E. C. DAPPLES Northwestern University, Evanston, III.

Stages of Diagenesis in the Development

of Sandstones1

Abstract: On the basis of mineral replacements and quence of occurrence three stages of diagenesis are intergrowths, four oxide series are recognized established: Stage (1), redoxomorphic, which domi- among sandstones: (1) alumina, lime-magnesia, nates the episodes of deposition and early burial. iron oxide, (2) silica, lime-magnesia, iron oxide, Reactions principally are reduction and oxidation (3) silica, alumina, iron oxide, (4) silica, alumina, which form the basis for the name of the stage. lime-magnesia. Typical reactions are: a, for series Stage (2), identified as locomorphic, involves re- (1), hematite + calcite + Mg <=t siderite + ferro- placement of one mineral by another, the two dolomite + dolomite; b, for series (2), hematite minerals not entering into mutual reaction. Such + clay minerals + silica «=i chlorite + greenalite; reactions are typical of the early burial stage and c, for series (3), diaspore + silica <=i clay minerals, are an important part of the process of hthification. and diaspore + silica + K <=t glauconite + clay Stage (3), identified as phyllomorphic, follows the minerals; also, during late burial illite + Na + K principal locomorphic replacements and involves + Ca —» muscovite + biotite + feldspar; d, for development of micas, principally from clays. series (4), illite or glauconite —> chlorite + musco- Among sandstones, the progress of diagenesis vite; and calcite + Mg + illite —> micas + feldspar may be arrested at either the locomorphic or + dolomite; also, with deep and late burial, clay phyllomorphic stages, in more or less parallel fashion minerals —> chlorite or sericite, K feldspar —> seri- to metamorphic grades. Also, the phyllomorphic cite, plagioclase —> chlorite + chert. stage overlaps the zeolite and chlorite grades of From the type of reaction and the principal se- metamorphism.

CONTENTS

Introduction 913 Sequential order of diagenetic stages . 930 Condition of early burial 914 Concluding remarks 931 Diagenetic progression 915 References cited 932 Thesis proposed 915 Oxide scries 916 Figure Mineral relations within the oxide series . 917 1. Alumina, lime-magnesia, iron oxide series. . . 919 Sediments of end-member composition . 917 2. Silica, lime-magnesia, iron oxide series .... 920 Alumina, lime-magnesia, iron oxide series 918 3. Silica, alumina, iron oxide series 922 Sil ca, lime-magnesia, iron oxide series . 921 4. Silica, alumina, lime-magnesia series 924 Si! ca, alumina, iron oxide scries .... 923 Sil ca, alumina, lime-magnesia series . . 925 Table Stages of diagenesis 928 1. Minerals showing mutual mtergrowth and re- Rede xomorphic stage 928 placement in poorly cemented sandstones Loconiorplnc stage. 929 of simple mineralogy 915 Phyllomorphic stage 929 2. Reactions characterizing diagenesis 931

Of primary concern is the extent to which INTRODUCTION detritus as deposited has been modified during Increasing information on the petrology of and after its lithification and the significance to sandstones has stimulated interest regarding the be placed on such postdepositional change. In- nature and magnitude of diagenetic alteration. deed, has alteration been important or minor, and are such modifications which can be recog- 1 Some of the findings reported herein were presented nized worthy of more than casual examination? orally as part of a symposium on Diagenesis held at the Does the evidence at hand cause one to regard 1959 meeting of The American Association of Petroleum authigenesis as of minor influence on the bulk Geologists. detritus and generally inconsequential in the

Geological Society of America Bulletin, v. 73, p. 913-934, 4 figs., August 1962 913

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classification of sandstone types; or, do certain mentally distinct, stages in their history. These sandstones exist which consist of minerals stages can be identified as depositional, early largely secondary in origin and in which the re- burial, and late burial or pre-metamorphic (See maining detrital component no longer repre- also Richardson, 1919; Pantin, 1958; Williams sents the composition of the original sediment? and others, 1955, p. 251-254; Dapples, 1959.) With the progress of investigations in sedi- The depositional stage is defined as that time mentary petrography, it has become necessary when the sediment reaches the site of deposi- to reappraise the definitions of diagenesis, and tion and forms part of the bounding interface currently a definition has been formulated from between the overlying transporting medium the older generalized notions (Correns, 1950). and the sediment below which is buried to a The effect has been to reduce divergences in sufficient depth to be preserved. A sediment opinion concerning what is to be included in that has completed the depositional stage enters the term, and to focus attention on the proc- the time of early burial. This is a period of esses involved. Among the most recent of these variable length and may be defined as the in- definitions is one proposed by Sujkowski (1958): terval during which lithification is in progress. Once lithification has been completed, the rock "Diagenesis is used to include all those processes enters the late-burial or pre-metamorphic stage which turn a fresh sediment into a stable rock of some hardness, under conditions of pressure and during which time it may undergo only mod- temperatures not widely removed from those exist- erate diagenesis. In other cases the rock is car- ing on the earth's surface . . . The time limits are ried to great depths, or is involved in intense the moment of deposition of a new sedimentary folding, and mineral and textural modifications layer at the one end and the onset of metamorphism ultimately recognized as metamorphic are at the other. The latter limit, of course cannot be thereby promoted. Each stage is characterized determined in precise terms of pressure or tempera- by unique physical conditions, important in the ture involved, and only to some degree by charac- nature of the final product, and in controlling teristic changes in mineral composition . . . Dia- the direction in which a chemical reaction is genesis, after all, is but the introduction to meta- morphism. On the other hand, there is no sharp displaced. boundary between diagenesis and weathering." The writer gratefully acknowledges financial support of the research by the Graduate School, Bearing the above concepts in mind, the in- Northwestern University. vestigator is led to conclude that the chemical reactions involved are in part those that gen- CONDITION OF EARLY BURIAL erate minerals ordinarily considered to be prod- At the time of initial burial, particles are in ucts of rock metamorphism, whereas other re- contact with interstitial fluids virtually identi- actions are those assigned to weathering. The cal in composition with the overlying water. reactions in question principally are solution In sands, both the solid particles and interstitial (i.e., ionization in water), hydration, hydroly- fluid are continuous phases, and mechanical sis, oxidation-reduction, and those controlled energy is restricted to shifting of fluids from by hydrogen-ion concentration or partial pres- voids during compaction, and to rearrangement sure of carbon dioxide. Each represents the bal- of the packing of grains by this process and by ance maintained by a steady state and can be the activity of burrowing organisms. Below the driven in either direction as the external condi- uppermost few feet of sediment, ejection of tions dictate. Indeed, the mineral products fluids is believed to be slow, and insofar as demonstrate the existence of the equilibria and certain solutions' rates are concerned, the chem- permit the interpretation that the chemical ical system is in effect closed; with respect to balance can be shifted as the thermodynamic the speed of other reactions, the system may conditions are modified. Some of these reactions be open. At this time certain minerals of the are to be regarded, therefore, as equilibria in- detrital fraction may attain chemical equilib- volving solids as well as interstitial liquids, and rium with the fluid fraction under the environ- the relative abundance of the products involved mental conditions which prevail. Quartz, cal- indicate the direction in which each equilib- cite, and certain clay minerals (e.g., glauconite) rium has been displaced. tend to come rapidly to equilibrium with the The progression of mineralogic change in interparticle fluids as indicated by overgrowths sandstones from the time of accumulation to observed on grains in the unconsolidated sedi- final lithification involves modification of the ment (See Siever, 1957). Sand that contains detritus during three interrelated, but funda- both calcite and quartz grains tends to show

