Trace-fossils and minor discontinuities in a marl limestone rhythmite, Lower–Middle Kimmeridgian, southern Spain

Geobios 35 (2002) 581–593 www.elsevier.com/locate/geobio

Trace-fossils and minor discontinuities in a marl limestone rhythmite, Lower–Middle Kimmeridgian, southern Spain Traces-fossiles et discontinuités mineures dans une rythmite de calcaires et de marnes (Kimméridgien inférieur-moyen, Espagne méridionale) Federico Olóriz *, Francisco Javier Rodríguez-Tovar Departamento de estratigrafía y paleontología, facultad de ciencias, universidad de Grenada, 18002 Grenada, Spain Received 6 July 2001; accepted 12 March 2002

Abstract In a fine-grained Lower to Middle Kimmeridgian marly limestone rhythmite succession, with scarce benthos, fluctuations in ecological and depositional conditions that affected the substrate are detected by analysing trace-fossil assemblages. The dominant grey mudstones with Chondrites and Planolites superimposed on lumpy matrix sediments indicate both late colonisation of dysaerobic mixed layers and the completeness of the stratigraphic record. Major disruptions in rhythmic deposition and substrate colonisation are recorded at the base of thick intercalations of marls in sedimentary packages that were related to tectonic instability. Minor disruptions affected only the deposition of the overlying limestone beds. The trace-fossils reveal changes in the ecological and depositional conditions affecting the substrate during deposition of monotonous marly limestone rhythmites at levels of resolution higher than biostratigraphic, ecostratigraphic and taphonomic resolution. This research demonstrates the application of trace-fossil analysis to approach stratigraphic completeness (maturation) in homogeneous marly limestone rhythmites © 2002 E´ditions scientifiques et médicales Elsevier SAS. All rights reserved. Résumé Dans une succession rythmique de calcaires à grains fins et de marnes, avec rares organismes benthiques, les fluctuations des conditions écologiques et du dépôt qui ont affecté le substrat sont étudiées à travers l’analyse des associations des traces fossiles. Les mudstones grises dominantes qui contiennent des Chondrites et des Planolites superposées à une matrice grumeleuse indiquent la colonisation tardive des couches de mélange (mixed layer) et la présence des horizons sédimentaires non affectés par l’érosion. Des interruptions majeures dans le dépôt rythmique et dans la colonisation des substrats sont identifiées à la base d’épaisses intercalations marneuses qui témoignent des intervalles d’instabilité tectonique. Des interruptions mineures affectent seulement le dépôt des niveaux calcaires sus-jacents. L’analyse des traces fossiles a montré des changements dans l’environnement de dépôt qui ont affecté le substrat pendant la sédimentation monotone des rythmites de calcaires et de marnes, contrôlés à des niveaux de résolution bien plus précis que ceux de la biostratigraphie, de l’écostratigraphie et de la taphonomie. Cette recherche démontre la validité de l’utilisation de l’analyse des traces fossiles pour l’évaluation de l’intégrité stratigraphique dans les rhytmites de calcaires et de marnes. © 2002 E´ditions scientifiques et médicales Elsevier SAS. Tous droits réservés. Keywords: Chondrites; Planolites; Rhythmites; Upper Jurassic; Betic Cordillera Mots clés: Chondrites; Planolites; Rythmite; Jurassique supérieur; Cordillère Bétique

* Corresponding author. E-mail address: [email protected] (F. Olóriz). © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 0 0 1 6 - 6 9 9 5 ( 0 2 ) 0 0 0 7 6 - 1

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Fig. 1. Location of the Betic Cordillera (A) with enlargements depicting the regional palaeogeography (B) and a geological map of the Puerto Lorente section (C). Fig. 1. Schéma de situation de la Cordillère Bétique (A) avec des agrandissements présentant la paléogéographie régionale (B) et une carte géologique de la région de Puerto Lorente (C).

1. Introduction Trace-fossil studies are useful in stratigraphic interpretations (Savrda, 1991; Pemberton, 1992; Goldring, 1995), since fluctuations in relative sea level can influence environmental factors (e.g. water energy, sedimentation rate, degree of oxygenation, consistency and chemistry of the substrate), which in turn can affect bioturbation. This study uses ichnological data to interpret changes in the ecological and sedimentary conditions at a level below biostratigraphic and ecostratigraphic resolution in homogeneous marly limestone rhythmites that were deposited on epicontinental shelves in southern Spain. In such cases, ichnological data may be the highest resolution signal of biotic–abiotic interactions in the eco-sedimentary context that affect the substrate (Olóriz, 1997) and a valuable tool for the investigation of depositional continuity and stratigraphic completeness.

