A MARINE LATE JURASSIC SYN-RIFT SUCCESSION IN THE LUSITANIAN BASIN, WESTERN PORTUGAL - TECTONIC SIGNIFICANCE OF STRATIGRAPHIC SIGNATURE

In the Lusitanian Basin, which is located along the western Iberian ma rgin, rapid subsidence in the Oxfordian-Tithonian resulted from extens ion related to the opening of the North Atlantic. In the Santa Cruz ar ea, located in the western part of the Lusitanian Basin, the Kimmeridg ian syn-rift, marine Abadia Formation and the overlying alluvial Kimme ridgian-Tithonian Praia da Amoreira member of the Lourinha Formation, were deposited in the middle part of the hangingwall dip slope of a la rger half-graben that constitutes the Turcifal subbasin. The marine pa rt of the succession consists of base-of-slope and lower slope mudston es and sandy turbidites, middle slope mudstones, upper slope to shelfa l/?shoreline mudstones and sandstones, and fan-deltaic deposits. Two i ntermediate-scale fining-to-coarsening (FUCU) units are present in the marine strata. The lower FUCU unit represents a normal, base-of-slope /lower slope to upper slope, shoaling-upward succession. The unit refl ects the progradation of a hangingwall shelfal ramp and infilling of a n inherited rift bathymetry during a period of relative tectonic quies cence. The upper FUCU unit represents an intermediate-scale deepening- shoaling-upward or backstepping-forestepping sequence which is related to a single rift event; the backstepping segment was deposited during a period of increasing basinal subsidence and basin floor tilt rates, the early syn-rotational stage. It consists of submarine channel-fill incised into slumped upper slope sediments, and adjacent overbank dep osits, The backstepping segment terminates with a clay-prone marine in terval with abundant soft-sediment deformation features, which represe nt a period of maximum sea-level stand during the rotational climax, T he overlying forestepping segment consists of a shoaling-upward, slope to fan-deltaic succession which terminates with alluvial deposits. It represents an interval with decreasing basinal subsidence and basin f loor tilt rates, and renewed overbalance of sediment supply, which led to a regression during the late syn-rotational stage. In medial reach es of the hangingwall dip slope, periods of increasing-to-decreasing r ift activity are recorded by a backstepping-forestepping package, or a threefold sandstone-mudstone-sandstone lithosome motif. The formation of smaller-scale units superimposed on the intermediate-scale FUCU sy n-rotational motif were controlled predominantly by minor fault-contro lled subsidence events or higher-frequency faulting events.