GEOLOGIJA 39, 193-214 (1996), Ljubljana
Interpretation of Depositional Environment Based
on Grain Size Distribution of Sandstones of the Val Gardena
Formation in the Area Between Cerkno and Smrečje, Slovenia
Interpretacija sedimentaci j skega okolja na osnovi porazdelitve
velikosti zrn peščenjakov grödenske formacije na območju med
Cerknim in Smrečjem, Slovenija
Dragomir Skaberne
Geološki zavod Ljubljana
Inštitut za geologijo, geotehniko in geofiziko
Dimičeva 14, 1000 Ljubljana, Slovenija
Key words: grain size, depositional environment, sandstone, Val Gardena For-
mation, Permian, Slovenia
Ključne besede: velikosti zrn, sedimentacijsko okoje, peščenjaki, grödenska
formacija, perm, Slovenija
Abstract
An attempt was made to interpret the mode of transport and the depositional
environment of sandstones of the Val Gardena Formation in the area between Cer-
kno and Smrečje by analysis of frequency distribution of lengths of long axes of
sections of quartz grains. The examined sandy fraction should have been trans-
ported by fluvial currents principally in the saltation and partly the suspension
population, and in a smaller degree in the rolling population of grains.
Kratka vsebina
Z analizo porazdelitve dolgih osi presekov kremenovih zrn peščenjakov grö-
denske formacije na območju med Cerknim in Smrečjem smo skušali podati inter-
pretacijo načina transporta in sedimentacijskega okolja. Analizirana peščena
frakcija naj bi bila transportirana z rečnimi tokovi pretežno v obliki poskakujoče,
deloma suspenzijske in v manjši meri kotaleče populacije zrn.
Introduction
The largest continuous belt of the clastic rocks of the Val Gardena Formation in
Slovenia extends in the area betv^een Cerkno and Smrečje where they take part of
194_Dragomir Skaberne
the Žiri-Idrija nappe unit (Mlakar, 1969). According to the twofold subdivision of
the Permian they are attributed to the lower part of the Upper Permian. The beds
underlying the Val Gardena Formation consist of dark grey clastic rocks of pre-
sumingly Carboniferous age, while their upper part might be of the Lower Permian
age. The contact with them is discordant. The passage into overlying beds of the
Upper Permian carbonate rocks is gradual and concordant.
The most frequent rocks of the Val Gardena Formation are developed in the red
sandstone facies, in which conglomeratic and muddy facies are interbedded. As a
consequence of their proportion the highest attention in the study of the Val Gardena
Formation was attributed to the sandstones.
A sandstone as a clastic sedimentary rock consists of terrigenous grains and of
chemically precipitated minerals. Its properties depend therefore upon the properties
of grains and their fabric. The characteristics of an aggregate of sediment grains (P)
can consequently be defined as a function of their composition (c), size (s), shape (sh),
and their fabric, described by orientation (o) and packing (p) (Griffiths, 1961, 1967).
P = f(c, s, sh, o, p)
Determination of one of the given parameters is usually interdependent upon the
others. The basic properties of solid cemented rocks that cannot be disintegrated into
primary particular grains without the influence on their primary size and shape, can
be determined only in thin sections. In this case, an additional variable, the direction
of the section across the grain aggregate of the rock must be considered.
In the following, only the size, or distribution of sizes, of terrigenous grains in the
considered rocks shall be dealt with.
The grain sizes in sandstones that can be disintegrated into their primary individ-
ual grains is usually determined by sieving. During disintegration the original fabric
of grains, that is their orientation (o) and packing (p), is destroyed. From this aspect,
the determination of grain size (P^) depends only upon the composition (c), size (s)
and shape (sh) of grains.
P, = f(c, s, sh)
Owing to the importance of grain size, or of its distribution in sandstones from the
technological, as well as from genetic aspects, various authors (Friedman, 1958,
1962; Adams, 1977; Harrell & Erikson, 1976; Johnson, 1994) attempted to
determine the correction factors for transformation of the lengths of long axes of
grain sections in thin sections into the grain sizes as determined by sieving. One
comes across the problems of the definition of grain size that is dependent upon the
method applied (Allen, 1968) and the determination of the "true" size in thin sec-
tions, which also represents a mathematical problem. Analysis of the mentioned
problems are beyond our frame. Lately they were discussed by Johnson (1994).
For a long time in sedimentary petrology, many attempts were made to use the
grain size of the sediment for determination of their depositional environment.
Grain size distribution in clastic sedimentary rocks is dependent upon the charac-
teristics of source rocks, mode of weathering, distribution of grain size in weathered
material, mode of transport, conditions of sedimentation and finally of postdeposi-
tional processes. An interpretation of the depositional environment by grain size dis-
tributions is based upon the assumption that they are predominantly affected by
Interpretation of Depositional Environment Based on Grain Size .■._195
Sampling and methodology
In total 209 point samples have been analyzed in detail from 18 local profiles-seg-
ments (OZS), which all together comprise a thickness of 1234 m. The local
profiles-segments can be assembled into five regional composite profiles (OZP):
Škofje, Sovodenj, Žirovski vrh, Goli vrh and Lavrovec, as arranged in the direction
from NW to SE. They represent the fairly well studied volume of the Val Gardena
Formation.
