Palaeoenvironmental models in the Eastern Karoo Basin
Date
1978
Authors
Hobday, D. K.
Journal Title
Journal ISSN
Volume Title
Publisher
Bernard Price Institute for Palaeontological Research
Abstract
Palaeoenvironmental models are based on a three-dimensional conception of sedimentary
rock units and their internal geometry. These models are process-oriented and are interpreted
by comparison of their attributes with those of modern sedimentary environments. Six models
are proposed as a result of observations in the eastern Karoo Basin, three in the Ecca and three
in the Beaufort, although some are common to both.
Both regressive delta and beach models are upward-coarsening, but they are readily distinguished
on the basis of sandstone composition, texture and sedimentary structures. Beaches
probably developed along a non-tidal or micro-tidal coast, but in most areas the relatively rapid
sediment influx favoured the formation of deltas which prograded across the shallow shelf. Incised
into the delta front sandstones are channels of distributary and alluvial origin. Large fluvial
channels were generally meandering, and their deposits record a vertical reduction in flow
energy from thalweg through point bar to levee, with the capping coal seams representing an
hiatus in detrital sedimentation.
Delta front sandstones within the Beaufort Group resemble superficially those of the Ecca, but
display differences in vertical sequence which are tentatively ascribed to changes in density of the
basin waters. Whereas the northern and eastern basin margins were characterized by persistent,
moderate energy fluvio-deltaic sedimentation, with small prograding lobes separated by shallow
embayments subject to crevasse splays, the southern part of the basin was the locus of major fluvial
deposition as a consequence of orogenic uplift to the south. High energy braided stream
conglomerates and sandstones were deposited contemporaneously with finer-grained meanderbelt
and floodplain sediments, which accumulated farther basinward in an area of reduced
gradient and more constant discharge.
The value of these models is that most outcrops in the study area can be explained in terms of
their relationship to one or more of the models. Future palaeoenvironmental synthesis should
incorporate the great variety of biological information available from the Karoo Basin.
Description
Main article
Keywords
Karoo basin; Palaeoenvironmental models; Ecca; Beaufort
Citation
None