Eustatic, tectonic and climatic controls on an early syn-rift mixed-energy delta, Tilje Formation (Early Jurassic, Smørbukk field, offshore mid-Norway) - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 381

separation from fine-sand to mud. This surface rep-
resents a major flooding event that drowned the
underlying deltaic system and was followed by the
prolonged deposition of shelf mudstone.

with distance away from the river mouth; the
complex interaction of these processes in turn
exerts a major control on the distribution of the
resultant sedimentary structures and deposits
(Bhattacharya, 2010). From a proximal-distal per-
spective, the Tilje channel deposits (T3.2 to T5.1;
Figs 21C to 22A) show the coarsest sand fraction
(medium to coarse) and are dominated by cross-
bedding and current-ripple cross-lamination
indicative of unidirectional, strong to moderate
currents. The channel deposits also preserve thick
fluid-mud beds (Ichaso & Dalrymple, 2009) and
other typical tidal signatures such as double mud-
drapes and indicators of bidirectional currents.
These tidally generated features were produced by
the landward increase of tidal energy (in a general
northerly direction) because of the compression of
flood currents into the smaller cross-sectional area
of the distributary channels (Dalrymple & Choi,
2007; Bhattacharya, 2010). The relative decrease
in the abundance of tidal indicators in unit T4,
coupled with the presence of the coarsest channel
deposits, indicates that this unit is the most proxi-
mal in the succession and was deposited near the
tidal limit. Tidal signatures increase in abundance
within the more distal tidal-fluvial channel depos-
its and proximal mouth-bars of reservoir units
T1.1 and T3.1 (Figs 20B & 21B), where deposition
was controlled by strong tidal currents in the outer
part of the mixed tidal-fluvial transition (Dalrymple
& Choi, 2007). The medial to distal distributary-
mouth and delta-front deposits of T1.2, T2 and T6
(Figs 20D, 21A & 22C) formed in the marine-
dominated deltaic zone and also preserve tidal sig-
natures, but in less abundance than their proximal
equivalents. Wave-generated structures become
important in these more distal reservoir zones
because of the rapid decrease of tidal-current
energy away from the river mouth. Similarly, the
fine sediments in areas laterally adjacent to the
river mouth (e.g. T5.2; Fig. 22B) are commonly
moulded by wave and/storm action (cf. Mángano
et al ., 1994).
As indicated by the above, the Tilje Formation
accumulated in a mixed-energy environment in
which fluvial, tidal and wave processes where all
active. However, based on the vertical distribution
and abundance of the preserved sedimentary
structures (Fig. 3 and Figs 20 to 22), the Tilje
Formation shows vertical changes in the apparent
intensity of these three processes: tidal and wave
processes are alternatively more intense in
sequence 2; tidal-fluvial processes dominate in

CONTROLS ON FACIES DISTRIBUTION,
PALAEOGEOGRAPHY AND SEDIMENT
DISPERSAL

Previous regional studies (Ehrenberg et al ., 1992;
Corfield & Sharp, 2000) have shown that significant
crustal extension had occurred in the Halten Terrace
area by the time of Tilje deposition, leading to the
development of subsidence and the generation of
accommodation. However, faulting was not suffi-
ciently active to cause surface rupture and to gener-
ate local fault-related topography (Marsh et al .,
2010). Thus, the Tilje is generally regarded to be an
early syn-rift deposit. The facies distribution pat-
terns reported above (Figs 20 to 22) indicate that
sediment input to the Smørbukk study area was
predominantly from the north. The environmental
interpretation proposed here indicates that the Tilje
Fm. accumulated in a deltaic system in which
fluvial and tidal processes predominated, but with
wave processes influencing and even dominating
deposition at certain times and places.
The stratigraphy of syn-rift successions is con-
trolled by a variety of autogenic and allogenic
processes which all leave distinctive signatures
in the deposits (Bosence, 1998). The stage of
rifting (early, main, or late) and the dynamics of
its structures affect considerably the depositional
patterns by creating sites of uplift and erosion
and by controlling the pathways of sediment
transport/dispersal (Gawthorpe & Leeder, 2000;
Withjack et al ., 2002), but more importantly,
together with relative eustatic sea-level changes,
by defining the accommodation for sediment
accumulation (Howell & Flint, 1996). The rela-
tionship between space, climate, hydrodynamic
processes and sediment supply determine the
depositional regime of marginal marine systems
in syn-rift settings where a variety of infill
patterns can be observed throughout the basin
evolution (Ravnås et al ., 2000).

Hydrodynamic processes

The relative importance of fluvial, tidal and wave
energy varies through the deltaic fluvio-marine
transition (Dalrymple & Choi, 2007), as well as