A eustatically driven calciturbidite sequence from the

FACIES

27 245-262

PI. 50-51

8 Figs.

ERLANGEN 1992

A Eustatically Driven Calciturbidite Sequence from the Dinantian II of the Eastern Rheinisches Schiefergebirge Hans-Georg Herbig and Peter Bender, Marburg KEYWORDS:

STRATIGRAPHY - CYCLICITY - EUSTASY - LIMESTONETURBIDITES - MICROFACIES - RHEINISCHES SCHIEFERGEBIRGE (RHENISH SLATE MOUNTAINS) (GERMANY) - CARBONIFEROUS (TOURNAISIAN, DINANTIAN II, GLADENBACH FORMATION, HORRE BELT)

SUMMARY

The Gladenbach Formation is an approximately 30 m thick, well- segregated calciturbidite sequence, restricted to the HOrre belt of the eastern Rheinisches Schiefergebirge. It is middle Tournalsian in age (lower Pericyclus Stage, lower ed II of the German Culm zonation) and is an equivalent of the Liegende Alaunschiefer. The sequence is composed predominantly of minor turbiditic fining-upward cycles. Cycles start with massive calciturbidite beds. They are composed off'me-grainedintraclastic-bioclasticgrainstone/ packstone, more or less ooid-bearing in the top of the formation, and/or radiolarian-rich packstone. Cycles continue with platy, dense limestones consisting ofradiolarianrich wackestone/packstone and microlithoclastic-microbioclastic wackestone/packstone. Different types of shales finish the fining-upward development. Minor cycles can be grouped into several 4th order cycles, composing a single 3rd order cycle. Towards the top, abundance of resedimented platform components, like ooids, calcareous smaller foraminifers,echinoderms,brachiopods, bryozoans and critical conodont genera, increases. Simultaneously, the thickness of the minor cycles decreases. This indicates a transgressive phase, characterized by increasing overproduction of carbonate on platform realms and a correlated increase in the frequency of resedimentation events in the basin. The transgression corresponds to the welldocumented global eustatic transgression of the Lower crenulata and isosticha-Upper crenulata Zone of the conodont chronology. Thus, the Gladenbach Formation is interpreted as a transgressive systems tract/highstand systems tract. The Liegende Alaunschiefer is the time-equivalent, starved basin facies. Predominating hemipelagic calciturbidites of the lower Gladenbach Formation derive from the deeper shelf slope or from an intrabasinal swell, which might constitute a

flexural bulge in front of the shelf slope. Turbidite sediments from the upper part of the formation derive from shelf-edge sands and the upper shelf slope. The source might be related to the ancient Devonian reef complex of LangenaubachBreitscheid in the southwest. 1 INTRODUCTION The Gladenbach Formation consists of an approximately 30 m thick sequence of dark siltstones, siliceous shales, alum shales and intercalated, mostly turbiditic limestones. The Formation is of Middle Tournaisian age (lower Pericyclus Stage of the German Culm zonation; Lower crenulata Zone, isosticha-Upper crenulata Zone and lower typicus-Zone of the conodont chronology; BENDER & HOMmGHAUSEN1979, BENDER& FIE~IG 1992). The Formation receives its special importance from the limestone turbidites, which represent the earliest allochthonous carbonate input into the Lower Carboniferous basin of the Rheinische Schiefergebirge. The Gladenbach Formation is restricted to the HOrre belt, a 2-8 km narrow, intensively faulted and imbricated tectonic unit separating the Lahn Syncline in the southwest from the Dill Syncline in the northwest. Longitudinally, the H6rre belt extends from the northwestern edge of the Westerwald to the Frankenberger Bucht northwest of Marburg over a distance of almost 50 km (Fig. 1). Only Upper Devonian and Lower Carboniferous sediments have been confirmed up to now. In addition to different facies development in the adjacent synclines and early, Upper Devonian greywacke sedimentation, this gave rise to a nappe concept, interpreting the H6rre belt as a southeast-derived, allochthonous tectonic unit (ENoEL et al. 1983). Based on facies transitions between the H6rre belt and the adjacent, marginal areas of Lahn and Dill synclines (BENDER& BPaNC~MaNN1969, BENDER1978, 1989), as well as microstructural investigations (ENTENMANN1990), a parautochthonous position seems to be reasonable. Recent deep seismic reflection data (F~Nr~ et al. 1990) produced no clear

Address: Priv.-Doz. Dr. H.-G. Herbig, Dr. P. Bender, Philipps-Universitlit Marburg, Institut ftir Geologie und Pal~iontologie, Hans-Meerwein-Strage, D-3550 Marburg

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principles successfully for interpreting the epicontinental South-German Muschelkalk basin. We would like to demonstrate that the first steps of dynamic stratigraphy can be successfully applied on a much smaller scale and to show that it is also a useful tool outside of epicontinental realms. 2 F R A M E W O R K OF THE G L A D E N B A C H FORMATION 2.1 Stratigraphy

Fig. 1. Position of the H6rre belt (stippled) at the eastern margin of the Rheinische Schiefergebirge (white), bordered by Neogene vulcanites of the Westerwald in the SW (random striping) and Mesozoicsedimentsof the Hessedepressionin the east (horizontally ruled). Triangles: known outcrops of the Gladenbach Formation; asterisk: type locality.

results concerning the tectonic position of the narrow H6rre belt. The lithological and biostratigraphical succession of the HOrre belt was elucidated by BENDER(1978) and BENDER& HOMRm~USSN (1979). HOMPJCFL~USEN(1979) and ENTENMANN(199 l) StUdied the siliciclastic sediments of the Upper Devonian and Lower Carboniferous and interpreted the depositional realms and paleogeography. The predominantly calcareous Weitershausen Formation (late Upper Devonian, Upper Hemberg - Wocklum Stages) and the Gladenbach Formation still lacked detailed descriptions. In the course of studies on the biostratigraphy and facies of Lower Carboniferous limestones in the Culm basin of the Rheinische Schiefergebirge, we describe here the lithology and microfacies of the Gladenbach Formation. An attempt is made to interpret the data according to the principles of dynamic stratigraphy (MAvrr~ws 1984) and sequence stratigraphy (VAIL et al. 1977). Biostratigraphic results (conodonts and foranlinifers) will be published separately (BENDER& HERBI~, in press). Like traditional lithostratigraphy, dynamic stratigraphy differentiates and describes sedimentary rocks as elementary building stones of a given succession. Facies analysis of the sediments, however, is the key to assemble facies sequences, which might be cyclic, and - in a next step - to combine them into sequence packages. Interpretation of facies sequences allows the main environmental factors in sedimentation history, like sea-level fluctuations or synsedimentary tectonics to be visualized. Through this process-oriented approach, dynamic stratigraphy yields a better understanding of basin dynamics. AmNF~(1984) used these

