Trans. geol. Soc. S. Afr.,87(1984), 169-179
TECTONOSTRATIGRAPHY, AS APPLIED TO ANALYSIS OF SOUTH AFRICAN
,
PHANEROZOIC BASINS
by
H. DE LA R. WINTER
"For now we see through a glass, darkly; but then face to face: now I know in part; but then I shall know even
as also I am known." 1 Corinthians 13: 12. K.l.V.
ABSTRACT
The case for recognition of tectono stratigraphy as an independent scheme of stratigraphic subdivision is
presented. These unconformity-bounded units of genetically related depositional facies are shown to be
the ideal stratigraphic scheme for basin analysis and for synthesis of the geological history of depositional
basins because tectonic control and depositional response can be related.
Models of tectonostratigraphic units are analysed, elucidating the principle that every depositional
discontinuity contains an isochronous element within its hiatus equal to the least break in time between
contiguous units. This principle enables the analyst to estabiish numerous relative time-surfaces through
any sedimentary basin without resorting to fossils or other methods of age-dating, thus creating the
essential time datums without which the palaeogeology cannot be accurately reconstructed. Practical
basin analysis is done from a combination of structural and chronostratigraphic sections across the basin.
A tectonostratigraphic unit is an unconformity-bound chronostratigraphic unit and is basin-restricted
unless the bounding unconformities are controlled by eustatic sea-level fluctuations.
It follows from the known hierarchy in dimensions of cyclic deposition and from chronostratigraphic
experience that a ranking of tectonostratigraphic units exist. Rank terms are recommended for formal use
of this stratigraphic scheme by SACS and that body will have to decide on the nature of the specific names
of units.
The Phanerozoic geological history is then reviewed employing tectonostratigraphic analytical
techniques on a regional scale to demonstrate the power of this analytical tool.
Obviously, these techniques could also be applied on a much smaller scale by sedimentologists
analysing the development and preservation of any environmentally and tectonically controlled
depositional facies.
CONTENTS
I. DEVELOPMENT OF TECTONOSTRATIGRAPHIC CONCEPTS. . . . . . . . . . . . . . . . .
A. Definition of a Tectonostratigraphic Unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Some Important Aspects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. ANALYSIS OF A TECTONOSTRATIGRAPHIC UNIT. . . . . . . . . . . . . . . . . . . . . . . . . .
A. Structural and Chronostratigraphic Basin Cross Sections. . . . . . . . . . . . . . . . . . . . . . . . .
B. Significance of Unconformities ..............................................
C. Time Significance of Depositional Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
III. RELATIONSHIP TO UNITS OF OTHER CLASSIFICATIONS . . . . . . . . . . . . . . . . . . .
IV. PROPOSED TECTONOSTRATIGRAPHIC SCHEME...........................
A. Cyclicity and Rank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Transgression and Regression ...............................................
C. Lateral Accretion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D . Nomenclature.............................................................
V. PHANEROZOIC APPLICATIONS.... . ... . . ...... . . . . . . . . . . . . ........ . . ... . .. .
A. Pre-Cape Events (Klipheuvel Sequence) ......................................
B. Cape Supersequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Karoo Sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Uitenhage Sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E. Post-drift Events ..........................................................
F. Synthesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VI. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ACKNOWLEDGMENTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES...............................................................
I. DEVELOPMENT OF TECTONOSTRATIGRAPHIC
CONCEPTS
Tectonostratigraphy is the study of depositional basins in
terms of the accumulation, periods of non-deposition,
erosion and deformation of its sedimentary and volcanic
rock fill in time progression. Tectonostratigraphic units are
tracts of genetically related depositional units stacked
cyclically into a hierarchy characterized by the increasing
hiatuses of their bounding unconformities. Sedimentary
basins are tectonically controlled, and so are their
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depositional fills; and tectonostratigraphy is thus the ideal
tool for the analysis of the tectonic control of
sedimentation.
The principle of tectono stratigraphy has recently been
brought to the notice of most South African geologists
(Winter, 1979a) after having been employed elsewhere in
various guises, as anew, therefore internationally still
informal, means of classifying sedimentary strata.
Examples are illustrated in publications by Wheeler
(1959a), Sloss (1963), Busch (1971a), Chang (1975),
170
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
Weimer (1975), Vail et al. (1977a, b, c), Brown and Fisher
(1977), and many others. Naturally, each have their
favourite informal approach in describing the units.
A. Definition of a Tectonostratigraphic Unit
A depositional sequence has been defined by Mitchum et
al. (1977) as "a stratigraphic unit composed of a relatively
conformable succession of genetically related strata and
bounded at its top and base by unconformities or their
correlative conformities". Others have referred to such
units as genetic, tectonic-genetic (Busch, 1971a, b) or
dynamic (Matthews, 1974). In South Africa the preferred
term is tectonostratigraphic.
The recent volume published by the South African
Committee for Stratigraphy (SACS) "Stratigraphy of
Southern Africa" (1980) serves to demonstrate that without
a time-based stratigraphic framework, an observational
descriptive classification such as lithostratigraphy cannot
be applied to stratigraphic synthesis. In Part I, therefore,
two concessions were made for the sake of better
understanding. A geochronological/chronostratigraphic
tabulation was employed as an introduction and
framework, and the tectonostratigraphic term, sequence,
was formally applied to units too large to be characterized
by a particular lithological aspect (p. 663).
B. Some Important Aspects
In this paper, the author wishes to expand upon the
recognition by SACS of the existence of formal
tectonostratigraphic units by demonstrating that:
1. the concept that unconformity surfaces are generally
markedly diachronous should be revised by a statement
of principle that each depositional discontinuity has an
isochronous element within the hiatus equal to the least
break in time between contiguous units,
2. the tectonostratigraphic unit is but a local
chronostratigraphic unit whose boundaries are limited
to unconformities or their correlative conformities,
3. a hierarchy of tectonostratigraphic units exist, requiring
(0 )
(b)
Figure 1
Model in space and time demonstrating relationships between
tectonostratigraphic sequences and the time significance of their
boundaries. (a) Simplified dip section across two major
tectonostratigraphic sedimentary intervals showing, by broken
lines, stippling, and the wavy unconformity surfaces, the
thicknesses eroded. Annotated isochronous surfaces of (b) shown
in spatial positions. (b) Simplified time or chronostratigraphic
section on same scale derived from (a) by construction of vertical
lines on to a vertical surface marked by an equal progression of
time-lines 1 to 5. Periods of erosion are indicated.
a set of specific formal rank terms, such as are required
by other kinds of stratigraphic classification, and
4. tectono stratigraphy is the most practical basis for basin
analysis, in that specific intervals of geological time can
be studied and in that the tectonic control of deposition
can be evaluated from unconformity analysis.
