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 basin configurations 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 palaeogeographic 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 depocentre, 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, whilst Fan 4 thins at a rate of 4 m/km. Marked differences between Fan 3 and 4 in sedimentary facies and architecture towards 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, whilst the upper part is dominated by structured sandstones and thin-bedded heterolithics. The variations in facies suggest that at the 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 architectures above slopes with subtle differences in gradient and orientation cautions against simple definition of reservoir input parameters for stratigraphic traps in submarine fan systems.
Highlights
This study focuses on the oblique up-dip pinchouts of Fans 3 and 4 in the Klein Hangklip area and the lateral pinchout of Fan 3 only in the Gemsbok valley area (Figure 2)
The difference in calculated slope gradients between the oblique up-dip and basinward lateral pinchouts indicate that the eastern margin of Fan 3 was less confined by seabed topography compared to the up-dip southern margin (Figure 12A)
The steeper seabed topography in the up-dip area meant there was limited accommodation. This is reflected in channels cutting proximal lobes (Hofstra et al, 2017), widespread scours mantled with mudclasts indicating sediment bypass (Sullivan et al, 2000; Hofstra et al, 2015; Stevenson et al, 2015), and the development of aggradationally stacked and amalgamated lobes (Figure 12D)
Summary
Understanding the architecture and sedimentology of basin-floor fans, and their component distributive channel-fills, channel-lobe transition zones, and lobes has evolved considerably in the last 30 years due to improved seabed imaging and extensive outcrop studies (Walker, 1978; Twichell et al, 1992; Bouma and Rozman, 2000; Carr and Gardner, 2000; Dudley et al, 2000; Satur et al, 2000; Hodgson et al, 2006; Brooks et al, 2018). Interest in the architecture and facies distribution at basin-floor fan fringes, and the characteristics of fan pinchouts, have been driven by a need to improve prediction of pinchouts as stratigraphic trap targets for hydrocarbon reservoirs (Pickering, 1981; Bouma and Rozman, 2000; Etienne et al, 2012; Bakke et al, 2013; Marini et al, 2015; Nagatomo and Archer, 2015; Spychala et al, 2017b,c). Quantification of the rate at which pinchout characteristics, such as thickness, sandstone content and facies variations change, is rarely constrained
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