Evolution of the northern Tethyan Helvetic Platform during the late Berriasian and early Valanginian

The Early Cretaceous period is characterized by widespread carbonate production in tropical and subtropical epicontinental seas, which was modulated by changes in sea‐level, detrital and nutrient fluxes, and the global carbon cycle. As a result, carbonate platforms were sensitive recorders of environmental change, which often anticipated global environmental perturbations. A good example is provided by the northern Tethyan carbonate platform, which is presently preserved in the central European Helvetic Alps. There, the latest early to late Valanginian Weissert episode of global change, which is defined by the first important positive shift in δ13C records of the Cretaceous, is expressed by a prolonged, stepwise drowning phase. In this contribution, a detailed reconstruction of palaeoenvironmental change before and during the Weissert episode is provided based on three representative sections of the Helvetic platform. The sections are placed along a deepening transect and correlated by means of ammonite and microfossil biostratigraphy, sequence stratigraphy and δ13C chemostratigraphy. In a first phase of palaeoenvironmental change during the latest Berriasian, photozoan carbonate production was stopped by a major and hitherto undetected drowning episode, which was followed by a phase of renewed carbonate production by heterozoan biota. This phase was linked to major sea‐level rise, a change to a more humid climate and strong regional subsidence associated with tectonic block tilting. During the Valanginian, the circulation of nutrient‐enriched sea waters prevented a return to oligotrophic conditions and two further drowning episodes occurred, which are both documented by condensed phosphate‐rich beds and dated as middle early Valanginian and late Valanginian to early Hauterivian. The exact causes of the three‐step deterioration in carbonate production are not established but a link to episodic volcanic activity is likely, eventually related to the formation of the Paranà‐Etendeka large igneous province.


INTRODUCTION
The Early Cretaceous was characterized by generally high pCO 2 levels and correspondingly reinforced greenhouse conditions, which were favourable to important carbonate production in tropical and subtropical shallow-water settings (Hay, 2008). During this time interval, carbonate deposition was modulated and in certain regions episodically interrupted by global sea-level change, changes in the global carbon cycle and corresponding pH and pCO 2 in sea water, which were often related to phases of major volcanic activity, and changes in detrital and nutrient flux rates. The resulting carbonate deposits represent first-order archives of palaeoclimate and palaeoenvironmental change, which recorded and often anticipated global perturbations (F€ ollmi et al., 1994Weissert et al., 1998;Huck et al., 2011).
During the Early Cretaceous, a first episode of major palaeoenvironmental change occurred during the latest early to the late Valanginian (Weissert episode), which is defined by the first important carbon-isotope excursion (CIE) of the Cretaceous. The mechanisms leading to this episode are likely to be sought in increased volcanic and hydrothermal activity, whichdepending on which time scale is usedmay be attributed to the formation of the Paran a-Etendeka Large Igneous Province (Lini et al., 1992;Erba et al., 2004;Martinez et al., 2015). The Weissert episode is preceded by a phase of significant palaeoenvironmental and palaeoclimate change, which occurred during the late Berriasian and early Valanginian. In the western European domain (England, Germany, France and Switzerland), a phase of enhanced humidity began during the late Berriasian, and probably reached a maximum in the earliest Valanginian (Hallam et al., 1991;Schnyder et al., 2005;F€ ollmi, 2012;Morales et al., 2013). A general increase in marine nutrient levels in ocean basins is recorded from the latest Berriasian to the late Valanginian (F€ ollmi, 1995;Duchamp-Alphonse et al., 2007) and on northern Tethyan platforms (F€ ollmi et al., 2007;Morales et al., 2013), which interfered with the evolution of carbonate platforms at that time. In addition, carbonate production was impacted by sea-level variations, the occurrence and timing of which are not wellconstrained and still under debate (Schlager, 1981;Haq et al., 1987;Hardenbol et al., 1998;Gr eselle & Pittet, 2010;Haq, 2014). Finally, and on a more regional scale, the change in platform morphology from a distally steepened ramp to a swell-dominated ramp and the disappearance of a barrier close to the Berriasian-Valanginian boundary documented in the Jura region probably influenced the distribution of continental fluxes on the northern Tethyan shelf and in the adjacent basin (Morales et al., 2013).
The Helvetic platform succession, which is presently preserved in the northern part of the central European Alps, documents the afore-mentioned palaeoenvironmental changes in great detail. A change from photozoan to heterozoan carbonate production has been observed near the Berriasian-Valanginian boundary (Ischi, 1978;Burger, 1985;Wyssling, 1986;Mohr, 1992;F€ ollmi et al., 1994, and two condensed phosphatic and glauconiticrich layers of early Valanginian (B€ uls Beds), and late early Valanginian to Hauterivian age (Gemsm€ attli Bed), highlight the occurrence of two successive incipient drowning phases (Haldimann, 1977;Wyssling, 1986;Kuhn, 1996). While the presence of ammonites within these two beds permits the two drowning episodes to be accurately dated, the stratigraphy of the platform carbonates deposited prior to the formation of these two condensed beds is less well-constrained, impeding its correlation with the general record of environmental change during the late Berriasian and earliest Valanginian.
In this contribution, a detailed study of three representative sections along a proximal-distal transect through the upper Berriasian and Valanginian Helvetic succession is presented. An improved stratigraphic framework based on ammonite, benthic foraminiferal and calpionellid biostratigraphy is established and combined with d 13 C bulkrock chemostratigraphy and sequence stratigraphy. The evolution in facies and microfacies associated with variations in mineralogical and phosphorus contents are used to examine changes in accommodation, ecology, nutrients and climate. The goal thereby is to trace the onset and evolution of the impact of palaeoenvironmental change prior to and during the Weissert episode on the northwestern Tethyan carbonate platform. In a second step, the Helvetic sedimentary succession is compared with sediments preserved in the Provence, Pyrenean and Jura Platforms, which allows a regional view of the palaeoenvironmental changes that occurred along the northern Tethyan margin to be developed. Finally, the potential influence of intense volcanic activity on palaeoecological and environmental changes is evaluated.

