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DATABASE AND METHODOLOGY
consistently picked on distribution characteristics
of the dinoflagellate cyst
Nannoceratopsis gracilis
and/or
N
.
deflandrei senex
(hereafter
Nannocer-
atopsis gracilis
/
senex
). Species of
Nannoceratopsis
are considered to have been derived from dinoflag-
ellates that were well-adapted to euryhaline
conditions (Riding, 1983, 2006) and particularly to
marginal marine (and even estuarine) settings. As a
result, it is reasonable to assume that the presence
of
Nannoceratopsis gracilis
/
senex
is correlateable
in time across the study area since they tolerate a
wide salinity range and are palaeo-environmentally
in situ
. In other words, basing stratigraphic events
on the distribution of species that are derived from
a more offshore setting will clearly result in corre-
lations with a palaeoenvironmental overprint
driven by sea-level change.
The highest stratigraphic occurrence of
Nannoceratopsis gracilis
/
senex
is used to place
the position if the Bajocian/Bathonian boundary,
whilst the highest consistent occurrence (or sig-
nificant increase) in the relative number of these
taxa is traditionally employed as a marker for the
end of the Early Bajocian. Occasionally, where
Nannoceratopsis gracilis
/
senex
are particularly
sporadic, the first down-hole occurrence of
Evansia granulata
, down-hole occurrence of com-
mon
Batiacasphaera
spp (including
B. rudis
) and
the last down-hole occurrence of common
Dissiliodinium willei
were used in conjunction
with
Nannoceratopsis gracilis
/
senex
to pick the
Bajocian/Bathonian boundary.
The significant increase in relative abundance
of
Nannoceratopsis gracilis
/
senex
(as a common
to abundant constituent of the marine assemblage)
is used to position the Aalenian/Bajocian boundary.
Although the placement of this boundary
is beyond the scope of the present study, this
relative abundance characteristic serves to high-
light the principle that a significant increase in
number or bloom of a species with respect to the
background assemblage, is characteristic of an
adaptation to stressful environments (e.g. low
salinity), further increasing confidence in the
biostratigraphic use of
Nannoceratopsis
in deltaic
palaeoenvironments of the Brent Group.
Palynological sample density is generally high
throughout the Bajocian and Bathonian, typically
much better than one sample per ten metres. Much
of the Bajocian sections were cored and sample
spacing is often on the sub-metre scale, indicating
that biostratigraphic precision is potentially high.
Over uncored sections, where the majority of
Well data
This study from the Gullfaks-Kvitebjørn area in
the central parts of the Northern Viking Graben is
based on 31 wells with wireline logs from blocks
33/12; 34/7, 8, 10, 11; 35/4, 10, 11 (Fig. 1). Slabbed
core material (2875 m) of the Brent Group has
been examined in terms of grain size, internal
sedimentary structures and bioturbation and
interpreted in terms of depositional environment
as identified from wireline logs. Palynological
data from consultant biostratigraphic reports have
been re-analysed to provide a chronostratigraphic
framework for the evolution of the basin and to
provide palaeoenvironmental support for the
sedimentological interpretations.
Timelines
Unlike in fully-marine shelf environments, where
high diversities and the continued basinward
transport of well-mixed dinoflagellate cyst assem-
blages result, characteristically, in the relatively
uniform distribution of marker taxa, care must be
taken in marginal marine areas, where changing
environmental conditions play a more significant
role in the distribution of key taxa. The palyno-
logical assemblages from the studied wells are
dominated by spores and pollen throughout, indi-
cating a clear proximity to land but the presence
of a generally moderate to high diversity of dino-
flagellate cysts in the Bathonian interval (that
broadly includes the lower Heather and upper
Tarbert formations), which suggests relatively nor-
mal marine conditions. As a result, confidence is
relatively high that the uppermost occurrence of
biostratigraphically significant dinocyst species
in the studied wells represents predictably corre-
lateable stratigraphic events (e.g. their last regional
occurrence/extinction); and industry-standard
palynological markers for the Upper and Lower
Bathonian can be used to pick these horizons.
Despite the marked evolutionary radiation of
dinocysts known to occur through the preceding
Bajocian, the number of dinocyst taxa in the studied
wells from the Bajocian sections is generally much
lower than expected, suggesting a significantly more
stressed, marginal marine environment than during
the Bathonian. Although confidence is inherently
reduced due to a reduction in number of marker
taxa, picks for the Late and Early Bajocian are almost
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