Analysis and modeling of contourite drifts and contour currents off promontories in the Italian Seas (Mediterranean Sea)
Introduction
Contourites, i.e., marine sediments deposited and/or reworked by the persistent action of bottom currents, have been recognized in a variety of settings, ranging from the abyssal plain to the outer-shelf/upper slope (Stow et al., 2002, Rebesco et al., 2008). Over the years, a fairly good characterization of contourite deposits has been achieved and several typologies have been defined (Faugères et al., 1999, Rebesco and Stow, 2001, Stow et al., 2002). On a worldwide scale, elongated-mounded drifts represent one of the most common typologies. Among these drifts, separated drifts are characterized by a convex-upward profile, a moat, an elongation trend sub-parallel to bathymetric contour and an upslope migration, and reach their largest extent in oceanic settings (1 000–10 000 km2; Stow et al., 2002). The overall geometry and distribution of contourites depend on the balance between seafloor morphologies, amount and types of sediments, and current characteristics, which are produced by Coriolis force, pressure gradients and frictional terms (McCave and Tucholke, 1986). Meteorological characteristics can play a significant role in such a dynamic as well (Crepon and Richez, 1982, Faugères et al., 1999, Viana et al., 2007).
Elongated-mounded drifts are also significant deposits in the Mediterranean Sea (Marani et al., 1993, Velasco et al., 1996, Ercilla et al., 2002, Reeder et al., 2002, Roveri, 2002, Amelio and Martorelli, 2008, Verdicchio and Trincardi, 2008). Relative to their oceanic counterparts however, they are rather scattered, smaller in size, and usually located at shallow-intermediate water depths (e.g. Marani et al., 1993, Roveri, 2002, Verdicchio and Trincardi, 2008). These differences are mainly related to the young age and complex setting of such a semi-enclosed basin, which leads to irregular distribution and underdevelopment of contourites. In addition, ocean currents here are complex, because of the high variability of seafloor topography and to meteorological effects (Lascaratos, 1993, Pinardi and Masetti, 2000). Beyond these problematic features, quantitative studies that fully describe the interaction between contourites and bottom currents are still lacking. However, these characteristics make the Mediterranean Sea an outstanding area to observe the interaction among the different factors that control contourite development, as they change considerably over short distances.
Taking into account the strong morphological control on contourite deposition in the Mediterranean Sea, we focus here on the relationships between hydrodynamic bottom-current characteristics offshore capes and the presence of elongated drifts occurring up- and/or downstream the tip of a cape. The main aim of this work is to provide new insights into the depositional response of bottom currents flowing around a cape. This is done here by means of an integrated analysis of numerical and tank simulations (Yin et al., 2003, Doglioli et al., 2004, Magaldi et al., 2008), analytical modeling (Smith, 1975, Killworth, 1977), and the observed variability of contourite distribution. In general, we recognize that the zone of maximal velocities, and shear, is near the tip of a cape; just upstream and downstream the cape, low velocity zones favour deposition of sediments, even though turbulence can inhibit deposition of sediments in the lee region, downstream the cape. Selected examples to illustrate this behaviour are taken from contourite deposits observed off Cape Vaticano and the Gargano Promontory (Fig. 1).
Section snippets
Cape Vaticano offshore
Cape Vaticano (Fig. 1, Table 1) is located within the Calabrian Arc and is largely made up of Palaeozoic crystalline-metamorphic units. The Cape is bordered by normal faults that extend along the Tyrrhenian offshore and show recent activity (Tortrici et al., 2003). In this area, the continental shelf is nearly absent (Fabbri et al., 1980); the continental slope is steep and structurally controlled by the offshore projection of the Cape Vaticano morpho-structure. This Cape separates two
Observed variability of contourites off studied capes
Here we describe the main results of the morphological and seismo-stratigraphic characterizations of contourite deposits located at intermediate depths offshore Cape Vaticano and Gargano Promontory (Fig. 1). The main characteristics of these deposits, collected by the existing literature (Marani et al., 1993, Gamberi and Marani, 2006, Verdicchio and Trincardi, 2006, Verdicchio and Trincardi, 2008), provide essential information regarding the genetic role played by such capes (Table 1), rather
Modelling and tank experiments
Since direct current measurements off the studied capes are not often available, tank and numerical experiments as well as analytical approaches can be useful in gaining realistic insights as to contouring currents and resulting contourite deposits. By using such simulations, we analyze here velocities, suspended sediment distribution and turbulence, related to currents flowing around a cape in the Northern-Hemisphere.
Dimensionless numbers, often used in controlling such experiments, are
Discussion
The results described earlier may provide some insights into contour-current sediment deposition and suspension. For this purpose, some considerations can indeed be taken regarding dimensionless numbers Ref, Ek and Bu. For a very low Ref (i.e., large friction), high Ek (i.e., no Ekman layer), and a mild density stratification (Bu ≪ 1), currents around capes faithfully follow the bottom contours, generally favoring sediment deposition in both up- and downstream zones. In this case, current
Conclusions
The compilation of contourite deposits considered in this work shows that the occurrence of wide capes strongly determines the development of intermediate water contourites. Our analysis is restricted to the Italian offshore and even if we considered different sub-basins (i.e. the Adriatic and the Tyrrhenian Sea), we remark that our conclusions might not be directly extrapolated to the entire Mediterranean Sea, since other oceanographic processes may play a significant role.
Seismic reflection
Acknowledgements
The authors thank Prof. Roman Kanivetsky for the fruitful comments and J.C Faugères for criticism and suggestions.
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2019, Marine GeologyCitation Excerpt :Under certain circumstances, they can reach the deep ocean (e.g., Schmitz, 1984; Gordon and Greengrove, 1986; Carpenter and Timmermans, 2012; Chen et al., 2015), modulate deep ocean circulation (e.g., Piola and Matano, 2001; Adams and Mullineaux, 2011; Chiang and Qu, 2013; Castelao, 2014), contribute to forming nepheloid layers, trapping and transporting suspended sediments for a long distance (e.g., Washburn et al., 1993; Viana and Faugères, 1998; Zhang et al., 2014; Gardner et al., 2018), and affect deep-water sedimentary processes (e.g., Hollister and McCave, 1984; Hollister, 1993; Preu et al., 2011; Zhang et al., 2014; Breitzke et al., 2017). Numerical modelling on both regional and global scales has further indicated that the mesoscale eddy process is a major driver of bottom current fluctuations (Martorelli et al., 2010; Hanebuth et al., 2015; Zhang et al., 2016b; Gardner et al., 2018). Eddies may also impart certain control on the global distribution of contourites (Falcini et al., 2016; Thran et al., 2018).