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overgrowths on one mineral, but not on both. authigenesis resulting from interaction between It is still questionable whether the clay- minerals. An example is the oxidation-reduc- mineral fraction undergoes substantial modifi- tion reaction between cation during early burial. Recent information illite + hematite <=* glauconite, supplied by Weaver (1958, p. 259) and Milne and Earley (1958) strongly suggests that very which shows a rearrangement among minerals, little modification occurs in the major struc- generally considered stable in the weathering tural groups of clay minerals in the depositional and depositional environments, to constitute a or early burial environments. However, Zen new assemblage, stable in the conditions of (1959a; 1959b) presents evidence that kaolinite, early burial. Other examples of similar oxida- calcite, and quartz derived from devitrification tion-reduction relations considered to be dem- of glass in deep-water marine sediments are in onstrated by mutual replacement are: equilibrium in the early burial environment. hematite + calcite «=t sidente, Grim (1958) considers it improbable that cer- chlorite -+- hematite «=* chamosite. tain illites (glauconites) are detrital, as they may be observed in various stages of develop- A list of associated minerals in poorly ce- ment from a siliceous, amorphous parent ma- mented sandstones of simple mineralogy reveals terial. the recurrence of certain types showing secon- TABLE 1. MINERALS SHOWING MUTUAL INTERGROWTH dary growth and mutual replacement. These AND REPLACEMENT IN POORLY CEMENTED SANDSTONES are sufficiently commonplace to warrant atten- OF SIMPLE MINERALOGY tion as representing reactions during early burial. In thin section some mineral assemblages (1) Quartz-hematite are seen to be intimately intergrown, or to (2) Quartz-calcite grade from large crystals of one mineral into (3) Quartz-hematite-calcite-siderite another mineral through a zone of admixture (4) Quartz-chert-clay mineral (5) Quartz-chert-clay mineral-siderite in an extremely finely divided state. In the (6) Quartz-glauconite-clay mineral zone of transition individual minerals are gen- (7) Quartz-calcite-clay mineral erally too small to identify; they are interpreted (8) Quartz-glauconite-clay mineral-calcite as representing the reactants of an equilibrium state between simple end members (Table 1). In certain weathering environments the at- In other assemblages the textural relation shows tainment of reversibility in the following re- grain replacement of one mineral by the other action has been reported (Goldman, 1955; (Siever, 1959, p. 75-77) and does not strictly Keller, 1958; Allen, 1952; Garrels, 1957, p. indicate chemical reaction between the min- 785): erals. In still other rocks no reaction between kaolinite <=t "bauxite" + silica. the end members minerals is indicated, as in- dividual mineral grains touch with point con- The same relationship should prevail in the tact along sharply defined boundaries. analogous depositional environment. Common occurrence of "floating" grains of DIAGENETIC PROGRESSION quartz in a fossiliferous-fragmental limestone and grains of quartz coated by sharply de- Thesis Proposed fined films of hematite in certain red-bed sand- In the burial environment minerals touch one stones indicates the absence of reaction be- another, and along such boundaries and in the tween these minerals under specific conditions interstitial openings, precipitation of mineral that prevailed. The same minerals occurring in matter (partial cementation) is initiated. This similar rocks show simple replacement relation- mineral matter is proof that either chemical ships, such as calcite, replacing quartz, or reaction is occurring between the mineral hematite replacing calcite (Walker, 1960; grains and fluids, or equilibrium in the fluids is Carozzi, 1960, p. 37-39). In such cases chemical being displaced (Carroll, 1953). Moreover, as reactions are indicated as well as a shift in ionic burial and subsidence continue, both the pres- equilibria to promote the solution of one sub- sure exerted and the temperature attained stance when the solubility product of another within the system increase; temperatures above is exceeded (Castano and Garrels, 1950). 100°C are not exceptional. The physical condi- A third variety of mechanism believed to tions gradually imposed upon the system are occur in the early burial environment is the those of higher pressure, increased temperature,

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and reduction in permeability. Reduction in ticle size react with the enclosed liquids and permeability permits ingress and egress of solu- generate new minerals; others, such as large tions under very low velocities only, and a film grains of quartz, are "seeds" and receive de- of solution must remain in surface contact with posits as overgrowths. mineral grains for long time intervals. With the (b) In the late-burial stage, these minerals progress of time a unit volume of mineral grains and some of the unreacted particles combine and interstitial fluid generally becomes more of chemically under the new conditions of a closed system than during the period im- burial. The reactions are between grains and mediately following deposition. The writer as- between minerals and interstitial solutions. sumes, therefore, that during this burial period (2) Diagenetic association can be recognized the solutions have had ample time in which to in the same manner as metamorphic facies, i.e., become saturated with respect to some of the by the appearance of minerals sensitive to re- common ions. Accordingly, such solutions must stricted physical and chemical conditions; cer- represent equilibrium systems, and the equi- tain diagenetic stages can be recognized in libria may be displaced in either direction as much the same way as metamorphic grades by the conditions dictate. One is led further to the characterizing mineral assemblages. conclude that the minerals precipitated during (3) Certain types of chemical reactions are this stage of burial history represent mineral more commonplace than others and may char- assemblages of reversible reactions which can acterize the stage of diagenesis. be identified by the relative order of their Implicit in the discussion to follow is the con- precipitation and their crystalline intergrowth cept that certain assemblages of authigenic or replacement relations. minerals represent the products of equilibria These assemblages can be regarded as end- involving interstitial fluids as well as crystals. member components sensitive to "equilibrium" The intent is not to designate equilibrium be- conditions, and therefore suited to characterize tween solid phases in the strict chemical sense, a stage of diagenetic advancement. Other implying that the minerals of the right and left detrital minerals remain as weakly reactive sides of the equation coexist in a stable assem- products (e.g., silicate rock fragments, feld- blage, but to show that the reversible reactions spars, and mica) which tend to be affected have not continued to completion. The min- slowly or under different conditions; in many eralogy shows certain minerals replacing others, sandstones of shallow burial and minor folding or developing as authigenic products from the extent of alteration is restricted to surface detrital or pre-existent authigenic minerals and margins of grains. In these cases the authigenic coexisting as an assemblage. In other rocks of minerals play a minor role in influencing the similar compositions, i.e., mineral assemblages, bulk composition of the rock. The unaltered mineral replacement is observed in reverse mineral grains may be considered a diluent of order. The summation of the relations is in- the burial reaction products, and their abun- terpreted as representative of reversible re- dance tends to be the major influence on the actions but not that all mineral phases of the composition of the sandstone. Other sand- assemblage can coexist. The reader should in- stones consist of important amounts of authi- terpret the equations presented as being only genic minerals which have been precipitated parts of the actual equations of reactions. The during early or late stages of burial, and, hence, equations represent the sequences of the solid the present composition of the rock is not that phases as recognized and do not attempt to in- of the sand as accumulated. Obviously such dicate the ions necessary to balance the reac- differences are important not only in the classi- tion, and which may enter or leave in solution. fication of sandstones but in reconstruction of Other reactions are considered irreversible the depositional environments and the condi- under conditions that characterize normal tions of diagenesis. cratonic sediments, but they may be reversible The writer, accepting the above assumptions, under particular environments of hydrothermal proposes the following thesis: action or weathering. The diagnostic mineral (1) Materials that pass through the deposi- relations showing interreaction or replacement tional environment and into the burial en- are considered characteristic of the reaction. vironment are subject to reaction and reach ' 'equilibrium" conditions primarily during each Oxide Series of two stages: On the basis of replacement and intergrowth (a) During deposition and early burial the relationships between minerals in hundreds of "stable" weathering products of small par- sediments and sedimentary rocks in various

