Tertiary Sea-Level Movements around Southern Africa
Author(s): William G. Siesser and Richard V. Dingle
Source: The Journal of Geology, Vol. 89, No. 4 (Jul., 1981), pp. 523-536
Published by: The University of Chicago Press
Stable URL: http://www.jstor.org/stable/30085066 .
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TERTIARY
SEA-LEVEL
MOVEMENTS
AROUND
SOUTHERN
AFRICA1
WILLIAM G. SIESSER and RICHARD V. DINGLE
Department of Geology, Vanderbilt University, Nashville, TN
Department of Geology, University of Cape Town, Rondebosch, South Africa
ABSTRACT
Sedimentological, micropaleontological and seismic-profiling data elucidate the history of Tertiary
sea-level movements around southern Africa. These new data show that landward movement of the sea
began in early Paleocene time and continued into the early Eocene. The sea probably reached its maximum Paleocene height during the early Eocene, and is today represented by outcrops up to at least 204 m,
and probably as high as 360m, above modern sea level. A brief regressive pulse occurred during the
middle Eocene, with renewed transgression in the late Eocene. A major regression followed, probably
spanning all of Oligocene and early Miocene times. This regression exposed much of the continental shelf
and is clearly represented on seismic-reflection profiles by a widespread unconformity. The major
Neogene transgression began in the middle Miocene and probably reached its greatest extent in the late
Miocene or early Pliocene. The overall middle Miocene to early Pliocene transgression was interrupted by
a brief regressive pulse near the Miocene-Pliocene boundary. Seas withdrew again in the late Pliocene.
Rocks deposited during the Miocene-Pliocene transgression are today found up to 330 m above sea level.
This scheme should be viewed as showing only the gross movements of the seas around southern Africa
during the Tertiary. Nevertheless, the timing of these southern African transgressions/regressions is
closely parallel to the timing recently established for sea-level movements in other parts of the world.
INTRODUCTION
Changes in sea level around southern
Africa during various epochs of the Tertiary
have been discussed by numerous authors
(e.g., King 1967; Haughton 1969; Truswell
1970; Tankard 1976). Opinions as to timing
are diverse. Dingle (1971, 1973) and Dingle
and Scrutton (1974) were perhaps the first
to present substantial evidence as to the
timing of sea-level movements for the entire
Tertiary Period. They postulated a late
Maastrichtian regression followed by a
regional, late Paleocene-early Eocene transgression. Another regression spanning late
Eocene to early Miocene time was followed
by an early Miocene transgression. A late
Miocene-early Pliocene regression was followed by a brief transgression, then regression again in the late Pliocene. These
times were based on older published reports
on the ages of onshore rocks, and on seismic
evidence combined with a small number of
dated rocks from the continental shelf.
Almost 200 new dates based on calcareous
1 Manuscriptreceived February 28, 1980; revised June 23, 1980.
[Journalof Geology, 1981, Vol. 89, p. 083-096 ;
publication revised, 1981, Vol. 89, p. 523-536]
© 1981 by The Universityof Chicago.
0022-1376/81/8901-006 $1.00
nannofossils and planktic foraminifers are
now available for Tertiary strata on the continental margin (Siesser 1977a, 1978a and
unpublished data) and for certain onshore
coastal rocks (Siesser 1977b, Siesser and
Miles 1979). The actual presence onshore, or
close inshore, of rocks deposited during
different time intervals is our strongest
evidence for transgression at those times.
Data based on lithologic facies and seismicprofile records add further information to
the history of sea-level movements around
southern Africa (fig. 1).
Based on these new data, we present here
a revised interpretation of relative sea-level
movements around southern Africa. Our
sea-level curve (fig. 2) should be viewed as a
"generalized" curve, showing the gross direction of sea-level movements during epochs
and sub-epochs of the Tertiary. Local uplift
or subsidence may have caused one area of
the coast to emerge or submerge slightly
earlier than another area.
STRATIGRAPHY
The sedimentation history of southern
Africa was dominated by erosion rather than
deposition during the Tertiary, and there
are relatively few onshore Tertiary outcrops.
523
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text.
the
in
mentioned
localities
the
showing
Map
1.
