Sedimentation and Tidal-Flat Development, Nilemah Embayment, Shark Bay, Western Australia1
Raymond G. Brown, Peter J. Woods, 1974. "Sedimentation and Tidal-Flat Development, Nilemah Embayment, Shark Bay, Western Australia", Evolution and Diagenesis of Quaternary Carbonate Sequences, Shark Bay, Western Australia, Brian W. Logan, James F. Read, Gregory M. Hagan, Paul Hoffman, Raymond G. Brown, Peter J. Woods, Conrad D. Gebelein
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Nilemah embayment lies in a land-locked arm of Shark Bay on the arid to semiarid Western Australian coastline. Although sheltered from prevailing wind and tidal influences, the embayment is infrequently exposed to effects of cyclones (hurricanes). A Quaternary sediment wedge several meters thick contains three marine sequences separated by calcareous soils. Each transgression was followed by buildup of seagrass banks with skeletal packstone and wackestone. During the Holocene-recent, the hydrologic regime became hypersaline and a prograding tidal-supratidal flat developed.
Beneath the tidal-flat sequence, skeletal packstone and wackestone units reflect the influence of seagrasss in upwardshoaling sequences. Intervening soil horizons with calcrete lithologies formed during periods of emergence; the soil profiles are differentiated according to substrate conditions. Reworked soil particles form distinctive lithologies when admixed with marine detritus in some units.
Major controls on tidal-flat facies have been basin-water hydrology, an arid climate, and regressive sedimentation across a low-gradient substrate. Hypersaline basin waters supported a restricted fauna that generated abundant skeletal debris but little mud. A low-energy hydrodynamic regime favored development of algal mats as continuous sheets in the intertidal zone; lithofacies with cryptalgal fabrics and structures developed as carbonate grains were recrystallized and cemented into diagenetic packstone and wackestone. In the supratidal zone, evaporation induced crystallization of gypsum as a disruptive fabric element. Storms reworked lithified sediments such as intraclast grainstone and transported skeletal detritus shoreward to beach ridges that prograded across the supratidal flat. The vertical succession of shallow-marine and tidal-flat units indicates progradation as sedimentation proceeded concurrently with marine regression.