On the origin of silt laminae in laminated shales

Experiments with mixtures of quartz silt and clays, under conditions where pure silt and clay suspensions form bedload ripples (non-cohesive silt, flocculated clay) show that coarse silt does segregate from clays during transport. As bedload transport proceeds, ripples of coarse silt and flocculated mud migrate over the bed surface simultaneously. Migrating ripples leave behind a thin veneer of silt grains or clay floccules, and over time a laminated deposit, consisting of thin parallel silt and clay laminae, builds up. If silt of a wide size range (60 μm to a few microns) is used, the coarse silt ends up in ripples and silt laminae, whereas the fine silt (20 μm or less) is integrated into clay floccules and stays mixed and suspended in the flocculated clay matrix. In natural laminated mudstones, coarse silt is typically found in thin laminae, whereas fine silt occurs mainly in the interlaminated clays. These experiments produce an analogous particle distribution and therefore suggest a likely way in which laminated mudstones of the rock record were formed.