Geoscience Reference
In-Depth Information
southern Central Graben, where it continued its
erosional and depositional activity (Surlyk et  al .,
2008). A northward flow of returning water seems
to have occurred through the Danish Basin
(Esmerode et al ., 2007; Surlyk & Lykke-Andersen,
2007) (Fig. 20). The postulated directions of these
palaeocurrents system in the North Sea and Danish
Basin are similar to those proposed in recent
palaeogeographic modelling (cf. Hart, 2007;
Fluteau et al ., 2007; Pearce et al ., 2009).
assemblages suggest that the channel formed
and continued to develop as a low-relief fea-
ture, owing its prolonged existence mainly to a
differential aggradational sedimentation along
the thalweg zone and the margins.
4 The channel is interpreted to have formed
during a phase of sea-level fall that intensified
bottom current activity and to have filled
during the subsequent sea-level rise and high-
stand phase. The correlative bipartition of
the  Maastrichtian chalk succession in the
Norwegian Central Graben, with autochtho-
nous and predominantly allochthonous
deposits separated by an extensive submarine
erosional surface, has led previous researchers
to the recognition of two seismic sequences.
The present study indicates another minor
unconformity within the upper sequence, the
base of which is defined by the erosive channel
feature described by this study.
5 The channel is considered to be a local
expression of a regional-scale system of bottom
currents that characterised the European Chalk
Sea reaching a marked activity during the late
Maastrichtian sea-level fall. The bottom current
flowing southwards along the North Sea
grabens probably originated from the Boreal
Sea and during its southern course it was
driven and constrained by the sea floor topo-
graphic relief in the Norwegian Central Graben.
CONCLUSIONS
1 Seismic sections of Maastrichtian chalk show a
large, WNW-ESE trending channel crossing
the southern part of the Ekofisk Field in the
Norwegian North Sea. The channel was ~ 100 m
deep, 2 km to 5 km wide and ~ 30 km long. The
channel has a funnel-shaped planform opening
to the SE and relatively steep southern margin,
which suggests that it was created by a
topographically-guided bottom current that
flowed towards the ESE parallel to the slope of
the Lindesnes Ridge. The current was probably
affected and deviated by the Coriolis force
towards its right-hand side leading to erosion
or non-deposition on the southern channel
margin, while sediment accumulation charac-
terised the northern channel flank.
2 A simple phase of major erosion is discounted
as a mechanism for the channel origin.
Although initiated by substrate erosion, the
channel is considered to have formed by
alternating episodes of sediment bypass, depo-
sition and minor erosion that occurred concur-
rently with the aggradation of the channel
margins. Well logs from the channel-fill and
channel-margin deposits indicate a two-phase
history of channel development: (1) an early
phase of erosion and pelagic chalk deposition,
with sediment bypass predominating in the
channel accompanied by channel margin
build-up; and (2) a later phase of allochthonous
chalk deposition by sediment gravity pro-
cesses, which draped both the channel and its
margins and eventually filled up the former.
3 Although the channel feature is recognisable in
seismic sections, the chalk facies associations
of the channel-fill and channel margins are
similar and show analogous two-phase accu-
mulation. The channel fill to channel margin
stratigraphic correlation and similarity of facies
ACKNOWLEDGEMENTS
This paper is dedicated to the memory of Professor
Michael R. Talbot, who passed away in November
2009. Mike was an exceptional person and scientist
and will be deeply missed. He planned the first
author's PhD study on which this paper is based
and supervised the research project at its early
stages. The study at the University of Bergen has
subsequently been supervised jointly by his two
colleagues, Gunnar Sælen and Wojciech Nemec.
Bruno Caline (Total E&P France), Michel Thomas
(Total E&P Norge) and Eric Lasseur (BRGM) are
thanked for sharing their sedimentological exper-
tise of the chalk. The comments, corrections and
suggestions by reviewers F. Surlyk and A. S. van
der Molen significantly improved the early version
of the manuscript and their contribution to the final
paper is highly appreciated. Total E&P Norge is
thanked for sponsoring this research at the
Search WWH ::




Custom Search