Pr. Gammon et Np. James, Palaeogeographical influence an Late Eocene biosiliceous sponge-rich sedimentation, southern Western Australia, SEDIMENTOL, 48(3), 2001, pp. 559-584
Late Eocene nearshore shallow-marine environments within the Bremer and wes
tern Eucla Basins of southern Western Australia were characterized by the t
hick deposition of spongolite and spiculite deposits. Epibenthic sponge com
munities dominated estuaries and topographically complex basin margin embay
ments-archipelagos, while cool-water carbonates with up to 10% sponges accu
mulated in open-shelf environments. The transition from a biosiliceous to c
alcareous epibenthos was related to the degree of palaeogeographical 'prote
ction'. Within basement-protected embayments there was an offshore gradatio
n from shoreface spongolite and pure spiculite to a muddy spiculite facies
towards central embayment areas. Calcareous fossils are rare throughout emb
ayment facies, but rapidly increase in more open outer archipelago areas. T
his depositional relationship occurred along 2000 km of the Late Eocene sou
thern Australian coastline. Palaeogeographical protection from strong curre
nts acted in concert with: (1) a planar, low-gradient inland topography wit
h sluggish run-off, supplying fine-grained sediment, nutrients, and abundan
t dissolved silica; and (2) a microtidal setting, weak to moderate swells a
nd opposing wind and Coriolis surface current forcing, which inhibited wate
r exchange between embayments-estuaries and the open shelf. This situation
led to an embayment water chemistry that encouraged prolific sponge growth.
Calcareous spiculites record the mixing front between these embayment wate
rs and normal open-shelf waters supporting cool-water carbonates.