Geoscience Reference
In-Depth Information
Well 34/8-1 is located at the footwall high of a
Permo-Triassic fault (Fig. 11) and shows predomi-
nantly wave-dominated strata in the Brent Group.
Here, the progradational portion extends as far
into the basin as well 34/6-1. An alternative inter-
pretation of the facies differences in the prograd-
ing 'Brent delta' would be to regard them as an
effect of lateral deltaic variability owing to the dis-
tance between the feeder systems. However, in the
context of the fault-block rotations at this time
(Figs 4 and 5), the related fault locations (Figs 1
and 11) and the manner in which the coastline
evolved later (see below) leads up to a strong
impression that these faults had an impact on the
shape of the coastline.
transforming to stacked bay-head deltas capped by
beach-barrier complexes in 34/10-A-11 (Figs 12C
and 13) that pass laterally into a wave-dominated
estuary in 34/7-22 (Figs 12B and 13; expressed as
fluvial-tidal channel; cf. Davis
et al.
, 2000). In light
of the argued influence of fault activity in the Ness
and Tarbert formations, the facies distribution
pattern described above for the Tarbert Formation
can best be explained by rotation of the Permo-
Triassic fault-blocks giving an asymmetric subsid-
ence pattern along the coastline. In this way the
estuary development represents high-subsidence
and accommodation space creation in the hang-
ingwall part (well 34/7-22) and the wave-reworked
area as the lesser subsidence (with low accommo-
dation-space creation) in the footwall part of the
Permo-Triassic fault block (34/8-1; Fig. 13). It is
typical for rift basins to show this kind of variabil-
ity in sediment infill patterns and facies types
along the coast (Gawthorpe & Leeder, 2000), as
observed here for the Tarbert Formation. Rift or
fault activity within the Tarbert Formation, with
some impact on the lateral facies variation, has
been reported by several authors (Fält
et al
., 1989;
Fjellanger
et al
., 1996; Ravnås
et al
., 1997; Davies
et al
., 2000; Ravnås
et al
., 2000).
Coastal morphology - retrogradational part
The Tarbert Formation represents the retreating
part of the 'Brent delta' and shows a variation in
depositional environments along the coast (east-
west; Fig. 12B, C and D) in the Middle-Late
Bajocian. The Tarbert Formation shows wave
reworking in well 34/8-1 as spit environment
(see also Ravnås
et al.
, 1997; Figs 12D and 13),
DISCUSSION
34/8-1
This study has demonstrated that Early Bajocian
fault-activation led to wedge-shaped stratal devel-
opment in the Ness and Tarbert formations in the
Tampen area. In terms of displacement and strain,
these fault movements are subordinate but suffi-
ciently large to have influenced the distribution
and lithological partitioning (net to gross ratio) of
Middle Jurassic strata at the scale of 100s of metres.
In this section, we discuss how our findings add to
the understanding of Jurassic rift-initiation in this
part of the North Sea rift system.
34/10-A-11
34/7-22
Rift initiation
~5 km
Well location not to scale
The period between the Late Permian-Middle
Triassic and Middle-Late Jurassic rift episodes is
traditionally characterised as a post-rift period (i.e.
a period of tectonic quiescence). By contrast, there
is evidence of fault activity at various stages dur-
ing this time interval that suggests this period is
better described as an inter-rift period during
which minor fault-slip accumulated due to subsid-
ence and compaction. It has also been speculated
Offshore
Coastal plain
Shallow marine sands
Bay
Fault
Well
Fig. 13.
Schematic illustration of the variation in deposi-
tional environments of the Ness and Tarbert formations
(during transgressive) in wells 34/7-22, 34/10-A-11 and
34/8-1 and their relationship with the faults. Well position
not to scale. Here, the shoreline at the footwall area extends
further into the basin, whereas the hangingwall area expe-
rienced transgression (see Gawthorpe & Leeder, 2000).
Search WWH ::
Custom Search