Geoscience Reference
In-Depth Information
0
Rate of sea level rise decreases
(allogenic event)
Inception of modern large deltas
Acropora palmata dates
(Barbados and Caribbean reefs)
10
Steady sea level rise
(autogenic conditions)
20
30
40
50
Number of Holocene
deltas initiated
60
12
70
10
80
90
6
4
100
3
Fig. 2.
Eustatic sea-level variation during last
17 ka (Fairbanks, 1989) and the timing of
Holocene delta formation (Stanley & Warne,
1994). Note that most modern deltas started to
form around 7.5 ka to 8 ka and sea-level has risen
steadily since then.
110
1
120
0
2
4
6
8
10 2 4 6 8
Age (ka)
change of one lobe from fluvial-dominated to
wave-dominated is caused by the decrease in
discharge to the active delta during avulsion
(Fig. 3) followed by compactional subsidence.
Older Mississippi delta lobes (Sale-Cypremort,
Teche, St. Bernard, Lafourche) that were fluvial-
dominated at the time of progradation also
turned into wave-dominated deltas and eventu-
ally to barrier islands. When the present Balize
delta lobe is abandoned and becomes wave-dom-
inated, the most recent delta lobe will be the
fluvial-dominated Atchafalaya Delta. Thus, each
past lobe was initially a fluvial-dominated delta
lobe of the Mississippi delta complex that was
abandoned and turned into a wave-dominated
delta lobe and later into a wave-dominated estu-
ary, as autogenic responses. The transition from
fluvial-dominated to complete abandonment and
transgression is a gradual process that is likely to
take hundreds of years (Roberts, 1997, his fig. 2).
The abandonment time (gradual decrease in flu-
vial discharge to one lobe) is the interval when
the delta will become wave-dominated before
being transgressed. Fig. 3 shows that the oldest
lobe (Sale-Cypremort) was abandoned about 4 ka
BP and was then transgressed for a greater dis-
tance than the younger lobes. Furthermore, it has
a larger area of sandy transgressive deposits than
the more recently abandoned St. Bernard lobe
(abandoned about 1.5 ka) and Lafourche lobe
(newly abandoned).
The Danube Delta also initiated about 7 ka ago
(Giosan
et al
., 2006) at the head of an embay-
ment in the north-western Black Sea. It formed
multiple lobes during overall progradation
(Panin, 1989) but at present has three main dis-
tributaries supplying lobes (Fig. 4) of varying
character because of differing fluvial discharge
(Panin & Jipa, 2002). The modern northern lobe
(Chilia) is fluvial-dominated and a historical
map from 1856 (Fig. 4) shows multiple distri-
butary channels, each bifurcated by mouth
bars as is typical for fluvial-dominated delta
morphology (Olariu & Bhattacharya, 2006). On
satellite images acquired in 2003 (Fig. 4) the
number of distri butary channels was reduced
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