Location via proxy:   [ UP ]  
[Report a bug]   [Manage cookies]                
SlideShare a Scribd company logo

La textura y clasificación en las areniscas folk

•
•
UO
UO

This document discusses the classification of sandstones based on texture and composition. It summarizes Bokman's criticisms of Folk's previous classification scheme, which considered these properties separately. Folk argues that texture (maturity) is primarily determined by environmental factors, not tectonism as Bokman claimed. Folk also disagrees with Bokman's hypothesis that immature sediments must be rich in feldspar/metamorphic rock fragments, while mature sediments are quartz/chert-rich. Folk provides a table comparing his classification terms to Bokman's and defends considering texture and composition as separate properties that can independently vary.

1 of 6
JOI:RNAL OF SE[nMENTAR~"PETRClI.OGY, VOL. 26, NO. 2, PP. 166 171 Fro. 1, Jq'xE. 1956 DISCUSSION T H E ROI.E OF T E X T U R E A N D C O M P O S I T I O N IN S A N D - STONE CLASSIFICATION ROBERT L. FOLK Univerity of Texas, Austin, Texas In a recent issue of this Journal (voh Before discussing the two a r g u m e n t s 25, pp. 201-206) John Bokman consid- above in more detail, I would like to con- ered the question of sandstone classifica- sider first the concept of textural ma tion in relation to mineral composition turity and its relation to tectonism versus and texture. In this discussion he sup- environment. Bokman sets up three high- ports the viewpoints proposed by Petti- Iv idealized e n v i r o n m e n t a l groups of john (1949), 1)al)ples, Krunll)ein. and sandstones based on the presence of Sloss ( 1953), and again Pettijolm (1954) ; three "energy levels" existing at the site but he vigorously criticizes the classifica- of deposition, with the resulting sedi tion pul+lished by this writer (Folk, 1954) ments possessing three (or really four) in ++hieh a ternary basis few rock nomen- stages of textural maturity. According to clature was introduced, inchlding grain Bokman, these are (l) the undisturbed size, textural maturily, and mineral con> enviroument, supl)osedly typical of rapid position. Apparently" Boktnan on p, 201 burial in geosynclines and characterized follows wholeheartedlv this writer's four by sands rich in clay (immature stage of stages of textural m a t u r i t y (Folk, 1951) Folk); (2) partially reworked zone. oc- - although he fails to cite the paper in curring on unstable s h e h e s and intra- which this classification was proposed- cratonic basins, and characterized by es- and he also agrees fully t h a t clay conlent sentially clay-free but still poorly-sorted should be considered a textural and not sediments (submature stage of Folk) ; and a compositional property. Furthernmre, (3) intensely reworked zone, typical of Bokman adopts a mineralogical classifi- stable shelves and marked by well- cation essentially similar to t h a t pro- sorted sands (mature stage of Folk), with posed by this writer (1954), and also like the most stable shelves also having well- those suggested t33" Krynine (1948) and rounded sands (stq)ermature stage of Dapples. Krumbein, and Sloss (1953). Folk). I?okman's only major criticism of this W h e n this writer first published the writer's classification thus comes down tu idea of textural maturity, he also at- (1) what shall we call the rock types t e m p t e d to tie it up with the rate of subsi- formed by combination of the three com- dence of the basin of deposition, hence positional end-members with the four tectonic activity. Now, however, he stages of textural maturity, and (2) what reat[zes t h a t this was a naive oversimpli- is the extent of variation from an alleged fication and t h a t textural m a t u r i t y is " m a i n sequence" of sandstone types .... much more a function of local environ- the hypothesis of a " m a i n sequence" t h a t m e n t r a t h e r than overall tectonism, es- supposes t h a t texturally i m n m t u r e sedi- pecially in the lower stages of maturity. ments are nearly all rich in feldspars or A flood-plain or neritic sediment u-ill metamorphic rock fragments ("crushed probably contain jnst as much clay if rock complex" of Dapples, Krnmbein, the depositional basin is sinking at the and Sloss), while texturally mature and rate of one inch per hundred ?'ears, or s u p e r m a t u r e sediments are almost ahvays one inch per ten thousand ?'ears. Simi- nearly pure quartz or chert. larly, a sample of beach sand will get just
D [SC USSIOX 167 as well sorted in one hundred years (or stability beach sands will be a b u n d a n t in even in a few weeks) as it will if given the section, and deltaic sediments scarce; one hundred thousand years. Texas Gulf while during a period of intense geosyn- coast beaches (and their buried Tertiary clinal deformation deltaic and neritic sed- equivalents) have excellent sorting values, iments will be very a b u n d a n t and beach with cr in the range of 0.25-0.35 ~; yet sands scarce. Yet beach sands deposited [ daresav no one wonld call this region a in b o t h tectonic frameworks will have es- "stable shelf." One swish of a miner's sentially similar sorting values, and neri- pan does a p r e t t y good job of sorting tic sediments will be just as immature. sand. and certainly of winnowing out Bokman also errs when he equates clay: hence no hmg period of stability is clayey sandstones with the undisturbed required to get well-sorted sediments, and (presumably deep-water) e n v i r o n m e n t good sorilng eannot be equated with tec- "typical of the eugeosyncline." Admit- tonic stability as I lokman has done tedly, clayey sands develop when very noa, and as the ~ r i t e r did in 1951. Only little energy is expended on the sediment in roundness does teeton[sm have much after its initial deposition, b u t this situa- Io do with textural maturity, and even tion can happen in numerous ways. Not here it has much less influence than com- only does it happen in deep waters, but monly thought. Sands in the carbonate it also happens in mudflows, m a n y allu- sequence of the Ellenlmrger group (Ordo- vial fans, flood plains, swamps, lagoons, vician