Sedimentation in an Arid Macrotidal Alluvial River System: Ord River, Western Australia

Abstract

In the Cambridge Gulf-Ord River region of Australia, an area that experiences a dry monsoonal climate and a high tidal range (5.9 m), a variety of tidal-plain and fluvial depositional environments were recognized. Near the coast, prograding shorelines are characterized by extensive mangrove successional sequences and scattered thin beach-ridge deposits. In the interfluve regions and farther inland along the coast, tidal-plain sedimentation has given rise to extensive flats that are bare of vegetation at approximately the high spring tide level. Evaporites, algal-layered silts and clays, and massive sandy deposits form the bulk of these tidal-flat sediments. Sinuous tidal channels, displaying low width/depth ratios and characterized by a progressive tide wave, are found throughout the coastal tidal-flat region. These channels migrate laterally, leaving preserved channel sand bodies isolated in tidal deposits. In the funnel-shaped channel of the Ord River, the major river in the region receiving freshwater runoff, the tide wave is symmetrical at the mouth but becomes deformed upstream owing to a high amplitude/depth ratio; thus the velocity of flood currents increasingly exceeds ebb velocities upstream and ebb flow increases in duration. In the upper portions of the tide-dominated river channel, the largest bedforms within the channel migrate upstream under the influence of dominant flood currents, choking the channel with sand deposits. Extensive channel migration and meandering are characteristic of this region of the channel. In addition, extensive overbank crevassing occurs, a process that forms the bulk of the fluvial deposition. In the lower Ord River, linear, elongate tidal ridges within and seaward of the channel form the major sand deposits of the delta. These environments and their distribution and geometrical relationships characterize arid, high-tidal-dominated river systems.