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 354

dominated by vertical burrows ( Skolithos and
Diplocraterion ) and locally (within the laminated
mudstones) by small Planolites . As with Facies 3,
the trace-fossil suite is suggestive of brackish-water
conditions.

because the variation in sand-layer grain size is
too large and the layer-thickness variations are not
sufficiently regular.

Subfacies 5.2.2: Wavy-bedded combined-flow
heterolithics

5.2) Mixed sand/mud heterolithics

Similarly to subfacies 5.2.1, wavy bedding is the
predominant bedding style in subfacies 5.2.2, but
with a mixture of wave, bipolar current and small-
scale combined-flow ripples. The presence of a
spectrum of pervasive small-scale combined-flow
structures suggests a variation of the type of energy
during deposition (Dumas et al ., 2005), ranging
from oscillatory currents forming wave (symmet-
ric) ripples to unidirectional currents (river flood
or tidal currents) superimposed on wave action to
produce asymmetric round-crested, combined-
flow ripples (Fig. 6F). Poorly laminated mud-
stones, associated with sandstones preserving
wave ripples and/or combined-flow ripples, show
local bottom-up cracks of mechanical origin
(diastasis or syneresis cracks; Cowan & James,
1992). However, a tidal influence is indicated by
the mud-drape pairs and local bipolar ripple cross
lamination. The low to moderate diversity of
the mixed Cruziana-Skolithos trace-fossil suite is
indicative of brackish-water conditions.
The nature of these complex heterolithic deposits
showing dominant combined-flow sedimentary
structures, local mantle and swirl structures
(suggesting soup-ground conditions) and normal
and inverse grading in the poorly laminated
mudstones suggest that deposition might have
occurred by means of hyperpycnal flows
(Bhattacharya & MacEachern, 2009). The alterna-
tion between moderately bioturbated intervals
showing small-scale asymmetric (current) ripples
and parallel-laminated mudstones and weakly
bioturbated intervals containing abundant current
and combined-flow ripples suggest the alternation
of periods of low river flow with periods of high
river discharge (floods), as was proposed for
subfacies 5.2.1 above.

These deposits consist of near equal amounts of
sandstone and mudstone (sandstone : mudstone
ratios between 40 : 60 and 60 : 40) and are predomi-
nantly wavy-bedded showing current, wave and/or
combined-flow structures (Fig. 5). Sand grain size
varies between fine and very-fine with occasional
quartz granule layers. Commonly, these deposits
show sparse to moderate bioturbation (BI 0 to 3).
Two subfacies are recognised.

Subfacies 5.2.1: Wavy-bedded
current-dominated heterolithics

These mixed sand-mud heterolithitcs show domi-
nantly wavy bedding styles (Figs 6D & 6E), together
with bidirectional current-ripple cross lamination
and paired poorly to more laminated mudstones,
suggesting that they were formed by tidal currents
with low to moderate speeds. The alternation bet-
ween more bioturbated, fine-grained thin-bedded
deposits (up to 30 cm thick) and weakly biotur-
bated, coarse-grained, thicker bedded deposits is
interpreted to reflect the alternation of times of low
and high fluvial discharge (Ichaso & Dalrymple,
2009). At times of high discharge (river floods) the
sand fraction is coarser grained and sedimentation
rates are higher, creating thicker beds. This, coupled
with a reduced salinity, minimises the degree of
bioturbation, which is dominated by Skolithos and
Diplocraterion .
Homogeneous mudstones associated with the
coarsest sands (high discharge) suggest high
suspended-sediment concentrations and the forma-
tion of fluid muds, because of the greater supply of
fine sediment by the river (SSC values commonly
correlate positively with river discharge; Xu, 2002)
and the seaward displacement of the turbidity maxi-
mum. By contrast, during times of low river dis-
charge, sand grain sizes are finer, sedimentation rates
are lower, creating thinner beds and salinities are
higher, allowing more intense bioturbation, which is
dominated by small Planolites , Palaeophycus ,
Chondrites and Teichichnus , with local Skolithos
and Monocraterion . Such decimetre-scale cycles are
unlikely to be generated by neap-spring tidal cycles

5.3) Mud-dominated heterolithics

This facies has sandstone:mudstone ratios between
10:90 and 40:60, and consists dominantly of len-
ticularly bedded heterolithics (Fig. 5). Sand grain
size varies between fine and very-fine with occa-
sional quartz granule layers. Commonly, these
deposits show sparse to moderate bioturbation (BI