PETROLOGY OF SEDIMENTARY ROCKS

0

ROBERT l. FOLK

Hemphill Publishing Company Austin, Texas 78703

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INTRODUCTION TO SEDIMENTARY ROCKS

Sedimentary rocks cover some 80 percent of the earth's crust. All our knowledge of stratigraphy, and the bulk of our knowledge of structural geology are based on studies of sedimentary rocks. An overwhelming percentage of the world's economic mineral deposits, in monetary value, come from sedimentary rocks: oil, natural gas, coal, salt, sulfur, potash, gypsum, limestone, phos­phate, uranium, iron, manganese, not to mention such prosaic things as con­struct ion sand, building stone, cement rock, or ceramic clays. Studies of the composition and properties of sedimentary rocks are vital in interpreting s tratigraphy: it is the job of the sedimentary petrologist to determine loca­tion, lithology, relief, climate, and tectonic activity of the source area; to deduce the character of the environment of deposition; to determine the cause for changes in thi c kness or lithology; and to correlate beds precisely by mineral work. Sedimentary studies are also vital in prospecting for economic mineral reserves, espe c ially as new deposits become harder to locate. Study of sediments is being pursued intensely by oil companies, phosphate, uranium, and iron mining companies in order to locate new deposits and explain the origi n of those already known.

Fundamental Classi fi cation of Sedimentary Rocks . Sediments consist funda­mentally of thr e e components, which ma y be m i xed i n nearly all proportions : ( 1) Terrigenous c o mponents, (2) A llochemica l components, and (3) Orthochemical components.

a. Terrigenous components are those substances deri ved from erosion of a lan~ a_Tea_outside the basin of deposition, and carried into the basin as solids; e xamples; qu a r tz or feldspar sand, heavy minerals, clay m ineral s, chert or limestone pebbles derived from erosion of older rock ou tc rops.

b. Allochemic a l constituents (Greek: "allo" m'.:!aning different from norma l ) are those substances precipitated from solution within the basin of deposi tion but which are "abnormal" chemical precipitates be cause in general they have been la ter move d as solids within the basin ; they ha ve a higher degree of organization than simple pre­cipitates. Examples : Broken or whole shells, oolites, calcareous fecal pellets, or fragments of penecontemporaneous carbonate sediment torn up and reworked to form pebbles.

c. Or thochemical constituents (Greek: "ortho" meaning proper or true) are"normal" chemical precipitates in the customary sense of the word. They are produced chemically within the basin and show little or no evidence of significant transpor tation or aggregation into more complex entities. Examples : microcrystalline calcite or dolomite ooze, probably some evapor ites, calcite or quartz pore­fillings in sandstones, replacement minerals.

Classes (b) and (c) are collectively referred to as "Chemical" constitu­ents ; classes (a) and (b) can be collectively termed "Fragmental." Some people use "detrital" or "elastic" as equivalent to "terrigenous"; other people use "detrital" or "elastic" as a collective term including both "terrigenous" and

1

"allochemical" above. Because of this confusion it is best that "detrital11

and 11 clastic 11 be dropped.

Sedimentary rocks are divided into five basic classes based on the P.ropor­tions of these three fundamental end members, as shown in the triangular diagram:

TERRIGENOUS

10% ALLOCHEMICAL ORTHOCHEMICAL

'-------- _______ _.,/ V

CHEMICAL

T. Terrigenous Rocks. Example: most mudrocks, sands tones , and conglomerates. Comprise 65% to 75% of the stratigraphic section. Most terrigenous rocks occur in the shaded area.

1..;. Impure Allochemical Rocks. Example : very fossiliferous shales ; sandy fossiliferous or oolitic limestones. Comprise 10 - 15% of the stratigraphic sectio n.

IO. Imoure Orthochemical Rocks. Example : clayey microcrystc;\lline limestones. Comprise 2-5% of the stratigraphic sect ion .

• ;\ . .A llochemica 1 Rocks. Example : foss iliferous, oolitic, pellet or intraclastic limestones or dolomites. Comprise 8-15% of the s tratigraphic section.

0. Orthochemical Rocks . Example: microcrysta lline limes tone o r dolomite; anhydrite; chert. Comprise 2-8% of the stra tigraphic section.

Collectively, 11 LA 11 and 11IO' ' are classed a s Impure Chemical Rocks, and

11.,;" and 11 0 11 as Pure Chemical Rocks.

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PROPERTIES OF SEDIMENT A RY ROCKS

1. Grain Size

Quantitative measurement of such grain size parameters as size and sort ­ing is required for precise work. To measure grain size, one must first choose a scale . Inasmuch as nature apparently favors ra tio scales over arithmetic scales, the grain size scale, devised by Udden, is based on a constant ratio of 2 between successive clas ses; names fo r the class intervals were proposed by Wentworth. Krumbein devised the phi (¢) scale as a logarithmic transforma t:.on of the Wentwor th scale, and modern data is near ly always s tated in¢ terms be ca use mathem:1. tic al co mpu tatio ns are much simplified.

Grain s ize of particles larger than several centimeters is usually dete r ­mined by direct measurement with calipers or meter sticks; particles down co about 4¢ (0. 062 mm) are analyzed by screening; and silts and clays (finer tha:-i. 4¢ ) are analyzed by pipette or hy drometer, utiliz ing differential settling rates in water . Sands can also be measured by petrographic microscope or by var :. ou s set tli ng devices.

Results of grain-size analysis may be plotted as histograms, cumulati·,e curve s or frequency curves . Curve data is sum:narized by means of m:1.the:;_a ti cal parameters allowing ready comparison between samples .. .\s measures of a·:erage size the median, mode, and mean are f requently used; as a measure of sorti:--.g the standard de viation is best. Skewness and kurtosis are also useful para­meters for some work .

Geological signifi ca nce o f the me a sures is not fully known . Many ger.e:-ali ­zat io ns have been made on fa r too little evidence, but s ignificant s tu dies are now going on. ..\s of our present s tate of knowledge, we c an make a few educated guesses about the meaning of grain - size s ta tistic s.

