THE SE AFRICA TRIPLE JUNCTION AND THE DRIFT OF MADAGASCAR

Journal of Petroleum Geology, 7, 4, pp, 403-418 403

THE SE AFRICA TRIPLE JUNCTIQN AND THE DRIFT OF MADAGASCAR

Giovanni Flares*

In a paper published in 1970, the Author expressed the view that the Mozambique Channel originated as Madagascar drifted to the NE from the Mozambique-Natal coast. A further analysis of that view in the light of recently acquired data now enables him to formulate the hypothesis set out in this paper, namely that:

(a). The geometry and tectonics of the Lebombo-Zoutspansberg-Buzi elongated fractures accompanied by Jurassic and Cretaceous effusives define a triple junction similar to the present-day Red Sea-Afar-Gulf of Aden triple junction;

(b). A n initial rifting of the Channel occurred as the swell originating the triple junction produced a stretching of the continental crust and the initial separation of the Indo- Malgache Landmass from Africa along the two eastern branches of the triple junction, while the middle one (Zoutspansberg) was being reduced to an abortive rift; and

(c). Only when the thinned continental crust was stretched to the point of rupture did the oceanization process set in, with the production of “classical” spreading centres and magnetic anomalies.

The latter process probably began in the Cretaceous, and was accompanied by impressive flows of alkaline lavas on the continental side and basalts in the oceanic area.

Considering that, ifthis hypothesis is correct, the proto-Channel in Early Jurassic time was a narrow seaway, as suggested also by the juxtaposition of the major Mozambique and Madagascar fault lineaments; and considering that Madagascar has numerous shows and seepages of petroleum, probably of Jurassic age, the hypothesis discussed may have considerable bearing on the petroleum possibilities of the deeper Mozambican, Jurassicsection, whose existence is indicated by several of the offshore seismic profiles recently run. Any oil generated in that section is likely to have migrated up-dip on both sides of the proto-Channel.

INTRODUCTION

The paleoposition of Madagascar is still a controversial subject in spite of the considerable amount of data recently acquired by geological and geophysical investigations in the Mozambican Channel and adjacent areas.

Students of the problem are divided into three groups: 1. Those who advocate a NE drift of the island from an original position close to

Mozambique;

Note added in proof stage: The Author is a ware that an increasing amount of geophysical data are in favour of a SE ward drift of Madagascar. Nevertheless, as no unequivocalans wer has been formulated as yet, the hypothesis discussed in the presentpaper is submitted as food for thought, and as an additional element for further discussion.

* Geological Consultant, 3 7 via Codacci 50023 Impruneta, Florence, Italy.

404 The SE Africa triple junction

2. Those who maintain that Madagacar drifted to the SE from the Kenya-Tanzania coast; 3. Those who believe that Madagascar has always been located essentially where it is now. In a previous paper (mores, 1970) it was submitted that the Mozambique Channel was

formed as Madagascar drifted to the east and NE from a position adjacent to Mozambique. This view, based on field observations on both sides of the Channel, stratigraphic correlations and subsurface data, is opposed to the View that Madagascar drifted to the SE from a position adjacent to the Kenya-Tanzania coast (e.g. Rabinowitz etal., 1983), as well as to the belief in the permanence of Madagascar in its present position throughout time (e.g. Kamen-Kaye, 1982).

Since the publication of the Author’s 1970 paper, several geophysical and geological investigations have been carried out in the Mozambique Channel and surrounding areas. They consist of seismic refraction and reflection lines, magnetic traverses by shipborne and airborne magnetometer, and the drilling of nine DSDP holes along Leg 25 of that Project.

Although the results of these investigations added valuable information to the knowledge of this part of the world, no unequivocal answer to the paleoposition of Madagascar was obtained. The various data resulting are graphically summarized in Fig. 5 and indicate the following:

(i). The main, meridionally-trending Fracture Zone with magnetic anomalies is interpreted as a spreading centre younger than Jurassic, with an age of about 150 million years (Segoufin, 1978).

(ii). The westernmost, weakly magnetic Mozambique Ridge is interpreted as an extinct spreading centre (Green, 1972).

(iii). The “weakly seismic and almost non-magnetic Davie Ridge seems.. . to be composed of compacted chalk and not crystalline rocks as expected” (Simpson, on Leg 25 in Geotimes, (1978)).

