ABSTRACT Pebbly sediments of the shallow marine Abrioja fan-delta show pockets (bowl-shaped structures, partly filled with pebbles) and pillars (elongate structures, filled with sand and pebbles). These structures are most abundant in pebbly sediments deposited on a steep slope (ca. 25°-10°) and are absent in conglomerates deposited on a slope of ca. 6° and less, although they are present in the pelitic top of these beds.

The pocket and pillar structures are interpreted as fluid escape structures originating from local liquefaction and fluidization, processes which are favoured by rapid deposition, rapid sediment accumulation, the presence of less permeable layers and an immature sediment texture.

These conditions are met in conglomeratic fan-deltas, which have steep slopes with immature sediments. It is concluded that the presence of fluid escape structures in conglomeratic sediments may indicate a steep depositional slope.