391
S.Afr.J .GeoI.1988,91 (3),391-398
Washover boulder fans and reworked phosphorite in the Alexander Bay Formation
P.G. Gresse
Geological Survey, P.O. Box 572, Bellville 7535, Republic of South Africa
Accepted 27 May 1988
Washover deposits which occur in bedrock-controlled channels at + 10 m, +35 m and +60 m elevations are
described in the Alexander Bay Formation. Transgressive and regressive back-barrier and shoreface
environments are recognized on grounds of textural and compositional properties, disconformities, and fossil
content. Reworked phosphorite material occurs as a lag gravel at the base of these washover deposits below 35
m a.s.1. and is correlated with the Varswater Formation of the Langebaanweg area. On grounds of transgressive
maxima and fossil content the Middle Terrace is correlated with the '30 m Package' of Pether (1986) of middle
to late Pleistocene age. Part of the Upper Terrace is compared to Pether's '50 m Package' dated as late Pliocene
to early Pleistocene while parts of the Upper Terrace and Grobler Terrace are tentatively correlated with his '90
m Package', deposited during early to middle Pliocene and possibly even Miocene times.
Oorspoelafsettings wat in vloerrots-gekontroleerde kanale by + 10 m, +35 m en +60 m elevasies voorkom in die
Formasie Alexanderbaai, word beskryf. Trangressiewe en regressiewe agterbarriere en strandvlakomgewings
word herken op grond van tekstuur, samestelling, diskordansies en fossielinhoud. Herwerkte fosforitiese
materiaal kom voor in 'n draalgruis aan die basis van hierdie oorspoelafsettings onder 35 m b.s.v. en word
gekorreleer met die Formasie Varswater van die Langebaanweggebied. Op grond van trangressiewe maksima
en fossielinhoud word die Middelterras met Pether (1986) se '30 m-Pakket' van middel- tot laatPleistoseenouderdom gekorreleer. 'n Gedeelte van die Hoerterras word vergelyk met Pether se '50 m-Pakket'
wat as laat-Plioseen tot vroeg-Pleistoseen gedateer is, terwyl gedeeltes van die Hoer- en Groblerterras tentatief
met sy '90 m-Pakket' wat tydens vroeg- tot middel-Plioseen en moontlik selfs in die Mioseen afgeset is,
gekorreleer word.
Introduction
Neogene and Pleistogene marine sediments along the
northwest coast of Namaqualand between Port Nolloth
and Alexander Bay occur as raised beaches and lagoonal
deposits at elevations ranging from 2 m to 90 m above
present sea level (De Villiers & S6hnge, 1959; Stocken,
1962; Hallam, 1964; Keyser, 1972, 1976). The older of
these deposits are generally found at successively higher
elevations up to 9 km inland from the present coastline
(Figure 1). Various periods of transgression can be
recognized. Some of these deposits abut against sea cliffs
while others form steep, transgressive beaches.
Successively older deposits have been subjected to
increasingly severe post-depositional erosion. No formal
stratigraphic nomenclature other than the Alexander
Bay Formation (SACS, 1980) has been proposed as yet
for any of these unconsolidated to semi-consolidated
marine-shoreline, coastal-alluvial, and associated eolian
deposits.
Cenozoic geology of the Namaqualand coast
The inland edge of the onshore marine sediments along
the West Coast follows a fairly straight line parallel to
and approximately 3 km east of the present coastline
(Figure 1) and extends to elevations ranging from 50 to
90 m a.s.l. The sediments were deposited under
conditions fitting the linear-clastic-shoreline model of
Selley (1978). This model contains a number of typical
facies including marine shelf, shoreface, barrier, backbarrier lagoon, and alluvial coastal plain. These facies
can be recognized in prospecting trenches and mining
N
1
Bay
T1 - T5 : Trench positions
o
10
20km
Figure 1 Onshore marine deposits between Alexander Bay
and Port Nolloth.
S.-Afr.Tydskr.Geol.1988,91(3)
392
blocks. Their lateral continuity is, however, quite often
broken by protruding headlands, long stretches of rocky
coastline and post-depositional erosion. The exact ages
of wave-cut terraces on land and their relation to the
directly overlying sediments are still uncertain. Two
major divisions of onshore marine deposits are found.
