Geological development of the Limpopo Shelf (southern Mozambique) during the last sealevel cycle

Abstract

Paleo-shorelines on continental shelves give insights into the complex development of coastlines during sealevel cycles. This study investigates the geologic development of the Limpopo Shelf during the last sealevel cycle using multichannel seismic and acoustic datasets acquired on the shelf in front of the Limpopo River mouth. A detailed investigation of seismic facies, shelf bathymetry, and a correlation to sea level revealed the presence of numerous submerged shorelines on the shelf. These shorelines are characterized by distinct topographic ridges and are interpreted as coastal dune ridges that formed in periods of intermittent sealevel still-/slowstand during transgression. The shorelines are preserved due to periods of rapid sealevel rise (melt water pulses) that led to the overstepping of the dune ridges as well as due to early cementation of accumulated sediments that increased the erosive resistance of the ridges. The high along-shelf variability of the submerged dune ridges is interpreted as a result of pre-existing topography affecting shoreline positions during transgression. The pre-existing topography is controlled by the underlying sedimentary deposits that are linked to varying fluvial sediment input at different points on the shelf. The numerous prominent submerged dune ridges form barriers for the modern fluvial sediment from the Limpopo River and dam sediment on the inner shelf. They may also facilitate along-shelf current-induced sediment transport.

Differentiating reefal ridges from relict coastal ridges: Lessons from the seismic geomorphologic study of buried Miocene buildups (North West Shelf, Australia)

“…Reefal ridges can, in some instances, be difficult to differentiate from drowned and/or buried coastal ridges, as both can form barriers that protect lagoons (Gardner et al, 2005; Figure 9). Examples of drowned non‐reefal ridges that form such structures on the present‐day seafloor are numerous (e.g., Alcántara‐Carrió et al, 2013; Brooke et al, 2010; De Falco et al, 2015; Lebrec et al, 2022a, 2022b; Mellett et al, 2012; Wenau et al, 2020), and they should be considered as possible analogues for seafloor ridges observed along continental shelves. Additionally, identification of corals or other reef‐building organism on the inside part of a ridge is not infallible evidence of a reefal origin, as loose corals or other sessile organisms can also be reworked to form ridges (e.g., Spiske, 2016), which may lead to their misinterpretation as a bioconstructed ridge.…”

“…Indeed, the front of Ridge 2 is overall transgressive, while it can be locally regressive (Figure 7a-d). The ridge also appears to be continuous over at least 80 km, except along the smaller stacked ridges (Figure 7g), which is typical of transgressive non-reefal barrier complexes protecting lagoons or tidal flats (Green et al, 2013;Otvos, 2012;Storms et al, 2008;Wenau et al, 2020). No platform reef morphologies or deep passes are observed along Ridge 2.…”

“…While it is becoming well documented that non‐reefal accumulations, such as stacked aeolianites and beachrocks, also have the ability to form bathymetric highs on the modern seafloor (e.g., Brooke et al, 2017; Bufarale et al, 2019; Green et al, 2020; Lebrec et al, 2022a, 2022b; O'Leary et al, 2020; Passos et al, 2019) and can misleadingly exhibit reefal morphologies in seismic‐reflection data (Bubb & Hatlelid, 1977; Salzmann et al, 2013), pre‐Quaternary carbonate aeolianites and other relict coastal features are rarely documented in the geologic literature (e.g., Abegg & Handford, 2001; Dodd et al, 2001; Kindler & Davaud, 2001; Loope & Abegg, 2001; McKee & Ward, 1983; Smith et al, 2001), and non‐reefal carbonate buildups are seldom described by seismic interpreters. This is particularly puzzling given the ability of drowned coastal features to exhibit buildup morphologies and to form both carbonate and siliciclastic barrier complexes—composed of beachrocks, aeolianites and other coastal sedimentary deposits preserved through early cementation—forming seafloor ridges enclosing lagoons, bays or estuaries (e.g., Alcántara‐Carrió et al, 2013; Brooke et al, 2010; De Falco et al, 2015; Gardner et al, 2007; Lebrec et al, 2022a; Locker et al, 1996; Mellett et al, 2012; Passos et al, 2019; Sade et al, 2006; Wenau et al, 2020). As an example, the islands of the Bahamas are largely formed by aeolianites (Carew & Mylroie, 2001; Nelson, 1853).…”

Antecedent geological control on transgressive delta and shoreline preservation: Examples from the SE African shelf

“…These ridges created a damming effect along their landward lee, restricting the basinward dispersal of fluvial sediment (e.g. Wenau et al, 2020). The landward flanks of the aeolianites at -105 m and -60 m coincide with the thickest accumulations of deltaic Units 5 and 6.…”

Fluid seepage on and offshore Western Greece

“…in multiple researches for detection and localization of sea floor seepage (e.g., Wenau et al, 2020;Geinert et al, 2010), the study of bubble characteristics and gas flow rates (e.g., Higgs et al, 2019), gas flux estimation (Leifer et al, 2017).…”