The long axes of 200 sections of quartz grains were measured with the micrometer
ocular at 150 x magnification in 206 thin sections. The measured quartz grains were
randomly selected by point counter The influence of composition on size determina-
tion was reduced by measuring the quartz grains only. They are also the principal
component in the composition of the Val Gardena sandstones and represent on aver-
age 63 % of the population of all terrigenous grains. The lengths of long axes, up to
5 Ф (0.03 mm), were subdivided into classes of 1/4 Ф wide, and below that into classes
of 1/2 Ф wied, down to the smallest measured size of about 7 Ф (0.008 mm). The rela-
tive and cumulative frequencies were calculated on the basis of these data for the use
in further analysis.
Results
The cumulative frequency distribution curves of the lengths of long axes of sec-
tions of quartz grains were drawn in the Ф units on the abscissa and the probability
scale on the ordinate. By using the cumulative frequency distribution curves the
graphic statistical parameters: graphic mean size (MZ), inclusive standard deviation
(SI), inclusive graphic skewness (SKI) and graphic kurtosis (KG) (Folk & Ward,
1957) were determinated. From the same curves also the metric sizes at 1 % (C) and
median (MD), as well as quartile deviation (QDa) (Buller&McManus, 1972) were
also obtained. The average size (M), standard deviation (S), skewness (SKEW), kurto-
sis (KURT), simple measure of skewness (ALFA) and mean cubic deviation (MCD)
(Friedman, 1967) were also calculated with the moment estimates. They were cal-
culated in spite of difficulties related with the partly open classes in the domains of
the smallest and the largest grain sizes, as warned by Folk (1966). The mentioned
parameters for individual analyzed samples are listed in table 1, and the descriptive
statistics of grain size data are summarized in table 2.
hydrodynamic properties of the currents during transport and sedimentation, which
would be typical for the distinct depositional environment. The latter represents the
most difficult problem, since equal or very similar depositional processes and corre-
sponding hydrodynamic conditions may occur in various depositional environments,
and thus affect the grain size distributions in a similar manner Therefore the studies
of grain size distributions are only in part successful in determining the paleodeposi-
tional environments and the depositional processes related with them.
In spite of the stated limitations an attempt was made to use the distribution of
lengths of long axes of sections of terrigenous quartz grains of the sandy fraction for
interpretation of the mode of transport and of the depositional environment of the
Val Gardena sandstones in the Cerkno-Smrečje area.
196
Dragomir Skaberne
Table 1. Granulometrie parameters
Interpretation of Depositional Environment Based on Grain Size ..._197
Tabela 1. Granulometrični parametri
198
Dragomir Skaberne
Interpretation of Depositional Environment Based on Grain Size ...
199
200
Dragomir Skaberne
Symbols for variables of granulometrie parameters are evident from the text; OZS - segments,
profiles on the galeries in the uranium mine of Žrovski vrh P 10, H53-54, H52 on the surface
KL, RAI, Ra2, RA3, GV, A, B, C, D, E, S and the cores B74, V2, V20, V25
Interpretation of Depositional Environment Based on Grain Size ...
201
Oznake spremenljivk, granulometričnih parametrov so razvidne iz tekasta; OZS - segmenti,
profili v rovih rudnika urana Žirovski vrh PIO, H53-54, H52, na površini KL, RAI, RA2, RAS,
GV, PR, A, B, C, D, E, S in vrtinah B74, V2, V20, V25
202 _Dragomir Skaberne
sprem. - symbols for variables of granulometrie parameters are evident
from the text; N - number of cases (samples); min - minimal value; M -
main; SD - standard deviation; SK - skewness; KU - kurtosis
sprem. - oznake spremenljivk so razvidne iz teksta; N - število enot
(vzorcev); min - minimalna vrednost; max - maksimalna vrednost; M -
srednja vrednost; SD - standardni odkoln; SK - asimetričnost; KU -
sploščenost
A relatively good correspondence of the graphic and the moment measures for the
mean and the standard deviation can be seen in the table 2. The lengths of long axes
of sections of quartz grains in the examined samples vary from 6.09 Ф (0.015 mm) to
-2.4 Ф (5.3 mm) Mrith the mean of 2.16 Ф (0.22 mm). The grains are from poorly to well
sorted, on the average moderately well sorted, and their distributions are mainly
symmetric. The distributions are very positively to very negatively skewed in the
extremes. With respect to the kurtosis, the curves are mostly mesokurtic, but some
very platykurtic or very leptokurtic curves could also be observed. The descriptive
terms used are related to the quantitative limits as reported by Folk and Ward
(1957). In general, the distributions of grain size in the analyzed samples are close to
normal with the mean size of 2.16 Ф (0.22 mm) and the average standard deviation of
0.68 Ф (0.094 mm).
Discussion and interpretation
The analysis of the distribution of grain size and its interpretation can be
approached in two ways. In the first one, the cumulative frequency distribution curve
of grain size is regarded as a whole, and it is attempted to be interpreted on the basis
of hydrodynamic characteristics. In the second way, the characteristics of statistic
parameters of distributions of grain size from known recent depositional environ-
ments are considered and attempts are made to determine empirically the discrimi-
nant functions between them.
