Orbital forcing recorded in subtidal cycles from a Lower Miocene siliciclastic–carbonate ramp system (Central Italy) Marco Brandano, 1 Laura Corda 1,2 and Goffredo Mariotti 1,2 1 Dipartimento Scienze della Terra, Universita ` di Roma ÔLa SapienzaÕ, Piazzale Aldo Moro 5, Rome 00185, Italy; 2 Istituto di Geologia Ambientale e Geoingegneria-CNR, Rome, Italy Introduction Metre-scale cyclicity is a characteristic feature which has long been recog- nized in carbonate rocks (Fischer, 1964; Wilson, 1975; James, 1984; Tucker and Wright, 1990; Osleger, 1991; Walker and James, 1992; Gold- hammer et al., 1993; Read, 1998; D’Argenio et al., 1999; Strasser et al., 1999; Lukasik and James, 2003) and it is a fundamental element in giving a better resolution and timing of third- order sequences. In the geological record, cycles are commonly composed of either lime- stones or siliciclastics, but more rare mixed carbonate–siliciclastic se- quences are also observed. These latter types are probably typical of icehouse periods, with terrigenous input being mainly controlled by significant sea- level and climate changes (Mount, 1984; Doyle and Roberts, 1988). They tend to be more common in Permo- Carboniferous and Quaternary succes- sions (Tucker, 2003) although they are recorded in other periods. As shown by Lukasik and James (2003) open- marine, wholly subtidal cyclicity is less well-documented outside the tropical peritidal realm and it is arguably not so easily interpreted because of the lack of specific palaeoenvironmental sedimentary features. The sections analysed in this paper have been measured in a Miocene marly-carbonate succession from cen- tral Apennines (Guadagnolo Forma- tion) that is mostly characterized by a regular repetition of metre-scale, mixed carbonate–siliciclastic se- quences (Madonna, 1996; Mariotti et al., 2002a; Barbieri et al., 2003/ 2004). The present contribution discusses the palaeoceanographic deposition model and the possible cycle-produ- cing mechanisms. Microfacies stack- ing pattern and spectral analysis were utilized for a better understanding of the interactions between changes in accommodation space, hydrodynam- ics, carbonate production and terri- genous input. Geological setting Measurements were made of two sec- tions from the Guadagnolo Forma- tion cropping out in the central Apennines (Fig. 1) between the Um- bria–Sabina basin (to the west) and the Latium–Abruzzi platform (to the east). The lower-middle Miocene Gu- adagnolo Formation is a marly-cal- careous succession whose thickness varies from tens of metres to more than 500 m in more basinal sections (Civitelli et al., 1986a,b; Barbieri et al., 2003/2004). The formation can be divided into two members: the lower one is characterized by several hundred metres of alternating marl- stones, marly limestones and calcare- nites; the upper member is composed of approximately 100 m of well-stra- tified bioclastic calcarenites. The pre- sent work focuses on the lower member. The main body of the lower member consists of a regular repeti- tion of shallowing-upward cycles, as defined by detailed stratigraphic and isotopic analyses given in Barbieri et al. (2003/2004). The depositional setting has been interpreted (see Mar- iotti et al., 2002a; Barbieri et al., 2003/ 2004) as an outer carbonate ramp and the cycles as subtidal cycles sensu Osleger (1991). During the Miocene the studied deposits were accumula- ting on the foreland as a mixed siliciclastic–carbonate outer-ramp sys- tem receiving terrigenous input from the north-west (from the developing Apennine accretionary wedge). At the same time it was a zone of both active carbonate production and accumula- tion of carbonate debris shed from the shallower sectors of the carbonate ramp (located south-eastward). Facies analysis Two sections, approximately 150 m thick each, from the upper part of the Guadagnolo Formation (lower mem- ber), were measured and analysed in detail (Fig. 2). The studied sections are at the same stratigraphic interval, as defined by an upper discontinuity surface, which has been isotopically dated at c. 16 Ma (Barbieri et al., ABSTRACT Metre-scale siliciclastic–carbonate cycles are the basic deposi- tional 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 con- trolled 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. Terra Nova, 17, 434–441, 2005 Correspondence: Laura Corda, Diparti- mento di Scienze della Terra, University of Rome ÔLa SapienzaÕ, Piazzale Aldo Moro 5, Rome 00185, Italy. Tel.: 00 39 06 4991 4859; fax: 00 39 06 445 4729; e-mail: laura.corda@uniroma1.it 434 Ó 2005 Blackwell Publishing Ltd doi: 10.1111/j.1365-3121.2005.00630.x