Detrital Smectite Research Articles

Chemical weathering of a marine terrace chronosequence, Santa Cruz, California. Part II: Solute profiles, gradients and the comparisons of contemporary and long-term weathering rates

The spatial and temporal changes in hydrology and pore water elemental and 87Sr/ 86Sr compositions are used to determine contemporary weathering rates in a 65- to 226-kyr-old soil chronosequence formed from granitic sediments deposited on marine terraces along coastal California. Soil moisture, tension and saturation exhibit large seasonal variations in shallow soils in response to a Mediterranean climate. These climate effects are dampened in underlying argillic horizons that progressively developed in older soils, and reached steady-state conditions in unsaturated horizons extending to depths in excess of 15 m. Hydraulic fluxes ( q h ), based on Cl mass balances, vary from 0.06 to 0.22 m yr −1, resulting in fluid residence times in the terraces of 10–24 yrs. As expected for a coastal environment, the order of cation abundances in soil pore waters is comparable to sea water, i.e., Na > Mg > Ca > K > Sr, while the anion sequence Cl > NO 3 > HCO 3 > SO 4 reflects modifying effects of nutrient cycling in the grassland vegetation. Net Cl-corrected solute Na, K and Si increase with depth, denoting inputs from feldspar weathering. Solute 87Sr/ 86Sr ratios exhibit progressive mixing of sea water-dominated precipitation with inputs from less radiogenic plagioclase. While net Sr and Ca concentrations are anomalously high in shallow soils due to biological cycling, they decline with depth to low and/or negative net concentrations. Ca/Mg, Sr/Mg and 87Sr/ 86Sr solute and exchange ratios are similar in all the terraces, denoting active exchange equilibration with selectivities close to unity for both detrital smectite and secondary kaolinite. Large differences in the magnitudes of the pore waters and exchange reservoirs result in short-term buffering of the solute Ca, Sr, and Mg. Such buffering over geologic time scales can not be sustained due to declining inputs from residual plagioclase and smectite, implying periodic resetting of the exchange reservoir such as by past vegetational changes and/or climate. Pore waters approach thermodynamic saturation with respect to albite at depth in the younger terraces, indicating that weathering rates ultimately become transport-limited and dependent on hydrologic flux. Contemporary rates R solute are estimated from linear Na and Si pore weathering gradients b solute such that R solute = q h b solute β S v where S v is the volumetric surface area and β is the stoichiometric coefficient. Plagioclase weathering rates (0.38–2.8 × 10 −15 mol m −2 s −1) are comparable to those based on 87Sr/ 86Sr mass balances and solid-state Na and Ca gradients using analogous gradient approximations. In addition, contemporary solute gradients, under transport-limited conditions, approximate long-term solid-state gradients when normalized against the mass of protolith plagioclase and its corresponding aqueous solubility. The multi-faceted weathering analysis presented in this paper is perhaps the most comprehensive yet applied to a single field study. Within uncertainties of the methods used, present day weathering rates, based on solute characterizations, are comparable to average long-term past rates as evidenced by soil profiles.

Fluid involvement in the active Helike normal Fault, Gulf of Corinth, Greece

Rock fabric and mineralogical composition from the fault core and the unaffected protolith have been used to define the role of the segmented Helike Fault to fluid flow. Sixty samples were investigated by XRD, SEM observation and SEM-EDS microanalyses. Detrital smectite, calcite, and quartz represent the mineral assemblage at the crest of the footwall block in Foniskaria sampling site. In this site smectite is enriched at the rims of the fault core. All other sampling sites located at the base of the fault scarp are characterized by detrital and newly formed minerals. Detrital minerals include plagioclase, quartz, calcite and illite in Nikolaiika sampling site, and smectite, illite, kaolinite, quartz, calcite in Selinous sampling site. In the latter sampling site erionite and cerussite are newly formed minerals with erionite considered as the hydrothermal alteration product of fluids at 80–100 °C. At the eastern fault segment illite, quartz and calcite (T13 site) corresponds to detrital minerals. Mineralogy in the fault core reflects its high permeability to down-flowing meteoric water and weak hydrothermal alteration. The rock fabric suggests mineral alignment parallel to the fault plane. Mineralogy indicates that the Aigion, Helike and Pyrgaki Faults in the Gulf of Corinth host hydrothermal activity at shallow levels.

