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Metre-scale siliciclastic–carbonate cycles are the basic depositional motif of the lower Miocene Guadagnolo Formation outcropping in the central Apennines. The mechanisms which formed the mixed-lithology cycles are still a matter of debate. The mixed siliciclastic–carbonate system discussed in this paper provides a new case study to illustrate the role of orbital forcing in controlling the facies evolution and cyclic stacking of small-scale sequences deposited on the outer sector of a ramp. Two sections are discussed that display mixed siliciclastics and carbonates arranged in upward-shallowing cycles. Each cycle shows an upward decrease in the terrigenous input and a parallel increase in benthic fauna. Time-series analyses indicate the cyclic carbonate-terrigenous pattern to be largely controlled by orbital forcing in the Milankovitch frequency band. Coupling of climate change and sea-level fluctuations in tune with orbital cycles are proposed as driving mechanisms.

In the western part of the central Apennines, Lower–Middle Miocene carbonates were deposited on a tropical–subtropical carbonate ramp. They record two long-term cycles, the first of which is illustrated in this paper. Between 21 and 17.5 Ma, Miocene carbonates, paraconformably overlying the Cretaceous limestones, record a transgressive event during a time of global (2nd order) sea-level lowstand. It is postulated that this deviation is related to an increase in tectonic subsidence. Between 17.5 Ma and 16–15 Ma, with a progressive relative sea-level rise, the inner–middle ramp facies belt stepped back, whereas the bryozoan-dominated outer ramp facies belt stepped back but simultaneously prograded. This bloom of suspension-feeding organisms is interpreted to reflect an increased nutrient availability, hence a change from oligotrophic to eutrophic conditions. Strontium-isotope dates constrain correlation of the second phase with a eutrophic event possibly linked to the influence of the neighbouring Apenninic accretionary wedge and foredeep system.