One of the pioneers of the study of cumulative frequency curves was D o e g 1 a s
(1946). He found that grain sizes follow the arithmetic probability function, and that
their distributions represent a mixture of two or more populations that are the conse-
quence of various mechanisms of transport. Inman (1949) distinguished three basic
types of transport: rolling, sliding and saltation on the bottom, and suspension. He
also contributed some thoughts on the graphic parameters of grain size distribution:
mean grain size, sorting (standard deviation) and skewness. Sindowski (1957) con-
Table 2. Descriptive statistics of some granulomertic parameters
Tabela 2. Opisne statistike nekaterih granulometričnih parametrov
Interpretation of Depositional Environment Based on Grain Size .■.__203
tinued Doeglas's w^ork, but he utihzed the log-probabihty diagrams for represent-
ing the cumulative frequency curves, and he empirically subdivided them into groups
that belong to sediments of various depositional environments: aeolic, limnic, estuar-
ine, littoral and shelf environment. Moss (1962, 1963) believed that the grain size
distribution consists of normally distributed subpopulations that are transported by
rolling-sliding, saltation and in suspension. The individual subpopulations have their
mean size and sorting. Later Vi s her (1965) studied the dependence of grain size dis-
tribution on the sedimentary structures in fluviatile deposits and the various deposi-
tional processes in diverse depositional environments (Vi s her, 1969).
Based on these concepts an attempt was made to determine the individual genetic
populations from the cumulative frequency curves. All three populations of grain
sizes, transported by either rolling and sliding (Ro), saltation (Sa) and in suspension
(Su), could be determined in the examined samples. Their presence is shown in table
1. In the same table the position of the inflection point between the saltation and sus-
pension populations is presented. This point is estimated according to the maximum
grain size in suspension (ФSu) and the amount of grains that were transported as a
bedload (BL %), by rolling or sliding and saltation.
The basic cumulative frequency distribution curves are shown in figure 1.
The presence of rolling and sliding populations could be established only in
5.26 % of the 209 analyzed samples. When present, their amount varied from 1.5 to
16.0 % with an average of 5.9 %. The minimum size of the mentioned subpopulation
is 0.75 Ф (0.6 mm) to 0.2 Ф (0.87 mm), with the mean of 0.43 Ф (0.74 mm).
The population of saltation grains was established in 99.04% of the examined
samples. This population occurs as a unique population in 40.8%, whereas it is
divided in the remaining 58.2 % into two subpopulations (Fig. lb). Their intersection,
the inflection point between the two subpopulations of saltation grains, is above the
line of unique saltation population in 46.2 % of the cases, and it is marked by a 2 in
table 1. This means that the saltation population is positively skewed in 46.2 % of the
cases, and contains more smaller grains with respect to their expected amount in a
symmetrical normal distribution. In 12 % of samples, this intersection between the
two subpopulations occurs below the line that represents the unique saltation popu-
lation, and is marked by a -2 in table 1. This suggests a negatively skewed saltation
population, which contains more larger grains with respect to their expected amount.
This indicates a certain hydrodynamic separation even within the population of
saltation grains itself, which was already noticed already by Vi s her (1965, 1969).
The distribution of grain size of the analyzed samples showing that the grains
transported as bedload by rolling, sliding and saltation in the thin layer in the bot-
tom, represents no less than 89.2 % of all grains.
The population of suspended grains was established in 52.15 % examined of the
samples, and of these only two samples, or 0.05 %, contain only this population of
grains. The average amount of suspended population is 10.8 %. The amount of sus-
pension population is 18.65 % on average, taking into account only the samples con-
taining it. The maximum grain size in the suspension population is 2 Ф (0.25 mm) to
4.50 Ф (0.04 mm), and on average 2.90 Ф (0.15 mm), with standard deviation of 0.51 Ф
(0.05 mm).
With respect to comparisons of presented data (Tab. 1) with those of Vi s h er
(1969, 1104, Tab.l), the analyzed distribution of grain sizes are interpreted as a prod-
uct of fluvial sedimentary environment. The detailed hydrodynamic interpretations,
as made on the basis of grain size distribution by Middleton (1976), Brey er
204
Dragomir Skaberne
Fig. 1. Basic groups of cumulative grain size distribution curves for the analyzed samples with
genetic interpretation: a) Ro -rolling-sliding, Sa - saltation, and Su - suspension populations of
grains; b) cumulative distribution curve represented by a single saltation population (Kl-7.2/1)
or by two subpopulations that form a positively skewed (Kl-83.9) and a negatively skewed (PIO
53/500) saltation population
SI. 1. Osnovne skupine kumulativnih porazdelitvenih krivulj velikosti zrn analiziranih vzorcev
z genetsko interpretacijo: a) Ro - populacija kotalečih - drsečih, Sa - poskakujočih, Su - sus-
pendiranih zrn; b) kumulativna porazdelitvena krivulja, predstavljena z eno populacijo (Kl-
7,2/1) ali dvema podpopulacijama, ki sestavljata pozitivno asimetrično (Kl-83,9) in negativno
asimetrično (PIO 53/500) populacijo poskakujočih zrn
(1979) and others, are beyond the frame of the present study. Only that Middleton
(1976) determined the average shear velocity of the transporting medium on the basis
of the interruption (position of inflection point) between the populations of grain
sizes that are transported by rolling and sliding, and the population of grain sizes in
intermittent suspension that coincides with the lower part of the population of salta-
tion grains shall be mentioned.
In study of dynamics during the transport and sedimentation. Pa s sega (1957,
1964) utilized the ratios between the largest grains at 1 % on the cumulative curve (C)
and the median (Md) in metric units, of a larger number of homogenous, but struc-
turally diverse samples. The importance of maximum size that should reflect the
maximum power of the current, was also recognized by other researchers.
