Diagenetic Chlorite Research Articles

THE SEDIMENTARY EVOLUTION OF THE RED SEA AND GULF OF ADEN

Onset of rifting, and flooding by marine waters, occurred in the late Oligocene in the Gulf of Aden and southern Red Sea. The northern part of the Red Sea may have been a largely continental rift at this initial stage, but continued rifting established marine conditions throughout the system by the early Miocene. Episodic isolation of the Red Sea system, leading to evaporite deposition in some basins, commenced in the mid‐Miocene and over two kilometres of salt had accumulated in most Red Sea basins by the end of the Miocene.Re‐establishment of persistently‐marine conditions occurred in the Pliocene, and marine recharge is now sufficiently high to permit vigorous carbonate build‐ups in shallow‐water areas.Clastic sediment textures suggest that marginal escarpments, which first developed during the onset of rifting, were strongly uplifted in the Pliocene‐to‐Recent period. Subsidence of basin floors seems to have been particularly rapid during the period dominated by salt deposition.If eruption of sea‐floor basalts in the axial rift areas is excluded, volumetrically important volcanism is centred on the present Afar triangle area, and is confined to the Oligocene and early Miocene. The amount of contemporary volcanic débris in the sandstones is consequently not particularly high. Some sandstones in northern Ethiopia, Sudan and Egypt do contain abundant acidic volcanic clasts, but these are derived from the Proterozoic basement and cause less diagenetic reservoir damage than contemporary glassy volcanic ash.Sandstones deposited in freely‐drained alluvial fan settings are characterised by early diagenetic kaolinite, whereas those of sabkha and marginal‐marine settings tend to show relatively early diagenetic chlorite. Those alluvial fan sandstones which were subsequently invaded by reduced pore waters expelled from the basin axis, and those in the basin axis, often developed later diagenetic chlorite.

Octahedral occupancy and the chemical composition of diagenetic (low-temperature) chlorites

AbstractThe chemical composition of about 500 diagenetic chlorites, determined by electron microprobe, has been studied in six different sedimentary sequences spanning conditions from early diagenesis to low-grade metamorphism, in the temperature range 40–330°C. The range of Fe/(Fe + Mg) is almost complete and is positively correlated with Al. Five sequences show the same compositional variation. In each, the most siliceous chlorites have the lowest R2+, substantially more octahedral than tetrahedral Al, and the lowest octahedral totals. Conversely, the least siliceous have the highest R2+, nearly equal octahedral and tetrahedral Al, and octahedral totals close to that for an ideal trioctahedral mineral. A dioctahedral substitution Si[]R2−2 (where [] represents a vacant octahedral site) which decreases with temperature, describes this variation. Low octahedral totals are, however, induced by the method of calculation and need not indicate vacancies; for published wet chemical analyses of metamorphic chlorites they may simply indicate oxidation of Fe. Intergrown dioctahedral phyllosilicates may partly account for apparent vacancies in diagenetic chlorites. Nevertheless, the correlation of composition with temperature and similarities to the temperature-related evolution of synthetic chlorites, suggest that diagenetic chlorites are compositionally distinct from, but metastable with respect to, fully trioctahedral metamorphic chlorites. Temperature-related trends are modified by bulk composition, complicating their potential use for low-temperature geothermometry.

Evidence of Ostwald ripening related recrystallization of diagenetic chlorites from reservoir rocks offshore Norway

AbstractChemical variations in individual chlorite crystals of diagenetic origin delineated by energy dispersive X-ray spectroscopy (EDS) in a transmission electron microscope (TEM) indicate a temperature dependent chemical zonation in each grain. Silicon decreases and Al increases with higher temperature resulting in a decreasing Si/Al ratio away from the crystal core reflecting the time and rate of the crystal growth. Chlorite particle-size distributions obtained by scanning electron microscopy (SEM) give steady state profiles which suggests that the chlorite growth is controlled by a grain coarsening process related to Ostwald ripening.

