Sequence Boundary:Its Significance in Stratigraphic Subdivision,Correlation and Classification

Sequence boundary is studied in detail in reference to the Permian Guadalupian Lopingian boundary, based on high resolution correlation of conodont zones. A conceptual synchronous stratigraphic boundary, corresponding to the age of the sequence boundary dated in the conformable portion of the sequence boundary, is defined as the sequence chronostratigraphic boundary (SCB). The sequence boundary is proved to be dual in nature in regards to tin-synchrony and diachroneity of stratigraphic boundaries. The merits of the sequence boundary in stratigraphic subdivision, correlation and classification are elucidated in comparison with the lithostratigraphic boundary, the biostratigraphic boundary and the traditional approach of the global stratotype section and point (GSSP ). By integrating the virtues of the sequence boundary and the GSSP, it is proposed that the GSSP should be established in the conformable portion of the related sequence boundary. The boundary established in light of this approach is defined as the best natural boundary. The analysis of the sequence boundary leads to the conclusion that sequence stratigraphy is a regional and transitional stratigraphic system between local lithostratigraphy and global chronostratigraphy. Therefore, a new tripartite stratigraphic classification scheme is tentatively proposed.

Transgressive Surface as Sequence Boundary

Analysis of the four cases of the sequence boundary (SB)-transgressive surface (TS) relation in nature shows that applying transgressive surfaces as sequence boundaries has the following merits: it improves the methodology of stratigraphic subdivision; the position of transgressive surface in a sea level curve is relatively fixed; the transgressive surface is a transforming surface of the stratal structure; in platforms or ramps, the transgressive surface is the only choice for determining the sequence boundary; the transgressive surface is a readily recognized physical surface reflected by seismic records in seismostratigraphy. The paper reaches a conclusion that to delineate a SB in terms of the TS is theoretically and practically better than to delineate it between highstand and lowstand sediments as has been done traditionally.

The Best Natural Boundary:a New Concept Developed by Combining the Sequence Boundary with the GSSP

Through a detailed study of sequence boundaries, it is concluded that sequence stratigraphy is an independent regional and transitional stratigraphic system between local lithostratigraphy and global chronostratigraphy. Therefore, a new tripartite stratigraphic classification scheme has been proposed. By combining organically the concept of sequence boundaries with the GSSP, it is suggested that the GSSP should be chosen in a conformale portion of a related sequence boundary, and the boundary established in light of this concept is defined as the Best Natural Boundary (BNB). The definition of the BNB points out the working area and stratigraphic level for the GSSP. By referring to a case study of the Permian Guadalupian/Lopingian boundary, the concept of the BNB has been elaborated in detail, and it is proposed that the BNB of the Guadalupian and the Lopingian lies between the Mesogondolella granti Zone and the Ctarkina postbitteri Zone, which is also the sequence chronostratigraphic boundary between the Transpecos composite sequence (chronozone)and the UAA- 1 composite sequence (chronozone).

Sequence Boundary and Sea Level Changes in Western Margin of Upper Yangtze Platform during Permian and Triassic

During Permian and Triassic the western margin of Upper Yangtze platform had experienced an evolution from early rift stage to thermal subsidence and then to foreland basin. The tectonic transformations have resulted in five supersequences and nineteen third-order se- quences.During the rift stage,transgeressive carbonate platform was built up,while in the thermal subsidence stage,regressive carbonate platform with foreslope was formed.The closure of the Garze-Litang narrow oceanic basin resulted in the migration of foreland flexure toward the ctaton.The carbonate ramp in foreland basin setting was formed during the relative sea level rise which was caused by the change in basin volume.

Study on Sequence Stratigraphy of Zhujiang and Zhuhai Formations, Zhu

The early Miocene in the Zhu Ⅲ subbasin, the Pearl River Mouth basin, includes two formations-Zhujiang and Zhuhai. There are 8 third-order sequences, S1, S2, S3, S4, S5, S6, S7 and S8 from the bottom of Zhuhai to the top of Zhujiang in thee two formations. There are only one transgressive systems tract (TST) and one highstand systems tract (HST) in each sequence because the whole Zhu Ⅲ subbasin was located updip the shelf break during sequence deposition. The boundaries and maximum flooding surfaces (mfs) are in good response to both gamma and acoustic log curves in the study area. In seismic profile 1249, sediments obviously onlap over the unconformity (SB0, the bottom of Zhuhai Fm), SB1 and SB2, but obviously over only SB2 in seismic profile 1283 since the well- devel-oped faults in the subbasin. The sand bodies with high porosity and permeability for Petroleum migration and accumulation had been reworked by tidal currents before their burial. Hence, the tidal influenced parasequence sets occur both in TST and HST. Through detailed analysis, the sand bodies in TST are more favorable for Petroleum to migrate and accumulate than those in HST.

