Introduction

Middle Palaeozoic reefs appear to have peaked in abundance during the Wenlock and the Givetian-Frasnian with some of the highest levels of reefal development in the entire Phanerozoic (Copper 2002a, b). Major proliferations of tropical carbonate reef environments were triggered by the intense global change as evidenced by palaeoclimate and volcanism, eustatic sea-level and regional tectonics, particularly during the Mid- and Late Devonian. Reefs flourished globally during the Mid-Devonian, expanding in the late Eifelian and reaching their maximum extension during the early to middle-Givetian. Givetian–Frasnian shallow-water carbonate facies of western Europe were deposited as a series of large-scale transgressions over sediments on the southern shelf of the Old Red Continent (ORC). The transgressions were diachronous, reaching the southern parts of the Ardennes/Eifel areas by the earliest Eifelian and the northern parts of the Rhenish Massif by the early and mid-Givetian. Widespread volcanic activity during the Givetian gave rise to submarine ridges and volcanic islands, particularly in the Lahn-Dill area, southeastern Rhenish Massif, separated into the Lahn Syncline and the Dill-Eder Syncline by the Hörre Nappe (e.g. Nesbor et al. 1993; Nesbor 2004, 2008; Königshof et al. 2010). Interactions of volcanic activity and reef development are also known from the Harz Mountains (e.g. Gischler 1995). Reefs became established on submarine rises and formed atolls or table-reef like bodies of smaller sizes of up to 20 km2, such as the Langenaubach/Breitscheid Reef in the Dill-Eder Syncline or the Hahnstätten Reef in the Lahn Syncline (Königshof et al. in press a). Reef development was diachronous and dependent from sea-level changes, tectonics (e.g. block faulting) and volcanic activity during the Mid-Devonian (e.g. Königshof et al. 2010; Flick 2020; Königshof et al. in press b). In some quarries in the southeastern Rhenish Massif large cut walls provide three-dimensional insights into the reef and post-reef history. Some of the reef limestones are covered by early Famennian cephalopod limestones (Krebs 1971, 1974; Oetken 1996). A number of examples show a complex interplay of volcanic activity, reef growth, internal sedimentation, and erosion. Spectacular examples are the Langenaubach/Breitscheid, Gaudernbach, and the Villmar Reef (e.g. Krebs 1974, 1979; Königshof et al. 1991; Braun et al. 1994; Garland 1997).

Long ago, Charles Darwin (1842) claimed that vertical growth on a subsiding foundation caused fringing reefs, which later transformed into barrier reefs and then atolls as the volcanic centre subsided. In the Rhenish Massif a number of different reef types occur but fringing reefs associated with subaerial volcanic buildups are less common. The principal factor that appears to have determined the growth and morphology of fringing reefs is the available accommodation space. Sea-level fluctuations are important, primarily because the sea surface determines the absolute accommodation space for a given reef. This means that reefs established during periods of sea-level rise such as in the Mid-Devonian (e.g. Johnson et al. 1985; Brett et al. 2011). Reefs were able to accrete vertically as space was created above them. If, however, the reef established at, or grew to the sea surface, thereby occupying all the available accommodation space, it could no longer accrete vertically and began to build laterally (Buddemeier and Smith 1988). Reef development during the Devonian required shallow-water facies and, thus, volcanic buildups not only gave rise to reef initiation but also controlled the principal modes of reef growth in deeper water facies settings (as long as they reached the sea-level or above). Nice examples occur in the southeastern Rhenish Massif, where reefs established around volcanic buildups (Lahn Syncline, Fig. 1a, b). The rate and pattern of sea-level change as well as hydro-isostatic effects such as cooling and compaction of volcanic buildups may have affected the ability of a reef to maintain growth close to the ocean surface. An example of this process occurs nearby the village of Balduinstein in the southwestern Lahn Syncline. Here, several sections provide insights into an emerged volcanic island, which was surrounded by a fringing reef.

Fig. 1
figure 1

a: Simplified geological map of Palaeozoic rocks of the Ardennes and the Rhenish Massif with the distribution of reefs in different settings within the shelf. Reef structures at the inner shelf and at the marginal inner shelf have been mainly established on siliciclastic sediments (not exclusively), whereas reef development in the southern Rhenish Massif is linked with volcanic sediments and volcanoes as a result of crustal extension. Cross section A – B see Figure 1b, rectangle shows Figure 3. Modified after Wehrmann et al. (2005) and Königshof et al. (in press b) b Simplified cross section (see Fig. 1; A – B) through the eastern Rhenish Massif during the Middle Devonian to early Famennian, showing the position of reef structures and cephalopod limestones, which developed upon submarine swells in more deeper facies settings on the outer shelf. In the Lahn Syncline reef development is linked with active rifting and volcanism at the outer shelf. ORC = Old Red Continent. Explanations in the text, modified after Nesbor (2019), cross section not to scale

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The fringing reef developed and flourished in the Givetian during inactivity of the volcano but finally was covered by thick felsic volcaniclastics (Flick and Schmidt 1987). Large fragments of land plants with roots were found in argillaceous volcanic epiclastics (Flick et al. 1988). Fringing reefs generally exhibit a relatively simple morphology, which can be subdivided into three zones, forereef, reef crest, and backreef but a detailed record of the Balduinstein sections reveals a complex interaction of sedimentology, reef growth and volcanic activity, which is described in this report. Volcanic rocks have not been the focus of this study but the existence of the Balduinstein volcano was a prerequisite for the establishment of the fringing reef.

