The thr ee main m arined epositionalc ycles of the N eogene of Portu gal

(

C ienci as da Terra (UN L) Lisboa N,"14 pp. 303-3 12 2000

J

.

.

.. . 2 Fig.

Th e three main marine depositional cycles of the

Neogene of Portugal

M. Cacha.

&

C. M. da Silva

Centre and Department ofGcolo gy, Faculty ofSciences, University of U sbon. RuadaEscclaP ol i l ~ n i c a, n- 58, P· 1294 Lisbca,Codex ,Portugal (mcaehao@ fc.ul.pt; Palco.Ca rloS@fc.ul.pt)

RESUM O

P a l . "r ~ bJl\le : Miodnico ; Pliocenicc; Estratigrafla; Paleogeografi a; Tect6nica; P lataforma costeira doAlgarve; Bacia do Sado; Bacia do Ba ixo Tej o; Bacia do Monde go .

o

modelo apresen tado correspondc: a uma stntese do estado actua l doconhecimentorelativo aos trabal hos estratigri.ficos e paleogeograficos que

tern

vindo • ser realizados nas reg iees , de norteparasuI: ( I) Bacia do Mond ego {litera l oest e de Porobel . CaldasdaRainha ), (2) regilo vesti bular da Bacia do Baixo Tej o (pen insu la de Setii.bal), (3) Bacia do Sad o e literal vicentino e (4 ) bo rdadura meridio nal algarvia.Ndese corrc:lacionam as principais sequ mcias sedimentares do Neogeni cc marinhoportugues tend o em vista0 seu enquadramcnto tectenc-eus taricc regional, no

"onshore"

e no

"offi llare".

o

modelo consideratrisciclos deposi cionais principa is, para 0 Neogenico marinho portu guCs, equivalentes a

sequencias

scdimen tares de 2- Ordem, comdura ~ o entre 5 e 8 Ma.

o

I Cicio esta completamente representado apenas na regilo vestibular da Bacia do Ba ixo Tejo. As sequ encias atnbuidas a este

ciclosse, esscncialmente , de jdade burdigaliana (Miodnico Inferior) e tcdo sido, provavelmcn te, originadas po r subsidencia isosLitica local. A aecn tuada variab ilidade das suas facies sugere a exis tencia na regilo de Lisboa de urn ambie nte dc transicac, de pequena pro fundidade, durante 0 Miocenicc Inferior. Instabilidade tcct6nica desta bacia ped e ter estadc na origem dc rapidos epis6dios regr essivos, de expressjc mais ou menos localizada . A s associecees f6sscis marin has (sir en ios . eorais, malac ofau na , macroforaminiferos bent6nicos) traduze m acentuado cani.ctcr terrn6fi1 o.

o

II Cicio esta muito bern representado na regiac vestibular da Bacia do Baixo Tejo , na plataforma costeira do Algarve e no litora l vicentino. TerA, ainda , influenciado a sedimentaeao no lite ral ocidental estremenho, mas de modo pouco significativo. A

sed i me n ~

e

essenc ialmcnte de idade langu iano-serravaliana (Mioceni co Medio) e tm sido dcsencadeada pelo acentuado eustatismo positive global que caractcri zou este intervale tempo ral. Atingiu maior expressli.o batimetrica na Bacia do Baixo Tej o, como0

atestam a maior uniformidadc e extensJ.o lateral das suas fi des e da.ssuas assoc ia¢es de micro e macrof6ssc is. A analise de lito e b io facies des sedimentos assoc iados eo II Cicio sugerem ter extsnd c accntuado gradient e termico das massas de aguaentria bo rdadura merid ional (facies forte mente carbonatadas com macroforam inifcros bentonicas) e a fachada oc iden tal (ficies detriticas) . Este cicjc termin a com importante fase co mpressiva betica que se mani festa a partir do final do Serravaliano , tendo resu ltado em importante lacuna dedepos i~ de sedimentos marinh os., ao lc ngo de todo 0 tenitono nae jonal , correspondente a ceres de 2,5 milh6es de mos.

o In

Cicio tern expresslo geogri.fica semelhante10anterior mas0inielo da sedimenlal,':1o marinha, condicionadaparabatimc:ntos

tcct6nicos diferenciados em cada bacia, migrade sui pan none, ao longodotempo.As primeiras unidades deste c iclo marinh o iniciaram a sua depos il,':1o, no sotavento algarvio (Fonnat;io de Caccia), no decurso do Tortoniano Superior (pri meira fase) enquanto que, mais a norte, naregiliode Pombal, a bese deste cicio (Formal,':!o de Camidel, data do Pticcenicc Superior, Placenc iano (tercei ra fase). Este facto est!provavelmente rc:lacionad o com arota~o dextrogira da Bacia de Guadalquivir e consequentemi gra ~ o da

d ef onna ~ o (subsiden cia) de sui para none, ao longo do territorio portugues.

