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Determinants of Clay and Shate Microfabric Signatures: Program Element No 61153NProcesses and Mechanisms

_ _,__ _ _ _ _,__ _ ," _Project No 032056. Author(s). .t 330

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Richard H. Bennett, Neat R. O'Brien, and Matthew H. Hulbert " Acso.DN70Accession No DN257003

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Ocean Science DirectorateNaval Oceanographic and Atmospheric Research LaboratoryStennis Space Center, MS 39529-5004 JA 360:027:89

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13. Abstract (Maximum 200 words).

The energy sources that result in sediment particle associations, reorientation, and disaggregation are presented in

terms of processes and mechanisms. Based on electron microscopy observations and theoretical considerations, theobserved and modeled microfabric forms and signatures are associated with processes and mechanisms operating invarious micro- and macroenvironments. The interplay of geological, chemical, and biological processes and mechanismsduring transport, deposition, and burial of particulate material Largely controls and ultimately determines thephysical nature, properties, and observable micro- and macrocharacteristics of soft sediments and their induratedequivalents. Discrete .vents such as suspended sediment transport, flocculation, and stumping may be identifiedand/or observed in the field or Laboratory. More often, the sedimentary material is studied to understand and inferprocesses and mechanisms responsible for its fundamental properties, origin, significance, and stratigraphic positionin the geological record. The particle-to-particte development and ultimate nature of a sedimentary deposit and itsvariability in time and space depend on multiple processes that include some important mechanisms that occurextremely fast and others that progress over eons. As defined in this study, mechanisms are the specific energysources that drive microfabric development. Two or more related mechanisms constitute that broader classificationtermed process. In the contimuum of microfabric development, the fundamental processes in which the individualmechanisms operate are described as (1) physicochemical, (2) bioorganic, and (3) burial diagenesis. (Fig. 2.1).

14. Subject Terms. 15. Number of Pages.(U) Sediment Transport; (U) Sediments: (U) Pore Pressure; (U) Clay 28

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CHAPTER 2Distribution/Availability Codes

Determinants of Clay and Shale Microfabric Signatures: o

Processes and Mechanisms Dist Special

Richard H. Bennett. Neal R. O'Brien, and Matthew H. Hulbert IIntroduction from detrital but also from biogenic sources Pos:tdepositional

authigenic mineraliza ion is important in some environments. TheThe energy sources that result in sediment particle associations, energy regimes characteristic of the particulaie transport path-reorientation, and di,aggr-egation are presented in terms of pro- ways and postdepositional environments significantly affect thecesses and mechanisms. Based on electron microscopy observa- time-dependent development of the microtabric of suspensionstions and theoretical considerations, the observed and modeled and sedimentary deposits. Discriminating analysis and discern-microfabric forms and signatures are associated with processes ment of the developmental stages of sediment microstructure. asand mechanisms operating in various micro- and macroenviron- a function of processes and mechanisms, are antecedent to gain-ments. The interplay of geological, chemical, and biological pro- ing a functional understanding of the relationship and importancecesses and mechanisms during transport. deposition, and burial of of microstructure to the developmental history of a deposit.particulate nmaterial largely controls and ultimately determines The microstructure is a crucial fundamental property thatthe physical nature, properties, and observable micro- and macro- largely determines the sediment's physical and mechanical prop-characteristics of soft sediments and their indurated equivalents. erties and behavior under static and dynamic stresses. Important)iscrete events such as suspended sediment transport, floccula- properties such as porosity. permeability, and stress-strain behav-

tion. and slumping may be identified and/or observed in the field ior are intimately tied to the microfatric and tile physicochemicalor laboratory. More often. the sedimentary material is studied to characteristics (Bennett et al.. 1977. 1989). Improvement inunderstand and infer processes and mechanisms responsible for predictive models for science as well as for the solution of practi-its fundamental properties, origin. significance, and stratigraphic cal problems is achieved by continual inwestigation into the com-position in the geological record. The particle-to-particle devel- plex interactive processes that influence the lundamental staticoprnent an.' ultimate nature ofa sedimentary deposit and its varia- and dynamic properties of sediments and rock. Important scien-bility in time and space depend on multiple processes that include tific and technical problems abound in the disciplines of geo-sonie important mechanisms that occur extremely fast and others acoustics, physical and historical geology, waste disposal, h'drol-that progress over cons. As defined in this study, mechanisns are ogy. agriculture, and the myriad of other activities that depend onthe specific energy sources that drive microfabric development, and can ultiumtely utilize a knowledge of the microfabric ofTwo or more related mechanisms constitute that broader classifi- sedimentary deposits.cation termed process. In the continuum of microfabric develop- The first objective of this paper is to formulate a rationalmenit, the fundamental processes in which the individual mechan- theme that describes the major interactive processes and mech-isms operate are described as (I) physicochemical, (2) bioorganic, anisms important in microfabric development temporally andand (3) burial diagenesis (Pig. 2. I). spatially. Various sources of energy are intrinsically coupled to

The properties of a sedimentary deposit at the time of initial the processes and mechanisms that determine clay sediment anddeposition are determined by (I) particle size. (2) mineralogy, shale microfabric signatures. Although some microlabric(31 particle sized distribution, anti (4) microstructure (fabric and models are reasonably well known in terms of the processcs thatphysicochemistry). The sediment particles are derived not only determine microlabric signatures. other models that describe

6 R.H. Bennent N. R. Olieni. antd N1,I1. tilheri

DETERMINANTS OF CLAY AND SHALE MICROFABRIC SIGNATURES:

PROCESSES AND MECHANISMS - A CONTINUUM

PHSC-CH MC III-R AI URA -IG NSS

ELECTRO-CHEMICAL BIO-MECHANICAL MASS GRAVITYTHERMO-MECHANICAL BIO-PHYSICAL DIAGENESIS-INTERFACE DYNAMICS1 BIO-CHEMICAL CEMENTATION

1 igtire 2.1. Diagram depicting thfe mt r piroesses and iechaitnim.ns Il hat de ri tic icrofthri ignattire% in thle macro and mi crogeoliogicatI env ironnments. Processes a nd mtechanismns represent a ContIinUuim d uring thle developmentalI history (if c Ia) sedimcnIit andshatle inicrost rut tire.

thc history ot inicrofabric developtmett are only in the formnative required for successful solution of practical problenm. I'l, fifthstages of' quantitative understanding. This study presents a is to reveal thle deficiencies in our knowledge of the interactingcotnceptual framtework of' the current state of understanding processes and mechanisms and to suggest Future areas oif poten-oft tilie processes and mechanisms that drive microfabric develop- t jal research. Thle main thrust of this study i.. to presetnt aI uniflieditent in order to set the stage for future understanditng of the approach to understanding the processes andi tuechanismis driv-complex microstructure processesa~ndl the evolution of advanced ing microf'abric development. An important underpinning of thisinicrofabric models. The second objective is to describe the study is to delimit the miacro- and rnicroenv ironmentslls %here tilemiacro- and] microeniviroinnetis in which the proc'esses and active energy regimes dotninate along the complex and oftenmechanisms and the particular energy regimes predominate convoluted sediment transport pathways: fromt "sedimetnt source-duritto microfhric development. The third is to describe thle to-sink."little atnd physical scales over which tile various processes andltmechanismts pm evail . The physicatl scales vary froin submlicronanid molecular to mieters and iti some case.% particular processes BackgroutndInav eXtend oirem distances of kilomecters. Thle fourth is to suggesthtow% these processes and mechanisms canl hc incorporated into Thle termi clay mnicrostructure refers% to two fundamental propt r-descript ive and predict ive models that reveal important intrinsic ties: thle fabric and physicochetnistry. Early usage and dlefinit it'lproperties ,ind] attributes of direct relevatnce to geology, geo- of, these terms are foutnd in Mitchell ( 1956), Lamibe ( 1959). andphysics. geoacoust ics, geotchlnique. and interpretat ion of the Foster and De (I 197 1I). Clay fabric is defitned as the orientat iongeologic record. Present qualitative and sentiquant itative tinder- and arrangement or spatial distribut ion of the solid particle,, andstatnditng of the mticrostructure of fine-grained selitment permits thie part ide-to-particle relationships. Clay nenrals are h~drous-otnly rudimentary miodels oif microf'abric (developtment. More aluminum silicates (classiflied as phyllosilicates) and are cetier-complex mlodels that include temporal atnd spatial scales are ally less than about 4 pin in siz~e as determined by standard tech-

