submarine lobes and feeder channels of redeposited ... · the monte ricardillo in cabo de gata...

Submarine lobes and feeder channels of redeposited, temperatecarbonate and mixed siliciclastic-carbonate platform deposits(Vera Basin, AlmerõÂa, southern Spain)

JUAN C. BRAGA*, JOSE M. MARTIN* and JASON L. WOOD *Departamento de EstratigrafõÂa y PaleontologõÂa, Universidad de Granada, Campus de Fuentenueva s.n.,Granada 18002, Spain (E-mail: [email protected], [email protected]) School of Earth and Environmental Sciences, University of Greenwich, Chatham Maritime, Kent ME44TB, UK (E-mail: [email protected])

ABSTRACT

Temperate carbonates and mixed siliciclastics-carbonates of Upper Tortonian

age were deposited on a narrow platform along the southeastern margin of the

Sierra de los Filabres on the western side of the Vera Basin. The temperate

carbonates were unlithi®ed or were only weakly lithi®ed on the sea¯oor and so

were easily prone to synsedimentary removal. Part of the shelf sediments were

eroded, reworked and redeposited in submarine lobes, up to 40 m thick and

1 km wide. The lobes consist of turbiditic carbonates (calcarenites and

calcirudites) and mixed siliciclastics-carbonates, which contain up to 30%

siliciclasts, derived from the Sierra de los Filabres to the northwest, and

abundant bioclasts of coralline algae, bivalves and bryozoans. In the inner

platform, the feeder channels of the lobes cross-cut beach and shoal deposits,

and are ®lled by strings of debris ¯ow conglomerates (up to 3 m thick and a few

metres wide). These channels presumably developed as the continuation of

river courses entering the sea. Further towards the outer platform, they pass

into large channels (up to several hundred metres wide and 20 m deep) steeply

cutting into the horizontally bedded strata of the platform. Signi®cant

quantities of platform sediment were removed by erosion during their

excavation. Once abandoned, they were ®lled by new platform sediments.

Further towards the basin, the channels associated with the lobes exhibit

lateral accretion and internal cut-and-®ll structures, and are intercalated

between hemipelagic deposits. The channel-®lling sediments are in this latter

case coarse-grained carbonates and mixed siliciclastics-carbonates. Lobe

development concentrated ®rst at Cortijo Grande on the western side of the

study area, and then to the east at MojaÂcar. This migration may relate to the

uplift of the Sierra Cabrera, a major high occurring immediately to the south of

the channel and lobe outcrops.

Keywords Feeder channels, Neogene basins, redeposited carbonates, south-east Spain, submarine lobes, temperate carbonates.

INTRODUCTION

Temperate carbonate platform sediments consist-ing of loose, sand- and gravel-sized skeletalparticles of `rhodalgal' (sensu Carannante et al.,1988)/`bryomol' (sensu Nelson et al., 1988) com-position have been widely reported from the

Neogene and the Quaternary of the Mediterranean(Carannante et al., 1988; Carannante & Simone,1988; Esteban et al., 1996; MartõÂn et al., 1996;Betzler et al., 1997; Spjeldnaes & Moissette, 1997;Vecsei & Sanders, 1999). They are typicallyunlithi®ed or were only weakily lithi®ed on thesea¯oor prior to burial (Nelson, 1988; James,

Sedimentology (2001) 48, 99±116

Ó 2001 International Association of Sedimentologists 99

1997), which makes them prone to synsedi-mentary removal, transport and redeposition.Although these characteristics have been exten-sively documented within the platform realm(Anastas et al., 1997; Betzler et al., 1997), upuntil now no examples have been given ofsubmarine fan systems with an unquestionabletemperate-carbonate platform source.

This paper describes Miocene (latest Tortonian)temperate carbonate depositional systems inwhich different palaeoenvironments from thecoast to the basin have been identi®ed within alateral distance of less than 5 km. It is a small-scaleexample in which shelf-to-basin transport of sedi-ments, with the ®nal deposition of temperate

carbonate sediments in submarine fan systems,can be documented. Although platform depositsare reported in the description of the sedimentarycontext, the focus of this work is on the architectureand distribution of the submarine fans that exhibitan exceptional preservation of feeder channel andlobe systems. This paper offers a model for asses-sing lateral transition from temperate carbonateplatform deposits to deeper water environments.

GEOLOGICAL SETTING

The study area is located along the southwestern¯ank of the Vera Basin, east of the Sorbas Basin,

Fig. 1. Geological (A) and geo-graphical (B) location, and detailedgeological map (C) (modi®ed fromMontenat, 1990) of the study area.Azagador limestones occur in fouroutcrop belts, three aligned NE±SW(one along the margin of the Sierrade los Filabres and two to the south-west of Turre) and one roughly E±W(west of MojaÂcar). Numbers insidecircles indicate localities referred toin the text. Localities 1±3 form partof the ®rst outcrop belt; localities4±6 belong to the second one;localities 7 and 8 relate to the thirdone, and localities 9 and 10 to thefourth one.

100 J. C. Braga et al.

Ó 2001 International Association of Sedimentologists, Sedimentology, 48, 99±116

close to the zone where these two Neogene basinsadjoin (Fig. 1). Basement rocks consist of Palaeo-zoic and older metamorphic rocks (primarilymica schists and quartzites) of the Nevado-FilaÂ-bride Complex, and Permo-Triassic phyllites andquartzites and Triassic carbonates of the AlpujaÂr-ride Complex. Both complexes are exposed onthe northern Sierra de los Filabres and on thesouthern Sierra Cabrera ranges (Nijhuis, 1964;Westra, 1969).

