Facies of Liassic sponge mounds, central High Atlas, Morocco

Liassic sponge mounds of the central High Atlas (Rich area, northern Morocc o) have a stratigraphic range from the Lower/Upper Sinemurian boundary inte rval 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 hardground s, ferruginous stromatolites, sponge spicules and ammonites. The top of Lia ssic sponge mounds is an irregular palaeorelief covered by cherty marl-lime stone 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 distinguishe d between three mound intervals. The lower mound interval shows only small, meter-scale sponge mounds consisting of bound-stones with lyssakine sponge s, commensalic Terebella and the problematicum Radiomura. This interval for ms a shallowing-upward sequence culminating in a bedded facies with Tubiphy tes, calcareous algae (Palaeodasycladus), sponge lithoclasts, coated grains , and thin rims of marine cement. The middle mound interval is aggradationa l with decametric mounds and distinct thrombolitic textures and reefal cavi ties. The mound assemblage here consists of hexactinellid sponges, lithisti d demosponges, non-rigid demosponges, Radiomura, Serpula (Dorsoserpula), Te rebella, encrusting bryozoa, and minor contributions by calcareous sponges, and excavating sponges (type Aka). Thrombolites are dendrolitic and may re ach sizes of several tens of centimeters, similar to the maximum size of si liceous sponges. The upper mound interval appears retrogradational and geom etries change upsection from mound shapes to Bat lenses and level-bottom, b iostromal sponge banks. The biotic assemblage is similar to that of the mid dle mound interval and there is no difference between mound and bank commun ities. The demise of sponge mounds is successive from regional spread in th e Sinemurian to more localised spots in the Lower Pliensbachian. This reduc tion correlates with an increasing influence of pelagic conditions. At Foum Tillicht, sponge mounds lack any photic contribution and there is v irtually no differentiation into subcommunities between mound surface and c avity dwelling organisms. There is some evidence that the heterotrophic foo d web of mound communities was sourced by oxygen minimum zone edge effects, namely microbial recycling of essential elements such as N and P. Basin ge ometry suggests a waterdepth of several 100's of meters, well below the pho tic zone and possibly only controlled by the depth range of the oxygen mini mum zone. Palaeoceanographic conditions of well-stratified deeper water mas ses diminished gradually during widespread transgression across the Sinemur ian to Pliensbachian boundary culminating in the Lower Pliensbachian ibex a mmonite zone.