The investigated section “Hainberg” is a motorway cutting located in Northern Germany, Lower Saxony, approximately 18 km ESE of Hildesheim (Fig. 1) at the motorway A7 (Hannover-Kassel) immediately south of the resting place “Sillium-Ost”. The coordinates of the section, located on the topographic map 1:25000, sheet 3927 Ringelheim are 52°3.250781'N, 10°11.397949'E to 52°3.465281'N, 10°11.517376'E, at an elevation of ca. 150–155 m a.s.l. at the western foot of the “Hainberg”. The exposure of the motorway cutting was about 350 m long, with up to 4 m height.

The Hainberg is situated in the northern foreland of the Harz Mountains at the southern margin of the North German Basin. The area is composed of gentle synclines and anticlines of Mesozoic sedimentary rocks, overlying Permian evaporites (Zechstein Group) and the Variscan basement of the deep subsurface (Fig. 1). The latter, at 2.0–2.3 km depth, is composed of folded Palaeozoic greywackes and shales, which are exposed further SE in the Harz Mountains. The Permian comprises minor continental siliciclastics (70–140 m Rotliegend Group) and a thick sequence of marine evaporites (600–1200 m Zechstein Group), which commonly form salt diapirs. The Mesozoic cover sequence starts with continental siliciclastics of the Lower Triassic Buntsandstein Group (700–950 m), followed by marine carbonates of the Middle Triassic Muschelkalk Group (275–290 m) and again continental red beds of the Upper Triassic Keuper Group (ca. 350 m) (Schröder 1912; Dahlgrün 1939; Look et al. 1984, 1986; Jordan 1989).

At the western slope of the Hainberg, which is part of the Ringelheim Syncline, Lower Jurassic strata of the Schwarzjura Group (Franz et al. 2020) show a 300–350 m thick succession of dark calcareous claystones, with the 27–30 m thick Posidonienschiefer Fm. and a less than 1 m thick Jurensismergel Fm. at its top. Of the following Braunjura Group, up to 300 m dark marine claystones are preserved, cut at their top by an angular unconformity. Younger Jurassic strata as well as major parts of the Lower Cretaceous strata are absent. Albian to Santonian strata of the Hilssandstein Fm., Alfeld Fm., Plänerkalk Group, and Emscher Fm. (Hiss et al. 2005), together up to 520 m thick, follow from the top of the Hainberg to the centre of the Ringelheim Syncline, which is covered by Quaternary sediments (Look et al. 1984, 1986).

The folding of the Mesozoic strata into synclines and anticlines is due to a combination of tectonic faults in the Variscan basement and halotectonic movements of the Zechstein Group (Martini 1953; Jordan 1989). These combined ortho- and halotectonic movements were active during the Triassic, Late Jurassic and, with increasing intensity, during the Santonian to Campanian, when the Harz Mountains were uplifted (Voigt et al. 2004; von Eynatten et al. 2008). For the Early Jurassic, such tectonic movements in Northern Germany have been inferred from increased strata thickness at the margin of salt diapirs (e.g. drilling Etzel 24; Hoffmann 1968a; drilling Hamburg Allermöhe 1; Zimmermann et al. 2015) and thin, iron-oolitic deposits at salt pillows (e.g., Fallstein; Thomas 1924; Ott 1967). Indeed, the Hainberg section lies at the western margin of the Eichsfeld-Altmark High, a Permotriassic tectonic high (Paul 1993), which was active at least to Late Triassic times (Barnasch et al. 2005).

At the site of investigation, the strata were 10° inclined towards ENE (70°). Further uplift of the whole region, from a near sealevel position to the present day elevation took place after the Oligocene (e.g. Jordan et al. 1994: p. 68). The present-day geomorphology is essentially the result of an intense Miocene-Pliocene weathering, followed by Pleistocene erosion with frost wedging and solifluction (e.g. Brosche 1996), which also shaped the ridge of the Hainberg and covered its lower slopes with debris (Look et al. 1984, 1986; Heunisch et al. 2017).

Informal bed names are given in quotation marks. An overview of the section is provided in Fig. 2. Ammonites and belemnite findings are illustrated in Figs 3–26. The strata are described from bottom to top:

Figure 2. 

Overview on lithology and lithostratigraphy of the Hainberg section (top Amaltheenton Fm. to lower Opalinuston Fm.) with CaCO₃, organic carbon, and sulphur contents. For legend see Fig. 28.

Bed 1: >100 cm medium-grey, well bedded claystone with white-grey quartz silt and fine-grained mica layers;

Bed 2: 1 cm rust-brown layer of siderite nodules;

Bed 3: 230 cm medium-grey, well bedded claystone with white-grey quartz silt and fine-grained mica layers (with small-scale cross stratification);

Bed 4: 2 cm rust-brown layer of siderite nodules;

Bed 5: 60 cm medium-grey, well bedded claystone with white-grey quartz silt and fine-grained mica layers;

Bed 6: 8 cm light-grey to white-grey, well bedded silty claystone with fine-grained mica.

Bed 7: 15 cm rust-brown/yellowish-brown varved, laminated clay with fine-grained mica and black manganese coatings on bedding planes and fractures;

Bed 8: 5 cm light-grey/middle-grey varved, laminated clay with yellow-brown weathered layers of former iron sulphides, abundant fine-grained mica on bedding planes;

Bed 9: 40 cm grey to yellow-brown weathered, laminated bendable clay with few fish scales and teeth; ammonoids: Dactylioceras cf. crosbeyi (Simpson) (compressed; Fig. 3);

Bed 10: 40 cm dark-grey bituminous calcareous claystone with even lamination and minor fish scale debris; ca. 10 above basis one 8 cm thick lenticular limestone concretion (“Elegantulum Concretion”); ammonoids: Eleganticeras elegantulum (Young & Bird) (in concretion; Fig. 27), 10 cm above basis Eleganticeras sp. (compressed); other fossils: Lepidotes elvensis (de Blainville) (Fig. 27), Meleagrinella (Clathrolima) sp.; Coelodiscus minutus (Schübler in Zieten);

Bed 11: 0–18 cm “Boreale Concretions”: medium grey laminated bituminous limestone concretions (pellet packstone) up to 50 cm width, with scattered mm-sized holoplanktonic gastropods in layers, fine-grained shell debris and minor fish scale debris; ammonoids: Hildaites murleyi (Moxon) (Fig. 4); other fossils: Coelodiscus minutus (Schübler in Zieten), abundant Parainoceramya dubia (Sowerby);

Bed 12: 150 cm dark-grey laminated bituminous marl to calcareous marl; minor fine-grained shell and fish scale debris; ammonoids: Lytoceras sp. (compressed; 75 cm above basis; Fig. 5); other fossils: Meleagrinella (Clathrolima) sp. (layers with poorly preserved specimens 10–15 and 140 cm above basis; Parainoceramya dubia (Sowerby) (75 cm above basis; Fig. 5);

Bed 13: 15 cm dark-grey laminated bituminous argillaceous limestone with fine-grained shell debris and minor fish scale debris; ammonoids: Lytoceras sp. (compressed), Eleganticeras sp. (compressed), other fossils: several bedding planes with abundant Bositra buchi (Roemer) up to 8 mm in size, few small Parainoceramya dubia (Sowerby), Meleagrinella (Clathrolima) sp.;

Bed 14: 25 cm dark-grey laminated bituminous calcareous marl with fine-grained shell debris and minor fish scale debris; fossils: Meleagrinella (Clathrolima) sp.;

Bed 15: 45 cm “Lower Elegans Bed”: dark-grey laminated bituminous argillaceous limestone with fine-grained shell debris and rare fish scales; ammonoids: Eleganticeras elegans (Sowerby) (compressed; Fig. 6); other fossils: two aptychae, one fragmentary fish remain (Tetragonolepis?);

