Journal of African Earth Sciences 35 (2002) 365–384
www.elsevier.com/locate/jafrearsci
Remote sensing, field studies, petrography, and geochemistry of
rocks in central Zambia: no evidence of a meteoritic impact in
the area of the Lukanga Swamp
Crispin Katongo a, Christian Koeberl
b
a,*
, Wolf Uwe Reimold b, Singute Mubu
c
a
Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
Impact Cratering Research Group, School of Geosciences, University of Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa
c
Geological Survey Department, P.O. Box 50135, Lusaka, Zambia
Accepted 10 October 2002
Abstract
The Lukanga Swamp in central Zambia has previously been proposed as the site of a large (52 km diameter) impact structure on
the basis of alleged observation of shock-diagnostic planar deformation features (PDFs) in quartz of breccia from the southern
margin of the swamp. The southern margin of the swamp, marked by the Nyama Dislocation Zone, consists of quartzite and
silicified meta-siltstone, shale, sedimentary quartz breccia, and fault breccia. Structures and textures in the meta-sediments are synsedimentary, whereas fault breccia displays tectonic fabrics. In thin section, quartz in sedimentary quartz breccia displays widely
spaced, randomly oriented, subparallel, non-planar fluid inclusion trails, which were earlier misidentified to represent decorated
PDFs formed by meteoritic impact. Siderophile element abundances (<105 ppm) in the rocks from around the swamp are normal
for quartz-rich crustal rocks, and there are no relative enrichments in the breccias compared to other rocks to suggest extraterrestrial
contributions. Aeromagnetic data do not reflect an impact crater signature. We analyzed regional structural and seismic data in an
effort to account for the development of the swamp. From the structural synthesis it appears that reactivation of movements along
the Nyama and Kapiri-Mposhi Dislocation Zones may have led to the development of the Lukanga Swamp, probably during the
Cenozoic era.
Ó 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Zambia; Lukanga Swamp; Meteorite impact crater; Breccia; Planar deformation features; Strike-slip faults
1. Introduction
Out of the approximately 165 terrestrial impact craters known worldwide, only 19 African impact craters
are now confirmed (Table 1). The total number of observed impact craters on Earth falls, by far, short of the
number of expected craters from cratering rate estimates
(e.g., Grieve, 1987; Trefil and Raup, 1990; Grieve, 1998).
Most craters have been destroyed by active geological
processes (Grieve et al., 1995), but a large proportion of
those that may have survived are yet to be discovered,
especially in Africa (Koeberl, 1994). In Zambia, there
are three structures that have previously been proposed
to have formed by meteorite impact (Fig. 1): the
*
Corresponding author. Tel.: +43-1-4277-53110; fax: +43-1-42779531.
E-mail address: [email protected] (C. Koeberl).
Lukanga Swamp (Vrana, 1985), the Bangweulu basin
(Master, 1993), and the Chituli structure (Master, 2001).
None of these has, so far, been confirmed as being of
impact origin. The Lukanga Swamp, centered at 27°450
E and 14°240 S, is located in central Zambia, about 100
km west of Kabwe. The swamp, with an average diameter of about 52 km, is rhomb-shaped (Fig. 2) and
surrounded by monotonously flat terrain, which stands
at about 1100 m above sea level. The southern margin of
the swamp is bounded by a roughly east-west trending
fault zone, locally known as the Nyama Dislocation
Zone. Moore (1964) recognised the Nyama Dislocation
Zone and mapped it within the confines of the map sheet
1428 SW of the Geological map of Zambia. Later,
Vrana (1974) and Vajner (1998a) mapped the western
continuation of the zone.
Several years later, Vrana (1985) re-examined some
breccia samples from his map area (Vrana, 1974), and
0899-5362/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 8 9 9 - 5 3 6 2 ( 0 2 ) 0 0 1 5 0 - 1
366
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
Table 1
Confirmed impact craters in Africa (modified after Master and Reimold, 2000)
Name
Country
Latitude
Longitude
Diameter (km)
Age (Ma)
Amguid
Aorounga
Aouelloul
B.P Structure
Bosumtwi
Gweni-Fada
Highbury
Kalkkop
Kgagodi
Morokweng
Oasis
Quarkziz
Roter Kamm
Sinamwenda
Talemzane
Tenoumer
Tin Bider
Tswaing
Vredefort
Algeria
Chad
Mauritania
Libya
Ghana
Chad
Zimbabwe
South Africa
Botswana
South Africa
Libya
Algeria
Namibia
Zimbabwe
Algeria
Mauritania
Algeria
South Africa
South Africa
26°050 N
19°060 N
20°150 N
25°190 N
06°300 N
17°250 N
17°040 S
32°430 S
22°290 S
26°280 S
24°350 N
29°000 N
27°460 S
17°120 S
33°190 N
22°550 N
27°360 N
25°240 S
27°000 S
04°230 E
19°150 E
12°410 W
24°200 E
01°250 W
21°450 E
30°070 E
24°260 E
27°350 E
23°320 E
24°240 E
07°330 W
16°180 E
27°470 E
04°020 E
10°240 W
05°070 E
28°050 E
27°300 E
0.45
12.6
0.36
2.0
10.5
14
20
0.64
3.5
70–80
18
3.5
2.5
0.22
1.75
1.9
6
1.13
250–300
6 0.1
<350
3:1 0:3
<120
1:07 0:05
<345
Not known
0:25 0:05
<65
145 0:8
<120
<70
3:7 0:3
<10
<3
2:5 0:5
<70
0:22 0:05
2023 4
effort to confirm or discount the impact origin of the
swamp, we carried out fieldwork, collected rock samples
in the area, scrutinized regional data, and performed
petrographic and geochemical studies of these rocks.
2. Geologic framework
Fig. 1. Locations of proposed impact structures in Zambia.
reported several sets of decorated and non-decorated
deformation features in quartz from the breccia, and
interpreted them as planar deformation features (PDFs).
PDFs (e.g., French, 1998) are diagnostic shock deformation effects indicative of impact metamorphism.
Consequently, Vrana (1985) proposed that the 52-kmwide Lukanga Swamp was of impact origin. The setting
of the swamp in Proterozoic basement rocks and lack of
a terraced rim and a central uplift suggested to him that
the structure was old and deeply eroded. No other
breccia occurrences or, in fact, any other evidence in
support of Vr
anaÕs claim has since been described by
other workers around the Lukanga Swamp. However,
since the work of Vr
ana (1985), no dedicated follow-up
investigation has been carried out to investigate the
proposed impact origin of the Lukanga Swamp. In an
The swamp is located in the Neoproterozoic to early
Palaeozoic Lufilian orogenic belt. Detailed reviews and
interpretations of the geological evolution of the Lufilian belt are presented by Kampunzu and Cailteux (1999)
and Porada and Berhorst (2000). Here, we only present
a general geological framework of the area around the
swamp.
Basement rocks occur to the east and meta-sedimentary rocks to the west of the Lukanga Swamp (Fig.
