Do mudclast-rich hybrid event beds have thinner mudstone caps than classic turbidites?

Hybrid event beds (HEBs) are bi- or tripartite event beds with a lower clean sandstone division and an upper or central chaotic mud-rich division. Similarly to classic turbidites, the bed is usually overlain by a mudstone cap... [ view full abstract ]

Hybrid event beds (HEBs) are bi- or tripartite event beds with a lower clean sandstone division and an upper or central chaotic mud-rich division. Similarly to classic turbidites, the bed is usually overlain by a mudstone cap thought to represent deposition from a muddy cloud generated as part of the same event. In recent years research has been focussed on the character of the parent flow (turbulent, transitional or laminar) and on the origin of the mud deposited as part of the central chaotic and mud-rich division (updip vs. local mud acquisition). Ponded basins trap entire flow volumes and develop tabular bed geometries, making them effective natural laboratories to study the relationships between parent flows and the resulting event beds, as the thickness of the bed is thought to reflect the volume of the parent gravity flow. By contrasting the differences between turbidite and HEB bed thickness statistics in a ponded setting, this study aims to improve understanding of HEB depositional processes.
The Castagnola Fm (Early Miocene, Tertiary Piedmont Basin, NW Italy) records the ponded to unconfined transition of a tectonically-controlled minibasin. Bed thickness analysis from turbidites allows recognition of a fully to partially ponded lower unit 650m thick. The ratio of sandstone thickness to event bed thickness (i.e. sandstone plus mudstone cap) for turbidites thicker than 30 cm ranges from 0.1 to 0.5 in the interval between 50 and 450m from the lower observed depocentre. Mudclast-rich hybrid event beds constitute around 10% of the basin fill thickness. Their bed thickness analysis reveals a number of key differences with turbidites: 1) the distribution of thicknesses for individual event beds is enriched in events between 1 and 3 metres thick and is depleted in thinner events; 2) the ratio of the sandstone thickness to event bed thickness is higher (e.g. 0.5 to 0.7 in the 50-450m interval). We infer that these differences stem from a lower parent flow magnitude threshold on HEB formation and incorporation of eroded muddy substrate and turbiditic mud into the central mud-rich division of HEBs, hence increasing the final sandstone bed thickness and decreasing that of its mudstone cap. The reported findings help improve our understanding of HEBs depositional processes (e.g., the loci of mud acquisition and deposition) and associated architectures; they can be applied to inform and interpret the results of bed thickness analysis in outcrop, core or wireline in HEB-prone confined basins.