The Carboniferous succession of the H6rre belt overlies late Upper Devonian calcareous turbidites and associated marly and silty shales (Weitershausen Formation). It starts with mica-bearing, partly nodular and quartzitic sandstones, intercalated within yellowish to olive-green, siliceous and silty shales. This up to 20 m thick series comprises the Endbach Formation (Gattendorfia Stage). It is overlain by the approximately 30 m thick Gladenbach Formation, which is dealt with here. It consists of diversified, dark shales and intercalated limestones. Greenish-grey, siliceous shales, overlying the Gladenbach Formation, rapidly change into grey, platy, fine-grained sandy shales and siltstones of the Bischoffen Formation (higher lower and middle Pericyclus Stage). In the uppermost part of this approximately 100 m thick Formation, the first greywacke beds indicate the onset of a coarse-grained, up to 300 m thick greywacke succession (Elnhausen Formation). Its stratigraphical extension is unknown (BENDER1989, EYrENMA~ 1990). Platy limestones, intercalated in the predominantly sandy and shaly Devono-Carboniferous succession of the H0rre belt (Urfer Schichten of older authors), were always known as Gladenbach Limestones (KAYsER & H O W L 1894). They were first thought to be Middle Devonian, later Silurian and finally early Upper Devonian (KAYszR 1899, 1907a,b, 1915, K~CEL 1933, ComteNS 1934). Using conodonts, BIscno~ & ZIECLER(1956) proved the heterochrony of the limestones. They differentiated several Upper Devonian limestone levels, today embraced by the Ulmbach and Weitershausen Formations, as well as a Lower Carboniferous (cd II) limestone. ZaZCI.eR(1957) placed the limestones of the cd II at the base of the Schiffelborner Schichten (see also BENDER& BPaNCKM.~m1969, cum lit.). Excluding the siliceous shales at the top, BZNDER& HOMRIGHAUS~(1979) introduced the term Gladenbach Formation for the cd II limestones and associated dark shales. Lateral equivalents of the Gladenbach Formation are the Liegende Alaunschiefer and the basal parts of the Schwarze Lydite (BENDER& BmNCKM~a~'N1969, see also discussion in BRAUN& GURSKY1991). Both units are known all over the Culm facies of the Rheinische Schiefergebirge (ARSErrSGEMEINSCHAFT FL'rR DINANTIUM-STRATIGRAPHIE 1971). 2.2 Occurrence

The Gladenbach Formation is best exposed in its type locality, a small abandoned quarry southwest of height 346.1, east of the town of Gladenbach (Sheet Gladenbach 5217, r 3472440, h 5626270; BENDER & HOMm~rtAUS~ 1979; see our Fig. 1), some 15 km SW of Marburg. This

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locality, still unknown to CORRZNSet al. (1933), was repeatedly studied because of its conodonts and fomminifers, and often mentioned in field trip guidebooks (BtscnoFF 1957, KOCr,~L 1958, P. BENDER in BENDER et al. 1971, ZrEOLER 1971, GROESSnNS et al. 1982, CO~L & P~RO~t 1983). Additional larger outcrops with limestone intercalations are missing. SmaUer, sometimes temporary outcrops (Fig. 1)

were mentioned by BISC~OFF& ZmGLER(1956; localities 1 and 2: track NW Gol3felden, Kalkberg S Sterzhausen), H. BENDER(1960: Ulmbach valley near Greifenstein) and BErbeR & BPdNCKMANN( 1969; Localities 98-100: Caldern, Kirchberg SE Gladenbach, federal road W Bischoffen). Further localities are known in the vicinity of Caldem and Bischoffen (BENDER & HE~m, in press). In all these outcrops, the

248 percentage, as well as the thicknesses of the calciturbidite beds, are lower than in the type locality. This points to very distal settings. Considering the already very distal, finegrained calciturbidite facies observed in the type locality, a study of this section should be sufficient to cover the widest facies spectrum of the formation. Calcareously developed sections of the Gladenbach Formarion are restricted to the axial part of the HOrre belt. Towards its margins, the limestones are gradually, but completely replaced by alum shales and dark siliceous shales. This is, for example, well-observed at the western margin of the H0rre belt, in a quarry at the southern slope of the Streichenberg hill, west of Diedenshausen, some 13 km west of Marburg. Thus, a transition into the facies of the Liegende Alaunschiefer of the adjacent Lahn and Dill synclines is readily proved (KREBS 1968, BENDER & BVaNCKMANN1969).

2.3 Type locality The type locality comprises an approximately 14 m thick succession of black bituminous limestones and intercalated dark shales, both belonging to different lithofacies types (Fig. 2). A few eastward dipping faults intersect the profile (P. BENDERin BENDER et al. 1971, ZIEGLER1971: Fig. 16). The eastern part of the quarry is thrust down at least by 6 m; a correlation with the western, stratigraphically older part of the succession was not possible. The base and top of the formation are not exposed. Rubble of lighter, thin-bedded siliceous shales and very rare microcrystalline limestones at an embankment some 1.5 m above the upper ledge of the quarry already belong to the overlying, basal Bischoffen Formation. An isolated flute cast in the uppermost part of the section (Fig. 2) indicates sediment transport from SW 70 ~ This is in accordance with the general sediment transport in the H0rre belt. Only in the younger siliciclastic sediments of the Kammquarzit and the upper Elnhausen Formation were additional transport directions from NW to SE reported ( H o ~ o n A t : S ~ 1979, E ~ 1991). 5 LITHOLOGY 5.1 Limestones Limestones are the predominating rock type in the studied section of the Gladenbach Formation. The following types can be distinguished: (1) Massive, dark grey to black limestones (P1. 50/1-2) Microfacies: Fine-grained intxaclastic-bioclastic grainstone/ packstone (type A1), ooid-bearing intraclastic-bioclastic packstone (type A2), radiolarian-rich wackestone/packstone (type B 1). Field appearance: Usually 10-30 cm (rarely 60 cm) thick beds, platy, or erosively intersecting the underlying sediment (PI. 50/1-2). Flute casts are extremely rare. Sometimes channel-like interfingering beds form up to 75 cm thick limestone units (PI. 50/2). Small channel structures are probably somewhat reprinted and deformed by the load, as indicated by generally slight erosion of the underlying shale

