Use of Sedimentary Simulations for Dating Sequence
Boundaries and Measuring the Size of Eustatic Sea Level
Changes: an Example from the Neogene of the Bahamas
C. G. St. C. Kendall and A. Sen
1
Introduction
Two major problems which confront sequence stratigraphers interpreting seismic lines are (1) the ages of sequence boundaries (MiallI990); and (2) the size
of eustatic sea-level changes (Burton et al.I988). Sedimentary simulation can be
used in conjunction with sequence stratigraphy to interpret seismic cross sections to solve these problems. To this end, a seismic cross section that records the
Neogene carbonate fill of the West Andros Channel of the Bahamian Platform
was interpreted, and a sedimentary simulation based on empirical modeling reproduced this interpretation. This match between the interpretation and the
seismic has enabled the age dating of sequence boundaries and determined that
the size of the eustatic sea-level excursions for this time period matches the sealevel events on the Haq et al. (1987) chart.
At the heart of this study is the recognition of eustatic events, evidenced by
the presence of synchronous sedimentary sequences and the unconformities
that bound them (Vail et al. 1978). These eustatic signals produce changes in
the accommodation for sedimentary fill and have a worldwide extent. Their
chronostatigraphic correlation is dependent upon reliable time markers
spaced sufficiently close in time to bracket the sediment packages formed in
response to changes in sea level. The amplitude of these eustatic events
presents an enigma, since these cannot be determined independent of models
for tectonic behavior and sedimentation. The result is that, while sea-level
charts can be created, the amplitudes of given events on these charts are dependent on assumptions concerning the rates of subsidence and sediment accumulation. Unfortunately, there are no direct methods available to measure
the amplitudes of sea-level variations. This is because there is no datum available to measure from, since the earth surface constantly moves in response to
(1) sediment compaction, (2) isostatic response to sediment loads, and (3)
thermal tectonic movement (Burton et al.I988). Thus, the relative sea level position is dependent on tectonic behavior and eustatic position, so the size of
either of these two variables can be measured only by assuming a model for the
other's behavior. Methods which attempt to indirectly measure sea level have
to assume models of tectonic behavior. Such methods include tide gauges,
Boundaries and Measuring the Size of Eustatic Sea Level
Changes: an Example from the Neogene of the Bahamas
C. G. St. C. Kendall and A. Sen
1
Introduction
Two major problems which confront sequence stratigraphers interpreting seismic lines are (1) the ages of sequence boundaries (MiallI990); and (2) the size
of eustatic sea-level changes (Burton et al.I988). Sedimentary simulation can be
used in conjunction with sequence stratigraphy to interpret seismic cross sections to solve these problems. To this end, a seismic cross section that records the
Neogene carbonate fill of the West Andros Channel of the Bahamian Platform
was interpreted, and a sedimentary simulation based on empirical modeling reproduced this interpretation. This match between the interpretation and the
seismic has enabled the age dating of sequence boundaries and determined that
the size of the eustatic sea-level excursions for this time period matches the sealevel events on the Haq et al. (1987) chart.
At the heart of this study is the recognition of eustatic events, evidenced by
the presence of synchronous sedimentary sequences and the unconformities
that bound them (Vail et al. 1978). These eustatic signals produce changes in
the accommodation for sedimentary fill and have a worldwide extent. Their
chronostatigraphic correlation is dependent upon reliable time markers
spaced sufficiently close in time to bracket the sediment packages formed in
response to changes in sea level. The amplitude of these eustatic events
presents an enigma, since these cannot be determined independent of models
for tectonic behavior and sedimentation. The result is that, while sea-level
charts can be created, the amplitudes of given events on these charts are dependent on assumptions concerning the rates of subsidence and sediment accumulation. Unfortunately, there are no direct methods available to measure
the amplitudes of sea-level variations. This is because there is no datum available to measure from, since the earth surface constantly moves in response to
(1) sediment compaction, (2) isostatic response to sediment loads, and (3)
thermal tectonic movement (Burton et al.I988). Thus, the relative sea level position is dependent on tectonic behavior and eustatic position, so the size of
either of these two variables can be measured only by assuming a model for the
other's behavior. Methods which attempt to indirectly measure sea level have
to assume models of tectonic behavior. Such methods include tide gauges,