GEOLOGICAL SETTING AND DESCRIPTION OF STUDIED SECTIONS
The succession of Early Cretaceous platform deposits (Funk et al., 1993;F€ ollmi et al., 2006 starts with the Zementstein Formation, which is characterized by dark and monotonous marly carbonate deposits and dated from the Berriasella jacobi and Subthurmannia occitanica zones by ammonites and calpionellids (early Berriasian; Mohr, 1992). It is overlain by the € Ohrli Formation, which documents the development of a photozoan carbonate platform with the predominant deposition of oolitic and bioclastic sediments, containing a rich and diverse fauna of benthic foraminifera, corals, green algae and echinoderms (Burger, 1985(Burger, , 1986Mohr, 1992). The € Ohrli Formation includes two marly and calcareous intervalsthe Lower € Ohrli Marl Member, Lower € Ohrli Limestone Member, Upper € Ohrli Marl Member and Upper € Ohrli Limestone Member, respectively. The age of the € Ohrli Formation is poorly constrained. A maximum age is provided by calpionellids and ammonites found at its base, which indicate an early Berriasian age (S. occitanica zone) (Mohr, 1992). The € Ohrli Formation passes laterally into the monotonous hemipelagic marly succession of the Palfris Formation (Burger, 1985(Burger, , 1986Wyssling, 1986).
Both formations are overlain by the marly and sandrich Vitznau Formation, which was attributed to the early Valanginian based on palynomorphs (Pantic & Burger, 1981). The overlaying Betlis Formation is rich in echinoderms and bryozoans and marks the development of a heterozoan platform. The occurrence of the ammonite Thurmanniceras thurmanni s.l. (Wyssling, 1986) indicates a T. pertransiens age. Distal occurrences of the Betlis Formation include the condensed and phosphatic B€ uls Bed, which was dated as late T. pertransiensearly B. campylotoxus zone (Kuhn, 1996). The overlying condensed and phosphatic Gemsm€ attli Bed and its lateral equivalent, the sandy Pygurus Bed, provides a minimum age corresponding to the S. verrucosum zone (Wyssling, 1986;Kuhn, 1996;F€ ollmi et al., 2007).
Three sections were selected, which represent inner shelf, barrier and outer shelf settings along a proximaldistal transect (S€ antis, Dr€ ackloch, Vitznau; Fig. 1

Microfacies and sequence stratigraphy
The outcrops, samples and thin sections were analysed for their facies and microfacies, which were described and interpreted following the classification of Arnaud- Vanneau & Arnaud (2005). Twelve facies zones were thereby differentiated, from F1 corresponding to hemipelagic environments, to F10 attributed to shallow subtidal to tidal environments. The sequence stratigraphic framework was established using field observations and trends in facies and microfacies Catuneanu et al., 2009).

Carbon and oxygen isotope analyses
The carbon and oxygen isotope composition ( carbon and oxygen isotopes are reported in the delta (d) notation as the per mil (&) deviation relative to the Vienna-Pee Dee belemnite standard. Replicate analyses demonstrated an analytical reproducibility for the international calcite standard NBS-19 and the laboratory standards Carrara Marble of better than AE0Á05& for d 13 C and AE0Á1& for d 18 O.

Phosphorus content
Total phosphorus (P) contents were measured on 320 bulk-rock samples (129 from the S€ antis section, 143 from the Dr€ ackloch section and 48 from the Vitznau section) following the procedure described in Bodin et al. (2006). The concentration of P was obtained in ppm by calibration with known standard solutions using a UV/Vis photospectrometer (Perking Elmer UV/Vis Photospectrometer Lambda 10, k = 865 nm) with a mean precision of 5%.

Bulk-rock mineralogy
The bulk-rock mineralogy was determined with a Thermo scientific ARL X'TRA IP2500 X-ray diffractometer using a semi-quantification method using external standards and following the procedures of K€ ubler (1983, 1987) and Adatte et al. (1996). The precision was 5 to 10% for phyllosilicates and 5% for grain minerals. A total of 113 samples were run for the section at S€ antis, 147 for the section at Dr€ ackloch, and 47 for the section at Vitznau. Relative contents of phyllosilicates, quartz, K-feldspar, Na-plagioclase, calcite, dolomite, pyrite, goethite and ankerite were determined. Variations in Kfeldspar, Na-plagioclase, dolomite, pyrite, goethite and ankerite proportions were not significant enough to be shown in this publication. They were nevertheless taken into account in the calculation of the percentages of the other minerals.

S€ antis section
The lower part of the section (the first 53 m) corresponds to the upper part of the € Ohrli Formation (Figs 2 and 5). This unit includes abundant large benthic foraminifera accompanied by green algae, rudists, gastropods, calcareous sponges, corals, bivalves and echinoderms. Its microfacies ranges from F2 to F10 (Table 1), and is largely dominated by shallow-water limestone deposited at either side of a shoal. Three dissolution levels were observed within the Upper € Ohrli Limestone Member. The first is located 20 m above the base of the section, where dissolution features infilled by mud ( Fig. 6) affect 2 m of the underlying sediment. A second level occurs at 26 m above the base of the section, where muddy cavity infillings are present (Figs 5 and 7A). The € Ohrli Formation terminates with facies characterized by enhanced microbial activity (intense micritization of clasts) and an abundance of gastropods and thin miliolids (F10). Its top surface is a complex surface marked by dissolution vugs, macroscopic borings and infillings by mudstone of the overlying Vitznau Formation (Fig. 6). These borings are also present in the overlying bed, indicating the superposition of two hardgrounds. The three dissolution levels are interpreted as epikarstic layers.
The base of the Vitznau Formation (between 53 to 57 m above section base) is characterized by a microfacies rich in crinoids and bryozoans with an important degree of sedimentary reworking (F4/FT). This microfacies type is replaced at 57 m above the base of the section by peloidal, crinoid-rich microfacies showing a relatively scarce and poorly diversified fauna of bivalves, bryozoans and circalittoral foraminifera (F3). Based on these observations, the Vitznau Formation indicates a change towards heterozoan carbonate production.
At the base of the Betlis Formation (69 to 72 m above the base of the section), oncoids and reworked bioclasts were observed (F4/T). Numerous chert nodules occur between 67 and 86Á5 m above the base of the section. An erosive surface was identified at 86Á5 m. The upper part of the Betlis Formation (from 96 to 107 m above the base of the section) shows an important increase in detrital quartz, with the occurrence of well-rounded and broken quartz grains (0Á2 to 1 mm). The top of the Betlis Formation is characterized by a hardground with borings infilled by phosphatic and glauconitic sediment of the overlying condensed Gemsm€ attli Bed (Fig. 8B).

