M iocene se diments fr om Foz daF onte and Penedo sections

(1)

(

Cie ncias da Terra (lINL) Lisboa N." 14

pp.

19 1-20 2 2000

1 21 Figs.

Miocene sediments from Foz da Fonte and Penedo sections

(Lower Tagus basin): clay minerals and isotopic data

A.P. Stlva'!', F. Roch a'!', P. Legotnha'" ,

J . Pais'", M. Telles Antunes?'

&

C. Gomes'"

I - Departamento de Geociencias. Universidade de Aveiro , 3810 Aveirc , Portugal

2 • Centro de Estudos Geotogicos. Fac.C iencias Tec nolo gia, Quinta da Torre, 2825-114 Caparica, Portugal.

3 - Academia das Ciencias de Lisboa; Centro de Estudos Geo16gicos, Fac.Ciencias Tecnologia , Quinta da Torre, 2825- 114 Caparica, Port ugal.

ABSTRACT

K ey-wo rd s: Clay minerals; Isotopic data; Mioce ne; Lowe r Tagus Basin; Paleoenvironmental reco nst ruction.

Eight depositional sequences (DS) delimitedbyregional disconformities had been recognized in the Miocene of Lisbon and

Serubal Peninsu la areas . In the case of the western coast o f the Setu bal Pen insula, outcrops consisting of Lower Burd igalian to Lower Torto nian sed iments were studied. The strat igrap hic zonography and the environmental consi derations are main ly supported on data concerning to foraminifera, ostrac oda, verte brates and palynomorphs.

The first mineralogical and geochemical data determined for Foz da Fonte, Penedo Sui and Penedc Norte sedimentary seque nces are presented. These analyt ical data mainly correspond to the sedi ments' fine fractions. Mineralogical data are based on X-ray diffraction (XRD), carried ou t on bot h the less than3811mand2 11mfractio ns. Qualitative and semi-quan titativ edeterminationsof clay and non-clay minera ls were obtained for both fractions . The clay minerals asse mblages complete the lithos tratigra phic and paleoenvironmental data obtai ned by stratigraphic and palaeontological stud ies. Some palaeomagnetic and isotopic data are discussed and correlated w ith the mineralogical data . Multivariate data analys is (Princi pal Compo nents Analysis) of the minera logical data was carried out usi ng bo th R-mode and Q-mode factor analysis.

RES UM O

Patavras-c have: mincrais das argiles: dad os isotop icos: Miocenic o: Bacia do Baixo Tejo; reconstt nncao paleoambiental. No Miocenico das regioes de Lisboa e da Peninsula de Senib al havia m side reconhecidas o tto sequenctes dep os icionais (OS ) limitadas pa r disconfonnidades. A zonacao e as

reconsnrutcoes

paleoambientais foram obndas, essencialmente, atraves do estudo de foraminiferos, ostracodos, vertebrados e palinomorfos.

No prcsentc trabalho aprcsentam-se os primeiros resultad os referen tes

a

mincralogia e geoqulmicadasfrac~es finas des depositos mioce mcos de Foz da Font e, Pcnedo SuI e Penedo Norte. Os dados min eralogico s foram obtidos atraves ded i frac~il. o de raios -X das fracr;oes<38J.lm e<2 p m. For am feitas determin acoes semi-quantitativas e qualit ativas dos minerais argilosos e nao argilosos. As associa coes de mmerais argilosos foram utilizad as como com ple mento dos dados litos rratigra ficos e paleoarnbientais anter ionncnte definidcs atrav es de estudos de estratigra fia e paleontolo gia . Estabe lecem-se correlacocs com dado s paleomagneticos e de isotopes estaveis (C, 0 ) anterionnentcobtidos de alguns dos nlvcis estud ados. Foi levada a efeito ana lise mu ltivariada (analise de componentes principals] dos dados mineralogicos recorrendo aos facto res "R-mode" e "Q -mod e".

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I" Congresso sobre 0 Cenoz6ico de Portugal

~ R ESULTS AND DIS C USSION

GEOLOGI C AL SETTING INTRO DUCTION

Antunes

et al.

