Lower Jurassic (Pliensbachian–Toarcian) marine paleoenvironment in Western Europe: sedimentology, geochemistry and organic petrology of the wells Mainzholzen and Wickensen, Hils Syncline, Lower Saxony Basin

Abstract

Over the past few decades, Toarcian (Early Jurassic) black shale deposits of NW Europe have been extensively studied, and the possible global and regional mechanisms for their regional variation have been discussed. In this context, the black shales of the Northwest German Basin are still sparsely studied with regard to their palaeo-depositional history. This study aims to understand the connection between regional and global influences on the widespread Early Toarcian oceanic anoxic event by examining two wells covering the Upper Pliensbachian to Upper Toarcian sediments in the Northwestern German Basin. The core intervals were analysed using a multidisciplinary approach, including geochemistry, biostratigraphy and organic petrography. Marine palaeoenvironmental changes were reconstructed, and sediment sequences were stratigraphically classified to allow a supra-regional stratigraphic correlation. The results reveal complex interactions between sea level changes, climate warming, basin confinement, and Tethys–Arctic connectivity resulting in the Toarcian black shale deposition. Upper Pliensbachian sediments were deposited under terrigenous influence, shallow water depths, and predominantly oxic bottom water conditions. The deposition of black shale is characterized by algal organic material input and anoxic bottom water conditions. Strong correlations between water stratification, anoxia, and bioproductivity suggest that global warming and intensification of monsoonal rainfall, continental weathering, and increasing freshwater and nutrient inputs were the main factors controlling the formation of black shales. Prolonged deposition of OM-rich sediments in the NWGB may be related to intensified monsoonal precipitation in northern Europe and enhanced Tethys–Arctic connectivity at the serpentinum–bifrons transition.

Graphical Abstract

Introduction

Toarcian (Lower Jurassic) black shale deposits have been extensively studied in recent decades, as they constitute a prominent and widespread sedimentary feature in NW Europe. The scientific community has controversially discussed potentially underlying mechanisms of their trans-regional deposition. One widely established theory implies the impact of the global Toarcian Oceanic Anoxic Event (T-OAE) (Jenkyns 2010), as suggested by globally observed negative carbon isotope excursions (CIE) (Hesselbo et al. 2002). The negative CIE (nCIE) indicates carbon cycle perturbations associated with prevailing bottom water anoxia and global warming. Possible global mechanisms that had provoked enhanced greenhouse effects and long-term warming are assumed to be, e.g. volcanic activity in the Karoo Ferrar Large Igneous Province and severe thermogenic dissolution of gas hydrates (Kemp et al. 2005; Svensen et al. 2007; Krencker et al. 2014; Them et al. 2017a; Fantasia et al. 2019a).

Studies on Toarcian (black) shales have predominantly focused on the sedimentary succession of the Central European Basin System (CEBS) in NW Europe. These studies have shown significant regional variations in the timing and intensity of the nCIE, and the thickness and lithological characteristics of the shale deposits (e.g. Wignall et al. 2005; Gorbanenko and Ligouis 2015). Numerous studies have elaborated paleodepositional models to get insights into the regional controlling mechanisms (Röhl and Schmid-Roehl 2005; van de Schootbrugge et al. 2005; Wignall et al. 2005), such as variations of water mass exchange due to local hydrogeographic restrictions and changing freshwater influx (Dera et al. 2009). This aroused the interest in investigating regional lithological and geochemical variations of Lower Jurassic sediments throughout different sub-basins of the CEBS to gain insights into the regionally prevailing depositional conditions and to understand the main driving mechanisms. Recent approaches for the reconstruction of Early Toarcian paleodepositional conditions are commonly mixed scenarios, including (i) intense greenhouse conditions, possibly caused by multiple global and regional driving mechanisms, (ii) eustatic sea level changes, and (iii) modulation by regional factors such as water mass exchange limitations (Röhl and Schmid-Roehl 2005; French et al. 2014; Fantasia et al. 2019b; Remirez and Algeo 2020). Existing depositional models differ e.g. in terms of the intensity, duration, lateral extent, and timing of bottom water anoxia, timing, and extent of eustatic transgression/regression (T/R) cycles, and the role and significance of the different mechanisms driving and controlling the T-OAE and the related deposition of organic-rich shales.

Studies introducing models for the Toarcian black shale deposition in the CEBS have been mainly conducted on the sedimentary succession in the Southwest German Basin (SWGB) as these Jurassic deposits are well exposed in numerous outcrops along the Swabian Alb mountains. The outcrops reveal a rich assemblage of fossils, allowing detailed biostratigraphic classification with a high level of accuracy (Röhl et al. 2001; Schmid-Röhl et al. 2002; Frimmel et al. 2004; Schwark and Frimmel 2004; Röhl and Schmid-Roehl 2005; Montero-Serrano et al. 2015; Fantasia et al. 2019b; Hougard et al. 2021; Ajuaba et al. 2022). In contrast, the description of Jurassic sedimentary sequences in the Northwest German Basin (NWGB) is only based on few borehole profiles (Littke et al. 1991a; van de Schootbrugge et al. 2019). One northern German region, where Lower Jurassic deposits of the Posidonienschiefer Formation (Fm) have been studied in the past is the Hils Syncline in the southern CEBS, Lower Saxony Basin. Studies in this region have mainly focused on organic-rich black shales and the effect of thermal maturity on their geochemical and petrophysical properties (Rullkotter et al. 1988; Littke et al. 1991a; Stahl 1992; Sundararaman et al. 1993; Ghanizadeh et al. 2014; Song et al. 2017; Fang et al. 2019). While most previous investigations have target the assessment of the black shales as potential petroleum source rock, the underlying and overlaying organic lean clay and mudstones are sparsely described and studied. Consequently, the stratigraphic classification of Lower Jurassic deposits in this area is incomplete, hampering the correlation of the deposits with time-equivalent deposits from other sub-basins of the CEBS.

For the present study, a combined sample set from two nearby boreholes, 'Wickensen' and 'Mainzholzen', has been analysed, providing a profile of Upper Pliensbachian to Upper Toarcian deposits from the Hils Syncline, Lower Saxony Basin. Nearly 200 samples have been studied using a multidisciplinary approach, combining inorganic and organic geochemistry, carbon isotope geochemistry, sedimentology, and organic petrology. The objective is to reconstruct regional marine palaeoenvironmental conditions in the NWGB, taking into account global climatic perturbations and regional tectono-sedimentary evolution. Moreover, a trans-regionally consistent classification of the lithostratigraphic units in this region is provided.

Geological background

During the Toarcian, the CEBS was a large-scale epicontinental basin system with an extent of several hundred kilometres. The basin system formed as a result of extensive igneous activity, faulting, and subsidence in the Early Permian (van Wees et al. 2000), resulting in the breakup of the supercontinent Pangea and an extensional regime in the CEBS during the Triassic to Lower Jurassic (Fig. 1a). In the Late Cretaceous, the onset of the Alpine orogeny and the subsequent Atlantic Ocean extension caused inversion in the area of the CEBS (Bachmann et al. 2008; Kley and Voigt 2008; Kley 2018).

Fig. 1

a Lower Jurassic (Toarcian) global palaeogeography after Scotese (2013). b Paleogeography of the Central European Basin System (CEBS) showing the location of sub-basins and the study area; PB – Paris Basin, WNB – Western Netherland Basin, NWGB— Northwest German Basin, SWGB – Southwest German Basin, YB – Yorkshire Basin; RM – Rhenish Massif (after Ziegler 1990; Thierry et al. 2000; Rübsam et al. 2014)

While predominantly continental and only partially shallow marine sediments were deposited during the Permian and Triassic, extensive transgression during the Lower Jurassic established long-lasting shallow marine conditions in Central Europe (Bachmann et al. 2008; Stollhofen et al. 2008).

The area of the CEBS was located between 20 and 40°N during the Early Toarcian and was affected by monsoonal, warm and humid climatic conditions (Fig. 1b). Multiple islands and submarine sills subdivided the CEBS into several sub-basins (Ziegler 1990), resulting in hydrological restrictions in the deeper sub-basins. This favoured the widespread deposition of Toarcian organic-rich black shales (Fig. 1b), locally referred to as the Posidonienschiefer Fm (Röhl and Schmid-Roehl 2005; Song et al. 2015). The onset of Early Toarcian organic-rich shale deposition is associated with a pronounced nCIE at the base of the ammonite zone serpentinum/falciferum (e.g. nCIE of C_org of − 4 to − 7‰; Hesselbo et al. (2000)), indicating significant carbon cycle perturbations and abrupt environmental changes.

The NWGB is a sub-basin in the northwestern part of the CEBS, bordered by the Rhenish Massif to the southwest and the Bohemian Massif to the east (Fig. 1b). The wells analysed in this study were drilled in the southwestern limb of the Hils Syncline, a structure in the southeast of the Lower Saxony Basin (Fig. 2). The syncline formed as a result of inversion tectonics during Santonian times (Late Cretaceous) (Voigt et al. 2008) in the context of NE–SW directed compression. Additional salt movement strongly influenced the structural style of the CEBS (Kley 2013). Mesozoic units, specifically from the Posidonienschiefer Fm, cover a great thermal maturity range from 0.48 to 1.45% R_o (vitrinite reflectance) over a rather short lateral extent (< 70 km) along the western limb of the Hils Syncline (Littke et al. 1991a; Song et al. 2017; Fang et al. 2019).

