Depositional architecture of sand-attached and sand-detached channel-lobe transition zones on an exhumed stepped slope mapped over a 2500 km2 area

Abstract

The geomorphology and seismic stratigraphy of deep-water clastic systems from slope valleys through channel-levee systems to basin-fl oor fans have been observed and described in modern and ancient sub surface examples around the world. However, the distribution of sedimentary facies, grain size, and small-scale architectural elements remains poorly constrained. Extensive exposures (>2500 km 2 ) of four stacked deep-water composite sequences have been mapped from heterolithic channel-levee systems on the slope to sand-rich basin-fl oor deposits. The data set from Units C-F of the Fort Brown Formation in the Permian Laingsburg depocenter of South Africa permits a unique opportunity to document and compare their depositional architecture at a high resolution for tens of kilometers downdip.Isopach thickness maps indicate that compensational stacking across multiple stratigraphic scales occurs on the basin fl oor, whereas preferred axial pathways were present on the slope, leading to subvertical stacking patterns. Units C and D are sand-attached systems; slope valley systems are mapped to pass transitionally downslope through leveeconfi ned channels to lobe complexes over distances of >30 km. The slope valley fi lls of Units E and F, however, are separated from their downdip sand-rich lobe complexes by a thin, sand-poor tract several kilometers in length and are termed sand detached. Locally, this sand-poor tract is characterized by a distinctive facies association of thin-bedded turbidites with numerous scours mantled with rip-up clasts, and a top surface that includes megafl utes and remobilized sediments. This assemblage is interpreted to indicate a widespread area of sand bypass.This unique data set provides an exploration-scale insight and understanding of how different segments of a prograding slope evolved over time in terms of gradient, physiography, and hence the degree to which sand was stored or bypassed to the basin fl oor, and the evolution from sand-attached to sand-detached systems. The development of sand-detached systems suggests that a steeper gradient formed, possibly related to developing underlying structure, that led to the development of a stepped slope profi le. The study highlights that updip stratigraphic trapping at reservoir scale can occur with minor bathymetric changes.Gold Open Access: This paper is published under the terms of the CC-BY license.on December 9, 2014 geosphere.gsapubs.org Downloaded from Sand-attached and sand-detached slope to basin-fl oor systems on a stepped slope profi le Geosphere,

Palinspastic restoration of an exhumed deepwater system: A workflow to improve paleogeographic reconstructions

“…Overall, sediment gravity flows entered from the south-west with net sediment dispersal direction to the north-east Morris et al 2014). Locally, subtle basin bathymetry influenced the flows and modified their behaviour (Grecula et al, 2003;Sixsmith et al 2004;van der Merwe et al 2014;Spychala et al, 2015).…”

“…The extensive exposures of turbidite packages in the Fort Brown Formation have been mapped, from entrenched slope valleys, through channel-levee systems and intraslope lobe complexes to lobe complexes on the basin-floor, over a 2500km 2 area (van der Merwe et al, 2014).…”

“…Grecula et al, 2003, Sixsmith et al, 2004, Hodgson et al, 2011aFlint et al, 2011;Di Celma et al, 2011;Brunt et al, 2013a, b;Morris et al, Interpretation 5 2014). Detailed studies have culminated in the correlation of stacked deep water composite sequences across the Karoo Basin over a >2500km 2 area, allowing high resolution study of the evolution of a basin through the stages of its fill (Van der Merwe et al, 2014). The maps presented have not been restored despite the depocenter being subject to post-depositional north-south compression (e.g.…”

“…Grecula et al, 2003;Sixsmith et al, 2004;Di Celma et al, 2011;Van der Merwe et al, 2014). Palinspastic restoration will improve map view configurations such as sediment dispersal patterns, isopach maps and rates of thinning (e.g.…”

Palynologie et environnements des sédiments du Dévonien de Saïda (Algérie nord occidentale)

“…En effet, les éventails à lobes détachés des conduits distributaires ou chenaux sont souvent caracté-risés par une zone de by-pass ou zone de réajustement hydrodynamique des sédiments (zone de passage de flux de sédi-ments ou zone d'érosion). Cette zone est alors formée le plus souvent par une épaisse couche hémipélagique d'argile séparant les sédiments de lobes de dépôts et ceux des chenaux ( VAN DER MERWE et al, 2014). La taille des grains, la nature et la concentration des sédiments exercent des contrôles majeurs sur les écoulements de by-pass (SUMNER et al, 2012 ;STEVENSON et al, 2015).…”