A tale of two syn-rifts: new age information and depositional environment interpretation for the North Falkland Basin syn-rift and implications for the rifting of Gondwana

The timing of North Falkland Basin (NFB) rifting, and how the wider Falkland Plateau fits into the history of the breakup of Southern Gondwana, has been a topic of debate and uncertainty. This has been exacerbated by a complete lack of direct geochronological age constraint on the sedimentary fill of the Early Syn-rift in the NFB. This study presents an analysis of well data, core from the Early Syn-rift interval, seismic interpretation, and new zircon U-Pb age dating. The resulting depositional model supports a revised understanding of the sedimentary environment and structural setting of the syn-rift of the NFB. The Early Syn-Rift strata infill a series of NW–SE striking sub-basins, which together comprise the Southern North Falkland Basin (SNFB). The overstepping Late Syn-rift rests unconformably above the lower syn-rift sequence, and is accommodated by a series of syn-depositional N–S trending fault-bounded basins that together form the Central North Falkland Basin (CNFB). The NFB Early Syn-Rift is characterised by ephemeral fluvial–fluvio-lacustrine and volcaniclastic intervals, with nine identified sedimentary facies assigned to five distinct facies associations. An analysis of ashfall tuffs identified in core from the 14/24–1 well yields a new LA-ICP-MS zircon U-Pb age determination of 175 ± 5 Ma (early to middle Jurassic) for the upper part of the Early Syn-rift. These results provide the first absolute age determination for Early Syn-rift strata in the NFB, and implies that they are substantially older than previous models suggest (by between 13.9–41 Ma). This suggests that the NW–SE striking basins of the SNFB developed as a result of Gondwana rifting during the early to middle Jurassic. The findings provide clear evidence for two distinct syn-rift packages in the NFB, with early to middle Jurassic (Gondwana) depocentres and associated faults overprinted by later N–S trending early Cretaceous sub-basins.