Sedimentological implications of an unusual form of the trace fossil Rhizocorallium from the Lower Muschelkalk (Middle Triassic), S. Poland
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Specimens of the trace fossil Rhizocorallium isp. with retrusive limbs occur in 10–20-cm-thick composite beds: the bottom layers contain abundant Planolites and Thalassinoides trace fossils, and the top layers are built of horizontally bedded pelitic limestone and contain less bioturbational structures. The first specimens of Rhizocorallium isp. occur just 2–3 cm above the lower surfaces of the top layers. The horizontal protrusive sections are 1 cm high and 1 cm wide. At some sites, the horizontal tunnels end, and the limbs become first obliquely retrusive (45°) at a distance of 1.5–2.5 cm, and then vertically retrusive at a distance of 4–6 cm, or at once vertically retrusive at a similar distance. They reach to the upper surfaces of the top layers. These composite beds with unusual Rhizocorallium mark successive stages of distal tempestite sedimentation. The erosional events are recorded as uneven bases and a lack of Rhizocorallium, which suggests erosion to approx. 10 cm deep. The lower parts of the top layers record deposition of lime mud from a powerful flow. The protrusive sections of Rhizocorallium record relatively quiet periods resulting from significantly decreased sedimentation. The successive retrusive sections provide evidence that the quiet conditions suddenly came to an end, and that sedimentation was reactivated or accelerated. The last retrusive limbs of Rhizocorallium isp. mark the border of the storm event and the next erosion event. The composition of the fecal pellets shows that these burrowing animals were deposit-feeders, fast enough to find a sufficient amount of food, and also fast enough to escape being buried.
KeywordsIchnofossil Rhizocorallium Retrusive structure Lower Muschelkalk Upper Silesia
In pursuance of the last formal revision of the ichnogenus Rhizocorallium Zenker, 1836, proposed by Fürsich (1974), this ichnogenus includes three ichnospecies: Rhizocorallium jenense Zenker, 1836, representing straight, short, U-shaped spreiten-burrows commonly oblique to the bedding plane and only rarely horizontal; Rhizocorallium irregulare Mayer, 1954, representing long, sinuous, bifurcating or planispiral U-shaped spreiten-burrows, mainly horizontal; and Rhizocorallium uliarense Firtion, 1958, representing trochospiral U-shaped spreiten-burrows. Fürsich (1974) regarded Rhizocorallium commune Schmid, 1876 as a synonym of Rhizocorallium jenense, and included it into this ichnospecies. Knaust (2007) emphasized that Rhizocorallium commune and Rhizocorallium jenense should be described as two independent ichnospecies, because they represent two different morphologies. According to the original description, Rhizocorallium commune is a rather short and straight, horizontally oriented structure showing spreiten, whereas Rhizocorallium jenense is oblique to the bedding plane, and the spreiten are poorly developed. Fürsich’s (1974) division is used in this paper.
The most characteristic features of Rhizocorallium are the spreiten, which consist of stacked lamellae of reworked sediment, showing that succeeding tunnels were made at ever deeper levels of an individual bed (protrusive structure of Rhizocorallium jenense and Rhizocorallium uliarense) or continuously further along the bed top (protrusive structure of Rhizocorallium irregulare). According to previous interpretations, the various forms of Rhizocorallium result from the different modes of their producers’ lives: Rhizocorallium jenense was interpreted as a suspension-feeder’s burrow, whereas Rhizocorallium irregulare and Rhizocorallium uliarense were thought to record the activity of deposit-feeders (Fürsich 1974; Worsley and Mørk 2001). Modern views emphasize the function of the horizontally developed Rhizocorallium tunnel systems as fodinichnia and domichnia of deposit-feeding crustaceans and/or worm-like animals (Knaust 2007; Seilacher 2007).