of sul3surface Vest Texas) slaow a deltas, and is probably characteristic of repeated alternation of streaks of angular most neritie sands along normal coasts. sands with beds of superlfly rounded Shall these all be glibly considered eugeo- sands of the same grain size. all coming svnclinal deposits? Furthermore, on page directly from the same primary granitic 2(12 he implies t h a t most of these clayey and gneissic source area; if periods of tec- sands are deposited by t u r b i d i t y currents! tonic stability are called upon to explain Until the precise mode of deposition of the beds of well-rounded sand. then these clayey sands is supported by more con- stable periods must have each covered crete d a t a from recent sediments, 1 con- only a minute fraction of the Ordovician. tend t h a t most clayey floodplain and Similar brief stable periods are indicated neritic sands in all but quite deep waters for the Silurian sands of West Virginia, are simply deposited by ordinary sedi- where beds of perfectly rounded grains mentation, where clay b r o u g h t out to sea alternate with beds of angular grains, all by muddy rivers or set in motion by wave coming (toni the sanle source area. In the turbulence simply settles by some quite opinion of this writer, the alternations " n o r m a l " mechanism. By extension of arc! caused bx llnct:uating environnients the overworked t u r b i d i t y current hy- and arc' modified only slightly bv tec- pothesis, should we assume t h a t all clays lonisnl, and shales are deposited by this process in :lthough the environnlent of deposi- engeosynclinal loci? Of course n o t - - h e n c e tion is a p p a r e n t l y the immediate con- if clays and shales can form by ordinary trolling factor in textural maturity, the processes of deposition, then clayey sands e n v i r o n m e n t s themselves are a function can also form this way. of something much more basic. As Kry- My present feeling on m a t u r i t y is t h a t nine (1951) has shown, the degree and (l) it depends chiefly on the a m o u n t of type of techmic activity does determine energy applied to the sediment after it is a certain preferred association of source b r o u g h t to the site of deposition, hence area lithology and relief, geomorphic is almost entirely a function of environ- processes, and rate of subsidence of the ment and is virtually i n d e p e n d e n t of tec- deposilional area. These factors in turn tonism. Even in the most violently sub- integrate u~ produce preferred associa- siding basins the rate of subsidence is so tions of e n v i r o n m e n t s : during a period of slow in inches per year t h a t the environ-
168 DISCUSSION "FABLE l . - - S a n d s t o n e classification table based on the two parameters of textural maturity and mineral composition. Folk's (1954) terms in ordinary type, corresponding terms of B o k m a n (1955) in italics. A s t e r i s k s denote rock types not recognized by B o k m a n but considered to be of great importance by Folk State of Textural Feldspar-Rich Quartz- and Chert- Metamorphic- Rich Maturity Sands Rich Sands Sands Quartz grains well Supermature Supermature (Supermature rounded, well sorted, Arkose Orthoquartzite Gravwacke) no cla}" * * * * * Orthoquartzite * *" * * * Grains well sorted, Mature Arkose Mature Orthoquartzlte Mature Gravwacke no clay but not well * * * * * Orthoquartzite * * * +* * rounded. Very little or no clay, Submature Submature Submature but grams poorly Arkose Orthoquartzite Graywacke sorted. Arkose * * * * * Lithic Sandstone Over S percent clay, hnmature Arkose Immature Orthoquartzite Immature Graywacke poorly sorted, and Arkose * * * * * Lithic Sandstone angutar. Over 20 percent clay, Immature Arkose Immature Orthoquartzite ImmatureGraywacke poorly sorted, and Feldspathic * * * * * Lithie Graywacke angular. Gra ywaeke ment has ample opportunity to modify tion that textural maturity and mineral the detritus; (2) a surge of excess energy composition should be described in sepa- can actualh" decrease the maturity of a rate terms to avoid confusion and over- sediment (as when hurricane winds mix generalization. Bokman states on pp. well-sorted beach sands with ahnost pure 201-202, t h a t "Composition and texture lagoonal clays to form a very poorly are i n t e r d e p e n d e n t properties . . . conse- sorted mixture, or when sudden moun- quently, they are not susceptible to sepa- tain torrents shatter pebbles that were rate t r e a t m e n t " ; yet in the three para- previously rounded by more gentle graphs immediately preeeding this state- streams); and (3) stratigraphic forma- ment, he treats these properties inde- tions all show a range in maturity on the pendently himself. The differences in the individual sample level: hence, in many nomenclature are indicated in table 1 formations clayey sands alternate with with my terms in ordinary type and well-sorted sands, l would hardly call Bokman's terms in italics. For brevity, this an alternation between eugeosyn- only the e n d - m e m b e r s are shown here; clinal and stable shelf conditions when the rocks rich in both metamorphic rock frag- change soccur on the scale of a foot or so! ments and feldspars are omitted, as are Bokman's first major eritleism of my transitional types between the ortho- classification is in the names applied to quartzite and the two other end-mem- the rocks resulting from the various bers. Thus Bokman's " p r o t o q u a r t z i t e " combinations of textural maturity with oceupies the field on either side of the mineral composition, l feel t h a t names orthoquartzite in the i m m a t u r e to sub- should be subordinated to concepts, so mature range, and my "subarkose" and that if the principles of formation are "subgraywacke" occur also on each side- understood and the petrographer pub- of the orthoquartzite, but exist (in na- lishes adequate detailed descriptions, ture as well as in theory) in all stages of anyone is entitled tu give any name he textural maturity. The asterisks denote pleases to the rock under consideration. rock types apparently not recognized or lh~wever, I earnestly defend the proposi- specified by Bokman.
