Sedimentary Environments of Shark Bay, Western Australia

Abstract

Shark Bay is a large embayment at lat. 25°S on the western coast of Australia. Studies of the environment have been carried out in conjunction with investigations of carbonate sedimentation and diagenesis in the region. In dimension, Shark Bay approaches a small epicontinental sea; the area is approximately 5,000 sq mi and average depth is 30 ft. The water mass is cut off from the Indian Ocean by a ridge of calcareous eolianite and is subdivided internally into numerous inlets, gulfs, and basins by dune ridges and submerged banks (sills). Influx of oceanic water is only through openings in the northern part of the outer barrier. The embayment is adjacent to a low-relief, arid to semi-arid hinterland. Runoff influx is negligible and evaporation greatly exceeds precipitation. These factors, combined with the hydrologic structure of the water mass and restriction imposed by banks and sills, result in increasing gradients of salinity into the closed southern parts of the embayment. The salinity is from 36 ^pmil in the north to as high as 65 ^pmil in the south. The water mass has a layered structure with nearly vertical attitude. Major clines subdivide the water body into three major types: oceanic (36-40 ^pmil), metahaline (40-56 ^pmil), and hypersaline (56-70 ^pmil). The water types are limiting on the biota. There are three biotic zones, the distribution of which is essentially similar to the distribution of water types. Within the broad environmental zones, numerous local environments are limited by such factors as wave action, tides and tidal currents, and depth. Depth is of major importance because most other parameters are linked to it.