Page 25 - Carbonate Facies in Geologic History

12 Principles of Carbonate Sedimentation

Depositional Texture recognizable Depositional texture
not recognizable
Original components not bound together Original components
during depositions were bound together
during deposition ... as
Contains mud shown by intergrown
(particles of clay and fine silt size) Lacks mud skeletal matter, lami- Crystalline carbonate
and is grain- nation contrary to
Mud-supported Grain- supported gravity, or sediment-
supported
Less than More than floored cavities that
10% grains 10% grains are roofed over by (Subdivide according
organic or question- to classifications
ably organic matter designed to bear on
and are too large to physical texture or
be interstices. diagenesis.)
Mudstone Wackstone Packstone Grainstone Boundstone

Fig.I-5. Classification of Carbonate rocks according to depositional texture from Dunham
(1962, Table 1), with permission of American Association of Petroleum Geologists

3. Grain Kind

In addition to purely textural parameters, all modern sedimentary classifications
recognize more or less the same basic grain types. These are:
Intraclasts or lithoclasts: Large particles derived by desiccation breakage or
burrow disruption of penecontemporaneously deposited carbonate sediment.
(Lithoclasts may be also externally derived from older lithified rock and must be
specially designated.)
Ooids: Spherical, multiple coated particles in which the laminae are smooth
and constitute a relatively thick coating are termed "well-formed ooids" in the
volume. Oolitically coated particles (superficial ooids) in which only one or two
laminae exist and which commonly retain the original grain shape are also preva-
lent. Well-formed ooids are products oftidal action.
Bioclasts: Fragmented tests, shells, or skeletons.
Peloids or pelletoids: Fecal pellets and rounded micritic grains of other origin.
Aggregated lumps or grapestones: These are agglutinated lumps of peloids
and ooids; they also may be coated.
Onkoids: Algally coated grains, generally over 2 mm in diameter. Coats are
generally irregular or crinkly.
Definitions ofthese basic grain types are given by Powers, Folk, Leighton and
Pendexter (see A.A.P.G. Memoir I, 1962), and they are used by most other descri-
bers of carbonate rock textures. They are also defined and well illustrated by
Horowitz and Potter (1971, pp.7, 8) and by Milliman (1974). The framework
parameter of Dunham cannot be accurately employed without a good under-
standing of grain type and visualization of their shapes.
In naming carbonate textural types the application of terms of particle kind
varies somewhat. Dunham and Leighton and Pendexter preface textural terms
with a designation of principle particle kinds: pellet lime wackestone (Dunham,