AB STRA CT

Key. word s: Miocene; Pliocene; Stratigraphy; Paleogeography; Teclonics; Algarve Basin; Sado Basin ; Lower Tagus Basin; Mondcgo Basin.

1°Congressosobre 0Cenoz6ico de Portugal

to the Late Burdigalian the I Neogene Cycletestifya transgressive episode in the region of Lisbon and Serebel Peninsula. Rapid lateral facies variations suggestfIshallow marine basin. This cycle enos with fi n importantLateBurdigalian tectonic compressive event expressed by uplift of the surrounding areas and deformation affecting the Early Miocene deposits of the Arrib ida Chain.

TheIINeogene Cycle includes thick sedimentary sequences covering Paleozoic and Mesozoicformationsin the Algarve and Alvalade-Melides regions and it extends as far north as Santarem in the Lower Tagus Basin. Mainly controlled by global eustasy,it was generated by the important positive eustatic trend that characterized the Middle Miocene worldwidetowhich the Portuguese continental margin acted more or less passively. This cycle ended with a second and the most important compression event starting after the end of the Semva1lian affecting the entire Portuguese onshore and shelf'arees. This ledtoan important depositional hiatus of marine sediments for more than2.5Ma.

During the Early and the Middle Tortonianoccurredthe clockwise rotation of the Guadalquivir Basin. The thick marine units deposited afterwards in this basin produced a Iitostatic load, which seemstohave induced subsidence farther west resuming the Neogene marine sedimentation in theCacelaregion (Eastern Algarve), during the LateTortc nian.This marks the beginning of the IIINeogene Cycle. To the north, in the Sado Basin(Alvalade-Melidesregion), a similar depositional sequence starts itssedimen ta-tion during the Messinian. Further north, in the Pombal - Caldes da Rainha region, marine sedimentata-tion started during the Late Pliocene (Piacenzian). The migration in time, from south to north for the beginning of the marine sedimentation of this cycle is interpreted as reflecting a visco-elastic propagation of the deformation from the Betic chain northwards.

INTRODUCTION

For almost two centuries extens ive stratigra phic and pa leoecological studies have been pub lished a bout the Neoge ne o f Portu gal ma inly focus ing on the Lowe r Tagus Bas in (see Antuneset al.,1999). Howeve r, still few works try to approach theentirePortuguese marine sed imentary sequence s, their corre lation and paleogeogra phy in a

sin-gle frame work. Cunha ( 1992a , 1992b) pro po sed Tertiary alo strati grap hic units which related co ntine ntal and

rna-rine Neog ene se que nces fro m the Lowe r Tagus Ba sin nort hward s, dis tinguis hi ng de pos itional episo de s an d tectono-sed imenta ry stages. In this sa me year the entire mar ine Neogene of Portugal is integrated on a synthesis of the paleogeogra phic evolut ion of the Western coast of France, the Iberia and the Bcti c-Rifian realms (Alvinerie

et al.,1992) . Th e prese nt work propose s a synthesis for the mar ine

Neogene

un its of Portuga l foll owing the stra tigraphic model proposed byCecbao( 1995), whic h is her e updated . Based on a compilatio n ofthe a vailable data the pa leogeographic evo lution of the marine units of the N eogene of Portu gal is des cri bed an d some of its s tratigraphic and

tectonic

imp lica tions are

discussed.

A l-re ad y a tt e mpte d by pl-rev io us a ut hors (Bo urca rt &

Zbyszewski, 1940; Zbyszewski, 1964 ), the definition of major mar ine sedime ntary c ycles, herein des ign ated theI,

II andIIINeogene Cycles, is impo rta nt in order to filter lo c al i nt ra- b as in facies vari a t io n s from re gio na l depos itional sequences , this way enabling inter-bas in cor-relation . Conceptually these marine Neogene Cycles can

be

defined as 2nd order sedimentary

sequences

(sensu

Binsele& Ricken s, 199 1) and should be viewed as a step tow a rds a full in teg ra ti o n in to a s i ngle Seq uenc e Stratigraphy framework.

MAIN DEPOSITIONAL C YC LES Th e I NeogeneC ycle

The I Neogene Cycle is fully represented only in the Low er Tagus region. Lithostratigraph ic unit s 1 to III of

304

Co tter ( 1956), ran ging from the Early Aq uitanian to the Bu rd igalian (Antunes et af., 1999) testify a small scale tra nsgressive sequence in the region ofLisbon and Setubal Pen insula (Fig. I ,2).Inthis last regio n there is e vidence of ro cky shores on the western s ide ofthe Amibida region based on the bioe roded sur face ofFoz da Fonte (Silvaet

al.,

1999) most probabl y re lated with cretaceo us tectonic ac tivity aro und th e Sesimbra di ap iric s tr uc ture (se e Ku lberg et al., 1995 b; Kullberg & Ku llbe rg in press) .

