S.-Afr.Tydskr.Geol.,1993,96(4)
228
the literature (e.g. Rioult et aI., 1991; Southgate &
Shergold, 1991), but in no case has it led to the supplanting
of conventional lithostratigraphy, biostratigraphy, or chronostratigraphy.
Finally, it must be clearly realised by all concerned that if
a genuine chronostratigraphic classification is to be introduced for the Witwatersrand, then the nomenclature will
have to conform to the internationally accepted way of
writing the names in question. Should names based on
existing lithostratigraphic units turn out to be unacceptably
'cacophonous' (,Mainic, 'Birdian, 'Coronationian', etc.)
then other names will have to be proposed by those advocating the use of chronostratigraphic nomenclature.
- - - - - - - - - - - DISTANCE-----------
l
~
~
x
j
References
Christie-Blick, N., Mountain, G.S. & Miller, K.G. (1990). Seismic
stratigraphic record of sea-level change. In: Sea-level Change.
National Academy Press, Washington, 234 pp.
Cohen, CR. (1982). Model for a passive to active continental margin
transition: implications for hydrocarbon exploration. Bull. Amer.
Assoc. Petrol. Geo!., 66,708-718.
Johnson, M.R. (1991). Discussion on 'Chronostratigraphic subdivision of
the Witwatersrand Basin based on a Western Transvaal composite
column'. S. Afr. J. Geo!., 94, 401-403.
Rioult, M., Duguc, 0., Jan Du Chene, R., Ponso!, C, Pily, G. Moron, J.M. & Vail, P.R. (1991). Outcrop sequence stratigraphy of the AngloParis Basin Middle to Upper Jurassic (Normandy, Maine, Dorset).
Bull. Centres Rech. Explor.-Prod. Elf-Aquitaine, 15, 101-194.
Southgate, P.N. & Shergold, UI. (1991). Application of sequence
stratigraphic concepts to Middle Cambrian phosphogenesis, Georgina
Basin, Australia. BMR J. Aust. Geo!. Geophys., 12, 119-144.
Vail, P.R., Hardenbol, J. & Todd, R.G. (1984). Jurassic unconformities,
chronostratigraphy, and sea-level changes from seismic stratigraphy
and biostratigraphy. In: Schlee, J.S. (Ed.), Interregional
Unconformities and lIydrocarbon Accumu.lation. Amer. Assoc. Petrol.
Geol., Tulsa, 129-144.
Winter, II. de la R. (1991). Author's reply to discussion. S. Afr. J. Geol.,
94, 398-400.
---- & Brink, M.C (1991a). Authors' reply to discussions. S. Afr. J.
Geol., 94, 406-408.
---- & ---- (1991 b). Chronostratigraphic subdivision of the Witwatersrand
Basin based on a Western Transvaal composite column. S. Afr. J.
Geol.,94, 191-203.
All the rocks below an unconformity are older
than the rocks above it: the fundamental law of
sequence chronostratigraphy
H. de la R. Winter
Department of Geology, Rand Afrikaans University, P.O. Box
524, Auckland Park 200n, Repuhlic of South Africa
'Open thou mine eyes, that I may behold wondrous
things out of Thy Law'
(Psalm 119:18)
I am delighted that Dr Johnson (1993) has finally made me
realise the nature of the problems he has with the fundamental principles of sequence stratigraphy. It is hoped that this
response will clear the way towards acceptance of the new
powerful tool of basin analysis not only in South Africa, but
internationally. As Secretary of the South African Committee of Stratigraphy, his conversion may not only be of immense importance to local geology, but as our representative
at international stratigraphic commissions, he holds the
DISTAL
DEPOCENTRE
PROXIMAL
SOURCE
Figure 1 Wheeler diagram: the conversion of a schematic
palaeostructural cross-section above to a chronostratigraphic equivalent helow, constructed hy dropping vertical lines from the
intersections in the upper section to equivalent horizontal timelines 1 - 9 in the lower. The proximal unconformity hetween the
two sequences contain all the time-lines from 3 - 7 at the proximal limit as seen on the chronostratigraphic version, and only line
5 where it ha<; hecome conformahle, which is the only time-line
appearing along the full distance of the unconformity surface as
seen on the physical upper section. This tangible surface represents time 5 throughout the region where it can be detected. Similarly, the other two hounding unconformities are measurable as
times 1 and 9.