Material, methods, and definitions

This research is based on field observations, polished and etched slabs, microfacies analysis and biostratigraphy carried out in spring 2022 at different sites around the Balduinstein Castle, southwestern Lahn Syncline. All units were measured bed by bed at cm scale, and 46 rock samples were taken for microfacies analysis producing more than 80 thin sections and polished slabs. In order to get a thorough overview of the facies, thin sections of 7.5 x 11 cm in size were used. Microphotographs were taken using a Zeiss Discovery V20 stereoscopic microscope and the Canon EOS body with transmitted light, and using 24–85 mm and 100 mm macro lenses. For detailed large-scale images a book2net A1 scanner was used. Characterisation of Fazies Types (FT) is based on Fazies Zones (FZ) after Embry and Klovan (1971), Standard Microfacies Types (SMF) after Flügel (2004), and are compared with other descriptions and facies models (e.g. Strohmenger and Wirsing 1991; Wright 1992; MacNeil and Jones 2016). Thin sections are stored at the Senckenberg Research Institute and Natural History Museum Frankfurt, Germany, under repository numbers SMF 99299 to SMF 99345. Additionally, carbonate samples were dissolved for conodont biostratigraphy according the preparation method described for instance in Ta et al. (2022). Conodont slides are also stored at the Senckenberg Institute.

The definition of reefs used herein is that reefs are biologically constructed by variable consortium of in situ, usually calcareous, benthic organisms. Reefs represent laterally confined, three-dimensional features raised significantly above the sea floor. Buildups and bioherms are often used as synonymous with the definition by Flügel and Kiessling (2002). Two-dimensional features of skeletal builders grown on a single horizontal bedding surface, lacking cavity spaces, cements and cryptic and encrusting biota are described as biostromes (e.g. stratified biostrome). An incipient biostrome represents limestones, which are composed of dwarfed or otherwise disturbed reef-building organisms and did not form true bioherms.

Geological setting and age

During Devonian time, the Old Red Continent delivered siliciclastic sediments into the southern shelf area occupied now by the Ardennes to Rhenish Massif (Fig. 1), and Harz Mountains. The studied section is located at the outer shelf of the Rhenish Massif (“basinal facies setting”) in low palaeolatitudes (approx. 20° to 25° south of the palaeoequator). have been around 20° C to 25° C in the (Joachimski et al. 2009). Intensive volcanic activities accompanied the sedimentation preferably during the Givetian/Frasnian and the Mississippian on the southern shelf of Laurussia, where two fundamentally different types of magmatic evolution characterise the plate-tectonic environment: continental intraplate and subduction-related volcanism. According to Nesbor (2004, 2008) two intraplate volcanic cycles can be distinguished: an alkaline Devonian cycle and a tholeiitic Mississippian cycle. The main Devonian volcanic cycle is composed of two phases: a bimodal main phase in the Givetian/Frasnian and a primitive basaltic late phase in the Famennian. The bimodal main phase was dominated by alkali basalts. Of minor amount are alkali rhyolites, which exhibit a configuration of trace elements that defines them as pantellerites typical for continental rift systems. Sedimentation in the Lahn-Dill area was generally controlled by active rifting, block faulting and volcanism, both reflecting the thinning of the crust due to extensional tectonics (Salamon 2003; Nesbor 2008; Salamon and Königshof 2010; Flick and Nesbor 2021), which was accompanied by coeval reef development (Fig. 1b). According to several authors (e.g. Flick and Schmidt 1987; Nesbor 2004, 2008) volcanic activity of the Devonian cycle culminated during the Givetian-Frasnian time (Fig. 2) and the Lahn-Dill area was the site of several volcanic centres during the Givetian-Frasnian phase. These comprise submarine basaltic edifices that were topped locally by felsic islands.

Fig. 2
figure 2

Composition of melt and temporal development of main volcanic activity during the Givetian-Frasnian in the Lahn Syncline (old and new conodont zonation based on Becker et al. (2016); MN zones sensu Klapper (1989); figure changed after Königshof et al. (2010) in comparison to eustatic sea-level changes in parts of the Mid- Devonian (parts of the curve of Johnson et al. (1985) (in black) and by Brett et al. (2011); explanations in the text); light grey = events

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Each submarine volcanic buildup could reach thicknesses of several hundred metres and multiple buildups could overlap to produce a distinct development of three facies, which are referred to as central, proximal, and distal volcanic facies (Nesbor et al. 1993; Flick and Nesbor 2021). As the Rhenish basin never exceeded a water depth of several hundred metres (a water depth of less than 200 m is quite likely), it seems obvious that the amount of volcanic material was in near equilibrium between eruption supply and subsidence at the outer shelf. Thus, water depth remained rather shallow (Königshof et al. 2010) so that reefs could develop upon volcanic buildups. During the Givetian, reefal organisms such as stromatoporoids, tabulate and rugose corals flourished during times of inactivity of volcanoes.

As a result of eustatic deepening (in some cases related to the Taghanic Event while others are associated with eustatic pulses of the Frasne Event, Fig. 2; see Stichling et al. 2022) and subsidence of volcanic buildups, most reefs in the southeastern Rhenish Massif terminated around the lowermost Frasnian (Ancyrodella rotundiloba pristina Zone or lower Mesotaxis falsiovalis Zone [old conodont zonation], e.g. Braun et al. 1994; Oetken 1996). Drowned reefs were often overlain by condensed cephalopod limestones. Reefal carbonates and volcanic sediments underwent intensive erosion and redeposition, which led to widespread facies differentiation, particularly in the Lahn Syncline, southeastern Rhenish Massif.

