Geoscience Reference
In-Depth Information
calculated sedimentation rates represent low
estimates. The sandier nature of the Garn megase-
quence indicates that the difference between com-
pacted and decompacted thicknesses should be
less. Subsidence rates based on compacted thick-
nesses are less erroneous as accommodation space
creation also involves early compaction of the
substrate. The calculated rates thus are only rela-
tive estimates, but allow for comparison between
the various megasequences.
deltaic units. The locally sourced deltaic units are
present around large intrabasinal highs such as the
Sklinna Ridge (ichaso & Dalrymple, this volume)
and Frøya High (e.g. Dreyer, 1992, 1999), that was
tectonically rejuvenated during a precursor
Sinemurian rifting event.
The forward stepping, normal regressive litho-
some stack is capped by an erosively based, sand-
rich and laterally extensive fluviodeltaic to estuarine
sandstone sheet, informally referred to as the lower
Tilje member. On the Halten Terrace the sandstone
sheet is composed of amalgamated, mixed wave and
tidally-influenced, coarser-grained fluviodeltaics,
distributary channel complexes and capping mar-
ginal marine (i.e. estuarine and shoreline) sandy
lithosomes (Fig. 7A,B2). On the Trøndelag Platform
to the east, it is backed by lower delta-plain hetero-
lithics with isolate channel-fills and channel
complexes, the latter representing candidate incised
valley-fills. it is interpreted to represent a forced
regressive to lowstand normal regressive lithosomes
stack. 'intraformational' unconformities and bypass
surfaces are locally present across structural highs
within the Halten Terrace (e.g. the SB2 of Martinius
et al
., 2001; ichaso & Dalrymple, this volume),
as well as on the Trøndelag platform to the east.
The aggradational segment (the middle Tilje mem-
ber) is separated from the underlying progradational
segment by a bioturbated mudstone unit represent-
ing inner shelf to protected embayment hetero-
liths, which forms a higher-order flooding surface.
The segment has an overall aggradational character,
evidenced by a near-vertical superposition of facies
belts within the stacked deltaic to estuarine litho-
somes, though with a thin forward stepping basal
part. it consists of stacked, mixed wave and tide-
influenced, mud-prone and heterolithic fluviodeltaic
lithosomes (Dreyer 1992, 1993, 1999; Martinius
et al
., 2001; ichaso & Dalrymple, this volume),
representing the outbuilding of mixed lithology or
muddy deltas into protected embayments or bays
(Fig. 7A,B3). Based on distribution and orientation of
facies belts, a mixed sourcing from basin-marginal
and intrabasinal sources is envisaged (see also ichaso
& Dalrymple, this volume).
The retrogradational (transgressive) segment (or
the upper Tilje member) comprises a heterolithic
unit with significant lateral variability in infill pat-
terns. Across the central part of the Halten Terrace
it consists of mixed wave-structured and tidally-
structured estuary fills, representing the infill of
wave-dominated estuaries and offset backstepping
tidally-influenced, deltaic lithosomes, representing
The Tilje Megasequence
The Pliensbachian Tilje Formation (Fig. 2) together
with the upper part of the underlying Åre
Formation forms an up to 350 m thick megase-
quence (Figs 4 and 6), representing a timespan
of some 8 + million years. it consists of a series
of fluviodeltaic-estuarine lithosomes or higher-
order sequences, which stack to form an overall
progradational-aggradational-retrogradational
megasequence architecture. Delta types range
from sandy, low gradient, shoal-water, mixed tide-
wave influenced river-deltas transitional into
braidplain-deltas (Dalland
et al
., 1988; Martinius
et al
., 2001; ichaso & Dalrymple, this volume) to
mixed sandy-muddy, tide-influenced bay-head or
protected embayment deltas (see also Kjærefjord,
1999). Estuaries range from smaller-scale inactive
distributaries to larger, sub-basin wide, wave-
dominated estuaries (Fig. 6, see also Dreyer 1992,
1993, 1999; Saigal
et al
., 1999; Martinus
et al
.,
2003, 2005; LaFont
et al
., 2010).
The basal boundary is defined by a relatively
thick marine claystone unit within the uppermost
Åre to basal Tilje Formations, which represents a
major marine flooding. The upper boundary is
defined by organic-rich claystones in the lower-
most Ror Formation, representing a prolonged
interval of basin-marginal delta retreats and basi-
nal sediment starvation.
The progradational (regressive) segment (the
upper Åre and the lower Tilje members) of the Tilje
megasequence is relatively thick (>100m) and con-
sists of an irregular but overall forward stepping
stack of wave-dominated fluviodeltaic and shore-
line lithosomes, each some 5 m to 15 m thick. These
higher order lithosomes are interpreted as mixed
tide- and wave-influenced deltas and shorelines,
that most likely infilled a broad embayment
(Fig. 7A,B1; Kjærefjord, 1999). The lithosomes
represent a mixture of larger basin margin derived
and smaller locally derived or intrabasinal sourced
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