The Role of Rift Transection and Punctuated Subsidence in the SW Atlantic Margin

Author(s):	Tina Lohr	,
	John Underhill	Heriot Watt University, Edinburgh, United Kingdom

A new interpretation of an extensive offshore data set of seismic and well data revealed the existence of two superimposed buried rift systems north of the Falkland Islands. Although refered to as the North Falkland Basin, the area in fact contains two sub-basins, a Late Jurassic NW-SE striking horst and graben system and superimposed an Early Cretaceous N-5 striking one. Both sub-basins are tilted and partially eroded due to several post-rift phases of uplift, therefore seismic and well data were not able to detect both rift systems everywhere and it rather required a comprehensive study that analyses the basin as a whole.

We have carefully mapped both rift systems, something that has led to the recognition of their component mega-sequences, and identified pronounced periodes of uplift and inversion. Regional and detailed seismic interpretation has been undertaken in order to better understand the age relationships and kinematic interaction between differently oriented basins and intersecting fault systems, the role of inherited structures, the amount and timing of uplift and inversion and the extent and age of unconformities.

Both extensional basins comprise asymmetric half grabens containing a preserved sedimentary thickness of at least 10 km at its deepest part. The Cretaceous rift system initially developed in a fluvial and later lacustrine environment before becoming predominantely marine in the Tertiary. Rapid extension led to the development of arcute shaped fault segments with hanging wall sub-basins into which sediments where deposited as fans. A prograding delta system filled the main Cretaceous rift basin from the north during the early post-rift phase. Contemporaneously, sediment was shed off the segmented basin-bounding fault via long-established feeder drainage systems through breached relay ramps into the depocentre. The resultant sediment dispersion led to deposition of numerous lacustrine turbidites that created the Sea Lion fans and its affiliates, the location of which is probably controlled by the underlying syn-rift sub-basins.

At least three major phases of uplift affected the North Falkland Basin: the oldest detectable is the exhumation of the Jurassic rift during commencement of the Early Cretaseous rift, followed by the uplift during the Aptian in the post-rift phase of the Cretaceous rift, and the Cenozoic exhumation of the Falkland Islands that imparted a tilt both Jurassic and Cretaceous rifts northwards. The Aptian phase was short-lived and created a long and gentle north-south striking anticline that runs along the central basin axis punctuating post-rift subsidence without inhibiting petroleum prospectivi