Introduction

Within the Late Jurassic, the Kimmeridgian stage (157.3–152.1 Ma, Gradstein and Ogg et al. 2012) is considered as a time of global warmth (Valdes and Sellwood 1992; Hallam 1993; Abbink et al. 2001; Sellwood and Valdes 2008). Seawater temperature estimates based on oxygen isotope composition of low-Mg calcite shells indicate warm ocean surface waters in subtropical latitudes (Riboulleau et al. 1998; Malchus and Steuber 2002; Lécuyer 2003; Martin et al. 2014; Nunn and Price 2010; Alberti et al. 2017). Moreover, this interval corresponds to a worldwide second-order transgression, spanning the Late Oxfordian to Late Kimmeridgian (Hardenbol et al. 1998; Hallam 2001; Colombié and Rameil 2007). During this period of sea-level highstand, large parts of Europe were covered by shallow epicontinental seas, allowing the widespread deposition of subtropical shoal–water carbonates (Morgans-Bell et al. 2001; Reolid et al. 2005; Hesselbo et al. 2009; Pearce et al. 2010; Lathuilière et al. 2015). In the Lower Saxony Basin (LSB) of northern Germany, Kimmeridgian strata are represented by a succession of alternating limestone, marl and claystone, which were deposited in shallow-marine and brackish waters on a gently dipping carbonate ramp (Fischer 1991; Weiß 1995; Gramann et al. 1997; Baldermann et al. 2015). These deposits host well-preserved and diverse invertebrate and vertebrate assemblages including remains of fish, crocodilians, turtles, dinosaurs and even mammals (Mudroch and Thies 1996; Sander et al. 2006; Wings and Sander 2012; Carballido and Sander 2013; Jansen and Klein 2014; Marpmann et al. 2014; Lallensack et al. 2015; Gerke and Wings 2016; Martin et al. 2016). Of particular significance was the discovery and excavation of numerous specimens of the dwarf sauropod Europasaurus holgeri from the Langenberg quarry in 1998 (Sander et al. 2006).

However, previous sedimentary investigations of the Upper Jurassic shallow-water deposits in the LSB have focused predominantly on the Oxfordian limestones (Gramann et al. 1997; Helm and Schülke 1998, 2006; Helm et al. 2003; Betzler et al. 2007; Kästner et al. 2008, 2010; Cäsar 2012). In contrast, detailed stratigraphic and sedimentary analyses and correlations of the overlying Kimmeridgian strata are hampered by the difficult age assignment as a result of the notorious lack of open-marine marker fossils, the prevalence of sedimentary gaps and lateral facies changes induced by sea-level fluctuations and/or synsedimentary tectonics (Betz et al. 1987; Gramann et al. 1997; Petmecky et al. 1999; Kley et al. 2008). Various biostratigraphic methods have been investigated in order to provide a better stratigraphic age constraint for the Kimmeridgian deposits, including the rare occurrence of isolated ammonite specimens (Schweigert 1996, 1999) and selected vertebrate remains (e.g. fish teeth) (Karl et al. 2006; Thies et al. 2007; Diedrich 2009), as well as ostracods, foraminifera, charophytes, spores and pollen or dinoflagellates (Schudack 1994, 1993, 1996; Weiß 1995; Luppold 2003; Gramann et al. 1997). Unfortunately, most of these stratigraphic schemes suffer from facies-dependent limitations and provide conflicting results. Ostracod biostratigraphy, in contrast, has been found to be a suitable and useful biostratigraphic tool, especially with respect to the Kimmeridgian and Tithonian intervals under study (Schudack, 1994; Weiß 1995; Gramann et al. 1997). An ostracod biostratigraphic scheme has been established for the Upper Jurassic succession in northern Germany, which enables correlations of the Kimmeridgian strata in the LSB with the Boreal standard ammonite zonation of Hardenbol et al. (1998) (Schudack, 1994; Weiß 1995; Gramann et al. 1997).

The aim of this study is to develop a high-resolution sedimentary and sequence-stratigraphic framework for the Kimmeridgian deposits in the LSB which can be used for both regional and larger-scale regional correlation. Herein, an integrated approach combining carbonate microfacies, sequence stratigraphy and ostracod biostratigraphy is applied to two outcrop sections (Langenberg and Bisperode) located in the southeastern LSB. A sequence-stratigraphic framework is established based on the stratigraphic distribution of carbonate microfacies (MF) types and analysis of depositional environments. The sequence-stratigraphic correlation of the two studied sections, integrated with ostracod biostratigraphic constraints, provides a better understanding of the main factors controlling Kimmeridgian sedimentary evolution in the LSB. In addition, an attempt to compare this record to successions in other European basins is carried out using the integrated stratigraphic results of this study.

Geological setting

The elongate east–west-trending LSB is located on the southern margin of the Central European Basin, with a length of ~ 300 km and a width of ~ 65 km. During Kimmeridgian times, the LSB was located within the Sub-Boreal province (Wierzbowski et al. 2016) and occupied a palaeolatitude of about ~ 35°N (van Hinsbergen et al. 2015), bordered by the Rhenish Massif to the south and the Ringkøbing-Fyn High to the north (Fig. 1a).

Fig. 1
figure 1

a Kimmeridgian palaeogeography and general facies distribution of the Lower Saxony Basin, northern Germany, and adjacent areas (combined and modified after Ziegler 1990 and Pieńkowski et al. 2008). b Location of the study area in northwest Germany with the two outcrops (Langenberg and Bisperode quarries) analysed in this paper

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The evolution of the LSB started in the Permian, created by rifting and/or thermal subsidence (Senglaub et al. 2006). In the Late Jurassic, a shallow epicontinental carbonate ramp began to develop (Gramann et al. 1997). Differential subsidence due to synsedimentary rifting resulted in the development of graben and horst structures (Gramann et al. 1997). Consequently, the thickness of the deposits can vary by tens of metres over a short distance (Hoyer 1965). Stratigraphic sections located in the uplifted horst area may contain fewer small- and medium-scale cycles compared to the adjacent graben area (Kästner et al. 2008).

This study covers the uppermost part of the Korallenoolith Formation and the Lower to Middle Süntel Formation, exposed to the south of Hannover (Fig. 1b). The boundary between the two formations is diachronous, corresponding to the Oxfordian–Kimmeridgian boundary or Early Kimmeridgian (Schudack 1994; Weiß 1995; Gramann et al. 1997; Helm 2005). Reef-bearing carbonates and oolitic limestones of the Korallenoolith Formation are indicative of a shallow-marine subtropical environment with limited terrestrial influx (Betzler et al. 2007; Kästner et al. 2008, 2010; Cäsar 2012). The Early Kimmeridgian part of the Korallenoolith is also known as the Humeralis-Schichten in some areas, since it is partially characterized by mass occurrences of the brachiopod Zeilleria humeralis (Gramann and Luppold 1991; Hoyer 1965; Gramann et al. 1997). The Süntel Formation, which is equivalent to the “Kimmeridge” in older literature (Hoyer 1965; Gramann et al. 1997; Schweigert 1999), is subdivided into the Lower, Middle and Upper Süntel Formations and covers early to late Kimmeridgian time. The depositional environment changed from open-marine platform carbonate towards a more shallow, lagoonal to peritidal setting characterized by strong salinity changes within this interval (Gramann et al. 1997; Mudroch et al. 1999).

