Tectonic influence on the Jurassic sedimentary architecture in the northern North Sea with focus on the Brent Group - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 411

1°0 ′

210

211

34 36

34 35

Unst
Basin

61°0 ′ N

29

No r way

East
Shetland
Basin

2

32

3 30

31

East
Shetland
Platform

60°0 ′ N

4

Horda
Platform

10

28

8

9

24

25 26 27

Major intra-basinal highs
Offshore mudst. High s u bsid enc e
Offshore mudst. Less subsid e nce
Tarbert Fm. sands
Ness Fm. Alluvial plain
Main Jurassic faults

Utsira
High

0 50 km

59°0 ′ N

Fig. 14. Schematic illustration of the transgressed northern Viking Graben with the footwall islands in the early part of the
Late Jurassic Epoch.

which made the footwall area of this block a (tem-
porary) footwall island. The ongoing block rota-
tions created estuaries with amplified tidal
currents in the hangingwall and spits/shoreface at
the footwall highs (Figs 13 and 14).
Under such transgressive conditions the net
sediment transport is directed landward (van der
Molen & van Dijck, 2000), where barrier-inlets
(as shown in Fig. 13) will act as grain size-filters
(Oost, 1995). Finer-grained sediments pass
through the inlets whilst coarser-grained sedi-
ments will be kept within or outside the inlets.
These observations can explain why the back-bar-
rier area of the Tarbert Formation has finer-grained
sediments (Figs 9A, 9B, 12B and C) and may also
explain the origin of the enigmatic 'silk sand'

(Bruaset et al ., 1999; named after its textural prop-
erties) found in the Gullfaks Field. The 'silk sand'
unit is structureless and trough cross-bedded,
medium grained, 30 m to 54 m thick and about
1.5 km wide (E-W), tidally influenced sandstone
with high permeability (Bruaset et al ., 1999). Its
tidal character and the uniform grain-size trend,
typical of the back-barrier area (Oost, 1995), indi-
cate that the 'silk sand' may represent a tidal-bar
complex or a flood-tidal delta. A full analysis is
beyond the scope of the present study but it is
interesting to note its presence in our study area
as a consequence of the transgression caused by
the rotation of the Permo-Triassic megablock.
The Late Bajocian transgression of the Brent
Group with block-faulting along the pre-existing