Origin of the upper jurassic limestones of the Swabian Alb

The Upper Jurassic of the Swabian Alb is characterized by a special association of facies types: A sequence of micritic limestones and marls interfingers with bioherms. These bioherms were produced by algae and siliceous sponges. The algae formed either calcareous crusts without internal structures or stromatolites, growing mostly on the sponges. The sponges are in general preserved as calcareous matter. The bioherms did not produce true reef detritus. They were growing in relatively deep and quiet water. This is also confirmed by the presence of the siliceous sponges, by the ammonite fauna and by the relation to the micritic sediments. A shallowing of the sea took place towards the end of the Jurassic. This becomes obvious by the occurence of corals and even coral reefs, and the deposition of bioclastic and oolitic limestones. The micritic ? normal? facies sequence consists of a rhythmic succession of marls and limestones which is also represented to a certain degree within the bioherm areas. Within the older members of the Jurassic the rhythmic sequences can be correlated quite well over greater distances, but towards the younger beds the sedimentation was more and more influenced by local conditions. This is also an indication to the shallowing of the sea. Within the sequence of the Upper Jurassic there is a variety of other rocks and phenomena: slumping structures, resedimentary, intraformational breccias, dolomites and dedolomites, siliceous concretions, and calcirhythmites.