Geoscience Reference
In-Depth Information
During the Early and Middle Jurassic there was a
complex interplay between depositional styles, sed-
iment supply and accommodation creation related
to minor structuring or rifting events which occurred
intermittently during this inter-rift period. The
minor structuring events sustained the background
subsidence which, in combination with eustatic
sea-level changes, resulted in a gradual drowning of
the Norwegian-Greenland Sea rift-system (Fig. 5).
As a result, the gross depositional setting changed
from a fluviodeltaic, elongated embayment in the
earliest Jurassic, through a narrow shelfal strait and
into an epicontinental seaway in the earliest Middle
Jurassic. This change in depositional setting was
paralleled by a gradual increase in basin topography
and bathymetry, as a result of the continuous back-
ground structuring and a change to an increasingly
under-filled basin during the Middle Jurassic.
similar analysis of the underlying non-marine Åre
Formation (Svela, 2001).
This contribution focuses on the evolution of
the three mixed non-marine to marginal-marine
Tilje, Tofte-ile and Garn megasequences. The
sequential development and stratigraphy of these
megasequences are described separately below,
with the aim to determine the controls on the vari-
ability in their infill trends and resultant strati-
graphic architecture.
The analysis is based on all released and proprie-
tary exploration and appraisal wells within the area,
regional 2D seismic data as well as a semi-regional
merged 3D dataset consisting of 17 individual 3D
surveys. Core data have been selected to cover all
stratigraphic intervals from all parts of the Halten
Terrace, totalling about 5500 m of cores, split by
about 1600 m (30%) over the Tilje megasequence,
2200 m (40%) over the Tofte-ile megasequence and
another 1600 m (30%) over the Garn megasequence
(Appendices 1, 2 and 3, respectively).
Subsidence and sedimentation rates (Table 1)
are based on preserved compacted thicknesses, as
interpreted from seismic data and calibrated by
wells. The Tilje and ile-Tofte megasequences
consist of highly variable lithologies with locally
high proportions of mudstones, sometimes with
interbedded coals. Decompacted thicknesses thus
should be significantly thicker, suggesting that
Lower to Middle Jurassic megasequence analysis
The large number and geographic spread of well
penetrations, together with a series of well-defined
biostratigraphic time-lines readily identified and
correlateable within these marginal marine units,
has allowed a detailed semi-regional correlation
of these Lower to Middle Jurassic units. However,
the significantly fewer well penetrations and the
poorer biostratigraphic resolution prevents a
Table 1.
Calculated sedimentation and subsidence rates for the Tilje, Tofte-ile and Garn megasequences, based on
compacted thicknesses. Note that sedimentation and subsidence rates are nearly the same but with minor differences: The
Tilje megasequence is an example of an overilled basin, with calculated sedimentation rate equal to subsidence rates.
Sediment supply rates was however larger and resulted at times in sediment bypass across the Halten Terrace during the
formation of this megasequence. Sediment supply rates for the Garn megasequence vary from that equal to the subsidence
rates, e.g. in areas where the basin was completely inilled, to less than the subsidence rates in areas where the basin
remained under-illed (or submerged) during the formation of the megasequence. For all megasequences, both subsidence/
uplift and sedimentation rates varied spatially and through time, sometimes also involving non-deposition or erosion, in
the latter situation locally resulting in complete removal of the megasequence.
Sedimentation
rate
Thickness (m)
Subsidence rate
Duration/
timespan
Myr
Mega-
sequence
Basin infill
state
Maximum
Range
Maximum
Range
Maximum
Range
Garn
185
2 to 185
7 (7+)
Under-filled
< 0.026 mm a
−1
0.0003* to
0.026
mm a
−1
0.026 mm a
−1
0.0003* to 0.026
mm a
−1
Tofte-ile
355
20 to 355
9 (8 to 9)
Balanced
0.039 mm a
−1
0.002 to
0.039
mm a
−1
0.039 mm a
−1
0.002 to 0.039
mm a
−1
Tilje
330
0 to 330
8 (8+)
Over-filled
> 0.041 mm a
−1
0.000* to
0.041
mm a
−1
0.041 mm a
−1
0.000* to 0.041
mm a
−1
*Missing stratigraphy. Reflects intervals of erosion and non-deposition. includes intervals with uplift.
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