Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks

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PHOSPHORITES

525

envirottmental probletns (see Jarvis

etttf.

1994), among which

three are especially significant:

(1) Mining ami processing: inost phosphorites are extracted

from open-pit trtines with their attendant problems of

landscape disruption and disposal of mine tailings. In

addition, beneficiation processes commonly produce phos-

phatic "waste clays"' which are extremely fine-grained and

have poor settling properties, hence may need decades to

settle and thus require numerous settling ponds covering

large areas,

(2) Pluisphogvpsiit}i\ phosphogypsum is a by-product of the

conversion, using sulfuric acid, of phosphorite ore to

phosphoric acid, an intertiiediale compoutid in the

manufacture of phosphatie fertilizers. The amount of

phosphogypsum produced is substantial. Jarvis ct al.

(1994) estimate worldwide annual production on the order

of lOOmetricMt. Although this form of gypsum has

commercial applications in many parts of the world,

governtnental regulatiotts in the USA prevent its usage, in

part due to an association with radionuclides (apparently

in a separate phase) in the - ' U decay chain, especially

radon. Stockpiling of this material is thus necessary.

leading in turn to potential problems of air and water

contatnination in the vicinity of the stockpiles,

(3) Trace eletnctii.s: many phosphorites contain small but

signilicant amounts of potentially toxic elements (e,g,.

cadtniutn. selenium, arsenic) or radionuclides, particularly

iiianium. The possibility thus exists that harmi'u! amounts

of these elements could be released into the environment

during the processing of phosphorite ore or the applicaiion

of fertilii^ers produeed from phosphorites.

Summary

Phosphorites are unique sedimentary deposits that have

piqued the interest of academicians and economic geologists

for decades. Due to their high phosphorus content, they are

the world's tnost important economic resource of elemental

phosphorus and thus are an essential additive for fertilizers

and phosphate-based chemicals. l"hc most common phos-

phitte-bearing mineral in ihc majority of phosphorites punc-

tuating the geologic record is carbonate tluorapatite (CKA).

which has an abbreviated chemical formula of formula

Cani[(PO4)(, ^(COOxlFi^ ^. but actually also contains numer-

ous substitutions of both cations and anions. Field obsetva-

tions plus isotopic and pore water studies of tnodern marine

phosphorite occurrences demonstrate that most form initially

as "pristine,"" nonreworked deposits during early diagenesis

within the upper few tens of centimeters beneath the seafloor.

Most occtir beneath strong upwelling systems which provide a

high delivery rale of phosphorus-bearing seditnentary organic

matter to the sealloor: this material is subsequently broken

down by microbial degradation processes in pore waters.

Many are then in turn "upgraded"" to what are tertned

"condensed" or "allochthonous"" phosphorites by processes

such as sediment winnowing and reworking, largely by bottotn

currents. Most ancient exatnples of these deposits thtts usttally

occur as stratified grantilar phojiphorites containing admi.x-

tures of silt to very large sand-si/ed phosphatic grains, the

most common cements of whieh are silica or carbonate

minerals. Concretionary and hardground phosphorites also

oeeur in both tnodern and ancient deposits. Additionally, there

is evidence that phosphorus tnay also be delivered to the sites

of phosphogenesis by a redox cycle whereby phosphorus

sorbed to iron oxyhydroxides is released as soluble reactive P

in suboxic pore waters and thus becomes available for solid

phase fixation into the CFA compound. Very large phosphor-

ite deposits, termed "phosphorite giants." occur episodically

throughout the Phanerozoic ditring discrete time periods.

Most were deposited on oceanic slopes and shelves and in

epeiric seaways. In addition, phosphatic sediments are also

found on oceanic islands and atop seamounts, guyots and

submarine plateaus. Present consensus holds that past varia-

tions in the phosphorus cycle and phosphorite output of the

oceatis were responses to a variety of lactors including sea-

level fluctuations, changes in rates of continental chemica!

weathering, shifts in the paleolatitudes of continental margins

and in positions of coastal upwelling zoties, and in patterns of

oceanic circulation,

Craig R, Glenn and Robert E, Garrison

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Cross-references

Authigenesis

Diagenesis

Oceanic Sediments

Upwelling

'HYS!CS OF SEDIMENT TRANSPORT: THE CONTRIBUTIONS OE R. A. RAGNOLD 527

PHYSICS OF SEDIMENT TRANSPORT: THE

CONTRIBUTIONS OF R. A. BAGNOLD

Dtiring the twentieth century a huge atiiotint of time, money

and research effort was expended in attempting to develop

accurate and reliable models of sediment transport by wind

and water. Amongst Ihe numerous significant contributions

that resulted, thai of R. A. Bagnold is characterized

particularly by his appreciation of the pivotal importance

and great usefulness of fundamental physics in clescribinL' and

detining sediment transport mechanisms and processes.

It was as an officer in ihe British Army, stationed in Egypt

and India between 1^26 and 1932. that Bagnold's fascination

widi sediment movement hegan. Bagnuld and a small group of

like-minded ofticers began exploring the Libyan Sand Sea

using Model T Ford motor cars. Early tbrays quiekly evolved

into serious geographical exploration and the group were

responsible for mapping an area of desert the size of hidia,

work that earned Bagnold the Gold Medal of the Royal

Geographical Society in 1934.

During these expeditions Bagnold became fascinated by the

movement of sand by wind and he wished to understand the

physics of the processes responsible tbr the tbrmation of desert

dunes,

Bagnold was astonished and appalled to find that no

theory of dune Ibrmation existed and that no serious scienlitic

investigation of the movement of sand by wind had at that

lime been performed. He decided that, despite having no

formal training in research and no higher degree, he would

retire trom the army and devote all of his time and energy to

the study of wind blown sands.

After making preliminary measurements of the physical

|iropertics of desert sands and the near-boundary wind speeds.

Bagnuld btiilt himself a wind tunnel wilh glass sides, tbr easy

observation and photography, within which to perform

controlled experiments. He used high-speed cinematic photo-

graphy (then in its infancy) to observe and iraee the

entrainment of individual sand grains. His results showed thai

detaching grains left their pockets at very steep angles, proving

that lift tbrces were of equal importance to drag tbrces in

detaching grains. His movie-films demonstrated that entrained

sand grains move along ballistic trajectories, unaffected b>

turbtilenee a motion he described as "a series of bounds",

and that moving grains dislodge further grains when they

botince—a proeess later described as saltation. Bagnold

compared the hopping distance of saltating grains to the crest

to crest spacing of ripples formed in Ihe sand surface,

concluding that,

"The length of ihe characteristic path from impact lo impact

was tbund in all experiments to be the same as the measured

wavelengths of the ripples Ibrmed on the surfaee" (Bagnold.

