In: Reitner, J., Neuweiler, F. & Gunkel, F. (eds., 1996): Global and Regional Controls on Biogenic Sedimentation. I. Reef Evolution. Research Reports.
Göttinger Arb. Geol. Paläontl., Sb2, 227-248, Göttingen.
Reefs from the Late Jurassic comprise various types of coral reefs, siliceous sponge reefs and microbolite reefs. Upper Jurassic corals had a higher ratio of heterotrophic versus autotrophic energy uptake than modern ones, which explains their frequent occurrence in terrigenous settings. Coral communities changed along a bathymetric gradient but sedimentation exerted a stronger control on diversities than bathymetry. One coral community was adapted to brackish waters.
Reefal siliceous sponge biostromes and sponge microbolite mud mounds generally occur below the coral facies, and hexactinellid-dominated sponge communities generally occur below a zone of mixed "lithistid"-hexactinellid growth. This distribution mirrors differences in nutrient conditions, with coral facies related to stable, moderately oligotrophic to mesotrophic conditions whereas siliceous sponges could tolerate fluctuating levels and hence may range from extremely oligotrophic to strongly mesotrophic settings. This is due to the fact that hexactinellid sponges can largely live on osmotrophy and "lithistid" sponges develop deposits of living organic matter by hosting a huge mass of bacteria. Microbolite crusts demand strongly reduced sedimentation and are important framework contributors in many coral and sponge reefs. Eutrophication or oxygen depletion may exclude reef fauna, giving rise to pure microbolite reefs.
Most Upper Jurassic reefs developed in ramp settings. High-energy reefs contain little preserved framework, whereas low-energy reefs may have excellently preserved
Framework and pronounced relief whenever microbolite crusts provided stabilization. Reefs in steepened slope settings are generally rich in microbolites because of bypass possibilities for allochthonous sediment. Reef rimmed shallow-water plafforms did occur but only developed on preexisting uplifts. Upper Jurassic sponge-microbolite mud mounds grew in subhorizontal mid to outer ramp settings and reflect a delicate equilibrium of massive and peloidal microbolite precipitation and accumulation of allochthonous mud and fine allochems, determined by the distance to shallow-water carbonate factories. Disturbances in this equilibrium lead to the development of sponge biostromes or the disappearance of sponge facies.
The growth of Upper Jurassic reefs was largely restricted to, or strongly facilitated by, rising global or regional sealevel, reducing sediment influx occurring during 4th or 5th order transgressive pulses within the window of 3rd order sealevel rise. Consequently, transgressive/early highstand shallow-water reefs are rich in microbolite crusts and highly diverse, whereas the rare late highstand/lowstand coral reefs are of low diversity and have little framework preserved. In deeper waters, the frequency of sponge-microbolite mounds is correlated with the sealevel development. Together with basin configuration climatic and oceanographic response to sealevel rise account for a predisposition for eutrophication and oxygen depletion particularly around the Iberian Peninsula, giving rise to the occasional occurrence of pure microbolite reefs or repetitive successions of coral-microbolite to pure microbolite reefs at fairly shallow water depths. The origin of Upper Jurassic reefs can only be unraveled by taking the paleogeographical, structural and sequence stratigraphic framework into account. On the other hand, the lateral and bathymetric distribution patterns of reef types from the Late Jurassic provide valuable tools for the better understanding of shelf dynamics and climate of this epoch at a regional and even global scale.
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