The Neogene sequence of the study zone(Fig. 2) comprises small, disconnected bodies ofLower and Middle Miocene (Burdigalian to Ser-ravallian) marine limestones and sandstones,overlain by an up to 500-m-thick, Serravallian-Lower Tortonian, continental conglomerate unit(Rondeel, 1965), unconformably overlain by anup to 50-m-thick, Lower Tortonian, shallow-marine, bioclastic sandstone and conglomerateunit (BarragaÂn, 1997). A 200-m-thick, UpperTortonian, marine marl-marly limestone unit(the Chozas Formation of Ruegg, 1964) occursunconformably on top of this sequence. The studyrocks belong to the so-called Azagador Member ofthe Turre Formation (VoÈlk & Rondeel, 1964)which lies unconformably on top of the Chozasmarls (Fig. 2). In the type locality of the Barrancodel Azagador (see Fig. 1 for location), where theAzagador Member was formally de®ned (VoÈlk,1967), it is a platform deposit up to 70 m thick,comprising bioclastic-carbonate and mixed silic-iclastic-carbonate sediments, with abundantremains of coralline algae, bivalves (mainly oys-ters and pectinids) and bryozoans, together withminor amounts of echinoids, brachiopods, foram-inifera, barnacles and azooxanthellate corals.This biotic association is typical of a temperateclimatic setting (`foramol association' of Lees &Buller, 1972; `rhodalgal association' of Carannan-te et al., 1988; `bryomol association' of Nelsonet al., 1988). The temperate character of theAzagador limestone has been recently con®rmed

by isotopic studies (MartõÂn et al., 1999; SaÂnchez-Almazo, 19991 ).

The Azagador Member also crops out exten-sively in several other Neogene basins in theAlmerõÂa area of southern Spain and has beenstudied in some detail in the Agua Amarga Basin(Braga et al., 1996; MartõÂn et al., 1996; Brachertet al., 1998), the Las Negras/Rodalquilar Basin(Franseen et al., 1997) and the Monte Ricardillo(Betzler et al., 2000) in Cabo de Gata, and in theSorbas Basin (Wood, 1996) (see Fig. 1 for loca-tion). In all of these basins, as well as in the VeraBasin, limestone platform deposits pass laterallyto basinal silts and marls (the Abad Member ofthe Turre Formation of VoÈlk & Rondeel, 1964;speci®cally the Lower Abad marls of MartõÂn &Braga, 1994) (Fig. 2). The Azagador limestone hasbeen dated as uppermost Tortonian in the Vera(Benson et al., 1991), Sorbas (Sierro et al., 1993)and Agua Amarga (Braga et al., 1994) Basins, andas uppermost Tortonian±lowermost Messinian inthe Monte Ricardillo in Cabo de Gata (Betzleret al., 2000).

In the studied sections (this paper) in the VeraBasin (Fig. 2), the Azagador limestone bedsinter®nger with and/or intercalate between sandysilts and silty marls containing a planktonicforaminiferal assemblage (Neogloboquadrinahumerosa and dominant Globorotalia gr. menar-dii I, Sierro et al., 1993) that indicate a LateTortonian age. Nevertheless, on the western sideof the study area, uppermost Azagador limestonelevels change laterally to marls that can be datedas earliest Messinian (Fig. 2) by the presence of G.mediterranea. These marls in turn intercalatethin-bedded, terrigenous turbidite layers contain-ing a well-preserved graphoglyptid ichnofauna(Braga et al., 1999).

In the vicinity of the study area, Messinianreefs bordering the northern Sierra de los Fila-bres high crop out in both the Sorbas (Ridinget al., 1991; Braga & MartõÂn, 1996) and Vera

Fig. 2. Schematic W±E stratigraphicsection for the study area. Numbersinside circles have the same mean-ing as in Fig. 1. Burdigalian toLower Tortonian sediments occurinside half-grabens related to earlyrifting. Upper Tortonian to recenttectonic evolution of the area is thatof strike-slip faulting, which isconsidered to be responsible for thepresent-day basin con®gurationshown in Fig. 1A (Montenat et al.,1987, 1992).

Redeposited temperate carbonates 101


(BarragaÂn, 1997) basins. These reef deposits(Cantera Member of the Turre Formation of VoÈlk& Rondeel, 1964) pass laterally (basinwards) intoa series of yellow marls with diatomites (theUpper Abad marls of MartõÂn & Braga, 1994). TheAbad marls are up to 200 m thick in the VeraBasin (BarragaÂn, 1997). Uppermost Messiniansediments (Yesares & Sorbas Members of theCanÄos Formation of Ruegg, 1964), although wellrepresented just to the west of the study area inthe Sorbas Basin (Dronkert, 1977; Roep et al.,1979; MartõÂn et al., 1993; Riding et al., 1998;Roep et al., 1998; Braga & MartõÂn, 2000), arecompletely absent here (Fortuin et al., 1995).Extensive Pliocene-Pleistocene, shallow marineto continental conglomerate deposits accumulat-ed to the northeast of the study area in the VeraBasin (Stokes, 1997).

Sequence stratigraphy of the Azagadorcarbonates/Abad marls

The Azagador/lower Abad sediments constitute asa whole a single unit, representing the lowstandsystems tract of a third-order cycle that can becorrelated with the TB3á3 cycle of Haq et al.(1987). The reef deposits (Cantera Member of theTurre Formation) correspond to the transgressiveand highstand systems tracts (MartõÂn & Braga,1994). Most Azagador carbonates concentrate inthe lower half of the Azagador/lower Abad unitand were deposited during the latest Tortonian(Fig. 2). These carbonates can be tentativelyassigned to the lowstand systems tract of afourth-order sequence (Fig. 3), while the upperhalf, represented mainly by marls, correspond tothe transgressive systems tract. The highstand isnot present in the area due to tectonic causes(MartõÂn & Braga, 1996). The carbonates of thelower half, and their corresponding distal marls,constitute in turn a subsequence of immediatelyhigher frequency (®fth-order) that can be subdi-vided, on the basis of stratigraphic and geometri-cal ®eld relationships, into: (1) a lowstand (not

represented in the area except perhaps for a few,small and isolated outcrops of heavily boredconglomerates, which occur directly on top ofthe Chozas marls and below the Abad silts andmarls 2á5 km to the west of Cortijo Grande) (seeFig. 1 for location); (2) a transgressive (the plat-form deposits underlying the Abad silts and marlsoutcropping 2 km to the north of Cortijo Grande);and (3) a highstand event (comprising all theAzagador deposits studied in detail in this paper,including the redeposited carbonates) (Fig. 3).

