Page 44 - Sedimentology and Stratigraphy

Limestone 31

Fig. 3.3 Fossil gastropod shells in a limestone.
Fig. 3.2 Bioclastic debris on a beach consisting of the hard
calcareous parts of a variety of organisms.

arthropods, such as the trilobites, are made up of
Carbonate-forming animals
microscopic prisms of calcite that are elongate per-
The molluscs are a large group of organisms that pendicular to the edges of the plates. Although they
have a fossil record back to the Cambrian and com- may appear to be quite different, barnacles are also
monly have calcareous hard parts. Bivalve molluscs, arthropods and have a similar internal structure to
such as mussels, have a distinctive layered shell struc- their skeletal material.
ture consisting of two or three layers of calcite, or Another group of shelly organisms, the echinoids
aragonite, or both. Of the modern forms, some such (sea urchins), can be easily recognised because they
as oysters and scallops are calcitic, but most of the rest construct their hard body parts out of whole low-
are aragonitic: aragonite shells may have been the magnesium calcite crystals. Individual plates of
norm throughout their history, but no pre-Jurassic echinoids are preserved in carbonate sediments.
bivalve shells are preserved because of the instability Crinoids (sea lilies) belong to the same phylum as
of the mineral compared with the more stable form of echinoids (the Echinodermata) and are similar in the
calcium carbonate, calcite. Gastropods are molluscs sense that they too construct their body parts out of
with a similar long history: they also have a calcite or whole calcite crystals, with the discs that make up
aragonite layered structure, and are distinctive for the stem of a crinoid forming sizeable accumulations
their coiled form (Fig. 3.3). The cephalopod molluscs in Carboniferous sediments. In life the individual
include the modern Nautilus and the coiled, cham- crystals in echinoid and crinoid body parts are per-
bered ammonites, which were very common in Meso- forated, but the pores are filled with growths of
zoic times. Most cephalopods have a layered shell calcite that may also extend beyond the original
structure, and, in common with most other molluscs, limits of the skeletal element as an overgrowth
this is a feature that may be recognisable in fragments (18.2.2). These large single crystals that make up
of shells under the microscope. There is an important echinoderm fragments make them easily recognis-
exception in the belemnites, a cephalopod that had a able in thin-section.
cigar-shaped ‘guard’ of radial, fibrous calcite: these Foraminifera are small, single-celled marine
can be preserved in large numbers in Mesozoic sedi- organisms that range from a few tens of microns in
mentary rocks. diameter to tens of millimetres across. They are either
The brachiopods are also shelly organisms with floating in life (planktonic) or live on the sea floor
two shells and are hence superficially similar to (benthic) and most modern and ancient forms have
bivalves. They are not common today but were very hard outer parts (tests) made up of high- or low-
abundant in the Palaeozoic and Mesozoic. The shells magnesium calcite. Both modern sediments and
are made up of low-magnesium calcite, and a two- ancient limestone beds have been found with huge
layer structure of fibrous crystals may be completely concentrations of foraminifers such that they may
preserved in brachiopod shells. The exoskeletons of form the bulk of the sediment.