geology and regional gravity traverses of the nuba mountains, kordofan province, sudan

Journal of African Earth Sciences, Vol. 5, No. 4, pp. 32%338, 1986 0731-7247/86 $3.00 + 0.00 Printed in Great Britain Pergamon Journals Ltd.

Geology and regional gravity traverses of the Nuba Mountains, Kordofan Province, Sudan

ABDEL ATI SADIG

Department of Geology, University of Khartoum, Sudan

and

J. R. VAIL

Department of Geology, Portsmouth Polytechnic, U.K.

(Received 1 May 1985)

Abstract--The Nuba Mountains region comprises an inlier of Basement complex rocks surrounded by a cover of younger, unfolded sediments. Quartzo-feldspathic gneisses and migmatites, a Metasedimentary unit, and an assemblage of basic volcanics, sediments and ultramafic rocks are all intruded by syn- to late-orogenic batholithic granitoids. Anorogenic alkali syenite ring-complexes pierce the Basement units. Many of the latter are fault bounded, with at least two major N-S thrusts separating gneisses, metasediments and metavolcanics.

Two gravity traverses north and west from Kadugli suggest the western region to be underlain by thinned crust, and the gravity field to be characterized by a long wavelength gravity low upon which are superimposed short wavelength anomalies. This pattern is interpreted as a deep-seated batholith with roof pendants of gneisses and steep sided cupolas rising from it. The shallow nature of some of the metavolcanics and metasediments, indicated by the gravity work, supports the possibility of major thrust faulting.

INTRODUCTION

THE PREFACE to Bulletin 23 of the Sudan Department of Geological and Mineral Resources stated the hope that the stage was set for more detailed geological mapping and mineral exploration in the Nuba Mountains. That has been fulfilled in the decade since the first description of the area was published and much new geological and geophysical work has been completed. The present contribution summarizes a new interpretation of the regional geology and presents details of two gravity traverses in the western sector.

The Nuba Mountains are a distinct physiographic region situated almost at the centre of the Sudan, west of the White Nile river. Geologically they form an inlier of metamorphic and plutonic Basement complex rocks which are completely surrounded by the Mesozoic and Cainozoic sedimentary cover. In recent years considerable interest has been shown in the sedimentary basins surrounding these Basement outcrops in the search for petroleum accumulations in the cover rocks, and con- sequently geophysical traverses have been made up to and across the Nuba Mountains (see for example Van Overmeeren 1981, Browne and Fairhead 1983, Mohamed 1983, Browne et al. 1985). In addition, the Federal West German Geological Survey (Bundesanstat fiir Geowissenschaft und Rohstoff, Hannover) has car- ried out surveys of the numerous anorogenic alkaline ring-complexes (Heimbach and Curtis 1980, Curtis 1982) and of selected parts of the Basement complex (Brinkman 1982). University of Khartoum geologists have also been active in the area, and have so far published the results of mapping in the north-east (El

Ageed and El Rabaa 1981), of petrological studies (Har- ris et al. 1983), and of the regional geology (Shaddad et

al. 1979, Khalil 1984). The Department of Geological and Mineral Resources has also investigated parts of the area (Khalil et al. 1979) and continues to update its maps (Geological Map of Sudan 1980) and investigate the economic mineral potential.

The present paper attempts to bring together the results of some of this work and to provide an interpretation of the geology which can be used to explain the gravity traverses, and to elucidate the mineral potential.

GEOLOGY

The rocks of the Nuba Mountains Basement inlier can be divided into a number of units, the exact relationships between which have yet to be studied in detail in the field. Granodioritic quartzo-feldspathic gneisses and amphibolite grade metasediments are intruded by heterogeneous batholithic masses of granodioritic composition. Greenschist facies andesites with associated serpentinite lenses and intercalated sedimentary layers, together with the higher grade rocks, have been intruded by an extensive suite of anorogenic alkali granites and syenites. Numerous quartz veins and some small dyke swarms are also present.