1936,

p.23).

These early experiments turther established that, not only

does the rate of sand transport vary with the cube of the wind

speed, but also the presence of moving sand itself generates

drag that atTects the velocity protile in a transport-limiting

mechanism (Bagnold, 1936),

Having obtained controlled data elucidating the transport

process, Bagnold returned to the field to check his results

against reality. After spending eight days alone in the desert

waiting for a sandstorm, he obtained reliable measurements

that contirmed the wind-tunnel results. He was also able to

apply his understanding of the saltation process to explain why

sand moving over a rocky plain becomes size sorted and tends

to collect into patches. Patches occur because grains moving

over a bare roek surfaee bounce very efficiently, maintaining

motion, while the grain grain collisions ofa grain moving over

a sand surfaee sap energy from the moving grain, tending to

damp transport so that sand accumulates in a patch.

The complete results ol' his research on sand transport by

wind during the 1930s were published in The Pliwiic; of

Blown SutuI atid Desert Datu's (Bagnold. 1941). a book ihat

has been reprinted many times and which remains a stand-

ard text.

In 1937. Bagnold was persuaded to perform experiments on

the shock pressures generated by waves breaking against a sea

wall (Bagnold, 1939. 1940), but characteristically, he took the

opportunity to extend this work to the inter-action of waves in

shallow water with a mobile sand bed. This line of research

was interrupted by World War IL but resumed and published

immediately afterward (Bagnold, 1946, 1947). From his wave-

tank experiments. Bagnold was able to explain the tbrmation

of rolling-grain ripples under oscillatory tlow. their evolution

into vorte.\ ripples and the occurrence of '"brick pattern"

ripples under certain conditions. He further related ripple

tbrmation to increased drag on the moving water and

produced a simple e.xpression tbr ihe tirst disturbance of a

planar sand bed by oscillatory waler motion.

Recalled to the army at the outbreak of World War M,

Bagnold returned to Cairo and used the results of his desert

exploration and mapping expeditions to persuade General

Wavell that a real threat existed of an Italian raid on ihe

Aswan dam from a base deep within the Libyan Desert.

Commissioned to establish the Long Range Desert Group

(LRDG). identity any threat and nullify it. Bagnold then

pertbrmed what may be the quintessential example of

applieation of sedimentology to wartare. He used his knowl-

edge of the physics of wind blown sands and the morphology

of desert dunes to take LRDG vehicles deep behind enemy

lines,

scouting and reporting troop movements, mapping

enemy positions, collecting intelligence and creating havoc

through lightening raids on tbrts. convoys and airtields. When

enemy tbrces attempted lo tbllow and track down the LRDG

they quickly became stuck in the soft sand of dune troughs

(Bagnold knew that you must drive over the "accretion

deposits'" of the dune crest) or got hopelessly lost in the maze

of sand dunes. Without Bagnold's grasp of sedimentology and

geomorphology. pursuit was impossible. The formation and

leadership of the LRDG uniquely combined (iagnold's talents

as a scientist and soldier and must rank as one of his most

noteworthy achievements. A contemporary account of the

exploits of the LRDG may be tbund in Bagnold (1945) while

tbr a longer term perspective readers should consult his

autobiography (Bagnold. 1990).

Bagnold again retired from Ihe army in 1944 and, afler a

period as Director of Researeh for Shell, during which he was

instrumental in the establishment of the Hydraulics Research

Station at Wallingtbrd. England, he again decided to pursue

fundamenlal research full time.

Bagnold's post-war work focused on the movement of

sediment by water. His approach rested on the application of

theory to identify key variables and tbrmulate relationships

between them, supported by carefully conceived and e.xecuted

experiments to quantit~y physical constants and material

528

PHYSICS OF SFDIMENT TRANSPORT: THE CONTRIBUTIONS OF R, A, BAGNOI

properties. The conception and construction ol" experimental

apparatus and instrumentation continued lo be incidental to

his work, but stemmed from a Bagnold family tradition that,

"a properly equipped workbench was as important to study as

was a well stocked library".

He began by conceiving a series of fundamental experitiients

on the physics of sediment movement conducted in a rotating-

drum tesl rig he himself designed, to quantify the pliysics of the

gravity-free dispersion of large, solid spheres in a Newtonian

fluid (Bagnold. 1954). He demonstrated thai collisions between

suspended solids generate a "dispersive pressure" on the

boundary resisting further dispersion. This expcritnent estab-

lished Ibr the first titne that the eoeftlcient of solid-to-solid

friction for sheared dispersion of granular solids is virtually the

satne as for the same solids in continuous contact a piece of

knowledge that lies at the heart of practically every seditnent

transport equation in comtnon usage today. A fut"ther

discovery of some portent was siitnmed up eloquently by

Bagnold himself in his autobiography,

"The rate of tnass transport of an immersed weight of solids

is easily measured, in the laboratory at least. And the fluid

shear stress titnes the velocity of its action is the rate at which

the tluid does transporting work. So

now saw elearly that the

transport rate of solids tnusl depend pritnarily on a rate ol'

fluid energy supply, or. in other words on sireatnpower and not

as previously thought, on either streatn velocity or bed shear

stress,

but the product of the two. This, I felt, began to put the

subject of solids transport by water flows on a sound footing."

(Bagnold, 1990, pp. 163-164), In 1956 Bagnold was ap-

proached by Luna B, Leopold, then Head of Ihe Water

Resources Division of the United States Geological Survey to

•"help stir the pool of cotnplacent tradition with the stiek of

reality" eoneerning sediment Iransport, The flrst area of eo-

operative research concerned river tneandering (Leopold, etal,.