A biostratigraphic division of the Upper Jurassic in northern Germany has been established based on ostracod assemblages (Schudack, 1994; Weiß 1995; Gramann et al. 1997). The Kimmeridgian strata in the LSB correspond to the interval between ostracod zones 7 and 15, and the boundary between the Oxfordian and Kimmeridgian stages is located at the base of zone 7. The uppermost Korallenoolith Formation assigned to the lowermost Kimmeridgian (Humeralis-Schichten) corresponds to zones 7 and 8. The Lower Süntel Formation covers the interval up to the top of zone 12, and the Middle and Upper Süntel Formations encompass zones 13 to 14 and zones 15 to 16, respectively (Fig. 2).

Fig. 2
figure 2

Lithostratigraphic scheme of the Late Jurassic in the Lower Saxony Basin, NW Germany, with the numerical age (Gradstein and Ogg 2012), as well as the standard zonation of the Boreal ammonite division (after Gramann et al. 1997) and ostracod zonation (after Schudack 1994; Weiß 1995)

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Materials and methods

The Langenberg section (51°54′6.74″N, 10°30′27.73″E) is accessible in an active quarry about 5 km east of Goslar (Fig. 1b). The Bisperode section (52°04′00.09″N, 9°32′36.47″E), 60 km to the northwest, can be studied in an active quarry located about 2 km southwest of Lauenstein (Fig. 1b).

Both sections were logged and sampled bed-by-bed. A total of 443 samples (Langenberg: n = 266; Bisperode: n = 177) were collected in stratigraphic order, resulting in an average resolution of three samples per metre. The carbonate content of all samples was measured using a LECO CS230 carbon–sulphur analyser (LECO Corporation, Saint Joseph, MI, USA) after combustion of the samples in a high-frequency furnace at about 2000° C under an oxygen stream in the laboratories of the Federal Institute for Geosciences and Natural Resources (BGR). Macroscopic description of hand specimens was carried out, providing lithological and sedimentological information for the selection of samples for microfacies analysis. Carbonate microfacies analysis of 143 petrographic thin sections provides a semi-quantitative compilation of the main skeletal and non-skeletal grains (Flügel 2004) and of textural features. Classification follows Dunham (1962) and Embry and Klovan (1971). Facies belts and sedimentary models of Wilson (1975) and Flügel (2004) are used for interpretation of the depositional environment. The methodology and nomenclature proposed by Strasser et al. (1999) for shallow-water carbonates are used for sequence-stratigraphic interpretation. Following the approach of Kerans and Tinker (1997), facies proportion diagrams are established, which illustrate superimposed facies trends with stratigraphic height.

For ostracod taxonomy and biostratigraphy, 37 samples were collected from clay-rich horizons (Langenberg: n = 25; Bisperode: n = 12). Fresh samples (0.6–1.4 kg) were dried and soaked in hydrogen peroxide, disaggregated in water and subsequently washed through a 100-μm sieve. The residue was fractionated and quantitatively handled in a test sieve with a mesh width of 1.6 to 0.2 mm. The stratigraphic interpretation of the ostracod assemblages follows the schemes of Schudack (1994) and Weiß (1995) (Fig. 2).

Results and interpretation

Lithostratigraphy

Langenberg section

The Langenberg quarry provides a well-exposed section of Upper Jurassic shallow-marine deposits covering ~ 180 m, which was overturned due to tectonic uplift along the Harz boundary fault and now dips at an angle of 50–70° towards the south (Fig. 3a). Here, only the Kimmeridgian interval (uppermost 82.5 m) was investigated, which corresponds to bed numbers 24–154 in Fischer (1991) (Figs. 4 and 5).

Fig. 3
figure 3

Field view and sedimentary characteristics of the exposed Kimmeridgian successions at the Langenberg and Bisperode quarry. a General view of the Langenberg section with the mark of strata showing images c, d and e (red star) as well as strata where the dinosaur (Europasaurus holgeri) was discovered (yellow star); b general view of the Bisperode section with the location of images f, g and h (red star); c hardground with abundant oysters attached, 8.5 m in the Langenberg section; d Thalassinoides bioturbation, 18.9 m in the Langenberg section; e hardground with borings and oysters attached, 20 m in the Langenberg section; f hardground with wave surface and borings, 1.5 m in the Bisperode section; g transition from shale to sandstone, 16–16.5 m in the Bisperode section; h Thalassinoides bioturbation, 23.3 m in the Bisperode section

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Fig. 4
figure 4

Legend for figures in this paper

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The Humeralis-Schichten (0.0–21.0 m) represent a heterolithic unit composed of dark-brown sandy dolomite containing abundant shell debris with intercalated marl (0.0–5.3 m), thick-bedded oolitic limestone (5.3–12.0 m) and thin-bedded, dark-grey bioclastic limestone alternating with fossiliferous marl (12.0–21.0 m) (Fig. 5). Black-stained ooids and intraclasts (diameter < 2 mm) appear in the interval overlying the oolitic limestones. Upper bedding surfaces of oolitic and bioclastic limestones show signs of firm/hardground formation with colonization by oysters (Fig. 3c) and vertical borings (Fig. 3e). Intense bioturbation is indicated by the dense meshwork of Thalassinoides burrows (Fig. 3d). Above, the Lower Süntel Formation (21.0–36.5 m) is composed of well-bedded, grey bioclastic limestone with dark argillaceous interbeds (21.0–32.7 m) passing up-section into greenish sandy limestone (32.7–33.8 m) and greenish dolomitic limestone (33.8–36.5 m). The overlying Middle Süntel Formation (36.5–82.5 m) is dominated by well-bedded micritic limestone containing no or rare bioclastic material. The middle interval (54.2–66.4 m) is composed of fine-grained pale-grey limestone showing a distinct nodular appearance. Rare float- to rudstone layers comprising gastropod accumulations occur in the uppermost part (63.4–66.4 m). Between 66.4 and 68.8 m, a conspicuous dark-brown dolomitic bed occurs (“Wasserbank” of Fischer 1991). Above, six conglomeratic layers are exposed (70.7–80.2 m), varying between 0.1 m and 1.4 m in thickness. Scattered sub-angular to sub-rounded clasts (size: 1–20 mm) occur embedded in a fine-grained micritic matrix (73.7–77.9 m) or form a dense, clast-supported fabric (78.8–80.2 m). Intercalated pale, micritic limestone beds show abundant vertical tube-shaped structures interpreted as Skolithos burrows (76.9–78.8 m).

