in some beds). A closer view of this facies shows that sepiolite is present as both intraclasts and phyllosilicate groundmass. In general, sepiolite that is included in the intraclasts shows lower crystallinity than the surrounding clay aggregate.

Although not so frequent as sepiolite, the presence of palygorskite in Neogene lacustrine and alluvial sequences of the Madrid Basin has often been reported (Gal~in and Castillo, 1984; Leguey et al., 1985; Pozo et al., 1985). In the study area, palygorskite is commonly found asso- ciated with sepiolite but it also occurs in separate layers in the section (Fig. 6). Palygorskite shows a good correlation with the silicified carbonates and marls of Unit I, where the fibrous clay occurs as mats lining ped surfaces and solution channels. Besides this occurrence, palygorskite forms some purer clay beds towards the top of Unit I. In both cases palygorskite shows good crystallinity.

Palygorskite has been found in significant amounts within the intraclastic mudstones that form the basal part of Unit II. Both intraclasts and groundmass of this facies contain paly- gorskite, the former showing poorer crystallinity.

As observed in Fig. 6, a significative variation of the clay mineralogy is recorded upwards through the Esquivias section. Some of the iden- tified clay minerals (trioctahedral smectites, sepi- olite, palygorskite) are interpreted to be of authi- genic origin and they would characterize strong to moderately saline lake environments. The scarcity of these clay minerals in the upper part of the section, where detrital clays are predomi- nant, clearly suggests a drastic change in lake conditions (see discussion below).

lakes are very complex hydrological and biologi- cal systems (Buchardt and Fritz, 1980).

The 180 content of these carbonates is closely related to the isotopic composition of the lake waters and to the temperature at which CaCO 3 precipitation occurs. Under dry climates, the 6180 signature of a lake is mostly influenced by the evaporation process producing an increase in heavy isotopes (Gonfiantini, 1986; Gasse et al., 1987).

The carbon isotope composition of lacustrine carbonates depends on the relative amounts of carbon of various origins dissolved in lake waters. A 13C enrichment may reflect the photosynthetic fractionation effect which is enhanced by high biological productivity and amplified by the estab- lishment of stratification in lakes (Stiller and Ma- garitz, 1974; Deines, 1980; McKenzie, 1985b). Relatively heavy ~13C values similar to those of marine limestones may also be due to lake water reequilibration with the atmospheric reservoir (Gonfiantini, 1986; Hoefs, 1987). The carbon iso- topic ratio of the dissolved bicarbonate in the lake water is also affected by the contribution of soil-derived CO 2 which has a wide range of strongly negative 613C values (from -10 to -28%o) depending on the photosynthetic cycle used by the vegetation (Salomons and Mook, 1986). Finally, during very early diagenesis, varia- tions in the carbon signature of lacustrine carbon- ates may be the result of reactions with bottom or pore waters in which lZC-enriched bicarbonate produced by the destruction of organic matter was dissolved.

A clear covariance between (~180 and 3~3C values is characteristic of carbonates formed in closed lakes while carbonates from hydrologically open lakes show little or no correlations between oxygen and carbon variations (Talbot, 1990).

Stable isotopes Isotopic characteristics of the Madrid Basin car- bonates

Factors controlling the isotopic signature of lacus- trine carbonates

Lacustrine carbonates have a wide range of oxygen and carbon isotopic compositions since

Carbon and oxygen isotopic compositions of the Madrid Basin carbonates cover a broad range of generally negative values with a spread of 6180 and t513C. The results of the isotopic study can be conveniently discussed in terms of fluctua-

148 A. BELLANCA ET AL.

tions along the lacustrine sequence of the Madrid Basin (Fig. 6).

The high 180 and 13C contents (+5.5 and + 2.9 6%o, respectively) of the non-stoichiomet- ric dolomicrites at the bottom of the sequence are consistent with a precipitation from waters enriched in heavy isotopes as a result of isotopic fractionation processes occurring during evapora- tion. These dolomites, which exhibit desiccation features and fenestral cavities, are thought to have formed in shallow lake fringes subjected to repeated periods of exposure. The processes in- volved in dolomitization of the lacustrine sedi- ment are probably those described by McKenzie et al. (1980) for sabkha dolomites which are in- herent in the model of capillary evaporation and evaporative pumping. Upwards in the sequence, dolomite lower in both 6180 and @13C formed according to the same model under less severe conditions. In fact, more intense freshwater sup- ply is reflected in the light isotopic values of the calcite (6180 about -6.5%o; @13C around -9.5%0) from the limestones interbedded with the dolostones.

