Mixtite deposits of the Damara sequence, Namibia, problems of interpretation

Palaeogeography, Palaeoclimatology, Palaeoecology, 51 ( 1985): 159--196

159

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

MIXTITE DEPOSITS OF THE DAMARA SEQUENCE, NAMIBIA, PROBLEMS OF INTERPRETATION

H. MARTIN, H. PORADA and O. H. WALLISER

University of G~ttingen, Geologisch-Paliiontologisches Institut, Goldschmidtstr. 3, D-3400 G~ttingen (Federal Republic of Germany) (Received July 15, 1983; revised version accepted December 10, 1984 )

ABSTRACT Martin, H., Porada, H. and Walliser, O.H., 1985. Mixtite deposits of the Damara sequence, Namibia, problems of interpretation. Palaeogeogr., Palaeoclimatol., Palaeoecol., 51: 159--196. Mixtite deposits form extensive outcrops in the Pan-African Damara orogenic belt of Namibia. They occur at two different stratigraphic levels in the lower part of the Upper Proterozoic Damara Sequence (South African Committee for Stratigraphy, 1980; KrSner, 1981). The older Varianto mixtite is confined to the northern margin of the fold belt. No evidence indicating a glacigenic origin has been found, but on the southern foreland of the fold belt two, possibly coeval, mixtite deposits (Court and Blaubeker Formation) show fair evidence of such an origin. The younger Chuos mixtites occur in three separate zones of the belt. All these mixtites have been interpreted as glacigenic sediments, and the Chuos Formation has been used as a stratigraphic marker for the correlation between different parts of the fold belt. The present investigation shows that: (1) The mixtite deposits show no features indicating a glacial origin; (2) The sedimentary features indicate deposition by gravity-flow processes; (3) turbidites are in several areas closely associated with and even interbedded in the mixtite deposits; (4) mixtite deposition is in some areas not confined to the Chuos Formation proper, but began at lower stratigraphic levels or persisted to a higher level; (5) in some areas calcareous or dolomitic sediments are interbedded in mixtite, or mixtites are interbedded in such formations. The mixtites of the Chuos Formation are interpreted as various kinds of submarine gravity-flows (slump breccias, mass-flows, slurry-flows, grain-flows, turbidites) which were probably triggered by tectonic activity during a stage of differential subsidence that affected the whole geosyncline, but was concentrated in three rift zones. Under this assumption the majority of the mixtites may have been deposited during a limited time span. A different model assuming synorogenic deposition of the "pebbly schist" mixtites of the Southern Margin Zone is briefly discussed. It is concluded that the mixtite deposits are not of glacial origin and can therefore not be regarded as reliable chronostratigraphic markers for correlations between the different facies domains of the Damara sequence nor for correlation with other Upper Proterozoic sequences (e.g. Gariep belt, Katanga belt); such a correlation might, nevertheless, exist, if the extensive gravity-flows should have been caused by eustatic changes of the sealevel during a time of widespread glaciation o n other parts of the globe. In this case local m o u n t a i n glaciers might have contributed material to some of the Chuos mixtites.

0031-0182/85/$03.30

© 1985 Elsevier Science Publishers B.V.

160 INTRODUCTION

The existence of an extensive mixtite deposit in the Damam Supergroup was first recognized by Gevers (1931) at the Chuos Mountains (Figs.1 and 4) and described under the name Chuos Tillite. Similar mixtite occurrences were subsequently found in the northern part of the Damara Belt (Le Roex, 1941), and also interpreted as tillites. The northwestern branch of the belt (Kaokoveld, Fig.l) contains an extensive mixtite formation in the same 12 0

16 0

140

18 o

+

+

•+{imil{{iiiiii+ +++++++..++`

X

KAOK+O+V ELD

N

#%<,'"

~P~O~4tE

P L A T F O RN~

;.++.$;++ T $++; ~+;~%+ ;+++.,~; + ++ ;2~ ~I.+," $ *

uiJo

"-,

......

.. 21o-~-

' ,AFt.! P /

23°+

J~

o

~,,o,oE,e..~

o O.e.., ~ , + ~

/

~

I"

"',

FF J i,

--. -

] , ~ ~,

~I{

i

J'~B

i

~

OKONGUARRI94 $TEINECK109 n'v~U MUNSTERLAND 113

AU OTHER lOCKS

I

FRANSeONTEIN

OK ST'

AUeOS sgs

~

HARASIB 717

VA

VARIANTO

PRE- O'~MARA BA.~EMENT

B

BLAUBEKEI

C D

COUR~

DAMARAN MIXTITES

DIANA

Fig.1. The mixtites o f the Damara Sequence. After Geological Survey of South Africa and Southwest Africa/Namibia, 1980. Mixtites occur in the three major structural zones of the intracontinental arm o f the fold belt and also in the Kaokoveld. In the Southern Zone they are confined to the Southern Margin Belt which is 30--70 k m wide.

161 stratigraphic position. Widespread thick pebble- and boulder-bearing schists in the southern part of the Damara belt were also correlated with the Chuos Tillite (De Kock and Gevers, 1933). The supposed glacial origin seemed to make these formations excellent chronostratigraphic markers for correlations between different facies domains within the Damara belt, and possibly with other parts of the Pan-African belt system also (Martin, 1965; KrSner and Rankama, 1972). The glacial interpretation was accepted by all the geologists who have worked in these areas. The name Chuos Formation was applied to all these mixtites (South African Committee for Stratigraphy, 1980). If the glacial interpretation should be correct, these extensive and voluminous mixtites would play an important role in the discussions of the Upper Proterozoic glaciations. The lower part of the Damara Sequence (Nosib Group) also contains mixtite-bearing formations. These have also been interpreted as glacigenic deposits. Serious objections to this interpretation, based on general considerations, were raised by Schermerhorn (1974, 1975). In order to assess the validity of the climatic interpretation the authors have studied the sedimentological features of the Damaran mixtites and associated sediments, and compared the pebble bearing schists with pebbly mudstones associated with an Oligocene olisthostrome of the Apennines, where a subtropical climate prevailed at the time of deposition. This investigation and comparison tends to support Schermerhorn's arguments. THE STRATIGRAPHICSETTING AND GEOGRAPHIC DISTRIBUTION OF THE MIXTITES The part of the Damara Supergroup containing the mixtite formations is younger than about 1000--900 Ma (Key and Rundle, 1981) and older than a pretectonic diorite of 750 Ma (KrSner, 1981, 1982). Mixtites occur in the three main structural zones of the fold belt (Fig.l). The stratigraphically oldest mixtite, Varianto Formation (Hedberg, 1979): is confined to a small area in the Otavi Mountains of the Northern Zone, where it forms the top of the lower Damaran Nosib Group (Table I). Two mixtite occurrences (Court Formation and Blaubeker Formation) on the southern foreland of the Damara fold belt (Fig.l) are also assigned to the upper Nosib Group (South African Committee for Stratigraphy, 1980). The younger mixtites which have been correlated under the name Chuos Formation, occur in all three zones, but are not connected by outcrops. Their correlation can therefore not be taken for granted. These mixtites are, as a rule, separated from the Nosib Group, or where the latter is missing, from the basement by carbonate rocks (Abenab Subgroup; RSssing Formation; Corona Formation); they are also overlain by carbonate sequences: in the North by the Maieberg Formation, in the Central Zone by the Karibib Formation (Table I). The lithology of the mixtite formations varies considerably within each structural zone and from zone to zone.

COW,LInE,ATE ARXOSE, ARelLLI~E (30(~H)

WITH

CHERT

LI~STONE,

=

MIXTITE,

S~LE

(qSO , )

~NDSTONE

OOLITIC CHERT,

LIMESTONE,

(525 M)-

~KOSE*

COte~L~ERATE

~TZITE,

RHYOLITE, TUFF, AC~LOMERATE, ANDESITE* EPIDOSITE. BOSTG~ITE (6000 H)

, ] X T I T E . TUF~. ITAIXRITE

GREYWACKE

S T R ~ T O L I T E S . ARKOSE,

DOL~'IITE,

(750 ~)

LIMESTONE

$TROf4ATOLITES

DOLOMITE.

~RL.

~OLO~ITE.

LIMESTONE.

DOLOMITE, LIHESTONE. SANDSTONE. ] T A ~ ] R I T E , OOLITE CHERT (700~)

SLUMp BRECCIA (9~0 H)

DOL~ITE.

STRO~4ATOLITES ( 1 ] 0 0 ~ )

DOLOMITE

LOCAL D I S ~

LOCAL

MJ

DOL~ITE LEMSES

~U~RTZ~TE,

SHALE

(1(~

SHALE, HARL, $]LTSTONE,

SANDSTOI4E

~OLO~ITE .ITH CHERT SHALE, LIMESTONE, STRO~ATOLITES. OOLITES (°J0O M

NORTH

LOCAL

(700M)

~JARTZ - BIOTITE B I O T [ T E - GARNET - ( SCHIST, AMPHIBOLE • JARTZITE. HARBLE. CATE ROCK

LITHOLOGY ( M A X . THICKNESS)

-

RHYOLITE

ARKOSE,

( ~

M)

CONGLOMERATE ~SCHIST,

QUARTZITE,

(IlOOM)

GNEISS AND ~ T Z I T E

~pHEBOLE

ROCK,

(7C~ M ) '

PYROXENE

ROCK

SC. EST, i

BIOTITE SCH,ST.J

QUARTZITE, I

HORNBLERDE

CALCSIL[CATE

CALCSILICATE

BZO~ETE -

~BLE.

~C~RC~NC~E' ~]__~. :AL o,s

CENTRE

3RON A

GROUP

LOCAL

NAMA

SCHIST,

C~L~E~TE

~JARTZ I TE,

pHYLLITE*

( L I ~ M)

SCHIST,

(~0

(6700 , )

ARKO$E,

QUARTZITE.