Fig. 2

Simplified geological map of the Hils Syncline showing Mesozoic geological sequences. Investigated wells, `Mainzholzen` (MAI) and ‘Wickensen’ (WIC), are marked by yellow rectangles. Locations of further study wells, HAR – Harderode, DOH – Dohnsen, DIE – Dielmissen, and WEN – Wenzen are marked by black squares

Lithostratigraphic overview of Lower Jurassic deposits in the Northwest German Basin

The description of the Lower Jurassic sedimentary succession in North Germany is based on very few borehole profiles (Littke et al. 1991a; Song et al. 2017; van de Schootbrugge et al. 2019). Thus, detailed descriptions of lithological variations are sparser than for the SWGB, and inconsistencies regarding the presented stratigraphic classification exist.

Van de Schootbrugge et al. (2019) comprehensively described the Liassic sedimentary succession below the Lower Toarcian black shale deposits in North Germany. The study comprises detailed biostratigraphic and lithological descriptions of the complete core profile from well Schandelah-1, including approximately 120 m of Pliensbachian sediments. The Upper Pliensbachian Amaltheenton Fm comprises OM-lean claystones of marine origin and extends relatively homogeneously over a wide depth range. The Lower Amaltheenton Fm is characterized by medium–dark grey calcareous claystone with sideritic carbonate concretions. The Upper Amaltheenton Fm consists of homogenous light to medium grey claystone with carbonate concretions (often sideritic). Species richness and diversity increase from the Lower to Upper Pliensbachian, likely related to long-term sea level rise. The uppermost Pliensbachian is characterized by a relatively sharp diversity decline, likely connected to a seawater temperature decline and sea level fall (van de Schootbrugge et al. 2019).

Lower Toarcian deposits are commonly subdivided into three ammonite zones in Northwest Europe: tenuicostatum, serpentinum/falciferum, and bifrons (from base to top), corresponding to the Upper Liassic units ε₁, ε₂, ε₃. Lithologically, the Lower Toarcian tenuicostatum zone is similar to the Upper Pliensbachian margeritatus zone in the SWGB and NWGB with organic-lean claystone deposits containing carbonaceous concretions, with a gradual lithological transition at the Pliensbachian–Toarcian (P–T) boundary (van de Schootbrugge et al. 2019). In the NWGB, tenuicostatum zone deposits have been observed with small thicknesses (< 10 m) (van de Schootbrugge et al. 2019). Thus, it is suggested that major parts of the lowermost Lower Toarcian unit in the NWGB have been eroded, as observed analogously throughout the CEBS, e.g. in nearshore parts of the SWGB (Röhl and Schmid-Roehl 2005). Accordingly, the tenuicostatum–serpentinum transition represents an abrupt lithological change, marked by the start of the deposition of organic-rich black shales, in this study referred to as the Posidonienschiefer Fm. The onset of the black shale deposition correlates with a distinctive short-term nCIE associated with the beginning of the T-OAE (e.g. Hesselbo et al. 2000). The base of the overlaying bifrons zone is marked by a condensed interval, the “Monotis Bank”, a stratigraphic marker of finely laminated carbonate beds (Montero-Serrano et al. 2015; van de Schootbrugge et al. 2019).

In the NWGB, the black shale deposition proceeds into Late Toarcian times (van de Schootbrugge et al. 2019), described as “Dörntener Schiefer” (variabilis ammonite zone, Lias ζ₁) (Hoffmann and Jordan 1980). In the SWGB, organic-lean mudstones of the Jurensismergel Fm directly overlay Lower Toarcian black shales. Only a few locations exist in the SWGB and Switzerland where Upper Toarcian deposits of the variabilis zone are present, e.g. in the Wutach area, Nürtingen (Germany) (Riegraf et al. 1984) and the Riniken and Gipf section, Aargau (Switzerland) (e.g. Fantasia et al. 2019b). The deposits are bituminous mudstones with alternating bedding, with lower organic carbon contents (Riegraf et al. 1984) and lower HI values (e.g. Fantasia et al. 2019b) compared to black shales of the bifrons zone. In southern and northern Germany, the top of Toarcian black shale is marked by an erosional surface comprising conglomerates and reworked fossils of the Posidonienschiefer Fm black shale. Erosion is attributable to the following transgressive cycle (Röhl and Schmid-Roehl 2005).

At the Hils syncline, the Posidonienschiefer Fm has been subdivided into three subunits (Unit I, II, III) based on lithological variations. The lowermost “Unit I” is characterized as marlstone, distinguished by a higher carbonate content. “Unit II” and “Unit III” are classified as calcareous claystone (Littke et al. 1991a). “Unit II” is defined as the “low carbon zone”, characterized by lower total organic carbon (TOC) contents and hydrogen index (HI) values. Different interpretations exist regarding the stratigraphic classification of the organic-lean clay-/mudstones, underlying and overlaying the organic-rich shales of the Posidonienschiefer Fm. Littke et al. (1991a) found a sharp erosional contact between the black shales of the Posidonienschiefer Fm and the underlying organic-lean claystones. Song et al. (2017) interpreted these underlying organic-lean mudstones as Lower Toarcian deposits equivalent to the tenuicostatum zone, consistent with observations made for the well Schandelah-1 (van de Schootbrugge et al. 2019). Overlaying organic-lean claystones are classified as deposits from the Aalenian Opalinuston Fm (Littke et al. 1991a) or as part of the Lower Toarcian bifrons zone (Song et al. 2017).

Samples and methods

Two well sections, MAI and WIC, drilled in the western flank of the Hils Syncline in northern Germany (Fig. 2) were sampled. The well sections cover sediments of the Upper Pliensbachian Amaltheenton Fm, the overlaying Lower Toarcian Posidonienschiefer Fm, and the Upper Toarcian Jurensismergel Fm in depth ranges of 3.5 to 99.0 m and 10.5 to 65.6 m, respectively. The wells were sampled at intervals of approximately 0.5 to 1 m. 107 samples were taken from MAI and 87 from WIC. The sample nomenclature is based on the well abbreviation and the depth in cm (e.g. MAI_5500-depth 55 m, well Mainzholzen).

Elemental analysis

For X-ray fluorescence (XRF), 8 g sample material of 42 samples from MAI and 23 samples from WIC was mixed with 2 g Fluxana CEROX wax and pressed into an aluminium cup. The tablets were analysed using a SPECTRO XEPOS ED(P)-XRF instrument with a detection limit of ≤ 1.4 ppm. For evaluation of the relative enrichment and depletion of certain elements, XRF data is presented as concentration (ppm and %) and additionally presented in the form of enrichment factors (EF), by normalizing the element concentration to the Al content and dividing it into the Al-normalized average composition of the Upper Continental Crust (UCC) (McLennan 2001) [X_EF = (X/Al)_sample/(X/Al)_UCC)].

The sulphur contents were measured using a Leco S-200 Sulfur Analyzer. The measurement was carried out for the XRF-measured samples to gain more accurate information on these samples` sulphur contents. For this, 100 ± 2 mg of powdered rock material was combusted at 1800 °C in a pure oxygen atmosphere. The generated sulphur dioxide concentration was measured by infrared absorption with an accuracy of ±  < 5% and a detection limit of 0.001 wt%.

The mineral composition of eight samples from MAI and eight samples from WIC was determined by X-ray diffraction (XRD). For preparation, 4 g sample material was mixed with 1 g of corundum as an internal standard and ground using a McCrown mill. Measurements were carried out on a Bruker D8 diffractometer with a Cu-anode (CuKα radiation generated by 40 mA and 40 kV). The samples were scanned in the angular range of 2–92°2θ using a step size of 0.02°2θ and a counting time of 5 s per step. Concentrations were obtained through Rietveld refinement using the BGNM-based software PROFEX (Doebelin and Kleeberg 2015). A detailed description of the preparation and analysis procedure is given by Gaus et al. (2022).

93 bulk rock samples from MAI (analysed at the Federal Institute for Geosciences and Natural Resources (BGR, Hannover) and 38 samples from WIC (analysed at the Westphalian Wilhelms-Universiy Münster (WWU), Muenster) were analysed for their organic carbon isotope composition. Analyses were carried out on decarbonized, dried and homogenized samples.

At the BGR, the values were measured with the EA-IRMS. The samples were oxidatively combusted at a temperature of 1020 °C in the element analyser Flash EA 1112. The CO₂ produced by combustion was separated by a GC column and transferred into the Thermo Delta V Advantage isotope ratio mass spectrometer, where the gas is separated according to their mass-to-charge-ratio (m/z). For evaluation, a reference material with known δ¹³C values was used. Calibration of the system is routinely carried out using laboratory own standards and is checked daily by certified IAEA standards.

At WWU, an off-line technique was applied. CuO wire was added to the material, and the homogenized material was combusted in a sealed quartz tube at 850 °C for 3 h. The released CO₂ was cryogenically purified and collected in a sealed pyrex tube. Mass spectrometric measurements were carried out using a Delta V Advantage equipped with a dual inlet. Results were normalized to the international reference standard V-PDB (Vienna pee dee belemnite) and reported in the standard delta notation in per mil difference (δ¹³C). Reproducibility was better than 0.15‰.

