基于生物诱导碳酸钙沉淀的土体固化研究进展

基于微生物或脲酶诱导碳酸钙沉淀（MICP/EICP）的土体固化技术是近年来岩土和地质工程领域的研究热点之一。在系统回顾基于生物诱导碳酸钙沉淀的土体固化技术发展历程的基础上,重点阐述了MICP/EICP固化机制、土体孔隙结构、菌液和脲酶性质、胶凝液性质和固化方式等方面对碳酸钙特性影响的研究进展。研究结果表明：土体孔隙越小,越不利于微生物或脲酶入渗,固化均匀性越差;土颗粒接触点越多,可为碳酸钙提供的沉积点位越多,碳酸钙与土颗粒间的黏结和桥接作用越强,固化效果越好;一定菌液或脲酶浓度或脲酶活性范围内,碳酸钙的生成速率和生成总量随浓度及活性的增大而增大,但过高的浓度或活性易导致碳酸钙生成速率过快,从而在土体注入端发生堵塞;低浓度胶凝液得到的碳酸钙晶体更小,在土体中的分布更均匀;采用合适的注浆饱和度可提高具有黏结作用的碳酸钙的占比;采用多层交替注入或单相低pH值注入可提高碳酸钙在试样中分布的均匀性。基于碳酸钙沉淀特性的影响因素,提高固化土体的均匀性,验证其耐久性,室内试验结果在现场尺度的适应性和改进方案应该成为以后研究的重点。     

Shallow-marine carbonate cementation in Holocene segments of the calcifying green alga Halimeda

Early-diagenetic cementation of tropical carbonates results from the combination of numerous physico-chemical and biological processes. In the marine phreatic environment it represents an essential mechanism for the development and stabilization of carbonate platforms. However, diagenetic cements that developed early in the marine phreatic environment are likely to become obliterated during later stages of meteoric or burial diagenesis. When lithified sediment samples are studied, this complicates the recognition of processes involved in early cementation, and their geological implications. In this contribution, a petrographic microfacies analysis of Holocene Halimeda segments collected on a coral island in the Spermonde Archipelago, Indonesia, is presented. Through electron microscopical analyses of polished samples, this study shows that segments are characterized by intragranular cementation of fibrous aragonite, equant High-Mg calcite (3.9 to 7.2 Mol% Mg), bladed Low-Mg calcite (0.4 to 1.0 Mol% Mg) and mini-micritic Low-Mg calcite (3.2 to 3.3 Mol% Mg). The co-existence and consecutive development of fibrous aragonite and equant High-Mg calcite results initially from the flow of oversaturated seawater along the aragonite template of the Halimeda skeleton, followed by an adjustment of cement mineralogy towards High-Mg calcite as a result of reduced permeability and fluid flow rates in the pores. Growth of bladed Low-Mg calcite cements on top of etched substrates of equant High-Mg calcite is explained by shifts in pore water pH and alkalinity through microbial sulphate reduction. Microbial activity appears to be the main trigger for the precipitation of mini-micritic Low-Mg calcite as well, based on the presumable detection of an extracellular polymeric matrix during an early stage of mini-micrite Low-Mg calcite cement precipitation. Radiocarbon analyses of five Halimeda segments furthermore indicate that virtually complete intragranular cementation in the marine phreatic environment with thermodynamically/kinetically controlled aragonite and High-Mg calcite takes place in about 100 years. Collectively, this study shows that early-diagenetic cements are highly diverse and provides new quantitative constraints on the rate of diagenetic cementation in tropical carbonate factories.     

