JUZGADO PROMISCUO MUNICIPAL

The delivery of sediment beyond the shelf edge rollover during the interval of steepest rising trajectory is adequately explained by high sediment supply rates outpacing relative sea-level rise and a narrow shelf width in the north, which enabled the deltaic system to prograde to the shelf edge rollover and deliver sediment to the slope and basin floor. However, the ability to bypass sediment to the slope and basin floor during a segment of steeply rising trajectory highlights a weakness in the strict application of the trajectory model when using it as a tool for the prediction of down dip sand deposits (Fig. 5.10A). Sequence stratigraphic interpretation of the lower Waterford Formation (Fig. 5.10B) can be extended across the margin from that described by Jones et al. (2013) for the Baviaans North profile. Jones et al. (2013) suggested that the regional fine-grained unit above Unit G represent a long-term rise in relative sea-level consistent with a transgressive systems tract (TST). The overlying succession of WfC 1 – WfC 4 shows a clear progradational stacking pattern, consistent with a highstand systems tract (HST). WfC 5 marks a slight landward step relative to WfC 4 (retrogradational stacking), and is interpreted as a transgressive systems tract (TST). The overlying regional fine-grained interval (Fig. 5.4A) is interpreted to include the offshore expression of the next maximum flooding surface and is followed by progradational stacking of WfC 6 - 8, consistent with a HST. WfC 4, therefore, represents the  maximum  regression  in  the  lower  Waterford  sequence,  and  the  ‘turnaround’  in  stacking  

Chapter 5 - lateral variability in shelf edge process regime and shelf edge trajectory during differential subsidence subaerial exposure; this turnaround in stacking pattern is interpreted as a regressive surface of marine erosion (sequence boundary) (Vail et al., 1977b; Van Wagoner et al., 1990). The maximum supply of sand over the shelf edge is associated with the regressive surface of marine erosion situated at the top of the most regressive parasequence in sequence 1 (WfC 4). In this instance the sequence stratigraphic model using facies-scale observations more accurately predicts the bypass of sediment beyond the shelf with the presence of a regressive surface of marine erosion at the top of WfC 4 (Fig. 5.10B).

A notable feature of the three depositional dip (Fig. 5.3) panels is the significant basinward step of WfC 8 beyond the previously established, sand dominated, shelf edge rollover position of WfC 4, and the decoupling of the shelf edge trajectory path from the pinch-out of the sandy component of WfC 6 and 7. The basinward step in the deposition of shelf facies associated with WfC 8 requires progradation across a relatively flat shelf setting. The fact that WfC 8 progrades over the top of several hundred metres of fine-grained offshore and slope facies would suggest that progradation of the margin took place through the transport and accretion of fine-grained material by low concentration flows during WfC 6, 7 and 8 time. Correlations reveal that the shelf edge must have prograded a minimum of 15 km (the distance from WfC 5 shelf edge rollover position to the pinch-out of WfC 8 shelf facies) through the accretion of fine-grained sediment under highstand conditions, and therefore represents a significant component of margin progradation comparable to that driven by coarse-grained supply of sediment when deltaic systems are positioned at the shelf edge rollover. Despite the significant volumes of fine-grained sediment transported basinward during rising relative sea-level, neither trajectory nor seismic sequence stratigraphic models recognize or predict accretion of the basin margin without the supply of sand. Comparing relative rates of progradation by coarse-grained and fine-grained material has not been possible due to poor chronostratigraphic constraints, however it is possible to hypothesize that rates of fine-grained progradation are likely to be significantly slower than coarse-grained progradation, yet each may make a comparable contributing to margin development.

Chapter 5 - lateral variability in shelf edge process regime and shelf edge trajectory during differential subsidence

Figure 5.11: Generalized architecture showing Unit G and the 8 overlying clinothems of the lower Waterford Formation. (A) Trajectory interpretation of the lower Waterford Formation showing a flat to slightly rising trajectory with a steepening rise between WfC 3 & 5. Established trajectory model would predict little sediment delivery beyond the shelf edge. However, correlations show the most significant sediment bypass to the slope occurs during the steep rise in trajectory. Bypass of sediment without exposure of the shelf means that a new shelf edge did not develop; therefore the relative sea-level fall is overlooked by the trajectory method, which would fail to predict the presence of lowstand slope turbidites. (B) Sequence stratigraphic interpretation of the lower Waterford Formation places WfC 4 in the falling stage systems tract (FSST) with a sequence boundary overlying it followed by WfC 5, which is part of the transgressive systems tract (TST). The model predicts a lowstand systems tract (LST) further down dip, which is consistent with the strong evidence for sediment bypass of the upper slope in the form of large channels on the Baviaans North and Zoutkloof profiles. It is likely that lowstand deposits lie beyond the final log positions toward the north east of the study area.