Calciturbidite dynamics and endobenthic colonisation: example from a late Barremian (Early Cretaceous) succession in southeastern France

Palaeogeography, Palaeoclimatology, Palaeoecology 211 (2004) 221 – 239 www.elsevier.com/locate/palaeo

Calciturbidite dynamics and endobenthic colonisation: example from a late Barremian (Early Cretaceous) succession in southeastern France B. Savary*, D. Olivero, C. Gaillard UMR 5125 CNRS, Pale´oenvironnements et Pale´obiosphe`re, UFR Sciences de la Terre, Universite´ Claude Bernard Lyon 1, 69622 Villeurbanne cedex, France Received 23 July 2003; received in revised form 2 April 2004; accepted 14 May 2004

Abstract Three Lower Cretaceous calciturbidite bundles, well exposed in the southeastern France, are studied with a combined sedimentological and ichnological approach. They were deposited in a toe-of-slope setting in the western border of the Vocontian Trough, during the late Barremian. The bundles appear to be the result of a cyclic sedimentation, with recurrent ichnotaxa including common Thalassinoides, Planolites, Zoophycos, and less abundant Taenidium, Chondrites and Ophiomorpha. These trace fossils constitute some ichnoassemblages that follow and superimpose each other. Analysis of the tiering produced may help to estimate the time interval between two turbiditic events. Indeed, the presence of Zoophycos indicates a low frequency of the turbiditic supplies, which allowed the substrate to firm up, a necessary condition for the construction and the persistence of this spreite trace fossil. Therefore, a decreasing frequency of the turbiditic events is pointed out from the base to the top of each turbidite bundle. The trace fossil distribution may also be influenced by the grain-size. In fact, Zoophycos producers seem to have thrived in a fine-grained sediment (0.1–0.2 mm). Moreover, the organic matter exported from the platform is better preserved in these fine-grained substrates, and may be more available to opportunistic burrowers. In the studied succession, two different morphologies of Zoophycos have been observed. One of them (lobed form) probably indicates an opportunistic behaviour of an organism quickly exploiting the organic matter between two turbiditic events. By comparison, the other (unlobed form) is produced later by a more specialised organism, exploiting large amounts of organic matter during episodes of lower frequency of turbiditic supplies. The ichnofauna studied is characterized by a low diversity and this may be related to an inner submarine fan setting. D 2004 Elsevier B.V. All rights reserved. Keywords: Calciturbidites; Dynamics; Palaeoenvironment; Trace fossils; Lower Cretaceous; Southeastern France

* Corresponding author. Tel.: +33 4 72 44 58 69. E-mail address: [email protected] (B. Savary). 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.05.008

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1. Introduction Palaeoenvironmental and ecological conditions are not well understood in settings perturbed by episodic supplies of turbidites. Even if, during the last three decades, siliciclastic gravity systems have been intensively studied (Normark, 1970; Mutti and Ricci Lucchi, 1972; Walker and Mutti, 1973; Savoye, 1997; Johnson et al., 2001; Gervais et al., 2001; Babonneau et al., 2002), some problems like the frequency of the turbiditic supplies are still unresolved. Moreover, carbonate systems are still poorly studied in terms of export processes (Droxler et al., 1983; Sarg, 1988; Eberli, 1991; Schlager, 1992; Handford and Loucks, 1993; Betzler et al., 1999), and depositional patterns (Wright and Wilson, 1984; Colacicchi and Baldanza, 1986; Mullins and Cook, 1986; Eberli, 1987; Swart, 1992; Braga et al., 2001). The palaeoenvironmental conditions can be partly deduced by the trace fossils. The colonisation of the turbiditic deposits by the benthic fauna has been approached by several authors (Seilacher, 1962; Gaillard, 1988; Miller, 1986, 1993; Frey and Goldring, 1992; Monaco and Uchman, 1999; Wetzel and Uchman, 2001). Ichnology can be a useful tool in the turbiditic setting, but questions have still to be resolved. Some of them concern the changing behaviour of the burrowers inside and outside the turbiditic deposits, and the impact of the nutrient content and grain-size on the endofauna. The aim of this work is to improve our knowledge of calciturbidite dynamics and endobenthic colonisation based on the analysis of successions preserved in the late Barremian of southeastern France. The excellent outcrops allow a detailed and combined sedimentological and ichnological analysis. Sedimentological study allows the characterization of the grain-size and the type of material exported from the platform, which permits the interpretation of the dynamics of the turbiditic flows. On the other hand, the analysis of a well-preserved ichnofauna reveals some environmental factors that prevailed during the turbidite episodes.