(iv). A Mesozoic sequence of magnetic anomalies found to the north of Madagascar (about 121 to 153 million years old, not shown in the diagrams in this paper) is symmetric to a ridge trending about east-west and normal to the Davie Ridge. This situation was interpreted by Rabinowitz et al., (1983) as supporting the theory of a SE drift of Madagascar. However, these authors state that.. . “more magnetic data is required to confirm this and to determine if oblique sea-floor spreading occurred”. They also state that Madagascar could not have occupied a position adjacent to the Kenya-Tanzania coast in post-Jurassic times.

(v). DSDP Holes 242,243/244,248 and 249 (Fig. 9) have a bearing on the paleoposition problem. Holes 242 and 243/244 did not reach the basalt acoustic basement.

DSDP hole 248 found porphyritic basalts at 422m, overlain by the Middle Paleocene. Eocene, Miocene and Pliocene are present between the Paleocene and the Pleistocene. DSDP Hole 249, “situated on a small, sediment-filled basin near the crest of the Mozambique Ridge” (Simpson, op. cit.) found Early Cretaceous vesicular amygdaloidal glassy basalt overlain by volcanic claystone and clayey siltstone of Neocomian age (Vallier, 1974). In this hole, there is an unconformity between the Late Cretaceous and the Middle Miocene, while the Oligocene is also missing at site 248. An interesting observation by Simpson was that both holes show sections with turbiditic deposition, from the Early Cretaceous interval at site 249 which shows “lamination, cross-bedding and occasional graded bedding”, to the Tertiary section overlying the basalt at site 248, which is “dominantly terrigenous turbidite”. This point is discussed further in this paper.

While the shipboard Scientific Party of Leg 25 do not take definite sides on the paleoposition problem, they nevertheless offer the following considerations:

“Most of the authors concerned” (with the paleoposition of Madagascar) “have displayed their unfamiliarity with (or have found it convenient to disregard) the significance of both the similarities and differences between the structure and geological history of eastern Africa and western Madagascar” (Leg 25, Cruise Synthesis, 1974, p. 139).

In addition to the published data briefly reviewed above, unpublished data were made available by the Mozambican Secretariat for Coal and Hydrocarbons (SECH) from the

G. Flores 405

-30'

0

I 30'

I 35' I

..... Jjj. \ ..... ...:.:.:.:.:. .:... . . . . .

100 ZOO 300 400 h m - 0 100 zoo MI

166) K l A r age m y

0 Maior f a u l t , Mozambique

/ Malor f a u l t , Madagascar

I I Fig. 1. Lower Jurassic. The Mozambique Channel opening that had started in the Permian (Flores, 1972) is still underlain by thinned continental crust. Note K/Ar determinations of 166 MM years for Lebombo and Zambezi Graben (Lupata Gorge) basalts. Also note the statistically high parallelism of

Madagascar and Mozambique major fault lineaments.


v 2

Fig. 2. Genetic model of evolution of an aulacogen (sl. mod. after Hoffman et al., 1974). In the case discussed here: the rift to the west is the abortive Zoutspansberg Rift; the active rift arms of (2) are, from south to north, the Lebombo and Buzi; the opening ocean of

(3) is the Mozambican Channel area.

offshore geophysical surveys which were recently carried out along the Mozambican coast by GECO of Norway, and by Western Geophysical Company on behalf of SECH. Other important data consist of the information from offshore drilling in Mozambique, particularly on the K/Ar age determinations of the basalts into which some of the wells were bottomed (Table 1).

After a further analysis of the Mozambique tectonic framework submitted in 1973 (Flores, 1973) and a study of the more recent data, the formulation of the following hypothesis was arrived at.

THE TRIPLE JUNCTION HYPOTHESIS

The Permo-Triassic opening of the Mozambique Channel continued in the Early Jurassic and developed along a sequence of events similar to the more recent, and still active, evolution of the Red Sea-Afar-Gulf of Aden.

According to this uniformitarian view, the Lebombo-Zoutspansberg-Buzi fracture system in Mozambique-Zimbabwe, along which Early Jurassic basalts and extremely abundant Cretaceous alkali-basalts issued (Fig. 1) would represent a triple junction analogous to the Red Sea-Afar-Gulf of Aden triple junction (Fig. 3, slightly modified after Lowell et al., 1972). Accordingly, the present-day relative movement of the Arabian Peninsula is considered to be

No.