Programmed temperature (Rock–Eval) pyrolysis

Online open-system pyrolysis was carried out on all 194 samples using the Rock–Eval 7 device by Vinci Technologies. Samples of the organic-rich black shales were analysed using 20 mg homogenized ground material, while 100 mg was used for the remaining OM-lean samples. The samples were measured using the IFPEN (IFP Energies Nouvelles) BasicMethod described in Behar et al. 2001 and Grohmann et al. 2023. Besides the S1 (free hydrocarbons [mg HC/ g rock]), S2 (hydrocarbons formed during thermal pyrolysis [mg HC/g rock] and S3 (CO₂ yield up to 390 °C [mg CO₂/g rock]) peaks, and the TOC-normalized hydrogen index (HI = S2/TOC × 100 [mg HC/g TOC]) and oxygen index (OI = S3/TOC × 100 [mg CO₂/g TOC]), and total organic and mineral carbon contents (TOC and MinC) were derived from programmed temperature pyrolysis.

Organic petrography

For organic petrography, whole rock samples were embedded in epoxy resin, ground flat and polished using a Struers Tegra Pol-21 grinding and polishing machine according to sample preparation standards, e.g. ISO 7404-2 (2009).

Vitrinite reflectance was measured using a Zeiss Axio Imager.M2m microscope equipped with an integrated white light VIS/LED illuminator, Basler Scout colour, and monochrome digital cameras, an oil immersion objective (50x/1.0 Epiplan-NEOFLUAR), and a 546 nm interference filter. The microscope setup was calibrated with optically isotropic standards of leuco-sapphire (0.592 R_o%). Measurements were performed using the Diskus-Fossil software from Hilgers Technisches Büro under non-polarised light according to (ISO 7404–5, 2009). Maceral groups were identified according to the ICCP standards (ICCP 1998, 2001; Pickel et al. 2017). The maceral distribution was evaluated qualitatively. Details of the sample preparation and microscopic equipment are found in Burnaz et al. (2023).

Organic-geochemical analysis

For organic–geochemical analysis, the ground material of 20 samples (11 samples from MAI and nine samples from WIC) was extracted by accelerated solvent extraction (ASE) with dichloromethane (DCM) using a DIONEX ASE 150 device (Thermo Scientific Inc). For OM-rich Posidonienschiefer black shale samples, 2–4 g sample material was extracted. For OM-lean claystone of the Jurensismergel and Amaltheenton Fm, 10–30 g sample powder was extracted. After drying with sodium sulphate (Na₂SO₄) and removal of elemental sulphur with activated copper powder, the raw extracts were fractionated by column chromatography using eluents with increasing polarity: n-pentane for the aliphatic fraction and a mixture of n-pentane and DCM (40:60, v/v) for the aromatic fraction. The fraction extracts were reduced to a final volume of 10 to 250 µL prior to GC–MS (gas chromatography–mass spectrometry) and GC–irmMS (GC–isotope ratio monitoring mass spectrometry) analysis, based on extract concentration analysed by GC–FID (GC–flame ionization detector).

The fraction extracts were analysed on a GC6000 series GC by Fisons Instruments equipped with a Zebron EB-1 fused silica column (30 m × 0.25 mm, 0.25 µm film thickness) and an FID for evaluation of the concentration of the molecular compounds of interest. As carrier gas, hydrogen was used with a velocity of 40 cm/s. The instrument was set to an injector temperature of 270 °C and programmed with a starting temperature of 60 °C, increasing up to 310 °C with a steady heating rate of 5 °C/min and an isothermal hold of 3 min at 60 °C and 20 min at 310 °C. Splitless mode was used for injection. GC–FID chromatograms were used for n-alkane and isoprenoid identification and the determination of relative concentrations using the Atlas software.

GC–MS was performed on the aliphatic and aromatic fractions using a Mega series HRGC 5160 gas chromatograph by Carlo Erba with a ZB-5 column (30 m × 0.25 mm, 0.25 μm film thickness), coupled with a Trace MS (Thermoquest). The mass spectrometer was operated in electron impact ionization (EI+) mode with 70 eV electron energy and an ion source temperature of 200 °C. Approximately, 1 μl of each extract was injected in split mode. Single ion monitoring was applied to the aliphatic fraction using the m/z values 191, 205, 217, 218, 259, 370, 372, 386, 398, 400, 412 and 414. The aromatic fraction was measured in full scan mode from 35 to 700 m/z. Helium was used as carrier gas with a velocity of 30 cm/s. For all analyses, the initial GC temperature was set to 80 °C, held isothermal for 3 min, and an end temperature of 320 °C was reached, followed by a 20 min-isothermal hold. For the aliphatic fraction, the temperature programme included a rapid temperature increase from 80 °C up to 160 °C at a heating rate of 10 °C/min, followed by a slower heating to the final temperature of 320 °C at a heating rate of 3 °C/min. For the analysis of the aromatic fraction, the temperature was increased from 80 to 320 °C at a constant heating rate of 3 °C/min.

Relevant biomarkers and compounds were identified by comparing the measured chromatograms with elution orders of reference samples and published gas chromatograms. Biomarker ratios were calculated based on integrated peak areas.

Compound-specific stable carbon isotope analyses were carried out on the aliphatic fraction using a Finnigan Delta Plus XL MS, connected to a Fisons Instruments DC 6980A via a GCC III combustion interface to analyse the isotopic composition of n-alkanes, pristane, and phytane. The GC–irmMS was equipped with a Zebron ZB-5 fused silica column of 60 m length, 0.25 mm internal diameter, and 0.25 µm film thickness. Measurements were carried out with a temperature programme starting at 60 °C, with an isothermal hold for 3 min and a subsequent temperature increase of 3 °C/min up to 310 °C. Helium was used as carrier gas with a velocity of 35 cm/s. The eluting compounds were oxidized by a CuO/NiO/Pt-catalyst at 940 °C. Each sample was measured three times, and the δ¹³C value is expressed relative to the V-PDB standard. The methodology is described in detail by Schwarzbauer et al. (2013).

Results and interpretation

Lithological variations and stratigraphic classification

Inorganic geochemical composition and total organic carbon (TOC) quantities reveal significant variations throughout the studied well sections MAI and WIC (Table 1). Geochemical data, biostratigraphic information, and microscopic observations were used to determine the stratigraphic units. The respective lithological changes are described in the following from old to young.

Table 1 Results of the elemental analysis, including XRD, Rock–Eval (RE) and XRF data

Amaltheenton Formation (Upper Pliensbachian)

The lowermost MAI interval (36–99 m) is identified as the Upper Pliensbachian Amaltheenton Fm (Fig. 3), based on the recovered ostracods in MAI_3600, MAI_3800 and MAI_9800. The formation is characterized by low to moderate OM contents, indicated by a low average TOC value of 0.9%. The formation is lithologically defined as claystone, as indicated by high average Al₂O₃ values and low MinC values. Furthermore, XRD data of the samples MAI_5200 and MAI_4600, regarded as representative of most of the Amaltheenton sedimentary formation, reveal high clay contents greater than 70% (Table 1). Besides the relatively homogenous claystone facies, locally high carbonate contents were observed (AMA_sid; Fig. 3), classified as sideritic concretions based on XRD data and coevally elevated Fe₂O₃ values from XRF analysis. Correspondingly, hard reddish bands and concretions of up to 10–15 cm in diameter were observed. Microscopically, some siderite concretions revealed diagenetic pyrite overgrowth (MAI_8500; Fig. 3). The geochemical composition of the sideritic core intervals shows the enrichment of Mn and Mg (Fig. 4), as well as ¹³C_org (Fig. 5). Sulphur contents range from 0.2 to 3.6% (Table 1), showing poor correlation with TOC contents. The trace element composition is similar to that of the lowermost Toarcian OM-lean claystone interval (MAI—32–35 m; WIC—61–66 m) and the Upper Toarcian Jurensismergel Fm (MAI—4–21 m; WIC—11–30 m). Mo and U are depleted. Siderite concretions (AMA_sid) show high enrichments (EF > 5) of P, Mn, Co, Y, Ce, Pr and W (Fig. 4). Microscopic observations revealed a fine-grained matrix and the presence of foraminifera (MAI_9900; Fig. 3).

Fig. 3

Simplified lithostratigraphic overview of the Pliensbachian, Toarcian and Aalenian sedimentary succession in the NWGB at the Hils Syncline with representative photomicrographs of sedimentary units. For further information on micrographs, see text. Note that Upper Toarcian Jurensismergel and Aalenian Opalinuston sediments are lithologically similar and the biostratigraphic boundary is not well established

Fig. 4

Trace element enrichment (EF > 1) and depletion (EF < 1). EF is given as average values for the studied stratigraphic intervals: JUR – Jurensismergel Fm (Upper Toarcian); POS – Posidonienschiefer Fm (Lower Toarcian); TEN – tenuicostatum zone Lower Toarcian), AMA – Amaltheenton Fm (Upper Pliensbachian). JUR sediments may contain in the upper part some Aalenian Opalinuston Fm (see Fig. 3)

Fig. 5

Organofacies variations shown vs. depth. Hydrogen and oxygen index (HI = S2/TOC [mg HC/g TOC]; OI = S3/TOC [mg CO₂/g TOC]). δ¹³C of C_org of the bulk rock (C_org(bulk)), n-alkanes (nC₁₅, nC₁₇, nC₁₉ and nC₂₅, nC₂₇, nC₂₉), and pristane and phytane (Pr, Ph)

OM-lean claystone (Lower Toarcian)

The core interval from 32 to 35 m at MAI is identified as lowermost Toarcian deposits, based on ostracod species in MAI_3500. Accordingly, the well section from 61 to 66 m in core WIC was also classified as lowermost Toarcian deposits (Fig. 3). Lithologically, the interval is similar to Upper Pliensbachian deposits of the Amaltheenton Fm. In South Germany, equivalent Lower Toarcian OM-lean claystones have been identified by containing fossils representing the tenuicostatum zone. Since index ammonite fossils are missing in the studied well sections, the interval is lithologically defined in the following as the Lower Toarcian OM-lean claystone interval.