Bed 16: 30 cm dark-grey to black, highly bituminous laminated argillaceous marl with fine-grained shell debris; fossils: several bedding planes with pavements of Bositra buchi (Roemer) (up to 8 mm in size); one Parainoceramya dubia (Sowerby);

Bed 17: 340 cm dark-grey to black, bituminous laminated marl with fine-grained shell debris and rare fish scales; minor fine-grained carbonaceous plant debris; ammonoids: Harpoceras sp. (compressed fragment 50 cm above basis); other fossils: Parainoceramya dubia (Sowerby) (common 220 and 280 cm above basis);

Bed 18: 22 cm dark-grey, bituminous laminated calcareous limestone full of Bositra shell debris and abundant fish scales in layers; fossils: Bositra buchi (Roemer);

Bed 19: 10 cm dark-grey, bituminous laminated calcareous marl full of Bositra valves and shell debris; uneven bedding planes; fossils: Bositra buchi (Roemer);

Bed 20: 10 cm dark-grey, bituminous laminated calcareous marl to argillaceous limestone; fish scale debris in layers;

Bed 21: 40 cm dark-grey, bituminous laminated marl full of Bositra buchi valves up to 12 mm in size; uneven bedding planes;

Bed 22: 1 cm “Monotis Bed”: medium-grey, microcrystalline argillaceous limestone composed of numerous Meleagrinella (Clathrolima) substriata (Münster) (see Lutikov and Arp 2022 for taxonomy);

Bed 23: 2 cm “Commune belemnite battlefield”: dark-grey, rust-brown oxidized, bituminous laminated argillaceous marl with numerous, current-aligned belemnite rostra; belemnites poorly preserved due to pyrite oxidation; ammonoids: Dactylioceras sp. (compressed; Fig. 7); other fossils: Acrocoelites sp. (Fig. 8), Dactyloteuthis irregularis (Fig. 9), Stenopterygius sp. (rib fragments);

Bed 24: ca. 18 m dark-grey bituminous laminated calcareous claystone to argillaceous marl; scattered compressed marcasite nodules; even bedding planes; lower 10 m with bedding planes full of Bositra shell debris and few complete Bositra buchi valves (up to 5 mm in size); uppermost 8 m with decreasing fine-grained Bositra shell debris, increasing carbonaceous plant debris and mica flakes; partially fossil-free-layers; compressed ammonoids Dactylioceras cf. commune (Sowerby) (5 cm above basis); other fossils: Bositra buchi (Roemer), ostracods (uppermost 2 m), one mm-sized pellet composed of fish remains (2 m below top);

Bed 25: 60 cm medium-grey to brownish, bituminous laminated claystone; with minor fine-grained shell debris at its top;

Bed 26: 0–6 cm “fucoid bed equivalent”: medium-grey, laminated lenticular to irregularly shaped calcareous marl to argillaceous limestone concretions with calcite or marcasite-filled burrows (0.5 cm diameter); thin layers with white, fine-grained shell debris; sharp upper boundary, with impressions of ooids from the bed above.

Bed 27: 20 cm “Oolite marl with belemnite accumulation”: indistinctly bedded, medium-grey marl with numerous bioclasts, iron ooids and cm-sized white, ooid-bearing phosphorite nodules; iron ooids 1–2 mm in size and colonized by nubeculariid foraminifera; belemnite accumulation at the basis of the bed; ammonoids: Osperleioceras cf. beauliziense (Monestier) (5 cm above basis) (Fig. 10); Perilytoceras sp.; other fossils: Dactyloteuthis irregularis (Schlotheim) (mass accumulation), Chlamys textoria (Schlotheim), Liostrea erina (d’Orbigny), Parainoceramya sp., Chladocrinus sp., ostracods, Chondrites sp.;

Bed 28: 15 cm “Zeta Conglomerate“: indistinctly bedded, medium-grey marl with iron ooids, bioclasts and numerous reworked rust-brown, cm-sized concretions (intraclast rudstone); abundant phosphoritic ammonite casts and reworked fragments; abundant nubeculariid foraminifera on ooids, echinoderm debris and other bioclasts; belemnite accumulation at the basis of the bed; ammonoids: Phlyseogrammoceras dispansiforme (Wunstorf) (Figs 11, 12), Phlyseogrammoceras transiens (Ernst) (Fig. 13), ?Alocolytoceras sp. (Fig. 14); Dumortieria sp. (juvenile, fragment from top of the bed); other fossils: Dactyloteuthis similis (Seebach) (mass accumulation), Palaeonucula hammeri (Defrance), Sphenodus sp.; bryozoan colonies on shell fragments;

Bed 29: 10 cm medium-grey, massive calcareous marl full of echinoderm remains (echinoderm packstone with micritic matrix) with abundant iron as well as calcareous ooids, abundant nubeculariid foraminifera at the surface of ooids, echinoderm ossicles and rounded shell fragments, echinid spines, bivalve shell; indistinct lower boundary;

Bed 30: 2 cm fibrous calcite with cone-in-cone structures.

Bed 31: 230 cm medium-grey, well-bedded claystone; few layers with fine-grained white shell debris of Bositra 25–40 cm and 120–140 cm above basis; thin marcasitic burrows; ammonoids: indeterminable compressed ammonite with bundled sinuous ribs; other fossils: Bositra suessi (Oppel) (25 cm above basis);

Bed 32: 6–8 cm dark-grey, well bedded marl full of fine-grained shell debris, abundant limonitic iron ooids, belemnite rostra and cm-sized white-grey, corroded phosphorite nodules; near the basis rust-brown irregular argillaceous limestone concretions and large compressed ammonite shell fragments (> 15 cm) with mm-thin stromatolitic crusts; ammonoids: Cotteswoldia aalensis (Zieten) (Fig. 15), Pleydellia subcompta (Branco) (Fig. 16), Pleurolytoceras cf. hircinum (Schlotheim) (Fig. 17); other fossils: Hastites subclavatus (Voltz) (Figs 24–25), Acrocoelites rostriformis (Theodori in Bronn) (Fig. 23); Nicaniella voltzii (Hoeninghaus in Roemer), Palaeonucula hammeri (Defrance), fragments of Chlamys textoria, Costatrochus subduplicatus (d’Orbigny), Thecocyathus mactrus (Goldfuss), one terebratulid brachiopod, one serpulid tube, driftwood (jet);

Figures 3–10. 

Ammonites and Belemnites of the Toarcian Hainberg section 3. Dactylioceras cf. crosbeyi (Simpson), bed 9, Posidonienschieder Formation, Tenuicostatum Zone. GZG.INV.866: d = (21) mm, n = (7), wh = (8) mm, rb/2 = (20); 4. Hildaites murleyi (Moxon), bed 11 “Boreale Concretions”, Posidonienschiefer Formation, Exaratum Subzone. GZG.INV.867: d = 75 mm, di = 53 mm, u = 30 mm, wh = 23 mm, wb = (17) mm, rb/2 = (20); 5. Lytoceras sp. und Parainoceramya dubia (Sowerby), bed 12, Posidonienschiefer Formation, Exaratum/Elegans Subzone. GZG.INV.868: d = (33) mm, u = (10) mm, wh = 14 mm; 6. Eleganticeras elegans (Sowerby), bed 15, Posidonienschiefer Formation, Elegans Subzone. GZG.INV.869: d = 66 mm, di = (42) mm, u = (13) mm, wh = (36) mm; 7. Dactylioceras (Dactylioceras) cf. commune (Sowerby), 5 cm above basis of bed 24, Posidonienschiefer Formation, Commune Subzone. GZG.INV.870: d = 53 mm, di = (42) mm, u = (27) mm, wh = (12) mm; 8. Acrocoelites sp., left: lateral view, right: ventral view, bed 23, Posidonienschiefer Formation, Commune Subzone. GZG.INV.871: Length = 63.5 mm (incomplete: without alveolar region); 9. Dactyloteuthis irregularis (Schlotheim) [syn.: Dactyloteuthis digitalis (Blainville)], left: lateral view, right: ventral view, bed 23, Posidonienschiefer Formation, Commune Subzone. GZG.INV.872: Length = 40.5 mm; 10. Osperleioceras cf. beauliziense (Monestier), bed 27, Jurensismergel Formation, ?Thouarsense Zone. GZG.INV.873: d = 40 mm, di = (27) mm, u = 11 mm, wh = 18 mm, wb = 9.5 mm, rb/2 = 20.

