High resolution stratigraphy and miocene facies ... · (Cicncias da Terra (UNL) Lisboa N.o14 pp. 183-190 2000) 4 Figs. High resolution stratigraphy and miocene facies correlation

( Cicncias da Terra (UNL) Lisboa N.o 14 pp. 183-1902000 )4 Figs.

High resolution stratigraphy and miocene facies correlation in Lisbon andSetubal Peninsula (Lower Tagus basin, Portugal)

M. Telles Antunes'!', P. Legolnha'" , P. Proenea Cunha'» & J . Pa ts'"

Work supported by "Praxis XXI 2!2.1!CTA/ I06J94" and by "POCTV32 34SICTN2000 Proj ects (FCI).1 - Acade mia das Ctencus de Lisboa , Centro de Estudos Geol6gicos (JNICT), Faculdade de creociese Tecnologia, Quinta da Torre,

P·282S·114 Caparica, Portugal; mla@maiL fct.unl.pt;2 •Centrode EstudosGeologicos(INICT), Faculdade de Ciencus eTecnologia, QuintadaTorre, P-Z825-11 4Capanca, Portugal; [email protected],pt;3 - CentredeGeociercias. Departamentode CienciasdaTerra, Faculdade deciences e Tecnologia, LargoMarques de Pombal, Coimbra; [email protected];4. Centro deEstudos GeoI6gicos(JNICT),Faculdadede Ciencias eTecnologia,Quinta da Torre, P·2825-l14Caparica, Portugal; [email protected]

ABSTRACT

Key words: Sequencial stratigraphy; Miocene; Lower Tagus Basin; Lisbon; Setu bal Peninsula; Portugal.

Correlation between facies associations (marine, estuarine and distal fluviatile environments) and disconformities, observedbetween Foz da Fonte (SW of'Sctubal Peninsula) andSantaIria daAz6ia(NE of Lisbon) are presented, The precise definition of themarine-continental facies relationships improved verymuch the chronology of the depositional sequence boundaries, Tectonic andeustatic controlsarediscussed on thebasis of subsidence rates' variation,

RESUl\10

Palavras chave: Estrangrafia sequencial; Miocenicc; Bacia do BaixoTejo; Lisboa; Peninsula de Setlibal; Portugal.

Apresenta-se correlacgo entre assoctacees de facies (ambientes marinhos, estuarinos e fJu viais distais) e deseontinuidadesscdimentares desde Foz da Fonte (a SW da Peninsula de Setilbal) ate Santa Iria de Azcia (a NE de Lisboa). Adefini"ao precise dainterdig jtacjlu entre facies marinhase continentais permitiu precisar a cronclogia dos limitesdas sequenctasdeposicionais. Discute­se a influencia dos controlos tectonico e eustatico na geracao de espaco de acomodacao com base na analise das veoecees dasubsidencia.

INTRODUCTION

The Lower Tagus basin is situated in the western mar­gin of the Iberian plate, It presents an NE-SW alignment.Its almost siliciclastic neogene infilling shows several sedi­mentary unconformities. The most important are angularunconformities related to tec tonic events assoc iated tothe geod inamic evolution of the Iberian plate.

The Lisbon and Setubal Peninsula areas belong to thedistal part of the Lower Tagus basin where the predomi­nant estuarine sedimentation passes westwards to an oce­anic one (Antunes et al.. 1987; Cunha , 1992a; Cunha1992b; Pena dos Reis et al., 1992; Barbosa, 1995).

The Miocene (Aquitanian to Early Tortonian) succes­sion is bounded by angular unconformities. It reflects acompressive context related to the Betic collision . Basin'sdepocenter is placed in the Setubal Peninsula wherethickness attains about 900 m.

