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



Abstract 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 (‘Cabaços’ 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.