Fig.
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TERTIARY SEA-LEVEL MOVEMENTS
70
65
60
55
PALEOCENE
LATE
CRET.
LATE
EARLY
50
45
EOCENE
ARLY MIDDLE
EARLY
MIDDLE
40
/35
30
25
25
20
OLIGOCENE
L. EARLY
LATE
525
15
10
MIOCENE
EARLY
I MID.
5
PLIO
] LATE
0 m.y.
Q
E.[ L. A
- + 400
- + 300
TRA NSGRESSIONS
- +200
- +100
-200
REGRESSIONS
- -300
- -400
W = 610 m(m in)
AB= 1350 m
- -500
--600
Fig. 2. - Curve showing gross direction of Tertiary sea-level movements around southern Africa. Curve
maxima above coastline are elevations in metres at which rocks deposited during the relevant transgressions
now lie; below coastline indicates levels at which erosion surfaces cut during the relevant regressions now
lie. Dashed lines indicate uncertainty. Letters refer to chief outcrop localities on which interpretations are
made: AB - Agulhas Bank, B - Birbury, E - east coast, N - Namibia, NC - Needs Camp, S - south coast,
W - west coast, Z - Zululand. Time scale is from Vail et al. (1977).
Offshore, however, the surface of the continental margin is made up mostly of Tertiary
rocks, overlain by thin, unconsolidated
Quaternary sediments.
Paleogene. - Lowermost Paleocene rocks
have not been found either onshore or on
the surface of the continental shelf around
southern Africa, but "middle" Paleocene has
been identified in Mozambique (Flores 1973;
Forster 1975), at Richards Bay (middle
Danian - Maud and Orr 1975; Stapleton
1975) and on the continental shelf off Cape
St. Lucia (Siesser 1977a) (fig. 1).
This pattern suggests that a regression
initiated in late Cretaceous time continued
into the Paleocene. Lowermost Paleocene
marine sediments were not deposited in the
inshore zone, although a complete sequence
was deposited farther offshore, as indicated
by DSDP drilling on the outer continental
margin (Bolli et al. 1975).
Lower, middle and upper Eocene rocks
are known onshore in Mozambique (Flores
1973; Forster 1975), and lower Eocene at
Birbury in South Africa (Siesser and Miles
1979). In Namibia, possibly Paleocene/lower
Eocene rocks are exposed near Buntfeldschuh
(Siesser and Salmon 1979), and upper
Eocene rocks are found at Bogenfels (Siesser
1979b). Upper Eocene foraminifers reworked into Miocene rocks show the former
presence of upper Eocene deposits in Zulu-
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526
W. G. SIESSER AND R. V. DINGLE
land (Frankel 1968, 1972). Offshore, lower
and upper Eocene rocks are common on the
continental shelf (Siesser 1977a). Middle
Eocene rocks are definitely known on the
shelf only from deep oil-exploration boreholes (Du Toit and Leith 1974; Dingle et al.
in press). Nevertheless, a recognizable stratigraphic break occurs within the middle
Eocene sequence in the boreholes (Dingle et
al. in press).
The deposits suggest that the Paleocene
transgression continued at least into the
early Eocene. The absence onshore and
scarcity offshore of middle Eocene rocks
around most of southern Africa probably
represents a regressive pulse, with seas
moving shoreward again in late Eocene times.
Another regression followed, spanning all
of Oligocene time. From seismic records,
Dingle (1971) recognized this as a widespread Oligocene unconformity on the continental shelf. Locally, some marine Oligocene rocks are reported in Mozambique
(Flores 1973), but none onshore in South
Africa or Namibia. Oligocene rocks are
similarly rare on the continental shelf, being
found only in the J(c)-l borehole and on the
continental shelf south of Plettenberg Bay (a
single upper Oligocene sample). Du Toit and
Leith (1974) suggested an early Oligocene
transgression, since marine limestones overlie
deltaic deposits in the J(c)-l borehole. However, data from all other areas militate
against an Oligocene transgression. A possible
alternate explanation for limestone over
deltaic deposits is simple cessation of delta
growth and the inevitable local "transgressive" covering by the sea, owing to wave
erosion and delta destruction (e.g., see
Rainwater 1964).
