Basin analysis of the Beaufort group in the western part of the Karoo basin
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Date
1990
Authors
Jordaan, Marthinus Johannes
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
A basin analysis of the lower Beaufort Group in the Karoo Basin (Carboniferous-Jurassic) west of
24°E, cover an area of nearly 70000 km2. The Karoo Sequence reaches a composite thickness
greater than 7500 m, comprising diamictites, sandstones, shales and mudstones. Along the
southern margin of the Basin the entire succession is intensely folded, becoming flat or gently
undulating in the north. Minor displacements along dolerite dykes and small-scale thrusting are the
only evidence of faulting in the Basin.
The transition from the uppermost Ecca to the lower Beaufort Group reflects a change from deltaic
to fluvial depositional environments. The contact is taken above the uppermost deltaic sandstones,
where the succession becomes distinctly argillaceous, and where therapsid fossils appear in the
succession.
The lower Beaufort succession consists of alternating sandstones, and red and green mudstones
forming cyclical, upwards-fining, fluvial sequences. Channel sandstones (greywackes and arkosic
wackes) and intraformational conglomerates are overlain by red and green overbank mudstones
with intercalated crevasse splay sandstone sheets and lenses. The mudstone facies (Fg, Fl, Fr and
Fv) are commonly bioturbated and contain a variety of therapsid fossils, calcareous nodules, and
palaeosols. Tuff beds are sparsely preserved. Freshwater molluscans, fish imprints and plant
fossils occur at certain horizons.
The lower Beaufort comprises four upwards-fining megasequences, the uppermost incompletely
preserved. The basal megasequence (A) overlies and grades diachronously into the Ecca Group
from south to north across the Basin, thinning from more than 2500 m in the south to where it
pinches out on surface, just south of Carnarvon in the north. In the unit which consists of a basal
Combrinkskraal sandstone member overlain by the Leeu Gamka member, green and subordinate
red mudstones are interbedded with thick, tabular sandstones. Megasequence B contains thick,
tabular sandstones and green mudstones of the Koup member at the base, overlain by. the
predominantly red mudstones and thin sandstones of the Teekloof member. The unit is similarly
wedge-shaped, thinning from nearly 900 m in the south to where it pinches out in the northeastern
corner of the Basin. Megasequence C consists of a basal Nuweveld member, overlain by red mudstones of the Giflcop member. Megasequence D is sparsely preserved in the central part of the
Basin, comprising the Leeukop member overlain by the Elandsberg Member. These
megasequences correlate with similar stratigraphic units in the eastern sector of the Basin.
Sandstone-hosted, syngenetic uranium orebodies occur in the arenaceous intervals of each
megasequence.
The lowermost Beaufort shows northerly transport directions in the south, and subordinate,
east-northeasterly directions in the central and northern. parts of the Basin. These directions reflect
transverse alluvial fan and longitudinal alluvial plain drainage systems, respectively. This pattern is
repeated throughout the lower Beaufort, with the longitudinal drainage system becoming dominant
in the uppermost units. Sandstone:mudstone ratio maps confirm the major transverse drainage
system which entered the Basin from the south. Westerly to northwesterly transport directions in
the easternmost part of the study area represent a separate, coeval drainage system in the eastern
sector of the Basin.
The Beaufort Group was deposited as a molasse-type basin-fill in an orogenic foreland (retroarc)
basin, yoked to a fold-thrust belt in the south. Isopach maps show that the Basin was strongly
asymmetric initially, becoming more symmetric during deposition of the uppermost members.
Sediment was derived from a mixed source, and uranium was introduced from weathered tuffs and
granites. Uranium ore formation was controlled by the petrographic composition, presence of
carbon, and permeability and diagenetic history of the host sands.
Description
Keywords
Thesis (Ph.D. (Geology))--University of the Free State, 1990, Beaufort group, Basin analysis, Karoo basin