Preliminary Magnetostratigraphy of the Tertiary Sedimentary Rooks, Mae Moh Group from Chiang Muan in Northern Thailand

Abstract

The Mae Moh Group sedimentary rocks, Chiang Muan, Northern Thailand, host recent finds of the first Miocene hominoid fossils found in Southeast Asia. The age of these deposits is not well known except for Mammalian fossils that indicate deposition during the late Middle Miocene. We conducted a paleomagnetic analysis to establish magnetostratigraphy of the Mae Moh Group and increase the age resolution for these important deposits. Oriented minicore samples were drilled from nine horizons in this Group consisting of non-marine siltstones intercalated with coal layers. Thermal demagnetization of these cores indicates relatively stable remanences from five horizons, three of which are normal polarity and two reversed. Rock magnetic analyses indicate that titanomagnetite is the main magnetic carrier. We obtained a normal-reverse-normal polarity sequence, and correlated it to that from C5A to C5 chrons. This result suggests that the Mae Moh Group in Chiang Muan was deposited 12Ma to 10Ma, with a mean sedimentation rate of 4-10cm/ky.