The investigated sections are located in the southeastern Rhenish Massif close to the village of Balduinstein (Balduinstein Syncline), approximately 10 km southwest of the town Limburg, Germany. Structurally, this area belongs to the southwestern part of the Lahn Syncline (Fig. 3). The investigated sections # 1 to # 3 are composed of intraplate alkali-rhyolitic lavas, volcaniclastics and limestones of different lithofacies and belong to the Balduinstein Formation, which has a stratigraphical range from the mid-Givetian (Polygnathus rhenanus/varcus Zone = former lower varcus Zone) to the Givetian/Frasnian boundary (Requadt and Weddige 1978; Requadt 2008).

Fig. 3
figure 3

Simplified geological map of the southwestern Lahn Syncline (partly reproduced from Flick and Nesbor (2021), Fig. 3.3.1) and location of investigated reef sections 1 – 3, around the village of Balduinstein

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To the southwest, these successions lead to the alkali-rhyolitic Balduinstein volcano (section # 4), situated on a submarine rise, which was originally formed by basaltic, mostly volcaniclastic rocks (Flick and Schmidt 1987). The Balduinstein Formation is conformably underlain by the Rupbach-Schiefer Formation, which is mainly composed of fossiliferous, argillaceous slightly marly shales with intercalations of limestone lenses. According to the conodont record by Requadt and Weddige (1978) this formation ranges from the late Emsian Polygnathus serotinus Zone to the Givetian Polygnathus rhenanus-varcus Zone.

Conodont samples were taken in order to get a more precise age determination. Unfortunately, only rare conodonts were obtained in the upper part of section 2 (Fig. 4). Most conodont samples were not productive. In sample Bal 2-14 we found mainly the long-ranging taxa Polygnathus linguiformis linguiformis and Belodella but this sample also contains Polygnathus rhenanus and Polygnathus timorensis (Fig. 4).

Fig. 4
figure 4

Conodonts from the base of the Balduinstein section # 2: a Bellodella sp., scale bar = 200 μm; b Polygnathus linguiformis linguiformis Hinde, 1879, upper view, scale bar = 200 μm; c Polygnathus timorensis Klapper, Philip and Jackson, 1970, upper view, scale bar = 100 μm; d Polygnathus rhenanus Klapper, Philip and Jackson, 1970, upper view, scale bar = 100 μm; e Polygnathus linguiformis linguiformis Hinde, 1879, upper view, scale bar = 200 μm

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The latter species is recorded from the basal Polygnathus timorenis Zone and ranges to the Polygnathus disparilis Zone (Aboussalam 2003), whereas Polygnathus rhenanus is recorded from the Polygnathus rhenanus/varcus Zone to the Polygnathus ansatus Zone in Belgium and elsewhere (Bultynck 1987; Liao and Valenzuela-Rios 2008; Narkiewicz and Königshof 2018). The rare conodont findings point to the Polygnathus rhenanus/varcus Zone to Polygnathus ansatus Zone. The Givetian age of the section is also confirmed by the occurrence of the brachiopod Stringocephalus burtini, a classical index fossil of the Givetian stage, which was found in some layers.

Studied sections

The volcanic island and the fringing reef are exposed in several sections in and close to the village of Balduinstein/Lahn (Figs. 5, 6, sections #1 - #4). The reef is approximately 150 m wide and has a thickness of ca. 30 m (section #1). The major non-algal frame-builders in the studied sections were stromatoporoids and corals. Associated fauna mainly consists of algae, ostracods, calcispheres, foraminifera, and crinoids. Bivalves, brachiopods, bryozoans, and trilobites are less frequent and occur in distinct layers, whereas conodonts have been found in one layer only. Stromatoporoids of the Balduinstein Reef exhibit a variety of morphologies, such as dome-shaped, dendroid, finger-like, encrusting and tabular growth forms, which point to different environmental settings (e.g. Stearn 1982a, b; Stearn et al. 1999).

Fig. 5
figure 5

Location map of outcrops of the volcanic island and the fringing reef and field photographs of the sections: # 1 main section below the castle; # 2 calcareous cliff on the other side of the road; # 3 abandoned quarry on the western side of the creek; # 4 Balduinstein volcano. In blue river Lahn with a small island, grey colour marks the village of Balduinstein – compare correlation scheme (Fig. 6)

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Fig. 6
figure 6

Correlation scheme of sections # 1 – # 3. Section # 4 (volcanic rocks) is not shown in this figure. Large blocks of lime mudstone and lime mudstone/alkali-rhyolitic breccia which were found close to section # 3 are integrated in this figure. Yellow rectangles mark the sample positions (thin sections, slabs, conodont samples). Main bioclasts, sedimentary structures and diagenetic fabrics listed on the right side of each column. Section # 1 and section # 2 mainly represent rocks assigned to reef crest and back-reef settings, whereas lithofacies of section # 3 represents lagoonal settings. Stippled lines mark the correlation of the different successions. Regressive/transgressive sea-level changes are marked by yellow arrows

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The sizes of stromatoporoids vary from a few millimetres to massive structures with domes up to several decimetre in diameter, but they did not reach a size up to one metre in diameter or even larger, as observed in other sections in the Lahn Syncline (Braun et al. 1994; Königshof et al. in press a) and in Givetian reefs in southern Morocco (Königshof and Kershaw 2006), among others. Environmental reconstructions indicate that the stromatoporoids lived in warm, well oxygenated waters and generally with low rates of sedimentation, and preferentially grew over substrates of soft carbonates or volcaniclastic material, exposed in several sections in the Lahn Syncline (Königshof et al. 2010, and references therein). Descriptions start from the oldest to youngest units in each section (from the base to the top).

