4.10.5.1 In tro d u ctio n
This transition zone from coarse-grained packet to fine-grained packet is different to that described in Section 4.10.4.1, in that only the upper half of the coarse-grained packet is showing lateral facies variations from coarse, clean sands characteristic of the coarse-grained packet (facies B2.1, B2.2 - see Chapter 3) to a finer, thin- and very thin-bedded lithology (facies C2.2, C2.3 and D2.1). In this section the lowest 4-5 m of each log, consisting of compensating, amalgamated, CSRUs has not been drawn. The datum used to hang the section is the base of the lowermost CSRU of the sampled section
4.10.5.2 S edim entology
Lithologies typical of coarse-grained packets are represented by Log 28 (part), and are believed to be present in the area of missing outcrop. The top of this packet is based on field based observation and is defined as forming the top of this particular log. This particular coarse grained packet, when viewed in other locations not shown, bears a resemblance to the first packet described in Section 4.10.2.2, with several prominent and persistent shale horizons (10-30 cm thick). Net / gross ratios in this panel decrease from 0.96 in Log 28 (part) to 0.78 in Log A (net / gross figures from Cossey, 1993 pers. comm.). Some variation in this value is due to the occurrence of a large clastic dyke, sampled by Log C, and shown in Figure 4.25. Fine-grained lithologies in this correlated section are again noted by the abundance of parallel- bedded, thin- and very thin-bedded very fine-grained sand / silt / silty- mud turbidites. Current ripples, ripple form sets and liquefaction structures are common throughout the fine-grained sediments. 4.10.5.3 ’B e d d in g '
Geometry and bedding contacts of CSRUs within the described top of the packet comprise essentially 0.5-2 m-thick tabular, sheet-like units, which in the persistent examples in this panel, extend laterally for 40-50 m. The uppermost discrete beds of some of the CSRUs, however, are essentially lenticular, rapidly thinning out from typically 10-40 cm- thick over lateral distances of 20-30 m, again approximately
perpendicular to palaeoflow. These coarser beds are replaced by
sequences of thin-bedded turbidites, which at their greatest development in Log A, are at least 1 m-thick. Beds in this particular section are disrupted in some particular areas (the largest bed at the base of Log D and the irregular beds above this in the same log) by injection structures which are common in this section, and thus improve reservoir
connectivity in these thin-bedded successions.
4.10.5.4 D iscu ssio n
This sand-packet is essentially of two parts: the lower part (not represented on the Enclosure) is of a similar nature to previously
described examples. This is namely pervasively amalgamated, sheet-like composite sand-rich units of Facies Class B and Facies C2.1. These CSRUs extend laterally for 300-400 m before becoming amalgamated, have a basic overall coarse grain size, a lack of recognisable cyclicity, and an irregular original topography resulting from expression of packet compensation.
In contrast to this, the upper part demonstrates a quite
recognisable transformation with beds of typical facies of the coarse grained packets rapidly thinning (over 20-30 m) into fine-grained lithologies. This is again interpreted to be the result of a form of bed compensation. Again, the lack of genuine erosive contacts means that this cannot be the margin of a shallow channel or scour. It is believed that the fine-grained lithologies represented in Logs A and B possibly formed another example of a slight positive sea-floor feature or 'swell', again with beds both pinching and feathering out laterally. In a similar fashion to the examples discussed previously, this 'swell' was not a fixed object at any particular time, but was actively aggrading through
deposition of the fine-grained feather edges of high-concentration flows to the west. The effects of this 'swell' varied laterally causing some beds to pinch-out over several metres; this was probably a function of the thickness and density of the flow and also the very local slope angle, factors which will vary with each succeeding flow. Indeed some beds are shown to pinch out towards the west as well as the east. Thus the 'swell' should not be considered as a fixed object with known
dimensions, but can considered to be a very low-relief object of less than 1 m in relative height over 20-40 m laterally, that varied in it's
local effects for each succeeding flow at any one point in time. This 'swell' possibly had a lateral extent of 10-20 m, as thicknesses of beds and CSRUs between Logs F and 28 (part) appear to have attained a constant thickness; lack of exposure to the E and W means this cannot be resolved. Again, the alternative explanation that this particular area marked the lateral termination and lateral transition of a series of high- concentration flows unaffected by any topographic variation is
conceivable, but the rapid lateral variation of so many flows over such a short lateral distance would argue otherwise.
4.10.6 E n clo su re 4.9, Log 12 W est A-E 4.10.6.1 In tro d u c tio n
The datum used to hang the logs in this enclosure was the base of the log in Logs A-E, and then the surface at a vertical height of 2.45 m for Logs 12 (part) and Log A. This section details another transition from coarse-grained packet to fine-grained, thin-bedded turbidite 1 thologies, the initial geometry being altered by subsequent injection and then differential compaction. Note that this is a very local transition (panel is approximately 5 m-high and 15 m-wide).
4.10.6.2 S edim entology
Coarse-grained packet lithologies in this panel are identical to those described previously: amalgamated, coarse-grained CSRUs of Facies Class B (see C h a p ter 3) with minor thin shale horizons (<30 cm-thick), composed of thin- and very thin-bedded turbidites (Facies C2.1 and D2.1). The upper portion of this small sand-packet passes over a very short lateral distance into a relatively thick succession of thinly tedded turbidites, together with subordinate dislocated sediments of an identical lithology: this is the result of very localised sediment failure. 410.6.3 'B e d d in g '
The CSRUs predominant in the coarse-grained packet essentially form stacked tabular sheets, 0.5-1.5 m-thick, and these are of essentially miform or undulose thickness for the lateral distance of 15 m
represented within this panel. Shale horizons within this packet form viry minor impersistent lenses, <30 cm-thick and up to 3-4 m-wide.
These thin lenses are cut out by either amalgamation or erosion of the sandy facies.
4.10.6.4 D iscussion
The cause of this very rapid lateral transition is complicated by the lateral and vertical injection of several of the eastward thinning sands to produce sedimentary dykes. Net / gross for the upper portion of this coarse-grained packet decreases from approximately >0.9 in Log E to 0.5-0.7 in Logs A, B and C - a lateral distance of 5-6 m. After deposition of the lower part of the packet, with one bed thinning but eroding to the west, deposition of a sequence of thin-bedded facies occurred, possibly resulting in another upraised topography and lateral thinning of discrete sand beds over an inferred very gentle rise of 1-2 m in height. This possible rise is of unknown lateral extent: all the sandy CSRUs in the upper parts of Log E have thinned out completely in Log A, 6-7 m to the west, the terminations of these beds having been
obscured by lateral injection during compaction. It is possible that this was not a very-low relief 'swell', but one margin of a scour into the shale-rich sediments, with a relatively steep slope parallel to the palaeoflow; potential bed terminations against the scour margins are now obscured by the lateral injection. Instability of slopes on either the possible 'swell' or (infilling) scour has resulted in the slide horizons both to east and west. In the case of the westward trending slide, transport of sediment may have been away from the inferred 'swell' apex. The large slide unit in thin-bedded facies at the top of Logs C, D and E possibly came from a similar topographic high that may have developed above a possible "swell" in the underlying sand-packet, but this remains conjecture.
4.11 SCOURS
4.11.1 Introduction
The lack of major erosive-based features, or large-scale down cutting surfaces within the Nâlneset section forms one of the major criteria that has been used to re-evaluate this part of the Kongsfjord Formation in terms of sheet-like deposition. Despite this, there are several examples of local internal scours within the system, and several of these features are examined below.