Ma ximum mari ne incursion dur ing thi s cyc le see ms not to extend beyond the localities Alverc a (A) and Vila Franca (VF)(Fi g. 2). The Neogene unconfonnably overlies thick sequences of endorre ic Paleogen e co ntinental deposits (B enfica and S. Caetano Formations ; Az evedo, 1998) and other Mesozo ic un its . This cycle ends with the regressive facies of the uni ts

IVb,

Va and

Vb

de fined for the Lower Tagu s Ba sin by Cotte r ( 1956), whic h dates range fro m the Late Burdigalian to the Early Langhian according to Antuneset al .( 1999). Althoug h isotop ic ag es compatible with this time interval have been regi stere d (An tuneset aI.,1999) in theAJgarve platform stra tigraphica l evidences are sti ll co nt rad ic to ry (see

Cechao,

199 5,

Cachao

et al.. 1998).

Despite impo rtant thickness, rap id late ra l facies vari -atio ns testify that thi s marine basin was qu ite sha llow at this time . An tune set al. (thi s volum e) interpr et this as related to conti nuous tectoni c subsi dence during th is long time interval. Oth er authors s uggest that during the Early Mi ocene mar ine sed imentation inside the Low er Tagus Basin may ha ve been possible du e to the iso static re-equ i-Iibrium that followed the uplift of the surroun ding a reas during the Late Cre taceous and the Paleogene (K ullbe rg & Ku lberg , in p ress) .Howe ver , th e Lowe r M ioce ne depositional sequence includes severa l smaller sedimen-tary se quences (O S A, O S BO, O S B land O S B2 of Antunes et al., 19 99), whi ch may re flect 3rd ord er paraseq uences(sensuVailet al.,199 1). One such case is the ero sional pa raconfonnity that separates uni ts II and

layers display what may be interpreted as load-indu ced sinsed imentary deforma tion along an ero sional surface carve d upo n sand waves formed dur ing an early stage of the Burdigalian ma rine estuary.

Marine fossil assemblages contain Sire nia mammals, cora ls, mo lluscs, benthi c macroforami nifera all ind icating the existence of subtrop icalwarmwaters at the latitude of Lisbon, durin g the Early Miocene (Antunes, 1971). Durin g this cycle, the main dcpocentric area change d from Lisbon to the C bameca-Corroios reg ion (Antunes et al ., this vo lume ) . Interpret ed as due to a compr essive b loc k tectonics(op.cit.)we rela te this to a deformation along a synclinal struc ture defined in lite reg ion of lite Seruba l Peninsula. In litis basin a more rigid northe rn margin and a more de formable southern margin controlled lite rate of the subsidence gradually shift ing its dep ocent ric area southwards along an asymme trical synclinal with vergence towards the South.

Th is cycle ends with the onset of the impo rtant Late Burdigalian tectonic compressi ve eve nt in the area . Thi s even t is express ed by deformation of the Lower Miocene de posits of theArrabidaChain, south of the Formosinho thrust (Kullbergetal.,1995a).Itwas dated from around 17 Ma, in the southern sid e of the Arra b ida Chain, at Port inho da Arni bida - An ixa sector (Paiset al., 1991 ;

An tunesetal.,1995). Uplift of the Iberian Meseta and the s ubsequen t north ea st to sout hwes t progradation of terrigen ou s sedime ntation b y fe ldspa tic sandsto nes pur sued unt il the limit Burdiga lian-Langhian inside the proximal areas ofthe LowerTagus Basin. This led to strong lateral facies varia tions and possible diachronism of the limits of the lithostrat igraph ic units IV and V of Cotter ( 1956), between the Lisbon and the Setuba l Pen insul a areas (Fig. I) . Wh ile the near proximal areas display thick sandstone units with terre strial vertebrate faunas (Antunes

et al.;1996) the distal are as, south and west of the Cris to-Re i sec tor, always had shall ow marine sed imentation, throughout this regressive interva l.