II. ANALYSIS OF A TECTONOSTRATIGRAPHIC
UNIT
Tectonostratigraphic units will now be analysed in order
to understand how this discipline can be applied to the
synthesis of the geological history of a depositional basin.
A. Structural and Chronostratigraphic Basin Cross Sections
In order to analyse the essential elements of a
tectonostratigraphic unit it was decided to follow the
approach of Wheeler (1959b) and of Mitchum etal. (1977).
Wheeler made use of a comparison of area-depth and areatime block diagrams to illustrate his arguments. For our
purpose it would be sufficient to set up two profiles of a
conceptual depositional model of two superimposed
tectonostratigraphic units, following the example of
Mitchum and his associates.
The upper diagram in Fig. 1 can be considered as a
structural or palaeo structural dip section, and the lower as a
chronostratigraphic section.
The bounding unconformities have developed either by
eustatic lowering of sea-level, or by uplift of the basin
margins, or a combination of both. Note that the original
proximal limits are younger than those preserved after
erosion.
B. Significance of Unconformities
The unconformity separating the two units appears on
the time or chronostratigraphic profile (Fig. 1b) as a single
line at the depocentre during time T3 flanked by two lines of
increasing hiatus. The single horizontal line is clearly the
signal of conformity, indicating uninterrupted deposition,
just as line T4, for example, shows in the unshaded area,
where deposition was taking place at T4 time. Shape is
distorted on the chronostratigraphic profile.
The depth or structural profile (Fig. 1a) shows that time
surfaces are distorted, and that only those that can be
recorded in the sedimentary accumulation are available to
analyse this hiatus. Time-horizons converge as the hiatus
increases, merge and become part of the unconformity
surface. They can cut into an unconformity, but cannot cut
across it into the contiguous unit. The reason is that the
hiatus straddles isochron T3 in such a way that isochron T3
is the only time-line that exists throughout the heterochronous boundary between the two units. All younger
time-lines can only be seen in the upper unit and the hiatus
and all older time-lines in the lower. With isochron T3 lying
within the boundary surface, it is impossible for any other
time-line to cross the boundary and be present within both
units.
C. Time Significance of Depositional Surfaces
An unconformity surface is, therefore, not diachronous,
for this term means to cut across time, but isochronous
(the same time) with respect to the depocentre and
heterochronous (other times) with respect to its varying
hiatus. The discordant boundaries of the contiguous units
are diachronous.
This conclusion differs from the statement by the ISSC
under the editorship of Hollis Hedberg (1976, p. 92):
"Although unconformity surfaces are not isochronous and
continuously cut across time horizons, major regional
unconformities obviously have very important, though
broad, time significance". The italics are the author's to
accentuate a point which has been misinterpreted to the
detriment of geological history interpretations.
ANALYSIS OF SOUTH AFRICAN PHANEROZOIC BASINS
Hedberg (1978, pp. 36-37) himself, in a later study,
showed that he meant that time-lines can cut into but never
across an unconformity surface. This rule that all sediments
above an unconformity must be younger than all those
below sounds very obvious, but on more complex models it
is quite difficult to realize this and it is a constraint that has
often been violated in practical basis analysis.
The time significance of depositional surfaces can now be
expressed as a principle:
Each depositional discontinuity has an isochronous
element within the hiatus equal to the least break in time
between contiguous units. In the case of a deep marine
bentonite-clay contact the hiatus is nil over the whole area,
and the succession is fully concordant. A bedding plane is a
discontinuity with a relatively large isochronous element.
In the case of Fig. 1 the isochronous element of the
bounding surface between the two tectonostratigraphic
units is the time-surface T3. With this principle one can now
quantify the broad time significance attached to an
unconformity (Isse, 1976 p. 92; Mitchum et al., 1977).
If a measurable hiatus at the depocentre should occur
between the two sedimentary units in Fig. l(b), the time
span between the two units at the depocentre would
represent the isochronous element. Stratal surfaces in the
marine environment are sufficiently continuous to be useful
for regional correlation; those of continental environments
generally not, because of the much smaller dimensions of
lithotopes and the intermittent nature of deposition.
A large number of time-lines in chronological order can
now be established by application of this principle, even if
the exact ages of these lines remain unknown.
Sedimentologists should note that tectonostratigraphy
can be applied on the scale of analysis of the development
and preservation of any specific depositional facies in any
geological environment. They will understand the
interrelationships amongst facies. They should be able to
determine how a sedimentary body responds to different
tectonic controls. Some of these aspects will be discussed in
Section IV.
III. RELATIONSHIP TO UNITS OF OTHER
CLASSIFICATIONS
The tectonostratigraphic units analysed above conform
to the general definition of a chronostratigraphic unit
(Isse, 1976, p. 67): " ... a body of rock strata that is unified
by being the rocks formed during a specific interval of
geologic time".
However, it is clear from the Guide (Isse, 1976, p. 84)
that chronostratigraphic units are defined in successions of
continuous deposition. The boundaries can be extended by
correlation into area~ where one or both become
discordant, and the remaining deposits are still mapped as
chronostratigraphic units. For example, interval T2 to T4 of
Fig. 1, if so defined at the depocentre, can still be
considered a chronostratigraphic unit at the proximal and
distal limits, even though deposition was limited to a
fraction of its time span.
The tectonostratigraphic unit differs from a
chronostratigraphic unit in that its boundary must
correspond to its bounding unconformities, whereas the
boundaries of the chronostratigraphic unit can be anywhere
in the succession. Early stratigraphers lacked essential·
borehole and seismic data to enable them to follow what
they considered to be important (unconformable)
boundaries into a conformable succession. Later a school of
thought arose that chronostratigraphic boundaries should
be set as far as possible within a continuous succession. It is
obvious that the time span of a tectonostratigraphic unit
should be defined from a stratotype as close as possible to
its depocentre. It was gradually realized that periods of
deposition and episodes of orogeny did not come as worldwide events. Many Standard Global Scale (SGeS) unit
171
boundaries, as a result, are also tectonostratigraphic
boundaries in the world stratotype only, but appear
anywhere within tectonostratigraphic units in other
sedimentary basins. The SGeS is now employed largely as a
relative time scale and for regional to global correlation,
i.e. in its geochronological sense.