Controlling factors of volumetrically important marine carbonate cementation in deep slope settings

The controlling parameters of early marine carbonate cementation in shoal water and hemipelagic to pelagic domains are well‐studied. In contrast, the mechanisms driving the precipitation of early marine carbonate cements at deeper slope settings have received less attention, despite the fact that considerable volumes of early marine cement are present at recent and fossil carbonate slopes in water depths of several hundreds of metres. In order to better understand the controlling factors of pervasive early marine cementation at greater water depths, marine carbonate cements observed along time‐parallel platform to basin transects of two intact Pennsylvanian carbonate slopes are compared with those present in the slope deposits of the Permian Capitan Reef and Neogene Mururoa Atoll. In all four settings, significant amounts of marine cements occlude primary pore spaces downslope into thermoclinal water depths, i.e. in a bathymetric range between some tens and several hundreds of metres. Radial, radiaxial and fascicular optic fibrous calcites, and radiaxial prismatic calcites are associated with re‐deposited facies, boundstones and rudstones. Botryoidal (formerly) aragonitic precipitates are common in microbially induced limestones. From these case studies, it is tentatively concluded that sea water circulation in an extensive, near‐sea floor pore system is a first‐order control on carbonate ion supply and marine cementation. Coastal upwelling and internal or tidal currents are the most probable mechanisms driving pore water circulation at these depths. Carbonate cements precipitated under conditions of normal to elevated alkalinity, locally elevated nutrient levels and variable sea water temperatures. The implications of these findings and suggestions for future work are discussed.     

Early submarine cementation in fore-reef carbonate sediments, Barbados, West Indies

Bottom sediments from the sea floor west of Barbados between depths of 110 and 324 m are composed of nodular or crusted carbonate deposits. Individual biogenic sediment grains and the cemented aggregates, nodules and crusts are usually more or less altered by bioerosion and may support one or more generations of encrusting organisms. On the basis of component analysis of the topmost part of the bottom sediments it is possible to recognize three facies: (1) a proximal slope facies down to a depth of about 140 m, rich in mollusc fragments, benthic foraminifera and bryozoans; (2) an upper distal slope facies between about 140 m and about 215 m, rich in benthic foraminifera, molluses and crustaceans; and (3) a lower distal slope facies from about 215 m to at least 300 m, dominated by molluscs, especially pelecypods, with subordinate scleractinians and tubes of the polychaete Lygdamus asteriformis. The appearance and quantitative importance of the cemented aggregates is also related to these facles. In the proximal slope facies, only relatively few irregular and very porous nodules are found, whereas in the lower distal slope facies, aggregates are very common. Most aggregates are crust-like with a smooth upper surface and a more or less irregular, knobby lower surface. The crusts are massive compared with the nodules of the proximal slope facies and, unlike the latter, the lower surfaces and walls of larger cavities are usually coated with Fe and/or Mn oxides. In the upper distal slope facies a gradual transition between the two types of aggregates is found. Petrographical and morphological evidence, together with carbon and oxygen isotopic data, indicates that the nodules and crusts were formed in situ by submarine lithification processes. Radiocarbon dating of two bulk samples suggests that the cementation took place during late Pleistocene and/or early Holocene.     

Making diagenesis obey thermodynamics and kinetics: the case of quartz cementation in sandstones from offshore mid-Norway

Calculation of the quantity and distribution of quartz cement as a function of time and temperature/depth in quartzose sandstones is performed using a coupled dissolution/diffusional–transport/precipitation model. This model is based on the assumptions that the source of the silica cement is quartz surfaces adjoining mica and/or clay grains at stylolite interfaces within the sandstones, and the quantity of silica transport into and out of the sandstone by advecting fluids is negligible. Integration of the coupled mass transfer/transport equations over geologically relevant time frames is performed using the quasi-stationary state approximation. Results of calculations performed using quartz dissolution rate constants and aqueous diffusion coefficients generated from laboratory data, are in close agreement with both the overall porosity and the distribution of quartz cement in the Middle Jurassic Garn Formation only after optimizing the product of the effective surface area and quartz precipitation rate constants with the field data. When quartz precipitation rate constants are fixed to equal corresponding dissolution rate constants, the effective surface area required to match field data depends on the choice of laboratory generated quartz rate constant algorithm and ranges from 0.008 cm⁻¹ to 0.34 cm⁻¹. In either case, these reactive surface areas are ∼2 to 4 orders of magnitude lower than that computed using geometric models.     

