Modern carbonate and terrigenous clastic sediments on a cool water, high energy, mid-latitude shelf: Lacepede, southern Australia



The wide Lacepede Shelf and narrow Bonney Shelf are contiguous parts of the south-eastern passive continental margin of Australia. The shelves are open, generally deeper than 40 m, covered by waters cooler than 18°C and swept by oceanic swells that move sediments to depths of 140 m. The Lacepede Shelf is proximal to the ‘delta’of the River Murray and the Coorong Lagoon. Shelf and upper slope sediments are a variable mixture of Holocene and late Pleistocene quartzose terrigenous clastic and bryozoa-dominated carbonate particles.

Bryozoa grow in abundance to depths of 250 m and are conspicuous to depths of 350 m. They can be grouped into four depth-related assemblages. Coralline algae, the only calcareous phototrophs, are important sediment producers to depths of 70 m. Active benthic carbonate sediment production occurs to depths of 350 m, but carbonate sediment accumulation is reduced on the open shelf by continuous high energy conditions.

The shelf is separated into five zones. The strandline is typified by accretionary sequences of steep shoreface, beach and dune carbonate/siliciclastic sediments. Similar shoreline facies of relict bivalve/limestone cobble ridges are stranded on the open shelf. The shallow shelf, c.40–70 m deep, is a wide, extremely flat plain with only subtle local relief. It is a mosaic of grainy, quartzose, palimpsest facies which reflect the complex interaction of modern bioclastic sediment production (dominated by bryozoa and molluscs), numerous highstands of sea level over the last 80 000 years, modern mixing of sediments from relatively recent highstands and local introduction of quartz-rich sediments during lowstands. The middle shelf, c.70–140 m deep, is a gentle incline with subtle relief where Holocene carbonates veneer seaward-dipping bedrock clinoforms and local lowstand beach complexes. Carbonates are mostly modern, uniform, clean, coarse grained sands dominated by a diverse suite of robust to delicate bryozoa particles produced primarily in situ but swept into subaqueous dunes. The deep shelf edge, c. 140–250 m deep, is a site of diverse and active bryozoa growth. Resulting accumulations are characteristically muddy and distinguished by large numbers of delicate, branching bryozoa. The upper slope, between 250 and 350 m depth, contains the deepest platform-related sediments, which are very muddy and contain a low diversity suite of delicate, branching cyclostome bryozoa.

This study provides fundamental environmental information critical for the interpretation of Cenozoic cool water carbonates and the region is a good model for older mixed carbonate-terrigenous clastic successions which were deposited on unrimmed shelves.