The older sediments consist of a sequence of marine
terraces referred to as A. They are represented by a
number of transgressions that reached various elevations
betweeen 10 m and 90 m a.s.l. (main inner terrace
boundaries or base of cliffs at ±38 m, ±50 m, 58 - 60 m
and 85 - 90 m a.s.l., Figure 2) and contain at least one of
the fossil genera Crassostrea, Haliotis, Turbo and
Fissurella. Sequence A is overlain successively by
AGE(Ma)
o
6
50
.....
I
, ,~
0,1
Q2-
E
100 ~
~ .... Va --~,rfro. ..
~
:E Q4 ~
t---
Q3 ~
\2-
Donax rogersi
......... ~
,
~
W
~
Fissurella
~
"....
............. "
D
C
38m
....,
q:
:J::
><
q:
UJ
.r:.
(J
~
...J
2,0 ~
~o
Q1
~
U
., .,
...J
......
o
a.. :E
f~t
W rrp1~
,~
~O~
D
t;,
c:c
.... ~q,~ ..
:J
....::::>
"'
q:
....,
....,
c:c
::::>
No fossils
CI)
CI)
.... ~8;20..11l "IJ...
55-75m q:
I.lJ Ostrea
(OR 90)
c:c
....
Isognomon
a::
W
al
C
U
~o 0 _
90mP
...J
CI)
zw...J
a.a..a... Sea cliffs at Alexander Bay
W
a..
a..
I.lJ
w
:E :E TM?
a::
Equus capensis"
C
CO fAcheulian tools"
CI)
II
Ostrea
Cl
.a
E
~
Donax haughton!
C
-45m
cu
w
-
CI)
~
......
C
C
"'25m
CI)
~
w
...J
:E
.~
-..
Q2
PATELLA
I
CHOROMYTILUS LOWER
~u-L_A_N_U_S_ _ _-I-_ _I--_-+-__~
,.,15m
.. ,
-
2 m
~~6m
Washover deposits in the Alexander Bay Formation
The Cenozoic deposits described here overlie bedded
quartzites of the Precambrian Stinkfontein Formation
CHARACTERISPETHER KEYSER HALLAM CARRING.&
TIC FOSSILS
TERRACES
KENSLEY
TRANSGRESSIVE MAX.
SEA LEVEL(m)
reddish, poorly sorted and angular sand and gravel of
terrestrial origin, calcrete, and sand dunes. A younger
sequence, B, comprises a number of beaches below 10 m
elevation (main transgressive highs at ±2 m, 4 - 6 m and
8 - 12 m a.s.l., Figure 2) that transgress the marine
deposits of sequence A as well as the associated
terrestrial sediments and contain none of the fossil
genera mentioned above. This paper deals mainly with
some of the deposits in sequence A, but a summary of all
relevant data is provided in Figure 2.
0
No marine
fossils
a::
(!)
.. ~~-..9Q~
IIIII
Sea cliffs mentioned in literature
Figure 2 Transgressions and characteristic fossils in the Alexander Bay Formation compared to the classifications of Keyser
(19:2), Hallam (1964), Carrington & Kensley (1969) and Pether (1986), as well as known sea-level stands of the Tertiary and
PleIstocene (Beard et al., 1982; Vail & Hardenbol, 1979).
~!'!
CD(Q
Elevation
.s
Barrier
<: I:
Pol ..,
::t.CI)
W
g. ...,
'"'I
CD
::s
(")
::r
'i:j
'"'I
o
~
(i"
r/l
:::r
o
s
3
I
0-
~
'"'I
CD
a
+10m
CD
(i"
<:
~
O·
::s
r/l
S·
Bedrock barrier
:;CD
>
(i"
)<
Pol
::s
0-
I
w.
s
E.
I
~-------,----~ ~.~:=--.:.:-.:----.:----~~==.:-.::--~~-
+~,
!~~
l?f711~
~
CD
'"'I
I
Pol
'<
"Tl
o'"'I
Gravel deposits
I
I
to
(j)-(j)
S
I
d
---- ----- - -- --- - ----- .-..---
3
R
~
o·
::s
...,
iie
p-
.~~
T
R
+35m
.......
