Journal of African Earth Sciences 38 (2004) 383–400

www.elsevier.com/locate/jafrearsci

Fluvial style variations in the Late Triassic–Early JurassicElliot formation, main Karoo Basin, South Africa

Emese M. Bordy *, P. John Hancox, Bruce S. Rubidge

School of Geosciences, University of the Witwatersrand, Johannesburg, Private Bag 3, Wits 2050, South Africa

Received 2 April 2003; accepted 10 February 2004

Available online 10 May 2004

Abstract

Architectural element analysis of the Late Triassic–Early Jurassic Elliot Formation (South Africa) reveals two contrasting

sandstone body geometries resulting from different fluvial depositional styles. In the lower part of the formation, the sandstones are

multi-storey, asymmetrical channel-fills. They are interpreted as deposits of perennial, moderately meandering fluvial systems

characterised by trough and planar cross-stratification, massive beds and less commonly low-angle cross-stratification. In the upper

part of the formation, the mostly tabular, multi-storey sheet sandstones are internally structured by massive beds, horizontal lam-

ination, ripple cross-lamination, and rare trough cross-stratification. These sandstone bodies are interpreted to have been deposited

by ephemeral fluvial processes. The change in fluvial style is accompanied by changes in sandstone petrography and palaeocurrent

patterns, suggesting that this shift in the depositional style is predominantly controlled by tectonism rather than climate.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Meandering and ephemeral fluvial systems; Late Triassic–Early Jurassic; Elliot Formation; Main Karoo Basin

1. Introduction

The Elliot Formation (Karoo Supergroup) of South

Africa comprises continental red beds of fluvial, lacus-

trine and aeolian origin, and represents the Late Triassic

to Early Jurassic (Lucas and Hancox, 2001) fill of themain Karoo foreland basin (sensu Catuneanu et al.,

1998) (Fig. 1). Although there are some excellent Elliot

Formation outcrops, to date only a few field-based sed-

imentological investigations have been undertaken.

Previous workers agree that the Elliot Formation com-

prises red beds indicative of laterally continuous flood-

plain mudstones and associated fluvial sandstones (e.g.,

Johnson, 1976; Visser and Botha, 1980; Smith et al.,1993; Johnson et al., 1996; Johnson et al., 1997), however

the overall palaeoenvironmental reconstructions and the

general stratigraphic profile (Visser and Botha, 1980)

have been extrapolated from data collected at very few,

relatively localized study areas. For instance, in-depth

examinations are available only for the north-eastern

* Corresponding author. Address: Department of Geology, Rhodes

University, Grahamstown 6140, South Africa. Tel.: +27-46-603-8313;

fax: +27-46-622-9715.

E-mail address: emese_bordy@yahoo.com (E.M. Bordy).

0899-5362/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jafrearsci.2004.02.004

outcrop area (Eriksson, 1985) and for a few localities in

the north-western (Kitching and Raath, 1984; Smith and

Kitching, 1997) and southernmost (Botha, 1968; Visser

and Botha, 1980) regions. Other areas (e.g., the south-

eastern, south-western and western outcrop regions and

western Lesotho) were only briefly examined by Stockley(1947) and Visser (1984). The only basin-wide study of

the Elliot Formation was undertaken by Le Roux (1974)

who provided a general description and several vertical

log profiles. His work, which was undertaken some 28

years ago, is unfortunately of little help in modern facies

analysis, because it lacks information on alluvial stack-

ing patterns (architecture).

The aim of this paper is therefore to provide a de-tailed description and interpretation of the internal fa-

cies associations and architecture of the sandstone

bodies in the Elliot Formation, with a view to docu-

menting the palaeoenvironmental changes that took

place during the deposition of the formation. This paper

however concentrates only on the details of the fluvial

architecture, as other sedimentological aspects of the

Elliot Formation (colour, thickness variations, grainsize variations, petrographic and palaeocurrent data,

etc.) and their implications for basin development are

elaborated on in Bordy et al. (in press a, in press b). In

Fig. 1. Geological map of the Elliot Formation in the Republic of South Africa and Lesotho, modified after the 1:1000000 geological map of South

Africa, Swaziland and Lesotho, Geological Survey, 1984, showing the geographic localities of the sedimentary logs measured in the Elliot Formation.

The thickness figures are selected from a few localities. Palaeocurrents: 1 and 2––regional vector mean directions in the Elliot Formation based on

recent measurements; 3––local vector direction based on Le Roux (1985); 4––regional vector mean directions based on Eriksson (1985). For details

on thickness relations, and palaeocurrent as well as provenance data the reader is referred to Bordy et al. (in press a, in press b), and for data east of

29 degree longitude to Eriksson (1983, 1985). Inserts A and B: summary palaeocurrent rose diagrams for the Lower and Upper Elliot Formation.

Insert C: ternary diagram of mineral composition of sandstones in the Elliot Formation. LEF––Lower Elliot Formation; UEF––Upper Elliot

Formation. Qm:FP:RF (monocrystalline quartz: feldspar: rock fragments). Dashed line indicates the stratigraphic logs shown in Fig. 2.

384 E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400

conjunction with the results of Bordy et al. (in press a, in

press b), the present paper also discusses the controls of

autogenic (e.g., avulsion; lateral migration) and allo-

genic (e.g., climate; tectonic activity) processes on allu-

E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400 385

vial architecture during the deposition of the ElliotFormation.

2. Geological background

The red beds of the Elliot Formation, together with

the underlying and overlying Molteno and Clarens for-

mations (i.e., the informal ‘‘Stormberg Group’’), have

been considered to have formed in the distal sector of

the Karoo Basin during the final unloading phase of the

Cape Fold Belt (Catuneanu et al., 1998). Recent basinal

investigations by the present authors (Bordy et al., in

press a, in press b) have shown that the relative abun-dance and geometry of the preserved sandstone bodies,

and their sedimentary structures, palaeocurrent indica-

tors (Fig. 1A and B), petrographic composition (Fig.

1C) and local and regional relationships are different in

the lower and upper parts of the formation. Based on

these differences, the lower part can be separated from

the upper part of the formation, and these units are

referred here to as the Lower Elliot Formation (LEF)and Upper Elliot Formation (UEF), respectively. These

informal lithostratigraphic units show reasonable cor-

respondence with the biostratigraphic units defined by

Kitching and Raath (1984) as the Euskelosaurus and

Fig. 2. Stratigraphic logs measured in the southwestern (4), central (9, 10) an

Fig. 1 for locations of sections, an Table 1 for the applied lithofacies codes.

