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 377

diminishes towards the E, NE and SW where
wave, combined-flow and current (locally bipo-
lar) ripples are the most common sedimentary
structures (FA7). The sandiest intervals in the
central area show low to moderate levels of
bioturbation mainly composed of a stressed,
mixed Cruziana-Skolithos assemblage, whereas
the muddier successions in the SW and NE show
moderate bioturbation composed of small-scale
proximal Cruziana traces. The distribution of
wave and current sedimentary structures and
trace-fossil assemblages suggest an overall NW to
SW, E and NE transition from medial to a more
distal delta-front setting.
The deposits of zone T2 are overlain by zone
T3.1 (15 m to 20 m thick). The base of this zone
can be recognised in well logs as an inflection
point in the GR-log curve with a corresponding
increase of mudstone separation on the DEN/
NEU logs. This level correlates with the
mid-Tilje Formation Nd143/Nd144 marker of
Martinius et al . (2001) and is another promi-
nent, high-frequency flooding surface. The T3.1
zone displays at least two thick (5 m to 6 m)
coarsening-upward successions composed of
distributary-mouth bar (FA3) and proximal to
medial, tidally influenced delta-front (FA4)
deposits. As in the lower Tilje (T1.2), the dis-
tributary mouth-bar deposits (FA3) are mainly
composed of current-dominated, mixed sand-
stone-mudstone heterolithics that pass upward
into sandstone-dominated heterolithic deposits.
The mouth bars are punctuated by fining-
upward (1 m to 3 m thick) cross-bedded deposits
interpreted as terminal distributary channels
that show a decrease in thickness and grain size
toward the SW of the field (Fig. 21B). FA3
successions dominate the north and central
areas (Fig. 21B) and they change upward and
laterally (towards the SW and NE; Fig. 19) into
more laminated tidally influenced delta-front
deposits (FA4) and more locally into laminated
and muddy proximal prodeltaic deposits (FA5).
The delta-front units show a variety of current-
dominated and wave-dominated heterolithic
facies displaying small-scale HCS and intensely
bioturbated sandstones that are especially com-
mon towards the east side of the field (Fig. 21B).
Evidence of seasonal variations in river dis-
charge is present (cf. Ichaso & Dalrymple, 2009).
Overall, the middle Tilje succession shows a
'lobate' distribution of facies, similar to the
lower stratigraphic interval T1.2.

Sequence 3: Middle-upper Tilje Fm.
(T3.2 to T6)

Distributary mouth-bar (FA3), tidally-influenced
delta-front (FA4) and prodeltaic deposits (FA5) of
T3.1 are overlain by stacked fining-upward tidal-
fluvial channel fills (FA8) and heterolithic tidal
point-bar deposits (FA9) of reservoir unit T3.2
(20 m to 25 m thick). The stratigraphic contact
between T3.1 and T3.2 (SB3) is erosional, with
only a small amount of incision in the SW part of
the field (Fig. 19, cross section I). Nevertheless, it
has been interpreted as a regional hiatal surface
(SB3 of Martinius et al ., 2001). This interpretation
is based on: 1) the abrupt change on grain size from
silt, very-fine and fine-grained sand in T3.1 to
medium and coarse-grained sandstones and locally
conglomerates, in T3.2; 2) the pronounced vertical
change of facies associations, which expresses a
shift of facies from medial-distal deltaic deposits to
the most proximal deposits within the Tilje
Formation; 3) the presence of terrestrially derived
humic debris within T3.2, suggesting proximity to
a fluvial source and freshwater environments in
the catchment area; and 4) the change of bioturba-
tion styles from high-diversity trace-fossil assem-
blages in the underlying T3.1 units, to weak or
almost non-existent bioturbation above SB3.

Middle-upper Tilje (T3.2 to T4)

Reservoir unit T3.2 is characterised by erosively
based, fining-upward, thick, tidal-fluvial channel
deposits (FA8) with GR and DEN/NEU log signa-
tures that show either fining up or blocky patterns.
The channel style changes upward within the
succession (Fig. 19) from single channels (fining-
upward sandstones of FA8 overlain by or replaced
laterally by heterolithic tidal point-bar deposits
of FA9), to more amalgamated, stacked packages
of FA8 sandstones. The FA8 successions are
mainly composed of coarse-grained to medium-
grained cross-bedded sandstone that fines upward
into fine-grained cross-bedded sandstone, bipolar
ripple cross-laminated sandstone and sand-
dominated heterolithics. Basal conglomeratic
lags are more common towards the north of the
field, whereas large rip-up mud clasts and anom-
alously thick (up to 15 cm) fluid-mud deposits
(Ichaso & Dalrymple, 2009) are common across
the field and within the middle-upper Tilje inter-
val, reflecting periodic high fluvial discharge.
These tidal-fluvial channel units commonly change