Overall this picture indicates that, in a first stage (interval A in Fig. 6), the lake was low enough in level to be subject to strong water concentration and to desiccation. Such conditions imply periodic hydrological closure which is also indicated by the covariance between 6180 and @13C.

In the middle part of Unit I (B in Fig. 6), over the interval consisting of nodular carbonates, the @180 undergoes small excursions around a mod- erately low value (-5%0). It then rises abruptly to a value as high as 1.5 to drop to values of around -6%o in samples 21-23. In the same interval the @13C values exhibit wide and fre- quent excursions which commonly are not paral- lel to those of @180. A low @180 together with high @13C is interpreted as indicative of well- mixed shallow waters, low biological activity and short residence time. The peak in the positive excursion of the 6180 coinciding with a low @13C probably records a time interval in which the waters in the lake were very shallow, concen- trated by evaporation and stagnant enough to favour the local production of biogenic CO z.

The absence of a link between oxygen and carbon isotopes, in this interval of Unit I, proba- bly reflects a marked instability in the hydrology of the lacustrine system.

In the higher portion of Unit I (C in Fig. 6), the isotopic pattern is characterized by a parallel positive excursion in the oxygen and carbon iso- topes. The highest 6180 and @13C values in this interval are observed in a sample showing evi- dence of pedogenic activity and point to in- creased evaporation and exchange with atmo- spheric carbon dioxide as well as dry conditions during pedogenesis. This scenario is indicative of a lake with a slow turnover and more and more reduced water supplies leading to a shallowing of the lake level and culminating in a stage of sub- aerial exposure.

In the lower portion of Unit II (D in Fig. 6), dominated by conglomerates and intraclastic limestones (samples 34-46), a moderate variabil- ity in 3180 corresponds to a small spread in @13C. Even if fluctuating, the oxygen isotopic values define a trend towards more negative values up- wards in the interval, thus reflecting a progressive deepening of the lake.

Starting from the first appearance of the di- atom-rich deposits, the changes in both 180 and 13C become concomitant and mostly small and regular (see interval E in Fig. 6). The values of 3180 and 613C average rather negative, indicating respectively low evaporation and high primary productivity. The picture is that of a permanent shallow lake fluctuating in level depending on periodic (seasonal?) flood discharge. The abrupt rise in isotopic values at the top of the section, coinciding with the deposition of root-bio- turbated carbonates, records a sudden shallowing of the lake.

Discussion

Evidence for fluctuations in lake level from sedimentary sequences of palaeolakes constitutes a field of study that has been developed in recent years. Most of the published information con- cerns hydrologically closed lake basins which are characterized by large water-level variations re- lated to climatic shifting and/or tectonic or geo- graphic readjustments (Spencer et al., 1984; Gasse

RECOGNITION OF LAKE-LEVEL CHANGES IN MIOCENE LACUSTRINE UNITS, MADRID BASIN 149

et al., 1987; Stine, 1990; Currey, 1990). The Es- quivias section studied in this paper offers a good example of a sharp change within a lacustrine sequence which is interpreted as a result of a marked transgression of the Tertiary palaeolake. This interpretation is supported by sedimentolog- ical analysis as well as both isotopic and miner- alogical criteria. The study case is thought to be significant for modelling of past fluctuations in closed lake basins. The causes for the water-level rise may be found in a shifting towards more humid and probably cooler climatic conditions at the beginning of the Late Miocene (Lower Valle- sian) in the western Mediterranean area (Muller, 1984; L6pez-Martfnez et al., 1987).

In the Esquivias section, two different stages of a palaeolake are represented by Units I and II. Sediments of Unit I were deposited in a shallow- lake environment whose recent analogue may be found in some lake areas of the western United States (Jones et al., 1986; Hay et al., 1986). The

similarities with those lakes lie in the occurrence of rather specific clay mineral assemblages (fibrous clays, Mg,rich smectites) and in the re- semblance of general facies associations (Khoury et al., 1982; Hay et al., 1986; Ord6fiez et al., 1991). Apparently minor fluctuations in lake level took place during the deposition of Unit I as suggested by the common presence of features indicative of emersion of the lacustrine deposits. Changes in both clay mineral assemblages (see discussion below) and isotopic values determined in carbonates also support this assessment.