Note that all sediments preserved hetween times 1 and 5, and 5 to
9 are hy definition two chronostratigraphic units. It is unnecessary
to have sediments representing the interval everywhere: such an
ideal is hardly ever encountered naturally.
The boundary surfaces of each sequence are generally diachronous, hest seen on the chronostratigraphic section. The diachronous transgressive surface of a sequence is often mistaken for a
diachronous unconformity, especially when the hiatus is small.
potential to be honoured as the delegate to have resolved the
impasse concerning the terminology and nomenclature of
sequence stratigraphy, a matter of importance for effective
communication.
All through the ages, simple axiomatic truths have been
accepted with less alacrity than minor advances in scientific
understanding (Hubbert, 1963).
Let us first conduct an experiment to test the statement in
the title. Fill any receptacle steadily with any particulate
material. Then stop this process and remove some material.
Cover the surface with a thin plastic material to represent an
unconformity. Then resume the depositional process. However hard one can try, it is impossible for the material below
the plastic layer to have been put above that layer before the
plastic was in place.
The epigraph to the Winter & Brink paper of 1991 has the
same connotation:
'For everything there is a season, and a time for every
S.AfrJ.Gco1.,1993,96(4)
matter under heaven. A time to cast away stones and a
time to gather stones together.'
With this experiment, one can logically and axiomatically
accept the statement that all the rocks below the unconformity are older than all the rocks above it. It is not an assumption made by myself (Winter, 1984), independently of
Vail et al. (1984) but the statement of an axiomatic principle
or natural law. It has withstood the tests of countless geologists on numerous occasions where seismic stratigraphy is
but one of many applications.
The geometric display of this axiom is developed from
the teachings of Wheeler, and involves the conversion of a
physical cross-section to the same seen in time progression.
Hence the lower cross-section is a chronostratigraphic profile (Figure 1). From such, both Winter (1984) and Vail et
al. (1984) deduced that 'although many chronostratigraphic
surfaces may merge along an unconformity, none actually
cross the unconformity'. 'For these reasons, unconformities
are not diachronous, but are time boundaries that may be
assigned a specific geologic age dated in those areas where
the hiatus is least and/or where the rocks above and below
become conformable' (Vail et al. 1984, p. 131).
A third point made by Vail et al. (1984) is: 'A
depositional sequence is a chronostratigraphic interval
because it contains all the rocks deposited during a given
interval of geologic time limited by the ages of the sequence
boundaries where they are conformities'. They make it elear
that their sea-level refers to depositional base level and that
local tectonics and sea-level changes affect base level.
Whereas they rightfully emphasised the latter for global correlation, Winter remained more interested in investigating
the proximal limiL~ of deposition, where local tectonic
effects dominate, and in the application towards Precambrian stratigraphy, hence differences in application.
We have not assumed the correctness of the above three
principles (not presuppositions) but have demonstrated them
by analysis and therefore cannot be accused of circular
reasoning. It is a logical conclusion. Logic is the basis of
science. If it concerns rocks it is geological science.
Again, the approach has been applied successfully for
many years in petroleum exploration and elsewhere. The
third statement is added here as detailed in Winter & Brink,
1991, that sequence chronostratigraphy is as valid a subdivision as magnetochronostratigraphyand others. This fact
should be brought to the attention of national and international commissions debating the validity of unconformitybounded uniL~ (UBU's) or my conclusion that classes such
as diachronic should be scrapped.