DISC USSION 169 C o n t r a r y to the s t a t e m e n t s of Bokman namely, t h a t i m m a t u r e a n d s u b m a t u r e (as reflected also in his lack of a com- sediments must be rich in feldspars or plete set of terms), all these "pigetm- m e t a m o r p h i c rock fragments, and t h a t holes" are occupied by significant vol- m a t u r e and s u p e r m a t u r e sediments must umes of rock in the stratigraphic section consist almost entirely of quartz a n d with but one exception: the s u p e r m a t u r e chert. B o k m a n implies t h a t mineral con> graywacke which may well be a non- position and textural m a t u r i t y are in- existent type. The question of which capable of i n d e p e n d e n t variation, and scheme is more logical should now be left t h a t no samples occur outside of this to the j u d g e m e n t of time; I have previ- " m a i n sequence." To quote: " T o t r e a t o u s h listed (Folk, 1954, pp. 352-356) [composition and texture separately] . . . my reasons for objecting to the use of the leads to a classification which is meaning- term graywacke as a s y n o n y m for a less in the sense t h a t m a n y of the 'pigeon- clayey sandstone and will not go into holes' are unoccupied in a c t u a l i t y . . . . " Ihose reasons again here. I contend t h a t the close linkage between Two minor points which come up in composition and texture proposed by allotting rock constituents to the Q, F, Dapples, Krumbein, and Sloss and re- or M poles: (1) I st)ecitically excluded stated by Bokman is far too idealized to clays from the mineralogical triangle in fit the actual situation. The)" a t t e m p t to three separate s t a t e m e n t s (Folk. 1954, force sediments into a rigidly artificial pp. 352. 353. 355). although Bokman on mold to which they simply will not con- page 205 distinctly states t h a t I included form, as a n y examination of an a d e q u a t e it with the m e t a m o r p h i c pole. I did in- thin-section collection will show. On clude with the M pole all coarse-silt page 205 Bokman indirectly a t t a c k s size grains of chlorite, sericite, and other Krynlne (without mentioning his name) fine micas, for a very practical reason: for oversimplifying the linkage between since micas are included with the M pole, tectonism and mineral composition, b u t it is fatuous hairsplitting to draw an), t h r o u g h o u t his entire paper B o k m a n falls distinct line between the finer micas and into the very same alleged error by as- the silt-sized sericite or chlorite. Bokman suming a direct and vastly oversimplified sidesteps this slight difficulty by not link between tectonic state, textural ma- mentioning micas a t all in his classifica- turity, and mineral c o m p o s i t i o n - - a link- tion. (2) In most good graywackes there age apparently so tenuous t h a t he pre- is a complete transition between frag- sents no evidence, either from the ments of schist, quartzose schist, mica- stratigraphic column or from recent sedi- ceous metaquartzite, and " p u r e " meta- ment work, to back it up. quartzite; here no one would question al- | agree t h a t most sediments do tend to lotting stretched metaquartzite to the follow the main sequence--i.e, t h a t im- M pole. If one is to be consistent, meta- m a t u r e graywackes, as they undergo quartzite is a metamorphic rock frag- more and more abrasion, tend to pass up ment and should be counted as such the scale of m a t u r i t y as they lose their whether in a polar graywacke or an al- less durable m e t a m o r p h i c c o n s t i t u e n t s most pure orthoquartzite. Rock descrip- and eventually pass t h r o u g h subgray- tions should not be bent to conform to wackes into orthoquartzites. Similarly, our preconceived notions of how the rock arkoses, either through abrasion or should be classified. weathering, tend to lose their feldspar B o k m a n ' s most f u n d a m e n t a l a r g u m e n t as they pass up the scale of m a t u r i t y and against the classification of this writer is grade into subarkoses and ultimately in the relationshi 1) between the two vari- orthoquartzites. Yet, the f u n d a m e n t a l ables, textural m a t u r i t y and mineral difference is t h a t Bokman feels t h a t rocks coml)osition. He assumes the same gross- falling on the main sequence are the only Ix" overgeueralized hypothesis proposed ones worthy of classification, whereas l by l)at)ples , Krumbein, and Sloss (1954): have examined (and c i t e d - - F o l k , 1954,
170 D[SCUSSION Percent F Percent M 5% ~ Supermature Mature ,/ 4 !l :II L 71 ,ol I "~l: I /' /. l~'n. I I ', ql ,, ~1 I Submalure ' I : ,...,kay.,:/, Immature I • • " "''* "L' ~o oe ,> / Arkose Subarkose Ortho- Subgroy- Graywacke quartzite wacke Fro. l.--Mineral composition plotted against textural maturity for several sandstone for- mations. This diagram illustrates that almost any combination of the two variables may occnr. According to other authors, sediments occur ahnost entirely on the "main sequence," exteading in an inverted V-shaped trend from the immature arkose to the mature or supermature ortho- quartzite, and back down to the imnlature graywacke. The "main sequence" hypothesis is clearly, a profound oversimplification. 1. Lower Ordovican Beekmantown sands, Pennsylvania. Folk, manuscript in preparation. This field also applies to Upper Cambrian, Pennsylvania. 