Coastal sedimentary environments and sea -level changes

Coastal sedimentary environments and sea-level changes

Cristino J. Dab rio

Depanamenlode EstratigrnfJa, Facultad deCiencias Ge<llOgicas, Universidad ComplulenseandlnstinuodeG«llogla Economica,CSIC,

28040 - Madrid (Espai'la). dabrio@cucmax.sim.ucm.es

ABSTRACT

Key-words: coasta l sedime ntation; eustatic cha nges; stratigraphic architec ture; tecto nics; accommodation space; Ca inozo ic. A detai led knowledge of the 3-D arrangement and lateral facies relationships of the stac king patterns in coastal deposi ts is essentialto approach many geo logical problems such as prec ise tracing ofsca le vel changes. pa rticular ly duri ng small scale fluctua-tio ns. These are useful data regarding the geodynamic evol ufluctua-tion of basin marg ins and yield profi t in oil explorafluctua-tion. Sediment supply, wave-and tidal processes, coastal morphology, and accommodation space generated by eustasy and tectonics govern the highly variable architecture of sedimentary bodies deposited in coastalsettings.But these parameters change with time, andero -sional surfaces may play a prominent role in areas located towards land. Besides, lateral shift ofero-sional or even depositional loci very often results in destruction oflarge parts of thesediment record.Several case studies illustrate some commonly found arrange-ments offaeies and their distinguishing features. The final aim is to get the best results from the sedimentological analysis of coastal units.

RESUMO

Palavras-chave: sedimentaiVlio costeira;vari~ eustaricas;arquitectura estratigrifica; tect6nica; espaeode acomodacgo;Cenoz6ico.

o conhecimento dctalhado do acranjo tridimensional e das re!aiViks laterais de facies nos depositos costeiros sAo essenciais para definir, com precisao, as variaeoes do nlvel do mar, em especial as Ilutuacdes de pequena escala. Estas fomecem dedos uteis para0

reconhecimento daevclueacgeodinjmlca das margens des bacias berncomo para a pesquisa e explcracao de petrcleo. 0 acarreio sedimentar, os processes ondulatorios e de mares, a morfologia costeira e0 espaco de acornodaejc geradopelaeustasia e pela tectonica sAo os reepcnsaveis pela arquitectura muito variavel des corpos sedimentares depositados em ambientes costetros. Todavia, estes pardmetros variam no tempo, e as superficies de ercssc podemterpapel importante nas

mas

pr6ximas do oontinente. Alem dissc, a desjocaejo lateral da ercsac ou mesmo lacunas sedimentares conduzem, com frequencia,

a

destruiiVto de parte significativa do registo sedimentar. Apresentam-seexemplcs que ilustram algunsarranjos frequentesde facies e cs seus parametres caractertsticos com vista a obter os melhores resultados na analise sedimentol6gica das unidades costeiras.

INTROD UCTIO N

The litto ra l is an unstable zone th at comprises marine an d terrestri al do mains ve ry sensitive to a var iety of geologica l p ro c es se s . Waves an d tides playa most importan t role in th e coastal dynamics, and their mutua l inte raction ac co unts for redistri bution of the sedi ment budget causi ng accumulation o r erosion . Changes of sea level gre atly mod ify the are a reac hed by th ese agents in th e course of time .

Wave s move sedimen t and ten d to accumulate it in beac hes. T hus, the incoming waves are really a barrier to th e sea wa rd m ovement o f sedime nt de rived from the continents. Storm waves p ro du ce erosional surfaces. T he erosive effect is particularly great during transgressions due

to th e land ward displacement o f the surfzonc ac ross former emergent parts of the coast tha t were topograp hica lly mo re elevated.

Tides cau se per iod ic oscillations of sea level that are felt mainly in areas where geographical co nstrictions such as shoals and straits restrict the free movement of water m asses and gene ra te c urrents th ai sweep the bo tt om . Astronomical tides ind uced b y the attraction of the SWl and Moon in large bas ins ca use periodi c reversion o fflow a nd oscilla ting cu rren t vel o c itie s . I n contras t , meteorological tides caused by pi ling up of water on the coast, either periodically or not, are importantinmicrotidal or tid eless coasts because they increase the area reached by the surf zone. From the a sedimentologica l poi nt o f view, the m ain differences be tween beaches and tidal flats refer

1° Congresso s obre 0 Cenoz6ico de Portugal

to slope, so urce of sediment and dominance o f waves or tida l curren ts. But thi s simple scheme c hanges from cliffs to beaches, barrier islands and lagoons, tidal flats, and estuari es.

I n o the r words, the (morpho-) s edimentary units deposited in coasta l settings exhi bit a highl y va ria b le stratigraphic architec ture, that is mainly governed by fac tors as sedimen t supp ly, wave- and tidal pro cesses, c oas ta l mo rp h ology a nd a ccommod at ion spac e . Seaward, these coastal sedimen tary un its interfi nger with muddy she lf depo sit s, bu t la nd wa rd these eit her en d abru ptly, o r show ra pid facies changes to tida l, lagoo nal, was hover-fa n and fluvial depo sits. With time the pattern becomes more co mplica ted as the abo ve mentioned factors change , especially wh en landward erosional surfaces start to playa prom inent ro le as soon as a position ab ove base level is reached.

EUSTASY AND CO ASTAL ENVIRONM ENTS

Despite da ily or we ekly oscillations, it is possible to estab lish a mean sea leve l that is considered 's table' or ' fixed' at the scal e of human life and serves as refe rence datum for topographic purp oses. Th e theoret ical da tumin Spain is the mean level of th e Mediterran ea n Sea in AIicant e, but it actu ally fluctuates some 20 to 30 em/yr. These sma ll-scale oscilla tio ns are a response to variations of atm osph eri c pressure, evapo ration, vol ume of ocean ic currents, and changes of wat er de nsity d ue to varying temperature and salinity. In the Northern Hemisph ere the mean sea level is topographically higher inautumn and lowe r in early spring.

Suess first used the term e ustasy in late 1911l_{Ce ntury to} re fer to changes of level in ocean s ass uming that they were coev al and of gl o ba l ex te nt. Later realisatio n o f the

GLACIAL

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Fig.I-Types of eustasy (left) and rates of sea level change (right).

o

marine _ terrestrial

\.. coast (beginning) '\ coast (end) tra nsgression

--.

