A siliciclastic coastal Sabkha, capricorn coast, Queensland, Australia

Abstract

A small (3 km 2), modern, siliciclastic sabkha occurs landward of Kemp Beach on the Capricorn Coast of central Queensland, Australia. Five sedimentary facies were recognized in shallow cores: (1) surface sand layer and associated halite crust; (2) a mottled clayey peat unit displaying interstitial gypsum (this layer has formed by vertical accretion of algal mats); (3) a unit of mangrove peat and clay with abundant jarosite corresponding to the locations of previous mangrove-lined tidal channels; (4) a carbonaceous sandy unit which represents the sediment that accumulated in a lake bordered by mangroves; and (5) a basal, well-sorted sand unit. Evaporative pumping during dry winter months is the dominant hydrologic process in this sabkha. Water lost from subsurface brines is replenished by subsurface flow of sea water via a pre-existing tidal channel now covered by the prograding sequence. The main aquifer below the sabkha surface is the carbonaceous sandy unit. The construction of a weir across the tidal channel maintains a relatively uniform water level below the sabkha surface during the wet summer period. However, during dry winter months, the piezometric gradient is such that the water flow is primarily unidirectional from the ocean inlet, through the mangrove-lined channel, to the subsurface aquifer below the sabkha. Analyses of subsurface waters revealed a gradient from almost normal marine salinity in the lake to a four-fold increase in subsurface brines. Correspondingly, there is a linear increase in major cation concentrations, with the exception of Ca 2+ which becomes depleted due to precipitation of gypsum. During dry winter months a halite crust forms at the surface. In contrast to most previously described sabkha environments, aragonite does not occur. Its absence may be explained by the large organic matter content of the sediments within the aquifer, inhibiting calcium-carbonate precipitation.