1

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

Name

2

Balano 1

Domo 1

Zandamela 1

S.Massinga 1

Mambone 1

Nhamura 1

Nhachangue 1

NE Palmeira 1

NW Macia 1

Funhalouro 1

Nemo lx

Sunray I IA

Sunray 2 IA

Sunray 3

Sunray 4 IB

Sunray 7

Sunray 12 I

G. FIores

Table 1. Mozambican wells that encountered volcanicso

407

Elevation (sea level)

3

-2,861 1

-2,980.9

-1,770.2

-4,169 .O

-3,247.9

-5,140.1

-5,441.3

-4,479.6

-4,441.2

-2 * 122 .o

-4,054.1

-3,709.4

-3,507 .O

-1,105.2

-3,552.7

-1,325.9

-2,595.7

-2,159 -8

Thickness Radiometric (m) age, MM yrs.

4 5

256.1

57.0

926.0

27.4

353.6

301.2

27.4

46.9

13.7

152.2

49.7

396.3

38.4

-

K/Ar - 137.0

-

- -

(Karroo?)

(Precambrian)

- K/Ar - 285.6214.3

-

- -

K/Ar - 129.6z4.0

Rock type

6

Basalt

Olivine, basalt

Basalt, rhyolites

Basalt

Basalt, alkali basalt

Dolerite

Dolerite

Olivine basalt

Basalt

Basalt

Weathered basalt,tuffs

Tuffs,alkali basalt

Basalt

118.8 K/Ar - a)116.8+1.8 Weathered basalt b)8.47+1. o( ?

28.0 - Weathered basalt

19.5 K/Ar - 74.2t3.0 Basalt

463.3 - Basalt

282.8 K/Ar - 18.1:0.3 Tuffs, basalt

(*I Courtesy of SECH. For wellposition, refer to Fig. 7. Well NE Palmeira 1 at 4441.2m below sea level may have reached a Paleozoic igneous rock, if the KIAr determination of 285.6 f 14.3 M M y is valid.

analogous to the movement of the Indo-Malgache Landmass relative to the African coast. This latter movement probably occurred mainly along the major Pebane Transcurrent Fault Zone* (Figs. 1 and 5) .

* The inference for the PTFZ rests on the observed relations of the basement outcrop edge with the sedimentaries to thesouth of it; on the presence ofparallel, NE-trending basementfaults to the north of it and on the steep gravity gradient to the south. Linear geometry of sea-bottom along the NEprojection of the PTFZ tends to support the assumed transcurrent movement.


Fig. 3. Pre-rift restoration of the Gulf of Aden and Red Sea (sl. mod. after Lowell et al., 1972). Note the parellelism of the heavy dashed lines defining approximate limits of rifting. Arrows and accompanying values in km indicate relative directions and amounts of displacement. Normal faults bounding the Red Sea are parallel with coast-

lines. Compare with Fig. 1.

0 5 0 0 4 I

k m

DISCUSSION

To quote Hoffman et al. 1972): “It is hypothesized that 6 eep-mantle convective plumes produce three-armed radial rift

systems (“rrr triple junctions”) in continents stationary with respect to the plumes. If only two of the arms spread to produce an ocean basin, the third remains as an abandoned rift extending into the continental interior from a re-entrant on the new continental margin.. . .Inasmuch as new continental margins are predestined to become geosynclines, such abandoned right arms are juvenile aulacogens”.

Fig. 2, modified from Hoffman et al. schematically portrays this situation. Comparing this figure with Fig. 1 the following deductions are made:

The western arm of the Triple Junction is represented in our case by the Zoutspansberg “aborted rift”.

G. Flores 409

CARBONATE BUILDUPS SALT ’ ’ _ ’ ”

0 100 I 1

k m L I

Fig. 4. Diagrammatic representation of latest 3.5 MM year-to-present stages of the Red Sea structural evolution (adapted and reduced from Lowell etal., 1972), showing final crustal breaching and incipient oceanization. A similar development is hypothesized for the Mozambique Channel, Figs 1 and 5.

