Foraminiferal evidence for inner neritic deposition of Lower Cretaceous (Upper Aptian) radiolarian-rich black shales on the Western Australian margin

Abstract. Diverse foraminifera, Lingula-like brachiopods and the geological setting indicate that Aptian radiolarian-rich black shales forming the Windalia Radiolarite were deposited at water depths probably less than 40 m in the Southern Carnarvon Basin. Elsewhere in Australia, coeval radiolarian-rich deposits are widespread in other western-margin basins and in vast interior basins. The organic-rich mudstones containing the radiolaria include the foraminiferal Ammobaculites Association, a sparse benthic macrofauna and kerogens of mainly terrestrial plant origin. The deposits suggest that there was substantial high-nutrient freshwater input into the epeiric seas as well as high levels of dissolved silica resulting from marine flooding of a mature silicate-rich landscape bordered on the eastern and western continental margins by large volcanic provinces. The widespread presence of organic-rich muds through the broad, shallow Southern Carnarvon Basin and through the coeval interior basins suggests that regional geomorphology controlled the distribution of eutrophic facies in the Australian Aptian rather than any global expansion of the oceanic oxygen minimum zone. The foraminiferal assemblage from the Windalia Radiolarite consists of calcareous hyaline benthic types (diverse Lagenida as well as abundant Lingulogavelinella, Epistomina and Coryphostoma) and organic-cemented agglutinated species (including common Ammobaculites humei, Haplophragmoides–Recurvoides spp., and Verneuilinoides howchini). Planktonic foraminifera are very rare and present only in the northern, more open part of the basin.