Neogene. - Miocene sea-level movements
are a subject of some debate in southern
Africa. Most of the outcropping Miocene
rocks in Mozambique are middle and upper
Miocene (Frankel 1972; Flores 1973).
Frankel (1972) and Flores (1973) accordingly suggested a middle or late Miocene
transgression followed by a late MiocenePliocene regression. Conversely, Forster
(1975) believed transgression occurred in
early Miocene times, followed by regression
with short, local transgressions occurring in
middle Miocene-Pliocene times. The outcrop evidence supporting Forster's interpretation is not entirely clear.
A similar controversy exists in Zululand,
where King (1953 et seq.) favored an early
Miocene transgression, followed by erosion,
then a Pliocene transgression. Frankel (1968,
1972) implied a middle or late Miocene to
early Pliocene transgression for the same
rocks. A late Miocene-early Pliocene age has
recently been firmly established for these
rocks, based on onshore micro- and nannofossil dating (Maud and Orr 1975; Stapleton
1977; Siesser and Miles 1979). It may be
that the sea only reached Zululand in late
Miocene time, at the earliest.
Neogene limestones are also exposed
intermittently
along the Cape Province
coastline between Bathurst and Saldanha
Bay. These limestones are Miocene and/or
Pliocene, but their precise age is difficult to
establish (see Siesser 1971, 1972a for a
review of the ages assigned to these limestones).
On the west coast, Corvinus and Hendey
and Hendey (1978a)
reported
(1978)
serpulid polychaete worm tubes (cf. Mercierella) in middle Miocene river-terrace
deposits at Arrisdrift, on the Orange River
about 30 km from the river mouth. This is a
typical estuarine worm, implying that the
middle Miocene coastline was much closer
to Arrisdrift than it is today. However,
estuaries can extend a considerable distance
inland, and it is difficult to estimate the
actual height of sea level based on these
worm tubes. Nevertheless, the presence of
the estuary, together with the height of the
river terraces (some 50m above present sea
level), strongly suggest a middle Miocene
sea level higher than today. Further strong
support for a middle-late Miocene transgression is found on the continental shelf.
Middle and upper Miocene rocks are common on the shelf, but only one definitely
lower Miocene sample has been found
(Siesser 1977a).
We believe the seas began to move landward in middle Miocene time, perhaps
reaching the present coastline during middle
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TERTIARY SEA-LEVEL MOVEMENTS
Miocene, but certainly by late Miocene. This
overall transgression continued into early
Pliocene; it is represented in Mozambique
by Pliocene rocks (Flores 1973), and some
of the coastal limestones from the South
African south coast are Pliocene as well
(Siesser 1971; King 1973). Only lower Pliocene has been definitely recognized among
the Pliocene rocks on the continental shelf
(Siesser 1977a).
The Miocene-Pliocene transgression was
probably broken by a regressive pulse close
to the Miocene-Pliocene boundary. Vertebrate remains in the Varswater Formation at
Saldanha Bay (fig. 1) have a maximum age
range of 3.5 to 7 m.y. B.P. (Hendey 1978b),
although Hendey (1978c; pers. commun.,
1979) believes 4 to 5 m.y. may be a closer
estimate. These vertebrate remains occur in
regressive fluvial and estuarine sands. The
underlying marine Varswater deposits thus
can be no younger than early Pliocene and
could well be late Miocene in age. In any
case, they were almost certainly deposited
during the middle Miocene-early Pliocene
transgression. Siesser and Rogers (1976)
found evidence for a brief westward shift in
the offshore Benguela Upwelling System at
this time, which they suggested was caused
by a brief late Miocene-early Pliocene regression. Siesser (1978b) also suggested seafloor erosion sometime during the middle
Miocene-Pliocene interval, based on the
incorporation of Miocene phosphatic clasts
in Pliocene phosphorites.
Further regression, with strong erosion,
is inferred to have occurred in late Pliocene
time, as shown by the widespread regressive
deposits onshore in both Mozambique
(Frankel 1972) and South Africa (Siesser
1971, 1972a) and the destruction and extensive reworking of offshore phosphorite beds
(Dingle 1974; Siesser 1978b).