The contacts between some of these units are faulted, and major N-S, E-W, NE and NW trending fractures cross much of the area. Drilling and geophysical work on the surrounding cover (e.g. All and Whiteley 1981, Mohamed 1983) has revealed a well-developed graben structure on three sides of the Basement inlier which

329

330 A . A . SADIG and J. R. VAIL

NUBA MOUNTAINS

SUDAN

Alluvium

Mesozoic- Cainozoic cover

~-~ Anorogenic complexes

Synorogenic granitoids

~ '~ G a b bros - serpentinites

Metavolcanic sequence

Metasedimentary unit

Gneisses

Faults

Thrusts

.._.--- Gravity traverse

o Towns

?, , , . . . . ~ K m

Fig. 1, Geological sketch map of Nuba Mountains, central Sudan.

also affects the metamorphosed rocks, although the widespread superficial deposits mask much of the underlying geology (Fig. 1).

Quartzo-feldspathic gneisses

Gneissose foliated, banded rocks of predominantly quartzo-feldspathic composition underlie large parts of the Nuba Mountains area, particularly in the west. The gneisses represent metamorphosed sediments and sheared and altered granitoids, and hence although they are generally considered to be the oldest rocks in the area they might in fact be made up of any of the other Basement units.

They tend to form low lying ground with poor outcrop and where exposed are monotonous in their lithology. Generally their foliation trends N-S and they are cut by the regional linear fracture pattern. Mansour and Samuel (1957), Vail (1973) and Brinkman (1982) described hornblende augen gneisses, cataclastic tex- tures, and granodioritic gneisses as belonging to this group. Shaddad et al. (1979) reported muscovite, biotite and sillimanite gneisses and migmatites near Rashad, and it is clear the group comprises a very heterogeneous assemblage of rock types of complex origin.

Metasedimentary unit

Rocks ranging in composition and appearance from paragneisses to psammites and mica- and graphitic- schists can be distinguished in several places. They have

been mapped in the eastern Mountains (El Ageed and El Rabaa 1981, Heimbach and Curtis 1980), they occur in a narrow strip north of Talodi, and crop out as isolated patches within the gneiss group in the western exposures (El Nadi 1980).

The relationship of the metasedimentary unit to the gneisses is not clear, and in parts they probably grade into one another. Elsewhere they are in sharp contrast. No unconformity has been reported, although it is generally considered that the sedimentary unit lies upon an eroded gneissic basement. East of Rashad the metasediments are in thrust contact with gneisses (El Ageed and E1 Rabaa 1981), the fault plane dipping westwards beneath the over-riding gneiss block. There may be parallel thrust planes further west, or the contacts may be more steeply faulted. Evidence from the mylonitiza- tion and shearing suggests strong deformation and tec- tonized contacts. Relationships in the western occurrences are too poorly exposed to determine the precise nature of the contacts. In the NE the metasediments are apparently directly overlain by the metavolcanic and associated sedimentary sequences (El Ageed and El Rabaa 1981), and it may be that the two units are closely related and should be considered as one group as they probably are elsewhere in eastern Sudan (Vail 1983a).

Volcano-sedimentary unit

There are two areas within the Nuba Mountains where metavolcanic rocks with intercalated pyroclastic and sedimentary strata are to be found. Between Kadugli

Geology and regional gravity traverses of the Nuba Mountains, Kordofan Province, Sudan 331

and Lagowa they occur as small bands of meta-andesites surrounded by gneisses (El Nadi 1980), while further north, in the vicinity of Abu Zabed, and near Abu Tulu similar metavolcanic greenschists and low grade phyl- lites occur (Mansour and Iskander 1960, D. C. Almond personal communication 1983).

In the eastern Nuba Mountains, E1 Ageed and El Rabaa (1981) and Brinkman (1982) have mapped extensive areas of volcano-sedimentary rocks in the greenschist facies of metamorphism. They are adjacent to the metasedimentary unit rocks, and more significantly, contain lenses and irregular masses of chromitiferous serpentinites. The metavolcanic unit is in thrust contact with the metasedimentary unit in the west, the thrust- plane dipping westwards, to form a narrow (up to 25 km wide) but long (about 180 km) strip of volcano-sedimentary-ultramafic strata.

The general lithology, metamorphic grade, and structural disposition of these rocks is closely comparable to similar occurrences in the Kurmuk-Ingessana region along the Ethiopian frontier, 300 km to the east (Abdel Rahman 1983), and to most parts of the Red Sea Hills and Arabian Shield (Vail 1983a). It is therefore likely that these rocks represent a disrupted ophiolite sequence, dismembered and probably thrust faulted away from its suture. If this is the case then the concept of the regional geology of the area must be viewed from an entirely new position, and the implications for interpret- ing the deep structure of the region are considerable.

