Geology Reference
In-Depth Information
tive' of the limestone studied, but some empirical ten-
dencies do exist:
cies criteria and microfossil distribution, based on a
'one-centimeter style' sampling exhibits hidden aspects
of environmental controls (Noé 1993).
Hardgrounds and condensed pelagic limestones
should be studied in channel samples covering the to-
tal bed thickness and with particular consideration of
the bed surfaces, because microfacies types can change
within a distance of a few millimeters and centimeters.
Collecting
samples for paleoecological or biostrati-
graphical analysis
must take into account the strong
facies control of the frequency and distribution of fos-
sils.
Many microfacies studies aiming for biostratigraphi-
cal classifications are based on spot samples taken at
regular intervals or over a regular number of event beds
(e.g. calciturbidites). Channel samples continuous over
the total thickness of limestone beds are necessary to
understand fine-scale vertical variations in
event beds
.
These variations describe the hydraulic changes dur-
ing the formation of the calciturbidites.
Shallow-marine bedded carbonate sequences
need
a detailed sampling program, because they exhibit high
microfacies variability due to changing subtidal and
tidal depositional sites and the common occurrence of
cyclic sedimentation patterns. A very close sampling
of cyclic carbonates at an interval of 0.10 m and less is
recommended if high-order short-term cycles are to be
detected (Buggisch et al. 1994). Thin- to medium-bed-
ded platform and ramp carbonates are commonly
sampled at intervals between 0.20 and 0.50 m (corre-
sponding to a bed-to-bed sampling) or at intervals of
several meters. Bed-to-bed sampling is advised if the
study aims to understand of short-term changes in en-
vironmental controls of biota and sedimentation. Novak
and Carozzi (1972) studied a 5 m thick section of Late
Carboniferous-Permian platform carbonates by using
thin section samples taken at intervals of 20 cm and a
continuous series of samples. Although continuous sam-
pling provided more information and allowed a better
separation of microfacies types, the general trend in
the sequence of microfacies types was still recogniz-
able for sampling intervals of 20 cm.
Wider intervals are used for (1) search sampling (tak-
ing samples preferentially from conspicuous beds no-
ticeable because of a particular composition, structure
or cyclic pattern), and (2) obtain pilot studies aiming to
a first overall picture of the section studied and recog-
nize lithotypes which may be evaluated later by micro-
facies data. Cyclothems that are evident already from
field observations are often described using only a few
samples. This may provide false information because
shallowing- or deepening trends of the cycles could be
missed. In addition, cyclic sedimentation patterns may
be obscured by the amalgamation of beds, thus calling
for densely spaced samples (Gischler et al. 1994).
Massive to coarse-bedded carbonates (reefs, mounds)
require the study of many samples in order to consider
(1) the variability of sediment within and between
frame-building and baffling organisms, (2) the various
kinds of voids and open-space structures common in
mounds and reefs, (3) lateral microfacies changes on a
decimeter-scale, and (4) relations between mound fa-
cies and intermound facies.
Microfacies analysis of
bedded deep-water carbon-
ates
often is based on rather few samples because of
the assumed 'monotonous' microfacies spectra of these
limestones. The calculated sampling interval (profile
thickness/number of samples) of most case studies lies
between 1.00 and 2.50 m. Wheras the overall qualita-
tive composition of deep-water limestones may indeed
be uniform, the quantitative investigation of microfa-
3.1.2.3 Practical Advice for Microfacies
Sampling
1
Become clear about the principal aim of the investi-
gation and decide whether you will use random sam-
pling or systematic sampling strategies.
2
Start with a test section to understand the scale of
microfacies variations and choose the most appro-
priate vertical sampling distance.
3
Mark the orientation of the sample (sedimentary top
and, if necessary dip and strike, which may be im-
portant for the interpretation of grain orientation as
a current indicator) before removing the sample from
the bed.
4
Take samples of an appropriate size allowing for the
preparation of thin sections, the study of non-car-
bonates in acid residues and geochemical analyses.
5
Consider the negative effect of weathering and avoid
taking samples for geochemical and diagenetic stud-
ies at or near bedding surfaces.
6
Follow a strict sampling strategy, but do not exclude
sampling of beds showing rare but conspicuous cri-
teria (e.g. extremely fossiliferous beds, beds exhib-
iting differences in colors). But be careful not to fol-
low the 'family photo album effect' by sampling
pre-
dominantly
'extraordinary' lithotypes.
7
Locate microfacies sampling in those parts of beds
which will deliver a maximum potential on infor-
mation.
8
When working with bedded carbonate sequences,
do not forget to sample also non-carbonate inter-
layers as well.