A comparable sequence stratigraphy has beenestablished for the Azagador carbonates in theAgua Amarga Basin (MartõÂn et al., 1996). Inthat basin the ®fth-order highstand carbonates,those of the so-called Bedded Unit (MartõÂnet al., 1996), show an internal high-frequencycyclicity probably related to the 20 000 yearsprecession cycles, with associated sea-level¯uctuations of around 20 m (Brachert et al.,1998). These have not been recognized in thepresent study area.

METHODS

Geological mapping (1:5000 scale) of the area anddetailed logging at selected points have beencarried out as the main study techniques. TheAzagador sediments consist mainly of particulate,bioclastic carbonates and mixed siliclastics-carbonates, with grain sizes ranging from ®nesand to pebble. Due to the coarse grain size, visualchart estimations were used effectively both inthe ®eld and in thin section to determine therelative abundance of siliciclastic and bioclasticcomponents in all the sampled and logged sec-tions. Grain sizes were measured directly in the®eld, either with a ruler, in coarse to very coarse-grained sediments, or using charts and with thehelp of a lens, in ®ner-grained, sand-sized sedi-ments. The latter values have been checked witha micrometer lens while examining thin sectionsunder the microscope.

Fig. 3. Proposed sequence stratigra-phy for the Azagador carbonates andAbad marls in the study area. HST,high stand systems tract.



DESCRIPTION OF THE AZAGADOROUTCROPS

In the study area, the Azagador Member consists ofcarbonate (calcarenite/calcirudite) and mixed si-liciclastic-carbonate (bioclastic sandstone andconglomerate) sediments that to the S-SE inter®n-ger with and/or intercalate between sandy siltsand silty marls. It normally has a thickness ofaround 30±50 m, although locally it can be morethan 100 m thick, and occurs within four principaloutcrop belts, located close together but remainingpartly disconnected (Fig. 1C). The ®rst two mainoutcrops, located to the south of the Sierra de losFilabres, trend ENE-WSW and extend from Almo-caizar in the west, to Los Gallardos in the east. Thethird and the fourth ones, situated to the north ofSierra Cabrera, close to Turre and MojaÂcar, arealigned NE-SW and E-W respectively (Fig. 1C).

First outcrop belt

The northernmost sediments from this outcropbelt are calcarenites/calcirudites with up to 20%siliciclastic grains that exhibit a well-developedlow-angle, parallel lamination. Southwards thisstructure changes gradually to conspicuous,metre-scale (ranging from 0á2 to 3 m in height)trough cross-bedding pointing to the E-SE. In some

isolated localities, large siliciclastic blocks (up to1á5 m across) occur at the bottom of the carbonate/mixed siliciclastic-carbonate sequence, whichmay be up to 25±30 m thick. The boulder blockslie directly on top of eroded Nevado-FilaÂbridebasement and are embedded in a bioclastic graveland sand-rich matrix. Most blocks are extensivelybored by lithophagid bivalves (Braga et al., 1999).

Matrix-supported conglomerates, with mega-clasts up to several metres in size of metamorphicbasement rocks from the Sierra de los Filabresembedded in a sandy matrix containing somedispersed bioclasts, occur locally, at differentheights within the sequence, in the centre (RõÂoJauto bridge just north of Alfaix: Locality 1 inFig. 1) and in the eastern part (Rambla Alcornia:Locality 2 in Fig. 1) of the outcrop. They formnarrow strings (up to 3 m thick and a few metreswide), traceable for a few hundred metres andtrending approximately N140°, which cross-cutthe above-mentioned, low-angle parallel-lami-nated, and trough cross-bedded, sediments(Fig. 4). In the western part of the outcrop (VentaAnita near Almocaizar: Locality 3 in Fig. 1)boulder-bearing, matrix-supported conglomeratesforming mainly small lobate-shaped (a few hun-dred metres across) deposits become dominantand intercalate between bioclastic carbonate andmixed siliciclastic-carbonate sediments.

Fig. 4. Outcrop photograph exhibiting channelized debris ¯ow deposits (C) cross-cutting trough cross-bedded car-bonate and mixed siliciclastic-carbonate shoal deposits (S). Rambla Alcornia. J.M.M. is 1á75 m tall. Sections to theleft and to the right of J.M.M. are near perpendicular and slightly oblique to the trend of the channel respectively.



Second outcrop belt

In the second main outcrop belt, Azagadorsediments up to 100 m thick, although normallyaround 35±40 m, are horizontally bedded. To thenorth they locally exhibit decimetre- to metre-scale trough cross-bedding, while to the south adiffuse lamination parallel to bedding is the onlyvisible internal structure. Sediments consist ofcalcarenites (calcirudites) with a variable silic-iclastic content. Their grain size progressivelydiminishes to the south from pebble and coarsesand to ®ne sand. Further to the south thesesediments inter®nger with silts and silty marls.Siliciclastic sediments are notably abundantalong the western side of the outcrop (La Losa:Locality 4 in Fig. 1), and are mostly conglomer-ates with clasts up to several cm in size. Well-rounded, quartz pebbles predominate althoughclasts of mica schist, gneiss, amphibolite, marble,limestone and dolostone are also present. Theclast assemblage is clearly derived from theSierra de los Filabres to the north. Siliciclasticabundance quickly diminishes to the south andeast.