~ ~ ~ _ _ _ _ _ _ ~ _ _ _ _ _ ~ ~

The Zoutspansberg-Buzi abortive rift was previously referred to as the “Limpopo Rift” by Burke and Bewey (1973). They conclude that “the Limpopo Rift ceased to function without spreading”. These authors were also the first to suggest the existence of a triple junction here, and that the rifts associated with it contributed to the movement of Madagascar. Stage 1

Pre-Cretaceous. The NE and SE arms of the triple junction began to spread. The eastern landmass was moving eastward as the continental crust was thinning and stretching. Lowell et al. ’sdiagrammatic sequential structural model of the Southern Red Sea (Fig. 4) can be applied to the much older Mozambique-Madagascar case without undue difficulty: the tectonic pattern of rifting and faulting on either side of the opening Channel is exemplified by the grabens produced in Mozambique and by the Madagascar fault pattern. This latter, as shown in Fig. 1, is parallel and complementary to that of Mozambique, once Madagascar is brought into a position adjacent to Mozambique, at basement outline level. Stage 2

Cretaceous: opening ocean. At this point, the continental crust was broken through stretching, and oceanic crust was produced through the “classical” spreading-cum-magnetic anomalies pattern. Note, in this context, the (extinct) Mozambique Ridge and the 150 MM year old Mozambique Fracture Zone (Figs 5 and 10). At this stage (Lower Cretaceous), large amounts of anorthoclase basalts and alkaline lavas were poured from the Lebombo and Buzi fissures and in the recently-opened or incipient Zambezi Graben, and basalts in the oceanized areas. The Zambezi alkali-basalts of the Lupata section* and those of the Lebombo range have ages ranging from 137 to 110 MM years; the basalts found in two of the offshore wells drilled off the southern coast (Sunray 1 and 2, Figs 5 and 7, Table 1) have ages of 129 and 116 MM years, and those found at site 249 are of “Early Cretaceous age” (on the Mozambique Ridge)?. The outpouring of alkaline magmas to the west and of basalts to the east is in accordance with the inferred presence of thinned crust to the west and newly-formed oceanic crust to the east, as one would expect in the case of a spreading, subsequent in time to the crustal thinning.

The discovery that the Mozambique fracture zone is associated with symmetrical magnetic anomalies numbered from 0 to 22 (Segoufin, op. cit.) which are 150 MM years old and which

*For details of the Zambezi Graben, Lupata section, see Flores, 1964.

t There are two additional Sunray wells (Fig. 7) drilled along the south coast to the N E of those mentioned above. Thebasalts into which they were bottomedhaveagesof 74 and 18 MM yearsrespectively from west to east (Sunray 4 and 12). This appears to be in accordance with the early suggestion (Flores, 1973) that the igneous flows underlying the Mozambique “bulge” issued from fractures of ages decreasing from west to east.


Precambrian basem. Mar ine sediments C o n t i n e n t a l sed. 0 Volcanic8

Oil or bitumen Madagascar DSPD H o i e , L e ~ 2 5 K l A r age Of G a s ,/ faults * (approx.position) )* igneous rocks ' seeps Mozambique 0 tauits

Fig. 5. Cretaceous: oceanization stage. The Mozambique Ridge and the Mozambique Fracture Zone are interpreted as NE migrating spreading centres. Note the severe rifting in Mozambique, that is to continue into the Tertiary, as the southernmost extension of the Rift Valley System (Flores, 1973).

are present to the east of the now inactive Mozambique Ridge, is suggestive of an eastward migration of the spreading centre with time. The trend of the Hinge Line and of the Pebane Transcurrent Fault Zone, parallel to the Buzi trend (NE branch of the triple junction) could be an indication of the existence of still-undiscovered transforms that would be responsible for the eastward migration of the spreading centre, in Cretaceous and later times. Some support for movement in this direction is afforded by the NE projection of the Hinge Line as shown by the offshore seismic survey and portrayed in Figs 5 and 11.

411

b

GENERALIZED WELL INFORMATION

W e l l : N I H A M U R A 1 D a t e : S e p t 7 1 l M a r c h 72

Operator : AOUITAIN€ E l e v . KB: 68'

D e p t h b e l o w K E

1000 f t 100 n

1 - *

2 - : 5 .

-110

a-1 .