By contrast, sediments of Unit II characterize a deeper-lake palaeoenvironment. The abrupt contact between Unit I and II is interpreted to represent rapid lake rise. Initially, the lake rise was accompanied by energic water inflow lying onto a nearly desiccated lake area, accounting for extensive reworking of the substrate. As a result of lake transgression, open lacustrine facies com- prising well laminated marls and limestones accu-

UNIT II STAGE-Il l

KEY FACIES AND CLAY MINERAL

BULK MINERALOGY ASSEMBLAGES

MEAN AND STANDARD

DEVIATIONS OF ISOTOPIC DATA

5180 .~13C

GENERAL EVOLUTION

OF THE LAKE SYSTEM

laminated bioclastic deeper and limestones i = -5.83 ~ = -7.59 fresher lake

diatomaceous marls ~ conditions intraclastic mudstones Sm di - 1 - (Sp-Pk-K) cr = 1.47 o= 3.14 with minor

and gravel fluctuations low-Mg calcite (n = 25) biogenic opal

lake transgression

nodular carbonates ~ = -4.67 ~ = -5.99 ) and marls with

S'I'AGE-II pedogenic features Pk-Sp- Smdi t~ = 2.36 c = 3.14 ~ minor calcite fluctuations inorganic opal-CT (n = 17)

UNIT I / dilution

and marls with S'I-AGI::-I pedogenic features Sm tn" -Sp- (I) t~ = 3.56 t~= 5.08

dolomite (n = 4)* on

Fig. 7. Summary of the main evolutionary stages of the lake system and their correspondence with l i thostratigraphic units, Clay

assemblages for each stage are formed of predominant and subordinate (in brackets) clays. Isotopic values (8%0) are expressed as

mean and standard deviations; n = number of samples; asterisk indicates isotopic values obtained from dolomites . A good

correspondence between more negative values and more diluted or fresher stages can be deduced from the graph.

150 A. BELLANCA ET AL.

mulated in the deeper water body. This was ac- companied by a significant flourishing of biota, both fauna (gastropods, ostracods, siliceous sponges) and flora (diatoms, charophytes). Minor fluctuations in the lake level are also recorded during the deposition of Unit II. Relative low- stands of the lake in this period are indicated by intercalations of root,bioturbated carbonates and mudstones that were affected by incipient pedo- genic processes.

A summary of the main stages of lake evolu- tion from the Esquivias section is presented in Fig. 7 (see Fig. 6 to check lithostratigraphic log). Besides the two main stages represented by Units I and II, a differentiated episode of lacustrine sedimentation is determined in the lowermost part of the sequence in view of the mineralogical composition and geochemical insight. Some re- marks on the three main lake stages are as fol- lows.

Stage I. The presence of dolomite forming some calcareous deposits at the lowermost part of the section, the heaviest 3 1 8 0 and t~13C values, and the occurrence of trioctahedral smectites characterize this stage of the lake evolution. All these features are consistent with a precipitation from shallow lake waters submitted to severe evaporative conditions.

Trioctahedral smectites (saponites) are inter- preted to be of authigenic origin by transforma- tion of inherited dioctahedral smectites (Galfin and Castillo, 1984; Doval et al., 1985). The mech- anism of clay diagenesis would be similar to that proposed by Jones and Weir (1983).

The relatively low crystallinity index (B.I. = 0.50) of the trioctahedral smectites suggests insta- bility of the smectites, which is increased (B.I. = 0.28) in the transition to sepiolite, in levels where both clay minerals coexist. Detailed observation by SEM of this transition (Leguey et al., 1985, 1989) allows the conclusion that sepiolite forms from trioctahedral smectite through a dissolu- tion-precipitation process such as that described by Khoury et al. (1982). As pointed out by several authors (Khoury et al., 1982; Hay et al., 1986; Darragi and Tardy, 1987), this mechanism is favoured by lower salinities than those required to form Mg-rich smectites. These conditions, im-

plying progressive dilution of the lake waters, are also supported by the depletion of 180 and 13C

contents upwards in the section. Stage H. During this stage, carbonates, marls

and mudstones were deposited in a very shallow lake submitted to periodic fluctuations in the water level. The occurrence of pedogenically formed nodular chert partially replacing some carbonate levels also attests the shallow character of the lake area (Bustillo and Bustillo, 1988). Calcite is the predominant carbonate mineral in both carbonates and marls, which clearly show vadose diagenetic features. Variations of 3180 and ~13C values along the section are also indica- tive of minor fluctuations that are sometimes masked in field and petrographical analysis.

On the other hand, the clay mineral assem- blage is also helpful to interpret palaeoenviron- mental conditions during this stage of lake evolu- tion. Thus, the palygorskite~-sepiolite-dioc- tahedral smectite association leads to the follow- ing considerations. Palygorskite is the predomi- nant clay. This mineral is frequently associated in the section with degraded dioctahedral smectites, thus suggesting formation of palygorskite from these detrital clays. In fact, mudstones formed exclusively of dioctahedral smectites exhibit a higher crystallinity index. The transformation from smectite to palygorskite has been reported by several authors (Yaalon and Wieder, 1976; El-Prince et al., 1979), this transformation being accomplished through dissolution-precipitation processes (Velde, 1985). The presence of subordi- nate sepiolite, often associated with palygorskite, would be indicative of brief stages of higher arid- ity leading to the leaching of aluminium and subsequent formation of silica-magnesium gels.

