Homogeneous dolomite beds are usually 10 to 50 cm thick, embedded within clays and exhibiting sharp, plane-parallel joints. The beds consist of dolomicrite, which was previously described as aphanotopic dolomite by Breda and Preto (2011), according to the extended nomenclature for dolomite fabrics by Randazzo and Zachos (1983). The sediment is matrix-supported and contains irregular, partially rounded mud clasts (intraclasts) that consist of aphanotopic dolomite. Some of the mud clasts contain smaller and somewhat darker mud clasts or peloids (Fig. 4a, arrow). Soft sediment deformation is often not clearly visible due to the homogeneous structure of the mud, but it can be observed where the mud clasts are deformed within the matrix (Fig. 4b). Some of the homogeneous beds in the lower part of the section show sub-millimetre lamination that is only visible under the microscope, where it consists of alternating layers of light (locally coarser) and dark aphanotopic dolomite.

The clay content in the homogeneous beds is generally low. A few beds (e.g. at 33.5 m in the section) consist of silty or sandy dolomite, as reflected in a high abundance of detrital quartz in thin section. Pseudomorphs after gypsum occur in a dolomite bed at 120 m (Fig. 4c, d). Moldic porosity occurs within aphanotopic dolomite layers at 43, 65, and 89 m. These correspond to the tempestite beds observed in outcrop (see Breda and Preto, 2011).

One dolomite bed, located at 64 m in the section, appears homogeneous at outcrop scale, but consists of oolitic grainstone and lacks both an aphanotopic and a cement matrix (Fig. 4e). Ooids show concentric, micritic layers and are either hollow (where the cores may have been dissolved) or filled with sparite, and are surrounded with an isopachous cement rim.

Laminated dolomites occur in the upper part of the clay-rich interval, between 90 and 110 m in the section (Fig. 4f–i). In the field, the laminated dolomites show an alternation between light grey dolomite laminae and dark grey to black clay laminae. Some dolomite laminae are bent upward and are reminiscent of pseudo-teepee structures (Fig. 4f); the space within the teepee is sometimes infilled with sparry cement. In addition, the bending of the laminae towards the upward-directed cuspids is reminiscent of load structures (dish structures), but they may also represent desiccation cracks. The laminae are frequently ripped apart and fragments of laminae occur reworked as flat pebbles embedded in an aphanotopic dolomite matrix (Fig. 4g). Some laminae show a microsparitic appearance and laminar fenestral porosity. In some laminae a clotted peloidal fabric is observed (e.g. in Fig. 4f). Laminae are typically graded, whereby the upper part is darker, indicating an increase in the clay content (Fig. 4h, i). The top of the laminae is often truncated by an erosion surface, and rip-up clasts of the fine mud are embedded in the overlying coarse layer. Some laminated dolomites contain continuous layers with inclusions of celestine crystals in the 100 µm range, some of them with barite in their centre (Fig. 5a–c). Pyrite also occurs.

Under the SEM, laminated dolomites show an anhedral structure in the 1–5 µm range. No difference in mineral structure and grain size is observed between mud clasts and the surrounding, often lighter-coloured matrix. Dolomite crystals at the margins between dolomite and clay interlayers often coalesce into 5 µm-scale, round aggregates consisting of several subhedral crystals with different orientations (Fig. 6a, b; the crystals show orientation contrast under BSE mode). Dolomite crystals are often porous, showing a somewhat disordered appearance, but they are surrounded by syntaxial rims. In most cases, the rims entirely fill the intercrystalline space, forming almost hexagonal compromise boundaries (Fig. 6c, d). These rims occur both in homogeneous and laminated dolomites.

Nodular dolomites (Fig. 3b) often occur in beds of vertical peds linked to palaeosols, as indicated by horizons of vertical cracks showing green alteration fronts. Single nodules may also sporadically occur embedded within metre-thick beds of red and green clay. Nodules are usually 5 to 10 cm in diameter, consist of aphanitic dolomite or occasionally somewhat coarser microspar, and in cross section show both red and pale grey areas. Most nodules also show a deformed or brecciated internal structure with the interstices between the clasts mostly consisting of matrix and clay cutans.

