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Exploration Insights | 13
A quantification of the magnitude of sea-level
change is difficult to determine because of
significant uncertainties in establishing accurate
and precise measurements of sea-level change,
and uncertainties in determining the age
synchronicity of sea-level change between
geographically widespread studies. The most
useful approach for estimating magnitude ranges
is to make a comparative synthesis of published
3. The magnitude estimates are derived by
clearly documented, reliable methods
Commonly used methods for estimating
the magnitude of sea-level change include:
backstripping; δ 18 O analysis; sedimentological and
palaeontological observations, such as erosional
and depositional relief; facies juxtaposition; fossil
assemblages; and seismic and stratigraphic
geometries. Rygel et al. (2008) pioneered a
comparative synthesis methodology for the study
of Late Palaeozoic eustasy, and more recently
Ray et al. (2019) followed a similar approach to
determine Cretaceous eustatic magnitude limits,
supported by a data sensitivity analysis. The
principal advantages of these approaches were
that they overcame the caveats and assumptions
associated with each method of calculating
eustatic magnitude, and removed local and
High
s t a s y
o - e u
i
c
a
G l
10 ky
Short-term changes (Duration)
1
100 y
40
10
Miller et al. 2004
Transgression
Regresssion
Haq 2014
Transgression
Regression
100
2. The sea-level changes are accurately
calibrated to the geologic timescale
100
Sahagian et al. 1996
Transgression
Regression
120
1. The sea-level change is deemed to
be eustatic (because the location was
relatively tectonically stable, and the
event was short in duration)
1,000
200
140
1 My
Figure 1> A schematic representation of the duration, magnitude, and rate of known drivers of short-term eustasy, alongside the impact
upon the stratigraphic record. The curves for thermo-, aquifer-, and glacio-eustasy reflect the upper limits of the climatic drivers of the
eustasy (modified from Figure 7 of Ray et al., 2019).
80
60
40
Magnitude limits from
Ray et al. 2019
20
0
Maastrichtian
HOW IS THE MAGNITUDE OF SEA-
LEVEL CHANGE DETERMINED?
magnitude records. Ideally, such records are
derived from studies where:
only glacio-eustasy is capable of generating
magnitudes of short-term sea-level change that
are >20 m (Davies et al., 2020). Accordingly, in
an “icehouse” world, the magnitude of sea-level
change is related to the volume of global icecaps,
with cooler climates and larger icecaps resulting
in greater magnitude sea-level changes. In this
way, the short-term magnitude of eustasy can
contribute to our knowledge of Earth systems
science, including palaeoclimate evolution, orbital
forcing of sedimentary systems, geochemical
evolution of the oceans, and biological evolutions
and extinctions. It, thereby, helps validate the
Neftex Earth system models and the predictions
of source rock, reservoir, and seal rock facies
distribution derived from these models.
Campanian
San. Con. Turonian Cenomanian
Albian
Late
Aptian
Bar.
Hau. Valanginian Berriasian
Early
Cretaceous
Figure 2> A comparison of the eustatic sea-level changes calculated by Sahagian et al. (1996), Miller et al. (2004), Haq (2014), and Ray et
al. (2019), illustrating the marked difference in magnitude estimates. Estimates are shown of individual sea-level rises and falls derived from
Sahagian et al. (1996), Miller et al. (2004), and Haq (2014), alongside the magnitude limits from Ray et al. (2019). Note, maximum values
are shown where upper and lower limits were given, otherwise best estimate values were taken. The data points identify the age and
magnitude of sea-level rises and falls. San. = Santonian; Con. = Coniacian; Bar. = Barremian; and Hau. = Hauterivian.
regional biases from the data. This was only
achievable by means of a global synthesis of all
applicable data.
Both studies identified broad stratigraphic
intervals, which approximate stages in duration,
from which robust upper magnitude limits were
calculated. The stratigraphic intervals were
identified by step-shifts in the magnitude of
short-term sea-level change, thus grouping data
into intervals that can be globally correlated with
ease. The identification of intervals characterized
by short-term sea-level changes of particular
magnitudes strongly suggests the dominance of a
global eustatic signal, rather than the presumably
random signal that might be expected from relative
sea-level changes derived from local variations in
subsidence and sedimentation rates.
The potential pitfalls of more qualitative
approaches to estimating the magnitude of
eustasy can be seen by comparison with
the eustatic curves of Haq and Schutter
(2008) and Haq (2014). These studies derived
eustatic magnitude estimates by averaging
local measurements from a limited number
of stratigraphic sections. Haq (2014) has
acknowledged that such measurements are
approximations, but the magnitudes of eustatic
change suggested greatly exceed those of Rygel
et al. (2008) and Ray et al. (2019), respectively.
They also exceed estimates purely from
backstripping (e.g. Sahagian et al., 1996; Miller et
al., 2004) (Figure 2). A simple conclusion might be
that the magnitude of local water depth change in
the sections selected by Haq and Schutter (2008)
and Haq (2014) was overestimated.
ESTABLISHING THE MAGNITUDE
OF SHORT-TERM CRETACEOUS
SEA-LEVEL CHANGE
Ray et al. (2019) utilized records of Cretaceous
sea-level change with durations of 3 Myr or less.
By doing so, they reduced the influence of local
tectonics and long-term drivers of eustasy (e.g.
sea-floor spreading and sedimentation) in favor of
a short-term climate-driven eustatic signal linked
to orbital forcing (climate-linked) mechanisms.
Almost 800 individual estimates of sea-level rise
and fall were included in the synthesis following a
rigorous selection process.
A consensus view was extracted from these
data in a step-wise manner. First, a preliminary
statistical analysis of the entire dataset was
undertaken to identify robust temporal trends
in magnitude estimates, and to establish
intervals characterized by a particular range of
magnitudes. Once these intervals had been
established, an in-depth review was performed