Overall this picture indicates short-term varia- tions of the hydrochemistry and palaeomorphic features of the lake. Stage II represents a period of more diluted waters than those deduced from Stage I. Nevertheless, reduced drainage into the basin lake as well as relatively arid climatic condi- tions would also prevail.

Stage III. The facies association that forms the upper part of the section is clearly indicative of deposition in deeper-lake conditions. Besides iso- topic evidence, the clay mineral assemblage, con-

RECOGNITION OF LAKE-LEVEL CHANGES IN MIOCENE LACUSTRINE UNITS, MADRID BASIN 151

stituted by predominant dioctahedral smectites and illites, is also consistent with this interpreta- tion. The occurrence of sepiolite and palygorskite in intraclastic mudstones and laminated marls may be interpreted as a result of reworking of the underlying sediments, although a cementing phase made of these fibrous clays has also been recog- nized in these deposits. The fibrous clay cement can be explained by "abrading" of the intraclasts in periods of relative dryness at the beginning of Stage III. Such a mechanism of formation of fibrous clay cement has been described by Khoury et al. (1982) and Pozo and Martin de Vidales (1989).

More diluted conditions, consistent with the enlargement of the water body and establishment of a permanent lake environment, led to the development of a "freshwater" biotic community in both relatively marginal and central parts of the lake. The sedimentation of open lacustrine facies was mainly contributed by biogenic carbon- ate, represented by calcified charophyte debris and other skeletal remains, as well as by siliceous tests. Flood discharge into the lake accounted for the deposition of detrital clays (dioctahedral smectite, illite, minor kaolinite) and small amounts of mica, quartz and feldspar grains. The occurrence of fibrous clays as traces in uppermost levels of the section (Fig. 6) can be simply ex- plained by drainage of outer basin lake areas. Thus, the clay mineral assemblage recognized in this stage is remarkably different from those de- termined in underlying levels. No evidence of authigenic processes affecting clays has been recorded in deposits of Unit II, which is consis- tent with the dilute nature of the lake water during this stage.

Conclusions

The two lithostratigraphic units recognized in the Esquivias section correspond to different wa- ter-level conditions of a palaeolake system. This differentiation is based on the sedimentological features of the deposits that form each unit and on the variations observed in the isotopic record through the section. In addition, clay mineral assemblages show drastic changes that are in all

compatible with the previous assessment. The presence of dolomite and trioctahedral smectite, accompanied by high 180 contents of the carbon- ates, in the basal part of the section indicates a period of strong evaporative conditions in a low- stand lake (Stage I). A transition to a shallow-lake environment with more diluted water (Stage II) is recorded from the depletion of lSO contents as well as from the wide occurrence of fibrous clay minerals. In this setting, palygorskite formed by transformation of previously deposited dioctahe- dral smectites.

A lake transgression is marked by a sharply erosive surface in which gravel- and sand-sized reworked clasts were deposited. The resulting deeper and fresher lake conditions (Stage III) favoured the development of a flourishing com- munity and the accumulation of both laminated and massive bioclastic limestones and marls. The permanent water body underwent periodic flood discharge as indicated by concomitant fluctua- tions of the isotopic oxygen and carbon composi- tions through the section. Most of the clays of this stage are of detrital origin. The lack of authi- genically formed clays is consistent with the high- stand lake situation proposed for this stage.

To conclude, the Esquivias section constitutes an appropriate example to describe the effect of lake-level changes in the resulting sedimentary sequence and provides evidence of the combined variation of isotopic contents, clay mineralogy and facies related to such phenomena.

Acknowledgements

We thank Dr. S. Servant-Vildary for her valu- able comments on the flora of diatoms collected in the section. Dr. A.M. Alonso Zarza and E. Sanz are thanked for both scientific and technical help. We are also grateful to Dr. Blair F. Jones for his field assistance and for furnishing some chemical data. K. Kelts and V.P. Wright provided encouraging reviews of the paper. Dr. C.J. Dabrio assisted in the final improvement of the text. Help with photography came from J. Sfinchez, and A. Blanco made some of the drawings in- cluded in the paper. The work was supported by CICYT-CSIC Project PR-84-0078-C02-02, DGI-

\

152 A. B E L L A N C A E T AL.

CYT Project PB 87-0264 and by MURST (Ministero dell' Universit~ e della Ricerca Scien- tifica e Tecnologica, Italy).

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