1960;

Bagnold, 1960), Bagnold and his co-workers ehallenged

the ""square-law" then generally accepted as governing flow-

resistance in alluvial rivers. The "square law" argued that flow-

resistance should increase with the square of the tnean velocity

because this was known to be the ease for sJtnple boundary

layer

flow-

past a flat plate, provided that boundary conditions

remain unchanged. Their results showed that, while the square

law holds for straight channels, defortnation of the water

surface due to transverse defleetlons of all or part of the flow in

meandering rivers could lead to a doubling of the rate at which

resistanee increases with the square of the veloeity (Leopold

eiuL,

1960), Further theoretical analysis, coupled with care-

fully designed experitncnts. demonstrated that energy losses at

bends were minimized for ratios of bend radius to width

between 2 and 3, A great deal of subsequent empirical reseat"ch

has dctnonstrated that natural tneanders do. in fact, tend to

evolve to a bend ratio to width ratios in this range (Hickin and

Nanson. 1984),

Another Anglo-American researeh initiative, with Douglas

lnman of Seripps Oeeanographie Institute, led to Bagnold's

main contribution to the understanding of beach and near-

shore sedimentation. Applieation of knowledge gained

through earlier studies of wave mechanics, the transport of

grains by fluids, the movetnent of sand by waves and the

concept of stream power was etnployed to construct a

comprehensive e,xplanation of on-shore and long-shore sedi-

tnent dynamies in the littoral zone (lnman and Bagnold, 1963),

Embedded within this treatise were a power-based sediment

transport equation for waves, a theoretical explanation of the

mechanics of auto-suspension in off-shore turbidity currents, a

theoretieally-based explanation of the equilibrium beaeh-

profile, and a power-based mode! for the transport of sand

by long-shore eurrents.

It was in the realm of sediment transport by rivers that

Bagnold's final and possibly most lasting eontribution was

made. Adopting an approach to the sedimetit transport

probletn based in general physics, he was able to attribute

the suspension of granular tnaterial in a fluid shear flow to the

effect of anisotropic turbulence creating an upward supporting

stress (Bagnotd. 1966), He went on to differentiate deflnitively

between suspended and bed-load components of the total load,

recognizing that the itnmersed weight of the suspended load is

supported by turbulence, while the immersed weight of bed-

load is supported by (intermittent) solid solid contact with the

bed. This explained why bed-load can therefore occur even

under laminar flow, in the cotnplcte absence of turbulence

while suspension cannot (Bagnold. 1973),

Discovery of the difference between the physics of

suspended and bed loads allowed Bagnold to derive sediment

transport equations for eaeh process from a futidatnentally

sound basis (Bagnold. 1973; 1980). It is perhaps his bed-load

equation which has proven the most useful in practical

applications and this equation remains in widespread use.

espeeially in the UK,. His use of streatn power has also been

replieated in several of the most popular sediment transport

equations used in North America (see. e.g.. Yang. 1976 and

Chang, 1988),

Advances in fluid mechanics, turbulenee theory and the

sophistication of instrutnetitation in the late twentieth have

supported new flndings thai, to varying degrees replaeed some

of Bagnold's interpretatiotis. which were neeessary based on

incomplete knowledge of the nature of turbulent How

structures. However, his contribution to sedimenlology lies

not only in the original knowledge and useful sediment

transport equations that he crafted—both certain to be

superseded with ihe passage of time, but also in his exemplary

approach to conducting basic researeh in this difficult and

oflcn frustrating field of enquiry. In sum. his contributions to

science and engineering must place him in the flrst rank of

twetitieth century researchers in sedimentology and seditnent

transport.

Colin R, Thorne

Bibliography

Bagnold, R,A,. i936, Thi; movL'mcnt of desert sand. Geograpliicat

Journal. 85: 342-369.

Bagnold. R,A,, 193'J, Cotnmittee on wave pressures: inierim report on

v\;ivt'-prcssiire research, Jtnirnal of the Institution

Civil

Engineers. 12:

:tl

1-226,

Bagnold. R.A.. 194U, Beach lormiitlon by waves: some tnodel

experiments In a wave tank, Journat of

the

Institution Civil Engi-

neers, piipcr ntimber 52.37. 27 53,

Bagnold, R,,A,. 1941, The Phvsies of BUmn Sands and Desert Dunes,

William Morrow and Company in 1971. Republislied by Chapman

and Mall. Melhiien Haisted Press, ISBN X-76-()5()S7.VK,

Bajjnold. R,A,. 1S)45, Early days of the Long Range Desert Group,

GeographiealJournal. 105: 30 46.

Bagnold. R,A,. 1946, Motion of waves in shallow water. Inieraclion

between waves inul sand bottoms, ProeeedingsoftheRovalSoeietyoJ

London. Series A. 187: 1-18,

Biignold. R.A,, 1947, Sand movement by waves: some small-scale

c,\perimetils with sand of very low density. Journal of the Institution

Civil Engineers, paper number 5554. 27: 447 469.

PILLAR STRUCTURF

529

Bagnold. R,A,, 1954, F.xperiments on the gravny-lVee dispersion ol'

largo solid spheres in a Newtonian litiid under shear, Proeeedingsof

the Royat Soeiety of London, Series A. A225: 49 63,

Bagnold, R,A,. 1960, Some aspects of the shape of river meimders,

L'S

Geotogieat

Survey Professionat

Paper

2S2-E. 135- 144,

Biignold. R,A.. 1966, An approaeh to the sediment transport problem

from generai physics, USGeohigicat Survey

Professionat Paper

422-1.

1-37.

Bagnold. R,A,. 1973, The natureof saltation and of bed-load transport

in water, Proeeedingsof the Royai Society of London. Series A. 332:

number 1591.473 .MU,

Bagnold. R,A,. 1980, ,^n empirical correlation of bedload transport

rates in flumes and natural rivers, Proceeding.softhe RoyalSocietyof

London. Series A, 372 paper number 1751, 453 473,

Bagnold. R,A.,

199(1,

Sand,

Wind and

War:

Memoirsofa

De.serl

Explorer.

University ol' Arizona Press,

Chang. H.H.. 1988, Pluvial Processes in River Engineering. Wiley

Interscienee,

Hickin. E.J,. and Nanson. Ci,. 1984, Liiteral migration rates of river

bends.

Journal Hydivulic Engineering. 110: 1557-1567.

lnman. D,L,. and Bagnold. R,A,. 1963. Beach and nearshore

processes Pan 2, Lilioral processes. In Hill. M.N. (ed,). The

Sea— Ideas atid Observations on Progress in the Study of the Sea,s,

Volume3—The Earih Betieath the Sen. Interscience Wiley: pp. 529-

553.

Leopold. L,B.. Bagnold. R,A.. Brush. L,M, Jr. and Woman. M,G,.

1960,

Flow resistance in sinuous or irregular channels. V,S, Geohi-

gieal Survev Profes,sional Paper 2f<2-D.

111

-134.

Keiin, M.J,. 1988. Biography, In Thorne. C,R,. MacArthur. R,C,, and

Bradley. .I,B, (eds,). The

Physies

of Sediment

Tran,sport:

A Collection

of Hallmark Papers by R.A. Bagnotd, American Society of Civil

Engineers. Book Number 665. xiii ,Kvii,

Yang. C,T,. 1976, Minimum unit stream power and liuvial hydraulics.

Proceeditig.s

of the American Society of Civil

Engineers.