Application of Back-Scattered Electron Microscopy to the Quantification of Clay Mineral Microporosity in Sandstones

ABSTRACT Computer-assisted image analysis of back-scattered electron (BSE) micrographs of petrographic sections is useful for the quantification of clay mineral microporosity in sandstones. Authigenic clay minerals from North Sea clastic reservoirs generally have higher microporosities than detrital clay minerals. Diagenetic kaolinite has microporosities varying from 25 to 50%. Diagenetic chlorite has a generally uniform grain-coating texture and microporosities of about 50%. Dispersed diagenetic illitic clays are difficult to characterize using this method, because their delicate morphology is susceptible to damage during sample preparation. Despite this limitation, the analytical data indicate a minimum microporosity of 63% for dispersed illitic clays. Analytical data for clay-rich detrital lasts have much lower microporosity values of about 10%. Interpretation of images from detrital clasts is complicated by the effects of sample preparation.

Sparta B Sandstones (Eocene), Fordoche Field, Pointe Coupee Parish, Louisiana-A Compartmentalized, Barrier-Island Oil and Gas Reservoir

ABSTRACT Fine-scale correlation of sixty-two well penetrations of Sparta B sandstones (Middle Eocene) in Fordoche Field, Pointe Coupee Parish, Louisiana, shows the unit is compartmentalized into an upper and lower sandstone by a thin shale body that is resolvable on standard (1= 100 ft) E-logs. Sedimentary structures and trace fossils preserved in conventional core of this thin shale indicate that it consists of interbedded fine to medium-grained sandstones and carbonaceous mudstones deposited as storm washover lobes, and tidal-inlet channels associated with a barrier-island complex. The bulk of Sparta B production (estimated at 5.3 MM STB OIP and 2.6 BCF GIP) is from upper shoreface and storm-washover sandstones whose pore spaces contain diagenetic chlorite, kaolinite, and calcite. Enhanced oil recovery (EOR) techniques involving SHF (sequential hydrofluoric acid stimulation) have eliminated declining production rates caused by these diagenetic minerals. Remaining in-place oil and gas is 4,195,960 STB and 1,812 MMCF respectively. Sparta mudstone samples bear a pyrolysis signature confirming a barrier-island depositional regime. Tmax values indicate Sparta rocks are immature to marginally mature in south-central Louisiana. Combination of reservoir engineering and petrophysical techniques permits resolution of extremely thin, unconventional oil and gas reservoirs that contain large reserves of by-passed hydrocarbons.

Modeling the Distribution of Diagenetic Elements within Deltaic Reservoirs of NE Brazil

Lacustrine deltaic sandstones are important reservoirs in Potiguar and Reconcavo rift basins of northeastern Brazil. They commonly occur as delta front deposits accumulated in lows associated with growth faults. Diagenetic elements, including calcite, secondary porosity, dolomite, and chlorite, present specific distribution patterns at different reservoir levels. Such distributions have been modeled based on thin section, core, and petrophysical data. At the microscopic level, the most significant heterogeneity is the association between reduced primary (RP) porosity and carbonate dissolution (CD) porosity. Reduced primary porosity commonly occurs as islands of small pores surrounded by zones presenting larger CD pores. This type of pore structure causes high irreducible water saturation to appear in many reservoirs. At a larger scale level, dolomite cement is observed associated with micas and mud clasts concentrated in discrete stratification planes, a feature that increases permeability anisotropy, lowering the effective permeability of cross-bedded zones. Chlorite rims cause permeability reduction in fine-grained sandstones but have little influence in coarser sandstones. This grain size dependent effect produces significant alterations in the permeability structure of the reservoirs. Relating the distribution of diagenetic elements to the facies architecture of deltaic sandstones permits generating reservoir models that integrate depositional and diagenetic heterogeneities at different more » levels, an approach that leads to more accurate determination of effective properties used for performance prediction and numerical simulation. « less

