Pr. Gammon et al., Eocene spiculites and spongolites in southwestern Australia: Not deep, notpolar, but shallow and warm, GEOLOGY, 28(9), 2000, pp. 855-858
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.