Jinirnatofthe

HvdrauliesDivision. 102: 919-934.

Cross-references

Lolian '^I-;m^port and Deposition

Sediment Transport by Lnidirectional Waier Flows

Sedimeni Transport by Waves

Sedimentology. History

Sedimentologists

Figure P9 Small subvcrticdl pillar structures (light-colored streaks) in

crudely Idyert-d muddy sandstone deijosited by a sediment graviiy

tlow. Pillars mark sheet-like zones from which darker, argillaceous and

organic grains have been removed by water escape. Lower Cretaceous

Britannia Formation, North Sea. Scale on left in tenths of a fool

I3cni unilsl.

PILLAR STRUCTURE

Vertical water-escape channels, variously termed pillar

structures, clutriation pipes, clutriation columns, fluidization

pipes.

Huidization channels, tube dewatering structures, and

water-escape sheets, are elongated vertical to subvertieal zones

of seditnent that are usually lighter-eolored and eotitain less

elay. tnica, organie detrittts. and other hydraulically tnobilc

grains than surrounding sediments (Figure P9). They form

thtough localized fluidization of sediment along discrete

subvertieal zones and accompanying elutriation of fine-grained

components during either post- or syndepositional water

escape. They arc frequently assoeiated with dish slructures

(see DishStrtHittre. ligure D26).

The lertn "pillar structure" was introduced by Wentworth

(1966) frotn studies of turbidites in western California, He

interpreted the "pillars'" as vertical fluid-escape channels.

Similar structures had been previously described in fluvial

deposits of the Old Red Sandstone by Allen (1961), Vertical

water-escape channels have also been deseribed from

volcanielaslic mud flows (Best. 1989). Lowe and LoPieeolo

(1974) discussed pillar structures associated with dish struc-

tures,

and Lowe (1975) provided a broader classilication and

discussion of vertical fluid-escape channels. The formalion of

these struetures has been studied experimentally by Nichols

elal. (1994) and Druitt (1995).

Vertical water-escape channels oeeur in sediments ranging

from coarse silt to pebbly sand or. more rarely, flne gravel,

usually in diserete beds at least a few decimeters thick.

Although they were initially thought to have pillar- or eolutnn-

like geometries, based largely on two-dimensional exposures,

most vertical water-escape channels are isolated to anastomos-

ing subvertieal sheets. The widths of individual sheets or pillars

range from about a millimeter to a few centimeters with

heights ranging frotn a few milhmelers to more than a meter.

When scdimem is moved over signiflcant distances from one

bed to olhers, ihe tertns "seditnenl intrusion" or "dike" should

be used, Subvertieal sediment intrusions can be up lo several

tens of n-ieters high, and iheir fbrtnatioi-i is related both to

water escape and density instabilities. In tnost cases, however,

water-eseape through vertieal ehannels or conduits involves

only loeal fluidization and restricted sedimeni movement.

530

PLACERS, FLUVIAL

Sediment in water-escape channels (pillar structures) is usually

homogenized and the margins of the channels may cut

sharply across primary sedimentary structures in surrounding

sediments.

Localized fluidization can take place during either rapid

sedimentation or post-depositional dewatering, Druitt (1995)

investigated experimentally the formation of "elutriation

pipes"

during sedimentation of poorly sorted aqueous disper-

sions.

He observed that at concentrations of 25-55 percent

solids vertical water-escape channels propagated upward at the

same rate as the depositional surface, A strong upflow of

escaping fluid above each escape channel depleted the settling

dispersion in fines and resulted in the upward propagation of

the stable water-escape channel. At higher concentrations the

water-escape channels nucleated throughout the dense disper-

sion. This behavior resembles the aggregative behavior and

self-organized segregation of particles in sedimenting bidis-

perse systems that lead to the formation of streaming columns

and blobs (Weiland etal,, 1984; Batchelor and Van Rensburg,

1986),

Mud-rich deep-water sediment gravity flow deposits called

slurry beds (Lowe and Guy, 2000) often contain a variety of

vertical water-escape channels that appear to develop during

the dewatering of sediments possessing strength. Some

channels are a few centimeters long and develop in sets with

individual pillars spaced 1-2 cm apart ("stress pillars" of

Lowe, 1975), They might have formed through a proeess

similar to conveetive streaming (Weiland etal., 1984), Others

occur as sets of small columns, 1-2 mm wide and 1-25 cm long

(Figure P9), The wide range of rheological properties, particle

densities, and hydraulic sizes in such sediments favors

formation of a wide variety of water-escape structures,

Zoltan Sylvester and Donald R, Lowe

Bibliography

Allen, J,R,L,, 1961, Sandstone-plugged pipes in the Lower Old Red

sandstone of Shophire, England, Journat of Sedimentary Petrotogy,

31:

325-335,

Batchelor, G,K,, and Janse Van Rensburg, R,W,, 1986, Structure

formation in bidispersc sedimentation. Journal of Etuid Meelianics,

166:

379-407,

Best, J,L,, t989, Fluidization pipes in volcaniclastic mass flows, Volcan

hudson, Southern Chile,

Terra

Nova,

1: 203-208,

Druitt,

T,t^,,

1995, Settling behavior of concentrated dispersions and

some volcanological applications, Journat of Voteanotogy and

Geottiermat Research, 65: 27-39,

Lowe, D,R,, and LoPiccolo, R,D,, 1974, The characteristics and

origins of dish and pillar structures, Journat of Sedimentary

Petrotogy, 44:

484-501,

Lowe, D,R,, 1975, Water-escape structures in coarse-grained sedi-

ments, Sedimentotogy, 22: 157-204,

Lowe, D,R,, and Guy, M,, 2000, Slurry-flow deposits in the Britannia

Formation (Lower Cretaceous), North Sea: a new perspective on

the turbidity current and the debris flow problem, Sedimentology,

47:

31-70,

Nichols, R,J,, Sparks, R,S,J,, and Wilson, C,J,N,, 1994, Experimental

studies of the fluidization of layered sediments and the formation

of fluid escape structures, Sedimentotogy, 41: 233-253,

Wentworth, CM, Jr,, 1966, The Upper Cretaceous and Lower Tertiary

Roctis of ttie Guatata Area, Northern Coast Ranges. Catifornia,

Unpublished Ph,D, Thesis, Stanford University,

Weiland, R,H,, Fessas, Y,P,, and Ramarao, B,V,, 1984, On

instabilities arising during sedimentation of two-component

mixtures of solids, Journat of Etuid Mechanics, 142: 383-389,

Cross-references

Dish Structure

Fluid Escape Structures

Gravity-Driven Mass Flows

Liquefaction and Fluidization

Turbidites

PLACERS, FLUVIAL

Fluvial placer deposits form when mechanical sorting causes

concentration of heavy minerals of economic interest. The

heavy minerals must be chemically stable to survive so placers

usually consist of native metals such as gold (SG 15-19) and

the platinum group (14-21); resistate minerals, for example,

eassiterite (~7), chromite (4-5), ilmenite (4,5-5), rutile (4,2),

magnetite (5,2), zircon (4,7); and, diamonds (3,5) and other

gemstones. Placer deposits can be of considerable economic

importance—for example, the tin (eassiterite) deposits of

South East Asia (Taylor, 1986) and the gold paleoplaeers of

the Witwatersrand (Smith and Minter, 1980),

Small accumulations of heavy minerals can form as planar

laminae (Ljunggren and Sundborg, 1968); thin ripples on the

upstream slopes of dunes (Brady and Jobson, 1973); or

develop on topsets and foresets of dunes (Brady and Jobson,

1973),

At intermediate scales heavy minerals accumulate at

bar heads (Hanson, 1979; Day and Fletcher, 1991); in zones

of flow separation in the wake of obstacles and at channel

confluences (Best and Brayshaw, 1985); or where a channel

narrows and flow is constricted (Smith and Beukes, 1983;

Paopongsawan and Fletcher, 1993),

Cumulative concentration of heavy minerals at small to

intermediate scales can upgrade average concentrations in

fluvial sediments so they become greater than in their source

materials and continue to increase as drainage basin size

increases (Fletcher and Muda, 1999; Halfpenny and

Mazzueehelli, 1999), Weathering in the humid tropics is

probably especially favorable to this insofar as large volumes

of bedrock are converted to fine grained weathering products

that are easily removed in suspension to leave a coarse bed-

load enriched in resistate heavy minerals (Sirinawin et at,,

1987),

Provided sediment continues to be preferentially

transported, changes in stream gradient (Toh, 1978; Day and

Fletcher, 1991; Hou and Fletcher, 1996) are often sites of

accumulation of heavy minerals within fluvial systems. Exits of

rivers from highlands onto plains can be particularly favorable

for placers if the upstream basin contains an extensive source

of heavy minerals (Toh, 1978),

Placers form because the density of heavy minerals causes

them to lag behind similarly sized grains of light minerals

during entrainment, suspension and transport of

grains,

and to

settle, by dispersive shearing, within a moving bed (Slingerland,

1984;

Reid and Frostiek, 1985; Slingerland and Smith, 1986;

Fletcher and Loh, 1997), However, grain size of the heavy

mineral, and the roughness and nature of the streambed voids

are also important factors in their accumulation. For example,

eobble-gravels at the heads of point bars have greater

accumulations of coarse heavies than sands at bar tails but

the difference in concentration between these sites decreases

PLACERS, FLUVIAL

531

with decreasing grain size of the heavy mineral (Saxby and

Fletcher, 1986; Fletcher and Day, 1989), At larger scales

coarse heavy minerals form placers closest to the source

whereas finer heavy minerals tend to accumulate further

downstream (Toh, 1978),

The association of fine heavies with coarser sediments leads

to the concepts of hydraulie (or settling) equivalenee, entrain-

tnent equivalenee, and transport equivatenee, Hydrautie equiva-

lence, introduced by Rubey (1933), describes particles of

different densities and size that settle through a column of fluid

at the same velocity, whereas entrainmentequivaletwe considers

grains of light and heavy minerals that are entrained together

at the same critical shear stress Slingerland (1977), Transport

equivalence, as defined by Fletcher etal, (1992), refers to those

grains that, despite differences in their physical properties,

have the same net average transport rate.

Sorting, transport and grain size equivalence of mineral

grains can only be described quantitatively by theories of grain

settling, pivoting and entrainment under idealized conditions

(see Slingerland, 1984; Slingerland and Smith, 1986; Fletcher

and Loh, 1997), Transport of grains of different sizes and

densities by unsteady, nonuniform turbulent flows acting on

non-uniform streambeds cannot, as yet, be predicted quantita-

tively. Field observations show that fluvial processes concen-

trate heavy mineral grains that are small enough to hide in the

bed (di/Dm <0,2, where d\ and Dm are grain diameter and the

median diameter of the bed, respectively). For grain sizes finer

than about 100|im concentration of heavy minerals in bed-

load is increased by transfer of fine sand, silt and clay to

suspended load at the early stages of sediment transport

(Fletcher and Loh, 1997),

Preferential entrainment of light minerals grains becomes

less effective as a concentration process as the size of the

heavies approaches the upper limit of hiding (di/Dm~0,2) and

their exposure to the flow and the probability of their

entrainment increases. However, as flows increase scouring

of the bed releases buried heavy minerals and a wide range of

grain sizes and densities are mobilized. As discharge falls, and

the bed is re-established, coarse heavy minerals are among the

first to be trapped at favorable sites such as the voids at bar

heads. At the same time fine heavies travelling higher in the

flow partly overpass trap sites and develop less spatial

variation in concentration on the stream bed (Fletcher and

Wolcott, 1991),

Improved understanding of fluvial placer deposits, and

hence development of improved exploration guidelines for

economic placers, requires more detailed descriptions of their

characteristics, at grain to system scales, and the use of these

observations to test and refine theories of sediment transport

and grain sorting in fluvial channels,

W,K, Fletcher

Bibliography

Best,

J,L,, and

Brayshaw,

A,C,, 1985,

Flow separation—a physical

process

for the

concentration

heavy minerals within fluvial

channels, Quarterty Journal

Geologicat

Soeiety

London,

142:

747-755,

Brady, L,L,,

and

Jobson,

H,E,,

1973,

experimental study

heavy

mineral segregation under alluvial-flow conditions, US

Geologicat

Survey Professionat

Paper

562-K,

38pp,

Day,

S,J,, and

Fletcher,

W,K,, 1991,

Concentrations

magnetite

and gold

at bar and

reach scales

in a

gravel-bed stream,

British Columbia, Canada, Journat

Sedimentarv Petrotogy,

61:

871-882,

Fletcher, W,K,,

and

Muda,

J,,

1999, Influence

selective logging

and

sedimentological processes

geochemistry

tropical rainforest

streams, Journatof Geochemieat Exptoration,

67:

211-222,

Fletcher,

W,K,, and Loh, CH,, 1997,

Transport

and

deposition

eassiterite

by a

Malaysian stream, Journatof Sedimentary Research,

67:

763-765,

Fletcher, W,K,, Church,

M,, and

Wolcott,

J,,

1992, Fluvial transport

equivalence

heavy minerals

in the

sand size range, Canadian

Journatof Earth Sciences,

29:

2017-2021,

Fletcher,

W,K,, and

Wolcott,

J,,

1991, Transport

magnetite

and

gold

Harris Creek, British Columbia;

and

implications

for

exploration. Journal of

Geoctiemieat

E.\ptoratton,

41: 253-274,

Fletcher, W,1C,,

and Day, S,J,, 1989,

Behaviour

gold

and

other

heavy minerals

drainage sediments: some implications

for

exploration geochemical surveys. Institution

Mining

and

Metat-

turgyTransactions,

98:

B130-B136,

Halfpenny,

R,, and

Mazzueehelli, R,H,, 1999, Regional multi-element

drainage geochemistry

in the

Himalayan Mountains, northern

Pakistan, Journat of

Geoctiemieat

Exploration, 57: 223-234,

Hanson,

C,G,, 1979,

Geoehemieat

and

Mineratogicat Investigations

Mettwdsfor Detecting Kimbertites

ttie Area ofStoctidate Kimbertite,

Ritey County, Kan.ias, Unpub,

M,S,

Thesis, Pennsylvania State

University, 119pp,

Hou,

Z,, and

Fletcher,

W,K,,

1996,

The

relations between false gold

anomalies, sedimentological processes

and

landslides

Harris

Creek, British Columbia, Canada, Journat of Geoetiemieat Exptora-

tion,

57: 21-30,

Ljunggren,

P,, and

Sundborg,

A,,

1968, Some aspects

fluvial sedi-

ments

and

fluvial morphology,

IL A

study

some heavy mineral

deposits

in the

valley

the river Lule,

Geografistca

Annaten,

50A:

121-135,

Paopongsawan,

P,, and

Fletcher,

W,K,, 1993,

Distribution

and

dispersion

gold

point

bar and

pavement sediments

in the

Huai

Hin

Laep, northeastern Thailand, Journat

Geochemieat

Exploration, 47: 251-268,

Reid,

L, and

Frostiek,

L,E,,

1985, Role

settling, entrainment

and

dispersive equivalence

and of

interstice trapping

placer forma-

tion: Journat of ttie

Geotogieat

Society of London, 142: 739-746,

Rubey,

W,W,,

1933,

The

size distribution

heavy minerals within

a water-laid sandstone: Journat

Sedimentary Petrotogy,

3-29,

Saxby,

D,, and

Fletcher, W,K,, 1986,

The

geometric mean concentra-

tion ratio (GMCR)

as an

estimator

hydraulic effects

geochemical data

for

elements dispersed

heavy minerals, Jour-

natof Geoetiemieat Exptoration,

26:

223-230,

Sirinawin,

T,,

Fletcher, W,K,,

and

Dousset,

P,E,,

1987, Evaluation

geochemical methods

exploration

for

primary

tin

deposits: Batu-

Gajah-Tonjong Tualang area, Perak, Malaysia, Journat

Geo-

ctiemieat Exptoration,

29:

165-181,

Slingerland,

R,L,,

1977,

The

effects

entrainment

on the

hydraulic

equivalence relationships

light

and

heavy minerals

sands,

Journatof Sedimentary

Petrotogy,

47:

753-770,

Slingerland,

R,L,, 1984,

Role

hydraulic sorting

in the

origin

fluvial placers, Journatof Sedimentary

Petrotogy,

54:

137-150,

Slingerland,

R,L,, and

Smith, N,D,, 1986, Occurrence

and

formation

of water-laid placers, Annuat Review

Earth

and

Planetary

Sciences, 14: 113-147,

Smith, N,D,,

and

Beukes,

N,J,,

1983,

Bar to

bank flow convergence

zones:

contribution

to the

origin

fluvial placers. Economic

Geotogy, 75:

1-14,

Smith, N,D,,

and

Minter, W,E,L,, 1980, Sedimentological controls

gold

and

uranium

in two

Witwatersrand paleoplaeers. Economic

Geotogy, 75:

1-14,

Taylor,

D,, 1986,

Some thoughts

on the

development

of the

alluvial

tinflelds

the Malay-Thai Peninsula,

GEOSEA VProceedings

Geotogical

Society of Malaysia Bulletin 19,

pp,

375-392,

Toh, E,S,C,, 1978, Comparison

exploration

for

alluvial

tin

and gold,

tn Jones,

M,J,

(ed,). Proceeding of ttie llth Commonweahh Mining

and Metatturgy Congress, Hong Kong, Transactions

of the

tnstitution

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Metallurgy,

pp,

269-278,

532

PLACERS, MARINE

Cross-references

Grain Threshold

Heavy Minerals

Sediment Transport by tJnidirectional Water Flows

PLACERS, MARINE

Introduction

A placer can be defined as a surficial mineral deposit formed

by the mechanical concentration of mineral particles from

weathered debris (Bates and Jackson, 1984). Marine placers

are mostly metallic minerals or gems which have been

transported to the seafloor in a solid form. They are mainly

mechanically and chemically resistant minerals which have

been liberated on breakdown of their parent rocks. Such rocks

are usually igneous in origin but metamorphic and sedimen-

tary rock can also liberate placer minerals on breakdown.

Marine placers occur almost entirely in shallow continental

shelf environments.

Nature of marine placer minerals

Marine placer minerals can be classified by composition

(Table PI) or in terms of their specific gravity. Emery and

Noakes (1968) defined heavy heavy minerals as those with

specific gravities of

6.8-21,

light heavy minerals as those with

specific gravities of 4.2-5.3, and gems with specific gravities

from

2.9-4.1.

The heavy heavy minerals include native gold

and platinum and the tin bearing placer cassiterite, light heavy

minerals generally comprise the resistant accessory minerals of

igneous rock and include monazite, zircon, ilmenite and rutile

as important varieties together with magnetite. The most

important gem placer mineral is diamond.

Formation of marine placer deposits

The formation of marine placer mineral deposits requires

several separate processes. There must be a large primary

source of minerals. For this reason major marine placer

deposits are much more common off continental margins than

off islands because the latter are rarely large enough to contain

sufficient source rock. Resistant accessory minerals are

liberated on source rock weathering and are released for

transport. Chemical weathering is more important than

mechanical weathering in this regard, which is one of the

reasons why beach and marine placer mineral deposits are

more common off mid and low latitude coastal areas than they

are off high latitude areas. Mechanical weathering predomi-

nates in the latter. Transport of the placer minerals is largely

by water, during which the less resistant minerals are separated

from the heavier more resistant ones. On reaching the coast,

placer minerals are usually concentrated in sinks or traps

because of their high specific gravity, or concentrated on the

upper parts of beaches by wave action, sometimes leading to a

concentration of the deposits to economic grades.