2. Determinants of Clay and Shale Microfabric Signatures: Processes and Mechanisms 7

niques (Lambe, 1951; Krurnbein and Pettijohn. 1938). The composed of parallel or nearly parallel plates that may he stackedphysicochemistry relates to the interparticle forces of the sedi- either as sheets in a book or with an offset or stair-step arrange-ment. These forces result from both the physical interactions ment (Fig. 2.2). Diagrammatic examples of domains were givenarising from gravitational forces and the electrical nature of the by Moon (1972). and computer analysis verified that domainsparticle and the surrounding fluids (Bennett et aL., 1977). Dur- are important elements in submarine sediments (Bennett. 1976:ing geological time when sediment experiences increasing over- Bennett et al., 1977). A domain is considered to have significantburden with increasing subbottom depth, the gravitational structural integrity and to behave in a functional sense as a unitforces dominate electrical forces between particles and burial particle for a finite period of time under an applied stressdiagenesis ensues. Often organic material has a significant influ- regime. Thus, it is important to note that "a particle" can beence on the strength of interparticle bonds in clay sediments defined in terms of its morphology as well as its function. lmpor-(Pusch. 1973). Because of the ubiquitous nature of organic tant microfabric elements of a hypothetical sediment arematerials, clay-organic interactions are believed to be signifi- depicted in Figures 2.2 and 2.3.cant mechanisms in the developmental history of microfabric(Bennett et al.. 1988). Thus knowledge of the fabric and phys-icocheinistry. the fundamental "building blocks:' is essential to Discussionour understanding of the nature, properties, and large-scaledevelopmental history of sedimentary deposits. Throughout the microfabric development of a sediment, begin-

The importance of microfabric in the development of sedimen- ning with erosion of material from parent sources to the time oftary deposits was suggested by Sorby (1908). Terzaghi (1925) and deposition and burial, particulate material passes through aCasagrande (1932) proposed primitive fabric models to help wide variety of macro- and microenvironmental conditions. Toexplain the bonding and sensitivity of cohesive sediments. During study and trace the history of nicrofabric development, thethe following 50 years, geologists, soils engineers, and soil scien- environmental conditions have been organized here into a frame-tists proposed numerous simplified models of sediment fabric to work described as a continuum of procc,,,-s and mechanisms.account for factors such as the electrolitic environment surround- These sources of energy assotiated with the processes and mech-ing the clay particles (Goldschmidt, 1926; Lambe, 1953, 1958: anisms are the determinants of clay and shale microfabric signa-van Ophlen. 1963. 1977: Von Engelhardt and Gaida. 1963; tures. The major fundamental processes active in the continuumO'Brien. 1970a, 1971). the process of consolidation and compac- include (I) physicochemical. (2) bioorganic, and (3) burial dia-tion (Quigley and Thompson. 1966: Smart, 1967: Ingles, 1968), genesis. Processes include two or more important environmentaland soil dynamics and behavior (Pusch, 1970: Yong, 1972). A forcing functions or energy sources defined here as mechanismZ"quantum jump" in observational techniques was realized with (Fig. 2.1). Models depicted in Figure 2.4 reveal simplifiedthe advent of the transmission electron microscope (TEM) and graphic examples of their function. The mechanisms are large!yadvanced fabric models emerged (Rosenqvist, 1959); Pusch, responsible for the specific particle-to-particle interactions1966: Bowles, 1968: Bowles et al., 1969; Moon, 1972: Collins during a particular time in which a general process is active. Twoand McGown, 1974: Bennett et al.. 1977). The more recently or more mechanisms (such as electrocheniical and interfacedeveloped scanning electron microscope (SEM) has provided a dynamics) that drive inicrofabric development may operate con-means of observing the microfabric of shales and surfaces of clay temporaneously but typically only one fundamental processminerals (Keller, 1976. 1978; O'Brien, 1968, 1970b; O'Brien and dominates at a specific time.Ilisatomi, 1978: Weaver. 1984). Physicochemical processes play a major role in microfabric

Early models of microfabric were developed on the basis of development during fluvial and acolian transport stages of partic-simplified assumptions of the physical chemistry of the fine- ulates and on their contact with a depositional interface. The fun-grained .inerals (single platelets) and the electrolitic chemistry damental mechanisms operating on particulate materials in theof tile fluid suspension. Recent electron microscopy observa- physicochemical process include thermomechanical. electro-tions (Moon. 1972: Yong and Sheeran. 1973: Collins and chemical, and interface dynamics. Bioorganic processes areMcGown, 1974: Bennett. 1976: Bennett et al.. 1977, 1981) have important in marine and coastal environments, during transportrevealed (he presence of muhiplate particles (domains) as the and sedimentation of particulates in organic-rich waters, in areaspredominant fundamental particle type rather than the thin, sin- of high productivity, at the depositional interface, and in surficialgle plate particles proposed in early models (Terzaghi. 1925: sediments. The mechanisms that are important in the bioorganicCasagrande. 1932. Lambe, 1953. 1958). Thus, high-resolution regime include biophysical, hiomechanical. and biochemical.observational evidence has demonstrated that the single clay Processes of burial diagenesis drive microfabric developmentparticle fabric model of sediment is not wholly tenable and the when overburden or tectonic stresses dominate physicochemicalsc(limcnt i:; :,r,"',ipletely represented by domain, aggregate. and bioorganic bonding energies. Mass gravity effects (slumping,and linking chain particle arrangements (Bennett. 1976: Bennett sliding, creep, and consolidation) and diagenesis-cementation areand Hulbert. 1986). A domain is defined as a multiplate particle impoiltant p,)tdclpositioial/mechanogravity mechanisms in the

SR.H. Benncu., N.R. O'Brien. and Wil. Ilulticrf

AA-A

E A16

Figure 2.2. imagrain (if live fundamntnal particle units called domlains that coul- domains in stepped face-to-face irrangctuilt that form long chains (crosprise the "building blocks- of clay nmicrofahuic in sediments and rocks (single sectional view). (F) Plane view oif edge to-edge -ontacts of domains that alkoparticle% are rare; stipple indicates individual clay layer). (A) neatly slacked formt long chains. Note the difference in the Potential strength of the domainmultiplate particle called the domain swen in plane view. (R) Cross-sectional formed chains in F and . (G1) Cross-sectional view oif edge-to-face domain% that% iew of a neatly stacked domain as would hc observed in an ultrathin section pre- are commttonly found in marine environments. the microfahi it develops t-~rgepared for hIM. fC) A c ros -sect ionai view of a domain hut showing thc frequent void %pace between domains. (11) Plane view oif a chain of cla,, plates. formed h%ofkct arrangciiinil (it plates. (D)) rlancand cross-sctional views of the possible stepped facc-to-facc arrangements.shingle type arrangements oif a domain with offset plates. (E) Linkages of

J. Deterninints of Clay and Shale Microlabric Signatures: Processes and Mechanisms, q

process of burial diagenesis. Cementation at particle contactsmay alter fabric morphology and inhibit particle reorientationswithin a deposit during burial or when the sediments are sub-jected to tectonic stresses.

The microfabric signatures resulting from the various pro-cesses and mechanisms are often recorded in the sediments androck and are revealed by direct electron microscopy observation.Some of the signatures are difficult to capture in the undisturbedstate and thus they are difficult to evaluate and still remain to bestudied in detail. In addition, some mechanisms produce veryfragile and delicate microfabrics that stretch technology beyondpresent observational and.measurement limits. Thus simplifiedmodels, such as depicted in Figures 2.2, 2.3. and 2.4 are a greatasset in developing a conce1'lual understanding of particle-to-particle interactions and resulting microfabrics. The remainderof this chapter presents examples of the microfabric signatures ofsediment and argillaccous rock revealed by electron microscopyobservations. Simplified microfabric models are presented forthose particulate materials and sediments where direct observa-tions have not yet been made. Verification of these miodels awaitfuture technological developments.

Physicochemnical processes

As particles are transported through water and air and becomeincorporated into the sediment, physical and chemical processesoperate to bring them together into aggregates. to hold themrtogether, to break aggregates apart. and to reorient particleswithin aggregates and the sediment mass. The resultant fabric ofthe sediment depends on the balance between the mechanismsoperating to bring and hold particles together and those tending Figure 2.3. Plane and cross-'Nctional N icvs of the microllbric (t a h poihctic ial

to disrupt particle-to-particle contacts. Energy mechanisms sediment constructed of numerous domains in -random" arrangement.included in these processes include electrostatic interaction andchemical bonding (electrochenical mechanisms), thermallydriven movement oflparticles (therinomechanical mechanisms), interactions and van dcr Waal's attraction. The van der Waal'sand interactions occurring at the surfaces of contact between the attraction (force) exists between all particles. The force betwcenNarious materials and phases (interface dynamics mechanisms). two particles increases as the particles are moved closer together

and as their area of overlap increases. Electrostatic interactions'exist between electrically charged particles: particles of opposite

lh('ilroc'heni'al Mechaniss charge attract each other and those of the same charge repel. Thegreater the electrical charge and the closer the particles

The same forces responsible for the chemical bonding that holds approach each other, the stronger the electrostatic interaction.particles together internally also bind particles to one another. In an aqueous medium, the electrochemical interactionsThese forces are included in the mechanism termed clectrochem- become complex. Almost all materials develop an electricalical. At a point of contact between two particles of the same charge on their surfaces when immersed in water. The magni-nmerial. the bonding is similar to and possibly even indistin- tude and even the sign of the electrical charge on a particle ingtiishable from that within the bulk material. i.e.. covalent and water may be quite sensitive to the pH and El1, of the s\stem andionic bonds. I.ondon-van der Waals attraction, and extremely on the types and concentrations of dissohed salts (Stnumn and,shor't-rangc Bor rcpulsion Mennett and Ilulberl. 1986). Morgan. 1981 ). This sensitivity to the cosition (f the sur-

Ior particles in near contact (separation distances ranging rounding aqueous medium results from the sorption of ions fromi1in tonmic dialleters to dimensions of clay particles), the water onto the particle surfance and the release of ions frontl theelectrochemical forces of greatest importance are electrostatic particle to water. Hydrogen ions and hydroxide ions are olten of

10 R.H. Bennett, N.R. O*Brien. and M.H. Hulbert

I MECHANISMS

ELECTRO-CHEMICAL

DOMAIN CARDHOUSE

-OR- -OR-

BEFORE AFTER

THERMO-MF(CHANICAL 'K

+ ETC

BEFORE AFTER

GASBUBBLE

INTERFACE DYNAMICS

-OR-CURRENT cmne

Figure 2.4. Determinants of clay sediment and shale microfabric signatures: processes and mechanisms-a continuum.Diagrams depict the major processes and mechanisms that influence the microfabric signatures in the macro- andmicrogeological environments. (A) Physicochemical processes. (B) Bioorganic processes. (C) Burial-diagenesis pro-cesses.