Relations between shallow cataclastic faulting and cementation in porous sandstones: First insight from a groundwater environmental context

The interplay between fault zone cataclasis and cementation is important since both processes can drastically reduce the permeability of faults in porous sandstones. Yet the prediction of fault cementation in high-porosity sandstone reservoirs remains elusive. Nevertheless, this process has rarely been investigated in shallowly buried faults (<2 km; T°<80 °C) where its sealing capacity could be acquired early in the geological history of a reservoir. In this paper, the macro- and microscopic analysis of a fault zone in the porous Cenomanian quartz arenite sands of Provence (France) shows that silica diagenesis occurs in the most intensely-deformed cataclastic parts of the fault zone. This fault zone shows 19–48% of its total thickness occupied by low-porosity quartz-cemented cataclastic shear bands whose porosities range from 0 – ca. 5%. The analysis of the weathering profile around the fault zone reveals the presence of groundwater silcretes in the form of tabular, tightly silicified concretions cross-cut by the fault. Detailed transmitted light, cold-cathodoluminescence and scanning electron microscopy analyses of the silica cements (from the fault and the silcrete) reveal that all the silica cements originate from groundwater diagenetic processes. This study therefore shows that silica cementation can occur specifically in fault zones and as groundwater silcrete in the shallow context of a groundwater system, generated at the vicinity of an erosional unconformity.     

Microbially induced cementation of carbonate sands: are micritic meniscus cements good indicators of vadose diagenesis?

Characteristic fabrics such as micrite envelopes, calcified filaments and micritic grain-to-grain bridges are observed in a modern subtidal firmground (Wood Cay, Bahamas) and in a variety of firm- and hardgrounds of Lower Cretaceous and Upper Jurassic platform carbonates (Swiss and French Jura Mountains). Their similarity to microbial fabrics described in grapestones and in intertidal to continental vadose environments suggests that microbial activity played an important role in the initial stabilization and cementation of carbonate sands. 'Meniscus-type cements' (to distinguish them from vadose meniscus cements), which clearly formed in subtidal environments, are related to filament calcification, trapping of percolating micrite and microbially induced carbonate formation. Such meniscus-type cements are commonly micritic, but meniscus-shaped precipitation of fibrous aragonite or sparitic calcite around organic filaments is also observed. Therefore, an interpretation of vadose early diagenesis should not be based on meniscus cements alone. Similarly, subtidally formed filamentous structures can strongly resemble alveolar septal structures and be interpreted incorrectly as related to subaerial exposure.     

The main causes of the occurrence of carbonate cementation zones in the strata of oil- and gas-bearing basins

The results of studies of sedimentary strata carbonatization in oil- and gas-bearing basins are generalized. The sources of CO₂, H₂CO₃, and organic acids, which regulate the carbonate balance in stratum waters at the stages of diagenesis and catagenesis, as well as under the influence of superposed processes, including those with the participation of microorganisms, are considered. The scale of secondary carbonatization of deposits in the zone of water-oil contacts and in the interval of hydrothermal action is estimated.     

Calcium isotope evidence for suppression of carbonate dissolution in carbonate-bearing organic-rich sediments

Pore fluid calcium isotope, calcium concentration and strontium concentration data are used to measure the rates of diagenetic dissolution and precipitation of calcite in deep-sea sediments containing abundant clay and organic material. This type of study of deep-sea sediment diagenesis provides unique information about the ultra-slow chemical reactions that occur in natural marine sediments that affect global geochemical cycles and the preservation of paleo-environmental information in carbonate fossils. For this study, calcium isotope ratios (δ^44/40Ca) of pore fluid calcium from Ocean Drilling Program (ODP) Sites 984 (North Atlantic) and 1082 (off the coast of West Africa) were measured to augment available pore fluid measurements of calcium and strontium concentration. Both study sites have high sedimentation rates and support quantitative sulfate reduction, methanogenesis and anaerobic methane oxidation. The pattern of change of δ^44/40Ca of pore fluid calcium versus depth at Sites 984 and 1082 differs markedly from that of previously studied deep-sea Sites like 590B and 807, which are composed of nearly pure carbonate sediment. In the 984 and 1082 pore fluids, δ^44/40Ca remains elevated near seawater values deep in the sediments, rather than shifting rapidly toward the δ^44/40Ca of carbonate solids. This observation indicates that the rate of calcite dissolution is far lower than at previously studied carbonate-rich sites. The data are fit using a numerical model, as well as more approximate analytical models, to estimate the rates of carbonate dissolution and precipitation and the relationship of these rates to the abundance of clay and organic material. Our models give mutually consistent results and indicate that calcite dissolution rates at Sites 984 and 1082 are roughly two orders of magnitude lower than at previously studied carbonate-rich sites, and the rate correlates with the abundance of clay. Our calculated rates are conservative for these sites (the actual rates could be significantly slower) because other processes that impact the calcium isotope composition of sedimentary pore fluid have not been included. The results provide direct geochemical evidence for the anecdotal observation that the best-preserved carbonate fossils are often found in clay or organic-rich sedimentary horizons. The results also suggest that the presence of clay minerals has a strong passivating effect on the surfaces of biogenic carbonate minerals, slowing dissolution dramatically even in relation to the already-slow rates typical of carbonate-rich sediments.     

