Geoscience Reference
In-Depth Information
The top surface of the Åre 7 Zone is defined as
the base of a calcite cemented mudstone interval
(1 m to 1.5 m thick) characterised by intense bio-
turbation and marking a facies change into pro-
grading storm-influenced prodelta facies of the
Tilje Formation (cf. Martinius
et al
., 2001). This
surface is interpreted as a condensed maximum
flooding surface (MFS3; Table 1 and Fig. 4; cf.
Martinius
et al
., 2001) Biostratigraphic correlation
confirms that this MFS is associated with a
regional shift in the composition of megaspore/
mesofossil assemblages and is attributed to the
top Early Pliensbachian age, defining the onset of
Tilje Formation (Morris
et al
., 2009; Pedersen
et al
., 1989).
stratigraphic surfaces within the formation,
which are associated with bio-chronostratigraphic
events and correlateable across the Halten Terrace
Basin, are the maximum flooding surfaces: Top
Åre 1 MFS1; Top Åre 3 MFS2 and Top Åre 7 MFS3
(Table 1). The Top Åre 7 MFS3 and the Top Åre 1
Coal Marker MFS1 represent important regional
seismic horizons and the latter is the only
sequence stratigraphic surface which has been
recognised with confidence within the non-
marine part of the succession.
Tectonic controls on stratigraphic development
The overall depositional trend within the Åre
Formation is clearly transgressive, passing
upwards from fluvial to brackish water bay-fills,
marginal marine and ultimately open marine
deposits (Martinius
et al
., 2001; Svela, 2001).
This trend also coincides with the regional tec-
tono-eustatic development of the Early Jurassic
North Atlantic seaway (first-order framework;
Hallam, 1988; Hallam, 2001; Haq
et al
., 1987;
Surlyk, 1990).
The overall transgressive character of the Åre
Formation reflects regional tectonic subsidence
and long-term sea-level rise. Sequence develop-
ment within the succession is attributed tenta-
tively to second order cyclicity. The regionally
correlateable maximum flooding surfaces (MFS1,
MFS2 and MFS3; Table 1), related to relative sea-
level rise, were probably the result of periods of
more rapid tectonic subsidence within the Halten
Terrace Basin. According to biostratigraphic dat-
ing and correlations with the time-scale of
Gradstein
et al
. (2004) the time span between the
maximum flooding surfaces; MFS1 (top Åre 1
Coal Marker; middle Hettangian); MFS2 (top Åre
3; lowermost Sinemurian); and MFS3 (top Åre 7;
top of early Pliensbachian), could possibly imply
sequences of 4 +/−1 Ma to 3 Ma duration. Other
studies of Jurassic stratigraphic cyclicity within
the Atlantic region (cf. Hallam, 2001) have identi-
fied similar second order cycles interpreted to
result from sea-level change. Regional early
Sinemurian and early Pliensbachian transgres-
sions documented by Hallam (2001) are attributed
to plate tectonic processes, such as the opening of
rift basins. Glacio-eustatic influence on Jurassic
sea-level variations are considered unlikely due
to lack of evidence for significant quantities of
polar ice during Jurassic time (Hallam, 1988;
Hallam, 2001).
STRATIGRAPHIC SYNTHESIS
The stratigraphic framework presented here is
based upon a standard procedure of stratigraphic
analysis in terms of recognising genetically linked
packages of strata bounded by key surfaces (e.g.
Van Wagoner
et al
., 1990). On a field-scale, the
sequence stratigraphic scheme appears to be
robust; however, the chronostratigraphic signifi-
cance of many of the marker horizons remains
uncertain. Autocyclic processes have played a
major role in the development of the Åre
Formation stratigraphy, forming surfaces that
look as if they could have sequence stratigraphic
significance. Frequent autocyclic channel and
delta lobe switching in the lower delta plain and
interdistributary bay depositional setting, in addi-
tion to local subsidence variations (cf. Hammer,
2010) and related rapid shifts in relative sea-level
along the low-gradient shoreline, may have pro-
duced local regressions and transgressions (e.g.
Demko & Gastaldo, 1996; Read, 1995). The exact
nature of the marine incursions and erosional
surfaces, whether they are of autocyclic origin or
represent regional events, has in many cases not
been possible to determine. This has been ham-
pered by the fact that few published sequence
stratigraphic studies of the Åre Formation exist.
An on-going study of the Åre Formation from the
Åsgard Field, 30 km SE of Heidrun, together with
in-house data from the Norne and Urd Fields
(~90 km NNE of Heidrun) and the Njord Field
(~110 km south of Heidrun; Fig. 1A), confirms a
regional extent for several of the key marker hori-
zons and the development of similar stratigraphic
stacking patterns. The most reliable sequence
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