The sediment is classified as claystone based on high clay mineral contents and high Al₂O₃ concentrations (Table 1). Organic and inorganic carbon contents are low, similar to the Upper Pliensbachian Amaltheenton Fm (Fig. 3; Table 1). Trace elements show no significant excursions (Fig. 4). The sediment’s appearance under the reflected light microscope is similar to that of the Amaltheenton Fm (WIC_6560; Fig. 3).

At the top of the interval in well MAI (32–33 m), a breccia-like matrix has been observed, likely indicating the presence of a stratigraphic gap. Moreover, the transition between the Lower Toarcian organic-lean claystone to the organic-rich black shale deposits in MAI is marked by core loss (30 to 32 m). This may indicate the presence of disturbed material, suggesting the presence of a fault.

Posidonienschiefer Fm (Lower to early Upper Toarcian)

The Upper Pliensbachian and Lower Toarcian OM-lean claystone intervals are overlain by black shales of the Posidonienschiefer Fm. Biostratigraphic observation based on calcareous nannoplankton reveals that the interval has been deposited throughout Early Toarcian times proceeding up to the Late Toarcian (Fig. 3). This finding corresponds to the observation of long-lasting black shale deposition considerably proceeding the Lower Toarcian CIEs and OAE in NW Europe (Röhl and Schmid-Roehl 2005; van de Schootbrugge et al. 2019). The thickness of the Posidonienschiefer black shale varies significantly in the studied cores MAI and WIC, with 9 m thickness at MAI (21–30 m) and 31 m at WIC (30–61 m). Thickness variations corroborate the presence of at least one fault at well MAI. The sharp geochemical and lithological transition between the black shale intervals and the underlying and overlaying claystone suggests unconformable erosional contacts (Fig. 3).

The Toarcian black shale deposition duration was estimated to last about 3.2 Myr for the adjacent SWGB (Ruebsam et al. 2023). This would result in an extremely low average total sedimentation rate of less than 1 cm/kyr for the investigated borehole WIC, with a maximum black shale thickness of 31 m. The deposition within a continental basin is generally associated with high detrital influence, moderate to high sedimentation rates and high nutrient input and bioproductivity. Considering this, it can be assumed that such a low average sedimentation rate indicates irregular sedimentation and considerable sedimentation interruptions (hiati or erosion), as confirmed by abrupt facies changes at the top and bottom.

The Posidonienschiefer Fm is characterized by significantly higher TOC and carbonate contents. Macroscopically white carbonate concretions and vein fillings were observed, identified as calcite by XRD. The clay content is low compared to the organic-lean over- and underlying sedimentary units (Table 1). Thus, the sediment is classified as carbonaceous marlstone or organic-rich calcareous mudstone. Furthermore, the formation comprises the highest pyrite abundance (accessories—ACC; Table 1). The black shales are relatively enriched in sulphur, with 4.3% on average (Table 1), and in the trace elements, Ni, Cu, Ag, Cd, Mo, U, and Sr, as indicated by EF above one (Fig. 4). Microscopic observations reveal that the Posidonienschiefer Fm is generally rich in marine fossils, mainly fish remains, especially for sections with increased carbonate content (MAI_3000, WIC_6050; Fig. 3).

In core WIC, the lower part of the black shale Sect. (49 to 61 m; Unit I and Unit II) is characterized by high TOC and MinC variations (Fig. 3) accompanied by relative enrichments of TS, Mo, and U in Unit I and gradually decreasing concentrations in Unit II (Table 1, Fig. 4). The overlaying interval at 30–49 m (Unit III) shows a less variable geochemical composition, with moderate MinC values and high TOC contents (Fig. 3). Differences in the OM enrichment have been observed for well MAI, with higher TOC content in the upper part (12.8% on average; 21–26 m) and relatively lower TOC contents in the lower part (7.9% on average; 26–30 m) (Fig. 3). Increased TOC values are accompanied by relative enrichments in TS, Mo, and U.

Jurensismergel Formation (Upper Toarcian)

The uppermost sedimentary interval (4–21 m) of core MAI is ascribed to the Upper Toarcian Jurensismergel Fm, based on ostracod findings at a depth of 17 m. Accordingly, the organic-lean sediments overlaying the Posidonienschiefer Fm at well WIC (11–30 m) are likely also classified as Upper Toarcian Jurensismergel deposits (Fig. 3). At MAI, the transition between the Posidonienschiefer Fm and the Jurensismergel Fm is marked by an erosional contact with a breccia–conglomerate interval at 20.0–21.3 m, which is absent in well WIC (Littke et al. 1991a). Geochemically, the transition from the Posidonienschiefer black shales to the Upper Toarcian Jurensismergel Fm is marked by an abrupt decrease in TOC and MinC (Fig. 3, Table 1). The deposits are classified as claystone based on their high clay mineral content greater than 50% (Table 1). Total sulphur contents are low to moderate (Table 1). The trace element enrichment/depletion pattern is similar to the Lower Toarcian and Upper Pliensbachian OM-lean claystone (Fig. 4). Apart from slight variations in TOC and elevated TS contents at WIC at a depth of around 23 m (Fig. 3), the formation is relatively homogenous.

Organic geochemical variations

Besides vertical variations in OM content (Fig. 3), the studied well sections reveal variations in the composition and quality of the OM. Organofacies variations are delineated in the following for the different lithostratigraphic subunits.

Amaltheenton Formation (Upper Pliensbachian)

The Amaltheenton Fm is characterized by low TOC contents (Table 1) together with low HI values (71 mg HC/g TOC on average). The OI ranges from 21 to 75 mg CO₂/g TOC for the Amaltheenton claystone and is higher for Amaltheenton siderite concretions (Fig. 5). It should be noted that OI values of samples containing siderite concretions are biased due to the early thermal decomposition of siderite below the temperature limit of the S3 trapping (390 °C) (Hazra et al. 2022), resulting in S3 peaks and OI values that are too high and MinC concentrations that are too low (max. 4.7%).

The interval has the least negative δ¹³C_org values, gradually decreasing towards the top of the formation from − 24.4 to − 26.4 ‰ (Fig. 5). A similar δ¹³C_org gradual trend has been observed in the upper interval of the Upper Pliensbachian margaritatus ammonite zone in Schandelah-1, following positive CIE (CJ8; van de Schootbrugge et al. 2019). Compound-specific isotope data of n-alkanes, pristane (Pr), and phytane (Ph) reveal low δ¹³C variations (Fig. 5).

Pr/Ph ratios are relatively high, with up to 4.28. Corresponding to high Pr concentrations, Pr/nC₁₇ ratios are higher than Ph/nC₁₈ ratios. The terrigenous-aquatic ratio (TAR) (Bourbonniere and Meyers 1996), which expresses the relationship of n-alkanes linked to a terrigenous and an aquatic source, is relatively high, with up to 4.63. Compared to 17α-hopanes, the concentration of regular steranes is low, as indicated by low ster/hop ratios (C_27-29 regular steranes/C_31-35 17α-hopanes; Moldowan et al. (1985)). The homohopane index (HHI) values (Peters and Moldowan 1991), expressing the relative concentration of C₃₅ homohopane to C₃₁ to C₃₄ homohopanes, are low. Gammacerane and isorenieratane were not detected. The higher plant index (HPI), which is based on the relative abundance of higher land plant-derived polyaromatic hydrocarbons and bacteria-derived 1,3,6,7-TeMN (tetramethylnaphthalene) (van Aarssen et al. 1996), is high. The interval shows the highest C₂₉ regular sterane shares, relative to C₂₇ and C₂₈ regular steranes with 30–44% (Table 2).

Table 2 Overview of biomarker ratios

OM-lean claystone (Lower Toarcian)

The TOC content, HI values and OI values of the lowermost Lower Toarcian claystone interval are similar to those of the Amaltheenton Fm. δ¹³C_org ranges between − 25.9 and − 26.2 ‰ (Fig. 5).

The sample WIC_6559 has the lowest TAR and HPI. Moreover, the sample shows the lowest relative C₂₉ regular sterane concentration, with 27% (Table 2). Low ster/hop ratios indicate the dominance of hopanes. The relative concentration of C₃₅ homohopanes is low; thus, only detectable in WIC_6182. Furthermore, isorenieratane was only detected in sample “WIC_6182” with a low concentration near the GC–MS detection limit. Gammacerane was not detected. Pr/Ph ratios are lower at WIC compared to MAI (Table 2). Molecular geochemical differences between WIC and MAI are likely attributed to the fact that the wells cover different intervals of the Lower Toarcian OM-lean claystone interval, supporting the assumption of the presence of a fault resulting in lithostratigraphic gaps in the Lower Jurassic deposits in well MAI (Fig. 3). However, a reliable conclusion on paleodepostional conditions or stratigraphic classification of the lowermost Posidonienschiefer Fm in the WIC well cannot be established based on limited geochemical data alone without further biostratigraphic information.