2. Detern'inants of Clay and Shale Microfabric Signatures: Processes and Mechanisms

[MECHANISMS I

BIO-MECHANICAL-

BLO-RHYSICAL i zORGANIC

B10-CHEMICALI

HS BUBBLE OR BACTERIA COLONY B

Figure 2.4B

2 R.H. Bennett, N.R. O'Brien. and M.1. luttlcit

MECHANISMS

MASS GRAVITY

DIAGENESIS-CEMENTATION

SILT/SILT AUTHIGENIC

0 0 CLAY

C BEFORE AFTER

Figure 2.4C

particular importance in establishing a charge. The electrical As the salt content of the water increases, the effective distancecharge on the surface of a particle in a given aqueous solution of separation of the positive charges from the particle surfaceswill depend not only on the chemical identity of the particle. but decreases, and the effective reach of the electrostatic repulsionmay even be of opposite sign on different crystallographic sur- decreases along with an increase in the potential of %an derfaces of a single particle. Waal's attraction. Now the particles have a greater probability of

Clay minerals in natural waters typically have a net surface approaching sufficiently close to permit aggregation. Graphiccharge of negative sign (van Olphen. 1963; Grim. 1968). The examples of the above clay particle interactions in salinedistribution of charge on the surface of a clay particle is expected environments are given by Bennett and Hulbert (1986).to be patchy rather than homogeneous, and regions of positive Initial aggregation tends to be edge-to-face. probably con-charge may develop at edges. at defects, and at sites of sorption trolled by differences in charge density on the different portionsof positive ions. The surface charge of the particles is balanced of the particle exterior (Figs. 2.5-2.7). The resulting aggregatesby the net charge of the nebile ions surrounding it in the aqueous are open with a high water content and the areas of particle con-meditum. In the water nearby there is a net surplus of ions with tact and of particle overlap are minimal. In the absence of strongcharge opposite to the particle charge and of total charge equal bonding at the point of contact, reorientation to a face-to-faceto the particle charge. These excess positive ions surround the configuration with a resulting increase in van der Waal's attrac-negative particle in a diffuse cloud of charge. As two clay parti- tion may ne expected (Fig. 2.8). Rotation about the line of con-ties in water approach one another, the repulsive interaction tact between particles will result in an offset shingle-type fabric.between the two clouds of positive charge tends to prevent the Once such a fabric with a relatively large area of particle overlapparticles approaching one another sufficiently closely so that is created, a nuch larger amount of energy would be required toshorter range van der Waal's attraction may pull them together. convert it to a neatly stacked book-type fabric.

2. Determinants of Clay and Shale Microfabric Signatures: Processes and Mechanisms IA

- NUMEROUS CHAINS-"BRIDGING" c F-F CONTACT%- E-F CONTACT : E-E CONTACT

Figure 2.5. Transmission electron microscope (TEM) photonicrograph of the chains, and the various modes of part'icle association. E-E, F F. anti steppedclay fabric characteristic of the smectite-illite-rich Mississippi Delta sub- F-F and sone E-E contacts.marine sediments. Note tie delicate "bridging" between particles (domains).

Therromechanical Mechanisms B). When the ice melts the sediment tends to retain the modifiedfabric while salts excluded by the frcezing process are redis-

The motion of particles that results in their initial approach and solved (see for example Fig. 4.1 of Bennett and Hulbert. 1986).their reorientation after contact is one result of the mechanism Temperature differences between :adlacent water masses leadstermed thermornechanical (Fig. 2.9A). At ordinary environ- to microscale laminar flow and turbulence at the boundaries.mental temperatures, the thermal energy of water is observable Shear from this nticroturbulence ma, ,lisrupt suspended aggre-in the Brownian motion it imparts to suspended particles smaller gates or reorient particles within aggregates (Gibbs. 1981).than about 10 prin in diameter (Feynman et al., 1963). The Cearly the scale of the turbulence is critical-turbulence on akinetic energy of small particles is significant relative to repul- dimensional scale much greater than the size of the aggregatesion forces ct typical temperatures of the marine environment will merely translate it through space. A much less probableand becomes greater in high-temperature regions such as those result of microturbulence is to bring suspended particles intonear hydrothermal vents (Fig. 2.9A). effective contact to form larger particles (Koh, 1984).

Another thermomechanical mechanism is the modification ofsediment fabric by the freezing of interstitial water. Particles areexcluded as water freezes and may be squeezed together if they I,,erfice Dvamnics Mechanismshappen to be trapped between two approaching freezing fronts orbetween a freezing front and a barrier (Fig. 2.9B). The resulting Microturbulence also may be considered more generally as ansediment microfabric may be characterized by relatively dense aspect of mass and energy transfer at boundaries between con-sediment units surrounding large voids filled with ice (Fig. 2.9A, tacting water masses or contacting particles and the surrounding

14 R.H. Bennett. N.R. O'Brien. and N.11. Hulbert

E-F. I '

igure 2.6. Th pical microlahric of sirticial submiarinc sedinent front the I jum AMissS, ippi D)eIla St-Ni dpicts eaimple of a single h 'cctile composed ofih~lnoan M 1)1 iliariecd c,,ee to lace (1: F) in surrounding material of randomly Figure 2.8. ITFM sQi ing cleat exanmples of domains tspical of the mcrofabricairaliied domains. of Mississippi Dela sediment. Arrows point to f'ace-to-face (F-F) orientation ol

domains.

fluid. This mechanism is termed interface dynamics. Lnergy approaches zero. Therefore shear forces overcome interparticlcsources include mass fluid flow driven by wave. cuirent. and binding forces and gravity forces only in regions of rchili\chvgravity firces and gravitational settling of particles (Fig. 2. )t. rapid flow. For unconsolidated clay-size particles the tflinitimIn general. mass fluid flow has its greatest impact on microfabric flow for particle entrainment is about 2.0 cm/sec (Boggs. 1987).SIien it impinges on the surface of the sediment. It may serve to a range of flow rates (0.6-3.0 cm/sec) was suggested recently by

reorient particles attached to the surface or it may drive sus- Li and Bennett (this volume). An important factor not consid-pcitded particles onto the surface. In the vicinity of a surface of ered above and often omitted from discussions of fabric devhlop-significant area. the perpendicular component of fluid motion ment under dynamic conditions is that, althouh individual

grains may be of clay size, the kinetic unit is often a mulligrain

aggregate Thus. consolidated clay sediments may resist erosionunder flow rates of I m/sec (Boggs, 1987); and, conversely. mul-tigrain particles (chains, Fig. 2.2) may protrude many singlegrain diameters above the plane of the sediment surface, and bequite subject to disturbance at even minimal flow.

Particles may be brought into contact by any of a number ofmechanisms that result in differential rates of particle movetnent(Montgomery, 1985). Such a differential may arise under gravi-tational settling between particles of different sizes or densities,

for eximple, and is particularly characteristic of dynamic effectsat interfaces. In the vicinity of a static boundary, tile velocity offlow of water or air becomes less the nearer the boundary isapproached. Particles swcpl along in the fluid attain a sitmilarvelocity gradient and the rate with which they collide is approxi-mately proportional to their concentration and to the velckity gra-dient (Swift and Friedlander, 1964) Attachment of particles togas bubbles in w:, ,:r, which may be greatly enhanced by the pres-ence of organic matter, results in the particles being moved in the

I Pm ' direction opposite to the general gravitational settling and may bemoderately effective in bringing particles into contact (Ieia.