Dr Johnson was in a position to contribute his dissent in
writing from as early as 1984 when he reviewed my paper
as referee, the first manuscript of which had possibly
reached him in 1982. Yet he has only now been able to find
a reason to react, by making use of dissenting views in the
literature. It is natural and good for the advancement of
science for any new theory to be thoroughly scrutinised and
it is the responsibility of the scientist to attempt to improve
or overthrow the theory. But apparent discrepancies may be
pitfalls for those investigators, and in effect, enhance the
strength of the theory. I shall proceed to point out that
Cohen (1982) has modelled the filling of two depositional
229
basins (depobasins) separated by a landridge, and that the
rules enunciated above and analytical procedures apply to
each one. Christie-Blick et al. (1990), Christie-Blick (1991),
and others such as Johnson (1987) and Galloway (1989)
with his concept of flooding surfaces, may be confusing the
diachronous lower transgressively encroaching surface of a
sequence with the hiatus of an unconformity (Figure 1).
In the Cohen model (1982, figure 8) or alternatively that
of Johnson (1993, figures 2 & 3), a migrating uplift is stated
to give rise to a diachronous unconformity. To create a
major unconformity, the uplift has to be well above the
depositional base level. Sediments are shed from the arch
back into an older basin and forwards into a younger basin
ahead, the unconsolidated sediments subsequently removed
as the uplift advanced, invalidating Cohen's model. Without
exposure of the ridge to erosion, the chronostratigraphic
column cannot be affected by a mere thinning of the interval
as the arch migrates without leaving an unconformity, and
there will be only one basin, sediments being bypassed over
the high. With exposure, there will be a polarity difference
of sedimentation across the unconformity as evidence of the
filling of two separate depositional basins, and the unconformity would be a basin-bounding one (Winter, 1989). This
case therefore docs not demonstrate a diachronous unconformity within a depobasin and figure 1 of Johnson implies that
Christie-Blick et al. (1990) has also documented a basinseparating unconformity. Unfortunately, the reference
provided is inadequate to locate the article.
Johnson (op. cit.) envisages his illustrations to display a
regional crustal flexure migrating systematically parallel to
the flank of a subsiding basin under circumstances that probably prevailed in the marginal area of the developing Witwatersrand Basin. Figure 2, contrary to statement, is not a
basin cross-section as modelled by Wheeler and his disciples, which implies that the bulk of the sediments are
derived from out of the section. A three-dimensional model
would have illustrated the situation better.
However, Johnson wishes to illustrate the depositional
response to a diachronous uplift to demonstrate how a diachronous unconformity should develop as predicted by
several authorities previously quoted. The uplift presumes to
expose recently deposited sediments, which would wash
over into the two enbaymenL~ of sub-basins formed. Since
his figure 2 represents structural profiles, it is obvious that
the bulk of sediments filling the equally thick chronostratigraphic uniL~ are derived from the basin margin and not
from the migrdting arch, as the sediments derived therefrom
did not thicken the flanking units. The model shows with the
help of his figure 3 that it is theoretically possible for a
transverse migrating arch to create a diachronous
unconformity only under the limiting condition that there
has been no erosion, hence no unconformity. Alternatively,
the depo-basin has been separated into two and the Cohen
pitfall would apply.
Johnson's chronostratigraphic conversion of the Wheeler
diagram, figure 3, can only show sediments that are being
deposited into two separate receptacles. The discrepancy is
apparent only if the ridge is exposed.
But the question is whether the above model really represents a real-life situation which we are asked to diseuss.
Looking at figure 2e as requested,I see that strata D and E,
S.-Afr.Tydskr.Geol.,1993,96(4)
230
towards the right hand side of the diagram, are beneath the
unconformity whereas deposits of the same age appear on
the left hand side above the break. Note also that it is a
question of a time surface such as the DE contact directly
crossing the unconformity. Though it is physically displaced
along the unconformity, it is still crossing. I have mentioned
earlier that it is a clear indication of some conceptual error if
sequence analytical data suggests an interpretation of a
chronostratigraphic surface crossing an unconformity.
