Chapter VIII: 3. The Upper Benue sandstones. (A. L.)

Upon the Cretaceous rocks which occupy the Benue plain in Yola province there rests a thick series of sandstones which will be referred to as the “Upper Benue Sandstones.” The relation between these sandstones and their basement is extremely interesting and yields valuable information as to the history of this part of Nigeria.

The southern boundary of the sandstones is a very irregular one. Thus on passing from Gidan Jauro to Bamga the path proceeds over sandstones for a time, then passes on to granite for a short distance, then on to sandstone (an arkose) again and finally on to the crystallines. North of Bamga the sandstone partly wraps round a small hillock of granite. But in the cliff-like sides of the Yendam Hills the evidence that the sandstones have buried up an old and hilly land-surface is very clear. Thus in Plate XVI a hill some hundred feet high is seen to be composite. The bulk of the hill (off the plate to the left) is granite, but its base is hidden by conglomerates, which pass upwards into pebbly sandstones. The latter are banked up against the granite and overtop it, thus forming a tabular hill. Many other sections in the district show a similar relation between sandstones and crystallines. (Plate VI, Fig. 1.) There is here no question of a faulted margin, but evidence on all sides that an older and hilly surface has been buried up in sandstones. If now one stands amongst these hills, there is the Benue plain to the north, floored by the Cretaceous limestone-shales, with the “Upper Benue Sandstones” resting upon them; around are the composite hills with the sandstone banked up against the granite, while to the south is the crystalline plateau of southern Yola, with the highest of its hills capped by sandstone. The accompanying section[88] is approximately correct, though the maps will not permit of great accuracy.

Passing on towards Yola, there are crystallines at Bikola and again beyond Jelima, but in the lower ground between the two are pebbly sandstones and conglomerates running up from the plain into the crystalline area as a tongue. Here the present stream course seems to occupy the same position as an older drainage channel existing before the deposition of the detrital rocks. The mass of the Gumba hills is almost all of granite. The southern part, entirely of granite, rises to 1,886 ft. above sea-level, but the cone of the northern peak, 1,727 ft. high, is of sandstone which, as in the case of the Yendam hills, must have been banked against the granite at the time of deposition.

The observations made may be summarised by saying that if the “Upper Benue Sandstones” were stripped off, so as to lay bare the underlying basement, no great change in the physical geography would result. Some detached hills and outliers would disappear, but the essential features of the Benue plain with its Cretaceous rocks and the crystalline plateau of southern Yola would still remain. The sandstones evidently accumulated upon lower ground to the north, at first abutting against the granite hills of the south but eventually rising above them. The physical features were buried up and have been exhumed by later denudation with great modification in detail but with the essential characters persisting throughout.[89]

The sandstones are found to run up the Benue plain past Yola to Garua and they also extend some little way north and north-west of Garua. The main spread, however, appears to turn southward, and according to German writers,[90] they are found between Djerim and Rei-Buba on the uppermost Benue, and also on the hilly area to the west.

Bearing in mind this distribution of the sandstones and their field- relations to the underlying basement, there is seen to be an intimate connection between the sandstones and the river, and it appears fairly certain that the sandstones were accumulated in the valley or valley- plain of an “Older Benue.” The outliers of the Yendam Hills and the Bagele Hills at Yola, which rise a thousand feet or so above the river, bear witness to the former thickness of these accumulations, now so largely removed.

The Upper Benue Sandstones cannot yet be subdivided even locally. No clay or shale bands have been found which would serve as horizons. The material does not vary much. The bulk is very felspathic and has evidently been formed out of granitic debris. At Bamga it is not always easy to distinguish between the arkoses of the sandstone series and the compacted granite sand at the base of the hills. The base of the series may become conglomeratic and the sandstone is liable to be pebbly even in the highest layers. Most of the rocks are white or slightly stained with iron oxide, but there are also intercalated layers of very ferruginous sandstone which occur as definite bands a few inches thick, traceable across the entire section exposed. The ferruginous character of these bands was determined at the time of deposition, possibly by local concentration of the heavier minerals. In addition to such bands there are in places much thicker masses of highly ferruginous grit occurring at what is locally the base of the series.

Much of the sandstone is massive, without bedding planes, and it tends to form caps to the hills with vertical sides, fifty feet or more in height. Cross bedding is beautifully exhibited in places in the lower part of the series, especially in the Bagele Hills, around Tagumbilli, and at Lamurde. The laminæ succeed each other in a perfectly regular manner through considerable thicknesses of material, one set of laminæ being truncated and overlaid by another set with a different inclination. Cross bedding is frequently met with in the Cretaceous grits of the Protectorate, where the laminæ are oblique to the major bedding planes. The cross-bedded sandstones of the Upper Benue, however, are built up of sets of laminæ, indistinctly marked off from those above and below, and without any true bedding planes at all. The structure resembles in every way the cross bedding of the Benue sandbanks, and in size of grain, in the presence of occasional pebbles and even of pebbly layers, and in the material itself, a coarse felspathic sand, the sandstones closely resemble the sandbanks of the present day (p. 15). The oblique lamination has been likened to that of dunes, but the coarse size of grain and the presence of pebbles indicate running water rather than wind as the immediate cause.