There are three other small outcrops of basic rocks worth noting. One is adjacent to the west margin of the Kadugli alkali microgranite ring-complex, east of the Kadugli-Dilling road; El Nadi (1980) described the rocks as meta-basic intrusions. Vail (1973) noted a small, slightly foliated gabbro mass 60 km NW of Kadugli on the road to Lagowa, surrounded by gneisses, but with unknown contact relationships. Lynes and Campbell Smith (1921) reported a troctolite body about 10 km SE of Dubeibat in an area of poor exposure, and therefore unknown relationship.

Syn- to late-orogenic batholithic granitoids

Granitic or granodioritic masses intrude the gneisses and metasedimentary rocks, and some small bodies cut the metavolcanic sequences. They are in the form of irregular, foliated batholiths, the largest being in the vicinity of Dilling, and they tend to form some of the more resistant topography. These granitoids have not been studied petrographically or geochemically, but their general distribution is comparable to similar masses in the Red Sea Hills (Vail et al. 1984).

It is possible that some of the Gneiss unit includes rocks of the foliated and cataclased granitic batholiths, and hence their distribution might be more widespread than realized.

Anorogenic alkali ring-complexes

The last major magmatic event in the area was the intrusion of a suite of alkali granites and syenites,

associated extrusive trachytes and rhyolites, with rare foid syenites, and one carbonatite (Curtis 1982, Harris et al. 1983, Vail 1983b). They take the form of ring-complexes and small circular intrusions. Over 50 centres are present in the Nuba Mountains, mainly in the SW between Talodi, Kadugli and Lagowa, where they crop out in a NW trending band of complexes.

They have been emplaced into all the earlier units, and are unaffected by folding or metamorphism, and thus are considered to be anorogenic. Limited geo- chronological data indicate they are between 567 Ma and 246 Ma in age (Heimbach and Curtis 1980, Batyr- murzaev et al. 1982, Curtis 1982, Harris et al. 1983, Vail 1983b).

Cover rocks

The metamorphosed gneisses, metasediments and the volcano-sedimentary sequences, together with the synorogenic granitoids and the anorogenic alkaline intrusions are all partially covered by undeformed, and in places poorly consolidated, sedimentary rocks.

The oldest of these sedimentary units was considered to be a group of conglomerates, grits, mudstones and thin limestones known as the Nawa Formation (Andrew and Karkanis 1945, Vail 1978) and more recently refer- red to as the Abu Habil Formation (Shaddad et al. 1979). However, S. M. E1 Rabaa (personal communication 1980) drew attention to the similarity of these deposits with those in Egypt at Wadi Hammamat, and so it could be that the Nawa-Abu Habil deposits represent a molasse facies associated with the suggested ophiolite sequence east of Rashad, near Abu Gubeiha.

Cretaceous, arenaceous and rudaceous beds, the Nubian Sandstone Formation, crop out in a few places around the Basement inlier, but are buried beneath the widespread Quaternary deposits of the Umm Ruwaba Formation which forms an extensive blanket over all earlier formations. A thin covering of wind blown sands, river alluvium and Nile valley silts obscures most of the underlying structures and strata.

STRUCTURE

The dominant brittle structures in the Basement rocks of the Nuba Mountains are the fractures which affect all rock units, prior to the anorogenic ring-complexes.

Drilling and geophysical work across the cover (Mit- walli 1969, Ali and Whiteley 1981, Van Overmeeren 1981, Browne and Fairhead 1983, Mohamed 1983) have shown that NW trending, sediment-filled graben structures occur SW and NE of the Basement inlier; they were considered by some authors to form an extension to an early East African rift system (Mitwalli 1969, Khattab 1975, R. B. Salama personal communication 1984).

Many of the NW and NE trending faults and lin- eaments in the Basement rocks may also reflect these movements. Earlier fractures form an E-W pattern, but

332

- 2 0 0

- 3 0 0

- 4 0 0

g.u. -5oo

- 6 0 0

- 700

- 8 0 0

A. A. SADIG and J. R. VAIL

profile

(a)

N I t t ,oo, t6 Dubeibat Dilling Ghulfan Keiga Giraw Kadugli

S

Km depth

N

o

3

6 9

12

15

18 21

24

27

30

40

:I Km30t

~ .;ooKQ/m3

Dubeibat Dill ing Kadugli

lOOKm S

Fig. 2. (a) Bouguer anomaly profile Kadugli to Dubaibat, showing regional gravity gradient and geological section. (b) Crustal model interpretation for the extended Kadugli-Dubaibat regional gradient profile.