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_{composite offlap+to}

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A

regression

Fig.2 _ (A) Sediment input, transgression and regression during ariseof sea level.(B)Superimposition of eustatic curves with variable periodicity and resulting composite curve. (C) Effects upon coastal units of eustatic oscillations with different amplitudes

te ctonic and glacial components of eus tasy led to the incorpo rati on of the terms gl acio-eustasy and tectono-eustasy. Monier ( 1976) coined the term geoida l eustasy after measurem ents from satellites revealed that the irregu-larity of the geoid also causes eustatic cha nges. At present it is widcly ack no wle dged that the sea level mov es con tinuously both vertical and horizontally forced by changes in the lithosph ere and the hydrosphere and their diffe re nt velocity o f resp onse (Fig. I).Thus, the term eustasy applies to any change of sea level regar dless of it bein g or not global and coeval.

Global factors such as the "glacio-eustatic general sea-level rise" that took place after the Last G lacia l Maxi-mum until ca. 7 000

yr.

BP (Goyet al.,1996) co ntrol the present position of coastlines, b ut reg ional factors also contribute significantly to their evolution and trends .In the coasts of the Southern IberianPeninsula, there are three ma in region al fac to rs . (I) The geographic loc ation (between 36°-40° N and 3°E-9°W) largely controls the wea ther and climatic trends. (2) The tectonic framewor k (boundary betwee n the European and A frican p lat es) wherenumerousfaults ac tive during the Late Pleistocene and Holocene have affec te d the coastline producing upli fting or subside nce (Zazoet al. ,1994).(3)The effects of the interc hange of Atlantic and Medite rranean waters thro ughdieGibraltar Stra it.

D uring eustatic oscillations thc coastalenvironments move latera lly tens to hundreds of kilometres and verti-cally tens to hundreds of metres de positi ng sedimentary units that very often are disconnected laterally and far away from each ot her. Th e laterally shifting high-energy surf and bre aker zones cut erosional surfaces. Eustatic falls expose large areas to subaerial erosion.

Apart from presently known possibilities to discriminate the vertical upward and downward relative movement of se a level within sequences of coastal depos its, a more co mplica ted effort is to incorporate the three-dimensional evolution of the shore line . Another com plexity is the de termination of the timeframe in which the distinguished patterns ofcoastal seq uences havebeenformed. Generally, coastal depos its do not easily lend themselves for precise d at in g of successive de posi tio nal steps. Rec ent and subrecent depos its form an except ion,e.g. through 14C dating possi bilities. Altho ugh seq uence stratigraphy helps to dis tinguish related sedi ment packets deposited unde r conditions o f varying acco mmodation space, careful sedimentolog ical analys is sho uld precede a se quenc e stratigra phic scheme (Van Wagoner& Bertrams,1995). Even then it is good to keep in mind that the number of variati ons on the general sequence stratigraphic model is limited only by the imagination (posamentier& Allen, 1993).

EUSTATIC CHANG ES AND STRATIGRAPHI C

ARCH ITECTURE

Sea level rises do not necessarily imply transgression and sea-level falls regressions, because these terms refer to latera l shits of the shorel ine (the coast indicated in maps)

CienciasdaTerra (UNL), 14

towar d the contine nt or the sea re spe ct ivel y, becau se another essent ial element is the sedime nt supply (Fig. 2A) . High input can produ ce regression with stable or rising sea le ve ls; in contrast, re duced input can fav o ur transgression even with stab le sea levels.

Positive eustatic oscillations not always reach the same topo graphical elevation. Th is implies that sedimen tary units forming an apparently conformable succession of coastal deposits separated by erosional surfaces formed during "sea-level falls" record only a part of eustatic histor y bet ween the depo sition of the oldest and the youngest outcropping units . Most probably the erosiona l surfaces represe nt one or mor e osc illations (Dabric, Zazo

et at .,1996,Zazoet al.,199 6). Of course palaeontological

or radiometric controls may, at leas t theoretically, solve this problem, but very ofte n the eustatic events are too rapid as co mpared to organic evol ution, or there ar e not materia ls suita ble for radiometry. Besides, many eustatic event s either leave no recognisable deposits (even if sea levels reache d an adequate elevati on) or their depos its were eroded later on. Last, but not least, it is not always easy or possib le to di scriminate be tween adjacent sedimentary

units

due to simi larity of facies and fossil contents.

As eustatic oscillations take place superimposed at various scales and have diverse wavelengths (Fig. 2B), shifts of the shoreline may look chaotic and the resulting sedi mentary record more or less unfo reseeable (Fig. 2C) . Coastal sed iments occur spread on large areas ofthe basin margins as the shorel ine shifts bac k and forth, and a good part o f it is eroded by the moving shoreline, tidal channels and subaerial processes during lowstands.

Tecto nic up lift and/or subsidence also control the stratigraphica l architecture of coastal units (Fig. 3). For this reaso n it ismoreadequate to speak about relative changes of sea level. It is not difficult to recogn ise the tectonic trends andthevelocity of move ment by com par-ing the topographic elevations o f the coastli ne in two geological ages (Zazoet al., 1993). Generally speaking, coastal units are arrangedinoffiap or in staircase in stable or uplifted

areas

whereas they occur stac ked or onlapping

in

subs iding regions (F ig. 3).

Estimation of eus tatic magnitudes is relativel y easy when con tinuous seismic profiles are available and the lateral shift of the coastal

onlep

ca n be measured . How-ever, surface studies (and more specifically the study of Quaternary units) are based mostly in laterally disc ontin u-ous outcrops (cliffs, quarries, road cu ts) and vertical sec-tions . These req uire to study and interpre t the thickness , environmental and palaecological meani ng, and water depth at which facies were depo sited, and ca refuluseof the so-called Walter 's ' law' (Dabrio& Polo, 1987). Th en it is time to deduce the locat ion of the shoreli ne or ot her features ab le to indicate water-dep ths .

1° Congresso sobre0Cenozoicc de Portugal

MEDITERRANEAN (microtidal) ATLANTIC_imesotldai)

~-

AUCANTE • MURCIA MUR CIA Al.MEJUA -NfJAR

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., ,

_{PIei$tociene)}

Fig. 3 - Geometry and sedimentary models ofTyrrhcnian marineunits as relatedtotectonics and distance to thc sourceof sediment (afterZazoel af., 1993).