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states of lithification, and from beds known to during early and shallow burial, as paragenetic have been folded moderately or not at all, an relations suggest that the chert was not added organization of certain associations for arenites during the late-burial stage. Rather, the change can be formulated. One such system is to erect during late burial appears to be one in which four major oxide-end-member series, each chert undergoes an "aging" process and be- somewhat distinct, but mutually intergrada- comes increasingly coarsely crystalline, assum- tional. Each series is recognized by an assem- ing more and more the microscopic aspect of blage of minerals considered to represent the clear quartz. Such recrystallization is acceler- reactants, certain minerals formed at the ex- ated under conditions of elevated pressures and pense of others as driving forces direct re- temperatures, and the ultimate addition of clear versible or irreversible reactions. At the onset quartz to the detrital grain is considered to pro- of diagenesis the major controlling feature of duce some of the ragged outline of grains in the bulk composition is the nature of the sandstones that have been strongly folded detrital components, and this exerts a suf- (Siever, 1962). ficient influence to distinguish each series. The Pure quartz sandstones commonly are lat- four oxide series as recognized are: erally gradational with carbonates of oolitic or (1) alumina, lime-magnesia, iron oxide series, fossil-fragmental texture. Sediments of such (2) silica, lime-magnesia, iron oxide series, mixed carbonate and silica composition may (3) silica, alumina, iron oxide series, and show some clearly recognizable diagenetic (4) silica, alumina, lime-magnesia series. textures. Initially, grains of the two end-mem- ber components tend to rest against one Mineral Relations Within the Oxide Series another with well-defined boundaries outlining Sediments of end-member composition. Sand- the shape of the grains as finally deposited. stones composed principally of quartz undergo Mineral paragenesis may show early deposition modifications which involve primarily solution of minor amounts of silica, as chert or clear and reprecipitation of the quartz. Initial quartz in overgrowths and partial fillings of the changes that occur during deposition and early interstitial openings. Precipitation of such burial consist of the precipitation of over- quartz is considered to be a feature of early growths or solution pitting of grains (Lamar, burial (i.e., prior to lithification) as some quartz 1928). Such addition attains various propor- grains are isolated from their neighbors by the tions of magnitude from inconspicuous over- secondary quartz. Expansion in the volume of growths to abundant well-terminated crystals. the sediment has been effected by the raising As a whole the most important solution and of grains formerly in contact. Late burial, and reprecipitation occurs during the late-burial also deep burial, produces conditions during stage and is best observed in pure quartz sand- which there may be active replacement of stones in which virtually no carbonate is pres- quartz by calcite or dolomite. The degree of ent. Such differential solution and precipita- replacement displays an exceedingly wide tion is accelerated in direct proportion to deep range from minor to prominent removal of burial and folding; development of over- quartz and the development of a largely growths is arrested, and an interpenetration secondary-carbonate rock. Such substitution texture appears (Taylor, 1950; Thomson, 1959). of carbonate for quartz appears to be more At first the interpenetration tends to be broad common in strata of some central parts of and cuspate so that grain contacts are of the structural basins where waters of exceptional concavo-convex type, but under more elevated properties, such as high salinities and wide pressures (deeper burial) a microstylolitic ranges in pH, are known to exist (e.g., Ten- boundary appears. In turn this texture leads to sleep Sandstone, Pennsylvanian, in eastern an intensely sutured boundary along which in- parts of the Wind River Basin, Wyoming. See dividual grains tend to lose their identity and also Walker, 1960). The replacement does not to become elongate and so irregular that their appear to be a response to strong folding but original shape and roundness no longer are rather the result of an invasion of waters of pH recognized (Heald, 1955; 1956). approaching 9, by which solution of quartz and In the early stages during which silica is precipitation of calcite may occur under the added to grains gradually filling the interstices, elevated temperatures of the deeper parts of chert rather than clear quartz may be precipi- the basin. tated. This "cement" can be observed to grade Certain sands of carbonate composition tend into quartz grains and create a silica weld. Such to undergo significant diagenesis. Originally addition appears to be a phenomenon developed such sands are grains of oolites or shell-frag-

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mental debris, or mixtures of the two. Grain pense of small ones and, hence, can be truly sizes range within the sand limits, and indi- regarded as "blasts". vidual grains rest upon one another with point Alumina, lime-magnesia, iron oxide series. A contact because of the high degree of round- typical, present-day, depositional environment ness. Alteration during early burial involves of this series is the paralic swamp. Here, in the addition of clear calcite generally occupying most recent deposits, where iron sulfide and the interstices between grains. Not uncom- bog limonite are accumulating, the range in monly dolomitization occurs before the addi- physical-chemical conditions approximates tem- tion of much of the spar calcite. In certain de- peratures of 5°-30°C, pressures atmospheric, posits (e.g., reef-flank strata, Silurian, Rich pH 4-8, and Eh negative. If it is proper to Valley, Indiana) dolomite is considered to have combine observations made in several widely formed while the sediment was still loosely separated swamps, iron and alumina oxides, granular, as grains of dolomitized fossil frag- clay minerals, iron sulfides, and minor amounts ments or oolites are surrounded by the clear of silica are thought to constitute the reactants calcite. of a steady-state equation which can be ex- A parallel stage of diagenetic change is the pressed in its most simple fashion as: development of a recrystallization texture silica (minor) + hematite + "bauxite" <=* pyrite identified as saccharoidal. Such recrystalliza- + clay minerals. tion, considered to occur in early burial, does not alter certain gross primary structures of a Under conditions of pH 7, Eh strongly nega- sediment, such as ripple mark, cross-bedding, tive, and temperatures of about 15° C, the and thin bedding, but it does tend to destroy products at the right appear to be favored in minor structure as, e.g., the shells of organisms. abundance; where the Eh approaches zero, but Saccharoidal texture is recognized by the well- is still mildly negative, aluminum and iron developed rhombs of dolomite of approximately oxides are the precipitates, and the equilibrium uniform size resting one against the other with is displaced to favor the products at the left. point contact and, likewise, commonly sep- An increase or decrease in reducing conditions arated by exceptionally large as well as small appears to be the principal mechanism of driv- pore openings. Pore shapes suggest a loose pack- ing the reaction to the right or left. Moreover, ing framework, and dolomitization and re- it may be suggested that the common tendency crystallization are considered to have occurred of the reaction when driven to the left is when the grains were loose and before com- gradual loss of silica in solution wherever the paction could alter the original textural pattern system is physically open and water is con- (i.e., a packing typical of loose beach and shore- tinuously passing through. On the other hand, line sands). Recrystallization of the calcite if the system is closed, a genuine balance ap- grains to dolomite, however, tends to destroy pears to be reached (Bolger and Weitz, 1952). completely the original particle-size distribu- Some semiconsolidated, sandy underclays tion and substitutes a new, highly restricted, (Pennsylvanian), thought to represent deposits crystal-size distribution ranging from medium- similar to those of the swamp, contain isolated, to coarse-sand dimensions, (as, e.g., the strata layered, or concentric zones of pyrite, kaolinite, of the east flank Thornton Reef, Silurian, and some aluminum oxides; the association of Thornton, Indiana). This texture remains silica, kaolinite, aluminum oxides, and pyrite stable where burial is shallow and is common in is present in flint clay from western Pennsyl- Ordovician strata bordering the Canadian vania. Still other underclays contain concre- Shield; it becomes unstable under conditions tions of siderite and calcite containing rhombs of deep burial, particularly in the zones of fold- of dolomite and layers of hematite. Well-pre- ing. The exceedingly porous texture is sup- served plant remains in the centers suggest that planted by a gradual transition into a crystal the concretions were formed early, before the intergrowth with strongly sutured boundaries delicate plant could be destroyed (See also and irregular crystal size. Such a texture can be Weeks, 1957). Reversible reactions are con- identified as granoblastic, at which stage very sidered to exist between the carbonates and iron little pore space remains (e.g., dolomitized, as follows: coarse- to medium-crystalline carbonates in the hematite + calcite <=^ siderite, Ordovician of the Shenandoah Valley, Vir- 2 ginia). The ultimate texture appears to be hematite + calcite + Mg+ «=» siderite + ferro- genuinely metamorphic, in which certain dolomite + dolomite. crystals have grown relatively large at the ex- An association of minerals exists, therefore,

