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 522

Table 1. ( Continued )

Facies co de Facies name and description Genetic interpretation

S M Massive sandstones : These non-stratified arenitic

sandstones range from fine-grained to medium/
coarse-grained, occasionally with an admixture
of very coarse sand grains. This facies is rare,
forming isolated units 10 cm to 40 cm thick,
with sharp tops and mainly diffuse bases. Its
only significant occurrence is in well 6506/11-
N-3H (Table 2), where it forms a unit ~ 5 m thick,
with local traces of planar parallel strata and a
diffuse basal transition to the underlying unit
of facies SPS.

Depending on its stratigraphic context,
the origin of this facies can be attrib-
uted to sporadic dune collapses on
tidal-bar slopes or to local sea floor
liquefaction triggered by earthquakes
(Seed, 1979; Field et al ., 1982) or by
the cyclic loading by storm waves
(Seed & Rahman, 1978; Okusa &
Uchida, 1980). The relatively thick
unit of this facies in well 6506/11-
N-3H suggests major liquefaction
and may indicate proximity to a
seismically active fault.

Facies association A consists of facies S CS
(66 vol.% to 92 vol.%) intercalated with subordi-
nate thin units of facies S RL (3 vol.% to 23 vol.%)
and S PS (1 vol.% to 15 vol.%) and sporadic iso-
lated beds of facies S M (≤8 vol.%), S SS (≤ 2.5 vol.%)
and/or S HS (≤0.5 vol.%). The correlation of
sedimentological logs (Fig. 8) suggests that this
facies assemblage forms mound-shaped, broadly
lenticular sandstone bodies elongated in the
north-south direction, with measured in-well
thicknesses of up to 12 m and estimated lengths
of up to ~ 20 km. They may correspond to the
elongate sandstone bodies with subtle positive
relief recognised by Corfield et al . (2001). The
sandstone bodies of facies association A consti-
tute 30 vol.% to 68 vol.% of the Garn Formation
and are interpreted to have been deposited as
wave-modified complexes of vertically-stacked
2D dunes forming tidal sand bars. The lack of
large-scale foresets with comparably great thick-
nesses indicates flow-aligned sand ridges (Fig. 9),
rather than flow-transverse sand waves with ava-
lanching lee-side slopes (Off, 1963). Linear sand
ridges of this type, subject to frontal growth and
oblique lateral migration, have been reported
from continental shelves, narrow epicontinental
seaways and straits, coastal embayments and
large estuaries (Caston, 1972; Swift, 1975; Swift &
Freeland, 1978; Reineck & Singh, 1980; Swift &
Field, 1981; Huthnance, 1982; Cameron et al .,
1992; Harris et al ., 1992; Lanckneus & De Moor,
1995; Goff et al ., 2005; Liu et al ., 2007). The evi-
dence from cores combined with dip-picking
data suggests that the tidal system was asymmet-
rical, with the majority of dunes migrating south-
wards and subordinate ones formed by the
reversing current. The stratigraphic stacking of

semi-isolated sandstone ridges (Fig. 8) resembles
the imbricate pattern described by Evans (1970).
Facies association B consists of facies S PS
(7 vol.% to 83 vol.%) and facies S RL (12 vol.% to
70 vol.%, mainly current ripple cross-laminated),
which alternate with each other and are interca-
lated with single or double cross-sets of facies S CS
(5 vol.% to 21 vol.%) as well as isolated beds of
facies S M (≤7 vol.%), S HS (≤2 vol.%) and/or S SS
(≤2 vol.%). This facies assemblage constitutes
12 vol.% to 65 vol.% of the Garn Formation and
laterally interfingers with the sandstone bodies of
facies association A (Figs 8 and 10). Facies asso-
ciation B is interpreted to have been deposited by
tidal currents with wave influence in inter-ridge
swales (Swift & Field, 1981; Dyer & Huntley,
1999; Liu et al ., 2007). The swales are thought to
have funnelled the near-bottom water flow, boost-
ing it to the upper flow regime at tidal current
peaks and thereby causing plane-bed transport
and considerable downflow bypass of sand. The
swale-confined, helicoidal flow of the tidal cur-
rent swept sand as dunes onto the adjoining
flanks of sand ridges, whereas transport of sand
as current ripples predominated at the weakest
flow stages.
Little or no mud will be deposited in the tidal
system if wave-generated ambient turbulence per-
sists (Montenat et al ., 1987; Keene & Harris, 1995;
Longhitano & Nemec, 2005). Sand tends to be swept
by waves from the tidal ridges and redistributed
further by tidal currents in the swales. Depending
on local hydraulic conditions, the inter-ridge swales
may thus accumulate tide-worked and wave-
worked heterolithic deposits (Hein, 1987) or mud-
devoid sand (e.g. see the Nantucket sand-ridge field
in Swift, 1975, fig. 19; Houthuys & Gullentrops,