Middle Devonian Stratigraphy of Northeastern Spanish Sahara

Abstract

In support of petroleum exploration, four stratigraphic sections were measured and described in the Middle Devonian outcrop area of northeastern Spanish Sahara. These sediments are exposed in an east-west belt along the south flank of the Tindouf basin. Near the Mauritanian border the Middle Devonian is 106 meters thick, and about 140 kilometers farther west (10° 12^primeW. Long.) it is 356 meters thick, the increased thickness being due to a progressively greater amount of clastics entering the sequence from the west. It is postulated that Caledonian movements affected the West African Paleozoic geosyncline and that emergent areas existed in late Silurian and early Devonian time, located at the western end of the cratonic Tindouf basin. These uplifted areas furnished more Middle Devonian clastics than did the Reguibat arch, which was probably a low, peneplaned area forming the southern boundary of the Tindouf basin. Five major lithologic units are recognized. Unit 1: At the base of the sequence, which rests with slight angular unconformity on Lower Devonian beds, is a uniform shale unit, lower Eifelian in age. Unit 2: Shale with many thin beds of blocky, calcareous siltstone. Includes the only sandstone found in the Middle Devonian and a faunal zone characterized by Uncinulus lodanensis Lower Eifelian. Unit 3: Has characteristics of deposition in an environment of restricted circulation. The beds include two well developed phosphatic limestones, some shales with dwarfed and phosphatized fossils, and a coquinal limestone yielding phosphatic nodules. It is believed to be upper Eifelian in age. Unit 4: Fossiliferous limestone, including patch reefs, interbedded with yellowish gray, calcareous to marly, fossiliferous shale. The circular or elliptical reefs appear to have developed on topographic highs, with bottom sediment type not an important factor. The largest is 24 meters high and 500 meters in basal diameter. Two faunal zones are described from this unit. It is characterized by the presence of Calceola sandalina and is lower Givetian in age. Unit 5: Alternating shale or siltstone and biostromal limestone, characterized by the presence of Phillipsastraea. Upper Givetian. The contact with the Upper Devonian is conformable, but chemical and biological changes are believed to indicate a colder climate following the warm conditions of the Givetian.