A sample from the Weser Fm. (middle Lehrberg bed; clay pit Friedland, 12 km south of Göttingen, northern Germany; Seegis, 1997; Arp et al., 2004; Rieder et al., 2019) exhibits a brittle structure with high porosity. The material consists mainly of packed ooids with few peloids in a sparitic cement matrix. Under the SEM, subhedral to euhedral dolomites in the 5 µm range are observed within the ooids (not shown).

A sample from the Norian Arnstadt Fm. (formerly termed “Steinmergelkeuper”; middle grey series; locality of Krähenberg, 11 km SSW of Göttingen, northern Germany; Arp et al., 2005) shows millimetre-scale lamination and centimetre- to decimetre-sized laminated clasts, which were interpreted as a stromatolite breccia. The laminae contain abundant agglutinated siliciclastic grains (mainly quartz, subordinate albite) and phosphoritic fish scales. The dolomicrite exhibits a subhedral structure in the ≤5 µm range with a few larger, subhedral grains resulting in a porphyrotopic fabric.

Results of Sr-isotope measurements are available from Rieder et al. (2019). Results of sequential and non-sequential leaching tests of bulk samples TZ14-1, TZ14-7, and TZ14-9 are shown in Fig. 10a–c. 87Sr/86Sr ratios decrease in sample TZ14-1 from 0.708125±0.000012 to 0.707666±0.000004 with increasing strength of the leaching reagent, while the values remain almost constant in sample TZ14-9. The values of bulk dolomite extracted with 1 N acetic acid are slightly lower than in the fraction extracted with 0.1 N acetic acid; only microdrilled samples show higher values. However, repeating the 0.1 N acetic acid extraction (for 36 h) after a rather intense first extraction (4, 12, 4 h) results in extremely high values (0.715417±0.000250 in TZ14-1 and 0.7192266±0.000455 in TZ14-9; not shown in Fig. 10). Standard deviations are also higher than in the other fractions. Highest 87Sr/86Sr ratios of up to 0.730453±0.000005 in sample TZ14-7 are reached by extraction with 6 N HCl. At the same time, these fractions show the lowest Sr concentrations (see above).

Sequential extractions of the clay samples TZ16-1 and TZ16-19B with the lowest TIC of 0.02 wt % (Fig. 10d; Rieder et al., 2019) show a similar increase in the 87Sr/86Sr ratio with the sequential extraction steps from 0.1 N acetic acid to 6 N HCl, reaching similar values as in the HCl fraction of the dolomites (0.722998±0.000018 to 0.733910±0.000024).

Repeated extractions of chemically pure reference material (Fig. 10e, f) dissolved in 0.1 N acetic acid show a range of 87Sr/86Sr ratios in dolomite between 0.709942±0.000011 and 0.710831±0.000007. Pure single crystals of dolomite extracted sequentially show the highest value (0.708401±0.000040) in the 1 M NaCl fraction. Values in the 0.1 N acetic acid fraction (0.707735±0.000006) and the 1 N acetic acid fraction (0.707666±0.000006) are lower by almost 0.001 compared to the NaCl fraction.

In pure barite, 87Sr/86Sr ratios decrease by about 0.0013 in the extraction sequence from 0.1 N acetic acid to 6 N HCl. Celestine is highly soluble and was only measured in the 1 M NaCl fraction and once in 0.1 N acetic acid. Extracts of pure celestine show similar values as in the 1 M NaCl fraction of the barite–celestine–dolomite mixture (0.708038±0.000003), but the mixture shows higher values (0.709501±0.000040) in the 0.1 N acetic acid fraction.

Eleven dolomite samples were microdrilled from areas where dolomite was purest based on examination by SEM and dissolved in 0.1 N acetic acid. The values of the Travenanzes Fm. are in the range of 0.707672±0.000003 to 0.707976±0.000004 (Fig. 11). The highest value occurs in a dolomite nodule, while no systematic difference between homogeneous and laminated dolomite was observed. Dolomite of the Germanic Keuper samples shows significantly higher 87Sr/86Sr ratios of 0.709303±0.000006 and 0.709805±0.000005, respectively.

