Geoscience Reference
In-Depth Information
a series of advancing and retreating bay-head del-
taic units (Fig. 7A,B4; see also Dreyer 1992, 1993,
1999; Saigal
et al
., 1999; Martinius
et al
., 2001;
ichaso & Dalrymple, this volume). Alluvial fan to
fan-deltaic and braidplain-deltaic lithosomes
were re-established around structural highs, such
as the Sklinna Ridge (ichaso & Dalrymple, this
volume) and downdip of relay-ramps along mas-
terfault-bounded highs, such as the Frøya High.
These coarser-grained lithosomes have an overall
forestepping structure, although with considera-
ble local variations in their gross sedimentary
structure and internal architecture.
Synthesis: The Tilje megasequence is an exam-
ple of infill during an interval with high rates of
subsidence and sediment supply (Fig. 3). The
lower part of the progradational segment, com-
prised by the uppermost Åre Formation, formed
during normal regression with high sediment
supply and rising relative sea-level. Accord-
ingly, it represents a highstand systems tract.
The stratigraphic variations in the dominant reser-
voir architectures probably represent changing
basinal energy responses during infilling of the
broad late Sinemurian to early Pliensbachian or
'late Åre embayments'. The capping sandstone
sheet of the lower Tilje member is, based on the
nature of the basal surface, component facies and
coarser lithologies, interpreted to represent a
marked basinward shift in facies tracts. it accord-
ingly represents a forced regression and thus a
falling stage to early lowstand systems tract. The
aggradational segment is interpreted to represent
the late lowstand systems tract, with the overlying
retrogradational segment representing the trans-
gressive systems tract. Sediments were supplied
continuously from intra-basinal and basin marginal
sources. Changes in provenance and sediment dis-
persal during deposition of the upper Tilje mem-
ber is interpreted to reflect tectonic rejuvenation
(e.g. Dreyer 1999; Martinius
et al
., 2001; ichaso &
Dalrymple, this volume) related to minor rifting
(Corfield & Sharp, 2000; Marsh
et al
., 2010). This is
also reflected in the increased spatial variability in
infill patterns between and within the various
sub-basins on the Halten Terrace (Figs 6 and 7).
infill was from a mixture of large basin-marginal,
longitudinal (NE to SW oriented) and lateral (SE to
NW oriented) as well as a series of smaller, basin-
interior sourced depositional systems (Fig. 7).
During the main part of the megasequence devel-
opment, these depositional systems linked to form
an extensive, very low-relief, delta-plain to coastal
(tidal) flat with a predominant longitudinally
(i.e. NE to SW directed) sediment dispersal.
The Tofte-Ile Megasequence
The Toarcian to Aalenian Ror-Tofte and ile
Formations (Fig. 3) form another megasequence,
more than 350 metres thick, (Figs 4 and 8), which
represents a timespan of some 8 Myr to 9 Myr. The
Ror-Tofte Formations consists of a series of stacked
deltaic units, ranging from shelfal, deeper-water
to shoal-water, mixed wave and river-/fluvial-
influenced deltas, to braidplain deltas along the
south-eastern part as well as braidplain-deltas and
fan-deltas fringing highs along the western part of
the Halten Terrace (Fig. 9; see also Gjelberg
et al
.,
1987; Dalland
et al
., 1988; Ehrenberg
et al
., 1992).
Seismically identifiable, lozenge-shaped sandstones
embedded in offshore mudstones, interpreted as
shelfal sand-ridges, are locally present. A shelfal
sand-ridge origin is supported by well- and core-
calibration of seismic amplitudes in combination
with palaeocurrent data derived from re-oriented
cores and FMi-logs. The ile Formation has tradition-
ally been interpreted as a succession of tide-
dominated deltas and estuaries (Gjelberg
et al
.,
1987; Dalland
et al
., 1988; Harris, 1989; Pedersen
et
al
., 1989; Ehrenberg
et al
., 1992; Saigal
et al
., 1999;
Mcilroy, 2004; Martinius
et al
., 2005), though braid-
plain deltas are present locally towards the base of
this unit as well. The Tofte-ile megasequence has
an overall progradational-to-retrogradational char-
acter in the central and south-eastern part of the
Halten Terrace, changing to a simple aggradational
or highly irregular gross internal stacking pattern
along the north-western part of the Halten Terrace
(Fig. 8).
The basal boundary is defined by a relatively
thick marine, shelfal claystone unit representing a
major flooding interval within the lowermost Ror
Formation. The upper boundary is represented by
marine claystones in the basal part of the Not
Formation, which represent a major landward
flooding across the upper ile tidal flats.
The progradational (regressive) segment (the Ror-
Tofte Formations and parts of lower ile member)
consists, in the south-eastern and central part of
the Halten Terrace, of a series of intermediate-scale
coarsening-upwards motifs (up to > 25 m thick),
representing the prodelta to delta-top and inner
shelf to paralic (back-barrier) deposits of wave-
influenced and storm-influenced delta and shore-
line systems (Fig. 9A,B1-2). The basal shelfal
Search WWH ::
Custom Search