(199 9) point out a I'S rfl6S_{r age of 20Ma}

approximately, for the lower beds. Disconformitie s can be observed in the inte rmediate part of the sec tion.

The isotopic data (018

₀

_{and Ol lC val ues) show} _{cl ear}

changes between depositio na l sequences BO and BI (Fig. 4).

.L.

illite

+

smectite

+

kaol inite (15K );

b

illite

+

kaol inite

+

smec tite (IK S);

L

smectite

+

illite

+

kaolinite (S IK);

~ smectite +illite+kaolinite (S IK).

Foz da Fonte Sec tio n (Figs. 2 - 5)

Fine fractions of Lower Miocene (lower Burdigalian, depositional sequence BO) sediments from Foz da Fonte section c onsists o f non-clay minerals (quartz , phyllosilicates, calcite, plagioclase and K-feldspar), and clay minerals (illite, smectite, kaolinite and rare chlorite). The deposi tiona l seq uence B I mineral co mp osi tion

appears to be richer in carbonates and in opalClCT.It

shows sharp oscillations as far as smectite is conc erned and slight oscillations in illi te and kaolinite contents. Variations on the relative contents of the clay minerals (Fig . 3) allowed the definition offour mineralogical units in the Fe z da Fonte section (the most rep resented clay minerals arc underli ned) :

Fifty one sa mples, collected at the Foz da Fonte,

Penedo Sui and Pened o Nort e outc ro ps (Fig. 1), were

studied.

Th e mineralogical co mposition of the sedime nts, particularly of their fine fractions, was determined through

X creydiffraction (X RD) patterns correspondi ng to less

than

38

11m and

2

11m fractions. Qualitative and

semi-quan-titative determ ination of clay and non-clay minerals was carried out on both stud ied fractions.

Palaeomagnetic and isot opic p re vio usly obta in ed

(An tunes et al.,1996; 1998; 1999; 2000) are correlated

with mineralogical data.

Mul tivar iate dat a analyses (Principal Com po nents

Analysis) of the mineralogical data were carri ed out(R·

mode and Q-mode facto r analys is). M ATERIALS AND M ETHODS

Nort e section with the classical units "Hel vetian" Vla-b which occur in the Lisbon region. There is an erosion surface between beds 5 and 6, on which lies a conglomerate whose elements main ly consist of very abraded bivalve

casts with a black, ph osphat e-ri ch pa tina (L. Gaspar,

Marine Geology Lab., Portuguese Geological Survey). The conglomerate corresponding to bed 8 contains glauconite and fragments of phosphate crusts.

-Ir - ~-0.• ••

..

~

ATLAN TIC OCE AN

The oldest Miocene sediments outcrop on the cliffs at Foz da Fonte beach. Mioce ne sediments directly overlie Cretaceo us limestones at a low angle unconfonnity. They consist essen tialy of fossili fero us biocalcaren ites and marls. The lowermost beds include cobbles and pebb les of Cretac eous volcan ites and limestones. In the lower part of the section, a sandy bed corresponds to a regression event. The middle section presents detrital shallow facies and eroded su rfaces. One o f the se is overl ain by a remarkable concentration of oyster shells. The uppe r part of the section shows a marl/marly clay cyclic character.

Penedo Sui outcro p is exposed on the cliff at Bicas beach . Basa l b eds (fossil iferous biocalcarenites and sands tones with sedimentary discontinuities) are the same as in the intermediate part of Foz da Fonte section.

Penedo Norte outcrop is located about lKm to the north of Penedo Sui. Zbyszewski ( 1967) correlates the Penedo

Fig. 1- Geographic settingoftheareaunder study.

Detailed stratigraphic studies have been carried on in Lisbon and Set ubal Peninsula regions. Eight Miocene depos itional sequences (DS) delimited by regional disconformities had been established. In the western coast

o f the Setuba l Peninsula, Lowe r Bu rdigalian to

Lower Tortonian outcrops were studied. Environmental considerations that could be put forward were ma inly supported by data on foraminifera, ostracoda, verteb rates and palynomorphs.

