posted on 2020-04-08, 07:31authored byJulian S Marsh, Peter R Hooper, Jakub Rehacek, Robert A. Duncan, Alexander R. Duncan
The Lesotho remnant contains the type succession for Karoo low-Ti
basalts of central southern Africa. The 40Ar/39Ar dating indicates that
the sequence was emplaced within a very short period at about 180 Ma and
consists of a monotonous pile of compound basalt lava flows which lacks
significant palaeosols and persistent sedimentary intercalations. We
have used geochemistry to establish a stratigraphic subdivision of the
lava pile. Thin units of basalt flows, the Moshesh's Ford, Golden Gate,
Sani, Roma, Letele, and Wonderkop units, with diverse geochemical
character and restricted geographical distribution, are present at the
base of the succession. These are overlain by extensive units of
compositionally more uniform basalt, the Mafika Lisiu, Maloti, Senqu and
Mothae units, which build the bulk of the sequence. A single
palaeomagnetic polarity reversal occurs within the lower third of the
basalt succession and is consistently located within the Mafika Lisiu
unit. This and the persistent and relatively uniform thickness of the
stratigraphic units suggest that the pile was constructed in a uniform
manner by eruption of basalt onto a generally planar surface from a
widespread plexus of dykes. The stratigraphic sequence in Lesotho
closely resembles that in the thinner sequence of low-Ti basalts of the
Springbok Flats remnant, some 400 km to the north. A thin unit of
high-Ti basalt within the upper part of the Springbok Flats sequence can
be correlated with the thick high-Ti basalt suite along the
rift-related Lebombo structure on the eastern margin of the Karoo
province. This is the first established correlation between these two
important outcrops of Karoo volcanic rocks and demonstrates that the
low-Ti basalts of Lesotho and the cratonic interior are the approximate
time equivalents of the lower part of the Lebombo sequence. This
conclusion has important implications for models for the origin of the
Karoo flood basalt province.
Location of this section is described in Marsh et al. (1997) AGU
Geophysical Monograph, 100, 247-272.