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(c)

Siderike , tiematike Ah03 EO fiz 0, Figure 1. Alumina, lime-magnesia, iron ozitie series. (a) End-member minerals stable during deposition. (b) End-member minerals and llem authigenic minerals formed during reactions of early burial. (c) Modification of association by addition of slnall amounts of silica during late burial

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Siderjte Siderite C/o y minerub G~eena/de minor Hema tjte Calcite Hema ti Le o ici ie ,Do/omife A03 03 k? CQ~40 fe;! C~O,40

(a)

(c) hq Fez% Hem alike, fl~q~et/Le

Ch/o/ite Fer/odo/om;jfe Sfilpnom e/me Do/orn ite Si 0, CQ~490

Figure 2. Silica, lime-magnesia, iron nsidc serics. (a) End-member minerals stable during deposition, presence of small quantities of detrital clay minerals and silica assumed. (b) Compositionc of minerals in "equilibria" associations during early burial. One "equilibrium" is recognized betn.een siderite, ferrodolomitc, dolomite, calcite, and chlorite, whereas another involves hematite, chlorite, greenalite, dolomite, and calcite. 7.I he two separate "equ~libria" are recognized in association in the same rock. (c) Compositions of associated minerals of the late burial stage wit11 the addition of silica as an end member. More than one reversible reaction is considered to be involved in the association.

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which can be regarded as reactants of the par- ingly, a hypothecated association of minerals ticular series being considered, and these are has been formulated for each of the three major the minerals that are important when the silica diagenetic environments (i.e., deposition, early content of the deposits tends to be low (Fig. 1). burial, and late burial), each such association As long as silica content remains low, the deep- representing a mineral "equilibrium" series. burial environment and moderate folding do The relations (Fig. 2a) are considered to have not strongly influence the direction in which existed during the depositional stage, during the reactions proceed. They can be regarded as which the relative amounts of hematite, reactions insensitive to moderate increases of siderite, and calcite were controlled by the temperature and pressure. oxidation-reduction balance. Clay minerals Silica, lime-magnesia, iron oxide series. Sedi- thought to have been present in minor pro- ments of this group are low in aluminum, and portions behaved as stable compounds and did hence clay minerals are minor. The typical en- not constitute part of the group of reactants vironment of deposition passing into early during deposition. A similar behavior is at- burial appears to be one in which some agita- tributed to silica, which doubtless was abun- tion occurs by currents sufficiently intense to dant, but appears not to have become a re- form oolites of calcite, iron oxide, or siderite, actant until the early burial stage. Early burial, and also powerful enough to cause fragmenta- however, appears to have promoted two sep- tion of shells to sand-grain dimensions. In sand- arate but associated reversible reactions in stones considered to have been deposited in which silica and alumina are involved. One is such an environment, large concretions of cal- the siderite, ferro-dolomite, dolomite, calcite, cite, siderite, and iron oxides occur, thin sec- chlorite assemblage; the other is hematite, tions of which show crystalline intergrowths chlorite, greenalite, dolomite, and calcite (Fig. suggesting the reversible reaction 2b). An assemblage of siderite, dolomite, cal- calcite + iron oxide <=t siderite. cite, and chlorite has been recognized in early- burial environments, but greenalite and other The driving force may be displaced in either related minerals have not been formed under direction so that the reactants of one side may such conditions. Yet there is some evidence in dominate those of the other. As a general rule, the form of replacements and paragenesis in the concretions formed tend to remain small, oolites that some of these minerals are early in and the deposit may be buried rapidly as indi- development (See also Ailing, 1947). Hence, cated by the large-scale cross-bedding in sur- equations of reactants can be postulated as rounding sands. The reaction may shift to favor follows: precipation of siderite in the early burial +2 stage, and large concretions may form if the hematite + calcite + Mg «=t siderite + dolomite supply of iron is adequate. This interpretation + ferrodolomite, is made upon the basis that the concretions hematite + clay minerals + silica «=J chlorite tend to be siderite around a nucleus of calcite, + greenalite + Stilpnomelane (late). and that not uncommonly cross-bedding in the A late-burial association of quartz, Stilpnome- sandstone passes through the large concretions, lane, chlorite, greenalite, hematite, magnetite, but not through the nucleus. ferrodolomite, and dolomite is considered to In some Precambrian iron formations of exist, but the reactants are doubtless involved Michigan (James, 1954, p. 264-270) the associ- in separate groups (Fig. 2c). This condition is ation of minerals suggests that when small to marked also by the increase in amount of mag- moderate amounts of alumina are present the netite and recrystallization of chert to quartz; above reaction is superseded by another of the hence, it is inferred that, with increase of temp- following association: chlorite, greenalite, stilp- erature and pressure, slight changes in the nomelane, siderite, ferrodolomite, hematite, nature of the reactions also occurred. In effect, calcite, and dolomite. Of these all but stilp- however, the sandstones entered the chlorite nomelane are considered to have developed in grade of metamorphism without substantial the early-burial environment (James, 1954, p. change other than that the products on the 270). Stilpnomelane is representative of other right-hand sides of the above equations were ferriferous silicate minerals occurring in the favored. Modification in the crystallinity of same rocks and which can be considered to be quartz is considered unrelated to the mineral associated with late-burial diagenesis but pre- association but primarily a response to increase ceding a more intense metamorphism. Accord- in temperature and pressure. Similar recrystal-

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i Figure 3. Silica, alumina, iron oxide series. (a) Composition of cnd-member minerals in "equilibrium" during deposition. Two "equi- llbria" are consitiered present, i.e., clay minerals, quartz, hematite, and sideritc rcprcsenting a composition in which silica is promi- nent; and, diasporc, eir., hematile, siderite, and clay minerals representing a generally Ion silica composition. (b) Cornpositio~lal assemblage of minerals appearing during early burial and in "etl~~ilibrium"when alkalis are added to the bulk composition. (c) Condition of late burial shon.ing compositions of minerals considered to be rclatcd in some complex "equilibrium"