Massospondylus range zones, respectively. The informallithostratigraphic units (Lower, Middle and Upper El-

liot formations) defined by Kitching and Raath (1984)

could not be traced on a basin-wide scale, because the

tripartite subdivision of the Elliot Formation is not

apparent in the southern and central outcrop areas. It

seems, however, that the newly defined Lower Elliot

Formation correlates with Lower Elliot Formation of

Kitching and Raath (1984), and the new Upper ElliotFormation incorporates both the Middle and Upper

Elliot formations of these authors.

The Elliot Formation has a relatively sharp, regionally

traceable lower contact with theMolteno Formation and

a mostly gradational upper contact with the Clarens

Formation. The total thickness of the formation de-

creases from a maximum of 460 m in the south, to less

than 70 m in the north. This change in thickness isgradual from south to north, however an abrupt change

takes places in the area between Lady Grey and Zastron,

where the thicknesses are extremely variable (Figs. 1 and

2B) (Bordy et al., in press a). This major change occurs

only in the Lower Elliot Formation, with the rest of

the formation displaying a smaller and more constant

thickness decrease from south to north (Bordy et al., in

press a). Similarly, the grain-size trends are different inthe lower and upper parts of the formation in that the

d northern (14, 18, 19, 24) outcrop areas of the Elliot Formation. See

386 E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400

basinal, south to north grain-size decrease in the LEF isnot present in the UEF, and the slight vertical grain-size

increase in the upper part is not present in the LEF (Fig.

2B, C and E) (Bordy et al., in press b). In addition, the

coarse- vs. fine-grained sediment ratio in the sandier

Lower Elliot Formation decreases sharply from south to

north, a trend which is absent in the overall siltier Upper

Elliot Formation (Fig. 2A vs. D–G; Bordy et al., in press

a, in press b). These observations are in contrast withprevious interpretations which viewed the Elliot For-

mation (especially the lower part) as a distal facies

equivalent of the alluvial braidplains of the Molteno

Formation (e.g., Turner, 1986; Smith et al., 1993;

Anderson et al., 1998). According to these authors, the

progressive denudation of the source area caused the

Elliot Formation to totally backstep over the Molteno

Formation, implying that younging direction and grain-size reduction in the red beds are from north to south.

Palaeoenvironmental reconstructions (Visser and

Botha, 1980; Eriksson, 1985) depict an increasingly arid

fluvial environment where aeolian influences became

progressively dominant through time. The reconstructed

fluvial styles, based on one outcrop section in the south

Fig. 2 (cont

(Visser and Botha, 1980) and eleven sedimentary logs inthe north-east (Eriksson, 1985), are meandering in the

lower parts and ephemeral in the upper part. The change

in fluvial regimes are explained in general terms by the

above-mentioned aridification. Lines of evidence indic-

ative of progressive aridification during deposition of

the Elliot Formation include (1) palaeomagnetic data

showing that southern Africa was positioned in humid

temperate to dry subtropical climate zones during theLate Triassic and Early Jurassic (Parrish, 1990; Scotese

et al., 1999); (2) gradually increasing aeolian influence of

the uppermost part of the Elliot Formation (e.g., Botha,

1968; Le Roux, 1974; Visser and Botha, 1980; Eriksson,

1983, 1985; Smith and Kitching, 1997) and (3) sub-

stantial thickness of the overlying aeolian sandstones

(Clarens Formation) (Beukes, 1970, Eriksson, 1986).

In this study, we show the details and regional extentof the architectural element differences’ between the

Lower and Upper Elliot formations, and in addition, by

incorporating data from Bordy et al. (in press a, in press

b), that the shift in architectural stacking patterns is

related not only to climatic factors, but to foreland basin

tectonism as well.

inued)

E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400 387

3. Methods

The regional distributions of coarse- vs. fine-grained

units, as well as the architecture of the Elliot Formation

were examined in natural exposures and road cuts

Table 1

Applied lithofacies (modified after Miall, 1978, 1985, 1996)

Gmcm S

Mud-pebble conglomerates: massive, clast-

supported

Granule- to pebble-grade clasts: mudstone o

bone fragments � quartz pebbles

Common in facies I and II, rare in facies III

Gravel bars from eroded floodplain fines and/or

in-channel fines (abandoned watercourses)

L

M

R

I

H

Gcm, Gh, Gp, Gt

S

Pedogenic glaebule conglomerates: massive,

clast-supported (Gcm); horizontally stratified

(Gh); planar (Gp) and trough (Gt) cross-bedded

Granule- to pebble-grade clasts: glaebules o

sandstone; bone and teeth fragments � quartz

pebbles

Exclusive to facies II and UEF mudstone units

Gravel bars from eroded overbank paleosols and

other sediments

H

O

d

M

R

i

P

r

Gmm

S

Intraformational sandstone-clast breccias:

massive, matrix-supported

Granule- to pebble-grade clasts: red sandy

siltstone and mudstone

Exclusive to facies VII, rare in facies II

Hyperconcentrated flood flow deposits

R

F

V

f

R

Sc

F

Clast-rich fine sandstones

Granule- to pebble-grade clasts: red sandy

siltstone and mudstone

Exclusive to facies VII, rare in facies II

Hyperconcentrated flood flow deposits

M

F

v

P

c

C

m

O

s

St F

Trough cross-bedded sandstone

Medium to fine-grained sand

Common in facies I, common in facies III,

virtually absent in facies II

Sinuous-crested (3D) dunes

H

F

v

M

u

O

s

Sp

Planar cross-bedded sandstone

Medium to fine-grained sand

Common in facies I, rare in facies II and III

Straight-crested (2D) dunes

Sm

Massive sandstone

Medium- to very fine-grained sand

Common in facies I–VI

Deposition from hyperconcentrated flows and/or destruction

of primary structures by bioturbation, liquefaction, etc.

throughout the exposure area in the main Karoo Basinin South Africa. Firstly, small-scale lithofacies units

were defined (Table 1), and combined into larger-scale

facies associations (architectural elements) (Table 2).

