ABSTRACT
The main Karoo Basin of South Africa contains a relatively continuous sequence
of continental deposits that accumulated over a 100 million year period from
Permo-Carboniferous (280 Ma) to early Jurassic (180 Ma). In the southwestern
region of the basin the Karoo succession is approximately 4 000 m thick, the
upper half of which consists of vertebrate fossil-bearing fluvial rocks of the
Beaufort Group.
This study deals with Lower Beaufort (Adelaide Subgroup) strata belonging to
the Teekloof Formation which are exposed in the cast-west trending erosional
escarpment between the towns of Beaufort West and Fraserburg in the central
Cape Province. The 450-mctre succession consists mainly of vertically accreted
floodplain mudrocks with interbedded continuous sheets of fine-grained
sandstone that bear evidence of having accumulated by lateral accretion on the
inner banks of meandering channels.
The mudrocks contain numerous fossilized skeletons of therapsid "mammal-like"
reptiles as well as more primitive cotylosaurs and a few amphibians. Rarely,
impressions of Glossopteris leaves and equisetalcan stems are found although
root moulds are relatively abundant. Several types of calcareous nodules and
sheets occur in the mudrocks and arc interpreted as evidence of calcic palaeosols
that formed under semi-arid climatic conditions.
This is an interdisciplinary study that integrates a conventional sedimentary facies
analysis with investigations of the taphonomy of in situ therapsid fossils and the
nature and distribution of palaeosols. Such an approach has not previously been
applied to any of the Karoo strata. The results contain descriptive and
quantitative information on sedimentary processes, palaeohydrology, absolute
time represented in the stratigraphic record and the topography, soils and
habitats of the ancient landscape. These are summarized into a
palaeoenvironmental synthesis of the Teckloof Formation.
Two facies associations arc recognized within the Teckloof Formation. (1) The
channel facies association: thick (5 - 25 m) laterally accreted sandstones of both
single and mutlistoried geometry made up of discrete point-bar and channel-fill
sedimentary facies that are characteristic of large, Mississippi-sized, mixed-load
meandering rivers. Exceptional and previously undescribed three-dimensional
exposures of a large exhumed palaeomcandcrbelt were mapped and logged and
provide the basis for reconstructing the palaeohydrology and migration behaviour
of one of the "Reiersvlei rivers".
(2) The interchannel facies association: comprising 10 - 50 m thick intervals of
alternating siltstone and fine-grained sandstone sheets with mudstone confined
mainly to thin "veneers". The mudrocks represent extended periods of alluviation
on exposed semi-arid floodplains between major meanderbclt ridges. Much of
the alluvium was deposited episodically from sediment-laden flood discharge that
spilled over both banks of the main channels, across prograding crevasse-splay
fans, into narrow distributaries that drained the proximal floodplain and issued
into shallow playa-type lakes in the more distal floodbasin. Alluvium that was
deposited in each of these subenvironments gained specific sedimentary,
taphonomic and pedogenic features that were preserved and arc used to
distinguish them in the rock record.
Three facies and six subfacies arc recognized within the interchannel facies
association, each is given an environmental epithet based on comparison with
modern floodplain deposits.
Channel bank/levee deposits contain rapidly alternating sandstone to siltstone
flood couplets overlying scour surfaces. Distinction is made between inner-bank
and outer-bank subfacies based mainly on their inter-relationship with strata of
upper point-bar facies and the relief of internal scour surfaces. Vertebrate fossils
are generally sparse but locally abundant in some abandoned crevasse channelfills.
Proximal floodplain facies comprise vertically stacked sheets of fine-grained
sandstone, siltstone and minor mudstone with abundant, but dispersed,
vertebrate fossils. They often contain one or more tabular crevasse-splay
sandstones which are up to 3 m thick, mainly horizontally laminated and usually
contain evidence of discontinuous vertical aggradation with possible aeolian
reworking. Narrow "shoestring" sandstones form another distinctive subfacics
interpreted as the preserved channel-fill deposits of ephemeral, low sinuosity
distributaries.
Distal floodbasin facies arc made up of thinly bedded sandstone-siltstone
couplets with interbedded sharply bounded distal crevasse-splay sandstones.