2. Geological context In southeastern France, the Vocontian Trough (Fig. 1A) was a few hundred metres deep basin (Wilpshaar and Leereveld, 1994) located on the northwestern

margin of Tethys. During the Barremian–Aptian interval, this basin was adjacent to three main urgonian platforms (facies represented by coral-rudist assemblages): the Jura-Bas Dauphine´ Platform to the north, the Bas-Vivarais Platform to the west and the Provence Platform to the south (Fig. 1A). On the platforms, a high carbonate production allowed sedimentation to keep up with subsidence (Arnaud-Vanneau and Arnaud, 1990; Masse, 1993). In the basin, this resulted in a massive accumulation of allochtonous carbonates (Fig. 1A). Gravitational deposition, especially at the western margin of the basin, created a substantial pile of turbidites, slumps and debris flow deposits (Ferry, 1990). The studied area is the western flank of the bLa LanceQ anticline (Fig. 1B) where the Barremian interval contains numerous turbidites, mostly calcarenites, deposited in a toe-of-slope setting (Ferry, 1979). The carbonate material was exported via feeder channels from the Bas-Vivarais Platform towards the Vocontian Trough where it constitutes a lobe-shaped morphology (Ferry, 1976). Therefore, these gravity systems can be referred to submarine fans, which are less common than slope aprons in the carbonate sedimentary record (Ruiz-Ortiz, 1983; Wright and Wilson, 1984; Mullins and Cook, 1986; Savary and Ferry, in press). In the bPas-de-la-CluseQ locality, below and above a distinctive interval called bHeteroceras marlsQ, thick calcarenitic bundles are hosted by hemipelagic slope calcsiltites and pelagic basinal mudstones. Below the Heteroceras marls, three calcarenitic bundles (1, 2 and 3 in Fig. 1C), showing partial Bouma sequences with intervening intervals bearing bHummocky Cross StratificationQ (HCS)-like features, are well exposed and have been analysed in the present work. However, the lateral extension of these outcrops is limited and does not allow us to interpret which part of the submarine fan the deposits represent. The calcarenitic bundles 1 and 2 are separated by hemipelagic calcsiltites containing some ammonites. Seven of these specimens indicate a late Barremian age (determination by F. Cecca), instead of early Barremian as indicated in the Nyons geological map (Flandrin, 1975) (Fig. 1B). More precisely, Pachyhemihoplites gerthii (Sarkar, 1955) has been found just above the calcarenitic bundle 1 (Fig. 1C) and indicates the presence of the Sartousiana Zone (Delanoy, 1997).

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3. Sedimentology Two sections, approximately 100 m thick and 100 m apart, were measured and described in the Pas-de-la-Cluse locality. Each section is made of a

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hierarchical organisation of lithological units: section/bundle/cycle/bed/turbidite sequence (Fig. 2). Each section has been documented through a weathering profile and a grain-size profile combined with depositional structures and bioturbation features

Fig. 1. Geological setting, geographical location and stratigraphy. (A) Barremian palaeogeographical map (from Ferry, 1984 modified), the Vocontian Trough is adjacent to three main urgonian platforms, the study area is located in the western part of the Vocontian Trough in a base-of-slope setting supplied by the Bas-Vivarais Platform, the transition between platform and turbidite system does not outcrop. (B) Geological map of the western flank of the La Lance anticline, the study is located in the bPas-de-la-CluseQ locality displaying three calcarenitic turbidite bundles that are dated from Upper Barremian in this study. (C) Synthetic section of the Pas-de-laCluse locality, turbidite bundles 1 and 2 are separated by hemipelagic calcsiltites that contain at their base a Pachyhemihoplites gerthii (Sarkar, 1955) belonging to the Sartousiana Zone, calcarenitic bundle 3 is overlain by the Heteroceras marls regionally attributed to the Giraudi Zone.

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Fig. 2. Hierarchical organisation of the sections studied. On the basis of the weathering profile and the grain-size variation, each section shows a subdivision in bundles. Bundles are composed by the stacking of four cycles made of several beds. Usually, one bed is the result of the accumulation of several fining upward turbidite sequences.

(Fig. 3). Among the calcarenites, which compose the turbidite sequences, we propose four grain-size classes:

–

– –

These deposits are separated by hemipelagic deposits: marls (or bMQ) and limestones (or bLQ).

very coarse or bvcQ (grains diameter z1 mm), coarse or bcQ (0.6 mm=grains diameter b1 mm),

–

medium or bmQ (0.2 mm=grains diameter b0.6 mm), fine or bf Q (0.1 mm=grains diameter b0.2 mm).

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Fig. 3. Weathering profile and grain-size evolution of the two sections logged. Hemipelagic deposits: Marls (M) and limestones (L), turbidite deposits: fine calcarenites (f), medium calcarenites (m), coarse calcarenites (c), very coarse calcarenites (vc). The proposed correlation between Section 1 and Section 2 is indicated.