Fig. 5
figure 5

Integrated log of the Langenberg quarry showing lithology, texture, microfacies types and semi-quantitative results of component analysis and carbonate content, as well as the correlation with the bed numbering in Fischer (1991)

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Overall, carbonate content is high and varies between 13 and 99% (Fig. 5). The highest carbonate content (84–99%) is associated with the fine-grained limestones in the Middle Süntel Formation, whereas fluctuating and overall lower carbonate content occurs in the limestone–marl alternations of the Upper Humeralis-Schichten and Lower Süntel Formation.

Bisperode section

The active quarry comprising the Bisperode section exposes a ~ 160-m-thick succession covering the Korallenoolith and lowermost Süntel Formations. This study focuses on the uppermost part (55 m) constituted by the Humeralis-Schichten and Lower Süntel Formations (Fig. 3b).

The base of the studied section is composed of a 1.5-m-thick oolitic limestone (Figs. 4 and 6). This limestone is assigned to the Upper Korallenoolith Formation, which shows a well-developed hardground with an undulating surface and vertical, sediment-filled borings (Fig. 3f). The overlying Humeralis-Schichten (1.5–11.5 m) comprises mainly bioclastic limestone with marly interbeds, containing fragments of oysters, Trichites, serpulids and minor brachiopods. Above, the Lower Süntel Formation (11.5–54.3 m) is dominated by thick-bedded bioclastic and micritic limestone intercalated with sandy and clayish intervals. A dark-grey shale (13.0–16.2 m) with intercalated centimetre-thin sand layers occurs in the lower part, which is capped by a conspicuous, ochreous, fine-grained sandstone showing well-developed wave ripples and convolute bedding. The middle interval (16.9–43.2 m) is composed of alternations of bioclastic and micritic limestone separated by marly interbeds. Oysters, Trichites and rare brachiopods occur within several fossiliferous floatstone layers, some of which show well-developed Thalassinoides burrows (Fig. 3h). At 35.0 m and between 36.5 and 39.5 m, massive oolitic limestone with well-developed cross-bedding occurs. Up-section, the remaining part of the Lower Süntel Formation (43.2–54.3 m) is composed of fine-grained marl and claystone with intercalations of thin-bedded bioclastic and partly sandy limestone. The lowermost interval (43.2–49.6 m) is separated by a thin-bedded, yellowish limestone layer and shows conspicuous rhythmic changes in colour. Above, this interval is capped by a package of sandy and oyster-bearing limestone (49.6–51.1 m), which is overlain by an alternation of thick-bedded marl and thin-bedded micritic limestone (51.1–54.3 m).

Fig. 6
figure 6

Integrated log of the Bisperode quarry showing lithology, texture, microfacies types and semi-quantitative results of component analysis and carbonate content. Ostracod samples from the present study are marked with red arrows, and those from the work of Weiß (1995) with blue arrows

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The carbonate content shows significant variation, ranging between 2 and 96% (Fig. 6). Low carbonate content (< 50%) marks the clay-rich intervals (13.1–17.2 m; 43.2–54.2 m). The highest carbonate content occurs in the oolitic facies at the very base, and the thickest interval around 38 m.

Biostratigraphy

Conventional stratigraphic marker fossils (e.g. ammonites) are rare in the Upper Jurassic formations of the LSB due to the shallow and restricted nature of the deposits (Fischer 1991; Gramann et al. 1997). Instead, age assignment is based on ostracod assemblages in this study. Based on the ostracod zonation of Schudack (1994) and Weiß (1995), zones 6 to 14 are recognized in the two sections, encompassing the Humeralis-Schichten and the Lower and Middle Süntel Formations (Figs. 5 and 6). The ostracod biostratigraphy of the Langenberg section is based on new data (this study). For the Bisperode section, it refers mainly to the results of Weiß (1995), with 11 additional new samples from this study.

Zone 6 is only constrained to the lowermost 1.5 m of the Bisperode section (Weiß 1995), which is characterized by only two species, Galliaecytheridea dissimilis and G. mandelstami, and assigned to the Late Oxfordian. Ostracod zones 7–8 are characterized by the species Procytheropteron decoratum, Macrodentina pulchra, Paranotacythere interrupta and M. lineata, indicative of the earliest Kimmeridgian. Humeralis-Schichten corresponds to zones 7 and 8 in the Langenberg section, but it is limited only in the lower part of zone 7 in the Bisperode section. Zone 8 is much more expanded in the Bisperode section (21.2–43.2 m) than in the Langenberg section (16.7–21.0 m). The Süntel Formation is constrained by the ostracod genus Macrodentina, which is adapted to both marine and brackish waters (Weiß 1995). Ostracod zones 9 to 12, corresponding to the Lower Süntel Formation, are characterized by the following taxa: M. intercostulata, M. pulchra and M. lineata. The Bisperode section comprises only ostracod zones 9 and 10. Zones 11 and 12, which are stratigraphically higher, are not determined in this section. The boundary between zones 9 and 10 is adjusted from 48.5 m (Weiß 1995) to 47.3 m, since a new sample 47.6 m (Pr.12, ESM_2) from the present study is defined as zone 10. In the Langenberg section, ostracod zone 9 (21.0–32.7 m) is present in the Lower Süntel Formation, whereas ostracod zones 10 to 12 are lacking, probably due to erosion. The overlying ostracod zones 13 to 14 in the Langenberg section correspond to the Middle Süntel Formation, characterized by the ostracod species M. wicheri, M. rudis, M. ornata and Rectocythere iuglandiformis. The biostratigraphic boundary between the Lower and Middle Süntel Formations is constrained by findings of the species M. steghausi. Additional data for ostracod assemblages are given in supplementary material EMS_1 and EMS_2.

Microfacies associations

In total, 19 MF types have been differentiated for the Langenberg and Bisperode sections combined. MF types are described in Table 1 and illustrated in Figs. 7 and 8. All MF types are attributed to seven microfacies associations (MAs) representative of characteristic depositional environments (facies belts) ranging from mid-ramp to back-ramp settings. These MAs are described below from distal to proximal and in relation to their depositional characteristics.

