Geology Reference
In-Depth Information
Fig. 4.26.
Depositional sites and environmental controls of ooids
exemplified by a case study of shallow-water, near-coastal
Purbeck facies (lowermost Cretaceous) carbonates of the Swiss and French Jura mountains (after Strasser 1986). Purbeck
sediments comprise shallow subtidal, inter- and supratidal, normal marine, brackish, freshwater and hypersaline facies.
Top: Inferred primary mineralogy, water energy, salinity, and depositional environment of ooid types characterized by dif-
ferent morphological features and cortical structures. All types show transitions. Type 1 and Type 4 may develop into
superficial ooids, Type 2 to oncoids, and Type 3 to large asymmetric coated grains.
1 - spherical micritic ooids with thinly laminated tangential cortices forming well-sorted grainstones with up to 90 % ooids
and only a few fossils (intertidal), 2 - irregular micritic ooids with thinly laminated cortices forming packstones (shallow
marine to lagoonal), 3 - ooids with thinly laminated fine-radial cortices contributing to poorly sorted skeletal grainstones
(deposition in quiet-water environments), 4 - ooids with a few fine-radial laminae occurring in well-sorted packstones and
wackestones (brackish lagoonal environment), 5 - ooids with coarse-radial cortices contributing to well-sorted grain- and
packstones (lagoons with varying water energy).
Bottom: Environments where ooids may have been formed. Type 1 ooids formed in normal-marine shallow waters under
high-energy conditions in small sand bars under the influence of tidal currents. Type 2 ooids originate in a quiet-water
lagoonal environment populated by algae and cyanobacteria. The radial ooids of Type 3 indicate marine conditions differing
in water chemistry from the environment of type 1 ooids, and frequent stirring-up of the sediment. Type 4 and type 5 ooids
represent low-energy lagoons of varying salinity with charophycean algae and ostracods. Ooids were accumulated in sand
bars by storms.
Wilkinson et al. (1985) found peak abundances of
ooid formation during the Late Cambrian, Late Mis-
sissippian, Late Jurassic, and the Holocene. These pe-
riods coincide with periods of global transgressions and
regressions.
The reduction in ooid abundance in Middle Ordovi-
cian to Early Mississippian and Late Cretaceous to mid-
Tertiary might be explained by the absence of wave-
agitated environments during times of strong continen-
tal submergence or by a threshold formed by too high
P
CO
2
levels.
Significance of ooids
Two examples of the potential of ooid analysis in
microfacies studies are shown in Fig. 4.25 and Fig. 4.26,
and summarized in Box 4.17.
