Geoscience Reference
In-Depth Information
The vertical organisation of fluvial facies asso-
ciations within the Lunde Formation and the
Statfjord Group can be characterised in three major
motifs (Table 5). These major types of vertical
facies trends have been interpreted to represent
(a) braided stream deposits dominating in the
lower part of the Lunde Formation (LLF) and in
the upper part of the Statfjord Group (USG), (b)
sinuous to meandering major streams in the upper
part of the Lunde Formation (ULF) and (c) single,
isolated sinuous to straight stream deposits pri-
marily in the interval comprising the ULF and the
lower part of the Statfjord Group (LSG). Soils
formed on floodplains and proximal slopes of
alluvial ridges are represented in well cores by
various types of palaeosols (Müller
et al
., 2004).
No significant changes are recorded in the grain
size of channel sandstones through the Lunde
Formation and the LSG, but the sand grain-size
increases to coarse and very coarse in the USG.
In the Lunde Formation, conglomerates with
extrabasinal clasts are extremely scarce, except in
the lower part (L11-L09) where pebble-sized clasts
of granite, gneiss, quartz and quartzite occur in
thin conglomerate beds at, or close to, the base
of the allostratigraphic units. Similar extrabasinal
clasts also occur in the coarse-grained sandstones
in the upper part of the Statfjord Group. Channel
lag conglomerates are otherwise composed of
intrabasinal clasts of pedogenic carbonates and
mudrock.
The channel sandstones throughout the Lunde
Formation and the lowermost part of the Statfjord
Group are feldspathic with an average feldspar/
quartz ratio of 0.56. In the USG, the sandstones are
quartz arenites with an average feldspar/quartz
ratio of 0.18 (Table 2). Clay minerals, namely illite,
smectite, kaolinite and chlorite, occur as clastic
grains or diagenetic products from the alteration
of feldspar and mica, while calcite fills pore
spaces (Sørlie, 1996; Khanna
et al
., 1997).
The 'channel deposit proportion' (CDP), defined
as the proportion of channel deposits relative to
bulk volume (Bridge & Mackey, 1993), generally
decreases upwards through the Lunde Formation,
reaching a minimum within the transitional
boundary zone to the overlying Statfjord Group
(Fig. 4); from here the CDP increases once more.
The fluvial deposits of the Lunde and Statfjord
formations are subdivided into six 'fluvial sand-
stone assemblages' (FSA1-6), each consisting of
one or several allostratigraphic units characterised
by particular fluvial facies and facies associations,
CDPs and inferred channel types. Core photo-
graphs of representative facies are presented in
Fig. 5.
Fluvial sandstone assemblage 1 (FSA1) - mobile
braided stream channel belts
FSA1 comprises the allostratigraphic units L12-
L10 in LLF (Figs 3 and 6). The lowermost channel
sandstone body of unit L12 rests with a marked
erosional boundary on the Alke Formation (Fig. 6).
This is a regional unconformity in the Tampen
Spur area (see above). The lower boundary of L11
is characterised in several wells by an extrabasi-
nal conglomerate bed with pebbles of quartz,
quartzite, granite and gneiss. This type of conglom-
erate is also present in the upper part of unit L11.
In the L12-L10 units, the CDP exceeds 40%, the
average varying from about 66% in L12 to 53% in
L10 (Fig. 4). Floodplain fines occupy about 35%
on average and fine-grained overbank sandstones
about 10%.
Channel sandstone bodies, inferred to repre-
sent individual channel infills, range in thickness
between 6 m and 9.5 m and are composed domi-
nantly of plane-parallel to low-angle cross-stratified
sandstone, with mainly box-shaped vertical grain-
size curve, or slight upward fining in the upper-
most part towards the top. Internal erosional
surfaces, occasionally draped with mudstone
fragments, are frequent in some sandstone bodies.
Calcrete and mudstone clasts are common at the
base of many of the channel deposits in L12; a small
coal fragment has been recorded at the base of one
channel sandstone. Individual channel sandstone
bodies are frequently stacked, forming composite
20 m to 40 m thick sandstone units, or multi-storey
sandstone bodies (Fig. 6). A few channel infill
successions reveal distinct fining-upward grain-
size trends. Floodplain mudstones between channel
sandstone bodies are commonly reworked by
bioturbation and have root structures along
palaeosol horizons (see below). The bioturbation is
simple burrows of various orientation, most com-
monly with diameters of about 1 cm; some burrows
are irregularly shaped as chamber-like structures.
No detailed studies of the bioturbation have been
performed; its origin may be related to the burrow-
ing of worms and/or insects in moist or dry mud.
The fluvial sandstone bodies in L12-L10 are
characterised by the association of plane-parallel
and low-angle cross-stratified beds, no marked
change in vertical grain size except at the very top
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