Dr€ ackloch section
The Dr€ ackloch section (Figs 3 and 9) starts with the Lower € Ohrli Limestone Member, which shows a microfacies rich in quartz, circalittoral foraminifera, sponge spicules, echinoderms and containing sparse larger agglutinated foraminifera (F2/3) typical of platform slope deposits. This member evolves (22 to 55 m above the base of the section) towards coarser grainstone containing large benthic foraminifera, echinoderms and small ooids, together with numerous large rounded mud intraclasts and bioclasts (shallow-water photozoan facies F5/6). The few samples collected from the marly upper part of the Lower € Ohrli Limestone Member and the Upper € Ohrli Marl Member (55 to 130 m above section base) show a return to facies F2/3.

The microfacies of the overlaying Upper €
Ohrli Limestone Member shows an increasing proportion of large benthic foraminifera, echinoderms and green algae (F6/7) between 130 and 174 m above the base of the section. At 174 m, a rapid change towards facies F3 is noted, and an evolution towards coarser grainstone (up to F8) follows to a point 246 m above the base of the section. Fringing cements and dissolution vugs with mud and microsparite infillings ( Fig. 8C and D) were observed between 236 and 246 m above the base of the section. From 246 to 260 m above section base, a change in biota occurs: corals and calcareous sponge debris become dominant and benthic foraminifera nearly disappear with the exception of Andersenolina (Fig. 8E). The presence of dolomitic extraclasts indicative of a confined and very shallow environment mixed with mud of outer shelf origin (containing Lenticulina), hermatypic and ahermatypic coral debris, rudists and bryozoans ( The overlying Vitznau Formation starts with a layer containing abundant reworked bryozoans, corals, Gryphaea and serpulids floating in a clayey matrix (FT, Figs 10C and 12C), which documents a change towards heterozoan carbonate production. A second hardground with borings infilled with pyrite is observed at the top of this interval. The lower part of the Vitznau Formation at 246 to 300 m above the base of the section shows limestone-marl alternations containing abundant Gryphaea in life position or weakly transported, some brachiopods, circalittoral foraminifera and sponge spicules (deep subtidal facies F2/F3). Limestone layers are absent in the upper part of the Vitznau Formation (300 to 315 m above section base) and Gryphaea is less abundant.
The Betlis Formation begins 315 m above the base of the section with a layer containing ooids, abundant crinoids, sparse bryozoans, small benthic foraminifera, sponges and gastropods, together with reworked mud pebbles from the Vitznau Formation (FT). From 315 to 370 m, a peloidal microfacies containing crinoids and bryozoans is present (F3 to F4). Similarly to the S€ antis section, numerous chert layers and nodules are present in the interval between 315 and 357 m (Fig. 11B). At 370 m, a level containing crinoids, bivalves, small benthic foraminifera, Lenticulina and ostracods shows important reworking (FT). The Pygurus Member on top of the Betlis Formation includes millimetre-sized, rounded and fractured quartz grains, and bioturbation is important.

Vitznau section
The Vitznau section ( Fig. 13) starts with the upper part of the Palfris Formation, in which muddy facies containing sparse bioclasts of echinoderms and reworked circalittoral foraminifera (F1/F2) evolve towards a grainstone facies with peloids, sparse bryozoans and small benthic foraminifera (F3). In the overlying Upper € Ohrli Limestone Member, 12Á5 to 23Á4 m above the base of the section, a change towards coarser grainstone with mud pebbles, echinoderms, sparse ooids, large benthic foraminifera, corals and sponges (F5) is observed. The last 1Á6 m of the € Ohrli Formation shows a complex succession of erosive surfaces and hardgrounds. A first irregular erosive surface associated with limestone lag pebbles ( Fig. 11C) occurs at 23Á4 m above section base. The overlying calcareous layer is perforated and the borings are infilled by marls of the covering layer (Fig. 11D). This thin marly layer is rich in reworked and pyritized extraclasts (Fig. 10D). The next two layers have an erosive base (Fig. 11E). Their microfacies consists of peloids, echinoderms and small benthic foraminifera with sparse reworked platform debris (benthic foraminifera, ooids and extraclasts) (F3). In the uppermost part of the Upper € Ohrli Limestone Member, the microfacies additionally contains calcareous sponges, corals and ooids (F6). A

55
The Berriasian -Valanginian Helvetic Platform hardground is observed on top of the Upper € Ohrli Limestone Member, whose borings are often filled with pyrite ( Fig. 10E). A further marine hardground occurs on top of the first marl-limestone alternation of the Vitznau Formation. The corresponding microfacies shows intense reworking with ostracods, microbial mats, bryozoans, broken serpulids, micritized crinoids and corals, together with mud pebbles rich in quartz (FT), up to 26Á3 m above the base of the section (Fig. 10F). From 26Á3 to 56Á8 m, the microfacies consists of monotonous wackestone containing ostracods, sponge spicules and serpulids (F1/F2). Numerous Gryphaea were observed in life position or reworked. At 56Á8 m above the base of the section, an erosive bank rich in bivalves is present (Fig. 11F), which is followed by a recessive marly interval of 1 m. Finally, field observations indicate that the Betlis Formation is composed of a peloidal echinodermal carbonate (F3). The lack of samples in this interval is due to difficult access in steep terrain and the presence of vegetation hiding the transition between the Vitznau and Betlis formations.

