A Study On Cyclostratigraphy Of South Hole Of Slcore-Ⅰ In Songliao Basin

Chinese Cretaceous Continental Scientific Drilling Project-"SLCORE-I"is one significant part of"Major geological events in the Earth Surface System during the Cretaceous and greenhouse climate change"project,which is in the National Basic Research Program (973 Program)."SLCORE-I"is not only the first scientific well with coring entirely in China, focusing on Cretaceous strata, but also the first continental facies scientific well on Cretaceous strata, under the frame work of International Continental Drilling Project (ICDP).The south hole of SLCORE-I was completed on 4 November 2006 with a core length of 944.23 meters and cumulative recovery of 99.37 %. The highly precise section of cyclostratigraphy is one of ten series of the SLCORE-I drilling progenies, also being important research content of the 3th subject"continental facies sedimentary response of major geological events and climate change".The extent of the research on cyclostratigraphy in continental sedimentary basin is much poorer than on marine strata, especially on carbonate rock. Because of the complexity of tectonic activities and sedimentation in continental basin, factors of autocyclicity mainly control the sedimentation and we know a little about relationship between control factors of cycle forming and sedimentary records. About allocyclicity or autocyclicity, which one mainly resulted in the cyclicity presenting in the records of continental strata , has been beening a project of continental cyclostratigraphy study. So it is near to the leading edge to study the characteristic of cycles under the controlling of allocyclicity in the records of continental basin.1.Studies on sedimentary microfacies of South holeThere are four facieses are identified from the strata of the south hole,including meandering river -flooding basin facies, delta facies, shallow lake facies, semi-deep lake-deep lake facies. Meandering river-flooding basin facies is recognized in QuantouⅢ～Ⅳmember, including six types of sedimentary microfacieses, point bar, natural levee, crevasse splay, crevasse channel, concave flood plain, flooding basin. Delta facies is recognized at the top of QuantouⅣmember and the bottom of YaojiaⅠmember, including six types of sedimentary microfacieses, distributary mouth bar, subaqueous channel, distal bar, interdistributary bay,mud sediment,slump sediment. Shallow lake facies is recognized in Yaojia formation, the middle and top of QingshankouⅡ～Ⅲmember,including two types of sedimentary microfacieses, mud sediment, tubidity current deposit. Deep lake facies is recognized in Qingshankou, the middle and top of QingshankouⅠmember ,the bottom of QingshankouⅡ～Ⅲmember and NenjiangⅠ～Ⅱmember,including six types of sedimentary microfacieses, storm deposit in semi-deep lake, mud sediment, tubidity current deposit, deep lake marlite , deep lake dolomite, oil shale deposit.2. Studies on meter-scale cycle of South holeBased on the analyse of sedimentary microfacies , three types including sixteen subtypes of meter-scale cycle are recognized according the definition and genesis of meter-scale cycle. The first type is normal grading succession, which is recognized in Meandering river -flooding basin facies ,including six subtypes:A:point bar-natural levee,B:point bar- natural levee- concave flood plain- flooding basin, C:crevasse splay- concave flood plain, D:crevasse splay- flooding basin, E:crevasse channel- concave flood plain- flooding basin. The second type is inverse grading succession, which is recognized in delta and lake facies, including eight subtypes : A : interdistributary bay - distributary mouth bar-subaqueous channel, B:interdistributary bay- distributary mouth bar, C:mud sediment - distal bar,D:mud sediment - distal bar,E:mud sediment - slump sediment,F:mud sediment– Ostracoda clastic limestone,G:mud sediment– seismite , H : oil shale-mud sediment. The third type is deep lake carbonate-mudstone succession, which is recognized in deep lake facies, including two types:A:marlite - mudstone,B:mudstone- dolomite.3.Fischer plots of south holeThere are eight hundred and sixty two meter-scale(sixth- order cycles) cycles recognized from strata of south hole. Four Fischer plots are plotted with making one formation of south hole as a plotting unit. After analyzing Fischer plots, some conclusions are educed:sixty two meter-scale cycles are recognized in Quantou formation, which stacked into twenty four fifth-order cycles, among which eight fifth-order cycles belong to QuantouⅢmember and sixteen fifth-order cycles belong to QuantouⅣmember. Twenty four fifth-order cycles stacked into eight fourth-order cycles, among which two and one a half fourth-order cycles belong to QuantouⅢmember and five and a half fourth-order cycles belong to QuantouⅣmember. Five hundred and three meter-scale cycles are recognized in Qingshankou formation, which stacked into one hundred and thirty four fifth-order cycles, among which twenty two fifth-order cycles belong to QingshankouⅠmember and one hundred and twenty two fifth-order cycles belong to QingshankouⅡ～Ⅲmember. One hundred and thirty four fifth-order cycles stacked into twenty three and two a half fourth-order cycles, among which four and two a half fourth-order cycles belong to QingshankouⅠmember and eighteen and two a half fourth-order cycles belong to QingshankouⅡ～Ⅲmember. One hundred fifty one meter-scale cycles are recognized in Yaojia formation, which stacked into forty three fifth-order cycles, among which eight fifth-order cycles belong to YaojiaⅠmember and thirty five fifth-order cycles belong to YaojiaⅡ～Ⅲmember. Forty three fifth-order cycles stacked into ten fourth-order cycles, among which two fourth-order cycles belong to YaojiaⅠmember and eight fourth-order cycles belong to YaojiaⅡ～Ⅲmember. One hundred thirty two meter-scale cycles are recognized in Nenjiang formation, which stacked into thirty nine fifth-order cycles, among which eight fifth-order cycles belong to NenjiangⅠmember and thirty one fifth-order cycles belong to NenjiangⅡmember. Thirty nine fifth-order cycles stacked into thirteen fourth-order cycles, among which eight and one half fourth-order cycles belong to NenjiangⅠmember and four and one half fourth-order cycles belong to NenjiangⅡmember.4.Studies on genesis of cyclostratigraphy of the south holeThe formation of cyclostratigraphy recognized in south hole was controlled by Milankovitch cycles. Three types of cycles are found in the south hole. Sixth-order cycle has a precession period of 22.9ka～29.8ka,fifth-order cycle has a short eccentricity period of 75.0ka～104.7ka, and fourth-order cycle has a long eccentricity period of 225.0ka～460.0ka.