Spit-platform temperate carbonates: the origin of landward ...hera.ugr.es/doi/14976523.pdf ·...

Spit-platform temperate carbonates: the originof landward-downlapping beds along a basin margin(Lower Pliocene, Carboneras Basin, SE Spain)

J . C. BRAGA*, C. BETZLER� , J . M. MARTIN* and J. AGUIRRE**Departamento de Estratigrafıa y Paleontologıa, Universidad de Granada, Campus Fuentenueva, 18002,Granada, Spain (E-mail: [email protected])�Geologisch-Palaeontologisches Institut, Bundesstr. 55, 20146 Hamburg, Germany

ABSTRACT

Lower Pliocene temperate carbonates exhibit landward-downlapping beds at

the southern margin of the Carboneras Basin in south-eastern Spain. This

rarely documented stratal geometry resulted from the accumulation of bedded

bioclastic carbonate sand and gravel by longshore currents along a spit

platform located a few hundred metres from the palaeoshoreline. The top of

the spit platform was covered by shoals that extended over a gently dipping

ramp inclined to the north. On the landward slope of the spit, sediments

washed over from the shoal area were deposited in parallel-laminated beds

with a southward dip of 8–11�. These beds aggraded and retrograded after an

increase in accommodation space, probably related to an Early Pliocene

eustatic sea-level rise. As a result, the beds downlap onto the underlying

unconformity surface in a shoreward direction. Eventually, the depression

between the shoreline and the spit platform was filled, and a gentle ramp

became established. These Pliocene exposures in the Carboneras Basin and a

similar Upper Miocene example in southern Spain suggest that landward-

downlapping stratal geometries can be expected in nearshore temperate

carbonates along basin margins, and demonstrate a similarity in sedimentary

dynamics to siliciclastic sands and gravels.

Keywords Carboneras Basin, downlap, Pliocene, SE Spain, spit platform,temperate carbonates.

INTRODUCTION

Shallow-water, cool-water carbonates typicallyform on ramps where variation in facies dependson the local substrate geometry and hydrodynam-ic conditions (James, 1997). In addition to modelsdeveloped for settings dominated by strong wavesand currents in which significant sedimentationonly takes place below storm-wave base (e.g.James & Bone, 1991; Boreen & James, 1995),examples where significant nearshore temperatecarbonate accumulation occurs in much shal-lower water have been documented in recentyears. Several models highlighting nearshorenon-tropical carbonate production and accumula-tion have been recognized in the Neogene basinsof Almerıa in south-eastern Spain (e.g. Braga

et al., 1996; Martın et al., 1996; Betzler et al.,1997; 2000; Franseen et al., 1997; Brachert et al.,1998). According to these models, coastal depos-its can form either on beaches and shoals or asslope aprons related to clifflines. Skeletal tem-perate carbonates accumulated on beaches andshoals in the shallowest part of gentle ramps.Most organisms lived seaward of the shoals, andcarbonate particles were washed landward bywaves and/or currents during storms and incor-porated into the shoals and beaches (Martın et al.,1996; Brachert et al., 1998). Cliff-related non-tropical carbonates occur as buildups attachedto the walls or as aprons of skeletal fragments atthe foot of the cliffs (Betzler et al., 2000).

In this paper, a new model of nearshoretemperate carbonate deposition from the Lower

Sedimentology (2003) 50, 553–563

� 2003 International Association of Sedimentologists 553

Pliocene carbonates of the Carboneras Basin insouthern Spain is presented. At the southernmargin of this basin, carbonate particles accumu-lated on a spit platform formed by prevailinglongshore currents. The temperate-carbonate stra-ta downlap landwards onto an erosional surfacethat cuts underlying Miocene sedimentary andvolcanic rocks at the basin margin. In carbonatedepositional systems, landward-downlappingclinoforms were previously only known at theleeward margin of the shelf edge (Fig. 1) and onthe leeward side of isolated shelf mounds intropical carbonate platforms (Handford & Loucks,1993). In these cases, such stratal geometry resultsfrom the tendency of tropical carbonate systemsto create depositional topography by in situaccumulation as buildups. In the Pliocene rocksof the Carboneras Basin, landward downlapsresulted from the hydrodynamic accumulationof loose, carbonate particles lacking early cemen-tation. This model stresses the importance ofhydrodynamic control on non-tropical carbonate

deposition and its similarities to siliciclasticsystems in the nearshore environment.

