Mudcracks

by Frederick M. Soster, Department of Geology & Geography, DePauw University, 1998

Dessication Mudcracks

Description

In plan view, dessication mudcracks form a polygonal pattern of fractures or cracks in the sediment. Polygons may be 4, 5, 6, or even 7 sided. Individual polygons may range in size from a few millimeters to over 30 cm; individual cracks may range from less than 1 mm to 5 cm in width (Pettijohn, 1975). Dessication mudcracks penetrate downward into the sediment, typically displaying a crude V shape that is widest near the sediment surface, gradually tapering downward, and eventually pinching out. Cracks may penetrate a few centimeters to tens of centimeters. The host sediment can be either siliciclastic or carbonate, but must be fine-grained. Prior to burial and lithification, the system of cracks usually becomes filled with sandy or silty sediment, which provides a contrast with the host rock and makes the mudcracks easy to see. Mudcracks that form in siliciclastic sediments are often preserved on the underside of an overlying sandstone bed as sharp-crested ridges, whereas those that form in carbonates generally occur on the topside of beds (Pettijohn, 1975).

Interpretation

Dessication mudcracks form as a result of shrinkage caused by loss of water. This implies drying and consequently subaerial exposure.

Occurence

Dessication mudcracks have been reported from a wide range of environments including ponds, lakes, playas, river floodplains, intertidal areas, and supratidal areas.

Stratigraphical Applications

Dessication mudcracks are excellent "stratigraphic up" indicators. The downward tapering V shape in cross-section points downward, hence stratigraphic up is opposite. Furthermore, the surfaces in the polygons between cracks are commonly slightly concave upwards, which provides another indicator of stratigraphic up.

Illustrations

Copyright by Fred Soster, 1998. All rights reserved worldwide.

 Click on thumbnails to enlarge.

	Figure 1. Mudcracks developing in a modern floodplain. Note that a second, smaller set is starting to form (lower right). There is also some curling upward of the mud surface near some of the cracks. The rounded depressions are raindrop imprints (closeup). Their circular shape indicates that the rain was falling straight down. (Putnam County, Indiana)
	Figure 2. Mudcracks developing in a thin mud layer underlain by sand on a modern floodplain. The size of the mudcrack polygons is controlled by the thickness of the cracked layer (Collinson & Thompson, 1982). In this picture, the mudcracked layer is thin, so the size of the polygons is small. (Putnam County, Indiana)
	Figure 3. Mudcracks and mud chips developing in a lime mud on a limestone quarry floor. Mud chips can be suspended, transported, and redeposited to form an intraformational conglomerate. Note also that the polygons are small because the cracked layer is thin. (Putnam County, Indiana)
	Figure 4. Small pattern of cracks forming on a modern floodplain. The mud chips show the typical concave upward pattern that, if recognizable in ancient rock, gives a "stratigraphic up" indicator. (Putnam County, Indiana)
	Figure 5. Mudcracks in the Chinle Formation (Triassic). This portion of the Chinle Formation is interpreted as being a stream deposit. (Coconino County, Arizona)
	Figure 6. Mudcracks in the Chinle Formation (Triassic). (Coconino County, Arizona)
	Figure 7. Dessication mudcracks might sometimes be confused with other types of mudcracks, such as synaeresis cracks (see below). The presence of raindrop imprints or vertebrate footprints in association with mudcracked beds provides confirming evidence for subaerial exposure and thus an interpretation of dessication mudcracks. In this photograph, three-toed dinosaur footprints (three prints) are found with mudcracked beds, indicating subaerial exposure and dessication. Note blacks lens cap near center for scale. Chinle Formation, Triassic. (Coconino County, Arizona)
	Figure 8. Closeup of three-toed dinosaur footprint. This print is the one seen in the center of Figure 7. Chinle Formation, Triassic. (Coconino County, Arizona)
	Figure 9. Mudcracks in the Wills Creek Formation (Silurian) on the top of a bedding plane at the Roundtop railroad cut in Maryland. These are developed in an argillaceous limestone. Note smaller set of cracks within the large polygon in lower part of photograph. (Washington County, Maryland)
	Figure 10. Mudcracks in the Wills Creek Formation (Silurian) on the top of a bedding plane at the Roundtop railroad cut in Maryland. Ruler is 15 cm long. Compare to Figure 11. (Washington County, Maryland)
	Figure 11. Side view of mudcracks shown in Figure 10. This particular crack shows the classic V-shaped, tapering downward profile and penetrates to a depth of 14 cm. Note that the crack is not straight nor is it symmetrical. Cracks often get deformed and crumpled during compaction. The filling material is usually coarser grained and therefore less compactable than the host sediment. The filling material is accomodates compaction by deforming. (Washington County, Maryland)
	Figure 12. Mudcracks on the top of a bedding surface in the St. Louis Limestone (Mississippian). (Monroe County, Indiana)
	Figure 13. Intraformational conglomerate in the Rockwell Formation (Mississippian). The dark areas in the rock are mud chips, which may have originated as rip up clasts from mudcracked sediments. (Sideling Hill, Washington County, Maryland)
	Figure 14. Intraformational conglomerate in the Conococheague Limestone (Cambrian). A large pod of clasts is present near the center of the photo (closeup). These were likely derived from nearby intertidal or supratidal mudcracked sediments. (Washington County, Maryland)
	Figure 15. Another example of an intraformational conglomerate in the Conococheague Limestone (Cambrian) (closeup). (Washington County, Maryland)

Synaeresis Cracks

Description

Not all mudcracks are the result of shrinkage due to dessication. Some mudcracks can develop in sediments that remain completely submerged. The processes are not well understood, but apparently involve changes in volume of clay minerals induced by salinity changes (Collinson and Thompson, 1982), expulsion of fluids from colloidal suspensions (McLane, 1995), or removal of water from mud layers that are in contact with brines produced by evaporation (McLane, 1995). Mudcracks produced by these processes are termed synaeresis cracks.

Synaeresis cracks do not display the well-developed polygonal pattern seen in dessication mudcracks. Rather, the pattern is irreular or radiating. Cracks are lenticular in shape and pinch out in both directions, or interesect other cracks, which then pinch out. Finally, a set of cracks do not originate from a single surface, but originate at all levels of the bed.

Interpretation

Synaeresis cracks form in a subaqueous setting where highly porous clays, flocculations, colloidal suspensions, or brines are present. Sediments undergo dewatering because of changes in clay mineral structures or loss of water to brines in contact with the sediment. The resulting shrinkage causes the cracks to form.

Occurence

Marginal marine settings are probably most conducive to formation of syneresis cracks because of frequent changes in salinity that occur in these environments. Collinson and Thompson (1982) report that syneresis cracks occur in ancient sediments originating in marine and non-marine settings at all water depths, but do not specify particular environments.

 Illustration

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