Significance of pot and gutter casts in a Middle Triassic carbonate platform, Betic Cordillera, southern Spain

Signi®cance of pot and gutter casts in a Middle Triassiccarbonate platform, Betic Cordillera, southern Spain

A. PEÂ REZ-LOÂ PEZDpto. EstratigrafõÂa y PaleontologõÂa, Facultad de Ciencias, Universidad de Granada, Campus deFuentenueva, 18071-Granada, Spain (E-mail: [email protected])

ABSTRACT

Pot casts and gutter casts are described for the ®rst time in the lower part of the

Majanillos Formation, a Middle Triassic carbonate unit located in the External

Zones of the Betic Cordillera (southern Spain). Their identi®cation, as well as

their relation to tempestites, enables the better interpretation of the

depositional environments and the shoreline-to-offshore facies transition on

the Anisian muddy carbonate ramp of the southern Iberian Massif. The

Majanillos Formation contains three members, which become progressively

more marly towards the top. Well-preserved pot and gutter casts and thin

intercalations of calcarenite, which are interpreted as tempestites, are abundant

in the lowest member. Above the pot and gutter casts, thicker calcarenite beds,

which locally contain hummocky cross-strati®cation, predominate.

Bioturbated nodular limestones are prevalent at the top of the member. The

remaining succession, which records a long-term Triassic transgressive cycle,

consists mostly of ®ne-grained limestones deposited in very shallow-marine

environments. Calcarenitic sediments only accumulated within potholes and

gutters in the nearshore. They developed during storms when strong currents

transported sediment to the outer shelf, where it was deposited as tempestite

beds. Pot and gutter casts characterize sedimentation in the bypass zone. It is

concluded that storm deposits provide important constraints for the

interpretation of palaeobathymetry; it is proposed that gutter casts display a

trend of increasing width/thickness ratios towards the outer shelf. The

identi®cation of these structures in marine successions elsewhere should

prove useful in the interpretation of depositional environments.

Keywords Carbonate platform, gutter cast, Muschelkalk, pot cast, tempestitemodel, Triassic.

INTRODUCTION

A Middle Triassic carbonate unit of the SubbeticZone, the Majanillos Formation (PeÂrez-LoÂpez,1991), was deposited in the south-east margin ofa Hercynian massif (Iberian Meseta). The forma-tion outcrops discontinuously in the ExternalZones of the Betic Cordillera (southern Spain)over a distance of more than 250 km (Fig. 1) andis composed of Muschelkalk facies (Bertrand &Kilian, 1889; Blumenthal, 1927; LoÂpez-Garrido,1971; GarcõÂa Rossel, 1973; Sanz de Galdeano,1973; Busnardo, 1975), a succession of shallow-

marine carbonates deposited in European epicon-tinental basins during the Triassic period. PeÂrez-LoÂpez (1991) and PeÂrez-LoÂpez et al. (1991) havestudied the Triassic stratigraphy and interpretedthe Muschelkalk facies as shallow ramp/platformcarbonate deposits. This paper focuses on thelower part of these carbonate strata, Member 1 ofthe Majanillos Formation, where pot and guttercasts occur.

Pot and gutter casts are ®lled isolated erosionalstructures of variable sizes (Aigner & Futterer,1978). In the literature, gutter casts have beendescribed using a variety of terms (Myrow, 1994,

Sedimentology (2001) 48, 1371±1388

Ó 2001 International Association of Sedimentologists 1371

table 1). Pots are rounded structures, and guttersare linear structures, also known as rinnens, cut-and-®ll, scour-and-®ll and furrows (Plessman,1961; Greensmith, 1965; Nagtegaal, 1966; Bridges,1972). Various origins for these structures havebeen proposed and discussed in the literature(Aigner, 1985; Browne, 1994; Myrow, 1992a, 1994;Duringer & Vecsei, 1998). These sedimentarystructures, which had not previously been identi-®ed in the epicontinental Triassic of the BeticCordillera, are of particular sedimentological sig-ni®cance for the interpretation of depositionalenvironments; their description and interpretationshould prove useful to sedimentologists ®ndingsimilar structures in other successions elsewhere.

The present study uses a tempestite model(Myrow, 1992b) to establish facies±depth rela-tionships in the sedimentary interpretation of thelower Muschelkalk platform deposits. The appli-cation provides some validation for this model,which is not a standard one. Myrow (1992b)studied shoreline facies dominated by siliciclas-tic ®ne-grained sediment in a shallow subtidalzone, and his model described sediment depos-ited in a storm-generated sediment bypass zone.The present paper demonstrates that pot andgutter casts can be especially useful for theinterpretation of storm deposits and facies trendsin shallow-marine carbonate environments. In

this study, the diverse lithofacies of a storm-in¯uenced shallow ramp have been characterizedand related to each other, and it is proposed thatthere is a trend in the gutter cast width±thicknessratio from nearshore to outer shelf, with guttercasts from the nearshore being both smaller andnarrower than offshore examples.

