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The Miocene Marnoso-arenacei and Laga Formations form two of the largest outcropping turbidite units of the north-central Apennines,where extensive exposures make facies analysis feasible. Following a more general model proposed by Mutti and Ricci Lucchi (which utilizes bed geometry), sedimentary structures and texture, facies association (slope, submarine fan, basin plain) and sedimentary facies (conglomeratic sandstones, sandstones, sandstones with shale interbeds, shales with sandstone interbeds, chaotic deposits, hemipelagic pelites) were recognized. The value of sequential analysis for comparing stratigraphic subunits within a submarine fan system, and particularly for differentiating between inner fan, mid-fan, outer fan, and basin-plain environments, is emphasized. Recurrent patterns of sedimentation (megasequences or megarhythms, 10-100 m thick) are composed of sets of thick, mostly arenaceous, massive, graded, or crudely laminated layers, with alternate finer grained beds, which are thinly laminated in the lower part. Two main types of "ordered" megasequences are recognized: (A) positive, or thinning upward, interpreted as filling of fan valleys or channels; and (B) negative, or thickening upward, considered to be prograding outer fans along nonchanneled distributaries. Three monotonous, or random, sequences composed of finer grained sediments are also present: (C) slope deposits (thin layers, with a sandstone/shale ratio less than 1); (D) interdistributary fan deposits (thin to medium layers, sand/shale ratio less than 1; and (E) basin-plain dep sits (medium to thin layers, with sporadic isolated thick turbidite units, sandstone/shale ratio greater than 1). The vertical association of these subunits, and especially those of the A and B megasequences, show both similarities and differences in the sedimentary history and tectonic control of the Miocene basins. Further application of these criteria to other turbidite basins probably will aid in predicting the location and geometry of sandstone bodies.
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