The thick-bedded tail of turbidite thickness distribution as a proxy for flow confinement: Examples from tertiary basins of central and northern Apennines (Italy)

• We detail the stratigraphic variability of turbidite thickness statistics of four tertiary basins of Central and northern Apennine which share a common stratigraphic evolution from an early ponded to a late unconfined depositional setting.
• We carry out an assessment of likely sampling biases linked to location and thickness of the studied unit which likely affect turbidite thickness statistics in presence of stratigraphic and spatial trends.
• We document a systematic bias against thin beds which can lead to a significant modification of the actual turbidite thickness frequency distribution.
• We show how thickness statistics of the deposit of large flows are modified due to transition from a ponded to a partially ponded and an unconfined depositional setting.
• We highlight the role of flow stripping, sediment by-pass and bias against thin beds in modifying the initial frequency distribution of turbidite thickness of an initially ponded basin as a result of enlargement of the host basin and lowering of the height of the enclosing topography associated to basin infilling.
• We test the applicability of the our findings to assessment of ponding from open-hole logs of a single well in a ‘fill to spill’ turbidite minibasin.

This study reviews the thickness statistics of non-channelized turbidites from four tertiary basins of Central-Northern Apennines (Italy), where bed geometry and sedimentary character have been previously assessed. Though very different in terms of size and, arguably, character of feeder system, these basins share a common stratigraphic evolution consisting in transition from an early ponded to a late unconfined setting of deposition. Based on comparison of thickness subsets from diverse locations and stratigraphic heights within the studied turbidite fills, this paper seeks to answer the following questions: i) how data collection procedures and field operational constraints (e.g. measure location, outcrop quality, use of thicknesses data from single vs. multiple correlative sections, stratigraphic thickness of the study interval) can affect statistics of sample data? ii) how depositional controls of confined vs. unconfined turbidite basins can result in different thickness-frequency distributions?; and iii) is there in thickness statistics a ‘flow confinement’ signature which can be used to distinguish between confined and unconfined turbidites? Results suggest that: i) best practices of data collection are crucial to a meaningful interpretation of sample data statistics, especially in presence of stratigraphic and spatial trends of turbidite bed thickness; ii) a systematic bias against cm-thick Tcd Bouma sequence turbidites exists in sample data, which can result in the low-end tail of empirical thickness-frequency distributions to depart significantly from the actual distribution of turbidite thickness; and iii) thickness statistics of beds starting with a basal Ta/Tb Bouma division bear a coherent relationship to the transition from ponded to unconfined depositional settings, consisting in reduction of variance and mean and, consequently, parameters, or even type, of best fit model distribution. This research highlights the role of flow stripping, sediment by-pass and bed geometry in altering the initial thickness distribution of ponded turbidites and suggests how fully ponded mini-basins represent the ideal setting for further research linking turbidite thickness statistics and frequency distribution of parent flow volumes.