Deep‐sea avulsion and morphosedimentary evolution of the Rhône Fan Valley and Neofan during the Late Quaternary (north‐western Mediterranean Sea)

Abstract

The Petit‐Rhône Fan Valley (north‐western Mediterranean) is a broad, sinuous, filled valley that is deeply incised by a narrow, sinuous thalweg. The valley fill is differentiated into three seismic subunits on high‐resolution seismic‐reflection profiles. The lower chaotic subunit probably consists of channel lag deposits that seem to be in lateral continuity with high‐amplitude reflections representing levee facies. The intermediate transparent subunit, which has an erosional base and clearly truncates levee deposits, is interpreted to be mass‐flow deposits resulting from the disintegration of the fan‐valley flanks. The upper bedded subunit shows an overall lens‐shaped geometry and the seismic reflections onlap either onto the top of the underlying transparent subunit or onto the Rhône levees. Piston core data show that the upper few meters of this upper subunit consist of thin turbidites, probably deposited by overflow processes. The few available 14C ages suggest that the upper stratified subunit filled the Petit‐Rhône Fan Valley between 21 and 11 kyr BP. The upper bedded subunit is deposited within the Petit‐Rhône Fan Valley downslope of a major decrease in slope gradient. This upper subunit and the thalweg are genetically related and represent a small channel/levee system confined within the fan valley. Previous studies interpreted this thalweg to be an erosional feature resulting from a recent avulsion of the major channel course. Our interpretation implies that the thalweg is not a purely erosional feature but a depositional/erosional channel. This small channel/levee system is superimposed on a large muddy channel/levee system after the sediment supply changed from thick muddy flows during the main phase of aggradation of the Rhône Fan levees, to thin, mixed (sand and mud) flows at the end of Isotope Stage 2 (∼16–18 ka BP). The pre‐existing morphology of the Petit‐Rhône Fan Valley played a determinant role in the sediment dispersal leading to the creation of this small and confined channel/levee system. These mixed flows have undergone flow stripping resulting from the changes in the slope gradient along the thalweg course. The finer sediment overflowed from the thalweg and were deposited in the Petit‐Rhône Fan Valley. Coarser channelled sediment remaining in the thalweg were deposited as a ‘sandy’lobe (Neofan). As indicated by 14C dating, sedimentation on this lobe continued until very recently, suggesting a further evolution of the turbidity flows from small mixed flows to small sandy flows. the deposition of this study lobe and the sedimentary fill of the Petit‐Rhône Fan Valley may be related to widespread shelf edge and canyon wall failures with a resulting downslope evolution of failed sediment into turbidity currents.

Integrating outcrop and subsurface data to assess the temporal evolution of a submarine channel–levee system

“…A 1-km-thick (4600-ft-thick) exhumed progradational basin floor to upper slope succession van der Merwe et al, 2014) crops out along a series of east-west-trending, eastward plunging, postdepositional anticlines and synclines, near the town of Laingsburg (Figures 1, 2). Deep-water deposition began with distal basin floor deposits of the Vischkuil Formation (van der Merwe et al, 2009Merwe et al, , 2010, which is overlain by basin floor and base-of-slope fan systems of the Laingsburg Formation (units A and B; Grecula et al, 2003;Sixsmith et al, 2004).…”

“…Deep-water deposition began with distal basin floor deposits of the Vischkuil Formation (van der Merwe et al, 2009Merwe et al, , 2010, which is overlain by basin floor and base-of-slope fan systems of the Laingsburg Formation (units A and B; Grecula et al, 2003;Sixsmith et al, 2004). The muddy slope succession of the 0.5-km-thick (1640-ft-thick) …”

“…These mudstone units constrain the stratigraphy and have been mapped for up to 90 km (56 mi) downdip (van der Merwe et al, 2014). In this study, the focus is on unit C, primarily exposed in the Baviaans syncline (Figure 1).…”

“…A long-term entry point in the southwestern corner of the Baviaans syncline is interpreted as due to the presence of channelized axes of units B, C, and D Hodgson et al, 2011;Brunt et al, 2013b;van der Merwe et al, 2014). The coeval shelf edge has been removed because of later uplift of the Cape fold belt so the mechanism for this longterm focus of supply is unknown.…”

“…The architecture and evolution of deep-water channel systems has been of particular interest in both the hydrocarbon industry and academic study in recent years with detailed investigations using highresolution reflection seismic data sets (e.g., McHargue and Webb, 1986;Badalini et al, 2000;Babonneau et al, 2002Babonneau et al, , 2004Abreu et al, 2003;Deptuck et al, 2003Deptuck et al, , 2007Posamentier, 2003;Posamentier and Kolla, 2003;Schwenk et al, 2005;Mayall et al, 2006;Kolla, 2007;Cross et al, 2009;Catterall et al, 2010;Armitage et al, 2012;Jobe et al, 2015;Ortiz-Karpf et al, 2015) and seabed imaging techniques (e.g., Torres et al, 1997;Maier et al, 2011Maier et al, , 2013Covault et al, 2014), although these provide limited detailed information on subseismic-scale elements and the range and distribution of sedimentary facies. This gap has been addressed through the use of analogous systems at outcrops (Badescu et al, 2000;Blikeng and Fugelli, 2000;Campion et al, 2000;Clark and Gardiner, 2000;Gardner et al, 2003;Beaubouef, 2004;Pickering and Corregidor, 2005;Hodgson et al, 2011;Brunt et al, 2013b;Hubbard et al, 2014;Masalimova et al, 2016).…”