2. Geological setting and stratigraphic framework Relative tectonic movements between Iberia and Africa (Fig. 1) occurred during the Late Jurassic in the South Iberian palaeomargin, forcing oblique extension in the area (Comas et al., 1988). This palaeomargin includes epicontinental platforms (Prebetic Zone and lateral equivalents) as well as distal swell areas and troughs on the epioceanic fringe (Subbetic Zone, Penibetic; Olóriz et al., 1991). In the Prebetic Zone, limestone marl successions were widespread during the Early-Middle Kimmeridgian showing episodic siliciclastic intercalations, among which those close to the Oxfordian–Kimmeridgian boundary are a reference level

for correlation. Changes in fossil assemblages, lithofacies and bed thickness reveal differences in bottom topography over short distances (Rodríguez-Tovar, 1993). In general, epicontinental areas in the South Iberian palaeomargin record the interactions of tectonics and eustasy (Marques et al., 1991; Olóriz et al., 1991; Olóriz and Rodríguez-Tovar, 1998). The Puerto Lorente section is one of the best-known exposures of epicontinental Lower–Middle Kimmeridgian deposits in southern Iberia. The Puerto Lorente section (Fig. 1) is located about 18 km south of Cazorla, on the road through El Chorro to the source of the Guadalquivir River (Cazorla topographical sheet no. 21–37, 928: 2º59’25’’– 37º50’15’’) in the External Prebetic Zone. 2.1. Lithostratigraphy and palaeoenvironment The Kimmeridgian succession, composed mainly of marls, marly limestones and limestones, is 105 m thick (Fig. 2). The base of the Kimmeridgian is a ferruginised surface (lowermost arrow in Fig. 2C) that is extremely rich in fossils (i.e. ammonites, belemnites, bivalves, brachiopods, bryozoans, crinoids and sponges), including the ammonite Sutneria platynota (REINECKE), which is the index fossil for the basal Kimmeridgian in southern Europe in the classic interpretation (at present, the Oxfordian–Kimmeridgian boundary is placed at the Bimammatum–Planula Zone boundary). The overlying 3 m of siliciclastic-rich marls are followed by 5 m of thick-bedded marly limestones, and then by 18 m of alternating limestone and marly limestone beds (a in Fig. 2C). Directly above are 12 m-thick intercalations of dominant marls (b in Fig. 2C).

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Fig. 2. The section at Puerto Lorente. (A) Sequence stratigraphy (Rodríguez-Tovar, 1993), depositional discontinuities (DVIII–DXI), local sequences (K.I–K.II), highstand system tract (HST), shelf margin wedge (SMW), transgressive system tract (TST); (B) Chronostratigraphy; (C) Lithologic column (a–f are sedimentary packages, and vertical arrows mark interpreted boundaries). At right, note trace-fossil distribution in enlarged portion of the studied section at Puerto Lorente. Stratigraphic positions of major intercalations of marls and enlargement of the genetically linked sedimentary packages of marls, marly limestones and limestones. Fig. 2. La coupe de Puerto Lorente. (A) Stratigraphie séquentielle (Rodríguez-Tovar, 1993), discontinuités (DVIII–DXI), séquences locales (K.I–K.II), Cortège de haut niveau (HST), Prisme de bordure de plate-forme (SMW), Cortège transgressif (TST) ; (B) Chronostratigraphie ; (C) Colonne lithologique (a à f intervalles sédimentaires ; flèches verticales marquant les limites interprétées). A droite, noter la distribution des traces fossiles (agrandi) dans la coupe de Puerto Lorente. Les localisations stratigraphiques des intercalations majeures de marnes et l’agrandissement des intervalles sédimentaires de marnes, de marno-calcaires et de calcaires génétiquement reliés.

The overlying 15 m (c in Fig. 2C) are composed of 20–30-cm-thick limestone beds, in which the Platynota–Hypselocyclum Zone boundary (Fig. 2B) was identified within a 20 cm-thick marly intercalation. The succeeding 19 m of thick-bedded limestones (d in Fig. 2C) belong to the Hypselocyclum and lower Divisum Zones. The Hypselocyclum–Divisum Zone boundary was identified within a condensed and bioclastic interval (middle arrow in Fig. 2C) containing solitary corals, bivalves, brachiopods and above those the ammonite Crussoliceras, which is the index fossil for the upper Lower Kimmeridgian (Fig. 2B). The next 16 m (e in Fig. 2C) contain mainly marly deposits, including condensed horizons (uppermost arrow in Fig. 2C), which

represent the upper Lower to lower-Middle Kimmeridgian section. The ammonite Orthaspidoceras uhlandi (OPPEL) characterises the top of the Lower Kimmeridgian (Fig. 2B). The youngest marls and marly limestones (f in Fig. 2C), contain the ammonite Taramelliceras compsum (OPPEL) of Early-Middle Kimmeridgian age (Fig. 2B), and these are capped by an erosional surface. Sedimentary structures are not easily identifiable in the fine-grained deposits cropping out in the Puerto Lorente section. The monotonous appearance of the marly limestone rhythmite results from minor fluctuations in grain size, composition, and distribution of bioclasts (unrecorded shell