3). The basement rocks consist of Palaeoproterozoic
granite-gneisses (Ngambi et al., 1986), which are unconformably overlain by a sequence of alternating
pelitic schists, polymictic meta-conglomerates, and
quartzite of the Mesoproterozoic Muva Supergroup
(Moore, 1964; Cairney and Kerr, 1998). The meta-sedimentary rocks to the west comprise intercalated carbonates as well as argillaceous, arenaceous and
conglomeratic units of the Neoproterozoic Katanga
Supergroup (Vrana, 1974; Keppie, 1977; Vajner,
1998a,b). Syenites, granites and gabbros of various ages
intrude in both gneisses and meta-sedimentary rocks. In
the area of Map Sheet 1428 SW (south of the Lukanga
Swamp) Kundelungu rocks of the Katanga Supergroup
were reported by Vrana (1974). Rocks in the area to the
west of the swamp (NE of Mumbwa) are mainly of
greenschist-facies grade. A concentric array of faults is
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
367
Fig. 2. Landsat Thematic Mapper satellite image of the Lukanga Swamp, Bands 5 and 7. The swamp is rhomb-shaped and the area around it is
relatively flat. The light terrane on the eastern side of the swamp is at a relatively higher elevation than the adjacent dark area. The image is about 110
km wide. The Kafue River is located in the left part of the image.
located within about 50 km around the swamp (Fig. 3).
The swamp area is largely covered by Kalahari sands,
alluvium, colluvium and laterite.
bably marks the original margin of the depression in
which the swamp developed. Most of the streams are
seasonal and filled by thick sand deposits. The swamp
consists of isolated lakes and islands of Kalahari Group
(<70 Ma) sands.
3. Drainage and topography
The Lukanga Swamp is situated on the Central African plateau. Streams in the region around the swamp
are roughly parallel and trend NE and ENE. The parallel alignment of streams is controlled by regional
structural lineaments. A radial pattern of streams surrounds the swamp, indicating that the development of
the swamp diverted the streams from their original NE
or ENE trends (Fig. 4).
The area surrounding the swamp is monotonously
flat, especially at the western side that is drained by the
Kafue River. The flat topography stands at 1100 m
above sea level. Isolated, small ridges of less than 10 m
elevation above the surroundings occur as well. The
terrain rises gently by some 50 m towards the watershed
to the east of the swamp (Fig. 4). The watershed pro-
4. Fieldwork
Fieldwork was conducted in the dry season, at a time
when there is limited vegetation that could obscure
outcrops. In the generally flat terrain, rock exposure is
poor. However, 96 rock samples could be collected from
31 locations around the Lukanga Swamp, including 82
from within and 14 from outside of the Nyama Zone
(Fig. 3). Samples were, inter alia, collected from sample
locations described by Vrana (1985).
Field observations were mainly focused on the
Nyama Zone, from where samples with alleged shock
metamorphic features had been reported. Locations
outside of the zone, in the environs of the swamp, were
visited in a bid to locate new outcrops. It should be
368
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
Fig. 3. Simplified geological map showing sample locations around the Lukanga Swamp. At the scale of this map, closely spaced locations are
plotted on the same locality point (e.g., locations LUK 26, 27, 28, 29, and 30 are plotted as LUK 26-30). The area northwest of the swamp has no
known rock outcrops (after Thieme and Johnson, 1981).
Fig. 4. Drainage and topographic map of the area around the Lukanga Swamp. A radial pattern of streams flows into the swamp. The relief is flat on
the western side of the swamp, but rises gently from 1100 to 1200 m above sea level towards the eastern side.
noted that, in this paper, we use the term ÔclastÕ in a
general sense to include rock and mineral fragments. It
is difficult to classify the clastic rocks in the Nyama
Zone following classifications of clastic rocks by, for
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
369
Table 2
Petrographic descriptions of selected samples collected around the Lukanga Swamp
Sample no.
Description
LUK 1-02, 03, 04, 05, 08, 11, 15,
17, 18, 24
LUK 1-06, 07, 16, 25
Silicified, polymict, reddish-brown sedimentary quartz breccia consisting mainly of fractured, irregularly
shaped, white clasts and subrounded, gray clasts set into a reddish-brown matrix
Silicified, polymict, white sedimentary quartz breccia consisting of mainly fractured, irregular, white clasts
and subordinate, angular to subrounded, gray clasts set into white matrix
Silicified, massive, brown meta-siltstone occurring as layers or veins in other lithologies
Silicified, dark-gray, sheared quartz breccias consisting mainly of stretched, dark-gray and white clasts set
into white matrix
Brecciated shale consisting of vesicular tuff-like clasts, laminated clasts, dark-gray tabular clasts, and
equigranular, sand-sized, irregular clasts set into and cemented by argillaceous matrix
Finely foliated, dark gray slate
Vesiculated, aphanitic tuff-like black shale, in a shale breccia sample
Massive, black shale
Silicified, porphyroblastic shale. Porphyloblasts are prismatic pseudomorphs consisting of a fine-grained
aggregate of sericite set into a groundmass of shale
Brownish, ferruginous muscovite quartz schist, with preserved bedding
Cleaved vein quartz from outside of the Nyama Zone
Rounded, loose pieces of black quartzite from shallow pit
Fault breccia with black, angular clasts
Strongly crenulated, gray phyllite
Silicified, white, sheared quartzite
Fault breccia, with black, carbonaceous, angular clasts
LUK 1-09
LUK 1-01, 13, 14, 30, 31, 33, 34,
35, 36, 37, 38
LUK 2-01, 09, 11, 12
LUK
LUK
LUK
LUK
2-02
2-03
2-06
2-07
LUK
LUK
LUK
LUK
LUK
LUK
LUK
5-01
7-01
13-01
14-01
17-02
19-01
31-01
example, Pettijohn (1975) and Laznicka (1988). The
rocks contain more angular clasts than rounded clasts
and contain abundant secondary polycrystalline quartz
aggregates, which in some cases have masked the original clastic texture of a rock. For this reason, we classify
clastic rocks in the Nyama Zone as sedimentary quartz
breccia. However, we distinguish these sedimentary
quartz breccias from fault breccia (Sibson, 1977), which
consists of angular clasts set into an indurated, finegrained cataclastic matrix.
Outcrops along the Nyama Zone occur mainly as
continuous trends of loose boulders that form low
ridges or represent isolated boulders, and are sparsely
distributed. We mapped sheared quartzite, sedimentary
quartz breccias, meta-siltstone, shale and fault breccia.
Muscovite schists, muscovite-bearing quartzite, granites,
and gneisses were mapped outside of the Nyama Zone.
Brief descriptions of lithological units mapped around
the swamp are given in Table 2.
The most extensive outcrops were found at locations
LUK 1 and 2 (Fig. 3), where low (<10 m elevation)
ridges of approximately 50 200 m extent occur and
trend oblique to the Nyama Zone. Important sampling
sites of Vr
ana (1985) coincide with these locations.
