A quantitative compositional approach to the study of a Miocene carbonate shelf in the northern Apennines

The petrography of 147 samples of carbonate and mixed siliciclastic-carbonate rocks has been performed on shallow-water carbonates of early-middle Miocene deposited in a wedge-top basin in the northern Apennines. High-frequency sampling of the Torriana log (Val Marecchia valley, Romagna Apennines) covers the entire carbonate succession from the basal unconformity to the terrigenous-rich sediments that mark the demise of the shelf. The modal analysis has been performed by point counting (300 points) in order to quantify the main framework components: an intrabasinal carbonate fraction (biogenic), a terrigenous fraction (largely siliciclastic), and an authigenic intrabasinal component constitued by glauconitic grains. On the basis of the relative amounts of these framework components, four different petrofacies are distinguished and matched with lithofacies units. From the base to the top they are the following: - Rudstones and subordinate grainstones, made up almost exclusively of intrabasinal carbonate grains, represented by bryozoans and subordinate echinoid spines and plates. Cement is constituted by scarce sparry calcite. - Rudstones to grainstones, made up almost exclusively of intrabasinal carbonate grains plus very subordinate amounts of siliciclastic grains (quartz, feldspars). Intrabasinal carbonates consist of shallow-water bioclasts such as bryozoan fragments, echinoid spines and plates, red algae and benthic macroforaminifera (Amphisteginae, Gypsinae). Grains are cemented by sparry calcite. Siliciclastic grains are concentrated in thin laminae; isorientation is also occasionally observable in bioclastic grains (Amphisteginae). - Hybrid arenites: the intrabasinal carbonate component (biogenic) is associated with variable but subordinate amounts of terrigenous siliciclastic components dominated by quartz and feldspar grains (both plagioclase and k-feldspar). The biogenic component is still represented by bryozoans and echinoids but the bryozoan/echinoid ratio noticeably decreases. The siliciclastic component is mainly concentrated in thin laminae. The interstitial fraction includes pore-filling sparry calcite and micritic matrix. - Arenites, from medium to fine-grained; the terrigenous input noticeably increases and becomes dominant. It is made up of prevalent quartz and feldspars and few lithics (sedimentary, metamorphic and volcanic). Intrabasinal grains are subordinate, made up of fragments of algae, bryozoans, and planktonic foraminifera. Glauconitic grains are present (up to 7% of the whole rocks); calcite is the dominant pore-filling cement. The hierarchical cluster analysis of point counting data is in agreement with outcrop-scale facies observations. Our data indicate an almost exclusively intrabasinal supply during deposition of the basal portions. A vertical evolution is documented and it is evidenced by a progressive increase in the terrigenous supply that replaces the bioclastic fraction. The terrigenous input becomes significant in the upper unit marking the crisis of the shelf. Abstract