Bo urca rt & Zbyszewski (194 0 ) cons ide r ed th e litostratigraphi c unit IVb ofCotter ( 1956; ''Sands and clays withOs trea crassissimaand fossil plant remains of Quinta do Bacalhau") as correla tive of lite max imum of the re-gress ion associated to a first Miocene cycle, denominated "p remier etag e mediterraneen" following the termin ology ofEdouard Suess(inBourc art& Zbyszewski, 1940) . The age ofthis unit, around 18 Ma (Antun esetat. ,this volume) may relate its de position with th e ab ove-mentioned compressive event. Cunha (pers. conunun., 2000) sugges t that the correlative alostratigraphic unit SLD9 may end with the unit Va2 of Cotter ( 1956 ). However, the full extend of the consequen ces of this compressi on, inside the Low erTagus Ba sin, namely the trend towards the most arid conditio ns (interpr eted by other authors as a sequence ofuncorrelated short-term climatic events.Antunes&Pais, 1984) , is only ac h ieved during the deposition of the litostratigraphic unit Vb of Cotter ( 1956; " Sands and sands tones with Ostrea crassissima o f the va lley o f Chelas") which is here interp reted as the fmal depositional stage of the I Ne oge ne Cycle .

Cunctas da Terra (UNL), 14

In the A lentejo con tinenta l mar g in, an erosional surfac e detect ed between se is m ic un it s 4 and 5 is interpreted as rela ted to litis sa me compressive event (Alveset al.,this volume).

Due to the combination of all the se local fact ors inter-acting with the positive eustatic trend of the Burdigalian, eac h of the small-scale events inside the Lower Tagus Basin, here integrated into the I Neogene Cycle, may result difficult to correlate with other regional basins. Onl y the entire I Neogene Cycle may allow success ful corre lations. Ho wever, the significant distance between this basin and the active Bet ic tecton ic areas during the Aq uitanian and Ea rly B urdigalian (t he o pe ning of the Algeri an and Provencal basins and the anti-clockwise rotati on of the Cors ica and Sardinia; see Sanz de Galdea no, 1997) may explain the lack of more evident and corre lative tectoni c struc tures contro lling lite sedimentation of the I Ne ogene Cycle inside the Lower Tagus Basin . The timing for the intra-Burdigalian com pression event of Arrabide was p rob ab ly r e lated with th e p r o gr es sive wes tward advancement of the Betic-Rif deformat ion into the Western Mediterranean area by the end of the Burdigalian (see Sanz de Galdeano, 1997). At Arra bida , due to its westernmost Iberian positio n the tectoni c efforts, related to the genera l compressive betic reg ime, led to the deform ation of meso-cainozoic layers into several en echelon anticlin es associated to a thrust belt with displace ment toward s SSE (Ribe iroet al., 1979 ).

T he II Neogene Cycle

The II Neogene Cycle, in contrast with the previou s one, is represen ted in a vast region of the southern and western coastal areas of Portu gal. From South to North it incl udes: (I ) the th ick sed imentary sequences of the Lagoa- Po rtimao F ormati o n in the Algarv e B a sin, (re defined in Cachao, 1995) ; (2) the "Ca lca renite de

Me lides" unit (Ca cbao, 1995) and the Alcaccr do Sal Fo rmation (A nt unes et at.. 1986) in the Sadc Basin (Alvalade-M elides region); and (3) the lithostrat igraph ic units Vc , VI and VII of Cotter (19 56) in the Lower Tague Basin, corresponding to the deposition al sequences Ll (part iall y), S I , S2, Tl and T 2 of An tunes et al. (this volume) (Fig. I) . Already Bourcart& Zbyszews ki ( 1940) recognized that the un its V, VI and VII of Cotter (1956) could be globally rela ted to a same dep ositi onal seq uence, which these autho rs denominated as "deuxieme etage med iterraneen".

No rth of Lisbon, the marine sedi mentation was most p ro bably re d uce d to small and sha llow local g ulfs develop ing north ofS intra (Choffat, 1950 ; Madeir aet al.,

1983· 85), at Ma rinha Grande (M . T. Antu nesinRibeiro

10_{Congresso sobre 0 Cenozoico}

_{de Portugal}

The II Neogene Cycle resulted from a large

transgres-sion that covered Palaeozoic and Mesozoic basements over

a polygenic erosional unconfonn ity in the Algarve region

(sec Cac bao, 1995) and extended as far as Santa rem in

the Lower Tagus Basin (Bourcart

&

Zbyszewski, 1940;

Antunes

&

Pais, 1993). Facies uniformit y throughout the

Low e r Ta gu s Basi n and the in te rpre ted deep e r

palcobatimcrrics

derived from the macrofossil assemblages

(c.g. fishes, Antunes

&

Jonet, 1969. 70) of the unit VI of

Coller ( 1956) clearly demonstrate s the importance of this

transgressive event (Alvinerie

et al.,

1992). The

transgres-sion and the subsequent inshore coastline retreat led to a

deficiency in sedimentation in the dista l areas of the basin

such as the regi on of Penedo (southwest of Sctubal

Peni nsul a) , expresse d by conde ns ed sec t ions with

glauconite (isotope K-Ar ages ranging from 15 to 12.5

Ma

in

Antunes'

et at., 1999 ).