Fig.4 -Usc cf cross bedding generatedbythe plunge-step in the lower foreshore, to reconstru ct beach progradation and retrogradation. and short-term trcnds of sea-level oscillations

(modified after Bardaj let al.,1990).

predomi nant gra in size. In gravelly beaches it is marked by an accumulation of the coarsest grain sizes available, locally crude planar cross bedding po inting

seawards

is preserved.In sandy beac hes it is evidenc ed by a change from the sea ward-incl ined parallel lamination ofthe swas h zonetotabular crOSS beddi ng pointing to se award.

The limit between shoreface and foreshore is used in tidal coasts , but cons ide rably less p reci se. Finally, a correc tion for co mpaction mustbeincorpora ted. Parti cular attention must be paid to erosional surfaces. For instance, the landward retreat of a barrier island produces two erosiona l surfaces related to breaki ng wa ves in the foreshore andtothe base of tidal channels and inlets.

f iWl.l PUHl

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I5ol<>pic5139"5

lJQl U - _meas.u_ n1S(Hil a im-Ma..::ele1a t.. 1986)

Fig.5 -Sequence ofTyrrhenian beach deposits exposed along the western(righ t)margin ofR amblaAmoladeras. Offlapping facies arrangement and sinistral(N40-4S0E) and dextral(N140· 1600E) wrench fault systems with vertical movement (modified

Ciincias da Terra (UNL) ,14

EVIDENCE O F SEA ~ ~ E V E L C H AN G ES I N COA RS E-G RAINED BEACH DEPOSITS

One of the bestpreserved seq uences o f prograd ing Tyrrhenian deposits

in

the Western Mediterranean is nicely exposed along the western (right) marginof Rambla [wadi] Amo laderas(Zazo

et aI.,

1998). Th e

Last

Intergla cial (IS 5) is characterised by three see-level bigbstands during ISS 5e, represent ed by three

maiine

terraces in

Almeria

(Zazoet

al.•

1993) withamean age of 128 ka (Goyet

aI. ,

1986, H illa ire-Marcel

et al.,

1986), suggesting climatic instability. Amarin e terrace corresponding to ISS 5e(ca. 95 Ka) is recorded in a ll coas tal sectors, and anothe r marine un it representing ISS 5a (ca. 85 ka) in uplifting tre nding sec tors ofAlmeria and Alicante.

InRamb la Arno laderas, three Tyrrhenian morpho-sedi-mentary units correlated with the isotopic stages 7, 5e and 5c occur separated by eros ional surfaces with beach features and evidence ofearly cementation (Fig. 5). Th ese units rep resent highstand facies during the Next to

Last

andLastIn terglacials, and erosional surfaces ind icate the intervening falls ofsea level. Aeolian dunes separate these un its from the un derly ing un its o lder beach deposits (IS 11 or

9).

Sed imentary units

were

depo sited in gravelly bea ches with

a

wannfauna of

SlTombus bubonius,

between 180 and

100

Ka.Theoutcropisaffected by a system of normal faults directe d N 140 -1 60oE, that locally pr odu ces a di sp lacement o f

the

present coastline.A fa ult directed N40-60" Econ trol s both the elevatio ns at which beac h deposits occur at both sides of the ramb la and the sudden inflexions o f tbe present coas t.

The

offiapping pattern ofthe morpho-sedimentary units and their variable elevations reflect tectonic act ivity with mea n uplift rates o f 0 .068

mm1yr.

This fact, tog ether with isotopic da ta, sugg est that the sea level duri ng Isotopic Substages 5c, 5e, and 7a was very close. Beach deposits

c on s ist mo stly o f c on gl o mer ates, sands to n es a n d mudstones arranged in coarsening-upwards se quences (Dabrioet al,1984 , 1985) that include

in

ascending order (F ig. 6): ( I) A lower unit o f parall el-laminated an d wa ve -rip p le cross-lamina ted micaceou s sandsto nes, commonly burrowed, representing

the

lower shoreface and transition zones. (2) A middle uni t o f trough cross-bedded sand and gra vel corresponding to shore face zone und er wave action. Th ese two units

are

poorly expo sed. (3)An

acc wnulation of the coarsest gra in sizes availabl e on

the

coastl ine, decreasin g upwardsupto well sorted fine gra vel. (4)An upper interval of well sorted. parallel-laminated gra vel, gently incl ined toward s the sea, rep resenting the upper part of the foreshore and berm.

EUSTATIC C HANGES AND STRATIGRAP H I C ARC HITECTURE OF G IL BERT-TYPE DELTAS

Thc bay of Cuatro Calas (Aguilas, Murcia Province) wasgenerated during Late Pliocene times by a local down-ward bending of olde r Neogene materials (Zazo

et

al.,

1998) . Th e bay, ma rgin a l to the Med iterranean Sea, extended inland in an western direction and was filled with a co mplex succession ofunits separated by erosional s urfaces th at in some cases remo ved mos t of the sedi mentary record. The four older units ( I to 4, Fig. 7) a re la rge-sc a le cross st ra ti fie d, with la rge foresets (clinoforms) . Th e younger units occ ur under a gen eral tabular geometry made up o f

a

large nwnber of interbed-ded marine (M -O to M·5) and terrestri al (T-Oto T-3 ) depo sits arranged in a multi- storey trans ition zone. The verti cal stacking of all these units gen erated a Gilb ert-type delta which records a co mplex histo ry of relative sea-level changes, tectonic trends and modifica tions of sediment supp ly. Succe ssive delta lob es progressivel y filled the bay during bighs tand s, but ero sion removed part of the lobes during

lowstends.

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-Fig. 6 - (A) Typical beachprofile for Med iterranean gravelly coasts.(8)Compositesequenceoffac ies generated by progradationofgravelly beaches. (Mod ifiedafter Dabrioet a1.•1984. 1985).

1° Congresso sobre0Cenoz6ieo de Portugal

E

'"

···'M-2 ···...

poles01coastal limil

"

--

...

w

""

·a···._2

1 ~ . ::. .

Fig. 7 - Panorama of the Cuartelillo hill(modifi edafter Dabrio

et at.,

1991).Numbers indicate sedimentary units.