The signifi c an ce o f mean grain size is no t ye t well enough known to make any positi ve statements ; volumes of da ta on recent sediments must be collec:ed before we can say anything really meaningful. To be sure there is a certain correlation of grain size with environments (see page 107)--e. g. you usually do not find co nglomera te s in swamps or silts on beaches--but there is a great deal of overlaping . These questions can only be solved by integration of size analysis resu lt s with shape study, detailed field mapping, s tudy of se dimentary s tructures, fossils, thickness pattern change s, etc. It is extremely risky rn postulate anything based o n size analysis from one sam ple or one thin se ct:on, unless that sample is known to be representative of a large thickness of sec-tion. One thing that is an especially common error is the idea tha t if a sedi­ment is fine it m us t be far from the source, while if it i s coarse it must be near to a source. M1ny s tu dies of individual environments show sediments getting finer away from the source but these changes are so varied that they can be deciphered only by extensive field and laboratory work. For examp~e, strong rivers ma y ca rry large peb bl es a thousand or so miles away from their sour ce, while on the other hand the finest clays and silts are found in playa lakes a matter of a few miles from e nc ircling rugged mountains. Grain size depends largely on the current strength of the local environment (together v: it h size of available particles), not on distance. Flood plain clays may lie im­mediately adjacent to coarse fluvial gravels, both being the very same distance from the ir source. One must wo rk up a large number of samples before anything

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, much can be said about the significa nce of g rain size. Still, it is an important descriptive property and only by collecting data on grain size will we be able to learn the meaning of it.

Mean size is a functio n of ( 1) the size range of available materials and (2) amount of energy imparted to the sediment which de pe nds on current velocity or tu rbulence of the transporting medium. If a coastline is made up of out ­crops of soft, fine-grained sands, then no matter how powerful the waves are no sediments coarser tha n fine sands will ever be foun d on the beach . If a coastline is made up of well-jointed, hard rocks which occasionally tumble down during rains, then the beach sediment will be coarse no matter how gentle the waves of the water body. Once the limitations of source material are under­s too d, though, one can apply the rule that sediments generally become finer in the direction of transport; this is the case with most river sands, beach sands, spits and bars. This is largely the result not of abrasion, but of selective sorting whereby the smaller grains outrun the larger and heavier ones in a downcurrent dire ction. Pettijohn an d students have made excellent u se of maximum pebble size in predicting distance of transport quantitatively, Sedi­ments usually become finer with decrease in energy of the transporting medium; thus, where wave action is dominant sediments become finer in deeper water be­cause in deep water the action of wave s on the sea bottom i s slight, whereas this turbu lence is at a maximum in shallow waters at the breaker zone. Where current action dominates, particularly in ti dal channels, coars er sediments occur in deeper waters, largely because of scour. Research is needed to quantify these changes so that the rate of grain-size change with depth can be correlated with wave energy expenditure or other environmental factors.

Sorting is another measure whic h i s poorly understood . It depends on at least three major factors: (1) Size range of the material supplied to the en­vironment--obviously, if waves are attacking a coastline composed of gla cial till with everything from clay to room-sized boulders, the beach sediments here will not be very well sorted; or if a turbulent rive r is running thro ugh outc ro ps of a friable well-sorted Tertiary sand then the river bars will be well sor te d. (2) Type of deposition-- "bea n spreading", with currents working over thin sheets of grains continously (as in the swash and backwa sh of a beach) will give better sorting than the "city-dump" deposition in which sediment s a re dumped down the front of an advancing series of crossbeds and the n rapidly burie d by more se_dimenl ( 3) Current characteristics- -currents of relatively cons tant strength whether low or high, will give better sorting than currents which fluctuate rapidly from al-most slack to violent. Also very weak c urrents do not sor t grains well, neither do very strong currents. There is an optimum current velocity or degree of turbu­lence which produced best sorting. For best sorting, then, c urrents must be of intermediate strength and also be of constant strength. (4) Time--rate of supply of detritus compared with efficiency of the sorting agent. Beach sediments where waves are attacking continually caving cliffs, or are battling great loads of detritus brought to the shore: by vigorous rivers, will be generally more poorly sorted than beaches on a flat, stable coast receiving little sediment influx.

It is probable that in every environment, sorting is strongly dependent on grain size. This can be evaluated by making a scatter plot of mean size versus sorting (standard deviation) . In making many of these plots, a master trend seems to stand revealed: the best sorted sediments are usually those

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with mean sizes of about 2 to 3¢ (fine sand) (Griffiths; Inm3.n). As one measures coarser sediments, sorting worsens until those sediments with a mean size of Oto -1¢ (1 to 2 mm) show the poorest sorting values. From here sort -ing improves again into the gravel ranges (-3 to -5¢), and some gravels are as well sorted as the best-sorted sands (Folk and Ward). Followed from fine sand into finer sediments, the sorting worsens so that sediments with a mean size of 6 to 8¢ (fine silts) have the poorest sorting values, then sorting gradually improves into the pure clay range ( 10¢). Thus the general size vs. sorting trend is a distorted sine curve of two cycles. Work so far indicates that the apparent reason for this is that Nature produces three basic populations of detrital grains to rivers and beaches (W entworth). (1) _t,., pebble population, resulting from massive rocks that unde rgo blocky breakage along joint or bedding planes, e.g. fresh granite or metaquartzite outcrops, limestone or chert beds, vein quartz masses. The initial size of the pebbles probably de­pends on spacing of the joint or bedding planes. (2) A sand-coarse silt pop ­ulation, representing the stable residual products liberated from weathering of granular rocks like granite, schist, phyllite , metaquartzite or older sand­stones who se grains were derived ultimately from one of these sources. The in­itial size of the sand or silt grains corresponds roughly to the original size of the quartz or feldspar crystal units in the disintegrating parent rocks. (3) A clay population, represen ting the reaction products of chemical decay of un­stable minerals in soil, hence very fine grained. Clays may also be deri ved from erosion of older shales or slates whose grain size was fixed by the same soi l ­forming processes in their ultimate sourc e areas. Under this hypothesis, a granite undergoing erosion in a humid clima.te and with moderate relief should produce ( 1) pebbles of grani e or vein quartz from vigorous erosion and pluc k-ing of joint blocks along the stream banks; (2) sand-size quartz grains, and(3) clay particles, both as products formed in the soils during weathering.

Because of the relati ve scarcity in nature of granule - coarse sand (0 to -2 ¢) particles, and fine silt (6 to 8¢ grains , sediments with mean sizes in these ranges must be a mixture of either ( 1) sand with pebble~or (2) sand or coarse silt with clay, hence w i ll be more poorly sorted than the pure end­members (pure gravel, sand, or clay) . This is believed ::o be the explanation of the sinusoida l sor ting vs . size t r end. Of course exceptions to this ~xist, e. g. in dis i ntegra tion of a coarse-grained granite or a very fine phyllite which might liberate abundant quartz grains in these normally-rare sizes. If a source area liberates grains abundantly over a wide range of sizes, sorting will remain nearly constant over that s ize range (Blatt) and no sinusoidal relation will be produced.

