The Great Atlasian Reef Complex: An early Cambrian subtropical fringing belt that bordered West Gondwana

摘要

The so-called Great Atlasian Reef Complex developed during early Cambrian time throughout the Moroccan margin (Souss Basin) of West Gondwana. According to the syndepositional tectonic activity associated with its intracratonic Ediacaran-Cambrian rift, the great reef complex can be subdivided into four major archaeocyathan-microbial reef episodes:. (i) The Atdabanian episode is recorded by a SW-NE-trending, 400. km long barrier reef that extended across the western Anti-Atlas. It was controlled by large-scale reactivation of an inherited rifting branch, which resulted in the nucleation and growth of linear reef complexes located along its margin. The interplay of block tilting, sharp modifications in accommodation space, and relative sea-level rise led to a composite retrogradational-aggradational reef systems tract, characterized by archaeocyathan-microbial kalyptrate complexes (Tiout Member and Amouslek Formation) that protected stromatolite-dominated, back-barrier environments (lower member of the Igoudine Formation).(ii) The western Anti-Atlas recorded an early Botoman reactivation of the same rifting branch that triggered a lateral migration of frame-building centres of carbonate productivity. As a result, the involved grabens and half-grabens recorded the development of fringing mound complexes (lower Issafen Formation). These occupied some linear intra-platform, deeper depressions capped by marls and shales, whereas laterally equivalent shallower environments recorded the development of patch-reefs and bioherms.(iii) The western Anti-Atlas subsequently recorded a late Botoman interval of tectonic quiescence superimposed to a local interval of progradational pulses. This favoured the nucleation of dispersed archaeocyathan-microbial patch-reefs and bioherms.(iv) A distinct palaeogeographic area is recognized in the southern High Atlas, where the entire Atdabanian-Botoman interval recorded small-scale, synsedimentary block tilting and high rates of volcaniclastic input. As a result, this sector was characterized by the record of microbial and archaeocyathan-microbial patch-reefs and bioherms, preferentially developed on the uplifted parts of tilted blocks.The end of reef development and carbonate productivity in the Souss Basin is related to the progradation of siliciclastic depositional systems (Toyonian regression), considered to have caused the collapse of reef communities throughout West Gondwana.