Plate 79

Indicators of intraformational erosion: mudstone clasts

Sometimes the evidence of erosion is indirect only. When a current erodes a muddy bottom, the softer superficial mud is disgregated and dispersed in fine particles within the flow. The underlying, stiffer mud is more resistant and, when it yields, it does in the form of chunks and flakes, which can be incorporated by the current. These fragments, ripped up from the sedimentary environment itself, represent intraformational,  or intrabasinal  clasts (other generic terms are rip up clasts  and clay (chips).  They can be strewn on the erosional surface as lag deposits, or be carried some distance down current and deposited together with sand or gravel. Intraformational clasts will then be found within  beds of coarse sediments.

Mudstone clasts are useful indicators  of intrabasinal, penecontemporaneous erosion: in other words, they point out that erosion occurred at the expense of previous sediments, possibly not far from the place redeposition. The examples shown here are rounded and platy, and can be described as mudstone pebbles. Rounding can be attributed to bouncing and rubbing, or to abrasion by whirling sand. A flat shape is common, and can be either primary, if the clasts derive from laminated mud, or acquired by compaction under the load of overlying sediments.

Plate 79 A is a section view: the mudstone chips are aligned just above an erosion (amalgamation) surface between two turbidite sandstone beds of the Messinian Colombacci Formation in central Apennines. The contact is not well defined, but the two beds can be distinguished because the lower one is laminated (cross and convoluted laminae: see plate 116), and the other is structureless.

Plate B is a plan view of a parting plane, i.e., an intrastratal surface. Elongated clasts show variable orientations, with two slightly prominent modes, respectively parallel and orthogonal to the hammer direction. Some pebbles probably rolled along the bottom after being carried in suspension, others did not.

Marnoso-arenacea Formation, northern Apennines.

Photo: M. A. Bassetti 1992.