Section # 1 (GPS: N 50° 20`40,1``; E 07° 58`24,3203``± 4 m)

This succession has a thickness of approximately 30 m (Fig. 6) and is located directly underneath the Balduinstein Castle (Fig. 5/1). The succession starts with a lime mudstone to wackestone covered by a lime mudstone/alkali-rhyolitic breccia, which can be correlated with the lime mudstone/alkali-rhyolitic breccia at the top of section # 2. This characteristic rock type was also found west of section # 3 in an isolated block (Fig. 6). Limestones at the base are not contact metamorphosed. Bioclasts include rare corals, stromatoporoids and shell remains, representing incipient reef development (Fig. 7a). The wackestone layers alternate with lime mudstone. Limestones directly underneath the lava flow (Fig. 6) are contact metamorphosed (showing a white colour) and strongly recrystallised. The next unit above the 5.5 m thick felsic lava flow is composed of a wackestone with stromatoporoids and corals, which is overlain by a coral/stromatoporoid framestone. This succession is covered by a volcaniclastic tuff of 21 cm thickness. The overlying succession up to the next volcanic horizon is dominated by framestone lithofacies, rudstone- and grainstone lithofacies are less frequent. Skeletal grains of the stromatoporoid/coral framestone show micrite envelopes or biogenic encrustations; marine cement also occurs (Fig. 7b). Clusters of ferroan dolomite occur in the matrix, around and within skeletal grains, such as in corals (Fig. 7c). The matrix can be a mudstone and/or volcaniclastic material, or it is composed of marine and burial cements. Bioclasts are rare and are composed of crinoids and ostracods. Microbial crusts occur, but they are very thin (1–2 mm). Renalcis growths are rare. Geopetal structures occur mainly underneath stromatoporoids and show marine cement, crystals range from mm-scale to upwards 1 cm.

Fig. 7
figure 7

a Incipient biostrome: reworked main frame-work building organisms (corals and stromatoporoids), in the upper part large tabular morphology of stromatoporoids are dominant, shedding from left to right below (sample Bal 1-1; oriented polished slap); the occurrence of corals and tabular stromatoporoids in fine-grained substrate with pyroclastic fragments point to beginning reef building; b coral/stromatoporoid framestone in a mixed matrix (micritic and sparitic bioclasts show micrite envelopes (sample Bal 1-4); c coral stromatoporoid framestone; clusters of dolomite in the sediment, within the bioclasts (corals, see arrow) and around bioclasts (e.g. crinoid ossicles); (sample Bal 1-10); d plant remains in volcaniclastic rocks. The size of the plant remains are several centimetres in length in average, but also large fragments of up to 10 cm were found (material shown here was collected by Schmidt, Heidelberg, diploma thesis); e coral bafflestone composed of rugose corals Phacellophyllum sp., which belong to the Phillipsastraeidae (sample Bal 1-11); f tangential section of dome-shaped morphology (high domical, non-enveloping) of Actinostroma sp. of the reef crest (stromatoporoid framestone); variations in laminae spacing indicate growth disturbances; growth surfaces exhibit micritization (sample Bal 1-14); g wackestone/floatstone with gastropods, corals, foraminifera; matrix is a pelmicrite and the sample represents more restricted lagoonal settings (sample Bal 1-20)

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The rudstone lithofacies is characterised by laminar and tabular stromatoporoids and corals, such as Thamnopora. The reef building components range from mm-scale up to 4 cm in length and comprise 60 – 70% of the clasts. The rudstone lithofacies contain more than 20% grains larger than 5 mm, some grains are larger than 1.5 cm in diameter. Subordinate bioclasts are brachiopods, gastropods, bryozoans, and foraminifera. The clasts range from subangular to well rounded. Cemented lithoclasts also occur and some are biogenically encrusted. Generally, the sediments are poorly sorted. Graded bedding was not observed. The grainstone and packstone/grainstone lithofacies contain more crinoid ossicles and the size of the component is smaller in comparison to the rudstone lithofacies.

The following 1.5 m thick unit is composed of a volcaniclastic layer at the base and a felsic lava flow at the top, intercalated by a rudstone (Fig. 6). Land plants were found in this section (Fig. 7), which have been also described in earlier publications (Flick et al. 1988). The plant fragments occur in silicified argillaceous volcanic epiclastics and in volcanic debris and can exceed a length of 10 cm. The average length of fragments is about 4 cm. Based on the thickness of the main axis of plants, Flick et al. (1988) calculated a height of the plants of several decimetres. Some fragments are parallel-aligned at the sediment surface, which is most likely a result of currents.

Rooting was observed in two horizons (Flick et al. 1988), but the resampled material does not contain roots. The plant remains are not well enough preserved to allow for a detailed determination (Prof. Uhl, Frankfurt pers. comm.) but branching patterns (striated and monopodially branching axes) point definitely to land plants, such as progymnosperms (Flick et al. 1988).

The overlying unit is composed of autochthonous bafflestone and framestone lithofacies. The bafflestone shows characteristic features of in situ stalk-shaped fossils, which trapped lime mud during deposition. The sediment bafflers are dominated by rugose corals (Phacellophyllum sp.), belonging to the Phillipsastraeidae (Fig. 7e). Less frequent are low-domical stromatoporoids. The fine-grained sediment between the corals and domical stromatoporoids is composed of a micritic matrix and, less frequently, calcareous siltite. The overlying framestone lithofacies exhibits similar reef components. Frame builders are cemented together by marine and early diagenetic cement and lime mud does not play a significant role in this facies type. This unit is covered by a rudstone lithofacies as described above. A 20 cm thick wackestone overlies this succession. Rare bioclasts are composed of brachiopod shells (Stringocephalus burtini), ostracods, and trilobite remnants. This limestone shows bioturbation and a variable amount of peloids. The overlying coral/stromatoporoid floatstone is mainly composed of Thamnopora and stromatoporoids (Stachyodes). Stromatoporoids exhibit laminar and tabulate growth forms, which are preserved in life position. The matrix is a micrite with small ostracod, gastropod, and brachiopod fragments. This floatstone is covered by a coral/stromatoporoid framestone, as described above. Stromatoporoids often show a dome-shaped morphology (Fig. 7f). The overlying packstone/grainstone lithofacies marks the beginning of a deeper facies setting. This unit is overlain by several wackestone and wackestone/floatstone layers (Fig. 7g). These bioclastic wackestones/floatstones contain reef debris, small gastropods, ostracods, rare foraminifera, and fecal pellets. The limestones are in part strongly burrowed. The occurrence of gastropods points to more restricted lagoonal settings. The matrix is a fine-grained pelmicrite. The wackestones are very similar to those described in section # 3. The top of the section # 1 is composed of a stromatoporoid/coral framestone which is covered by a rudstone (Fig. 6)

