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 550

Channel development in the chalk of the Tor Formation,
North Sea: evidence of bottom current activity

MATTEO GENNARO* and JONATHAN P. WONHAM †
* Department of Earth Science, University of Bergen, 5007, Bergen, Norway
Corresponding author (E-mail: [e-mail])
† Total Norge A.S., Dusavika, 4029, Stavanger, Norway

ABSTRACT

The establishment of a large epeiric sea over NW Europe during the Late Cretaceous
sea-level highstand provided the ideal conditions for calcareous coccolithophorid
algae to flourish and become the primary components of pelagic sedimentation.
The coccoliths, the skeletal remains of the coccolithophorid algae, were deposited as
thick successions of calcareous chalk ooze on the sea floor. In tectonically active areas,
downslope mass movement of chalk occurred along basin margin slopes, uplifted
areas and around halokinetic structures. The resulting allochthonous deposits consist
of slides, slumps, debrites and turbidites. Simultaneously, bottom currents swept the
sea floor creating an assortment of topographic features. This study focuses on a
particular area of the Norwegian North Sea that includes the southern part of the
Ekofisk Field and adjacent areas. Seismic and well data are used to examine the
characteristics of the Maastrichtian Tor Formation, where a large channel structure has
been identified. The channel is shown to be a significant and extensive feature (~30 km
long). It is oriented WNW-ESE, following the bathymetric contours of major structural
features, notably the Lindesnes Ridge located to the south, and is interpreted to have
been formed by bottom currents that originated from the deepest part of the Norwegian
Central Graben to the WNW. Log and core data indicate a multiphase history of channel
development characterised by variable current energy and interaction with downslope
mass flow processes. The channel has similar features to other recently described
channels observed in offshore chalk deposits from the North Sea region (Germany,
Denmark) that are interpreted to have formed as a result of intensification of bottom
current strength during the Late Cretaceous.

Keywords: chalk, contour current, Ekofisk Field, Maastrichtian, North Sea,
Tor Formation.

INTRODUCTION allowed calcareous coccolithophorid algae to

flourish and accumulate in thick, laterally contin-
uous, chalk deposits at water depths of hundreds
of metres (Surlyk et al ., 2003). Chalk is composed
mainly of tiny low-Mg calcite plates (coccoliths),
the remains of the coccolithophorid algae.
Since the late 1970s, when many hydrocarbon
exploration and production wells began to be
drilled into the Chalk Group reservoirs of
the Norwegian sector and elsewhere in the North
Sea, evidence of both pelagic deposition and
syndepositional re-sedimentation of chalk has
been increasingly identified from the inspection

The Maastrichtian Tor Formation in the Norwegian
sector of the North Sea is an excellent high porosity
carbonate reservoir from which the giant Ekofisk
Field has produced for more than 40 years. This
formation constitutes part of the Chalk Group (Upper
Cretaceous to Danian), a thick carbonate succes-
sion deposited in a large epeiric sea that covered
most of NW Europe during a global sea-level
highstand. The sea was characterised by low terri-
genous sediment influx, normal salinity and warm-
temperate surface temperatures. These conditions