Geology Reference
In-Depth Information
and at least once in between, for the purpose of calibrating
Th and U concentrations. Data were processed using
GLITTER
TM
software package and plotted in ISOPLOT Ex
(Ludwig
2003
). A common lead correction was applied to all
grains
1988
), so without further study, they cannot constrain the
environment more precisely.
9.6.2 Conchostracans
<
800 Ma using the ISOPLOT function Age7Corr.
Overall, conchostracans (Branchiopoda) are very poorly
studied in Africa. Equatorial Africa is an exception to this:
several authors have produced important monographs detail-
ing the conchostracan faunas (Defretin
1953
; Defretin-
Lefranc
1967
; Marli
`
re
1948
,
1950
; Tasch
1987
). However,
those materials studied are often insufficiently illustrated by
modern standards, making them difficult to compare to other
specimens.
Conchostracan remains were identified in three cores: one
from the Mbuji-Mayi area (172-X146) and two from the
Kabinda area (173-X009 and 173-X030). Core 173-X009
had conchostracans present in only one horizon (32 m
level); core 173-X030 yielded conchostracans from two
horizons between the 31 to 45 m levels; and core 172-
X146 yielded conchostrans at two levels, between 35-39 m
and at 67 m. All identifiable remains seem to represent new
species of spinicaudatan conchostracans. The species pres-
ent show a high endemicity—although one genus occurs in
multiple horizons/cores, species were only identified in
restricted horizons and did not reoccur in other horizons/
cores.
Several cores (172-X146 (2 horizons), 173-X009, and
173-X030) contained species that likely belong to the
genus
Pseudestherites
. Much of the material was too frag-
mentary to determine species-level affinities easily; how-
ever, they all possess punctae (minute pits) that are as
much as an order of magnitude smaller (1-6
9.5.2 Results
The majority of the detrital zircons analyzed in this sample
were dominantly of late Paleozoic and Precambrian age,
however four Cretaceous-age grains were identified that
provide important maximum depositional age constraint
for this sample. The youngest zircon population includes a
Late Cretaceous population of three concordant grains that
yield a mean age of ~79.0
6.8 Ma (Fig.
9.15
) and is
consistent with the age of kimberlite magmatism in the
Mbuji-Mayi region (~70 Ma). In addition, a single Mid-
Cretaceous grain (~113 Ma) that is statistically insignificant,
but consistent in age with some of the younger kimberlites
(~120 Ma) in the Lundas region in Angola was identified.
The former population provides a maximum depositional
age constraint for this unit and a tie point for correlating
this portion of the stratigraphy. This finding also provides
context for evaluating the biostratigraphic ranges of concho-
stracan fossils from this part of the stratigraphy, suggesting
that several of the forms may have younger upper age limits
than previously thought. Moreover, these data suggest that
conchostracan biostratigraphy in particular should be used
with caution in the basin and should not form the sole basis
for stratigraphic correlation or age assignments (see
expanded discussion of biostratigraphy below). Given the
small number of grains analyzed, a more intensive detrital
zircon investigation of the Kasai cover sequences is
warranted to build a more robust detrital zircon provenance
dataset that can be used to test the stratigraphic correlations
presented in this study, and elsewhere in the Congo Basin.
m in diameter
μ
versus as much as 70
m) than other known species of
Pseudestherites
—suggesting an identity as a new species
(Fig.
9.16
). There are, at present, no described species of
Pseudestherites
from Africa, but there is one undescribed
species found during field work in Zimbabwe (EMR & HJ)
that is presently being studied (Hegna et al. in preparation).
Pseudestherites
is known from the Late Jurassic to Early
Cretaceous of China and Early Cretaceous of Argentina.
Specimens from cores 173-X009 and 173-X030 could repre-
sent the youngest occurrence of
Pseudestherites
(Late Creta-
ceous). The detrital zircon sample from near the base of core
173-X030 indicates that these new specimens are not older
than ~79 Ma, however further work is required to confirm
whether specimens from core 172-X146 are indeed the same
and of similar age.
Several specimens of a new species belonging to the
family Loxomegaglyptidae were found in core 173-X030
(45 m from top, Kabinda area, Kasai-Oriental province)
(Fig.
9.16
). The specimens are likely Late Cretaceous in
age, as the core is thought to correlate closely with core
μ
9.6
Paleontology
9.6.1 Ostracodes
Small, podocopid ostracodes were identified in three cores:
one from the Kananga area (157-X016) and two from the
Kabinda area (173-X009 and 173-X030). They are present in
multiple horizons in the two Kabinda cores, even abundant
enough to form local ostracode pavements. Podocopids are
typically freshwater ostracodes, and their presence here is
consistent with the interpretation of fully continental depo-
sitional environments. However, ostracodes occur in many
different types of freshwater environments (Carbonel et al.
Search WWH ::
Custom Search