Dolomites of Deep Springs Lake show strongly radiogenic values of 0.713086±0.000004 and 0.713207±0.000004 (Fig. 12), which are much higher than modern seawater values, with a 87Sr/86Sr ratio of 0.709234±0.000009 (DePaolo and Ingram, 1985). In contrast, dolomite from the Coorong lakes (Milne Lake; Fig. 12) exhibits ratios between 0.709251±0.000004 and 0.709275±0.000003, which is very close to the ratio of modern seawater. Different incubation times (5 min and 10 h) in 0.1 N acetic acid had no influence on the isotope ratios.

The homogeneous dolomite beds, which are mainly intercalated in the lower, clay-rich part of the Travenanzes Fm., consist of fine-grained dolomicrite (aphanotopic dolomite), with occasional intraclasts of the same aphanotopic dolomite. Soft sediment deformation and dolomicrite infill between mud clasts indicate that this sediment consisted of unlithified, albeit cohesive, carbonate mud. Based on the abundance of fine mud, water energy was probably not very high (Demicco and Hardie, 1994), although reworking and partial rounding of the mud clasts requires at least occasionally higher water energies. According to the standard microfacies (SMF) concept, homogeneous aphanotopic dolomite falls into SMF 23 (“non-laminated homogeneous micrite and microsparite without fossils”), indicating deposition in “saline and evaporative environments, e.g. in tidal ponds” (Flügel, 2010). In addition, SMF 24 (“lithoclastic floatstones, rudstones and breccias”) is observed in some of the beds where mud clasts are abundant. These facies types are consistent with supersaturation-driven precipitation of fine-grained authigenic carbonate in environments that were partially disconnected from open seawater and would match with a coastal sabkha environment and/or shallow ephemeral lake. Ephemeral lakes may have formed on extended coastal alluvial plains along the Tethyan margin during the Carnian, as suggested by Breda and Preto (2011). The fine mud may have been homogenized and redistributed due to minor wave action in the ponds (see Ginsburg, 1971), which is often observed in ephemeral lake settings, explaining the formation of homogeneous dolomite beds.

Episodic flooding of the alluvial plain by the dryland-river system may have supplied water to temporary evaporating ponds. Alternatively, the alluvial plain may have been sporadically flooded by seawater, explaining the intercalations of authigenic dolomite layers with alluvial clays (Breda and Preto, 2011). Homogeneous dolomites show a positive carbon isotope signature between 0.7 ‰ and 4 ‰ VPDB (except one outlier), which is consistent with formation from unaltered marine carbon in evaporative brine, with no significant contribution of 12C derived from organic matter. Evaporative conditions are also indicated by several gypsum beds that occur between 45 and 70 m in the section and pseudomorphs after gypsum, which are observed in a thin section of a dolomite at 120 m (Fig. 4c, d). However, evaporites may not always be preserved, as they are frequently dissolved due to seasonally wet conditions.

A bed of dolomitic ooid grainstone that is devoid of matrix occurs at 64 m (Fig. 4e), and tempestites with moldic porosity indicative of dissolved allochems and dissolved fossils occur at several levels in the section, always associated with homogeneous dolomites. These beds must represent events of higher water energy, contributing sediment from more open marine areas. The presence of marine fossils, such as Megalodon bivalves, indicates that the environment was influenced by marine conditions, at least episodically. The microfacies of the oolite falls into SMF 15, which indicates proximity to the seaward edge of the platform. Several beds containing abundant siliciclastic material (mainly angular quartz clasts) are likely due to a riverine flooding event, which provided detrital material from the continent. In general, the microfacies in the homogeneous dolomite beds reflects both marine and continental influences on the depositional environment.