Origin of Calcium carbonate granules found in buried soils and Quaternary deposits

The ovoid calcium carbonate granules of less than 2 mm in size commonly found by molluscan specialists and soil micromorphologists are usually interpreted as coming from either earthworms or arionid slugs, or both. In this paper the granules are examined in detail, with a view to better understanding their origins and dynamics. Through collection and experiments, it is shown conclusively that both animals produce calcium carbonate granules of the right size range. The two types have distinctive morphologies, however, and most of the granules observed in soils and sediments are clearly identifiable as coming from earthworms. This arises from a far greater production rate: worms are capable of shedding granules at the rate of 1–3 per week, whereas Arion ater only yields around 40 granules < 250 μm on death. The granules survive digestive processes, so heavy predation of earthworms by various animals and birds means that there is a constant recycling of the granules in the biosphere.     

Celestite formation, bacterial sulphate reduction and carbonate cementation of Eocene reefs and basinal sediments (Igualada, NE Spain)

Petrographic and geochemical studies of an Upper Eocene reef and associated basinal sediments from the mixed carbonate–siliciclastic fill of the south‐eastern Pyrenean foreland basin near Igualada (NE Spain) provide new insights into the evolution of subsurface hydrology during the restriction of a marine basin. The reef deposits are located on delta‐lobe sandstones and prodelta marls, which are overlain by hypersaline carbonates and Upper Eocene evaporites. Authigenic celestite (SrSO₄) is an important component in the observed diagenetic sequences. Celestite is a significant palaeohydrological indicator because its low solubility constrains transportation of Sr²⁺ and SO₄^2? in the same diagenetic fluid. Stable isotopic analyses of carbonates in the reef indicate that meteoric recharge was responsible for aragonite stabilization and calcite cementation. Sulphur and oxygen isotope geochemistry of the celestite demonstrates that it formed from residual sulphate after bacterial sulphate reduction, but also requires that there was a prior episode of sulphate recycling. Meteoric water reaching the reef and basinal areas was most probably charged with SO₄^2? from the dissolution of younger Upper Eocene marine evaporites. This sulphate, combined with organic matter present in the sediments, fuelled bacterial sulphate reduction in the meteoric palaeoaquifer. Strontium for celestite precipitation was partly derived in situ from dissolution of aragonite corals in the reef and basinal counterparts. However, ⁸⁷Sr/⁸⁶Sr data also suggest that Sr²⁺ was partly derived from dissolution of overlying evaporites. Mixing of these two fluids promoted celestite formation. The carbonate stable isotopic data suggest that the local meteoric water was enriched in ¹⁸O compared with that responsible for stabilization of other reefs along the basin margin. Furthermore, meteoric recharge at Igualada post‐dated evaporite deposition in the basin, whereas other parts of the same reef complex were stabilized before evaporite formation. This discrepancy resulted from the spatial distribution of continental siliciclastic units that acted as groundwater conduits.     

