Controls on turbidite sedimentation: Insights from a quantitative approach of submarine channel and lobe architecture (Late Quaternary Congo Fan)

Abstract

The role of internal and external forcing of sedimentation in turbidite systems remains a subject of debate. Here we propose new insights from the quantitative analysis of architectural parameters of the Congo Axial Fan.Fifty-two channel-levee-lobe systems, spanning the last ca. 200 ka, are visible on the seafloor, most of them having slightly elongated lobe complexes at their termination. Volumes of lobe complexes (usually 3 to 196 km3) are highly variable in time and space. The cumulative volume of the lobe complexes represents approximately 30% of the volume of the Axial Fan.The Axial Fan is sequentially divided into periods of increasing/decreasing channel lengths and basinward/landward migrations of avulsion points, representing successive prograding/retrograding architectural patterns called architectural cycles. These cycles are either symmetrical saw toothed and bell-shaped with progressive progradation and retrogradation phases, or asymmetrical, with long-lasting progradation phases and abrupt retrogradation phases that correspond to channel avulsions occurring high up on the fan.Our study points to the interplay between internal and external factors controlling the architecture of the Congo Axial Fan. The local topographic constraint is a major factor in the fan's stacking pattern. However, cyclic evolution of the architecture reveals major shifts in the deposition site that are linked to very upfan avulsion events. These events are interpreted to be driven by external factors (e.g. climate and/or eustatic sea-level change) that were able to drastically increase and/or coarsen the sediment supply to the fan. Highlights ► Exhaustive architectural evolution of the Congo turbidite system since 200 ka. ► Shifts of channellevee-lobe systems reveal cyclic patterns at different time scales. ► Dimensions of terminal lobe complexes do not exhibit cyclic evolution. ► Pointing to permanent internal control by inherited topography. ► Revealing possible exceptional periods of increased or coarsening Congo River inputs. At a system scale, for instance on the Amazon Fan, the chrono-stratigraphic framework provided by Leg 155 ODP drillings (Flood and Piper, 1997) delivered clues that uncovered a link between sedimentation and architectural evolution to climate and eustasy. Maslin et al. (2006) assumed that channel avulsions in the Amazon Fan could be triggered by pulses of sediment flux, and therefore be externally forced by factors such as sea level and/or climate fluctuations. Lopez (2001) also suggests that sea level variations influence the occurrence of the avulsion process, which is more frequent during periods of sea level lowering. Additionally, at the levee scale it was demonstrated that external forcing mechanisms such as sediment flux pulses or sea level fluctuations control the growth of the levees (Bonneau et al., 2014; Jorry et al., 2011; Toucanne et al., 2012) therefore potentially playing a role in triggering channel avulsions and thus influencing the distribution pattern of the channel-levee-lobe syste...

Quantification of the bed‐scale architecture of submarine depositional environments

“…proximal, medial, distal) and a confinement rating (0 -unconfined, 1 -semiconfined, 2 -confined) based on contextual information and the original author's interpretation (Table 1; also see Tőkés & Patacci, 2018). For lobe deposits, the term "effective confinement" (Brunt, McAffrey, & Kneller, 2004) was used and the following categories defined: an "unconfined lobe," where the incoming flow has no lateral or distal barriers and is able to freely expand (Jegou, Savoye, Pirmez, & Droz, 2008;Picot, Droz, Marsset, Dennielou, & Bez, 2016); a "semiconfined lobe" that has some degree of a lateral or frontal barrier, but the flow is able to freely expand in the other direction (Marini, Milli, Ravnås, & Moscatelli, 2015;Prather, Booth, Steffens, & Craig, 1998; the "frontally" or "laterally" confined lobes of Tőkés & Patacci, 2018); and a "confined lobe" that has barriers in all directions, which are able to fully contain the flow (Lamb, Toniolo, & Parker, 2006;Sylvester, Cantelli, & Pirmez, 2015; the "ponded lobe" of Tőkés & Patacci, 2018).…”

Volume and recurrence of submarine‐fan‐building turbidity currents

“…Specifically, “lobe” 2 has T P50 = 1.5 kyr, “lobe” 5 has T P50 = 2.5 kyr, and “lobe” 6 has T P50 = 0.9 kyr (Figure ). These lobe duration estimates compare reasonably to the Holocene Amazon (15 to 20 lobes in 10 kyr, Jegou et al., ) and Zaire (38 to 52 lobes in 210 kyr, Picot et al., ) submarine fan deposits (vertical gray bars in Figure ). The hierarchically larger package (“lobe complex” of Prélat et al., often informally referred to as “Fan 3”) consists of six “lobes”; if minimum and maximum values from Figure are used to estimate T for the lobe complex, the range of T for “Fan 3” is 1.6 to 68 kyr, with median values ranging from 5 to 15 kyr.…”

“…Submarine fans exhibit radial or cone-like morphologies in plan view and are composite features, consisting of channel and lobe deposits (Bouma et al, 1985;Dill et al, 1954;Heezen et al, 1959;Menard, 1955;Shepard & Emery, 1941). Submarine fans form in net-depositional environments of continental-margin sediment-routing systems, commonly associated with slope breaks that promote sudden deceleration of turbidity currents and localized deposition of sand beyond the slope break (Adeogba et al, 2005;Fernandez et al, 2014;Jobe et al, 2017b;Mutti & Normark, 1987;Picot et al, 2016;Spinewine et al, 2009). The focus of this paper is on the stratigraphic record of submarine fans, as their deposits contain a more complete record of turbidity-current volume and recurrence (Piper & Normark, 1983) as compared to channelized elements that are primarily conduits for turbidity-current bypass (Hubbard et al, 2014;Stevenson et al, 2015).…”

Quantification of the Bed-scale Architecture of Submarine Depositional Environments and Application to Lobe Deposits of the Point Loma Formation, California

“…For lobe deposits, we use 'effective confinement' (Brunt et al, 2004) and define categories as the following: an 'unconfined lobe' as the incoming flow has no lateral or distal barriers and is able to freely expand (e.g., Jegou et al, 2008;Picot et al, 2016); a 'semiconfined lobe' has some degree of a lateral or frontal barrier, but the flow is able to freely expand distally or laterally, respectively (e.g., Prather et al, 1998;Marini et al, 2015; the "frontally or laterally confined 6 lobes" of Tőkés and Pattacci, in press); a 'confined lobe' has barriers in all directions, which are able to fully contain the flow (e.g., Lamb et al, 2006;Sylvester et al, 2015; the "ponded lobe" of Tőkés and Pattacci, in press). …”

“…An unconfined lobe was defined as an incoming flow that had no lateral or distal topographic barriers, which allows for the flow to fully expand and for the mud fraction to be transported to the most distal reaches of the lobe regardless of basin shape or size (Damuth and Flood, 1983;Picot et al, 2016). In contrast, semiconfined and confined lobes have differing spatial distributions of sandstone and mudstone due to the presence of topographic barriers (e.g., Prather et al, 1998).…”