-(b)

show little evidence of movement, although quartz veins and a few dykes also follow this trend (Vail 1973).

The main N-S faults are considered to be major thrusts (El Ageed and El Rabaa 1981) in the eastern Nuba Mountains, and it is possible that some of the N-S lithological boundaries are also thrust planes or zones of faulting and shearing. In this respect there are similarities between the Nuba Mountains and the basement structures in the Hoggar of eastern Algeria.

Gravity traverses north and west of Kadugli should elucidate the deep structure.

GEOPHYSICS

Previous geophysical work

Information available about earlier gravity surveys has come from Browne and Fairhead (1983) and Browne et al. (1985). These authors revealed that the Nuba Mountains inlier is associated with a regional negative gravity anomaly which is bounded by steep gradients on its south-western flank (Southern Sudan rift) and northeastern margin (White Nile rift). Browne and Fairhead (1983) explained the high regional gradient (the gradient amounts to 3.3 g.u./km in the Nuba Mountains) by thinning of the crust beneath the Southern Sudan rift

due to crustal extension, with the residual anomaly caused by a granitic body (density contrast -200 kg/m 3) which extends from the surface to a depth of about 4 km.

Presentstudy

The present study consists of two gravity profiles, one northwards from Kadugli to Dubaibat railway station and the other north-westwards from Kadugli to Lagowa. The distance between stations along the Kadugli- Dubaibat road is 4 km on the average, whereas those along the Kadugli-Lagowa traverse are governed by the position of the Sudan Survey Department bench marks. The elevations of stations along the asphalt road linking Kadugli and Dubaibat were determined from bench marks constructed by the Sudan Roads and Bridge Department; the elevations are accurate to 1 cm.

The gravity measurements were made with a Worden gravity meter No. 649 having a calibration constant of 1.207 g.u./dial division. The observations were mea- sured with respect to Kadugli base station, having a gravity value of 97809562 g.u. (Isaev and Mitwalli 1974) relative to the Potsdam datum. Drift corrections were applied, and a combined free air and Bouguer correction corresponding to a density of 2670 kg/m 3 was incorpor- ated to reduce the gravity value to sea level. Terrain


g.u.

-200

-300

-400

-600

-600

-- observed profile ) ~

_ -

Regional gradient

(a)

W N W i~;i!~.~.~.~_.~ . ~ 6 ..~i~:~, ~;~.~ ~'i~!~;i~ :,~!~

I I I I I I I I | l i t l 90Km t 50 20 "T

Lagowa Kadugli

E S E

o

K m depth

3

6

9

12

15

18

21

24

27

30 I

lOOKm

• ~ ;, ; : / +600 Kg/m 3

t t L a g o w a K a d u g l i

(b)

Fig. 3. (a) Bouguer anomaly profile Kadugli to Lagowa, showing regional gravity gradient and geological section. (b) Crustal model interpretation for the extended Kadugli-Lagowa regional gradient profile.

corrections are small except near isolated jebels; theoretical gravity is calculated using tables by Lambert and Darling (1931). Thus, the overall error in the Bouguer anomaly is estimated to be less than 5 g.u.

The gravity lows are interpreted in terms of granitic bodies with basement density contrast of -200 kg/m 3, the same contrast has been used by Browne and Fairhead (1983). This contrast is adopted for the sake of compari- son. A density contrast of +300 kg/m 3 is used to interpret the positive anomaly which is believed to arise from a buried gabbroic body. The adopted density contrast is justified from the work of Qureshi et al. (1966) and Sadig et al. (1974). A density contrast of + 100 kg/m 3 is believed to exist between the graphitic schist and the basement.

The anomalies (regional and residual) are interpreted using SIGMA (System of Interpretation of Gravity and Magnetic maps with Automatic processing) developed by the Geophysics Department of the Bureau de Recherches Grologiques et Minirres, Orleans.