Laminated dolomites reminiscent of loferites (Fischer, 1964) occur in the upper part of the clay-rich interval. The change from more homogeneous to laminated dolomite intercalations correlates with the change from red to dark grey clay. The laminations consist of millimetre-scale dolomite–clay interlayers, suggesting alternating deposition of clay and fine dolomite. This microfacies falls into SMF 25 (“laminated evaporite-carbonate mudstone facies”), indicating an “upper intertidal to supratidal sabkha facies in arid and semiarid coastal plains and evaporitic lacustrine basins” (Flügel, 2010). Laminae showing soft sediment deformation cannot be attributed to stromatolitic bindstone facies (SMF 19 to 21). Only some layers that show a coarser fabric with interstitial dolosparite or dolomicrosparite containing putative peloids have been interpreted as microbial laminites (Preto et al., 2015). Graded bedding mostly indicates a direct sedimentation process rather than in situ precipitation of the primary carbonate within a microbial mat (Vasconcelos et al., 2006; Bouton et al., 2016; Court et al., 2017; Perri et al., 2018). A detrital origin of the clay in the dolomites is confirmed by a well-ordered illite–smectite mixed-layer composition, which is atypical for authigenic clay minerals. Frequent subaerial exposure and desiccation may explain why the sediment was not homogenized and the lamination is preserved. This is supported by the occurrence of pseudo-teepee structures as remnants of desiccation cracks. Rip-up clasts were formed during subsequent flooding, when angular flat pebbles formed as the sediment was desiccated or partially lithified. However, laminae commonly exhibit plastic deformation (e.g. in Fig. 3g) where the mud was still unlithified.

Some uncertainty exists as to whether this facies was peritidal or represents an ephemeral lake, as suggested for the homogeneous dolomites above. Episodic high water energy, as indicated by the rip-up clasts, combined with frequent desiccation, could point to evaporative tidal conditions that occur in a sabkha. What is atypical for a modern sabkha is the large amount of clay input. This is attributed to seasonally wet conditions during the Carnian, and the sediments can be considered to be a mixed facies of alluvial plain and coastal sabkha: a “dirty” sabkha (see discussion below). Under such conditions, large amounts of evaporites, in particular gypsum, could have been dissolved. Why the occurrence of laminated dolomites coincides with the transition from red to grey clays is not clear, but it may be related to more permanently water-saturated conditions in the subsurface, while the surface was exposed to periodic desiccation. These conditions would also be consistent with a sabkha environment.

During intervals of arid conditions, the clay beds were subject to strong evaporation and vadose diagenesis, causing oxidation and the red colour. Although red beds may also form in humid environments if drainage is rapid (Sheldon, 2005), drainage was certainly slow due to the large amounts of poorly permeable clay in the Travenanzes Fm., and the climate was clearly seasonally arid (Breda and Preto, 2011). Dolomite nodules that occur sporadically within certain intervals show internal brecciation, which must have occurred after sedimentation. Internal brecciation is a typical feature of present-day calcretes in arid environments (e.g. Mather et al., 2018). Slightly negative δ13C values indicate a contribution of carbon derived from organic matter degradation, further suggesting that they formed within the sediment. The formation of dolomite nodules could presumably be related to diagenesis in palaeosols. In the upper part of the section (between 80 and 105 m) dolomite nodules are associated with green reaction haloes along vertical peds in palaeosols of Vertisol–Calcisol type (Preto et al., 2015). Carbonate formation may have been related to reducing fluids in water-logged soils during humid intervals, while the cracks formed during desiccation in dry periods, perhaps facilitated by the presence of expandable clay minerals (smectite).

Radiogenic 87Sr/86Sr ratios can be indicative of the source of ionic solutions that the dolomite precipitated from (Müller et al., 1990a, b). Sr isotopes in selected dolomites from the Travenanzes Fm. at the Dibona section show values between 0.707672±0.000003 and 0.707976±0.000004. Ammonoids found at the base of the succession suggest a Tuvalian II age (subbullatus zone, 232.5–231.0 Ma; Ogg, 2012). The upper boundary of the Travenanzes Fm. is time-transgressive, and hence the exact age is not known. We assume that the sedimentation rate was at least as high, or higher, than in the peritidal carbonates of the Dolomia Principale. In this region, the Dolomia Principale includes a part of the Rhaetian (Neri et al., 2007) and, thus, its upper boundary is near the Triassic–Jurassic boundary at 201.3 Ma. Although the age interval of the Travenanzes Fm. is not precisely constrained, we correlate the Dibona section (Fig. 11) with the Carnian seawater Sr-isotope curve (Korte et al., 2003). The seawater curve was fixed at the lower boundary of the Travenanzes Fm. and the time axis was varied to fit the seawater curve parallel to the envelope of minimal 87Sr/86Sr ratios measured in the dolomites (Fig. 11). The base of the first massive dolomite at 110 m in the profile would therefore have an age of approximately 229 Myr.