Returning to the application of the model to depobasins:
the tectonic control of basins cannot accommodate the
concept of a steady migration of an uplift transverse to any
basin margin (Winter, 1989), with the possible exception of
a migrating hotspot, the diameter of which is generally of
the same dimensional order as the depobasin. Also, it is
unlikely that any valid example will ever be found.
A common interpretive error which has appeared in the
literature mistakes the diachronous contacts of an unconformity-bounded sequence for a diachronous unconformity. So
widespread is this misunderstanding that the N ACSN (1983)
has seriously proposed a diachronic stratigraphic classification scheme. Figure 1 illustrates how this misconception can
arise.
By going all the way to accept the concept that the hierarchy of sequence stratigraphic uniL~ of Vail and collaborators are but another kind of chronostratigraphic scheme, following Haq et al. (1988), we saw what Johnson himself
(1991) considered as a praiseworthy opportunity to provide
a meaningful framework for the detailed chronostratigraphic
analysis of all depobasins for which biostratigraphy and
conventional chronostratigraphy cannot be applied and
where a large data-base is available. This would include all
extensively explored Precambrian depobasins. If successful,
such applications would represent a break-through of major
impact towards the analysis of depobasins of economic significance, such as the Witwatersrand. It would probably also
be the first Archaean depobasin having been thus analyzed.
Brown (1991) recognised the validity of the approach even
though he would have preferred a closer adherence to his
own methods.
It is indeed unfortunate that the problem of diachronous
unconformities appears to a minority to prevent the application of an approach which is proven time and again to be
successful when exactly the same approach is applied to
Phanerozoic depobasins, granted that additional methods are
available to constrain the basin analysis. I would be only too
pleased to be supplied with apparent contradictions to our
(Winter & Brink, 1991) method, so as to clarify the matter
in order to validate continued research on the practical
application of sequence chronostratigraphy to Precambrian
geology.
Our simple translation of sequences into chronostratigraphic units have, unfortunately for the hopes of Dr
Johnson, already been accepted by overseas workers as
quoted above. We have taken this discovery a step forward
by applying it to a Precambrian basin. It is therefore not at
all inopportune to have suggested that sequences, sechrons,
synthems, or whatever the stratigraphic commissions are
asked to consider, should include the terminology of chronostratigraphy. My greatest fear is that they will make a hash
of things, because of unfamiliarity with the practical
problems involved. While they are undecided, there will be
no option for each school but to continue with its own
stratigraphic vocabulary.
There will be no objection should the SACS insist on
chronostratigraphic nomenclature for the Witwatersrand to
conform to international convention. A humble suggestion:
both terms need not point to general chronostratigraphy.
One can signify the type of chronostratigraphic unit, for
example, Upper Kimberlian Subsechron. A final suggestion:
current research suggests that the term: synthem be reserved
for those UBU that comprise the full preserved lithologies
of contemporary depobasins, because that appears to have
been the intention ofthe original proposer.
Current research is being centred on the application of the
approach of Winter & Brink (1991) to the Witwatersrand
Basin as a whole, based on the case history of a region east
of Johannesburg, followed by a consideration of the evolution of the Kaapvaal Province.
The application of sequence stratigraphy has confirmed
geologic histories arrived at before the method was known,
and in addition, provides a powerful means of analyzing the
evolution. Such results would not be expected if sequence
were not a type of chronostratigraphy. No single case of
transverse migratory uplift can be fitted into the evolutionary scenario.
In summary, the question of the existence or otherwise of
diachronous unconformities is fundamental to progress in
the application of sequence stratigraphy to Precambrian
geology, and its resolution is a matter of priority. Anyone
who has succeeded in experimentally placing pre-unconformity sedimenL~ above the surface representing the interruption before it was there, can report on a major break-through
in science.