The flat tops of the sandstone hills and the terrace-like features on their sides show that the Upper Benue sandstones have not been folded. They rest on the Cretaceous limestone-shale series, which is folded in Yola province as it is in Muri. This suggests that the Upper Benue sandstones rest unconformably on the Cretaceous and are therefore post- Cretaceous in age. The actual boundary between the sandstones and the Cretaceous was only met with in one place, between Murki and Jibaru, about twenty-five miles north of Yola. There a coarse, dark red grit rests directly on Cretaceous limestone without any transition beds between the two. Unfortunately the dip of the limestone is not known, and therefore the evidence of the contact is not conclusive. The immediate change from limestone to unbedded ferruginous grit is easily understood if the overlying series is unconformable to the lower one, but it is not easy to account for it on the view of a continuous sequence. This section is regarded as strongly supporting the argument drawn from the unfolded character of the sandstones.

Though the reliance to be placed on lithological character is uncertain, so far as it goes the evidence is all in favour of correlating the Upper Benue sandstones with the Lokoja series. It is true that the oolitic ironstones so frequently met with in that series are not known from the Upper Benue area, but the soft friable sandstones and the ferruginous character of much of the material strongly remind one of the Lokoja series.

The above conclusions, and certainly the map of this area, will require modification in details, for the following reason. Two groups of sandstone are known in the Benue area, one below the limestone-shales and therefore Cretaceous, the other resting upon the limestone-shales, in all probability unconformably, and therefore post-Cretaceous. In the absence of fossils from either group of sandstones, the only method of determining the age of any particular exposure is to prove continuity either with the sandstones passing below the limestone-shales or with the sandstones resting upon the limestone-shales. This would require careful work under the most favourable conditions, and is out of the question in a preliminary survey. Thus in the Yola province it may ultimately prove that Cretaceous (infra-limestone) sandstones have in places been mapped with the Upper Benue sandstones.

In a few places in Muri Province there are sandstones having lithological resemblances to the post-Cretaceous series. Near Mutum Biu, for example, there is a porous, very ferruginous grit, breaking up into small fragments, each coated with a skin of limonite—characters exactly paralleled, in many localities, by the Lokoja series. Again at Loko, sixteen miles north of Katsena Allah, there is a soft unbedded pebbly sandstone which is much more like the Upper Benue sandstones than the Cretaceous. It may yet prove that on the widespread Cretaceous rocks of Muri there are here and there patches of post-Cretaceous sandstone, serving to link up the Upper Benue sandstones with the Lokoja series.

In spite of such future modifications, it is believed that the history of the Upper Benue can be already sketched out as follows:

1. Crustal movements (folding and perhaps faulting) dropped down the Cretaceous rocks below the crystalline basement along the Lau-Yola line.

2. The soft character of the Cretaceous rocks marked them out for rapid denudation, and an “Older Benue” was established along this line with crystalline plateaux north and south.

3. For some reason the detritus carried by the river accumulated, and on consolidation it formed the Upper Benue sandstones. The unstratified character of the material and the absence of alluvial mud or clayey bands indicates a rapid accumulation, probably under somewhat different conditions from those now prevailing. The sands were banked up against the granites of the south and buried up at least the foot hills.[91]

4. Later denudation has largely removed the sandstone and has exposed the Cretaceous floor again in many places. (A. L.)

PLATE XIII

The great development of rocks of Eocene age in the region of the Upper Niger has been sufficiently emphasised by M. Chudeau.[92] The Niger sandstones of Dahomey[93] are undoubtedly continuous with the western members of the Sokoto series, and are properly ascribed by M. Chudeau to the Eocene. The sandstones of the Upper Benue valley in the northern territories of the Kameruns described by Dr. Passarge,[94] are likewise continuous with the Upper Benue sandstones of N. Nigeria, and therefore most probably also of Eocene age. From the Southern Kameruns Dr. Esch[95] has described Cretaceous and Eocene rocks whose mutual relations are very obscure. The locally folded character of the Cretaceous, however, suggests an unconformity at the base of the Eocene. In Southern Nigeria Mr. Parkinson[96] has indicated the occurrence of both Cretaceous and post-Cretaceous rocks whose relations are at present unknown. The clays and lignites of Asaba[97] may possibly, however, have been accumulated contemporaneously with the upper shallow-water portion of the Eocene of Northern Nigeria.

