Abstract

Jurassic reefs are composed of different biotic types (coral, siliceous sponge, microbial and mixed facies) and textural types (bafflestone meadows, framestones, debris reefs, mudmounds). They indicate water energy, depth, temperature, sedimentation rate, oxygen depletion and slope angle. The distribution of reefs hence reflects changes in shelf morphology, sea level, climate and oceanographic circulation in the course of the breakup of Pangea. Reefs diagnostic of steepened slopes (mixed coral-sponge reefs) mostly occur on the southern Tethyan shelf and in the Atlantic realm, whereas reefs diagnostic of homoclinal to moderately steepened ramps (coral-debris sands, extensive siliceous sponge facies) are largely restricted to the more stable northern shelf of the Tethys.

Temporal reef distribution on the southern Tethys shelf reflects increasing drowning of Triassic carbonate platforms during the Early and, particularly Mid Jurassic, and local uplift during the Late Jurassic. On the northern Tethyan shelf reefs generally expanded during the Mid and Late Jurassic. Rather than evolutionary radiation, this pattern mostly reflects increasing habitat availability caused by general sea level rise accompanying early drift. In the Atlantic realm reef distribution helps analyze the marine rifting pulse of the Late Jurassic.

High paleolatitude occurrences of coral reefs are indicative of a very equilibrated climate during the Late Jurassic, which reflects the buffer capacities of the wide sea cover. During the same epoch dysaerobic microbial crust reefs often grew at shallow depths in the western part of the northern Tethys shelf. Together with the frequent occurrence of Upper Jurassic bituminous deposits, this feature is interpreted as reflecting a strong reduction in oceanic circulation and weak upwelling on the northern Tethyan shelf. Both climatic leveling and sluggish circulation are interpreted as feedback effects of the continuous sea level rise which itself was caused by the increasing breakup of the northern hemisphere of Pangea.