Comparison with the seawater curve shows that the dolomites of the Travenanzes Fm. have largely marine 87Sr/86Sr ratios (Fig. 11). Only values from microdrilled samples extracted with 0.1 N acetic acid were used for this reconstruction, and the resulting values all lie within 0.00022 of seawater values (grey shaded area). This scatter towards more positive values, compared to seawater, may be due to a small influence by continental water. Indeed, during deposition of the Travenanzes Fm. sufficient continental water would have been available from rivers, and ions may have become concentrated while the water was evaporating in the distal alluvial plain. Alternatively, Sr desorbed from clay minerals could have added more radiogenic values to the brine. But even if a small influence of Sr of continental origin is present, the marine signal is dominant because of the much higher Sr concentrations in seawater.

The marine signature shown by the Sr isotopes does not support the classical Coorong model for dolomite formation, where alkalinity is largely derived from continental groundwater. The Coorong lakes in South Australia are ephemeral lakes largely supplied by groundwater (Von der Borch et al., 1975). Strangely, though, the 87Sr/86Sr ratios we measured from Milne Lake (one of the Coorong lakes) are similar to the ratio in modern seawater (Fig. 11), but this can be explained, as the local groundwater largely originates from a Pleistocene carbonate aquifer and, accordingly, carries a Pleistocene Sr-isotope signature. A similar scenario for the Travenanzes Fm. is unlikely as the only large-scale preceding carbonate platforms at that time were the upper Ladinian–Carnian Cassian dolomite platforms (Russo et al., 1997). Based on the stratigraphic context, all basins between these platforms were infilled by the Heiligkreuz Fm. and an extremely flat topography was later established that is stratigraphically overlain and sealed by the alluvial deposits of the laterally persistent Travenanzes Formation. Furthermore, the Travenanzes Fm. consists of 100 m of impermeable clay (including expandable clays), such that the long-distance transport of groundwater can be excluded.

We conclude that the 87Sr/86Sr ratios of the dolomites represent a predominantly marine influence. Presumably, seawater was transported to the interior of a coastal plain by episodic flooding (spring tide or storm) events. Even in a seasonally wet climate, the contribution of Sr from river water was insignificant compared to the influence of ions (including Sr) from seawater that was concentrated by evaporation. Laminated dolomites in the uppermost part of the section show values most similar to seawater composition, which is consistent with a greater influence of peritidal conditions.

Despite precautions to prevent contamination by other mineral phases by microdrilling and using mild reagents, some scatter occurs in the Sr-isotope data. Higher 87Sr/86Sr ratios in a dolomite nodule may be due to a continental influence or perhaps more seasonally wet and evaporative conditions with less of a marine influence. But higher values also may be due to contamination and partial leaching of clay minerals within the dolomite samples. Within the extraction sequence (1 M NaCl → 0.1 N acetic acid → 1 N acetic acid), the 87Sr/86Sr ratio generally remains constant or becomes slightly less radiogenic, i.e. more similar to seawater. However, the values strongly increase with leaching in 6 N HCl (Fig. 10). A modification of 87Sr/86Sr ratios due to contamination by 87Sr from the radioactive decay of 87Rb to 87Sr can be regarded as negligible since the concentrations of Rb was below the detection limit of 0.05 ppm (Rieder et al., 2019), and the half life is 48.8 billion years (Neumann and Huster, 1974). In addition, the influence of celestine and Sr-rich barite, which were observed under SEM, on Sr-isotope values can also be largely excluded. These mineral phases are bound to distinct layers of the laminated dolomites, and they could be avoided by microdrilling areas where the dolomite is pure. Only one value from sample TZ14-9 shows extremely high Sr concentrations. This sample was microdrilled near a celestine layer, and it is therefore not surprising that a celestine crystal may have inadvertently been sampled. The isotopic composition of the celestine is also similar to Carnian seawater.

In the NaCl fraction, only minimal amounts of dolomite are dissolved. The slightly more radiogenic 87Sr/86Sr ratio may be derived from Sr that is lightly adsorbed to clay minerals and finely dispersed in the clay matrix, although Sr2+ as a two-valent cation is more strongly adsorbed to clay minerals than Na+ and thus is not easily desorbed by NaCl. The values approach seawater values in the 1 N acetic acid fraction with increasing extraction efficiency and purity of the carbonate phase. Values from microdrilled samples are also generally more similar to seawater values, probably because more pure dolomite was sampled (Rieder et al., 2019). Usually, 1 N acetic acid is observed to not strongly attack interlayer ions in clay minerals.