SCHIST

CO~LOMERATE

DOL~ [ TE

QUARTZITE

H)

GRAPHITE SCHIST, QUARTZITE, QUARTZ - HICA SCHIST, C~LO'ErAtE, ~TAeJR~T~ (1700 M)

pEBBLES SCHIST. .[XTttE. ~JARTZITE, SCHIST. [ t A B [ R I T E . AMPHIBOL]TE. CALCS[LICATE roc~ (1650 e)

AMpHIBOL]TE* [TAB]R]

QUARTZITE,

BIOTITE SCHIST. BIOTITE QUARTZITE, GRAPH)TIC SCHIST, CALKS[L[CATE ROCK AMPHIBOLITE (MATCHLESS ~E~BER)

?

LITHOLOGY (MAX. THICKNESS)

PARTIALLY

SOUTH

Stmtigraphic nomenclature and proposed correlations between the carbonate she|f (North), the central and southern part of the fold belt (after South African Committee for Stratigraphy, 1980)

TABLE I

bO

C~

163 MIXTITES O F THE NOSIB GROUP

The Varianto Formation

The Varianto mixtite is confined to a small area in the Otavi Mountains (Fig.l), on the northern flank of an anticlinal structure of Nosib strata which overlie granitic pre-Damara basement. The mixtite occurs at the top of the Nosib sequence. It transgresses across the lower units of the group onto the basement, and is itself overlain by dark-grey laminated dolomite of the Abenab Subgroup (Table I). The Varianto Formation varies greatly from place to place. The about 40-m thick mixtite is poorly sorted. A local intercalation of laminated silty shales contains small isolated pebbles. Tectonic overprinting makes it impossible to decide whether these are dropstones. The matrix of the mixtite is ferruglnous and sericitic. On the farm Ghaub 47 a thin layer of laminated iron formation is associated with the mixtite. The polymict clasts have sizes of up to about 60 cm. In places, the majority of the clasts consists of fragments of trachy-leucoandesite and andesite (SShnge, cited by KrSner, 1981). At other places well-rounded quartzite pebbles derived from underlying Nosib conglomerates predominate. A diligent search did not produce a single pebble with a convincing facet. On the farm Elandshoek 771 the lower part of the mixtite contains well-rounded quartzite pebbles similar to those of Nosib conglomerates, whereas the uppermost part is packed with small granite fragments and detrital feldspars. There the distribution of the clasts seems to mirror the unroofing of the neighbouring basement inlier. The mixtite has probably been derived from the underlying formations of the near neighbourhood. The volcaniclastic material has been correlated with the Naaupoort volcanics of the Nosib Group (Hedberg, 1979). There are nb sedimentological features indicating a glacigenic origin. Mixtites on the southern foreland

The southern foreland contains two formations for which a glacial origin seems probable, and which could be approximately coeval with the Varianto Formation. They have been named Court Formation and Blaubeker Formation respectively (Schalk, 1970; South African Committee for Stratigraphy, 1980; Hegenberger, 1980). Both are situated (Fig.l) near the southern fringe of Damaran deformation, where the matrix of the mixtite still shows a cleavage, but the clasts have remained undeformed. The Court mixtite (Buschmannsklippe tiUite of Martin, 1965) has a thickness of 400--500 m. It is polymict and has a morainic habit. Some pebbles (e.g. amygdaloidal lava, red jasper) are exotic to the region. A high percentage of the pebbles is well-striated. Facets and striations with more than one preferred orientation indicate glacial activity (Figs.2 and 3). The Blaubeker mixtite is also polymict. Its very well rounded quartzite

164

Fig.2. Clast of quartzite with set of deep parallel striations which are at the right covered by a piece of still attached matrix. Court tillite, Farm Auros, Gobabis District.

Fig.3. Two striated pebbles from Court tillite. The large stone has two sets of parallel striations at right angles to one another. The well-rounded small pebble of hard quartzite shows a set of striations parallel to its long axis.

165 pebbles and larger angular blocks seem mostly to have been derived from the directly underlying conglomerates and arkosic quartzites. The percentage of striated clasts is lower than in the Court tillite, and the striae do not form such well oriented sets, but are more randomly oriented. The matrix shows a well-developed cleavage. The overall impression is more suggestive of a flowtill or a rock glacier than a lodgement till. Both formations unconformably overlie fluviatile arkosic quartzites and conglomerates which have been correlated with the lithologically similar lower Damaran Kamtsas Formation of the Nosib Group (Schalk, 1970) from which they are, however, separated by large faults. A chronostratigraphic correlation of the foreland platform cover with the lithologically similar Kamtsas succession in the fold belt seems probable. The time-span represented by the unconformity is unknown. MIXTITES OF THE SWAKOPAND OTAVI GROUPS It has been mentioned above that within the intracontinental branch of the Damara Belt (Martin, 1983; Porada, 1983) the Chuos Formation seems to be confined to three separate zones (Fig.l). The mixtite deposits of these zones will be described separately. The Chuos Formation of the Kaokoveld (Fig.l) has not been reinvestigated. The Chuos Formation in the southern part o f the Central Zone In the southern Central Zone (Fig.4) the Chuos mixtite (Gevers, 1931) overlies either a carbonate (marble) sequence (RSssing Formation) or the uppermost Nosib beds (Table I). The latter show a facies transition from cross-bedded quartzites of the Etusis Formation to well-bedded and laminated calc-silicate rocks (metamarls and calcareous greywackes) of the Khan Formation. The wedging-out of the R5ssing marble below the Chuos mixtite was attributed by Smith (1965, p. 22) to a regional disconformity. The mixtite is conformably overlain and widely overstepped by the marble beds of the Karibib Formation or the coeval calcareous turbidites of the Tinkas Formation (Porada and Wittig, 1977, 1983). The interpretation of the mixtites as a tillite (Gevers, 1931) was based on the tillite-like appearance o f the rock (Fig.5), its abundant matrix, and the polymict nature of the usually angular to subrounded clasts which are poorly sorted and range in size up to about 40 cm and exceptionally 80 cm. The clasts consist of quartzite, different types of granite, carbonate rocks and vein quartz. The mixtite occurs in the region of high-grade metamorphism as defined by Winkler (1974). The originally somewhat carbonatic matrix has been altered to a calc-silicate rich rock in which carbonate clasts are surrounded by dark-green amphibole-rich reaction rims. Everywhere the clasts have been tectonically deformed, and their original surface features have been obliterated by metamorphism and deformation.

166

7

/

/

f

11

/ /

/

ET" • FARM ETUSI$ NO. 75

NF. • FARM NAMIBFONTEIN NO 91 NV. • FARM NELSVILLE, PTN. OF FARM NORDENBURG NO. 91

OT • OTJIPATERA MTS LV. = LIEVENBERG RR, = RABENRLJCKEN

MR = MON REPO$ v

~, ,b,~..~Q ~~

/ /

/

/

I

I

7

NOS~B ~ROUP ROCKS ~

,RED GRANITE ~' (REMOglLIZED NOSIB GROUP ANO BASEMENT ROCKS )

I /

/I

j

Fig.4. Map of the mixtite outcrops (Chuos Formation) in the southern Central Zone.

Fig.5. Chuos mixtite at type locality, Farm Dorstrivier, Karibib District. Many clasts are surrounded by reaction rims formed during high-grade metamorphism.

167 The features of the mixtite proper neither support nor contradict a glacial

origin. Gevers (1931, p. 12) and Smith (1965, p. 22) regarded the mixtite as a basal moraine deposited by a major ice sheet in a partly marine environment. The relationship of the mixtite to the underlying formations in the surroundings of the pre-Damara Abbabis Inlier (Fig.6) seemed to support this interpretation, because the mixtite oversteps from W to E the underlying RSssing marble and comes to overlie the calc-silicate bearing quartzite of the Khan Formation and the feldspathic quartzite of the Etusis Formation (Nosib Group) (Figs.6 and 7). Smith attributed this relationship to an unconformity caused by glacial erosion. That erosion of some kind has taken place, is proved by the absence of the RSssing marble on the southeastern flank of the inlier and the presence of marble clasts, presumably derived by erosion and/or slumping from the RSssing Formation, in most of the mixtite bodies. In order to assess the validity of the Gevers-Smith interpretation a number of profiles ( A N on Figs.6 and 7) were studied. t5 ~

15o 45'

30'

TSABICHAS

.

HABI$

15 0 15'

220

15'7-

5'

[~

OTHERROCKS ~AMARA~ G~ANnE

[~

]

SYNSEmMENrA~V~RECC,A I ~AJJ~I~FOR~^TION I~SW~KOp G~OUP

mlm c.uos FOrMAT*ON

!

~SS~NG pOR~ATIO~

0 I

5 . . . .

s

m . . . .

'

15 k m I

~

I

I

|

220 30'~-

Fig.6. The development of the Damara Sequence around the pre-Damara Abbabis Inlier (after Smith, 1965, with some corrections).Note that the RSssing Formation wedges out between the points X and X' on the northwestern flank of the Inlier,and that the Chuos Formation is not represented by mixtite along the northeastern margin of the Inlier.

168

;i 1

~ 7 - -

6

3

2

~"

200 8

7

6

~3~

2

I00

0m

2

I

2

~ - - 3

'4 5 ~ 6 ~ 7 ~ - - 8 ~

®

Fig.7. Block diagram and profiles to illustrate the lithosome relationships between the northern flank of the Abbabis Inlier and the Namibfonteindome. 1 = gneissic pre-Damara basement of Namibfontein Inlier; 2 = Etusis Fm. (arenaceous, arkosic); 3 = Khan Fm. (calc-silicate rocks, calcareous metagreywackes and quartzite, para-amphibolite, biotite schist); 4 = layer, 1--2 m thick, of graded small-pebble conglomerate and grit; 5 = R6ssing Fro. (marble, impure marble, calc-silicate beds); 6 = Chuos Fm. (mixtite, calc-silicate beds, marble); 7 = Karibib Fro. (marble, calc-silicate beds); 8 = Kuiseb Fro. (biotite cordierite schist); on profile (F) the Kuiseb Fro. contains an intercalation of distal calcareous Tinkas turbidite beds.

