4.11.2 Local erosion within and at the base of m eso-scale coarse-grained packets.
This has been discussed at length in Section 4.9.2.3.6 above, in relation to amalgamation of high-concentration turbidity currents. Particularly good examples have been examined in Enclosures 4.5, 4.6, and 4.8 (see Sections 4.10.2, 4.10.3, 4.10.5). Erosion within these packets is limited to a maximum depth of 1-2 m, over lateral distances of 10-150 m. The nature of the erosive sediment is identical to that of the underlying strata, namely beds of Facies Class B and Facies C 2.1.
Similarly, the sharp, often erosional nature of the bases of coarse grained packets have been described previously - see Section
4.9.3.2.4. Erosion of fine-grained, thin-bedded turbidites at the base of coarse-grained packets is limited to a depth of 1-1.5 m, over lateral distances of 150-300 m.
Two good examples of scours within coarse-grained packets are shown in Figures 4.26 and 4.27. Both examples are filled by
amalgamated coarse-grained rocks of Facies Class B, the first (Figure 4.26) being incised to a depth of 1.5 m through a 80 cm-thick succession of 'within coarse-grained packet' thin-bedded, fine-grained turbidites (i.e. a 'shale'), and into underlying sand-rich beds of Facies C2.3 and C2.2. The second example (Figure 4.27) shows part of an erosion surface that incises into sand-rich beds only. Depth of incision in this example is approximately 1 m over a lateral distance of approximately 30 m. Note both these erosion surfaces are cutting down towards the east, and no corresponding westward directed incision surface has been identified.
4.11.3 M u d -/silt-filled scours
Scours of this type are an order of scale smaller in dimension than the sand-filled equivalents. Depths of incision usually do not exceed 1 m, and many are only 20-50 cm-deep. These small-scours only cut down very locally, typically only over lateral distances of 1-10 m. Mud-filled scours occur almost exclusively within fine-grained packets only, and can be difficult features to identify; only bedding attitude discordancies reveal these scours.
4.11.4 D ebrite- and slide-/slum p-filled scour
The only truly large-scale erosional feature within the Nalneset section is a large, erosive-based structure that occurs at the top of Logs
10 and 9. This feature is characterised by dark grey, honeycomb weathered, mud-rich sediments of Facies A1.2, A1.3, A L 4 and Facies Class F (constituent sediments are of Facies D2.1, D2.2 and D2.3). Note that the westernmost part of this erosion-based feature is not exposed.
This erosional based structure was logged in detail at lateral separations of 5-15 m, and where possible, individual events were walked out and correlated. The results of this logging are shown in Enclosure 4.10. Erosion of the underlying coarse-grained packet to a depth of at least 3.5 m has resulted in an apparent stepped-margin, with the majority of erosion occurring over a relatively small lateral
separation of approximately 10 m (area between Logs 10 (part) and Log IOC West. To the west of this, the erosional feature has a relatively flat base. Note that to the east of Log 10 (part), the major, apparently strike- slip fault described in Section 4.3.2.
Despite detailed field mapping, it has proven problematic to match meso-scale coarse-grained packet thickness across this fault, and thus the correlations proposed between Logs 10 and 11 in Enclosures 4.1, 4.2 and 4.11 remain 'probable'. These thickness changes however, are suggestive that this particular fault was possibly a syn-sedimentary feature with the downthrown side to the west (meso-scale sedimentary packets apparently thicken in this direction). Tentative correlation of mud-rich sediment east of the fault with the thick sequence of debrites and slides has been attempted, but this remains speculatory. It appears probable that this fault may have been a positive feature that influenced the development and fill of the erosion-based feature, which is proposed to be a scour upon the sea-floor (Figure 4.28). The alternative to this is that the erosion-based feature is in an infilled slump-scar, possibly initiated by (re-) activation of the fault.
Following the initial incision event, the 'hollow' created by the scour / slump-scar was progressively infilled by debris flows, and slides and slumps of mud-/silt-rich very thin-bedded turbidites. The first debris flow to fill the hollow is in fact extensive across the entire width of the feature, and has an irregular geometry that varies in thickness
from 3 cm (pebble stringers in a muddy sand matrix - Log lOD West) to 40 cm (Log lOG West). This debrite has a sharp commonly planar but also erosional contact with the underlying coarse-grained packet, except for the eastern-most part (Log 10 (part) to Log IOC West), where the debris flow is injected by coarse-grained sand from the underlying packet (Figure 4.29). In this injected zone, mud from the debris flow forms a mud-clast breccia in which the matrix (30%) is composed of sand, while the clasts are very wispy, contorted, densely concentrated mud-/silt flakes, from 0.5-10 cm in length (Figure 4.30). Sand from the coarse-grained packet also interfingers extensively into the debrite in the form of clastic injections. It is possible that fault activity (i.e. seismic activity?) caused local liquefaction of the coherent sand while triggering contemporaneous release of the debris flow, with subsequent sediment mixing and injection.
Following the deposition of this initial debris flow, the remaining vacuity was infilled by a succession of thin to thick (10 cm to 1.2 m- thick) slide and slump units. These slides are very local horizons: no individual slide was correlated for more than 3 m laterally and most are less than 1 m-wide. Some mud-rich debrites (Figure 4.31) can be
tentatively correlated up to 50 m, but outcrop quality makes such correlations tenuous.