华北板块南缘寒武系苗岭统碳酸盐岩硬底：缺乏生物扰动背景下的早期海底胶结作用

华北地台南缘寒武系苗岭统碳酸盐岩硬底发育在核形石灰岩和鲕粒灰岩之上,并明显截断下伏的碳酸盐沉积物。本文通过对硬底界面之下颗粒间的放射纤维状方解石胶结物和等厚环边的微晶胶结物分析,表明其形成于早期海底胶结作用,该时期早期胶结区靠近沉积物—水界面,易受潮汐和波浪冲刷而暴露海底,并在持续搅动的高能环境中经历磨蚀和平整,形成硬底。由于硬底形成后几乎没有受到强烈改造和持续生物侵蚀的影响,通常表现出简单、平坦的表面形态。研究区馒头组二段含硬底层段形成于低、高能交替的潮间—潮下水道环境,以微晶方解石为主要的胶结剂沉淀;张夏组含硬底层段形成于台内鲕粒滩高速建造期,以物理化学沉淀和早期海底胶结作用为主。研究区出现的硬底表明,该时期海水化学条件和海底生态环境利于碳酸盐沉积物的早期胶结。硬底作为早期海底胶结作用的突出证据,对于研究碳酸盐沉积物的早期成岩作用及岩化特征具有重要意义,其形成过程及成因也为古海洋化学条件和底栖生态系统的变化提供了主要依据。     

Calcium carbonate dissolution history in late quaternary deep-sea sediments,Western Gulf of Mexico

Calcium carbonate dissolution has been studied in eight piston cores from the western Gulf of Mexico ranging in depth from 965 to 3630. The degree of dissolution throughout the cores was determined by studies of foraminiferal test fragmentation, benthonic foraminiferal abudance, calcium carbonate concentration, and various relationships between solution-resistant and solution-susceptible species. The paleoclimatic history recorded in these cores is similar to those defined previously in the Gulf of Mexico and equatorial Atlantic. Two mesgascopically distinct ash layers and well-defined planktonic foraminiferal subzones permit precise intercore correlation of dissolution horizons. All cores demonstrate intense dissolution during several subzones, especially during the early-middle Y, X1, and W1. Other less consistent dissolution horizons occur in various cores. Sedimentation rates increase while calcite concentrations decrease during glacial episodes suggesting increased dilution by terrigenous materials. Despite this, glacial episodes show greater dissolution and worse preservation of foraminiferal tests. Therefore, increased dissolution of calcium carbonate during glacial episodes must be a function of some mechanism that more than compensates for the increased rate of burial by terrigenous sediments. Dissolution is dissolution processes are not responsible for the observed effects. The oxidation of organic material may be the primary mechanism controlling the dissolution of calcium carbonate in the western Gulf of Mexico.     

基于星点状方解石胶结与溶解的识别查明砂岩粒间孔隙类型——以雅布赖盆地新河组砂岩为例

目前含油气砂岩中粒间孔隙是原生孔隙还是次生孔隙的认识仍不一致,而对星点状方解石胶结与溶解的识别能够有效地查明粒间孔隙的类型。文章通过铸体薄片细致地观察雅布赖盆地新河组砂岩中的微观现象,以成岩环境演化和成岩序列分析为主线,重视方解石胶结物的赋存状态与物质来源和溶蚀流体来源的配置关系,精细解剖微观现象,从而弄清楚星点状方解石的成因,进而查明砂岩的粒间孔隙类型和储集空间类型。结果表明,粒间孔隙中的星点状方解石是成岩早期浸染状方解石胶结物的溶蚀残余,溶蚀流体为成岩中期的有机酸流体,溶蚀类型为一致性溶解,形成的粒间孔隙为次生孔隙。鉴于此,雅布赖盆地新河组砂岩的储集空间由次生粒间孔隙和次生粒内孔隙（长石、岩屑、方解石胶结物的溶蚀孔隙）组成。     

Stable isotope evidence for multiple fluid regimes during carbonate cementation of the Upper Tertiary Hazeva Formation, Dead Sea Graben, southern Israel