2. Lower Ordovician Ellenburger sands, Texas. Folk, manuscript in preparation. 3. Upper Permian (?) Pierce Canyon formation, Texas and New Mexico. D. N. Miller, Ph. D. Thesis, University of Texas, 19.55. 4. Triassic Newark Series, Connecticut- Krynine, 1950. 5. Recent Gulf Coastal sands, Texas. Folk, unpublished observations. 6. Upper Cambrian Potsdam sandstone, New York. Folk, unpublished observations. 7. Silurian System, West Virginia. Folk, research in progress. 8. Cretaceous Woodbine sand, Texas. A. S. Cotera, M. A. Thesis, University of Texas. 9. Pennsylvanian sandstones, central Texas. Folk, unpublished observations. 10. Recent Gulf Coastal sands, Mississippi and Alabama. Folk, unpublished observations. 11. Ordovlcian Juniata and Oswego formations, Pennsylvania. O. F. Turtle, M. S. Thesis, The Pennsylvania State College, 1940. An essentially coexistent field is occupied by the Devonian ;Fhird Bradford Sand, Krynine, 1940. 12. E~mene Carrizo and Newbv sands, Texas. T. W. Todd, M. A. Thesis, University of Texas, 1955. 13. Penusylvanian sandstones, eastern Pennsylvania. Folk, unpublished observations. 14. Numerous scattered samples of Mesozoic sands, Texas and New Mexico. Folk, personal observations.
D I S C {'SSIO N 171 p. 355, 357) a great many examples of terpreting such stellar types as red giants rocks which fall off this main sequence: or in tracing the evolution of star clus- perfectly rotmded, beautifully-sorted ar- ters. T h e ) do not ignore or fail to classify koses from the Cambro-Ordovieian of stars t h a t fall off the main sequence; the Appalachian area; well-sorted gray- these unusual ones are precisely the ones wackes, rich in metamorphic rock frag- t h a t furnish the most valuable clues to ments, from the Upper Paleozoic of the the origin of our universe! Appalachians and Texas; poorly-sorted, Similarly, sedimentary petrographers angular, clay-rich orthoquartzite sands obtain some of their most valuablein- from the Pennsyh'anian of Texas, Si- formation from sandstones that fall off lurian of West Virginia. and uther locali- the main ti'end. Thus well-sorted, well- ties. These rock types are plotted in fig- rounded arkoses are believed to indicate ure 1. These important and widespread aridity in the source area, for under al- sandstone varieties occupy the " v a c a n t most any other climatic conditions the pigeonholes" which Bokinan contends feldspar would have been destroyed by are present in my c l a s s i f i c a t i o n - r o c k weathering before it had a chance to get types recognized neither by him nor by rounded. The clayey, poorly-sorted or- I)apples, Kruml)ein, and Slnss. Bok- thoquartzite-type sands also imply spe- man's compilation of 6l literature cialized conditions: either fairly rapid analyses (to which no references are erosion and deposition of sediments from given) apparently was not sufficiently an area of older sedimentary rock out- comprehensive to include these types. crops, or a mixture of several environ- A fertile analogy nmy be drawn be- ments, e.g. well-sorted beach sands mixed tween sandstone classification and the with lagoonal clays. To ignore these im- classification of stars. Astronomers pos- portant rock types because the)- do not sess a wonderful tool in a graph known follow the theoretical main sequence is to as the Hertzsprung-Russell diagram, in view the complexities of sandstone types which the magnitude (brightness) of a with rose-colored glasses. Bokman's con- star is plotted against its color, just as I nection of tectonic state with textural have plotted textural maturity against maturity may be an interesting theoreti- composition in figure 1. On the Hertz- cal exercise, but there are so many ex- sprung-Russell plot, most stars follow the ceptions t h a t I seriously question its "inaiu sequence" iu that faint stars tend utility, and even question its ultimate to be reddish and bright stars bluish, the scientific basis. His contention t h a t min- magnitude verst,s color trend forming a eral composition is a direct function of compressed band. But astronomers obtain textural maturity is also shown to be some of their most vital information by erroneous and simplified beyond the studying the departures from this trend, point of realism by the specific examples thus using the diagram as a tool in in- I have cited. REFERENCES BOKMAN, JOItN, 195.5, Sandstone classilication: relation to composition and texture: Jour. Sedimentary Petrology, v. 25, pp. 201 206. DAPPLES, E. C., [~.RUMBEIN,~.'. C., AND SLOSS, L. L., 1953, Petrographic and lithologic attri- butes of sandstones: Jour. Geology, v. 61, pp. 291-317. FOLK, R. L., 1951, Stages of textural maturity in sedimentary rocks: Jour. Sedimentary Petro- logy, v. 21, pp. 127-130. , 1954, The distinction between grain size and mineral composition in sedimentary rock nomenclature: Jour. Geology, v. 62, pp. 344 359. KRYNINE, P. l)., 1948, The megascopic study and field classification of sedimentary rocks: Jour. Geology, v. 56. pp. 130-165. , 1951, A critique of geotectonie elements: Am. Geophys. Union Trans., v. 32, pp. 743- 748. PE'rTIJOHN, F. J., 1940, S, edimentary rocks. Harper & Bros., New York. 526 pp. , 1954, Classification of sandstones: Jour. Geology, v. 62, pp. 360-365.