The profiles A--G (Fig.7) show: (1) that no recognizable unconformity exists between the Nosib Group and the R~ssing Formation, confirming Smith's conclusion in this respect; (2) that a layer of partly graded conglomeratic grit which in places grades into a mixtite, is present below the R6ssing marble (4 on profiles E and F). This extensive layer is interpreted as a slurry-flow deposit as defined by Carter (1975). (3) that the R6ssing Formation consists of 2--3 marble bands separated by schists and/or calc-silicate beds, and that in profile E two mixtite beds occur in this zone, showing that carbonate deposition and mixtite deposition can alternate; (4) that the wedging-out of the R6ssing marble on the northeastern flank of the Abbabis Inlier (probably accentuated by some faulting) could have been caused either by an erosional disconformity, or by transition into a calc-silicate facies by non-deposition of the carbonate layers;

169

(5) that the Chuos mixtite proper peters out between the latter calcsilicate-bearing quartzites and the overlying marble of the Karibib Formation (between the points X and X' on Fig.6). It peters out also in a northerly direction across the Khan synclinorium, as shown on Profile G, where no mixtite is present between the RSssing and the Karibib marble. In fact no mixtite was found anywhere on the Namibfontein dome. On the southeastern flank of the inlier the RSssing marble is not represented, but no disconformity is recognizable (Figs.6 and 8). The profiles show instead interfingering relationships between the Nosib Group and the Chuos Formation, and between the latter and carbonate beds of the Karibib Formation. On Dorstrivier (Profile M) the first rudaceous rocks appear in the uppermost part of the Nosib sequence where a conglomerate is overlain by typical crossbedded Etusis facies quartzite. The quartzite is overlain by arkosic sediments containing scattered granitic clasts and several intercalations of mixtite and biotite--sillimanite schist. This unit is followed by the typical Chuos mixtite (Fig.5). The mixtite is overlain by a schist unit which grades upwards into well-bedded, dark calc-silicate beds which would be assigned to the upper Nosib Khan Formation, if they were n o t underlain by the mixtite. A marble intercalation near the top heralds the onset of the carbonate deposition of the eonformably overlying Karibib marble. The marble formation is well-bedded. It contains 3 zones in which the original bedding has been disturbed by incipient slumping (Fig.9). At the top of the marble a few calc-silicate layers indicate a facies transition to the overlying biotite schists of the Kuiseb Formation. In this profile, and also in the profiles J~ K, L, and N, every facies unit is connected by interfingering with the underlying and overlying units. There

LEGEND

TO

SECTIONS

Lithology

J-N Lilho focles

tithostratigraphy

KU~SE~ F ~

~2

"" ;

K^R,B,6

..........

F~

¢NUOS

'M

ETUSlS

~

Fig.8. S e c t i o n s across C h u o s F o r m a t i o n o n t h e s o u t h e r n f l a n k o f A b b a b i s Inlier b e t w e e n Nelsvil]e a n d Etusis. F o r p o s i t i o n o f t h e s e c t i o n s see Fig.6.

170

Fig.9. Slump breccia in Karibib Formation. Farm Dorstrivier, Karibib District (for position see Fig.6).

is no indication of a sudden facies break which might mark a major regional hiatus or disconformity that could be attributed to a phase of glacial erosion. Such a phase of erosion was deduced by Smith (1965) from isolated mixtite occurrences that directly overlie basement on the northeastern part of the inlier, on the farms Tsabichas, Mort Repos and Habis (Fig.6) On Tsabichas the mixtite is 20--30 m thick. It contains angular clasts of pegmatite, gneiss and coarse leucogranite. There are no carbonate clasts. The schistose matrix has no resemblance to the calc-silicate rich matrix of the typical Chuos mixtites. One cube-shaped block of pegmatite measures 60 X 60 X 60 cm. Along strikes, where Karibib marble is in direct contact with the basement, similar pegmatite dykes are sharply truncated against the marble. The contact is probably faulted. The cube-shaped pegmatite block may have been derived from a pegmatite of the immediate neighbourhood. The termination of the Etusis quartzites to the southwest of the mixtite occurrence seems to have been caused by faulting against the basement as well as the Karibib Formation. The bedding of the quartzites is truncated at both contacts. As the mixtite contains neither Etusis quartzite nor carbonate clasts a correlation with the Chuos Formation is improbable. This mixtite could have originated as a talus deposit close to a fault scarp. The mixtite on farm Habis has probably a similar origin. On farm Mon Repos the supposed tillite has an elongated outcrop which according to its position on Smith's map could have been formed as a channel filled with glacial material. A reinvestigation showed, however, that this

171 body is a dyke-like carbonatic intrusion containing abundant inclusions of fragmented basement rocks. Some of the clasts have been intruded along fractures by thin veins of the carbonatitic matrix. There is thus no evidence for the assumption that, in the surroundings of the Abbabis Inlier, the Chuos mixtite formed a transgressive blanket that overstepped Basement, Nosib Group and RSssing Formation. Combining the observations from both flanks of the Abbabis Inlier the following conclusions seem justified: (1) There is no regional disconformity or hiatus at the base of the Chuos Formation, and no evidence for erosive action by an ice sheet. (2) There is ample evidence in the sediments underlying the mixtites for transport and deposition by turbidity currents and mass-flow, and in the overlying marble also for synsedimentary slumping. (3) The Chuos Formation was deposited by mass-flows in comparatively deep water during a time of tectonic activity. The mass-flows interfinger with carbonate layers, probably deposited as turbidites. The above observations and conclusions raise questions as to whether the Chuos Formation can really be used as a chronostratigraphic marker~ whether the well-bedded to laminated calc-silicate rocks of the upper Nosib Group (Khan Formation) and the RSssing marble Formation in the Central Zone might not represent a siliceous, marly, and carbonate facies of the dolomitic Abenab Subgroup of the Northern Zone, and whether mass-flow deposition did not take place at different times during the certainly lengthy period of sedimentation of these formations, as advocated by Martin (1983}, Porada (1983) and Porada and Wittig (1983). The provenance o f the clasts The majority of the clasts can be matched by rocks cropping out in the pre-Damaran Abbabis Inlier (Gevers, 1931, p. 8; Smith, 1965, p. 26). It is unlikely, however, that they have been derived from the inlier itself, because at this time the inlier was entirely or mostly covered by the thick Etusis quartzites of the Nosib Group (Porada and Wittig, 1977) which are too homogeneous to be regarded as proximal first-cycle sands. On the other hand, the extensive area in which these mixtites are found and their considerable volume require source areas in the vicinity, because the massive parts of the mixtites indicate deposition by proximal mass-flows. Three possibilities are suggestive: {1) A floating ice sheet could have provided moraine material which was redistributed by repeated slumping. Such a mode of deposition differs from that proposed by Gevers (1931, p. 12) only in assuming subaqueous mass-flow instead of direct deposition. (2) Both the Nosib sequence and the mixtite formation may have been deposited in a rift zone or aulacogen (Martin and Porada, 1977). Under this assumption the elevated rift shoulders could have supported mountain glaciers which provided the moraine material for the subaqueous resedimentation.

172

(3) Non-glacial fanglomerates and talus derived from the rift shoulders could have provided the material for the subaqueous mass-flows. This possibility corresponds to the mode of origin suggested by Schermerhorn (1974, p. 809, 1975). This interpretation fits the observed features best. The process could have been enhanced by eustatic sealevel changes. In all three cases tectonic movements in the rift zone may have triggered the repeated slumping. Tilting of a fault-bounded block is probably responsible for the local angular unconformity between Nosib quartzite and mixtite discovered by Jacob (1974) at the Rabenruecken (Fig.4) (R.E. Jacob, personal communication, 1975). The Chuos Formation o f the Northern Zone In this zone mixtite is only present in discontinuous areas (Fig.l), but it appears to occur in the same stratigraphic position, for it is always overlain by dolomites or limestones of the Karibib Formation or the corresponding Tsumeb Subgroup (Table I). The mixtite occurrences were not systematically investigated by the authors, therefore only a few relevant observations will be reported here. The mixtite was discovered in the area between Otavi and Tsumeb (Fig.l) by Le Roex (1941) and interpreted as a tillite. He reported faceted and striated pebbles. Author H. Martin who has reported such features from mixtite deposits in the Kaokoveld (Martin, 1965, p. 28) reinvestigated the outcrops mapped by Le Roex. This region has only been subjected to lowgrade metamorphism and slight tectonic deformation. On some outcrops clasts, freed by weathering, can be picked up on the surface. Their occasionaUy flattish surfaces lack the characteristic shape of glacially bevelled pebbles. Pebbles with flat surfaces, similar to those picked up, are formed when broken pebbles are slightly abraded by further transport. Such pebbles can be found in most non-glacial gravels. The very rare observed "striae" consisted of single, random scratches that could have been received whilst the pebble was exposed on the surface. None of the supposed facets showed a set of parallel striations. With respect to the Chuos mixtites in the Kaokoveld (Fig.l) author H. Martin does no longer regard the above mentioned observations as convincing evidence for glacial activity. The supposed facets and rare scratches were similar to those encountered in Le Roex's area. The shapes and surface features of the clasts provide no convincing evidence for a glacial origin of the massive mixtites of the Northern Zone. One has therefore to ask whether other features of the mixtites, including facies relations, allow insights into processes involved in their formation. The nature of the mixtite varies greatly from place to place. In the Otavi Mountains, to the south of Tsumeb this is well demonstrated by a comparison between the outcrops on the farms Harasib 317, and Jakal Omuramba 677, situated about 10 km from one another.

173

Fig.10. Polymict mixtite on farm Jakal Omuramba, Otavi Mountains.

