The structural and sedimentary evolution of the Arruda and Lower Tagus sub-basins, Portugal

The study discusses the Jurassic structural and sedimentary evolution of the Arruda and Lower Tagus sub-basins, located in the central and southern part of the Lusitanian Basin of Portugal. In the last five decades, thousands of kilometres of 2D seismic reflection lines were acquired for oil exploration. Reprocessing and reinterpretation of over 700 km from the study area was carried out. After the seismic to well ties based on well logs, VSP analysis, synthetic seismograms, revised petrological analysis and stratigraphic reinterpretation of the wells, nine key seismic/geologic horizons were mapped and depth converted. Reinterpretation of the seismic profiles was carried out using outcrop data and reprocessed aeromagnetic and gravimetric data of the study area that was integrated in a GIS environment. Several aspects of the tectonic and sedimentary evolution of the study area were enlightened. From the Triassic until the Late Jurassic, two rift pleases were recognised, in agreement with data from other parts of the Lusitanian Basin. The initial rifting phase in the Triassic continued until the Callovian, with three prominent tectonic pulses: a first in the Late Triassic, a second in the Hettangian and a third from the Sinemurian onwards. The latter episode lasted until the Callovian in the Arruda sub-basin, and at least until the Bathonian, in the Lower Tagus sub-basin. During the Middle Jurassic, the Arruda sub-basin evolved to a NW-deepening carbonate ramp. The Lower Tagus sub-basin constituted a platform since the Early Jurassic, limited in the east by the Setubal-Pinhal Novo fault and in the west, by the Cadafais fault. The third rifting episode occurred in the Late Oxfordian-Early Kimmeridgian, producing a major N-S fault system already recognised by several authors, and another important NW-SE and NE-SW fault systems post-dating the former. (c) 2004 Elsevier Ltd. All rights reserved.