Paleomagnetic ChRM directions of the Early Cretaceous sedimentary sequence in the southern Simao Basin, SE Tibetan Plateau

Precise stratigraphic chronology is the basis of many studies (e.g., tectonic, paleoclimate and mineralization) in geoscience. Here, we carried out a detailed youngest detrital zircon U-Pb age-constrained magnetostratigraphic study on the middle-lower parts of a 2309 m-thick Late Jurassic-Early Cretaceous sequence from west Jiangcheng in the southern Simao Basin, southeastern Tibetan Plateau. A total of 2262 paleomagnetic cores were collected for magnetostratigraphy, and 5 siltstone/sandstone samples were collected for detrital zircon U-Pb analyses. Progressive thermal and alternating field demagnetizations isolated 1575 robust primary characteristic remnant magnetization directions, which yielded twenty-five normal-polarity (N7-N31) and 25 reversed-polarity (R8-R32) intervals. The adopted maximum depositional ages calculated using the youngest detrital zircon U-Pb ages are 159.0 Ma, 154.0 Ma, 161.0 Ma, 149.3 Ma and 139.5 Ma for the stratigraphic horizons at 2303 m, 1986 m, 1600 m, 1350 m and 1110 m, respectively, in the section. Together with the previous magnetostratigraphic results of the upper part of the section, the observed polarity zones were best correlated with chrons C34n-M17r on the geomagnetic polarity timescale (GPTS2012), yielding magnetostratigraphic ages of ~143.5 to ~65.0 Ma for the whole stratigraphic sequence of the section. Paleogeographic analysis revealed that the Simao Basin was in a low-latitude offshore environment during the Late Jurassic-earliest Cretaceous (>142.3 Ma), a dominant fluvial environment at 20~24°N during the early-middle Early Cretaceous (142.3-111.5 Ma), with widespread desertification at the end of this period, a saline lake sedimentary environment during the late Early Cretaceous-early Late Cretaceous (111.5-88.7 Ma), and a fluvial environment during the late Late Cretaceous (88.7-65.0 Ma). This evolutionary process was concordant with regional/global tectonic and paleoclimate changes.

精确的地层年代学（stratigraphic chronology）是地球科学诸多研究（如构造学、古气候学与成矿作用研究）的核心基础。本次研究针对青藏高原东南部思茅盆地南部江城西部一套厚度达2309米的晚侏罗世-早白垩世地层序列中下段，开展了基于最年轻碎屑锆石U-Pb年龄（detrital zircon U-Pb age）约束的高精度磁性地层学（magnetostratigraphy）研究。研究共采集2262块古地磁岩芯（paleomagnetic cores）用于磁性地层学分析，同时采集5块粉砂岩/砂岩（siltstone/sandstone）样品用于碎屑锆石U-Pb定年分析。通过渐进式热退磁与交变退磁处理，共分离出1575组可靠的原生特征剩磁方向，据此划分出25个正极性区间（N7-N31）与25个反极性区间（R8-R32）。基于各层位最年轻碎屑锆石U-Pb年龄计算得到的最大沉积年龄显示，剖面2303米、1986米、1600米、1350米及1110米处的地层最大沉积年龄分别为159.0 Ma、154.0 Ma、161.0 Ma、149.3 Ma与139.5 Ma。结合该剖面上部已有的磁性地层学研究结果，观测到的极性带可与地磁极性年表（geomagnetic polarity timescale，GPTS2012）中的C34n-M17r极性时进行最佳匹配，据此得出该剖面完整地层序列的磁性地层年代范围约为143.5 Ma至65.0 Ma。古地理分析结果表明，思茅盆地在晚侏罗世至早白垩世早期（>142.3 Ma）处于低纬度近海环境；在早白垩世早-中期（142.3-111.5 Ma），盆地主体位于北纬20°~24°的河流沉积环境，并在该时期末期出现广泛荒漠化；早白垩世晚期至晚白垩世早期（111.5-88.7 Ma）为盐湖沉积环境；晚白垩世晚期（88.7-65.0 Ma）则重回河流沉积环境。这一演化过程与区域及全球构造、古气候变化均具有良好的一致性。