Triassic to Middle Jurassic sequences from the Lusitanian Basin Portugal, and their equivalents in other North Atlantic margin basins.

Watkinson, Matthew Philip (1990). Triassic to Middle Jurassic sequences from the Lusitanian Basin Portugal, and their equivalents in other North Atlantic margin basins. PhD thesis The Open University.



The Mesozoic sedimentary succession of the Lusitanian Basin, west central Portugal has been subdivided into four unconformity bounded sequences whose origin can ultimately be linked to broader plate tectonic events. The earliest of these formed during the multistage rifting which preceded ocean opening between Iberia and Newfoundland (Aptian-Albian).

The sedimentary succession deposited during the first unconformity bounded sequence in the Lusitanian Basin, can be further subdivided into two subsequences;

1 Carnian to Hettangian (synrift); deposition of continental siliciclastics and marine evaporites. Two sub-basins can be recognised, north and south of an area of thin Triassic and Hettangian sediments, and with very low apparent basin subsidence rates. This region may have acted as an accommodation zone between the two active synrift depocentres. This succession is marked by high apparent basement subsidence rates, and marked lateral variability in thicknesses of sequences, suggesting fault controlled subsidence. This rifting event also affected many other North Atlantic margin basins and was the initial episode of extensional deformation south of the Labrador - Biscay transform. Basins adjacent to, and north of this feature have histories extending back to the Permian in some cases.

2 Sinemurian to latest Callovian (postrift); at the end of the Hettangian, sedimentary sequences become relatively uniform in thickness. The dominant subsidence mechanisms were driven by thermal re-equilibration and sediment loading. Widespread carbonate deposition occurred over the whole basin for the first time with the establishment of a carbonate ramp which was maintained, on the eastern basin margin, throughout the Early and Middle Jurassic. The presence of a ramp for around 40my was a function of low subsidence rates and ample sediment supply to the outer ramp environment. Some faulting was present during this stage but restricted to the western margin of the basin. This may have been flexural faulting resulting from vertical stresses associated with thermal subsidence rather than lithospheric extension. Faulting of this nature led to asymmetry of facies and subsidence from Sinemurian times onwards. A bypass margin developed on this western margin in the Toarcian, which fed a carbonate submarine fan sequence.

During the Bajocian the ramp became distally-steepened as the result of flexural reactivation of a basement structural trend. A fine-grained mass-flow system developed in the Rio Mondego region which died out as fault movement ceased and the homocline was restored.

Outer ramp facies (Brenha formation) are dominated by deposition of limestone marlstone alternations with a pelagic fauna and some bituminous shales. Inner ramp facies are dominated by barrier island systems (Coimbra formation, Candeeiros formation). Barrier islands were wave dominated, but during the Bajocian to Callovian (Candeeiros formation) there is evidence for subordinate tidal and storm activity. Also during this period, thick carbonate sand bodies developed in the Fatima Plateau region due to sandbody stacking processes resulting from sedimentation rate exceeding, or keeping pace with relative sea-level rise.

The sequences of the Triassic to Middle Jurassic from the Lusitanian Basin, show initial similarities to those of formerly adjacent North Atlantic margin basins; for example, carbonate ramps were established in several basins. However, a large degree of heterogeneity was present, including the timing of rifting episodes which varied greatly over the whole North Atlantic and western Tethys area This suggests that the regional extensional stress direction changed several times making some basins more prone to normal faulting than others because of the orientation of their inherited structural trends. Comparison of sequence stratigraphies also suggests that timings of transgressive and regressive events were not synchronous in the North Atlantic region, implying that intrinsic processes were important in basin development.

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