Th e proposed model is a delta co nnected to a coarse-grained ri ver plain with rapidly shifting channe ls, flowing mostly from the north (line source ) (Fig. 8). Terrestrial (fluv ial) deposits include coarse -grained (channel fill) and fine-grai ned (flood pain) facies. The coarser gra in sizes were reta ined in gravelly reflec tive beaches at the wave-worke d de lta front. Terrestrial and coarse-grained coasta l depo sits overlay the foreset unit with an intervening flat, erosional surface gently inclinedinthe average direc tion ofdelta p rogradation(ESE).Adelta platfonn cove red with burrowed sand and scattered pebb les and boulders extended seaward ofthe wave-worked delta front. Shallow marine to coasta l dep osits consis t of light yellow to grey, well so rte d , conglomerates with ro unded and notab ly spherical clasts deposited in coarse-gra ined beaches ofthe reflective type . A stee p slope with dips rang ing from25" to 12" connected the delta platform w ith the shelf. C linoforms evidence the progradation of units under the action of sediment -laden currents which supplied sand to

the basin

margin

and slope . A lternati on of short periods with hyperpicnical flows down the de lta slope (dominance of sedimentation) and longer periods of quietness (domi-nance of burrowing) conditione d the internal struc ture of the foreset. The Cuatro Calas G ilbert delta is comp arable to present-day de lta s of eph emer al strea ms feed ing gravelly beaches in SE Spa in. Progradation of the delta generated a two-fold large-scal e cross-stratification: a topograph ically lower high- ang le related to the advan cing delta-foresetand the upper one asso ciated to progradation of beac h units (Fig. 8).

Ev idence of eustatic changes co me from the seep-sided, large-scaleero sional surfaces in the forese t facies that progress deep into the previous foreset deposits with varia ble orie ntatio ns. These are interpre tedasincision and partia l destruction o f delta-front and fo reset de posits during relative falls ofsea level. New delta lobes deposi ted during later highstands had to adapt to inherited topogra-phies (Fig. 9) and prograded to E, ESE, SE, and Eas shown

HIGH ANGLE CROSS BEDDING progradationofdelta foreset

. ~

,. 0 '

_..

'

p1alfonn sand+pebbles

l OW ANGLE CROSS BEDD ING rogradationofbeach

foreshore berm gravel ...gravel

STAG EI

LOWSTAND AND EROSHJN

Ciincituda Terra (UNL),14

STAGE I

HIGHSTAND AND DEPOSITION OF A NEW LO BE

---Fig. 9 - Generation ofa complex Gilbert-type delta (modified after Dabrioet~l ., 199 1).

TRANSITION-ZONE UNITS

Ma rine ~

LS

Marine

? F

LS L: :T

~ . TST+HST

Marm

~

.=:: ""~ ~ ::: "" ~ ,~

TST+HST

Post-Gilbert

M-5

T-3 M-4 T-2

M-3 M-2

T-l

M-'

T-(,

M-<l

DELTA-FORESET UNITS

Fig. 10- Sequencestratigraphy in theCuetrcCalas Gilbert-type delta(modifiedafterDabrioelal., 1991).

by dip of

clinoforms

in units

I

to 4 respectively. Sequence stratigraphy of the foreset units is explained by fluctua-tions ofsea level (Fig.

10),

but only the highstand delta deposits are expo sed; lowstand wedges lay und er the present se a le vel. Ba cksets ov erl yin g the er osiona l surfaces were deposited during relative lowstand and early transgressive episod es.

Th us, the develo pment of the complex Gilbert-type delt a followed a repetitive pattern in response to fluctuations of sea level: prograda tion ofhighstand Gilbert-type deltawas follo wed by en trenchmen t and coas ta l wedge in lowstand and early transgression (Fig .

10).

The various parts of the del ta reacted differently to these

see-level

changes and generated variable facies

associations.

Prograda tion of complex de lta pr isms does no t imply a continuo us stillstand as previously

assumed

for simple Gilbert-type deltas . The rapid evol ution of the bay-fill

can

berelated to

hi gh-fr equen cy fluctuations

o f se a level already documented in SE Spain during La te Pl iocene and Q ua ternary, supe ri mposed on longer-lasting tec tonic activity.

T HE INFL UEN C E OFA CCOM MODATI ON SPACE GENERATED BY EUSTASY AN D T E CTONICS

It is well ill ustrated by the late Messinian Sorbas Member in Almeria prov ince aro und the town of Sorbas due to absence of burrowing by raised water salinity and expo sure along a network of up to 30·m deep canyons (Roe pet al., 1979, 1998). Th is unit consists inits type

area

of an up to 75 m thick para sequ ence set of three prograding coastal barri ers (sequences

I-III),

assoc iated with lagoon and was hove r sedi ments (Dabrio, Roepet al., 1996). Logging and car eful corre lation of fifteen vertical sec tions perm it to reco nstru ct and d iscuss pattern s of relat ive sea-level mo vements between

decimetres,

up to

15

m within a parasequence. Excellent examples of

non-tidal transgressive facies are characteris ed by lagoon and was hover sediments instea d o f the usual combination of washover and tida l deposi ts (channel and flood -tidal delta) .

The lower two sequences are deposited in a relatively large, tec tonical ly enhanced wedge shaped acconunoda-lion sp ace . T he y sho w b oth

fining-up,

de epening

sequences,

followed by prograd ing

coarsening-up

shoaling sequences and can be compared to th e c lassical parasequences of the Western In terior Basin (USA). Progradation o f seq uence II was interrupted by a majo r slide even t (most likely triggered by an earthquake). that caused mo re than 400- m seaward slumping of a stretc h of 1kmof coastal sands.

Thearchitec ture of seque nce III is more complex due tolimited acconunodation space characteristi c for the late highstand, with a wedge-shape d acco mmodation spaceto the east (" sea" ) that changed to a more box-shaped space towards the wes t ("land") (Fig. II A ). Therefore, th is setting was very sensitive to sea leve l fluctuations and the coastline reacted in a j umpy way.

Taking the thickness ofUnit

III

as an indicator of water depth and magnitude of see-level fluctuations involved in the cha nges of coas tlines, a water de pth o f ca. 10m is

_ _ W"O~OIl ~ ~ICC :

.ifhOdlIIIon...-INR.UENCE OF ACCOMMODATION sPACE

I-Congressosobrc:0 Cenoz6ico de Portugal

A ·_

_

----_

.~.-.

""""-'11-INFLUENCE OF SUBRE DEFOR...TIONS

Fig. 11 • (A) Influence ofthe available accorrunodation space in the generation c r coesiat units. 1 10 4: ideal steps during fluctuations (modi fied from Dabrio,Roepet af.,1996).(8 )Influence of tectonicelly induced irregularities in the substratum on the location of

beach ridges and lagoons (modified after Dabrio, Fortu inet af.,1998).

deduced (Fig. 12). This is further supported by the position that occupy in the vertical sequence certai n sedimentary featur es indicative of the coast line such as bird-foot tracks, swas h zo nes , and beach-rock leve ls.