5-:15

6 -' .20

7 -1

"125

9 -. I

10 -?O

- ll-:' - 3 5

9 1%:

13--40

14-.

-45 1s - - 16 -_ 17-.

-50

c

' 8 - -5s

G. Flores

l ithology

92 0

1885' lgaOGRUDJA F M

CHERINGOMA FM

-.- I - -'-I

W.D. : onshore

T . D . : 18O1Op

Location 18 '06 4 5

35'42 38

S E N A FM

. . . . . . . , . . . . . . . . . . . . El . . . . . . . . . . . . . . . . . ,

to910

U LUPATA FM

CC 1 3 5 2 0

L L U P A T A FM

1 6 9 3 2 K A R R O O

17920 'BASEMENT T D 18010'

c o n g l o m

a s a n d s t o n e

s l l t s t o n e

s h a l e

= m a r l

mi i m e s t o n e

= d o l o m i t e

LvIlyvl v o i c a n i c s

Fig. 6. The Nhamura 1 well section showing the Lower Jurassic (?) interval. (Courtesy of SECH, Secretaria do Estado Carvao e Hidrocarbonetos, Maputo, Mozambique). Age determination is

questionable; rock involved is of marine environment.


In conclusion, it is suggested that there were two main stages of the Mozambique Channel opening and the contemporaneous Madagascar drift. The first stage was connected with crustal stretching in the area between the NE and SE arms of the Lebombo-Zoutspansberg-Buzi triple junction. The second stage was characterized by the oceanization of the rift bottom, between Mozambique and the Indo-Malgache Landmass, once the continental crust was stretched to the point of rupture. This second stage is believed to have begun in the Lower Cretaceous and to have been accompanied by basalt intrusion in the Channel and by alkaline flows on the mainland. A situation of this type, incidentally, is not dissimilar to the sequence of events that occurred as the South American continent was drifting away from West Africa at the latitude of the Benue Trough. This similarity, suggested by Hoffman et al. for the Benue Trough and the Red Sea, is here also considered as valid in the case under consideration.

PETROLEUM GEOLOGY

If we assume that the hypothesis discussed above is correct in relationship to the available evidence, we can also assume that at one stage of rifting a narrow sea-channel was developed between Mozambique and Madagascar. In his careful analysis of the stratigraphy of western Madagascar and eastern Mozambique, Kamen Kaye (1982) submitted that the present-day Mozambique Channel represents an “open-ended, non-compressional NNE-SSW downwarp, 1,700 km long” that he interprets as a geosyncline. He believes that no horizontal movement has affected Madagascar since the geosynclinal condition was initiated. The data discussed in the present study, however, do not appear to agree with the permanence of Madagascar in its present position through time, and would, instead, indicate a progressive opening of the Channel, from the Early Jurassic onward.

Madagascar has abundant surface and subsurface oil seepages and “shows”; while Mozambique has at least three known gasfields, traces of oil in several wells, at least one gas seepage and one oil seepage*.

The question to be resolved is whether petroleum was generated in the Channel at one stage and distributed by migration on both sides of it. The situation shown in Fig. 1 suggests the possible existence of naphthogenic conditions in the narrow seaway, rather than if the Channel is assumed to have been a broad geosyncline since Karroo time.

As to sources, it is doubtful whether the Karroo sequence per se as it is at present known in the outcrops and subsurface could represent a source of hydrocarbons, except perhaps in the south of the Channel, where some suggestion of lacustrine and/or lagoonal facies in the Ecca- Sakamena (i.e. in the Natal-southern Morondova Basin) is present (Flores, 1970). In any event, the organic matter of the terminal Karroo is mostly of a vegetal type; and according to the hypothesis here discussed it was only in the immediately post-Karroo Jurassic that true marine, if restricted, conditions were established in the proto-Channel. Marine Jurassic (?) was reportedly reached in only one deep test in Mozambique - Nhamura 1 drilled at some 100 km upstream and on the NE side of the Zambezi River (Figs 6 and 7). It is here 214m thick, and underlain by basalt. Several other wells drilled in Mozambique were bottomed in basalt or other igneous rocks, on the assumption that they represented the top of the Karroo. As shown in Table 1, however, later K/Ar determinations showed this assumption to be correct in only a few cases.

