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Dating Archean sedimentary processes and rates in the
Moodies Basin, Barberton Supergroup, Barberton Greenstone
belt
J. Engelhardt, C. Heubeck, J. Kirstein, M. Gröschke
Department of Geological Sciences, Freie Universität Berlin, Berlin, Germany, jfengel@web.de>
ABSTRACT
The Moodies Group (ca. 3.23 Ga) is the youngest sedimentary sequence of the Barberton Greenstone Belt (BGB).
Within its approximately 2.5 km–thick sandy and conglomeratic stratigraphic column, abundant sedimentary
structures, e.g., crossbedding, ripple marks and desiccation cracks record alluvial, fluvial, deltaic and shallow
marine environments. Petrography and sedimentary style clearly indicate a synorogenic setting of the Moodies
Basin, and much its sediments may have been deposited within a short time, raising the tantalizing possibility of
preserving a high-resolution time signal of the Archean surface record.
We mapped at 1:2000 scale the Moodies Lava (MdL), a unique 10-60 m thick effusive basaltic flow rock that
forms a distinctive marker unit throughout the BGB in the middle Moodies Group. The MdL overlies large-scale
cross-bedded coarse-grained sandstones interpreted as an offshore subaqueous dune field, and in places also an
enigmatic discontinuous cobble and boulder conglomerate that may reflect a sudden base-level fall. The lava is
strongly altered in most places and consists of a nearly homogeneous mass of illite, sericite, chlorite and very-fine
grained quartz in thin section. Ghost phenocrysts of feldspar, however, are recognizable in well-preserved samples.
In the eastern part of the Saddleback Syncline, the unit can be subdivided in two members representing individual
effusive events, separated by shales and cherts 8 to 15 m thick. Amygdules are common towards the top of the
MdL; the rare presence of pillow lavas attests to a submarine origin. The lower MdL shows magnetite-rich zones
and overlies banded magnetic shale in at least one site. A thin tuff unit overlying the MdL be datable, yield a direct
age of the Moodies Group, and constrain duration of Moodies sedimentation.
Keywords: Barberton Greenstone Belt, Moodies Group, sedimentology, volcanism.
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