Paleoenvironmental significance of Celleporaria (Bryozoa) from modern and tertiary cool-water carbonates of southern Australia

Abstract

Certain members of the bryozoan genus Celleporaria form large, erect colonies of hollow branches (∼10-30 cm tall and 1-3 cm diameter). These are common and conspicuous in Pleistocene and Cenozoic neritic strata of the southern margin of Australia. Most of these basins are characterized by decimeter-scale cycles through subtidal, heterozoan, cool-water, carbonate sediments. Several intervals are further characterized by pervasive celleporarid bryozoan thickets (10-30 cm thick), individual units of which can be traced for many tens of kilometers. Study of modern, live celleporarid bryozoans from the continental shelf of the Great Australian Bight (GAB), Pleistocene celleporarid mounds stranded below the shelf break of the Australian margin, and Miocene celleporarid thickets from the Murray and Torquay Basins indicates that Celleporaria thickets form under specific environmental conditions: i.e., low-energy or sub-swell wave-base settings, mud-silt substrate, medium-high mesotrophy, and moderate sedimentation rate. These conditions, however, are not site specific. They are manifested here in two different environmental settings: (1) a deep-water (>200 m), upwelling nutrient source, below the shelf break during extreme sea-level low stands; and (2) shallow-water (<50 m), terrestrial nutrient source, in embayments, far inland from the shelf break during relative high stands of sea-level. The celleporarid growth habit (species specific-not an ecomorph) allows their populations to proliferate in spatially restricted, transitional, geologically short-lived environments where most other bryozoans cannot cope. Their environmental sensitivity means that Celleporaria have great potential for paleoenvironmental analysis of Cenozoic carbonate Tethyan sediments.