The occurrence and genesis of clay minerals associated with Quaternary caliches in the Mersin area, southern Turkey

AbstractCaliche in various forms, namely powdery, nodule, tube, fracture-infill, laminar crust, hard laminated crust (hardpan), and pisolitic crust, is widespread in the Mersin area in southern Turkey. It generally occurs within and/or over the reddish-brown mudstone of the Kuzgun Formation (Tortonian, Miocene) and alluvial red soils of the Quaternary. The mineralogical distribution along representative caliche profiles was examined by X-ray diffraction, scanning electron microscopy, differential thermal analysis-thermal gravimetry, and chemical techniques. Calcite is the most abundant mineral associated with minor amounts of palygorskite in caliche samples, whereas smectite is prevalent mainly in the reddish-brown mudstone and alluvial red soils of the caliche parent materials and is associated with appreciable amounts of palygorskite. These minerals are also accompanied by trace amount of illite, quartz, feldspar, and a poorly crystalline phase. Palygorskite fibers and fiber bundles were developed authigenically on euhedral or subhedral calcite crystals of the caliche units and at the edges of smectite flakes in the caliche host-rocks or sediments. Intense, continuous evaporation of subsurface soil-water resulted in an increase in pH and the dissolution of detrital smectite within the red mudstones and alluvial red soils that enclose the isolated caliche forms, and caused an increase in the Al+Fe and Mg/Ca ratio, favoring the formation of palygorskite under alkaline conditions. The calcium required for caliche formation may have originated from eolian dust, detrital carbonate minerals, and/or other caliche materials, which are dissolved by carbonic acid.

Clay mineralogy of the Old Red Sandstone and Devonian sedimentary rocks of Wales, Scotland and England

AbstractThe Devonian sedimentary rocks of the UK are made up of a continental red bed facies, the Old Red Sandstone (ORS), and sediments of a marine origin. The latter are confined to southwest England whereas the ORS occurs much more extensively, particularly in South Wales, the West Midlands, Northern England, the Midland Valley of Scotland and the Orcadian basin. The ORS also occurs extensively offshore where it contains important hydrocarbon reservoirs. Highly variable suites of clay mineral assemblages are characteristic of the ORS. In the West Midlands and Monmouthshire, the Downton Group is characterized by illitic, smectitic, and mixed-layer illite-smectite minerals. A tuff bed (Townsend Tuff) also contains expansible minerals but when the bed is traced westwards it is found that the clay mineralogy changes progressively to an illite-chlorite assemblage, suggesting the influence of diagenetic or metamorphic change. It is not known, however, whether such a transformation is typical of the Downton Group as a whole. The overlying Ditton Group in its eastern outcrops contains a high-spacing mineral identified as tosudite, together with regularly interstratified illite-smectite and well crystallized kaolinite. Further west this assemblage gives way to illite and chlorite, with the latter being trioctahedral or dioctahedral, while in Dyfed the Ditton Group may contain smectite and poorly crystallized kaolinite in addition to illite and chlorite. The geographical distribution of clay minerals in the Ditton Group may also be accounted for by progressive diagenetic to low-grade metamorphic reactions, although it is necessary to postulate retrogressive diagenesis to account for the smectite and kaolinite that occur in the Dyfed samples. The clay mineralogy of the overlying Brecon Group and the Upper ORS also consists of mixtures of illite and chlorite in the west and central parts of the region. No data are available for the Brecon Group in the eastern parts of the outcrop but the Upper ORS from around Monmouth and Portishead contains assemblages rich in kaolinite and mixed-layer illite-smectite with only minor amounts of chlorite. The distribution of clay minerals in the Upper ORS is again suggestive of a progressive westerly increase in the influence of diagenetic alteration, although the influence of provenance cannot be discounted.In Scotland the clay mineralogy of the Lower ORS in the Midland valley is characterized by a variety of interstratified minerals, including regularly interstratified trioctahedral chlorite-vermiculite, a tosudite mineral similar to that described from South Wales and illite-smectite, as well as occasional illite, chlorite and smectite. The oldest Stonehaven Group is kaolinitic but in the younger groups kaolinite is either completely absent or present in only minor amounts. It is clear that detrital inputs, particularly from associated volcanic rocks, have contributed to the clay minerals found in these rocks, although the contribution could be indirect with diagenetic clay minerals forming from volcanic detritus after deposition. Diagenetic alteration may also be important, particularly with respect to the origin of the tosudite mineral. The Middle ORS lacustrine sediments of the Orcadian Basin of Scotland are characterized by mixtures of trioctahedral chlorite and dioctahedral illite, with interstratified chlorite-smectite, illite-smectite, kaolinite and occasional reports of minor montmorillonite. The most recent interpretations of the origins of the clay minerals in these rocks emphasize the role of progressive diagenetic and low-grade metamorphic reactions, based on correlations of clay mineral assemblages and parameters such as illite crystallinity with organic maturation data. This interpretation argues that the illite-smectite in the shales is derived largely from a precursor detrital smectite. However, the finding of two populations of morphologically and structurally distinct illite-smectite particles in the Middle and Upper ORS sandstones suggests a more complex picture involving different diagenetic episodes. In addition, the likelihood of a smectitic-rich detrital input to the ORS may also be a point of debate. The clay mineralogy of the North Sea offshore is also described briefly, in addition to the marine Devonian in southwest England. The latter is characterized by chlorite and illite assemblages of low-grade metamorphic origin, although smectite and kaolinite are also found occasionally in these rocks. The offshore ORS, however, contains a variety of clay minerals, including an assemblage similar to that found in the Lower ORS south of the Highland Boundary Fault and notably contains a tosudite-like mineral.

