The geology and regional correlation of the Proterozoic Okwa Inlier, western Botswana

Precambrian Research, 56 (1992) 255-274

255

Elsevier Science Publishers B.V., Amsterdam

The geology and regional correlation of the Proterozoic Okwa Inlier, western Botswana" D.T. Aldiss and J.N. Carney British Geological Survey, Keyworth, Nottingham, NG I 2 5GG, UK (Received January 18, 1990; accepted after revision August 5, 1991 )

ABSTRACT Aldiss, D.T. and Carney, J.N., 1992. The geology and regional correlation of the Proterozoic Okwa Inlier, western Botswana. Precambrian Res., 56: 255-274. The small Okwa inlier exposes four unconformity-bounded Proterozoic lithostratigraphic assemblages in the Kalahari region of central southern Africa, where the Precambrian geology is otherwise largely obscured by Phanerozoic strata. The inlier provides the only exposures in the Okwa basement, a structural block between the Archaean Kaapvaal-Zimbabwe Craton and the Late Proterozoic Damara orogen. Felsites and sericitic quartzites of the Okwa basement complex were intruded by varied granites between 2.1 and 2.0 Ga ago. Deformation of the granites to form gneisses at ~ 1.8 Ga (D~) was accompanied by metamorphism of pre-tectonic mafic dykes in the epidote amphibolite facies. In the Mid-Proterozoic, felsic tufts and red sandstones of the lower Okwa group were intruded by dolerite sills, and a contemporary northeasterly dyke-swarm intruded the basement granitoids. A fracture cleavage was imposed on the lower Okwa group during folding and greenschist/low amphibolite facies metamorphism (D2) at ~ 1.15 Ga. The red-bed sediments of the upper Okwa group suffered only very mild deformation (D3). The youngest assemblage is assigned to the Late Proterozoic Nama Group. It mainly consists of gently dipping, fine-grained sandstones. The Okwa basement complex is confirmed as a segment of Eburnian-aged crust which was accreted to the Archaean craton at about 1.8 Ga, probably adjacent to epicratonic sediments allied to the Magondi Supergroup of Zimbabwe or to the Olifantshoek Sequence of South Africa, although such sediments are nowhere exposed in the Okwa sector. The lower Okwa group is thought to be equivalent to volcano-sedimentary sequences such as the Sinclair Group of Namibia. The upper Okwa group can be correlated either with immediately pre-Damaran units such as the Auborus Formation of the Sinclair area, or with the Nosib and Ghanzi Groups in the early Damara Supergroup. The occurrence of Nama sediments overlying the Okwa basement seems to confirm their suspected presence beneath the Karoo Supergroup in the NcojaneNosop Basin of western Botswana.

Introduction

Correlation between the deformed Proterozoic terranes in various parts of southern Africa is made difficult by the extensive cover of Phanerozoic strata. Evidence from isolated inliers of Precambrian rock within the younger basins can thus be of considerable value in constraining regional geological models. This ~This paper is a contribution to I G C P Project 288: Gondwana Sutures and Fold Belts.

0301-9268/92/$05.00

paper draws attention to the significance of the Okwa inlier, and of other small inliers and borehole intersections in northern and western Botswana. The Okwa inlier is situated within the Kalahaft region of central southern Africa, a large area in which the Precambrian geology is mostly obscured by the Cenozoic "Kalahari beds" and the Carboniferous to Jurassic Karoo Supergroup (Fig. 1 ). Sporadic exposures along a 40 km length of the Okwa dry valley reveal an Early Proterozoic, Eburnian-aged

© 1992 Elsevier Science Publishers B.V. All rights reserved.

256

D.T. ALDISS AND J.N. CARNEY

Fig. 1. Location and tectonic setting of the Okwa inlier, Botswana (after Reeves, 1978a; Mortimer, 1984; Meixner and Peart, 1984). Small rectangle within Okwa basement indicates the Okwa inlier, as shown in Fig. 2A. /=Limit of Phanerozoic cover (Karoo Supergroup + Kalahari beds ), ticks lie on the cover, 2 = Late Proterozoic sedimentary basin, 3 = PanAfrican (Damaran) orogenicbelt, 4 = "Eburnian-aged" fold-belt, 5 = Archaean craton. P= Passarge Basin, N - N = NcojaneNosop Basin, Gb = Gaborone, Gh = Ghanzi. Major tectonic boundaries: K - K = Kalahari Line, M - M = Makgadikgadi Line, T-T= Tsau Fault Zone, Z - Z = Zoetfontein Fault. crystalline basement complex overlain by cover sequences ranging through to latest Proterozoic age (Table 1 ). Regional gravity and aeromagnetic surveys show that the inlier forms part of a structural block of about 120 km by 180 km known as the Okwa basement (Reeves and Hutchins, 1976; Reeves, 1978a; Hutchins and Reeves, 1980; Meixner and Peart, 1984). This block is situated between the Archaean K a a p v a a l - Z i m -

babwe craton and the G h a n z i - C h o b e belt (part of the D a m a r a orogenic belt) (Fig. 1 ). Aeromagnetic lineaments suggest that the predominant structural trend within the Okwa basement is N E - S W , parallel to the Makgadikgadi Line (Fig. 1 ). Sedimentary basins, in which the magnetic basement is inferred to lie at depths of more than 15 km, occur to the northeast and southwest. The geophysical character o f the crust beneath these basins contrasts markedly

300+

Red-bed elastic sediments and minor mudstones; local calc-siltstones

Metadolerite and diorite dykes Porphyritic granites and angen gneisses, microgranites and leucogranites Porphyritic felsites (meta-tuffs) and sericitic quartzites

Dolerite sills (NE dolerite dyke swarm in OBC) Felsic tufts, siltstones, Pyroclastic and fine red sandstones, fluviatile medium wackes, mudstones

complex

basement

Okwa

lower Okwa group

upper Okwa group

Group

Nama

Name

D~: NE-SW to N - S foliation with subvertical. lineation; SE vergence

upright folding and fracture cleavage

D2: NE-SW

D~: NE-SW and E - W faulting

of NE-SW faults

Reactivation

Structures

U = unconformity. U 1: post ~ 1.8 Ga; U2: post ~ 1.15 Ga, U3 and U4: post ~ 0.63 Ga.

...... U I

A+B

...... U2

500+

?5-50

Breccio-conglomerate: Debris-flow High-energy to distal fluviatile; lacustrine

?5-50

Fine sandstones: Distal fluviatile/deltaic

F ...... U4 E ...... U3

C+D

Thickness (m)

Lithology: Environment of deposition

Unit

Proterozoie Okwa Inlier--summary of lithostratigraphy

TABLE 1

Dykes: ~ 2.0 Ga

D~: ~ 1.8 Ga

D2: ~ 1.15 Ga

Radiometric ages

L~ --4

z

gtl

m

Y

I'rl

o

o

o

t-n

258

D.T. ALDISSAND J.N. CARNEY

with that of the Archaean craton (Reeves, 1978a), but its identity remains enigmatic. The Okwa basement was included within an Eburnian crustal province by Stowe et al. (1984) and Hartnady et al. ( 1985 ). There have been previous geological, geophysical and radiometric studies of the Okwa area (Crockett and Jennings, 1965; Bosum and Liidtke, 1980; Key and Rundle, 1981; Aldiss, 1988). The present paper is intended to explore its implications for the regional geological framework.

present and previous interpretations of the local stratigraphy.

