The Southwestern sector of the Iullemmeden Basin in Nigeria is regarded as “Sokoto Basin”. [1] Preferred the name “Iullemmeden Basin” rather than “Sokoto Basin” as used by other authors such as [2,3,4]. It consists predominantly of a gently undulating plain with an average elevation varying from 250 to 400 m above sea-level. The Iullemeden Basin is an intracratonic basin which covers an area of approximately 800,000Km² encompassing parts of Algeria, Mali, Niger and the Benin Republic as well as northwestern Nigeria. According to [5] The basin is bounded to the Northwest by Basement outcrops of the Adrar des Iforas, Tamesna Basin to the North, the Gao trough to the west, it is bounded by Basement outcrops of the Ahaggar or Hoggar Massif which form part of the central Saharan Massif to the South by crystalline and metamorphic rocks of northern Nigeria to the east bounded by Chad Basin (Figure 1).

Figure 1: Generalized regional Geology of Iullemmeden Basin with Sokoto Basin at southwest (After Moody, 1997).

The Sokoto sector of the Iullemmeden Basin in Nigeria has received less attention in recent years based on assertion that there is paucity of outcrops and has not been subjected to any considerable degree of faulting according to [6,7,8]. The statement of [9] and [10] that “most of the WCARS (West and Central African Rift System) Basins share similar tectono-stratigraphic history with little disparity among individual basins, which resulted from the controlling tectonic regime” created the curiosity to study this basin.

The Sokoto sector of the Uillemmeden Basin, Benue Trough, east Niger rift basins and Sudanese rift basins are major elements of the West and Central African Rift System (WCARS). The Sokoto Basin comprises sediments of Late Jurassic to Miocene.

This account concerns the remapping of the northern part of the basin for which few advances have been made especially its major structural features and presentation of small scale detail geologic maps. Researches in terms of aeromagnetic data were carried out by [3,4,11,12,13] while [2] mapped two tectonic structures (reverse fault and roll over anticline at Goronyo for the first time) and they proposed that such structural and stratigraphic traps may upgrade the petroleum system of the basin.

The integration of interpreted aeromagnetic data and outcrop structural data obtained from extensive fieldwork allowed proposing the tectono-stratigraphic evolution for the Sokoto Basin and the effect of structures on sedimentation, especially the three major faults (NE-SW, NW-SE and NNE-SSW) which characterized a rift basin being modified by strike slip faults and basin inversions, this also affected the deposition of the Maastrichtian to Eocene Rima and Sokoto Group of sediments respectively.

At the preliminary stage, interpretation of high resolution satellites imagery was conducted aided with Global Mapper software which was imported to Avenza Mapper and was used during the fieldwork proper. The aeromagnetic data covering northern part of Sokoto Basin was processed and filtered using first vertical derivative and horizontal gradient maps. These were done to enhance the structures within the study area. Interpretation of the first vertical derivative maps shows fault systems within the study area (NE-SW, NW-SE), and were ground trotted and placed in the final geological map. Detailed field mapping was conducted, it involves description of exposed sections mainly along river channels and road cuts; outcrop sample collection, and photography of relevant features. Delineation of geological boundaries was done using Global Positioning System (GPS). Geological Maps of 7 topographic sheets of 1:50,000 scale were produced manually  and subsequently scanned, georeferenced using global mapper 2.0, imported to Arc map 10.4.1 software and digitized. Update of Names of Villages were imported from Nigerian shape file. Mapping colour index approved by NGSA (Nigerian Geological Survey Agency) was used to delineate formations e.g Taloka Formation (174, 214, 176), Dukamaje Formation (189,237,156), Wurno Formation (186, 232, 163), Dange Formation (180,100,0) Kalambaina Formation (170, 80, 30), Gwandu Formation (220, 100, 35) and Recent Alluvial (255, 255, 150).

The stratigraphy of the basin was established by [1,2,7,8] with the basic succession confirmed and the correct positioning of unconformities.

The Maastrichtian to Paleocene succession is overlain uncomfortably by the pre-Maastrichtian continental intercalary. It is made up of the Rima Group, consisting of mudstones and friable sandstones (Taloka and Wurno Formations), separated uncomfortably by the fossiliferous, calcareous and shaly Dukamaje Formation. The Paleocene Sokoto Group, consists of the Dange and Gamba formations (mainly shales) separated by the calcareous Kalambaina Formation, The Gwandu Formation (Eocene Continental Terminal) uncomfortably overlies the shale Gamba Formation (figure 2).