Figures 11–26. 

Ammonites and Belemnites of the Upper Toarcian of the Hainberg section 11. Phlyseogrammoceras dispansiforme (Wunstorf), bed 28 “zeta conglomerate”, Jurensismergel Formation, Dispansum Subzone. GZG.INV.874: d = 55.5 mm, di = (38) mm, u = 17 mm, wh = 24 mm, wb = 14 mm, rb/2 = (30); 12. Phlyseogrammoceras dispansiforme (Wunstorf), bed 28 “zeta conglomerate”, Jurensismergel Formation, Dispansum Subzone. GZG.INV.875: d = 49.5 mm, di = 34.5 mm, u = 13 mm, wh = 23.5 mm, wb = 13 mm, rb/2 = 31; 13. Phlyseogrammoceras transiens (Ernst), bed 28 „zeta conglomerate“, Jurensismergel Formation, Dispansum Subzone. GZG.INV.876: d = 34 mm, di = (23) mm, u = 9.5 mm, wh = 15.5 mm, wb = 10 mm, rb/2 = 24; 14. ?Alocolytoceras sp., bed 28, Jurensismergel Formation, Dispansum Subzone. GZG.INV.877: d = (59) mm, wh = 28 mm, wb = 24 mm; 15. Cotteswoldia aalensis (Zieten), bed 32, Opalinuston Formation, Aalensis Subzone. GZG.INV.878: d = (34) mm, wh = 17 mm, wb = 9 mm; 16. Pleydellia subcompta (Branco), bed 32, Opalinuston Formation, Aalensis Subzone. GZG.INV.879: d = 40 mm, di = (28) mm, u = 15 mm, wh = 14.5 mm, wb = 18 mm, rb/2 = (45); 17. Fragment of Pleurolytoceras cf. hircinum (Schlotheim), dorsal view showing v-shaped constriction; bed 32, Opalinuston Formation, Aalensis Subzone. GZG.INV.880; 18. Pleydellia cf. pseudoarcuata Maubeuge, bed 39, Opalinuston Formation, Pseudolotharingicum Subzone. GZG.INV.881: wh = 11.5 mm, wb = 7 mm; 19. ?Cotteswoldia sp., bed 39, Opalinuston Formation, Pseudolotharingicum Subzone. GZG.INV.882: wh= 20 mm, wb = 10 mm; 20. Leioceras cf. goetzendorfensis (Dorn), Opalinuston Formation, Opalinum Subzone. GZG.INV.883: d = (70) mm, di = (48) mm, u = 19 mm, wh = 31 mm, wb = 14 mm; 21. Dactyloteuthis irregularis (Schlotheim) [syn.: Dactyloteuthis digitalis (Blainville)], left: lateral view, right: dorsal view; bed 27, Jurensismergel Formation, ?Thouarsense Zone. GZG.INV.884: Length = 61 mm; 22. Dactyloteuthis similis (Seebach), left: lateral view; right: dorsal view; bed 28, Jurensismergel Formation, Dispansum Subzone. GZG.INV.885: Length = 59 mm; 23. Acrocoelites rostriformis (Theodori in Bronn) [syn.: Acrocoelites (Odontobelus) curtus (d’Orbigny)], left: lateral view, right: dorsal view; bed 32, Opalinuston Formation, Aalensis Subzone. GZG.INV.886: Length = 28 mm; 24. Hastites subclavatus (Voltz), left: lateral view, right: dorsal or ventral view; bed 32, Opalinuston Formation, Aalensis Subzone. GZG.INV.887: Length = 52 mm; 25. Hastites subclavatus (Voltz), left: lateral view, right: dorsal or ventral view; bed 32, Opalinuston Formation, Aalensis Subzone. GZG.INV.888: Length = 42.5 mm; 26. Acrocoelites rostriformis (Theodori in Bronn) [syn.: Acrocoelites (Odontobelus) curtus (d’Orbigny)], left: lateral view, right: dorsal view; bed 39, Opalinuston Formation, Pseudolotharingicum Subzone. GZG.INV.889: Length = 24.5 mm.

Bed 33: 1 cm fibrous calcite with cone-in-cone structures;

Bed 34: 50 cm medium-grey, yellow-brown weathered, well-bedded claystone; few thin shell fragments, oxidized lenticular marcasite nodules; ca. 10 m lack of exposure (claystones);

Bed 35: 40 cm medium-grey, well-bedded claystone with abundant fine-grained white Bositra shell debris on bedding planes; fossils: Bositra suessi (Oppel) (one complete valve);

Bed 36: 5 cm medium-grey calcareous claystone with reworked cm-sized, rounded to irregular siderite concretions; top of concretions corroded and covered by thin veneer of echinoderm and bivalve debris; ammonoids: one reworked phosphoritic fragment of a ?Cotteswoldia sp.; other fossils: Hastites sp. (three fragments), pectinid bivalve fragments, small gastropods, one serpulid tube fragment;

Bed 37: 1 cm fibrous calcite with cone-in-cone structures;

Bed 38: 25 cm medium-grey, well-bedded claystone with fine-grained Bositra shell debris on bedding planes; fossils: Coelodiscus minutus (Schübler in Zieten) (limonite cast);

Bed 39: 1 cm yellow-brown calcareous clay with reworked phosphorite nodules, siderite nodule fragments, belemnites and phosphoritic ammonite fragments; ammonoids: Leioceras/Pleydellia sp. (fragment), Pleydellia cf. pseudoarcuata Maubeuge (Fig. 18) , ?Cotteswoldia sp. (Fig. 19); other fossils: Acrocoelites rostriformis (Theodori in Bronn) (Fig. 26);

Bed 40: 200 cm medium-grey, well-bedded claystone; 80 cm above basis a layer with fine-grained Bositra shell debris; 100 and 120 cm above basis 1-cm-thin siderite nodule beds;

Bed 41: >50 cm medium-grey, yellow-brown weathered unstratified clay (solifluction deposit) with white-grey septarian nodules; few marcasitic burrows and mica flakes; ammonoids: Leioceras cf. goetzendorfensis (Dorn) (Fig. 20).

The Amaltheenton Formation (beds 1–6) is characterized by very low CaCO₃ contents (0.2 wt%) as well as low C_org contents (0.3–0.7 wt%) (Table 1, Fig. 2). Similarly, carbonate-free S_tot contents are low (0.3 wt%).

The lowermost Posidonienschiefer Formation (beds 7–9) is still very low in CaCO₃ (0.2 wt%) and C_org (0.7 wt%), despite the onset of lamination. Carbonate-free S_tot is even lower (<0.1 wt%) than in the Amaltheenton Fm. (Table 1, Fig. 2). Only at the base of the middle Posidonienschiefer Fm., i.e. in beds 10 and 11 (horizon of Elegantulum and Boreale Concretions) a marked increase in all three parameters (CaCO₃: 10 wt%, C_org: 22.1 wt%, carbonate-free S_tot: 1.9 wt%) can be observed. Carbonate-free C_org contents reach their maximum here with 24.7 wt%. Farther up in the middle Posidonienschiefer Fm. (beds 12 to 17), CaCO₃ contents remain comparatively high (around 52 wt%). Likewise, C_org and carbonate-free S_tot fluctuate at elevated values (around 10 and 1.8 wt%, respectively). Normalised to the carbonate-free fraction, beds 16 and 17 again show similarly high C_org contents (around 23 wt%) as in beds 10 and 11. A cyclicity of the C_org contents, indicated by maxima in beds 10, 12, 16 and 17 with subsequent decreases, is unfortunately not provable due to the too low sample density. Beds 18–23, which include the “Monotis event bed” and “Commune Belemnite Battlefield”, again show increased CaCO₃ contents (around 60 wt%) and high carbonate-free S_tot contents (around 2.1 wt%), but with tending declines in carbonate-free C_org (from about 20 to 10 wt%).

