STORM-DOMINATED DIATOMITE: PROCESSES AND DEPOSITION FROM MICRO-TEXTURE

Diatomite acts as a locally important reservoir rock, resource for biostratigraphy, and in Pisco Basin, Peru is often associated with the burial of fossil marine vertebrates. Unusually thick accumulations of diatomite occur throughout the Mio-Pliocene around the Pacific margin, and models accounting for their deposition focus on high diatom productivity sufficient to overcome siliciclastic dilution. However, the diatomite and diatomaceous mudstone deposits of the Pisco Basin show longitudinally bedded hummocky and swaley cross-stratification interpreted to reflect a dominance of storm deposition. Other notable facies include laminated swale drapings, sometimes associated with fossil cetacean burial, and faintly laminated to structureless diatomite. In addition to diatoms, these deposits are primarily composed of silt, smaller quartz and feldspars, and volcanic ash.

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.