Alt hough it appears tha t all sediments (except glacial tills) follow this sinusoidal relation, there is somedifferentiation between environments. It is believed that given the same source material, a beach will produce better sorting values fo r each size than will a river; both will produce sinusoidal

, trends , but the beach samples will have better sor ting values all along the ; trend because of the "bean spreading" type of deposition. Considering only i the range of sands with mean sizes between 1¢ and 3¢., most beach sands so j~ far measured here have sorting (o-'r) values between .25- . 50¢, while most river

sands nave value s of .35-1. 00¢ . Thus there is some overlap, and of course there are some notable exceptions; beach sands formed off caving cliffs are more

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beach or marine sand will have well-sorted sediments. Coastal dune sands tend to be slightly better sorted than associated beaches, though the difference is very slight, but inland desert dunes are more poorly sorted than beaches. Near shore marine sands are somewhat more poorly sorted than corresponding beaches, and tend to have values roughly similar to river sands. Flood-plain, alluvial fan, ::rnd offshore marine sediments are still more poorly sorted although this subject is very poorly known and needs a great deal more data. Beach gravel between

0 b and - 8 d, whether made of granite, coral, etc., have characteristic sorting values of O. 4 - 0. 6 d ( if they are composed mainly of one type of pebble); the re seems to be no difference in sorting between beaches with gentle wave action v s. those with v igorous surf. (Folk).

Skewness and kurtosis tell h ow closely the grain-size distribution ap­proaches the normal Gaussia n probability curve, and the more extreme the values the more non-normal the size curve . It has been found that single­source sediments (e. g. most beach sands, aeolian sands, etc.) tend to have fair ly normal curv es, while sediments from multiple sources (such as mix-ture s of beach sands with lagoonal clays, or rive r sands with locally -deriv ed pebbles) show pronounced skewness a nd kurtosis. Bimodal sediments exhibit extreme skewness and kurtosis value s; although the pure end-members of such mixtures hav e nearly normal curves, sediments consisting domina ntly of one end member with only a small amount of th e other end member are extremely leptokurtic and skewed, the sign of the skewness depending on which end member dominates; sediments consisting of subequa l amounts of the two end-members :ire extremely platykurt ic. ( F olk and Wa rd). Plots of skewness against kur ­tosis are a promising clue to envi ronment a l differentiation, for example on :\Iustang Island (M a son) beaches give nearly normal curves, dunes are positive­ly-skewed mesokurtic, and a eolian flats are positiv ely -skewed leptokurtic. Friedman showed that dunes tend to be positive skewed and beaches negative skewed for many areas a ll over the Ea r th; but Hayes showed on P adre Island that this is often modified by source of supply.

Fluvial enviroments consisting chiefly of t raction load (coarse) with some infiltrated suspension load (finer grains) are commonly positive-skewed lep­~okurtic; glacial marine clays with ice -ratified pebbles are negative-skewed, etc. It whould be emphasized that faulty s ampling may also cause erroneous skew­nes s and kurtosis values, i f a worker s amples two adjoining lay ers of different , size - i. e., a gravel s t reak in the sand. Each laye r should be sampled separ­:ltely .

Size analys i s has been used practically in correlation of formations; in ricterminin g if a sand will co nta in oil, gas o r water (Griffiths); in de termin ­::1s direction of sedimen t transpor t; and an in te ns ive study is being made to -:e:ermine if charac teristic grain size distributions are a ssociatE:d wit h cer-·.:-i:n modern environments of sedimentation, so that such deposi ts may be identi­:· :ed by analysis o f ancient sediments in the stratigraphic column. Furthermore =:_any physical propert ie s of sediments such as porosity, permeability, or firmness '. r--.rumbein) are dependent on the grain size.

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2. Particle ~orpholoQ

Under the broad term " particle morphology" ar e include d a t lea s t fo ur concepts. Listed approximately in decreasing order of magni tude, these are (1) form, (2) sphericity, (3) roundness, and (4) surface fea tures.

Form is a measure of the r ela ti on b etw een the three dimens ion s of a n ob jec t, and thus particles may be classed quan t i ta t i v e ly as compact (o r equ i dimens iona l), elongated (or rodlike) and pla ty (or di s c like ), with s eve r al i n te r m edi a te ca tegories, by plo tti ng the dimens io ns on a t ri a ngula r graph (S nee d an d F ol k: see p. 9).

Sphericity i s a property whose defini tion i s s imple , bu t which ca n be measured i n numerous very di fferent way s. It s ta t es quantita ti v e ly ho w nearly equal the three dimensions of an object are. C. K. Wentwor th made the f irs t

quantitative study of shapes. Later, Wadell defined spheric ity a s

..; rv:-- where V pis t he actu a l volume of th e particle

v~ (measured by i mm e rsi on in \Vate r ) and V c s i s the vo lume of the c i r cu ms c ribing

sphere (the sma lle s t s phe r e tha t wi ll ju s t e nclo s e the par t icle); ac tually the diame ter of this spher e i s conside re d a s equa l to the lo n ge s t dimens ion of the par ti cle .. A. sphere ha s a s phericity of 1. 00; mo s t pebble s or s an d grains ha v e a spheric ity of about 0 . 6 -0 . 7 w hen mea s u r e d by thi s sys te m . One c a n get

a n approxima tio n of this me a sure by th e fo r mula,~~ where L ,

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represent the long, in te rmediat e and short dimens ions respe ct ively (Krumbein).

The above fo rmu la , alt hough in co m mo n u s e to da y , do e s no t indica te how the par ti cle behaves upo n se ttli ng ,in a flui d me dium and i s thus ra the r uns a tisfac ­tory . .A.ctua l ly , .::. r od s e ttle s fas te r tha n a d i s k of the same volu m e, but Wadell ' s formu la wou ld ha ve u s be lieve th e opposite. A sphericity value whi c h shows betler the beha vio r of a part icle during tra ns po r t i s the :\<Iaxim 1Jm

Pro jection Sphericity (Sne e d and F olk, J . G e o l. 1958) , gi ve n by the fo rm ula

~ . Particles tend to se ttl e wit h th e m a ximum projection a r ea (the pl ane

