Geoscience Reference
In-Depth Information
succession is a massive sandstone (1 m thick) with
no apparent traction structures but over-steepened
lamination evident towards the top. The sand-
stone passes gradually upwards to a zone with
very poorly size-sorted shale clasts. This zone is
overlain by a well-cemented interval with an
upward increase in shale clast size and abun-
dance. The top of the sandstone bed is capped by
a laminated unit which is a few cm-thick. The
largest shale clasts are of the same type as the in
situ dark shales seen elsewhere in the core and are
considered to be typical for the Sele Formation (as
described by Brunstad
et al
., 2009). Most of these
clasts are 1 to 3 cm long, but some clasts outsize
the circa 10 cm-wide core. Folded and micro-
faulted silty lamination can be observed in the
larger clasts. a second shale clast family consists
of an association of mostly green and brown, small
(<1 cm) angular clasts.
Interpretation and discussion: The basal sandy
part of the cored interval in well 25/6-3 is inter-
preted as a turbidite bed. The presence of internal
de-watering sheets and a thin injectite in the shale
underneath suggest that sand may have been
injected into the turbidite after deposition. The
overlying shale-clast conglomerates are inter-
preted as debris-flow deposits. The small clasts of
various colours are clearly distinct from the diag-
nostic dark shales of the Sele Formation which
were deposited during a period of anoxia in
the deeper parts of the Northern North Sea (see
'Background Sediments' above; and Brunstad
et al
., 2009). The coloured clasts must therefore
have been eroded from sediments in a shallower
oxic part of the basin or from an older formation
that was deposited prior to the period of anoxia.
Both scenarios imply long transportation and
the fact that the clasts are still angular suggests
that they were ripped up by deep erosion into a
well lithified formation. It appears to be most
likely, therefore, that these clasts have been trans-
ported at least from the deeply incised feeder can-
yon more than 60 km away. The larger clasts were
eroded from the Sele Formation background sedi-
ments and became deformed in the sediment
transport process. These clasts have not necessar-
ily travelled very far in the flow. The debris-flow
interpretation concerns the mechanism for sedi-
ment transportation immediately prior to primary
deposition and does not preclude later sediment
remobilisation. Soft-sediment deformation after
deposition seems probable based upon the obser-
vation of injectites below (see Fig. 11) and some of
the local shale clasts may thus have been plucked
from the encasing shale during injection of sand
(Johansson & Stow, 1995). The increase in clast
size and abundance towards the top of the upper-
most cemented interval is interpreted as an indi-
cation of reworking in a slightly turbulent flow
where the clasts were free to move and become
sorted. The topmost laminated layer was depos-
ited from a low density turbidity current.
This facies is interpreted as a fan-fringe deposit
overlying the transition from S1 to S2, which
marks the turnaround from fan growth to fan
retreat (Figs 4 and 10). The channel-splay transi-
tion zone is believed to be landward stacking in
the fan retreat phase. Well 25/6-3 is drilled close
to the terminus of a S1 seismic-scale channel
(Fig. 5) and the overlying S2 sub-seismic scale
sandstone unit could therefore represent a more
distal facies type, deposited in the fan retreat
phase. a systematic co-occurrence between tur-
bidites and overlying debrites has been docu-
mented from the outermost splays of many ancient
and modern submarine fans. The basal sandstone
may be the deposit from a turbidity current which
was a forerunner to the debris flow that deposited
the shale-clast conglomerate on top of it (Haughton
et al
., 2003). an alternative interpretation is that
the big clasts and sandstones represent singular
debrite intervals (Talling
et al
., 2010).
Massive sandstone
description: This facies consists of tens of metres-
thick sandstones with 'blocky' petrophysical log
responses. In well 25/6-3, the uppermost (c. 20 m)
of seismically defined channel sandstone (Fig. 10)
has been cored (Fig. 11). The cored interval con-
sists of massive sandstone (i.e. sandstone with
no visible traction structures). The grain size is
mostly fine to medium (occasionally coarse to
very coarse) and the grains are angular to sub-
angular or sub-rounded and moderately sorted.
No systematic vertical grain-size trends have been
observed but some coarser grains appear to be
concentrated as lags. Sedimentary structures are
generally not visible; however, faint dish struc-
tures are present in some places. The upper
boundary of this mostly massive sandstone is
sharp and parallel to the layering in the overlying
dark shales (Fig. 9). The shale that immediately
overlies the sandstone has a dark colour and less
associated silt laminae than the background sedi-
ments seen elsewhere in the core. The dark shale
Search WWH ::
Custom Search