Fluvial Facies, Vertebrate Taphonomy And Palaeosols Of The Teekloof Formation (Permian) Near Beaufort West, Cape Province, South Africa

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.