The stratigraphy of the T’Goob Sequence (Aggeneys Subgroup) in the eastern Aggeneys Terrane, Namaqua Mobile Belt

Abstract Most of the deformed amphibolite-grade lithological units of the eastern Aggeneys Terrane are traditionally regarded as correlatives of the Wortel Formation of the Aggeneys Subgroup in the Bushmanland Group. Recent structural-stratigraphic mapping in the eastern part of the terrane identified a series of macroscopic thrust sheets that contain megascopic sheath folds, which preserve unique as well as communal stratigraphic characteristics. The T’Goob megascopic sheath fold in the Lekdam thrust sheet is host to Wortel-type lithologies as well as unusual and exclusive series of metavolcanic/sedimentary lithological units. The atypical nature of units precludes their direct incorporation in the traditional Wortel Formation and lends itself rather towards a separate classification using a lithostratigraphic/lithodemic approach. The term “T’Goob Sequence” is assigned to the rock units in the T’Goob mega sheath fold and is used in a structural-stratigraphic sense as the sequence is structurally detached from the type area of the Wortel Formation (farm Wortel 42). For compilation of the members of the various units of the T’Goob Sequence, sequence mapping and three detailed stratigraphic profiles (stratotypes) on selected sections of the upright limb of the megascopic T’Goob sheath fold illustrate the lateral and vertical variation and alternation of rock units. The sequence consists of eight units. Unit 8 (top): a massive lens of corundum-sillimanite-rutile rock and biotite-sillimanite schist. Units 7, 6 and 5 form a metavolcanite marker consisting of biotite gneiss; interbanded units of actinolite-diopside-hornblende-cummingtonite layers and feldspathic quartzite; biotite gneiss with interbanded units of dark-blue quartzite and biotite-sillimanite schist. Unit 4: Biotite-sillimanite schist with interbanded garnet quartzite. Unit 3: nodular sillimanite-quartz-feldspar rhythmite. Unit 2: massive poorly bedded white orthoquartzite. Unit 1 (bottom): nodular sillimanite-quartz-feldspar gneiss with interbanded biotite-sillimanite schist. When comparing the lithological assemblages of the Aggeneys Subgroup formations in the western Aggeneys Terrane, the T’Goob Sequence represents a new addition to the stratigraphic database of the Aggeneys Subgroup and also a facies variant of the Wortel Formation – not recognised elsewhere in the Mesoproterozoic Aggeneys Terrane.