Biostratigraphy and sequence stratigraphy of the Toarcian Ludwigskanal section (Franconian Alb, Southern Germany)

Extensive construction work at the canal cutting of the Ludwigskanal near Dörlbach, Franconian Alb, provided the opportunity to re-investigate a scientific-historical and biostratigraphically important reference section of the South-German Toarcian. The 16 m thick section, described bed by bed with respect to lithology and macrofossils, starts within the Upper Pliensbachian Amaltheenton Formation, covers the Toarcian Posidonienschiefer and Jurensismergel Formation, and ends in basal parts of the Opalinuston Formation. Carbonate contents are high in the Posidonienschiefer and successively decline within the Jurensismergel to basal parts of the Opalinuston. The high carbonate contents in the Posidonienschiefer are associated with comparatively low organic carbon contents. However, organic carbon contents normalized to the silicate fraction are similarily high if compared to other regions in Germany. Only the persistence of high organic carbon levels into middle parts of the Upper Toarcian differs from those of most regions in central Europe. Ammonite biostratigraphy indicates a thickness of >9 m for the Upper Pliensbachian, 1.15–1.20 m for the Lower Toarcian, 5.04 m for the Upper Toarcian, and >0.5 m for the Lower Aalenian. Despite the low sediment thickness, all Toarcian ammonite zones and almost all subzones are present, except for major parts of the Tenuicostatum Zone and the Fallaciosum Subzone. On the basis of discontinuities, condensed beds, and correlations with neighbouring sections in Southern Germany, a sequence stratigraphic interpretation is proposed for the Toarcian of this region: (i) The Posidonienschiefer Formation corresponds to one 3rd order T-R sequence, from the top of the Hawskerense Subzone to a fucoid bed at the top of the Variabilis Subzone, with a maximum flooding surface at the top of the Falciferum Zone. (ii) The Jurensismergel Formation exhibits two 3rd order T-R sequences: The first ranges from the basis of the Illustris Subzone (i.e., the Intra-Variabilis-Discontinuity) to the top of the Thouarsense Zone, with a maximum flooding surface within the Thouarsense Zone. The “belemnite battlefield” reflects a transgressive “ravinement surface” within the first Jurensismergel Sequence, not a maximum regression surface at its basis. The second sequence extents from the erosive basis of the Dispansum Zone to the top of the Aalensis Subzone, with a maximum flooding surface at the Pseudoradiosa-Aalensis Zone boundary. Finally, the Opalinuston starts with a new sequence at the basis of the Torulosum Subzone. Transgressive system tracts of these 3rd order T-R sequences are commonly phosphoritic, while some regressive system tracts show pyrite preservation of ammonites. The maximum regression surfaces at the basis of the Toarcian and within the Variabilis Zone reflect a significant submarine erosion and relief formation by seawater currents, while this effect is less pronounced at the basis of the Dispansum Zone and basis of the Torulosum Subzone (i.e., the boundary Jurensismergel-Opalinuston Formation).


Introduction
The Franconian Alb is a classical region of Jurassic geosciences in Europe, and specifically the area of Altdorf SE of Nürnberg has been of great importance in the early times of palaeontology (von Freyberg 1958a(von Freyberg , b, c, 1972Schmidt-Kaler 1974;Mayr 1995;Kursawe 1995Kursawe , 1996Mäuser 2001). Indeed, the construction of the Ludwigskanal cutting at Dörlbach south of Altdorf ( Fig. 1) lead 1840-1841 to the first large-scale temporary exposure of the Schwarzjura-Group in Southern Germany and corresponding fossil discoveries such as one of the worldwide first finding of a large, 1.6 m long, Temnodontosaurus skull (von Freyberg 1972). Furthermore, the Ludwigskanal outcrop delivered many invertebrate type fossils, among them a number of ammonoids, described in the monographs of Quenstedt (1845Quenstedt ( -1849Quenstedt ( , 1851Quenstedt ( -1852Quenstedt ( , 1856Quenstedt ( -1858Quenstedt ( , 1865Quenstedt ( -1866Quenstedt ( , 1872Quenstedt ( -1875Quenstedt ( , 1876Quenstedt ( , 1881Quenstedt ( -1884Quenstedt ( , 1882Quenstedt ( -1885a. However, the only contemporary description of the exposed strata was given by Beyschlag (1841), and it took over eight decades until more details on the section were provided by Reuter (1927), Kolb (1964) and Urlichs (1971), all of them focussing on the Posidonienschiefer Formation. Despite these previous descriptions, and despite that the sediment succession of the Schwarzjura Group in this region is generally well known (Reuter 1927;Meyer and Schmidt-Kaler 1996;Bloos et al. 2005), a number of crucial stratigraphic details are subject to controversial views and remained unclear to date. Above all, this applies to the extent and position of discontinuities and condensed beds. Consequently, no sequence stratigraphic interpretation of the Toarcian has been suggested for this region, except for the Posidonienschiefer (Röhl and Schmid-Röhl 2005).
Slope failure at the Ludwigskanal cutting near Dörlbach and following extensive construction activities re-exposed the section in 2010-2012, allowing a re-investigation of the complete succession from the middle part of the Amaltheenton, through the Posidonienschiefer and Jurensismergel, to the basis of the Opalinuston Formation ( Fig. 2A, B). An overview and preliminary description of the new exposure was already given in Arp et al. (2014). Gastropods of the Amaltheenton were described by Gründel and Nützel (2015).