Carbon and oxygen isotope data
Oxygen isotope values (see supplementary data) oscillate between À5 and À2& in the S€ antis section (with a mean of À3Á5& and a standard deviation of 0Á4&), and À6 and À2& in the Dr€ ackloch and Vitznau sections (with means of À3Á7 and À3Á5& and standard deviations of 0Á8 and 0Á7&, respectively). The d 18 O values reflect significant diagenetic overprint (Choquette & James, 1987) and are not further discussed here.
The d 13 C long-term trends are similar between the three sections (Figs 5, 9, 10 and 12): relatively heavy but variable values (with mean values of 1Á3, 1Á1 and 0Á9&, and standard deviations of 0Á7, 0Á6 and 0Á4&, in the S€ antis, Dr€ ackloch and Vitznau sections, respectively) are observed in the € Ohrli Formation and its distal equivalent, the Palfris Formation. In contrast, lower d 13 C values (with mean values of 1Á3, 0Á5 and 0Á3&, and standard deviations of 0Á2, 0Á4 and 0Á2&, in the S€ antis, Dr€ ackloch and Vitznau sections, respectively) are recorded in the Vitznau and the Betlis formations. The S€ antis section is the only section that records a positive The circulation of meteoric or altered marine pore waters tends to decrease the oxygen and carbon-isotopic values in carbonates during diagenesis (Choquette & James, 1987). As such, a minor overprint of d 13 C values is to be expected. However, the d 13 C vs. d 18 O plot (Fig. 14) exhibits a relatively poor correlation coefficient (R 2 < 0Á4). In general, carbon-isotope records in neritic carbonates have been shown to be only marginally affected by burial diagenesis, and have been used as a reliable stratigraphic tool (Ferreri et al., 1997;Hennig, 2003;F€ ollmi et al., 2006;Weissert et al., 2008). Exceptions are, however, possible in the presence of emersion surfaces and depending on mineralogy (for instance aragonite versus calcite; Ferreri et al., 1997;Swart & Eberli, 2005;Weissert et al., 2008). The consistent trends between the sections, as well as the comparable d 13 C trends and value ranges with correlated sections in the Jura Mountains and the Vocontian Basin (La Chambotte, Juracime, Montclus; Morales et al., 2013), suggest that the d 13 C records are rather well preserved.

Phosphorus content as a nutrient tracer
Low P contents were measured in the Lower € Ohrli Limestone Member, followed by higher values in the Upper € Ohrli Marl Member (100 and 250 ppm on average in the Dr€ ackloch section). In the Palfris Formation of the Vitznau section, P values oscillate around 200 ppm (Fig. 13). A decrease in P concentrations was measured in the overlying Upper € Ohrli Limestone Member with mean values of 60, 80 and 70 ppm in the S€ antis (Fig. 6, with the exception of two samples with values close to 1000 ppm), Dr€ ackloch and Vitznau sections (Fig. 12), respectively. In the € Ohrli Formation, variations in P concentrations are therefore associated with lithological changes. Therefore, the level of nutrients in the sea water was significantly controlled by depth.
Phosphorus contents abruptly increase at the base of the Vitznau Formation in the three sections. In this formation, mean values of 110, 250 and 300 ppm were measured in the S€ antis, Dr€ ackloch and Vitznau sections, respectively, which are similar to those in the Betlis Formation. In the S€ antis and Dr€ ackloch sections, the uppermost part of the Betlis Formation shows a progressive increase in P values up to 3700 and 2750 ppm, respectively. The increase in P levels in the Vitznau and Betlis formations is associated with the proliferation of suspension-feeding organisms (essentially crinoids and bryozoans), leading to the production of heterozoan carbonates. The relatively high P contents observed in the Betlis Formation indicate that depth was not the only factor controlling the nutrient level, and that additional sources were involved.

Bulk-rock mineralogy as a proxy for detrital input
The phyllosilicate and quartz contents increase in the Lower € Ohrli Limestone Member and Upper € Ohrli Marl Member in the Dr€ ackloch section (from 5 to 35% and from 10 to 25%, respectively), as well as in the Palfris Formation in the Vitznau section (up to 20% and 25%, respectively). They decrease below the detection limit of the XRD (<5%) in the overlying Upper € Ohrli Limestone Member in all three sections. Therefore, these trends show a good correlation with lithological changes and are comparable to trends in P values.
In the Vitznau Formation, the phyllosilicate and quartz contents increase to 17% and 18%, respectively in the Dr€ ackloch section, and up to 32 and 47%, respectively in the Vitznau section, indicating a strong increase in detrital material. In the S€ antis section, they remain below the detection limit (<5%), perhaps linked to the more proximal location of this section. In the Betlis Formation, the phyllosilicate content falls below 10% in all three sections, but a different behaviour for the quartz content is observed. In the Dr€ ackloch section, relatively high values occur at the base of the formation (36% at section meter 326). In the S€ antis and Dr€ ackloch sections, the quartz content falls below 5% and rises in the uppermost part of the formation to 44% and 36%, respectively. This goes along with the presence of large (>1 mm) rounded quartz grains and a concomitant increase in P values shortly before the drowning of the Weissert episode.