Passega (1957, 1964) distinguished two modes of transport: rolling and suspen-
sion. He further subdivided the suspension into the graded and homogenous suspen-
Interpretation of Depositional Environment Based on Grain Size ..■__205
Conclusion
The grain size and its distribution are influenced by a multitude of factors which
limits their application for the interpretation of the paleodepositional environment.
In case of their use also other relevant criteria, such as the sedimentary structures,
spatial succession of facies, fossils, other textural parameters, etc. must be taken into
consideration.
In the analyzed samples the graphic and the moment derived estimates of the
mean grain size and the standard deviation correspond relatively well in spite of
open classes on both ends of the distribution curve, whereas the estimates for the
skewness and the kurtosis correspond somewhat less well.
The distribution of lengths of the long axes of grain sections, as determined in
thin sections, can serve in certain cases as a satisfactory approximation to distribu-
tions of sizes of real grains. Their representations on log-probability diagrams can be
used for interpretation of the mode of transport. Most of the analyzed sandy fraction
of the Val Gardena sandstones belongs to the saltation population. The suspension
Sion at the bottom, and the pelagic suspension. The Passega's (1957, 1964) diagram
in which the data of analyzed samples were plotted, shows that the prevailing part of
the material was transported in form of the graded bottom suspension that could cor-
respond to the saltation population (Moss, 1962, 1963; Vi s h er, 1969).
Besides the hydrodynamic interpretation of the cumulative frequency distribution
function and its importance for the hydrodynamic of the depositional environment
the graphic and the moment statistical parameters were also considered.
The factor analysis (Skaberne & Smolej, 1981) indicated that the mean grain
size and the skewness are interrelated. They are loading the first factor with the
highest loadings by skewness. The second factor is defined by the standard deviation
and the kurtosis, with the highest loadings by the standard deviation. The scatter-
plots of independent variables of the graphic mean grain size (MZ) versus the inclu-
sive graphic standard deviation (SI) that separate the aeolic and the fluvial sediments
(Friedman, 1961), the beach and the fluvial sediments (Fri e dm an, 1967); the
shore bars and the fluvial sediments (Moiola& Weiser, 1986) were therefore test-
ed. According to the discriminant function of Moiola and Weiser (1968) all the
analyzed samples were attributed to the fluvial deposits, whereas the solution on the
ground of Friedman's diagrams (1961, 1967) is not significant.
A better resolution between the beach and the fluvial deposits provide Friedman's
diagrams of the inclusive graphic standard deviation (SI) versus the inclusive graphic
skewness (SKI) (Fig. 2a) (Friedman, 1967) by which 85.6 % of studied samples were
attributed to the fluvial deposits. An even better resolution was given by the bivari-
ate diagram of the moment derived standard deviation (S) versus the mean cubic
deviation (MCD) (Fig. 2b), (Friedman, 1967), in which no less than 95.7% of
analyzed samples plot in the field of fluvial deposits.
An interesting diagram for defining the depositional environments on the basis of
granulometrie parameters of the median (MD) versus the arithmetic quartile devia-
tion (QDa) in metric units was proposed by Bull er and Mc Manus (1972). The dia-
gram discriminates between the "quietwater", fluvial, eolie and beach deposits (Fig.
3). In the envelope of the fluvial sediments plot no less than 97.6 % of the analyzed
samples.
206
Dragomir Skaberne
Fig. 2. Scatterplot for distinguishing between the fluvial and beach clastic
sediments: a) plot of graphic standard deviation (SI) versus inclusive
graphic skewness (SKI); b) plot of standard deviation (S) versus mean
cubic deviation (MCD) (Friedman, 1967)
SI. 2. Bivariatna diagrama za ločitev med rečnimi in obalnimi klastičnimi
sedimenti: a) diagram med grafičnim standardnim odklonom (SI) in
inkluzivno grafično asimetričnostjo (SKI); b) diagram med standardnim
odklonom (S) in srednjim kubiranim odklonom (MSD) (Friedman, 1967)
Interpretation of Depositional Environment Based on Grain Size ...
207
Fig. 3. Scatterplot of metric median (Md) versus quartile deviation (QDa) and their envelopes
representing fluvial (1), shore (2), eolian (3) and quiet water (4) environments (BuUer &
McManus, 1972)
SI. 3. Bivariantni diagram med metrično mediano (Md) in kvartilnim odklonom (QDa) s polji, ki
opredeljujejo rečne (1), obalne (2), eolske (3) in mirnovodne (4) sedimente (Buller & McManus,
1972)
population is present in about one half of the analyzed samples, with the average
amount of about 20 %. The rolling and sliding population was established only in 5 %
of analyzed samples, with the mean amount of about 6 %.
On the basis of statistical parameters of the distribution of lengths of long axes of
sections of quartz grains of the Val Gardena sandstones in the area between Cerkno
and Sovodenj their depositional environment is interpreted as fluvial. This interpre-
tation is also confirmed by other criteria such as: sedimentary structures, vertical
succession of lithofacies and their lateral correlation, etc. (Skaberne, 1995).
Acknowledgements
I would like to thank colleagues J. Čar and B. Ogorelec for their valuable com-
ments on the manuscript as well as S. Pire for translating it. I would also like to
extend my thanks to M. Rarer for the drawings.