Fig. 3. Bouma division Td (more or less corresponding to Piper division El) sketched directly from thin-section. Stippled: Radiolarian-richlaminae(microfaciesB 1); white:very Free-grained intraclastic-bioclastic laminae (rnicrofacies A1). Inverse grading in sample 24-2 caused by hydrodynamicallydifferent behavior of echinoderm fragments. Scale: 10 mm. packages (PI. 50/2). Low-angle cross-bedding in the lower part of the beds is replaced by horizontal lamination in the upper part; other beds reveal horizontal lamination and a structureless top or they are completely structureless and dense. The type frequently occurs as a siliceous limestone, especially in the lower part of the section. This seems to be correlated with the abundance of radiolarian-rich wacke-/ packstones observed there. The basal parts of many beds, or bed complexes are often strongly silicified. Macroscopically, the limestone appears very fine-grained to dense. Only in a single bed was a basal pebble layer with intrabasinal components observed. These are often platy pebbles of silicified radiolarian wackestone and microlithoclastic mud-/wackestone up to 15 mm in diameter; mmsized sandstone and tuffite granules and angular detrital quartz grains are associated. Polished sections: Most of the sliced beds show general grading. An idealized bed starts with a very fine-grained sandy basal layer, mostly of microfacies A1. Somewhat bigger, interspersed grains are echinoderm debris. They can also occur in separate layers in higher parts of a bed, thus evoking inverse grading. The basal layer might be structureless or grade into indistinct horizontal bedding, rarely into low-angle cross-bedding. Higher parts of the beds are distinctly densely laminated. This is partly due to interbedcling o f radiolarian-rich laminae (microfacies B 1), which are strongly affected by pressure solution, and intraclasticbioc lastic laminae (m icrofacies A 1) (Fig. 3 ). Without exception, the uppermost layer of the sectioned beds consists of homogeneous mud. Deviating from this idealized sequence, most beds reveal only an indistinctly graded, massive lower part and homogenous top, or a lower laminated division and homogenous top. Only sample 9, a 60 cm thick bed displays a top with an ideally developed, 15 cm thick sequence (Fig. 4). It follows on top of a stylolite surface, which most probably accentuates an internal erosion surface. The lower 45 cm of the bed are very faintly laminated and apparently consist of ungraded silt. However, weathered surfaces in the outcrop show well-developed cross-bedding. Thin-sections from these parts of the bed consist of radiolarian-rich packstone. As opposed to field appearance, polished sections more often reveal the amalgamated character of certain beds,

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Fig. 4. Direct drawings from polished sections. Sample 9: Welldevelopedmassivelimestone(lithology1). Fine-grainedintraclasticbioclastic grain-/packstone with Bouma divisions Tb-e overlying a 45 cm thick, ungradedradiolarian-richpackstone; erosivecontact accentuated by stylolite. Stippled: coarser grained layers rich in echinoderm debris. Sample 20: Homogeneous lime mudstone (lithology 2) interl~reted as mud turbidite fie). Note increasing bioturbation (Planolites) towards the top. demonstrated above for sample 9. Another example is given by the 30 cm thick bed of sample 24. It consists of three graded units, each about 10 cm thick with a more or less horizontally laminated lower division overlain by a homogeneous mud unit. Interpretation: These limestones are considered to represent fine-grained calciturbidites. Fine-grained tttrbidites in general were recently reviewed by STOW & PIPER (1984), PICZ,mUN~et al. (1986), and PIPER& STOW(1991). A review of calciturbidites was presented by TucK~ (1990) and EBmu~(1991). Fine-grained carbonate turbidites, also called biogenic or bioclastic turbidites because of the predominance of redeposited organism fragments, are very similar to their siliciclastic counterparts. However, structural sequences tend to be less well-developed; in general, very f'me-grained calciturbidites tend to be almost homogenous or faintly laminated (Sxowet al. 1984, EBEatta1991). Hydraulic sorting of different components is common. Very f'me-grained sandy and silt-sized turbidites, like those described here, still exhibit typical Bouma divisions (PurR 1978), which are therefore used below. In summary, the following criteria emphasize the turbiditic origin of the described limestones: -- General grading. -- Presence of Bouma divisions, mostly Td-Te, more rarely Tb-Te (Fig. 4). Td laminations (Fig. 3) strongly resemble the E1 division of PiPm~ (1978). -- Concentration of radiolarians in discrete laminae (Fig. 3) due to different hydrodynamic behavior (EINS~E & K~a's 1982) and general replacement of intraclastic-bioclastic grain-/packstone by radiolarian-rich wacke-/packstone in the upper part (Td) of the turbidites (~iogenic grading', cf. E~stn_~ 1991). EBm~ta(1991) also stressed the bimodal distribution between biodetritus and lithoclasts because of differences in density and hydrodynamicbehavior;, lithoclasts

are preferentially concentrated at the base of calciturbidite beds. -- Missing bioturbation. -- Presence of rare flute casts (bottom marks are generally rare in calciturbidites, cf. EBm~u 1991, cum lit.). -- Minor channeling and erosion of underlying beds. -- Exaggerated thickness (up to several tens of centimeters) of fine-grained, homogeneous, radiolarian-rich beds (disorganized turbidites, STow & I>WER1984; ponded mud turbidites, PIPER& STow 1991): missing component and grainsize differentiation fail to produce sedimentary structures (see also Dzui~YNSZa& WALTON1965). -- Strong silicification ofbasalparts ofturbiditebeds (EBr~ta 1991). This is explained by synsedimentaryto early diagenetic cementation by ascending SiO2of free pore spaces within the relatively coarse grained basal turbidite layers. SiO2 most probably originates as skeletal (radiolarian) opal from the directly underlying hemipelagites. Sudden turbiditic sediment influx and resulting load might have caused extrusion of SiO2-rich pore fluids or mobilization of amorphic SiO 2 and subsequent precipitation in open pores of the newly shed calciturbidite. A similar migration of SiO2and formation of chert concretions within the clay-free lower divisions of calcareous turbidites was described by Meischner (1964). The observed siliceous basal layer can be compared with the micritic 'pre-phase' or 'zero-phase' ( M z l s ~ R 1964) at the base of many Devonian and Carboniferous limestone turbidites in the Rheinische Schiefergebirge. The diagenetic generation of that 'pre-phase'was studied in detail by EDER (1970, 1982). (2) Thin-bedded, dense limestones (PI. 50/1) Microfacies: Radiolarian-rich wackestone/packstone (type B1), microlithoclastic-microbioclastic wackestone/packstone (type B2) Field appearance: Black, brown weathering, impure lime mudstones, frequently argillaceous, rarely slightly siliceous. Laminations, clay partings and Planolites occur. The mudstones are usually split into 2-5 cm thin beds. They occur in up to 35 cm thick packages. The uppermost part of the section contains strongly weathered mudstones with faint cross-bedding in up to 20 cm thick beds. They always overly an 1-4 cm thick, dense or laminated siliceous limestone layer, showing a distinct, erosive base. Polished sections: Homogeneous mudstones without any sedimentary structures. Planolites, if present at all, become more frequent towards the top of the beds (Fig. 4). Interpretation: The lithofacies represents silty to muddy calciturbidites, usually consisting of Bouma division Te. A turbiditic origin is assumed because bioturbation is either completely missing or, alternatively, becomes more frequent towards the top of the beds. Thicker beds of faintly cross-bedded lime mudstones with a thin sandy basal layer are considered to represent single turbidite events. Such a feature is typical of proximal m ud-turbidites (En~sEL~1991). 3.2 Shales