Posidonienschiefer Formation (Toarcian)

The overlying black shale intervals are characterized by high TOC contents and high HI values of up to 785 mg HC/g TOC. Generally, the HI is slightly higher at MAI than at WIC. The OI shows the lowest values of the studied well sections within the black shale intervals, with 12 mg CO₂/g TOC on average (Fig. 5).

Pr/Ph, TAR and HPI display the lowest values within the studied sedimentary sections. Corresponding to the more marine character of the OM, the Posidonienschieder Fm is microscopically characterized by a predominance of liptinites, i.e. large tasmanites (MAI_2500, MAI_3000; WIC_3880, WIC_6050; Fig. 3) and smaller alginites. Pr/nC₁₇ and Ph/nC₁₈ ratios are the highest. The biomarker distribution reveals a dominance of steranes over hopanes. C₂₇ regular sterane concentrations are higher at the base of the black shale intervals, concurrent with lower C₂₉ concentrations (MAI_3000, WIC_6051; Table 2). In Unit III, C₂₉ and C₂₈ regular sterane proportions are higher than in the underlying Units I and II, with a relatively decreased share of C₂₇ regular steranes (Table 2). Gammacerane was detected in all black shale samples with moderate concentrations (GI; Table 2). The aromatic fractions of all Posidonienschiefer Fm samples comprise aryl isoprenoids and isorenieratane (Table 2).

The prominent Lower Toarcian nCIE is represented in the lowermost black shale interval in well WIC (60.7 to 60.9 m—Unit I; Fig. 5) with a δ¹³C_org value of − 30.5 to − 30.7 ‰ (Fig. 5). In northern Germany at the well Schandelah-1, negative values down to − 33.2 ‰ have been observed over a 140 cm thick well interval (CJ9; van de Schootbrugge et al. 2019). By comparison, WIC reveals a relatively thin nCIE interval with a low magnitude. This indicates that the nCIE and by this the T-OAE climax deposits are missing in the studied black shale interval. Above the nCIE, Unit I in WIC is characterized by less negative δ¹³C_org (Fig. 5), likely reflecting the post-nCIE positive CIE (pCIE), which has been previously observed in the NWGB (van de Schootbrugge et al. 2019), the SWGB (e.g. Montero-Serrano et al. 2015; Suan et al. 2015; Hougard et al. 2021; Rübsam et al. 2022), the Paris Basin (e.g. Rübsam et al. 2014), the Yorkshire Basin (e.g. Kemp et al. 2005) and trans-regional in North American Lower Toarcian deposits (e.g. Them et al. 2017a). The distinctive CIE-pattern suggests that Unit I, covering the sedimentary sequence affected by CIEs, may correlate with the upper interval of the Lower Toarcian ammonite subzone “serpentinum zone”. The overlaying intervals, Unit II and Unit III, show a relatively stable δ¹³C_org profile.

Jurensismergel Formation (Upper Toarcian)

The HI is generally moderate in the Jurensismergel interval but is higher than in the underlying Lower Toarcian OM-lean claystone and the Upper Pliensbachian Amaltheenton Fm, with 252 mg HC/g TOC on average. OI values are similar to those of the claystone underlying the black shale (34 mg CO₂/g TOC on average). The conglomeratic/breccia base of the Jurensismergel Fm in well MAI is accompanied by a negative (low) HI and a positive OI peak, which is absent in well WIC, complying with the lack of a conglomerate base (Littke et al. 1991b). The Jurensismergel interval in WIC is additionally characterized by an interval with relatively high HI and parallelly low OI values at around 23 m, followed by a small interval with higher OI and lower HI at around 20 m depth (Fig. 5).

The transition between the Posidonienschiefer Fm and the Jurensismergel Fm is marked by a sharp increase of δ¹³C_org in both wells. With δ¹³C_org of − 27.7 ‰ on average, the organic carbon is isotopically heavier than in the underlying Upper Pliensbachina/Lower Toarcian claystone interval. In WIC, HI and OI variations at 23 m depth are accompanied by δ¹³C_org variations (Fig. 5). No significant difference was observed for compound-specific δ¹³C values of Pr, Ph and n-alkanes compared to the Lower Toarcian Posidionenschiefer Fm (Fig. 5).

Pr/Ph is higher compared to the underlying black shales, with values up to 4.72. Generally, lower concentrations of Pr and Ph are indicated by lower isoprenoid to n-alkane ratios. In well MAI, the upper samples, MAI_0800 and MAI_1700, show increased TARs, whereas the remaining samples have low TARs in a similar range as the underlying black shales. The uppermost samples of both wells, WIC_1452 and MAI_0800, show high relative sterane concentrations (ster/hop). For the remaining samples, hopanes predominate. The formation has the highest concentrations of C₂₈ ster with up to 43%. HHI is low, and gammacerane was only detected for sample MAI_1700. Isorenieratane was found in all Jurensismergel samples in low concentrations. Aryl isoprenoids are less abundant than in the black shales of the Posidonienschiefer Fm. The HPI is slightly higher compared to the underlying black shales; however, significantly lower than observed for the Lower Toarcian and Upper Pliensbachian claystone (Table 2).

Thermal maturity

Sample MAI_5500 contains vitrinite particles suitable for vitrinite reflectance measurement, revealing a mean reflectance value of 0.51 R_o%. VR for samples from WIC (0.55 R_o%) and other wells drilled along the southwestern limb of the Hils Syncline was recorded by Littke et al. (1991a). Accordingly, the mean Tmax of all studied samples is relatively low, with a mean value of 429 °C. High HI values confirm the low maturity of the studied samples, revealing relatively higher HI for well MAI (769 mg HC/g TOC on average) compared to WIC (720 mg HC/g TOC on average). These observations are in line with the trend observed at the southern limb of the Hils Syncline, with increasing thermal maturities from the SE to the NW (Fig. 2) (Littke et al. 1991b; Fang et al. 2019).

The C₃₁ hopane isomerization ratio [C₃₁hop 22S/(22S + 22R)] is a thermal maturation proxy suitable for immature samples because isomerization equilibrium is reached at low thermal maturation levels (Peters et al. 2005). The ratios of the studied black shale samples show the increasing thermal maturation level from MAI to WIC (Table 2). The calculated isomerization values are significantly below the isomerization equilibrium (Seifert and Moldowan 1980, 1986), indicating immature OM. Low degree of thermal maturation is additionally suggested by the abundance of isorenieratane in all studied samples of the Posidonienschiefer and the Jurensismergel Fm, a compound exclusively found in immature samples (Damsté et al. 2001).

The thermal alteration of OM potentially affects multiple organic geochemical paleodeposition proxies; as the organic geochemical composition of the studied sample sets suggests that the studied sedimentary sequences, WIC and MAI, are thermally immature, the samples are considered suitable for this study.

Discussion

Chemofacies variations of the studied core sections point to deposition under varying environmental conditions, such as the magnitude of detrital influx, OM precursors, paleoredox conditions, freshwater influx, and the degree of bioproductivity moderated by the nutrient input. Variations in paleodepositional conditions are described in the following from bottom to top (Amaltheenton Fm to Jurensismergel Fm) to develop an interpretative model of the Lower Jurassic depositional environment in the NWGB. Furthermore, the depositional environment of the study area is correlated with equivalent observations on Lower Jurassic deposits in other European sub-basins derived from previous studies, taking into account different theories concerning possible global and regional processes related to OM accumulation and carbon isotope perturbations.

Paleodepositional conditions

Amaltheenton Formation (Pliensbachian)/OM-lean Lower Toarcian claystone

Low U_EF, Mo_EF, and low C_org/P ratios (Fig. 6) indicate oxic bottom and pore water conditions (Algeo and Ingall 2007; Little et al. 2015; Algeo and Liu 2020) during the deposition of the Upper Pliensbachian and Lower Toarcian OM-lean claystone underlying the black shales of the Posidonienschiefer Fm. High Pr/Ph ratios, the absence of isorenieretane, and low concentrations of aryl isoprenoids (Table 2) support the assumption of oxic depositional conditions. Correspondingly, low TS values (Table 1; Fig. 9a) in the Amaltheenton Fm indicate minor activity of sulphate-reducing bacteria and aerobic OM degradation, resulting in low TOC contents. However, the presence of C₃₅ homohopanes in four of eight samples (HHI; Table 2) suggests that at least short-term dysoxic/anoxic conditions periodically prevailed during sedimentation and early burial (Peters and Moldowan 1991). For the Lower Toarcian deposits, higher TS values (2.9% on average; Table 1, Fig. 9a) and lower Pr/Ph ratios in WIC, suggest the development of generally more reducing conditions towards the transition to the black shale interval. Correspondingly, sample WIC_6182 showed the highest HHI value as well as the presence of isorenieretane (Table 2). These deviations might, however, be affected by short-distance vertical HC migration out of the overlaying black shale.