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beds) and lithoclasts. Subtle, incomplete horizontal laminae occur locally. Mudstones predominate over wackestones or packstones throughout the Puerto Lorente section. Mudstones with 5% bioclasts and lithoclasts are the background microfacies, while packstones dominate the upper part of the section. Quartz dominates the siliciclastic fraction. Bioclasts are mainly small fragments of thin bivalves and radiolaria, with lesser amounts of ostracods, brachiopods, echinoid spines, foraminifera (primarily Epistominidae) and sponge spicules. Gastropods, belemnites, ammonite aptychi, corals, bryozoans and red algae are recorded in condensed horizons (Hypselocyclum–Divisum and Divisum–Acanthicum Zone boundaries). Thin laminar concentrations of bioclasts and lithoclasts with horizontal or slightly oblique orientations occur rarely. Previous research on the section concluded that there was a topographic high (in maximum water depth of 60 m, according to Olóriz et al., 1992), located on the mid-shelf and generally subjected to low-energy conditions and mixed carbonate–fine-grained siliciclastic deposition (RodríguezTovar, 1993). On the basis of a homogeneous mineral content (López-Galindo et al., 1991), minor climatic fluctuations were suggested. Invertebrate assemblages, with scarce benthos and dominated by ammonites, fluctuated with sea-level changes (Olóriz et al., 1991, 1992, 1994; Rodríguez-Tovar, 1993). The scarcity of aptychi, the rarity of epibionts on ammonites (internal mould preservation) and the absence of fossil-rich horizons all indicate limited postmortem transportation of ammonites in a low-energy environment. No evidence of post-burial exhumation of fossils was found and no reworking of fossils affecting ammonite biostratigraphy was detected. 2.2. Sequence stratigraphy Third-order depositional sequences at the Puerto Lorente section (Fig. 2A) were interpreted on the basis of trends in mineralogy, grain size and the composition of invertebrate fossil assemblages, complemented by ammonite biostratigraphy (Marques et al., 1991; Olóriz and Rodríguez-Tovar, 1993a, b; Rodríguez-Tovar, 1993; Olóriz et al., 1994). According to these papers, high sea levels occurred during the Platynota and the Divisum Chrons and low sea levels occurred during the Hypselocyclum Chron. The succession of the highstand system tract, shelf-margin wedge and transgressive system tract are represented in the Lower Kimmeridgian deposits in the Puerto Lorente section (Fig. 2A). Early-Middle Kimmeridgian sediments were deposited under highstand system tract conditions (Fig. 2A). Marly intercalations with increased grain size and/or illite and kaolinite content were found at the bottom and top of the Platynota Zone. The lowest marls were easily correlatable with an erosional event related to tectonic pulses affecting highstand deposits during the Early Kimmeridgian Platynota Chron (Olóriz and Rodríguez-Tovar,

1998) and resulting in increased subsidence rate in the entire South Iberian palaeomargin (Rodríguez-Tovar, 1993). The marly intercalation at the Platynota/Hypselocyclum Zone boundary was correlated with a Type-2 sequence boundary (Marques et al., 1991). Synsedimentary movements during the deposition of highstand sediments of the earliest Kimmeridgian have been documented in the Iberian subplate and northwestern Africa (see references in Marques et al., 1991). Moreover, in comparatively marly successions from the central Prebetic, tectonic instability in the middle of the Platynota Zone (middle part of the highstand system tract) is expressed through slumping that can be biostratigraphically correlated with a marly intercalation in the rather calcareous and comparatively thinner Puerto Lorente section. In this section, significant marly intercalations of latest transgressive to early highstand deposits, close to the Lower–Middle Kimmeridgian boundary, were correlated biostratigraphically with the interval of faulting on outermost shelves in the Prebetic Zone and also with the beginning of carbonate turbidite deposition in the adjacent borderlands (see references in Marques et al., 1991).

3. Trace fossils 3.1. Methods The recognition of macroscopic biogenic structures was limited, as might be expected from the monotonous record of grey, fine-grained deposits. We analysed polished blocks and thin sections in order to determine microfacies and to compare matrix with burrow fillings. To enhance the visibility of indistinct biogenic structures, we polished sections of bulk rock samples that were cut perpendicular to the bedding and we examined the polished sections with water and then with light machine oil. Samples were studied with X-ray radiography (Medical X-ray unit: 45–65 kV; 50–100 mA; 0.30–0.8 s) and SEM (General Electric CT-MAX: window 4,000; contrast level 25; 120 kW; 90 mA; 2.76 s). Excellent results (enhanced visibility of trace-fossils) in grey limestones of superficially monotonous appearance were achieved by means of thin-section analysis under low-angle illumination. 3.2. Ichnotaxa In the Puerto Lorente section, previous studies by Olóriz and Rodríguez-Tovar (1999, 2000) reported the existence of Chondrites, Planolites and Thalassinoides, as well as rare paired holes that were tentatively identified as Diplocraterion or Arenicolites (Fig. 2). In more than 90% of the thin sections, small Chondrites and Planolites are superimposed upon a lumpy (Berger et al., 1979) matrix in the Puerto Lorente section (Table 1). No differences other than colour (pale-grey tones of the lumpy

F. Olóriz, F.J. Rodríguez-Tovar / Geobios 35 (2002) 581–593 Table 1 Trace-fossils at the Puerto Lorente section. General features (upper), local features (lower) Tableau 1 Trace-fossiles dans la coupe de Puerto Lorente. Caractéristiques générales (haut) et locales (bas) Chondrites Ethology Fodinichnia Ecology Deposit feeders Softground r-strategies Upper dysaerobic/lowermost aerobic Low energy High nutrients content References Ekdale and Berger (1978) Bromley and Ekdale (1984) Savrda and Bottjer (1986, 1987,1988,1989) Ekdale (1988) Vossler and Pemberton (1988) Sageman (1989) Wignall (1991) Pemberton et al. (1992a, b) Tyszka (1994) Goldring (1995) Abundance Abundant Features Transverse-longitudinal sections Sinuous to straight tubes Diameter 1-2 mm Lithofacies Unrestricted Interpretation Low oxygenation Relatively high nutrients