208_Dragomir Skaberne
Interpretacija sedimentacijskega okolja na osnovi porazdelitve
velikosti zrn peščenjakov grödenske formacije na območju med
Cerknim in Smrečjem, Slovenija
Uvod
Klastične kamnine grödenske formacije grade v Sloveniji največji sklenjeni pas na
območju med Cerknim in Smrečjem, ki pripada idrijsko-žirovski narivni enoti (M 1 a-
k a r, 1969). Po dvodelni razdelitvi perma jo uvrščamo v spodnji del zgornjega perma.
Talnino grödenski formaciji predstavljajo temno sive klastične kamnine, ki jim pripi-
sujemo karbonsko starost, njihov zgornji del pa je lahko tudi spodnjepermske staro-
sti. Kontakt s talnino je diskordanten. Prehod v krovnino, ki jo sestavljajo zgornje-
permske karbonatne kamnine, pa je postopen in konkordanten.
V grödenski formaciji so najbolj zastopani peščeni faciesi. V menjavi z njimi se
javljajo še konglomeratni in mulj asti faciesi, v katerih so ponekod prisotne tudi
kalcitne in dolomitne konkrecije. Glede na zastopanost smo pri raziskavah grödenske
formacije posvetili največjo pozornost peščenjakom.
Kot klastična sedimentna kamnina je peščenjak sestavljen iz terigenih zrn in ke-
mično izločenih mineralov. Zato so njegove lastnosti odvisne od lastnosti zrn in nači-
na njihove povezave. Tako lahko značilnost agregata sedimentnih zrn (P) opredelimo
kot funkcijo njihove sestave (c), velikosti (s), oblike (sh), orientacije (o) in zgoščenosti
(p) (Griffiths, 1961, 1967).
P = f (c, s, sh, o, p)
Določitev enega podanih parametrov je običajno delno odvisna od drugih. Osnov-
ne lastnosti trdno vezanih kamnin, ki se ne dajo dezintegrirati na posamezna zrna
brez vpliva na njihovo prvotno velikost in obliko, lahko določamo le v zbruskih. V
tem primeru se navedenim pridruži še ena spremenljivka, to je smer preseka prek
agregata zrn kamnine.
V nadaljevanju se bomo od navedenih lastnosti omejili na velikost oziroma poraz-
delitev velikosti terigenih zrn v obravnavanih kamninah.
Velikost zrn peskov in peščenjakov, ki se dajo dezintegrirati na posamezna zrna,
običajno določamo s sejanjem. Pri dezintegraciji pa uničimo povezave med zrni ozi-
roma njihov zlog, ki ga opredeljujemo z orientacijo (o) in zgoščenostjo (p) zrn. Tako je
določitev velikosti zrn (P^) odvisna le še od sestave (c), velikosti (s) in oblike (sh) zrn.
Рз = f (c, s, sh)
Zaradi pomembnosti velikosti zrn oziroma njihove porazdelitve v peščenjakih
tako iz tehničnega kakor iz genetskega vidika so različni avtorji (Friedman, 1958,
1962; Adams, 1977; Harrell & Eriksson, 1979; Johnson, 1994) skušali določiti
korekcijske faktorje za spremembo v zbruskih določene velikosti presekov zrn v
velikosti zrn, opredeljenih s sejalno metodo. Pri tem se srečujemo s problemi definici-
je velikosti, ki je odvisna od uporabljene metode (Allen, 1968) in določitvijo "prave"
velikosti v zbruskih, ki predstavlja tudi matematični problem. Analiza omenjenih
problemov presega naš okvir in jih je v novejšem času obravnaval J o h n s o n (1994).
V sedimentni petrologiji se že od nekdaj prizadevajo uporabiti zrnavost sedimenta
za določitev okolja njihovega nastanka.
Interpretacija sedimentacijskega okolja na osnovi porazdelitve velikosti zrn .■._209
Vzorčevanje in metodologija
Analizirali smo 209 točkastih vzorcev iz 18 podrobno posnetih lokalnih profilih -
segmentih (OZS), ki zajemajo skupno debelino 1234 m in bolj ali manj reprezenta-
tivno predstavljajo raziskovani volumen grödenskih kamnin. Lokalne profile - seg-
mente lahko povežemo v pet regionalnih sestavljenih profilov (OZP): Škofje, Sovo-
denj, Žirovski Vrh, Goli Vrh in Lavrovec, ki so navedeni v smeri od NW proti SE.
V zbruskih smo pri povečavi 150 x z mikrometrskim okularjem merili dolge osi
200 kremenovih zrn. Kremenova zrna so bila slučajno določena s točkovnim števcem.
Z izborom ene vrste terigenih, kremenovih zrn smo zmanjšali vpliv sestave na
določanje velikosti. V sestavi grödenskih peščenjakov so kremenova zrna tudi najbolj
zastopana, saj predstavljajo poprečno 63 % populacije vseh terigenih zrn. Dolžine
presekov smo razdelili do velikosti 5 Ф (0.03 mm) na razrede, široke 1/4 Ф, nato pa na
razrede 1/2 Ф do najmanjše merjene velikosti 7 Ф (0.008 mm). Na osnovi teh podatkov
smo izračunali relativne in kumulativne frekvence, ki so služile za nadaljnjo analizo.