Fig. 6

Variations of paleodepositional conditions. Outliers in the depth profiles of C_org/P, Si/Al, Ti/Al, Zr/Rb and P_EF coincide with sideritic carbonate concretions

The elemental relationships Si/Al and Zr/Rb are proxies for grain size. Si and Zr are attributed to the abundance of quartz and zirconium, transported in the coarser silt- to sand-sized detrital fraction, whereas Al and Rb are related to clay minerals, as part of the finer grain size fraction (Dypvik and Harris 2001; Thöle et al. 2020). Furthermore, titanium is mainly found in heavy mineral phases, gravitationally fractionated during transport (Chen et al. 2013); thus, Ti/Al ratios are additionally linked to detrital material transport distance. Si/Al, Ti/Al and Zr/Rb ratios increase slightly upwards in the Upper Pliensbachian interval in MAI from 99 to 50 m (Fig. 6), indicating coarsening of the sediments’ detrital fraction and greater detrital influx. Slight but abrupt chemofacies changes in MAI at approximately 50 m depth suggest a change in the paleodepositional conditions. Above 50 m depth, detrital influx ratios are slightly lower, possibly related to variations of terrigenous clastic input. The entire Amaltheenton Fm is characterized by high Fe concentrations (Table 1) indicating significant input of terrigenous clastic material; these values are higher than those in the overlaying sediments.

Correspondingly, the evaluation of the OM composition reveals a high proportion of terrigenous OM, with high TARs, low sterane concentrations (ster/hop), and a high abundance of higher land plant-derived aromatic hydrocarbon compounds (retene, cadalene, and 1-isohexyl-2-methylnaphthalene (iHMN)–HPI; Table 2). Furthermore, a dominance of inertinites in the maceral distribution, high contribution of C₂₉ and low C₂₇ regular sterane concentrations, low TOC and HI values, and high OI values (Figs. 3; 5) are indicative of terrigenous OM origin (type III; Fig. 7a), aerobic OM degradation and unfavourable conditions for OM preservation. HI and OI values are stable in the core interval from 50 to 99 m (MAI), suggesting a relatively consistent OM source and stable redox state. Higher HI and lower OI values in the core interval of 45 to 50 m (MAI) are attributed either to OM source changes with a higher contribution of marine/aquatic OM or redox variations with enhanced OM preservation. In WIC, lower TARs in the Lower Toarcian OM-poor claystone interval indicate an increasingly marine/aquatic OM character, associated with slightly more reducing conditions (Pr/Ph; Table 2). A high C₂₇ regular sterane concentration in sample WIC_6182 corroborates a higher plankton/algae contribution (Table 2). High HPI, however, suggests a still significant contribution of higher land plant-derived OM to the total OM mix. The presence of hydrogen-poor type III kerogen (Fig. 7a) and the pronounced presence of inertinite underlines a considerable input of terrigenous OM and oxidative OM degradation.

Fig. 7

a HI versus OI with kerogen trendlines. b Relationship between δ¹³C_org and hydrogen index (HI). Linear trendlines indicate δ¹³C_org-HI covariations dominated by OM source (terrigenous-marine). High HI values for the Posidonienschiefer Fm are attributed to enhanced OM preservation under reducing bottom water conditions

In concurrence with increasing terrigenous clastic input, the δ¹³C_org profile shows a negative upward trend between 99 and 50 m, likely attributed to an OM-source shift as suggested by a strong linear δ¹³C_org-HI correlation (Fig. 7b). The overlaying interval shows slightly more negative δ¹³C_org values (Fig. 5), with a relatively abrupt decrease at 50 m depth. δ¹³C_org fluctuations in the depth range between 50 and 33 m are likely affected by both OM source and/or biodegradation variations, as suggested by a divergent δ¹³C_org-HI-relationship within the Lower Toarcian OM-lean claystone (Fig. 9b).

Siderite concretions are abundant within the claystone matrix, indicating their formation during early burial prior to compaction. Diagenetic pyrite overgrowth within the siderite concretions (MAI_8500; Fig. 3) suggests that cementation progressed at late diagenesis. Precipitation of Fe-containing carbonate requires preferential Fe-fixation to carbonate, enabled by sulphur limitation and/or iron excess (Curtis et al. 1986; Raiswell and Fisher 2000). The presence of siderite, therefore, suggests high Fe-availability, neutral to alkaline, sulphate-poor, and dysoxic to anoxic pore water conditions shortly after deposition. Fe excess is linked to high detrital influx during deposition of the Amaltheenton Fm claystone; concordant inorganic and organic geochemical data clearly indicate significant terrigenous clastic input and varying redox conditions. In addition, the abundance of siderite suggests that paleosalinity was relatively low (Coleman 1993; Roh et al. 2003).

Posidonienschiefer Formation (Toarcian)

The Toarcian black shales of the studied well sections are characterized by high TOC contents, HI values, the enrichment of redox-sensitive trace elements, such as Mo and U, and the highest C_org/P ratios (Fig. 6). The distinctive composition indicates anaerobic and at least periodically sulfidic/euxinic conditions, favourable for OM preservation. According to an increased reduction potential, HHIs are elevated (Table 2). Moderate enrichment of Mo and U, with Mo_EF from 18 to 145 (63 on average) and U_EF from 2 to 16 (6 on average) (Fig. 4), suggests intermediate euxinia (Scott and Lyons 2012) or suboxic conditions (Algeo and Tribovillard 2009). This suggests oscillating redox conditions and the absence of persistent bottom water anoxia. Correspondingly, strong enrichment of Mo relative to U, with Mo_EF/U_EF of 9.0 to 20.1 (13.4 on average) (above modern seawater molar Mo/U [~ 7.5–7.9; Algeo and Tribovillard (2009)] suggests that enhanced Mo accumulation may be the result of particulate shuttle (Fig. 8c). This phenomenon is associated with Mn–Fe redox cycling within the water column, triggered by strong oscillation of the water column redox conditions. TOC and TS enrichments show a good correlation near the present-day normal marine trendline (Fig. 8a). The TOC–TS–Fe relationship shows a slight sulfur excess (Fig. 8b), suggesting Fe-limited pyrite formation. This corresponds to reducing and sulfidic bottom water conditions probably under conditions of reduced terrigenous clastic input.

Fig. 8

a TOC versus sulphur plot after Berner and Raiswell (1984) with trendline for normal marine conditions. b Ternary plot of the relative concentrations of TOC–TS–Fe with interpretative areas for S-excess, anoxic, dysoxic, and oxic conditions after Dean and Arthur (1989). c Enrichment of Mo (MoEF) versus U (UEF) with trendlines related to variations in redox conditions, basin restriction and particulate shuttle (after Algeo and Tribovillard (2009)). Dotted lines represent the Mo/U molar ratios in seawater (SW). d Mo concentration versus TOC values, indicative of the degree of basin restriction and deepwater age (after Rowe et al. (2008))

Compared to the underlying and overlaying deposits, Pr/Ph ratios (Table 2) are lower in the black shale section, indicating more reducing conditions during the deposition of the Posidonienschiefer Fm. Pr/Ph values are moderate (Pr/Ph > 1), typically indicative of suboxic conditions. However, Pr/Ph ratios slightly above 1 do not necessarily exclude the possibility of prevailing anoxic paleodepositional conditions, as the ratio is potentially biased by the additional origin of Pr from other compounds than chlorophyll, such as tocopherol (Frimmel et al. 2004).

The presence of isorenieratane, which is derived from anaerobic, photosynthetic green sulfur bacteria, indicates photic zone anoxia and the presence of H₂S in the photic zone water column, as suggested for Toarcian black shales previously (Schouten et al. 2000; Schwark and Frimmel 2004). In support of this, the presence of gammacerane indicates water column stratification. However, low GI values (Table 2) point to weak water column salinity differences or periodic disturbance of water stratification.

The Si/Al trend deviates from the Ti/Al and Zr/Rb ratios, suggesting a considerable contribution of non-detrital silica in the black shale intervals attributed to intense biogenic silica precipitation. Ti/Al and Zr/Rb ratios are lower in the black shale interval compared to the underlying deposits (Fig. 6), suggesting grain size fining and a lower detrital influx.

Molecular geochemical OM-source parameters (Table 2) indicate a high contribution of marine/aquatic algae to the total OM mix, that is correlating with a reduced terrigenous clastic input, e.g. low Ti/Al ratios. In corroboration, high Pr and Ph concentrations compared to nC₁₇ and nC₁₈ indicate a higher contribution of chlorophyll-incorporating plant material and a dominant proportion of aquatic plants (Powell and Mckirdy 1975). Increased C₂₇ and reduced C₂₉ regular steranes reflect a shift to aquatic, algae-derived OM. High HI and low OI values characterize type I kerogen (Fig. 7a). The high HI values (Type I; Fig. 7a) are likely not exclusively attributed to the OM source, but are also significantly influenced by conditions favourable for preservation of H-rich OM. Organic petrographic observations revealed a high abundance of telalginites (i.e. tasmanites) (MAI_2500, MAI_3000, WIC_3880; Fig. 3) and the presence of fish bone fragments (MAI_3000, WIC_6050; Fig. 3), indicative of a marine paleoenvironment.

High Ni_EF, Cu_EF and P_EF values (Figs. 3; 6) are potentially related to increased C_org sinking flux/productivity (Tribovillard et al. 2006). Hence, the predominantly aquatic OM character may be the result of a substantial increase in aquatic biomass input (algae). However, the relative enrichment of these elements may be also associated with prevailing reducing conditions and pyrite formation. This would also be supported by the enrichment of other elements associated with pyrite formation (e.g. Ag; Fig. 3).