MAGNITUDEOF SEA-LEVELMOVEMENTS
Transgressions. - The available evidence
is somewhat scanty to prove which of the
Paleogene transgressions was most extensive.
The Paleocene-early Eocene transgression
deposited rocks now found at 204m at
527
Birbury. The Eocene outcrops at Needs
Camp (about 360m) and at Buntfeldschuh
(163 m) are also probably early Eocene, although this cannot be conclusively proved
(Siesser and Miles 1979; Siesser and Salmon
1979). The upper Eocene, on the other
hand, is only known from low-lying elevations, about 70 m at Bogenfels and about
28m in Zululand (Eocene reworked into
Miocene).
Transgression beginning in the middle
Miocene deposited rocks now resting at 90 m
in Zululand (Frankel 1972, and our own observations). Along the southern coast of
South Africa the marine Miocene-Pliocene
limestones reach a height of about 300m
near Bathurst (Siesser 1972); Pliocene limestones in the same region (at Paterson) are
found at about 330 m (King 1973).
The possibility of differential uplift along
the southern African coast (Siesser 1978a)
makes it difficult to assess the true height
sea level reached during the various Tertiary
transgressions, but the present elevations
quoted here give some idea of the relative
magnitudes (fig. 2).
The approximate water depth during each
transgression can be inferred based on the
lithology, paleontology, and sedimentary
structures of the rocks deposited. The
Tertiary marine rocks exposed onshore, and
most of those exposed on the continental
shelf, are limestones, phosphorites, and
calcareous sandstones
deposited
under
shallow neritic conditions (Frankel 1968;
Siesser 1971; Siesser 1972a; Du Toit and
Leith 1974; Forster 1975; Siesser and
Salmon 1979). Parker and Siesser (1972)
and Siesser (1972b) have suggested that
some of the limestones and phosphorites
exposed on the continental shelf were
deposited under deeper, but still neritic, conditions (probably in a middle or outer shelf
environment).
Regressions. - Offshore, the main regressions are represented by unconformities
on seismic records and breaks in the succession in boreholes. In table 1 and figures
2, 3 and 4 we have plotted a summary of
data from each continental shelf, including
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1:693
1:963
1:347
1:108
Coast
Gradient value)
km
m
130
West
(min.
285
210m
610m
Depth
467m
Arch
1:167
1:6001:1818
Block Gradient
data
km
Flank) m no
52
Stable
(Agulhas 195
540m
Depth 375m
Gradients
1:833(W)
Gradient
50km
150m
Depth
Bank
Erosion-Surface
1
and
Agulhas
TABLE
Surfaces
Erosion
to
1:74
1:269
1:556(E)
1:78km
Gradient
75
m m
150
350m
Depth
425
1350m
Depth
1:4441:488
Gradient
data data km
Coast
20
Maximum
m no no
East
80
2
Depth 532m
Surface
Paleocene
Pliocene
Pleistocene
Erosion
Miocene
Miocene-lower
Pliocene-lower
Cretaceous-lower
width
Shelf
Upper
Upper
Oligocene-lower
Upper
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TERTIARY SEA-LEVEL MOVEMENTS
Late
early
Cret./
Pal.
Olig./
early Mio
late Mio./
early
Plio
529
late
early
Plio./
Pleist.
Fig. 3. - Plot of Tertiary erosion surfaceson the continentalshelvesaroundsouthernAfrica: erosionsurface gradient (filled circles) and maximumdepth of erosion (crosses)versus age of surface. Note the
anomalously high value for the Oligocene-earlyMiocene gradient on the flanks of the Agulhas Arch,
which is a stable basementblock. Data from table 1.
the maximum seaward depths to which
individual erosion surfaces can be identified,
and the seaward gradients of the surfaces.
Although the paucity of data does not allow
a very comprehensive statement, a few
points do emerge clearly. As might be expected, there is an approximately inverse
relationship between erosion-surface gradients and maximum depth of erosion when
plotted against age (fig. 3). In general, the
deepest levels of erosion and steepest
gradients are oldest, and the shallowest levels
of erosion and lowest gradients are youngest.