Table 1 Microfacies association and microfacies types in the present study
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Fig. 7
figure 7

Microfacies types of Kimmeridgian successions in the study area. a MF type 2, wackestone with ramp-derived intraclasts, 16.2 m, Langenberg section. b MF type 3, bioclastic wacke to floatstone with oysters and serpulids, 13.9 m, Langenberg section. c MF type 4, bioclastic wackestone with abundant echinoderms and other bioclastic material, 9.8 m, Bisperode section. d MF type 5, bioclastic grainstone with abundant fossil fragments, 18.3 m, Bisperode section. e MF type 6, oolitic grainstone, 7.1 m, Langenberg section. f MF type 7, peloidal bioclastic wacke- to packstone with ostracods and foraminifera, 45.3 m, Langenberg section. g MF type 8, bioclastic floatstone with gastropods, 51.6 m, Langenberg section. h MF type 8, bioclastic wackestone with bivalves and gastropods 50.8 m, Bisperode section. i MF type 9, 17.3 m, sandy limestone with large mollusc fragments, Bisperode section

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Fig. 8
figure 8

Microfacies types of Kimmeridgian successions in the study area. a MF type 10, pelletal grainstone, 72.7 m, Langenberg section. b MF type 11, mudstone, 1.4 m, Bisperode section. c MF type 12, peloidal sandy limestone, 33.7 m, Langenberg section. d MF type 13, limestone conglomerate, 73.8 m, Langenberg section. e MF type15, wackestone with charophytes and ostracods, 54.3 m, Langenberg section. f MF type 15, wackestone with charophytes, 41.8 m, Bisperode section. g MF type 16, charophyte-rich marlstone, 38.8 m, Langenberg section. h MF type 18, evaporite–dolomite couplets, 34.1 m, Langenberg section. i MF type 19, dolomitic mudstone, 67.5 m, Langenberg section

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MA1: Mid-ramp depositional setting

This facies belt is present in the lowermost part of ostracod zone 7 in Bisperode and in ostracod zones 7 to 8 in Langenberg (Figs. 5 and 6). MA1 shows a gradual shift into MA2. It comprises two MF types including thin-bedded marlstone with bioclasts (MF type 1) and wackestone with ramp-derived intra- and/or bioclasts (MF type 2) (Table 1; Fig. 7a), with MF type 2 being restricted to the Langenberg section.

Some intraclasts show truncated ooids along the edge (Fig. 7a), evidence of erosion of the intraclasts from a landward oolitic shoal, most probably during episodic storm events. Small bioclastic debris finely dispersed within the matrix indicates calm hydrodynamic conditions below the fair-weather wave base (FWWB). Combined with the dense meshwork of Thalassinoides burrows, MF type 2 indicates reworking and low sedimentation rates in a mid-ramp setting (Flügel 2004; Betzler et al. 2007). Accumulation of brachiopods and strong bioturbation in the marlstone matrix of MF type 1, integrated with the gradual contact relationship with MF type 2, indicate open-marine conditions adjacent to MF type 2 (Strasser et al. 1999; Kästner et al. 2008).

MA2: Distal inner-ramp depositional setting

This facies belt is represented in the lower part of both sections (Figs. 5 and 6). MF types of MA2 include bioclastic wacke- to floatstone with brachiopods, oysters and serpulid debris (MF type 3) (Table 1; Fig. 7b), as well as bioclastic wacke- to packstone with abundant echinoderm fragments and other bioclastic material (MF type 4) (Table 1; Fig. 7c). This facies belt shows accumulations of skeletal debris (e.g. brachiopod and oyster shells), which result from episodic storm events. The dominant components in MF type 3 can also be found occasionally in MF type 4 and vice versa.

Both the overall high diversity of fossils and abundance of echinoderm fragments indicate a medium-energy, open-marine environment with normal salinity (Flügel 2004). Rare findings of fragmentary ammonites and remains of marine crocodilians in the Langenberg section (Fischer 1991) confirm its open-marine character. Serpulids encrusting oyster shells indicate a low- to moderate-energy environment with low sedimentation rates (e.g. Schmid 1996; Krajewski et al. 2016), which is in accordance with findings of authigenic glauconite (Betzler et al. 2007; Baldermann et al. 2012). MA2 is typically intercalating with shoal deposits, thus reflecting a shallow subtidal setting between shoal and FWWB on the distal inner ramp (equivalent to the off-shoal facies belt of Cäsar 2012). Subsequent dolomitization of the matrix in the lowermost part of the Langenberg section (0–5.3 m) results in a dolomitic cement, which formed at shallow burial depth by replacement (Baldermann et al. 2015).

MA3: High-energy shoal depositional setting

This association occurs in several intervals at both localities and comprises bioclastic pack- to grainstone with abundant fossil fragments (MF type 5) and oolitic pack- to grainstone (MF type 6) (Table 1; Figs. 5, 6 and 7d, e).

Ooids form in agitated waters influenced by waves and currents within shallow-marine subtropical waters above the FWWB (Tucker and Wright 1990; Flügel 2004; Betzler et al. 2007), which is further confirmed by the appearance of cross-bedding. Well-sorted bioclasts in high diversity and a pack- to grainstone texture further indicate moderate- to high-energy shallow-water conditions of a shoal setting for MF type 5 (Wilson 1975; Flügel 2004). Based on comparison with modern analogues (Hine 1977; Burchette et al. 1990; André et al. 2003), MA3 is interpreted to represent a submarine oolitic-bioclastic shoal in a storm- and/or tide-influenced subtidal setting, located next to MA2 (Betzler et al. 2007; Kästner et al. 2008; Cäsar 2012).

MA4: Semi-restricted lagoon depositional setting

MA4 is present in the middle and upper part of both sections and is typically overlain by MA5/6 (Figs. 5 and 6). MA4 incorporates three MF types (Table 1; Figs. 7f–i) including peloidal bioclastic wacke- to packstone with ostracods and foraminifera (MF type 7), bioclastic wacke- to floatstone with bivalves and gastropods (MF type 8), and sandy limestone with large mollusc fragments (MF type 9), the latter being restricted to a small number of thin horizons in the Bisperode section.

The presence of a low-diversity assemblage (e.g. miliolid foraminifera and ostracods) and sparse echinoderm fragments and ooids in MF type 7 reflects a moderate- to high-energy near-shoal environment (Flügel 2004; Colombié and Strasser 2005; Cäsar 2012). Randomly oriented recrystallized molluscan shell fragments embedded within a micritic matrix (MF type 8) indicate a semi-restricted lagoon setting with moderate hydrodynamic conditions (Wilson 1975; Flügel 2004). In addition, mass occurrences of gastropods or bivalves in MF type 8 point to episodic high nutrient supply, which could be induced by a restriction of the lagoon during relative sea level fall (Dauwalder and Remane 1979; Dupraz and Strasser 1999; Kästner et al. 2008; Cäsar 2012) or stratification during a sea-level rise. Finely dispersed angular quartz associated with large shell debris in MF type 9 suggests an upper subtidal setting under high-energy conditions with a certain terrestrial influx (Pettijohn et al. 1987; Wilson 1975; Bauer et al. 2002; Flügel 2004). The pseudo-spar matrix indicates formation of MF type 9 close to the coast. The association comprising MF types 7, 8 and 9 implies a transitional semi-restricted lagoon setting ranging from an open-marine to a restricted setting, which is characterized by moderate water energy and episodic terrestrial influx.

