Quantification of Basin-Floor Fan Pinchouts: Examples From the Karoo Basin, South Africa

Abstract

The topography of the seabed (orientation and gradient) and rheology of the flows greatly influences the character of basin-floor turbidity current deposits. Therefore, submarine fan pinchouts can help to constrain seabed topography and flow behavior at the time of deposition. Although the depositional architecture of submarine lobe pinchouts has been documented in various basin-fills, the quantification of the rates of change at pinchouts in different paleogeographic positions and basin configurations has not been attempted previously. Here, we utilize extensive outcrops and research boreholes from the oblique up-dip pinchout of Fans 3 and 4 and the lateral pinchout of Fan 3 in the Tanqua depocenter, Karoo Basin, South Africa, to compare sedimentary facies and to quantify the rates of change in gross interval thickness. At the oblique up-dip pinchout, Fan 3 thins abruptly at a rate of 12 m/km, while Fan 4 thins at a rate of 4 m/km. Marked differences between Fans 3 and 4 in sedimentary facies and architecture toward the up-dip pinchout, with termination of lobes in Fan 3 and a channel-lobe transition zone and external levee in Fan 4, suggests progradation of the system. The thinning rate of the lateral pinchout of Fan 3 is 2 m/km, with the presence of hybrid beds in the lower part of Fan 3, while the upper part is dominated by structured sandstones and thin-bedded heterolithics. The variations in facies suggest that lobe-scale frontal and lateral pinchouts are stacked at the lobe complex-scale lateral pinchout of Fan 3, highlighting the importance of a hierarchical understanding when studying basin-floor fan pinchouts. The quantified rates of change in fan thickness and sedimentology on the oblique up-dip and lateral fan pinchouts are markedly different. Contrasting pinchout architecture above slopes with subtle differences in gradient and orientation cautions against the simple definition of reservoir input parameters for stratigraphic traps in submarine fan systems.

Stratigraphic hierarchy and three‐dimensional evolution of an exhumed submarine slope channel system

“…These data suggest that KHKC was oriented north-east/south-west, with a north-eastward palaeoflow consistent with regional palaeocurrent observations (e.g. Hansen et al, 2019), and is essentially straight at the scale of the outcrop (Fig. 10A).…”

“…The Skoorsteenberg Formation is 450 m thick in the Tanqua Depocentre and comprises five sandstone-prone units. The lower four units are interpreted as a progradational succession of submarine fans (Fans 1 to 4), overlain by a fifth unit (Unit 5) interpreted as a base of slope to lower-slope system (Bouma & Wickens, 1991, 1994Johnson et al, 2001;Wild et al, 2005;Hodgson et al, 2006;Pr elat et al, 2009;Hansen et al, 2019). Each fan is interpreted as a lowstand systems tract and is overlain by regionally correlated fine-grained packages interpreted as the combined transgressive and highstand systems tracts (Goldhammer et al, 2000;Johnson et al, 2001;Flint et al, 2011).…”

“…The channelfills have an internal hierarchy and are interpreted as a series of channel complexes (Wild et al, 2005). A regional fine-grained unit separates Unit 5 from the underlying Fan 4, which is subdivided into sandstone-rich Upper and Lower Fan 4 (Hodgson et al, 2006;Spychala et al, 2017;Hansen et al, 2019).…”

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

“…Sediment gravity flows encountering seafloor topography in confinedbasin settings form a range of onlap geometries that are often associated with complicated sedimentary facies (Fig. 1) (e.g., Kneller 1991;Haughton 1994;Wynn et al 2000;McCaffrey and Kneller 2001;Amy et al 2004;Gervais et al 2004;Lomas and Joseph 2004;Puigdefàbregas et al 2004;Smith 2004aSmith , 2004bSmith and Joseph 2004;Gardiner 2006;Mayall et al 2010;Tinterri et al 2016;Hansen et al 2019). The effect of seafloor topography on sediment gravity flows, their deposits, and onlap styles has been studied through outcrop data (e.g., Kneller et al 1991;Sinclair 1994;Bakke et al 2013), subsurface data (e.g., Prather et al 1998Prather et al , 2012Covault and Romans 2009;Bakke et al 2013), and numerical models (e.g., Smith 2004b;Kubo 2004;Gardiner 2006;Sylvester et al 2015) and physical models (e.g., Kneller 1995;1995;Amy et al 2004;Brunt et al 2004;Kubo 2004).…”

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

“…Sediment gravity flows encountering seafloor topography in confined-basin settings form a range of onlap geometries that are often associated with complicated sedimentary facies (Fig. 1) (e.g., Kneller, 1991;Haughton, 1994;Wynn et al 2000;McCaffrey and Kneller, 2001;Smith, 2004a;Smith 2004b;Amy et al, 2004;Gervais et al 2004;Lomas and Joseph, 2004;Smith and Joseph, 2004;Puigdefàbregas et al, 2004;Gardiner, 2006;Mayall et al 2010;Tinterri et al 2016;Hansen et al 2019). The effect of seafloor topography on sediment gravity flows, their deposits, and onlap styles has been studied through outcrop data (e.g., Kneller et al 1991;Sinclair, 1994;Bakke et al, 2013), subsurface data (e.g., Prather et al, 1998; 2012; Covault and Romans, 2009;Bakke et al, 2013), and numerical models (e.g., Smith, 2004b;Kubo, 2004;Gardiner, 2006;Sylvester et al, 2015) and physical models (e.g., Kneller, 1995;1995;Amy et al, 2004;Brunt et al, 2004;Kubo, 2004).…”