The tectonostratigraphic interval, by definition, contains
a genetically related sequence of sedimentation and,
therefore, was recognized (Mitchum et al., 1977) as
superior to a chronostratigraphic interval for basin analysis.
Furthermore, interpretations relating to earth movements
can be made from the analysis of unconformities, which
enables the economic geologist to assess all the geological
factors that controlled the accumulation of ore· bodies
within the tectonostratigraphic units in their correct
sequence. It is hardly necessary to state that it is the timely
interplay of the correct sequence of favourable geological
factors that gives rise to a giant oil or gas field or a bonanza
gold field. Tectonostratigraphy provides the analytical
tool relating coeval tectonic processes to depositional
responses.
The seismic-stratigraphic framework, spectacularly
revealed by seismic reflection profiling, is an assemblage of
tectonostratigraphic units with isochronous reflectors
within them (Vail et al., 1977a, b, c). A geologist seeking a
framework of reference would not use the methodology
outlined in the Guide to set up a chronostratigraphic
framework from his seismic data. He would use the SGCS
only in its geochronological sense to obtain ages of some of
the isochrons he requires for basin analysis. He would
establish many isochrons by application of the principle of
time significance of stratal surfaces. A case can, therefore,
be made for a combination of facies analysis,
biostratigraphy, tectono stratigraphy , and structural
analysis that would satisfy the requirements for basin
analysis. Lithostratigraphy, being facies-bound, is too
cumbersome and cannot by itself provide the essential timelines necessary for basin analysis.
IV. PROPOSED TECTONOSTRATIGRAPHIC
SCHEME
With tectonostratigraphy being so useful as a framework
it follows that the stratigrapher should strive to erect a
tectonostratigraphical classification scheme as a vehicle for
the communication of his ideas. As with other stratigraphic
schemes, there will be a hierarchy ofranks of units.
A. Cyclicity and Rank
A number of smaller interruptions in sedimentation are
normally encountered within the major unit as the basin
margin is approached, and these are often noticed within
the basin as significant lithological changes, and seen on the
seismic profile as prominent reflectors, some without
onlapping or offlapping.beds, others with distinct lapping.
Many geologists have noticed a hierarchy in cyclicity of
sedimentary units, all the way down to the scale of a coset of
cross-bedded sandstone (Duff et al., 1967). Whilst the
smaller cycles of sedimentation can be attributed to local
pulses of sedimentation caused by changes in weather and
often controlled by the morphology of the depositional
environment, the larger cycles associated with marginal
disconformities seem to have tectonic significance.
An example of cyclicity of a larger dimension than that
normally analysed by means of seismic stratigraphy are the
coastal cratonic sequences of Sloss (1963).
It is, therefore, understandable that a hierarchy of
tectonostratigraphic units could exist, related to the
hierarchy of chronostratigraphic units. Vail et al. (1977a, b,
c) called all the units that they could define by seismic
stratigraphy depositional sequences and did not
systematically apply a ranking classification. They related
their depositional sequences to cycles of sea-level changes
172
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
as a different category of classification, not necessarily on a
one-to-one basis. They did, however, rank their sea-level
cycles (Vail et ai., 1977c, p. 64).
The author considers that in the frame of a
tectonostratigrapbic classification the concept of rank
should be included and, therefore, designed a model based
on a real succession (Winter, 1979a, Fig. 3) to study the
implications (Fig. 2).
Note that the onlapping and offlapping or truncation of
cycles serve to define the position of the major sequence
boundaries. Lower order marginal unconformities can
truncate major boundary unconformities, e.g. T3 on T1 and
T5AonT5.
(0)
+ - - - - - - - - - - DISTANCE
-------------.1
~
I
DISTAL
PROXIMAL
DEPOCENTRE
SOURCE
B. Transgression and Regression
Each sedimentary cycle commences with a transgression
and ends with a regression, though some of the regressive
facies can be removed by subsequent erosion at major
sequence boundaries, and the former can be nondepositional. The term transgressive unconformity (Ryer,
1981, Fig. 12) therefore does not make sense, since the
unconformity is a surface of reversal of direction. Reversals
in continuous deposition have long been recognized as
isochrons. Weimer (1966), in particular, made use of this
approach in basins where fossil dating is inadequate. We
can now realize that an R-T reversal may still be recognized
near the basin centre where the marginal unconformity
becomes part of a conformable succession.
C. Lateral Accretion
Weimer (1975) has emphasized that basin filling by
traction is largely a process of lateral accretion.
The lateral accretion model (Fig. 3) shows that
progradation relates to regression, therefore much of the
filling of a basin is accompanied by a regression of
the shore-line. During rapid transgressions, no lateral
accretion can occur within the basin. The so-called
transgressive unconformity and the regressive cycle
(b)
PURE PROGRADATION OFTHREE
GENETIC INCREMENTS OFSEDIMENTATION
LITHOSTRATIGRAPHIC
Figure 2
Cross section of a conceptual model of a major unconformitybounded stratigraphic unit in a marginal oceanic or coastal basin
filled with sediments from one side only. Minor unconformities or
disconformities within the unit, shown as wavy lines, are greatly
exaggerated to show depositional and erosional intervals to greater
effect. Note that even the bounding unconformities approach their
equivalent conformities as the depocentre is approached. It is
assumed that there has been no deep-sea current erosion or
slumping. (a) Structural, depth-distance or lithostratigraphic
profile showing isochronous lines which relate to specific time
levels Tl to T9 in Fig. 2(b). (b) Chronostratigraphic or
time-distance profile of the same unit as (a). Periods of erosion are
shown as coarse dots for the first erosional period and as inclined
lineation for subsequent erosional periods. Non-depositional times
are shown as finely-dotted shading. Depositional intervals
subsequently removed by erosion may even have included
marginal disconformities. On the distal side, non-depositional
hiatuses can drastically interrupt the deposition during times
coinciding with proximal marine encroachments.
CHRONOSTR ATIGRAPHIC
CROSS
SECTION
CROSS
SECTION
=:~----="""~"""...".""""s:;:;;;"""
=~---=~~~~~~~~--~-,"-"-"-"""-","-""-""",-","~,,"~","=""---------=
:~ ~
~~.,...=--=."