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While the hemipelagic beds can be traced over long lateral distances, the thickness and the grain-size of the calcarenitic turbidites vary greatly. The close vicinity of the two sections allows accurate correlations (Fig. 3), while sedimentological characteristics like grain-size quickly change laterally. This permits a test of the influence of the grain-size on coeval trace fossil assemblages. Visible beds do not usually correspond to a single turbidite sequence. These sequences are 1 mm to 2 m thick, the mean thickness being on average 30 cm. The base of some sequences is a sharp erosional surface. The studied turbidites are represented by partial Bouma sequences, mainly a–e, ab–e, abc–e and b–e. The fining-upward organization of the turbidite sequences is well expressed in the decimetre-thick sequences. The vertical decrease in grain-size is never linear but a rapid shift to fine grains is observed. Inverse grading has never been observed, but inverse then normal grading sometimes occurs. Sorting is usually rather poor and bioturbation destroys the grading in the upper part of sequences. Secondary silicification is the most common diagenetic feature of these calcareous turbidites. Both bundles 2 and 3 display two different types of isotropic HCS-like features: bscour and drapeQ and baccretionaryQ types (oscillatory to oscillatory-dominant combined flows; Cheel and Leckie, 1993). The upper surface of these beds sometimes features small-scale ripples (like the wave ripples associated with the shallow-marine HCS). The organisation of the three turbidite bundles needs special attention. In Section 1, bundle 3 is coarser than bundle 1, which in turn is coarser than bundle 2. Considering the grain-size, a clear vertical cyclic evolution occurs in turbidite bundles (Fig. 3). Each bundle contains four cycles. Each cycle is represented by a fine-grained base and top and a coarser middle part. The first cycle is composed of about 60% of fine-grained sequences and the fourth cycle is represented by 90% (in bundle 1), 100% (in bundle 2) and 40% (in bundle 3) of fine-grained sequences. The second and third cycles can reach 20% of very coarse calcarenitic sequences. The average thickness of the calcarenitic sequences is 24 cm. Bundles 1 and 2 are separated by 35 m of alternating hemipelagic deposits. A nodular bed containing ammonites is intercalated between two marl layers,

and a 2-m-thick calcarenitic interval is also present in this hemipelagic succession (Fig. 3). This latter is not only dominated by fine-grained sequences but also contains very coarse calcarenites. Section 2 displays the same general trend of the grain-size with the same cyclic organisation (Fig. 3). In each bundle, the coarse sequences are mainly observed in the second and third cycles, and are more abundant in Section 2 than in Section 1. The base of the succession corresponds to a channel, 1 m deep, filled by very coarse bioclastic material. The filling also contains a fragmented hemipelagic bed, which testifies to the intensity of the turbiditic current and of the resulting erosion of the sea-floor. The average thickness of the calcarenitic sequences is 35 cm. Between bundle 1 and bundle 2, the 35m-thick alternation of hemipelagic deposits is not intercalated with the calcarenitic interval present in Section 1. In conclusion, a clear organisation of the lithological units characterises both sections. This pattern could record a cyclic variation of the intensity of gravity flows or a cyclic variation of the carbonate (nature and/or quantity) produced on the platform.

4. Trace fossils Trace fossils have been observed and studied on a bed-by-bed scale in both the hemipelagic and turbidite deposits. In the turbidite beds, some ichnotaxa may be clearly identified and are, following their order of abundance: Planolites, Thalassinoides , Zoophycos , Chondrites , Taenidium , Ophiomorpha. The density of the burrowing by these trace fossils has been evaluated on bedding planes and in the vertical sections following the abundance scale proposed by Olivero (1994) for the study of Zoophycos. Six classes of density (d) are thus recognised: d=0 (absent); d=1 (very rare, b10%); d=2 (rare, 10–20%); d=3 (common, 20–50%); d=4 (abundant, 50–80%); d=5 (very abundant, 80– 100%). The hemipelagic limestones are thoroughly bioturbated. Well-defined ichnotaxa are very rare and the resulting bioturbation is mainly characterised by deep-sea biodeformational structures described by Wetzel (1984). The resulting bioturbation is described below as bburrow mottlingQ.

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vagile mid-tier deposit feeder, probably a crustacean (Frey et al., 1984).

4.1. Planolites Unlined and unbranched burrows. They are straight to tortuous, usually horizontal or slightly oblique to bedding. Cross sections are circular or elliptical, and tunnels have a visible length of 2–5 cm and a diameter of 1–2 cm. The fill is structureless. These burrows are vagile shallow-tier deposit-feeder structures (Pemberton and Frey, 1982) having no visible connection to the sea-floor. Planolites is usually observed in the turbidite beds, especially in the upper layers, in association with Thalassinoides (Fig. 4), but it can also be seen in the hemipelagic deposits. 4.2. Thalassinoides Three-dimensional burrow system, consisting of prevailing horizontal cylindrical tunnels, unlined and usually branched. When it is complete, this system has an open connection to the sea-floor through a vertical tunnel. The tunnels are 1–3 cm wide and 10– 55 cm long. The tunnels, usually observed in the turbidite beds, are visible because of differences in colour and grain-size compared to the host sediment. In some beds, Thalassinoides galleries are strongly silicified. The structure is made by a semi-vagile or

4.3. Zoophycos The name Zoophycos will be used here to describe complex three-dimensional spreite systems consisting of a thin layer of bioturbated sediment (the lamina), with a simple or lobate outline. The lamina is bordered by a tunnel of circular cross-section (the marginal tube), and it is characterised by a complex network of arched structures (lamellae). In vertical section, the lamina appears as an alternation of lighter and darker arcuate menisci. Traces related to Zoophycos are considered as characteristic of dysaerobic substrates, in deep-sea environments (Mesozoic and Cenozoic) or shallow-waters (Palaeozoic) (Bromley and Ekdale, 1984; Kotake, 1989, 1991; Olivero, 1994, 1996, 2003; Bromley, 1996). They seem to have been produced by a chemosymbiotic deposit feeder (Bromley, 1996; Bromley et al., 1999; Bromley and Hanken, 2003), exploiting the organic matter trapped inside the substrate and preserved because of the low oxygen content of sediment. Complex trace fossils related to Zoophycos have been observed only in the turbidite bundles. Two morphologies have been recognised in this study.