LI of the Land I axes) perpendicular to the di re ction of m otio n and hen c e res i s t -ing the movement of the par ticle. This formu la co mpare s the maximum pro j e ction area of a given particle with the maximum proje ct ion are a of a sphere o f the same volume; thus if a pebble has a spherici ty o f 0 . 6 i t means tha t a sphere of the same volume would have a maximum proj e ction area on ly 0 . 6 a s lar ge as tha t of the pebble. Consequently the pebble wou l d s ettl e a bo ut 0 . 6 as fas t as the sphere because of the increased sur fa c e a r e a re s i s ti ng downwar d mot ion. The derivation follows: assuming the par ticle to be a t r iaxia l e llipsoid, the maximum projection area of the particle is 7; (LI). The vo lume of the partic le

is 7, (LIS). Hence the volume of the equi valent sphere wi l l also be ¼- (LIS). -s--

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measurements have been applied. Riley Sphericity is given as iDi where DC

Dc is the diameter of the smalles t circumscribing circle and Di is the diameter

of the largest inscribed circle. These can be easily measured by a celluloid scale ruled off in a series of closely -spa ced concentric circles of known dia­meter, which can then be placed over the sand grain image. Another measure is

E longation which is simply width (actually Least projection width) over length ~easured by rectangular grid. This is probably the most satisfactory two dimensional measure (Griffiths; Dapples and Rominger).

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Although individual grains may have widely varying W /L values, sample means (obtained by counting 100 quartz grains in one thin section, for example) show a much more restricted range. Measurement of many sandstones has suggested the following scale: under . 60, very elongate; . 60-. 63, elongate; . 63-. 66, subelongate; . 66-. 69, intermediate shape; . 69-72, subequant; . 72-. 75, equant; and over. 75, very equant.

L«o..st Pr'""oj~hor-. .E lo"Jqt,·o~ Roundness was first quantitatively measured by Wentworth, who used the

curvature of the sharpest corner. Later, it was defined by Waddell as the aver­age radius of curv ature of all the corners divided by the radius of the largest in­scribed circle. This is impractical to measure, though, and now roundness values are obtained by comparison with photographic charts for sand grains (Powers). A perfect ball has a roundness of 1. O; most sand grains have roundnesses about 0. 3 - 0. 4 on the Waddell scale. Use of the Powers roundness images for sand grains is faci litated by a logarithmic (rho, P _) sca le in which the limits of the very

a n gu 1 a r c la s s a re ta k e n a s O . 0 - l . 0, 6 f angula r l. 0-2. 0, subangular 2. 0-3. 0, sub­round 3. 0-4. 0, round 4. 0-5. 0, and very round 5. 0-6. Op (Folk ). On this scale, perfect balls ha ve a roundness of 6. 0_,P and most sand grains have a ve rage roundness about 2. ~ (subangular).

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The concept of roundness sorting (uniformi ty of roundness) may 'be expressed by the roundness standard deviation, C?):>. This can be determined by plotting

the roundness da ta as a cumulative curve and determiningQ,p by the intercept method, as is done in grain size cu r ves. Plotting of these values for many samples gives the foll owing limits for roundnes~ s tandard deviation: under 0. 60, very good roundness sor ti ng; 0. 60-0 . 80, good; 0. 80-1. 00, moderate; l . 00 - 1. 20, poor; o ver 1. 20, very poor roundness sorting. St. Peter sand, uni­:'ormly well rounded, has go under 0. 60; the average value for Recent sands

is about 0. 90; and the Colorado River at Austin, with its mixture of well­r-ounded Cambrian and Cretaceous quartz with freshly broken Llano granitic qu artz has a~ of 1. 30.

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Pebble roundness can be measured more quantitatively by dividing the radius of curvature of the sharpest single corner, by the radius of the largest inscribed circle (see Dobkins and Folk, 197 0 J. S . P.)

Surface Features. As yet no way has been developed to measure these quan­titatively . Frosted surfaces are noticeable chiefly on round grains, although not all round grains are fro st e d. Frosting may be caused by chemical etching (as by dolomite replacement), by incipient tiny quartz overgrowths, or by aeolian abrasion (where the frosting is due to minute crescentic percussion marks ca used _by the greater impact and velocity in a ir). These may be dis­tinguis hed by examining the grain with petrographic microscope in water . Polished surfaces are cau sed by a fine smoothing of the tiny irregularities and are ascribed to rubbing of the grains in water; thus they are the fir st stage in rounding of aqueous sands. They are supposedly common on beaches but much less so in neritic or river sands where the grains are not rolled back and forth so much; but there i s considerable question as to the validity of thi s criterion , and some polis h may form chemically (Kuenen). Much vo lcanic phenocryst quartz has naturally polis hed faces. Unpoli shed surfaces ha ve neither frosting or polish, and are dull or jagged because of tiny angular irregularities or fresh fract ure surfaces. They are c hiefly found in river or neri tic sands and indicate lack of much abrasive action . Per cu ssion mar ks are found on pebbles (especially chert or quartzite ) and indi cate high - velocity impact. G lacial action sometimes produces scrat ches or striae on soft pebbles like lime stone . The entire subject of surface features needs much more re ­search to find out what happens in various environments and to what extent the observed surface features are a function of grain siz e.

As a possible explanation fo r the co nfli cting data o n surface feature s and environment, the follo,\·ing co mpletely un te sted wild idea is put fo r th: all abrasional environmen ts produce all types of sur face feature s, but o n different sizes of pa r ticles . In each environment the coar s e s t grains are fro s ted (probably by per c ussion marks), int ermedia te grains are polished, and the fi nest grains are dull (unmodified) . The size ranges over which these sur-face features develop on quartz var:ies with the envi ronment, perhaps a s below:

size - -± -z. 0 2.

.:-;eritic D u L L

River F r o s e d Po 1 i s he d D u

Beach F r 0 s t e d Pol l S h e d D u 1 1

Dune F r 0 s t e d P o Lis hed D ull

The most common size inspected for surface featur es is about medium sand, which would explain the feeling that most dunes a re fro s ted , most beach sands polished, and river sands dull. Of course , rate of deposition, time available, and rate of supply add complications, and some surface features are formed chemically . Mature desert quartz often has a greasy lus ter caused by a minutely pimply deposition of quartz "turtle-skin '' overgrow ths.

As a rule, it is difficult to study surface features in ancient sediments because of c ementation. _.;n a ttempt may be made by cleaning the grains thorou gh­ly in warm HCl to remove carbonat es and iron stains. They should be studied under highest power of the binocular microscope, on a black surface and under

12

str, witl but i gra

ana whi t gra : cho, gra : eve call

san, whe the sult left les:=

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I

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, strong light. After this, the grains should be mounted in water and examined with petrographic microscope. Electron microscopy has made great contri­butions to study of grain surface features (Krinsley ) and intimate details of grain history can be studied.