Next to relati ve sea-level chang es and differential uplift also related geometrical factors such as gen tleand loca lly occ urring synsed imentary fold ing appear to have influe nced the stratigraphic architecture of the Somas Me mber an d bad palaeogeo gra phic implications. The crea tion of a rough ly N-S tr en ding ridge and swell morpholo gy around Somas contro lledtheprominent N· S orien ta tion ofthebeach ridges and lagoons (Fig. I I B).The rising areas ("anticlines) acted as nuclea tion areas for barrier islands, where astheinterve ning gentle depress ions favour ed the development of lagoons. A crucial point is th at th e (p res en t) differen ce in elevat ion s between the se a reas is usu all y 3 or 4 m , and mo st o f the irre gul arity wa s com pe nsa ted a fte r deposition of Un itIII(Fig. 12).

In the field, Sequence IIIis readily recognised as a parase quence. Upon closer inspec tion, however, it appears to co nsis t ofj uxtaposed transgressive and pro grading parts, separa ted by erosional surfaces and floo ding surfaces due to sea leve l fluctuations within the limited accommoda-tion s pace. M inor trans gress ions induced formaaccommoda-tion of landw ard moving washove r fan systems intercalated in lagoonal mud. which finally were partly erod ed by the retrea ting barri er. Periods of forced regression caused the seaward stepping of tcmporary barriers coastline over more than J.5 km, lea ving behin d "s tranded" coastal barriers (Fig.

J

t Al . Duringthisprocessthesemay show landward ero sion. or sea ward progradation depe ndant ofthe smal ler sea -le vel flucruations.

The

lateral jumping and erosional surfaces in parasequenceIIIseemto be good indicato rs for a much more red uced accommodatio n sp ace, as is the exaggera ted infl uence o f wa ves in the for esh ore as d est ru ct ive agents. Du e to it s more co mpl ex facies architecture, Sequence III might fonn a nomencl atorial prob lem : is it one paraseq uence or in fact a composite of 4 stac ked parasequen ce s, i.e. of similar ran king as se quences I and II, given the largest sea-level fluctuations dedu ce d? A main conclusion is that erroneous sequential (stratigraphic) interpretations are eas ily made when acconunodation space is limited.

46

Til E RECORD OF ESTUARI NE INFILL DURIN G TI l E LASTPOSTGLACIAL T RAN SGRESSION

The late Pleistocene and Holocene evo lution of the estuariesinthe Gulf of Cadiz (SW Spain)has beenstudied using dri ll co res, logs , trenches, and 38 new radiocarb on data . The resu lts

were

compared with the shelf to obtain the sedimentary interp retati on of the estuarine fill of the incised valleys (Fig. 13) cre ated along the prese nt coast of the Gulf of Cadiz during the Last Glacial fall of sea level (Dah rioelal.,1999 , 2000 ).

The

studied estuaries reco rd repeated incision during falls of sea level with prese rved deposi tion duri ng at least two higbstan ds of Late Ple istocene (I S 3)andHolocene ages. The overall pattern co rrelates withthe4'-order eustatic o scillations an d th e s uperi mposed 5111_{-ord er cycles}

recorded in depositional sequences found in the sh elf off the study area (Som oza etal.,199 7, Nelsonetal., 1999, Roderoet al., 1999 ).

Th e Odiel, Tin to an d Gu ad alete rivers depos ited co nglomerates during a highstand that did not reach the present se a level dated at ca . 2 5- 3 0 Ka (I S 3), co rresponding to a relatively humid period in the area . Rivers incised these coarse-grained deposits durin gthe last main lowstand at ca. 18 Ka, when sea level dropped to - 120 mandthe coastline lay14kmseawards from the present. The erosiona l surface is a sequence boundary and also the flooding surface of the postglacial eusta tic rise, overlain by the valley-fill deposits of the transgressiveand highstand phases of thelast4111_{and SIII-order depo sitional}

sequences reco gnised in the shelf. Vanneyet at. (I98S) found erosional channels in the shelf of Pleistocene ag e that havebeencov ered by Holoce ne sediments

Acco rding to da ta from the shelf (Hernandez-Mo lina

et al.;1994),at thebeginning of the Holocene and the

F1andrian transgression (ca. 10,000

yr

BP) the sea level

in

the Gulf of Cad iz was 30-35 m below present MSL.The

land-Ciincias do Terra(UNL) ,14

Time

•

13

14

_ ~ -'{J.

₁₁

I

S

white limestone layer

SEQUENCE III

·1

94 8

1000

12

750

7

SEQUENCE II

500

3

250

6

0

2

50 m

0

0

10

20

30

40

50

m

Fig. 12 · Es timates of sea- leve l fluctuations in Sequences II and III through ti me rela ti veto the po sition of the Zorreras white limestone datu m plane. To facilitate descri ption th e sea- level pos itions1- 14are indicated o n inset. Because ofthe

unkno wn time constraints the distan ces on the time axisan: arb itrary (modified after Roepet al.; 1998).

ward migration of the estuarine barrierinthe Guadalete. Most of the input of fluvial sed iment was trapped in the bay-head deltas and the axial zones of the estuaries . After 6500yrSP the evolution of the estuarine filling is closely related to the decrease of the rate of sea-level rise from5.7mm

yr-'

between WOODand 6500yrBP, to 2.6 mm yrlbetween 6500 and 40 00 yrBP recorded inthe central estuarine basin. Th e rate offluvial input to estuaries surpasse d the rate of sea-level rise (l.e .• crea tion o f accommod ation). favouring the vertica l accretion of tidal flats and the accumulation of the spitunit~ colonised by human settlements (BOIj aet al.•1999). Vegetation changed 10more arid conditi ons. T he increase in vertical accretion caused a reductio n of the tidal prismsinthe estuaries that allowed the acc umulation of stable spit barri ers and the app arent effect of waves was magnifi ed. The effect is particular ly felt between2500-2700 yr BP. when the spit M it H)began to grow and the first associated aeolian foredun e systems acc umulated. In Guadal ete, two fluvial cou rses supplie d sediment to the active littoral drift and favoured the gro wth of a broadH)spit barrier crossed by a Roman way.Inthe last500years, the confluence ofl ittoral drift. large fluvial input magnifiedbydeforestation, increasing aridity favoured the accumu lation ofthe largeH ~ spit units. Historical data. particularly the present position of watch lowers built in the170, century AD, evidence rapid coastal progradat ion rates up to 3 m

ycl

(Larioet aI.,1995).

