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Senegal - Geography

Senegal is bounded by the Atlantic Ocean, Mauritania, Mali, Guinea, and Guinea-Bissau. The Gambia penetrates more than 320 kilometers (200 mi.) into Senegal. Well-defined dry and humid seasons result from northeast winter winds and southwest summer winds. Dakar's annual rainfall of about 61 centimeters (24 in.) occurs between June and October when maximum temperatures average 27oC (82oF); December to February minimum temperatures are about 17oC (63oF). Interior temperatures are higher than along the coast, and rainfall increases substantially farther south, exceeding 150 centimeters (60 in.) annually in some areas.

Located in southern Senegal is a feature that appears to be a meteor-impact-generated structure, possibly millions of years old. It is a circular, multiple ring structure with an overall diameter of 48 km (30 miles) and centered about 12 km (7 miles) south-southwest of the town of Velingara. The high rim structure of the Velingara Crater encloses the Anambe Basin. Water previously flowing out the south end of the basin was harnessed behind a dam in the mid-1970s as a source of irrigation for rice and other crops. The 1975 image predates the irrigation development. By 2001 intense agricultural systems had appeared near the center of the crater, contrasting sharply with the swampy areas nearby. The Velingara Crater was first detected using Landsat data in the early 1970s. Velingara Ring Impact Crater A 48 km-diameter multi-ring feature, the Velingara structure, has been discovered in Landsat and NOAA-AVHRR images of southern Senegal (centred on 14°7’40”W, 13°02’13.2” N)[1]. The structure is developed in Mid-Eocene marine sediments of the coastal Senegal Basin, andhas been buried by up to 90 m of post-Eoceneunfossiliferous continental sediments. Drilling andresistivity surveys have revealed the presence ofsubcropping Neoproterozoic or Palaeozoic metamorphicbasement rocks in the centre of the structure. There is also apositive Bouguer gravity anomaly associated with thestructure. We interpret these features as representing apossible shallowly-buried complex impact crater with acentral uplift, which may be linked with other impactevents of the Late Eocene, or the terminal Eocene massextinction.The feature, which appears in published Landsat MSS& TM images (Ref. 2, p. 370; Ref. 3, p. 100), as well as inNOAA-AVHRR images, is a circular multiple ringstructure with an overall diameter of 48 km. It is centred onan area about 12 km SSW of the town of Velingara inHaute Casamance Province, southern Senegal. Thenorthern rim of the structure just touches the border withthe Gambia, while the southern rim is about 20 km N of theborder with Guinea-Bissau. The region is one of relativelyflat topography and low elevations (about 70 m above sealevel), and is characterised by thick ferruginous lateriteswith no outcrops, and an open tree savannah vegetation. Photogeologically, the Velingara structure has a darkcore, c. 20 km in diameter, which is surrounded by a light-coloured ring (defined by land use patterns), which is about7 km wide. This is followed by an outer dark ring, and onthe perimeter, a discontinuous light ring, from which anumber of streams emerge in a radial centrifugal pattern.Four of the most prominent of these streams, to the W ofthe structure, form the source of the Casamance River.Other streams issuing to the NW, N and E flow into theGambia River or its tributaries. Streams from the SE, S andSW parts of the outer ring drain into the Kayanga River,which flows SW into the Geba estuary of Guinea-Bissau.There is a superficial similarity in terms of size andappearance to the Araguinha ring structure in Brazil, in which a basement core is exposed. The central part of ofthe Velingara structure is a broad topographic depression,the Anambé basin, in which centripetal drainageconcentrates in a central swampy area at 22m elevation,which has an outlet in the SE-flowing Anambé river, atributary of the Kayanga. The soils of the central Anambébasin are hydromorphic sandy clay loams with a heaviersubsurface horizon that thickens towards the centre of thedepression, where the whole profile may have a clayeytexture [4]. These wet soils, which are regularly inundatedduring the rainy season as well as part of the dry season,are responsible for the dark colour of the core, which isaccentuated in Landsat Band 5. The light and dark ringssurrounding the Anambé depression define a broad flattorus, with elevations between 40 and 70 m above sea level,which forms a watershed between the centripetal drainageof the Anambé basin and the centrifugal drainage arisingfrom the outer light ring. The inner light ring is producedby deforestation of the tree savannah related to farmingactivities in a series of small villages along main roadswhich follow the more elevated area surrounding thecentral depression.The country rocks are Paleogene (Paleocene and Earlyto Middle Eocene) marine carbonate sediments of theSenegal Basin, which are overlain by an average of 40 m ofunfossiliferous continental sandstones and shales of post-Eocene (Neogene) age (locally called “Continentalterminal”) [5]. The Paleogene sediments overlieNeoproterozoic to Paleozoic rocks of the Mauritanide belt,and were deposited in a passive margin following riftingand the opening of the Atlantic Ocean [6]. Themetamorphic Mauritanide basement rocks are exposed inthe core of the Velingara structure, which although it hasthe form of a structural basin (the Anambé depression), isclearly a zone of basement uplift.The central basement uplif The economic importance of the Velingara structure is evidenced by the major SODAGRI agricultural project being carried out in the Anambé basin, which owes itspeculiar hydrology and waterlogged soils to the presence ofan impermeable basement close to surface. As a buriedpostulated impact crater, the Velingara structure has thepotential for hosting hydrocarbons (oil or gas), as is the case with several buried impact structures in North America. Further field and petrographic studies (including the search for microscopic shock deformationfeatures) are in progress. Attempts are being made to locate the original drill cores of bore holes drilled in the Velingara structure in 1979-1982, and to sample Eocene sedimentary rocks which would have formed the target rocks during the postulated impact event.