Geology Reference
In-Depth Information
Fig. 4.25.
Model of the
possible relationships between water energy, maximum and average grain size and the thickness of
ooid cortices
(superficial versus normal ooids). The model was developed for Great Salt Lake ooids (Carozzi 1957) and
improved during the study of Mississippian shallow-marine ooid shoals in the Illinois Basin (Lacey and Carozzi 1967, Rao
and Carozzi 1971). The maximum size of ooids and non-coated micritic grains is taken as a measure of the water energy level
necessary to transport grains by traction or suspension processes. The numbers 1 to 6 describe different situations with
regard to the hydrodynamic intensity which could favor the formation of ooids.
Situation 1 and 2:
The local agitation is greater than the maximum current intensity which brought the grains. Ooid
formation affects all the grains available until the size of the largest ooid and that of the largest movable grain are (nearly)
equal. The corresponding deposit is an oolite with a constant maximum size of normal and superficial ooids independant
from the size of the available nuclei.
Situation 3:
The local water energy equals the maximum size of the non-coated grains.
All smaller grains will be coated until their diameter reaches that of the largest non-coated grains. The sediment consists of
normal and superficial ooids associated with equal-sized, rounded non-coated grains.
Situation 4:
The local water energy is
lower than that of the currents which carry the largest grains into the environment. Grains which can not be lifted by the local
agitation settle as non-coated grains. Other grains are coated, resulting in a sediment consisting of variously sized non-coated
grains and small, predominantly normal ooids.
Situation 5:
The local water energy can just move the smallest grain resulting
in very small superficial ooids, associated with non-coated rounded particles of different size.
Situation 6:
The local agitation
equals or is less than the water energy needed to put the smallest grain in motion. The resulting deposit consists of rounded
non-coated grains without ooids.
Black circles: Non-coated carbonate grains (transported litho- and bioclasts, commonly rounded and micritic, brought
into the area of ooid formation by currents); open circles: superficial ooids; circles with cross-hatched signature: normal
ooids. After Carozzi (1960).
dolomite precipitation along porous cortical laminae
(Zempolich and Bakker 1993; Pl. 40/8). The coales-
cence of dolomite crystals results in the formation of
mimetic dolomite laminae and a concentric dolomite
fabric (Pl. 40/6). Dolomite ooids without traces of con-
centric laminae may indicate dolomitization subsequent
to a calcitization stage.
mon in oolitic ironstones. Iron ooids in limestones are
generally explained by the replacement of carbonate
(aragonite?) ooids by iron and silica compounds, prob-
ably during very early diagenetic stages, perhaps pre-
ceding the inversion of aragonite to calcite. The on-
shore or offshore source of the iron (terrestrial weath-
ering products, volcanic material, biochemical enrich-
ment?) and the formation of oolitic ironstones is the
subject of considerable debate (Gygi 1981, Kimberley
1983, 1994, Sturesson et al. 1999). Major problems are
the process and environment of ooid formation (accre-
tionary or concretionary; shallow-marine or nonma-
rine), the nature of iron minerals (diagenetic or primary),
and the mechanisms of iron enrichment. Examples of
modern iron ooids are known from lacustrine and ma-
rine environments (e.g. Lake Chad: Lemoalle and
Dupont 1973; Mahengetang Island, Indonesia: Heikopp
et al. 1996). Oolitic ironstones are known from the Pre-
cambrian and the Phanerozoic; they are particularly
Ancient ooids
Non-carbonate ooids
comprise iron ooids, phos-
phatic ooids (Swett and Growder 1982; Pl. 110/2, 4 ,
6), siliceous ooids, and less frequent ooids consisting
of silicate minerals (Meiburg and Kaever 1974; Ehren-
berg 1993; Bloch et al. 2002), sulfates (Babel and
Kaprzyk 1990) or sulfides (Schieber 1999).
Ancient limestones with concentrically laminated
iron ooids
are of great economic importance. The iron
minerals may be limonite, hematite, magnetite, sider-
ite or chamosite. Iron oxides and chamosite are com-
