Geology Reference
In-Depth Information
control on ooid growth is the distribution and compo-
sition of organic matter and the activity of microbes
(Mitterer 1972; Suess and Futterer 1972; Reitner et al.
1997).
environments is replaced by calcite. Dissolution starts
with the dissolution within ooid laminae and continues
to the formation of solution pores following the course
of the laminae (Budd 1988).
The loss of internal structures in many ancient oo-
ids is attributed to a rapid solution of aragonite due to
subaerial exposure, and facilitated by the decay of or-
ganic matter. Ooid cortices consisting of coarse blocky
spar or small equant crystals are interpreted as a result
of calcitization including the destruction of an origi-
nally porous aragonitic fabric and the formation of a
coarse-grained calcite fabric (Rich 1982; Tucker 1984,
1985; Friedel 1995). Originally Mg-calcite radial-fi-
brous ooids transform to Low-Mg calcite via intergranu-
lar solution/precipitation films (Richter 1983; Heydari
et al. 1993).
How to describe ooids?
Box 4.15 summarizes the essential criteria for mi-
crofacies analyses of oolites.
Diagenesis and porosity of ooids and oolites
Early diagenetic changes of ooids include alterations
by microborings and abrasion and processes related to
the decay of organic matter. Dissolution processes oc-
cur during fabric-selective diagenesis which is con-
trolled by the primary mineralogy. Aragonite within
ooids deposited in shallow-marine shoal and shoreface
Box 4.15. How to describe ooids? Check your sample!
Field criteria
Depositional regime: Predominantly carbonate, mixed siliciclastic-carbonate or siliciclastic?
Geometry of the oolite body: Mound-shaped or sheet-like? Restricted to well defined horizons or comprising a series of
limestone beds?
Limestone texture: Grainstone, packstone, wackestone? Or even bindstone?
Sedimentary structures (lamination, bedding, cross-bedding, -> hydrodynamic conditions)?
Association with lithoclasts and fossils?
Laboratory criteria
Abundance (high, medium, low, -> high abundance may indicate autochthonous ooid deposits) and distribution of ooids
in the samples (concentrated and densely packed or scattered)?
Single or polyphase ooids (-> redeposition)?
Major structural ooid types (tangential, radial, micritic ooids; micritic ooids with microborings or signs of recrystalliza-
tion? Radial ooids with fibrous crystals cross-cutting the laminae -> diagenetic alteration)?
Specific ooid types (cerebroid; asymmetrical and eccentric -> changes in growth conditions; broken and regenerated
-> synsedimentary reworking and temporal breaks in ooid formation, potential hiati in sedimentation; distorted ->
disruption of normal ooid formation and/or diagenetic alterations; half-moon ooids -> clues to vanished evaporites;
deformed ooids -> indication and measure of tectonic stress)?
Dominating shape (spheroidal, ellipsoidal, other)?
Size and sorting (ranges, average; uni- or bimodal; median, skewness -> differentiation of autochthonous and allochtho-
nous ooids)?
Nuclei (skeletal grain, lithoclast, peloids, mineral grain; single nucleus or several nuclei; preservation and rounding of
nuclei; comparison of nuclei material with the non-ooid grains in the sample)?
Nucleus/cortex ratio (-> clue to hydrodynamic conditions)?
Cortex (homogeneous or inhomogeneous; thickness -> superficial or normal ooid)?
Ooid laminae (thickness; alternation of light and dark laminae; signs of mechanical abrasion -> pointing to high-energy
conditions; detached laminae -> compaction, burial diagenesis)?
Inferred primary mineralogy based on the preservation (aragonite, calcite, bimineralic)?
Fossils (acting as nuclei of the ooids, or encrusted on ooid laminae and surfaces; skeletal grains associated with the
ooids)?
Preservation of the ooids (partly or completely recrystallized, calcitized, dolomitized, silicified ooids; different preser-
vation of laminae -> originally bimineralic composition; conspicuously black colored laminae or completely black
ooids -> may indicate incorporation of organic matter under reduced conditions)?
Cracks or ruptures within the ooids (-> shrinkage structures or burial features)?
Oomolds (partly or completely leached ooids: abundance, distribution, internal sediment and cements; connecting pores).
 
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