Fig. 4. Thalassinoides (Th) burrows on the top surface of a calcarenitic turbidite bed. Section 1.

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4.3.1. Unlobed Zoophycos The lamina is large (up to 60 cm wide), and may be arranged as a spiral around a central axis (Fig. 5); the central apex points upward, as in the Cretaceous Zoophycos described elsewhere in the basin (Olivero, 1994, 1996, 2003; Olivero and Gaillard, 1996). However, the entire burrow system is usually not observed. Large and irregular lamellae are present. The width of the marginal tube may reach 5 mm. 4.3.2. Lobed Zoophycos These specimens are very large (maximum width=160 cm). They usually appear as well developed lobes (Fig. 6), radiating from a virtual central apex, which is never clearly visible. These lobes may be quite long, up to 60 cm, their width never exceeds 7 cm, and they are bordered by marginal tubes up to 2–3 cm wide. Lobed Zoophycos can be very abundant and cover large surface areas of the beds but they rarely crosscut themselves. Adjacent specimens can touch and turn around each other. Sometimes the lobes seem to start from a larger lamina, resembling the Zoophycos described before (Figs. 6 and 7). The thickness of the laminae is unusually large (3 cm). Sometimes extra-tubes of the same dimensions crosscut the laminae. These structures

may represent the reworking of the burrows by the same organism during periods of famine, a behaviour already suggested for other Zoophycos by some authors (Bromley et al., 1999; Miller and D’Alberto, 2001). It is not clear if these lobes represent the most distal part of a larger structure. Similar morphologies have been described by Venzo (1950) in the Tertiary flysch of northern Italy, Bromley et al. (1999) in the Late Cretaceous chalk of Denmark and Sweden, Bromley and Hanken (2003) in the Pliocene of Rhodes, and by Olivero (2003) in the Late Cretaceous of southeastern France. According to these descriptions, the lobed and unlobed forms could represent different parts of the same spreite burrow. Another possibility is that two different traces co-occur in the same beds. 4.4. Chondrites This ichnogenus is restricted to some beds of the turbidite bundles. Chondrites probably was produced by a non-vagile deep-tier chemosymbiotic depositfeeder (Seilacher, 1990; Fu, 1991; Bromley, 1996), and the ichnofossil is usually considered a good indicator of dysaerobic–exaerobic conditions (Wetzel, 1983; Seilacher, 1990; Fu, 1991).

Fig. 5. Large Zoophycos lamina in the upper part of a calcarenitic turbidite bed. Bundle 3, Section 1. The arrow displays the direction of construction of the lamina.

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Fig. 6. Zoophycos lobes (lob). They are long and narrow, bordered by a marginal tube (t). The specimen on the top of the photo starts from a larger lamina (lam). Bundle 1, Section 1.

Fig. 7. Upper surface of a turbidite bed, covered by Zoophycos structure (abundance 4). Large Zoophycos with lobed outlines and a large marginal tube. Bundle 1, Section 1.

4.5. Taenidium On the top surface of some beds, cylindrical burrows having a meniscate internal structure, usually with annular constrictions and sometimes with a

pelleted filling, have been observed. Such specimens are like the trace fossils included in the Taenidium group (Uchman, 1999). They are usually considered as endichnial-feeding burrows, produced by an unknown vermiform organism.

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4.6. (?) Ophiomorpha Sometimes, burrows resembling Ophiomorpha have been observed, but better examples are needed for a more definitive identification. Ophiomorpha should be produced by crustaceans. It is frequently reported from shallow-water near shore deposits (Weimer and Hoyt, 1964; Uchman, 1990), but also occurs in deep-sea deposits (Crimes et al., 1981; Uchman, 1991, 1995). 4.7. Burrow mottling All of these ichnotaxa usually crosscut a thoroughly bioturbated background in the upper part of some turbidite beds. Also very common in the hemipelagic deposits, this style of bioturbation consists of indistinct compressed and smeared burrows, producing the mottled aspect of the sediment. It is responsible for the complete biogenic homogenization of the hemipelagic sediments. The resulting ichnofabric is common in pelagic to hemipelagic deposits of the Vocontian Trough. A similar ichnofabric has been previously described in Valanginian to Hauterivian deposits (Gaillard, 1984). It is probably produced by different deposit feeders in an oxygenated, organicsrich, soupground to softground substrate.

