Upper Jurassic reef types and controlling factors. A preliminary report

Reefs occurred widespread during the Late Jurassic, particularly along the northern Tethyan shelf and the marginal basins of the young North Atlantic Ocean. They thrived in a variety of settings such as on intrabasinal tectonic and halokinetic uplifts, within lagoons or within siliciclastic fan deltas. Most frequently they grew in homoclinal to steepened ramp settings, where they occupied a wide bathymetric field from the innermost, partly even hypohaline, part down to outer ramp settings. Compositionally they comprise the end members 'coral facies', 'siliceous sponge facies' and 'microbial facies', but transitions and successions are frequent. Microbial crusts are important not only in the microbial facies where they build thrombolitic reefs up to 30 metres thick but also within the siliceous sponge and coral facies where they occur at variable quantities and are largely responsible for constructing a positive relief. Reef facies without crusts is mostly biostromal. Siliceous sponge facies is frequently devloped as sponge - microbial crust - mudmounds which occur in a belt from Romania down to Portugal. Important factors determining the occurrence, composition and fabric of reefs are bathymetry, background sedimentation rate and oxygen fluctuations. Bathymetric interpretation based on sequential analysis of shallowing upward successions and on comparative semiquantitative palaeoecology shows that coral facies, mixed coral - sponge facies and siliceous sponge facies follow each other along a deepening gradient, although the zones overlap broadly. Decrease and cessation of background sedimentation increased diversity and favoured the growth of microbial crusts. An increasing rate of oxygen/nutrient fluctuations excluded reef macrofauna and eventually led to thrombolitic reefs. These were more frequent in deeper settings but occurred over a wide bathymetric range.