Sand composition changes across key boundaries of siliciclastic and hybrid depositional sequences

Sand composition of arenite successions is sensitive to a suite of factors operating between initial grain production and final diagenesis on a variety of spatial and temporal scales. Seven allogenic factors, the relative importance of which relies upon the complex interaction between tectonics, eustasy and climate, play a decisive role in dictating petrofacies distribution within siliciclastic to hybrid depositional sequences. These factors include: i) tectonic exhumation, ii) physical and chemical rock breakdown, iii) change in sediment flux, iv) change in source/basin physiography, v) shelf colonization by organisms, vi) generation of chemical grains, and vii) volcanism. Autogenic processes may locally have a considerable influence on the type and amount of sediment supplied to the basin, thus interfering with the external control. Based upon literature data, a conceptual framework of expected compositional changes across the key surfaces for sequence-stratigraphic interpretation is outlined. Two case histories, from the Miocene shelf-to-turbidite deposits of the northern Apennines and the Quaternary alluvial-to-nearshore succession of the Adriatic coast, respectively, are used as references to illustrate how arenite petrofacies changes can be framed into a sequence-stratigraphic scheme on multiple timescales.