Late Cenozoic evolution of the Nankai trench-slope system: evidence from sand petrography and clay mineralogy

Abstract Submarine slope systems in subduction zones evolve in response to a combination of tectonic and sedimentary forcing. It can be difficult to determine how and when tectonic forcing affects sedimentation, especially when investigating ancient rock successions, but one of the more reliable indicators is a change in sediment composition. During Leg 190 of the Ocean Drilling Program, sandy turbidites were recovered from a Quaternary trench wedge (Nankai Trough), a Pliocene-Pleistocene slope basin, the underlying Pliocene-Miocene accretionary prism, and a Miocene turbidite facies in the Shikoku Basin. Differences in detrital provenance between the sand and clay-sized fractions indicate that turbidity currents did not follow pathways of suspended-sediment transport during the past 10 Ma. During the middle and late Miocene, the sand probably was eroded from a newly exposed accretionary complex (Shimanto Belt). In contrast, high contents of detrital smectite in Miocene mudstones (>50 wt% of the <2 μm size fraction, relative to illite, chlorite + kaolinite, and quartz) point to a strong volcanic component of suspended-sediment input (Izu-Bonin island arc). The sand in accreted Pliocene turbidites was also eroded from the Shimanto Belt and transported by transverse flow down the insular slope. The trench-wedge facies then switched to axial flow during the Quaternary, when the sand supply tapped a mixed volcanic-metasedimentary provenance in the rapidly uplifted Izu-Honshu collision zone. Progressive depletion of smectite during the Pliocene and Pleistocene (<20 wt%) points to increased movement of illite- and chlorite-rich clay toward the east and NE from sources on Kyushu and Shikoku. That shift in mud composition coincides with intensification of the North Pacific western boundary current (Kuroshio Current) at approximately 3 Ma. Overall, the depositional system in the Nankai Trough and Shikoku Basin shifted its sand sources because of regional tectonics, whereas the suspended-sediment budget was modulated by hemispheric changes in ocean-water circulation.

Mineralogical and geochemical characteristics (major, minor, trace elements and REE) of detrital and authigenic clay minerals in a Cenozoic sequence from Ross Sea, Antarctica

AbstractThe mineralogy and geochemistry of the clay fraction of Victoria Land Basin (Ross Sea, Antarctica) sediments was investigated, to determine the origin of clay minerals and the features of authigenic smectite. The investigated core (CRP-3) is ~800 m long, mostly of Oligocene age. The clay fraction of the upper sequence consists of mica, chlorite and detrital smectite, while that of the central and lower part is largely made up of authigenic smectite. Authigenic smectites are ditrioctahedral, with a composition close to saponite, while detrital smectites such as Al-Fe beidellites are dioctahedral. Authigenic smectites have no illite mixed layers, show a higher degree of crystallization, higher MgO, Fe2O3, V, Cr, Co, Ni and Sc contents and lower SiO2, Al2O3, K2O, TiO2, Ba, Rb and Zr contents with respect to detrital clay minerals, and a clear depletion of LREE with respect to HREE. Authigenic smectite formed from the alteration of volcanic materials and clay minerals.