Sequence Stratigraphic Features of Mesozoic－Cenozoic Nonmarine Sediments in Eastern China

The basic concepts of sequence stratigraphy can be applied to any cyclic successions regardless of their origin being marias or nonmarine .The storm base can be taken as the base level at the gentle slope side of a faulted basin for distinguishing the systems tractS as no shelf edge is present in nonmarihe fault6d basins. Most of coatinuous rethetions with high amplitude in seismic sections, misunderstood previously as the boundaries of stratigraphic unit, are maximum flooding surface (MFS)in fact. Abundant and diversified microfossils and nannofossils high content of organic carbon,various authigenic minerals such as glauconie,siderite are concentrated nearby these surfaces. It means that the open sea water invaded iato the faulted basins many times in Cretaceous and Paleogene, which make a restricted environment favourable for oil generation. The trend of lake level changes is similar to that of Haq's curve generally, however, the influence of local movements and autostrahgraphic events have to be colandered. The stratal patterns and models of nonmarine sequence are proposed.

Significance of Coal Seam in Recognition of Sequence Boundaries and Systems Tract of the Jurassic in East Fukang Slope, Junggar Basin, China

The Jurassic in the East Fukang Slope can be divided into six sequences based on sequence stratigraphy by combining logging, core and seismic data. The indicators of sequence boundaries include unconformity, coal seams, change of spore and pollen abundance, scour surfaces and base conglomerate, change of logging curve and sedimentary facies. How to determine the location of the first flooding surfaces and the maximum flooding surface is the key step to divide the systems tract. There occurred a topographic slope break in the East Fukang Slope when the Jurassic was deposited, and therefore we can recognize the location of the first flooding surface and establish the sequence stratigraphic framework with the slope break in the study area. Coal seams regionally distributed are correlatable and isochronic, and record the termination of a depositional event or episode. So, the regional coal seam （more than 60 percent coverage） can be used as the genetic stratigraphic sequence boundary, while locally distributed coal seam （less than 60 percent coverage） can be used as the systems tract boundary. The thick coal seams distributed regionally in the middle of the Badaowan Formation and the lower part of the Xishanyao Formation in the study area act as the sequence boundaries, while the thin and locally distributed coal seam acts as the systems tract boundary, which results in the correlation of the division of sequence stratigraphy of the Jurassic to the whole basin where coal seams are developed extensively.

Geological Characteristics and Manifestations of Geological Processes of Sequence Boundaries and Their Vicinities

This paper discusses the geological characteristics and architectures of sequence boundaries and their vicinities and has proposed a classification scheme for the sequence boundaries, which can thus be grouped into three types and eight categories: type I includes exposed truncated surface, palaeosol surface, palaeokarst surface and exposed surface; type Ⅱ boundaries include structural transitional surface during sea-level fall and transgressive onlap surface; and type Ⅲ includes submarine erosional diagenetic diastem and event surface. A study has been made for the three major boundaries lying between the Permian and pre-Permian, the Permian and Triassic, and the Middle and Lower Triassic respectively in terms of multiple disciplines such as lithostratigraphy, biostratigraphy, magnetostratigraphy and carbon and oxygen isotopic geology. These three boundaries are ascribed to type I , typeⅡ and typeⅢ, respectively.

A 32-million year cycle detected in sea-level fluctuations over the last 545 Myr

Spectral analyses of past relative sea-level oscillations as represented by the ages of 57 Phanerozoic(the last 545 Myr)stratigraphic sequence boundaries from the Canadian Arctic show a strong spectral peak at 32 Myr(>99.9%confidence).These findings concur with previous reports of significant cycles with periods of around 30 Myr in various records of fluctuations of sea level,and in potentially related episodes of tectonism,volcanism,climate,and biotic extinctions.Sequence boundaries commonly coincide with stage boundaries based on biostratigraphy,and are correlated with episodes of extinction and times of flood-basalt volcanism.The connection between tectonics and sea-level variations may come from changes in rates of ocean-floor spreading and subduction,intraplate stresses from plate-reorganizations,and pulsations of hotspot volcanism.These coordinated periodic fluctuations in tectonics,sea level and climate may be modulated by cyclical activity in the Earth’s mantle,although some pacing by astronomical cycles is suspected.