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the lower part of sequence 3; tidal and wave
processes are again more important toward the
top of sequence 3. These vertical hydrodynamic
variations are believed to be controlled by the
longitudinal shifting of depositional environ-
ments (proximal and distal) as the result of chang-
ing accommodation (see eustacy and tectonics
discussion below) and by the lateral shifting of
entry points from NW (sequence 2) to N-NE
(sequence 3). Thus, fluvial and tidal processes are
more strongly represented at times when more
proximal environments occupied the study area,
whereas wave processes are recorded more during
episodes when distal and/or flanking environ-
ments were present in the study area.
Overall, it is believed that the relative intensity
of the three processes did not change significantly
throughout Tilje deposition. On the other hand, the
dramatic switch from the wave/storm-dominated
upper Åre Fm. to the overall tide-influenced Tilje
Fm. might be related to major allogenic controls,
such as regional structural changes at the rift-basin
scale. The abrupt decrease in wave energy, coupled
with the increase in the importance of tidal cur-
rents can both be explained if structural move-
ments created a more funnel-shaped embayment,
which was partially sheltered from wave action
and in which tidal currents were enhanced.
the same depositional environment however,
these seasonal indicators are more prominent
within the middle Tilje T3.1 deposits and less
pronounced in T5.1. This suggests that, during the
early Tilje deposition, the rainfall and runoff
events were somewhat more intense than later in
the Tilje.
Accommodation (eustacy and tectonics)
Eustatic sea-level curves indicate the presence of
a relative sea-level rise of up to 100 m during the
Early Jurassic (Haq
et al
., 1987; Hallam, 1988;
Surlyk, 1990). This favoured the generation of
accommodation on a regional scale. However, the
simultaneous tectonic subsidence associated with
rifting during the opening of the North Atlantic
Ocean (Doré, 1991) was probably the most impor-
tant factor responsible for generating the space in
which the 150 m to 220 m thick Tilje Formation
accumulated. However, over the entire duration of
the studied succession, there was an overall
balance between the creation of accommodation
and sediment supply, as shown by the fact that
depositional environments remained within a
few kilometres of the coast throughout the Tilje.
Thus, the Smørbukk area represents a sediment-
overfilled to balanced basin. This is a common
characteristic of the rift-initiation (late pre-rift to
early syn-rift) phase (Ravnås & Steel, 1998;
Withjack
et al
., 2002).
In the Smørbukk area, interpretation of seismic
data (Marsh
et al
., 2010) indicates that rift initia-
tion occurred during deposition of the upper
Åre Formation (Fig. 23) and consisted on localised,
'slow' down-to-the-west movement on the NE-SW-
oriented normal faults, including the blind Testakk
and Smørbukk faults, forming NE-SW axially ori-
ented half-graben morphologies (Corfield & Sharp,
2000). In general, the tabular nature of the reservoir
units indicates that differential movement on these
faults was significantly less than the regional rate
of subsidence/sediment accumulation. Through
Tilje Fm. time, the Trestakk and Smørbukk faults
did, however, propagate towards the north and
south respectively (Marsh
et al
., 2010) creating iso-
lated, syn-depositional, shallow depocentres on
the east side of the Smørbukk field, in the hanging-
wall of the Smørbukk Fault (Fig. 23). These areas of
relatively more rapid subsidence exerted a subtle
control on the Tilje succession, causing the thick-
ening of the lower part of sequence 2 (T1.1.1 to T2)
and the lower part of sequence 3 (T3.2 to T4)
Climate
The studied area was situated in a mid-palaeolati-
tudinal location (Doré, 1991) with a warm climate
that experienced strong seasonal differences in
temperature and rainfall (Hallam, 1994). This sea-
sonality is reflected in the presence of distinct
flood-interflood bedding (see above and also dis-
cussion in Ichaso & Dalrymple (2009)) within the
distributary-mouth bar deposits of the middle part
of sequence 2 (unit T3.1) and the upper sequence
3 (T5.1). In these environments, the seasonal dis-
charge pulses (cf. Day
et al
., 1995) promoted the
formation of current-generated structures and the
input of coarser sand grains in locations that
during inter-flood periods were commonly domi-
nated by tides and/or waves. The presence of
thick fluid-mud layers within the distal mouth-
bar and proximal delta-front deposits of T3.1 and
T5.1 indicates that river floods caused the expul-
sion of the turbidity maximum from the river
mouth (cf. Bhattacharya & MacEachern, 2009),
depositing fluid muds in more distal locations
than was the case during interflood times. Within
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