RUDIST BIOSTROMES ON THE MARGIN OF AN ISOLATED CARBONATE PLATFORM - THE UPPER CRETACEOUS OF MONTAGNA-DELLA-MAIELLA, ITALY

In the Upper Cretaceous shallow water platform succession of Montagna della Maiella, Italy, different types of bivalve biostromes (rudists, oysters) are present from the external to the internal platform. From Jurassic to Miocene times. Montagna della Maiella was part of the marg in of the large, intraoceanic Apulian carbonate platform. Two types of biostrome fabric were distinguished, (1) an open fabric (isolated rud ists, supported by matrix) and (2) a packed fabric (densely packed, in terlocking rudist clusters). An open biostrome fabric may have resulte d from various factors, e.g. unstable substrate or environmental restr iction. A packed biostrome fabric probably records favorable environme ntal conditions. In the Maiella, the following types of biostrome were recognized (1) caprinid biostrome, (2) hippuritid biostrome, (3) radi olitid biostrome and (4) oyster biostrome. The caprinid biostromes and hippuritid biostromes characterize the external platform environment. Coelenterates are subordinate or absent in these biostromes; encrusti ng organisms (red algae, bryozoans) are rare. The radiolitid biostrome s and the oyster biostromes occupied the normal saline to restricted i nternal platform environment. The Upper Cretaceous external platform s uccession contains 70-85% of cross-bedded rudist bioclastic grainstone ; rudist biostromes, mostly 1-3 m thick, amount to between 8% to 18%. Here, the biostromes are commonly situated near the top of cycles that may be up to more than 10 m thick. The biostromes are present at or n ear the base of meter-scale cycles. The position of the biostromes wit hin the cycles and the vertically-associated lithologies indicate that the requirements for both biostrome growth and preservation were met only in a narrow range of environmental conditions. The large lateral extent and the well-defined lower boundary of the biostromes suggest t hat the rudists could rapidly colonize lime sand and lime mud substrat es. The rudist biostromes, however, were often buried by migrating car bonate sand bodies.