Planolites Pascichnia/Fodinichnia

Soupground/softground Dysaerobic/aerobic

Savdra and Bottjer (1986) Ekdale (1988) Sageman (1989) Bromley (1990) Pemberton et al. (1992a, b) Goldring (1995)

Common

Max. diameter = 1 cm Max. length = 25 cm Marly limestones and limestones Low erosion at top of depositional unit With Chondrites in interstitial environment dysaerobic

Stratigraphy Lower-Middle Kimmeridgian Sequence stratigraphy Unrestricted location

matrix) were discerned between indistinctly bounded burrow fillings and the surrounding matrix (Fig. 3). The ichnofabric index (Droser and Bottjer, 1986; Bottjer and Droser, 1991) was generally high (ii6; Fig. 4). Locally, superimposed Chondrites and Planolites show that substrate occupation ranged from low to high density of trace makers. 3.3. Trace-fossil distribution in genetically linked marls, marly limestones and limestones Marly intercalations ranging from 1 to 6 m thick in the Puerto Lorente section represent major episodic changes in depositional conditions. These intercalations appear below horizons transitional to limestones with no identifiable lithostratigraphic features that could reflect the existence of discontinuities within these genetically linked sedimentary packages.

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The succession consists of (i) thicker lower marls with sharp lower boundary on the underlying limestone package, (ii) thinner marly limestones and (iii) a limestone bed at the base of thicker limestone succession (Figs. 2 and 5), and it reflects a gradual but not necessarily continuous change in depositional conditions from episodes of abrupt disturbance to a supposedly non-episodic calcareous deposition. Within these genetically linked sedimentary packages, the outcrop appearance of the surfaces bounding the calcareous horizons is interpreted to be the result of limited diagenetic overprint and weathering. Locally, these sedimentary packages (‘Type II elemental sequence’ of Rodríguez-Tovar, 1990) show the most visible trace-fossil concentrations, which are mainly Chondrites and Planolites (Tables 1 and 2), in the middle part of the Platynota Zone as well as in the Divisum and the Acanthicum Zones (Fig. 3). Upwards in these sedimentary packages, the precise relationship between lithofacies and ichnofossils was as follows (Figs. 2 and 5): • In the lower part of the marls, macroscopic bioturbation is absent, but tiny Chondrites (<1 mm in diameter) appears in the upper part, where siliciclastics diminish (Figs. 5 and 8). • Marly limestone levels transitional to limestones show a clearly visible Chondrites–Planolites assemblage (Figs. 5, 6, 7B and 8). Upward Chondrites increases in size and uncompacted Chondrites and Planolites with pale grey lime fillings can be distinguished from compacted Planolites with darker marly filling. The possible presence of compacted Chondrites with marly filling was not recognised but cannot be ruled out. • In the overlying limestone package, the basal bed (Figs. 2, 5, 7(A), (C) and 8) is characterised by the lime-filled Chondrites–Planolites assemblage, dominated by Chondrites and superimposed over the poorly visible bioturbation typical of the limestone beds above. From bottom to top in this basal bed, the following are differentiated: (i) The lower interval shows trace-fossils with the same appearance as in the underlying marly limestone (i.e. dominance of clearly visible lime-filled Chondrites of uniform diameter and secondary Planolites) and there are no compacted Planolites with marly filling. Locally, the upper part of this lower interval has a lumpy matrix and contains rare, scattered, disarticulated bivalves and small corals (Fig. 7A), (ii) The middle interval has no body fossils and no macroscopic trace-fossils, but some scattered and poorly visible small Chondrites have been revealed through laboratory techniques, (iii) The upper part shows inconspicuous bioturbation in outcrop, but polished samples showed small Chondrites, as well as specimens resembling Chondrites or small Planolites, increasing upward. Locally, macroscopic Chondrites and scarce Planolites appear at the top surface of this basal limestone bed.

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Fig. 3. Burrows and relict sedimentary structures in thin section. (A, B, D). Small Chondrites and Planolites with no differences between burrow fillings and the surrounding matrix other than colour. (C) Relict of thin laminar concentrations of small clasts determining slight oblique orientation. Fig. 3. Bioturbations et vestiges de structures sédimentaires en lame mince. (A, B, D) Petits Chondrites et Planolites sans différence autre que de couleur entre les remplissages des galeries et la matrice encaissante. (C) Vestige de fine concentration laminaire de petits grains montrant une légère orientation oblique.