There is a change in facies from south to north , along

the Portuguese margin, during this cycle . In the A lgarve,

at the south, the litofacies arc more heavily calcified while

further north, in the Lower Tegus Basin, the correlative

litotypes tend to be less cemented by calcium carbonate .

Fossil assemblages also seem to indicate the existe nce of

a thermal gradient in the waters along the Portuguese shelf

already during the Middle Miocene (sec discus sion

in

Cachao, 1995).

These trangrcssivc sequences are interpreted as being

mainly controlled by global eustasy reflecting a response

to the important positive eustatic trend that charac terised

the Middle Miocene worldwide (cycles 2.3-2.4 of Haq

et

al.,

1988 ). During this phase the Portuguese continental

margin acted passively to the transgre ssive motion with

the exception of the Lower Tagus Basin, which continued

behaving as an important depocentric area.

This cycle ended with a second and the most important

compression even t, which started after the end of the

Scr ravallian. T his tectonic phase affec ted the ent ire

Portuguese onshore and shelf areas. In the onshore distal

areas of the Lower Tagus Basin uppermost marine units

revealed that marine sedimentati on had been significantly

redu ced or even finished a rou nd this age , alt hough

biostratigrap hic eviden ces are still controversial. The

co-occurrence of

Helicosp haera watbersdorfensis

and

H.

statts

in the Calcareous nannofossil assemblages from the

uppermost section of Foz do Rcgo (Costa da Caparica)

indicate that the marine sedimentation ended in this distal

sector of the Lower Tagus Basin during biozone NN7 of

Mart ini / CN 5b of Okad a

&

Bukry (Cacbao, 1995),

between 11 .8 and II 'Ma of Berggren

et al.

(1995), latest

Scrravallian. On the other hand, planktonic Foraminifera

seem to indicate that the marine sedimentation pe rsisted

until the limit N 15/N 16 of Blow (ea rlymost Tortonian,

Delgado

et al.,

1993; Legoinha, 1993).

During the Lower Torto nian, in the Bet ic region,

compressive efforts led to the westward movement of the

Albo ran ' s microplat e (Tappo nier, 1997) a nd to the

development of olist rostrome deposits over the a byssal

planes of the Cadiz and Horseshoe Golfs (Boninn

et al.,

1975) most probably by gravitational placement (Sanz de

306

Galdeano, pers . commun., 2000). Meanwhile, over the

entire Portuguese margin, a generalized emergence was

produced fo llowed by intense ero sion . Co nt inen tal

de posits (endorreic alluvial fans) fonncd in relation to

major tectonic accidents define the SLD11 allostatigraphic

unit (Cunha 1992a, 1992b, Cunha

et al. ,

this volume ).

Karsts develo ped in the coastal areas south of Melides,

previously subjected to higher calcification during the

Middle Miocene (Cachso, 1995). Th is led to an important

stratigraphic hiatus of more that 2.5 Ma (Fig. I ) that may

be tracked from the Algarve region to the Iberian Abyssal

Plane.

It

is well defined in all O DP Leg 149 sites, in

particular Site 900 (Fig. I), located 250 km west of'Aveiro

and at the present day depth of around 5 OOOm (Kaenel

&

Villa, 1996) (Fig. 2). According to Kaencl

&

Villa (199 6)

th is hia thus co rre spond

10

the bio zo na l inte r va l

NN9 · NN I 0 o f Ma rt ini ( 197 1) (CN7-8 o f Okada

&

Bukry, 1980, aprox. between 10.5 and 8.6 Ma, ofBergreen

et al.,

1995), equivalent to the entire cycle 3. 1 of Haq

et ai.( 1987).

However, on the shelf of the Sctubal Canyon only

s mall-sca le erosi ona l fea tures see ms to testify this

compressive phase (Coppier

&

Mougenot, 1982).

The above-mentio ned charac teris tic s of this cycle

allow the establishment of correlat ions within a broader

regional scale. The already mentioned progression of the

Be tic- Rif defo rma tion tow a rds West (se c Sanz de

Galdeano, 1997) allows the establishment of a more direct

structural and paleogeographic correlation between the

Portuguese Iberian Margin and the Betic Chain. This may

explain why the compressive efforts that can

be recognized

in the Arrab ida and Sin tr a reg ions (see Kullberg

&

Kullberg,

in

press) are much more intense by the end of

the II Neogene Cycle than during the end of the I Neogene

Cycle , as it was already recognized by ot her autho rs

(A.

