The depositional evolution of diapir- and fault-bounded rift basins: examples from the Lusitanian Basin of West Iberia

Abstract

New data on the evolution of rift basins is presented after analysing the Late Jurassic stratigraphy of the Central Lusitanian Basin (west Iberia). Well, outcrop and regional 2D seismic reflection profiles are used to investigate the differences in stratigraphic signature between diapir- and fault-bounded sub-basins. During the Late Jurassic syn-rift phase, surface rupturing in fault-bounded sub-basins resulted in the formation of tectonic scarps from which footwall-derived gravity flows were sourced. In contrast, the diapir-bounded Bombarral-Alcobaça sub-basin evolved as a distal bowl-shaped depocentre with an axis located up to 10 km away from its basin margins. Low-gradient marginal slopes developed in the Bombarral-Alcobaça sub-basin during the Late Jurassic rifting, while growing salt pillows limited the vertical propagation of basement normal faults. Differences in tectonic evolution, basin physiography and sediment input are the main factors responsible for the distinct sedimentary evolutions recorded in the study area: (1) transverse footwall-derived sediment fans, predominant in fault-bounded regions, give place to axial southwards-prograding fluvial to shallow-marine units in the diapir-bounded sub-basins; (2) growing salt pillows, absent in the fault-bounded sub-basins, formed barriers to and limited the development of transverse drainage systems.