4. Discussion Trace-fossils in the Puerto Lorente section developed in a low-energy mixed carbonate–siliciclastic environment without significant reworking (Rodríguez-Tovar, 1993; Olóriz et al., 1994). Sediments accumulated on topographic highs in the middle of the epicontinental shelf. Body-fossil preservation, content and distribution, and the assumed water depth are consistent with this interpretation, as well as with a mid-shelf environment as characterised by Droser and Bottjer (1988). Thus, the depositional setting probably was below fair-weather wave base and close to lower storm wave base, in accordance with the model proposed by Kidwell and Bosence (1991) within a shelf area between the lower-middle and the upper-outer ramp in the model of Burchette and Wright (1992). In this fine-grained succession, there are neither any sedimentary structures nor any

taphonomic features available to determine the continuity or discontinuity in deposition. Intense bioturbation occurred in dysaerobic soupy-soft sediments (predominantly mudstones; see ‘ecology’ in Table 1) with scarce skeletal benthos. Episodic concentrations of Chondrites and Planolites probably reflect a combination of lower sedimentation rates and relatively higher levels of oxygenation and/or nutrient concentrations (see ‘interpretation’ in Table 2). The potential relative increase in nutrients is envisaged as a secondary effect of the two mentioned factors. 4.1. Marly levels (Fig. 8A–D) The absence of trace-fossils in the lower part of the marls is consistent with the interpretation of high sedimentation rates accompanied by a sudden increase in fine siliciclastics (Fig. 8A), resulting in oxygen depletion of interstitial waters

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single successive colonisation, sensu Goldring, 1995) after the disruption of stable but unfavourable ecological and depositional conditions. 4.2. Marly limestone levels (Fig. 8D and E)

Fig. 4. Mixed-layer of a marly limestone showing lumpy texture with some slightly compressed burrows (white arrow). Thin section under natural illumination. Fig. 4. ‘Mixed layer’ de marno-calcaire montrant une apparence grumeleuse avec la superposition des bioturbations légèrement comprimées (flèche blanche). Lame mince sous lumière naturelle.

(Sethi and Leithold, 1994). However, small burrowers may have been present but not preserved in soupy substrates. Upwards in the marly interval are recorded incipient marly limestone horizons that reflect a lowered accumulation rate and show intense but diffuse bioturbation, resulting in lumpy appearance, with rare compressed burrows (Figs. 4 and 8B). We interpret these horizons as infrequent episodes of opportunistic bioturbation (single rather than successive colonisation, sensu Goldring, 1995) that were contemporaneous with intervals of decreased siliciclastic supply. They are interpreted as examples of mixed-layer preservation. The rapid return to conditions for deposition of thick marls (Fig. 8C) led to the “freezing” of these mixed layers (Fig. 8B). The remote factors influencing substrate conditions were tectonic instability and erosion during the middle part of the earliest Kimmeridgian. The local factors were the abrupt increases in the sedimentary rate and the concomitant lowered oxygenation. In addition, very soupy bottoms could have limited substrate colonisation. Episodic slowdown or pauses in deposition gave opportunity for events of intensive bioturbation, which were rapidly aborted by the return to siliciclastic deposition. The reversal of sedimentary conditions (lower sedimentary rate, higher oxygenation and substrate cohesion) favoured the observed increase in tracefossils upwards. In the overlying that grade into marly limestones (Fig. 8D), small Chondrites are the only discrete trace-fossils. Their occurrence resulted from the combined effects of a decreased rate of deposition and a longer time available to colonise the substrate in overlying horizons, and their restriction to thin horizons suggests limited substrate suitability. This Chondrites assemblage (Ch) reveals an opportunistic occupation of dysaerobic sea bottoms (single or

The significant occurrence of the dense Chondrites–Planolites assemblage (Ch–Pl) is coincident with an increase in the carbonate/siliciclastic ratio in the marly limestone deposits. A decreasing rate of siliciclastic deposition resulted in more hospitable substrate conditions for opportunistic endobenthos and larger Chondrites and Planolites trace makers appeared. Planolites (5 mm wide), a tracefossil usually recorded in the shallow tiers of tiered profiles (Bromley and Ekdale, 1986; Wetzel and Aigner, 1986; Savrda and Bottjer, 1987; Bromley, 1990; Tyszka, 1994), indicates a phase of moderate ichnofossil diversification. The differentiation at the marl limestone level of compacted and marl-filled Planolites (Fig. 8D) from uncompacted limy-filled Chondrites and Planolites (Fig. 8E) implies the appearance of two temporally separate generations of burrowers. It is difficult to estimate the time that elapsed before burrow compaction occurred, since no biostratigraphic, ecological and/or taphonomic changes were evident. However, some amount of time was necessary for the substrate consistency to change from soft to firm ground. The absence of unbioturbated beds at the marly limestone/limestone boundary, together with the assumed two phases of burrowing, suggests a minor discontinuity (omission?) in deposition with the later removal of a marly limestone mixed layer. In such a context, the small Chondrites at the top of the underlying marls could represent the deepest tier piped down from the marly limestone (Fig. 8D). Alternatively but less probably, the homogeneous bioturbation just above and below the marly limestone/limestone boundary could reveal weathering overprint on a gradually changing lithology. Factors controlling eco-sedimentary conditions and substrate occupation by burrowers were the significant lowering of the sedimentation rate and the associated oxygenation and sediment compaction. In addition, pauses in sedimentation (omission?) followed by the potential exhumation of historical layers determined the kinds of trace-fossils that were produced and preserved. 4.3. Limestone bed (Fig. 8E and F) The basal bed in the overlying package of limestones contains the same trace-fossil assemblage as the underlying marly limestone level. The lower boundary of the basal limestone is locally distinct, possibly enhanced from weathering and diagenesis. No significant ecological changes occurred, and burrowing continued without change. The Chondrites–Planolites assemblage (Ch–Pl), which appears only in the lower part of the basal limestone bed (Fig. 8E), showed no differences from the lime-filled equivalents in

