Ichnocoenoses of the Boulonnais trace fossils

Palaeoichnocoenoses and their use as environmental indicators
Nine frequently occurring (1-9) palaeoichnocoenoses and one exceptional (10) palaeoichnocoenosis can be identified in the Upper Jurassic rocks of the Grès de Châtillon and Grès de la Crèche. The term "ichnocoenosis" is understand as an equivalent of "palaeobiocoenosis" or "life assemblage" sensu BROMLEY (1996). Each palaeoichnocoenosis is characterised by a certain assemblage of trace fossils and is named after a dominant ichnogenus or ichnospecies or a group of ichnofossils.

Diplocraterion ichnocoenosis
The Diplocraterion ichnocoenosis is dominated by domichnia/equilibrichnia of suspension-feeders-such as Diplocraterion parallelum. Skolithos linearis may occur within this ichnocoenosis as an accessory trace fossil. The Diplocraterion ichnocoenosis is interpreted as indicator of a high energy environment (FÜRSICH 1975, HEINBERG & BIRKELUND 1984, DAM 1990). The tops of the burrows are frequently capped and thus indicate erosional processes. The Diplocraterion ichnocoenosis occurs in small- and large-scale, trough cross-bedded and swash cross-stratified sandstones of the Grès de la Crèche at Cap Gris Nèz, which comprises deposits of a mid-ramp to inner-ramp wave-influenced facies.

Rhizocorallium jenense type 1 ichnocoenosis
The Rhizocorallium jenense type 1 ichnocoenosis is characterised by domichnia/equilibrichnia of suspension-feeders such as steep, oblique specimens of Rh. jenense, described as type 1 in the systematical part. As accessory trace fossils, Skolithos linearis or Diplocraterion parallelum may occur. Like the Diplocraterion ichnocoenosis, the Rh. jenense type 1 ichnocoenosis is interpreted as an indicator of a high energy environment, supported by the frequent occurrence of thick burrow linings in Rh jenense. The Rh. jenense type 1 ichnocoenosis occurs in trough cross-bedded and swash cross-stratified sandstones of the Grès de la Crèche at Le Portel and Grès de Châtillon at Ambleteuse. Thesesandstones are made up of wave influenced mid-ramp to inner-ramp deposits. Like in the above described Diplocraterion ichnocoenosis, the burrow tops are frequently eroded.

Spongeliomorpha nodosa ichnocoenosis
This is a monospecific ichnocoenosis consisting exclusively Spongeliomorpha nodosa type B occurring in large- to small-scale trough cross-bedded sandstones of the Grès de la Crèche at Le Portel. The ichnocoenosis corresponds to the Ophiomorpha ichnocoenosis of HEINBERG & BIRKELUND (1984) and DAM (1990) and is interpreted as an indicator of high energy conditions and mobile substrates, which favour opportunistic behaviour (DAM 1990).

Spongeliomorpha suevica ichnocoenosis
This ichnocoenosis can be divided into two form groups: 1) the maze-group and the 2) the boxwork-group. The first group consists of Spongeliomorpha suevica variation B (see taxonomy), a domichnion/fodinichnion of crustaceans that follow various modes of feeding (the same is true for variation C). It occurs in large-scale trough or planar cross-bedded sandstones of the Grès de la Crèche at Cap Gris Nèz. Occasionally Planolites isp. may be found in association with S. suevica variation B. The second group includes S. suevica variation C, sometimes associated with Planolites ispp., which occur in calcareous, large-scale trough or planar cross-bedded sandstones of the Grès de la Crèche at Pointe de la Crèche. The ichnocoenosis is interpreted as an indicator of physically unstable conditions in accordance to the Ophiomorpha ichnocoenosis of HEINBERG & BIRKELUND (1984) and DAM (1990). However, since there was no need to stabilize the burrow wall, the sediment consistency must have been higher than that described by HEINBERG & BIRKELUND (1984) and DAM (1990). Sediment influx appears to have been periodically (e.g., storm events) and during most of the time the energy level was low to intermediate as is indicated by the high carbonate content of the sediment.

Asterosoma-Teichichnus ichnocoenosis
This ichnocoenosis is dominated by pascichnia/fodinichnia of deposit-feeders such as Asterosoma ludwigae n. isp., Teichichnus patens n. isp., Cochlichnus anguineus, Taenidium serpentinum, T. baretti, and Rhizocorallium irregulare. In addition to these traces, Chondrites intricatus, Spongeliomorpha nodosa, Palaeophycus striatus, 'concentrically laminated burrows', Planolites ispp., and Arenicolites statheri can be found. This ichnocoenosis corresponds to the "Teichichnus association" of FÜRSICH (1975) and is interpreted as an indicator of a low energy regime with comparatively low rates of deposition and a fairly stable substrate which allowed intensive bioturbation. Nevertheless, the sediments (trough cross-bedded sandstones) in which the Asterosoma-Teichichnus ichnocoenosis occurs are interpreted as wave-influenced, high energy deposits, which indicate that the ichnocoenosis was not contemporaneous. This leads to the conclusion that the environment was occasionally dominated by high sedimentation rates in combination with high energy conditions (e.g., storm events) which alternated with times of more-or-less quiet conditions during which the burrowers were active. The intensity of bioturbation changes within a section and, thus, may indicate changes in the environmental conditions or rates of erosion, or both. The occurrence of both Spongeliomorpha nodosa (indicating a soft, unstable sediment) and Asterosoma ludwigae n. isp. (indicating firm, stable sediment) at first appears to be contradictory. However, close examination suggests that S. nodosa was a member of the first colonisation relatively shortly after the sediment has been deposited and still had a high water content and thus was unstable. In contrast, Asterosoma ludwigae represents a later stage of colonisation during which the sediment cohesiveness was higher. The Asterosoma-Teichichnus ichnocoenosis exclusively occurs in the Grès de Châtillon at Ambleteuse.