Unweathered dark grey to black shales are the next

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Fig. 5. Cyclicity, distribution of carbonate microfacies types and faunas from acid residues, type section of the Gladenbach Formation (lithology strongly simplified). Quadrangles: no faunas. Dots 1-4 indicate increasing faunal frequency: 1: rare (1-25 condonts), 2: moderate (26-100 condoms), 3: frequent (101-250 conodonts), 4: dominant (250->1000 conodonts). important lithology in the studied section (PI. 50/1-2). Strong lithological similarities with the Liegende Alaunschiefer are noteworthy. Calcareous, siliceous and silty to fine-grained sandy shales occur in 0.5-3 cm thick, platy beds. Laminated varieties and varieties with sandy partings occur. Planolites and plant debris are scarce. Rare pure shales are characterized by their fissility. Transitions between all varieties are common. The shales represent basinal hemipelagites with extremely fine-grained detrital influx. The calcareous varieties, always grading from the underlying thin-bedded, dense limestones (Fig. 6) might be still influenced by nepheloid layers of calciturbiditic origin. 3.3 Further lithologies Bedded chert is restricted to an isolated, very thin bed in the lower part of the profile. It represents the autochthonous basinal sedimentation. Fine-grained sandstones occur only as rare, up to centimeter thick layers and are associated with f'me-grained sandy shales. 4 CARBONATE MICROFACIES Two main facies groups, further subdivided into facies types were distinguished according to structure and component contents: (A) Grainstones/packstones with reworked shallow-water platform material. (B) Wackestones/packstones with predominating hemipelagic biota and the absence of shallow-water components. For the distribution of the facies types distinguished within the prof'de see Fig. 5. A1 Fine-grained intraclastic-bioclastic Grainstone/Packstone (P1.50/3, 6-11; 51/5) The microsparitic matrix contains densely packed pseudopeloids (small intraclasts, FAm~AE~Set al. 1974), unidentifiable microbioclasts and small bioclasts. In general, maximum diameters reach 0.1-0.25 mm, exceptionally 0.4 mm. Echinoderm detritus might exceed the size limits because of its special hydrodynamic behavior. Among the bioclasts, fragments of echinoderms and brachiopods (small shells and frequent spines) dominate. Like the generally rare calcareous foraminifers, echinoderms and brachiopods become more frequent in the upper part of the section. Ostracods occur throughout. Echinoid spines, trilobites and conodonts - the latter frequent in solution residues - are rarely observed. Intraclasts, extraclasts and small cortoids are rare, too. Completely micritized cortoids are a potential source for some of the peloids. Extraclasts

251 consist of (in places frequent) detrital quartz and ferruginous shales, which sometimes show enigmatic, centripetally growing, sparitic cement rims (P1. 50/11) The current-oriented components reveal horizontal lamination, rarely faint cross-bedding. Normal grading frequently leads to micritic packstones (P1.50/7). Transitions are characterized by increasingly frequent radiolarians, sponge spicules, and ostracods, i.e. organisms typifying microfacies type B 1. Interbedded laminae of both microfacies types also occur (Fig. 3). Bedding-plane parallel pressure solution laminae (individual microstylolites or microstylolite swarms - PI. 51/5) and microscopically sized stylonodular structures, often accompanied by strong recrystallization, are frequent in very fine-grained and silty varieties of the microfacies type. The microfacies is limited to massive limestones (limestone lithology 1). It is especially frequent in the upper part of the section. A20oid-bearing intraclastic-bioclastic Packstone (PI. 50/4-

5) This coarse-grained variant of the foregoing type A1 is limited to the uppermost part of the studied section, also occurring in massive limestones. Components are 0.3-0.4 mm in diameter. The sparitic matrix contains densely packed micritic intraclasts, ooids, echinoderm cortoids, further undifferentiated extraclasts, and angular quartz grains. Ooids and coated grains might be almost completely micritized. Bioclasts consist of common echinoderms and brachiopods (shells and spines); echinoid spines and calcareous foraminifers occur rarely. B 1 Radiolarian-rich Wackestone/Packstone (PI. 51/1-2) The micritic matrix yields frequent calcified radiolarians, sponge spicules and anidentifiable microbioclasts. Ostracod shells, conodonts, and rare minute echinoderm and brachiopod remains occur. As opposed to the related type B2, microlithoclasts are usually missing. Some thin, elongated mud streaks are interpreted to represent compacted crosssections of Planolites. Horizontal layering, caused by current orientation, is common. Grain size, packing, and identifiable biota often diminish upwards, thus indicating transition into microfacies B2 (microlithoclastic-microbioclastic wackestone/packstone) Strong pressure solution (stylonodular to stylolaminar structure) and recrystallization is common. Frequently, the matrix consists completely of opaque stylocumulate or a f-me-grained mosaic of neomorphic calcite. The microfacies occurs in massive and thin-bedded limestones as well (limestone lithologies 1, 2). B2 Microlithoclastic-microbioclastic Wackestone/Packstone (PI. 51/3-4, 6) Bioclast assemblage is similar to microfacies type B 1. It differs by containing rare or even lacking radiolarians. Trilobite hash, ostracods (P1. 51/6), and supposed crosssections of Planolites occur sporadically. Microbioclasts and microlithoclasts (pseudopeloids, some quartz silt) are common.

