Mixed siliciclastic-carbonate shelf sedimentation-Lower Devonian sequences of the SW Anti-Atlas, Morocco

Two types of stacking patterns are observed in prograding siliciclastic wedges. A normal progradational shoreline pattern with well developed parasequence sets in the proximal ramp, and an abrupt-regressive succession in which the amalgamated shelf sandstones rest more or less directly on offshore siltstones in the distal ramp. In the latter case, the falling stage systems tract sandstones are overlain by a relatively thick cyclic alternation with brachiopod-rich storm-beds below a marked transgressive surface at the base of the capping transgressive limestones. The cyclic alternation is interpreted as lowstand systems tract deposits. Marked flooding surfaces on parasequence sets are Fe-enriched and contain abundant articulated brachiopods, indicating breaks in sedimentation and an overall deepening-upward facies from the underlying maximum regression and sequence boundary. These aggradational to retrogradational stacked lowstand systems tracts are thus rather 'transgressive', not regressive in character, as is commonly described in sequence stratigraphic ramp models.