Rezultati
Na osnovi kumulativnih relativnih frekvenc smo izrisali kumulativne porazdelit-
vene krivulje dolgih osi presekov kremenovih zrn s Ф skalo na abcisi in z verjetnostno
skalo na ordinati. Iz kumulativne porazdelitvene krivulje smo odčitali vrednosti za
izračun grafičnih statističnih parametrov (Folk & Ward, 1957): grafično srednjo
vrednost velikosti (MZ), inkluzivni grafični standardni odklon (SI), inkluzivno grafi-
čno asimetričnost (SKI) in grafično sploščenost (KG). Poleg tega smo podali velikosti
v mm pri 1 % (C) in mediano (MD) ter izračunali kvartilni odklon (QDa) (Bu 11 er &
M C Manus, 1972). Kljub težavam, ki izhajajo iz delno odprtih razredov v območju
največjih in najmanjših zrn, na kar opozarja Folk (1966), smo poleg grafičnih sta-
tističnih parametrov iz podatkov relativnih frekvenc v razredih 1/4 Ф z momentnim
računom izračunali statistične parametre: povprečno velikost (M), standardni odklon
(S), asimetričnost (SKEW), sploščenost (KURT), preprosto mero asimetričnosti
Porazdelitev velikosti zm v klüsticitííl secßmentnih kamninah je odvisna od zna-
čilnosti izvornih kamnin, načina preperevanja, porazdelitve velikosti zrn v preperin-
skem materialu, načina transporta, pogojev usedanja in končno od postsedimentacij-
skih procesov. Ugotavljanje sedimentacijskega okolja na osnovi porazdelitve velikosti
zrn temelji na predvidevanju, da nanjo najbolj vplivajo hidrodinamične lastnosti to-
ka med transportom in sedimentacijo, pri čemer naj bi v določenem sedimentacij-
skem okolju vladale zanj značilne hidrodinamične razmere. Prav slednje predstavlja-
jo enega največjih problemov, kajti enaki ali zelo podobni sedimentacijski procesi
oziroma hidrodinamične razmere nastopajo v različnih sedimentaci]skih okoljih in
tako podobno vplivajo na porazdelitve velikosti zrn.
Zato so določitve paleosedimentacijskega okolja in sedimentacijskih procesov v
njih na osnovi porazdelitve zrnavosti le delno uspešni.
Kljub navedenim pomislekom smo se odločili poskusiti uporabiti porazdelitve
velikosti presekov terigenih kremenovih zrn peščene frakcije za interpretacijo načina
transporta in opredelitve okolja sedimentacije grödenskih peščenjakov med Cerknim
in Smrečjem.
210_Dragomir Skaberne
Razprava
Analize porazdelitve velikosti zrn in njene interpretacije se lahko lotimo na dva
načina. V enem upoštevamo kumulativno krivuljo porazdelitve velikosti zrn kot celo-
to in jo skušamo interpretirati na osnovi hidrodinamičnih značilnosti. V drugem
primeru pa obravnavamo značilnosti statističnih parametrov porazdelitev velikosti
zrn iz znanih, recentnih sedimentacijskih okolij in skušamo med njimi empirično
ugotoviti diskriminantne funkcije.
Eden izmed pionirjev preučevanja oblike kumulativnih krivulj je bil Doeglas
(1946). Ugotovil je, da velikosti zrn slede aritmetični verjetnostni funkciji in da nji-
hove porazdelitve predstavljajo mešanico dveh ali več populacij, ki so posledice ra-
zličnih mehanizmov transporta. Inman(1949)jev transportu prepoznal tri osnovne
tipe: kotaljenje ali drsenje in poskakovanje po dnu ter suspenzijo. Poleg tega je podal
nekaj misli o grafičnih parametrih porazdelitve velikosti zrn: srednja velikost, sorti-
ranost (standardni odklon) in asimetričnost. Sindowski (1957) je nadaljeval delo
Doeglasa, vendar je uporabljal log-verjetnostne diagrame prikazovanja kumula-
tivnih krivulj in jih glede na obliko empirično razdelil v skupine, ki pripadajo sedi-
mentom različnih sedimentacijskih okolij: eolskemu, limničnemu, estuari]skemu,
obalnemu in šelfnemu okolju. Moss (1962, 1963) je ugotovil, da je porazdelitev veli-
kosti zrn sestavljena iz normalno porazdeljenih podpopulacij, ki se premikajo s kota-
Ijenjem in drsenjem, poskakovanjem in v suspenziji. Posamezne podpopulacije imajo
svojo povprečno velikost in sortiranost. Kasneje je Vi s her (1965) v rečnih sedimen-
tih preučeval odvisnost porazdelitve velikosti zrn od sedimentnih tekstur in različnih
sedimentacijskih procesov v raznolikih sedimentacijskih okoljih (Vi sher, 1969).
Na osnovi teh spoznanj smo skušali v kumulativnih krivuljah opredeliti posame-
zne genetske populacije. V raziskanih vzorcih smo lahko določili vse tri populacije
velikosti zrn, ki so se premikala s kotaljenjem in drsenjem (Ro), poskakovanjem (Sa)
in v suspenziji (Su). Njihova prisotnost je prikazana v tabeli 1. Poleg tega smo v isti
tabeli podali položaj prevojne točke med poskakujočo suspenzijsko populacijo. Ta
točka je določena z ocenjeno maksimalno velikostjo zrn v suspenziji (Ф Su) in odstot-
kom, količino zrn, ki so se transportirala po dnu z mehanizmi talnega transporta
(BL %), s kotaljenjem, drsenjem in poskakovanjem.