M. Chudeau’s[98] admirable attempt at the construction of a map of the central Sahara is somewhat marred by his admittedly audacious method of deducing the solid geology of certain areas from the average depth of the wells. A striking example of the inaccuracy thus introduced is the relegation by M. Chudeau of the Eocene rocks of Sokoto to the Lower Cretaceous. Another fruitful source of error is the failure of M. Chudeau to recognise, doubtless through the want of sufficiently suggestive exposures, the possibility of an unconformity between the Cretaceous and the Eocene in the Sudan. There is every reason to believe that the erosion which followed the post-Cretaceous elevation was active over the whole of the Sudan, and that the greater part of the hummocky and irregular surface thus produced was again under water during the period of the middle Eocene depression. Moreover, the deposits then accumulated were in turn subjected to erosion during a subsequent movement of elevation, with the result that the early Eocene surface of crystalline and Cretaceous rocks was again laid bare over considerable areas. In view, therefore, of the former probable extension of the soft sandstones and ironstones of Kerri Kerri and the Tagum Hills over the plains of Bornu, it is permissible to doubt the accuracy of much of M. Chudeau’s suggested stratigraphy of the Chad area. It seems more reasonable to regard the sandstones of Kanem and Manga as Eocene rather than Lower Cretaceous, and to look upon them as having been deposited, as in the case of the Eocene rocks of the Gongola valley, upon an earlier surface of Upper Cretaceous and crystalline rocks now buried beneath the superficial accumulations of Bornu. The rocks of Koutous and Alakhos, Ouame and Tirminy[99] present, moreover, characteristics which ally them more with the Eocene rocks of the south than with the Turonian of the Damergou, and it is possible that even the southern escarpment of the Tegama itself[100] where it overlooks the Damergou should be referred to the Eocene. The red, white, and yellow sandstones of Ennedi and of Snoussi on the Upper Shari are probably also of the same age, and in no way related (as suggested by M. Courtet[101]) to the Karroo beds of South Africa. (J. D. F.)

[Footnote 78: _Cf._ Suess, _The Face of the Earth_, Vol. IV, Oxford Trans., 1909, p. 89.]

[Footnote 79: Chudeau, _Sah. Soud._, 1909, p. 104.]

[Footnote 80: De Lapparent, _C. Rd. Ac. Sc._, 136, 1903, pp. 1, 118; Bather, _Geol. Mag._, [5], 1, 1904, p. 292.

The Eocene fossils from Sokoto and Jega are similar to those collected by Col. Elliot and Capt. Lelean from the Sokoto frontier, of which accounts have been published by Dr. F. A. _Bather_ (_Geol. Mag._, 1904, p. 292, Plate XI) and by Mr. R. Bullen Newton (_G. J._, XXIV, 1904, p. 522), who regard them as of Middle Eocene age. The fossils consist principally of echinoids and internal casts of mollusca. The latter are not sufficiently well preserved for exact determination.

From Sokoto numerous examples of _Hemiaster sudanensis_ (Bather were obtained, and also specimens of Lucina and gasteropods. At Jega _Hemiaster sudanensis_ is also abundant, and a portion of _Plesiolampas_ (probably _P. Sahara_, Bather) was obtained. The mollusca belong to the genera _Nautilus_, _Turritella_, _Voluta_, _Cardium_, _Lucina_, _Ostrea_, etc.

(H. W.)]

[Footnote 81: Fossiliferous limestone has, however, been reported from Yelu at the confluence of the Dallul Mauri and the Niger valley.]

[Footnote 82: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 384.]

[Footnote 83: Chudeau, _Sah. Soud._, 1909, p. 274.]

[Footnote 84: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 112.]

[Footnote 85: Chudeau, _op. cit._, p. 275.]

[Footnote 86: _Cf._ Gurich, _Zeit. Deut. Geol. Gesell._, XXXIX, 1887, p. 122.]

[Footnote 87: Barth, _Central Africa_, 1857, II, p. 368.]

[Footnote 88: See Section IV on margin of coloured map.]

[Footnote 89: The crystalline rocks at the base of the Bagele hills, mentioned by Barth (_Central Africa_, Vol. II, p. 478) may be taken to represent the outcrop of a portion of the earlier hummocky floor which was buried under the sandstones of Yola.]

[Footnote 90: Passarge, _Adamaua_, 1895, p. 363.; Edlinger, _Niger-Tsad Expedition_, 1904, p. 154.]

[Footnote 91: _Cf._ Passarge, _Adamaua_, 1895, p. 390. Passarge did not recognise the presence of Cretaceous rocks in the Upper Benue valley.]

[Footnote 92: Chudeau, _Sah. Soud._, 1909, p. 97.]

[Footnote 93: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 399.]

[Footnote 94: Passarge, _Adamaua_, 1895, p. 363.]

[Footnote 95: Esch, _Geologie von Kamerun_, 1904, p. 11.]

[Footnote 96: Parkinson, “The post-Cretaceous Stratigraphy of S. Nigeria,” _Q. J. G. S._, 63, 1907, p. 309.]

[Footnote 97: Parkinson, _op. cit._]

[Footnote 98: Chudeau, _Sah. Soud._, 1909, p. 85; and map.]

[Footnote 99: Chudeau, _Sah. Soud._, 1909, p. 81 _et seq._]

[Footnote 100: Barth, _Central Africa_, 1857, Vol. I, p. 531.]

[Footnote 101: Courtet, _C. Rd. Ac. Sc._, 140, 1905, p. 162.]