GSSP of the Valanginian Stage
Definition
The GSSP for the base of the Valanginian Stage is defined at the base of the Bed VGL-B136 (at 19.23 m from the section base) of the Vergol section (SE France). This level coincides with the First Occurrence (FO) of the ammonite ”Thurmanniceras” pertransiens which marks the base of the ”T.” pertransiens Zone.
Location
The GSSP section is located in the Montbrun-les-Bains commune in the Département of Drôme (France), about 50 km east of Orange and 40 km west of Sisteron. GPS coordinates of the section are 44°12’11’’N; 5°25’03’’E. The Vergol section is situated along the road D 159, in Parcel 218 (of the land register) that belongs to the municipality of Montbrun-les-Bains therefore the section is fully and permanently accessible.
Lithology of the Vergol section
The exposed stage boundary interval is 53.12 m thick, extending from bed VGL-B95 to bed VGL-V43. It is highly expanded and has a very good sedimentary record; the rate of sedimentation is around 23 m/Myr. The section comprises a uniform succession of marl – limestone alternations with no vertical facies changes at or near the boundary. The Vergol strata were deposited in an open marine environment and are constituted by a hemipelagic facies that makes the section suitable for long distance correlation. The succession is characterized by a continuous sedimentation (no gaps or condensation) and an absence of tectonic disturbance. Above bed VGL-V43, there is a synsedimentary slump (12 m thick) in the Neocomites neocomiensiformis Zone, around 34 m above the Berriasian/Valanginian boundary. Metamorphism and (strong) diagenetic alteration can be excluded.
The base of the studied interval (upper Berriasian) is made up of thick limestone beds (such as VGL-B100 and VGL-B101) alternating with thin marl layers. Limestone beds become thinner stratigraphically upwards while marl layers are thicker. The uppermost Berriasian is characterized by a limestone-dominated interval of 11 limestone beds (calcareous bundle VGL-B119 to VGL-B129) named the “Otopeta bundle” that is located just below the Berriasian/Valanginian boundary. Around the boundary and in the lowermost Valanginian, the limestone beds tend to be thinner while marlstones are thicker, even if the marl layers remain generally thinner than limestones. From bed VGL-V30 to VGL-V43, marl layers become generally thicker than limestones. The fine-grained limestones comprise mudstones or wackestones.
Primary Marker
The FO of the ammonite ”Thurmanniceras” pertransiens.
N.B.: Due to some problems of attribution, the generic name of this species appears between inverted commas. After a palaeontological revision made during the Valanginian formal proposal, a new genus will be defined in a taxonomic note in the paper that will be published in Episodes.
Secondary Markers
Primary and secondary biostratigraphic markers occurring through the Berriasian/Valanginian stage boundary interval in the Vergol section are listed below from the stratigraphic bottom to the top: (Abbreviations: FO = First Occurrence, LO = Last Occurrence, L= Limestone bed, M = Marl layer)
• Bed VGL-B96 (L), 0.38 m, FO Tirnovella alpillensis (ammonite)
• Bed VGL-B107 (L), 5.97 m, FO Colomisphaera vogleri (calcareous dinoflagellate cyst)
• Bed VGL-B110 (M), 7.28 m, FO Muderongia macwhaei (organic-walled dinoflagellate cyst)
• Bed VGL-B112 (M), 9.03 m, FO Percivalia fenestrata (calcareous nannofossil)
• Bed VGL-B116 (L), 11.33 m, FO ”Thurmanniceras” otopeta (ammonite)
• Bed VGL-B117 (L), 12.05 m, FO Praecalpionellites murgeanui (calpionellid)
• Bed VGL-B119 (L), 12.98 m, FO Colomisphaera conferta (calcareous dinoflagellate cyst)
• Bed VGL-B129 (L), 16.46 m, LO Tirnovella alpillensis (ammonite)
• Bed VGL-B133 (L), 17.89 m, FO Calpionellites darderi and C. coronatus (calpionellid)
• Bed VGL-B135 (L), 19.04 m, LO “Thurmanniceras” otopeta and Berriasella calisto (ammonite)
• Bed VGL-B136 (L), 19.23 m, FO “Thurmanniceras” pertransiens (ammonite) - PRIMARY MARKER
• Bed VGL-B137 (L), 19.80 m, FO “Thurmanniceras” gratianopolitense (ammonite)
• Bed VGL-B141 (L), 21.27 m, FO Neocomites premolicus (ammonite)
• Bed VGL-B142 (M), 22.15 m, occurrence Thalassiphora? charollaisii (organic-walled dinoflagellate cyst)
• Bed VGL-B146 (M), 23.57 m, FO Calcicalathina oblongata (calcareous nannofossil)
• Bed VGL-B149 (L), 24.54 m, FO Calpionellites major (calpionellid)
• Bed VGL-B150 (L), 25.27 m, FO Castellanibelus orbignyanus (belemnite)
• Bed VGL-V2 (L), 26.91 m, FO Neolissoceras (Vergoliceras) salinarium (ammonite) and FO Calpionellites caravacaensis (calpionellid)
• Bed VGL-V5 (L), 28.87 m, FO Berriasibelus exstinctorius (belemnite)
Chemostratigraphy of the Vergol section
The base of the Valanginian (bed VGL-B136) does not coincide with any significant stratigraphic change in CaCO3 content or Mg/Ca, δ13O or δ13C values in either marls or limestones.
The carbonate carbon-isotope profile (δ13Ccarb) spanning the boundary shows a 10 m thick plateau interval (~1.1‰ in limestone; ~0.8‰ in marls) in the upper part of the T. alpillensis Zone, and then falling δ13Ccarb values across the Berriasian/Valanginian boundary. Values reach a minimum in the lower part of the “T.” pertransiens Zone (~0.5‰ δ13Ccarb in limestone; ~0.3‰ in marls), rising back to a second plateau interval above.