Pressure solution (stylonodular and stylolaminar structure) and accumulation of stylocumulate is frequent (cf. L o c ~ & SE~w.r~xa~1976). Especially calcitized radiolarians are strongly deformed and overprinted by partial solution and concommitant precipitation of styloreactate (PI. 51/4). These processes may lead to a secondary, diageneticaUy produced lamination. Pressure solution fabrics similar to the closely related microfacies types B1 and B2 were observed by GtmsKY (1988) in Mesozoic radiolarites, which are quite comparable in component spectrum and grain size. The microfacies is confined to thin-bedded limestones (limestone lithology 2). 5 BIOTA OF THE INSOLUBLE RESIDUES

All limestone samples were dissolved in 10% formic acid. No heavy-liquid separation was done. The organisms encountered in the fraction >64 pan enabled important environmental interpretations to be made, which complete the microfacies analysis based on thin-sections. Distribution of the biota in the studied section is included in Fig. 5. Conodonts, the main stratigraphic index fossils, are frequent in many samples. Siphonodellids predominate. The corresponding siphonodellid biofacies (Ct.~USENet al. 1989) characterizes basinal, pelagic to hemipelagic environments. Only in the uppermost part of the prof'de (from sample 27 onward, cf. Figs. 2 and 5), are some associated gnathodids found. They typify the upper and middle parts of the shelf slope and vanish basinwards near the dysaerobic zone (gnathodid-pseudopolygnathid conodont biofacies of S~'OBERG& GUTSCI-nCK1984; compare alSOGUTSCI-IICK& SANDBERC1983). However, interpretation of the eonodont biofacies remains somewhat problematical in our study, since the first stratigraphic occurrence of Gnathodus coincides with the time span represented in the section. Nevertheless, it is remarkable that Protognathodus, the direct precursor of Gnathodus, and therefore most probably still thriving in the same environment, is missing in the older samples. In contrast, the second marker of the gnathodid-pseudopolygnathid conodont biofacies, the genus Pseudopolygnathus, occurs throughout the profile. Ostracods can be found with changing frequency throughout the section. Spiny forms of the Thuringian ecotype characterize basinal realms. Co-occurring smooth to ribbed forms (Bairdiaceans, Beyrichiaceans) lived in open marine platform areas (BLESS1983). Mixing of both biofacies clearly indicates the importance of resedimentation. Very small brachiopods (spiny shells, isolated spines and shells) are frequent throughout the section Bryozoans are rare and occur only in the upper part of the profile. Their occurrence is distinctly connected with an increased influx of resedimented shallow-water material, indicated by microfacies A1 and A2. Only a few samples yielded rare radiolarians, agglutinated foraminifers, vertebrate remains (teeth and scales, mainly of Selachians) and spores. Sponge spicules can be frequently observed in thin-

252

sections, but they could never be isolated, probably because of their minute size and their fragility. However, together with radiolafians they occur abundantly on etched rock surfaces.

6 CYCLICITY 6.1 Minor cycles Themacroscopically discemedlithologiescan be grouped into two idealized cycle types, which reflect interaction of turbiditic influx and autochthonous basinal sedimentation (Figs. 5, 6). (1) Turbiditic Fining-upward Cycle (Fig. 6/la). A basal massive, fine-grained calciturbidite unit (limestone lithology 1) documents one to (usually) several turbidite events, maximally showing Bouma divisions Tb-Te. Platy calcareous mudstones (limestone lithology 2) follow on top and represent homogenous mud turbidites (re). Calcareous shales on top originated from extremely finely suspended sediment clouds or nepheloid layers. They indicate a transition to hemipelagic basinal sedimentation. Overlying fine-sandy to silty shales still document detrital input. Its further diminuation gradually leads to autochthonous basinal sedimentation, ranging from pure, fissile shales to increasingly siliceous shales and finally to bedded cherts. Renewed calciturbidite influx usually amputated upper cycle mem-

Plate

50

Fig. 1.

Fig. 2. Fig. 3. Figs. 4.-5.

Fig. 6. Fig. 7. Fig. 8. Fig. 9. Fig. 10. Fig. I1.

Fig. 6. Idealized minor turbidite cycles from the Gladenbach Formation (not to scale; for signatures compare Fig. 2). 1: Turbiditic timing-upward cycle, in lb with ommission of thin-bedded lime mudstones and calcareous shales, but generally f'mer-grained, hemipelagic base compared to 1a. 2: Turbiditic coarsening-upward cycle. bers. The thickness of individual cycles usually varies between 50 and 125 cm. An important variant displays neither thin-bedded lime mudstones nor calcareous shales (Fig. 6/lb). They are missing when the basal massive calciturbidite unit already consists of the very fine-grained, radiolarian-rich microfacies type B1, apparently completely absorbing the available carbonate of the cycle. Further grading in the shaly part of the cycle is not influenced. (2) Turbiditic Coarsening-upward Cycle (Fig. 6/2). This very rare type is an inverted fining-upward cycle.The influx of allochthonous carbonate silt increases on top of a basal

Field aspects from the turbiditic limestone sequence of the Gladenbach Formation in its type locality. Microfacies of limestone turbidites (MF-types A1, A2) composed of reworked shallowwater components Boundary between two turbiditic fining-upward cycles. The upper part of the lower cycle is composed of platy lime mudstones (a) overlain by a thin veneer of shales (b). The overlying cycle starts with two massive beds of limestone turbidites (cl, c2 = beds 22, 23 in text-fig. 2). Note erosive base of the lower, cross-bedded turbidite bed cl. Knife for scale. S mall channel, composed of several interbedded, massive limestone turbidite beds, following on top of shales. Note accentuation of the channel by load. Hammer for scale. Conodont in fine-grained intraclastic-bioclastic grainstone (MF-type AI). Sample 22-1, Scale 0.2 mm. Ooid-bearing intraclastic-bioclastic packstone (MF-type A2). Sample 31-2. Fig. 4. Overview. Numerous, mostly micritized ooids, frequently with nuclei consisting of echinoderm fragments. Additionally micritic intraclasts and common angular detrital quartz grains. Scale 1 mm. Fig. 5. Detail. Radial fibrous (recrystallized!) ooids, calcareous smaller foraminifer (lower right), echinoderm fragments, cortoids, intraclasts and abundant detrital quartz. Scale 0.2 mm. Intraclastic-bioclastic grainstone (MF-type AI). Coarse-grained basal layer of a turbidite bed: shell and echinoderm fragments, numerous micritic intraclasts, quartz silt. Sample 26. Scale 1 mm. Fine-grained intraclastic-bioclastic grainstone (MF-type A1) changing abruptly into wacke-/packstone with abundant radiolarians (MF-type B1). Sample 19-1. Scale 1 mm. Transverse section of brachiopod spine. MF-type A1. Sample 25-2. Scale 0.2 mm. Two transverse sections of Earlandia ex gr. elegans (RAUZER-CHERNOUSSOVA8Z REITLINGER1937). MF-type A1. Sample 9-1. Scale 0.2 mm. Cross-section of a thick-shelled ostracod showing characteristic radial-fibrous, calcitic shell, Numerous pseudopeloids and small inlxaclasts (MF-type A1). Sample 19-1. Scale 1 ram. Fine-grainedintraclastic-bioclastic grain-/packstone (MF-type A 1). Numerous,current-orientedpseudopeloids and some larger iithoclasts (angular detrital quartz, shale with cenlripetally growing rim of sparry calcitic cement). Sample 19-2. Scale 1 mm.