Other outcrops are small and isolated. The base of the
ridge at LUK 1 is dominated by loose boulders of a
dark-gray, sheared quartzite. The dark-gray sheared
quartzite was not found on top of any ridge or hill, as
such, it was considered to underlie the less deformed
quartz breccia. The sheared quartzite consists of stretched, dark clasts of carbonaceous shale, quartzite clasts,
and milky-white, irregularly shaped, polycrystalline
quartz that are all set into a completely silicified matrix.
The polycrystalline quartz contains vugs lined with
small quartz crystals. The stretched clasts define a shear
fabric in the rock.
Sedimentary quartz breccia consists predominantly of
irregularly shaped, milky-white, polycrystalline quartz
with subordinate rock and mineral clasts of subrounded
shapes set into a fine-grained matrix, which may or may
not contain iron oxide cement (Fig. 5a). The quartz
breccias are subdivided into two intimately associated
types of reddish-brown and whitish breccias, which
grade into each other and are distinguished by an increase or decrease in the amount of one type of clast or
iron oxide cement in the matrix. The colour of the
reddish-brown breccia is imparted by the presence of
disseminated fine-iron oxide grains that are absent or
occur in lesser amounts in whitish quartz breccia (Table
2). The clast types in quartz breccia include quartzite,
meta-siltstone and carbonaceous shale (Fig. 5b). Some
white clasts possess vugs lined with small quartz crystals,
indicating secondary overprint by hydrothermal, silicious solutions. Clast sorting is poor, with grain sizes
ranging from 4–20 mm, rarely up to 15 cm. In some
cases, the clast sizes decrease perpendicular to bedding
within quartz breccia. The matrix of the breccias is
clastic and of silty grain size, and is cemented by silica
and/or iron oxide. Decrease in the amount of clasts in
some reddish-brown breccia occurrences changes the
texture of breccia to that of a meta-siltstone, which may
or may not contain isolated, milky-white, polycrystalline
quartz aggregates.
The outcrops of fault breccia trend parallel to the
Nyama Zone. The breccias consist of black angular
clasts derived from carbonaceous shale, set into a dark,
370
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
Fig. 5. Field photographs of some clastic rocks in the Nyama Zone. Knife for scale: 9 cm long. (a) Polymictic quartz breccia showing abundant
irregularly shaped, secondary milky-white polycrystalline quartz, with subrounded quartz and carbonaceous shale fragments not discernible in this
picture due to poor contrast against matrix (location LUK 1). (b) Polymictic quartz breccia consisting of mainly subrounded white clasts, with
subordinate gray clasts (not discernible in this picture due to poor contrast against matrix), set into brownish matrix largely composed of sand-sited
quartz grains (location LUK 1). (c) Fault breccia consisting of dark angular fragments set in a cataclastic matrix (location LUK 14). (d) Vesiculated,
tuff-like clast, angular slate and phyllite fragments set in argillaceous matrix. Lens cap for scale: 5 cm in diameter (location LUK 2). (e) Strongly
silicified (white) argillaceous breccia consisting of mainly angular fragments of shale and minor rounded clasts (location LUK 2). (f) Brecciated shale
consisting of fragmented shale beds. Matchstick in the center of the picture for scale: 4.3 cm long (location LUK 2).
fine-grained cataclastic groundmass composed of finegrained quartz (Fig. 5c).
Brecciated shale does not show much variation in
mineralogical and chemical composition (Table 3). One
type contains laminated and strongly vesiculated, ex-
tremely fine-grained, black, tuff-like, clasts (Fig. 5d).
Other brecciated shale occurrences are strongly silicified
and consist mainly of fragments derived from thin shale
beds and minor rounded clasts of shale (Fig. 5e). In
some cases, elongate to tabular fragments of thin shale
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
beds are set into a silicified matrix, giving the rock the
brecciated texture shown in Fig. 5f. This may be due to
syn-sedimentary deformation, as fragments of the same
composition are aligned more or less parallel, though
they have been slightly rotated with respect to each
other.
Some rocks found within and outside of the Nyama
Zone are either massive, foliated, or folded, and do not
exhibit clastic sedimentary textures. They include crenulated phyllite, granite, and granite-gneiss.
At locations LUK 1 and 2, well-preserved sedimentary structures were observed. The sedimentary quartz
breccia and meta-siltstone are complexly intercalated in
a manner that makes it difficult to separate one type
from the other or establish relative age relations (Fig.
6a). In some loose boulders, thinly bedded meta-siltstone is interfolded and, at places, interlayered with
reddish-brown quartz breccia (Fig. 6b and c). Contacts
are generally gradational between intimately associated
meta-siltstones of different appearance (Fig. 6d), but are
sharp between sedimentary quartz breccia and metasiltstone (Fig. 6e). Lamination and bedding are commonly observed in shale. Fault breccia was observed at
locations LUK 14 and 31.
5. Samples and analytical methods
Fifty representative samples were selected for petrographic analysis; 46 samples were crushed in an alumina
ceramic jaw crusher and pulverized in an agate mill at
the Institute of Geochemistry, University of Vienna.
Analyses for major and selected trace elements were
done on powdered samples using standard X-ray fluorescence spectrometry (XRF) procedures at the Department of Geology, University of Witwatersrand
(South Africa) following the procedures described by
Reimold et al. (1994).
6. Petrography and whole rock chemistry
We studied 50 thin sections of representative samples
from localities around the swamp (Fig. 3). In thin section, the sedimentary quartz breccias are composed of
irregularly shaped, coarse-grained, polycrystalline aggregates of quartz crystals and angular to subrounded
rock and mineral clasts. The polycrystalline aggregates
of quartz are composed of euhedral to subhedral grains
with straight grain boundaries meeting at 120° (triple
junctions). Many quartz grains may or may not contain
fine carbonate grains. The lithic clasts are composed of
quartz aggregates and single-grained quartz, with or
without carbonate or fine-grained black, carbonaceous
inclusions. Constituent mineral grains in rock fragments
display a range of grain sizes varying from crystals that
371
are smaller or larger than crystals in the matrix. In the
case where the grains in the clasts are of the same size as
those in the matrix, it is difficult to distinguish one unit
from the other. The matrix is composed of clastic quartz
grains, with grain sizes ranging from 40 to 200 lm, that
are cemented by crystalline quartz with or without
reddish-brown, iron oxide. Some clasts consist of
strained quartz grains, whereas others are not deformed.
The strained grains in clasts show undulose extinction,
deformation bands, and subgrain development as a result of dynamic recrystallisation. The matrix grains are
neither recrystallized nor strained, and show secondary
overgrowths of quartz in optical continuity with the
original quartz grains (Fig. 7a). The overgrowths are
outlined by fine, generally black inclusions thought to
represent carbonaceous material. Inclusions of small,
subhedral to euhedral carbonate grains are mainly
concentrated in the core or arranged in concentric rings
in the mantles of quartz grains (Fig. 7b). The fault
breccia at locations LUK 14 is composed of highly
strained, angular clasts set into a fine-grained cataclastic
groundmass composed of quartz and carbonaceous
material. The clasts contain randomly oriented fluid
inclusion trails of irregular forms.