Osnovne oblike kumulativnih porazdelitvenih krivulj so prikazane na sliki 1.
(ALFA) in srednji kubirani odklon (MCD) (Friedman, 1967). Navedeni parametri
so za posamezne analizirane vzorce zbrani v tabeli 1, opisne statistike granu-
lometričnih statističnih parametrov pa podajamo v tabeli 2.
Iz tabele 2 je razvidno, da se grafične in momentne vrednosti za srednjo vrednost
in standardni odklon sorazmerno dobro ujemajo. Velikost dolgih osi presekov kre-
menovih zrn se v raziskanih vzorcih spreminja od 6,0 Ф (0,015 mm) do -2,4 Ф (5,3 mm)
in znaša povprečno 2,16 Ф (0,22 mm). Zrna so slabo do dobro, poprečno srednje dobro
sortirana, medtem ko so njihove porazdelitve večinoma simetrične. V ekstremih so
porazdelitve zelo pozitivno do zelo negativno asimetrične. Če pogledamo sploščenost,
vidimo, da so krivulje večidel normalne (mezokurtične), zasledimo pa tudi zelo
sploščene ali zelo ošiljene krivulje. Navedeni opisni izrazi se nanašajo na kvantita-
tivne meje, ki jih podajata Folk in Ward (1957). V splošnem lahko rečemo, da so
porazdelitve velikosti zrn v raziskanih vzorcih relativno normalne s povprečno
velikostjo 2,16 Ф (0,22 mm) in povprečnim standardnim odklonom 0,68 Ф (0,094 mm).
Interpretacija sedimentacijskega okolja na osnovi porazdelitve velikosti zrn ..._211
Izmed 209 raziskanih vzorcev smo ugotovili le v 5,26 % prisotnost kotaleče in
drseče populacije. Ob njeni prisotnosti se njena količina spreminja od 1,5 do 16,0 %
in znaša povprečno 5,9 %. Minimalna velikost v omenjeni podpopulaciji je 0,75 Ф (0,6
mm) do 0,2 Ф (0,87 mm), povprečno 0,43 Ф (0,74 mm).
Populacijo poskakujočih zrn smo zasledili v 99,04% raziskanih vzorcih. Ta popu-
lacija sestavlja enotno populacijo v 40,8 %, medtem ko se v preostalih 58,2 % razdeh v
dve podpopulaciji (si. 1 b). Presečišče, prevojna točka med obema podpopulacijama
poskakujočih zrn je v 46,2 % primerih nad premico enotne podpopulacije v tabeli 1
označeno z 2. To pomeni, da je poskakujoča populacija zrn v 46,2 % primerih pozitivno
asimetrična, oziroma vsebuje več manjših zrn v primerjavi z njihovo pričakovano ko-
ličino. V 12 % vzorcev je to presečišče med podpopulacijama pod premico, ki predstav-
lja enotno populacijo poskakujočih zrn v tabeli 1 označena z 2, kar kaže na negativno
asimetrično populacijo poskakujočih zrn, oziroma vsebuje več večjih zrn v primerjavi
z njihovo pričakovano količino. To kaže na določeno hidrodinamično separacijo tudi v
sami populaciji poskakujočih zrn, na kar je opozoril že Visher (1965, 1969).
Če pogledamo porazdelitve velikosti zrn v raziskanih vzorcih, vidimo, da pred-
stavljata populaciji zrn, ki se premikajo s talnim transportom, s kotaljenjem, drsen-
jem in poskakovanjem v tanki plasti po dnu kar 89,2 % vseh zrn.
Populacija suspenzijskih zrn je bila ugotovljena v 52,15 % raziskanih vzorcev, od
tega vsebujeta dva vzorca ali 0,04 % le to populacijo zrn. Povprečna količina sus-
penzijske populacije znaša 10,8 %. V primeru, da analiziramo le vzorce, ki so vsebo-
vali suspenzijsko populacijo zrn, znaša njena količina povprečno 18,52 %. Maksimal-
na velikost v suspenzijski populaciji je 2 Ф (0,25 mm) do 4,50 Ф (0,04 mm), povprečno
2,90 Ф (0,15 mm) s standardnim odklonom 0,51 Ф (0,05 mm).
Glede na primerjavo prikazanih podatkov (Tab. 1) z Visher j evimi (1969, 1104,
Tab. 1) interpretiramo analizirane porazdelitve velikosti zrn kot produkt rečnega
sedimentacijskega okolja. Podrobnejše hidrodinamične interpretacije, kakor so jih na
osnovi porazdelitev velikosti zrn izvajali Middleton (1976), Viard in Breyer
(1979) in drugi, pa presegajo naš okvir interpretacije. Kljub temu naj omenimo, da je
Middleton (1976) na osnovi prekinitve (položaja prevojne točke) med populacija-
ma velikosti zm, ki se transportiraj o s kotaljenjem in drsenjem, in populacijo zrn v
prekinjeni suspenziji (intermitted suspension), ki sovpada s spodnjim delom popu-
lacije poskakujočih zrn, določil povprečno strižno hitrost transportnega medija.