Several logs allow high resolution correlations betweenmarine and continental levels dated by planktic forams,mammals, and/or isotopic ages (KlAr - glauconites; Sr iso­topes). Small and large mammals give precise datationfor flu vial/estuar ine beds. Pal aeontologie wealth ­dinoflagellates, palinofossils , nannoplancton, algae, plantmacroremains, forams, ostracods and other crustaceans,molluscs, echinoderms, fish. reptiles, mammals - as well

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10 Ccngresso sobre0 Cenoz6ico de Portugal

as Oxygen and Carbon isotopic studies provide a hugeamo unt of informa ti on on pa laeo geograph ic an dpalaeobiogeographic reconstructions.

Antunes et al, (1973) defined sedimentary cycles inthe Lisbon region based on the sucessive unit' s transgres­sive/regressive character. Antunes & Pais (1993) madecorrelations with Almada (Setubal Peninsula) and with theinland part of the basin (Ribatejo); seven transgressiveeventsalternating with six regressive ones were recognized.

Research on the Setubal Peninsula (Scn er al., 1992;Legoinha, 1993;A ntunes et al., 1995;Antunes et al. , 1996)improved the biostratigrafic knowledge ofsome units (pre­senting correlation difficulties because of facies differencesfrom Lisbon region).

The integration of all lithologic, biostratigraphic (firstand last occurrence of significant groups/species of foramsand mammals) and isotope (KiAr e ~1S rfl6Sr) dating al­lowed the chronostrat igraphic framework from LowerTagus basin Miocene series, as well as the characterizationof eight depositiona l sequences (Antunes et al. , 1999).

Analysis and interpretation of outcrops and boreholes(Fig. l ) from the Lisbon-Sctubal Peninsula area (Cotter,1950; Choffat, 1956;Antunes & Torquato, 1969; Antuneset al.; 1996a; 1996b; 1998; 1999; Sen et aI., 1992) arepresented . Taking into account the previously defineddepositional sequences, we present a detailed correlation

scheme between facies and discontinuities, as well as apalaeoenvironmental interpretation. Lateral and verticalthickness changes are discussed.

FACIES ASSOCIATIONS

Lithology, sedimentary structures, palaeontologie con­tents and sedimentary discontinui ties were used to char­acterize facies (marine, estuarine and fluviatile). Correla­tions from Foz da Fonte (SW, in the Sctubal Peninsula) toSanta Iria da Az6ia (NE, Lisbon) are shown (Figs 2 and3).

Vertical and lateral facies' shifts were used to interpretthe see level changes and 10 characterize transgressive­regressi ve cycles of different scale order. The eustaticchanges could be registered because of the continuousregional subsidence.

The thickness of the Miocene successio n increasesfrom Foz da Fontc-Ribeir a da Lage sector (about 85m) toCharneca (more than 494m). It diminishes from there toSanta Iria da Az6ia (about 162m; Choffat , 1956). In thesouthwestern and centra l sectors of the concerned area analtemance between marly limestones and marls predomi­nates. In the NE sector detrital sedimentation predomi­nates in result of the fluviatile influx.

Fig. I - t ccettzacsc dos cortesgeol6gicos, das sondagens e do perfil interpretativo.

184

Facies associations and sedi mentary environments areas follows:

An - infralittoral to circalittoral (conglomera tes!gla uco nite sands and san dy marls withplankt ic forams) ;

Af2 - Iirtoral to estuarine, infratidal (biocalcareniresand marls, with molluscs) ;

AO - tidal flats (silts , often with abundant organicmatter);

Af4 - delta/e stuary channels and bars (bi oclastic,fine to middle sandstones);

Af5 - dista l fluvial (coarse to middle grainedsandstones and clays, with plant and mam­mal remains).

DEP OS ITI ONAL SEQUEN CES AND T il E C II RO­:\,O LOGY OF DlSCO:"'TI :"'UITI ES

For each log, sea-level changes were intrepreted onsedi mentological chara cteristics and palaeontologie con­tents. Trans gressive/regr essive cycles were correlated be­tween all the studied stratigraphic columns . Ten main cy­cles have been recognized and named by depositiona l se­quences (SO). As the result of better correlation data ob­tained no w, the previous sequence SO-A is divi ded in SO­AI and SO-A2 as well as the SOoT in SO-Tl and So-TI.Th e last two have not been sufficiently characterized be ·cause ofme poor exposition and the scarce biostrati graph icmarkers (Figs 3 and 4).