More Related Content

La textura y clasificación en las areniscas folk

  • 1. JOI:RNAL OF SE[nMENTAR~"PETRClI.OGY, VOL. 26, NO. 2, PP. 166 171 Fro. 1, Jq'xE. 1956 DISCUSSION T H E ROI.E OF T E X T U R E A N D C O M P O S I T I O N IN S A N D - STONE CLASSIFICATION ROBERT L. FOLK Univerity of Texas, Austin, Texas In a recent issue of this Journal (voh Before discussing the two a r g u m e n t s 25, pp. 201-206) John Bokman consid- above in more detail, I would like to con- ered the question of sandstone classifica- sider first the concept of textural ma tion in relation to mineral composition turity and its relation to tectonism versus and texture. In this discussion he sup- environment. Bokman sets up three high- ports the viewpoints proposed by Petti- Iv idealized e n v i r o n m e n t a l groups of john (1949), 1)al)ples, Krunll)ein. and sandstones based on the presence of Sloss ( 1953), and again Pettijolm (1954) ; three "energy levels" existing at the site but he vigorously criticizes the classifica- of deposition, with the resulting sedi tion pul+lished by this writer (Folk, 1954) ments possessing three (or really four) in ++hieh a ternary basis few rock nomen- stages of textural maturity. According to clature was introduced, inchlding grain Bokman, these are (l) the undisturbed size, textural maturily, and mineral con> enviroument, supl)osedly typical of rapid position. Apparently" Boktnan on p, 201 burial in geosynclines and characterized follows wholeheartedlv this writer's four by sands rich in clay (immature stage of stages of textural m a t u r i t y (Folk, 1951) Folk); (2) partially reworked zone. oc- - although he fails to cite the paper in curring on unstable s h e h e s and intra- which this classification was proposed- cratonic basins, and characterized by es- and he also agrees fully t h a t clay conlent sentially clay-free but still poorly-sorted should be considered a textural and not sediments (submature stage of Folk) ; and a compositional property. Furthernmre, (3) intensely reworked zone, typical of Bokman adopts a mineralogical classifi- stable shelves and marked by well- cation essentially similar to t h a t pro- sorted sands (mature stage of Folk), with posed by this writer (1954), and also like the most stable shelves also having well- those suggested t33" Krynine (1948) and rounded sands (stq)ermature stage of Dapples. Krumbein, and Sloss (1953). Folk). I?okman's only major criticism of this W h e n this writer first published the writer's classification thus comes down tu idea of textural maturity, he also at- (1) what shall we call the rock types t e m p t e d to tie it up with the rate of subsi- formed by combination of the three com- dence of the basin of deposition, hence positional end-members with the four tectonic activity. Now, however, he stages of textural maturity, and (2) what reat[zes t h a t this was a naive oversimpli- is the extent of variation from an alleged fication and t h a t textural m a t u r i t y is " m a i n sequence" of sandstone types .... much more a function of local environ- the hypothesis of a " m a i n sequence" t h a t m e n t r a t h e r than overall tectonism, es- supposes t h a t texturally i m n m t u r e sedi- pecially in the lower stages of maturity. ments are nearly all rich in feldspars or A flood-plain or neritic sediment u-ill metamorphic rock fragments ("crushed probably contain jnst as much clay if rock complex" of Dapples, Krnmbein, the depositional basin is sinking at the and Sloss), while texturally mature and rate of one inch per hundred ?'ears, or s u p e r m a t u r e sediments are almost ahvays one inch per ten thousand ?'ears. Simi- nearly pure quartz or chert. larly, a sample of beach sand will get just
  • 2. D [SC USSIOX 167 as well sorted in one hundred years (or stability beach sands will be a b u n d a n t in even in a few weeks) as it will if given the section, and deltaic sediments scarce; one hundred thousand years. Texas Gulf while during a period of intense geosyn- coast beaches (and their buried Tertiary clinal deformation deltaic and neritic sed- equivalents) have excellent sorting values, iments will be very a b u n d a n t and beach with cr in the range of 0.25-0.35 ~; yet sands scarce. Yet beach sands deposited [ daresav no one wonld call this region a in b o t h tectonic frameworks will have es- "stable shelf." One swish of a miner's sentially similar sorting values, and neri- pan does a p r e t t y good job of sorting tic sediments will be just as immature. sand. and certainly of winnowing out Bokman also errs when he equates clay: hence no hmg period of stability is clayey sandstones with the undisturbed required to get well-sorted sediments, and (presumably deep-water) e n v i r o n m e n t good sorilng eannot be equated with tec- "typical of the eugeosyncline." Admit- tonic stability as I lokman has done tedly, clayey sands develop when very noa, and as the ~ r i t e r did in 1951. Only little energy is expended on the sediment in roundness does teeton[sm have much after its initial deposition, b u t this situa- Io do with textural maturity, and even tion can happen in numerous ways. Not here it has much less influence than com- only does it happen in deep waters, but monly thought. Sands in the carbonate it also happens in mudflows, m a n y allu- sequence of the Ellenlmrger group (Ordo- vial fans, flood plains, swamps, lagoons, vician of sul3surface Vest Texas) slaow a deltas, and is probably characteristic of repeated alternation of streaks of angular most neritie sands along normal coasts. sands with beds of superlfly rounded Shall these all be glibly considered eugeo- sands of the same grain size. all coming svnclinal deposits? Furthermore, on page directly from the same primary granitic 2(12 he implies t h