MA5-a/b: Restricted lagoon depositional setting

Depositional setting MA5-a/b is represented predominantly in the middle and upper parts of both sections. MF types of MA5-a include pelletal pack- to grainstone (MF type 10, Fig. 8a), bioclastic mud- to wackestone (MF type 11, Fig. 8b), peloidal sandy limestone (MF type 12, Fig. 8c) and limestone conglomerate (MF type 13, Fig. 8d), the latter being restricted to the uppermost part of the Langenberg section. MA5-b corresponds to shale–claystone alternations with sandy interbeds (MF type 14) restricted to a single interval in Bisperode (13.0–16.2 m).

A dominance of pellets and scarcity or absence of faunal elements indicate a hypersaline lagoonal setting with low-energy conditions and reduced sedimentation rates (Scholle and Ulmer-Scholle 2003; Flügel 2004). The occurrence of centimetre-scale woody debris (MF type 11) and sub-angular quartz (MF type 12) indicates a proximal setting. The intraclast-rich facies of MF type 13 (float- and rudstone in Baldermann et al. 2015) is composed of sub-rounded, centimetre- to decimetre-sized carbonate clasts. Both intraclasts and matrix contain rare bioclasts and compacted pellets and represent reworked debris flows. This is consistent with the interpretation of Fischer (1991), who suggested a storm-induced formation. In summary, MA5-a reflects deposition under low-energy, restricted subtidal lagoonal conditions, influenced by episodic storm events and reworking.

The shale–claystone alternations of MA5-b have a low carbonate content, a dark colour, thin bedding and intercalations of thin sandstone layers, which point to a subtidal lagoon setting receiving significant terrestrial influx during sea-level fall, climate change and/or tectonic activity (Bauer et al. 2002).

MA6-a/b: Intertidal back-ramp depositional setting

MA6-a/b occurs in the upper part of the Langenberg section and middle and upper parts of the Bisperode section and corresponds to mud- to wackestone with charophytes and ostracods (MA6-a: MF type 15, Fig. 8e, f), charophyte-rich marl or claystone (MA6-a: MF type 16, Fig. 8g) and fine-grained sandstone (MA6-b: MF type 17) (Table 1). Typically, MA6-a/b grades up into MA7 and overlies MA4 or MA5.

Ostracod carapaces can form substantial sedimentary constituents in restricted environments, especially in brackish, hypersaline, or freshwater settings (Scholle and Ulmer-Scholle 2003). Charophyte remains are considered as indicators for fresh or brackish waters in coastal and non-marine environments (Scholle and Ulmer-Scholle 2003; Flügel 2004). Even though the combination of charophyte and ostracod remains within a micritic matrix in MA6-a may indicate a lacustrine environment (Cäsar 2012), its stratigraphic intercalation with lagoonal deposits suggests a brackish-water intertidal setting under low to medium hydrodynamic conditions (Flügel 2004). A peritidal setting is supported by the abundant occurrence of Skolithos in the upper part of the Langenberg section, probably formed by large crabs (Fischer 1991). Furthermore, finds of fish teeth of Lissodus curvidens n. sp. in the Langenberg section (bed 153) indicate reduced marine salinity and restricted conditions (Mudroch and Thies 1996; Duffin and Thies 1997; Mudroch et al. 1999; Thies et al. 2007).

Tidal-flat deposition under shallow-marine intertidal conditions is suggested by well-developed wave ripples in fine-grained sandstones (MA6-b), reflecting pulses of terrigenous siliciclastic influx derived from the hinterland during sea-level fall, climate change or tectonic activity (Walker and Plint 1992; Cäsar 2012).

MA7: Supratidal back-ramp depositional setting

MA7 is characterized by dolomitic deposits including evaporite–dolomite couplets (MF type 18, Fig. 8h) and dolomitic mudstone (MF type 19, Fig. 8i). MA7 is recorded in only three horizons in the middle and upper part of the Langenberg section.

According to Baldermann et al. (2015) and Rameil (2008), dolomitic limestone (mainly type A–C of Baldermann et al. 2015) is formed via early diagenetic replacement of micrite under slightly evaporative and reducing conditions in a carbonate tidal-flat setting during sea-level lowstand. A tidal-flat environment is also indicated by the remnants of microbial lamination and fenestral fabrics. Endocasts of the bivalve Trigonia sp. support a hypersaline scenario (Fischer 1991). Similarly, the occurrence of gypsum pseudomorphs (Fig. 8h) suggests formation in a supratidal sabkha under arid conditions (Flügel 2004; Colombié and Strasser 2005).

Sedimentary model

Based on facies distribution and stratal architecture, the Upper Jurassic deposits in the LSB have been interpreted to be the product of a gently inclined carbonate ramp (Betzler et al. 2007; Kästner et al. 2008; Cäsar 2012). The sedimentary and carbonate microfacies inventory raised here enables the establishment of a detailed depositional model of the Kimmeridgian strata (Fig. 9). The spatial organization of different MAs along a proximal–distal transect reveals a hydrodynamic energy gradient (Kästner et al. 2008; Carcel et al. 2010; Sadeghi et al. 2011), which is a consequence of gradual depth changes and varying storm intensity (Tucker and Wright 1990; Wright and Burchette 1996; Flügel 2004).

Fig. 9
figure 9

Model of the depositional setting for the Kimmeridgian successions in the study area (Lower Saxony Basin)

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Massive oolitic limestone and bioclastic pack- to grainstone of MA3 are interpreted as shoal deposits of the inner ramp formed under high-energy conditions (Flügel 2004; Betzler et al. 2007; Kästner et al. 2008). In a seaward direction, moderate- to low-energy conditions are indicated for the distal inner-ramp deposits formed close to the FWWB. In MA2, the composition of the biota indicates unrestricted, normal-marine conditions affected by storms, as shown by the tempestite beds composed of discrete shell accumulations (e.g. brachiopods, oysters). Deposits formed below the FWWB (MA1) are characterized by higher clay content, reduced species richness and intensified bioturbation, as well as redeposited inner-ramp-derived material. The occurrence of intraclasts containing ooids attests to reworking of shoal deposits and basinward transport into the mid-ramp facies belt (MA1) (Betzler et al. 2007). The deepest facies recorded in the mid-ramp is represented by marlstone containing an open-marine fauna and abundant Thalassinoides burrows.