One has to conclude that the models of Cohen, ChristieBlick, and Johnson are ficticious.
References
Brown, L.F., Jr. (1991). Discussion on 'Chronostratigraphic subdivision of
the Witwatersrand Basin hased on a Western Transvaal composite
column'. S. Afr. J. Geo!., 94, 400--401.
Christie-Blick, N. (1991). Onlap, offlap, and the origin of unconformitybounded depositional sequences. Marine Geology, 97, 35-56.
----, Mountain, G.S. & Miller, K.G. (1990). Seismic stratigraphic record
of sea-level change. In: Sea-level Change: Studies in Geophysics. Nat.
Res. Coun., Nat. Acad. Sci., 116-140.
Cohen, C.R. (1982). \'todcl for a passive to active continental margin
transition: implications for hydrocarbon exploration. Bull. Amer.
Assoc. petrol. Geo!., 66, 708-718.
Galloway, W.E. (1989). Genetic stratigraphic sequences in basin analysis
1 : Architecture and genesis of flooding-surface bounded depositional
units. Bull. Amer. Assoc. petro!. Geol., 73,125-142.
Haq, B.V., Hardentx)I, J. & Vail, P.R. (1988). Mesozoic and Cenozoic
chronostratigraphy, and cycles of sea-level change. In: Wilgus, C.K.,
et al., (Eds.), Sea-Level Changes - An Integrated Approach. Spec.
Publ. Soc. Eeon. Paleont. Mineral., 42, 71-108.
Hubbert, M. King (1963). Are we retrogressing in Science? Bull. Geol.
Soc. Amer., 74, 365-378.
Johnson, J.G. (1987). Vnconformity-txmnded stratigraphic units:
Discussion. Bull. Geol. Soc. Amer., 99, 43.
Johnson, M.R. (1991). Discussion on 'Chronostratigraphic subdivision of
the Witwatersrand Basin hased on a Western Transvaal composite
column'. S. Afr. J. Geo!., 94,401-403.
----- (1993). Reply to author's reply to discussion of 'Chronostratigraphic
subdivision of the Witwatersrand Basin hased on a Western Transvaal
composite column'. S. Afr. J. Geo!., This volume.
231
S.Afr.J .Gcol., 1993,96(4)
NACSN. North American Commission on Stratigraphic Nomenclature
(1983). North American Stratigraphic Code. Bull. Amer. Assoc. petrol.
Ceol., 67, 841-875.
Vail, P.R, Hardenbol, J. & Todd, RG. (1984). Jurassic unconformities,
chronostratigraphy and sea-level changes from seismic stratigraphy
and biostratigraphy. In: Schlee, J.S. (Ed.), Interregional
Unconformities and lIydrocarbon Accumulation. Amer. Assoc. Petrol.
Ceol., Tulsa., Okla., 129-144.
Winter, H. de la R. (1984). Tectonostratigraphy, as applied to analysis of
South African Phanerowic basins. Trans. geol. Soc. S. Afr., 87,
169-179.
---- (1989). A tectonic classification of certain South African depositional
basins and criteria for recognition of major unconformity-bounded
sequences. S. Afr . .T. Geol., 92, 167-182.
---- & Brink, M.R (1991). Chronostratigraphic subdivision of the
Witwatersrand Basin based on a Western Transvaal composite column.
S. Afr . .T. Geol., 94, 191-203.
Errata
Special Issue on Carbonatites
South African Journal of Geology, 1993, 96(3)
p. 91: Figures 14 and 15's photographs have been
transposed.
p. 111: The end of the caption of Figure 6 should read: (cL
Figure 9).
p. 112: The second sentence of the caption of Figure 9
should read: AB is the section line of Figure 6.
p. 123: In the second column, line 7 from below, REE
should read REO.
p. 147: In the footnote of Table 3, insert after Gl = glass:
Ql = quenched liquid.