Abstract:
The Pretoria Group in the major outcrops of the study area comprises fourteen
formations, the five uppermost only being developed in the eastern part of the
Transvaal Basin. The fifteenth, the Makeckaan Formation occurs in the Makeckaan
'fragment' to the northwest of Pretoria.
The basal Rooihoogte Formation, comprising chert breccia, chert conglomerate,
cross-bedded sandstone and minor mudrocks, unconformably overlies the dolomites
and banded iron-formations of the Chuniespoort Group. Only the conglomerate
lithofacies is developed at Marble Hall and Dennilton. The bulk of the
succeeding Timeball Hill Formation consists of laminated, ferruginous mudrocks,
with basal carbonaceous mudrocks and a medial sandstone member. An
ironstone lithofacies is associated with the latter, whereas arkose and diamictite
lithofacies occur locally within the upper shale zone. The Boshoek
Formation overlies the Timeball Hill Formation with an erosive contact, and
comprises an overall upward-fining arrangement of conglomerates, cross-bedded
sandstones and mudrocks. It is not encountered at Marble Hall or Dennilton.
Andesitic lavas, with minor pyroclastic material and mudrocks, characterize
the Hekpoort Formation, which is covered by a thin bed of sericitic mudrock in
the study area, and throughout large parts of the Transvaal Basin. T_he Dwaa1=
Heuwel Formation,-consisting of major cross-bedded.sandstone.s, with interlc:J,yered
mudrocks, and, in the northern part of the study area, conglomerates,
rests unconformably on the Hekpoort Formation, and is succeeded by the rhythmically
interlayered mudstones and siltstones and minor sandstones of the
Strubenkop Formation. Mature quartz arenites characterize the Daspoort Formation
in the south of the main eastern Transvaal outcrop area, as well as at
Marble Hall and Dennilton; further north, mudrocks and less mature sandstones
predominate. U_p _ ~Q.._?Q90m_of mudro~~s, with a medial volcanic unit, comprise
th:._3verlying Silverton Formation, whereas mostly mature, cross-bedded sandstones,
intercalated with minor mud rocks, characterize the Magaliesberg Formation.
Predominant mudrocks occur in the Vermont Formation, with subordinate
carbonate rocks, sandstones and chert. The cross-bedded and planar-bedded
sandstones of the succeeding Lakenvlei Formation are mostly subarkosic, but
quartzitic varieties also occur; minor mudrocks are present. A mudrock lithofacies
characterizes the basal part of the Nederhorst Formation, whereas the
upper portion consists of fine-grained arkoses, with minor quartzitic to
argillaceous sandstones. Mature and subarkosic, cross-bedded sandstones comprise
most of the Steenkampsberg Formation; in the Houtenbek Formation, the
uppermost unit of the Pretoria Group in the eastern Transvaal, major mudrocks
are interlayered with mature sandstones and, locally, with carbonate rocks and
arkoses.
The five post-Magaliesberg formations are developed only in the southern half
to two thirds of the main eastern Transvaal outcrop area; they are absent at
Potgietersrus and Marble Hall, and are poorly preserved at Dennilton. The
Makeckaan Formation, outcroping as an isolated occurrence to the north of
Marble Hall, consists of major feldspathic arenites, with minor quartzitic,
argillaceous and lithic sandstones.
Geochemical analyses of the Pretoria Group mudrocks point to intensive weath-ering in the source areas of the Timeball Hill, Strubenkop and Silverton
Formations, whereas fresher source materials probably were available during
later stages of sedimentation in the Pretoria Group depository. The source
area composition is thought to have been mixed granitic-basaltic. Boron concentrations
indicate a relatively low palaeosalinity in the basin, especially
during pre-Magaliesberg times.
Alluvial fan depositional settings are proposed for the Rooihoogte, Boshoek
and Dwaal Heuwel Formations, whereas the Time ball Hill Formation is thought to
have been laid down in a deltaic palaeoenvironment. The Klapperkop Sandstone
Member of the Timeball Hill Formation, the Daspoort Formation and the Steenkampsberg
Formation probably represent delta front deposits. The subaerial
extrusion of the Hekpoort lavas and pyroclastics was succeeded by a period of
subaerial weathering. A palaeoenvironmental setting basinal to the Dwaal
Heuwel fans is postulated for the Strubenkop mudrocks, which overlie the
former conformably. Prodelta deposition is envisaged for the Silverton Formation,
with subaquaeous extrusion of the interbedded lavas and pyroclastics.
The Magaliesberg Formation is thought to represent regressive shoreline deposits,
which were succeeded by the mudflats of the Vermont Formation. Fan-delta
or delta sedimentation is suggested for the overlying Lakenvlei Formation. In
the Nederhorst Formation, basal mudflats were succeeded by a fluvio-deltaic
palaeoenvironment. A complex, regressive shoreline - mudflat - shallow water
depositional setting is suggested for the uppermost Houtenbek Formation. The
Makeckaan Formation, which is ten ta ti vely correlated with the Lakenvlei Formation,
or, alternatively, with the five uppermost formations of the Pretoria
Group as a whole, only developed in the eastern Transvaal, and is thought to
have been laid down in a fluvio-deltaic palaeoenvironment.
The alluvial fans, deltas and fan-deltas proposed here for the Pretoria Group
in the eastern Transvaal, probably were fed from source areas situated to the
north, northeast, east, and, to a smaller extent, to the south. For the occurrences
of Pretoria Group rocks at Marble Hall and Dennilton, provenance regions
to the northwest and southwest probably existed, possibly related to a
central palaeohigh situated to the north of Pretoria, and separating western
and eastern Transvaal sub-basins.