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.