Clay minerals leached in 6 N HCl show significantly more radiogenic values compared to dolomite samples. This finding is consistent with strongly radiogenic values in the 6 N HCl fraction of dolomite samples (up to 0.730453±0.000005) and suggests that the clay minerals are the carriers of an Sr pool significantly more radiogenic than the carbonate phase showing marine values. Sr is known to adsorb to illite–smectite mixed-layer clay minerals (Missana et al., 2008). The HCl fraction most likely includes adsorbed Sr and Sr occupying the interlayer positions of the clay minerals and presumably also structurally bound Sr in the clay mineral phase. In particular, illite–smectite mixed-layer clay minerals, as detected by XRD of the clay mineral separate in sample TZ14-9 (Fig. 7d), could have two different origins: burial diagenesis and continental weathering. Based on the tectonic setting and shallow burial depth of the Dolomites, the burial depth for a smectite–illite transition has not been reached. Therefore, these minerals are most likely derived from silicate weathering, with the Sr signature representing the crustal origin of the parent rock. Our finding of radiogenic Sr-isotope ratios supports the interpretation that clay minerals did not incorporate Sr from seawater during a sea-level highstand. It is therefore clear that Sr extracted from the dolomites is not derived from clay minerals.

The question is whether the Sr-isotope ratio reflects the conditions of dolomite formation or whether it is inherited from a precursor phase. Baker and Burns (1985) and Vahrenkamp and Swart (1990) documented very small distribution coefficients between aqueous and solid solutions, and high Sr contents measured in Abu Dhabi Sabkha dolomites (Müller et al., 1990a) may be derived from precursor aragonite. However, if dolomite in the Travenanzes Fm. is largely primary (Preto et al., 2015) and, thus, not formed from an aragonite precursor, the Sr content should derive from the dolomite phase. Although some Sr may have been released due to replacement of the dolomite and excess Sr can explain the occurrence of celestine and barite inclusions, nanocrystal structures imply that primary dolomite is partially preserved. Indeed, Sánchez-Román et al. (2011) demonstrate a protodolomite forming in culture experiments that contains Sr in the range of several thousand parts per million. The incorporation mechanism of Sr is still not entirely clear, since Sr is a large ion that should occupy the sites of Ca in the crystal lattice. However, in Sánchez-Román et al. (2011), Sr appears to correlate with the Mg content, and another incorporation mechanism may occur, such as surface entrapment. Also the correlation of Sr contents with K contents could be explained by such a mechanism of Sr incorporation. Even if only protodolomite formed in microbial culture experiments (Gregg et al., 2015), natural modern dolomites are often rich in Sr (e.g. Meister et al., 2007). The Sr could occur in disordered nano-structural domains that are not picked up in the bulk XRD signal. Non-classical nucleation and growth pathways, e.g. by nanoparticle attachment, could play a role in the abnormal partitioning of Sr in the dolomite lattice. Thus, a high Sr content in the Travenanzes Fm. or in Abu Dhabi Sabkha dolomites is likely a true signature of primary dolomites.

Several results support a largely primary origin of dolomite in the Travenanzes Formation. Formation temperatures reconstructed from oxygen isotopes (assuming Triassic seawater composition of -1 ‰ VSMOW) are between 28 and 33 ∘C. If a typical 18O enrichment of 3 ‰ due to evaporation in a sabkha is assumed (McKenzie et al., 1980; McKenzie, 1981), the calculated temperatures are between 40 and 50 ∘C, which is still within the possible range in a sabkha (see Hsü and Schneider, 1973). Both temperature and evaporation may have changed over time, which may explain the observed linear trend in oxygen isotopes across the section (Fig. 8b), although there is no covariation between δ13C and δ18O as it would be expected due to evaporation in hydrologically closed settings, such as the Germanic Keuper Basin (Reinhardt and Ricken, 2000; Arp et al., 2005). But also, the observed trend in δ18O would be too steep to be explained by overprinting within a normal geothermal gradient, and no signs of any hydrothermal activity occur in this region. In any case, the oxygen isotope data do not imply any post-depositional overprint, while nanocrystalline structures observed by Preto et al. (2015) preclude a later pervasive recrystallization during burial diagenesis. Sedimentary structures indicate that most of the homogeneous dolomite and laminae containing aphanotopic dolomite was unlithified, and dolomite was therefore deposited as fine-grained mud. This is further supported by millimetre-scale interlayering of clay and dolomite in the laminated dolomites near the top of the sequence, and some dolomite–clay couplets exhibiting fining-upward bedding. Based on the observation of nanocrystal structures, replacement did not take place, and it appears logical to assume that the primary phase was already dolomite.