5. Distribution of trace fossils In both sections, the burrow mottling is well developed in all the hemipelagic mudstones, in association with some Planolites. Locally, in Section 1, Thalassinoides occur in relatively coarser layers, as in the interval between bundles 1 and 2. The following description will focus on the turbidite bundles, where some beds may be lacking any burrows (Fig. 8).

the bundle, then they appear with limited abundance (abundance class, d=1–2) in the middle part, when Thalassinoides and Planolites are lacking. They can be observed again in the upper part, sometimes with a relatively higher abundance (d=2–3), but they disappear at the top, while Thalassinoides and Planolites continue to be present. Zoophycos are essentially lobed forms. Unlobed forms are rare and without visible connection with lobed forms. Some Taenidium are present in the lower to middle part of the bundle, while Chondrites seem to be limited to the upper part and the beginning of the hemipelagic interval. 5.1.2. Bundle 2 Thalassinoides and Planolites are occasionally developed at the base, middle and especially upper part of the bundle, where they are more abundant and frequent. Zoophycos are completely absent at the base while they appear with limited abundance (d=1–2) in the middle part. After a short interval where they are lacking, they appear again at the top, more abundant (d=3–4) and frequent. Lobed Zoophycos dominate again, but unlobed forms increase toward the top of the bundle, sometimes linked to the lobes and forming single and unique specimens. 5.1.3. Bundle 3 Thalassinoides and Planolites are generally present and abundant throughout most of bundle. Zoophycos have been observed at the base (d=1) and in the upper part, with higher abundances (d=3–5) and frequency. They are absent in the middle part and at the top. Lobed and unlobed forms co-occur in the whole bundle, sometimes joining together. Lobed ones seem to dominate in the lower part of the bundle, while the opposite is recorded toward the top. In particular, in some beds only unlobed forms have been observed. Taenidium have been observed at the base of the bundle.

5.1. Vertical distribution of trace fossils in Section 1 5.2. Vertical distribution of trace fossils in Section 2 5.1.1. Bundle 1 Abundant Thalassinoides and Planolites are observed at the base of the bundle and in the upper part, but seem to be lacking in the middle part. This couple is also recorded in the first metres of the superimposed hemipelagic interval, with nearly the same abundance. Zoophycos are absent at the base of

5.2.1. Bundle 1 Abundant Thalassinoides and Planolites are observed in the middle and upper part of the bundle, while they are completely lacking at the base, characterised by very coarse-grained sediment. Zoophycos are absent at the base of the bundle. A few

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Fig. 8. Illustration of the weathering profile, vertical grain-size distribution (M: Marls, L: calcsiltites, f: fine calcarenites, m: medium calcarenites, c: coarse calcarenites, vc: very coarse calcarenites), Zoophycos (Z) abundance, and Thalassinoides and Planolites (T+P) abundance of the two sections logged. Palaeoenvironmental characteristics like the frequency of the turbiditic supplies, the organic matter (OM) content and the consistency of the substrate (from soup to firm) are deduced from the ichnological assemblages and their abundance for the two sections.

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lobes have been observed just above the very coarse base, while they are relatively more abundant and frequent in the middle (d=1–2) and especially the upper part (d=3– 4), where some unlobed forms are also presents. A few Taenidium and Chondrites are recorded in some beds of the upper part of the bundle.

covered outcrop, a few lobed Zoophycos are present with low abundance (d=1), together with rare Thalassinoides. A few Chondrites and Taenidium were observed in the upper part of the bundle.

5.2.2. Bundle 2 Thalassinoides and Planolites occasionally occur at the base, middle and especially upper part of the bundle, where they are more abundant and frequent. Zoophycos are completely absent at the base. They appear with limited abundance (d=1–2) in the middle part; afterwards they disappear again but are present toward the top (d=1–3). They are essentially lobed forms occurring with very few unlobed forms. As for Thalassinoides and Planolites, Zoophycos are completely lacking in the interval between the middle to the upper part of the bundle. In addition, burrow mottling is less represented in the same levels.

The comparative analysis of the two sections reveals the recurrent distribution pattern of trace fossils. Whereas burrow mottling is recorded throughout both sections, Thalassinoides–Planolites and especially Zoophycos are mostly limited to the three turbidite bundles. Their frequency and abundance, however, varies. This is most striking for Zoophycos. Zoophycos is usually absent at the base of the bundles, while it is more developed in the middle and upper part where its abundance usually increases. Bundles 1 and 3 are the richest in Zoophycos, but it is in the third one that frequency and abundance are highest. By comparison, bundle 2 is less heavily burrowed. Concerning the morphology of the Zoophycos, the unlobed forms generally are not only highly concentrated in bundle 3, but are also present toward the upper part of each bundle. The lobed forms seem to characterise bundle 1 as well as the base and middle parts of each bundle. The comparison between the two sections indicates that the grain-size quickly changes laterally and that this appears to have been a limiting factor for the burrowers (Fig. 9). If it is not so evident for Thalassinoides and Planolites, it is certainly clear

5.2.3. Bundle 3 Relatively abundant Thalassinoides, Planolites and Zoophycos are developed at the base. They are absent in the following interval. They appear again in the middle to upper part of the bundle, but Zoophycos (d=2–3) have also been observed in beds where Thalassinoides and Planolites are lacking. Unlobed forms dominate over the lobed morphologies. In a very few beds, they can reach a high abundance (d=4). At the top, just above a thick interval of mostly

5.3. Horizontal distribution of trace fossils

Fig. 9. Relationship between Zoophycos occurrence and grain-size in calciturbidites.