-5~~----~=~------------------------~-=
~~-----------~~~"="-""-"""-""""-"~-""-"'""-"«"-:""-"""'-'","-:":~-"
__-----
.. '-":"-;:'"-"'"""-"""'-':'"-"""-""":'-""-"''-"""-"-------
Figure 3
Model of three accreting GISs (Busch, 1971) in a subsiding basin
deposited by a process of lateral accretion only and without
erosion, and followed by non-depositional transgressions. (See
Table I.)
TABLE I
Proposed Tectonostratigraphic Rank Terms
Regional
chronostratigraphic
rank terms
Proposed
tectonostratigraphic
rank terms
Erathem
System a
Seriesa
Stagea
Chronozonec
Basin or
Supersequence f
Sequence
Cycle a
Cyclothemc
a.
b.
c.
d.
e.
f.
Some previously suggested
tectonostratigraphic terms
and their approximate ranks
Synthem b
Interthem b
Sequencee
Cyclothemc
If additional ranks are needed, the prefixes sub and super may be applied to these terms.
Terms suggested by Chang (1975) and discarded by Mitchum et al. (1977) because the concept of correlative conformities was not
included.
The cyclothem would represent a tectonostratigraphic unit of low rank, roughly equivalent to a chronozone.
Genetic Sequence of Strata (GSS) and Genetic Increment.of Strata (GIS) of Busch (1971a).
Sloss (1963), SACS (1980).
One or more supersequences constitute what can be defined as a depositional basin.
ANAL YSIS OF SOUTH AFRICAN PHANEROZOIC BASINS
concepts probably arose from this model of basin fill by a
dominantly traction load.
Matthews (1974) showed that terrestrial deposition on a
large scale can be related to a relative rise in sea-level. A
shoreline transgression could, therefore, not only signify an
173
encroachment of the marine facies, but of a fluvial facies on
to the continent.
At sites of deltaic accumulation, the shoreline may
retreat in spite of an enlarging, often called transgressive,
basin (Weimer, 1970; Asquith, 1970, 1974).
(!)
a::
w
CD
(/)
Z
W
~
of
!1L
a::
o
EXAGGERATION
w
o
w
(!)
±50X
'"
o
o
+3
+2
U
S.L.
T
UK
MK
LK
AFR
:
:
:
:
:
Tertiary
Upper Cretaceous
M idd Ie Cretaceous
Lower Crelaceous/U. Jurassic
Agulhas Fracture RidQe
Displacement towards vie_
® : Displacement from vie_
Drakensberg Basalt and intrusive dolerite
Beaufort and Ecca Groups
Dwyka Tillite Formation
Wilteberg, Bokkeveld and Table Mauntoin Group.
"Pre-Cape" or Gamtoos Group
Klipheuwel Formation
Karoo dolerit..
o:
D
Be,E
Ow
Wb,Bo,Tb
"p - C"
K
® 0
:
:
:
:
:
:
=(:
-2
-3
-4
-5
-6
-7
Figure 4
Simplified schematic structural profile from the offshore beyond the Agulhas Fracture Ridge across the Drakensberg in a north-east direction
as far as Swaziland.
s.w
~
SHELF EDGE
SOUTH COAST
KAROO
DRAKENSBERG
Cen.
boundaries
U.K.
I~ Palaeocurrent
L.K.
directions
I
Cen.
U.K
LJ(
~f
J
J
Uplift
p
t
u.c.
L.C.
o
S
o
e
6
Figure 5
Chronostratigraphic profile of same section as Fig. 4 to assist in tectonostratigraphic interpretation of the geological history of the South
African Phanerozoic Eon. Local tectonostratigraphic units relate to their specific depositional basins. Several magnitudes of cyclicity are
evident.
174
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
The hiatuses of Fig. 3 apparently climb progressively
basinwards with time and they can be characterized by their
isochronous elements, times 1, 5,9, and 12.
The corollary test can now be applied: no time lines cut
across the unconformities, because each unconformity
contains its significant isochronous element.
D. Nomenclature
In proposing a tectonostratigraphic scheme with a
ranking of dimensions of units, the author is building upon
the foundation laid by SACS (1980, p. 663) where only one
rank, the sequence or synthem, is recognized. With a complete set of terms, the tectonostratigraphic classification can
henceforth be used independently by the stratigrapher as it
should be done, and an admixture of classifications such as
the Pretoria Group of the Transvaal Sequence should no
longer be applied. Far too many of the lithostratigraphic
terms applied to South African stratigraphy are essentially
tectonostratigraphic. For example, many of the subdivisions of the Central Rand Group are unconformitybound and only vaguely distinguished by their lithology,
and consequently a definitive lithological term cannot be
applied.
The tectonostratigraphic rank terms follow the concepts
of stacked depositional sequences and cycles of
sedimentation.
Table I presents the proposed rank terms and compares
these to some previously suggested terms as well as to
regional chronostratigraphic rank terms.
The specific name of the unit should preferably be a
geographic term, unless an entrenched early name can be
revived. In seismic stratigraphy it has become customary to
refer to mapped seismic reflections with symbols, e.g.
horizons C and E in the southern offshore, and the units
bounded by these are described by their boundaries, e.g.
the C to E interval.
It is left for SACS to decide whether the many lithostratigraphic terms that are in fact tectonostratigraphic
units should retain their geographic terms. I would
recommend this to avoid the proliferation of names. Where
the boundaries of lithostratigraphic units bear no
relationship to unconformities, new names should be found
or terms of the old single classification can be revived,
wherever they fit the definition of a tectonostratigraphic
unit.
1. Stratotypes
The type area and section of a tectonostratigraphic unit
should be as close as possible to the depocentre of a
depositional basin, where the strata tend to be conformable.
Correlating away from the type area, one finds that the isochronous boundaries become unconformities.
The surface of unconformity often represents an everincreasing time break or hiatus as the basin margin is
approached, as long as the underlying rocks are not
tectonically disturbed.
A regional structural lithostratigraphic profile across
South Africa (Fig. 4) is sufficient for a geological history to
be told, but when it is combined with a chronostratigraphic
profile (Fig. 5), many new aspects come to be understood,
even in these two greatly generalized illustrations.
The geographical name of nearest lithostratigraphic
equivalent will be used for the tectonostratigraphic units
that are apparent on Fig. 5 since it is not the intention to
propose new formal terms in this paper.
A. Pre-Cape Events (Klipheuwel Sequence)
Phanerozoic basins of the Cape appear to be founded
upon a magmatic arc built up of the metamorphosed and
tectonized Malmesbury Group and intruded just before the
Cambrian Period by the Cape Granite Suite of plutons. The
Kaaimans Group and perhaps the Cango Group may be
time equivalents ofthe Malmesbury (SACS, 1980).
