Giovanni Rusciadelli

Carbonate production of ancient debris-dominated reefs: An outcrop-based example from the Upper Jurassic reef complex of the central Apennines (Italy)

This study provides the first quantitative carbonate production estimate of an ancient debris reef, represented by the Upper Jurassic reef complex of the central Apennines. The components of carbonate production (Pnet, Psed, and Pgross) have been defined entirely on the basis of outcrop data. The reef volume (Pnet) was reconstructed based on the present distribution of the Upper Jurassic reef complex and its relationship with coeval successions of platform and slope. The volume of sediment exported along the slope (Psed) was obtained through the reconstruction of the thickness reduction pattern of the reef-derived sediments, at increasing distances from the platform margin. Several variables have been introduced to overcome the lack of data in some areas. The estimates obtained indicate that the Upper Jurassic reef complex produced a total amount of calcium carbonate equal to 1.7–2.3 times the amount of material retained in the reef complex. This excess volume of sediment produced was redistributed along the slope-basin system, up to a distance of ∼55 km from the platform margin. According to the indices commonly used for quantifying carbonate production, the Upper Jurassic reef shows values that are consistently lower than other ancient and Holocene reef systems. The interaction between the sedimentologic characteristics of the reef structure and the hydrodynamic processes is ultimately seen as the main mechanism responsible for the low growth potential and reduced export of reef material. The methodology and the procedure used were designed specifically for the site being studied but could potentially be adapted for use in other regions where the data required are difficult to access.

CORAL COMMUNITIES, ZONATION AND PALEOECOLOGY OF AN UPPER JURASSIC REEF COMPLEX (ELLIPSACTINIA LIMESTONES, CENTRAL APENNINES, ITALY)

This work describes and analyzes an original collection of fossil corals from the Ellipsactinia Limestones (Kimmeridgian-Tithonian), exposed in the Marsica area (central Apennines, Italy), focusing on taxonomy and paleoecology. 43 species grouped into 32 genera, 16 families and 9 suborders were identified. Astreoidogyra giadae nov. gen. nov. sp. (Rhipidogyridae) and Clausastrea eliasovae nov. sp. (Montlivaltiidae) are new taxa. Corals occur from the back reef to the reef crest, showing a marked zonation, expressed by a variation of coral cover and type, although the reef front and slope facies could not be sampled. The back reef is characterised by scattered medium-to-small colonies, with a relative high variety of colony shape, corallite arrangement types and high taxonomic diversity. Stylosmilia, Calamophylliopsis, Intersmilia, Pleurophyllia, Bracthelia, Heliocoenia, Ogilvinella occur here among others. The inner reef flat records the highest coral cover, with large robust branching, such as “Pseudocoenia”, Heliocoenia, Calamophylliopsis, and large dome-shaped meandroid, such as Psammogyra, Pruvostrastraea, Eugyriopsis) colonies. Within the external reef flat and the reef crest the coral cover is low and the stromatoporoid-bearing mounds dominate on the isolated coral bioconstructions. Controlling factors as bathymetry, hydrodynamic disturbances, abrasive currents, background sedimentation and morphological irregularities of the depositional profile are considered to explain the observed coral zonation. High diversity and low dominance indices are interpreted to result from reef complex heterogeneity, which should have influenced the formation of different ecological niches and consequently the proliferation of a greater number of taxa in a relatively small area.