On Jakal O m u r a m b a the polymict mixtite is massive and u n b e d d e d (Fig.10). The matrix consists of sand grains and small angular clasts in a dolomitic matrix. Amongst the large clasts angular dolomite blocks of different types and sizes predominate (one slab of 1 X 2 m was observed), but a few well-rounded dolomite inclusions are also present. Clasts derived from the Nosib sequence are subrounded and consist of arkosic quartzite. Some of the larger ones {max. size 80 X 45 X 40 cm) are cross-bedded. The clasts derived from the pre-Damara basement are mostly rounded. Various types o f granite, metaquartzite and rare p o r p h y r y pebbles are observed. The largest granite block measured I m X 80 X 30 cm. One rounded block of mixtite measuring 20 X 20 cm was observed. It could have been derived by erosion or slumping from a slightly older portion of the mixtite. Only very few hard clasts weather o u t intact. No facets or striations were observed. The mixtite is underlain by well-bedded dark-grey dolomite of the Abenab Subgroup. On Harasib the Chuos Formation is represented b y a dolomite slumpbreccia consisting of large and small, angular to subrounded dolomite clasts in a dolomitic matrix ( F i g . l l ) . Amongst the clasts all the rock types o f the underlying Abenab sequence are represented. Granite clasts are extremely rare (N.C. Thirion, personal communication, 1976). Author H. Martin was n o t able to find a single one. The breccia is overlain by dark-grey laminated limestone of the Tsumeb Subgroup. On the farm Auros 595, a b o u t 10 km to the south of the mixtite outcrops on farm Jakal Omuramba, the mixtite is missing. There, platy laminated limestone of Zone 2 of the Tsumeb Subgroup directly, and seemingly con-

174

Fig.ll. Dolomite breccia containing angular to subrounded clasts of different types of dolomite. The breccia occupies the stratigraphic position of the Chuos mixtite. Farm Harasib, Otavi Mountains.

formably, overlies the stromatolitic dolomite o f the Auros Formation (Table I). The mixtite development at these three places, situated so close to one another differs so greatly that deposition b y a large ice sheet can be discounted. Regarding the relationship of the three localities to the larger structures the following facts m a y have a bearing on the origin of the mixtites: -- The mixtite on Jakal O m u r a m b a with its large granite and Nosib arkose clasts is situated in a syncline close to the flank of an anticlinal structure in which Nosib beds and pre-Damara granite are exposed. The asymmetrical disposition of the Nosib beds in this structure makes it probable that the structure was, even during Nosib times, an elevated, probably fault~bounded feature. The complete absence of the mixtite or a related sediment on Auros, only a b o u t 10 km to the south of the above anticlinal structure is evidence that a differentiated topography of the basin floor persisted or was rejuvenated during the time o f deposition of the mixtite. The dolomite breccia on Harasib is also situated on the flank o f this anticline. A large basement inlier exists a few kilometres to the north. By itself the breccia would be interpreted as a rather proximal slump deposit

175 unrelated to glacial activity, the more so because smaller slumps are encountered at both stratigraphically lower and higher levels in the Otavi Mountains. We conclude: (1) that in the Otavi Mountains the Chuos mixtite was not deposited by a large ice sheet; (2) that the mixtites can be interpreted as the results of rather localized slumping triggered by tectonic activity in a zone where tectonic ridges existed in the basin; (3) glacial activity, if any, could only be attributed to small glaciers on small elevated fault blocks. The small size of the fault blocks speaks against this assumption. Only two small mixtite deposits have been mapped in the dolomite hills between Otavi and Outjo, a distance of about 130 km But as this area has not yet been mapped in detail, the possibility that other mixtite occurrences might be hidden underneath the extensive calcrete cover cannot be excluded. Outcrops are again found about 37 km west of Outjo, to the south of the Fransfontein Ridge (Fig.l) which forms the southern flank of the large Kamanjab basement inlier along the E--W-trending part of the inlier. The Chuos mixtite development seems to be confined to this flank. No mixtite has been observed on the northern flank. Within this part of the Northern Zone the sedimentary features of the Chuos Formation are as variable as in the Otavi Mountains. Four examples will be given. On the farm Steineck 109, 40 km west of Outjo the mixtite has a phyllitic matrix in which clasts of up to one meter in size are randomly distributed. The mixtite was originally deposited as a boulder mudstone. Bedding is indistinct. The larger clasts are angular to subrounded and consist predominantly of diverse types of granite. The base of the mixtite is not exposed. It is overlain by blue-grey laminated limestone of the Tsumeb Formation. Some 10 km to the east, on the farm Mfinsterland 113, this laminated limestone contains intercalations of graded, upwards-fining sedimentary breccias. The breccias consist of densely packed small angular fragments, predominantly of porphyry (average size less than 1 cm) in a blue-grey limestone matrix. A small channel contains at its base a boulder pack with subrounded to rounded granite pebbles up to 30 cm in diameter. The regular primary lamination of the blue limestone indicates deposition in the low energy environment of comparatively deep water. The angular basement debris must have been spread by turbidity currents. It cannot have undergone fluviatfle abrasion over more than a very short distance, or have remained on a beach for any length of time before being redeposited by slumping. The rounded pebbles in the small channel, however, must have been derived from a beach or fluviatfle source. These features, to which T. Clifford drew the attention of author H. Martin, show that conditions which may have been responsible for the formation of the mixtite, persisted into the time of basin-wide carbonate deposition. On the farm Saturn 103, 30 km west of Steineck, the Chuos Formation is well exposed in a local nappe structure. The nappe seems to have moved

176

in a southerly direction, i.e. from the platform margin towards the geosynclinal basin. Clifford has regarded the nappe as a gravity structure (Clifford, 1962, p. 44). The mixtite consists of a few beds of subrounded to wellrounded pebbles, mostly of quartzite, in a silty to sandy open frame-work matrix (Fig.12). It does not resemble a till. The interbeds consist of dolomitic sandstone and slate. The sequence also contains a siliceous iron formation; this iron formation forms a conspicuous member of the Chuos Formation in the coastal branch of the Damara Belt in the Kaokoveld (Martin, 1965; Hedberg, 1979). On farm Saturn the Chuos Formation is overlain by the blue-grey laminated limestone of the Tsumeb Formation. Some 25 km to the south-southeast, on the farm Okonguarri 94, the Chuos Formation is again exposed in a large anticlinorium (Fig.l). This mixtite is about 30 m thick. It is well-bedded and consists of phyllitic slates containing scattered matrix-supported angular to subrounded clasts of dolomite and various types of granitic rocks. There is no grading in the clast~ bearing layers, neither has a clast been found to penetrate the thin beds in a dropstone-like manner. The unit contains clast~free interbeds of thin alternating slate and carbonate layers. There is thus an intimate association of mixtite and carbonate deposition. This formation resembles the "pebbly schists" of the Southern Zone in many respects (see below). The mixtite formation is conformably underlain by about 30 m of well-

Fig.12. Mixtite beds consisting of subrounded to angular pebbles in a matrix of silty to sandy slightly dolomitic material, Farm Saturn 103, Outjo District.

177 bedded to laminated grey dolomitic limestone correlatable with the RSssing Formation. In order to assess the depositional environment in this part of the developing geosyncline the sedimentary features of the succession underlying the carbonate sequence must be briefly mentioned also. The RSssing carbonate formation is here conformably underlain by the Okonguarri Formation consisting of 3--4 km of bimodal siliceous and carbonate turbidites (Porada and Wittig, 1983)exhibiting upward-fining graded bedding, current ripples, soft-sediment slump structures and also two layers of mixtite which contain both dolostone and granite clasts. No sole marks are exposed. Other directional indicators suggest an easterly source for the siliceous and a northerly source for the calcareous turbidites, but the observations are not numerous enough to be really significant in this respect. It is, however, evident that in this part of the Damara geosyncline turbidite and slump deposition had started already long before the deposition of the Chuos mixtite. On Okonguarri the Chuos mixtite is conformably overlain by blue-grey well-bedded dolomite of the Karibib Formation. The above examples show that the transition zone (Martin and Porada, 1977) between the "miogeosynclinal" carbonate platform and the 'eugeosynclinal" basin is characterized by deposits of gravitational resedimentation ranging in age from Nosib into Tsumeb times. These sediments, which include mixtites, are not confined to the stratigraphic level of the Chuos Formation. The incorporation of granite clasts at different stratigraphic levels indicates recurring differential vertical movements in a broad rift zone, which was also the site of a voluminous Nosib time volcanism. Repeated uplift of the Kamanjab Inlier has certainly supplied some of the granitic material and triggered synsedimentary slumping, and probably also the synorogenetic basinward directed nappe formation described by Clifford (1962). We conclude that the sedimentary features of the Chuos Formation of the Northern Zone can be explained without the assumption of glacial activity.