While most of the dolomite may have been primary, micron-scale interstices between the dolomicrite grains must have been cemented after deposition. This cementation resulted in rims visible under SEM and results in near-hexagonal compromise boundaries. The cement may have contributed 13C-depleted carbon during early diagenesis. The lowest δ13C values of -3.4 ‰ VPDB occur in the nodules. These nodules formed within the sediment, probably due to reducing conditions and influenced by dissolved inorganic carbon from degrading organic matter in the palaeosols. Homogeneous and laminated dolomites are clearly distinct from nodules in their carbon isotope compositions (Fig. 8a), indicating only a minor contribution from pore-water-derived dissolved inorganic carbon. Carbon isotope values are thus largely consistent with a primary precipitation. The mode of dolomite formation as fine mud and subsequent cementation is comparable to several modern sites of dolomite formation.

While dolomite formation under Earth surface temperatures has been suggested to be catalysed by microbes, perhaps via a secreted extracellular polymeric substance (EPS; see Bontognali et al., 2013), this mechanism is currently under debate (see Gregg et al., 2015). The present study neither supports nor rules out such a mechanism. We can raise the question whether microbial EPS is enriched in the surface waters, where it may affect precipitation of fine dolomite mud.

The classical sabkha model involves dolomite formation under intertidal to supratidal conditions, the concentration of brines through either seepage reflux (Adams and Rhodes, 1960) or evaporative pumping (Hsü and Siegenthaler, 1969; Hsü and Schneider, 1973; McKenzie et al., 1980; McKenzie, 1981), and precipitation of dolomite as Mg/Ca ratios increases due to gypsum precipitation (see Machel, 2004, for a more detailed discussion of varieties of sabkha models). This sabkha model allows for a mixture of seawater and continental groundwater, with seawater mainly providing the ions for dolomite precipitation. Coastal sabkhas are typically characterized by laminated (Lofer-type) dolomites, where the laminae are largely unlithified after deposition (Illing, 1965; Bontognali et al., 2010; Court et al., 2017). In the sabkha of Abu Dhabi, both pathways, via replacement of precursor aragonite and by direct precipitation of dislocation-ridden primary dolomite, are observed (Wenk et al., 1993).

The sabkha model is thus a reasonable model for the uppermost parts of the Travenanzes section, which contain laminated dolomites, marine Sr-isotope values, and indications of frequent desiccation and flooding in a peritidal setting. Yet, the conditions differed from the modern sabkhas along the Persian Gulf due to the large amount of alluvial clay (dirty sabkha), as opposed to aeolian sand. Most of the fine laminations may therefore result from periodically varying conditions, perhaps with clay deposition during episodes of fluvial discharge and carbonate deposition during evaporative conditions.

The playa lake model was originally suggested by Eugster and Surdam (1973) for dolomite of the Green River Formation (Wyoming), but the primary formation of fine dolomite mud is observed in many alkaline playa lakes, such as Deep Springs Lake (Peterson et al., 1963; Clayton et al., 1968; Meister et al., 2011), Lake Acıgöl (Turkey; Balci et al., 2016), Neusiedler See (Austria; see Neuhuber et al., 2015), and Lake Van (Turkey; McCormack et al., 2018). For an overview see Eugster and Hardie (1978) and Last (1990). This type of setting has also been suggested for the Germanic Keuper deposits during the late Carnian and Norian, when the Germanic Basin was entirely disconnected from Panthalassa and was continental (Reinhardt and Ricken, 2000). The Travenanzes Fm., with its homogeneous dolomite intercalations in red and green clays, is strikingly similar to playa lake Keuper facies in the Germanic Basin. There, dolomite formed following evaporation and concentration of the continental brines under a semiarid climate.