Graphic and Statis ti cal Analysis of Shape Data . Graphic or statistical analysis is necessary to show ( 1) the variation of grain morphology with size, which is nearly always present, and (2) environmental, spatial, or strati-graphic differences between sample s. In comparing a set of samples, one should choose the same size interval throughout for his analyses, because coarser grains are usually better rounded, and show different surface features. How­ever on some of the s ample s, shape s tudy should be made on all sizes. This is called an H pattern of sampling.

If a significant difference in morphology is found, (for example if beach sands in an area are more angula r than duhe sands), you must always consider whether it is due to ( 1) an actual difference in the processes going on (that the dunes are rounding more effectively) or whether (2) it is simply the re-sult of selective sorting, where e.g. rounded and more spherical grains are left behind by wind cu rr ent s which selectively sweep away the more angular and less spheri ca 1 grains .

To determine if a difference :n For~ is present between two sets of samples, one can ( 1) using a moving ci r cular mask, contour the points on the triangular diagram as is done in contouring join t or petrofabric diagrams; (2) use the x 2 te s t by counting the number of particles in each shape "cell" and comparing the r esults; (3) obtain by counting a median shape for each se t of data; (4) superimpose a shee t of t ransparent triangular-ruled graph paper over the data, and assign each point a "percent elongation" and a "percent" platiness" by measuring its distance fro m the base line of the triangle; this data may be t rea ed by finding i ts mean and standard deviation and comparing by the t test.

Spherici ty differe nces between samples can be evaluated by finding a mean and standard deviation for each set of particles and comparing by means of the t test .

Roundness is analyzed by counting a large number of particles using com­parison charts; then , use the log tra nsforma tions of the roundness values ( t° scale) and compu t e means (average roundness) and standard deviations (roundness sorting), then compare by the t test. The roundness data may also be plotted by means of cumulative cu r ve s if probability paper is used. In doing a set of samples from two or mere environments, ,ormations, or localities, it is well to h3 ve an associate mix the samples up so that you do not know which set you are ·.•.-orking on when you count a s ample; thus vou avoid a bias and the data is more "honest."

Surface features may be compared by counting the number of frosted , :1o_lisl-ied grainsetc-. -and comparing by the x 2 test. Again, an associate should rni:.; the samples up so you do not know which sets you are counting, in order to avoid bias.

13

- - - - ----------------------

Significance 0 Grain Morphology

Form and Sphericity are the result of (1) Structure (internal properities, inherited from the source; (2) Process (work of the depositional enviroment e . g . glacier, river or beach; and (3) Stage (length of time available for modi­fy ing the particle) . This is the concept drawn from W . M . Dav is, who used it for landscape interpretation . Concerning Structure, bedding, schistosity and cleavage tend to make partic l es wear to discoids; directional hardness may have some effect in minerals like quartz and ky anite . Original shape of the particle may be somewhat retained, as in shapes of joint blocks, or in platy quartz derived from some schists.

The effects of Process and Stage are complex, and there is insufficient data on this aspect. In studying pebble shapes, most workers have used a grand mixture of rocks, including schists, sandstones, thin -bedded lime­stones , etc . Of course such data is useless; to get v alid enviromental data one must use isotropic wearing rocks such as gra~ite, basalt, or quartz. Further, the same geologic process (e . g . surf action) may work to different ends with different sized pebbles, so pebble size must be carefully controlled (i. e . use the· same size pebbles for the entire study) .

A study of pebbles in the Colorado Riv er was made by Sneed. He found that the main control o n sphericity is pebb l e lithol ogy , with chert and quartz hav ing much higher sphericity than limestone. Smaller pebbles dev e lop higher sphericity than larger ones with long transport. Going downstream, sphericity of quartz increased slightly , l imestone stay ed constant , and chert decreased . He found a weak te ndency for the Colorado Riv er to produce rod­like pebbles in the larger sizes.

Dobkins and Folk (1970 J. S . P. ) found on Tahiti beaches, using uni­formly -wearing basalt, that beach pebble roundness a v eraged . 52 while river pebble s were . 38 . Beach pebbles wer-e oblate with v ery low spheric i ty (. 60), w hile fluvial pebbles a v eraged much higher sphericity , . 68. Relations are complicated by wav e height, substrate charact er ( s andy v s . pebbly ), and pebble size; nevertheless, a sphericity line of . 6 5 appears t o be an excellent splitter between beach and fluvial pebble suites, providing isotropic r ocks are used . Apparently this is caused by the sliding action of the surf.

Roundness probably results from the chipping or rubbing of very minute particles from projecting areas of the sand grains or pebbles. Solut ion is not held to be an important factor in producing roundness, though some dispute this (Crook, 1968) . Impact fracturing of grains (i . e . breaking them int o several subequally - sized chunks) is not an important process except possibly in mountain torrents; "norma l" rivers car r y few fractured pebble s, and bea c h or river sands only rarely contain rounded and refractured grains. Rounded pebbles may break alo~g hidden joints if exposed tolo ng weathering . The " r oundability " of a particular mineral or rock fragme nt depends upon i ts hardness (softer g ra ins rounding faster), and the cleavage or toughn e ss ( large gra ins wi th good cleav -age te nd to fracture rather than round; br ittle pebbles like chert al~o te nd to fracture readily whereas quartz pebbles will round) . On the Colorado River , limestone pebbles attain their maximum roundness in only a few miles of trans­port and thereafter do not get any more rounded. Quartz also becomes well rounded but at a much slower rate , equalling the roundness of limestone after about 150 miles; chert shows only a slight increase in roundness in over 200 miles of transport. Coarser grains round ea s ier than finer ones, because

14

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ar

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m fo

g1 i r. be i !3

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they hit with greater im pact and also tend to roll along the surface while f.in er ones may be carried in suspe nsion . Aeolian sands round faster than aqueous sands becaus e the grains ha ve a greater di ffe rent ial density in air, therefore hi t harder ; also they ar e not cu shioned by a water fi lm. In experi­ments by Kuenen, wind - blown quartz rounded much more rapidly than water­transported grains. Beach sands also round fast e r than r iver sands because on a b each the grains are roll ed ba c k and fort h re peatedly. It i s thought tha t little or no rounding of sand grains takes plac e in rivers; pebbles get rounde d rapidly in rive rs, howev er . To get rounded sand grains takes a tremendous time and a very l arge expenditure of energy . For example, the beach sands of the Gulf Coast show very lit tle rounding taking place simply because the shore­line fluctuates so rapidly that time is not adequate . Balasz and Klein report rapid rounding in a "merry -go-round" tidal bar.