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Fig. 5. Genetically linked sedimentary package of marls, marly limestones and limestones related to major changes in depositional conditions. (A) Outcrop view. Note left-bottom marls and top-right limestone. Hammer for scale. (B) Close-up view of (A) showing limestone bed with bioclast (black arrows) in the middle. (C) Close-up view of (A) showing marly limestone horizon that is devoid of macroscopic burrowing. Pen for scale. (D) Close-up view of (A) showing macroscopic bioturbation of uncompacted, lime-filled Chondrites. White arrow indicates transverse section of Planolites in marly limestone just below the top limestone bed in (A). Fig. 5. Marnes, marno-calcaires et calcaires génétiquement reliés, provenant de changements majeurs dans les conditions de dépôt. (A) Vue à l’affleurement. On remarque des marnes en bas et des calcaires au-dessus. Marteau pour échelle. (B) Vue de détail du (A) montrant le banque calcaire avec un horizon bioclastique (flèche noire). (C) Vue de détail de (A) montrant un horizon marno-calcaire sans bioturbation macroscopique. Stylo pour échelle. (D) Vue de détail de (A) montrant des sections transversales de bioturbation macroscopique de Chondrites non compactés et remplis de calcaire. La flèche blanche indique les sections transversales des Planolites dans le marno-calcaire juste en-dessous de l’horizon calcaire supérieur en (A).

the underlying bed. Dominant Chondrites and common Planolites reflect dysaerobic substrates (see Tables 1 and 2). In the Acanthicum Zone (Middle Kimmeridgian), the upper half of the lower part of the basal limestone bed contains disarticulated, unabraded, fragmentary bivalves and corals scattered in a lumpy 10-cm-thick interval (Fig. 8E). This loose-packed wackestone (floatstone) is a distinct horizon within the enclosing deposits that otherwise contain very few body fossils, and therefore it could be related to a discontinuity surface (Kidwell, 1991). Disarticulated and/or fragmented bioclasts of variable size, shape, and density, show loose and irregular packing, selective bioerosion (sponge? white arrows in Fig. 7C) and mud coating, but neither sediment screening nor sheltered preservation or sediment-filled shells. These features are consistent with a

biogenically overprinted hydraulic concentration winnowed out of habitat from adjacent areas (Aigner, 1982; Kreisa and Bambach, 1982; Bromley et al., 1990; Kidwell, 1991; Kidwell and Bosence, 1991). This winnowing resulted in a rapid pulse of a time-averaged shell input during aggradational deposition (Fig. 7C). Thus, remains of shelly benthic communities, which inhabited more oxygenated sea bottoms areas, were left vulnerable to colonisation by borers in supersaturated low energy, protected environments (Boekschoten, 1966; Alexsandersson, 1972; Bromley et al., 1990) before transportation. Winnowed bioclasts from these environments were transported to the studied area, where they were biogenically mixed (diffusive mixing sensu Cutler, 1993) in the taphonomically active zone (TAZ; Davies et al., 1989). Thus, the lumpy matrix, together with the absence of

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Table 2 Trace-fossils in genetically linked sedimentary packages related to tectonic instability Tableau 2 Traces-fossiles dans des couches sédimentaires reliées génétiquement et dues à l’instabilité tectonique Chondrites (Ch) assemblage Location

Preservation

Interpretation

Upper part of marly levels

Rarity of three-dimensional structures Size < 1 mm Only detectable in laboratory

Low oxygenation Relatively high content of nutrients in the substrate Increasingly firm consistency in marls (the deepest tier piped from overlying carbonates) Low oxygenation

Mid-upper part of the limestone Mainly detectable in laboratory bed Size increases upwards

Increasingly firm substrate consistency (tiered from the top of the limestone bed)

Chondrites (Ch)–Planolites (Pl) assemblage Location

Preservation

Marly limestone levels

Vertical tubes and oblique branches (Ch)

Lowermost part and top of the limestone bed

Interpretation

Two phases of substrate occupation are separated by increasingly firm substrate consistency Circular transverse section with limy filling(Ch & Pl) Progressive improvement of substrate availability for endobenthos Compacted transverse section and marly filling (Pl & Ch?) Probable removal of mixed layer at the top of marly limestones Circular transverse sections and subhorizontal tubes with Lowermost assemblage piped during short-term limy filling (Ch & Pl) discontinuous deposition below preserved mixed layer Top assemblage piped from removed mixed layer Increasing substrate availability

Fig. 6. Polished and oil-treated surface of marly limestone (just below the top limestone in Fig. 5D) showing the Chondrites–Planolites assemblage. The abundant Chondrites is evident as light-coloured, branching shafts and tunnels. The much less abundant Planolites (white arrow in upper left corner) typically are penetrated by Chondrites. Black spots in the background are organic matter. Fig. 6. Section polie d’une surface de calcaire argileux traitée à l’huile juste en-dessous du calcaire supérieur dans la Fig. 5D, montrant l’association des Chondrites–Planolites. L’abondance des Chondrites est mise en évidence par des tunnels simples et bifurqués de couleur blanchâtre. Les Planolites beaucoup plus rares (flèche blanche dans le coin supérieur à gauche) sont typiquement pénétrés par des Chondrites. Des mottes noires à l’arrière-plan attestent de matière organique.

