Geoscience Reference
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48.2% and 4.1%, respectively (Fig. 4). In unit L02
the thickness range of individual channel sand-
stone deposits is about 5 m to 15 m; amalgamated
channel successions have given rise to multi-storey
bodies up to about 40 m in thickness. The lower
boundary of L02 is thus defined as the base of a
40 m thick to 60 m thick interval dominated by sev-
eral stacked composite sandstone bodies, underlain
by a 25 m to 35 m interval dominated by floodplain
mudstone in the upper part of L03 (Figs 4 and 8).
L02 and L01 together form a marked upward-fining
trend in the uppermost part of the Lunde Formation
(Figs 3, 4 and 8). The sandstone bodies of L01 occur
as single-storey channel infill successions.
Individual channel infill successions in both L02
and L01 are generally upward fining, dominated by
trough cross-stratification. The cross-stratified beds
are organised in several metres thick bedsets with
practically parallel bounding surfaces that dip at a
steeper angle than the structural dip in the cored
sections. Cross-stratified beds usually pass into
current-ripple laminated fine-grained and very
fine-grained sandstone, alternating with thin beds
of bioturbated mudstone in the uppermost part of
individual upward-fining successions. Coalified
plant debris and rip-up clasts of dark, organic-rich
mudstone are commonly present within the lower
part of the sandstone bodies in L02 (Fig. 8).
The individual upward-fining channel succes-
sions are thought to represent large compound
bedforms of superimposed 3D sand dunes formed
as lateral accretion or point bars, overlain by levée
and overbank to floodplain fines. The individual
FSA4 channel sandstone bodies are thus inter-
preted as meandering river deposits (Fig. 3,
Table 5). In L01, individual fining-upwards chan-
nel sandstone bodies within thick mudstone units
are interpreted as isolated, sinuous or straight chan-
nels within mud-dominated floodplains (Table 5).
Coalified plant debris and thin organic-rich mud-
stone beds point to at least periodically high
groundwater level in depressions and abandoned
channel segments.
and structureless (Fig. 9). A few fining-upward
sandstone units are 3 m thick to 4 m thick, under-
lain by erosional channel-floor surfaces. In the
upper part of the interval, in the S4 unit, this type
of channel sandstone body increases in frequency.
Mudstone units are dominantly reddish-brown
with some calcrete nodules, but grey and greenish-
grey mudstone units without calcrete occur too
(see below).
S5 and the lower part of S4, together with all of
L01, represent the stratigraphic interval in which
a turnaround takes place from fining- to coarsening-
upward grain-size trends and thus the transition
from the Lunde Formation to the Statfjord Group
(Figs 3, 8 and 9).
The dominating type of sandstone bodies of
FSA5 have been interpreted as having formed as
sheets of sand, spread onto the muddy floodplain
during floods. The less abundant, FU sandstone
bodies are considered to represent single and nar-
row, straight, low-sinuosity or meandering streams
on the floodplain. Overall, the CDP index of this
interval is very low, generally below c. 0.2. Variation
in mudstone colour is thought to reflect changes
in soil wetness from undersaturated and aerated
(reddish-brown) to saturated and wet (grey and
greenish-grey).
Fluvial sandstone assemblage 6 (FSA6) - mobile
braided stream channels
Fluvial sandstone assemblage 6 (FSA6) is present
in the USG in the allostratigraphic units S3 to
S1 (Figs 3 and 9). The fluvial sandstone units
here are dominated by multi-storey bodies of
medium-grained to coarse-grained quartz-rich
sandstone (Table 5). Individual channel infill
successions are characterised by an erosional
channel base, frequently covered by pebbles of
quartz, quartzite, metamorphic crystalline rocks,
intrabasinal mudstone clasts and coalified plant
fragments, succeeded by cross-stratified, massive
and parallel-stratified beds, forming blocky or
slightly fining-upward grain-size trends.
Grey and greyish-green mudstone are preserved
as thin beds or laminae within individual sand-
stone bodies or up to some metres thick units
between channel infill successions and with some
thin sandstone bed intercalations. Carbonaceous
root and rootlet structures penetrate a few centi-
metres down into some of the mudstone beds; a
few beds have high organic content. This organic-
rich mudstone facies is associated with thin coal
Fluvial sandstone assemblage 5 (FSA5) - sand
sheets and single floodplain channels
Sandstone bodies of FSA5 occur in the allostrati-
graphic units S5 to S4 in the lower Statfjord
Group (LSG). They are typically fine-grained and
generally thin, up to some decimetres in thick-
ness displaying internal subparallel lamination
or current-ripple lamination, unless bioturbated
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