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 231

strike translations from deltas to estuaries with
intervening shorelines. These were probably situ-
ated within a broad tectonic embayment that formed
across the wider Halten Terrace (Fig. 9A,B3).
Extensive tidal flats developed across the eastern
part of the Halten Terrace and represent the inner or
landward portions of the embayment. As a corol-
lary, most estuary-fills recorded in the western part
of the Halten Terrace probably represent abandoned
distributaries within broader, deltaic settings.
The retrogradational (transgressive) segment
(the middle (parts) and upper ile member), is highly
variable across the Halten Terrace (Fig. 9A,B4):
The south-eastern part is dominated by an offset
backstepping stack of tidally structured, fluvi-
odeltaic lithosomes each up to 10 m to 15 m thick.
These consist of tide-influenced deltaics, tidal
shoreline and tidal flat heteroliths. The north-
western part of the terrace is dominated by similar
facies (Ehrenberg et al ., 1992; Saigal et al ., 1999;
Mcilroy, 2004; Martinius et al ., 2005), although
forming an asymmetrical forestepping (thinner)
to backstepping (thicker) stack of offset backstep-
ping heterolithic lithosomes, that pass landward,
i.e. toward structural highs to the west and north-
west, into fluvial strata. The central part of the
Halten Terrace is dominated by tidally-structured
brackish-water heteroliths, representing protected
bay or 'central estuary basin' deposits, whereas
laterally extensive tidal flat heteroliths continued
to be deposited along the eastern Halten Terrace
and the western Trøndelag Platform. Sandy to
heterolithic tidal bar and barrier/shoreline deposits
are present across the south-western part of the
Halten Terrace, representing the outer part of an
alternating tide- and wave-dominated estuary,
respectively.
The overall transgressive character of the
segment is mostly evident by the backstepping of
the basin-margin attached depositional systems
present along the south-eastern part of the Halten
Terrace. The locally sourced depositional systems
around the intrabasinal highs such as the Sklinna
(Ehrenberg et al ., 1992; Saigal et al ., 1999; Mcilroy,
2004; Martinius et al ., 2005) and Nordland Ridges
(Harris, 1989; Pedersen et al ., 1989), in contrast,
shows a progradational to retrogradational struc-
ture suggesting local, tectonically induced con-
trols on accommodation creation and sediment
supply. Moreover, these locally sourced systems
developed out of phase, with a diachronous build-
out and retreat towards the Nordland Ridge, com-
pared to that around the Sklinna Ridge. This is

attributed to local variations in timing and style of
structuring across the area.
Synthesis: The Tofte-ile megasequence repre-
sents an interval with high rates of subsidence and
intermediate to high sediment supply (Fig. 3) with
highly variable but gradually decreasing sediment
supply during development of the transgressive
segment (Fig. 9). in the south-eastern and central
part of the Halten Terrace, the progradational seg-
ment represents a normal regression in response to
high but overall muddy sediment supply during
continuously rising relative sea-level. Preservation
is mainly as shelfal to delta-front and shoreface
facies tracts. The capping sandstone sheets of the
upper Tofte and lower ile members are interpreted
as detached, lowstand deltas and shorelines.
Within the regressive segment, prograding
basin-margin attached fluviodeltaic lithosomes were
paralleled by prograding to retrograding, coarse-
grained braidplain-deltaic to fan-deltaic units along
basin-interior highs within the western and north-
western parts of the Halten Terrace. There is
normally a non-correlative and diachronous
sequence development between the basin-marginal
and basin-interior, as well as between the various
basin-interior sourced depositional systems. This is
interpreted to reflect spatial variations in sediment
supply and basinal subsidence rates.
The variable sedimentary architecture of the
transgressive segment is indicative of rapid flood-
ing across and sediment shut-off, to the former
basin marginal lowstand shoreline. This was par-
alleled by a phase of uplift of basin-bounding
highs with rejuvenation of local topography and
renewed progradation of local, fault-block sourced
depositional systems. As such, there was a marked
change in uplift-subsidence patterns and sediment
supply from the regressive to the transgressive
segment, induced by renewed structuring of the
greater Halten Terrace area. The highly variable
infill character and lateral changes in depositional
subenvironments, contained architectural ele-
ments and component stratigraphic surfaces pre-
sent in the transgressive segment, is characteristic
for basins with lateral changes in subsidence-uplift
patterns and sediment supply. This suggests a
return to a basin with an increase in the rate of
tectonic activity, with the entire Halten Terrace
forming a large funnel-shaped embayment bor-
dered by structural highs and underlain by a series
of half-graben sub-basins.
The presence of shelfal sand-ridges in the upper-
most part of the regressive segment is related to