Pri preučevanju dinamike med transportom in sedimentacijo jePassega(1957,
1964) uporabil razmerja med največjimi zrni pri 1 % na kumulativni krivulji (C) in
mediano (Md) v metrskih enotah strukturno raznolikih vzorcev. Pomembnost mak-
simalne velikosti, ki naj bi odražala največjo moč toka, so prepoznali tudi drugi
raziskovalci.
Pa ssega (1957, 1964) je ločil dva načina transporta: kotaljenje in suspenzijo.
Suspenzijo je nadalje delil na graduirano in homogeno suspenzijo ob dnu in na pela-
gično suspenzijo. Na osnovi posameznih načinov transporta je opredelil obalne, rečne
in turbiditne tokove in pelagično suspenzijo. Iz Passegovega diagrama (1957,
1964), v katerega smo vnesli podatke analiziranih vzorcev, je razvidno, da se je
pretežni del materiala transportiral v obliki graduirane talne suspenzije (graded bot-
tom suspension), kar bi odgovarjalo populaciji poskakujočih zrn (saltation popula-
tion) (Moss, 1962, 1963; Visher, 1969).
Poleg hidrodinamične interpretacije kumulativne porazdelitvene krivulje in njeno
uvrstitev v hidrodinamično sedimentacijsko okolje smo preučevali tudi grafične in
momentne statistične parametre.
212_Dragomir Skaberne
Sklepi
Na velikost zrn oziroma njihovo porazdelitev vpliva mnogo dejavnikov, zato je
njihova uporaba pri interpretaciji paleosedimentacijskega okolja omejena. Ob njiho-
vi uporabi v te namene pa moramo upoštevati še vse druge relevantne kriterije, npr.
sedimentne teksture, prostorsko sosledje faciesov, fosile, druge strukturne parametre
itd.
V analiziranih vzorcih se grafični in momentno izračunani podatki srednje veliko-
sti in standardnega odklona, kljub odprtim razredom na obeh konceh porazdelitvene
krivulje, relativno dobro ujemajo, medtem ko so razlike pri asimetričnosti in
sploščenosti nekoliko večje.
Porazdelitve velikosti presekov zrn, določenih v zbruskih, so lahko v nekih prime-
rih zadovoljiv približek porazdelitvam pravih velikosti zrn. Njihovi prikazi na log -
verjetnostih diagramih lahko služijo za interpretacijo načina transporta. Večina razi-
skane peščene frakcije grödenskih peščenjakov pripada poskakujoči populaciji. Sus-
penzijska populacija je prisotna v približno polovici analiziranih vzorcev s povprečno
količino približno 20 %. Kotaleča in drseča populacija pa je bila ugotovljena le v 5 %
analiziranih vzorcev, s povprečno količino približno 6 %.
Na osnovi statističnih parametrov porazdelitev velikosti presekov kremenovih zrn
peščenjakov grödenske formacije med Cerknim in Smrečjem interpretiramo okolje
njihovega nastanka kot rečno. Tako interpretacijo potrjujejo tudi drugi kriteriji v
podrobno posnetih profilih: sedimentne teksture, vertikalno zaporedje litofaciesov in
njihova lateralna korelacija itd. (Skaberne, 1995).
S faktorsko analizo (Skaberne & Smolej, 1981) smo ugotovili, da sta
povprečna velikost in asimetričnost med seboj odvisni in sta vezani na prvi faktor, ki
je obremenjen predvsem z asimetričnostjo. Drugi faktor opredeljujeta standardni
odklon in sploščenost, pri čemer ga prvi najbolj obremenjuje. Zato smo pregledali le
bivariantne diagrame z neodvisnimi spremenljivkami med grafično povprečno
velikostjo (MZ) in inkluzivnim grafičnim standardnim odklonom (SI), ki ločuje
eolske sipine in rečne sedimente (Friedman, 1961), obalne in rečne sedimente
(Friedman, 1967); obalne (obrežne) sipine in rečne sedimente (Moiola& Weiser,
1968). Diskriminantne funkcije Moiola in Weiser j a (1968) opredele vse raziskane
vzorce kot rečne sedimente, medtem ko določitev na osnovi Friedmanovih dia-
gramov (1961, 1967) ni značilna.
Boljšo ločljivost med obalnimi in rečnimi sedimenti kažeta Friedmanova diagra-
ma med inkluzivnim grafičnim standardnim odklonom (SI) in inkluzivno grafično
asimetričnostjo (SKI) (si. 2a) (Friedman, 1967), v katerem je 85,6% raziskanih
vzorcev opredeljenih kakor rečni sedimenti. Še boljšo ločitev kaže bivariantni dia-
gram med momentno določenim standardnim odklonom (S) in srednjim kubiranim
odklonom (MCD) (si. 2b), (Friedman, 1967), v katerem je kar 95,7% raziskanih
vzorcev v polju rečnih sedimento v.
Zanimiv diagram za opredeljevanje sedimentacijskih okolij na osnovi granu-
lometričnih parametrov med mediano (MD) in aritmetičnim kvartilnim odklonom
(QDa) sta podala Buller in Mc Manus (1972). Ta ločuje "mirnovodne", rečne,
eolske in obalne sedimente (si. 3). V polje rečnih sedimentov pade kar 97,6%
razisl^nilivzorcev,........ .........      . . ... ......
Interpretacija sedimentacijskega okolja na osnovi porazdelitve velikosti zrn ..■_213
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