Tectonic control on sedimentation, erosion and redeposition of Upper Jurassic sandstones, Central Graben, North Sea - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 473

North Sea Central Graben. This region has been
explored for thirty years (Erratt et al ., 2005) and
tests ever deeper and hotter targets approaching
200 ºC and beyond 1100 bar pressure. The cost of
exploration and development in such an environ-
ment means that significant efforts have been
made to understand the sedimentology and reser-
voir architecture of Upper Jurassic sandstones.
These sandstones developed throughout the
Central Graben and are known by a variety of for-
mation names, although commonly referred to as
Fulmar Formation in the UK sector and Ula
Formation in the Norwegian sector. They range
from Oxfordian to Volgian in age and overlie a
varied subcrop stratigraphy ranging from Middle
Jurassic fluvial deposits with coals (Pentland
Formation in the UK sector, Bryne Formation in
the Norwegian sector) to Triassic continental/
lacustrine deposits (Skagerrak and Smith Bank
Formations) and Permian Rotliegend Group or
Zechstein salt. The reason for this variety is their
transgressive character, progressively overlying
older rocks that had been exposed following the
Middle Jurassic Mid-Cimmerian uplift. This uplift
defined a regional thermal doming focused on the
North Sea triple rift junction (Underhill &
Partington, 1993) and strongly influenced the sub-
sequent rift-related structuring of the basin (Errat
et al ., 1999). Greater erosion on the unconformity
surface occurs towards the centre of the dome.
Detailed study by Davies et al . (1999) based on
regional mapping across the triple rift junction
area identified this Mid-Cimmerian Unconformity
as intra-Aalenian in age, although intra-Callovian
and intra-middle Oxfordian age unconformities
were also recognised. The intra-Callovian uncon-
formity is developed between the Pentland
Formation and the overlying Heather Shales or
equivalent age Fulmar Formation deposits.
Broadly, the Upper Jurassic sandstones of the
Central North Sea can be characterised as belong-
ing to one of two depositional types: (1) shoreface
deposits and (2) turbidite deposits. The criteria for
recognising these different types of deposit are
discussed later. Both types of sandstone can pass
laterally into offshore shales. Coastal plain coal-
bearing horizons and lagoonal shales are locally
identified, but are generally rare.
In the Central North Sea basin, Upper Jurassic
shoreface sandstones can be locally very thick (up
to 300 m or more) while the turbidite sandstones
are generally thinner, forming units where the
gross thickness is rarely more than 100 m. For

both sandstone types, prediction of reservoir pres-
ence, thickness and quality often proves difficult
because of the complex underlying structural con-
trols on accommodation and facies development.
These structural controls have a variety of origins
and will be expanded upon in more detail in later
sections. Reservoir thickness may also reflect vari-
able preservation due to intraformational or post-
depositional truncation. In much of the region
discussed, the sandstones are at a depth of 4 km to
5 km so that seismic resolution is commonly poor
and such truncations are difficult to observe
seismically.
This paper has three main objectives: (1) to
show examples of the tectonic context for convert-
ing areas of former deposition into areas of ero-
sion and to propose a simple scheme for classifying
three recognised types of tectono-sedimentary set-
ting in which this occurs; (2) to examine the basis
on which major erosion surfaces can be identified
in sandstones of shallow marine Oxfordian age by
focusing on the importance of Rhaxella sponge
spicule-bearing sandstones as correlateable units;
and (3) to examine the products of erosion in the
form of turbidite sandstones.

STRUCTURAL FRAMEWORK

The stratigraphic complexity of the Upper Jurassic
sandstones in the Central Graben is well known
and primarily a function of the varied structural
controls on their deposition. The structural evolu-
tion of the Central North Sea Graben is the subject
of an extensive literature with many studies based
on 3D seismic data presented in Parker (1993) and
Fleet & Boldy (1999). The Central Graben can be
regarded as a series of N-S oriented sub-basins,
progressively offset along Tornquist basement lin-
eaments (Errat et al . 1999, Fraser et al ., 2002). The
development of extension along normal fault sys-
tems as seen in the Viking Graben area (e.g. Rattey &
Hayward, 1993) also occurred in the Central
North Sea. However, the variety and complexity
of tectonic structures was greater due to superpo-
sition of the Central North Sea Basin above the
older North Permian Basin, which displays a thick
development of evaporitic halite deposits, origi-
nally up to 2 km thick in the basin centre (Stewart &
Clark, 1999).
A variety of views exist on the structural impor-
tance of salt in the area. Eggink et al . (1996) con-
sider that salt withdrawal played a role in the