GEOLOGICAL SETTING

Pliocene deposits are extensively developedwithin the Carboneras Basin, a small sub-basinwithin the intermontane Neogene Almerıa–NıjarBasin in the Betic Cordillera of south-easternSpain (Fig. 2). The oldest sedimentary rocks inthe Carboneras Basin are thin, Upper Miocenemarine carbonates and marls overlying Miocenevolcanic rocks of the Cabo de Gata volcanic region(Montenat et al., 1990; Serrano, 1990). The ma-rine Miocene sediments locally fill neptuniandikes in the volcanics. Pliocene deposits aremainly poorly cemented packstones to rudstones,which overlie an erosional surface on top of theMiocene volcanic and sedimentary rocks. Theyare Zanclean (Early Pliocene) in age accordingto locally recorded planktonic foraminiferal

Fig. 1. Large-scale stratal patterns encountered across tropical carbonate platforms and slopes. Landward-down-lapping packages are known to exist at the leeward margin of the shelf edge deposits, on the leeward side of mounds(after Handford & Loucks, 1993) and in hemipelagic drift deposits (drawn after a seismic profile from Davies et al.,1991).

Fig. 2. Location and geological map of the Carboneras Basin in south-eastern Spain. The Carboneras Basin is asub-basin of the Neogene intermontane Almerıa–Nıjar Basin in the Betic Cordillera.

554 J. C. Braga et al.

� 2003 International Association of Sedimentologists, Sedimentology, 50, 553–563

assemblages (Montenat et al., 1990; Aguirre,1998). At the highest relative sea level duringthe Early Pliocene, the Carboneras Basin formed asmall passage within the Mediterranean Seasurrounded by emergent volcanic relief (Aguirre,1998; Martın et al., 2003) and was only partiallyconnected to the Almerıa–Nıjar Basin by narrowstraits at its western margin. Before the highstandof the Early Pliocene transgression, the Carbon-eras Basin was an embayment open to the Medi-terranean Sea to the east (Fig. 3).

SECTIONS

Pliocene carbonates are well exposed along thecoastal cliffs between Mesa de Roldan and thevillage of Carboneras (Fig. 2). Two sections providethe relevant information to reconstruct the carbon-ate geometries and facies: Cala de la Pelirroja to thesouth and Torre Vieja to the north (Fig. 2).

Cala de la Pelirroja

The Cala de la Pelirroja section is located at thenorthern end of the Playa de los Muertos, where

stratal geometries of the Lower Pliocene carbon-ates can be observed in sea cliffs (Figs 4 and 5).The cliffs are accessible enough for logging andsampling the rocks at most points. Sedimentarysurfaces and individual beds can be followedlaterally for 750 m in an approximately N–Sdirection.

The unconformity surface below the Pliocenecarbonates erosively cuts Messinian (Upper Mio-cene) deposits comprising reef blocks and brec-cias embedded in silty marls and marlscontaining planktonic foraminifera (Serrano,1990). These deposits overlie the volcanic rocksof the Mesa de Roldan dome, dated at 8Æ6 Ma(Tortonian; Di Battistini et al., 1987). On thenorthern side of the section, the surface is anear-horizontal abrasion terrace, whereas to thesouth, it climbs at a gradient of 10� (Fig. 4).Incisions and pockets in the unconformity surfaceare filled by a breccia comprising minor volcaniccobbles and Messinian carbonate clasts heavilybored by bivalves.

The overlying Lower Pliocene carbonates canbe divided into two units according to theirstratal geometries (Figs 4–6). In the lower unit,metre-scale rudstone/packstone beds downlaponto the unconformity surface. Downlap surfacesdip southwards, and beds have maximum dipsthat increase from 8� at the bottom to 11� at thetop of the unit (Figs 4 and 5). In each bed, towardsthe bottomset, the dip decreases to around 6–7�(Figs 4 and 5), and dip values diminish north-ward as well. The dip flattens out in the TorreVieja section (see below) and changes direction,dipping to the north at a much lower angle.Although the sedimentary package graduallyprogrades over the substrate in the southern partof the section, it aggrades up to 30 m in thecentral and northern parts. This set of downlap-ping beds can be traced laterally for more than800 m to the west of the Cala de la Pelirroja cliffs.