Mineralogía y génesis de las arcillas de la Unidad de Almarchal (series flysch del Campo de Gibraltar. S de España)

A study of the fine fraction of the Almarchal unit (Campo de Gibraltar flysch series) revealed several very characteristic mineralogical associations, closely associated with the types of the rocks in this area. 1. Kaolinite-(illite)-swelling minerals, characteristically associated with the lutitic leves, clearly preponderate in this unit. 2. Illite-chlorite, a well-developed association found only in calcareous rocks. 3. Chlorite and/or berthierine, a characteristic association of silicified levels. Thus, it is possible to determine a group of levels, typically turbiditic and another group of lutitic levels, probably hemipelagic deposits, in which the content of organic material is noteworthy. The control of the mineralogic evolution by lithology is a result of different permeabilities in alternating levels. In calcareous beds the mineralogy of the fine fraction could result from the diagenetic degradation of aluminosilicates. Diagenetic Fe-rich chlorites, preferably developed in siliceous rocks resulting from the evolution of detrital kaolinites. In lutitic beds, two types of mineral constituents are mixed: Typically detrital mineral species (like kaolinite) and diagenetic materials (swelling minerals). The abundance of organic matter in these levels can be related with oceanic anoxic events in Cretaceous time in North Atlantic basins.

Depositional environment, sand provenance, and diagenesis of the Basal Salina Formation (lower Eocene), northwestern Peru

The Basal Salina Formation is a lower Eocene transgressive sequence consisting of interbedded shales, siltstones, and conglomeratic sandstones. This formation occurs in the Talara basin of northwestern Peru and is one of a series of complexly faulted hydrocarbon-producing formations within this extensional forearc basin. These sediments were probably deposited in a fan-delta complex that developed along the ancestral Amotape Mountains during the early Eocene. Most of the sediment was derived from the low-grade metamorphic and plutonic rocks that comprise the Amotape Mountains, and their sedimentary cover. Detrital modes of these sandstones reflect the complex tectonic history of the area, rather than the overall forearc setting. Unlike most forearc sediments, these are highly quartzose, with only minor percentages of volcanic detritus. This sand is variably indurated and cemented by chlorite, quartz, calcite, and kaolinite. Clay-mineral matrix assemblages show gradational changes with depth, from primarily detrital kaolinite to diagenetic chlorite and mixed-layered illite/smectite. Basal Salina sandstones exhibit a paragenetic sequence that may be tied to early meteoric influx or late-stage influx of thermally driven brines associated with hydrocarbon migration. Much of the porosity is secondary, resulting from a first-stage dissolution of silicic constituents (volcanic lithic fragments, feldspar, and fibrous quartz) and a later dissolution of surrounding carbonate cement. Types of pores include skeletal grains, grain molds, elongate pores, and fracture porosity. Measured porosity values range up to 24% and coarser samples tend to be more porous. Permeability is enhanced by fractures and deterred by clay-mineral cements and alteration residues.

Diagenesis and Cement Fabric of Gas Reservoirs in the Oligocene Vicksburg Formation, McAllen Ranch Field, Hidalgo County, Texas: ABSTRACT

