Oligocene nearshore benthic assemblages and coral facies of the northern Gargano Promontory

Shallow water carbonate successions of Oligocene age are quite well distributed in the Apulia domain and are often associated with the development of luxuriant coral reef systems (Salento Peninsula, Maiella Mountain). The dominant biogenic components generally consist of rich associations of scleractinian corals and larger foraminifera. In the Gargano Promontory, the Oligocene has been recently reported for the first time and a new carbonate unit informally established and named as Grotta S.Michele limestone (Morsilli et al., 2005). The aim of the present study is thus to investigate the type locality of this unit in order to describe and interprete its lithologic composition and biotic content through facies analysis. The stratigraphic succession consists of grainstones-rudstones to wackestones-mudstones, reaches a maximum thickness of about 28 m and clearly shows a fining-upward trend. The fossil fauna is mainly characterized by abundant scleractinian corals, associated with benthic forams and coralline algae. Corals are represented by very few genera and basically consist of globose colonies of Goniopora near the base of the succession, followed by floatstones-rudstones rich in branched Acropora, Stylophora, Alveopora and phaceloid colonies of Caulastrea towards the top. Similarly, also the foraminiferal assemblages, dominated by miliolids and soritids, are rather poor, often with a small number of specimens and low taxonomic diversity. Some larger foraminifera such as Archaias, Austrotrillina, Nephrolepidina, and Nummulites also occur, thus indicating an Oligocene age (Chattian). Several facies types have been identified on the basis of macroscopic observations in the field and microfacies analysis. Data are provided in particular for the distribution and relative abundance of the main biotic components, including coralline algae, bivalve and gastropod fragments, together with textural features. The facies association suggests a nearshore, sheltered, shallow-water marine paleoenvironment with possible evidence of changes in hydrodynamic conditions from very low to moderate-high energy. Within this setting, the unusual low-diversified coral assemblages and the absence of a true framework can be interpreted as a response to stressed conditions probably controlled by the inherited paleotopography.