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A eustatically driven calciturbidite sequence from the Dinantian II of the Eastern Rheinisches Schiefergebirge

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The Gladenbach Formation is an approximately 30 m thick, well-segregated calciturbidite sequence, restricted to the Hörre belt of the eastern Rheinisches Schiefergebirge. It is middle Tournaisian in age (lowerPericyclus Stage, lower cd II of the
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  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 - LIMESTONE TURBIDITES - 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 Schiefer- gebirge. 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-up- ward cycles. Cycles start with massive calciturbidite beds. They are composed off'me-grainedintraclastic-bioclastic grainstone/ packstone, more or less ooid-bearing in the top of the formation, and/or radiolarian-rich packstone. Cycles con- tinue with platy, dense limestones consisting ofradiolarian- rich wackestone/packstone and microlithoclastic-micro- bioclastic wackestone/packstone. Different ypes 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 over- production of carbonate on platform realms and a corre- lated increase in the frequency of resedimentation events in the basin. The transgression corresponds to the well- documented 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 sys- tems tract. The Liegende Alaunschiefer is the time-equiva- lent, 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 Langenaubach- Breitscheid 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 HOMmGHAUSEN 1979, BENDER FIE~IG 1992). The Formation receives its special importance from the limestone turbidites, which represent the earliest allochthonous carbonate input into the Lower Carbon- iferous 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 sedimenta- tion, 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~MaNN 1969, BENDER 1978, 1989), as well as microstructural investigations (ENTENMANN 1990), a parauto- chthonous 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  246 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 Mesozoic sediments of the Hesse depression n 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 ENTEN- MANN 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 Carbon- iferous limestones in the Culm basin of the Rheinische Schiefergebirge, we describe here the lithology and micro- facies 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 elemen- tary building stones of a given succession. Facies analysis of the sediments, however, is the key to assemble facies se- quences, 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 syn- sedimentary tectonics to be visualized. Through this pro- cess-oriented approach, dynamic stratigraphy yields a better understanding of basin dynamics. AmNF~ (1984) used these principles successfully for interpreting the epicontinental South-German Muschelkalk basin. We would like to de- monstrate 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 FRAMEWORK OF THE GLADENBACH FORMATION 2 1 Stratigraphy 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, over- lying 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 un- known (BENDER 1989, 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 HOWL 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, BI- scno~ 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 Carbonifer- ous (cd II) limestone. ZaZCI.eR 1957) placed the limestones of the cd II at the base of the Schiffelborner Schichten (see also BENDER BPaNCKM.~m 1969, 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~'N 1969, see also discussion in BRAUN GURSKY 1991). Both units are known all over the Culm facies of the Rheinische Schiefergebirge (ARSErrSGE- MEINSCHAFT 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   47 locality, still unknown to CORRZNS et al. (1933), was repeat- edly 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 locali- ties are known in the vicinity of Caldem and Bischoffen (BENDER HE~m, in press). In all these outcrops, the   48 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, fine- grained 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 For- marion 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 BVaNCKMANN 1969). 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. BENDER n BENDER et al. 1971, ZIEGLER 1971: 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 (Ho~onAt:S~ 1979, E~ 1991). 5 LITHOLOGY 5.1 Limestones Limestones are the predominating rock type in the stud- ied 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 sedi- ment (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-rich aminae (microfacies B 1); white: very Free-grained intraclastic-bioclastic laminae (rnicrofacies A1). Inverse grading in sample 24-2 caused by hydrodynamically different 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 microlitho- clastic mud-/wackestone up to 15 mm in diameter; mm- sized 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 struc- tureless 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 interbed- cling o f radiolarian-rich laminae (microfacies B 1), which are strongly affected by pressure solution, and intraclastic- bioc lastic laminae (m icrofacies A 1 (Fig. 3 ). Without excep- tion, 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,   49 Fig. 4. Direct drawings from polished sections. Sample 9: Well- developedmassivelimestone lithology 1). Fine-grainedintraclastic- bioclastic grain-/packstone with Bouma divisions Tb-e overlying a 45 cm thick, ungraded radiolarian-rich packstone; erosive contact 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 homo- geneous mud unit. Interpretation: These limestones are considered to represent fine-grained calciturbidites. Fine-grained tttrbidites in gen- eral 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 predomi- nance 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, EBEatta 1991). 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 tur- biditic srcin 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 dis- tribution between biodetritus and lithoclasts because of differences in density and hydrodynamic behavior;, 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 (disor- ganized turbidites, STow I>WER 1984; ponded mud tur- bidites, PIPER STow 1991): missing component and grain- size differentiation fail to produce sedimentary structures (see also Dzui~YNSZa WALTON 1965). -- Strong silicification ofbasalparts ofturbiditebeds (EBr~ta 1991 ). This is explained by synsedimentary o early diagenetic cementation by ascending SiO2of free pore spaces within the relatively coarse grained basal turbidite layers. SiO 2 most probably srcinates as skeletal (radiolarian) opal from the directly underlying hemipelagites. Sudden turbiditic sedi- ment 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 SiO 2 and 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' (Mzls~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/pack- stone (type B2) Field appearance: Black, brown weathering, impure lime mudstones, frequently argillaceous, rarely slightly siliceous. Laminations, clay partings and Planolites occur. The mud- stones 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 srcin is assumed because bioturbation is either completely missing or, alternatively, becomes more fre- quent 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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