ABSTRACT McAllen Ranch field produces natural gas from 12 deep, overpressured sandstone reservoirs, each interpreted to be the deposit of a prograding shelf-edge delta. Correlation of petrographic and sedimentologic features reveals a predictable pattern of sandstone cementation. The sandstones are feldspathic litharenites containing subequal proportions of volcanic rock fragments (VRF), feldspar, and quartz grains. Grain size ranges from very fine to coarse sand. Porosity is mostly secondary, having formed through dissolution of VRF and feldspar grains. Four major diagenetic facies in cored reservoir rock can be grouped by the predominance of one diagenetic cement type and by appearance in hand specimen: (1) calcite-cemented; (2) chlorite-cemented, tight; (3) chlorite-cemented, porous; and (4) quartz overgrowths, porous. The calcite-cemented facies predominates in very fine-grained sandstones and siltstones and encroaches into adjoining sandstones irrespective of grain size. Core permeabilities are generally less than 0.01 md, and porosities range from 7% to 15%. Authigenic chlorite generally cements sandstones intermediate in grain size between those cemented by calcite and those cemented by quartz. Two types of diagenetic chlorite fabric are interbedded, forming distinct alternating bands 0.1 in (0.25 mm) to 3 ft (1 m) thick. In the tightly chlorite-cemented facies, permeabilities are less than 0.3 md, and porosities range from 8% to 16%. In the porous chlorite-cemented facies, dissolution of framework grains and early yellow chlorite cement increased porosity, and a second coarsely crystalline chlorite cement was precipitated. Core permeability ranges from 0.1 to 1 md, and porosities range from 15% to 20%. The quartz overgrowth facies occurs in 1- to 2-ft-thick (0.3- 0.6-m) zones within the coarsest grained sandstones. Although these overgrowths constitute only 5% of the rock volume, they are the predominant cement. A relative abundance of intergranular pores results in permeabilities ranging from 1 to 20 md. Porosities range from 15% to 20%. Three depositional facies are present in McAllen Ranch reservoirs: (1) mid-delta-front deposits that are characterized by thin upward-fining sequences; (2) massive upper delta-front sandstones; and (3) distributary channel-fill sandstones. The diagenetic facies are localized into a distinct pattern within each depositional element. Mid-delta-front upward-fining sequences 0.5 to 5 ft thick (0.2 to 1.6 m) are cemented by calcite at the base and top. Porous and tightly cemented chlorite bands occur in the middle of the upward-fining sequences, with the proportion of porous bands increasing toward the top. Within massive delta-front sandstones, porous and tightly chlorite-cemented bands are commonly parallel to sedimentary structures. Distributary channel fills consist of thicker (10 to 20 ft), poorly sorted, upward-fining sandstones. The basal 1 to 2 ft (0.3 to 0.6 m) of the distributary channel fills, where underlain by shales, are tightly cemented with calcite. Porous, quartz-cemented diagenetic facies form 1- to 2-ft-thick (0.3- to 0.6-m-) intervals in the coarsest grained sandstones. The rest of the coarse channel fill is dominated by porous, chlorite-cemented facies. The upper, finer grained portions of the channel fills are tightly cemented with chlorite and calcite. A predictable relationship between depositional facies and diagenetic facies allows extrapolation of the distribution of cements and the resultant reservoir characteristics to uncored wells and intervals with the objective of maximizing recovery of natural gas.

Compositional Variations in Diagenetic Chlorites and Illites, and Relationships with Formation-Water Chemistry

Authigenic illites and chlorites from elastic reservoirs, offshore Norway, have been studied by analytical transmission electron microscopy (ATEM). Present-day reservoir conditions range from 1600 m burial depth and 70~ to 4400 m and 160~ The reservoir units have experienced continuous subsidence since early Cretaceous, and are presently at maximum burial. Textural and morphological evidence indicates that the authigenic illites and chlorites investigated are late diagenetic products, except for an early-formed berthierine in the shallowest reservoir. While the illites show very limited chemical variability at temperatures between 140 ~ and 160~ the chlorites exhibit definite compositional trends with increasing temperatures. Tetrahedral Ai increases substantially from 100~176 and the octahedral vacancy decreases correspondingly. These observations indicate that continuous recrystalliza- tion of authigenic clays has taken place. This is also supported by the chemical composition of present-day porewaters, which are close to equilibrium with the clay mineral assemblage. Illites and chlorites are diagenetic minerals of considerable importance both because of their general geochemical reorganization during diagenesis and also their influence on the production of hydrocarbons from elastic reservoirs. A better understanding of the control and mechanisms of their formation is needed in order to model their distribution in sedimentary basins. This paper focuses on the formation of iUites and chlorites from kaolinites in sandstones. Illites forming from kaolinite precursors are normally characterized by low octahedral Mg and Fe (Srodon & Eberl, 1984), and the same is true for hydrothermal illites. No compositional trends have been demonstrated for these illites in the diagenetic regime, although Cathelineau (1987) has used the compositions of hydrothermal illites coexisting with chlorites as geothermometers. Velde & Medhioub (1988) have recently discussed the compositional range of diagenetic chlorites within both sandstones and mudstones from the equilibrium standpoint. This study was based on microprobe analyses of chlorites from a series of samples taken from two wells covering temperature ranges of respectively 30~ ~ and 80~176 They concluded that these diagenetic chlorites appear to form under equilibrium conditions, and that they repeatedly recrystallized during increasing diagenetic grade. We have adopted a similar approach in this study. Samples from Jurassic clastic reservoirs offshore Norway, representing a wide range of burial depths and temperatures, have been examined to establish both the occurrence of diagenetic chlorites and illites, and their compositional variation. Any thermodynamic discussion of diagenetic clay minerals must refer to the activity- composition relations of these minerals (Aagaard & Helgeson, 1983; Tardy & Fritz, 1981;