Interpretation of the regional anomalies

The regional gradient for both profiles is interpreted in terms of the parameters used by Browne and Fairhead (1983), i.e. the crust is assumed to be 30 km thick, the density contrast between the mantle and the crust is +600 kg/m 3 and the background gravity field has a value of - 700 g.u.

(a) Kadugli-Dubaibat. There is no good geological control to deduce the regional gradient. This adds more to the ambiguity of the interpretation of gravity anomalies. Thus, we are obliged to assume a constant rate of gradient as Browne and Fairhead (1983) have suggested; the profile shows a southward increase over its length at a rate of 2 g.u./km (Fig. 2a).

The regional gradient is by definition due to deep effects. Thus, the dashed line shown in Fig. 4a which is labelled as the effect of'granitic batholith' may represent a type of regional gradient. Since we have preferred (see below) the third possibility which combines the effect of this granitic batholith with the effect of cupolas that give rise to short wavelength lows, this point must be borne in mind.

The regional gradient probably indicates thinning of the crust to about 27.5 km (comparable to Browne and Fairhead's value) in the region of Kadugli (Fig. 2b). The crust presumably increases in thickness northwards to reach its standard thickness (30 km) about 25 km south of Dilling; beneath the Dilling massif it is approximately 28.8 km thick and reaches its standard thickness in a region about 160 km from Kadugli which marks the northern limit of the Nuba mountains.

(b) Kadugli-Lagowa. The regional gradient shows a westwards decrease at an average of 5 g.u./km. How- ever, the rate of decrease is not constant (Fig. 3a) as in


the case of the Kadugli-Dubaibat traverse (Fig. 2a), being steeper on the eastern flank than in the west. The maximum rate of the gradient is associated with the positive anomaly.

The crust is 27 km thick in the region of Kadugli and thickens gradually westwards (Fig. 3b) to reach approximately 29.5 km beneath the region of the positive anomaly, and then it thins again to about 28 km in the region of Lagowa.

Note that the thickening beneath the positive anomaly arises from the assumption that the positive anomaly and the flanking negative anomalies represent a compen- sated structure. A constant rate of the gradient will lessen the magnitude of the large positive anomaly and increase the magnitude of the negative anomalies, and may mask the small positive anomaly. In this case (constant rate of gradient) the granitic masses bordering the positive anomaly need to be accommodated by thicker granitic bodies, and the basic body responsible for the positive anomaly would be thinner than indicated later (Figs. 5b and 5c).

Quantitative intepretation of the residual anomalies

(a) Kadugli-Dubaibat. The residual anomaly (residual = Bouguer anomaly - regional gradient) along the profile is characterized by a long wavelength negative anomaly that extends from Kadugli to about 15 km north of Dilling, that is to the northern edge of the Nuba Mountains. Thus, the residual anomaly confirms the presence of the gravity low detected by Browne and Fairhead (1983). However, the profile indicates that several short wavelength gravity lows are superimposed on the long wavelength anomaly. These local lows are encountered in the vicinity of Kadugli, in the Kiega Giraw region, Ghulfan Jebels, and in the vicinity of Dilling and have relatively steep gradients, which is an indication of their shallow sources. These anomalies are mostly associated with surface exposures of granitic rocks, with the exception of that in the Kiega Giraw region, where the local anomaly has the largest amplitude compared to the other lows and where undif- ferentiated quartzo-feldspathic gneisses of the Base- ment complex crop out.

Along the northern sector the profile traverses across superficial deposits (Umm Ruwaba Formation) and the anomalies show two highs separated by a low which is flanked on both sides by steep gradients which may indicate faulting. The high near Dubaibat may be due to the presence of a partially buried troctolite mass with a density contrast of +300 kg/m 3, such as has been reported from the vicinity by Lynes and Campbell Smith (1921).

Models can be proposed to explain the gravity profile: (i) First possibility. The long wavelength anomaly

(Fig. 4b) is considered to be due to a sub-surface granitic body whose base lies at a depth of say 6.0 km and the local anomalies are ascribed to cupolas originating from this body. Parts of the granitic batholith would be within a few hundred metres of the ground surface north of

Kiega Giraw, implying that the Basement gneisses are preserved as a roof pendant. The batholith is also assumed to terminate at its northern end and to continue at its southern end. It has a maximum thickness (2 km) in the region of Kiega Giraw and a thickness in the range of about 1.2 km-l .7 km in the regions of Ghulfan and Dilling, respectively. In the vicinity of Kadugli the granitic body has only to be approximately 200 m thick. The surface geology indicates the presence of a microgranite ring-dyke in this region of thinning which explains the difference.