The field data, in the form of 25 sedimentary logs

low-angle cross-bedded sandstone

edium to very fine-grained

are in facies I and III, fairly common in facies

I

umpback or washed-out dunes, antidunes

h

orizontally laminated sandstone

ften associated with parting lineations and

ebris-filled flutes

edium to very fine-grained sand

are in facies I, dominant in facies II, common

n facies II and V

lane-bed from fast, shallow flow (upper flow

egime)

r

ipple cross-laminated sandstone

ine to very fine-grained

irtually non-existent in facies I, dominant in

acies II, common in facies II and V

ipples (lower flow regime)

sm

assive mudstones

ine silt and clay grain-size classes, very rare

ery coarse silty or sandy mudstones

edogenically altered (very rarely in LEF, but

ommonly in UEF mudstone units)

ommon in facies IV and VI and in all

udstone units, rare in facies I

verbank and/or waning flow deposits in

tanding waterbodies or abandoned watercourses

l

orizontally laminated mudstones

ine silt and clay grain-size classes, very rare

ery coarse silty or sandy mudstones

ore common in UEF than in LEF mudstone

nits, present in facies VI, rare in facies II

verbank and/or waning flow deposits in

tanding waterbodies or abandoned watercourses

Table 2

Facies associations and identified architectural elements of the Elliot Formation (modified after Miall (1985, 1996))

Facies type Major lithofacies Other sedimentary fea-

tures

Geometry Occurrence Architectural element

A. Lower Elliot Formation

Facies Ia St, Sp, Sm and Sm with

minor Gmcm, Sl, Sh and

Fsm

Parting lineations and

debris-filled flutes; very

rare (virtually non-exis-

tent) soft sediment

deformation, ripple

cross-lamination, bio-

turbation

Asymmetrical channel-

shaped units with steep

cutbanks on one side;

few tens of metres wide

(max. 100–150 m), few

metres (max. 20–25 m)

high; wedge out later-

ally against erosive,

concave-up basal

bounding surfaces;

multi-storey; isolated in

mudstone units; sharp

lower and fairly sharp

upper bounding sur-

faces

Throughout the LEF Channels (element CH)

St, Sp and Sm with

minor Gmcm, Sl, Sh and

Fsm

Very rare (virtually

non-existent) soft sedi-

ment deformation, rip-

ple cross-lamination,

bioturbation

Separated by lateral

accretion surfaces

Within element CH

throughout the LEF

Lateral Accretion Mac-

roforms (element LA)

Mainly St and Sp Bound by semi-hori-

zontal, concave-up sur-

faces

Within element CH

throughout the LEF

Sandy Bedforms (ele-

ment SB)

Facies IIa Fsm and rare Fl Rare pedogenic altera-

tions (irregular mottles,

few desiccation cracks

and rare calcareous

glaebules)

Average 20–30 m thick;

laterally fairly persistent

Throughout the LEF Flood Plain Fines (ele-

ment FF)

Facies IIb Sr, Sm and Fl, Fsm Small, asymmetrical

channel-shaped units

with laterally accreted

layers

Very rare, through-

out the LEF

Small Channels of

Flood Plain Area (ele-

ment CHS)

Facies IIc Sm, Sh, Sr Tabular sandstones and

siltstones; sharply-

bound straight lower

and uneven upper sur-

faces; thinning upward

successions

Always close proxim-

ity to major chan-

nelized sand bodies;

throughout the LEF

Crevasse Splays (ele-

ment CS)

Facies IId Sm and Fsm/Fl Rhythmically-bedded

units of thin sandstones

and mudstones

Immediate vicinity of

the major LEF chan-

nel sand bodies; only

in the LEF

Levees (element LV)

B. Upper Elliot Formation

Facies Ib Sm, Sh, Sr and Sl with

minor Gmcm, Gcm, Gh,

Gp, Gt, Fl and Fsm,

very rare Sp, St

Flaser and wavy bed-

ding, mud-draped sur-

faces, parting lineations,

small-scale soft sedi-

ment deformations

(water escape struc-

tures, load casts), desic-

cation cracks and

bioturbations

Sheet sand bodies; sev-

eral tens of metres

(>100 m) wide,

maximum 5–6 m high;

multi-storey; laterally

continuous; sharp

bounding

surfaces

Throughout the UEF Laminated Sand Sheets

(element LS)

Facies Ic St or combination of

Sh/Sl and Sr

Very rare mud-draped

surfaces

Simple, lenticular-

shaped sand bodies with

laterally

extensive wings

Only upper part of

the UEF

Channels (element CH)

388 E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400

Table 2 (continued)

Facies type Major lithofacies Other sedimentary fea-

tures

Geometry Occurrence Architectural element

Facies IIa Fsm and Fl Animal trace fossils,

carbonized and

calcretized root traces,

calcareous concretions,

septarian nodules,

large-scale calcretized

and clay-lined

shrinkage cracks, irreg-

ular mottles,

desiccation cracks and

falling-water

level marks

Average 0.5–10 m thick;

laterally

persistent

Throughout the UEF Flood Plain Fines (ele-

ment FF)

Facies IIb Sr, Sm and Fl, Fsm Small, asymmetrical

channel-shaped units

with laterally accreted

layers

Very rare, through-

out the UEF

Small Channels of

Flood Plain Area (ele-

ment CHS)

Facies IIc Sm, Sh, Sr Tabular sandstones and

siltstones; sharply

bounded straight lower

and uneven upper sur-

faces;

thinning upward

successions

Always close proxim-

ity to major chan-

nelized sand bodies;

only upper part the

UEF

Crevasse Splay

(element CS)

Facies IIe Gmm and Sc Massive very fine to

fine-grained sandstones

Lens or irregularly

shaped; isolated bodies;

0.2–2 m wide; <0.25–1

m thick

Rare association with

UEF sand bodies,

fairly common within

UEF mudstone units

Sediment Gravity Flow

Deposits

(element SG)

E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400 389

(Figs. 1 and 2), field sketches, photomosaics and maps,

were integrated in order to clarify the three-dimensional

relationships of the units of the Elliot Formation.

4. Description

In the southern outcrop area, the higher number of

sandstone bodies in the LEF, and the sharp grain-size

contrast between the more resistant medium- to fine-

grained sandstone bodies and intermittent mudstone

units of the LEF result in characteristic terraced hills-

lopes. On the other hand, the moderate grain-size con-trast between the more resistant fine to very fine, silty

sandstone bodies, and intermittent mudstone units of

the UEF, result in characteristic smooth hillslopes which

give a false impression of a ‘‘muddy’’ UEF sequence.

The grain size variations result in characteristic weath-

ering patterns, and allow for a simple differentiation

between the LEF and UEF sequences in the outcrops of

the southern region (Fig. 2). The higher coarse- vs. fine-grained sediment ratio of the UEF was also reported by

Botha (1968, p. 112) who states that the ratio is 40:60 in

the LEF and 46:54 in the UEF in the southern area. The

grain-size trend of the UEF shows a slight coarsening

from the predominantly fine to very fine sandstones of

the lower succession into fine- to medium-grained

sandstones towards the upper part (Fig. 2B, C and E).

However, parallel to the main depositional palaeoslope,

from south to north, the average thickness of the se-

quence (Bordy et al., in press a) and the number of the

LEF sandstone bodies decrease (Fig. 2A–C vs. D–G),

and thus in the northern outcrop areas the LEF is less

than 30–40 m thick, and often devoid of sandstonebodies (Fig. 2F and G).

4.1. Sandstone facies associations

The Lower Elliot Formation is characterised by ma-

jor channel-shaped sandstone bodies (facies Ia) (Table

2) that are a few tens of metres in lateral extent (max.

