The fine-grained texture of the mudslide material was critical.

Fine particles create a denser packing than coarse particles, reducing the pore spaces through which oxygen could diffuse into the buried material.

The reduction of oxygen diffusion in a fine-grained, waterlogged sediment is the foundation of the anaerobic preservation environment.

The waterlogged condition of the covering sediment maintained the oxygen exclusion over centuries.

In a dry environment, fine-grained material might eventually allow enough oxygen diffusion to support the aerobic decomposers that quickly destroy organic material.

The wet climate of the Olympic Peninsula maintained the waterlogged state of the slide deposit, keeping it anoxic across the centuries.