Mudboils on a drumlinized till plain
In 1973 a program of mapping and laboratory study was begun in order
to define physical and chemical properties of the various sediment
types that are associated with two distinct types of patterned
ground, mudboils and frost polygons. One or the other of these
varieties of patterned ground occurs on virtually all unconsolidated
deposits found south of Chesterfield Inlet, in the eastern
District of Keewatin (now the southwestern region of Nunavut,
Canada). The purpose of these studies was to ascertain
relationships between surface patterns and properties of underlying
unconsolidated sediment. The conclusions from the study lead to
the development of a detailed mapping strategy and legends which
were applied to mapping surficial sediments in this low arctic
environment and to the production of a number of detailed
1:125000-scale maps of surficial geology of southwestern Nunavut.
The legends also served as the basis for a 1:1000000 map of the
main surficial deposits of the western Canadian Shield
(Aylsworth, and Shilts, 1989b).
Images 0051, 0055, 0057, 0058, 0059, 0062, 0065, 0066, 0068, 0069, 0070, 0072, 0073, 0088, 0090, 0091, 0092, 0093, 0094, 0095, 0096, 0098, 0099, 0100, 0101, 0102, 0103, 0104, 0106, 0107, 0121, 0216, 0217, 0220, 0221, 0222, 0223, 0224, 0225, 0226, 0227, 0228, 0230, 0231, 0232, 0233, 0234, 0236, 0237, 0238, 0239, and 0240 show mudboils, the principal type of patterned ground typical of areas underlain by unconsolidated diamictons and muds. They also illustrate the internal structures of mudboils, their impact on the tundra landscape, and features related to them, as well as the physical properties of surficial sediment facies involved in mudboil formation.
Mudboils and frost or tundra polygons rarely occur together and almost always reflect the textural and engineering properties of the unconsolidated sediment in which they are developed. Mudboils are formed on poorly sorted surficial sediment with low liquid limits (<20%-see explanation of engineering properties below) and significant amounts of silt and clay. Frost polygons usually are formed on sandy or gravelly, water-sorted sediments of various types or in low-lying or other poorly drained areas. Mudboils are commonly underlain by till, marine silty clay, and colluvium containing significant fines. Frost polygons are characteristic patterns on eskers; isostatically raised or modern marine or lake beaches or deltas; sandy, shallow-water bottom sediments; grass-sedge meadows underlain by half a meter or more of peat; and on alluvium or peat-covered flood plains along modern streams.
Mudboils are referred to in the literature by various names, such as frost boils, soil medallions, sorted circles, nonsorted circles, tundra craters, mud volcanoes, etc. The term mudboil, as described in a paper by the author (Shilts, 1978) has been chosen to describe the features in this and related images, because permafrost is not absolutely necessary to form them, but muddy (poorly sorted) sediment is. It should be emphasized, however, that permafrost facilitates the formation of mud boils to the extent that mudboils and associated patterns are an important feature of perennially frozen areas underlain by muddy surficial sediment. In order for mudboils to form, diamicton in a plastic or liquid state must be confined between a rigid surface soil layer and the frost table (see diagram in image 0098). When this condition exists, the liquid mud phase is under hydrostatic pressure on any slope, and even on relatively flat surfaces mud can be mobilized by differential loading stresses related to density differences among various sediment components of the mudboil or to loading of the mudboil's surface by animals or man-made objects (see images 0063, 0065, 0157, 0205). Under these conditions the mud is under hydrostatic pressure and can be deformed by the pressure if disturbed (images 0062, 0220) or it can be injected in diapiric fashion to the surface through points of weakness in a desiccated or sandy, relatively rigid surface layer or "carapace" (images 0055, 0059, 0090, 0091, 0092, 0098, 0099, 0216). Thus, the system is analagous to an artesian groundwater system with mud that can easily become liquid instead of water. As mud is extruded to the surface, surface runoff winnows it, carrying away clay and fine silt (images 0223,0224), and the sandy, pebbly residue left behind adds to the thickness of the carapace.
