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 216

Megasequence 5
Megasequence 4
Megasequence 3
Megasequence 2
Megasequence 1

few 100s of km

Post-Rift
Syn-Rift
Pre-Rift

Fluviodeltaic lithosomes

Marine mudstones
w/shelfal turbidites

Fig. 1. Conceptual illustration showing characteristics of post-rift or inter-rift succession of rift-basin fills. Note stacking of
clastic wedges in a forestepping to backstepping manner. Modified from Steel (1993), Ravnås & Steel (1998) and Ravnås
et al . (2000). Post-rift strata are shown with a background colour of blue (illustrating marine mudstones) with component
large scale (low order) fluviodeltaic lithosomes representing individual megasequences shown in yellow.

infill (Ravnås et al ., 2000). The clastic wedges have
a regressive to transgressive structure, are bounded
at their base and tops by mudstones representing
peak transgressions (in marine successions) or
peak backstepping (in non-marine successions).
They normally represent a time-span of a few
(i.e. 3 to 4) to more than eight million years. As
such, they represent second order sequences and
have been awarded the name 'megasequences'
(Steel, 1993; Ravnås et al ., 2000).
The inter-rift megasequences (referred to as
post-rift by Steel, 1993) differ from their syn-rift
equivalents by their simpler sequence structure
and component sedimentary architectures, as well
as representing a longer time-span. This reflects a
less complex basin and sediment infill develop-
ment during the inter-rift period. Syn-rift megase-
quences, in contrast, often have a much more
complex sedimentary architecture (Ravnås et al .,
2000), largely related to the fundamental control
that tectonics exert on accommodation creation,
sequence development and the resultant strati-
graphic architecture (e.g. Ravnås & Steel, 1998;
Gawthorpe & Leeder, 2000). However, a growing
number of studies support the notion that minor
or subdued extensional tectonics persisted through-
out the inter-rift periods (Ravnås et al ., 2000
and references therein). Thus tectonics, at least
partly, influence the stratigraphic architecture of

the inter-rift megasequences and some similarities
with proper syn-rift megasequences should be
expected.
This contribution focuses on the development
of the non-marine through marginal-marine to
shallow-marine megasequences in the Lower to
Middle Jurassic on the Halten Terrace, offshore Mid-
Norway (Figs 2, 3 and 4). These megasequences
were deposited during an interval of slow stretch-
ing, resulting in minor extensional tectonics during
the inter-rift period separating the Permo-Triassic
and Late Jurassic rift-episodes. The various non-
marine to shallow-marine Lower to Middle Jurassic
megasequences of the Halten Terrace have a simi-
lar progradational-aggradational-retrogradational
gross structure. Minor departures from the
'norm' or differences between the stacked megas-
equences are attributed to temporal changes
in basinal conditions. These are predominantly
due to variations in sediment dispersal but are
also caused by some variability in gross basinal
subsidence and sediment supply rates between
the various megasequences. The Lower to Middle
Jurassic of the Halten Terrace thus provides an
opportunity, firstly, for the identification and
differentiation of the fundamental control(s) on
the megasequence development. Secondly, a com-
parable study of these megasequences offers the
opportunity to demonstrate how variations in