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 481

major depositional successions varies across
the basin, reflecting the changing geographical
localisation of structurally-controlled depocentres
through time. At the same time, this study relies
on geological and geophysical integration, for
example: identifying sections in wells that have
been removed by faults and correctly calibrating
seismic data to geological interpretations at wells.
Locally significant erosive surfaces, such as that
developed across the Elgin-Franklin Field (see
later discussion) are considered to be controlled
by local tectonics rather than eustasy. This implies
that regional correlation of such erosive surfaces
throughout the basin should be resisted. The aim
of recognising such erosion surfaces (and down-
dip basinward shifts of facies) is to be predictive
about the possibility of sandstone reservoirs pro-
duced by erosion and re-deposition lying further
down depositional dip. If the erosion is a product
of local tectonism, this predictability will also be
purely local. Thus the emphasis of this paper is
placed on recognising erosion surfaces where they
exist within or above the shoreface sandstones
and considering their development in a tectono-
sedimentary context.

(Lasocki et al ., 1999). These studies have some-
times been equivocal about the depositional origin
of the sandstones, pointing to the fact that primary
sedimentary structures, which might have indi-
cated a depositional process origin, have almost
always been entirely obliterated by bioturbation.
Johnson et al . (1986) considered that storm-
induced currents were responsible for depositing
a large part of the succession, while gravity-driven
turbidity flows deposited a minor component
(roughly 10%). These processes were interpreted
to have operated in either of two different deposi-
tional settings: (1) a shoreface setting and (2) a
shelf setting. The authors were not explicit about
which model was preferred or at which times one
model or the other might be operating; rather they
represented two possible scenarios.
Subsequent studies placed greater or lesser
emphasis on shoreface or shelf depositional
settings. While some subsequent studies preferred
a shelf and offshore bar model (Armstrong et al .,
1987; Price et al ., 1993), a growing consensus
emerged that the Fulmar Formation was domi-
nantly a shoreface deposit. Gowland (1996)
defined two categories of shoreface: a high energy,
storm-influenced shoreface with some preserva-
tion of sedimentary structures and a low-energy,
bioturbated shoreface. The shelf model was still
retained, although deposits of this type show little
difference sedimentologically to the low energy
shoreface model. Donovan et al . (1993) also pre-
ferred a model of a low-energy wave-dominated
shoreline for the Fulmar Formation and Lasocki
et al . (1999) interpreted the Oxfordian Franklin
Sandstone of the Elgin-Franklin Field to be a
shoreface deposit. The Franklin Sandstone shows:
(1) clear coarsening-upwards, progradational par-
asequences bounded at top and base by flooding
surfaces and (2) a clear and gradual facies change
from north to south across the Franklin field pass-
ing from shoreface succession in the north towards
interfingering offshore deposits in the south where
a major, salt withdrawal-related, depocentre had
been identified. Recent studies of the Fulmar
Formation type sections in the Fulmar field have
firmly ascribed a shoreface origin to the majority
of the sandstones (Kuhn et al ., 2003).
The suggestion of Johnson et al . (1986) that tur-
bidity current processes might be responsible for
some Oxfordian age sandstones of the Fulmar
Field has been followed by other reports of turbid-
ite deposits from elsewhere in the Central Graben,
notably studies by Robinson (1990), Howell &

DEPOSITIONAL MODELS FOR UPPER
JURASSIC SANDSTONES

A large number of studies of Upper Jurassic sand-
stones in the Central North Sea basin have contrib-
uted to the understanding of these deposits. Studies
of wells and cores have their limitations when
reconstructing depositional models and resulting
sand body geometries or depositional patterns.
Outcrop analogues for the Fulmar Formation have
been sought, one of the most cited being the well
exposed Liassic outcrop of the Wessex Basin in
southern England. These sandstones appear as
massive tabular units with many cemented shell
beds of large lateral extent (up to 2 km) in cliff out-
crops (Kantorowicz et al ., 1987). Low angle, metre-
scale undulating bedforms have also been described
(Pickering, 1995). More recent study has shown the
development of large-scale, shingled clinoforms
observed in 3D seismic data from the nearby Wytch
Farm oilfield (Morris et al ., 2006).
Detailed sedimentological studies of the Fulmar
Formation are mainly based on the analysis of
closely spaced wells in large fields such as Fulmar
(Johnson et al ., 1986), Ula (Spencer et al , 1986),
Gannet (Armstrong et al ., 1987) or Elgin-Franklin