Autogenic process change in modern deltas: lessons for the ancient - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 158

vs. temporal, is even more difficult in ancient
delta deposits because of the limited datasets
(outcrops show only strike or dip-oriented faces,
lack detailed dating and lack a map view image of
the delta).

Cretaceous Blackhawk Fm. shallow marine depos-
its of Cretaceous Interior Seaway (Howell & Flint,
2003). The Maastrichtian Fox Hills Formation in
the Washakie Basin, S. Wyoming (Fig. 7B) also
shows significant process changes. The outcrops
of the Fox Hills delta in Fig. 7B correspond to clin-
othem 10 of the fill of the Maastrichtian Washakie
Basin (Carvajal & Steel, 2009; Olariu et al ., 2012).
Clinothem 10 topset contains Fox Hills Delta
deposits that change over a distance of < 1 km from
a fluvial-dominated delta front dominated by sedi-
ment gravity flow deposits to a tide dominated
delta front characterised by stacked cross-strata,
abundant mud drapes and fluid mud layers and
bi-directional palaeocurrents (Carvajal & Steel,
2009). Further to the south the same outcrop belt
shows large-scale syn-depositional deformation of
the delta deposits suggesting proximity to the
shelf-edge rollover. However, the short distance of
the lateral transition between dominant sedimen-
tary processes in the same outcrop belt suggests
that the stratigraphic change is an autogenic
response analogous to that noted above for the
modern Mahakam Delta. The change from domi-
nantly fluvial to tidal processes observed in the
Fox Hills succession can be explained using the
observation from modern deltas that fluvial dis-
tributaries switch their discharge between differ-
ent channels (e.g. Fly & Mahakam deltas) (Fig. 6).
The Fox Hills deltas mapped in the subsurface,
using several hundred wells, show the change of
the sand geometries from dip elongated to strike
elongated sand bodies as deltas migrated from the
inner to outer shelf (Olariu et al ., 2012).
In the Upper Cretaceous Wall Creek Member of
the Frontier Formation (Fig. 7C) delta-front depos-
its have been documented that contain tidal-
dominated and fluvial-dominated deposits with
changes to minor wave-dominated deposits over a
distance less than 500 m (Gani & Bhattacharya,
2007). As in the previous examples, the transition
between different facies regimes on the delta front
occurs over a short distance, suggesting that this
was also an autogenic response of the delta. The
Wall Creek Delta had wave influence on the proxi-
mal part of its delta front and this turned to fluvial
influence and eventually into intervals with tidal
domination toward south-east. The combination
of proximal wave and tidal process alternation
resembles that noted above for the modern Mekong
delta (Fig. 5) but the presence of fluvial and tidal
processes is similar to that in the Mahakam delta
(Fig. 6). However, the Wall Creek outcrops are of

Examples of process change in ancient deltas

Ancient examples of process change have been
documented in outcrop records of the shorelines
of the Cretaceous Western Interior Seaway (Fig. 7).
The regressive part of the Campanian Chimney
Rock delta (Fig. 7A) is predominantly wave-
dominated but also contains two intervals of
fluvial-dominated deposits, in the form of gravity
flows on delta-front clinoforms (Plink-Bjorklund,
2008). The delta-front facies of the lower tongue of
the Chimney Rock changes from fluvial-domi-
nated to wave-dominated over a distance of 2 km
to 3 km. The dip-oriented outcrops have eight dis-
tinct delta lobes (not all depicted in Fig. 7A, clino-
form sets documented by Plink-Bjorklund, 2008),
each with a thickness of 10 m to 40 m and stacked
in a dominantly progradational to aggradational
pattern along a dip-oriented section (Fig. 7A). As
the lobes are lateral to each other, it is probable
that the wave-dominated to fluvial-dominated
process changes observed were an autogenic
response during the overall progradation of the
delta complex. The transitions in the lower (older)
lobes from fluvial-dominated to wave-dominated
character are probably analogous to the northern
distributary of the Danube Delta (Chilia lobe)
where process change from fluvial-dominated to
wave-dominated occurred because of progradation
into a more open, stronger wave regime. However,
the Chimney Rock lobes were mainly wave-domi-
nated, with only locally occurring fluvial domi-
nance, as illustrated by the presence of fluvial
mouth bar and gravity flow facies. Another ancient
delta shoreline which shows an interplay between
fluvial-dominated and wave-dominated shoreline
that is probably similar to Danube (Sf. Gheorghe
lobe) or Mississippi deltas is the Cretaceous
Aberdeen Member of the Blackhawk Formation
in Utah (Charvin et al ., 2010). As the wave-
dominated to fluvial-dominated changes within
the Aberdeen-1 Member were interpreted to be in
the same parasequence (Charvin et al ., 2010) it is
possible that the deposits record autogenic
responses of the sedimentary system. Shoreline
morphological changes without process changes
(wave-dominated) have been interpreted in