The aim of the present study is to provide a detailed description of the lithologic succession and its macrofossils.
The high-resolution biostratigraphy and sequence stratigraphy may form a basis for further investigations on seawater temperatures (δ 18 O of low-Mg calcite skeletons), seawater currents, sealevel changes, and causes for the persistence of the Toarcian Oceanic Anoxic Event (T-OAE;Jenkyns 1988) in the eastern part of the NW European Epicontinental Seaway. The present study focusses on the description and stratigraphic interpretation of this section.

Location and geological overview
The Ludwigskanal cutting is located in Southern Germany, Bavaria, approximately 20 km ESE of Nürnberg ( Fig. 1) in the western foreland of the middle Franconian Alb. The village Dörlbach lies 1 km W of the cutting, while Altdorf/ Mfr. is located 3 km N of it. The coordinates of the section, located on the topographic map 1:25000, sheet 6634 Altdorf b. Nürnberg, are 49°21.238938N, 11°21.534298E. The region is part of the South German Scarplands (Peterek and Schröder 2010 and references therein), and the escarpment of the Franconian Alb, i.e., the edge of the Upper Jurassic limestone plateau, is located approximately 5 km E of the investigated section (Schmidt-Kaler 1974).
Geologically, the deep subsurface of the region is formed by high-grade metamorphics and plutonites of the Variscan basement (Moldanubian gneiss and granite). These basement rocks are overlain by a Mesozoic cover sequence starting with Triassic fluvial coarse siliciclastics of the Buntsandstein (63 m), Muschelkalk (45 m) and Keuper Group (270 m) (drilling Eschertshofen: Sal ger and Schmid 1982). After a stratigraphic gap comprising the Rhaetian, the Lower Jurassic Schwarzjura Group (47-69 m) starts with fluvial arkoses of the Bayreuth Formation, followed by marine near-shore sandstones of the Gryphaeensandstein Formation, condensed dolomitic limestones of the Numismalismergel Formation, monotonous claystones of the Amaltheenton Formation, condensed bituminous limestones and shales of the Posidonienschiefer Formation, and finally fossiliferous marls of the Jurensismergel Formation (Schmidt-Kaler 1974;Salger and Schmid 1982;Fig. 3). The Middle Jurassic Braunjura Group (125 m) is composed of marine, monotonous claystones (Opalinuston Formation), iron-orebearing sandstones (Eisensandstein Formation) and a condensed, highly fossiliferous succession of iron-oolites and glauconitic siltstones (Sengenthal Formation). Up to 70 m of the Upper Jurassic Weißjura Group are preserved  Schwerd (1996). in the Altdorf region, with bedded marine limestones and sponge-microbialite mounds, some of them dolomitized (Schmidt-Kaler 1974). Subaerial exposure, erosion and karstification during Cretaceous and Tertiary times led to the present-day landscape (Wagner 1960;Hofbauer 2001; Peterek and Schröder 2010).

Material and methods
Fieldwork and sampling was carried out on 16 days between September 2010 and August 2014. Lithological descriptions are based on field observation and binocular observations on hand specimens, supplemented by 21 thin sections between 5 × 7.5 cm and 7.5 × 10 cm in size, and about 80 µm thick.
Total carbon (C tot ), nitrogen (N tot ), and sulfur (S tot ) of bulk rock samples were analysed with a Euro EA 3000 Elemental Analyser (HEKAtech, Wegberg, Germany) applying 2,5-bis(5-tertbenzoxazol-2-yl)thiophene (BBOT) and atropine sulfate monohydrate (IVA Analysetechnik, Meerbusch, Germany) as reference material. Organic and carbonate carbon (C org , C carb ) contents were determined by a LECO RC612 (Leco, St. Joseph, MI, USA) multi-phase carbon and water analyser. For calibration, Leco synthetic carbon (1 and 4.98 carbon %) and Leco calcium carbonate (12 carbon %) standards were used. All analyses were performed as duplicates. Analytical accuracy of all analyses was bet-  ter than 3%. The carbonate-free fraction was calculated from the total weight minus the CaCO 3 and C org content.
Repository: The material is stored in the Museum and Collection of the Geoscience Centre, University of Göttingen, under  Carbonate and organic carbon contents The Amaltheenton Formation is characterized by low CaCO 3 contents (2.5-3.3 wt%) as well as low C org contents (0.8-1.0 wt%) ( Table 1, Fig. 4).
The Posidonienschiefer Formation, however, shows a sudden and strong increase in CaCO 3 to 94 wt% at its basis, followed by consistently high values (73-98 wt%) in the lower part of the formation (i.e. limestone and argillaceous limestone), with only the Falciferum Shale showing a lower value (45 wt%). Higher parts of Posidonienschiefer exhibit marl equivalent values around 50-65 wt% CaCO 3 , with only two intercalations of reduced CaCO 3 content (35 wt%) in the upper third (Table 1, Fig. 4). C org contents of the Posidonienschiefer vary between 0.3 and 2.0 wt% in limestone beds, and higher values up to 5.3 wt% in less CaCO 3 -rich lithologies. The highest value (7.5 wt%) was measured in the Falciferum Shale. However, C org contents calculated for the carbonate-free rock fraction demonstrate a different trend: Very high contents characterize Laibstein I and II near basis of formation (i.e., bed 7 and 8, with up to 17.6 wt%), while fish scale debris bed 9 shows a slightly lower value (7.4 wt%). It follows a further maximum in the interval Falciferum Shale to Monotis Bed (up to 14.1 wt%). Higher parts of the formation finally show fluctuating C org contents (4.4-14.0 wt%), with two minima in top parts (1.5 and 1.8 wt%) ( Table 1, Fig. 4).
