I

BOLETIM DOS

SIiINIC;OS DE GEOLOGIA E MINAS DE ANGOLA

iN DICE

I!IN1'f!l)()S,

M. {j, 1'1, M'AH(JARENHAS NETO

A ,fAUDA DE VERTIWRADOS li'68SEIS DO F'AROL DAB LAGOSTAS /, aJ!)NERAL]DA.DES E IDADE DA JAZIDA

" dAUJ)A DE VERTEBRADOS F6SSEIS DO FAROL DAB LAGOSTAS Il. PALEONTOLOGIA

Il, IIflAltlllfl DID (JARVALHO

lIM 1'1WBLEMA DE GEOMORFOLOGIA APLlCADA -AS PQSSIBILI­/.)ADES PARA A PROSPECQJiO DE MINJ1;RIOS DE ALUMtNIO NA PROViNOIA DE ANGOLA .,.

h ,I, /4, M[JHlORMERHORN

tWIJ/MICNTARY OYCLES IN THE WEST CONGO GEQSYNCLINE OF

N.O de p!gilla

5

11

19

NORTHWEST ANGOLA ... 47

M (1, 1'1, MA.S(JARENHAS NETO

All J1ACIAS SEDIMENTARE8 DE BENGUELA E MO(/AMEDES 63

/IlW[,AT!STICA ]fINEIHA RE8PEITANTE .a ACTIVIDADE NA PRO· V/NaJA DE ANGOLA EM 1960 ... 95

.... -~--~~~-.---.... - ..... ----.---i !, i ,I I.UANDA-IMPRENSA NACIoNAL DE ANGOLA-Ig6r

Sedimentary cycles Geosyncline of

in the West Congo Northwest Angola

BY

L. j. C. SCH.E RMEIIH.OIlN

Empresa do Cobre de Angola

Introduction

Tlds paper is based upon the geological survey, mostly by using iwd,,1 photographs, of the 55,000 sq. km. mining concession of the Em­

do Cobre de Angola in Northwest Angola. It was first read at the , Uopoldville C. C. T. A./A. S. G. A. meetings (Commission for Tech-

, Cooperation in Africa and Association of African Geological Sur­." respectively) and has been revised and expanded to include the . s obtained from the completion of the mapping of the southern

of the E. C. A. concession, especially as regards the Sansikwa Series, si raligraphy of which became fully known in '959. The E. C. A. Concession affords a cross-section reaching up to 200

, ill width through the Late Precambrian (or possibly Infracambrian) Congo Geosyncline, from the underlying basement in the west

11", Karroo-Kalahari cover in the east, and a longitudinal section of , :100 km. (fig. I). Mapping waS done on a lithological basis, as is usual

I'recambrian 'treas, and it does not seem unlikely that in many cases "oek units distinguished may be taken to represent time units as

I, in a broad sense, but there arc exceptions. Most of the formations Ill"'"'" ILl arc remarkably uniform and persistent in development ,md can

followed from Angola up to Gaboon. Th" West Congo Geosyncline is at least 1200 km. long and 350 km.

In Gaboon the north end of the northwest-southeast trending 11 is found, with successive formations overstepping onto the basc­, while in the Malange-Salazar arca south of the E. C. A. Concession

'1O[10nS change in facie, as if the basin edge waS neared (un­shed report by the Longyear Company, I954).

'1'11<' West Congo System consists of four series, the Sansikwa, Haut !H1ltgo, Schisto~calcairc and Schisto-greseux, "vhose maximUlll thick-

111'111'1< ., reach over I4 km. in Angola *). In the southeast of thc conces­where the M'Pioka is thickest but some other fonn<ltions arc thin­

I he total thickness is about I3.5 km. The E. C. A. g"oiogists have

'" Until recently these series were considered to be systems but in a.. iorth-111{ pa.per the E. C. A. geologists will present t.he case for the creation of a Congo System composed of tllcsc four series.

48 BOLETIM DOS SERVIQOS DE GEOWGIA E MINAS

traced formations first deseribed in the ex-Belgian Congo along the !nto Angola an' many of the stratigraphic units in Northwest are lmo n by designations derived from the Congo (see L. Cahen, logic du Congo Belge«, 1954, for a review).

The stratigraphic table is as follows:

KARROO-KALAHARI in the east; CRETACEOUS to RECENT in the west

MAJOR UNCONFORMITY

WEST CONGO SYSTEM:

I Inkisi :

5chisto - greseux Series M'Pioka:

Schisto-calcaire Series.

I Sekelolo : H aut Shiloango Series. M . ouyonzl:

•

Sansihwa Series lTerreiro: Uonde: Lulumba:

1I, 12 MB (M'Bridge Formation)

f Pc, Pe, Pg, Pi, Pk, Pm t PO {local and of variable position)

I Cr, Cz, C3, C4, CS", CSb . CO (Upper Tilloid)

SeO, Set, Se2u, Sezb

111.1,112 MO (Lower Tilloid)

53a, 53b, 53C, 53d, 53e 52 51

MAJOR UNCONFORMITY

BASEMENT COMPLEX

The scope of the present paper is to show the existence of a sedimentary cycle, the \\7est Congo Geosyncline megaeyele, of the four lesser cycles of the above series in each of which initial sidenee of the basin is followed by infilling and final emergence.

