MAGNETOSTRATIGRAPHY AND DISCUSSION OF COUPLING RELATIONSHIP BETWEEN TECTONIC MOVEMENT AND CLIMATE CHANGE OF MT04 BOREHOLE IN SOUTHERN LUANHE RIVER DELTA
The northern coast of Bohai Bay is located within the north of Huanghua Depression which tectonic subsidence can include the rifting episodic in Paleogene, thermal subsidence episodic in Neogene and accelerated thermal episodic in Quaternary. The Quaternary sediment is ca.490m ; Late Pleistocene and Holocene sediments are mainly composed of the Luan River delta and alluvial fan of different periods. The northern coast of Bohai Bay acting as the sink of erosion of Yanshan Mountain and the transport of river, can record the uplift of Yanshan Mountain and the subsidence of Bohai Bay basin,and record the climate change in Quaternary. The borehole MT04 is located in the southern Luanhe River delta, and has a length of 383m (39°15'55"N, 118°49'51"E),and includes 6 units. Rock magnetic studies include X-T, hysteresis loops and isothermal remnant magnetization. Remanence measurements are made using a 2G Enterprises Model 755 three-axis cryogenic magnetometer installed in a magnetically shielded space (<300nT). Specimens are thermally demagnetized up to 680°C using a TD-48 furnace with temperature intervals of 10 ~ 50°C. The other specimens are subjected to alternating field (AF) demagnetization at peak fields up to 70mT with 3 ~ lOmT intervals. The results are revealed that : (1) magnetite is the major magnetic carrier at the depth interval of 0 ?90m and 244~ 383m, and hematite at the depth interval of 90?244m. (2) There are 5 normal magnetozones and 5 reverse magnetozones depend on 208 specimens (180 specimens subjected to thermal demagnetization and 28 to alternating field demagnetization) with the characteristic remanent magnetization (ChRM) directions. Comparing with the geomagnetic polarity timescale (Cande et al., 1995 ; Gradstein et al., 2004), the normal magnetozones N1, N2, N3,N4 and N5 identified here can be correlated with Cln, Clr.ln, C2n, C2An.ln and C2An.2n, respectively, the reverse magnetozones R1,R2, R3,R4 and R5 can be correlated with Clr.lr, Clr.2r+Clr.3r, C2r.lr+C2r.2r, C2An.lr and C2An.2r, respectively. The B-M and M-G boundaries in the borehole are located in 122.4m and 327.2m, respectively, and the age of the bottom is about 3.2Ma, while the Upper Pleistocene boundary in the borehole is located in 83m. According to this paleomagnetic result, three sedimentary phases have been then identified : (1) The I phase whose age is 1.0?3.2Ma and sedimentation rate is more than 92m/Ma has been dominated by lacustrine facies, indicating a stable tectonic period and climate controlled sedimentary process. (2) The II phase whose age is 0.8 ~ 1.OMa and sedimentation rate is 256 m/Ma has been dominated by fine-sand lacustrine deposition, indicating an active tectonic period which could be correlated with Kunhuang Movement. (3)The III phase whose age is 0 ~ 0.8Ma and sedimentation rate is 157 m/Ma has been controlled by climate change and tectonics. During MIS 5,the area has formed ca.30m sand layers by fluvial incision after the formation of the third terrace in Yanshan mountain,and part of layers controlled by tidal with the transgressions. During MIS 3,the area has formed ca. 15m sand layers after Luanhe River merging into Qinglong River into the sea. During MIS 1, the two period typical lagoon-sandbar coasts have formed with transgression in the Holocene.