Desiccation of Late Quaternary inner shelf sediments: microfabric observations

Abstract

Palaeo-desiccated crusts often form at the top of pre-Holocene shelf sediments during low sea-level stands resulting in significant changes in the mechanical properties. To assist in understanding these changes and the implications for Quaternary geology, the microfabric of sediment samples from the inner shelf off Hong Kong has been studied using scanning electron microscope techniques incorporating the latest methods of quantitative image analysis. These studies have been supported by petrographic observations and chemical analyses of the pore fluids. Holocene marine sediments usually have a honeycomb fabric while pre-Holocene marine sediments are more compact. In regions of high clay content, the palaeo-desiccated crust often possesses large planar voids filled with coarse silt-sized particles. This desiccated crust often contains relics of root traces filled with pyrite showing varying degrees of oxidation. The release of amorphous iron acts as a cementation agent and binds large silt/small sand grains giving a more compact fabric than that in neighbouring regions. Cementation in the pre-Holocene marine deposits creates strengthened open-fabric arrangements which may initially withstand moderate loads. Further evidence of desiccation comes from the presence of halite crystals in the pore fluid which has salt concentrations below that of normal sea water.