I
j
.?
::::
+
.......
o
3
~
+~
+
w
VI
3
...,
VI
+
0\
o
3
Fossils - marine
land
d- dune sand
gc - green clay
_lif>
Cross- bedding -trough
- planar
-ripple
~.."
.JIIP
~
Contacts -sharp
-erosive
-gradational
c - calcrete csg - continental sand, gravel
i-induration layer
m - marine sand
R - Re
T - Tra
gs - fine-gra
S.-Afr.Tydskr.GeoI.1988,91(3)
394
E
W
GROBLER
TERRACE
LOWER
TERRACE
MIDDLE
TERRACE
UPPER
TERRACE
m I - - - I......f---------------------. 1••_ _ _ _ _ _ _ _ _ _ _ _ _
· 1 · > 4 - - - - - - -......... 1 m
T,-~
,
~
~
100
100
'_c~~~~;=+~+~++:
+ + + + + +
....
_.~,rnJ.>'"'
o
o
C=:J
IIlIIIIIIIl
,~_+
, + + + + + + + +
........
++++++++++
...•.•.•
+ + +T+++++++++++++++++++++++
T + + + + + + + + + + + + + + + + + + + +
r + + + + + + + + + + + + + + + + + + + + + + +
r + + + + + + + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
T + + +'+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + + + + + + +
km
Sand dunes
Calcrete
0
1
0
0
0
0
_
0
0
1
Terrestrial deposits
Marine deposits
50
0
3-10
E::IJ Bedrock
Figure 4 Schematic profile through the Alexander Bay Formation showing the relative positions of trenches Tl - T5.
(Gariep Complex) which strike due north as opposed to
the NNW orientation of the coastline (Figure 1).
Bedding-parallel thrusts and faults are common in the
Stinkfontein rocks and, together with differential
erosion of softer beds, contributed to the formation of
bedrock channels parallel to bedding and sub-parallel to
the coastline. These channels may be open-ended on
both sides but normally they widen and intersect the
coastline towards the south. Their formation is
attributed to wave erosion enhanced by fluvial run-off
during periods of regression. They are partly protected
from the open sea by bedrock barriers or 'reefs' similar
to the situation existent at Port Nolloth Harbour today.
Figure 3 shows profiles from prospect trenches at + 10
m, +35 m and +60 m elevations that respectively
conform to the traditionally accepted Lower, Middle
and Upper Terraces (Keyser, 1972 and Figure 4). The
fossil content of the exposed sediments is summarized in
Table 1.
\
1'-'
I
\
,
....
-
N
,
'-...
1
j
' ..... ,
J
"-
.....
"- \,.
\,.
\,.
\
\
\
\
\
\
"- "\
,
"-
"\
\
I
\
\
\
\
\
\
\
\
~\
130m to
\
present beach
\
6
\
+ 10m Washover deposits
Figure 5 shows the irttersection of three trenches (T1 T3) across a NW-SE channel which, other than normal,
is open-ended towards the northwest. The trench
profiles are depicted in Figure 3. In trenches T2 and T3,
superposed boulder and gravel fans occur behind a
bedrock barrier which is up to 4 m high. At trench T1,
which is situated at the entrance of the channel, the fans
grade into gravel and shell beaches stacked in imbricate
fashion on a level floor. Three gravel units, each with a
distinct clast composition andlor fossil content, and the
upper one being separated from the others by a
disconformity, can be recognized in T2 and T3. All three
units are wedge-shaped and thin out and fine landward.
The basal unit (1 in Figure 3) consists of well-rounded,
polished pebbles and cobbles of quartz, quartzite and
phosphorite in massive green (lOY SIS) or brownish,
clayey sand. The gravel is usually poorly sorted and also
contains large, subrounded quartzite boulders. Sand
grains and clasts alike are coated with a shiny brown
veneer of iron-oxide. Phosphorite clasts comprise
composite cobbles of phosphatic gravel - with or
"
I
I
I
I
I
\
\
\
\
\
o
50
100m
Figure 5 Bedrock contours showing the shape of the channel
and approximate outline of the deposits intersected by
'trenches Tl - T3.
without bone fragments and shark's teeth (Figure 6) individually rolled phosphatic bone fragments, shark's
teeth and mollusc casts (Figure 7), and quartzite
fragments with remnants of a thin phosphorite coating.