The breaks in the succession are the following:

(5) between Inkisi :llld Karroo (great uneonlormity) (4) between C5 and M'Pioka (strong unconformity locally, conform~

ity elsewhere) (3) between Sekelolo and CO (disconforl11ity to slight unconfonn­

ity) (2) between Ten-eiro and MO (disconformity to slight

ity) (1) between Basement Complex and Lulul11ba (great unconforrnity), ,:

Only (1) and (5) rank as major unconfonnities caused by orogcny; the West Congo GeosyncJine cycles are separated by epeirogenie move­ments.

JANEIRO A JUNHO -1961- NOMERO 3 49

The information embodied in this paper is derived from the results "I Ihn field survey carried out by E. C. A. geologists H. R. Korpershoek, W, l. Stanton, P. Vasconcelos and L. ]. G. Schermerhorn between I953 ""d H)lir. Grateful acknowledgment is made to Empresa do Cobre de \"Hola for permission to publish this paper. I wish to express my gra­Iillldn to my colleagues H. R. Korpershoek and W. I. Stanton for much I"formation and many instructive discussions and helpful criticism with-11111 which this paper could never have been written; for its shortcom­illf/M however I remain responsible.

The Basement Complex

The Basement Complex crops out in a broad belt along the west IIIJl'<ier of the West Congo Geosyncline (further west it is covered by C!'<'l:iLccous and younger coastal sediments) and consists of gneisses, ,!'Idsts, granites and other igneous and metamorphic rocks. The gneis­"I'!; form a thick succession which has been shown to be of sedimentary "rigin by Korpershoek in the northwest; they are paragneisses, in part lIilglll'ltitic, and are the oldest known rocks in Northwest Angola.

The post-Basement Complex pre-Sansikwa orogeny

The strongly folded and metamorphosed Basement Complex gneis­M'II and the ignous rocks intrusive into them are cut by the sub-Sansikwa IIl1Conformity, showing that this unconfonnity is of orogenic rank and WIlS formed after folding, metamorphism, migmatization and granite ""'placement took place. After this major orogeny there occurred a

!"triod of denudation and planation, producing an erosion surface cut­Ing down deeply enough to expose the gneisses and granites; on this

""rface the West Congo Geosyncline rocks were laid down.

The Sansikwa

The Sansikwa forms the basal series of the West Congo System and I h" first sedimentary cycle. In the northwest the Basement Complex 1111' been thrust over the West Congo rocks, obscuring the stratigraphic ,,,htionships, but towards the south the thrusts die out and there is a lill'ge area where gently folded Sansikwa rests on basement.

The Luiumba or Lower Sansikwa (SI) reaches up to rooo metres Ill' more in thickness and consists of cross-bedded arkoses, feldspathic 'I"artzites, shales and siltstones, with subordinate polygenic conglomer­,Ii"s and locally rare limestone beds. Lateral facies changes are common. TIIC abundance of coarse fresh potash feldspar in the arkosos, predomi­Hating over plagioclase, points to derivation from a granitic source area, I. c. the .basement in the west.

50 BOLETIM DOS SERVI90S DE GEOWGIA E MINAS

The Lulumba is followed, generally by a rapid transition, some­times through a conglomerate bed, by the Middle Sansikwa or Uondt: (S2), .a succession up to a few thousand metres thick of carbonaceous siltstones and shales, thinly and evenly bedded, generally with grading. They may be rather quartzose at the bottom, with quartzite intercal­ations, becoming more pelitic and carbonaceous upward. Locally there have been found lenses of coarse arkosic grit, indicating an in flux of SI-type detritus.

The Upper Sansikwa or Ten'eiro is subdivided into five stages(S3a-e) which arc fairly persistent though lateral facies changes occur, especially as regards the increase of quartzites in a westerly direction. It may reach up to 2500 metres in thickness.

53a, up to 1200 metres thick, is well and extensively exposed in the southeast of the E. C. A. Concession (Stanton). Its lower part con­sists of carbonaceous shales (grading occurs) with intercalations of silt­stone, greywacke and limestone lying conformably on the S2 shales, and the lower boundary of the formation is put at the incoming of beds of shaly and carbonaceous limestone. In the middle of the formation there is an extensive lenticular bed of cross-bedded feldspathic quartzite up to zoo metres thick. This bed grades upwards into a great thickness of interbedded quartzites, greywackes, siltstoncs, carbonaceous shales and limestones. The quartzites and some of the greywackes are crosS· bedded; other greywackes show graded bedding as do the shales and siltstones. The limestones arc mostly thin-bedded calcilutites and cale. arenites, the latter sometimes with small-scale cross-bedding. intercalations arC common and there occur beds of calcirudite and stone microbreccia. Some greywacke beds overlying limestones limestone fragments indicating contemporary erosion.

S3a is overlain conformably, perhaps with a rapid transition, 53b, consisting of up to 300 metres of carbonaceous slightly Hllcac;eou, thinly laminated mudstones and silt 0' mudstones, generally graded, occasional beds of siltstone, quartzite and greywacke.

- S3b passes rapidly up into 53c, a horizon made up of well and bedded and graded, blue (carbonaceous), very micaceous ~r,ev'wa.cke, siltstones and shales interbedded with thick feldspatbic thickness is about 250 metres.