The cupola near Kiega Giraw attains a maximum thickness of approximately 2.0 km. The Kiega Giraw and part of Ghulfan cupolas lie a few hundred metres beneath the surface, indicating the Basement gneisses in this region form a roof pendant.

It is not easy to attribute the negative anomaly north of Dilling to the Umm Ruwaba fill. The anomaly is ascribed to a body of a density contrast of -200 kg/m 3, and suggests it reflects buried granites.

It is to be noted (Fig. 4c) that most of the contacts, except along the northern flank of Dilling massif which is vertical, start with gentle dips at the surface and steepen with depth.

(ii) Second possibility (Fig. 4d). In contrast to Browne and Fairhead's (1983) interpretation, the base of the granitic body could have variations in depth. However, the maximum depth, beneath the Kiega Giraw, of approximately 3.8 km is the same as that calculated by Browne and Fairhead. Also the northern contact is vertical (Browne and Fairhead suggested a gentler dipping contact).

(iii) Third possibility (Fig. 4e). Presuming the base of the granitic body is at a uniform depth of 3.5 km, the calculated profile can be adjusted to match the residual profile if steep cupolas with near vertical or possibly stepped contacts rise up close to ground level. It is assumed in calculating the model that the batholith terminates at its northern end but continues southwards from Dilling to the vicinity of Keiga Giraw. Other anomalies can be explained by the ring dykes near Kadugli and the gabbroic body near Dubeibat.

Conclusion: The first two models contrast in having the main granitic batholith either separated from the exposed or near surface bodies, or with a very uneven lower limit. It seems more likely, on geological grounds, that a better explanation combines these contrasting models, in a third possibility such that a granitic source area of possibly uniform depth gives rise to heterogeneous cupolas which rise to variable heights beneath the present surface, some now eroded, having been emplaced to higher levels than others, which are still covered by roofs of granitic gneiss country rocks. In addition, the secondary high near the northern end of the traverse might also be due to a more deeply buried or smaller gabbroic body.

(b) Kadugli-Lagowa. The regional gradient (dashed line in Fig. 3a) shows a westward decrease (average 5 g.u./km) reaching its minimum value in a region of


g.u,

+100

-100

-200

N -300

=_ _~ca lcu la ted

" - - ' ~ - - - 7> ~ f e f f e c t of granitic batholith

1 I I I

(a)

S

Km

2

4

6

8

10 T 1 T 1 1- r

(b)

2

4

6

8

10

[ - - J - J -200

T I l i " i

(c)

2

4

6

8

10 L I I I I .1 1

(d)

Fig. 4. (a) Residual anomaly profile Kadugli-Dubaibat. (b) Interpretation. First possibility (effect of batholith). (c) In- terpretation. First possibility. (d) Interpretation. Second possibility. (Continued overleaf.)


100

0

g u 100

200

300

/ \ \ / 3 \

(e)

1

K m 2

3

,L

N 5 200

/3\

Dube iba t

I I I I 150 100 50 0

D i l l i n g Ghul fan Keiga G i r a w

Fig. 4 (Contd.) (e) Profile and third possibility interpretation.

Kadug l i

S

high gravity. The gradient is steeper on the eastern flank than on the western flank.

The main features of the residual profile are the pronounced positive anomaly and the negative anomalies flanking it. Another small gravity high is present west of the main high. Both highs correlate with outcrops of meta-andesites of the volcano-sedimentary sequence. The main anomaly shows steep gradients that suggests a mass concentration within the upper crust.

The low in the Kadugli region is accommodated by a granitic body about 2 km thick (Fig. 5b, c). The body adjacent to the granitic body is constructed on the assumption that it has a density contrast of + 100 kg/m 3 associated with the metasediments which crop out in this region (Fig. 1); graphitic schist is exposed at the El Kholiat Jebels about 10 km west along the road from Kadugli.