Several kinds of deposits and sedimentarystructures typical of storm-in¯uenced ancientand modern muddy shelves have been taken intoconsideration in this study (Reineck & Singh,1972; Aigner, 1984; Pedersen, 1985; Handford,1986; Hill & Nadeau, 1989). In particular, thecarbonate facies studied are compared with theshallow-water limestones of the upper Muschel-kalk of eastern France, which have been inter-preted as a shallow subtidal shelf facies depositedunder the in¯uence of relatively strong and moreor less persistent currents, with intermittentsedimentation under the in¯uence of storms(Duringer & Vecsei, 1998).

STRATIGRAPHY OF THE MAJANILLOSFORMATION

The Majanillos Formation of the Middle Triassicconsists of an overall upward-thinning carbonatesuccession of limestone and marly limestone

Fig. 1. Geological map of southern Spain showing location of study sections. The epicontinental Triassic outcropsare composed of disrupted sections of Buntsandstein, Keuper and Muschelkalk facies (Majanillos Formation).

1372 A. PeÂrez-LoÂpez

Ó 2001 International Association of Sedimentologists, Sedimentology, 48, 1371±1388

alternations generally dominated by marlstonetowards the upper part (Fig. 2). The formation isdivided into three members. Stratigraphic studyof these strata is dif®cult because almost all theoutcrops are bounded by faults, including thebases of the nappes of the External Zones. Thelower parts of these nappes consist largely ofTriassic rocks (Azema et al., 1979; GarcõÂa Her-naÂndez et al., 1980), which also constitute thematrix of the Tertiary olistostrome units on whichthe tectonic units (nappes) have slid (GarcõÂaCorteÂs et al., 1991; PeÂrez-LoÂpez & Sanz deGaldeano, 1994).

Member 1 is 20±50 m thick and consists ofmassive grey limestones, dolostones, thin-beddedlimestones and nodular limestones. The thin-bedded limestones occur in intervals »10 m thick,which in some cases contain thin beds of biotur-bated nodular limestone and marl. In addition, awide variety of pot and gutter casts and calcare-nite beds occurs in this member. Member 2, up to50 m thick, consists of a more marly succession ofcarbonate beds with burrows (Thalassinoides,Chondrites, Planolites) and thick intercalationsof bioclastic limestone. The upper member,Member 3 (20±35 m thick), is mainly composed

Fig. 2. Stratigraphy of the MajanillosFormation (Muschelkalk facies). Themain lithostratigraphic units areshown. The interval of gutter castfacies studied and the two deepening-upwards cycles of Member 1 aremarked. The top of the upper cycle isplaced at a hardground surface.BUNT, Buntsandstein facies.

Signi®cance of pot and gutter casts in a Middle Triassic carbonate platform 1373


of marl and shale with thin carbonate beds, whichgrade upwards into gypsum and terrigenousclastic deposits of the Keuper facies (UpperTriassic).

In spite of dif®culties with outcrop exposure, itis possible to correlate the carbonate intervals ofMember 1, which contain signi®cant gutter casts,between distant outcrops (Figs 1 and 3). Thethicknesses of four important sections of theMajanillos Formation range from 18 to 125 m.The thinnest sections are those of Cambil (18 m)and Calasparra (90 m). These sections are inter-preted as corresponding to areas located in a moremarginal position within the Triassic basin. Thepresent geographical positions (Fig. 1) are tecton-ically controlled.

In this study, only data from Member 1 of theMajanillos Formation are considered. Member 1corresponds to the lower Muschelkalk platformdeposits. Member 2 displays a different faciesdevelopment and is interpreted as belonging toanother depositional system. Member 1 has beeninterpreted as a transgressive systems tractdeposit (PeÂrez-LoÂpez, 1997). It contains brownoolitic dolostone of shallow high-energy origin atthe base and bioturbated nodular limestone cor-responding to the deepest facies at the top. Thelower transgressive surface of this sequence isoverlain by the brown oolitic dolostone facies.The upper member boundary represents a maxi-mum ¯ooding surface (hardground) regarded asbeing the lower boundary of the highstandsystems tract (HST). The HST deposits of Member2 consist of progradational stacked facies (PeÂrez-LoÂpez, 1997) of a carbonate platform. Moreover,in Member 1, two higher order, deepening-up-wards cycles occur (10±20 m thick). Pot andgutter cast facies are generally present in theupper cycle and are less common in the lowercycle (Fig. 2). Member 1 is assigned to theAnisian stage based on an assemblage of ceratitesthat occur in the Calasparra and Valdepenassections (Goy & MartõÂnez, 1996; Goy & PeÂrez-LoÂpez, 1996).

