Sedimentary and foraminiferal facies in Exmouth Gulf, in arid tropical northwestern Australia

Exmouth Gulf is a major U-shaped embayment on the northwestern coast of Western Australia, at a latitude of 22 degrees S. Water temperatures are 18-31 degrees C and normal oceanic salinity is maintained by strong tidal currents despite the hot, arid climate. A series of sediment grab samples were collected and analysed for particle-size and foraminiferal diversity. Samples contained mud, quartzose fine sand and coarse carbonate sand fractions. The muddiest facies are located in the most sheltered areas of the gulf: mangrove channels, tidal flats, southwestern flanks and the deeper axial region. Quartzose fine sands probably have mixed origins which might include: southern aeolian dunes; cyclone-related reworking of beach and near-shore deposits; and reworked relict shelf alluvium. The shallow-water fair-weather wave climate may play a significant role in localised sediment dispersal and sorting along the eastern margin of the gulf. Sediment distributions within the gulf are complicated by low sedimentation rates through much of the central and western areas of the gulf, significant mixing, and possible inheritance of pre-Holocene alluvium. The Holocene foraminiferal assemblage recorded from Exmouth Gulf is overwhelmingly dominated by benthic species: agglutinated, calcitic-porcellaneous, and calcitic-hyaline groups. The distribution of individual foraminiferal species shows relatively simple patterns, governed by environmental parameters. Live individuals are rare.