100–150 m) and a few metres (max. 20–25 m) in height.

Channel-fills wedge out laterally towards erosive basal

bounding surfaces which are concave-up, with reliefs of

about 5–10 m. In certain outcrops, the basal bounding

surfaces are deeply incised (Plate 1A). Although theoverall appearance of the sandstone bodies is lenticular,

in extensive outcrops an asymmetrical cross-section is

often visible (Fig. 3) with clearly identifiable channel

margins. In such cases, a steep cutbank occurs on one

margin of the sandstone body, and a set of gently in-

clined, thinner sandstone and mudstone strata on the

Plate 1. (A) Deeply incised scour hollows (element HO) at the base of a major channel-shaped sandstone body (facies Ia) in the Lower Elliot

Formation. Note the fairly sharp upper bounding surfaces of the major sandstone body (Barkly Pass, north of Elliot). Person for scale (1.92 m). (B)

Mud-pebble conglomerate (lithofacies Gmcm), and carbonized wood fragment-filled gutter cast on lower bedding plane of medium-grained, massive

sandstone (lithofacies Sm) in the Lower Elliot Formation (Vegkop farm, east of Zastron). See Table 1 for lithofacies description and interpretation.

Lens cap for scale (5.8 cm). (C) Sheet sand bodies and interbedded mudstones of the Upper Elliot Formation (facies Ib) in a roadcut on the R392

road (Ono farm, SE of Lady Grey). Vehicle for scale (�2 m). (D) Virtually flat internal erosion surfaces within horizontally laminated (lithofacies Sh)sheet sand bodies (facies Ib) in a roadcut on the R56 road (Withoogte farm, south of Lady Grey). See Table 1 for lithofacies description and

interpretation. Hammer for scale (28 cm). (E) Amalgamated lenses of sandstones in the uppermost part of the Upper Elliot Formation (facies Ic)

along the slopes of the Aasvoelkrans hill (Mooifontein farm, north of Zastron). Hill is capped by Clarens Formation cliffs. Total height of hillslopes

�250 m. (F) Wavy bedding and mud-draped surfaces associated with ripple cross-laminated sandstones (facies Ib) (Withoogte farm, south of Lady

Grey). Tape measure for scale (50 cm).

390 E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400

other side that grade laterally into pure mudstone. The

major sandstone bodies comprise multiple sandstone

beds separated by numerous laterally non-persistent

internal erosion surfaces with a relief of �1 m, giving

them a multi-storey appearance. Some of the internal

erosion surfaces are gently inclined towards the thickest

part of the sandstone bodies (Fig. 4). Other surfacesare semi-horizontal or slightly concave-up. Within the

sandstone units, well-developed upward-fining units

(Fig. 5), commencing with mud-pebble conglomerate

lags (Plate 1B), are rarely capped by extensive mud-

stones. The upper bounding surfaces of the major

sandstone bodies are horizontal and fairly sharp (Plate

1A), rarely showing gradational contacts with the

overlying mudstone units.

The sedimentary structures within these majorchannel-shaped sandstones of the LEF (facies Ia) are

predominantly trough (St) and planar (Sp) cross-strati-

Fig. 3. Cross-section of an asymmetrical channel fill with steep cutbank on the right (facies Ia) in the Lower Elliot Formation (Baffels Fontein farm,

south of Jamestown, east of the junction of N6 and R56 roads). Person indicated by an arrow for scale (1.92 m).

Fig. 4. Channel-shaped, multi-storey sand bodies (facies Ia) with numerous laterally non-persistent, gently inclined internal erosion surfaces are

typical of the Lower Elliot Formation. Some well-developed point bar successions are present (east of the junction of N6 and R56 roads, Palmiet

Fontein farm). Person for scale (1.92 m).

E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400 391

fication, massive beds (Sm) and less commonly low-an-gle cross-stratification (Sl) and horizontal lamination

(Sh) (Table 1). Deposits bound by semi-horizontal or

slightly concave-up surfaces are dominated by lithofa-

cies St and Sp. Stringers and beds less than 0.2 m thick

of mud-pebble conglomerates (Gmcm) (Plate 1B) are

common, but pedogenic glaebule conglomerates are

absent. Parting lineations (Plate 1C) and gutter casts

filled with mudstone clasts and wood-fragments (Plate1D) are present. Soft sediment deformation structures,

ripple cross-lamination and bioturbation are a rare

occurrence.

The sheet sandstones of the Upper Elliot Formation

are different to these of the LEF; however, a few len-

ticular-shaped sandstone bodies are present, especially

in its uppermost part (see below described as facies Ic).

The most abundant sandstone bodies have a sheet-likegeometry several tens of metres wide (average >100 m),

and a maximum 5–6 m high (facies Ib) with individual

sandstone bed thicknesses of 0.2–1 m (Plate 1E). Both

basal and upper bounding surfaces are laterally persis-

tent, sharp and parallel to each other, lacking topo-

graphic irregularities larger than a few tens of

centimetres. Laterally, the margins of these tabular sheet

sandstone bodies are difficult to identify. Internally, theycontain virtually flat internal erosion surfaces which are

similar in geometry to the basal bounding surface and

give the units a multi-storey appearance (Plate 1E and

F). Interbedded mudstones are finely laminated or

massive and seldom exceed 0.5 m in thickness (Plate 1E

and F). Locally, the frequency of these intercalated

mudstone layers is so high that it becomes difficult to

define the individual sheet sandstone bodies.Channelized sandstone bodies (facies Ic) in the UEF

are exclusive to the uppermost part of the sequence (Fig.

5), and form simple, lenticular-shaped features with

laterally extensive, 0.5–1 m thick wings which taper

away from the <8 m thick medial parts of the fine- to

medium-grained sandstone bodies. Locally, facies Ic

may display amalgamated lenses of sandstones (Plate

1G) with total thickness up to 15 m. Lateral accretionsurfaces have not been documented, and the internal

erosion surfaces are semi-horizontal (Fig. 6). The dis-

tribution of facies Ic is limited to the southern outcrop

areas with its northernmost occurrences in the region

between Lady Grey and Zastron.

The sheet sandstone bodies of the UEF (facies Ib) are

characterised structured by the following sedimentary

Fig. 5. Channelized sand bodies (facies Ic) are found only in the uppermost part of the Upper Elliot Formation. Filled by medium-grained sand-

stones, the simple, lenticular-shaped features have wings which taper away from the thicker medial parts (Withoogte farm, south of Lady Grey).

