Vent area and depositional mechanisms of the Upper Member of the Neapolitan Yellow Tuff (Campi Flegrei, Italy): new insights from directional fabric through image analysis
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In order to provide new information about the source area and depositional mechanisms of the Upper Member of the Neapolitan Yellow Tuff (NYT), a prominent pyroclastic deposit of the Campi Flegrei Volcanic District (southern Italy), statistics on directional fabric, by means of computer-assisted image analysis on 32 rock samples, were compiled. Seventeen samples were collected along vertical direction on two selected exposures and fifteen were taken from outcrops widely distributed all around the Campi Flegrei Volcanic District. Fabric measurements within the investigated successions reveal a vertically homogeneous direction of the mean particle iso-orientation, with considerable variability in the strength of particle iso-orientation even at cm-scale. The existence of particle iso-orientation can be related to continuous sedimentation from a concentrated bedload region beneath suspension currents, producing massive or inversely graded beds by traction carpet sedimentation. The considerable vertical variability in the strength of iso-orientation is the result of very unstable flow regimes, up to the extreme condition of discrete depositional events, with a variable combination of traction carpet and/or direct suspension sedimentation. The vertical homogeneity in the mean orientation values, found in the investigated sections, may derive from the sequential deposition of laminae to thin beds, whose relatively flat upper surfaces were unable to significantly deflect the depositional system of the following currents. According to the observed homogeneous mean particle orientation values along the investigated vertical profiles, samples collected through areal distribution are considered representative of the local paleo-flow directions of the whole deposit. The mean directions of the samples collected areally show two different coherent patterns which point to the existence of two different source areas. The first, which includes all samples from the northern outcrops, appears to converge in a narrow area about 2 km NE of the town of Pozzuoli, largely in coincidence with the inferred area on the basis of the pumice fall distribution. The second, which includes samples from Capo Miseno and Posillipo areas, points to the central part of the Pozzuoli Bay, about 4 km offshore the town of Pozzuoli.