The Upper Cretaceous in the Tagus Basin (Central Spain): sequential analysis based on oil-well data and outcrop correlation
Manuel Segura, Teresa Polo, José F. García-Hidalgo, Javier Gil, Beatriz Carenas, Álvaro García, 2006. "The Upper Cretaceous in the Tagus Basin (Central Spain): sequential analysis based on oil-well data and outcrop correlation", Tectonics of the Western Mediterranean and North Africa, G. Moratti, A. Chalouan
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The logs from six oil wells drilled between 1960 and 1980 in the Upper Cretaceous succession of the Tagus Basin were analysed and correlated with surface outcrops, to estimate sedimentary environments and to detect cyclicity in sedimentation. Log interpretation has been carefully checked with outcrop data, because high peaks in gamma-ray logs, usually considered to represent open marine facies, may correspond either to open marine or to muddy coastal deposits. The former correspond to the maximum flooding surfaces of depositional sequences, whereas the latter correspond to sequence boundaries. Three second-order depositional megasequences have been recognized (MS-2, MS-3 and MS-4). The basal megasequence onlaps older rocks, grading upwards from continental to marine deposits. The megasequence MS-3 shows a basal marly transgressive interval and a thick carbonate pile at the top, grading to the SE to sabkha environments. The top megasequence is hardly recognized in surface outcrops, but in well logs it is a thick evaporite-claystone unit of mainly sabkha environments that grade westwards to coastal deposits. Sedimentation of megasequences MS-3 and MS-4 extended farther west than previously considered, covering areas considered as part of the exposed Hesperian Massif. These were areas of marine and coastal sedimentation where organic matter accumulation could be potentially high, and can be considered potential areas for oil or gas exploration.
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This book provides an insight into the overall tectonic evolution of the Western Mediterranean region and North Africa. The tectonic setting of the region reflects a long-lived and complex evolution, mainly related to the Alpine Orogeny. This inheritance is expressed by an intricate pattern of arc-shaped mountain chains, the Alps, the Betic–Rif Cordilleras and the Apennine–Maghrebian belt, whose southern branches mark the present limit between the African and Eurasian plates. The volume covers the Maghrebian chains in North Africa, from Tunisia to Morocco and the Western and Central Mediterranean, from Spain to Italy from the pre-orogeric phases (Palaeozoic–Mesozoic) to the post-collisional neotectonic and Quaternary development. It includes both original research papers and syntheses dealing with the aspects of structural, sedimentary, metamorphic and marine geology.