Vertical grain-size variability within a turbidite levee: Autocyclicity or allocyclicity? A case study from the Rhône neofan, Gulf of Lions, Western Mediterranean

Abstract

The fining upward trend commonly described on levees of turbidite systems can be either attributed to changes in the sediment supply (external forcing = allocyclic) or changes in the spillover processes related to the growth of the levee (internal forcing = autocyclic). However the real causes remain speculative and difficult to demonstrate. Knowledge of vertical change in flow velocity (or grain size) in turbidity currents, of the turbidity current's height, as well as the growth rate of a levee are theoretically sufficient to describe the evolution of the grain size of a leveed turbidite sequence deposited by spillover processes.A piston core (MD99-2344) retrieved on the right levee of the Petit-Rhône neofan, sampled an 8.50 m long turbidite sequence containing more than one hundred turbidite beds and showing a marked fining upward trend. With the help of two profiles describing respectively the vertical velocity [Stacey, M. and Bowen, A.J., 1988b. The vertical structure of turbidity currents and a necessary condition for self-maintenance, Journal of Geophysical Research, pp. 3543-3553.] and the vertical grain size [Migeon, S., 2000. Dunes géantes et levées sédimentaires en domaine marin profond: approche morphologique, sismique et sédimentologique. PhD Thesis, Université Bordeaux 1, Talence, France, 288 pp.] of a turbidity current, and a simple graphical method, we test several hypotheses on the height of the turbidity current and on the grain size at the bottom of the turbidity current and we attempt to reproduce the vertical fining upward trend.Our results show that the fining upward trend of the turbidite facies can be explained by the aggradation of the turbidite levee and the gradual confinement of the turbidity currents in the channel. This is, therefore, an autocyclical phenomenon. However a high frequency variability of the grain size overprinted on the trend cannot be explained by internal forcing and is believed to be allocyclical, depending on changes to the sediment input. The vertical grain-size profile allows more realistic flow conditions to be determined and shows that the vertical velocity profile does not describe the ability of turbidity current to transport sediment. This simple graphical method allows a better understanding and quantification of the forcing on turbidite deposits and improves description of the characteristics of the turbidity currents from the grain size of the deposits.

Post-glacial persistence of turbiditic activity within the Rhône deep-sea turbidite system (Gulf of Lions, Western Mediterranean): Linking the outer shelf and the basin sedimentary records

“…Coarse-grained and silt-sand deposits have already been reported in the study area (Méar and Gensous, 1993;Beaudouin et al, 2004;Bonnel et al, 2005;Dennielou et al, 2006) and interpreted as originating from the PRC and its prolongation as a turbiditic channel. Dispersal of spillover flows from the turbiditic neochannel is believed to be the main pathway until it ended around 15 000 14 C yr BP (low stand and initial stages of post-glacial) (Bonnel et al, 2005;Dennielou et al, 2006).…”

“…Open circles indicate sediment cores. from the neochannel have stopped (Beaudouin et al, 2004;Bonnel et al, 2005;Dennielou et al, 2006). 12 kHz (EM12) and 30 kHz (EM300) multi-beam backscatter data, from previous studies (Droz, 1983;Bonnel et al, 2005) revealed two lobe-shaped adjacent areas at the outlet of the Sète canyon network and at the outlet of the La Fonera Canyon (LFC), with respectively heterogeneous low backscatter and lineated moderate backscatter (Droz et al, 2001;Bonnel et al, 2005) (Fig.…”

“…Near the neochannel it is attributed to the Neofan Transparent Unit of Bonnel et al (2005). More details on this facies are found in Dennielou et al (2006). Facies 4 corresponds to silt-sand turbidites corresponding to Td term base cut out and top cut out Bouma sequences.…”

“…3). The source of sediment is believed to be the Rhône River (Beaudouin et al, 2004;Bonnel et al, 2005;Dennielou et al, 2006) but heavy minerals indicate an exclusive Languedoc-Pyrenees source mainly carried by the abundance of epidote and andalusite. Although important mixing from Languedocian and Pyrenean rivers cannot be discarded, the sieving at 50 µm has preferentially extracted a minor coarse fraction from this dominantly silty sample, and may have oversampled andalusite (Languedoc-Pyrenees source), that has a positive correlation with the grain-size and that is documented as rare or absent from the fine fractions (Duplaix, 1972), and under-sampled the aegirine-augite (Alps source), that has a negative correlation with the samples grain-size.…”

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

“…However, the sands are only deposited by flows that are suspending sand grains higher than the active levee crest, whereas muds are suspended above the levee crest during nearly every flow. Therefore, sandstone bed thickness is indicative of a single flow event, whereas the mud event bed boundaries are likely indistinguishable (Dennielou et al, 2006), producing higher characteristic mudstone bed thickness. The higher mudstone-thinning rate is due to the high degree of thin bedded sandstone beds lumping into a single mud bed, inability for complete sandstone correlation, and indistinguishable mudstone bed boundaries within the original panel creating a rapid mudstone bed thickness change and increased thinning rate.…”