Late Cenozoic Seismic Onlap-Offlap Sequences And Sea Level Changes On The Northern Sunda Shelf, South China Sea

Research on sea level is of great significance for understanding mechanisms ofinteraction among the spheres of the earth, revealing origins of some importantgeological events, and studying the occurrence of natural resources and sedimentarydeposits, as well as predicting future trends of sea level changes. Sunda shelf, one ofthe largest shells in the world, is believed to be a good site to interpret sea levelfluctuations during Late Cenozonic age. In this thesis, about 2200 km high-resolutionseismic lines from German Cruise Sonne-ll5 and China Guangzhou MarineGeological Survey are used to reconstruct the framework of sequence stratigraphy, andto document the history of sea level changes, based on principles of seismicstratigraphy and sequence stratigraphy.Reviews on key surfaces of sequences and system tracts defined by modernsequence stratigraphy, including correlative conformity with unconformity, maximumflooding surfaces, transgressive surfaces, suggest that these surfaces are potentialdiachronous surfaces on global or inter-regional correlation. These surfaces areisochronous only in regions with no significant contrasts on subsidence and sedimentsupply.In this study, we use transits of onlap and offlap points to define seismicsequences and system tracts. Transits of onlap and offlap points are easily recognizedon regional seismic profiles across basin margins. They record the history of theexpending and shrinking of basin sedimentation. A seismic onlap-offlap sequence isdefined as a seismic unit bounded by minimum depositional surfaces or theircorrelative unconformities (including onlap/tmnction unconformities and downlapunconformities). A minimum depositional surface is a conformable stratigraphicsurface separating underlain offlapping unit from overlain onlapping unit. It formswhen the extent of basin sedimentation shrinked to the least. A seismic onlap-offlapsequence can be subdivided into two system tracts: onlapping system tract andof-flapping system tract. They are separated by maximum depositional surfaces,generating when stratigraphic onlapping transiting to offlapping.23 seismic onlap-offlap sequences are recognized on northern Sunda Shelf. Inmost of these sequences, the system tracts are developed asymmetrically. In the LowerPliocene, sequences are dominated by onlapping system tracts, while sequences in theUpper Pliocene and Pleistocene mainly consist of oftlapping system tracts. Progradingis the general trend of evolution of Pliocene and Pleistocene sequences.A quantitative model is developed for calculating magnitudes of global sea levelrising and falling based on seismic data. The model incorporates erosion, compaction,isostastic loading subsidence of basement, tectonic subsidence, and water depth. Usingthis model, a eustatic sea level curve is compiled based on study of onlap-offlapseismic sequences on northern Sunda shelf. Our curve is in good accordance with bothHaq et al.'s (1987, 1988) eustatic sea level curve and benthic fomminiferal stable isotopeδ¹⁸O curves of ODP Leg 138 in tropical Pacific Ocean and ODP Leg 184 inthe South China Sea. It suggests that our sea level model and the parameters used inthe calculation are reasonable.According to our sea level curve, four stages of eustatic sea level changes can bediscerned during last 5.33 Ma: 1) rapid rising stage during 5.33～5.1 Ma, 2) stablehigh-stand stage during 5.1～3.7 (±0.1) Ma, 3) gradual falling stage during 3.7～0.9 (±0.1), and 4) fluctuating stage with general low stand during the time interval from 0.9Ma to the incipient of Holocene transgression.On our original eustatic sea level curve, about 36 cycles of sea level:fluctuationcan be recognized with periods from 0.08 to 0.29 Ma, which are fallen into 4th order ofcycle.s defined by Vail et al. (1991). After processed by moving averaging with thesame window length, good matches between our curve andδ¹⁸O curves are found.This result suggests the 4th order of eustatic sea level fluctuations during last 5.33 Mamay be mainly controlled by changes of the size or volume of ice caps.