Geology Reference
In-Depth Information
4.2.5 Ooids
Box 4.13.
Selected case studies of ancient rhodoid lime-
stones
. A comprehensive list of references dealing with
modern and ancient corallinacean red algae can be found
in the Internet (http//paleoweb.net/algae).
As an introduction first let us show you some espe-
cially beautiful examples from the Early Triassic of Ger-
many. Diameter of ooids: 0.4 mm.
Pleistocene
: Hills and Jones 2000; Minoura and
Nakamori 1982; Richter and Sedat 1983.
Tertiary
: Adey 1979; Adey and Macintyre 1973; Bassi
1995; Borrouilh-Le Jan and Hottinger 1988; Bosence
and Pedley 1982; Boulanger and Poignant 1975;
Braga and Martin 1988; Buchbinder 1977; Burgess
and Anderson 1983; Dabrio et al. 1981; Dullo 1983;
Esteban 1996; Fravega et al. 1993, 1998; Friebe 1988;
Llombart 1982; Manker and Carter 1987; Martin et
al. 1993; Maslov 1962; Moussavian and Höfling
1993; Nebelsick and Bassi 2000; Orszag-Sperber et
al. 1977; Pedley 1996; Perrin 1992; Perrin et al. 1995;
Pisera and Studencki 1989; Plaziat and Perrin 1992;
Rasser 1994; Simone and Carannante 1985; Studencki
1988; Vanucci et al. 1993, 1994a, 1994b, 1996.
Cretaceous:
Reitner 1987.
Carboniferous and Permian
: Johnson 1946; Toomey
1975, 1983, 1985.
Ooids have long been recognized as the most in-
triguing constituents of carbonate rocks (Sorby 1879;
Kalkowsky 1908; Fig. 4.21). For a long time ooids were
regarded as chiefly 'inorganic' grains, but strong bio-
logical controls on the formation of ooids are evident
(Fig. 4.24). Microfabrics, the mineralogy, abundance
and size of ooids reflect physical and chemical condi-
tions of depositional environments in marine and non-
marine settings. Ooids, therefore, are valuable paleoen-
vironmental proxies for water energy, temperature, sa-
linity and water depths. The paleoenvironmental inter-
pretation of ancient ooids, however, should be done with
some caution because (a) present-day marine Baha-
mian-type ooids formed in turbulent environments are
not necessarily analogues of ancient ooids, and (b) the
transport of ooids from the areas in which they were
produced to the depositional areas is often underesti-
mated. Changes in mineralogy and abundance of ooids
over time have been related to changes in atmospheric
carbon dioxide or to the rate of sea-level change and
used as indicators of global secular variations in Phan-
erozoic climates. Valuable reviews on modern and an-
cient ooids were provided by Simone (1981) and Rich-
ter (1983). Oolites (calcareous rocks composed of oo-
ids) formed on marine platforms and ramps comprise
more than 50% of the world's carbonate hydrocarbon
reservoirs.
The environmental interpretation of rhodoid-rich
limestones in the geological record must necessarily
start with the appropriate choice of the present-day cli-
matic model. The composition, growth forms and zo-
nation of rhodoids and macroids are excellent facies
indicators, but the risks of a too simple use of rhodoid
data for paleoecological studies should be considered
(Reid and Macintyre 1988).
Rhodoids have been successfully used for estimat-
ing paleo-water depths (e.g. Perrin et al. 1995; Basso
1998), and evaluating sea-level fluctuations (Adey
1986; Bourrouilh-Le Jan and Hottinger 1988; Freiwald
et al. 1991; Basso and Tomaselli 1994). In a geological
section, variations in shape, size, algal communities and
associated sedimentary features of rhodoids commonly
reflect changes and temporal trends in depths and wa-
ter energy (Braga and Martin 1988). The biotic com-
position of macroids reflect depth-controlled distribu-
tional patterns (Hottinger 1983; Piller and Pervesler 1989;
Plaziat and Perrin 1992; Moussavian and Höfling 1993).
Economic importance:
Limestones with abundant
rhodoids are popular decorative stones for exterior and
interior use because of their pleasing appearance and
resistance to abrasion. Rhodoid floatstones, formed in
lagoonal and reef environments, are common in Oli-
gocene and Miocene platform carbonates containing
hydrocarbon rocks, for example in Indonesia, the Phil-
ippines and the South China Sea (e.g. Grötsch and
Mercadier 1999).
Terminology
Ooids are spherical and egg-shaped carbonate or
non-carbonate coated grains exhibiting a nucleus sur-