It is evident that the Klipheuwel Formation represents
graben infills (De Villiers, 1980), setting the plate tectonic
scene for the growth, after separation and the development
of a proto-Atlantic ocean, of a marginal continental
miogeoclinal basin, the Cape Basin, on the Dewey and Bird
(1970) model of Appalachian evolution. Though the
Klipheuwel sequence has not been recognized in the
Gamtoos area it is illustrated on Fig. 5. The model implies a
southward subduction of oceanic floor beneath a southern
continent (Fig. 6) or island arc; otherwise it is identical with
the model of Rhodes (1974). The version of Tankard et ai.
(1982) of Cape sedimentation within an aborted rift is not
supported. The Rhodes model and the flat-plate
refinement of Lock (1980) would predict greater instability
during Witteberg times than is evident from the geology.
Note that the collision has resulted in a typical reversal of
sedimentation direction from a northerly source during
deposition of the Cape Supersequence to a southerly
provenance for sediments deposited into the foredeep of
the Karoo foreland cratonic basin.
B. Cape Supersequence
Note the significance of a concordant Gamtoos Group,
previously named "pre-Cape" sediments, as an initial cycle
EARLY PALAEOZOIC TIME
'" 350 Ma
s
CAPE
..0
BASIN
~
•• dim.nts
N
l.v.1
Continental
Crus'
•• o-floor
cru.t
PRE - PANGAEA I
LATE PALAEOZOIC TIME
~250 Ma
v. PHANEROZOIC APPLICATIONS
Some practical examples of application to the South
African Phanerozoic can be suggested without proposing
new terminology and with suggestions to further work.
Such regional application is designed to relate tectonic
styles to depositional facies and patterns, and to define the
plate tectonic settings of the depositional basins. An
attempt is also made to compare the South African tectonic
styles with the sequences of Sloss (1963) and to bring
attention to possible local responses to worldwide eustatic
events (Vail et ai., 1977c).
The brief synthesis will illustrate how the tectonostratigraphic method of analysis forces one to recognize
aspects in the unfolding of the geological history of an area
that are not readily apparent to the field worker.
J
s
FOLDED CAPE
BASIN
N
Koroo
•• dim.nt.
(GONDWANA)
PANGAEA
n
Figure 6
Crustal profiles prior to and after a collision between continents
which caused the Cape orogeny and the Karoo basining, based on
an hypothesis of southward subduction of oceanic floor.
ANALYSIS OF SOUTH AFRICAN PHANEROZOIC BASINS
175
TABLE II
Phanerozoic Lithostratigraphy of South Africa Compared to Sea-level Changes after Vail et al. (1977) and the North American Cratonic
Interior Sequences of Sloss (1963) with the Help of a Dated Global Geochronological Scale
PERIODS
EPOCHS HIGH
SEA
I
TERTIARY
MIOCEq
OLIGOCENE
EOCENE
PAL.EOCENE
L.ATE
CRETACEOUS
I
JURASSIC
MIDDL.E
EARL.Y
TRIASSIC
PERMIAN
r--~E
L.
PENNSYLVANIAN
M
E
MISSISSIPPIAN
r--~=
DEVONIAN
E
SILURIAN
M
L.
ORDOVICIAN
M
E
L.
CAMBRIAN
M
E
GROUPS AND
I
( 1963)
0
TEJAS
-
""-..........
f---------
AGULHAS
SHALE
100
UITENHAGE
.......
SUURBERG
PRESENT"~
GROUP
GROUP
DRAKENSBERG
DAY MODAL
SHELF
EDGE
SEQUENCES OF SLOSS
"SUPERGROUPS"
FORMATIONS
L?W
~
EARL.Y
L.ATE
GEOLOGIC
TIME
LEVEL
GROUP
200- MOLTENO - ELLIOT - CLARENS F.
"
~
~
f~,
-
BEAUFORT
-
ECCA
300-
GROUP
---
ABSAROKA
-
DWYKA TILLITE
400-
NARDOUW
-------
-
, PAKHUIS
500-
KASKASKIA
GROUP
SANDSTONE
CEDARBERG
SHALE
CONGLOMERATE
--------
KAROO
WITTE BERG GROUP
BOKKEVELD
r-~
GROUP-
Z UN I
f--------TABLE
MOUNTAIN
GROUP
CAPE
TIPPECANOE
f---------
PENINSULA SANDSTONE
GRAAFWATER SANDSTONE
_ PIEKENIER
CONGLOMERATE
of Cape Basin deposition (Winter, 1979b) where the
Witteberg represents a final sub-cycle. Such a large
accumulation may warrant the rank of Supersequence or
Basin for the whole, and of Sequence for each of the four
tectono-units shown, as it is well known that subsidiary
sedimentary cycles can be mapped in all of the named units.
The important discordance between the main bodies of
Peninsula and Nardouw Sandstone (Rust and Theron,
1964) does not appear at all in the structural section, yet the
unconformity coincides with a world-wide eustatic change
in sea-level (Vail et al., 1977c) near the end of the
Ordovician (Table II). An Ordovician age for the sequence
containing the Cedarberg Shale, at the base of the next
sequence is not inconsistent with fossil dating (Cocks et al.,
1970; Moore and Marchant, 1981). The locally concordant
Gamtoos pre-Cape sediments, like the Nama, contain
abundant worm burrows (Germs, 1974, Shone, 1979,
Winter, 1979b) and limestones that indicate an early warm
climate, but does not prove contemporaneity of these
tectonostratigraphic units.
Table II compares the sequences of Sloss (1963) with the
eustatic sea-level changes of Vail etal. (1977a, b, c), and the
general Phanerozoic lithostratigraphy of the profile.
Comparing with Sloss's (1963) sequences, we find that our
Gamtoos and Peninsula sequences correlate with the Sauk,
and the Nardouw sequence with the Tippecanoe
Sequence. The Kaskaskia Sequence may well be
represented by our Bokkeveld and Witteberg groups.
The top of the Tippecanoe Sequence roughly coincides
with the upper part of the Nardouw Sandstone, where a
break coinciding with another global unconformity of late
Silurian age can be expected (Table II). Evidence of such a
break could have been removed by pre-Dwyka erosion, but
is suggested by the landward encroachment of marine
Bokkeveld shales. Note that a regression followed by a
transgression is the basinward indication of a marginal
unconformi ty.