53d overlies S3c with a rapid transition. It consists of inter))en:nel limestones, cherts, quartzites, shales and greywackes. There is a in the centre and along the southwest border of the geosyncline the limestones are well developed; they form caleilutites (often similar to C5 limestones) with beds and lenses of calcarenites and inate calcirudites, the latter products of contemporary erosion redeposition. CroSS-bedding occurs in the calearenites. Bands of types of cherts are common; ubiquitous are blue-black and grey (of C5 type) and there also occur oolitic and pseudo-ooliticcherts. beds of calcareous quartzite and intercalations of calc-shale arc of occurrence. In the southeast of the concession the limestones

JANEIRO A JUNHO -1961- NOMERO 3 51

Ihinner "nd mudstones, greywackes and quartzites come to make up IlIost of S3d. The formation is about 250 metres thick.

S3e is a horizon zoo metres in thickness of mudstone and silts tone with beds of quartzite and occasional layers of limestone and chert, the Ii!l.ter in grea.t variety. Towards the western border of the basin cross­\,elided and ripple-marked quartzites become dominant.

Three stages can be distinguished in the deposition of the Sansikwa. S I with its considerable thickness of arkoses, quartzites and conglomer­lites represents the basal level of the West Congo Geosyncline. It eon­Kists of the waste products laid down in a newly forming basin and deri­wc! from the erosion of adjacent crystalline areas. From the great quan­lily of feldspar detritus it may be concluded that the source area was heing uplifted at the same time as the trough started to subside. Where I he Sansikwa - Basement contact is exposed it is seen to be essentially II fhlt plane representing an old leveUed erosion surface. Thus, consider­IIbJe elevation and erosion of the border lands must have occurred, form­ing a high relief and supplying much fresh feldspar to the geosyncline. Moreover, the thickness of SI, with cross-bedded arkoses and qum·tzites occurring all througb it, sbows tbat subsidence took place during its deposition. Thu!" epeirogenic movements caused downwarping of the hasin and uplift of tbe crystalline border lands at the heginning of the history of the West Congo Geosyncline. Figure 2 attempts to show the relationship between subsidence and sedimentation in the geosyncline. The depth axis is marked in kilometres as the amount of subsidence can be deduced from the thickness of the deposits (compaction not taken into account), while the time axis is arbitrary and largely disproportional liS nothing is known about the duration of the stratigraphic stages. Gener­III considerations have led to the subdivision shown, e. g., it seems likely Ihat the depOSition of the Sansikwa took more time than the Haut Shi­loango and that thick shallow-water deposits such as the Lulumba, the Upper Schisto-calcaire and the Inkisi were laid down over a period much longer than was needed for the accumulation of the rather thinner tilloid formations whose lithology evidences a violent mode of sedimentation. The subsidence curve shows the depression of the basin in general, plot­

against time, while the sedimentation cu.rve is intended to show the reg­Ular alternation of deposition in deeper and shallower waters determined hy the rate of subsidence. Sea level is assumed to have remained station­nry. The two curves illustrate conditions in the central part of the geosyncline, where the formations are generally thickest, and do not tnke into account minor fluctuations of the rate of subsidence and the depth of sedimentation.

During SI, as during the deposition of the other shallow-water 10rm­lllions whlch reach considerable thicknesses, subsidence went on and {or the purpose of the figure the rate of depression has been arbitrarily (liken as linear though fluctuations may have occnrred.

While at first, during Lulum ba, the geosyncline received an abun­dlmt suply of quartzo-feldspathic detritus mingled with pe!itic matter,

52 BOLETIM DOS SERVII;OS DE GEOLOGIA E MINAS

the latter Came to dominate during Donde and part of Terreiro. 52. 53a. 53b and 53c together form a very thick succession in which carbon­aceous shales and siltstones with graded bedding prevail, displaying the character of turbidites laid down below wave-base. Carbonaceous shales occur already intercalated il, 51 and are found throughout the 5ansikwa, up to the very top of 53e. Although some of the carbonaceous shales might be of different origin, it seems that the bulk of these shales owes its con-

EO

" .S 1

'" ~ , ~ 0;' ~ (J"

! , 6

•

10

11

12

13

"

~ SCHISTO-GRfsEUX ~ crCLE • ~ CYCLE § CYCLE

<I: S£.XEIJ) <I:

SANSIKWA

p I I SI

CYCLE t: IiAUT SHILOANG~tl SCH/srO-CALCAIRE

'" WlKrcu'" 52 I 53 ~ M e ~ C

obt de 0 1 201 2 0112 345

TIME

--- Bunqo Basin

{I,

,/' \~ J ~

!

\/~---~ , , , , , \ , , ,

\

\

......... M'Bridge Uplift, N.E. •. - .• M'Bridqe Uplift, SW

\,~

Fig. 2, Subsidence and sedimentation in the West Congo Geosyncline, Northwest Angola,

tent of carbonaceous matter to a copious supply of organic substance on the shelf where the pelitic beds of 52 and 53 were first deposited before transport by turbidity currents and resedimentation in the geo­syncline. It seems less likely that they owe their carbon content to depo­sition in a stagnant, barred basin as the floor of the geosync1ine was

JANEIRO A JUNHO -1961 - NOMERO 3 53

I"" io<lically swept by turbidity curreuts originating along the margins III (he trough and bringing in aerated water; moreover carbonaceous ,d"",,, are found interbedded with graded greywackes, cross-bedded qlllll'Vites, and limestones (themselves often carbonaceous).