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from the analysis of Zoophycos. In bundle 1, Zoophycos are more abundant in Section 2, where fine turbidite sequences are more frequent (78%) compared to Section 1 (62%). This observation excludes the very coarse deposits filling the basal channel structure in Section 2. In bundle 2, grain-size is similar between the two sections or a little finer in Section 1 (76% of fine calcarenites in Section 1 and 80% in Section 2). Zoophycos are a little more abundant in Section 1. In bundle 3, the trace fossils seem to be less abundant in Section 2, which is finer than the other section (45% of fine calcarenites in Section 1 and 54% in Section 2). This contradictory result may be explained by the lack of sufficiently good outcrops in Section 2. In addition, the morphology of Zoophycos seems to be controlled by grainsize. In fact, unlobed forms are usually recorded in the finer-grained sediments, as in bundle 3 of Section 1, and bundle 1 of Section 2. Lobed structures are usually observed in the relatively coarser substrates. The detailed analysis of the two sections shows that the sedimentological and ichnological patterns are closely related. Two trends have been pointed out. First, there is a vertical trend inside each section, with generally an ichnofabric increasingly diversified from the lower to the upper part of the bundles and from bundle 1 to bundle 3. Secondly, there is a lateral variation of the coeval trace fossil assemblages between the two sections. These patterns reflect variations in the palaeoenvironmental conditions.

6. Discussion 6.1. Tiering The burrow mottling is visible either in the hemipelagic deposits (pre-turbidite ichnoassemblage) or at the fine-grained tops of the turbidites (postturbidite ichnoassemblage). In the hemipelagic limestones, the whole sediment is thoroughly bioturbated and the burrow mottling clearly indicates a permanent action of a precise kind of endofauna. Considering the turbidites, the burrow mottling corresponds to an intense bioturbation and to the shallow-tier because it is crosscut by all of the other traces. This probably indicates that the burrow mottling observed in the turbidites results from an early colonisation by the

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same organisms burrowing also in the hemipelagic sediments of the Vocontian sea-floor. Subsequently, the other traces are produced deeper in the sediment and commonly penetrate the coarser turbiditic material. Zoophycos is usually found in the deepest tier as observed in bed sections, several decimetres within the turbiditic bioclastic material. The well-preserved trace fossils such as Planolites, Thalassinoides and Zoophycos are typically post-turbidite ichnoassemblages. The tiering of the recognised ichnotaxa is, from shallowest to deepest traces: Planolites, Thalassinoides, Taenidium, Zoophycos and Chondrites. 6.2. Significance of ichnoassemblages in turbidites Two ichnoassemblages are evidenced: Thalassinoides–Planolites assemblage (Fig. 4) and Zoophycos assemblage (Figs. 5–7). As shown by their distribution in the studied calciturbidite bundles, these assemblages probably indicate different environmental conditions. The Thalassinoides–Planolites ichnoassemblage corresponds to trace-makers probably burrowing a softground substrate. They may be produced in coarser sediments compared to Zoophycos. Thalassinoides is probably produced by suspension feeders such as certain kinds of crustaceans (Frey et al., 1984). This suggests currents providing influx of food and a sea-floor sufficiently oxygenated. These burrows can be cross cut by Zoophycos, indicating a previous dwelling activity in a shallower tier at the same site. Thalassinoides and Planolites are classically considered as typical of the upper part of the transition layer in deep-ocean sediments (Wetzel, 1984; Bromley, 1996). Therefore, their presence at the bed surfaces suggests an erosion (Fig. 10). The Zoophycos ichnoassemblage is probably indicative of a slightly more cohesive substrate, according to observations made in other sites (Olivero, 1996; Olivero and Gaillard, 1996; MacEachern and Burton, 2000). It also indicates a high organics content inside the substrate, preserved owing to dysaerobic conditions. These conditions are possible if the organic matter is deeply stored and trapped in the sediment because of high deposition rates. The sedimentation rate is first high (favouring the trapping of the organic matter) and then reduced or stopped, allowing the substrate to become firm enough to permit construc-

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Fig. 10. Schematic relationships between ichnofabrics and calciturbidite dynamics. (A) Complete tiering (burrow mottling—Thalassinoides– Planolites–Taenidium–Zoophycos–Chondrites), because of enough time between two turbidite events; (B+C) increasing erosion of the shallower tiers (e), which allows the appearance, on the top surface, of Thalassinoides–Planolites, or of only Zoophycos. (A1) Partial tiering, without Zoophycos, because of insufficient time between two turbidite events; (B1+C1) increasing erosion of the shallower tiers (e), with Thalassinoides–Planolites on the top surface. (A2) Partial tiering, with only the burrow mottling. The time between two turbidite events is too reduced to allow the installation of the Thalassinoides–Planolites+Zoophycos. (B2+C2) Increasing erosion of the shallower tiers (e).