The rudstones/packstones are made up ofpoorly cemented fragments of molluscs, benthicforaminifera, bryozoans, barnacles, coralline redalgae and serpulid worm tubes. Volcaniclasticgrains account for <1% of the rock volume. Grainsize is coarsest at the bottom of the clinoforms,where large bivalve fragments and algal nodules(rhodoliths) occur together with cobbles andboulders of volcanic rocks (Fig. 6A). The mainstructure in the southward-dipping part of thebeds is a crude parallel lamination, whereasmetre-scale trough cross-bedding is locally con-spicuous towards the north, where the dip alsodecreases until beds become flat-lying.

Fig. 3. Palaeogeographic setting of the Carboneras areaduring the Early Pliocene sea-level highstand. Shadedareas indicate emergent land. The Carboneras Basinformed a passage through the volcanic islands of theCabo de Gata volcanic province, connecting the Medi-terranean Sea with the Almerıa–Nıjar Basin (afterMartın et al., 2003). During the Early Pliocene trans-gression and before the highstand, the CarbonerasBasin was an embayment of the Mediterranean Sea.

Spit-platform temperate carbonates 555


The stratal geometry in the upper unit ismarkedly different from the underlying one asthe bedding within the deposits is subhorizontalor slightly inclined to the north throughout.Within the upper unit, four sets of beds can bedistinguished, each one consisting of severallaterally changing lithofacies. In the most com-plete ones (sets 3 and 4), up to 4 m thick, thefollowing lithofacies can be recognized fromsouth to north: (1) Horizontal, greenish silts and

marls with land plant remains. (2) Parallel-lam-inated packstones/rudstones rich in barnaclesand bivalves. South-dipping packstones/rud-stones interfinger with and prograde on thegreenish silts to the south. (3) Low-angle paral-lel-laminated packstones and rudstones indecimetre-scale beds dipping north. (4) Troughcross-bedded packstones and rudstones. Metre-size troughs point to the W–SW (N200–270E) andNE (N20–40E). Volcanic pebbles and Miocene

Fig. 4. The Cala de la Pelirroja cliffs viewed from the east. Volcanic rocks of the Mesa Roldan dome (8Æ6 Ma) areoverlain by uppermost Miocene (Messinian) reef blocks and breccias. An unconformity surface erosively cuts thelatter deposits. Incisions and pockets in the erosion surface are filled by a breccia of Messinian carbonate clasts andminor volcanic cobbles. Overlying Pliocene carbonates can be divided into two units. In the southern half of thesection, rudstone/packstone beds in the lower unit downlap the erosional surface southward as they aggrade. Incontrast, in the northern half of the section, the dip diminishes and the beds become flat-lying to slightly inclined tothe north in the Torre Vieja section. In the upper unit, bedsets dip northward. The surface below the sets is sharp andclearly truncates the rudstone/packstone beds that dip to the south. Inset shows area in Fig. 5.

Fig. 5. View of the Cala de la Pelirroja section from the north-east. Lower Pliocene rudstone/packstone bedsdownlap the underlying erosion surface southward as they aggrade up to 30 m. Maximum dip recorded from anindividual bed is 8–11�. Steepest dips within the unit correspond to the uppermost layers. These layers are truncatedby beds of the upper unit that dip northward.



carbonate clasts occur locally at the bottom of thetroughs. (5) Rudstones rich in large bioclasts thatexhibit relatively low fragmentation. Bivalves andgastropods are common as well as bryozoans andbarnacles. The abundance of red algal rhodolithsand fragments increases to the north where theyare locally the main components.

The first set of beds in the upper unit overliesthe erosional unconformity on top of the Messin-ian reef carbonates that have been heavily boredby bivalves. The successive sets have an aggrada-tional to strongly progradational geometry. Thesurface below the sets is sharp and clearlytruncates the rudstone/packstone beds that dipto the south (Figs 4 and 5). The convergence offacies makes it difficult to separate the individual

bed sets in the northern part of the section.Lithofacies belts in this upper unit can be tracedto the W of the section with a N60–70E trend.