a t most of these clayey and gneissic source area; if periods of tec- sands are deposited by t u r b i d i t y currents! tonic stability are called upon to explain Until the precise mode of deposition of the beds of well-rounded sand. then these clayey sands is supported by more con- stable periods must have each covered crete d a t a from recent sediments, 1 con- only a minute fraction of the Ordovician. tend t h a t most clayey floodplain and Similar brief stable periods are indicated neritic sands in all but quite deep waters for the Silurian sands of West Virginia, are simply deposited by ordinary sedi- where beds of perfectly rounded grains mentation, where clay b r o u g h t out to sea alternate with beds of angular grains, all by muddy rivers or set in motion by wave coming (toni the sanle source area. In the turbulence simply settles by some quite opinion of this writer, the alternations " n o r m a l " mechanism. By extension of arc! caused bx llnct:uating environnients the overworked t u r b i d i t y current hy- and arc' modified only slightly bv tec- pothesis, should we assume t h a t all clays lonisnl, and shales are deposited by this process in :lthough the environnlent of deposi- engeosynclinal loci? Of course n o t - - h e n c e tion is a p p a r e n t l y the immediate con- if clays and shales can form by ordinary trolling factor in textural maturity, the processes of deposition, then clayey sands e n v i r o n m e n t s themselves are a function can also form this way. of something much more basic. As Kry- My present feeling on m a t u r i t y is t h a t nine (1951) has shown, the degree and (l) it depends chiefly on the a m o u n t of type of techmic activity does determine energy applied to the sediment after it is a certain preferred association of source b r o u g h t to the site of deposition, hence area lithology and relief, geomorphic is almost entirely a function of environ- processes, and rate of subsidence of the ment and is virtually i n d e p e n d e n t of tec- deposilional area. These factors in turn tonism. Even in the most violently sub- integrate u~ produce preferred associa- siding basins the rate of subsidence is so tions of e n v i r o n m e n t s : during a period of slow in inches per year t h a t the environ-
  • 3. 168 DISCUSSION "FABLE l . - - S a n d s t o n e classification table based on the two parameters of textural maturity and mineral composition. Folk's (1954) terms in ordinary type, corresponding terms of B o k m a n (1955) in italics. A s t e r i s k s denote rock types not recognized by B o k m a n but considered to be of great importance by Folk State of Textural Feldspar-Rich Quartz- and Chert- Metamorphic- Rich Maturity Sands Rich Sands Sands Quartz grains well Supermature Supermature (Supermature rounded, well sorted, Arkose Orthoquartzite Gravwacke) no cla}" * * * * * Orthoquartzite * *" * * * Grains well sorted, Mature Arkose Mature Orthoquartzlte Mature Gravwacke no clay but not well * * * * * Orthoquartzite * * * +* * rounded. Very little or no clay, Submature Submature Submature but grams poorly Arkose Orthoquartzite Graywacke sorted. Arkose * * * * * Lithic Sandstone Over S percent clay, hnmature Arkose Immature Orthoquartzite Immature Graywacke poorly sorted, and Arkose * * * * * Lithic Sandstone angutar. Over 20 percent clay, Immature Arkose Immature Orthoquartzite ImmatureGraywacke poorly sorted, and Feldspathic * * * * * Lithie Graywacke angular. Gra ywaeke ment has ample opportunity to modify tion that textural maturity and mineral the detritus; (2) a surge of excess energy composition should be described in sepa- can actualh" decrease the maturity of a rate terms to avoid confusion and over- sediment (as when hurricane winds mix generalization. Bokman states on pp. well-sorted beach sands with ahnost pure 201-202, t h a t "Composition and texture lagoonal clays to form a very poorly are i n t e r d e p e n d e n t properties . . . conse- sorted mixture, or when sudden moun- quently, they are not susceptible to sepa- tain torrents shatter pebbles that were rate t r e a t m e n t " ; yet in the three para- previously rounded by more gentle graphs immediately preeeding this state- streams); and (3) stratigraphic forma- ment, he treats these properties inde- tions all show a range in maturity on the pendently himself. The differences in the individual sample level: hence, in many nomenclature are indicated in table 1 formations clayey sands alternate with with my terms in ordinary type and well-sorted sands, l would hardly call Bokman's terms in italics. For brevity, this an alternation between eugeosyn- only the e n d - m e m b e r s are shown here; clinal and stable shelf conditions when the rocks rich in both metamorphic rock frag- change soccur on the scale of a foot or so! ments and feldspars are omitted, as are Bokman's first major eritleism of my transitional types between the ortho- classification is in the names applied to quartzite and the two other end-mem- the rocks resulting from the various bers. Thus Bokman's " p r o t o q u a r t z i t e " combinations of textural maturity with oceupies the field on either side of the mineral composition, l feel t h a t names orthoquartzite in the i m m a t u r e to sub- should be subordinated to concepts, so mature range, and my "subarkose" and that if the principles of formation are "subgraywacke" occur also on each side- understood and the petrographer pub- of the orthoquartzite, but exist (in na- lishes adequate detailed descriptions, ture as well as in theory) in all stages of anyone is entitled tu give any name he textural maturity. The asterisks denote pleases to the rock under consideration. rock types apparently not recognized or lh~wever, I earnestly defend the proposi- specified by Bokman.
