Modern siliceous sponge reefs in a turbid, siliciclastic setting: Fraser River delta, British Columbia, Canada

Siliceous sponges have formed reefs within the prodelta of the Fraser River, British Columbia, Canada, in an area where sedimentation rates adjacent to the reef site are greater than 2 cm per year. These newly discovered reefs differ in important ways from those described previously from the northern British Columbia continental shelf. The reefs consist of roughly circular inter-connected mounds up to 14 m in height and 200 m in diameter, found in water depths of 150-190 m and restricted to the top and flanks of an isolated promontory in the midst of the rapidly expanding Holocene prodelta. Two species of hexactinosidan sponge, Aphrocallistes vastus and Heterochone calyx, build a framework of densely packed sponge skeletons while several other species of hexactinellida and demosponges are accessory fauna. Morphologic differences in the framebuilding species are seen as responses to the extremely different environmental conditions of the sediment starved northern shelf and the turbid delta habitats.