Controls on the structural architecture and sedimentary character of syn-rift sequences, North Falkland Basin, South Atlantic

Abstract

The Falkland Islands lie in a critical location for our understanding of Gondwana supercontinent break-up. Subsurface data obtained during recent exploration activity provide important new insights into the structure and stratigraphy of the North Falkland Basin's Late Jurassic syn-rift fill history. Results of seismic interpretation indicate that deposition was largely controlled by prominent planar normal faults, which divide the basin into a series of north-south trending half-grabens and minor grabens. Well data (in the form of exploration wireline logs, cores, petrographic thin sections, major and trace element analyses) indicate that their syn-rift fill was dominated by abundant volcaniclastic sediment supplied through debris flows, hyperconcentrated flows and pyroclastic ash-fall. The geochemical affinity of the volcanic sedimentary components shows a direct link to the widespread, Middle Jurassic to Early Cretaceous, silicic Chon Aike Large Igneous Province, in southern South America and Antarctica. Diagenesis has obscured provenance information and severely reduced porosity within the syn-rift sediments, causing them to have poor reservoir potential. The results enable a radically new interpretation of this key period in the North Falkland Basin's evolution, which helps modify and unify previously disparate views. It is now suggested that basin development was the direct result of the Chon Aike back-arc stretching, influenced in part by the underlying Palaeozoic basement trends. This was followed by Cretaceous-Recent post-rift thermal subsidence of the basin, which was only arrested by local structural inversion related to Atlantic opening. Consequently, it is thought that the extension that created the North Falkland Basin was independent of both Early–Middle Jurassic rotation of the Falkland Islands microplate and the evolution of the South Atlantic Ocean in the Cretaceous and Cenozoic.