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SKELETAL
GRAINS
Foraminifera tests display a wide variety of shapes and sizes, but most are less than 1mm in diameter (figure above). Most are multichambered and these chambers are arranged serially (uniserial, biserial or triserial) or are coiled (planispiral or conically spired). Recognition of wall structure is critical for the identification of foraminifera because random sections of specimens do not reveal all the chambers or their true shape. Unfortunately, the wall structure, especially in Paleozoic species, is often obscured or destroyed by diagenesis. Brachiopods
Bryozoans Bryozoans are colonial
benthic organisms with calcareous skeletons that are encrusting,
branching or fenestrate (resembling a window screen). Trilobites
Ostracodes Tintinnids
Echinoderms
Ophiuroids and asteroids also contribute plates, spines and ossicles. In ancient limestones crinoid plates are easily distinguished in thin section by their reticulate pattern. Although these plates now have calcite mineralogy, they still behave as large single crystals. The plates usually appear dusty because the reticulate network of pores may be filled with micrite or inclusion-bearing cements. Pore filling or rims of cement on echinoderm plates are usually added in optical continuity. Red
Algae
Depending on the orientation of the section with respect to the growth surface, the calcified thallus of the alga has either a characteristic rectangular or rounded cellular pattern (above figure). Smaller fragments, only a few millimeters in diameter, may be distinguished by the structure, type of conceptacles (cavities where spore-bearing cells collect) and detail of the calcified tissue. These magnesium calcite algal fragments can be micritized to form a dark brown opaque mass. Corals
The fibers are grouped in sclerodermites that are aligned into trabeculae. The fibers are arranged somewhat differently in different parts of the coral, and some variation of fiber orientation exists between orders. The skeleton is characterized by an outer wall and by a series of internal vertical plates (septa), horizontal plates (tabula) and curved plates (dissepiments) (above figure). The patterns of internal plates and growth forms are used to identify the large variety of coral types. Alcyonarian corals, actually hydrozoans, are an exception in that they do not have massive skeletons but instead produce spicules. The spicules are varyingly shaped and range from 0.1 mm to 5 mm in length. Stromatoporoids
Molluscs
Mollusks inhabit nearly every marine environment. Shell growth is environmentally controlled; thin shells typify quiet-water or cold environments whereas thick shells occur in higher-energy environments. The shells are primarily aragonite or they may be a mixture of aragonite and calcite; thus, their manner of preservation is variable. In shells with mixed mineralogies, the outer layers are commonly calcitic and the inner layers are commonly aragonitic. Rudists represent a special case in that they underwent major changes in shell mineralogy and structure during the Cretaceous, involving an increase in the relative proportion of aragonite to calcite (and thus an increase in the likelihood of development of secondary porosity) and an increase in porosity and complexity of the shell. In contrast, oysters are shallow-marine benthic pelecypods having a three-layered calcite shell. Other pelecypods have similar mineralogy; an example is Inoceramus that is common to shelf and deep-shelf deposits of the Cretaceous. Green
Algae
Calcispheres are small hollow spheres that may be fruiting bodies of certain green algae. The spheres apparently were extremely buoyant and tended to accumulate in quiet settings like intraplatform basins. Cyanobacteria
In Recent environments, the blue-greens form slimy mats composed of filaments and carbonate. The carbonate may be precipitated through photosynthesis or trapped as grains washed onto the mat. The cyanobacteria form colonies that may be recognized by their external form (above figure). They may be stacked hemispheroids or saucers, etc. The crinkly banding of an ancient algal colony is difficult to distinguish from soil concretions or caliche in section because all of these deposits exhibit horizontal banding, spherical growths and heads. Occasionally algae infest the surface of grains and form an envelope of micrite. This envelope represents an alteration or addition to the perimeter of the grain. The alteration replaces the organized grain fabric with disorganized micrite in two steps: (1) the boring activity of the algae with dissolution of the grain followed by (2) reprecipitation or trapping of calcium carbonate in the void. Sponges
Sclerosponges are calcareous sponges that precipitate a massive aragonite skeleton and have siliceous spicules. These sponges are important contributors to the formation of deepwater reefs, living mainly in crevices and caves. Another important group of sponges are the boring varieties, such as Cliona, which produce large quantities of silt-size carbonate sediment that collects near reefs and on hard bottoms. The excavating cells of the sponge form characteristic hemispherical-shaped chips (above figure). Radiolarians
They can form thick widespread deposits over deep ocean bottoms or occur in silica-rich water associated with submarine volcanism. Radiolarians are an important biostratigraphic tool where they occur in deposits that commonly lack calcareous fossils. The radiolarian skeleton may be 2mm in diameter but is usually smaller. Skeletons vary greatly in shape and ornamentation. Often the shell is a hollow perforate sphere or vase. Most shells show low birefringence and fine-grained fibrous structure comparable with cherts or chalcedony. The porosity that characterizes the shells is usually occluded during this alteration. |
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Foraminifera
They
are either inarticulate and composed of calcium phospate, or articulate
and made of a chitinous periostracum covering thin outer prismatic
and thick inner lammelar calcite layers. As shown to left, the
shells may be (1) impunctate, (2) pseudopunctate with the inner
shell layer embedded with rod-like plugs, or (3) punctate in which
small rounded to elongate pores penetrate the shell. Some brachiopods,
like the late Paleozoic productids, grew spines which, when broken,
contribute separately to the sediment.