Interpretation: Section # 1 is dominated by autochthonous framestone lithology, whereas bafflestone occurs in one horizon. Other lithofacies are rudstone, packstone/grainstone, and wackestone. The wackestone above the volcanic rock at the base of this section exhibits reef building components in a fine-grained matrix, which is interpreted as an incipient biostrome. Wackestone at the base of the succession differs from those higher in the section. Whereas wackestone from the lower part shows incipient reef development, the wackestone/floatstone found in the upper part of the section represents lagoonal settings. In comparison to underlying sediments this succession suggests a regression (see Fig. 6).

The morphology of most stromatoporoids found in framestone lithofacies indicates medium-strength water turbulence (e.g. Machel and Hunter 1994; Kershaw and Brunton 1999) or rapid growth (e.g. dome-shaped morphology). The presence of marine cement in void spaces between stromatoporoids and corals suggests reef crest environments (Copper 2002a) with higher energy conditions. The packstone/grainstone lithofacies also points to similar environmental settings. In grainstone lithofacies crinoids are more frequent. Their occurrence points to a high-energy environment (Da Silva and Boulvain 2004; Flügel 2004). The absence of lime mud in these facies provides further evidence of high-energy palaeoenvironment, such as the reef crest or reef front (Wood 1999; Hofmann and Keller 2006). Rudstone lithofacies contains large numbers of Thamnopora, which is consistent with interpretations of high-energy, shallow-water environments (Wood 1999; Hofmann and Keller 2006; John 2012). Coarse gravels of biogenic material and lithified sediment, derived from the top of the reef crest or flanks, are a result of storm events or sea-level rise, and were then sedimented in low-energy settings within the reef. The lime mud between the clasts is consistent with this interpretation. This characteristic rudstone lithofacies of section # 1 corresponds to SMF 6 (packed reef rudstone) sensu Flügel (2004).

In contrast to the high-energy shallow water environments in the middle part of section # 1, a coral/stromatoporoid bafflestone occurs suggesting moderately agitated water conditions in a back-reef position, which is in accordance with facies zone IV-b of Machel and Hunter (1994) and SMF 7 of Flügel (2004). According to MacNeil and Jones (2016) this facies-type represents a metazoan-marine cement fabric. The dark-grey, organic-rich coral/stromatoporoid floatstone in the upper part of section # 1 also suggests a low-energy environment, for instance a back-reef setting (e.g. Malmsheimer et al. 1991). Several wackestone layers occur in the upper part of the section. Lithofacies of some layers show a transition from wackestone to floatstone. The faunal composition and sedimentological criteria (e.g. matrix, partly burrowed sediments, fecal pellets, gastropods) point to a lagoonal setting and correspond to SMF 9 (Flügel 2004).

Section # 2 (GPS: N 50° 20`37,5``; E 07° 58`21,9`` ± 4 m)

Description: Section # 2 (Fig. 6) is approximately 13.00 m thick and is composed of different limestone lithofacies and two thick-bedded felsic lava flow horizons. This section can be correlated with the two others based on the marker horizon of the lime mudstone/alkali-rhyolitic sediment (Fig. 6). The same rocks were found two metres west of section # 3 as a large isolated block on top of a lime mudstone. Section # 2 represents the oldest part of the investigated succession. This section starts with a contact metamorphosed, strongly recrystallized lime mudstone lithofacies. This succession is covered by a felsic lava flow, which has a thickness of 2.30 m (Fig 6). The next interval is covered by vegetation. The section continues with stromatoporoid/coral framestone (Fig. 8a, b) comparable to those in section # 1. In this lithofacies, laminar, tabular, and thick tabular stromatoporoids are the dominant growth forms and are typically several cm thick. Orientation of stromatoporoids is horizontal to sub-horizontal, as in other stromatoporoid-rich lithofacies (e.g. section # 1). This part is overlain by packstone and rudstone lithology (Fig. 8c). The bioclastic stromatoporoid/coral packstone has a matrix of wackestone, with fine-grained skeletal debris, or a lime mudstone. Close packing of lithoclasts is a result of pressure solution documented by stylolite seams. The micritic matrix is adjacent to sparry calcite, formed as cement within the remaining open pores. The following rudstone lithofacies is comparable to those described in section # 1. This lithofacies is characterised by abundant stromatoporoids, rare corals, and other subordinate grains, such as crinoids.

