Department of Petroleum Geology & Sedimentology,
Faculty of Earth Sciences,
King Abdulaziz University,
Jeddah, Saudi Arabia
(EPS 321)
CARBONATES AND EVAPORITES
Lecture 03: Components of carbonate sediments:
1) Non-skeletal components
(ooids, pisoids, oncoids)
Prof. Dr. Mahmoud A. M. Aref
Components of limestones
1)
2)
3)
4)
Non-skeletal (Ooids, Pisoids, Oncoids, Pellets, Peloids,
aggregates and intraclasts)
Skeletal
4
Micrite
3
Cement
1
1
2
1
1
2
3
EPS 321 Lecture 03
Non-skeletal components of limestone
„
„
„
Ooids, pisoids, oncoids
pellets, peloids
aggregates and intraclasts
EPS 321 Lecture 03
1. OOIDS (< 2mm) AND PISOIDS (> 2mm)
„
Composition: aragonite or high-Mg calcite
Modern ooids form in:
„
a. agitated shallow water
(concentric, random or tangential aragonite needles)
„
b. quiet water lagoon or ponds
(radial structure)
„
c. supratidal-intertidal setting
(concentric asymmetric ooids)
„
d. high-energy water
(dull, radial, aragonite or low-Mg calcite)
„
e. calcareous soil (concentric str.)
EPS 321 Lecture 03
Ancient ooids:
„
Composition: low-Mg calcite, dolomite, silica
„
Primary calcite ooids
(high or low energy) have radial wedge-shaped structure.
„
Primary aragonite ooids
(high energy; concentric tangential),
(quiet water; radial)
EPS 321 Lecture 03
ORIGIN OF OOIDS
„
Inorganic precipitation: agitated, shallow tropical area, CO2
degassing, high temperature.
„
biochemical precipitation: precipitation of carbonate within mucilage
of bacterial activity in the organic matter (quiet water condition).
Factors controlling primary mineralogy of ooids:
„
Water chemistry, PCO2, Mg/Ca ratio, carbonate saturation, water
agitation
„
Aragonite and high Mg-calcite form in: PCO2 low, Mg/Ca high (high
carbonate saturation favor aragonite than high Mg-calcite)
„
Low Mg-calcite form in: PCO2 high, Mg/Ca low
EPS 321 Lecture 03
EPS 321 Lecture 03
Oolite sands with some quartz from Utah
EPS 321 Lecture 03
Aragonitic Ooids
EPS 321 Lecture 03
Calcitic Ooids
EPS 321 Lecture 03
PISOIDS
Pisoids are spherical grains composed of
concentric layers of diameter greater than 2
mm, but typically 5-8 mm .
EPS 321 Lecture 03
ONCOIDS
They are layered spherical growth structures formed by
cyanobacterial growth. Oncolites are very similar to
stromatolites, but instead of forming columns form
approximately spherical structures
EPS 321 Lecture 03