We can expect a correspondence in tectonostratigraphic
units between the cratonic sequences of Sloss (1963) and
our Cape rocks because both accumulations were deposited
on continental shelves linked to a global ocean; therefore,
eustatic sea-Ie.vel changes probably controlled the positions
of unconformities, but the magnitudes of the hiatuses
probably depend upon local tectonics.
An explanation for the predominance of sandstone in
the Table Mountain Grouf' is offered on the chrono-
SAUK
stratigraphic diagram (Fig. 5). The basin encroaches the
land during that period. This supports an hypothesis of
coastal subsidence and downwarp of a mature continental
margin and is contrary to what one would expect of an
orogenic uplift of the dominantly granitic provenance.
Chemical weathering of the land surface built up a mature
regolith (Visser, 1974). Quartz grains and clays were
separated during transportation and much clay was
winnowed out in the shallow-marine depositional
environment. Coastal deposition remained above storm
wave-base and sand accumulated to a great thickness in a
fine balance between subsidence and sedimentation rate
during Peninsular and Nardouwian times. The clays and
silts would carry across the inner shelf and be deposited on
the outer shelf and slope as prodelta deposits, with sheet
sands accumulating on the shelf. Because of a fine balance
between subsidence and sedimentation great thicknesses of
sand accumulated during Peninsular and N ardouwian times
(Rust, 1973). Global eustatic sea-level fluctuations
probably disturbed the pattern of development of this onesided miogeoclinal accumulation. The distal succession
would be dominated by a "Bokkeveld" -type shale
lithology.
C. Karoo Sequence
The presumed Carboniferous collision was bound to
have had a profound effect on basin development. It spelt
the termination of Cape basining and initiated the Karoo
Basin, the asymmetric shape of which is typically that of a
foreland cratonic basin such as the Molasse Basin north of
the Alps and the Eastern Great Basin of the Rocky
Mountains. Dickinson's (1974) peripheral basin model (his
Fig. 11, p. 22) explains the lack of high-temperature
metamorphism and volcanism that would have been
expected had the polarity of subduction been reversed (Fig.
6). Orogenic effects of the collision continued until the end
of the Permian.
The Dwyka tillite could be envisaged as a post-collision
molasse deposit, had the evidence for glaciation not been
overwhelming, but the widespread White Band marked a
quiet period of clay deposition with some chemical
precipitation, suggesting a slow epicontinental downwarp
of the sea-floor prior to the turbid deposition of Ecca
sandstones into a more active foredeep, only the distal part
of which was preserved, since no coarse components are
present. Kingsley (1981) made use of time-stratigraphic
176
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
horizons in analysing the transition from the Ecca to the
Beaufort Groups in the Eastern Cape Province as the
gradual deltaic filling of the foredeep sea by a prograding
set of deltas-the beginning of the Beaufort being shown
to coincide with the delta-plain facies. Though McLachlan
and Anderson (1973) could not find undoubted evidence
for a connection of this early Karoo sea to the world ocean,
regional as well as local sedimentological studies provide
ample evidence for a large epicontinental sea in Dwyka
(Von Brunn and Gravenor, 1982) and Ecca times. Deepsea sands accumulate during low-stands of sea-level
(Matthews, 1974, pp. 306-308). The Pluto's Vale Member
may, therefore, provide an approximate time-level, which
correlates with Vail's Early Permian low sea-level. Marine
incursions, such as the Britskraal Shale Member (formerly
Middlewater Member, SACS, 1980) could be employed as
time-levels towards the end of Ecca time. It is clear that the
lithostratigraphic Beaufort-Ecca contact must become
younger northward, as indicated by Kingsley (1981).
Jordaan (1981, p. 24) came to the same conclusion relative
to the south-western Cape Province, but his proposal to
place the Beaufort-Ecca contact at the top of the delta plain
facies still leads to a diachronous boundary, unsuitable for
isopach mapping. One gains nothing by redefining the
lithostratigraphic boundary of SACS (1980, p. 538). Thin
beds like the Poortjie Sandstone Member and the
Britskraal Shale Member promise to be the type of timestratigraphic markers that the tectonostratigrapher must
have for isopach mapping. The distribution of Beaufort
reptile zones suggests that the fluvial red-bed environment
commenced earlier in the south-west than towards
the north-east. The establishment of time surfaces is
essential to understand the contemporaneous facies
distributions in the Karoo Basin. Figure 5, for example,
illustrates a Katberg to Molteno cycle, but it also shows that
we need to look for the equivalent cycle in the north, by
palynology perhaps, before we really can discuss the
evolution of the Karoo Basin. Tectonostratigraphy
disciplines the analyst to find the avenues of research
required to unravel its geological history.
Armstrong (1968) found that certain conglomerates can
date stages in the orogenesis related to sedimentation in the
Eastern Great Basin of the Rocky Mountains, another
example of a cratonic foreland basin. Figure 5 suggests that
the Katberg and Molteno conglomerates could similarly
relate to orogenic pulses.
As an example of economic application one may ask why
coal does not occur in the Beaufort delta-plain facies. It is
suggested that the essential requirement of still-stand is not
sufficiently met as these are flysch type sediments,
deposited concurrently with the Cape Orogeny to the
south.
The Vryheid Formation of the northern Karoo Basin has
been very aptly analysed by recent sedimentologists (Van
Vuuren and Cole, 1979; Le Blanc Smith, 1980) because
they incorporated cyclical depositional, and hence
tectonostratigraphic concepts, in their analyses. The timespan of these deposits, derived from a north-easterly
general source, is very difficult to establish, and would
probably have to rely heavily upon biostratigraphic agedating. Attempts at this have been made by palynologists
(Hart, 1966). The regional distribution of the
regressive-transgressive Vryheid "coal measures" cycles
are also not as well-known as the analogous models of
Hollenshead and Pritchard (1961) and of Ryer (1981). We
now know that the northern margin of the main Karoo
Basin is close to the original basin margin. What about the
more northerly Karoo basins? Are they discrete
depositional basins or structural remnants?
It is well known that Vryheid sandstones have generally
poor reservoir properties (Rowsell and De Swardt, 1974).
Nevertheless, certain well-sorted sandstones of shoreline
I
facies, especially when reworked by wave action (Hobday,
1982), could be commercially petroliferous at shallow
depths - a target for the small entrepreneur or
"wildcatter" (Hobday 1973; Ryan and Whitfield 1979).
With tectonostratigraphy as a framework, the localities of
winnowed sandstones can be predicted.