In the centre (Barranco del Aire: Locality 5 inFig. 1) and the east (RõÂo Chozas and Molino de laHiguera: Locality 6 in Fig. 1) of the secondoutcrop belt, large channelized structures arewell exposed along the vertical walls of theAzagador river cliffs (Fig. 5). Such channel struc-tures extend laterally for several tens or evenhundreds of metres; they have thicknesses of upto 20 m, and are aligned NW-SE. The channelscut down through, and are usually encasedwithin, horizontally bedded strata. Locally thechannels may erode right to the base of theAzagador sequence or even into the Chozas marls,and can extend vertically right up to the top of thelimestone. The sediments within the channelstructures are similar in composition to thesurrounding horizontal beds and correspond tocarbonates (calcarenites/calcirudites) and mixed,bioclastic, sand-to-pebble sized siliciclastics-carbonates (the siliciclastic content may be upto 30%). Large, bioclastic and siliciclastic clastsor conglomerate deposits are not present, even atthe base of the channels. In®lling beds dip steeply(up to 20°) towards the channel centre from bothsides of the structure and progressively ¯atten out

Fig. 5. A major channel eroding outer platform sediments and ®lled later, once it was abandoned, by new platformsediments at three different stages. Molino de la Higuera outcrop. The area shown in the photograph corresponds to aclose-up of the central part of the diagram.



upwards (Fig. 5), draping the channel feature. Insome cases several phases of in®lling can berecognized. The uppermost layers of the chan-nelized ®ll pass laterally into the upper beds ofthe interchannel deposits.

Third outcrop belt

In the third main outcrop belt (Cortijo Grandearea), the Azagador sediments occur as discon-tinuous, bioclastic limestone bodies intercalatedbetween sandy silts and silty marls (Fig. 6).Planktonic foraminiferal assemblages yield alatest Tortonian age for the silts and marls.

Some limestone bodies consist of irregularlybedded coarse bioclastic (oyster and corallinealgal-rich) and mixed siliciclastic-carbonate(lithoclast-rich) deposits, up to a few metres thickand tens to hundreds of metres wide, exhibitinglateral accretion and cut-and-®ll (channelized)structures. Clasts of metamorphic rocks up to150 cm in size and friable clasts of Chozas marlsand marly limestones up to 120 cm in lengthbored by lithophagid organisms (Gastrochaeno-lites) occur locally.

Other Azagador limestone bodies are thick andlobe shaped (up to 20 m thick and 400 m wide).They are made up of bioclastic calcarenites/calcirudites and mixed siliciclastics-carbonates,with a siliciclastic content up to 20% and grainsize ¯uctuating from ®ne sand to pebble. Frag-ments of bivalves (Ostrea spp. and pectinids),coralline algae (Lithophyllum, Mesophyllum,Lithothamnion) and bryozoans can make-up asmuch as 30% of the rock in the coarse granule andcoarser sediment classes. Individual beds extendlaterally for a few hundred metres and have aplano-convex lensoidal shape; they are eitheramalgamated or separated by very thin ®ne tovery ®ne sand/sandy silt seams. Parallel lamina-tion and graded bedding are the most conspicuousinternal structures. Bottom structures (¯ute casts)have only been found at one locality, indicating apalaeo¯ow to the SE. Seven of these lobate bodies

have been identi®ed. Two occur immediatelywest of Cortijo Grande on the western side of theoutcrop belt (Fig. 1), and ®ve are located in theBarranco del Gitano, 2 km to the SW of Turre.

The Azagador sediments are well exposedalong the Barranco del Gitano (Fig. 6), betweenCortijo La Cueva Sucia and Cortijo del Estrecho(Localities 7 and 8, respectively, in Fig. 1) in theCortijo Grande area. A detailed sedimentary log(25 m thick) at Cortijo del Estrecho (Figs 7 and 8),reveals three distinctive facies.

Facies a

Coarse- to medium-grained calcirudite beds, 20±150 cm thick, mostly 60 cm or greater, withhighly erosional and incised basal surfaces, thathave laterally impersistent (several metres) len-soid (channel-cut) shapes and are stacked one ontop of the other occur within the lowermost 6 mof section, and also in the upper part of the loggedsection from 17 to 22 m (Fig. 8). Coarse (>2 mm)bioclastic detritus makes up 20±30% of the rock.In contrast, coarse siliciclastic detritus (metamor-phic and Chozas Formation clasts) represent nomore than 15% of the total rock volume. Theremaining 60% is a calcarenite with an up to 20%siliciclastic content. Internal features of the indi-vidual stacked beds include a broad ®ning-upwards (positive grading) trend from the baseto the top, with small rip-up clasts, and large (upto cobble in size) Chozas and metamorphic clastscon®ned to the bottom. Towards the top ofindividual beds, ®ner-grained calcirudites andcoarse calcarenites show parallel lamination on acm scale, with orientation of elongate grainsparallel to the banding.

Facies b

Thinner (only 10±20 cm thick), well bedded andlaterally more persistent (several tens of metres)calcirudite beds de®ne the middle zone of thesection (from �7±17 m) (Fig. 8). Calciruditelayers show ¯at basal contacts and well-devel-

Fig. 6. A sketch of channel and lobe outcrops along the western side (left margin) of Barranco del Gitano, in the areaof Cortijo Grande. The northeasternmost outcrop, that of Cortijo del Estrecho (Locality 8 in Fig. 1), is the one shownin Fig. 7 and logged in Fig. 8. The underlying platform is thought to be part of the higher-order `Transgressive SystemTract' of the Azagador/lower Abad Unit (see Fig. 3).



oped parallel-lamination, have a massive char-acter or only very slight positive grading at thebase. These beds are often highly bioturbated atthe top by Scolicia (spreiten burrows) and mayalso contain the escape burrow Monocraterion.

Facies c

The uppermost part of the section (top 3 m inFig. 8) is de®ned by closely spaced or amalgama-ted laterally persistent (several metres to tens of

Fig. 7. Field view of the Cortijo del Estrecho outcrop (Locality 8 in Fig. 1) showing lobe sediments cut by channelswith conglomeratic ®ll, passing laterally to the south-west (left hand-side of the picture) into hemipelagic ®ne-grained deposits. See Fig. 8 for detailed facies interpretation.