In the p resent work, the firs t mine ra logical and

geochemical data for Fozda Fonte,

Pencdo

Sui and Penedo

Norte sedimentary sequences are presented

«

38

11m and

<

211m fraction s) (F ig.I). The clay mineral assemblages

were used as lithostratigraphic markers for correlation and paleoenvironmental reconstruction.

(3)

Ciencias da Terra (UNL), 14

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Fig. 4 - Foz da Fonte Sect ion. C and 0 isotopic composition (Antuneset aI.,1997 ).

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Fig. 2 - Foz da Fonte Section. Composition of the<38J,1m

sedimentary fraction.

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Once again, the isotopic curves(O I ~O and oUCvalues ) show clear changes between depositional sequences BO and B I and between B I and B2 (Fig. 7).

Pc ncdoSui_{Secti on (Figs. 6 - 9)}

Pen edo Nor te Sec tion (Figs. 10- 13) Lillite

+

kaolinite (IK) ;

bsmectite+illite+kaolinite (SIK); .l.smectite

+

illite

+

kaolinite (SIK ); :lillite

+

smcctite

+

kaolinite (ISK).

The mineral composition (fine fractions) ofthe Lower M io cene (B urdigalian ) depos itio na l sequence BO at Penedo Sui consis ts in calcite, q uartz, phyllosilicates, plagioclase and Kcfeldspar, and clay minerals suc h as smectite, illi te , ka olinite and rare chlorite . Mineral compo sition, on the passage to depositi onal sequence BI-, becomes richer in det rital minerals (quartz, phyllosilicates and plagioclase) and, in much less degree, in dolomite and opal C/CT. T he calcit e content diminishes; there are slight oscillations in smectite, illite and kaoli nite contents . Variations of the relative conten ts of clay minerals (Fig. 8) were used to recognize four mineralogical zones:

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-Fig. 3 - Foz da Fonte Section. Composition of the<2 urn

sedimentary fraction.

(4)

I" Congressc sobre 0 Cenozoico de Portugal

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Fig. 6 - Penedo SuI Section, -c381lm sedimentary fraction. Fig. 5 - Foz da Fonte: variations on the relative conte nts of the

main clay minerals K1J. Kaolin ite/Illite ; SII ·Smectite/Illite.

of quartz, phyllosilicatcs, plagi oclase and K-feldspar, and

opal ClCT and calc ite as accesso ry minerals . Smectite,

illite and kao linite are the clay minerals. The Burdigalian section becomes enriched in calc ite and (much less) in illite. The medium levels (Langhian, depositio nal sequence

LI)are characterised by a strong incre ase in do lomite in

the fin e frac tions, and by minor changes in the clay minerals asso ciations (slight increase in illite and loss of

smectite). TheScrravallianpartofthe section (depositional

sequences SI , S2 and T I) becomes much richer in calcite (almost monomineralic), with a strong increase of illite . T he relative contents of clay minerals were used to

characterize four mineralogical zones (Fig .11)on Penedo

Norte outc rop:

±.,. smectite

+

illite

+

kao linite (SIK);

i,illite

+

smecti te

+

kao linite (ISK );

2.,.illite

+

kaol inite

+

smecti te (IKS);

Lsmectite+illite+kaolinite (S IK).

Pectinid shells from bed

8

were87Srf':l6Sr dated(H.

Elderfield, Cambridge Univers ity) from 11.5 to 13Ma. Bed 9 is a medium grained, glauconite rich sandstone; K-Ar

age (C .

Regenc io

Macedo, Coimbra University) is

IO.97±0.25Ma(LowerTortonian) .Isotopic data(ol80 and

olle

values) do not show so shar p d isc onformities

between the depositional sequences as in the preceding outcro ps.

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Fig. 7 • Pencdo Sui Sectio n. Com position of the<Zum sedimentary fraction.