The depth of the oldest Tertiary erosion surface (Maastrichtian-lower Paleocene) varies
considerably from 1350 m off the east coast
and more than 610 m on the west coast, to
only 540 m on the flanks of a buoyant basement block (the Agulhus Arch). Elsewhere
the erosion surface is too deeply buried to
be picked up clearly on seismic records in
shallow-water areas.
The mid-Tertiary (Oligocene-lower Miocene) erosion surface is the most widespread
and easily recognized. Its maximum depth
occurs at a remarkably constant level (532 m
to 425 m) in the sediment basins: on the side
of the positive Agulhas Arch it is only
slightly shallower (375 m).
Regional information on the upper
Miocene-lower Pliocene erosion surface is
not complete. No evidence has been found
for it off the east coast, nor on the flanks of
the Agulhas Arch, but off the south and
west coasts its outer edge lies at 350 and
285 m, respectively. The last major period
of erosion to affect the southern African
continental margin cannot be dated accurately, but we consider it to be late Plioceneearly Pleistocene in age, with the resultant
erosion surface obviously being further
modified by later Pleistocene low sea-level
stands. The late Pliocene-early Pleistocene
erosion affected the continental shelves
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530
W. G. SIESSER AND R. V. DINGLE
Late
140
km.
Pliocene
- early
erosion
Pleistocene
continental shelf width
80
60
40
120
100
present
coast
20
0
20
surfaces
onshore width
80
40
60
r
-
100
km.
.50
100
-150
-200
1250
maximum
erosion level
in metres
Fig. 4. - Plot of Tertiary erosion surfaces on the continental shelves around southern Africa: Continental-shelf width versus depth of erosion for the late Pliocene-early Pleistocene erosion surface. Data
from table 1.
around the whole coastline and trimmed the
older rocks down to levels of between 80 m
(east coast) and 210 m (west coast).
Table 1 shows that those shelves with the
deepest levels of erosion are the widest and
have the lowest gradients (west coast),
whereas the narrowest shelf (east coast) has
the steepest gradient and shallowest erosion
level. This relationship seems to hold only
for those areas overlain by thick sediment
(i.e., not a basement high such as the
Agulhas Arch). Further investigation shows
an approximately linear relationship between shelf width and maximum erosion
level (fig. 4). This suggests that since the late
Pliocene-Pleistocene erosion surface was cut,
the shelves around southern Africa have all
subsided through the same angle about a
hinge that runs around southern Africa at a
common distance (somewhere between 80
and 120 km) inland from the present coastline. This may approximate the Great Escarpment axis recognized by King (1967).
The observation that present day elevations of the outer edges of the mid-Tertiary
erosion surfaces lie at roughly the same level
(532-425 m) after the subsidence that has
followed the cutting of the PliocenePleistocene surface produces the following
corollary: the Miocene-Pliocene subsidence
(deformation) of the mid-Tertiary surface
was greatest on the narrowest continental
shelves. (The rule does not, however, seem
to apply to a buoyant area such as the flanks
of the Agulhas Arch.) Although we do not
have sufficient data for a comparison (only
one reliable value in a sedimentary basin), a
similar situation may have existed between
early and mid-Tertiary times. Whether the
basic difference between Eocene-to-Pliocene
and post-Pliocene/Pleistocene
subsidence
was a change in location of hinge axis or
merely a difference of angular rotation from
place to place during the earlier period is not
clear and will require further investigation.
CAUSESOF SEA-LEVELMOVEMENTS
We stressed earlier that our sea-level curve
(fig. 2) is a generalized curve, showing the
gross movements of sea level around southern
Africa during the Tertiary. But are these
gross movements in themselves only regional
events or are they part of global eustatic
changes occurring during the Tertiary?
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TERTIARY SEA-LEVEL MOVEMENTS
70
60
65
55
50
PALEOCENE
CRET.
ARLY
LATE
45
35
40
EARLY
MIDDLE
25
30
20
L.
EARLY
ATE
15
EARLY
MID.
Om.y.
5
10
MIOCENE
OLIGOCENE
EOCENE
531
PLIO Q
ATE
ELL.
