Origin of Late Pleistocene Bryozoan Reef Mounds; Great Australian Bight

• DOI: 10.1306/062303740020
• Publication Date: 2007-12-17T10:52:13Z
• Authors: N. P. James; D. A. Feary; C. Betzler; Y. Bone; A. E. Holbourn; Q. Li; H. Machiyama; J.A. T. Simo; F. Surlyk
• Keywords: 550; 14. Life underwater; 01 natural sciences; 0105 earth and related environmental sciences

Bryozoan-rich biogenic mounds grew periodically on the prograding carbonate slope of central Great Australian Bight throughout Pliocene-Pleistocene time. Cores from three ODP Leg 182 drill sites provide a record mound growth during last 300,000 years over stratigraphic thickness similar to 150 m. These mounds, first such structures described modern ocean, between paleodepths 100 and 240 m; we infer that upper limit was established by swell wave base, lower boundary fixed an oligotrophic water mass. Detailed chronostratigraphy, based radiometric U-series dating, benthic foraminifer stable-isotope stratigraphy, planktonic abundance ratios, confirms buildups flourished glacial lowstands (even-numbered marine isotope stages) but were largely moribund interglacial highstands are not extant today. Mound floatstones compositionally mixture in situ bryozoans comprising 96 genera characterized fenestrate, flat robust branching, encrusting, nodular-arborescent, delicate branching forms. The packstone matrix comprises autochthonous allochthonous sand-size bryozoans, foraminifers, serpulids, coralline algae, sponge spicules, peloids, variable glauconite quartz grains, together with mud-size ostracods, tunicate bioeroded chips, coccoliths. Intermound, local grainstone contain particles, they conspicuously worn, abraded, blackened, bioeroded. An integrated model accretion sea-level begins bryozoan floatstone increases diversity upward 5-10 m, culminating thin intervals reduced locally abraded fossils. accumulation relatively rapid (30-67 cm/ky) punctuated rudstones firmgrounds. Intermound highstand deposition comparatively slow (17-25 typified meter-scale, fining-upward packages or burrowed packstone, firmgrounds overlain characteristically particles. periods is interpreted have resulted increased nutrient supply enhanced primary productivity. Such elevated trophic resources both regional local, thought be focused this area cessation Leeuwin Current flow, northward movement subtropical convergence related dynamic mixing.