Palaeogeographical considera tions To obtain additional arguments for or against a glacial origin of the Chuos Formation one has to look at the larger palaeogeographical context. In this respect, it is of interest that the Chuos mixtite occurrences of the southern Central Zone are separated from those in the north by an approximately 130 km wide zone in which no mixtite is exposed (Fig.l). How wide this zone was prior to folding cannot be estimated, because the complicated structure has not yet been unravelled satisfactorily. Recent investigations (Martin et al., in prep.) have shown that the stratigraphic level of the Chuos Formation is exposed in numerous antiforms (not shown on Fig.l), but no mixtite could be found. Instead the stratigraphic equivalent

178 of the Chuos Formation is represented by laminated calc-silicate rocks (originally calcareous greywackes and marls) and minor biotite schist which, further south, are known to inteffinger with mixtite layers (Fig.7, Profile E). No indication of an unconformity was found. The laminated metagreywackes and metamarls are underlain and overlain by dolomitic marble of marine origin, R6ssing and Karibib Formation respectively. The former consists of 5 to about 20 m of alternating thin marble and calc-silicate beds of probably turbiditic origin, and the basal marble layers of the Karibib Formation show a distinctly graded transition to calc-silicate intercalations. This entire sequence must have been deposited in a marine fairly deep, low-energy environment. Now, if this marine depository had been fringed on two sides by shelves on which glacial and glaciomarine deposition occurred, then surely dropstones should be present in the intervening part of the basin. This is not the case. The palaeogeographical configuration is a strong argument against the glacial interpretation of the Chuos Formation in the central and northern parts of the Damara mobile belt. The abandonment of the glacial interpretation makes the stratigraphic correlation (Table I) of the Chuos Formation of the central part with the mixtites ("pebbly schists") of the southern margin problematic, because in the intervening Khomas Trough (Fig.l) the stratigraphic level of the Chuos Formation is hidden by the younger Kuiseb Formation. THE "PEBBLY SCHISTS" OF THE SOUTHERN MARGIN ZONE The "pebbly schists" are confined to the Southern Margin Zone (Figs.1 and 15) which is characterized by thrusts and nappe tectonics (Hoffmann, 1981; Miller and Hoffmann, 1981; Kasch, 1983; Geological Survey Republic of South Africa and SW Africa/Namibia, 1980). This part of the later fold belt was during the initial stage of the geosynclinal development (Nosib Group) the site of a continental rift system in which fluviatile arenites and playa-lake evaporites had accumulated (Behr et al., 1981, 1983). The following marine transgression is documented by the dolomites of the overlying Corona Formation which is in turn overlain by the "pebbly schist" sequence. How can the "pebbly schists" be fitted into this geotectonic environment? The mixtite sequences of the Southern Margin Zone are lithologically quite dissimilar to those of the Central Zone. They consist of generally wellbedded polymict pebble- and boulder bearing micaceous or graphitic schists with interbeds of quartzite and pebble-free schist. The formation varies greatly in thickness. It is overlain, over a large part of its outcrop length, by two metatholeiite horizons (orthoamphibolites). The deformation is generally intense. The pebbles are flattened and elongated, and a pronounced schistosity, often in the form of a metamorphic transposition cleavage, is a conspicuous feature of most outcrops. The tec-

179

tonization has obliterated all the finer sedimentological features. The attribution of a glacial origin (De Kock and Gevers, 1933) seemed well-founded, being based on the interpretation of the pebbles and boulders as dropstones, and on thinly laminated rocks as glacial varves. In order to assess the validity of this interpretation author H. Martin has studied the sedimentary features of the "pebbly schists" in numerous places and sections throughout the outcrop belt of these deposits.

Lithology and sedimentary features Bedding can be recognized in all the more extensive exposures. It manifests itself in two kinds of changes of the lithology: (1) Distinct beds of a different sediment, usually quartzite, interbedded in the mixtite (Fig.13); (2) contacts of different types of mixtite with one another, e.g. sizesof the clasts, proportions of clasts to matrix (Fig.14). The bedding planes of the first category are usually quite sharp and regular. Those of the second category are less distinct because they have been overprinted by the transposition cleavage which usually forms an acute angle with the originalbedding.

Fig. 13. Well-bedded " p e b b l y schist" with intercalation o f graded quartzite. The average size of the clasts is different in adjacent mixtite layers. Note, softly deformed sand-ball (near centre of picture) enclosed in a layer containing only very small clasts. Farm Naos, Rehoboth District.

180

Fig.14. Widely separated quartzite boulders in quartz-biotite schist showing pronounced transposition cleavage. The large boulder has keeled over. The thickness of the quartzite interbeds varies from a few centimeters to several meters. In the Naos E m b a y m e n t (H~ilbich, 1977) layers of a b o u t 20--40 cm are most c o m m o n , b u t thicker and more numerous quartzite layers occur in some areas near the base o f the mixtite sequence. In the Nauaspoort-Kudis area (Fig.15) the quartzite beds are rare, b u t where present, have c o m m o n l y thickness of 10--20 cm. In the Dordabis area (farm Hatsamas 92), several thick layers of very hard quartzite are found. One of these contains small scattered pebbles. The uniform thicknesses of the thinner beds is a conspicuous feature. The individual thickness of these quartzite beds does, as a rule, n o t change over the length of an outcrop, i.e. over distances of several meters to 100 m. They contain no clasts. Most of the beds do n o t show any internal sedimentary features, b u t a few have been f o u n d which are distinctly graded. In these cases the grading is caused b y an upwards increasing percentage of micaceous material (Fig.13). These beds are in this respect similar to those of the Chausib Turbidite Member (Porada and Wittig, 1976, 1983) which interfingers with the quartzites of the Hakos Mountains (Fig.15). The facies relationships leave little d o u b t that the graded interbeds in the 'pebbly schists" are turbidites. This makes it highly probable that the numerous, non-graded quartzite layers, which have the same thicknesses as the graded ones, were deposited b y a similar, b u t less turbulent flow process, i e. grain flow. It is also possible that the quartz grains were originally graded according to size, b u t that recrystallization has obliterated this feature. If a glacigenic origin is assumed for the mixtite beds, h o w does the absence o f clasts in the quartzite interbeds affect this interpretation? Turbidite and grain-flow deposition takes place so fast that the likelihood of dropstones being incorporated m a y be very small. The absence of drop-

181

@ sE

sNLE

30 qDEKAREME;~to~

t*

18o 23°+

16° 23°+ KUDIS

[~ I

~ ~+

~++++

++~+++++ +~+

OTHERROCKS "PEBBLY SCHIST" OF CHUBSFORMATION NOSIB GROUP

~ - ' ~ BASEMENT m MAXIMUMSIZEOF CLASTSiN "PEBBLY SCHIST" ( cm) ( ~ ROTSTOCKSOUTH

( ~ NAOS

® BLAUBEKER

® Fig.15. Map showing the outcrop areas of the "pebbly schist" formation in the Southern Margin Zone. Maximum clast sizes in different areas indicated by numbers and white dots.

stones in the quartzite can therefore not be used as an argument against the dropstone interpretation for the clasts in the mixtite units. The "pebbly schists" contain, besides quartzite and schist intercalations, also two other types of sediment: (1) At a number of places thin beds of magnetite quartzite or hematite-rich schist were observed, and a 1 m thick lense of iron formation is exposed on the bank of the Usib River, on the farm Nauaspoort 261, Rehoboth District. It consists of laminated siliceous hematite ore and is directly overlain and underlain by polymict, boulder bearing schists. The scattered boulders attain sizes of up to 80 cm in the vicinity of the ore layer. (2) The other intercalation, on the farm Coas, Portion of Hatsamas 92, Windhoek District, consists of an 80 cm thick marble bed which is directly under- and overlain by pebble-bearing, distinctly bedded schist (Fig.16). Deformation and metamorphism have obliterated features which could have indicated the mode of deposition, however, in analogy to the quartzite layers, turbidite-deposition seems probable. There are no pebbles in the iron formation nor in the marble. The glacigenic explanation of the mixtite units seemed to find strong support by De Kock and Gevers' (1933, p. 116) description of 'banded varved rocks" containing scattered, occasionally faceted pebbles. The supposed valves consist of light and dark coloured laminae (quartz-rich and biotite-rich)

189.

Fig.16. Folded marble layer interbedded in mixtite. The orientation of the deformed clasts is parallel to the axial plane transposition cleavage. Farm Coas, (Portion of Hatsamas 92, Windhoek District).

which are several millimeters thick. These, however, are n o t of sedimentary origin, but define a metamorphic transposition cleavage (Fig.17) which can be mistaken for a sedimentary feature, where it forms a very acute angle with primary bedding. This is generally the case in the thrust belt, where So and $1 often form angles of only 5--10 degrees. The supposed facets which are found on quartzite pebbles only, are 'pseudofacets" that were produced by pressure-solution processes and are always oriented parallel to the transposition foliation (Fig.18). These reinterpretations do n o t exclude the possibility that some isolated pebbles and boulders could have been ice-rafted. The direct superposition of layers with fairly densely packed clasts by layers with more widely dispersed clasts can hardly be explained by the dropstone hypothesis. A more uniform distribution would be expected if the clasts are thought to have been dropped from floating ice. Furthermore, layers with densely packed, fairly uniformly sized clasts (Fig.19) cannot have been formed by in situ incorporation of dropstones. Such layers must have been formed, regardless of the origin of the clasts, by resedimentation processes. However, in spite of a continuous search, no indication of grading of the clasts was found in any mixtite layer. Another feature that cannot be explained by the dropstone hypothesis is a rough correlation between the overall thickness of the formation, the thickness of individual mixtite layers and the m a x i m u m size of clasts. The

183

Fig.17. Metamorphic transposition layering. In "pebbly schists" with widely scattered pebbles the light-coloured quartz-rich and dark-coloured biotite-rich layers were formed as an S I transposition cleavage which was later deformed by an $2 crenulation cleavage. The original bedding So is indicated by the light-coloured quartzite band. A small quartzite clast is visible near the centre of the picture. Farm Kudis 271, Rehoboth District.

IqIJfIJJ!tlJttptltitlIjlnjlllltlltll I 0

1-

2

3

4

Fig.18. Quartzite pebble with facet-like surface formed by deformation and pressuresolution. The faint lineation paralleling the long axis marks the intersection of the transposition foliation with the surface of the pebble. Remnants of quartz-fibre beards, parallel to the lineation, are still recognizable at the blunt ends of the pebble.

184

Fig.19. "Pebbly schist" containing a fairly well-sorted fraction of large quartzite cobbles suggesting fluviatile transport prior to resedimentation. thickest individual layers m a y reach more than 10 m in the Naos embayment, where the formation is probably more than 1000 m thick, and also in the Nauaspoort-Kudis area, whereas the thickest layers at the western out~ crops o f the Hakos Range (Fig.15), where the formation is only 20 m thick, do n o t exceed 2 m (Fig.20). A similar rough correlation exists between the m a x i m u m sizes of the clasts and the thickness o f the mixtite layer in which these occur. Thus the largest blocks (60 cm to a b o u t 1.5 m across) occur in layers which are many meters thick, whereas in the westernmost outcrops, where layers are only 50 cm--2 m thick, the largest pebbles do n o t exceed 20 cm in diameter, and layers 10--20 cm thick contain only pebbles with m a x i m u m diameters of a few cm (Fig.20). These rough estimates make no provision for the effects of tectonic deformation. The above rough relationship would be expected if the mixtite layers were deposited b y a mass-flow mechanism, for in this case the three parameters would depend on the distance from a source area (see below).