SEDIMENTARY FACIESAND ENVIRONMENTS

A study of lithofacies, sedimentary structures anddepositional cycles was carried out for Member 1in order to interpret the depositional history ofthe ®rst stages of Triassic carbonate platformdevelopment in the southern Iberian Massif. Fivemain lithofacies are distinguished in Member 1

(Fig. 3): brown dolostone, laminated limestone,thin-bedded marly limestone, marl and bioturb-ated nodular limestone. The thin-bedded marlylimestone is highly variable; it may be more orless marly, or it may consist of calcisiltite ormudstone. In addition, the thin-bedded marlylimestone facies unit contains bioclastic, ooliticand intraclastic calcarenite beds, as well as potand gutter casts.

Brown dolostone

The brown mesocrystalline dolostone faciesoccurs locally in the lower part of the Muschel-kalk sections. Bed thickness ranges from 1 to 3 m.Bedding is generally massive, but some bedsshow trough cross-bedding. Ooids can be recog-nized in thin section, suggesting an originalpackstone to grainstone texture. The highly crys-talline nature of the dolomite and the presence ofooids are interpreted as indicating that they wereoriginally (porous) calcarenitic sediments thatwere deposited under high-energy conditionsand later dolomitized. In most cases, the dolo-stone stratigraphically overlies red siltstone andsandstone of Buntsandstein facies. This dolo-stone was therefore the ®rst sediment of thetransgressive stage and corresponds to high-energy nearshore deposition.

Laminated limestone

This facies, which has been recognized in manyoutcrops of the Muschelkalk, consists of verythickly bedded (>2 m) laminated micritic lime-stone and calcisiltite, commonly bioturbated.Thin laminae of the micritic limestone are inter-preted in some cases as being microbial in origin,due to the occasional occurrence of irregularundulose laminations that are similar to smallstromatolite domes. However, the limestonesnormally contain thin to thick beds of calcisiltite,which display parallel lamination and locallyhave small wave ripples at their tops. Thesecalcisiltite beds range from several millimetres to60 cm thick, comprising more or less 50% of thelaminated limestone facies in some outcrops.They consist of microspar with a few smallbivalve skeletal grains. The microspar might haveoriginated from recrystallization during the dia-genesis of silt-sized carbonate grains such ascalcispheres or peloids (Duringer & Vecsei,1998; Vecsei & Duringer, 1998). In some cases,there are intraformational conglomerate beds(15±35% of the laminated limestone facies)



up to 10 cm thick that contain micritic clasts oflithi®ed carbonate sediment (mud chips).

The most common internal structure of thecalcisiltites is planar lamination, which is inter-

preted as upper plane bed strati®cation. Therelatively high energy of these deposits is corro-borated by the presence of Diplocraterion bur-rows. These facies are interpreted as indicating

Fig. 3. Main stratigraphic sections from Member 1, consisting mostly of ®ne-grained carbonates with pot and guttercasts and interbedded calcarenite beds. The Cambil section corresponds to Member 1, whereas the Calasparra, Juajaand Cabra sections correspond only to the upper deepening-upwards cycle of Member 1 (see Fig. 2). Their locationsare shown in Fig. 1.



deposition in a very shallow nearshore zone. Theavailable sediment was ®ne and sometimes madeup a cohesive laminated sediment.

Thin-bedded marly limestone and marl

The thin-bedded marly limestone facies is locallygradational with the laminated limestone facies.The thin-bedded marly limestone facies occurs inunits 10±15 m thick that contain interbeds (eachbed 1±5 cm thick) of grey limestone, marlylimestone and white marl. The grain of the greylimestone and the carbonate portion of the marlylimestone are mud or silt size.

This facies occurs in units in which the lowerpart consists of calcisiltite (30±75% of the lime-stone beds) or intraformational conglomerate(2±5%). The upper parts of these units areparticularly marly. Sedimentary structures inthe thin-bedded calcisiltite include parallel lam-ination, undulating lamination and tabular cross-bedding. The undulating, ¯aser-like aspect ofbeds in vertical sections results from small andalmost symmetrical ripples. The cross-beddingcorresponds to channel-®lling, lateral accretionbedding (Fig. 4), interpreted by Duringer & Vecsei(1998) as being typical of subtidal deposits.According to these authors, such channels arelikely to be of tidal origin, and the simultaneousoccurrence of small wave-ripples and channel-®lling, lateral accretion bedding is clear evidencefor a very shallow subtidal setting. Decimetre-scale, undulating erosion surfaces are more com-mon in these facies as a result of the in¯uence ofoscillatory currents on the sea¯oor.

A number of ceratites and scattered bivalveshells occur parallel to the bedding and, in rarecases, form a parting lineation, suggesting depos-ition under turbulent ¯ows (Allen, 1982). Thislineation corresponds to the alignment of skeletalgrains in a direction parallel to the ¯ow. Horizonsof aligned grains are separated from each other bybetween 15 and 50 cm. The tops of some very thinmudstone beds are marked by wrinkles withasymmetrical pro®les, and these are situatedperpendicular to the parting lineation structures,indicating the same direction for the palaeocur-rent. The wrinkles are interpreted as havingdeveloped as a result of the effects of the currentson the cohesive sediment of the sea¯oor (Fig. 5).

