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 paleo-bathymetric 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 bio-stratigraphic 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.

a. realize the relationships between

sedimentary facies and sea-level

b. transgression and regressionc . unconformity, diastem and hi

atusd. absolute sea-level change or t

ectonic movement

Column section

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 considercrustal subsidenceand sea-level

HST

HighstandSystemsTract

SB

SequenceBoundary

FSST

FallingStageSystemsTract

Different styles of sea-level falling

LST

LowstandSystemsTract

TS

TransgressiveSurface

TST

TransgressiveSystemTract

MFS

MaximumFloodingSurface

Using dating methods to know the ageCombining 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 geometryb. 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 c

omposed 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

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.

D. Summary

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 “Sea-level Changes”, National Academy Press, Washington D.C.

E. References