Geology reviewed for the Falkland Islands and their offshore sedimentary basins, South Atlantic Ocean

Abstract

The position of the Falkland Islands adjacent to the South American continental margin belies the close association of their geology with that of South Africa. A Mesoproterozoic basement is unconformably overlain by a Silurian to Devonian succession of fluvial to neritic and shallow marine, siliciclastic strata. This is disconformably succeeded by a largely Permian succession that, near its base, includes a glacigenic diamictite and, thence, passes upwards into a succession of deltaic and lacustrine strata. The lithological succession and the character of its deformation bear striking similarities to the Cape Fold Belt and Karoo retroarc foreland basin. Swarms of Early Jurassic dykes were coeval with the Karoo magmatism and the initial break-up of Gondwana; Early Cretaceous dykes were intruded during the opening of the South Atlantic Ocean. Offshore sedimentary basins surrounding the archipelago contain Late Jurassic to Palaeogene successions and are currently the focus of hydrocarbon exploration. Best known is the North Falkland Basin, a classic failed rift. To the SE, the passive margin, Falkland Plateau Basin may also be rift-controlled, whilst the South Falkland Basin is a foreland basin created at the boundary of the South American and Scotia plates. The role of the Falkland Islands during the breakup of Gondwana remains controversial. Compelling evidence from the onshore geology favours rotation of an independent microplate from an original position adjacent to the Eastern Cape, South Africa. Alternative interpretations, justified largely from offshore geology, favour extension of the Falkland Plateau as a fixed promontory from the South American margin.

A revised position for the rotated Falkland Islands microplate

“…This hypothesis is further supported by more recent palaeomagnetic, aeromagnetic, stratigraphic, palaeontological and structural data analysis (Mitchell et al 1986;Mussett & Taylor 1994;Marshall 1994;Curtis & Hyam 1998;Trewin et al 2002;Stone et al 2009). The palaeomagnetic measurements were carried out on dykes identified onshore the Falkland Islands (Mitchell et al 1986;Taylor & Shaw 1989;Stone et al 2008) and on Permian sediments (Thistlewood & Randall 1998in Stone 2016. The dykes…”

“…Their emplacement is related to the Karoo-Ferrar magmatism in South Africa and Antarctica for the Jurassic dykes (Mitchell et al 1999) and the opening of the South Atlantic for the Cretaceous swarm (Richards et al 2013). Five deformation phases (D1 to D5) were identified by Aldiss & Edwards (1999) onshore the Falkland Islands; the first four were interpreted as being synchronous to the Permo-Triassic CFB in South Africa (Curtis & Hyam 1998;Stone 2016). The West Falkland Group, which crops out on the West Falkland, the northern part of the East Falkland and the Beauchêne Island (Aldiss & Edwards 1999;Stone 2015), has been correlated with the Table Mountain, Bokkeveld and Witteberg groups in South Africa (Adie 1952bin Marshall 1994, whereas the Fitzroy Tillite Formation from the Falkland Islands is considered coeval with the Dwyka Group ( Fig.…”

“…The first to recognize a link between this fold and thrust belt and the Falkland Islands was Du Toit (1927) while Adie (1952a) further argued for a positioning of the islands east of South Africa in a rotated position as an extension of the Cape Fold Belt. Subsequent studies (Mitchell et al 1986;Marshall 1994;Mussett & Taylor 1994;Curtis & Hyam 1998;Thistlewood & Randall 1998in Stone 2016Thomson 1998;Trewin et al 2002) favoured this reconstruction but the associated uncertainties in interpretation and drawbacks of the model led also to the emergence of a rigid model (Richards et al 1996;Lawrence et al 1999;Ramos et al 2017) with the islands and the Falkland Plateau fixed to the South American plate.…”

“…However, there is little movement recorded along the AFFZ at this time (Broad et al 2006in Tankard et al 2009 in support of this hypothesis. In addition, the uncertainty around palaeomagnetic measurements (Richards et al 1996;Hodgkinson 2002in Stone 2016 and the absence of a pertinent mechanism to account for the rapid and substantial rotation of the islands resulted in numerous studies advocating for a contrasting rigid evolution of the Falkland Islands, in which the islands are part of a Falkland Plateau fixed to the South American plate…”

“…11). These are affected by open to isoclinal folds with symmetric to highly asymmetric limbs controlled at depth by thrusting (Aldiss & Edwards 1999;Stone 2016;Paton et al 2006). Towards the hinterland, thrusts active during the Cape orogeny underwent negative structural inversion during the Mesozoic rifting event.…”