Okwa basement complex The informal lithostratigraphic term "Okwa basement complex" (OBC) is here used to distinguish the observable rocks from the geophysically-defined Okwa basement structural block. The homogeneous porphyritic felsites in the OBC (Table 1) were interpreted by Aldiss (1988) as recrystallised ash-flow vitric tufts. Sericitic quartzite occurs as detrital fragments in Units C and E and is not otherwise exposed in the Okwa inlier, but as all the other clasts in the sedimentary units are composed of rocktypes which do crop out there, it is concluded that the sericitic quartzite is also locally derived, from a source now buried beneath the Kalahari beds. In thin section the metamorphic fabric in this quartzite appears more sim-

Stratigraphy Rocks in the Okwa inlier can be divided between four unconformity-bounded lithostratigraphic assemblages (Table l ). Ambiguities in the stratigraphic sequence arising as a result of multiphase faulting (Fig. 2 ) are discussed by Aldiss (1988), as are differences between the (a)

f 2°23'S ~'"~.

~56~0 ~1

63

78

5

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¢

5km

~-~2

I

13

N~

,,,J

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~ 5

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,~

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~

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.

B

,,~: ::..~!~: o--- /

.....

-~z-'t..-..z

~....

7

- --

- o

~--o'

9

Fig. 2. (A) Geology of the Okwa Inlier (after Aldiss, 19 8 8 ). I = N a m a Group, 2 = Okwa group, 3 = Okwa basement complex, 4 = limits to main exposures, 5 = bedding, 6 = foliation. Heavy line with arrow-heads is main road between Ghanzi (to the north) and Kang (south). (B) Detail in eastern section of (A). A - F as in Table l, 7=dolerite sill, 8 = O k w a basement complex granite, 9 = d o l e r i t e dyke. Other symbols as in (A). Faults dashed where inferred; tick indicates downthrow.

259

THE PROTEROZOIC OKWA INLIER, WESTERN BOTSWANA

ilar to that of the porphyritic felsites than to those in the lower Okwa group, and so the quartzite is interpreted as part of the OBC. Megacrystic granites and gneisses are the most widespread rock association exposed in the Okwa inlier. The predominant component is augen gneiss, which passes gradationally into a faintly foliated porphyritic, coarse monzogranite, representing the undeformed protolith. Two petrographic varieties of microgranite are present in these granites and gneisses. One forms intrusive bodies as much as one kilometre across, but also occurs as autolithic inclusions. The other forms sparse but widespread syn-tectonic aplite veins, and intrudes the porphyritic felsites. Deformed metadolerite and metadiorite dykes form sporadic exposures within the megacrystic granitoids (A1diss, 1988). The megacrystic granites, augen gneisses and associated lithologies in the centre and west of the inlier were named the "Okwa Gneiss" by Key and Rundle (1981), who differentiated them from the "Okwa Granite", which is the name they gave to coarse-grained but non-megacrystic leucogranites which are exposed east of the main road (Fig. 2). We argue that the differences between the leucogranites (Okwa Granite) and the "Okwa Gneiss" are no greater than the differences within the "Okwa Gneiss" itself, and therefore treat all the granitoids as variations within a single inhomogeneously deformed intrusive complex. Okwa group Unit A (Table 1 ) consists almost entirely of pale-coloured felsites containing scattered pseudomorphs after feldspar crystals. In one place these felsites enclose a lens of feldspathic quartzite, probably an alluvial channel-fill. Red sandstones, siliceous (probably tuffaceous) siltstones, and mudstones, together with felsic lapilli tuffs form Unit B. The sediments are mostly finely laminated, with some cross-lamination. Small mud-clasts, contorted laminae

and thin sandstone dykes occur sporadically, and some of the sandstone beds have scoured bases. Units C a n d D form a heterogeneous red-beds assemblage. The lower part comprises an upwards-fining sequence from pebbly arkosic sandstone to medium-grained feldspathic sandstone, and interlaminated mudstone and wacke. This is overlain by medium-grained sandstone. The common presence in the sandstones of trough cross-bedding, with local overturning within individual sets, and of layers containing mud rip-ups or dewatering structures suggest fluviatile deposition under relatively high energy conditions, with a high rate of sedimentation. A single exposure of bedded calcareous siltstones in the west of the inlier (Fig. 2A) could mark waning sedimentation and the development of lacustrine or marine conditions. The presence in Unit C of detritus from the OBC, from Unit A, and from the dolerite sills noted below, as well as a considerably less intense cleavage in Units C and D than in Units A and B, demonstrates that an unconformity exists between the lower and the upper portions of the Okwa group (Table 1 ). Dolerite dykes and sills

As well as the deformed mafic dykes within the granitoids, a later generation of metadolerite forms a northeasterly-trending dykeswarm in the OBC, and more extensive bodies (probably sills) within Unit B (Figs. 2B and 3 ). Although the contacts of these sills are now faulted, quartz-epidote veining in the adjacent Unit B sediments suggests an originally intrusive contact. Copper minerals are reported to occur in epidotised dolerite (Crockett and Jennings, 1965 ). Units E and F

The polymict red breccio-conglomerate of Unit E (Table 1 ) is a locally-derived debris-

260

D.T. ALDISSAND J.N. CARNEY \\\~-\. I

~( I IWIll;l'l

;x/I t ~ : :

.t; ' ~;:~:

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0

/'i.

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.

1(3,00m

,22Z~,~\~\~/'; ~ ~ ' . i ",~,,~\'/q..~'...':O~ ~?.9~~3.~-7-./'7/~ I "" -" ~/- f....~.~'Bu-~.~"t v vl ' '\','~ ~ 7 ~ ,

/\tl/#~t.U

~//~ ,,#'"': ~/b~'////I

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~ D

~ C

ESEi

B

~Doler-[SS~ ~ite ~

A

I

I

9 ~

r-;--4 3 I,f I

[/;//h/1 1

Fig. 3. Diagrammatic section across eastern part of the Okwa lnlier. A - F as in Table 1, / = f o l i a t i o n in Obc ( D l ) , 2 = cleavage in Units A and B (D2), 3 = normal fault (D3).

flow deposit which apparently developed at scarps within an E-W fault-zone (Fig. 2B). The extent of Unit F is controlled by a northeasterly graben which cross-cuts this E-W fault-zone (Figs. 2B and 3). It is composed of gently dipping laminated fine sandstones and siltstones, in places with cross lamination, and typically with redox spotting. These sediments contain calcareous diagenetic nodules up to 1 m in diameter, and also sparse pebble-lag deposits containing locally derived clasts, including pieces of the red breccio-conglomerate. The preponderance within Unit F of fine-grained sediments with calcareous horizons suggests that distal fluviatile, or possibly lacustrine environments mainly prevailed. Structure

The sequences in the Okwa inlier bear the imprint of at least three major phases of deformation. The earliest (D~) is recognised only in the Okwa basement complex. A pervasive ductile foliation was developed, albeit of varying intensity. Megacrystic granites were transformed into augen gneisses with sub-vertically plunging stretching lineations. The fabrics have been defined mostly by alignment of biotite and hornblende, and by dynamic recrystallisation of quartz into ribbons. L, SL and LS symmetries are all represented. The orientation of rotated feldspar megacrysts and of CS fabrics in the gneisses shows a relative upwards motion in western components, indicat-

Obc=Okwa

basement complex,

ing a southeasterly vergence during D~. A second and rather less ductile compressional deformation (D2) affected the OBC and Units A and B of the Okwa group. D2 structures in Units A and B comprise NE-SW trending upright folds and associated coarse fracture cleavage with subvertical mineral lineation (Fig. 3 ). D~ foliations in the OBC were folded, and cross-cut by D2 mylonite zones. The metadolerite dyke swarm and sills lack the main foliation of the gneisses, and therefore post-date the most pervasive phase of deformation in the OBC. They are, however, fractured, brecciated, and locally mylonitised, and this deformation is assigned to D2, implying that D2 compression was preceded by a tensional phase, when the northeasterly dykes were emplaced. During the D3 deformational event NW-SE and E-W faulting affected Units D and older. Cross-cutting northeasterly faults then displaced Okwa group strata by a minimum of 1500 m relative to the OBC (Fig. 3 ). The eastern margin of the half-graben thus formed is a faulted monoclinal structure in which Units C and D were down-flexured to dip at 40 to 60 ° to the northwest. The weak cleavage seen in the argillaceous components of these units is assumed to have formed during this event. Unit F oversteps the southeastern margin of the half-graben, and unconformably overlies tilted sediments of Unit D (Fig. 2B). Neither Unit E nor Unit F contains any cleavage.

