Desiccation Fissures Research Articles

Palaeogeographical and eodiagenetic settings of host‐replacing phreatic calcrete intervals developed in mud deposits of the Famennian Kinnesswood Formation in the Pennyseorach Subbasin of south‐west Scotland

ABSTRACTIn the Pennyseorach Subbasin of south‐west Scotland, the Famennian Kinnesswood Formation of the Inverclyde Group includes several, ca 1 to 3 m thick erosional remnants of host‐replacing phreatic calcrete hardpans developed in mud deposits. The latter are most unusual hosts for such calcretes due to their relative impermeability, which would normally prevent them from sufficiently accommodating the circulation of an aquifer. In previously documented phreatic calcretes, mud deposits clearly acted as aquicludes that sharply constrained calcrete development. In the Kinnesswood Formation at Pennyseorach, tall desiccation fissures allowed groundwater to circulate in thick, semi‐consolidated mud deposits. Because of the inferred development of an adjacent evaporitic basin, the mixing of fresh and evaporitic groundwaters raised the pH enough for the replacement of phyllosilicates by calcite to occur along the fissure walls, forming ‘fissure calcretes’. The latter gradually expanded and eventually coalesced into mature, ‘columnar host‐replacing phreatic calcrete hardpans’ in which 90 to 100% of the muddy host material was replaced by calcrete. At Pennyseorach, host‐replacing phreatic calcrete hardpan formation not only affected the transition zone between the Doughend Sandstone and Foul Port Members of the Kinnesswood Formation (the Dunagoil Calcrete interval), as in other localities of south‐west Scotland, but also affected the uppermost part of the formation, which is marked by several erosion surfaces downcutting into columnar host‐replacing phreatic calcrete hardpan intervals in association with the episodic rise and fall of base‐level. These newly recorded host‐replacing phreatic calcrete hardpan intervals from the upper part of the Kinnesswood Formation have a tightly constrained stable isotopic signature that is distinct from that of both the Dunagoil Calcrete and the successive host‐replacing phreatic calcrete hardpans of the Visean Clyde Sandstone Formation within the same group, underlining their potential as stratigraphic markers and palaeoenvironmental indicators.

SWELLING CLAY MINERALS AND SLOPE CUT FAILURES IN THE GARINONO FORMATION ALONG JALAN SUNGAI HITAM, LIBARAN, SANDAKAN

Jalan Sungai Hitam in the Sandakan Peninsula traverses a flat to gently undulating terrain of low hills and ridges surrounded by broad alluvial flats. Recent slope cuts expose bluish to dark grey mudstones of the Garinono Formation containing pebble to boulder sized, angular to rounded, blocks of sandstone and other rock types. Slump-mud flows have occurred at most of the slope cuts; the failed materials characterized by desiccation cracks and fissures. Soil index properties, including consistency limits and grain size analyses, indicate that the mudstones have a medium to high swell potential. X-ray diffraction analyses show the clay minerals present to be kaolinite, illite and randomly interstratified illite-montmorillonite (a swelling clay mineral). The mudstones are thus sensitive to atmospheric wet-dry cycles with repeated swelling and shrinkage giving rise to the desiccation cracks and fissures, The cracks and fissures reduce the shear strength of the mudstones and allow for the infiltration of rainwater which initiates the slump-mud flows. It is concluded that earthworks in areas of the Garinono Formation in eastern Sabah need to consider the presence of swelling clay minerals; an occurrence that can be inferred from evaluating soil index properties.

Mapping desiccation fissures using 3-D electrical resistivity tomography

Discontinuities which may form in a soil mass due to desiccation have been shown to be a source of engineering concern in situations where low permeability barriers are required. Current methods for assessing the condition of such geo-barriers, are mainly based on visual inspections, which are inadequate when the soil surface is obscured by vegetation. Electrical Resistivity Tomography (ERT) has been shown to be sensitive to the presence of fissures in 2-D with inversion models mapping the fissure's positions and enabling their growth to be monitored. This paper presents a method for mapping fissured networks in 3-D using miniature arrays under laboratory conditions. The results of two experiments using compacted clay from two separate embankments show a good comparison between the visible surface fissures and the surface model obtained from a 3-D inversion, as well as showing good agreement between the vertical sections of the model and the exposed model interior following dissection. Additionally the results confirm that the 3-D method is suitable for monitoring the onset of fissuring. A comparison of Schlumberger, Dipole–Dipole and combined arrays for visualising the fissures is also presented, indicating that the combined method produces the most accurate image of the subsurface, while the Schlumberger array provides a greater resolution than the Dipole–Dipole array, alongside the added benefit of shorter survey times.

Internal structure and origin of modern salt crust of Salar de Uyuni (Bolivia) salt pan based on tomographic research

This paper presents the results of a study on the internal structure of modern salt crust of Salar de Uyuni in SW Bolivia. Additionally, the structure of desiccation fissure infill was investigated. X-ray microtomography was used as the main analytical method because it enabled the complete structure of the sample’s interior to be imaged without its destruction. Based on microtomographic images three layers were distinguished in the crust between the surface and its base: a halite shell at the top, a cellular halite layer in the middle and a large-pore halite layer at the base. Results revealed that the desiccation fissures are filled with extremely porous bubble-halite. Distribution of mud in the sample as well as wash-out of the surfical halite crystals indicate the cyclicity of halite dissolution, its crystallisation and mud accumulation. These observations provide an explanation for low rate salt accumulation in the salt pan. This leads to the conclusion that the observed layering in the rocks does not reflect annual sedimentation-evaporation cycles but rather long-term cycles.

