Geology Reference
In-Depth Information
Significance of carbonate grains:
Grain types are paleo-
environmental proxies both for non-marine and marine
carbonates (e.g. water energy levels, sedimentation
rates); characterize and differentiate specific deposi-
tional settings and sea-level fluctuations, and provide
significant insights into global secular variations of car-
bonate mineralogy in Phanerozoic oceans. Grain asso-
ciation patterns are central to the reconstruction of
paleoclimate and paleolatitudinal zones (Sect. 12.2.2.1).
The 'compositional maturity' of limestones (the extent
to which a sediment approaches the constituent end-
members: synsedimentary clasts, ooids, fossils, peloids,
micrite, and terrigenous minerals) augment the com-
plexity of the processes that operate on carbonate sedi-
ments (Smosna 1987). Changing grain composition re-
flects cyclic sedimentation and assists in evaluating of
sequence stratigraphic models (Sect. 16.1.2.2). Grain
type, mineralogy and spatial variations in the distribu-
tion of grains are major controls on the porosity devel-
opment of reservoir rocks, as exemplified by Fig. 4.8.
high-resolution sequence stratigraphy and paleoenvi-
ronmental interpretation of shallow-marine and deep-
marine carbonates (Courtinat 1989; Steffen and Gorin
1993; Tyson 1995; Pittet and Gorin 1997; Rameil et al.
2000).
Reworking of whole or fragmented skeletal grains
(e.g. foraminifera, mollusks) from unconsolidated or
semiconsolidated sediments is common in shallow-
marine environments. Read (1974) termed those grains
lithoskels
and described modern examples from Shark
Bay, Australia, where banks of skeletal grainstone have
been formed by accumulation of grains eroded from
Pleistocene sediments. Ancient lithoskels are common
in lag deposits (Pl. 9/7).
Sizes of bioclasts
The average size of skeletal grains seen in thin sec-
tions varies widely, and for larger fossils depends on
the size of thin sections used. The sizes of skeletal grains
and fossils in the thin sections vary within a range of
<1 mm to a few millimeters to several centimeters. The
ordinary size ranges of skeletal grains are reported in
the context of the discussion of the fossil groups in
Chap. 10. The size of skeletal grains reflects growth
size (e.g. tests of planktonic foraminifera), and size pat-
terns; these are caused by transport processes leading
to the diminution of fossils or fragments of fossils, and
depend on hydraulic modifications.
The shape and bulk density of bioclastic carbonate
grains as well as the skeletal structure are important
factors affecting the composition and grain size distri-
bution of carbonate rocks (Maiklem 1968). The effect
of the hydraulic modification of the initial grain size
varies with the shape of the grain, whether blocks (cor-
als, coralline algae), rods (larger foraminifera, turreted
gastropods), plates (
Halimeda
segments, pelecypod
valves), or spheres (rounded foraminifera). In experi-
ments, plate-shaped grains settle significantly more
slowly than any of the other grain types. Spheres also
settle slowly. Block-shaped grains finer than about
2 mm settle slowly, grains coarser than 2 mm exhibit
higher and irregular settling velocities. Rods of all sizes
tend to settle quickly and smoothly with their long axis
horizontal, if the interior is not partly filled with sedi-
ment.
4.2.1 Bioclasts (Skeletal Grains)
Terminology
Originally, the term
bioclast
was used for fossils
which were transported, broken and abraded and which
became part of the organic debris (Grabau 1920). To-
day, the term has a rather vague meaning in the context
of microfacies studies, and is commonly used for fossils
seen in thin sections, regardless of whether the fossils
are fragmented (e.g. crinoid ossicles or broken shells)
or still completely preserved (e.g. double-valved bi-
valves). The latter have been assigned to
biomorpha.
The term
skeletal grain
is synonymous with the term
bioclast.
Organoclasts
are organic particles of various ori-
gin with or without structure. They include
phytoclasts
(Bostick 1971: clay- to fine sand-sized, land-derived
plant remains),
palynomorphs
(HCl and HF resistant
organic microfossils comprising cyanobacterial and al-
gal remains, e.g. dinoflagellates and acritarchs; spores
and pollen),
degraded organic matter
as well as
zoo-
clasts
of organic microfossils (e.g. organic relicts of
benthic foraminifera; scolecodonts: polychaete jaws).
Phytoclasts and other organoclasts commonly are stud-
ied in palynofacies slides after extracting the fossils
from sediments by chemical methods, but can rarely
also be recognized in thin sections. The analysis of
organoclasts found in Mesozoic and Cenozoic carbon-
ate rocks is of increasing importance for understand-
ing the relations between carbonate and terrestrial en-
vironments and provide very useful information on
Constraints on the preservation of skeletal grains
The record of carbonate bioclasts in limestones is
strongly controlled by (1) the primary skeleton miner-
alogy, (2) taphonomic and diagenetic processes and
(3) methodological biases.
(1)
Primary skeleton mineralogy and biomineral-
ization:
Skeletal grains consist of mineral phases and