Figure 27. 

Limestone concretion (“Elegantulum Concretion”) of bed 10 with Lepidotes elvensis (de Blainville) and Eleganticeras elegantulum (Young & Bird). GZG.INV.890ab: d = 28.7 mm, u = 8 mm, wh = 12 mm, wb = 7.8 mm.

The upper part of the Posidonienschiefer Fm., represented by beds 24–26, is characterised by only moderately high CaCO₃ contents (around 27 wt%) and consistently high carbonate-free S_tot contents (around 2.1 wt%), with C_org and carbonate-free C_org stabilising at values around 10 and 13.5 wt%, respectively. The uppermost, bioturbated bed 26 of the Posidonienschiefer Fm., however, already shows significantly reduced C_org (2.4 wt%) and S_tot contents (<0.1 wt%).

The Jurensismergel Formation with its strongly condensed, bioclastic and Fe-oolitic rocks (beds 27–29) shows clearly elevated CaCO₃ contents (up to 66 wt%) with now very low C_org (around 1 wt%) and S_tot (below 0.1 wt%) (Table 1, Fig. 2). Only the basal belemnite accumulation is still characterised by high S_tot contents (1.5 wt%, i.e., 2.3 wt% carbonate-free S_tot).

The Opalinuston Formation (beds 31–41) is composed of only slightly calcareous mudstones, with significant lower CaCO₃ values (around 2.0 wt%) compared to the Jurensismergel Fm. rocks, but slightly higher than the Amaltheenton Fm. rocks (0.2 wt%). Only the stromatolite-bearing condensed bed 32 and the conglomeratic bed 36 show increased CaCO₃ contents of 44 and 23 wt%, respectively. C_org and S_tot contents of the Opalinuston Fm. (total rock as well as carbonate-free fraction) are only 0.9 wt% and 0.3 wt%, respectively (Table 1, Fig. 2).

The orientation of the tip direction of belemnite rostra was measured in bed 23 (“Commune Belemnite Battlefield”) of the Posidonienschiefer Formation and bed 27 (“Oolite marl with belemnite accumulation”) of the Jurensismergel Formation (Suppl. material 1).

In bed 23, a total of 79 rostra with a length between 1 and 9 cm were analysed. The belemnite rostra tip direction pattern shows one maximum in the class 195–210° (i.e., 10.1%), a second and third maximum in the class 150–165° (i.e., 8.9%) and 105–120° (i.e., 8.9%). The average azimuth of all measurements is 156° (Fig. 28).

In bed 27, a total of 104 rostra with a length between 1 and 9 cm were analysed. While there is one maximum at 165–180° (i.e., 9.6%), several further maxima occur at 45–60° (i.e., 7.7%), 0–15° (i.e., 6.7%), 30–45°, 60–75°, 90–105°, 285–300°, and 300–315° (each 5.8%). The average azimuth of all measurements for bed 27 is 86° (Fig. 29).

Up to 9 mm thick laminated, sulphide-rich stromatolitic carbonate crusts were detected on cm-sized ammonite shell fragments in bed 32 of the Opalinuston Formation (Fig. 30). The crusts are composed of slightly undulating to flat hemispheroids. Thin sections demonstrate that the lamination results from an alternation of 10–40 µm thick microcrystalline to microsparitic and 15–40 µm thick dark, iron-sulphide-rich layers (Fig. 30). The stromatolitic crusts neither show a fenestral fabric nor morphological traces of microbial filaments or coccoidal structures. The same type of stromatolites has been described from discontinuities in the South-German Schwarzjura-Group by Keupp and Arp (1990).

Figure 28. 

Lithostratigraphy, biostratigraphy and biofacies of the Posidonienschiefer Formation, Lower Toarcian, exposed at the Hainberg section.

Figure 29. 

Lithostratigraphy, biostratigraphy and biofacies of the Jurensismergel Formation, Upper Toarcian, exposed at the Hainberg section. For legend see Fig. 28.

Figure 30. 

Thin section micrograph of a stromatolite crust of bed 32, Mactra/Aalensis Subzone, accompanied by marlstone with ooids showing mixed limonitic-calcareous cortices. Note the very dense lamination lacking a fenestral fabric and morphological microbial remains.

Although quantitative bivalve samples were not collected, qualitative descriptions provide some information on the general development of bivalve assemblages for the Toarcian along the section (Figs 28, 29; Suppl. material 2). No bivalves were recovered from the top parts of the Amaltheenton Fm. (beds 1–6) and lowermost parts (beds 7–9) of the Posidonienschiefer Fm. (i.e., gap of collection).

Lower parts of the middle Posidonienschiefer Fm. (beds 10–14) are dominated by Parainoceramya dubia (Sowerby) and Meleagrinella (Clathrolima) sp., which regularly occur on bedding planes. This “Parainoceramya Meleagrinella biofacies” is associated with cephalopods. Only in bed 13, first Bositra buchi (Roemer) covering single bedding planes were observed.

Further up in the middle Posidonienschiefer Fm., bed 16 is characterized by abundant bedding planes covered with Bositra buchi (Roemer) (“Bositra buchi biofacies”), while bed 17 regularly shows Parainoceramya dubia (Sowerby) but lacking Bositra. Beds 18–21 again show abundant bedding planes covered with Bositra buchi (Roemer) (“Bositra buchi biofacies”). The dense shell package results in a wrinkled pseudo-lamination of the rock.

Almost at the top of the middle Posidonienschiefer Fm., a monospecific Meleagrinella (Clathrolima) substriata (Münster) mass accumulation is developed, i.e. the “Monotis event bed” 22. Cephalopods are still present in this part of the section (Fig. 28).

Finally, the upper parts of the Posidonienschiefer Fm. (beds 24–25) are characterized by abundant beddings planes with Bositra buchi (Roemer) of reduced size (max. 6 mm) and its shell debris. Except for the immediate basis, this section is devoid of cephalopods (“cephalopod-free dwarf Bositra buchi biofacies”) (Fig. 28).

After a stratigraphic gap, iron-oolitic marls (beds 27–28) of the Jurensismergel Formation commonly show valves of the epibyssate Chlamys textoria (Schlotheim), associated with scattered Parainoceramya sp., Liostrea erina (d’Orbigny) and rare Palaeonucula hammeri (Defrance) (“Chlamys textoria biofacies”), while claystones of the lower Opalinuston Formation are characterized by Bositra suessi, locally forming accumulations on bedding planes (“Bositra suessi biofacies”; Fig. 29). Further up in the Opalinuston Formation, Bositra buchi (Roemer) significantly decreases in abundance.

Distinct lithofacies changes, marker beds and discontinuities provide a good allostratigraphic framework, which partly corresponds to lithostratigraphic formations (Figs 2, 28, 29). The application of the terms “formation” and “alloformation” follows the recommendations in NACSN (1983, 2021), Lutz et al. (2006) and Pratt et al. (2023):

Alloformation 1 (equivalent to Amaltheenton Formation): Bedded medium-grey claystones with mica and silt-layers (beds 1–6) represent the top parts of the Amaltheenton Formation.

