Recognition Of Cyclostratigraphy And Discussion Of The Genetic Mechanism Of Xiagou Formation In Early Cretaceous, Jiuquan Basin, West Of China

Cyclostratigraphy is the subdiscipline of stratigraphy that deals with the identification, characterization, correlation, and interpretation of cyclic variations in the stratigraphic record and, in particular, with their application in geochronology by improving the accuracy and resolution of time-stratigraphic frameworks. As such it uses astronomical cycles of known periodicities to date and interpret the sedimentary record. The most important of these cycles are the Earth’s orbital cycles of precession, obliquity, and eccentricity (Milankovitch cycles), which result from perturbations of the Earth’s orbit and its rotational axis. They have periods ranging from 20 to 400 ka, and even up to millions of years. These cycles translate into climatic, oceanographic, sedimentary, and biological changes that are potentially recorded in the sedimentary archives through geologic time. Cyclostratigraphy enable the sequence division to be a higher resolution and enhance the absolute age controls at a resolution of 10 to 100 ka. It also have a better understanding of the sedimentation, duration of sedimentary, accumulation rate and palaeoclimate changes through the cyclostratigraphy research. During the past two decades, there have been many efforts to construct an accurate geological timescale based on astronomical tuning of climatic response to astronomical solutions. But in China, only few people focus on this discipline, with even on systematic introduction of cyclostratigraphy in Chinese. In this dissertation, we introduce the fundamental conceptions, development history, reseach methods, and research status of cyclostratigraphy in detail, and then under take cyclostratigraphic research on several different geological times in the Qingxi Sag, Jiuquan Basin.1. With the data including well core, seismic data and well logging data, three third order sequences has been recognized in the Xiagou Formation which are Member 0 of the Xiagou Fomation (SQKig0), Member 1 of the Xiagou Formation (SQK1g1) and Member 2 and member 3 of the Xiagou Formation (SQK1g2+3). Stratigraphic sequence framework has been set up and the features of depositional systems of sequences have been released.2. The earth orbit parameters during 115 Ma to 120 Ma with the La (2004) solutions are calculated in this paper. Based on the earth orbital parameters, the eccentricity, obliquity and precession has been figured out by spectra analysis and wavelet analysis. The results showed that the eccentricity cycles mainly lie in 400 ka,124 ka,95 ka; and the obliquity cycles are mainly of 47 ka,37 ka,36 ka and 28 ka; the precession cycles are of 22 ka,21 ka and 18 ka. Moreover, the curve of variation in theory has been computed and the ratios for the cycles during 115 Ma to 120 Ma have been calculated.3. Seven wells which developed in the deep lake or semi-deep lake were chosen under the spectral analysis after the correction and normalization in Member 1 of the Xiagou Formation (SQKig1) and Member 2 and member 3 of the Xiagou Formation (SQK1g2+3). Milankovitch cycles have been recognized. It shown that SQKig1 were controlled by long eccentricity 400 Ka, and SQK1g2+3 were controlled by short eccentricity 124 ka and 95 ka.4. The study undertakes spectral analysis of high-resolution natural gamma-ray (NGR) well-logging records, carbonate content, TOC, magnetic susceptibility and main and trace elements content to determine the dominant frequency components and test whether Milankovitch orbital signals are recorded in rhythmic successions. Spectrum analysis results indicate the orbital cycles of precession, obliquity, and eccentricity, which provide the strong evidence for astronomically driven climate changes in the rhythmic alternation successions. The results from five wells all shows good correspond to the Earth’s orbit cycles ratios.5. Within U-Pb zircon age constraint, a high-resolution astronomical timescale was constructed through the astronomical tuning of the nature gamma ray records to recent astronomically calculated variation of Earth’s orbit. The astronomical ages for the geological events can be recognized by the astronomically tuned timescale throughout the period. Cyclostratigraphic analysis of Early Cretaceous terrestrial strata reveals significant 400 ka,124 ka and 95 ka eccentricity,47 ka,37 ka and 36 ka obliquity and precession cycles, indicating astronomically controlled sedimentation in the Qingxi Sag, Jiuquan Basin of northeastern China. An astronomical time scale (ATS) for the sedimentary record of is established by tuning the extracted theory cycles to La2004 astronomical solution. The astronomical orbital period can assure the time limit of SQKig2+3 about 1.97 Ma, SQK1g1 about 1.70 Ma and SQKig0 about 1.46 Ma, which corresponds to the astronomical period consisted of three parameters of Earth’s orbits. It reveals the correlations between the third order sequence and astronomical periods and indicates that the sequence development is controlled by astronomical and climatic period during the sedimentation. Moreover, study reveals the genesis of third sequences in theory and explains why its time duration is 1-2 Ma.6. Carbonate and oxygen isotopic compositions are analysis from five sections. Calculated by an indirect method which based on ATS, the ages of each sample were estimated. The δ13C values show a trend of positive excursion and δ18O values show a trend of negative excursion in the Xiagou Formtion, during the Early Cretaceous, the Qingxi Sag, Jiuquan Basin. It allows us to partially correlate the 813C and δ18O with the Early Cretaceous in other basins of NW China, the palaeoclimatic and palaeoenvironmental changes were come to light, and the relatively high δ13C and low δ18O were indicate the arid-evaporation climate.