The Middle-Late Jurassic forced regression and disconformity in central Portugal:: eustatic, tectonic and climatic effects on a carbonate ramp system

Successions across the Middle-Upper Jurassic disconformity in the Lusitanian Basin (west-central Portugal) are highly varied, and were probably developed on a large westward-inclined hangingwall of a half-graben. The disconformity is preceded by a complex forced regression showing marked variations down the ramp, and provides an example of the effects of rapid, relative sea-level falls on carbonate ramp systems. In the east, Middle Jurassic inner ramp carbonates ('Candeeiros' facies) are capped by a palaeokarstic surface veneered by ferruginous clays or thick calcretes. In the west, mid-outer ramp marls and limestones ('Brenha' facies) are terminated by two contrasting successions: (1) a sharp-based carbonate sandbody capped by a minor erosion surface, overlain by interbedded marine-lagoonal-deltaic deposits with further minor erosion/exposure surfaces; (2) a brachiopod-rich limestone with a minor irregular surface, overlain by marls, lignitic marls with marine and reworked non-marine fossils and charophytic limestones, with further minor irregular surfaces and capped by a higher relief ferruginous erosional surface. The age ranges from Late Bathonian in the east to Late Callovian in the west. This disconformity assemblage is succeeded by widespread lacustrine-lagoonal limestones with microbial laminites and evaporites ('Cabacos' facies), attributed to the Middle Oxfordian. Over the whole basin, increasingly marine facies were deposited afterwards. In Middle Jurassic inner-ramp zones in the east, the overall regression is marked by a major exposure surface overlain by continental sediments. In Middle Jurassic outer-ramp zones to the west, the regression is represented initially by open-marine successions followed by either a sharp marine erosion surface overlain by a complex sandbody or minor discontinuities and marginal-marine deposits, in both cases capped by the major lowstand surface. Reflooding led to a complex pattern of depositional conditions throughout the basin, from freshwater and brackish lagoonal to marginal- and shallow-marine settings. Additional complications were produced by possible tilting of the hangingwall of the half-graben, the input of siliciclastics from westerly sources and climate change from humid to more seasonally semi-arid conditions. The Middle-Late Jurassic sea-level fall in the Lusitanian Basin is also recorded elsewhere within the Iberian and other peri-Atlantic regions and matches a transgressive to regressive change in eustatic sea-level curves, indicating that it is related in part to a global event.