Shock metamorphism provides unambiguous evidence for conditions associated with impact cratering
(French, 1998; St€
offler and Langenhorst, 1994). The
most useful, generally accepted indicators for shock
metamorphism are features that can be studied using
the polarizing optical microscope, such as planar microstructures, optical mosaicism, changes in refractive
index, isotropism and phase changes (Grieve et al.,
1996; St€
offler and Langenhorst, 1994). PDFs in rockforming minerals (e.g., quartz, feldspar or olivine)
provide diagnostic evidence of shock deformation. In
quartz, PDFs consist of parallel zones of amorphous
silica with thicknesses of <1–3 lm that are spaced 2–
10 lm apart (Alexopoulous et al., 1988) and occur
in planes corresponding to specific crystallographic
orientations (French, 1998; St€
offler and Langenhorst,
1994).
In our petrographic study of rocks from around the
swamp, we did not find any features that are indicative
of shock metamorphism. We found up to three crudely
defined sets of fluid inclusion trails in some quartz
grains, both in the matrix and in lithic or mineral clasts.
The trails are generally subplanar to curved, or completely irregular (Fig. 7c, d and e). The spacings between
trails of any set are wide, ranging from 10 to 200 lm.
However, one quartz clast was found to show more
closely (2–20 lm) but unevenly spaced, intragranular,
subparallel fluid inclusions trails (Fig. 7f). Due to the
scarcity of grains with parallel or subparallel fluid inclusion trails in these thin sections, a statistically useful
universal stage study to establish possible textural controls did not seem justified.
372
Table 3
Chemical composition of 39 representative samples from around the Lukanga Swamp area
LUK1-01
S.Q.Br
LUK1-02
S.Q.Br
LUK1-03
S.Q.Br
LUK1-04
S.Q.Br
LUK1-05
S.Q.Br
LUK1-07
S.Q.Br
LUK1-13
S.Q.Br
LUK1-14
S.Q.Br
LUK1-16
S.Q.Br
LUK1-18
S.Q.Br
LUK1-20
S.Q.Br
LUK1-22
S.Q.Br
LUK1-24
S.Q.Br
LUK1-25
S.Q.Br
SiO2
TiO2
Al2 O3
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
LOI
97.34
0.02
0.20
0.58
<0.01
<0.01
0.97
<0.01
<0.01
0.04
0.37
97.75
0.11
0.38
0.90
<0.01
<0.01
0.13
<0.01
<0.01
0.21
0.45
97.41
0.02
0.08
0.90
<0.01
0.26
0.13
<0.01
<0.01
0.04
0.64
98.47
0.02
0.13
0.58
<0.01
<0.01
0.07
<0.01
0.15
0.02
0.24
99.36
0.02
0.02
0.54
<0.01
<0.01
0.11
<0.01
<0.01
0.01
0.31
98.07
0.04
0.24
0.42
<0.01
<0.01
0.16
0.39
<0.01
0.09
0.42
98.77
0.02
0.15
0.38
<0.01
<0.01
0.12
<0.01
<0.01
0.02
0.35
98.73
0.02
0.15
0.35
<0.01
<0.01
0.07
<0.01
<0.01
0.01
0.35
96.21
0.01
<0.01
0.39
<0.01
<0.01
0.13
<0.01
<0.01
0.02
3.44
97.86
0.04
0.16
0.53
<0.01
<0.01
0.18
0.16
<0.01
0.02
0.40
97.07
0.03
0.05
0.59
<0.01
<0.01
0.11
<0.01
<0.01
0.02
0.28
98.22
0.02
0.02
0.50
<0.01
<0.01
0.09
<0.01
0.02
<0.01
0.41
98.76
0.01
<0.01
0.44
<0.01
<0.01
0.10
<0.01
<0.01
<0, 01
0.06
97.89
0.02
<0.01
0.39
<0.01
<0.01
0.11
<0.01
<0.01
<0.01
0.23
Total
99.52
99.93
99.48
99.68
100.37
99.83
99.81
99.68
100.20
99.35
98.15
99.28
99.37
98.64
V
Cr
Co
Ni
Cu
Zn
Rb
Sr
Y
Zr
Nb
Ba
<15
<9
9
<9
<2
<9
<3
23
<3
12
4
12
17
12
10
<9
<2
<9
<3
65
6
32
6
42
<15
17
11
<9
<2
<9
<3
25
<3
11
5
7
<15
<9
10
<9
<2
<9
<3
14
<3
11
4
14
<15
<9
11
<9
<2
<9
<3
15
<3
12
4
37
<15
19
9
<9
<2
<9
<3
20
<3
15
5
13
<15
13
10
<9
<2
<9
<3
22
<3
11
4
7
<15
<9
10
<9
<2
<9
<3
18
<3
11
5
<5
<15
12
10
<9
<2
<9
<3
12
<3
11
4
<5
<15
14
11
<9
<2
<9
<3
20
3
13
5
64
<15
37
11
<9
<2
<9
<3
16
<3
13
5
22
<15
<9
10
<9
<2
<9
<3
10
<3
10
4
<5
<15
10
10
<9
<2
<9
<3
15
<3
10
4
14
<15
14
10
<9
<2
<9
<3
17
<3
12
5
35
LUK1-30
S.Q.Br
LUK1-31
S.Q.Br
LUK1-33
S.Q.Br
LUK1-34
S.Q.Br
LUK1-35
S.Q.Br
LUK1-36
S.Q.Br
LUK10a-1
S.Q.Br
LUK1602 S.Q.Br
LUK1801 S.Q.Br
LUK1-09
Meta-Sst
LUK2-01
Shale
LUK10b1 Shale
LUK1702 Shale
LUK2-02
Shale
SiO2
TiO2
Al2 O3
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
LOI
97.47
0.02
<0.01
0.56
0.04
<0.01
0.18
0.01
<0.01
<0.01
0.38
98.34
0.02
<0.01
0.35
<0.01
<0.01
0.10
<0.01
<0.01
0.04
0.26
98.87
0.02
<0.01
0.39
<0.01
<0.01
0.12
<0.01
<0.01
<0.01
0.49
97.81
0.02
<0.01
1.04
0.03
<0.01
0.13
<0.01
<0.01
<0.01
0.40
97.78
0.03
<0.01
0.69
<0.01
<0.01
0.09
<0.01
<0.01
<0.01
0.41
98.80
0.02
<0.01
0.47
<0.01
<0.01
0.16
<0.01
<0.01
<0.01
0.37
97.49
0.02
<0.01
1.00
<0.01
<0.01
0.06
<0.01
<0.01
0.12
0.37
87.06
0.21
4.26
2.63
0.03
0.23
1.16
0.05
1.28
0.07
2.61
96.46
0.14
0. 51
0.86
<0.01
0.31
0.09
0.03
<0.01
0.01
0.85
98.20
0.03
0.14
0.51