392 E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400

structures: massive beds (Sm); horizontal lamination

(Sh) (Plate 1F and H); ripple cross-lamination (Sr)

(Plate 1H) and low-angle cross-stratification (Sl).Trough cross-stratification is virtually absent, except in

lenticular-shaped sandstone bodies (facies Ic) that oc-

cur at the top of the UEF. The most common lithof-

acies combination in both sheet and lenticular

sandstone bodies is the association of horizontal-lami-

nation and ripple cross-lamination. Other sedimentary

structures observed in facies Ib are parting lineation,

flaser and wavy bedding and mud-draped surfaces(Plate 1H), small-scale soft sediment deformations (e.g.,

water-escape structures, load casts) (Plate 2A), desic-

cation cracks (Plate 2B) and bioturbation (both within

beds and on mud-draped upper bedding planes) (Plate

2C). Mud-pebble conglomerates (Gmcm) commonly

form stringers at the base of thicker sandstones, but

Fig. 6. Virtually flat internal erosion surfaces within a channelized sand bo

(Withoogte farm, south of Lady Grey). Person for scale (1.92 m).

thicker (up to 0.5 m) beds were occasionally encoun-

tered. Locally, these rip-up mudstone clasts exhibit

imbrication, and contain very rarely single, ±1 cmdiameter, rounded quartz pebbles (Fig. 7). In the

channel lags of the UEF sandstones, pedogenic glae-

bule conglomerates are quite common, especially in the

northern outcrop areas. They consist of granule- to

pebble-sized, white, well-rounded carbonate nodules,

septarian nodules, white to reddish, subangular to

subrounded mudstone clasts, red, subangular to sub-

rounded sandstone clasts, reddish, subangular to sub-rounded isolated, broken and abraded fossil bone and

teeth fragments, and very rare red to white, sub-

rounded to rounded quartz pebbles. Most conglomer-

ates lack stratification (Gcm) (Plate 2D), though some

display slight horizontal layering (Gh) or cross-strati-

fication (Gp, Gt).

dies (facies Ic) in the uppermost part of the Upper Elliot Formation

Plate 2. (A) Small-scale soft sediment deformation: load casts in the Upper Elliot Formation (facies Ib) sandstones (Fraaiuitsig farm, north–

northeast of Clocolan). Lens cap for scale (5.8 cm). (B) Massive, clast-supported pedogenic glaebule conglomerates (lithofacies Gcm). Other beds

display slight horizontal layering (Gh) or cross-bedding (Gp, Gt) (Leliekloof farm, east of Jamestown). See Table 1 for lithofacies description and

interpretation. Lens cap for scale (5.8 cm). (C) Calcretized root traces in the Upper Elliot Formation mudstone (Beatrix farm, northeast of Clocolan).

Hammer for scale (28 cm). (D) Calcareous concretions in the Upper Elliot Formation mudstone (La Roche farm, west of Ladybrand). Small tree for

scale in the foreground (1.70 m). (E) Large-scale calcretized shrinkage cracks (often clay-lined) in the Upper Elliot Formation mudstone (Aurora

farm, northeast of Ladybrand). Lens cap for scale (5.8 cm). (F) Red intraformational sandstone clast breccias (lithofacies Gmm) in the Upper Elliot

Formation mudstone (Damplaats farm, southeast of Ladybrand). Together with clast-rich sandstones (lithofacies Sc), this lithofacies forms lenticular

or irregularly shaped, narrow, isolated bodies (facies II). See Table 1 for lithofacies description and interpretation. Lens cap for scale (5.8 cm).

E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400 393

4.2. Mudstone facies associations

The sandstone bodies of the Lower Elliot Formationare separated by laterally persistent mudstone intervals

(facies IIa) on average 20–30 m thick. Sedimentary

structures are rare, and most of the mudstones are

massive (Fsm), or very rarely horizontally laminated

(Fl). The grain sizes are predominantly in the fine silt

and clay grain-size classes (very coarse silt or sand grains

are very rare). Pedogenic alteration is rare in the south,

and even in the north, it remains restricted to irregularmottles, a few desiccation cracks, and rare calcareous

glaebules. Small, asymmetrical channel-shaped succes-

sions (Fig. 8), with the contained mudstone (Fl, Fsm)

and very fine-grained sandstone (Sr, Sm) beds (facies

IIb) that dip towards the thickest part of the channels,

are rarely encountered in the LEF mudstone units bothin the south (e.g., Kraamberg, northwest of Jamestown)

and north (e.g., Fraaiuitsig farm, north–northeast of

Clocolan). Bioturbation is almost absent, with very few

unlined, tube-like burrows (Planolites isg.). Occasion-

ally, within the thick mudstone units, 0.2–1.2 m thick,

tabular layers of medium- to very fine-grained sandstone

and siltstone are also found (facies IIc) (Fig. 9). These

strata, which are laterally continuous for several tens ofmetres, have sharp contacts, but while their basal con-

tacts are straight, their upper contacts are often irregu-

lar. The thickness of the succeeding beds gradually

Fig. 7. Single, <1.5 cm diameter, rounded quartz pebbles (marked

with arrow) are occasionally found in both mud-pebble and pedogenic

glaebule conglomerates in the Upper Elliot Formation. Note reworked

bone fragment (circled) and pedogenic glaebules (Dam Plaatz farm,

southeast of Ladybrand). Lens cap for scale (5.8 cm).

394 E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400

decreases, forming thinning upward successions. FaciesIIc is internally structured by the following lithofacies:

massive (Sm), horizontal laminated (Sh) and ripple

cross-laminated (Sr) sandstone. In a few localities (e.g.,

Barkly Pass––north of Elliot, Baffels Fontein farm––

south of Jamestown), in the immediate vicinity of the

major channel-shaped sandstone bodies, the mudstones

contain <1.2 m thick, rhythmically bedded units of thin

(<0.3 m) medium to fine, massive sandstones (Sm) andlaminated to massive (Fl/Fsm) mudstones (facies IId).

Fig. 8. Small, asymmetrical channel-shaped successions (facies IIb). The con

side). Such small channels are found engulfed in overbank mudstones in the L

Person for scale (1.8 m).

Fig. 9. Tabular beds forming upward thickening successions (facies IIc) in m

Vehicle for scale.

In contrast to the sandstones of the LEF, the sand-stone sheets of the UEF are separated from each other by

mudstone units 0.5–10 m thick (facies IIa). In composi-

tion, the mudstones range from pure mudstone to muddy

fine sandstone or siltstones. Most of the beds are massive

(Fsm), but horizontal lamination (Fl) is more common

than in the LEF mudstones. Although more reduced in

vertical dimension, trace fossils, carbonized and cal-

cretized (Plate 2E) root traces, calcareous concretions(Plate 2F), septarian nodules, large-scale calcretized and

clay-lined shrinkage cracks (Plate 2G), irregular mottles,

small-scale and sandstone-filled desiccation cracks, and

falling-water level marks are more abundant in the UEF

mudstones than the LEF mudstone. Small, asymmetrical

channel-shaped successions (facies IIb) are similarly rare

features in the UEF as they are in the LEF mudstones

(Fig. 10). The laterally continuous, tabular sandstoneintercalations (facies IIc) described above are virtually

absent from the lower part of UEF mudstone units, but

they are quite common in the upper part, especially in

association with coarser grained, more channelized

sandstone units with lenticular geometries (facies Ic).

