The Garn Formation (Bajocian-Bathonian) in the Kristin Field, Halten Terrace: its origin, facies architecture and primary heterogeneity model - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 530

the nearby Smørbukk Field to the north (Corfield
et al ., 2001).
Isopach maps of the Garn Formation indicate
sand depocentres in the central zone of the Kristin
Field, along the eastern margin of the hypothetical
graben (Fig. 11). The sandstone succession reaches
its thickness maxima at different stratigraphic
levels in the individual wells before passing
upwards into the neritic mudstones of the Melke
Formation (Fig. 8). This relationship implies
short-distance facies diachroneity and indicates
an uneven sea floor topography evolving along the
graben's western margin during deposition of the
Garn Formation. An uneven sea floor topography
related to faults might explain both non-uniform
eastward advances of the sandy shoreface envi-
ronment and its non-uniform overall retreat
towards the west (Fig. 8). The graben margin was
probably defined by antithetic faults with local
relay ramps and/or relay-breach transverse anti-
clines, on which littoral sedimentation under
transgression would persist longer than in adja-
cent lower-lying areas (Fig. 11).
Subtidal sand ridges (Fig. 9) are considered to
have been an important morphodynamic ele-
ment of the depositional system, apparently
related to topography-enhanced tidal currents
and relative sea-level rise. The formation of
sand ridges is thought to be favoured by trans-
gression on continental shelves characterised
by the following four conditions (Snedden &
Dalrymple, 1999): (1) pre-existing irregularities
in sea floor topography; (2) sufficient supply of
sand; (3) sand-transporting tidal and/or storm-
driven currents; and (4) sufficient time for the
sand to be moulded into a ridge or ridge field.
The reported velocities of ridge-forming currents
are in the range of 0.5 m/s to 2.5 m/s, which may
mean a tidal prism in excess of 3 m for an open
continental shelf (McBride, 2003), but possibly
little more than 0.5 m for some narrow marine
straits (Montenat et al ., 1987). The morphology
and spatial configuration of sand ridges result
from the system's evolution towards equilib-
rium between tidal current dynamics, sea
floor topography and sediment supply (Dyer &
Huntley, 1999).
Modern case studies (Caston, 1972; Swift, 1975;
Kenyon et al ., 1981; Swift & Field, 1981; Cameron
et al ., 1992; Dyer & Huntley, 1999; Goff et al ., 2005;
Liu et al ., 2007) indicate that the height ( H ) of sand
ridges may reach 40 m and their length ( L ) and
width ( W ) are in the range of 5 km to 120 km and

0.5 km to 8 km, respectively (McBride, 2003), with
the following approximate relationship (Fig. 9D):

L ≈+ 36 10 4 . . W (1)

Sand ridges commonly form fields with a ridge-
and-swale topography (Swift, 1975; Hein, 1987;
Houthuys & Gullentrops, 1988; Le Bot et al ., 2005)
and with the lateral spacing ( S ) of the ridges scal-
ing both with their height (Fig. 9E):

(2)

≈ 2

2 SH

and with the water depth ( d w ) (McBride, 2003):

S ≈ 250 d (3)

w

The correlation of core logs (Fig. 8) indicates that
the sandstone ridges of facies association A are
up to 12 m thick and 21 km in length, which sug-
gests ridge widths of up to 1.7 km (eq. 1). Since
the log-derived greatest thickness is unlikely to
be the true H max and since not all ridges are likely
to have attained the same height, we assume a
conservative maximum thickness range of 10 m to
15 m for the sandstone ridges. Their inferred lat-
eral spacing would then be in the range of 2.9 km
to 6.5 km (eq. 2), suggesting a water palaeodepth
range of 12 m to 26 m (eq. 3). A fairweather wave
base at, say, ~5 m depth might set the upper limit
for ridge-top aggradation.
As the subtidal sand ridges grew in size and
aggraded, the vertical stacking of dunes was
increasingly interrupted by storm events and the
accommodation space became gradually filled,
leading to dune truncation, ridge-top erosion and a
temporal predominance of littoral wave action with
sand transfer both southwards and farther to the
east. The coeval tidal deposits of facies associations
A and B were thus covered with the wave-worked
deposits of facies association C, which itself tended
to be locally eroded by waves and become laterally
discontinuous at the accommodation limit (Fig. 10).
A significant rise in relative sea-level then created
new accommodation space and promoted develop-
ment of a new generation of sand ridges, some of
them superimposed directly on the wave-truncated
previous ones. The stratigraphic organisation of
facies assemblages (Fig. 8) indicates a repetitive
pattern of relative sea-level changes. The transgres-
sive-regressive cycles are attributed to pulses of
tectonic subsidence and their record defines a series
of sixteen recognisable parasequences bounded by
marine-flooding surfaces (Figs 8 and 10). The Garn