Approach to chemical equilibrium in diagenetic chlorites

Electron microprobe analyses were made on diagenetic chlorites in sandstones and mudstones from two deep wells according to the petrographic character of the chlorite occurrence: as pseudomorphic phases, rims on quartz or glauconite or as distinct phases in the clay matrix. Chlorite compositions do not depend upon crystallization site (reacting phases) making it apparent that new chlorites can form in an approach to chemical equilibrium at or near the surface (40° C, 1 km depth). Comparison of this data with that for late diagenetic and early metamorphic chlorites indicates that the compositional range for different grains in the same thin section is similar for the samples throughout the 40°–270° C temperature span. Compositional range decreases upon further metamorphism. Al content appears to be a more reliable indicator of temperature variations than other substitutions in the chlorite structure. The clay mineral assemblage which indicates sedimentary facies affects the trends in composition (Al increase or decrease) as a function of temperature. The octahedral site occupancy show a general increase in going from diagenesis to metamorphic conditions in pelitic rocks. The range of Fe-Mg ratios seems to depend more on the chemistry of each sample than the temperature of formation of the minerals.

Oxygen-isotope studies of clastic diagenesis in the Lower Cretaceous Viking Formation, Alberta: implications for the role of meteoric water

Summary The oxygen-isotope compositions and paragenetic sequence of diagenetic minerals from the Lower Cretaceous Viking sandstone and conglomerate from south-central Alberta, Canada have been used to identify changes in porewater composition during diagenesis and to relate these changes to major geological events within the western Canada sedimentary basin. Large-scale influx of meteoric water has played an important role in diagenesis of this unit, especially following uplift of the basin in early Eocene time. Diagenetic phases include early kaolinite ( δ 18 O SMOW = + 24.9 to +28.2 ‰), siderite ( δ 18 O SMOW = + 22.0 to + 23.9 ‰; δ 13 C PDB = −6.3 to −1.3 ‰), calcite ( δ 18 O SMOW = + 26.0 ‰; δ 13 C PDB = −0.4 ‰) and chlorite ( δ 18 O SMOW = + 10.7 − + 13.4 ‰), followed in order by dolomite ( δ 18 O SMOW = + 19.3 − + 22.1 ‰; δ 13 C PDB = −5.9 to −2.6 ‰), calcite ( δ 18 O SMOW = + 13.9 − + 15.9 ‰; δ 13 C PDB = −8.4 to −6.1 ‰), ankerite ( δ 18 O SMOW = + 16.3 − + 17.9 ‰; δ 13 C PDB = −12.4 to −3.5 ‰), kaolin group minerals ( δ 18 O SMOW = + 12.0 − + 15.7 ‰), and illite ( δ 18 O SMOW = + 14.2 − + 15.7 ‰) and illite/smectite ( δ 18 O SMOW = + 13.8 − + 16.5 ‰). Quartz overgrowths ( δ 18 O SMOW = + 16.2 − + 27.4 ‰) began crystallizing relatively early during burial diagenesis and continued to form at least up to the onset of late diagenetic formation of clay minerals. The interpretation of these results is that shallow diagenesis, early in the burial history (glauconite, pyrite, calcite, chlorite), occurred largely in the presence of seawater-derived fluids, although a freshwater influence is indicated where siderite and(or) early kaolinite cements are abundant. As compaction and burial diagenesis proceeded (diagenetic chlorite, quartz overgrowths, dolomite), the porewaters became enriched in 18 O due to water/rock interaction. Burial diagenesis was terminated in the early Eocene by uplift related to the major Laramide Orogeny. Recharge of the basin by low 18 O meteoric water occurred at this time. The meteoric water then became involved in the formation of diagenetic quartz, calcite and ankerite (and the dissolution and albitization of feldspar) at or near maximum burial temperatures, and in the crystallization at lower temperatures of kaolin group minerals, illite and illite/smectite as the post-Eocene erosion progressed.

