- 1Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre, 38000 Grenoble, France
- 2University of Perpignan Via Domitia, UMR 7194 Histoire Naturelle de l’Homme Préhistorique, CNRS, Muséum National d’Histoire Naturelle, 52 Avenue Paul Alduy, 66100 Perpignan, France
- 3University of Perpignan Via Domitia, Centre de Formation et de Recherche sur les Environnements Méditerranéens (CEFREM), UMR 5110, 52 Avenue Paul Alduy, 66100 Perpignan, France
- 4Université de Brest, CNRS, Ifremer, Geo-Ocean, F-29280 Plouzané, France
- 5Ellington Geological Services, 1414 Lumpkin Rd, Houston, TX 77043, USA
- 6Université de Guyane, UMR 5243 Geosciences Montpellier, 97300 Cayenne, Guyane
The Demerara Plateau is a submarine bathymetric high, 230 km-long and 170 km-wide, lying between 1000 and 3000 m-depth, and located north of French Guiana and Suriname shelves. On its northeastern border, the Bastille Plateau is a 16 km-long, 9 km-wide relief, at the intersection of the Cretaceous transform and divergent margins of the Demerara Plateau. It represents a crucial witness for understanding the early stages of the Equatorial Atlantic opening. Seismic profiles from GUYAPLACa (2003) and MARGATSb (2016) cruises reveal that the Bastille Plateau is a continentward tilted block with a planar top surface culminating at bathymetric depths of 3650 m, 15 km from the continent-ocean boundary. In 2016, the DRADEMc cruise dredged the rocks outcropping along the northern slope of the Bastille Plateau, retrieving mostly trachy-basalts and a single rudstone sample. During the DIADEMd campaign (2023), a dredge on the southern slope and two Nautile submarine dives confirmed that the Bastille Plateau was almost entirely made up of magmatic material. Three pelagic carbonates were sampled during one Nautile dive and came directly from the top of the Bastille Plateau, between 3745 m and 3685 m-depth.
We combine petrology with absolute U-Pb dating on calcite for the rudstone, and biostratigraphic dating of the pelagic carbonates deposited at the top of the Bastille Plateau to constrain the chronology of the rifting of the Equatorial Atlantic along the Demerara Plateau. We interpret the rudstone as deposited on a subaerial unconformity surface, similar in seismic lines to the post-rift unconformity. U-Pb analyses on calcite date this post-rift unconformity as Mid-Albian and constrain a continental break-up at 106 ± 9 Ma. Unexpectedly, post-rift subsidence did not follow the break-up, with marine transgression occurring circa 103 Ma on the Demerara Plateau, but later than 98 ± 3 Ma on the Bastille Plateau, closer to the continent-ocean boundary, possibly in relation with the vicinity of the Sierra Leone hotspot. Biostratigraphic ages indicate that subsidence was rapid from the Cenomanian onward, resulting in the early establishment of a deep-sea current acceleration zone along the outer margin of the Demerara Plateau.
a https://doi.org/10.17600/3010050
b https://doi.org/10.17600/16001400
c https://doi.org/10.17600/16001900
d https://doi.org/10.17600/18000672
How to cite: Coudun, C., Bienveignant, D., Basile, C., Girault, I., Giraud, F., Vezinet, A., Loncke, L., Graindorge, D., Klingelhoefer, F., Léger, J., Menini, A., and Heuret, A.: Unexpected post-breakup altitude of the distal continental margin of the Demerara Plateau (French Guiana): New constraints from LA-ICP-MS U-Pb calcite dating, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18237, https://doi.org/10.5194/egusphere-egu25-18237, 2025.