A tectonic origin for the southward dip of thebeds in the lower unit of the Cala de la Pelirrojasection can be discounted. Tectonically con-trolled southward tilting of the lower unit bedscould only have taken place during their depos-ition and before the formation of the upper unit,as beds in the latter unit are subhorizontal toslightly inclined to the north. Such a rotationwould only have been produced by roughly east–west trending faults. Structures reported in theMesa de Roldan–Carboneras area are subverticalfractures oriented N170E (Montenat et al., 1990),and faults trending N150–175E (Brachert et al.,2001). Furthermore, synsedimentary tectonic tilt-ing of the beds would have resulted in steeperdips in the lowest beds, whereas the steepest dipswithin the unit correspond to the uppermostlayers.

Torre Vieja

Beds of the lower unit can be traced laterally fromCala de la Pelirroja to Torre Vieja, 300 m to thenorth, located at the mouth of the Barranco delHondo ravine (Fig. 4). Two types of horizontaland metre-thick beds alternate in this 48 m highsection (Fig. 6B): (1) Massive and poorly cemen-ted rudstones to floatstones containing largebioclasts of bryozoans, bivalves, barnacles andcoralline algal rhodoliths. Clusters of the oysterNeopycnodonte navicularis occur locally. (2)Trough cross-bedded packstones and rudstonesmade up of poorly cemented fragments of mol-luscs, benthic foraminifera, coralline red algae,bryozoans, barnacles and serpulid worm tubeswith minor volcaniclastic particles. The troughcross-bedding is metric to decametric in scale,and sets dip towards the S–SE (150–190E). Coar-ser bioclasts are concentrated at the bottom of thetroughs. The relative proportion of massive cal-cirudites increases to the NW in the exposures onthe northern slopes of Barranco del Hondo ravine.

SEDIMENTARY MODELSAND EVOLUTION

The biotic components of these Lower Pliocenecarbonates are characteristic of the bryomol (Lees& Buller, 1972) and rhodalgal (Carannante et al.,1988) lithofacies that belong to non-tropical car-bonates (Nelson et al., 1988; James, 1997). The

Fig. 6. Stratigraphic columns of the Cala de la Pelirroja(location shown in Fig. 4) and Torre Vieja sections.



low degree of cementation is typical of temperatecarbonates (Nelson, 1988; James, 1997). Twocontrasting sedimentary models can be deduced,one for the aggrading and southward-retrogradinglower unit, and the other for the northward-prograding upper unit.

Lower unit

The geometry of the lower unit is that of a wedgewith a steeply dipping landward flank and agently dipping basinward surface (Fig. 7). Thecrest of the wedge was located several hundredmetres away from the southern palaeoshoreline ofthe Carboneras Basin. This geometry and faciesdistribution suggests that the sediment of thewedge accumulated on a spit platform (for ter-minology, see Nielsen et al., 1988) formed by alongshore current. This current was probablycaused by waves oriented at an oblique angleto the main Mediterranean palaeoshoreline. Thetrend of the spit platform and the palaeogeog-raphy indicate that the longshore currentsproducing the spit platform were driven bysouth-easterly winds (Fig. 8). Today, on the east-ern Almerıa coast, such a wind direction resultsfrom the deflection of westerly winds at thesouth-eastern extremity of the Iberian peninsula(Cabo de Gata). The eastern extremity of the Mesade Roldan upland was presumably the coastalpromontory that induced a deflection or deceler-ation of the longshore current and provided asubstrate surface for spit initiation (Fig. 8). Dur-ing the Early Pliocene, the Mesa de Roldanvolcanic upland extended to the east further thanit does today (Fig. 8). The present-day morphol-ogy of the coast is a post-early Pliocene featurerelated to N170E faulting that produced the

coastal cliffs in which the Lower Pliocene depos-its are exposed (Martın et al., 2003).

Carbonate production took place seaward ofthe spit platform both in the Carboneras Basinembayment and in the open Mediterranean.Bryozoans, coralline algae, molluscs, barnaclesand foraminifera grew in these areas (Fig. 7), asrecorded by the massive and poorly cementedrudstones to floatstones with relatively well-pre-served fossil remains at the Torre Vieja section.The high proportion of epiphytic benthic fora-minifera suggests that seagrass meadows werewidespread on the carbonate factory areas. Sea-grass probably played a role in baffling thebioclasts and thus promoting their preservation.

