MUD BED ACCRETION FROM MOVING SILT–KAOLINITE SUSPENSIONS – OBSERVATIONS FROM A FLUME STUDY

Previous experiments with clays in a racetrack flume (driven by paddle-belt to avoid destruction of floccules) have shown that pure clays can be deposited from swift-moving suspensions. At flow velocities that transport and deposit sand (20-35 cm/s, 5 cm flow depth) clay suspensions produce deposition-prone floccules that form migrating floccule ripples. When these floccule ripples accrete to form a clay bed, the resulting bed has a thinly laminated appearance upon compaction.

We modified these earlier experiments by blending a quartz silt component (20 to 63 microns grain size, modal size 20 microns) with kaolinite (80% finer than 5 microns). We explored this system from freshwater to 20 ‰ salinity, flow velocities ranging from 10 to 50 cm/s (5 cm flow depth), and suspended sediment concentrations up to 2.5 grams per liter.

Invariably, the coarsest silt component (40 to 63 microns) forms low-relief ripples even at 50 cm/s. At lower velocities ripples of flocculated clay and silt form, and begin to accrete into a bed at velocities between 20 and 25 cm/s (5 cm flow depth). Individual ripples show interleaving of clay-rich and silt-rich laminae, and the final deposit consists of alternating silt- and clay-rich laminae, and may show cross-stratification within these laminae. The latter attests to formation of bed laminae via migrating ripples. Floccules contain a mix of silt grains and clay platelets and were co-deposited with silt grains within individual laminae.

Beds of interlaminated silts and clays have previous been interpreted as quiet-water deposits, and alternatively also as the product of fine-grained turbidites. Our experiments show that mud accretion from migrating floccule ripples produces interlaminated clays and silts as well. Further studies should reveal petrographic criteria to differentiate between these possibilities in the rock record.