Downloaded from geology.gsapubs.org on 27 September 2009 Ichnological evidence for the environmental setting of the Fossil-Lagerstätten in the Devonian Hunsrück Slate, Germany

Owen E. Sutcliffe* Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK Derek E. G. Briggs Christoph Bartels Deutsches Bergbau-Museum, Am Bergbaumuseum 28, D-44791 Bochum, Germany

ABSTRACT The Hunsrück Slate represents part of the Rhen- Analysis of the ichnology and sedimentology of the Lower Devonian Hunsrück Slate, ish Lower Emsian (Mittmeyer, 1980), but its age is Germany, reveals that the distribution and preservation of the famous pyritized fauna were uncertain. Here we focus on the basinal, clay-rich controlled by the deposition of fine-grained turbidites that formed a firm substrate. The nature sequence that yields the pyritized fossils. It con- of this substrate is evidenced by the preservation of laminae and the finest details of arthropod tains the trilobite Chotecops ferdinandi and the trackways. The trace fossils are dominated by two ecological groups: those made by epifaunal goniatite Anetoceras, as well as the dacryoconarid organisms and those involving burrow systems connected to the sediment-water interface. Nowakia praecursor (Alberti, 1982), and it corre- Trace makers that moved through the sediment are poorly represented. The diversity of in situ sponds, in part, to the praecursor zone. body fossils and epifaunal traces confirms that conditions within the water column remained well oxygenated, even though the sediment rapidly became inhospitable. The Hunsrück Slate SEDIMENTOLOGY Konservat-Lagerstätten are remarkable in preserving soft tissues where unusual geochemical The Hunsrück Slate in the Eschenbach-Bocks- conditions prevailed in the environment where the animals lived, rather than following trans- berg quarry was deposited on a mud-rich slope port to a different setting. apron, its upper limit controlled by storm wave base. Progradation resulted in the coarsening up- INTRODUCTION fine-grained turbidites was critical to the preser- ward that characterizes the sequence. Fine- The Devonian Hunsrück Slate around Bun- vation of the fauna and to the generation of con- grained turbidites near the base indicate deposi- denbach, Germany, is renowned for preserving ditions favorable to a burrowing macrofauna tion on the distal part of a sedimentary fan. the nonbiomineralized tissues of organisms by dominated by the producer of Chondrites. Interbedded sandstones and claystones containing replacement with pyrite (Briggs et al., 1996; shelly coquinas indicate shallowing above storm Bartels et al., 1998). However, it is more remark- GEOLOGIC SETTING wave base toward the top. The slate-producing able that the pyritized soft-bodied fossils occur in The Hunsrück Slate crops out within the lithologies were deposited as turbidites and in- the same lithologies as trace fossils (Fig. 1). southern part of the Rhenohercynian zone. Depo- clude four levels (Konservat-Lagerstätten) that Analysis of the trace fossils permits a detailed sition occurred in the 150-km-long, northeast- yield exceptionally well preserved fossils (Fig. 2). synthesis of the setting of the Hunsrück Slate trending extensional, intrashelf central Hunsrück Fine-grained turbidites, such as those repre- Lagerstätten and the properties of the substrate in basin, which formed during the Early Devonian sented by the Hunsrück Slate, are interpreted as which the extraordinary pyritized fossils formed. (Langenstrassen, 1983; Winterfeld et al., 1994). the product of low-density turbidity currents

This is the only example of soft-bodied preserva- The basin was bounded to the northwest and (Stow and Shanmugam, 1980). The T0–T3 divi- tion so far analyzed in this way. The significance southeast by submarine paleogeographic highs sions (Stow and Shanmugam, 1980) are repre- of the trace fossils that occur at levels within the (Mittmeyer, 1980; Langenstrassen, 1983). sented by silt-rich lithologies, and the T3–T6 divi- Cambrian Burgess Shale has been considered (Allison and Brett, 1995), but they are not inti- mately associated with soft-bodied fossils. The spectacular pyritized fossils of the Hunsrück Slate are restricted to slate-producing horizons around the villages of Bundenbach, Gemünden, and Breitenthal (Bartels et al., 1998). Seilacher et al. (1985) considered that rapid sedi- mentation and oxygen deficiency were crucial for the preservation of the fossils. Like Richter (1941), they considered the oxygen deficiency to be re- Figure 1. Specimen of Lo- stricted to the sediment. Wollanke and Zimmerle riolaster mirabilis show- ing clear evidence of pyri- (1990) considered that volcanic material in the tized soft tissues beyond sediment gave it thixotropic properties. Brett and biomineralized skeleton Seilacher (1991) interpreted the Hunsrück Slate and associated pyritized as an obrution deposit, formed below storm wave burrows (Deutsches Berg- base beneath a stratified dysoxic water column. bau-Museum HS 642, X- radiograph W. Blind). Storm-induced turbidites buried the fauna that was asphyxiated when erosion released H2S from the sediment. This investigation, however, demonstrates that the mode of deposition of the

*Present address: Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK.

