Halten Terrace Lower and Middle Jurassic inter-rift megasequence analysis: megasequence structure, sedimentary architecture and controlling parameters - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 221

studies have documented variable structural
influence on the infill patterns of the Lower to
Middle Jurassic succession (e.g. Gjelberg et al .,
1987; Corfield & Sharp, 2000; Corfield et al .,
2001; Martinius et al ., 2001), testifying to the
prolonged but varying rates of subsidence and
rifting within this region. Structural styles
related to Early to Late Jurassic rifting were con-
trolled by the presence of Upper Triassic salt,
which on the Halten Terrace alternates with
mudstones. This interbedded salt-mudstone
package forms a detachment interval (up to a
few hundred metres thick), which commonly
results in decoupling of the shorter wave-length
Jurassic structures from the deeper-seated base-
ment structures (Withjack et al ., 1989; Pascoe
et al ., 1999; Marsh et al ., 2010).
Within the Halten Terrace area, seismically
mapable extensional normal faults which trend
NE-SW were active in the Early to Middle
Jurassic, whereas N-S trending faults were
active from the Middle Jurassic and onwards.
Gross subsidence rates of the terrace area were
relatively high in the Early Jurassic, gradually
decreased through the early Middle Jurassic but
increased again during the late Middle Jurassic
due to renewed rifting (Fig. 3). These variations
in subsidence rates reflect a decrease in gross
basinal rifting rates, in combination with a west-
ward shift and narrowing of the area of active
rifting to the deeper basins west of the Halten
Terrace. The westward shift in rift activity is
herein related to a narrowing of the rift-basin
proper; it resulted in the formation of a broad
platform area comprising the Halten-Dønna
Terraces and the Trøndelag Platform, along the
eastern margin of the Norwegian-Greenland Sea
rift-basin.
During the Early to Middle Jurassic, the Halten
Terrace was located at ca. 50° to 52° northern
palaeolatitude (Fig. 3; see also Dore, 1991) but
within a relatively warm and humid palaeocli-
mate setting (Hallam, 1994). Sediment supply was
very high during the Early Jurassic but gradually
waned throughout the Middle Jurassic. As a result,
a large-scale, overall transgression occurred dur-
ing the Early to Middle Jurassic, despite the fact
that the area experienced a gradual decrease in
gross subsidence rates. However, both rifting
and sediment supply rates fluctuated significantly
(Fig. 3), which during certain intervals show
marked departures from the overall gradual
decreasing trend.

Lower to Middle Jurassic stratigraphy
and palaeogeographies

On the Halten and Dønna Terraces, the Lower to
Middle Jurassic forms an overall retrogradational,
layered package of alluvial, fluviodeltaic, shallow
marine and shelfal strata (Figs 3, 4 and 5). This
contains the Rhaetian to Sinemurian, alluvial to
fluviodeltaic Åre Formation and the Pliensbachian,
fluviodeltaic and estuarine to shallow-marine
Tilje Formation. The Toarcian to Bathonian Tofte,
ile and Garn Formations form three sandy fluvi-
odeltaic, paralic, estuarine and shallow marine
units that are interbedded with the marine (off-
shore to shelfal) mudstones of the Ror, Not and
Melke Formations (Dalland et al ., 1988).
The Lower to Middle Jurassic can be organised
into four clastic wedges, which grossly corre-
sponds to the Åre, the Tilje, the Ror-Tofte-ile and
the Not-Garn Formations, respectively (Figs 3
and 4). These basin-marginal clastic wedges are
separated by relatively thick offshore mudstones
within uppermost Åre and basal Tilje, the lower-
most Ror and the basal Not Formations, which
represent pronounced marine flooding intervals.
The clastic wedges are herein referred to as megas-
equences (following the terminology of Steel,
1993). They all have a common architectural
motif comprising a basal coarsening or 'sandying'
upwards motif, a central sand-rich unit and an
upper part of more varied stratigraphy. The lower
and central part of these megasequences are inter-
preted as progradation of basin-marginal alluvial
to fluviodeltaic depositional systems during an
interval of slowly rising through still-stand to
falling base-level, typical of normal to forced
regressive situations. The upper part, in contrast,
shows more varied architectural motifs including
aggradation and back-stepping of the rift-marginal
deltas and locally developed clastic systems
around local intra-basinal highs, producing a rela-
tively thick transgressive succession.
Sediment provenance is inconclusive for the
Lower to Middle Jurassic of the Halten Terrace
and has been ascribed to easterly (e.g. Ziegler,
1988), westerly (e.g. Gjelberg et al ., 1987) or even
northerly (e.g. Dore, 1991) sources. Recent studies
have proposed repeated changes in provenance
through time (e.g. Morton et al ., 2009), while
other workers (e.g. Martinius et al ., 2001; Corfield
et al ., 2001; Elfenbein et al ., 2005) support an
overall Norwegian hinterland together with local,
intrabasinal sediment sources (Fig. 5).