Fig. 8
figure 8

a Stromatoporoid framestone with internal sediment: longitudinal section of Actinostroma sp. with slightly thickness variations in laminae and growth disturbances, reef crest, (sample Bal 2-1); the orange colour between latilaminae are iron-rich precipitations that are most likely a product of hydrothermal activity linked to volcanism; b stromatoporoid framestone (sample Bal 2-3), width of the thin section 5 cm; c stromatoporoid/coral packstone (sample Bal 2-5); close packing of grains has been enhanced by pressure solution (stylolite seams, see arrow); d mudstone to wackestone (sample Bal 2-14); e internal breccia; clast supported reef limestone breccia (sample Bal 2-15); f lime mudstone/alkali-rhyolitic breccia exposed in the upper part of section # 2; it is the same rock type, which was found west of section # 3; (compare Fig. 10b); hammer for scale; g contact between a thick felsic lava flow and contact metamorphosed brecciated reef limestone (sample Bal 2-15) underneath (upper part of section # 2), hammer for scale; h thin section of the felsic lava flow at the top of section # 2

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This lithofacies is overlain by a packstone lithofacies and a packstone/grainstone lithofacies. The following next youngest unit is composed of a mudstone/wackestone lithofacies (Fig. 8d). This lithofacies shows a characteristic stylolaminated to stylocumulate fabric as a result of diagenetic processes. Bioclasts are rare and mainly composed of shell hash (brachiopod and ostracod shells). This lithofacies is comparable to that described in section # 3 and at the base of section # 1. In this unit of section # 2 conodonts were found that indicate a Polygnathus rhenanus/varcus Zone to the Polygnathus ansatus Zone, or mid-Givetian age. The overlying lime mudstone yields the brachiopod Stringocephalus burtini. Overlying sediments are composed of an intraformational breccia (Fig. 8e). Stromatoporoid and coral clasts are subangular, angular to subrounded, and show variable grain sizes, ranging from 0.3 cm to 2.5 cm (Fig. 8e). The internal breccia is overlain by a lime mudstone/alkali-rhyolitic breccia (Fig. 8f). On top of this succession a felsic lava flow occurs (Fig. 8h) and forms a contact metamorphic aureole in the underlying lime mudstone/alkali-rhyolitic breccia with a thickness of 40 cm (Fig. 8f).

Interpretation: Section # 2 represents the oldest part of the studied sections. The uppermost part can be correlated with the other two sections. Sediments represent framestone, packstone, rudstone, and lime mudstone lithofacies alternating with volcanic rocks. The lithofacies of stromatoporoid and stromatoporoid/coral framestone is interpreted to have been deposited on reef crest palaeoenvironments. Stromatoporoids preferably show laminar and tabular growth forms, which grew in internal cavities, that were later filled with sediment. Larger tabular to low domical stromatoporoids, which occur in this lithofacies are interpreted to have generated structures with relief above the substrate (Wendte et al. 2009). The packstone/rudstone lithofacies is interpreted as a high-energy palaeoenvironment close to a deeper reef crest setting described in section # 1. Later, lime mud from low-energy settings infiltrated into the underlying sediment. Evidence for this interpretation is provided by the two matrix types found in these sediments, the lime mud and sparitic cements. A later phase of compaction led to the development of stylolite seams. Overlying sediments belong to mudstone/wackestone lithofacies with rare fossil content (including conodonts), representing more open marine conditions. This unit is overlain by a limestone breccia, which is interpreted as a mass-flow breccia resulting from submarine slides of semiconsolidated sediments derived from the reef crest in more back-reef settings. The youngest part of the section is the lime mudstone/alkali-rhyolitic breccia, which represents a lava flow or collapse breccia, which overspilled into the lagoonal setting. The latter breccia bed can be correlated with a similar bed west of section # 3 and at the base of section # 1 (compare Fig. 6).

Section # 3 (GPS: N 50° 20`28,9``; E 07° 58`17,3`` ± 4m)

Description: This section has a thickness of about 8 m and is primarily composed of lime mudstone and bioclastic wackestone/floatstone (Figs. 5, 6). In the upper part a 1 m thick, grainstone/rudstone lithofacies occurs. Intercalated in this succession are thin layers of volcaniclastic rocks. Felsic lavas are exposed close to this section further to the west (Fig. 6). The base of the section # 3 is composed of lime mudstone. The micritic matrix contains very small shell hash, ostracods, bivalves and gastropods. In one layer, spirorbiform or serpuliform tubeworms occur (Fig. 9a). The lime mudstone shows microbial lamination, which is characterised by planar, mm-scale lamination. Bioturbation occurs in some layers. Some metres to the west, close to this section, several blocks of lime mudstone and lime mudstone/alkali-rhyolitic breccia (Fig. 9b) were found, which can be correlated with section # 2 and section # 1 (see Fig. 6). The lime mudstone is overlain by brecciated limestone (Fig. 9c), volcaniclastic rock and coral/stromatoporoid framestone. This lithofacies is covered by an alternation of wackestone and two volcaniclastic tuff horizons. The latter rocks have a thickness of several cm. These rocks are overlain by stromatoporoid/coral floatstone/framestone (Fig. 9d). This lithofacies is characterised by laminar, bulbous and dendroid stromatoporoids, and tabulate corals. Subordinate bioclasts are bivalves and ostracods. The matrix is a lime-mudstone. A poorly sorted reef rudstone (Fig. 9e) covers this succession. The large coated bioclasts (stromatoporoids, corals and rare crinoids) are poorly sorted, exhibit mainly an angular to subrounded shape and correspond to reef debris. Pores are filled with submarine and burial cements. The next thicker unit is composed of a coral/stromatoporoid floatstone. The dark-coloured matrix is a fine-grained pelmicrite and contains fecal pellets. The bioclasts are micritised and iron-stained (Fig. 9f). Fossiliferous wackestone with bryozoans, ostracods, and rare bivalves cover this interval, which is overlain by coral/stromatoporoid floatstone as before with branched Amphipora. The overlying lithofacies is composed of a bioclastic stromatoporoid/coral rudstone. Grains are micritic lithoclasts, reef dwellers, peloids, and crinoids. Large volcanic extraclasts occur (Fig. 9g). The youngest unit (section # 3, sample Bal 22-3j) is represented by a fossiliferous wackestone with small, flat-growing stromatoporoids. Subordinate bioclasts are ostracods. Peloids and small-scaled, laminated fenestrae, small shell hash, and pyrite grains occur in the pelmicrite matrix (Fig. 9h).