Palaeophycus-Treptichnus ichnocoenosis
This ichnocoenosis is dominated by agrichnia/fodinichnia/domichnia of farming, trapping, predating, deposit-, detritus-, and/or suspension-feeding organisms. It consists of Palaeophycus tubularis, Treptichnus bifurcus, Rosselia socialis, 'concentrically laminated burrows', Spongeliomorpha nodosa type A, Beaconites coronus, and Arenicolites statheri. Palaeophycus, Treptichnus, and Spongeliomorpha nodosa type A are interpreted as premier colonisers of this ichnocoenosis since they can be observed exclusively within beds in which the Palaeophycus-Treptichnus ichnocoenosis occurs. In beds where all the traces of this ichnocoenosis occur together, Palaeophycus, Treptichnus, and Spongeliomorpha nodosa are usually cut or crossed by the others. This indicates that Palaeophycus, Treptichnus, and Spongeliomorpha nodosa were build at a previous stage of colonisation. The ichnocoenosis shows a variety of feeding modes. The sediments that bear the Palaeophycus-Treptichnus ichnocoenosis include current- and or oscillatory-rippled, partly flat-topped, shell-bed and conglomerate bearing, trough and planar cross-bedded sandstones and rare intercalations of silty clays. Occasionally dessication cracks can be found. These deposits are interpreted as storm- and tidally influenced nearshore, inner ramp deposits of the Grès de la Crèche. The ichnocoenosis predominantly occurs at Le Portel and Pointe de la Crèche and is interpreted as an indicator of low to intermediate energy conditions since suspension-feeders are only sparsely represented.

Rhizocorallium jenense type 2 ichnocoenosis
This ichnocoenosis is characterised by Rhizocorallium jenense type 2, Teichichnus rectus, Planolites ispp., Rhizocorallium irregulare, and Spongeliomorpha nodosa type B. The beds that bear this ichnocoenosis generally show a high degree of bioturbation. The uppermost 25 cm of a bed may be completely bioturbated with Spongeliomorpha nodosa reaching the deepest tier level with a depth of up to 75 cm. The Rh. jenense type 2 ichnocoenosis occurs in large-scale trough or planar cross-bedded sandstones with a relatively high content of shell debris (predominantly Nanogyra) and occasional occurrence of quartz pebbles. This ichnocoenosis is interpreted as an indicator of medium to low energy levels, although the sediments that bear this ichnocoenosis indicate a high energy environment. However, the occurrence of deposit-feeder structures, such as Teichichnus, Planolites, Rhizocorallium irregulare and maybe Rhizocorallium jenense type 2, indicate quiet conditions. As for the Asterosoma-Teichichnus ichnocoenosis, the traces represent stages of quiet conditions within a generally high energy environment. The Rh. jenense type 2 ichnocoenosis occurs exclusively in the Grès de la Crèche at Le Portel.

Gyrochorte-Protovirgularia ichnocoenosis
This ichnocoenosis is characterised by pascichnia of deposit-feeders such as Gyrochorte comosa and Protovirgularia. Alcyonidiopsis longobardiae, Planolites isp., Palaeophycus tubularis, and rarely Arenicolites statheri may also occur. This ichnocoenosis is dominated by burrows of deposit-feeders, however, some suspension-feeder burrows occur and indicate that enough energy was present to keep detritus in suspension. The ichnocoenosis is interpreted as an indicator of an intermediate to low energy environment. This ichnocoenosis occurs predominantly in isolated, oscillatory-rippled sandstone interbeds in silty clays of the Grès de la Crèche at Le Portel. These sediments are interpreted as deposits of the transitional zone between a low energy, outer-ramp environment and a wave and storm influenced, mid-ramp environment.. The ichnocoenosis can also be found in rippled sandstone beds that alternate with silty, organic-rich fine-sand beds of the Grès de Châtillon at Ambleteuse. These sediments are also interpreted as deposits of the transitional zone but with a more significant terrigenous influence than the first one.

Nereites ichnocoenosis
This ichnocoenosis is dominated by Nereites missouriensis, pascichnia of deposit-feeders. Other traces, such as Palaeophycus tubularis and Planolites are rarely associated with N. missouriensis. In some cases Arenicolites, Chondrites, and Teichichnus have been observed, but always cross and/or interpenetrate Nereites misssouriensis and are thus interpreted as non-contemporaneous and are no members of this ichnocoenosis. Although the sediments in which the Nereites ichnocoenosis occurs represent deposits of a high energy level (large-scale trough or planar cross-bedded sandstones of the Grès de Châtillon at Ambleteuse), the trace fossil Nereites missouriensis indicates a low energy environment. It is usually interpreted as a deep tier, single layer coloniser (UCHMAN 1995) and is commonly found in flysch deposits. SEILACHER (1967) chose Nereites as the name-giving trace of his archetypal Nereites ichnofacies, a typical deep sea ichnofacies. The exclusive occurrence of Nereites does not justify the assumption that a bed bearing this trace represents a deep-sea environment. However, Nereites appears to indicate low energy conditions during which the Nereites producer was active (see UCHMAN 1995 for detailed discussion on the palaeoecology of Nereites).

Teredolites ichnocoenosis
This ichnocoenosis exclusively consists of Teredolites clavatus, the trace of wood-boring bivalves. The ichnocoenosis was observed in a completely weathered piece of wood in the Grès de la Crèche at Le Portel.