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alternating laminae about 1 mm thick consisting slate-producing sequence, including the horizons of silt and clay. Cross and parallel laminae and that yield the pyritized fossils. fading ripples are present, as is Chondrites. Clay- The trace fossils were assigned to three eco- rich lithologies are more common than silt-rich logical categories: (1) epifaunal traces, (2) mobile ones and predominate in the lower slate-produc- infaunal traces, or (3) constructed infaunal traces ing horizons. They occur in beds 0.5–5 cm thick, with a connection to the sediment-water interface characteristically grading from lighter blue-gray (Fig. 3C). Traces occurring in the interbedded to darker blue-black claystones. The base of these clay- and silt-rich lithologies were separated. The beds is well defined by silt-rich laminae about latter were identified by the occurrence of silt- 1 mm thick. Fine laminae also occur in the clay- rich laminae more than 3 mm thick, resulting in a 4 stones, but not throughout. Chondrites is present granular and presumably less cohesive substrate. toward the top of beds, penetrating as deep as Where possible, the trace maker was identified. 3 3.5 cm. The burrow fill is darker than the host sedi- Although Chondrites is ubiquitous, its occur- ment and is similar to the hemipelagic material rence was omitted from the analysis because it is that separates some clay-rich beds. unremarkable, and therefore rarely represented in 50 m The sedimentation of fine-grained turbidites is the collections. characterized by gradual deposition of silt grains Epifaunal traces (Fig. 4A) dominate the ichno- punctuated by rapid accumulation and setting of fauna in both abundance and the range of be- clay aggregates (Stow and Bowen, 1980; McCave havior represented. The producers include trilo- and Jones, 1988) (see Fig. 3, A and B). Low cur- bites, an ophiuroid, and fish (Sutcliffe, 1997) rent velocities (10–20 cm/s: Stow and Bowen, (Fig. 3C); some fish trails reach a width of 12 cm. 1980) are consistent with the alignment and burial Most of the traces are preserved in the upper parts of crinoids and sponges in situ in the Eschenbach- of fine-grained turbidites where they were pro- 2 Bocksberg sequence (Bartels et al., 1998). En- duced after deposition, uninfluenced by currents. trainment and saltation of mobile epifauna, fol- Arthropod trackways, for example, display ex- 1 lowed by setting of the clays, results in the ceptional detail on a submillimeter scale (Bartels suspension of carcasses in variable orientations to et al., 1998, Figs. 221 and 224). The low current bedding (Bartels et al., 1998). velocities ensured that many organisms remained in contact with the substrate during deposition, TRACE FOSSILS generating syndepositional traces, which are The distribution of benthic communities is identified on the basis of their orientation parallel 0 m influenced by oxygen levels and the nature of to associated sole structures (e.g., Seilacher, ClaystoneSiltstoneFine MediumCoarseConglomerate the substrate (Ekdale, 1985). Variations in 1960, Fig. 13, b and c; 1962, Plate 24, Fig. 1). faunal diversity and the traces produced have Mobile infaunal traces are poorly represented been related to fluctuations in oxygen level in the ichnofauna (Fig. 4A). Their low abundance Sandstone Key: (Wignall, 1994), but only a few attempts have is interpreted as real, because they generally have Interbedded sandstone Volcaniclastic been made to integrate the effects of the sub- a higher preservation potential than epifaunal and claystone layer strate (Sageman, 1989; Wignall, 1993). Modes traces. The producers include protobranch bi- Interbedded siltstone Phosphatic of locomotion and burrowing are directly re- valves, mitrates (echinoderms), and polychaete and claystone nodules lated to sediment type. Hence a survey of the worms (Fig. 3C); burrow diameters range from 0.1 Siltstone and claystone Pyritized fauna fining couplets ichnofauna reveals important information about to 2 cm. Some examples of Protovirgularia show the environment of deposition. movement forward and upward from a bean- Silty claystone Fining upward The Hunsrück Slate trace fossils are not usually shaped resting trace, whereas other mobile in- Coarsening retained by the slate splitters because they have faunal traces spiral out of the sediment. These are Laminated claystone upward limited commercial value. There are only two interpreted as evidence of an escape response to Homogeneous major collections: at the Institut für Geologie und burial. Other traces, however, such as Pteridich- claystone Paläontologie, Universität Tübingen, and at the nites and cf. Scolicia, show meandering paths Figure 2. Generalized log of part of Hunsrück Deutsches Bergbau Museum, Bochum. The first suggestive of feeding. Slate sequence in Eschenbach-Bocksberg comprises 69 specimens from the Kaisergrube, In the total sample analyzed, infaunal construc- quarry, Bundenbach, including four Konservat- Gemünden (see Seilacher, 1960, 1962; Seilacher tion traces are only slightly more abundant than Lagerstätten. and Hemleben, 1966). The second consists of 39 their mobile counterparts (Fig. 4A). However, specimens accumulated by Bartels over a period field observations of the pervasive occurrence of sions are represented by clay-rich lithologies. Silt- of 30 yr. Additional material in the collections of Chondrites (Fig. 3C) show that construction rich lithologies dominate the higher slate-produc- the Senckenberg Museum, Frankfurt (Richter, traces are by far the most important in the ing horizons in the sequence analyzed in the 1931, 1941), and the Stürmer archive at the Hunsrück Slate. Burrow diameters range from 1 Eschenbach-Bocksberg quarry (Fig. 2) and that in Justus Liebig University, Giessen, together with to 2 mm for Chondrites and Treptichnus and to the Kaisergrube at Gemünden. Beds (representing specimens observed in the field, brings the total 4 cm for Ctenopholeus. The burrows remained individual depositional events) in these silt-rich number analyzed to 120. The precise levels open to the sediment-water interface while they lithologies range from 0.5 to 4.5 cm in thickness. within the sequence from which these fossils were occupied. The fill, which often displays a Sole structures preserved include tool marks, were collected is unknown (nor does the slate- series of subconcentric laminae (particularly in small flute casts, load structures, and biogenic producing process allow systematic sampling), Palaeophycus and Ctenopholeus), is interpreted impact marks. Basal silt-rich laminae more than but field observations have confirmed that Chon- as a product of the circulation of sediment-laden 3 mm thick typically grade within 15 mm into drites and other trace fossils occur throughout the currents (e.g., Goldring, 1996). Simple construc-

