Stromatolites of the Upper Precambrian in the Adrar of Mauritania (western Sahara)
The Adrar of Mauritania is the western part of the Paleozoic (s.l.) Taoudeni Basin. It is composed of three main units separated by two great unconformities which are overlain by glacial deposits. The first paleontologically dated stratigraphic unit is composed of the top of the second formation, with brachiopods sandstone of Late Cambrian to Early Ordovician age, and the base of the third formation, the graptolites shale of Silurian age. The first unit is supposed to be Infracambrian or Upper Precambrian and is characterized, in Adrar, by numerous stromatolitic limestones and dolomites. It is divided in eighteen members themselves subdivided into fifty nine levels.
The stromatolites are of various types: stromatolites with hemispheroidcl laminations (LOGAN et al., 1964), oncolites with spheroidal laminations, cono- phytons and stron-ntolites with flat and continuous laminations (LOGAN,1961). From the study c,f the mor: typical stromatolite formations we can make three main remarks on the taxonom;, the texture and the paleogeographical significance of stromatolites.
- 1The hemispheroidal-lamination stromatolites and the conophytons can be associated in a single structure. The branched stromatolites of Ij are built with a conophyton as trunk and a hemispheroïdal-lamination stromatolite of SH-V type (discrete, vertically, stacked hemispheroids in LOGAN et al., 1964, classification) as branches. Hence, there is no fundamental difference in the degree of development of these two kinds of stromatolites.
- 2In the conophyton trunk, the dark layers of the stromatolitic texture are formed by micrite and the light (or corpuscular) layers by dolsparite with few sparite crystals. The alternation of dark and light layers is irregular; the dark layers are often associated to form 1-mm thick rings, which are obvious in polished samples colored by copper nitrate or alizarine.
- 3The elongated “stromatolite reefs”, very numerous, have a constant direction of elongation, north-northeast to northeast, the same as the principal paleocurrent direction deduced from the study of ripple-marks, bounce-marks, groove-marks and striation-marks orientation of the interbedded clastic sediments. Hence, the “strornatolite reefs” could be used successfuly in drawing paleogeo- graphic maps.
The ecology of the Upper Precambrian stromatolites of Adrar is similar to that of the present stromatolites. Their wide distribution, from Atar to the Algerian frontier, involves the existence in western Africa, in Late Precambrian time, of a large shallow-marine basin with very scarce clastic sediments. Such a basin is unknown in present seas.