In summary, bedded calcisiltites of the thin-bedded marly limestone facies are interpreted assubtidal deposits, and mudstones and marls areinterpreted as inner shelf deposits. These ®ning-upward units, which increase in marl contentupwards, are upward-deepening cycles, likethose that occur in the Muschelkalk of theGermanic Basin, where marlstones characterizedthe deeper depositional areas of the ramp (Aigner,1985; Klein, 1985; Schwarz, 1985).

Bioturbated nodular limestone

This bioturbated mudstone facies displays a veryhomogeneous and generally massive appearance(bioturbation texture). Bed thickness varies from1 to 2 m and tones of grey colour predominate.Most of the burrows recognized in this facies arehorizontal Planolites. The original texture of thesediment has been completely destroyed by

Fig. 4. Lateral-accretion beddinginterpreted as a subtidal shallowchannel deposit (thin-bedded lime-stone facies, Jauja section). Black arr-ows show inclination of the laterallyaccreted beds. Hammer is 32 cm long.



bioturbation, re¯ecting slow and continuoussedimentation under low-energy conditions indeep water, mainly below storm wave base (outershelf). Thin beds (<5 cm thick) of bioclasticlimestone occur near the bottom of the bioturb-ated limestone succession and, in some cases,these include graded and laminated wackestoneand grainstone at their base and have ®ne-grainedsediment at their top. These thin beds could beinterpreted as turbidite-like distal tempestitesdeposited on the outer shelf.

STORM-GENERATED EROSIONALSTRUCTURES AND DEPOSITS

Member 1 facies have been interpreted as sedi-ment deposited on a muddy carbonate ramp,based on the facies sequence and on the absenceof high-energy deposits that could possibly cor-respond to a shelf barrier (PeÂrez-LoÂpez, 1991). It isdif®cult to determine speci®c depositional envi-ronments or the palaeobathymetry of these facies

because of a lack of diagnostic sedimentarystructures. However, the storm deposits provideimportant information regarding variations inramp relative palaeobathymetry, energy regimesand depositional mechanisms. In this respect, theMember 1 is characterized by a relative abun-dance of coarser grained deposits and of struc-tures indicating a sharp increase in energygenerated by storm currents (PeÂrez-LoÂpez, 1998).For example, erosion surfaces and ¯at pebbleconglomerates of micritic clasts (mud chips;Fig. 6A), skeletal and oolitic limestone beds(Fig. 6B) and rounded or channelled erosionalstructures (Fig. 6C) all occur.

Skeletal and oolitic beds

Beds of coarse-grained limestone are presentparticularly in the thin-bedded marly limestonefacies of the upper part of Member 1. Thesecoarse-grained limestones contain two kinds ofallochems, those of skeletal origin (bivalves,gastropods, echinoderm fragments) and those ofnon-skeletal origin (ooids, peloids, intraclasts),which indicate a shallow-water depositionalenvironment. The most common textures areskeletal ¯oatstone or rudstone. The skeletal andoolitic beds normally display thicknesses ofbetween 2 and 50 cm and have extensive lateralcontinuity. However, in a number of outcrops,some beds are lenticular and correspond to thein®llings of large scours. These beds usually havesharp erosive bases and display either gradedparallel lamination or cross-lamination. Manyoutcrops display amalgamated bioclastic bedswith these sedimentary structures. Wave ripplesoccur on the tops of some of these skeletal beds,and these are overlain in places by mudstonewith burrows. In some sections, skeletal lime-stone beds are found in close association withthin beds of micritic clast conglomerate. Inaddition, in the Cabra section, the thicker skeletalbeds are hummocky cross-strati®ed. The spacingof the hummocks is of the order of 1á5 m, andthicknesses of these beds average 20 cm. Thesebeds commonly contain amalgamation surfaces,usually with the hummocky cross-strati®ed skel-etal limestone truncating an underlying bed ofstructureless or cross-strati®ed oolitic limestone.The hummocky cross-strata sets are up to 60 cmthick and, in the Member 1 cycles, are locatedwithin the upper part of the thin-bedded marlylimestone, below the nodular limestone facies(outer shelf facies). This type of cross-strati®ca-tion can be generated by hurricanes and intense

Fig. 5. Bedding-plane view (bottom) of wrinkles pro-duced by high-energy currents, Cambil section (18 m).These currents partially eroded and deformed a sub-strate of muddy sediment. Cross-sections of verticalburrows (bu) are indicated. The large arrow shows thedirection of the inferred palaeocurrent. Scale is 5 cmwith 1 cm divisions.



winter storm multidirectional ¯ows at the sedi-ment±water interface (Swift et al., 1983; Duke,1985, Duke et al., 1991; Myrow & Southard,1996). In this manner, amalgamated storm bedsmay be formed (Dott & Bourgeois, 1982; Hand-ford, 1986).