Alloformation 2 (equivalent to Posidonienschiefer Formation): The onset of fine-laminated medium-grey, rustbrown weathered claystones (beds 7–9) represents the lower boundary of the Posidonienschiefer Formation with its 60 cm thick lower, still non-bituminous part. An increase to high C_org values, marking the onset of the middle Posidonienschiefer Fm. with the T-OAE, is observed in bed 10. This 7.48 m thick middle, bituminous, part of the Posidonienschiefer Fm. comprises beds 10 to 23, i.e. includes the Monotis event bed 22 and Commune Belemnite Battlefield (bed 23) at its top. The following upper part of the Posidonienschiefer Fm. (beds 24–26) consists of a 18.66 m thick monotonous succession of laminated bituminous marls with small Bositra buchi (Roemer) as almost the only fossils. The C_org contents are slightly lower than in the middle Posidonienschiefer Fm., similar to the trends in Southern Germany (Frimmel et al. 2004; Arp et al. 2021), but different from results in the Braunschweig area (Brockamp 1944). The top bed of the alloformation 2 is formed by bed 26, which is a laminated concretionary bed with distinct, straight burrows. This bed is considered as an equivalent of the “fucoid bed” at the top of the Posidonienschiefer Formation in Southern Germany (see e.g. Riegraf et al. 1984; Riegraf 1985). No deposits of the Dörnten Subformation with their fossiliferous limestone concretions were detected, consistent with former observations of Denckmann (1892: p. 107).

Alloformation 3 (corresponds to Jurensismergel and lowermost Opalinuston Formations): After a sharp boundary and discontinuity, the Jurensismergel Formation starts with a Fe-oolitic marls with a basal belemnite accumulation followed by a conglomeratic, Fe-oolitic calcareous clay bed with a second belemnite accumulation at its basis, and an echinoderm debris bed at its top. This rather marly, condensed and fossiliferous part of the Jurensismergel Fm. (beds 27–29) shows only 45 cm thickness. The higher, 13.3 m thick parts of this alloformation (beds 30–35) comprise medium-grey, bedded claystones, with more or less abundant Bositra suessi debris. This lithology already represents the Opalinuston lithofacies, so that earlier authors already assigned these beds in this region to the “Schichten des Am. opalinus” (von Seebach 1864), “Untere Zone des Ammonites opalinus” (Brauns 1865), or “Thone des Harp. opalinum” (Denckmann 1892). The thin bed 32 with reworked phosphorites and stromatolite crusts points to a minor discontinuity or condensation within this alloformation.

Alloformation 4 (corresponds to major parts of Opalinuston Formation): The lower boundary of this allostratigraphic unit is drawn with the erosional discontinuity of bed 36. Lithologically, the well-bedded carbonate-poor claystones are identical to those of the alloformation 3 top parts, except that Bositra shell debris layers are scarce. Only the lowermost 2.8 m of this formation were exposed.

Only a limited number (24) of determinable ammonites were recovered from the investigated section (Figs 3–26). A number of them were compressed and poorly preserved. However, well-preserved uncompressed specimens have been found in limestone concretions in the Posidonienschiefer Fm. and phosphorite-bearing beds of the Jurensismergel and Opalinuston Formations. For the biostratigraphic scheme of ammonite zones and subzones, the reader is referred to Gabilly 1976a, Knitter and Ohmert 1983; Riegraf et al. 1984; Riegraf 1985; Ohmert et al. 1996; Elmi et al. 1997, and Cresta et al. 2001, summarized in Arp et al. (2021).

Upper Pliensbachian: Top parts of the alloformation 1 (≙ Amaltheenton Fm.) are devoid of biostratigraphic relevant fossils in the investigated section, but Pleuroceras spinatum has been recovered from a former clay pit near Sillium 1.5 km NNW of the investigated section (specimen in BGR collection: BGR-H-STGR-000290034 and -000290039; leg. R. Jordan 1956). The youngest, in situ collected Pleuroceras spinatum in this region comes from an 1.85 m thick interval at the top of the Amaltheenton Formation of the Friederike Mine near Bad Harzburg (Jordan 1960: his Abb. 14). No findings at the immediate lithostratigraphic boundary Amaltheenton-Posidonienschiefer are known. The precise boundary to the Toarcian, therefore, cannot be drawn with confidence.

Lower Toarcian: The lowermost recovered ammonite from alloformation 2 (≙ Posidonienschiefer Fm.), a poorly preserved, compressed Dactylioceras cf. crosbeyi (Simpson) with low umbilical width in bed 9 points to the presence of the Tenuicostatum Zone (Figs 3). Unfortunately, the Siemensi and Capillatum Concretions of the Tenuicostatum Zone, elsewhere present in this region (Wunnenberg 1950; Hoffmann 1968a, b; Weitschat 1973), are not developed at the investigated site. Their absence, however, rather reflects different early diagenetic conditions in the former porewater, not a stratigraphic gap (Hoffmann 1968b: p. 20). Clear evidence for the Tenuicostatum Zone, therefore, can only be given by Dactylioceras semicelatum (Simpson) from Heinde (BGR collection, e.g., BGR-H-STGR-000292526) and Haverlah (Hoffmann and Martin 1960), both locations with well developed limestone concretions. One Dactylioceras sp. is mentioned from 22–29 cm above basis of the Posidonienschiefer Formation, Friederike Mine near Bad Harzburg (Jordan 1960: p. 109). Eleganticeras elegantulum (Young & Bird) within the limestone nodule of bed 10 (Elegantulum Concretions; Fig. 27) indicates the Elegantulum Subzone, i.e. the basis of the Serpentinum Zone. The limestone nodules of bed 11 (Boreale Concretions) show well-preserved, uncompressed Hildaites murleyi (Moxon) (Fig. 4), indicating the Exaratum Subzone, while bed 15 (Lower Elegans Bed) belongs to the Elegans Subzone due to findings of compressed Eleganticeras elegans (Sowerby) (Fig. 6). The following beds 16 and 17 did only reveal indeterminable fragmentary imprints of Harpoceras sp. and likely represent higher parts of the Serpentinum Zone, while beds 18–21 probably already belong to the basal Bifrons Zone. No ammonite findings were made in these beds 18–21 of the present section.

However, Vinken (1971, p. 61 f.) provides a partial section from Listringen (9 km WNW of the Hainberg section), showing the “interval of Monotis limestones” (i.e., bituminous calcareous marls with wrinkled lamination due to Bositra and Meleagrinella shells) with a basal limestone bed containing Dactylioceras commune (Sowerby) and ?Frechiella subcarinata (Young & Bird) (Suppl. material 3). This bed corresponds to bed 18 of the Hainberg section.

Differing from that, Riegel et al. (1986) and Loh et al. (1986) draw the lower boundary of the Bifrons Zone at Hildesheim-Itzum (Suppl. material 4) several meters deeper, which corresponds approximately to the middle of bed 17 in the Hainberg section. They support their stratigraphic assignment by the replacement of Harpoceras by abundant Dactylioceras in their section. However, the genus Dactylioceras occurs throughout the Posidonienschiefer Fm. in varying abundances. Many of these dactylioceratids cannot be attributed with confidence to a specific species if compressed (such as D. commune of the Bifrons, and D. toxophorum Buckman of the Serpentinum Zone), a fact already noted by Hoffmann (1968a, p. 452). The identification of the basis of the Bifrons Zone, therefore, appears more reliable using Frechiella subcarinata (Young & Bird), which appears identifiable even if compressed or less well preserved.

Indeed, Weitschat (1973) places the boundary Serpentinum-Bifrons zone in the lower part of the “interval of Monotis limestones” (which he defines as a 0.4–1.0 m thick interval of bituminous limestone beds partially with left valves of Meleagrinella (Clathrolima) substriata (Münster)), because of a Harpoceras falciferum (Sowerby) [not figured] finding within the “basal Posidonia pavements”. This bed corresponds to our bed 18 in the Hainberg section. It has to be noted, however, that Harpoceras falciferum (Sowerby) reaches into the Commune Subzone (Howarth (1992, p. 132 and text-figs. 2, 5 and 6), thus, overlaps with Dactylioceras commune (Sowerby) and Frechiella subcarinata (Young & Bird). We therefore place the basis of the Bifrons Zone, Commune Subzone at the basis of bed 18 in the Hainberg section.