In addition to the ichnospecies mentioned above, some modified Rhizocorallium have been found: (1) specimens with vertically retrusive limbs, known inter alia from the Early Carboniferous (Chisholm 1970), the Middle Triassic (Rodríguez-Tovar and Pérez-Valera 2008), the Late Triassic (Seilacher 2007), the Early Jurassic (Sellwood 1970), and the Late Jurassic (Schlirf 2000; Carvalho et al. 2010, and (2) lobate specimens which penetrate two or more levels in a sediment known from the Early Jurassic (Fürsich 1974; Seilacher 2007). The rarely reported first form is a modification of Rhizocorallium jenense, which was created in two stages: the animal initially constructed an inclined protrusive tunnel, and then switched to an upward retrusive position (Seilacher 2007). In the case of the Middle Triassic forms, the values of vertical shifts are approx. 2.0 cm (Rodríguez-Tovar and Pérez-Valera 2008); comparable values of the vertical shift of the Late Jurassic forms lie between 4.3 and 8.0 cm (Schlirf 2000). The retrusive limbs of Rhizocorallium are interpreted as an equilibrium structure developed as a response to slow but continuous sedimentation (Fürsich 1974; Schlirf 2000; Rodríguez-Tovar and Pérez-Valera 2008). The common Middle Triassic Rhizocorallium (without retrusive sections) known from the German Basin were always formed after deposition of a bed, starting at the top, and were developed during quiet conditions as a result of the sea floor colonization after storm activity (Aigner 1985; Worsley and Mørk 2001). The horizontally developed Rhizocorallium were formed just 1–3 cm beneath the bottom surface (Wetzel and Aigner 1986).
The present contribution reports for the first time the occurrence of Rhizocorallium isp. with retrusive limbs in the Gogolin Formation (Lower Muschelkalk, Upper Silesia, S. Poland). Such forms of Rhizocorallium isp. have to date not been reported from the other Muschelkalk formations in Poland. The specific position and the unusual form of the Rhizocorallium isp. with retrusive limbs provide new insight into the rate of development of the retrusive limbs in relation to the storm depositional processes.
The present observations of Rhizocorallium isp. and its relationship to other trace fossils make it possible to distinguish between the successive minor sedimentological processes during individual storm events, which are known from analogous carbonate formations.
The first horizon of the wavy-bedded limestone consists of several rocks types. The most characteristic are the approx. 40-cm-thick beds of crumpled, highly bioturbated pelitic limestones that contain Rhizocorallium jenense, Rhizocorallium irregulare, Planolites, and Thalassinoides, all preserved in full relief, enclosed in a more marly deposit (Fig. 2). The horizon also contains: (1) thin, platy pelitic limestone beds of approx. 1–3 cm thick with bioturbated tops, which are interbedded with marl laminae; (2) up to 40-cm-thick sets of wavy-bedded pelitic limestone laminae intercalated with very thin marls; and (3) composite beds consisting in their lower part of bioturbated pelitic limestone with erosionally truncated tops and in the upper part consisting of thin and medium-bedded bioclastic limestones, containing mainly crinoid ossicles and molluscs, or sandy limestones comprising mainly quartz and muscovite grains. The highly bioturbated pelitic limestones are interpreted as autochthonous sediments deposited in the mid-ramp zone, whereas the weakly bioturbated pelitic limestones, bioclastic limestones, and sandy limestones are interpreted as distal tempestites deposited on the autochthonous, semi-consolidated, usually bioturbated lime muds (Szulc and Głuchowski 1991; Kowal 1998).
The specimens of Rhizocorallium isp. occur in hard rocks, so isolation of individuals was not possible. Specimens have been cut to examine their three-dimensional forms. Most of the collected specimens are incomplete or show combined structures of two or more burrow systems. We present here the most complete and clear examples. According to the Fürsich’s division (1974), the collected specimens show features characteristic of Rhizocorallium jenense, but their protrusive tunnels are developed horizontally, which is rare. The long, horizontally developed Rhizocorallium irregulare are not known to have horizontally retrusive sections. We therefore use the name Rhizocorallium isp. to avoid incorrect identification of the specimens. Thin sections were prepared to examine the texture of the host rock and the composition of the fecal pellets. The specimens are deposited at the Institute of Geology, Adam Mickiewicz University, with catalogue numbers UAMIG-Zg-42-51.
Description of the Rhizocorallium burrows
A few escape burrows (fugichnia) leading from the bottom layers to the top layers have been found. They end 2–3 cm above the lower surfaces of the top layers and are cut by the protrusive sections of Rhizocorallium isp. (Figs. 3c, 4a).