Triassic–Jurassic weathering and clay mineral dispersal in basement areas and sedimentary basins of southern Sweden

The basement rocks of southern Sweden were deeply weathered in Late Triassic through Cretaceous times, and possibly even earlier. Sub-Mesozoic saprolites formed, presumably due to long-term warmth and humidity, in conjunction with decomposition of abundant organic matter and subsequent infiltration of acidic water. The upper parts of the weathering profiles were subjected to intense leaching and kaolinitisation whereas the less evolved deep weathering front is enriched in smectite. Thus, any erosion prior to full regolith development, or extraordinary deep erosion, released less developed, smectite-rich weathering material to the Late Triassic–Jurassic receiving basins. Mesozoic sedimentary successions in Southern Sweden show high detrital smectite and/or kaolinite contents, moderate illite contents and low chlorite contents, compatible with expected mid-latitude warm equable greenhouse conditions. Norian and older Triassic haematite-cemented continental arkoses and smectite-dominated clays (smectite≫kaolinite) were clearly formed in arid settings. From the Rhaetian and throughout the Jurassic, humid onshore conditions sustained abundant plant and peat accumulation, early diagenetic meteoric flushing, and intense chemical weathering. Kaolinite is therefore a dominating detrital clay mineral in these sedimentary successions (kaolinite≈illite≫chlorite and smectite (I/S)). Minor quantitative variations in clay mineralogy within the “humid climate” Rhaetian–Jurassic sedimentary successions are difficult to interpret. In deltaic coal-bearing successions, the detrital clay mineral composition is conspicuously constant (kaolinite≈illite≫chlorite), possibly due to post-depositional clay mineral transformation. Conversely, in shallow marine deposits, some quantitative variation is notable (kaolinite≈illite>variable smectite≫chlorite). This may reflect that the original detrital composition was better preserved in shallow marine settings. Smectitic clay minerals associated with gypsum and caliche nodules ambiguously indicate increased aridity towards the end of the Jurassic.

Paleomagnetic dating of the smectite‐to‐illite conversion: Testing the hypothesis in Jurassic sedimentary rocks, Skye, Scotland

A hypothesized connection between clay diagenesis and magnetite authigenesis is supported by the results of paleomagnetic, rock magnetic, geochemical, and petrographic studies on Jurassic sedimentary rock of Skye, Scotland. The results have implications for understanding remagnetization mechanisms and for the development of a paleomagnetic method to date clay diagenesis. Previous diagenetic studies indicate that rock in north Skye contains abundant detrital smectite, whereas the clays in the same age rock in south Skye have altered to illite because of Tertiary igneous activity. Geochemical (87Sr/86Sr, δ13C, δ18O) studies confirm that sedimentary rocks in south Skye are altered. The magnetization in the rocks in north Skye is weak and unstable. In contrast, the rocks in south Skye contain a multicomponent magnetization. At intermediate temperatures (225°–450°C) a magnetization with southerly declinations and negative inclinations or its antipodal equivalent is present. At higher temperatures (450°–580°C) a magnetization with northerly declinations and positive inclinations or its antipodal equivalent is removed. Both magnetizations have directions similar to Tertiary igneous rocks on Skye. Rock magnetic studies indicate the presence of pyrrhotite and magnetite. The intermediate‐temperature component resides in pyrrhotite and/or magnetite and is interpreted as a thermoviscous remanent magnetization or a thermochemical remanent magnetization related to the heat and/or hydrothermal activity associated with the igneous activity. The high‐temperature component resides in magnetite and is a chemical remanent magnetization (CRM). The presence‐absence test and the timing of acquisition for this CRM suggest that magnetite authigenesis is related to the smectite‐to‐illite conversion and that clay diagenesis is a viable remagnetization mechanism.