Locally, smaller (max. 1.5 m thick) coarsening-upward

units are present which commence with fine-grained,

ripple cross-laminated sandstones and terminate in ma-trix-supported mud-pebble conglomerates.

Red intraformational sandstone clast breccias (Gmm)

(Plate 2H) and clast-rich sandstones (Sc) (facies IIe), are

tained layers dip towards the thickest part of the channels (right hand

ower Elliot Formation (Fraaiuitsig farm, north–northeast of Clocolan).

udstones of the Lower Elliot Formation (Barkly Pass, north of Elliot).

Fig. 10. Small, asymmetrical channel filled by laterally accreted, thin layers of clay- and siltstone (facies IIb) in the Upper Elliot Formation (Barkly

Pass, north of Elliot). Hammer for scale (28 cm).

E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400 395

associated with massive very fine to fine-grained sand-

stones (Sm), and occur only in the UEF. These litho-facies are rarely found in UEF sandstone bodies, and

fairly often within UEF mudstone units. Lithofacies

Gmm and Sc form lenticular or irregularly shaped,

narrow (0.2–2 m), isolated, <0.25 to 1 m thick bodies,

and consist of red sandy siltstone and mudstone clasts,

but lack carbonate glaebules. These granule- to medium

pebble-sized particles are angular, have a red clay-film

coat and are set randomly in the mud-rich matrixforming mostly matrix- and rarely clast-supported fab-

rics. Petrographic studies (Bordy et al., in press b) of the

associated massive very fine to fine-grained sandstones

(Sm) showed that the lithofacies contains variable

amounts of wedge-shaped, silt-sized quartz grains.

5. Interpretation

The subdivision of the sandstones of the Elliot For-

mation presented here is comparable with that of Visser

and Botha (1980) and Eriksson (1985). The majorasymmetrical channel-shaped LEF sandstones (facies

Ia), with their medium- to fine-grained sandstones, are

analogous to Visser and Botha’s (1980) Type A and B

sandstones, and to Eriksson’s (1985) Facies 2. On the

other hand, the sheet-like UEF sandstones (facies Ib)

with their fine- to very fine-grained beds, are similar to

Visser and Botha’s (1980) Type C1 and C2, and Eriks-

son’s (1985) Facies 3. The stacked, lenticular-shapedsandstone bodies (facies Ic) in the upper part of the UEF

are similar to those classified as Type D sandstones by

Visser and Botha (1980). On the other hand, the above

subdivision of the mudstones of the Elliot Formation is

only partially comparable with the facies classification

of previous researchers, as Visser and Botha (1980) do

not define their mudstone facies, and Eriksson (1985)

groups all the muddy units in his Facies 3. Nevertheless,the laterally extensive sandstones (facies IIc) seem to fit

Visser and Botha’s (1980, Fig. 6) Type C2 facies

description (C2 has ‘‘definite, sharp to slightly transi-

tional upper contact’’ as opposed to facies IIc which

shows sharp, uneven upper contact). The very rare,small coarsening-upward units of the UEF are compa-

rable with Visser and Botha’s (1980) facies Type C3.

The sandstone units of the Lower Elliot Formation

are interpreted as deposits of moderately meandering

channels with various subenvironments in the overbank

area where mudstones were accumulated. In the Upper

Elliot Formation the laterally persistent sheet sandstone

bodies are interpreted as distal sheetflood deposits (sensuHogg, 1982), while the more channelized facies at the

top of the UEF are interpreted as predominantly

straight, single-thread, incised channels produced by

successive streamfloods (sensu Hogg, 1982). The mud-

stone units of the UEF are interpreted as distal flood-

plain deposits.

Using the method of architectural element analysis

(Miall, 1985, 1996), several three-dimensional buildingblocks were identified in the alluvial suite of the Elliot

Formation. The individual lithofacies and architectural

element interpretations, based on Miall’s (1978, 1996)

letter coding system, are summarized in Tables 1 and 2.

5.1. Meandering channel deposits

The major asymmetrical channel-shaped sandstone

bodies (facies Ia) in the LEF represent element CH

(channels) and contain further components such as

lateral accretion elements (LA) bound by lateral

accretion surfaces, as well as rare sandy bedform ele-ments (SB) which are bound by semi-horizontal, con-

cave-up surfaces. The asymmetrical channels, and the

contained architectural elements, are likely to represent

lateral accretion of pointbars, and vertical aggradation

of 2D and 3D dune fields in a mixed-load, meandering

system (e.g., Alien, 1965; Miall, 1985, 1996). The steep

cutbanks indicate that the channels were incised into

resistant mudstone units which were probably stabi-lized by riparian vegetation (Bridge, 1985) as evi-

denced, for example, by the fossil wood found in these

units. The cohesive nature of the mudstones is also

396 E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400

shown by the presence of rip-up mud-pebble con-glomerates (Gmcm). The large irregularities at the base

of larger channels might be interpreted as sites of

channel/tributary confluences where significant scour

took place. Similar features were observed by the first

author at confluences of active meandering rivers in

Hungary (Tisza River and its tributaries). Miall (1996)

named identical features as element Hollow (HO), and

explained them in similar fashion (Best, 1987, in Miall,1996: Fig. 6.45).

5.2. Sheetflood deposits

The laterally continuous, sheet sandstone bodies (fa-

cies Ib) of the UEF are identified as laminated sand sheetelements (LS) and interpreted as vertically stacked

products of high-energy, but rare and short-lived depo-

sitional events, in an otherwise low-energy environment

probably characterised by gentle depositional surfaces.

The individual sandstone sheets within the larger sand-

stone bodies formed in unconfined sheetfloods (sensu

Hogg, 1982) in either single events with multiple peaks,

or during several short-lived flood events separated byperiods of non-deposition and desiccation. These periods

of desiccation are also evidenced by the rip-up mud-

pebble conglomerates (Gmcm), with the mud-pebbles

thought to have survived the rigour of the short trans-

portation processes due to their desiccated, compacted

nature (Stear, 1985). The reworked, carbonate glaebules

found as lag conglomerates in the UEF sheet sandstone

bodies are very early diagenetic in origin (Bown andKraus, 1981 and references therein) and are a clear

demonstration of the fact that calcrete-bearing paleosols

were indeed more abundant during UEF than LEF

times, and that the sedimentation styles (striping/skim-

ming of floodplain by channels in UEF vs. more fixed

channels in LEF) were different.

