The Messinian primary monomagmatic turbidites in the Central Apennines: sedimentological and petrographic features from volcaniclastic layers in the new Amandola section (Marche, Italy)

An Upper Miocene volcaniclastic interval, up to about 2.5 m thick, is exposed at several locations along the deep-water foreland basin system of central Apennines. Geochronological dating converges to indicate that these volcanic sediments were deposited at about 5.5 Ma (e.g., Odin et al., 1997), whereas mineralogical analyses indicate that the component strata comprise 80% of blocky and vesicular glassy shards and that the mineralogical phases represent the remaining 20% (Potere et al., 2022). These features, and the observed silica and alkali content, suggest that the volcaniclastic layers can be considered as the product of a calc-alkaline rhyolitic magma (Guerrera et al., 1986; Potere et al., 2022; Trua et al., 2010). In order to determine transport directions and depositional processes, in this study the volcaniclastic strata exposed on a recently made road cut near the town of Amandola (FM) have been investigated. At this new outcrop, the volcaniclastic horizon is 1.90 m thick and it comprises twenty-three beds, each recording a discrete density flow depositional event. Individual beds are characterized by sedimentological features typical of the product of turbidity currents, such as: i) flame structures along the contacts; ii) overall normal grain-size grading formed during temporal flow deceleration; and iii) the occurrence of a massive (i.e., ‘‘structureless’’) Ta division overlain by planeparallel and cross-laminated Tb and Tc divisions, with the massive division indicating deposition at rates of suspended load fallout high enough to suppress tractional transport and the laminated divisions indicating final flow stages, in which flow density and velocity are sufficiently low for bedform development to occur. The absence of laminated Tb and Tc divisions in some of the beds can be explained by the narrow grain-size distribution (i.e. the presence of well-sorted sediment) or by the post-depositional erosion of these divisions by succeeding sediment gravity flows, thus forming a set of amalgamated, top-truncated Ta beds. Principal paleocurrent directions indicate deposition from NE- and NW-directed paleoflows. Collected samples have been investigated via optical microscopy, SEM and EPMA, through which both petrographic and chemical composition are described. Microscopic observations result in glass shard morphologies description (i.e., bubbles’ walls, vesiculation and stretched shapes). The high glass shard content, the glass coating of biotite and feldspar phenocrysts indicate an original primary deposition. The homogeneous composition of the volcaniclastic beds and the absence of interbedded hemipelagic marls is strong evidence that these turbidites were fed by a single eruptive event and that they may have originated either from flow transformation of pyroclastic flows, or rapid remobilization of stored volcaniclastic material.