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(B)
0
“tramline” (parallel) reflectors
(A)
1
Shelf
Seq 3
5 km
Seq 1
Seq 2
2
Seismic Courtesy TGS/GECO
Slope
Basin floor
(C)
D
9
15
19
8
13
11
17
5
12
18
4
C
14
10
16
3
7
B
5m
2
15 km
A
6
1
6
Predeltaic deposits
C
Delta lobes
Delta sub-lobes (numbers represents the successive depocenter locations)
Fig. 9.
Distinction between autogenic and allogenic generated deposits. (A) Sketch with the shelf to basin floor clinoform.
(B) Typical 'tramlines': seismic reflectors over the shelf of the Gulf of Mexico (modified after Mitchum
et al
., 1990). These
reflect the transgression-regression cycles at the large (shelf) scale. (C) Internal architecture of the Holocene Mississippi
Delta (modified after Frazier, 1967). Note the complex architecture generated by autogenic response with multiple loca-
tions of the delta depocentres at large scale (letters) or smaller scale (numbers). These changes happen over 8 ka and within
15 m thick deposits.
front, individual delta lobes or of smaller compo-
nents, and coeval changes will most probably not
be recognised in other delta systems in the same
basin. The lateral extent of the process changes
observed in Holocene deltas tends to scale with
the system; 5 km to 10 km (Mahakam), 20 km to
30 km (Danube), 40 km to 50 km (Mekong) or hun-
dreds of kilometres (Mississippi). The timing of
the process change is also system specific, as
for the specific time scale for shoreline autore-
treat (Muto, 2001). Nevertheless, we emphasise
that these apparently chaotic (Fig. 9C) autogenic
responses (causing great difficult for correlation at
this spatial and time scale) are usually contained
by more widespread flooding surfaces and
sequence boundaries (and more easily correlated)
caused by the longer time scale (few 100 ka) regres-
sive movement to transgressive movement of
the entire delta system across the shelf, in response
to allogenic unsteadiness of sea-level, tectonics or
climate (Figs. 9A, B). The landward-seaward
extent of these latter 'sequences' is dependent
mainly on the landward extent of marine and coe-
val brackish-water flooding back across the shelf,
which is especially probable to vary with Icehouse
and Greenhouse climates (Steel
et al
., 2008;
Sømme
et al
., 2009). These allogenic units create
the 'tramline' character of many shelf or topset
segments of shelf-margin sedimentary prisms
(Fig. 9B). Fig. 9 suggests the spatial and temporal
contrast between the allogenic and autogenic
components of shelf stratigraphy, at least until it
is demonstrated that autogenic responses can
occur at a time scale of several 100 ka.
CONCLUSIONS
Autogenic process changes are a common behav-
iour in delta systems during large-scale cross-shelf
progradation and retrogradation, as shown by
Holocene river deltas (Mississippi, Danube,
Mahakam, Mekong) that displayed autogenic
responses in the form of changes in dominant pro-
cess and attendant changes in morphology despite
steady Holocene external forcing conditions. The
process changes interpreted in these deltas hap-
pened with relative rapidity (hundreds to thou-
sands of years) and are demonstrably autogenic in
nature because, the external forcing factors were
constant for the last 7 ka to 8 ka. Process change
from fluvial-dominated or tidal-dominated to wave-
dominated deltas have been documented across
depositional dip or between fluvial-dominated to
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