Abstract

Accumulation of carbonate mud is minor on the Northwest Shelf (Western Australia), a broad (< 200–300 km wide), distally steepened, tropical carbonate ramp. It is negligible along most of the inner ramp (0 to 50 m water depth) owing to siliciclastic (riverine) input. It is also negligible along the mid (from 50 to 120 m) ramp, and where the entire ramp narrows to < 100 km, owing to constant reworking by storm waves and ocean swell. In these areas, carbonate mud is derived mostly from mechanical degradation of skeletal fragments, but this does not include calcified remains of calcareous green algae (e.g., Halimeda) although these plants grow locally. Only along the outer (120 to 200 m water depth) ramp and slope is there any relatively abundant accumulation of carbonate mud. Here the sediment consists of modern pelagic (foraminifer, pteropod, and nannoplantkon) ooze and aragonitic needle-rich (< 2 μm) micrite, the latter having an age (based on AMS ¹⁴C) of ~ 19 ka. Distribution of aragonite micrite defines an extensive (5.0 × 104 km²), yet relatively narrow (< 40 km), tract running ~ 600 km along the platform margin. Crystal morphology, mineralogy, and isotope (C, O) composition identify the micrite to be a seawater precipitate, its production and seaward export having occurred during the Last Glacial Maximum, when lowered sea level transformed the otherwise broad platform into a narrow epicontinental shelf margin, now the modern distal slope. Today, this sediment body lies stranded in deep water, about 200–300 km seaward of the shallow inner ramp. The volume of micrite produced appears equal to the present-day production potential of the Great Bahama Bank.

Rapid postglacial rise in sea level, and change in platform geometry, climate, and oceanography, were very influential factors, in addition to water depth and area of submergence, in the production and distribution of aragonite micrite along the Northwest Shelf. In general: (1) epicontinental carbonate platforms can lack substantial micrite accumulation when oceanic current and wave energies sweep unimpeded across a platform, and coastal clastics and absence of shoal barriers prevent development of protective lagoons and carbonate tidal flats; (2) not all highstand carbonate systems are associated with significant amounts of micrite in shallow water environments, nor do they necessarily export large volumes of micrite to the periplatform realm; and (3) some lowstand systems can produce and export volumes of carbonate mud that rival highstand systems.