Suan et al. (2015) delineated significant effects of OM origin variations on the magnitude of the Lower Toarcian nCIE, with more negative values correlated with predominantly marine algae OM input. Correspondingly, the marine OM character suggests that relatively negative δ¹³C_org of the Posidonienschiefer Fm, compared to the underlying and overlaying claystone, are dominantly attributed to OM source. Additionally, the δ¹³C_org-HI- covariation of the black shale interval is characterized by extraordinarily high HI values, likely attributed to high primary productivity, and fast OM accumulation enhanced OM preservation under reducing conditions. The negative and positive CIEs in Unit I (WIC; Figs. 5, 7b) are probably additionally attributed to superordinate global carbon cycle perturbations.

Variations of paleodepositonal conditions during the black shale deposition are, among other paleoredox condition proxies, supported by vertical variations of redox-sensitive trace element enrichments. In MAI, the upper black shale interval (27 to 21 m) shows higher Mo_EF, U_EF and TOC contents(Fig. 6) indicating more reducing conditions. In WIC, the most profound anoxia is suggested for the lowermost black shale Unit I (61 to 57 m), with highest HI, U_EF and MinC values. Low Mo concentration, compared to U and TOC, suggests strong basin restriction (Fig. 8c and d) (Algeo and Lyons 2006). Unit II (49 to 56 m) in well WIC represents a gradual decrease of the redox potential, e.g. indicated by decreasing Mo_EF and U_EF. The base of Unit II is characterized by extraordinarily high Mo_EF, compared to U and TOC (Fig. 6), potentially indicating deepwater renewal and increased basin connectivity (Algeo and Lyons 2006). A parallel negative excursion of TOC, HI, Mo_EF, Si/Al, P_EF, Cu_EF, and Ni_EF has been observed in this interval (Fig. 6). The interval corresponds to the low carbon zone (LCZ) described by Littke et al. (1991a), which is additionally characterized by the irregular appearance of bivalve-rich layers. The interval probably represents a period of sea level rise, with low detrital and nutrient influx, decreased bioproductivity and OM rain, and reduced activity of anaerobic OM degradation processes. This potentially led to increased oxygenation, allowing temporary benthos habitation. The overlaying Unit III (49 to 30) is characterized by increased HI, Mo_EF and U_EF values, suggesting the re-establishment of more stable and pronounced bottom water anoxia. Gradually decreasing Mo_EF in Unit III in WIC suggest reduced basin connectivity towards the Late Toarcian (Fig. 6).

The lowest TAR, HPI, and highest C₂₇ regular sterane and relative sterane over hopane concentrations were observed for Unit I and Unit II in WIC (WIC_5345 and WIC_6051; Table 2), indicating the most pronounced marine OM origin. Unit III is characterized by slightly higher TAR and HPI values, lower sterane concentrations and increased contribution of C₂₉ regular steranes (Table 2), indicating slightly increased contribution of terrigenous OM.

Jurensismergel Formation (Upper Toarcian)

The transition from the Posidonienschiefer Fm to the Jurensismergel Fm reveals abrupt changes in redox-sensitive proxies, indicating the onset of less reducing conditions (Fig. 6). However, depositional conditions remained reducing compared to the OM-lean Upper Pliensbachian–Lower Toarcian sediments (Figs. 6 and 8B). This is in agreement with the higher TOC contents and HI values (Table 2, Fig. 5). Additionally, the presence of isorenieretane implies at least periodic anoxia, sulfidic conditions, and photic zone anoxia.

Pr/nC₁₇, Ph/nC₁₈, ster/hop, TARs, C₂₉ regular sterane, and HPI values (Table 2) are similar to the underlying black shale interval. This suggests that less reducing conditions, i.e. less favourable for OM preservation, are the main reason for the decrease in TOC and HI values, rather than OM-source variations.

δ¹³C_org values are lower than in the underlying Upper Pliensbachian and Lower Toarcian OM-lean claystone. The variations are likely mainly related to a lower proportion of terrigenous higher land plant material (Fig. 7b), as e.g. indicated by lower HPI values (Table 2; Fig. 5).

Si/Al and Ti/Al ratios of the uppermost core section of MAI and WIC show contradictory trends within the Jurensismergel claystone interval (Fig. 6). This may indicate that the wells cover different stratigraphic intervals of the Jurensismergel Fm, making consistent interpretation of clastic input and silicate weathering difficult. However, the values are generally slightly higher than in the underlying Posidonienschiefer Fm, pointing to a coarser clastic influx.

Depositional model: Northwest German Basin

Based on chemo- and lithostratigraphic variations and interpreted paleoenvironmental changes, the following chronological depositional model has been developed for the NWGB (Fig. 9).

Fig. 9

Lithostratigraphic profile of WIC and MAI with schematic regression (R)—transgression (T) cyclicity (left). Schematic sketch of a chronological depositional model (right). For a detailed discussion and comparison with the SWGB, see the text

Conditions prior to the deposition of the Toarcian black shale were oxic with a high contribution of detrital input and dominance of terrigenous-derived OM. The combined information indicates deposition near the shoreline with low sea level, and mostly well-mixed water bodies. Monsoonal climate, suggested for NW Europe in Early Jurassic times (Parrish 1993), caused periods of seasonally increased freshwater inflow in summer. Seasonal climate variations may have resulted in periodical short-term reducing conditions near the sediment/water interface and low paleosalinity/low sulphate concentrations (Fig. 9a), as suggested by the abundance of siderite concretions.

A shift from a warm and humid climate during the deposition of the margaritatus ammonite zone to a dry and cold climate at the margaritatus–spinatum transition has been suggested in previous studies (Bailey et al. 2003; Korte and Hesselbo 2011; Storm et al. 2020). In this context, slightly increasing detrital influx proposed for the 99 to 50 m interval in the MAI borehole accompanied by gradually decreasing δ¹³C_org might be associated with the described climate shift: from a warm and humid climate with a sea level rise and widespread accumulation of ¹²C-rich OM to a cooler climate with gentle sea level drop.

The overlying interval (50 to 32 m MAI, 66 to 61 m WIC) is characterized by slightly lower detrital input (Fig. 6) and an abrupt change of δ¹³C_org values. These trends probably reflect the Late Pliensbachian cooling event (e.g. Korte and Hesselbo 2011); colder and dryer climate reduced the influx of terrigenous clastic material, and a sea level drop enforced sedimentary reworking and consequently enhanced ¹³C accumulation.

Sharp geochemical and lithological changes mark the transition of OM-lean claystone and black shales. This transition indicates the presence of either an extremely rapid turnover of the epicontinental sea conditions or a stratigraphic gap. Corroborative to the latter, the uppermost Lower Toarcian claystone interval in MAI (33 to 32 m) shows a breccia-like structure situated below the core loss interval (30 to 32 m), suggesting partial erosion of Lower Toarcian transgressive sediments (Fig. 9).

Significant geochemical and lithological differences of the Toarcian black shale interval (Fig. 9b to d) compared to the overlaying and underlying sedimentary successions are attributed to severe changes in paleodepositional conditions. High algae contribution to the total OM mix and lower detrital input, point to a more aquatic depositional setting, likely related to a strong and rapid preceded sea level rise in earliest Toarcian times (e.g. Röhl et al. 2001). Highly reducing and euxinic conditions (e.g. Mo and U; Fig. 6) and photic zone anoxia (isorenieretane; Table 2) indicate that restricted water circulation and a stratified water column with a shallow chemocline were associated with sea level rise. In addition, global greenhouse conditions with increased water temperatures during the Early Toarcian reduced O₂ solubility (Geng and Duan 2010) potentially favouring the development of anoxia in the lower water column. Considering a water temperature increase from 21 to 32 °C in a brine with 1 mol/kg NaCl, a reduction of the oxygen solubility in the bottom water column of > 10% is expected (Song et al. 2014).

The deposition of OM-rich sediments may be attributed to high bioproductivity, linked to enhanced nutrient supply caused by (i) intensified chemical weathering due to higher temperatures and (ii) intensification of monsoonal tropical cyclones due to significant climate warming of approximately 10 °C (Suan et al. 2010; Korte et al. 2015; Krencker et al. 2015) during the Early Toarcian. In contrast, recent studies suggest that the onset of Lower Toarcian black shale deposition during the T-OAE climax was associated with low C_org accumulation rates/sinking speed; i.e. a weakened biological pump (Rübsam et al. 2022). In this context, however, it is important to notice that the studied well section MAI and WIC do not fully cover the T-OAE climax, marked by CIEs (referred to as “CIE onset and core” in Rübsam et al. (2022). The studied deposits rather represent the nCIE recovery and post-nCIE intervals, which are characterized by increased C_org accumulation rates and the reestablishment of a “strong biological pump”.

Thick and relatively continuous Posidonienschiefer black shale intervals in the NWGB basin suggest that paleowater depths were shallow enough for the establishment of restricted water circulation, and deep enough to exceed the storm wave base to allow relatively undisturbed OM accumulation. Hence, the NWGB was probably characterized by low to moderate water depths, probably in the order of 100 to 200 m. Comparatively, the water depth in the NWGB was probably deeper than in the SWGB, where black shale deposits are often less thick and characterised by the presence of thin silt layers, and where periodically low paleowater depths of down to a few tens of metres are suggested (Röhl and Schmid-Roehl 2005).

