Geology Reference
In-Depth Information
The maximum rates of sea-level fall are taken
from the studies on the Pleistocene (Labeyrie
et al
., 1987; Shackleton, 1987) as discussed above.
The upper limit of subaerial erosion is set equal to
the maximum denudation rates of extant tropical
karst terrains, i.e. 200
the clastics lack the stabilizing effect of organic
frame-building and submarine lithifi cation.
(2) Dissolution is a dominant factor in erosion of
tropical carbonates and this means that the ero-
sion products are highly mobile and likely to dis-
appear (as ions) from the space considered here.
In siliciclastic systems, most eroded material can
be expected to be redeposited in falling stage or
lowstand systems tracts.
Siliciclastic case studies indicate high and
highly variable rates of erosion in siliciclastic
systems during sea-level falls. In particular,
complete erosion of delta topsets and severe
truncation of the adjacent prodelta foresets are
commonly observed (Anderson
et al
., 2004;
Roberts
et al
., 2004). Severe erosion may also
occur during the ravinement stage of transgression.
Erosion rates deduced from the carefully compiled
geohistory plots of Fillon
et al
. (2004) yield rates
of surface lowering of up to 1100
m yr
1
(White, 1984). The
maximum rate of marine erosion, arguably the
least constrained quantity of the plot, is assumed
to be 100
m yr
1
(see discussion above). This
yields a maximum total erosion rate of 300
m yr
1
in geologically probable space. The lower limits of
the rates of sea-level fall and erosion are assumed
to be indistinguishable from zero at the resolution
of the diagram.
The third important parameter, carbonate
production, is shown on the vertical axis of
Fig. 8c. The upper limit of geologically probably
space is set at a production rate of 40,000
m yr
1
.
However, this high rate is only probable for
time intervals of a few thousand years and shorter
(see discussion above). Only the low-rate part of
the production space has been explored by mod-
elling and the limit of the geologically probable
production space is not shown in Fig. 8c.
Finally, it must be emphasized that what is
'geologically probable' strongly depends on the
state of knowledge at a given point in time. At
present it seems very diffi cult to conceive of nat-
ural laws that would preclude rates of sea-level
fall, carbonate production or erosion to proceed
faster than observed up to now. In this regard,
numerical experiments remain heuristically valu-
able even where they lie outside the parameter
space currently supported by observation.
m yr
1
for the
Quaternary of the northeastern Gulf of Mexico.
The high rate of erosion may suppress the devel-
opment of a falling-stage systems tract and favour
the formation of a continuous surface of erosion
that connects highstand and lowstand tracts.
However, the intense erosion that creates the ero-
sive sequence boundary also severely truncates
the underlying highstand tract - a signifi cant
difference to the standard model of sequence
stratigraphy (Fig. 1) where erosional truncation of
the underlying highstand tract is minor.
Terminology
Since the fi rst reports on the existence of
signifi cant sediment accumulations formed dur-
ing sea-level fall, several proposals on terminology
have been made. Hunt & Tucker (1992) called them
'forced-regressive wedge systems tract' or 'forced
regressive systems tract' (Hunt & Tucker, 1995) to
emphasize that these sediment bodies were clear
evidence of forced regression in the sense of Plint
(1991) and Posamentier
et al
. (1992). Nummedal
et al
. (1992) coined the term 'falling sea level sys-
tems tract'. Nummedal
et al
. (1995) and Plint &
Nummedal (2000) simplifi ed this to 'falling-stage
systems tract'.
The position of the sequence boundary in the
presence of the FST is hotly debated. Should it
be put at the base of the FST, i.e. at the level
where sea-level fall commenced (Posamentier &
Morris, 2000), or at the top of the FST (Nummedal
et al
. 1995)? In a detailed review, Plint & Nummedal
(2000) succinctly summarize the pros and cons of
Tropical carbonates versus siliciclastics
The tropical carbonate factory studied here
differs in many aspects from siliciclastic systems
that provided the principal data base for the stan-
dard model. These differences need to be consid-
ered when applying the results of this study to
siliciclastics. The notion that FST development
is fundamentally determined by the rate of sea-
level fall and the rate of erosion should apply to
siliciclastics, too. Consequently, the distribution
of FST and STM anatomy of siliciclastics may
be described in the same parameter space (with
external sediment supply replacing carbonate
production).
Major differences can be expected with regard
to erosion and local reworking. (1) Mechanical
erosion in siliciclastic settings can be expected
to exceed those of tropical carbonates because
