Petrology of soft tertiary sedimentary rocks and its relationship to geomechanical behaviour Central North Island, New Zealand

Abstract

The petrology of soft sedimentary rocks (Miocene - Pleistocene) from the Wanganui Basin (western central North Island) and the East Coast Deformed Belt (Wairarapa and northern Hawke's Bay) is investigated and results are compared with geomechanical performance. The rocks fail by slabbing and slaking and geotechnical properties indicate that they have experienced stress relief. Dominant detrital components are quartz and albite immersed in a clay matrix of illite, chlorite and smectite. Accessory minerals include muscovite, biotite sphene and hornblende. Clay minerals occur as discrete species and as randomly interlayered chlorite-smectite, illite-smectite, illite-chlorite and illite-chlorite-smectite. Quantitative clay mineralogical techniques are discussed in detail. Presence of organic matter has commonly caused the formation of framboidal pyrite and glauconite. The rocks are uncemented and calcite is contributed by fossil fragments. Samples from northern Hawke's Bay contain large amounts of allophane, and small quantities of authigenic kaolinite and zeolite were found in some samples from the Wanganui Basin. Diagenetic influence on mineralogy and chemistry is, however, insignificant as are compositional changes during prolonged exposure. Breakdown by slaking is therefore a physical process. Compositional variation is small and trends do not readily correlate with geomechanical parameters. PI and LL tend to increase with smectite content, but both are determined on disaggregated material and of limited use in evaluating in situ characteristics. Swelling strain and stress show no clear relationship with smectite content. Rock fabric plays a decisive role in determining behaviour. Structural elements which form its basic building blocks are clay microaggregates, grains, microfossils and authigenic crystals. Their common distinctive feature is stability when suspended in water. Further features which are often found to act as structural elements are larger aggregates which are classified by origin and morphology. These are less stable and may assume the role of critical weak links in the fabric structure. Their breakdown decreases effective grainsize. Three fabric type endmembers - the matrix, turbostratic and skeletal fabric types - provide a framework within which the fabrics of soft rocks can be described adequately. Matrix and turbostratic microfabrics form an evolutionary series. Their relationship to compaction and stress relief and the evolution of fabric geometry and contact strength during loading and unloading is discussed. Skeletal microfabrics contain less clay which is organized into aggregates, grain coatings and connector assemblages. Strength and cohesion increase from matrix to turbostratic microfabrics, but decrease with increasing frequency of skeletal microfabric domains. Highest swelling characteristics are recorded from samples with continuous matrix which have experienced some degree of compaction. The fabrics are dynamic and can adjust to changing stress conditions. Their response to slow loading and unloading is quasi-elastic. Only during the fast final stages of unloading by erosion is a residual stress system set up in the fabric which results in stress relief fractures parallel to topography along which slabbing failure takes place. Fabric characteristics determining strength and cohesion of the rocks are the product of their geological history (no cementation, shallow burial, stress relief with high uplift rates persisting to the present).When designing projects involving soft rocks, it must be taken into account that many problems associated with them (low resistance to erosion and weathering etc.) are inherent and can not be entirely prevented.