Syneresis cracks are a sedimentary structure

Sedimentary structures

Sedimentary structures are those structures formed during sediment deposition.Sedimentary structures such as cross bedding, graded bedding and ripple marks are utilized in stratigraphic studies to indicate original position of strata in geologically complex terrains and understand the depositional...

 developed by the shrinkage of sediment without desiccation

Desiccation

Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...

. Syneresis

Syneresis (chemistry)

Syneresis , in chemistry, is the extraction or expulsion of a liquid from a gel, as when lymph drains from a contracting clot of blood. Another example of syneresis is the collection of whey on the surface of yogurt...

 is the expulsion of a liquid from a gel-like substance. Syneresis cracks are formed by the contraction of clay in response to changes in the salinity of a liquid surrounding a deposit. The markings would have been formed subaqueously

Subaqueous soil

Subaqueous Soils are soils that form in sediment found in shallow permanently flooded environments. Excluded from the definition of these soils are any areas permanently covered by water too deep for the growth of rooted plants....

 on the bedding surface and could resemble desiccation mudcrack

Mudcrack

Mudcracks are sedimentary structures formed as muddy sediment dries and contracts.- Formation :...

s, but are not continuous and vary in shape. They commonly occur in thin mudstone

Mudstone

Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...

s interbedded with sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

s, as positive relief on the bottom of the sandstone, or as negative relief on the top of the mudstone. Subaqueous shrinkage cracks can develop on and through a surface that has been continuously covered in water. Syneresis cracks in some shale

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

s and lime mudstones may initially be preserved as small cavities, which then usually fill with silt and sand from either the overlying or underlying beds

Bed (geology)

In geology a bed is the smallest division of a geologic formation or stratigraphic rock series marked by well-defined divisional planes separating it from layers above and below. A bed is the smallest lithostratigraphic unit, usually ranging in thickness from a centimeter to several meters and...

 and laminae

Lamination (geology)

In geology, lamination is a small scale sequence of fine layers that occurs in sedimentary rocks. Lamination is normally smaller and less pronounced than bedding. Lamination is often regarded as every planar structure smaller than 1 cm, and bedding as every planar structure larger than that...

.
Usually there is no pattern to the cracks, and they do not connect to form geometric shapes. Rather they are discontinuous and shaped in one of the following categories:

• polygonal - a closed plane figure, having three or more, usually straight, sides.
• sinuous - having many curves, bends, or turns.
• spindle - thickest in the middle and tapering at both ends.

A cross section of any of these would reveal that they are U- or V-shaped.

Desiccation v. syneresis

Some crack patterns resemble subaerial desiccation cracks (mudcracks), which in turn has caused some confusion as to the differences between desiccation cracks and syneresis cracks.

Desiccation mudcracks are usually continuous, polygonal, and have U- or V- shaped cross sections that would have been filled in with sediment from above. Syneresis cracks, however, are usually discontinuous, spindle or sinuous in shape, and have U- or V- shaped cross sections that have been filled in with sediment from above or below.

Experimental work

Experimental work done investigating subaqueously formed syneresis cracks have usually been carried out in one of two ways. One way to induce cracking was by sedimentation from a saline solution and a non-swelling clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

. However, when freshwater was used, no cracking could be induced in non-swelling clays, although in mud containing as little as two percent swelling clay, cracking was easily induced when salinity changes were introduced to the surrounding water.