<0.01
<0.01
0.12
<0.01
<0.01
0.02
0.30
92.20
0.15
2.75
0.87
<0.01
<0.01
0.64
0.01
0.90
0. 06
1.11
83.81
0.42
7.90
1.48
<0.01
0.30
0.23
0.02
2.61
0.20
1.91
91.04
0.28
2.69
2.36
<0.01
<0.01
0.06
0.02
0.76
0.01
1.97
90.46
0. 68
4.51
0. 50
<0.01
<0.01
0.05
0.07
1.26
0.01
1.31
Total
98.66
99.11
99.89
99.43
99.00
99.82
99.06
99.61
99.27
99.32
98.69
98.88
99.20
98.85
V
Cr
<15
<9
<15
<9
<15
<9
<15
<9
20
<9
<15
<9
20
14
34
34
20
18
<15
11
92
74
83
57
428
40
202
72
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
Sample
Co
Ni
Cu
Zn
Rb
Sr
Y
Zr
Nb
Ba
13
<9
<2
<9
<3
21
<3
11
4
12
10
<9
<2
<9
<3
12
5
11
4
<5
9
<9
<2
<9
<3
14
6
12
4
50
10
<9
<2
<9
<3
18
5
14
4
12
11
<9
<2
<9
<3
12
<3
10
4
27
11
<9
<2
11
<3
12
4
18
4
5
10
<9
<2
<9
62
44
7
43
8
240
12
11
<2
70
<3
20
37
28
7
32
10
<9
<2
<9
<3
19
<3
12
4
21
11
21
<2
<9
34
77
31
57
5
478
12
9
<2
<9
104
160
84
102
12
1404
11
15
<2
13
21
34
11
59
8
164
10
<9
<2
<9
51
74
25
132
30
408
LUK2-03
Shale
LUK2701 Shale
LUK2801 Shale
LUK2-07
Shale
LUK5-01
M.Q.Sc
LUK6-01
M.Q.Sc
LUK25-01
M.Q.Sc
LUK1301 Qzite
LUK1902 Qzite
LUK2601 Qzite
LUK3001 Qzite
LUK3004 Qzite
LUK1401 F.Br
LUK3101 F.Br
SiO2
TiO2
Al2 O3
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
LOI
90.03
0.18
3.28
2.96
0.02
0.23
0.77
0.07
0.67
0.28
1.86
95.59
0.14
1.32
1.11
0.02
<0.01
0.06
0.03
0.06
<0.01
0.73
95.38
0.16
0.79
1.45
<0.01
<0.01
0.26
0.01
0.01
0.26
1.40
93.33
0.50
3.05
0.64
<0.01
<0.01
0.16
0.06
0.89
0.01
0.90
82.44
0.39
8.63
4.31
0.07
<0.01
0.06
0.14
2.69
0.01
1.49
69.50
0.66
9.17
4.73
0.04
1.30
4.16
1.34
2.71
0.11
5.14
72.92
1.01
13.19
6.14
<0.01
0.38
0.06
0.12
2.71
<0.01
2.51
97.83
0.08
<0.01
1.49
<0.01
<0.01
0.04
<0.01
<0.01
<0.01
0.08
98.56
0.09
<0.01
1.13
<0.01
<0.01
0.03
0.02
<0.01
0.01
0.44
90.79
0.23
4.09
0.67
<0.01
1.59
0.16
0.10
1.59
0.06
1.13
97.28
0.08
0.63
0.32
<0.01
<0.01
0.76
0.06
<0.01
0.01
1.14
96.22
0.10
0.84
0.48
<0.01
<0.01
0.24
0.08
0.13
0.06
0.73
96.18
0.08
0.09
2.19
<0.01
<0.01
0.07
<0.01
0.00
<0.01
0.61
90.28
0.11
0.86
1.47
<0.01
<0.01
0.07
<0.01
0.27
0.05
5 51
Total
100.35
99.09
99.73
99.54
100.23
98.86
99.04
99.52
100.30
100.42
100.31
98.89
99.22
98.64
V
Cr
Co
Ni
Cu
Zn
Rb
Sr
Y
Zr
Nb
Ba
42
10
9
14
<2
11
31
159
24
87
7
274
21
21
11
<9
<2
19
12
24
4
26
5
51
29
23
11
<9
<2
9
6
55
13
31
5
277
72
40
10
<9
<2
<9
38
45
8
88
12
302
82
33
11
<9
<2
9
81
17
24
114
10
817
122
94
11
27
9
<9
100
39
50
217
12
362
101
103
<9
<9
<2
<9
122
10
16
134
12
486
15
15
9
<9
<2
<9
<3
38
<3
107
4
8
18
18
17
9
<2
<9
<3
20
4
25
5
54
23
28
10
<9
<2
23
87
49
8
53
8
195
<15
20
11
<9
<2
9
<3
41
<3
14
5
62
<15
30
12
<9
<2
26
4
44
<3
17
5
141
47
14
9
<9
9
26
6
13
4
15
5
11
67
25
<9
<9
<2
24
7
61
5
23
4
1040
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
10
<9
<2
<9
<3
14
3
10
4
30
Sample locations are shown in Fig. 3; sample 1/01 means location 1, sample number 1. Major elements in wt.%, trace elements in ppm. All Fe as Fe2 O3 . S.Q.Br: sedimentary quartz breccia,
sst: siltstone, M.Q.Sc: muscovite quartz schist, Qztite: quartzite, F.Br: fault breccia.
373
374
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
Fig. 6. Field photographs of some important structures observed in the Nyama Zone. Knife ¼ 9 cm long and lens cap ¼ 5 cm in diameter, for scale.
(a) Intercalated reddish-brown meta-siltstone (gray) and silicified white quartz breccia (location LUK 1). (b) Folded, bedded layers of brown metasiltstone alternating with reddish-brown quartz breccia (location LUK 1). (c) Intensely folded thin layer of reddish-brown meta-siltstone (right part
of photograph) in reddish-brown breccia (location LUK 1). (d) Gradational contact between inclusion-rich meta-siltstone and inclusion-poor metasiltstone. The inclusions are milky-white polycrystalline quartz aggregates (location LUK 1). (e) Sharp contact (bedding plane) between quartz
breccia with abundant milky-white polycrystalline quartz and meta-siltstone.
Results of major and some trace element analyses for
39 representative rock samples from around the swamp
are reported in Table 3. The samples analyzed are derived from sedimentary quartz breccia, meta-siltstone,
shale, quartz-muscovite schist, quartzite, and fault breccia. The sedimentary quartz breccia, meta-siltstone,
quartzite, fault breccia and some shale samples were
collected from the Nyama Dislocation Zone, whereas
other samples were collected outside the zone (Fig. 3).