Several seismic sections in the offshore indicate the existence of a sedimentary section below the known Lower Cretaceous, that could be ascribed to the Jurassic. On the Madagascar side, the marine Jurassic was found in some of the more than 40 wells drilled. The surface bitumen seepage in the Majunga Basin, significantly occurs in marine post-Karroo Jurassic shales, whereas all the seepages in the Morondova Basin are found within the Early Jurassic, terminal

* The oilseepage actually occurs at the mouth of the Rovuma River on the Tanzanian side at the N E end of the international boundary but still within the Northern Mozambique Sedimentary Basin, which extends across the Rovuma River.

G. Flores 413

3 1 E 3 4' 3 6' 3 8' I I I

4

I

Fig. 7. Location of major exploratory tests, Mozambique. (Courtesy of SECH).


Karroo beds. In the subsurface, over 30 wells had hydrocarbons shows, mostly in the Morondova Basin. In both Mozambique and Madagascar, on the other hand, gas shows and production are found in Upper Cretaceous sands. In Mozambique, Middle-Upper Cretaceous source rocks were identified in the Domo black shales that appear to occur in a narrow rift in southern Mozambique (Fig. 8). This formation does not appear to be present to the north of about 20’12’ S. Its north-south extension is almost coincident with the assumed narrow sea channel of Fig. 1. A 1952 unpublished report of the American Museum of Natural History in New York, on the depositional environment of the Domo shales (that are known only in the subsurface) reads:

“. . .stagnant conditions prevailed” (in the Domo) “because oxygenated ocean currents and surface waters could not flush-out the oxygen-deficient basin waters due to density differences (either thermal or salinity). The basin floors must have been too far below sea- level to be disturbed by wave action.. . . In conclusion, a moderately deep marine basin, perhaps 1,000 feet or more in depth, far from a source of clastic sediments, with a low oxygen content and ready access to the open sea is indicated as the depositional environment” (for the Domo shales). And, further, “. . . as large negative areas of the sea-floor were gradually filling with sediments, ocean currents flowed and flushed out the stagnant basin waters”. The above sequence of events is reflected in the sections of the Mozambique coastal wells that

penetrated the Domo formation. The “large negative areas of the sea-floor” do represent grabens, and the recent seismic surveys confirmed the early assumption that the Domo was deposited in a graben. The Domo shales, in a later source rocks study (unpublished), were considered to be “gas-prone in the near-shore area, oil-prone if buried deeper”. The pyrolysis of Domo 1 samples showed an S2 value of 0.08-0.47 kg/ton, which indicates poor-to-good oil potential. An “oil window” would be present at about 9,000 ft, according to known geothermal gradients. The Domo, having been deposited in a graben, indicates the persistence of elongated negative areas of the sea bottom into the Cenomanian-Turonian, the precise conditions that we have assumed to have begun at the time of early rifting. The section underlying the Domo is a Lower Cretaceous marine sandstone, the Maputo formation, that is found also in outcrops to the west of the city of Maputo lying directly above the Lower Cretaceous rhyolites (aged 137 MM years). The suggestion that the Maputo sandstone in the subsurface could be turbidites filling the early rift finds support in the findings of DSDP Hole 249 (see Introduction), at the Early Cretaceous level. The section overlying the Domo in the wells is the Grudja sand-shale sequence, in which the gas reserves of the three Mozambique gasfields Pande, Buziand Temane have accumulated. It might be speculated that the section filling the “Domo Graben” could be, from bottom to top: Jurassic shales (?); Lower Cretaceous (Maputo) turbidites; Domo shales; Grudja sand-shales. Small oil-shows are noted in the Grudja and in a sandy interval of the Domo formation in most wells, and it is generally believed that the Domo shales were the source of both the gas and the oil, depending on their depth of burial.

No information is available to the Author on the Upper Cretaceous of Madagascar where gas is reported to have been found in the offshore well Eponge 1 (Fig. 5) off the southern part of the Morondova Basin.