The filling follows a twofold pattern in respo nse to the progressive chan ge from vertical accretion to lateral (centripetal) pro gradation. First, subaqueous sedi

menta-tion greatly reduces water depth and the accommod amenta-tion space, but that does not have an appreciable effectinthe surface morpho logy, except near the bay-head delta. Then. the surface occ upied by the flood ed estuary diminishes rapid ly due to fast expansion of th e emergent area s. producing rapi d geo gra phical changes and land scape modifications.

A main deri vation is that onl y the radiocarbondata and depths for a single dri ll core can be used to co nstruct reliable curves of subsidence rate. A single curve reflects sedime nta tio n in a p articu lar spo t o f the est uar y. Comparison of curves obtained in an estuary will give a "m ean" curve of subs idence.

Three washover fans breaking the part ofthe spit active at ca.1750 AD may be the result of the tsunami associated to the so-ca lled 1755 Lisbo n earthquake that reached a Richt er magnitude8.5-9 (Dabrio, Goyet al.,1998. Luque

et

al. ,1999).Itis difficult to date accura tely the washover

fans becaus e of the lack of s ui ta ble fo ssil remains. Continued sou thwards migration of the San Pedro River channe l since 1755 has eroded the area immediately to the south of the spit barring the chances of findin g other preserved remains.

T H E RECORD O F EUSTATIC C HAN GES IN PLEISTOCENE AEOLIAN DEPO SITS. GULF OF CADIZ

Coasta l areas that re mained largely emerg ent during eustatic osci llations under aeolian influence(Menantcau,

j"Congressosobre oCcnozoico dePortugal

Fig. 13 - Four palaeogeographical steps of the changing landscapes in the Guadalete (above) and Odiel-Tinto (below)

incised valleys(modified after Dabrioet a/.• 2000).

1979, Borja 1997) also record the cha nges, although the stratigraphic record is some what different. The Quater. nary sa ndy de posits form ing th e E I Asp erillo cliffs (Hue lva ) exem p lifi es thi s case stu dy. H ere . the stra tigr aphic relat ionsh ip s, ge nesis and chronology . includi ng ra diocar bon da ting were studie d with spec ial emphas is on the influence of neotectonic activity, sea-level changes and cl imate upon th e evo lution of a subaerial coastal zone(Dabric,Borjaet al.,1996, Zaz oetal., 1999). Normal faults ac tive duri ng the Quaternary co nditioned the sed imentation in the coastal zones of SW Spain. The E-W trend ing Torre de l Lora norm al fault was a major control o n aeoli an sedi mentation in the area of the EI Asperi llo cliffs during the La te Pleistocene (Fig . 14) and on the coastl ine direc tions during the Mid dle and Late Pleistoc ene. The fault separates two tecton ic blocks but its movement stopped before the Holocene. Tiltin g ofthe upthrown fault bloc k (Torre de l Loro - M azag6n) to the No rthwest dur ing the Ea rly and Mi dd le Pl ei st ocene con trolled the displace men t towards the Northwest of flu viatile cha nne ls o f a trib utar y o f the p al ae o-Guada lquivir River. In this blo ck. mari ne de posits of probable Last Interglacial age have bee n preserved, and arc subaerially exposed nowadays. Their presence in the area is mentioned for the first time in this pape r. In our model, the Torre del Lora fault created accommoda tion space in the downthrown block where aeolian sedi ments accum ulated during the Last Glacial period , and probably during

part

ofthe Last Interglacial. The vertical offset along this fault is 80 m since the Early Pleistocene, and 18 to 20 m during the Late Pleistocene.

At present, the most active normal-fault system trends NW-SE, contro lling the direction of the shoreline and the pattern offluviatile valleys on land and of drowned flu vial valleys on th e she lf. Thi s sys tem and th e important NNE-SSW system separate blocks, like the Abalario sector, in 'domino tectonics' fashion along the Huelva co ast.

Six aeol ian and aeo lian-re lated sand deposits (U nits I to 6, Fig. 14) , stacked in the do wntbrown bloc k, ar e sepa-rated by bounding surfaces (supersurfaces) in response to major changes in sand supply and wetness of the substra-tum which contro lled the dep th of the water table. In contras t, the aeo lian depo sits below the iro n crust in the upthrown bloc k, thought to correspond to Units 1 to 3, are th inner and dep osited on drier s ubstrata, with severa l supersurfaces marked by palaeosoils indica ting repeated degradation and de flation.

Ciencias daTerra(UN LJ, 14

SE

__ surtaoewdh0l'gllIIicmane,

._ - surtacewill'lotJCl0tg80icman.< .... ~ 0ItlI'il;:hedin ironoUI8s

I

NW

BAsperilo{...tion 103m)

!

..

-

. . . .

_-- ~ - ~ ~

....

.,.,..

..

m

. ~

\1 " MAT· 'o U-6 •

IL

~ T -ll

"j.8:I

_{__I"}

. _.

_{1.44T- 18. J -}~ T .H _{U-5 --- . ...}

_.1'

U-S- - - ~ -':! ~ - - · _ - _ · - - _ · - U-4 ...- - - ... ~ .. -U-4 :.-.:; :.,-.: :.-,;;;.-,.;; :.: .: :.-.:-_ ____

+ ....

e - ... - - U-3 _{MA. T ~ ; ;;;'}

::::_!

Iia n

L

~_ l~ _ ~ ,...:-- ~ 4: E

• 1.4"" ' 4 " """" " ' " " ~ !Ii:

? ~~: - /' mir l...

-:!il\

~ ~ T~~ MA T" 5 '." . :-. : . _•• ·· U-2·._ •. •:".:" _ . ~

d

25 24 Z3 22 :n 20 " Ie ~ ': / l e \ 1 5 ,,-\ 4 13 12 n ' 0 • , 1 eo 5 4 3 2 1 0,,",

'" \ lolAT-t

,.Jl,

.,,,

l41

...

.-sooZ bl "' b4

Fig. 14 - Schematic profile showing di5lribution of un its in the EI Aspcri llocliffsand samplepositio ns,Distances inkmfrom the tower (Torre) o fLaHiguera. No te strong vertica l exaggeration. Palaeocurrent and wind direction s shown in mapview.The Torre

del Loro fault cited in the text is placed betweenKm16 and 17(mod ifiedafterZazoet al., 1999).