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lization is also noted in unmetamorphosed linite, constitute another. The percentage of sandstones in which quartz alters from the silica (either detrital or precipitated) is re- microcrystalline form as chert to quartz as sponsible for the difference in the associations, overgrowths or crystals welded and intergrown and the reactants of a reversible reaction are: with the larger grains. Often field inspection of diaspore or boehmite + silica <=> clay minerals. such rocks leads one to the opinion that much silica has been added to the bulk composition. On the basis of scattered occurrences in soils, In thin section, however, the various modifica- the reaction is believed to be displaced toward tions of silica are seen to be alteration from the left by a pH greater than 7; a low pH ap- chert to quartz, as noted by the increase in pears to favor increase in the reactants on the crystal size, overgrowths on quartz crystals ex- right (Allen, 1952; Keller, 1958). tending into the interstices between detrital Sandstones of this series deposited in the grains, and welding of quartz grains. In fact, marine environment commonly contain glau- the actual addition of silica may not have been conite formed after deposition of the sand. important at all. The glauconite has crystallized as an inter- Silica, alumina, iron oxide series. Sandstones stitial "cement", or as small platy aggregates, of this series are widespread throughout the replacing other clay minerals and siderite. In geologic column and appear to have been de- such rocks secondary chlorite has also been posited in rather diverse environments; some noted replacing clay minerals and gradational environments were strictly marine, whereas into glauconite. The association rather strongly others ranged from brackish to fresh water. The suggests an assemblage (Fig. 3b) representative rocks are characterized by abundant quartz, of a reversible system, an example of which clay minerals, moderate amounts of iron (in the may be regarded as: chemical analysis), and small quantities of kaolimte or illite + siderite ^ glauconite + chlor- detrital carbonate. A typical sandy deposit is +2 the delta accumulation resulting from long ite + Ca ^ (K+); stream courses in which the sediment is another reaction believed to exist but not dominated by clay and quartz. Mixtures of the demonstrated is: major clay-mineral groups prevail with mont- + morillonite almost invariably minor in quan- diaspore or boehmite + silica + K ^ glauconite + other clay minerals. tity, but reported as abundant in Mississippi River sands (Milne and Earley, 1958, p. 329- Other reactions mentioned earlier involving 330). As the percentage of silica is reduced, the glauconite, siderite, and hematite are consid- assemblage grades into the alumina, iron oxide, ered to be in effect at the same time. Conse- lime-magnesia series; hence, certain generally quently a complex "equilibrium" assemblage related associations are considered to occur in occurs during the stage of early burial (Fig. the depositional environment (Figs. 3a, 3b). A 3b). The distribution is most certainly related more or less silica-rich group consists of clay to the amount of K ion which the clay minerals minerals, quartz, hematite, siderite, and some may scavenge, and the precipitation of chlorite diaspore or boehmite; a silica-poor association is believed to be depressed accordingly. is boehmite or diaspore, hematite, siderite, and Minerals of compositions shown in Figure 3b calcite. Among the minerals of the silica-rich are believed to exist at the same time, but, if group, the occurrence of diaspore or boehmite the amount of alkali is increased beyond the without associated clay minerals does not ap- approximate limits, muscovite and biotite are pear to be common; among the silica-poor believed to form, and chlorite and perhaps group a paucity of silicate-clay minerals is glauconite will not crystallize. Also, conditions noted. Such distributions suggest that reactions for the precipitation of glauconite are generally are driven in preferred directions to establish acknowledged to be an oxidation-reduction certain mineral phases among groups of detrital reaction (Eh slightly negative) at a pH control and chemically precipitated components while of approximately 8. The extent of oxidation, they remain in the depositional environment. of course, is clearly marked by the hematite- One such "equilibrium" is represented in siderite proportion, and in beds of this general Figure 3a, in which quartz or chert, hematite, compositional series, transition or alternation siderite, and clay minerals constitute one as- of red deposits with those of various shades of sociation, and diaspore or boehmite, hematite, gray are noted in the field. siderite, and some clay minerals, such as kao- The siderite is most readily found as con-

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cretions which develop preferentially as ellip- as secondary products in the same type of sand- soidal or spherical masses of fist size or larger. stone, but in no case were all mineral species As such concretions may contain extremely recognized in the same specimen. Hence, the well-preserved remains of soft parts of organ- assemblage is not truly demonstrated as having isms and show evidence of having grown in formed together in the same rock. Occur- place while the sediment was soft, most of them rences of albite, calcite, dolomite, and clay are considered to belong to the early-burial minerals have been observed in sandstones in stage of diagenesis. Also they appear in greatest the late-burial stage. Separate occurrences of numbers in strata deposited in brackish water, biotite, muscovite, orthoclase, kaolinite and or where depositional conditions approached illite, and calcite or dolomite are rather com- the marine environment, but where the supply mon, but the clay minerals cannot be proved of carbonate was inadequate to produce beds as secondary. of carbonate. Silica, alumina, lime-magnesia series. In this During late burial, illite is considered to series matter interstitial between sand grains is react chemically as follows: dominated by clay minerals in different as- illite + Na+ + K+,+ Ca+2 -> muscovite + bio- semblages. There appears to be no satisfactory tite + K feldspar + Na-rich plagioclase; method of ascertaining if some of the minerals represent an "equilibrium" association con- the extent to which this reaction is driven trolled by the environment of deposition, or of toward the products at the right is dictated burial; or, whether the distribution represents primarily by the amount of K and Na ions merely an abundance ratio of detrital clays. In available and also by the pressure-temperature this connection the occurrence of montmoril- regime. Field relationships indicate that deep lonite is, as a whole, less abundant than burial, i.e., elevated pressure and temperature, kaolinite or illite, except in deposits connected favors development of feldspar over muscovite, with volcanic detritus. The association, there- reducing conditions favor crystallization of fore, shown as characterizing the depositional biotite in minor quantities, and muscovite is stage (Fig. 4a), cannot be demonstrated to found preferentially under conditions of mod- represent an "equilibrium" condition although erate burial and moderate to negligible folding. the aluminum oxides, silica, clay minerals Although the crystallization of feldspars ap- reaction described for the silica, alumina, iron pears to be enhanced by high temperatures and oxide series is believed to prevail. high pressures, such conditions are not uni- In the stage of early burial, however, glau- versally necessary as some feldspars developed conite crystallization can be reversible with in strata which experienced neither high temp- ordinary illite (Burst, 1958). The recognized erature nor high pressure (Cambrian sand- intergradation of biotite and glauconite is stones of Wisconsin; see Baskin, 1956). The similarly reversible, and the mineral association writer's impression, based upon thin sections (Fig. 4b) can represent mutual replacement examined, is that the feldspar association very and intergrowth relations in sandstones in the often is early burial stage. glauconite —> feldspar (K- or Na-rich), In a more advanced stage of burial associated with moderate folding, the development of whereas chlorite and muscovite from clay minerals is clay minerals + K+ muscovite noted, and a reaction is considered to exist with the displacement favoring the products at the appears to be much more common. right: Abundant large flakes of secondary mus- covite and some biotite are seen in sandstones illite or glauconite +± chlorite + muscovite. catalogued as quartz wacke by Williams and A complex intergrowth relation (Fig. 4b) of others (1955, p. 292) or subgraywacke by glauconite, illite, kaolinite, calcite, dolomite, Pettijohn (1957, p. 316). In some such sand- and chlorite in the Frontier Sandstone (Cre- stones the growth of mica in intergranular taceous) of the Wind River Basin, Wyoming, space during the early- and late-burial stages is a good example of the equilibrium assem- has prevailed over the crystallization of any blage of early burial. The association persists feldspar. However, an association is believed to also in zones where rocks show strong folding exist in some complex replacement reactions and where secondary minerals include feldspars, (Fig. 3c). These minerals have been observed muscovite, and biotite, i.e., the assemblage