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tion and persistence of the burrow system. Organic matter preservation is also favoured by a relative finegrained sediment. The burrowers probably are chemosymbiotic organisms that exploit this organic matter stored deep inside the substrate (Bromley, 1996). However, at the same time, they maintain an open connection to the sea-floor, as they need enough oxygen for their respiration and that of their symbionts. Therefore, the presence of Zoophycos does not necessarily suggest dysoxic conditions at the seafloor, but really a dysaerobic setting inside the sediment. 6.3. Endobenthic colonisation: a key to interpretation of dynamics of the turbiditic flows According to observations made in recent settings, the colonisation of turbidites is controlled by several factors: quantity of sediment eroded by the turbiditic flow, frequency of the turbiditic supplies, volume of sediment supplied, size and shape of grains (Gaillard, 1988). In the environments perturbed by turbiditic supplies, gravity-flow sedimentation is very discontinuous and the new substrate can be colonised by the trace-making organisms between two turbiditic events. Concerning the studied case, and according to the above-mentioned observations, the following scenario may be proposed (Fig. 10). First, opportunistic burrowers, partly represented by the common benthos of the Vocontian Trough, exploit the shallower tiers. The substrate is unstable and prevents the construction of more complex burrow systems like Thalassinoides and Zoophycos. When the time interval between two turbidites is sufficient, colonisation by Thalassinoides and Planolites occurred. In a later stage, only when the frequency of turbiditic supplies is low, the sediment may become firm enough to allow Zoophycos construction. This development is more easily and quickly achieved if the sediment is fine-grained. A coarse-grained substrate prevents penetration of some burrowers, and limits consolidation and organic matter preservation. The occurrence of the different ichnoassemblages can reveal the frequency of the sediment supplies (Fig. 10). When the frequency is too high, only the burrow mottling is present. A medium frequency involves the addition of the Thalassinoides–Planolites ichnoassemblage. A low frequency is indicated

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by the presence of a burrow mottling, Thalassinoides– Planolites ichnoassemblage and Zoophycos ichnoassemblage. The presence of the complete tiering is rare and usually corresponds to a bfrozen tieringQ, indicating a substrate largely preserved from erosion by sudden turbidite arrival (Wetzel, 1984; Goldring, 1995). According to the velocity of the gravity flow, the ichnofabric may be partially or fully destroyed by erosion (Fig. 10). For instance, the presence of Zoophycos on the bed surfaces indicates that the upper tiers have been removed by erosion. In each bundle, even if the grain-size first increases then decreases, the abundance of Zoophycos usually increases (Fig. 8). They are generally absent at the base, but are more developed in the middle and upper parts. This indicates that the environmental conditions, like the frequency of the turbiditic supplies, control the general trend of the abundance of the Zoophycos. The grain-size may reduce or enhance this trend, the fine-grained sediment enhancing it. Assuming the relation between the frequency of the turbiditic supplies and the abundance of the Zoophycos, the formation of each bundle is characterised by a decrease of the frequency of the turbiditic supplies (Fig. 8). In both sections, bundles 1 and 3 are the richest in Zoophycos, but in bundle 3 abundances are highest despite the comparatively coarser sediments. On the contrary, Zoophycos is less abundant in bundle 2 even though this last bundle is the finer-grained one. The turbidite frequency is medium in bundle 1 and shows a decrease within this bundle. Between bundle 1 and bundle 2, an episode of hemipelagic deposition indicates a pause in the turbiditic supply. Bundle 2 records a high frequency in turbiditic supplies. The end of the turbidite episode (bundle 3) displays a decrease in the supply frequency just before the deposition of a marl interval (Heteroceras marls). 6.4. Endobenthic colonisation: a key to interpretation of palaeoenvironmental setting The organic matter content is an important factor in palaeoenvironmental considerations. The food supply suddenly introduced in the basin by the turbidity currents would support efficient sediment feeders such as the Zoophycos producers. In this case, the Zoophycos trace-makers appear as opportunistic animals. In the studied case, the optimum grain-size for the

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Zoophycos is between 0.1 and 0.2 mm (Fig. 9). In coarser deposits, the size of particles is probably too high to be easily ingested by the Zoophycos producers and to efficiently preserve the organic matter. According to these patterns, we interpret the distribution of the ichnoassemblages as recording the following successive events: (1)

(2)

(3)

(4) (5)

the exportation of a significant amount of organic matter from the platform to the basin by the turbiditic flow; the efficient storage of this organic matter in the turbidites, mainly in fine-grained material where it is easily preserved; the rapid colonisation of the turbiditic material by indigenous upper tier burrowers producing the burrow mottling; the successive colonisation by other burrowers such as Thalassinoides; the late exploitation of the stored organic matter by the Zoophycos trace-makers, mainly when the enclosing sediment is not too coarse and provide a more consistent substrate.

