Page 45 - Sedimentology and Stratigraphy

32 Biogenic, Chemical and Volcanogenic Sediments

Some of the largest calcium carbonate biogenic important constituent of pelagic limestone, including
structures are built by corals (Cnidaria) which the Cretaceous Chalk.
may be in the form of colonies many metres across Cyanobacteria are now classified separately to
or as solitary organisms. Calcite seems to have been algae. The ‘algal’ mats formed by these organisms
the main crystal form in Palaeozoic corals, with ara- are more correctly called bacterial or microbial
gonite crystals making the skeleton in younger corals. mats. In addition to sheet-like mats, columnar and
Hermatypic corals have a symbiotic relationship domal forms are also known. The filaments and sticky
with algae that require clear, warm, shallow marine surfaces of the cyanobacteria act as traps for fine-
waters. These corals form more significant build-ups grained carbonate and as the structure grows it
than the less common, ahermatypic corals that do forms layered, flat or domed structures called stro-
not have algae and can exist in colder, deeper water. matolites (Fig. 3.4), which are some of the earliest
Another group of colonial organisms that may con- lifeforms on Earth. In contrast to stromatolites,
tribute to carbonate deposits are the bryozoa. These thrombolites are cyanobacterial communities that
single-celled protozoans are seen mainly as encrusting have an irregular rather than layered form. Oncoids
organisms today but in the past they formed large are irregular concentric structures millimetres to cen-
colonies. The structure is made up of aragonite, timetres across formed of layers bound by cyanobac-
high-magnesium calcite or a mixture of the two. The teria found as clasts within carbonate sediments.
sponges (Porifera) are a further group of sedentary Other cyanobacteria bore into the surface of skeletal
organisms that may form hard parts of calcite,
although structures of silica or protein are also com-
mon. Stromatoporoids are calcareous sponges that
were common in the Palaeozoic. Other calcareous
structures associated with animals are the tubes of
carbonate secreted by serpulid worms. These are a
type of annelid worm that encrusts pebbles or the
hard parts of other organisms with sinuous tubes of
calcite or aragonite.

Carbonate-forming plants
Algae and microbial organisms are an important
source of biogenic carbonate and are important con-
tributors of fine-grained sediment in carbonate envi-
ronments through much of the geological record
(Riding 2000). Three types of alga are carbonate
producers. Red algae (rhodophyta) are otherwise
known as the coralline algae: some forms are found
encrusting surfaces such as shell fragments and peb-
bles. They have a layered structure and are effective at
binding soft substrate. The green algae (chloro-
phyta) have calcified stems and branches, often seg-
mented, that contribute fine rods and grains of
calcium carbonate to the sediment when the organ-
ism dies. Nanoplankton are planktonic yellow-
green algae that are extremely important contribu-
tors to marine sediments in parts of the stratigraphic
Fig. 3.4 Mounds of cyanobacteria form stromatolites,
record. This group, the chrysophyta, include cocco- which are bulbous masses of calcium carbonate material at
liths, which are spherical bodies a few tens of various scales: (top) modern stromatolites; (bottom) a cross-
microns across made up of plates. Coccoliths are an section through ancient stromatolites.