CaCO 3 contents at the basis of the Jurensismergel Formation correspond to calcareous marl (65-71 wt%) and marl (44-53 wt%), then decrease to marls (37-62 wt%) and argillaceous marls (17-34 wt %). Near the top of the formation, one last bed of calcareous marl (66 wt%) is found. Increased C org values (3.0 and 4.3 wt%) were found near basis (bed 18, "Belemnite Battlefield", "Toarcensis Shale"), followed by low values of 1-2 wt% in middle and upper parts of the formation. Only two horizons, basis of bed 24 and bed 25, show slightly increased C org contents (2.5 and 2.8 wt%). Finally, the Opalinuston Formation revealed CaCO 3 contents only slightly lower than top parts of the Jurensismergel, and C org as low as in the Amaltheenton (Table 1, Fig. 4).

Discussion Lithostratigraphy
The assignment of beds to specific formations of the Schwarzjura Group follows the definitions given in Bloos et al. (2005), Mönnig et al. (2015), , with only minor modifications.
1. The lower part of section, i.e. beds 1-6, represent the top 9 m of the Amaltheenton Formation (Figs 2A, 5), which shows a total thickness of 36 m at this location (Fig. 3). The formation is characterized by monotonous bluish-grey claystones with disseminated pyrite, low CaCO 3 and low C org contents.

The Posidonienschiefer Formation (Figs 2B, 4), with
a total thickness of 1.85-1.90 m, is characterized by bituminous marls and limestones rich in fossils. The basis is drawn with the first bituminous and calcareous bed, i.e. "Laibstein I". The erosive discontinuity Table 1. Carbon (C tot , C org , C carb ), total sulphur (S tot ), and total nitrogen (N tot ) contents of sedimentary rocks of the Ludwigskanal section.   at its basis is indicated by reworked concretions of the Amaltheenton. Accordingly, the basal alternation of marls, bituminous marls and Chondrites-rich beds, as seen in the Swabian Posidonienschiefer (Urlichs 1977a;Riegraf et al. 1984;Riegraf 1985a;Bloos et al. 2005), is absent in this region. While the total rock C org contents are clearly lower than in the Swabian sections (mean C org = 2.45 wt% at Ludwigskanal versus 6.77 wt% at Dotternhausen, for top Semicelatum to Bifrons Zone), C org contents of the carbonate-free fraction are almost identical (mean C org carbonate-free = 11.18 wt% at Ludwigskanal versus 12.22 wt% at Dotternhausen ;Frimmel 2003;Frimmel et al. 2004). Therefore, the comparatively low C org values at Ludwigskanal section reflect a "dilution effect" by increased carbonate contents. 3. The Jurensismergel Formation (Figs 2B, 4), with a total thickness of 3.50 m, is formed by highly fossiliferous marls with phosphorite (lower part) or pyrite nodules (higher part). Major parts of the formation show abundant nubeculariid foraminifera as a significant component of the sediment. Increased C org contents were only detected near the basis ("Belemnite Battlefield", "Toarcensis Shale") and in bed 25, a condensed bed in the middle of the formation. Carbonate contents are generally lower (predominantly argillaceous marls; Table 1), if compared to sections of the Swabian alb (e.g. Göppingen-Ursenwang and Aselfingen: marls and argillaceous limestone beds: Bruder 1968, his tab 4.).
The lower boundary of the Jurensismergel has previously been drawn at the top of the "Belemnite Battlefield": Urlichs (1971: p. 70f) argued that earlier publications (von Gümbel 1891: p. 359;Reuter 1927: p. 56) assigned this belemnite accumulation to the Posidonienschiefer, and that its components are reworked from the Posidonienschiefer below. However, the phosphorite nodules with borings ( Fig. 2D) are not derived from bituminous shales below, and correlations with sections in the Swabian Alb (Figs 7,9) indicate an erosive discontinuity at the basis of the marl bed 18 below the "Belemnite Battlefield" (see discussion of sequence stratigraphy below). Therefore, the "Fucoid Bed" (bed 17) forms, in our view, the top of the Posidonienschiefer Formation, and the poorly bituminous marls of bed 18 form the basis of a new stratigraphic sequence, i.e., the basis of the Jurensismergel Formation.
4. The basis of the comparatively thick Opalinuston Formation (Figs 2B, 4) should be drawn with the lithological change from marl (Jurensismergel) to claystone (Opalinuston). While this change appears gradual in the northern Franconian Alb (e.g., Mistelgau; Schulbert 2001a, b), the Ludwigskanal section still shows a clear calcareous marl bed 308-318 cm above the basis of the Jurensismergel. A calcareous marl bed has also been found in a similar position in the neighbouring section Pölling and likewise assigned to the Jurensismergel (Arp 2010). Therefore, the change from marl to claystone occurs above this bed, and the belemnite accumulation of bed 31, marking a 3 rd order sequence boundary (see sequence stratigraphic discussion below) could be taken as the lithostratigraphic lower boundary of the Opalinuston Formation. This suggested boundary definition coincides with change from pyritic to phosphoritic or compressed preservation of ammonoids in this region.

