Changing provenance of the early Cretaceous rift and sag-related sediments of the Congo basin: Combining detrital zircon and apatite U-Th-Pb geochronology approach
Résumé
This study provides the first detrital zircon and apatite U-Th-Pb geochronology on the Berriasian prerift, Barremian synrift, and Aptian sag strata of the Congo basin, with the goal to reconstruct sediment provenance and relief evolution of the central South-Atlantic rifted- margin during the Early Cretaceous. Detrital zircons from 6 samples and detrital apatites from 3 samples collected from exploration wells and outcrops were analyzed for U-Th-Pb dating (LA-ICPMS). Six populations have been obtained: (1) Carboniferous- Permian, (2) Late Neoproterozoic-Ordovician, (3) early Neoproterozoic, (4) Mesoproterozoic, (5) Paleoproterozoic and (6) Archean.
The Berriasian prerift strata contain six populations of zircon (1 to 6) and five populations of apatite (1 to 5), interpreted as derived from various source terranes (e.g. Gondwanide, Brasiliano and Transamazonian orogenic belts) in South-America plate. This result implies a mixing of sediments from several distinct source regions and a long-distance transport, that is consistent with the deposition in a gentle proto-rift sag basin with a low relief.
Detrital zircon and apatite suites of Barremian synrift strata clearly contrast with those from prerift strata. These synrift strata contains three populations of zircons (4 to 6) and three populations of apatites (2, 3 and 5), which are consistent with local sediment sourcing from the marginal West-Congo belt, which was uplifted and exhumed along the rift flank. This imply a major change in provenance and a reorganization of the drainage pattern in response to fault bloc-tilting and rift-shoulder development during the Barremian main phase of the South- Atlantic rifting, resulting in the formation of significant topography relief.
Four samples from the Aptian sag strata yielded different age distributions, reflecting spatial and temporal changes in sediment provenance within the same stratigraphy unit. However, these populations also indicate local sediment sourcing from the West-Congo belts, implying no significant change in sediment provenance between the main rift phase and rift-drift transition phase. The different age distributions obtained for the Sag strata can be ascribed to the heterogeneity of the source terranes and to the occurrence of multiple catchment systems with small drainage areas.
This work emphasizes the importance of combining detrital zircon and apatite U-Th-Pb geochronology data for sediment provenance analyzing.