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A ECOLOGICAL SYNTHESIS OF THE LOWER DEVONIAN HUNSRÜCK SLATE, GERMANY

Deposition Minor Erosion

Waning turbidity Time current Saltation of benthic Smothering of fauna Colonization by the benthos constructors of Colonization by burrow systems epifaunal animals Firm seabed Passive filling of burrow system

B FORMATION OF LAMINAE IN Possible trace-makers EPIFAUNAL TRACES C FINE-GRAINED TURBIDITES AND 1. Arthropod; 2. Ophiuroid; 1 THEIR TAPHONOMIC 3. Fish; 4. Mitrate; Syn 5. Polychaete; 6. Bivalve SIGNIFICANCE Syn = Syndepositional preservation Turbidity Concave 1 epirelief current Convex Convex Syn hyporelief Boundary hyporelief layer Convex 1 hyporelief Concave Traction epirelief "Petalichnus" "Petalichnus" Monomorphichnus carpet Viscous sublayer 3 2 3 Concave Silt grains accumulate gradually within the 1 epirelief Convex viscous sublayer, forming a silty traction carpet 1 hyporelief at the base of the flow. Clay flocs are Syn disaggregated by shear and concentrate above Syn the silts. A live fauna may become entrained Concave within the flow and transported as saltating bed epirelief load. The traction carpet assists the Expanding preservation of syndepositional trackways. trace Undichna Merostomichnites Allocotichnus Arcichnus

INFAUNAL CONSTRUCTION 5 MOBILE INFAUNAL TRACES TRACES Full relief- Full relief Concave Concave hyporelief epirelief Epirelief Rapidly deposited clay layer Pteridichnites 6 Protovirgularia

Bean-shaped resting trace Convex On reaching a critical concentration, the silty Hyporelief traction carpet suppresses turbulence within the cf. Scolicia flow and causes it to stratify. The 6 hyperconcentrated clays aggregate en masse, Chondrites Ctenopholeus forming a dense clay blanket. The clays set All scale bars and weld onto the silts below. The rapid represent 2 cm deposition and viscous nature of the clays Full relief-Hyporelief encapsulate the benthic fauna, resulting in suffocation (except where organisms escape). 4 Concave Convex 4 Clay aggregates or Concave Silt grains flocs of various sizes Epirelief Concave Full Full relief epirelief Clay lamina relief Heliochone aff. Protovirgularia