Effets thermique et hydrothermal de la coulée de basalte triasico-liasique sur les argiles du bassin d'Argana (Maroc)

Thermal and hydrothermal effects of Triassic-Liassic basalt flow deposition on clays (Agana Basin, Morocco). Thermal and hydrothermal effects of Triassic-Liassic basalt flow deposition on sedimentary series of the Argana Basin are responsible for major modifications in detrital clays, until 20 m in depth. It expressed by transformation of detrital smectite to corrensite and moreover to chlorite, and by increasing illite crystallinity. On the 2 m of sediments located immediately under the flow, magnesium-rich hydrothermal fluids have caused precipitation of new mineral phases. To cite this article: L. Daoudi, J.-L. Pot de Vin, C. R. Geoscience 334 (2002) 463-468.  2002 Academie des sciences / Editions

Different rates of smectite illitization in mudstones and sandstones from the Niigata Basin, Japan

AbstractThe different rates of smectite illitization have been investigated in interbedded sandstone and mudstone samples from the Neogene to Lower Pleistocene stratigraphy of the Niigata sedimentary basin. For samples of approximately the same age and under the same temperature and pressure conditions, the analytical results of XRD and EDS show that differences in the transformation from R = 1 I-S to R≥3 I-S at a temperature of ~30–40°C and 1000 m in depth are determined by the initial smectite composition and the relative ion concentration of pore-fluid in the sedimentary rock system. The ordered I-S and illite in the mudstone have inherited compositional characteristics from the initial detrital smectite and illite. In the sandstone, the I-S composition was changed by interaction with pore-fluids and by precipitation and crystal growth during diagenesis.

Smectite Diagenesis in Neogene Marine Sandstone and Mudstone of the Niigata Basin, Japan

AbstractIllitization of smectite during progressive burial diagenesis occurs differently in sandstone and mudstone, which are interbedded in the sedimentary sequence of the Niigata basin. Reaction progress of illitization of smectite via mixed-layer illite-smectite (I-S) in the mudstone is more complete than in the sandstone. In sandstone, smectite converts to (Reichweit, R ≥ 3) I-S and illite via random (R = 0) I-S to ordered (R = 1) I-S, and authigenic chlorite and quartz form as products of the illitization of smectite.The original composition of detrital smectite and the occurrence of Na+, K+, Ca2+, Mg2+, and Fe2+ in pore fluids partly control both illitization of smectite and the resulting authigenic mineral products in the diagenetic process. In mudstone, detrital smectite is K- and Si-rich in composition, and the illitization of smectite indicates that the original composition is mostly inherited. Excess silica owing to illitization is released to produce authigenic quartz. In sandstone, smectite forms primarily by precipitation. The evolving compositions produced by early illitization form smectite, then random (R = 0) I-S, and then ordered (R = 1) I-S. These transitions are related to compositions of pore fluid. Changes in K/(K + Ca + Na) vs. K + Ca + Na imply that the increase of interlayer cations occurs by absorption and smectite transforms to random (R = 0) I-S, followed by the exchange of interlayer cations to ordered (R = 1) I-S with increasing diagenetic grade. Late illitization from (R = 1) I-S to R ≥ 3 suggests decomposition of smectite and early I-S with an increase in the number of illite layers. Dissolution experiments of host rocks with pure water and 0.01 M HCl solution reflect the differences in chemistry of the original pore fluid and authigenic carbonate in the process of diagenesis of clastic rocks. These results explain how chemical composition produces large variations in transformation temperature of smectite to illite in the diagenetic process.

Effects of burial on the clay assemblages in the Triassic Skagerrak Formation, Denmark

AbstractSandstones and mudstones of the Skagerrak Formation from the Danish Sub-basin have been studied to assess the origin of the clay minerals, their diagenesis and their transformation/ conversion temperatures (compared to modelled maximum burial temperatures). Detrital smectite and vermiculite occur only at shallow levels. Conversion of smectite into random S-I took place at >47°C and of random S-I into ordered I-S at >74°C. The omnipresent illite increases in amount with burial. The amount of authigenic kaolinite (kaolin) decreases with increasing temperature (>75°C), possibly due to the growth of chlorite. Random S-C is converted into ordered S-C between 70°C and 105°C and chlorite is most abundant at maximum burial temperatures (150°C). This investigation suggests that diagenesis of clay minerals begins at relatively low temperatures, and emphasizes the importance of the availability of necessary ions (K+, Mg2+, Fe2+) for the reactions, besides time and temperature, as controlling parameters.

