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stratigraphic products and responsible unsteady
dynamic external forcing (Fig. 6). Although strati-
graphic records are generally composed of both
autogenic and allogenic products, conventional
stratigraphy has been apt to ignore the importance
of autogenesis and thus to overrate allogenic pro-
cesses. Any large-scale stratigraphic analysis
without paying attention to the autostratigraphic
view can lead to a serious misinterpretation of
stratigraphic successions (Muto
et al
., 2007).
The deterministic nature of autostratigraphic
analysis is advantageous for detecting unsteady
dynamic external forcing because any significant
unsteadiness should come out as a discrepancy
from the theoretical prediction. An example of
the quantitative application of autostratigraphic
analysis has been given from an Eocene strati-
graphic succession of regressive shorelines in
the Central Tertiary Basin on Spitsbergen (Muto
& Steel, 2002a). This regressive wedge developed
as a delta-driven, shelf-margin system under a
regime of overall (i.e. long-term) rise of relative
sea-level, with a corresponding shelf-edge trajec-
tory available for analysis (Fig. 7). The reciprocal
relationship between R
slr
and Q
s
(Eq. 1) indicates
that an increase in Λ is possible both by increas-
ing Q
s
and decreasing R
slr
; and a decrease in Λ
can result from both decreasing Q
s
and increas-
ing R
slr
. Thus, unless a sufficient number of time
lines are provided in the stratigraphic section,
any change in R
slr
cannot be calculated without
specifying or assuming Q
s
beforehand; and vice
versa. Fig. 8 shows two different scenarios
obtained from autoretreat-based, numerical sim-
ulations; one for variable R
slr
with constant Q
s
and another for variable Q
s
with constant R
slr
. On
the assumption that Q
s
was constant, the analy-
sis of field data obtained from three sites (P:
proximal, M: middle, D: distal) within the basin
suggests a spectrum of temporal patterns of R
slr
that can account for the observed shoreline tra-
jectory (Fig. 8). A notion to be remembered here
is that even if R
slr
is constant, the resulting shore-
line trajectory will be a concave-upward and
concave-landward curve (Fig. 3). The deter-
ministic prediction by the theory of shoreline
autoretreat suggests that the initial water depth
in the basin was around 0.45 km as a most likely
scenario and that the overall relative sea-level
rise (c. 0.80 km) happened largely during an early
time period and was followed by a longer period
of much lower rate of rise. This pattern of relative
sea-level rise is consistent with the Palaeogene
EXTERNAL DYNAMIC FORCING
+
STEADY COMPONENT
UNSTEADY COMPONENT
STRATIGRAPHIC PRODUCTS
ALLOGENIC COMPONENT
+
AUTOGENIC COMPONENT
CONVENTIONAL STRATIGRAPHY
AUTOSTRATIGRAPHY
Fig. 6.
The strategy of autostratigraphy. Autostratigraphy
firstly tries to explain any stratigraphic features only in
terms of autogenic processes or steady dynamic external
forcing. Allogenic interpretation is not incorporated as
long as autogenic interpretation is valid. Without this pro-
cedure, the effect of unsteady external dynamic forcing can
be overrated. Conventional stratigraphy, usually assuming
a sinusoidal pattern for sea-level changes, tends to overes-
timate the allogenic effect of external dynamic forcing.
coastal plain & estuarine sandstones
(Aspelintoppen Formation)
shelf-slope clinoform including
turbidite sandstone wedges
(Gilsonryggen Formation)
WEST
EAST
D
Fig. 7.
Shelf-edge trajectory within the upper
part (Eocene) of the van Mijenfjorden Group,
the Central Tertiary Basin, Spitsbergen. P
(proximal), M (middle) and D (distal) are the
observation sites in which the parameters
needed for numerical simulations were meas-
ured. Adopted from Muto & Steel (2002a).
M
0.5 km
shelf / shoreface
(Battfjellet Formation)
P
10 km
deep-water shales
(Gilsonryggen Formation)
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