Shale lamination and sedimentary processes

Lamination is a common feature of shale and other argillaceous rocks. The type of shale lamination provides an important clue in identifying sedimentary processes when combined with other geological evidence. Presented here is an overview of some common lamination signatures in shale and the sedimentary processes responsible for their formation. Macro-and microfabric and textures of Proterozoic, Palaeozoic, and Mesozoic shales from various sedimentary environments are revealed by X-radiographs, thin-section photomicrographs, and scanning electron microscope micrographs. Characteristic fabrics associated with the following sedimentary processes are presented: (1) bottom flowing currents produce features of fine to thick laminae (approx 0.5–1.0 mm thick) in association with small-scale features of crossbedding, ripple marks, cut and fill and flame structures; (2) deposition from low-density bottom currents and turbidity currents leaves a fabric signature of graded bedding; (3) suspension settling is shown by alternating very fine (<0.5 mm thick) non-graded silt and clay laminae; (4) microbial mats are recognized by wavy lamination.