Cretaceous magnetostratigraphy of the southern Simao Basin, SE Tibetan Plateau and its paleogeographical significance (Supplementary data)

The Simao Basin has deposited thick continuous Cretaceous sedimentary sequence that contains abundant information about the regional tectonic, pre-Indian monsoon paleoclimatic condition and resource effect prior to the India-Asia collision. However, poor chronologic constraints on the long-scale stratigraphic records greatly hamper our ability to comprehensively understand these processes. Here we carried out a detailed chronological study on a Late Jurassic-Early Cretaceous sequence that consists of the upper Bazhulu, the Jingxing and the Mangang formations from west Jiangcheng (middle-lower parts of the JC section) in the south Simao Basin. A total of 2262 palaomagnetic cores were collected for magnetostratigraphy and 5 siltstone/sandstone samples for detrital zircon U-Pb analyses. Detailed rock magnetic and petrographic examinations suggest that the main magnetic carriers are likely detrital origin hematite and some magnetite in the brown-reddish sediments, and magnetite and a small amount of hematite in the grayish sediments. Progressive thermal and alternating field demagnetizations isolated 1575 robust characteristic remnant magnetization (ChRM) directions, which yield positive fold, reversals and detrital remnant magnetization model tests. All these together indicate primary magnetizations of the ChRMs, which yield twenty-five normal (N7-N31) and 25 reversed (R8-R32) polarity intervals. The adopted maximum depositional ages calculated using the youngest detrital zircon U-Pb ages are 159.0 Ma, 154.0, 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 of the JC section, respectively. Taking these youngest detrital zircon U-Pb ages and the previous magnetostratigraphic results of the Mengyejing Formation in the upper section into consideration, these polarity zones were best correlated to chrons C34n - M17r of the geomagnetic polarity timescale (GPTS2012). It generates magnetostratigraphic ages of ~143.5-~65.0 Ma for the whole stratigraphic sequence by connecting the previously reported results of the upper part of the JC section, yielding 143.5~142.3 Ma, 142.3~121.0 Ma, 121.0~111.5 Ma and 111.5~65.0 Ma for the upper Bazhulu, the Jingxing, the Mangang and the Mengyejing formations, respectively. Our results indicate that the J/T boundary lies in the upper Bazulu Formatiton. Paleogeographical analyses revealed that the Simao Basin was located at a lower paleolatitude of ~17° next to the Neo-Tethys Ocean that has frequently experienced marine and terrestrial interactions during the Late Jurassic-earliest Cretaceous, followed by northward movements to ~24°N characterized by a dominant fluvial environment during early-middle Early Cretaceous, and turned into a generally saline lake sedimentary environment during the late Early Cretaceous-early Late Cretaceous with insignificant latitudinal movement, and transitioned to a fluvial environment during the late Late Cretaceous. Paleogeographical evolution of the Simao Basin was concordant with regional/global tectonic and paleoclimate changes.