THE PROTEROZOICOKWAINLIER,WESTERNBOTSWANA

Metamorphism

261 TABLE 2 Radiometric age determinations----Okwa basement complex

Only two significant metamorphic episodes can be recognised. The first occurred during D, in the Okwa basement complex. Deformed mafic bodies within the granitoids show an epidote amphibolite facies metamorphic assemblage, while the foliated granitoids contain a syn-tectonic green biotite which is strongly altered in places to chlorite. The second metamorphic event is inferred to have accompanied the D 2 folding, although it might have been superimposed on pre-kinematic hydrothermal alteration of the sills in the lower part of the Okwa group. These contain the upper greenschist/lower amphibolite-facies paragenesis albite-green hornblendechlorite-sericite-zoisite-haematite, but no penetrative fabrics. The mineral assemblage in the northeasterly-trending dykes which intruded the OBC indicates a slightly higher metamorphic grade, with the additional local appearance of inclusion-filled garnet as small irregular masses within sericitised plagioclase, and of mylonite fabrics. This garnet-bearing assemblage suggests m i n i m u m pressures of 4 kbar (Winkler, 1979), corresponding to 15 km depth of burial a n d / o r tectonic thickening.

Age of the Okwa inlier assemblages Radiometric age data on formations in the Okwa inlier are given in Table 2. An errorchron (with M S W D = 2 4 ) yielding an age of 1.81 _ 0.07 Ga was derived from augen gneiss. This was interpreted by Key and Rundle (1981 ) as the age of syn-tectonic emplacement, whereas Cahen et al. (1984) considered it to reflect a major metamorphic event. Model ages calculated from the same data, assuming an initial strontium isotope ratio of 0.702+0.002, ranged from 2.12+0.03 Ga to 1.97 + 0.02 Ga. This demonstrates that whether the ~ 1.8 Ga event was a metamorphic re-setting or (less probable) a remobilisation of older crust, the precursor to the Okwa gneiss cannot

Leucogranite

1004 + 49 Ma (Model ages > 1150 Ma)

Rb-Sr WR errorchron Ri=ca. 0.7217

Augen gneiss Aplite in gneiss Augen gneiss Metadiorite Augen gneiss

1093 + 36 Ma 1115 + 20 Ma 1156 ___28 Ma 1193 __35 Ma 1813 + 68 Ma

Metadiorite

1971 _+31 Ma

K-Ar (biotite) K-Ar (biotite) K - A r (biotite) K - A r (hornblende) Rb-Sr WR errorchron Ri=ca. 0.7227 K - A r (hornblende)

Compiled from Key and Rundle ( 1981 ), Cahen et al. (1984).

be older than ~ 2.1 Ga, and was not derived by partial melting from Archaean crust (C.C. Rundle, pers. commun., 1989 ). K - A r analysis of hornblende from a pre-Dl metadiorite with relict igneous textures yielded an age of 1.97+0.03 Ga. Key and Rundle ( 1981 ) interpreted these bodies of metadiorite as xenoliths and so considered that this was the age of one component of a basement into which the Okwa gneisses had been intruded. There is, however, no field evidence to show that any xenoliths derived from older basement occur in the gneisses and Aldiss ( 1988 ) concluded that the metadiorite forms dykes. This K - A r determination of ~ 1.97 Ga can then be interpreted as the m i n i m u m age of emplacement of these dykes, and therefore also of the gneisses and foliated granites. This would be consistent with the Rb-Sr model ages of the Okwa Gneiss, and would imply that the porphyritic felsites and sericitic quartzites of the OBC are older than ~ 2.0 Ga. Another pre-D1 metadiorite, which had lost its igneous textures, gave a K - A r mineral age of 1.19___0.035 Ga. This lesser age was correlated by Key and Rundle ( 1981 ) with a re-setting event suggested by the three K - A r biotite ages from the gneisses of ~ 1.12 Ga (Table 2 ). Rb-Sr determinations of the "Okwa Granite" (i.e. the leucogranite ) indicate a relatively high initial strontium isotope ratio; and Key and Rundle (1981) suggest that the data re-

262

cord a metamorphic episode at 1.00 + 0.05 Ga. Although the reality of this metamorphic event is supported by four of the K-Ar determinations, model ages calculated from analysis of the leucogranite fall in a range from ~ 2.0 Ga to ~ 1.15 Ga (Key and Rundle, 1980; quoted by Cahen et al., 1984). There is little direct evidence bearing on the age of the Okwa group. We infer that because Units A and B were not affected by the pervasive ductile deformation seen in the OBC, their m a x i m u m age is that of the main basement metamorphic event, at ~ 1.8 Ga. Their minim u m age might be constrained by the four K Ar determinations (Table 2 ) which suggest an isotope re-setting event at ~ 1.15 Ga in the OBC, and which plausibly reflect the D2 folding and low-grade regional metamorphism. The Okwa group Units C and D are unconformable on Units A and B, and are in turn overlain unconformably by Units E and F. The latter are assigned to the Nama Group (below), which in Namibia commenced deposition at ~0.61 to 0.63 Ga (Miller, 1983; Cahen et al., 1984). Units C and D were therefore probably deposited between ~ 1.15 Ga and 0.63 Ga.

Discussion of regional correlations

Okwa basement complex Key and Rundle ( 1981 ) concluded that the granitic rocks (OBC) in the Okwa inlier are a continuation of the Kheis Belt (Kheis Subprovince of Stowe, 1986 ) in the northern Cape Province of South Africa, because of the northerly trends observed in the Kheis, its position immediately east of the Kalahari Line (Fig. 4), and the similar geochronology of the two terranes. A major part of the Kheis Belt is made up of the Olifantshoek Sequence. This is subdivided into various units distributed in three tectonostratigraphic terranes which were imbricated and transported eastwards towards the

D.T. ALDISS AND J.N. CARNEY

Kaapvaal Craton (Stowe, 1986). Gneissic granitoids of similar type to those in the Okwa basement occur in the Skeverberg Terrane, where they intrude metaquartzites of the Groblershoop Formation. They are, however, described as forming discontinuous sheets (Stowe, 1986) and are not as voluminous as the Okwa granitoids. Felsic volcanics are present in the Zonderhuis Formation of the Wilgenhoutsdrif Group, which is in the Diepklip Terrane, the tectonically highest part of the Kheis Belt. These felsic lavas occur in association with quartzites, and are perhaps similar to the Okwa basement volcanics, but are also accompanied in sequence by mafic lavas, schists and greywackes, which are not seen at Okwa. The Wilgenhoutsdrif Group is, moreover, only ~ 1.3 Ga old (Barton and Burger, 1983). Metamorphism took place in the Kheis Belt at 1.8 to 1.74 Ga (Stowe et al., 1984). The Belt was also affected by the Namaqua Province metamorphism between 1.35 Ga and 1.15 Ga (Barton and Burger, 1983; Cahen et al., 1984). Kheis structures cannot, however, be traced north of the Zoetfontein Fault (Fig. 4). They are present as a steep northerly-trending cleavage in the exposures at Mabuasehube Pan (Meixner and Peart, 1984; Carney, 1989 ), and as a coincident array of aeromagnetic anomalies which is truncated at the Zoetfontein Fault (Reeves, 1978a; Hutchins and Reeves, 1980). The rocks of the OBC are also comparable with assemblages seen in the Richtersveld Province, which is an Eburnian terrane to the west of the Kheis Belt (Fig. 4). The Richtersveld Province includes the Orange River Group, which is a thick metamorphosed volcano-sedimentary sequence with considerable components of felsic volcanic rocks, and of clastic sediments. The Orange River Group is thought to have been deposited at ~ 2.0 Ga, and was intruded by the Vioolsdrif Intrusive Suite, which includes granites and diorites, and which has yielded Rb-Sr age determinations in the range 1.9 to 1.7 Ga (Blignault et al., 1983;

