U-Pb and palaeomagnetism provide a Pleistocene age for Waypoint 160 (Bolt’s Farm) Cradle of Humankind, South Africa.

Abstract

Bolt’s Farm is a series of fossil bearing active caves and palaeokarst remnants in the Cradle of Humankind, Gauteng, which has been explored for palaeontological remains since the 1940s. An early Pliocene age of 5-4 Ma was suggested for one deposit at Bolt’s Farm, Waypoint 160, based on the discovery of a new species of rodent Euryotomys bolti (Senegas & Avery 1998). Given the rarity of early to mid Pliocene deposits in the area, and the importance of this time period in marking the arrival of hominins in South Africa, Waypoint 160 is an ideal site for geochronological analyses. A new multi-disciplinary study combining U-Pb dating, palaeomagnetism, petrographic analysis and SEM-EDS proposes a new age for the locality of Waypoint 160. U-Pb of the basal flowstone reveals a maximum depositional age of around 2.6 Ma and a minimum capping age of ~1.7 Ma. Alternating field demagnetisation (AFD) and thermal demagnetisation (THD) experiments were carried out on sub samples (n=40) with mineral magnetic studies revealing the presence of magnetite as a carrier of magnetic remanence. Magnetostratigraphy of the sequence supports deposition occurring within the Matuyama Reverse Chron with palaeomagnetic samples directly below the capping flowstone exhibiting a period of extended normal polarity which can now confidently be assigned to the Olduvai subchron. The capping flowstone itself exhibits a reversed polarity, thereby recording the transition period marking the end of the Olduvai subchron. With this information we can confidently say that the deposits and related fauna at Waypoint 160 are Pleistocene in age. This provides further evidence that the oldest sites we are seeing preserved in South Africa continue to be terminal Pliocene (Taung 3-2.6 Ma, Makapansgat 2.9-2.6 Ma) and calls into question the use of such preliminary biochronological analysis in attributing an age to a deposit with no paired geochronological analyses.

Full text

U-Pb and palaeomagnetism provide a Pleistocene age and constrain the end of
the Olduvai subchron at Waypoint 160 (Bolt’s Farm) Cradle of Humankind,
South Africa.
Edwards, T. R., Pickering, R., Mallet, T. L., Herries, Andy I. R.
Bolt’s Farm is a series of fossil bearing active caves and palaeokarst remnants in the Cradle
of Humankind, Gauteng, which has been explored for palaeontological remains since the
1940s. An early Pliocene age of 5-4 Ma was suggested for one deposit at Bolt’s Farm,
Waypoint 160, based on the discovery of a new species of rodent Euryotomys bolti (Senegas
& Avery 1998). Given the rarity of early to mid Pliocene deposits in the area, and the
importance of this time period in marking the arrival of hominins in South Africa, Waypoint
160 is an ideal site for geochronological analyses. A new multi-disciplinary study combining
U-Pb dating, palaeomagnetism, petrographic analysis and SEM-EDS proposes a new age for
the locality of Waypoint 160. U-Pb of the basal flowstone reveals a maximum depositional
age of around 2.6 Ma and a minimum capping age of ~1.7 Ma. Alternating field
demagnetisation (AFD) and thermal demagnetisation (THD) experiments were carried out
on sub samples (n=40) with mineral magnetic studies revealing the presence of magnetite
as a carrier of magnetic remanence. Magnetostratigraphy of the sequence supports
deposition occurring within the Matuyama Reverse Chron with palaeomagnetic samples
directly below the capping flowstone exhibiting a period of extended normal polarity which
can now confidently be assigned to the Olduvai subchron. The capping flowstone itself
exhibits a reversed polarity, thereby recording the transition period marking the end of the
Olduvai subchron. With this information we can confidently say that the deposits and
related fauna at Waypoint 160 are Pleistocene in age. This provides further evidence that
the oldest sites we are seeing preserved in South Africa continue to be terminal Pliocene
(Taung 3-2.6 Ma, Makapansgat 2.9-2.6 Ma) and calls into question the use of such
preliminary biochronological analysis in attributing an age to a deposit with no paired
geochronological analyses.