discrete macroscopic burrows (except for rare small Chondrites that were identified in the laboratory), provides evidence for preservation of a mixed layer showing a ‘frozen’ profile resulting from a later episode of comparatively rapid sediment accretion. No changes in the amount or kind of burrowing occurred between the lower part of the limestone bed and the latest bioturbation phase in the underlying marly limestone (i.e. the lime-filled Chondrites–Planolites assemblage). Thus, the record of an uncompacted and lime-filled Chondrites–Planolites assemblage (Ch–Pl) could be interpreted as a succession of transitional layers during sediment accretion before the preserved mixed layer (option a in Fig. 8E). Alternatively, the interval with an uncompacted and lime-filled Chondrites–Planolites assemblage (Ch–Pl) could indicate the piped zone from the preserved mixed layer at the upper lower part of the limestone bed (option b in Fig. 8E). Based on the occurrence of Planolites as a common component of shallower tiers, we believe that the former hypothesis (option a in Fig. 8E) offers the more plausible explanation for the complex nature of this limestone bed at the bottom of the limestone set. The middle part of the basal limestone contains neither body fossils nor macroscopic bioturbation and it is overlaid by a lumpy matrix (the indistinctly bioturbated upper part of the basal limestone) with scarce discrete burrows in the uppermost horizon (Fig. 8F top). The upward increasing occurrence of discrete burrows in the upper part of the basal limestone culminated in a distinct Chondrites assemblage and locally in the Chondrites–Planolites assemblage (Ch–Pl) at the top surface. The middle and upper parts of

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Fig. 7. Thin section of marly limestone and limestone (natural illumination). (A) Lowermost part of the limestone bed with uncompacted, lime-filled Chondrites and Planolites assemblage. (B) Uncompacted, lime-filled Chondrites and Planolites assemblage in the marly limestone horizon below the limestone bed. (C) Bioclastic interval in the upper lower part of the limestone bed showing fragmented disarticulated and unabraded bioclasts. There is no sediment screening, no sheltered preservation and no sediment-filled shells (white arrows for selectively bioeroded, mud-coated skeletal). Selective preservation is interpreted as the result of time averaging. Fig. 7. Lame mince de marno-calcaire et de calcaire argileux (lumière naturelle). (A) La partie la plus basse de la couche calcaire montre l’association de Chondrites et de Planolites non compactés et remplis de calcaire. (B) Association de Chondrites et de Planolites non compactés et remplis de calcaire dans l’horizon de calcaire argileux au-dessus de la couche calcaire. (C) Intervalle bioclastique dans le haut de la partie inférieure de la couche calcaire montrant des bioclastes fragmentés, désarticulés et non érodés sans filtrage synsédimentaire, sans particules préservées par des coquilles, sans coquilles remplis de sédiments (flèche blanche pour des bioérosions sélectives et pour des restes squelettiques avec boue enveloppante). La préservation sélective est interprétée comme résultant de ‘time-averaging’.

the basal limestone indicate rapid deposition from a very fine-grained suspension cloud during the late energywaning phase that follows distant storms when the turbulence event was nearly abated (Kreisa and Bambach, 1982). A depositional storm event (sensu Aigner, 1982) would explain the abrupt change between the winnowed bioclastic horizon below and the macrofossil–barren sediment above, which shows an upward increase in bioturbation. This abrupt change within a homogeneous lithofacies exemplifies the subtle record of a discontinuity surface, which is not always mantled by shelly horizons (Kidwell, 1986), and it also reinforces the fact that the lower the degree of omission

and storm disturbance, the less easy it is to recognise it in the sedimentary record (Fürsich, 1982). In addition, the comparison between trace-fossil patterns below and above this discontinuity surface (in terms of burrow size, degree of bioturbation, duration of substrate occupation and trace-fossil diversity) serves to identify pre-event and post-event communities of mud-burrowers (Seilacher, 1982). The upward increase in both carbonates deposition and bioturbation in the middle and upper parts of the basal limestone indicate transitional-layer preservation. The lumpy matrix reflects the background conditions that correspond to a previous mixed layer that was piped down from the topmost

