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 and paleogeographic or 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.

Evolution of a mixed siliciclastic-carbonate deep-marine system on an unstable margin: the Cretaceous of the Eastern Greater Caucasus, Azerbaijan

“…the up-dip conglomerates) to turbulent flows following the entrainment of ambient water (Potsma 1988;Haughton et al 2009), which punctuate slowly aggrading calcareous turbidites, interpreted to represent the remnants of dilute flows (Remacha & Fernández, 2003). The preservation of both structured and structureless sandstones suggests an off-axis location of deposition; similar preservation of both deposit types has been interpreted in the proximal lobe fringe elsewhere (Prélat et al 2009;Spychala et al 2017;Soutter et al 2019). FA 6 is differentiated from FA 5 based on its thinner beds and less frequent erosional events, and is therefore interpreted as being more distal and deposited within the proximal fringe.…”

“…10) (Al-Mashaikie & Mohammed, 2017;Chiarella et al 2017;Walker et al 2019). Structureless medium-bedded calcareous siltstones and sandstones are interpreted to represent deposition from medium density turbidity currents (Kneller & Branney 1995;Talling et al 2012;Soutter et al 2019) aggrading quickly enough to prevent tractional sedimentary structure development in their basal divisions (Kneller & Branney 1995;Sumner et al 2008). This depositional process is complicated within the calcareous medium-bedded deposits, which appear to have aggraded much more slowly than their siliciclastic counterparts, as evidenced by thinbedded and medium-grained siliciclastic beds being deposited within medium-bedded and finegrained calcareous beds.…”

“…It may also be possible that transitions from calcareous-dominated to siliciclastic-dominated deep-marine systems, which are commonly associated with the rapid arrival (progradation) of the siliciclastic system (e.g. Scott et al 2010;Kilhams et al 2012;Soutter et al 2019;Cumberpatch et al in prep. ), may have been overlooked as 'transition zones', and in fact represent short-lived mixed systems, which are often below the scale of seismic resolution.…”

“…These beds are interpreted as medium-density turbidites due to their bed thickness and common lack of structures in the lower part of the bed (e.g. Soutter et al 2019).…”

Evolution of a sand-rich submarine channel-lobe system and impact of mass-transport and transitional flow deposits on reservoir heterogeneity: Magnus Field, northern North Sea

“…Hodgson 2009;Fonnesu et al 2015;Spychala et al 2017b) and their occurrence in confined basins (e.g. Haughton et al 2009;Fonnesu et al 2018;Soutter et al 2019). Reservoir quality studies of transitional flow deposit and their impact on subsurface fluid flow are more limited (see Amy et al 2009;Porten et al 2016;Southern et al 2017).…”