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253

254

hemipelagic shale unit, until a massive calciturbidite unit is finally deposited. Such minor coarsening-upward sequences are not well documented in turbidite environments (cf. Pn'ER & STow 1991). Rare examples are described by LASH(1988), who noted uniform siliciclastic mudstone units overlain by siltstone turbidites.

SH

1

Each minor cycle represents a short period of major calciturbidite shedding, followed by gradual stabilization and successive establishment of hemipelagic to pelagic conditions. According to sequence stratigraphy, the minor cycles can be interpreted as 5th order cycles. They correspond to the punctuated aggradational cycles (PACs) of Gooow~ & A>rDERSON(1985).

e

ML

6.2 Superimposed cycles The minor cycles compile 4th order cycles, which reach thicknesses of more than 5 m. They again represent finingupward sequences. The frequency of erosive bases and the thicknesses of the massive calciturbidite units, which generally form the basal part of the included minor cycles, decrease towards the top of the 4th order cycles. By contrast, the higher, more distal units of the minor cycles become increasingly more completely developed and thicker. This development is clarified in Fig. 7, showing the shifting percentages of the fundamental l ithologies of minor cycles during 4th order cycles. The complete sequence of limestone turbidites within the Gladenbach Formation is framed by non-calcareous

Plate

51

Fig. 1.

Fig. 2. Fig. 3.

Fig. 4.

Fig. 5. Fig. 6. Figs. 7.-8.

/

--~

THL

Fig. 7. Paths of 4th order cycles nurnber 1-5 (cf. Fig. 5), showing minor cycles dominated by massive limestone (ML) dominated at the base and minor cycles dominated by shale (SH) or thin-bedded limestones (THL) at the top (circled). e: minor cycles with clearly erosive tops. hemipelagites. It is regarded as a superimposed 3rd order cycle. The abundance of echinoderms, brachiopods and foraminifers increases towards the top of the section. Ooids and frequent cortoids are limited to its uppermost part. Radiolarians and sponge spicules from pelagic environments and the frequency of the corresponding microfacies types B1 and B2, however, decrease (Fig. 5). These observations confirm the increasing resedimentation of shallowwater components. Moreover, conodont biofacies shift from

Microfacies of wacke-/packstones with predominating hemipelagic biota and missing shallowwater components (MF-types B1, B2: Gladenbach Formation and Liegende Alaunschiefer). Aspects of pressure solution Wackestone with abundant sponge spicules, some with axial canal (arrows). Additionally calcitized radiolarianghosts, strongly deformed by pressure solution (dirty white ellipsoids). Matrix strongly affected by pressure solution. MF-type B 1. Sample 25-2. Scale 0.2 mm. Wacke-/packstone with abundant, well-preserved radiolarians and sponge spicules (MF-type B 1). Sample 61. Scale 0.2 ram. Microlithoclastic-microbioclastic wacke-/packstone (MF-type B2). Unidentifiable bioclasts, strongly diminuated, or recrystallized through pressure solution. Some larger lithoclasts, mostly detrital quartz. Sample 1. Scale 0.2 mm. Microlithoclastic-microbioclastic wacke-/packstone (MF-type B2)? Calcitized radiolarian-ghosts, deformed by pressure solution and additionally overprinted by formation of styloreactate. Intensive pressure solution resulted in almost complete differentiation of the matrix into calcitic styloreactate (dirty white) and noncalcareous stylocumulate (black). Sample 8-1. Scale 0.2 mm. Fine-grained intraclastic-bioclastic grainstone (MF-type A1) with microstylolite swarms and resulting enrichment of stylocumulate. Sample 997, locality Bischoffen. Scale 1 mm. Microlithoclastic-microbioclastic wacke-/packstone (MF-type B2) with supposed transverse section of Planolites. Sample 7-1. Scale 1 mm. Graded radiolarian wacke-/packstones from a clay ironstone nodule of the Licgende Alaunschiefer. Sample 060990/17a, Kohlenbcrg near Eifa, about 10 km NNE Biedenkopf. Scales 1 mm. Fig. 7. Single lamina with packing decreasing towards the top and gradual transition into silt-bearing mudstone. Fig. 8. Overview with several laminae. Typical grading of turbidites composed of approximately equal-sized components ("delayed grading with poor separation", Dzt~Y.~SK1& WALTON1965).

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256 basinal towards slope-influenced environments. Thus, the 3rd order cycle seems to indicate a prograding platform. Smaller calciturbidite cycles might be controlled by autocyclic depositional variations or allocyclic variations, such as fluctuations in sea-level or the creation of shallowwater realms by tectonic movements. A discrimination has not been achieved to date (EBERLI1991). For the present case we favor a variation triggered by eustatic sea-level fluctuations. This means, 5th and 4th order cycles would indicate a decrease in sediment supply, i.e. sediment production in the source area. Autocyclic fining-upward caused by the abandoning of feeder/distributary channels, most frequent in generally point-source derived and therefore channelized siliciclastic mrbidite systems, seems to be less probable in carbonates (EBERLI 1991). This is due to the line source principle of carbonate resediments, which are commonly derived from along a carbonate shelf edge. (ScHLAGER& Ct~ERStaK 1979, COOK & MULLINS 1983, MtrLLIYS 1983, MLrLL~4S& COOK 1986). Moreover, the Gladenbach Formation with its microfacies composition, fine-grained appearance with predominance of higher Bouma divisions, faint basal erosion, almost completely missing conglomerates/ pebble layers, high percentage of associated hemipelagites and thin cycles indicates a distal sedimentation away from channels, fans or slopes. Tectonic forcing of the 5th and 4th order cycles cannot be ruled out (EDERet al. 1983), but seems to be unlikely, considering the eustatically driven, superimposed 3rd order cycle (cf. Chapter 7.3). 7 INTERPRETATION 7.1 Depositional realm The facies of the described sediments indicates deposition in a distal turbidite environment dominated by lowdensity turbidity currents. Besides the characteristics listed in chapter 3, turbiditic sedimentation is further emphasized by varying limestone percentages and bed thicknesses between the known outcrops, a feature contradicting pelagic sedimentation. Deposition from contour currents is excluded because of the presence of typical Bouma divisions and the occurrence of massive limestone beds with general fining-upward, which in spite of their fine-grained appearance reach exaggerated thicknesses of several tens of centimeters. The absence of a winnowed micritic matrix and absent to sparse bioturbation further contradicts contourite origin. However, an overprint of fine-grained turbidites is frequent and might be difficult to discern (lhCXERINGet al. 1986). The lateral transition of the Gladenbach Formation into the Liegende Alaunschiefer, in particular, points to a basin plain environment outside the reaches of slopes. Typical features of carbonate slopes (also characterized by finegrained sedimentation), like the predominance of thin-bedded, monotonous periplafform muds, small debrite-filled channels, or slides (Mtmt.rNs & Cook 1986), are not known from the formation. However, spatial limitation of the turbidites to the narrow H0rre belt remains somewhat enigmatic and seems to be controlled by a special topography of

