Magnetostratigraphic Analysis of Miocene Sediments in the Middle Bhuban Formation, Mizoram, Northeast India

ABSTRACT The magnetic polarity stratigraphy of the 320 m thick Turial− Seling section from the Middle Bhuban Formation (Surma Group), Mizoram, Northeastern India, is presented. The mean characteristic remanent magnetisation (ChRM) directions, determined through thermal and alternating field (AF) demagnetisation techniques, are recorded as D = 30.9°, I = 31.6° for normal polarity and D = 208.8°, I = −23° for reverse polarity. Magnetostratigraphic investigation reveals two normal (C6An.1n and C6n) and one reverse magnetic polarity zone within the studied section. These findings indicate an age range from approximately 20.73 Ma at the base of the section to ~19.05 Ma at the upper section, spanning a total duration of ~1.68 million years. The long normal magnetic sub-chron (20.13 – 19.05 Ma), correlated with the C6n event of the Geomagnetic Polarity Time Scale (GPTS), represents the primary polarity event recorded in this section, extending over a thickness of approximately 275 m. The sediment accumulation rate (SAR) for the long normal polarity event (C6n) is calculated to be 25 cm/1000 years, while the average SAR for the entire section is estimated at 19 cm/1000 years. The magnetic susceptibility (χ) of the Middle Bhuban sediments ranges from 9.3 × 10−5 SI units to a maximum of 235 × 10−5 SI units, with an average value of 20.6 x 10−5 SI units. Isothermal remanent magnetisation (IRM) and high-temperature susceptibility measurements (χ vs T, up to 700 °C) confirm that magnetite is the primary magnetic mineral responsible for carrying the remanent magnetisation. The coercivity of remanence (B0cr), which varies between 33 mT and 67 mT, further supports the presence of a low-coercivity magnetic mineral, consistent with magnetite, in the Middle Bhuban sediments.