(5)

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Fig. 8 • Pened o SuI Sec tion, C and 0 isotopic composit ion

(Antuneset at.,1997).

Fig . I I - Penedo Norte Section,<2 p m sedimentary fraction .

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Fig. 9· Penedo Sui: variations on the relative contents

of the main clay minerals. K1I . Kaolin ite/Illite; SII - Smectite/Illite.

(6)

Facto r 3 expresses the alternation o f more or less marine environments (with events characterized by an intense detri tal sup ply) with other ones marked by

chemical sedimentation. Factor I stresses that there were transitions towards more proximal environments oflagoon

type, under a more tempe-rate climate; Factor 2 explains

post-depositional parameters .

The vertical evo lution of the Factor scores shows the impact of each Factor on the characterized zones and allows to recognize a paleoenvironmental evolution. Along Unit I, the analysis of the evolution of the three Factors scores points out to a transition from a distal marine environment to a more proximal one, with some lagoon episodes . In Unit 2 oscillations of the relative impact of each Factor can be detected. This points out to an evolution towards more distal marine environments. The evolution of the Factors along Unit 3 reveals the occurrence of lagoon and fluvial episodes, alternating with marine ones. During the de position of the sed iments of the Unit 4, environment was again a more distal marine one .

The B-modc factor analysis of the mineralogical data of the Penedo Sui section fine fractions (Fig. 14) allows the following conclusions:

• Factor I shows zeo lites (Zeo l), pyrite [Pyr] and siderite (Sid) in front ofKcfeldspar (K.F); • Factor 2 shows hematite (Hem), opal C/CT (Opal),

plagioclase (Plag) and dolomite (Dol); • Factor 3 shows calcite(Calc)

in

front of the

quartz (Qz) and phyllosilicates (Kaol, Sm, 111).

• Factor I shows calcite (Ca lc) in front ofquartz (Qz), phyllosilicatcs (Kaol, Sm, lJI), K-feldspar (K. F), plagioclase (Plag), anhydrite(An),opal

Clcr

(Opal) and dolomite (Dol);

• Factor 2 shows siderite (Sid) in front of halite (Hal); • Factor 3 shows pyrite (Pyr) in front of zeolites

(2<01). Sta tis tical a na lysis

T he P -mode factor analysis o f the Fo z da Fonte Miocene profile fine fraction mineralogical data (Fig. 12) supports the following conc lusions:

lHT 6

Fig. 12 • Pencdo Nolte section , C and 0 isotopic compositio n (AnlUncset al., 1996).

1° Congresso sobre 0 Cenczoicc de Portugal

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Fig.13 •Penedo Nolte: main clay minerals.KII •Kaoli nite! Illite ;

sn

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Smecntezlttne.

Factor 1 expresses depositional environments of less marine influence, that are more prox imal and more siliciclastic. Continental influence has been greater, with a few lagoon episodes, charac terised by some chemical sedimentation. Factor 2 expresses an evolution towards lagoon environments; Factor 3 expresses a sharp evolution of the conditions expressed by Factor 2.

(7)

Ciencias daTerra(UNL ).14

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Fig. 14 · Foz da Ponte profile : R-mode factor analysis of the fine fraction mineralogical data .

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F ig. 15 - Foz da Fonte : vertical evolution of the Factor scores O. C - Co ntinental; 0 - Marine, Distal ; P - Marine, Prox imal; L -Lagoon.

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10_{Congresso sobre}

0Cenoz6ico de Portugal

Factor, pointing out a gra dual evo lution towards a more proximal marine env ironment, with lagoon events. The evo lution of the three Fac tors along Unit 3 reveals the

pers istency of alternating lagoon and marine events .

Dur-ing the deposition ofUn it 4 environments became cl early marine.

T he applica tion ofg -modefactor analysis to the fine

fraction minera logical data of Pe nedo No rte section (Fig.

