STORM-DOMINATED DIATOMITE: PROCESSES AND DEPOSITION FROM MICRO-TEXTURE
This study focused on formation and interpretation of storm-generated microtextures from a variety of diatomaceous mudstones of the Pisco Basin using data collected from oriented thin sections, SEMs of fractured rock surfaces, and in three dimensions using micro-CT (5 μm resolution). Low-angle and sometimes truncated micro-couplet laminae and lenses were observed within hummocky and swaley, and swale draping deposits. These contain a basal, normally graded siliciclastic silt component capped in diatomite. Large diatom fragments, complete valves, and broken-up diatom mats occur within a micromatrix (<30 μm) of highly fragmented diatomaceous debris. Micro-couplets often show reverse grading in pore size with greatest porosity in the intraskeletal pores of diatomaceous caps. Faintly laminated to structureless diatomite shows a greater quantity of complete valves, supporting sparse grains of floating, siliciclastic silt. These are interpreted as diatomaceous tempestites and a model is proposed for diatomite and diatomaceous mudstone deposition above storm wave-base under waning storm surge.