Geology Reference
In-Depth Information
Plate 13 Ooids: Key Criteria in Microfacies Studies
Ooids are spherical particles with concentric laminae coating a nucleus. Modern marine ooids consist of arago-
nite or High-Mg calcite, or they are bi- or polymineralic. Calcareous ooids are classified according to (a) the
primary microfabric (tangential, radial, micritic), (b) number of laminae (normal ooids with many laminae, -> 3;
superficial ooids with few laminae, -> 7), and (c) shapes (-> 5, 8; compound ooids). Primary microfabrics are
environmentally controlled: Tangential ooids originate predominantly in high-energy environments, radial oo-
ids preferentially as 'quiet-water ooids' in low-energetic marine and non-marine environments (e.g. salt lakes,
-> 2). Micrite ooids are caused by different processes (e.g. total micritization, -> 6; precipitation within biofilms,
-> 5). Transport processes and diagenesis can change shape and composition of ooids (-> 1, 4 , 7, 8). Ooids are
common constituents of shallow-marine carbonates, but also originate in non-marine settings, e.g. in hypersa-
line lakes and lagoons, and freshwater lakes. Wind-transported ooids form conspicuous eolianites. Genetic hy-
potheses focus on combined microbial-mechanical processes, chemical precipitation and chemical-physical pro-
cesses. Current research favors strong controls by organic compounds (Morse and Mackenzie 1990). Some
authors believe that microbes actively participate in the construction of ooid fabrics. Others suggest that mi-
crobes have a mainly destructive role or that they are only passively involved in building ooid layers by trapping
particles or by providing a favorable substrate for nucleation of crystals.
Ooids are of key importance in microfacies analysis: (a) They record paleoenvironmental conditions (water
energy, transport, salinity, depositional sites), if details of microfabrics, shape and sorting are considered (Fig.
4.26; Pl. 120/1-2); (b) Ooids indicate changes in carbonate mineralogy and oceanographic conditions through
the Phanerozoic; and (c) Oolitic limestones are important reservoir rocks for hydrocarbons, owing to high fab-
ric-selective inter- and intragranular porosities, and contribute to the formation of ore deposits.
1 Tangentially structured, primarily Mg-calcitic ooids. The picture shows spherical ooids and 'cerebroid' ooids (arrow)
with cones. The origin of cerebroid Mg-calcite ooids has been related to dissolution and precipitation by bacteria (Richter
1983). Cerebroid ooids are common in evaporitic settings (e.g. saline lakes), but also occur in marine environments. The
photograph shows an original thin-section of Kalkowsky (1908) from the
Rogenstein
. Early Triassic (Lower Bunter): Bad
Harzburg, Germany. The environment was a playa lake with variable salinity. Ooid-bearing beds are intercalated within
mainly siliciclastic successions and occur in close association with laminated stromatolites. This association and the
almost complete absence of fossils is characteristic for restricted brackish or saline environments.
2 Modern aragonitic radial and radial-concentric ooids. The small and partly distorted radial ooids have nuclei made of
micritic clots. The cortex consists of radially arranged crystals. The larger radial-concentric ooids (top left) are sub-
spherical in shape. Radial-fibrous crystals build the inner part of the ooid and occur within the concentrically arranged
outer laminae. Nuclei are lithoclasts and fecal pellets of the brine shrimp. The onset of radial-concentric growth is size-
controlled and indicates changes of low and high energy conditions. Great Salt Lake, Utah. This lake is regarded as a
modern scenario for ancient salt lake microbialites and a place where bimodally sorted aragonitic radial ooids are being
formed under hypersaline low-energy conditions. Ooid formation takes place close to the oxic/anoxic interface within the
sediment, and appears strongly controlled by the composition of organic matter of biofilms attached to the ooids.
3 Radial-fibrous ooids periodically encrusted by sessile foraminifera. The biogenic encrustations took place during phases
without movement of the ooids. Note the multi-stage development of the large ooid. Ongoing encrustation would result in
the formation of an oncoid. Late Triassic (Carnian): Carinthia, Austria.
4 Radial-concentric quiet-water ooids and purely radial ooids (PRO). Note the radial-fibrous structure of the large ooids
characteristic for formation in low-turbulence environments and the bimodal composition. Ooids exhibit moderate com-
paction (C) indicated by some interpenetrating grains. Breaks in ooid growth, regenerated broken ooids (left) and healed
cracks (desiccation?), differences in shapes, and meniscus cements (arrows) indicate reworking in tidal zones (Simone
1974). SMF 15-R. Early Cretaceous (Neocomian): Monte Camposauro, southern Apennines, Italy.
5 Microbial ooids, formed in situ within a microbialite (see Fig. 4.24). Thin wavy peloid laminae alternate with ooids
(arrows) and peloids. The small peloid nucleus is followed by a microspar rim and an outer micrite rim. The ill-defined
laminated structure and shape of these ooids in combination with the occurrence of variously sized grains is a common
feature of ooids formed in situ. Similar criteria are known from ooids of Late Jurassic sponge 'reefs'. Cretaceous (Al-
bian): Cantabria, Spain.
6 Micritized tangential ooids. The tangential fabric is only recognizable in peripheral parts of a few ooids. Note the very
weak compaction. The depositional style reflects the situation 1 of the Carozzi model (Fig. 4.25), characterizing ooid
formation and deposition in high energy zones. SMF 15-C. Oolite shoal, inner ramp environment. Early Carboniferous:
Tournai, Belgium.
7 Weakly compacted grainstone with partly dissolved superficial ooids ('oomolds'). Note the high intragranular porosity
caused by dissolution of the originally aragonitic cortices and/or nuclei. Intergranular pores are filled with granular and
drusy meteoric cements (see Pl. 29/3, 4). Pennsylvanian (Missourian): Utah, U.S.A.
8 Selectively dolomitized ooids. Note the distinct compaction of the grains indicated by the deformation of the ooids. The
arrow points to distorted ooids (see Fig. 4.23). Cambrian: Yoho Valley, Rocky Mountains, British Columbia, Canada.
-> 5: Neuweiler 1993