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characterizing late burial (Fig. 4c). In rocks of ing, and Mesaverde sandstones (Cretaceous) of this stage the paragenetic intergrowths suggest the eastern part of the Colorado Plateau. a reaction such as the following considered to Among such sandstones the principal differ- be driven toward the right as pressure and ences appear to be in the amount of matrix temperature increase: clay rather than gross mineralogy. Low-clay calcite + Mg+2 + clay minerals + glauconite —» varieties, in which clay occupies interstitial muscovite + biotite + feldspar + dolomite + space, tend to show important quartz over- chlorite. growths only, whereas high-clay varieties show quartz grains of irregular outline, generally The reactants occur repeatedly among cer- having undergone solution and common re- tain carbonate-rich graywackes in which sec- placement by clay. Such a relation suggests ondary chlorite, muscovite, feldspars, etc., can that clay minerals of low-silica content are be recognized clearly. In such cases the in- altering to those of higher-silica content (e.g., crease in chlorite appears to be related to a illite + silica <=i kaolinite). The same associa- corresponding addition in muscovite, biotite, tion may contain chert replacing quartz grains. and orthoclase. In other graywackes chlorite Such chert may be so intimately intergrown can be much later than muscovite; this relation with the clay that one gains the impression may indicate that, as long as sufficient K ion is that as quartz grains were being replaced by available, muscovite, biotite, and feldspar are the clay some of the silica was reprecipitated as formed, but when it is held in such minerals chert. Indeed if this is the case the relationship the remaining clay-mineral structures form is one of reversible reaction as indicated by the chlorite. Accordingly the reaction above may above equation. be regarded as occurring in several stages which A second and characteristic modification in- are presented below more as illustrations of the volves the transformation of clay minerals into reactions than the actual mechanisms: well-crystallized muscovite. Clusters of ex- calcite + Mg+2 + illite or glauconite —> micas + ceedingly minute crystals of sericite form in feldspars + dolomite, the clay matrix or in the interstitial clay. illite or glauconite + kaolinite + Mg+2 —> chlorite Sericite appears to increase proportionally as + K+, the folding of the strata intensifies. Strongly montmorillonite —> chlorite. folded sandstones of this variety tend to show well-developed sericite clusters, and some com- Crystallization of the minerals in complex pletely outline individual quartz grains. As the intergrowths is regarded as responsible for the shales associated with such sandstones generally characteristic microscopic texture of gray- contain some illite, the transformation illite wacke; i.e., the extraordinarily irregular and to muscovite probably explains the occurrence deeply interpenetrated grain boundaries along of authigenic mica. which the bonding is not cement, but an actual In sandstones of similar composition, but in intergrowth. Authigenic-mineral matter pene- gently inclined beds, localized occurrences of trates grains and forms a crystalline meshwork muscovite are also noted. This muscovite ap- welding several grains into a tough aggregate pears as relatively large individual flakes up to which fractures through as well as around the a half a centimeter in greatest dimension in con- grains. trast to the fine- and medium-sand size of the Mixtures of the lime-magnesia, alumina, quartz. Although the grain boundary relation- silica series range in composition from very ships are difficult to interpret, the irregular out- high silica values (i.e., a quartz sandstone) to line of the boundaries and the apparent inter- those with substantial amounts of alumina in gradation of the mica and clay indicate that the form of clay minerals constituting the such flakes are secondary. Such grains appear matrix in which the quartz grains are embedded to be too large to have been transported by the (quartz wackes or subgrayvvackes). Calcite, same current velocities that moved the quartz dolomite, and siderite are the principal lime- and clay. Biotite of similar dimension has also magnesia-iron minerals, and these may be been recognized, particularly in close associa- added to a significant degree during diagenesis. tion with coal fragments or carbonaceous Many of these sandstones are associated with matter, but the quantity is exceptionally small. coal-bearing sequences, e.g., the Pennsylvanian The recrystallization of such micas is be- or the Central Interior and Eastern coal basins, lieved to be associated with the addition of K and similar associations in the Frontier Sand- ion to the sediment during the early-burial stone (Cretaceous) Wind River Basin, Wyom- stage and the gradual addition of this ion to

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the clay-mineral lattice to generate well- feldspar are recognized in either the lime-mag- crystallized mica. Although one immediately nesia, alumina, silica series or the iron oxide, suspects that the more common muscovite has alumina, silica series. In broad terms such rocks been derived from illite, the abundance of constitute the arkoses and graywackes. some of the secondary muscovite suggests that Among the arkoses there is not uncommonly other clay minerals may have been involved a sequence of modifications, the general order also. As biotite and muscovite occur in the of which is indicated by mineral paragenesis. same specimens, and the distribution of biotite The earliest change involves addition of minor is related to the proximity to carbonaceous amounts of quartz as overgrowths; intense matter, biotite may form under somewhat carbonate replacement of feldspars may precede more reducing conditions than muscovite, and the addition of quartz. Feldspar is then altered muscovite may develop preferentially as the to clay minerals and chert. The degree to conditions of diagenesis approach an Eh of which this process progresses ranges from very zero. In this connection, a similar occurrence minor alteration of feldspar to replacement of of muscovite is found in some red-bed sand- the entire feldspar grain, the sharply angular stones (Keewenawan, Michigan; Wasatch For- outline of the "crystal" and small "islands" of mation, Wind River Basin, Wyoming) which, feldspar in the clay-chert mixture being the except for the oxidized state of the iron, are only identifying features of the original gram. similar in composition. The secondary mus- Such alterations do not appear to bear any well- covite and the rare biotite suggest that the marked relation to depth of burial but are con- biotite-muscovite equilibrium is displaced sidered to represent modifications occurring toward muscovite if conditions become in- during the early-burial stage. Indeed, it is pos- creasingly oxidizing in the burial environment. sible that "defeldspathization" of a sort has Sandstones of these varieties not uncom- occurred, and a formerly significantly feld- monly reveal late introduction of calcite, spathic rock has been modified to one which dolomite, or siderite replacing quartz, chert, contains less feldspar and considerably more and clay. This does not appear to be a reversible chert and clay than the original sediment. relation, but a reaction driven in one direction, Modifications which appear to be associated substituting carbonate for the earlier minerals. with deep burial or moderate folding, (i.e., Grains of quartz and chert show extremely ir- pressure-sensitive reactions) are the following, regular borders typical of solution. The clay which are considered to proceed more or less may be completely engulfed by the carbonate, at the same time but do not go to completion. and, furthermore, appears to have been locally quartz + clay mineral —> sericite, dissolved, as small "islands" remain in the +2 clay mineral + Fc —> biotite, carbonate crystalline aggregate. The areal dis- feldspar —> sericite. tribution of such secondary carbonate in the sandstones suggests very strongly that in some In short a somewhat more micaceous rock is cases at least it is distributed preferentially in produced. Such alteration is followed by those strata deposited in the marine environment. of a still later stage: Either the sandstones are marine or they are in clay mineral —> chlorite, close lateral proximity to calcareous shales of plagioclase — chlorite + chert. marine origin. As the shales underwent com- paction, solutions of high pH and others of high Lastly, there appears the addition of over- carbonate-ion concentration probably were growths to feldspar, principally potassium ejected from different shale beds, entered the feldspar, but also those as calcic as oligoclase. sandstones, and were reacted with the quartz In some arkoses (e.g., Keewenawan, Michi- and clay to cause the replacement relationships gan; Ocoee Series, Cambrian, Tennessee; and observed. Stockton Arkose, Triassic, Pennsylvania) sig- Sandstones of the above series tend to grade nificant amounts of chlorite develop in the in- into those of the iron oxide, alumina, silica terstitial space and invade surrounding min- series in which complex replacement relation- erals constituting a welded bond, making the ships between carbonates, quartz, iron oxides, original boundaries of detrital grains unrecog- clay minerals, and iron silicates can be de- nizable. Where such chlorite is abundant, the termined. The writer does not yet properly similarity between arkose and feldspathic gray- understand the chemical reactions involved. wacke is striking, and distinction between the There are, however, some more clearly outlined two becomes arbitrary. relations when significant amounts of detrital Among the acknowledged graywackes, the

