COOL-WATER CARBONATES IN AN EOCENE PALAEOESTUARY, NORSEMAN FORMATION,WESTERN-AUSTRALIA

Numerous palaeovalleys formed extensive drowned estuaries during Eocen e transgressions along the southwestern part of the southern margin of Australia. The Tertiary sediments of the Cowan palaeovalley have been extensively drilled, revealing deposition of the Norseman Formation d uring the Middle Eocene Tortachilla transgression. Initial deposition occurred during transgression of the valley to form a drowned estuary. Sediments consisted of coarse-grained muddy, lithic, iron and glaucon ite-rich sands and gravels of mixed carbonate and quartz. Pure carbona tes accumulated during the highstand, produced by a typical shallow te mperate water assemblage of bryozoans, coralline algae, echinoids and molluscs and were swept into shoals by strong tidal currents. Minor '' tropical'' components in the form of large benthic foraminifers and da sycladacean algae are present. Coarse bryozoan and trough cross-bedded carbonate sands accumulated in the margins of the estuary and fine br yozoan sands in the deeper parts. Rhodoliths accumulated to form shoal s in sheltered localities. The Spencer Gulf and Gulf St. Vincent of So uth Australia provide close modern analogues to the Cowan palaeovalley and the Norseman Formation. Modern carbonate sediments off Esperance on the south coast of Western Australia contain ''tropical'' faunal el ements within an otherwise temperate skeletal assemblage and also prov ide a modern analogue. The Norseman Formation thus provides an excelle nt example of cool-water carbonate deposition in near-shore, tide-domi nated environments. This study complements and contrasts existing cool -water shelf facies models based on Tertiary carbonates deposited on d eep shelves elsewhere in southern Australia.