Age control and correlation of sections
A combination of lithological, biological and sequence stratigraphic tools were used to correlate the sections. Stratigraphic sequences were determined on the base of key surfaces (sequence boundaries, transgressive and maximum flooding surfaces). Sequence boundaries in shallow-water deposits can be recognized by the presence of unconformities associated with erosion and subaerial exposure . Transgressive surfaces are often erosive and overlaid by reworked deposits, and maximum flooding surfaces are indicated by a maximum of accommodation space. In certain cases, accommodation maxima (between the transgressive and highstand systems tracts) correspond to a transitional interval termed maximum flooding zone (mfz). Parasequences were interpolated based on (micro-)facies and lithological observations, and are indicated in the corresponding figures. However, given the locally unequal sampling resolution due to limited access and fault zones, the determination of parasequences should only be considered as an approximation. The studied sections include a total of seven sequences, labelled from I to VII.
In addition, calpionellids were identified in the different sections. In the Vitznau section, a Berriasian age is assigned to the upper part of the Upper € Ohrli Limestone Member by the presence of Remaniella filipescui (Fig. 9), which extends from calpionellid zones B to D3 (Blanc, 1996). The presence of Calpionellopsis in the upper part of the Upper € Ohrli Limestone Member in the sections at S€ antis and Vitznau (Fig. 9) indicates calpionellid zone D2/D3 (Remane, 1963(Remane, , 1985Remane et al., 1986Remane et al., , 1998. Finally, an ammonite was found in the scree, immediately below the section of Vitznau (Fig. 9). The specimen is a Thurmanniceras thurmanni s.str. and indicates the early Thurmanniceras otopeta ammonite zone corresponding to the latest Berriasian (Blanc et al., 1992).
The first sequence (sequence I), is marked by the joint presence of Pseudotextulariella courtionensis and Montsalevia elevata, and as such is of late earlyearly late Berriasian age. Sequence I is well-documented in the Dr€ ackloch section (Fig. 15). Since this section is composed of outer shelf to outer platform deposits, sequence boundaries are not necessarily defined by emersion surfaces, and lowstand systems tracts (LST) can be preserved. There, sequence I starts 25 m above the base of the section, where the first sequence boundary (SB I) is placed  Table 2. Age attribution of stratigraphic sequences based on their marker fauna content. Biostratigraphies are based on the distribution of benthic foraminifera (principally established by Darsac, 1983 andBlanc, 1996), calpionellids (Blanc, 1996;Remane et al., 1998) and ammonites (Blanc et al., 1992) at a change from facies F5/6 to F2/3. The overlying shallowing-upward interval thickens towards the west (e.g. towards the basin, Fig. 3) and is interpreted as a LST. The deepening upward trend towards facies F2 constitutes the transgressive systems tract (TST), and the mfz is placed in the more recessive layers covered by vegetation. The HST is well developed and documents the progressive installation of the photozoan platform. In the more distal Vitznau section, Pseudotextulariella courtionensis is the only stratigraphic marker found in the first sequence (from 0 to 12 m above section base). Since this foraminifer is also present in the overlying sequence and since Pavlovecina allobrogensis is generally found in shallowwater deposits, this first sequence is attributed to sequence I. The following sequence (sequence II) contains also Pseudotextulariella courtionensis and Montsalevia elevata, but Pavlovecina allobrogensis is absent. This association is documented in the three sections. In the S€ antis succession, sequence II is present at the base of the section and continues 20 m up section. In the Dr€ ackloch section, SB II is placed at an abrupt change in facies from F5/F8 to F3 at 175 m above section base. Sequence II shows a shallowing-upward trend towards facies F8 up to 246 m above the base of the section, corresponding to a HST. In the Vitznau section, SB II is placed at the base of a more prominent calcareous bed showing reworked platform clasts (F5). The following interval with a deepening trend from facies F5 to F3 is interpreted as a TST. The mfs is placed where the most distal facies (F2/3) is observed 20Á6 m above the base of the section.
Sequence III is marked by the disappearance of Pseudotextulariella courtionensis. Sequence IV shows the first appearance of Pfenderina neocomiensis and is the last sequence observed in the photozoan platform succession of the Upper € Ohrli Limestone Member. In the S€ antis section, SB III corresponds to the first epikarstic level. Above, the facies shows a rather abrupt deepening to facies F3 corresponding to a TST, which is followed by a shallowing-upward trend to facies F9/10, typifying the HST. SBIV is placed 26 m above the base of the section, where a second epikarst level is observed. The upper part of the Upper € Ohrli Limestone Member is dominated by facies F8, but shows abrupt changes to facies F2 and F3. The presence of parasequence boundaries, local tectonic activity, and/or the transfer by storms (washover) may explain the occurrence of such outer platform deposits in the external lagoon.
In the Dr€ ackloch section, fringing cements and dissolution vugs indicative of conditions close to emersion are observed between 236 and 246 m above the base of the section. SB III is placed at 246 m, where the d 13 C record shows an abrupt shift to lighter values, indicating the presence of an important hiatus. Above this level (from 246 to 260 m above the base of the section), shallow-water organisms still largely dominate carbonates, but an abrupt change in the carbonate fabric is observed with a clear dominance of corals and calcareous sponges, and the near disappearance of benthic foraminifera (except for Trocholinas which are found in abundance, and Mohlerina basiliensis). Sequences III and IV are therefore not observed in the Dr€ ackloch section. This implies that sequence boundaries III, IV and V are combined, and a significant part of the late Berriasian is missing.
In the Vitznau section, an important erosional surface associated with lag pebbles marks a sequence boundary at 23Á5 m above section base. Between 23Á5 and 26Á3 m above the base of the section, a succession of erosional surfaces is present. The top of this interval shows the reworking of partly lithified sediments rich in ooids and bioclasts. Given the relatively distal position of the Vitznau section, this interval is interpreted as a falling-stage systems tract (FSST). The marker calpionellid Calpionellopsis simplex was found together with Calpionellopsis oblonga and Remaniella filipescui (Fig. 9), and with the benthic foraminifera Montsalevia elevata (Fig. 7). Pseudotextulariella courtionensis, which is abundant in the underlying sequence, is absent from these reworked deposits. Consequently, this FFST may belong either to the combination of sequences III, IV and V, or to sequence V alone.
Sequence V is marked by an increase in detrital minerals, a change towards heterozoan carbonate production, and a lower diversity and abundance in benthic foraminifera (large specimens are no longer observed). In the S€ antis section, SB V is documented by an epikarst overlapped by a hardground at 53 m above the base of the section, indicating that this sequence boundary is combined with the transgressive surface. The occurrence of a second and similar hardground in the immediately overlying bed, and of an interval containing reworked extraclasts of corals and calcareous sponges together with intraclasts of heterozoan organisms (58 m above section base) witness an important transgression (TST). A mfz is then placed where the deepest microfacies was observed (F2, outer shelf).
In the Dr€ ackloch section, SB V is placed where dissolution vugs indicate conditions close to the emersion. Sequence V starts with the uppermost beds of the Upper € Ohrli Limestone Member, where important sediment reworking is observed. This facies is interpreted as a lag on top of the transgressive surface, and the overlying interval as a TST. The phosphatic and pyritic infillings of the hardground on top of the Upper € Ohrli Limestone Member, as well as the presence of a second hardground on top of a reworked layer in the Vitznau Formation confirms the occurrence of major relative sea-level rise during sequence V. The mfs of this sequence is placed within the more recessive part of the Vitznau Formation, which is covered by vegetation (between 309 and 315 m above section base).
In the Vitznau section, this transgression of major amplitude is equally documented by two hardgrounds and sediment reworking, located on top of the FFST (26Á3 m above section base). The mfs of this sequence is also placed in the more recessive layers of the Vitznau Formation (facies F1). The ammonite Thurmanniceras thurmanni s.str. (Fig. 9) indicating the early part of the T. otopeta zone (latest Berriasian) was found in the scree. Ahermatypic corals were found in the host rock of the ammonite, which were also observed in the Dr€ ackloch section in the early TST of sequence V (lower part of the Vitznau Formation). Stable isotope and P analyses performed on the host rock of the ammonite indicate that its origin is from the Vitznau Formation (values of 0Á20& d 13 C, À2Á70& d 18 O, 137 ppm P are only found at 33 m). If the determination and the position of the ammonite are correct (Thurmanniceras thurmanni s.l. has an extended range into the T. pertransiens ammonite zone; Wippich, 2003;Bujtor, 2013), the lower part of the Vitznau Formation at Vitznau (e.g. early TST of sequence V) has a latest Berriasian age.
Sequence VI shows the development of heterozoan carbonates deposits and corresponds to the uppermost part of the Vitznau Formation in the Vitznau section, and to the lower part of the Betlis Formation in the Vitznau, Dr€ ackloch and S€ antis sections. In the three sections, its sequence boundary is mingled with the transgressive surface, and overlain by lag deposits. SB VI is placed at 69 m above the base of the S€ antis section, at 315 m above the base of the Dr€ ackloch section, and at 56Á8 m above the base of the Vitznau section where erosive banks showing intense sediment reworking are observed. Because no significant change in facies occurs within sequence VI in the S€ antis and Dr€ ackloch sections, the mfs probably coincides with the sequence boundary. In the Vitznau section, the mfz is placed within the overlying marly interval, which probably corresponds to a deeper depositional environment as it shows the most recessive layers of the sequence. There, only the lower part of the Betlis Formation is accessible, and interpreted as the base of a HST. Marker benthic foraminifera indicate an age close to the Berriasian-Valanginian boundary for sequence VI (Table 2).
Sequence VII corresponds to the top of the heterozoan Betlis Formation. In the S€ antis section, SB VII is placed at 86Á6 m above the base of the section, where an erosive surface associated with reworked oncoids was observed. The mfs is then placed within the hardground associated with the Gemsm€ attli Bed at the top of the Betlis Formation. In the Dr€ ackloch section, SB VII is noted at 370 m above the base of the section with a level of important reworking. The uppermost part of this interval corresponds to the Pygurus Member, which is part of the Pygurus-Gemsm€ attli complex documenting the major drowning phase of the Valanginian Helvetic platform (Haldimann, 1977;Wyssling, 1986;Kuhn, 1996;F€ ollmi et al., 2006. This sequence is thereby characterized by significant detrital input and ends with a major condensation phase. The duration of the condensation of this level (more than 3 Myr, from the late Valanginian to the early Hauterivian) implies the presence of several sequence boundaries within this level (Godet, 2013), therefore highlighting a complex sequence stratigraphic surface (SB VIII).