THE PROTEROZOIC

OKWA INLIER, WESTERN

263

BOTSWANA

28°E

18S

/

/

/

/

/ //

lfiE

r ,-

M

IVIp

AAAAI AAA

8 9

~

~o

-28S

Fig. 4. Regional setting of the Okwa Basement. Sources as quoted in text, with National Geological Maps of Botswana, Namibia and Zimbabwe. / = L i m i t of Phanerozic cover, 2 = N a m a Group outcrop, 3 = G h a n z i and Nosib Groups, 4 = Sijarira Formation, 5 = Nuwedam Subgroup, 6 = Tshane Complex, 7= Sinclair Group, 8 = Koras Group, 9 = Magondi belt (in the north) and Kheis belt (in the south), 10=Richtersveld Province, l l = p r e - S i n c l a i r Group basement, 12 = granitoids ( U = Urungwe area, Ke= Kheis area, V= Vioolsdrif Complex). K-K, M-M, Z - Z as in Fig. 1. Boreholes: 21= ACP21, V= Vreda. 0 I = Okwa Inlier, D = Dett Inlier, G = Gweta, Gh = Ghanzi, Mp= Mabuasehube Pan, Ns= Narubis, N-N= Ncojane-Nosop Basin, P = Passarge Basin, R = Rehoboth, S = Sinclair Mine, SP= Sua Pan.

Joubert, 1986). There was a deformation event at ~ 1.8 to 1.7 Ga in part of the Vioolsdrif (Barton, 1983). The Richtersveld Province and the Okwa basement are separated by a distance of some 800 kin, and by the N a m a q u a tectonic province, within which there was catazonal metamorphism and magmatism during the interval from -~ 1.3 Ga to 1.0 Ga (Barton and Burger,

1983). Blignault et al. (1983) point out, however, that there are small areas of "'basement complex" which lie immediately to the northeast of the N a m a q u a tectonic province, for example in the Kumbis Formation beneath the Sinclair Group, and in the Narubis area (Fig. 4). The age of these occurrences is poorly known b u t Blignault et al. (1974, 1983) suggest that together with the Kheis Belt and the

264

D.T. ALDISS A N D J.N. C A R N E Y

Richtersveld Province, they are representatives of Eburnian-aged terranes, formed ~ 2.0 Ga ago. It seems plausible that much of the area between the Okwa, Kheis and Richtersveld terranes, including the Ncojane-Nosop Basin, is underlain by Eburnian crust, wrapping around the western margin of the Archaean craton (Figs. 4 and 5 ). Although any immediate continuation of the Okwa basement to the northeast is obscured by sedimentary infill of the Passarge Basin, broad northeasterly-trending aeromagnetic anomalies do appear further along strike to the south I

[

p

I

I

I

of Gweta (Figs. 4 and 5), and continue towards Eburnian terranes in the Magondi Orogenic Belt in Zimbabwe (Hutchins and Reeves, 1980; Stowe et al., 1984). The Dett (Kamativi) inlier, which is in Zimbabwe some 650 km to the northeast of the Okwa inlier, includes amphibolite and cordierite-granulite facies metasediments (Lockett, 1979) which were intruded by late syntectonic granites at ~2.15 Ga (Priem et al., 1971 ). Still further to the northeast, the Deweras, Lomagundi and Piriwiri Groups of the Magondi Supergroup preserve a southeast to northwest progression from shelf sediments to

I

28E

20 [

22 S

ip ~'~S

--

-Ns

0

~KMB i

I

I

I

I

I

I

200km

]

Fig. 5. Proposed tectonic setting of Botswana: 2.0-1.77 Ga. l=Palapye Group (P), Soutspansberg Group (S), 2 = Granitoids (m = Mahalapye Granite, Ke= possible + 1.8 Ga granitic gneiss associated with Kheis belt), 3 = inferred distribution of epicratonic Magondi Supergroup and Olifantshoek Sequence, 4="Eburnian" volcano-sedimentary and plutonic terrains. MMB= Magondi Mobile Belt, KMB = Kheis Mobile belt. Arrows indicate vergence of thrusting. D = Dett Inlier, G=Gweta, K u = K u b u Hill and other small basement exposures southeast of Sua Pan, La=Lechana Fault, M = Mahalapswe fault zone, Ma = Mahalapye, Mp = Mabuasehube Pan, Ns = Narubis, 0 I = Okwa Inlier, P = Palala shear zone, R = Richtersveld Province, S = Sinclair Mine. The position of the northwesterly lineaments was inferred by interpretation of the National Aeromagnetic Map (Prakla-Seismos, 1987 ) and of gravity contour maps (Reeves and Hutchins, 1976; Coates et al., 1979).

265

THE PROTEROZOIC OKWA INLIER, WESTERN BOTSWANA

deep-water facies deposited across the cratonic margin at ~ 2.2 to 2.1 Ga. The Magondi Supergroup was metamorphosed, thrust southeast, and intruded by granitoids such as the Urungwe Granite, at ~ 2.0 Ga to 1.8 Ga, in the "Magondi orogeny" (Leyshon and Tennick, 1988; Treloar, 1988). The western and northern sectors of the distal Piriwiri Group were subjected to the highest metamorphic grades and the bulk of granitoid emplacement, and they can be correlated with the Dett inlier (Treloar, 1988). Thus the deduced age of the main metamorphism of the OBC, at ~ 1.8 Ga, is broadly consistent with that of deformation phases in both the Magondi and the Kheis terranes, which also share a craton-margin tectonic setting with the Okwa inlier (Fig. 4 ), and which contain structures that demonstrate tectonic transport towards the Kaapvaal-Zimbabwe craton (Stowe et al., 1984). We note that in neither the Magondi nor the Kheis terranes do syn-tectonic granitoids or felsic volcanic rocks appear in those units which are closest to the Archaean outcrops and which probably overlie the craton margin. A folded sequence of epicratonic sediments is therefore suggested to lie to the southeast of the OBC granitic basement (Fig. 5 ). The possible extent of this sequence is indicated by a swarm of northeasterly LANDSAT lineaments in a zone extending some 80 km southeast of the Makgadikgadi Line. This zone includes the intense magnetic anomalies in the Tsetseng and the Xade Complexes but is otherwise magnetically rather "quiet". Evidence for the nature of the Magondi Belt in the sector between the Okwa and the Dett inliers comes from the borehole recently drilled by the Botswana Geological Survey at Gweta (Figs. 4 and 5 ). This penetrated gneissic granitoids with the primary assemblage orthoclase-garnet-quartz-sillimanite-biotite, beneath Phanerozoic cover. These granitoids are comparable both in lithology and deformational history with certain of the Early Proter-

ozoic metamorphic units exposed in the Dett inlier (Carney and Dowsett, 1989). The Gweta-Dett-Magondi terranes apparently do not extend as far southeast as Sua Pan, where exposed granitoids are inferred to belong to the Zimbabwe Craton (Coates, et al., 1979; AIdiss, 1983). Okwa group