Many of the observed sedimentary features aretypical of storm deposits (e.g. Specht & Brenner,1979; Kreisa, 1981; Aigner, 1985), or tempestites,

which are intercalated with ®ne-grained fair-weather deposits. Skeletal and oolitic tempestitesare considered to be deposits of the inner shelf,above but near storm wave base. They mainlyoccur stratigraphically below the bioturbatednodular limestone facies of the outer shelf. Mudchip tempestites were deposited in a shallowerwater area, just below fairweather wave base. Thechips are interpreted as having undergone very

Fig. 6. Main storm-generated facies intercalated in fairweather deposits. (A) Close-up of the laminated limestonefacies from the lower part of the Cabra section showing erosive surfaces (arrows), laminated ®ne sediment and mudchips generated by storm currents. Scale in cm. (B) Tempestite bed (tem) in the bioturbated limestone of the upperpart of Member 1 (Cabra section). The tempestite bed ®nes upwards and is capped by wave ripples and ®ne-grainedlimestone (post-storm deposit) with burrows (bu). (C) Oblique view of thin-bedded marly limestone facies withnumerous cross-sections of gutter casts (dark colour), Calasparra section. Stratigraphic top to left. Pen is 13á5 cmlong.



little transport and were deposited in a veryshallow-water environment. Within the twodeepening-upward cycles of Member 1, they arelocated in the lower part of units of the thin-bedded marly limestone facies, above and withinthe shallow-water laminated limestone facies.

Gutter casts

In the thin-bedded marly limestone facies, narrowgutter casts occur with thicknesses of between 2and 60 cm, display U- and V-shaped cross sec-tions and have rectilinear or sinuous geometriesin plan view (Fig. 7A and B). Some gutter castshave more complex pro®les and may be bilobed,

irregular (Fig. 7C) or have ¯at bases. They areusually isolated, although some are amalgamated(Fig. 7D). The in®ll is composed of ®ne-grained(calcisiltite) or coarse skeletal grainstone withabundant bivalve and gastropod shells. This in®llis massive or displays an alternation of carbonatesilt and skeletal sand with weak normal gradingor planar lamination. In some cases, wave ripplesoccur on top of the gutter casts. A number of solemarkings can be distinguished along the sidesand bases of some of the gutter casts, includingcrescent and bounce casts, which are generallyparallel or subparallel to the long axis of thegutter casts. Gutter casts are thought to beproduced by helical ¯ows (Flood, 1983; Myrow,

Fig. 7. Gutter casts in the thin-bedded marly limestone facies, Calasparra section (gutter location is shown in Fig. 3).(A) U-shaped gutter cast of ®ne sediment with a lag of shells; scale in cm. (B) V-shaped cross-section of a sinuousgutter cast. (C) Complex gutter cast with an asymmetrical cross-section. (D) Slightly amalgamated irregular guttercasts with similar pro®les, indicating erosion and deposition from two successive ¯ows.



1992a). The cross-sections displayed by manygutter casts in the Majanillos Formation are veryincisive and asymmetrical (overhanging shapes).They display sinuous geometries in plan view(Fig. 7B). In addition, helical ¯ow is indicated bythe presence of skeletal lags that are displacedtowards the sides of the gutter cast.

The long axes of the gutter casts are unimodalfor each of the stratigraphical sections (Figs 8 and9). However, orientations differ as a result oftectonic rotation of outcrops, which can bedemonstrated by geological mapping. The palae-ocurrents indicated by gutter cast elongation aremore or less perpendicular to crest line orienta-tions of the oscillation ripples appearing on thetop of some beds or gutter casts. According to therelationship between trends of gutter casts andwave ripple orientations, the gutter casts wereprobably oriented perpendicular to the palaeo-shoreline. Myrow (1992b) described this sameorientation between gutters and the shoreline.

Similarly, in certain parts of the tidal ¯ats of theFlorida Keys (Bahia Honda Key), subtidal smallgutters with shells have been observed by theauthor that are perpendicular to the ®ne-grainedshore zone.

Fig. 8. Calasparra outcrop with multiple gutter casts.They occur at 24 m above Buntsandstein facies in thesection (90 m thick). Their axes are oriented in roughlythe same direction (arrows) at this outcrop. The arrowsshow the direction of the inferred palaeocurrent (seeFig. 9). Hammer is 32 cm long.

Fig. 9. Equal area rose diagram of long axes of guttercasts (striped area) taken from the Jauja (n � 14) andCalasparra (n � 23) outcrops and orientations of waveripple crests (black area) taken from the same outcrops(Jauja: n � 4; Calasparra: n � 7) immediately abovesome gutter casts. Directions of the gutters areapproximately perpendicular to the crests of the waveripples in each of the outcrops. However, gutter castorientations are 90° different between sections onaccount of variable tectonic rotation of each outcrop.



The gutter casts of the Majanillos Formationmainly occur in the thin-bedded marly limestonefacies, which in most cases records shorefacedeposition. Gutter casts occur in laminated calci-siltite, in some cases with Diplocraterion bur-rows, and they are never associated with mudcracks or with supratidal deposits. Thus, thegutter casts developed mainly in the shallowsubtidal zone, although they also occur in marlycarbonates of thin-bedded marly limestone facies.Speci®cally, the largest gutter casts are found inthe most marly parts of the thin-bedded limestonefacies, in some cases associated with ceratites,which are considered to be inner shelf depositsthat accumulated below fairweather wave base.Some gutter casts therefore originated in a deeperarea below fairweather wave base.