  • 4. DISC USSION 169 C o n t r a r y to the s t a t e m e n t s of Bokman namely, t h a t i m m a t u r e a n d s u b m a t u r e (as reflected also in his lack of a com- sediments must be rich in feldspars or plete set of terms), all these "pigetm- m e t a m o r p h i c rock fragments, and t h a t holes" are occupied by significant vol- m a t u r e and s u p e r m a t u r e sediments must umes of rock in the stratigraphic section consist almost entirely of quartz a n d with but one exception: the s u p e r m a t u r e chert. B o k m a n implies t h a t mineral con> graywacke which may well be a non- position and textural m a t u r i t y are in- existent type. The question of which capable of i n d e p e n d e n t variation, and scheme is more logical should now be left t h a t no samples occur outside of this to the j u d g e m e n t of time; I have previ- " m a i n sequence." To quote: " T o t r e a t o u s h listed (Folk, 1954, pp. 352-356) [composition and texture separately] . . . my reasons for objecting to the use of the leads to a classification which is meaning- term graywacke as a s y n o n y m for a less in the sense t h a t m a n y of the 'pigeon- clayey sandstone and will not go into holes' are unoccupied in a c t u a l i t y . . . . " Ihose reasons again here. I contend t h a t the close linkage between Two minor points which come up in composition and texture proposed by allotting rock constituents to the Q, F, Dapples, Krumbein, and Sloss and re- or M poles: (1) I st)ecitically excluded stated by Bokman is far too idealized to clays from the mineralogical triangle in fit the actual situation. The)" a t t e m p t to three separate s t a t e m e n t s (Folk. 1954, force sediments into a rigidly artificial pp. 352. 353. 355). although Bokman on mold to which they simply will not con- page 205 distinctly states t h a t I included form, as a n y examination of an a d e q u a t e it with the m e t a m o r p h i c pole. I did in- thin-section collection will show. On clude with the M pole all coarse-silt page 205 Bokman indirectly a t t a c k s size grains of chlorite, sericite, and other Krynlne (without mentioning his name) fine micas, for a very practical reason: for oversimplifying the linkage between since micas are included with the M pole, tectonism and mineral composition, b u t it is fatuous hairsplitting to draw an), t h r o u g h o u t his entire paper B o k m a n falls distinct line between the finer micas and into the very same alleged error by as- the silt-sized sericite or chlorite. Bokman suming a direct and vastly oversimplified sidesteps this slight difficulty by not link between tectonic state, textural ma- mentioning micas a t all in his classifica- turity, and mineral c o m p o s i t i o n - - a link- tion. (2) In most good graywackes there age apparently so tenuous t h a t he pre- is a complete transition between frag- sents no evidence, either from the ments of schist, quartzose schist, mica- stratigraphic column or from recent sedi- ceous metaquartzite, and " p u r e " meta- ment work, to back it up. quartzite; here no one would question al- | agree t h a t most sediments do tend to lotting stretched metaquartzite to the follow the main sequence--i.e, t h a t im- M pole. If one is to be consistent, meta- m a t u r e graywackes, as they undergo quartzite is a metamorphic rock frag- more and more abrasion, tend to pass up ment and should be counted as such the scale of m a t u r i t y as they lose their whether in a polar graywacke or an al- less durable m e t a m o r p h i c c o n s t i t u e n t s most pure orthoquartzite. Rock descrip- and eventually pass t h r o u g h subgray- tions should not be bent to conform to wackes into orthoquartzites. Similarly, our preconceived notions of how the rock arkoses, either through abrasion or should be classified. weathering, tend to lose their feldspar B o k m a n ' s most f u n d a m e n t a l a r g u m e n t as they pass up the scale of m a t u r i t y and against the classification of this writer is grade into subarkoses and ultimately in the relationshi 1) between the two vari- orthoquartzites. Yet, the f u n d a m e n t a l ables, textural m a t u r i t y and mineral difference is t h a t Bokman feels t h a t rocks coml)osition. He assumes the same gross- falling on the main sequence are the only Ix" overgeueralized hypothesis proposed ones worthy of classification, whereas l by l)at)ples , Krumbein, and Sloss (1954): have examined (and c i t e d - - F o l k , 1954,
  • 5. 170 D[SCUSSION Percent F Percent M 5% ~ Supermature Mature ,/ 4 !l :II L 71 ,ol I "~l: I /' /. l~'n. I I ', ql ,, ~1 I Submalure ' I : ,...,kay.,:/, Immature I • • " "''* "L' ~o oe ,> / Arkose Subarkose Ortho- Subgroy- Graywacke quartzite wacke Fro. l.--Mineral composition plotted against textural maturity for several sandstone for- mations. This diagram illustrates that almost any combination of the two variables may occnr. According to other authors, sediments occur ahnost entirely on the "main sequence," exteading in an inverted V-shaped trend from the immature arkose to the mature or supermature ortho- quartzite, and back down to the imnlature graywacke. The "main sequence" hypothesis is clearly, a profound oversimplification. 1. Lower Ordovican Beekmantown sands, Pennsylvania. Folk, manuscript in preparation. This field also applies to Upper Cambrian, Pennsylvania. 2. Lower Ordovician Ellenburger sands, Texas. Folk, manuscript in preparation. 3. Upper Permian (?) Pierce Canyon formation, Texas and New Mexico. D. N. Miller, Ph. D. Thesis, University of Texas, 19.55. 4. Triassic Newark Series, Connecticut- Krynine, 1950. 5. Recent Gulf Coastal sands, Texas. Folk, unpublished observations. 6. Upper Cambrian Potsdam sandstone, New York. Folk, unpublished observations. 7. Silurian System, West Virginia. Folk, research in progress. 8. Cretaceous Woodbine sand, Texas. A. S. Cotera, M. A. Thesis, University of Texas. 9. Pennsylvanian sandstones, central Texas. Folk, unpublished observations. 10. Recent Gulf Coastal sands, Mississippi and Alabama. Folk, unpublished observations. 11. Ordovlcian Juniata and Oswego formations, Pennsylvania. O. F. Turtle, M. S. Thesis, The Pennsylvania State College, 1940. An essentially coexistent field is occupied by the Devonian ;Fhird Bradford Sand, Krynine, 1940. 12. E~mene Carrizo and Newbv sands, Texas. T. W. Todd, M. A. Thesis, University of Texas, 1955. 13. Penusylvanian sandstones, eastern Pennsylvania. Folk, unpublished observations. 14. Numerous scattered samples of Mesozoic sands, Texas and New Mexico. Folk, personal observations.