Rare-earth elements as tracers of the genetic relationship between smectite and palygorskite in marine phosphorites

AbstractDetrital smectite in a sandy claystone and a phosphorite, and authigenic palygorskite in a dolomitic marl and a porcellanite from Cretaceous-Tertiary phosphorite deposits of the Ganntour Basin (Morocco) were purified using cation exchange resin, leached with dilute acid, and analysed for the contents and distribution patterns of their REE before and after acid treatment. The normalized patterns confirm a detrital origin for the smectite in the sandy claystone, whereas the origin of the smectite from the phosphorite is obscured by the addition of REE from the phosphogenic environment. The normalized REE patterns of the palygorskite suggest formation in non-oxidizing restricted environments. The Al2O3/ΣREE ratio of the two clay types suggests formation of diagenetic palygorskite (and mixed-layer illite-smectite) from Al-bearing detrital smectite by a dissolution-crystallization process.

Mg-Smectite Authigenesis in a Marine Evaporative Environment, Salina Ometepec, Baja California

AbstractFormation of authigenic trioctahedral Mg-rich smectite is common in evaporative lake sediments, but was not described previously in modern marine evaporative environments. This study documents formation of authigenic K-rich, Mg-smectite during very early diagenesis in the dominantly siliciclastic Salina Ometepec (Baja California), a large supratidal evaporative sabkha complex near the mouth of the Colorado River. Here, sediment pore waters are exceptionally Mg2+-rich relative to other marine evaporative environments due to suppressed sulfate reduction which limits production of carbonate alkalinity and, hence, carbonate (particularly dolomite) precipitation. Sediment cores were obtained along a five km transect seaward across the hypersaline mud flat to evaluate how these atypical geochemical conditions would affect the clay mineral compositions.Scanning transmission electron microscopy (STEM) observations show that the smectite from the marine Inlet, near the sediment source, consists of grains of irregular shape that give selected area diffraction (SAED) patterns reflecting dominant turbostratic stacking. Analytical electron microscopy (AEM) analyses indicate that K+ is the dominant interlayer cation; the mean composition is approximately K0.7(Al3.3Fe(III)0.3Mg0.5)(Al0.5Si7.5)O20(OH)4. Such smectite is implied to be detrital in part because it is similar to smectite known to be deposited by the Colorado River.Smectite from the hypersaline mud flat occurs as aggregates of small subhedral pseudohexagonal plate or lath-shaped crystals ≤250 nm in diameter, with thicknesses varying between three and ten layers. The SAED patterns reflect substantial turbostratic stacking, but with a greater frequency of interlayer coherency as compared with detrital smectite. Crystals from greater sediment depths are larger and more nearly euhedral. This smectite is dominantly trioctahedral, with mean composition approximately K0.7(Al0.7Fe(III)0.5Mg4.45)(Al1.2Si6.8)O20(OH)4 (saponitic). This smectite is inferred to be dominantly authigenic in origin.The X-ray diffraction (XRD) and STEM/AEM data collectively imply that detrital aluminous dioctahedral smectite reacts to form authigenic Mg-rich trioctahedral smectite, driven in part by the high Mg2+/ Ca2+ ratio of pore waters. Such early-formed Mg-rich smectite may be the precursor for the trioctahedral mixed-layer smectite, corrensite, and chlorite assemblages found in ancient marine evaporative sequences. These results also add to the accumulating evidence that interlayer K+ in marine smectite is fixed during the earliest stages of marine diagenesis near the sediment water interface.