17)allo wsusto recognize :

• Facto r1shows ca lcite(Calc) in front ofquartz(Qz),

phyllosilicates (Kaol, Sm.,lIl),p lagiocl ase (Plag),

opal ClCT(Opal ). hematit e (Hem) and anhydrite

(An);

• Factor 2 shows side rite (Sid) in front of Kcfeldspar

(K -F) andpyrite(Pyr) ;

• Factor 3 shows dolomite (Dol)infront of hal ite

(H.I).

. 2

1

.3

1

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c.s

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Fig. 16 - Pencdo SuI profile: R-mode factor analysisof the fine fraction mineralogical data.

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Fig. 17 · Pencdo SuI: vertical evo lution of' the Factor scores. D - Marine, Distal; P ,... Marine, Proximal;L ~ Lagoon.

(9)

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Fig. 18 - Pcncdo Norte profile:Rcrncdefactor analysis of the fine fraction mineralogical data.

3 ,-- - - ---,

2

1 o

-I

-2

PlLo..

, ,

_,

-0- FI

-0-

F2

o

UNIT 7

..•.. F3

14

34

21

23 UNIT 6

9

25

2 UNIT S

6.1 UNIT 4 L

1

Fig. 19 - Penedo Norte: vertical evolution oftheFactor scores

O.

D - Marine, Distal ; P - Marine, Proximal; L-Lagoon.

Factor I expr esses a depositional enviro nment

characterized by marin e influence. comprising episodes of intensive detrital supply. Th e presence of hematite in

this association (mo re clearlythaninthe other outcrop s)

may reflect some events of sub-aerian (flu vio-deltaic?) sedimentation. Fac tor 2 expresses a transition towards lagoon environments. Fac tor 3 exp lains pos t-de posit iona l variables typical of marin e environments.

The vertical evolution of the Factor sco res (F Ig. 17)

shows the impact of each Factor

in

thecharacterizedzones

recognizedinthe Penedo No rte section.Itpro vides

addi-tional data about pa leoenvironmental evolution. Along Un it 4 the evolution of the thre e Facto r scores points out to a transition from a proximal marine to a lagoon en

vi-ronment.In Unit 5, osci llations of the relative impact of

each Factor show an evolution towards a more marine environment. The evo lution of the Factors along Uni t 3 reveals a distal marine env ironment. During the depo si-tion of the sediments from Unit 4, environments reverted

tomore proximal mari ne one s.

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, 0Congresso sobre0Cenozcico de Portugal

CONCLUSIONS

I . Sedi mentsof Foz da Fonte section are characterized b y th e fo ll ow ing fe atu re s: illi te pr e dominat e s over smec tite, and kaol inite appea rs in significant amounts along all the sec tion . Clay minerals and non clay mineral asse mb lages po in t o ut to so urce areas o n whi ch a semi-arid clima te may have prevailed.

2. Sediments of Penedo SuI section arc charac terized by predomin ance o f smectite over illite. Kaolin ite becomes sign ificant in the lower strata of the section (chlori te is but vestigial).

3. Sediments ofPenedo Norte sec tion are characterized by smectite predominance. Ho wever, illite becomes a major clay mineral in the upp er strata of the sect ion just where dolomite contents increases. Compared to those ofthe other two sections, these sediments display features that are cha-racteristic of sedimenta tio n in a de eper sea envir onment.

4 . Clay mineral asse mblages allow to recogn ize seven mineralogical un its (Fig. 19). The vertica l evolution of the factor scores for eac h of the outcrops con firms the established zon ing.

It

also reveals the impact of ea ch Factor on the different d ist inctive zones, and therefore contributes to the characteriza tion o f a paleoenvironmental evolution (Fig. 20 ).

5. The time span of Unit 1 (Foz da Fonte and Penedo Sui sections) deposition is ch aracterize d b y tempe rate climate and weak hydro lysing conditi ons in the source areas. Environment was infralittoral.

6. Unit 2 ( Foz da F o nte an d Pe ne d o Sui) time span is cha rac terized by some cl ima te cha ng es a nd circalittoral deposition en vironments that evo lve to an infralitto ral one .

