Dingle, 1982 II. DISTRIBUTION OF BENTHIC OSTRACODS DURING THE EARLY
Dingle, 1982 II. DISTRIBUTION OF BENTHIC OSTRACODS DURING THE EARLY
Dingle, 1982 II. DISTRIBUTION OF BENTHIC OSTRACODS DURING THE EARLY
INTRODUCTION
This contribution summarizes some of the conclusions reached
in a micropalaeontological study of mid-Cretaceous ostracod
faunas obtained from DSDP sites 327 and 330 on the Falkland
Plateau, dredged samples on the Agulhas Bank, and outcrop samples
from Zululand. A detailed account of sampling localities,
ostracod systematics, and palaeoenvironment£:l analyses has beon
submitted to che South African Museum for publication in their
annals (Dingle 1983). Consequently, the formal status of some
taxa mentioned herein are provisional upon their publication
elsewhere. In addition, previously published data from Argentina
(Musacchio 1978), Madagascar (Grekoff 1963), Mozambique Ridge
(Simpson et al 1974), Tanzania (Bate & Bayliss 1969, Bate 1975),
India (Guha 1976), Australia (Oertli 1974, Krommelbein 1974) and
southern Africa (Dingle 1969, 1971, Brenner & Oertli 1976) have
been used in these syntheses.
In last year's report (Dingle 1981a), I gave an account of
the ostracod work that has been undertaken at UCT over the last
few years, and this contribution represents continuing activity
within a project concerned with Mesozoic Gondwana faunas.
Continental drift (as defined by the creation of oceanic
crust between the separating continental units) probably
commenced between southern Africa and South America at about
127my (Vaj.anginian) (Larson & Ladd 1973) and resulted in the
formation a.id progressive enlargement of the SE Atlantic and the
S Natal Valley. However, because of the large marginal offset
created by the Agulhas/Falkland Fracture Zone, the two continents
remained physically joined along a progressively shortening zone
7
DISCUSSION
SE Africa
26 species belonging to 22 genera (with two species
unallocated) have been recorded from the Aptian to Cenomanian
strata of SE Africa. The bulk of these (19 species in 15 genera)
are from the Zululand area, where relatively large numbers of
specimens were collected from rocks that range in age from Aptian
to Cenomanian. Collections from the Outeniqua Basin consist of
two samples only (TBD 1113 and 1266) which probably have a range
restricted to Upper Aptian-Albian. One species only (Sondagella
theloides) is common to these two areas in mid Cretaceous
material, although a further two species (Majur.gaella nematis and
M? hemigymnae) are common in pre-Aptian strata.
o
Pclaeoecology
In Zululand, a total of 21 samples yielded ostracods.
Plotting smoothed population data on a Cytheracea -
Cytherelloidea - Bairdiacea + Cypridacea diagram (CCBC triangular
diagram), the earliest populations (Aptian IV) lie within the
field predictive of water depths of 100-200 m (the 4a populations
of Dingle 1981b figs.68 & 75). This plots as the deepest water
population encountered in the Zululand mid-Cretaceous, and
younger populations suggest progressively shallower water depths
and higher energies of the sedimentary environments: Albian III
and Albian IV populations lie above the 'Cytheracea line' and
just within the <100 m predictive field; and the Albian VI and
Cenomanian II and III populations lie within the high energy
sector of the <100 m predictive field.
