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Orbital cyclicity in sedimentary sequence and climatic indications of C-O isotopes from Lower Cretaceous in Qingxi Sag, Jiuquan Basin, NW China

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Geoscience Frontiers 2019年第2期10卷 467-479页

作者： Siding Jin Haiyang Cao Hua Wang Michael Wagreich sylvain richoz State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation Chengdu University of Technology College of Energy Resources Chengdu University of Technology Institute of Sedimentary Geology Center for Post-doctoral StudiesChengdu University of Technology Key Laboratory of Tectonics and Petroleum Resources China University of Geosciences Department of Geodynamics and Sedimentology University of Vienna Institute of Earth Sciences NAWI GrazUniversity of Graz

Sedimentary deposits of the Lower Cretaceous Xiagou Formation form the most significant potential hydrocarbon reservoirs in the Qingxi Sag, Jiuquan Basin(NW China). Zircon U-Pb ages of the dated basalts at the top of the Xiagou Formation give an isochron age of 115.6 Ma, and the sedimentation interval of the Xiagou Formation was speculated to range from about 125/124 Ma to 115 Ma based on paleontological research and stratigraphic correlation analysis. Here we use GR logging data as a palaeoenvironmental and palaeoclimatic proxy to conduct a detailed cyclostratigraphic study of five selected wells. Power spectra, evolutionary fast Fourier transformation and wavelet analysis all reveal significant sedimentary cycles in the Xiagou Formation. The ratios of cycle wavelengths in these stratigraphic units are 33.82 m : 7.91 m : 3.06 m : 1.79 m, which is similar to the ratio of orbital targets of 20 : 5 : 2 : 1. The ratio of 20 : 5 : 2 : 1 is interpreted as Milankovitch cycles of 405 kyr long eccentricity,100 kyr short eccentricity, 37 kyr obliquity, and 22 kyr precession cycles respectively. A high-resolution astronomical time scale is constructed by tuning the stratigraphy into target curves of orbital cycles respectively. Based on the astronomical time scale, the absolute ages of 55 samples were estimated,which are used for subsequent stable carbon and oxygen isotope stratigraphy analysis. The analysis results of the five studied wells in the Qingxi Sag indicate:(1) a negative trend of δ^(13)C values upwards in the Xiagou Formation, and(2) negative δ^(18)O values with a positive trend upwards. Both relatively heavy values and pronounced covariances of δ^(13)C values and δ^(18)O values indicate an arid-evaporationcontrolled climate during the sedimentary period of the Lower Cretaceous Xiagou Formation, Qingxi Sag, Jiuquan Basin. Moreover, positive covariances of SQK_1g_(2+3) indicate extremely high temperature, and negative covariances of SQK_1g_1 indicate a relatively l

关键词： Jiuquan Basin Qingxi Sag Xiagou Formation Orbital cyclicity Stable isotope

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Western Tethyan Epeiric Ramp Setting in the Early Triassic： An Example from the Central Dinarides (Croatia)

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Journal of Earth Science 2018年第4期29卷 806-823页

作者： Dunja Aljinovic Micha Horacek Leopold Krystyn sylvain richoz Tea Kolar-Jurkovsek Duje Smircic Bogdan Jurkovsek University of Zagreb Faculty of Mining Geology and Petroleum Engineering Pierottijeva 6 Zagreb 10000 Croatia BLT Wieselburg HBLFA Francisco-Josephinum Rottenhauserstr. 1 Wieselburg 3250 Austria Institute of Paleontology Vienna University A lthanstrafle 14 Vienna 1090 Austria Lund University Lithosphere and Biosphere science Solvegatan 12 Lund 22362 Sweden Geological Survey of Slovenia Dimiceva Ulica 14 Ljubljana 1000 Slovenia

In the central part of the External Dinarides in Plavno, Croatia, near Knin, a remarkably thick （927.5 m） Early Triassic depositional sequence was investigated. The Plavno sequence starts in the EarlyGriesbachian and ends with a continuous transition into the Anisian strata. A complete 13C isotope curve has been achieved and combined with conodonts, bivalves and ammonoids to establish and correlate stage and substage boundaries. The 613C curve is consistent with former studies. It displays a general increase from the Griesbachian to a prominent maximum beyond the ＋8%0 amplitude around the Dienerian-Smithian boundary （DSB）, followed by a steep and continuous decline to low, negative values in the Smithian. Around the Smithian-Spathian boundary （SSB） a steep rise to a second maximum occurred. It is followed by a saw-tooth shaped decline in the Spathian and a similar increase to a peak at the Spathian-Anisian boundary （SAB）. Sedimentologically, the Plavno sequence is interpreted as having been deposited on an extensive epeiric ramp under long-term transgressive conditions, sharing depositional characteristics of both the epeiric platform and the carbonate ramp. The entire Plavno sequence was deposited above the storm-wave base and was storm influenced. Three informal members are differentiated： 1） the dolostone member （Early Griesbachian）; 2） the silicielastic member （red-coloured shale, siltstone, sandstone with oolitic/bioclastie grainstone intercalations）, which can be further divided into lower, middle and upper intervals （Late Griesbachian, Dienerian and Smithian）; and 3） the mudstone member （grey lime mudstones, marls and calcisiltites with common ammonoids and gastropods- Spathian）. The Plavno sequence is compared with other western Tethyan sections. Observed differences stem from local controls on deposition in the overall shallow marine environment.

关键词： Early Triassic Dinarides epeiric ramp 6Z3C isotope curve litho- and biostratigraphy.

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