As part of the quantitative assessment of lateral variability of the sheet-like sediments of the Nâlneset section, measurement of the
following four variables was carried out at each of the 30 logging points for the meso-scale coarse-grained packets: packet thickness, packet net / gross, packet mean grain size, packet maximum grain size. The
methodology used to assess these variables is described in Chapter 7
and tables of these data are presented in Appendix 1 It has been alluded to previously that the meso-scale coarse-grained packets of the Nâlneset section appear to exhibit a vertical change in depositional characteristics, an inference that is appreciated in Enclosure 4.1.
Analysis of the above statistics reinforces this point that the coarse-grained packets exhibit a vertical change (Figure 4.16). Two types of meso-scale coarse-grained packet appear to be preserved within the Nâlneset section. The lowermost eight packets are consistently
thinner, finer grained and apparently more sand-rich than the overlying fourteen packets. The first eight coarse-grained packets have a mean thickness of 2.19 m, and an overall mean grain size of fine-/medium- grained (2.7(|)). Most of the oldest packets have been shown to thin out or pass laterally into fine-grained, thin-bedded turbidites similar to the fine-grained inter-coarse-grained packets (qualitative analysis of sand packet lateral transitions are presented in Section 4.10), and thus
should not be regarded as true sheets sensuo stricto, but rather they have a more lens-like geometry.
Meso-scale coarse-grained packet 9 is the most heterogeneous of all these packets in terms of bedding nature, amalgamation, upper and lower contacts, and lateral thickness variability, and overall the packet probably represents amalgamation of a series of the underlying smaller coarse-grained sand packets. This packet marks the change in
depositional style of the meso-scale coarse-grained packets of the
Nâlneset section, from well-bedded, relatively thin, parallel-based lens like packets dominated by fine-grained sand, to a series of laterally- extensive, strongly amalgamated and coarser grained packets above. The upper fourteen packets, though strongly amalgamated, have an apparent lower net / gross than the lower eight packets: this is a function of the overall differences in thickness between the two types of packet. The
lower eight coarse-grained packets consist of three or four sandy turbidites with highly restricted mud/silt deposition. In contrast to this, the upper fourteen packets, despite being strongly amalgamated, do contain some appreciable development of mud and silt and hence have a marginally lower estimated average value of net / gross. These upper fourteen coarse-grained packets are also thicker (mean thickness of 8.33 m) and have overall coarser mean grain sizes (medium - fine-grained,
1.93(|)). It should be noted that the exposures in the coarsest examples of these packets have been destroyed by the new road construction (see Enclosure 4.12). Beds from this packet (which is identified as a section from packet 16) were locally very thick-bedded gravels and pebbly sandstones of Facies A 1.1 and A 1.4. The possible causes of this shift towards a coarser grained, thicker bedded, more amalgamated type of sand bed deposition is discussed at length in Section 4.16.
4.9.2.3.4 P a ck et u p p e r a n d low er contacts
The meso-scale coarse-grained packets of the Nâlneset section show both sharp-based and gradational upper and lower surfaces with immediately underlying and overlying meso-scale fine-grained packets. In many examples, individual coarse-grained packets vary laterally in the nature of their contact and have both sharp and transitional contacts. It should be noted that the lowest eight coarse-grained packets are
characterised by sharp, but non-erosive bases and tops (Figure 4.17). Sharp-based coarse-grained packets generally stand proud of the underlying fine-grained sedimentary rock, and thus can easily be
resolved. Examples of these sharp-based packets show a rapid change in facies types from beds of Facies C2.3 and D2.I into generally planar- based intervals of Facies B l .l, B2.I and rarely Facies A T I (Figure 4.18). These sharp-based examples suggest very subtle thickness variation of 1-2 m depth over lateral distances of 200-300 m. In rare examples, thinning of the coarse-grained packet is achieved by local onlap of a small number (2-5) of individual beds, typically 10-50 cm thick against a very low relief erosional surface. The majority of
examples, however, achieve this overall increase in packet thickness by a local increase in thickness of the basal thick-bedded bed only, infilling inferred pre-existing topography of the sea-floor. These topographic irregularities, of very low relief (up to 1 m vertical thickness variation
over 200-300 m laterally), are believed to represent slight variations in depositional thickness of the underlying fine-grained sediment.
Gradational-based coarse-grained packets show a transitional change from deposition of fine-grained turbidite packets (characterised by beds of Facies Classes D and E), through coarser grained and slightly more thickly bedded beds of Facies C2.2 and C2.3, into packets of
coarse-grained Facies C2.1, B l .l, B2.1 and B2.2 (Figure 4.19). Bed thickness increases progressively upwards through this transition, typically from 3-5 cm in the lower section, 10-30 cm in the centre and up to 50 cm or more in the coarse-grained packet (i.e. there is an
overall thickening - and - coarsening-upward sequence). Bed contacts in this transitional zone are planar (i.e. purely depositional without
erosion) and the transitional contact extends for 1-3 m of vertical section.
