Geoscience Reference
In-Depth Information
Ooids typically form in agitated water of no
greater depth than 15 m; more typically the
depth is 5 m; Most oolitic rocks are marine.
nucleus, skeletal fragment
or quartz grain
SUPERFICIAL
OOID
single lamella
radial fibrous calcite in
most ancient ooids
diameter
typically
0.2 to 1 mm
COMPOSITE
OOID
PISOID
(PISOLITH)
ONCOID
(ONCOLITH)
Oncoids are pisoids of algal origin.The algal
layers are wavy in cross-section and often
incorporate other bioclasts and grains.
diameter > 2 mm
diameter > 2 mm
PELOID
Peloids are pellets or amorphous grains composed of micrite, typically
0.1-0.5 mm diameter; most are micritized skeletal grains or faecal pellets.
BIOCLASTS
if originally
calcitic
BIVALVE
GASTROPOD
variable shape
structure
preserved
if originally
aragonitic
or
neomorphic calcite
with relics of internal
structure
typically drusy
sparite with no
internal structure
drusy sparite,
no internal structure
preserved
variable shape
single calcite
crystal
cloudy
appearance
syntaxial
overgrowth
of sparite
BRACHIOPOD
ECHINODERM
original structure
often preserved
endopunctae or
pseudopunctae
echinoid spines
variable shape
BRYOZOA
FORAMINIFERA
variable shape
Figure A6.10
Main limestone-forming grains.
