The mineralogy and fabric of 'Brickearths' in Kent, UK and their relationship to engineering behaviour Antoni E. MilodowskiKevin J. NorthmoreSimon J. KempDavid C. EntwisleDavid A. Gunn • Peter D. JacksonDavid I. BoardmanAris ZoumpakisChristopher D. F. Rogers • Neil DixonIan JeffersonIan J. SmalleyMichele Clarke
A. E. MilodowskiK.J. NorthmoreSimon J. KempDavid EntwisleDavid GunnP. JacksonD. I. BoardmanAris ZoumpakisNeil DixonIan SmalleyMichèle L. Clarke
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Mineralogical and petrographical investigation of two loessic brickearth profiles from Ospringe and Peg- well Bay in north Kent, UK have differentiated two types of brickearth fabric that can be correlated with different engineering behaviour. Both sequences comprise meta- stable (collapsing) calcareous brickearth, overlain by non- collapsing 'non-calcareous' brickearth. This study has demonstrated that the two types of brickearth are discretely different sedimentary units, with different primary sedimentary characteristics and an erosional junction be- tween the two units. A palaeosol is developed on the cal- careous brickearth, and is associated with the formation of rhizolithic calcrete indicating an arid or semi-arid envi- ronment. No evidence has been found for decalcification being responsible for the fabric of the upper 'non-cal- careous' brickearth. Optically-stimulated dates lend further support for the calcareous and 'non-calcareous' brickearth horizons being of different age or origins. The calcareous brickearth is metastable in that it undergoes rapid collapse settlement when wetted under applied stresses. It is char- acterised by an open-packed arrangement of clay-coated, silt-sized quartz particles and pelletised aggregate grains (peds) of compacted silt and clay, supported by an inter- ped matrix of loosely packed, silt/fine-grained sand, in which the grains are held in place by a skeletal framework of illuviated clay. The illuviated clay forms bridges and pillars separating and binding the dispersed component silt/ sand grains. There is little direct grain-to-grain contact and the resultant fabric has a very high voids ratio. Any applied load is largely supported by these delicate clay bridge and pillar microfabrics. Collapse of this brickearth fabric can be explained by a sequence of processes involving: (1) dispersion and disruption of the grain-bridging clay on saturation, leading to initial rapid collapse of the loose- packed inter-ped silt/sand; (2) rearrangement and closer stacking of the compact aggregate silt/clay peds; (3) with increasing stress further consolidation may result from deformation and break up of the peds as they collapse into the inter-ped regions. Smectite is a significant component of the clay assemblage and will swell on wetting, further encouraging disruption and breaking of the clay bonds. In contrast, the 'non-calcareous' brickearth already possesses a close-packed and interlocking arrangement of silt/sand grains with only limited scope for further consolidation under load. Minor authigenic calcite and dolomite may also form meniscus cements between silt grains. These have either acted as ''scaffolds'' on which illuviated clay has subsequently been deposited or have encrusted earlier- formed grain-bridging clay. In either case, the carbonate cements may help to reinforce the clay bridge fabrics. However, these carbonate features are a relatively minorKeywords:
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Oriented clay coatings (argillans, clay cutans, clay films, lamellae) are often interpreted to be caused by illuviation (pervection, lessivage) of fine clay particles. In montane meadow soils (Typic Humaquepts) of the northern Sierra Nevada Range, prominent clay cutans occur on ped faces of a paleosol 3Btgb horizon, which is contemporaneous or younger than 2840 ± 220 yr BP by radiocarbon dating. The unlikelihood of distinct clay cutans forming in such a short period of time via illuviation in a soil with an aquic moisture regime suggests an alternative mechanism. Laboratory studies show that silt- and sand-sized agglomerates can be produced by subjecting colloidal kaolinite (high Fe content), extracted from the montane meadow soil, to repeated freeze–thaw cycles. These agglomerates have a distinct microgranular texture and one to several birefringent domains. Thin section microscopy of the surface Oa horizon reveals that pores and root channels are coated by oriented birefringent zones resembling clay cutans. These clay cutans are similar in optical properties to laboratory-produced freeze-agglomerates. Optical microscopy of clay cutans, scraped from the paleosol 3Btgb horizon, suggests that they too may have formed via freeze-agglomeration. Freeze-agglomeration is proposed as a new mechanism to produce oriented clay films.
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ABSTRACT The Cheltenham fire clay of Missouri is relatively massive in structure while typical shale, exemplified by the Lagonda, is laminated. The microscopical texture of the Cheltenham fire clay is a pattern of clay minerals grown in random directions, whereas that of the associated shale is an oriented arrangement of clay-mineral flakes which lie parallel to the lamination. It is deduced that the Cheltenham fire clay was developed from redeposited weathered-soil clay and further hydrolyzed and dialyzed in nearly stagnant water of Pennsylvanian swamps. The crystals of the fire-clay minerals grew at random in this clay gel. Many shales, on the other hand, are believed to have been deposited directly after flocculation without a significant interim period of leaching. The clay floccule, having a flaky shape, settled to the bottom where countless other clay flakes in the same orientation gave rise upon compaction to a conventional laminated shale. Possibly these criteria may be extended more generally to other argillaceous rocks.
