Ephemeral rollover points and clinothem evolution in the modern Po Delta based on repeated bathymetric surveys

Abstract

Reconstructions of ancient delta systems rely typically on a two‐dimensional (2D) view of prograding clinothems but may miss their three‐dimensional (3D) stratigraphic complexity which can, instead, be best documented on modern delta systems by integrating high‐resolution geophysical data, historical cartography, core data and geomorphological reconstructions offshore. We quantitatively compare three precisely positioned, high‐resolution multi‐beam bathymetry maps in the delta front and pro delta sectors (0.3 to 10 m water depth) of Po di Pila, the most active of the modern Po Delta five branches. By investigating the detailed morphology of the prograding modern Po Delta, we shed new light on the mechanisms that control the topset to foreset transition in clinothems and show the temporal and spatial complexity of a delta and its pro delta slope, under the impact of oceanographic processes. This study documents the ephemeral nature of the rollover point at the transition between sandy topset (fluvial, delta plain to mouth‐bar) and muddy seaward‐dipping foreset deposits advancing, in this case, in >20 m of water depth. Three multibeam surveys, acquired between 2013 and 2016, document the complexity in space and time of the topset and foreset regions and their related morphology, a diagnostic feature that could not be appreciated using solely 2D, even very high‐resolution, seismic profiles. In addition, the comparison of bathymetric surveys gathered with one‐year lapses shows the migration of subaqueous sand dunes on the clinothem topset, the formation of ephemeral cut‐and‐fill features at the rollover point (few m below mean sea level), the presence of collapse depressions derived by sagging of sediments and fluid expulsion (possibly induced by storm waves) on the foreset, and splays of sand likely reflecting gravity flows on the lower foreset. Though the modern Po Delta is anthropogenic in many respects, its subaqueous clinothem can be studied as a scale model for ancient clinothems that are less resolved geometrically and far less constrained chronologically.

Natural Versus Anthropic Influence on North Adriatic Coast Detected by Geochemical Analyses

“…This trend seems in contrast with the decrease of sediment supply by the Po River recorded after 1945 (with a generalized phase of degradation and partial retreat of the entire Po River delta) caused by the construction of a dam [21,54], riverbed mining activities and the channelization of watercourses [55]. The most significant retreat phase, with rates in the order of tens of meters per year, reached its peak between 1954 and 1978 [56] when the E1 core recorded the highest Cr/Al 2 O 3 ratio ( Figure 7). The reduction in sediment supply, combined with the recent increase in storm events produced an increase of coastal erosion in the area [18][19][20]57].…”

Clinoforms and clinothems: Fundamental elements of basin infill

“…(b) Inset shows the case of changes in physiographic setting from single to compound clinoforms in subaerial delta clinoforms (e.g. Po River delta, Trincardi et al, 2019) | 197 EAGE PELLEGRINI Et aL. and 3-4 times shorter than wave-dominated clinothems of similar sandy sediment calibre, with mean gradients of ca 4° and ca 1°, respectively, and mean lengths of 150-230 m and 640-760 m. Similar relationships have been identified in the Early Cretaceous shelf margin of the North West Shelf (Australia) using seismic reflection data (Paumard et al, 2020).…”

“…These are the building blocks of many topsets and can be distinguished despite the otherwise tramline‐like seismic reflections of the shelfal parts of the topsets, particularly in their mid‐to‐outer reaches, where water depth can be greater and subsidence rate higher (Chen et al, 2019; Gamberi et al, 2019; Magyar et al, 2019; Steel et al, 2019; Zhang et al, 2019). In delta‐scale clinoforms, these cycles are commonly less easy to identify because of the ephemeral nature of elemental features constituting the clinoforms themselves (Trincardi et al, 2019), and because of the necessity of multidisciplinary approach in identifying key stratigraphic clinothem‐bounding surfaces (Amorosi et al, 2019; Playter et al, 2018).…”

“…The most typical examples of deltaic compound clinoforms show a genetically linked subaerial‐delta clinoform and a larger subaqueous‐delta clinoform as shown in Figure 1 (Cattaneo et al, 2003; Patruno et al, 2015; Pellegrini et al, 2015; Peng et al, 2018; Swenson et al, 2005). The occurrence of compound clinoforms seems to be associated with a period of highly efficient oceanographic currents, waves or tides leading to a thorough reshaping of sediment on the shelf, and giving rise to a de‐coupling of the subaqueous rollover point from its shoreline counterpart (Gamberi et al, 2019; Pellegrini et al, 2018; Trincardi et al, 2019). Compound clinoforms rarely occur also during still stands punctuating intervals of overall sea level rise (Pellegrini et al, 2015).…”

“…In this phase, up to five different genetically related clinoforms can be identified (note compound clinoforms in the subaerial delta; e.g. see Trincardi et al, 2019). All these clinoforms prograde outward at the same time, although with very different progradation rates.…”