Sz and 53a-C represent the phase of strong subsidence of the basin ,11,,1 its filling by turbidites. In figure 2 this is depicted in a general way; ill constructing the course of the curves during this period it was arbi-1I'II1'ily assumed that the depth of the sea at the middle of Vonde depo­"It io" waS one kilometre and that sedimentation took place at a steady III k. 53a,b,c continue the geosynclinal sedimentfdion of 52 but there Ill'" signs that the basin begins to get filled in. As it becomes shallower, ti:l:i with its shallow-water facies is formed. The depositional environ-1111'111: of the 53e mudstones is not well known; for the purpose of figure ; il has been assumed that 53e is a shallow-water formation as is the case ,lIolli; the western margin of the basin (see above). During the deposi-11011 of 53d and 53C subsidence went on, though at a lesser rate than 1",1' ore (again, the curves of figure z do not show the fluctuations that 1Ilily have occurred).

The 5ansikwa is sharply overlain by thc Haut 5hiloango and the hlls,,1 stage of the latter contains eroded 53 fragments, so that emergence IIr part at least of the 5ansikwa took place. While uplift and erosion of 1 h" 5ansikwa occurred in the axial and western regions of the geosyn-1'1111" in the northern part of the E. C. A. Concession, there is a large area 111 lhe centre of the geosyncline ill the southeast of the concession where Imsal Haut 5hiloango, the Lower Tilloid, overlies 53e mudstone with a ijllllrp contact but without any unconformity. Post-5ansikwa emergence, I liollgh widespread, was apparently not universal. During emergence I he sedimentation curve is interrupted.

Thus, the 5ansikwa starts with subsidence of the West Congo Geo­.Yllcline and the deposition of clastics deriving from the rapid erosion I)f '. crystalline borderlands, followed by strong subsidence and the Qn':;nrnu'lation of a thick geosynclinal series of shales and siltstones. When

Sansikwa period draws to a close the trough gets- filled in and shal-1"',,_,<>0 ter deposits are laid down. The cycle is ended by a phase of emer­H"nec affecting a great part of the geosyncline.

In each of the following three series we will see the cycle of subsi-. repeated, with slight variations.

The post-Sansikwa pre·Haut Shiloango movements

As shown, widespread emergence took place before the deposition lire Hant 5hiloango, and the unconformity is attested by the oceur­

of abundant debris deriving from the upper Terreiro rocks (e. g. 1I111\\15t,11<:1OJle limestone and chert fragments from 53d) among the stones

11lC MO tilloids, but there is no anguLtr discordance and the break t we en the two series is only a disconformity over most of the basin.

I I I , ! ~

I

54 BOLETIM DOS SERVIQOS DE GEOWGIA E MINAS

The junction is always sharp and well-marked. MO lies generally on S3e, only very locally on S3d and even S3C (in the west), from which it may be deduced that the uplift c'lUsing emergence increased gradually in amount tow,u'ds the basin edges. Everywhere the two series exhibit parallel folding, duc to the post-Inkisi orogeny that closed the history of the West Congo Geosyncline. The movements between 53 and MO are thns of epeirogenic character.

The Haut Shiloango

The deposition of this series starts with the Lower Tilloid (MO). Both this formation and the basal stage of the Schisto-calcaire, the Upper Tilloid (CO), are characterized by the occurrence of tillite-like pebbly rocks which are of non-glacial origin and have been termed tilloids. Both MO and CO wcre first described in thc ex-Belgian Congo and have been regarded as tillites there (sec Cahen, op. cit., r954) and in ex-French Equatorial Africa. However, in a recent study Dr. Stanton and the writer have shown the non-glacial origin of the {illoids and presented evidence in favour of deposition by submarine mudflows (<<Tilloids in the West Congo Geosyncline,{, read before the Geological Society of Lon­don on June :Z:Z, r960; see discussion in Proc. GeoL Soc. London No. r58r) .

11<[0, usually between 200 and 500 metres in thickness, consists of many beds of different lithology; typical are the tilloids, pebbly and gritty mudstones in beds up to many metres thick, generally showing a grey to black carbonaceous mudstone matrix, sometimes calcareous, containing scattered granules, pebbles and occasionally boulders of various rock types (limestone, chert, mudstone, spilite, metaqnartzite, granite and rare others). The limestone, chert and mudstone fragments, rang­ing from angular to rounded, are intrabasinal and derive from the eroded upper levels of the Sansikwa (the trough in which MO was deposited /, waS narrower than the Sansikwa basin and MU contains Sansikwa' ments all through its considerable thickness). The igneons and IIlt:caJ'Ilur., phic stones, always well-rounded, are extrabasinal and stem from crystalline basement outside the geosyncline (probably beach and shore deposits), while the spilite fragments derive temporary erosion of intraformational volcanics (lavas in MO in Congo just north of the frontier).

The Lower Tilloid forms a well-bedded succession and many of beds, including the finer tilloids, show grading. Apart from thL"ell~:~~:~g it inclndes greywackes, mudstones, conglomerates, breeeias, a and limestones.

MO waS deposited at the beginning of a new s~~~~r~~~70r~L~~~I~ in a rapidly subsiding basin, and the formation is followed by Mr, a succession of thinly and evenly laminated and graded C-"J,nnn. aceous shales, often pyritic. 1fr is generally between 200 and tres thick over the north and central parts of the Concession bnt pirLch{3.

JANEIRO A JUNHO -1961- NOMERO 3 55

11111 in the southeast. It grades into Mz which starts with thinly lamina­I ,q I shales very like MI but getting more and more calcareous upward; <11 higher levels they are interbedded with limestones, often stromato­lit le, siltstones and cross-bedded greywackes. Mz reaches 250 metres I11 thickness. In the extreme southeast of the concession MI and Mz 1,111"11 out completely owing to non-deposition.