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degree of authigenic growths and reaction be- seems to dominate the amount of authigenic tween mineral grains are extremely variable mineral produced; in some cases chlorite con- and dependent upon the detrital composition. stitutes nearly all the secondary interstitial ma- Also, as such rocks were accumulated in thick terial. In this connection the important altera- piles of sediments and tend to be moderately tion of detrital minerals and the generation of to strongly folded, it is likely that the former new minerals create a texture characterized elevated temperatures and pressures influenced typically by highly sutured and irregular grain mineral alteration more than in most arkoses. boundaries. Indeed, recognition of the boun- The ultimate modifications which are to be ob- daries of grains and former interstitial filling is served are transitional into unquestionable extremely difficult or impossible in many gray- characteristics of metamorphism, such as slaty wackes; hence, the texture is one of a reconsti- cleavage and mineral alignment. Among certain tuted rock. The graywacke as seen in thin sec- more or less recent sediments and poorly lithi- tion is only indirectly representative of the fied sedimentary rocks of presumed graywacke particle-size distribution and the mineralogy of detrital composition, significant constituents the sediment as deposited, and the properties are volcanic glass and opal. These appear to be of the original sediment could be very different rather unstable under conditions of early burial, from those of the final graywacke. and the following alterations are recognized The ultimate aspect of some of the gray- (See also Zen, 1959b): wackes in which feldspar is an abundant detrital constitutent may be very much like that of opal —> chert or quartz, arkoses which have experienced a similar condi- glass —> montmorillonite + chert. tion of diagenesis and in which abundant Silicification of the matrix of graywacke, in- chlorite has crystallized from the matrix. Also dicated by the addition of chert in particular, there are repeated instances in some of the appears to be a reasonably early process, but more strongly folded quartz wackes or sub- whether this is the result of the alterations of graywackes where chlorite and sericite have glass and opal or whether there is an actual formed in the matrix, detrital quartz has re- addition of silica is not established. It is reason- crystallized and has formed a continuous weld able to assume that in graywackes not sig- with quartz of the matrix, and such authigenesis nificantly rich in silica, silicification is a product has destroyed the original boundaries of detri- of alteration of other minerals, or glass and tal-mineral grains. Such a rock ultimately re- opal. Certain Precambrian graywackes, e.g., sembles feldspar-poor graywackes, and dis- beds in the Penokee Series, and Michigamme tinction between these types of rather different Slates, Michigan, contain much silica. The origin is uncertain. detrital-quartz grains grade without boundary into quartz precipitated in the matrix, and in STAGES OF DIAGENESIS such cases silica may have been introduced into the rock. This is suggested by the common Redoxomorphic Stage siliceous shales (slates), and concretionary and Among the diagenetic modifications observed bedded chert in shale (slate) units interbedded in sandstones, beginning with their deposition with the graywacke sandstones. However, pre- and continuing into the attainment of acknowl- cipitation of coarsely crystalline quartz and edged lowest grades of metamorphism, there chert appear to be processes of early burial, and appear to be preferential trends or tendencies the stability of the two forms of quartz appears for certain chemical reactions to prevail over to be clearly established during this stage of others. Such reactions are regarded as initially diagenesis. indicating equilibria in which the abundance Late burial, which involves deep burial and ratio of one set of minerals over another is con- folding, is dominated by the following reactions trolled by the partial pressure of CC>2, the listed in order of their prominence: available H-ion concentration, the domination clay minerals —> chlorite, of oxidizing or reducing conditions, and certain clay minerals —> sericite (major) + biotite (minor), reactions of hydrolysis. All such equilibria, K feldspar —> sericite, existing during deposition and early burial, are clay minerals + chert —> chlorite. those principally recognized in weathering. Note, for example, that the now classic table These reactions appear to proceed more or of Krumbein and Garrels (1952) representing less simultaneously, but the quantity of chlorite the conditions of precipitation of certain sedi-

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mentary minerals as controlled by Eh and pH in sandstones is considered by the writer to be actually considers only authigenesis in the an extremely important part of sediment depositional and early-burial environments. alteration. The direction in which such solution Reactions involving H-ion concentration or and precipitation is driven can be explained by oxidation-reduction appear to be most im- a mechanism currently favored, namely the portant in influencing the appearance and control of solubility by temperature of solu- characterizing mineralogy of the sediment as tions, which, being ejected from deeper sites of it is accumulating. These reactions are par- the sedimentary column, invade strata more ticularly evident during the stage of early recently deposited. However, the time of re- burial, as transition occurs from loose sediment placement, which generally appears to be prior to a lithified aggregate. The interval may be to lithification, but in some cases clearly occurs lengthy or short, the extent of cementation after initial lithification, indicates that perhaps variable; only in a general sense is lithification changes in the ionic concentrations of invading a function of the time involved. This period of solutions may be equally important in control- diagenesis is a time when certain detrital ling the direction of replacement (Siever, 1962, minerals enter into an equilibrium reaction p. 145). For the moment, as the thermo- with authigenic minerals in the environment of dynamics involved are not understood, this par- the acidity of the interstratal solutions and the ticular stage in the sedimentary rock history state (tendency) of oxidation. Some minerals may be identified by the general term, Loco- developed under these conditions tend to re- morphic, signifying the change in composition main in the rock, whereas others may be de- and modification of shape in situ by a young stroyed during later intense diagenesis. Indeed, mineral replacing an older one. such attributes as rock color and early "ce- ments," the order of precipitation of which is Phyllomorphic Stage indicated by mineral paragenesis, identify the Somewhat later than the favored conditions nature of the reactions of the depositional and for locomorphism there is a period of distinct early-burial stages. Not uncommonly such alteration among clay minerals. This alteration minerals tend to remain in the rock as it enters proceeds primarily toward recrystallization into a low grade of metamorphism. The tendency true micas. Recrystallization of chert to quartz for certain reversible reactions to dominate is included among such changes, as well as mild provides the basis for identification of this stage sericitization of feldspars and some authigenesis which the writer proposes to call Redoxomorphic of feldspars (e.g., precipitation of overgrowths after the most outstanding representative of on detrital grains, or growth of interstitial such reactions, namely oxidation and reduction. crystals). In part, the processes which bring about such changes involve the incorporation Locomorphic Stage of ions such as K and Mg into the structure of A following stage of diagenesis is recognized the lattice, whereas, earlier they were held as by complex replacement relations involving readily exchangeable ions (Garrett and Walker, reaction mechanisms. Some of these replace- 1959). Such modifications are believed to re- ments, such as calcite-silica, clearly are re- quire the absorption of heat in order to expand versible. For example, replacement of quartz the lattice to permit entrance of ions; hence, and chert by carbonates, particularly calcite, is these tend to be late-stage developments when commonplace, and replacements of carbonates long burial (aging), or deep burial and folding, by quartz likewise occur in abundance. More- provide the required total thermal input. As over, in the same specimen, replacement of the alteration of clay minerals to well-crystal- quartz by calcite and calcite by quartz is noted. lized mica is the most readily recognized of the There is reason, therefore, to consider the re- changes of this stage, and as such authigenic lation as representing some sort of precipitation mica is commonplace in sandstones, the writer mechanism driven in one direction or the op- proposes the term phyllomorphic to designate posite more or less in the sense of some equilib- this stage of diagenesis. Sandstones at this stage rium mechanism, but obviously one which show gradual transition into strata in which does not involve chemical combination of the there are distinct indications of lowest grades of two substances. metamorphism. For example, authigenic chlo- In this connection introduction of silica as rite in graywackes representing the phyllo- massive replacements of carbonates (e.g., con- morphic stage increases as cleavage is developed cretions and cherty beds), shales, and in matrix in the strata.