DISCUSSION
Global sea-level change during the late Berriasianearly Valanginian The stratigraphic distribution of marker foraminifer and calpionellid species allows for a correlation of the Helvetic platform with other shallow-water deposits in the northwestern Tethyan area (Fig. 16), and therefore permits differentiation between local and global factors controlling sedimentation. Faunal associations characteristic of sequences I and II were identified in the Pierre Châtel and Vions formations (Jura Mountains; Darsac, 1983;Morales et al., 2013) Blanc et al., 1992) and the Pyrenees (Marnes de Francazal;Peybern es & Combes, 1994).
Differences in the pattern of sedimentary deposits are, however, observed between the Helvetic and the northwestern Tethyan platforms during the early Valanginian. In the Jura Mountains, Provence, and the Pyrenees, photozoan carbonate facies are observed with the Upper Member of the Chambotte Formation, the Calcaire Blanc Sup erieur Formation, and the Calcaires Graveleux a Pfenderines, respectively. Conversely, these oligotrophic deposits are not seen in the Helvetic sections. This is related to an enhanced subsidence phase recorded in the Helvetic domain compared to the north-western Tethyan areas (Stampfli et al., 2002), which may be associated with the local influence of nutrient-rich waters onto the northern Tethyan shelf.
Stratigraphically above the lower Valanginian photozoan carbonates of the Jura Mountains, Provence and the Pyrenees, a second major transgression is recorded by the deposition of deeper marly facies, locally associated with hardgrounds and condensed layers containing ammonites (from the Busnardoides campylotoxus zone) in Provence (Marnes Grises; Virgone, 1997) and the Pyrenees (Peybern es & Combes, 1994). The onset of condensation may be correlated with the condensed phosphatic layer of the B€ uls Bed in the Helvetic Alps, which is dated from the late Tirnovella pertransiensearly Busnardoides campylotoxus ammonite zones (Kuhn, 1996). In the Pyrenees, heterozoan carbonate deposits are described on top of this condensed layer (Calcaires Jaunes a Bryozoaires). The latter formation is attributed to the Hauterivian based on brachiopod stratigraphy (Peybern es & Combes, 1994) but the 'low stratigraphic interest of micropalaeontological descriptors' mentioned by the authors suggests that this age attribution may need to be reviewed. In the Helvetic and the Jura regions, shallow-water heterozoan carbonates (sequence VI and VII, and Bourget Formation, respectively) appear below the highly condensed sediments of the Marnes a Astieria (Jura Mountains) and the Gemsm€ attli-Pygurus complex.
Sequence boundaries identified in this contribution have been correlated with global sequence boundaries of Haq (2014, Fig. 16). Following calpionellid determinations, SB III would correspond to KBe3 (early zone D), SB V to KBe4 (latest zone D); SB VI (mid zone E) to KVa1 (earliest zone E), and SB VII to KVa2 (Fig. 16). SB I (zone C) may correspond to KBe2 (at the boundary between zones B and C). Interestingly, SB III, SB V and SB VI are associated with biostratigraphic boundaries of foraminiferal faunas (discontinuities d1, d2 and d3 of Darsac, 1983), which might confirm their importance (medium cycle boundaries of Haq, 2014).