MI represents a phase of quiet in filling of the basin by turbidites ; 11,,· supply of coarser clastics (stones and grit in the tilloids) ceased abrupt­II' at the cnd of MO, so that no more tillaids were formed, but the depo­,11 ion of carbonaceous mudstonc (tillaid matrix in MO) continued in M1. TII" fine grain of the MI rocks and the thinness and great regularity of IllI'i,. lamination indicate deposition in a qniet fashion, as contrasted 11'11 It tilloid accumulation, by tnrbidite currents spreading out over a more Ill' 1"55 level sea floor. In Mz the turbidites get replaced by shallow-watcr d"posits, so that it can bc concluded that during MI and M2 sediment­III ion overtook subsidence.

From the special lithology of both tilloid formations, MO and CO, Ii lllay be deduced that their deposition took place in a violent fashion owing to sharp down warping of the basin at the beginning of cycles (see I'll per referred to above). As they are followed quite rapidly by turbi­diles of tranquil and regular accumulation, it appears that the steep illlhsidence causing tilloid deposition levelled off abruptly at the begin­lling of MI and Cl, 2. Thesc turbidites were laid down in a gently sub­"Iding or virtually static basin. If the combined thickness of MI (max. ~(I0 metres) and the lower turbidite part of M2 (50 to lOO metres up to i It<' first appearance of stromatolitic limestones) represent the infilling of a ."Iatic basin up to the transition into a shallow-water environment, then Ihis basin must have been slightly over 600 metres max. in depth, say I,.~o metres (not allowing for compaction). To deposit the remainder of Mz, up to ISO metres thick, slight subsidence to the same amount Illllst again bave set in. If it is supposed, on the other hand, that gentle ollbsidence went on at the same steady rate during MI and Mz, as is ~holVn in figure z, the sea at the beginning of MI must have been slightly ,h:t!lower, reaching 500 to 550 metres in the areaS where MI is thickest.

Given a depth of 400-600 metres for the post-MO sea over a large p:ut of the geosyncline and taking into account the thickness of MO, IIsllally between zoo and 500 metres, total subsidence during MO must Imve been in the order of 900 metres, taking place during a relatively »hort period.

M2 is sharply overlain by a widespread level of feldspathic quart-1.ile, Sea, reaching up to 40 metres in thickness. This is followed by SeI, Il p to ISO metres of mudstone which may show a thin lamination, gra­ding upwards through an alternation of thin limestone and shale layers (Sc2a, 30-40 metres thick) into blue-black limestone (Sezb), locally stro­Illatolitic. Sez is at least lOO metres thick but its top has been removed hy pre-Schisto-calcaire erosion.

56 BODETIM DOS SERVI90S DE GEOWGIA E MINAS

In the southeast of the concession SeO is developed in a different facics of greywacke and sh"le, "nd locally therc appear tilloids at the boundary between the two facies (Stanton). It seems that the Sekelolo may be considered to form a subcycle within the Haut Shiloango cycle, r. e., thc shallow-water accumulation of Mz was terminated by renewed though minor subsidence, with the influx of quartzo-feldspathic sand, presum<lbly deriving from the shelves and beaches bordering the geosyn­cline, rendered unstable by the movements and transported into the basin. Ser then represents the infilling of the basin with Sez marking its shallowing. By the same reasoning as applied in determining the depth of the postcMO sea the post-SeO sea may have been about 200 metres deep.

The post-Haut Shiloango pre-Schisto-calcaire movements

The Hau t Shiloango cycle was ended by widespread emergence and slight erosion after which the Schisto-calcaire begins with a basal level of tilloids (CO) resting unconformably on the Sekelolo and contain­ing eroded fragments of the latter. Like the MO /S3 junction, the CO/Se junction is a disconformity to slight unconformity, least in the axial zone of the geosyncline and increasing towards the edges. Thus, the earth movements apparently formed a broad very gentle syncline, with CO overlying Sezb in the axial zone of the basin while towards the borders and towards the southeast it rests on successively lower stages, first Seza, then Sel. In the southeast, where maximum pre-CO erosion took place, Cl) comes to rest on 11r and here at least 500 metres of Haut Shi­loango strata were eroded. Still, the distances involved are so great that angular unconformities are very slight.

The Schisto-calcaire

After the epeirogenic phase terminating the Haut Shiloango cycle had ended Sharp suhsidence caused once again the deposition of tilloids, opening the next cycle. The Upper Tit/oid, while lithologically similar to the Lower Tilloid, shows nevertheless some divergent features: CO is much thinner than MO (generally rO-50 metres, max. rOo-zoo metres in thickness), it does not contain interbedded volcanics or fragments deriving from them, and its tilloids have a calcareous mudstone matrix (very exceptionally carbonaceous). The formation is bedded and includ­es tilloids, arkosie and feldspathic quartzites, greywackes, mudstones and rare conglomerates, breccias and limestones. Grading may be pres­ent, as in MO. The tilloids are typically green-grey pebbly and gritty calcareous mudstones. The stones comprise intrabasinal Sez limestone fragments and extrabasinal well-rounded metaquartzite and other crys­talline pebbles and boulders.

JANEIRO A JUNHO -1961- NOMERO 3 57

Cr, following conformably on CO, is a thin but very persistent level of thinly laminated dolomitic limestone 5-r5 metres thick. CO marks It stage of strong subsidence, like NW, and Cr was probably laid down timing the lull between the cessation of rapid subsidence, terminating tilloid deposition, and the renewal of terrigenous supply in C2 (locally I here occurs at the top of 1ft) a level of dolomitic limestone very like Cri.