Ribei ro, pers. commun., 2000).

T he III Neoge ne Cycle

During the interval of time in which the mar ine

sedi-mentation completely cease d along the entire Portuguese

margin, the persistence of the deformation on the Betic

chain led to the developmen t of an importan t foreland

depocentric region. Related to a fault (Ribeiro

etal, 1990)

or to flexura l deformation (Sierra

et al.,

1990) an

elon-gated foredeep basin, the Guadalquivir Basin, started to

receive thick and deep marine sediments (Sierro

et al.,

1990). The tectonosedimentary depositional sequences of

the Gualdalquivir Basin (Olmo

et at. ,

1984

fide

Sierra

et

aI.,

1990) show evidences of a clockwise rotation with

the northwestward translation of its depocentre taking

place during the Early/Middle Tc rtonian transition (Sanz

de Galdcano

&

Vera, 199 1). Further west this rotation led

to ulterior litostatic-induced subsidence and to the

begin-ning or the marine sedimentation in the Algarve region,

associa ted to the III Neogene Cycle.

The III Cycle sedimentation starts with micaceous fine

poorly cemented sandstones, the

areolas

facies (Cachao

D

~. ~. i}

:;;

~

~

§::

~

.-e

? Algarve

. .

.

.

lag o

s--Pornmoo

Formation

• Lucio ' •

Form ati on

10)

Regressive sequence

~'

I

. .

.

. .

.

.

. .

. .

:---'.", r

_{. .}

_.

_.

_.

:.---coceo

(£)"

Formation

I I

Sil ic lastic

:J

seq uence Carbona te sequence

::jl,

"71 f

®

,

...

SW Alen tejo coast

Tranqressive

sequence

[ '9

-1

Penc surcc c

Forma tion

'Caicarenilo

leD

deA/jewr-

1

-Amare/a"

- - - - - -

1--

-

--

-

-.,

.

.

:-:- ~ - 1-

t

-

1-'Areolas de t-i\

Amelje/fa"

I..Y

.

....

~

®

? I ~

Esborronda

-oouo rm.

• A1valade • Forma tion •

.

-~ . _

--eD@ ---®

!

I

-'>.

?I@

Sensu:1 • cecnac ( 1995); 2 .Amended in cecnao (1995); 3 - Cachacel al.(1998); 4 - Pereira (1990): 5 - Antunesel al.(1986); 6 · Pimentel (1994); 7 - Azevedo(1983); 8 - Adapted fromAntunesetal.. this volume; 9 -Antunes& Mein, 1981; 10· Cachao"l,1989); 11- Kaenel & Villa,19 ~.

IVIV . Lithostratigraphic Units of Cotter (1956}.

Al ·1'2 _Depositional sequences of Antunesatal., this volume.

®

o

sw

-B2 .,.,

...

'

...

-S2 BO Sl A1feifa • Formation

' :"1 ~ ' ;';" :"1~ ' ; " _1 1_

..

-

...-..--...

"AreoJosdo

Penedo' - ?- - -

1-.

•

'., , '

1..., · ...- ... , ·...· ,

~ _ T1.,l:l

Lisbon

setc t et Peninsula

""-- ?

"-1-'-

:..?-Arne,

FOI~

tio ~

.I®

I. , ' . ,

- - -- - - Lithostrat igraphy

5

Alvalade--Melides

•

" .OffShore:WofAveiro ..,

.

l'"

ODP 149·~ i t e 900

if)- 8.6 Mo

®

:::>

!;(

I '_ 1O,5 Mo NE

', Rousso-Porede s Fm._.

,

cl , A ; ~ rn ide

Fm.

@

I

•

I

,

•

•

u -~ ­ U

•

e

•

..

°

•

Z

,

'.-

,

•

u ~ u

•

•

•

..

°

. ,41. Z

-.

_u ~ u

•

c

•

..

°

•

Z

rn

~

N

~

north Pombal.

c Caldas da Ralnna

? ?

I

1:

-Fig. I. Stratigraphic framewo rk of the dep os itional sequences of the Neogene of Portugal (Adapt. Cachao, 1995).

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Congressoscbre 0 Cenozcic o de Portugal

theareolaslower levels of the Cacela Formation enclose rich macrofossil and microfos sil assemblages allowing its stratig raphic po sitioning in the beginn ing of th e Late

Tortonian

(CN 9a Ok ada & Bukr y, 1980 in

Cacbsc.

1995) (F ig. 1). So, marine sedimentation in the Algarve Basin resu mes immed iate ly afte r the CN7·8 hiatus detec ted at the Ib eria Abyssal Pla in by Ka en nel &

Villa ( 1996).

