ABSTRACT

Facies characteristics are described for a 2300 m thick turbidite succession of the Macigno Formation (middle-upper Oligocene), which crops out as a continuously exposed section in the Gordana Valley (northern Apennines). In the study area the Macigno Formation consists of two sandy members separated by a shaly member. The sandy members contain lithic associations characteristic of submarine fan deposits and form distinct fan complexes. The middle shaly member represents a period of basin starvation.

Three main facies associations are recognized in the Macigno fan deposits, fan-fringe, outer-fan lobe, and braided midfan facies associations. The fan-fringe deposits consist of monotonous or small-scale cyclically arranged thin-bedded turbidites. The outer-fan lobe deposits are composed of medium-to very thick-bedded classical turbidites internally organized in thickening-upward cycles with minor intercalations of thin-bedded sandstone beds. Two distinct types of outer-fan lobe deposits, distal and proximal, are recognized on the basis of rock-type, internal organization, and vertical facies relationships. The midfan deposits consist of amalgamated massive sandstone and pebbly sandstone which display repeated thinning-upward cycles. The three main facies associations are superposed t define major thickening and coarsening-upward cycles reflecting distinct episodes of fan progradation. The relationships between the facies associations of the Macigno deposits and the existing models of submarine fan sedimentation are discussed.

After the deposition of the fan complexes, fan sedimentation in this part of the basin was replaced by movements of allochthonous units heralded by large olistostromes.