Description of the clasts Clasts of quartzite and granite predominate by far over other rock types. All the granitic boulders and the majority of the quartzite pebbles and

185

Fig.20. Top portion o f a 1 m thick mixtite bed containing several thin quartzite streaks. The largest pebble at the b o t t o m measures about 6 cm. Farm Chaibis 29, on northern limb of Hakos Mountain anticlinorium.

boulders are derived from pre-Damara basement. The largest granite boulders have sizes of up to 80 cm, exceptionally 150 cm. They are usuany subrounded. Where granite and quartzite clasts occur together in the same layer the latter are usually larger than the former. Carbonate clasts, probably of intrabasinal origin (Corona Formation), occur in the basal part of the formation on farm Naos. Carbonate clasts that were probably derived b y slumping o f intercalated carbonate layers, were observed on R o s t o c k South and in the Kudis-Langbeen area (Fig.15). The quartzite clasts may be well-rounded, subrounded or angular. Wellrounded clasts (discounting tectonic flattening and elongation) predominate in the size fraction smaller than about 50 cm. These must have received their shapes in rivers or on beaches before being incorporated in the mixtite (Fig.19). In some mixtite units quartzite blocks of 60--150 cm (longest axis) tend to be angular and even slab shaped. Occasionally these occur as trains o f lithologically similar blocks which are also similar to the quartzite interbeds in the mixtite. A good example of such a train was observed on farm Hatsamas 92, on the eastern flank of the Schaaf River Valley. There blocks of such a train, separated by distances of 5---10 m are strung o u t along a weakly indicated bedding plane. It seems probable that they were

186

formed by slumping and break-up of a quartzite layer. The quartzite must have been well consolidated prior to slumping, because the blocks are quite angular and show no signs of boudinage. It is also possible that some clasts are of tectonic origin. Fold-hinges of either slump-folds or tectonic folds may form boulder-like inclusions, if they have been isolated either by further slumping or by progressive tectonic deformation. Figure 21 shows, what is probably a slump-fold in the process of further tectonic disintegration. The hinges of folds whose limbs have been t o m or obliterated by pressure-solution processes, may occasionally on eroded surfaces, if the fold axes meet the surface at a high angle, have the appearance of boulders. Some of the large blocks in the Naos Embayment may have such an origin, but the number of pseudoclasts thus formed are considered to be very limited in number. "Pebbly schist" containing only granitic clasts is rare. On the Farm Dagbreek, in the Rotstock South synclinorium (Fig.15) clasts (max. size 1.5 m) are widely dispersed in a lithologically ill-defined zone. The schist matrix is indistinguishable from the underlying and overlying schists. A similar occurrence was observed on farm Kromhoek, to the south of Rotstock South. Tectonisation has obliterated the sedimentary structures of the schists. The general appearance suggests clast transport by ice-rafting.

Fig.21. Slump-folded quartzite layer in the process of disintegration surrounded by mixtire. Farm Coas, Portion of Hatsamas, 92.

187 In other parts of the synclinorium the "pebbly schists" are of the usual, bedded, polymict type. With, perhaps, the exception of the above observation neither the sedimentological context nor the shapes of the clasts can be regarded as compelling evidence for transport by floating ice. The Ondekaremba Formation

The Ondekaremba mixtite is lithologically quite different from the normal "pebbly schists" but seems to occupy the same stratigraphic level. It is confined to a limited area near the southern tip of the Seeis basement inlier, some 50 km to the east of Windhoek (Fig.15). The formation attains a maximum thickness of about 150 m in the most southerly exposures (K. Schalk, personal communication, 1983). It peters out northwards along the western rim of the inlier. The sediment is well-bedded, the bedding marked by feldspar-rich layers and streaks that alternate with more biotite rich material and mica schist. The clasts are irregularly scattered. They consist practically exclusively of well-rounded pebbles of various types of finegrained mostly leucocratic granitic rocks ranging in size from a few to 30 cm. No quartzite or vein quartz pebbles were observed. The absence of quartzite pebbles, the rather fine-grained nature of the granitic rocks and the feldsparrich composition of the matrix distinguish this formation from the typical "pebbly schists". The possibility of a tectonic origin was seriously considered but eventually abandoned. The general appearance of the Ondekaremba Formation does not suggest deposition by floating ice nor by gravity-flow processes. As the deposit is underlain and overlain by carbonate formations, the succession could form part of a delta complex derived from a small granitic terrain. Mixtites in the carbonate sequence o f the Kudis Subgroup

In the Southern Margin Zone mixtite deposition is not confined to the "pebbly schist" formation proper. Well-bedded polymict mixtite beds occur in some areas in the underlying Kudis Subgroup (Table I). The Kudis Subgroup consists of metacarbonate rocks (dolomite, siliceous dolomite, magnesian limestone) with occasional interbeds of schist and quartzite. On the western part of the farm Duruchaus 249, mixtite beds are intercalated in the carbonate rock sequence. Furthermore, mixtite beds with a carbonate matrix (De Kock, 1934, p. 44) alternate with mixtites with a schist matrix. The latter are indistinguishable from the typical "pebbly schist". The largest observed clasts (up to 1.5 m) consist of a grey quartzite containing small dispersed pebbles of veinquartz. A layer of a similar quartzite forms an interbed at one place. Pebbles of pre-Damara rocks form a large proportion of the inclusions. The size and spacing of the clasts may vary considerably in adjoining layers. One layer,

188

0.5 m in thickness, consists almost exclusively of dolomitic clasts. The larger of these (about 20 cm across) are slab-shaped, and some show indications of soft, sediment deformation. This layer must have been formed by intraforma. tional slumping and resedimentation by gravity-flow. This occurrence of mixtite beds in the Kudis Formation shows that in the Southern Zone, as well as in the Central and Northern Zones (Okonguarri anticline), mixtite deposition did not start at a well-defined stratigraphic level as would be expected, if a major climatic change had been the decisive factor for its formation. At this stage of the investigation the preliminary conclusion was reached that the evidence did not favour the dropstone interpretation for the pebbles, boulders and large blocks of the "pebbly schist" sequence. The observations seemed, instead, to indicate sedimentation by some kind of mass-flow process. This conclusion raised the question, as to what mechanism of sedimentation would spread thin regular sand layers over a mixtite deposit, and then cover the sand by mixtite without disrupting it. We hoped that a comparison with sedimentary features associated with non-glacial mass-flow deposits could help the interpretation. The Oligocene olisthostromes of the Apennines were chosen for comparison, because this region had a subtropical climate during the Oligocene, and because the very extensive olisthostromes show a great variety of sedimentary features. COMPARISON WITH SEDIMENTARY FEATURES ASSOCIATED WITH AN O L I S T H O S T R O M E IN T H E O L I G O C E N E R A N Z A N O FACIES O F T H E N O R T H E R N APENNINES

The olisthostromes are intimately connected with various facies developments. One of these which is well exposed near the confluence of the Rio Pecola and the Torrente Ceno to the southwest of the city of Parma, consists of a combination of lithologies resembling that of the "pebbly schists" and Kudis mixtite sequences. At this place an olisthostrome is overlain by a sequence of graded greywackes (Ranzano facies). The lowermost greywacke beds alternate with mixtite layers containing scattered pebbles in an argillaceous to silty-sandy matrix (Fig.22). The larger pebbles are widely scattered without any indication of grading. The graded greywacke beds contain no pebbles. The similarity to the combination of lithologies encountered in the bedded mixtites of the southern Damara belt is evident. Discussion

The above alternating mixtite and greywacke beds must have been deposited by two different transport mechanisms, the graded greywackes by turbidity currents, the mixtites by some kind of mass-flow. The base of the graded greywacke beds is slightly irregular, but the top, underneath the

189

Fig.22. Alternating layers of mixtite and graded greywacke. The pebbles in the mixtite are randomly distributed. The mixtite bed at the top is only 10 cm thick. Near confluence of Rio Pecola and Torrente Ceno, Northern Apennines. Oligocene. overlying mixtite, is quite level. Unfortunately no bedding planes that might show sole-marks are exposed. But the fact that the level tops of the turbidite beds have n o t been noticeably disturbed by the pebbly-bearing mass-flow can only be explained by the assumption that the gravity-flow moved slowly. Also, its theological properties must have allowed it to spread in thin sheets, as one of the layers in Fig.22 is only a b o u t 10 cm thick. The conclusion that some mass-flow processes can be very gentle is also indicated by the occurrence, within the underlying large olisthostrome, of two graded sandstone layers, only 0.5 cm thick. These have been slump-folded b u t not disrupted despite being interbedded in a part of the olisthostrome containing pebbles that have diametres which far exceed the thickness of the sandstone layers. A comparable p h e n o m e n o n was in places also found in the 'pebbly schist" sequence. The mixtite layers which alternate with the Oligocene greywacke turbidites show an internal structure. This consists of irregular, lensoid streaks of more sandy material which m a y envelop rolls of the clayey matrix, or of sand mixed with clay and small pebbles. These structures indicate a mass-flow mechanism which is neither laminar nor y e t turbulent enough to form a turbidity current. For this t y p e of mas~flow the term slurry-flow has been proposed (Carter, 1975). According to Lowe (1982) such deposits are formed b y cohesive debris flows in which water-saturated mixtures of

