LOWER KIMMERIDGIAN ECHINIDS OF THE CHELLA LA MOUNTAINS (WESTERN ALGERIA) - REVISION, PALEOENVIRONMENT AND EUSTASY

The rich Echinid fauna of the Malm, in the Chellala Mountains (Tellian fore-land), known since the last century is revised from the taxonomi c point of view, owing to new specimens (section of Djebel Keskess). I t is dated to the Lower Kimmeridgian after the discovery of Ammonites of the Hypselocyclum zone. The study of the sedimentary organization r elative to the Kimmeridgian series of Keskess (Hypselocyclum zone to A canthicum zone) makes it possible to distinguish a succession of paras equences (first silici-clastic, then carbonated) composing a depositio nal sequence of the 3rd order (according to Vail) with successive syst em tracts : lowstand, transgressive and hight-stand sea-level deposits . The environment which is characteristic of a shallow platform, in ge neral (with some episodic indices of deficient salinity at the base of the parasequences), is slightly liable to flooding. It is temporarily influenced by the open sea (eustatic oscillatious ?) when we note the development of carbonate facies and the stopping of terrigenous mater ial (= the more distal part of an immense fluvio-deltaic stretch of Sa harian origin). The sedimentary mechanism largely controlled by eustas y but also by subsidence (with an increased part during the principal deepening phases), leads to the accentuation of the infilling processu s of the Nador-Chellala preatlasic trough (well differentiated since t he Oxfordian) before its closing at the Upper Kimmeridgian (= installa tion of an extensive carbonate tidal-flat, uncovered at low tide).