How sampling procedures and sedimentary processes of confined basins can modify the observed frequency distribution of turbidite thickness? : examples from Tertiary confined basins of central and northern Apennine (Italy)

The assessment of turbidite thickness statistics, along with variability of bed geometry, represents an open research question for both applied and pure sedimentology. Empirical frequency distributions of turbidite thickness reported in the scientific literature are diverse (e.g. truncated Gaussian, lognormal, exponential and power law) and variously interpreted as resulting from modification of the input signal (i.e. distribution of parent flow magnitude) by depositional/erosional processes. Yet biases toward or against certain thickness classes due to bed geometry, erosion and data collection are commonly overlooked, casting doubts on thickness statistics as a tool for answering some of the key research questions of turbidite sedimentology. However, in situations where turbidites are confined by basin topography and erosion is negligible, some of the variables of the ‘bed thickness equation’ can be relaxed, making easier to investigate what the primary controls on turbidite thickness statistics are. This study reviews the bed thickness statistics of four confined to unconfined turbidite units (namely, the Castagnola Fm. and the Cengio-Bric la Croce-Castelnuovo turbidite systems of Tertiary Piedmont Basin and the Laga and Cellino Fms. of the Apennine Foreland Basin System) from the tertiary of Central-Northern Apennines (Italy), where bed geometry and sedimentary character have been previously assessed. Based on comparison of thickness subsets from turbidite systems with different degree of confinement and diverse locations within the basin fill, this contribution aims at answering the following questions: i) how data collection choices and field operational constraints (e.g. location, outcrop quality, use of thickness from single vs. multiple correlative sections, length of the stratigraphic section from which thicknesses were retrieved) can affect statistics of an empirical distribution of turbidite thicknesses so to induce biased interpretations? ii) how depositional controls of confined vs. unconfined basins can modify the initial thicknesses distribution of turbidites?; iii) is there in turbidite thickness statistics a ‘flow confinement’ signature which can be used to distinguish between confined and unconfined depositional settings? Results indicate that: i) best practices of data collection are crucial to a meaningful interpretation of turbidite thickness data; ii) a systematic bias against cm-thick beds deposited by small volume low density flows exists, which modifies the thin-bedded tail of turbidite thickness distributions and, iii) the thick-bedded tail of thickness distributions of the case studies bears some relationship to the transition from confined and unconfined depositional settings, suggesting its statistics can represent a proxy for the degree of flow confinement.