Integra ting all the avail ab le data , and the ir precisestratigraphic position in each sequence, discontinuitiesbetween depositional sequences are dated as follows:

SD·AI, 0 1 - 23 Ma;SO-A2, 0 2 - 21 Ma;SO-BO, 0 3 - 20 Ma ;SO. B l, D4 - 19 Ma;SD-B2, D5 - 17,8 Ma;SO-L1, 0 6 · 17 Ma;

Ciencias da Te"a (UNL), 14

SO- SI , 0 7 - 15,3 Ma;SO-S2, 0 8 - 12,7 Ma;So-T!, D9 - 11,6 Ma;SO-12, 010 - 9,5 Ma.

A correspondence with 3'" order eusta tic cyc les (Haqet at., 1987) is presented (Table I ). Sequences SO AI,A1, 8 2, L1, S I and Tl see m to correspond to 3rd ordereustatic cycles. Sequences SO-BO, ,SO-B I concern thesame eustatic cycle (TBl . I), while SO·S2 corresponds to

. two (TB2.5 and TB2.6 ) cycles . In some seq uences(SO.L1 , SO-5 1 and SO-52) lower order cycl es can berecognized.

As Antunes et 01. ( 1996) had referred, discrepanciesbetween the basin 's max imum floodi ng event s and theeustatic maxima (Haq el 01., 1987) may be rela ted to loca lchanges on subsiden ce (and/or inaccurate corre lationbetween different chrono and biostra tigraphic scales).

S UBSIDEl'"CE ANALYSIS

A ready aproacb to the subsidency rat e (Table 2) foreach depositional sequence was obtained by divid ingthickness by the age span between the upper and lowerdiscontinu ity boundaries . Backstrippi ng was no tcalcula ted ; th is was regarded as irrelevant due to thesuccession's weak burial.

Sedi mentation begins in the Lisbon area . Th is area isthe only one where the SO·A I sequence has been recog­nized. Subsidence rate increases everywhere till BI(Fig.4). After a general decrease during SD.B2, subsidenceinc re ases be tween Corroi os and Li sbon (SD-L 1).Subsidence rate increases again in the Ribeira da Lage-Chamcca region (SO-S2). .

The thickness of depositional sequences changes lat­erally very much . Migration ofthe depocenter also is sig­nificant. The A2, BOand B I sequences depocenters are inAlmada and Charneca . The 82 dcpocenter is confined toCharn eca. In se quences LI and S I the depocenrer islocated at Co rroio s, in sequence 52 it returns to Chameca.

C h r onos tra t igra phy JO or d . Cy cl es Depcs, Seq .

T ortonian TB 3.2 T2?TB3 .1 TITB2 .6 S2

S erra vallian TB2.sTB2.4 S I

Lana hian TB2 .3 L ITB2.2 B2

B urdig a li an TB 2.1 BIBO

TB U A2Aq uitania n TB l.4 A I

Table I • 3'"order cycles (Haq et al., 1987) and depositional sequences in the distal part of the LowerTagus Basin.

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I' Ccngressc scbre 0 Cenoz6ico de Portugal

Dep.Seq FF-RL CH-AC8 Corr·AC2 AL 1I 51AAl 0 0 0 0 19,5 0A2 0 38 18 46 17 6BO 12 48 30 37 20 14Bl 25,8 93,3 83,3 92,5 53,3 43 ,3

B2 7,1 45,7 27,1 21,4 19,3 11,4

Ll 0 34 ,S 56 ,4 13,6 32,7 25,5

51 1,2 32,3 46,2 13,8 12,3 6,952 29 70 ,9 38,2 12,7 10,9 10,9

T1 8,6 15,2 15,2 10,5 17,1 7,6T2 26 32

Table 2 - Subsidence rate (mlMa) for each deposicional sequence on studied logs (FF- Foz da Fonte; RL· Ribeira da Lagc: CH­Chameca; AC8 - Corrcios; AL- Almada; Li - Lisboa; Am - Ameixocira;SIA- Santa lriada Azoia). Darkbluecolour stresses the

greatest subsidence rate for eachdepositional sequence.