It has been shown that within the Okwa group there are two unconformity-bounded assemblages. The field relations and inferred ages of these assemblages provide some constraint on their regional correlation, but it is conceded that their remote setting and unusual tectonic position imply that precise equivalents may not be exposed elsewhere. Felsic tufts and fluviatile sediments form Units A and B in the lower part of the Okwa group. Mafic sills which intruded these sediments are correlated with a NE-SW trending dyke swarm within the OBC granitoids; the dykes and sills are petrographically identical and bear the same metamorphic and deformational imprint which distinguishes this lower part of the Okwa group from the later formations. We thereby infer that the dyke swarm was emplaced close to the end of Unit B deposition, and the orientation of the dyke swarm suggests that a NE-SW trending tensional regime prevailed at that time. During the D2 event, at ~ 1.15 Ga, Units A and B were folded and cleaved, and the dolerite dykes and sills were affected by greenschist to lower amphibolite facies metamorphism, consistent with rather shallow depths of burial a n d / o r low tectonic pressures. By analogy with relationships in the younger and better-exposed Proterozoic terranes of the Damara orogenic belt to the northwest (e.g. Borg, 1988), we suggest that the D 2 0 k w a event represents the closure of a NE-SW oriented volcano-sedimentary rift system. Bosum and Liidtke (1980) suggested that the lower part of the Okwa group (Units A and

266

B) could be the age equivalent of the Koras Group of the northern Cape Province (Figs. 4 and 6 ). The Koras Group consists of red-bed fluviatile sediments interlayered with felsic and mafic volcanic rocks. As inferred for Units A and B, it was deposited in graben-like depressions, but these had northerly orientations (Botha et al., 1979). Moreover, the Koras Group was deposited between ~ 1.15 Ga and 1.05 Ga (Barton and Burger, 1983) and is therefore probably somewhat younger than Units A and B. In further contrast, the Koras rocks are virtually unmetamorphosed. Better analogues for Units A and B can be found in volcano-sedimentary sequences of restricted distribution overlying inferred Eburnian crust elsewhere in the region, such as the Sinclair Group of southwestern Namibia (Fig. 4). The lower Okwa group might be the age-

D.T. ALDISS AND J.N. CARNEY

equivalent of the Guperas Formation in the upper part of the Sinclair Group, which was deposited prior to intrusion by the Rooiberg granite at ~ 1.25 Ga (Watters, 1976, 1977; Cahen et al., 1984). The lower Okwa group is older than the Nuwedam subgroup of the Rehoboth area (Fig. 4 ), which though pre-Damaran in age, rests on granites dated at 1.06 Ga (Cahen et al., 1984; summarising Hugo and Schalk, 1975). A correlation of Units C and D in the upper Okwa group with sediments in the Doornpoort or the Klein Aub Formations of the Nuwedam subgroup is, however, possible. Units C and D might also equate with the similar non-volcanic deposits of the Auborus Formation in the Sinclair area, which are dated at younger than 1.15 Ga (Watters, 1977; Cahen et al., 1984). Alternatively, the upper Okwa group could (B)

~0

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Fig. 6. Early development of rift systems during deposition and deformation of lower Okwa group. (A) + 1.2-I. 15 Ga. Formation of volcanic rift systems as "pull-apart" structures in Eburnian (and ?Irumide) crust north of dextral faulting in the Namaqua Front. / = A r e a s with volcano-sedimentary rift sequences, 2 = " E b u r n i a n " epicratonic sediments. Bb= Brakbos Fault, O = Okwa Inlier, S = Sinclair Mine, NF= Namaqua Front. (B) ~ 1.15 Ga. Closure of rift basins to northwest of Makgadikgadi Line. Dextral motion on Nam shear zone, and possibly on extensions of Brakbos Fault. Main metamorphism of Units A and B in the Okwa group. Symbols as (A). M - M = Makgadikgadi Line.

267

THE PROTEROZOIC OKWA INLIER, WESTERN BOTSWANA

be the same age as the Nosib Group (in the early Damara Supergroup of Namibia), and its lateral continuation in Botswana, the Ghanzi group. If so, the upper Okwa group would differ from these units only in being preserved as rifted outliers overlying the Eburnian Okwa basement. In looking for northeasterly continuations of Units C and D, the red-beds of the Sijarira Formation of western Zimbabwe perhaps provide a closer analogue than do components of the Ghanzi-Chobe belt. Like the upper Okwa group, the Sijarira is fault-bounded, and it overlies Eburnian-aged crust, which there forms the foreland to the southeast of the Irumides (Cahen et al., 1984; Borg, 1988).

Units E and F Hegenberger (1985) inferred that the Late Proterozoic N a m a Group is extensive beneath the Phanerozoic cover in southeastern Namibia, and it seems very likely that the Nama subcrop continues as far east as the Kalahari Line (Reeves, 1978a; Meixner and Peart, 1984). The nearest natural exposures of the N a m a Group to the Okwa inlier are ~ 230 k m to the west, in Namibia, but N a m a Group sediments have also been shown to occur beneath the Karoo Supergroup in a borehole at Vreda (Smith, 1984, 1985) and in borehole ACP21 (Fig. 4). The gently tilted sandstones in Unit F are more similar to the N a m a Group quartzites in ACP21 (Hegenberger, 1985) than to Karoo sediments found in cored boreholes within a similar radius of the Okwa basement (Meixner and Peart, 1984; Smith, 1984) or to the quartzites in the Ghanzi-Chobe belt (Litherland, 1982). Unit F is therefore correlated with the N a m a Group. It seems likely that Unit E is also part of that group. The presence of even a thin layer of N a m a sediments on top of the Okwa basement, albeit probably restricted to a graben, seems to confirm their presence in the Ncojane-Nosop

Basin. It also suggests that Nama sedimentation could have occurred in the Passarge Basin as well (Fig. 4). It is conceded, however, that the only boreholes to penetrate the Kalahari beds near the centre of the Passarge Basin encountered red sandstones which have been assigned to the Ghanzi group (Coates et al., 1979). Discussion of the Proterozoic tectonic framework

Early Proterozoic Figure 5 shows an interpretative model based on the above correlations, in which the Okwa basement represents part of a system of Eburnian-aged orogenic terranes that were accreted against the western margin of the KaapvaalZimbabwe craton at ~ 1.8 Ga. Lithological and structural differences between the Kheis, Okwa and Magondi terranes suggest a segmented orogenic system rather than a continuous belt. Part of the cause of this segmentation may have been the irregular shape of the western cratonic margin, but it is suggested here that major structures that were not parallel to the orogenic fronts were also generated at that time, and that some of these oblique structures formed lines of weakness extending into the craton interior. Structural discontinuities between the Okwa basement and the Kheis Belt may be marked by north-northwesterly elements of the Tshane aeromagnetic complex. This early expression of the Kalahari Line could have acted as a lateral ramp against which the Kheis Belt was terminated somewhat north of Mabuasehube Pan (Figs. 4 and 5 ). Although, like Treloar (1988), we believe that the Okwa basement was, in a broad sense, originally coextensive along the craton margin with the Magondi Orogenic Belt, exposures in the two terranes do differ in lithology, with particularly deep crustal sections represented in the Magondi Belt. We suggest that the two

