A TWT map of the top Utsira Horizon (TUH) shows several areas of low relief (Fig. 4.22a). However, due to their more subtle nature, the depressions are more difficult to identify on a TWT map than the underlying elliptical mounds. Nevertheless, the presence of two topographic linear ‘trenches’, roughly orientated SSW-NNE, is suggested by the azimuth map of the TUH (Fig.. 4.22b, Trench 1 and 2). These appear to correspond with the locations of the Base Utsira topographic high ‘ridges’
identified in Fig. 4.12a. The relationship between mounds and depressions is best demonstrated by overlaying a contour map of the TUH over a TWT map of the BUH (Fig. 4.23, a and b).
The negative relief of the depressions is measured in a similar manner to the positive relief of the mounds. The hinge points are defined as the point where the top Utsira reflection dips downward, while the maximum negative relief equates to the perpendicular distance between an arbitrary line connecting the hinge points, and the lowermost depth of the hanging TUH (Fig. 4.23a).
Generally, individual depressions cover a greater area but display a smaller vertical relief than their corresponding mound. The average area of the depressions exceeds 10 km2, and their negative relief ranges between 15 and 50 m, with an average relief of 24 m.
97 Fig. 4.22 a) TWT map of the Top Utsira Horizon demonstrating the subtle changes in topography
b) an azimuth map of the same horizon showing the distinct form of two ‘trenches’
orientated SSW-NNE
98 4.4.5.1. Comparison of depressions with mounds
Mound 1
Mound 1, as described in section 4.3.2.2, is an irregular shaped section of raised BUH which totals 3.9 km2 (Fig. 4.23b). A time depth map of the mound is overlain by the contour of the overlying depression, and it can be seen that the bold black contour line which denotes the extent of the depression exceeds the dimensions of the underlying mound in every extent except the north-west corner of the area (Fig. 4.23b). The depression is 4.4 km2 and its outline roughly conforms to the geometry of the underlying mound.
The TUH hangs by around 30 m within the depressed area while the mound displays a maximum positive relief of 90 m (Fig. 4.23b). Two areas of maximum relief can be seen along the mound profile, these are separated by a low section of the BUH. Although the data quality is fair, it appears that the depression mirrors this geometry, and consists of two low areas separated by a slightly raised middle section which directly overlies the low section of the mound
Elongated Mound
An elongated mound, close to the western extent of the Utsira Sand, can also be seen to be smaller in area than the overlying depression (Fig. 4.23d). The outline contour of the depression roughly follows the geometry of the mound. The mound has a positive relief of 90m while the depression hangs by 35 m, and the TUH and BUH are in contact in this example (Fig. 4.23e). A high amplitude, discordant reflection is apparent below the mound, and the BUH is broken in close proximity to the hinge points, while the TUH is intact across the whole profile of the depression. The sudden decrease in thickness of the Utsira Sand where affected by the mound and depression is in contrast with the gradual thinning of the unit towards the west.
99 4.4.5.2. Interpretation and discussion
Fig. 4.23 a) demonstration of how measurements for the Top Utsira depressions were taken b) time depth map of an irregular shaped mound overlain by a contour map of the Top Utsira depression c) seismic section through the mound, line location denoted by dashed black line d) time depth map of an elongated mound overlain by a contour map of the Top Utsira depression e) seismic section through the mound, line location denoted by red line
100 The spatial coincidence and similarity in geometry between the Base Utsira mounds and Top Utsira depressions suggests that there is a relationship between the features. Downturned and downlapping reflections at the flanks of the mounds have already been interpreted as indicators of downward movement affecting the mounds, caused by post emplacement aperture loss of the sand intrusions. It is therefore suggested that the depressions are a further manifestation of this downward movement, and are also caused by the sand intrusion aperture loss.
While the basal reflection often appears broken and detached at the hinge points of the Base Utsira, the Top Utsira reflection does not display this feature; this may be attributed to the smaller vertical movement. The Top Utsira reflection can also be followed across the whole of the profile of the depressions, unlike the basal reflection which dims, is displaced, broken and hard to follow across the fold profile. The irregular topography of the Utsira Sand has previously been noted by Zweigel et al., (2000), and has been described as consisting of several linked domal and anticlinal structures which are considered to be the result of uneven subsidence of the underlying strata.
Depressions which overlie mounds formed by the forced folding of the overburden during sand intrusion emplacement have previously been noted by Loseth et al., (2012). These depressions are interpreted as further evidence of an amount of settlement or downward movement affecting the mounds and overlying strata. As suggested in section 4.3.4.1. the presence of downturned and downlapping reflections onto the mound crests and flanks is evidence of downward movement which has affected the elliptical mounds. The downward curve of the reflections is interpreted to be the result of aperture loss within the underlying sand intrusions. This aperture loss is considered to be the result of mound settlement following grain reorganisation and post-emplacement abatement of the ‘jack up’ pressure within the intrusions following dewatering and sediment expulsion at the paleo-seafloor.
101