7 . Sedime nts of the Unit 3 (Fez da Fonte and Penedo Sui) reveal a sub tropical climate and a so mewhat more dista l infrali ttoral deposition environment.

8. Sedi ments o f the Unit 4 (a ll sections ) also reveal a subtro pical clima te and an infrali ttoral environment that changes to circalittora l.

9 . Uni t 5 ( Penedo N o rte) ind icate a d ep o sit ion environment similar to that for Unit 3 but under milder temperate conditions.

10 . Un it 6 (P ened o Norte) re vea l a c irca litto ral d ep osi t ion environ me nt character ized by still more temperate conditions.

I I . Finally, Un it 7 (Penedo Norte ) is re lated to a rnorc distal, infrali ttoral env ironmen t. Climate cond itions in the so urce a reas w ere typ ic a l o f mo re co ntras ting and hydrolysing conditions that were in favour ofsmect ic soils forma tio n.

[I]

tll ,

i

soz0 ... PONT I!

n S IK

.. ..

"

n

r. ""oIoNOM

5 " uNIT ,

H

"':ii' "

I " uN IT6

• n

I

"is"

PI!N Il DO SUI. 1 5 < U NIT ,

..~ ,J

_,

.. .

I 5 , UNIT .. 5 I ,

"

NIT" S I K

·-\v··f-- - - --1- - +!

I "

5 1 , UNIT3

UNIT 2

1'.;1.•.

f-

--1_ - +I

1 5< I , UN ITt

I -ILLITE

S - SMECTIT E

K • KAO LIN ITE

Fig. 20 - Clay mineral assemblages of the m.ineralogical units defin ed in the studied sections.

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:

Ciencias da Terra(UNL),14

FOZDA FONTE

PENEDOSUL

PENEDO NORTE

Units Marine

Hidrolysls

Marine

Hldrolysis

Marine

Hidrolysls

+

-

+ +

-

++

- -

+

I

s-Z

₇

c(

:::;

•

....J

:;

_I

ic(

a:

6 a:

w

Ul

\

z

_\

:$

J:

₅

e

Z

J

\

s

I

\

c

-,

z ·

₃

I?

/

c(

:::;

c(

Cl

c

_\

_~

a:

.;

::J

₂

In

• /

I

-

...

1 )

\

Fig. 21 • Paleoenvironmental evo lution.

(12)

10_{Congresso sobre}

0Cenozoic c de Portu gal REFERENCES

Antunes , M. T.; Civis,J.;Gonzalez-Delgado, 1.A.; Legoinha. P.; Nascimento, A.& Pais.J. (1996) - Miocene stable isotope s(011(),

~1lC) , biostratigraphy and environments in the southern limb of Albufeira syncline (SetUbal Penin su la, Portugal).Geogacera,

Madrid. 2 1:2 1 ~ 2 4 ,4 figs.

Antu nes, M. T.; Civis,J.;Gonzalez-Delgado,I.A.;Legoinh a. P.; Nascimento, A.&Pais,J. (199 7) - Lower Miocene stable isotopes

(SI_'O,_S"C),_{biostratigraphy and environments in the Foz da Fonte and}_Penedo_{sections (SetUba l Peninsula, Portugal).}_Geogaceta

23: 7-10 .

Antun es, M. T.; Elderfield , H.; Legoinha, P.; Nascimento,A.&Pais, 1. (1999) - A Stratigraphi c framewor k for the Miocene from the LowerTagus Basin (Lisbon, Setubal Pen insula, Portugal). Depositiona l sequences, biostratigraphy and isotopic ages.Bol. Soc.

oeot. Espana,

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Monte de Caparica : 159-160.

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U.S. Geol. Surv. Prof Paper,391-C ,1 ~ 3 \.

Thorez, 1. (1976) -Practical identification ofclay minerals.90pp . (Ed. G.Lelotte,Belg ique) .

Zbyszewski, G (1967) - Contributions

a

l'etude du Miocene de la serra da Arribida.Com. Servo GeotPormgal,Llsboa, LI: 37-148.