| Lingshan island is located in the sea area of southern Shandong Peninsula and its structural position belongs to the newly discovered Ri-Qing-Wei Basin situated in Sulu orogenic belt.A set of Mesozoic strata mainly composed of terrestrial clastic rocks,volcanic clastic rocks and volcanic lava have been developed in this area.The strata are also exposed in many places in Shandong Peninsula.However,the age of the strata has been controversial and a unified chronological framework has not been established,largely restricting the understanding of the Mesozoic tectonic evolution,basin sedimentary pattern,and petroleum geological conditions in this structural region.The 1352 m continuous coring from Cretaceous Continental Scientific Drilling borehole in the Lingshan Island(LK-1)reveals this set of strata,which provides favorable conditions for the establishment of high-precision chronology framework.Based on the detailed analysis of lithostratigraphic characteristics revealed by drilling cores,this paper carried out a comprehensive study of magnetostratigraphy: through microscopic identification and rock magnetic experiment,the main types of magnetic minerals in each section of LK-1were identified;through systematic demagnetization experiment,the characteristic remanence in samples was obtained,and the magnetic polarity sequence of LK-1 was established,based on the comparison with the Geomagnetic Polarity Time Scale(GPTS)and the previous isotopic dating data,a high-precision chronological framework of LK-1 is established;through the comparison with North China and intercontinental J/K boundary,the J/K boundary revealed by LK-1 is discussed.The results and understanding obtained are as follows:(1)Based on the systematic core observation of LK-1,the lithostratigraphic sequence is defined.LK-1 is mainly composed of gray medium-thick silty sand,fine sandstone and black thin mudstone,with multiple volcanic intercalations and three fracture zones.As far as sedimentary rocks are concerned,the grain size of sediments gradually becomes finer from bottom to top,and can be divided into fan delta,subaqueous fan,turbidite fan and deep water deposit from bottom to top;in terms of volcanic rocks,there are four sets of volcanic rocks developed in the LK-1,among which the tuff layer is extrusive rock,and the lamprophyre,andesite and cryptoexplosive breccia layer is intrusive rock;and three fracture zones in LK-1are at 499.5-509.1 m,1021.8-1024.9 m and 1317.7-1318.9 m respectively.Among them,the size of the fracture zone at 499.5-509.1 m is relatively large,which results in the repetition of the stratum with thickness of about 23 m.(2)Based on the microscopic identification and rock magnetic experiment,the main magnetic minerals of LK-1 are identified.The magnetic minerals are mainly pyrrhotite and a small amount of magnetite and greigite.Pyrrhotite is well developed in the whole section,mostly in euhedral-subhedral shape.Greigite occurs mainly in 300-800 m,and is often associated with pyrrhotite in scattered form.Magnetite is concentrated in lamprophyre with good crystal form.In the aspect of rock magnetism,the K-T curve decreases rapidly at about320 ℃,indicating that the main magnetic mineral in the sample is pyrrhotite.The hysteresis loop is closed when it is close to 0.5 T and the coercivity is medium,which indicates that the magnetic mineral in the sample is mainly pyrrhotite,this is consistent with the microscopic results,so it is considered that the characteristic remanence recorded by magnetic minerals in LK-1 is primary remanence.(3)Based on the system demagnetization experiment,the magnetic polarity sequence of LK-1 is set up,and then the high-precision stratigraphic framework of the research area is established.The thermal demagnetization experiments of 726 samples are completed,and most of the samples have obtained good characteristic remanence.The demagnetization temperature of characteristic remanence is concentrated between 280-340 ℃ and the dip angle of characteristic remanence is about 60°.Finally,27 normal polarity zones and 26 reverse polarity zones are identified,and the magnetic polarity sequence of LK-1 is established.Through comparison with the representative of the GPTS(2012),the high-precision chronostratigraphic framework of well LK-1 is obtained.The age of the top boundary of the borehole is 122.6Ma,the age of the bottom boundary is 147.6Ma,and the corresponding age of each magnetic polarity sequence can also be obtained.(4)Based on the comparison with North China and intercontinental J/K boundary,the J/K boundary revealed by LK-1 is discussed.The J/K boundary of LK-1 is located in the N26 normal polarity,about 1254 m,corresponding to the M19 n.2n magnetic polarity zone of GPTS;The magnetic polarity sequence at the J/K boundary of the borehole is consistent with the characteristics of the magnetic polarity sequence at the J/K boundary of the Puerto escano section in southern Spain,brodno section in Western Carpathian,Slovakia,nordvik section in Siberia,Russia and lokut section in Hungary.As far as North China is concerned,the stratum at J/K boundary of LK-1 is continuous,which makes up for the lack of stratum at J/K boundary of field section in West Liaoning area of North China and overcomes the problem that the stratum developed in field section in North Hebei area does not reach J/K boundary.Moreover,the magnetic polarity sequence of J/K boundary in LK-1 corresponds well with that of GPTS,which can provide a standard for further study on the development characteristics of J/K boundary in North China. |
PDF Full Text Request