Since the largest outcrop of bituminous sands in Madagascar, the Bemolanga tar-sands, occurs within the terminal Karroo sandstones (Isalo) which is a widespread continental- fluviatile deposit equivalent to the South African Stormberg “Cave Sandstone” of Early Jurassic-Late Triassic age, a source older than the Cretaceous one discussed above must be sought for the bitumen* at Bemolanga, especially since the reservoir sands lie directly on the crystalline basement, suggesting migration into the Isalo from a westerly source. Marine Lower Jurassic appears to be the most plausible source, particularly if we accept the theory of a narrow seaway existing at that time. If this is true, hydrocarbons should be expected to have also accumulated on the Mozambique side of the proto-Channel at Jurassic level. * The bitumen at Bemolanga is generally considered to be a polymerized petroleum.

.-__I_.--

G. Flores 415

/*s 19'

& DOMO Fm STRUCTURE CONTOUR I kF ( I N T E R P R E T E D )

CONTOUR INTERVAL 5 0 0 Ft BEIFA

2 2' \\\\\';::;::. A0 4

0.

....... \ , ~ ~ " O

\ ... ... ... ...... ... ... .. .. .. ... .. k:; ..

A .... .:.

...

~~ ... . . .* .

... ... ... ... ... ... ... .... ... .... .... ... ........ .....

AREA DROBAELY

D E E P E R THAN 750b'

0 4 0 8 0

k m

.... .... 2 5' 1

Fig. 8. Structure Map. Domo fm. black shales. Compare with Fig. 1.


0'

2oa

Fig. 9. Location of DSDP Holes, Mozambique Channel.

G. Flores 417

Fig. 10. Magnetic anomalies across the Mozambique Fracture Zone (after Segoufin, 1978). Note displacement at Anomaly 10N level, which suggests a NE movement.

The recent offshore seismic survey, as already mentioned, shows the occurrence of a pre- Cretaceous sedimentary section above the basal “volcanics” at several places, and this should represent a major objective of petroleum exploration operations in this area.

ACKNOWLEDGEMENTS The author is grateful to the Mozambican Secretariat for Coal and Hydrocarbons for

allowing him to have access to geological and geophysical information in their files in Maputo.

REFERENCES BURKE, K. and DEWEY, J. F., 1973. Plume - generated triplejunction. Key indicators in applying plate

FLORES, G., 1964. On the age of the Lupata rocks, Lower Zambezi River, Mozambique. Trans. Geol.

-, 1970. Suggested origin of the Mozambique Channel. Trans. Geol. SOC. S. Afr., LXXIII. -, 1973,The Cretaceous andTertiary basinsof Mozambique and Zululand. Ass. Afr. Geol. Surveys. In:

GREEN, A. G., 1972. Seafloor spreading in the Mozambique Channel. Nature Physical Sc., 236. HOFFMAN, P., DEWEY, J. F. and BURKE, K., 1974. Aulacogens and their genetic relation to

geosynclines, with a Proterozoic example from Great Slave Lake, Canada. SEPM Special Publ., 19. W E N - K A Y E , M., 1982. Mozambique-Madagascar geosyncline, I: deposition and architecture.

Journ. Petrol. Geol., 5 , (1).

tectonics to old rocks. Journ. Geology, 406-433.

SOC. S. A fr., LXVII.

Symposium of East Afr. Coast. Basins, Paris, 1973.


STRUCTURAL ELEMENTS Portly o f t r Flora6 1973

100 km -

Fig. 11. Major structural elements, southern Mozambique. Note the NE continuity of the Hinge Line and the offshore development of rifting as revealed by recent seismic work. (The offshore part of this

figure is the result of the seismic interpretation by Exploration Consultants Ltd.).

LOWELL, J. D. and GENIK, J., 1972. Sea-floor spreading and structural evolution of Southern Red Sea.

RABINOWITZ, P. D., COFFIN, M. F. and FALVEY, D., 1983. The separation of Madagascar and

SEGOUFIN, J., 1978. Anomalies magn6tiques MCsoziques dans le bassin de Mozambique. C. R. Ac. SC.

SHIPBOARD SCIENTIFIC PARTY, 1974. Leg 25, DSDP Initial Report, 25, Washington DC. p. 139. SIMPSON, E. S. W., 1978. DSDP LEG 25, Preliminary report, Geotimes, June 1978. VALLIER, T. L., 1974. Volcanogenic sediments and their relation to landmass volcanism and sea-floor-

- AAPG Bull., 56 ( 2 ) .

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THE SE AFRICA TRIPLE JUNCTION AND THE DRIFT OF MADAGASCAR