Biostratigraphy
The biostratigraphic subdivision of the investigated section follows the standard scheme by Dean et al. (1961), with revisions for the Lower Toarcian by Howarth (1973) and Riegraf et al. (1984: p. 19), and for the Upper Toarcian by Knitter and Ohmert (1983), Ohmert et al. (1996), Elmi et al. (1997), Cresta et al.(2001), and Schulbert (2001a) (Fig. 8). As a principal, the lower boundaries of subzones are drawn in the present paper by the first appearance datum (FAD) of the corresponding index species, with the exception of the Hawskerense Subzone (last appearance datum LAD of Pleuroceras solare: Dean et al. 1961). The distribution of each of the ammonite genera and species along the investigated section is given in Figs 5 and 6.
(i) Upper Pliensbachian (>9 m) The first Pleuroceras solare of the investigated section has been found 5 cm below the "Delta-Fossil Bed" (bed 4), indicating the Apyrenum Subzone of the Spinatum Zone. Abundant specimens of this species were found enclosed within reworked concretions of the "Delta-Fossil Bed" (bed 4), together with rare Amauroceras ferrugineum ( Fig. 10: 1, 2). Few of the Pleuroceras solare specimens already show minor tubercles, corresponding to var. solitarium ( Fig. 10: 2). The marl matrix of the "Delta-Fossil Bed" (bed 4) enclosing the reworked concretions already yielded compressed Pleuroceras spinatum with clear tubercles, while P. solare is absent. This suggests that the conglomeratic bed represents the base of the Hawskerense Subzone (sensu Dean et al. 1961: "lower boundary [...] drawn immediately above the highest Pleuroceras solare"; equivalent to "Upper Spinatum Zone" sensu Hoffmann et al. 2007). Besides, a juvenile Amaltheus sp. (possibly a Pseudoamaltheus) was recovered.
Higher parts of the Falciferum Zone are less well documented at the Ludwigskanal section as well as in the whole region. One compressed specimen of Harpoceras falciferum (Fig. 2C) has been found within the "Falciferum Shale" (bed 10), indicating that these bituminous marlstones above the "Fish Scale Bed" (bed 9) belong to the Falciferum Subzone.
The lower bedding plane of "Dactylioceras Bed" (bed 12) and the fish-scale-rich argillaceous limestone layer at its base (bed 11) exhibit poorly preserved specimen of Dactylioceras commune (Fig. 12: 1), marking the basis of the Bifrons Zone. The lowermost part of "Dactylioceras Bed" (bed 12) also revealed a poorly preserved Hildoceras cf. lusitanicum (Fig. 12: 2), consistent with basis of the Bifrons Zone. In addition, Frechiella subcari nata has been reported for this bed 5.5 km ESE of the Ludwigskanal section (Krumbeck 1932a;Weiß and Freitag 1991). No indication of the Ovatum Horizon (Howarth 1992;Page 2003) was found. Within the "Dactylioceras" to "Monotis Bed", however, Dactylioceras athleticum ( Fig. 12: 3) is most abundant. Therefore, the complete event bed (Arp and Gropengießer 2016) as well as the fish-scale-rich layer below, belongs to the Commune Subzone.
One compressed Haugia jugosa 3 cm below top ( Fig. 12: 9) and one compressed Denckmannia rude 1 cm below top of bed 16 are consistent with the Jugosa Horizon (Elmi et al. 1997) at the top of the Variabilis Subzone. A finding of Haugia ogerieni from Berg by Krumbeck (1943), which is a coarse ribbed variety of Haugia jugosa according to Lacroix (2011), may further support this interpretation. The following "Fucoid Bed" (bed 17), containing Haugia variabilis, Catacoeloceras raquinianum, Phylloceras heterophyllum, and Lytoceras cf. cornucopia may also belong to this horizon.
The "Belemnite Battlefield" (bed 19) is rather poor in ammonoids. Catacoeloceras raquinianum (d'Orbigny) has been recovered at the lower bedding plane (Fig. 13: 6), and deformed phosphoritic casts of Lytoceras cf. cornucopia occur within the bed. One fragmentary Pseudogrammoceras sp. is poorly preserved. This rather unspecific assemblage is, nonetheless, consistent with the view of Urlichs (1971) that the Belemnite Battlefield still belongs to the (upper) Variabilis Zone, with no elements of the Thouarsense Zone. However, no direct evidence of the Vitiosa Subzone was found at Dörlbach.
The "belemnite accumulation" of bed 22 yielded, except for Alocolytoceras cf. rugiferum, no determinable ammonoids. The range of A. rugiferum is not well constrained, but shows a maximum abundance in the Dispansum Zone in northern and southwestern Germany (Wunstorf 1904;Ernst 1923;Knitter and Riegraf 1984). Therefore, this bed is considered as erosive basis of the Dispansum Zone, and may contain reworked components of the Fallaciosum Subzone, as the ones described by Krumbeck (1943).