In a landward direction, the back-ramp area is composed of a broad spectrum of depositional environments, including semi-restricted lagoon, restricted lagoon and tidal-flat settings. In the semi-restricted lagoon environment (MA4), a decrease in the variety of bioclasts is observed towards the lagoon centre. Monospecific mass accumulation of gastropods and bivalves points to high nutrient supply and/or changes in salinity (Reiss and Hottinger 1984). A more restricted and hostile lagoon environment is indicated by the scarcity or total absence of faunal remains and the dominance of pellets and fine-grained carbonate mud (MA5-a). Facies belts MA5 and MA4 have been moderately influenced by periodic siliciclastic input (MF9 and MF12). In the peritidal environment, an intertidal facies belt (MA6-a) containing abundant ostracod and charophyte remains indicates brackish conditions. The shale–claystone–sandstone alternations (MA5-b and MA6-b) indicate particular episodes of enhanced terrestrial shedding into the lagoonal and peritidal environment (e.g. McLaughlin et al. 2004; Kästner et al. 2008). In addition, a supratidal facies belt (MA7) characterized by dolomite and/or evaporite precipitation indicates subaerial exposure under a dry climate.

Sequence stratigraphy

The sequence-stratigraphic interpretation of the Kimmeridgian succession in the LSB is based on the vertical stacking pattern of MF types, changes in bed thickness and the occurrence of diagnostic surfaces. This approach allows three orders of depositional sequence to be defined and provides a better understanding of the stratigraphic and spatial evolution of the sedimentary system. In view of the limited number of studied outcrops, the applied sequence-stratigraphic terminology follows Strasser et al. (1999), using lowstand, transgressive and highstand deposits instead of the systems tracts of Vail et al. (1991). In the following, only the Langenberg section is used as an example illustrating the small- and medium-scale sequences.

Small-scale sequences

Small-scale depositional sequences as defined in the Langenberg section (n = 30) vary between 1.3 and 4.9 m in thickness, which is in accordance with the scheme proposed by Strasser et al. (1999). They show shallowing-up or deepening–shallowing trends of facies evolution and are bound by three types of diagnostic surface, including firm or hardgrounds (SB1), subaerial exposure surfaces (SB2) and surfaces capping the shallowest facies (SB3) (Fig. 10).

Fig. 10
figure 10

Outcrop photograph, textures and microfacies as well as sequence stratigraphy and boundaries developed in the lower part of the Langenberg section. The black arrow points to the vertical direction in the outcrop. Please refer to Fig. 4 for the symbols used here

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Complete shallowing–deepening small-scale sequences mainly occur in the lower part of the section, especially in the transgressive and maximum-flooding intervals of medium-scale sequences where accommodation space was high (Colombié and Strasser 2005). The deepening trend is normally characterized by a facies evolution towards more open conditions, with a change from distal inner-ramp (MA2/MA3) to mid-ramp settings (MA1), and a reverse interval indicating a shallowing trend. An exceptional slightly shallowing-up sequence (small-scale sequence 2, Fig. 10) composed of shoal deposits (MF type 6) is present in this interval, which is indicated by upward-thinning bed thickness and changes in carbonate content. Firm or hardgrounds representing SB1 mainly appear in this interval and are interpreted to confine so-called subtidal cycles (Ma et al. 1999; Strasser et al. 1999). SB1-type surfaces (e.g. top of sequences 2, 3 and 7) show colonization by encrusting oysters or intense bioturbation and/or borings (Fig. 3c and e), indicative of reduced sedimentation rates during sea-level fall or subsequent submergence. SB1 is typically overlain by thin-bedded marl formed as the result of increased clay supply due to erosion of the hinterland during sea-level fall or, alternatively, reflecting concentration of argillaceous material due to reduced carbonate production following a rapid sea-level rise (Strasser et al. 1999).

Asymmetric shallowing-up sequences, which are commonly observed in many ancient shallow-marine carbonate systems, mainly appear in the middle and upper part of the section and are interpreted as peritidal cycles (Ma et al. 1999; Strasser et al. 1999). Depositional environments mainly show gradual transitions from semi-restricted lagoon (MA4) to restricted lagoon (MA5) or tidal flat (MA6), or from open-marine settings (MA2/MA3) to lagoon and tidal flat. Typical boundaries in these peritidal cycles are represented by SB2- and SB3-type surfaces. Exposure and development of SB2-type boundaries (e.g. top of sequences 1, 13 and 25) are indicated by the dolomitization of the underlying strata, and usually developed during the late highstand of larger-scale sequences when accommodation was low (Colombié and Strasser 2005). SB3 is represented by sub- or intertidal surfaces that are capping the shallowest facies of the corresponding sequence. For instance, the boundary between sequences 19 and 20 is represented by the abrupt shift from the shallower MF type 11 to the deeper MF type 8 (Fig. 11). SB3 may represent both the sequence boundary and the transgressive surface, an observation that has been reported from many shallow-water carbonate platform settings where the respective lowstand deposits are not preserved (Strasser et al. 1999; Betzler et al. 2007).

Fig. 11
figure 11

Correlation between Bisperode section and Langenberg section based on the sequence stratigraphy and microfacies as well as biostratigraphic constraints

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Medium-scale sequences

In the Langenberg section, a total of seven medium-scale sequences can be recognized, each of which comprises two to six stacked small-scale sequences, resulting in a thickness range of 5.3 to 17.7 m (Fig. 11). Influenced by the Late Jurassic second-order transgression, the typical asymmetric sequential characteristic in shallow-marine carbonate systems is distorted or even reversed in the Kimmeridgian medium-scale sequences under study.