the sea-floor, such as a shallow, channel-like depression (HoMRIGtlAUSEN1979). Besides the well-known stratigraphic equivalence and macroscopic facies connections (cf. 2.2) between the Liegende Alaunschiefer and the Gladenbach Formation, we would also like to describe microfacies similarities here. Early diagenetic clay ironstone nodules from the Liegende Alaunschiefer preserve the otherwise diagenetically destroyed primary bedding. The nodules reveal ram-thick laminae (PI. 5 lf7-8), consisting exclusively of radiolarians embedded within a compact matrix (cf. Boc,oisca et al. 1986, BRAUN 1990). The packing of the radiolarians decreases towards the top of the laminae of the studied nodules. This represents a special turbiditic grading of resedimented equalsized components (DzuLYNSK1& WALTON1965: Fig. 114: 'delayed grading with poor separation'), also described from closely comparable Mesozoic radiolarites (GURSKY1988). This is a strong indication that parts of the Liegende Alaunschiefer might be influenced by turbidity currents. Owing to the biota and sedimentation mechanisms, there is a closer relationship to the radiolarian-rich wacke-~packstones (m icrofacies B l) of the G ladenbach Formation, which, therefore, appear to be equivalents of a more proximal depositional area. 7.2 Position and environment of source

The paleogeographic position of the source of the Gladenbach calciturbidites is difficult to assess due to the frequent deviation of turbidity currents parallel to the basin axis. Owing to the general transport direction in the HOrre belt, one could well imagine a connection with the region of the Devonian Langenaubach-Breitscheid reef complex in the southwest, a still existing paleohigh during the early Lower Carboniferous (KREBS1966). Although no sediments of the early Pericyclus stage are recorded across most of the reef complex, as seen today, parts of it are unconformably overlain by the Liegende Alaunschiefer and a locally developed, black crinoidal limestone (KREaS 1966, 1968). The latter might be a remnant of the source sediment. The complete absence of calciturbidites from the Liegende Alaunschiefer of the southeastern Lahn syncline and the northwestern Dill syncline support this concept of a southwestern source. Reworked components from the calciturbidites allow the source environment and its development through time to be assessed. We rely heavily on the facies model of GtrrscrncK & SA~BERG (1983), dealing with uppermost Tournaisian continental margins of the conterminous United States. Our interpretations are further supported by the generalized facies model of lower Carboniferous carbonate platforms of the Canadian Cordillera and Alasca, worked out by M a ~ T (1976) and AkMs~oYo & MAMET (1977), and recently confirmed by Racnm~DSet al. (in SAr~DOet al. 1990). In the lower part of the Gladenbach turbidite cycle, redeposition of predominant pelagic organisms (radiolarians and conodonts of the siphonodellid biofacies) and deepbenthic organisms takes place - it also characterized the basal parts of turbidite beds. Deep-benthic organisms are

257 represented by ostracods of the Thuringian ecotype and, especially, by sponge spicules. According to R l ~ s et al. (in SAyOOet al. 1990), the latter occur from the upper part of the platform slope down into the basin. MAMEX(1976) and ARMSTRON~& MASWr (1977) noted spiculitic microfacies types especially in the lower part of the slope and the adjacent basin margin. Radiolarians are common in slope and basin sediments, but are absent on the carbonate platform proper (GtrrscmcK & S~oBm~o 1983). Using conodont biofacies in particular, the turbidites in lower parts of the Gladenbach cycle are considered to derive from the lower parts oftheplatform slope or from intrabasinal highs; a subordinate input from higher parts of the slope, sustained by some pseudopolygnathids, cannot be ruled out. The existence of Lower Carboniferous intrabasinal swells, which might constitute flexural bulges between shallow shelf and the basin proper, was already assumed by F R A ~ et al. (1975: Herzkamp syncline, NW Rheinisches Schiefergebirge; see also FRAr~m & WAt~ISER 1983) and JACKSON (1991: NW Devon, southern England). In the upper part of the Gladenbach turbidite sequence, echinoderms,bryozoans and brachiopods are common among the resedimented biota. In addition to sparse calcareous foraminifers, they indicate derivation from a grainstone belt at the shelf edge (MA~-'r 1976, MA~m'r& A~svRor~o 1977, Gtrrscmcz & SAr~BERO1983, R a i D S et al. in SAr~DOet al. 1990). Redeposited ooids in the uppermost part of the sequence, deriving from shelf edge sands (WmsoN 1976), sustain this interpretation. The resedimented biota can be compared with the 'Lower Reef Slope Communities' of MtmD'r (in RAMSBOTrOM1978). They consist mainly of molluscs, frequently spiny brachiopods, crinoids and bryozoans, but no algae. It has to be stressed that remains of spiny brachiopods are common in insoluble residues of the Gladenbach limestones. Accompanying gnathodids, inhabiting the upper and central parts of the platform slope, indicate more or less the same source. Most abundant

pseudopeloids (tiny carbonate lithoclasts) are interpreted to be reworked penecontemporaneous slope deposits (EBEPa_J 1991). Radiolarian-rich muds in the very fine-grained upper part of the turbidite beds were also upstirred from slope environments. In conclusion, the calciturbidites from the upper Gladenbach Formation have been derived from shelf edge environments; uptake of slope material or participation of higher slope derived turbidites is considerable. Very fine-grained distal calciturbidites, comparable to those of the Gladenbach Formation, were first described by Scnota.E (1971) from the Upper Cretaceous of the Apennines (see also STowet al. 1984). Microfacies similarities are striking. However, in the Cretaceous example radiolarians are replaced by pelagic foraminifers, which apparently are closely comparable in size and form. Similar hemipelagic calciturbidites were also described from theUpper Devonian of southern Spain (HERBIO 1985). A greater part of the Devonian to Lower Carboniferous 'Flinz' Limestones of the Harz Mountains, Germany (BucI~to~ et al. 1991) are quite comparable to the Gladenbach calciturbidites. According to ScHotJ_E(1971), hemipelagic turbidites are triggered by two mechanisms. These are destabilization of slope deposits through tectonic movements (see also MOYrANARIet al. 1989), or upstirring of deeper water muds by platform edge turbidites and successive replacement of the coarser-grained material. The eustatic origin of the Gladenbach turbidite cycle which we postulate below might offer a third trigger mechanism. This is overload and subsequent destabilization of periplatform muds deposited on the slope or on intrabasinal swells. Periplatform muds have their origin in the transgressive carbonate overproduction in shallow-water realms. They consist of very fine-grained, suspended carbonate particles, which are swept off the platform edge and can travel with surface currents or along density boundaries within the water column far into the basin (MtrLt~s 1983, Cooz & MtrLL~s 1983; cf. MAS~rI~ et al. 1991).