Fig. 8. Cortijo del Estrecho sectionat Barranco del Gitano (shown inFig. 7) and detailed log of its north-eastern margin. This is a compositesection consisting of channel de-posits at the bottom, followed bylobe deposits, and again channeldeposits. The uppermost sedimentscorrespond to levee deposits later-ally connected to the upper channelsystem. See text for detailed de-scription of facies. Mds, mudstone;wcs, wackestone; pcs, packstone; gs,grainstone; f, ®ne sand; m, mediumsand; c, coarse sand; cg, conglom-erate.



metres) calcirudite and calcarenite beds, withslightly lensoid shapes. These alternate with ®neto medium loose sands. Individual calciruditebeds show positive grading and usually haveparallel-laminated tops, similar to those of theFacies b units.

Fourth outcrop belt

The easternmost Azagador outcrop belt is locatedimmediately to the west of MojaÂcar and southeastof Turre (Fig. 1C). A detailed map of the area(MojaÂcar area) is shown in Fig. 9. On the westernside of the area (Cortijo GaÂtar: Locality 9 inFig. 1), a thick (up to 120 m) section of carbonatesand mixed siliciclastic-carbonate deposits inter-calated with sandy silts and silty marls occurs(Fig. 10). Planktonic foraminiferal assemblagesindicate a Late Tortonian age for the entiresection. Bedding dips steeply to the north(Fig. 11) as a result of post-depositional (probablyPlio-Quaternary) deformation. The sectionincludes a small lens-shaped deposit ofwell-bedded bioclastic carbonates and mixedsiliciclastics-carbonates at the base (lowermost12 m) (Fig. 10) followed by a major carbonatebody with similar characteristics some 40 m thick(Figs 10 and 11). To the east around CerroJoancho (see Fig. 9 for location) the main lensoidunit splits into three subordinate bodies thatinter®nger with sandy silts and silty marls.

In the upper part of the section (Fig. 10) twoother minor carbonate-mixed siliciclastic-carbon-ate, lens-shaped deposits occur (between 85 and100 m, and 113 and 117 m respectively), exhib-iting similar features to those of the main bodyand intercalated between sandy silts and silty

marls. However, in the middle part of the loggedsection (Figs 10 and 11) some other minor coarsesediment bodies (at approximately 58, 60, 66, 71,82 and 84 m) with distinctive internal beddinggeometries and highly irregular bases occur.These intervals show well-developed lateral ac-cretion bedding and contain abundant, pebble-sized bioclasts (oyster) and lithoclasts (mostly ofquartz and metamorphic rocks).

In the main carbonate body (Fig. 11) individ-ual strata have an extremely ¯attened, plano-convex lensoidal shape, and can be tracedlaterally for several tens to hundreds of metresbefore thinning out and disappearing. Well-bedded sediments within the body comprisebioclastic calcarenites and calcirudites, withabundant fragments of coralline algae (Litho-phyllum, Mesophyllum, Lithothamnion) andbivalves (Ostrea spp. up to 150 mm in size).Bryozoans, in contrast to the Cortijo Grandeoutcrops, are rare components. The grain size ofthe sediment mostly ranges from medium sandto granule, although pebble-sized bioclasts andsiliciclastic clasts do occur. In the calciruditebeds the coarse (granule and coarser) fractionvaries between 20% and 55%, and bioclastsusually outweigh siliciclasts. Detrital branchfragments of coralline red algae are particularlyabundant at the top of the unit. Parallel lamin-ation and sediment grading (both positive andinverse) are the most conspicuous internalsedimentary features.

At the eastern end of the outcrop (Cortijada deJoancho, Locality 10 in Fig. 1; see also Fig. 9 for amore precise location) carbonate and mixedsiliciclastic-carbonate bodies are embedded in®ne to very ®ne sands, sandy silts and silty marls

Fig. 9. Detailed geological map ofthe MojaÂcar outcrop belt.



(Fig. 12). Fine-grained sediments at the bottomand immediately overlying the limestone bedscontain Upper Tortonian planktonic foraminifera.These bodies have a thickness of several (up to10) metres, extend laterally for 100±200 m, andare characterized by an irregular base, internallateral accretion bedding and cut-and-®ll (chan-nelized) structures (Figs 12 and 13). Individuallateral accretion beds extend for some 10±15 mperpendicular to the channel trend. Channelizedstructures cross-cut one another with markedirregularity. Sediments consist of coarse-grainedcalcarenites and calcirudites with up to 25%siliciclastic content in the coarse (>2 mm) frac-tion. Large oyster (Ostrea spp.) fragments anddisarticulated valves, siliciclastics and friableclasts of Chozas Formation (up to 60 cm in size)are locally found.

INTERPRETATION

Coastal and inner platform deposits

Low-angle, parallel-laminated sediments alongthe northern margin of the ®rst outcrop belt thatpass laterally into trough cross-bedded deposits,are interpreted as beach deposits changingseawards (to the south) into a shoal area.Lithophagid-bored blocks correspond to coastalpalaeocliff deposits (Doyle et al., 1998; Bragaet al., 1999).

The strings of boulder-sized, matrix-supportedconglomerates in the centre and the east of the®rst outcrop belt correspond to channelizeddebris ¯ow deposits cross-cutting the inner plat-form sediments (Fig. 4). These channels occur atdifferent heights within the sequence. They do

Fig. 10. Cortijo GaÂtar stratigraphicsection. Note the existence ofseveral thick limestone bodies,distributed throughout the sectionand intercalated between sandysilts and silty marls, interpreted aslobe deposits. Some minor lime-stone beds, with highly irregularbases, are found in the lowerupper-half of the section andinterpreted as channel ®lls. Theenlarged log shows in detail thesequence corresponding to thelower-half of the section, includ-ing that of the main lobe. Abbre-viations as in Fig. 8.



not relate to a major exposure event as in the casedescribed by Hyden (1980) of channel-®llingconglomerates occurring within cross-strati®ed,inner-shelf bioclastic limestones in the Neogeneof New Zealand. The possibility does exist forthem being subaerial, in this case correspondingto ¯uvial courses crossing the inner platform

during precession-induced, eustatic sea-levelfalls. At these times at least part of the platformcould have emerged and been exposed. Never-theless, it must be pointed out that they arerandomly distributed within the sequence andappear not to be associated with any particularlevel, nor do they show any signs of subaerial

Fig. 11. Field view of the main lobe, overlying channels and some of the younger lobe deposits at GaÂtar, logged indetail in Fig. 10.