Figare 2.7. SENM of Mississippi Delta Sample depicting some edge-to edge (0-- 1982). These particle-to-particle contacts occur both during bnb-I-. and face to-face t V) particle contacts. Note large domain in upper center of ble movement through the water and on tie accumulation of btb-ligure and the large and randomly shaped voids. bles at tile surface. Differential motion of particles attachd to a

2. Detvvnamvs ol'Clay avid Shvale Microl~abric Signatures: Processes avid Mechanismis 15

WATER MOLECULESIN THERMO-DYNAMIC NMOTION

a CLAY PARTICLES CLAY PARTICLES

BEFORE

CLAY LIOUiD WATER

AFTERICE

Figuvre 2.9. (a) Fabric iliodificat ion by thermoiiechanical

mechani. -Ihermnal Iv driven tias;s flow (solar avid h~ dro)-thermal heating and atnosphcr ic cooling. Arrows depict

relatisc motion of vlie w\aler mass: crystallization (if ice in

selimcite fitm uncian mot01ion i' %&aver iiifcctlauic lvl rs ep l vaantalrev l c c s ad qurz

penici parficics (h)iThe effect% (if freezing (surrounding

vvaler (ion the reorienitation oif clay particles as the ice cr)ys-li

cprain or pebble. b

16 R.H. Bennett. N.R. O'Bricn, and N1I. ilulbert

sediment particles and, in shallow-water settings. can disruptsediment by bouyant release (see Wartel et al., this volume).

F= PG IBioniwclanical Alechanisins

-\ .Bioturbation. "tie churning and stirring of a sediment by organ-isms" (Bates and Jackson, 1987). is a significant mechanismresponsible fbr producing the particle orientation in a sediment

CURRENT MICRO- and ultimately in a rock. This biomechanical ichnlis,,m- - ROUGHNESS produces a clay microfabric that possesses a randomness similar

ATINTERFACE to the primary fabric of a flocculated (edge-to-face) clay. how,-ever, it is mainlv characterized by randomly oriented individualparticles (seen in SEM) rather than the random domains coni-mon in flocculated clays formed by other processes. Anotherdistinguishing feature observed by the second author. especiallyapparent in bioturbated shales and niudstones. is the abundanceof silt grains mixed in with the bioturbated clayey material.Iinirc 2.1. tichagaim dieeni iin ot mon o ti lin pahrt nl tinder the inhit In bioturbated sediment, the randomly oriented fabric of indi-

ti, n;,mics, il11c.-h;, lisml1s dii Icrenli a m ii oion ol S.en tin p Irtesc. trnder hc inllu-

cnou t gra% it%. dtlcrintial hov tf waler masses 1" dillering density, impact of vidual platelets is a result of mixing by organisms. Mixing dis-irlict!, o11 ,edintn inictlace, ant hoiw- at interface (nolte I o ilirtance of turbs the original primary flocculated fabric. Rhodes and Boyer

toicrorotiihne,% (n particle or,.'nlalion) (1982) indicate that "to facilitate burrowing and feeding. somemetazoa, especially bivalves, also liquify the sediment by inject-ing water anteriorly into the bottom . . . this causes an instan-

single bubble beco es even more pronounced as the water filn taneous local increase in pore water pressure and the liquid limit,sil.noding the bubble thins % hen it rises io the water surface or of the ,ediment is temporarily exceeded." At this point the pri-J, the gas of the submerged bubble dissolves. mary flocculated fabric is not only disrupted but silt size grains

are mobilized and mixed in with the liquified clayey sediment.On burial and lithification the bioturbatcd sediment olten

Iiotirganic PIocc,,cs retains its randlom fabric. Preservation of randotiness in exlen-sively bioturbated sediment is attributed to binding of particles

Bioorganic processes represent direct effects of living organisms by mucus secreted by burrowing organisms (Rhodes and Boyer.onit sediment properties and indirect effects mediated by the 1982; O'Brien, 1987).chcmicals (lorganic materials) they produce. The term bioor- Figure 2 II illustrates typical microfabric produced by theganic is constructed from /h meaning living and organic mean- biomechanical mechanism. Iland sample and X-ray radio-ing tle chemical products of the life process (Morrison and graphic viewing was first done on each of these poorly fissileBod. 1983). The mechanical alterations of sediment fabric by gray shales or mudstones to demonstrate that they had beenactivities of organims are classified as biomechanical. Included extensively bioturbated. In radiographs all samples show bur-are actii, ties that take place in the sediment such as bioturbation rows or other evidence of sediment mixing. It should be stressedand activities that take place in the water column such as inges- that to be certain of positive identificationofabioturbated fabriction and reorientation of particles and clusters of particles by one should combine radiography or thin-section analysis withlilter feeders. In the latter case. biophysical mechanisms such as SEM viewing. Body fossils are not common in the samples.the binding oI particles together by organic matter also may be however, various types of trace fossils are apparent. The twoimportant. Bioph,'sical mechanisms include the adherence of important features which characteri7e this nicrotabric are (I) aparticles to sticky organic mucus and particle binding by poly- randomness of individual clay flake,,. and (2) silt size grainsmer bridging. Biochemical influences on sediment microstruc- (quartz?) mixed in with the clay matrix.lure result Iront changes in the microenvironment brought about Fecal pellets leave another fabric signaltle representing aby the metabolic activities of organisms. These include the biomechanical mechanism of sediment aggi 'eation. An SI-Mbreakdown of naterials such as the polymers. which are impr- sludy by Syvilski and Lewis (1980) revealed the characteristicstant in polymer bridging and the production of biogenic of marine zooplankton fecal pellets. The ingestion of sedimentmaterials such as cementing precipitates of pyrite. Biogenic and its expulsion by organisms as fecal pellets are common.gases. including methane and carbon dioxide, caii have a locally Pryor (1975) studied tie feeding activities and excretoryiiliirtant impact on lie chemical environment surrounding products of the marine decapod Callianassa major and the

.2: lDeteripants of (lay and Shale Microlabric Signatures: Processes and Mechanisms 17

-i -N

I.I

aR

I'igtirt' 2.11i. St NI pholm iicr..graphs (it part ieasmi ii produiced by t he Mason ( ountl. V. Scale 1 ji (m. (C) iturhated tnudst one I ahr it Not iLC

htoliicchllical htih, ii echanisms. (A) Silt grains (smmill arrinss nmixed stt, ired nature (if plai, flakes. Pernn Yin shale ltDewmnan. IAmplgson C ommt.iti hill si idiial plaitk Li fLIMakes: Castiatma shale tiDewmaiill. Wiilning CounlN. NY). Scale il10 tin. li) Typical hioiurhalcd fabric :Irtows shos silt ilainls

NN) stalle it111 ) 111(11\t Ihidal ralndoiiI, mrienltcd tiakeN I large ariow0s cas lies forinelik occupied h%. grains dislodged during Prepim t t'll) Olurtawti.

iminse s ith %ilt grai, ismall arroisss inl the bioinrhalctd luruin shale (tewonian. gtav shale. Yorkshire. England).

mnarine annel id Onup/lis nuurclmeii/uil and foiund that in sonie is easily ident ified and (fhe faubric difference between pellet andareas thev removed argillaceous particles from mispension. and enclosing rock becomes obvious (Fi1g. 2. 14A). Figure 2. 14A anddeposited layers of lecal mud as, thick as 4.5 inun/year. producing B-2 show the tN pical tossil fecal pellet microf-abric characteri,edas much as 12 metric Ions (dry \%eight) of' pelleted mud per by dense packing of randoinl oriented nonplatI nmaterial that is

sqluare kilomeiter per yecar: in contrast to the preferred orientaltion offplaty miinerals cotlpos-Thle fabric of densely packed ratndomly orienited fitne particles ing the surroiundinig shale (see arrows in Fig. 2.1 4A. B-2).

tin a well defined ellipsoidal pellet is easily recogniied at lowttiagnificaiion in an SlIN1. lth recenit (Fig. 2.1I2A. 13). andancienit fe'cal pellets ( Figs. 2.1I 3A . B3. 2. 1 4A. 13) reveal biogenic Bily/i,,hovui /luai.smi

tnd Iii hi geic pafiti cles. lii weve r. \%hlen vi e~x d at inagifica -tnon greater thatn -1(1() it is (fteti ditlictilt to dlistinguishi fecal (lusters of rattdoml\ orienited particles ate preserved inl Nome

pcI let Ifabric Ifromtt that prodtuced by phiysicochiemiical processes organic-rich argillaceous rocks. Hlere I le terni biosedimetitWig: 2 .1I314). At 14 m~ er i agi I licat ion I lie sharp bo rde r offt lie pel let aggregate refers to cl usterIs that are hell e'ed to have 1 rin ed

18• R.H. Bennett, N.R. O'Brien. and MH. tlulbcr

Figure 2.12. Fabric of a recent lecal pellet. (At Recentfecal pellet (organism unknown) found in marine sdi

ment, Dixon Entrance. Queen Charlotte Is, BritishColumbia. Scale = 100 tim. (B) Close-up %,iew of pelletarea shown in box outlined in A. Notice the random mix

of biogenic (diatom fragments) and lithogenic claN?)matter. Scale = 10 rim.