The morphology of Zoophycos may help to support other interpretations. In fact, we have observed two dominant morphologies: the lobed and unlobed structures. We wondered if two kinds of Zoophycos could co-occur. Several observations suggest that, in most cases, a single kind of Zoophycos is likely. The variations of the morphology (essentially lobed or unlobed form) may be the consequence of different behaviour of the producing organisms. We suggest that the strongly lobed structure may represent the burrowing of an organism exploring a new substrate in search of nutrients. If enough organic matter is available and if the substrate is firm enough, the same organisms change behaviour and begin to construct a much larger lamina. These environmental conditions are achieved if sedimentation rate is high but discontinuous (Olivero and Gaillard, 1996) and if the material is not too coarse. Lobed structures could correspond to the rapid exploration of nutrient-depleted turbiditic sediments, and unlobed structures to a more methodical and longer exploitation. Therefore, lobed structures could correspond to an opportunistic behaviour and unlobed structures to a more specialised behaviour. Accepting this, distribution of the two morphol-

ogies can be explained. At the beginning of each bundle, the sudden arrival of allochthonous sediment, encouraged the burrowers to seek nutrients, first following an exploration behaviour (resulting in predominance of lobed structures). On the contrary, towards the top of each bundle, the frequency in turbiditic supplies decreases, and Zoophycos tracemakers could begin to more methodically exploit the substrate (resulting in increasingly unlobed forms). In particular, unlobed forms dominate in bundle 3 of Section 1. This could be explained by large amounts of organic matter trapped inside a nearly cohesive substrate, under conditions of low frequency of turbiditic supplies. Fig. 8 displays an interpretation about the organic matter content following the distribution of both kinds of Zoophycos. Trace fossils also have proved their utility in recognising various sub-environments of siliciclastic deep-sea fans (Seilacher, 1962, 1974, 1977; Crimes, 1973, 1976; Crimes et al., 1974; Ksiazkiewicz, 1977; Wetzel and Uchman, 2001). Crimes and Fedonkin (1994) noted that submarine canyons and the inner parts of fans commonly contain a low-diversity ichnofauna with a preponderance of shallow-water types; middle fan sub-environments have a mixed ichnofauna of shallow-water and deep-water ichnotaxa (in particular, high ichnofaunal diversity occurs in interchannel areas); and outer fan and fan-fringe subenvironments rarely have ichnotaxa typical of shallowwater, but deep-water forms are abundant and diverse. Ichnofaunal analysis could also be applied to the carbonate submarine fans and contribute to improve our knowledge about the sub-environments occurring here. The trace fossil assemblages documented in our study display a low-diversity. The dominant ichnogenera are Thalassinoides and Planolites, followed by Zoophycos. The other taxa (Taenidium burrows, Chondrites, Ophiomorpha) are scarce and occur in a few beds. In this context, the strictly deep-sea ichnotaxa are absent. Therefore, the sub-environment could correspond to the inner part of a fan. This is consistent with the only available palaeogeographic information which is the position of the slope break of the western margin of the basin during the Barremian (Ferry, 1976). The proximity of this slope break could be responsible for the frequent formation of HCS-type features in the turbidite sequences, and also for the presence of a channel and of the frequent erosion

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events. This is also consistent with the optimal bathymetric range for most of the Zoophycos observed during Mesozoic time (Olivero, 1994, 2003; Olivero and Gaillard, 1996).

7. Conclusion The detailed sedimentological and ichnological analysis of the turbidite bundles exposed in the Pas-de-la-Cluse locality leads to the following conclusions: (1)

A significant exportation of organic matter occurred from the Bas Vivarais platform to the Vocontian Trough during late Barremian age. This organic matter was efficiently stored in the turbidites, mainly in fine-grained material where it is more easily preserved. (2) First, a rapid colonisation of the turbiditic material by indigenous upper tier burrowers occurred, providing a partial and superficial exploitation of organic matter and a burrow mottling. The subsequent colonisation by other burrowers requiring a more consistent substrate produced firstly Thalassinoides and later Zoophycos. The Zoophycos trace-makers exploited the deeply stored organic matter, mainly when the enclosing sediment is fine-grained. (3) As a fundamental consequence, the occurrence of Zoophycos may be the result of bquietQ conditions at the sea-floor, with a low frequency of recurring gravity-flow events. This leads to a better knowledge of the depositional characteristics of this calcareous turbidites. The three bundles record three episodes of palaeoenvironmental perturbations, in a context of quiet sedimentation characterised by hemipelagic slope calcsiltites. The ichnofabric analysis in the three bundles suggests a decreasing frequency of the turbiditic supplies from the beginning to the end of each turbidite episode. (4) A significant change in the behaviour of the Zoophycos trace-makers is also revealed by two morphologies: an unlobed form for reduced frequency of the supplies and abundant organic matter (here dominant at the end of turbidite episodes); and a lobed, more opportunistic form,

(5)

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for higher sediment supply frequency and lower organic matter content (here dominant at the beginning of the turbidite episodes). Last, the trace fossils are characterised by a low diversity and a dominance of ichnogenera more typical of shallow-water environments, and suggesting that the observed succession was located in the inner part of a turbiditic fan.

Acknowledgements The French Centre National de la Recherche Scientifique (CNRS), the French National Programme GDR Marges and the University Lyon 1 provided funds for this research. We sincerely thank F. Cecca for the ammonite determinations and biostratigraphic framework. Thanks to the referees, R.G. Bromley and W. Miller III, for their constructive suggestions and for the kind review of the English text.

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