The compositions of the rocks are normal for rocks of
quartz-rich mineralogical compositions (Taylor and
McLennan, 1985). The major element chemistry of all
the rocks indicate that they were derived from quartzose
sedimentary protoliths (Fig. 8, Roser and Korsch, 1988).
Most sedimentary quartz breccias consist of >96 wt.%
of silica, with minor Fe2 O3 and Al2 O3 contributions.
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
375
Fig. 7. Photomicrographs of clastic rocks in the Nyama Zone. Width of field of view ¼ 6 mm. (a) Clastic quartz matrix of quartz breccia showing
silica overgrowths around quartz grains: sample LUK 1-08. (b) Concentric carbonate inclusions and quartz overgrowth marked by black material in
quartz clast: sample LUK 1-03. (c) NE and NW trending intergranular, irregular, widely spaced, and non-planar fluid inclusion trails in a quartz
grain of a quartzite clast: sample LUK 1-02. (d) Widely spaced, subparallel, intragranular fluid inclusion trails along NNW–SSE direction with
NE–SW trending fractures in quartz: sample LUK 1-02. (e) Randomly oriented, intragranular fluid inclusions trails in quartz: sample LUK 19-01.
(f) Relatively closely spaced (2–20 lm), non-planar, subparallel fluid inclusion trails in quartz: sample LUK1-05.
High silica contents determined for shale samples are
regarded to be due to silicification as evident in the
petrographic observations on these samples.
Siderophile (Ni, Co, and Cr), and platinum groupelements (Ir, Os, Re), which have high abundances in
meteorites but low abundances in terrestrial crustal
rocks are used to detect meteoritic components in sus-
pected or confirmed impactites (e.g., Morgan et al.,
1975; Palme, 1982; Koeberl, 1998; Montanari and
Koeberl, 2000). Detection of significant enrichment of
such elements provides evidence for the impact origin of
a crater structure, particularly high Ir. The samples from
the Lukanga Swamp show variable siderophile element
abundances (compare Table 3) that are comparable to
376
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
Fig. 8. Discriminant function for provenance signatures of sandstone–mudstone suites using major elements (after Roser and Korsch, 1988). The
chemical compositions of the rocks from the study area (Table 3) plot into the field of quartzose sedimentary provenance. The rock–name abbreviations are adopted from Table 3.
and, in some cases, slightly higher than the typical upper
continental crust values (i.e., Cr: 35 ppm; Co: 10 ppm;
and Ni: 20 ppm, Taylor and McLennan, 1985), whereas
the platinum group element are below the detection limit
of XRF. However, Co, Cr and Ni contents in terrestrial
rocks are variable but may be high due to local sources
Fig. 9. Structural map of central Zambia showing structural trends, main dislocation zones and the relative location of the Lukanga Swamp. Note
that the Nyama Zone terminates to the southwest of the Lukanga Swamp, but the Kapiri-Mposhi Zone extends further. In this arrangement, the
Nyama Zone is right-stepping with respect to the Kapiri-Mposhi Zone, i.e., to see the Kapiri-Mposhi Zone while facing in the direction of the trend
of the Nyama Zone, an observer has to look to the right (Mann et al., 1983). The arrows show the direction of movement of crustal blocks but not the
shear-sense displacement along the zones (after De Swardt et al., 1965).
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
such as sulphides and oxides of these elements (e.g.,
magnetite, chromite, carrolite, pyrite etc.) and would
give ambiguous results. There are no enrichments in
siderophile and platinum group element contents in the
rocks from the study area that could suggest an exotic
contribution (Table 3).
7. Structural and geophysical data analysis
7.1. Structural analysis
Structural mapping in the area was hampered by
poor exposure. For this reason, structural analysis and
interpretations attempted here are largely based on regional considerations from previous work, as well as
aerial photography interpretation.
There are several ENE trending, parallel strike-slip
faults across central Zambia. The most prominent ones
are the Mwembeshi, Kapiri-Mposhi, and Nyama Dislocation Zones (Fig. 9). The Mwembeshi Dislocation
Zone is well defined and has assumed an important role
in the geodynamic interpretation of the central African
Pan-African belts (De Swardt et al., 1965; Unrug, 1983;
Coward and Daly, 1984; Daly, 1986; Kampunzu and
Cailteux, 1999). The Lukanga Swamp, located north
of the Mwembeshi Zone, is straddled by the KapiriMposhi Zone, and bordered, on the southern margin, by
the Nyama Zone.
Transcurrent displacement has been inferred (De
Swardt et al., 1965) along these zones, but shear-sense
displacements are poorly constrained and are based on
regional shear-sense evaluations, using deflections of
structural trends along the fault zones (De Swardt et al.,
1965). Both dextral and sinistral strike-slip shear-sense
movements have been reported from along the
Mwembeshi Zone (Simpson, 1962; De Swardt et al.,
1965; Coward and Daly, 1984; Daly, 1986; Johns et al.,
1989), suggesting multiple movements in opposite directions during episodes of reactivation. The sense of
strike-slip movements along both the Kapiri-Mposhi
and Nyama Zones are dextral (De Swardt et al., 1965).
The Kapiri-Mposhi Zone is a composite zone comprising two belts of high shear zones, referred to as the
northern and southern shear zones (Smith, 1966).
Movements along these dislocation zones affected
both the Palaeoproterozoic basement rocks (Ngambi
et al., 1986) and Neoproterozoic Katangan metasedimentary rocks (Moore, 1964; Smith, 1966; Vr
ana, 1974;
Keppie, 1977; Vajner, 1998a). Synkinematic granites
along the Mwembeshi Shear Zone has been dated at
551 19 Ma (Hanson et al., 1993) and the zone is believed to be coeval with the Kapiri-Mposhi and Nyama
Zones on the basis of their parallelism (De Swardt et al.,
1965; Coward and Daly, 1984). This age has been
interpreted to suggest that initial movements along
377
the dislocation zones were contemporaneous with the
Lufilian orogenesis (Hanson et al., 1993; Porada and
Berhorst, 2000).
The northeastern segment of the Mwembeshi Zone in
the eastern part of Zambia is overprinted by the LuanoLukusashi-Luangwa Rift, which is filled by Mesozoic
Karoo sediments (De Swardt et al., 1965; Rosendahl,
1987) (Fig. 10a and b). In this area, Karoo-age faults
trend parallel to the Mwembeshi Zone, signifying that
Karoo rifting made use of weak zones established by the
Mwembeshi Zone (Reichwalder, 1978). The Mesozoic
Kariba Rift Zone (mid-Zambezi valley basin) and the
Cenozoic Okavango Rift occur along the same rift axis
(Modisi et al., 2000), whereas the Zambezi Rift originates from the Kariba Rift and trends east-west.