O10

when lithogenic sedimentary material adhered to sticky organic formation of aggregates of particles in recent sediment (Honjo.mucus nets and/or formed by a clay-polymer bridge interaction. 1982). Pierce and Siegel (1979) observed that the major portionHence. they represent another mechanisn by which sediment of suspended solids in estuarine and organic water is composedmay aggregate and settle through the water column, of aggregates of mineral grains, soft organic matter. biogenic

Our investigation of certain organic-rich marine shales reveals debris, and phytoplankton, all bound by a malix of organic rilat-the presetice of numerous aggregates scattered randomly through- ter. Alldredge (1986) has shown how the planktonic gelatinousout a matrix of predominantly preferred platy matter. Their pres- zooplanklon Appendicularia (Chordata. 'ltoicata) produces anence should not be surprising since McCave (1984) stated that external mucus structure that becomes cl(,ied with sedimentfine-grained marine sediment settles as aggregates produced by and is eventually discarded. Fungal mycelia and filamentousbiochemical bonding or electrostatic attractions. There is abun- bacteria in Lake Tahoe, Nevada, were found aggregated withdant evidence supporting the role oforganisms and organic matter detrital particles (Pearl. 1973).in promoting recent sediment aggregation (see for example, Riley. The work of Avninielech et al. (1982) is very significant to our1963: Kane. 1967: MacLean and Smart. 1978: Silver et al., 1978, study in illustrating the morphology of what we interpret asTrent ct al.. 1978: Mullins. 1980, Shanks and Trent, 1980: Syvit- recent biosediment aggregates (Fig. 2.16). They found thatski and Murray. 1981). The interaction between organic and inor- aggregation of clay was promoted by algae that secrete largeganic matter is considered respinsible for biosedintent aggregate amounts of polysaccharides and other polymers that pr(xlucefiortation described here. Examples of their microfabric are sticky surfaces. which in turn promote aggregation of clay onshown in Figure 2.15A and B. algal surfaces. Notice, in Figure 2.16. that the platy clay Ilakes

Biosediment aggregates fotund itl this study occur only in non- appear caught on the fine algal filaments much like a lIv trappedbiolurbated rocks that are well laminated, thus eliminating post- in a spider's web. The sticky algal surfaces that bind the clay con-depositional hiogenic mixing as a mechanism responsible for lain polymers that act as bridges via cat ions or anions to the claytheir fornation. They are relics of the original sediment fabric, particles. Avnimelech and Menzel (1984) and Reed (personalIccause of' their asociation with highly orgainic sedimentary communication) have used this aggregation mechanism to clar-tock Jall shale% in "hich they occur have a total organic content ify muddy ponds simply by stimulating algal growth by addingit)(CI of > 3," 1 it is cotidcd Ihat they record tie comtplex fertilizer to the water, which causes sedinientation of algal-clay

organic clay inlcrCicln which took place during sediment depo- clusters. Cyanobacterially induced flocs have been produced insition. Mode rn analogs of this interaction are nttmerous. claritying suspensions of hinely divided mineral %astes from

Coccolithophoroid mucus is reported to play a major role in phosphate ore beneficiation (Leslie el al.. 1984).

2. IDclerminants of Clay and Shale Microlabric Signatures: Processes and Miechanisms 1

IHgmre 2.13. Comparison oit the ,oicrolahric of a recenta nd Jujrassic fecal pellet. (A) Miagniflied view of theiuicrol.-O, i of recent tecal pellet in marine sediment.Fnierald Basin. off Nova Scotia. Note thie randomn particlereo rieritat ion. Scale = 1 0 pori. (H) Magniflied v'iew (if fos-sil lecal pellet lahric. Bitumninous Shale (Jurassic) 'thrk-shire. heland. Note the randlomt particle orientation and4dcnse packing. ('occolith fragments arc scattered through-out1. Scale =10 Vom.V

Organic polycicctrolyles have also been reported by numerous also Syvitski. this volutme). Rashid (1985) described thuisitwest igators to destatbilize colloidal suspensions by a "polymner mechanism as it applies to the seditmentatiotn of Iloccijies ol a%bridging mechanism- (Ruehrwcin and Ward. 1952: Lamer and and organic cotmplexes in the marine environment and stressedHlealey. 1963: Black et al. . 1965: Stunin and Morgan. 1981). (fhe role of saline water in promoting flocculation. It is signifi-The bridge fortms when part of thc polymner chain attaches to a cant that our observations show aggregates in marine organlic-solidl particle surface (at an adsorption site) whereas other see- rich argillaceous rocks.tiotus of the chain tmay extend out into the solution and become We propose that large masses of sediment could aggregate andattached to a free site oin a second particle (Fig. 2. 17). This settle ot in an organic-rich environmtentt by tfie hiopin sical (sedi-mechanism produces an aggregate of particles and polytmers (see menit aggregate) mechanism. Intiit ially large macromolecules

1)R.H. Bennett. N.R. 0 1, .nA. and NHI litill-

Figure 2.14. NI icril abric of 1(I Iccal 1c1% (A' I Mof fo.-il fecal pellet al loss maifidiLat ion Mile ctia't

ing fhbrics (if pc!let isitall arri''s I and mif i'iiiiilin Avile- matjriX (large :tro%). . Scale - IMK 1111 (ll I) t1OW1i 1lc ;I1

peiiet in ttminouis Shale liurassicl ttlu lei. I- tivland

Scale = 11M lii. (B3-2) ('lose-tp % iess Ai ;iica hmts 11 111

ouLttlied lox I,. PI. Note LabIc dilteICIRC hCtsscC~i pelle

- daire at ov. I mid pfelerred orictitation tin thc ii tomuidirig

shale I maill arrow). Scale 1(1 tin.

and organic mu11cus strings suspended in ocean watler act as sub- lacy network of organic and inorganic dctritus. Once incorpo-Wi ates onto which clay, silt. and other detritus aggregates (Fig. rated within the bottomn sediment,. the degree to which a bioscd-2.17). As this mass settles. other suspended particles are swept iment-aggregate resists compaction and particle reorientationup hy hcing caught in the sticky mucus web. The actual aggrega- relates to the amount of polymer hridgcs. which in turn relates tolion mechanism consists of an organic- sediment intcraction thc original organic concentration (and organic type?) in the sea-(i.e.. polymer bridging) as polymecr chains link particles into a water. Onc has to explain the numeirous aggregates scattered

' 2. Determinants of'Clay and Shale Microlahric Signatures: Processes and Mechanistits 21

Figure 2.15. Typical examrples ot hiosediiient aggregates, (A)L oose packing ot randoinly oriented ptaly part icle% in anorganic rich hiosedimnent a:ggregate shale. Swope Fin (Pennsyl-vanian, Adtair CounlYi, lowa). Scale -I jun. (13) Hisedimnnaggregate imicroIlmric charactcriied by an open texture of ramn-0domit oriented particle., Note the authigenic framboids (lowerr ighit) in the Nurrounding shale. (Jurassic, Ravensi. EIngland).Scale I111i1.

throughout a shale with dominantly preferred particle orienta- sites arc occupied, the bridging will be too weak even to with-tion. Thc preferred orientation may represent totally collapsed stand shearing forces by agitation. Thus, in a sedimentary'aggregates whereas thc clumps of random particles represent environment that is anoxic but contains a low contetnt of organicsoicl aggregates whose gel strength allowed theimi to resist matter. the stress exerted by bottomn flowing currents or sedi-reorientation. The concentration of organics is important. A mcnt overburden could be a factor in disturbing the stabilit\ oitsmall concentration oif polymners in suspension couldl produce most weakly bridged aggregates and thus produce a dotninantl\aggregates composed of only a limited number of weakly formed preferred particle fabric surrounding those few clumps of thebridges. Black et al. (1965) found, for example, that if too few original bioseditnent aggregates which resisted deformation.

•.22 R.H. Bennett, N.R. O Bricn. and M.H. Hulbrl.

A B

~ RIDGE

ORGANIC

S CLAY

C Dim

Figure 2.16. A hio.cdiient/algae-clay aggregate from As nimeleck et al. (1982).Note the fine algal tilanments and large (while) particle possibly some clay parti-cle and/or skeletal debris and the overall norphology ol the aggregate. Scale =I un. (Reprinted with permission from Y. Avnumclech el al., 1982. Fig. 2a,216. p. 63- 65. 2 April. Science, Mutual flocculation of algae and clay: evidenceand implication. Copyright Science, 1982 by the AAAS.)

Although the exact understanding of their preservation is notclear, aggregates of randomly oriented particles observed in this SEDIMENT SEDIMENT

s udy are in organic-rich rocks in the geologic record and pre-suniblythebioedientaggegats ocurals insom moern Figure 2.17. Steps in the formation of biosediment aggregates. (A) t~argcstlniably the biosedinent aggregates occur also in somne modern clumps of organic mucus sweep up suspended detritus during settling. (B) Thesedinents. It is suggested that biosedinient aggregates may be a actual contact between organics and sediment is a result of parlicle linking h%result of a polymer bridging mechanism. polymer bridges. Notice linkage of clay flakes by polymers. (C-D) Orientation

of biosedinient aggregates in loosely consolidated sediment.