Fig. 9
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Thin sections, polished slab and hand sample from section #3, showing characteristic facies types. a microconchids, tentaculitoid tubeworms with differentiated walls. Note the geopetal in the upper right of the thin section. (sample Bal 22-3a); b lime mudstone/alkali-rhyolitic breccia: the matching fragments reveal disruption by quenching and irruption of calcareous mud into the open spaces. (Sample southwest of section # 3; image width = 30 cm, photo: H. Flick); c collapse breccia: clast-supported breccia (note the corresponding boundaries of many clasts) most probably caused by an earthquake led to fracturing of slightly consolidated rocks. Sediments were brecciated after deposition (small pieces of mud fit well together) and final lithification. Poorly sorted angular to subangular clasts are composed of lime-mud, which are underlain by reef organisms, preferably stromatoporoid- and coral fragments. Some fragments show microbial coating of frame-building organisms (arrow). This succession is overlain by pyroclastic tuffs (polished slap cut vertical, sample Bal 22-3b, upper part); d accumulations of main framework-building organisms (floatstone/framestone). Rugose corals in soft sediment are overgrown by stromatoporoids (on the left side Trupetostroma sp. with tube-shaped corallites of ?Syringopora sp.) Low-energy environment (sample Bal 22-3d); e reef rudstone mainly composed of stromatoporoids, corals and crinoids. Some bioclasts exhibit microbial coating (sample Bal 22-3e); f stromatoporoid/coral floatstone. The dark matrix is composed of lime mud and microbial crusts. Bioclasts are iron stained and bioturbation occurs frequently (sample Bal 22-3f); g stromatoporoid/coral rudstone with rounded pyroclastic extraclasts. High-energy environment, shallow subtidal (sample Bal 22-3i); h wackestone with flat-growing stromatoporoids. Matrix is mainly lime mud with pelloids and irregular and laminated fenestrae (arrow); intertidal environment (sample Bal 22-3j)

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Interpretation: The overall sedimentary succession of section # 3 represents a rather shallow-water, lagoonal setting with intercalations of sediments, which were derived from reef crest or back reef settings (rudstone/grainstone). The section starts with a lime mudstone, which contains microconchids. These organisms colonised various palaeoenvironments but were also associated with reefal, particularly lagoonal facies settings (Burchette and Riding 1977; Dreesen and Jux 1995; Vinn 2006; Zaton et al. 2012; Zaton and Mundy 2020). This interpretation fits well to the other fauna, such as small gastropods and ostracods. Furthermore, the lime mudstone/alkali-rhyolitic breccia found in the vicinity of this section and in section # 2 is interpreted to represent a lava flow, which entered into the lagoonal mud suggesting a contemporaneous occurrence of volcanism and reef growth (Flick and Schmidt 1987; Flick 2020). The branched Amphipora limestone points also to lagoonal facies setting. Reef development and coeval volcanism took place. This is documented in sample Bal 22-3b (upper part), which represents a clast-supported collapse breccia (Fig 9c). Limestone clasts were eroded from the reef crest, whereas the reddish lime-mud belongs to a lower-energy protected environment. The sediments were deposited most probably after an earthquake in a back-reef/lagoonal setting. This rock type could also represent a dome collapse breccia, which is typically associated with felsic volcanoes. Later, the sediments were covered by volcaniclastic rocks and in a second step during a longer break of volcanic activity the soft sediment was settled again by reef organisms.

This succession is overlain by coral/stromatoporoid floatstone/framestone (Figs. 9d). Tabulate corals and small laminar and dendroid stromatoporoids occur in situ in a muddy matrix, accompanied by ostracods, mollusks, sessile foraminifera, and microbial crusts, which prove low-energy, lagoonal environments. Poorly sorted reef rudstone (Fig. 9e) covers this succession. The bioclasts were eroded from a reef crest or back reef setting and redeposited in a lagoonal palaeoenvironment. This could be linked to a transgression or a rather local event such as a tropical storm. Sediments referred to tropical storm layers within a reef are known from other sections in the Lahn Syncline (Königshof et al. 1991). The overlying sediments are composed of a coral/stromatoporoid floatstone (Fig. 9f), which are comparable to the succession described above and are interpreted to represent lagoonal facies (SMF 9, Flügel 2004). Dark-grey fossiliferous wackestone with rare ostracods, double-valved mollusks, and bryozoans represents the next younger lithofacies. A low-energy, shallow-water (lagoonal) environment is most likely. These rocks are overlain by coral/stromatoporoid floatstone/framestone as described before. Stromatoporoid/coral rudstone with volcanic extraclasts (Fig. 9g), represents a rather high-energy shallow-subtidal environment. The youngest succession of this section is represented by wackestone with laminoid fenestral fabric (Fig. 9g) and interpreted as an intertidal setting.

Section # 4

This section is the southwestern prolongation of the section # 3, and is composed of felsic lavas or a collapse breccia and volcaniclastic sediments (Fig. 5), representing the former Balduinstein volcano. These rocks were not the focus of this study but the existence of the Balduinstein volcano was the prerequisite for the establishment of the fringing reef. It is interesting to note that farther to the west, large blocks of limestone and alkali-rhyolite, up to some m3, were reported by Flick and Schmidt (1987), which were interpreted as mass-flow deposits intercalated with volcaniclastic rocks. This succession is more or less completely overgrown, except for small isolated loose blocks, which can be found along the road.

