Bed Thickness Statistics in Hybrid Event Bed Prone Ponded Minibasins, Castagnola System, Early Miocene, NW Italy

Abstract

Confined depocentres (e.g., salt withdrawal minibasins or structurally confined basins) are important depositional sinks for deep-water sediments. As such basins fill, successive turbidity currents may transition from being fully-, to partially-ponded and then to being effectively unconfined. An evolving ponding condition controls vertical changes in turbidite net-to-gross and may also control sedimentary architecture (e.g. sheets vs. compensational lobes), and therefore reservoir character. Statistical analysis of the distribution of sandstone thicknesses and the ratio between sandstones and their associated mudcaps can be used to characterise the degree of ponding of a confined basin. Hybrid event beds (HEBs) are bi- or tripartite event beds with a lower (turbiditic) sandstone division and an upper/central (debritic) mud-rich division and are commonplace in confined systems. This case study is the first to characterise the differences between turbidite and HEB bed thickness statistics in ponded settings. The Castagnola Fm (Early Miocene, Tertiary Piedmont Basin, NW Italy) records the tabular 800 m thick fill of a ponded minibasin. Bed thickness analysis from turbidites allows recognition of a fully ponded lower unit and a partially ponded upper unit with a continuously increasing trend of net/gross (e.g. 0.2 vs. 0.7) and decrease in mudcap to sandstone bed thickness (e.g. ∼2 vs. ≪1), as larger proportions of turbiditic mud escape the confining topography. Hybrid event beds constitute a significant proportion of the basin fill (up to 40% of the overall thickness in certain intervals). Their bed thickness analysis reveals a number of key differences with turbidites: 1) the shape of 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 event bed thickness to its associated mudstone cap is always higher (e.g. values of 2 vs. 0.5). We infer that these differences arise due to a lower threshold on the size of hybrid flows and incorporation of eroded muddy substrate and some of the turbiditic mud into the hybrid flows and hence into the HEB debritic division. The reported findings help improve our understanding of HEBs depositional processes 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.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016