The steep gradients of the main positive anomaly leave little doubt that it is caused by the basic rocks cropping out in the area. The ratio of width (20 km) of the basic body to its thickness (approximately 1.4 kin) is about 15:1 suggesting that it has a sheet-like structure. The negative anomalies on the limbs of the basic body are attributed to small granitic bodies (granitic gneisses) and the westernmost small positive anomaly is ascribed to the basic material (meta-andesites) cropping out in the region. The shallow nature of the metavolcanics and metasediments may be due to a shallow unconformity, near surface granitic intrusion, or thrust faulting. In view of the regional structure the latter possibility should be borne in mind.

CONCLUSIONS

The fourfold division of the Basement complex of the Nuba Mountains inlier into gneisses and migmatites, metasediments, a metavolcanic-sedimentary-ultramafic assemblage, and syn- to late-tectonic batholithic granitoids suggests it to be comparable to the continental margin-volcanic arc environment occurring along the western side of the Arabian-Nubian Shield in eastern Egypt and Sudan and adjacent Ethiopia and Kenya (Vail 1983a). The tectonic relationships of several of these units add to the similarity, but also confuse the interpretation of the underlying geology. Within-plate anorogenic magmatism in the form of syenitic alkaline ring-complexes post-dates these Basement events.

Preliminary geophysical information obtained from two recent gravity traverses and from reconnaissance work elsewhere has enabled models to be proposed concerning the crust beneath the Basement complex of central Sudan.

The gravity survey of the western Nuba Mountains confirms the results of Browne and Fairhead (1983) in two respects: (i) the crust is thin beneath the Nuba Mountains, and (ii) the gravity field is characterized by a long wavelength gravity low. Such a picture is similar to that described by Ajakaiye (1970) for the Nigerian Younger Granite Province, which has a comparable within-plate anorogenic magmatic environment to that of central Sudan. Detail from the Kadugli-Dubaibat traverse reveals the presence of short wavelength anomalies superimposed upon the low. This indicates

Geology and regional gravity traverses of the Nuba Mountains, Kordofan Province, Sudan

Km

2 4 - -

6 -

1 0

12

14

16

18

20

-200

g.u, - 300

- 4O0

W N W

/ ~ observed A 1

! I E S E I

Kadugli

}~'~0~ ~ +100 ~ _ 200

(a)

(b)

337

3

9

15

21

27

30 w

,,~l======,=lq

.._.___,_-.----~, + 600 Kg/m 3

(c)

Fig. 5. (a) Residual anomaly profile Kadugli to Lagowa. (b) Model interpretation. (c) Model interpretation.

the presence of a deep-seated batholith with steep cupolas originating from it.

Along the Kadugli-Lagowa traverse the effect of the different lithological units, namely the granites, metasediments and basic volcanics are reflected in the gravity profile, and thus the gravity method is shown to be a useful tool for distinguishing Basement complex rocks in the Sudan, a fact which encourages the conduct- ing of further geophysical measurements to cover the whole Nuba Mountains basement inlier. Moreover, the gravity work may reveal the presence of major structures, especially thrusts, beneath the cover rocks.

The Nuba Mountains basement inlier, like the adjacent sediment-filled Abu Gabra rift and Bara trough, is underlain by a thinned crust. The crustal attenuation might be an isostatic response in compensation for either the presence of lighter material in depth (the batholithic granitoids), regional uplift of the area, or most probably due to crustal extension across the bounding rift grabens.

Acknowledgements--The award of funds to the Saudi Sudanese joint commission for exploration of the Red Sea to A.A.S. to enable him to

visit the BRGM laboratories in Orleans and LERGA in Talence, France is acknowledged, The collaboration of these institutions is greatly appreciated. At Portsmouth Polytechnic, J. Davidson helped with the text figures and Jackie Duggua with the typing. Dr. J. D. Fairhead, University of Leeds, is thanked for providing unpublished data on Sudan and for critically reading the manuscript. This paper is a contribution to I.G.C.P. Project No. 164 'Pan-African crustal evolution in the Arabian-Nubian Shield'.

REFERENCES

Abdel Rahman, E. M. 1983. The geology of the mafic-ultramafic masses and adjacent rocks south of the Ingessana Igneous Complex, Blue Nile Province, E. Sudan. M. Phil. thesis, Portsmouth Polytechnic (unpublished).

Ajakaiye, D. E. 1970. Gravity measurements over the Nigerian Younger Granite province. Nature, Lond. 225, 50-52.

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geology and regional gravity traverses of the nuba mountains, kordofan province, sudan

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