Foraminiferal biofacies across mangrove-mudflat environments at Cocoa Creek, north Queensland, Australia

Contemporary and relict (subsurface) intertidal foraminiferal dead assemblages were collected from a site near to Cocoa Creek, north Queensland, Australia, and analysed for the purpose of understanding fossil assemblage development and preservation. A marked dichotomy was identified between dead foraminiferal assemblages within mangrove sediments and those within adjacent mudflat sediments. Mangrove assemblages were almost exclusively represented by agglutinated species (mainly Arenoparrella mexicana, Haplophragmoides wilberti, Milliammina fusca, M. obliqua, Trochammina inflata) while calcareous species dominated the mudflat (mainly Ammonia aoteana, Pararotalia venusta, Parrellina hispidula). Surface and subsurface test abundances were also an order of magnitude greater within mudflat sediments (1000–3000 tests per cm³) than in the sediments beneath the mangroves (100–300 tests per cm³). Statistically significant differences in the abundance of calcareous and agglutinated tests between wet and dry seasons within the mangrove suggest that tests are systematically lost from this part of the system on an annual basis. A model of assemblage formation under shoreface progradation is presented which makes a direct link between surface assemblages, post-depositional processes and the composition and environmental resolution of the fossil assemblages entering the stratigraphic record. Assemblages within the mangrove undergo rapid taphonomic loss and are then overprinted during burial by the deep infaunal communities associated with the prograding higher-elevation environments. As such, much of the eventual mangrove sequence is represented entirely by assemblages produced relatively recently within an upper mangrove environment, and therefore cannot be differentiated according to elevation at the time of deposition. In addition, organic enrichment of upper mudflat deposits by mangrove roots, which occurs during progradation, results in the dissolution of calcareous tests from an ~40 cm thick stratigraphic interval. It is argued that in order to reliably recognise the changes to foraminiferal assemblages which occur during burial, assemblages must be compared with respect to stratigraphy, rather than reflexively downcore.