T he externa l location of the Algarve regio n relative to the Gu ada lquiv ir basin e xp la ins the short ve rtic a l develo pment of the Caccia Form ation, correlativ e of the thick units A to D ofthe Guadalquivir Basin (see discussion

inCachno,1995). The common occurrence ofcondensed sectio ns with glauconi te (Antuneset al., 1984) is in agree-ment with this deficiency in sedimen ts (Cectao, 1995). At Central Algarv e(Ouraregion) theCacelaForm ation overlays the karsified upper units of the Lagos-Porti mao Format ion . T his unco nform ity is well mark ed by a bioerodcd marine abras ion surface that cuts an already litified substratum, testifying the importan t time gap that separates these two units (Silvaet af.,1999 ; Domenechet

al.,

1999). T he Cace la Format ion, toget her with the overlying silicic lastic Ludo Format ion (Cachao

et al.,

1998) defines the first phase of sedi mentation associated to theIIINeogene Cycle (Fig.2).

To the north, in the Sado Basin (Alvala de- Melides region), a similar de positional seq uence, initiating with the Esbarro ndado iro Formation,starts its sedimentation shortly afte rwards, dur ing the Messinian (Ant unesetal.,

1986). In the Lower Tagus Basin [Setubal Penins ula), ove rlaying the fossiliferous levels ofthe II Neogene Cycle, a tr a nsiti on a l seq uence p revio usly recognized an d denominated "Sobre da sands" (Dollfus& Colter, 1909) or "So breda layers" (Bourcart& Zbyszews ki, 1940) was subseq uently interpreted by T. Antunes(inRibeiroet al.,

1979) as a result of the calcium carbonate dissol ution of the uppermost Miocene layers of the units VII of Colter (1 956) . However, th ere is an unit, herei n des igna ted

"Areolas do Penedo"(Fig. I), that overlays foss ilifero us Middle Miocene (II Neoge ne Cycle) layers at the Penedo region by means of an unco nformity erosional surface. Although the existent stratigrap hic data are still imprecise, this unit seems to testify a shor t-term shall ow marine incursion in the Setuba l Peninsula that is assumed to be corre lative of the Esba rrondadoiro Formation. Together, the two units are considered as an evidence of a small-scale marine episode, the second phase of the III Neogen e Cycle in the Sado (Alvalade-Melides) and Lower Tagus basins (Fig. 2). Shortly afterwards this phase ended by transitional to fluvial regressive sequences represented by the Alvala de Formation, in the Sado Basin (Pimentel, 1994) and the"Serie arenosa do Alfeile" in the Lower Tagus Basin (Aze vedo , 1983).

Further north, in the Pombal - Caldas da Rainha region, an also similar depositional sequence may be observed (Cachso, 1989). The marine sedi mentation started with the deposition of the Carnide Sandst ones (Barbosa, 1983) during the Lower Upper Pliocene (the Carnide Form a-tion, Piace nzian, CN 12a Okada&Bukry, 1980inCachao, 1990). This unit is in depositional continuity with an upper

308

silic iclastic unit, the Roussa-Paredes Form ation , which contai ns pal in ological asse mblages in d ic a tiv e of Reuverian age (ante-Praetiglian)(i.e.Piacenzian) [Diniz, 1984). These two units evide nce a delayed third phase of marine sedimenta tion with in the III Ne oge ne Cycle (Fig. 2).

Onshore, du ring the IIINeogene Cycle , the marine sedimentation initiated in the south ofPortugal propa gated northwards to the Tagus Basin and further north toward s the west coast of Estre madura. This seems to sugges t migration of crustal subsidence from sou th to nort h in a visco-elastic rheological regime (Ribeiro, pers. commun., 2000). The change in the relative orien tation betw een the southern E· W elongateddepocentricare a of Algarve and the northern NS elonga ted de pocen tric area of Pombal -Caldas daRainhamay re flect a rotation in the or ientation of the maximum com pressi ve regi me from the Upp er Miocene (N NW· SSE ) to th e P liocene (WN W·ESE) (Ribeiroel af., 1996).

The marine sedi mentation on the Portuguese sector of the Iber ian Margin seemed to have been pro gressively more co ntro lled by tectonic de format ion, particu larly during this last cycle. T his seems to be in agreement with a most acce pted hypothesis for th e Neogene tec tonic structuration of the Betic-Rif chain, by which there was a prog ressive translation of the deformation from the inner zones of the chain towards West (Sanz de Galdeano, 1997).

COi"CLUSIONS

A new paleogeographic mode l for the evolu tion of the marine sedimentary units of the Neoge ne of Portugal is described based on the definition of three main marine depositional sequences. Defined as 2nd order sedi men-tary sequences the I, II and III Neoge ne Cycles have du rations rang ing from 5 to 8 Ma.