In the present investigation, poorly preserved Dactylioceras (Dactylioceras) sp. were only found in the “Commune Belemnite Battlefield” (bed 23) and at the very basis of the upper Posidonienschiefer Fm. (i.e., near the lower boundary of bed 24; Fig. 7). No further ammonites were found in beds 24–26. Likewise, no ammonite findings are reported from the upper Posidonienschiefer Fm. in adjacent sections Haverlahwiese and Hildesheim-Itzum. While the presence of the Fibulatum Subzone is likely (Hildoceras bifrons (Bruguiere) from Dörnten; Ernst 1923–1924), the presence of the Semipolitum Subzone appears questionable.

Upper Toarcian: No indication of the Variabilis Zone was found at the Hainberg section, consistent with its absence already shown by Denckmann (1892) and similar to adjacent sections Haverlahwiese and Hildesheim-Itzum (Maul 1984; Loh et al. 1986; Riegel et al. 1986). However, the Variabilis Zone, which forms major parts of the Dörnten Subformation, is well documented in Dörnten and Gallberg (Denckmann 1892; Ernst 1923–24; Dahlgrün 1928). It is probably present at the city centre of Hildesheim (Bischofskamp), from which Denckmann (1892) reports “Harp. quadratum Quenst. und Harp. bingmanni Denckm.” in the collection of the Roemer-Museum Hildesheim) (Fig. 32).

The stratigraphically lowest ammonite finding of alloformation 3 (≙ Jurensismergel and lowermost Opalinuston Formations) at Hainberg, an Osperleioceras cf. beauliziense (Monestier) in bed 27 (“oolite marl”; Fig. 10), points to the Fallaciosum Subzone (Lacroix and Le Pichon 2011). The specimen is probably reworked into the Dispansum Zone, although the latter cannot be proven for the bed 27 directly. Unfortunately, the belemnite Dactyloteuthis irregularis (Schlotheim) (Fig. 21), abundant in bed 27, also does not provide a precise biostratigraphic assignment, because it ranges from the Variabilis Zone into the Pseudoradiosa Zone (Kolb 1942; Riegraf 1996; Schlegelmilch 1998; Weis 1999; Arp 2010).

A clear assignment of bed 28 (“zeta conglomerate”) to the Dispansum Zone was possible due to findings of well preserved Phlyseogrammoceras dispansiforme (Wunstorf) (Figs 11, 12) and Phlyseogrammoceras transiens (Ernst) (Fig. 13). The single finding of a ?Alocolytoceras sp. (Fig. 14) is consistent with this interpretation. Similarly, the co-occurring belemnite Dactyloteuthis similis (Seebach) (Fig. 22) is consistent with the Dispansum Zone (Kolb 1942; Riegraf 1996). One poorly preserved Dumortieria sp. at the top of bed 28 indicates the transition to the Pseudoradiosa Zone. Bed 31, forming the lowermost parts of the Opalinuston Formation, unfortunately revealed only indeterminable compressed ammonoids and may represent younger parts of the Pseudoradiosa Zone.

However bed 32, with reworked phosphorite nodules, some ooids, stromatolite crusts and abundant belemnites, clearly represents a condensed lowermost part of the Aalensis Zone due to findings of Cotteswoldia aalensis (Zieten) (Fig. 15), Pleydellia subcompta (Branco) (Fig. 16), Pleurolytoceras cf. hircinum (Schlotheim) (Fig. 17), Hastites subclavatus (Voltz) (Figs 24, 25) and Acrocoelites rostriformis (Theodori in Bronn) (Fig. 23). The following 10 m gap of exposure and beds 34 to 35 probably still represent the Aalensis Subzone.

While only few fragmentary ammonoid remains (Figs 18, 19: Pleydellia cf. pseudoarcuata Maubeuge, ?Cotteswoldia sp.) were recovered from beds 36–41 of alloformation 4 (≙ major parts of Opalinuston Formation), Acrocoelites rostriformis (Theodori in Bronn) from bed 39 (Fig. 26) indicates that this bed is still Upper Toarcian in age. This belemnite species is known to occur from the uppermost Pseudoradiosa Zone (i.e. Moorei Subzone) to the top of the Aalensis Zone (Riegraf 1996; Rulleau 2007), if its short variety “Acrocoelits curtus (d’Orbigny)” (e.g. Arp 2010) is included in this taxon. Pinard et al. (2014) indicate its range into the lowermost Opalinum Zone, but the assignment of corresponding findings (Dumortier and Fontannes 1876; Quenstedt 1845–1849) cannot unequivocally assigned to the Aalensis or Opalinum Zone.

Lower Aalenian: Clear indication of Lower Aalenian provides Leioceras cf. goetzendorfensis (Dorn) (ex Leioceras comptum (Reinecke); see Dietze et al. 2021) from nodules of bed 41 at the top of the Hainberg section (Fig. 20), which is affected by solifluction. The ammonite, therefore, probably originates from further up in the section. In situ evidence for the Opalinum Subzone has not been found at the Hainberg. Leioceras opalinum (Reinecke), however, has been mentioned from Dörnten by Brachmann (1991) and from Haverlahwiese, with several specimens are hosted in the collection of the BGR in Hannover (e.g., BGR-H-STGR-000241142; leg. Kolbe 1955, BGR-H-STGR-000241149: leg. R. Jordan 1958). Borgmann (1990: p. 54, his Fig. 29) assigns a 15.4 m thick interval of claystones at Echte-Dögerode (30 km SSW of Hainberg) to the Opalinum Zone due to findings of Leioceras opalinum lineatum (Buckman). At Sehnde near Hannover, 30 km NNW of Hainberg, the Opalinum Zone is approximately 18 m thick (Hoffmann 1913).

In addition to the limited ammonite and belemnite findings discussed above, bivalve assemblages (Suppl. material 2) provide some supplementary information on the biostratigraphic position, when compared to the observations in Southern Germany (Röhl et al. 2001). The observations at Hainberg are also consistent with the distribution of bivalve assemblages mentioned in Maul (1984), Loh et al. (1986), and Riegel et al. (1986), except for Steinmannia, which has not been found at Hainberg. In Southern Germany, Steinmannia radiata (Goldfuss) is characteristic of the upper part of the Tenuicostatum Zone (Riegraf et al. 1984), i.e., and interval identical to the “Bositra buchi occurrence I” of Röhl et al. (2001).

With respect to the Serpentinum Zone (i.e., beds 10–17), bivalves are largely represented by Parainoceramya dubia (Sowerby) and Meleagrinella sp. As an intercalation, the Bositra buchi biofacies appears first in bed 16 of the Hainberg section, which corresponds to “occurrence II” of Röhl et al. (2001) in top parts of the Elegans Subzone in Dotternhausen. The Bositra buchi biofacies reoccurs in beds 18–21, which corresponds to the lower-middle Commune subzone in Dottenhausen (occurrence III of Röhl et al. 2001).

The Monotis event bed 22 corresponds to the Monotis bed in Southern Germany, i.e., a marker bed within Commune Subzone (Birzer 1936; Riegraf et al. 1984; Arp and Gropengießer 2016). From the sections Hainberg and Listringen it becomes clear that the Monotis event bed (i.e., the Monotis Bed s.str.) is located at the top of the “interval of Monotis limestones” sensu Weitschat (1973), which comprises a section of 40–100 cm thick shell-rich calcareous marls and marly limestones – possibly reflecting sediment condensation (Kaiser 2021). Finally, the cephalopod-free dwarf Bositra buchi biofacies is apparently time-equivalent to the Bositra buchi mass occurrence IV in Röhl et al. (2001) in the upper Commune and Fibulatum Subzones, however with cephalopods and normal-sized Bositra in Southern Germany. For the Upper Toarcian, with Bositra suessi dominating in bedded calcareous claystones and Chlamys textoria characterising condensed oolitic or conglomeratic beds, no ecostratigraphically relevant pattern has been recognized to date.