High-Resolution Imaging of Ordered Mixed-Layer Clays

AbstractHigh-resolution transmission electron microscopy (HRTEM) has been used to examine illite/smectite from the Mancos Shale; rectorite from Garland County, Arkansas; illite from Silver Hill, Montana; Na-smectite from Crook County, Wyoming; corrensite from Packwood, Washington; and diagenetic chlorite from the Tuscaloosa Formation. Thin specimens were prepared by ion milling, ultramicrotome sectioning, and/or grain dispersal on a holey carbon substrate. Some smectite-bearing clays were also examined after intercalation with dodecylamine hydrochloride (DH). Intercalation of smectite with DH proved to be a reliable method for HRTEM imaging of expanded smectite (d (001) = 16 Å) which could then be distinguished from unexpanded illite (d (001) = 10 Å). Lattice fringes of basal spacings of DH-intercalated rectorite and illite/smectite showed a 26-Å periodicity. These data support X-ray powder diffraction (XRD) studies which suggest that these samples are ordered, interstratified varieties of illite and smectite. The ion-thinned, unexpanded corrensite sample showed discrete crystallites containing 10-Å and 14-Å basal spacings corresponding to collapsed smectite and chlorite, respectively. Regions containing disordered layers of chlorite and smectite were also noted. Crystallites containing regular alternations of smectite and chlorite layers were not common. These HRTEM observations of corrensite did not corroborate XRD data. Particle sizes parallel to the c axis ranged widely for each sample studied, and many particles showed basal dimensions equivalent to more than five layers. For all illite, smectite, and illite/ smectite particles examined, crystallite sizes of about 20 Å in the basal dimension were not observed.

Oxygen Isotope Studies of Diagenesis in the Basal Belly River Sandstone, Pembina I-Pool, Alberta

ABSTRACT The oxygen-isotope compositions of authigenic clay and carbonate minerals from the Pembina I-Pool (basal Belly River sandstone) provide useful constraints on the diagenesis of this unit. Early diagenetic chlorite has 18O values (+ 4.2 to + 6.3, SMOW) compatible with low-temperature crystallization from brackish water. The 18O values of diagenetic calcite (+ 11.8 to + 14.7, SMOW) and kaolinite (+ 10.4 to + 12.8, SMOW) are best explained by crystallization at temperatures of at least 45° to 70°C from pore fluids similar in 18O to the existing formation water (-9, SMOW). The implicatio of such temperatures is that the sandstone was much more deeply buried in the past than it is at present. At least 1,300-1,400 m of overlying material may have been eroded since maximum burial.