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Experimental evidence suggests the detrital origin of underclays of the Illinois Basin. Flocculated clay materials were deposited in glass tubes and the resulting clay-mineral orientation determined by X-ray techniques. Random orientation in the flocculated clay was preserved in the dried clay, which had dewatered slowly under minimal overburden pressure. Samples were compressed to study the influence of water content and pressure on the preservation of random orientation. Clay with lower water content showed less reorientation due to compression than clay with a higher content, suggesting that the amount of liquid water in proportion to adsorbed water in the system may influence clay-flake orientation. The soil concept in explaining the origin of underclay is discussed. Evidence indicates that the nonbedded character of underclays is due to the random orientation of clay flakes, which could be a primary feature resulting from the slow deposition of flocculated clay during which the clay dewaters under minimal overburden pressure. Observations by the author and others do not support the concept that most underclays in the Illinois Basin are soils subjected to subaerial weathering and reqorking by plant rootlets.
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Experimental evidence suggests the detrital origin of underclays of the Illinois Basin. Flocculated clay materials were deposited in glass tubes and the resulting clay-mineral orientation determined by X-ray techniques. Random orientation in the flocculated clay was preserved in the dried clay, which had dewatered slowly under minimal overburden pressure. Samples were compressed to study the influence of water content and pressure on the preservation of random orientation. Clay with lower water content showed less reorientation due to compression than clay with a higher content, suggesting that the amount of liquid water in proportion to adsorbed water in the system may influence clay-flake orientation. The soil concept in explaining the origin of underclay is discussed. Evidence indicates that the nonbedded character of underclays is due to the random orientation of clay flakes, which could be a primary feature resulting from the slow deposition of flocculated clay during which the clay dewaters under minimal overburden pressure. Observations by the author and others do not support the concept that most underclays in the Illinois Basin are soils subjected to subaerial weathering and reworking by plant rootlets.
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Mineralogical and petrographical investigation of two loessic brickearth profiles from Ospringe and Pegwell Bay in north Kent, UK have differentiated two types of brickearth fabric that can be correlated with different engineering behaviour. Both sequences comprise metastable (collapsing) calcareous brickearth, overlain by non-collapsing 'non-calcareous' brickearth. This study has demonstrated that the two types of brickearth are discretely different sedimentary units, with different primary sedimentary characteristics and an erosional junction between the two units. A palaeosol is developed on the calcareous brickearth, and is associated with the formation of rhizolithic calcrete indicating an arid or semi-arid environment. No evidence has been found for decalcification being responsible for the fabric of the upper 'non-calcareous' brickearth. Optically-stimulated dates lend further support for the calcareous and 'non-calcareous' brickearth horizons being of different age or origins. The calcareous brickearth is metastable in that it undergoes rapid collapse settlement when wetted under applied stresses. It is characterised by an open-packed arrangement of clay-coated, silt-sized quartz particles and pelletised aggregate grains (peds) of compacted silt and clay, supported by an inter-ped matrix of loosely packed, silt/fine-grained sand, in which the grains are held in place by a skeletal framework of illuviated clay. The illuviated clay forms bridges and pillars separating and binding the dispersed component silt/sand grains. There is little direct grain-to-grain contact and the resultant fabric has a very high voids ratio. Any applied load is largely supported by these delicate clay bridge and pillar microfabrics. Collapse of this brickearth fabric can be explained by a sequence of processes involving: (1) dispersion and disruption of the grain-bridging clay on saturation, leading to initial rapid collapse of the loose-packed inter-ped silt/sand; (2) rearrangement and closer stacking of the compact aggregate silt/clay peds; (3) with increasing stress further consolidation may result from deformation and break up of the peds as they collapse into the inter-ped regions. Smectite is a significant component of the clay assemblage and will swell on wetting, further encouraging disruption and breaking of the clay bonds. In contrast, the 'non-calcareous' brickearth already possesses a close-packed and interlocking arrangement of silt/sand grains with only limited scope for further consolidation under load. Minor authigenic calcite and dolomite may also form meniscus cements between silt grains. These have either acted as "scaffolds" on which illuviated clay has subsequently been deposited or have encrusted earlier-formed grain-bridging clay. In either case, the carbonate cements may help to reinforce the clay bridge fabrics. However, these carbonate features are a relatively minor feature and not an essential component of the collapsible brickearth fabric. Cryoturbation and micromorphological features indicate that the calcareous brickearth fabric has probably been developed through periglacial freeze–thaw processes. Freezing could have produced the compact silt/clay aggregates and an open porous soil framework containing significant inter-ped void space. Silt and clay were remobilised and translocated deeper into the soil profile by water percolating through the active layer of the sediment profile during thawing cycles, to form the loosed-packed inter-ped silt matrix and grain-bridging meniscus clay fabrics. In contrast, the upper 'non-calcareous' brickearth may represent a head or solifluction deposit. Mass movement during solifluction will have destroyed any delicate grain-bridging clay microfabrics that may have been present in this material.
