Cementation and blackening of Holocene sands by peat-derived humates: A case study from the Great Dune of Pilat, Landes des Gascogne, Southwestern France

The present-day actions of waves and tides contribute to the erosion of the frontal parts of the humate-impregnated sandstone horizon, resulting in the formation of the dark rounded sandstone slabs and pebbles that are deposited on nearby beaches. This process provides a fine example of recent black pebble formation that derived from pre-existing organic matter-impregnated source rocks. In contrast to their tropical carbonate counterparts, however, the humate-induced blackening of the sandstone pebbles appears to be relatively unstable, as the impregnated fragments bleach when exposed to air and seawater for a few seasons.