Superposition of Cenozoic rifts on Mesozoic rifts is
common in the East African Rift system (Rosendahl,
1987). The Kariba, and Luano-Lukusashi-Luangwa
Rifts (Nyambe, 1999) and the Okavango Rift (Modisi
et al., 2000) exhibit structural patterns typical of extensional basins. An ENE trending, elongate strip of Karoo
sediments is superimposed on, and runs parallel to, the
southwestern reaches of the Kapiri-Mposhi Zone, north
of the Hook Granite Massif (De Swardt et al., 1965;
Thieme and Johnson, 1981). The basin is not bounded
by faults, as observed elsewhere (e.g., in the mid-Zambezi valley), to suggest Karoo rifting but may also be an
extensional rift zone similar to the other Karoo rift
basins in Zambia. The Lukanga Swamp is roughly
rhomb-shaped (Fig. 2) in contrast to elongate extensional basins in the East African Rift system (Rosendahl, 1987). The occurrence of an elongate Karoo basin
along the south-eastern extension of the Kapiri-Mposhi
Zone indicates that there has been reactivation along
this zone during the Mesozoic era.
No sedimentological, petrological and structural evidence or data exist to suggest that the Lukanga Swamp is
a site of an extensional Karoo or Cenozoic rift basin. No
drilling or seismic reflection studies have been done to
unravel the subsurface geology in the swamp area and
the structure of the faults. The surface geometric arrangement of the Kapiri-Mposhi and Nyama Zones in
the Lukanga Swamp area favors the hypothesis of development of a pull-apart (transtensional) basin during
strike-slip displacement. The Nyama Zone forms a rightstepping fault with respect to the Kapiri-Mposhi Zone,
and the overall shear-sense movement on both fault
zones is dextral (De Swardt et al., 1965). Movements
along right-stepping, dextral strike-slip fault systems can
develop pull-apart (transtensional) basins in overlapping
areas of faults (e.g., Mann et al., 1983; Christie-Blick and
Biddle, 1985, and references therein). Pull-apart basins in
such settings are bounded by normal faults inclined at an
angle to master faults, and are generally rhomb-shaped
(Mann et al., 1983). Normal faults bounding the swamp
have not been mapped or reported. However, the
378
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
Fig. 10. (a) Map of the East African Rift System and the rift zones in Zambia. The Lukanga swamp does not occur in any of the rift zones (after
Modisi et al., 2000). (b) Karoo sedimentary rocks along rift zones. Note Karoo sedimentary rocks southwest of the Lukanga Swamp presumed to
follow the Kapiri-Mposhi Zone (after Rosendahl, 1987).
Landsat Thematic Mapper Satellite image (Fig. 2) shows
a roughly rhomb-shaped swamp area bounded by linear,
NNW-trending boundaries. The eastern margin coincides with the Munkumpu Fault (Keppie, 1977), which
extends from the north of the swamp. The timing of
when the extensional event could have taken place is
difficult to constrain due to lack of sedimentological and
kinematic data. The swamp is filled by Tertiary to Recent
deposits and underlain by Neoproterozoic Katangan
metasedimentary rocks, which delimit the time when the
swamp could have formed.
7.2. Aeromagnetic data
Magnetic field measurements at confirmed impact
structures have not revealed any single unique signature
that could be related to an impact process (French,
1998; Pilkington and Grieve, 1992). Some impact
structures show no significant magnetic signature at all,
whereas others exhibit a strong local anomaly, which
may be positive or negative (usually 1000 nT), at or
near the center of the crater (Pilkington and Grieve,
1992). Vrana (1985) suggested that there was a magnetic
signature over the Lukanga Swamp that could be attributed to the presence of an impact structure.
The magnetic data used in this study were derived
from digital compilation of the contour maps of the
country-wide aeromagnetic mapping undertaken between 1967 and 1982 by the Geological Survey of
Zambia (Saviaro, 1980b). The data covering the Lukanga Swamp area are part of the Kabwe-West survey
flown by Canadian Aero Services in 1967. The survey
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
379
Fig. 10 (continued)
was carried out at a mean line spacing of 800 m and a
mean terrain clearance of 150 m with a Fluxgate Magnetometer. Manually contoured magnetic intensity
maps, at the 1:50,000 scale, were produced from data
obtained from this survey (Isaacs, 1968). These maps,
together with data from later surveys, were digitized,
with the help of the Council of Geoscience of South
Africa, and merged to produce a 250 m grid-cell digital
map of Zambia. The data covering the Lukanga Swamp
and its environs were processed using the Geosoft version 3.1 software.
The total magnetic field map (Fig. 11a) exhibits
positive and negative anomalies of þ230 and )237 nT
magnitudes, respectively, which are well below the
1000 nT anomalies observed at some confirmed impact
craters (Pilkington and Grieve, 1992). The map depicts a
subdued magnetic field in the central, northwestern and
southern parts of the swamp, whereas in the eastern and
northern parts high-frequency anomalies are noted.
Prominent anomalies include an ENE trending highamplitude anomaly in the northeast and the high-relief
NW–SE trending lineaments to the north. Other
anomalies include a west-east trending lineament traversing the southcentral part of the area, a NE–SW
trending lineament to the west of the swamp, and an
arcuate anomaly in the northwest. Whereas the southern
part of this area is dominated by west-easterly trending
anomalies, some NW–SE trends are discernible towards
the west and NE–SW trends through the central area. A
nearly N–S trending anomaly forms the eastern margin
of the swamp, though it is subdued in the north and
appears to mark the western termination of the highamplitude anomalies to the east of the swamp. The
Lukanga Swamp lies in the low magnetic zone bordered
to the north by a broad low-amplitude anomaly that
terminates in a NW–SE trend to the north.
The application of filters such as vertical derivative
and shaded relief, helps enhance subtle features, especially where the anomalies are weak compared to the
regional magnetic field (Fig. 11b). Many geological
features and structures are evident, though our interpretation is confined to the most prominent ones. A
detailed lineament interpretation of the shaded vertical
magnetic derivative map is presented in Fig. 11c.
The ENE–WSW trending anomaly in the eastern part
of the study area corresponds to the Kapiri-Mposhi
Dislocation Zone. It is cross-cut along its extension by
several lineaments, most of which trend NE–SW. These
features do not occur over the western part of the
Lukanga Swamp. NE–SW trending anomalies in the
southwestern part of the area have been interpreted as a
continuation of the Kapiri-Mposhi Dislocation Zone.
These anomalies continue northeasterly direction and
terminate at the western margin of the Lukanga Swamp.
The area in the north is dominated by NW–SE trending
features, which correlate with structures related to the
Lufilian belt (Porada and Berhorst, 2000).
NNW trending anomalies traverse the eastern margin
of the swamp and seem to be broader in the south than
to the north. This feature corresponds to the Munkumpu Fault Zone and may represent a reactivated
basement fault that controlled the sites of meta-gabbro
380
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
Fig. 11. (a) Total magnetic field anomaly map showing location of the Lukanga Swamp in a positive anomaly zone, stretching from the northwest,
through the central and southern parts. The map was extracted from the 120 m grid-cell digital total magnetic-intensity map of Zambia. (b) Shaded
vertical derivative map showing the position of the Lukanga Swamp and enhanced linear magnetic anomalies, which correspond to some known
structural and lithologic features in the area. (c) Magnetic lineament interpretation of the vertical derivative map. Some magnetic anomalies correspond to dislocation zones in the area. Earthquakes from the period 1967–1999 plot along the Kapiri-Mposhi Zone.