Figure 3. Hunsrück Slate ichnofauna. A: Ecological synthesis. B: Formation of laminae. C:Trace fossils. tion traces such as Ctenopholeus (Fig. 3C), a large proportion of epifaunal traces and correspond- abundant epifauna and the distribution and large (>1.2 m in diameter in the Herrenberg mine, ingly reduced proportion of infaunal construction size of constructed burrow systems indicate that Bundenbach) curved to circular burrow system traces where silt dominates. the water column was well oxygenated. The with vertical shafts, were occupied by suspension burial of crinoids in situ and the preservation of feeders. More complex forms such as Heliochone CONCLUSIONS syndepositional trackways show that the fauna (Fig. 3C), a circular or spiral system with radiat- The Hunsrück Slate paleoenvironment sup- of the Hunsrück Slate was alive during the en- ing burrows that terminates in a vertical shaft, or ported a diverse and complex community (Bartels trainment and deposition of the sediment (contra Chondrites, represent deposit feeders. A similar et al., 1998). Where the pyritized soft-bodied Brett and Seilacher, 1991, who inferred that the increase in complexity marks the transition from fossils occur, the slate is characterized by three organisms were killed during transport and accu- suspension to deposit feeding in modern burrow related features: (1) the preservation of thin sedi- mulated passively). Furthermore, not all the indi- systems (Griffis and Chavez, 1988). mentary laminae, (2) an absence of extensive bio- viduals perished during burial. There is evidence When the distribution of trace fossils is sepa- turbation, and (3) a low abundance of mobile of escape traces (Fig. 3C) and syndepositional rated into those in silt- and clay-rich lithologies infaunal traces. In addition, the constructed in- arthropod trackways as well as those preserved (Fig. 4, B and C) it is clear that there is a higher faunal trace Chondrites is very common. The on the tops of the turbidites.

