Plate 44

Vegetal debris on parting planes

Particulate vegetal matter of variable size is abundant in clastic sediments accumulating near swamps and marshes, for example in alluvial and deltaic areas. Owing to their low weight, even large fragments (dead twigs, etc.) are transported in suspension or float in the currents. Their deposition is delayed until they become waterlogged, and their sizes are larger than those of detrital grains with which they are associated (the hydraulic  size is more important than the geometric size: particles of different geometry and weight can have the same settling velocity). Plant remains can be resedimented from basin margins into deep water, as in the samples illustrated here, which derive from turbidite beds.

Vegetal debris is recognizable in Ancient, compacted sediments for its brown or black color resulting from carbonization (coalification) processes. Sulfur originally present in the organic matter is often found as iron sulfide, which, upon oxidative weathering, is altered into yellow-red oxides. The two specimens shown in the picture have different concentrations of vegetal particles spread on lamination planes. A preferred orientation, with long axes tending to be parallel to paleocurrent, is observable in the sample to the right.

In section view, laminae rich in vegetal matter are recognizable for their dark color. If carbonized debris is abundant in entire laminasets or intervals of sandstone beds, it can hamper cementation and make the rock more friable and erodible there. The resulting re-entrants in outcrops could thus be mistaken for clayey interbeds.

Marnoso-arenacea Formation, northern Apennines

Photo: G. Piacentini 1970.