The I Neogene Cycle is represented in the Lower Tagus Basin. Lithostratigraphic units I to III of Colter, ranging from the Lower Aquitanian to the Burdigalian testify a transgressive seq uence whic h maximum incurs ion not extended beyond Alverca-Vila Franca localities. Rap id lateral facies variations testify for a sha llow marine basin . This cycle ends by an important Late Burdigalian tectonic compressive event that induced terrigenous sedimenta tion inside the Lower Tagus Basin induced by uplift of the proxi mal surro unding areas and deformat ion affect ing the Early Miocene deposits of the Arra bida Chain. This led to the progressive change of the paleoenvironment of the region of Lisbon towards more terrestrial and arid condi-tions reac hing its maximu m during the B urd igalian-Langhian limit (unit Vb of Concr). It may be concl uded that the I Neogene Cycle resulted from the combination of local subsidence conditions pro bably induced by isos tatic crustal re-equ ilibrium associated to the with the positive eustatic trend of the Burdigalian.

Fig. 2 - Paleogeographic maps of the maximum transgressive events of the Neogene of Portugal. Shaded sectorsrepresentemerged areas surrounding the marine basins. I - I Neogene Cycleduring the Middle Burdigalian; II • II Neogene Cycleduring the MiddleSerravallian; III - III NeogeneCycleduringthe Upper Tortonian in theAlgarve region (1st phase),the Messinian-LowerPliocene in

the Alvalade-Melides-Setubal regions (2nd phase), and the Upper Pliocene-Piacenzian in the Caldas da Rainha - Pombal region (3rd phase). Abbreviations: L - Lisbon; A-Alverca; VP - Vila Franca.

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1° Congresso scbre0 Cenozcicc de Portugal

most prob ably reduce d to small and shallow local gulfs.It was generated by the important positive eustatic trend that characterised the M iddle M ioce ne worldwide (cycles2.3~

2.4 of Haq

et al.,

1988). The Portuguese marg in acted pas sively to the transgressive mov ement with the

excep-tion of the Lower Tagus Basin that behavedasan impo r-tan t depocentric area. This cycle ended with a seco nd and the most important compression event starting after the end ofthe Serravallian and pursu ing throughout the Lower and Middle Tortonian. This led to an important depositiona l hiatus of more that 2.5 Ma, which can be traced fro m the Algarve region to the Ib erian Abyssal Plane (O DP Leg

149).

T he cl ock wise ro tat io n of th e Guadalquivir basin between the Early and the M iddle Tortonian moved the depocentrie area closer to the Algarve region. The thick marine units deposited afterwards in this basin produ ced a litostati c load, which seems to have induced subsidence farther west res wning the Neogene marine sedime ntation

in the Cacela region (Eastern Algarve), during the Late Tortonien . T his marks the beginning of the marine III Neogene Cycle . To the north ,inthe Sado Basin (Alvalade-Mel ides region), a similar depositional sequence starts its sedimentation during the Messinian giving evidence for a second phase of sedimentation associated to the III

Neogene Cycle. Further north, in the Mondego Basin (Pombal - Caldas da Rainha reg ion), a similar depositiona l sequence (the third phase) started during the Late Pliocene (Piacenzian). It can be co ncl uded th at during the III Neogene Cycle , the marine sedimentation started first in Algarve and then propagated northwards sugges ting

mi-gration ofcrusta l subsidence, in this direction . T he change

in

the relative orientation between the southern depocentric

REFERENCES

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Dur ing the Neogene, the marine se dimentatio n on Portuguese sector of the Iberian Margin seemed to have been progres sively more con trolled by tectonic deforma-tion. This seems to be in agre ement with a most accep ted hypothesis for the Neogene tectonic structuration of the Betic-Rif chain by which there was a progressive trans la-tion ofthe inner zones of the chain towards the WesL While the most impo rtan t deformation in the Betic region occurs between the Upper Olig ocene till the Langhian, in th e Portuguese margin the intensity of the tecto nic deforma-tion seems to increase along the Neogene, whi ch may suggest a centrifugal transfere nce ofthe deformati on fro m the Inner Zone of the Betic- Rifregion out and westwards.

AKNOWL EDGEMENTS

Th e authors express thei r gratitude to Ant6nio Ribeiro and Carla Kull berg for fruitfu l discuss ions on M iocene tectonics and to Pedro Terrinha for the critical review of an earlier vers ion of the manuscript. Subseq uent versions of the manus cript also benefited from extensive disc us-sions with Ped ro

Proenca

e Cunha. Carlos Sanz de Ga ldeano and from

Cristino

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