The composite beds containing Rhizocorallium with retrusive limbs show sedimentary structures characteristic of distal calcareous tempestites. Such tempestites were identified and described in detail from, inter alia, the Upper Muschelkalk carbonate ramp of S. Germany, which was affected by storms and hurricanes (Aigner 1985). The “ideal” distal calcareous tempestite bed is 1–10 cm thick, shows an erosional base often with tool marks, consists of a thin skeletal lag at the base, and homogenous pelitic lime mud (calcisiltite to calcilutite), which shows horizontal bedding and low-angle cross-bedding in the upper part, and wave ripples on the top. These structures reflect changes in depositional processes during the one storm event. Deposition is followed by colonization on the bed tops. Animal activity manifested by bioturbations always starts at the bed tops (Aigner 1985; Wetzel and Aigner 1986).
The uneven, oblique upper surfaces of the completely bioturbated bottom layers of the composite beds, and the cut burrow-fillings record erosional truncation resulting from bottom current action (e.g., Myrow and Southard 1991). Lack of any Rhizocorallium in these bottom layers, which consist of numerous Planolites and Thalassinoides, allows estimation of the depth of erosion. According to infaunal tiering (the characteristic vertical zonation of the ichnofossils associations), Rhizocorallium is characteristic of the upper parts of bioturbated beds, thus the occurrence of Planolites and Thalassinoides on bed top records the erosion of more than 10 cm (Aigner 1985; Wetzel and Aigner 1986), as in this case. The oblique surfaces of the truncated bottom layers, which mark differences in levels of approx. 3 cm, additionally support deeper erosion.
The lowermost 2–3 cm of the top layers, without bioturbational structures other than the escape burrows, show that the first portion of the lime mud of the top layers was deposited relatively quickly, and just after the first horizontal Rhizocorallium isp. were formed. The sudden deposition is also evidenced by the escape structures leading from the bottom layers to the top layers. These observations suggest that the lime muds constituting the lower part of the top layers were deposited from the powerful flow resulting from storm activity (e.g., Walker et al. 1983; Myrow and Southard 1991). Because benthic animals start their burrowing activity after deposition at the tops of the beds (Aigner 1985), the horizontal protrusive sections of Rhizocorallium indicate that the deposition from the powerful flow had decreased significantly, and that a period of relatively quiet conditions had begun. The escape burrows mentioned above occur only to the level where the first horizontal Rhizocorallium systems are present, which also supports temporally decelerated deposition. Newly deposited lime mud a few centimeters thick, supplied from the shallower part of the carbonate ramp, must have been rich in organic matter, which made it very attractive for deposit-feeders; these started their activity at the top of the new bed, which contained abundant nourishment. The beige color of the fecal pellets in the protrusive tunnels, paler than the surrounding sediment, supports the hypothesis that the burrowing animals were deposit-feeders (e.g., Dworschak et al. 2006). The subsequent shift in direction of Rhizocorallium from horizontal to oblique and vertically retrusive indicate that the sedimentation of distal tempestite bed was reactivated or accelerated, although this fact is not recorded either in the sediment texture, or in sedimentary structures. The lowermost parts of the top layers continue into successive parts without any visible sedimentological boundary or other evidence of an interruption in sedimentation. This could mean that the whole tunnel systems (protrusive and retrusive sections) were created relatively quickly. This is not surprising, as research into modern mud shrimps shows that these animals can dig tunnels of tens of centimeters long in just a few hours, and a whole burrow system in only 1 week (Dworschak 2004). The shift in direction from horizontal to oblique and vertical was a response to the deposition from suspension (e.g., Walker et al. 1983), which started after deceleration or short cessation of sedimentation from the powerful flow. The animals had to protect themselves from burial, and therefore created vertically retrusive limbs. The composition and pale color of the fecal pellets in the tunnels, showing the same features regardless of whether they are placed in protrusive or retrusive sections, indicates that the whole Rhizocorallium systems occurring in the Gogolin Formation were made by deposit-feeders. These need to dig long tunnels to find sufficient food. Research into modern burrowing animals shows that they spend most of their time on burrowing activity, and that their tunnel systems are dynamic and change shape continuously for the purpose of obtaining food (Nickell and Atkinson 1995; Dworschak et al. 2006). Had sedimentation during the final stage of deposition been slow, the fast-digging deposit-feeders would have created an extensive horizontal burrow system in order to find enough food instead of vertically retrusive tunnels. The current hypothesis that vertical retrusive limbs reflect slight but continuous sedimentation, should therefore, in our opinion, be re-evaluated. This has been suggested earlier by Fürsich (1974) for Rhizocorallium jenense, and later, inter alia, by Schlirf (2000), who suggested that retrusive limbs were formed as a response to slow but continuous sedimentation, or in order to achieve sufficient nutrient exploitation of the sediment (the latter interpretation presumes that Rhizocorallium jenense burrows were also created by deposit-feeders, not suspension-feeders), and Rodríguez-Tovar and Pérez-Valera (2008), who also thought that the retrusive limbs were a response to slow but constant sedimentation. We postulate, in contrast, that the vertically retrusive limbs are a result of a relatively fast sedimentation rate, and that the animals continued burrowing for feeding, but also needed to avoid burial. Slow sedimentation is not a satisfactory explanation, because deposit-feeders digging only short vertically retrusive tunnels during slow sedimentation would not find sufficient food.