The investigated section Hainberg shows several discontinuities and condensed beds, separating continuous sediments with bedding or lamination, and beds with increased quartz silt and mica content (or even carbonaceous plant debris). Together with the biofacies and the comparison with sections along an offshore-coastal transect (Figs 1, 31), an interpretation of sequences controlled by sealevel changes can be made (Fig. 32). An important basis for comparison and discussion forms the sequence stratigraphic work of Zimmermann et al. (2015) for Northern Germany, as well as investigations from Southern Germany by Röhl and Schmid-Röhl (2005) and Arp et al. (2021) (Fig. 32). The transgressive-regressive sequences discussed here are considered to reflect changes in the shoreline trajectory, with the maximum regression surface at the end of a regression, and the maximum flooding surface at the end of a transgression (Embry and Johannessen 1992; Catuneau et al. 2009, 2011).

The Latest Pliensbachian sediments, i.e., the top of alloformation 1 (≙ Amaltheenton Formation) with quartz silt and mica, are considered as regressive, reflecting prograding siliciclastics of deltaic origin. This is in accordance with Zimmermann et al. (2015), who suggested a regression and mrs within Spinatum Zone. Seawater currents may have been directed from North to South, as indicated by the occurrence of cool water organisms (Arp and Seppelt 2012) and glendonites (Merkel and Munnecke 2023).

The following sequence boundary is located at or near the Amaltheen­ton/Posidonienschiefer Formation boundary, specifically within the lower Tenuicostatum Zone (Röhl and Schmid-Röhl 2005) or within the late Spinatum Zone (mrs Pli 2; Zimmermann et al. 2015). No sedimentological indication of a discontinuity is known from Hainberg and adjacent sections, so that a continuous sedimentation across the Pliensbachian-Toarcian transition appears likely. The lack of corresponding ammonites of the Paltum and Clevelandicum Subzones could reflect changing or reduced salinities during the lowstand conditions and proximity to deltaic influx.

For the 0.6 m thick Tenuicostatum and 6.6 m thick Serpentinum Zones, i.e. beds 7–17, a transgressive trend is indicated by first ammonites in bed 9, followed by ammonite-rich limestone concretions during the T-OAE (i.e., the negative d¹³C_org excursion in the Elegantulum and Exaratum Subzones; Hesselbo and Pieńkowski 2011; Xu et al. 2018) and continuing stagnant basin conditions with opportunistic bivalve assemblages during the Elegans and Falciferum Subzones (Röhl and Schmid-Röhl 2005).

Contrary to that, Zimmermann et al. (2015) suggest for the North-German Basin that this initial Toarcian transgression is followed by a maximum flooding surface (mfs Toa1) already within top parts of the Tenuicostatum Zone. This interpretation is based on a retrogradational shoreline shift of more than 200 km to the East (Zimmermann et al. 2015), with spreading of bituminous shale deposition to the Western Pomerania area (with Dactylioceras cf. semicelatum; Ernst 1967, 1991). Then, a regression from the latest Tenuicostatum to Bifrons Zone should follow, with the mrs Toa1 in the latest Bifrons Zone (Fig. 32).

Lowermost parts of the Bifrons Zone with the bivalve-shell-rich beds 18–22 (0.85 m “interval of Monotis limestones” sensu Weitschat 1973) can be interpreted as high-stand deposits with reduced rate of sedimentation (condensation according to Kaiser 2021), while the thin accumulation of Meleagrinella (Clathrolima) substriata (i.e., the Monotis bed s.str. at the top of this interval), traceable along 500 km to Southern Germany, is considered as an event bed (Arp and Gropengießer 2016). Indeed, Röhl and Schmid-Röhl (2005) place a mfs at the Serpentinum/Bifrons zone boundary in Southern Germany (Fig. 32).

The belemnite accumulation on top of the Monotis event bed, also known from the Franconian Alb, might reflect a SSE to NNW directed seawater bottom current though the “Hessian Seaway” (Fig. 1), after the possibly tectonic-induced “Monotis bed event” (Arp and Gropengießer 2016). Current-aligned belemnites have also been mentioned and figured at the same stratigraphic bed from the open pit mine Haverlahwiese by Hoffmann (1968a: p. 496, his pl. 34). Likewise, Maul (1984: p. 30) shows a rose diagram of 500 measurements from the Commune Belemnite Battlefield at Hildesheim-Itzum with a clear maximum at 165°, i.e., confirming a bottom current directed from SSE to NNW (Suppl. material 4). Later parts of the Bifrons Zone, i.e., the 18.66 m upper Posidonienschiefer Fm. (i.e., beds 24–26) with its cephalopod-free dwarf Bositra biofacies and increasing content of fine-grained mica and plant debris, is best explained as the regressive phase of a sequence (Fig. 32), with increasing terrestrial influx and decreasing salinities, in accordance with Zimmermann et al. (2015) and Röhl and Schmid-Röhl (2005). The lowering of carbonate-free C_org contents from the upper Falciferum (ca. 23 wt%) to the Bifrons Zone bituminous shales (about 13.5 wt%) reflects a dilution effect by the increased sedimentation rate. Anoxic bottom water conditions, therefore, continued unchanged in this region proximal to deltaic influx.

For the Upper Toarcian, a sequence stratigraphic interpretation remains difficult for the investigated section, because the absence of the Variabilis Zone and major parts of the Thouarsense Zone, a 10-m-lack of exposure, and limited ammonite findings. A preliminary interpretation, however, can be given on basis of the comparison with adjacent sections (Figs 31, 32).

Figure 31. 

Correlation of the investigated section Hainberg with sections from Northern Germany and Thuringia, with suggested T-R cycles for the sections Hainberg and Dörnten.

Figure 32. 

Sequence stratigraphic interpretation of the Toarcian succession in Northern Germany (including the Hainberg section) and Thuringia in comparison with 3^rd order South-German and the 2^nd order Boreal standard cycles. Sedimentary succession and ammonite biostratigraphic evidence according to Denckmann (1892), Ernst (1923–1924), Kumm (1941), Ernst (1970), Maul (1984), Brachmann (1991), Krause and Katzung (1999), and specimens from the BGR collection [https://gewis.bgr.de/, leg. K. Hoffmann, R. Jordan, K. Wiedenroth]. T-R cycles: ¹ third order sequences Northern Germany (Zimmermann et al. 2015), ² third order sequences Southern Germany (Röhl and Schmid-Röhl 2005; Arp et al. 2021); ³ second order sequences Boreal standard (de Graciansky et al. 1998; Jacquin et al. 1998).

A sealevel lowstand is inferred for the basis of the Variabilis Zone, i.e. just prior to the deposition of the Dörnten Subformation. The latter, while preserved in its type region and some basinal sections (e.g. Echte; Hoffmann 1968a), obviously has been eroded in the area Haverlahwiese-Hainberg-Hildesheim (except for Hildesheim-Bischofskamp: Denckmann 1892: p. 101).

