The Magnetostratigraphy and the Age of So’a Basin Fossil-Bearing Sequence, Flores, Indonesia

Abstract

Three fossil-bearing intervals have been recognized in the Pleistocene So'a Basin, with the upper one holding important evidence of hominin fossils. The sequence also contains numerous in situ stone artifacts and fossils of other vertebrate taxa. Therefore, multiple dating techniques are crucial to secure the age of the fossil and artifactbearing layers, especially the one with the hominin remains. This paper deals with the palaeomagnetic dating of the So'a Basin sequence to assist other dating methods that have been applied, and to refine the chronostratigraphy of the area. Palaeomagnetic sampling was conducted in four sections along a west to east transect. Four magnetozones can be recognized, consisting of two reverse and two normal polarity zones. By using the available radiometric ages as a guide and comparing the So'a Basin magnetostratigraphy with the Standard Geomagnetic Polarity Time Scale (GPTS), it became clear that both reverse magnetozones are part of the Matuyama Chron, while the normal magnetozones are the Jaramillo subchron and the Brunhes Chron. These palaeomagnetic dating results support the available radiometric dates and refine the age of the fossil-bearing deposits of the So'a Basin.

An integrative geochronological framework for the Pleistocene So'a basin (Flores, Indonesia), and its implications for faunal turnover and hominin arrival

“…In addition, these WST ages are further supported by palaeomagnetic polarity data from both the Tangi Talo and Mata Menge sections, which indicate a clear polarity change from normal to reverse just below WST, consistent with the base of the Jaramillo normal subchron, dated at 1.07 Ma (see Fig. 2; Yurnaldi et al, 2018).…”

“…At that time this boundary was thought to be 0.73 Ma, but is now considered to be 0.773 Ma (Cohen and Gibbard, 2019;Mark et al, 2017). These palaeomagnetic results have been confirmed in a more recent study (Yurnaldi et al, 2018), indicating that the base of MM-LOW is younger than 0.773 Ma (Fig. 2).…”

The Stegodon Bonebed of the Middle Pleistocene Archaeological Site Mata Menge (Flores, Indonesia): Taphonomic Agents in Site Formation

“…The Lower Fossil-bearing Interval (LFI), bracketed by a maximum age of 0.78 Ma based on palaeomagnetic evidence and a minimum age of 0.81 Ma [15,23], contains impoverished vertebrate fauna as well as in situ stone artefacts. The up to 2.2 m thick LFI represents a fining-upward sequence consisting of an amalgamated complex of sheets and lenses of sandstone alternating with tuffaceous siltstones and clay-rich mudflows.…”

“…A Zircon Fission-track age of 0.80 ± 0.07 Ma for the Lower Fossil-bearing Interval (LFI) has been reported [13]. A more recent palaeomagnetic study [15] revealed that the LFI has a normal polarity and corresponds with the base of the Brunhes Normal Epoch. The LFI is therefore thought to be slightly younger than the Matuyama-Brunhes boundary at 0.773 Ma, which is within the error range of the ZFT age.…”

“…The LFI is therefore thought to be slightly younger than the Matuyama-Brunhes boundary at 0.773 Ma, which is within the error range of the ZFT age. The Upper Fossil-bearing Interval (UFI) is bracketed by ages of 0.78 and 0.65 Ma [10,13,15]. At the nearby site Wolo Sege, stone artefacts occur directly below a widespread pumice tuff layer, dated at 1.0 Ma [9] and, combined with the evidence from Liang Bua [16], indicates hominin occupation on the island for at least one million years.…”