An Upper Jurassic Sponge-Algal Buildup from the Northern Frankenalb, West Germany
During Upper Jurassic time (middle-upper Oxfordian) a number of relatively small organic buildups, principally composed of siliceous sponges and algae, developed in the area of southern Germany. This study concentrates on one of these organic structures, the Müllersfelsen buildup (northern Franconian Alb near Streitberg). It is dem-onstrated that the dominant buildup constructional organisms are siliceous sponges, principally of two morphologies (cup-shaped and dish-shaped forms), and cyanophycean algae, which aided in forming the mound configuration. The Müllersfelsen buildup developed during several cyclic stages which occurred in relatively deeper water subtidal depositional environments, without strong current and wave action. Three facies types have been recognized, these are: 1. sponge-crust boundstone facies characterized by micritic boundstone rich in calcified siliceous sponges, tuberoids, and crusts; 2. lithoclastic packstone facies in which spheroidal sedimentary particles (lithoclasts, tu-beroids, and bioclasts) are the dominant grain-supported allochems; and 3. tuberolitic wackestone facies in which mud-supported sedimentary particles are dominant. The spatial distribution of these microfacies indicate that the sponge-crust boundstone facies is the mound constructional facies, whereas both the tuberolitic wackestone and packstone facies are only developed in those areas marginal to the main organic buildup. The uppermost portion of the Müllersfelsen buildup is dolomkized.