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Partial Ponding of Turbidity Currents, Castagnola Mini-Basin, Early Miocene, Northwest Italy

Abstract

Many deep-water settings are characterized by complex sea floor topography (e.g. salt-withdrawal mini-basins or structurally confined basins). Turbidity currents traversing such areas are affected by confinement, with associated changes to both flow behaviour and deposits. When some or all of the flow is trapped into a topographic low, partial or full flow ponding, respectively, occurs. While some processes are common to both partial and full ponding scenarios (e.g. flow deflection/reflection), full ponding results in distinct deposits (e.g. development of thick mud caps). The degree of ponding is controlled by a) basin geometry; b) range of flow volumes; c) flow structure (grain size range and concentration profile), influencing flow climbing capacity. This study aims to characterise the stratigraphic persistence of the depositional signature of partial ponding, contrasting it with that expected in fully ponded scenarios. The Castagnola sub-basin (Early Miocene, Tertiary Piedmont Basin, NW Italy) records the fill of a ponded minibasin, characterised by tabular deposits with no evidence of significant tectonic deformation during deposition. In the upper part of the basin fill a clear switch to unconfined conditions is recorded, marked by the disappearance of thick mud caps and sandstone beds with reflected facies. However, more than 400 m of the preserved 800 m basin fill records a continuous trend of slow increase in net/gross and decrease in mud cap to sandstone bed thickness, interpreted to reflecting different degrees of partial ponding. The significant thickness of deposits accreted under partially ponded conditions indicates that aggradation onto basin margins, including the Previous HitsillTop to the next basin, must have been ongoing synchronously with deposition in the basin. This is in agreement with the observed geometry of the southern margin, which is characterised by a feathered onlap with significant amount of sediment deposited above the confining topography. The unexpectedly high proportion of the stratigraphy laid down under partially ponded conditions has implication for classic fill-and-spill models and for techniques used in the subsurface to infer ponding conditions (such as bed thickness statistics). Understanding the factors controlling the degree of ponding, together with any associated diagnostic signatures in the resulting deposits, represents a key advance in subsurface predictive models for fill and spill minibasins.