Geology Reference
In-Depth Information
Cerebroid ooids
(Graf and Lamar 1960; Carozzi 1962)
Criteria: Indented periphery. Mottled appearance of the cortex. Sectors made of tangentially arranged lami
nae and radial micritic sectors which often start at former depressions of the nucleus surface.
Occurrence: Marine and nonmarine. Often associated with stromatolites.
Interpretation: The indented periphery of the ooids has been explained as a result of a peripheral replace
ment. The formation of the micritic sectors is ascribed to bacterial dissolution and precipitation (Kahlke 1974).
Asymmetrical and eccentric ooids
(Gasiewicz 1984)
Criteria: Small spherical and ovoid composite grains consisting of superficial ooids acting as eccentric nuclei
for successively formed ooids whose growth takes place preferrentially upward. Alternation of very thin oolitic
laminae (white) and micrite envelopes (black). Ooid size commonly < 0.30 mm. Rare in wackestones and
grainstones, common in packstones, associated with abundant peloids. Eccentric ooids should not be con
fused with oomolds with eccentric nuclei.
Occurrence: Lowenergy shallowmarine lagoonal and lacustrine environments, periodically without agita
tion.
Interpretation: The ooid laminae are explained as resulting from short periods of water agitation, the micritic
envelopes correspond to longer nonturbulent periods. Eccentric growth is controlled by changes from sus
pension to saltation and traction processes at the sea floor. Top: A superficial ooid formed in suspension and
deposited on the sea floor is the base of a micrite envelope and forms the eccentric nucleus of a subsequent
new superficial ooid. Bottom: Ongoing ooid formation subsequent to rolling and turning.
Broken and regenerated ooids
(Carozzi 1961a)
Criteria: Fragments of radial or tangential ooids that act as nuclei for new ooids are surrounded by ooid
laminae. The ooids are characterized by alternating concentric ooid laminae and micritic laminae (white).
Occurrence: Common in high and lowenergy settings, occurring together with partly cracked ooids and
regular ooids.
Interpretation: These ooids indicate multiple synsedimentary reworking and breaks in the formation of ooids,
commonly associated with redeposition. Also known from ferruginous ooids (hiatus ooids, Berg 1944).
Distorted ooids
(Carozzi 1961b; Conley 1977)
Criteria: Characterized by notched and stretched, sometimes flattened ooid grains, or grains connected by
narrow apophyses, or series of grains linked in zigzag chaines parallel to bedding. The distortion has com
monly preceded cementation and strong compaction. These ooids cooccur with other distorted grains (e.g.
micritized intraclasts) in irregular pockets or in zones parallel to the bedding.
Occurrence: Restricted to specific limestone horizons, affected by strong waves or currents. Com
mon also in noncarbonate oolitic deposits.
Interpretation: The shapes of distorted ooids illustrate a complete gradation from initial rupture
and plastic deformation of soft grains to the rupture of rigid bodies. Explanations include fracture
and plastic alteration of soft ooids in a turbulent environment, sediment sliding, or compaction.
Deformation occurs when uncemented or poorly cemented ooids are buried (Pl. 13/8, Pl. 36/5).
The term also is used for pitted ooids and cracked ooids which result from mechanical distortion
and pressure solution. These ooids exhibit displaced and sometimes broken laminae (Pl.150/2).
Half moon and shrunken ooids
(Wherry 1916; Mazzullo 1977)
Criteria: Ooids in which the interior cores have dropped to the bottom of the concentric outer layers, forming
a geopetal fabric. Cross sections commonly exhibit a halfmoon aspect characterized by an internal dividing
line convex upward which separates an upper light part (frequently calcitefilled) and a lower dark part.
Occurrence: Rare in carbonateevaporite series, but also known from meteorically influenced carbonates.
Interpretation: These ooids may be products of evaporitecarbonate or aragonite solution diagenetic pro
cesses (Carozzi 1963) and may indicate vanished evaporites in associated rocks (Folk and Pittman 1971;
Folk and Siedlecka 1974). Geopetal ooids, however, can also result from the selective aggrading recrystalli
zation of the ooid nuclei during a period of meteoric diagenesis (Mazzullo 1977).
Spiny ooids
(Davaud et al. 1990)
Criteria: Characterized by external cortices exhibiting spines. The external cortices are detached from the
underlying cortices near the points of contact between ooid grains, suggesting a postdepositional origin for
the spines. The external cortices (white) are not stretched or broken and fit perfectly into the underlying
cortices (densely arranged laminae) in undeformed areas.
Occurrence: Lagoonal beach sands cemented in a vadose environment.
Interpretation: The ooids are explained by tangential compressive deformation due to crystal growth in the
outer cortices during early subaerial deformation. Spiny ooids differ from distorted ooids formed during burial
compaction; these ooids display convex forms pointing into the pore space.
Deformed ooids
(Cloos 1947; Nissen 1964; Badoux 1970)
Criteria: Originally spheroidal ooids are distinctly elongated, flattened and stretched. Microfabric
structures recognizeable in traces only or completely destroyed.
Occurrence: Described from folded carbonate series. Indicate tectonic style and timing of plastic
deformations.
Interpretation: Intensive tectonic stress results in elongations parallel to the schistosity planes.
Fig. 4.23.
Common specific ooid types.