Diagenetic chlorite formation in some Mesozoic shales from the Sleipner area of the North Sea

AbstractDiagenetic chlorite is forming as a result of temperature-controlled burial diagenesis in shales from the Sleipner area of the North Sea. Accompanying chlorite diagenesis, kaolinite and illite-smectite decrease in abundance, and illite increases in abundance. These clay mineral transformations occur between 122–126°C at temperatures higher than normally expected for chlorite diagenesis. Kaolinite and ordered illite-smectite are largely unaffected by diagenesis below 100°C. It is proposed that chlorite diagenesis is thus delayed due to the absence of a source of ions resulting from smectite decomposition. Clay mineralogy is of no lithostratigraphic use in the Jurassic sediments of the Sleipner area. However, the zone of chlorite diagenesis is a reliable indicator of maximum burial temperature.

Transmission electron microscopic study of diagenetic chlorite in Gulf Coast argillaceous sediments : Ahn, Jung Ho and D.R. Peacor, 1985. Clays Clay Miner., 33(3):228–236. Dept. of Geol. Sci., Univ. of Michigan, Ann Arbor, MI 48109, USA

Transmission electron microscopic study of diagenetic chlorite in Gulf Coast argillaceous sediments : Ahn, Jung Ho and D.R. Peacor, 1985. Clays Clay Miner., 33(3):228–236. Dept. of Geol. Sci., Univ. of Michigan, Ann Arbor, MI 48109, USA

Gradational development of domainal slaty cleavage, its origin and relation to chlorite porphyroblasts in the Martinsburg Formation, eastern Pennsylvania

Slaty cleavage frequently consists of narrow folia or domains of phyllosilicates alternating with microlithons richer in quartz. Within cleavage domains the {001} planes of phyllosilicates are usually parallel or subparallel to the domain boundaries, but they may be at a small but persistent angle. In microlithons the phyllosilicates are commonly not parallel to the cleavage domains. Domainal cleavage may grade into pervasive slaty cleavage with few or no microlithons and a parallel orientation of all the phyllosilicates, or may grade within distances as short as a few millimeters into pelite with no cleavage and with the phyllosilicates either with no preferred orientation or a weak to strong orientation parallel to bedding. Study of cleavage specimens, mainly from the Martinsburg Formation of Pennsylvania and New Jersey, indicates that cleavage development is related to strain variation and to volume loss, at least as measured by deformed quartz-calcite veins in the XZ plane of a fold. Mass transfer of the more soluble constituents under conditions of low-grade metamorphism (chlorite zone) and under the influence of deviatoric stress leads to the development of “solution ways” initiated at heterogeneities in the fabric which result in solution cleavage that is extended by terminal and lateral propagation. Stress gradients result in gradational development of solution cleavage, with minimal development in “strain shadows.” The “solution ways” develop into cleavage domains by accumulation of inert materials, particularly carbonaceous grains, iron ores such as framboidal pyrite, and phyllosilicates. However, the preferred orientation of phyllosilicates is mainly the result of crystallization of new grains in the domains because of the breakdown of detrital or diagenetic phyllosilicates and of other minerals such as feldspar or epidote under low-grade metamorphic conditions. This interpretation is supported by lack of bent or deformed phyllosilicates in and along the boundaries of cleavage domains. (Larger detrital phyllosilicates, with {001} at a high angle to cleavage, commonly show deformation.) Further support is provided by the occurrence of chlorite grains with their {001} planes approximately parallel to the cleavage in “strain shadows” on competent grains such as framboidal pyrite and by the shape change of diagenetic chlorite porphyroblasts as traced from zones with no domainal cleavage into zones with well-developed domainal cleavage. Mechanical rotation appears to be of minimal importance in the development of cleavage domains that are unrelated to a crenulation structure; there will be grain-boundary sliding and other adjustments as a consequence of removal of material along the “solution ways.” Deformation of a rock possessing a strong preferred orientation of phyllosilicates (parallel to bedding or to a previous cleavage) may result in the development of a crenulation structure and thenec a differentiated crenulation cleavage. Mechanical re-orientation will in this case be a more important element in formation of cleavage, although mass transfer and crystallization are still necessary to produce a domainal cleavage.

