Statistical analysis of bed-thickness variation in a Tortonian succession of biocalcarenitic tidal dunes, Amantea Basin, Calabria, southern Italy.

The Tortonian biocalcarenitic succession in Amantea Basin was deposited in a ramp setting of a peri-Tyrrhenian shelf embayment, whose northern part was linked with the southern part by a tectonic graben referred to as the Monte Pellegrino strait. The graben was 1.5–2 km wide and 4 km long, had shallowly submerged margins and a south-sloping sublittoral floor dominated by strong, asymmetrical tidal currents. The water depth fluctuated due to episodes of rapid subsidence driven by extensional tectonics. The strait-fill biocalcarenites are up to 120 m thick and consist of planar cross-stratified beds that range in thickness from b0.2 m to N20 m and represent 2-D dunes accumulated as a mounded longitudinal complex. Bed thicknesses have been measured in four vertical logs along an outcrop section oblique to the palaeostrait. The stratigraphic pattern of bed-thickness variation has been analyzed by using two versions of the runs test, the Moore–Wallis test, the median-crossing test, the mean squared successive difference test, the Hurst statistic, the Spearman rank-correlation test and the Meacham rank-difference test. The statistical tests indicate that the axial zone and inner flank of the tidal complex show alternating clusters, 5–15 m thick, of thinner and thicker beds, whereas the outer flank and marginal zone show thinning-upward bed packages, 15–25 m thick. An overall thinning-upward trend characterizes the transitional flank zone. The bed packages are considered to form aggradational parasequences, whose varied lateral development is attributed to a differential response of the subtidal system to bathymetric changes. The data set indicates that the bed thicknesses are self-similar, but beds thinner than 250 cm have a different fractal dimension than the thicker beds. The pattern of bed-thickness variation is considered to be a result of internal forcing of a sedimentary system in a state of self-organized criticality, perturbed by bathymetric changes. Fractal property bears important implications for the spatial characteristics of sedimentary succession, allowing bed geometries and volumes to be assessed on the basis of thickness data from isolated logs or drilling cores.