Upper Jurassic reef types and controlling factors. A preliminary report. -
Profil, 5, 1-45; Stuttgart.
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 developed 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.
The success and broad compositional range of Upper Jurassic reefs is largely related to the high global sea level of this time. It provided wide epicontinental seas as well as the deeper shelf settings suitable for the expanse of sponge reefs. The much larger expansion of Upper Jurassic coral and sponge reefs relative to their Middle Jurassic counterparts is largely due to the generally rising Jurassic sea and only to a small proportion to evolutionary diversification of reef biota. The high sea level of the Late Jurassic also resulted in climatic buffering, giving rise to a general 'greenhouse'-type climate. This lowered atmospheric and oceanic circulation and resulted in the occurrence of dysaerobic bottom water in the deeper shelf, where thrombolitic reefs could thrive. Widespread occurrence of black shales and dysaerobic facies suggests that Upper Jurassic seas were widely stratified. The weak oceanic circulation was particularly driven by evaporization. It is suggested that weak but widespread upwelling along the northern Tethyan shelf was responsible for the occurrence of oxygen-controlled thrombolites on the shelf. Such oxygen-controlled reefs occurred even shallower when during times of intra-Upper Jurassic transgressions climate was aditionally buffered, leading to a rise of the dysaerobic zone. Comparison of occurrence pattern of reefs, condensed marker horizons and clay-rich intervals across the northern Tethyan shelf indicates that modifications of sequence stratigraphic concepts have to be made for ramp systems and 'greenhouse' times. The occurrence of microbial reefs and the lateral expanse of deeper water reef facies is considered to be of special significance for recognizing sea level rises, whereas due to climatic feedbacks and tectonic effects clay-rich intervals are not diagnostic for sequence stratigraphic interpretation. A model is suggested where climatic and oceanic feedback during sea level rise as well as lowstand condensation is considered. The causes for intra-Upper Jurassic sea level changes appear to be mostly of tectonic origin, although, for higher order oscillations, the observed climatic feedbacks point to an autocyclic component related to the carbon cycle.
Last changes Nov. 2004 by Reinhold Leinfelder