Redistribution and fractionation of rare-earth and other elements in a weathering profile

A weathering profile on a uniform Lower Cretaceous volcanogenic sandstone from southern Victoria, Australia is enriched in rare-earth elements (REE), Y and other elements including Ba, Sr and Rb. Enrichments of REE of up to 7 times parent-rock values are associated with Fe-leached members of alteration couplets with little or no enrichment in adjacent Fe-rich members. These alteration couplets are similar in appearance to Liesegang banding. The REE have been fractionated during accumulation, leading to relative enrichment of the light rare earths (LREE). The formation of an alteration couplet from fresh rock at the weathering front involves redistribution of only Fe (and P) and does not involve redistribution of REE, Y. Ba, Sr and Rb. Breakdown of original minerals in the present soil and degradation of vermiculite in the upper part of the weathering profile releases RE and related elements into solution. This solution moves down and along the profile until it comes into contact with neutral to alkaline conditions at the narrow weathering front. Here REE particularly are absorbed and apparently fixed in vermiculite forming at the expense of biotite and chlorite. (La + Ce + Nd + Y) up to 10.1 wt.% is recorded in degraded biotite grains with similar but lower contents in the degraded diagenetic chlorite cement. Such accumulation and fractionation have important implications for REE studies involving sedimentary rocks or for that matter outcrop samples of any rock type.

Pressure solution as a metamorphic process in deformed terrigenous sedimentary rocks

The most widespread record that terrigenous sedimentary rocks have deformed by a pressure solution mechanism is seen in the development of spaced cleavages and transposition structures under conditions of low grade metamorphism. Such structures are most common in immature sandstones and siltstones. Mineral reactions, involving modification of detrital grains and diagenetic minerals, and forming a logical extension to diagenetic processes, are an integral part of the deformation mechanism, and the cleavage stripes represent accumulations not just of inert particles, but mostly of newly crystallized micas, the products of these reactions. The mechanism of deformation by pressure solution is now seen to involve mmetamorphic reactions, change in volume of solid phases during reaction, removal of some silica from the rock, rearrangement of reaction products to produce fabrics, solution of some detrital grains in cleavage stripes. Formal reactions have been written to describe the alteration of detrital felspar and epidote to white mica, the modification of greywacke matrix to white mica, and the transformation of diagenetic chlorite to white mica, all of which are observed to occur during formation of the pressure solution cleavages. These reactions emphasize the importance of metamorphic processes during pressure solution deformation, suggest that pressure solution may involve removal of silica released as a product of the reactions, indicate that the pH of the aqueous phase may be buffered to a level that silica solubility is increased, involve a volume reduction that contributes to the overall shortening during deformation, and also involve dehydration, the large scale circulation of released water possibly being important to the removal and redistribution of silica during pressure solution.

Variations in clay mineralogy across facies boundaries in the Middle Devonian (Ludlowville) New York

A series of essentially time-equivalent shales and sandstones from the Middle Devonian Hamilton Group in New York State was traced and collected from those rocks representing the offshore marine environment through the nearshore and brackish water deposits into the nonmarine red bed facies to the E. The samples were studied by means of X-ray diffraction, X-ray fluorescence, and electron microscopy. The results of these analyses were treated statistically for reproducibility and significance of trend. The data indicate that the clay mineralogy of the shales and sandstones consists of dominant illite and subordinate chlorite. The chlorite is more susceptible to diagenetic change than is the illite. In the transition from the nonmarine to the marine environment, the evidence indicates that detrital chlorite is being supplemented by a diagenetic chlorite that seems to form more readily in the shales than in their associated sandstones. Subtle lateral variations in the clay mineralogy are discussed. Both illite and chlorite show a decrease in c-axis and an increase in a-b axis dimensions with distance from the source. Total Fe content decreases with distance.