Stable carbon- and oxygen-isotope compositions of calcite and dolomite cements have been used to understand porewater evolution in the Upper Tertiary Hazeva Formation within the Dead Sea Graben, southern Israel. Sandstone samples were obtained from four boreholes in three tectonic blocks of the graben over depths of 253–6448 m, a variation that largely reflects differential subsidence of individual fault-bounded blocks. Early carbonate cements dominate diagenesis. Calcite occurs at <1600 m, but was replaced by dolomite at greater depths. Dolomite at 1600–2700 m is Fe-poor (<0.8 mol% FeCO₃), and at 4700–6200 m, Fe-rich (0.5–7.2 mol% FeCO₃). Magnesite, anhydrite and halite are the final diagenetic phases. Calcite has positively correlated δ¹⁸O (+21‰ to +25‰) and δ¹³C (−6‰ to −2‰) values that generally decrease with depth. Dolomite has a wider variation in δ¹⁸O (+18‰ to +30‰) and δ¹³C (−8‰ to −1‰) values, which also generally are lower with increasing depth. However, the δ¹³C and δ¹⁸O values of dolomite from the uppermost 400 m of the Hazeva Formation in the Sedom Deep-1 borehole are anomalous in spanning the entire range of stable carbon and oxygen isotopic compositions over this relatively small interval.The decreasing dolomite δ¹³C values likely indicate an increased contribution of carbon from organic sources with increasing depth. Except for the uppermost 400 m, Hazeva Formation dolomite in the Sedom Deep-1 borehole has stable carbon-isotope compositions that imply initial dolomitization at much shallower levels, prior to the preferential subsidence of this tectonic block. The oxygen isotopic compositions of the calcite cement are best explained by equilibration at present burial temperatures (≤55 °C) with porewater of meteoric origin. Its δ¹⁸O values increased from −5‰ at the shallowest depths to 0‰ at 1600 m. The dolomite oxygen isotopic compositions also reflect equilibration at present burial temperatures with porewaters ranging from 0‰ at 1600 m to +7‰ at 3600 m (100 °C). In the deepest fault block (Sedom Deep-1 borehole), however, increasingly Fe-rich dolomite has (re)equilibrated with porewater whose δ¹⁸O values decreased from +9‰ at 4750 m (120 °C) to +1‰ to +2‰ by 6200 m (150 °C).Much of the dolomite likely formed at relatively shallow depths from saline brines derived from precursors to the Dead Sea. These infiltrated the Hazeva Formation, mixing with and largely displacing meteoric water, and dolomitizing calcite. Rock–water ratios tended to be high during these processes. However, the upper 400 m of the Hazeva Formation in the deepest fault block were likely deposited during its rapid tectonic subsidence, and largely escaped the initial style of dolomitization pervasive elsewhere in the study area. These sediments were also capped by evaporites. This relatively thin interval likely became a preferential conduit for brines that escaped underlying and overlying strata, including the Fe-rich, lower ¹⁸O fluids (evolved seawater?) present in the deepest part of the graben. These rocks present the most promising target for the passage and accumulation of hydrocarbons in the study area.     

Halite cementation and carbonate diagenesis of intra-salt reservoirs from the Late Neoproterozoic to Early Cambrian Ara Group (South Oman Salt Basin)

Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra‐salt ‘stringer’ reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self‐charging hydrocarbon system within partly highly overpressured carbonate bodies (‘stringers’). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post‐dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re‐precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack‐seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins.     

Analysis of faulting in porous sandstones

Faults in porous sandstones occur in three forms: deformation bands about 1-mm thick and tens of m long and across which offsets are a few mm; zones of deformation bands constituted of many closely spaced deformation bands across which offsets are a few cm or dm; and slip surfaces, that is, distinct surfaces within zones of deformation bands across which offsets are a few m to a few tens of m. Deformation bands represent highly localized deformation; analogous localization within a field of homogeneous deformation is theoretically possible in inelastic materials with certain ranges of constitutive parameters. Crushing and consolidation of sandstone within a band cause the material there to become stiffer than the surrounding porous sandstone. A zone of deformation bands behaves mesoscopically much as a stiff inclusion in a soft matrix. According to the constitutive model assumed to investigate the formation of deformation bands, an instability can develop, and strain increments within the zone of deformation bands can become boundlessly large when the far-field stresses reach critical values. This instability is here associated with the formation of slip surfaces.