All the phosphatic material observed represents
reworked deposits.
The basal pebble-gravel grades upward into bouldergravel (2 in Figure 3) consisting of sub rounded to
395
S.Afr.J. Geol.1988,91 (3)
Table 1 Fossil assemblages related to stratigraphic
units in trenches T1 - T5, Figure 2 (Identification by J.
Pether, S.A. Museum)
Trench Assemblage
TI-T5
Tl-T3
Al
A2
Fossils
Gastropoda
Trigonephrus sp.
: Patella hoffmani*
P. barbara
P. hendeyi*
Fissurella robusta*
Crepidula capensis
C. porcellana
Bivalvia
Tl-T3
A3
A4
T4
T5
A5
A6
Gastropoda
Nucella praecingulata*
Triumphus dilemma*
Bullia digitalis
Oxystele sinensis
Turbo cidaris
Afrocominella capensis
:Aulacomya ater
Perna perna
: Patella hoffmani*
Diodora elevata
Calliostoma depressa*
Turbo sp. - operculae
: Cancelloth yrislTerebratulina
Brachiopoda
Mollusca
(casts)
Chondrichthyes (rolled teeth)
Mammalia cetacea (rolled bone fragments)
: Patella hoffmani*
Gastropoda
P. barbara
Crepidula porcellana
Crepidula porcellana
Nucella praecingulata*
Triumphus dilemma*
Bullia digitalis
Oxystele sinensis
Turbo cidaris
Afrocominella capensis
Burnapena sp.
Conus mozambicus
Drillia tempestae*
Siphonaria aspera
Haliotis cf. H. saldanhae*
:Striostrea margaritacea
Bivalvia
Perna perna
: Fissurella robusta
Gastropoda
Bivalvia
:Striostrea margaritacea
Brachiopoda : Kraussina sp.
: Echinoid spines
Echinoidea
: Polychaete worm tubes
Polychaeta
Figure 6 Polished cobbles of phosphogenic material in
trenches, Tl - T3.
• _..
..........--.-....5cm
Figure 7 Rolled shark's teeth and mollusc casts, trenches Tl
-T3.
Figure 8 Back-barrier deposit in trench T2 with white calcrete
capping, overlain by younger wash over gravels. Note the
sharp, erosional contact on which the hammer rests. Crest of
bedrock barrier to the left.
* Extinct species
sub angular quartzite boulders and cobbles derived from
bedrock. The matrix consists of medium-grained
greenish (lOY 5/5) sand. This unit shows lateral grading
from very large boulders immediately adjacent to the
barrier, through cobble-gravel into medium to finegrained, greenish sand with clay layers, further
landward. Locally, unit 2 gravels exhibit some postdepositional erosion but elsewhere the unit is intact and
extends from the crest of the barrier into the channel.
Internal erosion surfaces are marked by thin, laminated,
calcrete horizons or thin lenses of angular, quartzite
rubble, the latter being derived from barrier and/or
gravel erosion either by storm tides or fluvial run-off
(e.g. trench T2). Units land 2 are normally devoid of
396
marine shells other than rolled, phosphatized molluscs.
However, rare lenses containing Diodora elevata, Patella
hoffmani, Calliostoma depressa, Turbo sp. and
CancellothyrislTerebratulina, (Table 1, assemblage A3)
testify to a shallow-marine environment, possibly in the
mid-to lowtide zone (Kilburn & Rippey, 1982).
The third gravel layer (unit 3 in Figure 3) overlies unit
2 or associated fine-grained sand, clay or calcrete with a
sharp erosive contact (Figure 8). It differs markedly in
composition, texture and fossil content from both units 1
and 2 and consists of a basal polymictic gravel;
containing rounded to subrounded pebbles and cobbles
that grade upward into coarse sand. The gravel contains
small pebbles of agate and jasper as well as an
abundance of fossil shell (Table 1, asemblage A2).