In studying r oundne ss watc h for two things : (1) an abno rmal re lation be­tween r oundness and minera l hardness, e.g. if tourmaline is round and so fter hornblende is angular it me a ns the r e is a multiple source area ; and (2) an abnormal r elation b etwee n size and rounding, e.g. if coars e grains are angula r and fine ones are round it again us ually means a multiple s ou r ce with the angular grains being primary, the rounder ones co ming from reworked sedimen ts . Al so, poor r ou ndness sor ti ng (i . e . wit h in the s ame grade s ize there are both very r ou nded and ve r y angula r grains) indicates a multiple sourc e. To det er ­mine how mu ch r ou ndin g is taking place i n the la s s ite of deposi ti on, look fo r the most angula r grains; for example a s and consisting of 70% well rounded grains and 30% angular grains indi cates little or no rounding i s actually t::ik-ing place in the present environme nt, ;:ind r. hat the rounded groins have simply been inheri ted f r om olde r sediments. Perhaps the 16 percentile o f roundn ess is the best parameter to evaluate the pres·ent r a e of r ounding.

E ffe<:__t .9J: !_ra~orta_E_~~ o~ g:_~ain _:i~~ an~ ~rpho l_?jl. Transportation does r educe he size of _eeb~.!._~ through chipping or rubb~ng and occasionally through fracturing; but it is thought that very ti tle size !"eduction of s a nd­~iz~ quartz i s effected through t ransport. The diffe r ences in s ize betw e en deposi s are chiefly due to selective sorting (where the finer particles , which travel in a most a_l curren s, outrun the coa rser pa rticles, whi ch c an t r avel only in th e strong cu rrents), rather tha n to abrasion . Thus one c anno t s ay that a ve r y fine sa nd ha s been abraded lo nger tha n a fine sand: simply it has been carr i ed farther or depo s ited by weake r currents. The effect of ab r a sion on s phericity of sand is slight hu t noticeable . Crushed quartz and many angu-lar sands have Vi / L values of about . 60 -. 64; ve r y well r ounde d sands ha ve W / L of over . 70 . Selective soring will also p r oduce form and sphericity dif fe rence s between sam ples .

It can be ce rtainly s ated that abrasion ,does cau s e rou nd ing of sand g r ains (albeit very slowly), and that i t even more rapidly will produce poli sh. Thus :he smalle st orde r features are c1ffected first: consider ·ng sand groins in ·.•.-ate r, s tarting ini tially with c r ushed quar z, the fir s t th in 5 that happens i s ,hat the grains become po lished ; afte r much more c1brosion, they beco m e rou nded; :::t er an extreme length of time they begin to ottDin higher sphericity val ue s ; :ind st ill later their size is redu ce d. Re3lly these proce sses a re all taki ng :ilnc e at once a nd he above li s t simply gives th e ra te s at ·.vhic h these changes ::re being effec ed . Surface featu r es a nd second;:ir ily r ou ndness, are the i m­:10 rtant clues to the la es t e nviro nm ent in which the s a nd '.Vas deposited; spheri city and form are the clue s to th e ea rli est environment in which the sand ·.•.-a s formed, name ly s ou r ce r ock .

15

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GRAIN SIZE SCALES FOR SEDIMENTS

The grade scale most commonly used for sediments is the Wentworth (1922) scale which i s a logarithmic scale in that each grade limit is twice as large as the next smaller grade limit. The scale starting at 1mm and changing by a fixed ratio or 2 was introduced by J . A . Udden (1898 ), who also named the sand grades we use today . However, Udden drew .the gravel/sand boundry at 1mm and us ed diffe rent terms in the gravel and mud divisions . For more detailed work, sieves have been constructed at intervals 2{2 and W . The t (phi ) scale , devised by Krumbein , is a much more convenient way of presenting data th an if the value s a re express ed in millimeters, and is used almost entirely in recent work .

r. s. Standard Millimeters Mic rons Phi(¢) Wentworth Size Class Sieve Me sh # (1 Kilometer) - 20

4096 -12 1024 -10 Boulde r ( -8 to - 12¢)

----- 256 - --------- - 8 - - - - ---(--,6- ---,,8~rl.,..,..)-6 6 Cobble - to - 'I-' ------ 4 -----------

Uae ;.rire squares 5 6 7 8

16 - 4 Pebble · ( - 2 to -6¢) 4 --------- - 2 3 . 36 - 1. 75 2 .83 - 1.5 Granule 2 . 38 - - . 25

_j w > <t a '--9

10 ------- 2 . 00 --------- - 1. 0 --------------12 14 16

1.68 - 0 . 75 1.41 -0 . 5 1.19 -0.25

- 18 - - ----- 1. 00 -------- 0 .0 20 25 30

- 35 ----40 45 50 60 70 80

O .84 0 . 25 0 . 71 0 . 5 0 . 59 O. 75

_/ 2 - 0 . 50 --- 500 ---- LO O . 42 20 1. 25 0 . 35 350 1.5 o . 30 300 _ 75

1/ - 0 . 25 - - - 250 ---- 2 .0 0 . 210 210 2 . 25 0 . 177 177 2 . 5

100 0 . 149 149 2,75 - 120 1/ 8 - 0 . 125 - - 125 - - -- 3 . 0

140 0 . 105 105 3.25 l 70 0 . 088 88 3 . 5 200 O . o 7 4 7 4 3 . 7 5

Very coarse sand

Coa rse sand

Medi u;n sand

Fi:1e sand

Ve rJ fine sand

0

z <1 u)

- 230 ----1/16 - 0 .0625 -- 62 . 5--- 4.0 -------------270 325

Analy zed

by

Pipette

or

Hydrometer

0 .053 53 4 .25 o .044 44 4. 5 Cc erse silt

0 .037 37 ---- 4. 75 1/32 - 0 . 031 -- 31 5 ,0 - --- -------1/ 64 0 .0156 15 .6 6 .o Medium silt

1/ 128 0 .0078 7 .8 7 .0 ? i :1e silt 1/256 0 .0039 -- 3 .9 --- 8 .o ------ Ve ::-y fi ne silt

0 . 0020 2 .0 ) . 0 0 .00098 0 .98 10 .0 0 .00049 0 . 49 11.0 0 .00024 0 . 24 12.0 0 .00012 0 . 12 13 .0 0 . 00006 0 .06 14 .o

25

(Some use 2)"' or 9~ as the cla v

I "

boundry )