190

mud, sand and clasts move by a kind of laminar flow (shearing). Deposition takes place by "cohesive freezing". Slurry-flow structures have not been observed in the "pebbly schist" formation, but such features cannot be expected to survive tectonic flattening and the formation of a pervasive metamorphic transposition foliation. The conclusion seems justified that the interbedded mixtite--turbidite sequences of the Damara belt and of the Apennines were deposited by similar processes. Differences exist in two respects: The Oligocene occurrence is small, and is associated with a typical olisthostrome, whereas the "pebbly schist" sequence is not associated with such olisthostromes and covers a very great area (Fig.15). It seems obvious that in both cases the slurry-flows and the turbidity currents (and perhaps grain-flows) were derived from different source areas. In the Oligocene sequence the mixtite interbeds are derived from, and probably interfinger with an olisthostrome and are confined to the basal part of an extensive turbidite formation which has a more distant source area. In the "pebbly schist" formation, on the other hand, the volume of the mixtite layers far exceeds that of the intercalated turbidite and (?) grainflow beds. The "pebbly schists" are nowhere associated with typical olisthostrome-like deposits. The comparison shows, not withstanding the differences, that the sedimentological features of the "pebbly schists" can be explained without recourse to a glacial environment. This conclusion makes the hitherto accepted correlation with the Chuos Formation in the Central and Northern zones problematical, because the correlation was largely based on the assumption of a glacial episode. GEOTECTONIC MODELS FOR THE DEPOSITIONAL ENVIRONMENT OF THE "PEBBLY SCHIST" FORMATION

The "pebbly schists" are confined to the Southern Margin Zone (Figs.1 and 15) which is characterized by thrusts and nappe tectonics (Hoffmann, 1981; Miller and Hoffmann, 1981; Kasch, 1983; Geological Survey Republic of South Africa and SW Africa/Namibia, 1980). This part of the later fold belt was during the initial stage of the geosynclinal development (Nosib Group) the site of a continental rift system in which fluviatile arenites and playa-lake evaporites had accumulated (Behr et al., 1981; 1983). The following marine transgression is documented by the dolomites of the overlying Corona Formation which is in turn overlain by the "pebbly schist" sequence. How can the "pebbly schists" be fitted into this geotectonic environment? A model has to explain: (1) the provenance of the polymict clasts, the great extent and volume of the formation, and its great regional variation in thickness; (2) the presence of orthoamphibolite intercalations with an outcrop length of more than 250 km. (1) In most of the outcrops pebbles and boulders of white and grey

191 metaquartzite o u t n u m b e r granite clasts by far. Similar quartzites of preDamara age form large outcrops on the southern foreland of the Damara belt, but are also found in some of the basement nappes of the thrust belt in which the " p e b b l y schist" formation occurs. Clasts of quartzporphyry and amygdaloidal lava are very rare. Similar rocks form extensive outcrops on the foreland, b u t have not been observed in the nappes. The percentage of granitic clasts varies greatly from area to area and between individual mixtite units. Pre-Damara granitic rocks occur both on the southern foreland and in basement nappes. Mixtite containing only granitic clasts was observed in two widely separated places: in the Ondekaremba area (see below), and in the R o t s t o c k South synclinorium (Fig.15), where granite boulders of up to 1.5 m size are e m b e d d e d in a 10--20 m thick zone of mica schist that is similar to the underlying and overlying clast-free schists. The regional distribution of ctast sizes (Fig.15) shows that the largest observed clasts occur near the southern margin and that the sizes diminish basinwards. This observation, combined with the above discussed relationship o f clast sizes with formation thicknesses would be a strong argument for southern source areas. The Ondekaremba mixtite forms an exception in this pattern and would seem to require a different source area. This could have been a renewed uplift o f the northern rim o f the Nosib time rift in which the playa-lake evaporites had accumulated, or a small elevated granitic terrain within this rift zone. The northern flank o f the hypothetical rift m a y subsequently. during the orogeny have become the source of the basement nappes (Martin et al., 1983). (2) The two extensive orthoamphibolite members have a tholeiitic withinplate geochemistry that suggests extrusion under a tensional regime on a continental crust (Miller, 1983). This interpretation favours the assumption that the deposition o f the " p e b b l y schists" was accompanied by renewed rifting. The combination o f these relationships can be explained b y a model assuming that the great volume of heterogeneous clastic material was derived by slumping, activated b y rapid subsidence in a rift zone of the developing geosyncline, and that most of the mixtites originated on the slopes of sizeable, north facing deltas. The material was spread and deposited b y different types o f gravity-flow on overlapping mid-fan slopes. The variation in formation, thickness and maximum clast sizes (Fig.15) suggests five delta areas: the R o t s t o c k South synclinorium; t h e Naos E m b a y m e n t ; the Nauaspoort-Kudis area; the Hatsamas area; the Ondekaremba area. From Hatsamas and Ondekaremba eastwards the formation is hidden b y a cover o f Kalahari sand. This model interprets the " p e b b l y schists" as pre-orogenic gravity-flows triggered by tectonic instability during a stage o f differential subsidence (Martin, 1983; Porada and Wittig, 1983). The superposition o f the voluminous " p e b b l y schists" on the shelf carbonate succession o f the Corona Formation requires n o t only a phase of

192

accelerated subsidence but also a corresponding uplift and denudation of the southern foreland. The fact that in the Naos Embayment carbonate clasts are confined to the basal part of the "pebbly schists" suggests that the uplift locally also affected parts of the shelf area. Extensive foreland areas were covered by fluviatile arenaceous and rudaceous Nosib age platform deposits and by the, perhaps also extensive, Court tillite of ill-defined age. Where uplifted, these deposits may have formed a voluminous, easy-to-erode, pebble-rich source for the "pebbly schists". The assumption of tectonic triggering would favour an approximate contemporaneity with the Chuos mass-flow deposits of the south Central and Northern zones which also occupy the sites of Nosib time rift zones (Martin and Porada, 1977; Porada, 1983). The close stratigraphic association of the "pebbly schists", over most of their outcrop length, with an orthoamphibolite (metatholeiite) formation (Miller, (1983) may support this tectonic interpretation. A widespread instability of deltas and shelves could also have been caused by eustatic sealevel changes connected with large glaciations on other parts of the globe. Under this assumption the "pebbly schists" might be coeval with one of the late Proterozoic glaciations for which palaeomagnetic studies have suggested low latitudes. In this case, mountain glaciers could have contributed morainic material to the deltas or locally reached the sea and formed icebergs. Lone clasts not closely associated with a cohesive debris flow -- e.g. the above mentioned large granite boulders in the Rotstock South s y n c l i n o r i u m - could indeed have been ice-rafted, though a lagstone origin cannot be excluded. A similar environment has been deduced by Cahen and Lepersonne (1981) for the diamictites of the Katanga Supergroup in Zaire, where there is convincing evidence for marine glacial deposition of icerafted debris. The glaciers descended from mountains formed by the 1100-1300 Ma old Kibaran fold belt. It is probably no accident that the coeval (Irumide) "Rehoboth Magmatic Arc" forms the southern foreland of the Damara Belt. It would, therefore, be tempting to correlate the Chuos Formation with the "Grand Conglom~rat" of the Katanga sequence, but the available, rather poorly constrained age estimates indicate a considerably younger age for the Chuos Formation. A different model assuming a synorogenic deposition of the "pebbly schist" sequence was tentatively proposed by Hartnady (1979). His model interprets the "pebbly schists" as a wildflysch facies deposited in the fore deep of a southwards advancing nappe front. The model would neatly explain the geographical position in the Southern Margin Thrust Belt, but faces great difficulties in explaining the stratigraphic relationships along the southern margin of the fold belt. The two models are more fully discussed in a wider context by Martin et al. (1983). A clear decision between the two models seems not possible at present, because telescoping by stacking and interslicing allows no unequivocal recognition of the facing direction of the depositional slopes. The

193

authors favour the first model which regards the western part of the 1100-1300 Ma old Irumide Belt as the main source of the large volume of coarse clastic material. CONCLUSION

The Damara Sequence contains in all its major structural zones mixtites which have been regarded as glacigenic deposits. The oldest mixtite forms the characteristic lithosom of the Varianto Formation which locally caps the lower Damaran Nosib Group. The clasts are of local origin. There is no convincing evidence of glacial activity. The Blaubeker and the Court Formations on the southern foreland of the Damara belt m a y be roughly contemporaneous with the Varianto Formation. For both there is fair evidence for a glacigenic origin. Their age is not well constrained. Stratigraphically younger mixtite containing formations were, because of their supposed glacigenic origin correlated with the Chuos Formation and used as stratigraphic markers for the correlation of the different facies domains o f the geosyncline. A study of the sedimentary features of these mixtites and other associated sediments has led to the following conclusions: (1) No features indicating a glacial origin were observed. Striae on pebble surfaces would have been obliterated by tectonic and metamorphic overprinting, if they ever existed. From the " p e b b l y schists" of the Southern Margin Belt faceted pebbles, embedded in laminated varve-like schists, had been reported. It was f o u n d that the flat facet-like surfaces of m a n y quartzite pebbles were formed by pressure-solution processes, and that the laminae of quartz-rich and biotite-rich material were the result of a metamorphic transposition cleavage. Large isolated clasts that are n o t stratigraphically associated with a debris flow zone m a y indicate ice-rafting. (2) The sedimentary features of the mixtites and other interbedded and interfingering sediments indicate deposition by different types of gravityflow processes: mass-flow, slurry-flow, grain-flow, turbidite-flow. The clasts contained in these deposits are usually of b o t h extrabasinal and intrabasinal origin. Slump structures and other features indicating deformation of soft sediments (schist blobs included in quartzite or vice versa; mixtite blobs in mixtite) have been observed. The mixtites can therefore not be regarded as glacigenic sediments and chronostratigraphic markers. (3) The mixtites which have been correlated under the name "Chuos F o r m a t i o n " could nevertheless have been formed more or less contemporaneously, if the gravity-flows were triggered by a basin-wide stage of differential subsidence, or by world-wide eustatic sealevel changes. Mountain glaciers on the southern foreland could have contributed morainic material to the " p e b b l y schist" formation. (4) Mixtite deposition is in some areas n o t confined to the Chuos Formation proper, b u t began already at lower stratigraphic levels or persisted to higher levels. In these cases the mixtites m a y have a carbonate matrix and may be interbedded in resedimented dolomitic or calcareous sequences.

