8012096-XEOLOXIA N 33.qxd:maquet. 1 7/1/09 19:47 Página 10 Cadernos Lab. Xeolóxico de Laxe Coruña. 2009. Vol. 34, pp. 13 - 26 ISSN: 0213-4497 As citas bibliográficas dentro do texto poránse en maxúscula e a abreviatura para os seguin- tes autores será: "et al.". Os textos enviados serán sometidos á crítica dos Censores Científicos designados pola Revista. Bibliografía: Autores en maxúsculas e as revistas ou textos citados en cursiva. Granite formations and granite cavities in Os autores dos traballos deberán incluir a súa dirección completa, incluído código postal. northern Nigeria Formaciones graníticas y cavidades Separatas graníticas en el Norte de Nigeria Enviaráse a cada autor un PDF co artículo na versión final. Envío de manuscritos SZENTES, G1. - Os traballos originais enviaránse a: (1) Alte Frankfurter Str. 22b, 61118 Bad Vilbel, Germany. E-mail: [email protected] Instituto Universitario de Xeoloxía Recibido: 9/8/2007 Revisado: 10/3/2008 Aceptado: 15/6/2008 Edificio Servicios Centrales de Investigación Campus de Elviña s/n Telephone: (+34) 981 167000 Ext. 2910 Fax: (+34) 981 167172 [email protected] 15071 A Coruña (Spain) incluindo dirección habitual e teléfono. Abstract — Unha vez terminada a impresión, os originais serán devoltos ao seu autor/es, si así o des- exan e expresan. The Author worked as geologist in Northern Nigeria during several years. This study presents — O número máximo de follas que se admitirá por traballo será quince (15) incluindo figu- his experiences concerning the granite formations of the region. The geological base of Northern ras, fotografías, mapas, etc.. Para traballos de maior extensión rógase consultar ao editor Nigeria is the so-called Basement Complex. A significant part of this basement is the Older Granite científico. Formation, which has been intruded in the late Precambrian and in the Lower Palaeozoic. The ring — O Instituto Universitario de Xeoloxía resérvase o dereito de devolver ao autor/es aque- tectonic complexes of Jurassic age - Younger Granites - intruded into late Precambrian basement les traballos que non se axusten a estas normas. rocks in a N - S trending zone. Most of the landscape is characterized by the occurrence of large, almost level peneplains formed by intensive erosion over long periods. The most spectacular landforms are the granitic inselbergs, the woolsack shaped boulders with cavities and the balanced rocks. The Author had studied the rock shelters and cavities, between and below the woolsack shaped granite blocks near the town of Birnin Kudu in Jigawa State. He describes the formations and the development of the three largest cavities. They contain rock paintings and may have provided shelters or religious places for the ancient inhabitants. These paintings are typical of those found along the sahel belt of West Africa and were probably made by the ancient Fulani cattle herders. Key words: Basement Complex, Older Granite Formation, Younger Granite Formation, peneplains, inselbergs, woolsacks, Birnin Kudu Rock Shelters, ancient rock paintings 14 ���������Szentes CAD. LAB. XEOL. LAXE 34 (2009) GEOLOGICAL SETTING The ring tectonic complexes of the Jurassic age - Younger Granites - intruded The Basement Complex in Northern into late Precambrian Basement rocks in a Nigeria is underlain by gneiss, migmatites N - S trending zone. The Younger Granite and metasediments of Precambrian Age, is 160 to 170 million years old. Its emplace- which have been intruded by a series of ment was associated with epirogenetic uplift. granitic rocks of late Precambrian to lower The ring structure and the petrology follow Palaeozoic. The plutonic rocks are known a general pattern. Their evolution can be as Older Granite and have been dated to summarized in three stages as the early vol- about 500 to 600 million years, represent- canic stage, the caldera and ring dyke stage ing the Pan-African orogeny in Nigeria. and the intrusive stage. The earliest rocks The granite bodies are widespread in the are volcanic, composed of rhyolite with mi- north and range in size from the smaller el- nor associated basalts and trachytes. They liptical plutons to the masses of batholithic are closely associated with the outer ring dimensions over 100 km in length. The con- dykes of granite porphyry. Within the ring tacts with the genisses are gradational pass- dykes there are stock-like intrusions of bi- ing from granite into metasomatic gneiss otite granite. The centre of the complexes is with marginal migmatites. The contacts composed mainly of massive biotite granite between granites and metasediments are with smaller intrusions of porphyry (KOB- sharp with no marginal migmatites (OYA- KE, 1976). VOYE, 1964). The granites are composed of alkali The granitic rocks are all composition- feldspar, quartz with a small proportion of ally similar, containing quartz, microcline, mafic minerals and are classed as alkali feld- plagioclase and biotite with accessory apa- spar granites. They are unfoliated. tite and zircon. The porphyritic granite is