The gutter casts were formed by storm-gener-ated turbulent ¯ows (Bridges, 1972; Kreisa, 1981;Aigner, 1985; Myrow, 1992a). The ¯ows erodedeither a ®ne-grained substrate or a cohesive bed(Leeder, 1999) and thus developed scours, whichwere then ®lled with sediment when the ¯owenergy decreased in intensity. These sedimentarystructures are directly comparable with the GutterCast Facies described by Myrow (1992a), whichwas also interpreted as being a shallow subtidaldeposit. However, gutter casts have been recog-nized in a wide range of environments (Myrow,1994). For instance, they were interpreted byAigner (1985) as being subtidal structures, and byDuringer & Vecsei (1998) as emersion structures;they have also been found in ancient ¯uvial (Rust& Gibling, 1990) and lacustrine deposits (Berry,1961; Daley, 1968).

Pot casts

Pot casts are formed from the in®lling of potholesor rounded non-linear erosional depressions(Myrow, 1992a) and are sometimes associatedwith gutter casts. They display various cylindri-cal forms and sizes (Fig. 10), and the deepest havea well-developed, downward-spiralling shape(Fig. 11). In most cases, the in®ll of the pot castsis ®ne grained (calcisiltite), although some arecomposed of skeletal sand, as is the case withgutter casts. The pot casts are very well developedin the Calasparra section, but do not occur atCambil. In addition, their stratigraphic position inthe sections is more restricted than that of thegutter casts. Pot casts co-occur with gutter castsand therefore developed in equivalent environ-ments. However, they are less frequent than guttercasts in most outcrops. For this reason, it may be

assumed that their formation required morespeci®c environmental conditions. Pot casts donot occur either within the shallower deposits ofthe thin-bedded marly limestone facies wherecalcisiltite predominates or within deeper depos-its where there are tempestite beds. Pot casts

Fig. 10. Oblique view of pot casts in the thin-beddedmarly limestone facies, Calasparra section. These potcasts are located in the middle of the section (Fig. 3),next to the gutter casts. (A) Interconnected pot casts. (B)Two pot casts of very different sizes; note the centralelevation of the base of the pot as a result of rotaryupwelling ¯ows. Pencil is 14 cm long.



therefore have a smaller bathymetric range thangutter casts and may require slightly more speci®cbathymetry conditions in marine environmentsthan in rivers (Arnborg, 1957; Allen & Friend,1968).

Potholes are produced by storm-generated ver-tical rotary ¯ows (Myrow, 1992a), and high-energy conditions are necessary for the generationof such rotary ¯ows. Myrow (1992a, 1994) inter-preted unidirectional ¯ows for the origin of potcasts. One hypothesis could be that these struc-tures originate in a depth interval in which twoopposed currents can interact and thus generatethe rotary ¯ows that ultimately produce thepotholes. Rotary ¯ows could be a consequenceof the interaction between a strong storm-gener-ated current that moves over the sea¯oor intodeeper water and another more super®cial cur-rent moving towards the onshore zone.

In this study, it can be concluded that potholesare storm-generated structures formed in thesubtidal zone probably just below fairweatherwave base, but above the depth at which tempes-tites are deposited. This interpretation does notrule out the possibility that, in some cases, thepotholes were produced by currents interactingwith obstacles forming horseshoe hollows thatlater developed into channels (gutters) in adownstream direction (Aigner & Futterer, 1978).However, many of the pot casts associated withgutter casts in the Calasparra outcrop appearwithin the length of the gutters (Fig. 12). After astorm, the potholes, as well as the gutters, were

not necessarily ®lled with sediment in theirentirety, not even by the silty sediment remainingsuspended at the end of the storm. The in®lling ofthese structures with ®ne sediment and smallquantities of skeletal fragments was graduallycompleted during fairweather conditions. Insome cases, the potholes were ®lled with skeletalsediment over successive stages of later storms.These are preserved as alternations of mudstoneand ¯oatstone/rudstone.

PROXIMALITY TRENDS IN STORMDEPOSITS

It is important to note that the size and frequencyof the pot and gutter casts vary substantially fromone outcrop to the next. For example, at Cambil,although there are no pot casts, gutter casts arefrequent, although quite small. This section is thethinnest and displays a predominance of veryshallow subtidal facies and abundant thin-bed-ded calcisiltite beds. The outcrop thus corres-ponds to an area located in a very marginalposition in the Triassic basin. The tempestites inthis outcrop are the thinnest (3 cm) studied. Inthe thicker Cabra and Jauja sections, a substantial

Fig. 11. Side view of an isolated pot cast with a well-developed downward-spiralling shape (Calasparrasection). Pen is 13á5 cm long.