Fig. 8. Genetic correlation between eustatically-driven shallowing-upward cycles of the platform realm and turbiditic fining-upward cycles in the basin.

258

7.3 Control of the Gladenbach turbidite cycle by eustacy - an evaluation

The Liegende Alaunschiefer, black alum shales with phosphate nodules, represent a starved basinal facies and drowned outer shelves during a well-developed transgression. The relation between the typical middle Tournaisian lithofacies of the Rhenohercynian realm and deepening of the basin, or concommitant transgression was already thoroughly described by KREBS(1968). In Europe, this transgression is also documented from the Saxothuringian (KREBS 1968, cum lit.), Poland (BELr,A 1985), Moravia (KALvOt)A 1989), and from the Northwest European Carboniferous Limestone province as well (RAMsBcrrroM 1973, CONtL& LVs 1977, PAr'ROa3aet al. 1983). It coincides with the global transgression-regression (TR) cycle of the Lower crenulata - isosticha-Upper crenulata Zone, which shows maximum coastal onlap at the top of the latter zone (Ross & Ross 1985, 1987). Since the Gladenbach calciturbidite sequence is the (somewhat longer ranging) equivalent of the Liegende Alaunschiefer, a eustatic forcing of the turbidite shedding seems likely; epecially, since the sequence is considered to represent a 3rd order cycle, which generally represent global TR cycles, if not influenced by synsedimentary tectonics (VAILet al. 1977). Generally, the frequency of limestone turbidites increases during sea-level highstands, because transgressions create extended shallow-water environments. Carbonate produced in excess spills off the shelf edge and is subsequently resedimented on adjacent slopes and basins (MuLLmS in MULL~Set al. 1983, ML~LtNS1983, DROXLER& SC3tLAGER 1985, EaERLI1991). Though accumulation rates of ancient carbonate platforms are much lower than Hotocene rates (which are not corrected for burial compaction,parasequence hiatuses, and other factors), carbonate highstand systems tracts evolved throughout the Phanerozoic (SARG1988) and thus enabled concommitant calciturbidite shedding. Therefore, Recent observations of eustacy - calciturbidite relations also seem to be valid in general for the Paleozoic. The following arguments strengthen the hypothesis of a eustatic forcing of the Gladenbach turbidites: (1) Calciturbidite shedding is directly linked and restricted to a known global eustatic sea-level rise. (2) The influx of shallow-water components increases significantly towards the top of the Gladenbach Formation, thus indicating flooding of a platform realm and marked onset of shallow-water carbonate production. In contrast, redeposition of predominantly hemipelagic material in the lower Gladenbach indicates exposed platform and still relatively low sea-levels (cf. REtrMERet al. 1991). (3) The predominant influx of shallow-water material (microfacies A1, A2, cf. Fig. 5) and distinctly decreased thicknesses of 5th order cycles in the upper part of the studied section can be correlated. This appears to be directly linked to continuously thinning minor shallowing-upward cycles on shallow-shelfrealms in the course of a eustatic TR cycle (JAMES 1984). During a transgression, only a few,

relatively thick minor cycles form, because rising sea-level parallels the aggradation of excessively produced shallowwater carbonates up to sea-level. During a sea-level highstand, slowly rising or slightly falling sea-levels cause unhindered and rapid shallowing-upward and result in thinner cycles. However, this means evidently reduced storage capacity of carbonate ,sedimenton the shelf and more frequent shedding of surplus sediment off the shelf edge. As a result, thin but abundant resedimentation cycles form (Fig. 8; cf. DROXLER & SCHLAGER1985, MASe~ et al. 1991). As a eustatically-driven calciturbidite sequence, facies equivalents of the Gladenbach Formation should be known from elsewhere in the Rhenohercynian Culm basin. In its eastern part, restriction to the HOne belt seems to be controlled by a special basin topography (7.1). A very comparable succession is known from the intrabasinal swell of the Herzkamp syncline (FRANr~et al. 1975), northwestern edge of the Rhenish Culm basin. In the Riescheid section (Wuppertal-Barmen), a few meters of very fine-grained, distal limestone turbidites (unit 3 of ZIMM~RLEet al. 1980) of the isosticha-Upper crenulata Zone (LANE& 7_aEO..ER1978) are intercalated within black shales, representing the Liegende Alaunschiefer. Thus, the limestones are equivalent to the upper part of the Gladenbach calciturbidite sequence and match the global transgressive highstand postulated by Ross & Ross (1987). The scarcity of calciturbidite sequences of this age in the Rhenohercynian Culm basin seems to be related to an important sea-level rise, drowning most of the potential platform sources. Correspondingly, unconformable onlap of the basinal facies of the Liegende Alaunschiefer across parts of the paleohigh of the Langenaubach-Breitscheid tee f complex was described by Krebs (1966, 1968). He also stressed the time-equivalent, short-lived onset of predominantly argillaceous sedimentation on the Lower Carboniferous Limestone shelf from the Velbert anticline to Belgium. In conclusion, Gladenbach Formation and Liegende Alaunschiefer are interpreted as transgressive systems tract/ highstand systems tract. The uppermost part of the Gladenbach Formation, characterized by vanishing calciturbidite influx and thus gradually merging into the shales of the B ischoffen Formation, and the correlative basal part of the Schwarze Lydite on top of the Liegende Alaunschiefer, might represent already falling sea-levels. Shallow-water carbonate platforms with steep slopes and adjacent starved basins seem to be generally frequent during the Upper Tournaisian - Middle Visean (Ross & Ross 1987). The studied example demonstrates that corresponding configurations already existed somewhat earlier, dating the early Pericyclus-stage (Middle Tournaisian). ACKNOWLEDGEMENTS The study originated from a research program concerned with multi-biostratigraphy and facies development of the German Lower Carboniferous culm facies. The program was initiated by the German Subkommission fiir Karbonstratigraphie (Dr. D. Stoppel, Dr. A. Rabitz) and was

259

financially supported by the Deutsche F o r s c h u n g s g e meinschaft (Project Be 591/8-1). Stimulating discussions with Dr. H.-J. Gursky (Marburg) are acknowledged. Priv.-Doz. Dr. C. B m u c k m a n n (Wuppertal) contributed unpublished excursion guidebooks. J. Kitsch (Marburg) prepared the photographs. The valuable comments of two anonymous reviewers improved the paper.

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Manuscript received February 25, 1992 Revised manuscript accepted May 30, 1992