Giant desiccation fissures filled with calcareous eolian sand, Hermosa Formation (Pennsylvanian), southeastern Utah

At two stratigraphic intervals within the upper member of the Upper Pennsylvanian Hermosa Formation, calcareous eolian sand fills downward-tapering fissures that are as much as 18 cm wide and 5.7 m deep. Fissure fillings define orthogonal polygons 10 m or more in diameter. One of the host beds is primarily composed of subtidally deposited limestone, the other is a thinly laminated, nonmarine red siltstone. Both systems of fissure fillings are directly overlain and underlain by large-scale cross-stratified, calcareous eolianites. The limestone host bed contains chert pseudomorphs after gypsum. Compaction of host rocks contorted fissure fillings and caused doming of eolian strata over each fissure. Platy mineral grains in fissure fillings are aligned subparallel to bedding in the host rocks, supporting the view that the fissures were passively filled rather than forcefully injected. These ancient fissure systems are similar in scale and pattern to those that define giant desiccation polygons in numerous Great Basin playas. The Pennsylvanian fissures, like their Holocene counterparts, probably formed when groundwater tables dropped from shallow levels within fine-grained, impermeable deposits into underlying aquifers, greatly decreasing the extent of the capillary fringe. Our study of the fissures and host rocks supports the hypothesis that carbonate grains within the eolianites were deflated from uncemented marine sediments that were broadly exposed during regressive intervals.

Some pedological features affecting the permeability of hillside soils in Caydale, Yorkshire

AbstractWeekly interflow and permeability data were obtained for 15 sites in two field plots in North Yorkshire. Both plots are similar in terms of lithology, slope, micro‐relief, climate and soil type, one being dominated by rough pasture grasses and the second by bracken. One factor which explained 75 per cent of the variation in interflow was the difference between topsoil and subsoil permeabilities, the former always being the greater. This permeability difference is discussed in terms of soil texture, porosity and plant‐rooting systems. Textural contrasts are considered insufficient to cause the large‐scale variations in permeability observed throughout the year. Being a fixed variable, texture cannot influence a characteristic which varies from one week to the next. Contrasts in total porosity between topsoil and subsoil also effected no discernible influence on measured permeability differences, although measures of association increased as smaller pores were excluded from the analysis. Utilizing only those pores drained at 20 cm of water tension, highly significant correlations were established. In the plot dominated by bracken, greater ‘lateral permeabilities’ are produced by well‐developed systems of Pteridium rhizomes. As the advancing front of such rhizomes extends forwards at quoted rates of up to 45 cm/yr, the rear portions decay, leaving horizontal ‘tubes’ in the soil. Recognized omissions from this analysis are the effects of soil fauna and desiccation fissures.

One case of the deformation of buildings constructed on quaternary clays

Deformations of buildings were recorded (at the end of a dry summer), which were due to shrinkage of the Quaternary clays making up their foundation bed. The continuous foundations of the main walls of the residential buildings were laid at a depth of 0.8 m. During the summer, desiccation fissures up to 1-15 cm wide extending with a gradual taper to a depth of 1.5 to 2m, usually form in the upper layers of the diluvial clays. At a depth of 1 - 1.2m the width of the fissures reaches 2cm. The confinement of the deformations to the end of the dry period, the dependence of the amplitude of the displacements on the depth of the foundations, the slowness of the development of deformations and the character of opening of the cracks in the buildings, and the state of the surface in unbuilt areas, left no doubt as to the shrinkage nature of the phenonomena. It is recommended that when erecting buildings under these conditions, the depth of the foundations should depend upon the period of construction and the water content of the soils during this period. When building during dry periods when the foundation soils are less wet, the depth of the foundations can be reduced to 0.8-1.2m.

Giant Desiccation Fissures on the Black Rock and Smoke Creek Deserts, Nevada

Open fissures, from 100 to several hundred feet apart, that have produced polygonal patterns on the Black Rock Desert, Nevada, are believed to be giant desiccation cracks resulting from a secular trend toward aridity in the last few decades. Similar features on the Smoke Creek Desert probably have the same origin.

PIMPLED PLAINS OF EASTERN OKLAHOMA

Patterns formed by the networks of furrows separating the natural mounds of pimpled plains in eastern Oklahoma are regarded as attributable to shrinkage-polygon systems of coarse texture comparable to those occurring (1) in mound-studded parts of the northwestern United States that are underlain by vertically jointed basalt, (2) in tracts of Alaskan tundra that are occupied by ice-wedge networks, and (3) in beds of playa lakes in the arid southwest that are cut by systems of giant desiccation fissures. While the inter-mound furrow networks characteristic of pimpled plains in the mid-continent region may owe their origin to former frigid climatic conditions, one of a number of alternative possibilities is that the peculiar configuration of these surfaces, as exemplified in eastern Oklahoma, has resulted from erosion of systems of giant soil polygons caused by desiccation. The time of origin of the pimpled plains of eastern Oklahoma is believed to have been not earlier than late Pleistocene.