268

sectors are separated by tectonic discontinuities in the complex embayment apparent in the aeromagnetic expression of the craton margin northeast of the Okwa basement. This embayment coincides with the extrapolated traces of northwesterly aeromagnetic and gravity lineaments (Hutchins and Reeves, 1980; PraklaSeismos, 1987), and west-northwest to northwesterly faults and shear zones in the Limpopo Belt around Mahalapye (Fig. 5 ). Intracratonic shear zones in the south of the Limpopo Belt were reactivated at about the same time as the Eburnian events on the western craton margin. For example, the Palala Granite, which has been correlated with the Mahalapye Granite, was emplaced at ~ 2.0 Ga and deformed by the Palala Shear Zone between ~2.0 and 1.8 Ga (McCourt and Vearncombe, 1987 ). Movement on the Lechana and Mahalapswe-Palala fault systems initiated a series of rifted basins, leading to the deposition of the Soutpansberg and Palapye Groups from ~ 1.77 Ga (Ermanovics et al., 1978; Barton, 1979; Barker, 1983). Stowe et al. (1984) suggested that the Magondi Belt curves to the southeast beneath Phanerozic cover in Botswana, terminating in a "Mahalapye terrane". This construction was based in part on an apparent curvature of aeromagnetic and gravity trends to the south of Gweta (cf. Pretorius, 1979). Our interpretation of the geophysical anomalies suggests, however, that NE-SW trending "Magondi" lineaments continue to the south of Gweta, although they have been segmented and so largely obscured by anomalies related to northwesterly and west-northwesterly structures, including the fault-zones extending from the Limpopo Belt, and Karoo dykes (Reeves, 1978b; cf. Pretorius, 1985). Sediments of the same age as the ~ 2.0 Ga Lomagundi and Olifantshoek Sequences (Treloar, 1988) are not present around Mahalapye. There the Palapye Group, which lies unconformably on the Mahalapye Granite (Skinner, 1978 ), is correlated with the ~ 1.77 Ga old Soutpansberg Group

D.T. ALDISS AND J.N. CARNEY

(Barton, 1979; Cahen et al., 1984). We therefore believe that during Eburnian times crustal growth was restricted to the terranes accreted along the western margin of the Kaapvaal-Zimbabwe craton. Eburnian events in the Mahalapye area did not contribute significant amounts of new crust but have been interpreted as a thermal re-setting of isotope systems (Cahen et al., 1984). Thus the Mahalapye area, although representing an intracratonic branch of the Eburnian tectono-thermal system, contains mainly Archaean crust. Middle to Late Proterozoic

The interpretation of the lower Okwa group (Units A and B ) as part of a rift-basin volcanic sequence of probable Middle Proterozoic age is both an extension and a complication to previous schemes such as those ofWatters ( 1976, 1977) and Borg (1988). The latter proposed that volcano-sedimentary sequences between 1.4 Ga and 0.9 Ga old formed in two sets of rifted basins. One of these was oriented N E SW, coincident with the present GhanziChobe belt, and the other, somewhat older, extended NW-SE just outside the Namaqua Front. The correlation of the lower Okwa group with sequences of Sinclair age, combined with the suggestion of contemporary northeasterly dyke fissuring, and the presence of northeasterly regional geophysical trends in the basement west of the Kalahari Line, leads to an alternative model (Figs. 6, 7, and 8). In this necessarily speculative construction, Units A and B were deposited in one of several rifted basin sequences which might lie within Eburnian basement to the southeast of the GhanziChobe belt, extending westwards from the Kaapvaal craton margin (Fig. 6A). We propose that the Sinclair rifts originated as pull-apart structures to the north of the shear zones along the Namaqua Front (Fig. 6A), as envisaged by Stowe (1983) for the formation of the Koras rifts. If the northeasterly-trending Okwa rifts

269

THE PROTEROZOICOKWAINLIER,WESTERNBOTSWANA (A) ~VV +vvvv *vvvvvv

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Fig. 7. Sequence of basin formation during deposition of the upper Okwa group. (A) ~ 1.1-0.8 Ga. Further rift-basin deposits, including volcanic rocks in the northwest, and Auborus Formation in the southwest. Possible setting for upper Okwa group. 1 = non-volcanicsedimentary associations, 2 = as l, but with volcanic components, R = Rehoboth, K = Kgwebe Hills, other symbols as Fig. 6. (B) ~ 0.8-0.65 Ga. Further migration of volcanic rift activity to the northwest. Accumulation of fluviatile facies of Nosib/Ghanzi Groups to the southeast of Damara Front. Alternative setting for deposition of upper Okwa group. Symbols as in (A).

developed as similar pull-apart structures, they are likely to be geometrically related to a more northerly-oriented shear system. This might have been a continuation of the Brakbos Fault (Fig. 6A), which is shown in the northern Cape Province to diverge north-northwesterly from the N a m a q u a Front (e.g. Stowe, 1986 ), or was perhaps a shear zone along the Kalahari Line (Figs. 4 and 5 ). Closure of the Okwa basin occurred during the D2 compression at ~ 1.15 Ga. Figure 6B shows the resulting basinal fold belt between the outcrops of the Okwa group and the Sinclair Group, b o u n d e d to the southeast by a frontal ramp coincident with the Makgadikgadi Line. As expressed by the linear gravimetric contours the Makgadikgadi Line loses its identity west of ~ 20 ° E, in Namibia. This coincides with a possible northerly extension of

fault zones such as the Brakbos. Although Pretorius ( 1985 ) interprets such a continuation of the Brakbos Fault as a front to northeasterly directed thrusting related to the N a m a q u a orogen, it also seems possible that it acted partly as a lateral ramp to the southeasterly vergent motion which deformed the lower Okwa group. The depiction here of rift/foldbelt axes on northeasterly trends, rather than in the northwesterly zone identified by Watters ( 1976 ) and Borg ( 1988 ), is in agreement with a suggestion made by Cahen et al. ( 1984, p. 78 ) that the present apparently narrow northwesterly-elongated outcrop of the rift sequences along the Sinclair-Koras axis is in large part a result ofpost-depositional structural control by activity on the N a m a q u a Front, rather than being a result of a regional-scale syn-depositional tectonic trend.

270

D.T. ALDISS AND J.N. CARNEY

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Fig. 8. Tectonic setting of Nama Group basins in Botswana, ~ 0.65-0.54 Ga. Folding and tectonic imbrication of Damara Supergroup and pre-Damara units, with accumulation of Nama Group in subsiding foreland basins. Major faulting preceded and accompanied deposition of Units E and F in the Okwa area. 1= Nama Group deposition. P= Passarge Basin, N-N= Ncojane-Nosop Basin, Ts=Tsau Hills, T-T=Tsau Fault Zone. Rifting later in the Proterozoic established basins in which were deposited sediments of the D a m a r a Supergroup. Volcanic activity after ,-~ 1.06 Ga was apparently confined to the area northwest of a prominent N E - S W trending line that was later to define the m a i n front o f PanAfrican deformation in the D a m a r a orogen (Fig. 7A). (The extension o f this line separates Irumide from Eburnian basement in northern Botswana and in the mid-Zambezi rift). Southeast of this line, only sediments were deposited, such as the Auborus Formation in the Sinclair Mine area (Cahen et al., 1984). This 2600 m arenaceous sequence (Watters, 1977 ) furnishes important evidence

for a phase o f pre-Damaran sedimentation which possibly includes Okwa group Units C and D. By early D a m a r a n times, between 0.84 and 0.73 Ga, a fluviatile facies of the Nosib Group, and its equivalent in Botswana represented in the Ghanzi group, was being deposited on the D a m a r a n foreland (Litherland, 1982; Miller, 1983). Figure 7B shows how these fluviatile facies o f the N o s i b / G h a n z i groups could have extended across western Botswana as an upwards continuation to the pre-Damara nonvolcanic sedimentary sequences, possibly including the Okwa group Units C and D, and towards the Sijarira Formation in western

THE PROTEROZOIC OKWA INLIER, WESTERN BOTSWANA

Zimbabwe. Late Proterozoic events in the region are depicted in Fig. 8. Deposition of the Nama Group, and any pre- to syn-Nama Group tectonism, was coeval with major compression within the Damara orogenic belt during the period 0.65 to 0.54 Ga (Miller, 1983), which culminated in the formation of upright to southeasterly verging folds and thrust zones. In Botswana the Tsau Fault Zone marks the southeastern limit of this compressive deformation, on the southeastern margin of the Ghanzi-Chobe belt. Immediately southeast of the fault zone, in the Tsau Hills, the Ghanzi group sediments show uniform dips of only 20 to 40 ° away from the orogen (Walker, 1974). This monodinal structure, which is also seen in Nosib Group sediments of similar tectonic setting in Namibia (Hegenberger, 1985 ), is in accordance with the suggestion by Germs ( 1983 ) that the Nama Group was deposited in syn-orogenic foreland-type basins. A similar model was proposed for Botswana by Pretorius (1985), though in his interpretation the foreland basin had developed rather earlier and was in part flled by Damaran-aged sediments. Indeed, sediments of any age between the 1.15 Ga deformation of the lower Okwa group and the deposition of the Karoo Supergroup could be present above the magnetic basement west of the Kalahari Line.