The depositional environments of marine placer minerals

have been reviewed in Cronan (1980, 2000) and need not be

considered in detail here. In the offshore area, placer minerals

can occur in several situations (Figure PIO). It should be

remembered however that some of the placer mineral deposits

currently offshore were formed in river valleys which subse-

quently became inundated during the post glacial rise in sea

level. Certain deposits of cassiterite, scheelite, gold and

platinum fall into this category (Cronan, 1992). Further, many

placer minerals have been deposited on beaches near the mouths

of the rivers down which the minerals were transported and

Table PI Principal placer nninerals and their composition (from Cronan, 1992)

Specific gravity

Composition and principal element

Gem.s

Diamond

Garnet

Ruby

Emerald

Topaz

Heavy

noble

metais

Gold

Platinum

Light heavy minerais

Beryl

Corundum

Rutile

Zircon

Chromite

Ilmenite

Magnetite

Monazite

Scheelite

Heavy

iieavy

minerais

Cassiterite

Columbite, Tantalite

Cinnabar

3.5

3.5-4.27

3.9-4.1

3.4-3.6

21.5

2.75-2.8

3.9-4.1

4.2

4.7

4.5-4.8

4.5-5.0

5.18

5.27

5.9-6.1

6.8-7.1

5.2-7.9

8-10

(CaMgFeMn)3 (FeAlCrTi)2 (Si

AI2O3

AI2O3 (Al)

TiO2 (Ti)

(Zr)

(Cr)

FeOTiO2 (Ti)

Fe3O4 (Fe)

(CeLaYt) PO4 ThO2 SiO2 (Th REE)

(W)

(Nb,

Ta)

(Sn)

(FeMn)

HgS (Hg)

PLACERS, MARINE

533

(a) placers

submerged beaches

(b) placers trapped

surface depressions

on the

sea floor

buried river valleys

Figure

P10

Environments

marine placer deposition (from Cronan,

1992),

have subsequently been concentrated

coastal dunes

wind

action. Placer minerals transported from

the

land into

the sea

ean

further transported

wave

and

current action,

but

when

the

energy level

of the

environment deereases

below

that

which

the

minerals

can be

transported, they

are

depo-

sited

on the

seafloor. Once deposited,

significantly higher

energy level

required

piek them

again.

For

example,

Hosking

and Ong

(1963) noted that

off

North Cornwall

in the

UK free eassiterite grains tend

to be

concentrated

elongate

zones below

low

water

and

parallel

to the

eoast. Further,

Moore

and

Welkie (1976) have suggested that very fine grained

gold

and

platinum placer deposits

can

occur

quiet bays

and

proteeted estuaries

off

Alaska, Initially supplied

to the

offshore area under higher energy conditions, their distribu-

tion would

circulation gyres

slackening

currents within protected embayments, coupled with agglom-

eration

particles

contact with electrolytes

seawater.

Marine placer mineral deposit types

Light heavy minerals

Certain light heavy minerals, partieularly rutile, ilmenite,

magnetite, monazite, zircon, siliimanite,

and

garnet

can be

concentrated

by the

huge panning system

sandy beaches

(Kudrass, 2000),

In the

surf zone, various processes tend

separate minerals according

their density, size,

and

shape.

Heavy minerals

are

eoncentrated

by the

combination

these

proeesses

in the

upper part ofthe beach where wind

may

erode

them

and

form heavy mineral rich coastal dune deposits.

According

Kudrass (2000), with rising

sea

level,

the

amount

of sand stored

on the

beach

and shelf, the

intensity

long

shore currents,

the

wind direction

and

wave energy,

and the

morphology

and

width

of the

shelf

are all

parameters

determining

the

dispersal

heavy mineral bearing sand

during marine transgression.

In the

case

large sand volumes,

and

wide gently sloping

shelf, the

transgressing

sea

cannot

move

the

total amount

sand stored

in the

eoastal system.

The transgression then results

in an

extensive mixture

of the

available mobile beaeh sand

and

dune placer deposits,

and the

latter become widely distributed

and

disseminated.

The

ilmenite-rutile-zircon placer deposit

on the

middle shelf

off

northern Mozambique, which

to the

Zambezi River,

one

example

this type

disseminated placer deposit.

Where there

are

moderate

to low

volumes

heavy mineral

bearing sand stored

in the

eoastal system,

and a

narrow steep

shelf, the

transgressing

sea

moves

the

deposits

storm

washover processes (Kudrass, 2000),

Due to the

vertical

differentiation

heavy mineral assemblages, some heavy

minerals

are

preferentially concentrated

in the

landward

migrating sand while others

are

predominantly left behind

the trailing sand sheet covering

the

transgressed

shelf. The

sand arriving

at the

coastline undergoes further differentiation

the

surf zone, which leads

to the

formation

important

beach

and

dune plaeer deposits, such

those

on the

south-

east coast

Australia,

Tin

As mentioned, drowned river valleys

can

contain alluvial

placer mineral deposits,

as for

example

in the tin

deposits

off

south east Asia, Often

these situations

the

plaeer minerals

are

at the

bottom

of the

sediment eolumn overlying bedroek.

the

formation

giant

tin

deposits

eontinental shelves,

the drowning

of old

river systems

recognized

to be the

most

important genetic process

by far

(Yim, 2000), Thus

the

bulk

the eeonomic

tin

deposits occurring

continental shelves

were formed under subaerial eonditions prior

submergence

below

sea

level. Consequently,

terms

understanding

the

geology

of tin

placer deposits

continental shelves,

it is

essential

learn from their comparatively well studied

onshore eounterparts

(Yim,

2000),

Cold

According

Garnett (2000a),

the

simplest form

offshore

gold concentration

is,

tin, in a

buried fluvial channel whieh

is usually covered

recent transgressive marine sediments.

The highest gold grades

are

usually

at the

base

of the

alluvial

sequence resting

on the

bedrock. Examples exist

in the

Philippines

and

other areas where tropieal

and

subtropical

weathering

has

eroded coastal primary gold deposits.

In Arctic regions, gold bearing source rocks have been

subjected

massive weathering

and

grain reduction

glaciation.

The

resulting sediments containing liberated gold

have been transported

glaeial

and

fluvial processes

to be

distributed over relatively large regions, Partieulate gold

was

then further transported

marine proeesses (Garnett, 2000a),

Marine gold deposits

such origin occur

off

Alaska, southern

Chile

and

Argentina,

New

Zealand,

and

elsewhere. Because

its high density, gold

less mobile than other heavy minerals

of equal partiele size. Only very fine partieles travel

far

under

marine conditions. Most economic concentrations

placer

gold

are

thus rarely situated more than

a few

kilometers from

the site

of the

original accumulation.