100

90

80

70..! 60

"~ 50,~ 40]",

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•

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/ \/

30

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Deposiliona l seq uence

Fig. 4 _ Subsidence rate (mlMa) for the studied logs(FF. Foz da Fonte; RL· Ribeirada Lagc; CH- Chameca; AC8 · Corroios;AL· Almada; Li - Lisboa; Am- Ameixoeira; SIA · Santa Iriada Azoia).

Tl and T2 sequences (T2 restricted to a narrow areabetween Fonte da Telha and Lisb oa) show greatersubsidence rate in Lisbon.

In a regional compressive context between the area tothe North of Lisbon and the Arrabida chain (defining alarge syncline with continuous subsidence) depocentermigration results from block's tectonics. Northwards mi­gration of the depocentcr between 82 and LI is a conse­quenceof a tectonic event (about 17 Ma).This event origi­natedan angular unconformitythat may be seen atPortinhoda Arrabida (Pais et al., 1991; Antunes et at., 1995).

CONCL USIONS

• Correlation of facies associations and deposicionalsequences in several Miocene logs (outcrops andboreholes) from Lisbon and Senlbal Peninsula hasbeen achieved.

• Vertical and lateral facies changes of the Aquitanianto Tortonian depositionalsequences have been char­acterized in detail. Redefinition of the previous SD­A and SD·T sequences is proposed.

186

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187

1°Congresso sobre 0 Cenoz6ico dePortugal

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• A chronostratigraphic framework of the facies,deposi tional sequences and corr es pondingboundaries is presented. Sequence depositional (SO)boundaries (0 ) are aged (Ma) as follows:

- Aquitanian: 01 - 23 Ma (beginning of Miocenesedimentation);

- Aquitanian: 0 2 - 21 Ma (SDA1-SDA2);- Burdigalian: 03 - 20 Ma (SDA2-SDBO);- Burdigalian: 0 4 - 19 Ma (SOBO-SDB I);- Burdigalian: D5 - 17,8 Ma (SDB I-SDB2);• BurdigalianlLangh ian : 0 6 - 16,4 Ma (SDB2-SDLl);

• Langhian: 0 7 - 15,3 Ma (SOLl -SDSl);- Serravallian: 0 8 - 12,7 Ma (SOSI-SDS2);- Serravallian: 0 9 - 11,6 Ma (SDS2-S0T1);- Tortonien: 0 10 - 9,5 Ma SDn -SOT2).

REFERENCES

Cunctas da Terra (UNL), 14

• Local subsidence rates have been quantified in theFoz da Fonte - Ribeira da Lage, Chameca, Corrolos,Almada, Lisboa.Ameixoeira and Santa Iria daAzoiasectors.

• Analysis of the sedimentary and palaeontologicalcontents and isotopic dating provides an improvedassessment of the influence of tectonics, eustatismand sedimentary supply in the generation of theaccomodation space.

• In a regio nal NW-SE comp ressive context, theAlbufeira syncline had continuous subsidence; thatallowed eustatic changes of diverse order to berecognized.

• Oepocenter migration and facies changes suggestthat tectonic blocks subsided at different rates.

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Antu nes, M.T.; Elderfield, H.; Legoinha, P. & Pais, 1. (1995) - Dataeces isotopicas com Sr do Miocenico do flanco SuI da Serra daArrabida. Com. Inst. Geol. e Mineiro. Lisboa, 81: 73-78.

Antunes, M. T.; Elderfi eld, R ; Legoinha, P.; Nascimento,A. & Pais, 1. (1998b) . A Stratigraphic fram ework for the Miocene fromthe Lower Tagus Basin (Lisbon , Setu ba j Peninsula , Portugal). Depositional sequen ces, biostratigraphy and isotopic ages.Proceedings of the 2d RCANS Congress ( 1997) . Bal. Soc. Geol. Espana, Madrid 12 (I ): 3-15.

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