GLOBA
Fig. 5. - Comparison of southern African, Western Australian and global sea-level changes. Vertical
scales are diagrammatic.
It is worth emphasizing that the timing of
such global events would not be expected to
coincide exactly. The same local subsidence
and uplift that might cause a slight variance
in timing around the coast of one area
(southern Africa for example), would be
multiplied when other continental masses
are considered. Nevertheless, the pattern of
"gross" movements should remain.
Vail et al. (1977) provided the most comprehensive and up-to-date synthesis of global
Tertiary sea-level changes, with a curve (fig.
5) based on the modal averages from many
areas around the world. Most of their data
come from oil wells and seismic sections in
the Northern Hemisphere; the only Southern
Hemisphere localities are around Australia
and New Zealand. Vail and his colleagues
also pointed out the expected distortion
from continent to continent caused by
regional subsidence, sedimentary loading,
deformation, etc., as well as the differences
in timing artifically created by local differences in techniques of age dating.
With these constraints in mind, it is
intriguing to note the similarities between
our southern African sea-level cycles and the
modal global cycles described by Vail et al.
(1977, p. 93). They proposed sea-level
highstands in the late Paleocene-early
Eocene, the late middle Eocene to early
Oligocene (with fluctuations), the middle
Miocene and the early to "middle" Pliocene.
Lowstands are mid Paleocene, early middle
Eocene, mid late Oligocene to early Miocene
(with fluctuations), late Miocene and late
Pliocene-early Pleistocene.
Their curves (Vail et al. 1977, fig. 3)
show gradually rising sea levels followed by
abrupt falls. This has probably also been the
nature of southern African sea-level changes.
However, we have no direct evidence to
prove this and have accordingly shown
smooth, rather than asymmetric, curves depicting the rises and falls in our area.
Major global transgressions in Eocene and
late Miocene times have also been documented by other workers, as has a global
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532
W. G. SIESSER AND R. V. DINGLE
Oligocene regression (Rona 1973; Flemming
and Roberts 1973).
In the Southern Hemisphere it is interesting to compare our sea-level curve with that
proposed by Quilty (1977) for Western
Australia (fig. 5). He found that the first
Tertiary transgression began in early Paleocene time (middle Danian) and continued
into early Eocene. Seas reached their maximum extent in the late Paleocene. Sedimentation ceased in late early and early middle
Eocene. Late Eocene was a time of major
transgression in Western Australia and was
followed by major regression during the
early Oligocene. Seas transgressed again from
late Oligocene to early Miocene time. Then,
after a brief regression in the late early Miocene, seas transgressed again in the middle
Miocene. This was the second great Tertiary
transgression in Australia. A hiatus spanning
late middle and early late Miocene time was
followed by further transgression in latest
Miocene, which more or less continued into
the Quaternary (the Pliocene is apparently
not as well documented as earlier epochs
in Western Australia). In southeastern
Australia, however, Carter (1979) has clearly
demonstrated the late Miocene transgression
was followed by a regression extending from
latest Miocene to earliest Pliocene. Renewed
transgression began in the early Pliocene, followed by another regression in the late Pliocene.
The similarities between southern Africa
and Australia with respect to Tertiary sealevel movements are striking. In both regions
the first major Tertiary transgression began
in middle Danian time and continued into
the early Eocene. Little middle Eocene sedimentation occurred in either region, but
both experienced a major transgression
during the late Eocene. Again, both regions
experienced a major regression during the
Oligocene. Here the only significant difference between the two areas arises: we
believe this regression continued into the
early Miocene around southern Africa
whereas Quilty reported renewed transgression in the late Oligocene. Both regions
experienced a second major transgression in
middle Miocene time. The Western Australian
sequence is broken by nondeposition in late
middle and late Miocene, whereas in southeastern Australia the regressive pulse spans
the Miocene-Pliocene boundary - exactly as
we have suggested for southern Africa. Moreover, an early Pliocene transgression is followed by the late Pliocene regression in both
regions.
If Tertiary sea levels changed in a grossly
uniform manner, the cause almost certainly
was related to some form of tectono-eustacy,
glacio-eustacy, or both (see Donovan and
Jones 1979 for a discussion of these and
second-order causes of
some
possible
eustacy).