The most important marine plaeer gold deposit

that

off

Nome, Alaska, According

Garnett (2000a)

all

writers

attribute

the

spatial distribution

gold

in the

sediments

covering

the

coastal plain there

glacial events combined with

marine transgression

and

regression. During

the

Pleistocene

glaciation

of the

region, changing

sea

levels caused

the

Bering

Sea

intermittently beeome

coastal plain. Primary gold

was

released from hard rock sources

extreme frost action

to be

deposited

down slope alluvial sediments

and in

fluvial

channels which were incised into

the

eoastal plain. These

deposits were later swept

together with eroded bedrock

advancing glaciers. Later,

as sea

level rose,

the

topography

was

molded, levelled

and

partially eroded.

The

high storm energy,

espeeially

in the

surf zone

and

shallow waters, reworked

the

sediments. Marine invasion alternating with retreat

of the

sea acted

on the

exposed glacial deposits, leaving relict thin

534

PLANAR AND PARALLEL LAMINATION

gold-bearing layers on the seafloor. Sediment-hosted gold

exposed on the seabed was then upgraded by the winnowing

aetion of marine eurrents to ereate a lag deposit.

Diamonds

The principal gem placer minerals occurring in the offshore

environment are diamonds. These were discovered in south-

west Afriea in raised beach sands in 1908 (Murray, 1969), the

diamond bearing beds extending down to the eoast and

offshore. The diamonds were derived from kimberlites in the

interior and had been transported to the eoast by fluvial

processes, Aecording to Garnett (2000b), longshore currents

combined with high energy wind and wave action during

periods of considerable sea-level change eoncentrated the

diamonds in trap sites on paleo coastlines and other marine

geologieal features. The host gravels generally exist as a thin

veneer on an irregular bedrock. Discontinuous deposits lie on

the inner and middle sections ofthe continental shelf along the

South African and Namibian coastlines,

Diamondiferous deposits on the inner shelf off southern

Africa are similar to those onshore. With a density of 3,5 g/cm''

diamond is the lightest of the economic placer minerals. Thus

it travels farther than gold and heavy placer minerals.

According to Garnett (2000b), the diamonds were redistrib-

uted from paleo river mouths along the coast and on ancient

beaches that developed in response to sea-level changes.

Powerful south-westerly winds, eombined with high energy

swells, created mostly northward moving longshore drift

currents. These transported the diamonds in waters less than

about

m deep. The resulting diamond-bearing deposits

have been continually upgraded by marine aetion. The

winnowing of the host sediments by waves has been the single

most important process that contributed to the enrichment (De

Decker and Woodborne, 1996), Present day diamond con-

centrations have resulted from the continual reworking of

older deposits and the subsequent redeposition of diamonds in

preferred trap sites throughout periods of sea-level change.

Deposits in the surf zone were eontinually sorted, which

concentrated the diamonds close to the bedrock. They were

trapped in gravels whieh collected in gulleys, potholes and

channels eroded into the wave cut terraces. Their final site of

entrapment was usually in a medium energy environment

(Garnett, 2000b),

On the middle shelf off southern Africa, resistant ridges up

to 8 m high trend generally north-northwest and provide a

rough bedrock on which diamonds have been trapped.

Cemented diamond bearing gravels on the ridges were

submerged during marine transgressive events. Their conse-

quent reworking produced a diamondiferous lag gravel.

According to Garnett (2000b), the diamonds are dispersed

neither uniformly nor totally at random. Irregularly shaped

well mineralized areas exist in places as widely separated

"islands" within extensive tracts which are either low grade or

barren. Inside sueh areas, the diamonds exhibit very localized

distribution characteristics,

David S, Cronan

Bibliography

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the Sunda Shelf essential for the exploration of submarine tin

placers,

Geologie en

Mijnhouw, 52:

79-91,

Bates,

R,L,, and Jackson, J,A, (eds,), 1984, Dictionary of

Geologieal

Terms, 3rd edn, American Geological Institute, Anchor Press,

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Cronan, D,S,, 1992, Marine Minerals in Exclusive Economic Zones.

Chapman and Hall,

Cronan, D,S, (ed,), 2000. Handhook of Marine Mineral Deposits. Bocca

Raton: CRC Press,

De Decker, R.H,, and Woodborne, M,W,, 1996, Geological and

technical aspects of marine diamond exploration in Southern

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OffshoreTechnology

Conference

Proceedings.

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Deposits of the

Continental Shelf Bangkok: ECAFE CCOP Tech, Bull 1, CCOP.

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Nome, Alaska. In Cronan, D.S. (ed.), Handhookof Marine Mineral

Deposits. Bocca Raton: CRC Press, pp,

67-101.

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reference to Southern Africa. In Cronan, D.S, (ed,), Handhookof

Marine Mineral Deposits. Bocca Raton: CRC Press, pp.

103-141,

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certain other heavy minerals in the superficial portions of the

Gwithian/Hayle beach of West Cornwall.

Transactions

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GeologicalSocietyof Cornwall, 19: 351-390.

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Cronan, D.S, (ed,), Handhookof Marine Mineral Deposits. Bocca

Raton: CRC Press, pp. 3-12.

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Cross-references

Grain Threshold

Heavy Minerals

Placers, Fluvial

Sediment Transport by Waves

Swash and Backwash, Swash Marks

PLANAR AND PARALLEL LAMINATION

Laminae are defined as sedimentary strata that are less than

mm thick, according to McKee and Weir (1953), Laminae

are recognizable beeause of variation in sediment texture and/

or composition within and between them (Figure Pll), For

example, the grain size within a lamina may increase or

decrease upward, or be different in adjacent laminae. Other

laminae may have distinctive concentrations of dark (heavy)

minerals. Planar laminae have planar bounding surfaces, Tlie

term planar lamination is commonly taken to indicate planar

laminae that are more-or-less horizontal (within a few degrees)

when originally deposited, and that have more-or-less parallel

bounding surfaces (but laminae do vary in thickness laterally).

Hence, planar lamination has also been called horizontal

lamination, even lamination and parallel lamination. The term

parallel lamination has also been used to describe laminae with

parallel but nonplanar boundaries (Figure Pll). Planar

laminated sandstones (popularly known as flagstones) are

valuable building materials. Commonly, these sandstones

contain near-parallel ridges of sand grains along lamina