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A A proportion of the organisms overwhelmed McCave, I. N., and Jones, K. P. N., 1988, Deposition of 80 by the turbidites was trapped by setting of the ungraded muds from high-density non-turbulent "Total" ichnofauna turbidity currents: Nature, v. 333, p. 250–252. clays and remained intact due to rapid elimina- 60 Mittmeyer, H.-G., 1980, Zur Geologie des Huns- n = 120 tion of infaunal scavengers (Fig. 3A). Where rückschiefers, in Stürmer, W., Schaarschmidt, F., unusual geochemical conditions prevailed (high and Mittmeyer, H.-G., eds., Versteinertes Leben 40 Syndepositional Fe and low C in the surrounding sediment: im Röntgenlicht: Frankfurt am Main, Verlag Briggs et al., 1996), pyritization of the carcasses Waldemar Kramer, p. 26–33. 20 Richter, R., 1931, Tierwelt und Umwelt im Huns- occurred. In contrast to other Konservat-Lager- rückschiefer; zur Genese eines schwarzen stätten (e.g., Burgess Shale, Solnhofen Lime- Schlammsteins: Senckenbergiana, v. 13, p. 229– 0 stone), there is no distinction between the envi- 242. Epifaunal Infaunal Infaunal mobile construction ronment in which the organisms were living and Richter, R., 1941, Marken und Spuren im Huns- rückschiefer: 3. Fährten als Zeugnisse des Lebens that in which they are preserved. Furthermore, B auf dem Meeres-Grunde: Senckenbergiana, v. 23, 80 the mineralization of soft tissues was possible in p. 218–260. Clay-rich lithologies spite of an oxygenated water column and the Sageman, B. B., 1989, The benthic boundary biofacies 60 presence of a burrowing macrofauna. model: Hartland Shale Member, Greenhorn n = 76 Formation (Cenomanian), Western Interior, North ACKNOWLEDGMENTS America: Palaeogeography, Palaeoclimatology, 40 Sutcliffe’s research was funded by Natural Environ- Palaeoecology, v. 74, p. 87–110. ment Research Council studentship GT4/93/116G. Seilacher, A., 1960, Strömungsanzeichen im Huns- rückschiefer: Notizblatt des Hessischen Lan- 20 Collaboration between Briggs and Bartels was facili- tated by a British-German Academic Research Collab- desamtes für Bodenforschung, v. 88, p. 88–106. oration grant funded by the Deutscher Akademischer Seilacher, A., 1962, Form und function des Trilobiten- 0 Austauschdienst (German Academic Exchange Or- Daktylus: Festband H. Schmidt: Paläontolo- infaunal infaunal epifaunal mobile construction ganization) and the British Council. This is contribution gische Zeitschrift, v. 36, p. 218–227. number 1 within the framework of Project Nahecaris. Seilacher, A., and Hemleben, C., 1966, Beiträge zur C Sedimentation und Fossilführung des Huns- 80 REFERENCES CITED rückschiefers: 14. Spurenfauna und Bildungstiefe Silt-rich lithologies Alberti, G. K. B., 1982, Nowakiidae (Dacryoconarida) der Hunsrückschiefer (Unterdevon): Notizblatt des Hessischen Landesamtes für Boden- 60 aus dem Hunsrückschiefer von Bundenbach n = 44 (Rheinisches Schiefergebirge): Senckenbergiana forschung, v. 94, p. 40–53. Lethaea, v. 63, p. 452–463. Seilacher, A., Reif, W. E., and Westphal, F., 1985, Sedi- 40 Allison, P. A., and Brett, C. E., 1995, In situ benthos mentological, ecological and temporal patterns of and paleo-oxygenation in the Middle Cambrian fossil Lagerstätten: Royal Society of London Burgess Shale, British Columbia, Canada: Geol- Philosophical Transactions, ser. 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E., 1997, An ophiuroid trackway from the cluding Chondrites. A:“Total” ichnofauna. B: raphy: Berlin, Springer-Verlag, p. 283–297. Lower Devonian Hunsrück Slate, Germany: Clay-rich lithologies. C. Silt-rich lithologies. Briggs, D. E. G., Raiswell, R., Bottrell, S. H., Hatfield, Lethaia, v. 30, p. 33–39. D., and Bartels, C., 1996, Controls on the pyriti- Wignall, P. B., 1993, Distinguishing between oxygen zation of exceptionally preserved fossils: An and substrate control in fossil benthic assem- Movement through recently deposited, and analysis of the Lower Devonian Hunsrück Slate blages: Geological Society of London Journal, hence oxygenated, sediment is indicated by ichno- of Germany: American Journal of Science, v. 150, p. 193–196. Wignall, P. B., 1994, Black shales: Oxford, Clarendon taxa such as Pteridichnites and cf. Scolicia. How- v. 296, p. 633–663. Ekdale, A. A., 1985, Paleoecology of the marine endo- Press, 127 p. ever, most of these infaunal traces were produced benthos: Palaeogeography, Palaeoclimatology, Winterfeld, C. V., Dittmar, U., Meyer, W., Oncken, O., by double-anchor techniques, indicating that the Palaeoecology, v. 50, p. 63–81. Schievenbusch, T., and Walter, R., 1994, Krusten- substrate was already sufficiently firm to provide Goldring, R., 1996, The sedimentological significance struktur des Rhenohercynischen Falten- und Überschiebungsgürtels: Göttinger Arbeiten über purchase. This activity was short lived and insuffi- of concentrically laminated burrows from Lower Cretaceous Ca-bentonites, Oxfordshire: Geologi- Geologie und Paläontologie, v. Sb1, p. 182–184. cient to homogenize the sediment. The cohesive- cal Society of London Journal, v. 153, p. 255–262. Wollanke, G., and Zimmerle, W., 1990, Petrographic ness of the substrate, which we attribute to the Griffis, R. B., and Chavez, F. L., 1988, Effects of sedi- and geochemical aspects of fossil embedding in rapid aggregation or setting of clays, is further evi- ment type on the burrows of Callianassa cali- exceptionally well preserved fossil deposits: Mit- denced by the preservation of very fine detail in forniensis Dana and C. gigas Dana: Journal of teilungen aus dem Geologisch-Paläontologischen Institut der Universität Hamburg, v. 69, p. 77–97. epifaunal trackways (Bartels et al., 1998, Figs. 221 Experimental Marine Biology and Ecology, v. 117, p. 239–253. and 224). The construction of open burrows is also Langenstrassen, F., 1983, Neritic sedimentation of the Manuscript received August 20, 1998 favored by rigid sediment (Ekdale, 1985). This Lower and Middle Devonian in the Rheinische Revised manuscript received October 26, 1998 preference is reflected in the dominance of Chon- Schiefergebirge east of the River Rhine, in Manuscript accepted November 19, 1998 drites and in the higher percentage of infaunal con- Martin, H., and Eder, F. W., eds., Intracontinental fold belts: Case studies in the Variscan belt of struction traces in the clay-rich lithologies than in Europe and the Damara belt in Namibia: Berlin, the granular, less-cohesive silts. The development Springer-Verlag, p. 43–76. of a firm substrate, and the onset of anoxia within it, rapidly eliminated mobile infaunal organisms in favor of those that constructed burrow systems retaining contact with the seawater.

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