Biostratigraphy
In the cytheracean component of the ostracod populations
found in the Zululand mid-Cretaceous rocks, two groups can be
distinguished: those that are known only from Zululand; and
those which nave been recorded from other localities in SE Africa
and Gondwanaland. The latter comprises Majungaella nematis and
Sondagella theloides, which range upwards into Cenomanian III and
Albian III, respectively. These are the youngest records of the
two species, both of which have extensive temporal and spatial
ranges outside the SE African region. The most diverse
assemblages occur in the late Aptian-early Albian strata which
are characterized by the presence of several apparently short-
range cytheracean taxa such as Makatinella inflata,
Pongolacythere striata, and two species of Pirileberis
(makatiniensis and mkuzensis). It is clear from this
distribution that despite the hiatus between the Makatini and
Mzinene formations (Albian I is missing), there was faunal
continuity in the ostracod populations across the Aptian/Albian
9
Regional considerations
During the time period with which we are concerned (Aptian
to Cenomanian), the palaeogeography in the vicinity of SE Africa
underwent significant changes. Reconstructions used here are
based on Dingle et al (1983), and are shown in Figure 2(a&b)
(pre-breakup Valanginian, and middle Albian/Cenomanian,
respectively). It is essential to consider the regional spatial
and temporal distribution of the various taxa in terms of these
palaeogeographies because they change from essentially inter
continental shelf seas (pre-breakup) to marginal seas separated
by deep ocean basins (Cenomanian). In addition, it must be borne
in mind that there was no connection between the North and South
sectors of the Atlantic Ocean across the Walvis Ridge before late
Cenomanian/early Turonian times, and that the Mid-Cretaceous
populations with which we are concerned can be seen as the last
representatives of an Upper Jurassic to Cenomanian ostracod
faunal province that occupied South Gondwanaland seas (Dingle
1982, Tambareau 1982). Figure 3 shows range charts for Mid-
Cretaceous ostracods that are common to more than one of the
regions in Figure 2, or have close relatives elsewhere in South
Gondwanaland.
Dingle (1982) has given details of the whole pre-Aptian
South Gondwanalcind ostracod fauna, hut here it is relevant to
emphasize the distribution of two important species in terms of a
pre-drift Valanginian palaeogeography (Figure 2a). In this
reconstruction, which is meant to illustrate the period
immediately prior to, and immediately after drifting in the Natal
Valley/South Atlantic, conditions of no stdimentation, or very
slow anoxic sedimentation obtained over the DSDP sites (327 &
330) on the soutnern Maurice Ewing Bank: no ostracods were
recovered from the Kimmeridgian-Albian condensed sequences (or
non-sequences) related to this period. However, because the
Agulhas Bank, Mozambique Ridge (DSDP 249) and Madagascar
11
South America) took place, and after about 100my (latest Aibian)
no shallow water (continental shelf depth) connections between
the two areas existed in the South Atlantic. As sea floor
spreading proceeded, the gap between the two areas progressively
increased, presumably preventing any further contact between
elements in the ostracod populations that were restricted to
shallow water environments.
REFERENCES
primary links *
Sondagella theloides OB-Z-N*
MajungaeTTa nematis. OB*-Z-N*-M*
Arculicythere tumida Ob-TP-Au
Isocythereis sealensi OB-tF
I. ndumuensis Z
Robsoniella falklandensis FP-Au
PirilebeTTs spp Z-M
secondary links
Rhadinocythere spp FP with U.Jur.Au,T
Procytherura a'erodynamica Z " Kimm. T
LL dinglei..TT7 FP " Haut/Val. OB
P. batei...FP similar to P.maculata from Haut.OB
FP similar to Paijenborchellina spl
(Damotte)
Cytherura oertlii FP " Alb. Au
* pre-Albian occurrences
b) between Zululand» Madagascar» Tanzania
Pirileberis Z-T-M
Pongolacythere Z similar tc T
FIGURE EXPLANATIONS
t>*
s
/
/
WA
Figure 1
to Mad. -o
'flC
249 JM
v w- A » 327 ,-' ]?
.' *
£ * j Falkland
Islands
Fícjure 2
Alblan
XI
| II | 1 | IV I
-ctaz
K
•*»•
• i n n f i i m MPWTI»
— — clal •MJMMMIL* H
ciarr MMCTTMMMU I
—»FP
h
• AB.Aas
I-
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[ ^-J' ttOCTTNIBII» M f t U M W
l. IMCTTMMtlS I W
CrTH'HUMT ftcrTUt
Figure 3