Figure 2: Lithostratigraphy of the Nigerian Sector of the Uillemmeden Basin (Sokoto Basin) Northwestern Nigeria (After Nwajide 2013).

NE-SW Maastrichtian Rifting

The NE-SW trending groups of faults in the Sokoto Basin are postulated to have controlled the trends of the major rivers in the Sokoto Basin. These groups of fault are normal faults and listric faults in some places and indicates rift/extensional regime. The faults are most importantly displayed by Sabon Gida – Kaffe faults (Figure 3), illela-Dawagare - Kirare Listric fault (Figure 4) in both areas the Early Maastricthian Taloka Formation is in direct contact with Late Maastrichtian Wurno Formation. An unconformity probably occurred due to the faults and probably led to the removal of Dukamaje Formation, which should be between the two formations. The Tunga Dankemo, Wurno- Sabon Gari fault (Figures 5, 14), Rabah, Gwadadi and Boyi Fault (Figure 6) these faults control the deposition early Maastrichtian to Eocene succession due to reactivation of the fault after some period of geological time. At Kaffe, Kagogo and Baredi hills the genesis of the anticline is related to NE-SW trending faults. At Kaffe hill all the sedimentary successions of the Maastrichtian Rima and Paleocene Sokoto Group were exposed and are attenuated. The vergence of the fold limb is exposed in this area and documented for the first time.

Figure 3: Geological Map of sheet 4 Gada SW and Cross Section E – F.

Figure 4: Geological Map of Rabah NW.

Figure 5: Geological Map of sheet 10 Sokoto NE and Cross Section C – D.

Figure 6: Geological Map of Rabah SW sheet 11.

The NW-SE trending group of faults are postulated to have control the trends of secondary rivers in the Sokoto Basin and are most importantly displayed around Dukamaje Hill, Gadabo, Gilbedi, Jargaba (Figure 7), Kaffe, Gidan Hashimu-Sahel Kaurawa-Gada (Figure 3), Taloka, Darbabiya, Tungayamma (Figure 4), Dundaye, Kalambaina, Adarawa-Asari (tulip), Arkilla (Figure 8) Choncha hill, Wurno hill (Figure 5), fault north of Gawakue here Early Maastrichtian Taloka Formation is in direct contact with Late Maastricthian Wurno Formation as a result of displacement created by NW-SE Kogogo faults (Figure 9). According to [14] during inversion normal faults become thrust and reverse faults. A regional geologic system formed in extension is forced to shorten; it does so in part by “reversing” the displacement on the pre-existing faults. Similar phenomenon was observed at Dukamaje hill, there was reactivation of the normal faults as a result of compressional regime which led to the thrusting, here late Maastrichtian Dukamaje Formation is overlain by early Maastrichtian Taloka Formation with upthrust of about 44m this is responsible for the basin inversion (Figure 10) which can be correlated with the late Maastrichtian Early Paleocene Basin inversion which affected Africa and Arabia of [15]. This phenomenon can also be associated with sinistral strike-slip fault occurring along a restraining bends which are marked by shortening which is achieved by thrusting and folding. Stratigraphic section is duplicated at Dukamaje Hill, which result in repetition of older Early Maastrichtian Taloka Formation overlying the younger Dukamaje Formation (Figure 10) similar tectonic basin inversion is proposed to have affected the Eocene Gwandu Formation around Adarawa, Asari, Arkilla and Kalambaina areas (Figure 8) this event also correlates well with the Eocene basin inversion of [15] (Figure 11)

Around Gawasu, Koliya (Figure 4) normal faulted half graben blocks affects the deposition of both Maastricthian Rima and Paleocene Sokoto Group. At Gidan Hashimu-Sahel-Gada fault series of listric faults and roll over anticlines (Figures 12, 13) are exposed which affected the deposition of Early Maastrichtian Taloka Formation conforming to extensional regime/rift [15] (Figure 12, 13 and 14). These faults are important in the structural/evolutionary framework of the Sokoto Basin, especially around Adarawa-Asari where a Tulip flower structure is displayed conforming to extensional - transtenssional regime affecting the post Paleocene lower Gwandu Formation, this was also interpreted as NW – SE faults traversing between Sokoto and Wammako by [3].

