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 355

1 to 3). Two subfacies can be distinguished: wave-
dominated and current-dominated.

due to complete post-depositional biogenic homog-
enisation. Sand grain sizes vary between very-fine
and fine. Occasional granules occur. Facies 6 has
been subdivided into five subfacies based on the
grain size, texture of the deposits, colour and the
degree and type of bioturbation.

Subfacies 5.3.1: Lenticular-bedded wave-
dominated heterolithics

The continuous to discontinuous, sandy lenticu-
larly-bedded nature of this subfacies, together with
the presence of parallel lamination, symmetric
(oscillatory) to round-crested ripples (Figs 7A & 7B)
and a dominantly horizontal trace-fossil suite
( Planolites, Palaeophycus and Chondrites ), suggest
deposition by weak oscillatory currents. Local small-
scale HCS reflects elevated oscillatory motion due to
sporadic storms (cf. Dumas et al ., 2005). The storm
waves appear to have re-suspended mud from the
sea floor, forming high-density suspensions that
then migrated offshore as a hyperpycnal fluid-mud
body (Mulder et al ., 2003), depositing some of the
thick, homogeneous and/or weakly laminated mud-
stones. Mudstone showing small-scale horizontal
and vertical burrows and continuous and distinct
parallel lamination is interpreted to be deposited by
settling from suspension in the presence of very
weak currents or wave action.

Subfacies 6.1: Reddish bioturbated sandstone

The homogenised aspect of the reddish silty fine/
medium-grained sandstone showing abundant
to complete bioturbation (high-diversity mixed
Skolithos-Cruziana suite; Figs 8A & 8B), together
with the presence of sub-angular limestone grains,
shell debris and mixing of sand and mud, suggest
extensive biogenic reworking at times of slow to
negligible sedimentation. The large vertical burrows
( Rosselia , Cylindrichnus , Skolithos and Berguaeria ;
Fig. 8A) and occasional tabular-stratified beds, cross-
bedding, low-angle lamination and coarsening
upward bedsets (up to 1 m thick), are indicative of
relatively high levels of wave or current energy. The
local presence of horizontal burrows ( Planolites ,
Palaeophycus and rare Thalassinoides ; Fig. 8B)
indicates intergradation with proximal expressions
of the Cruziana Ichnofacies suggesting moderate
energy levels, probably lying below fair-weather
wave base but above storm wave base (MacEachern
et al ., 2006).

Subfacies 5.3.2: Lenticular-bedded
current-dominated heterolithics

As is the case with Subfacies 5.3.1, sandy lenticu-
lar bedding is present in subfacies 5.3.2. However,
the predominance of asymmetrical ripples and
parallel lamination (Figs 7C, 7D & 7E) suggest that
Subfacies 5.3.2 was formed by weak unidirectional
currents, with tidal influence indicated by local
bipolar cross lamination and the mixed Cruziana-
Skolithos trace-fossil assemblage. Laminated mud-
stone layers showing moderate and more diverse
bioturbation suggest slow sedimentation from sus-
pension during periods of lower stress. The local
presence of coarse-grained sandstone lenses, with
abundant ripple cross-laminated fine-grained sand-
stone and poorly laminated mudstone showing
low bioturbation indexes by a low diversity assem-
blage, suggest periods of high river discharge, simi-
lar to what was seen in subfacies 5.2.1 and 5.2.2.

Subfacies 6.2: Disturbed sandstones

The fine-grained to medium-grained sandstones
(20 cm to 40 cm thick) showing common to abun-
dant bioturbation (low-diversity, archetypal
Skolithos Ichnofacies (cf. MacEachern et al ., 2006),
coupled with locally preserved current-dominated
structures, suggest moderate to high current energy.
U-shaped large dwellings (10 cm to 25 cm long
Diplocraterion ; Figs 8C & 8D) indicate that the
organisms had to respond to an ever shifting sub-
strate, probably related to tidal processes.

Subfacies 6.3: Disturbed heterolithics

These mud-dominated and/or mixed heterolith-
ics (20 cm to 40 cm thick) show common to
abundant bioturbation (high-diversity proximal
expressions of the Cruziana Ichnofacies
(MacEachern & Bann, 2008)). The trace-fossil
elements commonly tend to cross-cut each other
(Figs 8E & 8F); however, the original bedding
is still recognisable. The sand : mud ratio and

Facies 6: Intensely bioturbated sandstone
and heterolithics

This facies consists of intensely to completely bio-
turbated (BI 4 to 6) muddy sandstone (Fig. 8), in
which mudstone laminae are barely discernible