The next stage, C2, consists of a succession (300-500 metres thick) of thinly and evenly interbedded limestone and calc-shale layers, often graded. The content of terrigenous mud and silt in the shale bands of C2 decreases upwards.

The succeeding stages, C3 (roo-zoo metres), C4 (400-rooo metres) ond C5 (about 500 metres), are shallow-water limestone formations, often detrital, with oolites, stromatolites, cross-bedding and contem­porary erosion and redeposition; there occur beds of calc-sandstone ond calc-shale.

All six stages of this series are remarkably persistent and uniform in development.

C2 represents a phase of tranquil turbiditc infilling of the basin, nnalogous to Mr, and continuing the calcareous mudstone deposition of CO (tilloid matrix). In a similar way as has been done for the Mouyonzi (he depth of the post-CO sea can be calculated to have been about 500 metres, assuming, as in figure z, that subsidence went on all during C2 Itnd the following stages at the same rate. The amount of subsidence du­ring CO must ha vc been in the order of 500 metres. The following very thick sequence, C3, C4 and C5, in which there occur but slight amounts of terrigenous material, indicates that although a shallow-water environ­ment prevailed during their accumulation the basin continued subsiding, keeping pace with sedimentation, while the adjacent land masses stayed low and did not show any tectonic activity leading to rejuvenation of relief and the renewal of terrigenous supply.

The post-$chisto-calcaire pre-Schisto-greseu)( movements

The Schisto-calcaire cycle waS sharply brought to an end by move­ments of a character different from the preceding tectonic phases. Strong IJplift occurred in the borderlands, causing great quantities of terrigen­ous detritus to become available for deposition in the geosyncline, and on a broad geanticline crossing the basin, the M'Bridge Uplift (fig. r), and continued spasmodically dnring M'Pioka. In the geosyncline, sepamted into two troughs by the M'Bridge Uplift, pronounced subsidence set in, initiating the sedimentation of flysch-type deposits. On the geanticline 6trong differential uplift and erosion oce<lsioncd the appcamnce of an unconformity of variable magnitude between the Schisto-gr6seux and earlier strata, locally cutting down to the Upper S'U1sikwa.

58 BOLETIM DOS SERVIQOS DE GEOWGIA E MINAS

The Schisto-greseux

The Lower Schisto-grescux, the M'Pioha, lies conformably on Cs in the subsiding troughs formed on both sides of the central geanticline of the M 'Bridge Uplift. In the sou thcastern basin, or Bungo Basin, there is often a mpid transition from Cs upward into M'Pioka, as in the northern basin (in the Congo Republic and Gaboon). The Bungo Basin M'Pioka is composed of a well-bedded alternation of shales, siltstones and grey­wackes, all typically rather micaceous, with quartzite and rare conglo-111crate beds. Grading is COml110n and the formation was laid down by turbity currents. In the Bungo Basin the MTioka is coarser than the corresponding turbidites of the preceding cycles "nd reaches a much greater thickness (over 5000 metres where thickest and generally several kilometres in thickness). This indicates very strong subsidence and also, from the abundance of feldspar and white mica, strong uplift and vigor­ous erosion in the crystalline source area.

The M'Bridge Uplift (sce SchermerhornandStanton: La limite Schisto­calcaire-Schisto-greseux en Angola, en A. E. r. et ,m Congo, Ann. Soc, Geol. Belgique, 1960, t. 84, pp. 91-101 for a full discussion) is surrounded by coarse conglomerates derived from the rapid erosion of strata on the geanticline; the prevailing facies is onC of limestone-chcrt conglo111crates containing fragments of Schisto-calcaire rocks. These conglomerates, the PO facies of the M'Pioka (a wildflysch facies), "re the time equivalent of the Bungo Basin M'Pioka ; they continue up to the top of the M'Piok" on and around the geanticline (Stanton). The uplift was asymmetrical: along the southeast side PO crops out in a relatively narrow zone, becom­ing finer towards the basin where the conglomerates pinch out as the basin deposits get thicker. This shows that Pt) is derived from the uplift. To the northwest however PO is found in a very large area extending from Angola well into the Congo (13razzaville) Republic; here its compo­nents m"y derive in 1'"rt from the uplift, in part from the border zones of the geosyncline. Also, the deposits of the northern basin are rather thinner than the Bungo Basin M'Pioka in Angola.

In the northeast part of the M 'Bridge Uplift the basal unconfor­mitl' cuts down only to Cs 'lllc! C4 but inere"ses towards the southwest where Schisto-gri:scux straLL come to lie on Upper Sansikwa (mostly S3e, locally S3d but this is J"lrtly at least clue to pre-lI>1O erosion) ; here about 3 km. of sediments have been eroded (all. of the Schisto-calcaire and Haut Shiloango) during the M'Pioka. The unconformity attains this maximum value in a large area in the southwest part of the uplift, about 60 km. in length measured along the southwest-northeast axis of the geanticline ; the nearest Bungo Basin M'Pioka lying conformably on CS, in a parallel zone to the southeast, is found at 25 km. distanc" to the southeast. ConSidering that the Schisto-greseux in the southwesl part of the uplift (the M'Bridge Formation) was deposited only towards the cnd of the M'Pioka, when the Bungo Basin had alread been deeply depressed, some idea of the degree of tilting here may be formed. Whik there is a large area in the northeast of the M'Bridge Uplift that stayed

JANEIRO A JUNHO -1961 - NUMERO 3 59

"lIlerged all during the M'Pioka (cf. fig. 1 in Schermerhorn and Stanton l'Ii)O, referred to above), the southwest part started to subside again

III tIT on, after its great elevation earlier, and at least 800 metres of lithic 1;1"'ywackes, siltstones and shales, in part with grading, were deposited, Iflrllling the M'Bridge Formation which marks the transition from M'Pioka III Inkisi in the west (see below).