Bio(Ihemi al Mechanisms

this case, about 2 million cubic meters of sediment was exca-Organisms alter their environments by the production and des- vated and redeposited about the resulting blowout crater. Thetruction of many chemical entities. The impacts on sediment measured crater depth was 58 m. and the total amount of sedi-nticrostructure that these chemical alterations cause are ment excavated corresponded to a sedimentation time of aboutincluded in the term biochemical mechanisms. Some of the sim- 200,000 years. Much less spectacular but much more conmmonpier chemical conversions of interest are illustrated (Fig. 2.18). venting of biologically derived gases such as CO 2 and CH, inNot only are sugars. proteins, oils. and the rest of the complex shallow water churns and resuspends sediments.array of biotnolecules made by organisms, but also these corn- An example of a biochemical mechamni ielected throughpounds are decomposed and result in a variety of changes in inor- inorganic species is the formation of authigenic pyrite by theganic constituents. The dccay of polysaccharides and other poly- combination of ferrous ions and disulfide ions generated meta-mers responsible for polymer bridging discussed above, for bolically in anaerobic environments (Lynch. 1983). The forrma-example. niay allow particles in aggregates to be more easily tion of authigenic pyrite trainboids in fine-grained sediment alsomoved apart or reoriented after sedimentation. inlluences clay fabric. Although framboid growth does not pro-

Gases produced by organisms in sediment masses also may duce sediment aggregates. it does exert a significant influence onalter sediment micrtostructure as they are released. A spectacular microfabric by causing 1'article reorientation in specific areas ofexample of macroscale disruption of deep-sea sediment by pre- the sediment. The individual framboid (composed of iron stil-sumed gas release was reported recently (Prior et al.. 1989). In fide) found in shale is spheroidal (Fig. 2.19). The spheroidicily

• 2. Determinants ol Clay and Shale Microfabric Signatures: Processes and Mechanisms 23

S0= S0= N 221432

NO,

NO NO733

CH4

CO2

Fe*F

s04 F -S

Figure 2.19. Pyrite framboid in Bedftitd ,hale filtinalilion M sii,,pian. (),iw.so .4 Notice doming of platy particles (arro%) arnund framt'oid and %%cll dcsch'pcd

preferred orientation in surrounding shale. Sik = 10 t.n.

S FeS2t*3 2

Fe

Figure 2.18. I)iagran depicting simple biochemical conversions of materials, colony enlarged (generating sulfur in the process) the soft locCu-including represent ation for the formation of authigenic pyrite. Organisms are lated clay sediment was easily deformed adjacent to it causing claydepicted by stippled form showing uptake of chemical species such as nitrate, flakes immediately adjacent to the colony to wrap around its sur-which is reduced to nitrite. Nitrite can be processed hy an organism expelling the face and reorienting and doming the sediment farther away indiatomic molecule (N2 ). nitrogen gas, and also prolucing ammonia NH 3.

response to the growing sphere (Fig. 2.19). Subsequent sedimentloading due to deeper burial reoriented the surrounding floccu-

is attributed to pseudonorphisn of a p gspherical lated clay into the typical preferred fabric of shale. During earlyattriuted immiscbleorani gloules apreexisting spheous diagenesis. iron diffusion from the organic-rich anoxic sediment

body-e.g.. itmmiscible organic globules or infifling of gaseous it h atra aiyrsle nio ufd rcptto

vacuoles (Rickard, 1970). A biochemical origin for framboid into the bacterial cavity resulted in iron sulfide precipitation.This is only one possible explanation for the origin of pyritesphciciy hs ben popoed Kaliokski,197; lborand framboids in clayey sediment. What is important is that this

Mountjoy. 1976) and is discussed here. Bacteria or microflora eam ple i l at ey m ay form by ambiochemica eh-colonies may produce the spheroidal morphology of frarnboidsexmlilutasthy ayfr byaioeiclne-colonies may pdehe responsplefhei orhoily freaods anism that in turn exerts an influence on the final clay or shaleand also mtay hc responsible for the biochemical reactions lead- mcoarc

ing to their foIrmation in sediment.

The typical framboid found commonly in organic-rich shalesdisplays platy flakes wrapped around the sphere and is associ-ated with a doming of adjacent particles. The adjacent particle Burial Diagenesis Processesorientation is interpreted as indicating that the sphere (be it a gasbubble or bacterial colony) grew in soft, easily deformed, floc- Postdepositional alteration of microfabric during the proccs% ofculated clay sediment. Such a sediment would exist in the early burial diagenesis is driven largely by two ncchattisns: tiasstage of deposition. Framboids are found today at relatively shal- gravity and diagenesis-cementation. Both mechanisms ntaylow depth (< 3 n) forming as a feature of early sediment diagen- proceed simultaneously. However, mass gravity stresses within aesis (Love. 1967). It is proposed that some framboids found in deposit are ever present regardless of the degree of chemicalorganic-rich shales originally grew in soft flocculated clay under activity and mineralogical alterations and often mass gravitythe reducing conditions existing in the sediment soon after depo- acts independently without influence of significant chemicalsition. These conditions favored bacterial growth. As a bacteria activity associated with diagenesis-cementation. Burst (1976)

24 R.H. Bennett, N.R. O'Brien. and M.H. Hulbert

Figure 2.21. Microfahric of Missisippi Delta ,edirnen s (30 cm %subhitoml)Figure 2.20. Microlabric typical of the surlicial submarine sediment (1.3 m observed by SEM. Compare with TEM in Figure 20. Note domains, large ,.oid,,.

suhbolon) of the Mississippi Delia observed by TEM. Note random arrange- and short chains. Scale = I pr.ment of domains and large, irregular-shaped, voids. Scale = I ptn.

suggests that gravity is the dominant consolidation-compaction (Figs. 2.22, 2.23). Initial depositional porosities of 70-75% con-mechanism during dewatering of argillaccous sediment in the trasted sharply with consolidated sediment porosities ofapproxi-upper 914 in, but he contends thait mineral diagenesis of clays, mately 50% at burial depths of only 90-150 m (overburdenand osmotic and aquathernial pressuring are significant con- stresses of 6-12 kPa). Reorientation of particles is observed fortributing mechanisms at burial depth. both field and laboratory consolidation (Bowles et al., 1969:

Mass gravity mechanisms are used here to include not only Bennett et al., 1977, 1981). Hedberg (193(,- tecogni7ed theconsolidation and compaction. but also the dynamics of slump- importance of mechanical rearrangement of particles during theing. creep, orbital bed motion and deformation, and seismic early stages of cotnpaction and his observations appear to haveshock. For this study. diagenesis-cementation mechanisms been well documented by later studies (Bowles et al.. 1969: Ben-include mineral alterations, interstitial fluid transport, cementa- nett et al.. 1977, 1981; Faas and Crocket, 1983).tion and leaching, and organic-clay and gas interactions. A very The consolidation process involves volume reduction as alimited number of studies have addressed the significance of result of sediment dewatering under an imposed load. The geolo-these various mechanisms on microfabric development. The gist often refers to this process as compaction. The importancemost popular topics have included study of fissility versus depth of the mechanisms involved in sediment dewatering was recog-of burial (ledberg, 1936: Weller, 1959; White, 1961; Gillott, nized as early as 1908 by Sorby. He observed significant reduc-1969: Rieke and Chilingarian, 1974) and the association of tions in porosity as a function of compaction. As noted above.organic material with the development of preferred particle volume reduction on compaction also is associated with platyorientation in shales (Gipson, 1965, Odom, 1967; O'Brien, particle reorientation. Randomness in the original flocculated1968. 1989). clayey sediment changes to more preferred orientation with

depth. This volume reduction and particle reorientation areimportant early burial diagenesis processes that influence the

Mass Gravity Mechanisms final microfabric of resultant rocks. White (1961). Gipson(1965, 1966). and Gillott (1969) concluded that the high degree

Limited but revealing sediment microfabric signatures that of clay mineral preferred particle orientation was associateddeveloped as a function of post depositional/mechanogravity with fissile shales and that randon orientation of particles wasmechanisms have been observed in electron micrographs. The characteristic of the more massive argillaceous rocks. Scanningdepositional nicrofabric of high porosity sediment character- electron tmicroscopy observations of shales by O'Brien (1968.ized by randomly oriented domains (Figs. 2.20, 2.21) was 1970b) revealed strong preferred particle orientation in a fissileobserved to have responded to overburden stress by mechanical Pennsylvanian black shale (Fig. 2.14A) and random particlerearrangement of particles normal to the direction of stress orientation in nonfissile argillaceous rocks (Fig. 2.11 A).

• 2. Determinants of Clay and Shale Microfabric Signatures: Processes and Mechanisms 25

,, d

/j

fo..V. .-.