Conclusions The Okwa inlier comprises a granitoid basement complex intruded by mafic dykes and overlain by three unconformity-bounded supracrustal units. These various components together preserve evidence bearing on the nature of Early to Late Proterozoic events in a little-known and poorly exposed region of southern Africa, although in view of the isolated position of these admittedly limited outcrops, their interpretation is necessarily rather speculative.

271

The Okwa basement complex (OBC) is composed of metavolcanic rocks and metasediments which were extensively invaded by various granitoid phases between ~ 2. l Ga and 2.0 Ga. The whole complex was pervasively foliated, with granitoids converted to augen gneisses during a major metamorphic and deformational event dated at ~ 1.8 Ga. Fabric orientations and rotated megacrysts in the gneisses indicate that this was accompanied by tectonic transport of the OBC towards the southeast. In all these aspects of gross lithology and geological evolution, the OBC is believed to represent a fragment of Eburnian crust. It is part of a series of terranes that enclosed the western and northwestern margins of the Archaean Kaapvaal-Zimbabwe craton. Other components are the Urungwe/Piriwiri and Dett segments of the Magondi orogenic belt in Zimbabwe, and the Kheis Belt and, possibly, the Richtersveld Province of South Africa/ Namibia. These Eburnian terranes were accreted against contemporary craton margin sedimentary sequences of the Olifantshoek and Magondi successions, deforming them during the 1.8 Ga event. It is highly probable that analogous epicratonic sequences of sedimentary rock exist on the craton margin east and southeast of the Okwa basement. Intracratonic events at this time included the reactivation of earlier structures to form fault zones controlling the subsequent deposition of the Palapye, Soutpansberg and Waterberg Groups. A northeasterly-trending dyke swarm, and the metavolcanic and metasedimentary rocks of the lower Okwa group (Units A and B) are part of an unconformable sequence that was deposited in a rifted basin, and then folded and metamorphosed during a compressive event that locally re-set the Rb-Sr and K-Ar isotope systems in the OBC at ~ 1.15 Ga. The rift sequence was probably deposited during a Late Proterozoic tensional event, or

272

series of events which are also reflected by the Sinclair Group. The formation, and closure, of the rift systems may have occurred coevally with movements along northerly-trending strike-slip faults, perhaps associated with an extension of the Brakbos Fault. Thus a considerable area of southwestern Botswana and adjacent Namibia could be underlain at depth by rifted Eburnian basement and rift-bounded Mid-Proterozoic volcano-sedimentary rocks. Younger unconformable sequences in the Okwa inlier represented by the upper Okwa group (Units C and D) are correlated with either pre-Damara or Damara Supergroup rocks in Namibia and Botswana, and can also be correlated with the Sijarira Formation of Zimbabwe. They pre-date a major episode of normal faulting and monoclinal flexuring during the Pan-African which was associated with the formation of foreland basins within which the Nama Group accumulated, including Units E and F on the Okwa basement. These correlations indicate that the Ncojane-Nosop and Passarge sedimentary basins developed during the period 0.65-0.54 Ga, and were then either uplifted, or continued to accumulate Palaeozoic sediments through to deposition of the basal Karoo Supergroup in the Carboniferous. Underlying the Nama Group in these deep sedimentary basins one would expect to find downwarped but relatively undisturbed Damara foreland basinal sediments, probably equivalents of the fluviatile Ghanzi group, and the Nosib Group, in turn underlain by undeformed pre-Damara non-volcanic sequences younger than 1.15 Ga. These units would unconformably overlie earlier rifted-basin volcano-sedimentary sequences that are faulted and folded, with structural styles similar to those seen in Okwa group units A and B. At the base of the succession, and forming the basement within which the rift-related sequences were deposited, we infer the presence of an Eburnian granitoid/metamorphic complex.

D.T. ALDISS A N D J.N. C A R N E Y

Acknowledgements Some of the earliest ground follow-up to the 197 5 regional aeromagnetic survey of west and central Botswana was carried out by teams from the Bundesanstalt f'tir Geowissenschaften und Rohstoffe (BGR), Hannover. DTA benefited both from the use in the field of aerial photographs of the Okwa inlier annotated by Gerhard LiJdtke (BGR) during one of these surveys in 1978, and from numerous discussions with Dr. Liidtke and other past and present Geological Survey staff over the last few years. We are also grateful to C.C. Rundle of the NERC Isotope Geology Centre for his reassessment of the radiometric age data, and to D. Piper and R.M. Key (BGS), who also commented on an earlier draft of this paper. It also benefited from review by C. W. Stowe and M.J. de Wit, and by an anonymous referee. This paper is published with the authorisation of the Minister of Mineral Resources and Water Affairs, and of the Director of the Geological Survey Department, Botswana, and by permission of the Director of the British Geological Survey (NERC).

References Aldiss, D.T., 1983. The geologyof Kubu Hill and adjacent islandsin southeastSua Pan (2025D). Rep. DTA/ 5/82, Geol. Surv. Botswana, 5 pp., unpubl. Aldiss, D.T., 1988.The pre-Cainozoicgeologyof the Okwa Valley near Tswaaneborehole. Bull. Geol. Surv. Botswana, 34: 50. Barker, O,B., 1983. A proposedgeotectonicmodelfor the SoutpansbergGroup within the LimpopoMobileBelt, South Africa.In: W.J. van Biljon and J.H. Legg (Editors), The LimpopoBelt. Geol.Soc.S. Afr. Spec.Publ., 8: 181-190. Barton, E.S., 1983. Reconnaissanceisotopic investigations in the Namaquamobilebelt and implicationsfor Proterozoic crustal evolution-- Namaqualand Geotraverse. In: B.J.V. Botha (Editor), Namaqualand MetamorphicComplex.Geol. Soc.S. Afr. Spec.Publ., 10: 45-66. Barton, E.S. and Burger,A.J., 1983. Reconnaissanceisotopic investigations in the Namaqua mobile belt and implications for Proterozoic crustal evolution--Upington Geotraverse. In: B.J.V. Botha, (Editor), Na-