The event can be summarized (Figure 11): 1. During the Early Maastrichtian there was rifting of Taloka Formation, followed by, 2. Compressional and basin inversion during the late Maastrichtian affecting Dukamaje Formation. 3. Rifting commenced during early Paleocene affecting Wurno Formation. 4. Compressional episode resumed during late Paleocene affecting Dange, Kalambaina, Gamba and oolitic ferruginous sandstone expressed by folding of these formations. 5. Early Eocene Rifting/transtensional regime commenced and affects the lower Gwandu Formation. 6. The final tectonic episode was sealed in this area by compressional/transpressional regime and basin inversion which affected the upper Gwandu Formation during Eocene/Miocene.

Figure 7: Geological Map of sheet 4 Gada SE.

Figure 8: Geological Map of Sheet 10, Sokoto SW.

Figure 9: Geological Map of Sheet 10, Sokoto SE and cross section G – H.

Figure 10: Thrust Fault: older Taloka Formation (marron arrow) with listric faults and associated antithetic fault overlying younger Dukamaje Formation (blue arrow). The upthrust measure here is about 44m outcropping at Dukamaje Hill (Type section)  N13^o437′ 4.4′′  E005 ^o 47′ 47.6′′.

Figure 11: chronostratigraphic table for the Cretaceous and Tertiary with the correlation of tectonic events of Africa-Arabia and the Sokoto sector of Iullemmeden Basin. (Modified After Guiraud and Bosworth 1997).

Figure 12: lower Taloka Formation listric normal fault. At Gidan Hashimu N13^o 33′ 49.4″ E005^o 44′ 17.8″.

Figure 13: middle Taloka Formation roll over anticline / Listric normal fault with reverse drag. At Gidan Hashimu N13^o 33′ 50.8″ E005^o 44′ 20.5″.

Figure 14: Taloka Formation, Normal Fault outcrop at Wurno Hill N13^o 17′ 11.3′′ E005 ^o 25′ 50.3′′.

The N-S trending fault occur in few areas in the Sokoto Basin, these include  Durbawa-Alkamu, Gidan Mirabi, Tudun Dansema, Buji-Galadima Gari , Gidan Liman-Ubandoma and Gidan Banjo (Figure 9), these groups of faults affects late Maastrictian Wurno, Early Paleocene Dange and Kalambaina Formations and Early Eocene lower Gwandu Formations. In many areas Dange Formation and Kalambaina Formation were deposited vertical to each other. The effect of these fault probably in relation to NW-SE trending faults are responsible for the folded nature of the aforementioned Formations.

The present work has appraised four (4) unconformities in the Sokoto Basin, they include i) boundary between Late Jurassic/Early Cretaceous Illo/Gundumi Formation and Early Maastrichtian Taloka Formation, ii) Top of Middle Maastrichtian Dukamaje Formation, iii) Top of Middle Late Paleocene Kalambaina Formation/Gamba Formation and iv) Top of Eocene-Miocene Gwandu Formation. An unconformity between Dukamaje Formation and Wurno Formation is observed almost everywhere in the basin wherever they outcropped, especially an erosional unconformity occurring at Rabah, Maikujera, Gwadadi and Boyi (Figure 6). At Bari, Illela Dawagari and ungwan Jodo areas (Figure 4) Dukamaje Formation is missing and it occur very thin (few meters) wherever they outcropped in the basin, Therefore it is proposed that an unconformity occurred after the deposition of Middle Maastrichtian Dukamaje Formation as against the earlier postulated unconformity placed after the deposition of early Late Maastrictian Wurno Formation.

Folds directions in the Sokoto Basin vary in trends and direction and the fold axes show dominant E-W and N-S most probably in response to the end-Cretaceous compressional episode and post Paleocene extensional and compressional episodes, the strike - slip movement caused their progressive movements. [15] Interpreted the Santonian compressional event, as well as the Campanian-Maastrichtian rifting event and end Cretaceous compressional event as causally related aspects of a global tectonic event and critically connection between intraplate tectonic histories and processes occurring at sometimes very distant plate boundaries.