Figure 2.22. Microlabric of consolidated sediments from the Mississippi I)eleta Figure 2.23. High TEM magnification of consolidated %edimeni tecmeyed h'mrecovered front 120 in subbotion observed by TEM. Note the highly oriented 150ni subhottorn, Mississippi Delta. Notethe strong degree ot ptclerred patticllclay plates that lie in a direction normal to the direction of the overburden stress. orientation. Scale = I an.Larger particles prevent perfct! alignment of all the platelets. Voids beconle longand linear as compared to surficial sediments (see Figs. 2.21 and 2.22).

The most conclusive evidence to date indicates that fissility is basis of shear strength profiles with depth of burial. typicallyrelated to preferred particle orientation, but the degree of show a significant strength reduction or "cut back" in shearprelerred orientation is not directly related to depth of burial in strength at depths of about 8-14 m subbotton (Doyle et al..most cases. Thus the decrease in porosity with depth of burial 197 1: Bea and Arnold, 1973). These zones are related to subnma-varies considerably among various sediment types. Evidence rine slumping of soft, high porosity. deltaic sediments and thefromi numerous studies of the relation between seditnent and movement of massive amounts of sediment seaward on gentleshale microlabric and compaction has shown the importance of slopes. The microfabric of these smectite-illite-rich highother factors such as geochemical conditions, environments of porosity sediments above and below the "crusts" is chatacteri cddeposition. biological activity, and related geological and by randomly arranged clay particles and donains W [its.

dynamic factors that set the stage and influence the initial for- 2.24-2.26). This type of microlfabric is typical of the unconsoli-mation of inicrofabric in suspension and at the depositional dated surficial submarine sediments of the Mississippi Deltainterface. The initial depositional microfabric and effects ofbio- (Bowles, 1968: Bennett et al., 1977, 1981). Detailed micro-logical activity most certainly influence the response of the par- fabric studies of the "crusts" zones (Figs. 2.27. 2.28) have rev-tides in the postdepositional development of microfabric with ealed a significant difference in the particle arrangements andincreasing depth of burial and when the deposit is subjected to orientations compared to the fabric above and below theexternal environmental stresses (Bennett, 1976. 1977). "crusts.' The microfabric in the "crusts" zones is characterized by

l)ownslope movement and shearing of sedimentary deposits areas of preferred particle alignment but with an overall appear-also have a profound influence on the postdepositional develop- ance of remolding (Bennett et al., 1977: Bohlke and Bennett.tient of microfabric. Massive blocks of sedimentary material 1980). As the sediment shears. remolds. and dewaters. thecan move downslope without significant resuspension of parti- microfabric undergoes particle rearrangement. These micro-cles (McGregor and Bennett, 1981) and, conversely, slumping fabric signatures arc characteristic of remolded sediment thatcan produce massive amounts of resuspended particulates that has been verified by Bennett (1976) and Bennett et al. (1977,can travel many kilometers subaerially or on the sea floor as tur- 1981). Pusch (1970) studied the effects of seditnent shearing onbidity currents (Middleton and Bouna, 1973). microfabric and showed the distortion of links and chains and

The Mississippi Delta. a "natural laboratory" for the study of reorientation of particles in the direction of the principle shearcoastal sedimentary processes, is an excellent example of a stress. In shallow coastal environments, energy fron surfacesedimentary environment experiencing massive downslope waves reach the sea floor and produce orbital motion ofthe sedi-moventei of fine-grained deposits (Coleman and Garrison. ment (Yatnatnoto. 1982). These motions reduce the sediment1977: Coleman and Prior, 1978). "Crust" zones, defined on the strength and when shearing stresses exceed the seditent shear

20 R.H. Bennett, N.R. O*Brien. and N1.1. lulbert

41V/

Figu~re 2.24. SEN) o f %ur ficial sedints; recovered fromt 30 cm suhhottoom above Fiue25.TI (fncrlbcchatrkcofsdmtshetle"iw

Ike to thle sheared characteristic of the "crts. Note similar microfabric as to-epactice othFis. 2.24) Nhort hisghal po=iY aI m id. eiae tceobserved in other subenvironmients of the delta (compare Figs. 2.20 and 2.21).toprilcnatsadshrcai.Sae Itn.Scale = I muu.

.strength the deposit hatils and submarine sliding often occurs of* 70% pore water and relatively reactive, unstable minerals(Dunlap el al.. 1978; Henkel, 1970; Bea, 197 1). derived fromt soil profiles. As well as the clay minerals them-

selves, amorphous cotmpounds of iron. aluminutm. and siliconare important, as is organic matter. Carbonate. reactive silica.

Dit.g,c',,sis-C',t,'nuIuotn Me('cmfismns and more organic matter may be added from depositionalwaters" (Curtis. 1985. pp. 9 1-92).

IDiagcnesis of mineral phascs and cetmentation operate over a Dunoyer De SCgOn17ac (1970) poitied tont that mtineral traits-wide range oif thermal regimes that can alter the character of the formations, neoforniation, and recr> stallt,':tion are itccotpa-fabric. The literature is replete with studies of mineral transfor-mations. mud to shal~e diagenesis. compact ion (consolidation)versus depth of burial. atnd so on. however, meager attention hasbeen given to the relationships of mineral alterations andmicrot~abric. A recenit study by Howatrd (1987) pointed out theimportance ot riicrofatbric in controlling fluid flow properties,difkerences in permeability were shown to hatve a significantintluence on the reaction rates of mineral transformation inshales. Reaction rates clearly depend on complex interacting. . --

processes and] mechanisms that include not only fluid flow - ~properties but also the availability and presence of inorganicchemicatl species in solution. the mineratlogy oif the solid part i- -

cles. and the organic compounds present. The presence ofoirganics in sedlitmentary sequences was shown to be very sig-nit icant in butriatl diagenesis (Johns. 1979). Thle complexity ofhe geochemistry in cemnentat ion and mineral diagenesis was

clay miineral assemblages cannot he attributed to react ions1 m. tong clitys alone and indeed all the available evidence stuggests I'igtirc2.26. S5PM o i icrotabric characteristic of edimecnts belows the shecdit her iineralIs a re itnvo l ved . ('Iast ic sed imtenit sequences include sedillioents of the - rust- 7onc. Note similarit s with thle undisturbed sutiiac

shales. silts, and sands. Shales start life as muds with upward sediments.. Scale -- I fult.

D.Letermninant-, of Clay and Shale Microtabric Signatures-. Processes and Mechanisms 27

Figure 2.27. Nllerof-abric characteristic oif the "crtust" 7oflC of the Mississippi Figure 2.28. High niagni ficat ion oif reniolded Nediment charatcik 1 "nI.

D~elta as observed by TE~M. Note swirled features and locali7ed areas of the "crust- 7onc depicting a localiz~ed area of preferred particle orientation (8 iii

preferred patrt icle or ieliat i on. ('lay platelets orient in tilie forin ofan "onlion" skin subbottoimv) 16.5 111111 = I lull1.

around larger particles. Trhe inijerofabric is typical of'remnolded clay sedimient(lrnett el al- 1977. 1981). Scale = 3 full.

tijed by crystalline growth and changes in particle morphology. are physicochemnical. bioorganic. and burial dtagenests (igs.which translates directly to changes in mnicrol'abric. The complex 2.1, 2.4).tuicrofabric ol "red clays- frorn the Pacific Ocean Basin was rev- Mechanisms; associated with physicochernical processescaled in scanning and transmtssion electron micrographs (Bryant include (I) electrochemical inechanismns fromt the interact ion oftand Bennett, 1988), authigenic mineralization was shown to particles in response to the electrolytic nature of the sutrroundinghave a profound influence on not otnly the clay mnicrofabric fea- mediutm. the chcmnistry of associated organic materials, and thetures but also on the physical and mechanical properties of the "fixed" electrical character ot the specific minerals: (2) thermo-sediment (Fig. 2.29). rhe overconsolidation characteristics of mechanical mechanisms that arise from thermally driven foresthe soft clays were attributed to the strong bonding of argilla- in the water column (Fig. 2.4A and fromt dynamnic interaction (t'Miltls shale clasts and quartz grains by X-ray amorphous and particles driven by wave, current. and gravity forces (Pig. 2. 10):

wvell-developed atithigenic suteclite (Bryant and Bennett. 1988). and (3) interface dynamics mechanisms itn which particle-,Considerably more research is required to develop mecaningful itmpinging on the sediment-water interlace are in dynamiicmticrotabric models and to discern uticrolabric signatures that mot ion and collide with othier particles in 'iut tit contigura-are directly relatcd to diagenctic tmechanismts including authi- tions depending on the mnicrorelief'of tlic bottotin. Flectrochemnii-genic mineralizat ion. tmineral transliorutat ions, solution and cal and thermnorechanical mechanismts produce high intravoiddeposition of tmitnerals and cemtenting agents. (edge-to-face) floes and face-to face sheet like mutll iplate parti-

cle associations. l)onains are cotmmon (Fig. 2.2)1.Mechanisms associated with bioorgattic processes are(I

Sumumary biotttechanical or bioturbat ion, which produces a randoml clayinticrof'abric similar to the primary fabric of flocculated (edge-to-face) clay:. however, often it is characterized by randomly%

1Te miicrofahric signatures resulting frott the various processes oriented individual particles (as resolved by SEM) ratlher thanand ttechattismts are often recorded in the sedimetnts and rock thick domains and/or stepped face-to-face particles. (2) biti-and can be revealed by electron mticroscopy observations. Based physical mnechanismrs produce bioseditttent aggregates, ssltichon ntumterous dletailed studies of sediment and shale micro- are clusters of randonly oriented clay-silt particles formtedstructure and the related environmental factors affecting (he when lithoigertic miatter adhteres to sticky organic itttctts or isdevelopmtent of iticrofabric, three dottinant processes have bound together by polymner bridgitng: attd (3) biochemicalbeen identified and sftildied. The important processes that pro- mechanisms, which cause nticrofabric chainges itn sedittent dueduce inicrofabric developttetnt of clay sedittents and shales to chemtical traitsformiat ions mediated by organisitt.

