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 504

(1) sequence boundaries or their correlative con-
formities can be traced as synchronous surfaces
around the basin; (2) turbidite deposits are
introduced into the basin during synchronous,
eustatically controlled lowstand periods and (3)
turbidite deposits indicate a deep-water setting.
These conclusions are typically based on regional
studies but are not generally supported by the
findings of local case studies, such as those
reviewed in this paper, where a more complex
relationship between shoreface and turbidite mass
flow sandstones is documented.
In previous studies, sequence boundaries have
been used for regional correlation (e.g. Donovan
et al ., 1993); possibly with as many as eight sequence
boundaries indicated to be associated to lowstand
incision phases in the Upper Jurassic section
(Partington et al ., 1993b). However, it is consid-
ered here that the erosion surfaces previously pos-
tulated to be related to incision events were more
probably produced at different times due to local
tectonic changes, as supported by a number of
seismic-scale examples. In addition, it is considered
that in a strongly wave-influenced setting, such as
the Central Graben during the Oxfordian, preser-
vation potential for sequence boundaries may be
very limited due to later shoreface erosion during
transgression. This may also contribute to the lack
of preservation of time-equivalent continental
deposits.
The current study extends the stratigraphic
subdivision of the Upper Jurassic in the North
Sea Central Graben using four main concepts:
(1) sandstone depositional environment; (2) parase-
quence stacking trend analysis; (3) maximum
flooding surface correlation and (4) identification
of significant erosion surfaces. Detailed studies
based on numerous wells in the Elgin and
Franklin fields show that there is a close rela-
tionship between local tectonic changes related
to grounding of Triassic pods, development of a
significant erosion surface over the grounded
pod crest and the synchronous/subsequent depo-
sition of turbidite deposits in areas adjacent to
the pod. A similar relationship may be present
over the Fulmar Field where grounding of a
Triassic-Jurassic pod is interpreted to have
resulted in erosive reworking of shoreface sand-
stone at the crest of the structure (inverted high)
and deposition of turbidite sandstones on the
flanks of the structure.
Turbidite sandstones are observed in a number
of areas to immediately overlie the shoreface

(Shearwater Field UK-22/30b-11 well) or shore-
face-shelf transition deposits (Jacqui UK-30/13-3
well). Turbidite sandstone development appears
to occur widely post J54b Serratum MFS. While
local structural mechanisms may have created
local sources for these sandstones, it remains pos-
sible that turbidite deposition was linked to an
increase in the rate of rifting. This is also consist-
ent with the regional translation of shoreface dep-
osition from the central parts of the basin (e.g.
NO-2/4-20 well of Fig. 5) to the margins of the
basin (NO-2/1-4 well of Fig. 5). This translation of
the shoreface occurs above the widely recognised
J56 Rosenkrantzi MFS.
The link between increased turbidite deposi-
tion and increased rifting may have occurred via a
sequence of related events: (1) increased rates of
extensional fault movement; (2) increased flow of
salt up major faults and local differential salt
movement over fault block crests and (3) subse-
quent increased rates of pod grounding and ero-
sive reworking of shoreface sandstone as turbidite
sandstones. During the period between the
Serratum and Rosenkrantzi MFSs, the normal
shoreface depositional style became increasingly
disrupted by local tectonic controls. Factors that
enhanced the rapid deposition of shallow water
turbidites became increasingly common. The
most important of these might have been pod
grounding and associated erosion of previously
deposited shoreface deposits.
While a deepening of the basinal setting is
expected, this may not have been very substan-
tial. Equally, the distances over which the tur-
bidite deposits were transported may not have
been large, perhaps only 1 km to 5 km (Fig. 26).
Turbidite sandstone deposition is interpreted to
have taken place over a shelfal area with rapid
sediment supply due to a combination of initial
tectonic uplift and subsequent transgressive
shoreface erosion. The presence of lowstand
fluvial systems supplying turbidites to the shelf
via hyperpycnal flows cannot be ruled out, but
there is no evidence for such systems in the
examples studied. Rather, these studies show
that local sourcing of sediments eroded from the
crests of grounded structures was probably a
more important source of turbidite or tempestite
sandstones.
There has been, over the last few years, an on-
going reassessment of a number of littoral depos-
its that show signs of turbidite style deposition.
Pattison et al . (2007) reviewed a number of such