Eocene spiculites and spongolites in southwestern Australia: Not deep, notpolar, but shallow and warm

Siliceous spongolite and spiculite are generally interpreted as deep- and/o r cold-water deposits, largely because modern analogs are found mostly in c old- and/or deep-water environments. However, extensive shallow-water, bios iliceous accumulations are present in late Eocene age deposits of southern Western Australia. The attributes and paleoenvironmental setting of these a ccumulations frame the specific climatic and/or oceanographic constraints t hat allow such shallow neritic sediments to form. The generally fine graine d siliciclastic-biosiliceous sediments accumulated in a nearshore, shallow- water setting, around numerous islands, and offshore from a warm-temperate, rainforest-covered upland. Sponges compose a lithistid-hexactinellid assem blage of Mesozoic Tethyan affinity, confined to deep water in the modern oc ean, that migrated into neritic paleoenvironments during late Eocene time. Lithistids dominated shoreface environments and were reworked into sponge-c onglomerate beaches, while hexactinellids flourished offshore in muddy subs horeface environments. These deposits also contain common soft demosponge s picules, and a rare but otherwise normal marine calcareous biota. The biosi liceous sediments grade offshore into bryozoan-rich marls and limestones. S ponges out-competed the calcareous benthos in these inner shelf environment s because of elevated amounts of land-derived nutrients that included high dissolved silica and a calm hydrodynamic setting. These deposits suggest th at neither temperature nor water depth are critical factors for prolific sp onge colonization. The rarity of such deposits in the rock record likely re flects the necessity for the coincidence of all of these factors, or they a re misinterpreted.