This hypothesis is also supported by differences in the wall structure of the tunnels. The protrusive sections have indistinct wall boundaries, not consolidated by mucus, which shows that the horizontal section was created during relatively quiet conditions, and that the tunnels did not need to be protected against compaction. The retrusive sections have distinctly darker walls, consolidated by mucus, which suggests that the tunnels were made in loose, unstable and continuously falling sediment, and that they needed to be protected against compaction by the overlying sediment.
Detailed observations on the development of trace fossils, especially of those with unique features, are useful for the identification of relatively small events and the successive stages of single events of storm-induced sediment remobilization and deposition. In the case presented here, these stages of erosion, sedimentation from the powerful flow, relatively quiet condition, and renewed sedimentation from the suspension can be identified thanks to the trace fossils.
The erosion events are recorded as uneven tops of completely bioturbated bottom layers, with differences in levels up to 3 cm, and as a lack of Rhizocorallium, which indicates erosion to approx. 10 cm deep. The sudden deposition of the first amounts of the lime mud, remobilized in the inner ramp and transported to the middle ramp by the powerful flow, is manifested by escape burrows leading from the bottom layers to the top layers, and by lack of any other burrows in these parts of the beds.
The protrusive sections of Rhizocorallium isp. that occur approx. 2–3 cm above the bases of the top layers record the relatively quiet periods resulting from temporally decelerated sedimentation or even temporal cessation of sedimentation as the powerful flow waned. This is also supported by the length of the escape burrows, which end in the top layers and are cut by protrusive tunnels of Rhizocorallium isp. The succeeding vertically retrusive tunnels of Rhizocorallium isp. indicate that the relatively quiet conditions came suddenly to an end. The shift in direction from horizontal to oblique and vertical was a response to the deposition from suspension, which started after the deposition from the powerful flow of the same bed during the same storm event. Eventually, the last retrusive limbs of Rhizocorallium isp. occurring on the top of the beds mark the end of the storm event and the next erosion event (the last limb should occurs 1–3 cm beneath the bed top, and the last tempestite division with ripples is not preserved).
The composition and color of the fecal pellets (which are the same in both the protrusive and retrusive sections of Rhizocorallium) show that the burrowing animals were deposit-feeders, which presumably needed to dig relatively long tunnels to find enough food. We consequently propose that vertically retrusive limbs created by deposit-feeders record relatively fast sedimentation. Slow sedimentation is not a satisfactory explanation, because deposit-feeders digging only short vertically retrusive tunnels during slow sedimentation would not find sufficient food. The retrusive limbs must have been made by deposit-feeders fast enough to find sufficient food and also fast enough to avoid being buried.
We would like to thank Michał Jankowiak for preparing the rock slabs and thin-sections. The detailed and constructive comments by Eduardo Jesus Mayoral (University) and Prof. Alfred Uchman (Jagiellonian University, Poland) are highly appreciated. Prof. Antonius J. van Loon (Adam Mickiewicz University, Poland) and Stephen Mulraney are thanked for language editing. The research was financed by Adam Mickiewicz University.
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