The top 4 cm of bed 23 finally show an ammonite accumulation with Dumortieria pseudoradiosa (Fig. 14: 14), D. radiosa, and D. striatulocostata, marking the onset of the Pseudoradiosa Subzone. The first appearance of Dumortieria moorei (Fig. 15: 10), which decends from D. radiosa, is delayed relative to D. pseudoradiosa, and falls into the upper part of the Pseudoradiosa Subzone. The (chrono)species also shows an overlap with its successor Cotteswoldia mactra, in accordance with data from sections from France (Rulleau 2007: p. 32). Consequently, the Moorei Subzone sensu Dean et al. (1961) in England is, in our view, not equivalent to the Pseudoradiosa Subzone sensu Gabilly 1976a and Elmi et al. 1997 (Fig. 8). However, the Moorei Subzone sensu Knitter and Ohmert (1983) in SW Germany (defined by the FAD of Dumortieria pseudoradiosa) is identical to the Pseudoradiosa Subzone used in the present paper. Moreover, top parts of the Pseudoradiosa Subzone at the Ludwigskanal/Dörlbach yielded Paradumortieria cf. tectiforme ( Fig. 15: 5), forming the transition to the subsequent Cotteswoldia aalensis group.
Bed 25 clearly forms a minor condensation with the enrichment of phosphorite nodules, ammonites and belemnites. Cotteswoldia mactra (Fig. 15: 3) obtained from this bed indicates the basis of the Mactra Subzone, Aalensis Zone, still occuring together with the latest D. cf. moorei. This lowermost part of the Aalenis Zone, which is a comparatively short interval of only 20 cm thickness, is characterized by the persistent occurrence of coarse-ribbed Dumortieria, specifically D. costula und D. externicostata, which appear to be absent (or rare) in the upper part of the section. Also, first Pleurolytoceras wrighti were found in this bed.
(iv) Lower Aalenian (>0.5 m) No Leioceras opalinum has been found in the Ludwigskanal/Dörlbach section. However, compressed Leioceras opalinum found in a drill core (highway viaduct Pilsach, unpublished observations) immediately above the equivalent of bed 33 suggests that the Toarcian-Aalenian boundary is located at the basis of bed 34.

Sequence stratigraphy
Sequence stratigraphic interpretations require correlation and comparison of sections across the shelf with respect to discontinuities, facies trends, geometry and stacking patterns (e.g., Catuneanu et al. 2011). In the present paper, the definition of sequences as "transgressive-regressive cycles" (T-R cycles) follows Embry and Johannessen (1992), with "sequence boundaries" at maximum regression surfaces (mrs; for further discussion see Catuneanu et al. 2011 andSimmons 2012). However, it remains difficult to distinguish stratigraphic gaps at maximum regression surfaces (with reworked fauna in the following transgressive "ravinement surface"; Nummedal and Swift 1987) from condensation or non-deposition of maximum flooding surfaces (mfs) in sections distant from siliciclastic sediment influx.
A sequence stratigraphic framework, however, has been developed for the Toarcian of Northern Germany, based on a comprehensive analysis of drillings (sedimentology and gamma ray logs) covering the transition from marine to fluviodeltaic deposits of the NE-German Basin to fully marine deposits of the NW-German Basin (Zimmermann et al. 2015). Nonetheless, the biostratigraphic calibration of the drillings relies on a combination of microfossil records and comparatively rare ammonite records, with limited resolution at the subzone level. Furthermore, a sequence stratigraphic interpretation was given by Röhl and Schmid-Röhl (2005) for the Lower Toarcian in Southern Germany, specifically based on the biostratigraphically and sedimentologically well investigated sections Dotternhausen (Swabian Alb) and Schesslitz (Franconian Alb).
For the Upper Toarcian of Southern Germany, no such detailed analysis exists, but important information can be derived from proximal sections near Regensburg, where minor coarse siliciclastic influx during regressive phases intercalate between fine-grained open-marine sediments with ammonoids (Fig. 9). Furthermore, sequence stratigraphic interpretations based on well dated drillings and outcrops exist for the Toarcian of France , including Quercy (Cubaynes et al. 1984;Lezin et al. 1997) and the type region of the Toarcian (Galbrun et al. 1994). Indeed, the most recent interpretation of the Toarcian eustatic sealevel curve by Haq (2018) is largely based on European sections from France, United Kingdom, Poland, and Switzerland, with additional data from sections in Argentina, Tibet, and the Arabian Platform (partially with tentative correlations).
The total duration of the Toarcian is about 8.5 Ma (Gradstein et al. 2012: from 182.7 to 174.1 Ma, i.e. 8.6 Ma;Boulila et al. 2014: 8.3 Ma), while the duration of each of the ammonite zones is to date under discussion (Gradstein et al. 2012;Boulila et al. 2014;Rübsam and Al-Husseini 2020). In the following, vertical thickness and fa cies trends as well as discontinuities of the Ludwigskanal/ Dörlbach section are discussed in comparison with neighbouring sections (Fig. 7) and areas to reveal T-R cycles for the top Pliensbachian to basis Aalenian succession in Southern Germany (Fig. 9).
