Sedimentology and the stratigraphic sequence of a tropical tidal flat, north-western Australia

Abstract

King Sound, with its semiarid-tropical climate and large tides (11.5 m maximum), offers an opportunity to study unique mangrove-fringed tidal flats. Although much of the coastline is in an erosional phase there are local areas of deposition where the sedimentologic processes are active and stratigraphic sequences are developing. The tidal flats are zoned into several geomorphic/lithofacies units: (1) low tidal sand flats are underlain by megarippled sand and shelly sand; (2) mud/sand slopes encompass mid to low tidal levels and are underlain by mud/sand laminite and mud laminite; (3) mangal/mud flats, occurring between MSL and MHWS, harbour an abundant, diverse biota including mangroves, crustacea and molluscs; bioturbated mud is accumulating on these flats; (4) salt flats are the highest tidal unit and they are underlain by laminated and vesicular mud. The distribution of sediment types, sedimentary structures and biota across tidal flats is related ultimately to frequency of inundation and groundwater salinity. Physical processes dominate over biological in the frequently inundated, well-winnowed low tidal areas; upslope, sediments become finer and biogenic activity progressively becomes dominant, reaching a peak in the mangal. Thereafter to landward infrequent wetting and hypersalinity results in decrease in biota. With lateral progradation or vertical shoaling a stratigraphic sequence is generated that reflects the dynamic history of tidal flats. Laterally prograded sequences contain the four lithofacies in a stacked sheet-like array. Shoaled sequences are composed of three or four lithofacies and contain abundant erosional contacts, erosional products and shoal geometry bedforms within and between lithofacies. Regional tidal flat accretion appears to take place by an alternation of aceretion and erosion which results in laterally prograding sequences alternating with shoaling sequences.