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
381
Fig. 11 (continued)
intrusion and other lava feeder pipes (Keppie, 1977).
The feature affects both the Nyama and the Mwembeshi
Dislocation zones, as their anomalies are clearly subdued where NNW trends traverse them. The Munkumpu Zone marks the apparent termination of the
anomalies of the Kapiri-Mposhi and the topographic
eastern margin of the swamp. To the south, the
Munkumpu Zone broadens where it corresponds to a
line of gabbroic intrusions mapped by Moore (1964). In
the southern and southeastern parts of the area, the
Mwembeshi Dislocation Zone is represented by ENE
trending faults, which are cross-cut by NW lineaments
and NNW trends. To the southeast of the swamp margin, ESE trending faults are prominent. They are also
cross-cut by NNW and NE trending lineaments that
terminate against the Munkumpu Fault Zone. To the
southwest of the swamp margin, ESE directed lineaments are observed that continue westward where they
intersect the NE-trending Kapiri-Mposhi Zone. These
faults are, in turn, cut by NE-trending faults that have
been assigned to the Nyama Dislocation Zone (Moore,
1964; Vr
ana, 1974). Also to the southwest of the swamp,
arcuate anomalies (Fig. 11c) represent folded metasedimentary rocks of the Katanga Supergroup (Vajner,
1998a), also known as the Katanga-High of De Swardt
et al. (1965).
A small (2 km long) elliptical feature of high-magnetic frequency is located to the northeast of the Lukanga swamp (Fig. 11a, 28°100 E, 13°400 S). It coincides
with the location of an olivine-gabbro pluton mapped
by Gignoux (2000). The linear structures in the northeastern part of the map may represent faults and/or
mafic dykes. The NW–SE striking anomaly north of the
swamp may mark a faulted geological contact.
7.3. Seismic activity and gravity analysis
Relatively low seismic activity (<4.0) has been measured in the Lukanga Swamp area (Topfer, 1976; Saviaro, 1980a), compared to that observed in the East
African Rift zones (>5.0, Midzi et al., 1999). There has
been no focused seismo-tectonic study in Zambia, but
seismic activity is known to occur along the KaribaLuangwa and Mweru Rift Zones (Saviaro, 1980a) in the
eastern part of the African Superswell, a region of relatively elevated terrain (Nyblabe and Robinson, 1994;
Lithgrow-Bertelloni and Silver, 1998). The rifts are
characterized by central grabens with marginal uplifts,
382
C. Katongo et al. / Journal of African Earth Sciences 35 (2002) 365–384
and are associated with intervening depressions of low
and dispersed seismic activity (Gumbricht et al., 2001).
The Lukanga Swamp is located within the low-seismic
activity depression between the Mweru and LuangwaKariba Rifts. Earthquakes of 2–4 magnitude, on the
Richter Scale, have been recorded between 1967 and
1999 by the Zambia seismic-network (Geological Survey
of Zambia), which are shown in Fig. 11c. The highest
magnitude earthquakes fall along the Kapiri-Mposhi
Zone, whereas lower magnitude earthquakes are scattered along other structures. No work has been done to
monitor sediment loading in the swamp in relation to
the limited seismic activity reported from the area.
Regarding gravity signals simple impact craters generally have a circular, negative gravity anomaly due to
the lower density of brecciated rocks of the crater fill or
in the crater floor, compared to unbrecciated rocks. In
contrast, complex craters often have a positive gravity
anomaly associated with denser, uplifted rocks of the
central uplift zone that, in turn, may be surrounded by
an annular negative anomaly (Pilkington and Grieve,
1992).
No gravity data of the area around the swamp exist.
The sites measured during the reconnaissance gravity
survey of the country (Maz
ac, 1974) are too far from the
swamp. Similarly, the gravity survey by Sebagenzi
(1997) was restricted to the region adjacent to the
Copperbelt Province far to the north of the study area.
8. Summary and conclusions
Our results show that the rocks previously suspected
to be impact breccias and reported to display shock
metamorphic features (Vr
ana, 1985) display textures
characteristic of sedimentary quartz breccia and fault
breccia. Irregularly shaped polycrystalline quartz aggregates are dominant over mostly subrounded terrigenous clasts. The subrounded shapes indicate a certain
degree of transport and probably brecciation. The irregular shape of the polycrystalline quartz aggregates,
the granoblastic texture of constituent quartz, and the
presence of vugs lined with small crystals indicate that
silica is secondary and crystallized late in the evolution
of these rocks. Preservation of sedimentary structures,
such as contorted folding, lamination, and bedding, is
widespread. The fluid inclusion trails in quartz are nonplanar and much wider spaced than the 2–10 lm spacings of PDFs (e.g., St€
offler and Langenhorst, 1994;
Grieve et al., 1996). They are also generally of random
orientation and are irregularly spaced. These fluid
inclusion trails do not represent crystallographically
controlled, shock-diagnostic PDFs. Consequently, our
investigation failed to find any evidence that could
corroborate the earlier proposal by Vr
ana (1985) that
the Lukanga Swamp may represent an impact structure.
Regional geological and geophysical data were compiled and evaluated with a view of establishing possible
constraints on the origin of the swamp. Although the
strike-slip, dextral, shear-sense movements (De Swardt
et al., 1965), and right-stepping relation between the
Kapiri-Mposhi and Nyama zones could result in the
development of tensional stresses in the swamp area,
en echelon arrangement, which is critical in localizing
stresses (Mann et al., 1983) is lacking. Structural considerations, coupled with the fact that the swamp
straddles several strike-slip zones, present a strong case
for the tectonic origin of the swamp. However, there are
not enough well-constrained structural data to draw
firmer conclusions. We, thus, conclude that a tectonic
cause for the presence of the Lukanga Swamp in this
region is likely but cannot be completely constrained.
The aeromagnetic signature around the Lukanga
Swamp area can not also be attributed to the presence of
an impact crater. All the magnetic anomalies in the area
are due to faults and lithological units.
Acknowledgements
We thank the Austrian Academic Exchange Service
€ AD) for providing a Ph.D. stipend and partial fi(O
nancial support for fieldwork (to C. Katongo). This
work was supported by the Austrian Science Foundation, project Y58-GEO (to C. Koeberl). WURÕs research
is supported by grants from the National Research
Foundation of South Africa and the University of
Witwatersrand Research Council. We are grateful to the
Geology Department of the University of Zambia
for providing logistical support during fieldwork. The
Director of the Geological Survey of Zambia, Mr.
D. Mulela, is thanked for permission to use the aeromagnetic data. We appreciate the critical comments
by Dr. Vrana and an anonymous referee. This is University of the Witwatersrand Impact Cratering Research
Group Contribution No. xy.
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