8Pt the latter age corresponding to an acceleration of the rise of sea level. Unit

3

records wet conditions . The supersurfa ce separating Units

3

and

4

fossilised the Torre delLorafault and the two fault blocks. Units 4 (deposited before the 4" millennium BC),

5

(> 2100

1'"C

yr

BP to 16th century) and 6 (16th ce ntury to present) record re lati vely arid conditions . Prevailing wind directions changed with lime from W (Units 2--4)toWSW (Unit5)

and SW (Unit 6).

The generation ofthe aeolian depos its started prior the Last Glacial period and continued until the

present

day. Aeolian Unit s 4, 5 and 6 are probably rel at e d to progradation ofspit systemsinestuarine barriers. The sedi-mentat ion gaps found in the spits may correspond to the supersurfaces separating Units 4, 5 and 6. Both seem to be related to short-lived (250-300

yr),

relatively humid periods with a relative rise of sea level.

Wind directions changed during the deposition of the aeolian dunes: the oldest recorded (upthrown fault block) blew towards the north and south. Units 2, 3,and4 accu-mulated under prevailing westerly winds. Palaeowinds blew fromtheWSW during Unit 5 and, as at present, from the Southwest during Unit 6. The changeinpalaeowind directions from Unit 4toUnit 5 mayberelated to a change from wetter to more arid conditions since 4000 I·C

yr

BP, as suggested by pollen in

Las

Madres peat bog.

There are also three systems of dune systems related to estuarine barriers in the Gulf of Cadiz (Borj a

et al.,

1999). The oldest one, 0 1, accumulated under prevail ing WSW winds during the 1st millennium Be overlays both the occupational horizons of Late Neolithic-Early Copper (4th millennium BC) and the ' lithic workshop levels' (4th-2nd millennium BC). The middle dune system 02, con-taining both Roman and Medieval remains, accumulated between (?) 13" (1) -14* century and 17th century AD. The youngest 0 3 system occurs associated to the time of building of watchtowers in 17th century AD but extends to the present.It is related to prevailing winds from the

sw.

The absence of aeolian deposits prior to

- 2700

cal BP is the result of trapping ofa large

part

of the sediment supplyin the estuaries, which starved the neighbouring beaches and aeoli an settings. Aeolian accumulation reached significant values when sedimentation in the coastal zone changed from being mainly aggradational

in

theestuaries(-6500·nOO

cal

BP) to mainlyprogradational in spit barriers andrelated

dunes

(post - 2100 cal BP). The pre sent ana lysis of a eolian syste ms s uggests a non-direct correlation, at least in some cases. between coastal progradation of sp it ba rr iers and aridity as previously assumed.

CONCLUSIONS

Six co nc e ptua l mo de ls exe mp lify di verse in stratigraphic architectures in response to various types of relative sea-level change and associated lateral shift ofthe shoreline (Fig. 16). Illustrated are 6 (I - VI) hypothetical eustatic sea level curves (the sinuous lines in the middle) deduced from these deposits and associa ted shifts of the coastline that can be reconstructed from the lateral and vertical stacking of the coasta l sediments (left).

(I ) Coastal onlap records a steady rise of sea level (which. in principle, is considered tobecontinuous, see thick heavy line) during a single eustatic oscillation (sll , sl2, etc. indic ate succ ess ive position s of sea le vel ). However, a discontinuo us rise may produce the same effect, particularly if scrutiny of sectio ns is not careful enough. During th is process the shoreline moves land-and upward.

(II) Discontinu ou s coastal onlap may refl ect a continuous sea level rise

with

variable sediment input (thick line at the left), a two-step

rise

with an intervening interruption, or sudden jump upwards (line inthecentre), or(evcn) an intervening minor fall (line at the right). In the meantime. the shoreline progresses as in the former case.

1° Co ngresso sobre0Cenozeico de Portu ga l

t

•

•

Eo

!=-3QOO

1000 9C

' ~.~

_••

-.-

_••

:. ' - ~

• , _ 'v'

·

fiI~

\lo~

_,

•

•

• - iiit

.=-.--.

·1-

• <Q>

~

•.

, "

.

~

UttoicworI<ahop""'Db (4111 10 2ndmIJemiumBC)

LateNeoIilNc-ElUlyCoppe<(3&40-3130 Be)

0kIesI da le<lage~ 4 C) '"splIsedi_1lon

0lde9IrnIrlIrrulao- '"splIsedlmentalion

baseduponan:IIaeologicaldata SpItbatunits (t.lediI" ...Atlantic:)

1

T

••

•

I

~"- I

.-..

_••

-AYAUONTE

AEO LI AN P H ASES AN D H UM AN SE T T LE M EN T S

PUNTA UMaRIA ooNANA VAl.DELAGFlANA~ ~

PUNTAARENIUA LAALGAIDA CANTARRANAS

,

~

,

• iiit '

_~

_,

I, ,~ i - :l

,,

·

_••

-

.

~

1

T

I'iI

WatehlOW9. '7thcenlufyAD

~ ~a l

iiir:

Lal"Roman(2IIl1o5111cent<>riesAOj

~ Romanway (,..tcenlBeIo2ndeenlAOj <:Q;>Pre-Roman(8lh102nd cenluryBC)

"CJ

Bronz".

(1850- 1650Be)

Fig. IS - Chro nostrat igraphy of aeol ian phases and human senlements in the Gu lf ofC:id iz. No te that italic typing ind icate calibrated ages (after Borja elal.; 1999).

GEOMErRY

(ARCHITECT URE) OSClI.UTlONEUSTATIC COASTUNESHIFT OF

GEOMET1'IY (ARCHITECTURE)

SHIFTOf

COA STUNE

:7. ~?

;;.?'

.

-o f - - - - --- - ; . - ' ...·ll4o _ _·_W ...

...-or, __ __ _ _ _.0. ... 01. - ~ i ; ' ... ... -:;. ~ ... V

8-,.

... ...

.

"

DISC(lNT1HllOUSCOAST... OHLAo• OI'n.A o

- ~

_••

- ~

-~~

.----_:to'

_....

~

...1····

III

Fig. 16· Conceptual models ofslTat igraphic architecture induced by a variety ofsea -level cha nges, and associated lateral shift of shoreline ( modified after Dabrio&Polo. 1996)_

(III) Coastal o fflap rec o rds (q uasi) steady sea level. Seaward shift of s horeline may (or may not)be p receded by other shifts (dotted linewithquestion mark).