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In connection with the lower limits of genesis is not nearly so neatly defined, and the metamorphism there is the question of the time time and control of authigenesis is not so well of appearance of zeolites and significant growth understood. There does, however, appear to be of chlorite. Authigenesis of zeolites is reported an order to the progress of diagenesis in that by Coombs and associates (1959) as marking redoxomorphic processes exert far more in- the lowest grade of metamorphism, and the fluence during deposition and early burial than writer has noted zeolites in some of the sand- at a later stage, and, similarly, the replacements stones examined. However, these were so few typical of the stage of locomorphism reach their that their replacement relationships with other climax antecedent to the later changes identi- minerals were not clearly defined. The impres- fied as phyllomorphic. The reader must be well sion gained, nevertheless, is that the growth of aware, however, that each special group of zeolites may begin in early burial but is most reactions overlaps the other, and one finds it pronounced during the phyllomorphic stage rather difficult to identify rocks not advanced and may tend to follow the major recrystalliza- beyond the redoxomorphic stage, as some of tion of the clay minerals to micas and chlorite. the locomorphic processes will have already be- Indeed, the growth of authigenic feldspar and gun. Nevertheless, in an attempt to classify the zeolites appears to occur at about the same stages, the writer has grouped the diverse min- time. However, these relationships need to be eral reactions identified among the sandstones examined more thoroughly to clarify the posi- (Table 2). In Table 2 the reactions are listed tion which zeolites occupy in the diagenetic- according to the stage of diagenesis which they metamorphic sequential order. Similarly, crys- represent, rather than the particular type of tallization of chlorite begins early, is well de- reaction mechanism. Nevertheless, similar veloped during the phyllomorphic stage, but types of reactions tend to occur during the reaches its maximum growth during low-grade same stage, and generally in the progressive metamorphism. order of diagenesis. Indeed, virtually all re- actions listed under the locomorphic changes Sequential Order of Diagenetic Stages show by mineral paragenesis that they have The selected minerals used to illustrate the occurred after those described as redoxo- associations of the various sandstone diagenetic morphic. Likewise, many of the reactions de- series are examples known to exist in some as scribed as phyllomorphic are late in develop- yet poorly understood form of reversible re- ment, although the nature of the changes from actions. Those constituting one association are clay minerals to micas and feldspars makes it not to be interpreted as all mutually interactive difficult to be certain of the order of mineral but regarded as products of several reactions appearance. which can exist together under a given set of All sediments must at least attain the re- diagenetic conditions. Unfortunately, at this doxomorphic stage; sands and poorly con- stage of investigation the precise relationship solidated sandstones contain some of the sec- of one to the other is not manifest, nor are the ondary minerals, or show one or more of the sensitivities to displacement of the various changes due to oxidation or reduction. Ex- reactions understood. It appears that minerals ceptions are found among the pure quartz produced during the locomorphic and phyllo- sandstones which lack the compositional dif- morphic stages are likely to be associated with ferences to reveal the reactions. Among such irreversible reactions unless the burial environ- sands the earliest recognizable alteration in- ment is very drastically changed. In a sense, the volves addition or solution of quartz. This tendencies toward retrograde diagenesis depend process has been listed as a locomorphic change, upon the topographic and climatic conditions not because it does not begin early, but, be- imposed upon the rock, hence the rate at which cause it reaches its most prominent aspect dur- weathering conditions are brought in contact ing the stages of early and late burial when with the buried rock. grains are thoroughly intersutured and welded. Each stage of diagenesis proposed is, in a Similarly, among the calcareous sands, the mas- sense, parallel to a facies of metamorphism. sive replacement of calcite by dolomite is as- Each is characterized in part by the appearance signed to locomorphism as the processes of re- of certain minerals, and the secondary minerals placement reach their climax after the sediment produced are likewise controlled by the bulk is deposited, but probably before pronounced composition of the detrital components. lithification, i.e., during the stage of early However, recognition of the stage of dia- burial. For example, bulk replacement of

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ancient carbonate sands by dolomite in the ing. Nevertheless, the strong development of Silurian of Indiana is far more closely related micas and chlorite from the clay matrix is most to very shallow burial over the Cincinnati and definitely associated with deep burial and fold- Kankakee arches and the proximity to reefs ing and postdates the replacements of the than to deep burial. Such strata can be shown locomorphic stage. to pass by lateral transition into limestones in the direction of the Illinois Basin where they CONCLUDING REMARKS are much more deeply buried. Although the redoxomorphic, locomorphic, Widespread sandstones of essentially mixtures and phyllomorphic reactions take place at low

TABLE 2. REACTIONS CHARACTERIZING DIAGENESIS

Redoxomorphic stage Locomorphic stage Phyllomorphic stage Reversible reactions: Replacement reactions: Unidirectional reactions: Fc+2 «=» Fe+3 aragonite by calcite "clay minerals" —> muscovitc or biotite hematite -t- calcite <=^ sidente calcite by dolomite montmorillonite —> chlorite hematite + calcite + Mg+2 «=S sider- carbonates by quartz or chert "clay minerals" —> chlorite ite + ferrodolomite + dolomite hematite + clay minerals + silica quartz or chert by carbonates "clay minerals" + Fe+2 —> biotite <^ chlorite -f- grcenaltte -f- stilpno- melane (?) hematite + chlorite <=^ chamosite (?) feldspar by carbonates "clay minerals" + chert —» chlorite hematite + illite <=? glauconite quart/., chert, clay minerals by "clay minerals" + quartz —> sericite carbonates "bauxite" + silica ^ kaolinite opal by chert or quartz kaolinite + glauconite or illite + Mg+2 —> chlorite + K+ silica solution <=i chert or glauconite —> feldspar quartz diasporc or boehmitc + silica ^ clay illite or glauconite —> chlorite + musco- minerals vite + diaspore (?) + silica + K <^ clay kaolinite ++2 illite + glauconite + calcite minerals (glauconite) + Mg —> muscovite + biotite + feldspar + dolomite + chlorite +2 bauxite + hematite + silica (minor) illite or glauconite + calcite + Mg «=^ clay minerals + pyrite —> micas 4- feldspar + dolomite kaolinite + K+ <=* illite feldspar —* sericite biotite <=i glauconite plagioclase —> chlorite + chert feldspar <=J clay minerals + chert glass —> montmonllonite + chert

of quartz and clays (subgraywackes or quartz temperatures, some of the modifications in- wackes) clearly display the locomorphic and volved, e.g., modification of clay minerals to phyllomorphic stages. In basins where beds of feldspar, indicate that heat must be absorbed sandstones are inclined only slightly, the prog- by the system to accomplish the transforma- ress of diagenesis has terminated with the tion in the crystal lattice. One might speculate modifications of replacement, i.e., the loco- that the complex tektosilicate lattice could be morphic stages, whereas, where similar beds are generated through the transfer of large amounts deeply buried, or have been moderately or of heat via the normal geothermal gradient at prominently folded, the characteristics of the temperatures which at no time would rise ap- phyllomorphic stage are well defined. Excep- preciably above the boiling point of water. tion is noted among some beds in which isolated The mineral alterations suggest that the large flakes of mica have crystallized despite absorption by the system of sufficient heat the absence of deep burial and prominent fold- could bring about the development of an

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authigenic crystal of feldspar (See also Baskin, that diagenetic modification in clay minerals 1956, p. 152). In a crude sense this may be re- cannot be a major process in sedimentary-rock garded as an aging process, which begins in- alteration. As clay minerals may be an im- conspicuously soon after the sediment is de- portant part of a sandstone, the stability of posited. In view of their similarity - to clay such minerals is of paramount importance in minerals certain configurations, such as that of reconstruction of the composition of the muscovite, must be preferred over that of the original detritus. Many additional observations feldspar or zeolites. Presumably, such is the ex- are required to remove even a few of the un- planation for the early appearance of authi- certainties regarding the mechanism involved genie muscovite and/or chlorite; their de- in the equilibria and replacement reactions, velopment, however, is frequently over- The writer believes that one of the most sig- shadowed by the reactions typical of the nificant aspects of the present study has been redoxomorphic or locomorphic stages which recognition of the magnitude of the effect of can be accomplished with a much lower thermal diagenesis upon the composition of the sand- demand, stone at the locomorphic or phyllomorphic Important investigations remain to be made stages. There remains evaluation of the extent regarding the development of clay minerals and to which intense application of diagenetic their stability relationships after the sediment processes may alter the composition of the has been deposited. Current observations on sediment as it reached the depositional site, and rock thin sections disagree with some theoreti- the degree to which such modification must be cal crystallographic concepts employed to argue incorporated in the classification of sandstones.

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MANUSCRIPT RECEIVED BY THE SECRETARY OF THE SOCIETY, JUNE 26, 1961

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