Anatomy of a mixed bioclastic–siliciclastic regressive tidal sand ridge: Facies‐based case study from the lower Pleistocene Siderno Strait, southern Italy

Abstract

Sand ridges, a common feature of modern open shelves, reflect persistent currents and sediment availability under recent transgressive conditions. They represent the largest bedforms in the oceans and, as such, can yield information on long-term oceanographic processes. However, there is a limited number of tidal sand ridges documented from the rock record, examples of regressive tidal sand ridges are scarce and studies describing ridges in straits are even more rare. This study analyses a Gelasian succession within a structurally controlled, tide-dominated strait in the Siderno Basin, southern Italy. The strait connected two wider basins, and accumulated sediments reworked by amplified tidal (bi-directional) currents. A series of tidal sand ridges with superimposed dunes developed close to the south-eastern end of the strait, where bathymetry was deeper and flow expansion occurred. One of the best-exposed tidal sand ridges, 65 m thick, crops out along a ca 2 km long cliff. Large-scale, ESE-prograding, seaward-offlapping shingles contain sets of bioclastic-siliciclastic, coarse-grained, cross-stratified sandstones, erosionally overlying upper Pliocene shelf marls and fine-grained sandstones. Cross-strata show angular, tangential and sigmoidal foresets with compound architectures and a SSE migration, i.e. oblique to the main growth direction.Fossil content indicates open-marine conditions. The succession changes abruptly across an erosion surface to non-tidal, highly burrowed mixed siliciclastic-bioclastic fine-grained sandstones, less than 15 m thick. Documented features reflect stages of nucleation, active accretion and abandonment of an individual sand ridge, during a complete cycle of relative sea-level change. The ridge formed during a phase of normal regression, with accretion occurring during an initial highstand and the ensuing falling stage.

Dynamics and stratigraphy of a tidal sand ridge in the Bristol Channel (Nash Sands banner bank) from repeated high‐resolution multibeam echo‐sounder surveys

“…The processes operating during the lifetimes of tidal ridges and bars and their ultimate fates have been addressed through a combination of active monitoring (for example, time‐lapse bathymetric data, as here), sampling and geophysical surveys (Berné et al ., 1994, 1998; Fenies & Taslet, 1998; Reynaud et al ., 1999; Trentesaux et al ., 1999; Vecchi et al ., 2013; Franzetti et al ., 2015; Lockhart et al ., 2018), hydrodynamic modelling (De Vriend, 1990; Hulscher et al ., 1993; van Veelen et al ., 2018), and analyses of ancient examples preserved in cored subsurface sections (Folkestad and Satur, 2008; Schwarz et al ., 2011; Messina et al ., 2014; Wei et al ., 2018; Chiarella et al ., 2020) and in the rock record (Tillman & Martinsen, 1984; Gaynor & Swift, 1988; Mellere & Steel, 1995; Martinsen et al ., 1999; Yoshida, 2000; Plink‐Björklund & Steel, 2006; Plink‐Björklund, 2008; Steel et al ., 2008, 2012; Pontén & Plink‐Björklund, 2009; Hampson, 2010; Michaud, 2011; Martinius, 2012; Olariu et al ., 2012b; Scasso et al ., 2012; Chen et al ., 2014; López et al ., 2016; Michaud & Dalrymple, 2016; Sharafi et al ., 2016; Leszczyński & Nemec, 2020; Longhitano et al ., 2021). The latter potentially provide the finest spatial information but, before assessing it, confidence needs to be established that tidal ridges or bars have been correctly identified.…”

“…13‐31), likely also produced by accretion on one side of a swatchway. Channels crossing tidal‐strait sand ridges in south Italian outcrops have also been interpreted as swatchways (Longhitano et al ., 2021). The regions between the 10 m contours in East and West Nash away from the swatchway were more stable.…”

“…Willis (2005) also suggested that changes in sediment supply to tidal bars could lead to abandonment and Suter (2006) mentioned rapid sea‐level rise as potentially leading to preservation of tidal ridges, an effect implicit in the rapid subsidence of Michaud & Dalrymple (2016). In contrast, some researchers have described tidal‐strait sand ridges preserved by tectonic uplift and hence relative sea‐level fall (Leszczyński & Nemec, 2020; Longhitano et al ., 2021).…”

“…In the rock record, tidal ridge or bar deposits can be difficult to identify, although a number have been proposed (Tillman & Martinsen, 1984; Gaynor & Swift, 1988; Mellere & Steel, 1995; Martinsen et al ., 1999; Yoshida, 2000; Plink‐Björklund & Steel, 2006; Plink‐Björklund, 2008; Steel et al ., 2008, 2012; Pontén & Plink‐Björklund, 2009; Hampson, 2010; Michaud, 2011; Martinius, 2012; Olariu et al ., 2012b; Scasso et al ., 2012; Chen et al ., 2014; López et al ., 2016; Michaud & Dalrymple, 2016; Sharafi et al ., 2016; Longhitano et al ., 2021). In particular, there has been some debate concerning how tidal ridges can be discriminated from other tidally‐influenced deposits.…”

Early Pliocene Marine Transgression into the Lower Colorado River Valley, Southwestern USA, by Re-Flooding of a Former Tidal Strait

“…A growing body of literature shows that marine straits and seaways host a wide range of sedimentary processes, environments, and depositional products (Longhitano, 2013;Longhitano et al, 2014;Rossi et al, 2017;Longhitano and Steel, 2017;Telesca et al, 2020;de Weger et al, 2020;Longhitano et al, 2021). Little is known, however, about the processes by which fault-controlled marine connections influence the initiation and early development of large river systems.…”