Plate 12

Sedimentary bodies: base-of-slope tongues and downlap


This natural section was cut by waves in a volcanic island of the Eolian Archipelago in the Tyrrhenian Sea. The bedded deposits were emplaced by sedimentary processes but are not sedimentary in origin; they are pyroclastic, and were ejected by explosive eruptions. Ashes made of volcanic glass, crystals and fragments of minerals solidified before the eruption, and blocks of indurated volcanic rock tore apart from the conduit walls, are all pyroclastic materials. In this case, they fill a crater, partly visible in the background. Half of this crater, later on, subsided below the sea along a fault, or was simply eaten up by marine erosion under the attack of waves. Thus, in the cross-section, the pyroclastic beds leaning against the vertical wall of the crater on the right. This contact is a high-angle onlap.

The deposition of pyroclastic materials may have occurred in two ways: direct or delayed. Direct  deposition means that pyroclastic products are found where they touched the ground for the first time, after a ballistic trajectory and a free fall  or the entrainment by hot, dense suspensions flowing down the flanks of the crater (pyroclastic flows). In delayed  deposition, the particles have lost their heat: they fall after a long residence in the atmosphere or in seawater, being distributed over wide areas, or accumulate in the vicinity of the conduit but are remobilized afterwards. The final emplacement is thus delayed for two different reasons: 1) finer ash is pushed so high by rising, convection-driven gas columns as to reach the upper troposphere or the stratosphere and be involved in global circulation; it can stay there for years before returning to the ground with precipitation; 2) coarser and heavier particles, such as those forming the beds in the photo (see close-up in plate 69), are removed from the place of direct emplacement. The remobilizing agent can be rain, running water, wind, or simply gravity. Water-soaked ash is easily mobilized by gravity on steep volcano slopes and moves as a debris flow (which, in this specific case, is called lahar ).

Sedimentological criteria are useful for recognizing direct versus delayed deposition. Among them, the geometry of bedding is important. Fall deposits, for example, mantle a substratum, however irregular, with parallel drapes.  Lahar deposits, on the other hand, are lenticular  owing to their more localized character: the detrital mass flows as a viscous, cohesive substance that follows the underlying topography and fills depressions with elongated, tongue or lobe-shaped deposits. In this picture, it can be seen that the beds are not perfectly parallel, which suggests mass flows and remobilization. Furthermore, they can be grouped into two sets: a lower set, subhorizontal, and an upper one, slightly inclined. The lower set rests on the crater bottom (not visible because it is below sea level), while the upper set unconformably overlies the lower one with a tangential  contact (inclined beds that become gradually horizontal) called downlap.  Both sets lap on the steep crater flank.

The upper bedset is obviously nourished by the denuded slope to the right and on the background, and forms a base-of-slope  body (or a toeset ), which is tongue-shaped in cross-section. Base-of-slope bodies are point sourced and cone-shaped in 3-D, but frequently coalesce laterally and form aprons.  The more abrupt onlap of the lower bedset implies that the pyroclastics came down other, more distant slopes, then flowed along the crater bottom for a while before coming to rest.

Pleistocene pyroclastics, Salina, Eolian Islands, Tyrrhenian Sea.


Sedimentographica