Based on these observations, sediments of the Spinatum Zone in the Franconian Alb are considered to reflect a regression with prograding siliciclastics from NE, and a corresponding maximum regression surface in top parts of the Hawskerense Subzone (Fig. 9). This is in accordance with delta progradation and regression seen in N-Germany (Zimmermann et al. 2015) and a sequence boundary near the top of the Spinatum Zone (JPl8 sensu Haq 2018). Hence, the reduced thickness and increasing carbonate content towards the SW (Swabian Alb) mirrors the greater distance to the source of siliciclastics from NE (see Paul et al. 2008: fig. 5). An additional, minor, intermittent regression (with changes in bottom currents) might be seen in the erosional discontinuity at basis of the Hawskerense Subzone, while the coast-parallel lack of the total Amaltheenton mirrors a coast-parallel current intensification during the Pliensbachian-Toarcian transition (Fig. 9). Röhl and Schmid-Röhl (2005), however, suggest that the sequence boundary at the Pliensbachian-Toarcian transition in Southern Germany is located within the Tenuicostatum Zone, which might correspond to the medium sequence boundary JTo1 sensu Haq (2018). No decision can be made on that from the present data of the condensed Ludwigskanal section. In any case, the Upper Pliensbachian is generally known as a period of low sealevel, reflecting a cold climate interval with polar ice caps (Price 1999), glendonites (Teichert and Luppold 2013), and cool-water faunal elements (Arp and Seppelt 2012). Nonetheless, no evidence of subaerial exposure was found to date at basin margin sections (Regensburg area). However, these oolitic and laminated iron ores are poorly investigated with respect to sedimentology and geo chemistry.

(ii) Posidonienschiefer Formation
The Posidonienschiefer shows a very low thickness, combined with high carbonate contents. C org contents normalized to the silicate sediment fraction show that the T-OAE clearly peaks in the Exaratum Subzone (Fig. 4). A second maximum is developed in the Falciferum-Commune Subzones. Strikingly, first Posidonienschiefer beds (Laibstein I and II) show endo-and epibenthic bivalves (Nicaniella sp., Pleuromya sp., Goniomya rhombifera, Camptonectes subulatus), which are absent farther up the section, suggesting a delayed overstepping of anoxic bottom waters on the Altdorf High. Ammonite zones and subzones are densely spaced, however, with only one clear stratigraphic gap at the basis of the formation (Fig. 6). Here, the Posidonienschiefer Formation unconformably overlies the Hawskerense Subzone, with a stratigraphic gap comprising the Paltum-to midth of Semicelatum Subzone (Riegraf 1985a, b).
All other ammonite subzones from top of the Semicelatum to the Crassum Subzone are present in the bituminous and laminated sediments of the Ludwigskanal/ Dörlbach area (Fig. 6), although minor sedimentary gaps below the ammonite subzone resolution may be developed. The laminated bituminous marls continue into the lower Variabilis Zone with non-bituminous intercalations, fucoid beds, and scattered re-occurrence of benthic fauna (Grammatodon sp.). The fish scale-rich beds in the Falciferum Zone and at the basis of the Bifrons Zone, however, may reflect considerable sedimentological condensation, while the Dactylioceras-Monotis bed itself is an exceptional event bed with an erosional basis, possibly formed by a tsunami (Arp and Gropengießer 2015).
Towards the basin margin (Regensburg area), bituminous shales of the Falciferum Zone are overlain by prograding sandstones containing Dactylioceras commune and D. athlecticum (Pompeckj 1901;Putzer 1939;Krumbeck 1932b), followed by a discontinuity comprising the Fibulatum and Crassum Subzones (Fig. 9). Note that the "Crassum Bed" in this area does not contain Catacoeloceras crassum, but the younger species Cata coeloceras raquinianum, and is an equivalent of the "Belemnite Battlefield" (see below). In the Posidonienschiefer of Swabian Alb a number of fish scalerich beds ("Schlacken") at top of Falciferum Zone Riegraf 1985a) reflect a sedimentological condensation. Contrary to the Franconian Alb, bituminous sedimentation in the Swabian Posidonienschiefer ends with the Fibulatum Subzone.
Consequently, the sequence stratigraphic interpretation is as follows (Fig. 9): The transgression in the lower half of the Posidonienschiefer is documented by the successive onlap of subzones from SW to NE (Riegraf et al. 1984: p. 26, fig. 7;Riegraf 1985a: p. 55, fig. 27), delayed onset of sedimentation on the Altdorf High and basin margin (Regensburg area), and delayed benthos elimination on the Altdorf High. Minor sediment condensation (fish scale rich beds) at the Falciferum-Commune Subzone transition (bed 11; Fig. 6) may correspond to a maximum flooding surface, probably equivalent to fish-scale-rich in-tercalations ("Schlacken";Hauff 1921;Riegraf et al. 1984: p. 16 ff) in a similar lithostratigraphic position in the Swabian Posidonienschiefer (Fig. 7). Hence, higher Bifrons and Variabilis Subzone reflect regression with increasing siliciclastic influx, associated with pyrite preservation of ammonites (only Variabilis Subzone), and temporary re-oxygenation of the seafloor.
This interpretation is similar to the 3 rd order T-R cycle previously proposed for the Swabian and Franconian Posidonienschiefer by Röhl and Schmid-Röhl (2005) (Fig.  9). The only addition to be made is, that the regressive system tract of the cycle extents into the lower Variablilis Zone. In N-Germany, this T-R cycle appears slightly shifted, with a maximum flooding surface already in top parts of the Tenui costatum Zone (mfs Toa 1), followed by regression and maximum regression surface in top parts of the Bifrons Zone (mrs Toa 1) (Zimmermann et al. 2015). A possible explanation for this minor shift is the higher subsidence as well as higher sediment supply in North-German Basin at that time.