Contrary to Southern Germany, there is apparently no significant erosional discontinuity within (Arp et al. 2021) or at the top of the Variabilis Zone (Riegraf 1985; Jordan and Schmidt-Kaler 1985), because the presence of all its subzones is evident by corresponding ammonite findings at Dörnten (Denckmann 1892; Ernst 1923–1924; Brachmann 1991). This might be related to higher subsidence and sedimentation rates in Northern Germany. Likewise, the Thouarsense Zone at Dörnten is proven by Pseudogrammoceras doerntense (Denckmann), Ps. bingmanni (Denckmann), Ps. struckmanni (Denckmann), and Grammoceras striatulum (Sowerby), with only the Fallaciosum Subzone absent (Fig. 32). Continuous sedimentation with bituminous shales from the Bifrons to Variabilis Zone is also likely farther North in Schandelah (van de Schootbrugge et al. 2019: p. 272; Visetin et al. 2022). The precise nature of the sedimentological boundary Posidonienschiefer s.str. to Dörnten Member (i.e., sharp or gradationally) is, however, unknown.

In any case, after the regressive top of the Bifrons Zone (i.e., the top of the Posidonienschiefer Formation with cephalopod-free dwarf Bositra biofacies), the change to a cephalopod biofacies (with ammonite-rich limestone concretions at Dörnten, Gallberg, and corresponding relics at Hildesheim-Bischofskamp) could best be explained by a transgression. Deviating from that, Zimmermann et al. (2015) suggests transgressive conditions only during the early Variabilis Zone, with a maximum flooding surface (mfs Toa2) within the latest Variabilis Zone (Fig. 32).

The basis of the Dispansum Zone, i.e., the basis of alloformation 3, is the most obvious erosional sequence boundary in the working area, as indicated by conglomeratic oolitic marls and belemnite accumulations (i.e. beds 27–28) (Fig. 31). Indeed, submarine erosion removed the Variabilis and Thouarsense Zones as well as top parts of the Bifrons Zone in the area Haverlahwiese-Hainberg-Hildesheim, as already described by Denckmann (1892: p. 107). These conglomeratic deposits comprise reworked ammonites of the Variabilis and Thouarsense Zones (e.g. Haugia and Grammoceras sp. at Hildesheim-Itzum: Maul 1984; Osperleioceras cf. beauliziense (Monestier) at Hainberg), but are dominated by Phlyseogrammoceras dispansum (Lycett) and Phl. dispansiforme (Wunstorf), at least in their upper part (Hainberg). Unfortunately, no clear direction of potential bottom water currents was obtained from belemnite rostra alignment measurements (Fig. 29).

In accordance with that, Zimmermann et al. (2015) place their maximum regression surface mrs Toa2 within the latest Thouarsense Zone. Indeed, most of the described Jurensismergel Fm. sections in Northern Germany show a stratigraphic gap for the Fallaciosum Zone (Heidorn 1928), similar to Southern Germany, where a discontinuous Dispansum Zone belemnite accumulation overlies the Thouarsense Subzone with local erosion of the Fallaciosum Subzone (Arp et al. 2021). The conglomeratic-oolitic Dispansum Zone sediments themselves appear to be transgressive, with reworking at highly reduced siliciclastic influx.

The ongoing sealevel rise could be represented by strata dated to the Pseudoradiosa Zone, i.e., beds 29–31 of the Hainberg section. The condensed bed 32 with stromatolites might represent high-stand conditions, in analogy to the Pseudoradiosa/Aalensis zone transition in the Franconian Alb (Arp et al. 2021), if this correlation is correct. Unfortunately, the presence of this condensed bed has not yet been shown in adjacent sections due to limited detailed sedimentological observations (Fig. 31). Likewise, the following Aalensis Zone claystones (beds 34–35) are herein tentatively considered to represent a regressive phase. The discontinuity at bed 36 (i.e., the basis of alloformation 4), then possibly reflects a further sealevel lowstand and sequence boundary. Strikingly, a 1 m thick shelly carbonate bed has been reported from the Toarcian-Aalenian transition at Echte-Dögerode, 30 km SSW of the Hainberg (Borgmann 1990), where the Upper Toarcian is about 25 m thick (Hoffmann 1949, p. 122). Unfortunately, no definite ammonite proof of the presence and extent of the Pseudolotharingicum Subzone (i.e. top of Aalensis Zone) is available at the investigated section Hainberg. However, accepting Acrocoelites rostriformis (Theodori in Bronn) as an index fossil of the Upper Toarcian, the Pseudolotharingicum Subzone might cover the thin transgressive interval of bed 38, followed by a condensed bed 39 and thicker regressive claystones (bed 40 and younger) of the Opalinum Zone. In any case, the sequence stratigraphic considerations on the Toarcian-Aalenian transition currently remain hypothetical and further complete sections with sufficient ammonite findings are required for testing and potential corrections.

Minor differences appear in the positions of maximum regression and maximum flooding surfaces between Toarcian sections of the central North-German Basin to Poland (Zimmermann et al. 2015), southern part of Northern Germany (this paper) and Southern Germany (Röhl and Schmid-Röhl 2005; Arp et al. 2021). These may reflect limitations of biostratigraphic high-resolution dating in the ammonoid-poor North-German deposits proximal to deltaic foresets, and corresponding interpolations in stratigraphic dating and correlation (Fig. 32).

Furthermore, the high thickness of sections in the Northern Germany and Poland (i.e. 82 to 231 m Toarcian for Usedom to Hamburg; Baermann et al. 2000; Zimmermann et al. 2015; e.g. ca. 90 m Toarcian at Nowa Wieś 12-Praszka 1/81: Pienkowski 2004) indicates significant higher subsidence and sedimentation rates, if compared to the Hainberg-Hildesheim area. Significantly increased clastic deposition (delta front and prodelta) during eustatic sealevel fall and (forced) delta progradation may have caused an apparent earlier onset of relative sealevel fall, if compared to the low subsidence and low sedimentation rate areas in Southern Germany. In turn, stratigraphic gaps covering intervals longer than subzones obscure the precise timing of maximum regression in areas of low subsidence, while ammonoid findings in thicker but more proximal positions are rare.

A further complication for interpreting thickness pattern and sedimentary sequences in the working area may result from possible synsedimentary ortho- and halotectonic movements during the Lower Jurassic at the Eichfeld-Altmark-High. Indeed, the available thickness data for alloformation 2 (Posidonienschiefer Fm. exclusive Dörnten Member) show short-distance variations between 29.7 and 33.4 m in the Hildesheim area, between 23.3 and 30.4 m at the Hainberg (unpublished drilling reports BGR), and between 30 and 37 m at the Salzgitter anticline (Gallberg: Ernst 1923–1924; Schröderstollen: Dahlgrün 1928) (Fig. 31). Larger thickness variations have been reported from the Hils Syncline (16 to 40 m; Littke and Rullkötter 1987), while the Braunschweig area shows rather uniform thickness pattern (Brockamp 1944) around 36.5 m (Schandelah; Visetin et al. 2022). For the lower and middle Posidonienschiefer Fm. a minor reduction in thickness is evident for the Hainberg, Haverlah and Gallberg sections, if compared to the Hildesheim area (NW) and Dörnten area (SE) (Fig. 31), as well as the Braunschweig area (Wunnenberg 1950; Visentin et al. 2022). These data, however, may rather point to minor local halotectonic movements or erosion at the top of the formation. Nonetheless, reduced thicknesses of 12 m at Wefensleben (67 km ENE of the Hainberg section; Koert 1923), possibly 3 to 7 m at Bad Harzburg (30 km SE of the Hainberg section; Kumm et al. 1941; Brockamp 1944), and 12–16 m in the Altmark area (Bauss 1976) may point to a structural zone of reduced subsidence farther East, i.e. the Eichsfeld-Altmark-High.

For alloformation 3 and 4 (i.e., Dörnten Member and Jurensismergel plus lower Opalinuston Fm.), spatial thickness pattern are even less well known due to poor outcrop conditions and difficulties in recognizing lithostratigraphic boundaries in drillings. Thus, the relation of condensed sections of these allostratigraphic units (e.g., in the Gallberg-Dörnten area) to structural elements remains to be investigated.