 The East-West trending fold axis shown by the asymmetric anticlinal structure at Kaffe where Taloka Formation dip 12^o@060^oNE and 8^o@300 forming the core of the anticlinal structure and right vergence at the fold limbs were manifested, similar trend occur at Wurno where the core of the fold is Dukamaje Formations (Figure 15, 16), others include Dima Hill anticline and syncline, Salame syncline, Kagogo anticline and Baredi anticline (Figures 5, 9).

Figure 15: An Anticline exposed south of Wurno Village, where the Dukamaje Formation forms the core and the folds on both limbs are the Dange Formation see also Z-shaped minor folds N13^o17′ 0.5′′ E005^o 26′ 23.1′′.

Figure 16: Folded limy shale of Kalambaina Formation outcrop at Tambagarka stream N13^o 19′ 27.6′′  E005^o 22′ 46.8′′.

The effect of strike slip movement in the Sokoto Basin along the NE-SW and NW-SE trending faults gave rise to a tulip (negative) flower structure affecting the Early Maastrichtian lower Taloka Formation at Taloka Hill (Figure 17) and early Eocene lower Gwandu Formation at Arkilla and Adarawa hill, this is supported by the end Cretaceous extensional-transtensional stress regime of [15] (Figure 18).

Figure 17: Tulip (negative flower structure) in lower Taloka Formation, this form an exensional duplex N13^o 26′ 56′′  E005^o 41′ 15.1′′.

Figure18: Campanian – Maastrichtian Rift Trends.

Key to Figures 18 and 24 Senonian tectonic map of Africa – Arabia and adjacent regions. Reconsruction is Cretaceous/Paleocene times (?65 Ma).i=late Santonian fold belt; 2= major late Samtonian fault; 3= late Santonian trust (early Campanian; 4=Campanian-Maastrichtian rift; 5= latest Maastrichtian fold belt; 6=latest Maastrichtian thrust; 7=Santonian/Campanian unconformity; 8=Maastrichtian/Paleocene unconformity; 9=Senonian volcanism; After Guiraud and Bosworth (1997). The effect of these fault are consistent with the tulip (negative) flower structure at Adarawa affecting the lower Gwandu Formation, these are clearly displayed around Adarawa  (Figures 19,20) formed during the early Eocene extensional regime.

Figure 19: lower Gwandu Formation, Normal fault affecting the friable tabular cross bedded sandstone Arkilla Hill N13^o 01′ 31.6′′  E005^o 10′ 54.1′′.

Figure 20: Tulip (negative flower structure) in lower Gwandu Formation at Adarawa Hill, this form an exensional duplex N13^o 2′ 16.3′′  E005^o 8′ 28.5′′.

According to [16] Folds can also develop in strike-slip shear zones, typically before deformation is localized into discrete faults, Similar folds formed as a result of strike – slip movements which affected late Paleocene Dange and Kalambaina Formations and folded fine to conglomeratic bioturbated ferruginised sandstone beds of primary oolitic ironstone (Figure 21) This folded nature of the ironstone trends NE-SW and traverses 100’s of km basin wide. During the latest Paleocene to Eocene it affected the upper Gwandu Formation and is expressed by buckling (Figure 22).

Figure 21: Lower Gwandu Formation/irostone, Fold developed in strike-slip shear zones Maginawa village N13^o 07′ 35.2′′  E005^o 14′ 48.1′′.

Figure 22: upper Gwandu Formation, Buckling, and channel fillings sandstone outcrop at Arkilla Hill N13^o 01′ 31.6′′  E005^o 10′ 54.1′′.

The manifestation of palm tree (positive) flower structure (Figure 23) within the upper Gwandu Formation where there was a change from extensional-transtensional stress regime at the lower Gwandu Formation to compressional-transpressional stress regime affecting the upper Gwandu Formation as a result of the strike – slip faulting along a restraining bend. This episode was earlier mentioned by [15] to have been identified from West African Rift System basins (Figure 24). The effect of these strike slip are also consistent with the development of sigmoidal drag folds which occur within a closed spaced (Figure 25).

Figure 23: Positive flower (Palm Tree) structure developed due to transpressional movement at restraining bends Arkilla Hill N13^o 01′ 17.6′′  E005^o 10′ 51.9′′.

Figure 24: Latest Paleocene/Eocene Shortening direction (After Guiraud and Bosworth (1997).