Effects of tectonics on palaeogeography
The biostratigraphy of the outer shelf section at Dr€ ackloch indicates the occurrence of an important hiatus related to the absence of the late Berriasian sequences III and IV (Fig. 15). This hiatus is linked with an emersion; its duration corresponds to an important part of the S. boissieri ammonite zone, that is, of the late Berriasian. A relatively similar succession is observed in the Helvetic platform section of L€ ammerenplatten (Pasquier, 1995), consisting of nearly identical environments (close to or at the barrier) but with a less well-constrained temporal framework. During the Early Cretaceous, major fault zones affected the Helvetic region (Funk, 1985;Detraz et al., 1987) and an important subsidence phase is recorded from the Oxfordian to the Hauterivian (Funk, 1985;Stampfli et al., 2002). Thus, the different stratigraphic records of the three studied successions are probably linked to a phase of tectonic activity during the late Berriasianearly Valanginian, which is likely related to extensive movements affecting the northern Tethyan margin. The generally high subsidence rates documented in the Helvetic plateau was commonly linked to the opening of the Alpine Tethys and of the North Atlantic (Stampfli et al., 2002).
The Dr€ ackloch section, which is close to the platform margin, was probably located near the top of a tilted block, whereas the S€ antis section, which is composed of more lagoonal deposits, is thought to have been located within an intrashelf depression formed by the tilting process (Fig. 17). The topographic effect of tilting blocks may also be evident in the S€ antis section, where parasequences involving high amplitude relative sea-level variations were observed in the sedimentary succession and microfacies of the Upper € Ohrli Limestone Member. The slope section of Vitznau records a FSST, which may also be linked to the topographic effects of block tilting. Lithological logs represent synthetic successions as described by Peybern es & Combes (1994), Virgone (1997), Morales et al. (2013Morales et al. ( ), F€ ollmi et al. (2006 and this study. A correlation based on benthic foraminifera, calpionellids and ammonites is proposed, which permits variations in sea-level to be traced during this time interval. A major transgression started from the latest Berriasian onwards; however, TST are not fully recorded in shallow-water series. The Berriasian-Valanginian boundary is therefore placed at the base of the Vitznau Formation (Helvetic Platform) in this scheme, and similarly for time equivalent deposits of the Jura Mountains, Provence and the Pyrenean Platforms. A succession of three sea-level rises is highlighted from the Berriasian-Valanginian boundary to the late Valanginian, which led to the occurrence of three phosphate-rich condensed layers in the Helvetic area.

Palaeoenvironmental and palaeoclimate changes
In the late early and early late Berriasian, the increase in P and detrital input observed in sequence I (upper part of Lower € Ohrli Limestone Member and Upper € Ohrli Marl Member) are linked on the Helvetic platform to a period of transgression (Fig. 18). Outer shelf deposits typically show an increase in detrital minerals basinwards (Fig. 18). The following sequences do not show major changes in these parameters, which is different from the Jura areas, where an increase in quartz and P is observed in the Vions Formation. The sections examined in the Helvetic area were more distant from the continental coast, which may explain these different records.
An increase in P and detrital minerals is also recorded in the Vitznau Formation, which again corresponds to a sea-level rise of probably wider importance (Fig. 16). Platform carbonate production changed from photozoan to heterozoan assemblages in the Helvetic region. Less diagenetically altered, age equivalent sections in the Jura Mountains and Vocontian Basin (La Chambotte and Montclus; Morales et al., 2013) indicate that this interval also corresponds to a maximum in humidity, indicated by high kaolinite contents in both platform and basinal environments. Thus, with an important transgression and a highly hydrolysing climate, biotas were subjected to increasing stress leading to the disappearance of oligotrophic organisms. The presence of phosphates and reworked pelagic sediments on top of the Upper € Ohrli Limestone Member at Dr€ ackloch, associated with a series of superimposed hardgrounds in all three sections indicates a platform drowning phase associated with this important transgressive phase. Following the drowning phase, a mesotrophic fauna including bryozoans, crinoids and prolifering Gryphaea, brachiopods, ostracods and serpulids, was installed in the distal part of the platform.
With the Betlis Formation (sequences VI and VII), platform carbonate production recovered in heterozoan mode. Excess nutrients are also indicated by the presence of chert nodules, which are related to a higher proportion of filter-feeding siliceous sponges, and the presence of a phosphatic bed separating the Betlis Formation into two members in more distal sections (Kuhn, 1996). This phosphatic bed indicates a second drowning phase of the carbonate platform. The Jura Platform (where a similar heterozoan facies is observed) and the Vocontian Basin sections provide evidence, however, of a decrease in humidity during this period (Morales et al., 2013).
At the top of sequence VII, strong detrital input is recorded. The top of sequence VII is poorly documented in the Jura Mountains (Hennig, 2003) and clay mineral analyses were not performed. Nevertheless, the abundance of millimetre-sized quartz grains (Fig. 8C), sometimes containing a ferruginous coating, indicates an important phase of erosion on the continent. This is correlated with an increase in P contents, which suggest increased nutrient input to the ocean. This phase of enhanced weathering, combined with a sea-level rise was responsible for the third, long-lasting and most important drowning phase of the carbonate platform in the Helvetic area during the Weissert episode.
The early Valanginian negative CIE: a prelude to the Weissert Event?
The comparison of the d 13 C records of the S€ antis, Dr€ ackloch and Vitznau sections highlight similar trends, which can be correlated with the Vocontian Basin (section of Montclus, Morales et al., 2013), and which are in adequacy with the sequence stratigraphic interpretation (Fig. 15). A general increase in d 13 C values is observed during the Berriasian, whereas a decrease in d 13 C values is highlighted during the early Valanginian, finally followed by the global positive d 13 C shift characterizing the Weissert episode. On the Helvetic platform, which is marked by emersion and condensation phases, this results in a significant negative shift (of 0Á7&) at the boundary between the Upper € Ohrli Limestone Member and the Vitznau Formation. By comparison with the d 13 C record of the Vocontian Trough, where the amplitude of variations in d 13 C values should be reduced compared to the platform record (Morales et al., 2013), the duration of the hiatus occurring through the Berriasian-Valanginian boundary may be close to 800 kyr (counting Milankovitch cycles). Thus, our results show that in addition to biostratigraphy and sequence stratigraphy, the general trends of the d 13 C records may be used to correlate the sections within this Helvetic transect. The negative excursion is mainly attributed to a change in carbonate production from photozoan to heterozoan carbonate production on the platform Morales et al., 2013).
In the Helvetic area, this change in carbonate production coincides with a major relative sea-level rise linked to a transgression and local tectonics, a maximum in humidity, and associated higher nutrient levels. Similarly, a negative d 13 C shift and a change towards heterozoan carbonate deposits are observed in the Jura Mountains (Bourget Formation, Morales et al., 2013), but may not be entirely synchronic with the Helvetic record. This may be explained by different tectonic contexts. A likely similar succession is observed in the Pyrenees, where the heterozoan Calcaires Jaunes a Bryozoaires might be correlated with the upper part of the Betlis and the Bourget formations (of the Helvetic and Jura platforms, respectively). In the Provence area, shallow-water water heterozoan deposits are scarcely observed (with the exception of the Olioulles succession; Virgone, 1997;Schroeder et al., 2000;Masse et al., 2009;Bonin et al., 2012).
Higher nutrient levels control the settlement of heterozoan carbonates, characterized by suspension-feeding organisms (James, 1997). Enhanced humidity and runoff, however, are probably not the main factors driving nutrient fluxes, as the Bourget Formation (Jura Platform) is depleted in kaolinite (relative to smectite and chlorite; Darsac, 1983;Adatte, 1988). Instead, nutrient-rich currents associated with a major transgression may have played a role. The sea-level rise may have favoured watermass exchanges between the Boreal and Tethyan oceans (van de Schootbrugge et al., 2003). The opening of the Fig. 18. Stratigraphic variation in phosphorus and quartz contents along a proximal-distal transect (see Fig. 15 for correlation), comparison with the Montclus section (Vocontian Basin, Morales et al., 2013), and correlation with a recent global sea-level reconstruction (1st and 2nd order variations according to Haq, 2014). The phosphorus and quartz contents are expressed in ppm and per cent of the bulk-rock, respectively. The major relative sea-level rise occurring during the latest Berriasian -earliest Valanginian is accompanied by enhanced phosphorus and quartz contents and an ecological change towards heterozoan associations. Polish gateway was probably initiated during the early Valanginian, as testified by the deposition of open marine sediments belonging to the Tirnovella pertransiens ammonite zone on top of a Tithonian karstified limestone in the Polish Basin (Kutek et al., 1989;Morales et al., 2015), and the migration of Boreal calcareous nanofossils and ammonites to the Tethyan realm (Bulot, 1996).
These nutrient-rich water masses may result from an upwelling system explained by the inundation of continental margins, which would have triggered enhanced evaporation and wind velocities, resulting in stronger westerlies (Poulsen et al., 1998;Godet, 2013). There is, however, no strong evidence for cooler water during the Tirnovella pertransiens-lower Busnardoides campylotoxus interval. Nutrients may also come from the intense weathering of landmasses located north-eastwards, which may have been transported westwards by the circum-Tethyan current. A very humid climate is indeed known from the Polish Basin (Morales et al., 2015). The existence of these currents might also explain why the Helvetic platform turned to a heterozoan carbonate production earlier than other platforms located further to the west, and why the consistent association of phosphate and platform drowning in the Helvetic region (Kuhn, 1996) is not known in the Jura, Provence and Pyrenees platforms.
In this context, the third and major drowning phase of the Valanginian (corresponding to the Gemsm€ attli-Pygurus Beds in the Helvetic area) is related to a phase of intense weathering, as witnessed by the high quartz contents in the Gemsm€ attli-Pygurus complex, linked with a peak in humidity (higher kaolinite contents in the Vocontian Basin, Duchamp-Alphonse et al., 2011;F€ ollmi, 2012), and combined with a sea-level rise. This drowning phase affected a platform that was already weakened by high nutrient levels resulting in two incipient drowning phases and previous rapid relative sea-level changes.
A general increase in volcanic activity, eventually related to the eruption of the Paran a-Etendeka continental flood basalts is viewed as the main trigger of environmental changes, leading to an increase in pCO 2 levels and profound climate modifications. The increase in magmatic activity may have been linked to enhanced oceanic crust production that triggered eustatic sea-level rise. Martinez et al. (2015) recently re-calibrated the Valanginian astronomic time scale and suggested that the main volcanic pulse of the Paran a Etendeka may coincide with the onset of the positive d 13 C excursion. Their calibration also shows a rather good correlation between the initiation of the volcanism, and the early Valanginian decrease in d 13 C values recorded in the Vocontian Basin. Since it is not clear yet if the latter is of global significance, the decrease in stable carbon-isotope values is rather attributed to a change of carbonate production than to volcanic activity.

