The stratigraphic evolution of onlap in siliciclastic deep-water systems: Autogenic modulation of allogenic signals

Abstract

Seafloor topography affects the sediment gravity flows that interact with it. Understanding this interaction is critical for accurate predictions of sediment distribution, paleogeography, and structural reconstructions of deep-water basins. The effects of seafloor topography can be seen from the bed scale, through facies transitions toward intra-basinal slopes, to the basin scale, where onlap patterns reveal the spatial evolution of deep-water systems. Basin-margin onlap patterns are typically attributed to allogenic factors, such as sediment supply signals or subsidence rates, with few studies emphasizing the importance of predictable spatio-temporal autogenic flow evolution. This study aims to assess the autogenic controls on onlap by documenting onlap styles in the confined Eocene-to-Oligocene deep-marine Annot Basin of SE France. Measured sections, coupled with architectural observations, mapping, and paleogeographical interpretations, are used to categorize onlap styles and place them within a generic stratigraphic model. These observations are compared with a simple numerical model. The integrated stratigraphic model predicts that during progradation of a deep-water system into a confined basin successive onlap terminations will be partially controlled by the effect of increasing flow concentration. Initially thin-bedded low-density turbidites of the distal lobe fringe are deposited and drape basinal topography. As the system progrades these beds become overlain by hybrid beds and other deposits of higher-concentration flows developed in the proximal lobe fringe. This transition is therefore marked by intra-formational onlap against the underlying and lower-concentration lobe fringe that drapes the topography. Continued progradation results in deposition of lower-concentration deposits in the lobe off-axis, resulting in either further intra-formational onlap against the lobe fringe or onlap directly against the hemipelagic basin margin. Basinal relief is gradually reduced as axial and higher-volume flows become more prevalent during progradation, causing the basin to become a bypass zone for sediment routed down-dip. This study presents an autogenic mechanism for generating complex onlap trends without the need to invoke allogenic processes. This has implications for sequence-stratigraphic interpretations, basin subsidence history, and forward modeling of confined deep-water basins.

Deformation–sedimentation feedback and the development of anomalously thick aggradational turbidite lobes: Outcrop and subsurface examples from the Hikurangi Margin, New Zealand

“…15D). Similar effects have been documented in other confined deep-marine settings (e.g., Haughton, 2000;Soutter et al, 2019). The MTD emplacement across the basin may have been related to a wider modification of the margin structure and could also have blocked or diverted the sediment supply or otherwise destabilised the balance of sedimentation and deformation (e.g., Bull et al, 2020).…”

“…Understanding the interaction between autogenic and allogenic controls is a current research focus in sedimentology (Burgess et al, 2019). Autogenic or inherent controls are those within the system, such as flow process or local structure development (e.g., Covault et al, 2007;Picot et al, 2016), whilst allogenic controls are external to the sedimentary system, such as sediment supply or regional tectonics (e.g., Prelat and Hodgson 2013;Soutter et al, 2019). The geomorphology resulting from structural deformation influences accommodation and whether a deep-marine sedimentary system is fully confined (e.g., Sinclair and Tomasso 2002;Marini et al, 2015Marini et al, , 2016a, partially confined, with potential for downstream bypass (e.g., Deptuck et al, 2008;Casciano et al, 2019;McArthur and McCaffrey 2019), or unconfined, with unrestricted run-out of flows (e.g., Normark, 1978;Normark et al, 1983;Clift et al, 2002;Jegou et al, 2008;Savoye et al, 2009).…”

“…When confinement is tectonically controlled, it is thought that interaction of successive turbidity currents with evolving seafloor structures may result in the development of more complex deposits in terms of thickness and co-deposition of different grain size classes (e.g., Sinclair and Tomasso, 2002;Brunt et al, 2004, Noda, 2018Butler, 2019;Soutter et al, 2019). However, the sparsity of both outcrop and subsurface studies of such confined systems limits our understanding of the interplay between sedimentary and tectonic processes and the effects upon the resulting stratigraphic architecture.…”

Halokinetic modulation of sedimentary thickness and architecture: a numerical modelling approach

“…In the syn-kinematic stratigraphy, thick-bedded sandstones deposited in channels and lobes dominate topographic lows, where sedimentary flows were around salt-cored topographic relief ( Figure 12D; Mayall et al 2010;Sylvester et al 2012;Doughty-Jones et al 2017;Rodriguez et al 2020). Towards the highs, the lower-density part of the flows may run-up topography depositing thinlybedded muddier sandstones towards the pinchout ( Figure 12E; Kneller and McCaffrey 1999;Soutter et al 2019). Crestal deposition is limited due to elevation, until sedimentation period 3 (S3) where halokinetic bathymetry is healed.…”

“…thin layers are still influenced by topography associated with rising salt(Giles and Lawton 2002; Hudec and Jackson 2007; Giles and Rowan 2012;Sylvester et al 2015;Soutter et al 2019).Overall, halokinetic modification reduces with increasing sedimentation rate as halokinetic bathymetry is buried. Typically, such alteration decreases up stratigraphy, and laterally outwards from the diapir in agreement with outcrop and subsurface analogues globally(Figure 9, 10, 11; e.g Pratson and Ryan 1994; .…”