Considerable differences exist with respect to the proposed 3 rd order sequences in France. De  suggest four 3 rd order sequences for the Lower Toarcian, and a fifth sequence for the Fibulatum to Thouarsense Subzone. None of these T-R cycles appear recognizable in Germany, and may refer to minor superimposed changes (4 th oder sequences) only seen in areas of high sedimentation rate. Strikingly, there is also a continuous sedimentation from the higher Bifrons throughout the complete Variabilis Zone, in contrast to the erosive Intra-Variabilis-Discontinuity in S-Germany (Fig. 9).

(iii) Jurensismergel
Similar to the previous formation, the thickness of the Jurensismergel Formation is low (5 m). Ammonite zones and subzones are densely spaced and condensed at the phosphoritic and (slightly) bituminous basis of the formation (Fig. 6). Similar to the section Mistelgau (Schulbert 2001a, b), an increasing subzone thickness, decreasing carbonate content, and change from phosphoritic to pyritic ammonite preservation is observed towards the top of the formation (Fig. 7). A trend of increasing clay content is also recognizable from SW to NE, when compared to the increasingly carbonate-rich sections in the Swabian Alb (Fig. 7). This points to a general progradation of deltas in N-Germany at that time, affecting the Franconian realm, with the Swabian area more proximal to the warm Tethyan Ocean.
Therefore, we suggest that the erosive basis of the Jurensismergel Formation and sequence boundary lies within the Variabilis Zone, specifically at the basis of the Illustris Subzone (Figs 7,9). While no Intra-Variabilis-Zone mrs (maximum regression surface) is seen in N-German sections (Zimmermann et al. 2015), an equivalent mrs and sequence boundary at the basis of the Illustris Subzone was identified in the Toarcian type area in western France (Galbrun et al. 1994: their fig. 5).
In the Swabian Alb, submarine "swells" (i.e., remaining topographic highs after erosion within Variabilis Zone;  Stier 1922;Etzold et al. 1989), show aphotic microbialites in the Toarcensis Zone (e.g., Aalen-Weidenfeld: Dietl and Etzold 1977;Keupp and Arp 1990; Ohmden: Arp and Heyng 2013), indicating highly reduced sedimentation (Keupp and Arp 1990). These condensed deposits may represent a maximum flooding, while the Fallaciosum Subzone shows a very discontinuous distribution of sediments, in the Swabian as well as in the Franconian Alb. High sealevel, however is still indicated by the sporadic occurrence of P. fallaciosum at the basin margin ("nodule bed c", see above).
A clear and significant discontinuity is developed at the basis of the Dispansum Zone in the Franconian Alb. This erosive sequence boundary, commonly associated with a belemnite accumulation and aphotic microbialites (Dörlbach, Mistelgau: Fig. 7) is also recognizable in the Swabian Alb (Bad Boll : Wiedemann 1966: p. 91, Taf. 9;Bruder 1968: p. 15f.; middle part of bed 22 Rainau-Weiler, and KB4 Reutehau: Etzold et al. 1989). In N-Germany, the basis of the Dispansum Zone is a significant erosive surface (Heidorn 1928;Hoffmann 1968a: p. 465: "Zeta-Konglomerat") and coincides with the mrs Toa 2 proposed by Zimmermann et al. (2015). Likewise, the Fallaciosum-Insigne Subzone boundary represents a mrs and sequence boundary in SW-France (Cubaynes et al. 1984: their fig. 18). The widespread erosion of the Fallaciosum Subzone in the Franconian Alb explains the scarcity of Lytoceras jurense findings in this area, as this species is most common in this subzone.
Subsequently, condensation and phosphorites characterize the transgressive sediments of the Dispansum Zone at the Ludwigskanal/Dörlbach and elsewhere in the Franconian Alb, while in the following Pseudoradiosa Subzone with several beds of compressed Dumortieria accumulations may reflect further deepening. A flooding at the basin margin area might be indicated by findings of Pompeckj (1901: p. 143), who noted the occurrence of Phlyseogrammoceras cf. dispansum, Hammatoceras insigne and Dumortieria dumortieri (syn.: D. insignisimilis) from the Regensburg area.
Finally, the increasingly thick marls with pyritic ammonites of the Aalensis Subzone could reflect the regressive system tract of this 3 rd order cycle, with increasing clay supply from N. Indeed, at the basin margin (Regensburg area) quartz-pebble and feldspar-containing calcareous sandstones document this regressive interval (Putzer 1939: p. 93, 107) (Fig. 9). Consistent with this interpretation, the sedimentary succession in Quercy/France appears regressive for the Mactra to Aalensis Subzone, followed by a discontinuity and sequence boundary (Lezin et al. 1997).

(iv) Opalinuston
While basal parts of this formation, i.e., the Torulosum Subzone, still show a low thickness, discontinuities, phosphorite, and some carbonate, major parts are composed of a thick monotonous series of claystones. Discontinuities within the formation and lateral facies trends are poorly documented. However, the sections Grossenbuch (middle Franconian Alb) and Wittelshofen (southern Franconian Alb) described by Krumbeck (1943) demonstrate that the Aalensis-Torulosum Subzone boundary is associated with erosion, locally removing the complete Aalensis Subzone.
