Plate 50

Modified tractive structures: lozenge-shaped ripples

Bed forms and tractive structures have a highly variable preservation potential,  depending on both the dynamics of the environment and geologic factors such as subsidence, sea level changes, sedimentation rate, tectonic movements, etc. Once formed, they can be 1) canceled, 2) buried and preserved integrally, and 3) altered or modified, and preserved in this modified state.

Various processes can modify the structures: many of them are mechanical, but chemical and biological processes are also effective. Chemical and biological modifications will be examined in the sections dedicated to diagenetic and biogenic structures. Modification structures  could constitute a separate item, because they are often very useful as environmental indicators. What they indicate is a change in the environment, occurring soon after their formation or after a time lag. A structure formed under shallow water, for example, can emerge if the water evaporates or is drained away. Some environmental changes are more drastic than others; moreover, they can involve the whole environment or some parameters only (e.g., the rate of sedimentation, the ventilation, etc.).

Let us see how beach structures are modified when the sea retreats at low tide. Accretionary beaches, where the supply of sand is abundant, are wide and have a low topographic gradient. Their submerged portion is often characterized by a wavy morphology, with bars or ridges forming the relieved portions. These long shore bars and ridges are parallel or oblique to the shoreline; the shallower ones make the incoming waves break and emerge at low tide. During shallowing and emergence, subaqueous ripples tend to be canceled or to have their crest smoothed, as the thinning of the water cover changes the hydraulic regime from sub critical to super critical. In depressed areas between the bars (troughs, runnels), some water can remain; ripples are not smoothed out completely, but modified or distorted, as in the example shown here.

A small and shallow channel drains water toward the sea (to the right). The ripples maintain a transversal orientation in the central area and are oblique near the sides; their relief is minimum. Water films coming from different directions interfere and tend to split the ripples into rhomboid segments. On the far right, linguoid and spatulate (spoon-shaped) forms develop.