Geology Reference
In-Depth Information
A
B
Fig. 4.24.
Ooids originate
in low- and high-energy environments (Fig. 4/19), but also grow
in situ
within microbial mats
(
benthic ooids
), e.g. in hypersaline ponds (e.g. Friedman et al. 1985, Gerdes et al. 1994). The growth of the ooids is a result
of cummulative processes by biofilm attachment and subsequent aragonite encrustation.
A
- Microbial ooid surrounded by
sheats of filamentous mats. Dark laminae represent meshworks of degraded filament bundles of the cyanobacteria
Microcoleus
.
Light laminae are extracellular polymeric substances produced by coccoid cyanobacteria and diatoms.
B
- Benthic ooid as a
part of an aggregate grain. The ooid consists of microcrystalline aragonitic laminae with radially arranged crystals (light) and
organically enriched laminae produced by biofilms (dark). Dark laminae are usually thinner than light laminae. In contrast to
marine Bahamian ooids that are controlled by hydrodynamic conditions, the nuclei of benthic ooids are rarely allochthonous
grains; they normally originate from the adjacent microbial community (e.g. cell colonies) or as metabolic products, includ-
ing bubbles and liquid globules. Subrecent: Salina Janubio, Lanzarote, Canary Island, Spain. Scale is 100
m.
Courtesy of G. Gerdes, Wilhelmshaven.
Porosity:
Aragonite ooids are susceptible to disso-
lution and formation of oomoldic porosity during fresh-
water diagenesis; moldic porosity is generally accom-
panied by low permeability. Primary calcite ooids are
less susceptible to dissolution in meteoric waters, hence,
preserved porosity is interparticle porosity, and perme-
ability is generally higher.
The understanding of the complex time and space
relationships of different porosity types of oolitic lime-
stones is critical for reconstructing the depositional and
diagenetic history and for evaluating their importance
as potential hydrocarbon reservoirs (Cussey and Fried-
man 1977; Carozzi 1981; Handford 1988; Sellwood and
Beckett 1991). Primary porosity types of oolites include
intra- and interparticle porosity. Secondary porosity is
produced by early diagenetic intraparticle dissolution
(oomoldic porosity) and late diagenetic fracturing.
Oomolds (Friedman 1964): Dissolved aragonitic
ooids are common in subaerially exposed Pleistocene
limestones and are known from rocks as old as the Cam-
brian. Many oomolds exhibit an eccentric displacement
of the non-aragonitic nuclei, often perpendicularly to
the bottom of the mold, subsequent to the dissolution
of the aragonitic laminae (Pl. 13/7, Pl. 29/3, 4, Pl. 150/
1). The majority of observations indicates that oomoldic
porosity suggests the presence of former aragonite.
High-Mg calcite ooids transform to Low-Mg calcite
ooids with retention of fine fabric details and no indi-
cation of wholesale dissolution and creation of distinct
moldic porosity.
Compaction:
Most oolitic limestones are somewhat
compacted. The measurement of packing provides a
tool for quantitatively estimating the degree of grain
compaction before final cementation and lithification.
Compaction measures derived from thin-section data
of ooid grainstones include the packing density and the
packing index (Coogan 1970). Both measurements con-
sider the total number of grains and the number of grain-
to-grain contacts. Strong compaction of ooid carbon-
ates is indicated by the frequency of interpenetrating
grains, ooids with peripheral spalled-off laminae, flat-
tened ooids (Pl. 36/5) and abundant, very closely packed
'fitted' ooids. Experimental compaction of ooids un-
der deep-burial temperatures and pressure and varying
salinity conditions shows appreciable reduction of bulk
volume and porosity and the development of pressure-
solution contacts. Plastically deformed ooids exhibit
concav-convex and longitudinal contacts (Bhattacharyya
and Friedman 1984).
Dolomitization:
Many limestones exhibit selective
dolomitization of ooids starting with an initial precipi-
tation along the periphery of the grains, followed by
