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 235

The retrogradational (transgressive) segment (the
upper Garn member) is a sandy unit with marked
lateral variability in infill patterns and sedimentary
architectures (Fig. 11A,B4; see also Ehrenberg,
1991; Corfield et al ., 2001). The unit is represented
by an unconformity across most structural highs,
including local fault-block crests, sometimes with
a thin transgressive lag resting atop the unconform-
ity. Sub-basins within the central part of the Halten
Terrace are dominated by mixed wave-structured
and tidally-structured estuary fills, representing
the infill of wave-dominated estuaries, as well as
offset backstepping tide-structured, deltaic litho-
somes, representing advancing and retreating
bay-head deltas. Braidplain-deltaic lithosomes are
locally present along structural highs, such as the
Sklinna and Nordland Ridges (e.g. Harris, 1989).
Along the western Halten Terrace narrow straits
formed in swales between structural highs and
formed sites of active tidal sand ridge migration
(Messina et al ., this volume).
Synthesis: The Garn megasequence illustrates
infill during low rates of both subsidence and
sediment supply (Fig. 3). The lower part of the
progradational segment (the Not Formation) rep-
resents normal regression of a highstand systems
tract. The capping sandstone sheet of the lower
Garn member is interpreted as 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 represent-
ing the transgressive systems tract. The transgres-
sive systems tract has marked lateral stratigraphic
variability suggesting large spatial variations in
depositional sub-environments and infill trends.
its stratigraphic architecture is characteristic of
basins with lateral changes in subsidence-uplift
patterns and sediment supply, such as tectoni-
cally active basins. However, subsidence rates
appear to have remained low throughout the for-
mation of the megasequence, suggesting that
structural activity remained relatively low and
that any structurally formed topography was sub-
dued and removed by erosion and infilling.
infill was initially from larger basin-marginal,
longitudinal (NE to SW oriented) and a series
of smaller, basin-interior sourced depositional
systems, although with a predominant longitudi-
nally (i.e. NE-SW directed) sediment dispersal
(Fig. 11). With time, sediment supply from basin-
marginal systems gradually diminished and there
was a relative increase in the supply from basin-

interior sources (e.g. the Sklinna and Nordland
Ridges). This coincided with a change from domi-
nantly deltaic (i.e. progradational) to estuarine
(i.e. aggradational to retrogradational) develop-
ment of the megasequence.
The sandy nature of the aggradational to retro-
gradational segments of the Garn megasequence is
related to 1) sandy sediment supply during over-
all low rates of accommodation creation with
fines continuously partitioned offshore and 2)
uplift and erosion of older lower Garn sands to
form added sand-rich sources during the aggrada-
tional and retrogradational stages.

Megasequence structure and sedimentary
architecture

The Tilje, Tofte-ile and Garn megasequences
display similar gross stratigraphic structure,
external geometries and internal stacking patterns
(Fig. 12A), despite variability in subsidence, basin
physiography, sediment supply and dispersal pat-
tern, depositional systems and sediment calibre.
The duration of megasequence (some 7 Myr to
9 + Myr) suggests that they represent candidate
second order sequences.
Generic stratigraphic characteristics of, as well
as variations in, sedimentary architectures
between the various segments that comprise the
megasequences are described below. Then a dis-
cussion of the candidate controlling mechanism(s)
is presented.

Sedimentary architecture of the progradational
(regressive) segment

The regressive segments are dominated by a progra-
dational stack of higher-order, mixed wave- and
river-/fluvial-influenced fluviodeltaic lithosomes.
These represent delta advances and short-term
retreats or avulsions along open to semi-protected
(embayed) wave-dominated coasts, sometimes with
a subordinate tidal influence (Figs 12B and 13).
Normal regressive segment: The early stages of
delta-shoreline progradation were apparently slow
and during rising relative sea-level, with relatively
thick, gradually coarsening-upward, offshore-
shoreface or prodelta-delta front-distributary chan-
nel fill motifs. These motifs, with their gradual
facies tract transitions, represent highstand normal
regressions (Van Wagoner et al ., 1990; Helland-
Hansen & Gjelberg, 1994; Van Wagoner, 1995;
Helland-Hansen & Martinsen, 1996) of basin-margin