Exploration Insights May 2020 | Page 14

14 | Halliburton Landmark
Exploration Insights | 15
of the geologic setting and methods from which
the magnitude estimates were derived in each
interval. This geologic review identified the most
robust studies, discounted anomalous data,
and identified a robust maximum short-term
magnitude for each of the time intervals. Finally,
as a means of validating these geologically
defined maximum short-term magnitude limits,
a further statistical review focused on the upper
magnitude limits (Figure 3).
THE MAGNITUDE OF SHORT-TERM
CRETACEOUS SEA-LEVEL CHANGE
Coniacian
Interval 9
Turonian
95
Interval 8
Cenomanian
100
Interval 7
105
Albian
110
Magnitude limits (m)
based on review of
the associated
publications
Interval 6
120
125
130
115
67
70
75
Aptian
Median 3 million
years and 95%
confidence limits
80
Interval 5
85
90
95
Interval 4
Barremian
Median 6 million
years and 95%
confidence limits
Hauterivian
135
Ma Epoch
Valanginian
90 th percentile and
95% confidence
limits
Interval 3
Interval 2
Berriasian
© 2020 Halliburton
120
Interval 1
0
25
50
75 100 125 150
Sea-level change (m)
0
25
50
75 100 125 150
Median sea-level change (m)
from 3 and 6 million year
moving average
125
0
25
50
75 100 125 150
Magnitude limits (m) based
on the 90 th percentile of the
dataset and review of the
associated publications
Figure 3> A workflow for determining the magnitude of short-term Cretaceous sea-level change (modified from figures in Ray et al., 2019).
1. Identification of publications (n=37) that provide estimates of sea-level change (m); 2. Tabulation of point data (n= 791) according to the
age and magnitude of sea-level change; 3. Identification of patterns based on a statistical review of the short-term magnitude of sea-
level change (moving averages resulting from a data sensitivity analysis given here, see Ray et al., 2019); 4. Determination of maximum
magnitude limits based on the 90 th percentile of the entire dataset and a review of the associated literature.
130
135
140
Temperature Proxies
Ice Proxies
Campanian
Santonian
Coniacian
Turonian
Cenomanian
105
115
140
Magnitude Limit
Maastrichtian
100
110
Age/
Stage
Albian
Santonian
90
Interval 10
Aptian
Barremian
Hauterivian
Valanginian
Warm
Berriasian
© 2020 Halliburton
0
25
50
75
100
125
150
Magnitude limits (m) based on synthesis
of Cretaceous magnitude data
85
Campanian
80
75
Interval 11
Maastrichtian
70
4. Determination of upper
magnitude limits
The link between short-term sea-level
magnitudes and climate is supported by
broad trends within Cretaceous temperature
proxies, such as TEX 86 and δ 18 O. These proxies
illustrate that cooling is associated with larger
magnitude, short-term sea-level changes,
while globally warmer climates correspond to
smaller magnitudes; as would be expected if
eustasy were controlled by icecap volume. In
addition, sedimentological proxies for ice, such
as glendonites, a cool-water pseudomorph
of calcite, diamictites, and dropstones, are
reported for intervals of cooling (Ray et al.,
2019) (Figure 4). Thus, the magnitude and
rate of short-term eustatic change is strongly
3. Identification of
patterns
2. Tabulation of data
67
Age/
Stage
The initial review of the entire Cretaceous
dataset, weighing each data point equally, gave
a median value for short-term eustatic change of
12 m, hence the majority of sea-level estimates
are of relatively low magnitude with few
examples of large magnitude. Examining median
estimates at a stage level, following standard
statistical resampling procedures, demonstrated
that elevated magnitude values occurred during
the Valanginian, Barremian to Aptian, and
Santonian to Maastrichtian, with low magnitude
values in the Berriasian, Hauterivian, and Albian
to Coniacian. Furthermore, maximum magnitude
limits were derived that are in keeping with
some estimates derived from backstripping (e.g.
Sahagian et al., 1996; Miller et al., 2004), but are
in contrast with some of the greater magnitudes
suggested by Haq (2014) (Figure 2).
Early
Studies that estimate the
magnitude of short-term
sea-level change
Ma Epoch
Even though the Cretaceous eustatic limits
suggested by Ray et al. (2019) are relatively
modest (5 to 65 m), 50% of the Cretaceous
(Valanginian, Aptian, Albian, and Maastrichtian)
is associated with significant (>40 m) eustatic
changes that may be considered highly
characteristic of glacio-eustasy. Furthermore,
in the presence of significant eustatic change,
the immediately older and younger intervals of
modest magnitude changes (10 to 40 m) may
be interpreted as representing the growth and
demise of land-grounded icecaps. Based on
these criteria, it is only within the Berriasian
that glacio-eustasy may be considered
equivocal.
1. Identification of publications that provide estimates of the
magnitudes of short-term sea-level changes
0
8
0
4
Number of Number of
records
records
Figure 4> A comparison of the magnitude limits of Cretaceous short-term eustasy and Cretaceous climatic proxies (modified from Figure 9
of Ray et al., 2019).