5.3. Low sinuosity stream flood deposits

The lenticular sandstone bodies with laterally exten-

sive wings (facies Ic) in the uppermost part of the UEF

are identified as simple channels (CH). Their sedimen-

tary structures (either St or Sh/Sr combination) and the

lack of inclined internal erosion surfaces suggest that

they are vertically stacked products of confined, high-energy streamfloods (sensu Hogg, 1982) with relatively

straight courses.

5.4. Floodplain deposits

The mudstones units with contained sandy depositsare identified as various architectural elements of the

overbank environment. The mudstones of both the LEF

and UEF are regarded as floodplain fines (FF) formed

in standing water bodies or abandoned watercourses.The vertical and lateral distribution of the pedogenically

overprinted mudstone units and their significance are

explained later.

5.5. Triburary/distributary channels

Small, asymmetrical channel-shaped deposits (faciesIIb), with laterally accreted beds, found both in LEF and

UEF mudstones, are identified as small, secondary, sin-

uous channels of the overbank areas (element CHS). Due

to their small sizes, which are not comparable to the

major channels, and isolated nature, they are unlike

products of chute or neck cutoffs (element CH(FF) of

Miall, 1996). Small reticulate channels have been re-

ported by Knighton and Nanson (1997) from the back-swamps of the Channel Country (Australia), but their

sedimentology was not accounted for and the modern

analogue of facies IIb channels in the Elliot Formation

thus remains enigmatic. The mudstone fills of the UEF

small channels (e.g., Fig. 10) are thought to have been

deposited as sand-sized mud aggregates, an apparently

common mode of deposition in the modern dryland

fluvial systems of Australia (Rust and Nanson, 1989).

5.6. Crevasse splay deposits

The sharply bounded, tabular sandstones and silt-

stones (facies IIc) with uneven upper surfaces are

interpreted as crevasse splay deposits (element CS)

based on their geometries, internal sedimentary struc-tures and close proximity to major channelized sand-

stone bodies, both in the LEF and upper parts of the

UEF. The uneven upper boundaries are thought to

represent scouring by minor overbank sheetfloods in

which the flow conditions diminished at such fast rates

that bedforms could not develop. Similar uneven upper

boundaries of crevasse splay deposits are reported by

Stear (1983, Fig. 12) and are explained as products ofscouring by ‘‘anastomosing rivulets that flowed across

the splays’’.

5.7. Levee deposits

Rhythmically bedded units of sandstones and mud-

stones (facies IId) in the immediate vicinity of the majorLEF channel sandstone bodies are identified as rare le-

vees (element LV) which formed along the banks of the

major, temporarily fixed channels, that were probably

abandoned fairly rapidly (Brierley et al., 1997).

5.8. Colluvial mass movement deposits and associated

reworked aeolianites

The association of matrix-supported intraformational

sandstone breccias (Gmm) and clast-rich sandstones

E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400 397

(Sc) (facies IIe) of the UEF are interpreted as probablepassive colluvial fills of smaller, rainstorm-eroded gullies

and other irregular depression of the floodplain area

(element SG: sediment gravity flow deposits). Similar

lithofacies associated with semi-arid alluvial environ-

ments were described by Steel (1974) and Hartley (1993,

Facies association A) and interpreted as deposits of

hyperconcentrated flood flow, an intermediate between

normal stream and debris flows. The massive very fine tofine-grained sandstones are interpreted as reworked

aeolian deposits based on their petrographic properties.

Similar petrographic observations and the presence of

reworked aeolian (loess) deposits in the Elliot Forma-

tion in the north-eastern outcrop area (Fig. 1) were

documented by Eriksson (1983, 1985) as well.

6. Fluvial style variations

The narrow, moderately meandering channels of the

LEF were relatively stable rather than rapidly migrating(laterally). The limited lateral extent, stacked appear-

ance, fairly defined upper boundaries, poorly developed

levee deposits, and their total envelopment in floodplain

fines, are collectively taken as being indicative of rela-

tively rapid channel abandonment and relocation due to

avulsion (Leeder, 1978; Brierley et al., 1997). The low

frequency of the crevasse splay deposits might be due to

protective riparian vegetation, which prevented breach-ing of the river banks, and promoted the confinement of

the channels (Bridge, 1985; Brierley et al., 1997). The

presence of lateral accretion surfaces, and unimodal

palaeocurrents in the adjacent sandstone units (Bordy et

al., in press b), show that the meandering rivers were

only moderately sinuous. The relatively constant dis-

charge of the perennial, moderately sinuous rivers of the

LEF is indicated by the presence of small pointbars(gradual decrease of flow strength), and the absence of

pulsating discharge indicators such as upper flow regime

horizontal lamination coupled with ripple cross-lami-

nation (McKee et al., 1967; Stear, 1985; Langford and

Bracken, 1987; etc.), flaser bedding (Martin, 2000) and

desiccated mud-drapes, and rare pedogenic overprint-

ing. The depositional environment of the LEF maybe

summarized with some modification by the classicalarchitectural model of meandering rivers as presented in

Miall (1996, Fig. 8.34). One of the differences between

the sand-bed meandering stream model of Miall (1996)

and the LEF environment is that the latter streams were

less sinuous with less lateral migration.

Vertical accretion in a flood-dominated, low-sinuos-

ity fluvial system with wide, shallow and flat-based

watercourses and highly variable, pulsatory discharges,is envisaged during the deposition of the UEF. This

scenario is interpreted from the virtually flat external

and internal erosion surfaces, lateral continuity of

depositional units, dominance of couplets of upper andlower flow regime sedimentary structures (no interme-

diate flow structures like Sp or St), consistent palaeo-

currents (Bordy et al., in press b) within and in the

succeeding sheet sandstone bodies as well as lack of

pointbars, lateral or downstream accretion surfaces,

channel margins and crevasse splays. In addition, some

channelized sandstone bodies with largely similar

lithofacies associations (Sh/Sr) (e.g., the southern out-crops of the uppermost UEF) show that channel inci-

sion took place in the proximal parts of the depositional

slope.

A semi-arid environment is suggested by the presence

of calcrete-rich soil profiles (e.g., in situ and reworked

pedogenic carbonate glaebules and rhizoliths), desicca-

tion cracks, rubification in the mudstones (due to oxi-

dation of iron compounds above ground water level),intraformational conglomerates/breccias and the well

documented aeolian interbeds (e.g., Botha, 1968; Le

Roux, 1974; Visser and Botha, 1980; Eriksson, 1983,

1985; Smith and Kitching, 1997) in the uppermost UEF.