The Younger Granite is the source of the the most typical type of the Older Granite. tin mineralization in the Jos Plateau region. It is coarse to very coarse grained with large To the south and southeast and partly to white or pink prismatic phenocrystals of the west the Basement Complex is uncon- microcline. The granites are variably foli- formably overlain by the Cretaceous sedi- ated becoming almost gneissose in places. ments. In eastern Kano State, and in Borno Occasionally, medium to fine grained gran- and Bauchi States of the Quaternary limnic odiorite, quartz syenite and microgranite sediments of the Chad Formation are the also appear. characteristic covering series. The youngest rock of the Basement For- The weathered Basement rock and also mation is a series of rhyolites and rhyodac- the Younger Granite Formation is, in many ites that were intruded into the Older Gran- places, overlain by a 5 - 10 m thick laterite ite bodies during the lower Palaeozoic uplift crust, which had developed gradually in the following the Pan-African orogeny. They Tertiary and Quaternary. Further drift and occur in several places in North-East and fluvial deposits comprise overlying strata of North-West Nigeria. the Basement surface (Fig. 1.). CAD. LAB. XEOL. LAXE 34 (2009) Granite formations and granite cavities 15 Fig. 1: Theoretic geological section of Northern Nigeria Landforms of the Older Granite and granitic inselbergs, and hill ridges rise from these plains. Granitic inselbergs and Most of the landscape in West Africa ridges have resisted ongoing weathering is characterized by the occurrence of large, and erosion because of their hardness. They almost level plains (peneplains), formed by can be as high as 200 - 300 m. The mesas, intensive erosion over long periods. This be- which can rise to about 100 - 200 m above gan with the erosion during the Jurassic era the ground level, have formed because of the after the disruption of the Gondwana land, occurrence of ironstone hardpans on their followed by the Cretaceous Post-Gondwana surfaces. These hardpans have been formed erosion (e.g. Cameroon Highlands). through a downward movement and subse- Most of the plains and plateaux in north- quent accumulation of minerals in the lower ern Nigeria are the result of Plio-Pleistocene soil horizons notably iron oxides, under con- erosion, the so-called African and Post-Af- ditions of alternating moisture and drought rican planation. These plains and plateaux (THOMAS, 1974). have an undulating relief 400 - 600 m above On the level peneplains formed by the the sea level and are dissected by streams and various erosion cycles, massive sheets of la- rivers, usually in a dendritic drainage pattern teritic iron accumulation were able to devel- which, in some places, form deep gorges. Lo- op. The stripping of overlying soil material cally, steep-sided remnants of older plateaux during new erosion cycles, caused by either 16 ���������Szentes CAD. LAB. XEOL. LAXE 34 (2009) the uplifting or alternatively the lowering of The boulders were originally part of a the erosion base, proceeded down to the in- solid mass of coarse-grained granite which durated iron pan, which now appears at, or formed deep within the Earth’s surface near, the surface. about 1640 million years ago. The develop- Some remarkable geological formations, ment of these granite blocks is related to the such as woolsack shaped boulders with cavi- formation of the woolsack caves and it will ties, balanced rocks have been formed by be discussed in the chapter on the Birnin spherical weathering. They have developed Kudu Rock Shelters. from the combination of mechanical weath- I would now like to describe some char- ering (which cracked the rocks) and chemical acteristic examples of the Older Granite weathering (which flaked the off surface). landforms in Northern Nigeria (Fig. 2.). Fig. 2: Locations of the studied granite formations. Older Granite Formation: 1. Granite blocks near Gwoza village (Borno State), 2: Inselberg near the town of Gusau (Zamfara State), 3: Inselberg near the town of Tegina (Niger State), 4: Inselberg near the town of Bauchi (Bauchi State), 5: Granite surface near Rano village (Kano State), 6: Granite Surface near the Tiga Resrvoir (Kano State), 7:A shield inselberg near Ru- rum village (Kano State), 8: Partly disintegrated inselberg in the Kano River Forest (Kano State), ), 9: Chal- lawa Gorge (Kano State). Younger Granite Formation: 10: Ririwai Hills and tin ore mining (Kano State), 11:Disintegrated inselberg near the town of Dutse (Jigawa State), 12: Pile of granite blocks near Gwaram village (Jigawa State). 13: Birnin Kudu Rock Sheleters in Older Granite Formation (Jigawa State) CAD. LAB. XEOL. LAXE 34 (2009) Granite formations and granite cavities 17 The Gwoza Hills, along the Cameroon roundings are covered in scattered rock border in Borno State, are part of the larg- pieces and comprise spectacular inselbergs, er granite chain of the south-west part of balanced rocks and other related features Borno State.