RtPUBLIC OF

GEOLOGICAL SURVEY OF BOTSWANA

Director: G.c. Clark

District Memoir 5

THE GEOLOGY OF THE AREA AROUND AND , KWENENG, KGATLENG, SOUTHERN AND SOUTH EAST DISTRICTS

By

R.M. Key B.Sc., Ph.D. Geologist

Geological Survey Department Ministry of Mineral Resources and Water Affairs

1983

Coloured geological maps of quarter degree sheets 2425D and 2525B are contained in a pocket at the inside rear cover

Printed by the Government Printer, Gaborone 12. HYDROGEOLOGY

All the water courses of the a~~'ea aTe ephemera,l a1 thcu.gh ",rater is

retained for most of the year in the sandy beds of the lal'ger rivers. Ha6J-

dug wells in these riveTs, especially the Netsemotlhaba River, are used by

the rural population. The Kolobeng River which drains the porous sandstoncfl

of the Nannyelanong HilI Formation only ceases to flo\>I in ycarfl of poor Taill-

fall cmd 1s supplied by springs from the s2-ncis tones. H':Yh"eve:r, the water

only floltls as far as KWTlakW2"l8 and dOGS not reach the Metsemot1haba Ri1rel'.

A fault cuts the Kolobeng River course north of Kumak'dane and seepage

down the" fault plane -is thOlrght to t:ause t.hp. cessation of Wa,T.eT fl~w.

Annual rainfall is ahout 600 mil] irr,2tl'es/yc~ar wi ih a seasonal vaxia-

bility of 30 - 40 per cent. 'rhe hydrogeology of the Lo ba tse area is

describf-)d in more det.ail ':Jy Jennings (1974), FarT et .'1,1. (1979) and Davies

12.2 Descript-i on of th2 main aquifers

southwest part of the map-area. Therefo:':-8 the gT0und water charactey.'j i:~'~'LnrJ of these ancient rocks are not knm·m. HO\l2Ver, by cOTnparison "lith simi.lar rocks in eastern .Tjotswana, the main, aquifers will probf:l.bly be major frae ture zones, provid8d they aTe not 8..Dnea.led by quart.z 18 j 11S. According

to Jennings (1974) the igneous rocks of the Lobatse Volcanic Group are poor aquifers. rrhe ';ew boreholes sited j,n these rocks are along faults 8Jl(1 have low yields. Similarly the al'gilli'I-,es of i.~e Mogobai;,,: Fc·rm;-·dion have low yields due to the impermeability of the shalss. However west of

Ramo;:;swa along the major faults related to the Hoshaneng Lin8aTlent, cu ttinH throngh the Lo ba tse Volcanic Grollp, yields are compa.ra tive ly high. Sevo",,, J

210 important aquifers are present in the Transvaal Supergroup and have been

studied in detail in the Lobatse and RaDlotswa areas. The basal dolomites,

are poor aquifers when fresh but ,.,here weathered they are important. The

main aquifer appears to be at the base of the weathered, locally mangani­

ferous, rock which is up to 30 metres thick. Similarly cavities in the

dolomites du,3 to ka,rst solution features are important aquifers, especially

wh8re they develop along faults. Moderate to high transmissivi ties are

obtained, although the water tends to be hard. Total dissolved solid (TDS)

contents vary between 300 and 400 mgll with Ca, Mg and HC0 ions prevalent. 3 The overlying interlayered a:cgilli tes and "reni tes are generally good aqulfers with vcu:-iabl(~ yields and storage capacities depending to 8, large

"xten t on the amoun t of frac turing. In the Lobs tse area the two main water bearing horizons are the Ditlhojana Argillite and the Sengoma Quartzite.

The shales of the Di tlhojana Argillite have low coefficients of storage and

transmissiyi ty. Similarly the orthoquartzite near the top of the SentSoma

Quartzite, which is the main aquifer, has low coefficients of storage and

transmissivity except near fault.s. GroIX.'ldwater in this topmost aquif.... 'of the Transvaal Supergroup is extremely pure with less than 100 mgl£I!'DS.

Not a great deal is knmll1 about the hydrogeology of the various Waterberg

Supergroup rocks. Within the map-area the sandstones of the Ma..nnyelanong

Hill Formation are generally too thin to have any serious groundwater potential. Also they tend to form mesas where there is only lilili ted re­ charge potential. One borehole in the Maladiepe Hill Formation near Sunny­ side has a moderate transmissivity and high yield. No hydrogeological study has yet been made on the major faults such on the Ma..nnyelanong Fault which cuts the Waterberg Supergroup in the area. The various felsic rocks of the Gaborone Granites are poor aquifers and best possibility of finding

(,'Toundwater is along open fractures, or adjacent to large dolerite dykes.

211 TABLE 2<1: SUMr<1ARY OF AQUIFER TEST RESULTS AT SELECTE1J SITES

Distance of Specific Av. Trans- Av. Storage Duration of Av. Discharge observation capacity missivity Area Aquifer test in hrs. in j,/s holes in m (j,1 s/m) (m2 I d) coe ffic ient ,

Waterberg Lobatse 32 -2 Conglomera te 578 12.1 28 1.32 60 2 x 10

Transvaal Lobatse 3 Dolomi te 62 3.7 28 0.16 48 6.8 x 10-

Transvaal 0J Lobatse Lephala 3 Shales 113 0.6 33 - 20 3.4 x 10-

Transvaal -2 Dolomi te 312 11 .1 30 2.7 1170 5.7 x 10

Transvaal Ramotswa Lephala 4 Conglomerate + 223 28.0 - 3.5 492 8.7 x 10-

(after Farr etal (1979), Wellfield Consulting Services (pty) Ltd. (1980) The quality of groundwaLer is hOlt!eVel' gooe1. Table.24 fll)JIlllld,r,i.ses Lh~:; l'~,,:~:;;..:Jts

0-: aquifer tests carl'ied out in the Lobatse a::'10. Ramotsv.Ta area8~

12.3 Water supply

I}lhe water supply of Gaborone totally depr:;nds on the NotwaX1G Darn )."d.th

3 maximum storage of 35 mj.lJ i.on TIl • armual ",la.ter consumption of Gabo~Y.'one

3 for 1981 '"ras about 5 million m • ~'hc Lobatse \Jut

3 900 OOOm ) is met by the Notwane ram through :?ipeJ.j.n(; f:::orr~ Ga.bO:COIW; by the;

NnY1.",rane Dam be tv18en Otse and GaboTone, and bllO l,velJ..fie Ids near 1)0 ba r,s8,

\'lhich provide a."oout j60 OOOm" a year. Ramotswa, "Lhe tLi.rd major se (;t.l.2rnent on the map cheet, is Gupplied by a wellfield in the vicinity of -Lho villu,5'e. In the same area a successful g_I'oundwa ter paten tial asses[;;-;:cm t study has been conducted to f:ind addi tional underground \.,2,"ter for the

Gaborone-Lobat"" ;later supply.