The influence of basin setting and turbidity current properties on the dimensions of submarine lobe elements

Abstract

Submarine lobes have been identified within various deep-water settings, including the basin-floor, the base of slope and the continental slope. Their dimensions and geometries are postulated to be controlled by the topographic configuration of the seabed, sediment supply system and slope gradient. Ten experiments were conducted in a three-dimensional-flume to study the depositional characteristics of submarine lobes associated with: (i) different basin floor gradients (0 to 4°); (ii) different sediment concentrations of the parent turbidity current (11 to 19% vol); and (iii) varying discharge (25 to 40 m 3 h À1). Most runs produced lobate deposits that onlapped onto the lower slope. Deposit length was proportional to basin-floor angle and sediment volume concentration. A higher amount of bypass is observed in the proximal area as the basin-floor angles get steeper and sediment concentrations higher. Deposits of runs with lower discharge could be traced higher upslope while runs with higher discharge produced an area of low deposition behind the channel mouth, i.e. discharge controlled whether lobe deposits were attached or detached from their channel-lev ee systems. A particle-advection-length scale analysis suggests that this approach can be used as a first order estimation of lobe element length. However, the estimations strongly depend on the average grain size used for calculations (for example, silt is still actively transported after all sand has been deposited) and the method cannot be used to locate the main depocentre. Furthermore, attempted reconstructions of turbidity current velocities from natural systems suggest that the method is not appropriate for use in inversions from more complex composite bodies such as lobes.

Flow‐process controls on grain type distribution in an experimental turbidity current deposit: Implications for detrital signal preservation and microplastic distribution in submarine fans

“…The deposit widened from 80 cm at the base‐of‐slope, to 200 cm at a distance of 280 cm from the slope break. Between distances of 80 and 160 cm from the slope break there is a 50 cm wide anomalously thin zone ( ca 1 cm thinner than the surrounding deposits), similar to convex indents described by Spychala et al (2020), interpreted to represent a bypass‐dominated area in a channel to BFD transition zone (Figures 3 and 5). At the centroid, the axis of the deposit was >5 cm thick for a width of ca 80 cm.…”

“…Alexander et al (2008) showed that slope gradient was a primary control on the development of a lateral ridge with flows able to maintain forward momentum upon exiting the channel. Similarly, Pohl et al (2020a) document that the slope gradient is a stronger control on sediment bypass and depositional patterns at break‐of‐slope compared to basin floor gradient (see also: Spychala et al, 2020). Therefore, systems with steeper channel gradients, at least immediately up‐dip of lobes, may be more probable to develop finger‐like geometries due to focused axial flow components.…”

“…This suggests that microplastic fragments are likely probably strongly fractionated into mudstone caps (i.e. Bouma, 1962 T e division), or bypass to the siltstone fringe, sensu Spychala et al (2020), and are spread over vast expanses of the sea floor. As mudstone caps were not studied, this unknown is represented by question marks.…”

“…The experiment was carried out using the Eurotank flume at Utrecht University, which has dimensions of 11 m × 6 m × 1.2 m (Figure 2). The design of the basin followed Spychala et al (2020). From proximal to distal, the floor of the tank consisted of (a) an inlet pipe which discharged the sediment mixtures; (b) a channel 8 cm deep, 80 cm wide and 3 m long with sculpted levees on a slope dipping at 11°; (c) a 4 m long slope dipping at 4° and (d) a 4 m long, flat basin floor (Figure 2).…”

Fringe or background: Characterizing deep-water mudstones beyond the basin-floor fan sandstone pinchout

“…14). These results suggest that submarine lobes may be much larger than previously estimated, as also suggested by recent flume tank experiments (Spychala et al 2020). This opens the possibility of identifying (and predicting) the presence or absence of genetically linked deep-water basin-floor sandstone accumulations by the careful examination of deep-water mudstones along basin margin successions.…”