How relevant is the role of macrophytic vegetation in controlling peritidal carbonate facies?: Clues from the Upper Jurassic of Portugal

Stratigraphic variability in peritidal carbonates is generally interpreted in terms of sea-level changes, but little consideration is given to the effects of vegetation and of how long-term changes in macrophyte evolution have affected carbonate sedimentation in such settings. Many present day low energy tropical carbonate shorelines are dominated by macrophyte vegetation. Although several pre-Jurassic macrophytes have been interpreted to have lived in brackish-water zones, records of macrophyte-dominated carbonate shorelines in the stratigraphic record are apparently rare or even absent, which thus is an enigma. We use Upper Jurassic platform interior carbonates from central Portugal to emphasize these issues; these successions contain both classical tidal flat-bearing cyclothems and ones capped by marsh facies with evidence of macrophytes, the former having developed in more restricted settings. Although these late Jurassic shorelines were not affected by true mangroves, we propose that tidal flat deposits were not developed because the shorelines, at least locally, were dominated by macrophytes. We speculate that the paucity of classical tidal flat-bearing peritidal cyclothems in the Cainozoic may have been due, at least partially, to the spread of major macrophyte-dominated mangrove communities during the Paleogene.