Palaeoenvironment

The studied sections in the southeastern Rhenish Massif exhibit a rock succession, which is characterised by a complex interaction of volcanism, reef development, erosion and sea-level changes. Widespread volcanic activity during the Givetian and Frasnian, which was linked to crustal extension gave rise to the submarine ridges and volcanic islands, particularly at the outer shelf (“basinal facies”) of the Rhenish Massif. Submarine volcanism is known from several places in the Lahn Syncline such as the Philippstein volcano (Nesbor et al. 1993; Nesbor 2004, 2008), which did not grow further than pillow lava stage. In contrast, at the Balduinstein volcano a fringing reef developed. The general development of the studied section is characterised by a variety of carbonate facies settings and coeval volcanism. The Balduinstein volcano was situated on a submarine rise, mainly formed by volcaniclastic rocks (Flick and Schmidt 1987). The findings of land plants (Flick et al. 1988) provide evidence that the volcano was an emergent island. The island existed long enough so that land plants could settle and grow. Volcanism finally covered the plants and plant fragments were incorporated in volcaniclastic sediments.

Further evidence for an emerged island is given by facies analysis. The data provided in this report and earlier publications (Flick and Schmidt 1987) clearly show that a fringing reef surrounded a volcanic island formed by felsic lavas and volcaniclastics. Whether the entire volcano was completely rimmed by a fringing reef as shown herein (Fig. 10) is not known with certainty because of the limited outcrops and tectonic overprint due to the Variscan Orogeny. But it is clear that, at least a part of the volcano was surrounded by a fringing reef. Furthermore, microfacies analysis and rock-types prove that the fringing reef was backed by a shallow lagoon. Branched stromatoporoids, such as Amphipora, and microconchids, tube-bearing encrusting small organisms, which were found in distinct layers are common in lagoonal settings (e.g. Burchette and Riding 1977; Dreesen and Jux 1995; Zaton et al. 2012). Dark-grey, organic rich lime mudstone and wackestone with rare fossil content (e.g. small ostracods and gastropods) also point to lagoonal facies settings, whereas the coeval existing reef crest was mainly composed of stromatoporoids and corals. Evidently, lava did flow directly into the carbonate mud, which confirms contemporaneous occurrence of felsic volcanism and reef growth. Reef growth flourished during periods of inactivity of the volcano, but the reef was covered from time to time by felsic volcaniclastics and, thus, volcanism controlled the modes of reef development in that case. In a later stage the entire region was covered by volcaniclastics of the alkali-basaltic volcanism (Flick and Schmidt 1987).

Fig. 10
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Palaeoenvironmental reconstruction of the Balduinstein Reef and position of sections presented in this report (except section #4, which marks the position of the volcano)

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The Givetian is characterised by a number of eustatic sea-level changes of different scale (e.g. Johnson et al. 1985; Brett et al. 2011) particularly during the rhenanus/varcus Zone to the ansatus Zone, which also coincides with the period of main volcanic activity in the Mid-Devonian in the Rhenish Massif (Fig. 2). The rock succession of section # 3 is generally composed of facies associations that most likely represent lagoonal environments, except in the upper part where packstone and rudstone are the dominant lithofacies. Correlation of this specific unit with section # 1 is difficult. Thus, the general lithological change could be a result of sea-level changes or may be linked with compaction and cooling of the underlying volcano. The upper part of sections # 1 rather points to eustatic changes and may correlate with minor transgressions “Ig” (Giv-3) and/or “Ih” (Giv-4) (Brett et al. 2011), which is proven by a repeated succession of autochthonous reef building carbonates such as bafflestone and framestone alternating with bioclastic rubble deposits (e.g. rudstone and packstone/grainstone). From the microfacies point of view five transgressive phases and one regressive phase were recorded (see Fig. 6). Reef building kept up with sea-level changes during transgressive intervals. A succession of bioclastic wackestone and wackestone/floatstone lithofacies, more than 2 m thick, at the top of section # 1 reflects lagoonal setting, and a regressive trend seems plausible. But it remains unclear whether this regression can be correlated with the pre-IIa regression of Johnson et al. (1985) (mid Tully unconformity of Brett et al. 2011) or whether this is linked with rather local events, such as synsedimentary tectonics or even emersion. Overlying lithofacies types (floatstone and rudstone) point again to a transgressive phase (Fig. 6).

Conclusions

The studied sections provide insights into the development of a Givetian fringing reef, which is associated with coeval volcanism at the outer shelf of the Rhenish Massif. The interaction of volcanic activity, eustasy, erosion, and reef development led to a complex set of facies.

Facies analysis and fragments of plant remains prove subaerial occurrence of a Givetian isolated volcano, which was fringed by a reef. The volcano was situated on a submarine ridge, which was formed by basaltic, mainly volcaniclastic rocks that were traditionally named “Schalstein”.

Fauna, such as the delicately branched stromatoporoid Amphipora, and microconchids, as well as rock-types, such as the mixed lime mudstone/alkali-rhyolitic breccia, suggest that the Balduinstein Reef was backed by a shallow lagoon.

Reef growth flourished during several episodes of inactivity of the volcano, before being covered by felsic volcaniclastics or deposition of volcanic ash. Reef development continued, and in the upper part of the succession, reef development kept up with sea-level changes for a while. Sea-level changes may correlate with eustatic transgressions during the rhenanus/varcus to ansatus zones. In the upper part of the section # 1 a regression occurs, which is proven by lagoonal facies, whereas the youngest succession of floatstone and packstone indicates a transgressional trend (Fig. 6). In a later stage the whole area was covered by products of the alkali-basaltic volcanism.