The Ecca-Beaufort contact roughly defines the
beginning of the draining of an epicontinental sea from the
Karoo Basin. We now know that continent-continent
collision orogenies could be active for as much as 70 million
years, as exemplified by the Sevier Orogeny of the Rocky
Mountains (Armstrong, 1968). The Cape Orogeny
probably extended from Carboniferous (?) collision to Late
Permian/Early Triassic. Folding and thrusting, therefore,
continued until about the end of the Permian accompanied
and followed by uplift of the Cape Fold Belt, as indurated
coarse material was derived from this area during Katberg
and Molteno times.
Turner (1975) established seven disconformity-bounded
sedimentary cycles in the Molteno. Ryan and Whitfield
(1979) showed that palaeocurrents came from the southeast. The depositional environment of Molteno coal
measures is probably a low-level alluvial plain (delta plain,
coastal plain, or low-relief interior plain). The Beaufort
red-beds probably accumulated in a subsiding basin, the
rise in base level of the graded profile providing for fluvial
accumulation (Matthews, 1974, p. 161), though the rising
Cape mountains could also have been responsible.
Perhaps a Mid-Triassic eustatic rise in sea-level (Table II)
favoured deposition of coal measure facies during Molteno
times. Yet, the evidence is strong that renewed uplifts of the
source area introduced coarse clastic deposition in a
braided fluvial environment (Turner, 1980). Strong uplift
after collision accords with Dickinson's (1974, p. 22)
peripheral basin model. Criteria to distinguish between
diastrophism and eustasy requires further refinement. The
resumption of aridity evidenced by red-beds and the
thickness of the Karoo accumulation point to continued
basin subsidence with an equilibrium profile yoked to a
southern tectonic uplift. Emergence of the depositional
basin would not have been accompanied by deposition
(Matthews, 1974). With tectono stratigraphy one can better
analyse tectonic controls of sedimentation.
Isopachs of lithostratigraphic Ecca and Beaufort
intervals can provide only a distorted picture of the basin
shape. Isopachs of tectonostratigraphic intervals are
required for accuracy and to classify the type of basin
correctly.
The history of the period of crustal extension up to the
separation of east and west Gondwanaland is not yet well
understood. How does the final Drakensberg volcanism
relate to the Kalkrand Basalt, the Lebombo and the
Zuurberg periods of volcanism? What was the elevation of
Lesotho then and what of the isostatic effect of such a pile of
lavas? How does it relate to the opening of a southern ocean
and the transform motion along the Agulhas Fracture
Zone? Is the Agulhas Fracture Ridge (AFR) a product of
the opening, did it form during the ensuing period of
translational stress, or is it the result of Mid-Palaeozoic
collision tectonics? Perhaps the basalts represent the
southern limit of an aborted phase of the East African Rift.
Tectonostratigraphic analysis helps to relate these various
events in space and time.
D. Uitenhage Sequence
The chronostratigraphic diagram (Fig. 5) shows that the
Jurassic was a period of major change in tectonism and of
rapid sedimentation within Late Jurassic rifted troughs.
There appears to be an overlap of rifting and dolerite
intrusion. Dolerites of Mid to Late Jurassic age near East
London occupy rift faults thought to be similar to those of
the southern coastal area. The oldest rocks within the
graben that could be dated are the Kimmeridgian shales of
177
ANALYSIS OF SOUTH AFRICAN PHANEROZOIC BASINS
the offshore Gamtoos Basin. They are typical deep-marine
sediments and do not represent the basal deposits of the
trough. We do not know to what extent pre-rift sediments
are preserved in some of the grabens.
The rift faults of the south and east coasts predate the
unconformity known as horizon C (Fig 5), which is now
considered to be a break during the latest Valanginian (I.K.
McMillan, pers. comm.). This age is closer than the
previously held 121 Ma (McLachlan and McMillan, 1979)
and about equals a widely-accepted date 127 ± 2 Ma (Larson
and Ladd, 1973) for the initiation of drifting, the opening up
of the Atlantic Ocean and the initiation of lateral
movement along the Agulhas Fracture Zone. Tectono-,
stratigraphic subdivisions of the contemporaneous fill
provide the means of measuring periods of active' rifting.
E. Post-drift Events
The Falkland Plateau is calculated to have moved past
the south-eastern margin at a rate of some 60 km per Ma
(Le Pichon and Hayes, 1971) against the continental margin
and, as it cooled, it contracted and down-tilted the margin.
Miogeoclinal clastic sediments prograded across the
margin. Isostatic adjustment and compaction of the rift-fills
and of the miogeoclinal sediments accelerated the
downward movements locally and associated isostatic
rebound on the continent rejuvenated the sediment source
areas. By Tertiary times the seaward warping had not yet
ceased, but the rate was much reduced, as was the rate of
sedimentation. A nett fall of sea-level during the Tertiary
could have had much the same effect on coastal
geomorphology as a fringe upwarp with the escarpment as
axis. The several Tertiary terraces and related youthful
erosion may be associated with such sea-level falls, but it is
believed that sea-level fluctuations alone cannot account
for the present high levels of supposed Tertiary and
Cretaceous geomorphological features inland. They appear
to relate partly to eustatic equilibrium profiles and partly to
an upwarped rim around Africa (De Swardt and Bennet,
1974). It certainly is possible that the south coast peneplain
was sculpted by a number of transgressions (Figs. 4 and 5).
The seismic horizon named C in Fig 5 is the drift onset
unconformity (Falvey, 1974) and the lower boundary ofthe
miogeoclinal basin consisting of two Cretaceous and a
Cenozoic sequence. Note the correspondence with the
earlier Cape Basin.
Whereas sediments filling the graben did so from either
north or south, or both, depending on which flanks were
exposed to erosion, most post-C sediments accreted from
the north in a number of successive progradations,
concomitant with oceanward tilting, hinge-lines close to the
coast (Du Toit, 1979, McLachlan and McMillan, 1979).
One should be able to match the depositional cycles with
world-wide fluctuations of sea-level (Table II) because
these changes controlled the succession of progradational
cycles of sedimentation. At present we think that horizon Q
represents a sequence boundary because of a marked shift
of the palaeoshelf break.
Horizon L, near the base of the Tertiary is another
sequence boundary. The three sequences represent three
major sedimentary-tectonic phases in the development of
post-drift basins: the restricted oceanic phase, with
evaporites where silling has occurred and carbonates in the
warmer climatic areas; an open marine phase highly
susceptible to eustatic sea-level changes (transgressions and
regressions) in the shelf environment; and a late or mature
phase characterized by bypassing of sediments on to the
continental rise and by deltaic accumulations at major river
mouths (Dickinson, 1974). Such a history is certainly true
with respect to the bulk of the South African Cenozoic
deposits. All the subdivisions mentioned are tectonostratigraphic divisions.
