Geology Reference
In-Depth Information
Box 4.17.
Significance of ooids and oolites.
Paleoenvironmental proxies
Microfabrics, size distributions of ooids and sedimentary structures (e.g. cross-bedding, laminar bedding, fine
laminae) of oolitic limestones reflect environmental conditions dominating water energy, salinity and water depths.
Water energy levels and transport processes:
Differences in high- and low-energy settings control growth and
prevailing microfabrics of ooids, and determine the number of autochthonous ooids (deposited more or less at the
place of origin) and allochthonous ooids (deposited after greater transport). As many ooids form in current-swept
areas, they can accumulate in conditions different from those in which they were generated. Ooids of intertidal and
shallow subtidal areas are frequently transported to adjacent subenvironments, but also to areas at a greater distance
(e.g. slope, basins).
Autochthonous ooids
are characterized by rare or missing abrasion, the association of single and
broken ooids, and the occurrence of ooids with same maximum size. These ooids exhibit a relatively simple transport
history. Changes from radial to tangential concentric microfabrics combined with the crossing of a critical threshold
of grain size allow the paleocurrents to be estimated (Heller et al. 1980). A diameter of 0.6 mm appears to mark the
threshold for particles transported in suspension or as bed load (Miller et al. 1977).
Allochthonous ooids
are charac-
terized by mixing of various-sized ooids (Pl. 46/6) and abrasion of laminae. These ooids are distinguished from ooids
deposited in situ by microfabric, size, thickness of the cortices, and associated grains (Chow and James 1987).
Evidence of redeposition is (1) the size, sorting, and skewness of ooids and associated grains, (2) the ratio of ooid
content/mean diameter, and (3) the cortex/nucleus ratio (Carozzi 1983; Fig. 4.25). Low-energy conditions are gener-
ally inferred from the dominance of radial-fibrous microfabric, irregular and often asymmetrical shapes, and size
parameters. Changes from low- to high-energy and from bed-load to suspension transport are reflected by microfab-
rics of ooids (Poncet 1984).
Salinity:
Freshwater ooids often differ from marine ooids in having exceedingly irregular surfaces and shapes, and
commonly tangential or microsparitic laminae. Ooids formed in hypersaline settings appear to be characterized by
the predominance of radial ooids as compared with other ooid types.
Water depths:
Some workers use the amount of
ooids as measure of water depths (Fabricius 1967). Bahamian ooids are concentrated in very shallow waters. Their
abundance in mobile ooid fringes and on sand flats (depth <1 m) ranges between 75 and >95% (Harris 1979).
Sealevel fluctuations
Sea-level fluctuations influence circulation patterns and submarine morphology, which in turn control the local
conditions for ooid growth. This may be indicated by distinct compositional changes from skeletal to ooid grain-
stones or vice versa. Drops in sea level can lead to exposure and rapid cementation, causing the death of the ooid
factory (Bosellini et al. 1981).
Regional correlations
Oolitic beds which can be laterally followed for long distances are valuable stratigraphic markers. An example are
stratigraphically distinct Devonian ironstone horizons consisting of ferruginous ooids and bioclasts that were trans-
ported by storms several tens of kilometers over the open shelf (Dreesen 1989).
Depositional settings
Marine ooids are deposited in shoreface settings of ramps, on inner platforms, near outer margins of platforms, and
as allochthonous sediments on the slope and in proximal or distal parts of basins. Specific settings can be recognized
from the abundance, texture, ooid type, and the number of autochthonous and allochthonous ooids. Resedimented
ooids, deposited as calciturbidites on the slope and in coalescent deep-sea fans, are often bimodal, poorly sorted and
occur with lithoclasts and shallow-marine, platform-derived skeletal grains (Bosellini et al. 1981; Kolckmann 1992;
Zempolich and Erba 1999).
Paleoclimate and paleoceanography
Ooids are often used as proxies for warm-water environments or tropical settings (e.g. Opdyke and Wilkinson
1990, Kiessling et al. 2002). The main assumption is that carbonate ooids favor warm waters with high salinities
(Lees 1975). Oolitic deposits appear to be good paleoclimatic indicators. During the Phanerozoic they were usually
concentrated in the tropics and lower subtropics, but not at or very near to the equator. Differences in the diagenetic
style of ancient oolites are attributed to the prevailing climate. The paucity of early meteoric cements is explained as
a result of an arid climate, whereas abundant meteoric cements should reflect a more humid climate (Hird and Tucker
1988). Integrated paleoceanographic and paleoclimatic models have been developed based on the mineralogical and
geochemical criteria of ooids (Heydari and Moore 1993).
Economic importance of ooid carbonates
Hydrocarbon reservoir rocks:
Because aragonite and calcite ooids behave differently with regard to dissolution and
cementation, the original mineralogy as well as the spatial distribution of ooid sand bodies (sheets or shoals), and the
geometry of ooid-bearing sequences have a significant influence on reservoir qualities of carbonate rocks (Swirydczuk
1988; Burchette et al. 1990; Hovorka et al. 1993). Oolites consisting of
autochthonous and allochthonous ooids may
represent different reservoir types due to significant differences of primary fabric-selective intra- and interparticle
and secondary interparticle and fracture porosity (Carozzi 1983; Handford 1988). For important field case studies
and a discussion of oolitic reservoirs see Keith and Zuppann (1993).
Ore deposits:
Carbonates with ferruginous ooids form important iron ore deposits in Europe and North America.