The presence of a subordinate current system isindicated by the shoals occurring on the crest andon the gently inclined, basinward-dipping part ofthe spit platform. Carbonate sand and gravel fromthe spit platform and from the factory areas werereworked and incorporated into shoals by wave-generated currents flowing from the north,according to the orientation of trough cross-bedding directions (dipping to the S), probablydriven by northerly and north-easterly winds(Figs 7 and 8). The shoal facies extended to thenorth of the spit platform down a gentle ramp.

On the landward flank of the spit platform, it isestimated that the sea bottom descended about25 m with a depositional slope of about 8–11�.Here, parallel-laminated rudstones and grain-stones accumulated as washover deposits on thesheltered side of the spit (Fig. 7). The carbonateparticles were probably derived from sweeping ofthe shoals area by strong currents during storms.The lack of early cementation in temperatecarbonates clearly favoured the transport of loosecarbonate particles from the production areas

Fig. 7. Sedimentary model for the Pliocene lower unit. Carbonate particles removed from the carbonate factoryaccumulated on shoals on top of and seaward from a spit platform a few hundred metres away from the shoreline.Carbonate grains removed by storms from the shoals were deposited in parallel-laminated beds on the shorewardslope of the spit platform. A small carbonate factory developed in the depression left between the spit platform andthe shoreline.



onto the spit platform and down its shorewardslope. Landward-dipping sandstone foresetswith similar inclinations (averaging 12�) over-lain by trough cross-bedded sandstones aredescribed from the Campanian Seminoe 3 sand-stone of the Haystack Mountains Formation inWyoming, USA (Mellere & Steel, 1995; Mellere,1996). Landward-dipping beds consist of well-sorted, fine-grained, planar-laminated sand-stones. According to Mellere (1996), they formedon the landward flank of a spit by sedimenttransported over the spit crest by sheet flows,generated by storm waves and currents that partlyremoved the sediment in the dunes at the top.

There are no remains of deposits forming at thesouthern palaeoshoreline of the Carboneras Basin.On temperate carbonate platforms, beach depositsresult from the landward transport of bioclasticparticles produced in the factory (Martın et al.,1996). In the study example, the spit prevented thesupplyof sediment fromthe factory to the shoreline.Volcanic cobbles and boulders from the emergedMesa de Roldan volcanic dome concentrated in thelowest part of the sea floor at the toe of the spit slope.The largest bioclasts were concentrated in thissubenvironment as well (Fig. 7), protected fromfurther reworking after cascading down the shore-ward slope of the spit or, quite possibly, theybelonged to organisms that lived there too.

The top of the spit platform migrated south-ward after the displacement of the shoreline to

the south during a relative sea-level rise. Thisretrogradation produced landward-directeddownlapping of the steeply dipping spit-platformflank onto the erosional surface at the top of theMessinian carbonates. The increase in availableaccommodation favoured the aggradation of thespit system. In the seaward factory area, theincrease in accommodation during the relativesea-level rise promoted a similar aggradationalgrowth of the depositional system. However, thefacies cycles observed at Torre Vieja, whereshoals and factory facies alternate, suggest acertain periodicity in the availability of accom-modation.

The absence of subaerial spit deposits can beexplained by their reworking and erosion bywaves and currents as sea level rose. Thecarbonate sands and gravels that potentiallyaccumulated in the subaerial spit ridge, in sub-environments such as beach and aeolian dune,were probably submerged and incorporated intothe shoals, and eventually overwashed to formthe shoreward flank of the spit platform duringcontinued sea-level rise. Erosional truncationduring sea-level rise, stripping subaerial spitdeposits and creating similar stratal geometriesand facies to those in the lower unit at Cala de laPelirroja, has been reported in a Pleistocene spitcomplex from central Japan (Hiroki & Masuda,2000). As a possible alternative, spit beaches andsubaerial dunes may never have developed on the

Fig. 8. Palaeogeography of the Car-boneras Basin during the depositionof the lower unit, i.e. during thedeposition of the Lower Pliocenetransgressive systems tract. Whitearrows indicate prevailing winddirections for the present-dayeastern coast of Almerıa.



spit platform at the southern margin of thePliocene Carboneras Basin. Such a scenario mayoccur in a depositional system with low sedimentsupply rates. In this case, the longshore currenttransported carbonate particles to the spit plat-form from the open Mediterranean platform fac-tory. The sediment production of temperatecarbonate factories, however, is typically lowand, in this case, might have been insufficient toprovide enough sediment to build a subaerialspit.