Although the most striking geochemical differences are between the Posidonienschiefer Fm and the underlying and overlaying claystones, paleodepositional variations are also suggested for Toarcian black shale deposition.

The lowermost Unit I in well WIC represents the earliest phase of black shale deposition revealed in both studied wells (Fig. 9b), potentially corresponding to the upper serpentinum/falciferum zone based on the observed CIEs. The OM shows the highest HI values suggesting the most reducing conditions, related to sea level highstand (Röhl and Schmid-Roehl 2005; Song et al. 2014) (Fig. 8c and d). Furthermore, the highest relative C₂₇ regular sterane contribution in Unit I (Table 2) is ascribed to a different primary marine producer community structure. Consistent with this, the replacement of calcareous nannoplankton by green algae was suggested in previous studies in the course of a biocalcification crisis during the nCIE, i.e. the T-OAE climax (van de Schootbrugge et al. 2013; Rübsam et al. 2022).

There is controversy about the causes and their respective role and importance in the formation of anoxic and euxinic bottom water conditions during the Toarcian. As the cause of water stratification with anoxic lower water bodies, studies in southern Germany describe either (i) ostensibly sea level fall followed by rapid sea level rise together with the subordinate influence of increased rainfall and freshwater input (e.g. Frimmel et al. (2004); Röhl and Schmid-Roehl (2005); Rübsam et al. (2018)), or (ii) primarily the intensification of monsoonal rainfall and freshwater influx in interaction with basin confinement moderated by water level fluctuations (e. g. McArthur et al. (2008); Bodin et al. (2016); Van de Schootbrugge et al. (2020)). It is generally agreed that rapid and significant climate change played a crucial role, likely triggered by volcanic greenhouse gas emission in the Karoo Ferrar Large Igneous Province and/or severe thermogenic dissolution of gas hydrates (e.g. Kemp et al. 2005; Krencker et al. 2015; Them et al. 2017b). The positive δ¹³C_org excursion may be ascribed to a global warming-induced intensification of terrigenous chemical weathering causing increased ¹³C release (Them et al. 2017b) and/or the burial of great masses of ¹²C-enriched (marine) OM on a global scale, resulting in ¹³C-enriched atmospheric CO₂ (Rübsam et al. 2018). This supports the theory of global carbon cycle perturbations, associated with the observation of distinctive carbon isotope patterns of Lower Toarcian deposits worldwide (e.g. Bodin et al. 2016; Them et al. 2017a; Fantasia et al. 2019b). Another common theory for the development of the Lower Toarcian nCIE is based on a combination of high OM rain and OM degradation activity under reduced vertical water oscillation. These processes lead to ¹²C enriched aquatic CO₂ in the photic zone, causing the enhanced fixation of light carbon to marine biomass; commonly referred to as the Küspert model (Küspert 1982). However, the deposition of black shales in the NWGB goes far beyond Early Toarcian times. This suggests that CIEs may not have been exclusively controlled by regional processes, but that changes in the global carbon budget probably have played a role, as already discussed in previous publications (e.g. Hougard et al. (2021), Rübsam et al. (2018)).

Unit II is characterized by proxies indicating less reducing conditions, lower bioproductivity, and slightly reduced clastic input. This is probably linked to sea level rise, and enhanced water circulation attributed to enhanced hydrogeographical connection between the Tethys and Arctic (Fig. 9c). In South Germany, the transition from the serpentinum/falciferum zone to the bifrons zone is marked by a condensate section (Röhl and Schmid-Roehl 2005). This interval represents the maximum flooding surface deposition under enhanced oxygenation and temporally correlates possibly with the onset of the intensification of south-ward water influx from the Arctic through the Viking Corridor (van de Schootbrugge et al. 2019). The enhanced connectivity of the Tethys and Arctic Sea has likely enhanced water circulation, with the influx of high-density, cold water, causing weakening and migration of the chemocline and enhanced bottom water oxygenation. This event may correspond to the geochemical variation observed in Unit II, where the pronounced Mo-enrichment in sample WIC_5646 is possibly associated with deepwater renewal (Fig. 9d). Nutrient supply and paleobioproductivity possibly decreased due to increased distance from land and lower nutrient influx.

The overlaying black shale intervals (Unit III) suggest relatively stable detrital influx and the initiation of slow regression. A sea level fall consequently hampered water exchange between CEBS, the Arctic and the Tethys, allowing the reintroduction of stratified water bodies (Fig. 9e). Furthermore, shallower paleowater levels may have been associated with higher nutrient influx and promoting increased primary marine productivity. These conditions allowed the onset of a long-term proceeding black shale deposition that significantly endured Early Toarcian carbon isotope perturbations (Fig. 5).

Considering source-related δ¹³C_org variation (Fig. 7b), the black shale intervals show relatively high δ¹³C_org values. Therefore, a relatively high accumulation of heavier ¹³C might be related to fast OM accumulation and intense microbial biodegradation. Soluble bitumen shows increased C₂₈ and decreased C₂₇ regular steranes concentrations, compared to Units I and II in well WIC (Table 2). This may reflect the enhanced nannoplankton production that is suggested for the post-CIE period (Rübsam et al. 2022).

Due to erosional stratigraphic gaps, Upper Toarcian bituminous shales are often absent in more southerly regions. However, in a few southern wells and outcrops, the variabilis zone is present and characterized by relatively lower OM content compared to the underlying bifrons zone (Riegraf et al. 1984; Fantasia et al. 2019b). This supports that the Toarcian black shale deposition was relatively prolonged and intensified in the NWGB, compared to the SWGB. However, stratigraphic gaps in both profiles from northern and southern Germany prevent a clear stratigraphic correlation. Thus, differences in the lithology and geochemistry of Upper Toarcian Formations could also be explained by the fact that different sections of the variabilis zone are present.

The sharp lithological transition between black shale deposits and claystones of the Jurensismergel succession (WIC—30 to 11 m; MAI—21 to 4 m) is potentially related to sea level changes (Fig. 9e). The Jurensismergel marlstone composition suggests deposition under significantly increased oxygen availability in the bottom water. Compared to the Upper Pliensbachian claystone, the OM has a higher contribution of marine-sourced material; similar to Unit III of the Posidonienschiefer Fm. This may be attributed to increased connectivity between the Arctic and Tethys oceans starting at the end of the serpentinum/falciferum subzone. The climate shift to colder and dryer conditions suggested for the latest Toarcian to early Aalenian (Dera et al. 2009; Korte et al. 2015) may have reduced the influx of terrigenous OM. Moreover, changing climate conditions potentially led to reduced freshwater influx, increased O₂ water solubility and less reducing conditions.

Conclusion

Biostratigraphic and chemostratigraphic evaluation revealed that the combined studied sedimentary succession of the wells WIC and MAI represents a relatively complete sedimentary profile of Upper Pliensbachian to Upper Toarcian/Aalenian deposits. However, unconformities and considerable thickness variations indicate the presence of stratigraphic gaps. These are possibly attributed to sea level change-related erosion and non-depositional phases, or faults interrupting the sediment profile. Both wells are thermally immature in terms of petroleum generation with similar maturity levels, allowing the joint consideration for paleodepositional reconstruction. Due to the low thermal maturity, organic geochemical and organic petrological data are suitable for the reconstruction of paleo-environments.

During the Late Pliensbachian and earliest Early Toarcian, sediments in the study area were deposited in a shallow water environment with effective water circulation under predominantly oxic bottom water conditions. The input of terrigenous clastic mineral matter and OM was high according to the proximity to the coast.

The initiation of the black shale deposition probably was moderated by changes in sea level, with a sea level drop causing basin restriction initiated in the Late Pliensbachian, followed by a drastic sea level rise leading to the subsequent deposition of Toarcian black shales. Black shale deposition probably occurred at water depths of about 100 to 300 m, deeper than suggested for the SWGB. The TOC-rich Posidonienschiefer Fm deposits indicate a pronounced change in depositional conditions, with anoxic to euxinic bottom waters, a dominant contribution of aquatic OM and an elevated sea level. These factors allowed the development of water stratification, which, however, was periodically interrupted. Both organic and inorganic indicators suggest a close interaction between water stratification, bioproductivity and reducing conditions, linked to an intense greenhouse climate development. Rising temperatures probably led to a general reduction in the oxygen saturation of seawater, as well as an intensification of monsoonal rainfall with consequently increased freshwater and nutrient input. A period of maximum flooding at the beginning of the bifrons zone (Unit II) is related to enhanced water circulation attributed to an enhanced hydrogeographical connection between the Tethys and the Arctic. In the study area (NWGB), a particularly prolonged black shale deposition lasting up to the Late Toarcian can be observed. This may be attributed to increased nutrient influx due to greater monsoonal rainfall in northern Europe and/or a more intense influence of the inflow of Arctic water.

Black shale deposition ceased with the onset of Jurensismergel deposition, possibly triggered by a sharp temperature drop and sea level changes. The sharp lithological transition to the underlying black shale may indicate the presence of a stratigraphic gap and sea level change-related erosion. Compared to the Upper Pliensbachian Amaltheenton Fm, the Jurensismergel Fm is characterized by subordinate terrigenous input, with a higher contribution of marine-sourced OM and higher TOC and carbonate content.