At the base of VGL-B136 (19.23 m), the predicted value of 87Sr/86Sr is 0.707289 (± 0.000004).
Magnetostratigraphy of the Vergol section
Palaeomagnetic investigations reveal an exclusively normal polarity which indicates that the upper Berriasian – lower Valanginian of the Vergol section has been remagnetized.
Cyclostratigraphy and astrochronology of the Vergol section
The floating ages of the 405-kyr eccentricity cycles are anchored on a CA-ID-TIMS U-Pb age obtained from zircons separated from a tuff layer in the Neuquén Basin (Argentina, lower Hauterivian). This radioisotopic age is tied to the ammonite zonation recognised in this basin which in turn is correlated with the Mediterranean standard zonation. The age model allows the base of the Valanginian Stage (= base of the “T.” pertransiens Zone) to be dated at 137.05 ± 0.2 Ma.
Standard Auxiliary Boundary Stratotype
The Cañada Luenga section (Cehegín, SE Spain) is included as a Standard Auxiliary Boundary Stratotype (SABS). The Cañada Luenga section provides a robust magnetostratigraphic record of the Berriasian/Valanginian boundary interval, together with a rich, diverse and well-preserved fossil content that ensures precise correlation with the Vergol GSSP.
The section is located in the Cañada Luenga ravine, 3.1 km SSW of Cehegín and 750 m upstream from the confluence of the ravine with the River Quípar. The geographic coordinates of the middle part of the section are 38º03’55”N, 1º48’4”W. The entire Cañada Luenga section extends from the middle Berriasian to the lower Valanginian.
The Berriasian/Valanginian boundary is exposed in the upper part of the section within an interval about 13 m thick (beds CL-0 to 20). The base of this part of the section (beds CL-0 to 3, approximately 1.25 m thick), is assigned to the lower part of the Tirnovella alpillensis Subzone, and is made up of reddish to grey nodular calcilutites to calcirudites that correspond to the top of the Upper Ammonitico Rosso Formation. The remainder of the section consists of alternating light grey limestone and marly limestone beds (5 to 55 cm thick) and dark grey marlstone interbeds (2 to 110 cm thick) of the Miravetes Formation.
Primary and secondary markers
The primary marker of the base of the Valanginian Stage, the FO of the ammonite “Thurmanniceras” pertransiens, is recorded in the Cañada Luenga section in Bed CL-9 at 5.80 m.
The succession of primary and secondary biostratigraphic markers through the stage boundary interval of the Cañada Luenga section are listed below from the stratigraphic bottom to the top:
• Bed CL-0, 0 m, FO Tirnovella alpillensis (ammonite)
• Bed CL-3–4, 1.5 m, FO Percivalia fenestrata (calcareous nannofossil)
• Bed CL-5, 2.65 m, FO “Thurmanniceras” otopeta (ammonite)
• Bed CL-6, 3.35 m, FO Praecalpionellites murgeanui (calpionellid)
• Bed CL-8, 5.27 m, FO Calpionellites darderi (calpionellid) and LO C. coronatus (calpionellid)
• Bed CL-8, 5.50 m, LO Tirnovella alpillensis (ammonite)
• Bed CL-8, 5.50 m, LO “Thurmanniceras” otopeta (ammonite) and Berriasella calisto (ammonite)
• Bed CL-9, 5.80 m, FO “Thurmanniceras” pertransiens (ammonite), LO “Thurmanniceras” gratianopolitense (ammonite) and LO Neocomites premolicus (ammonite)
• Bed CL-12, 6.95 m, FO Calpionellites major (calpionellid)
• Bed CL-16, 10.25 m, FO Calpionellites caravacaensis (calpionellid)
• Bed CL-17, 11.90 m, FO Neolissoceras (Vergoliceras) salinarium (ammonite)
Magnetostratigraphy of the Cañada Luenga section
The base of polarity Chron M15r falls within the T. alpillensis ammonite Zone and the uppermost part of the NK2A calcareous nannofossil Subzone. The base of Chron M15n practically coincides with the base of the “T.” otopeta ammonite Subzone, and its top correlates with the middle part of the P. murgeanui calpionellid Subzone and the upper part of the “T.” otopeta ammonite Subzone. The base of the Valanginian, marked by the FO of “T”. pertransiens, lies within the lower part of Chron M14r.
Chemostratigraphy of the Cañada Luenga section
Carbon isotope values range between -0.24‰ and +1.34‰ δ13Ccarb through the boundary section. The carbon isotope curve shows fairly stable values, around 1‰ throughout the section, except for two well-defined, W-shaped negative excursions, one in the uppermost part of the T. alpillensis ammonite Subzone (upper part of Chron M15r) and one in the upper part of the N. premolicus ammonite Subzone (Chron M14r). Maximum values occur around the Berriasian/Valanginian boundary.
Links
Episodes paper in preparation: Reboulet S., Company M., Adatte T., Aguado R., Baudin F., Deconinck J-F., Duxbury S., de Gea G., Grabowski J., Granier B., Ivanova D., Janssen N.M.M., Klein J., Lakova I., Leng M.G., Lodowski D.G., Martinez M., Mattioli E., McArthur J.M., Olivero D., Reháková D., Tavera J.M., 2024. The Global Boundary Stratotype Section and Point (GSSP) for the base of the Valanginian Stage, the Vergol section (Drôme, southeast France) and its Standard Auxiliary Boundary Stratotype (SABS), Cañada Luenga section (Cehegín, Spain).