A marine Late Jurassic syn-rift succession in the Lusitanian Basin, western Portugal — tectonic significance of stratigraphic signature

Abstract

In the Lusitanian Basin, which is located along the western Iberian margin, rapid subsidence in the Oxfordian-Tithonian resulted from extension related to the opening of the North Atlantic. In the Santa Cruz area, located in the western part of the Lusitanian Basin, the Kimmeridgian syn-rift, marine Abadia Formation and the overlying alluvial Kimmeridgian-Tithonian Praia da Amoreira member of the Lourinhã Formation, were deposited in the middle part of the hangingwall dip slope of a larger half-graben that constitutes the Turcifal subbasin. The marine part of the succession consists of base-of-slope and lower slope mudstones and sandy turbidites, middle slope mudstones, upper slope to shelfal/?shoreline mudstones and sandstones, and fan-deltaic deposits. Two intermediate-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 reflects the progradation of a hangingwall shelfal ramp and infilling of an inherited rift bathymetry during a period of relative tectonic quiescence. 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 deposits. The backstepping segment terminates with a clay-prone marine interval with abundant soft-sediment deformation features, which represent a period of maximum sea-level stand during the rotational climax. The 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 floor tilt rates, and renewed overbalance of sediment supply, which led to a regression during the late syn-rotational stage. In medial reaches of the hangingwall dip slope, periods of increasing-to-decreasing rift 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 syn-rotational motif were controlled predominantly by minor fault-controlled subsidence events or higher-frequency faulting events.