( IV) A co mplex coastal offlap with

three

p ro gra d ing beach u nits

(B·1,

B -2, B·3) a t different to pographic elevations, eachcomposedof minor prograding un its(a, b,c), records at least a firs t seale vel rise followed by eus tat ic oscillations re achin g p ro gre s sivel y lo we r ele vations .Theshore line fo llo ws a zigzagging pattern.

(V) Two beach units(B.I ol de rthanB-2), each with minoroffla pping un its(a,b, c) record at leasttwo sea -level ma xima reach ing progre s sive ly higher el e vatio ns, with an int ervening stills tand, or minor fa ll that comple te these o scillatio ns. Th e shoreline fo llows a z igzaggi ng patt ern . (VI) A combinat io n offormer cases may p rodu ce m uch more co mp lex stratigraphical ar chite ctures, suc h as in the d isc on tin uo us co asta l on lap plus sim ple

offlap.

The se mo de ls c an be ap plied to the former ease stud -ies but it m ust be noted that differentiatio n

in

the field be tween si tua tion IV an d V may be d iffi cult when the bo undaries arc incomp lete ly e xpo sed. Furthcnnore, the lat e ral and vertical relat ionshi ps are m uch more compli-cated d ue totheinterplay of various o rders o f magnitude of sea leve l changes.

R E F ERENC ES

Ciincias da Terra (lINL), 14

The cases of gravelly beachesinRamblaAmoladeras andtheG ilbert-type delta of Cuatro Ca las areclassified as type IV forming a complex offiap o f several uni ts separated b y erosional surfaces. T he main d iffere nce betweenthetwo situations is tha t tectonic up lift

was

more p romi nen t in Amoladeras.

Sit ua tions I, II a nd III a nd th e ir com bi na tion in example VI a ll can be fo und

in

seq ue nce II ofSorbas, whereas examplesIVand V canbefound

in

sequence III . SituationVIapplie s to the po st -glaci al e ustatic

rise

in S W Iberian Peninsula . H ere , li mi ted p ro grada tio n o f Iithosomcs during te mporary stops o r times of reduced ra te o f r ise in termina l P leistoc ene ti mes produce d iscon-tinu ous coastal onlap plus o ffla p (V ), whereas progradat ion with qua si stab le sea leve l in Ho locene promotes sim ple offlap(III).

ACKNOWLEDG EME NT S

Fi n ancia l s u p port fr o m S p anish MEC proj e cts PB98-OS14andPB 98-0265. and ProjecrAreces: "Cambios climaticos y nivel d e l mar" . It is a contr ib u tio n t o IGCP437.

Bardaji, T.; Dabrio, C. J.; Goy, 1 L.; Somoza, L.& Zazo, c . ( 1990) - Pleistocene fan deltas is southeastern Iberian Pen insula: sedimentary controls and sea level changes.Spec. PubIs. lilt. Ass. Sedimem.,10: 129-1SI.

Borja, E ( 1997). Dunas litorales de Huelva (SW de Espana) Tipologla y secuencias P leistoceno Superior-Holocene . In: Rodriguez-Vidal, J. (Ed.),Cuaternario Iberica, Untverstdad de Huetva , 84 ·97.

Borja, F.; Zazo,

C.;

Dabrio, C.1.; Diu del Olmo, F.; Goy, l L.&Larin,J.(1999) - Holocene aeo lian phases and human settlements along the Atlantic coast o f soulhem Spain.The Holocene9 (3): 333·33 9.

Dabrio, C.1.; Bardajl, T.; Zazo, C.& Goy, lL. (1991) - Effects ofsc:a- Ievel changes on a wave-worked G ilberf.type delta (Late Pliocene . Aguilas Basin. SE Spain).Cuadernos de Geologia Iberica, IS : 103· 137 .

Dabrio, C.1.; Borja, E ;Zazo,C.;Boersma,I R.;Lario, 1.; Goy. 1.L.& Polo. M.D. (1996) - DImas eelicas y facies asociadas pleistocenas yholocenasen el acantilado del Asperillo (Huelva).Geogacela20(S): 1089- 1092.

Dabrio,C. 1.; Fortuin, A.R.; Polo, M. D.& Roep, T.B. (1998) - Subtle controls ofsedimen tation and sequence stratigraphyofcoastal settings: the Late Messinian Sorbas Member (SE. Spain). In: Caiiavc:ns.,l C , Garcia del CUTa,M.A. and Soria, J. (Eds.) ,

Sedim elltology at thedawnof the17IirdMille nllium .Abstracts. Publicaciones de la Universidad de Alicante: 279-28 1. Dabrio , C.1.; Goy. 1.L.&Zazo.C. (1984) • Dinirn ica litoral y ambientes sedimentarios en el Golfo de Almeria desde el«TtrrenienSClt

a la actualidad . 1CongresoEspaiiolde Geologia,I: S07-S22.

Dabrio, CJ .; Goy, It.&Zazo. C.( 198S)-A model ofcollglomerat ic beaches in tectonicallyaatve areas(Late Pleistocene-actual. Almeria, Spa in).6th European Meeting of SedimentologylA S.:104- 107.

Dabrio, C.1.; Goy, 1.L.&Zazo, C. (1998)· The record of the tsunami produced by the 17SS Lisbon earthquake in Valdelagrana spit (Gulf of Cadiz, southern Spain).Geoga ceta23: 3 1-34.

Dabrio ,

c.1.

&Polo, M.D. (1987) - Holocene sea-level changes, coastal dynam ics and human impacts in southern Iberian Peninsula. In: Zaze, C. (Ed.),Late Quaternary Sea-level Challges in Spai n.Musco Nal. Ciencias Natura les, CSIC., 10: 227· 247. Dabrio , C.1.& Polo, M.D. (1996) - Cambios eustariccs y arquitectura estratignifica de unidades costeras.Geog aceta 20(3) :

438-44 1.

\ - Congresso sobre0Cc:noz6ico de Portu ga l

D abrio, CJ .; Roep , T.o.; Polo , M.D.& Fortu in, A.R. ( 1996 ) - Late Messi nian coas tal barrier an d washover fan sed imentation in So rbas (SE Spai n).Geogaceta,2 1: 9 1-94.

Dabrio, CJ.;Zazo,C.;Goy, l L ; Sierrc, FJ .; Borj a, F.;Lario,l ; GonzAlez, lA.& Flores, l A (2000) - Depositional history of estuarine infill duri ng the last postglac ial transgression (Gu lf ofCidiz, sou thern Spain).Mar. Geo/.,162: 38 1-404 .

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