Tectono-eustacy. - Tectono-eustacy refers
to the change in ocean capacity owing to
This broad concept was
diastrophism.
originally suggested by Suess (1906), and has
since taken various forms. Tectono-eustatism
related to sea-floor spreading is a mechanism
which has probably received the most attention in recent years. In essence, the volume
of the ocean basins changes because of elevation or subsidence and/or accelerating or
decelerating spreading rates of the MidOceanic Ridge system. The basic idea is that
uplift of the Ridge system (a volume increase
of the Ridge) causes a volume decrease in
ocean-basin capacity. The displaced water
spills onto the continents and causes a transgression. Subsidence (volume decrease) of
the Ridge has the opposite effect. A refinement of this basic hypothesis links it to seafloor spreading.
Hallam (1963) was one of the first
workers to suggest that rapid spreading of
the Mid-Oceanic Ridge caused a global transgression. Hays and Pitman (1973) corroborated Hallam's ideas. Hays and Pitman (1973)
assumed that the volume of a ridge at any
time is a function of the spreading rate, since
cooling and subsidence of the ridge as it
moves away from the axis is time dependent.
They calculated the volume of the Ridge at
various times during the Mesozoic and Cenozoic and compared known spreading rates
for various ridge segments at those times.
Flemming and Roberts (1973) also suggested that eustatic changes are connected
with global spreading-rate changes. They
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TERTIARY SEA-LEVEL MOVEMENTS
Paleocene
80
Eocene
Oligocene
Miocene
533
PliojQ
70
Fig. 6. - Tertiary sea-floor spreading rates in the southeast Atlantic and their correlation with sea-level
movements on the continental shelves of southern Africa.
estimated the spreading rate during Tertiary
epochs and plotted this beside a generalized
global eustatic curve.
We have attempted to investigate the
relationship between southern African sealevel movements and regional ridge-spreading
rates. Average Tertiary spreading rates have
been calculated for the southeast Atlantic
(on a traverse north of Gough Island) using
the data given by Emery et al. (1975), and
are shown in figure 6, together with local
sea-level movements. Because of recent controversy over recognition of Cretaceous magnetic anomalies (e.g., Larsen and Ladd 1973,
Emery et al. 1975, Rabinowitz 1976, Du
Plessis pers. commun.) in the southeast
Atlantic, and in particular the older limit of
the quiet zone and the position of M.O., it is
not possible to calculate a Late Cretaceous
spreading rate.
Spreading rates progressively increased
during the Tertiary, from 13.5km/m.y. in
the Paleocene-early Eocene to 25.9 km/m.y.
during the late Miocene-Quaternary interval.
No correlation between spreading rates and
sea-level movements is apparent. Regional
fast spreading related to regional transgression, or vice versa, is certainly not indicated. For example, the main Paleogene
transgression occurred during the time of
slowest spreading, whereas the major Tertiary
regression occurred during a time of fast
spreading (fig. 6).
Glacio-eustacy. - Glacio-eustacy is undoubtedly the cause of Pleistocene sea-level
fluctuations, but its role in Tertiary eustacy
is debatable. Much new information on the
timing of Antarctic glaciation has been obtained by the Deep Sea Drilling Project
during the last few years. It is now known
that the Antarctic ice sheet began to enlarge
during the middle Miocene (about 13 m.y.
B.P.). This build-up was rapid and was
essentially complete by the early late Miocene (10-12 m.y. B.P.) (Kennett et al. 1975).
The first ice-rafted debris was derived from
this glacial accumulation. This ice sheet may
have been smaller than today, but expanded
between about 4.7 and 4.3 m.y. B.P. to become substantially greater (about 50%) than
today (Shackleton and Kennett 1975).
Shackleton and Kennett (1975) estimate a
glacio-eustatic sea-level lowering of about
40 m as a result. A lesser glacial advance may
have occurred about 3.5 m.y. B.P. Glaciation
in the Northern Hemisphere was a later
event, starting about 2.6 m.y. B.P.; more
extensive glaciation in the Northern Hemisphere occurred in the early Pleistocene
(Shackleton and Kennett 1975).