These soil profiles were often partially removed by

subsequent floods, incorporating the reworked, resistant

remnants into the bases of laterally extensive sand

sheets. The above interpretations are in line with mod-ern hydrological and geomorphological observations of

semi-arid fluvial systems (McKee et al., 1967; Stear,

1985; Langford and Bracken, 1987; Knighton and

Nanson, 1997; Reid and Frostick, 1997). In present

times, such systems are typical in regions with low an-

nual precipitation, especially in subtropical areas (e.g.,

North Africa), continental interiors (e.g., North Amer-

ica), along the western coasts of continents (e.g.,southern Africa, Australia, South America), and in rain

shadow areas, behind topographic barriers (e.g., South

America) (Nanson et al., 2002).

The stratigraphic distribution of facies Ib (unconfined

sheetflood deposits) and facies Ic (confined streamflood

deposits) within the UEF is suggestive of a flood-driven

ephemeral system, with a northward-prograding ten-

dency. With facies Ib as the distal and facies Icas the proximal part of this ephemeral system, the

contemporaneous orogenic unloading of the Cape Fold

Belt resulted in progressively steeper depositional slopes

(Bordy et al., in press a), which in turn caused the

overstepping of the distal facies Ib by the proximal facies

Ic.

In contrast to sandy meandering rivers (e.g., Jackson,

1978; Bridge, 1985; Miall, 1985, 1996), depositionalmodels of ephemeral fluvial systems of dry-land envi-

ronments (Fielding, 1999) are not well established. The

imbalance in knowledge between the existing models is

mainly due to the fact that semi-arid fluvial systems are

less accessible, and more unpredictable, than their

perennial counterparts, thus the monitoring of such

systems is challenging (Tooth and Nanson, 1995;

398 E.M. Bordy et al. / Journal of African Earth Sciences 38 (2004) 383–400

Knighton and Nanson, 1997). Although recent advancesin hydrological and geomorphological studies of dry-

land river systems, especially in Australia (e.g., Tooth

and Nanson, 1995; Thomas, 1997; Knighton and Nan-

son, 1997; Reid and Frostick, 1997), provide adequate

data for integration into depositional models, sedimen-

tological modelling of such systems is still in its infancy

due to the lack of empirical data on the preserved sed-

imentary structures and of the architecture of theseephemeral fluvial deposits. The sedimentary structures

and architectures of the Upper Elliot Formation have

much in common with those described from similar

studies of ancient ephemeral stream deposits (e.g.,

McKee et al., 1967; Steel, 1974; Alien and Williams,

1979; Turner, 1981; Sneh, 1983; Stear, 1983, 1985;

Tunbridge, 1984; Flint, 1985; Langford and Bracken,

1987; Dreyer, 1993; Luttrell, 1993; Olsen and Larsen,1993; Miall, 1996; Martin, 2000). Unfortunately, the

depositional models tend to be simplistic, and more

importantly, these models are, without exception, based

on examples that are areally restricted, and thus are not

really applicable for vast areas like major foreland ba-

sins. For instance, the depositional models of sandy

ephemeral streams and clay playa complexes (Tun-

bridge, 1984), distal and flashy, ephemeral sheetfloodrivers (Miall, 1996), terminal fans or fluvial distributary

systems (Friend, 1978; Parkash et al., 1983; Sneh, 1983;

Kelly and Olsen, 1993, among others), alluvial fans

(Hartley, 1993) and floodouts (Tooth, 1999) all illustrate

areas that are much smaller than the c. 87500 km3 ero-

sional remnant of the Upper Elliot Formation. A com-

prehensive depositional model for the Upper Elliot

Formation is therefore not justified at this stage. Suchmodelling shall have to await more sophisticated and

flexible models, which incorporate empirical data from

studies of large modern systems.

7. Conclusions

A regional facies analysis of the Late Triassic–Early

Jurassic Elliot Formation has shown that these conti-

nental red beds can be subdivided into two informal

units, which have distinct lithologies, and are traceable

throughout the basin. The two units are the products oftwo different sedimentological regimes. The lower part

of the formation (Lower Elliot Formation) with its

asymmetrical, isolated channel sandstones and thick

mudstones is reconstructed as a perennial meandering

fluvial system. The upper part (Upper Elliot Formation)

with its sheet-like, laterally continuous sandstones and

interbedded mudstones is interpreted as an ephemeral,

flash flood-dominated fluvial system. Because of theirdistinct lithologies and stratigraphic relations, we sug-

gest that the above bipartite lithostratigraphic scheme of

the Elliot Formation be utilized as a basis for formal

member classification by the South African Committeefor Stratigraphy.

Apart from the above practical application of the

facies analysis of the Elliot Formation, the contrasting

characteristics of the sandstone bodies and associated

mudstones are also instrumental in the estimation of the

relative rates of subsidence that took place during the

deposition of the Elliot Formation (Bordy et al., in press

a). The LEF with incised sandstone bodies engulfed bythick floodplain deposits (avulsion and abandonment of

channels) and lack of pedogenic overprinting, suggests a

higher rate of subsidence. In contrast to this, the pres-

ence of wide, shallow scours, the lack of significant

down-cutting and extensive pedogenic modification of

the UEF mudstones suggest that the floodwaters pro-

ceeded in unconfined, shallow sheets rather than chan-

nels, evoking a picture of a low rate of subsidence whichenabled more significant pedogenic alterations of the

floodplain areas.

In contrast to previous concepts, it is proposed that

the fluvial style change that took place during the

deposition of the Elliot Formation was caused not only

by climatic, but also tectonic factors as well (Bordy

et al., in press a). The tectonic control is evidenced,

among others, by the fact that the units of the ElliotFormation have different palaeocurrent trends and

petrography (denudation patterns and provenance)

(Fig. 1A–C) as demonstrated by Bordy et al. (in press b).

We also suggest that the previously described climatic

change across the proposed Triassic–Jurassic boundary

promoted the preservation of finer grained sediments,

and reduced the amount of fluvial input, which again

contributed to the change in the alluvial architecture.

Acknowledgements

The manuscript was prepared while EMB was a Na-

tional Research Found (South Africa) post-doctoral re-

search fellow at the School of Geosciences, University of

the Witwatersrand. EMB would like to thank her hus-

band, Mamadou Diop, for his enthusiastic field assis-

tance and companionship. The authors wish to thank the

late Prof J.W. Kitching and Dr M.A. Raath for intro-ducing them to numerous fossil sites, and for the

countless and stimulating discussions regarding the pal-

aeoenvironmental conditions of the Elliot Formation.

We thank reviewers Drs C. Heubeck, M. Johnson and

R.M.H. Smith for their thoughtful comments on the

original manuscript. Also, special thanks to Prof P.G.

Eriksson for his exceptional editorial support.

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