Quaternary stratigraphic architecture and sedimentaryevolution from borehole GB014 in the westernXiong’an New Area

Abstract

To scientifically utilize underground space and to explore the regional structural characteristics and processes of basin evolution of the North China Plain, we combine paleomagnetic and sedimentary analyses of sediments from borehole GB014, which extends 200 m deep, in the western Xiong’an New Area. Magnetostratigraphy and regional sedimentary characteristics were determined to establish a chronological framework. Sedimentary facies and sedimentary systems were reconstructed by considering the lithology, geophysical logs gamma ray ( GR ), and grain size, sediment color ( L *, a * and b *) and magnetic susceptibility measurements. To determine the remanence carriers and magnetic mineralogy of the sediments, rock magnetic measurements were performed on 8 representative samples from various depths. A total of 145 specimens were subjected to progressive thermal demagnetization up to a maximum temperature of 690 ° C, with intervals of 25−50 ° C below 585 ° C and 10−25 ° C above 585 ° C, using a Magnetic Measurements Thermal Demagnetizer (TD48) with a residual magnetic field less than 10 nT. Paleomagnetic measurements were made using a 2G Enterprises Model 760-R cryogenic magnetometer installed in a magnetically shielded space ( GR logs obtained at 20-cm intervals were used to analyze the sedimentary facies. For measurements of sediment color, 526 powder samples were air-dried and then disaggregated, passed through a 200-mesh sieve and measured using a Konica-Minolta SPAD503 spectrometer. The 526 powder samples were packed into 8-cm3 plastic boxes and used to obtain measurements of magnetic susceptibility ( χ ) using a Bartington Instruments MS2B sensor. Electronic scales with a resolution of 0.001 g were used to weigh the samples to enable the mass-specific susceptibility to be calculated. For grain-size measurements, 189 powder samples were pretreated with 10% H2O2 to remove organic matter and 10% HCl to remove carbonates. After standing for 24 h, the residues were dispersed with 10% (NaPO3)6 and ultrasonicated for 10 min. The grain size measurements were made using a Mastersizer-2000 laser analyzer (relative error The main findings are as follows: (1) Magnetite is the main carrier of the characteristic remanent magnetization in these sedimentary sequences, and the secondary carrier is hematite. (2) The borehole measurements recorded the Brunhes and Gauss normal chrons and the Matuyama reversed chron. The depths of the Matuyama/Brunhes and Gauss/Matuyama boundaries are 33.8 m and 135 m, respectively. (3) According to the chronological framework and sedimentary facies, five sedimentary units have been identified, from bottom to top: Unit V, with a depth of 200−146.6 m and sedimentary rate of 4.9 cm/ka, whose age is 3.90−2.81 Ma, is dominated by alluvial fan facies with coarse grain sediments. Unit IV, with a depth of 146.6−108.0 m and sedimentation rate of 2.76 cm/ka, whose age is 2.81−1.88 Ma, is dominated by flood plain facies with fine grain sediments. Unit III, with a depth of 108.0−57.6 m and sedimentation rate of 5.86 cm/ka, whose age is 1.88−1.02 Ma, is dominated by braided channel facies with coarse particle sediments. Unit II, with a depth of 57.6−35.8 m and sedimentation rate of 9.48 cm/ka, whose age is 1.02−0.79 Ma, is dominated by flood plain facies with fine grain sediments. Unit I, with a depth of 0−35.8 m and sedimentation rate of 4.53 cm/ka, whose age is 0.79−0 Ma, is dominated by flood plains interbedded with crevasse fan facies. At borehole GB014, evidence of two tectonic movements that occurred at 3.90−2.81 and 1.88−0.79 Ma have been preserved since the late Pliocene, and sediment provenance probably changed during 1.88−1.02 Ma. The piedmont area of Taihang Mountain hosts two widely deposited layers of coarse-grained sediments resulting from these two tectonic movements, and the Middle and Late Pleistocene sand layers are mainly related to the marine transgression in Bohai Bay and the enhancement of the East Asian monsoon system.