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--· _ __.-.; .. -=-~

GR-1\IN SIZE NOMENCLATURE FOR SEDIMENTS

The basis of the classification is a triangular diagram on which ar e plotted the ·--ooortions of gravel (material coarser than 2 mm), sand (ma teria l between 0. 0624 ' :c· ~mm.), and mud (defined as all material finer than 0. 0625mm, i.e., silt plus ~:·j\· ), as shown in the triangular diagram. Depending on the relative proportions of :·::~~e three constituents, fifteen major textural groups are defined- -for example, sandy .- :r. glomerace, sli~htlv conglomeratic mudstone, or sandstone. This classification is ~-csent ed in deta11 by Folk (Jour . Geol. 1954); and also Folk, Andrews & Lewis, :·>1 :---;wZd). -· To place a specimen i n one of the fif t een major groups, only two properties need '..) ':)e determined: (1) how much gravel (material coarser that 2 mm.) it contains-­::o:.Jndaries at 80, 30, 5 per c ent, and a trace; and (2) the ratio of sand to mud (s ilt plus ::j::) with boundaries a t 9: 1, 1: 1, and 1: 9 .

The proportion of gravel i s in part a function of the highest current velocity at the :::::e of deposition, together with the maximum grain size of the detritus that i s available; !".e:ice even a minute amoun t of gravel is highly significcint. For this reason the gravel :o:1:ent is given major emphasis, and it is the first thing to determine in describing : :-: e spec imen. This is bes t done on the outcrop by naked-eye examination, perhaps :i:ced by a percen tage comparis o n c har t ; thin sections and hand spe cimens commonly ~:-:e too small a sam ple to be representati ve of the gravel content. Using this scheme, l s;:,e c imen containing m o r e than 80 per c en t gravel is termed "conglomerate"; from 30 :o 8 0 per cent gra ve l, " sandy co ng lo mera te " or "muddy conglomerate"; from 5 to 30 ;:ier c ent grave l , " co nglom e r at i c sands tone" or "conglomeratic mudstone"; from a ::- :i ce (sa y 0. 01 per c en t ) up to 5 per c en t grav el, "slightly conglomeratic sandstone" or ·•s li ght ly conglomera tic m uds ton e" ; and a spec imen containing no gravel at all may :-:inge from sandstone th.rough mudstone, depending on the sand: mud ratio.

The propor tion of sand to mud is the next proper ty to be determined, reflecting ::-: e amount of winnowing a t the site of deposit i on. Four ranks are defined on the basis : : :he sand:mud rat io ; in the nonconglomeratic tier, these are sands tone (ratio of sand '.:) :nud o ver 9: 1), muddy s a nds tone (ra tio 1: 1 to 9: 1), sandy muds tone (ratio 1:9 to 1: 1), ~::d finally, muds tone (ratio under 1: 9) . The ratio lines remain at the same value through­o•.;: the triangle; e . g . sandy conglomerate is also di vided from muddy sandy conglomerate :,:,· a sand:mud ratio of 9: 1. This di vision is fairly easy to make wi th a hand lens, unless :i '.arge amount of coarse silt and very fine sand is present.

These two simple determinations are suffi cient to place a specimen in one of the I . f •

.. . . een major textura l groups shown in table 1. One might simply stop at this point and

.s,'.:.- :-io more about the grain size; yet a great deal of information is gained by specifying, -~ ::enever practicab l e, the median diame ter of each of the fractions present. Thus two ,;:~ c: mens belonging to the conglomerati c sandstone group have quite different signifi­~;'.:-: ce if one is a bouldery fine sandstone and the other is a pebbly very coa rse sandstone. · :: r: c:eca il ed breakdown will be used in all o ur class wo r k .

,_ _ _ These fine subdivisions are determined by specifying the median diameter of each ~--·:.('~\:on considered independe~ of any other fraction that may be present. For sane . · · ·- •:nens this is, of course, not possible; but in most it can be accomplished sufficient-

~. o , 1 r f. ld ., . · · ,or 1e purposes, especially if the material is bimodal. The size terms of ~ :·~-::·,,.-orth ( 1922) are used for the various c las ses. Thus, if gravel is present, one deter­~ ;.-~:<:>.s '.v hether the median of the gravel fractio n considered alone falls in the granule, ~.: :1_. e, cobb l e, or boulder class; for example, the major group of sandy conglomerate

· ·• 0 e s ubdivided into sandy granule conglomerate, sandy pebble conglomerate, sandy

N 00

G, jr'lll.~d; 9. j""v(: 11Y i

(j), )1,·,1...tly ,.,..._-.,clly

5. s 0.-'ct i 5 • .. .... Jy

M. ~\4J; h-1, w. ... Jdy

Sf<l'- ;fy i-1d, .... .sik of J""'"~ ~Lf'""' veJ p Y< 11::,<l

Spc~;(y .,,.c.i .-"" .J;4 .r sa ,J o"lr j.,. ·h,.c 1til'l'l.e.f ""e"' 5re .... (.,. ,_..,..f •Ji t-, ·._ of ,-J

( .... ~e·t1ov 1;lr,, ~-.J.lr . or

d, ... y •r I .,._, L. .. , • .....- r ,~ le ...t

E"" ... rtv: s6, 5,...,.t., c...-,"-'e 70.~I

('.lhM, 1l;':lhti1 yr-••~1•r

h....e.J"'"7 , ,"It

"'5, c.t ... rc.r .. c:J .-~., 1,., .. J

9

GRAVEL { '7 2. ...... ,

mG msG

gM . , ...

- .. l • • • , ,. f_

.. . _. ,

\ \ • t I • t 1 • , I \ ' I

·- ·() M- , . , . • ' ", \ 9 5 • f 'I • I \

\ "' \ t I I I\ \ ~ 1

0 I I 1 • t ' t I •

MUD (S.tt -t Cl"y,

4- .0'-ZS-)

rn .,

. :::•sM' ' t • • • 'I, I \ I

o I \ I . . . . r,; s '

•• 1:1

SAN!.>: MUD KA Tio

OQ OQ -" OQ - OQ

. .

OQ

5, :,,., .. J; s,s~ .. Jy

Z, i ; ft i -z, .s ,lty

M. ,.. .. J. i ,.,, . ,.. .. .uy C, <- 1 .. , ; ,, < 1"1'l>'

Sf'<'c.Jr M <"4 ,a n

.S,L<! of ..- ...... J t\,ya ~J '- o ...J-

C U\ I" 1 :l 5 I l T

'f; J

( ~r J"-W\fk' lo.Lk,.,, Gro. vel}

u., .-,.J .. ,".,.~ 11-..t..,-"1' J p,-H, k.

