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 574

Edda Field
2/7-4

Exploration well
2/7-14

Exploration well
2/4-12

Ekofisk Field
2/4-A8

Ekofisk Field
2/4-A6

50 50

30 30

clast size (mm) clast size (mm)

Southern margin Northern margin

8.5 km 6.8 km 4.8 km 0.2 km

Top Tor Fm

GR Ø D

GR Ø D

Lower
channel
unit

Ø D

GR
m TVDSS

Channel
outline

Base channel

?

GR Ø D

?

N

2/4

GR Ø D

Ekofisk

Well

2/7-14 2/4-12 Channel 2/4-A6

Top Tor Fm

2/7-4

2/4-A8 and A6

2/4-12

2/7- 4

Middle Tor SB 2/7- 14

Top Magne Fm

Edda

1km

Sesmic artifact-flat spot ESE

WSW ENE ENE WNW WNW NNE NNE 2/7 2/8

Fig. 15. Well correlation panel across the western segment of the channel. The cross-section is flattened at the top of Tor
Formation. The location of wells is shown in Fig. 4 and the legend is as given for Fig. 14. Depths are given in True Vertical
Depth Sub Sea (TVDSS).

A number of key factors point towards a contour
current origin and are discussed below.
Channel incised base: an incised character is
seen at the base of the channel on seismic data.
Similar incisions have been seen in the Chalk
Group of the Danish Sector and are interpreted
to have been produced by bottom currents
(Esmerode et al ., 2007). The channel is inter-
preted to have been initiated by a phase of sea
floor erosion, as indicated by the incised irregu-
lar base (Fig. 5, Sections A and B) and the pres-
ence of facies PGC interpreted to have been
produced by current winnowing and erosion
(Table 2). Sediment erosion would be within the
capability of bottom currents. Surlyk & Lykke-
Andersen (2007) consider that erosion of non

cemented chalk ooze could occur when currents
flow at 8 cm s −1 to 20 cm s −1 at approximately 20 m
above the sea floor. Studies of modern bottom
currents indicate a wide range of flow strengths,
largely independent of the current generative
mechanism. For instance, the velocities of ther-
mohaline bottom currents vary from 20 cm s −1
(Hollister & Heezen, 1972) to 300 cm s −1 (Gonthier
et al ., 1984) and also the velocity of deep-water
tidal currents, recorded down to 4000 m sea
depth, can vary from 25 cm s −1 to 280 cm s −1
(Shepard et al ., 1979; Shanmugam, 2008). Wind-
driven bottom currents are generally stronger,
with velocities of up to 200 cm s -1 recorded at
water depth of 100 m in the Gulf of Mexico
(Cooper et al ., 1990).