Geology Reference
In-Depth Information
3.1.2.1 Search Sampling and Statistical
Sampling
plan, e.g. at equal distances or according to a random
distribution.
Common methods of sampling are:
Size of samples
Microfacies sampling should provide (1) lithological
samples exhibiting textures, structures and fossils on a
hand specimen scale, (2) samples for laboratory analy-
sis (samples for geochemical investigations should con-
sider induration and degree of weathering!), and
(3) samples for paleontological studies. Because the
methods used in laboratory investigations may not yet
be certain at the time of sampling, large-sized samples
are recommended allowing several medium to large-
scale thin sections to be prepared and specific ques-
tions to be studied. The sizes 5 x 4, 5 x 5, and larger up
to 20 x 15 cm have proved useful as formats for thin
sections used in microfacies analyses.
•
Simple random sampling: Samples are taken at un-
evenly spaced intervals determined by random num-
bers, and the variability of the samples is checked
by variance tests (Eltgen 1970).
•
Systematic sampling: Samples are taken at evenly
spaced intervals and checked by significance tests;
(McCammon 1975). Sampling within a constant ver-
tical distance facilitates the statistical treatment of
distributional data (Grötsch et al. 1994).
•
Stratified sampling: At least two samples are taken
from each bed; the number of samples should be
proportional to bed thickness.
•
Cluster sampling: Several adjacent samples are taken
in random distribution. This approach was success-
fully used for microfacies studies of reef limestones
and shelf carbonates.
Large and oriented samples (showing the position
of the samples in spaces and marked in the field) are
necessary for
•
In practice, real possibilities of microfacies sampling
and theoretical statistical requirements differ widely due
to the outcrop situation, accessibility and physical con-
dition of the student carrying heavy sample bags. 'We
collect what we can...'
determining top-and-bottom criteria, allowing the
depositional and structural top to be recognized,
•
studying textural variations and fabrics (e.g. grain
orientation; bedding, discontinuity surfaces, biotur-
bation, breccia),
Beware of the 'family photo album effect'
In a family photo album extraordinary events like
birthdays and holidays are commonly overrepresented
as compared with photos of every day life. Similarly,
many microfacies samples are often selectively taken
because of a conspicuous abundance of fossils in some
beds or exceptional textural types. The 'daily life' re-
flected by more monotonous lithotypes (e.g. micritic
shelf limestones or hemipelagic limestones) is recorded
to a minor degree, because the vertical sampling dis-
tance is larger and the spacing more irregular. The depo-
sitional 'trends' (e.g. cycles) derived from samples rep-
resenting extraordinary events rather than 'daily life'
may display an artificial pattern.
•
describing biotic characters (e.g. growth forms, suc-
cession patterns, guilds). Reef limestones require
large and very large samples because of the high
compositional variations within centimeter and deci-
meter ranges. Only large samples allow synchronous
and non-synchronous associations to be differenti-
ated and sediment types and internal features (cavi-
ties, cryptic habitats) within bioherms or biostromes
to be discussed. Large samples, sometimes compris-
ing a whole bed, are also necessary for calciturbid-
ites (Herbig and Mamet 1994).
Equipment and first observations
Microfacies sampling requires the use of a hammer,
chisel and, sometimes specific equipment (angle grinder).
Hand lenses as well as a pocket microscope for grain
size measurements (Müller 1967) and textural obser-
vations in the field are important prerequisites.
•
3.1.2.2 How Many Samples?
The variability, diversity, and richness of microfacies
data are related to sample size effects. An increase in
sample size may lead to an increase of richness and
diversity. Various analytical techniques are known to
estimate sample size in geological, biological, and ar-
cheological studies. Some methods are also valuable
in microfacies analysis (curves of sample size against
diversity or richness, regression and simulation tech-
niques). There is, of course, no general rule for the num-
ber of microfacies samples that are 'truly representa-
Determine rock color, texture, composition and mean
size of grains, induration and weathering in hand
specimens.
Statistical sampling
Statistical sampling, based on random samples with
the objective of estimating the statistical probability of
facies types and patterns, is a common approach in mod-
ern microfacies studies (Carozzi 1988). Statistics re-
quire that the samples be taken according to a specific