While the PO conglomerates were being deposited up to the end IIf M'Pioka in the northeast of the M'Bridge Uplift, around the emerged ill'ea (since the Bungo Basin deposits thin and pinch out on nearing the geanticJine, interfingering with the PO beds which continue up to the lop of the M'Pioka), there were formed a few levels of PO conglomerate which reach fairly far out into the Bungo Basin, indicating phases of rllllewed uplift and erosion causing the spreading out of geanticlinal waste products into the southeastern basin.

In fig. 2 the curves show three branches representing the different COllrses taken by subsidence and sedimentation during the M'Pioka: (r) strong subsidence and sedimentation in the Bungo Basin (the depth of deposition here reached has been arbitrarily taken as exceeding one kilometre), (2) uplift and emergence in the northeast part of the M'Bridge Uplift, and (3) uplift and emergence followed by subsidence and sedi­mentation in the southwest of the M'Bridge Uplift.

The Upper Sehisto-greseux or Inkisi overlies the M'Pioka confor­mably. It has b\O stages, Ix (0 to 400 metres thick) and 12 (at least 800 metres thick; the top is never seen) ; hoth consist largely of cro'S-bedded Handstones in shallow-water facies. Inkisi is only found in the Ilorthvart of the concession, not in the south part where the Bungo Basin M!Pioka reaches a vcry great thickness. It is not kl1O\vn \vhcther this is due to non-deposition (with M'Pioka facies rocks being hlid down in lnkisi times, but there is no evidence for this) or erosion as the top of the M'Pioka has been removed here. The southernmost Inkisi outcrops clo not show facies changes.

The Lower Inkisi (h) is only developed in the northeast of the con­cession; it is composed of coarse pebbly arkoses sharply overlying the top M'Pioka slates. On the M'Bridge Uplift 11 rests both on PO and, in the area of non-deposition of M'Pioka, on the upper Schisto-calcaire; farther to the west it pinches out rapidly and there is a large area where ME grades rapidly into Iz, in the absence of 11. Towards the southeast MB overlaps onto eroded Bungo Basin deposits. The deposition of MB, Ilpparently starting well towards the end of M'Pioka times, waS caused by movements owing to which the southwest part of the M'Bridge Uplift, up to then above sea level, subsided again; shortly before, the last phase of uplift affected not only the geanticline but also the margin of the Bungo Basin, causing its border deposits to be eroded before thc dcposition of the overlying M'Bridge Formation. The re1ationsbip between lVIB ,wd th(~ 11 arkoses in the east is not known as exposures arc lacking, but since the former grade into I2 it seems that their deposition may llavc continued in Ix times, assuming that the base of 12 is not cliachrollous ; this

1 I ~

,

60 BOLETIM DOS SERVIQOS DE GEOWGIA E MINAS

is not certain. In the west the lithic greywackes of the M'Bridge Formation grade rapidly into 12, here composed of rather similar lithic greywackes, while in the cast Ir grades more slowly into 12, here consisting of rather feldspathic greywaekes more like the feldspathic Ix rocks. There were two different source areas: one in the northeast for the plutonic feldspar in Ir and overlying 12 and one in the northwest for the lithic fragments (mainly mudstone) in the M'Bridge Formation and overlying 12. As the cross· bedding developed everywhere in the 12 greywackes dips consist­ently to the southeast, the feldspar source area must be situated rather to the north and may be sought in the Chaillu Massif, a very large plu­tonic massif bordering the 'Vest Congo Geosyncline on the east in the Congo (Brazzaville) Republic.

In the east of the concession at least the Inkisi spreads out well over the Bungo Basin M'Pioka. Cross·bedding is ubiquitous in Ix and 12 (also, 12 greywackes often carry mudstone intraclasts derived from contem­porary erosion of mudstone beds), so that the Inkisi appears to have been laid down in shallow waters, after the Bungo Basin had largely been filled .. From the thickness of the Inkisi it follows that subsidence kept pace with sedimentation. In addition, the thickness of the Inkisi and the relative COarseness of its greywackes (coarser than most of the Bungo Basin M'Pioka) points to rejuvenation of relief in the Source areas, This must have been due to uplift outside the geosyncline taking place at the same time as widespread subsidence started in the Inkisi basin, including the M'Bridge Uplift whose role as a geanticline finishes with the M'Pioka. As the top of 12 does not show any noticeable diminishing of grain size in relation to the rest of the formation, it appears that extrabasinal uplift kept pace with su bsidence.

Thus the Schisto.greseux starts with strong subsidence in part of the geosyncline, together wich uplift forming a broad geanticline, and ends with a shallowing of the basin, before the post.Inkisi orogeny ends the history of the West Congo Geosyncline.