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ABSTRACT Many structural properties of argillaceous sediments correlate closely with clay fabric; the degree of preferred orientation of clay particles. Shaly structure is associated with a significant degree of clay particle orientation, massive structure with a more random clay particle arrangement. Fissile structure occurs in shales where there is a high degree of orientation of clay minerals and other flaky constituents. It is best developed in shales containing a high percentage of organic material and where organic material is concentrated in bands. Organic material and in some cases, carbonate minerals, exert a secondary influence on fabric and structural properties. In ancient sediments that contain a significant percentage of organic material, the preferred orientation of clay minerals generally increases as the percentage of organic matter increases. Sediments with more than 10 percent carbonate minerals commonly show a poor degree of clay particle orientation. Little relation appears to exist between fabric or structural properties and clay mineral composition, except perhaps in sediments with appreciable montmorillonite. Poor particle orientation has been shown to exist in sediments in which montmorillonite is the major clay mineral component. No apparent relation was found between fabric or structural properties and total clay content (above 20 to 30 percent), geologic age, or depth of burial. Variations in orientation of clay particles, from essentially parallel to nearly random, occur within a few vertical inches in some thin stratigraphic sections. This, and the close relation of particle orientation to structural properties suggest that the orientation of clay minerals (fabric) in argillaceous sediments is not dominantly controlled by compaction caused by the weight of overlying sediments. These relations appear to reflect physicochemical conditions in the environment of deposition that influence the mode of clay mineral sedimentation, and/or geochemical factors that may promote particle rearrangement soon after deposition.
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The presence of clay-sized particles and clay minerals in modern sands and ancient sandstones has long presented an interesting problem, because primary depositional processes tend to lead to physical separation of fine- and coarse-grained materials. Numerous processes have been invoked to explain the common presence of clay minerals in sandstones, including infiltration, the codeposition of flocculated muds, and bioturbation-induced sediment mixing. How and why clay minerals form as grain coats at the site of deposition remains uncertain, despite clay-coated sand grains being of paramount importance for subsequent diagenetic sandstone properties. We have identified a new biofilm mechanism that explains clay material attachment to sand grain surfaces that leads to the production of detrital clay coats. This study focuses on a modern estuary using a combination of field work, scanning electron microscopy, petrography, biomarker analysis, and Raman spectroscopy to provide evidence of the pivotal role that biofilms play in the formation of clay-coated sand grains. This study shows that within modern marginal marine systems, clay coats primarily result from adhesive biofilms. This bio-mineral interaction potentially revolutionizes the understanding of clay-coated sand grains and offers a first step to enhanced reservoir quality prediction in ancient and deeply buried sandstones.
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Primary porosity that is lost during burial through cementation by carbonate, evaporite, and some clay minerals can be regenerated during the stage of secondary porosity development that is typical of most basins. However, primary porosity that is lost through compaction is forever lost and cannot be regenerated. Thus, it is desirable to be able to predict the amount of porosity loss expected in sandstones buried to given depths. During progressive burial, terrigenous sandstones compact by (1) packing readjustments without changes in grain shape, (2) ductile deformation of clayey and micaceous grains, chiefly rip-up-clasts, fecal pellets, and fragments of shale, mudstone, slate, and schist, (3) bending of flexible micas, (4) pressure solution, and (5) fracturing of feldspar, quartz, and chert grains. Process 1 generally results in a 7-10% porosity loss and is independent of sandstone composition; processes 2, 3, and 4 are strongly dependent on framework composition and each by itself is responsible for producing tight sandstones; and process 5 is generally not important. Process 3 was modeled by Rittenhouse, who showed that sandstones with 35% ductile grains can compact to produce tight sandstones. Pressure solution becomes important at depths greater than 8,000 ft (2,400 m). Pressure solution at quartz grain contacts is enhanced where thick illite or chlorite clay coats develop and is most common in quartz-rich sandstones that lack much quartz cement. Quartz dissolved from grains generally exits the formation instead of being precipitated as cement. Stylolites develop at mica-rich and clay-rich laminae and develop conspicuous vertical permeability barriers. Wholesale dissolution of quartz grains leaves a residue of clay, micas, organic matter, and feldspar. End_of_Article - Last_Page 505------------
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