Geology Reference
In-Depth Information
A
B
Fig. 4.21.
The drawings, taken from the fundamental paper by Sorby (1879), demonstrate
two basic microfabrics found in
ancient ooids
and interpreted as being formed under different hydrodynamic conditions.
A
- Ferruginous
tangential concentric ooids
, which were repeatedly broken during formation. The nuclei are bioclasts
(network structure: bryozoans) and fragments of ooids. The microfabric of the ooids is characterized by tangentially oriented
crystals forming distinctly concentric laminae. The ooids occur together with small bioclasts and bivalve shells (top right)
and angular terrigenous quartz grains.
B
- Single and compound
radial-fibrous ooids
formed around bioclasts, rounded clasts and peloids (black) as well as small
oncoids (top left). The microfabric of the light-colored laminae is characterized by radially oriented fibrous crystals. Skeletal
grains are echinoderms (top right) and mollusk shells. Note grain-support fabric and high open interparticle porosity. Middle
Jurassic (A - Inferior Oolite, B - Great Oolite, Bathonian): Southern England.
Primary sedimentary structures observed in the oolitic limestones of the Great Oolite Series suggest deposition in a tidal
flat channel system (Klein 1965). The Great Oolite forms a hydrocarbon reservoir. The type of ooid fabric controls the
distribution and amount of intragranular porosity (Sellwood and Beckett 1991).
laminae or it occurs everywhere in the cortex of origi-
nally calcitic or originally aragonitic ooids. Aragonite
ooids can be transformed to micritic and microsparitic
ooids by in-situ calcitization (Popp and Wilkinson 1983,
Friedel 1995). In thin sections micrite ooids appear dark,
featureless and microcrystalline. The micritic fabric re-
sults from the activity of microborers (Pl. 13/6) or from
recrystallization, leading to a microgranular mosaic con-
sisting of small calcite crystals (2-10
m), and display-
ing neither concentric nor radial microfabrics.
blages can provide a powerful guide for estimating sedi-
ment stability when used in conjunction with the analy-
sis of microfacies and sedimentary structures.
Encrusting microfossils occurring upon ooids and
enclosed within ooids indicate short-term stable con-
ditions during the growth of the ooids (Fig. 4.22). Most
common are encrustations by foraminifera (Pl. 13/3).
Calcareous micro-algae in ooids are valuable environ-
mental indicators (Dragastan and Richter 1999).
Specific ooid types
Fig. 4.23 summarizes a few ooid types character-
ized by specific morphology. Syndepositionally formed
grains are cerebroid, eccentric, broken and regenerated
as well as part of the distorted ooids. Some distorted
ooids as well as half-moon and spiny ooids originate
from diagenetic processes, the latter two by early di-
agenesis. Significantly deformed ooids are of tectonic
origin.
Fossils in ooids and oolitic limestones
Oolitic limestones are either poor in fossils or they
contain specific fossil assemblages. The composition
of autochthonous fossil assemblages in grainstones is
constrained primarily by sediment stability. A decrease
in faunal diversity commonly correlates with increased
sediment mobility (Feldman et al. 1993). The common
absence or rarity of sessile or even mobile benthos in
oolitic limestones suggests that periods of stabilization
were not long enough for shelled organisms. Highly
unstable and constantly shifting substrates are inhab-
ited by only a few shelly benthos. Macrofossil assem-
Modern ooids
Modern ooids originate in marine and non-marine
settings. Marine ooids occur in intertidal and shallow