The post-Inkisi pre-Karroo orogeny

This orogeny caused folding and thrusting in the West Congo System strata, forming a broad belt of mainly parallel folds that increase in intensity in a westerly direction, culminating in a zone of overthrusts along which the Basement Complex overrides the System. This fold and thrust belt curves around the southwest end of the M'Bridge Uplift which acted as a rigid block around which the folds were dragged. Towards the east and northeast the thrusts disappear and the folds become gentler and die out. On the broad southwest part of the uplift two folding trends interfere, the post·Inkisi folds following a northwest·southeast course, and the northeast·southwest structures on the Uplift, apparently

JANEIRO A JUNHO -1961- NUMERO 3 61

Cilllsed by similar structures in the underlying basement (this direction I, common in the basement). The border thrusts, curving around the Uplift, die out towards the southeast.

Widespread regional metamorphism occurred, increasing in inten­Ilily towards the basement in the west and southwest (which was also IIffected), and granites and other igneous rocks were emplaced in the basement along the western border of the West Congo System or in the SIUlsikwa along this border.

Conclusions

The West Congo System megacycle is thus found to comprise four (:ycles, in part with su bcycles. There waS little volcanic activity in the gcosycline.

As can be seen in figure 2, the two main periods of subsidence and hence of sedimentation in the West Congo Geosyncline occurred in S2 Ilnd P, these two formations alone reaching up to half the thickness of the sedimentary column in the south part of the cor.cession. In contrast, the middle two cycles, marked by basal beds of the tilloid type, contrib­(J led much less to subsidence and sedimentation, which also took a liomewhat different course.

The movements separating the cycles are positive interludes in the general negative, subsiding trend of the geosyncline, and they, together which the levelling off of subsidence in the preceding cycles, CaUse a fourfold oscillation of the subsidence curve.

ABSTRACT

The Late Precambrian West Congo System in Northwest Angola is litlparated hy great unconformities denoting major orogenies from the underlying Basement Complex in the west and the Karroa cover in the east. It was laid down Itl a subsiding basin, the West Congo Geosyncline, which can be followed from Gaboon over the two Congo republiCS well into Angola. The combined maximum thicknesses of the beds reach over I4 km. The system is divided into four series, fhe Sansikwa, Haut Shiloango, Schisto-calcaire and Schisto-greseux, separated by !flight unconfonnities caused by epeirogenic movements. Each series forms a sedi­mentary cycle in which subsidence and infilling of the basin, first by turbidites, then by shallow-water deposits, is followed by emergence. Thus a fourfold oscillation JH superposed on the general subsiding trend of the West Congo Geosyncline.

SOMMAIRE

Le Systeme du Congo Occidental, d'age Precambrien superieur, est s6par6 par des grandes discordances, correspondant a des orogeneses majeures, du Complcxe do base sousjacent au ouest et de la couverturc de Karroo a fest. 11 s'est accumule {1ans un bassin subsident, le G60synclinal du Congo Occidental, s'etenclan t un Gabon, 011 passant par les deux rcpubliques du Congo, jusqu'en Angob. Les 6paisseurs lllaximales totalisees des couches atteignent plus de I4 km. Le systeme se subdivise (')n quatre series, le Sansikwa, le Haut Shiloango, le Schisto-calcairc et 10 Schisto­gr6scux, s6pan~es par des lacunes dues a des mouvemcnts 6pirog6niqucs. Chnquc

-

62 BOLETIM DOS SERVIQOS DE GEOWGIA E MINAS

serie constitue un cycle s6dimentaire dans lequel subsidence et comblement du ba.%in, d'abord par des turbidites, puis par des depots en eau peu profonde, soni: succ6d6s d'unc emersion. Ainsi unc oscillation quatel'naire se >iuperpose sur la subsi~ donee generale du Geosynclinal du Congo Occidental.

sUlVIAmo

o Sistema do Congo Ocidcntal, de idadc PrecambricQ superior, fiea separado por grandes discordancias atcstando orogenias impoI'tantcs do Complexo de Base subjacente no ocste e da cabertura de Ranoe no leste, Acnmulou-se este SiStCl113

nnma bacia de subsidencia, 0 Geossinclinal do Congo Ocidcntal, estendendo-se do Gabao, pass<lndo polas duas republicas do Congo, ate Angola. As espessuras maxim as so mad as dosanclares atingem para eima de 14 Ion. 0 sistema c subdividido em quatro series,o Sansikwa, 0 Alto Chiloango, 0 Xisto-calcario e 0 Xisto-grcsoso, separadas por lacunas estratigrMicas ocasionadas por movirncntos cpirogenicos. Cada scrio constitue um delo sedimentar cujo abaixamento e enchimento da bacia, primeiro par tl1rbiditos e depois par dep6sitos em aguas de pouea profundidade, e seguic10 duma emersao. Assim uma oscilay3.o quaternaria se sobrepoe na subsidencia geral do Geossinclinal do Congo Oddcntal.

r················

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•

! ++++++t++++++++++

• __ -I • • • . . . .

/ / / , , /

~

AMBRIZ

, , , / / / , , ,

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............ ~:;.;;. ;.;.;.;.;;.;.;.;..

c::::::::::JKARROO ANO KALAHARI

V'}!X;.Y) SCHISTO - GRESEUX

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§ SCHISTO-CALCAIRE

_ HAUT SHILOANGO

DSANSIKWA

WEST CONGO

SYSTEM

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D BASEMENT COMPLEX ANa YOUNGER ERUPTfVES

__ BOVNOARY OF E.C.A. CONCES'SION

Fig.i- The West Congo Geosyncline

100 Km

coP JO

in Northwest Angola Geology after E.C.A. survey {!953-196!1; in the MOlange - Solozar ore a

modified oft er mop by the Longyeor Company (unpublished report ,19541.