Facies of Liassic sponge mounds, central High Atlas, Morocco

Liassic sponge mounds of the central High Atlas (Rich area, northern Morocco) have a stratigraphic range from the Lower/Upper Sinemurian boundary interval up to the lower parts of the Lower Pliensbachian (Carixian). The base of Liassic sponge mounds consists of a transgressive discontinuity, i.e., a condensed section of microbioclastic wackestones with firm- and hardgrounds, ferruginous stromatolites, sponge spicules and ammonites. The top of Liassic sponge mounds is an irregular palaeorelief covered by cherty marl-limestone rhythmites, namely hemipelagic spicular wackestones with radiolaria. In the Rich area, section Foum Tillicht, the sponge mound succession has a total thickness of about 250 meters. Within this succession we distinguished between three mound intervals. The lower mound interval shows only small, meter-scale sponge mounds consisting of bound-stones with lyssakine sponges, commensalic Terebella and the problematicum Radiomura. This interval forms a shallowing-upward sequence culminating in a bedded facies with Tubiphytes, calcareous algae (Palaeodasycladus), sponge lithoclasts, coated grains, and thin rims of marine cement. The middle mound interval is aggradational with decametric mounds and distinct thrombolitic textures and reefal cavities. The mound assemblage here consists of hexactinellid sponges, lithistid demosponges, non-rigid demosponges, Radiomura, Serpula (Dorsoserpula), Terebella, encrusting bryozoa, and minor contributions by calcareous sponges, and excavating sponges (type Aka). Thrombolites are dendrolitic and may reach sizes of several tens of centimeters, similar to the maximum size of siliceous sponges. The upper mound interval appears retrogradational and geometries change upsection from mound shapes to Bat lenses and level-bottom, biostromal sponge banks. The biotic assemblage is similar to that of the middle mound interval and there is no difference between mound and bank communities. The demise of sponge mounds is successive from regional spread in the Sinemurian to more localised spots in the Lower Pliensbachian. This reduction correlates with an increasing influence of pelagic conditions. At Foum Tillicht, sponge mounds lack any photic contribution and there is virtually no differentiation into subcommunities between mound surface and cavity dwelling organisms. There is some evidence that the heterotrophic food web of mound communities was sourced by oxygen minimum zone edge effects, namely microbial recycling of essential elements such as N and P. Basin geometry suggests a waterdepth of several 100's of meters, well below the photic zone and possibly only controlled by the depth range of the oxygen minimum zone. Palaeoceanographic conditions of well-stratified deeper water masses diminished gradually during widespread transgression across the Sinemurian to Pliensbachian boundary culminating in the Lower Pliensbachian ibex ammonite zone.