From rift to foreland basin: A case example from the Magallanes-Austral basin, southernmost Andes

Abstract

This contribution characterizes primary lithologic and depositional components of the Magallanes-Austral basin and defines infill geometries and stacking patterns from seismic and well data. An integrated seismic model is proposed for recognition of rifting, thermal sag and foreland tectono-stratigraphic phases based on depositional geometries and its relation with the evolving deformational and geodynamic framework. Above a Middle–Late Jurassic extensional phase, evidenced by synrift depositional geometries, follow marine successions representing the subsidence thermal sag phase (Tithonian–Early Cretaceous) characterized by concordant and laterally extensive seismic reflectors. The following foreland phase is described through the evolution and lateral migration history of the foredeep depocentre and concomitant forebulge development. The foreland phase is represented by different stages characterized by asymmetric sedimentary wedges bounded by basal surfaces and/or major unconformities recording transitions from underfilled to overfilled conditions. The accumulated thickness due to lithospheric flexure reflects different foreland subsidence profile patterns across the southern depocentre of the Magallanes-Austral basin, producing asymmetrical westward and southward thickening wedges. The first Foreland I stage (Coniacian?–Maastrichtian) is recorded as an asymmetric wedge infill, that thins cratonward, with a NW-trending foredeep axis. The erosive basal foreland surface (BF) at its base deepens towards the west and south along the active margin of the basin, where subsidence was maximum. On top of it, along the western portion of the basin and with a source area from the north, deep-marine slope deposits and turbiditic complexes were deposited; while on the forebulge to the east, a clastic platform developed. The Foreland II stage (early-to-middle Palaeocene–middle Eocene) is characterized by renewed uplift and flexure, and increasing tectonic subsidence rates, building a new clastic wedge-shaped foreland succession next to the orogenic belt, and a well-represented forebulge to the east. Subsequently, an extensive diachronous G7 unconformity was generated, eroding locally the previous foreland deposits towards the eastern margin. A pronounced and continuous NW-SE trending deflection is established subtly to the east. The following Foreland III stage (middle to late-Eocene–Oligocene) is characterized by a reduction in thrust load along the western active margin, and progradational systems towards the NE, a time during which the subsidence rate decreased and accommodation space was reduced. Deposition occurred within a wide and continuous NW-SE trending foredeep without a marked forebulge. The top of this stage is the A1 unconformity, marking the beginning of the Foreland IV stage (early Miocene–Neogene), regarded as an overfilled basinal stage without a marked foredeep and major variations in thickness across the extent of the basin. The depositional pattern in this stage is largely conformal and tabular. The proposal model represents an evolutionary example for the internal geometry of deep-marine foreland basin system, including variables such as tectonic load and flexural subsidence, accommodation space, sediment supply variation, and relative sea-level fluctuations.