Palaeosurfaces on the upper surface of the sandstone interbeds are rarely
preserved beneath claystone veneers and display an array of sedimentary and
biogenic structures, including vertebrate tracks, which record shallow water
sedimentation followed by stillstand exposure and desiccation. Gypsum "desert
rose" crystals and silicified carbonate muds are interpreted as having precipitated
on the margins of playa type lakes in the axial depressions of the Teekloof
floodbasins.
Comparison of the morphology and petrology of palaeo-caliche nodules in the
Teekloof palaeosols with those of sub-recent calcretes suggests that the latter
were formed under a semi-arid climate of long warm to hot summers (mean
annual temperature 16 - 20 degrees C) and short wet winters (mean annual
rainfall 500 - 800 mm).
Maturity of the Teckloof palaeosols may be assessed from palaeocal iche
morphology, the degree of clay illuviation and the presence of ped structures in
the lower solum. Pedogenic maturity at any site appears to be controlled by the
rate of alluvium accumulation and, to a lesser extent, the topographic position.Taphonomic assessment of 940 vertebrate fossils collected from the study block
showed that rocks of the proximal floodplain facies were the most fossiliferous
and contained relatively more fully articulated therapsid skeletons than channel
bank or distal floodbasin facies. The occurrence of proximal floodplain deposits
containing numerous skull-only Diictodon specimens was investigated and
interpreted as the combined effect of carnivore attack in opening up the neck
area to early disarticulation and the fact that after death the lower jaw remained
locked in place by the curve of the horn-covered beak thus forming a single heavy
and compact unit that resisted entrainment and transportation during subsequent
floods. During this investigation it was discovered that some taphonomically
anomalous occurrences of curled-up and paired Diictodon skeletons resulted
from their entrapment and burial within underground burrows. The patterns of
bone weathering and skeletal disarticulation in the different floodplain facies is
interpreted as a function of the duration of post-mortem exposure on the
floodplain surface.
It is concluded that the main factor determining the style and rate of
sedimentation, the degree of skeletal disarticulation and weathering of bones and
the maturity of palaeosols in different parts of the floodplain was distance of the
floodplain site from the nearest major channel.
On a larger scale, two major first order fining-upward cycles make up the
Teekloof Formation comprising a lower channel-sandstone rich interval followed
by a dominantly mudrock interval. These variations arc interpreted as a
reflection of disequilibrium between alluvial plain morphology and total energy
supply brought about by differential rates of subsidence between adjacent regions
of the basin. It is concluded that because they result from natural shifts in the
drainage nets their controls are autocyclic rather than allocyclic.
SMITH, R (2021). Fluvial Facies, Vertebrate Taphonomy And Palaeosols Of The Teekloof Formation (Permian) Near Beaufort West, Cape Province, South Africa. Afribary. Retrieved from https://tracking.afribary.com/works/fluvial-facies-vertebrate-taphonomy-and-palaeosols-of-the-teekloof-formation-permian-near-beaufort-west-cape-province-south-africa
SMITH, R.M.H. "Fluvial Facies, Vertebrate Taphonomy And Palaeosols Of The Teekloof Formation (Permian) Near Beaufort West, Cape Province, South Africa" Afribary. Afribary, 15 May. 2021, https://tracking.afribary.com/works/fluvial-facies-vertebrate-taphonomy-and-palaeosols-of-the-teekloof-formation-permian-near-beaufort-west-cape-province-south-africa. Accessed 10 Nov. 2024.
SMITH, R.M.H. . "Fluvial Facies, Vertebrate Taphonomy And Palaeosols Of The Teekloof Formation (Permian) Near Beaufort West, Cape Province, South Africa". Afribary, Afribary, 15 May. 2021. Web. 10 Nov. 2024. < https://tracking.afribary.com/works/fluvial-facies-vertebrate-taphonomy-and-palaeosols-of-the-teekloof-formation-permian-near-beaufort-west-cape-province-south-africa >.
SMITH, R.M.H. . "Fluvial Facies, Vertebrate Taphonomy And Palaeosols Of The Teekloof Formation (Permian) Near Beaufort West, Cape Province, South Africa" Afribary (2021). Accessed November 10, 2024. https://tracking.afribary.com/works/fluvial-facies-vertebrate-taphonomy-and-palaeosols-of-the-teekloof-formation-permian-near-beaufort-west-cape-province-south-africa