Facies character and skeletal composition of heterozoan carbonates in a high-energy confined embayment (Miocene, Finale Ligure Limestone, NW Italy)

Abstract

During the early and middle Miocene, diverse photozoan to heterozoan carbonate and mixed carbonate-siliciclastic depositional systems characterized the Mediterranean region at palaeo-latitudes of 30–40°N, during a time of warm climate. These systems ranged from carbonate ramp to rimmed platforms to current-swept flooded incised palaeo-valleys, seaways and straits. This study focuses on the facies character, skeletal biota composition, diagenesis and stable carbon and oxygen isotope signature of a mixed carbonate-siliciclastic succession (lower-middle Miocene Finale Ligure Limestone and underlying Oligocene-lower Miocene siliciclastic deposits) accumulated in a high-energy coastal setting (Finale Ligure, NW Italy) to improve the knowledge on the variety and controlling factors of Miocene heterozoan skeletal carbonates.The 3D reconstruction of the geometry of the early-middle Miocene Finale Ligure basin identifies an embayment, about 35 km2 wide, limited by uplifted and eroded Alpine tectonic units and connected to the open sea through a strait. The Oligocene-Miocene sedimentary succession unconformably overlying the Alpine deformed substrate consists of 14 lithofacies (L1-L14). Besides pockets of karstic breccias (L1) preceding the marine transgression, discontinuous outcrops of glaucony-bearing litharenite, siltstone, conglomerate and wackestone/packstone with planktonic foraminifera (L2-L4) represent the preserved remnants of an eroded marine shelf affected by tectonic uplift and subsequent erosion, approximately in the Aquitanian-Burdigalian, overlain through an angular unconformity by transgressive conglomerates and bioturbated litharenites (L5-L7). The onset of carbonate skeletal production characterizes the overlying compositionally mixed carbonate-siliciclastic Langhian-Serravallian Finale Ligure Limestone succession (up to 100–150 m thick, L8-L14). These cross-bedded skeletal packstone and grainstone/rudstone are composed of barnacles, echinoderms, bryozoans, bivalves, scleractinian and stylasterid corals, and lack halimedacean algae as previously published. They accumulated in a high-energy setting with strong bottom currents, driven by the amplification of tidal and/or storm currents in a confined embayment, promoting the formation of seaward prograding metre-scale subaqueous dunes.Unlike other lower-middle Miocene carbonate systems, the Finale Ligure heterozoan carbonates contain rare coralline red algae and larger benthic foraminifera but are enriched in barnacles, scleractinian and stylasterid corals. The coastal palaeo-environmental conditions, with hard rocky substrates, strong bottom currents, high nutrients and water turbidity, influenced the composition of the skeletal carbonate producers. These distinctive lithofacies character and composition resulted from the confined embayment morphology inherited from the marine flooded Alpine bedrock that was subjected to land runoff driving nutrient input.