Figure 25: Sigmoidal drag folds along closed-spaced. Transpressional strike – slip fault occurring along a releasing bend resulting to pull apart basin. The down thrown here is about 47m. Arkilla Hill N13^o 01′ 25.6′′  E005^o 10′ 59.1′′ and Adarawa Hill N13^o 02′ 06′′  E005^o 10′ 43.2′′.

The outcropping part of the Sokoto Basin actually exhibits faults with three main trends established in this study: NE-SW, NNE-SSW and NW-SE this is similar to the tectonic characteristics of the Termit Basin in the Agadem Block of the east Niger rift Basin (Phoenix Wyoming International LLC June 2007), According to [15] the Termit basin registered slight transpressional deformation, illustrated by some NE – SW trending anticlines located along WSW – ENE or WNW – ESE oriented accommodation zones separated by rift sub-basins and folding was rejuvenated up to Late Eocene. This research has also established that the Sokoto Basin is of rift origin and modified by transpressional and transtensional strike – slip faults systems of Maastrichtian to Eocene. [17] And [18] associated the East Niger as complex rift system whose sedimentary fill ranges in age from Late Jurassic to Early Tertiary.

The following extensional stages are distinguished i) rifting in the Early Cretaceous, this gave rise to a series of large half-graben bounded by domino-like faults (corresponding to Illo/Gundumi Formation ). ii) Major angular unconformity separates the first rift from the second which took place between the Cenomanian and the Paleocene (cuts across both the Rima and Sokoto Group in the Sokoto Basin). Santonian – early Maastrichtian age mark the end of rift phase. During the Maastrichtian-Paleocene thermal sag phase occur. iii) Another episode of rifting began in the Eocene and lasted until Oligocene (corresponds to Eocene to Miocene Gwandu Formation).

According to [15] the thickness of the Termit Basin succession reached 14 km and include 1-3 km of Early Creataceous terrigenous clastics, 6-8 km of Late Cretaceous continental or shallow marine formation and 3-4 km of Cenozoic continental sand and shale. Two major rifting stages are evident, the first one is Early Cretaceous in age and characterized by NW – SE trending fault blocks; and second one is late Senonian – Paleogene and is represented by NNW – SSE trending normal fault, these corresponds to the episodes in the Sokoto Basin. In terms of the structures the basin is similar to the work done by [19] on the Lineamentary and Structural Cartography of Iullemmeden Basin in the Dosso Region (Southwest of Niger), they implemented methodology integrated remote sensing techniques and GIS tools to process and analyze satellite images. This approach has led to the identification, of two main directions of NE-SW and NW-SE fractures.

The Sokoto Basin evolves as rift basin during the Early Maastrichtian in response to NE-SW extensional/ transtenssional regime. During Late Maastrichtian a basin inversion affected the Dukamaje Formation in response to NW-SE horizontal shortening/thrusting and basin inversion. During early Paleocene the Wurno Formation was rifted. The Dange Formation, Kalambaina Formation and oolitic ferruginous sandstone are folded in response to Late Paleocene compressional event.

During earliest Eocene the lower Gwandu Formation is affected by NNW-SSE extensional/transtenssional event, while the upper Gwandu Formation was affected by NW-SE late Eocene compressional/transpressional inversion.

The Sokoto Basin was affected by extensional/transtensional rift setting, compressional/transpressional strike-slip setting. This findings will improve the quest of  offshore hydrocarbon resources since worlds offshore hydrocarbon resources are located in rift setting and many hydrocarbon traps are controlled by normal faults, listric faults, structural traps created by folds. Normal faults can form by reactivation of thrust faults and many low angle extensional faults especially in Dukamaje area. The migration of oil and gas occur within a sandstones and karstic limestone. The discovery of strike-slip fault structures in a petroleum reservoir can cause migration of oil across a fault that is elsewhere sealing. Number of the world’s oil resources are located in fold and thrust belts. Contractional deformation structures form when rocks are shortened by tectonic or gravitational forces and occur in various ways.

Acknowledgments are reserved for the Petroleum Technology Development Fund (PTDF), Abuja and the Professorial Chair (Petroleum Technology Development Fund) Usmanu Danfodiyo University, Sokoto for supporting different aspects of the Sokoto Basin hydrocarbon prospectivity evaluation.

The authors declare no conflict of interest.

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