Upper unit

Eventually, a final relative decrease in accommo-dation led to the filling of the depression in thesea floor between the spit platform and theshoreline by washover deposits. As soon as thisdepression was levelled, a new distribution offacies belts, which defines the upper unit of thesedimentary succession, replaced the previousone. Several subenvironments became estab-lished basinward from the margin on a gentleramp (Fig. 9). A small backshore lagoon wasinvaded at its seaward margin by washover fansthat incorporated calcarenites and calciruditesinto the fine-grained lagoon deposits. Foreshorelow-angle, parallel-laminated calcarenites chan-ged laterally to shoreface shoals, which in turnpassed into a factory zone in which most of thecarbonate production took place. Two majorcommunities of carbonate producers segregatedin the factory zone of the ramp. Coralline redalgae were the most significant components in thedeeper areas, while molluscs as well as bryozoanspredominated in the shallower ones (Fig. 9). Asin similar sedimentary models already proposedfor Cenozoic Mediterranean temperate carbonates(Martın et al., 1996), carbonate particles removedfrom the factory zone fed the onshore shoals andbeach deposits.

In its proximal reaches, the lower boundingsurface of the upper unit is erosional. It is a well-defined sharp boundary that truncates underlyingbeds. Sediments from the underlying unit werereworked and probably incorporated into theshoals of this new system. Overall, the accumu-lation pattern of the upper unit is stronglyprogradational, which indicates much lower ratesof relative sea-level rise than during the formationof the lower unit. Basinward, the convergence offactory facies from both units makes separation ofthe prograding from the underlying aggrading bedsets difficult.

In terms of sequence stratigraphy, the aggradingand retrograding beds can be assigned to thetransgressive systems tract, whereas the north-ward-prograding bed sets represent the highstandsystems tract of a sequence. According to the ageconstraints indicated by the planktonic foramini-fera assemblages, the increase in accommodationduring the deposition of the transgressive systemstract was probably the result of the Early Plioceneeustatic sea-level rise recorded in the short-termcurve of Haq et al. (1987).

DISCUSSION

The carbonate spit platform that formed along thesouthern margin of the Carboneras Basin offers anovel sedimentary model for nearshore depositsin non-tropical carbonate ramps (Figs 7 and 8).Previously reported nearshore non-tropical car-bonates formed on beaches and shoals in theshallowest portions of carbonate ramps or relatedto submarine cliffs. The local palaeogeographicconfiguration and hydrodynamic patterns influ-enced by prevailing winds in the Carboneras areaduring the Early Pliocene controlled the forma-tion and sedimentary dynamics of a temperatecarbonate spit platform. Previously reported

Fig. 9. Sedimentary model for the Pliocene upper unit. A decrease in accommodation forced a change in the faciesdistribution compared with the Pliocene lower unit (Fig. 7) with the development of a gentle ramp, a geometrytypical of Mediterranean Cenozoic temperate carbonates (Martın et al., 1996).



fossil examples of spits and spit platforms arealways composed of siliciclastic sand and gravel.The present example from the Carboneras Basinprovides additional evidence of the similaritiesbetween temperate carbonates and siliciclasticdeposits. The lack of significant early cementa-tion in non-tropical carbonates implies that looseskeletal particles remain upon the sea floor,available for transport, and can therefore accu-mulate under similar sedimentary dynamics tothose affecting siliciclastic deposits. Thus, non-tropical carbonates may exhibit any of the bed-form structures and geometries described formarine siliciclastic sands and gravels.

The most conspicuous stratal geometry of theCarboneras spit-platform deposits is a package oflarge landward-dipping clinoform foresets down-lapping the unconformity surface on top of theMiocene rocks. This package aggrades and retro-grades at the southern margin of the basin.Previously, in carbonate platforms, shoreward-dipping clinoforms have only been described inrelation to discrete lenticular buildups or alongthe leeward margin of shelf rims in tropicalexamples (Handford & Loucks, 1993). Sedimen-

tary bodies with shelf- or landward-directedprogradation resulting in an onland downlap arealso well known from deep-water settings. On alarge, kilometre-scale, such bodies are describedfrom seismic profiles in hemipelagic drift depos-its (Fig. 1) controlled by geostrophic or contouritecurrents (Mullins et al., 1980; Davies et al., 1991;Isern et al., 2002).

