| This paper probed into the sea-level changes from the latest Ordovician to the earliest Silurian and thesubdivision of the Ordovician-Silurian boundary based on the study of biostratigraphy, chemical stratigraphyand sequence stratigraphy.Analysis of the morphological feature, diversity and abundance of graptolite, brachiopod and radiolariaindicated that the sea-level in the Yangtze basin underwent the change from rising to falling, then to risingagain during the latest Ordovician to the earliest Silurian interval, and the maxium rising episode appeared inthe lower Tangyagraptus typicus Subzone and the maxium falling at the top of the Guanyinqiao bed.The variation of the microelement Ni, V, Sc, Rb, Ba and Sr was analogous to the change of the Ceanomaly from the Wufeng Formation to the basal Longmaxi Formation at the Goujiaya and Datangkousections, and 5 cycles, separately corresponding to the lower and upper part of the Dicellograptus complexusZone, the lower Pacificograptus pacificus Zone, the upper part of P. pacificus Zone to the Normalograptusojsuensis Zone, and N. persculptus to Parakidograptus acuminatus Zones, could be recognized during theinterval. In addition, the abundance changes of the radiolaria at the Goujiaya and Datangkou sections werecoincided with the sea-level changes, and correlatable with the synchorous 5 fluctuations of the sea-level inNorth America and Africa. So above-mentioned 5 cycles could represent the eustasies during the latestOrdovician to the earliest Silurian. Every fluctuation period of the earlier 4 cycles covered 0.75 to 1 Ma beingsimilar to that of the fourth-order glacial-eustasy with the interval of 0.1 to 1 Ma, imagining there probablyoccurred 4 fourth-order eustasies during the latest Ordovician from the D. complexus Zone to N. ojsuensisZone.In the light of the paleoenvironmental significance reflected by the radiolaria, graptolite, brachiopod,sponge spicule and the rock character, it's suggested that the greyish green mudstone at the basal WufengFormation of the Goujiaya section would be probably deposited in 60 to 100 meters deep water, theGuanyinqiao bed in 50 to 80 meters deep, and the radiolaria-bearing silicalite from Wufeng Formation inwater with 200 to 400 meters deep. Combining the above-mentioned water depth with the sea-level changecycles, the change range of every one of the 5 cycles could be further conjectured. They were in ascendingorder: 20～120 m, 80～130 m, about 150 m, 50～250 m, and in excess of 200 m.The boundary marks of the chronostratigraphic units which is determined on the InternationalStratigraphic Guide, weren't convenient for being identified, and all of them didn't coincide with thegeoevolutional rhythm. However, the studies of the Sinian-Cambrian, Ordovician-Silurian,Frasnian-Famennian, Permian-Triassic, Triassic-Jurassic and Cretaceous-Tertiary boundaries indicated that,the geological events at these boundary intervals generally comply with the following laws: (1) the age rangeof the carbon and oxygen isotope events was equal to or longer than that of the boundary clay or massextinction events. (2) The transgression events closely followed the other geological events and basicallycoincided with the organism recovery after extinction. (3) The peak of the geological events happened before transgression and the quiet time afar the transgression. (4) Two adjacent geological events above-mentionedcovered respectively 105 Ma, 66Ma, 122Ma, 45M and 140Ma. It can see that, the geological events withlonger period alternated with those events spanning shorter period, and the sum of a longer period and itsadjacent shorter period was identical (66Ma+122Ma=188Ma; 45Ma+140Ma=185Ma). (5) The geologicalevent scales across the Permian-Triassic and the Cretaceous-Tertiary intervals were bigger, occurred not onlymass extinctions but also global chemical anomaly events. These laws above-mentioned indicated the earthevolutional rhythm, and the global transgression event probably represents the turning point of the earthevolution from unbalanced to balanced condition, or the turnover of geohistory. Therefore present author holdsthat the natural events (catastrophic events), characterizing the geoevolution rhythm, especially thetransgression event above-mentioned, should be fully considered in the subdivision of the chronostratigraphicboundaries.On the basis of the irreversibility of the bioevolution, geoevolution rhythm and above-mentioned laws ofthe geological events at the turning point of the geohistory, it's suggested that the subdivision of thechronostratigraphic boundaries should be in line with the biostratigraphic classification at first, in associationwith the natural events and regard the latter as the supplementary indicators. Therefore the Ordovician-Silurianboundary should he defined by the FAD of N. persculptus displaying the geohistorical rhythm as the biomark,and by the not fully identical geological events as the supplementary marks for the global correlation. In viewof geological records preserved from different regions of the world are not identical, it's suggested that, whencorrelating the Ordovician-Silurian boundary, at the global boundary stratotype section, the FAD of N.persculptus be selected as the biomark, the transgression event identical to the FAD of N. persculptus as theauxiliary physical mark and the mass extinction and chemical anomaly events respectively as the auxiliarybiomark and chemomark. In some regions with few graptolite, the Ordovician-Silurian boundary should beindicated by the auxiliary marks, such as the transgression event, mass extinction or chemical anomaly event,with the other fossil such as conodont nearest to other geological events as the provincial biomark forcorrelation. In other regions with much stratigraphic hiatus originated from the regression, theOrdovician-Silurian boundary should he marked by the regression-transgression event nearly relevant to the N.persculptus Zone.
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