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 482

Flint (1996), Howell et al . (1996) and Jeremiah &
Nicholson (1999). Howell & Flint (1996) focus on
the role of tectonism in a sequence stratigraphic
context and identify a variety of depositional
responses to eustatically controlled sea-level fall
when comparing deposition within a single chron-
ostratigraphic interval in the hangingwall and
footwall succession. In particular, they interpret
the development of an incised valley (Curlew
Member) with a tidally-influenced sedimentary
fill, incision over an uplifted footwall crest (Puffin
fault block, location Fig. 1) and deposition of con-
temporaneous turbidite sandstones successions
(up to 50 m thick) in the adjacent footwall basin.

Facies S2: Strongly bioturbated sandy mud-
stone to very muddy sandstones. This facies is
strongly bioturbated and exhibits Chondrites ,
Helminthopsis , Phycosiphon , Planolites and
Terebellina trace fossils. This facies is interpreted
to reflect the shoreface to offshore transition
zone.
Facies S3: Bioturbated very fine grained
sandstones with abundant horizontal burrows.
This facies exhibits Palaeophycus , Teichichnus ,
Planolites , Terebellina , Asterosoma and Mac-
aronichnus traces. This facies is interpreted to
reflect the lower shoreface depositional setting.
Facies S4: Strongly bioturbated fine-grained to
medium-grained sandstones, with occasional rel-
ict stratification. Low angle discontinuities and
scour-and-fill structures are occasionally observed.
This facies exhibits Ophiomorpha and Skolithos
traces and is interpreted to be deposited in an
upper shoreface setting.
Facies S5: Stratified sandstone - consisting of
sandstones with cross-stratification and planar-
stratification and carbonaceous-rich mudstone,
ripple stratified sandstones and thin coal beds.
Bioturbation is uncommon and consists of Ophio-
morpha traces. This facies has a low preservation
potential in the Central Graben, probably due to
transgressive shoreface erosion, and it is rarely
observed. It is interpreted as a foreshore to coastal
plain deposit.
Sandstone deposits produced by stacked phases
of parasequence progradation generate thick
sandstone units that may often exceed 300 m in
thickness (e.g. Stockbridge & Gray, 1991; Lasocki
et al ., 1999). Parasequences may be stacked
progradationally, aggradationally or retrograda-
tionally, thereby indicating the interplay of
sediment supply and relative sea-level change.
Sandbody development at the basin margins
occurred simultaneously with deposition of more
basinal sandy turbidite facies (see later) and silt-
stone or mudstone of the Heather and Kimmeridge
Clay formations.
In a regional study of the Fulmar Formation,
Gowland (1996) made a further refinement of
the shoreface model by defining 12 facies that
were grouped into 6 facies associations and used
to construct three broad depositional models:
(1) Model 1: storm-influenced shoreface; (2) Model 2:
bioturbated shoreface and (3) Model 3: a specula-
tive bioturbated shelf model. Model 1 shorefaces
are said to have well-preserved event beds that
occur seaward of the upper shoreface, whereas

Shoreface depositional model

Thick, Oxfordian age, sandstones of the Central
Graben are now most commonly referred to as
shoreface, lower shoreface and offshore deposits.
Deposition is interpreted to have occurred by
the progradation of shoreface sandstone facies
into a mud-dominated basin. Fulmar Formation
deposits typically occur in the form of large-
scale coarsening-upward successions (10 m to
50 m thick) that evolve from claystone or silt-
stone into fine-grained to medium-grained sand-
stone. Such a succession can define a flooding
surface bounded parasequence that reflects
shoreline progradation during a period of fairly
constant (or gently rising) relative sea-level. The
coarsening upwards character of parasequences
reflects increasing wave energy when passing
from the offshore to the nearshore depositional
setting (Fig. 6).
The shoreface deposits of the Central Graben
typically comprise highly bioturbated sandstones
and argillaceous sandstones. The sandstones are
normally highly reworked by burrowing organ-
isms that produced a wide range of ichnofabric
types (Taylor & Gawthorpe, 1993; Fig. 6) and a
sedimentary fabric to the rock in which it is
usually difficult to identify primary sedimentary
structures (Fig. 7). A number of facies types (here
termed S1 to S5) are typically identified within
prograding shoreface successions:
Facies S1: Mudstone - consisting of silty clay-
stone or argillaceous siltstone, sometimes cal-
careous, ranging from well laminated to highly
bioturbated. Bioturbation when present consists
of Terebellina , Chondrites and Helminthopsis hor-
izontalis . This facies is interpreted as an offshore
shelf deposit.