+35 m Wash over deposits
The geometry of the deposit exposed in trench T4
(Figure 3) is interpreted in the same way as those just
described. It is situated on the Middle Terrace, about 3
km inland from the position of Trenches T1 - T3 (Figure
4). In this section a depression located behind a bedrock
ridge is filled with clay and fine-grained, greenish (lOY
SIS) sand with shell layers and some pebbles and cobbles
at the base. These sediments vary from massive to
laminated and fossil shells (Table 1, assemblage AS)
include a wide range of intertidal species (Kilburn &
Rippey, 1982). They are overlain by medium to coarsegrained sand and gravel, the latter consisting of
polymictic, very well-rounded and well-sorted pebbles
and cobbles (discoidal and roller types). The fossil
content is the same as that of the underlying deposits.
Trough and planar cross-bedded sand overlies gravel and
ripple cross-laminated sand regressively behind the
barrier. An indurated layer well below the calcrete
horizon consists of very hard, calcareous sandstone and
siltstone, and cuts across depositional boundaries to the
full extent of the bedrock depression. Such indurated
horizons (locally known as 'vaalbank') are probably the
result of post-depositional solution and redeposition of
CaC0 3 by groundwater, a process which is apparently
still in progress today.
+60 m Washover deposits
Trench TS (Figure 3), some 1 km further inland from T4
(Figure 4), contains remnants of a washover deposit
backed by a small cliff on the landward side. At least two
wash over events truncated by overlying terrestrial
deposits can be recognized. Very coarse sand and shellgrit at the base are overlain by gravel that dips and fines
towards the land. Subrounded boulders on the eastern
face of the barrier grade inland into moderately to
poorly sorted, well-rounded pebbles and cobbles in a
greenish, clayey matrix. Garnet, ilmenite and magnetite
are concentrated along the base of the gravel whilst
abundant Crassostrea and some small Fissurella and
Kraussina sp. (Table 1, assemblage A6) are also present.
Coarse-grained trough, planar and ripple crosslaminated sands overlie a second wash over gravel which
S.-Afr.Tydskr.GeoI.1988,91(3)
dips steeply towards the east and erodes the underlying
sediments. These coarse-grained sands are as highly
indurated as those in T4.
Environmental interpretation
The basal gravels (1 and 2) in trenches T1 - T3 (Figure 3)
represent wash over events in intertidal environments
during two regressive cycles. Field evidence, as depicted
on the profiles, indicates that the wedge-shaped boulder
layers were washed over the bedrock barrier from the
west. The marine faunal record of the units (Table 1)
furthermore suggests that deposition took place at
intertidal to lowtide level. Landward of the barrier, the
gravels grade into fine-grained, bioturbated sand and
clay of back-barrier origin. The phosphorite and bonebearing pebbly conglomerate at the base of each
sequence appears to be a remnant lag deposit left behind
on the channel floor by repeated transgressions and
regressions. The highly rolled nature of the mollusccasts, shark's teeth, bone, and phosphorite fragments
attests to multicycle erosion and deposition. Unit 3 is
interpreted as a lower shoreface gravel at the base of the
washover fan overlain by upper shoreface, pebbly beach
sand that was deposited after renewed transgression.
This sand forms a regressive beach complex with trough
and planar cross-bedding. Small wash over events,
perhaps deposited during storm conditions, are found
immediately behind the barrier in trench T2. In trench
T1 upper shoreface sands and lower shoreface gravels
are overlain regressively by fine-grained, white dune
sand and calcareous, poorly sorted and rounded
terrestrial sands with angular gravel lenses. These
deposits are overlapped by successively younger,
seaward-dipping beaches towards the channel entrance.
Comparable interpretations can be applied to trenches
T4 and TS although the basal phosphorite-bearing
gravels are absent in them. This may suggest that in situ
phosphorite deposits at Alexander Bay - if present at
all on land - probably only occur at elevations below 3S
m a.s.l. Fine-grained, clayey sediments with a rich faunal
record in trench T4 were deposited in an intertidal, backbarrier environment. Since exactly the same faunal
record is preserved in the upper and lower sedimentary
sequences, it would appear that the T4 material was
deposited during a single transgressiveiregressive cycle.