Their
binding ability makes them especially important as contributors
to the building framework in reef settings of all ages. Their
recognition and subdivision into classes is based primarily on
wall structure and the arrangement of internal round or polygonal
elongate tubes (zooecia) (left figure).
Laminated wall structure is a distinctive feature of bryozoan
debris in thin section. Most bryozoans are calcitic, but some
are partially or totally composed of aragonite.
In
cross section trilobite shells are long, sinuous and frequently
have a recurved shepherds crook shape (left
figure). Trilobite shells are generally less than 1mm thick.
The shell may be a few centimeters long but they are formed of
individual pieces that are millimeters or less in length. Their
shell is composed of calcite crystals that are oriented perpendicular
to the wall. However, when examined with a light microscope, the
fabric appears to be homogeneous because the crystals are too
small to be easily distinguished. The fabric exhibits undulose
extinction under cross-polarized light.
Tintinnids
are pelagic protozoan that have a microscopic cup-shaped calcite
test commonly .1 to .2 mm across (left figure).
In thin section the tests have U-shaped longitudinal cross-sections
and circular or elliptical transverse sections. Usually, the longitudinal
sections have characteristic flanges at the opening of the test
that create a horseshoe shape. Tintinnids are very common in Upper
Jurassic and Lower Cretaceous deep-water limestones of the Mediterranean
region.
The
plate fragments and spines are scattered through the marine sediments
of reefs, shallow shelves and deep-sea basins. Individual plates
and spines from echinoid tests are easy to identify in thin section
because the entire fragment extinguishes with cross-polarized
light. The spines exhibit characteristic internal radial patterns
(right figure). Holothurian spicules
(ossicles) are also single crystals. They are small, .07 to .1mm,
and occur in a variety of shapes.
Others,
which are encrusting or massive forms, grow on a variety of substrates
within varied energy conditions. They form nodules on unstable
sea bottoms and are important encrusters and cementers in modern
coral reefs, particularly in the Pacific where algal ridges form
a rim or lip to the reef. Red algae are important binders in reefs
of all ages. They play an especially important role in Mesozoic
buildups. Corallines can also be important in colder or deeper-water
sediments.
Corals
can be important contributors to the growth of carbonate platform
margins because of their rapid growth rates in reef settings and
their ability to produce large amounts of sediment. Stony solitary
or colonial corals have similar calcareous skeletons with basic
skeletal elements of aragonite or calcite fibers.
They
are possibly ancient equivalents of sclerosponges that coat ledges
in the deeper parts of modern coral reefs. Apart from siliceous
sponges the fossil skeleton is normally calcite, but their preservation
in limestones suggests that they were originally aragonite. The
skeleton consists either of overlapping dome-shaped, convex-outward
plates or of a trelliswork of concentric laminae with various
supports or pillars (right figure).
In sections parallel to growth direction, the stromatoporoid skeleton
has a rectangular grid pattern.
Pelecypods
have two shells that are hinged along one margin, while gastropods
are coiled tubes that can form a number of shapes (left
figure). Three or more layers form typical shells: an outermost
layer of chitinous periostracum and two inner calcareous layers.
Layers commonly have a nacreous or cross-laminated microstructure,
but up to six types of microstructures are possible. The appearance
of each structure varies according to its orientation. Molluscs
can be recognized by shell shape and their multilayered wall structures.
Broken pieces of fine mollusk debris may not be recognized as
such because the fragments are commonly leached and infilled or
replaced by calcite spar.
For
instance, the phylloid or plate-like dasyclad algae Ivanovia contributed
extensively to carbonate mound build-ups in the Mississippian
and Pennsylvanian. Green algae are plants that precipitate carbonate
in their flesh parts. They may be crustose, nodular growths or
erect plants commonly formed of segmented, flattened or cylindrical
branches (right figure). Green algae
are distributed worldwide, principally in warm shallow seas. They
are common in quiet lagoons, but are also found in less turbulent
parts of open shelves and reefs.
Stromatolites are layered structures formed by cyanobacterai and
carbonate sediment. They are commonly, but not exclusively, recognized
in ancient carbonates that were deposited in tidal flat settings.
Other blue-green structures include Girvanella, resembling an
intestine-like mass of tubes (left figure),
and Renalcis, a branching tree-like form. These forms are common
in reefal cavities and some slope sediments of the lower Paleozoic.
Other
forms produce abundant sediment during bioerosion of reefs and
other hard substrates. Most sponges live in water less than l00m
deep on hard bottoms where there is some water circulation. Sponge
spicules are commonly all that is preserved. These are composed
of silica, but they may be calcite or spongin (right
figure). Spicules vary in size and shape depending on their
position within the sponge and their function as structural framework
or as protection. Sponge spicules have smooth and simple geometrical
shapes and a hollow central axis; in contrast, spicules of other
organisms usually have a more complex shape.
Their
mineralogy ensures that the varieties of Radiolarian tests seen
in left figure are preserved below
depths at which carbonate grains dissolve.