.29 RI-I. Bennett. N.R. O'Brien. and MJ. H lbert

Figure 2.29. Thc tnicrotahri it autttige tie iin crali a

"k ton in a red clay of the nortltuei pacitic deep-ea hasin(22.5 ni subtoni as observed hr FFK. Note [lie ljts.finely divided crystals ffhai formt a %er; potrous neisorkThe incral "sineciite- appears to hase desciopetl b% expan

sin of the crystal network pressing larger cr~stals til tuneSand sinectite radially outward while reorienting the larger

particles in a direction normal to the direction oif stress

Note hoss the electron-dense particle,; appear to outline thecircunference ofthe authigenic mineral. Sample rccosecdby DSDP Leg 86. Hole 576 twater depth 62 17 to. 1-2fI38N. 1640 16.52'EI in the basin east ot Shatski Rise.

Scl I Mi

Blurial dihigencsis processes involve (I) postdepositional/mecch- enahlcs the investigator to determine or infer the dominanit pro-anogravily mechanisms that arise fromi overburden stress and ccsses and mechanisms responsihle for the observed micro-gravily-drivcn vertical and downslope forces (hat nmdify micro- fabric. Collectively, these mnacro- and mnicroproperlies provide afabric duiring consolidation and shearing. and (2) diagcesis- qualitative and quantitative data base for understanding the geo-cieentation 1iicchainistii5 that alter tie original character of the logical rcotrd. and environments of deposition. These proper-inicrofabric such as the formation of authfigcnic inerals, ties also providc crucial input properties for predictive modeling,

All of file above-tnentioncd tmcchanisms produce mnicrofahric or the static and dynamic behavior of sediments andl rocks Itir asignaltures. Some signatures may he revealed in sediments and wide variety of basic and applied research studies and for solvingrocks by detailed elctron microscopy observations. Recognition practical problems.oif a unique signature coupled with other observed properties The processes atid tmechanisms that determine the microlabric(e.g.. percent organics, porosity, and geochemlistry) and larger signatures operate over various physical and time scales that arescale features such as observed with X-ray radiography thus largely specific to a particular mechanism. For example. the

2. )cternin'nts of ('lay and Shilc Microfabric Signatures: Processes and Mechanisms 29

"able 2. 1. Summary of processes and of the fabric signatures and physical and temporal scales associated with various mechanisrms.

Fabric signatures ScalesProcesses Mechanisms (predominant)* physical time Remarks

l'h,,rcochenical ictrocheinical F-F: Atomic and molecular Ilsec to msec Two particles may rotate F-Tto - 4 in

lhernlonicchanecal F-F (sone F-F) Molecular to nsec to rain Initial contacts E-F then rotation to:O.2 mn F-F: common in selective ens iron-

mentsInterlace (IN nainics 1 F and 1;- F in to - ; 0.5 nit, sec Sonic large compound particles may

be possible at high concentrations

Itroorganic Bioinechanical Ft |. - 0J.5 nn to see to min Sotie F-F possible during bioturbation

>2.0 mmBioph).ical. E-lE and F-F pin to lm see to min Sonic very large clay organic coln-

plexes p',,sibleBiochenmical Nonunique pill to rilm hr to yr New chenicals formed, some altered

(unkno, n)

tirial Mass gravity F-F localiied cm to kin 5yr Can operate over large ph.sical scale%thicn nr,,i, swirl

I )igenries Nonunique molecular 5 yr New minerals formed. some altered.Cemenhltation (unkno. n) changes in morphology

1I. edge-tl o lace -. F -. cdge-to-edge: F-j:. face-to-face.

cnctg regincs it i% ing ticrofabric development can he impor- functional understanding of the relationship of microstructure toLitl ovcr ph\ ,ical scales that can range in siue from atomic the developmental history and the btlk physical and mechanicaldimentsions to as large as kilomctcrs and over time scales that are properties of a deposit.itpo tlatnt in the inicrosccond range or ip to periods of years or The microstructure is a crucial fundamental property thatgreater in the getlogical sense (Thble 2. 1). Examples of these plays a significant role in determining the sediment and rockextretles are found by comparing the electrochemical mechan- physical and mechanical properties and its behavior under staticistn. %,here altomic and tmolecular dimensitons are important over and dynamic loads. Many of the rock and sediment properties.micro- and milliseconds during nicrofabric development, with important to a variety of scientific and technical disciplines. arethe mass gravity nechanism, that occurs over dimensions of intimately tied to the rnicrofabric characteristics (Bennett et al.,centimeters and kilometers for periods of thousands of years. 1977, 1989). Delineation of the complex interrelationships

I-xamination of Table 2. I reveals that although the various among microfabric. processes and mechanisms is significant inniechanisnis produce predominant fabric signatures. similar terms of (I) understanding the role of specific energy regimesparlicle-to-particle associations are common to different pro- during the stages of tnicrofabric development and sediment dia-cesscs (conipare physicochenlical processes with bioorganic genesis and (2) development of predictive tuodel, (acoustical.processes). Obviously more than a single criterion is usually mechanical, geological) that depend critically on the nature ofrequired to interpret the processes of microfabric development, the nicrostructure and basic physical and mechanical proper-Such informtion may include percent organic carbon. intersti- ties. Future investigations of the subtle difference in microfabricial water chetiistry. porosity and/or permeability of the geolog- and its relationship to environmental proccsses and mechan-

ical material, position in the stratigraphic column, and associa- isms, including detailed geochemical factors (organic and inor-lion with other parliculates. ganic) and large and small scale features. could add imptortant

An imiprtant factor in understanding the microfabric of sedi- dimensions to scientific and technical kno%%ledge and the ulti-ments and rocks is the aspect of observational scale. This factor mate applications of microstructure to practical problems.can range front large-scale field observations, through the scaleof X-ray radiography. to the scale of angstronis revealed by tech- Acknowledgments

niques of electron microscopy. Often nany levels of observationare required to develop an adequate understanding of the partic- The author- appreciate the assistance of Ms. FL. Nastav in theular problems(s) being addressed whether it be in disciplines of preparation of the figures and in proofing of the manuscript. Thegeology, environmental ctigineering. geotechnical engineering, first author was supported by NORDA under Program Elementpetroleum exploration, or th~e recovery of hydrocarbons. Under- Number 61153N. The authors gratefully acknowledge the criticalstanding of the processes and mechanisms responsible for discussions with Drs. Huon Li and N. Currier regarding many of

microfabric development is an important factor in gaining a the ideas and concepts. The second author appreciates the support

11.) R.H. Bennett. N.R. O'Brien. and NIH. Hulbert

front donors of the Petroleum Research Fund, administered by the Bryant, W.R.. and R.H Bennett. 1988. Otiyin. physical. and mineralogicalAmerican Chemnical Society, and fromt the National Science Foun- nature of red clays, the Pacific Ocean basin a% a model. Gco-Marine Letters.dat ion Grant EAR-861 1608 Im funding. Ms. P.1. Burkett provided Special Issue, v. 8, p. 189-249.

the EM poloiicrgrap of he lay ampl frm th Shaski Burs(. .F. 1976. Argillaceous sediment dc~katering, Annual review (if Earththe EM holmicrgrah o th cla saplefromtheShaski and Planetary Science. v. 4. p. 293-318.

Rise. Field support. samples, and lahoratory support from Drs. Casagrande. A.. 1932. The structure of cla% and its imipotance in foundationMichael House. Philip Ileckel, John Gray. Richard Young, Wil- engineering. Contributions to Soil Mcehanits. Bost1on Socitsof is 0it11am Kirchgasser. Egotn Matijevic. James Syvitski, Mrs. Flona Engineers. (1940) p. 72. and Journal of Boston Society of Civil Fniinccrs.%Cole, Roy Madison. and (lhe Iowa Geological Survey are greatly I5 (April 1932).

apprecated.Coleman, J.M.. and L.E. Garrison. 1977. Geological aspects olt marine slpcapprecatedstability northwestern Gulf of Mexico. Marine Geotechnolog). %. 2. p. 9) 44

Coleman. J.M.. and D.B. Prior. 1978. Submarine landslides in the Mississ.ippiRiver Delta. Offshore Technology Conference. OTC paper 3170. p

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