Transformations and neoformations of clay in the cryogenic environment: examples from Transbaikalia (Siberia) and Patagonia (Argentina)

SummaryPermafrost desiccates and indurates soil horizons. During summer, the perennially frozen ground under the active thawed layer creates confined conditions, Whereas in winter the closed system between the permafrost and the freezing front migrating downwards leads to irreversible changes in the soil structure and to transformation and neoformation of minerals. Two examples were studied by field observation and laboratory analyses (optical microscopy, scanning electron microscopy, X‐ray diffractometry, transmission electron microscopy and analytical electron microscopy). In Transbaikalia (southeastern Siberia) a Pleistocene cryogenic slope deposit on syenite contains detrital clays (chlorite, illite), but neoformed ferroan smectites dominate. In Patagonia, centimetre‐sized silica concretions containing spongy and spheroidal opal occur in alluvium ascribed to cold Pleistocene periods. They seem to have formed from primary detrital smectite that was transformed to fibrous clay (sepiolite) then to acicular opal, and eventually to spheroidal opal. The neoformed opal contains 27% magnesium. These features occur systematically within fossil cryoturbations not related to present conditions. It appears that clay transformation and neoformation can happen at temperatures far lower than 0°C.

Detrital origin for smectite in soils and sediments from the coastal plain of Ariake Bay, northern Kyushu, Japan

The δ 18O values of smectite (+ 14.2 to + 20.5%o) from soils and sediments along the coastal plain of Ariake Bay, northern Kyushu, Japan, suggest that the smectites are detrital in origin. Samples of detrital smectite from the catchment of major rivers entering Ariake Bay have similar δ 18O values (+ 18.2 to + 22.8%o). The oxygen-isotope data rule out neoformation at low temperatures during interaction with seawater as a possible origin for most of this smectite.

Tem Observations of Coherent Stacking Relations in Smectite, I/S and Illite of Shales: Evidence for MacEwan Crystallites and Dominance of 2M1 Polytypism

AbstractTEM characterization of stacking relations in I/S of expanded shale samples from the Gulf Coast and Michigan Basin was carried out to address the issues of the degree of coherency and the nature of layer stacking sequences in smectite, I/S and illite. The two-dimensional lattice fringe images obtained from this study show that cross fringes are commonly observed to be continuous over at least 3–4 layers for smectite, 6–7 layers for ordered I/S and 9–10 layers for illite-rich I/S. This demonstrates that such sequences are coherent, or at least semi-coherent (in smectite) units (MacEwan crystallites). The observations demonstrate that so-called fundamental particles are fragments of MacEwan crystallites formed primarily as a result of disaggregation along weakly-bonded smectite interlayers. However, both 0k1 and h01 reflections may coexist in selected area electron diffraction (SAED) patterns. The frequency of occurrence of the coexistence in SAED patterns decreases in the order smectite, I/S and illite for Gulf Coast samples. This is consistent with the presence of turbostratically-related interfaces in packets of all of these materials. Therefore, any given layer sequence in smectite, ordered I/S and illite may have both turbostratic and coherent interfaces. The proportion of coherently-related layers increases with increasing proportion of illite-like layers. The concept of fundamental or elementary particles is not related to layer sequences in non-disaggregated, original rocks. Indeed, it implies relations that are not valid.The lattice fringe images, SAED and optical diffraction patterns demonstrate that where layers are coherently-related, 2M1 is the dominant polytypic sequence in all samples. However, this periodic 2M1 stacking is so frequently interrupted by stacking faults in smectite that it gives rise to apparent lMd polytypism. The degree to which the periodic 2M1 sequences are interrupted by stacking faults decreases with increasing proportion of illite-like layers. The SAED patterns of I/S and illite unmodified since its formation are diffuse parallel to c* and have poorly-defined, non-periodic reflections for indices k ≠ 3N as a measure of local ordering superimposed on poorly-ordered coherent sequences with a turbostratic component. X-ray diffraction (XRD) patterns, as integrated over domains with a range of heterogeneous stacking relations, do not represent simple mixtures of discrete IM and 2M1 polytypes.The observations of this study imply that dissolution-crystallization is a dominant mechanism for the smectite-to-illite transition. The semi-coherent stacking of smectite-like layers in smectite-rich samples implies that either a dissolution-crystallization process took place subsequent to deposition of detrital smectite or that Gulf Coast smectite is an in-situ alteration product of volcanic ash.

Stem/Aem Evidence for Preservation of Burial Diagenetic Fabrics in Devonian Shales: Implications for Fluid/rock Interaction in Cratonic Basins (U.S.A.)

