Channel development in the chalk of the Tor Formation, North Sea: evidence of bottom current activity - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 579

Sea-level
High

Low

7 Deposition of Ekofisk
dense zone

Channel

7

6 Resedimentation, channel is filled

6

Preset day
sea-level

5 Increased bottom current, channel formation

5

4 Sedimentation, increased mass-movements

7

4

6

5

3 Erosion and non-dep. Middle Tor SB 3

3

2

1

2 Sedimentation and onlap

2

Sea-level

1 1 Erosion - Top Magne unconf.

N S

Fig. 18. Simplified interpretation of the N-S seismic cross-section in Fig. 3 illustrating the sequence stratigraphic
interpretation of the deposition of the Tor Formation including the development of the channel described in the present
study. The successive development phases are as follows: (1) formation of an extensive erosional surface at the top of the
Magne Formation; (2) resumption of chalk sedimentation and progressive onlap of the erosional surface; (3) an increase in
bottom-current activity and formation of the mid-Tor Formation unconformity; (4) resumption of chalk sedimentation and
increase in its reworking; (5) another increase in bottom-current activity and formation of the channel; (6) a decrease in
current activity and filling of the channel; and (7) cessation of current activity followed by the deposition of clay-rich chalk
and formation of the Ekofisk dense zone. The sea-level curve is after Kominz et al . (2008).

on top of the slope it began to scour the poorly
consolidated chalk and incise the sea floor.
Although Fig. 19 depicts the present day
structural configuration of the study area, various
studies of chalk thickness maps and seismic
cross-sections (e.g. Bramwell et al ., 1999) show
that this 3D seismic map mirrors strongly the
physiography of the study area during the late
Maastrichtian. It therefore provides an illustra-
tion of the inferred current flow in relation to the
coeval topography.

The microfaunal assemblage of well 2/7-14
indicates that the channel margin deposits
identified here may reflect deposition under the
influence of deep water masses since they are
enriched in planktonic foraminifera and
nannofossils and radiolarians that could have
been transported by bottom currents of deep water
provenance. However, changes in the planktonic/
benthonic foraminifera ratio may also be due to
other factors, such as relative sea-level changes
or fluctuating saline/nutrient-rich plumes (e.g.