Therefore, the belemnite accumulation (bed 31) at the Aalensis-Torulosum Subzone boundary of the Ludwigskanal/Dörlbach section (Fig. 6), although unspectacular at first glance, can be interpreted as a mrs and sequence boundary (Fig. 7). Indeed, time-equivalent discontinuities have been described from N-Germany (e.g., Schlewecke, Dörnten : Ernst 1923;Heidorn 1928) and Quercy (Lezin et al. 1997). At the latter region, the discontinuity is associated with a lack of the Pseudolotharingicum (syn.: Lugdunensis) Horizon (Lezin et al. 1997), which also has not yet been detected in the Franconian Alb.
In the Ludwigskanal/Dörlbach section (Figs 6, 7), marls of bed 32 could reflect a subsequent sealevel rise, with condensed sediments, phosphorites, and the fossil accumulation in bed 33 representing a mfs, possibly equivalent to mfs Aal 1 in N-Germany (Zimmermann et al. 2015).
The monotonous and about 50 m thick claystone succession of the Opalinum Subzone, with only the lowermost 0.5 m exposed in the Ludwigskanal/Dörlbach section, reflects a progradation of siliciclastic influx from North, finally leading to shallow water conditions during deposition of the Upper Aalenian Eisensandstein Formation.

Conclusions
• The 16 m thick Ludwigskanal/Dörlbach section exposed upper parts of the Schwarzjura Group, from top parts of the Amaltheenton (>9 m), through the Posidonienschiefer (1.8-1.9 m) and Jurensismergel (3.5 m) to basal parts of the Opalinuston (>1.3 m). All formation boundaries are erosional discontinuities. Carbonate contents are low in the Amaltheenton, high in lower and middle parts of the Posidonienschiefer, followed by a successive decline to basal parts of the Opalinuston, with a last calcareous marl bed near the top of the Jurensismergel. The Posidonienschiefer shows increased organic carbon contents, which are, nonetheless, significantly lower than in other N-and SW-German sections due to the "dilution effect" by high carbonate contents. However, normalized to the sediment silicate fraction, organic carbon contents show a clear first maximum in the Exaratum Subzone and high values in the Falciferum and Bifrons Zone, similar to SW-German reference sections.
Highly fluctuating values characterize the Variabilis and Thouarsense Zone, while low organic carbon contents were found in the Dispansum and younger zones, with one single spike at the basis of the Aalensis Zone (Mactra Subzone). • Despite of the low thickness of the formations and a number of sedimentological gaps and condensation, all ammonite zones and subzones from the top of the Pliensbachian to the top of the Toarcian are present, with the following exceptions: Paltum and Clevelandicum (sedimentary gap), Vitiosa and Bingmanni (probably present, but no definite ammonite proof), and Fallaciosum Subzone (sedimentary gap). The erosive basis of the Torulosum Subzone may explain the lack of evidence for the Pseudolotharingicum (syn.: Lugdunensis) Horizon at the top of the Aalensis Subzone. The basis of the Aalenian is drawn in analogy to a neighbouring drill section that yielded Leioceras opalinum. Three subzones and horizons were detected for the first time in the investigated area: The Semipolitum Horizon, the Illustris Subzone, and the Dumortieri Horizon. The standard zone index ammonite Harpoceras falciferum, previously mentioned only by Urlichs (1971: p. 69), is figured for the first time for the Franconian Alb.
• The sequence stratigraphic standard Boreal 2 nd order cycle , with a transgression during the Lower Toarcian, maximum flooding at the basis of the Bifrons Zone, and a regressive succession in the remaining Toarcian to Aalenian, is clearly developed in Southern Germany. Superimposed to that, three 3 rd order T-R cycles are recognized at the Ludwigskanal/Dörlbach and adjacent sections, with a maximum regression near the basis of the Toarcian (i.e., within the lower Tenuicostatum Zon of Swabian Alb), within the Variabilis Zone (i.e., at the basis of the Illustris Subzone), at the basis of the Dispansum Zone, and less prominent at the basis of the Torulosum Zone.
In turn, maximum flooding surfaces are developed at the basis of the Commune Subzone, basis of the Thouarsense Subzone, basis of Mactra Subzone, and basis of Opalinum Zone. While transgressive sediments of the Franconian Toarcian commonly show phosphorites (similar to other epicontinental marine deposits: Loutit et al. 1988;Glenn et al. 1994: p. 767;Glenn and Garrison 2003: p. 524), regressive sediments frequently show pyrite preservation of fossils, especially ammonites. The extraordinary "Belemnite Battlefield" near the basis of the Jurensismergel is considered as transgressive sediment, consistent with the previous formation model by Urlichs (1971) of winnowing by a coast-parallel seawater current.
• The litho-, bio-, and sequence stratigraphic framework of the Ludwigskanal section may serve as a basis for further isotope and biogeochemical studies on the Toarcian Oceanic Anoxic Event, and its recovery phase, in this seawater current-affected part of the NW-European Epicontinental Seaway.
D2. The manuscript significantly benefited from discussions with René Hoffmann. Charlotte Kniest helped with the acquisition of rare papers. We thank Eckhard Mönnig and Matthias Franz (Göttingen) for their constructive and helpful reviews of the manuscript.