Atterberg limits and Moisture Contents
Atterberg Limits are derived from standard engineering laboratory tests that measure a sediment's propensity to behave as a solid, plastic substance, or a liquid at varying moisture contents. The Plastic Limit is the moisture content, in weight %, at which an unconsolidated sediment ("soil" in engineering terms) passes from a solid (non-deformable) to a plastic (deformable) state. The liquid limit is the moisture content, in weight % water, at which a plastic sediment passes from a plastic to liquid state, that is, the sediment loses its strength and behaves like a liquid. The plasticity index is the range of moisture contents, in weight % water, over which a sediment will behave as a plastic substance and represents the difference between the solid and liquid states, between the plastic and liquid limits.
It is apparent that liquid limits for Keewatin "muds" are very low
with respect to other arctic or subarctic "muds" and that plasticity
indices are low (<4%) or unmeasurable. This means that at very low
moisture contents, Keewatin "muds" pass from a solid state
possessing considerable shear strength to a liquid state with
virtually no shear strength, either directly, or after passing
through a very minor plastic phase. Thus, a very slight increase
in moisture content or an increase in pore-water pressure may
cause a seemingly solid soil to liquify or founder, or conversely,
very slight decrease of these stresses may cause an apparently
liquid, soft mud to become solid.
A dramatic example that illustrated these properties occurred on August 7, 1973, when a field assistant (C.I.D.A. student Aaron Villakazie) became trapped in a subaqueous mudboil on an island in Kaminak Lake (images 0063, 0157, 0196, 0197, 0198, 0157, 0198). His foot sank in mud which, because of increased pore-water pressure caused by his weight, was driven above its liquid limit; as his foot sank, the mud above the toe of his boot returned to the solid state, trapping him. The liquid and plastic limits of the sediment involved were nearly the same, so that the sediment could not behave plastically, but only as a solid or liquid. Thus, in two hours and despite the efforts of seven men, he sank almost to the frost table and was extricated only after the rigid sediment around his leg was excavated hydraulically by a portable, high-discharge pump.
Atterberg limits and natural moisture contents of unconsolidated sediments from Keewatin support the concept that mudboils are essentially soft-sediment deformation features, representing small-scale diapiric structures formed during the thaw season. Because of the low liquid limits and limited plasticity indices of sediments that characteristically are ornamented by mudboils, and because of natural moisture contents that are close to the liquid limits of typical surficial sediments on which mudboils form, slight increases in pore-water pressure due to loading by animals or man-made objects, small amounts of precipitation, cryostatic pressures created during fall freeze-up, or moisture increases during spring-summer thaw could all be causes of temporary liquefaction of the active zone with resulting hydrostatically driven diapirism or mud boiling. This seasonal activity would destroy other features, such as incipient frost polygons, so that these features are incompatible with sediment that forms mudboils. Organic growth would also be disrupted seasonally so that no persistent organic cover could be established.
In dry sands and gravels of ridged or hummocky stratified drift, organic cover is slight because the moisture content of the well-drained active zone is low and the liquid limit of the sediment is very high; because of these factors, the thawed zone is stable enough to perpetuate seasonal growth of vertical ice wedges that are expressed at the surface as frost cracks. Only locally, where moisture from spring thaw builds up temporarily behind snow banks, is the liquid limit likely to be exceeded and slumping or flowing to take place.
Frost cracks or polygons also occur where sediments derived from marine reworking or alluvial or lacustrine deposition have partially filled depressions that existed on the glaciated surface after retreat of the ice. These pockets of sediment have flat surfaces and occur in strips along major rivers and eskers, in post-glacial lake basins, and in numerous, smaller, random pockets at all altitudes up to marine limit in southwestern Nunavut, west of Hudson Bay. Three factors may cause mudboils to be absent and frost cracks to predominate in these poorly drained, flat areas: 1) the flatness of most basins would not allow for significant hydrostatic head to develop; 2) the sedimentary fill is commonly sandy or gravelly so that liquid limits are too high to cause sediment flow at normal moisture contents; 3) where the sedimentary fill is very wet, organic growth is lush, and a thick, insulating cover of peat develops on the sediment. In such areas the maximum depth of thaw is slight (15-30 cm) so that the active zone is never thick enough to develop the artesian system necessary for mudboil formation, no matter what the properties of the underlying mineral sediment might be.
Updated 05/06/2010 AW