Plate 58

Fallout deposits: mud drape on ripples

A tractive current with little or no material in suspension will leave bed forms exposed on the top surface of sand. When the sea retreats at low tide, for example, ripples, dunes and other forms are visible, as documented by previous pictures. If mud is present, it will settle on the tractive deposit when the water becomes calmer. An abundant suspension load tends to bury the bed forms under a relatively thick, flat-topped layer; in section view, a gradual upward transition from sand to mud will be observed (see plate 56). When, on the other hand, mud is more diluted, its deposition is slower or delayed; the mud forms a thin drape under which the underlying morphology is recognizable, as in this picture showing a river bed after the retreat of flood waters (pencil for scale to the lower right).

On drying, the mud shrinks and cracks in a typical polygonal pattern (see also plates 104-106). With further loss of water, the edges of the polygons curl up (plate 105, color photo 27) until clay flakes detach from the sandy substratum (color photo 28). Mud clasts  are thus prepared in place and can be carried away by wind or running water.

A couple of beds, made of sand and mud respectively, forms the basic element of rhythmic, repetitive successions, a sample of which is often called a heterolithic facies.  The pelitic  member of the couple is also referred to as interbed, interlayer, parting or drape. Pelite  is a broad term encompassing all mud-size (silt to clay) sediments, regardless of their consolidation state: mud, mudstone, clay, claystone, shale, argillite. As for silt and siltstone, their coarser types are usually grouped with sand and sandstone, the finer ones with pelites.