The lowermost medium-scale sequence 1 (0.0–5.3 m), composed of dolomitic and marly limestone, contains a thick maximum flooding deposit (MFD), which is characterized by MA1. The dolomite-rich interval indicates a period of exposure during relative sea-level fall (Sarg 2001; Rameil 2008; Baldermann et al. 2015). Medium-scale sequence 2 (5.3–12.0 m) comprises two small-scale sequences which are composed mainly of MF type 6 and capped by hardgrounds. Sequence 3 (12.0–20.1 m) is characterized by a well-developed transgressive part and a less-pronounced regressive interval. The lower deepening-upward subsequence is characterized by MF type 6, indicating shoal deposits, which are overlain by MF type 3 close to the FWWB. Relatively thick (~ 3 m) MFD show intense bioturbation and higher clay content. The overlying shallowing-upward subsequence shows a trend from a middle- to inner-ramp setting. The boundary between medium-scale sequences 3 and 4 is marked by a hardground and corresponds to a relative sea-level fall, resulting in a shift from open-marine towards more restricted facies, as indicated by MA4 to MA6. The maximum flooding interval of medium-scale sequence 4 (20.1–36.4 m) is composed of lagoonal and shoal facies (MA3 and MA4), whereas the overlying lagoon and tidal-flat deposits (MA4 to MA7) represent a well-developed regressive phase. The top is capped by a 2.5-m-thick dolomite interval, reflecting exposure during sea-level lowstand. A stratigraphic gap of ostracod zones 10 to 12 appears above this sequence, indicative of the major hiatus between the Lower Süntel and Middle Süntel Formations. Medium-scale sequence 5 (36.4–54.1 m) consists predominantly of lagoonal and intertidal facies (MA4 to MA6). A more distal setting is reflected by shoal sediments (MF type 5), which correspond to the MFD. Medium-scale sequence 6 (54.1–68.9 m) shows a well-developed deepening–shallowing trend, with its lower part reflecting a shift from tidal-flat to open-marine settings. MFD are represented by MF type 4, indicating a distal inner-ramp environment. This sequence is bound by replacement dolomite. Medium-scale sequence 7 (68.9–81.6 m) contains only a shallowing-upward interval consisting of shoal and back-shoal pack- to grainstone, with intercalated lagoonal facies containing several layers composed of MF type 13.

Large-scale sequences

Two large-scale sequences are defined by the stacking pattern of the medium-scale sequences in the Langenberg section. The lower one is bound by sequence boundaries KIM1 and KIM2 (Fig. 11). The deepening trend and the MFD correspond to medium-scale sequences 1–3, which are represented by MA1 to MA3, and the following shallowing trend is characterized by a facies evolution from semi-restricted lagoon to tidal flat (MA4–MA7) in medium-scale sequence 4. A stratigraphic gap caused by erosion (as indicated by the lack of ostracod zones 10–12) results in the formation of sequence boundary KIM2 at 36.4 m. Above this surface, the deepening trend in the upper sequence shows a change from a restricted to open-marine setting, and the MFD is represented by MA2. The shallowing trend above is indicated by the reverse change back to more restricted environments. Constrained by ostracod biostratigraphy, the upper large-scale sequence in the Langenberg section can be correlated to the large-scale sequence 4 in Bai et al. (2017). The entire Bisperode section is part of a single large-scale sequence, which corresponds to the lower one in the Langenberg section. Only the lower sequence boundary KIM1 can be identified. Given the limited number of large-scale sequences covered by the studied succession, the stratigraphic correlation discussed in the following sections is dependent mainly on medium-scale sequences.

Discussion

Sedimentary evolution and correlation

Based on the integrated sedimentological and sequence-stratigraphic analysis outlined above, a preliminary correlation scheme for the Langenberg and Bisperode sections is proposed (Fig. 11). In particular, the integration of ostracod biostratigraphic results with sequence-stratigraphic interpretations illustrates the stratigraphic and spatial facies variations within the LSB.

In the Bisperode section, the Oxfordian–Kimmeridgian boundary (between ostracod zones 6 and 7) is represented by a discontinuity surface located at ~ 1.5 m. Medium-scale sequence 1 is characterized by mid-ramp and distal inner-ramp deposits in both the Bisperode and Langenberg sections. A shift towards inner-ramp oolitic shoal (Langenberg) or proximal lagoonal and tidal-flat environments (Bisperode) during medium-scale sequence 2 indicates a drop in relative sea level (Kendall and Schlager 1981; Jones and Desrochers 1992). Up-section, a gradual transgressive trend in both sections is documented by deposits ascribed to the upper part of ostracod zone 7 and ostracod zone 8 (medium-scale sequences 3 and 4 at Bisperode; medium-scale sequence 3 at Langenberg). During this interval, deposition evolves from inner to middle ramp at Langenberg, whereas it shifts from restricted back ramp to a shoal setting at Bisperode. In general, the Bisperode section reflects a more proximal position than the Langenberg section, which is also supported by the higher siliciclastic influence observed at Bisperode (Figs. 5 and 6). Notably, the overall sediment thickness and the number of sequences show significant differences between the two sections, especially in the stratigraphic interval ascribed to ostracod zone 8. According to Bádenas and Aurell (2001), the differences in both stratigraphic thickness and the number of sequences suggest that regional tectonic activity acts as an important factor in sequence development in the LSB. This interpretation is further supported by the sharp change from an open-marine towards a restricted depositional environment between sequences 3 and 4 in the Langenberg section, indicative of tectonic control on this episode of rapid accommodation loss (Bádenas and Aurell 2001). Above, the medium-scale sequence 5 in the Bisperode section is characterized by a thick charophyte-bearing mixed carbonate siliciclastic interval reflecting increasing terrestrial input into a restricted, near-coastal environment. Coeval sediments at Langenberg (Lower Süntel Formation, medium-scale sequence 4) form a thick deepening–shallowing cycle capped by dolomitic supratidal deposits, which mark the uppermost part of a shallowing-upward peritidal sequence (Rameil 2008; Baldermann et al. 2015). Furthermore, the stratigraphic gap between the Lower and Middle Süntel Formations (top of sequence 4), as indicated by the absence of ostracod zones 10 to 12, supports tectonic processes as an important controlling mechanism on the sediment distribution patterns. The Middle Süntel Formation as represented in the Langenberg section is composed of restricted back-ramp facies, with local intercalations of shoal deposits. Up-section, an increase in carbonate content reflects a decreasing continental influence, probably due to more arid conditions during the Late Kimmeridgian versus Early Kimmeridgian (Abbink et al. 2001; Carcel et al. 2010).

A tectonic influence on sequence development fits with the tectono-sedimentary evolution of the LSB. According to Gramann et al. (1997), the development of graben and half-graben structures started in the Oxfordian and intensified during the Kimmeridgian, resulting in the subdivision of the LSB into small troughs and swells, accompanied by thickness variations and lateral facies changes. A partly tectonic control on the spatial and stratigraphic facies distribution pattern of the Late Jurassic successions in the LSB has also been proposed by other authors (Betz et al. 1987; Gramann et al. 1997; Petmecky et al. 1999; Kley et al. 2008). A similar situation has been observed in the Iberian Basin, where Upper Kimmeridgian inner-ramp oolitic and reefal facies prograded over Early Kimmeridgian outer-ramp mudstone and marl triggered by local synsedimentary tectonics (Bádenas and Aurell 2001; Aurell et al. 2003). In summary, the observations reported in this study confirm the important role of regional synsedimentary tectonics in Kimmeridgian sedimentary development in the LSB.