Two phases of transgression or storm-affected wash over
are indicated for the TS succession. Coastal plain sheetflood sands and gravels (terrestrial deposits), topped by
sand dunes, complete the prograding shoreline package
in each case.
Regional sediment correlations and sea level
movements
Correlation between sea level stands and marine and
phosphatic deposits along the west coast of South Africa
has generated considerable comment and controversy in
the past (e.g. Tankard, 1974, 1975; Smith, 1975; Birch,
1976; Hendey, 1981; Siesser & Dingle, 1981; Dingle et
al., 1983; Pether, 1986). This paper is by no means a
comprehensive report of the complex stratigraphy of the
S.Afr.J.Geol.1988,91(3)
Alexander Bay Formation, but some new information
published here may influence regional correlations with
other Cenozoic deposits along the west coast of southern
Africa and therefore needs mentioning.
The main morphological and palaeontological features
of the Alexander Bay Formation between Port Nolloth
and Alexander Bay are summarized in Figure 2. This
figure contains information compiled from personal
experience and work beyond the scope of this paper.
The most clearly defined transgressions and their
suggested relation to known sea-level stands of the lateTertiary and Pleistocene, as well as the various
classifications presented by Keyser (1972), Hallam
(1964), Carrington & Kensley (1969), and Pether (1986)
are shown. The older ( > 10 m a.s.l.) beaches are
characterized by thick-shelled, warm-water fossil species
of the genera Crassostrea, Fissurella, Turbo, Donax and
Haliotis. In contrast the younger « 10 m a.s.l.) beaches
contain thin-shelled cold water fossils, mainly
Choromytilus meridionalis (black mussel), Aulacomya
ater, (ribbed mussel), Patella sp. and Balanus sp.
No clear-cut palaeontological classification based on
index fossils has yet been done for the Port NollothAlexander Bay area, but the most characteristic fossils
for the different deposits are mentioned. These fossils
are not confined to the specified deposits; they are only
less abundant in others. Donax haughtoni and Donax
rogersi are the only index fossils (Carrington & Kensley,
1969; Pether, 1986) that can be recognized with ease on
the Upper and Middle Terraces respectively.
To date, phosphatic deposits have been described
onshore along the West Coast as far north as Hondeklip
Bay and offshore as far north as the Kunene River
(Bremner, 1978). They occur at elevations ranging from
below sea-level (Bremner, 1978; Birch, 1979) to +6 m
(Tankard, 1975) and +54 m (Tankard, 1974). Pether
(1986) has pigeonholed the various divergent opinions
regarding correlations between them based on his
research at Hondeklip Bay. He devided the Alexander
Bay Formation at Hondeklip Bay, some 200 km south of
Alexander Bay, into a '90 km Package, 50 m Package'
and '30 m Package'. They represent regressive packages
which overlap each other seawards in succession from
transgressive maxima at 90 m, 50 m, and 30 m a.s.l. His
'90 m Package' is identical to Carrington & Kensley's
(1969) '75 - 90 m Complex'. Pether regards the in situ
phosphatic deposits at Hondeklip Bay (at 18 m to 20 m
a.s.l.) as relicts of the seaward extension of his '90 m
Package' . No in situ phosphorite deposits like the
Saldanha Formation (Tankard, 1975) are known for the
Alexander Bay area although it is possible that remnants
may be present on the Middle Terrace.
The Alexander Bay phosphatic deposits at 6 m - 19 m
elevations in trenches Tl, T2 and T3 are reworked and
contain only rolled phosphorite pebbles, shark's teeth,
whale bones, and bivalve casts. They were probably
derived from the erosion of in situ phosphorite deposits
as described by Bremner (1978), Birch (1979), Tankard
(1974, 1975), and Dingle et al. (1983) by repeated
transgression and regression, and were deposited as lag
gravel in bedrock traps. They are correlated with the
397
nearshore shelf-facies (NSb) of the '50 m Package' of
Pether (1986) which includes the Varswater Formation
of Tankard (1974). There is, however, a very clear
distinction between the mollusc-rich shoreface sediments
in trenches Tl - T3 (Figure 3) and the underlying, nearly
non-fossiliferous lag deposits of brownish, iron-oxide
stained sands and gravels. Also present in places is a
sharp disconformity (e.g. trench T2, Figure 8). The
shoreface sediments above the disconformity are
therefore interpreted as representing a younger
transgressive/regressive cycle probably related to the '30
m Package' of Pether (1986).
