Seismic stratigraphy and tectonic evolution of the Flakland/Malvinas Plateau

Abstract

A new perspective on the geology of the FalklandIMalvinas Pl ateau (South Atlantic) is derived from a detailed synthesis of 14,000 km of mainly unpublished slngle-channel and multichannel seismic data, aided by redge-haul descri ptions and DSDP data. The Plateau is II foundered cornplex of continental blocks that were differentially rifted during the opening of the South Atlant ic in lhe Midd le Jurassic to Early Cretaceous, Up to 7 km of synrift, transitional, and hemipelagic sediments have been deposited since the Middle Jurassic, largely under the control of a northem marginal fracture ridge. Three major structural lineations outline basemenl geometry, They are: 1. a NE-SW tectonic trend marking the eastem edge of the Plateau; 2. the Falkland Escarpmenl marginal fracture ridge lo the north and nearby grabens thal are parallel or subparallel to the approximately E-W Falkland Fracture Zone; and 3. the westem edge of the Maurice Ewing Bank running NW-SE thal is related lo extensive rifting and volcanic activity in the central Falkland Basin, The complicated geometries of these structural trends are related lo triple-junction tectonics when the South American, African, and Antarctic blocks moved apart in the Late JurassicJEarly Cretaceous. We estimate that the Plateau had been lengthened by ai least 400 km in an east-west direction, before the initiation of drift at about anomaly MIO lime. From the acoustic stratigraphy we have defined four widespread depositional sequences. Three of these have been sampled by DSDP drilling and their bounding unconfonnities are interpreted to have resulted from: 1. an early Paleozoic pediplanation; 2. a post-rift erosional truncation; and 3. erosion by ocean currents that preceded the establishment of the Antarctic Circumpolar Current near the time of the Cretaceousffertiary boundary. Our seismic profiles reveal that in the early Paleogene, migrating sediment drift deposits several hundreds of meters thick were deposited over the whole Plateau. A fourth depositional sequence remains unsampled by drilling. It is bounded below by an intra-Cretaceous unconfonnity that originated from ocean current activity and its top by the above mentioned crr erosional surface. Six isopach sediment and basement structure maps which succinctly display the geometries of these sedimentary units are included. ReSUMO ESTRATIGRAFIA SíSMICA E EVOLUÇÃO TECTÓNICA DO PLATÓ FALKLAND/MALVINAS. Uma nova perspectiva da geologia do Platô Falk1andlMalvinas (Atlântico Sul) deriva de uma síntese detalhada de 14.000 km de sísmica de reflexão mono e multicanais, na sua maioria não publicados, em conjunto com descrições de rochas e de perfurações do DSDP. O Platô é um complexo de blocos continentais submersos que foram diferencialmente estendidos durante a abertura do Atlântico Sul no Mesojurãssico-Eocretâceo. Um máximo de 7 km de sedimentos sinrifte, transicionais e hemipelâgicos foram depositados desde o Mesojurãssico, largamente sob o controle estrutural de uma cadeia marginal de zona de fratura. A geometria do emba...

Tectono-stratigraphic evolution of the North Falkland region

“…It is correlated with the base of the Middle Inoceramus Formation in the Magallanes Basin which is placed within the Cenomanian by Biddle et al (1986). By extrapolation it is correlated with the U3 unconformity of the Falkland Plateau Basin, which corresponds to a Mid-Cretaceous lacuna identified in DSDP boreholes (Barker et al 1977;Lorenzo & Mutter 1988).…”

“…5a). Uplift of the same generation characterizes the Marginal Fracture Ridge adjacent to the Falkland Escarpment to the east (Lorenzo & Mutter 1988) where transform movement has taken place along the AgulhasFalkland Fracture Zone (Fig. 8).…”

“…This seismic horizon cannot be directly correlated with the U4 unconformity of the Falkland Plateau Basin, which represents a hiatus spanning the Cretaceous-Tertiary boundary (Maastrichtian-Palaeocene age) (Lorenzo & Mutter 1988). An Early Tertiary age is attributed to this unconformity for this study on the basis of its presumed causal relationship with the first phases of Scotia Plate indentation (see later).…”

“…Regional scientific seismic data acquired by the Lamont-Doherty Geological Observatory over the central and eastern Falkland Plateau (Lorenzo & Mutter 1988) have also been helpful in regional extrapolation. However, it should be recorded that there is no direct tie between the Lamont-Doherty and Spectrum datasets.…”

“…The eastern end of the Falkland Plateau is formed by a bathymetric feature, the Maurice Ewing Bank, which is separated from the main part of the Plateau by the Falkland Plateau Basin. Formation of this basin has been attributed to Mid-Late Jurassic rifting (Lorenzo & Mutter 1988). The southern margin of the Falkland Plateau is defined by another transform-related zone characterized by the Burdwood Bank, a transpressional feature (Platt & Philip 1995) formed during the indentation of the Scotia Plate beginning in the Early Tertiary.…”