Stratigraphic Record of
Sea-Level Change
Prof. Yue-Gau Chen
A. Conventional Stratigraphic Record of
Sea-level change
a. Oscillatory changes in sea level relative
to the continents, on time scales of <10
my, have long been inferred from paleobathymetric variations in facies
successions, and from stratigraphic
evidence.
b. degree of confidence depending on the
reliability and resolution of the facies
interpretation, the precision of the biostratigraphic correlation and the extent of
such correlation.
c. even if depositional base level is modulated
by eustasy, a given eustatic event may not be
evident in the facies that are preserved, and
max. and min. Water depth or
transgression/ regression may not be
precisely synchronous at different localities.
Column section
a. realize the relationships between
sedimentary facies and sea-level
b. transgression and regression
c. unconformity, diastem and hiatus
d. absolute sea-level change or
tectonic movement
B. Sequence Stratigraphy and Sea-level
Emphasizing the importance of the space
that is made available within a basin for
sediment to be deposited and the amount of
sediment supplied.
Sediment accommodation
space and its relationship
to eustatic sea-level
Shallow-marine equilibrium profile
The equilibrium
profiles of an
alluvial system
Parasequence
Long-term
changes over
two
parasequences
Only consider
crustal subsidence
and sea-level
HST
Highstand
Systems
Tract
SB
Sequence
Boundary
FSST
Falling
Stage
Systems
Tract
Different styles of sea-level falling
LST
Lowstand
Systems
Tract
TS
Transgressive
Surface
TST
Transgressive
System
Tract
MFS
Maximum
Flooding
Surface
Using dating methods to know the age
Combining the depths of identified SB, TS and MFS
To reconstruct the sea-level change curve,
but it is hard to understand the entire
picture
C. Seismic Stratigraphic Record of
Sea-level Change
a. Seismic stratigraphy is an approach to the
investigation of sea-level fluctuations that is less
sensitive than conventional stratigraphy.
b. Vail et al. (1977) suggested that many sequence
boundaries are of the same age in different parts
of the world and are therefore due primarily to a
global process, eustasy. They also developed a
technique for quantifying the amplitude of
relative sea-level change from the saw-tooth
patterns of onlap observed on seismic sections.
c. “relative change of coastal onlap” to “eustatic
curve” of Mesozoic and Cenozoic (Haq et al.,
1987).
d. seismic stratigraphy provides important
information about the timing of sea-level
fluctuations, on a time scale of m.y., but little
about magnitudes.
D. Seismic profile
a. seismic imaging of stratal geometry
b. seismic resolution
1. vertical resolution: acoustic velocity, burial depth,
acoustic frequency
*frequency: 20 Hz
*wavelength in shale: 210m
*layer: a single unit or composites
*layers as thin as 1/4 wavelength: reflections from
the upper and lower surfaces are discernible
*<1/4 wavelength: the distinct contributions from
the upper and lower surfaces cannot be identified
2. horizontal resolution: frequency of
the source, depth of the reflector
*frequency: 20 Hz
*acoustic velocity: 2.25 km/sec
*reflection from a depth 4 sec
－ diameter 500m
c. recognition of unconformities in seismic profile
1. depositional sequence: is a stratigraphic unit
composed of a relatively conformable
succession of genetically related strata and
bounded at its tops and base by
unconformities or their correlative
conformities
2. lower boundaries: baselap (onlap, downlap),
concordance
3. upper boundaries: erosional truncation,
toplap, concordance
d. interpretation of sea-level change
D. Summary
Basins should be selected for study on the
basis of stratigraphic completeness; simple
tectonic history; and the current or future
availability of high-resolution seismic sections,
fully cored bore-holes.
E. References
1. Haq, B.U., Hardenbol, J. and Vail, P.R. (1987)
Chronology of fluctuating sea levels since the Triassic.
Science 235, 1156-1167.
2. Sheriff, R.F. (1980) “Seismic Stratigraphy”, Published by
International Human Resources Development
Corporation, Boston, 227p.
3. Christie-Blick, N., Mountain, G.S. and Miller, K.G. (1990)
Seismic stratigraphic record of sea-level change. In “Sealevel Changes”, National Academy Press, Washington
D.C.