Fig. 12. Oblique bottom view of a gutter cast with acylindrical pot cast in the middle (Calasparra section).Arrow indicates direction of the palaeocurrent basedon prod casts. Hammer is 32 cm long.



number of gutter casts are observed, as well assome pot casts and, at Jauja, the size of the guttercasts is much larger. Finally, many large pot andgutter casts occur at Calasparra, which occupiedan intermediate palaeogeographic positionbetween Cambil and the outcrops at Cabra andJauja. However, it must be acknowledged that theoriginal positions of these outcrops were substan-tially different from their present locations,which have been changed by Tertiary tectonics:the foreland was located in the Hercynian Massif(Fig. 1), and the Triassic rocks have moved to thewest in different fold overthrusts.

According to the distribution, abundance, sizeand in®lls of the gutter casts in each section, andassuming that the gutter casts were perpendicularto the shoreline, it is interpreted that those guttersthat were formed closest to the shoreline were thesmallest, were more incisive and display acalcisiltite in®ll, whereas those that developedin deeper water areas were wider and displayskeletal grain in®lls with much greater frequency(skeletal ¯oatstone and rudstone; Fig. 13). One ofthe gutter casts studied at Calasparra is ofparticular interest for this interpretation(Fig. 14). It is a superposition of gutters that have

developed over various stages and during differ-ent storms. It was initially a bifurcating gutterwith an incisive cross-section ®lled with carbon-ate silt. Later, it became a progressively widergutter with a skeletal grain (¯oatstone) ®ll.Finally, above the fairweather laminated deposits,an even larger gutter developed in the same placeand was completely ®lled with skeletal grains(¯oatstone/rudstone). It is concluded that thissuccession displays an evolution from proximalto distal gutters, representing a high-order, deep-ening-upwards cycle; it is noteworthy that thethin-bedded limestone facies associated with thiscomplex gutter cast displays a ®ning-upwardssequence.

The presence of lenticular calcarenite beds, aswell as the wide gutter casts in the upper deepen-ing-upwards cycle of Member 1, suggest that thein®llings of these wider gutter casts pass intotempestite beds towards the deeper zone, abovestorm wave base. Most of the sediment bypassedthe very shallow subtidal zone and was depositedin deeper water (Myrow, 1992b); calcarenitebeds (tempestites) accumulated when the storm-generated currents, which were perpendicularto the coast, underwent strong deceleration.

Fig. 13. Outline of a muddy carbonate ramp showing proposed gutter and tempestite trends. These trends areinterpreted starting from the interval of the Member 1 gutter casts. Note the variation in the main features charac-terizing these trends: cross-section size of the gutter, incisiveness of these erosive structures and mean grain size ofthe in®lls of the gutters. FWWB, fairweather wave base; SWB, storm wave base.



TEMPESTITE MODEL

An idealized facies succession based on theSpanish sections (Fig. 15) starts with largely

laminated limestone and passes upwards intothin-bedded marly limestone and nodular lime-stone with bioturbation textures. It is suggestedthat the degree of bioturbation increases towardsthe central parts of the basin, a trend identi®edin many ancient (Seilacher, 1967; Pedersen,1985; Xian-Tao, 1982) and modern shelf sedi-ments (Hill & Nadeau, 1989). In the middle partof the thin-bedded marly limestone, pot casts,gutter casts and thin intercalations of calcareniteoccur, whereas in the higher part, there is apredominance of thicker calcarenite beds, whichin some cases are amalgamated or exhibithummocky cross-strati®cation. This facies suc-cession is interpreted as having accumulated ona carbonate ramp, deposition occurring from thenearshore to the outer shelf (Fig. 16), where thecoarse-grained storm deposits are signi®cant. Avery shallow and extensive subtidal zone devel-oped in the nearshore area. For this reason,high-energy ¯ows were generated and formedupper ¯ow-regime plane beds (Ashley, 1990)and erosional structures, especially duringstorms. Duringer & Vecsei (1998) described verysimilar facies, laminated limestones and intra-formational conglomerates, which were inter-preted as having accumulated on a lower tidal¯at. These authors suggested that the relativelystrong and more or less persistent in¯uencesof currents (other than those generated bystorms) on this shallow subtidal shelf are highlysigni®cant.