Trench T4 (Figure 3), located on the Middle Terrace,
is correlated with Pether's '50 m Package' (Figure 2). It
contains the fossil Haliotis (cf. H. saldanhae), which has
been recognized in the 'Gravel Member' of the
Varswater Formation as well as in the '50 m Package' at
Hondeklip Bay (PetheL pers. comm.). The deposits in
trench T5 (Figure 3) appears to be continuous along
strike with similar sediments found up to 90 m a.s.l.
(Figure 2) which contain Haliotis, Turritella and
Isognomon sp. in addition to the A6 fossil assemblage
(Table 1), and is therefore tentatively correlated with the
'90 m Package' of Pether (1986). However, the
relationship between T5 sediments and the 58 m
deposits, which normally abut against a sea cliff with
base at 58 m - 60 m a.s.l. (Figure 2), is not yet clear; it
may in fact represent washover material from the latter.
The relationship between the Upper Terrace, the
Grobler Terrace and the '90 m Package' will only be
properly understood when more information becomes
available from present studies under way at Alexander
Bay. These studies seem to indicate that the terrace
classification of Keyser could be wrong in a number of
places, especially at Alexander Bay itself and also north.
of Port Nolloth.
The postulated ages of the main recognizable
transgressions in the Alexander Bay Formation relative
to known sea-level stands TPI to Q5 (Vail & Hardenbol,
1979; Beard et al., 1982; Pether, 1986) are shown in
Figure 2. In assigning ages to the different
transgressions, consideration was given to relative
transgressive maxima, the presence of fossil remains of
Equus capensis and unidentified rhino and elephant sp.,
and Acheulian artefacts in dune, sheetwash and
estuarine-lagoonal sediments overlying the shallow
marine deposits of the Upper Terrace (50 m
transgression). These fossil bones and artefacts come
from below the uppermost calcrete horizon, which is
transgressed by the lower Pleistocene + 10 m beach at
the present coastline. It is inferred that the fossils
accumulated during middle Pleistocene times in an
estuarine-lagoonal environment behind the 38 m
transgression dune ridges (Figure 2, Q2) on raised
Upper Terrace beach and tidal deposits. From this line
of reasoning it follows that the 50 m and 38 m
transgressions correspond to Ql and Q2 respectively and
that the 58 m to 90 m sea-level stands occurred during
the TPI to TP2 events of early to middle Pliocene times.
Exact age relationships on the Grobler Terrace are still
obscure and it remains uncertain whether the 90 m cliff
398
represents a Miocene feature or not. Less clearly defined
transgressions on the Middle Terrace at approximately
20 - 25 m and 15 - 20 m elevations are assigned to Q3
and Q4 during the Middle Pleistocene whereas the oldest
of the younger beaches at between 8 - 12 m a.s.l. is
tentatively correlated with Q5 in the Lower Pleistocene.
Beaches at 4 - 6 m and 2 m a.s.l. span the Lower
Pleistocene to Holocene period. These ages are in fair
agreement with those proposed for various other
Neocene and Pleistogene West Coast deposits (cf.
Siesser, 1980; Hendey, 1981; Dingle et al., 1983; Kensley
& Pether, 1986).
Acknowledgements
The author wishes to thank his former colleagues O.
Genis and D. Ie Roux at Alexander Bay for their
interesting discussions and valuable contributions.
Special thanks are due to the General Manager of the
State Alluvial Diggings, Mr. S. Marais, for permission to
have the information in this paper published. Mr. John
Pether of the S.A. Museum, Cape Town, gave
invaluable assistance by identifying the marine fossils
and by discussing various correlation possibilities while
J. Rogers, R. de Decker, J.M. Bremner and R.S. Hill
helped to improve the contents of the paper by their
constructive criticism and suggestions. Mmes. C.
Roestoff and S. van Wyk of the Geological Survey,
Bellville, typed the manuscript.