CONCLUSIONS
The installation, growth and demise of the Berriasian-Valanginian carbonate platform have been documented using a transect of sections across the Helvetic Alps. A more accurate age control is proposed for the studied sections of the S€ antis, Dr€ ackloch and Vitznau locations, based on benthic foraminifera, calpionellid and ammonite biostratigraphy, chemostratigraphy, and sequence stratigraphy. This integrated stratigraphic approach provides a base for detailed correlation of the three studied sections, and enables the comparison with equivalent records from the Vocontian Basin, the Jura Mountains, Provence and the Pyrenean platforms. Thereby, this contribution provides a more balanced view of shallow-water ecological changes to global perturbations, by highlighting the superimposed influence of regional tectonic and palaeogeographic factors. The Helvetic sedimentological record is different from those of other northern and north-western Tethyan platforms, which is interpreted as reflecting the combined effect of palaeoceanographic differences, with the Helvetic platform being more exposed to the northern nutrient-carrying Tethyan current, and of enhanced subsidence in the Helvetic domain. Block tilting is the most likely mechanism to explain a major hiatus present in the upper part of the Upper € Ohrli Limestone Member in the Dr€ ackloch section, encompassing a significant part of the late Berriasian.
A major sea-level rise is documented, which started in the latest Berriasian and progressively flooded the platform. This sea-level rise, combined with a strongly subsiding setting and enhanced humidity led to the disappearance of photozoan faunas and provoked a first major drowning phase in the Helvetic domain. During the early Valanginian, suspension feeders dominated shallow-water organisms in the northern and north-western Tethyan area, as a response to higher nutrient rates in the ocean. The nearby continents, however, underwent a decrease in humidity and runoff. The turn to heterozoan carbonate production is interpreted as the consequence of palaeoceanographic changes, which may be related to the establishment of upwelling currents. During the late early and early late Valanginian, phases of enhanced detrital input linked with strong continental weathering, and sealevel rises were responsible for a second and a third, widespread demise of the already weakened carbonate platform: near the boundary of the pertransienscampylotoxus zone and during the verrucosum zone, respectively. In the Helvetic domain, the latter shallow-water carbonate crisis is evidenced by a quartz-rich phosphatic and glauconitic crust on top of a hardground (Gemsm€ attli Bed) or a quartz and phosphate-rich, highly bioturbated interval (Pygurus Member). These condensed horizons document the almost complete disappearance of shallow-water calcifying organisms for more than 3 Myr. The renewed development of a carbonate platform in the Helvetic region is only recorded from the middle early Hauterivian onwards.