Holocene sponge bioherms on the western Canadian continental shelf

Abstract

Bioherms dominated by hexactinellid sponges occur in the three major troughs which cross the north-central continental shelf of British Columbia, Canada. The preferred substrate of the bioherms is till, glaciomarine sediments or bedrock in 150 to 250 m water depth. These bioherms began to develop in the early Holocene and have expanded since that time to discontinuously mantle more than 700 km 2 today. The sponge colonies form mounds and sheet-like structures up to 15 m thick and several kilometres wide and accrete new material to the bioherm through the removal of silty clay from suspension by the baffling of bottom currents. The sedimentologic and morphologic characteristics of the bioherms are in part due to fused siliceous mainframe skeletons of the sponges that provide the framework for the construction of the bioherm. The locus of sponge habitation on the surface of the bioherms changes with time in response to sediment accretion and microtopographic changes which affect delivery of sediment and nutrients entrained in bottom currents.