Sr-isotope data, however, support a dominantly marine origin of ionic solutions to the Travenanzes Fm., whereas Sr isotopes are strongly radiogenic in the Germanic Keuper dolomites (or in Deep Springs Lake; Fig. 12). The two settings are thus fundamentally different. Even dolomite nodules, showing somewhat more radiogenic values than seawater in the Travenanzes Fm., still indicate a predominantly marine influence. The slightly more radiogenic influence could be due to clay minerals present in the nodules that were difficult to entirely separate from the carbonate. Also, dolomite nodules may have formed in relation to palaeosols, during somewhat more humid times and, thus, may have been slightly influenced by continental water input from rivers.

The Coorong model was proposed by Von der Borch et al. (1975), Von der Borch (1976), and Rosen et al. (1989) (see Warren, 2000, for detailed information) to explain the formation of primary and uncemented dolomite in the Coorong lakes of South Australia. The Sr-isotope values (Fig. 12) show that the contribution of ionic solutions, and hence alkalinity, of continental origin to the dolomitizing fluids was minimal, and that the dolomites are seawater derived. This may be distinct from the typical Coorong model, where alkalinity is provided from an inland karst system. But other coastal ephemeral lakes exist, including along the Brazilian coast, north of Rio de Janeiro. Partially unlithified dolomite occurs in Brejo do Espinho (Sánchez-Román et al., 2009), which is largely similar to the Coorong lakes, but ionic solutions are mostly derived from seawater.

A coastal ephemeral lake model would probably be most suitable to explain the homogeneous dolomite beds of the Travenanzes Fm., where hypersaline ponds may have formed in a dryland-river system. However, unlike recent ephemeral lakes (such as Lagoa Vermelha, Brejo do Espinho, and the Coorong lakes) the clay-rich sediment must have inhibited groundwater flow. Hence, while modern coastal ephemeral lakes receive their water largely through seawater percolating through porous dune sand, episodic flooding with seawater must have provided ionic solutions for dolomite formation on a coastal plain.

Overall, the depositional environment reconstructed for the Travenanzes Fm. shows similarities to modern systems where dolomite forms. Among all the modern scenarios, a coastal ephemeral lake model would be most similar to the conditions conducive to homogeneous dolomites, lacking signs of frequent desiccation, while a coastal sabkha model may explain the laminated intervals near the top of the studied succession. In contrast to modern systems, the clay-rich sediments of the Travenanzes Fm. preclude any input of groundwater, which plays a role for ionic transport in both the modern-day ephemeral lake model and the different versions of sabkha models. Although modern systems provide valid analogues for the mechanism of dolomite formation in the past, and probably throughout Earth history, none of them is a faithful environmental analogue. The Carnian coastal plains that covered an enormous area along the Tethys margin (Garzanti et al., 1995) represent a system without a single modern analogue in terms of their sedimentary, hydrological, and climatic boundary conditions. Also, the geochemistry of Tethys seawater may have been different from modern seawater, an issue that requires further investigation (see Burns et al., 2000; Li et al., 2018). These aspects need to be taken into account if we intend to understand the conditions that led to dolomite formation through Earth history.

In the light of the possibility of spontaneous precipitation of fine dolomite mud in the water column, perhaps via formation and aggregation of nanoparticles, further discussion of a nucleation and growth pathway of dolomite is necessary. While several modifiers may also play a role in the water column, such as dissolved organic matter (Frisia et al., 2018), microbial EPS (Bontognali et al., 2013), or suspended clay particles (Liu et al., 2019), fluctuating conditions inducing spontaneous nucleation and growth of dolomite, in agreement with Ostwald's step rule (Deelman, 1999), require further consideration as a factor favourable for dolomite formation on a seasonally variable platform (Meister and Frisia, 2019).

The main finding of this study is that most of the dolomite in the >100 m thick Travenanzes Fm. probably formed through direct precipitation from a seawater-derived solution. This mode of primary dolomite formation has rarely been considered in the study of dolostone formations, but may explain the genesis of many other large-scale, fine-grained dolomite units that preserve fossils and sedimentary structures.