The tempestite model applied to Member 1 ofthe Majanillos Formation is similar to that pro-posed previously by Myrow (1992b). The pres-ence of gutter casts and pot casts is associatedwith a bypass zone. Certain parts of the shallowzone were sites of sediment bypass resulting fromstorm currents. These currents developed ero-sional structures (gutters and potholes) on the

Fig. 14. Evolution of gutters that developed in succes-sive stages over the course of different storms. Thisinterpretation is inferred from a complex gutter cast atCalasparra in which two main stages can be observedfrom the formation of the proximal gutter to a distalgutter. The evolution of this gutter could be relatedto a high-order deepening-upwards cycle (40 cm thick).(A±C) Development of an incisive bilobed gutter,probably sinuous in the ®rst phase (A) and wide andmuch less incisive in the last phase (C). (D and E) Stagein which there is an increase in the bathymetry, anddevelopment of undulated surfaces and wave ripplespartially eroded by the strongest storm currents, thusforming a larger and wider gutter. This gutter has anin®ll of skeletal grains when the storm stops (E).



sea¯oor where the bypass zone was established.The in®ll of the gutters was laterally connected inthe deeper zone producing the overlying calcar-enite beds. The thickness of the tempestitestherefore increased towards the deepest zones ofthe ramp where hummocky cross-strati®cationdeveloped. During the greatest storms, thesetempestites were sometimes reworked and stormdeposits predominated. Below storm wave base,where the bioturbated nodular limestone wasdeposited, the thickness of the tempestitesdecreased quickly towards the central parts of thebasin. Thus, the distality trend is similar to that ofpreviously published tempestite models (Aigner,1982; Allen, 1982; Walker, 1984; Handford,1986).

CONCLUSIONS

The oldest carbonate deposits of the MiddleTriassic, which correspond to lower Muschelkalkfacies (Member 1 of the Majanillos Formation),were deposited on a shallow carbonate ramp.This ramp was situated on the margin of anepicontinental basin in which water depths werenever very far below storm wave base, as indica-ted by the common occurrence of storm deposits.Tidal processes dominated in the shallowestareas of the epicontinental platform.

The tempestite model developed here indicatesthat a storm bypass zone developed in nearshoreareas during a transgressive cycle. Features ofstorm deposits are considered to be more useful

Fig. 15. Deepening-upwards cycle inwhich the grain size of the fair-weather deposits decreases slightlyupwards (black arrow). The thick-nesses of the tempestite beds increasetowards the upper part of the suc-cession until reaching the bioturbatednodular limestone facies (whitearrow). This facies change corres-ponds to the beginning of sedimen-tation below storm wave base. Fromthis point upwards, tempestite thick-nesses decrease. The gutter cast faciesoccurs in the middle part of the cycleand is interpreted as representingsediment bypass zone depositionduring storms.



for the interpretation of marine environmentsthan associated fairweather deposits.

Signi®cant features of the Triassic carbonateramp are:

1 The existence of a very shallow-water andextensive nearshore zone, in which there wererelatively high-energy ¯ows. In this shallowdepositional system, in many cases, calcisiltgrains formed upper ¯ow-regime plane beds,and muddy sediment constituted a cohesivesubstrate.

2 Particularly during storms, the nearshorearea functioned as a bypass sediment zone, inwhich the offshore-oriented return currentsgenerated erosion structures (gutters and pot-holes) that were ®lled with sediment when theenergy ¯ow decreased at the end of the storm.In the shallowest area, the cohesive substratewas only partially eroded, and carbonate mudchips were formed and redeposited in some-what deeper areas, sometimes in narrow gutters.Most of the coarse sediment (ooid and skeletalsand) was deposited where storm current energybegan to decrease offshore towards deeper outershelf zones, forming tempestite beds. The thick-ness of the tempestite beds increased towardsthe deeper parts of the ramp. In the greateststorms, these calcarenite beds were reworked bywaves and developed hummocky cross-strati®-cation.

3 In the deepest areas below storm wave base,bioturbated nodular limestones accumulated. Inthese limestones, thin layers of distal tempestiteswere deposited.

Finally, it is concluded that the gutter castsformed in the shallowest areas of the subtidalzone but were also generated in deeper areasduring storms. These deeper water examples havecross-sectional pro®les that become progressivelywider and less incisive basinwards. Potholeswere formed by vertical rotary ¯ows in areas thatwere slightly deeper than those in which thesmaller gutters were generated. However, thedevelopment of the potholes extended into thedeeper areas where the wide gutters were formed.

ACKNOWLEDGEMENTS

The author would like to thank P. Myrow (Color-ado College), P. Duringer (Louis Pasteur Univer-sity) and I. Jarvis (Kingston University) for theircorrections to the manuscript, which substan-tially improved this paper. Thanks also to F. PeÂrezValera and J. A. PeÂrez Valera for their help in the®eld. This work has been carried out within theresearch group RNM 0l63 of the Junta de And-alucõÂa and has been supported by the DireccioÂnGeneral de EnsenÄanza Superior e InvestigacioÂnCientõÂ®ca, projects PB97-1201 and PB 98-0488.

Fig. 16. Tempestite model with a bypass zone for the lower Muschelkalk platform established during the Triassic onthe southern Iberian Massif. In this model, the nearshore potholes and gutters are related to the tempestites depositedin deeper areas (inner shelf). Note that most of the coarse-grained sediment transported during the storms isdeposited on the middle part of the ramp where the tempestite beds reach their maximum thickness.



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Manuscript received 2 December 1999;revision accepted 27 June 2001.



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Significance of pot and gutter casts in a Middle Triassic carbonate platform, Betic Cordillera, southern Spain