Fig. 12. Cortijada de Joancho sec-tion. Thick limestone beds andpackages of beds, interpreted aschannel ®lls, intercalate betweenhemipelagic ®ne-grained sediments.The enlargement shows the struc-ture of one of these packages, whichhas been restored to a horizontalposition, exhibiting an irregularbase and lateral accretion features.



exposure. In this respect a subaqueous originfor all of them is favoured. They most probablydeveloped at the continuation of river coursesentering the sea.

A small fan delta is located on the northwesternmargin of the shelf (Fig. 14) at Venta Anita(Locality 3 in Fig. 1) near Almocaizar, wherelobate-shaped, matrix-supported debris ¯ow con-glomerate predominate.

Outer platform deposits and erosionalchannels

Bioclastic calcarenites, locally exhibiting small-to-medium scale trough cross-bedding and with avariable siliciclastic content, characterize the sec-ond outcrop belt. They represent outer platformdeposits that inter®nger to the south with basinal

deposits. The abundance of siliciclastics at thewestern end of the outcrop (Fig. 14) is related to theVenta Anita fan delta, which was located immedi-ately to the north and supplied signi®cant quanti-ties of detrital material to that area. Terrigenousclasts are well rounded, and include abundantmature quartz pebbles, which indicate signi®cantmarine reworking. A possible longshore transportto the ENE is supported by a marked reduction ofsiliciclastic grain size and content in that direction.

The huge channelized structures cuttingthrough the outer platform sediments (Fig. 5) areinterpreted as major erosional channels crossingthe shelf (Fig. 14). Platform sedimentation andchannel erosion took place simultaneously. Thesituation that can be envisaged is very similar tothat exempli®ed by Carter & Lindqvist (1975)when referring to a large submarine channel

Fig. 13. Outcrop photograph showing in detail the central part of the channel displayed in Fig. 12, also restored to ahorizontal position. Bedding surfaces are indicated.

Fig. 14. Latest Tortonian palaeoge-ography of the Sorbas and Vera Ba-sins, at the time Azagador temperatecarbonate and mixed siliciclasticcarbonate platform sediments weredeposited (from Braga & MartõÂn,1997). The enlarged area corre-sponds to the south-easternmostcorner of the northern platform.Important features include a fandelta to the north-west, channelswithin the platform area and achannel and lobe system further tothe east. Channels cross-cut theplatform and supplied both coarsesiliciclastic and carbonate sedi-ments to the lobes located withinthe basin. Reconstruction is basedon preserved outcrops (solid lines)and inferred trends (dashed lines).



(40 m wide and 5 m deep) crossing the platformand feeding a submarine fan complex of Oligo-cene age in southern New Zealand. In thisexample, as in this case, the shelf-break wasfairly nearshore, and as the debris ¯ows spilledoff the landmass and across the narrow shelf theyprecipitated slope failure, slumping and ¯ow ofsand towards the inside of the channel, parti-cularly at the outer platform and the shelf break.These mass ¯ows gave way to turbidity currentsat locations further down slope. Carter & Lindq-vist (1975, 1977) described both the removedplatform sediments and those accumulating inthe submarine fan as siliciclastic deposits; in thiscase they are carbonates. Siliciclastic and bio-clastic, temperate-carbonate sediments are com-parable in many respects as they both consist ofloose, unlithi®ed grains (see Discussion below).The role played by sediment ¯ows in excavatingand enlarging submarine channels have also beenpointed out by Mullins et al. (1984) and byPratson et al. (1994), in the cases of the present-day Bahamas slope and of the New Jerseycontinental slope respectively.

In the example here, considering the size of thechannels, signi®cant amounts of the platformsediments were removed by erosion from thewalls of the steeply incised channels and mobil-ized downslope. The channels mainly acted asby-pass features as conglomerate deposits or largeconglomerate clasts have not been found insidethem. Once abandoned, they were ®lled bysediments from the surrounding platform. Indi-vidual channels may show several stages oferosion and in®lling. These distinct phases oferosion/in®lling could perhaps be related to theprecession cycles, erosion affecting mainly theouter platform area and taking place duringlowstands, and sedimentation, inside the chan-nels, during highstands. However, there is nodirect evidence to con®rm that submarine-fandeposition took place predominately during low-stands.

Channel and lobe systems of submarinefans and basinal deposits

In the third outcrop belt in the Cortijo Grandearea (Figs 6±8) the thin limestone bodiesshowing internal lateral accretion and cut-and-®ll (channelized) structures are interpreted asmigrating channels, which presumably connectedwith those crossing the platform. They exhibitgeometric features similar to those of ¯uvialchannels, which, as pointed out by Clark et al.

(1992) and Clark & Pickering (1996), is a directresult of similar physical processes operating insubmarine and ¯uvial systems. A large volume ofsediment was mobilized from the platform andtransported into the adjacent basin through thesechannels. During this process coarse-grainedsediment was selectively deposited, ®lling thechannels. The coarsest clasts of metamorphicrocks came all the way from the northern Sierrade los Filabres high, while friable clasts ofChozas Formation presumably originated fromerosion at the base of the deepest outer platformchannels.

The seven lobe-shaped bodies to be found inthe same area are interpreted as lobes of sedi-ment deposited at the channel mouths. Thesecarbonate bodies appear completely encased insilts and silty marls. The two small lobes just tothe west of Cortijo Grande seem to have formedfrom the same channel system (but presumablynot contemporaneously), while the ®ve lobes inthe Barranco del Gitano display a separatetemporal succession (Fig. 14). According to theirstratigraphic position and geometrical relation-ships, the two northernmost lobes in the Bar-ranco del Gitano were the ®rst to develop,followed by the easternmost and then the twosouthernmost ones. The feeder channels of thelast three lobes are visible on the left margin ofthe Barranco del Gitano ravine above the lower-most lobes (Fig. 6). Another lobe now removedby erosion probably existed further towards thecentre of this area, where the feeder channels,trending NW-SE, are still preserved (Fig. 14).The huge channel to the north of the ®ve lobesin the Barranco del Gitano, with a WSW-ENEtrend, seems to be the one feeding the MojaÂcarlobe placed approximately 2 km to the east(Fig. 14).