  • 6. D I S C {'SSIO N 171 p. 355, 357) a great many examples of terpreting such stellar types as red giants rocks which fall off this main sequence: or in tracing the evolution of star clus- perfectly rotmded, beautifully-sorted ar- ters. T h e ) do not ignore or fail to classify koses from the Cambro-Ordovieian of stars t h a t fall off the main sequence; the Appalachian area; well-sorted gray- these unusual ones are precisely the ones wackes, rich in metamorphic rock frag- t h a t furnish the most valuable clues to ments, from the Upper Paleozoic of the the origin of our universe! Appalachians and Texas; poorly-sorted, Similarly, sedimentary petrographers angular, clay-rich orthoquartzite sands obtain some of their most valuablein- from the Pennsyh'anian of Texas, Si- formation from sandstones that fall off lurian of West Virginia. and uther locali- the main ti'end. Thus well-sorted, well- ties. These rock types are plotted in fig- rounded arkoses are believed to indicate ure 1. These important and widespread aridity in the source area, for under al- sandstone varieties occupy the " v a c a n t most any other climatic conditions the pigeonholes" which Bokinan contends feldspar would have been destroyed by are present in my c l a s s i f i c a t i o n - r o c k weathering before it had a chance to get types recognized neither by him nor by rounded. The clayey, poorly-sorted or- I)apples, Kruml)ein, and Slnss. Bok- thoquartzite-type sands also imply spe- man's compilation of 6l literature cialized conditions: either fairly rapid analyses (to which no references are erosion and deposition of sediments from given) apparently was not sufficiently an area of older sedimentary rock out- comprehensive to include these types. crops, or a mixture of several environ- A fertile analogy nmy be drawn be- ments, e.g. well-sorted beach sands mixed tween sandstone classification and the with lagoonal clays. To ignore these im- classification of stars. Astronomers pos- portant rock types because the)- do not sess a wonderful tool in a graph known follow the theoretical main sequence is to as the Hertzsprung-Russell diagram, in view the complexities of sandstone types which the magnitude (brightness) of a with rose-colored glasses. Bokman's con- star is plotted against its color, just as I nection of tectonic state with textural have plotted textural maturity against maturity may be an interesting theoreti- composition in figure 1. On the Hertz- cal exercise, but there are so many ex- sprung-Russell plot, most stars follow the ceptions t h a t I seriously question its "inaiu sequence" iu that faint stars tend utility, and even question its ultimate to be reddish and bright stars bluish, the scientific basis. His contention t h a t min- magnitude verst,s color trend forming a eral composition is a direct function of compressed band. But astronomers obtain textural maturity is also shown to be some of their most vital information by erroneous and simplified beyond the studying the departures from this trend, point of realism by the specific examples thus using the diagram as a tool in in- I have cited. REFERENCES BOKMAN, JOItN, 195.5, Sandstone classilication: relation to composition and texture: Jour. Sedimentary Petrology, v. 25, pp. 201 206. DAPPLES, E. C., [~.RUMBEIN,~.'. C., AND SLOSS, L. L., 1953, Petrographic and lithologic attri- butes of sandstones: Jour. Geology, v. 61, pp. 291-317. FOLK, R. L., 1951, Stages of textural maturity in sedimentary rocks: Jour. Sedimentary Petro- logy, v. 21, pp. 127-130. , 1954, The distinction between grain size and mineral composition in sedimentary rock nomenclature: Jour. Geology, v. 62, pp. 344 359. KRYNINE, P. l)., 1948, The megascopic study and field classification of sedimentary rocks: Jour. Geology, v. 56. pp. 130-165. , 1951, A critique of geotectonie elements: Am. Geophys. Union Trans., v. 32, pp. 743- 748. PE'rTIJOHN, F. J., 1940, S, edimentary rocks. Harper & Bros., New York. 526 pp. , 1954, Classification of sandstones: Jour. Geology, v. 62, pp. 360-365.