Eustatic, tectonic and climatic controls on an early syn-rift mixed-energy delta, Tilje Formation (Early Jurassic, Smørbukk field, offshore mid-Norway) - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 374

Environmentally, previous workers have inter-
preted the lower half of the Tilje as the deposits
of an estuarine environment (Martinius et al .,
2001). We suggest, instead, that this interval in
the Smørbukk field formed in a tidally influ-
enced, open deltaic setting with widespread
development of distributary mouth-bar/delta-
front sub-environments, with small terminal
distributary channels. This interpretation is sup-
ported by several lines of evidence including:
the pervasive development throughout the basal
Tilje of sandier-upward (deltaic) successions
alternating with fining-upward (channel) succes-
sions that show a predominant palaeocurrent in
one direction; the widespread presence of deci-
metre-scale alternations in sand grain size, the
abundance of fluid-mud layers and the degree of
bioturbation that together are interpreted as rep-
resenting seasonal variations in river discharge;
and the presence throughout of a low-diversity,
mixed Cruziana-Skolithos ichnofacies suggestive
of a brackish-water assemblage.
Although this assemblage of features represents
clearly a deltaic environment, these deltas could
have been situated in the bayhead-delta region of a
wave-dominated estuary (cf. Dalrymple et al ., 1992;
Boyd, 2010). Although this cannot be discounted
unequivocally on the basis of this local study, we
think it is unlikely. The basal part of the Tilje, both
in the Smørbukk field (reservoir unit T1.1) and
regionally, is aggradational to progradational in
character, as is the upper part of sequence 2
(reservoir units T2 and T3.2); the only part of
sequence 2 that is retrogradational is reservoir unit
T1.2 (Ichaso, 2012). Such a stacking pattern is not
consistent with an estuary sensu Dalrymple et al .
(1992). Also, there are no facies that could repre-
sent such barrier-related environments as tidal
inlets and flood-tidal deltas, the parts of a barrier
that have the greatest preservation potential.
Therefore, we conclude that the lower part of the
Tilje is unlikely to have formed in a closed estuary.
We suspect that the widespread presence of a
brackish-water trace-fossil assemblage, coupled
with confusion over the two definitions of 'estuary'
(cf. Dalrymple et al ., 2011) contributed to the use of
the term 'estuarine' in previous work.

T1.1.1 and T1.1.2) based on the vertical increase
of grain size and thickness of cross-bedded sand-
stones and the abundance of tidally influenced
channel deposits (Fig. 3). These vertical changes
are recognised in well logs as a marked shift
from high to low GR readings and a change in
density-neutron separation from fine mixed
mudstone-sandstone to clean sandstone. Tilje
1.1.1 shows local development of a basal trans-
gressive shelf lag (FA2) where the thickest depos-
its (up to 25 cm thick) occur towards the south and
NE of the field. The shelf lags are commonly over-
lain by distributary mouth bar deposits (FA3) that
show a lateral transition from clean medium-
grained cross-bedded sandstones in the north-
central part of the study area to more fine
sand-mud heterolithics to very-fine sand-mud
heterolithics towards the S-SW part of the field.
The north and central deposits are punctuated by
local fining-upward terminal distributary-channel
deposits, whereas, toward the south, there is an
increase in the proportion of wave-influenced
mixed sand-mud heterolithics alternating with
HCS beds and bioturbated intervals (Fig. 20B). The
presence of the wave-generated structures and
abundant bioturbation towards the south indicates
less sheltering and deeper-water conditions with
salinities that were closer to normal marine.
The overlying sharp-based mouth-bar deposits
(FA3) of T1.1.2 show medium-grained, clean
cross-bedded sandstones that change laterally
into fine-grained sandstones and bioturbated
heterolithics toward both the NE and SW
(Fig. 20C). Locally, the base of T1.1.2 consists of
mudstone-dominated and bioturbated hetero-
lithic deposits, showing a more diverse marine
trace-fossil suite; thus, the contact between T1.1.1
and T1.1.2 represents a higher-order flooding
surface. Tidally influenced channel fills contain-
ing thick fluid-mud deposits are more prominent
in the central area. Wave-generated structures are
less common than in T1.1.1; however, small-scale
HCS occurs towards the E and SW margins of the
field. The facies distribution (Figs 20B & 20C) of
the basal Tilje, the regional variation of grain sizes
within the sandy portion and the southern
occurrences of wave-generated structures suggest
an overall NW to SE transition from proximal
(coarse-medium grained, cross-bedded, abundant
fluid-mud layers, sandstone-dominated hetero-
lithics and low bioturbation) to medial (medium-
fine grained, cross-bedded, HCS, sand and mixed
sandstone-mudstone heterolithics, fewer fluid

Basal Tilje (T1.1)

The basal Tilje Fm. comprises the reservoir pro-
duction zone T1.1 (15 m to 25 m thick), which has
been subdivided into two sub-units (subzones