CORAL-RUDIST BIOCONSTRUCTIONS IN THE UPPER CRETACEOUS HAIDACH SECTION(GOSAU GROUP, NORTHERN CALCAREOUS ALPS, AUSTRIA)

In the area of Haidach (Northern Calcareous Alps, Austria), coral-rudi st mounds, rudist biostromes, and bioclastic limestones and marls cons titute an Upper Cretaceous shelf succession approximately 100 meters t hick. The succession is parr of the mixed siliciclastic-carbonate Gosa u Group that was deposited at the northern margin of the Austroalpine microplate. In its lower part, the carbonate succession at Haidach com prises two stratal packages that each consists, from bottom to top, of a coral-rudist mound capped by a rudist biostrome which, in turn, is overlain by bioclastic limestones and, locally, marls. The coral-rudis t mounds consist mainly of floatstones. The coral assemblage is domina ted by Fungiina, Astraeoina, Heterocoeniina and Agathelia asperella (S tylinina). From the rudists, elevators (Vaccinites spp., radiolitids) and recumbents (Plagioptychus) are present. Calcareous sponges, sclero sponges, and octocorals are subordinate. The elevator rudists commonly are small; they settled on branched corals, coral heads, on rudists, and on bioclastic debris. The rudists, in turn, provided settlement si tes for corals. Predominantly plocoid and thamnasterioid coral growth forms indicate soft substrata and high sedimentation rates. The mounds were episodically smothered by carbonate mud. Many corals and rudists are coated by thick and diverse encrustations that indicate high nutr ient level and/or turbid waters. The coral-rudist mounds are capped by Vaccinites biostromes up to 5 m thick. The establishment of these bio stromes may result from unfavourable environmental conditions for cora ls, coupled with the potential of the elevator rudists for effective s ubstrate colonization. The Vaccinites biostromes are locally topped by a thin radiolitid biostrome. The biostromes, in turn, are overlain by bioclastic limestones; these are arranged in stratal packages that we re deposited from carbonate sand bodies. Approximately midsection, an interval of marls with abundant Phelopteria is present. These marls we re deposited in a quiet lagoonal area where meadows of sea grass or al gae, coupled with an elevated nutrient level, triggered the mass occur rence of Phelopteria. The upper part of the Haidach section consists o f stratal packages that each is composed of a rudist biostrome overlai n by bioclastic wackestones to packstones with diverse smaller benthic foraminifera and calcareous green algae. The biostromes are either bu ilt by radiolitids, Vaccinites, and Pleurocora, or consist exclusively of radiolitids (mainly Radiolites). Both the biostromes and the biocl astic limestones were deposited in a low-energy lagoonal environment t hat was punctuated by high-energy events. In situ-rudist fabrics typic ally have a matrix of mudstone to rudist-clastic wackestone; other bio gens (incl. smaller benthic foraminifera) are absent or very rare. The matrix of rudist fabrics that indicate episodic destruction by high-e nergy events contain a fossil assemblage similar to the vertically ass ociated bioclastic limestones. Substrata colonized by rudists thus wer e unfavourable at least far smaller benthic foraminifera. The describe d succession was deposited on a gently inclined shelf segment, where c oral-rudist mounds and hippuritid biostromes were separated by a belt of bioclastic sand bodies from a lagoon with radiolitid biostromes. Th e mounds document that corals and Late Cretaceous elevator rudists may co-occur in close association. On the scale of the entire succession, however, mainly as a result of the wide ecologic range of the rudists relative to corals, the coral-dominated mounds and the rudist biostro mes are vertically separated.