Orbital forcing recorded in subtidal cycles from a Lower Miocene siliciclastic-carbonate system (Central Italy)

ABSTRACT The metre-scale siliciclastic-carbonate cycles are the basic depositional motif of the lower Miocene Guadagnolo Formation outcropping in Central Apennines. The mechanisms which formed 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 the cyclic stacking of small-scale sequences deposited on the outer sector of a ramp. Two sections are discussed that display mixed siliciclastic-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.