ABSTRACT Fabrics, microstructures, and compositions of authigenic illite-rich clays in Devonian intracratonic basin shales were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and scanning transmission electron/analytical electron microscopy (STEM/AEM) methods in order to relate the extent of clay diagenesis to the timing of mudstone-system closure and to reconcile conflicting radiogenic and paleomagnetic ages obtained from these shales and associated limestones and bentonites. Authigenic illite in these Devonian shales is similar to post-transition illite-rich mixed-layer illite/smectite (I/S) in Gulf Coast and other young basin mudstones that have formed through replacement of detrital smectite or smectite-rich I/S without subsequent diagenetic modification. The clay matrix is composed of subparallel intergrowths of illite-rich crystals (30-200 A thick) with low-angle grain boundaries and defects at layer terminations. Contrast in some lattice fringe images is consistent with R1-, R2-, and R3-ordered, mixed-layer I/S. Electron diffraction patterns indicate partially disordered (1Md) stacking sequences with significant turbostratic stacking and mixed layering. The illite-rich material is K- and Aldeficient, and Si-enriched, relative to ideal muscovite. Preservation of burial diagenetic, authigenic, mixed-layer I/S microstructures in these Devonian shales implies that they have remained effectively closed systems following the smectite-to-illite transformation. Later Alleghenian (ca. 300 Ma) tectonic or fluid-flow events recorded by authigenic minerals in bentonites and carbonate lithologies throughout the U.S. Midcontinent are apparently not recorded in these Devonian shales. Preservation of diagenetic fabrics indicates that radiogenic isotope dating of cratonic basin shales should yield syndepositional to early burial diagenetic ages corresponding to the timing of I/S authigenesis.

Fibrous clays in the Almazan Basin (Iberian Range, Spain): genetic pattern in a calcareous lacustrine environment

AbstractThe Tertiary sediments of the southern part of Almazán basin (Province of Soria) exhibit two different facies with a sharp transition between them: (1) a coarse detrital facies; and (2) a lutite and carbonate facies. The vertical evolution of these facies in the different sections and boreholes is made up of several retrograde depositional sequences which evolve from proximal alluvial fans to carbonate lacustrine systems associated with the most distal fan sediments.The variation of the mineralogical association (carbonates, quartz, detrital clay minerals, palygorskite and, occasionally, sepiolite and smectites) allows four ‘mineralogical facies’ to be established. Each facies has a distinctive mineralogy related to the main genetic processes: (1) detrital facies; (2) transitional facies; (3) ‘chemical dolomite-palygorskite’ facies and (4) ‘chemical calcite’ facies. Although the carbonates (calcite and dolomite) are always the most abundant minerals, fibrous clay minerals are also important. The main mechanism of mineral authigenesis in this basin is chemical precipitation; the presence and distribution of carbonates and fibrous clay minerals is controlled by the relative concentrations of [Mg2+], [Ca2+], pCO2, pH and [SiO4H4]. Palygorskite occurrences, in these conditions, are related to dissolution-precipitation mechanisms of pre-existing detrital material.

Petrography, mineralogy and diagenesis of overpressured Tertiary and Late Cretaceous mudrocks from the East Shetland Basin

AbstractThe petrography, mineralogy and diagenesis of an overpressured Tertiary and Late Cretaceous mudrock sequence from a single well in the East Shetland Basin (North Sea) are described. The clay fraction is believed to be dominated by volcanic glass above 1400 m depth and by smectite below 1400 m. The smectite content increases with depth due to an increase in detrital smectite and toin situalteration of volcanic glass and grains; this commenced almost immediately after deposition and is still not complete at burial depths >2300 m. There is little evidence of alteration of the smectite to illite despite a local available supply of K+in the form of K-feldspar and the presence of K in the more deeply buried smectite. Very little leaching or precipitation of quartz has occurred. Microfossil tests filled with authigenic clays such as have been reported in other Tertiary mudrocks in the North Sea were not observed, though a few fossil cavities are filled with carbonate cements, and very rarely with microquartz. The suprisingly limited reactivity of this mudrock sequence may be due to the early development of overpressure, resulting in a “closed” system, and to the absence of interbedded higher permeability layers or open faults through which fluids could be exchanged.