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Ichnological Evidence for the Environmental Setting of the Fossil-Lagerstätten in the Devonian Hunsrück Slate, Germany

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Ichnological Evidence for the Environmental Setting of the Fossil-Lagerstätten in the Devonian Hunsrück Slate, Germany 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. Geology; March 1999; v. 27; no. 3; p. 275–278; 4 figures. 275 Downloaded from geology.gsapubs.org on 27 September 2009 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
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