Threshold of erosion of submarine bedrock landscapes by tidal currents

Abstract

Since sea level stabilized 7000 yr BP, shelf seas experiencing semi-diurnal tides will have been affected by streaming four times per day. If tidal erosion of bedrock were even only marginally efficient, the~10 million streamings since then should have left geomorphological imprints. We examine high-resolution multibeam sonar data from three areas with extreme tidal currents. The Minas Passage (Bay of Fundy) experiencing 8-knot surface tidal currents was surveyed in 2007 with a multibeam sonar. In an area near to transverse dunes, which are evidence for bedload transport, the data show local overhanging surfaces near to the sediment-rock contact, potentially created by abrasion by saltating particles. However, they are uncommon. In the Straits of Messina, where surface currents reach 10 knots, surveying revealed ridges lying oblique to the flow that are not obviously broken into separate outcrops by erosion. In the Bristol Channel, UK, sonar data collected where currents reach 3Á4 knots at 1Á5 m above the bed reveal outcrops of limestone with superimposed sand dunes, but only minor rounding of blocks.Holocene tidal currents have apparently been generally ineffective at eroding bedrock. We examine this issue further by compiling extreme tidal streams around the UK and from them estimate shear stresses, representing a macro-tidal environment where peak surface currents reach 9Á7 knots. Those data are compared with shear stresses in mountainous rivers where long-term rates of erosion are comparable with tectonic uplift rates and are thus geomorphologically significant. Whereas river stresses reach 10 2 -10 3 Pa, the largest tidal stresses are generally 10 1 and only rarely approach 10 2 Pa, too small for quarrying to operate generally. However, the vertical faces in the Minas Passage may represent the onset of abrasion. Given this limited evidence for abrasion, we explore conditions in the geological past for tides that may have locally eroded bedrock.

Streamlined islands and the English Channel megaflood hypothesis

“…Northern Palaeovalley due to tidal scour, as this was shown to have minor erosive potential in neighbouring 451 areas (Mitchell et al, 2013). The study area also shows preservation of subtle features such as the 452 secondary drainages which again points to tidal scour being ineffective at eroding bedrock.…”

Interannual Variability of Two Offshore Sand Banks in a Region of Extreme Tidal Range

“…Rapid sand transport across the Helwick Bank crest, in the form of reattaching dune crests, has been observed (Schmitt and Mitchell, 2014), which indicates storm waves could have resulted in our observations of a 'flattening sand bank crest'. Furthermore, work by Mitchell et al (2012) indicates sand dune trains overlying bedrock have been observed at the nearshore termination of Helwick Bank, which may be the outcome of an erosion phase. Therefore, it appears that storm wave events were responsible for the observed interannual changes to Helwick Bank.…”

“…A further two time-varying water-level scenarios of a flooding (MLWS to MHWS) and ebbing spring tide (MHWS to MLWS) were also simulated; see Table 3. Observations at Helwick Bank indicate highly mobile sand dunes with a grain size of 400 lm (Mitchell et al, 2012;Schmitt and Mitchell, 2014;Schmitt, Mitchell, and Ramsey, 2008), which, during times of extreme storm waves, could become washed out (i.e. flat bed).…”

“…Shorter timescale processes that affect sand banks include storm wave events and semidiurnal tidal currents (Collins et al, 1995;Neill, Hashemi, and Elliott, 2007;Whitmeyer and FitzGerald, 2008). During storms, near-bed wave orbital velocities (Uw) from waves can greatly exceed the critical velocity of sediment motion (UW crit ), even in water depths of tens of metres (Mitchell et al, 2012). This short-term (relative to the action of tidal currents) wave-induced sediment transport (e.g., Komar and Miller, 1974;Soulsby, 1997) can affect the evolution and maintenance of an offshore sand bank (Giardino, Van den Eynde, and Monbaliu, 2010;Van de Meene and Van Rijn, 2000).…”

Red Sea isolation history suggested by Plio-Pleistocene seismic reflection sequences

“…However, most geological processes are unlikely to reverse within a few ky; we know of no examples of normal fault movements reversing over such timescales in extensional environments such as that considered here. Furthermore, currents measured over Hanish Sill are ∼1 ms −1 (Pratt et al, 1999), which is too small to cause erosion of bedrock (Mitchell et al, 2012), so any emplaced volcanic materials are unlikely to been fully removed by currents. We therefore prefer to consider the short-period deviations from the other sealevel models in Fig.…”