Sequence-stratigraphic implications

Gramann et al. (1997) revised and refined the biostratigraphic correlation scheme between the North German ostracod zonation and the Boreal standard ammonite zonation of Hardenbol et al. (1998). Thus, the biostratigraphic framework established here can be used to compare the sequence-stratigraphic scheme of the LSB with sequence-stratigraphic interpretations proposed for other Late Jurassic basins in Europe (Fig. 12). Top boundaries of sequences 3, 5, 6 and 7 and the basal boundaries of sequence 5 identified in the LSB appear coeval with sequence boundaries described from the Paris Basin (Lathuilière et al. 2015), the Wessex Basin (Taylor et al. 2001), the Jura Platform and the Vocontian Basin (Colombié and Strasser 2005), the Iberian Basin (Bádenas and Aurell 2001) and Southern Germany (Ruf et al. 2005).

Fig. 12
figure 12

Comparison between the sequence-stratigraphic interpretation proposed in this study with those defined in other Late Jurassic basins in Europe (modified from Colombié and Strasser 2005; Ruf et al. 2005; Lathuilière et al. 2015). The uncertainty positions of some boundaries are noted as dashed lines and the greyish intervals represent the strata covered in this study

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Three medium-scale sequences can be recognized at the very onset (ostracod zones 7 and 8) of the Early Kimmeridgian in the LSB, but only a single sequence appears in comparable biostratigraphic positions in other European Basins (Fig. 12). Thus, top boundaries of sequences 1 and 2 in the LSB cannot be traced in other European Basins. According to Colombié and Strasser (2005) and Lathuilière et al. (2015), differences in the number of Kimmeridgian sequences among basins may result from the regional specificity of diagnostic sequence boundaries induced by differential synsedimentary tectonics and/or of deviating facies stacking patterns reflecting different sedimentary settings. In many European basins, the stratigraphic interval corresponding to the Cymodoce ammonite zone is marked by three to four medium-scale sequences. In contrast, the coeval interval in the LSB sections shows only a single sequence (sequence 4). The lack of more medium-scale sequences is best explained by erosion due to local tectonics in the LSB, which further underlines a regional tectonic control affecting the number of sequences. A major gap at this stratigraphic position is indicated by ostracod biostratigraphic results. The low biostratigraphic resolution of this particular interval (imprecisely assigned to ostracod zones 9 to 12) hampers the comparison and correlation of the top boundaries of sequence 4. Furthermore, the ambiguity in the correlation between Tethyan and Boreal ammonite zonation schemes may also explain the above-mentioned deviations or discrepancies to some extent (Colombié and Strasser 2005).

The Late Jurassic was characterized by a second-order transgression, which began in the Late Oxfordian and reached its maximum in the Late Kimmeridgian (Hardenbol et al. 1998; Hallam 2001; Miller et al. 2005; Ogg et al. 2012). In many western European basins, this major sea-level rise led to the formation of marly and/or condensed sections (e.g. Wessex Basin, England: Morgans-Bell et al. 2001; Taylor et al. 2001; Williams et al. 2001; Paris Basin, France: Lathuilière et al. 2015) and deeper-water carbonates (southern Germany, Ruf et al. 2005), all of which reveal a consistent deepening trend during the Kimmeridgian. However, the latter trend contrasts with the relative sea-level evolution in the LSB established herein. The observed overall shallowing trend from mid-ramp towards back-ramp settings implies that sediment production and supply in the LSB were faster than the formation of accommodation space during the second-order sea-level rise (Kendall and Schlager 1981; Jones and Desrochers 1992; Posamentier et al. 1992; Hunt and Tucker 1995; Carcel et al. 2010), resulting in progradation of the ramp during the Kimmeridgian. A similar scenario of ongoing carbonate platform progradation during the Early and early Late Kimmeridgian due to continuously high carbonate production was observed on the Swiss Jura platform as well as in the Vocontian Basin (Colombié and Strasser 2003, 2005). Therefore, high carbonate production (Figs. 5 and 6) must be an important factor controlling the shallowing trend of the Kimmeridgian strata in the LSB.

Conclusions

  1. 1.

    Detailed lithological logs are presented for two Upper Jurassic sections (Langenberg, Bisperode) located in the LSB in northern Germany. Although open-marine marker fossils (i.e. ammonites) are scarce in Kimmeridgian strata of the LSB, ostracod biostratigraphy enabled precise age assignment and correlation of the sections studied. Whereas the Langenberg section comprises strata ranging from the Humeralis-Schichten (Uppermost Korallenoolith, ostracod zones 7–8) to the Middle Süntel Formation (ostracod zones 13–14), the Bisperode section comprises only the Humeralis-Schichten (lower part of ostracod zone 7) and only part of the Lower Süntel Formation (ostracod zones 7–10).

  2. 2.

    Based on field observations and thin-section analysis, 19 microfacies (MF) types are defined and attributed to seven microfacies associations (MAs), which are assigned to a carbonate ramp model ranging from mid-ramp to restricted proximal inner-ramp settings. Facies reveal that lagoonal and peritidal deposits (MA4-7) and intercalated high-energy shoal sediments (MA3) dominate most of the Kimmeridgian successions under study. More open-marine facies representing distal inner-ramp (MA2) and mid-ramp deposits (MA1) appear only in the lower part of both sections. 

  3. 3.

    Stacking pattern of small-, medium- and large-scale sequences are defined based on the vertical microfacies stacking pattern, bed thickness and diagnostic sequence boundaries. Small-scale sequences in the Langenberg (n = 30) and Bisperode (n = 19) sections (thickness: 1.3–4.9 m) show shallowing-up and deepening-shallowing trends capped by three kinds of surface. Seven (Langenberg) and five (Bisperode) medium-scale sequences (thickness: 5.3–17.7 m) are identified and can be used for correlation based on the biostratigraphic framework. Two and one large-scale sequence are recognized in Langenberg and Bisperode sections, respectively.

  4. 4.

    The detailed sedimentary and sequence-stratigraphic analysis reveals that Bisperode reflects a more proximal setting than Langenberg. Regressive or transgressive trends within the medium-scale sequences correlate well between the two sections, both of which show an overall shallowing trend with stratigraphic height.

  5. 5.

    Regional synsedimentary tectonics is regarded as an important controlling factor for the differential sedimentation patterns of the Kimmeridgian deposits in the LSB. In addition, high carbonate production is suggested to have been responsible for keeping up with the creation of accommodation space and resulted in the progradation of Kimmeridgian successions (Süntel Formation) in the LSB.

  6. 6.

    Most of the medium-scale sequence boundaries defined in this study occur in similar biostratigraphic positions in other European basins. Observed deviations between different basins are probably caused by regionally differing tectonic activity, the inaccuracies of the applied biostratigraphic schemes and the ambiguity related to the correlation between the Tethyan and Boreal realms.