The upper contacts of the meso-scale coarse-grained packets have a similar nature to those of the basal contacts; i.e. they can be both sharp topped and gradational. Gradational contacts extend vertically over 1-2 m, although they are commonly poorly preserved as these contacts universally face southwards and inland and exposed to more severe winter weather conditions. Within this contact zone, beds thin- and - fine-up, and show a reverse facies sequence to that developed in the gradational lower contacts (i.e. Facies Classes A and B in the coarse grained packets through Facies Class C to predominantly Facies Class D and subordinate Facies Class E in the fine-grained packets). In contrast to this, the sharp upper contacts to packets are well preserved, and weather out to form a series of vertical, parallel 'walls' of beds of Facies Class B (see Figures 4.3 and 4.5). As noted previously, it is the top of these sharp-topped packets that have the greatest development of Facies B2.2 (cross-stratified sand) - these packet upper surface cross stratified facies are interpreted as a 'switching o ff of coarse-grained deposition within the system, and the reworking of the packet upper surfaces into subtle bedforms by bypassing, non-depositing turbidity currents.
4.9 2.4.5 B e d d in g an d a m a lg a m a tio n
In the Nâlneset section, bedding is predominantly vertical to sub vertical (Figure 4.6) varying from 60°S, up to very slightly overturned
beds adjacent to some of the major structural features. Where meso- scale folds are developed, beds can have local dips as low as 10°.
The extensive bed amalgamation (described below), in many of the meso-scale coarse-grained packets makes it difficult to identify individual beds, as individual flow units are commonly characterised by subtle grain size changes over sand-on-sand contacts. This is particularly true for beds of Facies Groups B1 and B2 and beds of Facies C2.1
(Figure 4.14). Note that for these examples erosive based flow units, interpreted as forming by erosive amalgamation, predominate over planar sand-on-sand contacts which can be interpreted to be the result of rapid sand flux in to the system. Such sand flux may be purely
depositional with no associated erosive contact. Where there is an absence in a major change in grain size between two flow units and no noticeable angular discordance between the two units, it is difficult to determine which process was the most important. As noted in Chapter
3, liquefaction structures, which are numerous in the Nâlneset section, would indicate that rapid sand flux was a major component of this system; thus both erosive amalgamation and rapid sand flux are key features of the meso-scale coarse-grained packets.
As a result of this, the concept of a 'mean bed thickness' becomes somewhat obsolete, and for the most amalgamated, sand-rich parts of coarse-grained packets, it is more appropriate to consider the preserved thickness of individual flows, which varies between 0.1 - 1.5 m in thickness (i.e. thin- to thickly 'bedded') with an approximate mean thickness of 0.5 m. It is important to note that in the less well-exposed outcrops it is very difficult or impossible to identify some of the more subtle bed contacts, and thus packets appear more thickly 'bedded'.
Amalgamation surfaces, where visible, are commonly irregular- based and effectively represent localised scouring by succeeding high- concentration turbidity currents to a depth of 1-20 cm. If these contacts are of a sand-on-sand nature, it is generally impossible to trace these contacts laterally for distances of more than a few metres. The overall effect of this pervasive amalgamation is to produce rapid lateral and vertical 'shingling' (or interdigitation) of individual flow units, associated with extensive removal of fine-grained sediment from the
upper parts of individual turbidites, and some limited development of cross-stratification.
Where amalgamation is less prevalent, individual beds are discernible with more confidence and even in the more weathered outcrops, bed contacts can be identified. This is particularly true for beds of Facies C2.2 and C2.3, which in some examples have well
developed Bouma Tabc, Tabcd and Tbcd divisions. In such cases, bedding is well defined as predominantly parallel upper and lower contacts (Figure 4.20). It is these beds that exhibit the best sole structures within coarse-grained packets and hence provide a majority of the data for palaeocurrent analysis. This more clearly defined bedding within
packets varies from 0.1 m-thick to rare examples that are 0.7 m-thick - beds of this type have a mean bed thickness of 0.3 m.
Bed amalgamation is present in all of the meso-scale coarse
grained packets identified upon Enclosures 4.1 and 4.2. It is particularly prevalent in packets with abundant development of Facies B l .l , B2.2 and 2.2 (Figure 4.21; other examples include Figures 4.6, 4.14, 4.17 and 4.18), but by no means is amalgamation exclusive to facies of this type. Within the coarse-grained packets, progressive change in grain size across the amalgamation surface should be considered to represent deposition from the next high-concentration flow in surge-type turbidity currents or next phase of increased suspended bedload in steady-type turbidity currents.
Amalgamation of beds has been noted in Facies C2.1, C2.2 and C2.3, and in these examples the silt/mud Bouma Tde of individual beds is commonly disrupted to form a series of mud clast horizons that pass laterally into sand-on-sand contacts. Thus, it is the locally erosive nature of succeeding flows and subsequent bed amalgamation in combination with reworking from non-depositional turbidity currents that are the two major causes of 'shale' removal from sand-rich sequences, with corresponding potential increases of net / gross and removal of lateral heterogeneities. A full discussion of the impact of shale barriers is presented in Section 4.10.