Geometry and Kinematics of Salt-detached Ramp Syncline Basins

Abstract

first recognized in the Gulf of Lyon, offshore France (e.g. Benedicto et al. 1999; 57 Sanchis and Séranne, 2000) and the Kvamshesten Basin, onshore Norway 58 (Osmundsen et al. 2000). They were initially described through conceptual (Gibbs, 59 1984) and physical models (Ellis and McClay, 1988; McClay, 1990 McClay, , 1996 McClay 60 and Scott, 1991), as forming above the hangingwall of ramp-flat extensional faults 61 whose basal detachments dip in the direction of tectonic transport. The hanging 62 wall is warped down above the ramp to create a relatively narrow basin. As the 63 hangingwall block moves, the locus of subsidence (located above the footwall 64 ramp) remains fixed in space, but its previous sediment fill is progressively moved 65 away from it, producing a characteristic asymmetric, shingled stratal unit b). 67A second type of RSB has been identified above salt-detached systems in which 68 the base of the moving unit is a salt layer ( Fig. 1c-d) (e.g. Kwanza Basin, Angola, 69 Peel et al. 1998; Marton et al. 1998; Jackson et al. 2001). Jackson and Hudec 70(2005) reviewed the processes and kinematics of RSBs on the Angolan margin 71 using highly schematized sections and 2D seismic data. These authors described 72 salt-detached RSBs as being formed by translation of sediments above a salt layer 73 with a basinward-dipping ramp at its base ( Fig. 1c-d 202In order to constrain the geometry, and to understand the 3D kinematics and 203 tectonostratigraphic evolution of RSBs, 3D seismic mapping of key surfaces within 204 the RSBs were produced to generate thickness maps of key stratigraphic intervals. 205We did not use primary well data, with the identification of key seismic stratigraphic ( Fiduk and Rowan, 2012; Guerra and Underhill, 2012; Jackson et al. 2015). Intra- 209RSB horizons were chosen based on their reflectivity and continuity. 24735% of total post-salt succession (Fig. 12a). 248The base-salt steps associated with the RSBs trend mainly NNE-NE, parallel to the 249 RSBs, although the northernmost high trends NNW and, thus, slightly oblique ( higher degree of translation and rotation of older RSB strata. 339A landward shift from abrupt to subtle transitional limits along the upper and lower 340 boundaries of the RSBs (Fig. 12b-c) is explained by an increase in the 341 aggradation/translation ratio (Ȧ/Ȧ). This is evidenced by landward steepening of 342 the depositional axial-trace in areas where the RSBs are less folded ( Fig. 7-11). (Figs. 10 and 11), typically in their uppermost sections (Fig. 12c). 362By summing the offsets between the thickest points on each isochron map in fig. 363 13, a total translation of 26.9 km was obtained for RSB 5. This, however, 364represents a minimum distance as we were not able to include isopachs of the first 365 and last onlapping intervals, due to intense faulting and folding at both their eastern estimated at 32 (+ 2) km ( Fig. 11 and 13). In many other examples, we were only 371 able to determine a minimum translation because they are l...

Base‐salt relief controls salt‐related deformation in the Outer Kwanza Basin, offshore Angola

“…The RSBs are therefore recognised by their characteristic pseudo-downlap surface and monoclinal geometry of the growth strata (Jackson and Hudec, 2005). Similar examples have previously been described on the Angolan margin using 2D seismic data (Peel et al, 1998;Marton et al, 1998;Jackson et al, 2001;Jackson and Hudec, 2005), and using 3D data from the conjugate margin in the Santos Basin, offshore Brazil (Pichel et al 2018).…”

“…The earliest onlap contained within the RSB defines the net distance of translation (Pichel et al, 2018). Our analysis shows that the oldest RSB, RSB 1, records 23.2 km of seaward translation, and 32° of clockwise rotation from its original position to its present day position ( Figure 8e).…”

“…Given that each RSB forms at the same position above the major base-salt ramp and is subsequently translated seaward as salt flow progresses, we can reconstruct the overburden translation through time by mapping-out the sedimentary depocentres and their onlaps, and laterally restoring them to their initial positions adjacent to the ramp. The lowermost, and thus earliest, onlap onto the base of each RSB is laterally restored to its original position above the updip hinge of the ramp ( Figure 6), and the associated depocentres are fitted as closely as possible to their (inferred) original positions directly overlying the ramp (Figure 8e-g), as shown by physical and numerical models Dooley et al, 2018;Pichel et al, 2018). This process is relatively straightforward for the large, elongate RSB depocentres, hence their original positions are quite well-constrained.…”

“…We interpret that the landward-thickening and -dipping packages described in Section 4.3 ( Figure 6) form a series of stacked 'ramp-syncline basins' (RSBs) that developed in response to the progressive downdip translation of the salt and its overburden across the major base-salt ramp located along the updip edge of the study area ( Figure 5a) (Peel et al, 1998;Marton et al, 1998;Jackson et al, 2001;Jackson and Hudec, 2005;Pichel et al, 2018;Pichel et al, 2019). A RSB is defined as a "translating growth syncline that gradually moves seaward on a dipping detachment as if on a conveyor belt" by .…”

“…RSBs have also been produced in physical analogue models Dooley et al, 2018) and numerical models (Pichel et al, 2018) (Figure 7).…”