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534
W. G. SIESSER AND R. V. DINGLE
The glacio-eustatic lowering between 4.7
and 4.3 m.y. may correspond to the regressive
pulse we postulate as occurring near the
Miocene-Pliocene boundary; the growth of
Northern Hemisphere glaciers from 2.6 m.y.
onward probably contributed to the late
Pliocene regression around southern Africa
and other parts of the world.
Epeirogenesis. - Epeirogenic uplift of
southern Africa occurred in early Late
Cretaceous times (King 1967; Truswell
1970), regenerating erosion and initiating
King's (1967) "African" erosion surface this major erosion cycle continued at least
until the end of the Oligocene. At the end of
that epoch, or in the early Miocene, gentle
uplift of a few hundred metres occurred
(King 1967, 1972; Truswell 1970). Stronger
upwarping occurred again at the end of the
Miocene, uplifting parts of Natal by 600 m
(King 1972). This phase of epeirogeny
initiated the "Post-African" erosion surface
which covers more area than any other in
Africa (King 1967). The final, and major,
Cenozoic uplift of southern Africa began at
the end of the Pliocene and continued into
the Quaternary. The interior of southern
Africa is believed to have been uplifted by
about 1200m during this phase (Truswell
1970).
Such major epeirogenesis must have had
an effect on sea level. The late Neogene uplifts of the continent are easiest to relate to
our sea-level curve. Uplifts at the end of the
Miocene and Pliocene appear to coincide,
respectively, with: 1) the regressive pulse we
have proposed close to the Miocene-Pliocene
boundary, and 2) the more widespread regression beginning in late Pliocene times.
Late Oligocene or early Miocene uplift
also coincides, at least in the later part, with
the major Oligocene-early Miocene regression
we have suggested, but the Eocene and
Paleocene regressions cannot be correlated
with any "uplift" during the long duration
of the African erosion cycle.
CONCLUSIONS
Tertiary seas first began to transgress the
southern African coastline during late early
Paleocene. Transgression continued into the
early Eocene, followed by regression during
the middle Eocene and transgression again
during the late Eocene. A major regression
spans all of Oligocene and early Miocene
times. This was followed by renewed transgression during the middle Miocene, which
continued into the early Pliocene (although
broken by a regressive pulse near the
Miocene-Pliocene boundary). The end of the
Tertiary was marked by a late Pliocene
regression.
Vail et al. (1977) have defined a "cycle"
of relative change of sea level as the time
interval during which a relative rise and fall
of sea level takes place. They recognize firstsecond- and third-order eustatic cycles. Their
second-order Tertiary cycles are as follows:
1) Ta supercycle: a late Paleocene to early
Eocene rise and a middle Eocene fall; 2) Tb
supercycle: a middle Eocene to late Oligocene rise and a late Oligocene fall; 3) Tc
supercycle: late Oligocene to late Miocene
rise and a late Miocene (Messinian) fall; and
4) Td supercycle: a latest Miocene to early
Pliocene rise and a late Pliocene fall (about
3 m.y. B.P.).
Using a similar method of designating
"cycles," we can divide our southern African
regional sea-level changes as follows: 1) Ta: a
late Paleocene-early Eocene rise and a
middle Eocene fall; 2) Tb: a late Eocene rise
and an Oligocene to early Miocene fall; 3)
Tc: a middle Miocene-late Miocene rise and a
latest Miocene or earliest Pliocene fall; and
4) Td: an early Pliocene rise and a late Pliocene fall. Our regional cycles correspond to
the global cycles to a surprising degree.
There is a paucity of information about
Tertiary sea-level changes in the Southern
Hemisphere compared with the Northern
(e.g., see Vail et al. 1977, p. 88); thus we
hope this paper will add yet another piece to
the puzzle by describing the history of
Tertiary sea-level movements in a previously
part of the world little-documented
southern Africa.
ACKNOWLEDGMENTS.
- This work began
while the senior author was on the staffs of
University of Cape Town and the South
African Museum, and was completed after
he joined Vanderbilt University. All three
organizations are thanked for their support.
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TERTIARY SEA-LEVEL MOVEMENTS
535
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