THE PROTEROZOIC OKWA INLIER, WESTERN BOTSWANA

maqualand Metamorphic Complex. Geol. Soc. S. Afr. Spec. Publ., 10: 173-192. Barton, J.M., Jr., 1979. The chemical compositions, RbSr isotopic systematics and tectonic setting of certain post-kinematic mafic igneous rocks, Limpopo Mobile Belt, southern Africa. Precambrian Res., 9: 57-80. Blignault, H.J., Jackson, M.P.A., Beukes, G.J. and Toogood, D.J., 1974. The Namaqua tectonic province in South West Africa. Precambrian Res. Unit, Univ. Cape Town, 15: 29-47. Blignault, H.J., Van Aswegen, G., Van der Merwe, S.W. and Colliston, W.P., 1983. The Namaqua geotraverse and environs: part of the Proterozoic Namaqua mobile belt. In: B.J.V. Botha (Editor), Namaqualand Metamorphic Complex. Geol. Soc. S. Afr. Spec. Publ., 10: 1-30. Borg, G., 1988. The Koras-Sinclair-Ghanzi Rift in southern Africa. Volcanism, sedimentation, age relationships and geophysical signature of a late middle Proterozoic rift system. Precambrian Res., 38: 75-90. Bosum, W. and Liidtke, G., 1980. Ground geophysical, geochemical and geological investigations in selected areas of the Kalahari. Final Report of Phase I (1979) of the GS17 BGR project (the German contribution to the Exploration of the Kalahari). Botha, B.J.V., Grobler, N.J. and Burger, A.J., 1979. New U-Pb age-measurements on the Koras Group, Cape Province and its significance as a time-reference horizon in eastern Namaqualand. Trans. Geol. Soc. S. Aft., 82: 1-5. Cahen, L. and Snelling, N.J., Delhal, J. and Vail, J.R., 1984. The Geochronology and Evolution of Africa. Clarendon Press, Oxford, 512 pp. Carney, J.N., 1989. Early Proterozoic rocks at Mabuasehube Pan, southwest Botswana. Rep. JNC/4/89, Geol. Surv. Botswana, unpubl. Carney, J.N. and Dowsett, J.S., 1989. The Geological Survey borehole near Gweta, northeastern Botswana: geophysical setting, description of the core samples and a discussion of the 'Gweta basement'. Rep. JNC/5/89, Geol. Surv. Botswana, unpubl. Coates, J.N.M., Davies, J., Gould, D., Hutchins, D.G., Jones, C.R., Key, R.M., Massey, N.W.D., Reeves, C.V., Stansfield, G. and Walker, I.R., 1979. The Kalatraverse One Report. Bull. Geol. Surv. Botswana, 2 l: 294 PP. Crockett, R.N. and Jennings, C.M.H., 1965. Geology of part of the Okwa Valley, western Bechuanaland. Rec. Geol. Surv. Botswana, ( 1961-62 ): l 01 - 113. Ermanovics, I., Key, R.M. and Jones, M.T., 1978. The Palapye Group, central eastern Botswana. Trans. Geol. Soc. S. Aft., 81: 61-73. Germs, G.J.B., 1983. Implications of a sedimentary facies and depositional environment analysis of the Nama Group in South West Africa/Namibia. In: R.McG. Miller (Editor), Evolution of the Damara Orogen,

273 SWA/Namibia. Geol. Soc. S. Aft. Spec. Publ., 11: 89ll4. Hartnady, C., Joubert, P. and Stowe, C., 1985. Proterozoic crustal evolution in southwestern Africa. Episodes, 8: 236-244. Hegenberger, W., 1985. Some aspects of Nama and Katoo sedimentation from a borehole south-east of Gobabis. Comm. Geol. Surv. SWA/Namibia, l: 63-67. Hutchins, D.G. and Reeves, C.V., 1980. Regional geophysical exploration of the Kalahari in Botswana. Tectonophysics, 69:201-220. Joubert, P., 1986. Namaqualand--a model of Proterozoic accretion? Trans. Geol. Soc. S. Afr., 89: 79-98. Key, R.M. and Rundle, C.C., 198 I. The regional significance of new isotopic ages from Precambrian windows through the Kalahari Beds in northwestern Botswana. Trans. Geol. Soc. S. Afr., 84: 51-66. Leyshon, P.R. and Tennick, F.P., 1988. The Proterozoic Magondi Mobile Belt in Zimbabwe--a review. Trans. Geol. Soc. S. Afr., 91: 114- 131. Litherland, M., 1982. The geology of the area around Mmamuno and Kalkfontein, Ghanzi District. Dist. Mem. Geol. Surv. Botswana, 4:145 pp. Lockett, N.H., 1979. The geology of the country around Dett. Bull. Rhod. Geol. Surv., 85. McCourt, S. and Veamcombe, J.R., 1987. Shear zones bounding the central zone of the Limpopo Mobile Belt, southern Africa. J. Struct. Geol., 9:127-137. Meixner, H.M. and Peart, R.J., 1984. The Kalahari Drilling Project. Bull. Geol. Surv. Botswana, 27:224 pp. Miller, R.McG., 1983. The Pan-African Damara Orogen of Southwest Africa/Namibia. In: R.McG. Miller (Editor), Evolution of the Damara Orogen, SWA/Namibia. Geol. Soc. S. Afr. Spec. Publ., 11: 431-515. Mortimer, C. 1984. The National Geological Map ( l: 1,000,000), Botswana Geological Survey. Prakla-Seismos, 1987. Anomalies of the magnetic total intensity of Botswana. 1: 1,000,000 map. Geol. Surv. Botswana. Pretorius, D.A., 1979. The aeromagnetic delineation of the distribution patterns of Karroo volcanics in Botswana and consequent implications for the tectonics of the sub-continent. In: G. McEwen (Editor), The Proceedings of a Seminar on Geophysics and the Exploration of the Kalahari. Bull. Geol. Surv. Botswana, 22: 93-140. Pretorius, D.A., 1985. The Kalahari Foreland, its marginal troughs and overthrust belts, and the regional structure of Botswana. In" D.G. Hutchins and A.P. Lynam (Editors), The Proceedings of a Seminar on the Mineral Exploration of the Kalahari (October 1983), Bull. Geol. Surv. Botswana, 29" 294-319. Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E.H., Verdurmen, E.A.Th. and Verschure, R.H., 1971. Isotopic dating in the Kamativi tin belt, southern Rhodesia. Geol. Mijnbouw, 50:619-624. Reeves, C.V., 1978a. The reconnaissance aeromagnetic

274 survey of Botswana, 1975- 1977. Final interpretation report. Terra Surveys Ltd., Geol. Surv. Botswana, 315 PP. Reeves, C.V., 1978b. A failed Gondwana spreading axis in southern Africa. Nature, 273: 222. Reeves, C.V. and Hutchins, D.G., 1976. The National Gravity Survey of Botswana, 1972-73. Bull. Geol. Surv. Botswana, 5:44 pp. Skinner, A.C., 1978. The geology of the Mahalapye area. Bull. Geol. Surv. Botswana, 9. Smith, R.A., 1984. The Lithostratigraphy of the Karoo Supergroup in Botswana. Bull. Geol. Surv. Botswana, 26:239 pp. Smith, R.A., 1985. The results of coal exploration in the Kalahari of Botswana. In: D.G. Hutchins and A.P. Lynam (Editors), The Proceedings of a Seminar on the Mineral Exploration of the Kalahari (October 1983). Bull. Geol. Surv. Botswana, 29: 6-38. Stowe, C.W., 1983. Explanation of the Upington Geotraverse, South Africa. In: N. Rast and F.M. Delany (Editors), Profiles of Orogenic Belts. Geodynamics

D.T. ALDISS AND J.N. CARNEY

Series. Am. Geophys. Union, 10: 35-43. Stowe, C.W., 1986. Synthesis and interpretation of structures along the northeastern boundary of the Namaqua Tectonic Province, South Africa. Trans. Geol. Soc. S. Aft., 89: 185-198. Stowe, C.W., Hartnady, C.J.H. and Joubert, P., 1984. Proterozoic tectonic provinces of southern Africa. Precambrian Res., 25:229-231. Treloar, P.J., 1988. The geological evolution of the Magondi Mobile Belt, Zimbabwe. Precambrian Res., 38: 55-73. Walker, I.R., 1974. North Ghanziland. 1:125 000 geological map with brief explanation (QDS 2121 B, 2122A and 2122B), Geol. Surv. Botswana. Watters, B.R., 1976. Possible late Precambrian subduction zone in South West Africa. Nature, 259:471-473. Watters, B.R., 1977. The Sinclair Group: definition and regional correlation. Trans. Geol. Soc. S. Afr., 80: 916. Winkler, H.G.F., 1979. Petrogenesis of Metamorphic Rocks, 5th ed. Springer, Berlin, 348 pp.