Identification Of Time Base Function Of Cretaceous Sediment Stratum Data From Sifangtai Formation In Songliao Basin

Dating and analyzing the strata are the groundwork for many problems in the earth science.Identification of the (time base) function between the depth and the age of the proxies reservedin the strata is needed by dating the strata. To solve this problem, our research object is the corefrom the Sifangtai Formation of SK1(a coring program)(North) in Songliao Basin and the proxyis the natural gamma-ray (GR) logging data which is found to closely and consistently track thelithological changes. Every step and matter worth paying attentions to build the initial model andthe modified model were described in detail based on the magnetostratigraphic results as thebase-points. The two kinds of models could identify the time base functions.At first, by frequency domain method usually used in signal process, the disturbances wereseparated and the main periods were identified. Then after applying the Inverse Discrete FourierTransform, the time base distortions were estimated by repeatedly tunings. So the initial timebase functions were established. From the time base functions we deduced the sedimentaccumulation rate formula. However the initial model was subjective. So the modified modelneeded to be parameterized in order to cover the shortage of the former model. Take the resultsin the initial model as the starting values of the modified model, optimize all of the modelparameters, at last by using model selection criterion, select the optimal model considering itscomplexity and fitting effect.The following results derived from the optimal modified model for the GR data of theSifangtai Formation of SK1(a coring program)(North) in Songliao Basin.1. The main periods of the upper and the lower GR series of Sifangtai Formation are estimatedas407.0103kyr and2318.6kyr, respectively, which are probably helpful to study continentaldeposit response to the global paleaoclimatic changes caused by different driving forces.2. The boundary age between the Sifangtai and the Mingshui Formatuion is72.1789Ma, whichis different from the magnetostratigraphic result obtained by linear interpolation.3. For the lower boundaries of chron R3, N3, the better relative errors compared with knownages are0.13%，0.14%, respectively. According to the time base from the modified model,the sediment accumulation rate formula is deduced. It can identify the unconformablesituation and the inflection points of the increase and decrease sediment accumulation rate，which can be verified by lithostrarigraphic and sedimentary environmental discoveries.These results indicate the adaptability of the modified model and the feasibility of the GRlogging data as the proxy. 4. The GR time series and its period object functions are recovered in favor of the analysisabout depositional environment and structure during that time.5. Different from common stepladder average sediment accumulation rates, we establisheddeposition rate continuous curve, whose tendency could match approximately with themagnetostratigraphic results, but whose numerical value were more than them.Viewed from the time series analysis, the models here are quantitative methods whichobjectively identify the time base relations hiding in the strata. The models could be combinedwith other base-points relative accurately dated by the cyclostratigraphic, magnetostratigraphic,biostratigraphic, sequence stratigraphic methods. It will improve the accuracy, the precision andthe resolution of chronostratigraphic analysis techniques; besides, they can enrich the methodsfor effectively analyzing the geologic chronology data in Cretaceous.