During the Middle to Late Oxfordian (locally ranging into the earliest Kimmeridgian), the epicontinental shelf that persisted in the southeastern paleomargin of Iberia (Prebetic Zone) was largely colonized by siliceous sponges (Dictyida, Lychniskida, and Lithistida, in descending order of abundance). Spongiolithic lithofacies (sponge-rich deposits in which buildups are rare and small) represent a marine environment extensively colonized by sponges in the form of large sponge meadows showing associated epibenthic organisms (brachiopods, bivalves, and echinoderms). Marl-limestone rhythmites represent muddy bottoms colonized by scarce endobenthos (bivalves and irregular echinoids). However, the occurrence of patchy sponge bioherms provided a favorable substrate for epibionts and microbial lithoherms (microbialites). The record of meter-scale buildups showing sponge bioherm-microbial lithoherm in marl-limestone rhythmites resulted from the preferred colonization of sponge patches by benthic microbial communities, the growth of which on surrounding muddy bottoms was very limited or impossible. Sedimentation rate, substrate, bathymetry, light, nutrients, oxygenation, and environmental (water) energy determined the eco-sedimentary conditions responsible for the occurrence and composition of spongiolithic facies in the area studied. Throughout the Late Oxfordian and the earliest Kimmeridgian, the progressive increase in the rate of sedimentation, especially influx of siliciclastics, forced the disappearance of sponge bioherms-microbial lithoherms from the Prebetic shelf.
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Vol. 18 • No. 6