194

(5) Two models are being discussed for the geotectonic setting of the "pebbly schist" sequence of the Southern Margin Belt. The formation could have been deposited either on mostly north-facing delta slopes in a rift basin during the subsidence stage of the geosyncline, or as a synorogenic wildflysch facies in front of southwards advancing nappe complexes during the orogenic stage. The authors favour the first interpretation. (6) The Chuos Formation could only be used for chronostratigraphic correlations with other Upper l>roterozoic sequences, if its extensive gravityflows have been caused by an eustatic change of the sealevel. The available age estimates make a correlation with the 'Grand Conglom~rat" of the Katanga Supergroup in Zaire improbable. ACKNOWLEDGEMENTS

The present work has greatly profited from discussions and joint excursions with R. Brand, T . N . Clifford, W. Hegenberger, K . H . Hoffmann, K.W. Kasch, J. Klein, R. McG. Miller, K. Schalk, N. Co Thirion, and N. Watson. For guidance in the Apennines we are indebted to K. J. Reutter. The work was funded by the German Research Association as a part of the investigations of the Special Research Group 48 of the University of GSttingen. REFERENCES Behr, H.J., Ahrendt, H., Schmidt, A. and Weber, K., 1981. Saline horizons acting as thrust planes along the southern margin of the Damara Orogen (Namibia/S.W. Africa). Spec. Publ., Geol. Soc. London, 9: 167--172. Behr, H. J., Ahrendt, H., Martin, H., Porada, H., R~hrs, J. and Weber, K., 1983. Sedimentology and mineralogy of upper Proterozoic playa-lake deposits in the Damara Oroge n. In: H. Martin, F. W. Eder (Editors), Intracontinental Fold Belts-- Case Studies in the Variscan Belt of Europe and the Damara Belt of Namibia. Springer, Heidelberg, pp. 755--610. Cahen, L. and Lepersonne, J., 1981. Upper Proterozoic diarnictitesof Shaba (formerly Katanga) and neighbouring region of Zambia. In: M. J. Hambrey and W. B. Harland (Editors), Earth's Pre-Pleistocene Glacial Record. Cambridge Univ. Press, Cambridge, pp. 162--166. Carter, R . M . , 1975. A discussion and classification of subaqueous mass-transport with particular application to grain-flow, slurry-flow and fluxoturbidites. Earth Sci. Rev., 11: 145--177. Clifford, T. N., 1962. Note on nappes in the Otavi facies of northern South-West Africa. Sixth. Annu. Rep. Res. Inst. Afr. Geol., Univ. Leeds, p, 44. De Kock, W.P., 1934. The geology o f the western Rehoboth. South West Aft. Dep. Mines, Mem., 1, 148 pp. De Kock, W.P. and Gevers, T.W., 1933. The Chuos Tillite in the Reh0both and Windhoek Districts, South West Africa. Trans. Geol. Soc. S. Aft., 35: 11--118. Geological Survey Republic of South Africa and South West Africa/Namibia, 1980. Geological Map 1:1,000,000. Government Printer, Pretoria. Gevers, T.W., 1931. An ancient tiUite in South West Africa. Trans. Geol. Soc. S. Aft., 34: 1--17.

195 Gevers, T. W., 1934. The geology of the Windhoek District in South West Africa. Trans. Geol. Soc. S. Afr., 37: 221--251. H~ilbich, I. W., 1977. Structures and tectonics along the southern margin of the Damara mobile belt, South West Africa. Ann. Univ. Stellenbosch, Ser. A1, 2: 149--247. Hartnady, C. J., 1979. Overthrust tectonics, stratigraphic problems and metallogenesis in the Khomas Ridge Province, Damara orogenic belt. 16th Annu. Rep. for 1978. Precamhrian Res. Unit. Univ. Cape Town, pp. 73--89. Hedherg, R . M . , 1979. Stratigraphy o f the Ovamboland basin South West Africa. Precambrian Res. Unit. Univ. Cape Town, Bull., 24, 325 pp. Hegenberger, W., 1980. The Geology o f the Gobabis Area. Expl. Sheet 2218 (Gobabis). Geol. Surv. South West Afr./Namibia, 11 pp. Hoffmann, K. H., 1981. Structural and stratigraphic relationships of overthrust basement rocks in the Oamites area, southern Damara margin, South West Africa/Namibia. Abstr. Vol. Geol. Soc. S. Afr., Geol. Congr. 1981, p. 113. Jacob, R. E., 1974. Geology and metamorphic petrology of part of the Damara Orogen along the lower Swakop River, South West Africa. Precambrian Res. Unit, Univ. Cape Town, Bull., 1 7 , 2 0 1 pp. Kasch, K. W., 1983. The structural geology, metamorphic petrology and tectonothermal evolution of the southern Damara Belt around Omitara. Precamhrian Res. Unit, Univ. Cape Town, Bull., 1 7 , 3 3 3 pp. Key, R. M. and Rundle, C. C., 1981. The regional significance of new isotopic ages from Precambrian windows through the "Kalahari Beds" in northwestern Botswana. Trans. Geol. Soc. S. Aft., 34: 51--66. KrSner, A., 1981. Late Precambrian diamictites o f South Africa and Namibia. In: M. Hambrey and W.B. Harland (Editors), Earth's Pre-Pleistocene Glacial Record. Cambridge Univ. Press, Cambridge, pp. 167--177. KrSner, A., 1982. Rb--Sr geochronology and tectonic evolution of the Pan-African Damara belt of Namibia, southwestern Africa. Am. J. Sci., 282: 1471--1507. KrSner, A. and Rankama, K., 1972. Late Precambrian glaciogenic sedimentary rocks in Southern Africa: a compilation with definitions and correlations. Precambrian Res. Unit, Univ. Cape Town, Bull., 11: 1--37. LeRoex, H.D., 1941. A tillite in the Otavi Mountains, S.W.A. Trans. Geol. Soc. S. Aft., 44: 207--218. Lowe,. D. R., 1982. Sediment gravity flows: II. Depositional models with special reference to the deposits of high-density turbidity currents. J. Sediment. Petrol., 52: 279--297. Martin, H., 1965. The Precambrian geology of South West Africa and Namaqualand. Precambrian Res. Unit, Univ. Cape Town, 159 pp. Martin, H., 1983. Overview of the geosynclinal structural and metamorphic development of the intracontinental branch of the Damara Orogen. In: H. Martin and F. W. Eder (Editors), Intracontinental Fold Belts - - Case Studies in the Variscan Belt of Europe and the Damara Belt in Namibi~ Springer, Heidelberg, pp. 473--502. Martin, H. and Porada, H., 1977. The intracratonic branch of the Damara Orogen in South West Africa. I. Discussion o f geodynamic models. Precambrian Res. Unit, Univ. Cape Town, Bull., 5: 311--338. Martin, H., Porada, H. and Wittig, R., 1983. The r o o t zone of the Naukluft Nappe Complex: Geodynamic implications. In: H. Martin and F. W. Eder (Editors), Intracontinental Fold B e l t s - Case Studies in the Variscan Belt of Europe and the Damara Belt in Namibia. Springer, Heidelberg, pp. 679--698. Miller, R. McG., 1983. Tectonic implications of the contrasting geochemistry of Damaran mafic volcanic rocks, South West Africa/Namibia. In: R. McG. Miller (Editor), Geodynamic Evolution of the Damara Orogen. Geol. Soc. S. Aft., Spec. Publ., 11: 115-138. Miller, R. McG. and Hoffmann, K. H., 1981. Guide to the excursion through the Damara Orogen. Geology o f the Damara Belt. Geol. Soc. S. Aft., 103 pp.

196

Porada, H., 1983. Geodynamic model for the geosynclinal development of the Damara Orogen, Namibia, South West Africa. In: H. Martin and F. W. Eder (Editors), Intracontinental F o l d Belts - - Case Studies in the Variscan Belt of Europe and the Damara Belt in Namibia. Springer, Heidelberg, pp. 503--540. Porada, H. and Wittig, R., 1976. Das Chausib-Turbiditbecken am Stidrand des Damara Orogens. Si~dwest-Afrika. Geol. Rundsch., 65: 102--119. Porada, H. and Wittig, R., 1977. Entwicklung der basalen Damana-Folge am Abbabis Inlier (Siidwest-Afrika) und Uberlegungen zur Geodynamik dieses Bereiches. Neues Jahrb. Geol. Pal~ontoh Monatsh., 8: 475--502. Porada, H. and Wittig, R., 1983. Turbidites in the Damara Orogen. In: H. Martin and F . W . Eder (Editors), Intracontinental F o l d B e l t s - Case Studies in the Variscan Belt of Europe and the Damara Belt in Namibia. Springer, Heidelberg, pp. 543--576. Schalk, K., 1970. Some late Precambrian formations in central South West Africa. Ann. Geol. Surv. S. Afr., 2: 29--40. Schermerhorn, L. J. G., 1974. Late Precambrian mixtites: glacial and/or nonglacial? Am. J. Sci., 274: 673--824. Schermerhorn, L. J. G., 1975. Tectonic framework of late Precambrian supposed glacials. In: A. E. Wright and E. Moseley (Editors), Ice ages, Ancient and Modern. Geol. J., 6: 241--274. Smith, D. A. M., 1965. The geology of the area around the Khan and Swakop Rivers in South West Africa. Geol. Surv. S. Afr., (South West Afr. Ser.), Mere. 3, 113 pp. South African Committee for Stratigraphy, 1980. Stratigraphy of South Africa, Part 1. Handbook 8. Geol. Surv. Rep. S. Aft., Government Printer, Pretoria, 690 pp. Winkler, H.G.F., 1974. Petrogenesis of Metamorphic Rocks. Springer, Heidelberg, 320 pp.