Silt Siln~r1e Silt· s L.a..l< t,.'- .... d f-',. .. cls h...,._ ('1 .. J-sL. ... I, C lo.y c1 .... yr tt>--. C<--, . si..._k

~..,- J.qt,.,l,, .£_,.,._ Fo lk ,

] , .. ,._ C~t . ..,.t 62, , . 3'<f· 3ff,

J-..\r I H'f.

SAND (Ol.ZS • 2.-)

tr ,

(.

•

j ;

'

TABLE 1*

Terms Applied to Mixtures of Gravel, Sand, and Mud Delimited in the triangular diagram page

Major Textura 1 Class Examples of Usage

G . Gra v el. ............................ Cobble gravel Conglomerate ....................... Granule conglomerate

sG. Sandy gravel ........................ Sandy pebble gravel Sandy c on glomerate . .............. .. . Sandy boulder conglomerate

msG. Muddy sandy gra v el ................. Muddy sandy granule gravel Muddy ~andy conglomerate . .... ... ... Clayey sandy pebble conglomerate

mG. Muddy gra vel. ...................... Silty boulder gravel Muddy conglomera te . ................ Muddy pebble conglomerate

gS. Grave llv sand ...................... Pebbly coarse sand ~on glo merat ic s a nds tone ... . . . ... .. . Granular very fine sandstone

gmS . Gra v e lly m uddy sand .. ....... .. .. .... Pebbly silty fine sand Co nglo m e r ati c muddy sandsto ne ....... Bouldery muddy coarse s andstone.

gM. Gra ve lly ~ ud .... ............... .. . . Cobbly clay Conglo m e ra tic m u ds to n e . . .. . . ... . ... Pebbly siltstone

(g) S . Slight ly g ra v el_!y sand ...... ... . . ..... Slightly granular medium sand Sligh tly co nglo merat i c sands to ne .. . . . . Slightly pebbly coarse sandstone

(g)mS. Sli gh tly =g_r_a_v_e_ll_y _m_u_d_dl _s_a_n_d ..... .. ... Slightly pebbly muddy medium sand Sli gh tly conglomera tic muddv

sands to ne .... ........ . .. ... . .. . ... Slightly cobbly s ilty fine sands tone

(g)s:'vI. Slightly gravelly sandy ~ud .... . ...... Slightly granukr fin e sandy mud Slightly con glomeratic ~

mudstone . ...... .... . .. .. . ........ . Slightly pebb ly coc!.rse sandy claystone

(g)M. Slightly gravelly m ud ......... . ..... . Slightly pebbly clay Slightly conglomeratic muds tone ... .. . Slightly cobbly mudstone

S. Sand (spe cify sort ing) ..... . .... .. . . .. Well-sor ted fine sand Sandstone (specify sort ing) .... . . ... .. Poorly sor ted medium sandstone

rnS. Muddy_ Sand ......................... Well -sorted silty very fine sand Muddy s a..r,.ds to ne . .... .. ..... . . .. . . .. .Muddy coarse sandstone

s:\I. Sandy mud . . .. . . ... . . .... .. .. . .. .... Fine sandy clay Sandv nwds tone (spec ify struc ture) .... Coarse sandy siltstone (if fissile,

coarse sandy s ilt -shale) :-.-I. M d l ~ . ..... . ......... ... ............ Sit

;\fods tone (specify struc ture) ......... Muds tone (if fi ssi le, mud-shale)

•3 ' . otn unconsolidated and consolida ted equivalents are shown in thi s table. The under-::ned terms are "u r ther spe cified as to their grain size, as shown in the examples.

......

cob.ble conglomerate, and sandy boulder conglomerate. For the sand fraction, the , median is also estimated separately, using the standard Wentworth grades of very

coarse sand, coarse sand, medium sand, fine sand, and very fine sand. This can be done very easily by reference to a comparison set of sand grains of the several sizes

For muds a somewhat different procedure is used, the name depending on the relative proportion of s ilt versus clay. This proportion is usually ve ry difficult to determine with a hand lens, and the only really satisfactory way is to make a thin ­section (preferable) or a grain-size analysis by pipette or hydrome ter. In many samples it might be best just to use the broad term " mud" and not attempt to split it any furthe r. But the mud fraction of many s ediments is obviously composed dominant­ly of silt, while the mud fraction of others is just as certainly composed largely of clay, therefore, it is considered worth while to make an attempt, if at all practicable, to estimate this ratio . . 0... threefold division is sugges ted : if the mud fraction contains mo:, than 67 per cent silt (i.e., sil~- to clay ratio greater than 2: 1) , the material should be called "silt" or "silty"; if more than 67 per cent clay is present , it should be called "clay" or "clayey" and for intermediate mixtures, the term "mud or "muddy" (u sed in a restricted sense) is proposed. Thus the major group of muddy s2.ndstone may be divided into clayey fine sandstone, silty very fi ne sands to ne, muddy coarse sands ton e, and so on, since bo th the grain size of the sand fraction and the inud composition are to be specified.

The complete ~eries of major textural groups i s pre sen ted in the table. Both unconsolidated and consolidated equivalents are gi ven for each group. At the right of the group name i s given an example of how the terms are to be further s pecified, depending on the median grain size of each fraction present. Sand, silty sand, and slightly conglomeratic sand may be fu rther described by mentioning the degree of sortir.

The major textural groups of "mudstone" and " sandy mudstone" should be modiii! according to their s truc ture, spe ci mens with a well - developed closely spa c ed pa r ting parallel with the beds being term•=d "shale," regardless of whether they are composed, clay, silt, sandy clay, mud, or any other mixture of materials, This additional modi­ficatio n is presented in tabl e 2.

T.,;BLE 2

Classificat ion of F ine -Grained Rocks Bc:ised on Grain Size, Induration, and Structure

Textural Class Unindurated Indura te d, Not F issile

Indurated and Fissi le

z Silt ( > 67 per ce nt silt) Siltstone Silt-shale :'\ I Mud (intermediate) Muds to ne Mud-shale C Clay ( --;? 6 7 per cent clay) Clays to ne Clay -shale sZ Sandy silt Sandy siltstone Sandy silt-sha le

sM Sandy mud Sandy muds tone Sandy mud-shal,

sC Sandy clay Sandy clays tone Sandy clay -shale

30

suff For 1

nen mic narr. cha : are pe r< of tl con t ind text · horn thor cyc l by a of g· size frag follc

-