Off the south coast the Early Cretaceous CQ Sequence is
characterized by at least five cycles of prograding
shelf-slope-basin sediments, with shelf facies dominating
inshore and deep-water organic-rich clays thickening
distally. Apparently there was no extensive restriction of
the sea in the Agulhas Bank area, so the Agulhas Fracture
Ridge (AFR) could not have been an important barrier.
Nevertheless, there is some doubtful seismic evidence that
early sediments could have prograded northwards from the
AFR.
The Late Cretaceous QL Sequence consists of an early
prograding cycle of mainly Turonian sediments restricted to
the southern part of the Agulhas Bank: this one would
expect after a large relative fall in sea-level (Table II). The
upper portion begins with a number of thin parallel cycles in
the Agulhas Bank area which overlap and sometimes
truncate underlying units proximally, but which built out as
a delta complex from the present mid-shelf position
towards the AFR as thick prograding units with steep slopes
and starved basin aprons. The general facies pattern on the
shelf is coarsening upward with a tendency to become more
continental in aspect with time, and with poor diversification of foraminifera.
The final sequence off Southern Africa is characterized
by shelf deposition and by bypassing into deep water, as
well as erosion of the continental slope, transportation, and
redistribution of sediments in the deep-sea environment as
contourites on the now extensive continental rise.
Alternating shelly and clay beds on the shelves can be
attributed to changing sea-level stands. Clay settling would
tend to occur on quiet shallow shelves of a transgressing
sea, the ocean currents mainly carving away the slope.
Coarser sediments would travel in dunes or be transported
in channels across the remaining shelf and be bypassed
through submarine canyons to the deep sea. During high
sea-level stands, shelf dunes can accumulate and fines will
cap the coarse deep-sea deposits. This tentative model is
but one of the possibilities presently being investigated
(Shanmugan and Moiola, 1982), to account for known
distributions of sands in the marine environment.
F. Synthesis
Table III summarizes the relationships between
tectonism and sedimentation and provides a plate-tectonic
interpretation, such as can be derived from the above
tectonostratigraphic analysis.
TABLE III
Relationship Between Tectonic Styles, Plate Tectonic Settings and
Sedimentary Basin Response During the Phanerozoic
SOUTH AFRICA
GEOCHRONOLOGICALI-----r---------,----------l
TECTONIC
PLATE TECTONIC
CONTEMPORANEOUS
SCALE
STYLE
SETTING
SEDIMENTARY BASINS
tIlt
!
Miogeoclinal
TERTIARY
Passive
CRETACEOUS
drift
Subsidence
accumulation
on
T continental margin
~-----II-Riit----f-'G_;;d~~;;;;;_-~--hmoHGamtOOs=tyPOmt1 - - - - - - - r----------I-~1!1.~'!!!!!!...-----
JURASSIC
Uplift
TRIASSIC
~---------
Peripheral
Foreland
foreland
Folding (FTB.)
PENNSYLVANIAN
ColliSion
Unstable
MISSISSIPPIAN
dolentes
Lorge
Subsidence
PERMIAN
--=--------Drakensberg Bosalt and
-
to
basin
linked
Cape
Orogeny (F T B.)
Korea
Dwyka Basin
~--------
- - Source Iwitch
subduction ,_ Witte berg
_ , Approaching
L!.o~,-I.£!!~<!l£e.!."LJ
DEVONIAN
Passive
SILURIAN
drift
SubSidence
ORDOVICIAN
LarQ8
tI*MIOgeOClinOI
accumulation
Cape
BoslO
continental
IT morQln
CAMBRIAN
1-------1---------'---+--------Rift
InitiatIOn
of
Pangaea I
Small Franschoek - type
~~~n~~~o~~-+'~~I~~~---Uplift
It
Late otage magmatic arc
t I
IncreaSing
effect of Isostatic subSidence
!T
Increasing
effect of Thermal subsidence
Cape Granite. plutanllm
178
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
The table demonstrates two passive continental margin
tectonic-sedimentary mega-sequences. The Cape event
ended in the elimination of an early Palaeozoic ocean and
plate-margin tectonism. This ocean was initiated after a
Klipheuwel rift sequence at the beginning of the Palaeozoic
Era. An intriguing question can be posed concerning the
geological future of our subcontinent. Is there any evidence
to suggest that the opening-up phase of the Atlantic Ocean
is approaching its end?
VI. CONCLUSIONS
The constraints placed upon interpretations by
tectonostratigraphic analysis are of great help in both
regional and local geological synthesis. As a result, a
restricted number of basic regional depositional models are
now becoming apparent (Matthews, 1974), and that again
increases the predictive power of analyses based on
tectonostratigraphy. This aspect is of great economic
importance for understanding the conditions of concentration of stratigraphically controlled ores, using this
term in its widest sense, including coal, petroleum, and
water, as well as of industrial minerals and rocks.
A strong point in favour of this scheme is its application
to structural analysis. We have briefly noted from Table III
how important it is to relate the correct style of deformation
to contemporary deposition.
The application of plate tectonic principles to construct a
genetically related geological history is obvious.
Tectonostratigraphy not only ties together genetically
related sedimentary facies, but also the correct tectonic
generations.
Tectonostratigraphy brings together cause and effect,
process and response. Hence, the importance of processresponse modelling in predictive analysis. We are dealing
with four dimensions, three defining space and one, time.
Because we cannot visualize all four together we need to
compare area-depth with area-time models of the region
being studied. In this presentation, the analysis was mainly
done on simplified profiles: for a more thorough analysis
one may require block diagrams.
ACKNOWLEDGMENTS
The management of Soekor (Southern Oil Exploration
Corporation (Pty) Ltd) is thanked for allowing time and
facilities for preparation of the paper. In this context
especially, the author wishes to acknowledge the following
services: typing, copying, library and drawing office. Many
senior officials contributed by reviewing earlier drafts. Two
of these whose discussions were most fruitful were Mr I.R.
McLachlan and Dr David Broad.
The author is especially indebted to referees Dr M.R.
Johnson and Prof. I.e. Rust for suggesting the streamlining
of two proposed manuscripts into the present version.
Mrs Parry of Anglovaal Limited kindly typed the final
manuscript.
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