Sediments within the lobes originated fromturbidity ¯ows. They show many characteristicstypical of turbidites, such as a well-developedand conspicuous internal parallel laminationand, sometimes, positive grading. Nevertheless,the absence or near absence of other signi®cantturbidite structures such as scour marks, ripplesand convolute lamination is remarkable. Thissituation is presumably due to the extremelycoarse grain size of the deposits. A signi®cant partof the mobilized skeletal and siliciclastic particlesderived from the shelf were of granule to pebblegrade. This fact no doubt introduced importantmodi®cations on the hydrodynamic behaviour ofthe grains. When sediments are ®ner than mediumsand, they can be maintained in suspension by



¯uid turbulence to give low-density turbiditycurrents. The deposits of these currents arecharacterized by graded beds showing well-developed Bouma sequences. High-density tur-bidity currents can transport much coarsesediment (from coarse sand to pebble and cobble).In this case ¯uid turbulence and dispersivepressure resulting from grain collisions helpstransport the coarse material. The resulting bedsshow grain-size gradation and parallel laminationas the most conspicuous structures (Lowe, 1982;Tucker, 1992a).

In the logged section at Barranco del Gitano(Figs 7 and 8) three separate facies occur: Facies(a) consists of amalgamated thick-bedded multi-lateral and multistorey lensoid beds with inter-erosional basal contacts, containing abundantcoarse bioclastic detritus, intraclasts and base-ment pebbles. These ®ning-upward lensoid pack-ages are considered as small- to medium-sizedintralobe channels. Facies (b) comprises thin-bed-ded laterally persistent ®ne-grained calcirudites,with prominent grading and parallel lamination.Bedform and sedimentology are consistent withtheir interpretation as proximal lobe-®llingturbidites. The lobe sampled is the northeastern-most one of the ®ve mentioned above. Facies (c)comprises the uppermost interval of calciruditesand calcarenites alternating with sands. Thisfacies is considered typical of levee deposits withan overall ®ning and thinning upwards stackingpattern, which can be traced laterally into chan-nel deposits.

In the MojaÂcar area (the fourth outcrop belt)(Figs 9±13) a prominent thick carbonate-mixedsiliciclastic carbonate body (Fig. 11) correspondsto a huge lobe that developed on the western sideof the outcrop (Fig. 9). Minor lobes and localchannels (represented by the deposits with lateralaccretion) formed immediately above it (Figs 10and 11), with another minor lobe developed inthe same place as, and just before, the main one.Further to the east (Fig. 9), the intervals of mixedsiliciclastic-carbonate sediment with irregularbases, internal lateral accretion bedding and cut-and-®ll (channelized) structures (Figs 12 and 13)are considered to be distributory channels. Mostof these channel levels occur at a slightly higherstratigraphic position than that of the main lobe atGaÂtar. Some of the channels presumably relate tothe minor lobes developed in the western area(Cortijo GaÂtar section), but most of them fed lobessituated further to the east, either now removedby erosion or not cropping out (present as subcropbelow the Mediterranean Sea). This seems to be

the case for the large channel feature at theeasternmost margin of the mapped area, at Cor-tijada de Joancho, whose trend is E±W (Fig. 14).In this particular channel metamorphic-rock andfriable clasts of Chozas Formation, up to half ametre long, together with large oyster bioclasts,can be found. It seems that lobe sedimentationceased in this area and the system migrated onceagain to the east.

AN INTEGRATED MODEL

Temperate carbonate and mixed siliclastic-carbonate sediments were deposited on a narrowplatform along the southeastern margin of theSierra de los Filabres during the latest Tortonian(Fig. 14). In the area of Almocaizar-Los Gallardos,some 15 km to the ENE of Sorbas and 10 km tothe SW of Vera, in the Vera Basin (Fig. 1), part ofthese sediments were removed, remobilized andredeposited in submarine lobes (up to 40 m thickand 1 km wide). Most of these lobes are stillpreserved as features resistant to weathering(Fig. 11) and can be easily recognized in the ®eldsouth of Turre and to the west of MojaÂcar(Fig. 14). Sediments in the lobes consist of turbi-ditic carbonates (calcarenites and calcirudites)and mixed siliciclastics and carbonates whichcontain up to 30% siliciclasts, mostly quartz andmetamorphic clasts from the Sierra de los Filabresto the northwest, and abundant bioclasts ofcoralline algae, bivalves and bryozoans.

The feeder channels of the lobes can beidenti®ed at many positions across a narrow shelfplatform. In the inner platform, channels cross-cut beach and shoal deposits (Fig. 4) and are®lled by long, thin (up to 3 m thick) strings ofconglomerate (channelized debris ¯ows). Thesechannels presumably developed as the continu-ation of river courses entering the sea. Furthertowards the outer platform, large channelizedstructures (up to several hundred metres wideand 20 m thick) can be identi®ed. They areconsistently orientated NW±SE and are steeplycut into horizontally bedded strata of the platform(Fig. 5). They represent by-pass channels crossingthe outer platform. The platform edge channelsare steeply incised structures, which suggests thatsigni®cant quantities of platform sediment wereremoved by erosion during their excavation, evento the point of eroding the underlying ChozasFormation. These channels, once abandoned,were ®lled in by new platform sediments, iden-tical in character to that of the surrounding outer



platform area. Several phases of in®lling can berecognized. Further towards the basin, the chan-nels associated with the lobes (Figs 6±8) exhibitlateral accretion (Figs 12 and 13) and internal cut-and-®ll structures, and are intercalated between®ne to very ®ne sands, sandy silts and silty marlsconsidered mainly as hemipelagic deposits. Thechannel-®lling sediments are in this latter casecoarse-grained carbonates and mixed siliciclas-tics-carbonates. Siliciclastic clasts up to 50 cm insize of metamorphic rocks occur locally.