The Pliocene landward downlaps of the Car-boneras Basin are not an exceptional phenom-enon. Similar structures occur in Messinian(uppermost Miocene) temperate carbonates in ElRicardillo, at the southern margin of the AguaAmarga Basin, in the Cabo de Gata region of SESpain (Figs 2 and 10) (see fig. 2 in Betzler et al.,2000). Individual shoreward-dipping beds areconspicuous at the outcrop scale in a package ofbeds up to 18 m in thickness that downlapprevious upper Miocene deposits. Away fromthe downlap area, stratal dip flattens and thenchanges to a gentle northward inclination. Theunderlying Upper Miocene carbonates formed atthe foot of submarine cliffs on the north-westernflank of the El Ricardillo volcanic dome. Thisdome constituted the southern margin of the

Fig. 10. View of the El Ricardillo section from the west. Messinian rudstone/packstone beds downlap southward theunderlying Upper Miocene cliff-related deposits. The inferred palaeoshoreline was to the right of the picture.



Agua Amarga Basin together with other volcanicbuildups (Betzler et al., 2000). However, the poorquality of the exposure makes the detailed recog-nition of the internal structure of the depositsdifficult. Trough cross-bedding does occur in theflat part of the beds, and parallel lamination canbe observed in the landward-dipping strata, sug-gesting that the features formed in a similarsetting to the lower unit in the Cala de la Pelirrojasection.

The Agua Amarga Basin was a small embay-ment of the Mediterranean surrounded by vol-canic uplands during the Miocene. The localpalaeogeography and current patterns leading tothe formation of a spit platform towards itssouthern margin could have been relatively sim-ilar to those that prevailed in the CarbonerasBasin during the Pliocene. The occurrence ofsimilar stratigraphically separated examples ofspit platforms, however, suggests that many moreare likely to exist in non-tropical carbonateplatforms elsewhere in the world.

CONCLUSIONS

Along the southern margin of the Pliocene Car-boneras Basin in southern Spain, temperate car-bonate sand and gravel accumulated on a spitplatform located a few hundreds of metres awayfrom the southern palaeoshoreline. The carbonateparticles that accumulated on the spit platformwere transported by longshore currents deflectedand decelerated by a basement prominence at thesouth-eastern extremity of the Carboneras Basinembayment. In cross-section, the spit platform isstrongly asymmetrical with a steeply dippingshoreward flank and a gently dipping seawardflank. The top of the platform was covered byshoals that extended over a gently dipping rampto the north. Parallel-laminated packstones torudstones, dipping 9–10� to the south, accumu-lated on the landward slope as washover depositsderived from the shoals at the top of the spitplatform. These shoals migrated landward, drivenby waves and currents flowing from the north.The spit-platform deposits reflect the alternatinginflux of flows driven by south-easterly winds(longshore current flowing to the north and west)and flows driven by north-easterly winds (wavesand currents responsible for the southward mi-gration of shoals and washover deposits onto thelandward slope). Increasing accommodation,probably related to the Early Pliocene eustaticsea-level rise, resulted in aggradation and

retrogradation of beds that produced landwarddownlaps at the basin margin. The eventualdecline in the availability of accommodationforced the complete filling of the depressionbetween the spit and the shoreline and replace-ment of the previous distribution of facies belts bya ramp that sloped gently towards the basincentre.

The only other known example of this stratalgeometry in fossil carbonate platforms occurs inuppermost Miocene deposits from the Cabo deGata area some 15 km south-west of Carboneras.The scarcity and spatial closeness of the exam-ples may be the result of the relatively smallnumber of studies considering coastal temperatecarbonates, many of which have been documen-ted from the Neogene basins in SE Spain.

ACKNOWLEDGEMENTS

We are indebted to M. L. Losada for his helpfulsuggestions about the origin of the studied de-posits, and to J. L. Wood for additional advice.R. G. Loucks and Ch. Montenat made constructivecomments on a previous version of this paper.The work of J.C.B., J.M.M. and J.A. was supportedby the Ministerio de Educacion y Ciencia ProjectsPB97-0809 and the Ministerio de Ciencia y Tec-nologıa Project BTE 2001-3023. C.B. was suppor-ted by the Deutsche Forschungsgemeinschaft(Be 1272/7). We are grateful to Christine Laurinfor correcting the English text.

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Manuscript received 27 March 2002;revision accepted 18 February 2003.



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