Abstract
M.Sc. (Geology)
The study was undertaken to investigate the stratigraphic
and sedimentological character of the transition between the Ecca
and Beaufort Groups in the northeastern region of the main Karoo
basin.
Detailed stratigraphic profiles demonstrate that the
transition comprises a regressional sequence of dark basinal
shale overlain by sandy delta-lobe deposits. This is overlain by
lenticular sandstones and shale representing meandering stream
deposits.
The deltaic sedimentary rocks vary markedly along strike and
record deposition in four different deltaic sUbenvironments,
namely:
(A) "Normal" central delta lobe areas characterized by distal
mouth bar sandstone-shale rhythmites gradationally overlain by
proximal distributary mouth bar sandstones.
(B) Interdistributary bay areas characterized by stacked thin
upward-coarsening shale-sandstone cycles.
(C) Areas of strong distributary channel erosion where
prodelta shale deposits are erosively overlain by distributary or
fluvial channel sandstones. No distributary mouth bar deposits
are present.
(D) Storm-dominated areas comprising prodelta sedimentary
rocks which are in turn overlain by storm and possible barrier
sand deposits.
Palaeocurrent directions indicate that sediment transport
was from the north-west, north and north-east.
The concentrations of the trace elements B, V, Zn and Ni in
prodelta shales indicate the possible prevalence of both marine
and freshwater conditions in the near-coastal waters of the Ecca
basin.
Lateral variation of depositional environments on the same stratigraphic level makes the placing of the Ecca-Beaufort
contact based on genetic criteria unsuitable. It is therefore
suggested that purely lithostratigraphic criteria be used. On
this basis the Ecca-Beaufort contact is taken at the base of the
first prominent sandstone which overlies the Volksrust Shale
Formation.