A tentative model connecting the channels andthe lobes is given in Fig. 14. This summary modelis partly speculative, as no actual physical con-nection can be traced today between all theAzagador outcrop belts (Fig. 1C). Two main areasof lobe development can be distinguished. In thewestern sector (Cortijo Grande area) most of thelobes concentrate to the NE, with a sequence ofdevelopment which has been reconstructedaccording to their geometrical and stratigraphicalrelationships. These lobes are presumably con-temporaneous with those developed further to theSW in this same area. At a later stage the wholesystem migrated to the east (MojaÂcar area) and anew lobe system, situated approximately 2 km tothe west of MojaÂcar, formed there. After a time,lobe sedimentation ceased at this latter point aswell and the system migrated once again to theeast. The easternmost outcrops (Cortijada deJoancho) correspond to channels ¯owing to theeast (Fig. 14) with no associated lobes recorded.

The successive migration to the east of thechannel-lobe system may relate to the uplift of theSierra Cabrera, a major high located immediatelyto the south of channel-lobe outcrops (Fig. 1C).Although the Sierra Cabrera clearly emerged inLate Messinian-Early Pliocene times (MartõÂn &Braga, 1996; Braga & MartõÂn, 1997), long after thedeposition of the study deposits, during Azagadortimes the relief presumably formed a submarineswell with a SW-NE orientation, which wasprogressively uplifted from the SW. This upliftno doubt affected the area immediately to thenorth of the swell where the lobes formed andconditioned their development. In this way thewesternmost lobes were the ®rst to develop, butwere subsequently abandoned, and replaced bynew lobes placed further to the east as thesouthern swell migrated. The uplift of the SierraCabrera diverted the ¯ow of the feeding channelsto the east, in a similar way to which the ¯uvialdrainage pattern is diverted today in some moun-tainous, tectonically active areas in California,where fault-related folds propagate (Keller et al.,

1998, 1999). This process caused abandonment ofancient lobes and development of new onesfurther to the east at the mouths of the morerecent channels.

All siliciclastic components originated from theSierra de Filabres to the north, even those presentin the most distal Azagador sediments (formingthe submarine lobes), which are the closest to theSierra Cabrera. This is consistent with very lateemergence of the Sierra Cabrera. Major siliciclas-tic inputs are concentrated at the margin of theSierra de los Filabres, in the westernmost part ofthe studied area (Almocaizar), where a fan deltadeveloped within the inner platform. Fan deltasediments were signi®cantly reworked towardsthe outer platform. Minor siliciclastic in¯uxesrelate to areas where inner platform conglomeratechannels appear.

DISCUSSION

The above model implies that river currentsoriginating from the Sierra de los Filabres enteredthe sea as mass ¯ows. Some of the sediment wasdeposited in the inner platform channels asconglomeratic debris ¯ows, but part of it contin-ued to be transported towards the shelf edge bycurrents traversing the platform, eroding andexcavating large channels within the outer plat-form zone. This scouring process supplied a largevolume of bioclastic-rich sediments that mixedwith the terrestrial siliciclastic sediments trans-ported within the channels to be ®nally depositedin the submarine lobes.

Redeposited bioclastic particles were derivedfrom all the different subenvironments across theplatform, from shallow to deep, but major erosionaffected mainly the outer platform. Bioclastsfound in the submarine-lobes and channels ori-ginated from coralline algal assemblages charac-teristic of a wide range of depths on the platform,from assemblages mostly made up of Lithophyl-lum typical of the shallowest areas (Adey, 1986)to deeper water assemblages dominated by mel-obesioids (Adey, 1986). Ostrea spp., which is asigni®cant bioclastic component in the lobes andchannels, is also an inhabitant of shallow-waterenvironments.

In the study example, platform sediments,formed by temperate carbonates and mixed sil-iciclastics-temperate carbonates, were prone toremoval and mobilization as the channels wereexcavated. Temperate carbonates consist mainlyof loose skeletal particles (Nelson, 1988) which



are unlithi®ed or show only very weak earlylithi®cation. Carbonate saturation of sea water islow in non-tropical settings (Nelson, 1988;Opdyke & Wilkinson, 1990), and consequentlyearly lithi®cation of the sea ¯oor occurs onlysporadically, under conditions of sediment star-vation (James & Bone, 1991). This lack of stabili-zation of the substrate provides sediment with aloose character and the skeletal particles areeasily mobilized as individual grains. In thisexample, once removed, the skeletal particleswere put into suspension and transported down-slope in turbidity ¯ows, together with siliciclasticgrains that behaved in a similar way.

Although it appears that temperate carbonatesare more prone to being mobilized than tropicalcarbonates because the latter commonly undergoearly sea¯oor diagenesis (Tucker, 1992b), most ofthe redeposited turbiditic carbonate examplesdescribed in the literature (e.g. Cook & Mullins,1983; Cook et al., 1983; Ruiz-Ortiz, 1983; Watts,1988; Cooper, 1989; Tucker, 1992a; Miller &Heller, 1994) have been interpreted as beingtropical in character. This suggests that someexamples may require reinterpretation to reassessthe climatic context in which they formed.

ACKNOWLEDGEMENTS

J.C.B. and J.M.M. were supported by `FundacioÂnRamoÂn Areces' Project: `Cambios climaÂticos en elsur de EspanÄa durante el NeoÂgeno' and DireccioÂnGeneral de EnsenÄanza Superior e InvestigacioÂnCientõÂ®ca (Ministerio de EducacioÂn y Ciencia,Spain) Project PB97-0809. J.W. thanks the Cons-ejo Superior de Investigaciones CientõÂ®cas/Brit-ish Council for funding his 10-month stay at theInstituto Andaluz de Ciencias de la Tierra (CSIC-Universidad de Granada), during the 1997±98academic year. Constructive, critical commentsby Sedimentology reviewers Noel P. James(Queen's University, Canada) and Campbell S.Nelson (University of Waikato, New Zealand),and Sedimentology editor Ian Jarvis (KingstonUniversity, UK) are greatly appreciated.

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