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Fig. 8. Distribution and genetic sequence (A–F) of trace-fossils in marl, marly limestone and limestone sedimentary packages related to episodic changes in deposition affecting a monotonous marly limestone rhythmite. (E) shows alternative interpretations (options a and b) for the sedimentary body made by the top of marls, the marly limestone and the lower part of the limestone bed. Note the following: (I) The interpreted changes in siliciclastic: carbonate ratio, relative oxygenation, and discontinuities in deposition (lower right). (II) The rare, calcareous intercalation rich in Chondrites and Planolites resulting from decreased sedimentary rates (lowermost horizontal arrow) increased oxygen levels and opportunistic colonisation of the substrate. (III) Discontinuity (D1) related to tectonic pulses, erosion and an increase in the siliciclastic content; discontinuity (“d2”) unrelated to ecological changes that affected the substrate and trace-fossil assemblages; discontinuity (d3) between two piped zones indicates discontinuous deposition and then rapid accretion of carbonates due to storm-back-flow deposition that corresponds to the overlying part of a limestone bed (note the abrupt freezing of a mixed layer showing active biogenic diffusive mixing within a taphonomically active zone oxygenated by winnowing); and discontinuity (d4) related to the removal of the mixed layer and the return to background rhythmic sedimentation. Fig. 8. Distribution et séquence génétique (A à F) des traces fossiles dans les marnes, calcaires argileux et les calcaires reliés à des changements épisodiques du dépôt affectant une rhytmite monotone de marnes et de calcaires (Kimméridgien inférieur-moyen). (E) montre des interprétations alternatives (options a et b) pour l’intervalle composé des marnes, des calcaires argileux et de la partie inférieure de la couche calcaire. Note : (I) Les changements interprétés dans le rapport siliciclastiques : carbonates, oxygénation relative et discontinuités sédimentaires (en bas à droite). (II) L’intercalation calcaire riche en Chondrites et Planolites, qui résulte de taux sédimentaires réduits (flèche horizontale inférieur), de niveaux d’oxygène plus élevés et de la colonisation opportuniste du substrat. (III) Discontinuité (D1) liée à l’instabilité tectonique, à l’érosion et à l’augmentation en siliciclastiques ; discontinuité (‘d2’) sans lien avec les changements écologiques qui ont affecté le substrat et les associations de traces fossiles ; la discontinuité (d3) entre deux intervalles bioturbés indique la sédimentation discontinue suivie d’une rapide accrétion des carbonates due au courant arrière de la tempête, qui correspond à la partie superjacente de la couche calcaire [remarque le ‘freezing’ soudain d’une couche de mélange (mixed layer) montrant le mélange (mixing) diffus actif dans une couche taphonomiquement active oxygénée par resédimentation (winnowing)] ; et discontinuité (d4) liée à la disparition de la couche de mixture (mixed layer) et au retour des conditions de ‘background’ sédimentaire.

surface of the basal limestone. The deepest tier is represented by the rare small Chondrites (Savrda and Bottjer, 1986, 1987), which are recorded in the bioclastic horizon below (Fig. 8F). We regard the top surface of this basal limestone as resulting from discontinuous deposition accompanied by removal of its contemporaneous mixed layer. Factors determining the trace-fossil pattern in the lower part of the limestone bed seem to be linked to the decreasing

influence of siliciclastics compared to the underlying horizons (Figs. 7 and 8F). Episodic winnowing brought exotic bioclasts into the area and probably oxygenated the substrate. This corresponds to the upper lower part of the basal limestone bed. Just above the winnowed horizon there is a lime mud horizon that is devoid of body fossils and macroscopic trace-fossils. The occurrence of this horizon is consistent with a storm surge that reached this distal setting

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below the fair-weather wave base. The combination of distance from shore, turbulence energy and shelf topography caused no abrupt sedimentary surface to occur at the bottom of the interpreted event. The overlying trace-fossil pattern in the upper part of the limestone bed reflects the decreasing sedimentation rate and the associated increase in sediment compaction and oxygenation far from a siliciclastic influx. A final episode of minor erosion interrupted sediment accretion.

Savrda (Auburn University), P.J. Orr (University of Bristol), A.A. Ekdale (University of Utah), G.C. Cadée (Netherlands Institut for Sea Research), R.G. Bromley (University of Copenhagen, Denmark), D. Olivero (Université Claude Bernard, Lyon 1, France), and an anonymous reviewer for valuable suggestions and comments on an early version of this paper.

References 5. Conclusions At a level below the resolution of ammonite biostratigraphy and the ecostratigraphy of ammonite and megabenthos, and in unfavourable conditions for the application of taphonomic analyses, trace-fossil analysis allows us to interpret the changing palaeoenvironmental conditions of the substrate in the middle of a low-energy epicontinental shelf in the unstable South Iberia palaeomargin during the Early-Middle Kimmeridgian. Chondrites and Planolites superimposed on a lumpy matrix background in monotonous marly limestone rhythmites, which contain scarce benthic body fossils, reflect the preservation of sediment mixed layers piped down in burrows under dysaerobic conditions. Rare calcareous intercalations rich in Chondrites and Planolites reflect decreased sedimentation rates, increased oxygen levels and opportunistic colonisation of the substrate by burrowers. In fine-grained carbonates, back-flow deposits from storms can be identified by the abrupt freezing of a sediment mixed-layer. Biogenic diffusive mixing occurred within a taphonomically active zone that was oxygenated by winnowing. Trace-fossil analysis reveals four discontinuities within the genetically linked marls, marly limestones and the basal bed of the overlying limestones (Fig. 8). These discontinuities were related to (i) tectonic pulses, erosion and an increase in the siliciclastic content (D1); (ii) only minor erosion (d2 and d4); and (iii) rapid accretion of carbonates due to storm-black-flow deposits (d3). The associated major change in depositional condition was related to tectonic pulses, which in turn induced higher ecologic impact. The analysis of trace-fossils has revealed sediment mixed-layer preservation and erosion in Upper Jurassic fine-grained epicontinental deposits. In these cases, tracefossil analysis can be useful for the evaluation of stratigraphic completeness in the absence of primary sedimentary structures and body fossils.

Acknowledgements This research was financed by Projects PB97-0803 and BTE2001–3029 (DGICYT, Spain) and the EMMI Group (RNM 0178, Junta de Andalucía, Spain). We thank C.E.

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