References
Beard, 1.M., Sangree, 1.B. & Smith, L.A. (1982). Quaternary
chronology, paleoclimate, depositional sequences and
eustatic cycles. Bull. Amer. Assoc. Petrol. Geol., 66,
158-169.
Birch, G.F. (1976). Surficial sediments of Saldanha Bay and
Langebaan Lagoon. Trans. geol. Soc. S. Afr., 79, 293-300.
---- (1979). Phosphatic rocks on the western margin of South
Africa. 1. sedim. Petrol., 49, 93-110.
Bremner, 1.M. (1978). Sediments on the continental margin
off South West Africa between latitudes 17' and 25' S.
Ph.D. thesis (unpubl.), Univ. Cape Town, 300pp.
Carrington, A.l. & Kensley, B.F. (1969). Pleistocene molluscs
from the Namaqualand coast. Ann. S. Afr. Mus., 52,
189-223.
De Villiers, 1. & S6hnge, A.P.G. (1959). The geology of the
Richtersveld. Mem. geol. Surv. S. Afr., 48, 219-240.
Dingle, R.V., Siesser, W.G. & Newton, A.R. (1983).
Mesozoic and Tertiary Geology of Southern Africa.
Balkema, Rotterdam, 375pp.
S.-Afr. Tydskr. Geo1.1988,91 (3)
Hallam, e.D. (1964). The geology of the coastal diamond
deposits of Southern Africa, 671-728, In:Haughton, S.A.,
Ed., The Geology of Some Ore Deposits in Southern Africa,
2, Geol. Soc. S. Afr., 739pp.
Hendey, Q.B. (1981). Geological succession at
Langebaanweg, Cape Province, and global events of the
late Tertiary. S. Afr. 1. Sci., 77, 33-38.
Kensley, B. & Pether, 1. (1986). Late Tertiary and Early
Quaternary fossil mollusca of the Hondeklip area, Cape
Province, South Africa. Ann. S. Afr. Mus., 97, 141-225.
Keyser, U. (1972). The occurrence of diamonds along the
coast between the Orange River and the Port Nolloth
Reserve. Bull. geol. Surv. S. Afr., 64, 1-23.
---- (1976). Diamonds - Marine (coastal) deposits, 27-30. In:
Coetzee, e.B., Ed., Mineral Resources of the Republic of
South Africa, 5th Edition.
Kilburn, R. & Rippey, E. (1982). Sea shells of Southern
Africa. South China Printing Co., Hong Kong, 249pp.
Pether, 1. (1986). Late Tertiary and early Quaternary marine
deposits of the Namaqualand coast, Cape Province: new
perspectives. S. Afr. 1. Sci., 82, 464-470.
Selley, R.e. (1978). Ancient Sedimentary Environments.
Chapman & Hall, London, 287 pp.
Siesser, W.G. (1980). Late Miocene origin of the Beguela
upwelling system off northen Namibia. Science, 208,
283-285.
---- & Dingle, R.V. (1981). Tertiary sea-level movements
around southern Africa. 1. Geol., 89, 83-96.
Smith, P.l.R. (1975). Discussion of 'Varswater Formation of
the Langebaanweg-Saldanha area, Cape Province' by
Tankard, A.l. and author's reply. Trans. geol. Soc. S. Afr.,
78, 373-375.
South African Committee for Stratigraphy (SACS) (1980).
Stratigraphy of South Africa, Part 1. (Comp. L.E. Kent)
Lithostratigraphy of the Republic of South Africa, South
West Africa/Namibia and the Republics of
Bophuthatswana, Transkei and Venda. Handbk. geol.
Surv. S. Afr., 8, 603-609.
Stocken, e.G. (1962). The diamond deposits of the
Sperrgebiet, S.W.A. Field excursion guide, 5th Ann.
Congr. geol. Soc. S. Afr., 76pp.
Tankard, A.l. (1974). Varswater Formation of the
Langebaanweg-Saldanha area, Cape Province. Trans. geol.
Soc. S. Afr., 77, 265-283.
---- (1975). The marine Neogene Saldanha Formation. Trans.
geol. Soc. S. Afr., 78, 257-264.
Vail, P.R. & Hardenbol, 1. (1979). Sea-level changes during
the Tertiary. Oceanus, 22, 71-79.