Geology Reference
In-Depth Information
Fig. 3.7.
Classification of coarse bio-
clastic fabrics
characterized by close-
packing of fossils (modified after
Kidwell and Holland 1991).
The classification can be used in the
field as well as in thin section studies.
Basic parameters are packing, size
sorting of the bioclasts larger than 2 mm,
and the biofabric which is bioclast-
supported ('densely packed') or matrix-
supported ('loosely packed and
'dispersed'). The fabrics are well-sorted
exhibiting little variations in the size of
bioclasts, bimodal sorted (with two
discretes modes), or poorly sorted
exhibiting great variations in bioclast
sizes. The packing and sorting descriptors
convey an immediate visual image of the
fossiliferous limestone. The criteria have
genetic implications. The categorization
by close-packing and size-sorting limits
the number of possible origins and leads
to basic hypotheses that can be tested
using taphonomic, palecologic, sediment-
ologic and stratigraphic features.
The sample exhibits a fabric characterized by large gastropod shells and abundant smaller bioclasts consisting of small gastropods
and bivalve valves. These bioclasts constitute an overall loosely packed fabric which would be classified as gastropod
floatstone following Dunham's classification (see 6.2). The finer bioclasts, however, are densely packed and these parts of
the thin section would be classified as fine-bioclastic mollusk packstone. Small gastropods often are regarded as 'dwarf'
forms pointing to unfavorable environments. It should be taken in mind, however, that the majority of gastropods is characterized
by mm-scale shells. Note the strong recrystallization of the large gastropods. The originally aragonitic shell has been replaced
by coarse-grained calcite crystals. The first whorls are completely obliterated; following whorls exhibit various cement
generations or are infilled by micritic sediment. Late Cretaceous pebble in Quaternary deposits: Graz/Styria, Austria.
scopic features of sedimentary rocks to be objectively
described and the data stored in a computer database.
3.1.2 Sampling
The following comments refer to outcrops and cores.
Sampling of cuttings requires specific methods dis-
cussed in Sect. 17.1.4.2.
Sampling depends on the nature of the project
(Griffith 1967). The scope of the problem must be de-
fined before collecting samples.
The standard lithologic logging system proposed by
Shell International Exploration and Production (Stan-
dard Legend 1995) offers a widely used possibility for
describing features recognized in outcrops, cores and
microfacies samples. Lithology, texture and composi-
tion, rock color, accessory minerals, fossils, stratifica-
tion and sedimentary structures, postdepositional cri-
teria and diagenetic features as well as physical criteria
(e.g. porosity and permeability) are expressed by sym-
bols, standard abbreviations and codes. Too many sym-
bols and criteria will mask trends in the stratigraphic
sequence. For the purpose of microfacies studies the
most important data should be shown in logs: e.g. depo-
sitional textures, fossils, grain types, matrix and car-
bonate cement. Logs are presented in a variety of scales
depending on the problems involved. Detailed micro-
facies logs may require a scale of 1 cm = 1 m or less.
Authors studying depositional patterns of carbonate
rocks use a scale of 1 cm =10 m.
Typical research objectives may be
•
depositional texture of carbonate rocks as an aid to
classifying rock types and assessing physical proper-
ties,
•
type, texture and size of carbonate grains as an aid
to determining depositional environment and paleo-
currents,
•
studies of grains, matrix, cement and pores for un-
derstanding diagenesis and physical/technological
properties of carbonate rocks,
•
paleontological criteria, for subdividing and corre-
lating stratigraphical units or providing data which
can help with paleoenvironmental reconstructions.
